Highly sensitive paper-based immunoassay using photothermal laser speckle imaging

Paper-based lateral-flow assay (LFA) is a simple and inexpensive point-of-care device that has become commonplace in medicine, environmental monitoring, and over-the-counter personal use. Some LFAs have demonstrated comparable analytical performance with laboratory-based methods, but the detection limit or sensitivity of most LFAs is significantly inferior to other molecular techniques by 10–100 ×. Consequently, LFAs are not viable for the early detection of disease-relevant biomarkers that are present in extremely small amounts in clinical specimens. Herein, we present a simple, cost-effective, and highly sensitive LFA sensor based on photothermal laser speckle imaging (PT-LSI). Under the illumination of a photothermal excitation light, gold nanoparticles (AuNPs), a common signal transduction medium in LFAs, absorb the light energy to produce heat, which subsequently induces modulation of the optical property and thermal deformation of the membrane. We measured these fluctuations through laser speckle imaging to quantify the concentration of AuNP-biomarker complexes. We experimentally demonstrate that the detection limit of our technique is superior to that of colorimetric detector by 68–125 ×. The capability of our sensor for highly sensitive detection of disease biomarkers is validated by using U.S. FDA-approved LFA kits for cryptococcal antigens (CrAg).