Observation and Analysis of Blinking Surface-enhanced Raman Scattering

Abstract

From a single molecule at a silver nanoaggregate junction, blinking surface-enhanced Raman scattering (SERS) is observed. Here, a protocol is presented on how to prepare the SERS-active silver nanoaggregate, record a video of certain blinking spots in the microscopic image, and analyze the blinking statistics. In this analysis, a power law reproduces the probability distributions for bright events relative to their duration. The probability distributions for dark events are fitted by a power law with an exponential function. The parameters of the power law represent molecular behavior in both bright and dark states. The random walk model and the speed of the molecule across the entire silver surface can be estimated. It is difficult to estimate even when using averages, autocorrelation functions, and super-resolution SERS imaging. In the future, power law analyses should be combined with spectral imaging, because the origins of blinking cannot be confirmed by this analysis method alone.

SERS Blinking on Anisotropic Nanoparticles

“…The first part of the SERS data analysis was carried out according to a conventional procedure . The theoretical background of the SERS blinking phenomenon is briefly described in the SI T1 part of the Supporting Information.…”

“…Video sequences of the blinking SERS were collected for 20 min using an objective lens (Olympus, LCPlanFl 60x, NA 0.7) coupled with an inverted microscope (Olympus, IX-70) coupled with a cooled digital charge-coupled device (CCD) camera (Hamamatsu, ORCA-AG) at a 61 ms time resolution. The recording method and analysis of the SERS blinking films are detailed elsewhere …”

“…The strongest enhancement is obtained in specific areas called hot spots, which can be placed in the junction between two nanoparticles (NPs) − or on the tops or edges of nonspherical NPs. − The strength of certain hot spots enables SERS spectra for even a single molecule to be recorded, − making this method a unique research tool for obtaining clear information regarding a single molecule in a defined place and time, and not simply studying the average chemical and structural states of an analyte. Nevertheless, this inhomogeneous SERS enhancement also results in a time fluctuation of the intensity and/or spectra, − which originates from a nanometer- or angstrom-sized hot spot, in which a single molecule is randomly present or absent. This incident manifests as a singular flashing and quenching and is repeated over time as a blinking phenomenon, which despite being a subject of interest in itself, hinders the recording of SERS spectra, making it irreproducible, particularly for single-molecule measurements.…”

“…Such blinking has prompted a thorough study of this effect in previous years. − The parameters describing a random walk of a molecule on an NP surface, the power law exponents for a probability distribution of dark and bright states, and a truncation of the log–log plot of a dark state were examined based on multiple factors. − , These factors include the intensity of the excitation light ( I exc ) , (also studied using an autocorrelation function), the wavelength of the excitation light (λ exc ) and its correlation with the localized surface plasmon resonances (LSPR) wavelength, , and the charges and concentration ratios of the walking molecules to a citrate anion (Cit – ) covering the NP surface as residues of NP synthesis substrates . The last of these factors was revealed to increase the probability distribution of a long duration for a dark state when an anionic capping agent remains on the NP surface, while simultaneously having no influence on the probability distribution of a bright-state duration .…”

“…The results not only allowed the assumed goal to be achieved but also facilitated the development of conventional analysis approach. The need for this development may arise from a disparity in the new SERS blinking set studied or may be prompted by an expansion/improvement of the analysis method for a conventional SERS blinking set − or even semiconductor quantum dot (QD) fluorescence blinking. − …”