# Cellulose-Based Colorimetric Test Strips for SARS-CoV-2 Antibody Detection

## TL;DR

This paper introduces low-cost, paper-based tests that detect SARS-CoV-2 antibodies using a new color-changing method, offering a simple and effective tool for monitoring immune responses.

## Contribution

The first use of glutaraldehyde as a protein dye in paper-based biosensors for antibody detection.

## Key findings

- Both sensors showed a linear correlation between antibody concentration and RGB values in buffer and serum.
- The VLP sensor detected antibodies in the range of 1.0–20 µg/mL in diluted serum.
- The N-protein sensor detected antibodies in the range of 1.0–40 µg/mL in diluted serum.

## Abstract

The COVID-19 pandemic highlighted the need for rapid, cost-effective tools to monitor transmission and immune response. We developed two novel paper-based colorimetric biosensors using glutaraldehyde as a protein dye—its first use in this context. Glutaraldehyde reacts with amino groups to generate a brown color, enabling detection of SARS-CoV-2 antibodies. Wathman filter paper was functionalized with (3-aminopropyl)triethoxysilane (APTES) to immobilize virus-like particles (VLPs) and nucleocapsid protein (N-protein) as biorecognition elements. Upon incubation with antibody-containing samples, glutaraldehyde enabled colorimetric detection using RGB analysis in ImageJ software. Both sensors showed a linear correlation between antibody concentration and RGB values in buffer and serum. The VLP sensor responded linearly within the range of 1.0–20 µg/mL (green coordinate) in 500-fold diluted serum and the N-protein sensor from 1.0–40 µg/mL (blue coordinate) in 250-fold diluted serum. Both sensors demonstrated good selectivity, with glucose causing up to 18% interference. These biosensors represent a paradigm shift, as they provide a sensitive, user-friendly, and cost-effective option for semi-quantitative serological analysis. Furthermore, their versatility goes beyond the detection of SARS-CoV-2 antibodies and suggests broader applicability for various molecular targets.