Biosensing stands to benefit immeasurably from techniques that enhance light, in particular the enhancement of "Raman signals", features in the spectra of scattered light that indicate a molecule’s vibrational modes. Raman signals offer a unique fingerprint of the molecule that is an invaluable resource for identifying substances and their surrounding medium. But with low signal strength – typically 10–14 that of fluorescence signals – the devil is in the detection.

Enhancing Raman signals

Fortunately the same light that activates vibrational modes in the molecule can trigger so-called lightning rod effects and resonant electron oscillations – plasmons – in metal nanostructures nearby. These enhance the local electromagnetic field by several orders of magnitude, while charge-transfer processes also contribute to the enhancements. The result is a detectable signal as first demonstrated by David Jeanmaire and Richard van Duyne.

Describing their results in 1977 they pointed out: "The ability to obtain resonance Raman spectra with good signal-to-noise with laser powers less than 1.0 mW, reported here for the first time, opens up possibilities of surface Raman studies with relatively inexpensive laser systems." These possibilities were neatly demonstrated 20 years later by Katrin Kneipp and colleagues at MIT and the Technical Institute in Berlin, who used surface-enhanced Raman spectroscopy (SERS) to detect a single molecule.

Since then, efforts to develop user-friendly SERS techniques have attracted intense research interest. One difficulty in applying the technique is that when Raman probes are attached to metal nanoparticles, the nanoparticles are prone to aggregating and the probes desorb from them. Ming Li and colleagues at West Virginia University and Ocean Nano Tech in the US and INRS-Énergie in Canada combatted these issues by sandwiching a Raman probe – in this case malachite green isothiocyanate (MGITC) – between gold nanoparticles and a silica coating. Their objective, as they reported in Nanotechnology, was "to develop and optimize a highly sensitive Raman probe that features high sensitivity, good water solubility and stability, low-background fluorescence, and an absence of photobleaching for biological applications." Experiments and supporting simulations demonstrated the success of their approach.

Combining approaches

Instead of avoiding nanoparticle aggregation Monica Potara and colleagues at Babes-Bolyai University in Romania embraced the tendency to aggregate by designing films of silver nanoparticle clusters coated in the biopolymer chitosan. The gaps in the nanoparticle arrangements generate hotspots for extremely high electromagnetic field enhancements, while the chitosan coating allows the analyte molecules to diffuse into the film and immobilize on the surface of the silver.

Yet with the inherent variations in geometry and mixing time, reproducibility in SERS is still an issue. In their review, Chao Wang and Chenxu Yu from Iowa State University describe the possible solutions offered by integrating SERS with microfluidics, a tool that has already proved useful for precise manipulation of small volumes of liquids. Despite the potential applications of combining the techniques in environmental surveillance and assay detection for DNA/RNA and living cells, the approach is not without challenges.

Microfluidics systems commonly use the polymer polydimethylsiloxane (PDMS), which is Raman active itself and could interfere with the measured signals. Reproducible intermixing control for colloids and analytes can also be tricky. Wang and Yu describe some of the ways around these and other aspects of SERS-microfluidics, as well as the potential of using continuous flow systems. These could further improve the reproducibility, and raise the demand for some sort of optical, electrical or mechanical trapping mechanism to hold samples in the detection area long enough for a signal to be collected.

Transparency in plasmonics

Plasmonics is an area of nanophotonics that has reached a level of maturity over the past decade. As well as biosensing there are many promising potential applications in communications and photonic circuits as the papers in our ‘Plasmonics in optoelectronics’ special issue highlight. Yet surprising characteristics in the behaviour of light in plasmonic systems continue to emerge.

"'What are light quanta?' Of course today every rascal thinks he knows the answer, but he is deluding himself," Albert Einstein

By combining plasmonic nanoparticles with photonic crystals, Ali Hatef and colleagues at the University of Western Ontario in Canada and the University of Alabama in Huntsville in the US have found that the absorption of the system can drop dramatically within a given frequency range in a state described as "plasmonic electromagnetically induced-transparency". Zhihua Zhu and colleagues from Tianjin University in China, King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, Osaka University in Japan and Oklahoma State University in the US, have also achieved "plasmon-induced-transparency", an analogue of electromagnetically induced transparency, by coupling dark-mode and light-mode resonators in a metamaterial. What’s more they achieved the effect over a broadband spectrum extending across the increasingly important terahertz frequency region of the electromagnetic spectrum.

In 2015 the world celebrates the International Year of Light with a diverse range of events that emphasizes the numerous guises of light that continue to attract interest. Nanotechnology is hosting a focus collection on nanophotonics that will be accepting and publishing papers throughout the year. The collection assembles results of research that continue to push the limits of our understanding and ability to control light interactions with nanomaterials, a topic that seems infinite in its scope for further investigation. As Albert Einstein is said to have commented in 1954, "All the fifty years of conscious brooding have brought me no closer to answer the question, 'What are light quanta?' Of course today every rascal thinks he knows the answer, but he is deluding himself."