Detecção Eletroquímica de Explosivos Nitroaromáticos em Nanotubos de Carbono: Efeito de Impurezas Metálicas

The application of carbon nanotubes (CNTs) in electroanalysis has been intensively investigated. The electrocatalytic ability of CNTs to enhance electron transfer kinetics results in improved electrochemical sensors with higher sensitivity and resistance to surface fouling [1]. The electrocatalytic activity of CNTs has been attributed to the presence of edge-plane-like sites located at the ends of tubes and in the defect areas of the CNTs [1]. Nevertheless, the presence of metallic impurities in CNTs [2] and the alteration of mass transport regime from linear (planar) diffusion to thin layer diffusion (if the CNT-modified electrode is considered as a conducting porous layer) are additional explanation for the lowered reduction or oxidation potentials observed in CNT-modified electrodes [3]. This work investigates the role of functionalization of multi-walled carbon nanotubes (MWCNTs) on the voltammetric determination of different nitroaromatic explosives, including 2-methyl-1,3,5-trinitrobenzene (TNT), hexahydro-1,3,5-trinitro- 1,3,5-triazine (RDX), and pentaerythritol tetranitrate (PETN). Glassy-carbon electrode (GCE) was modified with MWCNTs (non- and functionalized by acid treatment) by drop casting (10 µL of a suspension of 1.0 mg mL-1 in dimethylformamide sonicated for 10 min using an ultrasonic horn). Functionalization of MWCNTs was carried out with a 3:1 (v/v) mixture of concentrated H2SO4 and HNO3 acids (1000 mL) sonicated for 3 h at 40oC in ultrasonic bath, followed by filtration using a filter paper with a pore size of 0.05 µm. Voltammetric measurements were performed using a conventional cell filled with 10 mL of supporting electrolyte. The supporting electrolyte used was 0.1 mol L-1 KCl for the determination of all nitroaromatic compounds. Cyclic voltammetry of the MWCNTmodified GCE indicated higher background current for the electrodes modified with nonfunctionalized MWCNTs, however, a substantial faradaic current was obtained using this modified electrode. Moreover, square-wave voltammetric detection of TNT on the modified electrode with NF-MWCNTs presented a lower detection limit (0.9 µmol L-1 ) than the one obtained on the F-MWCNT-modified electrode (11.4 µmol L-1 ), which also indicates the effect of the metallic impurities on the electrochemistry of nitroaromatic explosives.