Department of Chemistry, ‡Department of Physics and Astronomy, §Department of Electrical and Computer Engineering, and ∥Laboratory for Nanophotonics and the Smalley-Curl Institute, Rice University , 6100 Main Street, Houston, Texas 77005, United States.
Nano Lett. 2017 Sep 13;17(9):5768-5774. doi: 10.1021/acs.nanolett.7b02736. Epub 2017 Aug 16.
Surface-enhanced infrared absorption (SEIRA) spectroscopy has outstanding potential in chemical detection as a complement to surface-enhanced Raman spectroscopy (SERS), yet it has historically lagged well behind SERS in detection sensitivity. Here we report a new ultrasensitive infrared antenna designed to bring SEIRA spectroscopy into the few-molecule detection range. Our antenna consists of a bowtie-shaped Au structure with a sub-3 nm gap, positioned to create a cavity above a reflective substrate. This three-dimensional geometry tightly confines incident mid-infrared radiation into its ultrasmall junction, yielding a hot spot with a theoretical SEIRA enhancement factor of more than 10, which can be designed to span the range of frequencies useful for SEIRA. We quantitatively evaluated the IR detection limit of this antenna design using mixed monolayers of 4-nitrothiophenol (4-NTP) and 4-methoxythiolphenol (4-MTP). The optimized antenna structure allows the detection of as few as ∼500 molecules of 4-NTP and ∼600 molecules of 4-MTP with a standard commercial FTIR spectrometer. This strategy offers a new platform for analyzing the IR vibrations of minute quantities of molecules and lends insight into the ultimate limit of single-molecule SEIRA detection.
表面增强红外吸收（SEIRA）光谱学作为表面增强拉曼光谱（SERS）的补充，在化学检测方面具有巨大的潜力，但在检测灵敏度方面，它的历史发展一直远远落后于 SERS。在这里，我们报告了一种新的超灵敏红外天线，旨在将 SEIRA 光谱学带入单分子检测范围。我们的天线由一个带有亚 3nm 间隙的蝴蝶结形 Au 结构组成，该结构位于反射衬底上方的空腔中。这种三维几何形状将入射的中红外辐射紧密限制在其超小的结中，产生一个热点，其 SEIRA 增强因子理论上超过 10，可以设计为跨越 SEIRA 有用的频率范围。我们使用 4-硝基硫酚（4-NTP）和 4-甲氧基硫酚（4-MTP）的混合单层来定量评估这种天线设计的红外检测极限。优化的天线结构允许使用标准的商业傅里叶变换红外光谱仪检测到多达约 500 个 4-NTP 分子和约 600 个 4-MTP 分子。该策略为分析微量分子的红外振动提供了一个新的平台，并深入了解了单分子 SEIRA 检测的极限。
