Trajectory-Profile-Guided Single Molecule Tracking for Assignment of One-Dimensional Diffusion Trajectories

Fluorescence Microscopic Investigations of Molecular Dynamics in Self-Assembled Nanostructures

Many analytical methods employ self-assembled nanostructured materials as chemical recognition media. Molecular permeation through these materials exhibits unique selectivity owing to nanoconfinement-induced enhancement of permeant-nanostructure interactions. This Personal Account introduces our efforts to investigate the detailed dynamics of single or a small number of molecules in nanostructured materials. We developed new experimental and analysis approaches built upon laser-based fluorescence microscopy to measure the detailed translational and orientational dynamics of molecules diffusing in horizontally-oriented, cylindrical nanostructures, including surfactant micelles, silica mesopores, block copolymer microdomains, and bolaamphiphile-based organic nanotubes. Our studies clarified nanoscale details on the structural/chemical heterogeneity of the nanostructures, and their impacts on molecular mass transport dynamics.

Transforming Separation Science with Single-Molecule Methods

Empirical optimization of the multiscale parameters underlying chromatographic and membrane separations leads to enormous resource waste and production costs. A bottom-up approach to understand the physical phenomena underlying challenges in separations is possible with single-molecule observations of solute-stationary phase interactions. We outline single-molecule fluorescence techniques that can identify key interactions under ambient conditions. Next, we describe how studying increasingly complex samples heightens the relevance of single-molecule results to industrial applications. Finally, we illustrate how separation methods that have not been studied at the single-molecule scale can be advanced, using chiral chromatography as an example case. We hope new research directions based on a molecular approach to separations will emerge based on the ideas, technologies, and open scientific questions presented in this Perspective.

Analysis of the Diffusivity Change from Single-Molecule Trajectories on Living Cells

With the wide application of live-cell single-molecule imaging and tracking of biomolecules at work, deriving diffusion state changes of individual molecules is of particular interest as these changes reflect molecular oligomerization or interaction with other cellular components and thus correlate with functional changes. We have developed a Rayleigh mixture distribution-based hidden Markov model method to analyze time-lapse diffusivity change of single molecules, especially membrane proteins, with unknown dynamic states in living cells. With this method, the diffusion parameters, including diffusion state number, state transition probability, diffusion coefficient, and state mixture ratio, can be extracted from the single-molecule diffusion trajectories accurately via easy computation. The validity of our method has been demonstrated with not only experiments on synthetic trajectories but also single-molecule fluorescence imaging data of two typical membrane receptors. Our method offers a new analytical tool for the investigation of molecular interaction kinetics at the single-molecule level.

Single-Molecule Investigations of Morphology and Mass Transport Dynamics in Nanostructured Materials

Nanostructured materials such as mesoporous metal oxides and phase-separated block copolymers form the basis for new monolith, membrane, and thin film technologies having applications in energy storage, chemical catalysis, and separations. Mass transport plays an integral role in governing the application-specific performance characteristics of many such materials. The majority of methods employed in their characterization provide only ensemble data, often masking the nanoscale, molecular-level details of materials morphology and mass transport. Single-molecule fluorescence methods offer direct routes to probing these characteristics on a single-molecule/single-nanostructure basis. This article provides a review of single-molecule studies focused on measurements of anisotropic diffusion, adsorption, partitioning, and confinement in nanostructured materials. Experimental methods covered include confocal and wide-field fluorescence microscopy. The results obtained promise to deepen our understanding of mass transport mechanisms in nanostructures, thus aiding in the realization of advanced materials systems.

Fluorescence Recovery after Photobleaching and Single-Molecule Tracking Measurements of Anisotropic Diffusion within Identical Regions of a Cylinder-Forming Diblock Copolymer Film

This work demonstrates ensemble and single-molecule diffusion measurements within identical regions of a cylinder-forming polystyrene-poly(ethylene oxide) diblock copolymer (PS-b-PEO) film using fluorescence recovery after photobleaching (FRAP) and single-molecule tracking (SMT). A PS-b-PEO film (∼4 μm thick) with aligned cylindrical PEO microdomains containing 10 μM sulforhodamine B (SRB) was prepared by directional solvent-vapor penetration (SVP) of 1,4-dioxane. The ensemble diffusion behavior of SRB in the microdomains was assessed in FRAP studies of circular photobleached regions (∼7 μm in diameter). The SRB concentration was subsequently reduced by additional photobleaching, and the diffusion of individual SRB molecules was explored using SMT in the identical area (∼16 × 16 μm(2)). The FRAP data showed anisotropic fluorescence recovery, yielding the average microdomain orientation. The extent of fluorescence recovery observed (∼90%) demonstrated long-range microdomain connectivity, while the recovery time dependence provided an ensemble measurement of the SRB diffusion coefficient within the cylindrical microdomains. The SMT data exhibited one-dimensional diffusion of individual SRB molecules along the SVP direction across the entire film thickness, as consistent with the FRAP results. Importantly, the average of the single-molecule diffusion coefficients was close to the value obtained from FRAP in the identical area. In some cases, SMT offered smaller diffusion coefficients than FRAP, possibly due to contributions from SRB molecules confined within short PEO microdomains. The implementation of FRAP and SMT measurements in identical areas provides complementary information on molecular diffusion with minimal influence of sample heterogeneity, permitting direct comparison of ensemble and single-molecule diffusion behavior.