Nanoplasmonic sensing for nanomaterials science and catalysis

Nanomaterials exhibit a number of phenomena that distinguish them from their bulk counterparts. For example, the reactivity of nanoparticles when interacting with molecules in the liquid or gas phase is size- and shape-dependent. This is widely exploited in catalysis, which is crucial in many areas of the chemical and energy industries. Developing efficient experimental strategies for in situ studies of catalytic nanomaterials under real application conditions is a major challenge.

Nanoparticles can be individually studied in situ by both atomic force- and scanning tunneling microscopy, yielding valuable information about adsorbed species and changes in catalyst structure. However, this is typically only possible in the absence of harsh chemicals typical of industrial catalysis: either under high vacuum, or in close-to-ambient conditions with mild chemistry. This is also true for electron-microscopy-based approaches, which are also known to potentially disturb or alter the sample during imaging due to ‘beam effects.’ As a completely new approach to circumvent these limitations, we have developed an optical spectroscopy platform—indirect nanoplasmonic sensing^{1, 2} (INPS)—that has the potential to contribute significantly towards closing the gaps between fundamental research and applied catalysis and to become a versatile spectroscopic tool for nanomaterials science in general.

When light interacts with a metallic nanoparticle, a resonant collective oscillation in the electronic system (called localized surface plasmon resonance; LSPR) is initiated.³ LSPR gives rise to a strong interaction of light with the nanoparticle and creates strongly-enhanced local electromagnetic fields. Consequently, the resonance wavelength is very sensitive to changes of the particle itself and to events occurring in the particle's immediate environment, such as electronic or refractive index changes. Optical nanosensors that use LSPR have been researched extensively since the 1990s, with a particular focus on label-free bioanalysis.⁴ Traditional LSPR sensing thus typically relies on measurements of changes in the LSPR spectral peak position or intensity caused by biomolecular adsorption on plasmonic gold nanoparticles, which occurs at room temperature in mild and often liquid environments.

Figure 1. Schematic of our indirect nanoplasmonic sensing (INPS) platform. Gold (Au) nanodisks are used as plasmonic nanosensor entities (with a thin dielectric spacer layer for protection) offering tailored surface chemistry, and a nanomaterial of interest is deposited onto the sensor. The inset to the right shows a scanning electron micrograph of an INPS sensor with the randomly arranged Au nanodisks clearly visible. Reprinted with permission from Insplorion AB.

Analogously, our INPS technology uses LSPR excitation, either individually⁵ or in arrays,¹ of plasmonic gold nanoparticles at visible light frequencies. As shown in Figure 1, it is possible to use our method to study processes and changes on or in adjacent nanomaterials. The INPS measurement principle relies on a dielectric spacer layer, a few nanometers to several tens of nanometers thick, physically separating the nanoplasmonic sensors from the nanomaterial under study. This provides a key step forward, facilitating nanoplasmonic sensing in highly demanding environments and allowing for a multitude of analyte materials. The spacer layer serves several key functions: the protection of the gold nanosensors from the environment and from structural reshaping at high temperature; the provision of a tailored surface chemistry of the support material for the nanomaterial or catalyst to be studied, and the prevention of direct interaction between the nanomaterial and gold sensors (which could lead to alloy formation). In principle, any other dielectric material that can be deposited as a thin flat or porous film, such as titanium oxide, silicon nitride, aluminum oxide, and even polymers, can be used as the spacer layer.

We successfully applied our INPS sensing platform to investigate structural changes of nanomaterials in catalyst sintering processes.⁶ We also scrutinized size effects in metal hydride nanoparticles less than 10nm in diameter,^{1, 5,7} and performed in situ measurement of changes in adsorbate surface coverage on heterogeneous catalysts at atmospheric pressure.² By exploiting the intrinsic temperature sensitivity of LSPR optical nanocalorimetry, we were able to measure local temperature changes at the nanometer scale, and related this to the activity of a catalyst.¹ Recently, we applied INPS to study dye molecule adsorption in thick mesoporous titanium dioxide (TiO₂) photoanodes for dye-sensitized solar cells by placing the INPS sensor at the internal interface between the support and the TiO₂.⁸ This approach provides a unique opportunity to selectively follow dye adsorption locally and inside the material, and it inspires a generic new type of nanoplasmonic hidden interface spectroscopy.⁸

So far, our results indicate that we can probe nanoparticles in the sub-10nm range at temperatures above 600°C in both reducing and oxidizing atmospheres, resolving both chemical and structural changes of ensembles of particles and at the single particle level. By placing INPS sensors at hidden material interfaces, optical spectroscopy inside a material has been shown feasible. The INPS technology is now commercially available from Insplorion AB.⁹ Our next steps will focus on the further development of INPS for single-particle in situ spectroscopy for catalysis, adapting the technology platform for operation in harsher industrial environments.