Development of a novel fluorescent nanobiosensor for the detection of cocaine, and its comparison to existing qualitative and quantitative methods for forensic drug testing

The trafficking and misuse of cocaine is a global problem. In 2022, it was estimated that over 23 million people globally used cocaine at least once during the year, highlighting its widespread use. Within Europe, cocaine was reported as the most commonly used illicit stimulant, with over 5.7 million users that same year (1). Given the high prevalence of use, quick and accurate qualitative and quantitative identification of cocaine in substances at points of intervention is of increasing importance. These points of intervention include the detection of cocaine in both seized drug samples, but also in biological samples obtained from individuals at workplaces, attending healthcare settings, or if requested by officers of the law.

The potential for the development of a novel and selective nanobiosensor for the qualitative and quantitative detection of cocaine has already been established (2–12), however, selectivity tests are often performed against a limited range of compounds, and no direct comparisons have been made with existing forensic analysis techniques.

The aim of this research was two-fold, firstly to develop a novel nanobiosensor for the detection of cocaine, and secondly to compare it with existing tests used by law enforcement, customs officers, prison officers, and within public health point of care initiatives, for the detection of cocaine, to determine if nanobiosensors have the potential to join the existing battery of tests available.

A novel amphiphilic polymer-functionalized cadmium-free ZnSe/In₂S₃ core/shell quantum dots (QDs)-cationic cetyltrimethylammonium bromide (CTAB)-capped gold nanoparticle (AuNPs) fluorescence nanoprobe for the detection of cocaine was developed. ZnSe/In₂S₃ QDs were synthesized using the hot organometallic synthetic route for group II – VI semiconductor QDs and was surface-capped with organophosphorous-based ligands and organic-based stabilizers. The amphiphilic polymer, synthesized using an anhydride-capped polymer backbone and a nucleophilic agent, was used as a capping agent to coat and stabilize the QDs surface. The amphiphilic polymer-coated QDs were electrostatically linked to CTAB-AuNPs to form the nanohybrid. The electrostatic interaction between the coated QDs and capped AuNPs quenched the fluorescence emission of the bound QDs. An MNS 4.1 anticocaine thiolated DNA aptamer was adsorbed on the nanohybrid’s surface. Upon interaction of cocaine with the nanohybrid, the fluorescence of the bound QDs was significantly enhanced based on localized surface plasmon resonance-induced fluorescence emission signal. In general, the QDs functioned as the fluorophore reporter, capped AuNPs as a plasmon amplifier and the aptamer as the receptor.

The novel nanobiosensor was then compared to currently used qualitative tests (point-of-care tests, colour tests, thin layer chromatography, microcrystal analysis, and ion trap mobility spectrometry) and the qualitative and quantitative analytical technique, gas chromatography – mass spectrometry. Comparing the nanobiosensor to existing techniques demonstrated several flaws which need to be addressed for the device to be applicable in a “real world” setting. The novel nanobiosensor cross reacted with two compounds – phenacetin and nicotine – and has an experimental limit of detection of 5 µg/mL, which falls within the range of LODs of the other currently available qualitative tests for cocaine. The variability in signal response between replicate cocaine detection by the nanobiosensor also needs to be improved as for some concentrations the %CV fell out with the acceptable ±20% (13). Results showed that the nanobiosensor has the potential to be another option within the battery of available tests, but further work is needed.