Functionalization of Poly(oligo(ethylene glycol) methacrylate) Films on Gold and Si/SiO2 for Immobilization of Proteins and Cells: SPR and QCM Studies

Glycopolymer brushes for the affinity adsorption of RCA120: effects of thickness, grafting density, and epitope density

The interactions between glycopolymer brushes and lectin are very important for the development of affinity membrane chromatography in protein separation. Here, we report the combination of surface-initiated atom transfer radical polymerization (SI-ATRP) and surface plasmon resonance (SPR) to investigate the relationship between the structure of glycopolymer brushes and the affinity adsorption of lectin. The glycopolymer brushes were fabricated from self-assembly of 11-mercapto-1-undecanol (MUD)/1-undecanethiol (UDT) mixture, immobilization of ATRP initiators, and then SI-ATRP of 2-lactobionamidoethyl methacrylate (LAMA). Brush thickness and grafting density were adjusted by controlling polymerization time and thiol ratio in MUD/UDT mixture, respectively. Sugar epitope density was also controlled through copolymerization of 2-hydroxylethyl methacrylate (HEMA) with LAMA. Ricinus communis agglutinin (RCA(120)), one kind of lectin that can bind galactose specifically, was chosen to study the effects of brush architectures on lectin adsorption. SPR results indicate not only the thickness but also the grafting density and the epitope density of glycopolymer brushes can achieve the best performance of sugar cluster effect in affinity adsorption of lectin. In addition, the mass transport effect is crucial in the adsorption process. We propose that it is important to keep the balance between the sugar cluster effect and the mass transport effect in the preparation of high-performance affinity membrane chromatography.

Structural and functional characteristics of chimeric avidins physically adsorbed onto functionalized polythiophene thin films

Stabilized bioreceptor layers are of great importance in the design of novel biosensors. In earlier work, chimeric avidins enabled immobilization of biotinylated antibodies onto gold surfaces with greater stability compared to more conventional avidins (wild-type avidin and streptavidin). In the present study, the applicability of chimeric avidins as a general binding scaffold for biotinylated antibodies on spin-coated functionalized polythiophene thin films has been studied by surface plasmon resonance and atomic force microscopy. Novel chimeric avidins showed remarkably increased binding characteristics compared with other avidins, such as wild-type avidin, streptavidin, and bacterial avidin when merely physically adsorbed onto the polythiophene surface. They gave the highest binding capacities, the highest affinity constant, and the highest stability for biotinylated probe immobilization. Introduction of carboxylic acid groups to polythiophene layer further enhanced the binding level of the avidins. Polythiophene layers functionalized with chimeric avidins thus offered a promising generic platform for biosensor applications.

Oligonucleotide and polymer functionalized nanoparticles for amplification-free detection of DNA

Sensitive and quantitative nucleic acid testing from complex biological samples is now an important component of clinical diagnostics. Whereas nucleic acid amplification represents the gold standard, its utility in resource-limited and point-of-care settings can be problematic due to assay interferants, assay time, engineering constraints, and costs associated with both wetware and hardware. In contrast, amplification-free nucleic acid testing can circumvent these limitations by enabling direct target hybridization within complex sample matrices. In this work, we grew random copolymer brushes from the surface of silica-coated magnetic nanoparticles using azide-modified and hydroxyl oligo ethylene glycol methacrylate (OEGMA) monomers. The azide-functionalized polymer brush was first conjugated, via copper-catalyzed azide/alkyne cycloaddition (CuAAC), with herpes simplex virus (HSV)-specific oligonucleotides and then with alkyne-substituted polyethylene glycol to eliminate all residual azide groups. Our methodology enabled control over brush thickness and probe density and enabled multiple consecutive coupling reactions on the particle grafted brush. Brush- and probe-modified particles were then combined in a 20 min hybridization with fluorescent polystyrene nanoparticles modified with HSV-specific reporter probes. Following magnetic capture and washing, the particles were analyzed with an aggregate fluorescence measurement, which yielded a limit of detection of 6 pM in buffer and 60 pM in 50% fetal bovine serum. Adoption of brush- and probe-modified particles into a particle counting assay will result in the development of diagnostic assays with significant improvements in sensitivity.

Development of a novel antifouling platform for biosensing probe immobilization from methacryloyloxyethyl phosphorylcholine-containing copolymer brushes

The immobilization of thiol-terminated poly[(methacrylic acid)-ran-(2-methacryloyloxyethyl phosphorylcholine)] (PMAMPC-SH) brushes on gold-coated surface plasmon resonance (SPR) chips was performed using the "grafting to" approach via self-assembly formation. The copolymer brushes provide both functionalizability and antifouling characteristics, desirable features mandatorily required for the development of an effective platform for probe immobilization in biosensing applications. The carboxyl groups from the methacrylic acid (MA) units were employed for attaching active biomolecules that can act as sensing probes for biospecific detection of target molecules, whereas the 2-methacryloyloxyethyl phosphorylcholine (MPC) units were introduced to suppress unwanted nonspecific adsorption. The detection efficiency of the biotin-immobilized PMAMPC brushes with the target molecule, avidin (AVD), was evaluated in blood plasma in comparison with the conventional 2D monolayer of 11-mercaptoundecanoic acid (MUA) and homopolymer brushes of poly(methacrylic acid) (PMA) also immobilized with biotin using the SPR technique. Copolymer brushes with 79 mol % MPC composition and a molecular weight of 49.3 kDa yielded the platform for probe immobilization with the best performance considering its high S/N ratio as compared with platforms based on MUA and PMA brushes. In addition, the detection limit for detecting AVD in blood plasma solution was found to be 1.5 nM (equivalent to 100 ng/mL). The results have demonstrated the potential for using these newly developed surface-attached PMAMPC brushes for probe immobilization and subsequent detection of designated target molecules in complex matrices such as blood plasma and clinical samples.

Surface patterning by microcontact chemistry

In this Feature Article we describe recent progress in covalent surface patterning by microcontact chemistry. Microcontact chemistry is a variation of microcontact printing based on the transfer of reactive "ink" molecules from a microstructured, elastomeric stamp onto surfaces modified with complementary reactive groups, leading to a chemical reaction in the area of contact. In comparison with other lithographic methods, microcontact chemistry has a number of advantageous properties including very short patterning times, low consumption of ink molecules, high resolution and large area patterning. During the past 5 years we and many others have investigated a set of different reactions that allow the modification of flat and also spherical surfaces in an effective way. Especially click-type reactions were found to be versatile for substrate patterning by microcontact chemistry and were applied for chemical modification of reactive self-assembled monolayers and polymer surfaces. Microcontact chemistry has already found broad application for the production of functional surfaces and was also used for the preparation of DNA, RNA, and carbohydrate microarrays, for the immobilization of proteins and cells and for the development of sensors.

Surface-grafted poly(acrylic acid) brushes as a precursor layer for biosensing applications: effect of graft density and swellability on the detection efficiency

Carboxyl groups along poly(acrylic acid) (PAA) brushes attached to the surface of a gold-coated substrate served as the precursor moieties for the covalent immobilization of amino-functionalized biotin or bovine serum albumin (BSA) to form a sensing probe for streptavidin (SA) or anti-BSA detection, respectively. Surface-grafted PAA brushes were obtained by acid hydrolysis of poly(tert-butyl acrylate) brushes, formerly prepared by surface-initiated atom transfer radical polymerization of tert-butyl acrylate. As determined by surface plasmon resonance, the PAA brushes immobilized with functionalized biotin or BSA probes not only showed good binding with the designated target analytes but also maintained a high resistance to nonspecific protein adsorption, especially those PAA brushes with a high surface graft density. Although the probe binding capacity can be raised as a function of the graft density of the PAA brushes or the amount of carboxyl groups along the PAA chains, the accessibility of the target analyte to the immobilized probe was limited at the high graft density of the PAA brushes. The effect was far more apparent for the BSA-anti-BSA probe-analyte pair than for the much smaller biotin-SA probe-analyte pair. The impact of the swellability of the PAA brushes, as tailored by the degree of carboxyl group activation, on both the sensing probe immobilization and analyte detection was also addressed. This investigation demonstrated that PAA brushes having a defined graft density have a promising potential as a precursor layer for biosensing applications.

A substrate-independent method for surface grafting polymer layers by atom transfer radical polymerization: reduction of protein adsorption

A general method for producing low-fouling biomaterials on any surface by surface-initiated grafting of polymer brushes is presented. Our procedure uses radiofrequency glow discharge thin film deposition followed by macro-initiator coupling and then surface-initiated atom transfer radical polymerization (SI-ATRP) to prepare neutral polymer brushes on planar substrates. Coatings were produced on substrates with variable interfacial composition and mechanical properties such as hard inorganic/metal substrates (silicon and gold) or flexible (perfluorinated poly(ethylene-co-propylene) film) and rigid (microtitre plates) polymeric materials. First, surfaces were functionalized via deposition of an allylamine plasma polymer thin film followed by covalent coupling of a macro-initiator composed partly of ATRP initiator groups. Successful grafting of a hydrophilic polymer layer was achieved by SI-ATRP of N,N'-dimethylacrylamide in aqueous media at room temperature. We exemplified how this method could be used to create surface coatings with significantly reduced protein adsorption on different material substrates. Protein binding experiments using labelled human serum albumin on grafted materials resulted in quantitative evidence for low-fouling compared to control surfaces.

Anti-fouling bioactive surfaces

Bioactive surfaces refer to surfaces with immobilized bioactive molecules aimed specifically at promoting or supporting particular interactions. Such surfaces are of great importance for various biomedical and biomaterials applications. In the past few years, considerable effort has been made to create bioactive surfaces by forming specific biomolecule-modified surfaces on a non-biofouling "base" or "background". Hydrophilic and bioinert polymers have been widely used as anti-fouling layers that resist non-specific protein interactions. They can also serve as "spacers" to effectively move the immobilized biomolecule away from the surface, thus enhancing its bioactivity. In this review we summarize several successful approaches for the design and preparation of bioactive surfaces based on different types of anti-fouling/spacer materials. Some perspectives on future research in this area are also presented.

Surface grafting of blood compatible zwitterionic poly(ethylene glycol) on diamond-like carbon-coated stent

Blood compatibility is the most important aspect for blood-contacting medical devices including cardiovascular stents. In this study, the surface of nickel-titanium (TiNi) stent was coated with diamond-like carbon (DLC) and then subsequently grafted by using zwitterion (N(+) and SO(3) (-))-linked poly(ethylene glycol) (PEG). We hypothesize that this coupling of zwitterion and PEG may significantly improve blood compatibility of DLC-coated TiNi stent. The surface modified TiNi stents, including PEG-grafted stent (DLC-PEG) and zwitterionic PEG-grafted one (DLC-PEG-N-S) were the main focus on the tests of surface characteristics and blood compatibility. The zwitterionic PEG derivatives were obtained from a series of chemical reactions at room temperature. The results exhibited that as compared to the DLC-PEG, the hydrophilicity was much better with DLC-PEG-N-S and significantly increased atomic percentage of oxygen and nitrogen proved the entity of zwitterions on the surface of DLC-PEG-N-S. Meanwhile, the adsorption of blood proteins such as, human serum albumin (HSA) and fibrinogen was found considerably down-regulated in DLC-PEG-N-S, due mainly to the protein-repellent effect of PEG and zwitterion. Microscopic observation also revealed that as compared with the other substrates without zwitterion, the degree of platelet adhesion was the lowest with DLC-PEG-N-S. In addition, DLC-PEG-N-S retained an extended blood coagulation time as measured by activated partial thromboplastin time (APTT). The present results suggested that surface grafting of zwitterionic PEG derivatives could substantially enhance the blood compatibility of TiNi-DLC stent. In conclusion, anti-fouling properties of PEG and zwitterions are expected to be very useful in advancing overall stent performance.

Essential Factors to Make Excellent Biocompatibility of Phospholipid Polymer Materials

Recently, much attention has been attracted to bio/blood compatible materials to suppress undesirable biological reactions that determine the fate of living organisms and materials. A phospholipid polymer composed of 2-methacryloyloxyethyl phosphorylcholine (MPC) unit, which is designed by inspiration of cell membrane surface structure, is the most promising polymer biomaterial with excellent bio/blood compatibility. Progress in living radical polymerization method initiated from the surface enables preparation of a dense polymer chains on the surface, which is called as a polymer brush. The polymer brush structure has narrow molecular weight distribution and controlled chain length. So, it is ideal surface to clarify the interactions between the biomolecules and biomaterial surface that has never done. In these regards, the poly(MPC) brush surfaces are expected to be a novel class of biomaterials, and have been extensively studied its unusual properties. In this review, surface-initiated living radical polymerization of MPC and the characteristics of the poly(MPC) brush surfaces are summarized from a viewpoint of biomaterials science.

Highly sensitive detection of protein toxins by surface plasmon resonance with biotinylation-based inline atom transfer radical polymerization amplification

Ultrasensitive detection of proteins is of great importance to proteomics studies. We report here a method to enhance detection sensitivity in surface plasmon resonance (SPR) spectroscopy by coupling a polymerization initiator to a biospecific interaction and inducing inline atom transfer radical polymerization (ATRP) for amplifying SPR response. Bacterial cholera toxin (CT) is chosen as the model protein that has been covalently immobilized on the surface for demonstrating the principle. The specific recognition is achieved by use of biotinylated anti-CT, which allows initiators with a biotin tag to be fixed at the protein binding site through a neutravidin bridge and triggers the localized growth of polymer brushes of poly(hydroxyl-ethyl methacrylate) (PHEMA) via an ATRP mechanism. To further enhance the signal, a second ATRP reaction is conducted that takes advantage of the hydroxyl groups of PHEMA brushes from the first step to form hyperbranched polymers onto the sensing surface. The two consecutive ATRP steps significantly improve SPR detection, allowing low amounts of CT that yield no direct measurement to be quantified with large signals. The resulting polymer film has been characterized by optical and atomic force microscopy. Ascorbic acid (AA) is employed as deoxygen reagent in the catalyst mixture that effectively suppresses oxygen interference, shortening the reaction time and making it possible for applying this ATRP approach to flow injection based SPR detection. A calibration curve of PHEMA amplification for CT detection based on surface coverage has been obtained that displays a correlation in a range from 8.23 x 10(-15) to 3.61 x 10(-12) mol/cm(2) with a limit of detection of 6.27 x 10(-15) mol/cm(2). The versatile biotin-neutravidin interaction used here should allow adaptation of ATRP enhancement to many other systems that include DNA, RNA, peptides, and carbohydrates, opening new avenues for ultrasensitive analysis of biomolecules with flow-injection assay and SPR spectroscopy.

ATRP grafting of oligo(ethylene glycol) methacrylates from gold surface — Effect of monomer size on grafted chain and EO unit densities

Oligo(ethylene glycol) methacrylate (OEGMA) was grafted from Au surface via surface-initiated atom-transfer radical polymerization (s-ATRP). The initiator density was adjusted by the mole fractions of surface-attachable ATRP initiator and diluting agent. Three OEGMA monomers of Mr 300, 475, and 1100 were used. The Au surfaces before and after modification were characterized by water contact angle and ellipsometry. The effect of monomer Mr and initiator density on surface chain density and EO unit density was investigated. It was found that at low initiator density (mole fraction of initiator of 2%–5%), poly(OEGMA) grafts had comparable surface chain density independent of OEGMA Mr. However, at high initiator density (mole fraction of initiator >10%), poly(OEGMA)1100 had obviously lower chain densities than poly(OEGMA)300 and poly(OEGMA)475, which could be attributed to the size difference of the monomers. The short chain length and low surface chain density of poly(OEGMA)1100 resulted in its relatively low surface EO unit density.

Plasma oxidized polyhydroxymethylsiloxane--a new smooth surface for supported lipid bilayer formation

A novel substrate for preparation of supported lipid bilayers (SLBs), smooth at the subnanometer scale and of variable thickness from ten to several hundred nanometers, was developed by surface oxidation of spin-coated poly(hydroxymethylsiloxane) (PHMS) films. The deposited polymeric thin films were modified by a combination of oxygen plasma and thermal treatment (PHMS(ox)), in order to convert the outermost surface layer of the polymer film to a stable SiO(2) film, suitable for SLB formation. The hydrophilic, SiO(2)-like surfaces were characterized by XPS, wetting angle, ellipsometry, and AFM. Lipid bilayers were formed on this surface using the well-known vesicle adsorption-rupture-fusion process, usually performed on glass or vapor-deposited SiO(2). Reproducible formation of homogeneous SLBs of different compositions (POPC, DOEPC, and POPC/DOPS) was demonstrated on the new SiO(2) surface by quartz crystal microbalance with dissipation (QCM-D), surface plasmon resonance (SPR), and optical reflectometry measurements. The SLB formation kinetics on the PHMS(ox)-coated sensors showed very similar characteristics, for all investigated PHMS thicknesses, as on reference sensors coated with vapor-deposited SiO(2). The good adhesive properties of the PHMS to gold allows for the preparation of thin PHMS(ox) layers compatible with SPR. The much smaller roughness at the nanometer scale of the PHMS(ox) surfaces, compared to standard vapor-deposited SiO(2)-coated sensors, makes them advantageous for AFM and optical experiments and promising for patterning. To benefit optical experiments with the PHMS(ox) surfaces, it was also investigated how the PHMS film thickness influences the SPR and reflectometry responses upon SLB formation.

Ultralow-fouling, functionalizable, and hydrolyzable zwitterionic materials and their derivatives for biological applications

In recent years, zwitterionic materials such as poly(carboxybetaine) (pCB) and poly(sulfobetaine) (pSB) have been applied to a broad range of biomedical and engineering materials. Due to electrostatically induced hydration, surfaces coated with zwitterionic groups are highly resistant to nonspecific protein adsorption, bacterial adhesion, and biofilm formation. Among zwitterionic materials, pCB is unique due to its abundant functional groups for the convenient immobilization of biomolecules. pCB can also be prepared in a hydrolyzable form as cationic pCB esters, which can kill bacteria or condense DNA. The hydrolysis of cationic pCB esters into nonfouling zwitterionic groups will lead to the release of killed microbes or the irreversible unpackaging of DNA. Furthermore, mixed-charge materials have been shown to be equivalent to zwitterionic materials in resisting nonspecific protein adsorption when they are uniformly mixed at the molecular scale.

Protein-resistant NTA-functionalized polymer brushes for selective and stable immobilization of histidine-tagged proteins

Protein-resistant polymeric coatings that allow highly selective immobilization of specific biomolecules are essential for biomedical applications such as microarrays, biosensing, heterogeneous catalysis, and bioengineering. Polymer brushes are particularly interesting for this purpose because their chemical structure and physical properties can easily be tailored to meet specific needs. This article explores the functionalization of two protein-resistant polymer brushes, poly(oligoethylene glycol methacrylate) (POEGMA) and poly(hydroxyethyl methacrylate) (PHEMA), with nitrilotriacetic acid (NTA) moieties that can complex histidine-tagged (His-tagged) proteins selectively and reversibly. Using fluorescence microscopy, IR spectroscopy, X-ray photoelectron spectroscopy, surface plasmon resonanace, and ellipsometry, we demonstrate that His-tagged green fluorescent protein can be immobilized on NTA brushes with high stability and loading. The loading saturation reached for NTA-POEGMA is higher than that for NTA-PHEMA because of increased swelling of the former brush. Despite this higher loading capacity, NTA-POEGMA remained highly protein-resistant, which shows its potential for "clean" and specific protein immobilization. Finally, we showed that the preserved protein resistance of NTA-POEGMA brushes can be used to generate well-defined binary biofunctional patterns via a simple protocol of incubations and washes. These patterns may find applications in cell arraying and screening.

Stamps, inks and substrates: polymers in microcontact printing

It's all about polymers! Polymers play a key role in the patterning and functionalization of surfaces by microcontact printing. Polymers are versatile stamps, inks and substrates and microcontact printing can provide microstructured polymer surfaces in a single printing step.

Selective collection and detection of leukemia cells on a magnet-quartz crystal microbalance system using aptamer-conjugated magnetic beads

A novel method for selective collection and detection of human acute leukemia cells has been proposed using aptamer-conjugated magnetic beads (apt-MBs) and a magnet-quartz crystal microbalance (QCM) system. The sgc8c aptamer-conjugated MBs specifically binding to CCRF-CEM cells were used for target cell extraction from complex matrixes, and the magnet-QCM system was successfully applied for quantitative cell detection, requiring no further labeling of cells. The accumulation of MBs-conjugated CCRF-CEM cells on a quartz crystal gold electrode surface under a magnetic field resulted in decreased resonant frequency. A linear relationship between the frequency shift and cell concentration over the range of 1 x 10(4)-1.5 x 10(5)cells mL(-1) was obtained, with a detection limit of 8 x 10(3)cells mL(-1). The applicability of the method for target cell detection from cell mixture was satisfactory.

Osteoconductive conjugation of bone morphogenetic protein-2 onto titanium/titanium oxide surfaces coated with non-biofouling poly(poly(ethylene glycol) methacrylate)

This paper describes a method for introducing osteoconductivity onto titanium, a widely used material for implants, as well as maintaining its non-biofouling ("bioinert") property, in the aim of increasing bioactivity of titanium for its wider applications to biomedical areas. Titanium substrates were coated with a non-biofouling poly(poly(ethylene glycol) methacrylate) (pPEGMA) by surface-initiated polymerization, and bone morphogenetic protein-2 (BMP-2) was chemically conjugated to the activated pPEGMA films. The non-biofouling property and increased bioactivity of titanium were confirmed by the maintenance of the cellular response of mesenchymal stem cells on the titanium substrates: the BMP-2-conjugated pPEGMA films induced the adhesion and differentiation of mesenchymal stem cells, while non-conjugated pPEGMA films showed the excellent resistance against the adhesion of the cells.

Reactive surface coatings based on polysilsesquioxanes: controlled functionalization for specific protein immobilization

The key designing in reliable biosensors is the preparation of thin films in which biomolecular functions may be immobilized and addressed in a controlled and reproducible manner. This requires the controlled preparation of specific binding sites on planar surfaces. Poly(methylsilsesquioxane)-poly(pentafluorophenyl acrylates) (PMSSQ-PFPA) are promising materials to produce stable and adherent thin reactive coatings on various substrates. Those reactive surface coatings could be applied onto various materials, for example, gold, polycarbonate (PC), poly(tetrafluoroethylene) (PTFE), and glass. By dipping those substrates in a solution of a desired amine, specific binding sites for protein adsorption could be immobilized on the surface. The versatile strategy allowed the attachment of various linkers, for example, biotin, l-thyroxine, and folic acid. The adsorption processes of streptavidin, pre-albumin, and folate-binding protein were monitored using surface plasmon resonance (SPR), Fourier transform infrared (FTIR) spectroscopy, fluorescence spectroscopy, and atomic force microscopy (AFM). The presented protein immobilization strategy, consisting of four steps (a) spin-coating of PMSSQ-PFPA hybrid polymer from tetrahydrofuran (THF) solution, (b) annealing at 130 degrees C for 2 h to induce thermal cross-linking of the PMSSQ part, (c) surface analogues reaction with different amino-functionalized specific binding sites for proteins, and (d) controlled assembly of proteins on the surface, may find various applications in future biosensor design.