Anand Gole

Water-dispersible tryptophan-protected gold nanoparticles prepared by the spontaneous reduction of aqueous chloroaurate ions by the amino acid.

The synthesis of water-dispersible amino-acid-protected gold nanoparticles by the spontaneous reduction of aqueous chloroaurate ions by tryptophan is described. Water-dispersible gold nanoparticles may also be obtained by the sequential synthesis of the gold nanoparticles by borohydride reduction of chloroauric acid followed by capping with tryptophan. Comparison of the proton NMR spectroscopic signatures from the tryptophan-protected gold nanoparticles obtained by the two processes indicated that the indole group in tryptophan is responsible for reduction of the aqueous chloroaurate ions. The reduction of the metal ions is accompanied by oxidative polymerization of the indole group of the tryptophan molecules and, consequently, some degree of cross-linking of the gold nanoparticles.

Iron Oxide Coated Gold Nanorods: Synthesis, Characterization, and Magnetic Manipulation

We report a simple process to generate iron oxide coated gold nanorods. Gold nanorods, synthesized by our three-step seed mediated protocol, were coated with a layer of polymer, poly(sodium 4-styrenesulfonate). The negatively charged polymer on the nanorod surface electrostatically attracted a mixture of aqueous iron(II) and iron(III) ions. Base-mediated coprecipitation of iron salts was used to form uniform coatings of iron oxide nanoparticles onto the surface of gold nanorods. The magnetic properties were studied using a superconducting quantum interference device (SQUID) magnetometer, which indicated superparamagnetic behavior of the composites. These iron oxide coated gold nanorods were studied for macroscopic magnetic manipulation and were found to be weakly magnetic. For comparison, premade iron oxide nanoparticles, attached to gold nanorods by electrostatic interactions, were also studied. Although control over uniform coating of the nanorods was difficult to achieve, magnetic manipulation was improved in the latter case. The products of both synthetic methods were monitored by UV-vis spectroscopy, zeta potential measurements, and transmission electron microscopy. X-ray photoelectron spectroscopy was used to determine the oxidation state of iron in the gold nanorod-iron oxide composites, which is consistent with Fe2O3 rather than Fe3O4. The simple method of iron oxide coating is general and applicable to different nanoparticles, and it enables magnetic field-assisted ordering of assemblies of nanoparticles for different applications.

Langmuir−Blodgett Thin Films of Quantum Dots: Synthesis, Surface Modification, and Fluorescence Resonance Energy Transfer (FRET) Studies

We describe herein studies on as-prepared hydrophobic ZnS-CdSe quantum dots (QDs) at the air-water interface. Surface pressure-area (pi-A) isotherms have been used to study the monolayer behavior. Uniform, lamellar multilayer thin films of QDs were deposited by the Langmuir-Blodgett (LB) technique. The role of two different surfactant systems commonly employed in the synthesis of these QDs (trioctylphosphine oxide-octadecylamine (TOPO-ODA) system and trioctylphosphine oxide-tetradecylphosphonic acid (TOPO-TDPA) system) on the monolayer behavior and the quality of thin films produced has been investigated. The thin films were characterized by quartz crystal microgravimetry (QCM), contact angle measurements, fluorescence spectroscopy, and transmission electron microscopy (TEM). These QD films were further modified by an amphiphilic polymer, poly(maleic anhydride-alt-1-tetradecene) (PMA). The hydrophobic interaction between the polymers and the surfactants attached to the QDs drove the self-assembly process. The carboxylic acid functional groups in the polymer were also used to immobilize avidin. We have demonstrated a proof of concept for the biosensing strategy wherein the avidin-coated QD films attracted biotinylated gold nanoparticles, resulting in fluorescence resonance energy transfer (FRET) quenching of the thin films.

One pot synthesis of magnetite–silica nanocomposites: applications as tags, entrapment matrix and in water purification

We report herein a novel one pot single step synthesis of magnetite–silica nanocomposites. The concept of co-precipitation of iron(II) and iron(III) salts by a base has been used. The main process differentiator is the use of an alkaline solution of sodium silicate instead of a base. This unique modification helps not only in the formation of iron oxide (due to alkaline conditions) but also introduces silica in the reaction to form a magnetite–silica nanocomposite. Having a silica surface is advantageous, in that it stabilizes the silica particles, makes them biocompatible and gives opportunity to further modify and tag functional groups via the well established silica surface chemistry. Furthermore, the process also yields porous silica, which increases the overall surface area of the nanocomposite. The nanocomposite samples have been characterized by a host of techniques, such as UV-vis spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), energy dispersive analysis using X-rays (EDAX), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), zeta potential measurements, dynamic light scattering and magnetic measurements. The utility of these high-surface area nanocomposites for different applications such as tagging (attachment of fluorophores Rhodamine, Rh B), entrapment matrix (zinc loading) and removal of arsenic for water purification has been explored.

Size separation of colloidal nanoparticles using a miniscale isoelectric focusing technique

A new method for the isolation of nanometer scale metal particles according to size is demonstrated in this paper. The colloids were derivatized with carboxylic acid groups using self-assembled bifunctional surfactant molecules. The pH at which onset of ionization occurs for the surface carboxylic acid groups was determined for silver and gold colloidal nanoparticles of different sizes using a miniscale isoelectric focusing unit. The pH at which ionization begins is a strong function of the surface curvature of the colloidal particles. It is demonstrated that this property can be used to separate mixtures of colloidal particles of different sizes. In addition to the ability to separate different sized nanoparticles, this technique may also be used to improve the polydispersity of colloids which is an important prerequisite for many applications.

One-pot synthesis of silica-coated magnetic plasmonic tracer nanoparticles

We demonstrate a one-pot procedure to synthesize and embed silver nanoparticles inside silica shells together with iron oxide nanoparticles and Raman reporter molecules, followed by fluorophore attachment to the silica, to form a class of tracer nanoparticles suitable for biological and environmental applications.

Lamellar Langmuir–Blodgett films of hydrophobized colloidal gold nanoparticles by organization at the air–water interface

The organization of hydrophobically modified colloidal gold nanoparticles at the air–water interface and the formation thereafter of lamellar, multilayer films of the nanoparticles by the Langmuir–Blodgett (LB) technique is described in this paper. The hydrophobization of the gold colloidal particles was accomplished by the electrostatic extraction of carboxylic acid derivatized gold particles (synthesized in an aqueous medium, 35 A in size) from solution into thermally evaporated fatty amine films by a simple immersion procedure. The acid–base complex formed by the association of the carboxylic acid groups bound to the colloidal particle surface and the amine groups in the lipid matrix leads to the formation of a strongly-bound hydrophobic sheath of fatty amine molecules around the particles. The colloidal gold particles can thereafter be dissolved in different organic solvents, dried and redispersed repeatedly without significant aggregation of the gold particles. The hydrophobic gold particles were dissolved in a spreading solvent and organized on the surface of water. The organization of the particles and the formation of multilayer films by the Langmuir–Blodgett technique was followed by surface pressure–area isotherm measurements of the colloidal particle Langmuir monolayer, quartz crystal microgravimetry, ultraviolet-vis spectroscopy and Fourier transform infrared spectroscopy. It was observed that a close-packed monolayer of the colloidal particles was formed on the surface of water and that excellent multilayer films of the colloidal nanoparticles can be grown on different supports by sequential transfer by the LB technique.

Penicillin G Acylase‐Fatty Lipid Biocomposite Films Show Excellent Catalytic Activity and Long Term Stability/Reusability

The formation of biocomposite films of the pharmaceutically important enzyme penicillin G acylase (PGA) and fatty lipids under enzyme‐friendly conditions is described. The approach involves a simple beaker‐based diffusion protocol wherein the enzyme diffuses into the lipid film during immersion in the enzyme solution, thereby leading to the formation of a biocomposite film. The incorporation of the enzyme in both cationic as well as anionic lipids suggests the important role of secondary interactions such as hydrophobic and hydrogen bonding in the enzyme immobilization process. The kinetics of formation of the enzyme‐lipid biocomposites has been studied by quartz crystal microgravimentry (QCM) measurements. The stability of the enzyme in the lipid matrix was confirmed by Fourier transform infrared spectroscopy (FTIR) and biocatalytic activity measurements. Whereas the biological activity of the lipid‐immobilized enzyme was marginally higher than that of the free enzyme, the biocomposite film exhibited increased thermal/temporal stability. Particularly exciting was the observation that the biocomposite films could be reused in biocatalysis reactions without significant loss in activity, which indicates potentially exciting biomedical/industrial application of these films.

Protein diffusion into thermally evaporated lipid films: role of protein charge/mass ratio

The kinetics of entrapment of proteins of different charge:mass ratios in thermally evaporated fatty lipid films (cationic, anionic and neutral) has been studied by quartz crystal microgravimetry (QCM) and analyzed in terms of a 1-D diffusion model. Under conditions where the proteins and immobilizing lipid matrix are oppositely charged, it is observed that the protein diffusivity increases with increasing charge:mass ratio. A notable exception to this rule was the membrane protein, Cyt c, indicating that interactions other than electrostatic dominate the immobilization of such proteins in the lipids. While entrapment of proteins also occurs in non-ionizable lipid films, the protein diffusivity did not show a meaningful correlation with the protein charge:mass ratio. Taken together with the observation that the extent of protein loading was directly related to the protein:lipid equilibrium molar ratio, the above findings support a predominantly electrostatic picture of protein immobilization in thermally evaporated cationic/anionic lipid films.

Glucose induced in-situ reduction of chloroaurate ions entrapped in a fatty amine film: formation of gold nanoparticle–lipid composites

The formation of gold nanoparticle–lipid composite films by glucose-induced reduction of chloroaurate ions entrapped in thermally evaporated fatty amine films is described. Simple immersion of films of the salt of octadecylamine and chloroaurate ions (formed by immersion of thermally evaporated fatty amine films in chloroauric acid solution) in glucose solution leads to the facile in-situ reduction of the metal ions to form gold nanoparticles in the fatty amine matrix. The formation of gold nanoparticles is readily detected by the appearance of a violet color in the film and thus forms the basis of a possible new, gold nanoparticle-based colorimetric sensor for glucose. The formation of the fatty amine salt of chloroauric acid and the subsequent reduction of the metal ions by glucose has been followed by quartz crystal microgravimetry, Fourier transform infrared spectroscopy, X-ray photoemission spectroscopy and transmission electron microscopy measurements.