Self-Healing Label Materials Based on Photo- Crosslinkable

Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China. KEYWORDS: self-healing, polyelectrolyte multilayers, surface ...
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Self-Healing Label Materials Based on PhotoCross-Linkable Polymeric Films with Dynamic Surface Structures ACS Nano 2018.12:8686-8696. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/09/18. For personal use only.

Xia-Chao Chen, Wei-Pin Huang, Ke-Feng Ren,* and Jian Ji* MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, P.R. China S Supporting Information *

ABSTRACT: Spatially controlling the evolution of surface structures may provide an effective strategy for patterning surface roughness and facilitating the construction of various functional surfaces. In this study, we report a photo-crosslinkable polymeric film with dynamic surface micro/ nanostructures. The surface structures of the un-cross-linked regions can be eliminated under saturated humidity, which can be utilized to create patterned roughness on the film. One potential application of this patternable platform is as a “smart” label material for graphical symbols. Various graphical symbols can be programmed onto this film by partially erasing its surface roughness, enabling visibility due to the difference in light scattering between different areas of the film. When a thus-prepared label was blurred by mechanical scratches, it could be healed under saturated humidity, and its original readability could be fully restored. Furthermore, the patterned rough surface created using our approach can also be very useful in many other research fields, such as surface wettability and cell behavior manipulation. KEYWORDS: self-healing, polyelectrolyte multilayers, surface roughness, photolithography, barcodes

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alternating deposition of interacting species onto substrates, can be performed at room temperature under mild conditions, being widely considered as an environmentally friendly coating technique.18−20 Due to the sensitivity of many assembled units toward environmental stimuli (pH, ionic strength, electric field, etc.), a morphological or structural control over the LbL multilayers can be flexibly imparted by selecting assembly conditions, assembled units (e.g., polymers, biomacromolecules, nanoparticles, micelles, liposomes, etc.), or postassembly treatments, thus generating various surface micro- and nanostructures.21−25 In particular, these morphological features of LbL films have intrinsically dynamic characteristics as the intermolecular interactions of these films can be readjusted.23,26,27 However, how to spatially control the evolution of these surface structures remains a challenge, and a study on this issue may provide a strategy for patterning surface rough structures and facilitate constructing various functional surfaces. In this study, cationic poly(ethylenimine) (PEI) was LbL assembled with a photoreactive poly(acrylic acid) derivative (PAA-N3) onto substrates to yield a conformal polymeric film.

t micro- and nanometer scales, many biological surfaces possess delicate roughness features that play a key role in the realization of their natural functions.1−3 In past decades, people have realized the great significance of roughness features on the performance of artificial surfaces and interfaces.4−6 Regulation over surface roughness plays a key role in developing various categories of functional surfaces and is helpful to tune optical, electrical, biological, and other physicochemical surface properties.7−9 The patterning of these rough surface structures, which can be used to provide a spatial control over the properties of artificial surfaces, has already emerged as a major research direction to develop various functional surfaces.10−12 A wide range of artificial surfaces with patterned surface roughness have been utilized in the fields of marine antifouling, cell behavior manipulation, tunable wettability, and so on.11−13 Multiple efforts including electrospinning, chemical vapor deposition, phase separation, laser microfabrication, and others have been carried out to yield various rough structures onto materials.14−17 All of the present methods have distinctive features and advantages; however, most methods result in a static rough surface, and few of them can achieve a dynamic one, which can be further modulated. In addition, many existing methods are performed with the aid of expensive instruments under harsh environments, such as vacuum and high temperature. Layer-by-layer (LbL) assembly, based on © 2018 American Chemical Society

Received: June 19, 2018 Accepted: August 14, 2018 Published: August 14, 2018 8686

DOI: 10.1021/acsnano.8b04656 ACS Nano 2018, 12, 8686−8696

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Figure 1. (a) 1H NMR spectrum of PAA-N3 in D2O. The content of phenyl azido groups in PAA-N3 was determined by 1H NMR according to the peak ratio of phenyl azido protons at 6.5−7.5 ppm and methylene protons of the main chain at 1.0−2.5 ppm. The inset shows the chemical structure of photoreactive PAA-N3. (b) Thickness of PEI/PAA-N3 films as a function of the number of bilayers. The inset shows the initial growth of film thickness up to 10 layers. (c) Top-down SEM images of PEI/PAA-N3 films with different numbers of bilayers.

method is that the occurrence of film patterning can be controlled precisely. In this study, photoreactive PAA-N3 was synthesized by grafting phenyl azido groups onto PAA via a carbodiimide reaction. By analyzing its 1H NMR spectra (Figure 1a), the grafting ratio of phenyl azido groups to PAA was calculated to be 5.4% according to the peak ratio of the phenyl azido protons at 6.5−7.5 ppm and the methylene protons of the main chain at 1.0−2.5 ppm. In our previous studies, we proposed a direct approach to amplify the exponential growth of polyelectrolyte multilayers by alternating the deposition of the polycation under an alkaline condition and the polyanion under an acidic condition.34,35 Here, cationic PEI (1 mg/mL, pH 9.0) was LbL assembled with anionic PAA-N3 (3 mg/mL, pH 3.5) to fabricate a PEI/PAAN3 film according to this pH-amplified exponential growth mode. The primary advantage of this method is that it can increase the deposited mass per layer and enhance the growth of polyelectrolyte multilayers significantly.36 Figure 1b shows the dependency of film thickness on the number of PEI/PAAN3 bilayers. The PEI/PAA-N3 film grows exponentially in the initial few bilayers (inset of Figure 1b) and then switches to an approximately linear growth in the subsequent bilayers. Evidently, the thickness increment of such exponentially growing polyelectrolyte multilayer (PEM) films in its linear growth regime is substantially larger than that of linearly growing LbL ones (typically hundreds of nanometers versus a few nanometers).35 Figure 1c shows the top-down scanning electron microscopy (SEM) images of PEI/PAA-N3 films with different bilayers. It can be found that many cavernous structures were spontaneously created on the surface of PEI/ PAA-N3 films. The size of these surface structures increases as more bilayers were deposited onto the substrates. The arithmetical mean roughness of samples with different bilayers was measured using a stylus profiler. It can be found that the surface roughness increased in the initial 8 bilayers but decreased when more bilayers were deposited onto the

Through alternating deposition of a polycation at pH 9.0 and a polyanion at pH 3.5, many rough structures were spontaneously created on the surface of PEI/PAA-N3 films. Such a rough surface can be smoothed at saturated humidity, under which condition the mobility of polyelectrolytes is activated. Upon UV irradiation, the PEI/PAA-N3 film can be covalently cross-linked, and its surface structures can be preserved at saturated humidity. By taking advantage of the different properties of PEI/PAA-N3 films before and after exposure to UV irradiation, one can readily fabricate various patterned surfaces composed of structured and flat regions with the aid of photolithography. One potential application of this patternable platform is to be a “smart” label material for various graphical symbols. For the purposes of demonstration, a series of barcode patterns, which are widely used in modern industry and people’s daily life, were programmed onto PEI/PAA-N3 films. These graphical symbols can be visible and machinereadable due to the striking difference in light scattering between different areas of the film. Interestingly, when such patterned films were blurred by mechanical scratches, they were healed at saturated humidity, and their original macroscopic appearance and machine readability were fully restored. This can be mainly ascribed to the evolution of scratches on the un-cross-linked regions and the full recovery of the difference in light scattering across the patterned film. Beyond that, the patterned rough surfaces created using our approach can be very useful in many other research fields, such as surface wettability and cell behavior manipulation.

RESULTS AND DISCUSSION Fabrication of PEI/PAA-N3 Films with Structured Surfaces. In the past decade, a variety of patterned LbL films have been developed through regioselectively crosslinking the films.28−33 This method is mainly operated with the aid of photolithography, in which photoreactive LbL films were assembled beforehand. One prominent advantage of this 8687

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Figure 2. (a) UV−vis spectra of a (PEI/PAA-N3)4 film after UV irradiation for various times. (b) Schematic of a possible mechanism for the photochemical reaction of phenyl azido groups.

Figure 3. (a,b) SEM and optical images of un-cross-linked (PEI/PAA-N3)10 films (a) before and (b) after exposure to saturated humidity for 12 h. (c) SEM and optical images of a photo-cross-linked (PEI/PAA-N3)10 film after exposure to saturated humidity for 12 h. (d) Simplified mechanism of how water molecules affect the interactions between polycation and polyanion. The blue dots correspond to the water molecules incorporated into the film at saturated humidity.

Photo-Cross-Linking of PEI/PAA-N3 Films. The introduction of phenyl azido groups onto PAA enables the PEI/ PAA-N3 films to be covalently cross-linked upon UV irradiation. The kinetics of this photochemical reaction was studied through UV−vis spectroscopy, utilizing a (PEI/PAAN3)4 film assembled on a quartz slide. Figure 2a shows a gradual decrease in absorbance at 270 nm with extended UV irradiation, which indicates the photoinduced decomposition of phenyl azido groups.29,40 At the same time, there are concomitant increases in absorbance near 238 and 354 nm, providing two isosbestic points. Upon UV irradiation, a phenyl azido group is known to be easily dissociated into a highly reactive phenyl nitrene, which can react with a surrounding group to form an intra- or intermolecular covalent bond.41,42 The nitrene intermediate can insert into a neighboring C−H bond to form a covalent bridge of aminobenzene, as illustrated in Figure 2b. It can also couple with another phenyl nitrene to form a covalent bridge of azobenzene. However, the photoinduced cross-linking of the PEI/PAA-N3 films can be mainly ascribed to the insertion reaction because the concentration of phenyl nitrenes is actually very low compared to that of C−H bonds. At the same time, a hypsochromic shift to lower wavelength is observed for the characteristic peak initially at

substrate (Figure S1). For the initial 8 bilayers, the increase in surface roughness can be ascribed to the increase in the size of surface structures. However, when their size further increased, these surface structures tended to combine with each other, and the grooves between them became shallower, which led to the decrease in the surface roughness. In our previous study on PEI/PAA films, many worm-like structures were found on the surface of PAA-ending films.34,37 The spontaneous formation of those worm-like structures is related to the swelling-induced deformation of films during the assembly process and is similar to the mechanical instability of hydrogels undergoing a volume phase transition.36 Very interestingly, the cavernous structures on the surface of PEI/PAA-N3 films are quite different from the worm-like structures on the surface of PEI/PAA films and are more similar to the honeycomb-like structures38,39 on the surfaces of PEM films treated with an acidic solution. This observation implies that the spontaneous formation of the surface structures of PEI/PAA-N3 films might be essentially analogous to the structural reorganization of PEM films in the process of postassembly treatments. In other words, the assembly process and structural reorganization of PEM films can occur synchronously under certain conditions. 8688

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Figure 4. (a) Schematic of the surface patterning of a PEI/PAA-N3 film. (b−d) SEM images of three patterned (PEI/PAA-N3)10 films. Each of these has an array of flat regions with varied geometries (circle, square, and triangle). Insets in images b, c, and d are the overall appearance of these three patterned (PEI/PAA-N3)10 films. (e) High-magnification SEM image shows the junction area between the flat region and the structured region in image d. (f) SEM image of a patterned surface with an array of stripes. (g) High-magnification SEM image shows the morphological features of the patterned surfaces in image f.

screening of charges along polyelectrolyte chains and increase the possibility of disrupting the electrostatic interaction between the polycation and polyanion.49 As both PEI and PAA-N3 are weak polyelectrolytes with a low charge density along the main chain, their LbL assembly leads to an electrostatically cross-linked network that is loose and readily disturbed.50 Thereafter, the effects of the incorporated water can significantly weaken the intermolecular interactions within the (PEI/PAA-N3)10 films, as illustrated in Figure 3d. In addition, increasing water content can also increase the free volume available for the motion of chain segments.51 As a result, the mobility of the polyelectrolyte chains is highly enhanced, and the polyelectrolyte interdiffusion enables the substance transfer between different sites on the un-crosslinked films, thus smoothing surface structures, reducing interfacial area, and minimizing surface energy. For the photo-cross-linked (PEI/PAA-N3)10 films, however, the chemical cross-linking process can generate covalent intermolecular interactions, which are able to restrict the mobility of polymer chains and thus prevent the structural evolution of surface structures. Surface Patterning of PEI/PAA-N3 Films. By taking advantage of the different properties of PEI/PAA-N3 films before and after photo-cross-linking, various patterned surfaces composed of structured and flat regions can be fabricated. This was mainly performed through a two-step approach (illustrated in Figure 4a). First, an as-prepared (PEI/PAA-N3)10 film was subjected to UV irradiation through a photomask, partially exposing the film and leaving some areas un-cross-linked. Then, the partially cross-linked (PEI/PAA-N3)10 film was exposed to saturated humidity for 12 h. This process led to the surface flattening of un-cross-linked areas, and finally, a distinct pattern with high fidelity emerged on the (PEI/PAA-N3)10 film. Figure 4b−d shows the SEM images and digital photographs of three patterned surfaces, each of which has an array of flat regions with varied geometries (triangle, square, and circle). The regions in the middle of these SEM images correspond to the areas where structured surfaces were smoothed at saturated humidity. In contrast, the other areas

270 nm, indicating a loss of conjugation between the disappearing azide groups and their corresponding benzyl rings.43 Humidity Sensitivity of PEI/PAA-N3 Films. In our recent studies, we indicated that humidity (gaseous water) can serve as an effective trigger for activating the transition of porous PEI/PAA films.44,45 Herein, we investigated whether the surface morphology of PEI/PAA-N3 films can also be reconstructed under similar conditions. To this end, both un-cross-linked and photo-cross-linked (PEI/PAA-N3)10 films were exposed to saturated humidity for various times, followed by SEM observation to reveal their structural changes. Figure 3a shows the typical surface of a (PEI/PAA-N3)10 film without any treatment. Figure 3b shows the top-down SEM image of an un-cross-linked (PEI/PAA-N3)10 film after exposure to 100% relative humidity (RH) for 12 h. It is found that the structured surface of the un-cross-linked (PEI/PAA-N3)10 film was smooth. Unlike the un-cross-linked ones, the surface structures of a photo-cross-linked (PEI/PAA-N3)10 film can be preserved mostly at saturated humidity (Figure 3c). The effects of saturated humidity on the (PEI/PAA-N3)10 films can also be reflected in the change of their optical properties. As shown in the inset of Figure 3b, an apparent change occurred to the transparency of the un-cross-linked (PEI/PAA-N3)10 film after it was exposed to saturated humidity for 12 h. In contrast, the transmittance as well as appearance of the photo-cross-linked (PEI/PAA-N3)10 film did not change considerably under the same condition (inset of Figure 3e). Evidently, the transparency of these films is highly related to the absence or presence of surface roughness and the accompanying light scattering. Both external and internal factors can contribute to the disappearance of these cavernous structures on un-cross-linked (PEI/PAA-N3)10 films. The effects of humidity on the composition and properties of PEM films have been previously investigated in many studies.46−48 One general result is that more water can be absorbed by the PEM films with an increase in ambient humidity. As a solvent with a high dielectric constant (78.36 F/m, 25 °C), water can also enhance the 8689

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Figure 5. (a−d) Four different 2D barcodes (Aztec code, DM code, PDF417 barcode, and QR code) were encoded onto (PEI/PAA-N3)10 films. All barcode patterns contained an information “barcode”. (e) Encoded film in image d was scanned and decoded successfully using a barcode-reading application (CortexScan) on a smartphone. The reading result is marked with the dotted box in image e.

visible 2D barcode patterns can be created through patterning the surface topographies of (PEI/PAA-N3)10 films, which were constructed on silicon substrates. As shown in Figure 5a−d, four types of 2D barcodes, which are an Aztec code, data matrix (DM) code, PDF417 barcode, and quick response (QR) code, were successfully encoded onto (PEI/PAA-N3)10 films. All of these 2D barcodes contained an information “barcode” and can be correctly decoded using appropriate barcode decoders. For the purposes of demonstration, the (PEI/PAA-N3)10 film encoded with a PDF417 barcode (Figure 5d) was successfully scanned and decoded using a barcodereading application (CortexScan) on a smartphone (Figure 5e). As no pigment was included for the visualization of these barcode patterns, they do not readily fade over time, thus offering long-term stability. For instance, a (PEI/PAA-N3)10 film encoded with a PDF417 barcode was kept for 1 year at room temperature and under normal humidity, and its appearance did not change and could still be decoded successfully (Figure S2). In many circumstances, 2D barcodes are used to store a large amount of information (e.g., a long webpage link and anticounterfeiting code) concerning the objects to which they are attached.52,58 For a 2D barcode with a given overall size, enhancing its information storage capacity implies increasing the number of modules, whereas decreasing the size of each module52 can add to the sophistication of the 2D barcode and challenge the definition of the final pattern on (PEI/PAAN3)10 films. Here, the QR code,59 an extremely useful 2D barcode, was chosen to be encoded onto the films. QR barcodes provide a variety of versions for users to choose.52 Each version has a different number of modules, and a greater number of modules bring about a larger information storage capacity. Using the method mentioned above, we encoded different versions (1, 4, 7, and 10) of four 2 cm × 2 cm QR codes onto (PEI/PAA-N3)10 films. The number of modules for these QR codes is 441, 1089, 2025, and 3249, and the size of each module was set to be 950, 600, 440, and 350 μm,

clearly retain a structured surface. The junction area between the flat region and the structured region in Figure 4d is shown in Figure 4e, and a clear boundary can be found. This indicates that the selective photo-cross-linking and surface smoothing can be precisely controlled. Apart from the fabrication of arrays of triangles, squares, or circles, an array of stripes (Figure 4f,g) can also be prepared with the aid of the corresponding photomask. Programming Barcode Patterns onto PEI/PAA-N3 Films. Symbology, which involves using certain graphics or patterns to represent information, is one of the most important approaches to convey data and communicate between individuals.52−54 For example, Arabic numerals are symbols for quantitative values, and trademarks are symbols representing individual companies. Another typical symbology is the two-dimensional (2D) barcode,52 which has become extremely popular in today’s digital age due to its fast machine readability and considerable storage capacity. Various 2D barcodes have gained increasing acceptance in many application fields, such as batch identification, product tracking, and mobile device operating systems, thereby promoting the efficiency of modern industries and daily life.55−57 Generally, a 2D barcode conveys information by arranging light and dark elements in two dimensions and can hold a larger amount of data in a smaller space than can traditional one-dimensional (1D) barcodes.52 For a (PEI/PAA-N3)10 film, various patterns created on it can be visible due to the difference in light scattering between different areas of the films. The insets in Figure 4b−d are the overall appearance of three patterned (PEI/PAA-N3)10 films. The light-colored areas on the patterned film are structured areas, whereas the dark-colored areas correspond to flat areas. The light color observed for the structured areas was due to the high amount of light scatter between surface structures and air, whereas the dark color seen on the flat areas originates from the silicon substrate, which became visible because of the high light transmittance of the overlying flat regions. Based on the color contrast between structured regions and flat regions, 8690

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Figure 6. (a−d) QR codes with the same size (2 cm × 2 cm) but of different versions (1, 4, 7, and 10) were encoded onto four (PEI/PAAN3)10 films. The maximum information capacity for each QR code is noted below each image. (e) Five (PEI/PAA-N3)10 films encoded with a millimeter-sized QR code (3 mm × 3 mm). (f) Imaging of the QD code in image e using a detachable device, which was set up by clipping a commercially available portable microscope (60×) on a smartphone.

Figure 7. (a−c) Optical images showing the recovery process of a (PEI/PAA-N3)10 film, which was encoded with a QR code. (a) As-prepared film without any treatment. (b) Damaged film after being scratched with sandpaper. (c) Healed film after being exposed to saturated humidity for 12 h. (d−g) Top-down SEM images of (d,e) the structured regions and (f,g) the flat regions of a scratched (PEI/PAA-N3)10 film (d,f) before and (e,g) after being exposed to saturated humidity.

PAA-N3)10 films. The size of each module for such small QR codes is only 150 μm. However, a clear QR code pattern (Figure 6f) can still be observed on the encoded films, demonstrating the reliable patterning accuracy on (PEI/PAAN3)10 films. Self-Healing of Patterned PEI/PAA-N3 Films. In this study, our method is to pattern the surface roughness of films and make various graphics visible through regulating the interactions between light and these morphological features. This approach is operated without the use of pigments,

respectively. As shown in Figure 6a−d, all of the QR codes on the resulting patterned films can be clearly observed and successfully decoded using a QR code reader. For binary data, the maximum capacity of these QR codes was 17, 78, 154, and 271 bytes. Although 2D barcodes in the centimeter-size range are adopted in many situations, smaller 2D barcodes might be quite useful in some situations, especially when marking and identifying small objects. However, further reducing the size of each module may be needed. As shown in Figure 6e, we encoded a QR code in the millimeter-size range onto (PEI/ 8691

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Figure 8. Changes in the macroscopic appearance and machine readability of an encoded (PEI/PAA-N3)10 film during three scratching− healing cycles.

thereafter rendering the resulting pattern stable and fadeless.60 However, thus-prepared graphical labels can be susceptible to damage, such as accidental scratches, which can destroy the original morphology and generate undesired defects.52,61 As a result, the graphical symbols on these labels become blurred or difficult to identify. Interestingly, an outstanding feature of these patterned (PEI/PAA-N3)10 films is their capacity of selfhealing when faced with mechanical abrasion. For the purposes of demonstration, a (PEI/PAA-N3)10 film encoded with a QR code pattern (Figure 7a) was rubbed with 2000 grit sandpaper, making the barcode pattern blurred and unreadable through decoding devices (Figure 7b). Afterward, the damaged film was placed in a 100% RH environment for 12 h, and then the QR code pattern became visible and machine-readable once again (Figure 7c). For an encoded (PEI/PAA-N3)10 film, the rubbing process can generate many scratches on both structured regions and flat regions. The scratches on the structured regions dramatically changed their original surface profile (Figure 7d). However, the arithmetical mean roughness of these regions changed only slightly from 665.8 ± 59.3 to 682.6 ± 74.6 nm, and the low light transmittance of the structured regions did not change because of the high light scattering between the scratches and air. On the flat regions, the scratches (Figure 7f) made the roughness value increase from 33.5 ± 7.4 to 484.6 ± 55.4 nm, bringing about strong light scattering and compromising the light transmittance of these originally transparent areas. Therefore, the damaged flat areas became whitish, and the dark color of underlying silicon substrate was obscured. As a result, the distribution of lightness and darkness on the encoded film changed significantly after it was scratched (Figure S3). Because a barcode conveys information by arranging light and dark elements in a certain area, such a change made the 2D barcode on the scratched film unreadable. In a 100% RH environment, the macroscopic appearance and machine readability of this disabled (PEI/PAA-N3)10 film can be well restored (Figure 7c and Figure S3). This recovery process was related to the evolution of the scratches at a microscopic level. As shown in Figure 7e, the scratches on the structured regions were transformed but did not disappear with saturated humidity, leaving many structures on the surface and thus keeping these regions rough (surface roughness: 599.7 ± 189.1 nm) and visibly opaque. On the damaged flat regions,

however, the scratches can be fully repaired in the 100% RH environment (Figure 7g), and the surface roughness was reduced to 20.5 ± 3.4 nm. The healing of these scratches restored the original transparency of the flat regions and enabled visualization of the dark silicon substrate. As a consequence, the original arrangement of light and dark elements on the encoded (PEI/PAA-N3)10 film was fully recovered, making the 2D barcode pattern visible and machine-readable once again. Furthermore, when an encoded (PEI/PAA-N3)10 film was kept for 1 year at room temperature and under normal humidity, it remained self-healable after being mechanically scratched (Figure S4). As the flat regions of encoded (PEI/PAA-N3)10 films were not chemically cross-linked, the mobility of polyelectrolyte chains in these areas can be highly activated under saturated humidity. Subsequently, the interdiffusion of polyelectrolytes enables the substance transfer between different sites and heals the scratches. The structured regions of patterned (PEI/PAAN3)10 films were chemically cross-linked, but the scratches on these regions can also change under saturated humidity. This implies that a portion of polyelectrolytes on such regions can turn mobile under this condition. These mobile polyelectrolytes could be primarily PEI because PAA was modified by a photoreactive group and covalently linked to surrounding polymer chains upon UV irradiation. Both PEI and PAA are weak polyelectrolytes with low charge densities,36,44 and their electrostatic LbL process leads to a loose assembly in which not all PEI chains are located next to the photoreactive groups on PAA chains. Therefore, some PEI chains may not be covalently restricted, and their mobility can be then activated under saturated humidity. However, the scratches on the structured regions cannot disappear completely, which illustrates that a stable covalent network was developed within these cross-linked regions, thus limiting the mobility of most polymer chains. Although the surface profile of the healed film is not exactly the same as that of an undamaged one, the readability of these encoded films can be fully restored, and this is the important result. Actually, the recovery of functionality, rather than the exact external or internal microstructure, is very common in natural biological systems.62 For example, many injuries on human skin will lead to scars but will restore primary sensing functionalities.63 8692

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this film and became visible and machine-readable due to the difference in light scattering between different areas of the film. When the encoded film was mechanically scratched, the resulting scratches dramatically changed the original surface profiles and disturbed the difference in light scattering across the film, making the graphical symbols fuzzy and unreadable. Interestingly, the scratched film can be healed under saturated humidity, and its original readability can be fully restored. This can be mainly ascribed to the healing of scratches on the uncross-linked regions and the full recovery of the difference in light scattering across the encoded film. In addition, such a healing process can be repeated multiple times for different scratch events in the same area. In essence, both the patterning process and the healing process of this film are highly related to the mobility of its constituent polymeric chains, which were manipulated through selective cross-linking and environmental stimulus. Furthermore, the patterned rough surfaces created using our approach can also be very useful in many other research fields, such as surface wettability and cell behavior manipulation.

In this study, the grafting degree of PAA-N3 can directly affect the structural transition and self-healing of a PEI/PAAN3 film. We examined three grafting degrees, 2.8, 5.4, and 11.5%. When using PAA-N3 with an 11.5% grafting degree, the assembly of multilayers was very slow, and it was timeconsuming to obtain a film of several micrometers in thickness, which restricted its further application and characterization. This can be ascribed to the significant consumption of negatively charged carboxylic groups during the grafting reaction and the weakening of intermolecular static interactions during the LbL assembly process. When using PAA-N3 with a 2.8% grafting degree, the consumption of negatively charged carboxylic groups was significantly less and had very little impact on the assembly of multilayers. The structural transition and self-healing of this resulting PEI/PAA-N3 film can be readily realized under saturated humidity. However, we found that a 2.8% grafting degree was insufficient to fix the morphological structures of the PEI/PAA-N3 film. When using PAA-N3 with a 5.4% grafting degree, the assembly of multilayers is slower than the situation using PAA-N3 with a 2.8% grafting degree but still substantially faster than the LbL assembly process when we used PAA-N3 with an 11.8% grafting degree. The self-healing of the resulting PEI/PAA-N3 films can also be enabled under saturated humidity. More importantly, such a grafting degree was sufficient to fix the morphological structures of the PEI/PAA-N3 film. After comprehensive consideration, a grafting degree of 5.4% was chosen for our study. The recovery of a scratched (PEI/PAA-N3)10 film mainly stems from the generation of mobile polyelectrolytes under saturated humidity. One advantage of this kind of intrinsic selfhealing material is its ability to heal multiple damages.64,65 For the purposes of demonstration, the healing process of an encoded (PEI/PAA-N3)10 film was repeated three times for different scratch events in the same region (Figure 8). Along with surface scratches, these damage events can also result in a partial loss of materials from the film. The thickness of the encoded (PEI/PAA-N3)10 film was reduced by 60.7% (from 2701.9 ± 374.1 to 1062.7 ± 50.5 nm) after three scratch events. However, we found that the macroscopic appearance and original readability of this encoded film can still be fully recovered (Figure 8). The healing capability of a PEM film is highly dependent on its film thickness.65 Considering that, a thicker film is essential if plenty of scratch events are likely to happen. This can be easily implemented by increasing the number of bilayers.34,36 In addition to saturated humidity, the healing of a scratched (PEI/PAA-N 3 ) 10 film can be accomplished by immersing it into a bath of deionized water (Figure S5). This is of great practical significance because liquid water is more achievable than a 100% RH environment in many situations.

EXPERIMENTAL SECTION Materials. Branched poly(ethylenimine) (PEI, Mw = 25 000), poly(acrylic acid) (PAA, Mw = 100 000), and 4-azidoaniline hydrochloride were purchased from Sigma-Aldrich (Germany). 1Ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC) and N-hydroxysulfosuccinimide sodium salt (NHS) were obtained from Aladdin (Shanghai, China). Hydrogen chloride (HCl) and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent (Shanghai, China). The deionized water used in all experiments was acquired through a Milli-Q water purification system (Millipore, Billerica, America). The pH values of various aqueous solutions were adjusted utilizing 1.0 M HCl or 1.0 M NaOH as needed. Synthesis of PAA-N3. Photoreactive PAA-N3 was synthesized by grafting phenyl azido groups onto PAA via a carbodiimide reaction. Briefly, 500 mg of PAA, 71.08 mg of 4-azidoaniline hydrochloride, 239.61 mg of EDC, and 479.23 mg of NHS were dissolved in 50 mL of deionized water. The pH value of the mixed solution was adjusted to 7.0 by adding 1.0 M NaOH. The mixture was then stirred at 4 °C for 24 h. Afterward, the mixed solution was dialyzed against deionized water, and the final solution was freeze-dried to obtain a white solid. This end-product was analyzed using 1H NMR (DMX-500, Bruker, Switzerland) to acquire its chemical structure and composition. Film Buildup. Glass, quartz, and silicon substrates were immersed in a fresh piranha solution (30% H2O2/98% H2SO4 = 3/7 v/v) for 40 min and then thoroughly rinsed with deionized water. PEI/PAA-N3 films were constructed by alternately dipping substrates into a PEI solution (1 mg mL−1, pH 9.0) for 15 min and a PAA-N3 solution (3 mg mL−1, pH 3.5) for 15 min. After each dipping step, the substrates were flushed using deionized water and blown dry by a nitrogen stream. These processes were repeated until the desired number of bilayers were assembled onto the substrates. In this study, PEI/PAAN3 films are referred to as (PEI/PAA-N3)n, where n is the number of bilayers. Without any postassembly treatment, many structures were created spontaneously on the surface of PEI/PAA-N3 films. Surface Smoothing. In a sealed container, the equilibrium water vapor over purified water at room temperature was employed to establish a 100% RH environment. The surface smoothing of PEI/ PAA-N3 films was enabled by simply exposing the samples to the 100% RH environment for a period of time. In a small sealed container, the equilibrium water vapor over deionized water (25 °C) was employed to build the 100% RH environment. Film Patterning. Photo-cross-linking of PEI/PAA-N3 films was initiated under UV irradiation (365 nm, 1165 μW/cm2), which was provided by a portable UV lamp (EN-180L, Spectroline, America). The kinetics of the photoinitiated reaction was studied via UV−vis

CONCLUSIONS In summary, we demonstrated an approach to generate a patterned rough surface by using photo-cross-linkable polymeric films with dynamic structures. Such polymeric films were LbL assembled from PEI and a photoreactive PAA derivative. The surface roughness on the film’s un-cross-linked regions can be eliminated under saturated humidity, which can be utilized to program various patterned roughness regions onto the film. One potential application of this patternable platform is as a “smart” label material for various graphical symbols. Two-dimensional barcodes were chosen to be encoded onto 8693

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ACS Nano spectroscopy (UV-2550, Shimadzu, Japan) using quartz slides as the substrate. A two-step approach was performed to render the surface patterning of PEI/PAA-N3 films. First, an as-prepared PEI/PAA-N3 film was exposed to UV irradiation through a chrome-coated quartz mask for 60 min, leading to the site-selective photo-cross-linking of the sample. Second, the surface structures on un-cross-linked regions were eliminated by exposing the sample to 100% RH, and thus, a patterned surface composed of structured and flat regions was obtained. Structural Analysis. Scanning electron microscopy (S4800, Hitachi, Japan) was performed as needed to reveal the surface morphology of the samples. The thickness of PEI/PAA-N3 films was estimated from their SEM cross-sectional images. Before the film thickness was measured, the samples were exposed to a 100% RH environment for 24 h, leading to the smoothing of the structured surface. The arithmetical mean roughness of the surface of a sample was measured using a stylus profiler (DektakXT, Bruker, Germany). Optical Analysis. The optical absorbance of PEI/PAA-N3 films was recorded via a UV−vis spectrophotometer (UV-2550, Shimadzu, Japan).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b04656. Arithmetical mean roughness of PEI/PAA-N3 films as a function of the number of bilayers; images illustrating the long-term stability of a (PEI/PAA-N3)10 film; optical images showing the distribution of lightness and darkness on an encoded film during its recovery process; optical images showing the self-healing of an encoded (PEI/PAA-N3)10 film after it was kept for 1 year under room temperature and normal humidity; optical images showing the water-enabled recovery of a (PEI/PAAN3)10 film (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ke-Feng Ren: 0000-0001-5456-984X Jian Ji: 0000-0001-9870-4038 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the National Key Research and Development Program of China (2017YFB0702500), the National Natural Science Foundation of China (51333005, 51573162), the Zhejiang Provincial Natural Science Foundation of China under Grant No. LR15E030002, and the 111 Project under Grant No. B16042. REFERENCES (1) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Adhesive Force of a Single Gecko Foot-Hair. Nature 2000, 405, 681−685. (2) Chen, H.; Zhang, P.; Zhang, L.; Liu, H.; Jiang, Y.; Zhang, D.; Han, Z.; Jiang, L. Continuous Directional Water Transport on the Peristome Surface of Nepenthes Alata. Nature 2016, 532, 85−89. (3) Zheng, Y.; Bai, H.; Huang, Z.; Tian, X.; Nie, F. Q.; Zhao, Y.; Zhai, J.; Jiang, L. Directional Water Collection on Wetted Spider Silk. Nature 2010, 463, 640−643. 8694

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