| Description |
The development of techniques to probe molecular transport and the dynamics of molecular interactions at interfaces is important for understanding and optimizing surface-based technologies including surface-enhanced spectroscopies, biological assays, sensors, catalysis, and chemical separations. In particular, the efficiency and resolution of separation via reversed-phase liquid chromatography is governed by the interaction of analytes with the solution/stationary phase interface. Most commonly, the stationary phase material consists of high surface area, micron-sized, mesoporous silica particles functionalized with n-alkane ligands. Understanding the timescales at which analyte molecules are transported through the interior of the particle, as well as adsorbed and desorbed from the particle surface, is of fundamental importance in the development of new, more efficient chromatographic materials. Probing chemical interactions at interfaces is difficult due to the selectivity needed to measure the small population of molecules at an interface versus bulk solution. Measuring interfacial chemical interactions within chromatographic particles has the added challenge that the majority of the surface area is contained within the particle making it difficult to measure interfacial processes directly. In this work, single-molecule spectroscopic techniques are used to measure the transport and adsorption/desorption kinetics of molecules at planar reversed-phase chromatographic interfaces and within reversed-phase chromatographic particles. Fluorescence imaging with single-molecule tracking is used to track the locations of fluorescent molecules during their retention within chromatographic particles. This yields information regarding their diffusion rates and their residence time within the particle. Statistical criteria based on the single-molecule localization resolution are also developed to characterize the population of strongly adsorbed molecules and their effect on intraparticle molecular residence times. Fluorescence imaging is also combined with fluorescence-correlation spectroscopy and used to measure fast interfacial transport and sorption kinetics at planar models of chromatographic interfaces. This technique has higher temporal resolution relative to imaging and is capable of measuring transport approaching free solution diffusion rates of small molecules. Finally, a comparison is made between interfacial transport rates and surface populations measured at planar chromatographic interfacial models versus within porous particles. It is found that n-alkyl modified planar interfaces are reasonable models for reversed-phase chromatographic particles with proper interpretation of measured parameters. |
