Dynamic Imaging of Multiple SERS Hotspots on Single Nanoparticles

Abstract

Signal intensity fluctuations are a ubiquitous characteristic of single-molecule surface-enhanced Raman scattering (SERS). In this work, we observed SERS intensity fluctuations (SIFs) from single nanoparticles fully coated with an adsorbate layer. Fluctuations from dry, fully coated nanoparticles are assigned to a dynamic molecule/metal environment wherein atomic-scale reconstructions support SERS. Using super resolution imaging techniques, we were able to pinpoint the positions of the fluctuations with subparticle precision. We observed that the fluctuation events were separated spatially, temporally, and were unique to different laser excitation wavelengths and polarizations. Dual-wavelength super-resolution SERS imaging with green and red lasers reveal various classes of SIFs that occur either simultaneously or nonsimultaneously and from either the same or different location on a single nanoparticle. Similar results were seen when the particle is excited with different polarizations. This suggests that single molecule responses from several different hotspots in the same nanoparticle were readily probed. Furthermore, each nanoparticle contains multiple unique hotspots of different strengths, and resonance conditions, which are accessible by the different illumination conditions. The plasmon resonances localized by the roughness features at the nanoparticle's surface play a significant role in the fluctuation events. Our experiments show that SERS hotspots that support single molecules are not a static feature of the nanoparticle. This information should be useful to guide future single-molecule SERS experiments.

High-Speed Fluctuations in Surface-Enhanced Raman Scattering Intensities from Various Nanostructures

“…Strong SERS intensity fluctuations (SIFs) have been reported for single metallic nanoparticles fully coated by a molecular probe. 18,19 Figure 1 presents similar observations from a common SERS substrate consisting of spherical silver nanoparticles deposited on a gold surface (System 1).

Figure 1.High-speed SERS with Ag nanoparticle clusters on Au film.

…”

“…The typical maximum acquisition rate for these systems is one spectrum every ∼100 ms (10 Hz) and SIFs have been observed in those conditions even from fully coated surfaces. 18 Interestingly, the fluctuations become even more evident when the acquisition time is decreased further, into the μs time scale. 19 For instance, using an acquisition rate of 800 Hz, enabled by an AiryScan (Zeiss) detector, it was possible to observe strong SIFs from fully covered single nanoparticles.…”

“…Beyond spatial confinement and control of surface concentration, another parameter that can be explored to reveal SIFs is the acquisition time. 18,19 Individual SIFs might be observed from experiments where several hotspots covered by a monolayer are simultaneously illuminated, as long as the acquisition time is very short. However, SERS experiments are commonly performed using spectrometers connected to either charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) cameras.…”

“…The sub-ms experiments were realized using photomultiplier tubes (PMTs), rather than CCDs and CMOS cameras, which allowed faster acquisition time with single-photon sensitivity. 18,19 On the other hand, the PMT detection system did not have the spectral resolution to define individual vibrational features, which is a drawback of the technique. In any case, the fast SIFs observed from various hotspots even when the surface was completely covered by a molecular layer reveals new dynamical SERS effects on the microsecond scale that have not yet been fully explored.…”

“…All the experiments reporting high speed SIFs have so far 18,19 been realized only using silver nanoshells. Therefore, it is not clear if those types of high-speed intensity fluctuations are in fact a fundamental characteristic of SERS or if the phenomenon is specific for that nanoparticle substrate.…”