The Effect of Swelling on Charge’s Transmission of Chitosan and Reduced Graphene Oxide Films Electrodes
Authors: Nakagawa Milena Arruda , Brito de Souza Camila , Gonçalves Emerson Sarmento 
Published: February 2020

Chitosan (CS) and reduced graphene oxide (rGO) films are suitable materials for matrix electrochemical devices. However, CS_rGO films show a swelling effect after water permeation yielding into electrochemical disturbs measurement and a challenge for the point-of-care of sensor uses. In order to optimize the transducional measures, further investigations were conducted to analyze the appropriate CS:rGO ratio obtained by CS acetic hydrogel and several contents (0.2%, 0.4%, 0.6%, 0.8% and 1%) of rGO (obtained by Hummer’s method and subsequently reduced thermally at 400º C). After sonication of 3 hrs to ensure a homogenous phase, dispersion was poured into dish plate and dried for 24hrs at 60º C. Subsequently, CS:rGO films were neutralized by 3 different solutions: (A) ethanol solution 80%, (B) ethanol 80%/sodium hydroxide 0,02M (50:50 v/v) and (C) sodium hydroxide 0,02M and left dried on room temperature. Neutralized and non-neutralized films were compared and analyzed at Electrochemical Impedance of Spectroscopy. The electrochemical impedance spectra were recorded using buffer acetate pH 6 and 0.1 M of sodium chloride. Potential used was open circuit potential (OCP), signal amplitude, 10mV, and frequency varying from 10kHz down to 10mHz. The OCP range was -0,1 to -0,18 V. The swelling degree level performed the wettability analysis. Chemical, structural and morphological surface’s alterations were analyzed at FTIR, XRD and FEG-SEM microscopy, respectively. CS_rGO films non-neutralized after wetted with buffer solution at pH 6, shows instability and a great impedance value even in higher rGO contents. The addition of rGO into CS polymer decreased the swelling degree and the neutralized method (B) seems to show the lowest swelling level (Table 1), turning films more stable after water contact, followed by an unchanged surface morphology, and a non- shrinkage aspect. Due to the high rGO hydrophobic portion for the samples contents at rGO 0.8% and at 1%, the nanocomposite agglomeration character related a low portion of coating area being in a real contact with electrolyte solution resulting in a high impedance value, even for low swelling results. The neutralizations methods might provide charge reorganization on films surface, as showed the bands of carboxylate ion, –COO-, important group of hydroxyls linkage,shifted from two strong bands at 1560 cm-1and 1410 cm-1 to 1543 cm-1and 1315 cm-1vibrations, detailed in Figure 1. The crystalline structure of CS_rGO films changed for an amorphous on after the rGO contents but kept unchanged after the neutralization methods, as described by diffratograms in Figure 2. The addition of rGO provides a lamellar disorder and after certain rGO proportion the films presented agglomerations and an increase in the porous sizes. After neutralizations, there was a pronounced of the porous size and was noticed a more sponge aspect of films. The porous and wrinkles formation may be analyzed by FEG-SEM analysis in Figure 3. Conductivity and capacitive measurements were analyzed for each rGO content and after neutralization treatments are described in Figure 4 and the Nyquist plots were obtained and described in Figure 5. The circuits fittings were described in Figure 6.There was a decreasing of resistance behavior as much as rGO increased and inversely, as low as the swelling level. The NaOH treatment neutralized the cationic films surface, controlled the water entrance and minimized the repulsion electrostatic allowing the charges to pass. The NaOH/EtOH treatment seems to result in non-uniform charges distribution on the films surface loading in unstable material for charges percolation. EtOH treatment presented the lowest swelling level due to water extraction of films but the hydroxyls groups linked to the surface may resulted in a hindrance of charges percolation on the surface and in the bulk of films. It was noted that for rGO 0.8% content or more, after all neutralization treatments promoted, resulted ina high resistance value due to the formation of hydrophobic portions agglomerated which may interfered the double layer formation. At high rGO contents, EIS spectra showed at lower frequencies an increasing of Randles circle diameter, indicating higher double layer impedance for sample of rGO 0.8%. The circuit fittings was R(RQ)([RW]Q)for all films except for NaOH/EtOH treatment whose circuits fitting was given by R(RQ)(RQ)([RW]Q).The physical interpretation may relate to a more porous and grooves formation. After the ethanol neutralization, the films aspect was similar to a veil whereas chitosan seems to overlaps the rGO. Thus, the CS_rGO films neutralized with NaOH assures more stability to films until the rGO 0.6%.

Figure 1