To overcome the surface damage of ZnO-based biosensors operated in strong acidic environments, similar to the stomach environment, the Zn 3Ta2O5 stoichiometric thin films were applied in the field-effect-transistor (FET)-based calcium ion biosensors as the sensing membrane. Since the Zn3Ta2O5 sensing membrane possessed the property of high corrosion resistance and high enzyme immobilization, the resulting calcium ion biosensors can be stably operated in the strong acid environments. The sensing sensitivity of the Zn 3Ta2O5 FET-based biosensor was 17.1 μA/pCa within calcium ion concentration range from pCa = 1 to pCa = 6 under the solution of pH = 2 compared to the 12.8 μA/pCa within the pCa range from 1 to 5 of the FET-based biosensor using the Ta2O5 sensing membrane. The superior results were attributed to the better immobilization between the calmodulin (CaM) and the Zn3Ta2O5 sensing membrane. Consequently, the Michaelis-Menten constant of 1.08 mM for the Zn3Ta2O5 sensing membrane was much better than 1.45 mM of the Ta2O5 sensing membrane.
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