Eletrodos de carbono cerâmico modificados com óxidos de estanho e nióbio para aplicação como sensores eletroquímicos

Abstract

In this work, ceramic carbon electrodes modified with SnO2 (ECC/SiO2-SnO2), Nb2O5 (ECC/SiO2-Nb2O5) as well as with mixed oxides of SnO2 and Nb2O5 (ECC/SiO2-SnO2-Nb2O5), were prepared using as inorganic precursors tin dibutyldiacetate and niobium chloride, respectively. The study of the variables in the synthesis process are of fundamental importance for the development of voltammetric devices. In this way, the synthesis of the ECC/SiO2-SnO2 was optimized through a factorial design 23, where the following factors were studied: amount of catalyst (HCl), proportion between graphite: organic precursor and amount of inorganic precursor (tin dibutyldiacetate). The statistical treatment of the data showed that only the interaction of second order effect, amount of catalyst x inorganic precursor, was significant with a confidence level of 95%, for the electrochemical response of ECC/SiO2-SnO2. For the other systems, ECC/SiO2-Nb2O5 and ECC/SiO2-SnO2-Nb2O5, their syntheses were based on the values optimized for ECC/SiO2-SnO2. The materials obtained were characterized by scanning electron microscopy (SEM), X ray diffraction (XRD), RAMAN spectroscopy, X ray photoelectron spectroscopy (XPS) and voltammetric techniques (cyclic and differential pulse voltammetry). From the XPS analysis, the formation of Si-O-Sn and Si-O-Nb bonds was confirmed from the displacements in the Sn energy 3d3/2 and Nb 3d, due to the interaction of Sn or Nb with the silanol groups. These results corroborated with the FTIR measurements, which showed the presence of low intensity bands in the region of 615 cm-1- 620 cm-1, related to the Si-O stretch of the Si-OH group, which were shifted to low wavenumbers due to formation of the Si-O-Sn bond or Si-O-Nb bond. Raman spectra showed that the ratio of the intensity of the D and G bands related to the graphite (ID/IG) increased as the dopants were added in the electrode structure. The values obtained were: ECC/SiO2= 0.0452; ECC/SiO2-SnO2= 0.066; ECC /SiO2-Nb2O5= 0,1224 and ECC/SiO2-SnO2-Nb2O5= 0.3962, indicating that the inclusion of these oxides increases the degree of system disorder, therefore increasing the number of defects. From MEV/FEG analysis, it was observed that the niobium and tin oxides were homogeneously dispersed in the silica matrix. The incorporation of the metal oxides provided an increase of the electrode response to levofloxacin (LEVO) with Ipa = 147.0 μA for ECC/SiO2 and Ipa = 228.8 μA for ECC/SiO2-SnO2, after optimization. For the others electrodes, the current values were Ipa = 196.0 μA for ECC/SiO2-Nb2O5 and Ipa = 214.0 μA for ECC/SiO2-SnO2-Nb2O5, indicating that these mixed oxides when incorporated in the silica network provide an increase of current facilitating the electron transfer process. These ceramic carbon composites were used for detection of LEVO and in all systems a linear increase of the peak current was observed with the increase of LEVO concentration in the range of 6.21 × 10-5 to 6.97 × 10-4 mol L-1. For the ECC/ SiO2-SnO2, the detection (LD) and quantification limits (LQ) were equal to LD = 1.13 X 10-5 mol L-1 and LQ = 3.80 X 10-5 mol L-1, for ECC/SiO2 - Nb2O5, LD = 3.48 x 10-5 mol L-1 and LQ = 1.16 X 10-4 mol L-1, and for ECC/SiO2-SnO2-Nb2O5, LD = 5.05 X 10-4 mol L-1 and LQ = 1.68 X 10-3 mol L-1.