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Herein, we report in-situ electrochemical derivatization of the intrinsic iron species in a pristine-multiwalled carbon nanotube (MWCNT) as Prussian blue (PB) modified MWCNT hybrid (MWCNT@PB) using a dilute solution of ferricyanide as a derivatization agent in pH 2 HCl-KCl solution. The PB hybrid system showed a defined redox peak at an apparent standard electrode potential, Eo’ = 0.18 V vs Ag/AgCl with an excess surface value, 1.71 × 10−10 mol cm−2. A discreet EQCM study on the electrochemical preparation of MWCNT@PB using MWCNT and ferricyanide precursors reveal the specific stripping of iron species and uptake of iron species, potassium and ferricyanide ions upon the electrochemical preparation condition. In-situ imaging of MWCNT@PB was carried out using SECM with ferricyanide as a redox mediator under a feedback-current mode. It has been identified that a mixed-potential based electrochemical reaction involving oxidative stripping of iron to iron ion species (step-1) coupled with reduction of ferricyanide to ferrocyanide (step-2) followed by a chemical interaction between the iron ion and ferricyanide (step-3) have occurred for the overall formation of MWCNT@PB hybrid. Electrocatalytic and electroanalytical performance of the MWCNT@PB hybrid towards H2O2 reduction and sensing were demonstrated by performing cyclic voltammetric, amperometric i-t and batch injection analysis.

Pencil graphite electrode (PGE) is an alternative, commercially available, ready-to-use, screen-printed electrode for a wide range of electroanalytical applications. Due to the complex-matrix composition and unpredictable electro-inactive nature of PGE in its native form, a surface pre-treatment/activation procedure is highly preferred for using it as an electroactive working electrode for electroanalytical applications. In this article, we review various surface pre-treatment and modification procedures adopted in the literature with respect to the sensitive and selective detection of dopamine as a model system. Specific generation of the carbon–oxygen functional group, along with partial surface exfoliation of PGE, has been referred to as a key step for the activation. Based on the Scopus® index, the literature collection was searched with the keywords “pencil and dopamine”. The obtained data were segregated into three main headings as: (i) electrochemically pre-treated PGE; (ii) polymer-modified PGEs; and (iii) metal and metal nanocomposite-modified PGE. This critical review covers various surface activation procedures adopted for the activation for PGE suitable for dopamine electroanalytical application.

We report an easy electrochemical route for preparing a catechol derivative (di-ferulyl sesamol derivative, designated as CA-Fer) functionalized low-cost carbon black (CB) modified electrode in a neutral pH buffer solution. As synthesized precursor organic compound, CA-Fer has been electrochemically oxidized over the CB by potential cycling treatment in pH 7 PBS that leads to a high redox active CA-Fer-catechol derivative (CB@CA-Fer-Redox), which exhibited a well-defined and stable surface-confined redox response at Eo’ = 0.150 V vs Ag/AgCl with a surface excess value = 50.46 nmol cm−2. Unlike the conventional redox active mediators, GCE/CB@CA-Fer-Redox modified electrode has failed to show any mediated oxidation/reduction response to various electro-active biochemicals and chemicals, viz., ascorbic acid, glucose, cysteine, caffeic acid, hydrazine, hydrogen peroxide, uric acid, dopamine, creatinine, urea, nitrite, sulfide and sulfate ions, depicting a clear advantage of using it as a voltammetric pH sensor for real-time applications. The modified electrode showed a linear voltammetric potential signal against pH in a window, 3–11, with a slope value of (∂Ep/∂pH) = −59 ± 3 mV pH−1. As an independent study, a CA-Fer-Redox modified three-in-one screen printed electrode was developed, and a sensitive voltammetric pH analysis of some chemical biochemical real sample systems were demonstrated.

A simple, one-step and facile method has been introduced to prepare fluorescent and electrochemically active carbon nanoparticles with single-size distribution and good long-term stability by electrochemical exfoliation of polyacrylonitrile-based carbon fibers in an alkaline solution-phase condition. The preparation condition was systematically optimized by studying the effect of temperature and electrolytes. It has been found that an electrochemical exfoliation reaction carried out at an applied potential of 2 V vs. Ag/AgCl in a phosphate-ion-containing alkaline solution at a temperature of 40 °C is an ideal condition for the preparation of 14 ± 4 nm-sized carbon nanoparticles. Unlike the literature protocols, there are no filtration and membrane dialysis-based off-line sample pretreatments adopted in this work. The as-prepared carbon nanoparticles were characterized by fluorescence, Raman spectrum, transmission electron microscope, and X-ray photoelectron spectroscopic characterization methods. It was found that the carbon–oxygen functional group rich in graphene–oxide quantum dots (GOQDs) such as carbon nanoparticles were formed in this work. A preliminary study relating to simultaneous electrochemical oxidation and the sensing of uric acid and ascorbic acid with well-resolved peaks was demonstrated as a model system to extend the new carbon material for electroanalytical applications. Furthermore, in situ synthesis of 2 nm-sized gold nanoparticles stabilized by GOQDs was presented. The carbon nanoparticles prepared by the direct method in this work have shown good stability over 6 months when stored at room temperature. The electrochemical exfoliation reaction has been found to be highly reproducible and suitable for bulk synthesis of luminescence-effective carbon nanoparticles to facilitate fundamental studies and practical applications.

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...

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 [Mn

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...

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.

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.

As an alternate for the conventional glass-based pH sensor which is associated with problems like fragile nature, alkaline error, and potential drift, the development of a new redox-sensitive pH probe-modified electrode that could show potential, current-drift and surface-fouling free voltammetric pH sensing is a demanding research interest, recently. Herein, we report a substituted carbazole-quinone (Car-HQ) based new redox-active pH-sensitive probe that contains benzyl and bromo-substituents, immobilized multiwalled carbon nanotube modified glassy carbon (GCE/MWCNT@Car-HQ) and screen-printed three-in-one (SPE/MWCNT@Car-HQ) electrodes for selective, surface-fouling free pH sensor application. This new system showed a well-defined surface-confined redox peak at an apparent standard electrode potential, Eo′ = − 0.160 V versus Ag/AgCl with surface-excess value, Γ = 47 n mol cm−2 in pH 7 phosphate buffer solution. When tested with various electroactive chemicals and biochemicals such as cysteine, hydrazine, NADH, uric acid, and ascorbic acid, MWCNT@Car-HQ showed an unaltered redox-peak potential and current values without mediated oxidation/reduction behavior unlike the conventional hydroquinone, anthraquinone and other redox mediators based voltammetry sensors with serious electrocatalytic effects and in turn potential and current drifts. A strong π–π interaction, nitrogen-atom assisted surface orientation and C–C bond formation on the graphitic structure of MWCNT are the plausible reasons for stable and selective voltammetric pH sensing application of MWCNT@Car-HQ system. Using a programed/in-built three-in-one screen printed compatible potentiostat system, voltammetric pH sensing of 3 μL sample of urine, saliva, and orange juice samples with pH values comparable to that of milliliter volume-based pH-glass electrode measurements has been demonstrated.

Due to their explicit properties, screen-printed electrode (SPE) based technologies are superior to other conventional electro-analytical methods. SPE technology can be extended to a miniaturized system, which requires only a few microlitres of the test sample. The surface of SPEs is feasible for a variety of modifications and it eliminates the possibility of tedious cleaning, which is needed for traditional electrodes. Similarly, carbon nanotubes (CNT), one of the more advanced carbon nanomaterials, have a unique graphitic tubular structure with excellent electronic properties. In this chapter, we will focus on CNTs chemically modified SPEs for biochemical sensing applications. We have sub-divided the articles under the following headings: (i) direct surface modification of CNT on SPE, (ii) functionalized CNTs modified SPE, (iii) electroactive species immobilized CNTs modified SPE, (iv) bulk modified SPE, (v) enzyme-immobilized CNT modified SPE, (vi) flow injection analysis of CNT-modified SPE and (vii) separation technique coupled electroanalysis using CNT based SPEs. Details of the preparation, characterization and electrochemical sensor for biomedical applications are included along with 160 references.

A naturally occurring water-soluble cobalt-complex cyanocobalamin (Vitamin B12) has been identified as a new and efficient electrocatalyst for the CO2 -to-CO reduction reaction in aqueous solution. Heterogeneous B12-electrocatalysts prepared by a simple electrochemical immobilization technique on graphene-oxide (GO)-modified glassy carbon and carbon paper (CP) electrodes, without any non-degradable polymer-binders, showed a highly stable and well-defined surface-confined redox peak at E'=-0.138 V vs. RHE with a surface-excess value, ΓB12 =4.28 nmol cm-2 . This new electrocatalyst exhibits 93 % Faradaic efficiency for CO2 -to-CO conversion at an electrolysis potential, -0.882 V vs. RHE (an optimal condition) with a high current density, 29.4 mA cm-2 and turn-over-frequency value, 5.2 s-1 , without any surface-fouling problem, in 0.5 m KHCO3 . In further, it follows an eco-friendly, sustainable and water-based approach with the involvement of biodegradable and non-toxic chemicals/materials like B12, GO and CP.

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

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)

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...

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.

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.