Department of Applied Physical Science, University of North Carolina at Chapel Hill, Caudill Labs, Chapel Hill, North Carolina, 27514, U.S.A.
Department of Chemistry, University of North Carolina at Chapel Hill, Caudill Labs, Chapel Hill, North Carolina, 27514, U.S.A.
Chemphyschem. 2020 Feb 4;21(3):188-193. doi: 10.1002/cphc.201901002. Epub 2020 Jan 8.
Tip-enhanced Raman spectroscopy (TERS) is a promising technique for structural studies of biological systems and biomolecules, owing to its ability to provide a chemical fingerprint with sub-diffraction-limit spatial resolution. This application of TERS has thus far been limited, due to difficulties in generating high field enhancements while maintaining biocompatibility. The high sensitivity achievable through TERS arises from the excitation of a localized surface plasmon resonance in a noble metal atomic force microscope (AFM) tip, which in combination with a metallic surface can produce huge enhancements in the local optical field. However, metals have poor biocompatibility, potentially introducing difficulties in characterizing native structure and conformation in biomolecules, whereas biocompatible surfaces have weak optical field enhancements. Herein, a novel, biocompatible, highly enhancing surface is designed and fabricated based on few-monolayer mica flakes, mechanically exfoliated on a metal surface. These surfaces allow the formation of coupled plasmon enhancements for TERS imaging, while maintaining the biocompatibility and atomic flatness of the mica surface for high resolution AFM. The capability of these substrates for TERS is confirmed numerically and experimentally. We demonstrate up to five orders of magnitude improvement in TERS signals over conventional mica surfaces, expanding the sensitivity of TERS to a wide range of non-resonant biomolecules with weak Raman cross-sections. The increase in sensitivity obtained through this approach also enables the collection of nanoscale spectra with short integration times, improving hyperspectral mapping for these applications. These mica/metal surfaces therefore have the potential to revolutionize spectromicroscopy of complex, heterogeneous biological systems such as DNA and protein complexes.
尖端增强拉曼光谱（TERS）是一种很有前途的技术，可用于生物系统和生物分子的结构研究，因为它能够提供具有亚衍射极限空间分辨率的化学指纹。由于在保持生物相容性的同时难以产生高场增强，因此 TERS 的这种应用迄今为止受到限制。TERS 能够实现高灵敏度，这是因为在贵金属原子力显微镜（AFM）尖端中激发局部表面等离激元共振，该共振与金属表面结合可以在局部光场中产生巨大的增强。然而，金属的生物相容性差，可能会在生物分子中对天然结构和构象的表征带来困难，而生物相容性表面的光场增强较弱。在此，基于机械剥离在金属表面上的几单层云母薄片，设计并制造了一种新颖的、生物兼容的、高度增强的表面。这些表面允许形成耦合等离子体增强，用于 TERS 成像，同时保持云母表面的生物相容性和原子平整度，以实现高分辨率 AFM。通过数值和实验确认了这些基底用于 TERS 的能力。我们证明，与传统云母表面相比，TERS 信号提高了五个数量级，从而提高了 TERS 对具有弱拉曼截面的广泛非共振生物分子的灵敏度。通过这种方法获得的灵敏度提高还能够实现具有短积分时间的纳米级光谱的收集，从而改善了这些应用的高光谱映射。因此，云母/金属表面有可能彻底改变复杂、不均匀的生物系统（如 DNA 和蛋白质复合物）的光谱显微镜检查。
