Chapter 1: Introduction of Nanotechnology and Sustainability

The importance of sustainability has never been felt so greatly before, especially with the recent occurrences of extreme weather, stronger storms, global warming, etc. The depletion of resources, rising sea water levels and ozone layer depletion are also signs of concern, as they could threaten the survivability of future generations. Yet, the current efforts that aim to promote sustainable development, such as the implementation of carbon tax, are not very fruitful. This could be accounted to the relatively heavier emphasis on result efficiency in this fast-paced world, which merely results in the shift of the companies to countries with less stringent regulations. Furthermore, cost and inefficient processes also contribute to unsuccessful attempts.

Nanotechnology is defined by the National Nanotechnology Initiative (NNI) as “the understanding and control of matter at the nanoscale, at dimensions between approximately 1 and 100 nanometers, where unique phenomena enable novel applications”.¹  The ability to control matter at the nanoscale presents new possibilities, such as more efficient processes and accessibility to remote regions, which would be advantageous for new development. At the nanoscale, the surface area of the material is greatly increased, enabling higher reactivity. A smaller scale also suggests that less energy is required for operation, thereby leaning toward sustainability. Granted with small size, nanoscale materials could reach regions that were once impossible to reach, which is beneficial to the clinical field, where human anatomy is the limitation for treatment. Furthermore, materials in the range of nanoscale are found to possess different sets of properties from the bulk material itself. For example, the material in bulk form could be non-toxic but could become toxic when reduced to the nanoform.²  A possible explanation for the change could be attributed to the enhanced reactivity arising from the increased surface area. Besides the divergence in properties between bulk and nanoscale materials, property variation among the nanomaterials itself is not uncommon. For instance, nanoparticles (NPs) consisting of the same material but of different sizes were shown to possess different absorption wavelengths.³  The size of the NPs was also found to be a parameter that affects absorption performance.⁴  Additionally, the morphology of the NPs determines their reactivity and other properties.⁵  This divergence in the properties according to the morphology and size of the NPs could potentially be adopted as a controlling element for new unique applications. With the broad spectrum of properties exhibited by nanomaterials in magnetic, electrical, mechanical and catalytic domains, nanotechnology is no doubt indispensable in various sectors, ranging from transport, energy and agriculture to healthcare and information and communication⁶  to achieve both new development and sustainability. For instance, nanocapsules are useful for drug delivery and pharmaceutical purposes, whereas quantum dots are sought after for biosensors and cell imaging.⁵  Nanobased technology is also identified with the potential to promote sustainability in agriculture and food systems through various means but not limited to nanobased fertilizers and nanobased water treatment for agricultural fields.⁷ 

Nanomaterials exist in various forms and dimensions, such as NPs, nanofibers, nanotubes and nanoplates (Figure 1.1). Their properties are heavily dependent on the structure, size and shape, and therefore, different forms of nanomaterials are suited for distinct applications. More details of each type of nanomaterial will be delved into in the following sections.

NPs are defined as “particulate dispersions or solid particles with a size in the range of 10–1000 nm”.⁸  Their synthesis could be categorized into two main branches, namely, the bottom-up and the top-down approach. In the former approach, NPs are formed through the building up of atoms and molecules, whereas in the latter, NPs are obtained through the breaking down of bulk material. The bottom-up approach is more efficient as it enables large-scale and fast synthesis with a more homogeneous structure and lower defect rate. Some of the commonly used processes are green synthesis, the sol–gel process, spinning and the biochemical process.⁵  In contrast, the top-down approach is more destructive as it involves etching out existing crystal plates that are present on the substrate. Inevitably, the derived NPs are not as consistent.⁵  NPs could be further divided into three major clusters based on their material type – inorganic, organic and carbon-based – and the different types of NPs will be discussed in the subsequent sections.

Gold, silver and copper are some of the more established elements in this category, populated by metal and metal oxides. The reason for the attention garnered by these materials could be narrowed down to the tunability of optoelectronic properties, achievable through the manipulation of size and shape of the inorganic NPs. Within the group, gold NPs are considered one of the most highly regarded materials. Their biocompatible nature appeals greatly to biomedical usage, and their presence in this field could be perceived in various forms, such as carriers for drugs⁹  and genes,^10,11  tracers for bioimaging^12,13  and agents for cancer therapy.¹⁴  While other inorganic NPs have also been examined for similar functions, toxicity and poor degradability remain a persisting concern,^15,16  which would have to be resolved before inorganic NPs could be utilized in the aforementioned applications. In response to the concern, numerous studies have emerged over the years with different techniques to improve the biocompatibility of inorganic NPs.¹⁷  At present, researchers have identified surface modification as a probable means to overcome the incompatible status.¹⁸  Specifically, PEGylation appears to be a good strategy to improve the compatibility of NPs by concealing their foreign identity and, ideally, avoiding evoking the immune response.¹⁸  In the circumstance that toxicity is caused by non-specific accumulation, selective biodegradable NP¹⁹  carriers could be used such that they would not remain in the system for an excessively long duration. Alternatively, means to promote renal excretion^19,20  would be relevant in circumventing the non-specific accumulation and the consequent toxicity. The methods to produce these inorganic NPs fall under three big branches – physical, chemical and biological. Microwave irradiation,²¹  laser ablation²²  and hydrothermal process²³  are examples of the physical methods that have been used to obtain the NPs. In terms of chemical techniques, there are the sol–gel and phase-transfer methods.²⁴  Recently, copper oxide NPs were successfully synthesized using Aspergillus terreus,²⁵  in which a biological means is exploited to generate inorganic NPs. With the continuous development in the field, inorganic NPs would remain relevant and no doubt an invaluable asset to further the frontier of existing technology.

Organic NPs are solid particles with diameters in the range of 10 nm to 1 µm that are made up of organic compounds.²⁶  Considering the work performed on inorganic NPs, the studies conducted on their organic counterpart seem to pale in comparison. For example, existing persistent luminescence agents still mainly rely on inorganic NPs.^27–29  The fewer explorations could be attributed to complicated preparation procedures and poor solubility of the organic compounds, which inevitably cause it to be less efficacious than intended.¹⁵  Yet, research in this area is of paramount importance as organic NPs possess high potential waiting to be unleashed. Their inherent biocompatibility is highly relevant to biomedical applications. In addition, organic NPs, with the flexibility to be modified into different structures and sizes³⁰  and be equipped with unique functions,³¹  have unlimited possibilities. In fact, recent research has seen a surge in the development of organic NPs. Dong and coworkers designed an organic NPs by introducing triphenylamine to diketopyrrolopyrrole molecule functionalized with thiophene group.³²  As a result, these organic NPs demonstrated improved intersystem crossing ability through the heavy atom effect and they also had refined charge transport capacity and bathochromic shift absorption.³²  Researchers have also attempted to streamline the synthesis process. In their study, Wei and their team successfully simplified the synthesis reaction and attained amphiphilic luminescent polymers through a “one-pot” mercaptoacetic acid locking imine reaction.³³  At present, some of the techniques that have been developed to synthesize organic NPs are nanoprecipitation,²⁷  microemulsion,³⁴  self-assembly,^35,36  electrospraying³⁷  and photoablation of aggregation-induced enhanced emission luminophores.³⁸  Among the new functionalities introduced to the organic NPs, the organelle-targeting function seems to have become popular in recent years. Studies have succeeded at targeting organic NPs to selected regions such as lysosomes,³⁹  mitochondria⁴⁰  and kidney.³⁰  With the developments to date, it would not be surprising to encounter the extensive use of organic NPs in the fields of drug delivery, bioimaging and photodynamic/photothermal therapy in the near future.

Among the different types of carbon-based nanostructures, carbon nanoparticles (CNPs) seem to be the most attractive. Being endowed with good electrical conductivity, high sensitivity, large specific surface area and impressive electrochemical activity, CNPs have infiltrated and transformed the electrochemical sensing platform. CNP-modified electrodes, which combine CNPs and electrochemical techniques, have been shown to be capable of determining a broad range of analytes ranging from chemicals^41,42  to bacteria.⁴³  Higher efficiency was also detected for these CNP-modified electrodes.⁴¹  Besides the revolutionary impact in the electrochemical sensing field, the presence of CNPs is also apparent in the biomedical field. Their biocompatible and non-toxic nature is the main factor that has seen the expansion of its scope of applications to the biomedical field.⁴⁴  Their bright fluorescence and tunable photoluminescence characteristics appeal to biosensing,⁴⁵  photodynamic therapy,^46,47  drug delivery⁴⁸  and bioimaging purposes.^49,50  With their dimensions in the nanoscale range, CNPs easily traverse cell membranes, which is not impeded even by binding with drug molecules,⁵¹  proteins⁵²  or genes.⁵³  This advantage, coupled with their biocompatible properties, makes CNPs well suited as carriers. In addition, it is not surprising to find the presence of CNPs in the photo-/electrocatalysis arena.^54,55  The adjustable band gap that arises from the introduction of CNPs empowers us to tap into the visible light⁵⁴  and near-infrared⁵⁶  range of solar energy to promote the catalysis process. Techniques that have been explored for the synthesis of CNPs include but are not limited to microwave irradiation,⁵⁷  pulsed laser irradiation,⁵⁸  hydrothermal^59,60  and ultrasonic treatment.⁶¹ 

Nanomaterial can exist in the form of nanofibers, which is made possible by progress in the arena of electrospinning. Particularly, the thickness of the fibers is found to vary along with numerous parameters, which include but are not limited to the distance between the nozzle and the collector, flow rate of fluid, surface tension of fluid and dielectric permittivity.⁶²  Through careful manipulation of these variables, nanofibers could be achieved. This process utilizes electrostatic force to direct the charged polymer solution across the charged field to the collector. According to the requirement in the fiber orientation, the collector may be a flat surface or a rotating drum. These nanofibers demonstrate the potential to be implemented in diverse applications ranging from filters,⁶³  food packaging⁶⁴  and electrocatalysts⁶⁵  to carriers for therapeutic agents⁶⁶  and wound-healing applications.⁶⁷ 

With a high length-to-diameter ratio, nanotubes are regarded as one-dimensional structures. Briefly, nanotubes are formed via rolling sheets into tubes, and this group of materials could be further classified according to the layers of sheets utilized to wrap into the cylindrical structure. When a single sheet is used, the obtained structure is known as a single-walled nanotube (SWNT), whereas multi-walled nanotubes (MWNTs) are attained by wrapping layers of concentric single-walled nanotubes into a cylinder. Owing to the difference in the layers of sheets that contribute to the formation of nanotubes, these structures consequently possess distinct properties. For instance, MWNT was revealed to exhibit better fluorescence quenching ability than SWNT, whereas the latter demonstrates higher sensitivity to the base sequences of Fluor-ssDNA.⁶⁸  In comparison with the SWNT arrays, MWNT arrays were found to experience higher friction and adhesive force.⁶⁹  As such, nanotubes appeal to a broad range of applications ranging from sensors,⁷⁰  electrodes⁷¹  and filters^72,73  to biomaterials⁷⁴  and energy-harvesting applications.⁷⁵  Carbon nanotubes in particular are extremely alluring within this group of materials. Apart from having desirable mechanical properties, carbon nanotubes also possess good electrical conductivity and thermal stability, which are prerequisites for applications such as electrodes and sensors. Recently, carbon nanotubes were investigated for use in tissue engineering. Promising results were attained as osteogenic differentiation of the mesenchymal stem cells was shown to be stimulated.⁷⁴ 

Nanoplates can be considered two-dimensional structures, given that the third dimension is in the nanometer range. As a higher-order nanostructure, there are numerous approaches to modify its structure. For instance, the morphology of the nanostructure could be lamellar, fragmented or compact.⁷⁶  Intercalation and exfoliation are also two of the commonly used techniques to vary nanoplates. Specifically, the manipulation of temperature for exfoliation can lead to several changes in the morphology of the nanoplates. It was shown that a higher temperature used for the treatment generally reduced the Langmuir surface area and average pore diameter of graphene nanoplates.⁷⁷  The pore volume and mesopore volume in the graphene nanoplates were found to increase with temperature for up to 300 °C, and beyond this temperature, a decline was observed.⁷⁷  Besides the thermal exfoliation, many other methods have been developed to generate the nanoplates, such as seed-mediated growth,⁷⁸  template-assisted self-assembly⁷⁹  and chemical vapor deposition.⁸⁰  With a large specific surface area, these materials are widely investigated for use in sensors due to their high sensitivity. To date, nanoplate-based sensors have been shown to be capable of detecting a large spectrum of analytes, ranging from glucose⁸¹  and DNA⁸²  to cancer biomarkers⁸³  and gas.⁸⁴  This technology has also been explored as an alternative drug delivery system.⁸⁵  Recently, it was reported that silver nanoplates with a very narrow gap were successfully constructed and this development will improve future analysis performed using surface-enhanced Raman scattering.⁷⁸ 

In recent years, efforts have been devoted to discovering and optimizing greener synthetic methods that follow one or more of the Twelve Principles of Green Chemistry. Existing methods that have been developed with green chemistry in mind can be extrapolated to nanotechnology development in terms of synthetic methodologies and green manufacturing processes (Table 1.1).

Mechanochemistry is the use of mechanical force to break and make new chemical bonds. It has been employed not only in inorganic solid-state syntheses and phase transformations⁸⁶  but also increasingly in organic syntheses, such as in forming and breaking carbon–carbon, carbon–heteroatom and metal–ligand coordination bonds.⁸⁷  The synthesis of high-surface-area carbon⁸⁸  as well as metal–organic frameworks⁸⁹  has also been shown to be possible.

Mechanochemical transformations are usually carried out via ball milling, with ball bearings commonly made of hard ceramics such as yttria-stabilized zirconia (YSZ), alumina or stainless steel, but other types of mills such as planetary mills, vibration mills and rolling mills also exist.⁸⁶  Small amounts of solvent are sometimes added to reduce friction and improve the degree of mixing of solids.⁸⁶  Sonochemistry can be considered a subcategory of mechanochemistry and involves the use of ultrasound energy for chemical reactions.⁹⁰ 

Mechanochemical synthesis is advantageous in that it can be performed in the absence of a solvent, which is usually a significant source of chemical waste, and requires relatively unsophisticated equipment. More interestingly, new modes of reactivity have been unlocked using sonochemical methods. For example, force-responsive polyladderene has been shown to undergo a cascade-type “unzipping” reaction to form polyacetylene upon sonication.⁹¹ 

The side effects unique to a mechanochemical approach must also be taken into consideration.⁸⁶  For example, the temperature within the reactor can rise over the course of milling, up to 200 °C or higher. Contamination originating from the wear and tear of the mill or ball bearings or residue from previous reactions can be an issue for applications requiring high purity. Finally, milling has also been known to lead to the deformation and loss of faceting of crystallites, as well as the aggregation of particles, both of which are processes that can have adverse effects on nanoparticulate products.

Microwave irradiation involves the application of electromagnetic radiation of between 0.3 and 300 GHz in frequency to a sample and is an alternative to traditional convective heating.^92,93  It works on the principle of dielectric heating, whereby dipoles in the molecules of the solvent or reactant align with the electric field component of incoming microwaves, causing molecular rotational velocities to increase and producing heat via molecular friction between rotating molecules.⁹³  Heating throughout the bulk of the sample then occurs via conduction or radiation. As microwaves are non-ionizing and are not in the energy range to break chemical bonds, any induced chemical transformations are purely a result of temperature increases.⁹³ 

Microwave heating has the advantage of being fast compared to conventional heating.⁹³  Since microwave heating onsets throughout the bulk rather than the edges (volumetric heating), thermal gradients are also gentler and heating is more uniform, suppressing local temperature fluctuations and unwanted side reactions and engendering greater reproducibility in general. Simple microwave reactions can be performed in home microwave ovens, but more sophisticated models that include features such as stirring and temperature and pressure monitoring exist.⁹³ 

However, microwave heating is limited to solvents that possess a net permanent dipole moment, such as water, ethanol, acetone and dimethylsulfoxide (DMSO). Solvents such as carbon tetrachloride and benzene lack a dipole moment and are essentially microwave transparent. In the case where the bulk solvent is not a microwave absorber, an additional component that is capable of microwave heating (e.g. inert metal oxides, carbides) known as a susceptor can be added as a heating element.⁹² 

Microwave synthesis has been broadly applied to both inorganic and organic syntheses.^93,94  Of note is the successful demonstration of its application to metallic NPs.⁹⁵  Due to more homogeneous thermal profiles compared to conventional heating, microwave irradiation has allowed the synthesis of NPs at smaller sizes, lower polydispersities, and a greater degree of crystallinity than conventional heating.

Photochemistry commonly refers to the use of electromagnetic radiation in the visible or ultraviolet spectrum to drive chemical reactions.^96,97  Light is perhaps one of the ‘cleanest’ reagents available, since it leaves no side products and, in the case of solar light, is readily available and does not leave a carbon footprint.⁹⁶  Photochemistry's role in discussions of sustainability has traditionally been in renewable solar power generation,^98,99  but the development of direct photochemical synthesis and photoredox catalysts to drive organic transformation has made significant progress in recent years.^100,101  Direct photochemistry involves reactant molecules absorbing photons and entering a more reactive high-energy excited state,¹⁰²  whereas, in photoredox catalysis, photons are absorbed by catalysts, which are then activated and able to oxidize or reduce reactant molecules.¹⁰¹  The development of photoredox catalysts was motivated by the drive to make use of freely available sunlight, since most organic molecules do not absorb light within the solar spectrum, but transition metal–based catalysts, such as tris(bipyridine)ruthenium(ii) chloride, do.¹⁰⁰  Examples of photoredox catalysis include reductive dehalogenation and the α-trifluoromethylation of aldehydes.¹⁰³ 

In the context of nanotechnology, photochemical methods have most commonly been employed for silver NP synthesis due to the well-known photoreduction of Ag⁺ ions.^104,105  Gold nanorods of controllable aspect ratios have also been synthesized using Ag⁺ ions as photomediators.¹⁰⁶ 

Electrochemistry generally refers to the use of electricity to make and break chemical bonds.¹⁰⁷  Electrocatalysis refers to the use of a catalyst that is reduced or oxidized by the electrode prior to interaction with the substrate and may be either an extraneous species that is attached to the electrode or dissolved in an electrolyte solution or the electrode surface itself.¹⁰⁷  At the heart of much of modern electrochemical research is meeting sustainable development goals, whether that is in the form of higher-capacity, longer-lasting batteries for practical electric vehicle or grid storage applications or efficient electrolyzers capable of splitting water to produce hydrogen to use as renewable and clean fuel.¹⁰⁸ 

Electrochemical methods have seen extensive use in manufacturing. The Hall–Héroult process for aluminum production and adiponitrile synthesis are perhaps the two most well-known and largest processes by production volume.¹⁰⁹  In the context of materials production, the electrodeposition of metals and compounds such as metal oxides is well known and exploited in industrial processes such as electroplating and electrowinning.¹⁰⁷ 

However, a recent trend has been the general application of electrochemical methods to organic synthesis in the field that has come to be known as organic electrosynthesis, which involves the use of electrodes to supply oxidizing or reducing equivalents to substrate molecules in place of chemical agents.^110–112  Using electrochemistry to drive organic transformations has the advantages of avoiding hazardous redox reagents and their associated waste products, being able to generate reactive intermediates in situ and accessing new reactivities.¹¹⁰  For example, a safer electrochemical analog to the Birch reduction has been developed with excellent chemoselectivity and functional group tolerance.¹¹³ 

Electrochemistry has also been widely explored for nanomaterial synthesis. Nanostructures of metals such as copper, palladium, platinum, gold and metal oxides such as cerium oxide and tin oxide have been made with varying morphologies.^114–117  The electrosynthesis of nanostructured polymers has also been demonstrated but has been limited to electronically conductive polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT), poly(aniline) (PANI), poly(pyrrole) (PPy) and poly(thiophene) (PT).¹¹⁸  Resultant polymer morphologies (nanospheres, nanowires, nanotubes, nanosheets, etc.) can be controlled via monomer concentration, applied potential, applied current density or the use of templates.¹¹⁸  Here, the green advantage offered by electrosynthesis is the substitution of harmful redox reagents with electrodes that can be polarized to different potentials to provide matching driving forces. In some cases, the discovery of new reactivity may also pave the way toward safer reactions compared to traditional chemistry.

Flow chemistry refers to the use of continuously flowing streams of liquid in place of batch reactors to perform reactions.^119,120  Flow reactions are often performed in microreactors that contain microchannels with diameters of 1 mm and below and accommodate the use of small volumes of solvents and reagents. Due to the array of advantages afforded, flow methods have seen a rise in both academic and industrial interest in recent years. To begin, the narrow channels in flow microreactors in combination with flow rate and channel geometry control enable the implementation of well-defined flow regimes such as laminar, slug and annular flow, as opposed to the turbulent flow present in most, if not all, batch reactors.¹¹⁹  This allows more precise control of heat transfer and reactant mixing times and is beneficial for reproducible and fine optimization of chemical reactions. For instance, side reactions in general and the thermal runaway of exothermic reactions in particular are suppressed due to more homogeneous thermal profiles. The larger surface-area-to-volume ratio in flow reactors also greatly increases phase transfer and reaction rates in multi-phasic systems involving gas–gas, gas–liquid and liquid–liquid systems, for example.¹¹⁹  The scale-up of flow processes is easily achievable by either increasing the number of reactors running in parallel (numbering up) or simply running a process continuously for the needed duration (scaling out).¹¹⁹ 

Flow methods also work hand in hand with the other synthetic approaches outlined here, such as photochemistry and electrochemistry. In photochemistry, the yield is strongly dependent on the irradiation efficiency, which is dependent on the path length l according to the Beer–Lambert Law, A = εcl, where A is absorbance and ε the molar extinction coefficient. Hence, flow reactors with narrow channels maximize the irradiation efficiency by exposing a greater proportion of reactants to incident light.¹²¹  In electrochemistry, energy losses can occur via resistive heating between the two electrodes due to solution resistance, which necessitates the addition of a supporting electrolyte to increase conductivity. Since this solution resistance R_solution, is a strong function of distance (R_solution = d/κ, where d is the distance between driving electrodes and κ the solution conductivity), the short lateral distance across a flow microchannel is again beneficial for minimizing energy losses and can even accommodate forgoing the use of supporting electrolytes in this case.¹²² 

The precise control of reaction conditions afforded by flow chemistry is particularly advantageous for NP synthesis.¹²³  The bottom-up synthesis of NPs often requires the hot injection method, wherein organometallic reagents are added to a solvent after it has been heated up in order to favor homogeneous nucleation and produce monodisperse particles. However, this method is not amenable to scaling up using batch methods. The more precise heat and mass transfer control that flow chemistry offers allows better control over nucleation kinetics, enabling high-quality NP synthesis. In addition, the separation and purification of NPs can be streamlined by coupling the continuous flow process with a diafiltration setup, where the product stream is pumped through a nanoporous membrane, simplifying the recovery process and minimizing solvent use.¹²⁴  Unfortunately, despite its many advantages, flow chemistry is not without its drawbacks: heterogeneous reaction media, for example, can severely clog tubing.¹²⁵ 

Biological synthesis, or biosynthesis, entails the use of living organisms, often plants or microbes, as reactors for generating chemical products.^126,127  Biosynthesis using plants, for example, can be carried out in vivo, whereby the plant absorbs and processes reactant ions, or in vitro, whereby plant extract is used as the reaction medium.¹²⁶ 

The biosynthesis of NPs has been shown to be possible in a variety of eukaryotic (fungi, algae and plants) and prokaryotic (bacteria) organisms and has been a burgeoning field.¹²⁸  To date, the biosynthesis of NPs of gold, silver, palladium and platinum, as well as an assortment of metal oxides such as cupric oxide, zinc oxide, and iron oxide, has been demonstrated.¹²⁸  The properties of biosynthesized NPs such as size, size distribution and shape can be tuned by controlling the organism type (which in turn influences the enzymes involved) and reaction conditions. An advantage of biosynthesis is the avoidance of harmful chemicals, which is an advantage in terms of waste avoidance and the safety profile of resultant NPs. Reducing agents such as sodium borohydride are substituted with naturally derived compounds such as polyphenols and catechins, and the use of organic solvents is avoided.¹²⁸  Different classes of organisms also pose different advantages and disadvantages: for instance, plant-based in vivo NP biosynthesis has been observed to occur at faster rates than in microbes, and using plants circumvents the need for culture media.¹²⁶  However, research is still ongoing to better understand the mechanisms of NP formation and growth, especially using in vivo methods, and to improve control over the polydispersity of biosynthesized NPs.

Many of today's industrial processes were developed in the previous century, meaning that the choice behind many feedstocks and chemicals was driven by economic pragmatism. As a key example, the chemical industry heavily relies on crude petroleum not just as fuel but also as feedstock, with 96% of all organic chemicals made being derived from petroleum.¹²⁹  This is problematic for two reasons: the first being the continual net emission of greenhouse gases such as carbon dioxide from the incineration of carbon-containing waste exacerbating the climate change issue, and the second being the non-renewable nature of fossil fuels. Both reasons sound the death knell on the long-term sustainability of using crude petroleum as feedstock and indicate the urgent need to find sustainable and greener alternatives.

Biomass has and will continue to play a major role as an alternative source of chemical feedstock. Biomass can be a source of renewable and new precursors¹³⁰  that are nominally carbon neutral due to the carbon present having originally been sourced from the atmosphere via the photosynthetic process in plants.¹²⁶  It is hence inherently compatible with green chemistry, since it already adheres to a number of its principles: biomass is, by definition, biodegradable and does not leave persistent contaminants in the environment; it is far less likely to be toxic in general compared to the man-made chemicals used in many of today's processes; it is renewable within timescales relevant for human societies.

Polysaccharides are comprised of a group of macromolecules called carbohydrates that consist of repeating monosaccharides. Naturally occurring polysaccharides, such as cellulose, pectin, alginate and chitin, are structural building blocks derived from plants, animals or microbes. It was reported that the interest in polysaccharides has increased significantly with the goal of designing and developing novel renewable materials for future applications. Polysaccharides possess several advantages, such as low cost, hydrophilicity, stability, safety, biodegradability and non-toxicity. The presence of several functional groups such as hydroxyl, carboxyl and amino groups on polysaccharides allows for easy chemical modifications resulting in many types of polysaccharide derivatives. In recent years, nanomaterials derived from natural polysaccharides have received significant interest from academic and industrial research laboratories.

Lignin constitutes about 15–35% of lignocellulosic biomass obtained from the processes of wood and paper processing industries, and it is the second most abundant natural material on earth. It consists of cross-linked polyphenolic structures, which provide structural support for plants, and it plays an important role in the formation of cell walls. Lignin has been long known to be environmentally friendly and possesses antioxidant and anti-UV properties, as well as good antimicrobial activities. This makes lignin exceptionally interesting and a strong candidate for the development of new and sustainable nanomaterials.

Polynucleotides and polypeptides are also important and abundant biomacromolecules in nature. Proteins, composed of amino acids, are the most versatile macromolecules in the body and possess many functionalities. One of the unique characteristics of polypeptides is their self-organization behavior, where the polymer chains can self-assemble into secondary, tertiary or quaternary structures due to the amphiphilic characteristics of various functional groups on the polymer backbone. On the other hand, nucleic acids are the most important biological macromolecules found in living organisms. The functions of nucleic acids are mainly encoding, transmitting and expressing genetic information in living systems. They include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNAs can also be self-assembled via bottom-up construction to serve as scaffolds for many inorganic molecules and biopolymers.

In the context of sustainable development, the massive and increasing volumes of waste modern industrial societies generate a genuine cause for concern.¹³¹  Improper disposal of waste can lead to its persistence in the natural environment for prolonged periods of time, having potentially devastating effects. The pollution of oceans by plastic waste and its repercussions on wildlife is a sobering reminder of this. Incineration, another popular tactic, is also an inherently unsustainable method of waste management, since it can release toxic compounds into the environment and is a significant contributor to carbon emissions.¹³²  Hence, efforts to achieve sustainable development must reconcile with the status quo of waste generation. The exploitation of waste as a resource for the chemical industry is a tremendous opportunity for reassimilating waste products into supply chains and making progress toward a more sustainable circular economy model.¹³¹ 

Carbon dioxide has increasingly been viewed as an attractive feedstock for carbon-containing products as it is the terminal product of carbon oxidation and its incorporation theoretically mitigates the carbon footprint of the product (barring considerations of energy input).¹³³  Carbon dioxide has been used as a reagent for the production of bicarbonate and carbonate salts, as well as in the electrocarboxylation of organic compounds such as halides.¹³³  In addition, the synthesis of copolymers incorporating carbon dioxide such as polycarbonates and polyurethanes has been shown to be possible with the use of reactive monomers such as cyclohexene oxide, hence overcoming the thermodynamic stability of carbon dioxide.¹³³  Supercritical carbon dioxide has also found use as an environmentally friendly, non-toxic solvent with good solvating properties and has been used for decaffeination and in dry cleaning.

Though exciting, the abovementioned developments represent specialized applications that represent a small market share of all carbon-derived chemicals. In terms of upending the current petroleum-dependent, net carbon–emitting paradigm, the valorization of carbon dioxide into higher-order hydrocarbons has been gaining significant traction.^134,135  The electrochemical conversion of carbon dioxide allows the process to be coupled with renewable sources of electricity (such as solar and wind) and avoids the need for fossil fuel–derived energy.^136,137  There is promise for the production of carbon monoxide (a component of the industrially useful syngas), ethylene (useful for plastic production), and various alcohols and longer alkanes (useful as solvents and fuels) from carbon dioxide, but challenging issues such as product selectivity (especially for higher-order products) and the ability to make energy-efficient electrolyzers that can run at practical current densities remain.¹³⁸ 

Plastic is a blanket term for a wide-ranging assortment of versatile polymer-based materials, such as poly(vinyl chloride), poly(styrene) and poly(ethylene) of varying densities. Traditionally, plastic waste has been managed in one of three ways: mechanical recycling, whereby plastics are shredded, melted and reconstituted; feedstock recycling, whereby plastics are converted into hydrocarbons and hydrogen gas; and energy recovery, whereby plastics are incinerated at power plants to generate electricity.¹³⁹  Incineration is increasingly becoming an unviable option due to its high carbon footprint and concerns regarding the release of toxins such as dioxins, furans, mercury and polychlorinated biphenyls.¹⁴⁰  Mechanical recycling is on paper an attractive option, but due to the variety of plastics tailor made with different thermal, chemical and mechanical properties for different applications, there is a need to sort plastics by type prior to their disintegration and remolding, as mixing different polymers leads to inferior mechanical properties.¹⁴¹  Even so, the properties of recycled plastics are still compromised after several cycles. This makes mechanical recycling inherently labor intensive and inefficient, disincentivizing the process.¹⁴² 

Feedstock recycling entails the pyrolysis of plastics to produce hydrocarbons and hydrogen gas^143,144  but requires high temperatures (500–800 °C) and emits significant quantities of carbon dioxide,¹⁴⁵  leaving the viability of the process in question for now. Recently, there has been promising research on the solar-driven reforming of various plastics using a photocatalyst at ambient temperature and pressure as a more sustainable alternative.¹⁴⁶ 

Chemical recycling is an emerging concept that involves chemically degrading plastic polymers into their constituent monomers, which can then be used as the starting material to produce pristine, high-quality products.¹³⁹  However, this method is only amenable to condensation polymers with reactive linkages, such as polyesters and polyurethanes, and is not applicable to most addition polymers such as poly(styrene) and poly(ethylene). The urgent need for a sustainable plastics program worldwide has spurred campaigns for designing new types of chemically recyclable plastics to overhaul existing ones.¹⁴²  Design strategies have been inspired by dynamic covalent chemistry, which features linkages that can be reversibly broken and formed, such as imines, Diels–Alder adducts and hemiaminals; self-immolative polymers that undergo a cascade reaction and completely disintegrate into their constituent polymers when exposed to a trigger, such as heat or light; and vitrimer networks, which are able to respond to stimuli without depolymerizing.¹⁴²  Assuringly, there has also been progress in the development of catalysts for the chemical recycling of challenging existing plastics such as polyethylene.¹⁴⁷ 

With an annual growth rate of 3–5%, electronic waste or e-waste represents the fastest-growing type of municipal waste worldwide.¹⁴⁸  This trend is likely to continue with the current pace of technological advancement in consumer devices such as smartphones and computers. Unfortunately, effective and large-scale recycling programs do not exist even in developed nations, and recycling rates are low, with only 17% of all e-waste recycled in 2019.¹⁴⁹  Instead, e-waste is often exported to developing countries, where it is simply incinerated in open air or recycled in premises ill equipped to handle the toxic chemicals released, compromising the health and safety of the residents of that community.¹⁴⁹  For example, circuit boards are often crudely recycled by burning to recover copper, but this also releases toxic mercury and cadmium vapors.¹⁴⁹ 

Due to the large cumulative quantities of precious metals such as gold hidden within used electronics, e-waste has been labeled an “urban mine”, and it is estimated that around US$10 billion worth of precious metals is currently buried within the global e-waste stream.¹⁴⁹  However, e-waste recycling is fraught with difficulty. For one, the valuable precious metals (gold, palladium, platinum and ruthenium) are found at minute ppm-level quantities within the bulk material of the appliance, which is often composed mostly of plastic, copper, aluminum and iron.¹⁵⁰  Sorting through and dismantling electronics is also time- and labor-intensive, and the need to provide facilities that are able to handle the toxic vapors generated adds to the overall cost.¹⁴⁹  Hence, e-waste recycling businesses require sufficiently large economies of scale to turn a profit.

Current methods to recover metals in e-waste recycling are also problematic. Hydro- and pyrometallurgy represent the current paradigm, but these are highly energy-intensive processes that also generate significant amounts of toxic liquid and gaseous waste.¹⁵¹  Fortunately, there has been significant progress in developing greener alternative methods, such as bioleaching, the use of supercritical carbon dioxide as extraction solvent, and electrochemistry.¹⁵¹  An example is the use of electrochemistry to recover rare earth metals such as europium, neodymium, gadolinium and scandium using carbon nanotubes via their selective electrodeposition at different applied potentials.¹⁵² 

Needless to say, the precious and rare earth metals present in e-waste are highly relevant for nanotechnological applications, and improvements in e-waste recycling create downstream benefits in securing a reliable stream of raw materials for sustainable nanotechnology.

Nanotechnology can be considered the golden child of scientific advancements in our modern era. The major breakthroughs in this research field have revolutionized the understandings of science and laid the foundations for new possibilities. Thereafter, a plethora of research areas and applications of nanotechnology skyrocketed. Its potential role in progressing science and economic prosperity attracted not only commercial but also political attention. Widespread efforts to create a positive impact through nanotechnology undeniably raise concerns regarding its consequences and adverse effects, thus raising the question of, is it really the better choice?

The presence of nanotechnology is ubiquitous in our daily life, albeit unnoticeable. In approximately half a century, this emerging technology has rooted itself in various industrial applications and established advanced solutions. The extent of its reach comprises a diversity of fields, including cosmetic, electronic, medical, food packaging, automotive and sensing. Nanotechnology boasts many advantages to each unique application. For example, silver NPs (AgNPs) are an active constituent found in many consumer products due to their wide coverage of antifungal, antimicrobial and antiviral properties.^153,154  In the medical field, NPs are combined with insoluble drugs to facilitate drug delivery in the body.¹⁵⁵  In the early 2000s, nanomaterials were manufactured in hundreds of thousands of tonnes, and the global market of them was estimated to reach an optimistic value of 5 million tonnes in 2025.¹⁵⁶ 

According to the European Commission (EC), an estimated US$67 billion has been invested in nanotechnology by governments around the world since then. In the Framework Programmes 7 (a research funding initiative supported by the EC), nanotechnology makes up the largest fraction, with an estimated value of €896 million (2007–2011).¹⁵⁷  It is no surprise that companies are investing in nanoscience and securing this as a competitive edge over others. Despite its significance, a recent survey conducted in five selected European countries found that the general awareness of nanotechnology and its effect on our daily lives is relatively low.^158,159  This can be similarly observed in other parts of the world, where a large percentage of citizens know very little about nanotechnology.^160–162  On the other hand, countries such as China and Singapore demonstrated relatively high awareness of nanotechnology.^163,164  Public sentiments toward nanotechnology are generally positive and cautious.

It is imperative to bring awareness of nanotechnology to the community and ensure members of the public can make informed decisions about their choices. Nanotechnology is commonly defined as the control of matter in the nanoscale range of approximately 1–100 nm.¹⁶⁵  With the rapid development of nanotechnology, the rate of utilization seems to have outpaced the necessary evaluations and regulations required to monitor their usage and effects.

As part of the key enabling technologies under Horizon 2020, Europe actively sees this domain as an important field that would play a crucial role in the European industries for the many years to come.¹⁶⁶  Sustainability is quickly becoming the key strategic priority for the government and private stakeholders. The conscious assessment of industrial processes from the extraction of raw materials to their end life provides crucial information to conceptualize a sustainable future. Nanotechnology itself is considered by many in the industry a promising solution toward a green future. This can be attributed to the size of the nanoscale materials, which would naturally tackle the issues of limited resources, space and costs. However, this field has yet to fully mature and the implications of such use are yet to be known.

Manufacturing nanoproducts involves either a top-down or bottom-up approach.¹⁶⁷  It is commonly believed that top-down methods such as carving, lithography and etching would generate more waste. The bottom-up approach is still in its infancy but with good promises to minimize waste. To ensure the sustainability potential of nanotechnology, various studies have been made to evaluate the manufacturing considerations and the adverse impacts on the environment and human health.

The systems-based approach of industrial ecology (IE) covers multi-disciplinary fields and takes the engineering sciences, economic values and environmental studies into concern. It provides an overview of the relationship between industrial systems and the environment. This method focuses on developing a circular system where one aims to optimize the output while maximizing recycling of the by-products and minimizing waste materials.^167–169  Suffice to say, this approach addresses the issues of sustainability by studying the industry through multiple perspectives. IE attempts to evaluate the processes and flows in order to ensure a sustainable and green industry.¹⁶⁷  A defining characteristic of IE is the use of systems to categorize the different aspects in the industry and identify the relationships in between. Ideally, all waste outputs from a process can be inputs for other processes, thus resulting in a closed-loop system. The concept of a circular economy originates from the idea of IE and industrial metabolism.¹⁷⁰  IE comprises many tools to help with the evaluation of a particular industry, including but not limited to life cycle assessment (LCA), material flow analysis (MFA), substance flow analysis (SFA) and technology assessment (TA). These methods are generally complementary to each other to comprehensively assess the industry.

A notable methodology is LCA, which is often deployed to evaluate the environmental impact of a product or technology from the extraction of raw materials to its end life. The most holistic assessment is the cradle-to-grave assessment, which takes into account the entire life cycle of a product as opposed to other variants of LCAs.¹⁷¹  It is tremendously challenging to establish a well-rounded framework applicable to all products of nanotechnology due to their diverse applications. Manufacturing nanotechnology in the industry can be classified into two groups: nanotechnology as a singular product and nanotechnology as a supporting role in a product. By definition, only an intentionally produced material with at least 50% of it made up of NPs that have one or more external dimensions in the nanoscale range of 1–100 nm is considered a manufactured nanomaterial.¹⁷² 

There are four main phases of an LCA in accordance with the existing standards ISO 14040 and 14044, as indicated in Figure 1.2.^173–175  The International Organization for Standardization (ISO) uphold standards that are considered the benchmark of standardized procedures to accurately assess the life cycle of a product. The most critical aspect is to determine the functional unit and the system boundaries at the goal and scope phase.¹⁷⁶  In the second phase, the units are expanded to quantitatively identify the necessary value of materials, energy and products in the process flow. Subsequently, in the Life Cycle Impact Assessment (LCIA) phase, potential toxicology impact on the ecosystem and human health is identified and derived from the Life Cycle Inventory (LCI). Finally, the last phase interlinks all other three phases to summarize and conclude all findings. The system boundaries should clearly be defined in phase 1 to understand the extent of the assessment. The structured methodology builds on a relative approach where all inputs and outputs are critical to the function of the product.

However, conducting a comprehensive cradle-to-grave assessment is challenging as there are too many uncertainties and variables to the technology.¹⁷⁷  Table 1.2 summarizes a number of recent LCA studies in various fields of nanotechnology. The study of an LCA commonly involves a few key databases to model the potential impacts on the environment, such as Ecoinvent 3, European reference Life Cycle Database (ELCD) and the US Life Cycle Inventory (USLCI).¹⁷⁸  The Cumulative Energy Demand (CED) and the Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI 2.1) contribute to the impact assessment phase.¹⁷⁸ 

There are a few shortcomings that can be constantly highlighted from these studies, for example, the uncertainties in the end-of-life (EOL) phase. The release of NPs to the environment during their use or the EOL phase is a pressing concern. Large assumptions have been made in LCA studies, such as that these NPs are released as unadulterated particles. However, in reality, NPs may undergo further processes that can transform their properties and interactions with the surroundings.¹⁸⁶  There is also very little understanding of the waste management process and release of NPs.^186,187  Despite its limitations, LCA provides a comprehensive analysis to identify environmental hotspots and a platform for comparison to other technologies.

MFA and SFA are both important methods to assess the sustainability characteristics of an industry. Both types of assessments are based on the mass balance principle.¹⁸⁸  SFA is utilized to quantify the input and output flow of a particular substance in production. It focuses on the usage effect of the necessary substance in a region or its entire life cycle and thus is able to identify hidden hotspots of unexpected flow.¹⁸⁹  This is significantly useful for waste management systems to account for the movement of hazardous compounds. The roles of MFA and SFA can significantly strengthen and complement other analyses to develop a holistic evaluation.¹⁹⁰  According to Udo de Haes et al., MFA is a broader concept covering both SFA and bulk flow of materials.¹⁹¹ 

In a study by Arvidsson et al., it was found that different uses of titanium dioxide (TiO₂) NPs in paint, sunscreen and self-cleaning cement result in different analysis outcomes.¹⁹²  The emission of TiO₂ NPs is the highest from sunscreen despite the higher mass flows in the other applications.¹⁹²  Wang et al. modeled the flow of nanometal oxides and quantum dots (QDs) to the environment and quickly discovered that different waste management techniques between countries was a limitation to the assessment.¹⁹³  As highlighted by the authors, there are existing gaps in the current knowledge of NPs. However, through modeling the flow of NPs in an industry, the study can be a foundation to build on for further assessments and to support crucial policy-making.

This assessment aims to evaluate new technology to forecast potential futures and repercussions that it might face. This will provide information on critical decision making concerning the technology and implementing it in society.^194,195  Through the combinations of research assessments, TA is able to convey the consequences of the technology and provide knowledge to better design strategies. Often it has been regarded as a significant step to shape the technology's acceptance in the societal aspect, for example, regulations and public sentiments.^194,196  In emerging technologies such as nanotechnology, this assessment plays a major role in its future.

The TA development has grown over the years, with various modifications being made to this assessment and adopting TA for sustainability assessments.^197,198  A prominent example of the further development of TA is the constructive technology assessment (CTA) and the real-time technology assessment (RTTA).^197,198  Retèl et al. assessed the introduction of nanotechnology in oncology through a modified TA:CTA.¹⁹⁹  In order to study the dynamics and impact of nanotechnology, it is recommended to start during the early development phase. This projects potential roadblocks in the system and facilitates decision making.

The exponential growth of nanotechnology has led to various benefits and solved multiple limitations. However, its fast-expanding reach brings about complications and uncertainties to its impact on the environment and human health. This is especially critical when it comes to selected industries such as healthcare, agriculture and food and beverage. The risks and hazards of these NPs are an immense challenge to identify in the ecosystem and the biological system.^200,201  Ironically, the desirable small nature of nanotechnology creates many uncertainties in its behavior at the EOL phase. Little is known about the toxicity and exposure of NPs to the environment or to humans.

Various modeling studies have been made to understand the direction and flow of the particles. However, these studies are with assumptions that the NPs maintain the same chemical state as that during the production.¹⁸⁶  Their physicochemical properties may undergo transformations and form interactions that may contribute to their toxicity. According to Oberdörster et al., the properties of NPs may modify chemical responses and cellular interactions in the biological systems.²⁰²  In addition, exposure to NPs can involve many different channels such as inhalation, ingestion and skin contact.²⁰³  The risks of short-term and long-term exposure to NPs remain in the dark due to the limited data. Conventional methods to monitor exposure typically utilize the mass and bulk chemistry but this may not be accurate for NPs.

Hence, studies have been made to understand the direct impact of nanotechnology on the environment. According to Ottoni et al., increasing concentrations of AgNP had harmful side effects on the germination of rice seedlings.²⁰⁴  The effects of NPs on soil have also been investigated by Kumar et al. Unfortunately, all manufactured silver, silica and copper NPs had a certain degree of toxic impact on Artic soil.²⁰⁵  Similar studies were also conducted on animals and it was found that the increase in doses of AgNP greatly affected the oxygen consumption in zebrafish.²⁰⁴  Suffice to say, studies have shown that there is a detrimental direct impact of NPs to the environment. However, the effect of manufactured NPs in reality is difficult to comprehend. Their impact on human health is exceedingly critical due to the direct application of NPs on people.

Rushton et al. have developed a new approach to predicting toxicity through the analysis of in vitro and in vivo studies.²⁰⁶  Here, the results demonstrated the ability of NPs (copper-, titanium dioxide – and gold-based NPs) to produce reactive species (ROS) that would lead to oxidative stress in the biological environment.²⁰⁶  In the human body, the translocation of the NPs is largely unknown and would be in low quantities. However, accumulation over time may change and affect the internal organs. The combination of properties in NPs such as particle size, surface area and ROS generation are the main suspects for inducing lung damage.²⁰⁷  Another concern of nanotoxicity is dermal exposure and penetration, especially in the current application of AgNP and titanium dioxide NPs (TiO₂) in cosmetics and consumer care. In vitro studies on the penetration of AgNP have demonstrated localization of these particles at the stratum corneum.^208,209  Furthermore, the surface charge of AgNP was not found to drastically affect this migration on skin samples.²¹⁰  Several studies have concluded that damaged and wounded skin will facilitate the penetration of AgNP, but it will not interfere with the healing rate of the skin.^211,212  However, a theoretical study by Watkinson et al. showed that NPs are not large enough to pass the skin barrier by passive diffusion or absorption.²¹³  The behavior of NPs varies depending on their characteristics. Therefore, standardized protocols would be a step forward to understand and manage the impact of NPs on the body.

At present, there is plenty of research about nanotechnology and its possible applications. It appears researchers believe that nanotechnology has not been fully exploited as they continue to push the frontier of science and technology with further development in the field. In view of the government's efforts to address the issue of sustainability, nanotechnology can contribute to the protection of the environment. For instance, the enhanced sensitivity and reactivity that comes with the small scale would be useful for monitoring and treating pollution. Additionally, it would be a big leap if nanotechnology could transform the current manufacturing processes to be leaner with fewer pollutants and more effective processes. While nanotechnology seems very promising, it is also necessary for us to acquire further knowledge of its interaction with the environment to ensure that the implementation is safe.

Also there have been various studies to address the importance of nanotechnology and its impact on our economy, our environment and our health. Although there are still gaps and assumptions made in the research, the emphasis is on understanding and the motivation to uncover the unknown sets us in the correct direction. Long-term studies on the exposure and effect of nanotechnology on the surroundings are one way we can shed light on this aspect. Further scientific studies and classification of NPs are crucial to compare their physicochemical properties across different applications. With a sustainable future in mind, it is ideal to develop a roadmap to identify the positive and negative contributions of nanotechnology. This would contribute to decision making and policy developments at national levels.