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Anisotropic metal nanoparticles for ultrasensitive plasmon sensors : application for hydrogen sensing


Metallic nanoparticles (NPs), such as silver, gold or palladium, exhibit collective electron oscillations when illuminated with light. This property, known as localised surface plasmon resonances (LSPRs), bestows specific optical absorptions of the NPs and thus different colours from those of the bulk material (e.g. red or purple for gold). The spectral position of this absorption depends on the metal and on the immediate environment of the NPs. This property is at the root of plasmonic sensors : a spectral shift of the resonance, related to the quantity of adsorbed entities, is induced when the molecular species one wishes to analyse, build up on the surface or within the NPs. Yet, conventional plasmonic sensors have several limiting factors influencing their sensitivity and use : mainly the need of a monochromator inducing limitation in resolution, sensitivity to fluctuations (mechanical or lamp), as well as parasitic signal due to light room. INSP researchers have developed an original differential method, based on anisotropic samples, which makes it possible to overcome the need of a monochromator by working at a single wavelength, leading to an unequalled sensitivity.

For this, partially aligned gold or palladium NPs were elaborated using oblique incidence vacuum deposition on glass (Fig.1.a). The samples exhibit an optical dichroism, that is an optical transmission, T, dependent on the polarisation of light, due to different plasmon resonances according to the orientation (Fig.1.c). It is this difference in transmission which is directly measured, and is modified when molecules interact with the metal NPs. This is illustrated in Fig. 1.d, which shows the relative change in transmission, ΔT/T, when the sample is exposed to hydrogen. By recording the variation of the signal, ΔS, at a fixed wavelength, the sensitivity is increased by a factor of more than 100 compared to the conventional method of spectral displacement analysis of the resonances. This method has been applied to two studies.

The first is a fundamental study, related to the questions of heterogeneous catalysis : how does dihydrogen (H2) interact with gold NPs ? Unlike the case of solid gold, dihydrogen can indeed bind with gold NPs and dissociate into atoms. Though neither the quantity that was deposited nor the fate of the H atoms was known. Through analysis various parameters (size of the NPs, temperature, partial pressure of H2 ...) the research team showed that the molecules dissociate on the edges of the NPs, followed by the diffusion on the facets of H atoms where they form weak chemical bonds with the gold atoms (Fig.1.b). Moreover, an electronic charge transfer from the NP to the Au-H bond equal to -0.2 e was measured, which had not been determined so far.

JPEG Figure 1
a) : scanning electron microscopy image showing the preferentially aligned NPs of Au. (b) Diagram showing the adsorption of H2 molecules on the edges, their dissociation and the diffusion of H atoms on the facets.(c) Optical transmission for polarisations perpendicular and parallel to the NPs alignment axis (d) Transmission anisotropy spectrum, ΔT / T. The insert shows the effect of exposure to H2. Rather than measuring the very low spectral shift (well below 1 nm), it is the variation of the signal ΔS at a fixed wavelength that is measured with a much higher accuracy.

The second study focuses on the development of an ultrasensitive hydrogen sensor, which can detect as few as ppm (parts per million) amounts of H2 in an ambient gas, a proportion well below the explosive mixing threshold of 4% in air. For this, the sensor is formed of an anisotropic porous nanostructure composed of palladium (Pd) NPs prepared in a similar manner to the gold sample. Pd reacts with H2 to form a hydride in bulk, but the dense phase (β - phase) which leads to significant changes in LSPR of the Pd NPs is only obtained for proportions of H2 in the gas greater than 1%. For lower proportions, only the diluted phase (α - phase) is obtained, which leads to a small variation of the LSPR. Thanks to the differential method, the variation of the signal ΔT/T can be measured over the entire H2 concentration range, from a few ppm to 100% (Fig. 2), measurable quantities a 100 to 500 times lower what the best conventional plasmon sensors are capable of.

JPEG Figure 2
(a) ΔT/T signal recorded at a fixed wavelength, during alternating cycles of pure Ar and decreasing concentrations of H2 in Ar (grey areas). (b) Transition between α and β phases corresponding to the values x in PdHx measured for all the partial pressure range of H2.

After these preliminary feasibility studies that have demonstrated the outstanding sensitivity of the method, research will continue towards applications in aqueous media for the detection of heavy metals pollution and detection of bio-molecules.

« Mechanism of hydrogen adsorption on gold nanoparticles and charge transfer probed by anisotropic surface plasmon resonance », W. L. Watkins et Y. Borensztein ; Phys Chem Chem Phys 19, 27397 (2017)
« Ultrasensitive and fast single wavelength plasmonic hydrogen sensing with anisotropic nanostructured Pd films », W. L. Watkins et Y. Borensztein, submitted to Sensor and Actuators (2018)


William Watkins
Yves Borensztein