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.
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