Using Atom Transfer Radical Polymerization To Amplify Monolayers of Initiators Patterned by Microcontact Printing into Polymer Brushes for Pattern Transfer

Construction of 3D polymer brushes by dip-pen nanodisplacement lithography: understanding the molecular displacement for ultrafine and high-speed patterning

Dip-pen nanodisplacement lithography (DNL) is a versatile scanning probe-based technique that can be employed for fabricating ultrafine 3D polymer brushes under ambient conditions. Many fundamental studies and applications require the large-area fabrication of 3D structures. However, the fabrication throughput and uniformity are still far from satisfactory. In this work, the molecular displacement mechanism of DNL is elucidated by systematically investigating the synergistic effect of z extension and contact time. The in-depth understanding of molecular displacement results in the successful achievement of ultrafine control of 3D structures and high-speed patterning at the same time. Remarkably, one can prepare arbitrary 3D polymer brushes on a large area (1.3 mm × 1.3 mm), with <5% vertical and lateral size variations, and a patterning speed as much as 200-fold faster than the current state-of-the-art.

Selective patterning of gold surfaces by core/shell, semisoft hybrid nanoparticles

The generation of patterned surfaces with well-defined nano- and microdomains is demonstrated by attaching core/shell, semisoft nanoparticles with narrow size distribution to microdomains of a gold-coated silicon wafer. Near monodisperse nanoparticles are prepared using reversible addition-fragmentation chain transfer (RAFT) polymerization, initiated from a silica surface, to prepare a polystyrene shell around a silica core. The particles are then used as-prepared, or after aminolysis of the terminal thiocarbonyl group of the polystyrene shell, to give thiol-terminated nanoparticles. When gold-coated silicon wafers are immersed into very dilute suspensions of these particles (as low as 0.004 wt%), both types of particles are shown to adhere to the gold domains. The thiolated particles adhere selectively to the gold microdomains, allowing for microdomain patterning, while particles that contain the trithiocarbonate functionality lead to a much more even coverage of the gold surface with fewer particle aggregations.

Fabrication of polymer brush surfaces with highly-ordered perfluoroalkyl side groups at the brush end and their antibiofouling properties

The protein-resistant performance was enhanced greatly by constructing a polymer brush surface with perfectly close-packed perfluoroalkyl groups.

Massively parallel patterning of complex 2D and 3D functional polymer brushes by polymer pen lithography

We report the first demonstration of centimeter-area serial patterning of complex 2D and 3D functional polymer brushes by high-throughput polymer pen lithography. Arbitrary 2D and 3D structures of poly(glycidyl methacrylate) (PGMA) brushes are fabricated over areas as large as 2 cm × 1 cm, with a remarkable throughput being 3 orders of magnitudes higher than the state-of-the-arts. Patterned PGMA brushes are further employed as resist for fabricating Au micro/nanostructures and hard molds for the subsequent replica molding of soft stamps. On the other hand, these 2D and 3D PGMA brushes are also utilized as robust and versatile platforms for the immobilization of bioactive molecules to form 2D and 3D patterned DNA oligonucleotide and protein chips. Therefore, this low-cost, yet high-throughput "bench-top" serial fabrication method can be readily applied to a wide range of fields including micro/nanofabrication, optics and electronics, smart surfaces, and biorelated studies.

Patterning of hyperbranched polymer films

This Review describes new methods for patterning functional hyperbranched poly(acrylic acid) thin polymer films. "Hyperbranched polymer" is a generic term used to describe a wide variety of polymeric materials that contain a high percentage of functional groups, that are highly branched, and that are irregular in structure. Hyperbranched polymer films (HPFs) are prepared by an iterative three-step process: activation of an acid functionalized surface, surface grafting of amine-terminated poly(tert-butyl acrylate), and hydrolysis to regenerate the acid surface. The resulting materials have a high density of acid groups, which can be functionalized with moieties that introduce interesting optical, electrochemical, biological, and mechanical properties to the films. HPFs can be patterned with micron-scale resolution using either a template-based approach or photolithography. Templates consist of self-assembled monolayers prepared by microcontact printing, whereas photolithographic patterning relies on selective hydrolysis using photoacids. Biocompatibility can be introduced by grafting a conformal layer of poly(ethylene glycol) atop the HPFs. Such patterns serve as templates for spatially segregating viable mammalian and bacterial cells. In addition to the PAA HPFs, another family of patternable HPFs consisting of dendrimers and an active anhydride copolymer is described.

Amplification of surface-initiated ring-opening metathesis polymerization of 5-(perfluoro-n-alkyl)norbornenes by macroinitiation

This article reports the enhanced rate of the surface-initiated polymerization (SIP) of 5-(perfluoro-n-alkyl)norbornenes (NBFn) by combining two SIP techniques, namely surface-initiated atom-transfer polymerization (SI-ATRP) to grow a macroinitiator and surface-initiated ring-opening metathesis polymerization (SI-ROMP) to produce the final coating. This polymerization approach promotes the rapid growth of dense partially fluorinated coatings that are highly hydrophobic and oleophobic and yield thicknesses from 4-12 μm. Specifically, the growth rate and the limiting thickness of pNBFn with different side chain lengths (n = 4, 6, 8, and 10) at various monomer concentrations and temperatures are evaluated through two approaches: growing the polymer from an initiator-terminated monolayer (control) or from a modified poly(2-hydroxyethyl methacrylate) (PHEMA) macroinitiator. X-ray photoelectron spectroscopy (XPS) analysis shows that 38% of the hydroxyl termini in the macroinitiator react with a norbornenyl diacid chloride (NBDAC) molecule, and 7% of such anchored norbornenyl groups react with a catalyst molecule. The kinetic data have been modeled to determine the propagation velocity and the termination rate constant. The PHEMA macroinitiator provides thicker films and faster growth as compared to the monolayer, achieving a 12 μm thick coating of pNBF8 in 15 min. Increasing the monomer side chain length, n, from 4 to 10 improves the growth rate and the limiting polymer thickness. Performing the polymerization process at higher temperature increases the growth rate and the limiting thickness as evidenced by an increase in the film growth rate constant. Arrhenius plots show that the reactions involved in the macroinitiation process exhibit lower activation energies than those formed from a monolayer. Electrochemical impedance spectroscopy reveals that the films exhibit resistance against ion transport in excess of 1 × 10(10) Ω·cm(2).

Straightforward, one-step synthesis of alkanethiol-capped silver nanoparticles from an aggregative model of growth

Classical nucleation theory and Derjaguin, Landau, Verwey, Overbeek (DLVO) theory for colloidal stability were applied to gain insight into the synthesis of dodecanethiol (DDT) functionalized silver nanoparticles (NPs) by reduction of silver nitrate with sodium borohydride in ethanol. This analysis indicated the importance of quickly establishing a dense DDT ligand brush on inherently unstable primary particles to achieve colloidal stability. The DLVO calculations also indicated that the electrostatic potential was a minor contributor to repulsive interactions, signifying that it would be possible to control NP size and uniformity in solutions with high ionic strength, as long as sufficient DDT was available to form a densely packed ligand layer on the NPs. These insights were applied to design a new straightforward, one-step, one-phase synthesis for the production of alkanethiol-functionalized silver NPs. To test the insights from DLVO theory, 16 samples were synthesized in the parameter space R = 3-12, S = 1-12 where R = [NaBH4]/[AgNO3], S = [DDT]/[AgNO3], and [AgNO3] = 10 mM. In general, samples with R = 3 or S = 1 were polydisperse; however, samples in the R = 6-12 and S = 3-12 range had uniform particle sizes with average diameters between 3.5 and 4.7 nm. Additionally, samples with R = 72-108 and S = 12 were synthesized to test particle stability at high ionic strength; again, uniform NPs with average diameters from 3.5 to 3.8 nm were produced. Ultimately, the insights gained from DLVO theory successfully guided the development of a one-step, one-phase technique for the synthesis of uniform, spherical alkanethiol-functionalized silver NPs.

Affinity separation of lectins using porous membranes immobilized with glycopolymer brushes containing mannose or N-acetyl-d-glucosamine

Porous membranes with glycopolymer brushes were prepared as biomaterials for affinity separation. Glycopolymer brushes contained acrylic acid and D-mannose or N-acetyl-D-glucosamine, and were formed on substrates by surface-initiated atom transfer radical polymerization. The presence of glycopolymer brush was confirmed by X-ray photoelectron spectroscopy, contact angle, and ellipsometry measurements. The interaction between lectin and the glycopolymer immobilized on glass slides was confirmed using fluorescent-labeled proteins. Glycopolymer-immobilized surfaces exhibited specific adsorption of the corresponding lectin, compared with bovine serum albumin. Lectins were continuously rejected by the glycopolymer-immobilized membranes. When the protein solution was permeated through the glycopolymer-immobilized membrane, bovine serum albumin was not adsorbed on the membrane surface. In contrast, concanavalin A and wheat germ agglutinin were rejected by membranes incorporating D-mannose or N-acetyl-D-glucosamine, respectively. The amounts of adsorbed concanavalin A and wheat germ agglutinin was increased five- and two-fold that of adsorbed bovine serum albumin, respectively.

Thin film hydrophilic electroactive polymer coatings for bioelectrodes

Hybrids of conducting polymers (CPs) and hydrogels have been explored as soft electroactive coatings for improving the mechanical and electrical performance of metallic implant electrodes. However, hydrogel fabrication methods pose a significant challenge to producing thin (sub-micron) coatings, resulting in bulky implants, which displace a large volume of tissue. To address this issue, polymer brushes of poly(2-hydroxyethyl methacrylate) (pHEMA) were covalently bound to a gold electrode using surface initiated atom-transfer radical-polymerization (SI-ATRP). The CP poly(3,4-ethylene dioxythiophene) (PEDOT) was electropolymersied through the brush layer to form a thin hydrophilic coating. The electrical properties of the hybrid were shown to be superior to homogenous CPs and the surface chemistry was varied as a function of PEDOT deposition time to present a graded composition of pHEMA and PEDOT. The resulting material was shown to support the attachment and differentiation of model neural cells, signifying the potential of these hybrid coatings for bioelectrode applications.

Poly(N-isopropyl acrylamide) brush topography: dependence on grafting conditions and temperature

The topography of poly (N-isopropyl acrylamide) brushes end-grafted from initiator-terminated monolayers was imaged by atomic force microscopy, as a function of the area per chain and of solvent quality. Measurements were done in air and in water, below and above the lower critical solution temperature. At low grafting densities and molecular weights, area-averaged ellipsometry measurements did not detect changes in the volume of water-swollen, end-grafted polymer films above the lower critical solution temperature. However, atomic force microscopy images revealed surface features that suggest the formation of lateral aggregates or "octopus micelles". At high grafting densities and molecular weights, the films collapsed uniformly, as detected by both AFM imaging and ellipsometry. These findings reconcile in part prior results suggesting that some poly(N-isopropyl acrylamide) chains do not collapse in poor solvent, and they also reveal more complex collapse behavior above the lower critical solution temperature than is commonly assumed. This behavior would influence the ability to tune the functional properties of poly(N-isopropyl acrylamide) coatings.

SiO2@Au core-shell nanospheres self-assemble to form colloidal crystals that can be sintered and surface modified to produce pH-controlled membranes

We prepared colloidal crystals by self-assembly of gold-coated silica nanospheres, and free-standing nanoporous membranes by sintering these colloidal crystals. We modified the nanopore surface with ionizable functional groups, by forming a monolayer of L-cysteine or by surface-initiated polymerization of methacrylic acid. Diffusion experiments for the cationic dye Rhodamine B through L-cysteine-modified membranes showed a decrease in flux upon addition of an acid due to the nanopore surface becoming positively charged. Diffusion experiments for the neutral dye, ferrocenecarboxaldehyde, through the PMAA-modified membranes showed a 13-fold increase in flux upon addition of an acid resulting from the protonated polymer collapsing onto the nanopore surface leading to larger pore size. Our results demonstrate that SiO2@Au core-shell nanospheres can self-assemble into colloidal crystals and that transport through the corresponding surface-modified Au-coated colloidal membranes can be controlled by pH.

Cell membrane-inspired phospholipid polymers for developing medical devices with excellent biointerfaces

This review article describes fundamental aspects of cell membrane-inspired phospholipid polymers and their usefulness in the development of medical devices. Since the early 1990s, polymers composed of 2-methacryloyloxyethyl phosphorylcholine (MPC) units have been considered in the preparation of biomaterials. MPC polymers can provide an artificial cell membrane structure at the surface and serve as excellent biointerfaces between artificial and biological systems. They have also been applied in the surface modification of some medical devices including long-term implantable artificial organs. An MPC polymer biointerface can suppress unfavorable biological reactions such as protein adsorption and cell adhesion - in other words, specific biomolecules immobilized on an MPC polymer surface retain their original functions. MPC polymers are also being increasingly used for creating biointerfaces with artificial cell membrane structures.

Bimetallic multifunctional core@shell plasmonic nanoparticles for localized surface plasmon resonance based sensing and electrocatalysis

Smart bimetallic core@shell nanoparticles were fabricated based on gold nanoparticles (AuNPs) decorated with pH-sensitive polymer shell. Concretely, AuNPs having poly(4-vinylpyridine) (P4VP) on the surface were first fabricated through surface-initiated atom transfer radical polymerization (SI-ATRP). Then, they were mixed with selected metal precursor solutions followed by reduction using reducing agent. The metal NPs thus introduced were uniformly distributed in P4VP polymer shells. In order to explore the diversity and viable function of the resultant nanostructures, we controlled the size of AuNP, pH, selectivity of metal precursors, etc. We investigated the structural alteration during the sequential synthetic process. The bimetallic nanostructures of AuNP@P4VP nanocomposites containing another type of metal NP at the P4VP periphery exhibit a controlled sensing property in terms of the change in the refractive index of surrounding media and a typical electrocatalytic activity for methanol oxidation reaction.

Facile synthesis of thermally stable poly(N-vinylpyrrolidone)-modified gold surfaces by surface-initiated atom transfer radical polymerization

Well-controlled polymerization of N-vinylpyrrolidone (NVP) on Au surfaces by surface-initiated atom transfer radical polymerization (SI-ATRP) was carried out at room temperature by a silanization method. Initial attempts to graft poly(N-vinylpyrrolidone) (PVP) layers from initiators attached to alkanethiol monolayers yielded PVP films with thicknesses less than 5 nm. The combined factors of the difficulty in the controllable polymerization of NVP and the instability of alkanethiol monolayers led to the difficulty in the controlled polymerization of NVP on Au surfaces. Therefore, the silanization method was employed to form an adhesion layer for initiator attachment. This method allowed well-defined ATRP polymerization to occur on Au surfaces. Water contact angle, X-ray photoelectron spectroscopy (XPS), and reflectance Fourier transform infrared (reflectance FTIR) spectroscopy were used to characterize the modified surfaces. The PVP-modified gold surface remained stable at 130 °C for 3 h, showing excellent thermal stability. Thus, postfunctionalization of polymer brushes at elevated temperatures is made possible. The silanization method was also applied to modify SPR chips and showed potential applications in biosensors and biochips.

Programmable light-controlled shape changes in layered polymer nanocomposites

We present soft, layered nanocomposites that exhibit controlled swelling anisotropy and spatially specific shape reconfigurations in response to light irradiation. The use of gold nanoparticles grafted with a temperature-responsive polymer (poly(N-isopropylacrylamide), PNIPAM) with layer-by-layer (LbL) assembly allowed placement of plasmonic structures within specific regions in the film, while exposure to light caused localized material deswelling by a photothermal mechanism. By layering PNIPAM-grafted gold nanoparticles in between nonresponsive polymer stacks, we have achieved zero Poisson's ratio materials that exhibit reversible, light-induced unidirectional shape changes. In addition, we report rheological properties of these LbL assemblies in their equilibrium swollen states. Moreover, incorporation of dissimilar plasmonic nanostructures (solid gold nanoparticles and nanoshells) within different material strata enabled controlled shrinkage of specific regions of hydrogels at specific excitation wavelengths. The approach is applicable to a wide range of metal nanoparticles and temperature-responsive polymers and affords many advanced build-in options useful in optically manipulated functional devices, including precise control of plasmonic layer thickness, tunability of shape variations to the excitation wavelength, and programmable spatial control of optical response.

Nanostructured bio-functional polymer brushes

Structured poly(glycidyl methracrylate) (poly-GMA) brushes have been grafted onto flexible fluoro-polymer films using a radiation grafting process. The reactive epoxide of poly-GMA provides the basis for a versatile biofunctionalization of the grafted brushes. Structure definition by extreme ultraviolet (EUV) exposure allowed nanometer-scale resolution of periodic patterns. By variation of the exposure dose the height of the grafted structures can be adapted in a wide range. Derivatization of the grafted brushes included reaction with various amines with different side chains, hydrolysis of the epoxide to diols to increase protein resistance and introduction of ionic groups to yield poly-electrolytes. As an example for biofunctionalization, biotin was linked to the grafted brush and biofunctionality was demonstrated in a competitive biotin-streptavidin assay. In this article we also present a brief review of other approaches to obtain structured biofunctional polymer brushes.

Carbohydrate structure dependent hemocompatibility of biomimetic functional polymer brushes on surfaces

Glycocalyx mimicking glycopolymer brushes presenting mannose, galactose and glucose residues in the pyranose form, similar to those present on cell surfaces, were synthesized on planar substrates (Si wafer, gold chip) and monodispersed polystyrene (PS) particles, and the interaction of blood to these surfaces were studied using various methods with the goal of producing a hemocompatible surface. Surface plasmon resonance (SPR) spectroscopy and gel analyses showed that the total protein adsorption from plasma was greatly reduced, as low as 24.3 ng/cm(2) from undiluted plasma on the glucose carrying brush. The protein adsorption decreased with increasing grafting density of the brushes. It was also found that the protein adsorption varied with the anticoagulant used for blood collection; much higher amount of protein was adsorbed from heparinzied plasma than citrated plasma. Proteomics protein identification analysis revealed that protein adsorption from plasma depended on the type of sugar residue present on the surface as well as the type of anticoagulant. All the three types of glycopolymer brushes showed similar level of platelet activation as that of buffer control irrespective of the nature of carbohydrate residue. However, the number of adhered platelet and their morphology depended on the type of carbohydrate residue present on the brush. On glucose brush, the extent of platelet adhesion and spreading was significantly lowered compared to other brushes. All the glycopolymer brushes were neutral to blood coagulation as indicated by thromboelastography analysis. The glucose brush gave a slightly longer initial coagulation time suggesting that this surface may be more biocompatible. Our data demonstrate that the structure of carbohydrate residue is an important factor in the design of synthetic blood contacting surface based on glycopolymer.

Surface functionalization of aluminosilicate nanotubes with organic molecules

The surface functionalization of inorganic nanostructures is an effective approach for enriching the potential applications of existing nanomaterials. Inorganic nanotubes attract great research interest due to their one-dimensional structure and reactive surfaces. In this review paper, recent developments in surface functionalization of an aluminosilicate nanotube, "imogolite", are introduced. The functionalization processes are based on the robust affinity between phosphate groups of organic molecules and the aluminol (AlOH) surface of imogolite nanotubes. An aqueous modification process employing a water soluble ammonium salt of alkyl phosphate led to chemisorption of molecules on imogolite at the nanotube level. Polymer-chain-grafted imogolite nanotubes were prepared through surface-initiated polymerization. In addition, the assembly of conjugated molecules, 2-(5''-hexyl-2,2':5',2''-terthiophen-5-yl)ethylphosphonic acid (HT3P) and 2-(5''-hexyl-2,2':5',2''-terthiophen-5-yl)ethylphosphonic acid 1,1-dioxide (HT3OP), on the imogolite nanotube surface was achieved by introducing a phosphonic acid group to the corresponding molecules. The optical and photophysical properties of these conjugated-molecule-decorated imogolite nanotubes were characterized. Moreover, poly(3-hexylthiophene) (P3HT) chains were further hybridized with HT3P modified imogolite to form a nanofiber hybrid.

Coatings from micropatterned sulfobetaine polymer brushes as substrates for MC3T3-E1 cells

In the last decades, polymer brush coatings have proven to be excellent anti-fouling materials by preventing protein adhesion. When using this property to restrict cell growth laterally in cell culture, it is crucial to ensure that other cell functions remain unaffected. The present study therefore examines MC3T3-E1 cell growth and morphology on patterned PSBMA brush substrates and probes their proliferation potential at mRNA level. The osteoblastic cells display a more elongated morphology than cells on the control substrates, but show no sign of elevated levels of the apoptosis marker p53 or diminished levels of Ki-67 or H4, which serve as indicators of proliferation. Therefore, patterned polymer brushes do not seem to influence cells in their proliferation state and are suitable cell culture substrates. Nevertheless, the use of polymer brush surfaces in long-term cell culture was found to be limited by their instability in cell culture medium.

ATRP in the design of functional materials for biomedical applications

Atom Transfer Radical Polymerization (ATRP) is an effective technique for the design and preparation of multifunctional, nanostructured materials for a variety of applications in biology and medicine. ATRP enables precise control over macromolecular structure, order, and functionality, which are important considerations for emerging biomedical designs. This article reviews recent advances in the preparation of polymer-based nanomaterials using ATRP, including polymer bioconjugates, block copolymer-based drug delivery systems, cross-linked microgels/nanogels, diagnostic and imaging platforms, tissue engineering hydrogels, and degradable polymers. It is envisioned that precise engineering at the molecular level will translate to tailored macroscopic physical properties, thus enabling control of the key elements for realized biomedical applications.

Facile synthesis of thick films of poly(methyl methacrylate), poly(styrene), and poly(vinyl pyridine) from Au surfaces

Atom transfer radical polymerization (ATRP) is commonly used to grow polymer brushes from Au surfaces, but the resulting film thicknesses are usually significantly less than with ATRP from SiO(2) substrates. On Au, growth of poly(methyl methacrylate) (PMMA) blocks from poly(tert-butyl acrylate) brushes occurs more rapidly than growth of PMMA from initiator monolayers, suggesting that the disparity between growth rates from Au and SiO(2) stems from the Au surface. Radical quenching by electron transfer from Au is probably not the termination mechanism because polymerization from thin, cross-linked initiators gives film thicknesses that are essentially the same as the thicknesses of films grown from SiO(2) under the same polymerization conditions. However, this result is consistent with termination through desorption of thiols from noncross-linked films, and reaction of these thiols with growing polymer chains. The enhanced stability of cross-linked initiators allows ATRP at temperatures up to ∼100 °C and enables the growth of thick films of PMMA (350 nm), polystyrene (120 nm) and poly(vinyl pyridine) (200 nm) from Au surfaces in 1 h. At temperatures >100 °C, the polymer brush layers delaminate as large area films.

Programming nanostructures of polymer brushes by dip-pen nanodisplacement lithography (DNL)

We report a facile and versatile scanning probe based approach-dip-pen nanodisplacement lithography (DNL)--for manipulating nanostructures of polymer brushes. Nanostructured polymer brushes with sizes as small as 25 nm are made by DNL patterning of the initiator molecules and subsequent surface-initiated polymerization. Nanoconfinement effects including chain collapsing and spreading are observed in the nanopatterned polymer brushes. In addition to chemical structure, size, topography and shape, our approach can also readily program the grafting density, chain configuration, hierarchical structure and multiplexing of the polymer brushes, which allows for the realization of complex chemical surfaces.

Surface initiated polymerization from substrates of low initiator density and its applications in biosensors

Surface initiated polymerization (SIP) has become an attractive method for tailoring physical and chemical properties of surfaces for a broad range of applications. Most of those applications relied on the merit of high density coating. In this study, we explored a long overlooked field of SIP: SIP from substrates of low initiator density. We combined ellipsometry with AFM to investigate the effect of initiator density and polymerization time on the morphology of polymer coatings. In addition, we carefully adjusted the nanoscale separation of polymer chains to achieve a balance between nonfouling and immobilization capacities. We further tested the performance of those coatings on various biosensors, such as quartz crystal microbalance, surface plasmon resonance, and protein microarrays. The optimized matrices enhanced the performance of those biosensors. This report shall encourage researchers to explore new frontiers in SIP that go beyond polymer brushes.