Monocamada de 1,4-benzenoditiol sobre ouro contendo nanopartículas de ouro funcionalizadas com um derivado de manose: uma proposta para detecção de Escherichia coli

Abstract

This work deals with the results on the formation of a platform composed of a self-assembled monolayer (SAM) of 1,4-benzenodithiol (BDT) molecules on gold (Au/BDT) on which it was immobilized gold nanoparticles (AuNPs) functionalized with 4-thiobenzene amminomanose (PTAM) producing the electrode Au/BDT/AuNPs/PTAM. The modification process of the gold surface was performed in subsequent steps, all of them spontaneous, with the produced electrodes being characterized by cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman scattering (SERS). At the first step (Au/BDT), a gold electrode was immersed in a dimethyl formamide solution containing BDT for different times reaching an optimum time of 2h according to the surface coverage data (  0.95 for immersion times of 2h or higher) determined by EIS. The SERS spectra of Au/BDT indicated the BDT molecules undergo adsorption on gold through one of the sulfur atoms assuming a configuration close to perpendicular in respect to the surface. For the Au/BDT/AuNPs electrode, the SERS spectra showed an intense Raman scattering signal due to the high concentration of electric field formed by the hot spots, indicating the incorporation of AuNPs through the binding of thiol fragments to substrates. After the functionalization with PTAM, the SERS spectrum of Au/BDT/AuNPs/PTAM indicated the presence of the sugar based on the similarity with the spectrum of the molecule in the solid state. The Nyquist diagram of Au/BDT/AuNPs/PTAM showed a meaningful increase of the charge transfer resistance (RCT = 99.80  cm2) in respect to the electrodes Au/BDT/AuNPs (RCT = 63.50  cm2) and Au/BDT (RCT = 50.00  cm2). The variation of RCT (RCT) of the [Fe(CN)6]4−/3− redox pair was used as parameter to determine the ability of Au/BDT/AuNPs/PTAM to quantitatively detect the Escherichia coli (E. coli) bacterium. The Nyquist diagrams obtained after immersion of Au/BDT/AuNPs/PTAM in solutions of different concentrations of E. coli ([E. coli]), allowed the construction of the RCT vs log[E. coli] plot from which the limit of detection (LOD) was determined as 8.0 x 105 UFC/mL, which is 11 times lower than the Au/PTAM biosensor (8,8 x 106 CFU/mL).