1 Department of Electrical and Computer Engineering, Concordia University, 1455 de Maisonneuve Blvd West, Montreal, Quebec, H3G1M8, Canada; Centre for Applied Synthetic Biology, Concordia University, 7141 Sherbrooke St. West, Montreal, Quebec, H4B1R6, Canada.
2 Centre for Applied Synthetic Biology, Concordia University, 7141 Sherbrooke St. West, Montreal, Quebec, H4B1R6, Canada; Department of Biology, Concordia University, 7141 Sherbrooke St. West, Montreal, Quebec, H4B1R6, Canada.
3 Department of Electrical and Computer Engineering, Concordia University, 1455 de Maisonneuve Blvd West, Montreal, Quebec, H3G1M8, Canada; Centre for Applied Synthetic Biology, Concordia University, 7141 Sherbrooke St. West, Montreal, Quebec, H4B1R6, Canada; Department of Biology, Concordia University, 7141 Sherbrooke St. West, Montreal, Quebec, H4B1R6, Canada. Electronic address: steve.shih@concordia.ca.

We present a novel "on-off", cost-effective, rapid electrochemical aptasensor combined with a microfluidics cartridge system for the detection of ?⁹-THC (?⁹-tetrahydrocannabinol) in human saliva via differential pulse voltammetry. The assay relied on the competitive binding between the ?⁹-THC and a soluble redox indicator methylene blue, using an aptamer selected via FRELEX. We found that the aptasensor can detected 1 nM of ?⁹-THC in PBS in a three-electrode cell system, while the sensitivity and both the dissociation constant (K_d) and association constant (K_b) were dependent on the aptamer density. The aptamer also showed great affinity towards ?⁹-THC when tested against cannabinol and cannabidiol. The same limit of detection of 1 nM in PBS was achieved in small volume samples (~60 µL) using the aptamer-modified gold screen-printed electrodes combined with the microfluidic cartridge setup, however, the presence of 10% raw human saliva had a negative effect which manifested in a 10-fold increase in the LOD due to interfering elements. Filtering the saliva, improved the tested volume to 50% and the LOD to 5 nM of ?⁹-THC which is lower than the concentrations associated with impairment (6.5-32 nM). The aptasensor showed a good storage capability up to 3 days, however, the reusability significantly dropped from 10 cycles (freshly prepared) to 5 cycles. The results clearly demonstrate the feasibility of the aptasensor platform with the microfluidics chamber towards a point-of-care testing application for the detection of ?⁹-THC in saliva.