Electronic Tongue Based on Laser-Induced Graphene Electrodes for Monitoring Ions in Aqueous Media

This work describes the use of a portable 450 nm wavelength laser system to fabricate an electronic tongue (e-tongue) comprising an array of potentiometric laser-induced graphene (LIG) sensors on polyimide. The sensing units were modified with ion-selective polymer membranes for the detection of Ca2+, Na+, and K+. The sensors exhibited pseudo-Nernstian behavior, with sensitivities of 32.7 ± 0.8, 63 ± 3, and 52 ± 2 mV/dec for Ca2+, Na+, and K+, respectively, and limits of detection of 4.5 μM for Ca2+, 606 μM for Na+, and 66 μM for K+. The qualitative response of the e-tongue was evaluated using principal component analysis (PCA), which allowed a clear distinction between monovalent and divalent ions based on the first two principal components. Discrimination among the three ions at concentrations of 20 μM, 220 μM, and 4.0 mM was achieved using the K-means clustering algorithm, with a silhouette coefficient of 0.946, close to the ideal value. Quantitative analysis using artificial neural networks (ANNs) was applied to ternary mixtures of the three ions, enabling simultaneous and accurate prediction of individual ion concentrations down to 10 μM. Furthermore, we demonstrate the capability of the e-tongue to provide reliable measurements even at trace ion concentrations in mineral water samples, confirming its suitability for precise and sensitive ion monitoring in complex, real-world applications.