Drug trafficking is a major worldwide problem. In this context, cocaine is one of the most commonly used drugs of abuse. In addition, street cocaine is commonly seized adulterated with pharmaceutical compounds, and the composition of the mixture provides a chemical fingerprint that can assist the police in tracking the distribution route of the drug; hence, the development of facile, cost-effective methods for determining the composition of street cocaine is an important objective. Herein we report a simple strategy for the fabrication of paper-based analytical devices (PADs) for the dual electrochemical and surface-enhanced Raman-scattering (SERS) determination of cocaine samples. Accordingly, a 2-μm-thick Au film was prepared by depositing gold nanoparticles (AuNPs) on office paper with wax-barrier templates to create nanostructured gold tracks that are mainly formed by Au(111) fcc planes as electrodes and SERS transducers. These devices were characterized by scanning electron microscopy, X-ray diffractometry, electrochemical impedance spectroscopy, and energy-dispersive X-ray spectroscopy. The optimized device is simple and inexpensive to prepare and exhibited a Raman-scattering enhancement factor of 3 × 106, a 15-fold superior electroactive area, and a 2.6-fold decrease in charge-transfer resistance when compared with a conventional Au electrode. In addition, these PADs were successfully used in a forensics scenario to screen and analyze a seized street cocaine sample, determine its chemical profile, and to identify simultaneously caffeine, paracetamol, and levamisole adulterants.
Plasmonic catalysis takes advantage of the surface plasmon resonance (SPR) excitation to drive or accelerate chemical transformations. In addition to the plasmonic component, the control over metal-support interactions in these catalysts is expected to strongly influence the performances. For example, CeO2 has been widely employed towards oxidation reactions due to its oxygen mobility and storage properties, which allow for the formation of Ce3+ sites and adsorbed oxygen species from metal-support interactions. It is anticipated that these species may be activated by the SPR excitation and contribute to the catalytic activity of the material. Thus, a clear understanding of the role played by the SPR-mediated activation of surface oxide species at the metal-support interface is needed in order to take advantage of this phenomenon. Herein, we describe and quantify the contribution from active surface oxide species at the metal-support interface (relative to O2 from air) to the activities in green SPR-mediated oxidation reactions. We employed CeO2 decorated with Au NPs (Au/CeO2) as a model plasmonic catalyst and the oxidation of p-aminothiophenol (PATP) and aniline as proof-of-concept transformations. We compared the results with SiO2 decorated with Au NPs (Au/SiO2), in which the formation of surface oxide species at the metal-support interface is not expected. We found that the SPR-mediated activation of surface oxide species at the metal-support interface in Au/CeO2 played a pivotal role in the detected activities, being even higher than the contribution coming from the activation of O2 from air.
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