See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/221709145

ChemInform Abstract: Complex Brush Gradients Based on Nanolithography and Surface-Initiated

ARTICLE in CHEMICAL SOCIETY REVIEWS · MARCH 2012 Impact Factor: 33.38 · DOI: 10.1039/c2cs15316e · Source: PubMed

CITATIONS READS 23 89

3 AUTHORS, INCLUDING:

Xiankun Lin Qiang He Harbin Institute of Technology Harbin Institute of Technology

18 PUBLICATIONS 248 CITATIONS 90 PUBLICATIONS 2,752 CITATIONS

SEE PROFILE SEE PROFILE

Available from: Qiang He Retrieved on: 28 October 2015 View Online / Journal Homepage / Table of Contents for this issue Chem Soc Rev Dynamic Article Links

Cite this: Chem. Soc. Rev., 2012, 41, 3584–3593

www.rsc.org/csr TUTORIAL REVIEW

Complex polymer brush gradients based on nanolithography and surface-initiated polymerization

Xiankun Lin,a Qiang He*a and Junbai Li*b

Received 25th November 2011 DOI: 10.1039/c2cs15316e

Confined surface gradients consisting of polymer brushes have great potential in various applications such as microfluidic devices, sensors, and biophysical research. Among the available fabrication approaches, nanolithographies combined with self-assembled monolayers and surface- initiated polymerization have became powerful tools to engineer confined gradients or predefined complex gradients on the nanometre size. In this tutorial review, we mainly highlight the research progress of the fabrication of confined polymer brush gradients by using electron beam, , and probe-based nanolithographies and the physical base for these approaches. The application of these polymer brush gradients in biomedical research is also addressed.

Introduction groups, and responsive behavior under external stimuli. A promising way to render solid surfaces with diverse Accessing surfaces with tailorable properties is very important for functionalities is to integrate functional macromolecules such diverse applications from daily life to advanced technologies. as self-assembled monolayers, polymer brushes, block copolymers, A lot of approaches have been employed to engineer surfaces and layer-by-layer polyelectrolyte multilayers onto various solid with a variety of surface energies, topography, patterns, functional substrates.1–3 Particularly, patterned and/or stimuli-responsive polymer brushes have attracted considerable attention from a Key Laboratory of Microsystems and Microstructures industrial and academic areas for advanced technologies such Manufacturing, Ministry of Education, Micro/Nano Technology as microfluidic devices, biological sensors, tissues engineering, Downloaded by Harbin Institute of Technology on 17 April 2012 Research Centre, Harbin Institute of Technology, Yikuangjie No. 2, 4,5 Harbin 150080, China. E-mail: [email protected]; and anti-biofouling. Published on 16 March 2012 http://pubs.rsc.org | doi:10.1039/C2CS15316E Fax: +86-451-86403605; Tel: +86-451-86403605 Additionally, surface gradients have recently become a hot b Beijing National Laboratory for Molecular Sciences, Institute of research area, especially in material sciences and biophysical Chemistry, Chinese Academy of Sciences, Zhong Guan Cun, Bei Yi Jie No. 2, Beijing 100190, China. E-mail: [email protected]; research. Gradients on surfaces mean that chemical or physical Fax: +86 10 82614087; Tel: +86 10 82614087 properties of components attached on substrates or substrates

Xiankun Lin received his PhD Qiang He graduated from the degree from the State Key Inner Mongolia University and Laboratory of Supramolecular received his PhD degree in 2003 Structure and Materials, Jilin from the Institute of Chemistry, University in 2010 under the the Chinese Academy of supervision of Professor Lixin Sciences (ICCAS). He then Wu, when his research mainly joined the ICCAS as an focused on the self-assembly of assistant professor and block molecule–polyoxometalate became an associate professor composites. He currently is an in 2006. He spent four years as assistant professor in Prof. a research fellow of the Qiang He’s group at Micro/ Alexander von Humboldt Nanotechnology Research Foundation in the Max Plank Center, Harbin Institute of Institute of Colloids and Inter- Xiankun Lin Technology, China. Qiang He faces, Germany. Currently he is a full Professor at the Micro/Nanotechnology Research Center, Harbin Institute of Technology, China. His research interests include self-assembled active biomimetic systems, stimuli-responsive surface patterning for biomedical application.

3584 Chem. Soc. Rev., 2012, 41, 3584–3593 This journal is c The Royal Society of Chemistry 2012 View Online

themselves gradually vary along one or more given directions. Gradients play an important role in vivo, they drive a range of biological processes from matter transport across biological membranes to the motion of proteins. Engineering gradients on surfaces could thus provide not only in vivo models for better understanding of biological processes, but also tools to mimic the biological functions. Moreover, gradients are also important characteristics or requirement of advanced materials. As a special type of surface, polymer brush gradients could provide one or more controllable parameters on a substrate, and render a powerful platform for high-throughput analysis to cut down the experimental time and deviation.6–11 Usually, the surface gradient of polymer brushes is divided into a physical gradient and chemical gradient (Fig. 1a). The physical gradient means the variations of physical properties such as topography and surface wettability. As an example for physical gradient, Zhang et al. prepared a topography gradient through applying a temperature field to uniform polystyrene (PS) brushes, due to the difference of the chain mobility under different temperatures.12 Such a topography gradient shows a gradual variation of wettability. In most cases, researchers concentrate more on the chemical gradient of polymer brushes and in particular on the gradual variations of grafting densities and/or molecular weights of polymer chains along one or more directions. They are generally prepared by employing various gradient fields on the initiator densities or polymerization processes for polymer brushes (Fig. 1a). These gradient fields include a temperature gradient,13 a concentration gradient derived from the diffusion in gas14,15 or in solution, and a gradient of the exposure time under UV light, ozone, and monomer solutions. The latter can be realized through a mobile mask,16 pulling the substrates from the reaction solutions,17 or draining the reaction solutions gradually.18,19 They provide

Downloaded by Harbin Institute of Technology on 17 April 2012 simple and cheap methods for the primary control to surface

Published on 16 March 2012 http://pubs.rsc.org | doi:10.1039/C2CS15316E gradients. For instance, Genzer et al. fabricated orthogonal polymer brush gradients with varied grafting densities and molecular weights in orthogonal directions and have shown the Fig. 1 (a) Possible routes for the fabrication of polymer brush tailored adhesion of cells on such gradient substrates.20,21 These gradients; (b) (AFM) image of a typical approaches for the fabrication of surface-bound gradients patterned polymer brush gradients prepared by nanolithography have been reviewed previously.6–8,11 These gradients are enough combined with surface-initiated living polymerization. for those applications which do not need a precise boundary of polymer brush gradients. However, many other specific studies or applications require elaborate confined gradients or predefined Junbai Li is a full Professor of complex gradients on the nanometre size. Obviously, the above- Chemistry and Director of the mentioned methods cannot satisfy the request and thus, new CAS Key Lab of Colloid and approaches such as various lithographical techniques have to be Interface Science, Institute of introduced. Several lithographical techniques combined with the Chemistry, the Chinese Acad- surface-initiated living polymerization have actually been proved emy of Sciences (ICCAS). He to be powerful tools for tailorable gradient polymer brushes received his BSc and PhD (Fig. 1b). degrees from Jilin University. Regular preparation of polymer brushes uses the ‘‘grafting He then spent two years as a from’’ strategy, but two other methods, self-assembled mono- Postdoctoral fellow at the Max layers (SAMs) and surface-initiated polymerization (SIP), are Planck Institute of Colloids and also crucial in the tool box for fabricating gradient surfaces by Interfaces in Germany and later he had got a collaborative project lithographies. SAMs afford diverse advantages for the preparation for some years. His research of polymer brushes, including simple preparation, well-defined 22 Junbai Li interests involves molecular structures, and easy modification and patterning. After SAMs assembly of biomimetic systems, are modified with initiators, SIP can be performed to synthesize biointerfaces and nanostructures. polymer brushes with controllable structures and polymerization

This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 3584–3593 3585 View Online

degrees. An example of a typical SIP is surface-initiated atom surfaces.28 However, they are not suitable for the fabrication transfer radical polymerization (SI-ATRP). It has been used of gradient surfaces due to their dependence on master widely, because it can provide surface-bound (including patterns. In general, the lithographies which are compatible block copolymers) with controllable molecular weights and low with the direct-writing systems are preferred for the fabrication polydispersities from a wide spectrum of monomers under mild of gradient or arbitrary patterned polymer brushes. In view of conditions.23–27 this, the lithograhies based on electron beams, X-rays, , Generally, two strategies can be used to combine SAMs, and probes have been developed to design confined polymer SIP, and lithographies for the fabrication of the gradient brush gradients. polymer brushes: (i) performing SIP after the lithography process of SAMs and (ii) processing pre-prepared polymer brushes by lithographies. In the beginning of the former case, Electron beam lithography SAMs are treated with lithographies to produce patterns or Electron beam lithography (EBL) uses a focused electron 2D gradients, and the treated areas can be directly activated, beam (e-beam) to fabricate nanostructures with feature sizes chemically modified or backfilled with initiator-bearing molecules. below 50 nm.29 Usually, a scanning electron microscope Subsequently, SIP is performed to generate polymer brushes with (SEM) with a direct-writing system is employed. Although complex morphologies. In contrast, in the latter case, the polymer EBL requires expensive instruments, complex operations (such brushes are prepared on the SAMs through SIP, and then are as ultrahigh vacuum), and the long processing time, it is still modified through lithographies to give gradient surfaces. In most one of the most powerful techniques to prepare arbitrary or preparations of polymer brushes using lithographies, the relations gradient nanostructures. of the brush heights with the grafting densities and lateral sizes are Ahn et al.30 and Kaholek et al.31 have employed so-called crucial (Fig. 2). Under the same polymerization condition, the lift-off EBL to produce defined Au patterns which can be conformation of polymer brushes can undergo a change from a further modified using initiators for the synthesis of polymer ‘‘pancake’’ to a ‘‘brush’’ one through an intermediate ‘‘mushroom’’ brushes by SI-ATRP (Fig. 3). By using this method, the state when increasing the grafting densities, while the brush height poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopro- will reduce when the lateral size is decreased. We will discuss these pylacrylamide-co-methacrylic acid) brushes with complex patterns relationships in detail in the following sections. (e.g., a line and dot mixed pattern) have been prepared. The In this review, we will mainly focus on the progress of the former polymer is temperature-sensitive, while the latter fabrication of gradient or arbitrary nanostructured polymer copolymer is pH- and salt-sensitive. Although complex patterns brushes through various lithographies and SIP techniques. In can be obtained, using such a method requires the effective addition, the application of gradient polymer brushes in the cooperation of multiple steps. biology-related fields will also be discussed. The general intro- Combining specific chemical reactions (such as electron 24,25 7 duction about SIP, gradient surfaces, and patterned polymer induced crosslinking or reduction) and EBL, electron beam 4,5 brushes can be found in previously published reviews. chemical lithography (EBCL) has been developed for well- Downloaded by Harbin Institute of Technology on 17 April 2012 defined functional SAMs or gradient polymer brushes.30–37 In Published on 16 March 2012 http://pubs.rsc.org | doi:10.1039/C2CS15316E a typical process (Fig. 4),32 when exposing the well-defined Lithographies for polymer brush gradients SAMs to e-beam, the molecules (such as 40-nitro-1,10-biphenyl- Some lithographies, such as microcontact printing (mCP), 4-thiol (NBT)) in the irradiated areas can be crosslinked to , and , have been enhance the stability of films for the following steps and be employed broadly for patterning functional polymers on various activated through the conversion of the terminal nitro groups

Fig. 3 (a) Schematic of the preparation of PNIPAM brushes with complex patterns by lift-off EBL and SI-ATRP. (b) AFM height image and the corresponding height profile of a combined PNIPAM-brush Fig. 2 The dependence of the height of patterned polymer brushes on line micropattern and a dot nanopattern. Reproduced with permission (a) the grafting density and (b) the lateral size. from ref. 30. Copyright 2004 Wiley–VCH.

3586 Chem. Soc. Rev., 2012, 41, 3584–3593 This journal is c The Royal Society of Chemistry 2012 View Online

Fig. 4 Scheme of electron induced chemical lithography: (a) An electron beam converts the terminal nitro groups of a NBT monolayer to amino groups while the underlying aromatic layer is cross-linked. (b) The cross- Fig. 5 AFM image of nanostructured PNIPAM brushes prepared by linked aminobiphenylthiol region is used for the further modification. EBCL. Reproduced with permission from ref. 33. Copyright 2007 Reproduced with permission from ref. 32. Copyright 2001 Wiley–VCH. American Chemical Society.

to amino groups for binding the ATRP initiators (such as laterally, which leads to the lower height of polymer brushes bromoisobutyryl bromide (BIBB)) or other functional groups. under the uniform polymerization condition. Under the same At last, polymer brushes can be grown from the patterned electron doses, the brush heights have a dependence on the initiators through SIP. Although the EBL techniques which diameters of the brush dots, with smaller diameters producing work with masks are powerful tools for patterning polymer lower heights. This effect is pronounced when the length of brushes, to access arbitrary nanostructures or gradients requires the polymer chains is analogous to the lateral size of the the combination of a direct-writing system with the appropriate nanostructured brushes. Based on these relationships and the chemistry. procedure developed by He et al., complex patterned polymer He et al. fabricated PNIPAM brushes through combining brushes with tailorable gradients can be produced with the EBCL and SI-ATRP and investigated the adhesion of fibroblasts resolution on the nanometre scales. 33 Downloaded by Harbin Institute of Technology on 17 April 2012 on the patterned polymer brushes. In this work, two methods Schmelmer et al. reported a similar process by using surface-

Published on 16 March 2012 http://pubs.rsc.org | doi:10.1039/C2CS15316E were employed: (1) a flood gun with low energy (50 eV) and a initiated photopolymerization (SIPP) and showed that nano- stencil mask were used to prepare the array of micrometre-size structured polymer brushes with feature sizes below 50 nm can polymer brush dots; (2) a direct-writing system with a high be prepared.38 The authors also found the merger of the energy e-beam (3 KeV) was used to produce arbitrary patterns. neighboring nanostructured polymer brushes when their spacings The authors found that the circumference heights of the brush are below the threshold of ca. 50–70 nm. Steenackers et al.39 dots are larger than the inner heights, which should derive from prepared PS brushes by SIPP and showed that the grafting the higher electron dose due to the electron scattering near the densities of initiators and thus the resulting brush heights can be rims of holes in the mask. This result suggests that the brush controlled by tuning the electron doses under the specific threshold heights can be tuned through adjusting the electron doses. values. Through fabricating a structure array of polymer brushes Using the direct-writing method, 70-nm line-width brushes with varied lateral sizes and irradiation dosages, the brush heights have been obtained (Fig. 5). These nanostructured brushes have been shown to be dependent on their lateral sizes. have larger lateral sizes than the underlying exposed SAMs It is very important to explore the mechanical properties of and the brush heights have a dependence of the lateral sizes. polymer brushes, especially for cell culture studies. Besides the The thermosensitivity of PNIPAM brushes has also been dependence of the polymer brush heights on the electron explored. The brush heights can increase more than double dosage and their lateral sizes, He et al.34 have also found that when the temperature changes from 40 to 20 1C. the swelling behavior and Young’s modulus of nanopatterned Based on the previous finding, He et al.34 studied the polymer brushes are very similar to those of bulk PNIPAM dependence of the polymer brush heights on electron dosages materials. According to the force–distance curves measured by or grafting densities by employing EBCL and SI-ATRP AFM, the Young modulus E of PNIPAM brushes was (Fig. 6) In this study, the brush height increases with the determined as 10.6 kPa at 25 1C, while the E values at 40 1C electron dosage under a saturated value of about 40 mC cm2 cannot be estimated by this method because of the rough (Fig. 7). This phenomenon should derive from two points: (1) surface of the collapsed brushes. the surface density of the amino groups and thus the initiator The scaling relation between heights with lateral sizes, chain density increase with the electron dose; (2) at a lower initiator lengths, and grafting densities of nanostructured polymer density, the polymer chains have more space to spread brushes has also been demonstrated theoretically using molecular

This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 3584–3593 3587 View Online

Generally, the polymer brushes formed by EBCL have different chemical compositions from the surrounding areas, which results in complicated surfaces with varied morphologies and surface energies. Schilp et al.43 reported a technique to fabricate full- coverage polymer brushes with chemically homogeneous surfaces. The key to this technique is the 11-aminoundecanethiol hydro- chloride SAMs. The amino groups can be used as the initiators for the growth of PNIPAM by SI-ATRP, however their activity is repressed partly by some oxygen-containing quenchers as proposed by the authors. But the electron irradiation can remove the quenchers and activate the amino sites for binding ATRP initiators, which results in the different activities for SI-ATRP between the irradiated and non-irradiation areas. As a result, full-coverage polymer brushes with complicated gradients can be prepared. EBCL has also shown great potential for the fabrication of protein chips. Winkler et al. have realized the fabrication of micrometre-size protein-resistance gradients by combining the irradiation-promoted exchange reaction (IPER) and EBCL (Fig. 9).44 In this study, dodecanethiolate (DDT) SAMs on Au substrates were employed. In the common condition, the substitution of DDT by a heptaethylene glycol terminated

undecane-thiol (HO(CH2CH2O)7(CH2)11SH, EG7UT) is very difficult. However, electron irradiation-induced defects are readily substituted by EG7UT. Moreover, exorbitant electron dosages can induce the cross-linking of molecules, which is disadvantageous to the exchange reaction. Thus, by combining EBL, the controllable irradiation strengths and locals can result in the tailorable EG7UT gradients. This example suggests a possibility to prepare polymer brush gradients by combining EBCL and a displacement strategy. As an impressive progress of EBL, Steenackers et al. have developed a new technique called carbon templating (CT)

Downloaded by Harbin Institute of Technology on 17 April 2012 through combing electron-beam induced carbon deposition

Published on 16 March 2012 http://pubs.rsc.org | doi:10.1039/C2CS15316E (EBCD) and the self-initiated photografting and photopoly- merization (SIPGP) of vinyl monomers (Fig. 10).45,46 The obvious advantages of such a method are resist-free and suitable to wide kinds of substrates. In the EBCD, the carbonaceous deposits mainly derive from the residual hydro- carbons in the vacuum chamber of an electron microscope. The carbon template gradients produced by controlling the e-beam irradiation can be further magnified into polymer Fig. 6 (a) Schematic of the fabrication of polymer brushes by EBCL brush gradients by SIPGP. The high thermal and chemical and SI-ATRP: i) exposing a NBT SAM using e-beam, ii) binding the stability of the carbonaceous deposits is beneficial to the initiators, iii) polymerization. (b, c) AFM images of complex patterned further modification of the grafting polymer brushes even polymer brushes. Reproduced with permission from ref. 34. Copyright 2007 Wiley–VCH. under harsh conditions. The authors have prepared polymer brush gradients on Si, Si3N4, Ge, GaAs, GaN, and Al with dynamics simulation.40 Moreover, Lee et al. have verified a native oxide layer. Recently, Steenackers et al. have experimentally the modeling results (Fig. 8).41 These relations realized the preparation of gradient polymer brushes on among the brush heights, the electron dosages, and the lateral Si–OH terminated (0001) 6H-SiC surfaces through the CT sizes provide the physical base for the ability to fabricate technique, although Si–OH terminated (0001) 6H-SiC itself is complex or gradient topographic surfaces based on polymer not suitable for SIPGP of styrene and N,N-dimethylamino- brushes. ethyl methacrylate.47 The same technique has also been used In the following studies, the method that uses EBCL to to produce polymer brush gradients on the hydrogenated fabricate polymer brush gradients has been improved and nanocrystalline diamond (NCD) and the obtained brushes expended. Ballav et al. changed the resist materials from can be conjugated with green fluorescent protein (GFP) to aromatic to aliphatic SAMs for EBCL.42 The method used give a protein density gradient.48 The CT technique has been the commercially available molecules and has reduced the proved to be compatible with graphene and can give the required electron dosage by at least an order of magnitudes. gradients of polymer brushes on graphene.49

3588 Chem. Soc. Rev., 2012, 41, 3584–3593 This journal is c The Royal Society of Chemistry 2012 View Online

Fig. 7 a) AFM image of PNIPAM brushes obtained after writing in a NBT SAM with different electron doses and the corresponding height profile. The colors of red, blue, and green represent the brush heights located in the ranges of 0–25 nm, 25–50 nm, and 50–75 nm, respectively. b) The relation between the electron dose and the brush height of PNIPAM dot patterns. c) Brush height as a function of dot diameter etched using different electron doses. Reproduced with permission from ref. 34. Copyright 2007 Wiley–VCH. Downloaded by Harbin Institute of Technology on 17 April 2012

Published on 16 March 2012 http://pubs.rsc.org | doi:10.1039/C2CS15316E A promising method for patterning polymer brushes by EBL developed recently by Ober et al., is to directly write the pre-prepared polymer brushes.50 This method can produce the polymer brushes with uniformity and sharp edges. To use suitable developers (e.g., some organic solvents) is important to reduce the degree of chain relaxation. When the block polymer brushes serve as the resists, complex structures with channels can be obtained.51 In principle, such a method could also be used to fabricate gradient polymer brushes.

Laser-based lithography Employing the interactions of photons with surfaces is a rapid and simple method to fabricate micro- and nanostructured patterns. Although photolithography has become a routine method for the semiconductor industry, it is not suitable for the fabrication of gradient surfaces due to the dependence of masks. However, some novel laser-based lithographies, such as direct-write multiphoton lithography,52 mass spectrometry assisted lithography,53 and interference lithography,54 have been developed, showing considerable applicability for the Fig. 8 a) SEM images of initial Au nanopatterns and AFM images of fabrication of gradient surfaces and arbitrary patterns. resulting PNIPAM polymer brushes. Here, n is the width of the Au Femtosecond laser pulses are able to induce multiphoton nanopatterns and h is the sum of heights of polymer brushes and the Au absorption and have been employed in the microfabrication of patterns. b) The result from molecular-dynamics simulations. Reproduced microelectromechanical systems (MEMS) due to their non-linear with permission from ref. 41. Copyright 2007 Wiley–VCH.

This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 3584–3593 3589 View Online

Fig. 9 The mechanism of the irradiation-promoted exchange reaction. Reproduced with permission from ref. 44. Copyright 2008 Wiley–VCH. Downloaded by Harbin Institute of Technology on 17 April 2012 Published on 16 March 2012 http://pubs.rsc.org | doi:10.1039/C2CS15316E

Fig. 10 a) Schematic of the preparation of carbon template gradients by EBCD and the formation of polymer brush gradients by SIPGP. b) An AFM image and the corresponding height profile of a carbon template gradient. c) An AFM image and the corresponding height profile of the resulting PS brush gradient. Reproduced with permission from ref. 45. Copyright 2009 Wiley–VCH.

processing characteristics.55 Femtosecond laser-induced ablation substrate areas are readily modified for imaging and the control of can achieve the modified structures with sub-diffraction-limit the cell adhesion. In principle, this lithography should be able to resolutions. Jeon and Schmidt et al. showed that direct-write directly produce the gradients of polymer brushes themselves. multiphoton lithography can be used to form the density gradients Mass spectrometry assisted lithography is actually a kind of of ablated defects in ultrathin polymer films.52 The authors -based lithography. It is performed by the produced an ultrathin polyethyleneglycol monomethacrylate brush matrix-assisted laser desorption/ionization time-of-flight mass film with low surface roughness through SI-ATRP, and then spectrometry (MALDI-TOF MS) using an erasing–backfilling desorbed locally the polymer brushes to expose the underlying strategy. The advantages of this lithography include employing quartz substrate with the femtosecond laser pulse. This strategy is a more freely available equipment and the ability to directly post-processing method to give the patterns or gradients of the monitor and control the erasing–backfilling process by the defects. Because the ablation threshold of polymer brushes is MALDI-TOF MS. The authors have shown that arbitrary smaller than that of the quartz, the polymer can be removed with microstructures and gradients of the cell adhesion ligands on the quartz substrate intact. Both the laser energy and the film SAMs can be fabricated, although nanostructures are difficult thickness are important factors to determine the pore sizes the to access.53 This lithography should also be available for minimum values of which can reach ca. 80 nm. The exposed gradient polymer brushes.

3590 Chem. Soc. Rev., 2012, 41, 3584–3593 This journal is c The Royal Society of Chemistry 2012 View Online

Fig. 11 Schematic of (a) UV interference lithography and (b) the resulting concentration gradient of active initiators. c) Overlay of an AFM image with a scheme of the gradient polymer brushes. Reproduced with permission from ref. 54. Copyright 2009 Wiley–VCH.

Schuh et al. have combined UV interference lithography and SI-ATRP to fabricate gradient polymer brushes.54 Using a Lloyd setup, sinusoidal light intensity distribution can be produced. The UV light with higher energies can decompose more initiators, which results in the sinusoidal concentration gradients of the active initiators for the polymer brush preparation (Fig. 11). Although such lithography is a simple and fast method to create the arrays of nanostructures with tunable and steep gradients, arbitrary gradients are difficult to access.

Scanning probe lithography Fig. 12 a) Schematic of the fabrication of polymer brushes by DNL. Besides serving as powerful investigation tools in surface b) 3D topographic views and average cross-sectional profiles of three Downloaded by Harbin Institute of Technology on 17 April 2012 sciences, scanning probe microscopy, typically including typical morphologies of the polymer brushes with different lateral 56,57 Published on 16 March 2012 http://pubs.rsc.org | doi:10.1039/C2CS15316E scanning tunneling microscopy (STM) and AFM, are also spacings. c) Gray-scale image of the Mona Lisa. d) AFM topographic powerful tools for surface patterning, named as scanning image of the Mona Lisa made by polymer brushes. Adapted with probe lithography (SPL). SPL has been employed widely for permission from ref. 58 (Copyright 2010 Royal Society of Chemistry) patterning SAMs and polymer brushes on the nanometre and ref. 59 (Copyright 2011 Wiley–VCH). scale, which has been summarized by Zauscher et al.4 SPL has recently shown its advantages for fabricating polymer Application in biological-related fields brush gradients and a typical example is the dip-pen nanodis- placement lithography (DNL). The research on gradient polymer brushes based on lithographies Liu et al. have reported the fabrication of nanostructured is still in its early stage and most studies have only focused on the and gradient polymer brushes by DNL.58 As a special preparation methods. Here, we will briefly describe the potential development of dip-pen lithography, tips in DNL can shave application of gradient polymer brushes on biological-related away the thiol-containing SAMs and backfill the uncovered fields by highlighting some typical examples. areas with the thiol-bearing initiators from inks at the same Bhat et al. showed that cell adhesion could be controlled time (Fig. 12a). In this process, the applied force value is a using gradient polymer brushes.20 In this study, an orthogonal crucial factor and can influence the grafting densities. Unlike gradient of poly(2-hydroxyethyl methacrylate) (PHEMA) with EBL, DNL can operate in ambient conditions with compared varied polymer molecular weights (MW) and grafting densities resolutions. (s) in two orthogonal directions was prepared, using a method Similar to the dependence of brush heights on their lateral developed by the authors (Fig. 13). The hydrophilic polymer sizes, the spacing distances between nanostructured brushes PHEMA is able to prevent adsorption of proteins to its are also able to determine the brush heights, which has been surface. It behaves like polyethylene glycol (PEG) but has a proved experimentally by Zhou et al. through creating the lower efficiency. Fibronectin (FN) is an extracellular matrix arrays of uniform lines or dots by DNL.59 Based on the same (ECM) protein, which contains the amino acid sequence RGD principal, the authors have developed a method to produce (Arg–Gly–Asp) and can facilitate cell adhesion through the arbitrary patterned polymer brushes from a gray-scale image interaction with the transmembrane integrin receptors on cell such as the Mona Lisa (Fig. 12c). surfaces. The study demonstrated that an increase in the

This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 3584–3593 3591 View Online

can be modulated through changing the slope and the direction of cell migration. The experimental cells tend to migrate faster down the gradients than up the gradients. In fact, SAMs are employed in this example. However, polymer brushes should also be suitable for the fabrication of dynamic gradient surfaces and can realize the cell migration under more kinds of stimuli and stable surface modification. So, confined gradient polymer brushes present a powerful platform to investigate adhesion, migration, culture, and differentiation of cells as well as the adsorption of proteins.

Conclusion and outlook Nanolithographic techniques have proved to be powerful tools in the fabrication of confined or predefined polymer brush gradients through combining SAMs and SIP techniques. The interactions of electron beams, lasers, and probes with molecules can be employed to modify SAMs and produce the gradients of initiator grafting densities or directly write Fig. 13 Left: contour plots of dry thicknesses of (a) PHEMA and (b) pre-prepared polymer brushes. The knowledge derived from FN. Right: fluorescence microscopy images of labeled MC3T3-E1 cells previous work on the dependence of brush heights on the cultured on PHEMA/FN gradients. Reproduced with permission lateral sizes of brushes themselves, electron dosages, forces from ref. 20. Copyright 2005 Wiley–VCH. applied on tips, and spacing distances between nanostructures, can be used to direct the transition from 2D grafting gradients PHEMA thickness led to a decrease in the FN adsorption and to 3D topographical nanostructures. thus a decrease in the density of adhered osteoblastic cells However, the related research is still in its infancy and there MC3T3-E1. Interestingly, with the increase of the PHEMA arealsomanyareaswhichneedtobeexplored.Theseareas thickness, the cell shape changes from a well-spread polygonal include: (1) fabrication of ‘‘smart’’ gradients with stimuli-responsive phenotype to an elongated spindle-shaped phenotype. In the properties for use with cell cultures;60,61 (2) combining more SIP latter case, the elongated cells are prone to from multilayers. The methods to realize the preparation of polymer brush gradients with study demonstrated that gradient polymer brushes are a useful new kinds of monomers;62 (3) to apply new nanolithographic tool for fundamental research into surface texture and nature on techniques, most of which have been employed for patterning the control of protein adsorption and cell behavior (i.e., adhesion SAMs and polymer brushes, but not for polymer brush gradients; and differentiation). This can be useful for practical applications

Downloaded by Harbin Institute of Technology on 17 April 2012 (4) deeper research into the cell behavior on the gradient polymer such as implantable man-made materials and tissue engineering. Published on 16 March 2012 http://pubs.rsc.org | doi:10.1039/C2CS15316E brushes and screening specific gradients to control the behavior of Besides controlling the densities and shape of adherent cells, the specific cells; (5) transition the polymer brush gradients to other gradient surfaces also have the ability to modulate the materials such as inorganic materials for broader applications; (6) migration of cells through the combination of patterning and developing methods to reduce the total cost of fabrication of stimuli-responsive functional molecules. Luo et al. have shown polymer brush gradients. that a gradient pattern can be electrochemically activated in the presence of cells to covalently attach the adhesive peptide RGD and trigger the tissue migration along the resulting RGD Acknowledgements gradient (Fig. 14).60 Meanwhile, the speed of tissue morphing This work was supported by the National Nature Science Foundation of China (91027045 and 21103034), 100-talent Program of HIT, the Fundamental Research Funds for the Central University (HIT.ICRST.2010003), Hei Long Jiang Postdoctoral Foundation. Q. He thanks Prof M. Grunze and Dr A. Kueller for valuable discussion and Dr R. Cook to polish the English.

References 1 R. C. Advincula, W. J. Brittain, K. C. Caster and J. Ru¨he, ed., Polymer Brushes: Synthesis, Characterization, Applications, WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2004. 2 S. T. Milner, T. A. Witten and M. E. Cates, Macromolecules, 1988, 21, 2610. Fig. 14 Fluorescent micrographs showing spatial and temporal con- 3 B. Zhao and W. J. Brittain, Prog. Polym. Sci., 2000, 25, 677. trol of tissue morphing by using dynamic gradients. Reproduced with 4 R. Ducker, A. Garcia, J. Zhang, T. Chen and S. Zauscher, permission from ref. 60. Copyright 2011 American Chemical Society. Soft Matter, 2008, 4, 1774.

3592 Chem. Soc. Rev., 2012, 41, 3584–3593 This journal is c The Royal Society of Chemistry 2012 View Online

5 S. V. Orski, K. H. Fries, S. K. Sontag and J. Locklin, J. Mater. 37 N. Ballav, A. Shaporenko, A. Terfort and M. Zharnikov, Adv. Chem., 2011, 21, 14135. Mater., 2007, 19, 998. 6 R. R. Bhat, M. R. Tomlinson, T. Wu and J. Genzer, Adv. Polym. 38 U. Schmelmer, A. Paul, A. Ku¨ller, M. Steenackers, A. Ulman, Sci., 2006, 198, 51. M. Grunze, A. Go¨lzha¨user and R. Jordan, Small, 2007, 3, 459. 7 J. Genzer and R. R. Bhat, Langmuir, 2008, 24, 2294. 39 M. Steenackers, A. Ku¨ller, N. Ballav, M. Zharnikov, M. Grunze 8 M.S.Kim,G.KhangandH.B.Lee,Prog. Polym. Sci., 2008, 33,138. and R. Jordan, Small, 2007, 3, 1764. 9 I. Luzinov, S. Minko and V. V. Tsukruk, Soft Matter, 2008, 4, 714. 40 M. Patra and P. Linse, Nano Lett., 2005, 6, 133. 10 J. Zhang and Y. Han, Chem. Soc. Rev., 2010, 39, 676. 41 W. K. Lee, M. Patra, P. Linse and S. Zauscher, Small, 2007, 3, 63. 11 S. Morgenthaler, C. Zink and N. D. Spencer, Soft Matter, 2008, 42 N. Ballav, S. Schilp and M. Zharnikov, Angew. Chem., Int. Ed., 4, 419. 2008, 47, 1421. 12 J. Zhang, L. Xue and Y. Han, Langmuir, 2004, 21,5. 43 S. Schilp, N. Ballav and M. Zharnikov, Angew. Chem., Int. Ed., 13 L. Ionov, B. Zdyrko, A. Sidorenko, S. Minko, V. Klep, I. Luzinov 2008, 47, 6786. and M. Stamm, Macromol. Rapid Commun., 2004, 25, 360. 44 T. Winkler, N. Ballav, H. Thomas, M. Zharnikov and A. Terfort, 14 M. K. Chaudhury and G. M. Whitesides, Science, 1992, 1539. Angew. Chem., Int. Ed., 2008, 47, 7238. 15 T.Wu,K.EfimenkoandJ.Genzer,J. Am. Chem. Soc., 2002, 124, 9394. 45 M. Steenackers, R. Jordan, A. Ku¨ller and M. Grunze, Adv. Mater., 16 J. H. Lee, H. G. Kim, G. S. Khang, H. B. Lee and M. S. Jhon, 2009, 21, 2921. J. Colloid Interface Sci., 1992, 151, 563. 46 M. Steenackers, A. Ku¨ller, S. Stoycheva, M. Grunze and 17 S. Morgenthaler, S. Lee, S. Zu¨rcher and N. D. Spencer, Langmuir, R. Jordan, Langmuir, 2009, 25, 2225. 2003, 19, 10459. 47 M. Steenackers, I. D. Sharp, K. Larsson, N. A. Hutter, 18 M. R. Tomlinson and J. Genzer, Macromolecules, 2003, 36, 3449. M. Stutzmann and R. Jordan, Chem. Mater., 2009, 22, 272. 19 L. H. Li, Y. Zhu, B. Li and C. Y. Gao, Langmuir, 2008, 24, 13632. 48 N. A. Hutter, M. Steenackers, A. Reitinger, O. A. Williams, 20 R. R. Bhat, B. N. Chaney, J. Rowley, A. Liebmann-Vinson and J. A. Garrido and R. Jordan, Soft Matter, 2011, 7, 4861. J. Genzer, Adv. Mater., 2005, 17, 2802. 49 M. Steenackers, A. M. Gigler, N. Zhang, F. Deubel, M. Seifert, 21 R. R. Bhat, M. R. Tomlinson and J. Genzer, J. Polym. Sci., Part L. H. Hess, C. H. Y. X. Lim, K. P. Loh, J. A. Garrido, R. Jordan, B: Polym. Phys., 2005, 43, 3384. M. Stutzmann and I. D. Sharp, J. Am. Chem. Soc., 2011, 22 J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo and 133, 10490. G. M. Whitesides, Chem. Rev., 2005, 105, 1103. 50 A. Rastogi, M. Y. Paik, M. Tanaka and C. K. Ober, ACS Nano, 23 D. M. Jones and W. T. S. Huck, Adv. Mater., 2001, 13, 1256. 2010, 4, 771. 24 S. Edmondson, V. L. Osborne and W. T. S. Huck, Chem. Soc. 51 M. Y. Paik, Y. Xu, A. Rastogi, M. Tanaka, Y. Yi and C. K. Ober, Rev., 2004, 33, 14. Nano Lett., 2010, 10, 3873. 25 R. Barbey, L. Lavanant, D. Paripovic, N. Schu¨wer, C. Sugnaux, 52 H. Jeon, R. Schmidt, J. E. Barton, D. J. Hwang, L. J. Gamble, S. Tugulu and H.-A. Klok, Chem. Rev., 2009, 109, 5437. D. G. Castner, C. P. Grigoropoulos and K. E. Healy, J. Am. Chem. 26 J. Pyun, T. Kowalewski and K. Matyjaszewski, Macromol. Rapid Soc., 2011, 133, 6138. Commun., 2003, 24, 1043. 53 Y.-K. Kim, S.-R. Ryoo, S.-J. Kwack and D.-H. Min, Angew. 27 D. M. Jones, A. A. Brown and W. T. S. Huck, Langmuir, 2002, Chem., Int. Ed., 2009, 48, 3507. 18, 1265. 54 C. Schuh, S. Santer, O. Prucker and J. Ru¨he, Adv. Mater., 2009, 28 Z. Nie and E. Kumacheva, Nat. Mater., 2008, 7, 277. 21, 4706. 29 A. del Campo and E. Arzt, Chem. Rev., 2008, 108, 911. 55 Y.-L. Zhang, Q.-D. Chen, H. Xia and H.-B. Sun, Nano Today, 30 S. J. Ahn, M. Kaholek, W. K. Lee, B. LaMattina, T. H. LaBean 2010, 5, 435. and S. Zauscher, Adv. Mater., 2004, 16, 2141. 56 D. S. Ginger, H. Zhang and C. A. Mirkin, Angew. Chem., Int. Ed., 31 M. Kaholek, W.-K. Lee, J. Feng, B. LaMattina, D. J. Dyer and 2004, 43, 30. S. Zauscher, Chem. Mater., 2006, 18, 3660. 57 K. Salaita, Y. Wang and C. A. Mirkin, Nat. Nanotechnol., 2007,

Downloaded by Harbin Institute of Technology on 17 April 2012 32 A. Go¨lzha¨user, W. Eck, W. Geyer, V. Stadler, T. Weimann, 2, 145. P. Hinze and M. Grunze, Adv. Mater., 2001, 13, 806. 58 X. Liu, Y. Li and Z. Zheng, Nanoscale, 2010, 2, 2614. Published on 16 March 2012 http://pubs.rsc.org | doi:10.1039/C2CS15316E 33 Q. He, A. Ku¨ller, M. Grunze and J. Li, Langmuir, 2007, 23, 3981. 59 X. Zhou, X. Wang, Y. Shen, Z. Xie and Z. Zheng, Angew. Chem., 34 Q. He, A. Ku¨ller, S. Schilp, F. Leisten, H.-A. Kolb, M. Grunze and Int. Ed., 2011, 50, 6506. J. Li, Small, 2007, 3, 1860. 60 W. Luo and M. N. Yousaf, J. Am. Chem. Soc., 2011, 133, 10780. 35 W. Eck, V. Stadler, W. Geyer, M. Zharnikov, A. Go¨lzha¨user and 61 L. Ionov, N. Houbenov, A. Sidorenko, M. Stamm, I. Luzinov and M. Grunze, Adv. Mater., 2000, 12, 805. S. Minko, Langmuir, 2004, 20, 9916. 36 U. Schmelmer, R. Jordan, W. Geyer, W. Eck, A. Go¨lzha¨user, 62 M. Ballauff and O. Borisov, Curr. Opin. Colloid Interface Sci., M. Grunze and A. Ulman, Angew. Chem., Int. Ed., 2003, 42, 559. 2006, 11, 316.

This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 3584–3593 3593