Nanocomposite of polypyrrol and silica rods-gold nanoparticles core-shell as an ammonia sensor.

Ammonia is widely needed in the chemical industry as well as in fertilizers for agriculture. However, in small as well as large quantities, it is not only hazardous for human health but also for our ecosystem. Therefore, ammonia sensing at low concentration with high sensitivity, selectivity and low response time as well as recovery time is important. Here, various nanosensors are fabricated using gold nanoparticles (∼15 nm), silica-gold nanoparticles coreshell particles and coreshell particles embedded in polypyrrol. Comparisons with bare polypyrrol and coreshell particles are also made. In fact, two types of coreshell particles with rod (∼300 nm × 2 µm) shape and spheres (200 nm) of silica were used to anchor gold nanoparticles on them. A comparison showed that silica-gold core-shell particle with silica rods had the highest sensitivity (∼166% @ 130 ppm) amongst all. The sensor is simple to operate (only resistance change is measured), requires no heater as the sensing occurs at room temperature, and showed no response, except for ammonia, to other gases or humidity. It also has a low response time (4 s) and recovery time (10 s) at the lowest (10 ppm) ammonia concentration measured here. Thus, a simple, economical ammonia sensor has been demonstrated here.

Synthesis and Characterization of Controlled Size CdSe Quantum Dots by Colloidal Method.

Monodispersed and highly luminescence cadmium selenide (CdSe) quantum dots (QDs) have been prepared in a single pot by colloidal reaction method. The QDs were characterized using X-ray diffraction (XRD), Raman Spectroscopy, transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), UV-visible absorption spectroscopy and photoluminescence (PL) spectroscopy to study the structural, morphological, compositional and optical properties. The growth temperature played an important role to control the particle size. The optical wavelength was found to be shifted systematically from 460 nm to 575 nm upon increasing the reaction temperature from 110 °C to 260 °C. The size of CdSe QDs, ~2-4 nm was estimated from absorption data. The emission tail exhibited at higher wavelength in PL measurement for the QDs synthesized for lower reaction temperature revealed the presence of surface trap-states. A cubic crystal structure of CdSe QDs was revealed by XRD analysis. The spherical QDs of size 2 to 4.5 nm were observed from TEM analysis for the samples prepared at 140 °C, 200 °C and 260 °C. The sizes of QDs obtained by TEM are in good agreement with the results obtained from optical and XRD data. High resolution transmission electron microscopy (HRTEM) confirmed the cubic crystal structure of CdSe QDs. The Selected area diffraction (SAD) pattern exhibited diffused ring corresponds to (111), (220) and (311) reflections of cubic structure of CdSe. The compositional analysis studied by EDS revealed the growth of nearly stoichiometric CdSe QDs. The LO1 vibrational mode observed about 202-205 cm-1 decreases the broadening systematically upon increasing the reaction temperature.

Ultrasensitive Gold Nanostar-Polyaniline Composite for Ammonia Gas Sensing.

Gold in the form of bulk metal mostly does not react with gases or liquids at room temperature. On the other hand, nanoparticles of gold are very reactive and useful as catalysts. The reactivity of nanoparticles depends on the size and the morphology of the nanoparticles. Gold nanostars containing copper have rough surfaces and large numbers of active sites due to tips, sides, corners, and large surface area-to-volume ratios due to their branched morphology. Here the sensitivity of the gold nanostar-polyaniline composite (average size of nanostars ∼170 nm) toward ammonia gas has been investigated. For 100 ppm ammonia, the sensitivity of the composite increased to 52% from a mere 7% value for pure polyaniline. The gold nanostar-polyaniline composite even showed a response time as short as 15 s at room temperature. The gold nanostars act as a catalyst in the nanocomposite. The stability and sensitivity at different concentrations and the selectivity for ammonia gas were also investigated.

Electroless Ni plated nanostructured TiO2 as a photocatalyst for solar hydrogen production.

Herein, we have demonstrated a facile electroless Ni coated nanostructured TiO2 photocatalyst for the first time. More significantly the photocatalytic water splitting shows excellent performance for hydrogen production which is hitherto unattempted. The structural study exhibits majorly the anatase phase along with the minor rutile phase of TiO2. Interestingly, electroless nickel deposited on the TiO2 nanoparticles of size 20 nm shows a cubic structure with nanometer scale Ni coating (1-2 nm). XPS supports the existence of Ni without any oxygen impurity. The FTIR and Raman studies support the formation of TiO2 phases without any other impurities. The optical study shows a red shift in the band gap due to optimum nickel loading. The emission spectra show variation in the intensity of the peaks with Ni concentration. The vacancy defects are pronounced in lower concentrations of Ni loading which shows the formation of a huge number of charge carriers. The electroless Ni loaded TiO2 has been used as a photocatalyst for water splitting under solar light. The primary results manifest that the hydrogen evolution of electroless Ni plated TiO2 is 3.5 times higher (1600 µmol g-1 h-1) than pristine TiO2 (470 µmol g-1 h-1). As shown in the TEM images, nickel is completely electroless plated on the TiO2 surface, which accelerates the fast transport of electrons to the surface. It suppresses the electron-hole recombination drastically which is responsible for higher hydrogen evolution using electroless Ni plated TiO2. The recycling study exhibits a similar amount of hydrogen evolution at similar conditions which shows the stability of the Ni loaded sample. Interestingly, Ni powder loaded TiO2 did not show any hydrogen evolution. Hence, the approach of electroless plating of nickel over the semiconductor surface will have potential as a good photocatalyst for hydrogen evolution.

Ensemble modeling of very small ZnO nanoparticles.

The detailed structural characterization of nanoparticles is a very important issue since it enables a precise understanding of their electronic, optical and magnetic properties. Here we introduce a new method for modeling the structure of very small particles by means of powder X-ray diffraction. Using thioglycerol-capped ZnO nanoparticles with a diameter of less than 3 nm as an example we demonstrate that our ensemble modeling method is superior to standard XRD methods like, e.g., Rietveld refinement. Besides fundamental properties (size, anisotropic shape and atomic structure) more sophisticated properties like imperfections in the lattice, a size distribution as well as strain and relaxation effects in the particles and-in particular-at their surface (surface relaxation effects) can be obtained. Ensemble properties, i.e., distributions of the particle size and other properties, can also be investigated which makes this method superior to imaging techniques like (high resolution) transmission electron microscopy or atomic force microscopy, in particular for very small nanoparticles. For the particles under study an excellent agreement of calculated and experimental X-ray diffraction patterns could be obtained with an ensemble of anisotropic polyhedral particles of three dominant sizes, wurtzite structure and a significant relaxation of Zn atoms close to the surface.

Highly sensitive, room temperature operated gold nanowire-based humidity sensor: adoptable for breath sensing.

A novel, highly sensitive gold nanowire (AuNW) resistive sensor is reported here for humidity sensing in the relative humidity range of 11% to 92% RH as well as for breath sensing. Both humidity and breath sensors are widely needed. Despite a lot of research on humidity and breath sensors, there is a need for simple, inexpensive, reliable, sensitive and selective sensors, which will operate at room temperature. Here we have synthesized gold nanowires by a simple, wet chemical route. The nanowires synthesized by us are 4-7 nm in diameter and a few micrometers long. The nanowires are amine functionalized. The sensor was prepared by drop casting gold nanowires on an alumina substrate to form a AuNW layer with different thicknesses (10, 20, 30 µm). The AuNW sensor is highly selective towards humidity and shows minimum cross sensitivity towards other gases and organic vapors. At an optimum thickness of 20 µm, the humidity sensing performance of the AuNW sensor over 11% to 92% RH was found to be superior to that of 10 and 30 µm thick layers. The response time of the sensor is found to be 0.2 s and the recovery time is 0.3 s. The response of the AuNW sensor was 3.3 MΩ/% RH. Further, the AuNW sensor was tested for sensing human breathing patterns.

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.

Photothermal effects in connective tissues mediated by laser-activated gold nanorods.

We report a study on the application of laser-activated nanoparticles in the direct welding of connective tissues, which may become a valuable technology in biomedicine. We use colloidal gold nanorods as new near-infrared chromophores to mediate functional photothermal effects in the eye lens capsules. Samples obtained ex vivo from porcine eyes are treated to simulate heterotransplants with 810-nm diode laser radiation in association with a stain of gold nanorods of aspect ratio approximately 4. This stain is applied at the interface between a patch of capsule from a donor eye and the capsule of a recipient eye. Then, by administration of laser pulses of 40 msec and approximately 100-140 J/cm(2), we achieved the local denaturation of the endogenous collagen filaments, which reveals that the treated area reached temperatures above 50 degrees C. The thermal damage is confined within 50-70 mum in a radial distance from the irradiated area.

Ruthenium-decorated vanadium pentoxide for room temperature ammonia sensing.

Layer structured vanadium pentoxide (V2O5) microparticles were synthesized hydrothermally and successfully decorated by a facile wet chemical route, with ∼10-20 nm sized ruthenium nanoparticles. Both V2O5 and ruthenium nanoparticle decorated V2O5 (1%Ru@V2O5) were investigated for their suitability as resistive gas sensors. It was found that the 1%Ru@V2O5 sample showed very high selectivity and sensitivity towards ammonia vapors. The sensitivity measurements were carried out at 30 °C (room temperature), 50 °C and 100 °C. The best results were obtained at room temperature for 1%Ru@V2O5. Remarkably as short a response time as 0.52 s @ 130 ppm and as low as 9.39 s @ 10 ppm recovery time at room temperature along with high selectivity towards many gases and vapors have been noted in the 10 to 130 ppm ammonia concentration range. Short response and recovery time, high reproducibility, selectivity and room temperature operation are the main attributes of the 1%Ru@V2O5 sensor. Higher sensitivity of 1%Ru@V2O5 compared to V2O5 has been explained and is due to dissociation of atmospheric water molecules on 1%Ru@V2O5 as compared to bare V2O5 which makes hydrogen atoms available on Brønsted sites for ammonia adsorption and sensing. The presence of ruthenium with a thin layer of oxide is clear from X-ray photoelectron spectroscopy and that of water molecules from Fourier transform infrared spectroscopy.

Sulfite reductase-mediated synthesis of gold nanoparticles capped with phytochelatin.

An enzymatic synthesis route to peptide-capped gold nanoparticles has been developed. Gold nanoparticles were synthesized using alpha-NADPH-dependent sulfite reductase and phytochelatin in vitro. The gold ions were reduced in the presence of the enzyme sulfite reductase, leading to the formation of a stable gold hydrosol of dimensions 7-20 nm and were stabilized by the capping peptide. The nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and UV-visible optical absorption. These studies will help in designing a rational enzymatic strategy for the synthesis of nanomaterials of different chemical compositions, shapes and sizes as well as their separation.

Nitrate reductase-mediated synthesis of silver nanoparticles from AgNO3.

Synthesis of silver nanoparticles using alpha-NADPH-dependent nitrate reductase and phytochelatin in vitro has been demonstrated for the first time. The silver ions were reduced in the presence of nitrate reductase, leading to the formation of a stable silver hydrosol 10-25 nm diam. and stabilized by the capping peptide. The nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and UV-Vis absorption. These studies will help in designing a rational enzymatic strategy for the synthesis of nanomaterials of different chemical composition, shapes and sizes as well as their separation.

Rapid detection of Escherichia coli by using antibody-conjugated silver nanoshells.

Silver shells of 20 nm thickness have been deposited on silica particles of 200 nm diameter with narrow size distribution. Silver nanoshells dispersed in water exhibit a strong surface plasmon resonance band at 443 nm. This band was found to be very sensitive to rabbit immunoglobulin G antibodies, which were anchored on the nanoshells. These in turn could be utilized to detect the presence of small (approximately 5) to large numbers (approximately 10(9)) of Escherichia coli in water. The protocol presented here proves to be a specific, rapid, reliable, and inexpensive method to detect E. coli.

Chemical mapping of individual semiconductor nanostructures.

We demonstrate experimentally the power of a novel analytical tool for X-ray spectromicroscopy. This provides a minimally intrusive elemental mapping of surfaces at the nanoscale and holds the promise of remarkable versatility. We have applied our procedure to the characterization of Ge(Si) islands on Si(111) substrates, with the aim of investigating the surface stoichiometry gradients and gaining insight into the intermixing dynamics. By identifying Si-richer edges with respect to the centers, we are able to associate alloying in these islands to surface transport processes.

A facile and fast approach for the synthesis of doped nanoparticles using a microfluidic device.

The microfluidic approach emerges as a new and promising technology for the synthesis of nanomaterials. A microreactor allows a variety of reaction conditions to be quickly scanned without consuming large amounts of raw material. In this study, we investigated the synthesis of water soluble 1-thioglycerol-capped Mn-doped ZnS nanocrystalline semiconductor nanoparticles (TG-capped ZnS:Mn) via a microfluidic approach. This is the first report for the successful doping of Mn in a ZnS semiconductor at room temperature as well as at 80 °C using a microreactor. Transmission electron microscopy and x-ray diffraction analysis show that the average particle size of Mn-doped ZnS nanoparticles is ∼3.0 nm with a zinc-blende structure. Photoluminescence, x-ray photoelectron spectroscopy, atomic absorption spectroscopy and electron paramagnetic resonance studies were carried out to confirm that the Mn(2+) dopants are present in the ZnS nanoparticles.