Control of lysozyme adsorption by pH on surfaces modified with polyampholyte brushes

Effects of Cationic and Anionic Surfaces on the Perpendicular and Lateral Forces and Binding of Aspergillus niger Conidia

The binding of conidia to surfaces is a prerequisite for biofouling by fungal species. In this study, Aspergillus niger subtypes 1957 and 1988 were used which produced differently shaped conidia (round or spikey respectively). Test surfaces were characterised for their surface topography, wettability, and hardness. Conidial assays included perpendicular and lateral force measurements, as well as attachment, adhesion and retention assays. Anionic surfaces were less rough (Ra 2.4 nm), less wettable (54°) and harder (0.72 GPa) than cationic surfaces (Ra 5.4 nm, 36° and 0.5 GPa, respectively). Perpendicular and lateral force assays demonstrated that both types of conidia adhered with more force to the anionic surfaces and were influenced by surface wettability. Following the binding assays, fewer A. niger 1957 and A. niger 1988 conidia bound to the anionic surface. However, surface wettability affected the density and dispersion of the conidia on the coatings, whilst clustering was affected by their spore shapes. This work demonstrated that anionic surfaces were more repulsive to A. niger 1998 spores than cationic surfaces were, but once attached, the conidia bound more firmly to the anionic surfaces. This work informs on the importance of understanding how conidia become tightly bound to surfaces, which can be used to prevent biofouling.

Durability Assessment of a Plasma-Polymerized Coating with Anti-Biofilm Activity against L. monocytogenes Subjected to Repeated Sanitization

Biofilm formation on food-contact surfaces is a matter of major concern causing food safety and spoilage issues to this sector. The aim of this study was to assess the durability of the anti-biofilm capacity of a plasma-polymerized coating composed of a base coating of (3-aminopropyl)triethoxysilane (APTES) and a functional coating of acrylic acid (AcAc). Coated and uncoated AISI 316 stainless steel (SS) plates were subjected to five sanitization cycles with sodium hypochlorite (0.05%) and peracetic acid (0.5%). The effectiveness of the coating for the inhibition of multi-strain Listeria monocytogenes biofilm formation was confirmed using a three-strain cocktail, which was grown on the SS plates at 12 °C for 6 days. Compared to the uncoated SS, relative biofilm productions of 14.6% on the non-sanitized coating, 27.9% on the coating after sanitization with sodium hypochlorite, and 82.3% on the coating after sanitization with peracetic acid were obtained. Morphological and physicochemical characterization of the coatings suggested that the greater anti-biofilm effectiveness after sanitization with sodium hypochlorite was due to the high pH of this solution, which caused a deprotonation of the carboxylic acid groups of the functional coating. This fact conferred it a strong hydrophilicity and negatively charged its surface, which was favorable for preventing bacterial attachment and biofilm formation.

Tunable multi-responsive nano-gated mesoporous silica nanoparticles as drug carriers

Tunable multi-responsive mesoporous silica nanoparticles were prepared by post-condensation/surface modification of MCM-41 nanoparticles. Surface grafting of a poly(N,N-dimethylaminoethyl methacrylate)-based polymer containing disulfide bonds was achieved by a click reaction. Chemical modification, morphological characteristics, and textural properties of the nanoparticles were studied using multiple characterization techniques such as Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, small-angle X-ray scattering, and nitrogen adsorption/desorption behavior. The nanoparticles retained the meso-structural integrity of MCM41 and particle size < 100 nm after grafting with the polymer. The pH and redox-responsive behavior of the nanoparticles were also studied. The nanoparticles possess excellent drug-loading capacity owing to their large surface area and 'closed gate' mechanism of the grafted polymer chains. The release profile of doxorubicin at two different pH (7.4 and 5.5) and in the presence of dithiothreitol showed a dual response behavior. The nano drug carrier device exhibited efficient intracellular uptake in cancer cells with suitable cytotoxicity and pharmacokinetic behavior, and may therefore be considered a good candidate for cancer therapy.

Adsorption of lysozyme on gold surfaces in the presence of an external electric potential

Adsorbed protein films consist of essential building blocks of many biotechnological and biomedical devices. The electrostatic potential may significantly modulate the protein behaviour on surfaces, affecting their structure and biological activity. In this study, lysozyme was used to investigate the effects of applied electric potentials on adsorption and the protein structure. The pH and the surface charge determine the amount and secondary structure of adsorbed lysozyme on a gold surface. In-situ measurements using polarization modulation infrared reflection absorption spectroscopy indicated that the concentration of both the adsorbed anions and the lysozyme led to conformational changes in the protein film, which was demonstrated by a greater amount of aggregated β-sheets in films fabricated at net positive charges of the Au electrode (E_ads > E_pzc). The changes in secondary structure involved two parallel processes. One comprised changes in the hydration/hydrogen-bond network at helices, leading to diverse helical structures: α-, 3₁₀- and/or π-helices. In the second process β-turns, β-sheets, and random coils displayed an ability to form aggregated β-sheet structures. The study illuminates the understanding of electrical potential-dependent changes involved in the protein misfolding process.

Potential false-positive reasons for SARS-CoV-2 antibody testing and its solution

Coronavirus disease 2019 (COVID-19) has brought a huge impact on global health and the economy. Early diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is essential for epidemic prevention and control. The detection of SARS-CoV-2 antibodies is an important criterion for diagnosing COVID-19. However, SARS-CoV-2 antibody testing also has certain false positives causing confusion in clinical diagnosis. This article summarizes the causes of false-positive detection of SARS-CoV-2 antibodies in clinical practice. The results indicate that the most common endogenous interferences include rheumatoid factor, heterophile antibodies, human anti-animal antibodies, lysozyme, complement, and cross-antigens. The exogenous interference is mainly incomplete coagulation of the specimen, contamination of the specimen, and insufficient optimization of the diagnostic kit's reaction system.

Superhydrophilicity and strong salt-affinity: Zwitterionic polymer grafted surfaces with significant potentials particularly in biological systems

In recent years, zwitterionic polymers have been frequently reported to modify various surfaces to enhance hydrophilicity, antifouling and antibacterial properties, which show significant potentials particularly in biological systems. This review focuses on the fabrication, properties and various applications of zwitterionic polymer grafted surfaces. The "graft-from" and "graft-to" strategies, surface grafting copolymerization and post zwitterionization methods were adopted to graft lots type of the zwitterionic polymers on different inorganic/organic surfaces. The inherent hydrophilicity and salt affinity of the zwitterionic polymers endow the modified surfaces with antifouling, antibacterial and lubricating properties, thus the obtained zwitterionic surfaces show potential applications in biosystems. The zwitterionic polymer grafted membranes or stationary phases can effectively separate plasma, water/oil, ions, biomolecules and polar substrates. The nanomedicines with zwitterionic polymer shells have "stealth" effect in the delivery of encapsulated drugs, siRNA or therapeutic proteins. Moreover, the zwitterionic surfaces can be utilized as wound dressing, self-healing or oil extraction materials. The zwitterionic surfaces are expected as excellent support materials for biosensors, they are facing the severe challenges in the surface protection of marine facilities, and the dense ion pair layers may take unexpected role in shielding the grafted surfaces from strong electromagnetic field.

Design of Ultra-Thin PEO/PDMAEMA Polymer Coatings for Tunable Protein Adsorption

Protein adsorption on solid surfaces provides either beneficial or adverse outcomes, depending on the application. Therefore, the desire to predict, control, and regulate protein adsorption on different surfaces is a major concern in the field of biomaterials. The most widely used surface modification approach to prevent or limit protein adsorption is based on the use of poly (ethylene oxide) (PEO). On the other hand, the amount of protein adsorbed on poly(2-(dimethylamine)ethyl methacrylate) (PDMAEMA) coatings can be regulated by the pH and ionic strength of the medium. In this work, ultra-thin PEO/PDMAEMA coatings were designed from solutions with different ratios of PEO to PDMAEMA, and different molar masses of PEO, to reversibly adsorb and desorb human serum albumin (HSA), human fibrinogen (Fb), lysozyme (Lys), and avidine (Av), four very different proteins in terms of size, shape, and isoelectric points. X-ray photoelectron spectroscopy (XPS), quartz crystal microbalance (QCM), and atomic force microscopy (AFM) were used to characterize the mixed polymer coatings, revealing the presence of both polymers in the layers, in variable proportions according to the chosen parameters. Protein adsorption at pH 7.4 and salt concentrations of 10-3 M was monitored by QCM. Lys and Av did not adsorb on the homo-coatings and the mixed coatings. The amount of HSA and Fb adsorbed decreased with increasing the PEO ratio or its molar mass in a grafting solution. It was demonstrated that HSA and Fb, which were adsorbed at pH 7.4 and at an ionic strength of 10-3 M, can be fully desorbed by rinsing with a sodium chloride solution at pH 9.0 and ionic strength 0.15 M from the mixed PEO5/PDMAEMA coatings with PEO/PDMAEMA mass ratios of 70/30, and 50/50, respectively. The results demonstrate that mixed PEO/PDMAEMA coatings allow protein adsorption to be finely tuned on solid surfaces.

Grafting density dominant glass transition of dry polystyrene brushes

The effects of the grafting densities (σ_p), molecular weights (M_n) and thicknesses of dry polystyrene (PS) brushes on their glass transition temperature (T) were investigated by ellipsometry. The results show that T strongly depends on the grafting density of the PS brushes. The T of the PS brushes with σ_p > 0.30 increases with decreasing M_n (or brush thickness) and is mainly dominated by entropic effects, in which the grafted chains are highly extended along the film thickness direction resulting in a sharp reduction in configurational entropy. The T of PS brushes with σ_p < 0.30 decreases with decreasing M_n (or brush thickness) which is mainly dominated by surface effects. For intermediate-density brushes (σ_p = 0.30), T becomes independent of M_n or brush thickness. The reason for this grafting density dependence of T is attributed to the transition of the PS brush conformation from mushroom-to-brush.

Silica nanofibers as a new drug delivery system: a study of the protein–silica interactions

Drug delivery from silica nanofiber based materials for intervertebral disc regenerative medicine.

pH-Responsive polymers

This review summarizes pH-responsive monomers, polymers and their derivative nano- and micro-structures including micelles, cross-linked micelles, microgels and hydrogels.

Surface charge-bias impact of amine-contained pseudozwitterionic biointerfaces on the human blood compatibility

This work discusses the impact of the charge bias and the hydrophilicity on the human blood compatibility of pseudozwitterionic biomaterial gels. Four series of hydrogels were prepared, all containing negatively-charged 3-sulfopropyl methacrylate (SA), and either acrylamide, N-isopropylacrylamide, 2-dimethylaminoethyl methacrylate (DMAEMA) or [2-(methacryloyloxy)ethyl]trimethylammonium (TMA), to form S_nA_m, S_nN_m, S_nD_m or S_nT_m hydrogels, respectively. An XPS analysis proved that the polymerization was well controlled from the initial monomer ratios. All gels present high surface hydrophilicity, but varying bulk hydration, depending on the nature/content of the comonomer, and on the immersion medium. The most negative interfaces (pure SA, S7A3, S5A5) showed significant fibrinogen adsorption, ascribed to the interactions of the αC domains of the protein with the gels, then correlated to considerable platelet adhesion; but low leukocyte/erythrocyte attachments were measured. Positive gels (excess of DMAEMA or TMA) are not hemocompatible. They mediate protein adsorption and the adhesion of human blood cells, through electrostatic attractive interactions. The neutral interfaces (zeta potential between -10mV and +10mV) are blood-inert only if they present a high surface and bulk hydrophilicity. Overall, this study presents a map of the hemocompatible behavior of hydrogels as a function of their surface charge-bias, essential to the design of blood-contacting devices.

Charged hydrophilic polymer brushes and their relevance for understanding marine biofouling

The resistance of charged polymers to biofouling was investigated by subjecting cationic (PDMAEMA), anionic (PSPMA), neutral (PHEMA-co-PEG10MA), and zwitterionic (PSBMA) brushes to assays testing protein adsorption; attachment of the marine bacterium Cobetia marina; settlement and adhesion strength of zoospores of the green alga Ulva linza; settlement of barnacle (Balanus amphitrite and B. improvisus) cypris larvae; and field immersion tests. Several results go beyond the expected dependence on direct electrostatic attraction; PSPMA showed good resistance towards attachment of C. marina, low settlement and adhesion of U. linza zoospores, and significantly lower biofouling than on PHEMA-co-PEG10MA or PSBMA after a field test for one week. PDMAEMA showed potential as a contact-active anti-algal coating due to its capacity to damage attached spores. However, after field testing for eight weeks, there were no significant differences in biofouling coverage among the surfaces. While charged polymers are unsuitable as antifouling coatings in the natural environment, they provide valuable insights into fouling processes, and are relevant for studies due to charging of nominally neutral surfaces.

Elastic Properties of Protein Functionalized Nanoporous Polymer Films

Retaining the conformational structure and bioactivity of immobilized proteins is important for biosensor designs and drug delivery systems. Confined environments often lead to changes in conformation and functions of proteins. In this study, lysozyme is chemically tethered into nanopores of polystyrene thin films, and submicron pores in poly(methyl methacrylate) films are functionalized with streptavidin. Nanoindentation experiments show that stiffness of streptavidin increases with decreasing submicron pore sizes. Lysozymes in polystyrene nanopores are found to behave stiffer than the submicron pore sizes and still retain their specific bioactivity relative to the proteins on flat surfaces. Our results show that protein functionalized ordered nanoporous polystyrene/poly(methyl methacrylate) films present heterogeneous elasticity and can be used to study interactions between free proteins and designed surfaces.

Lysozyme adsorption at a silica surface using simulation and experiment: effects of pH on protein layer structure

Hen Egg White Lysozyme (HEWL) is a widely used exemplar to study protein adsorption on surfaces and interfaces. Here we use fully atomistic Molecular Dynamics (MD) simulations, Multi-Parametric Surface Plasmon Resonance (MP-SPR), contact angle and zeta potential measurements to study HEWL adsorption at a silica surface. The simulations provide a detailed description of the adsorption mechanism and indicate that at pH7 the main adsorption driving force is electrostatics, supplemented by weaker hydrophobic forces. Moreover, they reveal the preferred orientation of the adsorbed protein and show that its structure is only slightly altered at the interface with the surface. This provides the basis for interpreting the experimental results, which indicate the surface adsorbs a close-packed monolayer at about pH10 where the surface has a large negative zeta potential and the HEWL is positively charged. At higher pH, the adsorption amount of the protein layer is greatly reduced due to the loss of charge on the protein. At lower pH, the smaller zeta potential of the surface leads to lower HEWL adsorption. These interpretations are complemented by the contact angle measurements that show how the hydrophobicity of the surface is greatest when the surface coverage is highest. The simulations provide details of the hydrophobic residues exposed to solution by the adsorbed HEWL, completing the picture of the protein layer structure.

Polyampholyte Brushes Grafted on the Surface of a Spherical Cavity: Effect of the Charged Monomer Sequence, Grafting Density, and Chain Stiffness

Molecular dynamics simulations are used to study the conformational behaviors of the flexible and semiflexible polyampholytes coated onto the internal surface of a spherical cavity. Dependences of the brush structure and the local conformation of grafted chains on the sequence of charged monomers, the grafting density, and the chain stiffness are addressed. In the range of parameters studied, it was found that a significant transition of the brush structure occurs due to the variation of the charged monomer sequence. As the number of repeat charged monomers increases, both the flexible and semiflexible polyampholyte brushes change to the collapsed conformation. The spherical concave geometry tends to exclude the conformation of chains perpendicular to the grafting surface for the semiflexible case. In addition, we find that most counterions are depleted in the polyampholyte brush due to the strong electrostatic correlation between the oppositely charged monomers.