RAFT‐mediated synthesis of poly[(oligoethylene glycol) methyl ether acrylate] brushes for biological functions

The synthesis of poly[(oligoethylene glycol) methyl ether acrylate] [poly(OEGA)] brushes was achieved via reversible addition‐fragmentation chain transfer (RAFT) polymerization and used to selectively immobilize streptavidin proteins. Initially, gold surfaces were modified with a trithiocarbonate‐based RAFT chain transfer agent (CTA) by using an ester reaction involving a gold substrate modified with 11‐mercapto‐1‐undecanol and bis(2‐butyric acid)trithiocarbonate. poly(OEGA) brushes were then prepared via RAFT‐mediated polymerization from the surface‐immobilized CTA. The immobilization of CTA on the gold surface and the subsequent polymer formation were followed by ellipsometry, X‐ray photoelectron spectroscopy, grazing angle‐Fourier transform infrared spectroscopy, atomic force microscopy, and water contact‐angle measurements. RAFT‐mediated polymerization method gave CTA groups to grafted poly(OEGA) termini, which can be converted to various biofunctional groups. The terminal carboxylic acid groups of poly(OEGA) chains were functionalized with amine‐functionalized biotin units to provide selective attachment points for streptavidin proteins. Fluorescence microscopy measurements confirmed the successful immobilization of streptavidin molecules on the polymer brushes. It is demonstrated that this fabrication method may be successfully applied for specific protein recognition and immobilization. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Controlled grafting of well-defined epoxide polymers on hydrogen-terminated silicon substrates by surface-initiated ATRP at ambient temperature

Controlled grafting of well-defined epoxide polymer brushes on the hydrogen-terminated Si(100) substrates (Si-H substrates) was carried out via the surface-initiated atom-transfer radical polymerization (ATRP) at room temperature. Thus, glycidyl methacrylate (GMA) polymer brushes were prepared by ATRP from the alpha-bromoester functionalized Si-H surface. Kinetic studies revealed a linear increase in GMA polymer (PGMA) film thickness with reaction time, indicating that chain growth from the surface was a controlled "living" process. The graft polymerization proceeded more rapidly in the dimethylformamide/water (DMF/H(2)O) mixed solvent medium than in DMF, leading to much thicker PGMA growth on the silicon surface in the former medium. The chemical composition of the GMA graft-polymerized silicon (Si-g-PGMA) surfaces were characterized by X-ray photoelectron spectroscopy (XPS). The fact that the epoxide functional groups of the grafted PGMA were preserved quantitatively was revealed in the reaction with ethylenediamine. The "living" character of the PGMA chain end was further ascertained by the subsequent growth of a poly(pentafluorostyrene) (PFS) block from the Si-g-PGMA surface, using the PGMA brushes as the macroinitiators.     

NHS-ester functionalized poly(PEGMA) brushes on silicon surface for covalent protein immobilization

Poly(PEGMA) homopolymer brushes were developed by atom transfer radical polymerization (ATRP) on the initiator-modified silicon surface (Si-initiator). Through covalent binding, protein immobilization on the poly(PEGMA) films was enabled by further NHS-ester functionalization of the poly(PEGMA) chain ends. The formation of polymer brushes was confirmed by assessing the surface composition (XPS) and morphology (atomic force microscopy (AFM), scanning electronic microscopy (SEM)) of the modified silicon wafer. The binding performance of the NHS-ester functionalized surfaces with two proteins horseradish peroxidase (HRP) and chicken immunoglobulin (IgG) was monitored by direct observation. These results suggest that this method which incorporates the properties of polymer brush onto the binding surfaces may be a good strategy suitable for covalent protein immobilization.     

Polymer brushes grafted to "passivated" silicon substrates using click chemistry

We present herein a versatile method for grafting polymer brushes to passivated silicon surfaces based on the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition (click chemistry) of omega-azido polymers and alkynyl-functionalized silicon substrates. First, the "passivation" of the silicon substrates toward polymer adsorption was performed by the deposition of an alkyne functionalized self-assembled monolayer (SAM). Then, three tailor-made omega-azido linear brush precursors, i.e., PEG-N3, PMMA-N3, and PS-N3 (Mn approximately 20,000 g/mol), were grafted to alkyne-functionalized SAMs via click chemistry in tetrahydrofuran. The SAM, PEG, PMMA, and PS layers were characterized by ellipsometry, scanning probe microscopy, and water contact angle measurements. Results have shown that the grafting process follows the scaling laws developed for polymer brushes, with a significant dependence over the weight fraction of polymer in the grafting solution and the grafting time. The chemical nature of the brushes has only a weak influence on the click chemistry grafting reaction and morphologies observed, yielding polymer brushes with thickness of ca. 6 nm and grafting densities of ca. 0.2 chains/nm2. The examples developed herein have shown that this highly versatile and tunable approach can be extended to the grafting of a wide range of polymer (pseudo-) brushes to silicon substrates without changing the tethering strategy.     

Branched fluoropolymer-Si hybrids via surface-initiated ATRP of pentafluorostyrene on hydrogen-terminated Si(100) surfaces

Linear, branched, and arborescent fluoropolymer-Si hybrids were prepared via surface-initiated atom transfer radical polymerization (ATRP) from the 4-vinylbenzyl chloride (VBC) inimer and ClSO(3)H-modified VBC that were immobilized on hydrogen-terminated Si(100), or Si-H, surfaces. The simple approach of UV-induced coupling of VBC with the Si-H surface provided a stable, Si-C bonded monolayer of "monofunctional" ATRP initiators (the Si-VBC surface). The aromatic rings of the Si-VBC surface were then sulfonated by ClSO(3)H to introduce sulfonyl chloride (-SO(2)Cl) groups and to give rise to a monolayer of "bifunctional" ATRP initiators. Kinetics study indicated that the chain growth of poly(pentafluorostyrene) from the functionalized silicon surfaces was consistent with a "controlled" or "living" process. The chemical composition and functionality of the silicon surface were tailored by the well-defined linear and branched fluoropolymer brushes. Atomic force microscopy images revealed that the surface-initiated ATRP of pentafluorostyrene (PFS) had proceeded uniformly on the Si-VBC surface to give rise to a dense and molecularly flat surface coverage of the linear brushes. The uniformity of surfaces with branched brushes was controlled by varying the feed ratio of the monomer and inimer (VBC in the present case). The living chain ends on the functionalized silicon surfaces were used as the macroinitiators for the synthesis of diblock copolymer brushes, consisting of the PFS and methyl methacrylate polymer blocks.     

Synthesis and stability of BODIPY‐based fluorescent polymer brushes at different pHs

We report a simple strategy for the grafting of poly(methacrylic acid) [poly(MAA)] brushes from silicon substrate by surface‐initiated RAFT polymerization and the subsequent coupling of BODIPY to these brushes to render them fluorescent. The poly(MAA) brushes were first generated by functionalization of hydrogen‐terminated silicon substrate with methyl‐10‐undecenoate which both leads to the formation of an organic layer covalently linked to the surface via Si? C bonds without detectable reaction of the carboxylate groups and couples to the polymerization initiator, followed by surface‐initiated RAFT polymerization of tert‐butyl methacrylate from these substrate‐bound initiator centers, and finally conversion of tert‐butyl groups to carboxylic acid groups. The poly(MAA) brushes were then made fluorescent by grafting a BODIPY derivative via an ester linkage. The stability of the BODIPY‐based fluorescent polymer brushes in buffer solutions at pH 6.0 to 12.0 with added salt was investigated by ellipsometry, fluorescence microscopy, grazing angle‐Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy. The results of these measurements indicated that the organic molecule‐initiator bond (ester linkage) is unstable and can be hydrolyzed resulting in detaching of the immobilized polymer from the silicon substrate. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3586–3596     

High density cationic polymer brushes from combined “click chemistry” and RAFT‐mediated polymerization

A simple method for preparing cationic poly[(ar‐vinylbenzyl)trimethylammonium chloride)] [poly(VBTAC)] brushes was used by combined technology of “click chemistry” and reversible addition‐fragmentation chain transfer (RAFT) polymerization. Initially, silicon surfaces were modified with RAFT chain transfer agent by using a click reaction involving an azide‐modified silicon wafer and alkyne‐terminated 4‐cyanopentanoic acid dithiobenzoate (CPAD). A series of poly(VBTAC) brushes on silicon surface with different molecular weights, thicknesses, and grafting densities were then synthesized by RAFT‐mediated polymerization from the surface immobilized CPAD. The immobilization of CPAD on the silicon wafer and the subsequent polymer formation were characterized by X‐ray photoelectron spectroscopy, water contact angle measurements, grazing angle‐Fourier transform infrared spectroscopy, atomic force microscopy, and ellipsometry analysis. The addition of free CPAD was required for the formation of well‐defined polymer brushes, which subsequently resulted in the presence of free polymer chains in solution. The free polymer chains were isolated and used to estimate the molecular weights and polydispersity index of chains attached to the surface. In addition, by varying the polymerization time, we were able to obtain poly(VBTAC) brushes with grafting density up to 0.78 chains/nm² with homogeneous distributions of apparent needle‐like structures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

硅表面分子刷图案化及生长DNA纳米管

结合"自上而下"和"自下而上"技术构建微纳米器件是目前纳米科学和技术领域追逐的目标之一。本文首先采用硅氢化反应在硅表面共价偶联引发聚合的活性基团,接着实施表面原子转移自由基聚合(ATRP)反应形成高分子刷poly(PEGMA),采用"自上而下"的光刻技术在硅表面制备功能化的图案,最后利用"自下而上"的DNA自组装技术在图案部分原位生长DNA纳米管。上述组装过程通过多次透射反射红外光谱、凝胶电泳、透射电镜和扫描电镜进行了检测,证实了硅芯片表面定位生长DNA纳米管的可行性。     

Wettability and antifouling behavior on the surfaces of superhydrophilic polymer brushes

The surface wettabilities of polymer brushes with hydrophobic and hydrophilic functional groups were discussed on the basis of conventional static and dynamic contact angle measurements of water and hexadecane in air and captive bubble measurements in water. Various types of high-density polymer brushes with nonionic and ionic functional groups were prepared on a silicon wafer by surface-initiated atom-transfer radical polymerization. The surface free energies of the brushes were estimated by Owens-Wendt equation using the contact angles of various probe liquids with different polarities. The decrease in the water contact angle corresponded to the polarity of fluoroalkyl, hydroxy, ethylene oxide, amino, carboxylic acid, ammonium salt, sulfonate, carboxybetaine, sulfobetaine, and phosphobetaine functional groups. The poly(2-perfluorooctylethyl acrylate) brush had a low surface free energy of approximately 8.7 mN/m, but the polyelectrolyte brushes revealed much higher surface free energies of 70-74 mN/m, close to the value for water. Polyelectrolyte brushes repelled both air bubbles and hexadecane in water. Even when the silicone oil was spread on the polyelectrolyte brush surfaces in air, once they were immersed in water, the oil quickly rolled up and detached from the brush surface. The oil detachment behavior observed on the superhydrophilic polyelectrolyte brush in water was explained by the low adhesion force between the brush and the oil, which could contribute to its excellent antifouling and self-cleaning properties.     

硅表面分子刷图案化及生长DNA纳米管

结合“自上而下”和“自下而上”技术构建微纳米器件是目前纳米科学和技术领域追逐的目标之一。本文首先采用硅氢化反应在硅表面共价偶联引发聚合的活性基团,接着实施表面原子转移自由基聚合（ATRP）反应形成高分子刷poly（PEGMA）,采用“自上而下”的光刻技术在硅表面制备功能化的图案,最后利用“自下而上”的DNA自组装技术在图案部分原位生长DNA纳米管。上述组装过程通过多次透射反射红外光谱、凝胶电泳、透射电镜和扫描电镜进行了检测,证实了硅芯片表面定位生长DNA纳米管的可行性。     

Synthesis of dual‐functional poly(6‐azidohexylmethacrylate) brushes by a RAFT agent carrying carboxylic acid end groups

Novel types of dual‐functional surface‐attached polymer brushes were developed by interface‐mediated reversible addition‐fragmentation chain transfer (RAFT) polymerization of 6‐azidohexylmethacrylate using the surface‐immobilized RAFT agent and the free initiator. The interface‐mediated RAFT polymerization produced silicon substrate coated with dual‐functional (azido groups from monomer and carboxylic acid groups from RAFT agent) poly(6‐azidohexylmethacrylate) [poly (AHMA)] with a grafting density as high as 0.59 chains/nm². Dual‐functional polymer brushes can represent an attractive chemical platform to deliberately introduce other molecular units at specific sites. The azido groups of the poly(AHMA) brushes can be modified with alkyl groups via click reaction, known for their DNA hybridization, while the carboxylic acid end groups can be reacted with amine groups via amide reaction, known for their antifouling properties. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1696–1706     

Improvement of hemocompatibility of polycaprolactone film surfaces with zwitterionic polymer brushes

Polycaprolactone (PCL) has been widely adopted as a scaffold biomaterial, but further improvement of the hemocompatibility of a PCL film surface is still needed for wide biomedical applications. In this work, the PCL film surface was functionalized with zwitterionic poly(3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate) (P(DMAPS)) brushes via surface-initiated atom transfer radical polymerization (ATRP) for enhancing hemocompatibility. Kinetics study revealed an approximately linear increase in graft yield of the functional P(DMAPS) brushes with polymerization time. The blood compatibilities of the modified PCL film surfaces were studied by platelet adhesion tests of platelet-rich plasma and human whole blood, hemolysis assay, and plasma recalcification time (PRT) assay. The improvement of hemocompatibility is dependent on the coverage of the grafted P(DMAPS) brushes on the PCL film. Lower or no platelet and blood cell adhesion was observed on the P(DMAPS)-grafted film surfaces. The P(DMAPS) grafting can further decrease hemolysis and enhance the PRT of the PCL surface. With the versatility of surface-initiated ATRP and the excellent hemocompatibility of zwitterionic polymer brushes, PCL films with desirable blood properties can be readily tailored to cater to various biomedical applications.     

Controlled grafting from poly(vinylidene fluoride) films by surface‐initiated reversible addition–fragmentation chain transfer polymerization

A reversible addition–fragmentation chain transfer (RAFT) polymerization technique was applied to graft polymerize brushes of poly(methyl methacrylate) (PMMA) and poly(poly(ethylene glycol) monomethacrylate) (PPEGMA) from poly(vinylidene fluoride) (PVDF) surfaces. PVDF surfaces were exposed to aqueous LiOH, followed by successive reductions with NaBH₄ and DIBAL‐H to obtain hydroxyl functionality. Azo‐functionalities, as surface initiators for grafting, were immobilized on the PVDF surfaces by esterification of 4,4′‐azobis(4‐cyanopentanoic acid) and the surface hydroxyl groups. The chemical composition and surface topography of the graft‐functionalized PVDF surfaces were characterized by X‐ray photoelectron spectroscopy, attenuated total reflectance‐FTIR spectroscopy, and atomic force microscopy. Kinetics studies revealed a linear increase in the graft concentration of PMMA and PPEGMA with the reaction time, indicating that the chain growth from the surface was consistent with a “controlled” or “living” process. The living chain ends were used as the macroinitiator for the synthesis of diblock copolymer brushes. Water contact angles on PVDF films were reduced by surface grafting of PEGMA and MMA. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3071–3082, 2006     

Fabrication of thermoresponsive polymer gradients for study of cell adhesion and detachment

A poly(N-isopropylacrylamide) (PNIPAAm) gradient covalently anchored on a silicon substrate with a linear variation of thickness was fabricated by continuous injection of the reaction mixture (NIPAAm, CuBr and its ligand, methanol, and water) into a glass chamber containing a silicon wafer, whose surface had been homogeneously immobilized with bromoisobutyryl bromide (BIBB). Because of the good control of the surface-initiated atom transfer radical polymerization (SI-ATRP) technique, the thickness of the PNIPAAm brushes was linearly proportional to the polymerization time. As a result, the gradient length and sharpness could be easily controlled by the experimental parameters such as the polymerization time and the injection rate. The as-prepared PNIPAAm gradients were characterized by ellipsometry, water contact angle, and atom force microscopy to detect their alteration of the thickness, surface wettability, and morphology, confirming the gradient structure. X-ray photoelectron spectroscopy confirmed the surface composition of the PNIPAAm. In vitro culture of HepG2 cells was implemented on the gradient surfaces, revealing that the cells could adhere at 37 degrees C and could be detached at 24 degrees C when the gradient thickness was in the range of 20-45 nm. The work thus develops a method to fabricate the stable gradient surface with better quality control, and clarifies in a facile manner the appropriate thickness of the PNIPAAm brushes in terms of cell adhesion and detachment.     

Reversible Addition‐fragmentation Chain Transfer Polymerization Grafted from Silicon Surface to Prepare Poly(methyl methacrylate) Brushes

通过共价键锚固链转移剂4-氰基-4-二硫代苯甲酰基戊酸琥珀酯于硅片表面,然后采用可逆加成-断裂链转移（RAFT）自由基聚合方法制备了聚甲基丙烯酸甲酯（PMMA）高分子刷. 聚合动力学研究表明,在反应进行2小时后,PMMA的厚度随聚合时间的增大而几乎呈线性增大,具有明显的活性聚合特征. 用椭圆偏光仪、X-光电子能谱（XPS）、原子力显微镜（AFM）及接触角测试对硅片表面的PMMA高分子刷进行了表征.     

Functionalization of hydrogen-terminated silicon via surface-initiated atom-transfer radical polymerization and derivatization of the polymer brushes

Surface-initiated atom-transfer radical polymerization (ATRP) of poly(ethylene glycol) monomethacrylate (PEGMA) was carried out on the hydrogen-terminated Si(100) substrates with surface-tethered alpha-bromoester initiator. Kinetic studies confirmed an approximately linear increase in polymer film thickness with reaction time, indicating that chain growth from the surface was a controlled "living" process. The "living" character of the surface-grafted PEGMA chains was further ascertained by the subsequent extension of these graft chains, and thus the graft layer. Well-defined polymer brushes of near 100 nm in thickness were grafted on the Si(100) surface in 8 h under ambient temperature in an aqueous medium. The hydroxyl end groups of the poly(ethylene glycol) (PEG) side chains of the grafted PEGMA polymer were derivatized into various functional groups, including chloride, amine, aldehyde, and carboxylic acid groups. The surface-functionalized silicon substrates were characterized by reflectance FT-IR spectroscopy and X-ray photoelectron spectroscopy (XPS). Covalent attachment and derivatization of the well-defined PEGMA polymer brushes can broaden considerably the functionality of single-crystal silicon surfaces.     

Influence of nitroxide structure on polystyrene brushes “grafted‐from” silicon wafers

Alkoxyamine derivatives based on 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO), N‐tert‐butyl‐N‐(1‐diethylphosphono‐(2,2‐dimethylpropyl)) nitroxide (SG1) and N‐tert‐butyl‐N‐(2‐methyl‐1‐phenylpropyl) nitroxide (TIPNO) containing a C₁₁ hydrophobic spacer and a reactive triethoxysilyl polar head, were synthesized and anchored to silicon wafers by the Langmuir–Blodgett reactive deposition technique at surface pressures ranging from 15 to 32 mN/m. Polystyrene brushes (M_n ～ 8500–66,400 g/mol) were grown from the alkoxyamine functionalized silicon wafers by nitroxide mediated radical polymerization and characterized by ellipsometry and water contact angle measurements. The main parameters influencing the grafting density and the degree of stretching of the brushes are the nitroxide polarity and, therefore, the behavior of the corresponding alkoxyamines at the air/water interface of the Langmuir–Blodgett trough. Depending on the alkoxyamine chemical structure and the surface pressure during Langmuir–Blodgett deposition, polystyrene brushes with grafting densities of 0.3–1.0 chains/nm² and stretching values of 40–70% were obtained. Regarding alkoxyamines deposited at high surface pressures, size exclusion chromatography experiments performed on both cleaved polystyrene brushes and chains simultaneously grown in the bulk revealed that the polymerization degree of the bulk and surface chains are significantly different, suggesting that steric constrains affect the polymerization kinetics occurring at the silicon surface. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3367–3374, 2008     

Electrokinetic characterization of poly(acrylic acid) and poly(ethylene oxide) brushes in aqueous electrolyte solutions

Surfaces carrying hydrophilic polymer brushes were prepared from poly(styrene)-poly(acrylic acid) and poly(styrene)-poly(ethylene oxide) diblock copolymers, respectively, using a Langmuir-Blodgett technique and employing poly(styrene)-coated planar glass as substrates. The electrical properties of these surfaces in aqueous electrolyte were analyzed as a function of pH and KCl concentration using streaming potential/streaming current measurements. From these data, both the zeta potential and the surface conductivity could be obtained. The poly(acrylic acid) brushes are charged due to the dissociation of carboxylic acid groups and give theoretical surface potentials of -160 mV at full dissociation in 10(-)(3) M solutions. The surface conductivity of these brushes is enormous under these conditions, accounting for more than 93% of the total measured surface conductivity. However, the mobility of the ions within the brush was estimated from the density of the carboxylic acid groups and the surface conductivity data to be only about 14% of that of free ions. The poly(ethylene oxide) (PEO) brushes effectively screen the charge of the underlying substrate, giving a very low zeta potential except when the ionic strength is very low. From the data, a hydrodynamic layer thickness of the PEO brushes could be estimated which is in good agreement with independent experiments (neutron reflectivity) and theoretical estimates. The surface conductivity in this system was slightly lower than that of the polystyren substrate. This also indicates that no significant amount of preferentially, i.e., nonelectrostatically attracted, ions taken up in the brush.     

Electrostatic force microscopy measurements of CdSe-PS nanoparticles and CdSe-PS/poly(styrene-b-butadiene-b-styrene) nanocomposites

Electrostatic force microscopy (EFM) measurements were performed to analyze the conductive properties of CdSe nanoparticles functionalized with polystyrene (PS) brushes and embedded in a poly(styrene-b-butadiene-b-styrene) triblock copolymer. CdSe nanoparticles were synthesized aqueously and functionalized with polystyrene chains by the grafting through technique. CdSe-PS nanoparticles obtained after 5 and 8 h of polymerization were analyzed, in order to study the effect of the molecular weight of PS chains on conductive properties. EFM results showed the maintenance of the conductive properties of CdSe nanoparticles through functionalization reactions and even when they were confined in the block copolymer. Due to the low differences between the values obtained in the response of the samples to the charged tip, no effect of the molecular weight of brushes was confirmed.     

Surface Modification of Polydimethylsiloxane Substrates with Nonfouling Poly(Poly(ethylene glycol)methacrylate) Brushes

This contribution presents a new strategy to grow nonfouling poly (poly(ethylene glycol)methacrylate) (PPEGMA) brushes from polydimethylsiloxane (PDMS) substrates. The strategy presented here is based on the use of a sequence of vapor deposition/hydrolysis cycles to generate a surface-confined atom transfer radical polymerization (ATRP)-initiator functionalized interpenetrating polymer network (IPN) layer. In contrast to most other approaches that have been developed to graft thin polymer layers from PDMS substrates, this technique obviates the need for UV/ozone pretreatment of the PDMS substrate. It is shown that the surface-confined ATRP-initiator functionalized IPN layer can be used to grow PPEGMA brushes in a controlled fashion and that the resulting PPEGMA coating significantly reduces nonspecific protein adsorption as compared to unmodified PDMS substrates.