Osmium Bipyridine-Containing Redox Polymers Based on Cellulose and Their Reversible Redox Activity

A π-π Bonding-Assisted Molecular-Wiring of Folded-Cytochrome c and Naphthoquinone and Its Electron-Relay-Based Bioelectrocatalytic H2O2 Reduction Reaction Visualized by Redox-Competitive Scanning Electrochemical Microscopy

The electron-transfer (ET) reaction of cytochrome c (Cytc) protein with biomolecules is a cutting-edge research area of interest in understanding the functionalities of natural systems. Several electrochemical biomimicking studies based on Cytc-protein-modified electrodes prepared via electrostatic interaction and covalent bonding approaches have been reported. Indeed, natural enzymes involve multiple types of bonding, such as hydrogen, ionic, covalent, and π-π, etc. In this work, we explore a Cytc-protein chemically modified glassy carbon electrode (GCE/CB@NQ/Cytc) prepared via π-π bonding using graphitic carbon as an underlying surface and an aromatic organic molecule, naphthoquinone (NQ), as a cofactor for an effective ET reaction. A simple drop-casting technique-based preparation of GCE/CB@NQ showed a distinct surface-confined redox peak at a standard electrode potential (E°) = -0.2 V vs Ag/AgCl (surface excess = 21.3 nmol cm-2) in pH 7 phosphate buffer solution. A control experiment of modification of NQ on an unmodified GCE failed to show any such unique feature. For the preparation of GCE/CB@NQ/Cytc, a dilute solution of Cytc-pH 7 phosphate buffer was drop-cast on the GCE/CB@NQ surface, wherein the protein folding and denaturalization-based complication and its associated ET functionalities were avoided. Molecular dynamics simulation studies show the complexation of NQ with Cytc at the protein binding sites. The protein-bound surface shows an efficient and selective bioelectrocatalytic reduction performance of H2O2, as demonstrated using cyclic voltammetry and amperometric i-t techniques. Finally, the redox-competition scanning electrochemical microscopy (RC-SECM) technique was adopted for in situ visualization of the electroactive adsorbed surface. The RC-SECM images clearly show the regions of highly bioelectrocatalytic active sites of Cytc-proteins bound to NQ molecules on a graphitic carbon surface. The binding of Cytc with NQ has significant implications for studying the biological electron transport mechanism, and the proposed method provides the requisite framework for such a study.

Redox Polyelectrolytes with pH-Sensitive Electroactive Functionality in Aqueous Media

A framework of ferrocene-containing polymers bearing adjustable pH- and redox-active properties in aqueous electrolyte environments was developed. The electroactive metallopolymers were designed to possess enhanced hydrophilicity compared to the vinylferrocene (VFc) homopolymer, poly(vinylferrocene) (PVFc), by virtue of the comonomer incorporated into the macromolecule, and could also be prepared as conductive nanoporous carbon nanotube (CNT) composites that offered a variety of different redox potentials spanning a ca. 300 mV range. The presence of charged non-redox-active moieties such as methacrylate (MA) in the polymeric structure endowed it with acid dissociation properties that interacted synergistically with the redox activity of the ferrocene moieties to impart pH-dependent electrochemical behavior to the overall polymer, which was subsequently studied and compared to several Nernstian relationships in both homogeneous and heterogeneous configurations. This zwitterionic characteristic was leveraged for the enhanced electrochemical separation of several transition metal oxyanions using a P(VFc_0.63-co-MA_0.37)-CNT polyelectrolyte electrode, which yielded an almost twofold preference for chromium as hydrogen chromate versus its chromate form, and also exemplified the electrochemically mediated and innately reversible nature of the separation process through the capture and release of vanadium oxyanions. These investigations into pH-sensitive redox-active materials provide insight for future developments in stimuli-responsive molecular recognition, with extendibility to areas such as electrochemical sensing and selective separation for water purification.

Improved Electrical Wiring of Glucose Oxidase Enzyme with an in-Situ Immobilized Mn(1,10-Phenanthroline)2Cl2-Complex/Multiwalled Carbon Nanotube-Modified Electrode Displaying Superior Performance to Os-Complex for High-Current Sensitivity Bioelectrocatalytic and Biofuel Cell Applications

The search for a new and efficient transducer that can electrically connect enzyme active sites, like flavin adenine dinucleotide in glucose oxidase (GOx), with the electrode surface is a cutting-edge research area. Currently, Os(bpy)-complex pendent polyvinylpyridine/polyvinyl imidazole/pyridinium hydrogel based chemically modified electrodes have been widely used for this purpose (bpy = 2,2'-bipyridine). Herein, we report, a [Mn₂^III(phen)₄(O)(Cl)₂]²⁺ complex/Nafion-immobilized carboxylic acid-functionalized multiwalled carbon nanotube modified glassy carbon electrode (GCE/f-MWCNT@Mn₂(Phen)₄O(Cl)₂-Nf, phen = 1,10-phenanthroline), prepared by an in-situ electrochemical method using the precursor, Mn(phen)₂Cl₂, as an efficient and low cost alternate to the Os-complex transducer, for the glucose oxidase enzyme (GOx) based bio-electro-catalytic system. The existence of the key active site, [Mn₂^III(phen)₄(O)(Cl)₂]²⁺, on the modified electrode was confirmed by physicochemical characterizations using transmission electron microscope, Raman, infrared, and UV-vis spectroscopes and electrospray ionization mass spectrometry techniques. The Mn-complex modified electrode showed a redox peak at E°' = 0.55 V vs Ag/AgCl in neutral solution with a surface excess (Γ_Mn) value of 5.6 × 10^-9 mol cm^-2. The GOx enzyme bioanode prepared by adsorbing GOx on the Mn-complex modified electrode has shown an efficient bioelectrocatalytic oxidation of glucose with a Tafel slope value of 111 mV dec^-1. Amperometric i-t analysis of glucose showed a calibration plot in a linear range of 50-550 μM and with current sensitivity of 316.7 μA mM^-1 cm^-2. The current sensitivity value obtained here is about 2-80 000 times higher than that of the Os(bpy)-complex based transducers used for GOx based bio-electro-catalytic applications. Utilizing this new bioanode system along with a Pt-based oxygen reduction electrode, a new biofuel cell was constructed and achieved a power density value 7.5 μW cm^-2.

Organometallic polymers for electrode decoration in sensing applications

Macromolecules containing metals combine the processing advantages of polymers with the functionality offered by the metal centers. The developments in the area of electrochemical chemo/biosensors based on organometallic polymers are reviewed.