SPP sensing. Nanohole films can be used in two different configurations to sense molecules in a water solution. In the reflection mode (top), light is directed at the sample from the water side. In the transmission mode (bottom), light is directed at the sample from the back, leading to different SPP properties. The SPP field intensity is represented by the color plot. The optical fields on the top and bottom are calculated for different resonance frequencies. Credit: 2012 IEEE

The
detection of small quantities of molecules is important for a myriad of
applications, ranging from gas sensing to biomedical diagnostics. The
majority of these applications require the sensors to be cheap and
disposable, yet sensitive enough to detect molecules down to the
single-molecule level. Ping Bai and co-workers at the A*STAR Institute
of High Performance Computing and the Institute of Materials Research
and Engineering have now studied the properties of thin metallic films
with holes in them that are particularly promising for molecular
sensing.

Metallic
thin films with nanometer-sized holes in them are known to transmit
light of particular wavelengths very efficiently. The efficiency arises
from surface plasmon polaritons (SPPs)—the collective movements of
electrons on the metal surface—which are able to focus light into tiny
spots much smaller than the wavelength of light used (see image).

These
SPPs can be used to detect the molecules through the fluorescence of
tracer molecules attached to them. This fluorescence is also strongly
enhanced by the SPP and can easily be detected by a microscope even for
small quantities of molecules.

“The whole setup is ultra-compact to support a point-of-care sensing system,” explains Bai.

Bai
and his colleagues studied two sensing arrangements. In the first
arrangement, light is directed at a film with nanoholes at an oblique
angle from the same side as the sample. In the second arrangement, the
film is illuminated from the back so that light is travelling through
the holes first. The researchers found that each scheme has its own
advantages.

In
the ‘reflection’ scheme, the SPP effect is stronger as the light is
directly aimed at the sample and does not have to cross the metal film.
However, a thicker film is needed so that the light does not pass
through. In the ‘transmission’ scheme, the intensity of the light
emitted by the molecules is weaker, but the advantage there is that
filters and other sensors can possibly be included with the metal film,
and the film thickness can be much thinner.

“There
is therefore no clear advantage for either sensing modes of such
films,” says Bai. “One thing that is clear from the study, however, is
the clear benefits of using metal films with nanoholes as a molecular
sensing platform,” says Bai.

“This
is merely a snapshot of our whole project. Ultimately, our sensing
technology will be utilized in hospitals and test centers, for example,
in prostate cancer screening, or even used at home just like glucose
test kits,” adds Bai.

Reflection and Transmission Modes in Nanohole-Array-Based Plasmonic Sensors

Source:  A*STAR