Abstract
Simple and fast detection of small molecules is critical for health and environmental monitoring. Methods for chemical detection often use mass spectrometers or enzymes; the former relies on expensive equipment, and the latter is limited to those that can act as enzyme substrates. Affinity reagents like antibodies can target a variety of small-molecule analytes, but the detection requires the successful design of chemically conjugated targets or analogs for competitive binding assays. Here, we developed a generalizable method for the highly sensitive and specific in-solution detection of small molecules, using cannabidiol (CBD) as an example. Our sensing platform uses gold nanoparticles (AuNPs) functionalized with a pair of chemically induced dimerization (CID) nanobody binders (nanobinders), where CID triggers AuNP aggregation and sedimentation in the presence of CBD. Despite moderate binding affinities of the two nanobinders to CBD (equilibrium dissociation constants K(D) of ∼6 and ∼56 μM), a scheme consisting of CBD-AuNP preanalytical incubation, centrifugation, and electronic detection (ICED) was devised to demonstrate a high sensitivity (limit of detection of ∼100 picomolar) in urine and saliva, a relatively short sensing time (∼2 h), a large dynamic range (5 logs), and a sufficiently high specificity to differentiate CBD from its analog, tetrahydrocannabinol. The high sensing performance was achieved with the multivalency of AuNP sensing, the ICED scheme that increases analyte concentrations in a small assay volume, and a portable electronic detector. This sensing system is readily applicable for wide molecular diagnostic applications.