Eletrodos modificados pela técnica LbL para determinação eletroquímica de iodo

Abstract

In this work, modified electrodes with Layer-by-Layer (LbL) films were applied for electrochemical determination of iodine. The construction of films by LbL technique used the starch-stabilized silver nanoparticles (AgNPs-Am) as polyanion, alternating with polycation 3-n-propylpyridiniumsilsesquioxane (SiPy+Cl-). The AgNPs-Am were synthesized by reduction of AgNO3 using the NaBH4. The formation of AgNPs-Am was monitored by UV-Vis spectroscopy and dynamic light scattering (DLS). The AgNPs-Am synthesis was performed in bath ice with final concentrations of 3.6 x 10-3 mol/L NaBH4, 0.9 x 10-3 mol/L AgNO3 and starch 0.6 % (w/v). The average size of the AgNPs-Am was between 21 and 77 nm. Transmission electron microscopy (TEM) confirmed the AgNPs-Am formation inside and outside of the starch chains, however the smaller sizes were referred to the NPs stabilized by starch. Measurements of Potential zeta indicated stability of the particles, confirmed by DLS monitoring that demonstrated low agglomeration of NPs in a period of 115 days. For the characterization of AgNPs-Am it was also used infrared spectroscopic (FTIR) and X-ray diffraction (XRD). In order to obtain the films, pH and immersion time of the polyelectrolytes were optimized, as well as the concentration of SiPy+Cl-, monitoring the deposition by UV-Vis. The optimum parameters were immersion time 240 seconds, 2 mg/mL SiPy+Cl- pH 6.5 and AgNPs-Am pH 9.0. Atomic force microscopy (AFM) images showed that film thickness increases linearly and the roughness decrease with the bilayers number. FTIR spectra confirmed the interaction between the polyelectrolytes in the assembly of LbL films. The LbL films with architecture (SiPy+Cl-/AgNPs-Am)n (n = bilayers number) were applied as modified electrodes for iodine detection, using differential pulse voltammetry (DPV). It was verified that interaction of the components in the LbL films improved the current intensity. The film (SiPy+Cl-/AgNPs-Am)n (n=5) showed better current response in phosphate buffered saline (PBS) 0.1 mol/L pH 7.0. The instrumental parameters Epulse,  and tpulse were optimized by 23 factorial design. It was verified a significant effect for third order interaction for the intensity of iodine redox peak currents, so the instrumentals parameters were evaluated together. The results of optimization were tpulse = 0.05 s,  = 40 mV/s e Epulse = 50 mV. Accordingly, the modified electrode obtained a linear response for iodine concentrations ranging from 4.34 x 10-5 to 3.47 x 10-4 mol/L (R=0,9936) and from 4.40 x 10-4 to 4.24 x 10-3 mol/L (R=0,9938). It was obtained limit of detection (LD) 5.56 x 10-6 and 1.51 x 10-5 mol/L and for limit of quantification (LQ) 1.85 x 10-5 and 5.04 x 10-5 mol/L, respectively. The results presented confirm the potential use of AgNPs-Am in the modification of electrodes for electrochemical determination of iodine.