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Porous materials are used extensively in industrial filtration and mass separation processes, but it is often difficult to predict their mass transport behavior because porous materials are an inherently heterogeneous medium and multiple microscopic mechanisms can lead to macroscopic changes in transport. To provide a microscopic view of hindered porous transport, we present the results of single-particle tracking experiments in which we followed the diffusive motion of individual nanoparticles in commercial filtration media. We compared two materials, glass fiber and nitrocellulose, with similar nominal characteristics, but we found that the diffusion behavior of the embedded particles differed significantly. While diffusion in the glass fiber material was nearly unhindered, the dynamics were heterogeneous and significantly slowed in the nitrocellulose. We rationalized the observations based on differences in geometric hindrance, particle binding, and hydrodynamic interactions. Our results highlight the ...

The efficiency of chromatographic separations decreases markedly when peaks exhibit asymmetry (e.g., "peak tailing"). Theoretically, these effects can arise from heterogeneous adsorption kinetics. To investigate the nature and consequences of such heterogeneity, we used a combination of single-molecule imaging and reversed-phase liquid chromatography (RPLC). In both single-molecule and macroscopic RPLC experiments, the stationary phase was hydrophobic end-capped (trimethylsilyl-functionalized) silica, which we exposed to different methanol/water solutions (50%-62% methanol), containing a fluorescent fatty acid analyte. Super-resolution maps based on single-molecule observations revealed rare, strong adsorption sites with activity that varied significantly with methanol concentration. The adsorption and desorption kinetics on the strong sites were heterogeneous and positively correlated, suggesting a broad underlying distribution of site binding energies. Adsorption equilibrium on the strong sites was more sensitive to solution conditions than overall retention measured in RPLC experiments, suggesting that the effect of strong sites on the overall adsorption kinetics should change with solution conditions. Interestingly, in RPLC experiments, peak tailing had a nonmonotonic dependence on methanol concentration within the range studied. Using the stochastic model of chromatography, we showed quantitatively that our single-molecule kinetic results were consistent with this macroscopic trend. This approach to identifying and quantifying adsorption sites should be useful for designing better chromatographic separations and for identifying the role of heterogeneous surface chemistry in molecular dynamics.

We used single-molecule tracking experiments to observe the motion of small hydrophobic fluorescent molecules at the interface between water and a solid surface that exhibited periodic chemical patterns. The dynamics were characterized by non-ergodic, continuous time random walk statistics. The step-size distributions displayed enhanced probability of steps to periodic distances, consistent with theoretical predictions for diffusion in an atomic/molecular scale periodic potential. Surprisingly, this general behavior was observed here for surfaces exhibiting characteristic length scales three orders of magnitude larger than atomic/molecular dimensions, and may provide a new way to understand and control solid-liquid interfacial diffusion for molecular targeting applications.

The dynamics of polymers adsorbed to a solid surface are important in thin-film formation, adhesion phenomena, and biosensing applications, but they are still poorly understood. Here we present tracking data that follow the dynamics of isolated poly(ethylene glycol) chains adsorbed at a hydrophobic solid-liquid interface. We found that molecules moved on the surface via a continuous-time random walk mechanism, where periods of immobilization were punctuated by desorption-mediated jumps. The dependence of the surface mobility on molecular weight (2, 5, 10, 20, and 40 kg/mol were investigated) suggested that surface-adsorbed polymers maintained effectively three-dimensional surface conformations. These results indicate that polymer surface diffusion, rather than occurring in the two dimensions of the interface, is dominated by a three-dimensional mechanism that leads to large surface displacements and significant bulk-surface coupling.

Texas Red-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (TR-DHPE) is a frequently used experimental tool in the study of lipid membranes by fluorescence microscopy techniques. Although it is usually incorporated at low concentration, several experimental studies suggest that fluorescent labels could alter the membrane's behavior. To investigate this possibility, we developed and validated a molecular dynamics model for TR-DHPE. The model was used to simulate TR-DHPE in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers of 127 lipids and 511 lipids. We also simulated a pure 128 DPPC bilayer under the same conditions and found that the results compared well with other simulations and experiments. Texas Red was found to reside in the upper acyl chain region of the bilayer. Coinciding with its position in the bilayer, we found that Texas Red decreased the upper acyl carbon order parameters of neighboring lipids. We analyzed area per lipid at varying distances from the Texas Red and discovered finite size effects in the 1:127 system. In both systems, the Texas Red bound to a DPPC lipid. The model we have developed should also be useful for other biomolecular simulations which try to mimic experimental systems.

We study the behavior of charged spherical Au nanoparticles in a nanofluidic slit as a function of the separation of the symmetrically charged confining surfaces. A dedicated setup called the nano-fluidic confinement apparatus (NCA) allows us to parallelize the two confining surfaces and to continuously approach them down to direct contact. Interferometric scattering (iSCAT) detection is used to measure the particle contrast with 2 ms temporal resolution. We obtain the confinement gap distance from the interference signal of the glass and the oxide-covered silicon wafer surface with nanometer accuracy. We present a three parameter model that describes the optical signal of the particles as a function of particle height and gap distance. The model is verified using nanoparticles immobilized at the glass and the substrate surface. For freely diffusing particles, the envelope of the particle signal as a function of gap distance and the known particle height at tight confinement is used to calibrate the particle signal in-situ and obtain all free model parameters. Due to the periodic contrast variation for large gap distances we obtain a set of possible particle heights for a given contrast value. For a range of small gap distances this assignment is unique and the particle height can be measured directly with high accuracy. The high temporal resolution allows us to measure the height occupation probability which provides a direct link to the free energy landscape the particles are probing via the Boltzmann distribution. Accordingly by fitting the results to a physical model based on the linear superposition approximation (LSA), the physical parameters governing the particle-glass interaction are quantified., 28 pages, 6 Figures

Quantum dots at the hexane-glycerol interface exhibited unexpected behavior including highly dynamic adsorption/desorption, where the lateral nanoparticle motion was anomalously fast immediately after adsorption and prior to desorption. At the interface, particles exhibited pseudo-Brownian lateral motion, in which the instantaneous diffusion coefficient was temporally anticorrelated, in agreement with our simulations involving fractional Brownian motion in the surface-normal direction. These phenomena suggest that, in contrast to the conventional picture for colloidal particles, nanoparticles explore a landscape of metastable interfacial positions, with different exposures to the two adjacent phases.

The inherent pore-scale heterogeneity of many natural and synthetic porous materials can make it difficult to model and predict porous transport because the underlying microscopic processes are often poorly understood. Here we present the results of single-particle tracking experiments in which we followed the pore-scale diffusion of individual nanoparticles, deep within a three-dimensional porous material of moderate porosity. We observed significant hydrodynamic damping of particle motion at subpore length scales, resulting in heterogeneous and spatially dependent mobility. The accessibility of the void space was strongly dependent on particle size, and related to the heterogeneous hydrodynamics. Our results suggest that pore-scale diffusion is more heterogeneous and volume accessibility more limited than previously expected. The method demonstrated here will enable studies of a broad new class of materials including porous polymers of technological interest.

Many fields of research have adopted self-assembly of colloidal spheres as an easy and reliable method to produce macroscopic structures with nanoscale periodicity. The field of soft lithography in particular has used colloidal self-assembly to fabricate lithographic masks and templates. We developed a colloidal lithography method that uses the colloidal assembly directly to produce submicrometer topographic and chemical surface patterns. The method does not require any specialized equipment, making it particularly useful in biological and chemical laboratories without lithography expertise. The technique involves the curing and solvent removal of a self-assembled colloidal crystal from an inorganic surface. The result is a triangular array of polymer features with submicrometer periodicity that covers square centimeters of surface area. The feature size and spacing is easily controlled, and the features serve as reactive sites for biomolecule immobilization.

The mobility of molecules on a solid surface plays a key role in diverse phenomena such as friction and self-assembly and in surface-based technologies like heterogeneous catalysis and molecular targeting. To understand and control these surface processes, a universally applicable model of surface transport at solid-liquid interfaces is needed. However, unlike diffusion at a solid-gas interface, little is known about the mechanisms of diffusion at a solid-liquid interface. Using single-molecule tracking at a solid-liquid interface, we found that a diverse set of molecules underwent intermittent random walks with non-Gaussian displacements. This contrasts with the normal random walk and Gaussian statistics that are commonly assumed for molecular surface diffusion. The molecules became temporarily immobilized for random waiting times between surface displacements produced by excursions through the bulk fluid. A common power-law distribution of waiting times indicated a spectrum of binding energies. We propose that intermittent hopping is universal to molecular surface diffusion at a solid-liquid interface.

We present a new quantitative imaging method, based on accumulated single molecule trajectories, to map and probe surface charge heterogeneity. Using a fluorescent cation as a probe molecule, we found that surface charge has a dramatic affect on the local adsorption rate, primarily through its influence on near-surface ion concentrations. Other physical measures, including surface residence time and diffusion coefficient, provided complementary information about the probe–surface interaction that can be obtained only through a single molecule approach. We present spatial maps of surface charge based on each of these dynamic probe properties.

Anomalous diffusion has been observed abundantly in the plasma membrane of biological cells, but the underlying mechanisms are still unclear. In general, it has not been possible to directly image the obstacles to diffusion in membranes, which are thought to be skeleton bound proteins, protein aggregates, and lipid domains, so the dynamics of diffusing particles is used to deduce the obstacle characteristics. We present a supported lipid bilayer system in which we characterized the anomalous diffusion of lipid molecules using single molecule tracking, while at the same time imaging the obstacles to diffusion with atomic force microscopy. To explain our experimental results, we performed lattice Monte Carlo simulations of tracer diffusion in the presence of the experimentally determined obstacle configurations. We correlate the observed anomalous diffusion with obstacle area fraction, fractal dimension, and correlation length. To accurately measure an anomalous diffusion exponent, we derived an expression to account for the time-averaging inherent to all single molecule tracking experiments. We show that the length of the single molecule trajectories is critical to the determination of the anomalous diffusion exponent. We further discuss our results in the context of confinement models and the generating stochastic process.

Biophysical research has shown that membrane phospholipids and proteins diffuse not only under normal Brownian diffusion, but with confined and anomalous behavior. We are motivated to understand this anomalous diffusion because it may be involved in many cell functions. We have used single molecule tracking on phase separated, supported lipid bilayers to investigate the origin of this unusual diffusion. DSPC forms ∼250 nm, gel phase domains that act as obstacles to diffusion in the DOPC continuous liquid phase. Incorporated into the fluid phase at very low concentration is DMPE labeled with Alexa Fluor 647. When combined with the low background of a single supported bilayer on a quartz substrate, this high quantum yield dye yields a signal to noise ratio greater than 10. By controlling the gel domain morphology and characterizing it with atomic force microscopy, we can correlate the observed single molecule diffusion with the obstacle characteristics. We also compare our results to simulated Brownian diffusion in the presence of experimentally determined obstacle fields. An understanding of this correlation will aid studies that cannot directly characterize the obstacles to diffusion.