Complex Salt Dependence of Polymer Diffusion in Polyelectrolyte Multilayers

Extreme dependence of dynamics on concentration in highly crowded polyelectrolyte solutions

Charge-carrying species, such as polyelectrolytes, are vital to natural and synthetic processes that rely on their dynamic behavior. Through single-particle tracking techniques, the diffusivity of individual polyelectrolyte chains and overall system viscosity are determined for concentrated polylysine solutions. These studies show scaling dependences of D ~ c-6.1 and η ~ c7.2, much stronger than theoretical predictions, drawing the applicability of power law fits into question. Similar trends are observed in concentrated solutions prepared at various pH and counterion conditions. These hindered system dynamics appear universal to polyelectrolyte systems and are attributed to the large effective excluded volumes of polyelectrolyte chains inducing glassy dynamics. The framework of the Vrentas-Duda free-volume theory is used to compare polyelectrolyte and neutral systems. Supported by this theory, excluding counterion mass from total polymer mass results in all environmental conditions collapsing onto a common trendline. These results are applicable to crowded biological systems, such as intracellular environments where protein mobility is strongly inhibited.

Equilibration of precipitants in a counter-diffusion apparatus for protein crystallization

A cost-effective capillary dialysis apparatus (Toledo Capillary Box, TCB) developed for biomacromolecule crystal growth in microgravity and unit gravity environments can provide slow equilibration between the precipitant reservoir and capillary solutions, nurturing growth of neutron-diffraction-quality crystals. Under microgravity conditions, mass transfer of precipitants and biomacro-mol-ecules occurs under diffusion-controlled conditions, promoting slow growth and suppressing defect formation. The equilibration of common precipitants (polyethyl-ene glycol and salts such as ammonium sulfate) between capillary and reservoir solutions was measured for capillaries oriented horizontally or vertically with respect to the gravitational field at unit gravity. Precipitants equilibrated less rapidly in the vertical orientation when capillary solution densities were lower than those of the reservoir solutions. A plug filled with agarose gel was introduced in the TCB apparatus for salt precipitants since salts often exhibit relatively high free diffusion. Equilibration of the capillaries with reservoir solutions was significantly delayed for many of the salt precipitants tested. Analytical and semi-analytical models allow the prediction of precipitant equilibration of capillary and reservoir solutions under diffusion-controlled transport and show good agreement with experimental results.

A Novel Family of Stable Polyelectrolyte Complexes Based on Mixed Olefins-Maleic Anhydride Copolymer

In the present study, the copolymer of mixed olefins included in unetherified gasoline and maleic anhydride (PUGM) was prepared by self-stabilized precipitation polymerization (2SP) and employed for the synthesis of a new family of stable polyelectrolyte complexes (PECs). Polyanionic saponified PUGM partially grafted with methoxy poly(ethylene glycol) (PUGMS-g-mPEG) and polycationic quaternized PUGM (PUGMQ) were both derived from PUGM via the facile modification of anhydride groups. The particle size, zeta potential, morphology, and stability of self-assembled PEC particles were investigated thoroughly. Strikingly, the introduction of long mPEG side chains (M_n = 4000) had a remarkable effect on the self-assembled particles, which displayed a constant particle size of ∼200 nm regardless of varying n+/n-. Moreover, it also enhanced the salt tolerance and long-term stability of PEC particles significantly. Our work not only provides an effective approach to PECs from petroleum resources with low cost but also deepens the understanding of the relationship between the chain structure of polyelectrolytes and the stability of PECs.

Predicting the outcomes of interpolyelectrolyte neutralization at surfaces on the basis of complexation experiments and vice versa

This study was carried out with the aim of establishing how the outcomes of polyelectrolyte multilayer formation can be predicted on the basis of the results of complexation studies in solution and vice versa. For this purpose, the correlation between the processes of complex and multilayer formation involving three pairs of vinylic polyions in solutions of binary 1 : 1 sodium salts (NaX; X = F, Cl, Br, I, NO₃, ClO₄) was explored by means of dynamic and electrophoretic light scattering, potentiometry, microcalorimetry, spectrophotometry and quartz crystal microbalance. The gradual reactant mixing in solution at lower salt concentrations resulted in a Fuoss-Sadek sequence of events (primary complexes → secondary complexes → 1 : 1 flocculate), whereby the obtained nano-complexes could be successively overcharged. At high salt concentration and with excess polycation present, metastable nano-complexes and precipitates containing surplus of positively charged monomers were formed. The amount of extrinsically compensated charge was in accord with the polycation affinities toward counteranions, established by monitoring the electrolyte-induced aggregation of positively charged nano-complexes. Perfect analogy with respect to counteranion influence on the amount of adsorbed polycation was noticed for corresponding multilayers. Aside from providing a deeper understanding of interpolyelectrolyte neutralization, the gained insights can also be used to steer the polyelectrolyte multilayer composition and properties.

Simulating stochastic adsorption of diluted solute molecules at interfaces

This report uses Monte Carlo simulations to connect stochastic single-molecule and ensemble surface adsorption of molecules from dilute solutions. Monte Carlo simulations often use a fundamental time resolution to simulate each discrete step for each molecule. The adsorption rate obtained from such a simulation surprisingly contains an error compared to the results obtained from the traditional method. Simulating adsorption kinetics is interesting in many processes, such as mass transportation within cells, the kinetics of drug-receptor interactions, membrane filtration, and other general reaction kinetics in diluted solutions. Thus, it is important to understand the origin of the disagreement and find a way to correct the results. This report reviews the traditional model, explains the single-molecule simulations, and introduces a method to correct the results of adsorption rate. For example, one can bin finer time steps into time steps of interest to simulate the fractal diffusion or simply introduce a correction factor for the simulations. Then two model systems, self-assembled monolayer (SAM) and biosensing on the patterned surface, are simulated to check the accuracy of the equations. It is found that the adsorption rate of SAM is highly dependent on the conditions and the uncertainty is large. However, the biosensing system is relatively accurate. This is because the concentration gradient near the interface varies significantly with reaction conditions for SAMs while relatively stable for the biosensing system.

Single molecule characterization of anomalous transport in a thin, anisotropic film

The diffusion of small, charged molecules incorporated in an anisotropic polyelectrolyte multilayer (PEM) was tracked in three dimensions by combining single-molecule fluorescence localization (to characterize lateral diffusion) with Förster resonance energy transfer (FRET) between diffusing molecules and the supporting surface (to measure diffusion in the surface-normal direction). Analysis of the surface-normal diffusion required model-based statistical analysis to account for the inherently noisy FRET signal. Combining these distinct single-molecule methods, which are inherently sensitive to different length-scales, permitted simultaneous characterization of severely anisotropic diffusion, which was more than three orders of magnitude slower in the surface-normal direction. We hypothesize that the anomalously slow surface-normal diffusion was related to the periodic distribution of charge in the PEM, which created electrostatic barriers. The motion was strongly subdiffusive, with anomalous temporal scaling exponents in lateral and normal directions, suggesting a connection to the transient, random fractal conformation of polymer chains in the film's matrix.

Single-Molecule Observations Provide Mechanistic Insights into Bimolecular Knoevenagel Amino Catalysis

While single-molecule (SM) methods have provided new insights to various catalytic processes, bimolecular reactions have been particularly challenging to study. Here, the fluorogenic Knoevenagel condensation of an aromatic aldehyde with methyl cyanoacetate promoted by surface-immobilized piperazine is quantitatively characterized using super-resolution fluorescence imaging and stochastic analysis using hidden Markov modeling (HMM). Notably, the SM results suggest that the reaction follows the iminium intermediate pathway before the formation of a fluorescent product with intramolecular charge-transfer character. Moreover, the overall process is limited by the turnover rate of the catalyst, which is involved in multiple steps along the reaction coordinate.

Nanoparticle Tracking to Probe Transport in Porous Media

From the granular and fractured subsurface environment to highly engineered polymer membranes used in pharmaceutical purification, porous materials are ubiquitous in nature and industrial applications. In particular, porous media are used extensively in processes including water treatment, pharmaceutical sterilization, food/beverage processing, and heterogeneous catalysis, where hindered mass transport is either essential to the process or a necessary but undesirable limitation. Unfortunately, there are currently no universal models capable of predicting mass transport based on a description of the porous material because real porous materials are complex and because many coupled dynamic mechanisms (e.g., adsorption, steric effects, hydrodynamic effects, electrostatic interactions, etc.) give rise to the observed macroscopic transport phenomena.While classical techniques, like nuclear magnetic resonance and dynamic light scattering, provide useful information about mass transport in porous media at the ensemble level, they provide limited insight into the microscopic mechanisms that give rise to complex phenomena such as anomalous diffusion, hindered pore-space accessibility, and unexpected retention under flow, among many others. To address this issue, we have developed refractive index matching imaging systems, combined with single-particle tracking methods, allowing the direct visualization of single-particle motion within a variety of porous materials.In this Account, we summarize our recent efforts to advance the understanding of nanoparticle transport in porous media using single-particle tracking methods in both fundamental and applied scenarios. First, we describe the basic principles for two-dimensional and three-dimensional single-particle tracking in porous materials. Then, we provide concrete examples of nanoparticle transport in porous materials from two perspectives: (1) understanding fundamental elementary particle transport processes in porous media, including pore accessibility and cavity escape, which limit transport in porous media, and (2) facilitating applications in industrial processes, e.g., by understanding the mechanisms of particle fouling and remobilization in filtration membranes. Finally, we provide an outlook of opportunities associated with investigating other types of mass transport in confined environments using single-particle tracking methods, including electrophoretic and self-propelled motion.

Cadherin clusters stabilized by a combination of specific and nonspecific cis-interactions

We demonstrate a combined experimental and computational approach for the quantitative characterization of lateral interactions between membrane-associated proteins. In particular, weak, lateral (cis) interactions between E-cadherin extracellular domains tethered to supported lipid bilayers, were studied using a combination of dynamic single-molecule Förster Resonance Energy Transfer (FRET) and kinetic Monte Carlo (kMC) simulations. Cadherins are intercellular adhesion proteins that assemble into clusters at cell-cell contacts through cis- and trans- (adhesive) interactions. A detailed and quantitative understanding of cis-clustering has been hindered by a lack of experimental approaches capable of detecting and quantifying lateral interactions between proteins on membranes. Here single-molecule intermolecular FRET measurements of wild-type E-cadherin and cis-interaction mutants combined with simulations demonstrate that both nonspecific and specific cis-interactions contribute to lateral clustering on lipid bilayers. Moreover, the intermolecular binding and dissociation rate constants are quantitatively and independently determined, demonstrating an approach that is generalizable for other interacting proteins.

A closer physico-chemical look to the Layer-by-Layer electrostatic self-assembly of polyelectrolyte multilayers

The fabrication of polyelectrolyte multilayer films (PEMs) using the Layer-by-Layer (LbL) method is one of the most versatile approaches for manufacturing functional surfaces. This is the result of the possibility to control the assembly process of the LbL films almost at will, by changing the nature of the assembled materials (building blocks), the assembly conditions (pH, ionic strength, temperature, etc.) or even by changing some other operational parameters which may impact in the structure and physico-chemical properties of the obtained multi-layered films. Therefore, the understanding of the impact of the above mentioned parameters on the assembly process of LbL materials plays a critical role in the potential use of the LbL method for the fabrication of new functional materials with technological interest. This review tries to provide a broad physico-chemical perspective to the study of the fabrication process of PEMs by the LbL method, which allows one to take advantage of the many possibilities offered for this approach on the fabrication of new functional nanomaterials.

Polyelectrolyte Multilayers Enhance the Dry Storage and pH Stability of Physically Entrapped Enzymes

Polyelectrolyte multilayers (PEMs) are attractive materials for immobilizing enzymes due to their unique ionic environment, which can prevent unfolding. Here, we demonstrated that the stability to dry storage and elevated pH were significantly enhanced when negatively charged nitroreductase (NfsB) was embedded in a PEM by depositing alternating layers of the enzyme and polycation (PC) onto porous silica particles. The PC strength (i.e., pK_a) and the surface charge of the film were varied to probe the effects that internal and surface chemistry had on the pH stability of the entrapped NfsB. All films showed enhanced activity retention at elevated pH (>6), and inactivation at reduced pH (<6) similar to NfsB in solution, indicating that the primary stabilizing effect of immobilization was achieved through ionic interactions between NfsB and the PC and not through changes to the surface charge of the NfsB. Additionally, films that were stored dry at 4 °C for 1 month retained full activity, while those stored at room temperature lost 30% activity. Remarkably, at 50 °C, above the NfsB melting temperature, 40% activity was retained after 1 month of dry storage. Our results suggest that internal film properties are significantly more important than surface charge, which had minor effects on activity. Specifically, immobilization with the weak PC, poly(l-lysine), increased the optimal pH and the activity of immobilized NfsB (which we attribute to greater permeability), relative to immobilization with the strong PC, poly(diallyldimethylammonium chloride). However, NfsB was leached from the PLL film to a greater extent. Overall, these observations demonstrate that internal ionic cross-linking is key to the stabilizing effects of PEMs and that the pH response can be tuned by controlling the number of cross-links (e.g., by changing the strength of the PC). However, this may be at the cost of reduced loading, illustrating the necessity of simultaneously optimizing enzyme loading, internal ionic cross-linking, and substrate transport.

Destruction of Polyelectrolyte Microcapsules Formed on CaCO3 Microparticles and the Release of a Protein Included by the Adsorption Method

The degradation of polyelectrolyte microcapsules formed on protein-free CaCO₃ particles consisting of polyallylamine (PAH) and polystyrene sulfonate (PSS) and the resulting yield of protein in the presence of various salts of different concentrations, as well as at two pH values, was studied by fluorescence spectroscopy; the protein was incorporated into prepared microcapsules by adsorption. It was found that a high concentration of sodium chloride (2 M) leads to considerable dissociation of PAH, which is apparently due to the loosening of polyelectrolytes under the action of ionic strength. At the same time, 0.2 M sodium chloride and ammonium sulfate of the same ionic strength (0.1 M) exert less influence on the amount of dissociated polymer. In the case of ammonium sulfate (0.1 M), the effect is due to the competitive binding of sulfate anions to the amino groups of the polyelectrolyte. However, unlike microcapsules formed on CaCO₃ particles containing protein, the dissociation of polyelectrolyte from microcapsules formed on protein-free particles increased with increasing temperature. Apparently, a similar effect is associated with the absence of a distinct shell, which was observed on microcapsules formed on protein-containing CaCO₃ particles. The high level of the presence of Fluorescein isothiocyanate (FITC)-labeled Bovine Serum Albumin (BSA) in the supernatant is explained by the large amount of electrostatically bound protein and the absence of a shell that prevents the release of the protein from the microcapsules. In 2M NaCl, during the observation period, the amount of the released protein did not exceed 70% of the total protein content in the capsules, in control samples, this value does not exceed 8%, which indicates the predominantly electrostatic nature of protein retention in capsules formed on protein-free CaCO₃ particles. The increase in protein yield and peeling of PAH with increasing pH is explained by the proximity of pH 7 to the point of charge exchange of the amino group of polyelectrolyte, as a result of the dissociation of the microcapsule.