Liquid‐Metal‐Based Nanophotonic Structures for High‐Performance SEIRA Sensing

Abstract

applications. As molecular vibrational absorption is generally proportional to the intensity of electric field experienced by the molecules, [2] surface-enhanced infrared absorption (SEIRA) sensors based on a variety of resonant nanophotonic structures have been demonstrated to significantly enhance molecular vibrational absorption and consequently achieve high sensitivity performance. [3] Resonant nanophotonic structures excited by incident light can confine highly enhanced electric field in deepsubwavelength regions, known as "hot spots," in which the interactions between light and analyte molecules can be drastically enhanced, leading to significant improvement of the sensing performance. Designing nanophotonic structures with smaller hot spots is a widely utilized strategy to increase field enhancement and improve sensor performance. A variety of nanophotonic structures with nanometric gaps have been demonstrated for sensing applications, such as dimer antennas, [4,5] split-ring resonators, [6] coaxial disk resonators, [7,8] and nanopatch antennas. [9][10][11] However, with the gap size decreasing down to the nanometric scale, it becomes increasingly difficult to deliver analyte molecules into these gaps (i.e., the hot spots), especially when the gap size is comparable to typical sizes of molecules. This issue fundamentally limits the further performance improvement of nanophotonic sensors. An effective approach to addressing this issue is to deliver the analytes before forming the nanometric gaps (or other types of hot spot structures). For example, metallic nanoparticles coated with analyte thin films can form supercrystals with nanometric separation gaps. [12,13] In a recent study, such supercrystals of gold nanoparticles coated with thiolated poly styrene molecules were demonstrated to function as SEIRA sensors for sensing the polystyrene molecules with high performance. [13] Another previous demonstration of SEIRA sensors based on graphene acoustic plasmon resonators realized effective delivery of analytes into nanometric gaps by first spin-coating a thin analyte film on gold nanoribbons, and subsequently transferring graphene onto the analyte film, which led to the successful detection of SEIRA signals from sub-nanometerthick analyte films. [14] Nevertheless, assembling supercrystals of metal nanoparticles or transferring graphene to form the complete sensor structures is not a simple and straightforward process, and hence may not be suitable for point-of-care applications.Surface-enhanced infrared absorption (SEIRA) spectroscopy can provide label-free, nondestructive detection and identification of analytes with high sensitivity and specificity, and therefore has been widely used for various sensing applications. SEIRA sensors usually employ resonant nanophotonic structures, which can substantially enhance the electric field and hence light-matter interactions by orders of magnitude in certain nanoscale hot spots of the devices. However, as ever, smaller hot spots are employed to further enhance the fi...

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