Given the sometimes complex composition of drugs street samples and biological samples (such as saliva from drugs users) strategies have been developed to accurately detect illicit drugs in such samples. These strategies involved changing the experimental conditions (such as pH of the solution), modifying the sensor surface or developing biomimetic materials that capture only the target drug close to the sensor surface, thus leading to an improved performance of the sensor. Computational design was used to model the interaction between the target illicit drug and the compounds (monomers) used to prepare biomimetic materials. Computational modeling helped to produce biomimetic materials with recognition cavities that bind specifically to the illicit drugs, and exclude other compounds present in the sample. The sensitive and selective detection detection of illicit drugs was achieved by electrochemical methods, by applying a potential at the working electrode (sensor) and measuring the current that results from the oxidation of the illicit drug. Cocaine, heroin, amphetamine, methamphetamine, MDMA and cannabinoids were selected as target drugs.
Electrochemical fingerprinting of illicit drugs was done on unmodified and modified electrodes (with nano- and biomimetic-materials). In terms of designing biomimetic materials with specific recognition cavities two strategies have been followed (i) designing biomimetic receptors (molecularly imprinted polymers-MIPs) in form of nanoparticles (nanoMIPs) followed by integration onto electrodes and (ii) direct electrosynthesis onto electrodes in form of thin MIP-layers. The first strategy was developed based on a novel solid-phase synthesis procedure within the Biotechnology group of University of Leicester, during a secondment and served for developing nanoMIPs for cocaine, morphine and tetrahydrocannabinol. The synthesis of MIPs in the form of thin layers directly onto electrodes was efficient for the selective detection of cocaine, heroin and MDMA. Another strategy for electrodeposition of MIPs in the form of nanoparticles was developed based on multipulse amperometry and successfully applied for heroin and MDMA detection. Amphetamine is a primary amine and does not show an electrochemical signal in the potential window of the single use electrodes. A novel strategy was developed to make possible the electrochemical detection of amphetamine.
Illicit drugs and some of their metabolites have been detected in standard (buffer) solution by square wave voltammetry. The unique electrochemical fingerprint of the target drugs and some of their metabolites has been established. Quantification has been achieved for illicit drugs on bare and (imprinted) polymers/nanomaterials modified graphite electrodes. The sensors’ performance for illicit drugs in terms of the lowest concentration detectable (detection limit), the range of concentrations that can be determined (linear range) and reproducibility (relative standard deviation) was established. Detection limits between 5 and 125 µM were obtained, and the relative standard deviation varied amongst drugs from 0.7-15.8%.
The developed electrochemical strategies were applied on the analysis of drug street samples seized in Belgium, both in the lab and on-site using a portable, miniaturized potentiostat (at the National Institute of Criminalistics and Criminology and Port of Antwerp). The electrochemical screening allowed the accurate detection of illicit drugs in complex street samples, in the presence of most adulterants/cutting agents. The polymer based sensor for cocaine was applied further to detect cocaine in spiked saliva samples and river water.
