S. Annapoorni

Gas and humidity sensors based on iron oxide–polypyrrole nanocomposites

Abstract Nanocomposites of iron oxide and polypyrrole were prepared by simultaneous gelation and polymerization process. This resulted in the formation of mixed iron oxide phase for lower polypyrrole concentration, stabilizing to a single cubic iron oxide phase at higher polypyrrole concentration. The composites in the pellet form were used for humidity and gas sensing investigations. Their sensitivity to humidity was found to increase with increasing concentration of polypyrrole. Gas sensing was performed for CO2, N2 and CH4 gases at varying pressures. The sensors showed a linear relationship between sensitivity and pressures for all the gases studied. The sensors showed highest sensitivity to CO2 gas.

Humidity-sensing properties of nanocrystalline haematite thin films prepared by sol-gel processing

Nanocrystalline α-Fe2O3 thin films prepared by sol-gel processing were used for studying the variation of electrical resistance with changes in humidity. The resistance of these films decreased by an order of 3 from dry (20%) to humid (85%) conditions. The sensitivity to humidity decreased with increasing temperature as well as with grain size. A sudden increase of resistance from 1.2 × 108 Ω to 1.6 × 1010 Ω was observed for a film of grain size ~30 nm at 100°C. The reaction between water and lattice iron was found to be reversible, suggesting the reusability of the films.

Immobilization of urease on poly(N-vinyl carbazole)/stearic acid Langmuir–Blodgett films for application to urea biosensor

Urease was immobilized in mixed monolayers of poly(N-vinyl carbazole) (PNVK) and stearic acid (SA) formed at an air-water interface. The monolayers were transferred onto indium-tin-oxide (ITO) coated glass plates using Langmuir-Blodgett (LB) film deposition technique. Urease immobilized on PNVK/SA LB films, characterized using FTIR and UV-visible spectroscopy, was found to exhibit increased stability over a wide pH (6.5-8.5) and temperature (25-50 degrees C) range. Potentiometric measurements on these urease electrodes were carried out using an ammonium ion analyzer. Two values for K(m)(app) were obtained at lower and higher concentrations of substrate urea.

Langmuir–Blodgett films of poly(3-dodecyl thiophene) for application to glucose biosensor

Abstract Monolayers of poly(3-dodecyl thiophene) (P3DT) have been obtained on indium–tin-oxide (ITO) coated glass plates by dispensing mixed solution of P3DT and stearic acid (SA) prepared in chloroform onto water subphase by a microsyringe. The pressure–area isotherms of these P3DT–SA monolayers were studied as a function of temperature and pH. The monolayer stability onto the water subphase has been experimentally studied at different temperatures, pH and surface pressure. These P3DT–SA monolayers fabricated onto the ITO-coated glass plates were characterized using FTIR and cyclic voltammetry studies. The desired enzyme monolayers were fabricated by dispensing glucose oxidase mixed with P3DT/SA in chloroform and were transferred onto desired ITO-coated glass. An attempt has been made to utilize these P3DT/SA/GOX LB films for fabrication of a glucose biosensor.

Phase change induced by polypyrrole in iron-oxide polypyrrole nanocomposite

Nanocomposites of polypyrrole and iron oxide were prepared using simultaneous gelation and polymerization processes. Varied amounts of pyrrole monomer were added to a solution containing iron nitrate as precursor and 2-methoxy ethanol as solvent. The presence of oxide and polypyrrole was confirmed by using X-ray and FTIR techniques. Some of these nanocomposites exhibited magnetic behaviour. SEM studies of powders indicated presence of nanosized particles. Electrical conductivity studies of powders showed a slight variation in conductivity for lower concentration of pyrrole, with a sudden increase in conductivity at 15% of pyrrole concentration. A transition from a nonmagnetic to magnetic phase was also observed at the same concentration.

DIODE LIKE BEHAVIOUR OF AN ION IRRADIATED POLYANILINE FILM

Abstract PN junction diode formation has been achieved for the first time in a single polyaniline film, cation doped chemically on one side and anion doped by irradiation of ions from dense plasma focus on the other side. The diode behaviour is confirmed by I-V characteristics. The results of ESCA, XRD and SEM for chemical composition, structure and surface morphology, respectively, are also reported for these films.

Characterization of electrochemically synthesized poly(2-fluoroaniline) film and its application to glucose biosensor

Abstract Poly(2-fluoroaniline) has been electrochemically deposited on ITO coated glass plates in the form of thin films using 4 M perchloric acid as electrolyte. These polymeric films have been spectroscopically characterized by FT-IR and UV-visible spectrophotometric methods. Room temperature electrical conductivity of the polymeric films was measured and found to be lower than that of polyaniline films. The SEM studies show the blunt needle like surface morphology of the polymeric film. Glucose oxidase (GOX) has been immobilized on to these electrochemically deposited conducting poly(2-fluoroaniline) films by physical adsorption method. The redox characterization of poly(2-fluoroaniline) and poly(2-fluoroaniline)/GOX films has been carried out by cyclic voltammetry technique. Amperometric detection of β-glucose was carried out by using the above enzyme immobilized polymeric film as a working electrode and Ag/AgCl as a reference electrode at a bias voltage of 0.2 V over a physiological range. The electrode carrying GOX was found to be stable up to 32 days.

Dielectric relaxation in thin conducting polyaniline films

The results of dielectric measurements carried out on polyemeraldine salt in an Al-polyemeraldine salt-Al configuration as function of frequency and temperature are reported. The analysis of the dielectric data obtained for Al-polyemeraldine salt-Al configuration shows the space-charge phenomenon. The mobility was obtained as a function of temperature and thickness of the sandwiched emeraldine salt, respectively.

Oxygen vacancy induced phase formation and room temperature ferromagnetism in undoped and Co-doped TiO2 thin films

TiO2 and Co-doped TiO2 (CTO) thin films deposited at various oxygen partial pressures by pulsed laser deposition exhibit room temperature ferromagnetism (RTFM) independent of their phase. Films deposited at 0.1?mTorr oxygen partial pressure show a complete rutile phase confirmed from glancing angle x-ray diffraction and Raman spectroscopy. At the highest oxygen partial pressure, i.e. 300?mTorr, although the TiO2 film shows a complete anatase phase, a small peak corresponding to the rutile phase along with the anatase phase is identified in the case of CTO film. An increase in O to Ti/(Ti+Co) ratio with increase in oxygen partial pressure is observed from Rutherford backscattering spectroscopy. It is revealed from x-ray photoelectron spectroscopy (XPS) that oxygen vacancies are found to be higher in the CTO film than TiO2, while the valency of cobalt remains in the +2 state. Therefore, the CTO film deposited at 300?mTorr does not show a complete anatase phase unlike the TiO2 film deposited at the same partial pressure. We conclude that RTFM in both films is not due to impurities/contaminants, as confirmed from XPS depth profiling and cross-sectional transmission electron microscopy (TEM), but due to oxygen vacancies. The magnitude of moment, however, depends not only on the phase of TiO2 but also on the crystallinity of the films.

Magnetic properties of polypyrrole - coated iron oxide nanoparticles

Iron oxide nanoparticles were prepared using the sol–gel process. In situ polymerization of a pyrrole monomer in the presence of oxygen in an iron oxide–ethanol suspension resulted in an iron oxide polypyrrole nanocomposite. The structure and magnetic properties of the nanocomposites with varying pyrrole concentrations are investigated. X-ray diffraction studies indicate the presence of the γ-Fe2O3 phase for the concentrations investigated. FTIR studies confirm the presence of polypyrrole. TEM studies show agglomeration in uncoated samples and in samples with a lower concentration of polypyrrole. Agglomeration is greatly reduced for samples coated with a higher concentration of polypyrrole. The ac susceptibility measurements performed in the temperature range 77–300 K show the presence of blocking, indicating the superparamagnetic phase. The blocking temperature is found to depend on the pyrrole concentration. Monte Carlo studies for an array of polydispersed single domain magnetic particles, based on an interacting random anisotropy model, were also carried out, and the blocking temperatures obtained from the simulation of the field cooled–zero field cooled magnetization compare favourably with experimental results.