Such sensors contain a prism with one face covered in a thin film of gold (Au), whereby a laser light shines through the prism and thereafter mostly reflects off the Au into a detector. However, if the light reflects the Au layer at a particular angle, then some of it couples with electrons in the metal to produce electromagnetic waves dubbed surface plasmon polaritons. Higher coupling leads to less light being reflected towards the detector.
For instance when a liquid sample flows across a Au film, then it changes the refractive index in that region and slightly alters the angle at which the light arrives at the metal. Such minor alteration ultimately hampers the formation of polaritons - meaning that more of the light is reflected toward the detector. Thereafter varying the angle of the laser beam and monitoring the intensity of the reflected light reveals the composition of samples flowing over the metal surface.
Other groups have shown that AuNPs can enhance the sensor's responsiveness, whereby incoming light sparks localised plasmon resonances (LPR) in around the nanoparticles that couple to the sensor surface. Such processing causes larger changes in the intensity of the reflected light - making the device more sensitive to the light's angle of arrival, and, therefore able to detect lower concentrations of the chemicals being tested.
Yu's team calculated the optical responses of four different AuNPs - ranging in diameter from 40-80 nm - by determining that they would be most effective when held ca. 5 nm above the Au surface. Subsequently the researchers mounted the different nanoparticles onto Au films employing a sulfur-containing molecule called dithiothreitol, which provided the optimum 5 nm gap.
The calculations had suggested that the electric field of the surface plasmon polaritons would be hundreds of times greater when 40 nm particles were added to the surface. As continued by Yu - "the stronger the electric field, the more sensitive the sensors,". Further testing employing different concentrations of glycerin and formamide solutions reaffirmed that the 40 nm particles did in fact offer the greatest increase in sensitivity. Again, as stated by Yu - "the detection limit is at least three orders of magnitude higher than current commercial SPR sensors."
Moving forward - Yu hopes to apply the discovery onto ultra-sensitive sensors, which can detect traces of cancer bio-markers. Original article available here
Interestingly DCN Corp finds the above research extremely interesting, and wishes to find-out if the same thin film porosity/fabrication can be achieved from the company's homogeneous dip coating displacement protocol? If so, and you or your colleagues are interested in making the above a reality - please ensure to contact the company as soon as practicably possible.
 Zeng, S., Yu, X., Law, W.-C., Zhang, Y., Hu, R., Dinh W.-C., Ho, H.-P. and Yong, K.-T. - Size dependence of Au NP - enhanced surface plasmon resonance based on differential phase measurement. Sensors and Actuators B: Chemical 176, 1128–1133, 2013. Journal citation available here