SERS: A New Tool on the Roads and in the ER
A 2007 US national survey found drugs to be present more than 7x as frequently as illegal levels of alcohol among weekend nighttime drivers1. Drugged driving is a growing concern, as many users mix drugs and alcohol, amplifying their debilitating effects. Prescription drugs were the most common drug found in drivers involved in fatal crashes, closely followed by marijuana and then cocaine in smaller numbers.
Unfortunately, there is no way to quickly test for the presence of drugs in a driver’s system. In fact, some countries require a medical practitioner or other expert to evaluate the suspect prior to ordering drug testing. When added to the hour or more it can take to process a sample via GC-MS (gas chromatography – mass spectrometry) for identification and quantification, the entire process is cumbersome at best, prohibitive at worst.
The need for rapid drug testing is apparent in hospitals as well, where the number of drug-related visits to emergency rooms in the U.S. nearly doubled from 2004 to 2009, split equally between illegal drugs and pharmaceuticals2. Some of these visits are unintentional drug intoxication by older adults mistakenly taking a prescription drug more or less often than recommended or in the wrong amount. Regardless of the cause, drug intoxication and overdose can present symptoms that can confuse diagnosis. Standard testing for drug use, meanwhile, delays diagnosis and administration of appropriate medical care.
A non-invasive, portable system capable of providing accurate, quantitative results for drug intoxication roadside and bedside would facilitate more aggressive anti-drug driving campaigns by authorities, and could significantly improve patient outcomes in emergency rooms. The answer, not surprisingly, is an outgrowth of the same spectroscopy technique now widely used by law enforcement for identification of bulk narcotics in the field.
Raman Takes the Next Step in Detection
Raman spectroscopy has become a well-established technique for first responders seeking to identify unknown substances suspected to be narcotics. Raman spectroscopy relies on laser light scattering by molecules to provide unique spectral fingerprints that allow the identification of chemical substances. It is based on matching of the unknown’s spectral signature against a known library, allowing it to adapt quickly to new drug variants entering the market through the expansion of the library.
Surface Enhanced Raman Spectroscopy (SERS) is an extension of Raman spectroscopy in which gold or silver nanoparticles provide amplification of the Raman signals. The technique works via an electromagnetic effect where molecules come into close proximity with gold or silver particles. When incident laser light strikes the nano-particulate surface, localized surface plasmons can be excited, greatly enhancing Raman signals. The enhancement can be a factor of up to 1011, making SERS well-suited to trace level detection of materials.
In addition to enhancing the Raman scattering intensity, the metallic surfaces of the SERS materials also have the ability to quench endogenous fluorescence and produce much higher quality data allowing improved spectral analysis. SERS substrates are being actively developed for forensic and homeland security applications. They have the potential to provide molecular-level, chemical fingerprint spectra to enable the detection of many different species in a single measurement, with the added benefit of requiring very small volumes of analyte (as little at 0.1 mL).
A Special Kind of Substrate
Testing for drug intoxication is somewhat simplified by the fact that drugs are present in saliva at roughly the same concentrations as in blood. Saliva is easily collected from subjects, and is easily applied to a substrate. Researchers have used this to demonstrate the effectiveness of SERS for testing saliva for drugs, but their method required a two-step procedure combined with a solid phase extraction step to separate the drug concentrate from the saliva3.
The second technical hurdle to be surmounted is cost per test. Many good quality commercial SERS substrates cost up to $60 U.S. per measurement, and are not re-usable. In response to this industry gap, Ocean Optics has developed a low-cost, high performance alternative SERS substrate using gold or silver nanoparticles deposited through an innovative process onto paper or glass-fiber substrates. Our technique produces a substrate that is low cost and can be produced in large volumes. It can even be optimized to achieve better attachment of a target analyte to the nanoparticle surface for enhanced SERS effect. Both highly sensitive and selective, our technology makes cost-efficient SERS possible for volume consumable applications like drug testing.
When compared with other SERS substrates on the market, the Ocean Optics substrates show better sensitivity for a wide range of low concentrations of BPE (trans-1,2-bis (4-pyridyl) ethylene), a commonly used molecule for benchmarking SERS performance. Additionally, the Ocean Optics SERS substrate is far more robust and simpler to use with sample preparation and measurement – taking seconds as opposed to minutes or hours.
Detection of Cocaine with the IDRaman mini
To test the ability of the Ocean Optics SERS substrates to detect trace drugs in saliva, measurements were performed with the IDRaman mini (discontinued product as of July 2017). The IDRaman mini is a miniaturized, integrated Raman spectroscopy system containing a laser, spectrometer and collection optics together with a microprocessor for onboard library matching. This robust portable unit contains a number of unique features including a laser scanning option and runs from two standard AA batteries. The overall format of this device in combination with the possibility of low-cost SERS substrates has the potential to be a solution for roadside saliva drug measurements.
Samples of cocaine were prepared in manufactured synthetic saliva at various concentrations, then tested with the IDRaman mini using the “point-and- shoot” free 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.