Interaction Mechanisms and Predictions of the Biofouling of Polymer Films: A Combined Atomic Force Microscopy and Quartz Crystal Microbalance with Dissipation Monitoring Study.

Biofouling of polymeric membranes is a severe problem in water desalination and treatment applications. A fundamental understanding of biofouling mechanisms is necessary to control biofouling and develop more efficient mitigation strategies. To shed light on the type of forces that govern the interactions between biofoulants and membranes, biofoulant-coated colloidal AFM probes were employed to investigate the biofouling mechanisms of two model biofoulants, BSA and HA, toward an array of polymer films commonly used in membrane synthesis, which included CA, PVC, PVDF, and PS. These experiments were combined with quartz crystal microbalance with dissipation monitoring (QCM-D) measurements. The Derjaguin, Landau, Verwey, and Overbeek (DLVO) and the extended-DLVO (XDLVO) theoretical models were applied to decouple the overall adhesion interactions between the biofoulants and the polymer films into their component interactions, i.e., electrostatic (El), Lifshitz-van der Waals (LW), and Lewis acid-base (AB) interactions. The XDLVO model was found to predict better the AFM colloidal probe adhesion data and the QCM-D adsorption behavior of BSA onto the polymer films than the DLVO model. The ranking of the polymer films' adhesion strengths and adsorption quantities was inversely proportional to their γ- values. Higher normalized adhesion forces were quantified for the BSA-coated colloidal probes with the polymer films than the HA-coated colloidal probes. Similarly, in QCM-D measurements, BSA was found to cause larger adsorption mass shifts, faster adsorption rates, and more condensed fouling layers than HA. A linear correlation (R2 = 0.96) was obtained between the adsorption standard free energy changes (ΔGads°) estimated for BSA from the equilibrium QCM-D adsorption experiments and the AFM normalized adhesion energies (WAFM/R) estimated for BSA from the AFM colloidal probe measurements. Eventually, an indirect approach was presented to calculate the surface energy components of biofoulants characterized by high porosities from Hansen dissolution tests to perform the DLVO/XDLVO analyses.

Microscale characterization of abiotic surfaces and prediction of their biofouling/anti-biofouling potential using the AFM colloidal probe technique.

Given its detrimental consequences, biofouling is a central problem in industrial applications that affects submerged artificial abiotic surfaces. For instance, it shortens the service life of surfaces operating in marine and industrial environments, such as ship hulls, pipelines, heat exchangers, water filtration membranes, biosensors, and medical implants and concomitantly increases maintenance demands. In recent years, research has been propelled by the urge to minimize the replacement and maintenance costs related to the biofouling of abiotic surfaces. Colloidal probe atomic force microscopy (AFM) is a crucial analytical technique for quantifying interfacial interactions and unveiling the underlying biofouling mechanisms for a broad range of substrates. This technique is used to measure surface and colloidal forces by retracting individual colloidal particles immobilized at the apex of the AFM cantilever away from a planar surface under the conditions of choice. These forces are believed to dominate the biofouling behavior of surfaces. Several studies have utilized the colloidal probe AFM technique to estimate the adhesion of biofoulants to surfaces and elucidate the various aspects governing this process. This review summarizes the applications of colloidal probe AFM, including but not limited to characterizing the properties of abiotic surfaces, such as adhesive interactions, mechanical properties, and biofouling propensities. In addition, recent applications of colloidal probes relevant to biofouling are outlined. Furthermore, studies using various modified colloidal probes to test the biofouling resistance/anti-biofouling capacity of modulated abiotic surfaces are included.

Shifting to Transparent/Hazy Properties: The Case of Alginate/Network Cellulose All-Polysaccharide Composite Films.

Light-management films made entirely from natural polymers with tunable haze properties are developed via a facile approach. A novel green method based simply on the blending of network cellulose (NC)/water suspension with alginate (CaAlg) aqueous solution is proposed. The unique NC suspension created by a controlled hydrolysis of microcrystalline cellulose acts as the scatterer media while alginate serves as the transparent host matrix. NC features isotropic intertwined network of nanofibers that contributes to light scattering and produces optical haze. The opaque but hazy NC is dispersed purposefully in the alginate film, where its original properties are preserved owing to its poor solubility in water. Additionally, the dispersion notably increases the roughness of the composite film surface and acts as a light scatterer. Eventually, composite CaAlg/NC film with high transparency (>94%) and customized haze (15-73%) at 550 cm-1 wavelength is fabricated. Herein, the transparent alginate is successfully combined with the hazy cellulose of uniformly distributed nanofibers by blending to fabricate transparent/hazy all-natural films. The fabricated films exhibit high transparency with tailored transmission haze. The film is highly fitting for large-scale production and adequate to meet different haze requirements to accommodate different applications such as privacy protection films and antiglare/antireflection coatings.

Force-Averaging DLVO Model Predictions of the Adhesion Strengths Quantified for Pathogenic Listeria monocytogenes EGDe Grown under Variable pH Stresses.

The roles of the bacterial surface biopolymers of pathogenic Listeria monocytogenes EGDe grown under variable pH conditions in governing their adhesion to a model surface of silicon nitride were investigated using atomic force microscopy under water. Our results indicated that the adhesion forces were the highest for cells cultured in media adjusted to pH 7 followed by 1.39, 1.49, 1.57, and 2.18-fold reductions at pH 6, 8, 9, and 5, respectively. Adhesion energies followed the same trends with 1.35, 1.67, 2.20, and 2.79-fold reductions in energies at pH 6, 8, 9, and 5, respectively, compared to the energy measured at pH 7. Furthermore, the structural properties of the bacterial surface biopolymer brush represented by the biopolymer brush thickness (Lo) and the molecular density (Γ) were determined by fitting a steric model of repulsion to the approach force-distance data. The Lo values followed the same trends as adhesion forces and energies, with thickness being highest at pH 7 followed by 1.82, 2.99, 3.11, and 4.66-fold reductions at pH 6, 8, 9, and 5, respectively. Γ was the highest at pH 5 and was followed by 1.26, 1.27, 1.70, and 2.82-fold reductions at pH 8, 9, 6, and 7, respectively. Our results indicated that bacterial adhesion forces and energies increased linearly with the product of Lo and Γ representing the number of biopolymers per unit length of the bacterial surface. To predict the adhesion forces and energies measured, a force-averaging model of the soft-particle analysis of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used. In addition to the standard parameters accounted for in the soft-particle analysis of the DLVO theory such as surface potential, hydrophobicity, and size, this averaging model incorporates in it structural bacterial parameters such as Lo and Γ as well as a surface coverage factor (ϕ) that represents the fraction of the bacterial surface covered by biopolymers. When the soft-particle analysis of DLVO was considered, repulsive hydrogen bond strengths were predicted at close distances of approach (<0.3 nm). In comparison, the force-averaging model predicted that attractive hydrogen bonds dominate the bacterial adhesion strengths quantified. The highest adhesion quantified for cells grown at pH 7 was related to longer and more spaced biopolymers, higher contents of cellular carbohydrates, and more hydrophilic biopolymers, each of which contributes to higher possibilities for hydrogen bonding formation. These results are significant in designing new strategies that aim at controlling bacterial adhesion to surfaces.

Thioether-Crown-Rich Calix[4]arene Porous Polymer for Highly Efficient Removal of Mercury from Water.

A rational design of adsorbents with high uptake efficiency and fast kinetics for highly toxic pollutants is a key challenge in environmental remediation. Here, we report the design of a well-defined thioether-crown-rich porous calix[4]arene-based mesoporous polymer S-CX4P and its utility in removal of highly relevant toxic mercury (Hg2+) from water. The polymer shows an exceptional, record-high uptake efficiency of 1686 mg g-1 and the fastest initial adsorption rate of 278 mg g-1 min-1. Remarkably, S-CX4P can effectively remove Hg2+ from high concentration (5 ppm) to below the acceptable limit for drinking water (2 ppb) even in the presence of other competitive metals at high concentrations. In addition, the polymer can be easily regenerated at room temperature and reused multiple times with negligible loss in uptake rate and efficiency. The results demonstrate the potential of rationally designed thioether-crown-rich polymers for high performance mercury removal.

Synthesis of Mesoporous/Macroporous Microparticles Using Three-Dimensional Assembly of Chitosan-Functionalized Halloysite Nanotubes and Their Performance in the Adsorptive Removal of Oil Droplets from Water.

Halloysite nanotubes (HNTs) were assembled into mesoporous/macroporous microparticles (c-g-HNTs MPs) using Pickering template-assisted approach. To unravel the stabilization mechanism in Pickering emulsion form, several emulsions and microparticles were prepared at various conditions and visualized using confocal laser scanning microscopy. The prepared c-g-HNTs MPs were used to treat emulsified oil solutions resulting in a maximum removal efficiency of 94.47%. The kinetics data of oil adsorption onto c-g-HNTs MPs was best fitted by the pseudo-second-order kinetic model ( R2 = 0.9983). The maximum monolayer adsorption capacity of oil onto c-g-HNTs MPs as predicted by the multilayer Brunauer-Emmett-Teller model was found to be 788 mg/g. Compared with pristine HNTs, c-g-HNTs MPs exhibited higher self-settleability rates in aqueous solutions as well as in emulsified oil solutions, demonstrating their candidacy for practical water treatment applications. The c-g-HNTs MPs were repeatedly used for five adsorption-desorption cycles with minimal losses noticed in their performance.

A new approach to decoupling of bacterial adhesion energies measured by AFM into specific and nonspecific components.

A new method to decoupling of bacterial interactions measured by atomic force microscopy (AFM) into specific and nonspecific components is proposed. The new method is based on computing the areas under the approach and retraction curves. To test the efficacy of the new method, AFM was used to probe the repulsion and adhesion energies present between L. monocytogenes cells cultured at five pH values (5, 6, 7, 8 and 9) and silicon nitride (Si3N4). Overall adhesion energy was then decoupled into its specific and nonspecific components using the new method as well as using Poisson statistical approach. Poisson statistical method represents the most commonly used approach to decouple bacterial interactions into their components. For all pH conditions investigated, specific energies dominated the adhesion and a transition in adhesion and repulsion energies for cells cultured at pH 7 was observed. When compared, the differences in the specific and nonspecific energies obtained using Poisson analysis and the new method were on average 2.2% and 6.7%, respectively. The relatively close energies obtained using the two approaches demonstrate the efficacy of the new method as an alternative way to decouple adhesion energies into their specific and nonspecific components.

Cellular and molecular investigations of the adhesion and mechanics of Listeria monocytogenes lineages' I and II environmental and epidemic strains.

Atomic force microscopy (AFM) was used to probe the mechanical and adherence properties of eight Listeria monocytogenes' strains representative of the species' two phylogenetic lineages I and II. From a functional perspective, lineage' I strains were characterized by lower overall adhesion forces and higher specific and nonspecific forces compared to lineage' II strains. From a structural perspective, lineage' II strains were characterized by higher Young's moduli and longer and stiffer biopolymers compared to lineage' I strains. Both lineages' I and II strains were similar in their grafting densities. Finally, our results indicated that epidemic and environmental strains of L. monocytogenes and irrespective of their lineage group were characterized by similar Young's moduli of elasticities and adhesion forces at the cellular level. However, at the molecular level, epidemic strains were characterized by higher specific and nonspecific forces, shorter, denser, and more flexible biopolymers compared to environmental strains.