Sputter coated ZnO thin films on glass and polycarbonate: Evaluation of stability and interaction with Flavin adenine dinucleotide-dependent oxidases.

Aqueous stability of sputter coated ZnO thin films were studied on two base materials, viz., polycarbonate (PC) and glass. The films showed higher stability on PC compared to glass, when exposed to aqueous buffered solution at pH-7.4, as studied by x-ray diffraction, surface reflectometry, and inductively coupled plasma-optical emission spectroscopy. Glucose oxidase (GOx) and cholesterol oxidase (Chl.Ox.) were used as model enzymes to study their electrochemical interaction with ZnO/PC. GOx showed a higher immobilization on ZnO/PC with an activity of 9.2 ± 1.7 mU cm compared to Chl.Ox. with an activity of 2.79 ± 0.5 mU cm. This is attributed to the larger crystallite size and higher Zn per unit area on PC as compared to glass which enabled a higher activity of GOx on ZnO/PC compared to ZnO/glass. Immobilization was mainly dependent on the surface residue and the charge of the enzyme as indicated by zeta potential which showed -23 mV for GOx compared to -6 mV for Chl.Ox. under physiological conditions. Further under unstirred condition, the reaction was limited by diffusion of the substrate for the enzyme. Chl.Ox. showed a lower activity as compared to GOx on the surface due to low diffusional coefficient of the bulky cholesterol molecule as compared to glucose. It was confirmed by low charge transfer resistance in electrochemical impedance spectroscopy for GOx (1.51 ± 0.072 × 10 Ω) as compared to Chl.Ox. (1.98 ± 0.09 × 10 Ω). But under stirring condition, the diffusion limitation was overcome, and the sensitivity for Chl.Ox./ZnO was 11.2 μA cmmM as compared to GOx/ZnO/PC with 3.5 μA cmmM. Thus, sputter coated ZnO thin films appeared to be good quality transducers for immobilization of oxidases with sensitivity dependent on the substrate diffusion and its potential application in biosensors.