Structure−Activity Relationships in Gold Nanoparticle Dimers and Trimers for Surface-Enhanced Raman Spectroscopy

Abstract

Understanding the detailed relationship between nanoparticle structure and activity remains a significant challenge for the field of surface-enhanced Raman spectroscopy. To this end, the structural and optical properties of individual plasmonic nanoantennas comprised of Au nanoparticle assemblies that are coated with organic reporter molecules and encapsulated by a SiO(2) shell have been determined using correlated transmission electron microscopy (TEM), dark-field Rayleigh scattering microscopy, surface-enhanced Raman scattering (SERS) microscopy, and finite element method (FEM) calculations. The distribution of SERS enhancement factors (EFs) for a structurally and optically diverse set of nanoantennas is remarkably narrow. For a collection of 30 individual nanoantennas ranging from dimers to heptamers, the EFs vary by less than 2 orders of magnitude. Furthermore, the EFs for the hot-spot-containing nanoparticles are uncorrelated to aggregation state and localized surface plasmon resonance (LSPR) wavelength but are crucially dependent on the size of the interparticle gap. This study demonstrates that the creation of hot spots, where two particles are in subnanometer proximity or have coalesced to form crevices, is paramount to achieving maximum SERS enhancements.

Optically and elastically assembled plasmonic nanoantennae for spatially resolved characterization of chemical composition in soft matter systems using surface enhanced spontaneous and stimulated Raman scattering

“…As compared to regular 5CB spectra, the average enhancement factor of Raman signal from 5CB molecules in the presence of NB particles is ~500, although the values varies from 200 -700 for different NB 5 particles. The calculations of this factor were performed by taking the ratios of integrated intensities of SERS signal corresponding to the characteristic line at 1158 cm -1 collected from the NB location and Raman signal collected after moving the beam away from NB particles in LC medium, normalized with respect to the number of 5CB molecules in the excitation volume, as commonly done in SERS literature and described in more details elsewhere 7 . The achieved enhancement factor is somewhat limited by the fact that the Raman spectra measured from the NB locations are convoluted because of the larger volume of the excitation beam as compared to NB assembly size, which is only about several hundreds of nanometer along the light propagation direction.…”

“…11 Such metal nanoparticles have been frequently used for SERS. [6][7][8][9] Nano structures of noble metals like gold and silver are especially suitable for SERS applications because of their tunable optical responses in the visible region, through surface plasmon resonance (SPR) effects. Raman enhancement of several orders of magnitude can be obtained by suitably arranging metal particles in dimers, trimers and so on, [5][6][7] as well as by using particles with sharp edges.…”

“…[6][7][8][9] Nano structures of noble metals like gold and silver are especially suitable for SERS applications because of their tunable optical responses in the visible region, through surface plasmon resonance (SPR) effects. Raman enhancement of several orders of magnitude can be obtained by suitably arranging metal particles in dimers, trimers and so on, [5][6][7] as well as by using particles with sharp edges. 9 However, manipulating such particles in different media and controlling the position within the sample where the enhancement occurs has been a challenge, although certain metal nanoparticles can be manipulated with optical tweezers and can be localized in topological defects in liquid crystalline (LC) host media.…”

Fundamental behavior of electric field enhancements in the gaps between closely spaced nanostructures

“…Structures that generate large electric field enhancements relative to the incident field, hereon referred to as |E| 2 enhancements, have recently received a great deal of attention [1][2][3][4][5][6][7][8][9][10] . This is because such enhancements are central to a number of physical processes, including such surface-enhanced spectroscopy techniques as surface-enhanced Raman scattering (SERS), second harmonic generation, and enhanced absorption and fluorescence 1 .…”

“…This is because such enhancements are central to a number of physical processes, including such surface-enhanced spectroscopy techniques as surface-enhanced Raman scattering (SERS), second harmonic generation, and enhanced absorption and fluorescence 1 . Often the enhanced field is generated in the small crevices of a roughened metal surface 2 or at the junctions of closely spaced nanoparticles [3][4][5][6][7][8][9][10] . Herein we focus on the latter structures, and while much is known about the |E| 2 enhancements in them, there is still confusion over fundamental principles.…”