Calibration-free Approach Using a Nanoporous Gold Microelectrode for Paracetamol Quantification in Viscous Medium

Direct quantification methods are suitable for samples that have chemical composition different from the solutions employed in the calibration step. In this work, we propose developing a calibration-free method through a single chronoamperometry experiment. A nanoporous gold (NPG) microelectrode was used to enhance the electron transfer process, allowing for a potential shift and analysis under milder conditions. The calibration-free method relies on the Mahon and Oldham equations for chronoamperometric experiments with disk-shaped microelectrodes. This approach provides a linear regression of the current i(t) with respect to t-1/2, and through the intercept and slope, it is possible to obtain the concentration and the diffusion coefficient, this latter, depending on the physical-chemical environment. The microelectrode radius, as the only variable, can be easily obtained by electrochemical techniques using an appropriate electroactive probe. The Au microelectrode surface modification consisted of creating a nanoporous structure (NPG) by anodization in a 0.5 M H2SO4 solution. The formation of NPG creates electrocatalytic sites owing to surface defects and/or edges, providing a potential shift in redox reactions. This approach allows for a 154 mV potential shift and a reliable quantification of paracetamol. The paracetamol analysis can be performed at +0.8 V using the modified electrode rather than +1.15 V for the bare microelectrode. Standard 4 mM paracetamol solutions with different viscosities and a real sample (medication) were used in this study (n=3). Using the proposed calibration-free protocol, paracetamol concentration values within 5% error were achieved, i.e., (4.0 ± 0.3) mM for aqueous medium, (3.9 ± 0.3) mM and (4.19 ± 0.05) mM for solutions containing 5% and 10% thickener (ethylene glycol), respectively, and (204 ± 3) mg/mL for the medication whose label concentration is 200 mg/mL.
The authors thank FAPESP, CNPq and CAPES for the generous funding.