Small-Molecule Adsorption Energy Predictions for High-Throughput Screening of Electrocatalysts.

Predicting adsorption energies of small molecules (e.g., OH, OOH, CO) on electrocatalysts involved in electrochemical reactions aids in accelerating the design and screening of electrocatalysts. Avoiding computationally expensive electronic structure calculations increases the speed of such predictions. Geometric and electronic descriptors have been reported to characterize the environment around surface active sites and predict adsorption energies. However, these descriptors cannot be used to predict adsorption energies of small molecules on various substrates, e.g., metal-oxide and nonmetal electrocatalysts. We compare the performance of these descriptors in predicting adsorption energies of small molecules on various electrocatalysts with adsorption energies calculated from density functional theory. We show that two recently developed machine learning algorithms, Crystal Graph Convolutional Neural Network (CGCNN) and Atomistic Line Graph Neural Network (ALIGNN), outperform the reported descriptors based on geometric (coordination number of the active site and its nearest neighbors) and electronic (the bond-energy-integrated orbitalwise coordination number, the electronegativity, and the number of valence electrons of the active site) properties in predicting the adsorption energies. Our results suggest that ALIGNN is almost always more accurate than CGCNN in adsorption energy predictions. The improvement ranges from 0.02 to 1.0 eV in the mean absolute errors (MAEs). We also compare the performance of CGCNN and ALIGNN algorithms in predicting the overpotentials of the oxygen evolution reaction occurring on various electrocatalysts with MAEs of 0.06 and 0.05 V, respectively.

Prediction of zwitterion hydration and ion association properties using machine learning.

Molecular dynamics simulations were performed to study the hydration and ion association properties of a library of zwitterionic molecules with varying charged moieties and spacer chemistries in pure water and with Na+ and Cl- ions. The structure and dynamics of associations were calculated using the radial distribution and residence time correlation function. Resulting association properties are used as target variables for a machine learning model, with cheminformatic descriptors of the molecule subunits used as descriptors. Prediction of hydration properties revealed that steric and hydrogen bonding descriptors were of greatest importance and there was influence from the cationic moiety on the anionic moiety hydration properties. Ion association properties prediction performed poorly, which is attributed to the role of hydration layers in ion association dynamics. This study is the first to quantitatively describe the influence of subunit chemistry on hydration and ion association properties of zwitterions. These quantitative descriptions supplement prior studies of zwitterion association and previously described design principles.

Shape Transformation Mechanism of Gold Nanoplates.

Shape control is of key importance in utilizing the structure-property relationship of nanocrystals. The high surface-to-volume ratio of nanocrystals induces dynamic surface reactions on exposed facets of nanocrystals, such as adsorption, desorption, and diffusion of surface atoms, all of which are important in overall shape transformation. However, it is difficult to track shape transformation of nanocrystals and understand the underlying mechanism at the level of distinguishing events on individual facets. Herein, we investigate changes of individual surface-exposed facets during diverse shape transformations of Au nanocrystals using liquid phase TEM in various chemical potentials and kinetic Monte Carlo simulations. The results reveal that the diffusion of surface atoms on nanocrystals is the governing factor in determining the final structure in shape transformation, causing the fast transformation of unstable facets to truncated morphology with minimized surface energy. The role of surface diffusion introduced here can be further applied to understanding the formation mechanism of variously shaped nanocrystals.

In Situ Liquid Phase TEM of Nanoparticle Formation and Diffusion in a Phase-Separated Medium.

Colloidal nanoparticles are synthesized in a complex reaction mixture that has an inhomogeneous chemical environment induced by local phase separation of the medium. Nanoparticle syntheses based on micelles, emulsions, flow of different fluids, injection of ionic precursors in organic solvents, and mixing the metal organic phase of precursors with an aqueous phase of reducing agents are well established. However, the formation mechanism of nanoparticles in the phase-separated medium is not well understood because of the complexity originating from the presence of phase boundaries as well as nonuniform chemical species, concentrations, and viscosity in different phases. Herein, we investigate the formation mechanism and diffusion of silver nanoparticles in a phase-separated medium by using liquid phase transmission electron microscopy and many-body dissipative particle dynamics simulations. A quantitative analysis of the individual growth trajectories reveals that a large portion of silver nanoparticles nucleate and grow rapidly at the phase boundaries, where metal ion precursors and reducing agents from the two separated phases react to form monomers. The results suggest that the motion of the silver nanoparticles at the interfaces is highly affected by the interaction with polymers and exhibits superdiffusive dynamics because of the polymer relaxation.

Molecular View of the Distortion and Pinning Force of a Receding Contact Line: Impact of the Nanocavity Geometry.

We present a molecular view using many-body dissipative particle dynamics simulations to unravel the pinning phenomenon of a liquid film receding over a solid substrate with a nanocavity. We find that the pinning force and distortion of the pinned contact line vary across different nanocavity shapes. We show that the mechanism of a caterpillar motion, which has previously been proposed for advancing precursor films, persists in a partially pinned receding contact line. Our results also demonstrate a localized clamping effect, which is originated from the variation of the dynamic contact angle along the pinned contact line. The simulation results suggest that the clamping effect can be controlled by the geometry of the nanocavity and hydrophilicity of the underlying substrate.

Uptake and Distribution of Administered Bone Marrow Mesenchymal Stem Cell Extracellular Vesicles in Retina.

Cell replacement therapy using mesenchymal (MSC) and other stem cells has been evaluated for diabetic retinopathy and glaucoma. This approach has significant limitations, including few cells integrated, aberrant growth, and surgical complications. Mesenchymal Stem Cell Exosomes/Extracellular Vesicles (MSC EVs), which include exosomes and microvesicles, are an emerging alternative, promoting immunomodulation, repair, and regeneration by mediating MSC's paracrine effects. For the clinical translation of EV therapy, it is important to determine the cellular destination and time course of EV uptake in the retina following administration. Here, we tested the cellular fate of EVs using in vivo rat retinas, ex vivo retinal explant, and primary retinal cells. Intravitreally administered fluorescent EVs were rapidly cleared from the vitreous. Retinal ganglion cells (RGCs) had maximal EV fluorescence at 14 days post administration, and microglia at 7 days. Both in vivo and in the explant model, most EVs were no deeper than the inner nuclear layer. Retinal astrocytes, microglia, and mixed neurons in vitro endocytosed EVs in a dose-dependent manner. Thus, our results indicate that intravitreal EVs are suited for the treatment of retinal diseases affecting the inner retina. Modification of the EV surface should be considered for maintaining EVs in the vitreous for prolonged delivery.

Thermoplastic polyurethane with controllable degradation and critical anti-fouling properties.

Bioresorbable polymers, including polyesters and polypeptides, are being widely used in the medical field. However, these materials still suffer from some long-standing challenges, such as material-induced blood coagulation, foreign body response, non-adjustable degradation rate, and absence of elastic properties. In this work, we explored a new approach to address these challenges by incorporating critical anti-fouling, improved mechanical and controllable degradation properties into the existing bioresorbable polymers. We synthesized a set of zwitterionic thermoplastic polyurethanes, which consist of degradable polycaprolactone diols as soft segments and faster hydrolyzable carboxybetaine (CB) diols as chain extenders. Differential scanning calorimetry and temperature sweep rheology revealed thermal transition performance and thermoplastic behavior of the polymers. The calorimetric study observed that CB-based chain extender played a critical role in the crystallization process by affecting the structure and crystallization temperature. Cell attachment study demonstrated that the degradable zwitterionic polyurethane surfaces highly resist cell attachment even after being submerged in 100% fetal bovine serum for two weeks. The gold standard PEG-based degradable polyurethane showed the initial resistance to the cell attachment for one day and then failed after three days. This work clearly shows that the adaption of existing materials with slightly better anti-fouling properties is unlikely to solve these long-lasting challenges. Our design approach and the material platform with critical anti-fouling properties and other desired tunable properties show the potential to address these complications associated with existing bioresorbable polymers. This method can be adapted to design customized bioresorbable polymers for a wide range of applications, including implantable biomedical devices and drug delivery.

Unraveling the Mechanism of a Rising Three-Phase Contact Line along a Vertical Surface Using Many-Body Dissipative Particle Dynamics.

We present coarse-grained molecular (many-body dissipative particle) dynamics simulations to unravel the wetting mechanism of spontaneous rise of a liquid thin film along vertical flat and rough surfaces. We show that the displacement of the rising contact line, in single- and double-wall geometry, exhibits a ballistic motion (∼t) followed by a diffusive dynamics (∼[Formula: see text]) during the rise of the liquid thin film against gravity. Dynamic contact angle decreases as the contact line transitions from ballistic to diffusive regime. Explicit analysis of the velocity and vorticity profile in the bulk and in the proximity of the contact line suggests an unsteady flow field behind the rising three-phase contact line. Furthermore, our simulation results indicate that contact line dynamics and the flow field behind the contact line are independent of the surface roughness.

Winning the fight against biofilms: the first six-month study showing no biofilm formation on zwitterionic polyurethanes.

Biofilms have been a long-standing challenge for healthcare, water transport, and many other industries. They lead to bacterial growth and infections in animals, food products, and humans, cause premature removal of the implanted materials or devices from patients, and facilitate fouling and corrosion of metals. Despite some published and patented methods on minimizing the effects of biofilms for a short period (less than two weeks), there exists no successful means to mitigate or prevent the long-term formation of biofilms. It is even more challenging to integrate critical anti-fouling properties with other needed physical and chemical properties for a range of applications. In this study, we developed a novel approach for combining incompatible, highly polar anti-fouling groups with less polar, mechanically modifying groups into one material. A multifunctional carboxybetaine precursor was designed and introduced into polyurethane. The carboxybetaine precursors undergo rapid, self-catalyzed hydrolysis at the water/material interface and provide critical anti-fouling properties that lead to undetectable bacterial attachment and zero biofilm formation after six months of constant exposure to Pseudomonas aeruginosa and Staphylococcus epidermidis under the static condition in a nutrient-rich medium. This zwitterionic polyurethane is the first material to demonstrate both critical anti-biofilm properties and tunable mechanical properties and directly validates the unproven anti-fouling strategy and hypothesis for biofilm formation prevention. This approach of designing 'multitasking materials' will be useful for the development of next generation anti-fouling materials for a variety of applications.

Role of Near-Electrode Solution Chemistry on Bacteria Attachment and Poration at Low Applied Potentials.

This research investigated mechanisms for biofouling control at boron-doped diamond (BDD) electrode surfaces polarized at low applied potentials (e.g., -0.2 to 1.0 V vs Ag/AgCl), using Pseudomonas aeruginosa as a model organism. Results indicated that electrostatic interactions between bacteria and ionic electrode functional groups facilitated bacteria attachment at the open-circuit potential (OCP). However, under polarization, the applied potential governed these electrostatic interactions and electrochemical reactions resulted in surface bubble formation and near-surface pH modulation that decreased surface attachment under anodic conditions. The poration of the attached bacteria occurred at OCP conditions and increased with the applied potential. Scanning electrochemical microscopy (SECM) provided near-surface pH and oxidant formation measurements under anodic and cathodic polarizations. The near-surface pH was 3.1 at 1.0 V vs Ag/AgCl and 8.0 at -0.2 V vs Ag/AgCl and was possibly a contributor to bacteria poration. Interpretation of SECM data using a reactive transport model allowed for a better understanding of the near-electrode chemistry. Under cathodic conditions, the primary oxidant formed was H2O2, and under anodic conditions, a combination of H2O2, Cl•, HO2•, Cl2•-, and Cl2 formations likely contributed to bacteria poration at potentials as low as 0.5 V vs Ag/AgCl.

Modeling electrochemical oxidation and reduction of sulfamethoxazole using electrocatalytic reactive electrochemical membranes.

In this research, degradation of the antibiotic sulfamethoxazole (SMX) was studied using electrochemical reduction and oxidation in single pass, flow-through mode using porous titanium suboxide (Ti4O7) reactive electrochemical membranes (REMs) and Pd-Cu doped Ti4O7 REMs (Pd-Cu/Ti4O7 REMs). Electrochemical reduction of SMX increased from 3.8 ±â€¯0.3% for the Ti4O7 REM to 96.1 ±â€¯3.9% for the Pd-Cu/Ti4O7 REM at -1.14 V/SHE and at a permeate flux of 300 L m-2 h-1 (LMH) (liquid residence time: ∼1.8 s). By contrast, electrochemical oxidation using Ti4O7 REMs achieved 95.7 ±â€¯1.0% removal of SMX at 2.03 V/SHE and a permeate flux of 300 LMH (liquid residence time: ∼9.0 s) without the catalyst addition. We developed a reactive transport mathematical model and calibrated it to the SMX experimental data. The calibrated model predicted SMX permeate concentrations at fixed potentials and as a function of permeate flux. Based on products from SMX reduction, we proposed that SMX was reduced by a hydrogen atom transfer reaction that was mediated by the Pd-Cu/Ti4O7 REM. Toxicity tests indicated that electrochemical oxidation/reduction lowered solution toxicity. The results of this work indicate that a tandem electrochemical reduction/oxidation approach using the REM-based technology is a potential treatment strategy for sulfonamide-contaminated pharmaceutical wastewater.

Predictive Model to Probe the Impact of Gravity and Surface Tension on Rising Wetting Thin Films.

Utilizing kinetic Monte Carlo simulations, we developed a three-dimensional Ising lattice gas model to reveal the wetting mechanism of a liquid film rising along a vertical substrate. The model takes into account the impact of surface tension, gravity, and interaction energy between liquid particles and between liquid and substrate on the rise of the liquid film. We verify that in low gravitational acceleration regime, the growth of the liquid film follows the universal law of [Formula: see text]. As gravitational acceleration and surface tension vary, the simulation results show the detailed dynamics of the solid-liquid interface. Explicit analysis of the interface displacement and roughness under different gravitational accelerations and surface tensions is also presented.

Large Area Directed Self-Assembly of Sub-10 nm Particles with Single Particle Positioning Resolution.

Directed self-assembly of nanoparticles (DSA-n) holds great potential for device miniaturization in providing patterning resolution and throughput that exceed existing lithographic capabilities. Although nanoparticles excel at assembling into regular close-packed arrays, actual devices on the other hand are often laid out in sparse and complex configurations. Hence, the deterministic positioning of single or few particles at specific positions with low defect density is imperative. Here, we report an approach of DSA-n that satisfies these requirements with less than 1% defect density over micrometer-scale areas and at technologically relevant sub-10 nm dimensions. This technique involves a simple and robust process where a solvent film containing sub-10 nm gold nanoparticles climbs against gravity to coat a prepatterned template. Particles are placed individually into nanoscale cavities, or between nanoposts arranged in varying degrees of geometric complexity. Brownian dynamics simulations suggest a mechanism in which the particles are pushed into the template by a nanomeniscus at the drying front. This process enables particle-based self-assembly to access the sub-10 nm dimension, and for device fabrication to benefit from the wealth of chemically synthesized nanoparticles with unique material properties.

Directed Self-Assembly of sub-10 nm Particles: Role of Driving Forces and Template Geometry in Packing and Ordering.

By comparing the magnitude of forces, a directed self-assembly mechanism has been suggested previously in which immersion capillary is the only driving force responsible for packing and ordering of nanoparticles, which occur only after the meniscus recedes. However, this mechanism is insufficient to explain vacancies formed by directed self-assembly at low particle concentrations. Utilizing experiments, and Monte Carlo and Brownian dynamics simulations, we developed a theoretical model based on a new proposed mechanism. In our proposed mechanism, the competing driving forces controlling the packing and ordering of sub-10 nm particles are (1) the repulsive component of the pair potential and (2) the attractive capillary forces, both of which apply at the contact line. The repulsive force arises from the high particle concentration, and the attractive force is caused by the surface tension at the contact line. Our theoretical model also indicates that the major part of packing and ordering of nanoparticles occurs before the meniscus recedes. Furthermore, utilizing our model, we are able to predict the various self-assembly configurations of particles as their size increases. These results lay out the interplay between driving forces during directed self-assembly, motivating a better template design now that we know the importance and the dominating driving forces in each regime of particle size.

Rheology and simulation of 2-dimensional clathrin protein network assembly.

Clathrin is a three-legged protein complex that assembles into lattice structures on the cell membrane and transforms into fullerene-like cages during endocytosis. This dynamic structural flexibility makes clathrin an attractive building block for guided assembly. The assembly dynamics and the mechanical properties of clathrin protein lattices are studied using rheological measurements and theoretical modelling in an effort to better understand two dynamic processes: protein adsorption to the interface and assembly into a network. We find that percolation models for protein network formation are insufficient to describe clathrin network formation, but with Monte Carlo simulations we can describe the dynamics of network formation very well. Insights from this work can be used to design new bio-inspired nano-assembly systems.

Membrane fluctuations destabilize clathrin protein lattice order.

We develop a theoretical model of a clathrin protein lattice on a flexible cell membrane. The clathrin subunit is modeled as a three-legged pinwheel with elastic deformation modes and intersubunit binding interactions. The pinwheels are constrained to lie on the surface of an elastic sheet that opposes bending deformation and is subjected to tension. Through Monte Carlo simulations, we predict the equilibrium phase behavior of clathrin lattices at various levels of tension. High membrane tensions, which correspond to suppressed membrane fluctuations, tend to stabilize large, flat crystalline structures similar to plaques that have been observed in vivo on cell membranes that are adhered to rigid surfaces. Low tensions, on the other hand, give rise to disordered, defect-ridden lattices that behave in a fluidlike manner. The principles of two-dimensional melting theory are applied to our model system to further clarify how high tensions can stabilize crystalline order on flexible membranes. These results demonstrate the importance of environmental physical cues in dictating the collective behavior of self-assembled protein structures.

Template-induced structure transition in sub-10 nm self-assembling nanoparticles.

We report on the directed self-assembly of sub-10 nm gold nanoparticles confined within a template comprising channels of gradually varying widths. When the colloidal lattice parameter is mismatched with the channel width, the nanoparticles rearrange and break their natural close-packed ordering, transiting through a range of structural configurations according to the constraints imposed by the channel. While much work has been done in assembling ordered configurations, studies of the transition regime between ordered states have been limited to microparticles under applied compression. Here, with coordinated experiments and Monte Carlo simulations we show that particles transit through a more diverse set of self-assembled configurations than observed for compressed systems. The new insight from this work could lead to the control and design of complex self-assembled patterns other than periodic arrays of ordered particles.

Correlating Non-Geminate Recombination with Film Structure: A Comparison of Polythiophene: Fullerene Bilayer and Blend Films.

The morphology of the active layer in polymer:fullerene solar cells is a key parameter in determining their performance. In this study we monitor the charge carrier dynamics in bilayers of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (fabricated by sequential spin coating and vacuum deposition) before and after thermal annealing, and compare these against conventional solution processed bulk heterojunction (BHJ) blend films. Transmission electron microscopy images, supported by field effect mobility data show that while not-annealed P3HT/PC61BM bilayers possess a sharp interface, intermixing proceeds upon annealing. Transient absorption studies indicate that the not-annealed bilayer yields fewer, but longer lived, charge carriers compared to the BHJ. Monte Carlo (MC) simulations further suggest that the difference in non-geminate recombination dynamics observed for the BHJ and bilayer films could be related to the confinement of charge carriers to the interface, with the lower dimensionality for the flat interface bilayer films relative to the intercalated donor-acceptor network BHJ films leading to lower recombination.

Ultralow doping in organic semiconductors: evidence of trap filling.

Tail states in organic semiconductors have a significant influence on device performances by acting as traps in charge transport. We present a study of the controlled passivation of acceptor tail states in fullerene C(60) by the addition of electrons introduced by molecular n doping. Using ultralow doping, we are able to successively fill the traps with charges and examine the changes in conductivity, activation energy, mobility, and Fermi-level position. Passivation of the traps leads to an increase of the electron mobility in C(60) by more than 3 orders of magnitude, to reach 0.21 cm(2)/(V s).

Intrinsic fluctuations lead to broad range of transduced forces in tethered-bead single-molecule experiments.

We build a theoretical platform for predicting the behavior of tethered-bead single-molecule experiments, accounting for bead translational and rotational fluctuations, the specific type of experimental setup, and the detailed application of tension to the tether molecule. Within this framework, the external force applied to the bead is distinguished from the instantaneous force transduced to the tether molecule, resulting in a distinction between the observable response of the bead and the underlying force fluctuations felt by the tether that directly affect the biomolecular processes being studied. Our theoretical model indicates that the spread of the distribution of tether forces increases with applied external force, resulting in substantial deviations between the external and tether forces. We find that the impact of rotational and translational fluctuations of the bead motion is larger in magnetic tweezers than optical tweezers. However, this distinction diminishes at large external forces, and our asymptotic expressions offer a simple route for experimental analyses. Overall, our theory demonstrates that fluctuations in the tether molecule due to bead rotation and translation lead to a broad range of tether forces.