Joachim Dzubiella

Thermosensitive Au-PNIPA yolk-shell nanoparticles with tunable selectivity for catalysis.

Metallic nanoparticles have been the subject of intense research recently because of their catalytic properties, which may differ considerably from the bulk metal. As the free nanoparticles tend to aggregate and are difficult to handle in catalytic applications, colloidal carrier systems have been developed that encapsulate and stabilize the particles. 5] More recently, so-called smart carrier systems, such as thermosensitive microgels, have become the focus of research. These hybrids react on external stimuli and allow the catalytic properties to be altered accordingly. Thus, thermosensitive polystyrene (PS)-poly(N-isopropylacrylamide) (PNIPA) core–shell microgels were applied as the active nanoreactor for the immobilization of metal nanoparticles. The catalytic activity of immobilized metal nanoparticles can be tuned by the swelling and shrinking of the microgels. Liz-Marz n et al. have developed a AuPNIPA core–shell colloidal system. They found that the thermoresponsive PNIPA shell with limited cross-linking allows for particularly efficient control of the catalysis of encapsulated Au nanoparticles. Recently, yolk–shell structures that consist of a single metal nanoparticle within an inorganic or polymeric shell 19] have become the subject of intense research. These systems can be used to tune the catalytic activity of the enclosed nanoparticle by a suitable architecture of the shell. Yolk–shell structures have the clear advantages in that individual metal nanoparticles are enclosed in a compartment that prevents aggregation with other nanoparticles. Furthermore, the embedded gold nanoparticle has a free surface that is not blocked by any surface group or polymer compared to the Au-PNIPA core–shell system. Moreover, the permeability of the shell may be tuned to a certain extent. Therefore, yolk– shell systems may be regarded as true nanoreactors that allow the catalytic activity of single nanoparticles to be studied in a defined environment. Herein we present a thermosensitive yolk–shell system that uses temperature as a trigger for reaction. Figure 1a shows the underlying principle of these systems: A single Au nanoparticle is encapsulated in a hollow thermosensitive PNIPA shell. The porosity and the hydrophobicity of this shell can be tuned in a well-defined manner by temperature while the colloidal stability of the entire hybrid is fully maintained. We show this by monitoring the reduction of hydrophilic 4-

Coupling Hydrophobicity, Dispersion, and Electrostatics in Continuum Solvent Models

An implicit solvent model is presented that couples hydrophobic, dispersion, and electrostatic solvation energies by minimizing the system Gibbs free energy with respect to the solvent volume exclusion function. The solvent accessible surface is the output of the theory. The method is illustrated with the solvation of simple solutes on different length scales and captures the sensitivity of hydration to the particular form of the solute-solvent interactions in agreement with recent computer simulations.

Coupling nonpolar and polar solvation free energies in implicit solvent models

Recent studies on the solvation of atomistic and nanoscale solutes indicate that a strong coupling exists between the hydrophobic, dispersion, and electrostatic contributions to the solvation free energy, a facet not considered in current implicit solvent models. We suggest a theoretical formalism which accounts for coupling by minimizing the Gibbs free energy of the solvent with respect to a solvent volume exclusion function. The resulting differential equation is similar to the Laplace-Young equation for the geometrical description of capillary interfaces but is extended to microscopic scales by explicitly considering curvature corrections as well as dispersion and electrostatic contributions. Unlike existing implicit solvent approaches, the solvent accessible surface is an output of our model. The presented formalism is illustrated on spherically or cylindrically symmetrical systems of neutral or charged solutes on different length scales. The results are in agreement with computer simulations and, most importantly, demonstrate that our method captures the strong sensitivity of solvent expulsion and dewetting to the particular form of the solvent-solute interactions.

Ion specificity at the peptide bond: molecular dynamics simulations of N-methylacetamide in aqueous salt solutions.

Affinities of alkali cations and halide anions for the peptide group were quantified using molecular dynamics simulations of aqueous solutions of N-methylacetamide using both nonpolarizable and polarizable force fields. Potassium and, more strongly, sodium exhibit an affinity for the carbonyl oxygen of the amide group, while none of the halide anions shows any appreciable attraction for the amide hydrogen. Heavier halides, however, interact with the hydrophobic methyl groups of N-methylacetamide. Using the present results for a model of the peptide bond we predict that the destabilizing effect of weakly hydrated Hofmeister ions, such as bromide or iodide, is not due to direct interactions with the backbone but rather due to attraction to hydrophobic regions of the protein.

Application of the level-set method to the implicit solvation of nonpolar molecules

A level-set method is developed for numerically capturing the equilibrium solute-solvent interface that is defined by the recently proposed variational implicit solvent model [Dzubiella, Swanson, and McCammon, Phys. Rev. Lett. 104, 527 (2006); J. Chem. Phys. 124, 084905 (2006)]. In the level-set method, a possible solute-solvent interface is represented by the zero level set (i.e., the zero level surface) of a level-set function and is eventually evolved into the equilibrium solute-solvent interface. The evolution law is determined by minimization of a solvation free energy functional that couples both the interfacial energy and the van der Waals type solute-solvent interaction energy. The surface evolution is thus an energy minimizing process, and the equilibrium solute-solvent interface is an output of this process. The method is implemented and applied to the solvation of nonpolar molecules such as two xenon atoms, two parallel paraffin plates, helical alkane chains, and a single fullerence C(60). The level-set solutions show good agreement for the solvation energies when compared to available molecular dynamics simulations. In particular, the method captures solvent dewetting (nanobubble formation) and quantitatively describes the interaction in the strongly hydrophobic plate system.

Nonequilibrium sedimentation of colloids on the particle scale

We investigate sedimentation of model hard-sphere-like colloidal dispersions confined in horizontal capillaries using laser scanning confocal microscopy, dynamical density functional theory, and Brownian dynamics computer simulations. For homogenized initial states we obtain quantitative agreement of the results from the respective approaches for the time evolution of the one-body density distribution and the osmotic pressure on the walls. We demonstrate that single-particle information can be obtained experimentally in systems that were initialized further out of equilibrium such that complex lateral patterns form.

Sensing Solvents with Ultrasensitive Porous Poly(ionic liquid) Actuators

We introduced a new concept for fabricating solvent stimulus polymer actuators with unprecedented sensitivity and accuracy. This was accomplished by integrating porous architectures and electrostatic complexation gradients in a poly(ionic liquid) membrane that bears ionic liquid species for solvent sorption. In contact with 1.5 mol% of acetone molecules in water, the actuator membrane (1 mm x 20 mm x 30 um) bent into a closed loop. While the interaction between solvents and the polymer drives the actuation, the continuous gradient in complexation degree combined with the porous architecture optimizes the actuation, giving it a high sensitivity and even the ability to discriminate butanol solvent isomers. The membrane is also capable of cooperative actuation. The design concept is easy to implement and applicable to other polyelectrolyte systems, which substantially underpins their potentials in smart and sensitive signaling microrobotics/devices.

Core–shell microgels as “smart” carriers for enzymes

We present a thermodynamic study of the adsorption of lysozyme on a negatively charged core–shell microgel at pH 7.2. The carrier particles consist of a polystyrene core onto which a charged poly(N-isopropylacrylamide-co-acrylic acid) network is attached. Isothermal titration calorimetry (ITC) is used to investigate the temperature and salt dependence of lysozyme binding. Our ITC analysis unequivocally shows that the adsorption of lysozyme onto the charged gel is driven by entropy. The addition of salt strongly decreases the binding affinity, indicating significant electrostatic contributions to the adsorption process. However, at high salt concentrations, substantial protein binding with unaltered entropies is still observed pointing to large contributions from hydrophobic interactions. Furthermore, the calorimetric analysis suggests that protonation of lysozyme takes place upon binding. This is directly shown by analysis of the enzymatic activity of adsorbed lysozyme. It was found that the activity is enhanced about ∼3.5 times, indicating that lysozyme has taken up approximately one proton when entering the gel. The entire set of data demonstrates that core–shell microgels present “smart” colloidal carriers for lysozyme that enhance its activity.

Electrostatic Free Energy and Its Variations in Implicit Solvent Models

A mean-field approach to the electrostatics for solutes in electrolyte solution is revisited and rigorously justified. In this approach, an electrostatic free energy functional is constructed that depends solely on the local ionic concentrations. The unique set of such concentrations that minimize this free energy are given by the usual Boltzmann distributions through the electrostatic potential which is determined by the Poisson-Boltzmann equation. This approach is then applied to the variational implicit solvent description of the solvation of molecules [Dzubiella, Swanson, McCammon, Phys. Rev. Lett. 2006, 96, 087802; J. Chem. Phys. 2006, 124, 084905]. Care is taken for the singularities of the potential generated by the solute point charges. The variation of the electrostatic free energy with respect to the location change of solute-solvent interfaces, that is, dielectric boundaries, is derived. Such a variation gives rise to the normal component of the effective surface force per unit surface area that is shown to be attractive to the fixed point charges in the solutes. Two examples of applications are given to validate the analytical results. The first one is a one-dimensional model system resembling, for example, a charged solute or cavity in a one-dimensional channel. The second one, which is of its own interest, is the electrostatic free energy of a charged spherical solute immersed in an ionic solution. An analytical formula is derived for the Debye-Hückel approximation of the free energy, extending the classical Borns formula to one that includes ionic concentrations. Variations of the nonlinear Poisson-Boltzmann free energy are also obtained.

Solvent fluctuations in hydrophobic cavity-ligand binding kinetics.

Water plays a crucial part in virtually all protein–ligand binding processes in and out of equilibrium. Here, we investigate the role of water in the binding kinetics of a ligand to a prototypical hydrophobic pocket by explicit-water molecular dynamics (MD) simulations and implicit diffusional approaches. The concave pocket in the unbound state exhibits wet/dry hydration oscillations whose magnitude and time scale are significantly amplified by the approaching ligand. In turn, the ligand’s stochastic motion intimately couples to the slow hydration fluctuations, leading to a sixfold-enhanced friction in the vicinity of the pocket entrance. The increased friction considerably decelerates association in the otherwise barrierless system, indicating the importance of molecular-scale hydrodynamic effects in cavity–ligand binding arising due to capillary fluctuations. We derive and analyze the diffusivity profile and show that the mean first passage time distribution from the MD simulation can be accurately reproduced by a standard Brownian dynamics simulation if the appropriate position-dependent friction profile is included. However, long-time decays in the water–ligand (random) force autocorrelation demonstrate violation of the Markovian assumption, challenging standard diffusive approaches for rate prediction. Remarkably, the static friction profile derived from the force correlations strongly resembles the profile derived on the Markovian assumption apart from a simple shift in space, which can be rationalized by a time–space retardation in the ligand’s downhill dynamics toward the pocket. The observed spatiotemporal hydrodynamic coupling may be of biological importance providing the time needed for conformational receptor–ligand adjustments, typical of the induced-fit paradigm.