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Biomedical subjects

K Vijayamohanan

Publications and source records attributed to K Vijayamohanan.

13 recordsLinked to original sources
PubMed2006

ZnO multipods, submicron wires, and spherical structures and their unique field emission behavior.

A simple method of vapor deposition for the shape selective synthesis of ZnO structures, namely, multipods, submicron wires, and spheres, has been successfully demonstrated. A plausible growth mechanism based on the studies of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) is proposed. Our studies suggest that the growth of a multipod structure is governed by the screw dislocation propagation while the vapor-liquid-solid (VLS) mechanism is responsible for the formation of submicron wires and spheres. Moreover, the flow rate of the carrier gas plays a crucial role in governing the morphology. Further, these structures exhibit an enhanced field emission behavior. The nonlinearity in the Fowler-Nordheim (F-N) plot, a characteristic feature of electron emission from semiconductors, is explained by considering the contributions from both the conduction and the valence bands of ZnO.

Journal Article↗
PubMed2006

Silver nanocluster redox-couple-promoted nonclassical electron transfer: an efficient electrochemical Wolff rearrangement of alpha-diazoketones.

In this work we report the unique electrocatalytic role of benzoic acid protected silver nanoclusters (Ag(n), mean core diameter 2.5 nm) in the Wolff rearrangement (Scheme 1) of alpha-diazoketones. More specifically, the presence of a Ag(n) (0)/Ag(n) (+) redox couple facilitates a nonclassical electron-transfer process, involving chemical reaction(s) interposed between two electron-transfer steps occurring in opposite directions. Consequently, the net electron transfer between the electron mediator (Ag(n)) and alpha-diazoketone is zero. In-situ UV-visible studies using pyridine as a nucleophilic probe indicate the participation of alpha-ketocarbene/ketene as important reaction intermediates. Controlled potential coulometry of alpha-diazoketones using Ag(n) as the anode results in the formation of Wolff rearranged carboxylic acids in excellent yield, without sacrificing the electrocatalyst.

Journal Article↗
PubMed2006

Electrochemical organization of monolayer protected gold nanoclusters on single-walled carbon nanotubes: significantly enhanced double layer capacitance.

This paper reports a novel electrochemical route for anchoring monolayer protected gold nanoclusters (size 8 +/- 0.2 nm) on single-walled carbon nanotube bundles, resulting in the formation of hybrid materials. Monolayer protected gold nanoclusters prepared by modified Brust synthesis route were organized on SWNT bundles by cycling the potential in dichloromethane between -1 to +1 V at a scan rate of 50 mV/s. Monolayer protected nanoclusters in electrolyte solutions possess ionic space charge around them (double layer charging), making them suitable for organization on nanotube bundles, by tuning the electrostatic interactions. More significantly, analysis of the double layer capacitance of these hybrid materials shows almost ten times increase in capacitance compared to that of bare SWNT bundles. We believe that these hybrid materials are potentially useful in nanoelectronics.

Crystallization↗
PubMed2005

Wet-chemical templateless assembly of metal nanowires from nanoparticles.

We describe a new, simple, room-temperature wet-chemical approach for assembling Au and Ag nanoparticles into nanowire networks, without the use of lithographic templates. Five to 35 nm-diameter nanowires passivated with a thin organic layer were synthesized by mechanically agitating a biphasic liquid mixture of an aqueous hydrosol containing the nanoparticles, and toluene. Nanowire structure and surface chemistry are discussed based on electron microscopy, UV-visible spectroscopy and thermogravimetric analyses.

Journal Article↗
PubMed2004

Templateless room-temperature assembly of nanowire networks from nanoparticles.

We demonstrate a new, room-temperature approach to assemble two-dimensional and three-dimensional networks of gold nanowires by agitating nanoparticles in a toluene-aqueous mixture, without the use of templates. The nanowires have a uniform diameter of about 5 nm and consist of coalesced face-centered cubic nanocrystals. Toluene molecules passivate the gold surfaces during nanoparticle coalescence, rendering the nanowires hydrophobic and enabling their transfer into the toluene layer. Such templateless low-temperature assembly of mesostructures from nanoscale building blocks open up new possibilities for creating porous self-supporting nanocatalysts, nanowires for device interconnection, and low-density high-strength nanofillers for composites.

Journal Article↗
PubMed2004

Controlled interlinking of Au and Ag nanoclusters using 4-aminothiophenol as molecular interconnects.

This work describes the formation of interlinked gold and silver nanoclusters at controlled pH using 4-aminothiophenol (ATP) as a molecular interconnect. UV-visible spectra give on intercrystal plasmon resonance band in the region 550-580 nm. The crystalline heteroassembly formation is also evident from the transmission electron microscopic (TEM) images, whereas X-ray photoelectron spectroscopic (XPS) analysis of the aggregates shows the presence of charged -N species, indicating electrostatic interaction of -N with Ag nanoclusters. Furthermore, electrochemical studies of these heteroassembled systems suggest that silver nanoclusters are not fully passivated by the monolayers of ATP and are accessible for redox reactions.

Journal Article↗
PubMed2002

Formation of Cu and Cu2O nanoparticles by variation of the surface ligand: preparation, structure, and insulating-to-metallic transition.

Copper and copper (I) oxide nanoparticles protected by self-assembled monolayers of thiol, carboxyl, and amine functionalities [X(CH(2))(n)-CH(3), where X can be -COOH, -NH(2), or -SH] have been prepared by the controlled reduction of aqueous copper salts using Brust synthesis. The optical absorption spectrum (lambda(max)=289 nm) is found to be invariant with the nature of the capping molecule while the particle shape and distribution are found to depend strongly on it. A comparison of the protection efficiency for different capping agents such as dodecanethiol (DDT), tridecylamine (TDA), and lauric acid (LA) suggests that although zerovalent Cu is initially formed for dodecanethiol, all other cases allow oxidation to Cu(2)O nanoparticles. Despite the variation in particle size and relative stability, nanoparticles have been found to form oxides after a few days, especially for the case of LA and TDA surface capping. For all the samples studied, the size has been found to be 4-8 nm by high-resolution transmission electron microscopy. The protective ability is found to be better for dodecanethiol SAM (similar to the case of Au and Ag nanoparticles), while the order of capping efficiency varies as Cu-DDT>Cu-TDA>Cu-LA. In the present study we also demonstrate a reversible metal-insulator transition (MIT) in capped nanoparticles of Cu using temperature-dependent electrical resistivity measurement. However, the LA-capped sample does not show any such transition, possibly due to the oxide formation.

Journal Article↗
PubMed2002

Self-assembled monolayers as a tunable platform for biosensor applications.

Considerable attention has been drawn during the last two decades to functionalize noble metal surfaces by forming ordered organic films of few nm to several hundred-nm thickness. Self-assembled monolayer (SAM) provides one simple route to functionalize electrode surfaces by organic molecules (both aliphatic and aromatic) containing free anchor groups such as thiols, disulphides, amines, silanes, or acids. The monolayer produced by self-assembly allows tremendous flexibility with respect to several applications depending upon their terminal functionality (hydrophilic or hydrophobic control) or by varying the chain length (distance control). For example, SAM of long chain alkane thiol produces a highly packed and ordered surface, which can provide a membrane like microenvironment, useful for immobilising biological molecules. The high selectivity of biological molecules integrated with an electrochemical, optical or piezoelectric transduction mode of analyte recognition offers great promise to exploit them as efficient and accurate biosensors. It is demonstrated with suitable examples that monolayer design plays a key role in controlling the performance of these SAM based biosensors, irrespective of the immobilisation strategy and sensing mechanism.

Biosensing Techniques↗
PubMed1999

Self-assembled Monolayers of Two Aromatic Disulfides and a Diselenide on Polycrystalline Silver Films: An Investigation by SERS and XPS.

Self-assembled monolayers of diphenyldisufide (DDS), naphthalenedisulfide (NDS), and diphenyldiselenide (DDSe) on polycrystalline silver films have been investigated by surface enhanced Raman spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS). DDS adsorbs on Ag through a homolytic cleavage of the S-S bond and resultant thiolate at the surface decomposes upon prolonged exposure to air. The geometry of the molecule is such that the benzene ring is almost horizontal to the surface. The Raman spectrum has been assigned in the light of ab-initio molecular orbital calculations. In DDSe, the Se-Se bond is retained upon adsorption and the molecule sticks up. In contrast, NDS is highly reactive on the microscopically rough surface so that a stable monolayer could not be prepared. A temperature dependent Raman study of the DDS monolayer shows the absence of any reorientation at the surface as one would expect from the adsorption geometry. XPS study complements the SERS data and shows the presence of Ag2S on an NDS exposed surface. Copyright 1999 Academic Press.

Journal Article↗
PubMed1998

Effect of Co-Adsorbed Surfactant on the Structure of Self-Assembled Monolayer of Thiol on Polycrystalline Gold.

The structure and formation kinetics of a self-assembled monolayer (SAM) of long-chain alkane thiol in the presence and absence of an anionic surfactant like sodium dodecyl sulfate (SDS) has been studied using impedance, cyclic voltammetic, and quartz crystal microbalance (QCM) measurements. The comparison of these results, especially the ionic permeability of the monolayer, suggests a composite monolayer where both the surfactant and thiol molecules are cooperatively adsorbed to form distinct surfactant patches. This is further substantiated by the voltammetric experiments with redox probe in solution. A gradual change in the double layer capacitance value for the thiol monolayer alone (1.4 µF/cm2) to the composite monolayer (6.7 µF/cm2) and to the surfactant alone (11 µF/cm2) further supports the nature of the mixed monolayer. The kinetics of monolayer formation also shows interesting changes as revealed by the QCM studies, where a phase transition from simple to composite monolayer has been observed around an approximate coverage of 0.44. The same growth scheme is also observed for 1-pentanethiol, naphthalene disulfide, and diphenyl disulfide, suggesting that the mechanism appears to be general. Copyright 1998 Academic Press.

Journal Article↗
PubMedDate not supplied

Applications of self-assembled monolayers for biomolecular electronics.

Preparation and characterization of ordered ultrathin organic films (a few nanometers to several hundred nanometers) has recently attracted considerable attention because of the possibility of controlling order and interactions at the molecular level and has triggered several innovative applications ranging from molecular electronics to tribology. Monomolecular films prepared by self-assembly are attractive for several exciting applications because of the unique possibility of making the selection of different types of terminal functional groups as well as length scales more flexible. The present article discusses various applications of self-assembled monolayers (SAMs) in molecular electronics ranging from biosensors to optoelectronic devices with specific examples. Similarly, SAMs and multilayers of bifunctional molecules on polycrystalline substrates can be effectively used to carry out specific reactions between pendent functionalities and solution or gaseous species to produce new hybrid materials for devices such as molecular diodes. The importance of SAMs in controlling nucleation and growth is also illustrated using biomimetic synthesis of ceramic thin films (biomineralization) of zirconia.

Biosensing Techniques↗