Chapter 1: Ionic Liquids for Gas and Vapor Sensing Applications

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).

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

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.

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).

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.

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 NO_X 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.

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.

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.

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.

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.

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 sensors^10–18  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 

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.

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).

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).

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.

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).

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 (P_666,14 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)₆^3−/4− solution containing 0.1 M NaClO₄ by cyclic voltammetry (CV) and electrochemical impedance spectrometry (EIS) at each step of the above modifications.

The EIS data reveal that the electron transfer resistance (R_et) 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 R_et increased further. This result confirmed that a strong immobilized IL layer was made, otherwise the R_et 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 images^65,66  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).

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.

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 (P_666,14 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).

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).

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 P_6,6,6,14LABSO₃/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.

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.

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.

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.

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.

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.

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.

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).

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.