echnical 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