Liquid-Metal-Based Nanophotonic Surface-Enhanced Infrared Absorption Sensors

Surface-enhanced infrared absorption (SEIRA) is a powerful technique for optically detecting and sensing substances of interest with high sensitivity and specificity. To improve SEIRA sensing performance, various types of resonant nanophotonic structures have been employed to enhance light-matter interactions. Such nanophotonic structures are usually designed to channel the energy of the incident light into deep-subwavelength volumes, often referred to as hot spots, to achieve exceedingly high field enhancement. However, the drawback of this commonly used strategy is that the delivery of target analytes into the hot spots becomes increasingly difficult and inefficient as the hot spots are designed to be ever smaller, especially when the hot spot dimensions are comparable to typical sizes of molecules. This drawback associated with deep-subwavelength hot spots has become a key factor that limits the performance of various types of SEIRA sensors in practical application settings. An effective approach to addressing this issue is to deliver the target analytes before forming hot spot structures. For example, metallic nanoparticles coated with analyte thin films can form super-crystals with nanometric gaps which function as the hot spots [1, 2]. Another demonstration of SEIRA sensors based on graphene acoustic plasmon resonators realized effective delivery of analytes into nanometric gaps by first spin-coating the thin analyte film on gold nanoribbons and subsequently transferring graphene onto the analyte film to form the complete SEIRA sensor structure [3]. Although these SEIRA sensor designs and device preparation methods effectively addressed the analyte delivery issue, they involve relatively complex procedures and hence may not be suitable for point-of-care applications.<br/><br/>Here, we demonstrate a high-performance SEIRA sensor based on nano-patch antennas which employ liquid metal (i.e., liquid gallium) as the ground plane. Target analytes are first introduced onto the surface of lithography-defined gold nano-strips on the sensor chip, and then the sensor chip is simply brought into contact with a droplet of liquid gallium at room temperature to form the complete nano-patch antenna structures, so that the target analytes are sandwiched between the liquid gallium and the gold nano-strips and experience highly enhanced electromagnetic field. Our liquid-gallium-based SEIRA sensors demonstrate state-of-the-art sensing performance for nanometric analyte thin films, such as monolayer 1-octadecanethiol (ODT) and thin PMMA films. The operation procedure is simple and suitable for point-of-care applications. Our experimental results also suggest that liquid gallium does not damage or affect the properties of the target analytes. Moreover, the liquid gallium can be conveniently and completely removed from the sensor chip surface, so that the sensors can be reused.<br/><br/><b>Reference</b><br/>[1] K. Bian, H. Schunk, D. Ye, A. Hwang, T. S. Luk, R. Li, Z. Wang, H. Fan, <i>Nature Communications</i> <b>2018</b>, 9, 2365.<br/>[2] N. S. Mueller, E. Pfitzner, Y. Okamura, G. Gordeev, P. Kusch, H. Lange, J. Heberle, F. Schulz, S. Reich, <i>ACS Nano</i> <b>2021</b>, 15, 5523.<br/>[3] I. H. Lee, D. Yoo, P. Avouris, T. Low, S. H. Oh, <i>Nature Nanotechnology</i> <b>2019</b>, 14, 313.