An optical pancreatic beta-cell based biosensor for measuring cytosolic [Ca2+] is presented in this work. An in vitro bio-electronic system is built for testing various applications and characterizing features of the biosensor. A panoply of applications such as effect of temperature and response of the cells in culture media with 11 mM glucose vs. KRBH with 0 mM glucose are explored. The effect of the excitation light intensity and integration time on the fluorescence performance of the biosensor FRET probe are elucidated experimentally.