(−)-
trans
-Δ-Tetrahydrocannabinol
(THC) is a major psychoactive component in cannabis. Despite the recent
trends of THC legalization for medical or recreational use in some
areas, many THC-driven impairments have been verified. Therefore,
convenient, sensitive, quantitative detection of THC is highly needed
to improve its regulation and legalization. We demonstrated a biosensor
platform to detect and quantify THC with a paper microfluidic chip
and a handheld smartphone-based fluorescence microscope. Microfluidic
competitive immunoassay was applied with anti-THC-conjugated fluorescent
nanoparticles. The smartphone-based fluorescence microscope counted
the fluorescent nanoparticles in the test zone, achieving a 1 pg/mL
limit of detection from human saliva samples. Specificity experiments
were conducted with cannabidiol (CBD) and various mixtures of THC
and CBD. No cross-reactivity to CBD was found. Machine learning techniques
were also used to quantify the THC concentrations from multiple saliva
samples. Multidimensional data were collected by diluting the saliva
samples with saline at four different dilutions. A training database
was established to estimate the THC concentration from multiple saliva
samples, eliminating the sample-to-sample variations. The classification
algorithms included
k
-nearest neighbor (
k
-NN), decision tree, and support vector machine (SVM), and the SVM
showed the best accuracy of 88% in estimating six different THC concentrations.
Additional validation experiments were conducted using independent
validation sample sets, successfully identifying positive samples
at 100% accuracy and quantifying the THC concentration at 80% accuracy.
The platform provided a quick, low-cost, sensitive, and quantitative
point-of-care saliva test for cannabis.