A novel optical biosensing platform utilizing the unique solubility and chemochromic properties of polyaniline is presented. A facile, ion-induced reprecipitation method leads to the entrapment of a chosen oxidoreductase enzyme, which, in the presence of its associated substrate, catalyzes a reversible redox change in the host polymer. This change is monitored via the UV–vis absorption and subsequently analyzed to fit a Michaelis–Menten model. Here, in vitro prototype devices demonstrate selective sensing of glucose, choline, and uric acid, and the potential to be adapted for use as part of real-time in vivo monitoring systems is discussed.