Noble Metal Nanosystems for the Detection and Removal of Pollutants in Drinking Water

Noble metal nanostructures are widely explored as they have unusual size- and shape-dependent physical, chemical, and optical properties. Silver and gold nanosystems were synthesized in a variety of forms-including spherical, rod, wire, triangle, star, and flower shapes. These systems were used for applications in catalysis, sensing, surface-enhanced Raman scattering, and many others. When the size of the particles of noble metal nanosystems (NMNs) reaches in-between molecules and nanoparticles (NPs), they exhibit molecular properties such as discrete optical absorption and photoluminescence. Use of bulk noble metals in water treatment and medicine is very well documented since ancient times. In recent history, water pollution has become one of the major problems throughout the world. The intensity of water contamination is becoming more complex with time because of the addition of a large number of chemicals and biological species, mainly because of anthropogenic activities. Permissible limits for most of the contaminants have decreased as time progressed. Technologies to be developed must be able to detect ultralow concentrations and remove large amounts of pollutants. NMNs promise to be efficient adsorbents for the treatment of polluted water as they exhibit high absorption and scattering cross-sections. In the recent past, noble metal clusters, a new category of materials, were employed in selective and sensitive detection of heavy metals and anions using their optical properties, efficient adsorption capacities, and high reactivity. Some of them were demonstrated as practical devices for these purposes. NMNs were found to exhibit affinity toward pesticides and novel mineralization reactions were seen with halocarbons. NPs of silver, palladium, and gold were also used for the removal of other organic molecules such as pharmaceuticals, dyes, explosives, and for the catalytic degradation of chlorinated solvents. Silver NPs and clusters have been shown to be useful as antimicrobial agents. In all the above applications, NMNs were used alone or as a component of the composite. In some cases, NMNs were supported onto metal oxide surfaces such as Fe2O3, TiO2, MgO, Al2O3, and Mn2O3 as the separation of materials and purified water is easy after treatment. Alloy NPs, alloy clusters, and composites are emerging materials, and they need to be explored as they provide new opportunities in comparison to the known materials.