2.1.2. Glucose Oxidase Enzyme Layer

The proposed glucose sensor is based on the optical detection of oxygen consumed by the GOD-catalyzed oxidation of β-d-glucose [19]. It uses glutaraldehyde (GA) to crosslink GOD with bovine serum albumin (BSA). β-d(+) glucose (EC 207-756-2), GOD (EC 1.1.3.4, 15,500 units/g), BSA (EC 232-936-2), and GA (EC 203-856-5) were obtained from Sigma (Sigma-Aldrich Co., St. Louis, MO, USA). In general, a thicker enzyme layer shows better linearity in a wider concentration range, but the response time is longer. A smaller ratio of glutaraldehyde may result in insufficient immobilization, while a higher ratio may reduce the efficiency of measurement due to more immobilization than necessary. The glucose enzyme solution included 0.75 mg of BSA and 0.75 mg of GOD in a 15 μL of 10 mM phosphate buffer solution. To promote adhesion between the enzyme membrane and the PSP surface, 20 μL of 1 wt% 3-aminopropyltriethoxysilane (3-APTES) was applied and cured for 30 min at 80 °C for surface silanization. Next, 15 μL of the enzyme solution was cast on the silanized area with a microsyringe. Then, 15 μL of 5 wt% GA was applied to initiate the chemical crosslinking reaction of BSA on the PSP surface. All other chemicals were of analytical reagent grade. Deionized water was used throughout the experiments for the preparation of samples, buffers, and other solutions.