Use of x-ray photoelectron spectroscopy to study protein adsorption to mica surfaces

How the dynamics of subsurface hydration regulates protein-surface interactions

The role of water structure near surfaces has been scrutinized extensively because it is accepted to control protein-surface interactions, however, often avoiding effects of hydration dynamics. Relating to this, we have recently discussed how the amount and state of water, accumulated within various hydrophobic-to-hydrophilic subsurface gradients of plasma polymer films, influence the magnitude of adsorbed bovine serum albumin, spurring the hypothesis of the presence of a subsurface dipolar field. This study now analyzes the kinetics of hydration by systematically introducing modified gradient architectures and relating different hydration times to the adsorption of a dipolar probing protein. We find that dry-stored subsurface gradients, owing nominally identical surface characteristics, exhibits comparable surface potential and protein adsorption values, while they behave in a different manner at transient hydration times of few hours, before reaching near-equilibrium state of the hydration. A characteristic hydration time is found where protein adsorption on gradient films is minimal, unveiling the transient nature of the effect. In general, protein adsorption is sensitive to the time allowed for hydration of the adsorbent surface, supporting our initial hypothesis inasmuch as the quantity as well as quality of water inside the subsurface matrix is crucial for controlling protein-surface interactions.

Plasma-modified nitric oxide-releasing polymer films exhibit time-delayed 8-log reduction in growth of bacteria

Tygon(®) and other poly(vinyl chloride)-derived polymers are frequently used for tubing in blood transfusions, hemodialysis, and other extracorporeal circuit applications. These materials, however, tend to promote bacterial proliferation which contributes to the high risk of infection associated with device use. Antibacterial agents, such as nitric oxide donors, can be incorporated into these materials to eliminate bacteria before they can proliferate. The release of the antimicrobial agent from the device, however, is challenging to control and sustain on timescales relevant to blood transport procedures. Surface modification techniques can be employed to address challenges with controlled drug release. Here, surface modification using H2O (v) plasma is explored as a potential method to improve the biocompatibility of biomedical polymers, namely, to tune the nitric oxide-releasing capabilities from Tygon films. Film properties are evaluated pre- and post-treatment by contact angle goniometry, x-ray photoelectron spectroscopy, and optical profilometry. H2O (v) plasma treatment significantly enhances the wettability of the nitric-oxide releasing films, doubles film oxygen content, and maintains surface roughness. Using the kill rate method, the authors determine both treated and untreated films cause an 8 log reduction in the population of both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. Notably, however, H2O (v) plasma treatment delays the kill rate of treated films by 24 h, yet antibacterial efficacy is not diminished. Results of nitric oxide release, measured via chemiluminescent detection, are also reported and correlated to the observed kill rate behavior. Overall, the observed delay in biocidal agent release caused by our treatment indicates that plasma surface modification is an important route toward achieving controlled drug release from polymeric biomedical devices.

Surface modification of a polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU) nanocomposite polymer as a stent coating for enhanced capture of endothelial progenitor cells

An unmet need exists for the development of next-generation multifunctional nanocomposite materials for biomedical applications, particularly in the field of cardiovascular regenerative biology. Herein, we describe the preparation and characterization of a novel polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU) nanocomposite polymer with covalently attached anti-CD34 antibodies to enhance capture of circulating endothelial progenitor cells (EPC). This material may be used as a new coating for bare metal stents used after balloon angioplasty to improve re-endothelialization. Biophysical characterization techniques were used to assess POSS-PCU and its subsequent functionalization with anti-CD34 antibodies. Results indicated successful covalent attachment of anti-CD34 antibodies on the surface of POSS-PCU leading to an increased propensity for EPC capture, whilst maintaining in vitro biocompatibility and hemocompatibility. POSS-PCU has already been used in 3 first-in-man studies, as a bypass graft, lacrimal duct and a bioartificial trachea. We therefore postulate that its superior biocompatibility and unique biophysical properties would render it an ideal candidate for coating medical devices, with stents as a prime example. Taken together, anti-CD34 functionalized POSS-PCU could form the basis of a nano-inspired polymer platform for the next generation stent coatings.

Cellulose morphology and enzymatic reactivity: A modified solute exclusion technique

An expeditious and accurate simplification of Stone and Scallan's solute exclusion technique was developed, thereby avoiding several sources of experimental error coupled with the determination of cellulose pore volume. Using this method, it is shown that cellulolytic enzymes do not enter into the micropores of five studied celluloses. These results suggestes that hydrolysis occurs initially at the external surface of the fibers. This surface area was calculated with the help of adsorption isotherms of bovine serum albumin. The obtained values for the different samples agree with the microscopically observed cellulose morphology. The correlation obtained by several authors relating cellulose porosity and its digestibility is explained as a consequence of the lower crystallinity and easier fragmentation of the more porous celluloses during hydrolysis. (c) 1994 John Wiley & Sons, Inc.

Antibody linking to atomic force microscope tips via disulfide bond formation

Covalent binding of bioligands to atomic force microscope (AFM) tips converts them into monomolecular biosensors by which cognate receptors can be localized on the sample surface and fine details of ligand-receptor interaction can be studied. Tethering of the bioligand to the AFM tip via a approximately 6 nm long, flexible poly(ethylene glycol) linker (PEG) allows the bioligand to freely reorient and to rapidly "scan" a large surface area while the tip is at or near the sample surface. In the standard coupling scheme, amino groups are first generated on the AFM tip. In the second step, these amino groups react with the amino-reactive ends of heterobifunctional PEG linkers. In the third step, the 2-pyridyl-S-S groups on the free ends of the PEG chains react with protein thiol groups to give stable disulfide bonds. In the present study, this standard coupling scheme has been critically examined, using biotinylated IgG with free thiols as the bioligand. AFM tips with PEG-tethered biotin-IgG were specifically recognized by avidin molecules that had been adsorbed to mica surfaces. The unbinding force distribution showed three maxima that reflected simultaneous unbinding of 1, 2, or 3 IgG-linked biotin residues from the avidin monolayer. The coupling scheme was well-reproduced on amino-functionalized silicon nitride chips, and the number of covalently bound biotin-IgG per microm2 was estimated by the amount of specifically bound ExtrAvidin-peroxidase conjugate. Coupling was evidently via disulfide bonds, since only biotin-IgG with free thiol groups was bound to the chips. The mechanism of protein thiol coupling to 2-pyridyl-S-S-PEG linkers on AFM tips was further examined by staging the coupling step in bulk solution and monitoring turnover by release of 2-pyridyl-SH which tautomerizes to 2-thiopyridone and absorbs light at 343 nm. These experiments predicted 10(3)-fold slower rates for the disulfide coupling step than actually observed on AFM tips and silicon nitride chips. The discrepancy was reconciled by assuming 10(3)-fold enrichment of protein on AFM tips via preadsorption, as is known to occur on comparable inorganic surfaces.

Study of adsorption and viscoelastic properties of proteins with a quartz crystal microbalance by measuring the oscillation amplitude

The adsorption kinetics of protein A, BSA, IgG, and fibronectin has been investigated using a homemade quartz crystal microbalance. Information about the energy losses appearing in the system is measured by the maximal oscillation amplitude and the dissipation factor. Only the maximal oscillation amplitude allows us to distinguish the different contributions of liquid and mass to the total frequency shift. The adsorption of proteins has been performed on Ti and Au surfaces at different concentrations. The amount of irreversible adsorbed protein A and IgG increases with increasing bulk concentrations. On Au more proteins adsorb, but their biological activity is reduced in comparison to Ti. Protein A forms a first monolayer in a few seconds, which shows practically no energy losses, and following this a second monolayer is formed. The adsorption rate for the second monolayer is much smaller and energy losses are present. Fibronectin is forming a very viscoelastic system, whose mechanical properties are affected by immersion in different buffer solutions.

X-ray photoelectron spectroscopic and Raman analysis of silk fibroin–Cu(II) films

There is evidence to suggest that Cu(II) is involved in the natural spinning process of a silkworm helping to convert the concentrated silk fibroin (SF) solution (or dope) into tough insoluble threads. To investigate the interaction between SF and Cu(II), a series of regenerated SF (RSF) films with different mass ratios of Cu(II) to SF were prepared. X-ray photoelectron spectroscopy (XPS) was employed to study the chemical interaction between Cu(II) and SF in these Cu(II)-RSF films. A significant change in the binding energy of Cu 2p(3/2) demonstrated that the chemical state of Cu(II) in the Cu(II)-RSF films was affected by the interaction between Cu(II) and SF. Moreover, chemical shifts of N 1s and O 1s of SF were also detected, implying that Cu(II) may coordinate with both N and O atoms in the SF. In addition, Raman spectra of the same series of Cu(II)-RSF films recorded the conformation transition of SF that may occur by the coordination of Cu(II) and SF macromolecular chains.

Limits of detection for time of flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS): detection of low amounts of adsorbed protein

Characterization of biomaterial surfaces requires analytical techniques that are capable of detecting a wide concentration range of adsorbed protein. This range includes detection of low amounts of adsorbed protein (<10 ng/cm2) that may be present on non-fouling biomaterials. X-ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) are surface sensitive techniques capable of detecting adsorbed proteins. We have investigated the lower limits of detection of both XPS and ToF-SIMS on four model substrates each presenting unique challenges for analysis by XPS and ToF-SIMS: mica, poly(tetrafluoroethylene), allyl amine plasma polymer and heptyl amine plasma polymer. The detection limit for XPS ranged from 10 ng/cm2 of fibrinogen (on mica) to 200 ng/cm2 (on allyl amine plasma polymers). The detection limit for ToF-SIMS ranged from 0.1 ng/cm2 of fibrinogen to 100 ng/cm2, depending on the substrate and data analysis. Optimal conditions provided detection limits between 0.1 ng/cm2 and 15 ng/cm2 on all of the substrates used in this study. While both techniques were shown to be effective in detecting protein, the sensitivity of both XPS and ToF-SIMS was shown to be dependent on substrate surface chemistry and the organization of the adsorbed protein film. This study specifically highlights the applicability of ToF-SIMS in the characterization of low level protein adsorption.

XPS and surface-MALDI-MS characterisation of worn HEMA-based contact lenses

XPS and MALDI-MS were used to analyse initial adsorption events in the fouling of HEMA-based contact lenses. All of the lenses tested accumulated tear film deposits within 10 min of wear. XPS indicated the presence of mainly proteinaceous deposits, with indications of some contributions by mucins or lipids on some lenses and the nature of the deposit being influenced by the lens chemistry. MALDI-MS detected the presence of surface-adsorbed species with molecular weights < 15 kDa. While lysozyme could be identified by comparison of MALDI-MS signals with known protein mass and assignments are suggested for some other signals, several other species, with MWs less than that of lysozyme, could not be identified as no ocular proteins with corresponding MWs had been reported in previous biochemical tear film analyses. These species, and others, were also detected in MALDI-MS analysis of reflex tear film, suggesting that the adsorbed unidentified species were not simply products of surface-induced dissociation of adsorbing higher-MW proteins. This short-term wear study detected rapid interface conversion and demonstrated the utility and surface sensitivity of XPS and MALDI-MS in characterising contact lens deposits at the initial stages when sub-monolayer adsorbed amounts are present on lenses.

A Direct Method of Studying Polymer Adsorption onto Mica Surfaces Using a Commercial Mettler Ultramicrobalance

This paper deals with a simple and direct method of determining absolute values of adsorbance, i.e., mass per unit area, of polymers adsorbed from solution onto mica surfaces. The method is based on direct weighing of mica sheets using a Mettler ultramicrobalance UMT2 or UM3 (readability 1 x 10(-4) mg), which is commercially available. A set of mica sheets is weighed twice: before and after the immersion of mica sheets in polymer solution for a given period of time. The difference in weight of the mica sheets gives the mass of adsorbed polymers, which is divided by the total area of mica surface to derive the adsorbance. Measurable change in adsorbance was 0.1 mg m-2 in consideration of the size of mica sheets (50 cm2 in total surface area) and the scatter of readings of the ultramicrobalance. A detailed description is given on the apparatus and procedures for weighing and adsorption experiments. The present method has been applied to the experiments on the adsorption kinetics of polystyrene from cyclohexane and of bovine serum albumin from aqueous solution onto mica surfaces. Some of the experimental results are presented to show the practical examples of the application of this method. Copyright 1999 Academic Press.

Structure and reactivity of water at biomaterial surfaces

Molecular self association in liquids is a physical process that can dominate cohesion (interfacial tension) and miscibility. In water, self association is a powerful organizational force leading to a three-dimensional hydrogen-bonded network (water structure). Localized perturbations in the chemical potential of water as by, for example, contact with a solid surface, induces compensating changes in water structure that can be sensed tens of nanometers from the point of origin using the surface force apparatus (SFA) and ancillary techniques. These instruments reveal attractive or repulsive forces between opposing surfaces immersed in water, over and above that anticipated by continuum theory (DLVO), that are attributed to a variable density (partial molar volume) of a more-or-less ordered water structure, depending on the water wettability (surface energy) of the water-contacting surfaces. Water structure at surfaces is thus found to be a manifestation of hydrophobicity and, while mechanistic/theoretical interpretation of experimental results remain the subject of some debate in the literature, convergence of experimental observations permit, for the first time, quantitative definition of the relative terms 'hydrophobic' and 'hydrophilic'. In particular, long-range attractive forces are detected only between surfaces exhibiting a water contact angle theta > 65 degrees (herein defined as hydrophobic surfaces with pure water adhesion tension tau O = gamma O cos theta < 30 dyn/cm where gamma O is water interfacial tension = 72.8 dyn/cm). Repulsive forces are detected between surfaces exhibiting theta < 65 degrees (hydrophilic surfaces, tau O > 30 dyn/cm). These findings suggest at least two distinct kinds of water structure and reactivity: a relatively less-dense water region against hydrophobic surfaces with an open hydrogen-bonded network and a relatively more-dense water region against hydrophilic surfaces with a collapsed hydrogen-bonded network. Importantly, membrane and SFA studies reveal a discrimination between biologically-important ions that preferentially solubilizes divalent ions in more-dense water regions relative to less-dense water regions in which monovalent ions are enriched. Thus, the compelling conclusion to be drawn from the collective scientific evidence gleaned from over a century of experimental and theoretical investigation is that solvent properties of water within the interphase separating a solid surface from bulk water solution vary with contacting surface chemistry. This interphase can extend tens of nanometers from a water-contacting surface due to a propagation of differences in self association between vicinal water and bulk-phase water. Physicochemical properties of interfacial water profoundly influence the biological response to materials in a surprisingly straightforward manner when key measures of biological activity sensitive to interfacial phenomena are scaled against water adhesion tension tau O of contacting surfaces. As examples, hydrophobic surfaces (tau O < 30 dyn/cm) support adsorption of various surfactants and proteins from water because expulsion of solute from solution into the interphase between bulk solid and solution phases is energetically favorable. Adsorption to hydrophobic surfaces is driven by the reduction of interfacial energetics concomitant with replacement of water molecules at the surface by adsorbed solute (surface dehydration). Hydrophilic surfaces (tau O > 30 dyn/cm) do not support adsorption because this mechanism is energetically unfavorable. Protein-adsorbing hydrophobic surfaces are inefficient contact activators of the blood coagulation cascade whereas protein-repellent hydrophilic surfaces are efficient activators of blood coagulation. Mammalian cell attachment is a process distinct from protein adsorption that occurs efficiently to hydrophilic surfaces but inefficiently to hydrophobic surfaces. (ABSTRACT TRUNCATED)

Contact activation of the plasma coagulation cascade. II. Protein adsorption to procoagulant surfaces

A study of blood protein adsorption to procoagulant surfaces utilizing a coagulation time assay, contact angles, Wilhelmy balance tensiometry, and electron spectroscopy (ESCA) is presented. Using a new contact angle method of measuring protein adsorption termed "adsorption mapping" it was demonstrated that protein-adsorbent surfaces were inefficient activators of the intrinsic pathway of the plasma coagulation cascade whereas water-wettable, protein-repellent surfaces were efficient procoagulants. Repeated use of fully water-wettable (spreading) glass procoagulants in the coagulation time assay demonstrated that putative "activating sites" were not consumed in the coagulation of platelet-poor porcine plasma. Furthermore, these procoagulant surfaces retained water-wettable surface properties after incubation with blood proteins and saline rinse. The interpretation of these observations was that plasma and serum proteins were not adsorbed to water-wettable surfaces. However, ESCA of these same surfaces revealed the presence of a thin protein layer. Wilhelmy balance tensiometry resolved these seemingly divergent observations by demonstrating that protein was "associated" with a bound hydration layer, but not formally adsorbed through a surface dehydration or ionic interaction mechanism.

Structural characteristics of a globular protein investigated by X-ray photoelectron spectroscopy: comparison between a legumin film and a powdered legumin

Films of legumin, a pea protein, were deposited onto a glass support using the Langmuir-Blodgett method, at various surface pressures. XPS study of these films show that their thickness increases with the deposition pressure. At the pressure limits of films stability, the thickness values (respectively 73 and 110 A) are close to the protein dimensions. Layered at low pressure, the oblate protein stands up when pressure increases. Furthermore, XPS study shows that the orientation of the external flexible loops depends on the obtention conditions. Thus, in the case of Langmuir-Blodgett films, hydrophobic residues are turned towards the external surface, and the hydrophilic ones towards the glass substrate. But, in the opposite, when protein is obtained by lyophilization, the hydrophilic residues are orientated outsides. It seems possible to determine by XPS the nature of the residues which give to the protein its reactivity, since they are located at its external surface.

Inhibition of platelet spreading from plasma onto glass by an adsorbed layer of a novel fluorescent-labeled poly(ethylene oxide)/poly(butylene oxide) block copolymer: characteristics of the exclusion zone probed by means of polystyrene beads and macromolecules

We have investigated the anti-adhesive properties of a newly synthesized fluorescent triblock copolymer containing poly(ethylene oxide). This adsorbs from aqueous solution onto glass that has been rendered hydrophobic. When the polymer-treated surface was exposed to human platelet-rich plasma (PRP) or whole blood at 37 degrees C, platelet adhesion and spreading were prevented. Avid adhesion and rapid platelet spreading occurred along tracks scraped in the adsorbed polymer coating, as seen by video-enhanced interference reflection microscopy. Leukocytes from whole blood are eventually able to adhere to the polymer-treated surface and were seen to remove labeled polymer from their vicinity and accumulate it at the cell body. Interferometry using polystyrene spheres showed that they do not adhere to polymer-coated glass and are unable to approach closer than 70-95 nm. On scraped tracks, beads make molecular contacts with the glass. Because the fully extended solvated (EO)400 arms may extend up to 100 nm from the glass, this suggests that the polymer forms a monolayer with the hydrophilic arms projecting into the water, whereas the hydrophobic (BO)55 segment binds the molecule to the hydrophobic surface. Another tri-bloc copolymer with shorter hydrophilic arms allows particles to approach more closely.