High-affinity tags fused to s-layer proteins probed by atomic force microscopy

Two-dimensional, crystalline bacterial cell surface layers, termed S-layers, are one of the most commonly observed cell surface structures of prokaryotic organisms. In the present study, genetically modified S-layer protein SbpA of Bacillus sphaericus CCM 2177 carrying the short affinity peptide Strep-tag I or Strep-tag II at the C terminus was used to generate a 2D crystalline monomolecular protein lattice on a silicon surface. Because of the genetic modification, the 2D crystals were addressable via Strep-tag through streptavidin molecules. Atomic force microscopy (AFM) was used to investigate the topography of the single-molecules array and the functionality of the fused Strep-tags. In high-resolution imaging under near-physiological conditions, structural details such as protein alignment and spacing were resolved. By applying molecular recognition force microscopy, the Strep-tag moieties were proven to be fully functional and accessible. For this purpose, streptavidin molecules were tethered to AFM tips via approximately 8-nm-long flexible polyethylene glycol (PEG) linkers. These functionalized tips showed specific interactions with 2D protein crystals containing either the Strep-tag I or Strep-tag II, with similar energetic and kinetic behavior in both cases.     

S-layer templated bioinspired synthesis of silica

The current understanding of the molecular mechanisms involved in the bioinspired formation of silica structures laid foundation for investigating the potential of the S-layer protein SbpA from Lysinibacillus sphaericus CCM 2177 as catalyst, template and scaffold for the generation of novel silica architectures. SbpA reassembles into monomolecular lattices with square (p4) lattice symmetry and a lattice constant of 13.1 nm. Silica layers on the S-layer lattice were formed using tetramethoxysilane (TMOS) and visualized by transmission electron microscopy. In situ quartz crystal microbalance with dissipation monitoring (QCM-D) measurements showed the adsorption of silica in dependence on the presence of phosphate in the silicate solution and on the preceding chemical modification of the S-layer. An increased amount of precipitated silica could be observed when K₂HPO₄/KH₂PO₄ was present in the solution (pH 7.2). Further on, independent of the presence of phosphate the silica deposition was higher on S-layer lattices upon activation of their carboxyl groups with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) compared to native S-layers or EDC treated S-layers when the activated carboxyl groups were blocked with ethylene diamine (EDA). Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy revealed the formation of an amorphous silica gel (SiO₂)_x·yH₂O on the S-layer. The silica surface concentrations on the S-layer was 4 × 10^?9 to 2 × 10^?8 mol cm^?2 depending on the modification of the protein layer and corresponded to 4–21 monolayers of SiO₂.     

Self-assembly of S-layer-enveloped cytochrome c polyelectrolyte multilayers

We report a study of the electrostatic layer-by-layer self-assembly of electroactive polyelectrolyte multilayers incorporating the redox protein cytochrome c (cyt c) combined with recrystallization of the bacterial cell wall surface layer from Bacillus sphaericus CCM 2177 SbpA (S-layer). The polyelectrolyte multilayer assembly was prepared on flat gold electrodes with a nanometer-scale roughness that allowed monitoring of the film formation throughout all the assembly stages by atomic force microscopy measurements in liquid with respect to topography and forces. The deposition of alternating layers of sulfonated polyaniline and cyt c was carried out by adsorption from the corresponding solutions on a cyt c monolayer electrode. The electroactivity of cyt c within the assembly was confirmed by cyclic voltammetry. We showed that the surface properties of the electrode terminating layer change after each adsorption step accordingly. We also found that S-layer recrystallization on the top of the multilayer film was feasible while electroactivity of cyt c within a polyelectrolyte matrix was partially maintained. This approach offers a new strategy to design a biocompatible and permselective outer envelope of a polyelectrolyte multilayer, promising sensor applications.     

Methods for research on immune complement activation on modified sensor surfaces

We have developed a methodological system consisting of a new surface sensitive quartz crystal microbalance with dissipation monitoring (QCM-D) sensor surfaces together with different surface modification methods for the investigation of surface associated complement activation in human sera. The QCM-D surface, 10 mm in diameter, was modified by spin-coating of poly(urethane urea) (PUUR) and polystyrene (PS). Some sensor surfaces were also sputtered with titanium (Ti) or modified by hydrophobic self-assembled monolayer (SAM) of an 18-carbon alkane thiol with a ---CH₃ end group. The amount of surface deposited complement protein was investigated by incubation of the modified sensor surfaces in human sera, followed by incubation with antibodies directed against complement factor 3c (C3c). The amounts of bound anti-C3c were then used as an arbitrary measure of surface induced complement activation. The order of complement activation of the different surfaces, as judged by three separate measurements per surface modification, was PUUR>PS=SAM>Ti. The Ti surface had a similar low degree of anti-C3c binding as the negative controls (heat inactivated sera). The novel QCM-D methodology was found to be very simple, accurate, sensitive and well suited as a screening method for complement activation and protein adsorption on different materials. We also compared the sensitivity of QCM-D method with surface plasmon resonance (SPR) for the quantification of protein adsorption and complement activation on gold sensor surfaces. The QCM-D method was equally sensitive as the SPR for the detection of protein adsorption from a solution independently if low flow rate (5 μl/min) was used. A slight increase in sensitivity was found at higher flow rate (30 μl/min). However, we found it difficult to use the SPR method on the Ti, PS and PUUR surfaces due to decreased light penetration of the modified SPR sensor chip.     

耗散型石英晶体微天平在生物医用高分子材料中的应用

石英晶体微天平(QCM)是一种基于石英晶体压电效应的分析检测技术,可实时在线提供石英晶体表面吸附层质量、厚度、粘弹性等信息,由此获得表面分子相互作用关系。 耗散型石英晶体微天平(QCM-D)因其独特的对粘弹性的解析,使其在高分子材料中的应用迅速发展,尤其是生物医用高分子材料领域,已用来评价生物医用高分子材料的表界面相互作用,力学和生物相容性等。 本文简单介绍了耗散型石英晶体微天平的基本原理及理论模型,重点综述了近几年QCM-D在高分子链构象、蛋白质吸附、生物大分子相互作用、药物释放以及水凝胶中的应用,并且展望了QCM-D的未来发展趋势。     

Crosslink effect and albumin adsorption onto chitosan/alginate multilayered systems: an in situ QCM-D study

The adsorption of HSA onto CHI/ALG multilayer assemblies was assessed in situ using QCM-D. It was found that the behavior of HSA on biomaterials surface can be tuned by adjusting parameters of the polyelectrolyte system such as pH, layer number, crosslinker and polymer terminal layer. Our results confirmed the key role of electrostatic interactions during HSA adsorption, since oppositely charged surfaces were more effective in promoting protein adhesion. QCM-D data revealed that crosslinking (CHI/ALG)(5) CHI films allows HSA to become adsorbed in physiological conditions. Our results suggested that the biological potential of biopolymers and the mild conditions of the LbL technique turn these natural nanoassemblies into a suitable choice to be used as pH-sensitive coatings.     

Use of cellulose derivatives on gold surfaces for reduced nonspecific adsorption of immunoglobulin G

This study addresses the design of protein-repellent gold surfaces using hydroxyethyl- and ethyl(hydroxyethyl) cellulose (HEC and EHEC) and hydrophobically modified analogues of these polymers (HM-HEC and HM-EHEC). Adsorption behavior of the protein immunoglobulin G (IgG) onto pure gold and gold surfaces coated with cellulose polymers was investigated and described by quartz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM) and contact angle measurements (CAM). Surfaces coated with the hydrophobically modified cellulose derivatives were found to significantly outperform a reference poly(ethylene glycol) (PEG) coating, which in turn prevented 90% of non-specific protein adsorption as compared to adsorption onto pure gold. HEC and EHEC prevented around 30% and 60% of the IgG adsorption observed on pure gold, while HM-HEC and HM-EHEC were both found to completely hinder biofouling when deposited on the gold substrates. Adsorption behavior of IgG has been discussed in terms of polymer surface coverage and roughness of the applied surfaces, together with hydrophobic interactions between protein and gold, and also polymer-protein interactions.     

Protein-associated water and secondary structure effect removal of blood proteins from metallic substrates

Removing adsorbed protein from metals has significant health and industrial consequences. There are numerous protein-adsorption studies using model self-assembled monolayers or polymeric substrates but hardly any high-resolution measurements of adsorption and removal of proteins on industrially relevant transition metals. Surgeons and ship owners desire clean metal surfaces to reduce transmission of disease via surgical instruments and minimize surface fouling (to reduce friction and corrosion), respectively. A major finding of this work is that, besides hydrophobic interaction adhesion energy, water content in an adsorbed protein layer and secondary structure of proteins determined the access and hence ability to remove adsorbed proteins from metal surfaces with a strong alkaline-surfactant solution (NaOH and 5 mg/mL SDS in PBS at pH 11). This is demonstrated with three blood proteins (bovine serum albumin, immunoglobulin, and fibrinogen) and four transition metal substrates and stainless steel (platinum (Pt), gold (Au), tungsten (W), titanium (Ti), and 316 grade stainless steel (SS)). All the metallic substrates were checked for chemical contaminations like carbon and sulfur and were characterized using X-ray photoelectron spectroscopy (XPS). While Pt and Au surfaces were oxide-free (fairly inert elements), W, Ti, and SS substrates were associated with native oxide. Difference measurements between a quartz crystal microbalance with dissipation (QCM-D) and surface plasmon resonance spectroscopy (SPR) provided a measure of the water content in the protein-adsorbed layers. Hydrophobic adhesion forces, obtained with atomic force microscopy, between the proteins and the metals correlated with the amount of the adsorbed protein-water complex. Thus, the amount of protein adsorbed decreased with Pt, Au, W, Ti and SS, in this order. Neither sessile contact angle nor surface roughness of the metal substrates was useful as predictors here. All three globular proteins behaved similarly on addition of the alkaline-surfactant cleaning solution, in that platinum and gold exhibited an increase, while tungsten, titanium, and stainless steel showed a decrease in weight. According to dissipation measurements with the QCM-D, the adsorbed layer for platinum and gold was rigid, while that for the tungsten, titanium, and stainless steel was much more flexible. The removal efficiency of adsorbed-protein by alkaline solution of SDS depended on the water content of the adsorbed layers for W, Ti, and SS, while for Pt and Au, it depended on secondary structural content. When protein adsorption was high (Pt, Au), protein-protein interactions and protein-surface interactions were dominant and the removal of protein layers was limited. Water content of the adsorbed protein layer was the determining factor for how efficiently the layer was removed by alkaline SDS when protein adsorption was low. Hence, protein-protein and protein-surface interactions were minimal and protein structure was less perturbed in comparison with those for high protein adsorption. Secondary structural content determined the efficient removal of adsorbed protein for high adsorbed amount.     

Fibronectin adsorption on gold, Ti-, and Ta-oxide investigated by QCM-D and RSA modelling

The adsorption of fibronectin on gold, Ti-, and Ta-oxide surfaces is investigated by means of the quartz crystal microbalance with dissipation (QCM-D) technique. The surface chemistry (gold, Ti-, and Ta-oxide) is found to influence the frequency shift observed during adsorption of the fibronectin layer with the magnitude being Delta f Au>Delta f Ti-oxide approximately Delta f Ta-oxide. Corresponding variations in the dissipation change normalised to frequency change (Delta D/Delta f) for the layer are observed. The QCM-D data are further analyzed by the random sequential adsorption (RSA) model, and adsorption rate parameter ka and footprint (a) determined, which supported the trend seen in the Delta f and Delta D/Delta f values. The value of ka found by the RSA modelling of the QCM-D resonance frequency data is found to match the ratio between the mass measured by QCM-D and the mass reported by optical techniques in literature. We conclude that comparison of the adsorption rate parameter (ka) obtained by RSA modelling of the QCM-D data with ka values obtained from RSA modelling of data obtained using optical techniques can be a route to determine the degree of hydration of the adsorbed protein layer.     

Solution and solid-state models of peptide CH...O hydrogen bonds

Fumaramide derivatives were analyzed in solution by (1)H NMR spectroscopy and in the solid state by X-ray crystallography in order to characterize the formation of CH...O interactions under each condition and to thereby serve as models for these interactions in peptide and protein structure. Solutions of fumaramides at 10 mM in CDCl(3) were titrated with DMSO-d(6), resulting in chemical shifts that moved downfield for the CH groups thought to participate in CH...O=S(CD(3))(2) hydrogen bonds concurrent with NH...O=S(CD(3))(2) hydrogen bonding. In this model, nonparticipating CH groups under the same conditions showed no significant change in chemical shifts between 0.0 and 1.0 M DMSO-d(6) and then moved upfield at higher DMSO-d(6) concentrations. At concentrations above 1.0 M DMSO-d(6), the directed CH...O=S(CD(3))(2) hydrogen bonds provide protection from random DMSO-d(6) contact and prevent the chemical shifts for participating CH groups from moving upfield beyond the original value observed in CDCl(3). X-ray crystal structures identified CH...O=C hydrogen bonds alongside intermolecular NH...O=C hydrogen bonding, a result that supports the solution (1)H NMR spectroscopy results. The solution and solid-state data therefore both provide evidence for the presence of CH...O hydrogen bonds formed concurrent with NH...O hydrogen bonding in these structures. The CH...O=C hydrogen bonds in the X-ray crystal structures are similar to those described for antiparallel beta-sheet structure observed in protein X-ray crystal structures.     

Self-assembled monolayers of an oligo(ethylene oxide) disulfide and its corresponding thiol assembled from water: characterization and protein resistance

Self-assembled monolayers (SAMs) of the disulfide [S(CH2CH2O)6CH3]2 ([S(EO)6]2) on Au from 95% ethanol and from 100% water are described. Spectroscopic ellipsometry and reflection-absorption infrared spectroscopy indicate that the [S(EO)6]2 films are similar to the disordered films of HS(CH2CH2O)6CH3 ((EO)6) and HS(CH2)3O(CH2CH2O)5CH3 (C3EO5) at their protein adsorption minima. The [S(EO)6]2 SAMs exhibit constant film thickness (d) of 1.2 +/- 0.2 nm over long immersion times (up to 20 days) and do not attain the highly ordered, 7/2 helical structure of the (EO)6 and C3EO5 SAMs (d = 2.0 nm). Exposure of these self-limiting [S(EO)6]2 SAMs to bovine serum albumin show high resistance to protein adsorption.     

Chitosan adsorption on hydroxyapatite and its role in preventing acid erosion

Polymer adsorption onto an artificial saliva (AS) layer is investigated using quartz-crystal microbalance with dissipation (QCM-D) and chitosan as the model polymer. QCM-D is utilized in an innovative manner to monitor in situ adsorption of chitosan (CH) onto a hydroxyapatite (HA) coated crystal and to examine the ability of the adsorbed layer to "protect" the HA upon sequential exposure to acidic solutions. After deposition of a thin AS layer (16nm), the total thickness on the HA substrate increases to 37nm upon exposure to CH at pH 5.5 for 10min. Correspondingly, the surface charge changes from negative (i.e., AS) to positive, consistent with the adsorption the polycationic CH onto or into the AS layer. Upon exposure to an oxidizing agent, the chitosan cross-links and collapses as noted by a decrease in thickness to 10nm and an increase in the shear modulus by an order of magnitude. Atomic force microscopy (AFM) is used to determine the surface morphology and RMS roughness of the coated and HA surfaces after citric acid challenges. Both physisorbed and cross-linked chitosan are demonstrated to limit and prevent the erosion of HA, respectively.     

Chemical modifications of Au/SiO2 template substrates for patterned biofunctional surfaces

The aim of this work was to create patterned surfaces for localized and specific biochemical recognition. For this purpose, we have developed a protocol for orthogonal and material-selective surface modifications of microfabricated patterned surfaces composed of SiO(2) areas (100 μm diameter) surrounded by Au. The SiO(2) spots were chemically modified by a sequence of reactions (silanization using an amine-terminated silane (APTES), followed by amine coupling of a biotin analogue and biospecific recognition) to achieve efficient immobilization of streptavidin in a functional form. The surrounding Au was rendered inert to protein adsorption by modification by HS(CH(2))(10)CONH(CH(2))(2)(OCH(2)CH(2))(7)OH (thiol-OEG). The surface modification protocol was developed by testing separately homogeneous SiO(2) and Au surfaces, to obtain the two following results: (i) SiO(2) surfaces which allowed the grafting of streptavidin, and subsequent immobilization of biotinylated antibodies, and (ii) Au surfaces showing almost no affinity for the same streptavidin and antibody solutions. The surface interactions were monitored by quartz crystal microbalance with dissipation monitoring (QCM-D), and chemical analyses were performed by polarization modulation-reflexion absorption infrared spectroscopy (PM-RAIRS) and X-ray photoelectron spectroscopy (XPS) to assess the validity of the initial orthogonal assembly of APTES and thiol-OEG. Eventually, microscopy imaging of the modified Au/SiO(2) patterned substrates validated the specific binding of streptavidin on the SiO(2)/APTES areas, as well as the subsequent binding of biotinylated anti-rIgG and further detection of fluorescent rIgG on the functionalized SiO(2) areas. These results demonstrate a successful protocol for the preparation of patterned biofunctional surfaces, based on microfabricated Au/SiO(2) templates and supported by careful surface analysis. The strong immobilization of the biomolecules resulting from the described protocol is advantageous in particular for micropatterned substrates for cell-surface interactions.     

Monomolecular reassembly of a crystalline bacterial cell surface layer (S-layer) on untreated and modified silicon surfaces

Scanning force microscopy was used to investigate the recrystallization of isolated bacterial cell surface layer (S-layer) proteins of Bacillus stearothermophilus NRS 2004/3a variant V1 on untreated, cleaned, silanized and photoresist-coated silicon wafers. With the exception of the hydrophilic surface of cleaned wafers, all other surfaces showed hydrophobic surface characteristics. Recrystallization occurred only at the hydrophobic surfaces, and, with respect to the bacterial cell, the S-layer was always oriented with its more hydrophobic outer face against the interface. Monolayer formation was initiated by crystal growth from several distant randomly oriented nucleation points and terminated by neighbouring, also growing, crystalline areas. The size of the individual crystalline domains formed in this way was in the range of 5–10 μm in diameter. The entire silicon surface was covered by a coherent monolayer after a recrystallization time of approximately 1 h.     

Experimental studies on the adsorption of two surfactants on solid-aqueous interfaces: adsorption isotherms and kinetics

A quartz crystal microbalance with dissipation (QCM-D) was used to measure the adsorption from aqueous solutions of CTAB (cationic) and C(12)E(6) (nonionic) surfactants on gold and silica surfaces. QCM-D allows for the determination of adsorption isotherms and also the monitoring of the dynamics of adsorption in real time. By considering the atomic-scale roughness of the solid surfaces and the surface area per head group at the air/water interface, our experiments indicate that at bulk concentrations above the critical micelle concentration adsorbed C(12)E(6) forms a monolayer-like structure on both surfaces and CTAB yields a bilayer-like structure. Although our measurements do not allow us to discriminate between the morphology of the aggregates (i.e., between flat monolayers, hemicylinders, or hemispheres in the case of C(12)E(6) and between flat bilayers, cylinders, or spheres in the case of CTAB), these results are particularly significant when compared to recent QCM-D data reported by Macakova et al. (Macakova, L.; Blomberg, E.; Claesson, P. M. Langmuir 2007, 23, 12436). These authors reported that QCM-D overestimates the amount of CTAB adsorbed on silica by as much as 30-40% as a result of entrapped water. Our analysis suggests that the effect of entrapped solvent is not as important as previously assumed and, in fact, QCM-D may not overestimate the amount of CTAB adsorbed when roughness is considered. Results for the kinetics of adsorption suggest that the aggregate structure as well as whether micelles are present may influence the adsorption mechanism. We discuss our results in the perspective of molecular theories for both the equilibrium and kinetics of surfactant adsorption.     

Real-Time Characterization of Fibrinogen Interaction with Modified Titanium Dioxide Film by Quartz Crystal Microbalance with Dissipation

The adsorption of fibrinogen can be used as a quick indicator of surface haemocompatibility because of its prominent role in coagulation and platelet adhesion. In this work the molecular interaction between fibrinogen and a modified titanium oxide surface/platelet has been studied by quartz crystal microbalanee with dissipation （QCM-D） in situ. In order to further characterize the conformation of adsorbed fibrinogen, αC and γ-chain antibody were used to check the orientation and denaturation of fibrinogen on solid surface. QCM-D investiga- tions revealed the fibrinogen have the trend to adsorb on hydropllilic surface in a side-on orientation by positively charged αC domains, which would reduce the exposure of platelet bonding site on γ chain and enable less platelet adhesion and be activated. These obser- vations suggest that certain conformations of adsorbed fibrinogen are less platelet adhesive than others, which opens a possibility for creating a non-platelet adhesive substrates.     

静电力驱动蛋白质在疏水蛋白表面的吸附

疏水蛋白是丝状真菌产生的一种外泌蛋白质, 它们可以在不同表面形成双亲性蛋白膜. 疏水蛋白也是一种优良的蛋白质固定化基质, 然而蛋白质在疏水蛋白表面吸附的驱动机制却是未知的. 本文系统研究了不同pH和离子浓度下蛋白质在疏水蛋白表面的吸附. 首先, 用石英晶体微天平技术研究了不同pH和离子浓度下, Ⅰ型疏水蛋白HGFI和Ⅱ型疏水蛋白HFBI在聚苯乙烯表面的吸附. 结果发现, pH和离子强度对HGFI在聚苯乙烯表面的吸附影响较大, 对HFBI的吸附影响与HGFI相比则较小; HGFI在聚苯乙烯表面主要形成的是弹性膜, 而HFBI在聚苯乙烯表面主要形成的是刚性膜. 随后又研究了不同pH和离子浓度下牛血清白蛋白(BSA)和亲和素(Avidin)在HGFI和HFB上吸附, 结果表明, pH和离子强度对BSA和Avidin在HGFI和HFB上吸附有显著影响, 说明BSA和Avidin在两种疏水蛋白上吸附的主要驱动力为静电力. 本文研究结果为实现疏水蛋白表面可控地固定蛋白质提供了理论指导.     

Ultrafiltration membranes prepared from crystalline bacterial cell surface layers as model systems for studying the influence of surface properties on protein adsorption

Crystalline bacterial cell surface layers (S-layers) were used for the preparation of the active filtration layer of ultrafiltration membranes (S-layer ultrafiltration membranes; SUMs). Since the S-layer is uniform in its pore size and morphology and its functional groups are aligned in well-defined positions, the SUMs provide ideal model systems for studying protein adsorption and membrane fouling. Due to the presence of surface-located carboxyl groups the standard SUMs have the net negative charge but exhibit basically a hydrophobic character. In order to change the net charge, the charge density and the accessibility of charged groups of the SUMs as well as their hydrophobicity, free carboxyl groups of the S-layer protein were modified with selected low molecular weight nucleophiles under conditions of preserving the crystalline lattice structure. SUMs with 1.6 to 7 charged or functional groups exposed per nm² of the membrane area were used for adsorption experiments. After solutions of differently sized and charged test proteins were filtered, the relative flux losses of distilled particle free water were measured. The results showed that the adsorption capacity of the SUMs increased with the extent of their hydrophobicity. Test proteins showed their own specific adsorption characteristics, which clearly demonstrated the difficulties in determining parameters controlling the membrane fouling. Independent of the net charge of the test proteins and that of the SUMs, the flux loss of SUMs increased with the increased charge density and an improved accessibility of the charged groups on the S-layer surface. No essential differences in the adsorption characteristics were observed between the zwitterionic SUMs of slightly surplus of free carboxyl groups and the standard SUMs of net negative charge.     

Adsorption of bacteria and polycations on model surfaces of cellulose, hemicellulose and wood extractives studied by QCM-D

Quartz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) were used as the tools to study the adsorption of bacteria onto surfaces of silica and polystyrene coated with materials related to papermaking. Cationic polyelectrolytes used as fixatives and retention aids in paper industry were found to promote irreversible adsorption of the ubiquitous white water bacterium, Pseudoxanthomonas taiwanensis, onto model surfaces of cellulose (pH 8). The high charged low molecular weight polyelectrolyte, poly(diallyldimethyl) ammonium chloride (pDADMAC) adsorbed to silica surface as a flat and rigid layer, whereas the low charged cationic polyacryl amide (C-PAM) of high molecular weight adsorbed as a thick and loose layer. AFM images showed that the polyelectrolytes accumulated as layers around each bacterial cell. In the presence of wood hemicellulose (O-acetyl-galactoglucomannan) the bacteria adsorbed massively, as large, tightly packed rafts (up to 0.05mm in size) onto the polystyrene crystal surface coated with wood extractives (pH 4.7). AFM and FESEM micrographs also showed large naked areas (with no bacteria) in between the bacterial rafts on the crystal surface. In this case, QCM-D only incompletely responded to the massiveness of the bacterial adsorption. The results indicate that cationic polymers can be used to increase the retention of bacteria from the process water onto the fibre web and that, depending on the balance between hemicelluloses and wood extractives and pH of the process waters, bacteria can be drawn from process waters onto surfaces.     

In situ kinetic study of zinc sulfide activation using a quartz crystal microbalance with dissipation (QCM-D)

We have studied the activation kinetics of zinc sulfide (ZnS) using silver as an activator by a quartz crystal microbalance with dissipation (QCM-D). The zinc sulfide coating on QCM-D sensor was shown to have similar crystallographic structure, composition, and surface properties as nature sphalerite through the characterization of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and xanthate adsorption measurement using QCM-D. The activation of ZnS sensor by silver was confirmed by the mass increase in ZnS sensor coupled with subsequent xanthate adsorption during QCM-D measurement, the change of surface wettability, and the presence of Ag(2)S on the surface. Two distinct stages on the silver uptake vs. time curve were identified and fitted well by a logarithmic function for the initial stage and a parabolic law in the later stage, which agrees with the two-stage zinc-silver reaction kinetics reported previously. Argon sputtering followed by XPS measurement on the ZnS surface demonstrated the penetration of silver into the bulk ZnS after activation. The present study is the first of its kind to apply the QCM-D technique to investigate sphalerite activation, which introduces a new in situ approach to investigate surface adsorption and activation in many mineral processes and surface modifications.