While the effects of the morphology and composition of plasmonic substrates in Surface-Enhanced Raman Spectroscopy (SERS) are widely studied, surface chemistry and, more specifically, the role of preadsorbed species on colloidal substrates (i.e., stabilizers and synthesis byproducts) are typically less explored. In this paper, a surfactant-free synthesis of sparingly capped bimetallic colloidal AuAg nanostars was selected as a basis to (1) examine the effect of varying stabilizers and (2) systematically assess the impact of the resulting surface environment on SERS intensity. The latter entailed the characterization of the colloidal formulations in terms of optical reproducibility, suitability for analytical applications, long-term colloidal stability, and SERS performance. Emphasis was given to the elucidation of the stabilizer–metal interactions, which were studied by electrophoretic light scattering and infrared spectroscopy. It was found that the capping process is the result of chemisorption to an essentially neutral alloy and that the capping environment has effects on the SERS response that can overtop those caused by nanoparticle morphology. The model stabilizer, citrate, was found to weakly chemisorb (−4.36 ± 0.08 and −4.58 ± 0.05 kJ/mol at 10 and 20 °C, respectively) to the bimetallic surface in a positively cooperative fashion (n Hill > 1) via the unidentate mode.

We examined fentanyl and its six analogs using wB97XD/cc‐pVTZ density functional theoretical (DFT) calculations as well as Raman and Surface‐enhanced Raman spectroscopy (SERS). The in silico DFT calculations provided the vibrational frequencies, Raman activities, and normal mode assignment for each analog. Raman spectroscopy can detect crystalline fentanyl analogs but cannot obtain bands for samples in solution. Therefore, we utilized gold/silver nanospheres and gold/silver nanostars to examine them. The gold/silver nanostars provided stronger signals for the fentanyl analogs, and their SERS spectra can easily distinguish these fentanyl analogs from nonfentanyl opioids and other common drugs of abuse using principle component analysis and other statistical tests. Overall, our results demonstrate that SERS shows great potential to distinguish fentanyl analogs and detect trace quantities of these compounds in mixtures of seized drugs.

An analytical protocol based on surface-enhanced Raman spectroscopy (SERS) and aimed at the detection of toxicologically relevant concentrations of JWH-018 in oral fluid is presented for the first time. A DFT-supported in-depth vibrational characterization of the drug in the solid state and in solution was also performed, providing a body of literature for future spectroscopic work on the compound. A Langmuir adsorption model was used to derive quantitative parameters such as the affinity of JWH-018 for citrate-capped gold nanospheres as well as the LOD. The application of the implemented method to the analysis of extracts from fortified oral fluid samples demonstrates the feasibility of SERS as an alternative to current immunoassays as a screening tool for use in emergency room settings.

Recently there has been upsurge in reports that illicit seizures of cocaine and heroin have been adulterated with fentanyl. Surface-enhanced Raman spectroscopy (SERS) provides a useful alternative to current screening procedures that permits detection of trace levels of fentanyl in mixtures. Samples are solubilized and allowed to interact with aggregated colloidal nanostars to produce a rapid and sensitive assay. In this study, we present the quantitative determination of fentanyl in heroin and cocaine using SERS, using a point-and-shoot handheld Raman system. Our protocol is optimized to detect pure fentanyl down to 0.20 ± 0.06 ng/mL and can also distinguish pure cocaine and heroin at ng/mL levels. Multiplex analysis of mixtures is enabled by combining SERS detection with principal component analysis and super partial least squares regression discriminate analysis (SPLS-DA), which allow for the determination of fentanyl as low as 0.05% in simulated seized heroin and 0.10% in simulated seized cocaine samples.

In previous studies, AuAg colloidal nanostar formulations were developed with the two-fold aim of producing optimized surface-enhanced Raman spectroscopy (SERS) substrates and investigating the nature of the capping process itself. Findings demonstrated that the nanoparticle metals are alloyed and neutral, and capping by stabilizers occurs via chemisorption. This study utilizes citrate as the model stabilizer and investigates the mechanistic aspects of its interaction with mono- (Au20) and bimetallic (Au19Ag) surfaces by density functional theory (DFT) calculations. Citrate was modeled according to the colloid’s pH and surrounded by a water and sodium first solvation shell. A population of stable cluster–citrate structures was obtained, and energies were refined at the uB3LYP//LANL2TZ­(f)/cc-pVTZ level of theory. Solvation was accounted for both explicitly and implicitly by the application of the continuum model SMD. Results indicate that both direct binding and binding by water proxy through the charge-transfer complex formation are thermodynamically favorable. Water participation in citrate adsorption is supported by the adsorption behavior observed experimentally and the comparison between experimental and DFT-simulated IR spectra. Vibrational mode analysis suggests the possible presence of water within a crystal in dried nanostar residues. All ΔG ads(aq) indicate a weak chemisorptive process, leading to the hypothesis that citrate could be displaced by analytes during SERS measurements.