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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 elec...

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 of10-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

Carbazole-1,4-quinone is a key structural unit of naturally occurring carbazole-quinone alkaloids, for instance, 3-methyl carbazole-1,4-quinone (Murrayaquinone), that are widely used in pharmaceuti...

Herein we report a simple and selective open-circuit potential-time (OCPT) based preparation of nickel hexacyanoferrate (NiHCF) on nickel and iron impurity containing carbon nanotube (CNT*, * = intrinsic metal impurity) modified glassy carbon electrode (GCE/CNT*NiHCF) using pH 2 ferricyanide solution. Unlike potentiodyanamic cycling, OCPT allowed the selective preparation of NiHCF with a distinct redox features at an equilibrium potential = 0.38 V in pH 7 phosphate buffer solution (PBS). Kinetic parameters such as transfer coefficient and rate constant of GCE/CNT*NiHCF were calculated from cyclic voltammetry (CV) studies of the redox peak. Characterization of CNT* and CNT*NiHCF by XRD, TEM, FTIR and Raman spectroscopy techniques revealed the presence of iron and nickel impurities in CNT* and confirmed the formation of CNT*NiHCF and CNT*Fe-NiHCF (Fe-NiHCF = FeHCF + NiHCF) by OCPT and potentiodynamic cycling, respectively. Furthermore, electrocatalytic activity of CNT*NiHCF modified electrode was explored with a model analyte, hydrazine (Hz) in pH 7 PBS. Likewise, electron number involved during the rate determining step, complete electrocatalytic reaction and its heterogeneous rate constant values were calculated using CV technique. At optimal amperometric i-t conditions, GCE/CNT*NiHCF showed a linear calibration plot with a current linearity on a range of 20–200 μM with current sensitivity and detection limit (signal-to-noise = 3) values of 1217.39 nA μM−1 cm−2 and 0.8 μM respectively. It also displayed zero interference from oxalic acid, uric acid, ascorbic acid, sulphate, nitrite, nitrite, magnesium and sodium displaying feasibility to Hz detection in polluted water bodies.

Development of a new pH sensor system, which is simple to prepare, sensitive, selective and workable with low volume, is demanding research in biomedical and environmental studies. In the literature, organic molecules like methylene blue, toluidine blue, hydroquinone, catechol, anthraquinone and polyaniline-based redox probes have been widely used for this purpose. In general, these redox probes have easily interfered with common biochemicals such as dopamine, ascorbic acid, NADH, H2O2, cysteine, hydrazine, and some transition metal ions, etc., and in turn to marked potential and current drifts (pH-false positive response). In this work, a highly redox-active, stable and interference-free redox polymer based on poly(4-chloroaniline) (PANI(4-Cl)) modified graphitized mesoporous carbon (GMC), designated as GMC@PANI(4-Cl), has been prepared using 4-chloroaniline as a monomer in pH 7 phosphate buffer solution. The new redox polymer system showed a distinct redox peak at Eo’= 0.15 V vs Ag/AgCl with a stable voltammetric response. Transmission electron microscope analysis of the redox polymer composite had shown adhesion of black polymeric solid due to polyaniline like the molecular system as a surface layer on the GMC material. The constructed calibration plot was linear in the pH window 2-11 with a slope and regression values − 58 mV pH−1 and 0.9997, respectively. The GMC@PANI(4-Cl) modified electrode showed a sensitive and selective pH monitoring without any interference from the common biochemicals as listed above. As a practical application, pH sensing of commercial pH solutions, undiluted urine and saliva samples were demonstrated. Also, a three-in-one screen-printed carbon modified GMC@PANI(4-Cl) was explored for pH monitoring of a bacterial (E.coli) growth, which showed a comparable response with the conventional pH electrode.

Owing to its biological significance, preparation of stable surface-confined catechol (CA) is a long-standing interest in electrochemistry and surface chemistry. In this connection, various chemical approaches such as covalent immobilization (using amine- and carboxylate-functionalized CA, diazotization-based coupling, and Michael addition reaction), self-assembled monolayer on gold (thiol-functionalized CA is assembled on the gold surface), CA adsorption on the ad-layer of a defect-free single-crystal Pt surface, π–π bonding, CA pendant metal complexes, and CA-functionalized polymer-modified electrodes have been reported in the literature. In general, these conventional methods are involved with a series of time-consuming synthetic procedures. Indeed, the preparation of a surface-fouling-free surface-confined system is a challenging task. Herein, we introduce a new and facile approach based on electrochemical demethylation of 2-methoxyphenol as a precursor on the graphitic surface (MWCNT) at a bias potential, 0.5 V vs Ag/AgCl in neutral pH solution. Such an electrochemical performance resulted in the development of a stable and well-defined redox peak at Eo’ = 0.15 (A2/C2) V vs Ag/AgCl within 10 min of preparation time in pH 7 phosphate buffer solution. Calculated surface excess (16.65 × 10–9 mol cm–2) is about 10–1000 times higher than the values reported with other preparation methods. The product (catechol) formed on the modified electrode was confirmed by collective electrochemical and physicochemical characterizations such as potential segment analysis, TEM, Raman, IR, UV–vis, GC–MS, and NMR spectroscopic techniques, and thin-layer chromatographic studies. The electrocatalytic efficiency of the surface-confined CA system was demonstrated by studying hydrazine oxidation and sensing reactions in a neutral pH solution. This new system is found to be tolerant to various interfering biochemicals such as uric acid, xanthine, hypoxanthine, glucose, nitrate, hydrogen peroxide, ascorbic acid, Cu2+, and Fe2+. Since the approach is simple, rapid, and reproducible, a variety of surface-confined CA systems can be prepared.

The measurement of time evolution of electrochemical impedance enables enzymatic kinetic studies in real-time, and obviates the need of using additional reagents as in many popular spectroscopic methods. This can eventually lead to the development of enzyme biosensors. We have used the urea–urease system as a model for this study. The usage of a free enzyme (without any immobilization steps) in this work makes the technique very simple and unique for electrochemical measurement on urease. The impedance vs. time measurement of urease exhibits Michaelis–Menten (MM) behaviour with the MM constant ( $$K_{\mathrm {{m}}}$$ ) of 0.8 mM and maximum velocity ( $$V_{\mathrm {{max}}}$$ ) of $$5000\hbox { ohms min}^{{{-1}}}$$ . This $$K_{\mathrm {{m}}}$$ value closely matched the one, which is obtained from the conventional colorimetric method (values). The enzyme kinetics was performed in a standard three-electrode system and reproduced in a fabricated mini electrochemical cell in an Eppendorf tube, which could pave the way for the development of impedimetric biosensors for a variety of enzyme systems, especially the ones for which spectrometric techniques cannot be readily applied.

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.

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 [Mn2III(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, [Mn2III(phen)4(O)(Cl)2]2+, on the modified electrode was confirmed by physicochemical characterizations using transmissi...

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 Eo = 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.

DPPH (2,2-diphenyl-1-picrylhydrazil), a free radical-containing organic compound, is used widely to evaluate the antioxidant properties of plant constituents. Here, we report an efficient electroactive DPPH molecular system with excellent electrocatalytic sensor properties, which is clearly distinct from the traditional free radical-based quenching mechanism. This unusual molecular status was achieved by the electrochemical immobilization of graphene oxide (GO)-stabilized DPPH on a glassy carbon electrode (GCE). Potential cycling of the DPPH adsorbed-GCE/GO between − 1 and 1 V (Ag/AgCl) in a pH 7 solution revealed a stable and well-defined pair of redox peaks with a standard electrode potential, E0′ = 0 ± 0.01 V (Ag/AgCl). Several electrochemical characterization studies as well as surface analysis of the GCE/GO@DPPH-modified electrode by transmission electron microscopy, Raman, and infrared spectroscopy collectively identified the imine/amine groups as the redox centers of the electroactive DPPH on GO. The use of different carbon-supports showed that only oxygen-functionalized GO and MWCNTs could provide major electroactivity for DPPH. This highlights the importance of a strong hydrogen-bonded network structure assisted by the concomitant π-π interactions between the organic moiety and oxygen function groups of carbon for the high electroactivity and stability of the GCE/GO@DPPH-NH/NH2-modified electrode. The developed electrode exhibited remarkable performance towards the electrocatalytic oxidation of NADH at 0 V (Ag/AgCl). The amperometric i-t sensing of NADH showed high sensitivity (488 nA μM−1 cm−2) and an extended linear range (50 to 450 μM) with complete freedom from several common biochemical/chemical interferents, such as ascorbic acid, hydrazine, glucose, cysteine, citric acid, nitrate, and uric acid.

Triazophos, O,O-diethyl O-(1-phenyl-1H-1,2,4-triazol-3-yl)phosphorothioate (TPZ), an organophosphorus pesticide, has been widely used in agriculture to control pests, insects, and some nematodes (roundworms). Unfortunately, it has been found that a significant trace of the pesticide residue enters into the agricultural products and creates a major health threat to human. In order to selectively detect the TZP pesticide in real samples, several indirect and time-consuming analytical assays based on acetylcholinesterase enzymes, affinity-based antibody systems, and molecularly imprinted polymers, apart from the separation-coupled mass spectrophotometer, have been demonstrated. For the first time in the literature, we report a direct electrochemical method for the selective and quick detection of TZP based on electrocatalytic oxidation by a Coomassie brilliant-blue dye surface-confined carbon-black-nanoparticle-modified glassy carbon electrode (GCE/CBnano@CoomBB) in a pH 7 phosphate buffer solution. GCE/CBna...

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...

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.

Hematin is a hydroxyl group linked heme site (hydroxyl heme) of the natural enzymes/proteins like hemoglobin, cytochrome c, catalase, and horseradish peroxidase, and it has an important role in the physiological function. Because of problems like poor electron-transfer functionality (on solid electrodes), poor solubility, and molecular aggregation in aqueous solution, limited electrochemical studies have been reported in the literature. A new electrode modification method for hematin using graphitized mesoporous carbon nanomaterial and chitosan for enhanced redox-active and efficient electrocatalytic reductions of hydrogen peroxide and dissolved oxygen in neutral pH was demonstrated in this work. The hematin-modified electrode showed a highly stable redox peak at E°′ = −0.390 V versus Ag/AgCl with a heterogeneous rate constant value of 1.34 s–1. Calculated hematin-active loading concentration (Γhemat = 126 × 10–10 mol cm–2) is ∼20 times higher than the reported values. Physicochemical and electrochemical ...

Indole and its derivatives are important core constituents of several natural, biological and pharmaceutical relevant compounds. In general, electrochemical oxidation of indole on solid electrodes in acid and non-aqueous conditions results in the formation of polyindole like compounds as an end product. Selective and controlled electrochemical oxidation of indole and its derivatives to redox active intermediate compound/s without over-oxidation to the polymeric product is a challenging research task. Herein, we report an electrochemical oxidation of electro-inactive indole to a multi-redox active Indole Tetraone (1H-Indole-2,3,4,7-Tetraone)-a new organic redox species (Ind-Tetraone) and entrapment as a surface-confined redox active species on multiwalled carbon nanotube modified glassy carbon electrode (GCE/MWCNT@Ind-Tetraone) in physiological pH solution. GCE/MWCNT@Ind-Tetraone showed a well-defined surface-confined redox peaks at E1/2, −0.270 V (A1/C1) and +0.270 V (A2/C2) vs Ag/AgCl. From the physicochemical characterizations by Raman and IR spectroscopy, XPS, LC-MS (an ethanolic extract) and control electrochemical experiments with various substituted indole derivatives, it is confirmed the formation of Ind-Tetraone species without any polyindole formation upon the electrochemical oxidation of indole on MWCNT surface. Electrochemical oxidation of nitrogen atom as a radical species and subsequent electron-transfer/water addition reaction is proposed as a possible mechanism for the Ind-Tetraone product formation. A simultaneous electrocatalytic oxidation of hydrazine and reduction reaction of hydrogen peroxide at two discreet potentials has been demonstrated as a bifunctional application of the GCE/MWCNT@Ind-Tetraone system. In further, the GCE/MWCNT@Ind-Tetraone as a electrochemical detector, simultaneous flow injection analysis of hydrazine and hydrogen peroxide was also demonstrated as a proof of concept for the bifunctional application.

A human whole blood-chemically modified electrode prepared using graphitized mesoporous carbon and Nafion, as an in-vitro model system, has been studied for the specific interaction of hydrogen peroxide at biased potentials by cyclic volumetric technique. A blood-chemically modified electrode prepared by modifying a few drops of human whole blood with the carbon nanomaterial and Nafion had showed a well-defined redox peak at an apparent standard electrode potentials, E°′~− 0.38 V vs Ag/AgCl in N2 purged pH 7 phosphate buffer solution, similar to an isolated hemoglobin protein redox feature. When the blood modified electrode is exposed with dilute solution of hydrogen peroxide, selective electrochemical reduction behaviour where the heme site redox potential exists was noticed. The electrochemical reduction reaction is found to follow diffusion controlled mechanism. The H2O2 reduction peak current was linear with the concentration in a range of 100 to 800 μM with a current sensitivity of 0.048 μA μM−1. Qualitative cyclic voltammetric patterns of the blood modified electrode in the entire concentration window of H2O2 are same indicating intact biomolecular arrangement of the heme site. Kinetic parameters of the electron-transfer reaction of H2O2 were estimated using rotating disc technique and Michaelis-Menten reaction approaches. The electron-transfer function of the heme in the RBC is not influenced by any other electroactive biochemicals such as glucose, nitrate, ascorbic acid, uric acid and dopamine, except nitrite. As an independent study, the blood-chemically modified electrode has been used as an electrochemical sensor system for the flow injection analysis of H2O2.

Mn-Pseudocatalase is a non-heme catalases family enzyme produced by various bacteria that involves in a two-electron H2O2 catalytic cycle in a manner similar to that of the heme-based Catalase enzymes. Herein, we report a bio-mimicking functional model system prepared by in-situ electrochemical oxidation of Mn(bpy)2Cl2 precursor to a surface-confined [MnII(bpy)2(H2O)2]2+ complex, wherein, bpy = 2,2′-bipyridyl, on a carboxylic acid functionalized multiwalled‑carbon nanotube (f-MWCNT)/Nafion modified glassy carbon electrode for biomimicking H2O2 disproportionation reaction. The modified electrode showed a well-defined redox peaks at an apparent standard electrode potentials (Eo′), 0.65 ± 0.05 V and 0.2 V vs Ag/AgCl with surface-excess (ΓMn) values, 6.24 × 10−9 mol cm−2 and 0.43 × 10−9 mol cm−2 for the MnIV/III and MnIII/II sites of the Mn-complex in neutral pH solution respectively. Physico-chemical characterizations of the system by FTIR, UV–Vis and ESI-MS (ethanolic extract of the electrode) confirming the conversion of [Mn(bpy)2(H2O)2]2+ complex (m/z, 403.09). A strong π-π interaction between the f-MWCNT's graphitic sp2 carbons and the bpy's aromatic electrons, hydrogen bonding between the oxygen and water molecules and ionic interaction between the complex and sulphonic site of nafion favor the stability of the complex. The hybrid system showed selective current signals for mediated oxidation and reduction reactions of H2O2 (disproportional reaction) without any dissolved oxygen interference. As an independent study, selective electro-catalytic oxidation and amperometric detection of H2O2 was demonstrated.

RuO2 is an unparalleled electrode with wider applications. Chloride, bound water, and surface stoichiometry are the inherent residues of RuO2 left upon the high-temperature pyrolysis of a RuCl3⋅xH2O precursor. Although the electrocatalytic properties of RuO2 for specific applications have been studied extensively, there is a paucity of studies linking the intrinsic electron-transfer (ET) activity of RuO2 with the oxide preparation temperature-dependent inherent residual parameters. This paper presents the intrinsic ET activity-oxide residue correlations for RuO2 electrodes. The ET kinetic parameters were estimated using a surface oxide-sensitive Fe3+/Fe2+ redox probe by rotating disc electrode voltammetry. Oxide powder-based electrodes (RuO2 powder-PVC/Pt-modified electrodes), which were fabricated conveniently at room temperature even with high-temperature oxides, were used instead of the traditional thermally prepared electrodes to circumvent the primary problems at the coating|support interface and inefficient chloride removal. RuO2 powders prepared at five temperatures (Tprep), i.e., 300, 400, 500, 600, and 700 °C, were used for electrode fabrication. The results showed that the electron exchange rate was highest for the 400 °C RuO2 electrode, and it was independent of the Tprep in the range 500 to 700 °C. The oxide powders were characterized using a range of techniques. The measured intrinsic ET activity and the associated structural correlation over the Tprep range 300–700 °C suggest that the best activity of the 400 °C electrode can be attributed to the optimal chloride and bound water contents in a completely formed rutile surface layer containing the catalyst sites of a particular nature with the highest electroactivity.

Isomers of dihydroxy benzene (DHB) such as 1,2-DHB, 1,3-DHB and 1,2,3-trihydroxy benzene (1,2,3-THB) are the functional constituents of tea polyphenols. Herein, we report an elegant 6B pencil lead for real-time tea quality assessment via a simple and quick procedure to electrochemically detect polyphenols in a mixture without any adsorption complication. A pre-anodised 6B-PGE, denoted as 6B-PGE* has showed well-defined anodic signals without any separation technique for the dihydroxy (DHB) and trihydroxy benzene derivatives present in tea polyphenols at distinct potentials. Differential pulse voltammetry was adopted for the quantification procedure. The constructed calibration plots were linear in the windows, 10–100 μM, 50–350 μM and 10–70 μM for 1,2-, 1,3- and 1,4-DHB isomeric derivatives, respectively. The calculated sensitivity and detection limit (signal-to-noise ratio = 3) values were 0.2234, 0.0725 and 0.0627 μA μM−1 and 0.719, 2.469 and 5.072 μM, respectively. Quantification of the polyphenolic content in two different tea dust samples was demonstrated as representative examples.

Amongst various isomers of dihydroxy benzene, 1,2-dihydroxy benzene (Catechol, CA) isomer and its derivative based natural compounds are considered to be the key functional group for various health benefits. In electrochemistry, pulse voltammetric techniques combined with chemically modified electrodes (CMEs) have been often reported for simultaneous detection of 1,2- (CA); 1,3- (Resorcinol, RE) and 1,4-dihydroxy benzene (hydroquinone, HQ) isomers at discreet potentials, ∼0.1, ∼0.2 and ∼0.4 V vs Ag/AgCl, respectively, in a neutral pH condition. Indeed, the above technique and the reported CMEs were not suitable for selective amperometric i-t based detection of CA without interference from HQ and RE. In fact, at CA detection potential, ∼0.2 V, HQ also got co-detected. Herein, we report a palladium nanoparticles-Chitosan indium tin oxide modified electrode (ITO/CHIT@Pd nano ) as a selective amperometric sensor system for CA isomer detection without any interference of HQ and RE. A specific interaction between Pd 2+ and CA as {Pd 2+ -CA complex} is proposed as a key factor for the selectivity achieved in this work. As a proof of concept, flow injection analysis of CA functional group in wine and tea real samples was demonstrated.

Understanding the electron-transfer (ET) functionality of enzymes in relation to their molecular orientation and dynamics is a cutting-edge research interest in the interdisciplinary areas of chemistry and biology. In this work, we demonstrate how the molecular structure and orientation of a redox polymer influence the ET behavior and specific catalytic functionality to target substance, ascorbic acid (AA) or cysteine (CySH) in a physiological condition. In the literature, Fe(CN)63−, a well-known benchmarking redox system, based chemically modified electrodes (CME) prepared by the ion-exchange method, has been widely used as an electrocatalyst for ascorbic acid oxidation reaction without the interference of CySH. In this work, a Fe(CN)63− bearing copoly(ionic liquid)-ethanol solution modified glassy carbon electrode, designated as GCE@{Fe(CN)63−}-coPIL, prepared as a homogenous solution followed by CME formation, has shown a unique and selective CySH oxidation reaction, rather than expected AA mediation, similar to the Thiol Oxidase enzyme-based biomimetic functionality. Andrieux and Saveant and Michaelis-Menten kinetic models were adopted to explain the reaction mechanism. The polymer network orientation and dynamics are found to influence strongly on the electrocatalytic functionality of the new CME. It has been revealed that underlying surface, functional groups, and solvent nature impact the molecular architectures of the {Fe(CN)63−}-caped polymer back-bone (insulating) network as a fully (H2O and CHCl3) or partially (C2H5-OH and CH3CN) shielded structures and decide its electron-transfer and selective mediated oxidation reaction functionalities. This observation is similar to the protein folding of enzymes at various external stimuli conditions and its unfolded structures for direct electron transfer and selective mediated oxidation/reduction reaction. As a proof of concept, electroanalytical application of thiol functional group (CySH) oxidation to disulfide bond (CyS-SyC) catalyzed by the GCE@{Fe(CN)63−}-coPIL in neutral pH solution has been demonstrated.

The development of an alternate environmental friendly electrocatalyst suitable to replace the function of the ferrocene, ferricyanide and prussian blue-based redox-mediators is a challenging research interest. Herein, we introduced a novel method for synthesis of a highly redox-active surface-confined Hydroquinone on MWCNT modified glassy carbon electrode surface, GCE/MWCNT@H2Q using the earth-abundant lignocellulose-biomass product, syringaldehyde (Syn) by one-step potential cycling and potentiostatic polarization method in pH 7 phosphate buffer solution that can show an efficient ascorbic acid electrocatalytic signal better than the conventional redox-mediators suitable to use as an anode in biofuel cell and selective electrochemical sensor applications in neutral pH solution. This new electrode showed a well-defined redox peak at Eo′ = −0.15 V(A2/C2) and 0.0 V(A3/C3) vs Ag/AgCl corresponding to the redox-active molecular species of H2Q and its polymerized product, Poly-H2Q. Based on various physicochemical techniques like Raman, IR, TGA, DTA, TEM, GC–MS, H1-NMR and scanning electrochemical microscope imaging using substrate generation/tip-collection mode, it has been revealed that Syn-precursor underwent a demethoxylation and hydration upon the electrochemical preparation condition. It has been proposed that the Poly-H2Q is an active site for the selective electrocatalytic oxidation of AA in a neutral pH solution.

A unique gold(111) oriented nanoparticles trapped carbon nanofiber-chitosan modified electrocatalyst coated glassy carbon electrode(GCE/CNF-CHIT@Aunano) was prepared by an in-situ electrochemical procedure for efficient bifunctional electrocatalytic oxidation and reduction of formaldehyde and hydrogen peroxide in neutral pH solution. In the typical preparation, a microliter quantity of Au3+ solution was drop-casted on GCE/CNF-CHIT surface and potential cycled in pH 7 PBS. The GCE/CNF-CHIT@Aunano showed well-defined electrochemical response of gold nanoparticles of calculated electrochemically active surface area, 0.1347 cm2, which is about 3–3000 times higher than that of the surface area of the respective unmodified electrodes such as polycrystalline Au (0.0407 cm2), GCE/CNF@Aunano (0.0034 cm2) and GCE@Aunano (0.0005 cm2). Physicochemical characterizations such as FESEM, EDAX, TEM, XRD, Raman, FTIR and XPS spectroscopic techniques revealed stabilization of 10 ± 5 nm sized Au(111) phase oriented nanoparticles by the amino functional group of chitosan in the composite matrix. The detailed electrochemical characterization by cyclic voltammetry(CV), rotating disk electrode (RDE) and in-situ CV-electrochemical quartz crystal microbalance (EQCM; Mw = 44 ± 1 g mol−1 species was identified) techniques showed direct oxidation of formaldehyde to formate as an intermediate (45 g mol−1) without any CO poisoning, unlike the conventional Pt and Au based electrodes. Using a bipotentiostat, selective and simultaneous flow injection analyses of formaldehyde and hydrogen peroxide in a mixture at discreet applied potentials (Eapp = 0.5 V/0.15 V and −0.15 V vs Ag/AgCl) were demonstrated. The applicability of the GCE/CNF-CHIT@Aunano was tested by detecting formaldehyde and hydrogen peroxide in a commercial hair dye formulation with about 100% recovery values.

Lactoferrin (LAF), an iron-binding glycoprotein has several health benefits ranging from anti-cancer, anti-inflammatory and antimicrobial activities against large number of microorganisms and is present abundantly in milk. Enzyme linked immunosorption assay based analytical protocol has been often referred for the selective detection of LAF in real samples. Herein, we report a simple electrochemical methodology for the direct recognition of LAF in raw milk using a methylene blue (MB) immobilized iron impurity (2.1 wt.%) containing multiwalled carbon nanotube/nafion modified glassy carbon electrode system. It is interesting to observe that upon potential cycling of GCE/Nf-MWCNT in MB dissolved milk system, both MB and LAF got co-immobilized on the electrode surface, designated as GCE/Nf-MWCNT@MB-LAF and showed a selective bio-electrocatalytic reduction signal for H2O2 at −0.4 V vs Ag/AgCl in milk or pH 7 PBS system. From control electrochemical experiments such as loading MB in pH 7 phosphate buffer solution and simulated milk sample and effect of carbon material (activated charcoal, graphite nanopowder and functionalized MWCNT) on preparation of the LAF modified electrode, it is revealed that iron impurity in the MWCNT, MB and LAF of the modified electrode involved in the electron-shuttling process to facilitate the H2O2 reduction reaction. By utilizing this novel process, selective bio-electrocatalytic sensing of H2O2 with current sensitivity and detection limit values 0.035 μA μM−1 and 3.2 μM respectively and specific recognition of LAF in raw and water diluted milksamples have been successfully demonstrated.

Probing electron transfer behaviour of natural products and trapping of its intermediate species is a challenging research task in the cross-disciplinary area of phytochemistry and electrochemistry. Ginger is a concoction of zingiberene, shogaol and gingerol (responsible for flavour and pungency) which breaks down to zingerone (less pungent) upon drying or cooking. Several reports in the literature deal with the anti-inflammatory and therapeutic uses of ginger, but very scant reports are available on the electrochemical feature of ginger chemicals. Owing to the complex nature, selective recognition of a redox active compound in a phytochemical is a difficult task. Herein, we report an electrochemical study of redox properties of ginger juice on a graphitized mesoporous carbon modified glassy carbon electrode surface by cyclic voltammetry technique (CV) in pH 2 KCl–HCl solution. CV of the modified electrode in presence of dilute ginger solution showed a distinct surface-confined redox peak at E 1 ∕ 2 , 490 ± 20 mV vs Ag/AgCl with peak-to-peak separation and surface-excess values of 60 ± 2 mV and 20 . 66 × 1 0 − 9 mol cm − 2 respectively. Physico-chemical characterizations of ginger extract modified electrode by Raman, UV–Vis, FT-IR and TEM reveal electrochemical transformation of 2-methoxyphenol derivatives of ginger compounds such as shogaol and gingerol to respective 1,2-dihydroxy benzene derivatives of gingerol and further anchoring as a surface-confined redox system on the GMC surface via π - π interaction. Ginger based redox system was tuned for complex formation with Cu 2 + ion and further to sense electro-catalytic activity towards hydrogen peroxide in neutral pH solution.

This article documents the unexpected optical property observed in chemically modified BPO 4 which is carried out by partial substitution of P 5+ ion by V 5+ ion and redox behaviour of the resultant new phases. Structurally, the extent of bigger V 5+ ion substitution for smaller P 5+ ion in the phosphate lattice is found to be restricted. Powder X-Ray diffraction and FT-IR analysis confirmed BP 0.95 V 0.05 O 4 and BP 0.9 V 0.1 O 4 as optimized compositions and were further characterized by UV-VIS-NIR and SEM-EDS techniques. Nevertheless, such small replacement of PO 4 moiety by VO 4 moiety in BPO 4 lattice resulted in greatly enhanced optical absorption and the absorption edge values were found to be around 569 and 591 nm in the case of BP 0.95 V 0.05 O 4 and BP 0.9 V 0.1 O 4 respectively against the absence of such absorption in the parent BPO 4 . In addition, the new phosphovanadates exhibited redox behaviour on interacting with hydrazine and hydrogen peroxide.

The authors report on the electrochemical degradation of the dye Methyl Orange (MO) on a glassy carbon electrode (GCE) modified with multiwalled carbon nanotubes. Continuous electrochemical cycling of the modified electrode in pH 7 solution leads to reductive cleavage of the azo bond of MO to form intermediate amines such as aniline-4-sulfonic acid and 1,4-diaminobenzene. These are further converted to a highly redox-active composite consisting of quinone and polyaniline derivative respectively on MWCNT. Cyclic voltammetric experiments display two well-defined redox peaks at an equilibrium potential (E1/2) of about 0 V (A1/C1) and 0.2 V (A2/C2) vs Ag/AgCl. Physicochemical characterizations such as FT-IR and in-situ UV-vis spectroelectrochemistry support the mechanism of cleavage of MO. The composite modified electrode is shown to be a viable sensor for use in flow injection amperometric analysis of ascorbic acid (AA; vitamin C) at a potential of −0.15 V (vs Ag/AgCl). No interferences were observed with cysteine, glucose, dopamine, citric acid, nitrite, and uric acid. The measured current is linearly related to the AA concentration in the range from 1 μM to 700 μM, with a 115 nM limit of detection (at an S/N ratio of 3). The method was successfully applied to the selective quantification of AA in two pharmaceutical samples.

Studies related to redox reaction of human whole blood with hydrogen sulfide is highly important in biological system (e.g., detoxification of sulfide ion) and there have been several indirect experimental evidences for their interaction. This is the first and direct report for electrochemical oxidation and reduction of sulfide ion on a human whole blood chemically modified electrode system in physiological solution. A specific diffusion controlled electrochemical oxidation signal at 0 V vs Ag/AgCl and reduction signal at − 0.5 V vs Ag/AgCl corresponding to the electrochemical conversion of sulfide ion to sulfate and sulfide to sulfur atom respectively by the Blood-Hemoglobin-Fe III/II redox site were noticed. These reactions are similar to the red blood cell functional behaviours with hydrogen sulfide (exist as HS − in a neutral solution) as detoxification mechanism in the human body. Control results of electrochemical sulfide oxidation/reduction reactions with commercial hemoglobin and heme derivative (hematin) modified electrodes support the observation. Calculated electrochemical sulfide oxidation parameters such as Tafel slope, transfer coefficient (α) and heterogeneous rate constant (Andrieux and Saveant model) values are 120 mV decade − 1 , 0.5 and 4.77 ± 1.51 × 10 − 6 mol − 1 cm 3 s − 1 respectively. Selective flow injection analysis of sulfide ion (HS − ) using the blood chemically modified electrode was demonstrated as a proof of concept for the applicability of the working electrode to analytical application.

Electrochemical oxidation of aniline and its derivatives like aminobenzene sulfonic acid (SA) on solid electrodes (Pt, GCE, ITO and Au) lead to formation of conducting polyaniline(PANI) and its derivatives as a sole product in aqueous solution. We report here for the first time, a unique and unexpected electrochemical transformation of ortho-, meta- and para-SA into highly redox active adsorption-controlled quinone isomers (catechol, CA; resorcinol, Re; hydroquinone, HQ), without any PANI like product formation on multiwalled carbon nanotube modified glassy carbon electrode (GCE/MWCNT@Q, Q = quinone) at an restricted potential cycling window (-0.4 to 0.4 V vs Ag/AgCl) in a neutral pH solution. It is proposed that upon potential cycling of a 2.1 wt.% iron-impurity containing MWCNT modified GCE with mM concentration of SA isomers, benzene (after deamination and desulfonation of SA) and hydrogen peroxide (from oxygen reduction reaction at -0.4 V vs Ag/AgCl) are formed as intermediate species which then converted to quinone/s like product via electro-Fenton reaction (utilizing the iron impurity and H2O2). One of the reaction intermediate, benzene (Mol. wt. (Mw) = 78) was identified (Mw = 77.8±0.5 g mol-1) by in-situ electrochemical quartz crystal microbalance coupled cyclic voltammetric technique. Using GCE/MWCNT@HQ as a model system, efficient electro-catalytic oxidation and sensing of reduced form of nicotinamide adenine dinucleotide (NADH) was successfully demonstrated.

Pencil lead is composed of clay and graphite and it has been widely used as a low-cost electrode system in electrochemistry. There are a few reports on chemical modification of PGE as oxygen functionalized surface that has been prepared via a pre-anodization technique for electro-analytical applications. For the first time in this work, we introduce a new strategy for electrochemical derivatization of PGE as Prussian blue (PB) and Fe(bpy) 3 2 + functionalized PGEs (i.e., PGE-PB and PGE-Fe(bpy) 3 2 + ), utilizing its intrinsic iron (1.31 wt.%), in 0.1 M KCl-HCl medium. Potential cycling of the PGE with Fe(CN) 6 3 − (as solution phase system) and bipyridyl (as surface adsorbed system) resulted in a facile formation of the hybrid complexes on the PGE surface. Physico and electrochemical characterization of PGE-PB and PGE-Fe(bpy) 3 2 + modified electrodes by Raman, FT-IR and UV–Vis spectroscopies, cyclic voltammetry and several control experiments (graphite nano powder, clay, Fe 2 O 3 modified electrodes) evidenced the formation of surface confined complexes on PGE. It was found that the intrinsic iron in clay is responsible for the complexation reactions. Selective electrocatalytic function for the H 2 O 2 reduction reaction was demonstrated as an application of the PGE-PB system.

Residual metal impurities in the pristine carbon nanotubes (CNT) have been found to show an exceptional electrochemical and electro-catalytic properties. To quantify the impurities, sophisticated instrumental techniques and time consuming electrochemical methods have been adopted. Herein, we report, a new strategy for simple and quick detection of nickel impurity in pristine single walled carbon nanotube (SWCNT*Ni, *Ni = nickel impurity) as redox active nickel hexacyanoferrate hybrid (SWCNT*Ni-HCF) by potential cycling of the pristine SWCNT modified glassy carbon electrode (GCE/SWCNT*Ni) with ferricyanide in pH 2 KCl-HCl solution at a scan rate 50 mV s−1 (t = 12 ± 1 min). Quantitative stripping of the nickel as nickel ion that combines with ferricyanide to form Ni-HCF hybrid on the SWCNT surface (i.e., GCE/SWCNT*Ni-HCF) is found to be the mechanism for the observation. In-situ derivatization of NiO modified GCE with ferricyanide was studied as an important control experiment. A calibration plot was constructed by plotting surface excess value of GCE/Ni-HCF obtained by cyclic voltammetric experiment against NiO loading on unmodified GCE surface. Unknown concentration of nickel in the pristine SWCNT was then determined by substituting surface excess value of SWCNT*Ni-HCF in the calibration plot. Obtained value is comparable with the result of continuum source electro-thermal atomic absorption spectrometry method.

Analyzing catabolism of the fish cycle, wherein hypoxanthine (Hx) and xanthine (X) as intermediates and uric acid (UA) as the end product are formed, provides vital information about the freshness of fish. Biosensors based on xanthine oxidase have often been used for this purpose. Herein, we introduce an enzyme-free electrochemical sensor developed using an ultra-low cost 4B grade pencil graphite electrode (PGE) pre-anodized at 2 V vs. Ag/AgCl (4B-PGE*, where * means pre-anodized) as a novel electrode system for separation-free and simultaneous differential pulse voltammetric (DPV) detection of three purine bases, Hx, X and UA, in a pH 7 phosphate buffer solution. Stable and well-defined peaks at 0.95, 0.65 and 0.3 V vs. Ag/AgCl were noticed upon electrochemical oxidation of Hx, X and UA respectively at the 4B-PGE*. The 4B-PGE* is found to show about a twenty five times higher electrochemical response than the 4B-PGE (non-preanodized) for the purine oxidations. Under optimal DPV conditions, the 4B-PGE* showed linear calibration plots with current linearities in the ranges 6–30 μM, 8–36 μM and 3–21 μM with current sensitivities of 0.921 μA μM−1, 1.742 μA μM−1 and 0.499 μA μM−1 for Hx, X and UA respectively. Ten consecutive detections of 10 μM Hx, X and UA showed a relative standard deviation (RSD) of 2.14%, 4.95% and 0.32%, respectively. In order to validate the analytical approach, separation-free and simultaneous electrochemical detection of Hx, X and UA in five freshly dead fish samples, stored at different temperatures and for different storage times, was successfully demonstrated.

Owing to their enhanced electrical conductivity, flexibility, cost-effective and simple sampling methods, polymer-carbon nanocomposite material composed of conducting polymers have been referred as an elegant system for supercapacitor applications. Indeed, integration of engineering polymer, which has an insulating property, such as Polyvinyl butyral (PVB), into the conducting matrix is a challenging task. Herein, we introduce a ternary polymer nanocomposite composed of PVB, PANI, poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT: PSS) and green-synthesized GQD based functionally stable and optically active system for an efficient supercapacitor application. The polymer-carbon nanocomposite has been prepared as a film by the solution-casting method. The “as prepared” polymeric film was characterized using XRD, TEM, Raman, IR, UV-Vis spectroscopic, laser-optical profilometric imaging and electrochemical techniques. From the collective results, it has been revealed that there is a chemical interaction between the GQD and polymeric systems, presumably due to strong intermolecular hydrogen bonding. Supercapacitor application of the polymer nanocomposite was demonstrated by simply modifying the composite mixture on a conducting substrate-disposable screen-printed carbon electrode. Cyclic voltammetry, electrochemical impedance and galvanostatic charge/discharge experiments were performed with the modified electrode. 1 wt. % GQD loaded ternary polymer-nanocomposite is found to be an optimal system for supercapacitor application. This material showed a capacitance value of 4998 Fg−1cm−2

The electron-transport chain that involves a proton-coupled electron-transfer (ET) reaction between hydroquinone (H2Q) and Cytochrome c (Cyt c) in the acid pool is one of the key processes of the respiratory complex 1 in cytoplasmic membrane of the bacterium and mitochondrial complexes, complex III (cytochrome bc1 or ubiquinol:cytochrome c oxidoreductase) of photosynthesis systems. In the literature, several spectroscopic techniques in association with electron-transfer inhibitors have been reported for this purpose. Till now, there is no straight forward voltammetric proof to reveal the mechanistic feature of the direct ET process. In the electrochemistry point of view, it is a challenging task to prepare a chemically modified bio-electrode composed of H2Q and Cytc redox systems with proper ET channels between them and to probe the mixed-potential reaction by direct voltammetric technique. Herein, we report a carboxylic acid functionalized hydroquinone (2,5-dihydroxy benzoic acid; H2Q-COOH) immobilized multiwalled carbon nanotube modified glassy carbon electrode (GCE/MWCNT@H2Q-COOH), prepared by in-situ electrochemical oxidation of 2-hydroxy benzoic acid (Ph-COOH), as a molecular wiring system for unfolded Cytc protein and further to study the direct voltammetric ET reaction using H2O2 as a specific probe in pH 2 KCl-HCl medium. The formation of surface-confined H2Q-COOH was confirmed by physicochemical and spectroscopic characterization techniques including IR, Raman, UV-Vis, Thin-layer chromatography, NMR and GC-MS. The GCE/MWCNT@H2Q-COOH showed well-defined redox peaks corresponding to consecutive one-electron-transfer reactions of H2Q-COOH/•QH-COOH (Eo'= 0.410 V vs Ag/AgCl, A1/C1) and •QH-COOH/Q (Eo'= 0.750 V vs Ag/AgCl, A2/C2), similar to one occurs with a membrane-bound ubiquinone and plastoquinone systems in the biological systems. The unfolded Cytc modified electrode, GCE/MWCNT@H2Q-COOH@Cytc prepared by chemical modification technique, has exhibited a specific and selective bioelectrocatalytic reduction current signal in between the redox potentials of QH2/•QH and Cytc in pH 2 KCl-HCl biomimicking the biological electron-transport chain reaction. In further amperometric techniques have been used to successfully demonstrate the ET reaction.

Electrochemical detection of Hydrogen peroxide in neutral pH is an important analytical problem that has been often worked out using horseradish peroxide (HRP) enzyme coupled with organic redox-active molecules as a transducer, and metallic systems in the form of metal nanoparticles and complexes chemically modified electrodes. Owing to non-amenable characteristic, direct electrochemical oxidation/reduction of H2O2 by redox-active organic molecules has been rarely described in the literature. Herein, we report naphthalene (NaP) and phenanthrene (PhenE) moieties immobilized MWCNT as advanced organic nanomaterial systems for enzyme-free, selective (dissolved oxygen, cysteine, citric acid, ascorbic acid, uric acid and glucose-interference free) and direct electrocatalytic reduction and sensing of H2O2 in neutral pH. These new organic-materials have been prepared by multiwall carbon nanotube (MWCNT) surface-bound electrochemical oxidation of NaP and PhenE at ~1.2 V vs Ag/AgCl in pH 2 KCl-HCl solution. The “as prepared” organic molecular materials showed a highly symmetrical voltammetric signal in cyclic voltammetric technique (peak-to-peak separation potential, ΔEp = 10 mV at scan rate = 10 mV s−1) and proton-coupled electron-transfer in characteristic. These new materials were characterized using several physicochemical techniques such as FTIR and Raman spectroscopes, transmission electron-microscope and electrochemical characterization (using radical quenching molecular system, 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO)). From the collective experimental results, it has been revealed that cationic radical species like organic molecules are stabilized on the organic nanomaterial modified electrode surface (via strong π-π interaction) and showed the redox peak. Amperometric i-t responses of the modified electrodes showed a systematic variation in the H2O2 detection current signal with current linearity in a range of 25–300 μM. Calculated current sensitivities and regression coefficient values are 4.8 nA μM−1 and 0.9996 with NaP-Redox and 4.20 nA μM−1 and 0.9994 with PhenE-Redox systems, respectively.

Owing to its importance in environmental pollution and human toxicity, the development of a simple and selective sensor for heavy metal ions is a continued research interest in cross-disciplinary areas of analytical chemistry. The colorimetric analysis based conventional complexation assay of Pb2+ is a cumbersome procedure for practical applications. Herein, we report a novel electrochemical sensor approach based on a high index facets (HIF)‑silver nanoflower modified glassy carbon electrode (AgNF@GCE) for anodic stripping voltammetric detection of lead ion in a pH 4.5 acetate buffer solution. Physicochemical techniques such as XPS, FESEM, and electrochemical studies with Fe(CN)63 were adopted for the surface characterization of the AgNF@GCE. Unlike the conventional Ag-nanoparticles, this new system provides a highly crystalline and large surface area with HIF's {422} and {111} for sensitive and selective electrochemical analysis of Pb2+ ion. Under an optimal experimental condition, AgNF@GCE showed a linear calibration plot in the range of 10–700 ppb of Pb2+ ion with a detection limit 0.74 ppb. Eight repetitive measurements of 50 ppb Pb2+ yielded a relative standard deviation value of 2.8%. The sensor showed tolerable interference with other metal ions such as Cu2+, Fe2+, Mg2+, Ni2+, K+, and Na+. As practical applicability, selective detection of lead concentration in blood-serum and cosmetics were successfully analyzed with appreciable recovery values.

Tea, one of the most consumed beverages in the world, has several health benefits which include antioxidant and reducing the risk of diabetes, heart-attack and cancer etc. Polyphenols such as catechin, epicatechin, epicatechin gallate, epigallocatachin and epigallocatechin gallate (EGCG), which generally contain combination of 1,2-dihydroxy benzene (1,2-DHB), 1,3-dihydroxy benzene (1,2-DHB) and 1,2,4-trihydroxy benzene (1,2,3-THB) functional groups are rich in green tea. Among the various polyphenols, EGCG (1,2,3-trihydroxybenzene functional group), is viewed as the most active component for the health benefits and hence it has been assayed in the tea quality assessment. Here in, we report a simple, rapid and separation-less electro-analytical technique based on a flow injection analysis coupled dual electrochemical detector system, in which graphitized mesoporous/Chitosan modified glassy carbon electrodes (GCE/Chit@GMC) set at two different applied potentials ( E app ), 0.1 (case-I) and 0.7 V vs Ag/AgCl (case-II), controlled by a bipotentiostat, with pH 7.0 phosphate buffer as a carrier solution has been reported for the selective and simultaneous detection of 1,2,3-THB and {1,2,3-THB + 1,2-DHB + 1,3-DHB} contents in tea samples.

Phenol and its derivatives are important aromatic compounds and that have been widely used in pharmaceutical, cosmetic, industrial applications for many years. Selective and direct detection of phenol, especially monophenol, is a challenging research interest in analytical chemistry. Although electrochemical methodology offers simple and straightforward route for phenol oxidation reaction, complications like tarry polymeric products formation and electrode surface fouling problems limit the electrodes for further extension to practical applications. In order to solve the problems, indirect electrochemical and biosensor based protocols have been often employed. Here in, we report an elegantly designed flow-injection analysis coupled dual electrochemical detector (FIA-DECD) made-up off a screen-printed carbon ring (SPCE-R*) and disk (SPCE-D*), which have been pre-anodized at 2 V vs Ag/AgCl at 300 s in pH 7 phosphate buffer solution, for direct and simple detection of monophenols using 0.05 M H2SO4 solution as a carrier solution. In this protocol, phenol got first electro-oxidized on SPCE-D* (disk part) at Eapp = 1.5 V vs Ag/AgCl to quinone like intermediates (1,2-dihydroxy, 1,4-dihydroxy and 1,2,4-trihydroxy benzene) that have been subsequently detected on SPCE-R* at Eapp = 0.1 V vs Ag/AgCl. The SPCE-D* was found to trap fraction of the intermediates (quinone like derivatives) on its surface. Discreet electrochemical and physico-chemical characterizations of the phenol exposed SPCE-D* by cyclic voltammetry, Raman spectroscopy and Gas chromatography coupled mass spectroscopy confirmed the formation of different types of quinone like products on its surface. A complicated phenolic compound, zinc phenolsulfonate (ZPS), which has been widely used as an antimicrobial agent in cosmetics, was taken as a model phenolic system for the FIA-DECD. Selective detection of ZPS in different cosmetic real samples has been demonstrated as a validation for the present protocol.

Phenothiazine (PTZ), a thiazine class heterocyclic compound, is a well-known electron donating system and has been widely used as a starting compound to prepare various phenothiazine dyes and pharmaceutically important compounds. Quinones and its derivatives are constituents of biologically active molecules serve as excellent electron-acceptor systems. Oxidation of PTZ by chemical and electrochemical methods often resulted into monohydroxylation of benzene ring moiety, S-oxidized and polymerized compounds as end products. Electrochemical oxidation of PTZ on a multiwalled carbon nanotube (MWCNT) modified glassy carbon electrode in pH 7 phosphate buffers solution (PBS) has been investigated in this work. A highly redox active surface confined PTZ-bi-1,4-quinone derivative (PTZ-biQ) on MWCNT modified glassy carbon electrode, designated as GCE/MWCNT@PTZ-biQ, as a product was unusually observed. The GCE/MWCNT@PTZ-biQ showed well-defined redox peaks at E1/2 = −0.07 and +0.29 V vs Ag/AgCl corresponding to surface confined electron-transfer behavior of the bi-quinone (acceptor) and PTZ-cationic radical species (donor) respectively. No such electrochemical characteristics were noticed when unmodified GCE was subjected to the electrochemical oxidation of PTZ. Existence of PTZ-biQ was confirmed by XRD, Raman spectroscopy, FT-IR and GC-MS (methanolic extract of the active layer) analyses. Position of biQ in PTZ-biQ as 1,4-quinone isomer was confirmed by observation of absence of copper-complexation with 1,4-quinone and H2O2 electrochemical reduction reactions at −0.1 V vs Ag/AgCl unlike to the specific copper-complexation and H2O2 reduction with 1,2-quinone isomer in pH 7. Cysteine (CySH) oxidation was studied as a model system to understand the electron-transfer function of the MWCNT@PTZ-biQ. A highly selective electrocatalytic oxidation and sensing by amperometric i-t and flow injection analysis of CySH at low oxidation potential, 0.3 V vs Ag/AgCl in pH 7 PBS with detection limit values (signal-to-noise ratio = 3) of 11.10 μM and 110 nM respectively, without any interference from other biochemicals like uric acid, dopamine, nitrite, citric acid and H2O2, unlike the conventional chemically modified electrodes with serious interference's, have been demonstrated.

Owing to its importance in the fuel cell, biofuel and bioinorganic chemistry, since 1981, there has been an immense interest in the development of surface-confined ruthenium-oxo complex and to utilize it as a model bio-mimicking system for oxidation of water and alcohol reactions. In this connection, ruthenium-oxo complex has been attached on solid-surfaces by various chemical approaches. Indeed, the preparation of stable and fouling-free surface-confined ruthenium-complex system is a challenging task. Herein, we report a new methodology based on a modification of [Ru(tpy)(DMSO)Cl2]-Nafion (Nf) precursor, wherein, tpy = 2,2′:6′,2″-terpyridine ligand, system on a graphitized mesoporous carbon (GMC) modified electrode followed by electrochemical treatment for in-situ conversion to a high-valent ruthenium oxo complex electrode, designated as GCE/GMC@[(tpy)RuIV/III=O]-Nf, in pH 7 phosphate buffer solution. It showed a stable and well-defined redox peak at a standard electrode potential, Eo’ = 0.780 V vs Ag/AgCl with a surface-excess value, 5.5 × 10−9 mol.cm−2. Some of the kinetic parameters such as transfer coefficient, α and heterogeneous electron-transfer rate constant, ks values were evaluated as 0.54 and 2.43 s−1 respectively for the redox couple. Collective physicochemical characterizations of the surface-confined system by TEM, Raman, UV–Vis and FTIR spectroscopic techniques and ESI-MS analysis, it has been revealed that [RuIII(tpy)(DMSO)(Cl)(OH)]+ complex is surface-confined on the electrode via π-π interaction, pore-encapsulation, and ionic interaction and further transformed to [(tpy)(DMSO)RuIVO]2+ like complex at an applied bias potential, 0.8 V vs Ag/AgCl. This new surface-confined electrode showed high efficient and enzyme-free electrocatalytic oxidation of methanol with a current sensitivity of 39 μA mM−1 in a neutral pH solution.

Bay-region containing polyaromatic hydrocarbons (PAHs) like Benzo(a)pyrene (BaP), which comprise several strained benzenoid rings, are representative organic compounds for the carcinogenic and mutagenic activities in physiological system. In general, cytochrome c, peroxidase and certain soil-bacteria oxidize these compounds to respective hydroxylated metabolites like BaP-7,8-diol (BaP–2OH), that can interact with DNA, RNA and protein, and in turn makes the cellular system dysfunctional. The structure - activity relationship of these compounds is still unclear and therefore, it is necessary for a new analytical approach to delineate the intricate mechanism behind the oxidation of the PAHs. Herein, we report, a simple electrochemical approach of surface-confined oxidation of BaP on multiwalled carbon nanotube (MWCNT) modified glassy carbon electrode (GCE/MWCNT) in physiological condition (pH 7 phosphate buffer solution). It has been found that MWCNT-surface adsorbed BaP (electro-inactive compound) gets electro-oxidized to highly redox active BaP–2OH compound at high positive potential, 1.2 V vs Ag/AgCl, in which, the water molecule was oxidized to molecular oxygen via hydroxyl radical intermediate. From the collective electrochemical and physicochemical studies using Raman, FTIR, GC-MS and 1,2-dihydroxy redox active probe, cysteine (CySH), it has been observed that the hydroxyl radical species produced on the surface has assisted the BaP oxidation to BaP-7,8-diol product, which is similar to the biocatalyzed oxidation of BaP observed in the physiological system. The BaP-7,8-diol surface confined MWCNT modified GCE showed a well-defined and stable redox peak at an apparent standard electrode potential, Eo’ = 0 V vs Ag/AgCl. The redox process is found to be proton-coupled electron-transfer in nature. As an independent study, selective electrocatalytic oxidation of CySH has been demonstrated as an application.

Owing to the high demand of conducting molecular systems for various electronics and technological applications, development of new polyaniline (PANI)/carbon nanomaterial is a continued research interest in cross-disciplinary area of material chemistry. In the literature, for the preparation of PANI/carbon nanomaterial composite electrode systems ex-situ preparation method, in which, chemical oxidation of aniline in presence of carbon nanomaterial, followed by modification on solid substrate has been widely adopted. In this work, a systematic study has been carried out for in-situ electrochemical polymerization of 4-chloroaniline and other substituted aniline monomers to respective PANI on graphitized mesoporous carbon (GMC), that show porous structure and enormous sp2 sites to aniline and PANI for π-π interaction, unusually in neutral pH condition. The new GMC@PANI modified electrode showed a highly redox active peak at Eo’ = 0.12 V vs Ag/AgCl in pH 7 phosphate buffer solution. No such behaviour was noticed when an unmodified glassy carbon was used as working electrode for the polymerization in the above mentioned condition. The redox peak is found to be stable, surface-confined and Nernstian type of proton-coupled electron-transfer in nature. Based on the physicochemical characterization techniques using Raman, IR, X-Ray photoelectron-spectroscopy, scanning electron-microscope and scanning electrochemical microscope (SECM) instruments and electrochemical studies with several ortho, para and meta substituted aniline monomers, mechanism for the in-situ polymerization has been revealed that 4-chloroaniline monomer has undergone a surface-confined electrochemical oxidation reaction with involvement of aniline(4-Cl)-cation radical species, anti-orientation of the monomer (that exist in solution phase), head-to-tail collision (C-N bonding), ejection of the para-substituents as Cl- and Cl•, partial chlorination and PANI(4-Cl) formation mechanism. Such a PANI(4-Cl) film showed a wave-like 2D molecular film structure with enhanced redox and electrical property over the conventional PANI system.

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 (

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.

Developments of new decomposition or degradation methods of environmentally hazardous azo dyes from textile industries are very important. Usually, strong acid-based chemical/electrochemical and neutral pH-based bacterial decomposition methods were widely used. Here, we report a mild, simple, and facile electrochemical method for decomposition of azo dye (Sudan yellow; SY) into a highly redox active copolymer of polyanilines via aniline derivatives as intermediates on a MWCNT modified glassy carbon electrode (GCE/MWCNT) surface unusually in a neutral pH using phosphate buffer solution (PBS) (GCE/MWCNT@SY-CoPANIpH7). One of the intermediate products, aniline (Mw = 93 mol g–1, calculated) was identified by an in situ cyclic voltammetry-electrochemical quartz crystal balance experiment. No such SY electrochemical reaction was observed on a naked GCE surface. Physico-chemical characterizations by TEM, Raman, IR, and UV–vis spectroscopic methods supported the formation of polymeric product on MWCNT surface (GC...

Electrochemical oxidation of phenol on carbon electrodes has often been associated with problems such as serious adsorption, formation of electro-inactive tarry polymers and surface fouling. Thus, it is highly challenging to develop a phenol electrochemical sensor without encountering such problems. Alternately, biosensors, which comprise of enzymes such as tyrosinase and polyphenol oxidase, were widely used for the aforesaid purpose. Herein, we introduce an ultra-low cost 6B grade pencil graphite, pre-anodized at 2 V vs. Ag/AgCl, designated as 6B-PGE*, where * = preanodized, as a novel electrochemical sensor for surface fouling-free and efficient differential voltammetric (DPV) detection of phenols (meta-cresol and phenol) in pH 7 phosphate buffer solution (PBS). A well-defined cyclic voltammetric peak at 0.65 ± 0.02 V vs. Ag/AgCl, which is stable under multiple electrochemical cycling, was noticed upon electrochemical-oxidation of meta-cresol and phenol at 6B-PGE*. The 6B-PGE* showed eight times higher DPV current signal and 60 mV lower oxidation potential than non-preanodized electrode (PGE) for the phenol detection. Under optimal DPV conditions, the 6B-PGE* showed a linear calibration plot with current linearity in a range of 40–320 μM with current sensitivity and detection limit (signal-to-noise = 3) values of 1.43 μA μM−1 cm−2 and 120 nM, respectively. Six repeated detections of 80 μM meta-cresol without any interim surface cleaning process showed a relative standard deviation (RSD) value of 0.21%. This electro-analytical approach was validated by testing total phenolic contents in three different insulin formulations with an electrode recovery value of ∼100%.