Smartphone-integrated lateral flow assay for robust detection of Δ9-Tetrahydrocannabinol

The increasing prevalence of cannabis use necessitates accurate and accessible monitoring methods to mitigate substance-associated risks and enhance public safety. We present iLine (integrated lateral-flow assay with intensity normalization and enhancement) for sensitive and on-site quantification of tetrahydrocannabinol (THC), the primary psychoactive component of cannabis. The iLine platform incorporates i) gold nanoclusters for enhanced optical signals and ii) a smartphone-based imaging system with intelligent algorithms to ensure consistent signal acquisition. These features enable a portable, hardware-minimal THC detection while maintaining laboratory-grade performance in real-world conditions. Specifically, iLine achieved a detection limit of 0.12 ng/mL for THC, representing an 85-fold improvement over conventional lateral-flow assays. The analytical performance was highly robust, with coefficient of variation values < 2 % under diverse imaging conditions, including daylight, direct lamp illumination, and shadow environments. When applied to human saliva samples, iLine successfully differentiated cannabis users (n = 10) from non-users (n = 10) within 10 min. The iLine approach facilitates the use of smartphones as ubiquitous detectors, broadening access to THC testing in field applications.