ree space accessory positioned such that the SERS substrate would be at the focal point of the sample/collection lens. Spectra were collected using the IDRaman’s Raster Orbital Scanning mode to average the measurement from a small area of the SERS substrate to improve measurement-to-measurement reproducibility.
The Raman spectrum of pure cocaine is known to be dominated by peaks at 872, 999, 1026, 1273 and 1597 cm-1, peaks which can shift slightly when operating in SERS mode depending on the extent of interaction with the gold or silver surface and the plasmon field at each wavelength. The symmetric 999 cm-1 peak tends to show the most enhancement, its intensity increasing with concentration.
To validate that the signal is indeed from cocaine and not the synthetic saliva, the two lowest concentrations were measured in comparison to a blank of synthetic saliva. As can be seen in the highlighted region, the Raman peak at 999 cm-1 is still resolvable even at concentrations of 0.0039 g/L.
Conquering Cannabis with SERS
The same SERS system was also tested with various concentrations of THC (tetrahydrocannabinol), the chemical responsible for most of marijuana’s psychological effects. There is very little Raman data available for THC in the scientific literature, mainly due to the high level of fluorescence from concentrated samples. When measured at trace levels with Ocean Optics SERS substrates, however, several distinctive peaks were observed, and potential interference from fluorescence seemed to be effectively suppressed.
Raman peaks were observed at 710, 999, 1130, 1232, 1352 and 1390 cm-1 and were seen to decrease in height with decreasing concentration of THC. The lowest concentration of THC measured was 6×10-4 M (equivalent to 180mg/L THC), although it may be possible to identify presence versus absence even at lower concentrations.
SERS Shows Strong Promise
Our tests showed that both cocaine and THC display very distinctive SERS peaks using Ocean Optics SERS substrates and the IDRaman mini handheld system. In both cases, the intensity of key Raman peaks scaled via a well-defined relationship with concentration, even down to very low concentrations.
When considered in combination with the field-ready performance of the IDRaman mini and the sensitivity of Ocean Optics SERS substrates at a low cost-per-use, these results indicate a very promising future for use in both the legal system and hospitals.
1 R. Compton and A. Berning, ‘‘Results of the 2007 National Roadside Survey of Alcohol and Drug Use by Drivers,’’ National Highway Trafﬁc Safety Administration’s National Center for Statistics and Analysis, Report Number DOT HS 811 175 (2009).
2 Highlights of the 2009 Drug Abuse Warning Network (DAWN) findings on drug-related emergency department visits; The DAWN Report; Center for Behavioral Health Statistics and Quality, Substance Abuse and Mental Health Services Administration: Rockville, MD, USA, 28 December 2010; Available online http://dawninfo.samhsa.gov. (accessed on 26 April 2011).
3 Farquharson, Stuart, et al. “Rapid detection and identification of overdose drugs in Saliva by surface-enhanced Raman scattering using fused gold colloids.” Pharmaceutics 3.3 (2011): 425-439.