Total internal reflection ellipsometry for kinetics-based assessment of bovine serum albumin immobilization on ZnO nanowires

Abstract

ZnO materials exhibit a rich family of nanostructures, which show great potential for the sensitivity improvement of optical detection systems. In this work, ∼350 nm ZnO nanowires (ZnO-NWs) were electrochemically deposited on ZnO and an indium tin oxide coated glass (ZnO-NWs/ZnO/ITO/glass) substrate. ZnO-NWs were modified with N-(3-aminopropyl)triethoxysilane (APTES) for covalent bovine serum albumin (BSA) immobilization. The studies were performed using a spectroscopic total internal reflection ellipsometry (TIRE) setup based on the Kretschmann configuration. The refractive index dispersion of ZnO-NWs in the air was obtained from the optical model applying a Bruggeman effective medium approach. It was determined that the ZnO-NWs effective layer consists of 30% ZnO and 70% void. Reflectance difference before and after ZnO-NWs modification with BSA was 6.6 times higher than in the case of the plain ZnO layer. The Δ kinetics of covalent BSA immobilization on the APTES/ZnO-NWs/ZnO/glass substrate contains two phases. Evaluation of the diffusion coefficient for BSA in PBS filled APTES/ZnO-NWs/ZnO/ITO/glass was performed using numerical calculation, and the obtained diffusion coefficient was 2.4 × 10⁻¹⁷ m² s⁻¹. Simulation of BSA immobilization on a flat ZnO layer and on a ZnO-NWs modified surface showed that 13 times higher sensitivity was observed for the substrate with ZnO-NWs.