A special interaction between light and matter, plasmons, will increase the sensitivity and ease in using biosensors for detecting disease biomarkers.

Pietro Giuseppe Gucciardi, a physicist at the Institute for Chemical-Physical ProcessesUntil now, few biosensors have had the required sensitivity to detect single molecules. A novel approach for improved biosensor sensitivity has opened new avenues for developing new kinds of biosensors. It relies on light irradiation of tiny gold nanorods, dubbed nanoantennas, leading to waves of electrons, known as plasmons, that strongly amplify the light. Specifically, an EU funded research project, called NANOANTENNA, completed in March 2013, focused on how this property can enhance the sensitivity of biosensors. Pietro Giuseppe Gucciardi, a physicist at the Institute for Chemical-Physical Processes, affiliated with the Italian National Research Council CNR, in Messina, Sicily, tells about the kind of improvements this new technology could bring to biosensors.

Read: Designing Ultra-Sensitive Biosensors for Early Personalized Diagnostics

What has been your role in developing this new type of biosensors?
Our goal as physicists was to investigate the amplification properties of nanoantennas. And we were also interested in providing continuous feedback to those who produce them so that they can improve and increase the signal amplification.

How do the nanoparticles enhance the sensitivity of the bioprobe?
When you shine light on the nanoparticles, the resulting plasmonic waves go back and forth over the nanoparticle. They, thus, amplify the magnetic field of the nanoantenna in so-called hotspots, which typically are located at the edges. Therefore, the molecules located near these hotspots are subjected to an enhanced electromagnetic field that causes a much stronger vibrational infrared signal. This, in turn, can be detected with a technique called Raman spectroscopy.

Do the molecules to be detected have to be trapped by these nanorods?
Yes, we make the nanoantennas functional by attaching bioreceptors to their surfaces. We use fragments of DNA, so-called haptomers, which are engineered to recognize and trap specific proteins, such as manganese superoxide dismutase (MnSOD), a biomarker for several types of cancers. The nanoantennas are incubated with a solution containing biomarker molecules that become attached to the bioreceptors on the nanoantennas. When they are illuminated with light, we get the vibrational fingerprints of both the bioreceptors and the attached biomarkers.

As a proof of concept, we incubated the functionalized nanoantennas with MnSOD, and with BSA (bovine serum albumin, which does not attach to the bioreceptors). We found that we could only detect vibrational fingerprints of MnSOD.

What is the advantage of using such approach compared to other testing methods?
Typically, kits that are now available are based on fluorescence and you have a sensitivity going to extremely small scale, down to one nanomolar, or even one picomolar. But the real advantage is that with the nanoantennas we can reach a sensitivity of a femtomolar. And what is fascinating is that we even have proof of concept that you can detect single molecules by their vibrational fingerprint. The other advantage of this approach is that the detection method, Raman spectroscopy, does not require staining of the target molecules, it is label free.

Are biosensor kits using nanoantennas now available for clinical tests?
No, our project finished in March 2013. And our aim was to deliver a proof of concept. We will first have to implement the technology in a microfluidic circuit, a biochip, in which we can test biofluids, such as saliva or blood. This biochip should be combined with a spectrometer that should be portable. For the moment we have a table-top spectrometer linked to a computer. We should aim at developing a suitcase spectrometer based on optical fibers.