Jun Ren

Stability of polydopamine and poly(DOPA) melanin-like films on the surface of polymer membranes under strongly acidic and alkaline conditions

This study investigated the stability of polydopamine and poly(3,4-dihydroxyphenylalanine) (poly(DOPA)) melanin-like films on the surface of polymer substrates. Three polymer membranes, polypropylene (PP), poly(vinylidenefluoride) (PVDF) and nylon, were modified with polydopamine or poly(DOPA), and then immersed in 0.1M HCl or NaOH, followed by UV-vis spectrometry analysis to detect the presence of film detachment. The results showed that the outer parts of both polydopamine and poly(DOPA) films were detached, probably due to electrostatic repulsion between the polymers within the film, when the modified membranes were washed in HCl or NaOH solution. These two films were more stable in strongly acidic solution, but the stability of poly(DOPA) film was better than that of polydopamine film. Compared to the films on the surface of PVDF or nylon membrane, films on PP surface showed the lowest stability, possibly because of the hydrophobic property of PP. The process of film detachment was analyzed by GPC, which showed that unreacted dopamine or DOPA monomers were still present in the freshly formed films. The unreacted monomers, as well as polydopamine or poly(DOPA) that were incorporated in the film via noncovalent interactions, became detached when the film was exposed to strongly acidic or alkaline solution. Oxidation of freshly formed films could significantly enhance their stability. The results therefore provide us with a better understanding of the stability of melanin-like films, and allow us to develop an effective strategy for constructing stable films.

Fouling-resistant behavior of silver nanoparticle-modified surfaces against the bioadhesion of microalgae.

Unwanted adhesion of microalgae on submerged surfaces is a ubiquitous problem across many maritime operations. We explored the strategy of developing a silver nanoparticle (AgNP) coating for antifouling applications in marine and freshwater environments. In situ growth of AgNPs was achieved by a polydopamine (PDA)-based method. A range of most used industrial materials, including glass, polystyrene, stainless steel, paint surface, and even cobblestone, were employed, on which AgNP coatings were built and characterized. We described the fouling-resistant behavior of these AgNP-modified surfaces against two typical fouling organisms: a marine microalga Dunaliella tertiolecta and a freshwater green alga community. The PDA-mediated AgNP deposition strategy was demonstrated applicable for all the above materials; the resulting AgNP coatings showed a significant surface inhibitory effect against the adhesion of microalgae by above 85% in both seawater and freshwater environments. We observed that contact killing was the predominant antifouling mechanism of AgNP-modified surfaces, and the viability of the microalgae cells in bulk media would not be affected. In addition, silver loss from PDA-mediated AgNPs was relatively slow; it could allow the coating to persist for long-term usage. This study showed the potential of preparing environmentally friendly surfaces that can effectively manage biofouling through the direct deposition of AgNP coatings.

Highly flexible heparin-modified chitosan/graphene oxide hybrid hydrogel as a super bilirubin adsorbent with excellent hemocompatibility

As a pathogenic toxin, bilirubin is generally removed from blood by hemoperfusion for the remission of liver disease or to gain time for patients waiting for liver transplantation. However, the development of bilirubin adsorbents with excellent mechanical properties, adsorption performance and hemocompatibility is still a considerable challenge. In this work, a heparin-modified chitosan/graphene oxide hybrid hydrogel (hep-CS/GH) has been developed for bilirubin adsorption using a lyophilization-neutralization-modification strategy. The as-prepared hybrid hydrogel displayed a unique foam-like porous structure and excellent mechanical flexibility. It was revealed that the incorporation of graphene oxide into the chitosan matrix enhanced both the compressive strength and the Youngs modulus of the hybrid hydrogel, as well as its adsorption capacity for bilirubin. The maximum adsorption capacity of hep-CS/GH for bilirubin was 92.59 mg g-1, according to the Langmuir isotherm model. It was demonstrated that hep-CS/GH successfully competed with albumin, and could effectively adsorb bilirubin from a bilirubin-enriched serum. After the hydrogel was modified with heparin, protein adsorption, platelet adhesion and hemolysis were reduced, and the plasma clotting time was prolonged from 4.1 to 23.6 min, indicating the superior hemocompatibility of hep-CS/GH. Therefore, this study may pave the way for improving the performance of the adsorbent in removing blood toxins.

A Label-Free Optical Biosensor Built on a Low-Cost Polymer Platform

Planar integrated optical biosensors are becoming more and more important as they facilitate label-free and real-time monitoring with high sensitivity. In this paper, the systematic work on the development of this kind of optical biosensor built on a novel polymer platform named PSQ-Ls will be presented. The material itself is of very low cost, and the optical devices with high performances are fabricated through a simple UV-based soft imprint lithography technique. Especially for ring resonator, Q value as high as 5 × 104 and 2.7 × 104 are achieved in air and water condition. These optical chips are functionalized efficiently with protein A molecules through physical immobilization, after careful investigation of the physicochemical and chemical properties of their surface. Both bulk sensing and surface sensing are performed. The proposed optical biosensor exhibits good response not only to its surrounding environments change but also to the small amount of targeted molecules appearing in the buffered solution, which is human-IgG in our study.

Anticoagulant surface coating using composite polysaccharides with embedded heparin-releasing mesoporous silica

Release of heparin from the surface of biomaterials is a feasible and efficient manner for preventing blood coagulation because of the high bioactivity of free heparin and a low application dosage compared to intravenous injection of heparin. Here we report a novel method featuring a blend of heparin-loaded SBA-15, catechol-modified chitosan (CCS), and heparin as a heparin-releasing film. The release of heparin was based on its leakage from heparin-loaded amino-functionalized mesoporous silica SBA-15 (SBA-15-NH2), which was controlled by the amino density of the SBA-15-NH2. Heparin-loaded SBA-15-NH2, CCS, and heparin were mixed together, and the mixture was cast onto the surface of a polydopamine-modified substrate, forming a heparin-releasing film on the surface of the substrate. The polydopamine acted as an adhesive interlayer that stabilized the film coated on the substrate. The sustained release rates of heparin from the film ranged from 15.8 to 2.1 μg/cm(2)/h within 8 h. The heparin-releasing film showed low fibrinogen adsorption, platelet adhesion, and hemolysis rate, indicating that it has good blood compatibility. This new approach would be very useful for modifying the surface of versatile blood-contacting biomaterials and ultimately improve their anticoagulation performance.

Removal of autoantibodies by 4-mercaptoethylpyridine-based adsorbent

Extracorporeal immunoadsorption (ECI) therapy using Staphylococcal Protein A columns has proven effective for removing autoantibodies and circulating immune complexes from patients selectively, providing a promising treatment for autoimmune diseases. However, due to the drawbacks of Protein A in terms of cost and stability, the widespread use of Protein A based ECI is limited. In this study, we investigated the feasibility of 4-mercaptoethylpyridine (MEP, MW 139 Da), a simple and inexpensive synthetic compound, as an alternative to Protein A for autoantibody removal therapy. MEP-based adsorbents were prepared by coupling MEP to Sepharose CL-6B. We found that ligand density was an adjustable parameter for the synthesis of adsorbents aiming at different pathogenic factors, depending on the class of antibody. MEP-Sepharose with a ligand density of 98.8 micromol/ml could remove 80% of the anti-double-stranded DNA antibodies from human serum, whereas a ligand density of 64.5 micromol/ml was enough to remove 96% of the rheumatoid factor (RF) in the serum. Moreover, MEP-based adsorbents showed a lower degree of individual differences compared to Protein A-Sepharose. RF removal of 90% was achieved for all 12 serum samples from different individuals. Among the 14 serum samples derived from systemic lupus erythematosus patients, 11 samples had markedly reduced antinuclear antibody titers. In addition, non-specific adsorption of plasma components to MEP-Sepharose was limited, and the binding capacity of the absorbent for IgG was still about 20 mg/ml of gel after 10 cycles. The results indicated that MEP-based adsorbent could offer a new type of adsorber for the treatment of autoimmune diseases.

Organic silicone sol-gel polymer as a noncovalent carrier of receptor proteins for label-free optical biosensor application.

Optical biosensing techniques have become of key importance for label-free monitoring of biomolecular interactions in the current proteomics era. Together with an increasing emphasis on high-throughput applications in functional proteomics and drug discovery, there has been demand for facile and generally applicable methods for the immobilization of a wide range of receptor proteins. Here, we developed a polymer platform for microring resonator biosensors, which allows the immobilization of receptor proteins on the surface of waveguide directly without any additional modification. A sol-gel process based on a mixture of three precursors was employed to prepare a liquid hybrid polysiloxane, which was photopatternable for the photocuring process and UV imprint. Waveguide films were prepared on silicon substrates by spin coating and characterized by atomic force microscopy for roughness, and protein adsorption. The results showed that the surface of the polymer film was smooth (rms = 0.658 nm), and exhibited a moderate hydrophobicity with the water contact angle of 97°. Such a hydrophobic extent could provide a necessary binding strength for stable immobilization of proteins on the material surface in various sensing conditions. Biological activity of the immobilized Staphylococcal protein A and its corresponding biosensing performance were demonstrated by its specific recognition of human Immunoglobulin G. This study showed the potential of preparing dense, homogeneous, specific, and stable biosensing surfaces by immobilizing receptor proteins on polymer-based optical devices through the direct physical adsorption method. We expect that such polymer waveguide could be of special interest in developing low-cost and robust optical biosensing platform for multidimensional arrays.

Application of cyclodextrin-based eluents in hydrophobic charge-induction chromatography: Elution of antibody at neutral pH

Hydrophobic charge-induction chromatography (HCIC) has emerged as a useful addition to Protein A chromatography for antibody purification due to its remarkable merits in cost and stability. However, the instability of antibody during acidic elution, which may cause inactivation and aggregation, is still a major concern for the efficiency of this method. The aim of this study is to develop a new strategy of competitive elution with inclusion complexes in HCIC, and to apply it to antibody elution under neutral pH conditions. Interactions between 4-mercaptoethylpyridine (MEP), a typical ligand of HCIC, and four different types of cyclodextrins (CDs) were investigated by molecular docking; immunoglobulin G (IgG) elution capacities of CDs were characterized on MEP-based HCIC mediums. The results demonstrated the general effectiveness of CD-based eluents for HCIC. This type of displacement eluents could allow an efficient elution of bound antibody over a broad range of pH and ion strength. With 15 mM β-CD, elution of human IgG was achieved at physiological pH, with an average IgG recovery of 87%. When this elution strategy was used to separate antibody directly from human serum, substantial elution of bound IgG could be obtained at pH 7.4, with product purity comparable to traditional method with an acidic buffer. We expect such method can be of special interest in developing HCIC elution strategy for the proteins like antibody that are sensitive to acidic conditions.

Direct site-specific immobilization of protein A via aldehyde-hydrazide conjugation

Immobilization of affinity ligands on supporting matrices is a key step for the preparation of affinity chromatography resins, and an efficient coupling strategy can significantly improve the validity and cost of the affinity system, especially for systems that employ expensive recombinant proteins or antibodies as affinity ligands. This study described a simple method for obtaining site-specific immobilization of protein A (the ligand) via aldehyde-hydrazide conjugation and its application in antibody purification via protein A chromatography. An aldehyde group was generated at the N-terminus of protein A in vivo by co-expressing a formylglycine-generating enzyme (FGE) and recombinant protein A containing a FGE recognizing sequence (aldehyde tag) in Escherichia coli. The resulting aldehyde allowed direct immobilization of protein A onto the hydrazide-modified agarose matrices under mild condition. We found that 100mM aniline was most effective for catalyzing the coupling reaction, and the recombinant protein A could be coupled with high selectivity, directly from a crude cell extract. The site-specific immobilized protein A showed good capacity for antibody purification. The specificity of the aldehyde-hydrazide reaction not only allowed site-specific immobilization of affinity ligands, but also improved the cost of the process by employing unpurified ligands, a method that might be of great use to industrial applications.

Salt-independent hydrophobic displacement chromatography for antibody purification using cyclodextrin as supermolecular displacer

Hydrophobic interaction chromatography (HIC) offers an orthogonal selectivity to ion exchange chromatography and the combination of the two processes can provide a potential cost-effective alternative to protein A chromatography in industrial antibody purification. However, the application of HIC is limited by its close dependence on high concentrations of kosmotropic salts to achieve desired separation. These salts can cause antibody precipitation and induce the corrosion of manufacturing facilities. Here, we report a new strategy of salt-independent HIC, which can capture antibody at the physiological salt concentration and allow the recovery of bound proteins through cyclodextrin (CD)-based displacement elution. Hydrophobicity-intensified HIC media with different coupling amount of phenyl ligands were prepared and assessed for their antibody binding capacity and selectivity. β-CD was investigated for its supermolecular interaction with phenyl ligands and elution capacity as a displacer. The results clarified a nearly linear correlation between binding capacity of human immunoglobulin G (IgG) and phenyl coupling density in the range of 44-159 μmol/mL. The host-guest interaction between β-CD and the phenyl ligands revealed a modest binding strength (Ka=4.1×10(3) M(-1)), and 15 mM β-CD solution showed a general effectiveness as displacement eluent for these HIC media, with IgG recovery varying with the ligand density. This strategy allowed the direct purification of human IgG from serum with satisfactory purity. The whole procedure of this method, including loading and elution, can be performed under physiological conditions. We expect such a salt-independent mode of HIC could be used as a capture or intermediate step in industrial antibody purification.