Low-cost, highly sensitive medical diagnostic assays inspired by hair dye chemistry

Lateral flow assays (LFAs), such as the COVID-19 Rapid Antigen Tests or home pregnancy tests, are quick and easy. But they frequently fail to detect trace quantities of illness indicators (biomarkers) like antibodies or proteins, leading to a high number of false negatives. A novel strategy based on hair colour chemistry could address this issue.

New research at the Department of Chemical Engineering, IISc, led by Bhushan Toley, and his PhD student, Karan Saxena, describes the use of innovative enzymatic substrates inspired by permanent hair dyes. These substrates enhance the performance of lateral flow assays by employing a two-component system. The first component, known as the primary intermediate, reacts with a second component called the coupler. In the presence of horseradish peroxidase (HRP) enzyme and a small amount of hydrogen peroxide, these two components undergo an oxidative coupling reaction that produces an intensely coloured, insoluble dye. This dye precipitates at the test lines on the LFA strip, allowing very low concentrations of a target substance (analyte) to be detected.

(A) Conventional, commercially available LFAs employing gold nanoparticles (B) Enzymatic LFA with traditional DAB substrate (C) Novel approach from this study (Image: Karan Saxena)

The optimised substrate combination, using N,N-Dimethyl-p-phenylenediamine as the primary intermediate and 1-Naphthol as the coupler, achieved an unprecedented sensitivity of 0.16 ng/mL in LFAs for human IgG – matching the sensitivity of gold-standard lab-based ELISA tests. This method not only improves sensitivity by over 40 times compared to conventional gold nanoparticle-based tests but is also about 10,000 times more cost-effective. The researchers also demonstrated that the system is stable and simple to use, which makes it ideal for point-of-care diagnostics in resource-limited settings. This approach can contribute to early detection of diseases, facilitate timely treatment, and ultimately contribute to better public health outcomes.

                         
Karan Saxena                                                                       Bhushan J Toley

REFERENCE:
Saxena K, Toley BJ, Alternative chromogenic substrates for horseradish peroxidaseenhanced lateral flow immunoassays provide higher sensitivity at lower cost, Sensors and Actuators B: Chemical (2025).
https://www.sciencedirect.com/science/article/abs/pii/S0925400524017477?via%3Dihub

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