Annamalai Senthil Kumar

Graphitized mesoporous carbon modified glassy carbon electrode for selective sensing of xanthine, hypoxanthine and uric acid

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%.

Room Temperature Aerobic Oxidation of Amines by a Nanocrystalline Ruthenium Oxide Pyrochlore Nafion Composite Catalyst

The aerobic oxidation of primary amines to their respective nitriles has been carried out at room temperature using a highly reusable nanocrystalline ruthenium oxide pyrochlore Nafion composite catalyst (see figure).

Highly selective immobilization of amoxicillin antibiotic on carbon nanotube modified electrodes and its antibacterial activity

An electrochemical route for highly selective immobilization of a β-lactam family antibiotic, amoxicillin (AMX), from the other drugs, penicillin and ampicillin, on multiwalled carbon nanotube modified glassy carbon electrodes (GCE/AMX@MWNT), without any linkers and surface functionalization, has been successfully demonstrated. The electrochemical response of the AMX on GCE/MWNT showed an irreversible oxidation peak at 0.5 V vs. Ag/AgCl (A1), followed by the growth of a new redox peak at 0 V vs. Ag/AgCl (A2/C2) in pH 7 phosphate buffer solution, which is in parallel to a control phenol electrochemical response, revealed that the phenoxy radical electrogenerated at A1 gets subsequently adsorbed on the underlying MWNT modified electrode with a specific surface confined A2/C2 redox peak with proton-coupled electron transfer behaviour. Physicochemical characterization from X-ray diffraction, transmission electron microscopy and scanning electron microscopy collectively evidenced the immobilization of AMX both on the inner and outer (surface) walls of the carbon nanotubes. Further, the AMX@MWNT hybrid material was found to show enhanced antibacterial activity against three bacterial pathogens, Escherichia coli, Staphylococcus aureus and Bacillus subtilis, over the unmodified AMX and MWNT. Finally, as an environmental pollution remedy, the uptake of the AMX drug from five different simulated sources: river water, sea water, river soil, sea soil and farm milk, was successfully demonstrated by this new electrochemical methodology.

Simple method for simultaneous detection of uric acid, xanthine and hypoxanthine in fish samples using a glassy carbon electrode modified with as commercially received multiwalled carbon nanotubes

A simple electrochemical sensor was designed, based on a glassy carbon electrode modified with “as commercially received” multiwalled carbon nanotubes (GCE/MWCNT), for the simultaneous detection of three purine bases: uric acid (UA), xanthine (X) and hypoxanthine (Hx) by differential pulse voltammetry (DPV). Comparison amongst various carbon nanotubesviz.; as commercially received-MWCNT, functionalized-MWCNT, purified-MWCNT and single walled carbon nanotubes for modified electrode preparation, and in turn for the simultaneous electrochemical detection of the purines by DPV, the “as received MWCNT” modified GCE showed the best performance in terms of well-separated peaks and higher peak current values. Scan rate experiments on discreet oxidations of UA, X and Hx suggested that the electron-transfer mechanism for both UA and X follows a mixed diffusion–adsorption controlled pathway, while in the case of Hx, a diffusion controlled route is followed. Calibration responses for the simultaneous detection of UA, X and Hx were linear up to 700, 200 and 150 μM respectively. Corresponding detection limit values were 141 nM, 134 nM and 2.87 μM. Finally analysis of UA, X and Hx content in three different fresh dead fish samples was successfully demonstrated with recoveries of around 100%.

Ni Nanoparticles Stabilized by Poly(Ionic Liquids) as Chemoselective and Magnetically Recoverable Catalysts for Transfer Hydrogenation Reactions of Carbonyl Compounds

Imidazolium‐based poly(ionic liquids) with hydroxide as the counter anion were employed to prepare stable aqueous dispersion of Ni nanoparticles. The synthesized poly(ionic liquid) stabilized Ni nanoparticles (PIL‐Ni‐NPs) were characterized by thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), powder XRD, TEM, Brunauer–Emmett–Teller (BET) surface area measurements, X‐ray photoelectron spectroscopy (XPS), EPR, and UV/Vis spectroscopy. The PIL‐Ni‐NPs possess good catalytic activity towards transfer hydrogenation (TH) reactions of carbonyl compounds to their alcohol derivatives, in isopropanol at 80 °C in the absence of any additional base. This catalyst system chemoselectively reduces only the carbonyl group of α,β unsaturated carbonyl compounds. The magnetically separable PIL‐Ni‐NPs were recycled and reused for further TH reactions.

An electrochemical immunosensor for efficient detection of uropathogenic E. coli based on thionine dye immobilized chitosan/functionalized-MWCNT modified electrode

Uropathogenic Escherichia coli (UPEC) is the major cause of 150 million Urinary Tract Infections (UTI) reported annually world-wide. High prevalence of multi-drug-resistance makes it dangerous and difficult to cure. Therefore simple, quick and early diagnostic tools are essential for effective treatment and control. We report an electrochemical immunosensor based on thionine dye (Th) immobilized on functionalized-multiwalled carbon nanotube+chitosan composite coated on glassy carbon electrode (GCE/f-MWCNT-Chit@Th) for quick and sensitive detection of UPEC in aqueous solution. This immunosensor was constructed by sequential immobilization of UPEC, bovine serum albumin, primary antibody and Horse Radish Peroxidase (HRP) tagged secondary antibody on the surface of GCE/f-MWCNT-Chit@Th. When analyzed using 2.5mM of hydrogen peroxide reduction reaction using cyclic voltammetry in phosphate buffer, pH 7.0, the immunosensor showed excellent linearity in a range of 10(2)-10(9)cfu of UPEC mL(-1) with a current sensitivity of 7.162μA {log(cfumL(-1))}(-1). The specificity of this immunosensor was tested using other UTI and non-UTI bacteria, Staphylococcus, Klebsiella, Proteus and Shigella. The clinical applicability of the immunosensor was also successfully tested directly in UPEC spiked urine samples (simulated sample).

Facile Electrochemical Oxidation of Polyaromatic Hydrocarbons to Surface‐Confined Redox‐Active Quinone Species on a Multiwalled Carbon Nanotube Surface

Polyaromatic hydrocarbon (PAH) oxidation: PAHs, which are considered major environmental pollutants, are carcinogenic, and cannot be electrochemically oxidized on conventional electrodes (gold, platinum, and glassy carbon), can be electrochemically oxidized on multiwalled carbon nanotube surfaces at a potential of 1 V versus Ag/AgCl at pH 7. This results in the formation of stable surface-confined quinone systems (see scheme; AN = anthracene; AQ = anthraquinone).

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.

Enzyme-less and selective electrochemical sensing of catechol and dopamine using ferrocene bound Nafion membrane modified electrode

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.