Ionic Strength-Dependent Attachment of Pseudomonas aeruginosa PAO1 on Graphene Oxide Surfaces

Graphene oxide (GO) is a widely used antimicrobial and antibiofouling material in surface modification. Although the antibacterial mechanisms of GO have been thoroughly elucidated, the dynamics of bacterial attachment on GO surfaces under environmentally relevant conditions remain largely unknown. In this study, quartz crystal microbalance with dissipation monitoring (QCM-D) was used to examine the dynamic attachment processes of a model organism Pseudomonas aeruginosa PAO1 onto GO surface under different ionic strengths (1-600 mM NaCl). Our results show the highest bacterial attachment at moderate ionic strengths (200-400 mM). The quantitative model of QCM-D reveals that the enhanced bacterial attachment is attributed to the higher contact area between bacterial cells and GO surface. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and atomic force microscopy (AFM) analysis were employed to reveal the mechanisms of the bacteria-GO interactions under different ionic strengths. The strong electrostatic and steric repulsion at low ionic strengths (1-100 mM) was found to hinder the bacteria-GO interaction, while the limited polymer bridging caused by the collapse of biopolymer layers reduced cell attachment at a high ionic strength (600 mM). These findings advance our understanding of the ionic strength-dependent bacteria-GO interaction and provide implications to further improve the antibiofouling performance of GO-modified surfaces.

Influence of Dispersed TiO2 Nanoparticles via Steric Interaction on the Antifouling Performance of PVDF/TiO2 Composite Membranes

Herein, the influence of various contents of polyethylene glycol (PEG) on the dispersion of TiO2 nanoparticles and the comprehensive properties of PVDF/TiO2 composite membranes via the steric hindrance interaction was systematically explored. Hydrophilic PEG was employed as a dispersing surfactant of TiO2 nanoparticles in the pre-dispersion process and as a pore-forming additive in the following membrane preparation process. The slight overlap shown in the TEM image and low TSI value (<1) of the composite casting solution indicated the effective dispersion and stabilization under the steric interaction with a PEG content of 6 wt.%. Properties such as the surface pore size, the development of finger-like structures, permeability, hydrophilicity and Zeta potential were obviously enhanced. The improved antifouling performance between the membrane surface and foulants was corroborated by less negative free energy of adhesion (about −42.87 mJ/m2), a higher interaction energy barrier (0.65 KT) and low flux declination during the filtration process. The high critical flux and low fouling rate both in winter and summer as well as the long-term running operation in A/O-MBR firmly supported the elevated antifouling performance, which implies a promising application in the municipal sewage treatment field.

Nonequilibrium Uptake Kinetics of Molecular Cargo into Hollow Hydrogels Tuned by Electrosteric Interactions

Hollow hydrogels represent excellent nano- and microcarriers due to their ability to encapsulate and release large amounts of cargo molecules (cosolutes) such as reactants, drugs, and proteins. In this work, we use a combination of a phenomenological effective cosolute-hydrogel interaction potential and dynamic density functional theory to investigate the full nonequilibrium encapsulation kinetics of charged and dipolar cosolutes by an isolated charged hollow hydrogel immersed in a 1:1 electrolyte aqueous solution. Our analysis covers a broad spectrum of cosolute valences ( z_c) and electric dipole moments (μ_c), as well as hydrogel swelling states and hydrogel charge densities. Our calculations show that, close to the collapsed state, the polar cosolutes are predominantly precluded and the encapsulation process is strongly hindered by the excluded-volume interaction exerted by the polymer network. Different equilibrium and kinetic sorption regimes (interface versus interior) are found depending on the value and sign of z_c and the value of μ_c. For cosolutes of the same sign of charge as the gel, the superposition of steric and electrostatic repulsion leads to an "interaction-controlled" encapsulation process, in which the characteristic time to fill the empty core of the hydrogel grows exponentially with z_c. On the other hand, for cosolutes oppositely charged to the gel, we find a "diffusion-controlled" kinetic regime, where cosolutes tend to rapidly absorb into the hydrogel membrane and the encapsulation rate depends only on the cosolute diffusion time across the membrane. Finally, we find that increasing μ_c promotes the appearance of metastable and stable surface adsorption states. For large enough μ_c, the kinetics enters an "adsorption-hindered diffusion", where the enhanced surface adsorption imposes a barrier and slows down the uptake. Our study represents the first attempt to systematically describe how the swelling state of the hydrogel and other leading physical interaction parameters determine the encapsulation kinetics and the final equilibrium distribution of polar molecular cargo.

Interaction between two parallel plates covered with a polyelectrolyte brush layer in an electrolyte solution

An approximate analytic expression is derived for the interaction energy between two parallel plates covered with a polyelectrolyte brush layer in an electrolyte solution. The interaction energy has three components: electrostatic interaction energy between two brush layers before and after their contact, steric interaction energy between two brush layers after their contact, and the van der Waals interaction energy between the cores of the plates. It is shown that these three components are of the same order of magnitude and contribute equally to the total interaction energy between two polyelectrolyte-coated plates in an electrolyte solution. On the basis of Derjaguin's approximation, an approximate expression for the interaction energy between two spherical particles covered with polyelectrolyte brush layers is also derived.

Chiral High-Speed Counter-Current Chromatography: Future Strategies for Chiral Selector Development

In conventional high-performance liquid chromatography, chiral separations are performed by chiral column with a chiral selector (CS) chemically boned to the solid support. In contrast, high-speed counter-current chromatography (HSCCC) performs chiral separations by dissolving CS in the liquid stationary phase. During the past two decades, several CSs were developed to successfully carry out chiral HSCCC which include N-dodecanoyl-L-proline-3,5-dimethylanilide, β-cyclodextrin derivatives, vancomycin, cinchona alkaloid derivatives, cellulose and amylose derivatives, tartaric acid derivatives, etc. Compared to HPLC which uses over hundred different kinds of CSs, the number of CSs effectively used in HSCCC is limited to several compounds. This may be due to the violent molecular movement of CS dissolved in the liquid stationary phase which reduces chiral selectivity based on steric affinity. Future development strategy of CS for HSCC proposed here is to suppress the molecular movement of the CS in the liquid stationary phase by the following three ways: 1) using viscous stationary phase such as aqueous-aqueous polymer phase system; 2) attaching a long hydrophobic chain to the asymmetric carbon, or 3) chemically bonding CS onto hydrophobic small particles such as carbon nanotubes, gold colloidal particles, and submicron silica particles.