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Understanding mass transport mechanisms in nanopores is essential for developing advanced materials for chemical separations, chemical sensing, and energy storage. This paper reports a novel imaging platform that is employed for the first time to investigate the detailed diffusion dynamics of single rhodamine B (RhB) dye molecules confined within solution-filled cylindrical anodic aluminum oxide (AAO) nanopores. The imaging platform relies on illumination of horizontally-oriented AAO nanopores in a highly inclined and laminated optical (HILO) light sheet microscopy geometry. The method was used to investigate the translational and orientational dynamics of single rhodamine B (RhB) molecules within horizontally-oriented 5- and 10-nm diameter AAO nanopores filled with water–ethanol mixtures. The established platform enabled the observation of one-dimensional motion along the pore axis involving occasional short- or long-term immobilization at the single-molecule level. Analysis of cumulative squared-displacement distributions revealed fast (5 – 30 µm²/s), intermediate (1 – 5 µm²/s), and slow (< 1 µm²/s) diffusion components. From the effects of mixture composition and pore size on the contributions of these three components, we inferred that the fast, intermediate, and slow components could be assigned to desorption-mediated hopping, crawling, and wiggling motions, respectively. The platform based on the horizontally-oriented AAO nanopores also permitted single-molecule emission polarization measurements that revealed the negligible steric confinement of individual diffusing RhB molecules within the AAO nanopores. The imaging platform based on AAO membranes and HILO microscopy provided a unique means to investigate how solvation-mediated surface interactions and nanoconfinement govern molecular transport in nanoporous environments. 

It is well known that toxic organic micropollutants (OMs) accumulate on the surfaces of microplastics. However, much remains to be learned about the exact molecular level mechanisms of OM accumulation and how these evolve as the plastics age. In this work, super-resolved single-molecule tracking (SMT) is used for the first time to investigate the accumulation of Rhodamine B (RhB) dye on fresh and artificially aged polyethylene terephthalate (PET) surfaces. PET thin films serve as models for microplastics, while RhB serves as a proxy for the OMs they accumulate. Artificial aging of the films is accomplished by exposing them in a UV-ozone chamber. Water contact angle, spectroscopic ellipsometry, and carbonyl index measurements reveal a gradual decrease in film hydrophobicity, thickness, and carbonyl content with age. Atomic force microscopy (AFM) data reveal an increase in surface roughness and confirm the films remain largely intact and continuous across the aging times explored. In SMT experiments, wide-field fluorescence videos acquired from the water/PET interface under 7.5 pM RhB reveal both mobile and immobile dye molecules. Measurements of the frame-to-frame displacements of the dye show that diffusion occurs by a desorption-mediated mechanism and that the diffusion rate varies with PET film age. The surface density of mobile dye molecules decreases with increasing PET age, while the population of immobile molecules becomes relatively larger, suggesting an age-dependent transformation of the mechanism(s) by which the dye is accumulated. SMT data reveal that both mobile and immobile molecules repeatedly adsorb over the same surface sites, consistent with the emergence of nanoscale PET surface heterogeneity also revealed by AFM. Estimates of the adsorption coefficients are obtained using a nearest-neighbor analysis, giving values from 9.9 × 105 M-1 to 2.1 × 106 M-1 for immobile molecules and from 1.8 × 105 M-1 to 2.5 × 105 M-1 for mobile molecules on fresh and five-minute aged PET, respectively. Anomalous age-dependent variations in the velocity of molecular motion on the PET surface and in the population of immobile molecules are shown to correlate with changes in the strength of RhB adsorption. 

Variable area fluorescence correlation spectroscopy is used to study diffusion by three Nile red derivatives within aqueous C₁₂EO₁₀lyotropic liquid crystal gels. The dyes exhibit different levels of interactions with the micelle cores in the gels. 

The synthesis of a new Nile red derivative incorporating a reactive aldehyde moiety (NR-CHO) is reported and its use in spectroscopic studies of heterogeneous catalyst activity in crossed aldol reactions is demonstrated. 1 H and 13 C NMR, and high-resolution mass spectrometry confirmed the desired NR-CHO was obtained. Mg-Zr-Cs doped silica (Cs(Zr,Mg)-SiO 2 ) was employed as the catalyst and its performance was compared to that of commercially available MgO. Fumed silica was used as a control. Aldol reactions with acetone and acetophenone were run in 4 : 1 (v/v) DMSO : ketone solutions in the presence of both dilute (1 μM) and concentrated (1 mM) NR-CHO. NR-CHO fluorescence spectra were acquired as the reactions progressed. Shifts in its emission spectrum are used to distinguish the products formed and to characterize the reaction rate. The dye exhibits different behavior that defines whether the reaction stops at the addition (alcohol) product, or forms both addition and condensation (olefin) products, providing valuable initial information on catalyst activity. The assignment of addition and condensation products is supported by thin layer chromatography, high performance liquid chromatography (HPLC), and HPLC-mass spectrometry data. Product formation is shown to depend upon the catalyst employed, with the Cs(Zr,Mg)-SiO 2 yielding both addition and condensation products, while MgO yields primarily addition products. The advantages of NR-CHO in spectroscopic studies of aldol reactions are also demonstrated relative to commercially available 3-perylenecarboxaldehyde. The NR-CHO reported here and the results obtained will facilitate a broad range of both ensemble and single molecule spectroscopic investigations of heterogeneous catalysis in crossed aldol reactions in the future.