Exploration on Solvatochromic Sensing Probe of Phenol Blue for L-Lactic Acid Detection: Effective Strategy to Obtain Reliable Capacitive Sensor

This study systematically investigates a capacitive sensor applied with phenol blue (PhB)-based sensing medium for detection of L-lactic acid (LA), as a health monitoring indicator. PhB is a substance with solvatochromic effect, inducing the change in capacitance by exposure to polar molecules. However, the capacitive LA sensor with a flat-structured PhB/polyvinylchloride (PVC) composite-sensing medium is observed to have a problem in that sensing capacitance variation saturate quickly with increasing the LA solution concentration. This main cause can be analyzed that the interaction of proton from LA molecule with the lone pair electrons of the PhB molecule acts as a major factor on the sensing characteristics rather than the solvatochromic behavior of PhB molecule. Therefore, a strategy is adopted to introduce a porous structure to the PhB/PVC composite-sensing medium to maximize the interaction of PhB with protons, which is implemented through solvent and non-solvent exchange methods. Consequently, the sensitivity and linearity of the porous-structured LA sensor are 2.99 pF mm⁻¹ and 0.966 over LA concentrations ranging from 0 to 100 mm, respectively, which is a significant improvement over that of the flat-structured one. Notably, the sensing performance remained unchanged even after a month of storage under normal ambient conditions.