Water is one of the most essential resources for living things. Although 75% of the earth’s surface is covered with water, only 2.5% of the same is potable. Further, less than 1% of freshwater is accessible to humans. With extensive industrialization, the per capita water requirement has increased enormously. To produce one ton of steel about 215,000 litres of water will be needed. Hence, the need for effective water management.
Although several conventional water treatment technologies like chemical treatment, mechanical separation, ultraviolet radiation, biological treatment and desalination are currently available in the market, nanotechnology-enabled products are expected to have a few distinct advantages for large-scale application, to economise and perform with greater efficiency.
Nanotechnology can contribute to effective water management in many ways: • Nano-membranes and nano-clays for water filtration and desalination • Nanoparticle-activated wastewater reuse systems
• Nanosensors to monitor water quality against bacteria, heavy metals and toxins Nano-enabled water treatment technologies include the use of nano-membranes and filters based on carbon nanotubes, nano-porous ceramics, magnetic nanoparticles and other nanomaterials. It has been shown that nano-filtration can lead to remediation of
even brackish water. Carbon nanotube–based water filters have been found to be effective tools for nano-filtration. Carbon nanotubes function as molecular filters and allow water molecules to pass through them. Molecules that are bigger than the diameter of the CNT are filtered out. Also, as a consequence of their electronic state, a few smaller ions are also not permitted to enter through the carbon nanotube. Carbon nanotube membranes can reduce the cost of desalination sigificantly. It is reported that the use of nano-titanium dioxide and magnetic nanoparticles can decompose organic pollutants and remove salts and heavy metals, enabling wastewater reuse.
Different molecules can be separated based on their size during nano-filtration using membranes. The technique is mainly applied for the removal of organic substances, such as micropollutants and multivalent ions. In industrial processes, nano-filtration is applied for the removal of specific components, such as colouring agents.
Other applications of nano-filtration are: • Removal of pesticides from groundwater • Removal of heavy metals from wastewater • Wastewater recycling in laundries
• Water softening
There are several conventional technologies in practice today to remove bioorganisms, toxins and impurities from water. It is well known that Saudi Arabia produces nearly 70% of its potable water by the desalination technology. Nanotechnology is expected to result in economic solutions capable of reaching a wider cross section of people in the longer run. For effective remediation of contaminated water, particularly for removing heavy metal ions, various nanoparticles and nanomaterials like zeolites, carbon nanotubes, self-assembled monolayers on mesoporous supports (SAMMS), biopolymers, single-enzyme nanoparticles, zero-valent iron nanoparticles, bimetallic iron nanoparticles and nanoscale semiconductor photocatalysts are in use.
Removal of pathogens from water is essential to avoid several waterborne diseases. It is believed that the use of nanomaterials like silver and titanium dioxide with antimicrobial characteristics can provide a viable alternative to the use of chlorine treatment. Air pollution can be monitored using nanotechnology with filtration. Nano-filters could be applied to automobile and other exhausts in industry to filter out contaminants before the exhaust gases are let into the atmosphere, so that the build-up of greenhouse gases in the atmosphere is prevented. Nanocatalysts can be used in catalytic converters in automobiles to remove contaminants and increase driving performance. Nanosensors could also be developed to detect toxic gases at very low concentrations in the atmosphere.
4.10 NANO-MEDICAL APPLICATIONS
Nanotechnology is promising to revolutionise healthcare technologies in a more patient- friendly direction. Nanotechnology and nanomaterials are being used in diagnosis, therapy and prevention. Significant developments have been illustrated in the field of nano-enabled targetted drug delivery, cancer and TB therapy, disease diagnosis, biosensors for health
monitoring, surgical tools, implant materials, tissue engineering, molecular imaging, biodetection of disease markers, etc. Nanotechnology has also enabled the production of ‘laboratories on a chip’ that perform multiple medical tests (in vitro or in vivo).
When these particles function as nano-medibots that release anti-cancer pharmaceuticals into the cells or penetrate the tissue and deconstruct them mechanically, then they treat cancer. Further, particles may absorb infrared radiation, which is converted to heat to ablate the target (cancer) tissue. Finally, when administered prophylactically (as a nano-vaccination), they can also prevent cancer.
Nano-pharmacology is the use of nanotechnology in pharmacology applications—assembly
of current molecular entities; exploring and matching specific compounds to particular patients for maximum effectiveness; and advanced molecular compound delivery systems. Nanoparticles may yield targetted and sustained delivery of pharmaceuticals to specific tissues with a minimum of systemic side effects. In case of nanoshells, manipulating the ratio of wall to core dimensions, they can be precisely tuned to scatter or absorb any particular wavelength of light. Gold-coated nanoshells could convert light into heat, enabling the destruction of tumours.
Nano-pharmaceuticals and nanotechnology drug delivery systems provide greater and
more controlled pharmaceutical uptake in tissues throughout the the body. This is critical for oncological applications. ‘Trojan Horse’ capsules can be used to sneak in a biological compound payload through the blood–brain hurdle to treat ophthalmologic diseases such as macular degeneration, glaucoma and diabetic retinopathy. Nano-pharmacology delivery systems are not limited to internal use—they also enable the absorption of pharmaceuticals through nano-emulsions spread on the skin. Newly developed contrast dyes make it possible to examine patients at a molecular level. Nanotechnology can also be used to partially repair neurological damage. For example, it can improve the correctness of cochlear implants that turn sound into electrical impulses and assemble light-activated implants in the retina to partially restore lost vision. Biomemetic scaffolds are used to support damaged nerves to regrow and reconnect.
4.11 TEXTILES
There are many novel applications of nanotechnology in the textile industry to provide multifunctional attributes to fabrics. For example, the fabric can be made stain resistant, water repellent or absorbing, light emitting, antibacterial, release fragrance in a controlled manner, etc. Antimicrobial properties have been imparted to fabrics by incorporating suitable nanoparticles into nylon and other fabric polymers. It is also possible to coat nanocrystalline zinc oxide particles on synthetic fibres, to impart antimicrobial effect without much change in the colour and gloss of the fabric. Plasma technology is being used to modify the top few (nanometres) layers of textiles, allowing them to be made antibacterial, antifungal and water repellent. Other areas of interest include heat resistance and mechanical resilience to work wear, ballistic protection, sensors and camouflage.
Application of nanotechnology has enabled the development of intelligent textiles that are capable of sensing the environment or health of the personnel, to change colour in response to stimuli, and to generate heat. The major motivation for developing intelligent textiles is again perhaps nature. The skin is distributed with a sensor network, to detect pressure, heat, etc. The skin sweats on a hot day to cool the body, and enforces blood circulation on a cold day. Today, scientists have succeeded in developing fabrics with fibres coated with a variety of nanomaterials that can be used as sensors. Fibre sensors, which are capable of measuring temperature, strain/stress, gas and smell, are typically smart fibres that can be directly applied to textiles. These are expected to find use in skiwear, shoes, sports helmets and insulation devices. Development of textiles impregnated with sensors that are integrated with global positioning system (GPS) can help the wearer navigate to the desired destination. Fabrics and composites integrated with optical fibre sensors have been used to monitor the soundness of major bridges and buildings. The first generation of wearable motherboards has been developed, which have sensors integrated inside garments and can detect information regarding injury to the health of the wearer, and transmit such information remotely to a hospital.
Shape memory polymers exhibit much higher recoverable strain limits (~100%) in contrast
to shape memory metallic alloys. These are expected to have potential application in non- invasive surgery. Intelligent textiles that can change colour and provide camouflage are being developed for military applications. Scientists have developed an innovative process to combine extremely thin layers of two materials: plastic and glass. This results in a new fibre that can reflect all the light that hits it, from any direction. Uniforms woven from these fibres have an optical bar code that will help soldiers distinguish friend from foe on night patrol, or during the smoke and confusion of an attack with firearms.
4.12 PAINTS
Incorporating nanoparticles in paints could improve their performance, for example, by making them lighter and giving them different properties. Thinner paint coatings can reduce the weight. The solvent content of paints may also be substantially reduced with nanopaints. New types of fouling resistant marine paint could be developed and are urgently needed as alternatives to tributyl tin (TBT), now that the ecological impact of TBT has been recognised. Anti-fouling surface treatment is also valuable in process applications such as heat exchange, where it could lead to energy saving. If they can be produced at sufficiently low cost, fouling- resistant coatings could be used in piping for domestic and industrial water systems. It remains speculative whether effective anti-fouling coatings could reduce the use of biocides, including chlorine. Other novel, and more long-term, applications of nanoparticles might lie in paints that change colour in response to change in temperature or chemical environment, or paints that have reduced infrared absorptivity and therefore reduced heat loss. Owing to concerns about the health and environmental impacts of nanoparticles, the durability and abrasive behaviour of nano-engineered paints and coatings have to be addressed, so that abrasion products take the form of coarse or microscopic agglomerates rather than individual nanoparticles.
4.13 ENERGY
Nanomaterials are bound to find a place in green energy technologies too. The most common nanostructured energy enabling technologies that are emerging are:
• Nanostructured photovoltaic systems • Nanostructured fuel cells
• Hydrogen storage systems • Efficient light emitting devices
Conventional solar cells suffer from the limitations of poor efficiency and high cost compared to other large-scale energy resources. Nanomaterials-based solar panels can have increased performance and the technology could soon compete with conventional power plants.
The three main types of nano-solar cells being developed are: • Flexible polymer-based photovoltaics
• Nanoparticle solar cells
• Sprayable self-assembling photocells
Scientists are developing cheap and easy-to-apply plastic solar cells, composed of tiny nanorods dispersed in a polymer that can be easily applied to any surface. These nanorods absorb a particular wavelength to generate electrons. Their efficiency is presently only about 2%, though they are much cheaper. However, it is expected that by tuning the dimensions of the nanotube, it is possible to absorb a wide range of energy from light which would result in improved efficiency. It has been observed that the addition of a small fraction of carbon nanotubes to nanocrystalline TiO2 film almost doubles the efficiency.
Another promising area for the application of nanotechnology is the ‘hydrogen economy’. Hydrogen can be a good alternate fuel of the future not only because it is readily available in water but also because it is a non-polluting source of energy. Low-cost techniques of obtaining hydrogen from water may be worked out with the help of nanocatalysts. Nano-pyramids of (5–15 nm) iridium have been found to be highly efficient in aiding hydrogen generation from ammonia.
Nanotechnology can help in the development of the following: • Nano-engineered hydrocarbon carbon surface membranes • Spray depositing platinum on porous alumina
• Replacement of platinum catalysts using less expensive nanomaterials
Nanotechnology has also resulted in a significant about turn in the performance of of Li ion batteries. Application of certain nanomaterials has resulted in a decrease in organic electrolyte reduction during recharging. Nanoparticles quickly absorb and store vast quantities of lithium ions, without causing any deterioration in the electrode. This nano-enabled Li battery is also 60 times faster than the typical lithium ion batteries that are widely used today. In addition, the battery has a long life cycle, losing only 1% of capacity after 1,000 cycles of discharging and recharging, and can operate at very low temperatures. At –40°C the battery can discharge 80% of its capacity, against 100% in an ambient temperature of 25°C. This speedy and highly effective recharge characteristic of the battery will support CO2 reduction resulting in eco- friendly batteries.
Armchair quantum wire (AQW) is a SWNT-based wire with physical properties conducive
to flowing electrons that AQW cables could literally let electricity (in the form of electrons) glide across a grid for 1,000 miles with virtually no resistance or power loss. In comparison to superconductors, the SWNT-based AQWs have an advantage as they do not have to be cooled to cryogenic temperatures. According to Dr Adams,“The armchair quantum wire can simply by its structure propagate an electron down the length of a nanotube, much like light waves travel down an optical fiber.” The AQW cables would be a revolutionary leap beyond copper, providing lighter, stronger and more conductive cables with vastly more capacity. AQWs are tiny, and about 1014 wires would be present in 1 cm. Although each AQW can conduct only 20 μA of electricity, it would be possible to bundle 10–14 of them together to get 10 million amps down a single filament. It should also be possible to have multiple filaments together in a single cable, like multiple filaments in an optical cable, resulting in a capacity of over billions of amps, in contrast to copper cables that carry about 2000 amps.
Many significant efforts are in progress to identify and utilise new energy sources, to increase the production of existing sources, to increase conversion and storage efficiency, and, equally important, to reduce pollution. In MIT’s Laboratory for Electromagnetic and Electronic Systems, they are exploring a nanostructured ultracapacitor electrode that has the potential to increase a capacitor’s energy storage density to approach that of a chemical battery. Nanostructured emissive coatings and filters that significantly increase the efficiency of direct thermophotovoltaic (TPV) generation of electricity from heat are also under development.