Efeito do tamanho de nanotubos de carbono de paredes múltiplas em sensores eletroquímicos para moléculas de interesse farmacêutico

Abstract

This work evaluates the development of electrochemical sensors based on multiwalled carbon nanotubes (MWCNTs) of two different sizes (110-170 nm x 5-9 μm, LD-MWCNTs, and 6-9 nm x 5 μm, SD-MWCNTs), chemically treated with concentrated acids, and deposited on a glassy-carbon surface for the determination of naproxen, ibuprofen, chlorhexidine, and 4-chloroaniline (present as a degradation product) in pharmaceuticals products. By Raman spectroscopy, it was verified that SD-MWCNTs presented higher density of structural defects, which corroborated with the higher electrocatalytic effect observed using amperometric and voltammetric techniques toward the oxidation of the analytes. X-ray diffraction confirmed the absence of metals (that could contribute to the electrocatalytic oxidation of the molecules) on the outer wall of the carbon tubes as well as inside the tubes. The greater electroactive area was observed for the SD-MWCNT-modified electrode, however this parameter was not the predominant one to infer the best performance of these electrodes. Experiments using voltammetry and amperometry, the latter associated with batch-injection analysis (BIA) and flow-injection analysis (FIA), showed that modified electrodes presented electrocatalytic properties, overall when using SD-MWCNTs. Decrease in the oxidation potential of the molecules, reduction of adsorption effects of analytes or their oxidation products, widening of the linear working ranges, and reduction of the detection limits, compared to unmodified and LD-MWCNT-modified electrodes, were observed. The SD-MWCNT-modified electrode showed the following results using amperometric detection, respectively, for naproxen / ibuprofen / chlorhexidine / 4-chloroaniline: detection limits of 0.4 μmol L-1 / 1.9 μmol L-1 / 0.2 μmol L-1 / 0.02 μmol L-1, relative standard deviations of 3.4 % / 4.2 % / 1.8 % / 1.2 %, analytical frequency of 120 h-1 / 100 h-1 / 60 h-1 / 130 h-1, and working linear ranges from 10 to 75 μmol L-1 / 10 to 1000 μmol L-1 / 1 to 300 μmol L-1 / 1 to 300 μmol L-1. All the results indicate better performance of the SD-MWCNT-modified electrode, which can be associated with the higher density of defects in the structure of these MWCNTs, which in turn, contributed to the higher electrocatalytic activity of this material.