Potentiometric Detection of Calcium Ions Using an Organic Electrochemical Transistor

Excreted biological fluids represent a matrix rich in useful biomarkers that can be correlated with a vast number of medical conditions and physiological processes, for both diagnostic and therapeutic purposes. Among the plethora of medically relevant analytes, the calcium ion is a target of prime interest, as it is present in many biofluids such as saliva, sweat, and wound exudate. Its concentration can be associated with a variety of afflictions, such as osteoporosis, periodontal diseases, kidney stone formation, parathyroid gland activity, and impaired wound healing. In this work, we present a novel device architecture for the potentiostatic, reference electrode-free detection of calcium, renovating the original organic electrochemical transistor (OECT) model by Wrighton. The sensing event takes place on the gate, functionalized with a Ca²⁺-selective membrane, whose potentiometric shift is used to modulate the current flowing through a poly-(3,4-ethylenedioxythiophene) perchlorate (PEDOT:ClO₄) interdigitated channel. Calcium sensing was performed in buffer solutions, reporting a linear range between 0.002 and 19 mM. The sensor proved to cover the biologically relevant range for calcium ion concentrations in complex media, using synthetic wound exudate as a model biomatrix, and it reported excellent selectivity under exposure to high concentrations of interfering species.