Illicit and licit drugs continue to have a negative impact on the public’s health even with new regulations and laws. At local, state, and federal levels, crime laboratories analyze samples in a variety of mixtures and concentrations. Raman spectroscopy is a molecular fingerprinting, multiplex technique, which is considered a category A forensic characterization tool by the Scientific Working Group for the Analysis of Seized Drugs (SWGDRUG). The sensitivity of the Raman technique can be further enhanced down to the single-molecule detection level when analytes of interest are located at the interstitial sites of aggregated silver nanoparticles (i.e., the “hot spots” of enormous electromagnetic fields). This is the surface-enhanced Raman spectroscopy (SERS) effect, which was exploited in this work for the enhanced detection of sucrose, caffeine, acetaminophen, and quinine sulfate dihydrate, prevalent cutting agents in “street” samples. To achieve this, tangential flow filtration (TFF) was utilized to purify, size-select, and concentrate two types of synthesized silver nanoparticles (AgNPs). The physicochemical properties of AgNPs were then characterized by UV-Vis, ICP-OES, and Raman spectroscopy before SERS measurements.
The limit of detection for each cutting agent in solution was first determined and used as a starting point for SERS measurements. The resulting LODs visually were, 0.2 M, 0.0089 M, 0.08 M, and 5.10 x 10^-6 M, for sucrose, caffeine, acetaminophen, and quinine sulfate dihydrate solutions, respectively, and calculated LODs using a specific characteristic band were 0.02 M, 0.002 M, 0.008 M, 1.7 x 10^-4 M, respectively. The resulting SERS spectrum of each cutting agent demonstrated detection of characteristic bands that were indistinguishable from noise in the normal Raman measurements. By considering band patterns and ratio of intensities, indications of the electromagnetic and chemical enhancement mechanisms could be discussed.
The SERS enhancement factor was estimated for the quinine sulfate dihydrate solution, and the lowest limit of detection was established. The visual LOD was 6.4 x 10^-8 M and a calculated LOD using the 1367 cm-1 band was 9.09 x 10^-9 M. The SERS enhancement factor calculated from the 6.4 x 10^-8 M spectrum and 1367 cm-1 characteristic band and water bands was 5.14 x 10^4, which is an average amount of signal enhancement.
SERS has many variables that are necessary to be considered and kept consistent to ensure reproducibility and future use for crime laboratories. The resulting enhancement will be affected by nanoparticle size, geometry, homogeneity, surface chemistry/surface charge and concentration causing the resulting plasmonic resonance to vary. Through comparison of resulting spectra, AgNPs size, concentration, synthesis method, and pH of analyte solution were considered and discussed within this work.
In addition to SERS techniques, handheld Raman devices has been of interest due to quick and easy detection of controlled substances. Two handheld devices, Thermo Scientific Gemini and Rigaku Progeny ResQ were compared to the benchtop instrument. The handheld devices produced characteristic spectra for all solids and most aqueous samples in addition to various mixtures. Student t-tests indicated there were statistical differences from the resulting S/N ratios of the benchtop instruments and the handheld devices for the solids and aqueous solutions, but comparable S/N ratios for the mixtures.
Future studies could focus on implementing this SERS-based multiplex sensing technique in the analysis of “street” samples, with handheld devices for immediate public health benefits.