Parameswar Krishnan Iyer

Aqueous polyfluorene probe for the detection and estimation of Fe3+ and inorganic phosphate in blood serum

A novel anionic polyfluorene derivative, poly(9,9-bis(6′-sulfate)hexyl) fluorene-alt-1,4-phenylene sodium salt (P1) is synthesized. P1 exhibits exemplary activity towards the selective detection of Fe3+ and phosphates (Pi) under physiological conditions. On binding to Fe3+, exceptional fluorescence quenching of P1 occurred, demonstrated by a >97% reduction in the fluorescence intensity. Furthermore, the P1–Fe3+ assay is highly selective for inorganic phosphate (Pi) anions at biological pH values, observed by complete fluorescence dequenching and confirmed through a >95% fluorescence enhancement. In order to validate its diagnostic potential, this assay was employed to monitor the Pi levels in a competing biological environment like blood serum. At pH 7.4 this assay showed a high specific activity to detect Pi in the bioassay environment, observed by the unique enhancements in fluorescence intensities for varying and low Pi concentrations. Since this assay performed Pi detection at very low concentrations we utilized it successfully for the fluorometric estimation of Pi in blood serum with high accuracy and within short duration. This remarkable ability of P1 to accomplish in situ monitoring and estimation of indispensable biological targets like Fe3+ and Pi rapidly and in label-free conditions, corroborates the extension of this assay system for clinical application.

Diameter dependence of oxidative stability in multiwalled carbon nanotubes: Role of defects and effect of vacuum annealing

While the oxidative stability of single walled carbon nanotubes has been studied extensively, very little is known about the diameter dependence of oxidative stability in multiwalled carbon nanotubes (MWNTs) and the effect of vacuum annealing on such stability. We have investigated six sets of samples with different diameters in the range of 5–100 nm systematically by thermogravimetric analysis (TGA) before and after vacuum annealing. High resolution transmission electron microscopy studies provide evidence for structural defects in the nanotube walls. Further, it reveals that with increasing diameter, interlayer d-spacing between concentric tubes decreases. Experimental TGA profile is found to constitute multiple components of oxidation as revealed from reverse Sigmoidal fitting. Analysis of the TGA profile shows that the oxidation temperature follows an exponential recovery function with increasing diameter, while width of the differentiated TGA spectra decreases. It is shown that oxidative stability of...

Improved molecular architecture of D–π–A carbazole dyes: 9% PCE with a cobalt redox shuttle in dye sensitized solar cells

Two new D–π–A dyes (SK2 & SK3) based on carbazole, and vinylene-phenylene (π-bridge) with rhodanine-3-acetic acid and cyanoacrylic acid as electron withdrawing–injecting as well as anchoring groups were designed and synthesised under conditions that were free from precious metal-catalysts and well characterized for dye-sensitized solar cell (DSSC) applications. A high power conversion efficiency (PCE) of 9% (AM 1.5 G, 100 mW cm−2) has been achieved using cyanoacrylic acid as an acceptor in D–π–A carbazole dye (SK3) with a cobalt based redox shuttle, while a PCE of 7.1% was exhibited in triiodide based redox mediators. A short-circuit current density, Jsc of ∼18.2 mA cm−2, an open-circuit voltage, Voc of ∼725 mV, and a fill factor, FF of ∼67% have been afforded by the SK3 based DSSC incorporating the Co2+/Co3+ electrolyte as the one-electron redox mediator. In contrast, SK2 dye based DSSCs with a cobalt based redox mediator have shown a Jsc ≈ 8.4 mA cm−2, a Voc ≈ 587 mV, and a FF ≈ 48%, yielding a PCE of 2.4%. The devices based on SK3 showed outstanding stability performance without significant degradation even after 1000 h of illumination under standard conditions in the Co2+/Co3+ electrolyte.

Chiral Ru(II) Schiff base complex-catalysed enantioselective epoxidation of styrene derivatives using iodosyl benzene as oxidant. II

Abstract Six-coordinated chiral Ru(II) Schiff base complexes of the type [RuLX(Y) 2 ] where L=terdentate chiral Schiff bases derived from l -tyrosine, l -phenylalanine with salicylaldehyde, 3- tertiary -butyl-, 3,5-di- tertiary -butyl-, 3,5-dichloro- and 3,5-dinitrosalicylaldehyde, X=PPh 3 and Y=H 2 O have been investigated as catalysts for enantioselective epoxidation of styrene, 4-chloro-, 4-nitro- and 4-methylstyrene in fluorobenzene in order to explore the efficiency of catalytic system by varying the substituents on the ligand moiety of the catalysts as well as on the substrates using iodosyl benzene as terminal oxidant. Much better results were obtained with catalyst 5 and 10 with 4-nitrostyrene. The enantiomeric excess of the resulting epoxide was evaluated by chiral capillary column.

Synthesis of catalytically active polymer-bound Mn(III) salen complexes for enantioselective epoxidation of styrene derivatives

Abstract Catalytically active metal-complexing polymer-containing chiral Mn(III) salen moieties derived from (1 R , 2 R )-(−)-diphenylethylenediamine, (1 S , 2 S )-(+)-cyclohexanediamine, and ( S )-(+)-diaminopropane with α-naphthyl salicylaldehyde anchored to the polymeric matrix obtained from styrene-4-vinyl pyridine-divinyl benzene (PVPD) have been synthesised. These catalysts were used for enantioselective epoxidation of styrene and substituted styrenes, viz. 4-chloro-, 4-methyl and4-nitrostyrene using iodosyl benzene as terminal oxidant by GLC. The enantiomeric excess of the resulting epoxide was determined by GLC using chiral capillary column or by 1 H-NMR using chiral-shift reagent Eu(hfc) 3 . Each catalyst/substrate combination was examined under epoxidation condition and the results for catalysts 1–3 are presented as Hammet plots. A mechanism involving formation of an Mn-oxo complex and oxygen transfer from a reactive Mn-oxo intermediate to styrene was also proposed for the reaction. These catalysts can be recycled at least ten times without loss of its activity.

Chiral Ni(II) Schiff base complex-catalysed enantioselective epoxidation of prochiral non-functionalised alkenes

The synthesis, characterisation and single crystal X-ray structures of square planar Ni(II) chiral Schiff base complexes with N2O2-type ligands have been described. These catalysts were used for enantioselective epoxidation of non-functionalised alkenes viz. 1-hexene, 1-octene, styrene, 4-chloro-, 4-nitrostyrene and 1,2-dihydronaphthalene using NaOCl as oxidant, giving excellent conversions with long chain alkenes while ees were moderate to good. A mechanism for the Ni(II)-catalysed epoxidation with NaOCl is proposed.

Synthesis, physicochemical studies and aerobic enantioselective epoxidation of non functionalized olefins catalyzed by new Co(II) chiral salen complexes

Abstract Co(II) chiral salen complexes 1 – 3 derived from α -naphthyl salicylaldehyde with 1 S ,2 S (+) diaminocyclohexane, 1 R ,2 R (−) diaminodiphenylethane and S (+) 1,2-diaminopropane have been prepared. The characterization of the complexes was done by microanalysis, magnetic moment, IR-, UV/Vis-, CD spectral studies, optical rotation, conductance measurements and cyclic voltammetry. Epoxidation of non-functionalized prochiral olefins viz. styrene, trans 3-nonene and trans 4-octene was achieved by the combined use of an atmospheric pressure of molecular oxygen and sacrificial reductant isobutyraldehyde catalyzed by the above synthesized Co(II) chiral salen complexes with and without pyridine N -oxide as cooxidant. Good yields of the desired epoxide were obtained with the substrate trans 3-nonene and trans 4-octene by GLC. Enantiomeric excess of the epoxide were evaluated by 1 H NMR using chiral shift reagent Eu(hfc) 3 and by chiral capillary column.

Enhancing the Photostability of Poly(3-hexylthiophene) by Preparing Composites with Multiwalled Carbon Nanotubes

Poly(3-hexylthiophene) (P3HT) degrades in organic solvents containing dissolved molecular oxygen when irradiated with ultraviolet light. Hence, it is important to develop strategies that can enhance the photostability of P3HT and enhance the device performance. In this work, we report that preparing composites of P3HT with appropriate amounts of multiwalled carbon nanotube (MWCNT) results in superior photostability of P3HT. UV-visible and fluorescence spectroscopy have been used as primary tools to study the photostability of P3HT and its composites. Scanning electron microscopy (SEM) images of the composites display dispersed CNTs being well coated by P3HT. Transmission electron microscopy (TEM) micrographs along with SAED patterns reveal that P3HT coats the CNTs, which is the reason for superior dispersion of the composite. ESR spectroscopy was also performed to pursue and follow the degradation of P3HT. Ten weight percent of MWCNTs in P3HT was found to be the optimum loading amount that results in maximum photostability of the P3HT as compared to the other ratios. This enhanced photostability of P3HT on preparing composites with MWCNT in addition to its easy processability directly from solution makes these composites immensely important for optoelectronic applications.

Development of solution, film and membrane based fluorescent sensor for the detection of fluoride anions from water

The synthesis and application of a neutral polymer, poly(1,4-bis-(8-imidazole-octyloxy)-benzene) (PPI), is performed by economical and simple reaction steps. The PPI polymer demonstrates exemplary activity to be used as a film on a TLC plate, or as a membrane by blending it with a desired polymer or in a solution phase to detect fluoride anions from contaminated water in the presence of competing anions at ppb levels easily and rapidly. This polymer PPI works on the simple displacement principle where fluorescence turn-on/turn-off are observed as signals. On selectively binding Cu2+ it displays extraordinary fluorescence quenching, resulting in >97% reduction in the fluorescence intensity. This effect could be visualized in solution phase, on a TLC plate and on a blended polymer membrane. Furthermore, the fluorescence of this PPI–Cu2+ assay showed 81% enhancement on selectively binding F− anions in contaminated water in the presence of other competing anions with higher positive free energies of hydration. Polymeric systems with such robust fluorescence dequenching activity are novel, providing a unique platform for detection and possible removal of fluoride anions. To validate this potential, two experiments were performed: (a) preparation of a film on a TLC plate and (b) preparation of a membrane by mixing 1% PPI in polystyrene and casting as a membrane film of desired shape and thickness. Our results confirm that PPI–Cu2+ films and membranes described above have the highest specific activity to sense fluoride in a competitive environment, observed by the unique enhancements in fluorescence intensities at varying and extremely low quantities of 1 ppm, 10 ppm and 50 ppm of fluoride. The detection limit of fluoride in contaminated water for the TLC plate and membrane methods was very low and was in the range of 2.5–10.0 ppb. We have further used these methods for the detection of fluoride in natural ground water samples and ascertained the percentage of fluoride.

Vapor phase sensing of ammonia at the sub-ppm level using a perylene diimide thin film device

The fabrication of a two terminal sensor device based on a histidine substituted perylene diimide (PDI-HIS) thin film for the sensitive detection and quantification of ammonia (NH3) vapors by monitoring the changes in its current intensity is reported at room temperature under ambient conditions. The thin film morphological variations of the drop cast PDI-HIS films before and after exposure to NH3 vapors are characterized by FESEM and TEM confirming the diffusion/adsorption of the NH3 vapors. The solution cast PDI-HIS thin film gas sensor device exhibited rapid, highly sensitive and selective vapor phase response towards NH3 with a detection limit as low as 0.56 ppm which is much lower than the maximum permissible limit set for NH3 (25 ppm) for prolonged exposure. Furthermore, control sensing experiments performed using alkyl substituted PDI (PDI-n-octyl) demonstrated that the presence of histidine groups at the imide position of PDI-HIS drastically affects the solid-state aggregation mode as well as redox potential that ultimately enhances the sensing response of the device. The key performance parameters of the device such as sensitivity, response/recovery time, selectivity, recyclability, stability and detection limit demonstrated the protocol as simple, reliable, cost-effective and most efficient in performing NH3 detection under very realistic conditions.