S.K. Mehta

Synthesis and capping of water-dispersed gold nanoparticles by an amino acid: bioconjugation and binding studies.

We report a novel strategy for the synthesis of aqueous stable, carboxylated gold nanoparticles (GNPs) by using glutamic acid as the reducing agent. The ratio of chloroaurate ions, AuCl(-)(4) to glutamic acid was optimized in the reaction medium to obtain monodispersed GNPs. Glutamic acid reduced gold nanoparticles were characterized by UV-visible, FTIR, dynamic light scattering and transmission electron microscopy, which demonstrated high stability in aqueous solution over a period of time indicating stabilization via surface-bound amino acid. Functionalized nanoparticles were conjugated with protein molecules through electrostatic attraction between the surface-terminated negatively charged carboxylate groups (COO(-)) of glutamic acid and the positively charged amino groups (NH(+)(3)) of the protein. The conjugation efficiency of the GNP:protein conjugates was confirmed qualitatively and quantitatively through gel electrophoresis and critical flocculation concentration analysis. The interaction between functionalized GNPs with protein molecules was investigated using fluorescence spectroscopy showing the fluorescence quenching of the tryptophan residues of protein molecules after conjugation. Circular dichroism (CD) studies of the conjugates confirmed that the protein undergoes a more flexible conformational state on the boundary surface of GNPs after conjugation. There was substantial conformational transition from alpha-helix to beta-sheet structure after conjugation of protein to GNPs.

Evolution of ZnS Nanoparticles via Facile CTAB Aqueous Micellar Solution Route: A Study on Controlling Parameters

Synthesis of semiconductor nanoparticles with new photophysical properties is an area of special interest. Here, we report synthesis of ZnS nanoparticles in aqueous micellar solution of Cetyltrimethylammonium bromide (CTAB). The size of ZnS nanodispersions in aqueous micellar solution has been calculated using UV-vis spectroscopy, XRD, SAXS, and TEM measurements. The nanoparticles are found to be polydispersed in the size range 6–15 nm. Surface passivation by surfactant molecules has been studied using FTIR and fluorescence spectroscopy. The nanoparticles have been better stabilized using CTAB concentration above 1 mM. Furthermore, room temperature absorption and fluorescence emission of powdered ZnS nanoparticles after redispersion in water have also been investigated and compared with that in aqueous micellar solution. Time-dependent absorption behavior reveals that the formation of ZnS nanoparticles depends on CTAB concentration and was complete within 25 min.

Time dependence of nucleation and growth of silver nanoparticles generated by sugar reduction in micellar media

Synthesis of silver nanoparticles in homogeneous aqueous solutions of the precursors silver nitrate and three saccharides (two mono saccharides i.e. glucose and fructose and one disaccharide such as sucrose) has been performed. These Ag-nanoparticles can be derivatized by other surface treatments, as required. The anionic surfactant SDS was tested to investigate its effect on the dependence of size, growth rate, photoluminescence (PL) emission and polydispersity of the nanoparticles. The time evolution of UV-vis absorbance suggested that nucleation and growth rate markedly vary in a first order fashion w.r.t. Ag(+) salt concentration. The differences in the reducing ability of the saccharides were discussed with reference to their structure dependent adsorption behavior onto the particles. Changes in UV-vis, PL and FT-IR spectra during nucleation and growth of the nanoparticles were used to establish plausible mechanisms for the adsorption of surfactant on the particle surface, so as to restrict the growth. The results revealed a simple and easy strategy for synthesizing metal nanoparticles with well controlled shapes, sizes and structures.

Highly effective Fe-doped TiO2 nanoparticles photocatalysts for visible-light driven photocatalytic degradation of toxic organic compounds

This paper reports the synthesis of various molar concentrations of iron (Fe)-doped TiO2 nanoparticles and their efficient use as potential photocatalysts for photocatalytic degradation of toxic and harmful chemical, paranitrophenol. The nanoparticles were synthesized by a novel and facile ultrasonic assisted hydrothermal method and characterized in detail by various analytical techniques in terms of their morphological, structural, compositional, thermal, optical, pore size distribution, etc properties. The photocatalytic activities of the as-prepared Fe-doped TiO2 nanoparticles were examined under visible light illumination using para-nitrophenol as target pollutant. By detailed experimental findings revealed that the Fe dopant content crucially determines the catalytic activity of TiO2 nanoparticles. The maximum degradation rate of para-nitrophenol observed was 92% in 5 h when the Fe(3+) molar concentration was 0.05 mol%, without addition of any oxidizing reagents. The prepared nanoparticles demonstrated excellent photocatalytic response because of their small size, excellent crystalline structure, increase in threshold wavelength response and maximum separation of photogenerated charge carriers. Further, the determination of reaction intermediates has also been carried out and plausible mechanism of photocatalytic degradation of para-nitrophenol has been proposed.

Growth, stability, optical and photoluminescent properties of aqueous colloidal ZnS nanoparticles in relation to surfactant molecular structure.

The interaction between organic molecules and the surface of nanoparticles (NPs) strongly affects the size, properties and applications of surface-modified metal sulfide semiconductor nanocrystals. From this viewpoint, we compared the influence of cationic surfactants with various chain lengths and anionic surfactants with different head groups, as surface modifiers during synthesis of ZnS NPs in aqueous medium. The surfactant adsorbs on the surface of the particles as micelle-like aggregates. These aggregates can form even at the concentration lower than critical micelle concentration (cmc) due to interaction between the polar groups and the NPs. The nature of interaction depends specifically on the surfactant polar group. The ability of surfactant to form the micelle-like aggregates on the surface of the NPs correlates with their cmc. This leads to the fact that the surfactant with longer tail stabilizes the NPs better since its cmc is lower. The adsorption of the surfactant on the NPs also stabilizes them by the change of their charge which is in accordance with the correlation of zeta potential with the particles stability. The energetics of surface states generating interesting photoluminescence (PL) properties in ZnS NPs has been governed by the nature of surfactant molecules. In general, the size, structure, and stability of the ZnS NPs can be controlled by the choice of suitable surfactant.

Tween-based microemulsions : a percolation view

Abstract Results obtained on electrical conductivity, density, ultrasonic velocity and viscosity for the macroscopically homogeneous, stable, optically transparent and isotropic media — generally called microemulsions, are presented for a number of systems incorporating non-ionic surfactant: Tween 20, water, benzene and alkanol (C 1 –C 8 ,C 10 ) as a function of volume fraction of water ( φ ) at 30°C. The experimental path followed in each system has been characterized by a fixed molar ratio of chosen alkanol to the surfactant ( n a / n S )=2. Water is used as titrating component hence varying the value of ω from 0 to 180 within the system, where ω is the molar ratio of the aqueous phase to Tween 20+alkanol. In this paper, a phenomenological theory for the effects of interactions on the conductivity of water in oil microemulsions in the dilute limit of spherical droplets is analyzed. The density ( ρ m ) and the isentropic compressibility ( k s,m ) of the micellar phase are derived from the experimental density ( ρ ) and ultrasonic velocity ( u ) data. The results indicate a trend towards an enhanced water like character of the dispersed phase at high volume fraction of water ( φ ). Viscosity varies in a non-monotonic way, giving two peaked plots. In the phase diagram, the realms-of-existence of single phase (i.e. microemulsion formation), double phase and mesophase have been delineated for the system containing propanol as co-surfactant. A simple structural model has been applied for the calculation of the various parameters i.e. aggregation number ( n ), core radius ( r n ) and surface number density of the surfactant molecules at the interface ( α S ).

Ultrasonic velocity and apparent isentropic compressibilities in mixtures of nonelectrolytes

Ultrasonic velocities have been determined for binary mixtures of pyridine + n-alkanol (C1-C10) over the whole composition range at 25‡C. The excess isentropic compressibilities KSE and apparent molar isentropic compressibilities KΦ,s are estimated from these measurements. The KSE values are negative for all the systems over the complete mole fraction range except pyridine + decanol for which small positive values are obtained. The standard partial molar isentropic compressibilitiesK‡ of the alkanols are positive and increase linearly with the chain length of the alkanol molecules. It indicates that a methylene functional group makes a positive contribution to the expansion coefficient of a solute in these mixtures.

Solubilization, microstructure, and thermodynamics of fully dilutable U-type Brij microemulsion

A U-type microemulsion of Brij 96 has been characterized with respect to the change in cosurfactant, oil chain length on dilution, water solubility, and water solubilization capacity. The phase behavior of the systems has been mapped with different oils. Several techniques, viz., conductivity, optical microscopy, dilution method, absorption, and FT-IR spectroscopy, have been used for microemulsion analysis. The equilibrium within the microemulsion droplets and liquid crystals has been visualized using optical microscopy. The microemulsions have evidenced volume-induced conductance percolation in all the cosurfactants (n2-n6 alcohols). The energetics of transfer of alcohol from the bulk oil to the interface has been determined through dilution method. To gain insight into the microenvironment of microemulsion, two optical probes, hydrophilic (Methyl orange) and hydrophobic (Nile red), have been utilized in absorption spectroscopy. Lastly, FT-IR has been explored to observe the state and dissolution behavior of water with increasing weight fraction.

Synthesis of Highly Stable, Water‐Dispersible Copper Nanoparticles as Catalysts for Nitrobenzene Reduction

We report an aqueous-phase synthetic route to copper nanoparticles (CuNPs) using a copper-surfactant complex and tests of their catalytic efficiency for a simple nitrophenol reduction reaction under atmospheric conditions. Highly stable, water-dispersed CuNPs were obtained with the aid of polyacrylic acid (PAA), but not with other dispersants like surfactants or polymethacrylic acid (PMAA). The diameter of the CuNPs could be controlled in the range of approximately 30-85 nm by modifying the ratio of the metal precursor to PAA. The catalytic reduction of p-nitrophenol to p-aminophenol takes place at the surface of CuNPs at room temperature and was accurately monitored by UV/Vis spectroscopy. The catalytic efficiency was found to be remarkably high for these PAA-capped CuNPs, given the fact that at the same time PAA is efficiently preventing their oxidation as well. The activity was found to increase as the size of the CuNPs decreased. It can therefore be concluded that the synthesized CuNPs are catalytically highly efficient in spite of the presence of a protective PAA coating, which provides them with a long shelf life and thereby enhances the application potential of these CuNPs.

Well-crystalline porous ZnO-SnO2 nanosheets: an effective visible-light driven photocatalyst and highly sensitive smart sensor material.

This work demonstrates the synthesis and characterization of porous ZnO-SnO2 nanosheets prepared by the simple and facile hydrothermal method at low-temperature. The prepared nanosheets were characterized by several techniques which revealed the well-crystallinity, porous and well-defined nanosheet morphology for the prepared material. The synthesized porous ZnO-SnO2 nanosheets were used as an efficient photocatalyst for the photocatalytic degradation of highly hazardous dye, i.e., direct blue 15 (DB 15), under visible-light irradiation. The excellent photocatalytic degradation of prepared material towards DB 15 dye could be ascribed to the formation of ZnO-SnO2 heterojunction which effectively separates the photogenerated electron-hole pairs and possess high surface area. Further, the prepared porous ZnO-SnO2 nanosheets were utilized to fabricate a robust chemical sensor to detect 4-nitrophenol in aqueous medium. The fabricated sensor exhibited extremely high sensitivity of ~ 1285.76 µA/mmol L(-1)cm(-2) and an experimental detection limit of 0.078 mmol L(-1) with a linear dynamic range of 0.078-1.25 mmol L(-1). The obtained results confirmed that the prepared porous ZnO-SnO2 nanosheets are potential material for the removal of organic pollutants under visible light irradiation and efficient chemical sensing applications.