Super-Resolution SERS Imaging beyond the Single-Molecule Limit

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Letter pubs.acs.org/NanoLett

Super-Resolution SERS Imaging beyond the Single-Molecule Limit: An Isotope-Edited Approach Eric J. Titus, Maggie L. Weber, Sarah M. Stranahan, and Katherine A. Willets* Department of Chemistry and Biochemistry, The University of Texas at Austin, Welch Hall 2.204, 105 E. 24th St. STOP A5300, Austin, Texas 78712-1224, United States S Supporting Information *

ABSTRACT: Super-resolution imaging of single-molecule surface-enhanced Raman scattering (SM-SERS) reveals a spatial relationship between the SERS emission centroid and the corresponding intensity. Here, an isotope-edited bianalyte approach is used to confirm that shifts in the SERS emission centroid are directly linked to the changing position of the molecule on the nanoparticle surface. By working above the single-molecule limit and exploiting SERS intensity fluctuations, the SERS centroid positions of individual molecules are found to be spatially distinct. KEYWORDS: Super-resolution imaging, SERS, single molecule, bianalyte, isotope-edited ince the first reports in 1997, single-molecule surfaceenhanced Raman scattering (SM-SERS) has attracted many researchers in an attempt to understand the mechanism behind this phenomenon.1−6 SM-SERS is thought to originate from molecules located in the tightly confined junctions between adjacent nanoparticles, known as “hot spots,” in randomly aggregated noble metal nanoparticle clusters.7−10 However, observation of the hot spot is complicated by the diffraction limit of light, which prevents objects smaller than ∼λ/2 from being resolved in a far-field optical microscope.1 As a result, the true location of the hot spot, its size, the location of the molecule on the nanoparticle surface, and the nanoscale features of the nanoparticle that are required for SM-SERS enhancement are obscured by this optical diffraction limit. Super-resolution optical imaging has recently emerged as a powerful tool for studying SM-SERS hot spots in an effort to understand what hot spot properties enable the highest electromagnetic field enhancements.1,11−13 In super-resolution imaging, the point spread function (PSF) of a diffractionlimited spot is approximated as a two-dimensional Gaussian function in order to extract the spatial origin of the emitter.14,15 This technique is well-established in biological systems, where single fluorescent dyes are used to label and spatially locate spectroscopically silent species, like proteins or DNA, with