Point-of-care diagnostics for drugs of abuse in biological fluids: application of a microfabricated disposable copper potentiometric sensor.

The major objective of this work was to develop a portable, disposable, cost-effective, and reliable POC solid-state electrochemical sensor based on potentiometric transduction to detect benzodiazepine abuse, mainly diazepam (DZP), in biological fluids. To achieve that, microfabricated Cu electrodes on a printed circuit board modified with the conducting polymer poly(3-octylthiophene) (POT) have been employed as a substrate. This polymer was introduced to enhance the stability of the potential drift (0.9 mV/h) and improve the limit of detection (0.126 nmol mL-1). Nernstian potentiometric response was achieved for DZP over the concentration range 1.0 × 10-2 to 5.0 × 10-7 mol L-1 with a slope of 55.0 ± 0.4 mV/decade and E0 ~ 478.9 ± 0.9. Intrinsic merits of the proposed sensor include rapid response time (11 ± 2 s) and long life time (3 months). In order to enhance the selectivity of the potentiometric sensor towards the target drug and minimize any false positive results, calix[4]arene (CX4) was impregnated as an ionophore within the PVC plastic ion-sensing membrane. The performance of the POC sensors was assessed using electrochemical methods of analysis and electrochemical impedance spectroscopy as a surface characterization tool. The studied sensors were applied to the potentiometric determination of DZP in different biological fluids (plasma, urine, saliva, and human milk) in the presence of its metabolite with an average recovery of 100.9 ± 1.3%, 99.4 ± 1.0%, 101.8 ± 1.2%, and 99.0 ± 2.0%, respectively. Graphical abstract.