High-sensitivity interpretation of lateral flow immunoassays using thermophotonic lock-in imaging

Lateral Flow Immunoassay (LFIA) strips are among the most widely-used and cost-effective biodetection tools that have applications in disease diagnosis and food safety monitoring. In this paper, we demonstrate the novel application of Thermophotonic Lock-In Imaging (TPLI) in quantitative and sensitive interpretation of LFIA strip results used to detect the pregnancy hormone human chorionic gonadotropin (hCG) as a proof-of-concept analyte. Existing optical immunoassay readers, based on image acquisition and processing algorithms, rely on reflective signals (color intensity) emanating from the surface of the LFIA. Such contrast mechanism results in missing a large amount of signal from gold nanoparticles (GNP) trapped inside the bulk of the LFIA nitrocellulose strips and leads to suboptimal detection thresholds and sensitivities. TPLI, on the other hand, operates based on diffusion of thermal waves and as such does not require optical transparency to reach GNPs in the bulk of the strip. Our experimental results indicate that the developed TPLI system is able to detect GNP-hCG conjugates at concentrations as low as 0.2 mIU, while the detection threshold of human visual inspection and commercial optical immunoassay readers were both found to be 2 mIU. Miniaturization of TPLI technique will enable a highly sensitive, quantitative, low-cost, and truly point-of-care total analysis test interpretation system for LFIAs, facilitating critical early stage detection of diseases or biochemical conditions.