The monitoring of volatile organic compounds (VOCs) has become an important issue, particularly in the control of air quality, industrial processes, public safety, healthcare and so on. Electronic noses are promising candidates for analysis and monitoring of these ubiquitous compounds, some of which are harmful to the environment and sometimes even dangerous to human health. Surface Plasmon Resonance (SPR) is an optical technique widely used in the development of biosensors for the detection of chemical and biological compounds. The use of gas-phase SPR imaging is innovative and interesting because of its multi-parameter detection capabilities, particularly suitable for the detection of VOCs.

In an effort to improve the performance of the optoelectronic nose, researchers at l'IRIG propose to study fundamental aspects related to optics such as wavelength, temperature and thickness of metallic film deposited on the SPR prism. To this end, they studied the influence of the wavelength of the light source (usually a LED) on the sensitivity of the system ^[1]. A comprehensive theoretical study has been corroborated by a related experimental study. In the wavelength range (530 nm to 740 nm), they showed an increase in sensitivity as the wavelength increased, with good consistency between the theoretical and experimental results. At the optimal wavelength of LEDs, the detection limits of their optoelectronic nose are in the order of ppb (parts per billion) for VOCs such as 1-butanol, limits that are similar to those of the human nose.

To go further in optical characterization, the researchers analyzed the effect of the different metallic layers deposited on the SPR prisms in terms of optical performance ^[2]. They developed a theoretical model considering each thickness of the metallic layers measured during the deposition (carried out at the CEA Advanced Technology Platform) as well as the surface roughness as analyzed by atomic force microscopy (carried out at the CEA Nano-Characterization Platform). In order to compare theory versus experiments, they established a new method for characterizing the optical performance of gas-phase SPR imaging prisms. This method makes it possible to obtain a generic measure of sensitivity, independent of the nature of the carrier gases. The experimental sensitivities thus obtained are quantitatively comparable to the theoretical results without adjustable parameters over a temperature range from 5 °C to 45 °C.

These results are of importance for the use of optoelectronic noses in real conditions and especially on-field in order to consider corrective measures during seasonal or regional temperature variations.


Schematic diagram of the optical device for SPR imaging.
Chromium (Cr) and titanium (Ti) have been tested as metals for the adhesion layer of gold deposited on the SPR prism. Surface roughness information, analyzed by atomic force microscopy, is used in the model to predict the reflectivity versus incidence angle curve. Optical index sensitivity is obtained as a function of temperature between 5 °C and 45 °C. A quantitative agreement between theory (red curves) and experiments (black dots) is observed.