This study demonstrates that spectral fingerprint patterns for a weakly scattering biological sample can be obtained reproducibly and reliably with tip-enhanced Raman spectroscopy (TERS) that correspond well with the conventional confocal Raman spectra collected for the bulk substance. These provided the basis for obtaining TERS images of individual self-assembled peptide nanotapes using an automated, objective procedure that correlate with the simultaneously obtained scanning tunneling microscopy (STM) images. TERS and STM images (64x64 pixels, 3x3 μm2) of peptide nanotapes are presented that rely on marker bands in the Raman fingerprint region. Full spectroscopic information in every pixel was obtained allowing post-processing of data and identification of species of interest. Experimentally, the "gap-mode" TERS configuration was used with a solid metal (Ag) tip in feedback with a metal substrate (Au). Confocal Raman data of bulk nanotapes, TERS point measurements with longer acquisition time, atomic force microscopy images, and an infrared absorption spectrum of bulk nanotapes were recorded for comparison. It is shown that the unique combination of topographic and spectroscopic data that TERS imaging provides reveals differences between the STM and TERS images, e.g. nanotapes that are only weakly visible in the STM images, a coverage of the surface with an unknown substance, and the identification of a patch as a protein assembly that could not be unambiguously assigned based on the STM image alone.
[Show abstract][Hide abstract] ABSTRACT: Typically limited by the diffraction of light, most optical spectroscopy methods cannot provide the spatial resolution necessary to characterize specimens at the nanoscale. An emerging exception to this rule is tip-enhanced Raman spectroscopy (TERS), which overcomes the diffraction limit through electromagnetic field localization at the end of a sharp metallic tip. As demonstrated by the Zenobi group in this issue of ACS Nano, TER imaging is an analytical technique capable of providing high-resolution chemical maps of biological samples. In this Perspective, we highlight recent advances and future applications of TER imaging as a technique for interrogating biology at the nanoscale.
[Show abstract][Hide abstract] ABSTRACT: We propose a practical method to obtain near-field infrared absorption spectra in apertureless near-field scanning optical microscopy (aNSOM) through homodyne detection with a specific choice of reference phase. The underlying mechanism of the method is illustrated by theoretical and numeric models to show its ability to obtain absorptive rather than dispersive profiles in near-field infrared vibrational microscopy.The proposed near-field nanospectroscopic method is applied to obtain infrared spectra from regions of individual multiwall boron nitride nanotubes (BNNTs) in spatial regions smaller than the diffraction limit of the light source. The spectra suggest variations in inter-wall spacing within the individual tubes probed.
The Journal of Physical Chemistry A 03/2013; 117(16). DOI:10.1021/jp4008784 · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Finite-Difference Time-Domain (FDTD) calculations are used to characterize the electric field in the vicinity of a sharp silver or gold cone with an apex diameter of 10 nm. The simulations are utilized to predict the intensity and the distribution of the locally enhanced electric field in tip-enhanced Raman spectroscopy (TERS). A side-by-side comparison of the enhanced electric field induced by a radially and a linearly polarized light in both gap-mode and conventional TERS setup is performed. For this purpose, a radially polarized source is introduced and integrated into the FDTD modeling. Additionally, the optical effect of a thin protective layer of alumina on the enhancement of the electric field is investigated.
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