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Robust miniaturized low-cost and low-power gas sensors that can accurately detect and quantify important gaseous analytes in real-time and be in continuous use are needed for a broad range of applications. In this book chapter, we discuss the challenges and requirements for real-time gas sensing. We focus on elucidating ionic liquids (ILs), the non-volatile environmentally benign media, and their multiple benefits for real-time gas sensing applications. In particular, we demonstrate nanomaterials coupled with ILs, as electrolytes and solvents, which offer great opportunities in electrochemical gas sensors with high sensitivity and selectivity for real-time and continuous-use gas detection in real-world harsh conditions. We discuss various methods of immobilization of ILs on transducer electrodes to prepare IL thin films for chemical analysis (gas detection) in which ILs’ unique solvation properties, varying with the nature of the constituent ions, render them ideal for selective gas detection strategies in array-based piezoelectric mass sensors. We show examples of using ILs coupled with low-cost, low-power, and miniaturized mass sensors for high temperature gas sensing applications. In addition, we illustrate an example to show that the absorption of analytes and the redox behavior of electro-active analytes in IL sensing films allow simultaneous detection of two properties of the same target analyte, i.e., sorption/partition and redox activity via electrochemical and piezoelectric mass transducers in real-time allowing one to cross-validate the measurement results, enhancing selectivity and accuracy for gas detection.

Our natural environment, the biosphere (i.e., the thin layer of life and life support) contains the earth’s air, soil, water, and living organisms. Human health and well-being are directly linked to the state of our environment. We depend on clean air and water, fertile land for food production, and energy and material inputs for its production. After thousands of years of societal evolution, we are at the peak of technology. However, the rippling of consequences of our technological success is that we are also at the peak of pollution manifested by the extinction of species due to air pollution, the melting of glaciers due to greenhouse gas emission, and the need for alternative clean energy to sustain our environment. One of the biggest challenges of the 21st century lies in meeting the ongoing demand of the growing population and continuous economic growth which has, in turn, caused irrevocable environmental damage. For example, we are now being exposed to hundreds or even thousands of chemicals every day. Many of these substances appear to be almost completely harmless, others potentially harmful but unlikely to get into our bodies in a large enough quantity, while others may be incredibly harmful, even in very small quantities. The most common way for harmful chemicals to get into our body is through the air that we breathe. As a result, air pollution is commonly perceived as one of the world’s largest health and environmental problems.

The most common gases in the natural environment can be classified into two major types: gas molecules and volatile organic compounds. The periodic table is redrawn to depict the physical states of the various elements (Figure 1.1).

Figure 1.1

Periodic table of elements highlighting the solids (green), liquids (blue), and gases (red) at room temperature (about 22 °C).

Figure 1.1

Periodic table of elements highlighting the solids (green), liquids (blue), and gases (red) at room temperature (about 22 °C).

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There are common gaseous and volatile compounds in our environment that require constant monitoring for various applications (Table 1.1).

Table 1.1

Common gas analytes and their categories.

Gas targets Categories of gases Ref.
CO2, O2, N2, CO  Atmospheric gases  3 and 4   
CO2, CH4, N2O, H2 Greenhouse gases  6   
CH4, H2   Flammable gas leaks  3   
VOCs, SO2, NOx   Toxic gases  4   
H2   Energy fuel  7   
O2, CO2, N2, He  Asphyxiates and oxygen  8   
Amines, ethylene, alcohols, food aromas  Food quality and storage  3   
O2, NH3, CO2, CH4, H2, VOCs, acetate  Bioreactors and landfill  9   
CO, VOCs  Cabin air quality  3   
HCs, O2, CO2, CO, NOx   Exhaust air gases  7   
CO2, NOx   Outdoor emissions  6   
O3, NOx, formaldehyde, cleaning solvents  Indoor air quality  4   
NH3, chlorofluorocarbon (CFCs)  Refrigerants and intensive farming  3   
O3, NOx, bioparticulates  Asthma at home  7   
VOCs, CH4   Nuisance odors  8   
TATP, HMTD, nitro-explosives, bio-hazardous chemicals  Explosives, chemical and biological attacks  9   
Gas targets Categories of gases Ref.
CO2, O2, N2, CO  Atmospheric gases  3 and 4   
CO2, CH4, N2O, H2 Greenhouse gases  6   
CH4, H2   Flammable gas leaks  3   
VOCs, SO2, NOx   Toxic gases  4   
H2   Energy fuel  7   
O2, CO2, N2, He  Asphyxiates and oxygen  8   
Amines, ethylene, alcohols, food aromas  Food quality and storage  3   
O2, NH3, CO2, CH4, H2, VOCs, acetate  Bioreactors and landfill  9   
CO, VOCs  Cabin air quality  3   
HCs, O2, CO2, CO, NOx   Exhaust air gases  7   
CO2, NOx   Outdoor emissions  6   
O3, NOx, formaldehyde, cleaning solvents  Indoor air quality  4   
NH3, chlorofluorocarbon (CFCs)  Refrigerants and intensive farming  3   
O3, NOx, bioparticulates  Asthma at home  7   
VOCs, CH4   Nuisance odors  8   
TATP, HMTD, nitro-explosives, bio-hazardous chemicals  Explosives, chemical and biological attacks  9   
For example, gaseous molecules including hydrogen (H2), nitrogen (N2), oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), water vapor (H2O), helium (He), neon (Ne), argon (Ar), ozone (O3), nitrogen dioxide (NO2), nitrous oxide (N2O), nitric oxide (NO), sulfur dioxide (SO2), hydrogen sulfide (H2S), hydrocarbons (e.g., methane (CH4) and ethane (C2H4)) are either released naturally or emitted in certain applications. Among common gaseous molecules, methane, the major constituent of natural gas, is one of the most abundant US energy resources. Its use is being expanded to meet the rising energy needs in industry, homes, and truck transportation over the next decade. Methane is an explosion hazard in enclosed areas. Since methane is also the constituent of landfill gas and mine gas, methane explosions occur in mines when a build-up of methane gas, a by-product of coal, comes in contact with a heat source, and there is not enough air to dilute the gas to levels below its explosion point. The lower and upper explosive limits of methane in air are 5% (LEL) and 15% (UEL), respectively. The warning percentage is often set to 0.5–1%. Besides methane, CO, CO2, and H2S are present in underground coal mines. The high concentration of CO2 in confined spaces causes a diminution of the oxygen available to breathe. The suffocating mixture of CO2 and other unbreathable gases built-up in mines causes poisoning, asphyxiation, and ultimately death. According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the optimum CO2 concentrations range from 350–800 ppm for outdoor spaces and 1000 ppm for indoor spaces, respectively. Above this critical limit, people can experience headaches, sore throats, and nasal irritation. Due to the need for CO2 monitoring in places such as food industries related with carbonated beverages, agricultural industries related with the production of fertilizers, medical centers where incubators are employed, and indoor places such as schools, offices, etc., the demand for CO2 sensors with high accuracy, fast response, and low cost has increased during the last five years. Additionally, it is of the utmost importance to regulate and monitor CO2 levels in wide operating temperatures. Sulfur dioxide (SO2) is another such harmful gas, which is colorless with a strong odor. The primary SO2 sources are anthropogenic such as the burning of high-sulfur coals and heating oils in power plants. Natural causes such as volcanoes contribute anywhere from 35–65% of total sulfur dioxide emissions annually. When released into the atmosphere, sulfur dioxide can react to form acid rain according to the following reactions:
SO 2 + OH ˙ HOSO 2 ˙
(1.1)
HOSO 2 ˙ + O 2 HO 2 ˙ + SO 2
(1.2)
SO 3 ( g ) + H 2 O H 2 SO 4 ( aq )
(1.3)
Carbon dioxide, methane, nitrous oxide, and water vapor (which all occur naturally), and fluorinated gases (which are synthetic) are greenhouse gases. Greenhouse gases cause harmful effects in the environment by absorbing infrared radiation. One of the main greenhouse gases highly produced in the environment due to the consumption of fossil fuels is carbon dioxide (CO2). The excessive presence of CO2 affects the energetic equilibrium of the earth because it absorbs the short wavelength radiation reflected from the earth to space, causing an increase in the temperature of the earth. Consequently, this affects climate patterns producing droughts, floods, and ice melting at the poles. N2O is a greenhouse gas, and it can also be toxic for human health since it interacts with vitamin B12 resulting in selective inhibition of methionine synthase, a key enzyme in methionine and folate metabolism. Long-term exposure to high concentrations of N2O may cause megaloblastic bone-marrow depression and neurological symptoms. Exposure to higher doses for less than six hours, as in clinical anesthesia, are considered harmless.

Oxygen (O2), nitrous oxide (N2O), helium (He), nitrogen (N2), and carbon dioxide (CO2) are of the utmost importance in the regulation and operation of medical laboratories and hospitals. Such gases used in laboratories are colorless, odorless, and tasteless, which makes them hard to detect if there is a gas leak. These gases are used for a wide range of medical procedures, e.g., N2O is used for anesthetic purposes, N2 is used as a freezing agent. Artificial ventilation with medical air and oxygen is required by patients in intensive care, with restricted lung function, or during an operation under anesthetic. CO2 is essential during surgery as the stomach of a patient is filled with the gas to create space for examining an affected area. Hence, a gas leak from a cylinder or fixed piped gas system can cause potentially fatal accidents. Also, long-term exposure to anesthetics is extremely dangerous since they gradually cause chronic effects of the respiratory, cardiovascular, reproductive, and central nervous system. It is known to displace oxygen, causing asphyxiation, thus affecting respiration. Another major type of gaseous compound at room temperature is volatile organic compounds (VOCs). An organic compound is defined as any compound containing at least the element carbon and one or more of hydrogen, halogens, oxygen, sulfur, phosphorus, silicon, or nitrogen. Carbon dioxides, inorganic carbonates, bicarbonates, methane, ethane, CO, CO2, organometallic compounds, and organic acids are excluded from this definition. VOCs have a high vapor pressure and are emitted as gases from certain solids or liquids, and they typically have low water solubility.

Air pollution is often classified into two contexts: indoor (household) air pollution and outdoor air pollution. Concentrations of many VOCs are consistently higher indoors (up to ten times higher) than outdoors. VOCs can be emitted by a wide array of sources including paints and lacquers, paint strippers, cleaning supplies, pesticides, building materials and furnishings, office equipment such as copiers and printers, correction fluids and carbonless copy paper, graphics and craft materials including glues and adhesives, permanent markers, and photographic solution. As a result, VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects. The monitoring of common gases and VOCs is important in air quality control, occupational health and safety, biomedical diagnostics, industrial process control, and military and civilian counterterrorism (Figure 1.2). A gas detection system for medical laboratories and hospitals can stretch from the basement, where the gases are stored and produced, via a conduit system, to the corridors, to terminal units in operating rooms, intensive care areas, laboratories, and treatment rooms.

Figure 1.2

Schematic depicting the release of gases into the atmosphere from various anthropogenic activities. The primary pollutant gases are directly emitted from sources such as factories, towns and homes, vehicle exhausts, agriculture, shipping, airplanes or from natural causes such as wildfires and volcanoes from the mountains. The secondary air pollutants on the other hand, are a result of reactions between the primary pollutants in the atmosphere.

Figure 1.2

Schematic depicting the release of gases into the atmosphere from various anthropogenic activities. The primary pollutant gases are directly emitted from sources such as factories, towns and homes, vehicle exhausts, agriculture, shipping, airplanes or from natural causes such as wildfires and volcanoes from the mountains. The secondary air pollutants on the other hand, are a result of reactions between the primary pollutants in the atmosphere.

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A traditional gas analysis technique is gas chromatography (GC), which separates gaseous samples into its components followed by detection using various detectors including a flame ionization detector (FID), an atomic emission detector (AED), and mass spectrometry (MS). Among the techniques, gas chromatography–mass spectrometry (GC–MS), in which a MS detector provides the sensitive and selective detection of compounds after chromatographic separation, is the most powerful gas analysis technique. GC–MS has high selectivity and can provide specific compound structural information based on the analyte’s fragmentation pattern and mass ions allowing for the quantification of a compound in the presence of co-eluting compounds. Another similar hyphenated analytical method for gas analysis is the GC–IMS (the ion mobility spectrometry (IMS) coupled to GC), providing compound selectivity by measuring unique gas phase mobilities of characteristic analyte ions. In addition, spectroscopic methods such as infrared spectroscopy (IR) for the sensing and identification of gases have shown great promise, owing to their inherent non-invasive nature, relative simplicity, and good selectivity. The selectivity of gas sensing with IR is based on the “fingerprint” absorption in the mid-infrared (2–20 mm), where molecular vibrations often provide a unique signature. Both incoherent (e.g., Fourier-transform infrared, FTIR) and laser-based coherent sources have been commonly employed. They are sensitive for the detection of many gases including greenhouse gases such as CO, CO2, and CH4, chemical etchants such as HCl and HF, and common smokestack pollutants such as SO2 and N2O. Despite these promising results, significant challenges remain in the spectroscopy-based gas analysis technique, particularly in expanding the range of gases amenable to spectroscopic detection.

Gas analytes have a high rate of diffusion and gas sampling is needed for most gas analysis. Since most gas and VOC samples encountered in a laboratory are not in a form that can be directly placed into an analytical instrument, sample preparation is an essential part of gas analysis using chromatographic and spectroscopic techniques (Figure 1.3).

Figure 1.3

Flowchart depicting the sample analysis workflow in traditional gas analysis instruments. Sample preparation is an essential part of gas analysis using gas chromatographic and spectroscopic techniques. Improper collection methods or handling in transit may destroy sample integrity.

Figure 1.3

Flowchart depicting the sample analysis workflow in traditional gas analysis instruments. Sample preparation is an essential part of gas analysis using gas chromatographic and spectroscopic techniques. Improper collection methods or handling in transit may destroy sample integrity.

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The sample preparation could be as simple as “dilute and inject” or as complex as multistage sample handling. Typically, a gaseous sample is pulled into an evacuated glass, metal bulb or canister, or by a syringe; the gas can also be pumped into a plastic bag or another inert container. Solid phase trapping and liquid phase trapping have also been developed. In solid phase trapping, a gaseous sample is passed through a tube packed with adsorbent (e.g., silica gel, activated carbon), and the trapped analytes are eluted with a strong solvent. In liquid phase trapping, a gaseous sample is bubbled through a solution that is a good solvent for the analytes in which the analyte should have a higher affinity for the solvent than it does for the gas. Sample collection in the field and transportation of samples to a lab are major steps for the proper quantification of samples. Improper collection methods or handling in transit may destroy sample integrity. Sample collection in the field, transporting samples to labs, sample processing, and quantification using analytical tools such as gas chromatography are not only time-consuming but may also produce inaccurate results due to changes in the sample conditions. As shown in Figure 1.4(a), in the case of chromatographic analysis, sample processing takes of 61% of the time period whereas collection and analysis require 6% of the total time followed by data management, which takes up 27% of the time period.1  Figure 1.4(b) shows a histogram depicting errors in conventional gas detection techniques. Not only is the sample processing step time-consuming but it also contributes to 30% of the sources of error in chromatography analysis. Volatile analytes that are labile, thermally unstable, or prone to adsorb to metal surfaces in the vapor state present more challenges for gas sampling and sample preparation.

Figure 1.4

(a) Histogram showing the time taken for a typical analysis of a sample by gas chromatography (GC); (b) errors of each step in sample analysis by GC.

Figure 1.4

(a) Histogram showing the time taken for a typical analysis of a sample by gas chromatography (GC); (b) errors of each step in sample analysis by GC.

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There is an increasing demand for miniaturized gas monitoring technology that allows the detection of the concentrations of various gaseous components in the field without sampling and sample transports for a broad range of applications, especially for environmental health, greenhouse gas emission and climate change, and industry safety applications. A few examples are briefly summarized below.

The mine explosions due to methane at the Sago (2006), Darby (2006), and Crandall Canyon (2007) mines in the USA are clear evidence of the danger to workers that may worsen as mining expands to meet rising energy demands. To prevent explosions and exposure to toxic gas concentrations, real-time gas sensors are needed that can quickly and accurately measure explosive gases within the mix of gases and variable environmental conditions in mines. In another example, hydrogen is the most important molecule for energy, and has been widely used in many applications from aerospace and transportation to fuel cells and satellite power supply. Although hydrogen is not toxic, it is flammable and dangerous because when hydrogen is mixed even in small amounts with ambient air, ignition/explosion can occur at a volumetric ratio of hydrogen to air as low as 4% due to the oxygen in the air. Thus, monitoring hydrogen leaks is essential for the use of hydrogen in hydrogen production and transportation, and all hydrogen-based applications.

Many respiratory diseases such as asthma can be linked to environmental exposure to hazardous pollutants. For example, it is now well established that pollutants such as NO2, SO2, ozone, and formaldehyde are asthma-triggering agents, and are often found in more than tolerable concentrations in ambient environments. Health risks due to these environmental polluting agents are often compounded by other factors such as genetic predisposition, smoking, obesity, and high blood pressure, and can pose serious health problems for individuals with persistent exposure to such pollutants. The current methods for identifying links between individual environmental exposures and their disease states are typically performed by periodically collecting air samples of living and working environments such as in the home and office, and by performing individual exposure surveys. Such offline analysis on periodically collected samples suffer from the following shortcomings: (a) they fail to capture fine grain temporal and spatial variation of the pollutant concentrations often leading to inconclusive results; (b) they are insufficient for establishing a causal relationship between an individual’s exposure profile of specific pollutant agents and their resulting disease symptoms and severity; and (c) since the monitoring process is decoupled from an individual’s vulnerabilities to specific pollutant agents, it is not possible to use such a generalized analysis system for alerting patients in an individual-specific and customized manner.

Current climate change and carbon cycle models consider the levels of CO2 and CH4 as critical parameters to predict future greenhouse gas emissions in the Arctic. To understand the changing properties of permafrost in the tundra ecosystem and to provide reliable data for model prediction and to advance our understanding of biogeochemical processes in the warming Arctic, long-term continuous and in situ measurements of greenhouse gas emission from tundra soils must be conducted. Traditional methods to quantify soil-gas turnover involve sampling of soil-gas at different depths in the soil profile. In most cases, the gas samples are collected with a syringe, transferred to an airtight container, and measured ex situ using analytical tools such as gas chromatography. These methods are not only time-consuming but can also produce inaccurate results due to changes in soil conditions. Besides, since soil is a complicated multi-phase system, quantification of gases in soil water and soil pores requires not only temporal resolution but also spatial resolution. To address these critical gas sensing needs, sensor technologies are needed to perform the analysis not only in real-time, but also in a distributed manner, so that through the spatial-temporal distribution of gas analytes we can understand the link between their short- and long-term impacts for a broad range of applications.

The increasing need for gas sensors in a wide range of applications drives the substantial growth in the commercialization of gas sensing technologies. The global gas sensor market size was valued at $823.1 million in 2019, and is projected to reach $1336.2 million by 2027, growing at a CAGR of 6.4% from 2020 to 2027.2  The gas sensing market is expanding into several areas related to human healthcare and environment, security, and sustainable economy. However, the technological priorities and applicability in real-time environments are associated with cost and reliability of the sensing device.

The current gas sensing market is estimated to be $6.3 billion with a 10% growth rate for new technologies with the major share of the market being focused on the development of gas sensing instruments for new and improved gas sensors.4,20  A lesser growth rate (4%) in existing technologies is representative of the challenges that the market is facing in terms of application in complex backgrounds, the ability to identify abnormal variations, relating to the stability of the sensing materials.

According to recent statistics (Figure 1.5), the NOX sensors contribute to the highest share in the market of gas sensors followed by differential pressure sensors (21%). The mass airflow sensors (MAF) and manifold absolute pressure (MAP) sensors contribute to 25% of the market share. The oxygen sensors, on the other hand, contribute to 8% of the revenue generated from gas sensors.3  Hence, the motivation of present research lies in the development of sensors which have the potential to offer advantages over existing sensors in terms of volume, cost, compactness, power consumption, and responsiveness, thus having the potential to open up new application areas, displacing existing technologies or creating new markets.

Figure 1.5

Schematic representing the statistics of the gas sensing market in terms of generated revenue and expected growth for varying technologies.

Figure 1.5

Schematic representing the statistics of the gas sensing market in terms of generated revenue and expected growth for varying technologies.

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Gas sensors work on the principle of determining the concentration of gas analytes through the chemical and physical effects of gas analytes that interact with an energy input or a gas sensing material. A typical gas sensor is an analytical device comprising three major components: (i) a transducer that can convert the presence of a gas analyte into some form of readable signal; (ii) a sensing material which is usually in contact with the transducer allowing the detection and quantification of the gas analyte through a specific interaction mechanism; and (iii) signal processors and readout devices (data acquisition, lodging, and a processing unit) that can convert the sensor’s input signal to a readable signal (Figure 1.6). The sensing interface allows the interaction of the gaseous analytes with the sensing material and the transducer allows the conversion of the corresponding chemical information of the analyte into an electrical signal.3  The sensor system typically integrates the sensor detection components with associated electronics and readout devices (transducers) allowing a user to directly obtain information on the analyte. The operating principles of diverse transducers can be classified into the following groups: optical sensors,5–7  electrical sensors,8–10  mass (or gravimetric) sensors,11–14  magnetic sensors,15,16  thermometric sensors,17,18  radiation sensors,19  and electrochemical sensors.20–24  Optical sensors are typically based on UV-visible absorption, fluorescence, phosphorescence, chemiluminescence, surface plasmon resonance, and surface-enhanced Raman scattering techniques.

Figure 1.6

Schematic of the components of a typical gas sensor.

Figure 1.6

Schematic of the components of a typical gas sensor.

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The gas sensor performance factors are very important for a specific application. Table 1.2 summarizes those key analytical factors that need to be established for any gas sensor including sensitivity (i.e., the slope of the response), limit of detection (LOD), drift, offset of the response, and range of linearity, etc. Gas sensors allow monitoring and measuring of gaseous and volatile chemical molecules under various conditions in the local environment. To be broadly applicable, these sensors also need to be small, accurate, reliable, and long-lasting. The information collected by such miniaturized gas sensors, when combined with communication and information technology, allows application to diverse areas which had not previously been possible. The next-generation gas sensor technology is gradually moving from bulky and expensive instruments to miniaturized low-cost and low-power gas sensors. The miniaturization of gas sensors will be fundamentally important as low-cost/low-power solutions for incorporation into smart electronic devices. This will enable sensors to provide immediate feedback related to the molecular and cellular microenvironments – just like our own five senses that can provide immediate feedback of the world around us – for a broad range of applications including critical healthcare, safety, industrial hygiene, process controls, product quality controls, food safety, climate change, manufacturing, human comfort controls, emission monitoring, automotive, clinical diagnostics, home safety alarms, and homeland security.

Table 1.2

Performance factors of gas sensors for real world applications (example applications).

Three characteristics = reliable measurement: precision, accuracy, validity
Precision  The degree of mutual agreement among the data (e.g., standard deviation) 
Accuracy  The deviation from the true value (e.g., absolute error) 
Validity  The validation process of proving that an analytical result is acceptable 
Three characteristics = reliable measurement: precision, accuracy, validity
Precision  The degree of mutual agreement among the data (e.g., standard deviation) 
Accuracy  The deviation from the true value (e.g., absolute error) 
Validity  The validation process of proving that an analytical result is acceptable 
Signal (S), Sx = S − S0, signal provides data for the five analytical “S” of a sensor
Sensitivity  The slope (m) of the calibration curve (Sx vs. [X] curve at [X]i: dS/dx), the reproducibility or precision of the measuring device 
Selectivity  The relative sensitivities: mx /my ability to discriminate between different chemical species. The function of the selective component of a sensor 
Speed of response  t90 or t95 (the necessary time for achieving signals of 90% or 95% of the final equilibrium value) 
Stability  Stability of the signal vs. time, concentration, matrix, temperature, pressure, etc., over short time = noise, over long time = drift 
Cost  Logistics (size/shape, weight, power, application specific) 
Signal (S), Sx = S − S0, signal provides data for the five analytical “S” of a sensor
Sensitivity  The slope (m) of the calibration curve (Sx vs. [X] curve at [X]i: dS/dx), the reproducibility or precision of the measuring device 
Selectivity  The relative sensitivities: mx /my ability to discriminate between different chemical species. The function of the selective component of a sensor 
Speed of response  t90 or t95 (the necessary time for achieving signals of 90% or 95% of the final equilibrium value) 
Stability  Stability of the signal vs. time, concentration, matrix, temperature, pressure, etc., over short time = noise, over long time = drift 
Cost  Logistics (size/shape, weight, power, application specific) 

Due to advances in portable electronics and microsensor technologies, a tremendous opportunity exists to develop gas sensor systems that can address the need to monitor key gas analytes in real-time in a lightweight, low-power, highly autonomous platform that overcomes the many practical limitations in traditional gas analysis techniques (gas chromatography combined with thermal energy analysis (GC/TEA), or mass spectrometry (GC/MS) and chemical ionization coupled to tandem mass spectrometry, IR and Raman spectroscopy, ion mobility spectrometry, H1 and C13 NMR, HPLC that have relative high costs, require extensive operator skill, and have limited field portability). The application demands of current gas sensor design specifications include:

  1. Utility: reliable and accurate measurement of one or multiple gases in a single module.

  2. Portability: small, lightweight, battery (or externally) powered for long durations.

  3. Practicality: low cost, easy to install, autonomous operation (no user training or inputs required).

  4. Compatibility: usable with existing methods and adaptive to future instruments/networks.

There have been significant efforts to develop miniaturized and real-time gas sensors. For example, miniaturized optical sensors such as non-dispersive infrared (NDIR) sensors have been developed since they provide high sensitivity when coupled with very sensitive optical detectors, and high selectivity, because absorption lines are usually unique for the gas under detection. However, NDIR sensors necessitate a relatively complicated optical system and typically require operation by trained technical personnel, making them expensive and bulky. Furthermore, optical sensors typically cannot provide real-time monitoring. The three most common candidates for miniaturized gas sensors in mine safety applications are electrical sensors using metal oxide sensing materials (e.g., stannic oxide), mass sensors using an acoustic transducer, and electrochemical sensors based on the interface of electrochemical reactions. Metal oxide sensors6  are based on the catalytic oxidation of reducing gases on the catalytic metal oxide surface. They have limited selectivity because all reducing gases in the atmosphere will inevitably be detected. Some metal oxide semi-conductive sensors measure a resistance change due to analytes such as gas adsorption; however, they take a relatively long time to reach equilibrium for the sorption of analytes from the gas phase onto the metal oxides, especially for porous materials. In addition, the dependency of metal oxide resistance on the analyte (e.g., vapor) concentration is not linear, which reduces the accuracy of quantitative analysis.3,4  As mass is one of the fundamental properties of an analyte, mass sensors measure the mass change by the accumulation of the analyte at a specific surface. As a result, a mass sensor requires the use of sensing materials to provide sensitive and selective detection of an absorbed gas on the coating surfaces using a variety of transducers including Bulk Acoustic Wave, Surface Acoustic Wave, Capacitive Micro-machined Ultrasonic Transducer, and Micro/Nano cantilevers. The Microsystems or Micro/Nano ElectroMechanical Systems (M/NEMS) based mass sensors can be manufactured en masse and can provide a multi-gas analyzing platform. Mass sensors can exhibit superior selectivity if suitable recognition materials are selected. Meanwhile, the high sensitivity of a mass sensor also casts a limitation on mass sensitive chemical sensors as true signals may encounter false alarms from interferences that have similar physicochemical properties. Electrochemical gas sensors are based on the measurement of the properties of the electrode and/or electrolyte as they interact with a gaseous analyte. They have been developed for effective measurement of airborne trace compounds such as CH4, CO, CO2, NO, NO2, SO2, H2, and O2 targets since 1950.5–8  Compared to solid state based metal oxide sensors, electrochemical gas sensors provide good sensitivity and selectivity of detection based on the unique interface reactions. They possess an impressive capacity to achieve the analytical requirements of real-time monitoring of analytes within a small footprint, are constructed with low-cost materials and fabrication techniques, and are controlled by compact, low-power microelectronic circuits. In the following sections, we will focus on the electrochemical and acoustic based mass sensor principles and examples, especially those using ionic liquids in the sensor design.

An ideal gas sensor would be compact, inexpensive, portable, and have low power requirements – but also provide real-time monitoring of multiple gas concentrations. In recent decades, gas sensor technologies have grown tremendously. Since the transducer and readout components of gas sensors are well-advanced, the majority of research and growth is in the gas sensing materials and interfaces to meet the growing standards for accuracy, cost, portability, and accessibility of current gas sensing applications. While the transducer and readout components can often be interchangeable for a specific target analyte, the gas sensing interface must be specifically tailored to a specific analyte. Improvements in the sensitivity, specificity, and stability of the sensing material and sensing interface design, as well as reduction of the cost for their mass production, are regarded as the most critical parts of the gas sensor technology in both the technical and commercial sense.9 

Table 1.3 lists the common gas sensing materials that are used in gas sensor development. Among them, room-temperature ionic liquids (ILs) are an emerging class of compounds that have been receiving increased attention in recent years as new gas sensing materials and interfaces. ILs are organic salts that are liquid at room temperature. An early group of ionic liquids was reported by Osteryoung et al., who described a mixture of 1-(1-butylpyridinium) chloride and aluminum chloride that was liquid at room temperature. Soon after, a series of ILs based on the cations of alkylpyridinium or dialkylimidazolium were developed. The anions vary from halides, such as Cl, Br, or AlCl4, to coordinates such as BF4, PF6, or SbF6, NO3, SO4, CuCl2, and organics, such as CH3SO3, or NTf2. In the last few decades, ILs based on organic cations of tetraalkyl-ammonium or tetraalkylphosphornium and anions of alkylsulfonate were developed. Those “pure organic” ILs are more stable, especially at relatively higher temperature, less toxic, and more hydrophobic. Ionic liquids were initially developed as “green” designer solvents to replace many conventional volatile organic solvents in reaction and separation processes. In contrast to conventional organic solvents that are composed of molecular entities such as DMSO, DMF, CH2Cl2, CHCl3, and THF, ionic liquids have unique properties. They have no significant vapor pressure, thus allowing chemical processes to be carried out with essentially zero emission of toxic organic solvents into the environment. Consequently, they have been the possible environmentally-friendly, recyclable media for synthetic organic chemistry, separation sciences, and other chemical sciences and engineering. For example, ILs have been used as solvents for organic reactions (nucleophilic and electrophilic reactions including acidic catalyzed reactions), transition mental catalyst reactions, and biotransformation. Not only were enhanced reaction rates and improved chemo- and regioselectivities relative to other organic solvents observed, but also potential solutions for biphasic separation of reaction products via extraction were provided (i.e., products can be obtained through distillation from these non-volatile reaction media which eliminates the need for noxious organic solvents). ILs have low miscibility in several organic solvents (such as ether, hexanes, ethyl acetate) as well as in supercritical carbon dioxide; consequently, organic compounds can be extracted into supercritical carbon dioxide layers from ILs. The significant advance in the synthesis of a broad range of ionic liquids has made them available for sensor applications, especially in mass sensing and electrochemical sensing. Compared with the three hundred organic solvents widely used in the chemical industry, there are over a trillion possible ionic liquids, providing tremendous potential for their new analytical applications.

Table 1.3

The most common gas sensing materials for mass and electrochemical sensing.

Gas sensing materials Properties for gas sensing Advantages Disadvantages
Metal oxides  Porous structure and particle size of sensing film improves sensitivity3   
  • High sensitivity

  • Compact design

  • Low cost

 
  • Requires high operating temperatures and high voltage

  • Low selectivity

 
Carbon nanotubes 
  • High surface area

  • High electrical conductivity

  • Large carrier mobility21 

 
  • Low limit of detection

  • Easily integrable into devices

  • Real-time gas sensing possible

 
  • Unfunctionalized CNTs can agglomerate by van der Waals interaction

  • Reproducibility of detection is a concern

 
Polymers  Easily processable by dispersion20    High sensitivity and rapid response 
  • Poor stability, low surface area, and low sensitivity at room temperature

  • Low thermal stability

 
Ionic liquids 
  • Organic salts that are liquids at room temperature23 

  • Very low vapor pressure

  • High thermal stability24 

 
  • Gases are soluble in ionic liquids

  • Acts as both solvents and electrolytes

  • Excellent reversibility and fast response

 
  • High cost of ionic liquids

  • Trace water in the ionic liquids can change the properties of ionic liquids

 
Gas sensing materials Properties for gas sensing Advantages Disadvantages
Metal oxides  Porous structure and particle size of sensing film improves sensitivity3   
  • High sensitivity

  • Compact design

  • Low cost

 
  • Requires high operating temperatures and high voltage

  • Low selectivity

 
Carbon nanotubes 
  • High surface area

  • High electrical conductivity

  • Large carrier mobility21 

 
  • Low limit of detection

  • Easily integrable into devices

  • Real-time gas sensing possible

 
  • Unfunctionalized CNTs can agglomerate by van der Waals interaction

  • Reproducibility of detection is a concern

 
Polymers  Easily processable by dispersion20    High sensitivity and rapid response 
  • Poor stability, low surface area, and low sensitivity at room temperature

  • Low thermal stability

 
Ionic liquids 
  • Organic salts that are liquids at room temperature23 

  • Very low vapor pressure

  • High thermal stability24 

 
  • Gases are soluble in ionic liquids

  • Acts as both solvents and electrolytes

  • Excellent reversibility and fast response

 
  • High cost of ionic liquids

  • Trace water in the ionic liquids can change the properties of ionic liquids

 

The specific properties of ILs which are of prime importance in gas sensing include the following:

  • ILs have zero or negligible vapor pressure and work in a very large temperature range: the higher end can reach ∼400 °C, which is very important for industrial high temperature sensing applications such as in-car, on-road, real-time measurement of tailpipe emissions. There is no drying out of the ILs, which overcomes the serious problem for electrochemical sensors using aqueous electrolytes. Furthermore, it reduces the hazards associated with flash points and flammability.

  • Since ILs are organic, most VOCs are soluble in them. The analyte molecules can adsorb or partition quickly into the ILs’ sensing materials and reach the equilibrium state very quickly after the sensor is exposed to the VOC vapors. This ensures a fast response and excellent reversibility of the sensor. At equilibrium state, the distribution of organic vapors in the ILs’ phase and the gas phase will depend on the partial pressure of the vapors. This provides the possibility of a quantitative measurement.

  • ILs provide media capable of dissolving a vast range of inorganic molecules at high concentrations for organic volatile sensing. They suppress conventional solvation and solvolysis phenomena. ILs are complex solvents that can support many types of solvent–solute interactions (hydrogen-bond, π–π, dipolar, ionic, etc.). In any IL, different types of interaction will be simultaneously present, and the resulting properties of the IL depends on the dominant interactions. Consequently, the surface design of ILs can be used to fit a particular sensing application.

  • Due to significant advances in synthetic strategies of ionic liquids, functionalized ionic liquids are being developed for specific applications that provide them with new physicochemical properties either as solvents or analytical reagents.

Table 1.4 lists the names and chemical structures of some commercially available ILs. Our lab has made significant contribution in using ILs as new gas sensing materials for the development of real-time electrochemical and piezoelectric acoustic gas sensors. The technical details of sensor design principles of ionic liquids sensing materials for electrochemical sensors and mass sensor development are discussed in the following sections.

Table 1.4

Structure of common ILs commercially available.a

Structure of cationsStructure of anionsName or abbreviation of nameb
   N7,7,7,7SO3-ph-C12H25, N4,4,4,4SO3-ph-C12H25  
 , , CH3–SO3  P6,6,6,14SO3-ph-C12H25, P8,8,8,8SO3-ph-C12H25, P4,4,4,14SO3-ph-C12H25, P6,6,6,14CH3SO3, P6,6,6,6(+)camphorsulfonate, P4,4,4,4CH3SO3  
 , (CF3SO2)2N, CH3SO3, BF4, HSO4  bmi(CF3SO2)2N, bmi(+)camphorsulfonate, bmiBF4, bmiHSO4, bmiCH3SO3  
 (CF3SO2)2N, PF6  bbi(CF3SO2)2N, bbiPF6  
 CH3SO3,  beiCH3SO3, bei(+)camphorsulfonate 
 CH3SO3  pmiCH3SO3  
 PF6  hpPF6  
 CH3SO3  bpCH3SO3  
Structure of cationsStructure of anionsName or abbreviation of nameb
   N7,7,7,7SO3-ph-C12H25, N4,4,4,4SO3-ph-C12H25  
 , , CH3–SO3  P6,6,6,14SO3-ph-C12H25, P8,8,8,8SO3-ph-C12H25, P4,4,4,14SO3-ph-C12H25, P6,6,6,14CH3SO3, P6,6,6,6(+)camphorsulfonate, P4,4,4,4CH3SO3  
 , (CF3SO2)2N, CH3SO3, BF4, HSO4  bmi(CF3SO2)2N, bmi(+)camphorsulfonate, bmiBF4, bmiHSO4, bmiCH3SO3  
 (CF3SO2)2N, PF6  bbi(CF3SO2)2N, bbiPF6  
 CH3SO3,  beiCH3SO3, bei(+)camphorsulfonate 
 CH3SO3  pmiCH3SO3  
 PF6  hpPF6  
 CH3SO3  bpCH3SO3  
a

(a) bmiBF4, bmiN(SO2CF3)2 and hpPF6 can be prepared via metathesis of the corresponding imidazolium chlorides with appropriate salts.77  (b) Water-immiscible ionic liquids, such as bbiN(SO2CF3)2 and bbiPF6, can be prepared based on a process known as “one-pot synthesis of ionic liquids”.78  By mixing aqueous formaldehyde with two equivalents of 1-butylamine, hexafluorophosphoric acid, or bis(trifluoromethanesulfon)imide and aqueous glyoxal solution, the hydrophobic ionic liquid (lower layer) thus formed can be separated directly from the reaction mixture.79  (c) Sulfonate ionic liquids with various cations can be made via an alcohol-to-alkyl halide conversion method, which is also a one-pot synthesis of ionic liquids.80  By using primary alcohols (ROH), suitable acids (HA), the 1,3-dialkylmidazolium halides, pyridinium halides, tetraalkylammonium halides and tetraalkylphosphonium halides (all designated as Q+X) can be converted to the new ionic liquids (Q+A), with the anions being the conjugated bases of the acids used.

b

Nl,m,n,j and Pl,m,n,j represent the tetraalkylammonium and the tetraalkylphosphonium, respectively. The subscripted numbers, l, m, n, and j represent the numbers of carbons in each alkyl substitutes. For example, N7,7,7,7 is tertraheptylammonium. The anion, dodecylbenzenesulfonate (SO3-ph-C12H25), was also abbreviated as DBS in the text. bmi and bbi are 1-butyl-3-methylimidazolium and 1,3-dibutylimidazolium, respectively. bei and pmi are 1-butyl-3-ethyl-imidazolium and 1-propyl-3-methyl-imidazolium, respectively. hp and bp are hexylpyridinium and butylpyridinium, respectively.

Electrochemical sensors measure the potential, the conductivity/capacitance change at an electrode–electrolyte interface or the electrical current generated by the reaction of an analyte at a fixed or variable potential for quantitative analysis of the gas analyte concentrations. They are classified as potentiometric, impedance, capacitance, or amperometric sensors based on the readout signals. The majority of electrochemical sensors use a potentiostat to measure the signals due to analyte interactions or reactions at the electrode–electrolyte interface with or without the applied electric stimuli. Due to the large market for glucose sensors which use electrochemical transducers, commercial single-chip potentiostats are made by many leading integrated-circuit manufacturers with sizes (5+ mm on a side) and power requirements (4–5 mW) that can be easily adapted for many electrochemical gas sensor instrument electronics. Thus, electrochemical sensors10–18  are typically small, low power and low cost, and they have been widely successful for miniaturized chemical and biosensors. The commercial success of electrochemical gas sensors can also be attributed to the linearity of their output, low-power requirements, and good resolution. Moreover, once calibrated to a known concentration of the target gas, the repeatability and accuracy of measurement is also excellent. Electrochemical sensors can offer very good selectivity to a particular gas type.

However, there are several major challenges to developing miniaturized electrochemical gas sensors. First, electrochemical sensors require the use of aqueous or non-aqueous (i.e., organic solvents) electrolytes that have a limited applicable potential window. Aqueous electrolytes also suffer from electrolyte volatility and drying out problems. These pitfalls shorten the sensor’s lifetime, limit the analyte choice, and make continuous gas sampling difficult or impossible. Second, miniaturization of electrochemical sensors often requires the use of smaller sensing elements, such as electrodes. However, the signals of the electrochemical sensors are typically proportional to the electrode area (except potentiometry): smaller electrodes result in low sensitivity. Furthermore, some of the targeted gaseous analytes are often in very low concentrations in the presence of many other high concentration compounds/molecules (e.g., water and oxygen), thus high sensitivity and selectivity of the sensor are required. Finally, sensor signal drift is a common problem for real-time and continuous sensing in the field where the sensing environments (e.g., temperature, humidity variations) could change dynamically depending on the time of the day.

Ionic liquids are promising new electrolytes in electrochemical sensor development since they are non-volatile and possess high ion concentration, high heat capacity, and good electrochemical stability. Non-volatile ILs overcome the drying out problems of traditional electrolytes and they also facilitate the redox activity by viscosity, conductance, and protonation dictated electrochemical effects as well as the adsorption and dissolution phenomenon by variable physiochemical interactions which enhance electrochemical sensing characteristics. They have been used in electrochemical devices including supercapacitors, fuel cells, lithium batteries, photovoltaic cells, electrochemical mechanical actuators, and electroplating.19  Our lab has demonstrated that utilizing metal nanocrystals as electrode materials and ILs as electrolyte materials could address current gas sensing challenges. Nanocrystals with controlled surface structures are able to maximize the sensitivity and selectivity of electrochemical interface reactions since metal nanocrystals could provide both high surface areas and high catalytic activity through the judicious choice of crystal facets to enable highly sensitive and selective sensing reactions.20  In addition, metal nanocrystals are compatible with microprinting and batch fabrication techniques for sensor miniaturization. Advances in nanocrystal syntheses make these materials readily available for sensor development at relatively low cost and high batch-to-batch reproducibility.21,22  Analogously, ILs containing organic cations or anions are liquid at room temperature. They are non-volatile, and chemically and thermally stable. These characteristics of ILs can overcome the problem of aging and poisoning of the sensing materials in ambient conditions. Additionally, ILs are electrically conductive and have a wide stable potential window that enables sensitive and selective detection of many common gaseous analytes due to their unique electrochemical sensing reactions, which is not possible in conventional solvents.23–25  Thus, the combination of metal nanocrystals as electrode materials and ILs as electrolyte materials can produce new miniaturized low-cost gas sensors that satisfy the demanding requirements for their usage in various real-world applications. Some examples to illustrate this electrochemical sensing approach have been described below.

Nanocrystals are nanoparticles with well-controlled shapes related to their surface structure (facets) (Figure 1.7).26  For example, cubic particles are enclosed with six facets.27  Many reactions are surface sensitive, which means the reaction kinetics and even the pathways depend on the surface atomic arrangements. The catalytic activities of nanocrystals have been extensively explored, especially in fuel cell related reactions.26,28–37  Reports have shown that the oxygen reduction reaction (ORR), the cathode reaction of the polymer electrolyte membrane fuel cells and lithium air batteries, is much more facile on facets of Pt3Ni than that of Pd{100}.29,38,39 

Figure 1.7

(a–c) TEM images of Pd nanocrystals: (a) cubes; (b) octahedral; (c) rhombic dodecahedra. Scale bar: 200 nm. (d–f) SEM images of Au nanocrystals: (d) cubes; (e) octahedral; (f) rhombic dodecahedra. Scale bar: 100 nm.

Figure 1.7

(a–c) TEM images of Pd nanocrystals: (a) cubes; (b) octahedral; (c) rhombic dodecahedra. Scale bar: 200 nm. (d–f) SEM images of Au nanocrystals: (d) cubes; (e) octahedral; (f) rhombic dodecahedra. Scale bar: 100 nm.

Close modal

However, this surface structural dependence of reactivity is seldom applied to sensor applications.40  By taking advantage of the higher catalytic activity of a given facet to a reaction, we have shown that the detection limit, selectivity, and sensitivity of sensors can be significantly improved. For example, our study of the ORR on a Pd nanocrystal/IL interface is the first exploration of redox reactions at the nanocrystal/IL interface and supports their use for IL-based electrochemical sensor development.41,43  By exploiting the benefits of a bimetallic platinum–nickel (Pt–Ni) alloy nanosphere and an IL (i.e., 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BmpyNTf2)), we demonstrate that an organic–inorganic hybrid interface of IL/Pt–Ni allows better control over the chemistry and charge transfer reactions at the electrode–electrolyte interface for electrochemical sensing of oxygen and hydrogen gases in miniaturized electrochemical gas sensor development.42 

As shown in Figure 1.8, palladium nanocrystals enclosed by {100} and {110} crystal facets, respectively, were synthesized through an aqueous one-pot synthesis.43  A thermal annealing method for fabricating Pd nanocrystals as the working electrode on a Teflon gas permeable membrane (GPM) was developed.43  Figure 1.8 shows a nanocrystal electrode that was drop-coated on the membrane and annealed at 300 °C in atmosphere. The thermal annealing process has been shown to effectively remove residual surfactants from the nanoparticle synthesis, so clean catalyst surfaces are obtained.44,45  No obvious size or morphology changes were observed on the nanocrystals annealed in this process, as shown by the SEM image of the annealed Pd nanocrystals.

Figure 1.8

SEM images (inset: drop-coated Teflon membrane) of Pd{110} nanocrystals annealed at 300 °C.

Figure 1.8

SEM images (inset: drop-coated Teflon membrane) of Pd{110} nanocrystals annealed at 300 °C.

Close modal

In addition, immobilized Pd nanocrystals show very good adhesive characteristics on the membrane surface, which is critical for their utility as the working electrode in sensors. Electrochemical methods were used to validate the facets of Pd nanocrystals coated on the GPM and a glassy carbon electrode. Different crystal facets of Pd present different electrochemical features of surface oxidation and hydrogen adsorption/desorption in aqueous electrolytes due to different surface atomic arrangements.38  Hydrogen desorption peaks were observed at more positive potential on Pd{100} (+0.2 V vs. RHE) compared to Pd{110} (+0.1 V), which is consistent with the reported electrochemical features of Pd nanocrystals.22  The oxidation of Pd{110} at 0.7 V also showed much larger current density that is consistent with bulk Pd single crystal electrodes. These electrochemical results combined with the SEM images confirmed the preservation of the specific facets ({100} and {110}) on the Pd nanocrystals after they were immobilized on the GPM or glassy carbon electrode.

The Pd nanocrystal electrode was fabricated on a GPM (Figure 1.8) with a nanocrystal electrode/IL electrolyte. Due to the high viscosity of ILs, a backflow Clark-type electrochemical cell was used to characterize the analytical parameters of the IL-Pd nanocrystal electrode gas sensor46  (Figure 1.9).

Figure 1.9

Clark-type back flow nanocrystal/IL electrochemical sensor cell. WE: working electrode; RE: reference electrode; CE: counter electrode; CS: cellulosic spacer; GPM: gas permeable membrane; IL: ionic liquid; O: O-ring. Reproduced from ref. 46 with permission from American Chemical Society, Copyright 2011.

Figure 1.9

Clark-type back flow nanocrystal/IL electrochemical sensor cell. WE: working electrode; RE: reference electrode; CE: counter electrode; CS: cellulosic spacer; GPM: gas permeable membrane; IL: ionic liquid; O: O-ring. Reproduced from ref. 46 with permission from American Chemical Society, Copyright 2011.

Close modal

It is shown that the electrochemical gas sensor assembly with the micro-porous Teflon gas permeable membrane (e.g. porous PTFE, PM231, or PM131) was coated with Pd nanocrystals as the working electrode and a thin layer of IL electrolyte sandwiched between the working/Pt quasi reference electrodes and the counter electrode beneath the IL.43  The small volume of the thin layer cell allows fast solubility equilibrium of the analytes to be established, which necessitates a fast response in an analytical technique.47–50  This electrochemical cell structure has been shown to significantly improve sensor response time compared to existing devices since gaseous analytes will enter from the gas membrane side to quickly reach the electrode/electrolyte interface without passing through the IL diffusion barrier, minimizing the slow mass transport effects typically observed on electrochemical reactions in ILs. Using nitrogen as a diluting gas with no redox activity will not give any signal. The amperometric sensor responds to oxygen at varying concentrations. A Pd nanocrystal electrode and an IL electrolyte were compared with Pt polycrystalline electrodes (Figure 1.10).

Figure 1.10

(a) Current density vs. time plots for Pd nanocrystals and Pt polycrystal in different oxygen concentrations (1%–20% v/v). (b) Calibration curves of current density vs. oxygen concentration for the three different electrodes in A. Reproduced from ref. 43 with permission from the Royal Society of Chemistry.

Figure 1.10

(a) Current density vs. time plots for Pd nanocrystals and Pt polycrystal in different oxygen concentrations (1%–20% v/v). (b) Calibration curves of current density vs. oxygen concentration for the three different electrodes in A. Reproduced from ref. 43 with permission from the Royal Society of Chemistry.

Close modal

Pd{100} presents similar sensitivity to polycrystalline Pt, but interestingly, Pd{110} shows about two-fold sensitivity improvement over Pt or Pd {100}. This is the first demonstration of the facet-dependence of the analytical performance of oxygen sensing in an IL/nanocrystal electrode interface. In addition, the reduction of the oxidation product, superoxide radical, also depends on the Pd crystal facets. These observations reinforce the importance of controlling the surface structure of the sensing materials. Furthermore, Pd nanocrystal-based sensors present a much lower detection limit compared with Pt polycrystalline electrodes, which confirms that they are suitable and superior as the working electrode in sensor applications. Over a two-month testing period, the Pd nanocrystal electrodes maintained their electroactivity and reproducibility.

Electrochemical sensors for continuous sensing are based on unique chemical reactions that detect an analyte with sensitivity and specificity. However, chemical reactions result in the formation of product(s) which, if not inert or removed, can potentially change the sensing environment and cause sensor signal drift. We addressed this challenge by identifying and selecting sensing reactions in the IL/electrode interface that are kinetically fast and reversible, permitting the response signal to depend only on the analyte concentration. This promotes accuracy, reproducibility, and long sensor lifetime.

Our IL-based hydrogen sensor utilizes the fast hydrogen oxidation reaction (HOR) at a Pt electrode51  (Figure 1.11(left)). This excellent performance is the result of protons being formed by the hydrogen oxidation which are quickly removed from the electrode surface at the counter electrode via reduction within the time period of the measurement, thus eliminating the most common sensor drift mechanism, product accumulation at the sensing/working electrode interface. A linear response to hydrogen concentration was obtained in both nitrogen and air backgrounds (Figure 1.11(middle)), which showed that the gases in air did not interfere for hydrogen concentration below 0.5%. This high selectivity is due to the low (+0.4 V) hydrogen detection potential where there are no redox reactions from oxygen, CO2, or other trace gases in air. A much larger linear range of hydrogen sensing was found in [Bmim][NTf2], but with lower sensitivity than in [Bmpy][NTf2] due to the influence of cations in the hydrogen electrode adsorption step.

Figure 1.11

(Left) Multiple measurements of the amperometric hydrogen response of sensors in ionic liquids [Bmpy][NTf2] (black) and [Bmim][NTf2] (red), constant potential at E = +0.4 V vs. Fc+/Fc is the potential used for hydrogen sensing, sampling time is 20 seconds for each measurement. (Middle) Calibration curves of current density vs. hydrogen concentration under four different conditions. (Right) The reversible electrochemical reaction loop that results in reproducible and continuous sensing with little drift. Reproduced from ref. 51 with permission from the Royal Society of Chemistry.

Figure 1.11

(Left) Multiple measurements of the amperometric hydrogen response of sensors in ionic liquids [Bmpy][NTf2] (black) and [Bmim][NTf2] (red), constant potential at E = +0.4 V vs. Fc+/Fc is the potential used for hydrogen sensing, sampling time is 20 seconds for each measurement. (Middle) Calibration curves of current density vs. hydrogen concentration under four different conditions. (Right) The reversible electrochemical reaction loop that results in reproducible and continuous sensing with little drift. Reproduced from ref. 51 with permission from the Royal Society of Chemistry.

Close modal
Besides serving as unique, non-volatile electrolytes for electrochemical gas sensors, ILs can act as stable solvents and/or adsorbing materials for mass-based gas sensors. The most common transducers for mass sensing are acoustic quartz crystal microbalance (QCM) transducers that are based on piezoelectric or “pressure electric” effects, i.e., pressure exerted on a small piece of piezoelectric material such as quartz crystal causes an electrical potential difference between the deformed surfaces. QCM mass sensors comprise a thickness-shear mode planar quartz crystal. The essence of QCM lies in its ability to direct sense mass deposited on a crystal surface. Interfacial mass changes can be related to changes in the QCM oscillation frequency by applying the Sauerbrey equation,52 
Δ f = 2 Δ m n f 0 2 A ( μ q ρ q ) 1 / 2
(1.4)
where n is the overtone number, μq is the shear modulus of the quartz (2.947 × 1011 g cm−1 s−2), and ρq is the density of the quartz (2.648 g cm−3). It is assumed that the foreign mass is strongly coupled to the resonator. By obtaining the damping resistance through fitting the Butterworth–Van Dyke circuit, one can also explore the validity of the Sauerbrey equation and obtain the dissipative properties, if the surface layer shows viscoelastic characteristics. Thus, QCM,53–55  like any other gravitational probe, has universal sensitivity and there is usually an unambiguous relationship between its frequency shift and the changes in surface populations. QCM is an extremely sensitive mass sensor with sensitivity to picograms and has been used widely in studying solid/liquid interfacial phenomena in physical science (e.g. redox chemistry), biosensor systems,56–60  and in detecting the change of cell adhesive properties including cell growth and attachment as well as cell–receptor and cell–cell interactions.61–63  Furthermore, QCM as a transducer only requires oscillator circuits. Oscillator circuits are relatively simple and inexpensive to fabricate, which make QCM mass sensors suitable for field instruments. Digital frequency counters to measure the oscillator signal are equally suitable for fabrication as lightweight components of field instruments. The frequency measurements are also beneficial because frequency is one of the most precisely measurable quantities with a precision of ∼1 : 10.10  These devices can be easily automated or combined with flow injection systems extending their capability for continuous and repeated assays. This raises an exciting possibility of using QCM crystal arrays to assay different analytes in complex samples with on-line display of the results. There are other types acoustic sensors such as SAW (surface acoustic wave) sensors. A microcantilever sensor is another type of mass sensor. Microcantilevers are micromechanical beams that are anchored at one end, such as diving springboards. Molecular adsorption results in the bending of the beams that allow extremely sensitive measurement of molecular adsorption. They have been demonstrated as gas sensing transducers.

Mass sensors are very sensitive, but they require the use of sensing materials to provide the sensitivity and selectivity. The key component of a mass sensor for chemical sensing is the design of thin films made of sensing elements. For example, sensor arrays using sensing materials with different functionalities are common approaches to enhance the selectivity via pattern recognition64  in mass sensors.

Among all the available materials, biological recognition elements (e.g., antibody, aptamer, enzyme, nucleic acid, receptor) show excellent selectivity and sensitivity, but biosensor devices often cannot be used for longer than a few weeks or in harsh environments due to their limited stability. They are rarely used in gas sensing applications. Inorganic materials (e.g., metal oxides and semiconductors, etc.) are commonly used in commercial gas sensors because of their durability and suitability for operation at elevated temperatures. But they have limited specificity and also require operation at high temperature. Organic materials (e.g., polymers) are getting more attention because of the vast menu of physical and chemical properties they provide and for their synthetic flexibility. However, most organic materials have the same problems as biological materials for their limited stability, especially in harsh environments.

The use of mass sensing devices depends on whether the sensing layer immobilized on the mass transducer forms a thin uniform surface and is chemically stable during the measurement processes. A thin film allows a fast response time and reproducible sensing performance. We have shown that ILs combine the best properties of organic and inorganic materials for sensing gaseous and VOCs using mass transducers. Since the immobilization procedure for the IL sensing materials is the most important step for the formation of stable robust thin IL films on mass transducers, in the following section, we describe the methods to make ILs or IL composites (e.g., IL-SAM (self-assembled monolayer)/polymer) sensing interfaces. Example results of analytical performances with these IL-based sensor and sensor arrays when interacting with VOC analytes using acoustic QCM transducers are also presented.

Self-assembled monolayers (SAMs) have been studied for a diverse set of thin-film applications.81,82  The beauty of SAMs is in the spontaneous association of molecules under equilibrium conditions that gives stable, structurally well-defined, two-dimensional aggregates. The vast majority of the alkanethiolate SAMs provide a simple, reproducible, relatively well-ordered materials platform with chemically diverse terminal groups.83,84  The ease and flexibility of the self-assembly process provides a convenient method for modifying the electrodes with additional functionalities. By varying the terminal group, the interfacial functionality of the monolayer can be changed. Even though a SAM may either have the function of performing some aspect of gas sensing in its own right, using it as a “primer” onto which the IL can be “grafted” on it, it provides more complex bilayers with additional control over selectivity and sensitivity.

A SAM with a carboxylic acid end group can be used as a surface modifier to change the physical and chemical nature of an Au substrate (Figure 1.12).

Figure 1.12

Immobilization of ILs via electrostatic interaction with SAMs.

Figure 1.12

Immobilization of ILs via electrostatic interaction with SAMs.

Close modal

Then, the surface is treated with alkali solution and the carboxylic acid will deprotonate to become carboxylate rendering the surface negatively charged. When the surface is immersed into an IL solution, the ILs are immobilized on the electrode surface via the interaction between the carboxylate group and the tetraalkylammonium or tetraalkylphosphoernium cations.

Alternatively, the electrode surface can also be made positively charged using a SAM with a pyridine terminal group treated with iodoalkane solution.85  The pyridine can react with the iodoalkane to produce pyridinium cations.86  Afterwards, when the surface is immersed into an IL solution, it is immobilized on the electrode surface via the interaction between the pyridinium groups and the organosulfonate anions. Ethanol can be used as a solvent for n-alkanethiols up to a chain length of about 18 methylene units (n = 18). Above 18 methylenes, the compound displays a tendency to precipitate, then hexane, dimethyl ether, or tetrahydrofuran instead of ethanol can be used as solvents. For shorter chain thiols which are water soluble, aqueous solutions can be used as solvents.

As shown in Figure 1.13, a freshly prepared gold bead was immersed in 1 mM HS(CH2)10COOH/THF solution for 3 days. Then it was treated with 0.1 M KOH solution for 15 min. Finally, it was soaked in 5 mM IL (P666,14 LBASO3)/EtOH solution for 2 days. After this treatment, the Au bead was rinsed in EtOH for 24 hours. The gold bead was characterized in 1 mM Fe(CN)63−/4− solution containing 0.1 M NaClO4 by cyclic voltammetry (CV) and electrochemical impedance spectrometry (EIS) at each step of the above modifications.

Figure 1.13

Nyquist plots of an EIS study of 1 mM Fe(CN)63−/4− in 0.1 M NaClO4 on a gold electrode modified by soaking sequentially in: 1 mM HS(CH2)10COOH/THF solution for 3 days (open triangle), 0.1 M KOH for 15 min (solid circle), 5 mM IL P666,14DBS/EtOH solution for 2 days (solid triangle) solutions and ethanol (open square). The gold electrode was prepared by annealing in a gas/O2 flame, to produce a smooth surface with predominant Au (111) facets. Note: after each treatment, the gold electrode was rinsed in EtOH and stored for 24 hours before the EIS study was carried out.

Figure 1.13

Nyquist plots of an EIS study of 1 mM Fe(CN)63−/4− in 0.1 M NaClO4 on a gold electrode modified by soaking sequentially in: 1 mM HS(CH2)10COOH/THF solution for 3 days (open triangle), 0.1 M KOH for 15 min (solid circle), 5 mM IL P666,14DBS/EtOH solution for 2 days (solid triangle) solutions and ethanol (open square). The gold electrode was prepared by annealing in a gas/O2 flame, to produce a smooth surface with predominant Au (111) facets. Note: after each treatment, the gold electrode was rinsed in EtOH and stored for 24 hours before the EIS study was carried out.

Close modal

The EIS data reveal that the electron transfer resistance (Ret) value increases after each step of modifications (Figure 1.13).

Immobilization of ILs results in a more passive surface. After the thiol/IL modified electrodes were rinsed with THF solvents, the Ret increased further. This result confirmed that a strong immobilized IL layer was made, otherwise the Ret value would have decreased if the IL was removed by the solvents. The thiol/IL modified Au surface might be at its dynamic state when soaking in the solvents, which may allow further organization of the thin film.

Time-dependent AFM images65,66  have revealed that self-assembly from solution follows a multi-step process that depends on the chain length of the thiol and the cleanness of the gold: (1) growth of the lying-down phase in combination with the displacement of surface contaminants. The rate of this step depends upon the chain length of the thiol and the cleanness of the gold surface; (2) relatively fast 2D phase transition to a nearly complete SAM; and (3) a slow step of scar-healing and domain enlarging. In a short time, a loose monolayer will form at a clean surface in an alkylthiol solution. Normally, a compact lattice-like monolayer will form after immersion in the solution for about 24 hours.66 

As shown in Figure 1.14, co-adsorption of ILs on the SAM of thiols offers a great opportunity to have greater control over the structure of the monolayer (Figure 1.14).

Figure 1.14

Immobilization of ILs via co-assembly.

Figure 1.14

Immobilization of ILs via co-assembly.

Close modal

More importantly, alkane thiols and ILs differ in their chemical nature. Consequently, greater diversity over the structure and properties of the monolayer by co-adsorption of thiols and ILs could be obtained. Mixed monolayers of thiols and ILs on gold containing different chain lengths and functionality can also provide a convenient means of introducing new properties into interfaces.

A fresh gold electrode was immersed in an alkylthiol solution for a short time (5 minutes to 3 hours) (Figure 1.15). Then, the electrode was treated in an IL solution. Alternatively, a gold electrode may be immersed directly in a solution of alkylthiol and an IL mixture to promote the spontaneous co-assembly of ILs with the alkylthiols.

Figure 1.15

CV results of the gold electrode at different surface states: bare gold, soaked in 1,2-thiol, soaked in IL, and rinsed in CH2Cl2.

Figure 1.15

CV results of the gold electrode at different surface states: bare gold, soaked in 1,2-thiol, soaked in IL, and rinsed in CH2Cl2.

Close modal

The length of the alkyl chain of alkylthiol and ILs, the concentration of thiol and IL, the time, or the sub-monolayer coverage of the alkylthiol monolayer, the solvents and other conditions will be investigated and optimized.

As shown in Figure 1.15, gold beads were prepared by annealing in an oxygen flame, to produce a smooth surface with predominant Au (111) facets. This freshly prepared gold bead was soaked in dodecanethiol/CH2Cl2 1 mM solution for 3 days. Then it was immersed in a 1.6 mM IL (P666,14 LBASO3) solution for 3 days. Finally, it was rinsed in CH2Cl2 three times. After each step, the gold bead was characterized in 1 mM Fe(CN)63−/Fe(CN)64− solution by CV and EIS.

ILs provide a unique environment for nano chemistry and material design. For example, anions can control the solvent’s reactivity with water, coordinating ability, and hydrophobicity. ILs containing the cations or anions with sulfide, disulfide, or even a thiol group were reported.87,88  Therefore, these functionalized ILs can be immobilized directly on an Au electrode surface (Figure 1.16).

Figure 1.16

Immobilization of ILs via sulfide groups.

Figure 1.16

Immobilization of ILs via sulfide groups.

Close modal

Polymers are often used in sensory devices as passive supports or structure materials to provide stability. Conductive or conjugated polymers (CPs) have been used as sensing materials for the detection of VOCs, oxygen, nitrogen oxide gases, amine, hydrazines, pH, humidity, various ions, and cytochrome C because films can be produced rapidly with controlled thickness and porosity and can respond readily to changes in their local environment.67  However, conductive polymer sensing materials tend to show very little specificity and they are not useful in “stand alone” sensors, they are typically used in sensor array design.89  Since many conductive polymers, such as polyaniline, polypyrrole, polythiophene,90–95  polyamic acids,96  and poly(o-aminophenol)97  can be deposited on an electrode electrochemically from their solution or their monomer, they often are regarded as polyions even without the functional group after they were doped (Figure 1.17).

Figure 1.17

(a) Layer by layer deposition of polysolfonate styrene and IL; (b) PVF embedded with ILs.

Figure 1.17

(a) Layer by layer deposition of polysolfonate styrene and IL; (b) PVF embedded with ILs.

Close modal

It is possible to embed ILs inside these polymer films through electrostatic interactions to increase the film’s specificity. Using ILs which consist of multi-cations, multi-anions, or zwitterions could even further increase the complexity of the mixed IL/CP films and lead to great choices of films with enhanced specificity.68 

As an example, polyvinylferrocene (PVF) was electrochemically deposited on Au substrate. Then, 10 mM P6,6,6,14LABSO3/EtOH solution was added to a bare Au and PVF film for 30 min, followed by rinsing with EtOH three times. For each step, the modified surfaces were characterized by reflectance FT-IR and/or cyclic voltammetry (Figure 1.18) showing characteristic peaks at 1010 cm−1, 1035 cm−1, 1207 cm−1, 1597 cm−1, 2860 cm−1, and 2929 cm−1 for the IL casted film on a bare Au electrode.

Figure 1.18

PVF film is made by electrochemical oxidation in CH2Cl2 containing 0.1 M tetrabutyl ammonia perchlorate (TBAP) at 0.8 V vs. SCE. A Bio-Rad FTS 175C FTIR spectrometer mounted with a Harrick Seagull was utilized to obtain the IR spectra of the IL, PVF and PVF/IL thin film on the Au substrate respectively.

Figure 1.18

PVF film is made by electrochemical oxidation in CH2Cl2 containing 0.1 M tetrabutyl ammonia perchlorate (TBAP) at 0.8 V vs. SCE. A Bio-Rad FTS 175C FTIR spectrometer mounted with a Harrick Seagull was utilized to obtain the IR spectra of the IL, PVF and PVF/IL thin film on the Au substrate respectively.

Close modal

The early literature of ionic liquid QCM gas sensors quantified the analyte concentration by viscosity induced frequency change. The sensor response is more complicated and can vary depending on experimental conditions. Mass detection by using Sauerbrey’s equation assumes that the foreign mass is strongly coupled to the resonator. This condition can be met when the device is operating in the gas or the vacuum phase, and the added mass binds tightly to the surface. It is important to make thin, rigid IL film so that the Sauerbrey equation is valid.

The film thickness (11 nm) can be determined by QCM measurements. Its rigidity can be characterized by simultaneously measuring the damping resistance and the frequency change during the vapor detection experiments using QCM.

Figure 1.19 shows that the change of damping resistance (ΔR) decreases with increasing temperature. At room temperature, the ΔR% values are relatively large (e.g., ethanol (11%); dichloromethane (12%); benzene (3.7%); and heptane (1.6%)), indicating a slight viscosity change of the film upon the adsorption of organic vapors (VOCs). At 120 °C, the ΔR% was less than 2.6% for the four VOC samples tested. This is consistent with the thermodynamics, i.e., the partition coefficient of gas molecules in liquid film reduces with increasing temperature. At high temperature, the change of viscosity caused by the gas adsorption on the IL film is very small and the frequency changes were contributed mainly from the mass loading in the IL film. As a result, the Sauerbrey equation relating frequency change to pure mass loading is valid. This enables us to quantify the thermodynamic and kinetic parameters of the interaction of IL film with volatile organic molecules by the QCM technique.

Figure 1.19

Change of damping resistance of the QCM signal (ΔR%) vs. temperature curve.

Figure 1.19

Change of damping resistance of the QCM signal (ΔR%) vs. temperature curve.

Close modal

Table 1.5 shows the Henry constants of various VOC vapors in ILs obtained from our experimental results. Ethanol, benzene, and heptane have similar vapor pressure but heptane has a higher Henry constant. This result is consistent with the data which indicate that IL thin films have high sensitivity to polar analytes. Since all ILs studied have both polar and non-polar groups, the difference in Henry constants could provide structural information about IL immobilization on the substrates. Orientating the immobilized ILs with either a polar terminal or non-polar terminal could lead to selective response of IL film to various compounds. Figure 1.20 shows the AFM morphology of a polished gold electrode before and after immobilization of an IL.

Table 1.5

Empirical Henry constant.a

Ethanol Benzene CH2Cl2 Heptane
beiOCS  3.25 × 105   1.05 × 106   1.15 × 106   1.87 × 106  
bmiOCS  5.57 × 105   2.03 × 106   1.55 × 106   2.58 × 106  
P6666OCS  5.80 × 105   5.51 × 105   1.31 × 106   1.42 × 106  
P66614OMS  3.85 × 104   4.33 × 105   2.25 × 105   0.31 × 106  
P66614DBS  2.00 × 105   8.26 × 105   8.75 × 105   1.26 × 106  
P8888DBS  7.23 × 105   1.25 × 106   9.71 × 105   1.88 × 106  
N7777DBS  1.27 × 106   1.37 × 106   NA  2.50 × 106  
Ethanol Benzene CH2Cl2 Heptane
beiOCS  3.25 × 105   1.05 × 106   1.15 × 106   1.87 × 106  
bmiOCS  5.57 × 105   2.03 × 106   1.55 × 106   2.58 × 106  
P6666OCS  5.80 × 105   5.51 × 105   1.31 × 106   1.42 × 106  
P66614OMS  3.85 × 104   4.33 × 105   2.25 × 105   0.31 × 106  
P66614DBS  2.00 × 105   8.26 × 105   8.75 × 105   1.26 × 106  
P8888DBS  7.23 × 105   1.25 × 106   9.71 × 105   1.88 × 106  
N7777DBS  1.27 × 106   1.37 × 106   NA  2.50 × 106  
a

Calculated according to equation pi= Cimi, where pi is the partial pressure of the samples in gas phase, Ci is the Henry constant (unit: Pa), and mi is the molar fraction of the samples in IL. Saturated vapor pressures were obtained from vapor pressure data. mi was calculated from the Sauerbrey equation (for a 10 MHz crystal, the sensitivity is 1.02 ng cm−2 Hz−1) and the total weight of the IL film.

Figure 1.20

Contact mode AFM images. (a) A polished Au QCM surface; (b) after it was modified with P66614DBS thin film.

Figure 1.20

Contact mode AFM images. (a) A polished Au QCM surface; (b) after it was modified with P66614DBS thin film.

Close modal

The abovementioned IL thin film fabrication methods allow the design of various unique IL sensing layers at mass transducers for gas sensor development, especially for harsh conditions. For example, ILs have high thermal stability (e.g., typical decomposition temperature is about 350 °C by thermogravimetric analysis (TGA)).56,98  Reports also show that ILs are able to protect biological films from thermal degradation.56,99  We have demonstrated that both tetraalkylphosphonium and tetraalkylammonium IL thin films show enhanced sensitivity and selectively to the organic vapors (ethanol, dichloromethane, heptane, or benzene) at room temperature and elevated temperatures as high as 200 °C when compared to a bare gold electrode. Figure 1.21 shows a typical sensorgram of benzene adsorption on P66614DBS IL thin film at 120 °C as a function of the concentration of benzene in gas phase.

Figure 1.21

QCM frequency change vs. benzene at various concentrations, at 120 °C.

Figure 1.21

QCM frequency change vs. benzene at various concentrations, at 120 °C.

Close modal

Linear relationships of the frequency changes and the concentrations of vapor were obtained over the 0% to 100% saturation vapor pressure range at 120 °C for all the organic vapors tested (Figure 1.22). When the system was cooled down to 24 °C, the IL/QCM sensor gave a reproducible response at 24 °C, again indicating high stability and reversibility. This procedure has also been used to remove the volatile impurity in the ionic liquid coatings. Figure 1.23 shows that the sensitivity of IL sensors decreases with increasing temperature. However, at 200 °C, the sensors still kept relatively strong sensitivity. At 120 °C, the detection limit currently is about 5% (e.g., 7 mg L−1 for ethanol); at room temperature the detection limit is about 100 ppm for CH2Cl2.

Figure 1.22

Frequency change vs. concentration of the P66614DBS/QCM sensor exposed to ethanol (square), heptane (triangle), benzene (star), and dichloromethane (circle) at 120 °C.

Figure 1.22

Frequency change vs. concentration of the P66614DBS/QCM sensor exposed to ethanol (square), heptane (triangle), benzene (star), and dichloromethane (circle) at 120 °C.

Close modal
According to the Sauerbrey equation,
Δ f = 2 Δ m n f 0 2 A ( μ q ρ q ) 1 / 2
(1.4)
where n is the overtone number, μq is the shear modulus of the quartz (2.947 × 1011 g cm−1 s−2), and ρq is the density of the quartz (2.648 g cm−3), it can be predicted that reducing the gold electrode area (A) of the quartz crystal and increasing the frequency (f0) or using overtone (n) can dramatically reduce the detection limits. For example, just increasing the operation frequency to 250 MHz (the typical surface acoustic wave (SAW) operation frequency) can reduce the detection limits of CH2Cl2 to about 0.05 ppm at room temperature and to 16 ppm at 120 °C. The excellent linear relationship in our study and the potential of low detection limits offered by the QCM transducer illustrate that the IL sensors present significant advantages over conventional metal oxide sensors that are non-linear for high temperature industrial sensing applications. Figure 1.23 depicts the change in frequency with increasing temperature for the IL sensors using four different solvents such as ethanol, heptane, benzene, and dichloromethane.
Figure 1.23

(a) Frequency changes of two ILs/QCM sensors exposed to 80% ethanol, heptane, benzene, and dichloromethane at various temperatures. (b) One with P66614MS IL and another with P6666OCS IL.

Figure 1.23

(a) Frequency changes of two ILs/QCM sensors exposed to 80% ethanol, heptane, benzene, and dichloromethane at various temperatures. (b) One with P66614MS IL and another with P6666OCS IL.

Close modal

Mass sensors show small size and high sensitivity at low power providing an ideal kind of sensing option in contrast to many spectroscopic techniques for which factors like molecular orientation and the nature of solvent can influence the response, in addition to associated engineering challenges and expensiveness. However, the downside of this universality is the lack of absolute selectivity. A QCM sensor array using various IL sensing materials can provide additional selectivity and significantly increase the accuracy of detection at little or no power cost. IL’s unique solvation properties, varying with the nature of the constituent ions, render them ideal for selective detection strategies. The interaction abilities can be varied by judicious selection of ILs compatible with target analytes. The key to a sensor array is to develop chemically selective interfaces which exhibit a high level of chemical independence and structural order. Consequently, the information about which function groups of ILs interact with the organic volatiles is critical for the controlled configuration of IL on the surface to generate IL films with a great diversity in structural and chemical properties. ATR FT-IR was used to characterize the gas/IL interaction to guide the surface design of IL selective interfaces. Figure 1.24(A) shows the absorbance spectrum of a P666OCS thin film. The peak at 1730 cm−1 originates from the C═O group. Peaks at 1187 cm−1 and 1035 cm−1 come from the O═S═O group. The other peaks come from the alkyl groups. Figure 1.24(B) shows the spectrum of ethanol when there is no IL film on the ATR crystal. When the IL film is exposed to ethanol, its absorbance spectrum is shown in Figure 1.24(C). The negative peaks of the C═O and O═S═O groups of P6666OCS indicate their interactions with ethanol vapor. Additionally, the intensity of the ethanol peaks was enhanced about 50-fold when interacting with only the 10 µg cm−2 IL film. This study shows that the intensity of ethanol peaks depends on the thickness of the film and on the concentration of the ethanol vapor in the gas phase. ATR-FT-IR alone or in combination with other techniques will be invaluable to obtain information on IL orientation, kinetics, concentration of the vapor, and the physicochemical interactions of ILs with the gas analytes to facilitate the configuration of ILs on a surface.

Figure 1.24

ATR-FTIR spectra of ionic liquid P66614OCS film (A), ethanol vapor exposed to bare substrate (B), and to a P66614OCS film covered substrate (C).

Figure 1.24

ATR-FTIR spectra of ionic liquid P66614OCS film (A), ethanol vapor exposed to bare substrate (B), and to a P66614OCS film covered substrate (C).

Close modal

Statistical modeling such as regression analysis can be used to obtain the classification and quantitative relations between the frequency response and other experimental factors (e.g., temperature, chemical structures, molecular weights, vapor pressure, etc.) from the experimental data. The model will be powerful to quantify the contributions of different experimental factors to the frequency response and could be used to predict the responses of similar analytes. Figure 1.25 shows the different patterns when four different IL coating materials respond with ethanol, benzene, heptane, and dichloromethane vapors by QCM. In summary, our study and those of others have shown that successful immobilization of ILs and preparation of IL thin film can provide a platform for mass-based gas sensors, particularly in array-based gas sensing and high temperature gas sensing.

Figure 1.25

Normalized relative response pattern of IL sensors (coated with BmiOCS, P66614DBS, P66614OMS, and P66614OCS) for ethanol (red), heptane (green), CH2Cl2 (blue), and benzene (cyan) at 120 °C. The signals are normalized by the weight of IL coatings and the vapor pressure of each analyte.

Figure 1.25

Normalized relative response pattern of IL sensors (coated with BmiOCS, P66614DBS, P66614OMS, and P66614OCS) for ethanol (red), heptane (green), CH2Cl2 (blue), and benzene (cyan) at 120 °C. The signals are normalized by the weight of IL coatings and the vapor pressure of each analyte.

Close modal

Inspired by the individual merits of mass and electrochemical sensing via ILs under their standard protocols, we have demonstrated the first integrated electrochemical quartz crystal microbalance (EQCM) gas sensor onto a single miniaturized platform.69  The piezoelectric electrodes for mass sensing and the electrochemical electrodes for amperometric detection were fabricated on a single quartz plate (Figure 1.26).

Figure 1.26

Single EQCM sensor design and the combined mass and electrochemical signals for accuracy of measurement. Reproduced from ref. 69 with permission from Elsevier, Copyright 2009.

Figure 1.26

Single EQCM sensor design and the combined mass and electrochemical signals for accuracy of measurement. Reproduced from ref. 69 with permission from Elsevier, Copyright 2009.

Close modal

The geometry was optimized to ascertain the stability of all the electrodes, and to reduce IR-drop. Cyclic voltammetry was used to study electrode reaction mechanisms while differential pulse voltammetry and square wave voltammetry were used for quantitative analysis. Simultaneous sensing provided two completely orthogonal responses with ON/OFF switching of an explosive target 1-ethyl-2-nitrobenzene (ENB). The adsorption of the analyte led to viscoelastic changes while at the same time, the redox reaction generated an increase in current. Adsorption of any possible interferent would not have generated such a combined response. If the analytes had been redox active, the current and the potential values would be entirely different, otherwise the analytes would have shown only a mass signal and that can be totally different too. This additional selectivity of the sensor can significantly increase the accuracy of the detection at little or no power cost. Alongside this, the high thermal stability of ILs also allowed easy elimination of volatile contaminants and water by heating. However, the lateral expansion of ILs when coated on the electrode can lead to non-reproducible films. Electrochemical measurement requires the IL films to be in contact with all three electrodes. Thick and/or confined IL films will thus be highly beneficial, which can increase the sensitivity of the IL QCM sensor and reduce IR drop for electrochemical measurement, especially in the case of EQCM arrays that take advantage of the higher order sensing,69,70  where multiple transduction principles (electrochemical and QCM) and plural sensors (EQCM arrays) will be simultaneously interrogated for the solution of the same detection problem.

Gas sensors, especially in their miniaturized form,71  provide convenient analytical tools for quantifications of various analytes in a broad range of applications (e.g., ambient air monitoring,27  military and civilian counter-terrorism,72,73  occupational health and safety,74,75  biomedical diagnostics,76  and industrial process control77 ). Precision, accuracy, and validity are the key criteria of a reliable sensor, which are often described as the five “S” of the sensor: sensitivity, selectivity, speed, stability, and cost. The most sought-after gas sensors are those that are small, low cost, low power, and can accurately detect important gaseous analytes in the concentration range of interest in real-time and continuously. In this chapter, we have summarized some of our own work and those in the literature regarding gas sensing using IL sensing materials coupled with low-cost, low-power, and real-time electrochemical and acoustic QCM mass transducers. ILs allow continuous sampling and preconcentration of low vapor pressure gases. An IL sensor array with a plurality of sensors can further enhance the information content obtained from a specific analyte by these sensors, increasing the accuracy of detection and reducing the background noise levels. The unique properties of ILs enable orthogonal detection of key gaseous analytes with both E-QCM transducers. The IL sensing material interfaced with E-QCM design will allow the optimization of sensitivity, reproducibility, and isolation from various interferences while the micro-mechanical-electrochemical system implementation will suppress inherent chemical and electronic noise, permitting the extreme accuracy and fast response time needed for modern “selective” detection equipment. Furthermore, the small size, low-cost electrochemical and piezoelectric sensor devices will set up the foundation for intelligent wireless sensing protocols in wide area surveillance with real-time reporting, source locating, and the opportunity to enhance the detection process by itself through sensor-to-sensor networking.

We would like to acknowledge the support provided by the following funding agencies: National Science Foundation EAGER SitS 1841301 and Sits 2034323 and Department of Energy DE-SC0021753 for our current ionic liquid gas sensor developments.

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Figures & Tables

Figure 1.1

Periodic table of elements highlighting the solids (green), liquids (blue), and gases (red) at room temperature (about 22 °C).

Figure 1.1

Periodic table of elements highlighting the solids (green), liquids (blue), and gases (red) at room temperature (about 22 °C).

Close modal
Figure 1.2

Schematic depicting the release of gases into the atmosphere from various anthropogenic activities. The primary pollutant gases are directly emitted from sources such as factories, towns and homes, vehicle exhausts, agriculture, shipping, airplanes or from natural causes such as wildfires and volcanoes from the mountains. The secondary air pollutants on the other hand, are a result of reactions between the primary pollutants in the atmosphere.

Figure 1.2

Schematic depicting the release of gases into the atmosphere from various anthropogenic activities. The primary pollutant gases are directly emitted from sources such as factories, towns and homes, vehicle exhausts, agriculture, shipping, airplanes or from natural causes such as wildfires and volcanoes from the mountains. The secondary air pollutants on the other hand, are a result of reactions between the primary pollutants in the atmosphere.

Close modal
Figure 1.3

Flowchart depicting the sample analysis workflow in traditional gas analysis instruments. Sample preparation is an essential part of gas analysis using gas chromatographic and spectroscopic techniques. Improper collection methods or handling in transit may destroy sample integrity.

Figure 1.3

Flowchart depicting the sample analysis workflow in traditional gas analysis instruments. Sample preparation is an essential part of gas analysis using gas chromatographic and spectroscopic techniques. Improper collection methods or handling in transit may destroy sample integrity.

Close modal
Figure 1.4

(a) Histogram showing the time taken for a typical analysis of a sample by gas chromatography (GC); (b) errors of each step in sample analysis by GC.

Figure 1.4

(a) Histogram showing the time taken for a typical analysis of a sample by gas chromatography (GC); (b) errors of each step in sample analysis by GC.

Close modal
Figure 1.5

Schematic representing the statistics of the gas sensing market in terms of generated revenue and expected growth for varying technologies.

Figure 1.5

Schematic representing the statistics of the gas sensing market in terms of generated revenue and expected growth for varying technologies.

Close modal
Figure 1.6

Schematic of the components of a typical gas sensor.

Figure 1.6

Schematic of the components of a typical gas sensor.

Close modal
Figure 1.7

(a–c) TEM images of Pd nanocrystals: (a) cubes; (b) octahedral; (c) rhombic dodecahedra. Scale bar: 200 nm. (d–f) SEM images of Au nanocrystals: (d) cubes; (e) octahedral; (f) rhombic dodecahedra. Scale bar: 100 nm.

Figure 1.7

(a–c) TEM images of Pd nanocrystals: (a) cubes; (b) octahedral; (c) rhombic dodecahedra. Scale bar: 200 nm. (d–f) SEM images of Au nanocrystals: (d) cubes; (e) octahedral; (f) rhombic dodecahedra. Scale bar: 100 nm.

Close modal
Figure 1.8

SEM images (inset: drop-coated Teflon membrane) of Pd{110} nanocrystals annealed at 300 °C.

Figure 1.8

SEM images (inset: drop-coated Teflon membrane) of Pd{110} nanocrystals annealed at 300 °C.

Close modal
Figure 1.9

Clark-type back flow nanocrystal/IL electrochemical sensor cell. WE: working electrode; RE: reference electrode; CE: counter electrode; CS: cellulosic spacer; GPM: gas permeable membrane; IL: ionic liquid; O: O-ring. Reproduced from ref. 46 with permission from American Chemical Society, Copyright 2011.

Figure 1.9

Clark-type back flow nanocrystal/IL electrochemical sensor cell. WE: working electrode; RE: reference electrode; CE: counter electrode; CS: cellulosic spacer; GPM: gas permeable membrane; IL: ionic liquid; O: O-ring. Reproduced from ref. 46 with permission from American Chemical Society, Copyright 2011.

Close modal
Figure 1.10

(a) Current density vs. time plots for Pd nanocrystals and Pt polycrystal in different oxygen concentrations (1%–20% v/v). (b) Calibration curves of current density vs. oxygen concentration for the three different electrodes in A. Reproduced from ref. 43 with permission from the Royal Society of Chemistry.

Figure 1.10

(a) Current density vs. time plots for Pd nanocrystals and Pt polycrystal in different oxygen concentrations (1%–20% v/v). (b) Calibration curves of current density vs. oxygen concentration for the three different electrodes in A. Reproduced from ref. 43 with permission from the Royal Society of Chemistry.

Close modal
Figure 1.11

(Left) Multiple measurements of the amperometric hydrogen response of sensors in ionic liquids [Bmpy][NTf2] (black) and [Bmim][NTf2] (red), constant potential at E = +0.4 V vs. Fc+/Fc is the potential used for hydrogen sensing, sampling time is 20 seconds for each measurement. (Middle) Calibration curves of current density vs. hydrogen concentration under four different conditions. (Right) The reversible electrochemical reaction loop that results in reproducible and continuous sensing with little drift. Reproduced from ref. 51 with permission from the Royal Society of Chemistry.

Figure 1.11

(Left) Multiple measurements of the amperometric hydrogen response of sensors in ionic liquids [Bmpy][NTf2] (black) and [Bmim][NTf2] (red), constant potential at E = +0.4 V vs. Fc+/Fc is the potential used for hydrogen sensing, sampling time is 20 seconds for each measurement. (Middle) Calibration curves of current density vs. hydrogen concentration under four different conditions. (Right) The reversible electrochemical reaction loop that results in reproducible and continuous sensing with little drift. Reproduced from ref. 51 with permission from the Royal Society of Chemistry.

Close modal
Figure 1.12

Immobilization of ILs via electrostatic interaction with SAMs.

Figure 1.12

Immobilization of ILs via electrostatic interaction with SAMs.

Close modal
Figure 1.13

Nyquist plots of an EIS study of 1 mM Fe(CN)63−/4− in 0.1 M NaClO4 on a gold electrode modified by soaking sequentially in: 1 mM HS(CH2)10COOH/THF solution for 3 days (open triangle), 0.1 M KOH for 15 min (solid circle), 5 mM IL P666,14DBS/EtOH solution for 2 days (solid triangle) solutions and ethanol (open square). The gold electrode was prepared by annealing in a gas/O2 flame, to produce a smooth surface with predominant Au (111) facets. Note: after each treatment, the gold electrode was rinsed in EtOH and stored for 24 hours before the EIS study was carried out.

Figure 1.13

Nyquist plots of an EIS study of 1 mM Fe(CN)63−/4− in 0.1 M NaClO4 on a gold electrode modified by soaking sequentially in: 1 mM HS(CH2)10COOH/THF solution for 3 days (open triangle), 0.1 M KOH for 15 min (solid circle), 5 mM IL P666,14DBS/EtOH solution for 2 days (solid triangle) solutions and ethanol (open square). The gold electrode was prepared by annealing in a gas/O2 flame, to produce a smooth surface with predominant Au (111) facets. Note: after each treatment, the gold electrode was rinsed in EtOH and stored for 24 hours before the EIS study was carried out.

Close modal
Figure 1.14

Immobilization of ILs via co-assembly.

Figure 1.14

Immobilization of ILs via co-assembly.

Close modal
Figure 1.15

CV results of the gold electrode at different surface states: bare gold, soaked in 1,2-thiol, soaked in IL, and rinsed in CH2Cl2.

Figure 1.15

CV results of the gold electrode at different surface states: bare gold, soaked in 1,2-thiol, soaked in IL, and rinsed in CH2Cl2.

Close modal
Figure 1.16

Immobilization of ILs via sulfide groups.

Figure 1.16

Immobilization of ILs via sulfide groups.

Close modal
Figure 1.17

(a) Layer by layer deposition of polysolfonate styrene and IL; (b) PVF embedded with ILs.

Figure 1.17

(a) Layer by layer deposition of polysolfonate styrene and IL; (b) PVF embedded with ILs.

Close modal
Figure 1.18

PVF film is made by electrochemical oxidation in CH2Cl2 containing 0.1 M tetrabutyl ammonia perchlorate (TBAP) at 0.8 V vs. SCE. A Bio-Rad FTS 175C FTIR spectrometer mounted with a Harrick Seagull was utilized to obtain the IR spectra of the IL, PVF and PVF/IL thin film on the Au substrate respectively.

Figure 1.18

PVF film is made by electrochemical oxidation in CH2Cl2 containing 0.1 M tetrabutyl ammonia perchlorate (TBAP) at 0.8 V vs. SCE. A Bio-Rad FTS 175C FTIR spectrometer mounted with a Harrick Seagull was utilized to obtain the IR spectra of the IL, PVF and PVF/IL thin film on the Au substrate respectively.

Close modal
Figure 1.19

Change of damping resistance of the QCM signal (ΔR%) vs. temperature curve.

Figure 1.19

Change of damping resistance of the QCM signal (ΔR%) vs. temperature curve.

Close modal
Figure 1.20

Contact mode AFM images. (a) A polished Au QCM surface; (b) after it was modified with P66614DBS thin film.

Figure 1.20

Contact mode AFM images. (a) A polished Au QCM surface; (b) after it was modified with P66614DBS thin film.

Close modal
Figure 1.21

QCM frequency change vs. benzene at various concentrations, at 120 °C.

Figure 1.21

QCM frequency change vs. benzene at various concentrations, at 120 °C.

Close modal
Figure 1.22

Frequency change vs. concentration of the P66614DBS/QCM sensor exposed to ethanol (square), heptane (triangle), benzene (star), and dichloromethane (circle) at 120 °C.

Figure 1.22

Frequency change vs. concentration of the P66614DBS/QCM sensor exposed to ethanol (square), heptane (triangle), benzene (star), and dichloromethane (circle) at 120 °C.

Close modal
Figure 1.23

(a) Frequency changes of two ILs/QCM sensors exposed to 80% ethanol, heptane, benzene, and dichloromethane at various temperatures. (b) One with P66614MS IL and another with P6666OCS IL.

Figure 1.23

(a) Frequency changes of two ILs/QCM sensors exposed to 80% ethanol, heptane, benzene, and dichloromethane at various temperatures. (b) One with P66614MS IL and another with P6666OCS IL.

Close modal
Figure 1.24

ATR-FTIR spectra of ionic liquid P66614OCS film (A), ethanol vapor exposed to bare substrate (B), and to a P66614OCS film covered substrate (C).

Figure 1.24

ATR-FTIR spectra of ionic liquid P66614OCS film (A), ethanol vapor exposed to bare substrate (B), and to a P66614OCS film covered substrate (C).

Close modal
Figure 1.25

Normalized relative response pattern of IL sensors (coated with BmiOCS, P66614DBS, P66614OMS, and P66614OCS) for ethanol (red), heptane (green), CH2Cl2 (blue), and benzene (cyan) at 120 °C. The signals are normalized by the weight of IL coatings and the vapor pressure of each analyte.

Figure 1.25

Normalized relative response pattern of IL sensors (coated with BmiOCS, P66614DBS, P66614OMS, and P66614OCS) for ethanol (red), heptane (green), CH2Cl2 (blue), and benzene (cyan) at 120 °C. The signals are normalized by the weight of IL coatings and the vapor pressure of each analyte.

Close modal
Figure 1.26

Single EQCM sensor design and the combined mass and electrochemical signals for accuracy of measurement. Reproduced from ref. 69 with permission from Elsevier, Copyright 2009.

Figure 1.26

Single EQCM sensor design and the combined mass and electrochemical signals for accuracy of measurement. Reproduced from ref. 69 with permission from Elsevier, Copyright 2009.

Close modal
Table 1.1

Common gas analytes and their categories.

Gas targets Categories of gases Ref.
CO2, O2, N2, CO  Atmospheric gases  3 and 4   
CO2, CH4, N2O, H2 Greenhouse gases  6   
CH4, H2   Flammable gas leaks  3   
VOCs, SO2, NOx   Toxic gases  4   
H2   Energy fuel  7   
O2, CO2, N2, He  Asphyxiates and oxygen  8   
Amines, ethylene, alcohols, food aromas  Food quality and storage  3   
O2, NH3, CO2, CH4, H2, VOCs, acetate  Bioreactors and landfill  9   
CO, VOCs  Cabin air quality  3   
HCs, O2, CO2, CO, NOx   Exhaust air gases  7   
CO2, NOx   Outdoor emissions  6   
O3, NOx, formaldehyde, cleaning solvents  Indoor air quality  4   
NH3, chlorofluorocarbon (CFCs)  Refrigerants and intensive farming  3   
O3, NOx, bioparticulates  Asthma at home  7   
VOCs, CH4   Nuisance odors  8   
TATP, HMTD, nitro-explosives, bio-hazardous chemicals  Explosives, chemical and biological attacks  9   
Gas targets Categories of gases Ref.
CO2, O2, N2, CO  Atmospheric gases  3 and 4   
CO2, CH4, N2O, H2 Greenhouse gases  6   
CH4, H2   Flammable gas leaks  3   
VOCs, SO2, NOx   Toxic gases  4   
H2   Energy fuel  7   
O2, CO2, N2, He  Asphyxiates and oxygen  8   
Amines, ethylene, alcohols, food aromas  Food quality and storage  3   
O2, NH3, CO2, CH4, H2, VOCs, acetate  Bioreactors and landfill  9   
CO, VOCs  Cabin air quality  3   
HCs, O2, CO2, CO, NOx   Exhaust air gases  7   
CO2, NOx   Outdoor emissions  6   
O3, NOx, formaldehyde, cleaning solvents  Indoor air quality  4   
NH3, chlorofluorocarbon (CFCs)  Refrigerants and intensive farming  3   
O3, NOx, bioparticulates  Asthma at home  7   
VOCs, CH4   Nuisance odors  8   
TATP, HMTD, nitro-explosives, bio-hazardous chemicals  Explosives, chemical and biological attacks  9   
Table 1.2

Performance factors of gas sensors for real world applications (example applications).

Three characteristics = reliable measurement: precision, accuracy, validity
Precision  The degree of mutual agreement among the data (e.g., standard deviation) 
Accuracy  The deviation from the true value (e.g., absolute error) 
Validity  The validation process of proving that an analytical result is acceptable 
Three characteristics = reliable measurement: precision, accuracy, validity
Precision  The degree of mutual agreement among the data (e.g., standard deviation) 
Accuracy  The deviation from the true value (e.g., absolute error) 
Validity  The validation process of proving that an analytical result is acceptable 
Signal (S), Sx = S − S0, signal provides data for the five analytical “S” of a sensor
Sensitivity  The slope (m) of the calibration curve (Sx vs. [X] curve at [X]i: dS/dx), the reproducibility or precision of the measuring device 
Selectivity  The relative sensitivities: mx /my ability to discriminate between different chemical species. The function of the selective component of a sensor 
Speed of response  t90 or t95 (the necessary time for achieving signals of 90% or 95% of the final equilibrium value) 
Stability  Stability of the signal vs. time, concentration, matrix, temperature, pressure, etc., over short time = noise, over long time = drift 
Cost  Logistics (size/shape, weight, power, application specific) 
Signal (S), Sx = S − S0, signal provides data for the five analytical “S” of a sensor
Sensitivity  The slope (m) of the calibration curve (Sx vs. [X] curve at [X]i: dS/dx), the reproducibility or precision of the measuring device 
Selectivity  The relative sensitivities: mx /my ability to discriminate between different chemical species. The function of the selective component of a sensor 
Speed of response  t90 or t95 (the necessary time for achieving signals of 90% or 95% of the final equilibrium value) 
Stability  Stability of the signal vs. time, concentration, matrix, temperature, pressure, etc., over short time = noise, over long time = drift 
Cost  Logistics (size/shape, weight, power, application specific) 
Table 1.3

The most common gas sensing materials for mass and electrochemical sensing.

Gas sensing materials Properties for gas sensing Advantages Disadvantages
Metal oxides  Porous structure and particle size of sensing film improves sensitivity3   
  • High sensitivity

  • Compact design

  • Low cost

 
  • Requires high operating temperatures and high voltage

  • Low selectivity

 
Carbon nanotubes 
  • High surface area

  • High electrical conductivity

  • Large carrier mobility21 

 
  • Low limit of detection

  • Easily integrable into devices

  • Real-time gas sensing possible

 
  • Unfunctionalized CNTs can agglomerate by van der Waals interaction

  • Reproducibility of detection is a concern

 
Polymers  Easily processable by dispersion20    High sensitivity and rapid response 
  • Poor stability, low surface area, and low sensitivity at room temperature

  • Low thermal stability

 
Ionic liquids 
  • Organic salts that are liquids at room temperature23 

  • Very low vapor pressure

  • High thermal stability24 

 
  • Gases are soluble in ionic liquids

  • Acts as both solvents and electrolytes

  • Excellent reversibility and fast response

 
  • High cost of ionic liquids

  • Trace water in the ionic liquids can change the properties of ionic liquids

 
Gas sensing materials Properties for gas sensing Advantages Disadvantages
Metal oxides  Porous structure and particle size of sensing film improves sensitivity3   
  • High sensitivity

  • Compact design

  • Low cost

 
  • Requires high operating temperatures and high voltage

  • Low selectivity

 
Carbon nanotubes 
  • High surface area

  • High electrical conductivity

  • Large carrier mobility21 

 
  • Low limit of detection

  • Easily integrable into devices

  • Real-time gas sensing possible

 
  • Unfunctionalized CNTs can agglomerate by van der Waals interaction

  • Reproducibility of detection is a concern

 
Polymers  Easily processable by dispersion20    High sensitivity and rapid response 
  • Poor stability, low surface area, and low sensitivity at room temperature

  • Low thermal stability

 
Ionic liquids 
  • Organic salts that are liquids at room temperature23 

  • Very low vapor pressure

  • High thermal stability24 

 
  • Gases are soluble in ionic liquids

  • Acts as both solvents and electrolytes

  • Excellent reversibility and fast response

 
  • High cost of ionic liquids

  • Trace water in the ionic liquids can change the properties of ionic liquids

 
Table 1.4

Structure of common ILs commercially available.a

Structure of cationsStructure of anionsName or abbreviation of nameb
   N7,7,7,7SO3-ph-C12H25, N4,4,4,4SO3-ph-C12H25  
 , , CH3–SO3  P6,6,6,14SO3-ph-C12H25, P8,8,8,8SO3-ph-C12H25, P4,4,4,14SO3-ph-C12H25, P6,6,6,14CH3SO3, P6,6,6,6(+)camphorsulfonate, P4,4,4,4CH3SO3  
 , (CF3SO2)2N, CH3SO3, BF4, HSO4  bmi(CF3SO2)2N, bmi(+)camphorsulfonate, bmiBF4, bmiHSO4, bmiCH3SO3  
 (CF3SO2)2N, PF6  bbi(CF3SO2)2N, bbiPF6  
 CH3SO3,  beiCH3SO3, bei(+)camphorsulfonate 
 CH3SO3  pmiCH3SO3  
 PF6  hpPF6  
 CH3SO3  bpCH3SO3  
Structure of cationsStructure of anionsName or abbreviation of nameb
   N7,7,7,7SO3-ph-C12H25, N4,4,4,4SO3-ph-C12H25  
 , , CH3–SO3  P6,6,6,14SO3-ph-C12H25, P8,8,8,8SO3-ph-C12H25, P4,4,4,14SO3-ph-C12H25, P6,6,6,14CH3SO3, P6,6,6,6(+)camphorsulfonate, P4,4,4,4CH3SO3  
 , (CF3SO2)2N, CH3SO3, BF4, HSO4  bmi(CF3SO2)2N, bmi(+)camphorsulfonate, bmiBF4, bmiHSO4, bmiCH3SO3  
 (CF3SO2)2N, PF6  bbi(CF3SO2)2N, bbiPF6  
 CH3SO3,  beiCH3SO3, bei(+)camphorsulfonate 
 CH3SO3  pmiCH3SO3  
 PF6  hpPF6  
 CH3SO3  bpCH3SO3  
a

(a) bmiBF4, bmiN(SO2CF3)2 and hpPF6 can be prepared via metathesis of the corresponding imidazolium chlorides with appropriate salts.77  (b) Water-immiscible ionic liquids, such as bbiN(SO2CF3)2 and bbiPF6, can be prepared based on a process known as “one-pot synthesis of ionic liquids”.78  By mixing aqueous formaldehyde with two equivalents of 1-butylamine, hexafluorophosphoric acid, or bis(trifluoromethanesulfon)imide and aqueous glyoxal solution, the hydrophobic ionic liquid (lower layer) thus formed can be separated directly from the reaction mixture.79  (c) Sulfonate ionic liquids with various cations can be made via an alcohol-to-alkyl halide conversion method, which is also a one-pot synthesis of ionic liquids.80  By using primary alcohols (ROH), suitable acids (HA), the 1,3-dialkylmidazolium halides, pyridinium halides, tetraalkylammonium halides and tetraalkylphosphonium halides (all designated as Q+X) can be converted to the new ionic liquids (Q+A), with the anions being the conjugated bases of the acids used.

b

Nl,m,n,j and Pl,m,n,j represent the tetraalkylammonium and the tetraalkylphosphonium, respectively. The subscripted numbers, l, m, n, and j represent the numbers of carbons in each alkyl substitutes. For example, N7,7,7,7 is tertraheptylammonium. The anion, dodecylbenzenesulfonate (SO3-ph-C12H25), was also abbreviated as DBS in the text. bmi and bbi are 1-butyl-3-methylimidazolium and 1,3-dibutylimidazolium, respectively. bei and pmi are 1-butyl-3-ethyl-imidazolium and 1-propyl-3-methyl-imidazolium, respectively. hp and bp are hexylpyridinium and butylpyridinium, respectively.

Table 1.5

Empirical Henry constant.a

Ethanol Benzene CH2Cl2 Heptane
beiOCS  3.25 × 105   1.05 × 106   1.15 × 106   1.87 × 106  
bmiOCS  5.57 × 105   2.03 × 106   1.55 × 106   2.58 × 106  
P6666OCS  5.80 × 105   5.51 × 105   1.31 × 106   1.42 × 106  
P66614OMS  3.85 × 104   4.33 × 105   2.25 × 105   0.31 × 106  
P66614DBS  2.00 × 105   8.26 × 105   8.75 × 105   1.26 × 106  
P8888DBS  7.23 × 105   1.25 × 106   9.71 × 105   1.88 × 106  
N7777DBS  1.27 × 106   1.37 × 106   NA  2.50 × 106  
Ethanol Benzene CH2Cl2 Heptane
beiOCS  3.25 × 105   1.05 × 106   1.15 × 106   1.87 × 106  
bmiOCS  5.57 × 105   2.03 × 106   1.55 × 106   2.58 × 106  
P6666OCS  5.80 × 105   5.51 × 105   1.31 × 106   1.42 × 106  
P66614OMS  3.85 × 104   4.33 × 105   2.25 × 105   0.31 × 106  
P66614DBS  2.00 × 105   8.26 × 105   8.75 × 105   1.26 × 106  
P8888DBS  7.23 × 105   1.25 × 106   9.71 × 105   1.88 × 106  
N7777DBS  1.27 × 106   1.37 × 106   NA  2.50 × 106  
a

Calculated according to equation pi= Cimi, where pi is the partial pressure of the samples in gas phase, Ci is the Henry constant (unit: Pa), and mi is the molar fraction of the samples in IL. Saturated vapor pressures were obtained from vapor pressure data. mi was calculated from the Sauerbrey equation (for a 10 MHz crystal, the sensitivity is 1.02 ng cm−2 Hz−1) and the total weight of the IL film.

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