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.
Molecular sensors based on nanoholes in metallic films are shown to be
ideal for medical diagnosis
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 sensing1.
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.
The A*STAR-affiliated researchers
contributing to this research are from the Institute of High Performance Computing and
the Institute of Materials
Research and Engineering
References
- Wu, L., Bai, P., Zhou, X. & Li, E. P.
Reflection and transmission modes in nanohole-array-based plasmonic
sensors. IEEE Photonics Journal 4, 26–33 (2012). | article
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