Fast Strategy to Functional Paper Surfaces - ACS Applied Materials

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Surfaces, Interfaces, and Applications

A Fast Strategy to Functional Paper Surfaces Min Wang, Yuli Wang, Bingbing Gao, Yifeng Bian, Xiaojiang Liu, Zhenzhu He, Yi Zeng, Xin Du, and Zhongze Gu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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ACS Applied Materials & Interfaces

A Fast Strategy to Functional Paper Surfaces Min Wang,1, 2, 3, † Yuli Wang,4, 5, † Bingbing Gao,1, 2, 6 Yifeng Bian,4, 5 Xiaojiang Liu,1, 2 Zhenzhu He,1, 2 Yi Zeng,1,2 Xin Du1, 2* and Zhongze Gu*1, 2 1 State key laboratory of Bioelectronics, Southeast University, Nanjing, 210096, China 2 School of Biological Sciences&Medical Engineering, Southeast University, Nanjing, 210096, China 3 Department of Oncology, Nanjing First Hospital, Nanjing Medical University, Nanjing 210006, China 4 Jiangsu Key Laboratory of Oral Diseases, Nanjing Medical University, Nanjing, 210029, China 5 Department of Oral and Maxillofacial Surgery, Affiliated Hospital of Stomatology, Nanjing Medical University, Nanjing, 210029, China 6 School of Pharmaceutical Sciences and School of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, 211816, China KEYWORDS: photopatterning; photochemistry; surface functionalization; thiol-ene; cellulose paper

ABSTRACT: Paper, with advantages of low-cost, easy fabrication and disposal, flexibility and renewability, are suitable substrate materials for various applications. Functionalization and patterning on paper substrates are commonly required in many applications. Although many methods have been developed to achieve this, they typically suffer from some drawbacks such as time-consuming process, specific devices dependence, lack of flexibility, low patterning resolution, etc. Herein we present a general and fast method to functionalize paper sheets and

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create patterns. The whole modification process can be completed in 10 minutes and can be applied on various types of paper substrates and other natural materials such as natural fabrics. By our method many common-used functional groups can be covalently attached and patterned on paper substrates, while the characteristic features of the original paper substrates, for example, color, transparency and conductivity, can be perfectly retained after modification, to allow these properties to be incorporated into resulted materials. High resolution patterns can be created on paper by applying a photomask during the modification or controlling the time of modification, to precisely control the functionality at any area on the obtained paper substrates. We also show the potential applications of our method in the fabrication of superhydrophobic coatings and biomaterials.

Introduction Owing to its significant advantages such as low-cost, easy fabrication and disposal, flexibility and renewability, paper has played a very important role through the propagation and development of human civilization since ancient times.1–4 Various types of paper with different physical and chemical properties were created in the past thousands of years, making a huge library of substrate materials for both industry production and scientific research. The easy deformation properties (by cutting or folding) of the paper have also led to various innovative technologies and applications.5–7 Due to these superiorities, paper has been widely applied in publications, packages, Point-of-Care test (POCT) devices, filters, etc., in every corner of our daily life.8–14 It has also been frequently employed in scientific research, as substrate materials to grow nanomaterials,15–17 perform bioanalysis,12,18–20 culture cells21–23 and fabricate various paper-based devices.24–28 In many applications, paper has to be functionalized to alter its physical/chemical properties or bioactivities, for example, hydrophilicity, reactivity, biocompatibility, antifouling and antibacterial performances.11,18 Patterning of physical and chemical properties on paper is also


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commonly required in the fabrication of paper based microarrays and microfluidics, which are widely used in cell incubation devices,21–23 POCT settings19,20 and paper-based electronic devices.26,29 Many techniques have been used to functionalize paper and create patterns. Chemical functionalization on pure cellulose paper could easily be achieved due to the existence of numerous hydroxyl groups on cellulose molecules, which can react with various functional groups, allowing the covalent attachment of designed molecules on the paper substrate. Although many chemical modification methods, such as esterification,30,31 acetylation,32 silane based chemical vapor deposition (CVD)33,34 or chemical liquid deposition (CLD),2,35 and grafting polymerization,1 have been developed for paper functionalization, many of them suffer from the time-consuming and poorly controllable process. In addition, most methods only worked well on pure cellulose papers, while many types of commercial available paper sheets contain filling materials such as French chalk, TiO2, CaCO3 and dyes, that exhibit distinct reactivity from cellulose and can only be poorly functionalized by those methods. Many reported methods can only introduce alkyl or fluorinated chains onto paper substrates, which significantly limited the potential application of the functionalized paper substrates. To patterning on paper, methods such as wax printing, photolithography, micro-contact printing and plasma treatment have been developed.17,23,27,28 However, these methods usually require time-consuming steps and specific devices, and most of them can only generate patterns with low resolution (> 200 μm), far from the requirement for high density cell arrays and precision microfluidic/electronic devices. A paper-functionalization method that is general (can be applied on different types of paper, and allows various functional groups to be attached on paper substrates), highly efficient, low-cost, and patternable is highly demanded.


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Herein we present a general and fast method to functionalize paper and create patterns. The method is based on a two-step process: 1) paper were treated with trichlorovinylsilane (TCVS), a substance that is reactive to both organic and inorganic hydroxyl groups, to introduce vinyl groups onto any kind of paper substrate in several minutes. 2) the resulted vinyl paper were further functionalized by different thiols through photo triggered thiol-ene chemistry, to introduce and pattern various functionalities onto the paper in seconds. The whole process can be completed in 10 minutes. This method can be applied on different types of paper and is compatible to most common-used functional groups. The characteristic features of the original paper substrates, for example, color, transparency and conductivity, can be perfectly retained after modification. Patterned paper sheets with very high resolution (~ 70 μm) can be created by applying a photomask or by controlling the irradiation time during the modification, to precisely control the functionality at any area on the obtained paper substrates. We also applied this method on natural fabrics functionalization, and show the potential application of the obtained functional paper/fabrics. Experimental section Materials 2,2-dimethoxy-2-phenylacetophenone (DMPAP, 98%), 1-dodecanethiol (DT, 98%), 3mercaptopropionic acid (98%), cysteamine (CA, 95%) and 1H,1H,2H,2H-perfluorodecanethiol (FDT, 97%) were obtained from Aladdin (China). 4-(dimethylamino)pyridine (DMAP 99%), mPEG-SH (Mw = 550, 95%), triethylamine (TEA, 99%), ethanol (99.5%), dichloromethane (DCM, 99.5%), acetone (99.5%), dimethylformamide (DMF, 99.5%), toluene (99%) and other solvents were obtained from Macklin (China). Trichlorovinylsilane (TCVS, 99%) was obtained from Adamas (China) Reagent Co., Ltd. 2-mercaptoethanol (ME, 99%) was obtained from Hefei Bomei (China) Biotechnology. Co., Ltd. Whatman grade 1 chromatography paper (Whatman#1 paper) was purchased from GE Healthcare (Hangzhou, China) and used as received. The kit for


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the fluorimetric urea assay was obtained from Sigma-Aldrich, and the kit for the fluorimetric lactate assay was purchased from AAT Bioquest. The fabrics used were raw fabrics directly bought from the market. During the experiments the chemicals were weighted by a METTLER TOLEDO MS205DU electronic balance. The silicate glass plates used in the experiment were 7101 microscope slides of Sail Brand (China). The quartz glass plates were obtained Jingrui Photonics (China). A GY-6 point light source (Tian Jin Tuo Pu Instruments Co., Ltd, China) fitted with a 200 W Hg lamp was utilized for high intensity UV irradiation. The lamp was calibrated to 10 mW cm-2 at 365 nm with the UV-A UV power meter (Photoelectric Instrument Factory of Beijing Normal University, China). The intensity of the resulted light at other wavelengths: 400-1000 nm, 70.2 mW/cm2; 297 nm, 1.4 mW/cm2; 254 nm, 3.16 mW/cm2. The microscopy images were obtained by an Olympus BX53 microscope (Olympus GmbH, Japan). Mandible samples were obtained from the patients undergoing Sagittal Split Ramus Osteotomy (SSRO) and human bone marrow mesenchymal stem cells (HBMSCs) were collected from these samples following a previously described approach.36 Cells were cultured in Alpha modified Eagle’s medium (HyClone, UT, USA) supplemented with 10% fetal bovine serum (Gibco; Thermo Fisher Scientific, Inc., MA, USA), 100 U/L penicillin, and 100 mg/L streptomycin (both Gibco; Thermo Fisher Scientific, Inc.) in a humidified atmosphere of 5% CO2 at 37˚C. The conﬂuent cells were transferred to the next passage using 0.25% trypsin for up to three passages, and the culture medium was changed every 3 days. The study protocols were approved by the Ethics Committee of Nanjing Medical University, China (NO. 2018-190). The informed consent received from patients was written and the study methodologies conformed to the standards set by the Declaration of Helsinki.

Formation of vinyl paper The vinyl paper was formed by silanization of TCVS on various paper in a fume cupboard. First, 200 μL of TEA and 25 mg of DMAP were added into a 50 ml Falcon tube containing 45 ml of DCM, followed by the addition of 100 μL of TCVS. Then, the Whatman#1 paper was placed


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into the tube, and the solution was stirred with a small magnetic stirrer for 30 min. The Whatman#1 paper was finally washed with ethanol and acetone, and dried by N2. Thiol−ene modification of the cellulose paper The vinyl paper was positioned on a glass slide and then wetted with 30 μL of thiol solution (typically, for liquid monomers, 20% vol in DMF or toluene, with 10 mg/mL DMPAP as photoinitiator; for solid monomers, 50 mg/mL in DMF or toluene, with 2.5 mg/mL DMPAP as photoinitiator). To improve the reaction efficiency, the sample was covered with a quartz slide. The covered vinyl paper was irradiated under UV for several seconds. After irradiation, the vinyl paper was thoroughly washed with acetone or ethanol (depending on the solubility of the modification molecules, for example, when FDT is used, the paper should be washed by acetone; when cysteamine hydrochloride is used, the paper should be washed by ethanol), and dried by N2. Preparation of superhydrophobic−superhydrophilic patterns via thiol−ene reaction. Vinyl paper were positioned on a glass slide that was wetted with hydrophilic thiol solution and covered with a photomask. After exposure for 5 s under UV light, the cellulose paper was rinsed with acetone in and dried under nitrogen flow. To prepare superhydrophobic areas, the vinyl paper was repositioned on a glass slide and covered with a quartz slide after addition of 100 μL of FDT or 1-Dodecanethiol solution. After 5 s exposure to UV light, the film was completely rinsed with acetone and dried with a nitrogen flow. Scanning electron microscope (SEM) test SEM images were obtained using a field emission scanning electron microscope (Zeiss Ultra Plus, Germany). The samples were sputtered with a gold layer for 10 s using a Hitachi E-1010 ion sputter (Hitachi, Ltd., Japan) before the measurement. RAMAN spectrometry test The Raman spectra were obtained by an inVia confocal Raman Microscope (Renishaw Inc., UK) equipped with a 633 nm laser. All measurements were carried out with a laser power of 3.4 mW and sequential spectral collection of 10 s. The spectra from 3200 to 100 cm−1 were collected


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from at least five different positions of samples and then the average signal intensity was calculated. Apparent water contact angle (WCA). WCAs of the samples were measured by a contact angle measuring instrument (JC2000D, POWEREACH, Shanghai). A 3 μL droplet of ultra-pure grade water was deposited on the sample. The shape of each droplet was recorded and analyzed using ImageJ software with a DropSnake plugin (http://bigwww.epfl.ch/demo/dropanalysis/). At least five measurements were taken for each sample and then the average WCA was calculated. X-ray photoelectron spectroscopy (XPS) analysis The XPS spectra of the paper surface before and after treatment were obtained by a scanning XPS microprobe instrument (PHI-5000 VersaProbe, Ulvac-Phi, Japan, monochromated Al Kα source, hv =1486.6 eV). The ATR-FTIR measurements The ATR-FTIR measurements were recorded using a Thermo Scientific Nicolet iS50 FTIR spectrometer (Thermo Fisher, USA) using a diamond single reflection attenuated total reflectance (ATR) accessory equipped by a zinc selenide crystal. The diamond was cleaned between samples using ethanol. Other characterizations UV-Vis spectrometry was performed on a Mapada UV-6100 spectrometer. Conductivity test was performed on a KEITHLEY semiconductor characterization system. The stiffness measurement was performed on a Piuma nanoindenter (Optics 11, Netherlands), each sample was measured for 10 times. Protein-adhesion test A human IgG/Cy3 protein with yellow/orange fluorescence was used and the protein adsorption was calculated by following method: Circular samples (6 mm) were carefully fixed on the glass slide by using 3M double-faced gum. Then, 1 μL human IgG/Cy3 solutions, diluted with PBS


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(0.01M, Sigma-Aldrich) to the concentration of 5 μg/ml, were added on the circular samples respectively and dried by N2. Then, circular samples absorbed protein were placed into the cell culture dish (35mm × 10mm) and immersed in 5 mL PBS for washing 24 h. Then, circular samples were dried by N2 and fluorescent images were obtained by fluorescence microscope. Photoshop image editor was used to convert fluorescent images to black-and-white and calculate mean gray values of each black-and-white image. Five different areas were conducted for each image to decrease error. Flow velocity test Rectangular samples (35 mm × 5 mm) were carefully fixed on a retort stand along vertical direction. Then, the samples were dipped into a cell culture dish (35 mm × 10 mm) with 5 mL ultra-pure grade water. The capillary flow time was recorded when the flowing distance reached 30 mm. At least five measurements were taken for each sample. Cellular morphology Cellular morphology of adhesion on cellulose paper, ME modified vinyl paper and CA modified vinyl paper was observed and analyzed by an Olympus BX53 fluorescence microscope at 1, 3 and 5 days. HBMSCs were labeled GFP by using lentivirus before this assay. In vivo wound healing evaluation Male Wistar rats (200–230g) were purchased from the Experimental Animal Center of Nanjing Medical University and were maintained in a conventional animal housing facility throughout the experiment. All animal experimental were performed with the approval of the Ethics Committee of Nanjing Medical University, China (NO. 2018-190). All procedures were carried out according to the guidelines of the Animal Care Committee of Nanjing Medical University. A skin defect was made using a circular puncher (6 × 4 cm) after the hair on the back was shaved. Then the wounds were covered with cotton gauze or CA gauze. The wound dressings were replaced every other day. Macroscopic images of the wound site of each rat were taken at 0, 7 and 14 days.

Results and discussion


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1. Formation of vinyl paper and their further functionalization A pure cellulose paper (Figure 1i), Whatman grade 1# chromatography paper, was first used as the sample substrate. The vinyl paper (Figure 1ii) was obtained by immersing bare paper substrates into a solution containing trichlorovinylsilane (TCVS, 100 μL), triethylamine (TEA, 200 μL) and 4-dimethylaminopyridine (25 mg) in 40 mL dichloromethane in a 50 mL Falcon tube. TCVS quickly react with the hydroxyl groups on cellulose to introduce vinyl groups onto the paper. This is confirmed by the change of WCA on paper before and after modification (from < 5o to 132 ± 3o). The kinetics of the modification could be monitored by measuring the WCA of the substrate at different time points. As shown in Figure 2a, the WCA of the modified paper increased as the silanization proceeded, and became stable after 5 min immersion, indicating that the vinylization was completed after this time point. Thus the vinylization process exhibit very fast kinetics. In previous report, trimethoxyl silane (TMVS) was used to functionalize cellulose fibers to obtain vinyl paper,37 while the process was time-consuming (4h immersion process and 2h 120 ℃ thermal treatment). To compare the modification kinetics between TCVS and TMVS, TMVS was used in our experiment to functionalize cellulose paper, either by our method (40 mL DCM, 200 μL Et3N, 25 mg DMAP) or by the method reported in the literature.37 As shown in Figure 2a, after 40 min immersion treatment, no WCA change was found on TMVS-treated cellulose paper with both methods, indicating that the kinetics of TMVS modification is very slow. The huge difference between the kinetics of TCVS modification and TMVS modification can be easily explained by the mechanism of the reaction: as shown in Figure S1a, during the modification, TCVS reacts with surface hydroxyl groups to form HCl, which quickly react with Et3N to result triethylamine hydrochloride. The excess of Et3N in the system can continuously remove the produced HCl, pushing the equilibrium of the reaction to the product side. Thus in TCVS solution the surface hydroxyl groups can be easily modified with very high efficiency. For


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TMVS functionalization, however, the reaction leads to methanol (Figure S1b) that can not be removed from the system. Therefore the kinetics of TMVS modification is slow and inefficient. From the proposed TCVS modification mechanism (Figure S1), the reaction should lead to a mono coating layer on cellulose fibers. To confirm this, paper with 0, 0.5, 1, 2, 5, 10, 20 and 40 min TCVS treatment were investigated by SEM. As show in Figure S2, almost no difference could be observed on the morphologies of the cellulose fibers on these paper. Similar results were also observed on Raman and FTIR test on these paper (Figure 2b, c), showing that the coated silane layer is very thin, while XPS measurements on vinyl paper (Figure 2d) confirmed the attachment of vinyl silane layers by the appearance of clear peaks at binding energy of 101 eV and 152 eV (referring to the peak of Si2p and Si2s, respectively), and the change of C1s/O1s peaks (Figure 2e, f).37 The attachment of silane layer on cellulose can be confirmed by the appearance and strengthen of Si/C ratio as well as the decrease of O/C ratio along with the TCVS treatment (Figure S3a). The Si/C ratio and O/C ratio of the 2 min and 5 min sample are very similar, indicating that TCVS modification on cellulose is almost completed after 2 min. The spectra of C1s peak of these samples are shown in Figure S3b. It is clear that, as the functionalization goes on, the peak at 283.2 eV (C=C, C-C and C-H peak) was significantly strengthened, describing the attachment of C=C double bond on cellulose fibers, while the signal of C-O peak (284.9 eV), which comes from cellulose molecules, can be clear seen on all samples, showing that the thickness of the coated silane layer should be lower than the detection depth of XPS (< 10 nm).38 All results showing above confirmed that TCVS modification leads to a very thin layer on cellulose fibers, which is coincident to our hypothesis. The C1s peak on sample with 20 min TCVS treatment shows obvious difference from other samples, that the peak


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at 283.2 eV significantly strengthened. This can probably be attributed to the exist of some polysilane nanoparticles on cellulose fibers, which affects the XPS results (as discussed later). The obtained silane layer on cellulose paper in our experiment is different from previous reports, in which functionalized cellulose fibers or paper sheets were obtained by CVD or CLD of trimethoxysilanes and trichlorosilanes.2,33,35,37,39–42 In those reports, huge differences were observed on the SEM images and IR spectra of the cellulose before and after modification, indicating the existence of a thick polysilane layer on cellulose fibers. For example, Tingaut et.al. coated cellulose fibers with TMVS by immersing cellulose fibers in hydrolyzed TMVS solution for 2h and then treating the cellulose under 120 ℃ for 2h. The obtained cellulose shown significantly difference on FTIR spectrum compare to that of unmodified cellulose;37 Guo et.al. modified cellulose nanocrystals with TCVS by immersing the cellulose in water/toluene mixture for 48h, and they obtained a thick polysilane layer with micro-nano structures on the cellulose nanocrystals.35 The difference can probably be understood when considering the effect of water in the system. Silanes are able to react with water molecules, and unlike surface hydroxyl groups, this reaction initials the self-polymerization of silanes to form polysilanes (Figure S1). In all previous reports, little amount of water was added into the system to trigger the polymerization and deposition of polysilanes. Therefore thick polysilane layers were obtained on cellulose in these cases, while in our experiments, the modification was carried out in hydrophobic solvent (DCM), and no water was added during the reaction. Thus it is quite difficult for TCVS to polymerize in this case, the modification prefers to form a monolayer, only little polysilane could be formed by trace of water dissolved in DCM. This can be confirmed by the observation of few polysilane particles on vinyl paper (Figure S2, highlighted by red cycles).


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After the formation of vinyl paper, further functionalities could easily be introduced through photo triggered thiol-ene click chemistry, to covalently attach various types of functional groups onto the paper (Figure 3a). As thiol-ene chemistry is compatible with many commonly-used functional groups, thiols with various functionalities can be used in the modification process, to introduce different functional groups onto paper. Examples are given by converting vinyl paper to fluorinated paper, alkyl paper, hydroxyl paper, carboxyl paper, amino paper and PEG paper (Figure 3a). The successful attachment of different thiols can be confirmed by the change of contact angle of different liquids on the obtained paper. For example, as shown in Figure 3b, after fluorination (modified by 1H, 1H, 2H, 2H-perfluorodecane thiol, FDT), the vinyl paper became superhydrophobic and oleophobic. Modifying vinyl paper with mercapto ethanol (ME) leads to a highly hydrophilic paper which is very similar to the unmodified paper, while using dodecanethiol to modify the vinyl paper generates a hydrophobic and oleophilic paper due to the hydrophobicity and oleophilicity of the dodecyl chains. To investigate the kinetics of thiol-ene modification process, we used several thiols to modify the vinyl paper to introduce strong hydrophobicity or hydrophilicity onto the vinyl paper through UV irradiation (20 mW/cm2 at 365 nm). WCAs of the modified vinyl paper were measured at different time points to monitor the kinetics of the modification (Figure 3c). The thiol-ene modification on vinyl paper is a very fast process that all modifications were finished in 15 seconds. SEM (Figure S2f), Raman (Figure 2b) and FTIR (Figure 2c) results on FDT modified paper (Whatman#1 paper) are very similar to that of vinyl paper, almost no difference could be observed on these two kinds of paper sheets. This is because that thiol-ene reaction only result to a mono modified layer on vinyl substrates,43 which is undetectable by the above characterization methods. The XPS analysis on FDT paper clearly shows the appearance of F1s peaks, C-F peaks and S2p peaks, as well as the further


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decrease of O1s peaks and C-O peaks compare to vinyl paper (Figure 2d-f), describing the formation of fluorinated alkyl chain monolayer on vinyl paper after thiol-ene modification. Combination of the TCVS treatment and thiol-ene modification can lead to a highly efficient modification method, which can convert a bare cellulose paper to almost any functional paper in 10 min, much faster than most reported strategies for chemical functionalization of paper (3~12 h).1,31,44,45 Due to the general reactivity of trichlorosilane group to various functional groups, for example, organic and inorganic hydroxyl groups, amino groups and carboxyl groups,43,46,47 our method can be applied on many types of commercial available paper substrates. To prove this, TCVS treatment was applied on copy paper (containing inorganic fillers such as TiO2), “Post it” paper (containing dyes), tracing paper and toner paper (containing a conductive toner covering layer), the obtained vinyl paper substrates were further modified with FDT or ME, and the contact angle of the modified paper (water, DCM and EtOH) were recorded. As shown in Table 1, the wettability of all paper substrates can be significantly varied from superhydrophobic (SH) to superhydrophilic (SL, the water drop is quickly spread and become invisible. In this case the WCA of the paper is set to be < 5o) by applying our method, indicating that the functionality of all kinds of paper can be easily controlled using this strategy. Kinetics test on these paper showing that the TCVS modification on all paper substrates were very fast and can be completed in 5 min (Figure 4a), thus our method is a general modification strategy to fast functionalize different types of paper. Moreover, since the modification was achieved on a mono layer on the paper, many characteristic features of the original paper, for example, the color (Figure 4bi, ii, Figure S4a), transparency (Figure 4biii and Figure S4b) of the paper and conductivity of toner paper (Figure 4biv and Figure S5), could be retained after modification, allowing the feature


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properties of these paper substrates to be included in the obtained functional paper sheets. The flexibility of the paper can also be retained after modification, for example, the filter paper before and after TCVS modification exhibit similar stiffness (303.8 ± 36.5 Mpa and 349.4 ± 51.4 Mpa, respectively). The modification process can also be applied on other natural fiber based materials. Natural fabrics such as cotton (composed of cellulose) and silk (composed of natural proteins), could easily be functionalized by TCVS, as confirmed by the change of the wettability of the fabrics from hydrophilic to hydrophobic after functionalization (Figure 4c). The stability of the coated layer on functionalized paper is very important for real applications. To test the stability of the silane coating, vinyl paper and FDT paper (substrate: Whatman #1 paper) were kept in indoor environment for 90 days (25-30 ℃, humidity ~60%, in dark) and the WCA of the paper were measured at different time points. The results show that the coated silane layer is rather stable over time, that the WCA of both vinyl paper and FDT paper only decreased a little after 90 days (Figure 4d). The vinyl paper still kept its reactivity after 90 days and can be easily functionalized with FDT (Figure 4d), thus the vinyl paper can potentially be used as user-designable paper substrates. The stability of the silane coating to different solvents was also tested, the vinyl paper (1 cm ×1 cm) was immersed in 40 mL solvent (EtOH, acetone, DCM, THF and toluene, respectively) for 24h, then the WCA of the paper was tested after removing the solvent. The results confirm the stability of the coated silane layer to these solvents (Figure 4e), thus the functionalized paper sheets can be used for in-solvent applications such as microfluidic chips, cell incubation, micro-container for chemical reactions, etc. 2. Patterning on vinyl paper


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In many applications, patterning on paper substrate is usually required to generate microchannels and microarrays. Although wax printing, photolithography, microcontact printing and plasma treatment can be used to perform the patterning, the resolution of these methods are typically low (200-500 μm). Meanwhile most of these methods can only create hydrophobic borders or physically attach molecules/polymers on paper,48 and the chemical modification and patterning on the paper is still difficult. The photo-triggering nature of thiol-ene reaction enables the spatial and temporal control of the functionalization on vinyl paper. Thus patterned papers can be easily generated by this method. For example, the spatial controllability of the method allows 2D patterns (patterns on the X-Y direction of the paper substrate) to be generated by applying a photomask during the irradiation. The patterning properties of the method were simply investigated through the formation of superhydrophobic-superhydrophilic (SH-SL) patterns on paper. To do this, we first modified vinyl paper with hydrophilic mercaptoethanol (ME) under a photomask (365 nm UV light, 2.5 mW/cm2) to create superhydrophilic areas, followed by flood irradiation in the presence of hydrophobic FDT to result SH-SL patterns (Figure 5a). As shown in Figure 5b-e, SH-SL patterns with different geometries could easily be created by applying different photomask during the UV irradiation. Our patterning process allows not only SH-SL paper, but also the precisely control of desired functionalities on different area of the paper, examples are given by modifying vinyl paper with fluorescein isothiocyanate thiol (FITC-SH, Figure 5f) and Rhodamine-SH (Figure 5g), to pattern fluorescence molecules on paper substrates. The resolution of the patterning process was investigated by forming SH-SL lines using a photomask with different line width (1000, 500, 200, 100 and 50 μm). The results (Figure 5h) indicated that, on vinyl paper (Whatman #1 paper substrate with 11 μm average pore size), the obtained SH-SL line exhibit width of 1183, 635, 297, 125 and 72 μm, respectively,


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confirming that patterns with high resolution can be generated on paper by this process. The obtained patterning resolution is much higher than reported methods, which typically exhibit resolutions around 200-500 μm.31,49 Microscopy image on the cross section of the SH-SL channel shows that the modification is not only achieved on the top of the surface, but also through the whole depth of the paper substrate (Figure S6). In recent years, Janus modified paper sheets, namely, paper sheets with different properties (such as wettability) on the two sides, have been reported to be an advanced material to construct complicate paper-based microfluidic devices, to create better paper-based cell culture systems and to be potentially employed as an advanced device for separation, detection, quantification, and analysis,34,50–52 while the fabrication of this type of material typically are device-dependent and are limited to create (super)hydrophobic-(super)hydrophilic Janus paper sheets. The temporal controllability of thiol-ene chemistry allows the control of the depth of the modification by varying the time of UV irradiation. Therefore paper with multiple functionalized layers (Janus paper as example) can be fabricated (Figure 6a). To confirm this, we fabricated SH-SL Janus paper by modifying the vinyl paper with ME with limited UV time (with 2.5 mW/cm2 UV at 365 nm), followed by flood modify the paper with FDT for sufficient time (2 min). The modification process was applied on filter paper with different thickness (1 mm and 180 μm, respectively). WCA on both sides of the paper were tested, it can be seen that it is possible to generate multilayer paper by controlling the time of UV during the ME modification (Figure 6b and Figure S7). The existence of multilayers in modified paper were further confirmed by adding Rhodamine B aqueous solution on the paper and analyze the cross section under microscope. As shown in Figure 6c and Figure S7, it’s clear that on paper with different UV times, Rhodamine B solution can only penetrate part of the paper, indicating the existence of hydrophilic-hydrophobic


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multilayers in the modified paper. The relationship between the thickness of the hydrophilic layer and UV time is shown in Figure S8, as UV time increases, the thickness of the hydrophilic layer increases gradually. However, the controllability of the patterning on Z direction is not as good as that of XY direction, as indicated by the large error bar in Figure S8. This is probably due to the dynamic change of the intensity of UV lamp during the irradiation and the possible gradient formed when adding modification solutions on the paper substrate. By combining the spatial and temporal controllability of thiol-ene photochemistry, we can fabricate 3D patterns on vinyl paper. Examples are shown in Figure 6d, a SH-SL microchannel was generated on filter paper by applying a photomask during the irradiation, while varying the UV time during ME modification can control the position of the microchannel: either penetrate the whole paper, or stay on the top of the surface. The exist of the 3D patterned microchannel can be observed from the cross-section of the patterned paper sheets under microscope (Figure S9). The spatial and temporal controllability of the method allows us to precisely control the chemical properties on every point on the paper substrate, therefore it is possible to construct complicated paper based interfaces and devices by this method. 3. Applications of the method Our method can be simply applied on different paper to form various functional paper and patterns. For example, after fluorination the vinyl paper can be (super)hydrophobic and oleophobic, thus they can be used as flexible liquid-proof covers (Figure 7a-c, Video S1). Since many characteristic features of the paper substrates can be retained after the modification process, functional (super)hydrophobic/oleophobic surfaces can be easily obtained by using specific paper. Examples are shown by forming colored superhydrophobic surfaces (Figure 7d) and conductive superhydrophobic surface (Figure 7e) by using colored or conductive paper


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substrates. Meanwhile, due to the flexibility and porous nature of the paper, writing and printing can still be performed on these (super)hydrophobic paper (Figure 7f, g, Figure S10), extending the applications of these paper sheets as water repellent publications, packages and information storage materials. (Super)hydrophobic fabrics can also be produced by the same manner, as shown in Figure 7h, after TCVS-FDT modification, superhydrophobic or highly hydrophobic fabrics (cotton, silk, linen and woolen) were obtained. These (super)hydrophobic fabrics were placed in outdoor environment for 1 month, and WCA test on the cloth (Figure S11) shows that their water repellency are well kept after exposing to the real-life conditions. The coated silane layers also exhibit good stability to mechanical damage, after treating the FDT cotton with sandpaper for 50 cycles, (Figure 7i, the cotton fiber is partly damaged in this case), the cotton still keeps water-repellency, describing the potential of our method in the fabrication of waterproof clothes. Since paper substrates are commonly included in bio-applications, for example, as microfluidic substrates for paper based analytical devices (µPADs) and cell incubation scaffolds, we also investigated the possibility to apply our method in bio-applications. Paper based microfluidic analytical devices (µPADs) have been applied in many fields such as clinical diagnostics, food safety and environmental monitoring, due to their portability, ease-of-use, lowcost and minimal sample consumption features. Many techniques including wax printing, photolithography and CVD, have been developed to produce patterned paper microfluidic devices. However, these methods suffered from some restrictions such as fabrication speed, cost, simplicity and feasibility. In addition, most of these methods are convenient for creating hydrophobic barriers on paper substrates, but exhibit limitations when chemically functionalizing the hydrophilic channels. With our method, properties of both the hydrophobic barriers and the


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hydrophilic channels can be precisely designed and controlled. For example, multichannel microfluidics with different geometries can be easily fabricated on paper by using different photomask (Figure 8a, b). The functionalities (e.g., hydroxyl group, carboxyl group and amino group) and their positions inside the hydrophilic channels can be precisely assigned (Figure 3a, b). By modifying the vinyl paper with hydrophobic/hydrophilic thiol mixtures (dodecanethiol and ME as examples) with predesigned ratios, the liquid flowing speed on hydrophilic channels can be controlled, to govern the interaction time between the paper substrate and the test liquids during the analysis (Figure 8c). The interaction between biomolecules and hydrophilic channels can be significantly decreased by attaching anti-fouling PEG chains (Figure 8d), resulting antifouling channels that can decrease the unexpected adsorption of analytes.1 To confirm that the modified paper can be used as µPADs, we fabricated a microfluidic chip on Whatman 1# filter paper for fluorescent detection of urea and lactate, which are known as two biomarkers in human sweat (the details of the microchip and the testing process can be seen in Figure S12). As shown in Figure S12, the fluorescent intensity of the detection reservoir (2) was linearly correlated to the urea concentration from 0.595 to 7.14 mM, and the fluorescent intensity of the detection reservoir (3) was linearly correlated to the concentration of lactate from 1 to 300 μM. The limit of detection (LOD), which was calculated as 3 times the standard deviation of the testing results of the blank divided by the slope of the calibration curve, is 0.497 mM for urea and is 0.331 μM for lactate, respectively, which are consistent with the test results of normal filter paper. Thus the modified paper can be employed as substrate materials for µPADs. Paper have been commonly employed as cell incubation substrates because they are lowcost, patternable and reshapable, uniform in 3D dimensions and allows multilayer incubation. The improvement of biocompatibility of the paper substrate can lead to better cell adhesion,


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survival, proliferation and progenitor differentiation.53 We analyzed the biocompatibility of the paper substrate with Human bone mesenchymal stem cells (HBMSCs), by culturing green fluorescent protein (GFP) labeled HBMSCs on unmodified filter paper, mercapto ethanol (ME) modified vinyl paper and cysteamine (CA) modified vinyl paper (Figure 8e). The cellular morphology on these paper sheets were observed after 1, 3 and 5 days’ incubation. As shown in Figure 8e, unmodified paper substrate does not exhibit favorable biocompatibility with HBMSCs, the cells were oval-like with few pseudopodia formed, while cells on both ME and CA modified paper sheets presented long spindle-shaped adherent growth with more pseudopodium formed than those on unmodified paper, indicating that both ME and CA modified paper are capable of providing a preferential environment for driving HBMSCs adhesion (Figure 8e). Thus the biocompatibility of the paper substrate can be significantly improved by simply applying a modification process with our method. The method can also be applied on clinical cloth to improve their biomedical performances. For example, we applied TCVS-CA modification on cotton gauze (CA gauze), which was commonly used in wound healing treatment, and investigated the effects of this gauze on mouse wound healing model (unmodified cotton gauze as comparison). As shown in Figure 8f, wound healing was accelerated by using CA gauze, after 14 days, the mouse in the CA gauze group showed larger wound closure than that observed in the unmodified gauze group. This demonstrated that CA gauze could protect cutaneous wounds and promote wound healing. These results suggest this modified method have great potential in promoting the development of tissue regeneration via favoring cell adhesion and wound reepithelialization.

Conclusions


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As conclusions we developed a general and fast method to chemically functionalize paper and create patterns. We show that by combining trichlorovinylsilane treatment and thiol-ene photochemistry, a paper substrate can be arbitrary functionalized in minutes. The method is simple, fast and can be applied on different types of paper sheets as well as natural fabrics. Various types of functionalities could be covalently attached onto vinyl paper through phototriggered thiol-ene reaction, meanwhile keeping the characteristic feature of the paper substrates such as color, transparency and conductivity. Due to the spatial and temporal controllability of the thiol-ene photochemistry, 2D and 3D patterns with high resolutions can be easily generated by applying photomask or controlling the time of UV. We also show the successful application of our method in the fabrication of superhydrophobic coatings and as advanced biomaterials. We expect this simple functionalization method to be widely applied in paper based materials and devices.


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Figure 1. Schematic illustration of the functionalization of paper surfaces. First, the paper substrates were treated with trichlorovinylsilane (TCVS) for 5 min, to introduce a reactive vinyl monolayer on cellulose fibers. Second, vinyl groups were modified with various thiol-containing molecules via the photoclick thiol−ene reaction, to introduce different functionalities at the designed areas.


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Figure 2. Characterizations of the unmodified and modified cellulose paper. (a) WCA of the vinyl paper obtained by treating the cellulose paper with TCVS for0.5, 1, 2, 5, 10, 20, 30 and 40 min. The result show that TCVS modification could be completed in 5 min. As comparisons, trimethoxyl vinyl silane (TMVS) was also used to modify cellulose paper by our method (TMVS 1) and reported method (TMVS 2), the results show that after 40 min incubation in TMVS solution, no WCA change could be observed on paper under both conditions. The bare paper and TMVS modified paper are all superhydrophilic, that the WCA can not be measured and the values were set to be 5° in the figure. (b) Raman spectra of the bare paper, vinyl paper and FDT paper (FDT modified vinyl paper). (c) ATR-FTIR spectra of the bare paper, vinyl paper and FDT paper. (d) XPS spectra of the bare paper, vinyl paper and FDT paper, and (e) the detailed scan on (i) C1s peak, (ii) O1s peak and (iii) F1s peak. (f) Detailed analysis on C1s peaks of the XPS spectra.


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Figure 3. Introducing further functionalities on vinyl paper through thiol-ene photochemistry. (a) Schematic representation of thiol-ene modification on vinyl paper. Various types of functionalities could be attached onto vinyl paper through phototriggered thiol-ene reaction, as confirmed by the WCA change before and after modification. (b) Contact angle of water, DCM and ethanol on bare paper and different modified paper. The modifying molecules are 1H,1H,1H,2H-Perfluorodecanethiol

(FDT),

dodecanethiol

(DT)

and

mercaptoethanol,

respectively. In many case the paper is superwettable, the contact angle is not measurable and is set to be < 5°. (c) Kinetics test of the thiol-ene modification process. The WCA of the modified vinyl paper were measured at different time points to monitor the kinetics of the modification process, the results shown that for all thiols, the modification were completed in 15s. For superhydrophilic paper surfaces the WCA was set to be 5° in the figure.


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Figure 4. Modification on different types of paper and fabrics. (a) WCA change of the paper substrates during vinylization process. For all types of paper, the modification processes were finished in 5min. (b) The modification does not change the appearance and some feature properties of the paper substrates, for example, the color of (i) Whatman#1 paper and (ii) “Post it” paper, the transparency of (iii) tracing paper, and the conductivity of (iv) toner paper. (c) The method can also by applied on other natural fiber based materials, examples are shown by performing TCVS functionalization on cotton (i) and silk (ii), the significant WCA change after vinylization confirmed the successful attachment of vinyl silane layer on these materials. (d) Long-term stability test on vinyl paper and FDT paper indicates the excellent stability of the coated silane layers, as revealed by the small WCA change over 90 days. The vinyl paper still keeps the reactivity after 90 days and can be easily modified by FDT. (e) The coated vinyl silane


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layer shows good stability upon various solvents. As confirmed by the tiny WCA change after 24h immersion in EtOH, acetone, DCM, THF and toluene.

Figure 5. Spatial control of functionalities on paper through photopatterning process. (a) Schematic illustration of the patterning process. Vinyl paper was modified with one type of thiol under UV through a photomask to partly functionalize the surface, then the paper was flood irradiated with another thiol to result the final patterns. Scale bar: 1 cm. (b) - (e) Photo of different superhydrophobic-superhydrophilic patterns on Whatman#1 paper. Scale bar: 5 mm. (f) Patterning FITC-SH on the paper, resulting a fluorescence pattern under 365 nm UV light. (g) Patterning Rhodamine-SH on the paper, resulting a red fluorescence pattern under fluorescence microscope. (h) Resolution test on Whatman#1 paper by forming superhydrophobicsuperhydrophilic lines with different width.


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Figure 6. Patterning on Z direction of the paper substrate. (a) Schematic illustration of the patterning process. Vinyl paper was modified with one type of thiol (hydrophilic thiol as example) under UV with limited time, to partly functionalize the paper. Then the paper was flood irradiated with another thiol, to result a modified paper with multilayer functionalities. (b) WCA on both sides of the modified filter paper with different time of hydrophilic modification. The results show that paper with anisotropic wettabilities can be formed by controlling the time of UV irradiation. (c) Visualization of the anisotropic wettabilities on modified paper by adding Rhodamine B solution on modified paper and analyze the substrate from cross section. Scale bar: 1 mm. (d) Formation of 3D patterns on paper substrate by combining the spatial (use photomask) and temporal (control UV time) controllability of the thiol-ene chemistry. Scale bar: 5 mm.


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Figure 7. Application of the method on the formation of (super)hydrophobic paper and fabrics. Different types of paper, for example, (a) filter paper, (b) copy paper and (c) “Post it” paper, can be easily transferred to highly hydrophobic or superhydrophobic paper by this method. The characteristic feature of the paper, such as (d) color and (e) conductivity, can be retained after modification, to result functional superhydrophobic surfaces. (f) Writing can be performed on SH paper, and (g) the chirography can be retained after rinsing, due to the porous nature of the paper. (h) Various types of cloth can be modified to highly hydrophobic or superhydrophobic via TCVS functionalization and FDT modification. (i) After treating FDT cotton fabrics with sandpaper for 50 cycles (cotton fibers are partly destroyed), the damaged FDT cotton fabrics still shows high water-repellent properties, confirming the excellent stability of the silane coating to mechanical damage.


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Figure 8. Applying the method on bioapplications. µPADs with predesigned channels can be easily obtained by using corresponding photomask, examples are shown in (a) and (b). (c) The liquid flowing speed in µPADs can be exactly controlled, either faster or slower than normal paper, by modifying the paper with hydrophobic/hydrophilic thiols with different ratios. (d) The adhesion of paper substrate to biomolecules can be controlled by modifying PEG-SH on the vinyl paper. A human IgG/Cy3 protein with yellow/orange fluorescence was used as a model biomolecule in the protein adsorption tests. (e) HBMSCs incubated on cellulose paper, ME modified vinyl paper and CA modified vinyl paper at 1, 3 and 5 days. Long spindle-shaped adherent growth with more pseudopodium formation was observed on both ME and CA modified paper. Scale bar: 200 μm. (f) Mouse wound healing models by cotton gauze and CA gauze were observed on 0, 7 and 14 days. CA modified gauze induced more re-epithelialization than cotton gauze.


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Table 1. Contact angle of different paper surfaces before and after modification.

Substrate

Type

Filter paper

Copy paper

“Post it” paper

Tracing paper

Toner paper

Static Contact Angle

Adv

Rec

Water

DCM

EtOH

Water

Water

Normal

< 5°

< 5°

< 5°

< 5°

< 5°

FDT

158 ± 4°

93 ± 2°

76 ± 5°

164 ± 3°

152 ± 4°

ME

< 5°

< 5°

< 5°

< 5°

< 5°

Normal

116 ± 3°

< 5°

< 5°

131 ± 2°

< 5°

FDT

135 ± 2°

81 ± 4°

27 ± 11°

147 ± 4°

102 ± 5°

ME

< 5°

< 5°

< 5°

< 5°

< 5°

Normal

114 ± 4°

< 5°

< 5°

122 ± 4°

< 5°

FDT

140 ± 3°

82 ± 2°

64 ± 4°

157 ± 5°

98 ± 3°

ME

< 5°

< 5°

< 5°

33 ± 4°

< 5°

Normal

102 ± 4°

< 5°

< 5°

133 ± 3°

< 5°

FDT

134 ± 1°

53 ± 3°

38 ± 3°

165 ± 2°

112 ± 4°

ME

84 ± 3°

< 5°

< 5°

117 ± 3°

< 5°

Normal

< 5°

< 5°

< 5°

< 5°

< 5°

FDT

152 ± 9°

79 ± 3°

34 ± 2°

163 ± 6°

147 ± 4°

ME

< 5°

< 5°

< 5°

< 5°

< 5°

Adv: advancing water contact angle Rec: receding water contact angle Normal: unmodified paper substrates. FDT: modified by 1H, 1H, 2H, 2H-perfluorodecanethiol ME: modified by mercaptoethanol


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ASSOCIATED CONTENT . Supporting Information. Experimental information, supporting figures and video S1. The following files are available free of charge. Experimental information and supporting figures (PDF) Video S1 (mp4) AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †:

M. Wang and Y. L. Wang contributed equally.

Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT The research is supported by the National Key R&D Program of China (No. 2017YFA0700500). X. Du thanks the Fundamental Research Funding from Southeast University (3207048422) and


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Jiangsu province (BK20170662). Z. Z. Gu thanks the funding from National Science Foundation of China (No. 21327902). Y. L. Wang thanks the support from National Natural Science Foundation of China (81800936), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, 2018−87), the China Postdoctoral Science Foundation Funded Project (2018M640503), the Natural Science Foundation of Jiangsu Province (BK20180668), and the Jiangsu Postdoctoral Science Foundation Funded Project. X. Du and Y. L. Wang thanks the Southeast University-Nanjing Medical University Cooperative Research Project (2242018K3DN17) for funding support.

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Fast Strategy to Functional Paper Surfaces - ACS Applied Materials

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