Molecular Understanding on the Underwater Oleophobicity of Self-Assembled Monolayers: Zwitterionic versus Nonionic

Bioinspired Zwitterionic Nanowires with Simultaneous Biofouling Reduction and Release

Marine biofouling is a complex and dynamic process that significantly increases the carbon emissions from the maritime industry by increasing drag losses. However, there are no existing non-toxic marine paints that can achieve both effective fouling reduction and efficient fouling release. Inspired by antifouling strategies in nature, herein, a superoleophobic zwitterionic nanowire coating with a nanostructured hydration layer is introduced, which exhibits simultaneous fouling reduction and release performance. The zwitterionic nanowires demonstrate >25% improvement in fouling reduction compared to state-of-the-art antifouling nanostructures, and four times higher fouling-release compared to conventional zwitterionic coatings. Fouling release is successfully achieved under a wall shear force that is four orders of magnitude lower than regular water jet cleaning. The mechanism of this simultaneous fouling reduction and release behavior is explored, and it is found that a combination of 1) a mechanical biocidal effect from the nanowire geometry, and 2) low interfacial adhesion resulting from the nanostructured hydration layer, are the major contributing factors. These findings provide insights into the design of nanostructured coatings with simultaneous fouling reduction and release. The newly established synthesis procedure for the zwitterionic nanowires opens new pathways for implementation as antifouling coatings in the maritime industry and biomedical devices.

Impact of Membrane Modification and Surface Immobilization Techniques on the Hemocompatibility of Hemodialysis Membranes: A Critical Review

Despite significant research efforts, hemodialysis patients have poor survival rates and low quality of life. Ultrafiltration (UF) membranes are the core of hemodialysis treatment, acting as a barrier for metabolic waste removal and supplying vital nutrients. So, developing a durable and suitable membrane that may be employed for therapeutic purposes is crucial. Surface modificationis a useful solution to boostmembrane characteristics like roughness, charge neutrality, wettability, hemocompatibility, and functionality, which are important in dialysis efficiency. The modification techniques can be classified as follows: (i) physical modification techniques (thermal treatment, polishing and grinding, blending, and coating), (ii) chemical modification (chemical methods, ozone treatment, ultraviolet-induced grafting, plasma treatment, high energy radiation, and enzymatic treatment); and (iii) combination methods (physicochemical). Despite the fact that each strategy has its own set of benefits and drawbacks, all of these methods yielded noteworthy outcomes, even if quantifying the enhanced performance is difficult. A hemodialysis membrane with outstanding hydrophilicity and hemocompatibility can be achieved by employing the right surface modification and immobilization technique. Modified membranes pave the way for more advancement in hemodialysis membrane hemocompatibility. Therefore, this critical review focused on the impact of the modification method used on the hemocompatibility of dialysis membranes while covering some possible modifications and basic research beyond clinical applications.

Role of chemistry in bio-inspired liquid wettability

Chemistry and topography are the two distinct available tools for customizing different bio-inspired liquid wettability including superhydrophobicity, superamphiphobicity, underwater superoleophobicity, underwater superoleophilicity, and liquid infused slippery property. In nature, various living species possessing super and special liquid wettability inherently comprises of distinctly patterned surface topography decorated with low/high surface energy. Inspired from the topographically diverse natural species, the variation in surface topography has been the dominant approach for constructing bio-inspired antiwetting interfaces. However, recently, the modulation of chemistry has emerged as a facile route for the controlled tailoring of a wide range of bio-inspired liquid wettability. This review article aims to summarize the various reports published over the years that has elaborated the distinctive importance of both chemistry and topography in imparting and modulating various bio-inspired wettability. Moreover, this article outlines some obvious advantages of chemical modulation approach over topographical variation. For example, the strategic use of the chemical approach has allowed the facile, simultaneous, and independent tailoring of both liquid wettability and other relevant physical properties. We have also discussed the design of different antiwetting patterned and stimuli-responsive interfaces following the strategic and precise alteration of chemistry for various prospective applications.

Implications of surfactant hydrophobic chain architecture on the Surfactant-Skin lipid model interaction

Although surfactants have been widely used in skin care and other related applications, our knowledge about how surfactants interact with stratum corneum (SC) lipids remains limited. This work reports how surfactants interact with a lipid SC model by neutron diffraction and molecular dynamics (MD) simulations, focusing on examining the impact of surfactant molecular architecture. The surfactant-SC mixed membrane was constructed by an equimolar mixture of ceramide/cholesterol/fatty acids and surfactant at 1% molar ratio of total lipids. The arrangements of water and surfactant molecules in the membrane were obtained through neutron scattering length density (NSLD) profiles via contrast variation method, meanwhile, MD simulation clearly demonstrated the mechanism of hydration change in the surfactant-model SC mixed membrane. No drastic difference was detected in the repeating distance of the short periodicity phase (SPP) upon adding surfactants, however, it significantly enhanced the membrane hydration and reduced the amount of phase separated crystalline cholesterol, showing a strong dependence on surfactant chain length, branching and double bond. This work clearly demonstrates how surfactant architecture affects its interaction with the SC membrane, providing useful guidance for either choosing an existing surfactant or designing a new one for surfactant-based transdermal application.

Heat Conduction Performance over a Poly(ethylene glycol) Self-Assembled Monolayer/Water Interface: A Molecular Dynamics Study

This study examined the interfacial heat condition between a poly(ethylene glycol) (PEG) self-assembled monolayer (SAM) and water using molecular dynamics simulation. It was found that the PEG SAM has higher thermal boundary conductance (TBC) than the traditionally used alkane-based SAM. The TBC conditionally varied with the length of the PEG molecules, where interfacial thermal resistance was a key factor. Our results reveal that the TBC of the PEG SAM/water interface is greatly influenced by its structural properties rather than the matching of vibrational properties between the SAM terminal and water. The structural analysis shows that the water structure around the terminal oxygen atom of the SAM plays a vital role in controlling the TBC. In this study, the concept of free volume has also been exploited, and the result suggests that the reduction of the free volume fraction accommodates a higher TBC. The model was precisely validated against experimental data by calculating the tilt angle and dihedral angle of the PEG SAM, the persistence length of the PEG chain in the water medium, and the sulfur position of the PEG SAM headgroup on the gold surface using quantitative scanning transmission electron microscopy image simulation.

Computer Simulations on a pH-Responsive Anticancer Drug Delivery System Using Zwitterion-Grafted Polyamidoamine Dendrimer Unimolecular Micelles

Unimolecular micelles have attracted wide attention in the field of drug delivery because of their thermodynamic stability and uniform size distribution. However, their drug loading/release mechanisms at the molecular level have been poorly understood. In this work, the stability and drug loading/release behaviors of unimolecular micelles formed using generation-5 polyamidoamine-graft-poly(carboxybetaine methacrylate) (PAMAM(G5)-PCBMA) were studied by dissipative particle dynamics simulations. In addition, the unimolecular micelles formed using generation-5 polyamidoamine-graft-poly(ethyleneglycol methacrylate) (PAMAM(G5)-PEGMA) were used as a comparison. The simulation results showed that PAMAM(G5)-PCBMA can spontaneously form core-shell unimolecular micelles. The PAMAM(G5) dendrimer constitutes a hydrophobic core to load the doxorubicin (DOX), while the zwitterionic PCBMA serves as a protective shell to improve the stability of the unimolecular micelle. The DOX can be encapsulated into the cavity of PAMAM(G5) at the physiological pH 7.4. The drug loading efficiency and drug loading content showed some regularities with the increase in the drug concentration. At the acidic pH 5.0, the loaded DOX can be released gradually from the hydrophobic core. The comparison of DOX-loaded morphologies between the PAMAM(G5)-PCBMA system and PAMAM(G5)-PEGMA system showed that the former has better monodisperse stability. This work could offer theoretical guidance for the design and development of promising unimolecular micelles for drug delivery.

Computer simulation of zwitterionic polymer brush grafted silica nanoparticles to modify polyvinylidene fluoride membrane

Dissipative particle dynamics (DPD) simulations was adopted to investigate the modification of polyvinylidene fluoride (PVDF) membrane by adding zwitterionic polymer brush poly(sulfobetaine methacrylate)- tetraethyl orthosilicate (PSBMA-TEOS) grafted silicon nanoparticles (SNPs) to the casting solution. The effects of polymer concentration and grafting architecture (PSBMA length and SNPs grafting ratio) on membrane morphology are discussed. When the polymer concentration reaches 40%, part of the SNPs is embedded in the membrane; the optimal polymer concentration is around 25-30%. In the SNPs system with the grafting ratio of 1, some SNPs are eluted into solution during phase separation. Compared with different grafting architectures, M_8-5, M_10-5 and M_12-5 system (M_x-y, where x represents the length of the zwitterionic polymer brush and y represents the grafting ratio of the silica nanoparticles) exhibited stable membrane morphologies. This work can provide guidance for the design and modification of organic-inorganic composite membrane and help understand the distribution of modified materials on the membrane surface.

Biomimicking properties of cellulose nanofiber under ethanol/water mixture

The two types of cellulose nanofiber (CNF) surface characteristics were evaluated by oil contact angle under ethanol-water solution at several concentrations as well as in air. Wood pulp-based 2,2,6,6-tetramethylpiperidine-1-oxylradical (TEMPO)-oxidized cellulose nanofiber (TOCNF) sheets and bamboo-derived mechanical counter collision cellulose nanofiber (ACC-CNF) sheets were fabricated by casting followed by drying. The CNF shows underwater superoleophobic mimicking fish skin properties and slippery surface mimicking Nepenthes pitcher. The underwater superoleophobic properties of CNF was evaluated theoretically and experimentally. The theoretical calculation and experimental results of contact angle showed a large deviation. The roughness, zeta potential, and water absorption at different concentrations were key factors that determine the deviation. Antifouling investigation revealed that CNF was a good candidate for antifouling material.

Understanding the Antifouling Mechanism of Zwitterionic Monomer-Grafted Polyvinylidene Difluoride Membranes: A Comparative Experimental and Molecular Dynamics Simulation Study

The antifouling process of the membrane is very vital for the highly efficient treatment of industrial wastewater, especially high salinity wastewater containing oil and other pollutants. In the present work, the dynamical antifouling mechanism is explored via molecular dynamics simulations, while the corresponding experiments about surface properties of the zwitterionic monomer-grafted polyvinylidene difluoride membrane are designed to verify the simulated mechanism. Water can form a stable hydration layer at the grafted membrane surface, where all the simulated radial distribution function of water/membrane, hydrogen bond number, water diffusivity, and experimental oil contact angles are stable. However, the water flux across the membrane will increase first and then decrease as the grafting ratio increases, which not only depends on the reduced pore size of the zwitterionic monomer-grafted membrane but also results from water diffusion. Furthermore, the dynamical fouling processes of pollutants (taking sodium alginate as an example) on the grafted membrane in water and brine solution are investigated, where both the high grafting ratio and electrolyte CaCl₂ can enhance the fouling energy barrier of the pollutant. The results show that both the enhanced hydrophilic property and the electrostatic repulsion can affect the antifouling capability of the grafted membrane. Finally, the ternary synergistic antifouling mechanisms among the zwitterionic membrane, electrolyte, and pollutant sodium alginates are discussed, which could be helpful for the rational design and preparation of new and highly efficient zwitterionic antifouling membranes.

Ultrastable Underwater Anti-Oil Fouling Coatings from Spray Assemblies of Polyelectrolyte Grafted Silica Nanochains

Surfaces that have superhydrophilic characteristics are known to exhibit extreme oil repellency under water, which is attractive for applications including anti-fogging, water-oil separations, and self-cleaning. However, superhydrophilic surfaces can also be easily fouled and lose their extreme oil repellency, which limits their usage in practical applications. In this work, we create an anti-oil fouling coating by spray coating poly(acrylic acid) (PAA)-grafted SiO₂ nanochains (approximately 45 nm wide and 300 nm long) onto solid surfaces, forming a nanoporous film exhibiting superhydrophilicity (water contact angle in air ≈ 0°) and underwater superoleophobicity (dichloroethane contact angle ≥ 165°). The polymer-grafted nanochain assemblies exhibit extremely low contact angle hysteresis (<1°) and small adhesion hysteresis (-0.05 mN m^-1), and thus, oil can readily roll off from the surface when the coating is immersed in water. Compared to other superhydrophilic surfaces, we show that both the unique structure of spray-assembled nanochains and the hygroscopic nature of PAA are essential to enable ultrastable anti-oil fouling. Even after the PAA-grafted nanochain coating is purposely fouled by oil, oil can be readily and completely expelled and lifted-off from the coating within 10 s when placed under water. Further, we show that our coating retains underwater superoleophobicity even after being subjected to shearing under water for more than 168 h. Our approach offers a simple yet versatile method to create an ultrastable superhydrophilic and anti-oil fouling coating via a scalable manufacturing method.

Molecular Dynamics Simulation of the Effect of Carbon Space Lengths on the Antifouling Properties of Hydroxyalkyl Acrylamides

Surface hydration has been proposed as the key antifouling mechanism of antifouling materials. However, molecular-level details of the structure, dynamics, and interactions of interfacial water around antifouling polymers still remain elusive. In this work, using all-atom molecular dynamics (MD) simulations, we studied four different acrylamides (AMs) for their interfacial water behaviors and their interactions with a protein, with special attention to the effect of carbon spacer lengths (CSLs) on the hydration properties of AMs. Collective MD simulation data revealed that although all four AMs displayed strong hydration, N-hydroxymethyl acrylamide (HMAA) and N-(2-hydroxyethyl)acrylamide (HEAA) with shorter CSLs displayed a longer residence time, slower self-diffusion, and lower coordination number of interfacial water molecules than N-(3-hydroxypropyl)acrylamide (HPAA) and N-(5-hydroxypentyl)-acrylamide (HPenAA) with longer CSLs. The shorter CSLs allow water molecules to form bridging hydrogen bonds with different hydrophilic groups in the same AM chain, thus enhancing the hydration capacity of AMs. Consequently, different from HPenAA, which had a weak but detectable interaction with the protein, HMAA, HEAA, and HPAA had almost zero interactions with the protein. This computational work provides a better fundamental understanding of the surface hydration and protein interaction of different AMs with subtle structural changes from structural, dynamic, and energy aspects at the atomic level, which hopefully will guide the design of new and effective nonfouling materials.

Zwitterionic Membrane via Nonsolvent Induced Phase Separation: A Computer Simulation Study

Dissipative particle dynamics (DPD) was adopted to study the nonsolvent induced phase separation (NIPS) process during a pH-responsive poly(ether sulfone) membrane preparation with a zwitterionic copolymer poly(ether sulfone)- block-polycarboxybetaine methacrylate (PES-b-PCBMA) as the blending additive. The membrane formation process and final morphology were analyzed. Simulation results show that the hydrophilic PCBMA segments enrich on the membrane surface by surface segregation and exhibit pH-responsive behavior, which is attributed to the deprotonation of the carboxylic acid group. With the polymer concentration increasing, both the shrinkage of the membrane and the flexibility of the system decrease, which also reduce the effect of surface segregation. By adjusting the blend ratio of PES-b-PCBMA with PES from 5% to 15%, the surface coverage of PCBMA segments on the membrane can be regulated. This work contributes to a better understanding on the mechanism of NIPS and can serve as a guide for the design of the polymer blend membrane.

Atomistic Study of Zwitterionic Peptoid Antifouling Brushes

Using molecular dynamics (MD) simulations, we study the molecular behavior and hydration properties of a set of zwitterionic "peptoid" brushes, grafted on a rutile surface, that has been previously reported to exhibit excellent resistance against protein adsorption and cell attachment. Peptoids are novel poly( N-substituted glycine) peptide mimics with the side chains attached to amide nitrogens. They constitute a unique model polymer system because hundreds of side chains have been demonstrated, and the exact chain length and sequence order of the residues/monomers may be specified in experiments. In this report, we vary the brush grafting density as well as the side chain/polymer molecular volume. We include in our study polysarcosine as an uncharged comparison with a small polymer chain cross-section. Sarcosine is the simplest peptoid residue with only a nominally hydrophobic methyl group as side chain, but is also reported to exhibit high antifouling performance. Overall, we show in detail how molecular volume and hydration effects are intertwined in a zwitterionic polymer brush. For example, the zwitterionic design significantly promotes extended chain conformations and could actually lower the overall electrostatic potential. Some properties promoted by the balanced charges, such as chain flexibility and hydration, increase more prominently at "low" to "intermediate" chain densities. These and other observations should provide insight on the molecular behavior of peptoids and inform the design of zwitterionic antifouling polymer brushes.

pH-Responsive Zwitterionic Copolymer DHA-PBLG-PCB for Targeted Drug Delivery: A Computer Simulation Study

In this work, the self-assembled behaviors of zwitterionic copolymer docosahexaenoic acid- b-poly(γ-benzyl-l-glutamate)- b-poly(carboxybetaine methacrylate) (DHA-PBLG-PCB) and the loading and release mechanism of the anticancer drug doxorubicin (DOX) was investigated via computer simulations. The effects of polymer concentration, drug content, and pH on polymeric micelles were explored by dissipative particle dynamics (DPD) simulations. Simulation results show that DHA-PBLG₁₅-PCB₁₀ can self-assemble into core-shell micelles; in addition, the drug-loaded micelles have a pH-responsive feature. DOX can be encapsulated into the core-shell micelle under normal physiological pH conditions, whereas it can be released under acidic pH conditions. The self-assembled behaviors of copolymer DHA-PBLG-PEG were also studied to have a comparison with those of DHA-PBLG-PCB. The DHA-PBLG₁₅-PCB₁₀ system has a stable structure and it has a great potential to serve as drug delivery vehicles for targeted drug delivery.

Fabrication of Janus Membranes for Desalination of Oil-Contaminated Saline Water

Desalination of oil-contaminated saline water using membrane distillation requires hydrophobic membranes with underwater superoleophobic surfaces. For designing such membranes, the chemistry and morphology of the interfacial domains in contact with the contaminated water need to be adjusted such that a stable water layer, adhering to the surface, prevents oil droplets from wetting the membrane. In this article, we present an approach that relies on the controlled functionalization of the surface of polyvinylidene fluoride (PVDF) membranes; we adjust the surface topography of the membranes and introduce chemical heterogeneity to them. We show that the morphology of the PVDF surface can be altered by adjusting the composition of the nonsolvent bath used for the phase inversion process. Also, we render the surface of the membranes hydrophilic by using an alkaline chemical bath solution. The membrane morphology and effectiveness of our chemical treatment were confirmed by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Fourier-transformed infrared spectroscopy (FTIR), and zeta potential measurements. A stable underwater contact angle, higher than 150°, was observed for both canola oil (ρ ≈ 0.913 g cm^-3, γ ≈ 31.5 mN m^-1) and hexane (ρ ≈ 0.655 g cm^-3, γ ≈ 18 mN m^-1). We evaluated the performance of both pristine and functionalized membranes in a laboratory-scale direct contact membrane distillation (DCMD) setup and desalinated a saline solution contaminated with 500 ppm canola oil. Our results show that oil does not wet the functionalized membrane during the desalination process. The average permeate flux and salt rejection values for the functionalized membranes were 45 ± 5 Lm^-2h^-1 ( T_feed = 70 °C, T_distillate = 20 °C) and 99.99%, respectively.

Progress in ligand design for monolayer-protected nanoparticles for nanobio interfaces

Ligand-functionalized inorganic nanoparticles, also known as monolayer-protected nanoparticles, offer great potential as vehicles for in vivo delivery of drugs, genes, and other therapeutics. These nanoparticles offer highly customizable chemistries independent of the size, shape, and functionality imparted by the inorganic core. Their success as drug delivery agents depends on their interaction with three major classes of biomolecules: nucleic acids, proteins, and membranes. Here, the authors discuss recent advances and open questions in the field of nanoparticle ligand design for nanomedicine, with a focus on atomic-scale interactions with biomolecules. While the importance of charge and hydrophobicity of ligands for biocompatibility and cell internalization has been demonstrated, ligand length, flexibility, branchedness, and other properties also influence the properties of nanoparticles. However, a comprehensive understanding of ligand design principles lies in the cost associated with synthesizing and characterizing diverse ligand chemistries and the ability to carefully assess the structural integrity of biomolecules upon interactions with nanoparticles.

Understanding the Cellular Uptake of pH-Responsive Zwitterionic Gold Nanoparticles: A Computer Simulation Study

Surface functionalization of nanoparticles (NPs) with stealth polymers (e.g., hydrophilic and zwitterionic polymers) has become a common strategy to resist nonspecific protein adsorption recently. Understanding the role of surface decoration on NP-biomembrane interactions is of great significance to promote the application of NPs in biomedical fields. Herein, using coarse-grained molecular dynamics (CGMD) simulations, we investigate the interactions between stealth polymer-coated gold nanoparticles (AuNPs) and lipid membranes. The results show that AuNPs grafted with zwitterionic polymers can more easily approach the membrane surface than those coated with hydrophilic poly(ethylene glycol) (PEG), which can be explained by the weak dipole-dipole interaction between them. For zwitterionic AuNPs which can undergo pH-dependent charge conversion, different interaction modes which depend on the polymer protonation degree are found. When the protonation degree is low, the particles just adsorb on the membrane surface; at moderate protonation degrees, the particles can directly translocate across the lipid membrane through a transient hydrophilic pore formed on the membrane surface; the particles are fully wrapped by the curved lipid membrane at high protonation degrees, which may lead to endocytosis. Finally, the effect of polymer chain length on the cellular uptake of zwitterionic polymer-coated AuNPs is considered. The results demonstrate that longer polymer chain length will block the translocation of AuNPs across the lipid membrane when the protonation degree is not high; however, it can improve the transmembrane efficiency of AuNPs at high protonation degrees. We expect that these findings are of immediate interest to the design and synthesis of pH-responsive nanomaterials based on zwitterionic polymers and can prompt their further applications in the field of biomedicine.

Mesoscopic Structures of Poly(carboxybetaine) Block Copolymer and Poly(ethylene glycol) Block Copolymer in Solutions

The antifouling property of exogenous materials is vital for their in vivo applications. In this work, dissipative particle dynamics simulations are performed to study the self-assembled morphologies of two copolymer systems containing poly(ethylene glycol) (PEG) and poly(carboxybetaine) (PCB) in aqueous solutions. Effects of polymer composition and polymer concentration on the self-assembled structures of the two copolymers (PLA-PEG and PLA-PCB) are investigated, respectively [PLA represents poly(lactic acid)]. Results show that whatever the copolymer composition is, PLA-PEG systems will self-assemble into core-shell structures, whereas onion-like and vesicle structures are also found for the PLA-PCB systems. Different morphologies are obtained at different polymer concentrations in both copolymer systems. Simulation results demonstrate that PCB is more stable than PEG in maintaining self-assembled spherical structures of copolymer systems because PLA-PEG forms dumbbell-like structures whereas PLA-PCB is spherical under the same polymer concentration. Although both copolymer systems can self-assemble into core-shell nanoparticles when the block ratio of PLA:PEG or PLA:PCB is 80:20, the core-shell structures of the nanoparticles are quite different. The shell layers formed by PEG in PLA-PEG nanoparticles are inhomogeneous in size because of the amphiphilicity of PEG, whereas the shell layers in PLA-PCB nanoparticles are homogenous because of the strong hydrophilicity of the zwitterionic PCB polymer block.