π-Self-Assembly of a Coronene on Carbon Nanomaterial-Modified Electrode and Its Symmetrical Redox and H2O2 Electrocatalytic Reduction Functionalities

The structure-electroactivity relationship of graphene has been studied using coronene (Cor), polyaromatic hydrocarbon (PAH), and a subunit of graphene as a model system by chemically modified electrode approach. In general, graphene and PAH do not show any redox activity in their native form. Herein, we report a simple electrochemical approach for the conversion of electro-inactive coronene to a highly redox-active molecule (Cor-Redox; E°' = 0.235 ± 0.005 V vs Ag/AgCl) after being adsorbed on graphitic carbon nanomaterial and preconditioned at an applied potential, 1.2 V vs Ag/AgCl, wherein, the water molecule oxidizes to dioxygen via hydroxyl radical (^•OH) intermediate, in acidic solution (pH 2 KCl-HCl). When the same coronene electrochemical experiment was carried out on an unmodified glassy carbon electrode, there was no sign of faradic signal, revealing the unique electrochemical behavior of the coronene molecule on graphitic nanomaterial. The Cor-Redox peak is found to be highly symmetrical (peak-to-peak potential separation of ∼0 V tested by cyclic voltammetry (CV)) and surface-confined (Γ_Cor-Redox = 10.1 × 10^-9 mol cm^-2) and has proton-coupled electron-transfer (∂E°'/∂pH = -56 mV pH^-1) character. Initially, it was speculated that Cor is converted to a hydroxy group-functionalized Cor molecule (dihydroxy benzene derivative) on the graphitic surface and showed the electrochemical redox activity. However, physicochemical characterization studies including Raman, IR, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), redox-site selective oxidation probe, cysteine (for dihydroxy benzene), radical scavenger ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl, TEMPO), and scanning electrochemical microscopy (SECM) using ferricyanide redox couple have revealed that coronene cationic radical species like electroactive molecule is formed on graphitic material upon the electrochemical oxidation reaction at a high anodic potential. It has been proposed that ^•OH generated as an intermediate species from the water oxidation reaction is involved in the coronene cationic radical species. Studies on coronene electrochemical reaction at various carbon nanomaterials like multiwalled carbon, single-walled carbon, graphite, graphene oxide, and carbon nanofiber revealed that graphitic structure (without any oxygen functional groups) and its π-π bonding are key factors for the success of the electrochemical reaction. The coronene molecular redox peak showed an unusual electrocatalytic reduction of hydrogen peroxide similar to the peroxidase enzyme-biocatalyzed reduction reaction in physiological solution.

Molecular wiring of glucose oxidase enzyme with Mn polypyridine complex on MWCNT modified electrode surface and its bio-electrocatalytic oxidation and glucose sensing

A simple method for molecular wiring of glucose oxidase (GOx) enzyme with a low cost Mn polypyridine complex, Mn(phen)₂Cl₂, carboxylic acid functionalized multiwalled carbon nanotube (f-MWCNT) and Nafion (Nf), which is useful for glucose oxidation and sensing application in pH 7 phosphate buffer solution, has been demonstrated. In the typical preparation, f-MWCNT, Mn(phen)₂Cl₂, Nafion and GOx solution/suspension were successfully drop-casted as layer-by-layer on a cleaned glassy carbon electrode and potential cycled using cylic voltametric (CV) technique. In this preparation procedure, the Mn(phen)₂Cl₂ complex is in-situ converted as a dimer complex, Mn₂(phen)₂(O)(Cl₂). A cooperative interaction based on π-π, covalent, ionic, hydrophilic and hydrophobic are operated in the bioelectrode for molecular wiring and electron-transfer shutting reaction. The modified electrode is designated as GCE/f-MWCNT@Mn₂(phen)₂(O)(Cl₂)-Nf@GOx. CV response of the bioelectrode showed a defined redox peak current signal at an apparent standard electrode potential, E^°'=0.55V vs Ag/AgCl. Upon exposure of glucose, the modified electrode showed a current linearity in a range, 0-6mM with a current sensitivity value, 349.4μAmM^-1cm^-2 by CV and a current linearity in a window, 50-550μM with a current sensitivity, 316.8μAmM^-1cm^-2 at applied biased potential, 0.65V vs Ag/AgCl by amperometric i-t methods. Obtained glucose oxidation current sensitivity values are relatively higher than Os-complex based transducer systems.

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 E_1/2 = 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.

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.

In Situ Immobilized Sesamol-Quinone/Carbon Nanoblack-Based Electrochemical Redox Platform for Efficient Bioelectrocatalytic and Immunosensor Applications

Most of the common redox mediators such as organic dyes and cyanide ligand-associated metal complex systems that have been used for various electrochemical applications are hazardous nature. Sesamol, a vital nutrient that exists in natural products like sesame seeds and oil, shows several therapeutic benefits including anticancer, antidiabetic, cardiovascular protective properties, etc. Herein, we introduce a new electrochemical redox platform based on a sesamol derivative, sesamol-quinone (Ses-Qn; oxidized sesamol), prepared by the in situ electrochemical oxidation method on a carbon nanoblack chemically modified glassy carbon electrode surface (GCE/CB@Ses-Qn) in pH 7 phosphate buffer solution, for nontoxic and sustainable electrochemical, electroanalytical, and bioelectroanalytical applications. The new Ses-Qn-modified electrode showed a well-defined redox peak at E ^o = 0.1 V vs Ag/AgCl without any surface-fouling behavior. Following three representative applications were demonstrated with this new redox system: (i) simple and quick estimation of sesamol content in the natural herbal products by electrochemical oxidation on GCE/CB followed by analyzing the oxidation current signal. (ii) Utilization of the GCE/CB@Ses-Qn as a transducer, bioelectrocatalytic reduction, and sensing of H₂O₂ after absorbing the horseradish peroxidase (HRP)-based enzymatic system on the underlying surface. The biosensor showed a highly selective H₂O₂ signal with current sensitivity and detection limit values 0.1303 μA μM^-1 and 990 nM, respectively, with tolerable interference from the common biochemicals like dissolved oxygen, cysteine, ascorbic acid, glucose, xanthine, hypoxanthine, uric acid, and hydrazine. (iii) Electrochemical immunosensing of white spot syndrome virus by sequentially modifying primary antibody, antigen, secondary antibody (HRP-linked), and bovine serum albumin on the redox electrode, followed by selective bioelectrochemical detection of H₂O₂.

In Situ Structural Elucidation and Selective Pb2+ Ion Recognition of Polydopamine Film Formed by Controlled Electrochemical Oxidation of Dopamine

Owing to the versatility and biocompatibility, a self-polymerized DA (in the presence of air at pH 8.5 tris buffer solution) as a polydopamine (pDA) film has been used for a variety of applications. Indeed, instability under electrified condition (serious surface-fouling) and structural ambiguity of the pDA have been found to be unresolved problems. Previously, pDA films (has hygroscopic and insoluble property) prepared by various controlled chemical oxidation methods have been examined for the structural analysis using ex situ solid-state NMR and mass spectroscopic techniques. In this work, a new in situ approach has been introduced using an electrochemical quartz crystal microbalance (EQCM) technique for the improved structural elucidation of pDA that has been formed by a controlled electrochemical oxidation of DA on a carboxylic acid functionalized multiwalled carbon nanotube-Nafion (cationic perfluoro polymer) modified electrode (f-MWCNT-Nf) system in pH 7 phosphate buffer solution. Key intermediates like 5,6-dihydroxy indole (DHI; 150.7 g mol^-1), dopamine (154.1 g mol^-1), Na⁺, PO₄^2-, and polymeric product of high molecular weight, 2475 g mol^-1, have been trapped on f-MWCNT-Nf surface via π-π (sp² carbon of MWCNT and aromatic e-s), covalent (amide-II bonding, minimal), hydrogen, and ionic bonding and identified its molecular weights successfully. The new pDA film system showed well-defined peaks at E°' = 0.25 V and -0.350 vs Ag/AgCl corresponding to the surface-confined dopamine/dopamine quinone and DHI/5,6-indolequinone redox transitions without any surface-fouling complication. As an electroanalytical application of pDA, selective recognition of Pb²⁺ ion via {(pDA)-hydroquinone-Pb⁰} complexation with detection limit (signal-to-noise ratio = 3) 840 part-per-trillion has been demonstrated.

A blood-serum sulfide selective electrochemical sensor based on a 9,10-phenanthrenequinone-tethered graphene oxide modified electrode

The sulfide ion and its associated species (H₂S and HS^-) are widely referred to as toxic chemicals. However, at concentrations of ∼10-100 μM, it serves as a neurotransmitter and signaling agent in biological systems. Abnormalities in blood serum sulfide can be an indication of several diseases, including diabetes, wherein there is a significant reduction in the sulfide ion concentration (<10 μM). Herein, we wish report a 9,10-phenanthrenequinone (PQn) tethered graphene oxide (GO) modified glassy carbon electrode (GCE/GO@PQn) for the highly selective and stable electrocatalytic oxidation and flow injection analysis (FIA) of sulfide ions. The electrode exhibits a detection range of 1-100 μM, and is suitable for the common biochemical interference-free detection of blood serum sulfide in pH 7 phosphate buffer solution. The modified electrode was found to be tolerant of interfering chemicals such as cysteine, uric acid, ascorbic acid, nitrate, glucose, hydrogen peroxide, nitrate, nitrite and dissolved oxygen. This is unlike conventional redox mediator modified electrodes, which all show marked interference with the above-mentioned chemicals during sulfide detection. A constructed FIA calibration plot (applied potential, E_app = 0.15 V vs. Ag/AgCl) was linear in the sulfide concentration ranges of 1-100 μM (1^st region) and 300 μM-5 mM (2^nd region) with a detection limit value of 700 nM. The selective and quick FIA of sulfide ions in three diabetic patient blood samples along with a control was demonstrated as a proof of concept.

Axial Coordination Site-Turned Surface Confinement, Electron Transfer, and Bio-Electrocatalytic Applications of a Hemin Complex on Graphitic Carbon Nanomaterial-Modified Electrodes

Understanding the relation between the chemical bonding and the electron-transfer (ET) reaction of surface-confined hemin (a five-coordinated Fe-porphyrin-with-chlorine complex) is a special interest in the biomimicking studies of heme proteins. Owing to the difficulty in ET function, scanty electrochemical reports of hemin in aqueous solution were reported. It has been noticed that in most of the reported procedures, the sixth axial coordination position of the hemin complex has been unknowingly turned by attaching with water molecules (potential cycling in alkaline conditions or heating), solvents such as ethanol and dimethyl sulfoxide, and nitrogen-donating compounds that have helped for the heme ET reaction. In this work, a systematic effort has been taken to find out the contribution of hemin and its axial bond coordination with π-π interaction, hydrogen bonding, and hydrophobic binding systems toward the ET reaction. Various graphitic carbons such as graphitized mesoporous carbon (GMC), mesoporous carbon-hydrophilic and hydrophobic units, graphite nanopowder, graphene oxide, single-walled carbon, multiwalled carbon nanotube (MWCNT), and carboxylic acid-functionalized MWCNT (as a source for π-π interaction, hydrogen bonding, and hydrophobic environment) along with the amino functional group of chitosan (Chit; as an axial site coordinating system) have been tested by modifying them as a hemin hybrid on a glassy carbon electrode (GCE). In addition, a gold nanoparticle (Aunano) system was combined with the above matrix as a molecular wiring agent, and its role was examined. A highly stable and well-defined redox peak at an apparent formal potential (Eo') of -320 mV versus Ag/AgCl with the highest surface excess of 120 × 10-10 mol cm-2 was noticed with the GCE/Aunano-GMC@hemin-Chit hybrid system, wherein all interactive features have been utilized. Omitting any of the individual interactions resulted in either decreased (with Aunano) or nil current response. As applications, efficient bio-electrocatalytic reduction and sensing of dissolved oxygen and hydrogen peroxide have been demonstrated.

Undiluted human whole blood uric acid detection using a graphitized mesoporous carbon modified electrode: a potential tool for clinical point-of-care uric acid diagnosis

Direct sensing of uric acid (UA) in an undiluted whole blood sample is reported here taking human whole blood as an analyte and a self-supporting electrolyte. Among various solid electrodes (Pt, Au, GCE, and GCE/Nafion) and carbon nanomaterials (carbon nanofibers, graphene oxide, graphite nanopowder, graphitized mesoporous carbon (GMC), single-walled carbon nanotubes, and multiwalled carbon nanotubes) tested, a GMC-modified glassy carbon electrode, designated as GCE/GMC, showed a remarkable response towards direct electrochemical oxidation of blood uric acid at ∼0.25 V vs. Ag/AgCl, unlike the poor and/or feeble current signals with the other unmodified and modified electrodes. It is plausible that the mesoporous nature of the GMC favours the formation of a blood-GMC bio-corona through internalization and provides straight access to blood-matrixed uric acid. Furthermore, the effects of the scan rate and interference with various biochemicals on the GCE/GMC were analysed. The electrochemical oxidation reaction is found to be diffusion controlled in nature and there is no interference from common biochemicals like ascorbic acid, glucose, tryptophan, H2O2, xanthine, hypoxanthine, cysteine, nitrate, nitrite, and sulfide in blood. Real blood UA sample analysis was demonstrated with comparable UA analysis results from the clinical measurement.

Morphology and Admittance Spectroscopy of Cellulose Acetate/Graphene Quantum Dots Nanocomposites

Graphene quantum dots (GQDs) are considered as fascinating materials feasible for biological, optoelectronic devices, energy and environmental applications. Casting nanocomposite films for technological application is a challenging research interest. Cellulose acetate (CA) is one of the most abundant, economic, environmental friendly and biodegradable biomaterials. It has been found that CA is a preferred composite matrix to prepare recasting films, due to its efficient antifouling feature. In the present investigation, we exhibited preparation of CA/GQD nanocomposite by solution blending as a function of GQD loading 0.1–0.5[Formula: see text]wt.%. Morphology and electrical properties were examined as a function of GQD loading. The nanocomposite was characterized by impedance spectroscopy, and the measured admittance ([Formula: see text]) was plotted against temperature across broadband frequency. The magnitude of [Formula: see text] exhibits direct relation under the varying temperature. The morphology of the nanocomposites was observed by atomic force microscope technique in contact mode. Collective observation from our results is that it can be revealed that CA/GQD nanocomposites are suitable for thermal sensing applications.

Core-shell heterostructured multiwalled carbon nanotubes@reduced graphene oxide nanoribbons/chitosan, a robust nanobiocomposite for enzymatic biosensing of hydrogen peroxide and nitrite

A robust nanobiocomposite based on core-shell heterostructured multiwalled carbon nanotubes@reduced graphene oxide nanoribbons (MWCNTs@rGONRs)/chitosan (CHIT) was described for the fabrication of sensitive, selective, reproducible and durable biosensor for hydrogen peroxide (H2O2) and nitrite (NO2-). The excellent physicochemical properties of MWCNTs@rGONRs such as, presence of abundant oxygen functionalities, higher area-normalized edge-plane structures and chemically active sites in combination with excellent biocompatibility of CHIT resulting in the versatile immobilization matrix for myoglobin (Mb). The most attractive property of MWCNTs@rGONRs which distinguishes it from other members of graphene family is its rich edge density and edge defects that are highly beneficial for constructing enzymatic biosensors. The direct electron transfer characteristics such as, redox properties, amount of immobilized active Mb, electron transfer efficiency and durability were studied. Being as good immobilization matrix, MWCNTs@rGONRs/CHIT is also an excellent signal amplifier which helped in achieving low detection limits to quantify H2O2 (1 nM) and NO2- (10 nM). The practical feasibility of the biosensor was successfully validated in contact lens cleaning solution and meat sample.

An Elegant Analysis of White Spot Syndrome Virus Using a Graphene Oxide/Methylene Blue based Electrochemical Immunosensor Platform

White spot syndrome virus (WSSV) is a major devastating virus in aquaculture industry. A sensitive and selective diagnostic method for WSSV is a pressing need for the early detection and protection of the aquaculture farms. Herein, we first report, a simple electrochemical immunosensor based on methylene blue dye (MB) immobilized graphene oxide modified glassy carbon electrode (GCE/GO@MB) for selective, quick (35 ± 5 mins) and raw sample analysis of WSSV. The immunosensor was prepared by sequential modification of primary antibody, blocking agent (bovine serum album), antigen (as vp28 protein), secondary antibody coupled with horseradish peroxidase (Ab2-HRP) on the GCE/GO@MB. The modified electrode showed a well-defined redox peak at an equilibrium potential (E_1/2), -0.4 V vs Ag/AgCl and mediated H₂O₂ reduction reaction without any false positive result and dissolved oxygen interferences in pH 7 phosphate buffer solution. Under an optimal condition, constructed calibration plot was linear in a range of 1.36 × 10^-3 to 1.36 × 10⁷ copies μL^-1 of vp28. It is about four orders higher sensitive than that of the values observed with polymerase chain reaction (PCR) and western blot based WSSV detection techniques. Direct electrochemical immunosensing of WSSV in raw tissue samples were successfully demonstrated as a real sample system.