A Green and Sustainable Approach to Toluidine Blue Detection: Electrochemically Polymerized l-Alanine Modified Carbon Paste

Toluidine blue (TBL) is a redox active cationic dye whose potential toxicity and heavy industrial use make sensitive, sustainable detection essential in biomedical and environmental work. In this study we developed a green, low-cost electrochemical sensor by electropolymerising L-alanine onto a carbon paste electrode, producing the EP(L/AN) MCPE. Cyclic voltammetry (CV), differential pulse voltammetry (DPV), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) confirm that the modified electrode delivers much higher redox currents and faster electron transfer than the bare carbon paste electrode. The sensor shows a linear response to TBL from 10 to 140 mu M, with a detection limit of 16.26 mu M and a quantification limit of 54.19 mu M. It also demonstrates excellent precision, with repeatability (RSD = 1.33%) and reproducibility (RSD = 1.09%), and it retains 93.88% of its signal over several days. Spiked tap water tests yield recoveries between 98.0 and 99.6%, confirming its practical value. Optimal results are obtained in 0.2 M phosphate buffer at pH 7.5 using DPV. This eco-friendly sensor represents one of the earliest voltammetric platforms specifically designed for TBL detection, offering significant potential for deployment in environmental and biomedical monitoring.