Novel approach to urinary 11-nor-9-carboxy-delta-9-tetrahydrocannabinol analysis in forensic applications using disposable, graphene-based electrochemical sensor coupled with monolithic micro-solid-phase extraction

Abstract

Background: The escalating global use of cannabis as a narcotic demands the development of a rapid, cost-effective, and portable method for assessing cannabis intake in forensic contexts. A major cannabis metabolite, 11-nor-9-carboxy-Δ⁹-tetrahydrocannabinol (THC-COOH), is commonly targeted for cannabis drug testing. However, the quantification of urinary THC-COOH typically relies on chromatographic methods, which require bulky, expensive instruments and skilled expertise. This study presents a novel approach for the simple, selective, and sensitive detection of THC-COOH in human urine samples by integrating monolithic pipette-tip solid-phase extraction (monolithic PT-SPE) with an electrochemical sensor based on disposable screen-printed graphene electrodes (SPGEs).

Results: A C₁₈-functionalised monolithic backbone, poly(stearyl methacrylate-co-ethylene dimethacrylate), is in situ synthesised within micro-pipette tips for effective urine sample cleanup and pre-concentration of THC-COOH before quantification using the SPGE-based sensor in a few minutes. Critical parameters for PT-SPE and electrochemical detection are systematically optimised. The method exhibits a wide linear range of 50-1000 ng mL^-1 (R² = 0.9980), with both a low limit of detection (5.2 ng mL^-1) and a low limit of quantification (17.5 ng mL^-1). Under optimal conditions, THC-COOH is effectively isolated from complex urine matrices, achieving a 14-fold signal enhancement and good recovery (80 %-110 %), with intra- and inter-day precisions of 5.9 % and 7.1 %, respectively. Both for authentic urine specimens and spiked samples, analysis results obtained using the method developed demonstrate strong correlation with liquid chromatography with tandem mass spectrometry (LC-MS/MS) results.

Significance: We successfully employed an electrochemical sensor for the determination of THC-COOH in human urine specimens for the first time. By coupling it with the developed monolithic PT-SPE, we achieved direct detection of THC-COOH without requiring additional electrode surface modifications. The method is environmentally friendly, requiring only a minimal volume of reagents for both extraction and detection. Furthermore, it is efficient, simple and cost-effective.

Keywords: 11-Nor-9-carboxy-Δ(9)-tetrahydrocannabinol; Illicit drug; Micro-solid-phase extraction; Monolith; Screen-printed graphene electrode; Urine analysis.