Renjis T. Tom

Porosity of core-shell nanoparticles

The porosity of titania and zirconia covered Ag and Au nanoparticles has been investigated using the metal core reactivity as a probe. The presence of pores was confirmed by a newly discovered reaction between halocarbons and core–shell nanoparticles, in which the core gets converted into ions, which are leached out through the shell. Halocarbons having different alkyl chain lengths react with metal cores at different rates due to the differences in the accessibility of the core. It is also observed that the electrochemical accessibility of the core can be reduced by blocking the pores by adsorbates such as cis-dithiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)ruthenium(II) dye (popularly called N3 dye). With the adsorbed dye molecules on the oxide shell, metal cores are stable for extended periods of time even after the addition of halocarbons. The porosity of the Au@SiO2 system, in which a silica shell is formed over the metal clusters through monolayers, has also been studied. Our studies show that the porosity of different kinds of shells is largely similar, allowing molecular and ion penetration.

ZrO2 bubbles from core–shell nanoparticles

Selective removal of metal cores from core–shell Ag@ZrO2 (ZrO2 coated Ag) and Au@ZrO2 (ZrO2 coated Au) nanoparticles result in stable and freely suspendable oxide nanobubbles of varying dimensions, both in thickness and in diameter. The metal core is removed by a newly found reaction in which halocarbons, generally chlorides, oxidise the metal core and leach out the metal ions. Reduction in the surface plasmon excitation intensity and decrease in the voltammetric current during metal core removal were used to study the process.

Detection and extraction of endosulfan by metal nanoparticles

One of the most common pesticides in the developing world, endosulfan, can be detected in ppm levels using gold nanoparticles. Endosulfan adsorbs on the nanoparticle surface and upon interaction for a long time, the nanoparticles precipitate from the solution. Interaction with silver is weak, yet adsorption occurs leading to removal of endosulfan from the solution. A multilayer assembly of gold nanoparticles prepared on a glass substrate shows excellent spectrophotometric response suggesting potential applications.

CHEMICAL INTERACTIONS AT NOBLE METAL NANOPARTICLE SURFACES — CATALYSIS, SENSORS AND DEVICES

In this paper, a summary of some of the recent research efforts in our laboratory on chemical interactions at noble metal nanoparticle surfaces is presented. The article is divided into five sections, detailing with (i) interactions of simple halocarbons with gold and silver nanoparticle surfaces at room temperature by a new chemistry and the exploitation of this chemistry in the extraction of pesticides from drinking water, (ii) interaction of biologically important proteins such as Cyt c, hemoglobin and myoglobin as well as a model system, hemin with gold and silver nanoparticles and nanorods forming nano–bio conjugates and their surface binding chemistry, (iii) formation of polymer–nano composites with tunable optical properties and temperature sensing characteristics by single and multi-step methodologies, (iv) nanomaterials-based flow sensors and (v) composites of noble metal nanoparticles and metallic carbon nanotubes showing visible fluorescence induced by metal–semiconductor transition.

Nanoparticles-chemistry, new synthetic approaches, gas phase clustering and novel applications

In this paper, an overview of the synthesis, chemistry and applications of nanosystems carried out in our laboratory is presented. The discussion is divided into four sections, namely (a) chemistry of nanoparticles, (b) development of new synthetic approaches, (c) gas phase clusters and (d) device structures and applications. In ‘chemistry of nanoparticles’ we describe a novel reaction between nanoparticles of Ag and Au with halocarbons. The reactions lead to the formation of various carbonaceous materials and metal halides. In ‘development of new synthetic approaches’ our one-pot methodologies for the synthesis of core-shell nanosystems of Au, Ag and Cu protected with TiO2 and ZrO2 as well as various polymers are discussed. Some results on the interaction of nanoparticles with biomolecules are also detailed in this section. The third section covers the formation of gas phase aggregates/clusters of thiol-protected sub-nanoparticles. Laser desorption of H2MoO4, H2WO4, MoS2, and WS2 giving novel clusters is discussed. The fourth section deals with the development of simple devices and technologies using nanomaterials described above.