Morphology control of structured polymer brushes

Tracking down the origin of peculiar vibrational spectra of aromatic self-assembled thiolate monolayers

Several studies have previously observed surprisingly low frequencies for the C-H stretching modes of self-assembled monolayers (SAMs) prepared from aromatic thiols. The reason for this property has so far remained elusive. Therefore, we report a novel study of the vibrational spectra of SAMs prepared on Au from two different aromatic thiols, namely, 4'-nitro-1,1'-biphenyl-4-thiol (NBPT) and 4-aminothiophenol (ATP). The SAMs were prepared by vapor deposition (VD) in ultrahigh vacuum (UHV) as well as by the solution method (SM) and their quality was controlled by X-ray photoelectron spectroscopy (XPS). In addition, amino terminated SAMs were also obtained by electron irradiation and by chemical reduction of NBPT SAMs. Beside infrared reflection absorption spectroscopy (IRRAS), we have employed high resolution electron energy loss spectroscopy (HREELS), by which VD SAMs can be studied in situ, i.e. without exposing them to air. Hence, we can exclude possible contributions of solvent molecules to the vibrational spectra. Nonetheless, HREELS in fact reveals the same large red shift of the C-H stretching modes in the SAMs as also observed in ex situ IRRAS experiments. In contrast, HREELS for physisorbed ATP and ATP in a KBr pellet measured by transmission infrared spectroscopy exhibit the expected aromatic bands. Using a computational approach, we can exclude molecular packing effects as origin of this shift. Therefore, we propose chemical changes in the aromatic rings during SAM formation as an alternative explanation for the observed frequency shift. As another striking effect, the N-H stretching vibrational modes of the amino-terminated SAMs are extremely weak in both IRRAS and HREELS despite the fact that XPS confirms the presence of amino groups. A very weak signal is observed only in the case of an electron irradiated NBPT SAM. In contrast, an energy loss ascribed to the N-H stretching vibrations is clearly observed in HREELS of ATP physisorbed on an ATP SAM and on graphite as well as in the transmission infrared spectrum of ATP in KBr. The extremely low intensity of these vibrations in the SAM is traced back to the inherently low transition dipole moment for the excitation of N-H stretching modes in free N-H groups. Furthermore, the calculations suggest that the much stronger signals of N-H stretching modes involved in hydrogen-bonding with adjacent amino groups are suppressed because these vibrations are oriented parallel to the surface.

Nanopatterned polymer brushes by reactive writing

Polymer brush patterns were prepared by a combination of electron beam induced damage in self-assembled monolayers (SAMs), creating a stable carbonaceous deposit, and consecutive self-initiated photografting and photopolymerization (SIPGP). This newly applied technique, reactive writing (RW), is investigated with 1H,1H,2H,2H-perfluorooctyltriethoxysilane SAM (PF-SAM) on silicon oxide, which, when modified by RW, can be selectively functionalized by SIPGP. With the monomer N,N-dimethylaminoethyl methacrylate (DMAEMA), we demonstrate the straightforward formation of polymer brush gradients and single polymer lines of sub-100 nm lateral dimensions, with high contrast to the PF-SAM background. The lithography parameters acceleration voltage, irradiation dose, beam current and dwell time were systematically varied to identify the optimal conditions for the maximum conversion of the SAM into a carbonaceous deposit. The results of this approach were compared to patterns prepared by carbon templating (CT) under analogous conditions, revealing a dwell time dependency, which differs from earlier reports. This new technique expands the range of CT by giving the opportunity to not only vary the chemistry of the created polymer patterns with monomer choice but also vary the chemistry of the surrounding substrate.

Nanopatterned polymer brushes: conformation, fabrication and applications

Surfaces with end-grafted, nanopatterned polymer brushes that exhibit well-defined feature dimensions and controlled chemical and physical properties provide versatile platforms not only for investigation of nanoscale phenomena at biointerfaces, but also for the development of advanced devices relevant to biotechnology and electronics applications. In this review, we first give a brief introduction of scaling behavior of nanopatterned polymer brushes and then summarize recent progress in fabrication and application of nanopatterned polymer brushes. Specifically, we highlight applications of nanopatterned stimuli-responsive polymer brushes in the areas of biomedicine and biotechnology.

Fabrication of Two-Component, Brush-on-Brush Topographical Microstructures by Combination of Atom-Transfer Radical Polymerization with Polymer End-Functionalization and Photopatterning

Poly(oligoethylene glycol methyl ether methacrylate) (POEGMEMA) brushes, grown from silicon oxide surfaces by surface-initiated atom transfer radical polymerization (SI-ATRP), were end-capped by reaction with sodium azide leading to effective termination of polymerization. Reduction of the terminal azide to an amine, followed by derivatization with the reagent of choice, enabled end-functionalization of the polymers. Reaction with bromoisobutryl bromide yielded a terminal bromine atom that could be used as an initiator for ATRP with a second, contrasting monomer (methacrylic acid). Attachment of a nitrophenyl protecting group to the amine facilitated photopatterning: when the sample was exposed to UV light through a mask, the amine was deprotected in exposed regions, enabling selective bromination and the growth of a patterned brush by ATRP. Using this approach, micropatterned pH-responsive poly(methacrylic acid) (PMAA) brushes were grown on a protein resistant planar poly(oligoethylene glycol methyl ether methacrylate) (POEGMEMA) brush. Atomic force microscopy analysis by tapping mode and PeakForce quantitative nanomechanical mapping (QNM) mode allowed topographical verification of the spatially specific secondary brush growth and its stimulus responsiveness. Chemical confirmation of selective polymer growth was achieved by secondary ion mass spectrometry (SIMS).

Advances in the surface modification techniques of bone-related implants for last 10 years

At the time of implanting bone-related implants into human body, a variety of biological responses to the material surface occur with respect to surface chemistry and physical state. The commonly used biomaterials (e.g. titanium and its alloy, Co-Cr alloy, stainless steel, polyetheretherketone, ultra-high molecular weight polyethylene and various calcium phosphates) have many drawbacks such as lack of biocompatibility and improper mechanical properties. As surface modification is very promising technology to overcome such problems, a variety of surface modification techniques have been being investigated. This review paper covers recent advances in surface modification techniques of bone-related materials including physicochemical coating, radiation grafting, plasma surface engineering, ion beam processing and surface patterning techniques. The contents are organized with different types of techniques to applicable materials, and typical examples are also described.

Nanopatterned smart polymer surfaces for controlled attachment, killing, and release of bacteria

Model surfaces with switchable functionality based on nanopatterned, thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) brushes were fabricated using interferometric lithography combined with surface-initiated polymerization. The temperature-triggered hydration and conformational changes of nanopatterned PNIPAAm brushes reversibly modulate the spatial concealment and exposure of molecules that are immobilized in the intervals between nanopatterned brushes. A biocidal quaternary ammonium salt (QAS) was used to demonstrate the utility of nanopatterned PNIPAAm brushes to control biointerfacial interactions with bacteria. QAS was integrated into polymer-free regions of the substrate between nanopatterned PNIPAAm brushes. The biocidal efficacy and release properties of these surfaces were tested against Escherichia coli K12. Above the lower critical solution temperature (LCST) of PNIPAAm, desolvated, collapsed polymer chains facilitate the attachment of bacteria and expose QAS moieties that kill attached bacteria. Upon a reduction of the temperature below the LCST, swollen PNIPAAm chains promote the release of dead bacteria. These results demonstrate that nanopatterned PNIPAAm/QAS hybrid surfaces are model systems that exhibit an ability to undergo noncovalent, dynamic, and reversible changes in structure that can be used to control the attachment, killing, and release of bacteria in response to changes in temperature.

Liquid-mediated three-dimensional scanning probe nanosculpting

3D functional polymer brushes are fabricated by liquid-mediated scanning probe nanosculpting (LSPN). Surface-tethered functional polymer brushes, which are immersed in their good solvent, are mechanically cleaved away from the substrate by the AFM tip at high forces, and immediately imaged in situ with the same AFM tip at low applied forces.

Noncovalent microcontact printing for grafting patterned polymer brushes on graphene films

This article describes a simple and universal approach to prepare patterned polymer brushes on graphene-based substrate surfaces by microcontact printing (μCP) of initiator molecules and subsequent surface initiated atom transfer radical polymerization (SI-ATRP) method. Four different initiators are designed and have strong adhesion with graphene-based substrates through noncovalent interaction. Optical and fluorescence microscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the successful polymerization of vinyl monomers on substrate surfaces. To demonstrate the broad applicability of this strategy, polymer brushes with different functionalities including cationic and anionic polyelectrolyte, thermally and pH responsive polymers, as well as polymer patterns on different graphene-based surfaces are fabricated. Binary polymer brushes can also be easily prepared by further initiating the initiator backfilled in the bare areas.

Modification of nitrile-terminated biphenylthiol self-assembled monolayers by electron irradiation and related applications

Here we describe the behavior of self-assembled monolayers (SAMs) of 4'-cyanobiphenyl-4-thiol (CBPT) on Au(111) upon electron irradiation. Under such a treatment, the aromatic framework of CBPT SAMs is laterally cross-linked while the nitrile groups, located at the SAM-ambience interface, are reduced to active amine moieties which can be used as docking sites for the coupling of other species. This makes CBPT monolayers as a promising system for conventional and chemical lithography as well as for nanofabrication. Along these lines, we demonstrate the preparation of complex polymer brushes, patterning of the underlying substrate, and fabrication of molecule-thin, free-standing membranes on the basis of CBPT SAMs. The balance between the application-favorable processes and defragmentation in these films is studied in detail, and comparison to the well-established (for the relevant applications) system of 4'-nitrobiphenyl-4-thiols is performed. Taking CBPT SAMs as a model system, the effect of the energy of the primary electrons on the extent of the chemical transformation and cross-linking in substituted aromatic SAMs is investigated.

Polymer nanogels grafted from nanopatterned surfaces studied by AFM force spectroscopy

Nanopatterned cross-linked polymers are important for applications with controlled mechanical properties. Grafted linear and cross-linked polydimethylacrylamide gels on micro- and nanopatterns were created using iniferter-driven quasi-living radical polymerization combined with conventional photolithography and nanosphere lithography. Micropatterned linear polymers reproduce the expected scaling behavior at moderate grafting density. The addition of cross-linker to the polymerization solution leads to an increased tendency of early termination as determined by AFM force spectroscopy. Similarly, nanopatterned linear polymers show reduced thickness in agreement with the expected scaling relationship for nanoisland grafts that have reduced lateral confinement. The addition of cross-linker reintroduces some of the lateral confinement for the length of polymers reported here. The mechanical properties of both the micro- and nanopatterned linear as well as cross-linked polymers were analyzed using an algorithm to objectively determine the contact point in AFM force spectroscopy and two independent Hertz-based analysis approaches. The obtained Young's moduli are close to those expected for homogeneous thick polymer films and are independent of pattern size. Our results demonstrate that polymeric nanopillars with controlled elastic modulus can be fabricated using irreversible cross-linkers. They also highlight some of the factors that must be considered for successful fabrication of grafted nanopillars of defined mechanical and structural properties.

Patterned polymer carpets

For the development of polymer carpets as active devices for micro- and nanotechnology, a control of the polymer carpet morphology and especially control of the stimulus responsive polymer brush is needed. Here, we report on the first example for the fabrication of patterned polymer carpets. On a two-dimensional framework of fully crosslinked and chemically patterned nanosheets, polymer brushes of styrene and 4-vinyl pyridine were grafted by self-initiated surface photopolymerization and photografting (SIPGP). It was found that polymer grafting by SIPGP occurred over the entire nanosheets but with a preferred grafting on the amino functionalized nanosheet areas. This results in continuous polymer carpets with an intact nanosheet framework but with amplification of the chemical patterning into a three dimensional topography of the grafted polymer brush. In the case of negative patterned nanosheets, the patterned carpet could be prepared as freestanding ultrathin membranes. Furthermore, swelling experiments with poly(4-vinyl pyridine) carpets showed that the patterns induces a directional buckling of the flexible polymer carpet. This may open the possibility of the development of micro- or nanoactuator devices with anisotropic responds upon environmental changes.

Tandem surface microfluidic lithography and activation to generate patch pattern biospecific ligand and cell arrays

We report a rapid, inexpensive, and flexible methodology that combines microfluidic lithography and oxidative activation to pattern and chemically alter selective regions of SAMs on gold for subsequent chemoselective ligand immobilization. We demonstrate that PCC, a mild oxidant, can be used to convert hydroxyl-terminated SAMs to aldehydes and decorated with a variety of oxyamine-containing molecules. This strategy is compatible with cell culture and was employed to create a biospecific ligand platform for peptide-mediated, cell adhesion arrays. By using a number of different ligands and characterization tools, we showed that the generation of both cell patterning and ligand microarray patterning can be achieved. SAM formation, activation, ligand immobilization, and biospecific cell patterning are characterized by contact angle, cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) (Supporting Information), scanning electron microscopy (SEM), and fluorescence microscopy.

Comparison of chemical lithography using alkanethiolate self-assembled monolayers on GaAs (001) and Au

We have investigated the efficiency of bifunctional pattern formation in alkanethiolate self-assembled monolayers (SAMs) adsorbed on GaAs (001) and Au, using time-of-flight secondary ion mass spectrometry. Two patterning techniques were employed: electron beam lithography and UV photopatterning. Previous work has always assumed that complete degradation of the SAM was necessary for the formation of well-defined multifunctional patterned surfaces, requiring large electron doses or long UV irradiation times. We demonstrate that well-defined multifunctional patterned surfaces can be produced on GaAs (001) with only partial degradation of the SAM, allowing greatly reduced electron beam doses and UV irradiation times to be used. Using electron beam lithography we observe that sharp well-defined patterns can form after an electron dose as low as 450 microC cm(-2). We also demonstrate that only 50% of the monolayer must be photooxidized in UV photopatterning, reducing the exposure time needed by a factor of 3. In contrast, patterning of alkanethiolate SAMs adsorbed on Au requires much higher electron doses (> or = 1250 microC cm(-2)) and photooxidation times (2 h). The substantial differences observed on these two substrates appear to arise from differences in the SAM structure on GaAs and Au. These results suggest that alkanethiolate SAM resists may be a suitable technology for nanometer scale lithography of GaAs and possibly other semiconductors.

High aspect ratio constructive nanolithography with a photo-dimerizable molecule

A major challenge in constructive nanolithography is the preservation of the lateral resolution of a monolayer-thick template pattern while amplifying it to a structure with a thickness above 10 nm. So far, the most successful approach to achieve this is surface-initiated polymerization (SIP) from e-beam structured monolayer templates in a multistep process. However, spreading of the polymer on the substrate leads to a rapid line-widening. Therefore, structures with lateral resolutions well below 100 nm and thicknesses above 10 nm (aspect ratio: 0.1) were not reported yet. Our approach of photoinduced, constructive, reversible nanolithography, is based on nanografting within a coumarin-derivative thiol (CDT) solution using the tip of an atomic force microscope (AFM). By photodimerization and the formation of disulfide bonds, the CDT polymerizes in a single-step process. We demonstrate the highest lateral resolution in constructive nanolithography at thicknesses above 10 nm (40 nm lateral resolution at 12 nm thickness, aspect ratio: 0.3).

Microstructured poly(2-oxazoline) bottle-brush brushes on nanocrystalline diamond

We report on the preparation of microstructured poly(2-oxazoline) bottle-brush brushes (BBBs) on nanocrystalline diamond (NCD). Structuring of NCD was performed by photolithography and plasma treatment to result in a patterned NCD surface with oxidized and hydrogenated areas. Self-initiated photografting and photopolymerization (SIPGP) of 2-isopropenyl-2-oxazoline (IPOx) resulted in selective grafting of poly(2-isopropenyl-2-oxazoline) (PIPOx) polymer brushes only at the oxidized NCD areas. Structured PIPOx brushes were converted by methyl triflate into the polyelectrolyte brush macroinitiator for the living cationic ring-opening polymerization (LCROP) of 2-oxazolines. The LCROP was performed with 2-ethyl-2-oxazoline (EtOx) as well as 2-(carbazolyl)ethyl-2-oxazoline (CarbOx) as monomers, resulting in structured bottle-brush brushes (BBB) with different pendant side chains and functionalities. FT-IR spectroscopy, fluorescence microscopy, and AFM measurements indicated a high side chain grafting density as well as quantitative and selective reactions. Poly(2-oxazoline) BBBs containing hole conducting carbazole moieties on NCD as electrode material may open the way to advanced amperometric biosensing systems.

Adjustment of the bioresistivity by electron irradiation: self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiols with embedded cleavable group

The bioresistivity of protein-repelling films, such as e.g. oligoethyleneglycol (OEG) bearing self-assembled monolayers (SAMs), can be adjusted by electron irradiation. We have studied the effect of an embedded irradiation-sensitive functional group (so called "weak link") on the irradiation sensitivity of such films. As test systems, we used two OEG-substituted alkanethiolate SAMs with different lengths of the OEG chain and a sulfone group serving as a "weak link" moiety; this group was embedded between the OEG and alkyl chains of the target molecules. The expected cleavage of the molecular chains at the predetermined "weak link" position was found to be accompanied by direct damage of the OEG matrix, with a dominance of the latter process in the case of a long OEG chain. At the same time, in the case of short OEG chain, the insertion of sulfone group resulted in a noticeable gain in the irradiation sensitivity of the respective SAMs and, consequently, in enhanced tunability of their protein-adhesive properties. These films, along with other OEG-substituted SAMs and polymer layers with and without an embedded weak link moiety, can be used as primary matrices for the fabrication of different protein patterns by electron beam lithography.

Interaction forces and reversible collapse of a polymer brush-gated nanopore

Nanopores are ubiquitous in nature and technology, yet relatively little is known about how surface-grafted polymers can affect the interaction forces at the pore. By fabricating Au nanorings on Si substrates, we have constructed a unique experimental platform that allows for direct atomic force microscope (AFM) measurements to be made on polyethylene glycol (PEG) chains locally anchored onto a geometric pore surface. Force measurements show that the PEG gives rise to a steric repulsive barrier that envelops the entire nanoring, signifying polymer brush formation. This is confirmed by a direct imaging of the PEG brush, which reversibly collapses by switching between poor and good solvent conditions to "open" and "close" the pore, respectively. From the view of interaction forces, these results highlight possible functionalities in which polymer brushes may play a role in minimizing fouling/clogging effects in synthetic nanopores and biological nuclear pore complexes (NPCs). By the mechanism of a reversible collapse, this work illustrates how polymer brush-gated nanopores may be used as nonfouling sieves for small molecules and/or solvent-controlled chemical valves that regulate solute traffic.

Surface-initiated polymerization on laser-patterned templates: morphological scaling of nanoconfined polymer brushes

Nonlinear laser processing of silane-based monolayers is used to fabricate nanostructured chemical templates for the selective growth of polymer brushes in confined domains via surface-initiated polymerization (SIP). Upon varying the laser parameters, reactive domains with lateral dimensions from several micrometers down to the sub-100-nm range are fabricated. This provides a versatile means for studying the morphological scaling behavior of confined polymer brushes. Here, the surface-initiated growth of a stimuli-responsive polymer, poly(N-isopropylacrylamide) (PNiPAAm), via atom transfer radical polymerization (ATRP) is investigated. Polymer chains at the domain boundaries extend into the surrounding polymer-free areas. For this reason the width of confined polymer brushes is significantly larger than that of the underlying domains. Within experimental error, though, the excess width does not depend on the domain size. In contrast, the brush height decreases more and more when the domain size falls below a certain value. Simple considerations point to a geometrical scaling relation between height and width of the polymer brushes. These results are considered as essential for implementation of SIP routines in laser-assisted fabrication schemes targeting micro- and nanofluidic applications.

Structured and gradient polymer brushes from biphenylthiol self-assembled monolayers by self-initiated photografting and photopolymerization (SIPGP)

The self-initiated photografting and photopolymerization (SIPGP) of styrene, methyl methacrylate, and tert-butyl methacrylate on structured self-assembled monolayers (SAMs) of electron beam cross-linked omega-functionalized biphenylthiols SAMs on gold was investigated. Polymer brushes with defined thickness can be prepared on crosslinked benzyl-, phenyl-, hydroxyl-, and amino-functionalized SAMs, whereas non-cross-linked SAM regions desorb from the surface during the SIPGP process. By the preparation of brush gradients on different functionalized SAMs, it was demonstrated that the resulting polymer brush layer thickness is determined by the locally applied electron beam dosage. Defined micro-nanostructured polymer brush patterns can be prepared down to a size of 50 nm. Finally, it was shown that polymer brushes obtained by the SIPGP process have a branched architecture.

Effect of nanoconfinement on the collapse transition of responsive polymer brushes

Nanopatterned brushes of a thermo-responsive polymer, poly(2-(2-methoxyethoxy)ethyl methacrylate) (PMEO2MA), displaying a collapse temperature in the physiological range were synthesized for grafting diameters from a few micrometers down to 35 nm. The reversible collapse transition of the nanobrushes was studied in water as a function of their lateral confinement, down to ensembles of brushes containing only approximately 300 chains. The confinement results in a considerable broadening of the collapse transition and in an increase of the degree of vertical swelling, which can be explained by the internal structure of the nanodroplets derived from a theoretical model of dry nanobrushes. These results enable the rational design of responsive surfaces having a tunable topography engineered at the nanometer scale, which is of direct interest for the development of soft nanoactuators and new substrates for cell adhesion studies.

Adhesion forces between Staphylococcus epidermidis and surfaces bearing self-assembled monolayers in the presence of model proteins

Self-assembled monolayers (SAMs) are being developed into coatings to reduce microbial biofilm formation on biomaterials. To test anti-adhesion properties, SAMs can be easily constructed on gold, and used to represent a coated biomaterial. However, coatings that prevent bacterial adhesion must also resist protein adsorption. We explored the competitive effects of bacteria and protein for adsorption to SAMs, choosing fetal bovine serum (FBS) to represent protein non-specific binding, and fibronectin (FN) to evaluate ligand/receptor binding. Staphylococcus epidermidis were immobilized on an atomic force microscope (AFM) tip and used as a force probe to detect the interaction forces between bacteria and gold-coated SAMs. The SAMs tested were alkanethiol molecules terminating in isophthalic acid (IPA) or isophthalic acid with silver (IAG). While S. epidermidis showed weak interactions with FBS, the bacteria showed strong adhesion with FN, due to ligand/receptor binding. Bacterial retention and viability experiments were correlated with the force measurements. S. epidermidis interacting with IAG SAMs showed a loss of viability, due to the mobility of silver ions. For most substrata, there was a link between high adhesion forces with bacteria and a high percentage of dead cells being retained on that substratum (even in the absence of a specific biocidal effect, such as silver). This may suggest that high adhesion forces can cause stress to the bacteria which contributed to their death. The relationship between highly adhesive SAMs and bacterial inactivation may be useful in future biomaterial design. When evaluating coatings for biomaterials, it is important to consider the interplay between bacteria, proteins, and the coating material.

Patterned layers of a semiconducting polymer via imprinting and microwave-assisted grafting

Enhancements in both the rate and extent of grafting of poly(9,9'-n-dihexyl fluorene) (PDHF) onto flat and nanopatterned crosslinked photopolymer films are described. Reactivity of the surfaces toward grafting via the Yamamoto-type Ni(0)-mediated coupling reaction is increased by synthesizing and incorporating 2,7-dibromo-9-fluorenyl methacrylate (DBFM, 2) as a new grafting agent. Varying the concentration of surface-embedded DBFM is shown to control both overall graft formation and fluorescence with a maximum thickness of up to 30 nm and peak emission at 407 nm for 40 wt% loading. In addition, microwave irradiation is introduced as an effective means to drive graft formation and thus allows fabrication of PDHF-functionalized surfaces in as little as 30 min. Both forms of improvement are extended to DBFM-embedded, nanocontact-molded features ranging in size from 100 microm to 100 nm in width and 60 nm in height. Microwave-assisted grafting from these patterned surfaces produces fluorescent features as imaged by optical microscopy and a corresponding increase in feature height as measured by atomic force microscopy.