Institut des
NanoSciences de Paris
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Capteurs plasmoniques

Yves Borensztein

Development of ultra-sensitive plasmonic sensors

 

Nanoparticles of different metals (gold, silver, palladium, etc.) have particular optical properties, related to the so-called localized surface plasmon resonances, which are collective oscillations of the conduction electrons confined within the particles, as illustrated in Fig.1. For example, plasmon resonance for Au is in the visible optical range, and gives a red or purple color to Au particles instead of the usual yellow color (Fig.2).

 

 

Fig.1 Localized surface plasmon resonance of a metal nanoparticle : collective oscillation of conductions electrons induced by the electric field of light

Fig.2 Solution of gold nanoparticles in suspension

 

 

This resonance is very sensitive to the immediate environment of the particles and can be strongly affected when the particles interact with molecules or ions. Thanks to this very high sensitivity, gas or biological sensors based on nanoparticles of gold, gold-based alloys or other metals [1] are being developed (see Fig. 3). We have developed an original optical technique, the reflectance anisotropy spectroscopy (RAS), which allows us to achieve a higher sensitivity than conventional plasmonic sensors, making it possible to demonstrate the adsorption of very small quantities of molecules. We used it to study the reaction of hydrogen with gold nanoparticles [2] or to make a prototype hydrogen sensor [3]. For this purpose, specific samples with optical dichroism are prepared by evaporation of gold and / or palladium on a glass substrate (fig 4).

 

 

Fig. 3 Shift of the plasmon resonance induced by the adsorption of molecules

Fig. 4 Scanning electron micrograph of gold nanoparticles

 

 

 

An illustration of the sensing sensitivity down to a few ppm of H2 is shown in Fig. 5. The top panel shows the modification of the RAS signal measured in real time, when switching the gas from pure Ar to H2 diluted in Ar and back, for decreasing concentration of H2, from pure (100%) to highly diluted (0.0002%, i.e 2 part per million). On the left part, from 100% to 4%, the change of signal is large, and corresponds to the formation of the dense ’beta’ phase of Pd hydride. On the right part, at and below 0.25%, the change of signal is much smaller, and corresponds to the formation of the dilute ’alpha’ phase of Pd hydride. At intermediate concentration of H2 (1%), there is coexistence of the two phases. The zoom shown in the bottom panel shows that, except for the intermediate case, the kinetics for the detection of H2 is fast, even at very low concentration, showing that this method provides a very competitive H2 sensor. More details can be found in 2018 PhD defense by William Watkins

 

Fig.5 Effect of H2 diluted in Ar on the RAS signal measured on an anisotropic plasmonic Pd sample. Up : change of the RAS signal measured as a function of switching between pure H2 in Ar and pure Ar, for decreasing concentrations of H2, from pure H2 (100%) to 2 ppm. Middle : zoom on three cycles (16%, 1% and 0.015%), the first and last ones showing a fast saturation Bottom : scheme of PdHx for dense, dilute and intermediate phases

 

 

 

 

References