Electrochemical reduction of the hexavalent chromium compounds, a serious environmental pollutant and carcinogen, to a nontoxic Cr(III) species is of significant research interest in interdisciplinary areas. Due to its high oxidation potential (1.36 V vs RHE) and associated surface fouling problems with organic-electrocatalyst systems, expensive precious metalbased electrodes like Au, Ag, Pt, and Pd have been widely used for this purpose. Herein we report, an in situ electrochemically prepared highly redox-active Binol species (Binol-Redox)-impregnated carbon nanofiber-modified electrode (CNF@Binol-Redox), E o ′ = 0.50 V versus Ag/AgCl and surface excess value (Γ Binol-Redox ) = 32.5 × 10 −9 mol•cm −2 , as a low-cost, stable, and highperformance molecular electrocatalyst for selective electrochemical reduction of Cr(VI) species in pH 2 HCL/KCl. The new electrode material was characterized using SEM, FTIR, Raman, GC-MS, 1 H NMR (active site isolated as an ethanolic extract), and scanning electrochemical microscopy (SECM) techniques. It has been revealed that an intermediate electrochemical oxygen reduction reaction to H 2 O 2 occurred at CNF@Binol ads , which is a key step for the surface-adsorbed Binol-precursor oxidation to redox-active molecular electrocatalyst. CNF@Binol-Redox showed a high-performance reduction reaction at a reduction potential of 0.5 V versus Ag/AgCl (low overpotential, η = 0.5 V), which is much better than that of several conducting polymers and Pd-based electrocatalysts (high η = 1.5−0.6 V) and closer to the performance of a bulk gold electrode system. Further, this new electrode does not show any dissolved oxygen interference or surface-fouling problem. The Andrieux-Savent electrochemical kinetic model was adopted to calculate the heterogeneous rate constant (k chem ) as 4.29 × 10 5 mol −1 •dm 3 •s −1 . As practical applications, a prototype electrochemical Cr(VI) sensor operatable by one-drop analysis and bulk electrochemical detoxification of Cr(VI) to Cr(III) have been successfully demonstrated.

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.

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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.

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