Influence of Order within Nonpolar Monolayers on Hydrophobic Interactions

Inverse design of pore wall chemistry and topology through active learning of surface group interactions

Design of next-generation membranes requires a nanoscopic understanding of the effect of biologically inspired heterogeneous surface chemistries and topologies (roughness) on local water and solute behavior. In particular, the rejection of small, neutral solutes, such as boric acid, poses a heretofore unsolved challenge. In prior work, a computational inverse design technique using an evolutionary optimization successfully uncovered new surface design strategies for optimized transport of water over solutes in smooth, model pores consisting of two surface chemistries. However, extending such an approach to more complex (and realistic) scenarios involving many surface chemistries as well as surface roughness is challenging due to the expanded design space. In this work, we develop a new approach that uses active learning to optimize in a reduced feature space of surface group interactions, finding parameters that lead to their assembly into ordered, optimal patterns. This approach rapidly identifies novel surface functionalizations that maximize the difference in water and boric acid transport through the nanopore. Moreover, we find that the roughness of the nanopore wall, independent of its chemistry, can be leveraged to enhance transport selectivity: oscillations in the pore wall diameter optimally inhibit boric acid transport by creating energetic wells from which the solute must escape to transport down the pore. This proof-of-concept demonstrates the potential for active learning strategies, in concert with molecular simulations, to rapidly navigate complex design spaces of aqueous interfaces and is promising as a tool for engineering water-mediated surface interactions for a broad range of applications.

Interactions of SARS-CoV-2 and MERS-CoV fusion peptides measured using single-molecule force methods

We address the challenge of understanding how hydrophobic interactions are encoded by fusion peptide (FP) sequences within coronavirus (CoV) spike proteins. Within the FPs of severe acute respiratory syndrome CoV 2 and Middle East respiratory syndrome CoV (MERS-CoV), a largely conserved peptide sequence called FP1 (SFIEDLLFNK and SAIEDLLFDK in SARS-2 and MERS, respectively) has been proposed to play a key role in encoding hydrophobic interactions that drive viral-host cell membrane fusion. Although a non-polar triad (Leu-Leu-Phe (LLF)) is common to both FP1 sequences, and thought to dominate the encoding of hydrophobic interactions, FP1 from SARS-2 and MERS differ in two residues (Phe 2 versus Ala 2 and Asn 9 versus Asp 9, respectively). Here we explore whether single-molecule force measurements can quantify hydrophobic interactions encoded by FP1 sequences, and then ask whether sequence variations between FP1 from SARS-2 and MERS lead to significant differences in hydrophobic interactions. We find that both SARS-2 and MERS wild-type FP1 generate measurable hydrophobic interactions at the single-molecule level, but that SARS-2 FP1 encodes a substantially stronger hydrophobic interaction than its MERS counterpart (1.91 ± 0.03 nN versus 0.68 ± 0.03 nN, respectively). By performing force measurements with FP1 sequences with single amino acid substitutions, we determine that a single-residue mutation (Phe 2 versus Ala 2) causes the almost threefold difference in the hydrophobic interaction strength generated by the FP1 of SARS-2 versus MERS, despite the presence of LLF in both sequences. Infrared spectroscopy and circular dichroism measurements support the proposal that the outsized influence of Phe 2 versus Ala 2 on the hydrophobic interaction arises from variation in the secondary structure adopted by FP1. Overall, these insights reveal how single-residue diversity in viral FPs, including FP1 of SARS-CoV-2 and MERS-CoV, can lead to substantial changes in intermolecular interactions proposed to play a key role in viral fusion, and hint at strategies for regulating hydrophobic interactions of peptides in a range of contexts.

Structural features of interfacial water predict the hydrophobicity of chemically heterogeneous surfaces

The hydrophobicity of an interface determines the magnitude of hydrophobic interactions that drive numerous biological and industrial processes. Chemically heterogeneous interfaces are abundant in these contexts; examples include the surfaces of proteins, functionalized nanomaterials, and polymeric materials. While the hydrophobicity of nonpolar solutes can be predicted and related to the structure of interfacial water molecules, predicting the hydrophobicity of chemically heterogeneous interfaces remains a challenge because of the complex, non-additive contributions to hydrophobicity that depend on the chemical identity and nanoscale spatial arrangements of polar and nonpolar groups. In this work, we utilize atomistic molecular dynamics simulations in conjunction with enhanced sampling and data-centric analysis techniques to quantitatively relate changes in interfacial water structure to the hydration free energy (a thermodynamically well-defined descriptor of hydrophobicity) of chemically heterogeneous interfaces. We analyze a large data set of 58 self-assembled monolayers (SAMs) composed of ligands with nonpolar and polar end groups of different chemical identity (amine, amide, and hydroxyl) in five mole fractions, two spatial patterns, and with scaled partial charges. We find that only five features of interfacial water structure are required to accurately predict hydration free energies. Examination of these features reveals mechanistic insights into the interfacial hydrogen bonding behaviors that distinguish different surface compositions and patterns. This analysis also identifies the probability of highly coordinated water structures as a unique signature of hydrophobicity. These insights provide a physical basis to understand the hydrophobicity of chemically heterogeneous interfaces and connect hydrophobicity to experimentally accessible perturbations of interfacial water structure.

Identifying nonadditive contributions to the hydrophobicity of chemically heterogeneous surfaces via dual-loop active learning

Hydrophobic interactions drive numerous biological and synthetic processes. The materials used in these processes often possess chemically heterogeneous surfaces that are characterized by diverse chemical groups positioned in close proximity at the nanoscale; examples include functionalized nanomaterials and biomolecules, such as proteins and peptides. Nonadditive contributions to the hydrophobicity of such surfaces depend on the chemical identities and spatial patterns of polar and nonpolar groups in ways that remain poorly understood. Here, we develop a dual-loop active learning framework that combines a fast reduced-accuracy method (a convolutional neural network) with a slow higher-accuracy method (molecular dynamics simulations with enhanced sampling) to efficiently predict the hydration free energy, a thermodynamic descriptor of hydrophobicity, for nearly 200 000 chemically heterogeneous self-assembled monolayers (SAMs). Analysis of this dataset reveals that SAMs with distinct polar groups exhibit substantial variations in hydrophobicity as a function of their composition and patterning, but the clustering of nonpolar groups is a common signature of highly hydrophobic patterns. Further molecular dynamics analysis relates such clustering to the perturbation of interfacial water structure. These results provide new insight into the influence of chemical heterogeneity on hydrophobicity via quantitative analysis of a large set of surfaces, enabled by the active learning approach.

The Interplay of Ligand Properties and Core Size Dictates the Hydrophobicity of Monolayer-Protected Gold Nanoparticles

The hydrophobicity of monolayer-protected gold nanoparticles is a crucial design parameter that influences self-assembly, preferential binding to proteins and membranes, and other nano-bio interactions. Predicting the effects of monolayer components on nanoparticle hydrophobicity is challenging due to the nonadditive, cooperative perturbations to interfacial water structure that dictate hydrophobicity at the nanoscale. In this work, we quantify nanoparticle hydrophobicity by using atomistic molecular dynamics simulations to calculate local hydration free energies at the nanoparticle-water interface. The simulations reveal that the hydrophobicity of large gold nanoparticles is determined primarily by ligand end group chemistry, as expected. However, for small gold nanoparticles, long alkanethiol ligands interact to form anisotropic bundles that lead to substantial spatial variations in hydrophobicity even for homogeneous monolayer compositions. We further show that nanoparticle hydrophobicity is modulated by changing the ligand structure, ligand chemistry, and gold core size, emphasizing that single-ligand properties alone are insufficient to characterize hydrophobicity. Finally, we illustrate that hydration free energy measurements correlate with the preferential binding of propane as a representative hydrophobic probe molecule. Together, these results show that both physical and chemical properties influence the hydrophobicity of small nanoparticles and must be considered together when predicting gold nanoparticle interactions with biomolecules.

Chemical and topographical patterns combined with solution shear for selective-area deposition of highly-aligned semiconducting carbon nanotubes

Selective deposition of semiconducting carbon nanotubes (s-CNTs) into densely packed, aligned arrays of individualized s-CNTs is necessary to realize their potential in semiconductor electronics. We report the combination of chemical contrast patterns, topography, and pre-alignment of s-CNTs via shear to achieve selective-area deposition of aligned arrays of s-CNTs. Alternate stripes of surfaces favorable and unfavorable to s-CNT adsorption were patterned with widths varying from 2000 nm down to 100 nm. Addition of topography to the chemical contrast patterns combined with shear enabled the selective-area deposition of arrays of quasi-aligned s-CNTs (∼14°) even in patterns that are wider than the length of individual nanotubes (>500 nm). When the width of the chemical and topographical contrast patterns is less than the length of individual nanotubes (<500 nm), confinement effects become dominant enabling the selective-area deposition of much more tightly aligned s-CNTs (∼7°). At a trench width of 100 nm, we demonstrate the lowest standard deviation in alignment degree of 7.6 ± 0.3° at a deposition shear rate of 4600 s^-1, while maintaining an individualized s-CNT density greater than 30 CNTs μm^-1. Chemical contrast alone enables selective-area deposition, but chemical contrast in addition to topography enables more effective selective-area deposition and stronger confinement effects, with the advantage of removal of nanotubes deposited in spurious areas via selective lift-off of the topographical features. These findings provide a methodology that is inherently scalable, and a means to deposit spatially selective, aligned s-CNT arrays for next-generation semiconducting devices.

Molecular Features for Probing Small Amphiphilic Molecules with Self-Assembled Monolayer-Protected Nanoparticles

The sensing of small molecules poses the challenge of developing devices able to discriminate between compounds that may be structurally very similar. Here, attention has been paid to the use of self-assembled monolayer (SAM)-protected gold nanoparticles since they enable a modular approach to tune single-molecule affinity and selectivity simply by changing functional moieties (i.e., covering ligands), along with multivalent molecular recognition. To date, the discovery of monolayers suitable for a specific molecular target has relied on trial-and-error approaches, with ligand chemistry being the main criterion used to modulate selectivity and sensitivity. By using molecular dynamics, we showcase that either individual molecular characteristics and/or collective features such as ligand flexibility, monolayer organization, ligand local ordering, and interfacial solvent properties can also be exploited conveniently. The knowledge of the molecular mechanisms that drive the recognition of small molecules on SAM-covered nanoparticles will critically expand our ability to manipulate and control such supramolecular systems.

Recent Advances in the Quantification and Modulation of Hydrophobic Interactions for Interfacial Applications

Hydrophobic interaction is responsible for a variety of colloidal phenomena, which also plays a key role in achieving the desired characteristics and functionalities for a wide range of interfacial applications. In this feature article, our recent advances in the quantification and modulation of hydrophobic interactions at both solid/water and air/water interfaces in different material systems have been reviewed. On the basis of surface forces apparatus (SFA) measurements of hydrophobic polymers (e.g., polystyrene), a three-regime hydrophobic interaction model that could satisfactorily encompass the hydrophobic interaction with different ranges was proposed. In addition, the atomic force microscope (AFM) coupled with various techniques such as the colloidal probe, the electrochemical process, and force mapping were employed to quantify the hydrophobic interaction from different perspectives. For the hydrophobic interactions involving deformable bubbles, the bubble probe AFM combined with reflection interference contrast microscopy (RICM) was used to simultaneously measure the interaction force and spatiotemporal evolution of the thin film drainage process between air bubbles and hydrophobized mica surfaces in an aqueous medium. The studies on the interactions of air bubbles with self-assembled monolayers (SAMs) demonstrated that the range of hydrophobic interactions does not always increase monotonically with the hydrophobicity of interacting surfaces as characterized by the static water contact angle; viz., surfaces with similar hydrophobicity can exhibit different ranges of hydrophobic interaction, while surfaces with different hydrophobicities can exhibit a similar range of hydrophobic interactions. It is found that the hydrophobic interaction can be modulated by tuning the surface nanoscale structure and chemistry. Moreover, the long-range "hydrophilic" attraction that resembles the hydrophobic interaction was discovered between water droplets and polyelectrolyte surfaces in an oil medium, on the basis of which polyelectrolyte coating materials were designed for oil cleaning, oil/water separation, and demulsification. The interfacial applications, remaining challenges, and future perspectives of hydrophobic interactions are discussed.

Recent progress in the science of complex coacervation

Complex coacervation is an associative, liquid-liquid phase separation that can occur in solutions of oppositely-charged macromolecular species, such as proteins, polymers, and colloids. This process results in a coacervate phase, which is a dense mix of the oppositely-charged components, and a supernatant phase, which is primarily devoid of these same species. First observed almost a century ago, coacervates have since found relevance in a wide range of applications; they are used in personal care and food products, cutting edge biotechnology, and as a motif for materials design and self-assembly. There has recently been a renaissance in our understanding of this important class of material phenomena, bringing the science of coacervation to the forefront of polymer and colloid science, biophysics, and industrial materials design. In this review, we describe the emergence of a number of these new research directions, specifically in the context of polymer-polymer complex coacervates, which are inspired by a number of key physical and chemical insights and driven by a diverse range of experimental, theoretical, and computational approaches.

From Langmuir-Blodgett to Grafted Films

A locally organized monolayer film strongly attached to a gold surface is obtained by transfer of a Langmuir-Blodgett (LB) film of octadecylamine (ODA) or alcohol (ODOH) onto a Au surface and simultaneous oxidative electrografting of this film still in contact with the aqueous subphase. As opposed to LB films, these films resist ultrasonication; and unlike electrografted films, they are organized monolayers by construction. They are characterized by AFM (atomic force microscopy), water contact angle, ellipsometry, XPS (X-ray photoelectron spectroscopy), IRRAS (infrared reflection absorption spectroscopy), and GIXD (grazing incidence X-ray diffraction).

Alkanethiol Monolayer End Groups Affect the Long-Term Operational Stability and Signaling of Electrochemical, Aptamer-Based Sensors in Biological Fluids

Electrochemical aptamer-based (E-AB) sensors achieve highly precise measurements of specific molecular targets in untreated biological fluids. This unique ability, together with their measurement frequency of seconds or faster, has enabled the real-time monitoring of drug pharmacokinetics in live animals with unprecedented temporal resolution. However, one important weakness of E-AB sensors is that their bioelectronic interface degrades upon continuous electrochemical interrogation-a process typically seen as a drop in faradaic and an increase in charging currents over time. This progressive degradation limits their in vivo operational life to 12 h at best, a period that is much shorter than the elimination half-life of the vast majority of drugs in humans. Thus, there is a critical need to develop novel E-AB interfaces that resist continuous electrochemical interrogation in biological fluids for prolonged periods. In response, our group is pursuing the development of better packed, more stable self-assembled monolayers (SAMs) to improve the signaling and extend the operational life of in vivo E-AB sensors from hours to days. By invoking hydrophobicity arguments, we have created SAMs that do not desorb from the electrode surface in aqueous physiological solutions and biological fluids. These SAMs, formed from 1-hexanethiol solutions, decrease the voltammetric charging currents of E-AB sensors by 3-fold relative to standard monolayers of 6-mercapto-1-hexanol, increase the total faradaic current, and alter the electron transfer kinetics of the platform. Moreover, the stability of our new SAMs enables uninterrupted, continuous E-AB interrogation for several days in biological fluids, like undiluted serum, at a physiological temperature of 37 °C.

Quantifying the Extent of Hydration of a Surface-Bound Peptide Using Neutron Reflectometry

Establishing how water, or the absence of water, affects the structure, dynamics, and function of proteins in contact with inorganic surfaces is critical to developing successful protein immobilization strategies. In the present article, the quantity of water hydrating a monolayer of helical peptides covalently attached to self-assembled monolayers (SAMs) of alkyl thiols on Au was measured using neutron reflectometry (NR). The peptide sequence was composed of repeating LLKK units in which the leucines were aligned to face the SAM. When immersed in water, NR measured 2.7 ± 0.9 water molecules per thiol in the SAM layer and between 75 ± 13 and 111 ± 13 waters around each peptide. The quantity of water in the SAM was nearly twice that measured prior to peptide functionalization, suggesting that the peptide disrupted the structure of the SAM. To identify the location of water molecules around the peptide, we compared our NR data to previously published molecular dynamics simulations of the same peptide on a hydrophobic SAM in water, revealing that 49 ± 5 of 95 ± 8 total nearby water molecules were directly hydrogen-bound to the peptide. Finally, we show that immersing the peptide in water compressed its structure into the SAM surface. Together, these results demonstrate that there is sufficient water to fully hydrate a surface-bound peptide even at hydrophobic interfaces. Given the critical role that water plays in biomolecular structure and function, these results are expected to be informative for a broad array of applications involving proteins at the bio/abio interface.

Forces between zinc sulphide surfaces; amplification of the hydrophobic attraction by surface charge

Smooth Zinc Sulphide (ZnS) surfaces were prepared by magnetron sputtering and the interaction forces were measured between them as a function of pH. At the isoelectric point (iep) of pH 7.1 the attractive force was well described by the van der Waals interaction calculated using Lifshitz theory for a layered system. Away from the iep, the forces were fitted using DLVO theory extended to account for surface roughness. At pH 9.8 the surfaces acquire a negative charge and an electrostatic repulsion is evident. Below the iep the surfaces acquire a positive charge leading to electrostatic repulsion. The forces in the range 3.8 < pH < 4.8 show an additional attraction on approach and much greater adhesion than at other pH values. This is attributed to the hydrophobic attraction being amplified by a small degree of charge on the surface as has previously been reported for adhesion measurements. The range of the measured forces is attributed to the long-range orientational order of water (>5 nm).

Spatially Heterogeneous Water Properties at Disordered Surfaces Decrease the Hydrophobicity of Nonpolar Self-Assembled Monolayers

Understanding the relationship between hydrophobicity and the properties of functionalized surfaces is vital to the design of materials that interact in aqueous environments. In this Letter, we use atomistic molecular dynamics simulations to investigate the effects of surface order on the hydrophobicity of self-assembled monolayers (SAMs) containing nonpolar ligands. We find that the interfacial hydrophobicity is highly correlated with SAM order and, strikingly, poorly correlated with the solvent-accessible surface area, which typically has been related to interfacial hydrophobicity. Analysis of spatial variations in both SAM and water properties reveals that the SAM-water interface is pinned near regions of disordered SAM surfaces with increased free volume, decreasing the overall interfacial hydrophobicity. Spatial variations in ligand end group positions at disordered SAM surfaces thus translate to spatial variations in hydrophobicity, yielding heterogeneous surface properties. These findings provide new insights into how surface order can alter the hydrophobicity of chemically uniform surfaces.

Molecular Order Affects Interfacial Water Structure and Temperature-Dependent Hydrophobic Interactions between Nonpolar Self-Assembled Monolayers

Understanding how material properties affect hydrophobic interactions-the water-mediated interactions that drive the association of nonpolar materials-is vital to the design of materials in contact with water. Conventionally, the magnitude of the hydrophobic interactions between extended interfaces is attributed to interfacial chemical properties, such as the amount of nonpolar solvent-exposed surface area. However, recent experiments have demonstrated that the hydrophobic interactions between uniformly nonpolar self-assembled monolayers (SAMs) also depend on molecular-level SAM order. In this work, we use atomistic molecular dynamics simulations to investigate the relationship between SAM order, water structure, and hydrophobic interactions to explain these experimental observations. The SAM-SAM hydrophobic interactions calculated from the simulations increase in magnitude as SAM order increases, matching experimental observations. We explain this trend by showing that the molecular-level order of the SAM impacts the nanoscale structure of interfacial water molecules, leading to an increase in water structure near disordered SAMs. These findings are consistent with a decrease in the solvation entropy of disordered SAMs, which is confirmed by measuring the temperature dependence of hydrophobic interactions using both simulations and experiments. This study elucidates how hydrophobic interactions can be influenced by an interfacial physical property, which may guide the design of synthetic materials with fine-tuned interfacial hydrophobicity.

Vesicle-like assemblies of ligand-stabilized nanoparticles with controllable membrane composition and properties

Here, we report that nanoparticles modified with simple end-functionalized alkyl thiol ligands show interesting directional self-assembly behavior and can act as an effective surfactant to encapsulate other functional molecules and nanoparticles. Gold nanoparticles modified with the mixture of alkyl thiols and hydroxyl-terminated alkyl thiols organize into unique vesicle-like structures with controllable membrane thicknesses. Molecular dynamics simulations showed that the ligand segregation and the edge-to-edge ligand binding are responsible for the two-dimensional assembly formation. Furthermore, the nanoparticle assemblies can encapsulate other functional nanoparticles into the membrane or inside the cavity, generating multicomponent inorganic vesicles with various morphologies. The light-induced release profiles of encapsulated dye molecules showed that the membrane properties can be controlled by varying the membrane thickness and ligand composition.

Alkyl-Modified Gold Surfaces: Characterization of the Au-C Bond

The surface of gold can be modified with alkyl groups through a radical crossover reaction involving alkyliodides or bromides in the presence of a sterically hindered diazonium salt. In this paper, we characterize the Au-C(alkyl) bond by surface-enhanced Raman spectroscopy (SERS); the corresponding peak appears at 387 cm^-1 close to the value obtained by theoretical modeling. The Au-C(alkyl) bond energy is also calculated, it reaches -36.9 kcal mol^-1 similar to that of an Au-S-alkyl bond but also of an Au-C(aryl) bond. In agreement with the similar energies of Au-C(alkyl) and Au-S-(alkyl), we demonstrate experimentally that these groups can be exchanged on the surface of gold.

Modulation of Hydrophobic Interaction by Mediating Surface Nanoscale Structure and Chemistry, not Monotonically by Hydrophobicity

The hydrophobic (HB) interaction plays a critical role in many colloidal and interfacial phenomena, biophysical and industrial processes. Surface hydrophobicity, characterized by the water contact angle, is generally considered the most dominant parameter determining the HB interaction. Herein, we quantified the HB interactions between air bubbles and a series of hydrophobic surfaces with different nanoscale structures and surface chemistry in aqueous media using a bubble probe atomic force microscopy (AFM). Surprisingly, it is discovered that surfaces of similar hydrophobicity can show different ranges of HB interactions, while surfaces of different hydrophobicity can have similar ranges of HB interaction. The increased heterogeneity of the surface nanoscale structure and chemistry can effectively decrease the decay length of HB interaction from 1.60 nm to 0.35 nm. Our work provides insights into the physical mechanism of HB interaction.