Nandimalla Vishnu

A preanodized 6B-pencil graphite as an efficient electrochemical sensor for mono-phenolic preservatives (phenol and meta-cresol) in insulin formulations

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%.

Unusual neutral pH assisted electrochemical polymerization of aniline on a MWCNT modified electrode and its enhanced electro-analytical features†

Unusual electropolymerization of aniline to polyaniline (PANI) in a neutral pH solution has been successfully demonstrated using a multiwalled carbon nanotube (MWCNT) modified gold electrode (Au-MWCNT@PANIpH7). The modified electrode showed highly redox active surface confined peaks corresponding to the molecular transitions of leucoemeraldine-emeraldine and emeraldine-pernigraniline in pH 7 phosphate buffered solution (PBS), along with a low capacitance behavior, in contrast to the conventional acidic solution PANI systems. Control experiments in the absence of MWCNTs (i.e., Au/PANIpH7) and in an acidic medium, pH 2 (i.e., Au-MWCNT@PANIpH2), resulted in nil and poor redox features respectively. The physicochemical characterization of the MWCNT@PANIpH7 film by TEM, Raman spectroscopy, FTIR and UV-Vis revealed the presence of polaron type PANI structures on the MWCNT surface. More interestingly, MWCNT@PANIpH7 showed a highly selective electrocatalytic signal to ascorbic acid (AA) at a low oxidation potential, -15 mV vs. Ag/AgCl, without interference from nitrite, uric acid, dopamine, glucose, cysteine and citric acid in pH 7 PBS. Extended flow injection analysis yielded an excellent AA sensing response with a detection limit (signal-to-noise ratio = 3) of 42 nM, which is better than that of the conventional acid assisted MWCNT@PANIpH2 and MWCNT systems.

Pencil graphite as an elegant electrochemical sensor for separation-free and simultaneous sensing of hypoxanthine, xanthine and uric acid in fish samples

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.

Single step grown MoS2 on pencil graphite as an electrochemical sensor for guanine and adenine: A novel and low cost electrode for DNA studies

Herein we report a simple, one-step approach to prepare a low-cost and binder free MoS2-pencil graphite electrode (i.e., MoS2-PGE) for the electrochemical oxidation of DNA nucleobases i.e., guanine (G) and adenine (A) in physiological pH (7.4) buffer solution. MoS2-PGE was synthesised by hydrothermal method and the morphology of such hybrid was characterized by field emission scanning electron microscopy, X-ray diffraction, Raman and X-ray photoelectron spectroscopy. In cyclic voltammetry, MoS2-PGE displays two well-seprated and well-defined irresversible peaks at 0.58 and 0.90 V for electrochemical oxidation of G and A respectively when compared to bare PGE. Likewise, differential pulse voltammetry of MoS2-PGE showed well-seprated and sharp peak current responses for G and A at 0.56 V and 0.85 V respectively. Under optimized conditions, DPV was further adopted for simultaneous and separation-free determination of G and A in physiological pH. MoS2-PGE shows good stability with linear range of 15-120 μM and 15-120 μM for G and A detection respectively. Obtained sensitivity and limit of detection (signal-to-noise = 3) are comparable with the previous literature. As an immediate practical applicability, MoS2-PGE was used for quantification of G and A concentration in calf-thymus DNA and detected ratio of G and A (i.e., [G]/[A]) ratio is 0.85. The current approach provides a new avenue towards the development of affordable electrodes for a wide range of bioanalytical applications.

Tea quality testing using 6B pencil lead as an electrochemical sensor

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.