Sulabha K. Kulkarni

Extracellular synthesis of silver nanoparticles by a silver-tolerant yeast strain MKY3

Silver nanoparticles in the size range of 2-5 nm were synthesized extracellularly by a silver-tolerant yeast strain MKY3, when challenged with 1 mM soluble silver in the log phase of growth. The nanoparticles were separated from dilute suspension by devising a new method based on differential thawing of the sample. Optical absorption, transmission electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy investigations confirmed that metallic (elemental) silver nanoparticles were formed. Extracellular synthesis of nanoparticles could be highly advantageous from the point of view of synthesis in large quantities and easy downstream processing.

Grain size effects on H2 gas sensitivity of thick film resistor using SnO2 nanoparticles

The effect of ppm level H2 on the d.c. resistance of SnO2 thick film planar resistor with SnO2 particle size variation (∼ 20 to 50 nm) is reported. SnO2 nanoparticles were synthesized using the sol-gel method. The films were prepared using standard screen printing technology. The SnO2 crystallite size, D, is varied in the range of 20–50 nm and sensitivity for H2 is found to increase steeply as D decreases.

Green luminescence from copper doped zinc sulphide quantum particles

Free‐standing powder of zinc sulphide quantum particles has been synthesized using a chemical route. X‐ray diffraction analysis shows that the diameter of the particles is ∼21±2 A which is smaller than the Bohr exciton diameter for zinc sulphide. UV absorption shows an excitonic peak centered at ∼300 nm corresponding to an energy gap of 4.1±0.1 eV. These particles show a luminescence band at ∼424 nm. The quantum particles could be doped with copper during synthesis without altering the UV absorption or x‐ray diffraction pattern. However, doping shifted the luminescence to 480 nm, green wavelength in the visible region.

Microbial Synthesis of Semiconductor PbS Nanocrystallites

The use of microbes as producers of semiconductor nanocrystals is demonstrated. When torulopsis yeast is challenged with lead, it builds intracellular spherical crystallites of PbS, 2-5 nm in diameter (see Figure for an HR-TEM image) and pure by X-ray diffraction. The crystals, which can be isolated by freeze-thawing, show a sharp absorption maximum at 330 nm, corresponding to a bandgap of 3.75 eV.

High Coercivity of Oleic Acid Capped CoFe2O4 Nanoparticles at Room Temperature

High coercivity (9.47 kOe) has been obtained for oleic acid capped chemically synthesized CoFe(2)O(4) nanoparticles of crystallite size approximately 20 nm. X-ray diffraction analysis confirms the formation of spinel phase in these nanoparticles. Thermal annealing at various temperatures increases the particle size and ultimately shows bulk like properties at particle size approximately 56 nm. The nature of bonding of oleic acid with CoFe(2)O(4) nanoparticles and amount of oleic acid in the sample is determined by Fourier transform infrared spectroscopy and thermogrvimetric analysis, respectively. The Raman analysis suggests that the samples are under strain due to capping molecules. Cation distribution in the sample is studied using Mossbauer spectroscopy. Oleic acid concentration dependent studies show that the amount of capping molecules plays an important role in achieving such a high coercivity. On the basis of above observations, it has been proposed that very high coercivity (9.47 kOe) is the result of the magnetic anisotropy, strain, and disorder of the surface spins developed by covalently bonded oleic acid to the surface of CoFe(2)O(4) nanoparticles.

Influence of biomass and gold salt concentration on nanoparticle synthesis by the tropical marine yeast Yarrowia lipolytica NCIM 3589

Cell-associated gold nanoparticles and nanoplates were produced when varying number of Yarrowia lipolytica cells were incubated with different concentrations of chloroauric acid (HAuCl(4)) at pH 4.5. With 10(9)cells ml(-1) and 0.5 or 1.0 mM of the gold salt, the reaction mixtures developed a purple or golden red colour, respectively, and gold nanoparticles were synthesized. Nanoparticles of varying sizes were produced when 10(10)cells ml(-1) were incubated with 0.5, 1.0 or 2.0 mM chloroauric acid salt. With 3.0, 4.0 or 5.0 mM HAuCl(4), nanoplates were also observed. With 10(11)cells ml(-1) nanoparticles were synthesized with almost all the gold salt concentrations. The cell-associated particles were released outside when nanoparticle-loaded cells were incubated at low temperature (20 degrees C) for 48 h. With increasing salt concentrations and a fixed number of cells, the size of the nanoparticles progressively increased. On the other hand, with increasing cell numbers and a constant gold salt concentration, the size of nanoparticles decreased. These results indicate that by varying the number of cells and the gold salt concentration, a variety of nanoparticles and nanoplates can be synthesized. Fourier transform infrared (FTIR) spectroscopy revealed the possible involvement of carboxyl, hydroxyl and amide groups on the cell surfaces in nanoparticle synthesis.

Manganese doped zinc sulphide nanoparticles by aqueous method

Zinc sulphide nanoparticles in the size range ∼10–40 A diameter have been synthesized using the aqueous chemical method. Scanning tunneling microscopy showed that particles are indeed nanosize particles. The size dependent band gap could be varied from a bulk value of 3.68 to 4.5 eV. X‐ray diffraction indicated that nanoparticles are crystalline except for those with band gap ∼4.5±0.1 eV. Nanoparticles with particle size ∼21×2 A diameter or energy gap 4.1×0.1 eV were doped with manganese. The photoluminescence peak at ∼600 nm corresponding to yellow light emission was observed. Atomic absorption studies show that maximum luminescence intensity is achievable with 0.12 at. wt % of Mn doping.

Investigations on chemically capped CdS, ZnS and ZnCdS nanoparticles

CdS, ZnS and ZnCdS nanoparticles of various sizes (1-10 nm) are synthesized using a wet chemical route including passivation by organic capping molecules. The particles can also be doped with transition metal ions. The optical properties of the particles are investigated using UV absorption and photoluminescence spectroscopies. High resolution photoelectron spectroscopy using variable photon energy from a synchrotron source is performed to obtain detailed information about the nanoparticle surfaces. These investigations are able to reveal the termination of the nanoparticles and the nature of bonding between the surface atoms and stabilizing organic molecules.

Plasmon-assisted photonics at the nanoscale

Plasmons are collective oscillations of electrons that have been exploited in many applications by manipulating and guiding light at resonant frequencies. The transition from understanding the origin and fundamentals of surface plasmon resonance to its many revolutionary applications has been intriguing. Advances in nanofabrication techniques over the last few years have led to variety of applications such as high-resolution plasmon printing, nanoscale waveguides, biodetection at the single-molecule level and enhanced transmission through sub-wavelength apertures, which are all examples of plasmon-assisted nanophotonics. Fundamental aspects of the surface plasmon resonance underlie enticing applications in nanophotonics.

Luminescence quenching in ZnS nanoparticles due to Fe and Ni doping

Nanoparticles of zinc sulphide have been synthesized by a chemical method. Mercaptoethanol is used to passivate the surface of the particles. Under certain conditions highly luminescent particles emitting blue light at ∼425 nm can be synthesized. This blue light emission in nanoparticles of zinc sulphide is observed to be completely quenched when doped with iron or nickel metal ions.