Jahangeer Ahmed

Biosynthesis, structural characterization and antimicrobial activity of gold and silver nanoparticles

An eco friendly simple biosynthetic route was used for the preparation of monodisperse and highly crystalline gold and silver nanoparticles using cell free extract of fungus, Candida albicans. Transmission electron microscopic studies show the formation of gold and silver nanocrystals of average size of 5 nm and 30 nm with the specific surface areas of 18.9 m(2)/g and 184.4 m(2)/g respectively. The interaction of gold and silver nanoparticles with proteins has been formulated by FT-IR spectroscopy and thermal gravimetric analysis. The formation of gold and silver nanoparticles was also confirmed by the appearance of a surface plasmon band at 540 nm and 450 nm respectively. The antimicrobial activity of the synthesized gold and silver nanoparticles was investigated against both Staphylococcus aureus and Escherichia coli. The results suggest that these nanoparticles can be used as effective growth inhibitors against the test microorganisms. Greater bactericidal activity was observed for silver nanoparticles. The E. coli, a gram negative bacterium was found to be more susceptible to gold and silver nanoparticles than the S. aureus, a gram positive bacterium.

Development of a microemulsion-based process for synthesis of cobalt (Co) and cobalt oxide (Co3O4) nanoparticles from submicrometer rods of cobalt oxalate.

Rod-shaped nanostructures of cobalt oxalate dihydrate were synthesized at room temperature by the microemulsion (reverse micellar) route. These rods are highly uniform in length and can be modified with temperature (from approximately 6.5 microm at 50 degrees C to approximately 2.5 microm at 150 degrees C) while keeping the diameter nearly constant (200-250 nm). Thermal decomposition of these rods in a controlled atmosphere (air and H(2)) leads to nanoparticles of Co(3)O(4) and Co, respectively, while in a helium atmosphere a mixture of Co and CoO nanoparticles is obtained. Co(3)O(4) nanoparticles (approximately 35 nm) were slightly agglomerated, while Co nanoparticles were monodispersed and highly uniform (approximately 25 nm). The oxalate rods and Co(3)O(4) nanoparticles show an antiferromagnetic ordering at 54 and 35 K, respectively.

Microemulsion-mediated synthesis of cobalt (pure fcc and hexagonal phases) and cobalt-nickel alloy nanoparticles

By choosing appropriate microemulsion systems, hexagonal cobalt (Co) and cobalt-nickel (1:1) alloy nanoparticles have been obtained with cetyltrimethylammonium bromide as a cationic surfactant at 500 degrees C. This method thus stabilizes the hcp cobalt even at sizes (<10 nm) at which normally fcc cobalt is predicted to be stable. On annealing the hcp cobalt nanoparticles in H(2) at 700 degrees C we could transform them to fcc cobalt nanoparticles. Microscopy studies show the formation of spherical nanoparticles of hexagonal and cubic forms of cobalt and Co-Ni (1:1) alloy nanoparticles with the average size of 4, 8 and 20 nm, respectively. Electrochemical studies show that the catalytic property towards oxygen evolution is dependent on the applied voltage. At low voltage (less than 0.65 V) the Co (hexagonal) nanoparticles are superior to the alloy (Co-Ni) nanoparticles while above this voltage the alloy nanoparticles are more efficient catalysts. The nanoparticles of cobalt (hcp and fcc) and alloy (Co-Ni) nanoparticles show ferromagnetism. The saturation magnetization of Co-Ni nanoparticles is reduced compared to the bulk possibly due to surface oxidation.

Microemulsion route to the synthesis of nanoparticles

Nanoparticles of several titanates and zirconates in the range of 20-60 nm have been obtained using the reverse micellar route. Important oxides like CeO2 (mixture of nanorods; 7 nm diameter and 30 nm length and nanoparticles; 10 nm), ZrO2 (3-4 nm) and SnO2 (8 nm) have also been synthesized. Nanorods and nanoparticles of CaCO3 in all three forms (aragonite, vaterite, and calcite) have been obtained using reverse micelles as nanoreactors. The specific reactions vary depending on the nature of the target nanomaterial. For synthesis of ternary oxides like BaTiO3, a modified and convenient route using microemulsions (avoiding Ba-alkoxide) has evolved. Monophasic tin dioxide (SnO2) was obtained when liquid NH3 was used as precipitating agent. Transmission electron microscopy (TEM) studies show that the SnO2 nanoparticles are highly uniform and particle size was found to be 6-8 nm at 500 °C. The gas sensing characteristics of SnO2 have also been investigated using n-butane, which shows high sensitivity and fast recovery time. Reverse micelles have been used, for the first time, to mimic the conditions suitable for the room-temperature synthesis of the high-temperature and -pressure orthorhombic phase of calcium carbonate (aragonite). Other forms of calcium carbonate (vaterite and calcite) could be obtained by varying the atmospheric conditions. At a lower temperature (5 °C), homogeneous and monodisperse spheres of vaterite are obtained. The spherical particles aggregate after longer aging (168 h) to form nanorods, and the self-assembly is clearly seen at various stages by electron microscopy images. The samples were well characterized using powder X-ray diffraction (PXRD), line-broadening studies, TEM, variation in the dielectric properties with frequency and temperature, were measured on disks sintered at high temperature.

Controlling the Size, Morphology, and Aspect Ratio of Nanostructures Using Reverse Micelles: A Case Study of Copper Oxalate Monohydrate

This study focuses on understanding the growth and control of nanostructures using reverse micelles. It has been earlier realized that parameters like surfactant, cosurfactant, and aqueous content influence the size and shape of the nanostructures obtained using reverse micelles. However, a concerted effort to understand the role of these factors on the growth of a specific nanomaterial is missing. In this study we have focused on one nanomaterial (copper oxalate monohydrate) and determined how the above-mentioned factors control the size, shape, aspect ratio, and growth of these nanostructures. Our results show that cationic surfactants (CTAB, TTAB, and CPB) favor the formation of nanorods of copper oxalate. The aspect ratio of these rods could be controlled to obtain nanocubes (approximately 80-100 nm) and nanoparticles (approximately 8-10 nm) in the CTAB system using longer chain cosurfactants like 1-octanol and 1-decanol, respectively. Nanocubes of approximately 50-60 and approximately 60-80 nm were obtained using nonionic surfactants Triton X-100 and Tergitol, respectively. The size of the nanostructures could also be controlled by varying the molar ratio of water to surfactant (W0) by using a nonionic (Triton X-100)-based reverse micellar system. The study espouses the versatility of the microemulsion method to realize a variety of nanostructures of copper oxalate monohydrate. Our results will be of use in extending these ideas to other nanomaterials.

Structural characterization and antimicrobial properties of silver nanoparticles prepared by inverse microemulsion method

Silver nanoparticles have been synthesized in the inverse microemulsions formed using three different surfactants viz., cetyl-trimethyl ammonium bromide (CTAB), Tergitol and Triton X-100. We have done a systematic study of the effect of the surfactants on the particle size and properties of the silver nanoparticles. Microscopic studies show the formation of spheres, cubes and discs shaped silver nanostructures with the size in the range from 8 to 40 nm. Surface plasmon resonance (SPR) peak was observed around 400 nm and 500 nm. In addition to SPR some extra peaks have also been observed due to the formation of silver metal clusters. The surface area increases from 3.45 to 15.06 m(2)/g with decreasing the size of silver nanoparticles (40-8 nm). To investigate the antimicrobial activity of silver nanoparticles, the nanoparticles were tested against the yeast, Candida albicans and the bacterium, E. coli. The results suggest very good antimicrobial activity of the silver nanoparticles against the test microbes. The mode of action of the antimicrobial activity was also proposed.

Controlled growth of nanocrystalline rods, hexagonal plates and spherical particles of the vaterite form of calcium carbonate

Pure vaterite (hexagonal) phase of calcium carbonate has been obtained at room temperature (20 °C) using a microemulsion route. The process resulted in uniform micron-sized hexagonal plates (∼1 µm) of vaterite where 9.375 µmol of Ca2+ and CO32− ions was used while at lower concentration (0.625 μmol) cubes of calcite phase were obtained. High magnification TEM images of the hexagonal plates shows an assembly of spherical nanoparticles (∼50 nm). At an optimal concentration, rod-shaped vaterite particles were obtained at 40 °C. This is the first report of the rod-shaped morphology of the vaterite phase. The length and diameter of the rods have been found to be ∼250 and ∼30 nm, respectively. The surface charge of the vaterite phase (rod-shaped) was found to be +31 mV which is consistent with the high ionic concentration used in the synthesis.

Bifunctional electro-catalytic performances of CoWO4 nanocubes for water redox reactions (OER/ORR)

In this paper, we report the synthesis of cube shaped nanoparticles of CoWO4 (∼30 nm) by molten salts and their bifunctional electro-catalytic activities in water redox reactions for oxygen evolution and oxygen reduction reactions (OER and ORR). Bifunctional performances of CoWO4 nano-cubes are explored for water electrolysis in an alkaline medium (1.0 M KOH) vs. reversible hydrogen electrode (RHE) under various atmospheres (N2, air and O2). Low overpotential (η10 = 0.45 V) of CoWO4 nano-cubes is accomplished at the current density of 10 mA cm−2. Tafel polarization curves (potential vs. log current density) reveal relatively lower slope values for OER (∼82 mV dec−1) and ORR (∼68 mV dec−1) compared to previous reports. Stability test of electrode materials has been performed using chrono-amperometry (CA) at fixed potential for 500 seconds. Kinetics and mobility of electrons have also been studied during the water redox reactions. Stable nature and enhanced bifunctional electro-catalytic performances of earth abundant CoWO4 electrodes could be used as the replacement of expensive electroactive noble electrode materials (Pt, Ir, Ru etc.) for water electrolysis (OER and ORR) in near future.

Synthesis, characterization, and enhanced photocatalytic properties of NiWO4 nanobricks

Brick shaped nanoparticles of nickel tungstate (NiWO4) were synthesized magnificently using the molten salts process at 500 °C. Powder X-ray diffraction (PXRD) and electron microscopy measurements were carried out to investigate the phase purity, crystal structure, and morphology (size and shape) of the NiWO4 nanobricks (diameters of ∼20 nm). The BET surface area of the NiWO4 nanobricks was found to be ∼25 m2 g−1. The optical properties of the NiWO4 nanobricks demonstrate a direct band gap of 2.95 eV. Thereupon, the photocatalytic activities of the NiWO4 nanobricks have been investigated for the degradation of methylene blue (MB) dye in neutral and basic media under solar light irradiation (SLI). The NiWO4 nanobricks show significant enhancement in the photo-degradation of MB dye solution in alkaline (∼100% in 50 minutes) and neutral media (∼80% in 330 minutes) as compared to bulk NiWO4 and previous reports of metal tungstate. ESI mass spectrometry was also carried out to confirm the photocatalytic degradation of dye molecules.

An efficient and cost-effective tri-functional electrocatalyst based on cobalt ferrite embedded nitrogen doped carbon

The development of efficient, cost-effective and long-lived electro-catalyst is necessary for the realization of practically viable water-splitting systems. A trifunctional electrocatalyst for water splitting (hydrogen evolution, oxygen reduction and oxygen evolution reaction, HER/ORR/OER) was designed via eco-friendly and facial way. CoFe2O4 nanoparticles embedded in nitrogen doped mesoporous carbon were prepared using chicken egg white/albumin after pyrolysis at different temperatures, 700, 800, 900 and 1000 °C. The specific surface area, pore size and the interaction between CoFe2O4 nanoparticles and carbon matrix were tuned via pyrolysis temperature. The catalyst prepared at 900 °C, (N/CF-EC-900) exhibit superior catalytic activity as well as the superior stability than that other nanocomposites prepared and other commercial catalyst (Pt/C, RuO2) for water splitting. Our findings emphasize the importance of CoFe2O4 nanoparticles embedded in the carbon and suggest the catalytic activities with low onset potential, high current densities, small Tafel slope in basic medium.