Submicro-Nanopatterned

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Highly Tunable Complementary Micro/Submicro-Nano Patterned Surfaces Combining Block Copolymer Self-Assembly and Colloidal Lithography Tongxin Chang, Bin-Yang Du, Haiying Huang, and Tianbai He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07730 • Publication Date (Web): 10 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016

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ACS Applied Materials & Interfaces

Highly Tunable Complementary Micro/Submicro-Nano Patterned Surfaces Combining Block Copolymer Self-Assembly and Colloidal Lithography

Tongxin Chang,a,b Binyang Du,c Haiying Huang,*a,b and Tianbai He*a,b

a

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China b

University of Chinese Academy of Sciences, Beijing, 100039, P. R. China

c

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of

Polymer Science & Engineering, and Department of Chemistry, Zhejiang University, Hangzhou, 310027, P. R. China E-mail: [email protected]; [email protected] Phone: +86-431-85262875; Fax: +86-431-85262126

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ABSTRACT: Two kinds of large-area ordered and highly tunable micro/submicro-nano patterned surfaces in a complementary manner, were successfully fabricated by elaborately combining block copolymer self-assembly and colloidal lithography. Employing monolayer of

polystyrene

(PS)

colloidal

spheres

polystyrene-block-poly(2-vinylpyridine)

assembled

(PS-b-P2VP)

on or

top

as

etching

mask,

polystyrene-block-poly(4-

vinylpyridine) (PS-b-P4VP) micelle films were patterned into micro/submicro- patches by plasma etching, which could be further transferred into micropatterned metal nanoarrays by subsequent metal precursor loading and a second plasma etching. On the other hand, micro/submicro-nano patterns in a complementary manner were generated via preloading metal precursor in initial micelle films before the assembly of PS colloidal spheres on top. Both kinds of micro/submicro-nano patterns showed good fidelity at micro/submicroscale and nanoscale, meanwhile they could be flexibly tuned by the sample and processing parameters. Significantly, when the PS colloidal sphere size was reduced to 250 nm, a high resolution submicro-nanostructured surface with 3~5 metal nanoparticles in each patch, or single-nanoparticle interconnected honeycomb network was achieved. Moreover, by applying gold (Au) nanoparticles as anchoring points, micro-nano patterned Au arrays can serve as a flexible template to pattern bovine serum albumin (BSA) molecules. This facile and cost-effective approach may provide a novel platform for fabrication of micropatterned nanoarrays with high tunability and controllability, which are promising in the applications of biological and microelectronic fields.


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KEYWORDS: block copolymer, self-assembly, micro/submicro-nano pattern, colloidal lithography, metal nanoarray, complementary structure

INTRODUCTION Micro-nano patterned structures are ideal candidates for fabricating smart miniaturized devices and architectures with high efficiency and novel capabilities in applications spanning from microelectronics to biological areas,1-6 since they enable high-density integration of regulable functional nanoscopic building blocks. For example, in the cell behavior study, the microstructured arrays are able to control the extracellular environment of a single cell or a group of cells,5-8 meanwhile the nanostructured array can elaborately regulate the ordering, density and orientation of the extracellular factor, controlling cell adhesion, spreading and differentiation.9-15 Therefore, the design and precise control over the micropatterning and nanopatterning simultaneously, is highly demanded to meet the practical requirements. To this end, combination of different patterning methods at disparate lateral length scales has been put forward in the past two decades.16-20 So far micropatterning depends mostly on the traditional lithography techniques, namely top down approaches, while nanopatterning is generally fabricated either by advanced top-down lithography procedure6, 21-23 or bottom-up self-assembly approaches.24-25 Among the various ways to generate nanostructured patterns, block copolymer (BCP) self-assembly is proved to be a promising alternate for the creation of ordered nanostructures with high resolution and good tunability,26-27 which has also been combined with a rich variety of micropatterning strategies to generate micro-nano patterned



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structures.28-43 Various top-down methods, photolithography,28-30, 36 microcontact printing31 and E-beam lithography,32-33 as well as some bottom-up controlled self-assembly strategies such as controlled dewetting,37-39 controlled evaporative process,40-42 and breath figure approach43 have been applied to position the block copolymer nanostructure at larger scale. Nevertheless emerging methods have been demonstrated for micro-nano patterning, there remains some limitations more or less encountered during the fabrication process, restricting the wide application. Microcontact printing of block copolymer micelle solution can fabricate micro-nano patterned surface directly, while the resolution is limited by the printing template and the diffusion of ink may result in rough edge and disordered inner structure.31 During photolithography process, multistep operation is needed and the coating and rinsing process of photoresist may destroy the underneath structure,29 and large-area submicro- or nanophotomask are always expensive. Although precise placement of BCP nanostructure can be realized by electron beam lithography, its wide application is confined by its high cost and low throughput.32-33 When applying a low-cost controlled assembly strategy to direct the assembly of BCP into multiscale pattern, it is generally hard to obtain high resolution patches of submicroscale.37-43 Hence a simple and effective method to achieve large area micro-nano patterned structures maintaining the ordering both at microscale and nanoscale with low cost and high resolution is an urgent and critical task. In this work, we report for the first time, a facile and effective approach for fabrication of large-area ordered micro/submicro-nano patterned arrays through the combination of colloidal lithography and block copolymer micelle self-assembly. Colloidal lithography is widely used


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in surface patterning at micro/submicroscale, since it relies on a cost-effective, flexible technology with the ability to produce patterned samples in parallel and fabricate micro/submicro- arrays with long-range order.44-45 Applying two dimentional (2D) self-assembled periodic colloidal particles and voids as the mask during colloidal lithography process, various ordered structures can be fabricated on different surfaces, such as nanoparticle arrays, nanopore arrays, nanotip arrays, nanoring arrays, etc.44-49 Although, the ordered micro-nanostructured arrays based on the monolayer colloidal crystals have been well developed, to the best of our knowledge, no fabrication strategies integrated colloidal lithography with the block copolymer micelle self-assembly, to generate well-defined micro-nano patterned arrays, providing control over the ordering both at the micro/submicroscale and nanoscale. Herein, with such patterning strategy, complementary micro/submicro-nano patterned arrays with tunable size and morphologies were successfully achieved by changing the sample and processing parameters. Ordered monolayer of PS colloidal spheres assembled at the water-air interface was lifted off onto the pre-treated hydrophilic block copolymer micelle films, afterwards the micelle film was patterned using PS colloidal spheres as etching mask through oxygen plasma treatment. In this approach, microstructures are provided by the colloidal particles, which enables the control over the ordering, patch size and periodicity at micro/submicroscale, meanwhile the ordering, density and morphology at nanoscale can be significantly altered by the different BCP parameters and micelle preparation conditions.



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Furthermore, by modifying the surface of micro-nano patterned Au arrays, it can be used as a flexible template to pattern BSA molecules. EXPERIMENTAL SECTION Materials. PS102k-b-P2VP97k with polydispersity index (PDI) of 1.13 and PS9.8k-b-P4VP10k with PDI of 1.08 were purchased from Polymer Source Inc. The number in subscripts stands for the number average molecular weight of each block. Analytical pure o-xylene and N,N-dimethylformamide (DMF) were purchased from Beijing Chemical Works. Dodecyl sodium sulfate, Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4·3H2O), Hydrogen hexachloroplatinate (IV) hydrate (H2PtCl6·6H2O) and mercaptopropionic acid (MPA) were obtained from China National Medicine Corporation Ltd. Polystyrene (PS) microsphere aqueous suspensions (10 wt%) were purchased from Duke Scientific Corporation. 3-aminopropyltriethoxysilane

(APTES)

was

purchased

from

Acros

Organic.

N-hydroxysuccinimide (NHS)-terminated polyethylene glycol (PEG2k-NHS) was kindly supplied

by

Prof.

Xuesi

Chen.

N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide

hydrochloride (EDCI) and NHS were purchased from Sigma Aldrich. Bovine serum albumin (BSA) and fluorescein isothiocyanate (FITC) were purchased from Shanghai DEMO Medical Tech Corporation Ltd. All chemicals were used as received without further purification. Preparation of block copolymer micelles. The block copolymers were dissolved in o-xylene to give a concentration of 5 mg/mL. The solutions were kept at 80 °C for 5 h and then naturally cooled down to room temperature. The micelle solutions were used after keeping at room temperature for at least 24 h. Silicon and quartz wafers were cut into pieces and


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immersed in Piranha solution (7:3 mixture of H2SO4 and H2O2) for 30 min at 120°C, and then cleaned with acetone, ethanol and deionized water, respectively, in an ultrasonic bath for 15 min. A monolayer of micelles was obtained by spin coating the corresponding micelle solutions onto the cleaned substrates at 3500 rpm (if without specific instruction). The crosslinking of PS9.8k-b-P4VP10k micelle was accomplished by exposing the micelle thin film to 1,4-diiodobutane (DIB) vapor at 50 °C for 2 h. Assembly of PS sphere. PS microspheres with diameters of 3.8 µm, 1.9 µm, 1 µm, 500 nm and 250 nm were assembled at the water-air interface, respectively, by using a method reported by Rybczyncoski.50 Briefly, PS microsphere aqueous suspensions (10 wt%) were first diluted with ethanol to 5 wt %. Afterwards, 10 µL diluted suspension was uniformly dropped onto a 2 cm × 2 cm cleaned silicon wafer. The microsphere coated silicon wafer was then slowly immersed into distilled water surface placed in a clean glass dish. An disordered discrete monolayer of PS microspheres started to appear. Several drops of dodecyl sodium sulfate solution (10 wt %) were then dripped into the distilled water to consolidate the microspheres. Finally, a large area of ordered monolayer of PS microspheres was formed at the water-air interface. Then, the ordered monolayer of PS microspheres was lifted off from the water-air interface by the substrates with the monolayer of regular BCP micelles on top, which were slightly pretreated by air plasma (18 W, 20 Pa, 10s) to increase the hydrophilicity of the surface. In order to fabricate micro-patterned nanostructure in a complementary way (as shown in Figure 1, Route 2), the monolayer of block copolymer micelles could be firstly



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immersed in H2PtCl6 aqueous solution (10 mM) for 10 min and then dried with nitrogen gas flow before assembling the monolayer of PS spheres. Fabrication of micro/submicro-nano patterned metal arrays. The micelle array covered with ordered PS microspheres was exposed to oxygen plasma at constant etching condition (50 W, 10 Pa) for various times in order to obtain predesigned micro-patterns. After plasma etching, the substrates with micro-patterns were cleaned with deionized water in an ultrasonic bath for 10 s to remove the upper PS spheres. In Route 1, after plasma etching and removal of spheres, uniform monolayer PS-b-P2VP or PS-b-P4VP micelle films were patterned into independent micro-patches, then the patterned micelle films were immersed in H2PtCl6 or HAuCl4 aqueous solution (10 mM) for 10 min and dried with nitrogen gas flow. Followed by a second time oxygen plasma etching (50 W, 10 Pa, 60 s), block polymers were etched away with metal precursors reduced to metal nanoparticles. Hence micro-nano patterned Au or Pt array were fabricated. In Route 2, after plasma etching and removal of spheres, uniform monolayer PS-b-P2VP micelles films pre-loading with H2PtCl6 were patterned into periodic alternate micelles and Pt nanoparticles. To reveal the patterned Pt nanoarray, the patterned films were immersed in DMF under an ultrasonic bath for 20 s or keeping still for 12 h to remove

the

remaining

polymers.

Followed

by

nitrogen

gas

flow

drying,

micro/submicro-patterned Pt nanostructures were developed. Patterning of proteins on Au micro/submicro-nano patterns. The BSA was patterned on Au micro-nano pattern generated by Route 1. The quartz substrates with micro patterned Au nanoparticles were firstly treated with APTES vapor at 50 °C for 1 h, and rinsed with ethanol


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and deionized water. The APTES-treated substrates were subsequently immersed in PEG-NHS dichloromethane solution (0.3 wt %) at 4 °C for 24 h, MPA aqueous solution for 4 h, and EDCI/NHS in phosphate buffer saline (PBS) solution (10 mM, pH = 7.4) for 1 h in sequence. Afterwards, the substrates were exposed to the BSA PBS solution (0.2 mg/mL) for 1 h, and then rinsed with PBS solution thoroughly to remove the physically adsorbed BSA. Finally, the BSA molecules were labeled with FITC by immersing into FITC PBS solution (0.2 mg/mL) at 4 °C for 24 h. Characterizations. The morphologies of the micelle array, PS microsphere monolayer and patterned nanostructures were characterized by various techniques. Atomic force microscopy (AFM) images were obtained by using a commercial SPA-300HV/SPI3800N AFM (Seiko Instruments Inc., Japan) operated in tapping mode. Optical images were observed by using a Carl Zeiss A2m optical microscope equipped with a CCD camera under reflectance mode. Scanning electron microscope (SEM) measurements were carried out by using a Hitachi S-4800 field emission SEM operating at 10 kV. X-ray photoelectron spectra (XPS) were recorded on a THERMO ESCALAB 250 using Al Kα X-rays with photon energy of 1486.6 eV. Fluorescent images were obtained by using a confocal laser scanning microscope (CLSM) (LSM700-Zeiss, Germany) equipped with an InGaN semiconductor laser (405 nm), an Ar laser (488 nm), and a He-Ne laser (555 nm). Grazing-incidence small-angle X-ray scattering (GISAXS) experiment was conducted at the Shanghai Synchrotron Radiation Facility (SSRF, beamline BL16B1) with λ= 0.124 nm (E = 10 keV). The incidence angle is 0.2 °, the exposure time is 60 s and the sample-to-detector distance was 5.07 m.



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RESULTS AND DISCUSSION As schematically shown in Figure 1, two main routes are applied for the fabrication of micro/submicro-nano patterned arrays. For Route 1, pure BCP micelle monolayer is patterned into discrete micro/submicro- patches after oxygen plasma treatment and the removal of PS colloidal sphere mask, followed by metal precursor loading and a second plasma etching, it is further transferred into micro/submicropatterned metal nanoarray. For Route 2, BCP micelle monolayer preloading with metal precursor is patterned into periodic alternate micelle and metal nanoparticle arrays, followed by the rinse with DMF, subsequently, micro/submicropatterned metal nanoparticle array complementary to that generated by Route 1 is obtained. Firstly, a monolayer of block copolymer micelles was prepared by spin coating the micelle solution of either PS102K-b-P2VP97K or PS9.8K-b-P4VP10K on the cleaned silicon substrates (Figure 1a, 1a’). O-xylene was chosen as solvent since it was a good solvent for PS and P2VP/P4VP blocks at high temperature but a poor solvent for P2VP/P4VP at room temperature. When spin coating the micelle solution onto the substrates, weak volatility of o-xylene facilitated the well arrangement of micelles and leaded to the formation of initial ordered micelle monolayer with a P4VP/P2VP core and PS corona (Figure 2a, 2d). The center-to-center distance of the micelles can be well controlled by varying the solution concentration, micelle size and spin-coating speed. For PS102k-b-P2VP97k micelle monolayer， the average center-to-center distance was 45 nm, 55 nm (Figure 2b), and 57 nm at a spin-coating speed of 2000 rpm, 3500 rpm and 6000 rpm respectively (Supporting Information, S1), since the micelles were spaced farther apart at high speed.50 While for

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PS9.8k-b-P4VP10k micelles, the spin-coating speed has weaker influence on the center-to-center distance of micelles due to the smaller micelle size, the average center-to-center distance was 29 nm for PS9.8k-b-P4VP10k micelles at a spin-coating speed of 3500 rpm (Figure 2a). Moreover, to avoid the destroying of the low molecular PS9.8k-b-P4VP10k micelle film during the further treating process, it is pre-crosslinked with DIB to enhance the film stability. Since P2VP/P4VP can chelate metal ions, the micelle arrays can be transferred into metal nanoarrays. Micelle films were first immersed into the given metal precursor solution for 10 min and then subjected to oxygen plasma etching. During plasma etching, polymers were etched away with the loaded metal precursors reduced to metal nanoparticles, which is demonstrated by the X-ray photoelectron spectroscopy (XPS) analysis (Figure 2c, 2f). It is clearly shown that the peak for oxidized metal atom in [PtCl6]2- (78.3 eV and 75 eV) and [AuCl4]- (91.2 eV, 87.4 eV and 85 eV) disappear after plasma reduction, replaced by the peak of metallic Pt 4f (74.4 eV and 71 eV) and Au (87.7 eV and 84 eV) 4f electron. According to the SEM characterization (Figure 2b, 2e), Au nanoarray with average nanoparticle diameter about 11 ± 2nm (Figure 2b) was obtained using PS9.8K-b-P4VP10K as template, Pt nanoarray with an average nanoparticle diameter about 21 ± 2nm (Figure 2e) was obtained using PS102K-b-P2VP97K as template. Furthermore, grazing-incidence small angle X-ray scattering (GISAXS) was employed to offer a statistical average of the ordering over large-area. The 2D GISAXS data are given in Figure 3. Bragg rods are observed along the out-of-plane scattering direction in both the Au nanoarrays prepared from PS9.8K-b-P4VP10K and PS102K-b-P2VP97K



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respectively (Figure 3a, 3b). Then qx profiles were created from the corresponding 2D GISAXS images (Figure 3c, 3d), Bragg rods (vertical streaks) up to fourth order and the ratio of peak positions relate as 1:√3:√4:√7 ，reflecting the hexagonally ordered Au nanoparticles. The period of Au nanoarray can be calculated by d = 2π/q. For Au nanoarray prepared from PS9.8K-b-P4VP10K , the first order peak ranges from 0.195 nm-1 to 0.232 nm- 1 with a peak value of 0.215 nm-1, indicating that the period of the Au nanoarray is 29 ± 2 nm, similarly, the period of the Au nanoarray from PS102K-b-P2VP97K is 55 ± 6 nm. Both period of Au nanoarrays is in good agreement with the measured average center-to-center distance of PS9.8K-b-P4VP10K and PS102K-b-P2VP97K micelles, respectively. Secondly, an ordered monolayer of PS colloidal spheres was assembled on the micelle film (Figure 1b, 1b’). PS spheres are widely used as a mask in colloidal lithography owing to its mature preparation technique and low cost. 2D assembly of PS colloidal spheres can be realized by solvent evaporation, spin-coating and Langmuir-Blodgett techniques.45 In the present study, PS spheres with a diameter of 3.8 µm, 1.9 µm, 1 µm, 500 nm and 250 nm were employed respectively. They were first assembled at the water-air interface according to the method reported by Rybczynski.51 During the assembly process, disperse PS spheres on the water surface were excluded by the small surfactant molecules, driven by the minimization of free surface energy, and ultimately self-assembled into an ordered compact monolayer on the water surface over tens of square centimeters. The ordered monolayer of PS colloidal spheres was then scheduled to lift off onto the micelle arrays, however this was hindered due to the poor spreading ability of hydrophilic PS spheres on the micelle film which was covered by


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hydrophobic PS molecules. To circumvent this, the micelle films were exposed to a short air plasma etching (18 W, 20 Pa, 10 s). After that, ordered monolayer of PS colloidal spheres with various sizes successfully lifted off onto the micelle film since the introduction of carboxyl and hydroxyl on the film surface (Supporting Information, S2). Thirdly, oxygen plasma etching was applied to pattern the micelle array using PS colloidal sphere as mask (Figure 1b, 1c). Oxygen plasma went through the voids between two adjacent PS colloidal spheres, resulting in the degradation of the micelles in the exposure area, while the shielded micelles underneath the PS colloidal spheres were protected and reserved. After plasma etching, the hydrophilic PS spheres could be easily removed from the film surface by sonicating the substrates in deionized water for 3~5 s, promoting the development of micro/submicro- micelle pattern. Figure 4a-c show the AFM images of representative micro/submicropatterned PS9.8K-b-P4VP10K micelle array after using PS colloidal sphere as etching mask with respective sphere diameters of 1.0 µm, 500 nm and 250 nm, micelle monolayers were site-selectively removed, generating discrete micro/submicro- patches with nanopatterned micelles in each patch. Moreover, our results demonstrate that the ordering from BCP micelles and PS colloidal spheres are both well maintained after the aforementioned treatment no matter micro- (3.8 µm, 1.9µm and 1 µm) or submicro- (500 nm and 250 nm) spheres are employed (Figure 4, Supporting Information, S3-S5). It is noteworthy that limited micelles were presented on a single patch when employing 250nm PS colloidal spheres (Figure 4c). Additionally, prolonging plasma etching, the PS colloidal spheres were gradually etched to a smaller size, and more micelles were etched away, finally



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leading to smaller patches (Supporting Information, S4). Briefly, micro/submicro-nano patterned surfaces are successfully prepared and they are highly tunable by different parameters, such as plasma treatment time, PS sphere size, micelle size and micelle film preparation condition. In the final step, the micro/submicro-nano patterned micelle arrays were transferred into patterned metal nanostructures (Figure1c, 1d). The patterned micelle films were loaded with the given metal precursor solution and subjected to a second oxygen plasma etching, hence metal micro/submicro- nanoarrays were obtained eventually. Micro/submicropatterned Au nanoarrays fabricated from the corresponding micro/submicropatterned PS9.8k-b-P4VP10k micelle arrays (Figure 4a-c) were illustrated in Figure 4a’-c’. Apparently, the nanoscale ordering was well kept compared with the uniform Au nanoarray prepared from PS9.8k-b-P4VP10k micelle monolayer (Figure 2b), meanwhile the micro/submicro- ordering derived from PS colloidal sphere was also well preserved over a large area (Supporting Information, S5), despite some defects on the surface, which is inevitable due to the BCP self-assembly and colloidal lithography process. Naturally, tuning the first plasma etching time, Au nanoarrays of different patch size can be obtained, as shown in Figure 5. Interconnected patches were obtained when 60 s plasma etching was applied (Figure 5a) using 1 µm PS sphere as etching mask, while discrete patches with a diameter about 857 nm, 540 nm and 270 nm were acquired when 120 s, 160 s and 200 s plasma etching were applied, respectively (Figure 5b-d). It is worth noting that the number of metal nanoparticles in each submicro- patch can be drastically reduced when prolonging the first plasma etching time and


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applying submicro- PS colloidal spheres. 16 ± 2 nanoparticles were observed in each patch when 500 nm PS colloidal sphere and an increased plasma etching time of 120 s were applied (Figure 5f), while only 4 ± 1 nanoparticles located in each patch when 250 nm PS colloidal sphere and 80 s plasma etching were applied (Figure 4c’, 5h). Such precise placement of nanoparticles with sophisticated resolution is hard to achieve using microcontact printing technology and conventional photolithography method. Onses et al. reported the successful modulation of number of Au nanoparticles per patterned feature over several micrometers by electron beam lithography of polymer brushes,22 whereas its patterning can only be realized in a small area. Herein, through characterizing Au micro-nanoarrays at different location of 2 cm ×2 cm substrate, large-area patterning ability of such fabrication approach is proved (Supporting Information, S5). Besides fabricating large-area ordered multiscale patterned metal nanodots, we can also generate multiscale patterned metal nanotoroids. When PS102k-b-P2VP97k micelle film is treated in deionized water under sonication or in acidic water solution, spherical micelle morphology will be reconstructed into nanotoroids through swelling and reconstruction of P2VP domain,52 which accordingly results in the formation of micro/submicropatterned metal nanotoroids (Supporting Information, S6). Pre-crosslinking can prevent such reconstruction process for PS-b-P4VP micelle film,53 which resulted in the micro/submicro patterned Au nanodots array, as shown in Figure 4a’-c’. It means that the morphology in each patch can also be manipulated by modifying the underlying BCP films.



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On the other hand, micro/submicro-nanostructures in a complementary manner were also fabricated by preloading metal precursors in BCP micelle monolayers before the assembly of PS sphere monolayer on top, as illustrated in Figure 1 (Route 2). In this manner, micelles loading with metal precursors within the plasma exposure area would be reduced into metal nanoparticles, accompanying with the degradation of BCPs, whereas the shield micelles would remain intact during the oxygen plasma etching process (Figure 1b’, 1c’). Thus periodic alternate micelle and metal nanoparticle arrays were observed after the removal of PS colloidal sphere mask (Supporting Information, S7). The remained micelles could be removed by sonicating the film in DMF for 20 s or immersing the film in DMF for 12 h, accordingly a honeycomb-like metal nanoparticle patterns were developed (Figure 1d’). As for Route 2, H2PtCl6 instead of HAuCl4 was chosen as preloading metal precursor, since the weaker interaction of Au nanoparticles with substrate compared to Pt nanoparticles, which might result in the destruction of metal nanoarray during the removal process of residual micelles by sonication.54 PS102k-b-P2VP97k instead of PS9.8k-b-P4VP10k was preferred because the residual micelle could be easily removed by DMF compared to PS9.8k-b-P4VP10k, even if PS9.8k-b-P4VP10k has much smaller molecular weight, which is probably due to the stronger intermolecular force of P4VP in the presence of metal ions. Figure 6 presents the AFM height images (Figure 6a-c) and SEM images (Figure 6a’-c’) of micro/submicropatterned Pt nanoparticle arrays corresponding to the Figure 4a’-c’ in a complementary way, after using PS colloidal sphere as etching mask with respective sphere diameters of 1.0 µm, 500 nm and 250 nm. The micro/submicropatterned Pt nanoarrays maintain both the ordering from PS


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colloidal spheres and uniform Pt nanoarray prepared from PS102k-b-P2VP97k micelle monolayer (Figure 2e). The ability to obtain large-area ordered patterning has been further certified by the large-area AFM and SEM characterization (Supporting Information, S8). It should be emphasized that when 250 nm PS colloidal sphere was applied as etching mask, high resolution single nanoparticle interconnected honeycomb-like network was obtained (Figure 6c and Figure 6c’), which was rarely achieved via other patterning methods. The micro/submicro-nano patterned Au arrays fabricated by the present method, can also serve as 2D scaffolds to pattern the biomolecules using Au nanoparticles as anchoring points. The ordering of metal nanoparticles at sub-50 nm scale have a great potential in control of surface orientation of biomolecules, which is essential in maintaining bioactivity of biomolecules,9 and the ordering at microscale facilitates the practical application in biosensors and cell behavior research.11,

44

Herein BSA was chosen as a model protein to test the

biomolecule patterning ability of the prepared micro-nano patterned Au arrays. First we successfully grafted APTES onto the SiO2 area (none-Au area) from the gas phase at elevated temperature (50 °C). A protein-repellent background of PEG molecules was then introduced by chemically grafting PEG-NHS to the amidogen at the end of APTES molecules.55 Following that MPA molecules were anchored on the Au nanoparticles through the thiol-Au bond, then the carboxyl were further activated using EDC/NHS method for chemically bonding BSA molecules. FITC was lastly chemically bonded to BSA molecules to facilitate the CLSM measurement by the reaction between the residual amidogen in BSA with FITC. To facilitate the CLSM measurement, 3.8 µm PS sphere was employed as etching mask to



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fabricate the ordered micro-nano patterned Au arrays via Route 1 (Figure 7a). The success of each surface modification processes was confirmed by characterizing the surface elements content through X-ray photoelectron spectra (XPS) measurement, as listed in the Table 1 (see detail analysis in Supporting Information, S9). Figure 7b shows the CLSM image of the patterned BSA molecules on the micro-nano patterned Au array, apparently, the FITC labeled BSA molecules have been accumulated exclusively on each patch maintaining the ordering at microscale. It is certified that Au nanoparticles have successfully bind the protein molecules, while in the none-Au area proteins were repelled by the PEG molecules.56 Similarly, the BSA molecules have also been successfully patterned when applying 1 µm PS colloidal sphere as mask (Supporting Information, S10).

CONCLUSION We have demonstrated a simple and versatile method for fabricating highly tunable micro/submicro-nanostructured surface by combining colloidal lithography and block copolymer assembly. Owing to the well development of BCP assembly and colloidal lithography, this method shows good feasibility and controllability of the micro-nano structure. On the one hand the patterning size and periodicity of the micro/submicro patch can be effectively tuned by changing the PS colloidal sphere size and plasma treatment time, on the other hand the periodicity and morphologies of nanoarray can be adjusted by altering the block copolymer parameters and spin-coating conditions. Meanwhile through varying the loading sequence of metal precursors, two kinds of complementary micro/submicro-


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nanostructures have been successfully prepared. Most importantly, high-resolution submicro-nano pattern with 3~5 nanoparticles in each patch or a complementary single-nanoparticle connected network has been achieved, when the colloidal particle size is down to 250 nm. The proposed patterning approach is proven to be a promising technique for the creation of well-ordered micro/submicro- nanostructured arrays, which can be realized in large area with good reproducibility. Significantly, the successful immobilization of BSA molecules on the micro-nano patterned Au arrays would further promote the advantages of current fabrication strategy, which is applicable for various combinations of nanomaterials and biomaterials to satisfy the requirements for the evolving field of nanotechnology and bioengineering. However there are still some naturally formed defects during such a manual operation process, it is expected that some sophisticated control over the assembly process of colloidal spheres and BCP film (such as the combination with top-down methods or external fields) can be applied to enhance the ordering to further promote its application.

ASSOCIATED CONTENT Supporting Information Characterization of PS colloidal sphere monolayer and large-area micro/submicro-nano patterned polymer or metal array by AFM, SEM and Optical microscopy. XPS analysis of the BSA immobilization process on the micro-nano patterned Au array (PDF) AUTHOR INFORMATION Corresponding Authors - 19 ACS Paragon Plus Environment


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*Email: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (Grant 21474108, 21274148, 21574133)

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Figure 1. Schematic illustration of the two fabrication routes of ordered micro/submicro-nano patterned arrays in a complementary manner. Block copolymer micelle monolayer without (a) or with (a’) metal precursor preloading. (b, b’) Micelle films with PS monolayer assembled on top were exposed to plasma etching. Micro/submicro-nano patterned micelle array (c) and periodic alternate micelle and metal nanoparticle arrays (c’) after removing the PS colloidal sphere mask. (d, d’) Micro/submicro-nano patterned metal array.


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Figure 2. Characterization of the uniform micelle and metal nanoparticle monolayer. AFM height images of initial micelle monolayer obtained from PS9.8K-b-P4VP10K (a) and PS102K-b-P2VP97K (d) diblock copolymer; SEM images of Au (b) and Pt (e) nanoparticle array fabricated from micelle monolayer of PS9.8K-b-P4VP10K and PS102K-b-P2VP97K respectively; (c, f) XPS spectra of the BCP film loading with metal precursors before and after the plasma etching (50W, 10Pa, 60s).



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Figure 3. Representative 2D GISAXS images of Au nanoarrays prepared from PS9.8K-b-P4VP10K (a) and PS102K-b-P2VP97K (b) micelle film. (c, d) qx profile extracted from the GISAXS patterns in (a) and (b) respectively, the arrows mark the positions of the first-, second-, third- and fourth-order Bragg reflections.


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Figure 4. (a-c) AFM height images of micro/submicro-nano patterned PS9.8K-b-P4VP10K micelle arrays after plasma treatment (50W, 10Pa, 80s) using PS colloidal sphere as etching mask with respective sphere diameters of 1.0 µm, 500 nm and 250 nm. (a’-c’) SEM images of micro/submicro-nano patterned Au arrays fabricated from micro/submicro-nano patterned micelle arrays in (a-c).



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Figure 5. SEM images of the Au micro-nano arrays prepared from patterned PS9.8K -b-P4VP10K micelle film using 1 µm (a-d), 500 nm (e-f), 250 nm (g-h) PS sphere as etching mask for different plasma etching time (50W, 10Pa): (a-d) 60s, 120s, 160s, 200s; (e-f) 80 s, 120s; (g-h) 60 s, 80s.


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Figure 6. (a-c) AFM height images and (a’-c’) SEM images of micro/submicro-nano patterned Pt nanoparticle arrays after plasma treatment (50W, 10Pa, 80s) using PS colloidal sphere as etching mask with respective sphere diameters of 1.0 µm (a, a’), 500 nm (b, b’) and 250 nm (c, c’), corresponding to the Figure 4 in a complementary way.



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Figure 7. (a) SEM image of micro-nanopatterned Au array prepared by using PS9.8K-b-P4VP10K micelle monolayer and 3.8 µm PS colloidal spheres. (b) CLSM image of immobilized BSA molecules array labeled with FITC on the micro-nano patterned Au array in (a).


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Table 1. Representative Surface Element Content after Each Surface Modification Step

1)

Au1)

APTES2)

PEG3)

MPA4)

BSA5)

C-1s

41.39

64.38

59.98

58.57

61.04

N-1s

1.01

10.14

10.31

9.43

14.38

S-2p

0.00

0.05

0.03

0.39

0.68

Au-4f

3.69

1.05

0.30

0.07

0.04

O-1s

53.90

24.30

29.36

32.54

23.68

Au micro-nano array on quartz substrate; 2) after the silanization with APTES; 3) after PEG

grafting; 4) after MPA modification; 5) after immobilization of BSA molecules.



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Table of Contents (TOC) Image


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