Mediator-free interaction of glucose oxidase, as model enzyme for immobilization, with Al-doped and undoped ZnO thin films laser-deposited on polycarbonate supports

Al doped and undoped ZnO thin films were deposited by pulsed-laser deposition on polycarbonate sheets. The films were characterized by optical transmission, Hall effect measurement, XRD and SEM. Optical transmission and surface reflectometry studies showed good transparency with thicknesses ∼100 nm and surface roughness of 10 nm. Hall effect measurements showed that the sheet carrier concentration was −1.44 × 10¹⁵ cm⁻² for AZO and −6 × 10¹⁴ cm⁻² for ZnO. The films were then modified by drop-casting glucose oxidase (GOx) without the use of any mediators. Higher protein concentration was observed on ZnO as compared to AZO with higher specific activity for ZnO (0.042 U mg⁻¹) compared to AZO (0.032 U mg⁻¹), and was in agreement with cyclic voltemmetry (CV). X-ray photoelectron spectroscopy (XPS) suggested that the protein was bound by dipole interactions between AZO lattice oxygen and the amino group of the enzyme. Chronoamperometry showed sensitivity of 5.5 μA mM⁻¹ cm⁻² towards glucose for GOx/AZO and 2.2 μA mM⁻¹ cm⁻² for GOx/ZnO. The limit of detection (LoD) was 167 μM of glucose for GOx/AZO, as compared to 360 μM for GOx/ZnO. The linearity was 0.28–28 mM for GOx/AZO whereas it was 0.6–28 mM for GOx/ZnO with a response time of 10s. Possibly due to higher enzyme loading, the decrease of impedance in presence of glucose was larger for GOx/ZnO as compared to GOx/AZO in electrochemical impedance spectroscopy (EIS). Analyses with clinical blood serum samples showed that the systems had good reproducibility and accuracy. The characteristics of novel ZnO and AZO thin films with GOx as a model enzyme, should prove useful for the future fabrication of inexpensive, highly sensitive, disposable electrochemical biosensors for high throughput diagnostics.