"Our platform could be easily adapted for point-of-care diagnostics that can detect a broad range of viral pathogens in resource-limited clinical settings, in defence and homeland security applications as well as in civilian settings such as airports," team leader Hatice Altug told nanotechweb.org.

Recent years have seen a number of viral disease outbreaks, such as H1N1, H5N1 and SARS, raising fears that such viruses could rapidly spread and turn into a pandemic like the 1918 Spanish flu that killed more than 50 million people worldwide. Controlling future epidemics will require rapid and sensitive diagnostic techniques capable of detecting low concentrations of viruses in biological solutions.

Exploiting plasmonics
Plasmonics is a new branch of photonics that exploits surface plasmon polaritons, which arise from the interaction of light with the electrons that oscillate at a metal's surface. The new sensor exploits such resonances in plasmonic nanohole arrays. These are arrays of nanoapertures or holes just 200–350 nm across spaced 500–800 nm apart on nanolayers of noble metal films, such as those made of gold.

At certain wavelengths, the nanohole arrays can transmit light much more strongly than predicted by classical aperture theory. This phenomenon is called extraordinary optical transmission (EOT) and it occurs thanks to surface plasmon polariton resonances.

Measuring red-shifts
The resonance wavelength of the EOT depends on the dielectric constant of the medium surrounding the plasmon sensor. As pathogens bind to the sensor surface, the refractive index of the medium increases, so red-shifting the plasmonic resonance, explains Altug. This red-shift can then be measured to identify the virus.

Different viruses can be detected by attaching highly specific antiviral immunoglobulins to the sensor surface. Different immunoglobulins can capture different viruses from a sample solution (see figure).

The researchers have already used their device to detect pseudo viruses that look like highly lethal viruses, such as Ebola and smallpox.

Simpler and better
The technique has many advantages over conventional methods, such as PCR (polymerase chain reaction) and cell culturing, says team member John Connor of Boston University School of Medicine. Cell culturing is a highly specialized labour-intensive process and PCR, while robust and accurate, cannot detect new or highly divergent strains of viruses – unlike our new sensor.

And that's not all. "The detection platform is also compatible with physiological solutions (such as blood or serum) and is not sensitive to changes in the ionic strengths of these solutions. It can reliably detect viruses at medically relevant concentrations," added team member Ahmet Yanik.

Next on the list for the researchers, who are working with the United States Army Medical Research Institute for Infectious Diseases (USAMRIID), is to make a portable version of their platform using micofluidics.

The current work was published in Nano Letters.