Surface engineering of plasmonic gold nanoisland platforms for high-sensitivity refractometric biosensing applications

Abstract Plasmonic chips comprising gold nanoisland structures that are fabricated by solution-phase seeding, have demonstrated excellent promise as high-sensitivity substrates for molecular detection and medical diagnostic applications. Even so, there still remains an outstanding need to examine the potential utility of these plasmonic chips for label-free refractometric biosensing and to understand how nanostructure design principles affect measurement sensitivity. Herein, we developed a thiol-based surface functionalization strategy to fabricate gold nanoislands on a functionalized glass surface with improved fractional surface coverages and inter-island gap distances of 80–85% and 5–10 nm, respectively, as compared to values of 50–65% and 15–20 nm for gold nanoislands on bare glass substrates. By tuning the gap distance, it was possible to adjust the bulk refractive index sensitivity of the measurement signal from ∼99 nm per refractive index unit (nm/RIU) for gold nanoislands on a non-functionalized glass surface to ∼180 nm/RIU for gold nanoislands on a functionalized glass surface. The nanoplasmonic biosensing capabilities of the latter plasmonic chip were further investigated and demonstrated larger measurement responses for detecting bovine serum albumin (BSA) protein adsorption compared to other types of plasmonic gold nanostructures. It was also possible to detect antigen-antibody interactions related to coronavirus disease-2019 (COVID-19), especially binding events that occurred near the sensor surface. These findings demonstrate the broad application possibilities of gold nanoisland platforms for refractometric biosensing and emphasize the importance of finetuning nanostructure dimensions to optimize sensing performance.