Adsorption and viscoelastic properties of fractionated mucin (BSM) and bovine serum albumin (BSA) studied with quartz crystal microbalance (QCM-D)

Relative importance of the humic and fulvic fractions of natural organic matter in the aggregation and deposition of silver nanoparticles

As engineered nanoparticles (NPs) are increasingly used, their entry into the environment has become an important topic for water sustainability. Recent investigations point to the critical role of natural organic matter (NOM) in altering the persistence of NPs by complexing with their surfaces. The NP-NOM complex, in turn, is the new entity that may potentially influence subsequent fate of NPs. To understand the relative impact of humic (HA) and fulvic fraction of NOM on the stability and mobility of silver nanoparticles (AgNPs), a combination of dynamic light scattering and quartz crystal microgravimetry with dissipation monitoring was used. In the absence of unbound NOM, (1) surface modification on either AgNP or silica substrate by different NOM fractions could lead to substantial changes in the extent and kinetics of AgNP aggregation and deposition, and (2) HA has a greater capability to enhance the transport of AgNPs by reducing their aggregation and deposition. With unbound NOM, HA seems to compete more successfully for binding sites on the substrate under electrostatically favorable conditions and formed a steric layer to prevent subsequent deposition of AgNPs. These findings highlighted the importance of NOM fraction in the overall environmental partitioning of AgNPs.

Cations as switches of amyloid-mediated membrane disruption mechanisms: calcium and IAPP

Disruption of the integrity of the plasma membrane by amyloidogenic proteins is linked to the pathogenesis of a number of common age-related diseases. Although accumulating evidence suggests that adverse environmental stressors such as unbalanced levels of metal ions may trigger amyloid-mediated membrane damage, many features of the molecular mechanisms underlying these events are unknown. Using human islet amyloid polypeptide (hIAPP, aka amylin), an amyloidogenic peptide associated with β-cell death in type 2 diabetes, we demonstrate that the presence of Ca(2+) ions inhibits membrane damage occurring immediately after the interaction of freshly dissolved hIAPP with the membrane, but significantly enhances fiber-dependent membrane disruption. In particular, dye leakage, quartz crystal microbalance, atomic force microscopy, and NMR experiments show that Ca(2+) ions promote a shallow membrane insertion of hIAPP, which leads to the removal of lipids from the bilayer through a detergent-like mechanism triggered by fiber growth. Because both types of membrane-damage mechanisms are common to amyloid toxicity by most amyloidogenic proteins, it is likely that unregulated ion homeostasis, amyloid aggregation, and membrane disruption are all parts of a self-perpetuating cycle that fuels amyloid cytotoxicity.

Surface chemistry to minimize fouling from blood-based fluids

Upon contact with bodily fluids/tissues, exogenous materials spontaneously develop a layer of proteins on their surface. In the case of biomedical implants and equipment, biological processes with deleterious effects may ensue. For biosensing platforms, it is synonymous with an overwhelming background signal that prevents the detection/quantification of target analytes present in considerably lower concentrations. To address this ubiquitous problem, tremendous efforts have been dedicated over the years to engineer protein-resistant coatings. There is now extensive literature available on stealth organic adlayers able to minimize fouling down to a few ng cm(-2), however from technologically irrelevant single-protein buffered solutions. Unfortunately, few coatings have been reported to present such level of performance when exposed to highly complex proteinaceous, real-world media such as blood serum and plasma, even diluted. Herein, we concisely review the surface chemistry developed to date to minimize fouling from these considerably more challenging blood-based fluids. Adsorption dynamics is also discussed.

Real-time investigation of the muco-adhesive properties of Lactococcus lactis using a quartz crystal microbalance with dissipation monitoring

This work was devoted to probe, at the entire population level, interactions between mucins and Lactococcus lactis, using QCM-D. Real-time monitoring of adsorption on polystyrene of PGM (Pig Gastric Mucin) and subsequent adhesion of L. lactis was performed for IBB477 and MG1820 strains. Measuring simultaneously shifts in resonance frequency and dissipation on the polystyrene-coated crystal demonstrated a two-phase process for PGM adsorption. XPS analysis confirmed the presence of adsorbed mucin. The Voigt-based model was used to describe the QCM-D outputs. The predicted thickness of the PGM layer was consistent with the AFM experimental value. Adhesion of L. lactis to bare or PGM-coated polystyrene was then monitored, in combination with DAPI cell counting. Positive frequency shifts were caused by adhering bacteria. The presence of adsorbed PGM strongly reduced bacterial adhesion. However, adhesion of IBB477 to the PGM coating was greatly increased in comparison with that of MG1820. Muco-adhesion may be a highly variable and valuable phenotypic trait among L. lactis strains.

Relation between EPS adherence, viscoelastic properties, and MBR operation: Biofouling study with QCM-D

Membrane fouling is one of the main constraints of the wide use of membrane bioreactor (MBR) technology. The biomass in MBR systems includes extracellular polymeric substances (EPS), metabolic products of active microbial secretion that adversely affect the membrane performance. Solids retention time (SRT) in the MBR is one of the most important parameters affecting membrane fouling in MBR systems, where fouling is minimized at optimal SRT. Among the operating parameters in MBR systems, SRT is known to strongly influence the ratio of proteins to polysaccharides in the EPS matrix. In this study, we have direct evidence for changes in EPS adherence and viscoelastic properties due to changes in the sludge removal rate that strongly correlate with the membrane fouling rate and EPS composition. EPS were extracted from a UF membrane in a hybrid growth MBR operated at sludge removal rates of 59, 35.4, 17.7, and 5.9 L day(-1) (corresponding SRT of 3, 5, 10, and 30 days, respectively). The EPS adherence and adsorption kinetics were carried out in a quartz crystal microbalance with dissipation monitoring (QCM-D) technology in several adsorption measurements to a gold sensor coated with Polyvinylidene Fluoride (PVDF). EPS adsorption to the sensor surface is characterized by a decrease of the oscillation frequency and an increase in the dissipation energy of the sensor during parallel flow of aqueous media, supplemented with EPS, above the sensor surface. The results from these experiments were further modeled using the Voigt based model, in which the thickness, shear modulus, and shear viscosity values of the adsorbed EPS layers on the PVDF crystal were calculated. The observations in the QCM-D suggested that the elevated fouling of the UF membrane is due to higher adherence of the EPS as well as reduction in viscosity and elasticity of the EPS adsorbed layer and elevation of the EPS fluidity. These results corroborate with confocal laser scanning microscopy (CLSM) image analysis showing thicker EPS in close proximity to the membrane surface operated at reactor conditions which induced more fouling at elevated sludge removal rates.

Osteoconductive protamine-based polyelectrolyte multilayer functionalized surfaces

The integration of orthopedic implants with host bone presents a major challenge in joint arthroplasty, spinal fusion and tumor reconstruction. The cellular microenvironment can be programmed via implant surface functionalization allowing direct modulation of osteoblast adhesion, proliferation, and differentiation at the implant--bone interface. The development of layer-by-layer assembled polyelectrolyte multilayer (PEM) architectures has greatly expanded our ability to fabricate intricate nanometer to micron scale thin film coatings that conform to complex implant geometries. The in vivo therapeutic efficacy of thin PEM implant coatings for numerous biomedical applications has previously been reported. We have fabricated protamine-based PEM thin films that support the long-term proliferation and differentiation of pre-osteoblast cells on non-cross-linked film-coated surfaces. These hydrophilic PEM functionalized surfaces with nanometer-scale roughness facilitated increased deposition of calcified matrix by osteoblasts in vitro, and thus offer the potential to enhance implant integration with host bone. The coatings can make an immediate impact in the osteogenic culture of stem cells and assessment of the osteogenic potential of new therapeutic factors.

Probing fundamental film parameters of immobilized enzymes--towards enhanced biosensor performance. Part I--QCM-D mass and rheological measurements

Enzyme immobilization is an ever-growing research-area for both analytical and industrial applications. Of critical importance in this area are the effects of immobilization procedures upon the functionality of the immobilized biomolecules. Both beneficial and detrimental effects can be conferred through the selection and tuning of the immobilization procedure. Quartz-crystal microbalance with dissipation (QCM-D) has been previously used to great effect in tracking alterations to thin films of biomolecules immobilized onto quartz transducers. In this study, we investigate the ability of QCM-D to track and monitor film parameters of a monolayer of laccase immobilized on a series of self-assembled monolayers (SAMs), differing in lateral density of binding residues on the SAM and height of the SAM from the quartz surface. Both mass gains and rheological parameters for these varying surfaces were measured and trends later compared to the apparent enzyme kinetics of the immobilized laccase films, assessed electroanalytically (Paper II in this two part study). For covalent attachment of proteins, both shear and viscosity were increased relative to physically adsorbed proteins. An increase in lateral density of protein-binding surface of the SAM components was shown to increase the shear/viscosity of the resultant film while an increase in distance from the electrode (through incorporation of lysine linkers) was shown to decrease the shear/viscosity while simultaneously increasing the wet mass gain of the films. Shear and viscosity may be indicative of both enzyme denaturation and increased lateral protein packing within the film structure hence it is assumed that less distortion occurs with the inclusion of linkers which allow for more optimal protein immobilization.

α-Hemolysin pore formation into a supported phospholipid bilayer using cell-free expression

Cell-free protein synthesis is becoming a serious alternative to cell-based protein expression. Cell-free systems can deliver large amounts of cytoplasmic recombinant proteins after a few hours of incubation. Recent studies have shown that membrane proteins can be also expressed in cell-free reactions and directly inserted into phospholipid membranes. In this work, we present a quantitative method to study in real time the concurrent cell-free expression and insertion of membrane proteins into phospholipid bilayers. The pore-forming protein α-hemolysin, fused to the reporter protein eGFP, was used as a model of membrane protein. Cell-free expression of the toxin in solution and inside large synthetic phospholipid vesicles was measured by fluorometry and fluorescence microscopy respectively. A quartz crystal microbalance with dissipation was used to characterize the interaction of the protein with a supported phospholipid bilayer. The cell-free reaction was directly incubated onto the bilayer inside the microbalance chamber while the frequency and the dissipation signals were monitored. The presence of pores in the phospholipid bilayer was confirmed by atomic force microscopy. A model is presented which describes the kinetics of adsorption of the expressed protein on the phospholipid bilayer. The combination of cell-free expression, fluorescence microscopy and quartz crystal microbalance-dissipation is a new quantitative approach to study the interaction of membrane proteins with phospholipid bilayers.

Is the viscoelasticity of Alzheimer's Abeta42 peptide oligomers a general property of protein oligomers related to their toxicity?

The largest group of protein misfolding diseases is associated with the conversion of specific peptides or proteins from their soluble functional states into highly organized fibrillar aggregates named amyloid fibrils or plaques. The amyloid-beta peptide (Abeta) is involved in pathogenesis of Alzheimer's disease (AD), being the main constituent of the amyloid plaques found in AD brains. Abeta is a proteolytic product of a transmembrane protein and due to its amphipathicity it may be retained in the membrane, and this has been shown to be crucial for neurotoxicity. Hydrophobic and electrostatic interactions strongly influence its conformation and aggregation both in solution and at interfaces. Appropriate solid sorbent surfaces were used to study the different interactions independently. Quartz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM) and attenuated total reflection infrared spectroscopy (ATR-IR) were employed for the investigation of the behavior of Abeta peptides on planar surfaces. Abeta peptides have high affinity for hydrophobic and rough surfaces that promote aggregation. QCM-D measurements indicate that the oligomers are soft when compared to monomers, and this property might be related to the bioactivity of protein oligomers in general.

Probing in vitro interactions between Lactococcus lactis and mucins using AFM

This work was devoted to the first AFM investigation of the adhesion force to pig gastric mucin (PGM) using Lactococcus lactis as the model for lactic acid bacteria. The PGM coating on polystyrene was characterized using a complementary set of multiscale analytical methods, including AFM (HarmoniX mode), XPS, and the sessile drop method. The PGM layer, which was mainly composed of C-O, C-N, COOH, CONH, and sulfur-related species (protein core and oligosaccharide side chains), was quite homogeneous and hydrophilic, with an estimated thickness of 3.4 nm. L. lactis cells were immobilized on the AFM tip (lacto probe) and used as a force probe to measure the interaction forces between bacteria and PGM-coated polystyrene on the nanoscale. After mucin adsorption, adhesion force levels were lower because of the interplay of electrostatic, hydrophilic, and steric repulsions. For example, the adhesion forces of the lacto probe to bare and PGM-coated polymer were 0.74 +/- 0.10 and 0.12 +/- 0.06 nN, respectively. The shape analysis of retraction force-distance curves highlighted the contribution of both nonspecific and specific forces (ligand/receptor bonding). The lacto probe concept and the associated AFM measurements may now provide a powerful framework for understanding interaction mechanisms between mucins and lactic acid bacteria.

Sequential adsorption of bovine mucin and lactoperoxidase to various substrates studied with quartz crystal microbalance with dissipation

Mucin and lactoperoxidase are both natively present in the human saliva. Mucin provides lubricating and antiadhesive function, while lactoperoxidase has antimicrobial activity. We propose that combined films of the two proteins can be used as a strategy for surface modification in biomedical applications such as implants or biosensors. In order to design and ultilize mixed protein films, it is necessary to understand the variation in adsorption behavior of the proteins onto different surfaces and how it affects their interaction. The quartz crystal microbalance with dissipation (QCM-D) technique has been used to extract information of the adsorption properties of bovine mucin (BSM) and lactoperoxidase (LPO) to gold, silica, and hydrophobized silica surfaces. The information has further been used to retrieve information of the viscoelastic properties of the adsorbed film. The adsorption and compaction of BSM were found to vary depending on the nature of the underlying bare surface, adsorbing as a thick highly hydrated film with loops and tails extending out in the bulk on gold and as a thinner film with much lower adsorbed amount on silica; and on hydrophobic surfaces, BSM adsorbs as a flat and much more compact layer. On gold and silica, the highly hydrated BSM film is cross-linked and compacted by the addition of LPO, whereas the compaction is not as pronounced on the already more compact film formed on hydrophobic surfaces. The adsorption of LPO to bare surfaces also varied depending on the type of surface. The adsorption profile of BSM onto LPO-coated surfaces mimicked the adsorption to the underlying surface, implying little interaction between the LPO and BSM. The interaction between the protein layers was interpreted as a combination of electrostatic and hydrophobic interactions, which was in turn influenced by the interaction of the proteins with the different substrates.

Potential use of mucins as biomaterial coatings. II. Mucin coatings affect the conformation and neutrophil-activating properties of adsorbed host proteins--toward a mucosal mimic

In continuation of our recent fractionation and characterization study on mucins of bovine salivary (BSM), porcine gastric (PGM), and human salivary (MG1) origin, this study evaluates the effect of mucin precoating on the conformation and neutrophil-activating properties of host proteins adsorbed to a polyethylene terephthalate-based model biomaterial. Microscopy combined with assays for the neutrophil releases of reactive oxygen species and human neutrophil lipocalin showed that mucin precoating greatly reduced the strong immune-response normally induced by adsorbed immunoglobulin G (IgG) and secretory immunoglobulin A (sIgA), respectively. A similar finding was made for the proinflammatory fibrinogen. Although the total uptakes of these proteins depended on the mucin surface concentration, a detailed composite analysis suggested the fraction of surface-exposed protein to be a stronger determinant of coating performance. The unexpectedly low neutrophil activation showed by composites containing near-monolayer concentrations of exposed IgG and sIgA, respectively, suggested that these act synergistically with mucin on the surface. In support of this hypothesis, quartz crystal microbalance with dissipation monitoring measurements revealed that a preadsorbed BSM layer stabilizes IgG through complexation on a polymeric model surface. Our findings link well to the complex in vivo situation and suggest that functional mucosal mimics can be created in situ for improved biomaterials performance.

Influence of ionic strength changes on the structure of pre-adsorbed salivary films. A response of a natural multi-component layer

Salivary films coating oral surfaces are critically important for oral health. This study focuses on determining the underlying nature of this adsorbed film and how it responds to departures from physiological conditions due to changes in ionic strength. Under physiological conditions, it is found that pre-adsorbed in vitro salivary film on hydrophobic surfaces is present as a highly hydrated viscoelastic layer. We follow the evolution of this film in terms of its effective thickness, hydration and viscoelastic properties, as well as adsorbed mass of proteins, using complementary surface characterisation methods: a Surface Plasmon Resonance (SPR) and a Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). Our results support a heterogeneous model for the structure of the salivary film with an inner dense anchoring layer and an outer highly extended hydrated layer. Further swelling of the film was observed upon decreasing the salt concentration down to 1mM NaCl. However, upon exposure to deionised water, a collapse of the film occurs that was associated with the loss of water contained within the adsorbed layer. We suggest that the collapse in deionised water is driven by an onset of electrostatic attraction between different parts of the multi-component salivary film. It is anticipated that such changes could also occur when the oral cavity is exposed to food, beverage, oral care and pharmaceutical formulations where drastic changes to the structural integrity of the film is likely to have implications on oral health, sensory perception and product performance.

Quantitative and qualitative evaluation of adsorption/desorption of bovine serum albumin on hydrophilic and hydrophobic surfaces

We studied the adsorption of bovine serum albumin (BSA) from phosphate-buffered saline (pH 7.4) to hydrophilic and hydrophobic surfaces. Attenuated total reflection Fourier transform infrared spectroscopy, supported by spectral simulation, allowed us to determine with high precision the amount of BSA adsorbed (surface coverage) and its structural composition. The adsorbed BSA molecules had an alpha-helical structure on both hydrophobic and hydrophilic surfaces but had different molecular conformations and adsorption strengths on the two types of surface. Adsorption of BSA was saturated at around 50% surface coverage on the hydrophobic surface, whereas on the hydrophilic surface the adsorption reached 95%. The BSA molecules adsorbed to the hydrophilic surface with a higher interaction strength than to the hydrophobic surface. Very little adsorbed BSA could be desorbed from the hydrophilic surface, even using 0.1 M sodium dodecyl sulfate, a strong detergent solution. The formation of BSA-phosphate surface complexes was observed under different BSA adsorption conditions on hydrophobic and hydrophilic surfaces. The formation of these complexes correlated with the more efficient blocking of nonspecific interactions by the adsorbed BSA layer. Results from the molecular modeling of BSA interactions with hydrophobic and hydrophilic surfaces support the spectroscopic findings.

Low biofouling chitosan-hyaluronic acid multilayers with ultra-low friction coefficients

Resistance to biofouling is an advantageous material property in a variety of biomedical and biofluid processing applications. Protein-resisting surface coatings must also be resistant to wear and degradation and in certain applications good aqueous lubricating properties are required. We show that cross-linked polyelectrolyte multilayers, consisting of chitosan and hyaluronan on polydimethylsiloxane (PDMS) surfaces, form a highly lubricating film that is resistant to wear and protein adsorption. The multilayer film shows much stronger resistance to protein adsorption from human whole saliva than both hydrophobic and hydrophilic PDMS surfaces; the latter two showed identical adsorbed salivary film thicknesses. The boundary friction coefficient under aqueous conditions was extremely low (mu approximately 0.01) between multilayer-coated PDMS substrates and the film is robust against dry rubbing and many hours of tribological experiments in a range of aqueous lubricants. The origins of the assembly's low friction coefficients and robustness are discussed. In addition, we found that the addition of negative phosphate ions to water lowers the boundary lubricating properties of negatively charged hydrophilic PDMS surfaces by 1 order of magnitude to mu approximately 0.01. We consider this to arise from the large hydration sheaths and resulting "ball-bearing" properties of the hydrated phosphate ions, which form a lubricating barrier against asperity contact. These findings offer new insights toward biolubrication processes and suggest that chitosan-hyaluronan polyelectrolyte multilayer films have the potential to be used in (bio-) applications requiring low friction as well as resistance to biofouling and wear.

Comparison of the adsorption kinetics and surface arrangement of "as received" and purified bovine submaxillary gland mucin (BSM) on hydrophilic surfaces

The effect of bovine serum albumin (BSA) as impurity in a commercial bovine submaxillary gland mucin preparation (BSM; Sigma M3895) on the adsorption of BSM to hydrophilic surfaces (mica and silica) has been studied in terms of adsorption kinetics, amount and structure of the formed adlayer. The Surface Force Apparatus (SFA) was used to gain information about the extended and compressed structure of adsorbed "as received" BSM, purified BSM, BSA extracted from the "as received" BSM and mixtures of the latter purified proteins. The adsorbed amount was estimated using a combination of X-ray Photoelectron Spectroscopy (XPS), Enzyme-Linked Immuno Sorbent Assay (ELISA), Enzyme-Linked Lectin Assay (ELLA), Dual Polarization Interferometry (DPI) and Quartz Crystal Microbalance (QCM-D) measurements. Under the used conditions, purified BSM showed very low affinity for silica and only small amounts were found to adsorb on mica. Initially, the BSM molecules adopted an extended conformation on the mica surface with tails extending into the bulk phase. These tails were irreversibly compressed into a very thin (10A) layer upon applying a high load. "As received" BSM formed considerably thicker compressed layers (35A); however, the extended layer structure was qualitatively the same. When mixtures of purified BSM and BSA were coadsorbed on mica, a 9wt-% albumin content gave a comparable layer thickness as the "as received" BSM and from XPS data we draw the conclusion that the albumin content in the layer adsorbed from "as received" BSM was approximately 5wt-%. Adsorption from an equal amount of BSM and BSA revealed that even though the amount of BSM is scarce in the mixed layer, the few BSM molecules have a drastic effect on the adsorbed thickness and structure. Clearly, this study shows the importance of characterizing the mucin used since differences in purity give rise to different adsorption behaviours in terms of both adsorbed amount and layer structure.

Viscous boundary lubrication of hydrophobic surfaces by mucin

The lubricating behavior of the weakly charged short-side-chain glycoprotein mucin "Orthana" (Mw=0.55 MDa) has been investigated between hydrophobic and hydrophilic PDMS substrates using soft-contact tribometry. It was found that mucin facilitates lubrication between hydrophobic PDMS surfaces, leading to a 10-fold reduction in boundary friction coefficient for rough surfaces. The presence of mucin also results in a shift of the mixed lubrication regime to lower entrainment speeds. The observed boundary lubrication behavior of mucin was found to depend on the bulk concentration, and we linked this to the structure and dynamics of the adsorbed mucin films, which are assessed using optical waveguide light spectroscopy. We observe a composite structure of the adsorbed mucin layer, with its internal structure governed by entanglement. The film thickness of this adsorbed layer increases with concentration, while the boundary friction coefficient for rough surfaces was found to be inversely proportional to the thickness of the adsorbed film. This link between lubrication and structure of the film is consistent with a viscous boundary lubrication mechanism, i.e., a thicker adsorbed film, at a given sliding speed, results in a lower local shear rate and, hence, in a lower local shear stress. The estimated local viscosities of the adsorbed layer, derived from the friction measurements and the polymer layer density, are in agreement with each other.

Interactions between human neutrophils and mucin-coated surfaces

Recently, we showed microscopically that bovine (BSM), porcine (PGM) and human (MG1) mucin coatings could suppress the adhesion of neutrophils to a polyethylene terephthalate-based model biomaterial (Thermanox). Here, using the release of reactive oxygen species (ROS) as a marker of material-induced neutrophil activation, the strong surface-passivating effects of these mucin coatings were corroborated. Under optimal adsorption conditions, all mucin species performed equally well, thus indicating a high degree of functional homology between the mucins. Cell adhesion and morphology correlated well with the release of ROS. Quartz crystal microbalance (QCM-D) analysis linked low neutrophil activation to efficient mucin surface-shielding. Interestingly, the shielding power appeared equal for thick expanded and thin compact mucin coatings. Combined mucin-serum coatings were found to be highly surface-passivating. Particularly, since our data suggested partly synergistic mucin-serum action, we highlight the possibility that pre-adsorbed mucins could provide favorable support for adsorbing host components.

Effects of casein, ovalbumin, and dextran on the astringency of tea polyphenols determined by quartz crystal microbalance with dissipation

Tea polyphenols (TPPs) can bind with proteins and peptides through hydrophobic interaction and hydrogen bonding. Casein, ovalbumin, and dextran were used to investigate their influence on the interactions between TPP and gelatin and, therefore, to investigate their influence on TPP taste. Casein-g-dextran (CgD) and ovalbumin-g-dextran (OgD) grafting conjugates were prepared through the Maillard reaction. Dispersible CgD/OgD-TPP complexes formed in acidic pH solution even after a heating process. At the same weight ratio of protein to TPP, about 20-30% of TPP was bound to the proteins. TPP affinity for dextran is much lower. Gelatin, a model of the salivary proteins in buccal cavity, was immobilized on quartz crystal sensor surface through covalent bond. By use of a quartz crystal microbalance with dissipation, we found that the complexation of TPP with gelatin causes a dehydration and collapse of the gelatin layer on the sensor surface that is similar to the sensation of dryness and constriction on oral membranes caused by polyphenols. The complexation between TPP and casein/ovalbumin/dextran can decrease the interaction between TPP and gelatin by decreasing the free TPP molecules and shielding gelatin surface from TPP. Casein has stronger binding ability on the gelatin surface compared to ovalbumin and dextran, and therefore casein is more effective to decrease the sensation of astringency caused by TPP.

QCM-D analysis of the performance of blocking agents on gold and polystyrene surfaces

With today's developments of biosensors and medical implants comes the need for efficient reduction of nonspecific binding. We report on a comparison of the ability of traditionally used blocking agents and poly(ethylene glycol) (PEG) derivatives to prevent protein adsorption on both gold and polystyrene surfaces. The adsorption kinetics of blocking molecules and proteins was monitored gravimetrically using quartz crystal microbalance with dissipation (QCM-D). The resistance to nonspecific adsorption was evaluated on gold and polystyrene surfaces coated with bovine serum albumin (BSA) or casein, gold coated with three different 6-11 ethylene glycol (EG) long hydroxyl- or methoxy-terminated PEG-thiolates and polystyrene blocked with a PLL-g-PEG or three different 12 EG long benzyl-PEG-derivatives. The prevention of protein adsorption on the coated surfaces was evaluated by monitoring the mass uptake at the addition of both pure prostate specific antigen (PSA) and seminal plasma. We demonstrate that on pure gold the PEG-thiols are superior to the other blocking molecules tested, with the end group and length of the PEG-thiols used being of minor importance. On polystyrene surfaces blocking with PLL-g-PEG, BSA and casein gave the best results. These results have an impact on further development of an optimized immunoassay protocol.

Mucin-electrolyte interactions at the solid-liquid interface probed by QCM-D

The interaction between mucin and ions has been investigated by employing the quartz crystal microbalance technique with measurement of energy dissipation. The study was partially aimed at understanding the adsorption of mucin on surfaces with different chemistry, and for this purpose, surfaces exposing COOH, OH, and CH(3) groups were prepared. Mucin adsorbed to all three types of functionalized gold surfaces. Adsorption to the hydrophobic surface and to the charged hydrophilic surface (COOH) occured with high affinity despite the fact that in the latter case both mucin and the surface were negatively charged. On the uncharged hydrophilic surface exposing OH groups, the adsorption of mucin was very low. Another aim was to elucidate conformational changes induced by electrolytes on mucin layers adsorbed on hydrophobic surfaces from 30 mM NaNO(3). To this end, we investigated the effect of three electrolytes with increasing cation valance: NaCl, CaCl(2) and LaCl(3). At low NaCl concentrations, the preadsorbed layer expands, whereas at higher concentrations of NaCl the layer becomes more compact. This swelling/compacting of the mucin layer is fully reversible for NaCl. When the mucin layer instead is exposed to CaCl(2) or LaCl(3), compaction is observed at 1 mM. For CaCl(2), this process is only partially reversible, and for LaCl(3), the changes are irreversible within the time frame of the experiment. Finally, mucin interaction with the DTAB cationic surfactant in an aqueous solution of different electrolytes was evaluated with turbidimetry measurements. It is concluded that the electrolytes used in this work screen the association between mucin and DTAB and that the effect increases with increasing cation valency.