A simple and sensitive method was developed for simultaneous determination of methotrexate (MTX) and folic acid (FA) by ion chromatography with electrochemical detection (IC-ECD). Quaternary amine functionalized multi-wall carbon nanotubes (q-MWNTs) modified glassy carbon electrode (GCE) was used as an amperometric sensor to determine MTX and FA. The electrochemical behaviors of MTX and FA at the q-MWNTs/GCE were studied by cyclic voltammetry (CV). Results indicated that this modified electrode exhibited electrocatalytic oxidation toward MTX and FA with high sensitivity, stability and long life. Good separation of MTX and FA was demonstrated by IC on an anion-exchange column with phosphate buffer solution (PBS) as eluent. Under the optimal conditions, the linear ranges were from 0.01 to 20 mg/L for both MTX and FA with correlation coefficients r ≥ 0.9994. The obtained detection limits (LODs) for MTX and FA were 0.2 and 0.4 μg/L (S/N = 3), respectively. The method has been successfully applied to the determination of MTX and FA in plasma and urine of patients of rheumatoid arthritis.Highlights► Fabricating novel quaternary amine functionalized MWNTs modified electrode. ► Employment of q-MWNTs modified electrode as amperometric sensor in IC-ECD. ► Simultaneous determination of methotrexate and folic acid.

•Preparing positively charged MWNTs with quaternary amine functionalization.•Fabricating poly-ABSA/MWNTs composite film modified electrode as voltammetric sensor.•Simultaneous determination of methotrexate and calcium folinate.A highly sensitive electrochemical sensor was fabricated by electropolymerizing p-aminobenzene sulfonic acid (ABSA) on quaternary amine functionalized multi-wall carbon nanotubes (Q-MWNTs) modified glassy carbon electrode (GCE). Q-MWNTs provide a conductive and stable network for the poly-ABSA (PABSA) immobilization. The voltammetric behaviors of methotrexate (MTX) and calcium folinate (CF) at the PABSA/Q-MWNTs/GCE were investigated by cyclic voltammetry (CV). Compared with bare GCE, Q-MWNTs/GCE and PABSA/GCE, the PABSA/Q-MWNTs/GCE exhibited excellent electrocatalytic oxidation toward MTX and CF. Separation of the oxidation peak potentials for MTX and CF was 0.26 V in phosphate buffer solution (pH 7.0), which made it suitable for simultaneous determination of these two compounds by differential pulse voltammetry (DPV). Under the optimized conditions, the anodic peak current of MTX was linear with the concentration from 0.1 to 8.0 μM in the presence of 8.0 μM CF, with a detection limit of 0.015 μM (S/N = 3). At the same time, the anodic current of CF was linear with the concentration from 0.1 to 6.5 μM with a detection limit of 0.020 μM in the presence of 8.0 μM MTX. The proposed sensor showed advantages of simple preparation, high sensitivity, good reproducibility and stability. It was successfully applied to simultaneous determination of MTX and CF in urine samples.Graphical abstract

The electrosynthesis of polyaniline (PANI) film has been achieved on glassy carbon electrode (GCE) in a novel ionic liquid medium by means of cyclic voltammetry (CV). Then a copper nanoparticles (Cu-NPs) film was in situ electrochemically deposited on the surface of PANI/GCE. Electrochemical behavior and surface characteristics of the Cu-NPs/PANI/GCE were studied using cyclic voltammetry, scanning electron microscopy (SEM) and the results confirmed the presence of PANI and Cu-NPs on the electrode surface. Cu-NPs were highly dispersed and firmly stabilized by surface attachment of the PANI, which was prepared in ionic liquid and strongly attached to the electrode surface. The Cu-NPs/PANI/GCE showed excellent electrocatalytic activity toward the oxidation of ascorbic acid (AA) under weakly basic conditions. Amperometry was carried out to determine the concentration of AA at 0.2 V, and a good linear concentration range from 0.005 to 3.5 mM was found. The Cu-NPs/PANI composite film on GCE surface showed good reproducibility and stability.

A La3+–Cu/Pt modified electrode was fabricated by electrodepositing process in CuSO4 solution by adding a small amount of lanthium compound, and it was employed for direct current (DC) amperometric detection of spectinomycin by anion-exchange chromatography. Without derivatization, this method can simultaneously determine the main component and impurities in spectinomycin pharmaceutical raw material. Ease of preparation, being applied in DC detection mode and good catalytic stability confirmed the interests of this modified electrode as amperometric sensor for the determination of spectinomycin.