Surface grafted sulfobetaine polymers via atom transfer radical polymerization as superlow fouling coatings

One of the sulfobetaine methacrylate (SBMA) monomers, N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine, was polymerized onto initiator-covered gold surfaces using atom transfer radical polymerization (ATRP) to form uniform polymer brushes. Self-assembled monolayers (SAMs) with ATRP initiators were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The thickness of grafted poly(SBMA) films was measured by ellipsometry. Fibrinogen adsorption on poly(SBMA) grafted surfaces was measured with a surface plasmon resonance (SPR) sensor. Two approaches were compared to graft ATRP initiators onto gold surfaces for surface polymerization and subsequent protein adsorption on these polymer grafted surfaces. The first was to prepare a SAM from omega-mercaptoundecyl bromoisobutyrate onto a gold surface. Superlow fouling surfaces with well-controlled poly(SBMA) brushes were achieved using this approach (e.g., fibrinogen adsorption <0.3 ng/cm2). The second approach was to react bromoisobutyryl bromide with a hydroxyl-terminated SAM on a gold surface. Although protein adsorption decreased as the density of surface initiators increased, the surface prepared using the second approach was not able to achieve as low protein adsorption as the first approach. Key parameters to achieve superlow fouling surfaces were studied and discussed.     

Adsorption isotherms and dissipation of adsorbed poly(N-isopropylacrylamide) in its swelling and collapsed states

Poly(N-isopropylacrylamide) (PNIPAM) physisorbed on gold surfaces in aqueous solutions has been studied using a quartz crystal microbalance with dissipation monitoring (QCM-D). The adsorption isotherms of the polymer, that is, the adsorbed mass versus the concentration of PNIPAM in solution, show distinctly different behaviors at temperatures below and above a lower critical solution temperature (LCST). Below the LCST, PNIPAM forms a single compact layer in solutions with concentrations up to 100 ppm in weight; above the LCST, much thicker films of PNIPAM form in the same concentration range. Changes in the dissipation factor versus solvent concentration show a behavior similar to those in the isotherms. The difference in the adsorption behavior below and above the LCST can be qualitatively explained in terms of the conformation difference of the polymer in its swelling and collapsed states.     

A new temperature-sensitive polymer: Poly(ethoxypropylacrylamide)

In this study, a new temperature sensitive polymer was obtained by the solution polymerization of ethoxypropylacrylamide. The monomer, N-(3-ethoxypropyl)-acrylamide was synthesized by the nucleophilic substitution reaction of 3-ethoxy-propylamine and acryloyl chloride. The solution polymerization was performed in ethanol at 70 °C, by using azobisizobutyronitrile as the initiator. Poly(N-(3-ethoxypropyl)acrylamide), PEPA, exhibited a reversible phase transition by the temperature. The effects of polymer and salt concentrations on the lower critical solution temperature, (LCST) behaviour were investigated. LCST was found to be strongly dependent on the polymer concentration. The dynamic light scattering (DLS) measurements confirmed the formation of aggregates by the association of nucleated polymer chains at the temperatures higher than LCST. However an unusual behaviour, a marked decrease in the hydrodynamic diameter by the increasing PEPA concentration was observed below the LCST. The effect of salt concentration on the critical flocculation temperature of PEPA was reasonably similar to poly(isopropylacrylamide), PNIPA. In the ethanol-water media, the reversible phase transition behaviour was observed up the ethanol concentration of 30% v/v. This study indicated that PEPA was a new alternative thermally reversible material for PNIPA. With respect to the well-defined temperature-sensitive polymers like PNIPA, polymer concentration dependent LCST of PEPA can provide significant advantages in the applications like drug targeting, affinity separation and immobilization of bioactive agents.     

Capture and release of proteins on the nanoscale by stimuli-responsive elastin-like polypeptide "switches"

This article describes the fabrication and characterization of stimulus-responsive elastin-like polypeptide (ELP) nanostructures grafted onto omega-substituted thiolates that were patterned onto gold surfaces by dip-pen nanolithography (DPN). In response to external stimuli such as changes in temperature or ionic strength, ELPs undergo a switchable and reversible, hydrophilic-hydrophobic phase transition at a lower critical solution temperature (LCST). We exploited this phase transition behavior to reversibly immobilize a thioredoxin-ELP (Trx-ELP) fusion protein onto the ELP nanopattern above the LCST. Subsequent binding of an anti-thioredoxin monoclonal antibody (anti-Trx) to the surface-captured thioredoxin showed the presentation of the immobilized protein in a sterically accessible orientation in the nanoarray. We also showed that the resulting Trx-ELP/anti-Trx complex formed above the LCST could be reversibly dissociated below the LCST. These results demonstrate the intriguing potential of ELP nanostructures as generic, reversible, biomolecular switches for on-chip capture and release of a small number (order 100-200) of protein molecules in integrated, nanoscale bioanalytical devices. We also investigated the molecular mechanism underlying this switch by measuring the height changes that accompany the binding and desorption steps and by adhesion force spectroscopy using atomic force microscopy.     

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

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

Adsorption behavior of water soluble polymers with lower critical solution temperature

The adsorption behavior of hydroxylpropyl cellulose (HPC), ethyl hydroxylethyl cellulose (EHEC) and poly-vinylalcohol (PVA) polymers, which have a lower critical solution temperature (LCST), have been studied in comparison with the behavior of hydroxylethyl cellulose (HEC) with no LCST. The saturated amount of adsorption (A _s ) for the polymers with LCST depended significantly on the adsorption temperature and theA _s , e. g., for HPC obtained at the LCST, the amount was 1.5 times as large as the value at room temperature. The highA _s values obtained at the LCST were maintained over a long period at room temperature, and the dense adsorption layer formed on the latex particles at the LCST showed a strong protective action against flocculation. Furthermore, the effect of the surface nature of the adsorbent on the polymer adsorption at the LCST has been investigated using six kinds of synthetic latices with different surface natures. It was found that the hydrophobic interaction between the polymer and the adsorbent plays an important role in inducing the adsorption, and the trend of increasing the hydrophilic character of the latex surface prevents the formation of the adsorption layer of the polymer.     

Temperature-controlled flow switching in nanocapillary array membranes mediated by poly(N-isopropylacrylamide) polymer brushes grafted by atom transfer radical polymerization

We report actively controlled transport that is thermally switchable and size-selective in a nanocapillary array membrane (NCAM) prepared by grafting poly(N-isopropylacrylamide) (PNIPAAm) brushes onto the exterior surface of a Au-coated polycarbonate track-etched membrane. A smooth Au layer on the membrane surface, which is key to obtaining a uniform polymer film, was prepared by thermal evaporation of approximately 50 nm Au on both exterior surfaces. After evaporation, the inner diameter of the pore is reduced slightly, but the NCAM retains a narrow pore size distribution. PNIPPAm brushes with 10-30 nm (dry film) thickness were grafted onto the Au surface through surface-initiated atom transfer radical polymerization (ATRP) using a disulfide initiator, (BrC(CH3)2COO(CH2)11S)2. Molecular transport through the PNIPAAm polymer brush-modified NCAMs was investigated by real-time fluorescence measurements using fluorescein isothiocyanate (FITC)-labeled dextrans ranging from 4.4 to 282 kDa in membranes with variable initial pore diameters (80, 100, and 200 nm) and different PNIPAAm thicknesses. Manipulating the temperature of the NCAM through the PNIPAAm lower critical solution temperature (LCST) causes large, size-dependent changes in the transport rates. Over specific ranges of probe size, transport is completely blocked below the LCST but strongly allowed above the LCST. The combination of the highly uniform PNIPAAm brush and the monodisperse pore size distribution is critical in producing highly reproducible switching behavior. Furthermore, the reversible nature of the switching raises the possibility of using them as actively controlled filtration devices.     

Adsorption and thermoresponsive behavior of poly(N-isopropylacrylamide-co-N,N'-cystaminebisacrylamide) thin films on gold

We describe the synthesis of thermoresponsive polymers made from N-isopropylacrylamide and varying amounts of a thiol-containing co-monomer, N,N'-cystaminebisacrylamide (P(NIPAm-co-CBAm)). Infrared spectroscopy revealed a backbone similar to that seen with pure PNIPAm. UV-vis spectroscopy showed that P(NIPAm-co-CBAm) undergoes a thermoresponsive phase transition around 32 degrees C in aqueous solution. The presence of the thiol groups enabled the polymer to adsorb onto gold surfaces. Following adsorption onto a gold surface, X-ray photoelectron spectroscopy showed a carbon/gold atomic ratio of 0.93 for a sample without CBAm and a ratio of 1.61 for a P(NIPAm-co-CBAm) sample with 0.20% CBAm. Quartz crystal microbalance (QCM) analysis showed increases in the mass of polymer adsorbed when the CBAm content in the polymer increased. The thermoresponsive behavior of the thin films on gold was investigated with contact angle and dissipative QCM analysis. Contact angles were measured for polymer films at both 25 and 60 degrees C. The largest temperature-induced alteration in the contact angle was seen with the 1.00% CBAm sample. Similarly, QCM-D results showed a significantly greater change in frequency and dissipation following a temperature change when CBAm was present than in pure NIPAm polymers.     

Redox-responsive Inclusion Complexation between β-Cyclodextrin and Ferrocene-functionalized Poly(N-isopropylacrylamide) and its Effect on the Solution Properties of this Polymer

Ferrocene-functionalized polymers (poly(NIPAM/FCN)) and their β-CD (β-Cyclodextrin) complex have been prepared. The inclusion complexation between them was investigated by several techniques, including ¹H NMR spectra, cyclic voltammetry, UV–vis spectra and dynamic light scattering measurements. The results showed that β-CD could interact with the reduced ferrocene side groups and hardly affect the oxidized form. Thus a redox-responsive inclusion complexation system based on β-CD and poly(NIPAM/FCN) was obtained. In addition, the effect of this inclusion complexation on the solution properties of this polymer was also investigated. LCST (lower critical solution temperature) increased and the viscosity decreased upon addition of β-CD into the reduced polymer solution due to the disruption of the hydrophobic interaction between the ferrocene side groups by the inclusion complexation. Yet LCST and viscosity of the oxidized polymer solution changed slightly, which resulted from the weak interaction exerted by β-CD.     

新型钴酞菁接枝温敏聚合物的制备及性能

采用2,4,6-三氯-1,3,5-三嗪对四氨基钴酞菁进行改性,并以共价键接枝到聚N-异丙基丙烯酰胺上制得一种新型温敏性高分子催化剂——钴酞菁接枝温敏聚合物,并采用UV-Vis、TG等对其进行表征.对钴酞菁接枝温敏聚合物、温敏聚合物和小分子金属酞菁进行溶解性测试,结果表明与四氨基钴酞菁相比,所合成的钴酞菁接枝温敏聚合物能溶解于水和大多数有机溶剂,且该聚合物水溶液具有良好的温敏性,其最低临界溶解温度(LCST)为34.5℃.采用浊度法考察了不同比例的混合溶剂(乙醇/水、DMF/水)对LCST的影响,结果表明随着有机溶剂含量的增加,LCST先下降后升高,而当有机溶剂增加到一定程度时温敏性消失.本文还考察了钴酞菁接枝温敏聚合物对2-巯基乙醇的催化活性,结果表明随着温度升高,催化活性也不断提高,而当温度超过LCST时催化活性急剧下降,聚合物从溶液中析出.基于这些特性,该温敏聚合物负载酞菁作为一种新型的催化剂可实现均相催化、异相分离.     

Tuning of thermo-responsive self-assembly monolayers on gold for cell-type-specific control of adhesion

Self-assembled monolayers (SAMs) on gold containing a thermo-responsive poly( N-isopropylacrylamide)-poly(ethylene glycol)-thiol copolymer were formed. These layers show considerable potential for inducing enzyme-free and gentle detachment of cultivated cells. In an effort to optimize detachment of cells, including strongly adhering ones, two approaches are presented. First, two thermo-responsive copolymers with different poly(ethylene glycol) (PEG) contents of 15 wt % ("P15") and 19 wt % ("P19") were grafted to Au surfaces. Second, mixed monolayers were formed containing P19 and various concentrations of thiol bearing PEG. X-ray photoelectron spectroscopy (XPS) on pure and mixed P19 containing layers confirmed the expected layer compositions. Contact angle measurements showed good functionality of all surfaces prepared. Upon a temperature decrease below the lower critical solution temperature (LCST), the duration until cultivated fibroblasts detached from pure P19 surfaces was half of the one determined on P15. Strongly adherent human osteosarcoma cells could not be detached from pure P19 layers. Through co-adsorption of P19 and thiol-bearing PEG of a molar composition of 1:6, layers were formed that allowed good spreading of osteosarcoma cells above LCST and their efficient detachment below LCST.     

Polymer bilayer formation due to specific interactions between beta-cyclodextrin and adamantane: a surface force study

The purposes of this study are to utilize the interactions between an adamantane end-capped poly(ethylene oxide) (PEO) and a cationic polymer of beta-cyclodextrin to build polymer bilayers on negatively charged surfaces, and to investigate the interactions between such layers. The association of this system in solution has been studied by rheology, light scattering, and fluorescence measurements. It was found that the adamantane-terminated PEO (PEO-Ad) mixed with the beta-cyclodextrin polymer gives complexes where the interpolymer links are formed by specific inclusion of the adamantane groups in the beta-cyclodextrin cavities. This results in a higher viscosity of the solution and growth of intermolecular clusters. The interactions between surfaces coated with a cationized beta-cyclodextrin polymer across a water solution containing PEO-Ad polymers were studied by employing the interferometric surface force apparatus (SFA). In the first step, the interaction between mica surfaces coated with the cationized beta-cyclodextrin polymer in pure water was investigated. It was found that the beta-cyclodextrin polymer adsorbs onto mica and almost neutralizes the surface charge. The adsorbed layers of the beta-cyclodextrin polymer are rather compact, with a layer thickness of about 60 A (30 A per surface). Upon separation, a very weak attractive force is observed. The beta-cyclodextrin solution was then diluted by pure water by a factor of 3000 and a PEO-Ad polymer was introduced into the solution. Two different architectures of the PEO-Ad polymer were investigated: a four-arm structure and a linear structure. After the adsorption of the PEO polymer onto the beta-cyclodextrin layer reached equilibrium, the forces were measured again. It was found that the weak repulsive long-range force had disappeared and an attractive force caused the surfaces to jump into contact, and that the compressed layer thickness had increased. The attractive force is interpreted as being due to a specific recognition between the hydrophobic adamantane groups on the PEO-Ad polymer and the hydrophobic cavity in the beta-cyclodextrin molecules. Furthermore, the attractive force observed on separation has increased significantly, which is a further indication of a specific interaction between the beta-cyclodextrin polymer and the adamantane groups.     

Temperature-responsive phase transition of polymer vesicles: real-time morphology observation and molecular mechanism

Novel thermosensitive polymer vesicles with controlled temperature-responsive phase transition at the lower critical solution temperature (LCST) varying from 8 to 81 degrees C were prepared via self-assembly of amphiphilic hyperbranched star copolymers having a hydrophobic hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (HBPO) core and many hydrophilic polyethylene oxide (PEO) arms. Real-time optical microscopic observation revealed that the polymer vesicles have undergone sequential morphology changes including enrichment, aggregation, fusion, and vesicle-to-membrane transformation near the LCST. Molecular-level investigation indicates that the LCST transition results from the decreasing water solubility of the polymer vesicles with increasing temperature based on the partial dehydration of the PEO vesicle corona. On the basis of these results, a LCST transition mechanism, in view of the molecular configuration, balance of hydrophilic and hydrophobic moieties, and the vesicle morphology transformations, was proposed. As far as we know, the work presented here is the first demonstration of thermosensitive vesicles based on PEO, and the finding may be useful to design the thermosensitive core-shell structures by introducing the PEO segments.     

Surface modification of microfluidic channels by UV-mediated graft polymerization of non-fouling and ‘smart’ polymers

Microfluidic channels prepared from polydimethylsiloxane (PDMS) have been modified by UV-mediated graft polymerization of temperature-responsive polymers (poly[N-isopropyl acrylamide] or pNIPAAm), temperature- and pH-responsive copolymers (P[NIPAAm-co-acrylic acid (AAc)]), and a non-fouling hydrogel (polyethyleneglycol diacrylate, or PEGDA). This was done by presorbing a photosensitizer (PS) within the PDMS channel surface regions, contacting the different monomer solutions with the PS-containing surface under nitrogen, and irradiating with UV. The pNIPAAm-grafted surface was hydrophilic below its lower critical solution temperature (LCST), resisting non-specific adsorption, while it was hydrophobic above its LCST, now binding pNIPAAm-coated nanoparticles. Combined temperature- and pH-responsive surfaces were also prepared by UV radiation grafting a monomer mixture of pNIPAAm with AAc. The surfaces have been characterized by advancing water contact angle measurements. These smart microfluidic channels should be useful for many applications such as affinity separations and diagnostic assays.     

Thermosensitive gels incorporating polythioether units for the selective extraction of class b metal ions

Novel temperature-responsive copolymers of N-isopropylacrylamide and monoaza-tetrathioether derivative, were synthesized for the selective extraction of soft metal ions such as silver(I), copper(I), gold(III) and palladium(II) ion. The ratio between N-isopropylacrylamide group and monoaza-tetrathioether group in the copolymer was determined. The ratio between N-isopropylacrylamide group and monoaza-tetrathioether group varied in the range of 66:1–187:1. Each lower critical solution temperature (LCST) of the polymer solution was determined spectrophotometrically by the relative absorbance change at 750 nm via temperature of the polymer solution. Metal ion extraction using the copolymer with appropriate counter anions such as picrate ion, nitrate or perchlorate ion was examined. Soft metal ions such as silver(I), copper(I), gold(III) and palladium(II) ion were extracted selectively into the solid polymer phase. The extraction efficiency of a metal ion such as silver ion increased as the increase of the ratio of the monoaza-tetrathioether group to N-isopropylacrylamide group in the polymer. The quantitative extraction of class b metal ions as well as the liquid–liquid extraction of metal ions with monoaza-tetrathioether molecule was performed.     

The collapse transition of poly(styrene-b-(N-isopropyl acrylamide)) diblock copolymers in aqueous solution and in thin films

The thermal behavior of a poly(styrene-b-N-isopropyl acrylamide) diblock copolymer was studied in aqueous solution as well as in thick and in thin films. The polymer was synthesized using reversible addition–fragmentation chain transfer. The critical micelle concentration in aqueous solution was determined using fluorescence correlation spectroscopy. The lower critical solution temperature (LCST) of micellar solutions was detected using microcalorimetry and turbidimetry at 31 °C. Using dynamic light scattering, the collapse of the micelles at the LCST as well as their clustering above was observed. These findings were corroborated with small-angle X-ray scattering. In thick films immersed in water, similar findings were made. In a thin film, however, the LCST is depressed and is found at 26–27 °C.     

Encapsulation of polystyrene latex with temperature-responsive poly(N-isopropylacrylamide) via a self-assembling approach and the adsorption behaviors therein

Temperature-responsive poly(N-isopropylacrylamide) (PNIPAm)-encapsulated polystyrene latex was prepared via a layer-by-layer self-assembly of cationic and anionic polyNIPAms alternately. Studies showed that the size of PNIPAm-encapsulated polystyrene particles (m-PS) increased with the encapsulation manipulation, as verified by both transmission electron microscopy and dynamic light scattering measurements. The m-PS underwent a dramatic volume decrease at the lower critical solution temperature (LCST) of PNIPAm, with the onset temperature shifting to a higher temperature range as the number of encapsulating layers increased. A qualitative study on the adsorption behavior of Ag nanoparticles revealed that while the pristine (p) Ag nanoparticles were predominantly adsorbed on the m-PS below the LCST, the hydrophobically modified one (m-Ag) was preferentially adsorbed on the same PS above the LCST, which corresponded to the hydrophilic to hydrophobic transition of the m-PS at the LCST.     

Switchable surface traps for injectable bead-based chromatography in PDMS microfluidic channels

We report here a reversible microchannel surface capture system for stimuli-responsive grafted bioanalytical beads. Poly(N-isopropylacrylamide) (PNIPAAm) was grafted onto polydimethylsiloxane (PDMS) surfaces by a UV-mediated graft polymerization from a photoinitiator that was preadsorbed in the channel wall. The surface grafting density and resulting switchable hydrophilic/hydrophobic properties were controlled by varying the photo-illumination times and/or the initiator concentration. At limiting PNIPAAm-graft densities, the surfaces demonstrated minimal contact angles of 35 degrees below the lower critical solution temperature (LCST) and maximal contact angles of 82 degrees above it. These contact angles could be varied depending on the graft density. The surface grafts are spatially limited to the photo-illuminated region to define where the trap is constructed. The surface traps capture PNIPAAm-grafted nanobeads uniformly above the LCST and facilitate their rapid release as the temperature is reversed to below the LCST. This dual surface trap and injectable chromatography system could be useful in many applications, such as affinity separations, immunoassays, and enzyme bioprocesses, by providing for the controlled capture and release of chromatography beads.     

LCST and UCST behavior of poly(N-isopropylacrylamide) in DMSO/water mixed solvents studied by IR and micro-Raman spectroscopy

Temperature-responsive phase separations of poly(N-isopropylacrylamide) (PNiPAm)/dimethylsulfoxide (DMSO)/water mixtures have been investigated by infrared and confocal micro-Raman spectroscopy. The ternary mixtures exhibited lower critical solution temperature (LCST) and upper critical solution temperature (UCST) phenomena at low and high DMSO concentrations, respectively. The amide I band of PNiPAm consists of two components; the intensity of the 1650 cm-1 component increased, and that of the 1625 cm-1 component decreased with increasing temperature during both LCST and UCST phase transitions. Gradual red shifts of the C-H stretching and the amide II bands with increasing temperature or increasing DMSO concentration indicate a removal of water molecules from the alkyl and N-H groups. Raman microscopic measurements showed that DMSO is excluded from the polymer-rich phases upon both LCST and UCST phase separation. On the basis of the experimental results and the quantum chemical calculations, a model that explains the solvation change of the polymer during phase transitions was proposed.     

Enzymatic activity of trypsin adsorbed on temperature-sensitive composite polymer particles

Two kinds of temperature-sensitive composite polymer particles were prepared by seeded emulsion copolymerizations of (dimethylamino)ethyl methacrylate and ethylene glycol dimethacrylate with 0.14 μm-sized polystyrene and 0.26 μm-sized poly(methylmethacrylate) seed particles. To evaluate the usefulness as a carrier for biomolecules, the enzymatic activities of trypsin adsorbed on these two composite polymer particles were measured at temperatures above and below each lower critical solution temperature (LCST). In both cases, adsorbed trypsin retained its enzymatic activity during repeated adsorption/desorption measurements. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 883–888, 1998