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Search for simple and economical electrocatalyst for the hydrogen gas evolution reaction (HER), which can resemble to the performance of Pt and other precious metals, is a challenging research interest. In this work, a systematic effect of pre-treatment potential of screen-printed carbon (SPCE) surface on the HER performance in 0.5 M H2SO4 was carried out. A new observation of a low potential HER (onset potential, Eonset = −0.02 V vs. RHE) at a cathodic potential, −0.5 V vs. Ag/AgCl on 1 hr pre-treated screen-printed carbon electrode (SPCE*, * = pre-treated) was observed. Physicochemical and electrochemical characterizations of the SPCE* by field emission scanning electron-microscope, Raman, IR and X-Ray photoelectron spectroscopes reveals specific generation of carboxylic acid functionalized carbon surface and in turn for the enhanced HER on the modified electrode surface. Electrochemical characterization of SPCE* with Fe(CN)63− support the observation. A marked decrement in the peak current and significant increase in the peak-to-peak separation potential response due electrostatic repulsion between the anion sites of Fe(CN)63− and –COO– were noticed. This observation is in parallel with the reduced electrical double layer capacitance value of the SPCE* system. The Eonset and Tafel value (54.7 mV dec−1) obtained here are comparable to those at Pt, MoS2, MoSe2 and superior over the N- and P-doped graphene/carbon electrocatalysts for HER. A prototype HER system was developed and demonstrated for H2 gas production at a rate of 0.0053 μM s−1 (Operating potential = −0.5 V vs Ag/AgCl), which is comparable to that of precious metal and metal compound electrocatalysts based HER performance.

An ex-situ pharmacokinetic of Triamterene (Trim) drug has been investigated by electrochemical technique using multiwalled carbon nanotube modified glassy carbon electrode, GCE/MWCNT (GCE/MWCNT@Trimads) as a biomimicking carbon network system. Trim is a phenyl ring containing electro-inactive drug that has been widely used for the treatment of diuretic and antihypertensive related diseases. Upon cyclic volumetric measurement of Trim-drug adsorbed GCE/MWCNT at an optimal potential window, −1 to 0.9 V vs Ag/AgCl showed formation of a well-defined surface-confined redox peak at Eo' = 0.1 V vs Ag/AgCl in pH 7 phosphate buffer solution. Calculated surface-excess value is 0.85 × 10−9 mol cm−2 and it is proton-coupled electron-transfer in nature. Collective characterization results of the modified electrode by physicochemical techniques (Raman, IR and Mass spectrophotometer) and electrochemical methods (effect of potential window) reveal hydroxylation of the phenyl ring of the Trim-drug to phenolic derivative (intermediate) in association with involvement of H2O2 intermediate species that have been generated by oxygen reduction reaction at negative potential and subsequent oxidation to trihydroxy benzene-Trim drug (Trim-3HQ). The GCE/MWCNT@Trim-3HQ showed selective mediated oxidation current signals for cysteine in relation with importance of the free thiol molecules and its conversion to disulfide (CyS-SCy) at pathogenic condition in the biological system like human serum albumin. As an independent study, amperometric i-t and flow injection analysis of CySH have been demonstrated with a detection limit value 695 nM.

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.

A three-dimensional hierarchical network of bismuth nanoparticles decorated graphene-carbon nanotubes nanocomposite (Bi NPs@Gr-CNTs) was synthesized and employed for electrocatalytic detection of mercury (Hg (II)). The electrocatalyst was characterized via scanning electron microscopy, transmission electron microscopy, Energy-dispersive X-ray spectroscopy, X-ray diffraction, FT-IR, electrochemical impedance spectroscopy, and cyclic voltammetry. The electrocatalytic activity of Bi NPs@Gr-CNTs modified screen-printed carbon electrode (SPCE) toward Hg (II) was studied using cyclic voltammetry, and differential pulse voltammetry. The Bi NPs@Gr-CNTs/SPCE exhibited excellent electrocatalytic ability to Hg (II) in comparison to control electrodes. Under optimized conditions, Bi NPs@Gr-CNTs/SPCE exhibits excellent Hg (II) sensing attributes in the range of 1.0 nM–217.4 µM with 0.2 nM of detection limit. The electrode was specific for Hg (II) in presence of other metal ions ascribe excellent selectivity. Practicality of the method was demonstrated in tap water, fish oil tablet, human serum, and urine samples (spiked method), which presented acceptable recoveries.

Metal and heteroatom-free electro-active carbon soot nanoparticles of average size 50 ± 10 nm have been prepared by simple atmospheric combustion of abundant naphthalene compound. Physicochemical characterizations by Transmission electron-microscope, Infrared, Raman spectroscopy, X-Ray diffraction, and CHN analysis techniques revealed that the soot has a graphitic core with highly defective oxygen structure composed of carbonyl, hydroxyl, ether, furan, pyran, pyrone and carboxylic acid along with aliphatic carbons on the surface. Electrochemical characterization with a bench mark redox system, Fe(CN)63− reveals the efficient conductive behaviour of the new soot material even after mixing with the anionic-Nafion membrane (Nf). The GCE/Soof-Nf showed a well-defined oxygen reduction reaction (ORR) current signal at cathodic peak potential, −0.39 V vs Ag/AgCl similar to that of the heme, hemoglobin and quinone based electrochemical systems for oxygen reduction reaction (ORR) in neutral media. Obtained ORR peak current signal is found to be 2–5 times higher than that of the current signals noticed with activated charcoal, graphite nanopowder, functionalized MWCNT and bulk gold electrodes. Rotating disc electrode and bio-potentiostat coupled flow-injection analysis techniques have been adopted to find out the kinetics and mechanism of the ORR. Utilizing the new carbon soot material, sensitive detection of dissolved oxygen with detection limit 8.6 ppb was demonstrated in neutral buffer solution.

Biomass waste, a good candidate for advanced carbon materials for sustainable electrodes, is receiving more and more attention for high value-added materials because of its promising contribution to economic growth and sustainable development. We proposed a green co-hydrothermal approach to prepare lotus seedpods biochar (BC) decorated molybdenum disulfide (MoS

pH value of a non-invasive sample is an important biomarker in diagnosing certain clinical disorders of a human body. Herein, we report, a stable, surface-confined and redox active poly(1-naphthylamine) (PNPA) on multiwalled carbon nanotube modified gold and pencile graphite electrodes (Au/MWCNT@PNPA and PGE@PNPA) prepared via electrochemical polymerization of 1-naphthylamine (1-NPA) in pH 7 phosphate buffer solution for sensitive and selective pH monitoring of non-invasive samples (saliva, tears and urine). Control experiment with 2-NPA isomer failed to show such redox feature, indicating the selective electrochemical polymerization of 1-NPA. Physicochemical characterizations of MWCNT@PNPA by SEM, TEM, XPS, Raman, FTIR, UV–vis (an ethanolic extract) and several control electrochemical experiments revealed that 4th (para) position of 1-NPA isomer is involved in the initiation of electro-polymerization on a carbon surface and followed by a stable redox polymer formation. Interestingly, the redox peak responsible for pH sensing showed negligible to permissible level of alteration with several biochemicals. As as a diverse application, selective monitoring of bacterial (E. coli) growth was demonstrated using this new electrochemical pH sensor system with result comparable to that of conventional pH sensor.

Chromium(III) species is an essential micronutrient, whereas, its hexavalent form, Cr(VI) is considered to be a carcinogen. For the selective detection of Cr(III) in presence of Cr(VI), separation coupled spectroscopic techniques have been often used. For the first time in this work, we report a flow injection analysis coupled dual electrochemical detector (FIA-DECD) for separation-free detection of Chromium (III) and (VI) species. A gold nanoparticles decorated carbon nanofibres-chitosan modified electrode has been prepared by a quick and in-situ electrochemical deposition of Au3+ ion in pH 7 phosphate buffer solution (PBS) and has been used as a dual electrochemical detector for Cr(VI)-reduction and Cr(III)-oxidation reactions in pH 2 PBS. Under an optimal hydrodynamic FIA-DECD condition, i.e., at applied potentials 0.1 V (for Cr(VI)-reduction) and 1 V vs Ag/AgCl (for Cr(III)-oxidation) and at a flow rate = 0.8 mL min−1, calculated linear range and detection limit values are; 0.1–100 ppm and 0.69 ppb (0.72 ppt for 20 μL sample loop volume) for Cr(III) and 0.1–100 ppm with detection limit, 0.32 ppb (0.33 ppt for 20 μL) for Cr(VI). No marked interference from other cations and anions like Cu2+, Zn2+, Cd2+, Pb2+, Co2+, Ca2+, Fe2+, Mg2+, NO3−, NO2−, SO32− and SO42− were noticed. Selective detection of Cr species (Cr(III) and Cr(VI)) in industrial waste water samples with data comparable to ICP-OES was demonstrated.

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.

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.

Studies related to interaction of whole blood with dissolved oxygen with a new analytical method is a continued research interest in the interdisciplinary areas of biomedical, biochemistry and analytical chemistry. There has been significant number of reports for spectroscopic and electrochemical investigation for the interaction of heme/hemoglobin with dissolved oxygen. Indeed, there is no direct study relating to electron-transfer behaviour of whole blood and oxygen reduction reaction. Complex nature of the blood and difficulty in transferring the electron from the buried hemoglobin in the blood cell are the limitations for the observation. Herein, we report, a human whole blood modified electrode system using graphitized mesoporous carbon and Nafion as a matrix for direct-electron-transfer reaction with dissolved oxygen in physiological solution. The blood modified electrode exhibits a specific redox peak at an apparent standard electrode potential, E°′ = − 0.38 V vs Ag/AgCl corresponding to the electron-transfer reaction of hemoglobin-Fe(III)/Fe(II) redox system and shows a mediated oxygen reduction reaction similar to a commercial Hb modified electrode response. Using the new bio-analytical system, interaction between the blood and dissolved oxygen was studied in terms of oxygen reduction reaction (ORR). Cyclic voltammetry, rotating disc electrode and flow injection analysis coupled with a dual electrode system, wherein, blood and MnO 2 modified electrode systems are used for signals corresponding to the oxygen reduction and H 2 O 2 oxidation reactions, kinetics and mechanism. As an extension to practical applicability, quantification of dissolved oxygen using the blood modified electrode was demonstrated with result comparable to that of a commercial dissolved oxygen measurement kit.

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 mystery of > 2000 years of Hindu culture holy basil plant-Tulsi (Ocimum tenuiflorum)’s active ingredient for health benefit has been revealed by performing a blind cyclic voltammetric based electrochemical experiment (without pre-targeted chemicals) with a crude solution of the Tulsi-plant-water extract (TE) using a graphitized mesoporous carbon nanomaterial modified glassy carbon electrode (GCE/GMC) in pH 2 solution. The as-prepared chemically modified electrode was characterized using TEM, FTIR, Raman, UV–Vis, HPTLC, UPLC and with several control samples to identify the active molecular species trapped by GMC. It has been revealed that amongst various phytochemicals-ingredients, the rosmarinic acid (RA) in the TE-extract trapped selectively on the graphitic sites and showed an exceptionally efficient redox behavior at an apparent standard electrode potential, Eo’ = 0.550 V vs Ag/AgCl with a peak-to-peak potential difference, ΔEp ∼ 0 V. A specific interaction between the π-electrons of the aromatic compound and sp2 carbon of GMC is found to be an influencing parameter for the selective electrochemical micro-extraction of the RA from the TE (GMC@TE-RA). This electrochemical methodology helps for qualitative and quantitative analysis of the phytochemical, RA in the TE. The GMC@TE-RA is found to involve selective electrocatalytic oxidation of phenylhydrazine-pollutant in pH 2 KCl-HCl medium. Electrochemical impedance spectroscopic analysis of the modified electrode showed a superior electronic activity over the respective unmodified electrode. A scanning electrochemical microscopy (SECM) has been used to image the active site of the electrocatalytic surface. As a sustainable and green electrochemical approach, a highly selective electrocatalytic oxidation and flow injection analysis-based sensing of phenylhydrazine using the GCE/GMC@TE-RA has been demonstrated without any interference from hydrazine and other common electroactive chemicals and biochemicals.

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.

Curcumin, a natural pigment of the turmeric, has been widely used as a phytonutrient for various health benefits. There are several electrochemical reports relating to curcumin-metal complex modified electrodes for electroanalytical applications, and sensing of curcumin utilising the electrochemical behaviour of the phenolic site using different carbon-based chemically modified electrodes, in the literature. Herein, we report, a curcumin-quinone (Cur-Q) derivative immobilised carbon black (CB) modified glassy carbon electrode, designated as GCE/CB@Cur-Q, prepared by an in-situ electrochemical oxidation method, for efficient electrocatalytic oxidation and electrochemical sensing application of sulfide in pH 2 KCl-HCl solution. The “as prepared” GCE/CB@Cur-Q showed a sharp redox peak at an apparent standard electrode potential, Eo′ = 0.55 V vs. Ag/AgCl with surface excess and peak-to-peak potential values of 12.37 × 10− 9 mol cm− 2 and 19 ± 5 mV respectively. No such electrochemical behaviour was noticed when unmodified GCE was subjected to the electrochemical oxidation of curcumin. The existence of Cur-Q was confirmed by Liquid chromatography-mass spectrometer and in-situ electrochemical quartz crystal microbalance measurements. Amongst various carbon materials like CB, multiwalled carbon nanotube (MWCNT), single-walled carbon nanotube, oxygen functionalized MWCNT, graphite nanopowder, graphitised mesoporous carbon, carbon nanofiber, graphene oxide and activated charcoal investigated, CB was showed an excellent electrochemical reaction for the in-situ formation of Cur-Q. A highly sensitive electrocatalytic oxidation and sensing by amperometric i-t and flow injection analysis of sulfide at operating potential, 0.55 V vs. Ag/AgCl with detection limit values (signal-to-noise ratio = 3) of 2.4 × 10− 6 and 7.12 × 10− 6 M respectively, without any interference from other biochemicals like uric acid, dopamine, nitrite, cysteine, NADH and H2O2, unlike the conventional chemically modified electrodes with serious interferences, have been demonstrated.

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.

Isoniazid (INZ) is an effective anti-tuberculosis drug that has been widely used for chemotherapy of tuberculosis. Development of new and simple electroanalytical method for routine analysis of INZ is a continued research interest in the field of analytical and pharmaceutical chemistry. Although there have been several electroanalytical reports, flow injection analysis coupled electrochemical detection of INZ, which is an advanced electrochemical technique useful for practical applications, is rarely reported in the literature. Instability problem associated with the underlying electrode by the amino-functional group of the INZ is the prime reason for the limitation. Herein, we report a gold nanoparticles decorated carbon nanofibers-chitosan modified carbon screen printed electrode, designated as Aunano@CNF-CHIT/SPE, prepared by drop-casting of Au3 + ion directly on CNF-CHIT/SPE underlying electrode followed by electrochemical potential cycling, as an elegant electrochemical detector system for high stable FIA of INZ in pH 7 phosphate buffer solution. The modified electrode showed about fifty times higher electrocatalytic current and 700 mV reduction in the over-potential values than that of a polycrystalline gold modified electrode towards INZ. Under an optimal hydrodynamic FIA condition, the present ECD showed a wide linear range from 1 μM to 1 mM with regression coefficient and sensitivity values of 0.9958 and 16.1 nA μM− 1 respectively. The calculated limit of detection and limit of quantification values are 172 nM (i.e., 472 pg/20 μL) and 570 nM (i.e., 1.57 μg/20 μL), respectively. The present FIA-ECD was successfully applied for the real sample analysis by detecting INZ in two pharmaceutical formulations with satisfactory good 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.

Owing to facile electron-transfer reaction, metal complex based molecular architecture has attracted much interest in electrochemistry, especially for bioinspired electrocatalytic and electrochemical sensor applications. Indeed, preparation of stable surface-confined molecular system is a challenging task. In general, derivatization methodology, in which, a specific functional groups such as thiol, carboxylic acid, pyrene and amino bearing inorganic complexes synthesized discreetly by chemical approach have been attached suitably on electrode surface via any one of the following techniques; self-assembly, covalent immobilization, electrostatic interaction, ionic exchange and encapsulation. Herein, we report a copper-bipyridyl complex immobilized multiwalled carbon nanotube (MWCNT)-Nafion (Nf) modified glassy carbon electrode (GCE/Nf-MWCNT@bpy-Cu2+) prepared by a new strategy in which sequential modification of bipyridyl (bpy) ligand on MWCNT via π-π interaction followed by in-situ complexation with copper ion for efficient electrochemical reduction of H2O2. The copper species chemically modified electrode showed highly stable and well-defined surface-confined Cu2+/1+ redox peak response, without any Cu1+/0 redox transition, at an equilibrium potential, E1/2 = −135 mV vs Ag/AgCl in a pH 7 phosphate buffer solution. Detailed physico-chemical characterization by SEM, FT-IR, Raman and ESI-MS and electrochemical characterization reveals that [Cu(bpy)2(H2O)2]+ (molecular weight 413.4) like species was immobilized as a major species on the modified electrode. A bioinspired electro-catalytic reduction of H2O2 was studied using cyclic voltammetric and rotating disc electrode techniques. In further, electrochemical sensing of H2O2 by amperometric i-t and flow injection analysis methods with a detection limit values 4.5 and 0.49 μM respectively were demonstrated.

Development of new sensor material suitable for simple, cost effective and quick practical application is a demanding research interest in electroanalytical chemistry. Carbon based inks prepared using different type carbon materials and binders as a gel or suspension have been often used as active sensor materials. Unfortunately, those systems are involved with complicated preparation route and usage of hazardous chemicals. Herein, we report a water based carbon ink composed of low cost carbon black powder, chitosan and acetic acid (prepared within 15 ± 2 min) for electrochemical sensor applications. A new carbon ink modified electrode was fabricated in this work by drop-casting of micro-litre quantity of the CB-Chit ink on a cleaned glassy carbon electrode (GCE) followed by air drying in room temperature for 10 ± 2 min. The CB-Chit ink modified GCE (GCE/CB-Chit) showed enhanced electrical conductivity, surface area and electrochemical activity than that of the unmodified GCE. Investigated simultaneous electrochemical oxidation and sensing of three biologically important molecules such as ascorbic acid, dopamine and uric acid on GCE/CB-Chit displayed excellent peak current signals at well-defined peak potentials with linear concentration ranges of 25–1600, 0.1–1400, and 5–1800 μM and detection limit of 0.1 μM (S/N = 3) for the all analytes. The newly fabricated sensor was validated by applying to the detection of AA, DA, and UA in vitamin C tablets, dopamine hydrochloride injection and human urine real samples. The sensor can be operated without any surface pre-treatment and analyte adsorption, unlike to the carbon nanotube and graphite based electrode systems with serious adsorption complications.

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.

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.

Ellagic acid (EA) is a lactone and polyphenolic functional groups-containing phyto-chemical that has been widely used as an anti-oxidant, anti-cancer and anti-aging cosmetic agent. EA is known as a stable redox active system only in organic medium, but irreversibly oxidized with coupled chemical reactions showing ill-defined redox peaks in aqueous solutions. Interestingly, we report here that electro-oxidation of EA tethered multiwalled carbon nanotube-modified glassy carbon electrode (GCE/MWCNT@EA) showed a well-defined pair of redox peaks with a surface-confined characteristics at Ef = 0.020 V vs Ag/AgCl corresponding to ortho-quinone moiety of oxidized EA in pH 7 phosphate buffer solution (PBS). No such behavior was noticed with EA adsorbed GCE. The GCE/MWCNT@EA was characterized by cyclic voltammetery (CV) and the transfer coefficient (α) and electrode to redox surface layer electron transfer rate constant (ks) were calculated. Physicochemical characterization of MWCNT@EA by FTIR, XRD and Raman Spectroscopy techniques revealed immobilized EA in its native form on MWCNT. Effect of various CNTs on EA electro-immobilization and the features that distinguish each other was highlighted. The GCE/MWCNT@EA showed excellent electrocatalytic activity toward N2H4 oxidation. The mechanism and kinetics of the catalytic reaction was investigated by CV, and the kinetic parameters ca., number of electrons in the rate determining step (na′), total number of electrons (n′), reaction order with respect to N2H4, catalyst reaction rate constant (kchem) were evaluated. Finally, amperometric i-t and flow injection analysis for highly selective sensing of hydrazine without any interference from other biochemicals were validated.

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.

Most of the conventional electrochemical cells require a minimum sample volume of 2–5 mL for the analyses. However most of the times, availability of biological sample in this much quantity is challenging and may be the reason for misreading. Besides, a lower volume cell is always preferred for analyzing rare or expensive materials. Thus, development of a miniaturized electrochemical cell setup (MECS) with a lower volume capacity at an affordable cost is a paramount of interest. In this present work, a home-made MECS is developed using a disposable graduated microcentrifuge tube vial as a low-volume (150 μL) holding electrochemical cell and 0.7 mm HB-pencil graphite electrode (PGE), Ag wire and PGE as working, reference and counter electrode respectively. These three electrodes are fused into the disposable graduated microcentrifuge tube vial. In our preliminary investigations, kinetic parameters and electro-catalytic response of uric acid (UA) on working electrode (PGE* = predanodized PGE) was demonstrated as a model system for the application of this newly developed MECS. Furthermore, the developed electroanalytical approach was validated by testing the concentration of UA in non-invasive bio-samples (urine and saliva) with an electrode recovery value of ~ 100%.

Coenzyme Q10 (2,3-dimethoxy-5-methyl-6-decaprenyl-1,4-benzoquinone, Q10) is a fat soluble, hydrophobic, vitamin-like quinone present in the cell membranes, regulates metabolic pathways via redox signalling. There have been considerable reports relating to the structural bio-mimics of Q10, wherein, lipid type molecular matrix stabilized Q10 modified micro-porous electrode systems have been used. Most of the reported procedures showed either nil or very poor redox functional activity of the Q10 in cyclic voltammetry (CV) analysis. Here in, we report, a Q10 stabilized functionalized-multiwalled carbon nanotube modified glassy carbon electrode (GCE/f-MWCNT@Q10) as a functional biomimetic system for Q10. CV response of the GCE/f-MWCNT@Q10 showed a well-defined redox active at E 1/2 = −138 mV vs Ag/AgCl with peak-to-peak separation (Δ E p ) and surface excess ( Γ Q10 ) values of 275 mV (at v = 10 mV s −1 ) and 12.57 n mol cm −2 respectively. Amongst various carbon nanomaterials investigated, f-MWCNT was found to be the best for the Q10 functional activity. From the physicochemical characterizations it was identified that interactions such as pi-pi, hydrogen-bonding and iron metal impurity-oxygen existing in f-MWCNT@Q10 aided to stabilize molecular structure of the Q10 on f-MWCNT surface. Using CV technique, pro-oxidant activity of Q10 with NADH, ascorbic acid, cysteine, glucose and hydrazine were tested and found that observed interactions were similar to one existing with the real biological system. The f-MWCNT@Q10 system showed efficient pH sensing ability with Nernstian type proton-electron transfer mechanism.

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.

In the present work, we report a simple chemical precipitation method for the preparation of different levels of Cu doped MnO2 nanocrystals. X-ray diffraction, field emission transmission electron microscope and X-ray photoelectron spectroscopy were used to study the material properties. To demonstrate the suitability of the doped products for electrode applications electrochemical properties were evaluated by Cyclic Voltammetry (CV), galvanostatic charge–discharge studies and impedance spectroscopy. The results indicate that the MnO2 electrode modified with 0.1 M of Cu has a better electrode property with a specific capacitance of 583 F/g and an energy density of 80 W h kg−1.

Organic redox mediator functionalized carbon nanomaterials has been considered the future of advanced nanomaterials owing to their exemplary behaviors. It is highly challenging to prepare stable functional carbon nanomaterials. Herein, we report a simple preparation of highly redox active and stable quinoline quinone (QLO) functionalized multiwalled carbon nanotube (MWCNT) modified glassy carbon electrode (GCE/MWCNT@QLO) by in-situ electrochemical oxidation of 8-hydroxyquinoline (QL) on a GCE/MWCNT in pH 7 phosphate buffer solution. Unlike the formation of electro-inactive and tarry polymeric products with electrochemical oxidation of QL at glassy carbon electrode, a multi-redox active QLO derivative intermediate was electro-generated as intermediate species and was trapped on a MWCNT modified electrode in this work. Specific pi-pi interaction between QL and MWCNT and its diffusion restrictive electrochemical oxidation process are keys for the selective entrapment of QLO on the underlying electrode. The QLO trapped MWCNT modified electrode (GCE/MWCNT@QLO) was characterized by Transmission electron microscope, X-ray photoelectron spectroscopy, Raman spectroscopy, Infrared spectroscopy, gas liquid chromatography coupled mass spectrometry and in-situ cyclic voltammetry (CV) electrochemical quartz crystal microbalance. CV of the GCE/MWCNT@QLO has displayed three well-defined redox peaks at E1/2 = -0.45 V (A1/C1), -0.1 V (A2/C2) and 0 V vs Ag/AgCl (A3/C3) corresponding to the electron-transfer behaviors of pyridinium ion, 2,3 and 5,8 diquinone functional groups of the MWCNT@QLO. The intermediate trapped modified electrode showed excellent electrocatalytic behavior and amperometric current-time sensing response to an environmental pollutant, hydrazine at 0 V vs Ag/AgCl in pH 7 phosphate buffer solution with a calibration plot linearity and detection limit values of 25-450 μM and 12 μM respectively without any interference from ascorbic acid, uric acid, cysteine and nitrate. In further, flow injection analysis coupled electrochemical of hydrazine at 0 V vs Ag/AgCl with a detection limit value 0.7 μM was also demonstrated.

Hydrazine is one of the hazardous chemicals present in tobacco and known to be human carcinogen. It is highly challenging to detect hydrazine present in the tobacco selectively. Since, hydrazine molecule didn’t have any chromophore, indirect methods like separation coupled derivatization technique have been reported for the detection. Herein, we report a direct and separation-less flow injection analysis (FIA) coupled electrochemical detection technique (ECD) for hydrazine in tobacco. Quercetin (Qn, a plant-derived flavonoid found in fruits, vegetables, leaves and grains) tethered pristine-multiwalled carbon nanotube modified glassy carbon electrode (GCE/pristine-MWCNT@Qn) has been developed as a selective electrochemical detector for FIA and characterized by electro-chemical and physico-chemical techniques. At an optimal condition the new FIA-ECD showed a hydrazine calibration plot in a range 5–3000 μM with a detection limit value 136 nM/20 μL. This electrochemical detector is found to be tolerable to several electro-active chemical entities such as ascorbic acid, uric acid, dopamine, cysteine, nitrite, benzene, ammonium chloride, nitrate, citric acid, oxalic acid (negligible interferences), aromatic amine and sulfide. The applicability of this technique is further tested by analyzing the hydrazine content in different brand of cigarettes with appreciable recovery values.

Prussian blue (PB), an inorganic iron-hexacyanoferrate polymer, is a potential redox system. In general, it can be prepared on any conducting substrate by electrochemical deposition technique in which a working electrode is potential cycled with millimolar quantities of a mixture FeCl 3 and [Fe(CN) 6 ] 3− in an alkaline metal ion containing acid solution (pH 6 ] 3− is subjected to the electrochemical potential cycling technique with a Iron(III) adsorbed multiwalled carbon nanotube modified glassy carbon electrode (GCE/MWCNT–Fe 2 O 3 ), as a template, in pH 2 KCl–HCl. The PB electrode thus designated as GCE/MWCNT–Fe 2 O 3 @PB showed well-defined redox peaks at apparent standard electrode potentials, 190 ± 5 and 850 ± 5 mV vs Ag/AgCl and synergistic effect of PB, MWCNT and Fe 2 O 3 systems. As prepared MWCNT–Fe 2 O 3 @PB was characterized by various physico-chemical techniques such as Scanning electron microscopy, Energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. Presence of trace amount of the precursor, Fe 2 O 3 within the inner core of the hybrid PB system was evident from the characterization results. In further, it was found that one of the metallic impurities, iron in the MWCNT has influenced the PB formation. The new hybrid PB electrode exhibited 17 times higher reduction current value over the conventionally prepared GCE/PB for the H 2 O 2 electrocatalysis. Applicability of the electrode to sensing of H 2 O 2 in real sample such as milk, cream bleach and clinical solution was successfully demonstrated.

Adenine (A) and guanine (G), important bases of nucleic acids, are often analyzed by separation coupled spectroscopic detection methods. Herein, we are demonstrated a new flow-injection analysis (FIA) coupled dual electrochemical detector (DECD), where a chitosan-carbon nanofiber (Chit-CNF) modified glassy carbon electrode prepared by a simple technique and pH 7 phosphate buffer solution as a carrier system, for separation-less quantification of G and A. This method is highly selective and no interference by the presence of the other DNA bases (Thymine and Cytosine). The FIA-DECD was operated at two different operating potentials, E1 = 0.80 V and E2 = 0.95 V vs Ag/AgCl, where G and {G + A} get oxidized, respectively. Amount of A was calculated from the difference between the FIA current signals, measured at E20.95V and E10.80V. The GCE/Chit-CNF was characterized by cyclic voltammetry with potassium ferricyanide system and Raman spectroscopy. The modified electrode showed unique electron-transfer feature with metal like conductivity. Under an optimal condition, FIA-DECD showed linear calibration plots for G and A in a concentration range, 200 nM—50 μM with current sensitivity values 13.83 ± 0.48 and 4.84 ± 0.11 nA μM−1 respectively. Calculated detection limit (signal-to-noise ratio = 3) values were 46.8 nM and 73.8 nM for G and A respectively. Applicability of the present technique was further demonstrated by detecting G and A in beef kidney sample and DNA hybridization process.

Riboflavin (vitamin B2) immobilized multiwalled carbon nanotubes modified electrode (GCE/MWCNT@RB) has been prepared and demonstrated as a selective and sensitive electrocatalytic flow injection analysis electrochemical sensor for iodate detection. The GCE/MWCNT@RB modified electrode showed a highly stable and well defined surface confined redox peak at an E 1/2 = 155 ± 20 mV vs. Ag/AgCl with a surface excess value of 4.72 nmol cm −2 . Physicochemical and electrochemical characterizations reveal the presence of surface confined riboflavin sites on GCE/MWCNT system. At an applied potential = −0.1 V vs Ag/AgCl, hydrodynamic flow rate = 800 μL min −1 and phosphate buffer (pH 2) as a carrier solution condition, the analytical characteristics such as stability, repeatability, linear range and detection limit were described. The relative standard deviation of successive 29 injections of 25 μM of iodate was 3.61%. Iodate calibration plot was linear in a range of 25–750 μM with regression coefficient, sensitivity and limit of detection (S/N = 3) of 0.9998, 0.77 μA mM −1 and 2.7 μmol L −1 (9.4 ng/20 μL) respectively. The new system showed absence of interference with iodide, chloride, nitrate, bromate and perchlorate. Quantitative detection of iodate in iodized table salts was further demonstrated.