Quartz crystal microbalance study of ionic strength and pH-dependent polymer conformation and protein adsorption/desorption on PAA, PEO, and mixed PEO/PAA brushes

Modular and Substrate-Independent Grafting-To Procedure for Functional Polymer Coatings

The ability to tailor polymer brush coatings to the last nanometer has arguably placed them among the most powerful surface modification techniques currently available. Generally, the synthesis procedures for polymer brushes are designed for a specific surface type and monomer functionality and cannot be easily employed otherwise. Herein, we describe a modular and straightforward two-step grafting-to approach that allows introduction of polymer brushes of a desired functionality onto a large range of chemically different substrates. To illustrate the modularity of the procedure, gold, silicon oxide (SiO₂), and polyester-coated glass substrates were modified with five different block copolymers. In short, the substrates were first modified with a universally applicable poly(dopamine) primer layer. Subsequently, a grafting-to reaction was performed on the poly(dopamine) films using five distinct block copolymers, all of which contained a short poly(glycidyl methacrylate) segment and longer segment of varying chemical functionality. Ellipsometry, X-ray photoelectron spectroscopy, and static water contact angle measurements confirmed successful grafting of all five block copolymers to the poly(dopamine)-modified gold, SiO₂, and polyester-coated glass substrates. In addition, our method was used to provide direct access to binary brush coatings, by simultaneous grafting of two different polymer materials. The ability to synthesize binary brush coatings further adds to the versatility of our approach and paves the way toward production of novel multifunctional and responsive polymer coatings.

Understanding Nanocellulose-Water Interactions: Turning a Detriment into an Asset

Modern technology has enabled the isolation of nanocellulose from plant-based fibers, and the current trend focuses on utilizing nanocellulose in a broad range of sustainable materials applications. Water is generally seen as a detrimental component when in contact with nanocellulose-based materials, just like it is harmful for traditional cellulosic materials such as paper or cardboard. However, water is an integral component in plants, and many applications of nanocellulose already accept the presence of water or make use of it. This review gives a comprehensive account of nanocellulose-water interactions and their repercussions in all key areas of contemporary research: fundamental physical chemistry, chemical modification of nanocellulose, materials applications, and analytical methods to map the water interactions and the effect of water on a nanocellulose matrix.

Versatile Coating Platform for Metal Oxide Nanoparticles: Applications to Materials and Biological Science

In this feature article, we provide an overview of our research on statistical copolymers as a coating material for metal oxide nanoparticles and surfaces. These copolymers contain functional groups enabling noncovalent binding to oxide surfaces and poly(ethylene glycol) (PEG) polymers for colloidal stability and stealthiness. The functional groups are organic derivatives of phosphorous acid compounds R-H₂PO₃, also known as phosphonic acids that have been screened for their strong affinity to metals and for their multidentate binding ability. Herein we develop a polymer-based coating platform that shares features with the self-assembled monolayer (SAM) and layer-by-layer (L-b-L) deposition techniques. The milestones of this endeavor are the synthesis of PEG-based copolymers containing multiple phosphonic acid groups, the implementation of simple protocols combining versatility with high particle production yields, and the experimental evidence of the colloidal stability of the coated particles. As a demonstration, coating studies are conducted on cerium (CeO₂), iron (γ-Fe₂O₃), aluminum (Al₂O₃), and titanium (TiO₂) oxides of different sizes and morphologies. We finally discuss applications in the domain of nanomaterials and nanomedicine. We evaluate the beneficial effects of coatings on redispersible nanopowders, contrast agents for in vitro/vivo assays, and stimuli-responsive particles.

Distinctive Adsorption Mechanism and Kinetics of Immunoglobulin G on a Nanoscale Polymer Surface

Elucidation of protein adsorption beyond simple polymer surfaces to those presenting greater chemical complexity and nanoscopic features is critical to developing well-controlled nanobiomaterials and nanobiosensors. In this study, we repeatedly and faithfully track individual proteins on the same nanodomain areas of a block copolymer (BCP) surface and monitor the adsorption and assembly behavior of a model protein, immunoglobulin G (IgG), over time into a tight surface-packed structure. With discrete protein adsorption events unambiguously visualized at the biomolecular level, the detailed assembly and packing states of IgG on the BCP nanodomain surface are subsequently correlated to various regimes of IgG adsorption kinetic plots. Intriguing features, entirely different from those observed from macroscopic homopolymer templates, are identified from the IgG adsorption isotherms on the nanoscale, chemically varying BCP surface. They include the presence of two Langmuir-like adsorption segments and a nonmonotonic regime in the adsorption plot. Via correlation to time-corresponding topographic data, the unique isotherm features are explained with single biomolecule level details of the IgG adsorption pathway on the BCP. This work not only provides much needed, direct experimental evidence for time-resolved, single protein level, adsorption events on nanoscale polymer surfaces but also signifies mutual linking between specific topographic states of protein adsorption and assembly to particular segments of adsorption isotherms. From the fundamental research viewpoint, the correlative ability to examine the nanoscopic surface organizations of individual proteins and their local as well as global adsorption kinetic profiles will be highly valuable for accurately determining protein assembly mechanisms and interpreting protein adsorption kinetics on nanoscale surfaces. Application-wise, such knowledge will also be important for fundamentally guiding the design and development of biomaterials and biomedical devices that exploit nanoscale polymer architectures.

Online preconcentration of lysozyme in hen egg white using responsive polymer coating in CE

A mixed polymer brushes material based on poly (2-methyl-2-oxazoline)- and poly (acrylic acid)-coated capillary with switchable protein adsorption/desorption properties was applied for online preconcentration of lysozyme in hen egg white during capillary electrophoresis performance. First, lysozyme in simulated egg white was successfully online preconcentrated and the detection signal of lysozyme was amplified. Ovalbumin, ovomucoid, and conalbumin in egg white were verified show negligible interference on the online preconcentration of lysozyme according to the study on electroosmotic flow mobility. Second, a series validation procedure was carried out to evaluate the proposed method performance. There was a good linearity behavior range from 0.1 to 5.0 ng/mL, limit of detection was 20 pg/mL, and limit of quantity was 50 pg/mL, the accuracy and robustness of this method were also excellent. Last, the proposed method has been successfully used to detect and analyze lysozyme in hen egg white, the determined amounts of lysozyme in hen egg white were consistent with reported normal levels and recoveries were in the range of 96.0-99.2%. After 75 consecutive runs, this prepared capillary was still stable for online preconcentration and determination of lysozyme in hen egg white without being affected by complex matrix.

Interaction of Proteins with a Planar Poly(acrylic acid) Brush: Analysis by Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D)

We describe the preparation of a poly(acrylic acid) (PAA) brush, polymerized by atom transfer radical polymerization (ATRP) of tert-butyl acrylate (tBA) and subsequent acid hydrolysis, on the flat gold surfaces of quartz-crystal microbalance (QCM) crystals. The PAA brushes were characterized by Fourier transform infrared (FT-IR) spectroscopy, ellipsometry and water contact angle analysis. The interaction of the PAA brushes with human serum albumin (HSA) was studied for a range of ionic strengths and pH conditions by quartz-crystal microbalance with dissipation monitoring (QCM-D). The quantitative analysis showed a strong adsorption of protein molecules onto the PAA brush. By increasing the ionic strength, we were able to release a fraction of the initially bound HSA molecules. This finding highlights the importance of counterions in the polyelectrolyte-mediated protein adsorption/desorption. A comparison with recent calorimetric studies related to the binding of HSA to polyelectrolytes allowed us to fully analyze the QCM data based on the results of the thermodynamic analysis of the binding process.

Generic high-capacity protein capture and release by pH control

Techniques for immobilization and release of proteins are of general interest but challenging to develop. Here we show a new method for high-capacity (several μg cm^-2) immobilization of proteins in polyelectrolyte brushes by multivalent hydrogen bonds. Upon increasing pH, the proteins are fully released with preserved structure and activity.

Characterizing protein G B1 orientation and its effect on immunoglobulin G antibody binding using XPS, ToF-SIMS, and quartz crystal microbalance with dissipation monitoring

Controlling how proteins are immobilized (e.g., controlling their orientation and conformation) is essential for developing and optimizing the performance of in vitro protein-binding devices, such as enzyme-linked immunosorbent assays. Characterizing the identity, orientation, etc., of proteins in complex mixtures of immobilized proteins requires a multitechnique approach. The focus of this work was to control and characterize the orientation of protein G B1, an immunoglobulin G (IgG) antibody-binding domain of protein G, on well-defined surfaces and to measure the effect of protein G B1 orientation on IgG antibody binding. The surface sensitivity of time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to distinguish between different proteins and their orientation on both flat and nanoparticle gold surfaces by monitoring intensity changes of characteristic amino acid mass fragments. Amino acids distributed asymmetrically were used to calculate peak intensity ratios from ToF-SIMS data to determine the orientation of protein G B1 cysteine mutants covalently attached to a maleimide surface. To study the effect of protein orientation on antibody binding, multilayer protein films on flat gold surfaces were formed by binding IgG to the immobilized protein G B1 films. Quartz crystal microbalance with dissipation monitoring and x-ray photoelectron spectroscopy analysis revealed that coverage and orientation affected the antibody-binding process. At high protein G B1 coverage, the cysteine mutant immobilized in an end-on orientation with the C-terminus exposed bound 443 ng/cm² of whole IgG (H + L) antibodies. In comparison, the high coverage cysteine mutant immobilized in an end-on orientation with the N-terminus exposed did not bind detectable amounts of whole IgG (H + L) antibodies.

How Dissociation of Carboxylic Acid Groups in a Weak Polyelectrolyte Brush Depend on Their Distance from the Substrate

A weak polyelectrolyte brush is composed of a layer of polyacids or polybases grafted by one end of their chains to a substrate surface. For such brush layers immersed in an aqueous solution, the dissociation behavior of the acidic or basic groups and the structural and physical properties of the brush layer will thus be strongly dependent on the environmental conditions. For a polyacid brush layer consisting of, e.g., poly(acrylic acid), this means that the chains in the brush layer will be charged at high pH and uncharged at low pH. However, theoretical scaling laws not only foresee the structural changes occurring in response to the pH-induced dissociation behavior but also how the dissociation behavior of the brush layer depends on the ionic strength of the aqueous solution and the density of acidic groups within the brush layer. We have herein employed spectroscopic ellipsometry and a quartz crystal microbalance with dissipation monitoring (QCM-D) to experimentally evaluate the theoretically predicted dissociation and structural behavior of PAA brushes. Spectroscopic ellipsometry allows us to study the brush thickness as a function of pH and ionic strength, while QCM-D gives us an opportunity to investigate the swelling behavior of PAA brushes at various penetration depths of propagating acoustic waves. Our studies show that the dissociation degree of the carboxylic acid groups in a PAA brush increases with increasing distance from the substrate. Moreover, the ionic strength enhances carboxylic acid dissociation, such that a higher ionic strength leads to a narrower distribution and higher average dissociation degree. In conclusion, our results provide an experimental verification of the theoretically predicted gradient in the degree of dissociation of the acid groups in weak polyacid brush layers and shows that at a pH value equal to approximately the average pK_a value of the brush, the state of the acid groups varies from being almost uncharged to almost fully dissociated depending on the ionic strength and vertical position in the brush.

Amplification of lysozyme signal detected in capillary electrophoresis using mixed polymer brushes coating with switchable properties

In this work, a mixed polymer brushes based on poly(2-methyl-2-oxazoline) (PMOXA) and poly(acrylic acid) (PAA) coated capillary with switchable protein adsorption/desorption properties was developed and applied for on-line extraction and preconcentration of lysozyme. The study of electroosmotic flow (EOF) and fluorescence microscope showed that the inner surface charge of PMOXA/PAA mixed brush coated capillary displayed the switchable behavior toward the change of pH value and ionic strength (I), and PMOXA/PAA mixed brushes coated capillary could adsorb high amounts of lysozyme at pH 7 (I = 10^-5 M), and the most of adsorbed lysozyme could then be desorbed at pH 3 (I = 10^-1 M). Subsequently, this coated capillary with switchable lysozyme adsorption/desorption ability was applied for on-line extraction and preconcentration of lysozyme during capillary electrophoresis (CE) performance. Under the process of on-line preconcentration, the detection signal (peak area) of lysozyme obtained in PMOXA/PAA coated capillary was 26 times that obtained in bare capillary under normal CE while the contour chain length of PAA was 1.56 times that of PMOXA. Moreover, the value of low detection limit (LOD) of lysozyme using above coated capillary under on-line preconcentration method reached to 4.5 × 10^-9 mg/mL, and 1 × 10⁵-fold sensitivity enhancement was realized for lysozyme as compared with the bare capillary under normal CE.

Mixed Polymer Brushes for the Selective Capture and Release of Proteins

Selective protein adsorption is a key challenge for the development of biosensors, separation technologies, and smart materials for medicine and biotechnologies. In this work, a strategy was developed for selective protein adsorption, based on the use of mixed polymer brushes composed of poly(ethylene oxide) (PEO), a protein-repellent polymer, and poly(acrylic acid) (PAA), a weak polyacid whose conformation changes according to the pH and ionic strength of the surrounding medium. A mixture of lysozyme (Lyz), human serum albumin (HSA), and human fibrinogen (Fb) was used to demonstrate the success of this strategy. Polymer brush formation and protein adsorption were monitored by quartz crystal microbalance, whereas protein identification after adsorption from the mixture was performed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) with principal component analysis and gel electrophoresis with silver staining. For the ToF-SIMS measurements, adsorption was first performed from single-protein solutions in order to identify characteristic peaks of each protein. Next, adsorption was performed from the mixture of the three proteins. Proteins were also desorbed from the brushes and analyzed by gel electrophoresis with silver staining for further identification. Selective adsorption of Lyz from a mixture of Lyz/HSA/Fb was successfully achieved at pH 9.0 and ionic strength of 10^-3 M, while Lyz and HSA, but not Fb, were adsorbed at ionic strength 10^-2 M and pH 9.0. The results demonstrate that by controlling the ionic strength, selective adsorption can be achieved from protein mixtures on PEO/PAA mixed brushes, predominantly because of the resulting control on electrostatic interactions. In well-chosen conditions, the selectively adsorbed proteins can also be fully recovered from the brushes by a simple ionic strength stimulus. The developed systems will find applications as responsive biointerfaces in the fields of separation technologies, biosensing, drug delivery, and nanomedicine.

Making strong polyelectrolyte brushes pH-sensitive by incorporation of gold nanoparticles

Doping polymer brushes with gold nanoparticles (AuNPs) results in composite materials with colorimetric sensor properties. The present paper addresses the effect of electrostatic particle-particle interaction and the effect of the polymer brush type on particle assembly formation within the polymer matrix. The prospect for long-term use as colorimetric sensors is tested. Therefore, two different types of brushes of pH-insensitive polymers, non-ionic poly(N-isopropylacrylamide) (PNIPAM) and cationic poly-[2-(methacryloyloxy)ethyl] trimethylammonium chloride (PMETAC), are studied. After incubation of the non-ionic PNIPAM brush in an aqueous suspension of AuNPs with a pH-sensitive carboxylic acid capping, hydrogen binding led to attachment of the AuNPs, but they were easily detached at high pH due to loss of the hydrogen binding. In contrast, the anionic AuNPs adhere well to cationic PMETAC brushes even after post-treatment at low pH where the charge density of the AuNPs is strongly reduced. Therefore, the PMETAC/AuNP composites were further tested with respect to their stability against pH variations and their impact for colorimetric sensors. Although the neat PMETAC brush is not pH-sensitive, after embedding pH-sensitive AuNPs, the PMETAC/AuNP composite becomes pH-sensitive in a reversible manner. This is detectable by the reversible shift of the plasmon band and the reversible thickness change of the composites by exposing them to different pH.

Salt-Induced Control of the Grafting Density in Poly(ethylene glycol) Brush Layers by a Grafting-to Approach

In this work, a method to obtain control of the grafting density during the formation of polymer brush layers by the grafting-to method of thiolated poly(ethylene glycol) onto gold is presented. The grafting density of the polymer chains was adjusted by adding Na₂SO₄ in concentrations between 0.2 and 0.9 M to the aqueous polymer solution during the grafting process. The obtained grafting densities ranged from 0.26 to 1.60 chains nm^-2, as determined by surface plasmon resonance. The kinetics of the grafting process were studied in situ by a quartz crystal microbalance with dissipation, and a mushroom to brush conformational transition was observed when the polymer was grafted in the presence of Na₂SO₄. The transition from mushroom to brush was only observed for long periods of grafting, highlighting the importance of time to obtain high grafting densities. Finally, the prepared brush layer with the highest grafting density showed high resistance to the adsorption of bovine serum albumin, while layers with a lower grafting density showed only limited resistance.

Reversible Protein Adsorption on Mixed PEO/PAA Polymer Brushes: Role of Ionic Strength and PEO Content

Proteins at interfaces are a key for many applications in the biomedical field, in biotechnologies, in biocatalysis, in food industry, etc. The development of surface layers that allow to control and manipulate proteins is thus highly desired. In previous works, we have shown that mixed polymer brushes combining the protein-repellent properties of poly(ethylene oxide) (PEO) and the stimuli-responsive adsorption behavior of poly(acrylic acid) (PAA) could be synthesized and used to achieve switchable protein adsorption. With the present work, we bring more insight into the rational design of such smart thin films by unravelling the role of PEO on the adsorption/desorption of proteins. The PEO content of the mixed PEO/PAA brushes was regulated, on the one hand, by using PEO with different molar masses and, on the other hand, by varying the ratio of PEO and PAA in the solutions used to synthesize the brushes. The influence of ionic strength on the protein adsorption behavior was also further examined. The behavior of three proteins-human serum albumin, lysozyme, and human fibrinogen, which have very different size, shape, and isoelectric point-was investigated. X-ray photoelectron spectroscopy, quartz crystal microbalance, atomic force microscopy, and streaming potential measurements were used to characterize the mixed polymer brushes and, in particular, to estimate the fraction of each polymer within the brushes. Protein adsorption and desorption conditions were selected based on previous studies. While brushes with a lower PEO content allowed the higher protein adsorption to occur, fully reversible adsorption could only be achieved when the PEO surface density was at least 25 PEO units per nm². Taken together, the results increase the ability to finely tune protein adsorption, especially with temporal control. This opens up possibilities of applications in biosensor design, separation technologies, nanotransport, etc.

Dominant Albumin-Surface Interactions under Independent Control of Surface Charge and Wettability

Understanding protein adsorption behaviors on solid surfaces constitutes an important step toward development of efficacious and biocompatible medical devices. Both surface charge and wettability have been shown to influence protein adsorption attributes, including kinetics, quantities, deformation, and reversibility. However, determining the dominant interaction in these surface-induced phenomena is challenging because of the complexity of inter-related mechanisms at the liquid/solid interface. Herein, we reveal the dominant interfacial forces in these essential protein adsorption attributes under the influence of a combination of surface charge and wettability, using quartz crystal microbalance with dissipation monitoring and atomic force microscopy-based force spectroscopy on a series of model surfaces. These surfaces were fabricated via layer-by-layer assembly, which allowed two-dimensional control of surface charge and wettability with minimal cross-parameter dependency. We focused on a soft globular protein, bovine serum albumin (BSA), which is prone to conformational changes during adsorption. The information obtained from the two techniques shows that both surface charge and hydrophobicity can increase the protein-surface interaction forces and the adsorbed amount. However, surface hydrophobicity triggered a greater extent of deformation in the adsorbed BSA molecules, leading to more dehydration, spreading, and resistance to elution by ionic strength changes regardless of the surface charge. The role played by the surface charge in the adsorbed protein conformation and extent of desorption induced by changes in the ionic strength is secondary to that of surface hydrophobicity. These findings advance the understanding of how surface chemistry and properties can be tailored for directing protein-substrate interactions.

Dopamine assisted PMOXA/PAA brushes for their switchable protein adsorption/desorption

PMOXA/PAA mixed brushes with switchable protein adsorption/desorption properties were prepared by sequentially grafting PMOXA-NH₂ and PAA-SH onto PDA-coated substrates.

Immobilization of Aluminum Hydroxide Particles on Quartz Crystal Microbalance Sensors to Elucidate Antigen-Adjuvant Interaction Mechanisms in Vaccines

Aluminum hydroxide (AH) salts are the most widely used adjuvants in vaccine formulation. They trigger immunogenicity from antigenic subunits that would otherwise suffer from a lack of efficiency. Previous studies focusing on antigen-AH interaction mechanisms, performed with model proteins, suggested that electrostatic interactions and phosphate-hydroxyl ligand exchanges drive protein adsorption on AH. We however recently evidenced that NaCl, used in vaccine formulation, provokes AH particle aggregation. This must be taken into account to interpret data related to protein adsorption on AH. Here, we report on the successful development and use of a stable AH-coated surface to explore the mechanisms of protein adsorption by means of ultrasensitive surface analysis tools. Bovine serum albumin (BSA) adsorption was studied at different pHs and ionic strengths (I) using quartz crystal microbalance. The results show that protein adsorption on the AH adjuvant cannot be explained solely by electrostatic interactions and ligand exchanges. Hence, a higher adsorption was observed at pH 3 compared to pH 7, although AH and BSA respectively undergo repulsive and attractive electrostatic interactions at these pH values. Almost no effect of I on adsorption was moreover noted at pH 7. These new developments and observations not only suggest that other mechanisms govern protein adsorption on AH but also offer a new platform for the study of antigen adsorption in the context of vaccine formulation. Immobilizing particles on QCM sensors also enriches the range of applications for which QCM can be exploited, especially in colloid science.

Antimicrobial and Osseointegration Properties of Nanostructured Titanium Orthopaedic Implants

The surface design of titanium implants influences not only the local biological reactions but also affects at least the clinical result in orthopaedic application. During the last decades, strong efforts have been made to improve osteointegration and prevent bacterial adhesion to these surfaces. Following the rule of “smaller, faster, cheaper”, nanotechnology has encountered clinical application. It is evident that the hierarchical implant surface micro- and nanotopography orchestrate the biological cascades of early peri-implant endosseous healing or implant loosening. This review of the literature gives a brief overview of nanostructured titanium-base biomaterials designed to improve osteointegration and prevent from bacterial infection.

Serum Protein-Resistant Behavior of Multisite-Bound Poly(ethylene glycol) Chains on Iron Oxide Surfaces

Recent surveys have shown that the number of nanoparticle-based formulations actually used at a clinical level is significantly lower than that expected a decade ago. One reason for this is that the physicochemical properties of nanoparticles fall short for handling the complexity of biological environments and preventing nonspecific protein adsorption. In this study, we address the issue of the interactions of plasma proteins with polymer-coated surfaces. With this aim, we use a noncovalent grafting-to method to functionalize iron oxide sub-10 nm nanoparticles and iron oxide flat substrates and compare their protein responses. The functionalized copolymers consist of alternating poly(ethylene glycol) (PEG) chains and phosphonic acid grafted on the same backbone. Quartz crystal microbalance with dissipation was used to monitor polymer adsorption kinetics and evaluate the resistance to protein adsorption. On flat substrates, functionalized PEG copolymers adsorb and form a brush in moderate or highly stretched regimes, with densities between 0.15 and 1.5 nm-2. PEG layers using phosphonic acid as linkers exhibit excellent protein resistance. In contrast, layers prepared with carboxylic acid as the grafting agent exhibit mitigated protein responses and layer destructuration. The present study establishes a correlation between the long-term stability of PEG-coated particles in biofluids and the protein resistance of surfaces coated with the same polymers.

The internal structure of PMETAC brush/gold nanoparticle composites: a neutron and X-ray reflectivity study

The amount and distribution of gold nanoparticles within a polymer brush matrix can be changed by altering the electrostatic interaction between particle–particle as well as particle–brush.

Monitoring the Switching of Single BSA-ATTO 488 Molecules Covalently End-Attached to a pH-Responsive PAA Brush

We describe a novel combination of a responsive polymer brush and a fluorescently labeled biomolecule, where the position of the biomolecule can be switched from inside to outside the brush and vice versa by a change in pH. For this, we grafted ultrathin, amino-terminated poly(acrylic acid) brushes to glass and silicon substrates. Individual bovine serum albumin (BSA) molecules labeled with fluorophore ATTO 488 were covalently end-attached to the polymers in this brush using a bis-N-succinimidyl-(pentaethylene glycol) linker. We investigated the dry layer properties of the brush-protein ensemble, and it is swelling behavior using spectroscopic ellipsometry. Total internal reflection fluorescence (TIRF) microscopy enabled us to study the distance-dependent switching of the fluorescently labeled protein molecules. The fluorescence emission from the labeled proteins ceased (out-state) when the polymer chains stretched away from the interface under basic pH conditions, and fluorescence recurred (in-state) when the chains collapsed under acidic conditions. Moreover, TIRF allowed us to study the fluorescence switching behavior of fluorescently labeled BSA molecules down to the single-molecule level, and we demonstrate that this switching is fast but that the exact intensity during the in-state is the result of a more random process. Control experiments verify that the switching behavior is directly correlated to the responsive behavior of the polymer brush. We propose this system as a platform for switchable sensor applications but also as a method to study the swelling and collapse of individual polymer chains in a responsive polymer brush.

Role of Ionic Strength in Staphylococcal Cell Aggregation

Cell aggregation plays a key role in biofilm formation and pathogenesis of Staphylococcus species. Although the molecular basis of aggregation in Staphylococci has already been extensively investigated, the influence of environmental factors, such as ionic strength, remains poorly understood. In this paper, we report a new type of cellular aggregation of Staphylococci that depends solely on ionic strength. Seven strains out of 14, all belonging to staphylococcal species, formed large cell clusters within minutes in buffers of ionic strength ranging from 1.5 to 50 mM, whereas isolates belonging to other Gram-positive species did not display this phenotype. Atomic force microscopy (AFM) with chemically functionalized tips provided direct evidence that ionic strength modulates cell surface adhesive properties through changes in cell surface charge. The optimal ionic strength for aggregation was found to be strain dependent, but in all cases, bacterial aggregates formed at an ionic strength of 1.5-50 mM were rapidly dispersed in a solution of higher ionic strength, indicating a reversibility of the cell aggregation process. These findings suggest that some staphylococcal isolates can respond to ionic strength as an external stimulus to trigger rapid cell aggregation in a way that has not yet been reported.

Uptake of pH-Sensitive Gold Nanoparticles in Strong Polyelectrolyte Brushes

The impact of electrostatic attraction on the uptake of gold nanoparticles (AuNPs) into positively charged strong poly-[2-(Methacryloyloxy) ethyl] trimethylammonium chloride (PMETAC) polyelectrolyte brushes was investigated. In this work, PMETAC brushes were synthesized via surface-initiated atom transfer radical polymerization (Si-ATRP). PMETAC/AuNP composite materials were prepared by incubation of the polymer brush coated samples into 3-mercaptopropionic acid-capped AuNP (5 nm in diameter) suspension. The electrostatic interactions were tuned by changing the surface charge of the AuNPs through variations in pH value, while the charge of the PMETAC brush was not affected. Atomic-force microscopy (AFM), ellipsometry, UV/Vis spectroscopy, gravimetric analysis and transmission electron microscopy (TEM) were employed to study the loading and penetration into the polymer brush. The results show that the number density of attached AuNPs depends on the pH value and increases with increasing pH value. There is also strong evidence that the particle assembly is dependent on the pH value of the AuNP suspension. Incubation of PMETAC brushes in AuNP suspension at pH 4 led to the formation of a surface layer on top of the brush (2D assembly) due to sterical hindrance of the clustered AuNPs, while incubation in AuNP suspension at pH 8 led to deeper particle penetration into the brush (3D assembly). The straightforward control of particle uptake and assembly by tuning the charge density of the nanoparticle surface is a valuable tool for the development of materials for colorimetric sensor applications.

Synthesis and Biomedical Applications of Poly((meth)acrylic acid) Brushes

Poly((meth)acrylic acid) (P(M)AA) brushes possess a number of distinctive properties that are particularly attractive for biomedical applications. This minireview summarizes recent advances in the synthesis and biomedical applications of P(M)AA brushes and brushes containing P(M)AA segments. First, we review different surface-initiated polymerization (SIP) methods, with a focus on recent progress in the surface-initiated controlled/living radical polymerization (SI-CLRP) techniques used to generate P(M)AA brushes with a tailored structure. Next, we discuss biomolecule immobilization methods for P(M)AA brushes, including physical adsorption, covalent binding, and affinity interactions. Finally, typical biomedical applications of P(M)AA brushes are reviewed, and their performance is discussed based on their unique properties. We conclude that P(M)AA brushes are promising biomaterials, and more potential biomedical applications are expected to emerge with the further development of synthetic techniques and increased understanding of their interactions with biological systems.

Probing the adjustments of macromolecules during their surface adsorption

Thiol-terminated polymers poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC-SH), poly(N,N-isopropylacrylamide) (PNIPAM-SH), and poly(tert-butyl acrylate) (PtBA-SH) were synthesized, and the polymers were grafted on the gold surfaces of quartz crystal microbalance with dissipation (QCM-D) and surface plasmon resonance (SPR) sensor chips to form brushes. The grafting process of the polymer brushes as well as protein adsorption onto the brush layers was monitored by in situ QCM-D and SPR techniques. By examining the changes in frequency and dissipation factor as well as the value of ∂D/∂f from QCM-D measurements, different stages of the polymer grafting and protein adsorption are distinguished. The most interesting discovery is the conformation change of BSA protein adsorption from a weakly adsorbed native state to a strongly immobilized denatured state on the polymer brushes. The corresponding change in BSA adsorption from a reversible state to an irreversible state was confirmed by SPR measurements. The adsorption of protein on the polymer brushes' surface relies largely on interaction between the protein and the polymers, and the stronger hydrophilicity of the surfaces is proved to be more effective to suppress the protein adsorption. Analysis of the D-f plot of QCM-D measurements helps to characterize different binding strength of protein and the underlying polymer surface.

Protein adsorption can be reversibly switched on and off on mixed PEO/PAA brushes

Adsorption of proteins on surfaces placed in biological fluids is a ubiquitous and mostly irreversible phenomenon, desirable or not, but often uncontrolled. Adsorption of most proteins on poly(ethylene oxide) (PEO) brushes is very limited, while the amount of proteins adsorbed on poly(acrylic acid) (PAA) brushes varies with the pH and ionic strength (I) of the protein solution. Mixed brushes of PEO and PAA are designed here to reversibly adsorb and desorb albumin, lysozyme, collagen and immunoglobulin G, four very different proteins in terms of size, solubility and isoelectric point. Protein adsorption and desorption are monitored using X-ray photoelectron spectroscopy, as well as with quartz crystal microbalance for in situ and real-time measurements. Large amounts of protein are adsorbed and then nearly completely desorbed on mixed PEO/PAA brushes by a simple pH and I trigger. The mixed brushes thus nicely combine the properties of pure PAA and pure PEO brushes. These adsorption/desorption cycles are shown to be repeated with high efficiency. The high-performance smart substrates created here could find applications in domains as diverse as biosensors, drug delivery and nanotransport.

Electronic control over detachment of a self-doped water-soluble conjugated polyelectrolyte

Water-soluble conducting polymers are of interest to enable more versatile processing in aqueous media as well as to facilitate interactions with biomolecules. Here, we report a substituted poly(3,4-ethylenedioxythiophene) derivative (PEDOT-S:H) that is fully water-soluble and self-doped. When electrochemically oxidizing a PEDOT-S:H thin film, the film detaches from the underlying electrode. The oxidation of PEDOT-S:H starts with an initial phase of swelling followed by cracking before it finally disrupts into small flakes and detaches from the electrode. We investigated the detachment mechanism and found that parameters such as the size, charge, and concentration of ions in the electrolyte, the temperature, and also the pH influence the characteristics of detachment. When oxidizing PEDOT-S:H, the positively charged polymer backbone is balanced by anions from the electrolyte solution and also by the sulfonate groups on the side chains (more self-doping). From our experiments, we conclude that detachment of the PEDOT-S:H film upon oxidation occurs in part due to swelling caused by an inflow of solvated anions and associated water and in part due to chain rearrangements within the film, caused by more self-doping. We believe that PEDOT-S:H detachment can be of interest in a number of different applications, including addressed and active control of the release of materials such as biomolecules and cell cultures.