Zwitterionic Surface Research Articles

A zwitterionic surface with general cell-adhesive and protein-resistant properties

A choline phosphate functionalized surface is proved to have general cell-adhesive and protein-resistant properties, which give it potential for biomaterials.

PH-Responsive and selective protein adsorption on an amino acid-based zwitterionic polymer surface

An amphiphilic polymer bearing glutamic acid in the polymer side chain was used as a surface modifier to produce an amino acid-based zwitterionic surface with pH-responsive and selective protein properties.

One-step functionalization of zwitterionic poly[(3-(methacryloylamino)propyl)dimethyl(3-sulfopropyl)ammonium hydroxide] surfaces by metal-polyphenol coating.

The non-biofouling properties of a zwitterionic sulfobetaine polymer surface were easily made attractive to bioentities (e.g., proteins and cells) by metal-polyphenol coating, and spatio-selective functionalization of the zwitterionic polymer surface was achieved by using a soft lithographic technique.

Constructing a zwitterionic ultrafiltration membrane surface via multisite anchorage for superior long-term antifouling properties

A novel zwitterionic membrane surface was constructed which exhibited stable antifouling ability by the formation of multisite anchorage covalent bonds.

Surface characterization and antifouling properties of nanostructured gold chips for imaging surface plasmon resonance biosensing

Surface Plasmon Resonance (SPR) optical sensing is a label-free technique for real-time monitoring of biomolecular interactions. Recently, a portable imaging SPR (iSPR) prototype instrument, featuring a nanostructured gold chip, has been developed. In the present work, we investigated the crucial first steps, prior to eventual use of the nanostructured iSPR chip, i.e., its surface modification, in-depth surface characterization and the antifouling performance. Results were compared with conventional flat (i)SPR gold chips having the same surface chemistries, viz. different types of polyethylene glycol and zwitterionic polymers. Characterization of the (i)SPR chips before and after surface modification was performed using atomic force microscopy (AFM), scanning electron microscopy (SEM), water contact angle (WCA), X-ray photoelectron spectroscopy (XPS) and direct analysis in real time high resolution mass spectrometry (DART-HRMS). The antifouling properties were then studied using the nanostructured chip in the portable iSPR instrument and the flat gold chip in conventional SPR setup. The zwitterionic polymer surface chemistries showed the best antifouling properties. Comparison of the nanostructured iSPR chips with conventional flat (i)SPR gold chips showed that the latter perform slightly better in terms of surface modification as well as antifouling properties. The portable iSPR instrument is almost as sensitive as conventional iSPR (IBIS) and nine times less sensitive than conventional SPR (Biacore 3000). The nanostructured iSPR chip, along with the portable instrument, offers the advantage of about ten-fold reduction in instrument size, weight and costs compared to conventional (i)SPR instruments using flat gold, thus making it highly interesting for future biosensing applications.

Surface zwitterionization of hemocompatible poly(lactic acid) membranes for hemodiafiltration

Poly(lactic acid) (PLA) hemodiafiltration membranes were grafted by zwitterionic polymer to improve the fouling resistance and hemocompatibility. The surface zwitterionization was accomplished by dopamine inspired bromoalkyl initiator anchoring and subsequent atom transfer radical polymerization (ATRP) of zwitterionic poly(sulfobetaine methacrylate) (PSBMA). Attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were applied to analyze the surface chemistry and morphology evolution of the fabricated membranes, which confirmed that zwitterionic PSBMA was successfully grafted on PLA membrane. The results of water contact angle and zeta potentials demonstrated that the hydrophilicity and negativity of the modified PLA membranes were highly improved, which effectively alleviated the surface adsorption of bovine serum albumin (BSA) and palates. Moreover, the modified PLA membrane exhibited good dialysis performances. When the polymerization time of PSBMA was 1h, the water flux was 184Lm−2h−1, while BSA retention reached up to 90%, and the clearance of urea and creatinine was maintained at 66% and 60%, respectively. The present work provides a convenient strategy to improve the hydrophilicity, anti-fouling and hemocompatibility of PLA membranes for their biomedical and blood-contacting applications such as hemodialysis.

Synergism in Mixed Zwitterionic Surface Activity Ionic Liquid and Anionic Surfactant Solution: Analysis of Interfacial and Micellar Behavior

Synergistic interactions between zwitterionic surface active ionic liquids (SAILs), N-alkyl-N′-carboxymethyl imidazoliumm inner salts ([N-C n , N′-CO2-Im], n = 12, 14), and a traditional twin-tail anionic surfactant (dioctyl sulphosuccinate sodium salt, AOT) in aqueous solution has been studied by surface tension and fluorescence measurements at 298.15 K. The results imply that the mixed systems have superior surface activity in comparison to individual surfactant, and they all present a non-ideal behavior with synergism. The strength of interactions between two surfactants increases with the enhancement of the hydrocarbon chain of the zwitterionic SAILs. Compared with interaction parameters in mixed monolayer, the larger value of interaction parameters in mixed micelles indicates that the mixed micelle formation is more favorable. Additionally, a series of thermodynamic parameters such as the standard Gibbs free energy of adsorption (), the standard Gibbs free energy of micellization (), and excess free energy of micellization (ΔG ex) have been calculated. The negative values of and show that both micellization and adsorption of surfactants are spontaneous, while the negative value of ΔG ex indicates the formation of stable mixed micelles for [N-C n , N′-CO2-Im]/AOT mixture, which can potentially be useful for industrial preparations and medicinal formulations.

Theoretical vibrational sum-frequency generation spectroscopy of water near lipid and surfactant monolayer interfaces. II. Two-dimensional spectra.

The structural stability and function of biomolecules is strongly influenced by the dynamics and hydrogen bonding of interfacial water. Understanding and characterizing the dynamics of these water molecules require a surface-sensitive technique such as two-dimensional vibrational sum-frequency generation (2DSFG) spectroscopy. We have combined theoretical 2DSFG calculations with molecular dynamics simulations in order to investigate the dynamics of water near different lipid and surfactant monolayer surfaces. We show that 2DSFG can distinguish the dynamics of interfacial water as a function of the lipid charge and headgroup chemistry. The dynamics of water is slow compared to the bulk near water-zwitterionic and water-anionic interfaces due to conformational constraints on interfacial water imposed by strong phosphate-water hydrogen bonding. The dynamics of water is somewhat faster near water-cationic lipid interfaces as no such constraint is present. Using hydrogen bonding and rotational correlation functions, we characterize the dynamics of water as a function of the distance from the interface between water and zwitterionic lipids. We find that there is a transition from bulk-like to interface-like dynamics approximately 7 Å away from a zwitterionic phosphatidylcholine monolayer surface.

Spraying quantum dot conjugates in the colon of live animals enabled rapid and multiplex cancer diagnosis using endoscopy.

The detection of colon cancer using endoscopy is widely used, but the interpretation of the diagnosis is based on the clinician's naked eye. This is subjective and can lead to false detection. Here we developed a rapid and accurate molecular fluorescence imaging technique using antibody-coated quantum dots (Ab-QDs) sprayed and washed simultaneously on colon tumor tissues inside live animals, subsequently excited and imaged by endoscopy. QDs were conjugated to matrix metalloproteinases (MMP) 9, MMP 14, or carcinoembryonic antigen (CEA) Abs with zwitterionic surface coating to reduce nonspecific bindings. The Ab-QD probes can diagnose tumors on sectioned mouse tissues, fresh mouse colons stained ex vivo and also in vivo as well as fresh human colon adenoma tissues in 30 min and can be imaged with a depth of 100 μm. The probes successfully detected not only cancers that are readily discernible by bare eyes but also hyperplasia and adenoma regions. Sum and cross signal operations provided postprocessed images that can show complementary information or regions of high priority. This multiplexed quantum dot, spray-and-wash, and endoscopy approach provides a significant advantage for detecting small or flat tumors that may be missed by conventional endoscopic examinations and bestows a strategy for the improvement of cancer diagnosis.

Molecular fouling resistance of zwitterionic and amphiphilic initiated chemically vapor-deposited (iCVD) thin films

Biofouling is a universal problem in various applications ranging from water purification to implantable biomedical devices. Recent advances in surface modification have created a rich library of antifouling surface chemistries, many of which can be categorized into one of the two groups: hydrophilic surfaces or amphiphilic surfaces. We report the straightforward preparation of antifouling thin film coatings in both categories via initiated chemical vapor deposition. A molecular force spectroscopy-based method is demonstrated as a rapid and quantitative assessment tool for comparing the differences in antifouling characteristics. The fouling propensity of single molecules, as opposed to bulk protein solution or bacterial culture, is assessed. This method allows for the interrogation of molecular interaction without the complication resulted from protein conformational change or micro-organism group interactions. The molecular interaction follows the same trend as bacterial adhesion results obtained previously, demonstrating that molecular force probe is a valid method for the quantification and mechanistic examination of fouling. In addition, the molecular force spectroscopy-based method is able to distinguish differences in antifouling capability that is not resolvable by traditional static protein adsorption tests. To lend further insight into the intrinsic fouling resistance of zwitterionic and amphiphilic surface chemistries, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, advancing and receding water contact angles, and atomic force microscopy are used to elucidate the film properties that are relevant to their antifouling capabilities.

Zwitterionic hydrogel thin films as antifouling surface layers of polyethersulfone ultrafiltration membranes anchored via reactive copolymer additive

The fabrication and properties of anti-fouling and hemocompatible polyethersulfone (PES) membranes having zwitterionic hydrogel thin films as surface layers were described in this paper. PES membranes containing a reactive block copolymer PES-b-PHEDMA as an additive were first prepared by traditional non-solvent induced phase separation process. The hydrogel thin films were then grafted from membrane surface by free radical cross-linking polymerization participated by reactive ethylenic bonds in the PES-b-PHEDMA additive. The analysis of attenuated total reflectance-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the successful immobilization of zwitterionic hydrogel thin films on PES membrane surfaces. Due to the hydrophilic and low-fouling characteristics of zwitterionic surfaces, the hydrogel-modified PES membranes showed significantly improved anti-fouling ability. More importantly, the zwitterionic membranes exhibited excellent hemocompatibility and the hydrogel thin films showed good durability in long-term tests. This work provides us with a facile and versatile strategy of covalently anchoring hydrogel thin films onto polymer membranes to achieve excellent anti-fouling properties and hemocompatibility.

Detecting kallikrein proteolytic activity with peptide-quantum dot nanosensors.

Contamination and adulterants in both naturally derived and synthetic drugs pose a serious threat to the worldwide medical community. Developing rapid and sensitive sensors/devices to detect these hazards is thus a continuing need. We describe a hydrophilic semiconductor quantum dot (QD)-peptide Förster resonance energy transfer (FRET) nanosensor for monitoring the activity of kallikrein, a key proteolytic enzyme functioning at the initiation of the blood clotting cascade. Kallikrein is also activated by the presence of an oversulfated contaminant recently found in preparations of the drug heparin. Quantitatively monitoring the activity of this enzyme within a nanosensor format has proven challenging because of inherent steric and kinetic considerations. Our sensor is designed around a central QD donor platform which displays controlled ratios of a modular peptidyl substrate. This peptide, in turn, sequentially expresses a terminal oligohistidine motif that mediates the rapid self-assembly of peptides to the QD surface, a linker-spacer sequence to extend the peptide away from the QD surface, a kallikrein recognized-cleavage site, and terminates in an acceptor dye-labeling site. Hydrophilic QDs prepared with compact, zwitterionic surface coatings were first evaluated for their ability to self-assemble the dye-labeled peptide substrates. An optimized two-step protocol was then utilized where high concentrations of peptide were initially digested with purified human kallikrein and samples collected at distinct time points were subsequently diluted into QD-containing solutions for assaying. This sensor provided a quantitative FRET-based readout for monitoring kallikrein activity and comparison to a calibration curve allowed estimation of the relevant Michaelis-Menten kinetic descriptors. The results further suggest that almost any protease should be amenable to a QD-based FRET assay format with appropriate design considerations.

Influence of zwitterionic SAMs on protein adsorption and the attachment of algal cells

Zwitterionic polymers are non-fouling materials with immense potential for a range of biological applications. Here, we describe the resistance of zwitterionic self-assembled monolayers prepared from different solution ratios of positively and negatively charged thiols towards the adhesion of proteins, zoospores of the green alga Ulva linza, and cells of the unicellular alga Navicula perminuta. While mixed zwitterionic surfaces with a high hydrophilic nature significantly reduced the adhesion strength of the two algae, the positively and negatively charged components were far less effective.

Surface zwitterionization of titanium for a general bio-inert control of plasma proteins, blood cells, tissue cells, and bacteria.

Surface coating of antifouling materials on the substrates offers convenient strategies and great opportunities to improve their biocompatibility and functions of host substrates for wide biomedical applications. In this work, we present a general surface zwitterionization strategy to improve surface biocompatibility and antifouling properties of titanium (Ti) by grafting zwitterionic poly(sulfobetaine methacrylate) (polySBMA). This method also demonstrates its general applicability to graft polySBMA onto Ti surface using different anchoring agents of dopamine and silane. The resulting polySBMA grafted from dopamine- (pTi-D-pSBMA) and silane-anchored titanium surfaces (pTi-Si-pSBMA) surfaces exhibit superlow fouling ability to highly resist the adhesions of plasma proteins, platelets, erythrocytes, leukocytes, human fibroblast (HT1080), E. coli, and S. epidermidis. The interfacial properties of the surface-modified Ti surfaces are analyzed and correlated with their antifouling properties. The new method and materials provide a more general, flexible, and robust way to produce an excellent nonfouling surface with adjustable interfacial structures of grafted polymers, which hopefully can be expanded to wider applications based on both the structure and surface superiorities.

Hemocompatibility of zwitterionic interfaces and membranes

This focused review addresses the fundamental development of nonfouling zwitterionic poly(sulfobetaine methacrylate) (polySBMA) coatings on a wide range of interfaces and membranes in our laboratory for use in the development of hemocompatible medical devices. The molecular designs of zwitterionic interfaces (for example, surface packing density, grafting coverage, polymer chain conformation and chemistry) as well as the evolution of ‘intelligent’ interfaces are scrutinized and discussed. Correlations between polymeric membrane surface modification methods, such as thermal- and plasma-induced polymerizations, the conformation of grafted polySBMA chains and hemocompatibility features, are also delineated. Fundamental development of nonfouling zwitterionic poly(sulfobetaine methacrylate) (polySBMA) coatings on a wide range of interfaces and membranes for use in the development of hemocompatible medical devices was discussed. The molecular designs of zwitterionic interfaces as well as the evolution of ‘intelligent’ interfaces and correlations between zwitterionic polymeric membrane surface modifications are scrutinized and delineated.

Nonfouling property of zwitterionic cysteine surface.

Applications of implantable bioelectronics for analytical and curative purposes are currently limited by their poor long-term biofunctionality in physiological media and nonspecific interactions with biomolecules. In an attempt to prolong in vivo functionality, recent advances in surface modifications have demonstrated that zwitterionic coatings can rival the performance of conventional poly(ethylene glycol) polymers in reducing nonspecific protein fouling. Herein, we report the fabrication of a very thin layer of nonfouling zwitterionic cysteine surface capable of protecting implantable bioelectronics from nonspecific adsorption of plasma proteins. This work is the first of its kind to fabricate, through solution chemistry, a cysteine surface exhibiting zwitterionic state as high as 88% and to demonstrate antibiofouling under the exposure of bovine serum albumin (BSA) and human serum. The fabricated surface utilized a minimal amount of gold substrate, approximately 10 nm, and an extremely thin antifouling layer at 1.14 nm verified by ellipsometry. X-ray photoelectron spectroscopy assessment of the nitrogen (N1s) and carbon (C1s) spectra conclude that 87.8% of the fabricated cysteine surface is zwitterionic, 2.5% is positively charged, and 9.6% is noncharged. Antibiofouling performance of the cysteine surface is quantitatively determined by bicinchoninic acid (BCA) protein assay as well as qualitatively confirmed using scanning electron spectroscopy. Cysteine surfaces demonstrated a BSA fouling of 3.9 ± 4.84% μg/cm(2), which is 93.6% and 98.5% lower than stainless steel and gold surfaces, respectively. Surface plasmon resonance imaging analysis returned similar results and suggest that a thinner cysteine coating will enhance performance. Scanning electron microscopy confirmed the results of BCA assay and suggested that the cysteine surface demonstrated a 69% reduction to serum fouling. The results reported in this paper demonstrate that it is possible to achieve a highly zwitterionic surface through solution chemistry on a macroscopic level that is capable of improving biocompatibility of long-term implantable bioelectronics.

Preparation, Characterization and Antibacterial Adhesion Performance of the Biomimetic Surfaces via Zwitterionic Self-assembly†

A sulfobetaine zwitterionic compound,N,N-dimethylamino ethyl carbamate propyl triethoxysilane sulfonate(SiNNS),was designed and synthesized and its composition and molecular structure were characterized by means of FTIR and1H NMR spectroscopy.Furthermore,the biomimetic surface imitating the chemical features of cellular outer membrane were constructed via the self-assembly of SiNNS molecules on the hydroxylated glass surface.The morphology structure,chemical composition and wettability of the prepared biomimetic surface were characterized by atomic force microscopy(AFM),X-ray photoelectron spectroscopy(XPS) and contact angle measurement.The antifogging and antibacterial adhesion performances of the biomimetic surface were investigated using an untreated glass surface as a control sample.The results indicate that the zwitterionic self-assembled biomimetic surface can possess superhydrophilicity with a water contact angle of9.2° and underwater superoleophobicity with an oil contact angle of 175.6°.Compared to the corresponding control sample,the zwitterionic self-assembled biomimetic surface can exhibit an excellent antifogging property and an obvious antibacterial adhesion performance.

Modification of Ti6Al4V substrates with well-defined zwitterionic polysulfobetaine brushes for improved surface mineralization.

Osteoconductive mineral coatings are beneficial for improving the osteointegration of metallic orthopedic/dental implants, but achieving adequate structural integration between the surface minerals and underlying metallic substrates has been a significant challenge. Here, we report covalent grafting of zwitterionic poly(sulfobetaine methacrylate) (pSBMA) brushes on the Ti6Al4V substrates to promote the surface-mineralization of hydroxyapatite with enhanced surface mineral coverage and mineral-substrate interfacial adhesion. We first optimized the atom transfer radical polymerization (ATRP) conditions for synthesizing pSBMA polymers in solution. Well-controlled pSBMA polymers (relative molecular weight up to 26kD, PDI = 1.17) with high conversions were obtained when the ATRP was carried out in trifluoroethanol/ionic liquid system at 60 °C. Applying identical polymerization conditions, surface-initiated atom transfer radical polymerization (SI-ATRP) was carried out to graft zwitterionic pSBMA brushes (PDI < 1.20) from the Ti6Al4V substrates, generating a stable superhydrophilic and low-fouling surface coating without compromising the bulk mechanic property of the Ti6Al4V substrates. The zwitterionic pSBMA surface brushes, capable of attracting both cationic and anionic precursor ions during calcium phosphate apatite mineralization, increased the surface mineral coverage from 32% to 71%, and significantly reinforced the attachment of the apatite crystals on the Ti6Al4V substrate. This facile approach to surface modification of metallic substrates can be exploited to generate multifunctional polymer coatings and improve the performance of metallic implants in skeletal tissue engineering and orthopedic and dental care.

Preparation of Sulfobetaine-Grafted PVDF Hollow Fiber Membranes with a Stably Anti-Protein-Fouling Performance.

Based on a two-step polymerization method, two sulfobetaine-based zwitterionic monomers, including 3-(methacryloylamino) propyl-dimethyl-(3-sulfopropyl) ammonium hydroxide (MPDSAH) and 2-(methacryloyloxyethyl) ethyl-dimethyl-(3-sulfopropyl) ammonium (MEDSA), were successfully grafted from poly(vinylidene fluoride) (PVDF) hollow fiber membrane surfaces in the presence of N,N′-methylene bisacrylamide (MBAA) as a cross-linking agent. The mechanical properties of the PVDF membrane were improved by the zwitterionic surface layers. The surface hydrophilicity of PVDF membranes was significantly enhanced and the polyMPDSAH-g-PVDF membrane showed a higher hydrophilicity due to the higher grafting amount. Compared to the polyMEDSA-g-PVDF membrane, the polyMPDSAH-g-PVDF membrane showed excellent significantly better anti-protein-fouling performance with a flux recovery ratio (RFR) higher than 90% during the cyclic filtration of a bovine serum albumin (BSA) solution. The polyMPDSAH-g-PVDF membrane showed an obvious electrolyte-responsive behavior and its protein-fouling-resistance performance was improved further during the filtration of the protein solution with 100 mmol/L of NaCl. After cleaned with a membrane cleaning solution for 16 days, the grafted MPDSAH layer on the PVDF membrane could be maintain without any chang; however, the polyMEDSA-g-PVDF membrane lost the grafted MEDSA layer after this treatment. Therefore, the amide group of sulfobetaine, which contributed significantly to the higher hydrophilicity and stability, was shown to be imperative in modifying the PVDF membrane for a stable anti-protein-fouling performance via the two-step polymerization method.

‘Should I stay or should I go?’ Bacterial attachment vs biofilm formation on surface-modified membranes

A number of techniques are used for testing the anti-biofouling activity of surfaces, yet the correlation between different results is often questionable. In this report, the correlation between initial bacterial deposition (fast tests, reported previously) and biofilm growth (much slower tests) was analyzed on a pristine and a surface-modified reverse osmosis membrane ESPA-1. The membrane was modified with grafted hydrophilic polymers bearing negatively charged, positively charged and zwitter-ionic moieties. Using three different bacterial strains it was found that there was no general correlation between the initial bacterial deposition rates and biofilm growth on surfaces, the reasons being different for each modified surface. For the negatively charged surface the slowest deposition due to the charge repulsion was eventually succeeded by the largest biofilm growth, probably due to secretion of extracellular polymeric substances (EPS) that mediated a strong attachment. For the positively charged surface, short-term charge attraction by quaternary amine groups led to the fastest deposition, but could be eventually overridden by their antimicrobial activity, resulting in non-consistent results where in some cases a lower biofilm formation rate was observed. The results indicate that initial deposition rates have to be used and interpreted with great care, when used for assessing the anti-biofouling activity of surfaces. However, for a weakly interacting ‘low-fouling’ zwitter-ionic surface, the positive correlation between initial cell deposition and biofilm growth, especially under flow, suggests that for this type of coating initial deposition tests may be fairly indicative of anti-biofouling potential.