Abstract
Surface-enhanced Raman scattering is a powerful approach to detect molecules at very low concentrations, even up to the single-molecule level. One important aspect of the materials used in such a technique is how much the signal is intensified, quantified by the enhancement factor (EF). Herein we obtained the EFs for gold nanoparticle dimers of 60 and 80 nm diameter, respectively, self-assembled using DNA origami nanotriangles. Cy5 and TAMRA were used as surface-enhanced Raman scattering (SERS) probes, which enable the observation of individual nanoparticles and dimers. EF distributions are determined at four distinct wavelengths based on the measurements of around 1000 individual dimer structures. The obtained results show that the EFs for the dimeric assemblies follow a log-normal distribution and are in the range of 106 at 633 nm and that the contribution of the molecular resonance effect to the EF is around 2, also showing that the plasmonic resonance is the main source of the observed signal. To support our studies, FDTD simulations of the nanoparticle’s electromagnetic field enhancement has been carried out, as well as calculations of the resonance Raman spectra of the dyes using DFT. We observe a very close agreement between the experimental EF distribution and the simulated values.
Highlights
The Atomic force microscopy (AFM) images of the self-assembled nanoparticle dimers are shown in Figure 2B for 60 nm dimers and Figure 2C for 80 nm dimers, indicating the appropriate assembly of the dimers while there are still single nanoparticles present over the silicon chip surface
The Surface-enhanced Raman scattering (SERS) results showed that the distribution of intensities of around 1000 individually measured dimers follows a log-normal distribution, with some dimers presenting a very strong enhancement, probably due to smaller gaps between particles or other possible defects from the nanoparticle surface
From the SERS data, we can conclude that plasmonic effects of the dimers occur mainly at one wavelength, 633 nm, while at the other wavelengths only the plasmonic enhancement of the single nanoparticles contributes as well as the molecular resonance enhancement
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. SERS spectra from single nanoparticles, the minimal structure to obtain a plasmonic hot spot is a nanoparticle dimer, where it is possible to achieve enhancements up to 1011 in the nanometric gap between the particles [5,6]. We will discuss the calculation of enhancement factors for dimers composed of 60 and 80 nm gold nanoparticles (AuNPs) modified with the dyes TAMRA (5carboxytetramethylrhodamine) and Cy5 (cyanine 5) Both dyes have absorption bands in the regions between 488 and 700 nm, which can give rise to strong SERS signals, both in single nanoparticles and in dimeric assemblies, which enables us to compare the differences in intensity due to the resonance and the hot spot effect. The obtained results show that the contribution of the resonant effect can increase the EF compared to a non-resonant condition by 2 times, while the hot spot contributes to further enhance the Raman signal by up to 106 compared to the signal obtained from a single nanoparticle
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