Tokeer Ahmad

Nanorods of manganese oxalate: a single source precursor to different manganese oxide nanoparticles (MnO, Mn2O3, Mn3O4)

Nanorods of anhydrous manganese oxalate were prepared by the reverse-micellar method using CTAB as the surfactant. Manganese oxalate precursor was used to synthesize single phase nanoparticles of various manganese oxides such as MnO, Mn2O3 and Mn3O4 under specific reaction conditions. Both MnO (28 nm) and α-Mn2O3 (50 nm) are stabilized as cubic phase. α-Mn2O3 shows a weak antiferromagnetic transition (TN = 80 K), while the spinel Mn3O4 (100 nm) particles show a ferrimagnetic transition at 43 K.

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.

Zinc oxalate nanorods: a convenient precursor to uniform nanoparticles of ZnO

Nanorods of zinc oxalate dihydrate have been synthesized using the reverse micellar route. These nanorods were decomposed at 450 °C in air to obtain nanoparticles of zinc oxide. Transmission electron microscopy shows the nanorods to be 120 nm in diameter and 600 nm in length. The ZnO nanoparticles are 55 nm in diameter. The photoluminescence studies show two peaks at 370 and 403 nm which can be ascribed to free excitonic transition and donor-acceptor pair transition respectively. The temperature dependent PL intensity shows an anomalous non-monotonous temperature dependence probably due to two different optical processes.

Size and shape dependant antifungal activity of gold nanoparticles: A case study of Candida

A simple and economical sonochemical approach was employed for the synthesis of gold nanoparticles. The effect of the reducing agents has been studied on the particle size, morphology and properties at the same ultrasonic frequency under ambient conditions. Gold nanodiscs of average diameter of 25 nm were obtained using tinchloride (SnCl(2)) as a reducing agent, while sodium borohydride (NaBH(4)) produced polyhedral structures of the average size of 30 nm. The time evolution of the UV-visible absorption spectra of the gold nanostructures shows the origin of peaks due to higher order quadrupolar modes apart from the peaks of the in plane and out plane dipolar surface plasmon modes. Surface area studies reveal the much higher surface area of the gold nanodiscs (179.5 m(2)/g), than the gold nanoparticles (150.5m(2)/g) prepared by the sodium borohydride as the reducing agent. The gold nanoparticles exhibit excellent antifungal activity against the fungus, Candida. We investigated the effect of the gold nanoparticles on the H(+)-ATPase mediated H(+) pumping by various Candida species. Gold nanodiscs displayed the stronger fungicidal activity compared to the gold polyhedral nanoparticles. The two types of gold nanoparticles inhibit H(+)-ATPase activity at their respective MIC values.

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.

Alpha amylase assisted synthesis of TiO2 nanoparticles: Structural characterization and application as antibacterial agents

The enzyme alpha amylase was used as the sole reducing and capping agent for the synthesis of TiO2 nanoparticles. The biosynthesized nanoparticles were characterized by X-ray diffraction (XRD) and transmission electron microscopic (TEM) methods. The XRD data confirms the monophasic crystalline nature of the nanoparticles formed. TEM data shows that the morphology of nanoparticles depends upon the enzyme concentration used at the time of synthesis. The presence of alpha amylase on TiO2 nanoparticles was confirmed by FTIR. The nanoparticles were investigated for their antibacterial effect on Staphylococcus aureus and Escherichia coli. The minimum inhibitory concentration value of the TiO2 nanoparticles was found to be 62.50 μg/ml for both the bacterial strains. The inhibition was further confirmed using disc diffusion assay. It is evident from the zone of inhibition that TiO2 nanoparticles possess potent bactericidal activity. Further, growth curve study shows effect of inhibitory concentration of TiO2 nanoparticles against S. aureus and E. coli. Confocal microscopy and TEM investigation confirm that nanoparticles were disrupting the bacterial cell wall.

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.

Reverse micellar based synthesis of ultrafine MgO nanoparticles (8–10 nm): Characterization and catalytic properties

Anisotropic nanostructures of magnesium oxalate dihydrate were synthesized using cationic surfactant based microemulsion method. The cationic surfactant plays an important role in forming the anisotropic structures. The oxalate nanostructures acts as an excellent precursor for the synthesis of fine magnesium oxide nanoparticles (~10 nm). Both the precursor and the oxide were characterized by using PXRD, IR, surface area and HRTEM. The surface area of these surfactant free oxide nanoparticles was found to be 108 m(2)/g. The catalytic activity of this basic oxide was examined for the Claisen-Schmidt condensation reaction and was found to be comparable to the best reported for the conventionally prepared MgO. Chalcone formation was found to increase with time as observed using gas chromatography-mass spectrometry (GC-MS). The reusability of the catalyst was checked by using the same catalyst twice which showed a reduced percentage (50% compared to first cycle) conversion.

Nanostructured barium titanate prepared through a modified reverse micellar route : Structural distortion and dielectric properties

A modified and convenient route using microemulsions (avoiding Ba-alkoxide) was evolved for the synthesis of uniform and monodisperse nanoparticles of BaTiO3 at low temperature (800 °C). X-ray line broadening and transmission electron microscopy studies show that the particle size varies in the range of 20–25 nm. Evidence for tetragonal distortion was found in these nano-sized (20–25 nm) particles of barium titanate from careful x-ray diffraction studies as well as from Raman spectroscopy. Our study showed that the critical size of the cubic to tetragonal transition in barium titanate may be much lower than suggested theoretically. The grain size showed an increase on sintering of 35 nm at 900 °C to 120 nm at 1100 °C, which was much lower than the grain size obtained at this temperature by the normal solid state route. The dielectric constant depends on sintering temperature and was found to increase from 210 (900 °C sintering) to 520 (1100 °C sintering) at 100 kHz. The dielectric constant was highly stable with temperature as well as frequency.