Picomolar-Level Sensing of Cannabidiol by Metal Nanoparticles Functionalized with Chemically Induced Dimerization Binders

Simple and fast detection of small molecules is critical to 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 successful design of chemically conjugated targets or analogs for competitive binding assays. Here, we developed a generalizable method for 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 (nano-binders), where CID triggers AuNPs aggregation and sedimentation in the presence of CBD. Despite moderate binding affinities of the two nano-binders to CBD (K(D)s of ~6 and ~56 μM), a scheme consisting of CBD-AuNP pre-analytical 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 assay time (~2 hours), 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 coupled to other binders for wide molecular diagnostic applications.