Spatially Resolved Scattering Correlation Spectroscopy Using a Total Internal Reflection Configuration

Resonance Light-Scattering Correlation Spectroscopy and Its Application in Analytical Chemistry for Life Science

Resonance light-scattering correlation spectroscopy (RLSCS) is a new single-particle detection method with its working principle being like fluorescence correlation spectroscopy (FCS). RLSCS is obtained by autocorrelation function analysis on the measured fluctuation of the resonance light scattering (RLS) intensity occurring within a subfemtoliter volume when a single nanoparticle (such as gold nanoparticles (NPs) or silver (SNPs)) freely diffuses through the volume. The RLSCS technique can detect such parameters as concentration, diffusion coefficient (translation and rotation), etc. Compared with the FCS technique, the correlated fluorescence intensity signal in RLSCS is replaced with the RLS signal of the nanoparticles, overcoming some limits of the fluorescent probes such as photobleaching under high-intensity or long-term illumination. In this Account, we showcase RLSCS methods, theoretical models at different optical configurations, and some key applications. First, the RLSCS optical detection system was constructed based on the confocal optics, its theoretical model was proposed, and the diffusion behaviors of the nanoparticles in the solution were studied including the rotational and translational diffusion. And, methods were developed to measure the concentration, size, aspect ratio, and size distribution of the NPs. Second, based on the RLSCS methods, some detection strategies were developed for homogeneous DNA detection, immunoassay, apoptosis assay, self-thermophoresis of the nanomotor, and quantitative assay in single living cells. Meanwhile, a new fluorescence/scattering cross-correlation spectroscopy (FSCCS) method was proposed for monitoring the molecule-particle interaction. This method enriched the conventional fluorescence/fluorescence cross-correlation spectroscopy (FCCS) method. Third, using the EMCCD with high sensitivity and rapid response as an optical detector, two temporospatially resolved scattering correlation spectroscopy methods and their theoretical models were developed: total internal reflection (TIR) configuration-based spatially resolved scattering correlation spectroscopy (SRSCS) and dark-field illumination-based scattering correlation spectroscopy (DFSCS). These methods extended single-spot confocal RLSCS to imaging RLSCS, which makes RLSCS have the ability for multiple channel detection with temporospatial resolution. The method was successfully used for investigating the dynamic behaviors of gold NPs in live cells and obtained its temporospatial concentration distribution and diffusion behaviors. The final section of this Account outlines future directions in the development of RLSCS.

The Theoretical Model, Method, and Applications of Scattering Photon Burst Counting Based on an Objective Scanning Technique

Scattering photon burst counting (SPBC) is a single-particle detection method, which is based on measuring scattering photon bursting of single nanoparticles through a detection volume of <1 fL. Although SPBC has been used for bioassays and analysis of nanoparticles, it is necessary to establish its theoretical model and develop a new detection mode in order to further enhance its sensitivity and enlarge its application fields. In this paper, we proposed a theoretical model for the confocal SPBC method and developed a novel SPBC detection mode using the fast objective scanning technique. The computer simulations and experiments documented that this model well describes the relation between photon counts and experimental parameters (such as nanoparticle concentration and diameter, temperature, and viscosity). Based on this model, we developed a novel SPBC detection mode by using the fast objective scanning technique. Compared to the current confocal SPBC method, the sensitivity of this new method was significantly increased due to the significantly increased photon counts per sampling time, the linear detection range is from 0.9 to 90 pM, and the limit of detection is reduced to 40 fM for 30 nm gold nanoparticles. Furthermore, this new method was successfully applied to determine the enzyme activity of caspase-3 and evaluate the inhibition effectiveness of some inhibitors.

A near-infrared upconversion luminescence total internal reflection platform for quantitative image analysis

A near-infrared upconversion luminescence total internal reflection platform was developed for quantitative image analysis of ClO⁻.

Fluorescence correlation spectroscopy: The technique and its applications in soft matter

Fluorescence correlation spectroscopy (FCS) is a well-established single-molecule method used for the quantitative spatiotemporal analysis of dynamic processes in a wide range of samples. It possesses single-molecule sensitivity but provides ensemble averaged molecular parameters such as mobility, concentration, chemical reaction kinetics, photophysical properties and interaction properties. These parameters have been utilized to characterize a variety of soft matter systems. This review provides an overview of the basic principles of various FCS modalities, their instrumentation, data analysis, and the applications of FCS to soft matter systems.

Total Internal Reflection-Based Extinction Spectroscopy of Single Nanoparticles

Herein we report a reflection-mode total internal reflection microscopy (TIRM) to measure the extinction spectrum of individual dielectric, plasmonic, or light-absorbing nanoparticles, and to differentiate absorption and scattering components from the total optical output. These capabilities were enabled via illuminating the sample with evanescent wave of which the lightpath length was comparable with the size of single nanoparticles, leading to a dramatically improved reflectance change (ΔI/I₀ ) up to tens of percent. It was further found that scattering and absorption of light contributed to bright and dark centroids, respectively, in the optical patterns of single nanoparticles, allowing to distinguish scattering and absorption components from the extinction spectrum by the use of an appropriate image processing method. In addition, wide-field feature of TIRM enabled the studies on tens of nanoparticles simultaneously with gentle illumination.

Gold Nanorods as Plasmonic Sensors for Particle Diffusion

Plasmonic gold nanoparticles are normally used as sensor to detect analytes permanently bound to their surface. If the interaction between the analyte and the nanosensor surface is negligible, it only diffuses through the sensor's sensing volume, causing a small temporal shift of the plasmon resonance position. By using a very sensitive and fast detection scheme, we are able to detect these small fluctuations in the plasmon resonance. With the help of a theoretical model consistent with our detection geometry, we determine the analyte's diffusion coefficient. The method is verified by observing the trends upon changing diffusor size and medium viscosity, and the diffusion coefficients obtained were found to reflect reduced diffusion close to a solid interface. Our method, which we refer to as NanoPCS (for nanoscale plasmon correlation spectroscopy), is of practical importance for any application involving the diffusion of analytes close to nanoparticles.

Imaging fluorescence fluctuation spectroscopy: new tools for quantitative bioimaging

Fluorescence fluctuation spectroscopy (FFS) techniques provide information at the single-molecule level with excellent time resolution. Usually applied at a single spot in a sample, they have been recently extended into imaging formats, referred to as imaging FFS. They provide spatial information at the optical diffraction limit and temporal information in the microsecond to millisecond range. This review provides an overview of the different modalities in which imaging FFS techniques have been implemented and discusses present imaging FFS capabilities and limitations. A combination of imaging FFS and nanoscopy would allow one to record information with the detailed spatial information of nanoscopy, which is ∼20 nm and limited only by fluorophore size and labeling density, and the time resolution of imaging FFS, limited by the fluorescence lifetime. This combination would provide new insights into biological events by providing spatiotemporal resolution at unprecedented levels.