Getting the signal (from Raman)

DOUGLAS FARMER, SENIOR EDITOR DOUG.FARMER@PHOTONICS.COM

To capitalize on the specificity of Raman spectra in biomedicine and the life sciences, some researchers have turned to surface-enhanced Raman spectroscopy (SERS) to clearly ascertain the vibrational signal of molecules. The SERS technique is based on the principle that the vibrational spectra of molecules are magnified when in the vicinity of a structured metal nanomaterial, and thus they become more readable. But it has been a considerable challenge to translate this capability into a miniaturized wearable format that can be used to track components of blood and other samples. A team from the University of Tokyo and other institutions is attempting to overcome this challenge, using the flexibility of diode lasers and silicon integrated photonics in the process.

A major limitation of Raman spectroscopy — and what has hindered its use in many laboratory and clinical settings — is the equipment needed to reliably capture the inherently weak Raman signal. Given its success in analyzing a variety of biofluid samples, not to mention unknown chemicals, the potential is present for Raman to be used in diagnostics. Keisuke Goda, a chemistry professor at the University of Tokyo, said that smaller spectrometers typically offer lower resolution and sensitivity than full lab versions. But the collaboration has experimented with on-chip spectrometers, enabled by silicon photonics, and tested the way that gold and silver materials interacted in film, microfluidic, and mesh forms. The difficulty in mass-producing SERS-quality substrates has been a barrier to commercialization, though startup companies have achieved some success in this area.

While Goda acknowledged that a prototype has not been fully developed yet, he said a wearable SERS system could evaluate biomarkers in bodily fluids, including sweat, saliva, interstitial fluid, and urine, establishing real-time information on a patient’s health, outside of a doctor’s office. This could pave the way for not only tracking therapeutic treatment but also practicing preventive medicine, providing early warning signs of disease before symptoms have appeared.

Similarly, our cover story this edition comes from authors at Johns Hopkins University, who have modeled Raman probes after the natural structure of DNA, providing an innovative way to detect cancer in low concentrations. Using specific structures, such as optical cavities, light can be confined within deep-subwavelength regions. And by mimicking the precise folding of single-stranded DNA, metallic nanoparticles are folded accurately, allowing for plasmonic coupling, which heightens the Raman signal. As the authors outline here, this method was shown to not only distinguish cancer cells but also determine their severity.

And SERS is also at the center of two presentations in our upcoming virtual BioPhotonics Conference, on the detection of gold colloid aggregates in Alzheimer’s disease and capturing deep-seated tumors with surface enhanced spatially offset resonance Raman spectroscopy. Read the preview here.

Enjoy the issue!
Douglas J. Farmer