Characterization of carfentanil and thiofentanil using surface-enhanced Raman spectroscopy and density functional theory

Fentanyls are synthetic opioids up to 10,000 times more potent than morphine. Although initially developed for medical applications, fentanyl and its analogues have recently grown synonymous with the ongoing opioid epidemic. To combat the continued spread of these substances, there is a need for rapid and sensitive techniques for chemical detection. Surface-enhanced Raman spectroscopy (SERS) has the potential for trace detection of harmful chemical substances. However, vibrational spectra obtained by SERS often differ between SERS substrates, as well as compared with spectra from normal Raman (NR) spectroscopy. Herein, SERS and NR responses from two fentanyl analogues, carfentanil (CF) and thiofentanil (TF), were measured and analysed with support from density functional theory (DFT) modelling. Using commercially available silver nanopillar SERS substrates, the SERS signatures of samples diluted in acetonitrile between 0.01 and 1000 µg/mL were studied. Relative SERS peak intensities measured in the range of 220–1800 cm⁻¹ vary with concentration, while SERS and NR spectra largely agree for CF at higher concentrations ( $\ge$100 µg/mL). For TF, three distinct NR peaks at 262, 366 and 667 cm⁻¹ are absent or strongly suppressed in the SERS spectrum, attributed to the lone-pair electrons of the thiophene's sulphur atom binding to the Ag surface. The concentration dependence of the Raman peak at $\sim$1000 cm⁻¹, assigned to trigonal bending of the phenyl ring, approximately follows a Langmuir adsorption isotherm. This work elucidates similarities and differences between SERS and NR in fentanyl detection and discusses the chemical rationale behind these differences.