Pratap Mukherjee

Electronic properties of poly(o-methoxy aniline)-silver nanocomposite thin films: influence of nanoparticle size and density

A tailor-made synthesis of Ag nanoparticles in chloroform is made through an interfacial redox reaction between poly(o-methoxy aniline) (POMA) and an aqueous AgNO3 solution. Particles of three different sizes and densities are prepared. Different current–voltage (I–V) behavior is observed in the thin films of POMA-Ag nanocomposites for different sizes and densities of nanoparticles. If the nanoparticle size is large (diameter = 21.9 ± 1.7 nm), and the density is low (density = 2.6 × 1014/m2), they exhibit switching behavior. And if size is low (8.9 ± 0.8 nm) and density is very high (60 × 1014/m2), they exhibit rectification properties. Possible reasons for rectification and switching characteristics are discussed.

Growth of different shape Au nanoparticles through an interfacial redox process using a conducting polymer

An interesting interfacial redox method is developed for the preparation of Au nanoparticles of various shapes in organic medium using Au seeds in aqueous medium without using phase-transfer reagent. The conversion, stabilization, and the transfer of gold nanoparticles to the organic phase is accomplished using the reducing and solubility properties of poly(o-methoxy aniline) in chloroform. The preparatory method is simple, and the Au nanoparticles are free from excess oxidant and external stabilizer. The characterizations of the metal nanoparticles are made using transmission electron microscopy, UV-vis spectra, electron diffraction and energy-dispersive X-ray techniques. Au nanoparticles of various shapes, for example, hexagonal, pentagonal, triangular, rod-shaped, etc. are produced together. A mechanism of the formation of differently shaped nanoparticles is proposed from catalytic reduction of Au(3+) ion by POMA on the Au seed surface followed by the growth of nanoparticles from the exposed Miller planes of seed surfaces.

Bimetallic Aucore–Agshell nanoparticles from interfacial redox process using poly(o-methoxyaniline)

Stable Au(core)-Ag(shell) bimetallic nanoparticles are produced in organic medium through an interfacial redox method using the redox property of an organically soluble conducting polymer poly(o-methoxyaniline) (POMA). The transition of the emeraldine base (EB) form of POMA to its pernigraniline base (PB) form occurs during nanoparticle formation and also the nitrogen atoms of POMA (PB) stabilize the nanoparticles via coordination. TEM images indicate that the interfacial redox replacement reaction between Au (III) in aqueous medium and Ag (0) in organic medium produces bimetallic nanoparticles with Au(core)-Ag(shell) morphology. The formation of such type of core-shell morphology is due to the different standard reduction potentials of the metal ions. The core-shell thickness of the bimetallic nanoparticle can be regulated by changing the POMA (EB) concentration in the organic layer.

Biomolecular hybrid of poly(3-thiophene acetic acid) and double stranded DNA: optical and conductivity properties.

A new biomolecular hybrid of poly(3-thiophene acetic acid) (PTAA) and double stranded deoxyribonucleic acid (ds-DNA) is prepared. The transmission electron microscopy (TEM) images exhibit fibrillar network morphology making a nanostructured self-assembly of PTAA-DNA hybrid. The confocal fluorescence image of PTAA shows green fluorescence exhibiting agglomeration in the pure state but the spreading of green fluorescence over the network superstructure in the hybrids indicating the immobilization of PTAA on DNA surfaces. Fourier transform infrared (FTIR) spectra indicate hydrogen bonding between -COOH groups of PTAA and P=O groups of Na-DNA. Circular dichroism (CD) spectra denote that DNA conformation remains unaltered during hybrid preparation. A blue shift of the pi-pi* absorption peak of PTAA in the hybrid solutions occurs with aging time. The photoluminescence intensity in the hybrid solution increases with a concomitant blue shift of the emission peak with aging time, and it is faster with increased DNA concentration. Possible reasons of different optical behavior are discussed in the light of duplex and triplex hybrid formation. Dynamic light scattering study indicates an increased particle size of PTAA with addition of DNA favoring the hybrid particles to remain in solution. The dc-conductivity of the hybrids decreases from that of PTAA with an increase of Na-DNA concentration, and the current (I)-voltage (V) curves indicate a semiconducting nature of the hybrids.

Suppression of ferroelectricity in ultrathin barium titanate films grown within mica-4 interlayer space

The critical thickness for ferroelectricity has assumed importance because of the integration of ferroelectric perovskite films into microelectronic devices. Recent calculations reported by Junquera and Ghosez (J. Junquera and Ph. Ghosez, Nature, 2003, 422, 506, ref. ) have shown the critical thickness in the case of BaTiO3 thin films to be ca. 2.4 nm. We have grown BaTiO3 films of thickness ca. 1.2 nm within the crystal channels of sodium fluorophlogopite mica of chemical composition Na4Mg6Al4Si4O20F4·H2O. Dielectric permittivity measurements on the composites do not show any transition at 393 K. Several possible causes for the suppression of this dielectric anomaly have been examined e.g., reduction of BaTiO3, sodium ion introduction into BaTiO3 and generation of thermal stress on these films. All of these have been ruled out on the basis of available experimental data. We believe our results are consistent with the recently reported theoretical predictions.

Concomitant synthesis of polyaniline and highly branched gold nanoparticles in the presence of DNA

The reduction of chloroauric acid using aniline adsorbed on DNA produces highly branched dendritic gold nanoparticles with concomitant formation of polyaniline (PANI) in contrast to the formation of spherical Au nanoparticles in the absence of DNA. The conformation of DNA remains intact in the process as evident from circular dichroism (CD) spectra. The UV-Vis spectrum exhibits a broad absorption peak at 520-900 nm, for a combined effect of the gold surface plasmon and π band to localized polaron band transition of DNA-doped PANI. Both the dendritic Au-PANI-DNA and the spherical Au-PANI systems emit two peaks for excitation with radiation of 300 nm and the intensity ratio of the emission and FRET peak is higher in the dendritic Au-PANI than that in the spherical Au-PANI system. The dc-conductivity values of spherical Au-PANI and dendritic Au-PANI-DNA systems are 1.2×10(-10) and 1.7×10(-8) S/cm at 30°C, respectively.

Enhanced optoelectronic properties of RNA-poly(o-methoxyaniline) hybrid containing monodispersed Au nanoparticles

In situ, gold nanoparticles are produced in poly(o-methoxy aniline) (POMA)–RNA (R) hybrids; POMA (P) acting both as reductant and stabilizer. The POMA (emeraldine base, EB) is oxidized into a mixture of POMA (emeraldine salt, ES) and POMA (pernigraniline base, PB) in the course of reaction as evident from FTIR and UV-vis spectra. The Au nanoparticles are of uniform size (diameter ∼ 14 nm) and the density of Au nanoparticles increases with increase in POMA (EB) concentration showing a maximum of 150 × 1010 particles per square centimetre in PRAu31 (the number indicates respective weight ratio). The electron diffraction pattern indicates the polycrystalline nature of the Au nanoparticles and CD spectra suggest a small conformational distortion of the A helix of RNA towards the B helix in the nanobiocomposite. The FTIR spectra indicate the presence of H-bonding, π–π and ionic interactions between POMA (ES) and RNA, and the Au nanoparticles are stabilized by complexation through nitrogen atoms of POMA (PB). The π band to polaron band transition peak of POMA shows a gradual red shift with aging time and the shift is greater in the case of POMA–RNA–Au nanobiocomposites than that of the POMA–RNA hybrid. The larger shift in the PRAu nanocomposites compared to the POMA–RNA hybrid is attributed to the more uncoiled state of POMA for stabilizing Au nanoparticles. The POMA–Au system generates new photoluminescence properties when excited at 500 nm and PL intensity increases abruptly with the addition of RNA in the hybrid. The excitation of plasmon electrons and the stabilization of excitons in the conjugated chain of POMA is attributed to the new PL property and its enhancement with addition of RNA is due to the increased conjugation length of POMA on the RNA surface. PRAu13 shows rectification property, while the other compositions do not exhibit such behavior. The rectification is explained from the band energy diagram, and its absence in PRAu11 and PRAu31 hybrids is attributed to the increased concentrations of Au and POMA causing a better conducting composite material.

Self assembly of poly(o-methoxy aniline) with RNA and RNA/DNA hybrids: Physical properties and conformational change of poly(o-methoxy aniline)

Biomolecular hybrids of a conducting polymer [poly(o-methoxy aniline) (POMA)] and RNA are prepared at the three different compositions by mixing aqueous solutions of diethyl, 2-hydroxy ethyl, ammonium salt of RNA (type IX from Torula Yeast) and POMA (ES, emeraldine salt; doping level [Cl]/[N]=0.52). A slow increase of pH up to 30 h of aging occurs in the mixture till it levels up. The TEM micrographs indicate a fibrillar network structure in all the hybrid compositions (POMA: RNA=1:3, 1:1, 3:1, by weight). In the complexes three types of supramolecular interactions, viz. (i) electrostatic, (ii) H-bonding and (iii) pi-pi interactions, are evident from the FTIR spectroscopy. The CD spectra indicate a small distortion of A-RNA conformation towards its B form during the hybrid formation. Time and temperature dependent UV-vis spectral studies indicate a slow red shift of the pi-band to polaron band transition peak (lambda(max)) for the uncoiling of the POMA (P) chain on the RNA (R) surface. The repulsive interaction between the radical cations of POMA (ES) absorbed on the RNA surface is attributed to the conformational change causing the uncoiling of POMA chain. UV-vis spectral study indicates that the uncoiling and attachment of POMA on RNA surface is much faster than that on DNA (D). In POMA-RNA-DNA (PRD) hybrid solutions slower red shift of lambda(max) indicates more disordered array of the phosphate groups than that in PR and PD systems. The conductivity values of the PR hybrids (10(-)(6) S/cm(-1)) are three orders higher than that of RNA, rendering the PR hybrids to be useful for fabricating good biosensors. In the PRD hybrids conductivity decreases by two orders than those of PR and PD hybrids suggesting a disorder arrangement of POMA chains in the PRD hybrids. The I-V characteristic curves of the PR and PRD hybrids indicate a semiconducting nature of the hybrids.

Nondestructive characterization of Li+ ion-doped multifunctional poly(vinylidene fluoride)-g-poly(dimethyl amino ethyl methacrylate) by impedance spectroscopy.

Poly(vinylidene fluoride) (PVDF)-graft-poly(dimethyl amino ethyl methacrylate) (PDMAEMA) (PD copolymer) is produced via atom transfer radical polymerization from PVDF solution in N-methyl-2-pyrrolidone. PD copolymer is doped with 1% and 5% (w/w) Li(+) ion to produce PDLi1 and PDLi5 samples, respectively. In PD copolymer, the crystalline structure of PVDF changes from α polymorph to a mixture of α and β polymorph, and it transforms completely to piezoelectric β polymorph on doping with 1% (w/w) Li(+) ion. The impedance behavior of PVDF changes on grafting, and that of the PD graft copolymer also changes with increasing Li(+) ion dopant concentration. In the Nyquist plots, PVDF exhibits a straight line character, and a curvature has appeared in the PD graft copolymer; on doping the latter with Li(+) ion (1% w/w), the curvature increases and a semicircle is completed on 5% Li(+) doping. Fitting the data from the Z-view program, the Ohmic resistance of PDLi1 is found to be 78 MΩ having capacitance with constant phase element (CPE) = 1.38 nF while for the PDLi5 sample the resistance decreases to16.1 MΩ with a small increase in CPE to 1.46 nF. The modulus plane plots for PDLi1 and PDLi5 samples also exhibit only one peak supporting the presence of only one equivalent resistance-capacitance circuit with constant phase element in both PDLi1 and PDLi5 samples. Both the impedance and modulus vs frequency plots of PDLi1 and PDLi5 samples exhibit a single Debye peak suggesting isotropic nature of the samples. For PVDF and PDMAEMA, ac-conductivity increases linearly with angular frequency, but in the case of PDLi1 and PDLi5 samples, it remains at first invariant in the frequency range 1-10(2) Hz, and above 10(2) Hz, an increase in conductivity with frequency occurs obeying the double power law. In the temperature variation of conductivity, PVDF exhibits its typical insulating nature, and in the PD graft copolymer, the conductivity decreases with increase of temperature (metallic-like behavior) due to gradual breaking of supramolecular interaction. The temperature variation of ac-conductivity of the Li(+)-doped PD graft copolymer suggests that both the ionic and supramolecular contributions of conductivity operate; the former increases and the latter decreases with rise in temperature showing a maximum. The temperature-dependent FTIR spectra of PDLi1 and PDLi5 samples support the gradual breaking of supramolecular interactions with increase of temperature.

Nanojacketing and Dejacketing of ds-DNA: A Nondestructive Characterization of a Nanojacketed Sample by Impedance Spectroscopy

A facile approach of nanojacketing DNA in intact conformation is evolved by the in situ polymerization of o-methoxyaniline (OMA) at 30 °C using HAuCl4 as an oxidant and DNA as a soft template. It concomitantly produces poly(o-methoxyaniline) (POMA) and a Au nanojacket encapsulating the double stranded DNA (ds-DNA). The POMA chains remain adhered to the Au nanojacket, facilitating the dissolution of nanojacketed DNA (DNA-Au-POMA) in organic solvent without affecting its conformation. Digestion of the nanojacketed system with saturated iodine solution dejackets the ds-DNA with retention of its conformation, leaving the POMA nanotube. The nanojacketing and dejacketing phenomena are established by transmission electron microscopy (TEM), UV-vis spectroscopy, and CD spectroscopy, and the nanostructure is further characterized by FTIR, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The impedance study of the DNA-Au-POMA sample suggests the Cole-Cole plots at both the impedance and modulus planes and the values of capacitance and electron-transfer resistance of the material (R(et)) are calculated to be 13.74 pF and 388 kΩ, respectively. The presence of a single Debye peak in both the impedance and modulus vs frequency plots suggests an isotropic nature of the system, and the frequency dependent ac-conductivity suggests the presence of short-range translational and reorientational (localized) hopping of charge carriers at lower and higher frequency region.