Electrocatalysis and simultaneous determination of catechol and quinol by poly(malachite green) coated multiwalled carbon nanotube film

Electrochemically active composite film that contains multiwalled carbon nanotubes (MWCNTs), Nafion (NF), and poly(malachite green) (PMG) has been synthesized on glassy carbon electrode (GCE), gold, and indium tin oxide (ITO) electrodes by potentiodynamic method. The presence of MWCNTs in the composite film (MWCNT–NF–PMG) enhances the surface coverage concentration (Γ) of PMG by fivefold. Similarly, an electrochemical quartz crystal microbalance study revealed enhancement in the deposition of PMG at MWCNT–NF film when compared with bare and only NF modified electrodes. The surface morphology of the composite film was studied using atomic force microscopy, which revealed that the PMG incorporated on MWCNT–NF film. The composite film exhibited enhanced electrocatalytic activity toward the mixture of biochemical compounds catechol and quinol. The electrocatalytic responses of analytes at MWCNT–NF–PMG composite film were measured using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV). From electrocatalysis studies, well-separated voltammetric peaks were obtained at the composite film for catechol and quinol with a peak separation of 147 mV. The sensitivity values of the composite film toward catechol and quinol by the DPV technique were 0.4 and 3.2 mA mM⁻¹ cm⁻², respectively, which are higher than the values obtained by the CV technique. Similarly, the above-mentioned values are better than the previously reported electroanalytical values for the same analytes.     

Electrochemical nitrite biosensor based on the immobilization of hemoglobin on an electrode modified by multiwall carbon nanotubes and positively charged gold nanoparticle

The report is on an electrochemical biosensor with remarkably improved sensitivity toward nitrite. In this strategy, positively charged gold nanoparticle (PCNA) is used in combination with multiwall carbon nanotubes (MWCNT) by electrostatic adsorption for fabricating PCNA/MWCNT films. Then hemoglobin (Hb) biocatalyst will easily be attached to the surface of the combination films aforementioned. After that, the Hb/PCNA films are immobilized onto the Hb/PCNA/MWCNT films through layer-by-layer assembly technique. The (Hb/PCNA)₂/MWNT/GC electrode thus prepared exhibits enhanced electrocatalytic behavior to the reduction of nitrite at −0.10 V versus SCE in 0.05 M H₂SO₄ solution_. On condition of the low detecting potential and low pH, interference caused by direct electrochemical oxidation or oxidizable substances can be prevented. Therefore, the modified electrode shows fast response time, very high sensitivity, good selectivity and stability. The current response of the sensor increases linearly with nitrite concentration from a range of 3.6 × 10⁻⁶ to 3.0 × 10⁻³ M with a detection limit(S /N = 3) of 9.6 × 10⁻⁷ M.     

Vitamin B(12) incorporated with multiwalled carbon nanotube composite film for the determination of hydrazine

Electrochemically active composite film containing multiwalled carbon nanotubes (MWCNTs) and vitamin B₁₂ was synthesized on glassy carbon, gold, and indium tin oxide electrodes by the potentiodynamic method. The presence of MWCNTs in the composite film (MWCNT–B₁₂) modified electrode mediates vitamin B₁₂’s redox reaction, whereas vitamin B₁₂’s redox reaction does not occur at bare electrode. The electrochemical impedance spectroscopy studies reveal that MWCNTs present in MWCNT–B₁₂ film enhance electron shuttling between the reactant and electrode surface. The surface morphology of bare electrode, MWCNT film. and MWCNT–B₁₂ composite film was studied using atomic force microscopy, which reveals vitamin B₁₂ incorporated with MWCNTs. The MWCNT–B₁₂ composite film exhibits promising enhanced electrocatalysis toward hydrazine. The electrocatalysis response of hydrazine at MWCNT film and MWCNT–B₁₂ composite film was measured using cyclic voltammetry and amperometric current–time (i–t) curve techniques. The linear concentration range of hydrazine obtained at MWCNT–B₁₂ composite film using the i–t curve technique is 2.0 μM–1.95 mM. Similarly, the sensitivity of MWCNT–B₁₂ composite film for hydrazine determination using the i–t curve technique is 1.32 mA mM⁻¹ cm⁻², and the hydrazine’s limit of detection at MWCNT–B₁₂ composite film is 0.7 μM.     

Au-nanoclusters incorporated 3-amino-5-mercapto-1,2,4-triazole film modified electrode for the simultaneous determination of ascorbic acid, dopamine, uric acid and nitrite

A novel biosensor has been constructed by the electrodeposition of Au-nanoclusters (nano-Au) on poly(3-amino-5-mercapto-1,2,4-triazole) (p-TA) film modified glassy carbon electrode (GCE) and employed for the simultaneous determination of dopamine (DA), ascorbic acid (AA), uric acid (UA) and nitrite (NO₂⁻). NH₂ and SH groups exposed to the p-TA layer are helpful for the electrodeposition of nano-Au. The combination of nano-Au and p-TA endow the biosensor with large surface area, good biological compatibility, electricity and stability, high selectivity and sensitivity and flexible and controllable electrodeposition process. In the fourfold co-existence system, the linear calibration plots for AA, DA, UA and NO₂⁻ were obtained over the range of 2.1–50.1 μM, 0.6–340.0 μM, 1.6–110.0 μM and 15.9–277.0 μM with detection limits of 1.1 × 10⁻⁶ M, 5.0 × 10⁻⁸ M, 8.0 × 10⁻⁸ M and 8.9 × 10⁻⁷ M, respectively. In addition, the modified biosensor was applied to the determination of AA, DA, UA and NO₂⁻ in urine and serum samples by using standard adding method with satisfactory results.     

Novel 2,2'-[1,2-ethanediylbis(nitriloethylidyne)]-bis-hydroquinone double-wall carbon nanotube paste electrode for simultaneous determination of epinephrine, uric acid and folic acid

The electro-oxidation of epinephrine (EP), uric acid (UA), folic acid (FA), and their mixture has been studied by modified carbon nanotube paste electrode of 2,2'-[1,2-ethanediylbis(nitriloethylidyne)]-bis-hydroquinone using cyclic voltammetry, chronoamperometry and differential pulse voltammetry. This modified electrode exhibited potent and persistent electron mediating behavior followed by well-separated oxidation peaks towards EP, UA and FA with activation overpotential. For the ternary mixture containing EP, UA and FA the three compounds can be well separated from each other at the scan rate of 20mVs(-1). The obtained catalytic peak current, was linearly dependent on the EP, UA and FA concentrations in the range of 0.7-1200muM, 25-750muM and 15-800muM and the detection limits for EP, UA and FA were 0.216+/-0.004, 8.8+/-0.2 and 11.0+/-0.3muM, respectively. The diffusion coefficient (D), and the kinetic parameters such as electron transfer coefficient, (alpha) and heterogeneous rate constant, (k') for EP were also determined using electrochemical approaches. The modified electrode showed good sensitivity, selectivity and stability, and was employed for the determination of EP, UA and FA in the real samples.