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A Universal Coating Strategy for Controllable Functionalized Polymer Surfaces

Shihua Mao, Dong Zhang, Yanxian Zhang, Jintao Yang,* and Jie Zheng*

Development of a universal and stable surface coating, irrespective of surface chemistry or material characteristics, is highly desirable but has proved to be extremely challenging. Conventional coating strategies including the com-monly used catechol surface coating are limited to either a certain type of substrates or weak and unreliable surface bonding. Here, a simple, robust, and universal surface coating method capable for attaching any stimuli-responsive glycidyl methacrylate (GMA)-based copolymer, consisting of one surface-adhesive moiety of epoxy groups and one stimuli-responsive moiety, to any type of hydrophobic and hydrophilic surfaces via a one-step ring-opening reaction is proposed and demonstrated. The resultant GMA-based copolymers are not only strongly adhered on different substrates (e.g., silicon, polypropylene, polyvinyl chloride, indium tin oxide, polyethylene tere-phthalate, aluminum, glass, polydimethylsiloxane, and even polyvinylidene fluoride with low surface energy), but also are possessed distinct thermal-, pH-, and salt-responsive functions of bacterial killing, bacterial releasing, tunable multicolor fluorescence emission, and heavy metal detection. This coating method is also compatible with the directional quaternization of GMA-based copolymers for further improving surface adhesion and function-ality. This study provides a simple yet universal coating method to solve the long-standing challenge of robust integration of stimuli-responsive polymers with strong adhesion between various polymers and substrates.

DOI: 10.1002/adfm.202004633

S. Mao, Prof. J. YangCollege of Materials Science & EngineeringZhejiang University of TechnologyHangzhou 310014, P. R. ChinaE-mail: yangjt@zjut.edu.cnD. Zhang, Y. Zhang, Prof. J. ZhengDepartment of ChemicalBiomolecularand Corrosion EngineeringThe University of AkronAkron, OH 44325, USAE-mail: zhengj@uakron.edu

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202004633.

desirable properties (e.g., physical, chem-ical, mechanical, magnetic, electronical, and biological properties) and property changes in response to temperature,[2] ionic strength,[3] chemical or biological analytes,[4] electric field,[5] light at dif-ferent wavelengths,[6] and other external stimuli[7] in either reversible or irreversible manner.[8] Intuitively, surface function-alization by stimuli-responsive polymers is considered as a common strategy to realize stimuli-responsive polymer films and coatings. There are two types of sur-face functionalization strategies, i.e., chemical anchoring and physically attach-ment, to coat stimuli-responsive poly-mers and retain their stimuli-responsive properties on the surfaces in a control-lable way, including surface-initiated atom transfer radical polymerization,[9] catechol derivatives codeposition,[10] thiol-ene click-chemistry,[11] and diels-alder reaction.[12] Chemical anchoring strategies require the premodification of a surface with ini-tiators for polymerization, but it is easier to achieve a higher grafting density and a stronger surface adhesion.[13]

Physical attachment strategies (e.g., host–guest interaction,[14] layer-by-layer assembly,[15] supramo-lecular interaction,[16] and metal coordination interaction[17]) are simple and easy to carry out, but often suffer from weak and unstable polymer coatings on the surfaces due to intrinsic nature of interfacial physical bonds. Inspired by natural marine mussels, 3, 4-dihydroxy-l-phenylalanine (DOPA), as a surface-binding protein expressed by mussels, has been extensively studied and used as a universal adhesive group for conjugating with different polymers to coat DOPA-based polymers onto a variety of hydrophobic and hydrophilic surfaces, including glass, ceramic, titanium, gold, and Teflon.[10a] However, the challenge still remains. Due to the nature of DOPA-induced physical adhesion of polymers on surfaces, DOPA-based polymer coatings often have weak surface adhesion as evi-denced by weak adhesion energy of 100 kJ mol−1.[18] Even small external environmental changes (e.g., ultrasound, acid solution) can cause DOPA-based coatings to be fall off from the surfaces. Another roadblocker is that both chemical and physical coating strategies usually lack the ability of on-demand bonding and debonding in a reversible way.[19]

Currently, most of surface coating methods are only spe-cific to a certain type of surfaces. There is lack of a general

1. Introduction

Bulk stimuli-responsive polymers are known to exhibit con-formational transitions from the disordered coils to the more ordered conformations[1] (e.g., alignment and organiza-tion of polymer chains, formation of secondary structures, occurrence of crystallizations) for achieving a wide variety of

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surface coating method applicable to any surfaces. To address this challenge, we proposed and developed a simple, robust, and universal surface coating method, which enabled to attach the epoxy-modified functional polymers onto different aminated surfaces via “one-step” ring-opening reaction. Alter-native to DOPA-based polymers that used a catechol group as an adhesive motif, we discovered a stronger and more stable adhesive motif of epoxy groups, which in principle can attach to any type of surfaces in the presence of amine groups. To prove this design, we copolymerized GMA with a wide variety of functional polymers to form antibacterial poly(GMA-co-METAC), salt-responsive poly(GMA-co-DVBAPS), tem-perature-responsive poly(GMA-co-NIPAM), pH-responsive poly(GMA-co-DMAEMA), fluorescence-emission poly(GMA-co-SPMA), poly(GMA-co-NDBCB), and poly(GMA-co-PyMA), all of which can be coated on different aminated substrates via the nucleophilic attack-induced ring opening reaction between epoxy groups of copolymers and aminated groups of the substrates, including silicon, polypropylene (PP), poly-vinyl chloride (PVC), indium tin oxide (ITO), polyethylene terephthalate (PET), aluminum, glass, polydimethylsiloxane (PDMS), and even polyvinylidene fluoride (PVDF) with low surface energy. This GMA-assisted coating strategy allows the direct attachment of different functional polymers onto different substrates in a single step with uniform and highly densed polymer topologies. The resultant GMA-coated polymer coatings achieved distinct functions of highly effi-cient sterilization (>95%), controlled release of bacteria (≈96%), tunable multicolor fluorescence emission, and fast detection of hypertoxic Hg2+ ions. Furthermore, directional quaternization derived from tertiary amine residues also endowed different GMA-based coatings to possess a common antibacterial property, while still remaining other functions. This work demonstrates a new universal coating and func-tionalization method, allowing to attach a wide variety of polymers to any surface, irrespective of surface chemistry, or material characteristics, which offers new impacts on a wide range of surface coating and engineering applications from medical devices to consumable products.

2. Results and Discussions

2.1. Design, Synthesis, and Coating of Functional GMA-Based Copolymers

Figure  1a shows a one-step, universal, ring-opening surface coating method for anchoring different GMA-based, stimuli-responsive copolymers on any aminated-substrate to achieve distinct antibacterial functions and highly stable coatings. Briefly, our design strategy is twofold: we first copolymerized GMA with stimuli-responsive polymers to form GMA-based, stimuli-responsive copolymers, where GMA acted as a surface adhesion motif, while stimuli-responsive polymers served as surface functionality. Second, epoxy groups of copolymerized GMA residues interacted with amine groups of aminated-substrate via a nucleophilic attack-induced ring-opening reac-tion to form robust GMA-based stimuli-responsive coating with controllable thickness and high packing density. To prove our

hypothesis, we synthesized seven different stimuli-responsive monomers to copolymerize with GMA, including salt-respon-sive 3-(dimethyl (4-vinylbenzyl) ammonium) propyl sulfonate (DVBAPS), pH-responsive diethylaminoethyl methacrylate (DMAEMA), temperature-responsive N-isopropylacrylamide (NIPAM), antimicrobial 2-(methacryloyloxy) ethyl] trimeth-ylammonium chloride (METAC), and fluorescent 2-(acryloy-loxy)ethyl 4-(3a1,5a1-dihydropyren-1-yl)butanoate (PyMA), N-((2-((2-allyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)amino)ethyl)-carbamothio-yl)-benzamide (NDBCB), and 2-(3′,3′-dimethyl-6-nitrospiro[chromene-2,2′-indolin]-1′-yl)ethyl acrylate (SPMA) using traditional free radical polymerization to produce poly(GMA-co-DVBAPS), poly(GMA-co-DMAEMA), poly(GMA-co-NIPAM), poly(GMA-co-METAC), poly(GMA-co-PyMA), poly(GMA-co-NDBCB), and poly(GMA-co-SPMA) copolymers (Figure  1b). These functionalized coatings later demonstrated their distinct functions to prevent bacterial adsorption on the coatings, release dead bacterial from the coatings, fluorescent emission from the coatings, and heavy toxic metal detections on the coatings, all of which had poten-tial applications in antibacterial or related bioapplications (Figure 1c).

The structural compositions of the GMA-based copolymers were confirmed by 1H-NMR spectra in Figure 2a–g. Obviously, all polymer coatings showed a symmetrical chemical shift at 2.7–3.5 ppm, corresponding to the methylene protons of epoxy groups. Specifically, chemical shifts at ≈3.2 ppm were attributed to the methyl resonance of (CH3)3N+ (poly(GMA-co-METAC)) and N(CH3)2 (poly(GMA-co-NIPAM)) groups, while char-acteristic peaks located at 7.8–8.6, ≈8.0, 6.8–7.2, and ≈7.0  ppm were assigned to the proton resonance on aromatic heterocycte (poly(GMA-co-NDBCB)), pyrene (poly(GMA-co-PyMA)), and ben-zene groups (poly(GMA-co-DVBAPS) and poly(GMA-co-SPMA)), respectively. Scheme S1 (Supporting Information) also shows the synthesis procedure of four uncommercial monomers of PyMA, NDBCB, SPMA, and DVBAPS, whose chemical structural char-acterizations by 1H-NMR spectra were shown in Figures S1–S4 (Supporting Information). In parallel, Fourier-transform infrared (FT-IR) spectra in Figure S5 (Supporting Information) showed that all of GMA-based copolymers shared three characteristic adsorption peaks at 1740, 1012, and 906 cm−1, assigning to the stretching vibration of carbonyl and epoxy groups in GMA. Moreover, every GMA-based copolymer also had its own dis-tinct peak, i.e., ≈1644 cm−1 peak from the stretching vibration of CONH groups of poly(GMA-co-NIPAM), ≈1364 cm−1 peak from the stretching vibration of tertiary amine groups of poly(GMA-co-DMAEMA), ≈827 cm−1 peak from the stretching vibration of aromatic groups of poly(GMA-co-DVBAPS), ≈1259  cm−1 from the strongly bonded primary amine group (RNH2) of poly(GMA-co-NDBCB), and ≈1340 and ≈1447  cm−1 from the stretching vibration of tertiary amine (NR3) and pyrene groups of poly(GMA-co-SPMA) and poly(GMA-co-PyMA). We also characterized the molecular weight distribution of seven functional copolymers using gel permeation chromatography, where linear polystyrene was used as a reference (Table S1, Supporting Information). Figure  2h shows a high-resolution survey scan of X-ray photoelectron spectroscopy (XPS) spectra to characterize GMA-based copolymer coatings. The Cl2p scan showed a peak at ≈198  eV for poly(GMA-co-METAC) coating,

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Figure 1. a) A one-step, universal surface coating method for anchoring different glycidyl methacrylate (GMA)-based, stimuli-responsive copolymers on any aminated-substrate via the ring-opening reaction between epoxy groups of GMA and amine groups of the substrates. b) GMA and seven stimuli-responsive monomers to be copolymerized into different surface adhesive and functional polymer coatings, including salt-responsive 3-(dime-thyl (4-vinylbenzyl) ammonium) propyl sulfonate (DVBAPS), pH-responsive diethylaminoethyl methacrylate (DMAEMA), temperature-responsive N-isopropylacrylamide (NIPAM), antimicrobial 2-(methacryloyloxy) ethyl] trimethylammonium chloride (METAC), and fluorescent 2-(acryloyloxy)ethyl 4-(3a1,5a1-dihydropyren-1-yl)butanoate (PyMA), N-((2-((2-allyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)amino)ethyl)-carbamothio-yl)-benza-mide (NDBCB), and 2-(3′,3′-dimethyl-6-nitrospiro[chromene-2,2′-indolin]-1′-yl)ethyl acrylate (SPMA). c) GMA-based copolymer coatings to achieve their distinct functions for surface resistance to live bacteria, surface releasing of dead bacteria, surface killing and releasing of the bacteria, and surface detection of heavy metal ions.

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but not for pristine silicon substrate. Further composition devo-lution of this new peak into two peak binding energy at ≈197.1 and ≈198.7  eV confirmed the Cl2p spin–orbit doublets, i.e.,

the successful modification of poly(GMA-co-METAC) coating. Similarly, a distinct peak at ≈168.2 eV, corresponding to sulfate groups, was observed in poly(GMA-co-DVBAPS).

Figure 2. Surface and composition characterizations of seven GMA-based copolymer coatings. a–g) 1H-NMR spectra of GMA-based copolymers, including poly(GMA-co-METAC), poly(GMA-co-NIPAM), poly(GMA-co-DMAEMA), poly(GMA-co-DVBAPS), poly(GMA-co-NDBCB), poly(GMA-co-PyMA), and poly(GMA-co-SPMA). h) High-resolution scans of XPS spectra (Cl2p and S2p) for poly(GMA-co-METAC) and poly(GMA-co-DVBAPS) coat-ings as compared to pristine silicon. i) Water contact angle of pristine silicon, aminated-silicon, and seven GMA-based copolymer coatings. j) AFM images of poly(GMA-co-METAC) coating and pristine silicon.

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Generally, surface modification always changes surface hydrophilicity at the first place. As shown in Figure  2i, initial amination-treated silicon significantly increased its hydrophi-licity as evidenced by a large reduction of water contact angle from 68.3° to 22.3°. Then, upon coating seven GMA-based poly-mers onto amine-modified silicon substrates, the consump-tion of amine groups by interacting with epoxy groups, surface hydrophilicity of stimuli-responsive poly(GMA-co-METAC), poly(GMA-co-DVBAPS), poly(GMA-co-NIPAM), and poly(GMA-co-DMAEMA) coatings was reduced to 49.5°, 46.1°, 64.3°, and 56.5°, respectively. In sharp contrast, poly(GMA-co-SPMA), poly(GMA-co-NDBCB), and poly(GMA-co-PyMA) coatings with fluorescent groups exhibited high surface hydrophobicity, as indicated by the much higher contact angles of 87.3°, 72.1°, and 83.4°, respectively. Atomic force microscope (AFM) images confirmed that poly(GMA-co-METAC) coatings, as an typical example, completely covered the silicon substrates and largely remained surface continuity (Figure  2j). Small surface rough-ness of all polymer coatings was ≈1.4–3.9 nm, indicating homo-geneous polymer coatings at nanometer scale. Taken together, we have demonstrated this simple and universal coating method capable of attaching any copolymer consisting of one functional moiety and one surface-adhesive moiety to any amine-modified surface with a uniform and smooth morphology.

2.2. Stimuli-Responsive Polymeric Coatings

Next, we tested the distinct function of two types of stimuli-responsive polymer coatings, i.e., antibacterial coatings and fluorescence coatings in response to different stimuli. First, as a proof-of-concept, we investigated the antibacterial perfor-mance (including bacterial resistance, killing, and release) of poly(GMA-co-METAC) coatings using Escherichia coli (E. coli, Gram-negative) and Staphylococcus aureus (S. aureus, Gram-pos-itive). A series of poly(GMA-co-METAC) coatings were prepared under different copolymer concentrations of 0.2–1.0 g mL−1, and their typical surface morphologies were provided in Figure S6 (Supporting Information). Figure  3a,b showed that after 24 h incubation of E. coli and 12 h incubation S. aureus with poly(GMA-co-METAC) at 37  °C, the increase of poly(GMA-co-METAC) concentration from 0.2 to 1.0 g mL−1 led to the higher killing efficiency from ≈8.9% to ≈95.1% of E. coli bacteria and from ≈20% to ≈94.8% of S. aureus bacteria, as compared to extremely low killing efficiency of E. coli (2.0%) and S. aureus (1.8%) by pure aminated-silicon (Figure S7, Supporting Infor-mation). The superior bacterial killing property of poly(GMA-co-METAC) coating is likely attributed to the contact killing capacity of quaternary ammonium groups, which interrupts bacterial metabolism. However, surface hydrophilicity is a nec-essary but not sufficient condition for antibacterial surfaces.

Apart from the bacterial killing function of poly(GMA-co-METAC) coatings, we tested the bacterial releasing function of three salt-responsive poly(GMA-co-DVBAPS), thermo-respon-sive poly(GMA-co-NIPAM), and pH-responsive poly(GMA-co-DMAEMA) coatings. After 24 h coculture with E. coli and 12 h coculture with S. aureus, salt-responsive poly(GMA-co-DVBAPS) released 91.1% of attached E. coli and 93.5% of attached S. aureus after switching DI water (4.6 × 105 and

7.4 × 105 cells  cm−2) to 1 m NaCl solution (4.1 × 104 and 4.8 × 104 cells cm−2) (Figure 3c,d; and Figure S8, Supporting Informa-tion). Thermo-responsive poly(GMA-co-NIPAM) released 87.4% of E. coli and 93.2% of S. aureus after switching warm water of 40 °C (2.7 × 105 and 3.3 × 105 cells cm−2) to cold water of 4 °C (3.4 × 104 and 2.6 × 104 cells cm−2). pH-responsive poly(GMA-co-DMAEMA) coatings released ≈98.8% of E. coli and ≈92.1% of S. aureus after switching acidic solution of pH = 1 (1.9 × 106 and 3.8 × 105 cells cm−2) to alkaline solution of pH = 12 (2.2 × 104 and 3.0 × 104 cells  cm−2). Such bacterial releasing property of three polymer coatings was mainly stemmed from stimuli-responsive-induced phase transition of polymer coatings. Specifically, polyNIPAM chains exhibited a sharp phase tran-sition by changing its hydrophilicity to hydrophobicity as temperatures were above LCST (lower critical solution tem-perature), as evidenced by water contact angle changing from 51.3° to 76.3° (Figure S9, Supporting Information). Differently, poly electrolyte/antipolyelectrolyte properties in zwitterionic DMAEMA and polyDVBAPS chains made them undergo con-formational changes between collapsed and extended states in response to external changes of pH and ionic strength.

In parallel, the proposed strategy can also construct fluores-cent polymer coatings, which can be directly visualized under the excitation of 365 nm UV light. Figure 3e showed that both coatings and solutions of poly(GMA-co-SPMA), poly(GMA-co-NDBCB), and poly(GMA-co-PyMA) emitted intense red, green, and blue color, respectively. Quantitatively and consistently, each coating also exhibited distinct strong fluorescence band around 550–600, 450–600, and 380–400  nm (Figure  3f). Three copolymer coatings displayed the same fluorescence emission as the bulk copolymers in dimethylsulfoxide solution. The con-sistent bright emission is believed to stem from the spiropyran, naphthalimide, and pyrene luminogens, which are excited by ultraviolet light with specific wavelength. This phenom-enon also demonstrates that such mild ring-opening reaction does not destroy apparent spacing between those luminogens. Specifically, poly(GMA-co-NDBCB) coatings also endowed sensing ability in response to soluble Hg2+ ions in aqueous solution. As shown in Figure 3g, in the presence of Hg2+ ions, poly(GMA-co-NDBCB) coatings can rapidly switched green-light-emission to blue-light-emission and emission peaks were also shifted from ≈520 to ≈425 nm, due to the formation of guani-dine derivatives via desulfurization and recyclization of thiourea groups.[20] Further, different concentrations of Hg2+ were tested for the sensing ability of poly(GMA-co-NDBCB) coatings. A two-stage linear relationship between Hg2+ concentrations and fluorescence intensity revealed a correlation coefficient value R2 = 0.98 and determined ≈0.001 × 10−6 m as the lowest detection limit of Hg2+ ions (Figure S10, Supporting Information). To fur-ther investigate the detection selectivity of poly(GMA-co-NDBCB) coatings, we further examined the detection selectivity of poly(GMA-co-NDBCB) coatings in response to different com-binations of heavy metal ions in the presence and absence of Hg2+. As shown in Figure S11 (Supporting Information), when poly(GMA-co-NDBCB) coatings were treated with different combinational ion solutions without Hg2+ ions (i.e., Pb2++Ba2+, Cu2++Zn2+, Fe2++Cu2+, Al3++Cr3+, and Pb2++Ba2++Cr3+), a sharp and stable emission peak was observed at ≈520  nm without any change, indicating no interaction between ions and

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poly(GMA-co-NDBCB) coatings. In contrast, when ion solutions contained Hg2+ (Hg2+, Hg2++Cu2+, and Hg2++Zn2++Cr3+), the emission peak of poly(GMA-co-NDBCB) coatings was blueshift to ≈425 nm immediately, indicating that poly(GMA-co-NDBCB) coatings were only specifically responsive to Hg2+ ions.

To guide a rational design GMA-based coatings with suf-ficient surface adhesion and favorable functions, we used poly(GMA-co-METAC) as an example to determine its func-tional range of GMA:monomer ratios for retaining its bacterial killing property. A series of poly(GMA-co-METAC) surfaces were prepared by varying GMA:METAC ratios from 20:1, 10:1, 5:1, 1:1, 1:5, 1:10, to 1:20. Figure S12 (Supporting Information) sum-marizes the representative microscopic images to show E. coli killing efficiency of poly(GMA-co-METAC) coatings up to 24 h coculture. Clearly, there existed an optimal GMA:METAC ratio

of 1:1 to achieve the maximal bacterial killing efficiency of 97.0%. Too high or too low GMA:METAC ratios led to the reduction in bacterial killing efficiency. Specifically, at high GMA:METAC ratios of 20:1–10:1, poly(GMA-co-METAC) coatings exhibited the relatively low bacterial killing efficiency of 4–54%. These results indicate that surface adhesion property from GMA and bacterial killing property from METAC work in a cooperative way to optimize the coating functions. Based on these data, we prepared a normalized phase diagram to show the relation-ship between coating properties and GMA:monomer ratios for GMA-based functional coatings in Figure  3h. It can be seen that sufficient surface adhesion is required to achieve the desir-able function of the GMA-based surfaces, i.e., GMA:monomer ratios of 0–0.2 (red area) lead to insufficient surface adhesion, while GMA:monomer ratios of >5.0 (green area) result in

Figure 3. Distinct functions of different GMA-based copolymer coatings. Representative fluorescence microscopy to show the killing efficiency of poly(GMA-co-METAC) coatings of different coating concentrations (0–1.0 g mL−1 for a) E. coli for 24 h coculture and b) S. aureus for 12 h coculture. Representative fluorescence microscopy to show the releasing efficiency of c) E. coli and d) S. aureus for thermo-responsive poly(GMA-co-DVBAPS) (40 °C → 4 °C), pH-responsive poly(GMA-co-NIPAM) (pH = 1 → pH = 12), and salt-responsive poly(GMA-co-DMAEMA) (0 m → 1.0 m). Scale bar in (a–d) is 20 µm. e) Visual inspection of fluorescence emission of red, green, and blue from both solutions and coatings of respective poly(GMA-co-SPMA), poly(GMA-co-NDBCB), and poly(GMA-co-PyMA) under the excitation of 365 nm UV light. Scale bar is 1 cm for glass bottle, but 20 mm for silicon wafer. f) Fluorescence spectra of poly(GMA-co-SPMA), poly(GMA-co-NDBCB), and poly(GMA-co-PyMA) coatings to show dominant peaks corresponding to red, green, and blue color. g) Application of poly(GMA-co-NDBCB) coatings for Hg (II) detection via green-to-blue fluorescence switch. h) Normalized phase diagram to show the relationship between coating properties and GMA:monomer ratios for GMA-based functional coatings.

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insufficient desirable function groups, both of which will fail the coatings and functions of GMA-based surfaces. Moreover, it is worth pointing out that this normalized phase diagram actually is not suitable for fluorescent polymer system. A cut off GMA:fluorescent monomer ratio can be very high, because a fluorescence monomer is very sensitive and thus a very small amount of fluorescence monomers is sufficient enough to achieve its light-responsive function.

2.3. Universal Surface Bonding on Any Substrate

Another feature of our design is to realize the universal sur-face bonding of GMA-based copolymer to any surface ranging from hydrophobic to hydrophilic, including silicon, PP, PVC, ITO glass, PET, PVDF, aluminum, glass, and PDMS. Figure 4a shows a very simple dipping operation to achieve coating of GMA-based copolymers on different types of solid substrates.

As a proof, poly(GMA-co-DVBAPS) was first selected to dip-coat on different substrates, because polyDVBAPS possessed excellent salt-responsive and antipolyelectrolyte behavior that induced chain transformation from a collapse state in water to an extension state in salt solution. As shown in Figure  4b, upon dip-coating poly(GMA-co-DVBAPS) on nine substrates, water contact angle of poly(GMA-co-DVBAPS) coatings was 79.9°–133.3° on hydrophobic substrates of PP, PVC, PVDF, PDMS, and PET, in contrast to 64.7°–76.8° on hydrophilic substrates of silicon, glass, aluminum, and ITO glass. Clearly, hydrophobic substrates induce the higher water contact angle than those hydrophilic substrates. Differently, when switching from pure water to salt solution, all poly(GMA-co-DVBAPS) coatings in salt solution showed different degrees of reduction in contact angles. Among different substrates, poly(GMA-co-DVBAPS) coatings on hydrophobic polypropylene and PVDF exhibited the largest and least drops in water contact angle by 68.4° and 21.5°, respectively. Such different salt-responsive

Figure 4. Universal coatings of GMA-based copolymers on different substrates. a) Schematic of a “one-step” dip-coating method for attaching any GMA-based functional polymer to any aminated surface. b) Salt-induced contact angle changes on different poly(GMA-co-DVBAPS)-coated substrates, including silicon, polypropylene (PP), polyvinyl chloride (PVC), ITO glass, polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), aluminum, glass, and polydimethylsiloxane (PDMS). c) Polymer anchoring mechanism via single, double, and multiple anchoring sites between multiple epoxy groups in copolymer chains and aminated substrates. d) Comparison of different coating strategies between polydopamine codeposition,[10a,21] Au-S crosslinkers,[13a] polysisesquioxanes,[22] host–guest chemistry (layer-by-layer assembly),[23] metal coordination interaction,[10b,24] and this work in terms of their coating preparation, property, and performance.

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property could be due to the mismatch between amine groups on the substrates and epoxy groups in poly(GMA-co-DVBAPS) chains. It is also possible for multiple random epoxy groups located at copolymer chains to react with amine groups in a nonselective way, resulting a single, double, or multiple anchoring sites on substrates (Figure 4c).

We compared our coating strategy with the existing ones in terms of six different coating properties. In Figure 4d, polydo-pamine-based polymer coatings have advantages of mild reac-tion condition, simple operation, and universal coating, but surface adhesion (binding energy) was too weak to prolong their functions.[10a,21] Polysiloxane-based polymer coatings pos-sess averaged properties in universality and diversity of adap-tive substrate, multifunctions, and favor to strongly bond to hydrophilic soft materials, but slight moisture environment will greatly compromise the siloxane condensation polymeriza-tion process, thus damaging their surface coatings and coating functions.[22] Other surface coating strategies, including Au-S crosslinker-assisted,[13a] host–guest-induced,[23] metal coordi-nation interaction-assisted coatings,[10b,24] always require spe-cific linkers or interactions between polymers and substrates, showing substrate-dependent coating behaviors. Our simple, robust, and universal coating strategy outperforms the others in

overall performance from ease of preparation, strong binding energy, and versatile adapted substrates to multifunctions with desired service duration. More importantly, functional poly-mers are independent of coating preparation process, which will greatly enhance their applications in surface engineering.

2.4. Antibacterial Applications of Polymer Coatings

We further proposed a general quaternization modification of GMA-based polymer coatings by oxidation of tertiary amine groups, which was expected to improve specific bacterial killing and releasing properties of the GMA-based coating poly-mers. To test this hypothesis, we further modified salt-respon-sive poly(GMA-co-DVBAPS) coatings on silicon substrate to improve antibacterial performance by introducing quaternary ammonium groups by nucleophilic substitution with halohy-drocarbon under the bromoethane/DMF (v:v = 1:5) (Figure 5a). Water contact angles (water or 1.0 m NaCl solution) and FT-IR spectra of poly(QGMA-co-DVBAPS) coatings were presented in Figures S13 and S14 (Supporting Information), demon-strating that such quaternization modification did not affect salt-responsive performance of polyDVBAPS segment. Thus,

Figure 5. a) A general quaternization modification of GMA-based polymer coatings by oxidation of tertiary amine groups. Poly(GMA-co-DVBAPS) coating was selected as an example to demonstrate its improved antibacterial performance. b) Representative fluorescence microscopy images and c) quantitative statistics of killing and releasing E. coli on the modified poly(QGMA-co-DVBAPS) coatings on silicon with different molar ratios of DVBAPS:GMA = 3:1, 4:1, and 6:1 before and after the treatment of NaCl solution (1.0 m). d) Representative fluorescence microscopy images and e) quantitative statistics of killing and releasing E. coli on the poly(QGMA-co-DVBAPS) coatings on three different substrates of polydimethylsiloxane (PDMS), silicon, and bistoury before and after the treatment of NaCl solution (1.0 m) (Scale bar: 20 µm).

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poly(QGMA-co-DVBAPS) surface is expected to kill and release bacteria, in which QGMA containing quaternary ammonium group acts as bacterial killing agents, while salt-responsive DVBAPS serves as antifouling material to release dead bacteria using steric repulsion force. Figure 5b,c shows the representa-tive fluorescence microscopic images and quantitative statistics (including killing and releasing efficiency) of E. coli attachment on poly(QGMA-co-DVBAPS) coatings with different monomer feed molar ratios (e.g., DVBAPS:GMA = 3:1, 4:1, and 6:1) before and after the treatment of 1.0 m NaCl solution. The results showed the highest bacterial killing efficiency of ≈71% and bac-terial releasing efficiency of ≈95% at an optimal molar ratio of DVBAPS/GMA of 4:1.

We further coated this modified poly(QGMA-co-DVBAPS) on different substrates of PDMS, silicon, and bistoury to dem-onstrate its general bacterial killing and releasing capacity. It can be seen in Figure 5d,e that as compared to poly(QGMA-co-DVBAPS) coating on silicon wafers, three new coatings exhib-ited the higher killing efficiency of ≈72.7–81.5%, but similar bacterial releasing property of 86.6–92.3%. This indicates that on one hand, bacterial killing efficiency was highly correlated with the degree of quaternization (the amount of initial amine groups), while salt-responsive bacterial releasing property was not affected by this quaternization process. On the other hand, the quaternization process was substrate-dependent capable for anchoring the GMA or QGMA-based copolymers and their anchoring density.

3. Conclusions

In summary, we present a general yet simple coating method to achieve strong, universal adhesion of different stimuli-responsive GMA-based copolymers on diverse substrates via a one-step ring-opening reaction. The proposed method can provide robust interfacial integration of various stimuli-respon-sive polymers including temperature-responsive poly(GMA-co-NIPAM), pH-responsive poly(GMA-co-DMAEMA), and salt-responsive poly(GMA-co-DVBAPS) with surface adhe-sion epoxy groups in GMA on a wide range of hydrophobic and hydrophilic aminated solid substrates, including silicon, polypropylene, polyvinyl chloride, ITO, polyethylene tereph-thalate, polyvinylidene fluoride, aluminum, glass, and polydi-methylsiloxane. The resultant GMA-based copolymer coatings adhered on substrates achieve robust adhesion performance and distinct stimuli-responsive functions, including bacterial killing efficiency of >95%, thermal-, pH-, and salt-responsive bacterial releasing efficiency of ≈96%, tunable fluorescence emission, and ion-responsive detection of hypertoxic Hg2+ ions. The surface-adhesive moiety is based on a reductive group, whose epoxy ring-opening reaction in principle can be chemi-cally bonded to any type of surfaces in the presence of amine groups. Different from other physical coating methods gov-erned by self-adhesive polydopamine, host–guest molecular interactions, and metal coordination interactions, our polymer coatings demonstrated their ultraphysicochemical stability to resist surface disruption by stripping force, UV radiation, and even erosion by most organic solvents, greatly outperforming other coating methods. This work provides a different coating

strategy to realize the attaching of any kind of functional poly-mers to any surface, and thus has far-reaching implications for surface engineering applications.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsS.M. and D.Z. contributed equally to this work. J.Z. thanks financial supports from NSF (Nos. 1825122 and 1806138). J.Y. thanks financial supports from Natural Science Foundation of China (No. 51673175), and Natural Science Foundation of Zhejiang Province (Nos. LY16E030012 and LZ20E030004), and Special Foundation for M. E. of Zhejiang Provincial Education Department (No. GZ19621250015).

Conflict of InterestThe authors declare no conflict of interest.

Keywordsfunctionalized polymer surfaces, polymeric coatings, ring-opening reaction, stimuli-responsive polymers

Received: May 30, 2020Revised: June 30, 2020

Published online: August 6, 2020

[1] a) L.  Tang, D.  Zhang, L.  Gong, Y.  Zhang, S.  Xie, B.  Ren, Y.  Liu, F. Yang, G. Zhou, Y. Chang, J. Tang, J. Zheng, Macromolecules 2019, 52, 9512; b) D.  Zhang, B.  Ren, Y.  Zhang, L.  Xu, Q.  Huang, Y.  He, X. Li, J. Wu, J. Yang, Q. Chen, Y. Chang, J. Zheng, J. Mater. Chem. B 2020, 8, 3171; c) D. Zhang, B. Ren, Y. Zhang, Y. Liu, H. Chen, S. Xiao, Y. Chang, J. Yang, J. Zheng, J. Colloid Interface Sci. 2020, 578, 242.

[2] a) Y.  Tu, F.  Peng, X.  Sui, Y.  Men, P. B.  White, J. C. M.  van  Hest, D. A. Wilson, Nat. Chem. 2017, 9, 480; b) J. Wieme, K. Lejaeghere, G. Kresse, V. Van Speybroeck, Nat. Commun. 2018, 9, 4899.

[3] a) F.  Tantakitti, J.  Boekhoven, X.  Wang, R. V.  Kazantsev, T.  Yu, J. H.  Li, E.  Zhuang, R.  Zandi, J. H.  Ortony, C. J.  Newcomb, L. C.  Palmer, G. S.  Shekhawat, M. O.  de  la Cruz, G. C.  Schatz, S. I. Stupp, Nat. Mater. 2016, 15, 469; b) B. Wu, X. W. Wang, J. Yang, Z. Hua, K. Z. Tian, R. Kou, J. Zhang, S. J. Ye, Y. Luo, V. S. J. Craig, G. Z.  Zhang, G. M.  Liu, Sci. Adv. 2016, 2, e1600579; c) D.  Zhang, Y.  Fu, L.  Huang, Y.  Zhang, B.  Ren, M.  Zhong, J.  Yang, J.  Zheng, J. Mater. Chem. B 2018, 6, 950.

[4] a) L.  Yuan, W.  Lin, K.  Zheng, L.  He, W.  Huang, Chem. Soc. Rev. 2013, 42, 622; b) M. D.  Hammers, M. J.  Taormina, M. M.  Cerda, L. A.  Montoya, D. T.  Seidenkranz, R.  Parthasarathy, M. D.  Pluth, J. Am. Chem. Soc. 2015, 137, 10216.

[5] a) J. Ge, E. Neofytou, T. J. Cahill, R. E. Beygui, R. N. Zare, ACS Nano 2012, 6, 227; b) Y. Y. Yuan, C. Q. Mao, X. J. Du, J. Z. Du, F. Wang, J. Wang, Adv. Mater. 2012, 24, 5476.

[6] a) X.  Qian, Y.  Zhao, Y.  Alsaid, X.  Wang, M.  Hua, T.  Galy, H.  Gopalakrishna, Y.  Yang, J.  Cui, N.  Liu, M.  Marszewski, L.  Pilon, H.  Jiang, X.  He, Nat. Nanotechnol. 2019, 14, 1048;

Adv. Funct. Mater. 2020, 30, 2004633

www.afm-journal.dewww.advancedsciencenews.com

2004633 (10 of 10) © 2020 Wiley-VCH GmbH

b) L. F.  Kadem, M.  Holz, K. G.  Suana, Q.  Li, C.  Lamprecht, R.  Herges, C.  Selhuber-Unkel, Adv. Mater. 2016, 28, 1799; c) P.  Li, D.  Zhang, Y.  Zhang, W.  Lu, J.  Zhang, W.  Wang, Q.  He, P.  Théato, T. Chen, ACS Macro Lett. 2019, 8, 937.

[7] a) Z. Liu, W. Wang, R. Xie, X. J. Ju, L. Y. Chu, Chem. Soc. Rev. 2016, 45, 460; b) M. A. C.  Stuart, W. T. S.  Huck, J.  Genzer, M.  Muller, C.  Ober, M.  Stamm, G. B.  Sukhorukov, I.  Szleifer, V. V.  Tsukruk, M. Urban, F. Winnik, S. Zauscher, I. Luzinov, S. Minko, Nat. Mater. 2010, 9, 101; c) D. Roy, J. N. Cambre, B. S. Sumerlin, Prog. Polym. Sci. 2010, 35, 278; d) P. M. Mendes, Chem. Soc. Rev. 2008, 37, 2512.

[8] a) E. S. Gil, S. M. Hudson, Prog. Polym. Sci. 2004, 29, 1173; b) F. Liu, M. W. Urban, Prog. Polym. Sci. 2010, 35, 3.

[9] L. A.  Navarro, A. E.  Enciso, K.  Matyjaszewski, S.  Zauscher, J. Am. Chem. Soc. 2019, 141, 3100.

[10] a) S. M. Kang, N. S. Hwang, J. Yeom, S. Y. Park, P. B. Messersmith, I. S.  Choi, R.  Langer, D. G.  Anderson, H.  Lee, Adv. Funct. Mater. 2012, 22, 2949; b) E.  Filippidi, T. R.  Cristiani, C. D.  Eisenbach, J. H.  Waite, J. N.  Israelachvili, B. K.  Ahn, M. T.  Valentine, Science 2017, 358, 502.

[11] G. Herwig, A. P. Dove, ACS Macro Lett. 2019, 8, 1268.[12] M.  Vauthier, L.  Jierry, J. C.  Oliveira, L.  Hassouna, V.  Roucoules,

F. Bally-Le Gall, Adv. Funct. Mater. 2019, 29, 1806765.[13] a) W.  Li, X.  Liu, Z.  Deng, Y.  Chen, Q.  Yu, W.  Tang, T. L.  Sun,

Y. S.  Zhang, K.  Yue, Adv. Mater. 2019, 31, e1904732; b) X.  Wang, Y.  Feng, L.  Zhang, I.  Protsak, R.  Jamali, Y.  Shu, P.  Pal, Z.  Wang, J.  Yang, D.  Zhang, Chem. Eng. J. 2020, 382, 122927; c) Q.  Zhao, D. W.  Lee, B. K.  Ahn, S.  Seo, Y.  Kaufman, J. N.  Israelachvili, J. H.  Waite, Nat. Mater. 2016, 15, 407; d) C.  Peng, Z.  Chen,

M. K.  Tiwari, Nat. Mater. 2018, 17, 355; e) T.  Ghidini, Nat. Mater. 2018, 17, 846.

[14] a) L. M. Cao, Y. C. Qu, C. M. Hu, T. Wei, W. J. Zhan, Q. Yu, H. Chen, Adv. Mater. Interfaces 2016, 3, 1600600; b) D. H.  Qu, Q. C.  Wang, Q. W. Zhang, X. Ma, H. Tian, Chem. Rev. 2015, 115, 7543.

[15] a) J.  Borges, L. C.  Rodrigues, R. L.  Reis, J. F.  Mano, Adv. Funct. Mater. 2014, 24, 5624; b) H. P. Xu, M. Schonhoff, X. Zhang, Small 2012, 8, 517.

[16] W. J. Zhan, T. Wei, L. M. Cao, C. M. Hu, Y. C. Qu, Q. Yu, H. Chen, ACS Appl. Mater. Interfaces 2017, 9, 3505.

[17] H. Ejima, J. J. Richardson, K. Liang, J. P. Best, M. P. van Koeverden, G. K. Such, J. W. Cui, F. Caruso, Science 2013, 341, 154.

[18] W. Wang, Y. Zhang, W. Liu, Prog. Polym. Sci. 2017, 71, 1.[19] S. T. Wang, H. J. Liu, D. S. Liu, X. Y. Ma, X. H. Fang, L. Jiang, Angew.

Chem., Int. Ed. 2007, 46, 3915.[20] a) D.  Zhang, Y.  Zhang, W.  Lu, X.  Le, P.  Li, L.  Huang, J.  Zhang,

J. Yang, M. J. Serpe, D. Chen, T. Chen, Adv. Mater. Technol. 2019, 4, 1800201; b) D. Zhang, Y. Yao, J. Wu, I. Protsak, W. Lu, X. He, S. Xiao, M. Zhong, T. Chen, J. Yang, ACS Appl. Bio Mater. 2019, 2, 906.

[21] J. H. Waite, Nat. Mater. 2008, 7, 8.[22] a) H. Yuk, T. Zhang, S. Lin, G. A. Parada, X. Zhao, Nat. Mater. 2016,

15, 190; b) S. H. Kim, S. Jung, I. S. Yoon, C. Lee, Y. Oh, J. M. Hong, Adv. Mater. 2018, 30, 1800109.

[23] a) T. Ogoshi, S. Takashima, T. A. Yamagishi, J. Am. Chem. Soc. 2015, 137, 10962; b) L. Cao, Y. Qu, C. Hu, T. Wei, W. Zhan, Q. Yu, H. Chen, Adv. Mater. Interfaces 2016, 3, 1600600.

[24] M. J. Harrington, A. Masic, N. Holten-Andersen, J. Herbert Waite, P. Fratzl, Science 2010, 328, 216.

Adv. Funct. Mater. 2020, 30, 2004633