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Development of simple and selective detection of poisonous arsenic species [As(III)] in drinking water is the subject of continued research in the multi-disciplinary areas of chemistry and biology. In this paper is reported, a well dispersed gold nanoparticle decorated-carbon nanofiber-chitosan modified carbon screen printed electrode (SPE/CNF-CHIT@Aunano) prepared using an inexpensive and simple electrochemical method for electro-catalytic oxidation and flow injection analysis (FIA) of inorganic As(III) species in a neutral (pH 7) phosphate buffer solution. The modified electrode has about a four times higher current signal for the arsenic oxidation than that of a gold polycrystalline electrode. The SPE/CNF-CHIT@Aunano system is highly stable in vigorously stirred solutions. Rotating disc electrode (RDE) studies performed using a glassy carbon electrode–rotating disc electrode (GCE–RDE) with its surface modified using the CNF-CHIT@Aunano film gave a heterogeneous rate constant, kcat = 1.92 × 104 mol−1 dm3 s−1 for the electrochemical oxidation of As(III). Furthermore, SPE/CNF-CHIT@Aunano was used as an efficient electrochemical detector (ECD) for the FIA of arsenic(III) in a neutral pH solution. The present FIA-ECD shows zero interference with other common cationic and anionic species. As an analytical application, selective detection of As(III) in arsenic polluted industrial effluent and tap water samples were determined satisfactorily with good recovery values. The analysis results from FIA-ECD were comparable with those obtained using the inductively coupled plasma – optical emission spectrometry method.

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

The detection of thiol-containing amino acids, particularly L-cysteine (L-CySH), without any interference from other biochemicals is a challenging research interest in electroanalytical chemistry. Amongst various electrodes, the nanogold modified electrode has been reported to be effective for low potential electrochemical oxidation (∼0 V vs. Ag/AgCl) and sensing of L-CySH. Herein, we report a conductive carbon black (CB, CL-08) modified glassy carbon electrode (GCE/CL-08) with a high surface area (1000 m2 g−1), prepared by a simple drop-casting technique for efficient electrocatalytic oxidation at the lowest oxidation potential, −0.13 V vs. Ag/AgCl, and sensing of L-CySH in a pH 6.65 phosphate buffer solution. The trace metal impurities such as Ni (0.18 wt%) and Fe (0.42 wt%) in CL-08 are found to be key for such unique and unusual electrocatalytic property observed in this study. Compared with the multiwalled carbon nanotube and graphite powder modified electrodes, the GCE/CL-08 showed about 400 mV reduction in the oxidation potential and twenty times enhancement in the current signal for the CySH. A chronoamperometry detection of L-CySH on the GCE/CL-08 at an applied potential = −0.13 V vs. Ag/AgCl yielded a current linearity from 50 to 700 μM with a LOD = 45.87 nM. There are no interferences by common biochemicals such as ascorbic acid, dopamine, uric acid, xanthine, hypoxanthine and homo-cysteine on the L-CySH oxidation potential. Selective chronoamperometric detection of L-CySH in the blood serum demonstrated ∼100% recovery value as a validation for the present protocol.

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

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

An efficient electrochemical sensor for simultaneous electrochemical sensing of three purine compounds, uric acid (UA), xanthine (X) and hypoxanthine (HX), using a graphitized mesoporous carbon (GMC) modified glassy carbon electrode (GCE/GMC) has been demonstrated in pH 7 phosphate buffer solution without any enzyme, prior to electrode activation, surfactant and sample pre-concentration step. Electrochemical investigation of the GCE/GMC with [Fe(CN)6]3− indicates metallic conductor like surface features of the modified electrode. A diffusion controlled reaction mechanism was identified for the electro-oxidation of the three purine compounds with an electrocatalytic pathway, except for the UA, where it shows a surface area effect with a mixed-diffusion and adsorption controlled mechanism at higher scan rates (>70 mV s−1). Calculated full-width of the half maximum values for the simultaneous detection of the three purine compounds are 42, 53 and 64 mV respectively and these are the lowest values ever reported in the literature, suggesting effective electron-transfer behaviour of the modified electrode for the purine oxidations. Calibration plots for the simultaneous detection of the purine compounds were linear in the concentration range of 20–400 μM, 20–320 μM and 20–240 μM for UA, X and HX with detection limit values of 110 nM, 388 nM, and 351 nM respectively. Selective sensing of the purine compounds in human blood-plasma, urine and fish samples was successfully demonstrated with recovery values ∼100%.

Simultaneous detection of 1,4-, 1,2- and 1,3-dihydroxy benzene (DHB) isomers on a glassy carbon electrode modified with graphitized mesoporous carbon (GCE/GMC) in pH 7 phosphate buffer solution was demonstrated. GCE/GMC showed absence of any strong adsorption of the DHB isomers on the working electrode surface. The electrochemical response of 1,4-DHB resembled that of 1,2,3-trihydroxybenzene (gallo group) on the GCE/GMC. Differential pulse voltammetry was used as a tool for quantification of the DHB isomers. Constructed calibration plots were linear in the window 4 × 10−5 to 2.5 × 10−4, 2.5 × 10−5 to 2.0 × 10−4 and 2.5 × 10−5 to 2.0 × 10−4 mol L−1 for the 1,4-, 1,2- and 1,3-DHB, respectively. Corresponding detection limits (S/N = 3) were 0.91, 1.31 and 0.67 μM. Polyphenols extracted from three different tea samples using hot water were successfully analyzed with recovery values ∼100%.

Here in, we are reporting a new membrane modified electrode based on ferrocene (fc) mixed Nafion (Nf) system for enzyme-less and selective electrochemical sensing of catechol and dopamine without any interference from cysteine, uric acid, hydrogen peroxide and ascorbic acid at an applied potential of 200 mV vs. Ag/AgCl in pH 7 phosphate buffer solution. The working electrode, ferrocene-bound Nafion membrane modified glassy carbon electrode (GCE/{Nf-fc}-MME) was prepared by manual mixing of 10 mg of ferrocene with 2% of Nafion solution (optimal) followed by drop coating on the GCE surface, and electrochemical pretreatment by continuous potential cycling in pH 7 PBS. The modified electrode showed well-defined redox behavior at an equilibrium potential (E1/2) of 150 mV vs. Ag/AgCl in pH 7 PBS due to the electron-transfer reaction of fc/fc+ redox. The calculated exchange current density (Io) and apparent heterogeneous rate constant (khapp) were 6.16 × 10−5 A and 4.51 × 10−3 cm s−1 respectively for catechol by electrochemical impedance spectroscopy, which were about 100 times higher over the unmodified electrodes. Different domains of Nafion, namely hydrophilic and hydrophobic sites have a specific role to immobilize the ferrocene species and in turn to selectively mediate catechol and dopamine in a solution mixture containing common physiological interferents. Calibration plots were in the range of 250 μM–2.5 mM and 250 μM–5 mM for catechol and dopamine, respectively, with the corresponding detection limit (S/N = 3) values 10.8 μM and 22.7 μM. The current sensitivity was 1.1 μA mM−1 for both the compounds. The working electrode showed negligible variation in the detection current up to 2 days of measurement.