Francis D’Souza

Electrochemically synthesized polymers in molecular imprinting for chemical sensing

This critical review describes a class of polymers prepared by electrochemical polymerization that employs the concept of molecular imprinting for chemical sensing. The principal focus is on both conducting and nonconducting polymers prepared by electropolymerization of electroactive functional monomers, such as pristine and derivatized pyrrole, aminophenylboronic acid, thiophene, porphyrin, aniline, phenylenediamine, phenol, and thiophenol. A critical evaluation of the literature on electrosynthesized molecularly imprinted polymers (MIPs) applied as recognition elements of chemical sensors is presented. The aim of this review is to highlight recent achievements in analytical applications of these MIPs, including present strategies of determination of different analytes as well as identification and solutions for problems encountered.

Selective Histamine Piezoelectric Chemosensor Using a Recognition Film of the Molecularly Imprinted Polymer of Bis(bithiophene) Derivatives

A histamine piezoelectric (acoustic) sensor using a molecularly imprinted polymer (MIP) film has been devised and tested. The sensor comprises an electrodeposited MIP film as the recognition element and a 10 MHz AT-cut shear-thickness-mode bulk-acoustic-wave quartz crystal resonator with Pt film electrodes as the signal transducer. Preparation of the sensing film involved two consecutive electrochemical polymerizations, performed under cyclic voltammetric conditions, with the use of a supporting electrolyte of 0.1 M tetra-n-butylammonium perchlorate in acetonitrile. First, a poly(bithiophene) barrier film was deposited by electropolymerization on the Pt/quartz resonator to prevent histamine electro-oxidation and avoid possible contamination of the Pt electrode surface. Next, the histamine-templated MIP film was deposited by electropolymerization on top of this barrier film. For that purpose, two functional monomers of bis(bithiophene) derivatives, i.e., one bearing the 18-crown-6 and the other dioxoborinane substituent, were copolymerized in the presence of the histamine template. The consecutive growth of both these overlaid films was monitored with an electrochemical quartz crystal microbalance (EQCM). Subsequently, the histamine was extracted from MIP with 0.01 M NaOH for 12 h. The UV-vis and X-ray photoelectron spectroscopic measurements confirmed the completeness of the removal of the histamine template from the MIP film. The analytical performance of the chemosensor was assessed under flow injection analysis (FIA) conditions using the carrier 0.5 M HEPES buffer (pH = 7.5) solution and the piezoelectric microgravimetry detection at QCM. The negative peaks of resonant frequency linearly decreased with the increase of the histamine concentration in the range 10-100 mM for 150 microL/min flow rate, and 100 microL volume of the injected sample. The sensitivity of the chemosensor (0.33 Hz/mM) was more than twice as that of the chemosensor without the poly(bithiophene) barrier film (0.15 Hz/mM). The chemosensor performance was superior for selective histamine recognition if the poly(bithiophene) barrier film thickness exceeded 200 nm. The chemosensor discriminated histamine from functionally or structurally similar compounds, such as dopamine, tryptamine, and imidazole. Stability constants of the affinity complexes of MIP and analyte or the interfering agent were determined from kinetic studies. For the MIP-histamine complex, the stability constant thus evaluated was equal to 57.0 M(-1) being much higher than those for the MIP-tryptamine and MIP-dopamine complexes determined to be 10.7, and 6.4 M(-1), respectively. The concentration limit of detection was as low as 5 nM histamine if the carrier solution flow rate was as low as 35 microL/min and the injection sample volume as large as 1 mL.

Melamine Acoustic Chemosensor Based on Molecularly Imprinted Polymer Film

A melamine piezomicrogravimetric (acoustic) chemosensor using a molecularly imprinted polymer (MIP) film has been devised and tested. The MIP films were prepared by electropolymerization of the melamine complexed by the functional monomer of the bis(bithiophene) derivative bearing an 18-crown-6 substituent 4. The structure of the MIP-melamine complex was visualized by the DFT B3LYP/3-21G(*) energy optimization calculations. The sensitivity and selectivity of the MIP film was improved by cross-linking the polymer with the bithianaphthene monomer 5 and the presence of the porogenic ionic liquid in the prepolymerization solution. After electropolymerization, the melamine template was extracted from the MIP film with an aqueous strong base solution. The measurements of UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), DPV, and EIS as well as scanning electrochemical microscopy (SECM) imaging confirmed extraction of the melamine template from the MIP film and then rebinding of the melamine analyte while the film relative roughness and porosity was determined by atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging, respectively. The analytical as well as kinetic and thermodynamic parameters of the chemosensing were assessed under flow-injection analysis (FIA) conditions with piezoelectric microgravimetry (PM) detection. The linear concentration range for melamine detection was 5 nM to at least 1 mM with a limit of detection of approximately 5 nM. The chemosensor successfully discriminated the cyanuric acid, cyromazine, and ammeline interfering agents.

Phenothiazine-sensitized organic solar cells: effect of dye anchor group positioning on the cell performance.

Effect of positioning of the cyanoacrylic acid anchoring group on ring periphery of phenothiazine dye on the performance of dye-sensitized solar cells (DSSCs) is reported. Two types of dyes, one having substitution on the C-3 aromatic ring (Type 1) and another through the N-terminal (Type 2), have been synthesized for this purpose. Absorption and fluorescence studies have been performed to visualize the effect of substitution pattern on the spectral coverage and electrochemical studies to monitor the tuning of redox levels. B3LYP/6-31G* studies are performed to visualize the frontier orbital location and their significance in charge injection when surface modified on semiconducting TiO₂. New DSSCs have been built on nanocrystalline TiO₂ according to traditional two-electrode Grätzel solar cell setup with a reference cell based on N719 dye for comparison. The lifetime of the adsorbed phenothiazine dye is found to be quenched significantly upon immobilizing on TiO₂ suggesting charge injection from excited dye to semiconducting TiO₂. The performances of the cells are found to be prominent for solar cells made out of Type 1 dyes compared to Type 2 dyes. This trend has been rationalized on the basis of spectral, electrochemical, computational, and electrochemical impedance spectroscopy results.

Porphyrin-Sensitized Solar Cells: Effect of Carboxyl Anchor Group Orientation on the Cell Performance

The effect of the orientation of the porphyrin sensitizer onto the TiO2 surface on the performance of dye-sensitized solar cells (DSSCs) is reported. Free-base and zinc porphyrins bearing a carboxyl anchoring group at the para, meta, or ortho positions of one of the meso-phenyl rings were synthesized for application in Grätzel-type photoelectrochemical cells. The remainder of the meso-phenyl rings was substituted with alkyl chains of different length to visualize any aggregation effects. Absorption and fluorescence studies were performed to characterize and observe spectral coverage of the thirteen newly synthesized porphyrin derivatives. Photoelectrochemical studies were performed after immobilization of porphyrins onto nanocrystalline TiO2 and compared with DSSC constructed using N719 dye as reference. The performance of DSSCs with the porphyrin anchoring at the para or meta position were found to greatly exceed those with the anchoring group in the ortho position. Additionally, cells constructed using zinc porphyrin derivatives outperformed the free-base porphyrin analogs. Better dye regeneration efficiency for the zinc porphyrin derivatives compared to their free-base porphyrin analogs, and for the meta and para derivatives over the ortho derivatives was evaluated from electrochemical impedance spectroscopy studies. Femtosecond transient absorption spectroscopy studies were performed to probe the kinetics of charge injection and charge recombination with respect to the orientation of porphyrin macrocycle on TiO2 surface. The ortho porphyrin derivative with an almost flat orientation to the TiO2 surface revealed fast charge recombination and suggested occurrence of through-space charge transfer. The overall structure-performance trends observed for the present porphyrin DSSCs have been rationalized based on spectral, electrochemical, electrochemical impedance spectroscopy, and transient spectroscopy results.

Comparison of Amorphous Iridium Water-Oxidation Electrocatalysts Prepared from Soluble Precursors

Electrodeposition of iridium oxide layers from soluble precursors provides a route to active thin-layer electrocatalysts for use on water-oxidizing anodes. Certain organometallic half-sandwich aqua complexes of iridium form stable and highly active oxide films upon electrochemical oxidation in aqueous solution. The catalyst films appear as blue layers on the anode when sufficiently thick, and most closely resemble hydrous iridium(III,IV) oxide by voltammetry. The deposition rate and cyclic voltammetric response of the electrodeposited material depend on whether the precursor complex contains a pentamethylcyclopentadieneyl (Cp*) or cyclopentadienyl ligand (Cp), and do not match, in either case, iridium oxide anodes prepared from non-organometallic precursors. Here, we survey our organometallic precursors, iridium hydroxide, and pre-formed iridium oxide nanoparticles. From electrochemical quartz crystal nanobalance (EQCN) studies, we find differences in the rate of electrodeposition of catalyst layers from the two half-sandwich precursors; however, the resulting layers operate as water-oxidizing anodes with indistinguishable overpotentials and H/D isotope effects. Furthermore, using the mass data collected by EQCN and not otherwise available, we show that the electrodeposited materials are excellent catalysts for the water-oxidation reaction, showing maximum turnover frequencies greater than 0.5 mol O(2) (mol iridium)(-1) s(-1) and quantitative conversion of current to product dioxygen. Importantly, these anodes maintain their high activity and robustness at very low iridium loadings. Our organometallic precursors contrast with pre-formed iridium oxide nanoparticles, which form an unstable electrodeposited material that is not stably adherent to the anode surface at even moderately oxidizing potentials.

Development of Nanopatterned Fluorine-Doped Tin Oxide Electrodes for Dye-Sensitized Solar Cells with Improved Light Trapping

Transparent conductors (TCs) are an important component of optoelectronic devices and nanoscale engineering of TCs is important for optimization of the device performance through improved light trapping. In this work, patterned periodic arrays of nanopillars and nanolines of pitch size of ~700 nm were created on fluorine-doped tin oxide (FTO) using nanoimprint lithography and reactive ion etching using environmentally friendly gases. The patterned FTO exhibits enhanced light trapping as compared to the unpatterned FTO, which agrees well with simulations based on Finite-Difference Time-Domain method for up to a distance of 4 μm. Dye sensitized solar cells (DSSCs) fabricated on the patterned FTO exhibited improved performance (fill factor and power conversion efficiency), which can be attributed to enhanced light absorption in the range 400-650 nm. Further, electrochemical impedance measurements revealed lower recombination resistance for the patterned FTO/TiO(2) electrode compared to the unpatterned FTO electrode/TiO(2) electrode as a result of better light capturing properties of patterned FTO. The direct fabrication of nanopatterns on TCs developed in the present study is expected to be a viable scheme for achieving improved performance in many other optoelectronic devices.

Thieno-Pyrrole-Fused 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene–Fullerene Dyads: Utilization of Near-Infrared Sensitizers for Ultrafast Charge Separation in Donor–Acceptor Systems

Donor-acceptor dyads featuring near-IR sensitizers derived from thieno-pyrrole-fused BODIPY (abbreviated as SBDPiR) and fullerene, C60 have been newly synthesized and characterized. Occurrence of ultrafast photoinduced electron transfer (PET) leading to the formation of charge-separated state in these dyads, capable of harvesting light energy from the near-IR region, is established from femtosecond transient absorption studies.

Two-Component Films of Fullerene and Palladium as Materials for Electrochemical Capacitors

The redox-active films, C 60 /Pd, formed by electrochemical reduction of solutions containing palladium(II) acetate and C 60 fullerene have been studied as active components for electrochemical capacitors. The capacitance properties of these materials have been investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The structure and electrochemical properties of films deposited on an electrode surface depend on the composition of the solution from which they are grown. Films formed in a solution with a low concentration of palladium(II) acetate exhibit conductivity in the potential range of the film reduction. The faradaic process of C 60 reduction gives rise to pseudocapacitance. The capacitance of this polymer depends on the solvent and the size of the cations in the supporting electrolyte. For an acetonitrile solution containing only tetra(methyl)ammonium perchlorate, the film displays a high specific capacitance, close to 300 F/g. Films formed in a solution with a high concentration of palladium(II) acetate also contain metallic palladium nanoparticles. Such systems exhibit conductivity at potentials less negative than the potentials for film reduction, and these films can be considered as double-layer capacitors. The specific capacitance of these films is much smaller (about 20 F/g) but a large potential window (from +800 to-2000 mV) is available for the performance of these capacitors.

Syntheses, Charge Separation, and Inverted Bulk Heterojunction Solar Cell Application of Phenothiazine–Fullerene Dyads

A series of phenothiazine-fulleropyrrolidine (PTZ-C60) dyads having fullerene either at the C-3 aromatic ring position or at the N-position of phenothiazine macrocycle were newly synthesized and characterized. Photoinduced electron transfer leading to PTZ(•+)-C60(•-) charge-separated species was established from studies involving femtosecond transient absorption spectroscopy. Because of the close proximity of the donor and acceptor entities, the C-3 ring substituted PTZ-C60 dyads revealed faster charge separation and charge recombination processes than that observed in the dyad functionalized through the N-position. Next, inverted organic bulk heterojunction (BHJ) solar cells were constructed using the dyads in place of traditionally used [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) and an additional electron donor material poly(3-hexylthiophene) (P3HT). The performance of the C-3 ring substituted PTZ-C60 dyad having a polyethylene glycol substituent produced a power conversion efficiency of 3.5% under inverted bulk heterojunction (BHJ) configuration. This was attributed to optimal BHJ morphology between the polymer and the dyad, which was further promoted by the efficient intramolecular charge separation and relatively slow charge recombination promoted by the dyad within the BHJ structure. The present finding demonstrate PTZ-C60 dyads as being good prospective materials for building organic photovoltaic devices.