Rapid, trace-level direct cathodic voltammetric determination of dopamine by oxidized multiwalled carbon nanotube-modified carbon paste electrode in selected samples of pharmaceutical importance

Selected carbonaceous nanomaterial-modified carbon paste electrodes (CPEs) were compared for trace-level determination of dopamine (DA) by direct cathodic square wave voltammetry (SWV). The advanced nanomaterials containing electrodes were obtained from homemade multiwalled carbon nanotubes (MWCNTs) in their native and functionalized (ox-MWCNTs) forms, commercially available MWCNTs, single-walled carbon nanotubes, and fullerene, by a very simple drop coating method, and from classical CPE which consists of graphite powder and paraffin oil only. The comparative SWV measurements confirmed the priority of ox-MWCNT/CPE as it exhibited the best analytical performance with a well-shaped and intensive DA signal, which strongly depended upon pH and exhibited the most favorable characteristics at pH 6.3 (0.1 M phosphate buffer solution). The cyclic voltammetric (CV) characterization of DA on ox-MWCNT/CPE confirmed the reversible process and much higher sensitivity of this electrode in comparison with the other ones. Fourier transform infrared (FTIR) and Raman spectroscopic measurements indicated a significant difference between MWCNTs and ox-MWCNTs, which is the consequence of the chemical oxidation/functionalization of the latter one having a positive influence on its sensing properties. The optimized SWV method was characterized by a linear concentration range of 16.15-192.70 ng mL(-1) DA, a relative standard deviation (RSD) lower than 1.3%, and a detection limit in the model solution of 4.89 ng mL(-1). Also, interferences from ascorbic acid were negligible. The developed method was successfully applied for DA determination in injection/selected infusion matrix solutions, whereby the obtained results were in good agreement with DA content declared by the producer with RSD of the method below 1.0%.