Breath-Taking Patterns: Discontinuous Hydrophilic ... - ACS Publications

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“Breath-taking” Patterns: Discontinuous Hydrophilic Regions for Photonic Crystal Beads Assembly and Patterns Revisualization Xuemin Du, Juan Wang, Huanqing Cui, Qilong Zhao, Hongxu Chen, Le He, and Yunlong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10359 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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ACS Applied Materials & Interfaces

“Breath-taking” Patterns: Discontinuous Hydrophilic Regions for Photonic Crystal Beads Assembly and Patterns Revisualization ‡

Xuemin Du,*,† Juan Wang,† Huanqing Cui,† Qilong Zhao,† Hongxu Chen, † Le He, Yunlong Wang † †

Research Centre for Micro/Nano System & Bionic Medicine, Institute of Biomedical & Health

Engineering, Shenzhen Institutes of Advanced Technology (SIAT), Chinese Academy of Sciences (CAS), Shenzhen 518055, China. ‡

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-

Based Functional Materials & Devices, Soochow University, Suzhou 215123, Jiangsu, China. KEYWORDS: hydrophilic-hydrophobic, microdroplet array, photonic crystal beads, breathtaking, invisible patterns.

ABSTRACT: Surfaces patterned with hydrophilic and hydrophobic regions provide a robust and versatile means for investigating the wetting behaviors of liquids, surface property analysis and producing patterned arrays. However, the fabrication of integrity and uniform arrays onto these open systems remains a challenge, thus restricting them from being used in practical applications. Here, we present a simple yet powerful approach for the fabrication of water droplet arrays and the assembly of photonic crystal bead arrays based on hydrophilichydrophobic patterned substrates. Various integrity arrays are simply prepared in a high-quality output with a low cost, large scale and uniform size control. By simply taking a breath, which brings moisture to the substrate surface, complex hydrophilic-hydrophobic outlined images can be re-visualized in the discontinuous hydrophilic regions. Integration of hydrogel photonic

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crystal bead arrays into the “breath-taking” process results in breath-responsive photonic crystal beads, which can change their color upon a mild exhalation. Not only this state-of-the-art technology provides an effective methodology for the preparation of patterned arrays, but also demonstrates intriguing applications in information storage and bio-chemical sensors.

INTRODUCTION Hydrophilicity of a surface is always an intriguing topic as it explains the microscopic view of many molecular interactions and, at the same time, gives rise to macroscopic daily norms such as water condensation on a cold surface. Surfaces patterned with hydrophilic and hydrophobic regions provide a robust and versatile means for investigating the wetting behavior of liquids, for example, dropwise condensation and droplet arrays formation.1-7 Not only can the hydrophilic and hydrophobic patterned surfaces be widely used for water collection and surface property analysis,8, 9 but also attractive for producing patterned arrays,5 such as geometry controllable liquid arrays,7, 10 cell microarrays,11 hydrogel arrays and particle arrays.6, 12-14 In spite of the low cost, facile manipulation and high throughput for the hydrophilicity patterning, the application of which is rare to date.5 One of the major challenges is that these open systems face an issue of solvent evaporation, which restricts them from being employed in applications involving aqueous systems such as analytical screening tests and crystallization.15 Fast evaporation of volatile solvents in open microsystems may induce problems, for example, unreliable analytical results or inset vacancy defects in particles deposition, as a result of the variation in solvents volume and the change in concentration of components with time.13, 16, 17 Several approaches have been proposed to solve this issue. One of the most facile methods is to control the experimental temperature and humidity.18 However, it is still challenging for low boiling point


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solvents, for example, acetone, ethanol and hexane.7 Another suggested solution is to introduce non-volatile solvent into the open system for micropatterns’ formation, for example, ionic liquids.9,

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These ionic liquid based micropatterns are limited to non-aqueous applications,

which may not be suitable for analysis methodologies as they require water as a reaction medium.11, 16, 19, 20 An alternative approach to address the issue is to constrain the solvent within specified structures, such as walls, reservoir or channels. However, these structures induce problems such as complex technique of preparation, large dead volume, and poor control of hydrodynamic pressure.16, 21, 22 By a wise use of lab-on-a-chip techniques with appropriate non-volatile solvent, however, a hydrophilic-hydrophobic regionalized device can be potentially developed into a state-of-the-art technology which is practical for different functions. Herein we report a simple but effective strategy for fabrication of patterned surfaces with hydrophilic and hydrophobic regions, and demonstrate two of their “breath-taking” applications, the assembly of precise 2D large-scale photonic crystal bead arrays and information storage triggered by exhalation vapor. Photonic crystals, with band-gap properties, are constructed from periodically arranged nanoparticles, which can be widely used for in vivo imaging,23, 24 displays,25-27 sensors,28, 29 and anti-counterfeiting labels.30 At this stage, however, there are too few reported methods performing the fabrication of uniform photonic crystal bead array in a high-quality output with a low cost, large scale and precise size control. In this study, photonic crystal beads have been assembled onto a surface with well-defined hydrophilic and hydrophobic regions by immersing the array into a polystyrene (PS) suspension followed by a stabilized oil phase. The microdroplets attached are of precise size and shape which enable the formation of high quality photonic crystal beads. To the best of our knowledge, this would be the first reported method


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which allows a fast and simple dip-and-pick process for the massive fabrication of photonic crystal beads with a regular and controllable size. Furthermore, patterned hydrophilichydrophobic regionalized surfaces have also been designed for information storage application by a simple technique, inducing exhalation visualization of invisible images. As an unprecedented attempt, integration of hydrogel photonic crystal beads into this “breath-taking” process results in breath-responsive photonic crystal beads, which can potentially be used for information storage and bio-chemical sensors. EXPERIMENTAL SECTION Materials. 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFTS) were purchased from SigmaAldrich. AZ 5214 and AZ 300 MIF were purchased from AZ Electronic Materials (USA). Styrene (St), acrylic acid (AA), N-isopropylacrylamide (NIPAM), N,N’-methylenebisacrylamide and potassium persulfate (KPS) were purchased from Sigma-Aldrich. Hexadecene (HE) was obtained from Acros. All chemicals, except styrene, were used as received. Styrene was treated with 5.0 wt% aqueous sodium hydroxide (NaOH, Sigma-Aldrich) to remove the inhibitor. Ultrapure water was used in all the experiments. Characterization. The photographs were taken with a digital camera (Canon, EOS Kiss X4). The SEM images were recorded using field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 450) to determine the photonic crystal beads morphology. All samples for SEM characterization were coated with thin layers of gold (~25 nm thick). Transmission electron microscope (TEM) image was carried out on a FEI/Philips Tecnal 12 Bio TWIN transmission electron microscope. The optical images of the droplet arrays and photonic crystal beads were recorded using an optical microscope (Nikon Ni-U, Japan). The reflection spectra of the photonic crystal beads were recorded using an optical microscope (Nikon Ni-U, Japan) and a fiber optic


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spectrometer (HR 2000+, Ocean Optics, USA). The profile height of photonic crystal bead array was recorded with a profilometer (AlphaStep D-100). An X-ray photoelectron spectroscopy (XPS) was used to detect the functional groups of the treated and untreated silicon substrates. XPS measurements were performed on ESCALAB 250Xi (Thermo Fisher Scientific, America) using a monochromic Al Kα source operating at 200 W with an X-ray beam diameter of 650 µm. The pass energy was 20 eV. The binding energy of C1s (284.6 eV) was used as a reference. Static contact angles (CAs) were measured using the contact angle meter (DSA 30, Kruss Gmbh) at room temperature, and each contact angle value was measured at least 5 times. Fabrication of the hydrophilic-hydrophobic patterns. Firstly, a layer of AZ 5214 photoresist was spin-coated and patterned on the clean substrates including silica wafer and plastic film using the standard photolithography. After the AZ 300 MIF development, the substrates were washed with ultra-pure water and dried in a stream of nitrogen gas. Then, the substrates were treated by oxygen plasma (Weike PDC-M, China) for 10 s.30 In a typical experiment, the wafers were further treated by vapor-phase self-assembled monolayer (SAM) using PFTS.31 Finally, the substrates were rinsed with ethanol to remove patterned AZ 5214 photoresist, and dried in a stream of nitrogen gas. Preparation of patterned droplet array. The 2-inch treated silica wafers with various patterns of square, hexagon, and round geometries (size from 50 µm to 200 µm) were immersed into a plastic Petri dish partially filled with water for 30 s and then taken out immediately. In addition, to ensure the integrity of the tiny droplets array, silica wafers with a fluorescence dye dyed green water droplets were immersed into a Petri dish containing 10 mL hexadecene to prevent the fast evaporation. The bright-field and fluorescence microscopy images were recorded with a microscope.


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Fabrication of the photonic crystal bead array. PS particles (ca. 174 nm and ca. 225 nm) and core-shell hydrogel particles bearing a poly(styrene-co-acrylic acid) (PS-co-PAA) core and poly(acrylic acid-co-N-isopropylacrylamide) (PAA-co-PNIPAM) hydrogel shell (ca. 182 nm) were prepared by emulsifier-free emulsion polymerization according to our previous work.23,30 The particle suspensions were diluted with ultra-pure water to the desired concentration (typically 2.0 wt%~4.0 wt%). The patterned silica wafers were then immersed into the colloidal suspension and withdrawn after 30 s. Subsequently, the substrates were immersed into a glass Petri dish containing 10 mL hexadecene, which was warmed to 50 ℃ for 72 h to allow the water within these droplets to slowly evaporate.32 During the evaporation, the particles were selfassembled into photonic crystal beads and pinned onto the wafers. The resultant beads were washed with n-hexane to remove the hexadecene, and dried at room temperature. Exhalation performances. The simplest way to bring water vapor to the substrate is by exhalation, which was how we discovered this interesting phenomenon. However, the water vapor condensed on the substrate was easily evaporated. To guarantee visible and completed images, we put some ice underneath the substrate that lengthens the lasting time of the condensed water droplets. All images were captured at 25 ℃ with a relative humidity of 75%. RESULTS AND DISCUSSION The preparation process of the hydrophilic-hydrophobic patterned surface is illustrated in Scheme 1. In a typical experiment, Si wafer was employed as the substrate and finally processed with standard photolithography. After oxygen plasma treatment, patterned Si wafers were modified with PFTS by chemical vapor deposition methods respectively.31 As shown in Figure 1, the CA value increases from 58.2°± 1.5° to 108.1°±0.4°, which is attributed to the vapor-phase


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SAM using PFTS. Without modification with PFTS, the non-patterned zone of Si wafer is hydrophilic and clean due to the previous plasma treatment which brings about -OH functionalities and increases wettability observed as a thin film of water (CA

Breath-Taking Patterns: Discontinuous Hydrophilic ... - ACS Publications

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