Lateral flow immunoassays (LFAs) have received much attention in recent years for detecting THC (a psychoactive ingredient of the cannabis plant) in oral fluids for point-of-care (POC) diagnostics. Specific advantages of screening oral fluids for THC include ease of sample collection in public and correlation of presence of THC in oral fluid with recent use of cannabis. However, despite their popularity, the detection limit of LFA is normally limited to greater than 25 ng/ml of THC in oral fluid which impedes the implementation of per se regulations in many jurisdictions (i.e., 1-5 ng/ml). To address this shortcoming, several LFA reader technologies have been developed in recent years but none of them have satisfied the required performance criteria of <80% sensitivity, specificity, and accuracy at per se limit, set by Driving Under the Influence of Drugs, Alcohol, and Medicines (DRUID). In this work, we explore Lock-In thermography (LIT) method for detecting THC in saliva-based LFA strips, utilizing thermal signatures of gold nanoparticles (GNPs) for interpretation of LFAs. Our results suggest that LIT enhances the limit of detection of the commercially available LFA by over an order of magnitude and promises an affordable solution that allows for proper enforcement of per se regulations worldwide.
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