SpectraFluidics takes SERS into the Field

Elegant improvements to surface-enhanced Raman spectroscopy from a UCSB spin-out are opening up strategic real-world applications.

A reliable way to detect the presence of toxins or explosive vapor in the open atmosphere without the need for lengthy sample preparation and laboratory work would be a valuable tool in many current contexts. The trick is developing one that can reliably spot trace concentrations of the target molecules in air that is already well-stocked with potential contaminants, and deliver a definitive verdict.

Raman spectroscopy is a strong candidate for the task. It is non-destructive, can usually work without a great deal of sample preparation, and the vibrational energy levels it examines are quite distinct for different molecules. This means it can be extremely good at spotting individual substances of interest and provide a quantitative fingerprint of their presence.

The fly in the ointment is the inherently weak signal generated by the Raman effect, with perhaps only one incident photon in every ten million experiencing the inelastic scattering that manifests itself as a Raman shift. Consequently, Raman's practical use is considered to be limited to analysis of bulk solids and liquids.

Boosting the signal

A solution that combines nanotechnology and plasmonics has existed for many years. When incident light strikes particular nanostructures of gold or silver, the structure is able to harvest the incoming radiation, much as an antenna collects radio waves, and concentrate the electromagnetic energy in "hotspots". Often these hotspots are nanoscale structural features such as clefts, gaps and fissures.

This effect is the principle behind surface-enhanced Raman spectroscopy (SERS). If analyte molecules find themselves adsorbed onto a metallic substrate and caught in one of the nanoscale hotspots, then the localized enhancement of the electric field creates surface plasmon oscillations. An incident laser photon meeting a correctly oriented plasmon oscillation gives rise to a dramatically boosted Raman response.

"SERS has been studied for 35 years and the mechanism is now fairly well understood," said Phil Strong, CEO of SpectraFluidics, a spin-out from the University of California, Santa Barbara (UCSB). "The plasmonic resonance effect is particularly pronounced when two or three metal nanoparticles sandwich the analyte molecules, leading to a tremendous amplification of the Raman signal by between 10⁵ and 10⁹. Such sensitivity makes SERS ideally suited to detect trace chemicals at concentrations ranging from parts per billion to parts per trillion."

Find the hotspots

SERS has historically been reliant upon the use of certain active substrates or carefully manufactured colloids as the detection media, and using them to reliably detect trace concentrations of atmospheric vapor has proven difficult.

SpectraFluidics has developed an answer. "Free-surface microfluidics and a great deal of in-house expertise in the manufacture of SERS colloids are the keys," explained Strong. "Within our sample cell is an open micro-channel, approximately ten microns wide and less than ten microns deep, along which a fluid flows.”

“In effect, this is a thin micro-scale film of fluid, open to the atmosphere and hence open to direct contact with the vapor molecules we are looking for. It acts like a natural pre-concentrator, leveraging Henry's Law to great effect." Henry's Law controls the solubility of gases into liquids, and at this scale effectively compels vapor molecules at trace concentrations to partition into the fluid film.

In the SpectraFluidics detector, this flowing liquid is a carefully engineered colloid of nanoparticles, absorbing analyte molecules from the air and delivering them into the beam of a standard Raman spectrometer just as they form the optimal dimer or trimer arrangements. The result is a drastic jump in sensitivity, allowing the detection of trace vapor molecules present in the air at their natural vapor pressure.

The technique elegantly sidesteps some of SERS' usual limitations. The constant renewing flow of fresh fluid dissipates any heat caused by the laser, as well as minimizing the contamination build-up that can hinder the performance of solid-state substrates. It also enhances the probability of an analyte molecule finding itself sandwiched by nanoparticles in a SERS-favorable configuration.

"In a liquid carrier, the SERS hotspots are by nature dynamic, able to flow within the liquid and meet the analyte," explained Strong."This gives you a much greater probability of detecting low volumes of analyte molecules than you would have with a solid metal substrate, where the chances of a molecule in the air landing on a stationary SERS hotspot are not that great."

Out of the laboratory

SpectraFluidics' origin as a UCSB spin-out was seeded by The Institute for Collaborative Biotechnologies, an interdisciplinary center led by UCSB. The company has also secured venture funding from Cycad Group and In-Q-Tel, a strategic investment firm that identifies technology of value to the US intelligence community.

"Our primary target has been explosives detection, in both aviation security and army field deployments," Strong commented. "But we recognise that this is a platform technology and have started to apply it to different markets. We see a big opportunity in food inspection, both for food safety and food quality, where most testing requires either expensive lab-based mass spectrometry or a coarse litmus test out in the field. The SERS technique could bring the sensitivity of gas- and liquid-chromatography mass-spectrometry (GCMS and LCMS) to a point-and-shoot operation carried out on the spot."

SpectraFluidics anticipates launching its first analytical instrument, designed for laboratory use, at the end of Q2 2011. OEM products targeted at aviation security and hand-held systems will follow.

"We have solved some of the fundamental application issues associated with SERS and with making it a robust technology," said Strong. "Now we are taking it out into the field, and actively looking for application partnerships with companies who want to take this technology and apply it to their standard instrumentation."