Electrochemical Reaction Assisted 2D π-Stacking of Benzene on a MWCNT Surface and its Unique Redox and Electrocatalytic Properties.

Turning the π-structure and electronic properties of carbon nanotubes (CNTs) is a cutting-edge research topic in interdisciplinary areas of material chemistry. In general, chemical functionalization of CNT has been adopted for this purpose, which has resulted in a few monolayer thickness increment of CNT diameter size. Herein, we report an interesting observation of >10-fold increment in the apparent diameter of multiwalled carbon nanotubes (MWCNTs) brought about by a process of self-assembly of the BZ moiety on MWCNT, which is formed by electrochemical oxidation of a surface-adsorbed benzene-water cluster, {BZ-nH2O}. From physicochemical characterizations by transmission electron microscopy (TEM) and Raman and IR spectroscopic techniques and electrochemical characterizations by several radical scavenger species, it has been revealed that benzene radical moieties as a series of π-stacked layers ([BZ]-π-stack) were self-assembled on the MWCNT surface. A possible mechanism for their formation was proposed to be electrochemical oxidation of H2O from the MWCNT@{BZ-nH2O}ads layer to oxygen gas via hydroxyl radical formation and benzene cationic radical species at 1.2 V vs Ag/AgCl followed by its self-assembly into a unique MWCNT@[BZ]-π-stack network. The scanning electrochemical microscopic (SECM) technique was used to identify the in situ •OH radical formation. The electrochemical studies of a glassy-carbon-modified MWCNT@[BZ]-π-stack system showed a well-defined and highly symmetrical redox peak at an equilibrium potential E1/2 = 0.2 V vs Ag/AgCl (pH 2 HCl/KCl), with a peak-to-peak potential separation of 0 V, highlighting the ideal-surface-confined electron-transfer nature of the redox couple. Furthermore, enhanced electrical conductivity over the unmodified MWCNT was observed when testing the surface-sensitive redox couple Fe3+/Fe2+ with the modified electrode. This new redox material showed a specific electrocatalytic reduction of hydrogen peroxide at neutral pH (pH 7 phosphate buffer solution) unlike the quinone and other organic redox mediators, which show the reduction signal only in the presence of horseradish peroxidase enzyme.

Automatic Detection of the Optic Disc of the Retina: A Fast Method.

Localizing the optic disc (OD) in retinal fundus images is of critical importance and many techniques have been developed for OD detection. In this paper, we present the results obtained from two fast methods, correlation and least square, to approximate the location of optic cup. These methods are simple and are not complex, while most of the OD detection algorithms are. The methods were tested on two groups of data (a total of 100 color fundus images) and were 98% successful in the detection of the optic cup. An algorithm using the vessel mask of fundus images is proposed to be run after correlation to ensure that the localization of OD in all images is successful. It was tested on 40 of the test images and had a 100% rate of success.

In situ fabrication of electrochemically grown mesoporous metallic thin films by anodic dissolution in deep eutectic solvents.

We describe here a simple electrodeposition process of forming thin films of noble metallic nanoparticles such as Au, Ag and Pd in deep eutectic solvents (DES). The method consists of anodic dissolution of the corresponding metal in DES followed by the deposition on the cathodic surface. The anodic dissolution process in DES overcomes the problems associated with copious hydrogen and oxygen evolution on the electrode surface when carried out in aqueous medium. The proposed method utilizes the inherent abilities of DES to act as a reducing medium while simultaneously stabilizing the nanoparticles that are formed. The mesoporous metal films were characterized by SEM, XRD and electrochemical techniques. Potential applications of these substrates in surface enhanced Raman spectroscopy and electrocatalysis have been investigated. A large enhancement of Raman signal of analyte was achieved on the mesoporous silver substrate after removing all the stabilizer molecules from the surface by calcination. The highly porous texture of the electrodeposited film provides superior electro catalytic performance for hydrogen evolution reaction (HER). The mechanisms of HER on the fabricated substrates were studied by Tafel analysis and electrochemical impedance spectroscopy (EIS).

Mutually ordered self-assembly of discotic liquid crystal-graphene nanocomposites.

The room temperature anthraquinone discotic 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-dimethyloctyloxy)-9,10-anthraquinone (RTAQ) self-assembles in the presence of octadecylamine functionalized graphene (f-graphene) into an ordered sandwich like structure, where the discotic molecules form columnar structures on graphene sheets. Cryo-SEM and SEM images provide evidence for this ordering. This behaviour is also supported by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and conductivity studies of nanocomposites.

Gold nanorods embedded discotic nanoribbons.

It is shown that gold nanorods (GNRs) can be easily inserted into the supramolecular order of discotic liquid crystals (DLCs) along the director without disturbing their mesomorphism. GNRs embedded discotic nanoribbons, useful for constructing devices like thin film transistors, sensors, etc., are prepared by simple solution processing of GNR-DLC nanocomposites.

Electro-catalytic activity of multiwall carbon nanotube-metal (Pt or Pd) nanohybrid materials synthesized using microwave-induced reactions and their possible use in fuel cells.

Microwave induced reactions for immobilizing platinum and palladium nanoparticles on multiwall carbon nanotubes are presented. The resulting hybrid materials were used as catalysts for direct methanol, ethanol and formic acid oxidation in acidic as well as alkaline media. The electrodes are formed by simply mixing the hybrids with graphite paste, thus using a relatively small quantity of the precious metal. We report Tafel slopes and apparent activation energies at different potentials and temperatures. Ethanol electro-oxidation with the palladium hybrid showed an activation energy of 7.64 kJmol(-1) which is lower than those observed for other systems. This system is economically attractive because Pd is significantly less expensive than Pt and ethanol is fast evolving as a commercial biofuel.

Electron transfer studies of redox probes in bovine milk.

In this work, we show that milk can act as an electrolytic medium to study electrochemical processes in the absence of any supporting electrolyte. The electron transfer properties of three different redox systems in bovine homogenized whole milk, skimmed milk, and reconstituted milk powder have been studied by cyclic voltammetry and impedance spectroscopy using a three-electrode system with a gold disk working electrode, a platinum sheet counter electrode, and a standard calomel reference electrode. It has been shown that the milk incredibly sustains the redox reactions in the absence of any supporting electrolyte and the electrochemical responses are comparable to those obtained when the same reactions were carried out in standard solvent preparations containing supporting electrolytes. The study clearly demonstrates the potential of developing new innovative techniques based on the intricate concepts of electrochemistry to study various aspects of milk that may help in the development of analytical sensors for the diary industry.

Microemulsion phase as a medium for electrodeposition of nickel and electron-transfer study of ferrocyanide-ferricyanide redox system.

We report our electrochemical studies in a W/O microemulsion phase consisting of a ternary mixture of water, Triton X-100, and toluene. The microemulsion phase plays the dual role of a template in the electrodeposition of nickel and as an electrolytic medium in the study of electron-transfer kinetics. The nickel electrodeposits obtained using this microemulsion phase as a template were characterized by surface analysis techniques such as scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies while cyclic voltammetry (CV) was used to determine the electro-active true surface area of the template-deposited nickel. For electron-transfer studies, CV and electrochemical impedance spectroscopy were employed using potassium ferro/ferricyanide as a redox probe. In contrast to the diffusion-controlled process of the redox probe in aqueous medium, a charge-transfer control was observed in the W/O microemulsion phase. We found that the rate constant value for this particular redox reaction in the microemulsion phase is decreased by about four orders of magnitude when compared to the corresponding value in aqueous medium. The observed phenomenon has been correlated to the structure of the W/O microemulsion phase at the interface, exhibiting a microelectrode array behavior.

Novel benzene-bridged triphenylene-based discotic dyads.

We report the synthesis and characterization of two series of novel triphenylene-based benzene-bridged symmetric discotic dimers. Two triphenylene discotics have been connected to a rigid benzene ring via flexible methylene spacers. In one series, triphenylene moiety was tethered with benzene via an ester linkage, while in the second series it is via an ether linkage. Within each series, the orientation of the linkage of the triphenylene core around the benzene core has been changed by substituting the benzene ring at o-, m-, and p-positions. These materials have been characterized from their spectral and elemental analysis. The thermotropic liquid crystalline properties were investigated by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction studies. All the virgin compounds do not display any mesomorphism; however, their charge transfer complexes with trinitrofluorenone, an electron acceptor, exhibited columnar mesophases. The direct current (dc) conductivity of charge transfer complexes at two ratios has been studied at variable temperature. The small conductivity value demonstrates that long spacers as well as connectivity to the rigid benzene ring dilute the column packing, hence making pi-pi interaction less efficient for the entire column length. A hexagonal assembly of triphenylene-benzene dimer 12 on a highly oriented pyrolytic graphite (HOPG) surface has been visualized with scanning tunneling microscopy (STM).

Electrical conductivity studies on discotic liquid crystal-ferrocenium donor-acceptor systems.

The dispersion of electron-deficient ferrocenium ions was studied in the electron-rich media of two different triphenylene-based columnar hexagonal liquid-crystalline phases. These composites were characterized using polarizing optical micrography (POM), differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), visible absorption spectroscopy, and dc and ac conductivity measurements. It was found that these composites form donor-acceptor systems that enhance the quasi-one-dimensional conductivity of the discotic system without altering the hexagonal columnar mesophase. The absorbance spectra confirm the formation of a charge-transfer complex between the electron-rich discotic molecules and the electron-deficient ferrocenium ions.

Electrochemical synthesis of thiol-monolayer-protected clusters of gold.

We have synthesized for the first time thiol-monolayer-protected gold nanoparticles (MPCs) by the process of electrochemical dissolution of gold in an ethanol-water mixture. The MPCs have been formed both with and without NaBH4 as a reducing agent. The well-dispersed thiol-capped MPCs were characterized by UV-visible absorption and transmission electron microscopy (TEM) studies.

Electron transfer studies on cholesterol LB films assembled on thiophenol and 2-naphthalenethiol self-assembled monolayers.

We have formed the cholesterol monolayer and multilayer LB films on the self-assembled monolayers of 2-naphthalenethiol (2-NT) and thiophenol (TP) and studied the electrochemical barrier properties of these composite films using cyclic voltammetry and electrochemical impedance spectroscopy. We have also characterized the cholesterol monolayer film using grazing angle FTIR, scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Cholesterol has a long hydrophobic steroid chain, which makes it a suitable candidate to assemble on the hydrophobic surfaces. We find that the highly hydrophobic surface formed by the self-assembled monolayers (SAM) of 2-NT and TP act as effective platforms for the fabrication of cholesterol monolayer and multilayer films. The STM studies show that the cholesterol monolayer films on 2-NT form striped patterns with a separation of 1.0 nm between them. The area per cholesterol molecule is observed to be 0.64 nm2 with a tilt angle of about 28.96 degrees from the surface normal. The electrochemical studies show a large increase in charge transfer resistance and lowering of interfacial capacitance due to the formation of the LB film of cholesterol. We have compared the behavior of this system with that of cholesterol monolayer and multilayers formed on the self-assembled monolayer of thiophenol.

Dispersion of thiol stabilized gold nanoparticles in lyotropic liquid crystalline systems.

A new method of forming stable dispersions of alkanethiol and aromatic thiol stabilized gold nanoparticles in two different lyotropic liquid crystalline mediums, namely, a columnar hexagonal phase made up of a Triton X-100/water system and an inverse columnar hexagonal phase made up of pure AOT, are presented. The dispersions have been characterized using small-angle X-ray scattering (SAXS) and polarizing optical microscopy. Our studies show that the gold nanoparticles are distributed outside the columns formed by both the surfactants. Such dispersions can find applications in the study of nanoparticles as well as in the development of devices based on some unique properties of metal nanoparticles.

Electron-transfer studies in a lyotropic columnar hexagonal liquid crystalline medium.

We have studied the electron-transfer properties of some redox systems on a gold electrode in a lyotropic hexagonal columnar liquid crystalline phase (H1 phase) using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The liquid crystalline medium consists of the nonionic surfactant Triton X-100 and water. The redox reactions that have been studied are ferrocene/ferricenium, [Fe(CN)6]3-/4-, and [Ru(NH3)6]3+/2+. We have confirmed by polarizing optical microscopy that the liquid crystalline nature of the medium is maintained even after the addition of the redox species and the supporting electrolyte. The CV studies show a significant shift in the half-peak potentials of these redox reactions in the liquid crystalline medium. From the EIS studies, we have measured the diffusion coefficients and the kinetic parameters for the redox species. These results are discussed and compared with the values obtained in the conventional solvent medium. The CV and impedance studies demonstrate that the hexagonal columnar phase provides a novel controlled environment for the study of electron-transfer reactions in biological and physiological media.

Simultaneous purification and spectrophotometric determination of nickel present in as-prepared single-walled carbon nanotubes (SWCNT).

The quality of single-walled nanotubes (SWCNT) is usually assessed by means of electron microscopic techniques or Raman spectroscopy. However, these sophisticated techniques are not widely available and do not reliably estimate the impurities in highly heterogeneous samples containing metal particles, fullerenes and other carbonaceous materials. We have developed a simple, inexpensive and convenient spectrophotometric method to assess the purity of arc-discharge grown as-prepared SWCNT. Purification process consists of initial gas phase oxidation and refluxing with nitric acid at the optimal conditions including short time period during acid refluxing. We have shown that this method could remove the metal particles effectively with a good yield of high quality SWCNTs, as shown by the spectrophotometric and scanning tunneling microscope studies described here. The extent of removal of the nickel present in as-prepared carbon nanotube sample is followed by spectrophotometeric analysis of the dissolved nickel analyte. The composition of nickel in the SWCNT sample is found to be 17.56%. The method is based on the chelating of Ni(2+) with dimethylglyoxime in ammoniacal citrate medium to form nickel dimethylglyoxime complex. A second stage purification of SWCNT eliminates the residual metal particles. The purified SWCNT has been studied using scanning tunneling microscopy which shows clearly resolved individual carbon nanotubes.

Novel conducting nanocomposites: synthesis of triphenylene-covered gold nanoparticles and their insertion into a columnar matrix.

Gold nanoparticles fully covered with triphenylene-based discotic liquid crystals (DLCs) were synthesized and dispersed in a columnar matrix. The thermophysical properties of these nanocomposites, studied using polarizing optical microscopy, differential scanning calorimetry and small angle X-ray diffraction studies (SAXS), confirm their insertion into the columnar matrix. The presence of the gold nanoparticles in the triphenylene-based DLCs does not disturb the nature of the mesophase other than altering the transition temperatures. We propose that the gold nanoparticles are distributed between the domain gaps of the DLCs in random disordered manner. Interestingly the DC conductivity measurements show an enhancement of the electrical conductivity by more than a million times upon doping of the discotic liquid crystals with the triphenylene-capped nanoparticles under ambient conditions.

Self-assembled monolayers of alkanethiols on gold prepared in a hexagonal lyotropic liquid crystalline phase of Triton X-100/water system.

In this paper, we have reported a new method of preparing self-assembled monolayers (SAMs) of decanethiol and hexadecanethiol on gold surface by using a lyotropic liquid crystalline phase as an adsorbing medium. The stability and blocking ability of these SAMs were characterized using grazing angle Fourier transform infrared (FTIR) spectroscopy and electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy. The lyotropic liquid crystalline medium possesses a hexagonal structure consisting of a nonionic surfactant Triton X-100, water, and the corresponding thiol, which provides a highly hydrophobic environment to solubilize the alkanethiols and later to facilitate their delivery to the gold surface. We find that the SAMs formed from the hexagonal liquid crystalline phase are highly compact and have excellent electrochemical blocking ability towards the redox probes compared to conventional SAMs prepared from commonly used organic solvents such as ethanol. From the impedance studies, we have determined the capacitance of the monolayer-coated electrodes and the surface coverage of the SAM, which has been found to be >99.98% on gold surface. We have also estimated the extent of ionic permeability through the film and measured the rate constants for the redox reactions on the SAM-modified electrodes. Our results show that the rate constants of [Fe(CN)6](3-/4-) and [Ru(NH3)6](2+/3+) redox couples are very much lower in the case of monolayers prepared in liquid crystalline phase compared to the SAM formed in 1 mM thiol in ethanol solution, suggesting a better blocking ability of the SAMs in the former case. From the grazing angle FTIR spectroscopic studies and capacitance measurements, we have ruled out any coadsorption of surfactant molecules on the Au surface. These results suggest that SAMs of very low defect density and extremely low ionic permeability can be obtained when a hexagonal lyotropic liquid crystalline phase is used as an adsorbing medium.

Self-assembled monolayers (SAMs) of alkoxycyanobiphenyl thiols on gold--a study of electron transfer reaction using cyclic voltammetry and electrochemical impedance spectroscopy.

Self-assembled monolayers (SAMs) of liquid crystalline thiol-terminated alkoxycyanobiphenyl molecules with different alkyl chain lengths on Au surface have been studied for the first time using electrochemical techniques such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The barrier property of the SAM-modified surfaces was evaluated using two different redox probes, namely potassium ferro/ferri cyanide and hexaammineruthenium(III) chloride. It was found that for short-length alkyl chain thiol (C5) the electron transfer reaction of hexaammineruthenium(III) chloride takes place through tunneling mechanism. In contrast, redox reaction of potassium ferro/ferri cyanide is almost completely blocked by the SAM-modified Au surface. From the impedance data, a surface coverage value of >99.9% was calculated for all the thiol molecules.

Scanning tunneling microscopy, Fourier transform infrared spectroscopy, and electrochemical characterization of 2-naphthalenethiol self-assembled monolayers on the Au surface: a study of bridge-mediated electron transfer in Ru(NH3)6(2+)/Ru(NH3)6(3+) redox reactions.

We have studied the structure, adsorption kinetics, and barrier properties of self-assembled monolayers of 2-naphthalenethiol on Au using electrochemical techniques, grazing-angle Fourier transform infrared (FTIR) spectroscopy, and scanning tunneling microscopy (STM). The results of cyclic voltammetric and impedance measurements using redox probes show that 2-naphthalenethiol on Au forms a stable and reproducible, but moderately blocking, monolayer. Annealing of the self-assembled monolayer (SAM)-modified surface at 72 +/- 2 degrees C remarkably improves the blocking property of the monolayer of 2-naphthalenethiol on Au. From the study of kinetics of SAM formation, we find that the self-assembly follows Langmuir adsorption isotherm. Our STM and FTIR results show that the molecules are adsorbed with the naphthalene ring tilted from the surface normal by forming a square root 3 x 3 R30 degrees overlayer structure. From our studies, we conclude that the electron-transfer reaction of ferro/ferricyanide in the freshly formed monolayer occurs predominantly through the pinholes and defects present in the monolayer. However, in the case of thermally annealed specimen, although the ferro/ferricyanide reaction is almost completely blocked, the electron-transfer reaction of hexaammineruthenium(III) chloride is not significantly inhibited. It is proposed that the electron-transfer reaction in the case of the ruthenium redox couple takes place by a tunneling mechanism through the high-electron-density aromatic naphthalene ring acting as a bridge between the monolayer-modified electrode and the ruthenium complex.

Inclusion of gold nanoparticles into a discotic liquid crystalline matrix.

The thermophysical properties of mixtures of hexanethiolate-capped gold nanoparticles and three types of discotic liquid crystals, investigated using polarizing optical microscopy, differential scanning calorimetry and DC conductivity, indicate inclusion of gold nanoparticles into a matrix of triphenylene-based discotic liquid crystals.