 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” f