Amit Paul

Evidence for a Near-Resonant Charge Transfer Mechanism for Double-Stranded Peptide Nucleic Acid

We present evidence for a near-resonant mechanism of charge transfer in short peptide nucleic acid (PNA) duplexes obtained through electrochemical, STM break junction (STM-BJ), and computational studies. A seven base pair (7-bp) PNA duplex with the sequence (TA)(3)-(XY)-(TA)(3) was studied, in which XY is a complementary nucleobase pair. The experiments showed that the heterogeneous charge transfer rate constant (k(0)) and the single-molecule conductance (σ) correlate with the oxidation potential of the purine base in the XY base pair. The electrochemical measurements showed that the enhancement of k(0) is independent, within experimental error, of which of the two PNA strands contains the purine base of the XY base pair. 7-bp PNA duplexes with one or two GC base pairs had similar measured k(0) and conductance values. While a simple superexchange model, previously used to rationalize charge transfer in single stranded PNA (Paul et al. J. Am. Chem. Soc. 2009, 131, 6498-6507), describes some of the experimental observations, the model does not explain the absence of an enhancement in the experimental k(0) and σ upon increasing the G content in the duplexes from one to two. Moreover, the superexchange model is not consistent with other studies (Paul et al. J. Phys. Chem. B 2010, 114, 14140), that showed a hopping charge transport mechanism is likely important for PNA duplexes longer than seven base pairs. A quantitative computational analysis shows that a near-resonant charge transfer regime, wherein a mix of superexchange and hopping mechanisms are expected to coexist, can rationalize all of the experimental results.

A robust iron oxyhydroxide water oxidation catalyst operating under near neutral and alkaline conditions

Efficient electrochemical splitting of water to hydrogen and oxygen using cheap and abundant metal ion based catalysts is of fundamental significance to solar devices. For an efficient water splitting reaction, the development of a highly active, robust and cost-effective catalyst is desirable. Herein, we report iron oxyhydroxide thin films as an efficient water oxidation catalyst. The films have been electrochemically deposited applying anodic potential in the presence of a nonaqueous solvent, using ferrocene as the metal ion precursor and exclude interference from the problems of precipitation of iron hydroxide during the deposition process. The as-prepared films exhibit high catalytic activity towards the oxygen evolution reaction under alkaline as well as under near neutral conditions. Long term testing results showed that the films were able to oxidize water for almost 8 h of continuous operation with a current density of 10 mA cm−2 at an overpotential of 600 mV under near neutral conditions. The facile method of electrodeposition reported here with outstanding catalytic efficiency is of great significance for the large scale production of hydrogen.

Role of Nucleobase Energetics and Nucleobase Interactions in Single-Stranded Peptide Nucleic Acid Charge Transfer

Self-assembled monolayers of single-stranded (ss) peptide nucleic acids (PNAs) containing seven nucleotides (TTTXTTT), a C-terminus cysteine, and an N-terminus ferrocene redox group were formed on gold electrodes. The PNA monomer group (X) was selected to be either cytosine (C), thymine (T), adenine (A), guanine (G), or a methyl group (Bk). The charge transfer rate through the oligonucleotides was found to correlate with the oxidation potential of X. Kinetic measurements and computational studies of the ss-PNA fragments show that a nucleobase mediated charge transport mechanism in the deep tunneling superexchange regime can explain the observed dependence of the kinetics of charge transfer on the PNA sequence. Theoretical analysis suggests that the charge transport is dominantly hole-mediated and takes place through the filled bridge orbitals. The strongest contribution to conductance comes from the highest filled orbitals (HOMO, HOMO-1, and HOMO-2) of individual bases, with a rapid drop off in contributions from lower lying filled orbitals. Our studies further suggest that the linear correlation observed between the experimental charge transfer rates and the oxidation potential of base X arises from weak average interbase couplings and similar stacking geometries for the four TTTXTTT systems.

Role of graphite precursor and sodium nitrate in graphite oxide synthesis

Graphite oxides were synthesized following the Hummers method using three different graphite precursors. The role of the graphite precursor and sodium nitrate during graphite oxide synthesis has been investigated. Different analytical techniques, namely powder X-ray diffraction, thermal gravimetric analysis, infrared spectroscopy, and ultraviolet-visible absorption spectroscopy have been implemented to characterize the synthesized graphite oxides. The study revealed that a longer c-axis (axis perpendicular to the carbon layer) in the graphite crystallite favored basal plane oxidations over sheet edge oxidations on the graphitic sheets during graphite oxide synthesis. These basal plane oxidations caused a strain in the graphitic sheets. Due to this strain, graphitic layers were cracked along the a-axis (axis in the carbon layer planes) and also layers were peeled off along the c-axis. Hence, crystallite sizes of the synthesized graphite oxides were significantly reduced compared to their graphite precursors. Furthermore, basal plane oxidations and consequently reduction in the crystallite sizes were enhanced by the addition of sodium nitrate during the synthesis.

Uniform spheroidal nanoassemblies of magnetite using Tween surfactants: influence of surfactant structure on the morphology and electrochemical performance

We report solvothermal synthesis of uniform spheroidal nanoassemblies of magnetite (Fe3O4) in ethylene glycol medium by using polyethoxylated surfactants viz. Tween 80 and Tween 20, having oleyl (18 carbon) and lauryl (12 carbon) hydrophobic tails, respectively as structure directing agents. Uniform nanoassemblies could be obtained only in a narrow range of water content in the reaction medium (between 20 and 50 ppt). Outside this window, heterogeneous assemblies were obtained. Within the optimized water content regime, varying the water content allowed easy modulation of assembly dimensions between 80 and 200 nm. Under similar synthetic conditions, Tween 80 yielded smaller nanoassemblies having larger crystalline domain sizes when compared to Tween 20. Dynamic light scattering (DLS) confirmed the dimensional uniformity and stability of these assemblies in ethanol dispersions. High resolution scanning electron microscopy (HRSEM) showed the presence of smaller nanoparticles within each assembly while transmission electron microscopy (TEM) revealed the densely packed internal architecture of these assemblies. Powder X-ray diffraction (PXRD) confirmed that the nanoassemblies were exclusively composed of the magnetite phase. The size of crystalline domains present within the nanoassemblies (as determined by the Scherrer equation) could be varied from ∼10 to 25 nm based on synthetic conditions employed. Formation of small nanoparticles precludes the assembly formation that requires ≥8 h to mature. Magnetic measurements revealed very high saturation magnetization values even at room temperature, as well as hysteresis behavior for all the nanoassemblies. Electrochemical measurements showed that Tween 20 derived samples had the highest specific capacitance of 75 F g−1 and their performance was also more stable in cyclic tests compared to Tween 80 derived materials. This is attributed to the greater physical stability of Tween 20 derived nanoassemblies over Tween 80 derived assemblies, as indicated by SEM studies.

Electrochemical formation of FeV(O) and mechanism of its reaction with water during O-O bond formation.

A detailed electrochemical investigation of a series of iron complexes (biuret-modified tetraamido iron macrocycles FeIII -bTAML), including the first electrochemical generation of FeV (O), and demonstration of their efficacy as homogeneous catalysts for electrochemical water oxidation (WO) in aqueous medium are reported. Spectroelectrochemical and mass spectral studies indicated FeV (O) as the active oxidant, formed due to two redox transitions, which were assigned as FeIV (O)/FeIII (OH2 ) and FeV (O)/FeIV (O). The spectral properties of both of these high-valent iron oxo species perfectly match those of their chemically synthesised versions, which were thoroughly characterised by several spectroscopic techniques. The O-O bond-formation step occurs by nucleophilic attack of H2 O on FeV (O). A kinetic isotope effect of 3.2 indicates an atom-proton transfer (APT) mechanism. The reaction of chemically synthesised FeV (O) in CH3 CN and water was directly probed by electrochemistry and was found to be first-order in water. The pKa value of the buffer base plays a critical role in the rate-determining step by increasing the reaction rate several-fold. The electronic effect on redox potential, WO rates, and onset overpotential was studied by employing a series of iron complexes. The catalytic activity was enhanced by the presence of electron-withdrawing groups on the bTAML framework. Changing the substituents from OMe to NO2 resulted in an eightfold increase in reaction rate, while the overpotential increased threefold.

Cobalt phosphonates as precatalysts for water oxidation: Role of pore size in catalysis

We report a simple approach for the synthesis of cobalt phosphonate (CoOP) nanocages with two distinct types of pore diameters by utilizing a novel tetra-constituent assembly of CoCl2 ⋅6 H2 O, nitrilotris(methylene)triphosphonic acid (NMPA), F127 surfactant, and polyvinyl alcohol (PVA, co-surfactant). Transmission electron microscopy images revealed the formation of large nanocages in spheres (pore diameter: 20-60 nm) and the existence of narrow micro/mesopores (pore diameter: 1.5-5 nm) on their walls. Brunauer-Emmett-Teller adsorption/desorption experiments led to the observation of dual porosity and indicated that the contribution of micro/mesopores increased gradually with increasing concentration of PVA during synthesis from CoOP-0 to CoOP-15 (where the number gives the wt % of PVA used in CoOP synthesis). These materials acted as precatalysts for heterogeneous water oxidation at pH 13.9 (1 m KOH) and electrochemical studies revealed that the reactivity improved remarkably with increasing contribution of narrow micro/mesopores. Among these catalysts, the best catalyst (CoOP-15) exhibited an overpotential of 380 mV and turnover frequency of 1.6×10-2  s-1 . The improvement of reactivity was due to significant enhancement of electrochemically accessible surface area with increasing contribution of narrow micro/mesopores, which facilitated contact between the catalyst and water molecules by improving mass transport inside the nanomaterials. Hence, this study suggests narrow micro/mesopores are beneficial towards enhancement of water oxidation catalysis.

Proton conduction through oxygen functionalized few-layer graphene.

The first report of oxygen functionalized few-layer graphene (OFG) having an interlayer distance of 3.6 Å as an excellent proton conductor (8.7 × 10-3 S cm-1 at 80 °C, 95% RH) utilizing hydrophilic oxygen functionalities present at sheet edges bypassing the theoretical limitation of proton conduction through a basal plane. The synthesized OFG also exhibited excellent supercapacitor performance (296 F g-1).

Highly conducting reduced graphene synthesis via low temperature chemically assisted exfoliation and energy storage application

We report a facile approach towards mass production of few layered reduced graphene by low-temperature (160 °C) exfoliation of graphite oxide under ambient atmospheric conditions with the aid of formic acid in a short duration (∼14 min). The obtained reduced graphene showed very high bulk electrical conductivity (1.6 × 103 S cm−1) at room temperature due to restoration of extended conjugation of the sp2 network during rapid exfoliation. Protonation followed by the hydride transfer mechanism has been proposed for the restoration of extended conjugation and high electrical conductivity. BET surface areas of 789 and 1130 m2 g−1 with narrow mesopore distribution (1.9–2.5 nm) were obtained for two different samples prepared by modification in the synthetic methodology. The reduced graphenes were tested as supercapacitors and specific capacitances of 152 and 157 F g−1 with excellent cyclic stability were observed for two samples in aqueous electrolytes.

Importance of Electrode Preparation Methodologies in Supercapacitor Applications

The work reported here aims toward the optimization of electrode preparation methodologies for superior performance of supercapacitors through a rigorous understanding of underlying physical parameters. Oxygen-functionalized few-layer graphene was employed as an active material while binders [Nafion, polyvinylidene fluoride (PVDF), and polytetrafluoroethylene], solvents for active material dispersion [ethylene glycol and N-methyl-2-pyrrolidone (NMP)], and electrode-drying temperatures (100, 170, and 190 °C) were varied. Maximum specific capacitances at different electrode preparation conditions ranged from 240 to 318 F g–1 at 1 mV s–1 scan rate of cyclic voltammetry for the same active material. The study revealed that the electrodes prepared using the PVDF binder, the NMP solvent for active material dispersion, 170 °C electrode-drying temperature (slightly below the boiling temperature of the solvent) provided the best electrochemical performance. Electrochemical impedance spectroscopy revealed that the resistance for electron transfer at the electrode/electrolyte interface can be minimized while mass transport and pseudocapacitive charging can be improved significantly by tuning electrode preparation methodologies which resulted in smaller time constants and hence better capacitor performances. Scanning electron microscopy images revealed that graphene layers were properly stacked much similar to the synthesized nanomaterial wherein better electrochemical performances were achieved, avoiding the agglomeration of nanomaterials on the electrode surface. Low viscosity of the solvent for active material dispersion and better solubility of the binder in the solvent helped to reduce the agglomeration of nanomaterials by minimizing the strong van der Waals interaction which causes agglomeration.