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Porous materials such as cellular cytosol, hydrogels, and block copolymers have nanoscale features that determine macroscale properties. Characterizing the structure of nanopores is difficult with current techniques due to imaging, sample preparation, and computational challenges. We produce a super-resolution optical image that simultaneously characterizes the nanometer dimensions of and diffusion dynamics within porous structures by correlating stochastic fluctuations from diffusing fluorescent probes in the pores of the sample, dubbed here as "fluorescence correlation spectroscopy super-resolution optical fluctuation imaging" or "fcsSOFI". Simulations demonstrate that structural features and diffusion properties can be accurately obtained at sub-diffraction-limited resolution. We apply our technique to image agarose hydrogels and aqueous lyotropic liquid crystal gels. The heterogeneous pore resolution is improved by up to a factor of 2, and diffusion coefficients are accurately obtained through our method compared to diffraction-limited fluorescence imaging and single-particle tracking. Moreover, fcsSOFI allows for rapid and high-throughput characterization of porous materials. fcsSOFI could be applied to soft porous environments such hydrogels, polymers, and membranes in addition to hard materials such as zeolites and mesoporous silica.

Surface morphology, in addition to hydrophobic and electrostatic effects, can alter how proteins interact with solid surfaces. Understanding the heterogeneous dynamics of protein adsorption on surfaces with varying roughness is experimentally challenging. In this work, we use single-molecule fluorescence microscopy to study the adsorption of α-lactalbumin protein on the glass substrate covered with a self-assembled monolayer (SAM) with varying surface concentrations. Two distinct interaction mechanisms are observed: localized adsorption/desorption and continuous-time random walk (CTRW). We investigate the origin of these two populations by simultaneous single-molecule imaging of substrates with both bare glass and SAM-covered regions. SAM-covered areas of substrates are found to promote CTRW, whereas glass surfaces promote localized motion. Contact angle measurements and atomic force microscopy imaging show that increasing SAM concentration results in both increasing hydrophobicity and surface roughness. These properties lead to two opposing effects: increasing hydrophobicity promotes longer protein flights, but increasing surface roughness suppresses protein dynamics resulting in shorter residence times. Our studies suggest that controlling hydrophobicity and roughness, in addition to electrostatics, as independent parameters could provide a means to tune desirable or undesirable protein interactions with surfaces. [ABSTRACT FROM AUTHOR]

Plasmon-induced charge transfer has been studied for the development of plasmonic photodiodes and solar cells. There are two mechanisms by which a plasmonic nanoparticle can transfer charge to an adjacent material: indirect transfer following plasmon decay and direct transfer as a way of plasmon decay. Using single-particle dark-field scattering and photoluminescence imaging and spectroscopy of gold nanorods on various substrates, we identify linewidth broadening and photoluminescence quantum yield quenching as key spectroscopic signatures that are quantitatively related to plasmon-induced interfacial charge transfer. We find that dark-field scattering linewidth broadening is due to chemical interface damping through direct charge injection via plasmon decay. The photoluminescence quantum yield quenching reveals additional mechanistic insight into electron–hole recombination as well as plasmon generation and decay within the gold nanorods. Through these two spectroscopic signatures, we identify charge transfer mechanisms at TiO2 and indium doped tin oxide interfaces and uncover material parameters contributing to plasmon-induced charge transfer efficiency, such as barrier height and resonance energy. [ABSTRACT FROM AUTHOR]

Electrogenerated chemiluminescence (ECL) is a promising technique for low concentration molecular detection. To improve the detection limit, plasmonic nanoparticles have been proposed as signal boosting antennas to amplify ECL. Previous ensemble studies have hinted that spectral overlap between the nanoparticle antenna and the ECL emitter may play a role in signal enhancement. Ensemble spectroscopy, however, cannot resolve heterogeneities arising from colloidal nanoparticle size and shape distributions, leading to an incomplete picture of the impact of spectral overlap. Here, we isolate the effect of nanoparticle-emitter spectral overlap for a model ECL system, coreaction of tris(2,2′-bipyridyl)dichlororuthenium(ii) hexahydrate and tripropylamine, at the single-particle level while minimizing other factors influencing ECL intensities. We found a 10-fold enhancement of ECL among 952 gold nanoparticles. This signal enhancement is attributed exclusively to spectral overlap between the nanoparticle and the emitter. Our study provides new mechanistic insight into plasmonic enhancement of ECL, creating opportunities for low concentration ECL sensing. [ABSTRACT FROM AUTHOR]

We have evaluated the effect of varying three key parameters for Fluorescence Correlation Spectroscopy analysis, first in the context of a one species/one environment system, and then in a complex system composed of two species, or conversely, two environments. We establish experimentally appropriate settings for the (1) minimum lag time, (2) maximum lag time, and (3) averaging times over which an autocorrelation is carried out, as a function of expected diffusion decay time for a particular solute, and show that use of appropriate settings plays a critical role in recovering accurate and reliable decay times and resulting diffusion constants. Both experimental and simulated data were used to show that for a complex binary system, to extract accurate diffusion constants for both species, decay times must be bounded by adequate minimum and maximum lag times as dictated by the fast and slow diffusing species, respectively. We also demonstrate that even when constraints on experimental conditions do not permit achieving the necessary lag time limits for both of the species in a binary system, the accuracy of the recovered diffusion constant for the one species whose autocorrelation function is fully time-resolved is unaffected by uncertainty in fitting introduced by the presence of the second species.

Single-molecule fluorescence resonance energy transfer (SMFRET) was used to study nucleocapsid protein (NC)-induced fluctuations of different DNA hairpins over a broader range of conditions and in more depth. Results reveal a complex mechanism for secondary structure fluctuations with dynamic processes happening over a wide time range that is ~5 to >250 ms and with the involvement of long-lived intermediates.