Many molecules, especially biomolecules, vibrate when excited with infrared light. The way the molecules vibrate, or their vibrational modes, shows which types of bonds are present in a sample. Being able to probe these modes can therefore be used to help elucidate the sample’s molecular structure. As well as helping researchers better understand basic biological functions, such information could also allow them to monitor how diseases progress, and so develop improved treatments.

Plasmonic resonances

In recent times, a new branch of photonics, known as plasmonics, has been a great boon in this field of research with the discovery that specifically engineered metallic nanoparticles can be made to resonate in the mid-infrared part of the electromagnetic spectrum. The nanoparticles interact strongly with light via localized surface plasmons (collective oscillations of electrons on a metal's surface) and so act as efficient optical nanoantennas that capture more light. Such plasmonic resonances in the infrared can be used as probes that can be tuned to and away from various vibrational modes in a molecule.

The main problem in performing IR spectroscopy in aqueous environments, however, is that water molecules also absorb strongly in this part of the electromagnetic spectrum. This means that water produces a background signal that overwhelms and obscures the signal produced from the (often) tiny biological sample being studied. Ideally, researchers would like to be able to very sensitively probe extremely thin layers (between 5 and 100 nm across) of a sample and reject the signal from anywhere else, explains team leader Hatice Altug.

Problem solved

"Our new device allows us to accomplish exactly this," she told "It is made of a small array of specially designed nanoscale gold antenna structures patterned on the topside of an optically transparent calcium fluoride chip. The sample being studied is placed in a fluidic chamber on top of the antenna arrays. When we shine light on the pattern through the backside of the chip, the antennas capture, store and concentrate it in the form of a plasmonic resonance at the surface."

"The antennas then redirect the light very efficiently back in the same direction to the detector. In this way, light does not need to pass through the entire sample and only interacts (very strongly though) with the small group of biological molecules we are interested in."

Only a thin layer at the surface of the antennas interacts with the plasmonic resonance and the information contained within the molecules in this layer is "imprinted" on the backscattered light signal. The technique is thus very sensitive while rejecting the background of the surrounding water medium.

Exquisite sensitivity

Using their technique, the researchers say that they were able to monitor a series of protein-protein binding interactions involving molecules that participate in the body’s immune response. "We also studied the interactions between molecules and nanoparticles of different sizes," explained team member Ronen Adato. "Here, we were able to track how various molecular groups in the sample moved."

In both of these studies, but especially the second, the exquisite sensitivity of our IR-based approach is a key advantage over other label-free methods that often measure a secondary, nonspecific bulk property such as molecular mass, he added. The technique is also better than traditional IR spectroscopy techniques that require cumbersome macroscopic optics and sample volumes that are several thousands of times larger.

The EPFL-Boston researchers, who report their work in Nature Communications, say that they now hope to apply their technique to study biologically important and disease relevant samples. "In particular, we will be looking at events that do not involve molecular binding – something that has proven to be difficult to measure with existing techniques," said Altug.

"It will also be interesting to take advantage of the chip-based, compact nature of our platform and develop new functionalities. For example, combining the device with advanced microfluidic systems might help dramatically improve sample throughput and the ability to better manipulate samples."

Further reading

Nanoplasmonic sensor detects viruses (Nov 2010)
Understanding gold and silver nanoshells: plasmonics analysis using finite element method and Mie theory (Sep 2010)
Plasmonics ramp up nanocrystal photosensor performance (May 2013)
Enhancing IR spectroscopy (Sep 2012)
Seeing biomarkers with the naked eye (Aug 2011)