Selective and ultrasensitive citric acid assay in urine samples using a MWCNT/Fe3O4 nanoparticle-modified carbon paste electrode.

A carbon paste electrode (CPE) modified with multiwalled carbon nanotubes and magnetite nanoparticles is introduced (CNT/Fe3O4-CPE) as the most sensitive citric acid (CA) electrochemical sensor reported so far. The structural and spectroscopic characterization by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy is presented. The electrocatalytic performance of the electrode was substantiated using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Performing voltammetric experiments at different pH values and various scan rates gave some evidence for the electro-oxidation mechanism of CA. The EIS study gave an obvious indication for the enhancement of the charge transfer rate of CA by the nanocomposite. Various parameters affecting the electrode performance were studied. Using differential pulse voltammetry it was found that the CNT/Fe3O4-CPE showed a linear dynamic range of 5.0 × 10-7-1 × 10-4 mol L-1, an excellent sensitivity of 5184 µA mM-1 cm-2, and a detection limit of 3.6 × 10-7 mol L-1 for CA determination. The relative standard deviation was found to be 3.1% (n = 3). The method was utilized for CA determination in urine samples without utilizing complicated sample pretreatment.

An enzyme-free sensing platform based on molecularly imprinted polymer/MWCNT composite for sub-micromolar-level determination of pyruvic acid as a cancer biomarker.

Pyruvic acid (PA) has been demonstrated to be an important cancer biomarker. Herein, carbon/carbon nanotube paste electrode was modified with the newly synthesized PA-imprinted polymer (MIP) and used as an enzyme-free sensor for PA assay. Methacrylic acid and ethylene glycol dimethacrylate were copolymerized in the presence of PA to prepare PA-IP. The MIP was characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. To analyze PA by the MIP/CNT-CP electrode, the electrode was incubated in the PA solution for a constant time and then, the anodic differential pulse voltammetry signal was recorded. Both extraction and electrochemical determination solutions were the same, making the procedure simple and fast. Presence of the CNT in the MIP electrode led to a great enhancement in the PA signal. The MIP material not only pre-concentrated PA at the electrode surface but also increased the electron-exchange rate. This was confirmed by electrochemical impedance spectroscopy. The effects of electrode composition, extraction condition, and voltammetry parameters on the sensing efficiency were optimized. Dynamic linear range, detection limit, and RSD of the sensor were estimated to be 0.1-200 µM, 0.048 µM (S/N), and 3.6% (n = 3), respectively. The utility of the method was confirmed by appropriate analysis results obtained for the determination of PA in the plasma and urine samples. Graphical Abstract.

Graphite/Ag/AgCl nanocomposite as a new and highly efficient electrocatalyst for selective electroxidation of oxalic acid and its assay in real samples.

Herein, graphite/Ag/AgCl nanocomposite is introduced as a new electrocatalyst material for the electrocatalytic oxidation of oxalic acid. Graphite/Ag/AgCl was synthesized by electroless deposition of nano-sized metallic silver and then silver chloride on graphite powder. The material obtained was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Energy-dispersive X-ray spectroscopy. The nanocomposite was mixed with n-eicosane as binder and used as carbon paste electrode for electrocatalytic oxidation of oxalic acid (OA). The graphite/Ag/AgCl nanocomposite electrode showed good catalytic activity for the electroxidation of oxalic acid in H3PO4 solution (0.05 mol L-1), leading to a distinct decrease in anodic overpotential (100 mV) and a substantial increase in anodic peak current (about 10 times), in comparison with the unmodified carbon paste electrode. Using the developed nanocomposite electrode and differential pulse voltammetry method, it became possible to determine oxalic acid in the concentration range of 0.01-0.75 mmol L-1 with detection limit of 3.7 × 10-6 mol L-1. The electrode showed very high sensitivity of 1341.3 µA mM-1 cm-2 which is remarkably better than the previously reported oxalic acid sensors. Thanks to high sensitivity and good selectivity of the electrode, the proposed method was successfully applied for the determination of OA in human urine and spinach samples. The satisfactory results obtained, confirmed the applicability of this sensor in the practical analysis.

Graphitic carbon nitride (g-C3N4)/graphite nanocomposite as an extraordinarily sensitive sensor for sub-micromolar detection of oxalic acid in biological samples.

Nanosized graphitic carbon nitride (nano-g-C3N4) was synthesized using the thermal polymerization of melamine and utilized as a novel electrocatalyst for electrooxidation of oxalic acid (OA). The nano-g-C3N4 was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The electrocatalytic performance of the g-C3N4-modified carbon paste electrode (g-C3N4/CPE) was investigated by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The modified electrode showed excellent electrocatalytic activity towards the oxidation of OA. The effects of electrode composition, pH and scan rate on the electrooxidation response of OA were studied. Under optimized conditions, the differential pulse voltammetric response of the electrode was linearly related to OA concentrations between 1 and 1000 µM, with a limit of detection (LOD) of 7.5 × 10-7 M. The electrode exhibited very high sensitivity of 1945 µA mM-1 cm-2 for OA assay. The developed method was successfully applied for the determination of OA in urine samples with satisfactory results.

A high performance potentiometric sensor for lactic acid determination based on molecularly imprinted polymer/MWCNTs/PVC nanocomposite film covered carbon rod electrode.

A novel nano-sized imprinted polymer/multi-walled carbon nanotube (MWCNTs)-based potentiometric sensor is introduced for lactic acid (LA) sensing in dairy products. The imprinted polymer was synthesized using allyl amine (AA) and ethylene glycol dimethacrylate as functional monomer and cross-linker, respectively. It was demonstrated that the amide linkage was created between LA and AA during copolymerization reaction which was finally hydrolyzed when removing template from the synthesized MIP. It was also shown that the MIP cavities, compatible with LA anion, were created during polymerization reaction which influenced the potentiometric response behavior of the MIP-based electrode. This novel potentiometric sensor is a carbon rod electrode, coated with a membrane consisting of the MIP nanoparticles (2.5%), MWCNTs (2%), dibutylphthalate (DBP) (65%), poly-vinyl chloride (PVC) (28.5%) and tetra phenyl phosphonium bromide (TPPB) (2%). The active ion sensed by the electrode is the LA anion formed at elevated pH condition. The sensor exhibited Nernstian slope of 30.3 ±â€¯0.4 mVdecade-1 in the working concentration range of 1.0 × 10-1to 1.0 × 10-6 mol L-1 with detection limit of 7.3 × 10-7 mol L-1. The sensor displayed a stable potential response in the pH range of 5-8 and fast response time of less than 60 s. It exhibited also high selectivity over the interfering species. The proposed sensor was successfully applied for the determination of LA in real samples (milk and yoghurt).