Single-molecule observations of surfactant diffusion at the solution-solid interface

Three-Dimensional Single Particle Tracking and Its Applications in Confined Environments

Single particle tracking (SPT) has proven to be a powerful technique in studying molecular dynamics in complicated systems. We review its recent development, including three-dimensional (3D) SPT and its applications in probing nanostructures and molecule-surface interactions that are important to analytical chemical processes. Several frequently used 3D SPT techniques are introduced. Especially of interest are those based on point spread function engineering, which are simple in instrumentation and can be easily adapted and used in analytical labs. Corresponding data analysis methods are briefly discussed. We present several important case studies, with a focus on probing mass transport and molecule-surface interactions in confined environments. The presented studies demonstrate the great potential of 3D SPT for understanding fundamental phenomena in confined space, which will enable us to predict basic principles involved in chemical recognition, separation, and analysis, and to optimize mass transport and responses by structural design and optimization.

Diffusion behavior of peptide amphiphiles containing different numbers of alkyl tails at a hydrophobic solid-liquid interface: single molecule tracking investigation

Using the single molecule tracking technique, the diffusion behavior of peptide amphiphiles (PAs) with different numbers of alkyl tails at a hydrophobic solid-liquid interface has been investigated. The effect of the number of alkyl tails of PAs on molecular trajectories at the hydrophobic solid-liquid interface has been systematically studied. PA molecules display an intermittent motion consisting of immobilization and hopping processes, which has been well simulated by the continuous time random walk (CTRW) model. The results reveal that the hydrophobic interaction between the PAs and hydrophobic surface plays an important role in the diffusion behavior of PAs. Increasing the number of alkyl tails in PAs systematically reduces the mobility of PAs on the hydrophobic surface. Moreover, the diffusion behavior of PAs at the hydrophobic interface also shows pH dependence. A decrease in pH is beneficial to the motion of all PAs on the hydrophobic surface, which can be ascribed to the protonation of PAs in acidic solutions. Therefore, the hydrophobic interaction is crucial to the transport of peptide amphiphiles at hydrophobic interfaces which would be important for the design of peptides in biological applications.

Polymer Free Volume Effects on Protein Dynamics in Polystyrene Revealed by Single-Molecule Spectroscopy

Protein-polymer interactions are critical to applications ranging from biomedical devices to chromatographic separations. The mechanistic relationship between the microstructure of polymer chains and protein interactions is challenging to quantify and not well studied. Here, single-molecule microscopy is used to compare the dynamics of two model proteins, α-lactalbumin and lysozyme, at the interface of uncharged polystyrene with varied molecular weights. The two proteins exhibit different surface interaction mechanisms despite having a similar size and structure. α-Lactalbumin exhibits interfacial adsorption-desorption with residence times that depend on polymer molecular weight. Lysozyme undergoes a continuous time random walk at the polystyrene surface with residence times that also depend on the molecular weight of polystyrene. Single-molecule observables suggest that the hindered continuous time random walk dynamics displayed by lysozyme are determined by the polystyrene free volume, a finding supported by thermal annealing and solvent quality studies. Hindered dynamics are dominated by short-range hydrophobic interactions where the contributions of electrostatic forces are negligible. This work establishes a relationship between the microscale structure (i.e., free volume) of polystyrene polymer chains to nanoscale interfacial protein dynamics.

Stabilization of Immobilized Enzymes via the Chaperone-Like Activity of Mixed Lipid Bilayers

Biomimetic lipid bilayers represent intriguing materials for enzyme immobilization, which is critical for many biotechnological applications. Here, through the creation of mixed lipid bilayers, the retention of immobilized enzyme structures and catalytic activity are dramatically enhanced. The enhancement in the retention of enzyme structures, which correlated with an increase in enzyme activity, is observed using dynamic single-molecule (SM) fluorescence methods. The results of SM analysis specifically show that lipid bilayers composed of mixtures of 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl- sn-glycero-3-phospho-(1'- rac-glycerol) (DOPG) stabilize the folded state of nitroreductase (NfsB), increasing the rate of refolding relative to unfolding of enzyme molecules on the bilayer surface. Remarkably, for optimal compositions with 15-50% DOPG, over 95% of NfsB remains folded while the activity of the enzyme is increased as much as 2 times over that in solution. Within this range of DOPG, the strength of the interaction of folded and unfolded NfsB with the bilayer surface was also significantly altered, which was evident by the change in the diffusion of folded and unfolded NfsB in the bilayer. Ultimately, these findings provide direct evidence for the chaperone-like activity of mixed DOPG/DOPC lipid bilayers, which can be controlled by tuning the fraction of DOPG in the bilayer.

Mapping the Functional Tortuosity and Spatiotemporal Heterogeneity of Porous Polymer Membranes with Super-Resolution Nanoparticle Tracking

As particles flow through porous media, they follow complex pathways and experience heterogeneous environments that are challenging to characterize. Tortuosity is often used as a parameter to characterize the complexity of pathways in porous materials and is useful in understanding hindered mass transport in industrial filtration and mass separation processes. However, conventional calculations of tortuosity provide only average values under static conditions; they are insensitive to the intrinsic heterogeneity of porous media and do not account for potential effects of operating conditions. Here, we employ a high-throughput nanoparticle tracking method which enables the observation of actual particle trajectories in polymer membranes under relevant operating conditions. Our results indicate that tortuosity is not simply a structural material property but is instead a functional property that depends on flow rate and particle size. We also resolved the spatiotemporal heterogeneity of flowing particles in these porous media. The distributions of tortuosity and of local residence/retention times were surprisingly broad, exhibiting heavy tails representing a population of highly tortuous trajectories and local regions with anomalously long residence times. Interestingly, local tortuosity and residence times were directly correlated, suggesting the presence of highly confining regions that cause more meandering trajectories and longer retention times. The comprehensive information about tortuosity and spatiotemporal heterogeneity provided by these methods will advance the understanding of complex mass transport and assist rational design and synthesis of porous materials.

Cadherin Diffusion in Supported Lipid Bilayers Exhibits Calcium-Dependent Dynamic Heterogeneity

Ca²⁺ ions are critical to cadherin ectodomain rigidity, which is required for the activation of adhesive functions. Therefore, changes in Ca²⁺ concentration, both in vivo and in vitro, can affect cadherin conformation and function. We employed single-molecule tracking to measure the diffusion of cadherin ectodomains tethered to supported lipid bilayers at varying Ca²⁺ concentrations. At a relatively high Ca²⁺ concentration of 2 mM, cadherin molecules exhibited a fast diffusion coefficient that was identical to that of individual lipid molecules in the bilayer (D_fast ≈ 3 μm²/s). At lower Ca²⁺ concentrations, where cadherin molecules were less rigid, the ensemble-average cadherin diffusion coefficient was systematically smaller. Individual cadherin trajectories were temporally heterogeneous, exhibiting alternating periods of fast and slow diffusion; the periods of slow diffusion (D_slow ≈ 0.1 μm²/s) were more prevalent at lower Ca²⁺ concentration. These observations suggested that more flexible cadherin ectodomains at lower Ca²⁺ concentration alternated between upright and lying-down conformations, where the latter interacted with more lipid molecules and experienced greater viscous drag.

Toehold-Mediated Displacement of an Adenosine-Binding Aptamer from a DNA Duplex by its Ligand

DNA is increasingly used to engineer dynamic nanoscale circuits, structures, and motors, many of which rely on DNA strand-displacement reactions. The use of functional DNA sequences (e.g., aptamers, which bind to a wide range of ligands) in these reactions would potentially confer responsiveness on such devices, and integrate DNA computation with highly varied molecular stimuli. By using high-throughput single-molecule FRET methods, we compared the kinetics of a putative aptamer-ligand and aptamer-complement strand-displacement reaction. We found that the ligands actively disrupted the DNA duplex in the presence of a DNA toehold in a similar manner to complementary DNA, with kinetic details specific to the aptamer structure, thus suggesting that the DNA strand-displacement concept can be extended to functional DNA-ligand systems.

Probing Heterogeneity and Bonding at Silica Surfaces through Single-Molecule Investigation of Base-Mediated Linkage Failure

The nature of silica surfaces is relevant to many chemical systems, including heterogeneous catalysis and chromatographies utilizing functionalized-silica stationary phases. Surface linkages must be robust to achieve wide and reliable applicability. However, silyl ether-silica support linkages are known to be susceptible to detachment when exposed to basic conditions. We use single-molecule spectroscopy to examine the rate of surface linkage failure upon exposure to base at a variety of deposition conditions. Kinetic analysis elucidates the role of thermal annealing and addition of blocking layers in increasing stability. Critically, it was found that successful surface modification strategies alter the rate at which base molecules approach the silica surface as opposed to reducing surface linkage reactivity. Our results also demonstrate that the innate structural diversity of the silica surface is likely the cause of observed heterogeneity in surface-linkage disruption kinetics.

Interfacial Molecular Searching Using Forager Dynamics

Many biological and technological systems employ efficient non-Brownian intermittent search strategies where localized searches alternate with long flights. Coincidentally, molecular species exhibit intermittent behavior at the solid-liquid interface, where periods of slow motion are punctuated by fast flights through the liquid phase. Single-molecule tracking was used here to observe the interfacial search process of DNA for complementary DNA. Measured search times were qualitatively consistent with an intermittent-flight model, and ∼10 times faster than equivalent Brownian searches, suggesting that molecular searches for reactive sites benefit from similar efficiencies as biological organisms.

Influence of Protein Surface Coverage on Anomalously Strong Adsorption Sites

Serum albumin is commonly used as a blocking agent to reduce nonspecific protein adsorption in bioassays and biodevices; however, the details of this process remain poorly understood. Using single molecule techniques, we investigated the dynamics of human serum albumin (HSA) on four model surfaces as a function of protein concentration. By constructing super-resolution maps, identifying anomalously strong adsorption sites, and quantifying surface heterogeneity, we found that the concentration required for site blocking varied dramatically with surface chemistry. When expressed in terms of protein surface coverage, however, a more consistent picture emerged, where a significant fraction of strong sites were passivated at a fractional coverage of 10(-4). On fused silica (FS), "non-fouling" oligo (ethylene glycol) functionalized FS, and hydrophobically modified FS, a modest additional site blocking effect continued at higher coverage. However, on amine-functionalized surfaces, the surface heterogeneity exhibited a minimum at a coverage of ∼10(-4). Using intermolecular Förster resonance energy transfer (FRET), we determined that new anomalous strong sites were created at higher coverage on amine surfaces and that adsorption to these sites was associated with protein-protein interactions, i.e., surface-induced aggregation.

Anisotropic molecular hopping at the solid-nematic interface

Single molecule tracking was used to observe intermittent and anisotropic molecular motion at the solid-nematic interface. Although the interfacial diffusion was dramatically slower than self-diffusion in the nematic, the diffusion anisotropy was the same at the interface and in bulk, supporting the desorption-mediated mechanism of interfacial diffusion, where molecules sample the physical properties of the vicinal fluid phase during flights, and the magnitude of the interfacial diffusion coefficient is primarily determined by the distribution of waiting times between flights.

Capturing Conformation-Dependent Molecule-Surface Interactions When Surface Chemistry Is Heterogeneous

Molecular building blocks, such as carbon nanotubes and DNA origami, can be fully integrated into electronic and optical devices if they can be assembled on solid surfaces using biomolecular interactions. However, the conformation and functionality of biomolecules depend strongly on the local chemical environment, which is highly heterogeneous near a surface. To help realize the potential of biomolecular self-assembly, we introduce here a technique to spatially map molecular conformations and adsorption, based on single-molecule fluorescence microscopy. On a deliberately patterned surface, with regions of varying hydrophobicity, we characterized the conformations of adsorbed helicogenic alanine-lysine copeptides using Förster resonance energy transfer. The peptides adopted helical conformations on hydrophilic regions of the surface more often than on hydrophobic regions, consistent with previous ensemble-averaged observations of α-helix surface stability. Interestingly, this dependence on surface chemistry was not due to surface-induced unfolding, as the apparent folding and unfolding dynamics were usually much slower than desorption. The most significant effect of surface chemistry was on the adsorption rate of molecules as a function of their initial conformational state. In particular, regions with higher adsorption rates attracted more molecules in compact, disordered coil states, and this difference in adsorption rates dominated the average conformation of the ensemble. The correlation between adsorption rate and average conformation was also observed on nominally uniform surfaces. Spatial variations in the functional state of adsorbed molecules would strongly affect the success rates of surface-based molecular assembly and can be fully understood using the approach developed in this work.

Electrostatic Interactions Influence Protein Adsorption (but Not Desorption) at the Silica-Aqueous Interface

High-throughput single-molecule total internal reflection fluorescence microscopy was used to investigate the effects of pH and ionic strength on bovine serum albumin (BSA) adsorption, desorption, and interfacial diffusion at the aqueous-fused silica interface. At high pH and low ionic strength, negatively charged BSA adsorbed slowly to the negatively charged fused silica surface. At low pH and low ionic strength, where BSA was positively charged, or in solutions at higher ionic strength, adsorption was approximately 1000 times faster. Interestingly, neither surface residence times nor the interfacial diffusion coefficients of BSA were influenced by pH or ionic strength. These findings suggested that adsorption kinetics were dominated by energy barriers associated with electrostatic interactions, but once adsorbed, protein-surface interactions were dominated by short-range nonelectrostatic interactions. These results highlight the ability of single-molecule techniques to isolate elementary processes (e.g., adsorption and desorption) under steady-state conditions, which would be impossible to measure using ensemble-averaging methods.

Unbiased Clustering of Molecular Dynamics for Spatially Resolved Analysis of Chemically Heterogeneous Surfaces

A technique is described for resolving and interpreting molecular interactions with a chemically heterogeneous surface. Using total internal reflection fluorescence microscopy, dynamic single molecule trajectories were accumulated simultaneously for fluorescently labeled fatty acid (interacting primarily via hydrophobic interactions) and dextran (interacting via hydrogen-bonding interactions) probe molecules at the interface between an aqueous solvent and a photopatterned solid surface with distinct regions of amine-terminated and poly(ethylene glycol) self-assembled monolayers. Using dynamic properties of the probe molecules (adsorption rate, surface diffusion coefficient, residence time), an unsupervised Gaussian mixture model algorithm was used to identify areas of the surface that were chemically related to each other, and the dynamic behaviors of the probe molecules were studied statistically on these distinct regions. The dynamic data were compared to data from homogeneous surfaces of known chemistry to provide a chemical identification of each location on the surface. Spatial maps were also constructed, allowing for spatial visualization of surface chemistry on a hydrophilic substrate. This work enables the direct study of interactions between single-molecule probes and distinct surface chemistries, even in the presence of spatial heterogeneity, without human bias, assumptions about surface structure, or model-dependent analysis.

Molecular trajectories provide signatures of protein clustering and crowding at the oil/water interface

Using high throughput single-molecule total internal reflection fluorescence microscopy (TIRFM), we have acquired molecular trajectories of bovine serum albumin (BSA) and hen egg white lysozyme during protein layer formation at the silicone oil-water interface. These trajectories were analyzed to determine the distribution of molecular diffusion coefficients, and for signatures of molecular crowding/caging, including subdiffusive motion and temporal anticorrelation of the instantaneous velocity vector. The evolution of these properties with aging time of the interface was compared with dynamic interfacial tension measurements. For both lysozyme and BSA, we observed an overall slowing of protein objects, the onset of both subdiffusive and anticorrelated motion (associated with crowding), and a decrease in the interfacial tension with aging time. For lysozyme, all of these phenomena occurred virtually simultaneously, consistent with a homogeneous model of layer formation that involves gradual crowding of weakly interacting proteins. For BSA, however, the slowing occurred first, followed by the signatures of crowding/caging, followed by a decrease in interfacial tension, consistent with a heterogeneous model of layer formation involving the formation of protein clusters. The application of microrheological methods to single molecule trajectories described here provides an unprecedented level of mechanistic interpretation of interfacial events that occurred over a wide range of interfacial protein coverage.

Single-molecule insights into retention at a reversed-phase chromatographic interface

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.

Mechanisms of surface-mediated DNA hybridization

Single-molecule total internal reflection fluorescence microscopy was employed in conjunction with resonance energy transfer (RET) to observe the dynamic behavior of donor-labeled ssDNA at the interface between aqueous solution and a solid surface decorated with complementary acceptor-labeled ssDNA. At least 100,000 molecular trajectories were determined for both complementary strands and negative control ssDNA. RET was used to identify trajectory segments corresponding to the hybridized state. The vast majority of molecules from solution adsorbed nonspecifically to the surface, where a brief two-dimensional search was performed with a 7% chance of hybridization. Successful hybridization events occurred with a characteristic search time of ∼0.1 s, and unsuccessful searches resulted in desorption from the surface, ultimately repeating the adsorption and search process. Hybridization was reversible, and two distinct modes of melting (i.e., dehybridization) were observed, corresponding to long-lived (∼15 s) and short-lived (∼1.4 s) hybridized time intervals. A strand that melted back onto the surface could rehybridize after a brief search or desorb from the interface. These mechanistic observations provide guidance for technologies that involve DNA interactions in the near-surface region, suggesting a need to design surfaces that both enhance the complex multidimensional search process and stabilize the hybridized state.

A bottom-up approach to understanding protein layer formation at solid-liquid interfaces

A common goal across different fields (e.g. separations, biosensors, biomaterials, pharmaceuticals) is to understand how protein behavior at solid-liquid interfaces is affected by environmental conditions. Temperature, pH, ionic strength, and the chemical and physical properties of the solid surface, among many factors, can control microscopic protein dynamics (e.g. adsorption, desorption, diffusion, aggregation) that contribute to macroscopic properties like time-dependent total protein surface coverage and protein structure. These relationships are typically studied through a top-down approach in which macroscopic observations are explained using analytical models that are based upon reasonable, but not universally true, simplifying assumptions about microscopic protein dynamics. Conclusions connecting microscopic dynamics to environmental factors can be heavily biased by potentially incorrect assumptions. In contrast, more complicated models avoid several of the common assumptions but require many parameters that have overlapping effects on predictions of macroscopic, average protein properties. Consequently, these models are poorly suited for the top-down approach. Because the sophistication incorporated into these models may ultimately prove essential to understanding interfacial protein behavior, this article proposes a bottom-up approach in which direct observations of microscopic protein dynamics specify parameters in complicated models, which then generate macroscopic predictions to compare with experiment. In this framework, single-molecule tracking has proven capable of making direct measurements of microscopic protein dynamics, but must be complemented by modeling to combine and extrapolate many independent microscopic observations to the macro-scale. The bottom-up approach is expected to better connect environmental factors to macroscopic protein behavior, thereby guiding rational choices that promote desirable protein behaviors.

Single-molecule diffusion in a periodic potential at a solid-liquid interface

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.

Interfacial protein-protein associations

While traditional models of protein adsorption focus primarily on direct protein-surface interactions, recent findings suggest that protein-protein interactions may play a central role. Using high-throughput intermolecular resonance energy transfer (RET) tracking, we directly observed dynamic, protein-protein associations of bovine serum albumin on polyethylene glycol modified surfaces. The associations were heterogeneous and reversible, and associating molecules resided on the surface for longer times. The appearance of three distinct RET states suggested a spatially heterogeneous surface - with areas of high protein density (i.e., strongly interacting clusters) coexisting with mobile monomers. Distinct association states exhibited characteristic behavior, i.e., partial-RET (monomer-monomer) associations were shorter-lived than complete-RET (protein-cluster) associations. While the fractional surface area covered by regions with high protein density (i.e., clusters) increased with increasing concentration, the distribution of contact times between monomers and clusters was independent of solution concentration, suggesting that associations were a local phenomenon, and independent of the global surface coverage.

Intermittent molecular hopping at the solid-liquid interface

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.

A mesoporous silica nanocomposite shuttle: pH-triggered phase transfer between oil and water

With a simple protocol, we synthesize a novel mesoporous silica nanocomposite shuttle that can reversibly transfer between an organic phase and water in response to the pH, due to the switchable surface hydrophobicity/hydrophilicity. Our synthesis protocol allows the phase transfer ability to be tuned in a controllable fashion. This nanocomposite shuttle exhibits a good ability to load various cargoes such as Pd(OAc)2, Pd nanoparticles, and organic molecules. The built-in properties of the nanocomposite shuttle lay the foundations for many innovative applications. As a proof of concept, we successfully demonstrate its application in separating and recycling Pd nanoparticle catalysts. The composite shuttle can take Pd nanoparticles to an organic phase for catalyzing hydrogenation of olefins and come back to an aqueous phase at the end of reaction, making in situ separation and recycling of nanocatalysts possible. This pH-driven round trip for catalysis can be repeated several times. Our investigations not only supply a novel nanocomposite shuttle with controllable properties but also open an innovative avenue to in situ separation and recycling of nanocatalysts, which can address the obstacles of the conventional methods such as centrifugation and filtration.

Enhanced transport through desorption-mediated diffusion

We present a master equation approach to the study of the bulk-mediated surface diffusion mechanism in a three-dimensional confined domain. The proposed scheme allowed us to evaluate analytically a number of magnitudes that were used to characterize the efficiency of the bulk-mediated surface transport mechanism, for instance, the mean escape time from the domain, and the mean number of distinct visited sites on the confined domain boundary.

Effects of molecular size and surface hydrophobicity on oligonucleotide interfacial dynamics

Single-molecule total internal reflection fluorescence microscopy was used to observe the dynamic behavior of polycytosine single-stranded DNA (ssDNA) (1-50 nucleotides long) at the interface between aqueous solution and hydrophilic (oligoethylene glycol-modified fused silica, OEG) and hydrophobic (octadecyltriethoxysilane-modified fused silica, OTES) solid surfaces. High throughput molecular tracking was used to determine >75,000 molecular trajectories for each molecular length, which were then used to calculate surface residence time and squared displacement (i.e., "step-size") distributions. On hydrophilic OEG surfaces, the surface residence time increased systematically with ssDNA chain length, as expected due to increasing molecule-surface interactions. Interestingly, the residence time decreased with increasing ssDNA length on the hydrophobic OTES surface, particularly for longer chains. Similarly, the interfacial mobility of polynucleotides slowed with increasing chain length on OEG, but became faster on OTES. On OTES surfaces, the rates associated with desorption and surface diffusion exhibited the distinctive anomalous temperature dependence that is characteristic of hydrophobic interactions for short-chain species but not for longer chains. These combined observations suggest that long oligonucleotides adopt conformations minimizing hydrophobic interactions, e.g., by internal sequestration of hydrophobic nucleobases.

Identifying mechanisms of interfacial dynamics using single-molecule tracking

The "soft" (i.e., noncovalent) interactions between molecules and surfaces are complex and highly varied (e.g., hydrophobic, hydrogen bonding, and ionic), often leading to heterogeneous interfacial behavior. Heterogeneity can arise either from the spatial variation of the surface/interface itself or from molecular configurations (i.e., conformation, orientation, aggregation state, etc.). By observing the adsorption, diffusion, and desorption of individual fluorescent molecules, single-molecule tracking can characterize these types of heterogeneous interfacial behavior in ways that are inaccessible to traditional ensemble-averaged methods. Moreover, the fluorescence intensity or emission wavelength (in resonance energy transfer experiments) can be used to track the molecular configuration and simultaneously directly relate this to the resulting interfacial mobility or affinity. In this feature article, we review recent advances involving the use of single-molecule tracking to characterize heterogeneous molecule-surface interactions including multiple modes of diffusion and desorption associated with both internal and external molecular configuration, Arrhenius-activated interfacial transport, spatially dependent interactions, and many more.

Single molecule dynamics on hydrophobic self-assembled monolayers

The interactions between adsorbate molecules and hydrophobic surfaces are of significant interest due to their importance in a variety of biological and separation processes. However, it is challenging to extrapolate macroscopic ensemble-averaged force measurements to molecular-level phenomena. Using total internal reflection fluorescence microscopy to image individual molecules at hydrophobic solid-aqueous interfaces, we directly observed dynamic behavior associated with the interactions between fluorescently labeled dodecanoic acid (our probe molecules) and self-assembled monolayers (SAM) comprising n-alkyltriethoxysilanes with systematically increasing chain length (from n = 4-18). In all cases, we observed at least two characteristic surface residence times and two diffusive modes, suggesting the presence of multiple distinct adsorbed populations. In general, the mean surface residence time increased and the mobility decreased with increasing SAM chain length, consistent with stronger probe-surface interactions. However, these trends were not primarily due to changes in characteristic residence times or diffusion coefficients associated with the individual populations but rather to a dramatic increase in the fraction associated with the long-lived slow-moving population(s) on long-chain SAMs. In particular, on longer (16-18 carbon) alkylsilane monolayers, the probe molecule exhibited far fewer desorption-mediated "flights" than on short (4-6 carbon) monolayers. Additionally, probes on the longer chain surfaces were much more likely to exhibit extended surface residence times as opposed to short transient surface visits.

Apparent activation energies associated with protein dynamics on hydrophobic and hydrophilic surfaces

With the use of single-molecule total internal reflection fluorescence microscopy (TIRFM), the dynamics of bovine serum albumin (BSA) and human fibrinogen (Fg) at low concentrations were observed at the solid-aqueous interface as a function of temperature on hydrophobic trimethylsilane (TMS) and hydrophilic fused silica (FS) surfaces. Multiple dynamic modes and populations were observed and characterized by their surface residence times and squared-displacement distributions (surface diffusion). Characteristic desorption and diffusion rates for each population/mode were generally found to increase with temperature, and apparent activation energies were determined from Arrhenius analyses. The apparent activation energies of desorption and diffusion were typically higher on FS than on TMS surfaces, suggesting that protein desorption and mobility were hindered on hydrophilic surfaces due to favorable protein-surface and solvent-surface interactions. The diffusion of BSA on TMS appeared to be activationless for several populations, whereas diffusion on FS always exhibited an apparent activation energy. All activation energies were small in absolute terms (generally only a few kBT), suggesting that most adsorbed protein molecules are weakly bound and move and desorb readily under ambient conditions.

Connecting rare DNA conformations and surface dynamics using single-molecule resonance energy transfer

A mechanistic understanding of single-stranded DNA (ssDNA) behavior in the near-surface environment is critical to advancing DNA-directed self-assembled nanomaterials. A new approach is described that uses total internal reflection fluorescence microscopy to measure resonance energy transfer at the single-molecule level, providing a mechanistic understanding of the connection between molecular conformation and interfacial dynamics near amine-modified surfaces. Large numbers (>10(5)) of ssDNA trajectories were observed, permitting dynamic correlation of molecular conformation with desorption and surface mobility. On the basis of dynamic behavior, molecules could be designated as members of the more common coiled population or a rare, weakly bound conformation. Molecules in the coiled state generally exhibited slow diffusion and conformational fluctuations that decreased with increasing average end-to-end distance. Lattice simulations of adsorbed self-avoiding polymers successfully predicted these trends. In contrast, the weakly bound conformation, observed in about 5% of molecules, had a large end-to-end distance but demonstrated conformational fluctuations that were much higher than predicted by simulations for adsorbed flexible chains. This conformation correlated positively with desorption events and led to fast diffusion, indicating weak surface associations. Understanding the role of the weakly bound conformation in DNA hybridization, and how solution conditions and surface properties may favor it, could lead to improved self-assembled nanomaterials.

Super-resolution surface mapping using the trajectories of molecular probes

The surface characterization of 'soft' materials presents a significant scientific challenge, particularly under 'wet' in situ conditions where a wide variety of non-covalent interactions may be relevant. Here we introduce a new chemical imaging method, MAPT (mapping using accumulated probe trajectories) that generates images of surface interactions by distributing different aspects of molecular probe trajectories into distinct locations and then combining many trajectories to generate spatial maps. The maps are super-resolution in nature, because they are accumulated from highly localized single-molecule observations. Unlike other super-resolution techniques, which report only photon or point counts, our analysis generates spatial maps of physical quantities (adsorption rate, desorption probability, local surface diffusion coefficient, surface coverage/occupancy) that are directly associated with the molecular interactions between the probe molecule and the surface. We demonstrate the feasibility of this characterization using a surface patterned with various degrees of hydrophobicity.

Single molecule observations of desorption-mediated diffusion at the solid-liquid interface

By directly observing molecular trajectories on a chemically heterogeneous surface, we have identified two distinct modes of diffusion involving (1) displacements within isolated surface islands (crawling mode), and (2) displacements where a molecule desorbs from an island, diffuses through the adjacent liquid phase, and readsorbs on another island (flying mode). The diffusion coefficients corresponding to these two modes differ by an order of magnitude, and both modes are also observed on chemically homogeneous surfaces. Comparison with previous results suggested that desorption-mediated diffusion is the primary transport mechanism in self-assembled monolayer formation.

Single-molecule resolution of interfacial fibrinogen behavior: effects of oligomer populations and surface chemistry

Through the use of single-molecule total internal reflection fluorescence microscopy, the dynamic behavior of fibrinogen was observed at the interface between aqueous solution and various solid surfaces. Multiple populations of objects were observed, as characterized by surface residence times, interfacial diffusion, and fluorescence intensity. On all surfaces, populations exhibited direct links between surface residence time, rate of diffusion, and fluorescence intensity. In particular, longer-lived populations diffused more slowly and exhibited greater fluorescence intensity, leading to the conclusion that the objects represented fibrinogen monomers and discrete oligomer populations (dimers, trimers, etc.), and that these oligomer populations play an important role in the protein-surface interaction because of their long surface residence times. Two or three diffusive modes were observed for most populations, indicating that protein aggregates have multiple mechanisms for interaction with solid substrates. In addition, the fastest diffusive mode is believed to represent a hopping mode that often precedes desorption events. Surprisingly, a monolayer of 5000 Da poly(ethylene glycol) (PEG5000) increased surface residence time and slowed diffusion of fibrinogen relative to bare fused silica or hydrophobically modified fused silica, suggesting that the mechanism of PEG resistance to protein adhesion is more sophisticated than the simple repulsion of individual proteins.

Kinetic measurements on single-molecule disulfide bond cleavage

We use single-molecule force clamp spectroscopy (SMFCS) to explore the reactivity of tris(2-carboxyethyl)phosphine (TCEP), 1, 4-dl-dithiothreitol (DTT) and hydrosulfide anion (HS(-)) on disulfide bonds within a mechanically stretched polypeptide. The single-bond level bimolecular nucleophilic substitution (S(N)2) events are recorded at a series of precisely controlled temperatures so that the Arrhenius kinetic parameters, that is, the height of the activation energy barrier (E(a)) and the attempting frequency (A) of the chemical reactions, can be determined. The values of A are typically at the order of 10(7) M(-1) s(-1), which is far lower than that predicted by the transition-state theory, in which A is given by k(B)T/h and around 10(12) M(-1) s(-1) at room temperature. Furthermore, E(a) is derived to be 30-40 kJ/mol, which can be lowered by ∼6-8% with every 100 pN mechanical force applied. The correlation of the A and E(a) with the molecular structures reveals that the relative magnitude of these two parameters cannot be simply judged from the size of the molecule or the nucleophilicity of the attacking atom. The comparison of the influences on the reaction rate induced by force and temperature indicates an equivalent accelerating effect by every 50 pN or 10 K increment, giving for the first time the relationship between mechanical and thermal effects on a single-molecule S(N)2 chemical reaction.

Forced and spontaneous imbibition of surfactant solution into an oil-wet capillary: the effects of surfactant diffusion ahead of the advancing meniscus

A previous paper (Hammond, P.; Unsal, E. Langmuir 2009, 25, 12591-12603) reported a simplified model for the flow of a surfactant solution into an oil-wet capillary. Results were computed by neglecting the spreading of surfactant molecules ahead of the moving oil/water meniscus onto the hydrophobic surface. We now present a more thorough version of the theory where such spreading is considered. Both spontaneous and forced imbibitions are studied. As the differential pressure across the capillary increases, a slow increase in the meniscus velocity is observed until the capillary threshold pressure is reached. At this point, the pattern changes and the velocity increases dramatically. The surfactant concentration did not have a significant effect on the speed under differential pressures greater than the capillary threshold. For lower pressures, there is a critical surfactant concentration below which the interface was not able to advance into the capillary even under positive differential pressure.

Directed nanoparticle motion on an interfacial free energy gradient

Using total internal reflection fluorescence microscopy (TIRFM), we have observed the directed motion of 20 nm probe particles on specific regions of surfaces that exhibited strong gradients of hydrophobicity. Patterned surfaces were prepared by selective photodegradation (using a contact photomask) of a hydrophobically modified fused silica surface. The lateral distribution of hydrophobicity was characterized in situ using the selective affinity of amphiphilic probes (i.e., hydrophobic interaction microscopy). Probe particles were observed to move unidirectionally from regions of lower to higher to hydrophobicity over distances of approximately 1 microm when the hydrophobicity gradient was greater than d(cos theta)/dx = 0.05 +/- 0.02 microm(-1), where theta is the water contact angle on the bare surface. Only adsorption events were observed on energetically homogeneous surface regions.

Role of vapor-phase mass transport during the spreading of a long-chain alkane drop

The spreading of liquid alkanes over solid surfaces has important applications in painting, coatings, lubrication, and petroleum tertiary recovery. The role of the vapor-phase mass transport accompanying liquid spreading has not been well studied because it is difficult to separate the contributions from the liquid spreading and the vapor-phase transport that occurred at the same time. We used the engineered surface patterns to study the vapor-phase mass transport during liquid spreading. First, we fabricated several hydrophilic, carboxylic acid-terminated patterns (OTSpd) on a hydrophobic, methyl-terminated octadecyltrichlorosilane (OTS) surface. These OTSpd patterns did not connect to each other. Next, we let an alkane drop spread within one OTSpd pattern. The liquid alkane could not spread to other OTSpd patterns because OTS separated them; however, the alkane molecules in the vapor phase could migrate and adsorb on other OTSpd patterns. Therefore, the contributions from the liquid spreading and the vapor-phase transport were separated and could be investigated independently. We found that during the spreading of the liquid alkane, mass transport through the vapor phase cannot be ignored. Alkane molecules adsorbed on the OTSpd surface with their backbones parallel to the surface in the first few layers. Additional alkane molecules adsorbed on these parallel layers to form the seaweed-shaped layers in which the alkane molecules stood up. Our study showed that the parallel layers formed from the vapor-phase mass transport before the liquid alkane spread. Therefore, the liquid alkane does not spread over the more strongly binding OTSpd surface. It actually spreads over the parallel alkane layer, which formed from its own vapor.

Solvent dependence of the activation energy of attachment determined by single molecule observations of surfactant adsorption

Single-molecule total internal reflection fluorescence microscopy was used to obtain real-time images of fluorescently labeled hexadecanoic (palmitic) acid molecules as they adsorbed at the interface between fused silica and three different solvents: hexadecane (HD), tetrahydrofuran (THF), and water. These solvents were chosen to explore the effect of solvent polarity on the activation energy associated with the attachment rate, i.e., the rate at which molecules were transferred to the surface from the near-surface layer. Direct counting of single-molecule events, made under steady-state conditions at extremely low coverage, provided direct, model-independent measurements of this attachment rate, in contrast with conventional ensemble-averaged methods, which are influenced by bulk transport and competing detachment processes. We found that the attachment rate increased with increasing temperature for all solvents. Arrhenius analyses gave activation energies of 5+/-2 kJ/mol for adsorption from HD, 10+/-2 kJ/mol for adsorption from THF, and 19+/-2 kJ/mol for adsorption from water. These energies increased systematically with the solvent polarity and, therefore, with the expected strength of the solvent-substrate interaction. We hypothesize that the adsorption of amphiphilic solute molecules from solution can be regarded as a competitive exchange between solute molecules and surface-bound solvent. In this scenario, adsorption is an activated process, and the activation energy for attachment is associated with the solvent-substrate interaction energy.

Probing hydrophobic interactions using trajectories of amphiphilic molecules at a hydrophobic/water interface

Individual molecules of fluorophore-labeled alkanoic acids with various chain lengths, BODIPY-(CH(2))(n)-COOH (abbreviated as fl-Cn), were observed to adsorb and move at the methylated fused silica-water interface as a function of temperature using total internal reflection fluorescence microscopy. The statistical analysis of squared-displacement distributions indicated that the molecular trajectories were consistent with a diffusive model involving two intertwined modes. The slower mode, typically responsible for <50% of the molecular diffusion time, had a diffusion coefficient of <0.005 mum(2)/s and could not be distinguished from the apparent motions of immobilized molecules because of the limitations of experimental resolution. The faster mode exhibited diffusion coefficients that increased with temperature for all chain lengths, permitting an Arrhenius analysis. Both the effective activation energies and kinetic prefactors associated with the fast-mode diffusion coefficients increased systematically with chain length for fl-C2 through fl-C10; however, fl-C15 did not follow this trend but instead exhibited anomalously small values of both parameters. These observations were considered in the context of hydrophobic interactions between the adsorbate molecules and the methylated surface in the presence of water. Specifically, it was hypothesized that fl-C2, fl-C4, and fl-C10 adopted primarily extended molecular conformations on the hydrophobic surface. The increases in activation energy and entropy with chain length for these molecules are consistent with a picture of the transition state in which the molecule partially detaches from the surface and exhibits greater conformational freedom. In contrast, the small activation energy and entropy for fl-C15 are consistent with a scenario in which the surface-bound molecule adopts a compact/globular conformation with limited surface contact and conformational flexibility.

Hydrophobic interaction microscopy: mapping the solid/ liquid interface using amphiphilic probe molecules

The adsorption, interfacial mobility, and desorption of amphiphilic molecules are extremely sensitive to the chemical properties of the interface at which they adsorb; we demonstrate here that this sensitivity can be used to map subtle spatial variations of surface hydrophobicity. We have used total internal reflection fluorescence microscopy to observe the dynamic behavior of individual fluorescently labeled fatty acid molecules at the interface between water and a hydrophobically modified fused silica surface. Patterned surfaces were prepared by photodegradation of trimethylsilane-modified surfaces using a contact photomask; the degree of hydrophobic contrast was varied by controlling the dose of ultraviolet radiation. Cumulative images of single-molecule fluorescence, integrated over various exposure times, exhibited structural features consistent with the photopattern, and the fluorescence contrast was systematically related to the hydrophobic contrast. Lateral force microscopy was also used to characterize the patterned surfaces and provided qualitative images when the hydrophobic contrast was relatively high. However, the fluorescent probes provided more sensitive, reproducible, and reliable images of the lateral hydrophobic variations.