# Hybrid Polystyrene–Plasmonic Systems as High Binding Density Biosensing Platforms

## TL;DR

This paper introduces a new biosensing platform combining polystyrene and plasmonic systems to detect biomarkers with high sensitivity and stability.

## Contribution

The novel contribution is a stable, ultra-thin polystyrene layer on plasmonic gratings for enhanced fluorescence and single-molecule detection.

## Key findings

- The hybrid system showed 63.8 times brighter fluorescence than commercial wellplates.
- The platform enabled single-molecule counting of the spike protein for COVID-19.
- The polystyrene layer improved shelf-life and binding stability compared to plasma-activated surfaces.

## Abstract

Sensitive, accurate, and early detection of biomarkers is essential for prompt response to medical decisions for saving lives. Some infectious diseases are deadly even in small quantities and require early detection for patients and public health. The scarcity of these biomarkers necessitates signal amplification before diagnosis. Recently, we demonstrated single-molecule-level detection of tuberculosis biomarker, lipoarabinomannan, from patient urine using silver plasmonic gratings with thin plasma-activated alumina. While powerful, biomarker binding density was limited by the surface density of plasma-activated carbonyl groups, that degraded quickly, resulting in immediate use requirement after plasma activation. Therefore, development of stable high density binding surfaces such as high binding polystyrene is essential to improving shelf-life, reducing binding protocol complexity, and expanding to a wider range of applications. However, any layers topping the plasmonic grating must be ultra-thin (<10 nm) for the plasmonic enhancement of adjacent signals. Furthermore, fabricating thin polystyrene layers over alumina is nontrivial because of poor adhesion between polystyrene and alumina. Herein, we present the development of a stable, ultra-thin polystyrene layer on the gratings, which demonstrated 63.8 times brighter fluorescence compared to commercial polystyrene wellplates. Spike protein was examined for COVID-19 demonstrating the single-molecule counting capability of the hybrid polystyrene-plasmonic gratings.