What You Will Learn
- 1 Nanotechnology for the Environment Foundations
- 2 Remediation and mitigation
- 3 Pollution prevention
- 3.1 Materials
- 3.2 Other coatings
- 3.3 Nanocatalysis
- 3.4 Green manufacturing
- 4 Environment sensing
- 5 Food packaging and monitoring
Nanotechnology for the Environment Foundations
In industrialised nations, the air is filled with numerous pollutants caused by human activity or industrial processes, such as carbon monoxide (CO), chlorofluorocarbons (CFC), heavy metals (arsenic, chromium, lead, cadmium, mercury, zinc), hydrocarbons, nitrogen oxides, organic chemicals (volatile organic compounds, known as VOCs, and dioxins), sulphur dioxide and particulates.
The presence of nitrogen and sulphur oxide in the air generates acid rain that infiltrates and contaminates the soil. The high levels of nitrogen and sulphur oxide in the atmosphere are mainly due to human activities, particularly the burning of oil, coal and gas. Only a small portion comes from natural processes such as volcanic action and the decay of soil bacteria. Water pollution is caused by numerous factors, including sewage, oil spills, leaking of fertilisers, herbicides and pesticides from land, by-products from manufacturing and extracted or burned fossil fuels.
Contaminants are most often measured in parts per million (ppm) or parts per billion (ppb) and their toxicity defined by a ‘toxic level’. The toxic level for arsenic, for instance, is 10 ppm in the soil whereas for mercury it is 0.002 ppm in water. Therefore, very low concentrations of a specific contaminant can be toxic.
In addition, contaminants are mostly found as mixtures. Consequently, there is a need for technologies that are capable of monitoring, recognising and, ideally, treating such small amounts of contaminants in air, water and soil.
In this context, nanotechnologies offer numerous opportunities to prevent, reduce, sense and treat environment contamination. Nanotechnologies can enhance and enable pre-existing technologies and develop new ones.
What can nanotechnologies do?
Nanotechnologies offer the ability to control matter at the nanoscale level to create materials with specific properties that can serve specific functions. This is particularly important in environmental issues where pollution often arises from the presence of a specific contaminant within a mixture of materials, in solid, liquid or gas form.
The small size of nanomaterials, together with their high surface-to-volume ratio, can lead to very sensitive detection. These properties allow the development of highly miniaturised, accurate and sensitive pollution-monitoring devices (nano-sensors).
Nanomaterials can also be engineered to actively interact with a pollutant and decompose it into less toxic species. Thus, in the future, nanotechnology could be used not only to detect contaminated sites but also to treat them. Finally, this technology can be used to reduce the production of harmful wastes in manufacturing processes by reducing the amount of material used, and by employing less toxic compounds.
Another application area is the engineering of coatings that are nanostructured in such a way that they resist the attack of pollutants or have self-cleaning properties so that are easily cleaned by rain water and, therefore, require less detergent to be washed.
The starting point for any discussion on the applications of nanotechnologies to the environment is the ability of nanoscience to create new nanostructured materials with specific properties to serve specific functions.
Remediation and mitigation
Soil and groundwater contamination arising from manufacturing processes are a matter of great complexity and concern. Affected sites include contaminated industrial sites (including lakes and rivers in their vicinity), underground storage tank leakages, landfills and abandoned mines.
Pollutants in these areas include heavy metals (e.g. mercury, lead, cadmium) and organic compounds (e.g. benzene, chlorinated solvents, creosote). Nanotechnology can develop techniques that will allow for more specific and cost-effective remediation tools. Currently, many of the methods employed to remove toxic contaminants involve laborious, time-consuming and expensive techniques. A pretreatment process and removal of the contaminated area is often required, with consequent disturbance of the ecosystem.
Nanotechnology facilitates developing technologies that can perform in situ remediation and reach inaccessible areas such as crevices and aquifers, thus eliminating the necessity for costly ‘pump-and- treat’ operations. In addition, as a result of its ability to manipulate matter at a molecular level, nanoscience can be used to develop remediation tools that are specific to a certain pollutant (e.g. metal), therefore increasing affinity and selectivity, as well as improving the sensitivity of the technique.
Drinking water quality and its contamination from pollutants is another matter of concern. Mercury and arsenic are, in particular, two extremely toxic metals that pose very high health risks. Remediation methods that allow the fast, economic and effective treatment of water polluted with such contaminants is urgently needed. Nanotechnology can introduce new methods for the treatment and purification of water from pollutants, as well as new techniques for wastewater management and water desalination.
Nanomaterials currently being investigated for remediation use include iron and bimetallic nanoparticles, semiconductor nanoparticles, magnetic nanoparticles and dendrimers. Some detailed examples follow.
Remediation using metal nanoparticles
The use of zero-valent (Fe0) iron nanoparticles for the remediation of contaminated groundwater and soil is a good example of how environmental remediation can be improved with nanotechnology. When exposed to air, iron oxidizes easily to rust; however, when it oxidizes around contaminants such as trichloroethylene (TCE), carbon tetrachloride, dioxins, or PCBs, these organic molecules are broken down into simple, far less toxic carbon compounds.
Since iron is non-toxic and is abundant in the natural environment (rocks, soil, water, etc.), some industries have started using an ‘iron powder’ to clean up their new industrial wastes.
However, the ‘iron powder’ (i.e. granular zero-valent iron with dimensions in the micron range) is not effective for decontaminating old wastes that have already soaked into the soil and water.
Moreover, bioremediation using granular iron powder is often incomplete: some chlorinated compounds, such as PCE or TCE, are only partially treated and toxic by-products (such as DCE) are still found after treatment. This effect is due to the low reactivity of iron powders.
Another matter of concern is the decrease in reactivity of iron powders over time, possibly due to the formation of passivation layers over their surface.
Nanotechnology has offered a solution to this remediation technology in the form of iron nanoparticles. These nanoparticles are 10 to 1 000 times more reactive than commonly used iron powders. They have a larger surface area available for reacting with the organic contaminant and their small size (1–100 nm) allows them to be much more mobile, so they can be transported effectively by the flow of groundwater. A nanoparticle water slurry can be injected into the contaminated plume where treatment is needed (Figure 1).
The nanoparticles are not changed by soil acidity, temperature or nutrient levels, so they can remain in suspension maintaining their properties for extended periods of time to establish an in situ treatment zone. Experimental results collected both in the laboratory and in the field have shown that nanoscale iron particles are very effective for the complete transformation and detoxification of a wide variety of common environmental contaminants, such as chlorinated organic solvents, organochlorine pesticides and PCBs.
When nano-sized iron powders are used, no toxic by-products are formed, a result of the increased reactivity and stability of the nanoparticles compared to the granular iron powder. Contaminant levels around the injection level are considerably reduced in a day or two and nearly eliminated within a few days. As a result of their stability, nano-iron particles remain active in a site for six to eight weeks before they become dispersed completely in the groundwater and become less concentrated than naturally occurring iron.
Researchers are assessing whether the technique could also be used for the remediation of dense non-aqueous phase liquid (DNAPL) sources within aquifers, as well as for the immobilisation of heavy metals and radionucleotides.
Bimetallic iron nanoparticles, such as iron/palladium, have been shown to be even more active and stable than zero-valent iron nanoparticles, thus further improving this remediation technology. Finally, iron or bimetallic nanoparticles could be anchored on solid supports such as activated carbon or silica for the ex situ treatment of contaminated water and industrial wastes.
Remediation using semiconducting nanoparticles
Semiconducting nanoparticles made of TiO2 and ZnO are used in photocatalytic remediation. Being semiconductors, these materials produce an electron-hole pair when irradiated with light having energy in the order of the material bandgap. TiO2 has a bandgap of 3.2 eV so when the material is irradiated with UV light an electron-hole pair is formed. Both TiO2 and ZnO are capable of transferring the charge to organic pollutants (such as halogenated hydrocarbons) and induce their oxidation to less harmful by-products, such as CO2, H2O and other species.
Since TiO2 and ZnO are readily available and inexpensive, their use for remediation has been studied for many years. Recently, nano-sized TiO2 and ZnO have been considered, as these have more active surface given the same volume of material. The vision is to create solar photocatalysis remediation systems where TiO2 or ZnO are used to convert toxic contaminants, such as chlorinated detergents, into benign products using the radiation.
There is evidence that these semiconductors can photodegrade numerous toxic compounds, but the technology requires improvements in term of efficiency since TiO2 or ZnO only absorb UV light which represents only 5 % of the solar spectrum. In this context, nanotechnology could bring an improvement in two ways.
- When noble metals like gold and platinum are chemisorbed to the TiO2 and ZnO nanoparticles, the photocatalytic activity is accelerated. The reason is that the presence of the metal helps to keep the electrons and holes from recombining in the semiconductor and thereby increases the efficiency of the
- To increase the photoresponse window of TiO2 and ZnO from UV to visible light, the nanoparticles can be modified with organic or inorganic This is an area of intensive research.
Nanomaterials have also been found able to remove metal contaminants from the air. For example, silica-titania nanocomposites are being investigated for use in the removal of elementary mercury (Hg) from vapours such as those from combustion sources. In these nanocomposites, silica acts as a support material and titania transforms mercury to a less volatile form (mercury oxide).
Remediation using dendrimers
Dendrimers are highly branched polymers with controlled composition and nanoscale dimensions. Chelating agents in the form of dendrimers are also studied for the removal of metal contaminants. These can be designed so as to able to act as ‘cages’ and trap metal ions and zero-valent metals, making them soluble inappropriate media or able to bind to certain surfaces. The vision is to use dendrimers as nanoscale chelating agents for polymers supported ultrafiltration systems.
Remediation using magnetic nanoparticles
Another class of nanoparticles that have environmental applications is magnetic nanoparticles. For example, researchers from Rice University’s Centre for Biological and Environmental Nanotechnology (CBEN) have recently shown that nanoparticles of rust can be used to remove arsenic from water using a magnet.
The concept is simple: arsenic sticks to rust which, being essentially iron oxide, tends to be magnetic so it can be removed from water using a magnet. Nano-sized rust, about 10 nm in diameter, with its high surface area, has been found to improve removal efficiency while reducing the amount of material used.
Compared to other techniques currently used to remove arsenic from contaminated water, such as centrifuges and filtration systems, this method has the advantage of being simple, and most importantly, not requiring electricity. This is very important, given that arsenic-contaminated sites are often found in remote areas with limited access to power. Magnetic nanoparticles modified with specific functional groups are also used for the detection of bacteria in water samples.
Arsenic and arsenate may also be precipitated using nanoscale zero-valent iron (Fe0) as indicated by recent studies. The removal mechanism, in this case, involves the spontaneous adsorption and co-precipitation of arsenic with the oxidized forms of Fe0. As already noted, zero-valent iron is extremely reactive when it is nano-sized, so it is currently considered a suitable candidate for both in situ and ex situ groundwater treatment.
Remediation using aerogels and solid absorbents
The problem of oil spills in seawater is of great concern and has detrimental environmental consequences. Currently, there are numerous bioremediation strategies that use microbial cultures, enzyme additives or nutrient additives to clean up oil spills. The purpose of these additives is to boost the natural nanotechnology of the microbial community to decompose the oil material.
Another method gaining acceptance is the use of aerogels (a nanomaterial) modified with hydrophobic molecules to enhance the interaction with the oil. These aerogels have very large surface areas so they can absorb 16 times their weight of oil. They act as a sponge: once the oil has been absorbed, the ‘oil-soaked sponge’ can be removed easily. The problem is that these materials are expensive, so alternatives are under study.
The company Interface Scientific Corporation has developed a new nanomaterial modified with self-assembled monolayers (SAMs) which appears to be very effective in remediating oil spills. The company does not provide details of the material but claims that the nanomaterial can absorb 40 times its weight in oil a method that exceeds any other currently available and that the oil can be recovered.
As outlined, the use of nanoparticles is very promising in the field of environmental remediation and treatment, precisely because of their small size and reactivity. Nevertheless, some concern exists regarding their use in soil and water treatment: once dispersed in a contaminated site, would the nanoparticles be mobile to a point that they could be taken up by plants or animals at the site and adversely affect them?
Biodegradable nanoparticles are likely to be less problematic; nevertheless, there is a need to investigate these safety aspects, and this is the subject of numerous international research programmes. These concerns belong to the more general field of environmental impact assessment of the use of nanoparticles, which includes both risk assessments and life-cycle analysis to understand the short-term and long-term effects of nanoparticles in the environment.
Nanomembranes and nanofilters
Nanotechnology can also be employed for the fabrication of nano filters, nano-adsorbents and nanomembranes with specific properties to be used for decontaminating water and air. As with other applications, it is the ability to manipulate matter at a molecular level that makes nanotechnology so promising in this field, together with the small size and high surface-to-volume ratio of nanomaterials that are employed in the fabrication of these products.
In principle, ‘nanotraps’ designed for a certain contaminant can be produced, for example with a specific pore size and surface reactivity. An example is given by the work carried out at Rice’s CBEN, where researchers are developing reactive iron oxide ceramic membranes (ferrocene membranes) that are capable of remediating organic waste in water (Figure 2).
Filters and membranes can also be engineered to be ‘active’ in the sense of being able not only to trap a certain contaminant but also to chemically react with it and convert it to a non-toxic product. For example, researchers at the University of Tennessee are investigating a new type of nanofibre for the removal of micro-organisms via filtration that can also kill them on contact.
An interesting application of nanomembranes has been developed by researchers at the University of California Los Angeles (UCLA) in the form of new reverse osmosis (RO) membrane for seawater desalination and wastewater remediation. The membrane is made of a uniquely cross-linked matrix of polymers and engineered nanoparticles designed to draw in water ions but repels contaminants. This is possible due to the nano-size of the holes forming the membrane, which are ‘tunnels’ accessible only to the water molecules. Another distinctive feature of this nanomembrane is its ability to repel organics and bacteria, as a result of the chemical composition of the nanoparticles embedded in the membrane. Compared with conventional RO membrane, these membranes are thus less prone to clogging, which increases the membrane lifetime with an obvious economic benefit.
In many circumstances, access to clean and safe water is a problem. Nano filters allow contaminants such as arsenic and other heavy metals to be filtered from the water. One commercial reality is the LifeSaver® bottle that has a super hydrophilic filter inside blocks material up to 15 nm in size, which includes viruses and bacteria. The filter is inserted in a plastic bottle and enables contaminated water to be cleaned on-site.
Nanotechnologies offer many innovative strategies to reduce pollution in numerous processes including the reduction of waste in manufacturing processes, a reduction of the use of harmful chemicals, reduced emission of greenhouse effect gases in fuel combustions and the use of biodegradable plastics. These are only a few of the many approaches that can be taken to reduce pollution of the environment. Nanotechnologies are already actively involved in this sector, either as a technology producing advanced materials that pollute less or as a method to increase the efficiency of certain industrial processes (e.g. catalytic processes).
Materials that are manufactured in a more environment-friendly way using nanotechnologies include biodegradable plastics made of polymers that have a molecular structure optimal for degradation; non-toxic nanocrystalline composite materials to replace lithium-graphite electrodes in rechargeable batteries; and self-cleaning glasses, such as Activ™ Glass, a commercial product available worldwide from Pilkington. The glass has a special coating made of nanocrystals of TiO2 which, when exposed to daylight, reacts in two ways. First, it breaks down any organic dirt deposits on the glass and, secondly, when exposed to water, it allows rain to ‘sheet’ down the glass easily and wash the loosened dirt away. In this product, TiO2 is found in the form of a thin film 2–20 nm deposited by a high-temperature gas phase. The thickness of the film is essential for ensuring maximum photocatalytic activity and transparency (Figure 3).
The coating is hydrophilic (water contact angle (CA) is 20° compared to conventional soda glass for which the CA is 40°). When dirt is deposited, the contact angle of the surface increases, but is then reduced again upon irradiation. The photochemical reaction, which requires oxygen, is quite complex and involves a number of radical sub-products. Titanium oxide is not consumed in the reaction but acts as a catalyst. As a result, organic material is decomposed to CO2. Concurrently, the contact angle of the surface is further reduced upon irradiation (from 20° to about 15°). After irradiation, dirt can be more easily removed from the glass by rain. The result is that water spreads very effectively (forming a ‘layer’ over the glass), washing the surface easily. The coating is partially durable to abrasion. Although the name of the product suggests otherwise, this is not a truly ‘self-cleaning’ layer since it requires water to allow the surface to be cleaned.
Lotus effect® surfaces and textiles
Sometimes, the term ‘self-cleaning’ is also associated with surfaces that have been engineered to imitate the natural ‘self-cleaning’ effect found in some leaves, such as the lotus leaf. In this case, the coating is not a uniform layer with specific chemical functionality (as in the case of photocatalytic coating), but a surface with an engineered topography at the nanoscale level. This leads to a surface which is superhydrophobic (extremely water repellent). Water droplets roll off the surface and in doing so collect and remove dirt deposited on the surface.
The Lotus effect® has been an inspiration for several innovative materials, such as coatings and textiles. The realisation that certain surface properties can induce water repellence is important in numerous applications. Materials scientists are now engineering numerous types of materials to render them superhydrophobic.
There are many instances where avoiding the wetting of a surface is an advantage, for instance in textiles, which are routinely stained by liquids (juices, coffee, etc.) and solids (mustard, ketchup, etc.). Some companies such as Nano-Tex, Inc., are now commercialising textiles that are engineered to confer superhydrophobic properties on their textiles (Figures 4 and 5). This effect is obtained by the presence of ‘nano-sized whiskers’ on the surface of the fibres that make up the fabric.
The inclusion of TiO2 nanoparticles in textiles is also being investigated, as this material catalyses the degradation of organic dirt.
Surfaces and materials engineered to mimic the Lotus effect® are useful in construction as they allow a reduction in the need for cleaning. Currently, there are various products commercialised or under research that make use of this principle, for example, Lotusan®, an exterior paint from the firm Sto launched in 1999. The application of this exterior paint reduces the attack of dirt on the façade to which it is applied, and induces self-cleaning properties when rain droplets roll off and drag dirt away with them.
The above-mentioned applications are examples of cases where the superhydrophobic properties of engineered material, such as a textile or a coating, can reduce cleaning needs, with a consequent reduction in water usage and obvious environmental benefit.
It should be noted that, in contrast to photocatalytic coatings, those based on the Lotus effect® are non-transparent: light is scattered due to the rough nature of the coating. Researchers are thus also investigating transparent superhydrophobic surfaces.
Antimicrobial coatings, textiles and other products
Antimicrobial coatings are needed in many applications, for example, to protect medical surfaces and tools, or to reduce microbial attack on the hulls of boats. Sprays and coatings for this use already exist but improvement in this area is needed as many microbes are becoming resistant to the antibiotic treatments that have been used so far. To prevent bacteria attachment, surfaces with nanocoating with specific functionalities and topographies are under investigation. Antifouling surfaces are being investigated for use in the coating of medical utensils and instruments and household appliances, as well as for coating boats. A nanomaterial that is becoming widely used is silver nanoparticles.
Silver is a metal with a long history used for its antibacterial properties — even the Romans used it to dress wounds. This property explains why silver has been used to produce the highest-quality cutlery (silverware) or to store water in vessels in antiquity (even by the Phoenicians). In medicine, 1 % silver nitrate was used in the past as an eye solution to prevent infections in newborn children, and until antibiotics were discovered, silver nitrate was added to germicides and antiseptics as a disinfectant.
The antibacterial properties of silver are due to the silver ions (Ag+) released by the bulk metal once this is oxidized. In fact, silver tableware or dishware has antimicrobial activity only if oxidized species are present on their surface. Silver ions induce the oxidative stress of the bacteria cell wall, where many cellular functions are performed, affecting the bacteria’s ability to respire and to maintain an intracellular environment suitable for life. Silver ions inhibit bacteria growth, suppress respiration and metabolism and basically induce cell death. Silver toxicity has been shown towards many strains of bacteria, both gram-negative and gram-positive, and to fungi (less towards viruses).
Silver is not considered toxic to the cardiovascular, nervous or reproductive systems in humans. In some people, exposure to silver leads to argyria (or argyrosis) which is due to a process of sequestration of silver ions in an innocuous form which is not reversible and leads to pigmentation or discolouration of the skin. The few cases of death due to silver intoxication have been related to very high concentrations of silver.
In recent years, silver nanoparticles (often called ‘nanosilver’) have been added to numerous consumer products to give them antimicrobial properties. Because of their antibacterial effectiveness and low toxicity towards mammalian cells, silver nanoparticles have become one of the most common nanomaterials used in consumer products. The range of products is quite wide and includes kitchen utensils (pots, pans, etc.), personal wear (socks, shoe liners, underwear), outerwear and sportswear, bed linen (sheets and mattress covers), appliances (refrigerators, washing machines, air filtration devices, computer keyboards), disinfectant sprays (deodorants) and cosmetics. Nanosilver is incorporated into these different materials through various impregnation techniques (sprayed, painted over the product, incorporated into plastics, etc.).
A number of products are appearing on the market that can confer some specific property to the surface and which contain nanoparticles (e.g. silica) or other nanomaterials. Examples are anti-graffiti coatings, anti-fog coatings, anti-fingerprint coatings, etc.
Fertilisers and wood treatment products
Another area where nanotechnologies are making a contribution is in the development of fertilisers and wood treatment products that are more stable and leach less into the environment. For example, researchers at Michigan State University have incorporated biocides for wood treatment inside polymeric nanoparticles. The small size of the nanoparticles allows them to travel efficiently inside the very fine, sieve-like structure of wood and, at the same time, the biocide, being safely trapped inside a ‘nanoshell’, is protected from leaching and random degradative processes.
Biomimetic water harvesting
Natural nanomaterials are inspirational for the fabrication of advanced materials. One example is biomimetic water-harvesting materials. Some plants and insects have the ability to capture water from fog. For example, the Namibian desert-dwelling beetle Stenocara has bumps on its wing scales with superhydrophobic nanostructured surfaces. The peaks of the bumps are extremely hydrophilic, whereas the slopes of the bumps and the area between them are covered with hydrophobic wax. As a result of this fine nanostructure, as drop let us accumulate in size, they roll from the peaks to the waxy channels to a place in the beetle’s back that supplies its mouth. A UK company, QinetiQ Ltd, has developed sheets that capture water vapour from cooling towers and industrial condensers based on the nanostructure of the beetle’s wing. These materials can capture 10times more water than conventional technology.
A catalyst is a substance that increases the rate of a chemical reaction without being consumed or chemically altered. Conventional catalysts are rare earth metals such as palladium (Pd) and platinum (Pt), which are very expensive. One of the most important properties of a catalyst is its ‘active surface’ where the reaction takes place. The active surface increases when the size of the catalysts is decreased (Figure 6). The greater the catalyst’s active surface, the greater the reaction efficiency. Research has shown that the spatial organisation of the active sites in a catalyst is also important. Both properties (nanoparticle size and molecular structure/distribution) can be controlled using nanotechnology. Hence, one area of intense nanoscience research is the development of new nanostructured catalytic surfaces.
In the environmental field, nanocatalysis is being investigated, for example, in the desulphurisation of fuels, with the aim of developing ‘clean’ fuels containing very low sulphur products (produced in the fuel during its refining process and responsible for generating sulphuric acid during fuel combustion). Recent nanotechnology research at iNANO has also aided the Danish company Haldor Topsøe A/S in implementing a new generation of hydro desulphurisation catalysts (BRIM™ Technologies) to be used for sulphur clean-up of fossil fuels worldwide. The hydro desulphurisation (HDS) catalytic reaction is a reductive hydrogen treatment of fuels to clean up sulphur-containing oil compounds, preventing the emission of many tons of harmful sulphur into the environment on a daily basis.
Another example is Oxonica’s Envirox fuel, which uses nano-sized cerium oxide as a catalyst to enhance the efficiency of the fuel combustion. This enhanced-fuel was tested in 2003 and 2004 in 1 000 buses in the United Kingdom (another 500 buses were tracked as control). It was found that the test buses used 5 % less fuel than the controls and that the fuel savings more than paid for the additive.
Nanoscale catalysts are also showing promise in improving air quality and for treating particularly challenging contaminants in water that must be reduced to a very low level.
As a bulk material, gold is notably inert: it does not react with many chemicals (including strong acids and bases). However, when in a nano-size form, gold becomes extremely reactive and this has opened the way for its use in catalytic processes. Conventional materials used in catalysis are rare earth mater- ials such as platinum and palladium, which are extremely expensive. There is great need for catalysts and alternatives are in huge demand. Nano-gold has been shown to be an extremely efficient catalyst in numerous pollution control studies. For example, it has been shown that it is able to remove carbon monoxide from room air under ambient conditions and from fuel cell hydrogen feed gas. Another study has shown that an Au-Pt co-catalyst was able to break down trichloroethylene (TCE) 100 times faster than a catalyst made of a traditional material. Recently, the company Nanostellar Inc. has announced an engineered nano-gold oxidation catalyst which can reduce diesel hydrocarbon emission by 40 % more than commercially available materials. Considering that there are over 14 million light-duty diesel vehicles worldwide, and two million heavy-duty vehicles, the impact of this nanotechnology could be enormous.
Recovery of catalytic material
One of the problems associated with catalysis is that it often makes use of rare earth materials such as palladium (Pd). Natural Pd resources are limited so there is a need to recycle industrial waste from processes that have used Pd as a catalyst. Recycling Pd requires reduction of Pd(II) to Pd(0). For an environmentally friendly alternative to chemical recycling processes, iNANO is investigating the use of bacteria to mediate Pd reduction. The enzyme hydrogenase found in the membrane of many bacteria has the potential to transfer electrons from an organic substrate to Pd(II). In the presence of some bacteria, the Pd is produced as nanoparticles located on the cell surface and in the periplasmic space of Gram-negative bacteria (known as ‘bio-Pd’). Figure 7 shows an example. The catalytic properties of the bio-Pd are similar to commercially available Pd nanoparticles. The hope is to use bacteria to recover catalytically active Pd from industrial waste and to use recovered Pd for other catalytic processes.
Manufacturing processes are always accompanied by the production of diverse waste products, many of which pose a threat to the environment and thus need to be removed and treated. Ideally, manufacturing processes should be designed to minimise material usage and waste production, while ensuring the use of the least amount of energy possible. ‘Green manufacturing’ is a generic name to broadly cover methods and technologies that are directed towards achieving this goal. It includes the development of new chemical and industrial processes (e.g. water-based rather the solvent-based processes); reduction in the use of unsafe compounds (such as metals); development of ‘green’ chemicals that are more environment-compatible; and the efficient use of energy. In terms of its application to the reduction of manufacturing waste, nanotechnology can contribute in two ways: by helping the manufacturing to be more controlled and efficient, and by using nanomaterials (such as catalysts) that can raise the manufacturing efficiency while reducing or eliminating the use of toxic materials. Overall, nanotechnology has the potential to make industrial processes more efficient in terms of energy usage and material usage, while minimising the production of toxic wastes. Sometimes the application of nanotechnology to ‘greener’ manufacturing processes is referred to as ‘green nanotechnology’ and this includes, for example, bottom-up atomic-level synthesis for developing improved catalysts; inserting information into molecules to build new materials (such as DNA) through highly specific synthesis routes; scaling down material usage during chemical reactions by using nanoscale reactors; and improving manufacturing to require less energy and less toxic materials.
An example of ‘green nanotechnology’ is the development of aqueous-based microemulsions as an alternative to volatile organic compounds (VOCs) in the cleaning industry. These toxic and potentially carcinogenic compounds, such as chloroform, hexane and perchloroethylene, are conventionally used in the cleaning, textile and oil extraction industries. Microemulsions contain nano-sized aggregates that can be used as ‘receptors’ for extracting specific molecules at a nanoscale level. Researchers from the University of Oklahoma have synthesised microemulsions with water-attractive and water-repellent ‘linkers’ inserted between the head and tail parts of a surfactant molecule. The result is a surfactant that has a very low interfacial tension with a wide range of oils. When tested for cleaning textiles from motor oil residues and for extracting edible oil from oilseeds, the microemulsions were found to be very competitive with conventionally used VOCs, both in terms of extraction yield and simplicity of the process.
Protection of human health and the environment requires the rapid, sensitive detection of pollutants and pathogens with molecular precision. Accurate sensors are needed for in situ detection, both as miniaturised portable devices and as remote sensors, for real-time monitoring of large areas in the field. Generally speaking, a sensor is a device built to detect a specific biological or chemical compound, usually producing a digital electronic signal on detection. Sensors are now being used for the identification of toxic chemical compounds at ultra-low levels (ppm and ppb) in industrial products, chemical substances, water, air and soil samples, or in biological systems. Nanotechnologies can improve current sensing technology in various ways. By using nanomaterials with specific chemical and biological properties, the sensor selectivity can be improved, thus making it possible to isolate a specific chemical or biological compound with little interference. Hence, the accuracy of the sensors is improved. As with other nano-engineered products discussed in this document, the high surface-to-volume ratio of nanomaterials increases the surface area available for detection, which, in turn, has a positive effect on the limit of detection of the sensor, thereby improving the sensitivity of the device. Nanosensors are generally faster, as they can detect the targeted analyte (e.g. bacteria) at a lower concentration than a conventional sensor, so the positive response arrives quicker. Scaling down using nanomaterials allows detection sites to be packed into the same device, thus allowing the detection of multiple analytes. This scaling-down capability, together with the high specificity of the detection sites obtainable using nanotechnology, will allow the fabrication of super-small ‘multiplex’ sensors, thus lowering the cost of the analysis and reducing the number of devices needed to perform the analysis with an economic benefit. Advances in the field of nanoelectronics will also allow the fabrication of nanosensors capable of continuous, real-time monitoring.
Research in the field of nanosensors encompasses various areas such as: synthesising new nanomaterials with specific detection sites able to recognise a certain pollutant; developing new detection methods to increase the limit of detection of the sensors while ensuring a ‘readable’ electrical signal; and miniaturising the size of the sensor elements while integrating them with larger parts of the device.
An example of how nanoscience can be applied to sensing technology is shown in Figure 8, which schematises the operational principle of a heavy-metal nanosensor developed for monitoring heavy metals in drinking water. The sensor is made of an array of electrode pairs fabricated on a silicon chip and separated by a few nanometres. When the electrodes are exposed to a solution of water containing metal ions, these deposit inside the nano-gap between the electrodes. Once the deposited metal bridges the gap, a ‘jump’ in conductance between the electrodes is registered. The size of the gap, being only a few nanometres, allows the detection of a very low concentration of metal ions. This type of sensor is called a ‘nanocontact sensor’.
Nanowire and nanotube-based sensors
Some nanomaterials in the form of nanowires or nanotubes offer outstanding opportunities as sensor elements in chemical and biological sensors. Individual single-walled carbon nanotubes (SWNTs) have been demonstrated to exhibit a faster response and a substantially higher sensitivity, for example towards gaseous molecules (such as NO2 and NH3) than that of existing solid-state sensors. In this case, the mechanism involved in sensing is the direct binding of the gaseous molecule to the surface of the SWNT, upon which the electrical resistance of the SWNT dramatically increases or decreases. Moreover, this sensitivity has been registered at room temperature, whereas conventional solid-state sensors operate at very high temperatures (200–600 °C) to achieve enhanced chemical reactivity between molecules and the sensor material.
Although SWNTs are promising candidates as nanosensors, they also have some limitations that could curb their development. First, existing synthesis methods produce a mixture of metallic and semiconducting nanotubes with only the latter being useful as sensors. Second, in order to be able to sense a variety of chemical and biological species, the surface of nanotubes needs to be modified to have specific functionalities to bind those species. Flexible methods to modify the surface of nanotubes to bind a large variety of analytes are not yet well established. Conversely, nanowires of semiconductors such as silicon do not have these limitations: they are always semiconductors and there is established knowledge for the chemical modification of their surface. Boron-doped silicon nanowires (SiNWs) have been used for the sensitive real-time electrical detection of proteins and antibodies. The small size and the capability of these semiconductor nanowires to detect a wide range of analytes in real-time could be used to develop sensors to detect pathogens, chemical and biological agents in water, air and food.
A cantilever sensor is a device made of an array of silicon cantilevers, each coated with a nano-layer which is sensitive to a specific pollutant. The cantilevers are typically 10–500 µm long, but have a thickness of a few micrometres or less. The pollutant-specific layer on top of the cantilever ‘arm’ is at the nanoscale level. The interaction of the pollutant with the ‘arm’ of the cantilever causes this to bend as a consequence of a change in surface stress. A laser beam detects this minute bending, which can also lead to a quantitative mass measurement of the pollutant detected. Cantilever sensors have been developed to detect VOCs, heavy metals, pesticides and harmful bacteria like salmonella.
Food packaging and monitoring
Nanotechnology may have a tremendous impact on the way food is produced, packaged, stored and transported. Applications include improved processing and packaging, enhanced flavour and nutrition, tracking of products and ingredients from farm to shelf, and monitoring of taste, ripening, and microbiological contamination.
These areas are of major public and industrial interest and technical solutions require that profound insights from materials science are combined with a thorough understanding of the chemical, molecular, and physical composition of foods and their nutritional effects.
Hopes are high that nanotechnology can provide solutions to many of these diverse challenges. Meeting these challenges is, for example, the mission of the NanoFOOD consortium at iNANO (Aarhus University).
One project, in particular, focuses on the characterisation and exploitation of proteins immobilised on surfaces. Immobilisation strategies need to be developed to ensure the surface attachment of functional proteins in order to modify surface properties to include enzymatic activity. Such enzymatic surfaces have huge potential in many applications including food production, healthcare and environmental control. An example of the latter is the development of substitutes for the ecotoxic tributyltin antifouling paint used on ships. This paint must not be used after 2008, and the race is on to find effective and environmentally friendly alternatives.
Scientists are working to develop surfaces and paint formulations with antifouling properties. Apart from marine vessels, such surfaces and paint may find applications in food production and hospitals. The approach is to engineer surfaces with antifouling effects against a wide range of organisms by immobilising enzymes that either degrade key components of biofilms or have toxic effects on the fouling organisms. The hope is that such properties will inhibit bacterial adhesion and growth. Long-term functionality and sustained release of active compounds are key goals.
Another area where nanotechnology will have an important impact is food packaging and tracking. The vision is to be able to create packaging systems that are ‘smart’ in the sense of being able to detect the freshness of the food they contain, report any spoilage, and allow tracking of the package through the entire supply chain.
Some technologies already exist that combine the plastic of the package with a dye which changes colour in the absence or presence of a specific chemical, such as oxygen or ethanol (two gases that are indicative of spoilage), therefore visually alerting the consumer that the package is compromised, even before there are visible signs of spoilage. Latest advancements in this field consider the use of nanomaterials, such as nanoparticles, or nanofibres, embedded in the inner side of the plastic used in the packages and in combination with dyes.
Nanosensors such as the one described in the previous section could be used, capable of detecting ultra-small quantities of material. Nanomaterials have the advantage of having high surface areas, so they are very reactive and can be functionalised to be very sensitive to detect a specific chemical species, such as oxygen, and provide a fast response. In a similar way, nanoparticles could be used to detect other chemicals that indicate food spoilage, such as ethanol, or even the presence of bacteria. This nanosensor could even be engineered to continuously wireless communicate the status of the product it contains to the manufacture.