A Cost-effective Strategy for Surface Modification via Complexation of

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A Cost-effective Strategy for Surface Modification via Complexation of Disassembled Polydopamine with Fe(III) Ions Pei-Bin Zhang, Wenjihao Hu, Min Wu, Lu Gong, Anqi Tang, Li Xiang, Baoku Zhu, Li-Ping Zhu, and Hongbo Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00245 • Publication Date (Web): 23 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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A Cost-effective Strategy for Surface Modification via Complexation of Disassembled Polydopamine with Fe(III) Ions

Peibin Zhang†⊥, Wenjihao Hu‡⊥, Min Wu‡, Lu Gong‡, Anqi Tang†, Li Xiang‡, Baoku Zhu†, Liping Zhu, †*and Hongbo Zeng‡*

†

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer

Science and Engineering, Zhejiang University, Hangzhou 310027, PR. China. ‡

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, T6G 1H9,

Alberta, Canada. ⊥P.Z.

and W.H. contributed equally to this work.

* Email: [email protected] (L. Zhu) or [email protected] (H. Zeng)

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ACS Paragon Plus Environment

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ABSTRACT Mussel-inspired polydopamine (PDA) deposition provides a prominent approach for constructing functional coatings, which has received much research interest over the past decade. However, large PDA aggregates often formed and precipitated from the solution during the deposition process, significantly lowering the utilization efficiency of dopamine for surface modification. It is of both fundamental and practical importance to “re-activate” and reuse the precipitated aggregates to achieve higher usage efficiency of PDA in surface modifications. In this work, we report a facile, substrateindependent and cost-effective coating strategy, by disassembling the precipitated PDA aggregates, to achieve the coating deposition through the complexation of disassembled polydopamine (d-PDA) species with Fe(III) ions on various substrates. Adsorption tests determined by quartz crystal microbalance with dissipation monitoring (QCM-D) technique indicated that the pH of the solution and the ratio of d-PDA to Fe(III) significantly influence the deposition behaviour of d-PDA/Fe(III). Force measurements using a surface forces apparatus (SFA) demonstrated that the coordination interaction between d-PDA and Fe(III) was the major force leading to the formation of coatings. The deposited d-PDA/Fe(III) coatings featured controllable nanoscale thickness, uniform surface morphologies and light color. Furthermore, the d-PDA/Fe(III) coating could act as an intermediate layer in the preparation of hydrophobic polyurethane (PU) sponge for highly efficient oil/water separation. This work provides a useful strategy to realize the reusability of PDA aggregates for versatile surface functionalization, with implications for the fundamental understanding of the formation mechanism in the metal-phenolic complexation systems and development of new coating approaches in various engineering applications.

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INTRODUCTION Surface modification of different materials has become imperative in a wide range of engineering and bioengineering applications.1 Over the past decades, mussel-inspired catecholic surface deposition method has attracted much attention for functionalizing the surfaces of various materials due to its convenient operation under moderate conditions.2-3 Dopamine, a representative of catecholic derivatives, can be processed into a multifunctional polydopamine (PDA) coating over a wide range of substrates by oxidative polymerization in a weak alkaline aqueous solution.4-5 The derived materials were endowed with a large variety of functions, such as hydrophilicity, biocompatibility, antibacterial property, and reducibility.6-8 Generally, the coating could be generated through the deposition of PDA aggregates formed by the covalent polymerization and non-covalent assembly of dopamine and its intermediates.9 The PDA aggregate size has been demonstrated to significantly influence the deposition capability, in which relatively smaller aggregates dispersed in the aqueous media (i.e., 2 to 50 nm) could deposit onto the substrate and assemble to form the coating layers via a variety of possible intermolecular interactions such as van der Waals interaction, hydrogen bonding, quadrupole-quadrupole and monopole– quadrupole interaction; while the bigger ones tend to precipitate from the solution instead of adhering to the surfaces.10-13 Since the PDA aggregate size can gradually increase with the polymerization time, the growth of PDA coating on substrate often accompanied with the generation of a considerable number of large PDA aggregates as the by-products precipitated in the solution during the coating deposition process. These large PDA aggregates are generally thought to have weak adhesion capability and be useless for surface modification. The utilization ratio of dopamine, an expensive raw materials, is often lower than 5% during the deposition process.14-15 Although researchers have devoted 3


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much effort to tuning the size of PDA particles by controlling the oxidized self-polymerization process to increase the utilization ratio,16-22 the aggregation and precipitation of large PDA particles cannot be evitable after a long reaction time.14-15, 23-24 Therefore, it is of both fundamental and practical important to develop a cost-effective and facile approach to “re-activate” and reuse the precipitated PDA aggregates, achieving versatile coating fabrication. In order to realize the reproducibility of the deposition behaviour, decomposing the large PDA aggregates to the suitable size are regard as an essential step. A recent study has reported the delamination and dissociation of PDA coatings under strong alkaline condition (pH > 12),25 offering a feasible method to dissociate the precipitated PDA aggregates into small size disassembled PDA (dPDA) nanoparticles which could disperse in the solution. The d-PDA moieties have been demonstrated to contain an abundance of catechol groups.25 Inspired by the coating formation using coordination complexes of catecholic polyphenols (e.g., tannic acid) and multi-valent metal ions (e.g., Fe(III)) in previous works,26-28 we report an easy-to-implement strategy to reuse the d-PDA moieties for achieving coating deposition and surface modification through the coordination interaction by introducing Fe(III) ions. The complexation of d-PDA and Fe(III) ions in solution are illustrated in Figure 1. The deposition behaviour, coating performance, and post-functionalization capability are systematically investigated. Our work provides a useful methodology through the recycle and reuse of wasted PDA to fabricate multifunctional coatings, and the results provide quantitative information on the fundamental understanding of deposition mechanism underlying the metal-phenolic complexation systems.

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Figure 1. (A) Schematic diagram of d-PDA and d-PDA/Fe(III) complexes; the creation of capsules (B) and coatings (C) by the complexation of d-PDA and Fe(III) ions.

EXPERIMENTAL SECTION Materials.

Dopamine

hydrochoride,

iron

(III)

chloride

hexahydrate

(FeCl3·6H2O),

Tris(hydroxymethyl)aminomethane (Tris), sodium hydroxide (NaOH), hydrochloric acid (HCl), dodecyl mercaptan and silver nitrate(AgNO3) were purchased from Sigma-Aldrich, Canada. Silicon wafer was obtained from Alfa-Aesar, Canada. Polypropylene (PP) microfiltration membrane was purchased from GmBH Co, Germany. Poly(vinylidene fluoride) (PVDF) and polytetrafluoroethylene (PTFE) membranes were obtained from Millipore Co, USA. Polystyrene (PS) substrate and polyurethane (PU) sponge were obtained from Fisher Co, Canada. Ultra pure water with a resistivity 18.2 MΩcm was prepared by a thermo water purification system (BARNSTEAD Smart2Pure, Thermo Scientific, Canada). Preparation of d-PDA/Fe(III) coatings on various substrates. Dopamine (4.0 g/L) was oxidized and self-polymerized into PDA aggregates in Tris-buffer (pH = 8.5, 50 mmol/L) solution under stirring in air. Afterwards, the pH value of the PDA suspension was changed to 12.5 with the addition of NaOH 5


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solution (1.0 mol/L). Then the resulted d-PDA solution was filtered with a Millipore syringe filter (Millex-GP, 0.22 μm pore size) and the pH was adjusted to 3.5 with HCl solution (1.0 mol/L) for subsequent use. Next, a clean substrate, e.g. PP, PS, PVDF, PTEFE, mica, quartz or PU sponge, was put into a 20-mL glass bottle with 8.5 mL of water followed by the simultaneous addition of 250 μL of d-PDA solution (~4.0 g/L) and 250 μL of FeCl3•6H2O (5.4 g/L) under vigorous stirring for 10 s. Subsequently, 1 mL of Tris solution (pH 8, 100 mmol/L) was added and the deposition last for 1 min. The concentrations of d-PDA and FeCl3•6H2O in the mixture were about 0.10 and 0.135 g/L, respectively. Finally, the substrate was taken out and rinsed fully with deionized water for characterizations. CaCO3 particles were used as a particulate substrate in the preparation of d-PDA/Fe(III) coatings. CaCO3 particles with an average diameter of 3.0±0.2 um were beforehand prepared following a reported procedure.29-30 Spherical CaCO3 particles were dispersed into pure water to obtain a suspension with a concentration of 0.5 % (w/w). Then, 250 μL of d-PDA (4.0 g/L) and 250 μL of FeCl3•6H2O (5.4 g/L) solution were rapidly added and the mixture was vortexed for 10 s. The pH was raised by addition of 1 mL of Tris solution (pH= 8, 100 mmol/L) under stirring. The resulted mixture was centrifuged (1000 rpm, 2 min) and the precipitate was collected as the d-PDA/Fe(III)-coated CaCO3 particles. The modified particles were washed and centrifuged repeatedly to remove residual d-PDA, FeCl3. Then the CaCO3 cores of the d-PDA/Fe(III)-coated CaCO3 particles were etched and removed with an EDTA-NaOH solution (pH 8, 200 mmol/L) and the d-PDA/Fe(III) capsules were obtained. Hydrophobic surface modification of PU sponge was conducted by dipping d-PDA/Fe(III) coated PU sponge into 1-dodecanethiol solution (1.0 wt.% in ethanol) overnight. Then the modified PU sponge 6


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was completely rinsed with ethanol and dried in an air-blow oven. In the oil absorption tests, a piece of modified PU sponge was immersed into a variety of organic liquids for about 1 min and then taken out and weighed. The weight gains of the sponge were calculated by the weights before and after absorption. Surface force measurements. The intermolecular and interfacial force measurements were conducted using a surface forces apparatus (SFA).31-32 Briefly, thin silver-backed mica sheets (around 1-5 μm) were glued onto two cylindrical silica disks with a radius of ~ 2 cm. In the measurements of adhesion force, only one mica surface was coated with d-PDA/Fe(III) complexes. After washed thoroughly, the coated mica and the bare mica were placed into the SFA chamber in a cross-cylinder configuration. A 0.1 mol/L of acetate buffer (pH 5.5) containing 0.25 mol/L of KNO3 was injected into the gap between two mica surfaces. The coating thickness and the interaction forces between two surfaces were monitored in situ using an optical technique named multiple beam interferometry (MBI). In the measurements of cohesion force in the d-PDA/Fe(III) complexes, both mica surfaces were coated with d-PDA/Fe(III) complexes and the interaction forces between two surfaces also determined using the SFA. The effect of extra Fe(III) concentration on the interaction forces was investigated by injecting a FeCl3 solution with various concentrations (in 0.1 mol/L of acetate buffer (pH 5.5) containing 0.25 mol/L of KNO3 and 1.0 mmol/L of Bis-Tris).33 It has been reported that the transitional pH for bistype complexation between catechol and Fe(III) is about 5.5,34 and the buffer with pH 5.5 was used here. Higher pH may lead to the oxidation of catechol groups in d-PDA as well as the precipitation of Fe(III) ions. The electrostatic interaction between two d-PDA/Fe(III) coated surfaces was suppressed by the high concentration of salts in the buffer. Characterizations and oil absorption tests. Ultraviolet-visible (UV-vis) absorption spectra of the 7


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solutions and the d-PDA/Fe(III) coated quartz substrate were recorded using a UV-vis spectrophotometer (UV5500-PC, Jing-Ke Instrument, China). A Malvern Nano-ZS zetasizer (Malvern Instruments, U.K.) was used to measure the hydrodynamic radius of PDA aggregates by dynamic light scattering (DLS) method. The Matrix-Assisted Laser Desorption/ Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS) was used to monitor mass change by a MALDI-TOF mass spectrometer (Bruker Daltonics, Billerica, MA) with 2,5-Dihydroxybenzoic acid as the matrix. A Dimension Icon Atomic Force Microscope (AFM) equipped with Nanoscope Analysis software (Bruker, USA) equipment was used to scan the surface roughness of the coated surfaces. The surface morphologies of the d-PDA/Fe(III) coated surfaces were observed by scanning electron microscopy (SEM) (Hitachi S4800, Japan). Quartz crystal microbalance with dissipation (QCM-D, Q-sense E4, Sweden) was used to monitor the increment of adsorption of d-PDA/Fe(III) complexes. The thicknesses of the coatings on silicon wafer were measured using an ellipsometer (M-2000, J.A. Woollam, USA). The contact angles of water droplet on the coatings were measured using a contact angle goniometer (Ramé-Hart Instrument Co., New Jersey, USA) to characterize the surface wettability. Surface chemistry of the coatings were analyzed using X-ray photoelectron spectroscopy (XPS, PHI 5300, Perkin Elmer, USA), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet iS50) and energy dispersive X-ray (EDX) spectroscopy (Hitachi S4800, Japan). The XPS spectrometer was equipped with Mg Kα excitation radiation (1253.6 eV; 250 W, 14 KV) and survey and narrow spectra were recorded and all XPS spectra were evaluated by using Auger Scan software. Oil absorption capacity test was conducted by forcing hydrophobic modified PU sponge into the organic liquids for about 1 min and then taking out to be weighed. The weights before and after 8


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absorption were calculated to obtain the weight gain of the sponge. After oil absorption, the sponge could be regenerated by squeezing or distillation ways. RESULTS and DISCUSSION Disassembly of PDA aggregates and complexation with Fe(III) ions In this work, PDA aggregates were beforehand prepared following a conventional solution oxidation method.35 Briefly, a 4 g/L of dopamine solution was prepared with Tris buffer solution (50 mmol/L, pH 8.5) and vigorously agitated overnight. Then PDA aggregates were visually observed and collected from dopamine solution. It has been proved that both covalent linking and non-covalent interaction are involved in self-polymerization and self-assembly of dopamine.36-37 Due to the complicated structures as well as the insolubility of PDA aggregates, their accurate analysis and description still remains unsolved. The as-synthesized PDA aggregates in water lost their ability of deposition on a substrate because their adsorption on a substrate was inhibited by the great and random Brownian motion.38 The polydisperse PDA aggregates are easily trapped kinetically and generated amorphous precipitates rather than ordered structures.37, 39-40 As the pH of PDA suspensions was elevated to higher than 12, the suspensions became gradually clearer and nearly no precipitates could be collected after centrifugation (Figure S1). The strong alkaline environment often leads to the dissociation of PDA aggregates and generates d-PDA species with smaller size.41-42 In PDA suspensions (pH 8.5), some black PDA precipitates can be observed. The average dynamic hydration diameter of PDA particles in supernatant was about 200 nm determined by DLS (Figure S1A-S1C). Whereas, the d-PDA species exhibited sub-10 nm of size, which was much smaller than that of PDA aggregates, and thus were well dispersed in water (Figure S1D). From the MALDI-TOF spectra (Figure 2A and 2B), it can be seen that the mass-to-charge (m/z) ratio of the PDA ranges from 9


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582 to 2500, and the space between adjacent peaks is about 149, which is just the molecular weight of dopamine or 5,6-Dihydroxyindoline (DHI). Comparatively, the m/z value of the d-PDA is lower than 724, indicating the average unit number of dopamine or DHI in each d-PDA molecule is less than 5. In another word, only low molecular weight of oligomers was detected in the d-PDA. These results substantially confirmed the disassembly of PDA in alkaline condition. Some possible structural models in the d-PDA are shown in Figure 2C. It is worth noting that the peak clusters can be observed in the MALDI-TOF spectra of both d-PDA and PDA, indicating the components are much complicated than those as shown in Figure 2C. Some other m/z values may be attributed to the molecules generated from -OH leaving in melanin-like PDA aggregates or other covalent linking forms, e.g., Michaeladdition or Schiff-base linking between two oligomer units.24, 43 Some typical DHI-based oligomers structures reported in literatures are used to represent the d-PDA (Figure 2 and Figure S2).44

Figure 2. MALDI-TOF MS spectra of PDA (A) and d-PDA (B), respectively. (C) Possible structures 10


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of dopamine/DHI based oligomers with their corresponding molecular weights.

The d-PDA was used to create functional d-PDA/Fe(III) coatings over a variety of substrates by coordination with Fe(III) ions. In order to avoid the precipitation of Fe(III) ions in alkaline condition, the pH value of d-PDA solution was first adjusted to 3.5 to make d-PDA bind with Fe(III) ions. Then the pH value was elevated to 8.5 in order to convert the chelation type from mono-type into tris-type (Figure S3).35 Except for the dynamic hydration diameter and the MALDI-TOF spectra, the formation of d-PDA/ Fe(III) complexes was also confirmed by the FTIR spectra (Figure S4B). Compared with the spectrum of pure PDA sample, the peak at 1508 cm-1 shifts to 1538 cm-1 in that of d-PDA/Fe(III) complexes. This is attributed to the transformation of phenolic hydroxyl groups in d-PDA to -C-OFe(III) in d-PDA/Fe(III) complexes.45-46 After the preparation of the d-PDA solution, a substrate was placed in the solution for coating. The adsorption and deposition of d-PDA/Fe(III) complexes on substrate could be attributed by the hydrogen bonding, coordination interaction, cation-π interaction, hydrophobic interaction and van der Waals force.34, 38, 47-49 The deposition of d-PDA/Fe(III) complexes on substrate was a rapid coordinative self-assembly process with the deposition rate comparable to that of PDA deposition reported previously.18 The assembly of dPDA/Fe(III) complexes led to the amorphous state of aggregation, which was confirmed by XRD analysis (Figure S4A).50 The generation of iron hydroxide by-products, that would exhibit crystalline peaks in XRD spectrum, was successfully avoided. In this work, we defined the process of primary d-PDA/Fe(III) complexes formation and further deposition of the complexes on a substrate as one deposition cycle. This process can be finished within one minute, benefitted from the rapid adsorption of functional d-PDA/Fe(III) complexes on various 11


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substrates. If necessary, multilayer coatings can be obtained through multiple deposition cycles. It is known that the traditional PDA coating is often in deep color.51 On the contrary, the d-PDA/Fe(III) coatings were nearly colourless, as shown in Figure 3A. The substrates coated with 10 deposition cycles exhibit only a slight change in color. Various substrates with different shapes and dimensions including CaCO3 nanoparticles, mica, silicon wafer, PS flat sheet, PP, PVDF, PTFE flat sheet, and PU sponge were applied in the coating of d-PDA/Fe(III) complexes. It was found that the d-PDA/Fe(III) complexes could be deposited on all these substrates, indicating the substrate-independent characteristic of this strategy for material surface modification (Figure 3A and 3B). In the case of CaCO3 nanoparticles as the substrate, d-PDA/Fe(III) capsules were successfully obtained after etching and removing the CaCO3 templates with an EDTA-NaOH solution. This result indicates that the coating has a continuous structure. The d-PDA/Fe(III) coatings on various substrates exhibit similar water contact angle (~ 60º), showing that the substrates were fully covered by the d-PDA/Fe(III) complexes (Figure S5). According to the AFM images analysis (as in Figure S6), it is found that the d-PDA/Fe(III) coated mica with one deposition cycle had an extremely smooth surface with a rootmean-square (RMS) roughness value of 0.19 nm. After ten deposition cycles, the RMS roughness of the resultant coating increased to 1.21 nm (Figure S6). Compared with traditional PDA coating,52-53 the d-PDA/Fe(III) coating in this work is more smooth and uniform. In the case of PP porous membrane as the substrate, nearly no large aggregates was observed in the d-PDA/Fe(III)-coated sample (Figure S7). Then surface chemistry of PP and modified PP was analysed by XPS (Figure 3). C1s, O1s, N1s, and Fe2p peaks occurred in the survey spectrum of modified PP membrane, verifying the successful deposition of d-PDA/Fe(III) coating at the membrane surface. The Fe2p3/2 signal appears at 712 eV with a 2p peak separation, which is consistent with the presence of Fe(III) species.54 The O1s 12


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peak is de-convoluted and the component peaks at 531.2 eV and 533.0 eV are assigned to Fe-O and Fe-OH species, respectively,45,

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proving the coordination of d-PDA with Fe(III) ions. The atom

content of each element is listed in Table 1 and the atomic compositions ratio of N: Fe is calculated to be ~ 2.4:1. Considering that d-PDA is a mixture of oligomers with various unit numbers, the actual molar ratio value of d-PDA: Fe(III) in the coating is smaller than 2.4, suggesting the absence of tristype d-PDA/Fe(III) complexation. The element composition of other coated substrates, e.g., mica and PVDF, were also obtained by XPS analysis (Figure S8). The calculated N: Fe atomic ratio is 2.5 and 2.4 for d-PDA/Fe(III) coated mica and PVDF substrate, respectively (Table S1 and Table S2).

Figure 3. (A) Photo images of various substrates coated by d-PDA/Fe(III) complexes with one and ten deposition cycles, respectively.(B) SEM image of d-PDA/Fe(III) capsules. Scale bar is 1 μm. (C) XPS spectra of modified PP and original PP surfaces with survey spectra (upper side), and high resolution Fe2p and O1s spectra of the modified PP substrate. Table 1. Chemical compositions of d-PDA/Fe(III) coating on PP substrate from XPS analysis. 13


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Element (mol %) PP PP@d-PDA/Fe(III)

C 97.6 81.4

O 2.4 13.3

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N \ 3.6

Fe \ 1.5

N/Fe \ 2.4

Growth behaviour of d-PDA/Fe(III) coating To investigate the adsorption and deposition behavior of the d-PDA/Fe(III) coating, QCM-D was used to monitor the deposition process on silica surface under the effects of different pH conditions and molar ratios of d-PDA to Fe(III). As the d-PDA solution without Fe(III) was introduced into the QCMD chamber, no obvious frequency change was observed under all the pH conditions investigated (i.e., pH 3, 5.5 or 8) (shown in Figure S9) due to the small size of d-PDA and their Brownian motion. Figure 4A shows the results of frequency changes with time after the d-PDA/Fe(III) complex solutions were pumped into the QCM-D chamber. The molar ratio of d-PDA to Fe(III) is 1:1 and the pHs are 3, 5.5, and 8, respectively. The frequency change at pH 5.5 is found to be much more remarkable compared with those at pH 3 and 8. It has been reported that the coordination type of catechol groups with Fe(III) is mainly mono-type at acidic conditions and thus the deposition ability of d-PDA/Fe(III) complexes at pH 3 is not sufficiently strong due to the small size and Brownian motion.35 Thus, almost no obvious change in the frequency was observed in Figure 4A. With increasing the pH to 5.5, the catechol-Fe(III) coordinative cross-linking occurred spontaneously and larger d-PDA/Fe(III) complexes were formed, indicating lower PDA concentration in the solution. Therefore, the bridging effect of Fe(III) ions, hydrogen bindings and other interactions with the silica surface could result in the adsorption of d-PDA on silica surface. As the pH is further elevated to 8, the d-PDA/Fe(III) complexes tended to aggregate and precipitate from the solution (some visible precipitates was observed in the solution). The precipitated aggregates led to low concentration of the solution, resulting in the less adsorption amount of d-PDA/Fe(III) complexes. The results demonstrated that the 14


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adsorption of d-PDA/Fe(II) complexes varied with the pH of the solution, and the highest adsorption amount was achieved at pH 5.5. The results on the effect of molar ratio of d-PDA to Fe(III) on the QCM-D frequency changes at pH 5.5 are shown in Figure 4B. It is found that the frequency change is most significant with the molar ratio of d-PDA/Fe(III) 1:1. Increasing the proportion of d-PDA or Fe(III) results in the reduction of the adsorption amount of the complexes on silica surface. The optimal stoichiometry of d-PDA/Fe(III) in this work is close to that of the reported phenolic system, such as garlic acid/Fe(III).26, 50 As the molar ratio of d-PDA to Fe(III) is high (i.e., 3:1), the Fe(III) ions are able to rapidly coordinate with d-PDA in the solution and generate large size of precipitates of d-PDA/Fe(III) complexes, resulting in a low concentration of d-PDA in the solution. As a result, the deposition of d-PDA/Fe(III) on the sensor is decreased. As the mole ratio of d-PDA to Fe(III) is relatively low (e.g., 1:3), plenty of Fe(III) ions are available in the solution, which is disadvantageous to the formation of cross-linked d-PDA/Fe(III) network. The excess of Fe(III) ions could form the chelate bonds with d-PDA, and reduce the active sites for the formation of hydrogen bonds between the d-PDA and silica surface. Thus, the deposition of d-PDA/Fe(III) on substrate was also decreased. Therefore, a modest molar ratio of d-PDA to Fe(III) is required to achieve a desirable deposition ability of the d-PDA/Fe(III) complexes, which was found to be strongest at the molar ratio of 1:1 according to the results.

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Figure 4. (A) The deposition of d-PDA/Fe(III) complexes on silicon sensor at various pH

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monitored

in situ by QCM-D (the molar ratio of d-PDA to Fe(III) is 1:1). (B) The QCM-D frequency changes in d-PDA/Fe(III) deposition with various molar ratio of d-PDA to Fe(III). (C) UV-vis spectra of dPDA/Fe(III) coatings with different deposition times and fitting result of adsorption peaks at 280 nm (insight) (molar ratio of d-PDA to Fe(III) is 1:1). (D) The thicknesses of the d-PDA/Fe(III) coatings with different deposition steps measured by ellipsometry (molar ratio of d-PDA to Fe(III) is 1:1).

Ultra-violet measurements were also conducted to monitor the coating growth (Figure S10). The maximum absorbance peak of d-PDA/Fe(III) complexes occurred at 280 nm, which is attributed to the 16


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PDA species. The solution color became deeper when Fe(III) ions encountered with the ligand d-PDA. However, the ligand-to-metal charge transfer (LCMT) band, generally appears at visible portion in the UV-vis spectrum of catechol and metal complexes,56 has not been found in the UV-vis spectra, This may be due to the low ligand concentration relative to metal ions, or the overlapping effect of d-PDA broadband absorption throughout the visible regime.45,

50, 57

In addition, d-PDA with low

polymerization degree also suppresses the election delocalization compared with conventional polydopamine system.58 It is believed that, compared with traditional PDA coatings, the π-π stacking between PDA oligomers in d-PDA/Fe(III) complexes is weakened, and thus the stacking freedom of d-PDA decreases due to the coordination of d-PDA and Fe(III) ions.59 Therefore, the deposited dPDA/Fe(III) complex film exhibits light color. As shown in Figure 4C, the absorbance of the dPDA/Fe(III) coatings increase with the number of deposition times and the peak values at 280 nm increases linearly. The d-PDA/Fe(III) coatings with different molar ratio of d-PDA to Fe(III) exhibited similar absorption spectra (Figure S11), indicating that the UV absorption of the d-PDA/Fe(III) complexes is mainly attributed to the d-PDA. From Figure 4C, it can also be observed that the UV absorbances are very weak, which is in agreement with the light color of the coatings as shown in Figure 3A. From the results of both UV adsorption spectra and ellipsometric tests (Figure 4D), it can be seen that the thicknesses of the d-PDA/Fe(III) coatings increased linearly with the number of deposition times. Calculated from the data in Figure 4D, the thickness of the first d-PDA/Fe(III) layer is about 1.9 nm and the subsequent layers is about 1.4 nm per layer. These results indicated that the thickness of d-PDA/Fe(III) coating can be regulated and controlled by the number of deposition cycles on substrate. To further reveal the adsorption and deposition mechanisms of d-PDA/Fe(III) coatings on substrate, 17


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SFA was used to investigate the adhesion and cohesion interaction mechanisms between d-PDA/Fe(III) complexes and mica substrate.24 The d-PDA/Fe(III) coatings onto mica surface was prepared following a process described in experimental section (d-PDA/Fe(III) ~ 1:1). The force-distance profiles during approaching and separation process between two surfaces were obtained using the SFA. The dPDA/Fe(III) complex coating at the mica surface was extremely flat and stable in buffer solution (0.1 mol/L sodium acetate buffer, pH=5.5, with addition of 0.25 mol/L potassium nitrate and 1 mmol/L Bis-Tris). To investigate the deposition of d-PDA/Fe(III) complexes on mica surface, the adhesion force between a d-PDA/Fe(III) coated mica and a bare mica was measured in an asymmetric configuration. Only repulsion was detected during surface approaching, which might be due to the electrostatic repulsion and the steric repulsion. After 3 minutes of contact, the separation of these two surfaces was conducted and a weak adhesion (F/R ~ -0.58 mN/m) was observed (Figure 5A). This detected adhesion between d-PDA/Fe(III) complexes and mica surface is most likely due to the hydrogen bonds. To understand the growing deposition and adsorption of d-PDA/Fe(III), interfacial interactions between two mica surfaces coated with d-PDA/Fe(III) complexes were measured. No obvious repulsion was detected during the approaching process, while a strong adhesion (F/R ~ -7.1 mN/m) was detected associated with separation (Figure 5B). The approaching-separation cycle was conducted several times and the forces between two d-PDA/Fe(III) surfaces were recorded (Figure 5C). It was found that the cohesion force decreased after the first cycle, which might be attributed to the conformation rearrangement of the deposited molecules associated with contacts. A buffer solution containing Fe(III) ions was used as the liquid medium in SFA measurements to investigate the effect of metal ions. As a low concentration of Fe(III) ions (10 μmol/L) was introduced, the cohesion force drastically decreased to ~ -2.5 mN/m. The adhesion could be attributed by the coordinative interactions 18


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between the catechol groups of d-PDA and Fe(III) ions. When the concentration of Fe(III) further increased to 100 μmol/L, the cohesion became weaker to ~ -1.0 mN/m (Figure 5D). Such weakened cohesion indicates a much reduced effect of Fe(III) ions on the surface interaction, which is consistent with the formation of the monocatecholato-Fe(III) complex in previous study.34 The measured adhesion and cohesion could be the major driving force for the fast adsorption and deposition of dPDA/Fe(III) complexes on a substrate. These results show that Fe(III) ions are of great importance in the continuous deposition of d-PDA/Fe(III) complexes on substrate surfaces. (B) 20

12 8 4 0

Approach

F/R~ 0.58 mN m-1

-4 0

20 30 Distance (nm)

40

50

8 4 0

Approach

-4 -8

F/R~ - 7.08 mN m-1 0

(D)

20

12

10

20 30 Distance (nm)

16

8 4

Force/Radius, F/R (mN m-1)

Separation

1st load (3min) 2nd load 3rd load 4th load

10

Approach

0 -4

5

10

20

30

Distance (nm)

40

50

Approach

10  Fe(Ⅲ ) 100  Fe(Ⅲ )

0 F/R~ - 0.96 mN m-1

-8 0

50

0.1 M acetate buffer, pH =5.5 with 0.25 M KNO3

15

12

40

20

Separation

(C)

10

16 Separation


16

Separation


(A) 20


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-5

F/R~ - 2.52 mN m-1 0

10

20 30 Distance (nm)

40

50

Figure 5. (A) SFA measurements for surface adhesion force of d-PDA/Fe(III) coating with mica substrate. The embedded images illustrate the process of adhesion force measurements. (B) SFA measurements for cohesion force inside d-PDA/Fe(III) complexes. (C) Several approaching separation cycles for cohesion force measurements. (D) SFA measurements were conducted between 19


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symmetrical coatings with 0.1 mol/L of acetate buffer (pH 5.5) containing 0.25 mol/L of KNO3 and 10 or 100 μmol/L Fe(III) ions.

Surface functionalization of d-PDA/Fe(III) coated materials by post modification In order to explore the functionality and application of d-PDA/Fe(III) coatings, post modification was performed based on the reactivity of the coatings.60 As an example, a piece of hydrophobic PU sponge was prepared by d-PDA/Fe(III) deposition followed by the dip-coating step with 1-dodecanethiol solution. It has been reported that thiol groups can react with catechol groups by Michael addition,61 which provide a feasible way to immobilize functional molecules or polymer chains onto material surface. The water contact angle of the modified sponge reached up to 135.2º, and the water droplet stayed as quasi-sphere on the sponge surface (more details are described in Figure S12). The hydrophobic sponge was used as a candidate material to remove spilled oils from water (Figure 6A, B). The oil uptake capacities of the sponge ranges from 109 to 163 times of their own weights towards various organic liquids (Figure 6C). Moreover, the sponge can be used in continuous oil/water separate process with a flow-through mode (Figure S13). After the absorption saturation, the loaded organic liquids inside sponge can be removed by manually squeezing or distillation process (Figure 6D). In addition, the recyclability of the hydrophobic sponges prepared in this work was comparable to most oil absorbents reported previously.62-63

20


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Figure 6. Chloroform (A) and toluene (B) (dyed with Oil Red O) absorption from water using hydrophobic PU sponge modified by d-PDA/Fe(III) coating and further alkylation. (C) The absorption capacities of the sponge towards various organic liquids. (D) The recyclability of the hydrophobic sponge by squeezing method (hexadecane as the adsorbate). CONCLUSIONS In summary, we have developed a facile, substrate-independent and cost-effective coating strategy, by disassembling the precipitated PDA aggregates, to achieve the coating deposition through the complexation of disassembled polydopamine (d-PDA) species, which has usually been regarded useless as waste materials in conventional PDA deposition processes, with Fe(III) ions on various substrates. The obtained d-PDA/Fe(III) coatings is smooth and uniform on various substrates, and exhibit a much lighter color than traditional PDA coating. The deposition experiments indicate that the coating thickness can be modulated by the number of deposition cycles. Besides, the adsorption tests using QCM-D indicate that the pH of the solution and the ratio of d-PDA to Fe(III) significantly influence the deposition behaviours of d-PDA/Fe(III). The measured results of the frequency changes suggest that the pH of 5.5 and the d-PDA/Fe(III) ratio of 1:1 were the optimal deposition conditions 21


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for d-PDA. The measured interaction force results reveal that the coordination interaction plays an important role in the deposition and film growth of d-PDA on substrate surfaces. Furthermore, the dPDA/Fe(III) coating could act as an intermediate layer in the preparation of hydrophobic polyurethane (PU) sponge for highly efficient oil/water separation. This work demonstrates a facile and useful approach for the re-activation and reuse of PDA aggregates for the functionalization of various substrate surfaces. Our results provide useful information regarding the fundamental understanding of metal-phenolic complexation interaction mechanism and the development of versatile coating methods with different engineering applications. ASSOCIATED CONTENT Supporting Information The supporting information is available on the ACS publication website. DLS tests, TEM images, XRD images, FTIR spectrum of PDA aggregates and dPDA/Fe(III) complexes, UV-vis spectrum of PDA and dPDA/Fe(III) solution, CA measurements, AFM images and XPS spectrum of dPDA/Fe(III) coatings, and QCM experiments for determining deposition behavior. AUTHOR INFORMATION Corresponding Author *[email protected] (Liping Zhu). *[email protected] (Hongbo Zeng). ORCID Liping Zhu: 0000-0002-1553-4190. Hongbo Zeng: 0000-0002-1432-5979. Author Contributions 22


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⊥P.Z. and W.H. contributed equally to this work.

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51773175, 51573159, and 51828301), the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chairs Program (H.Z.). P.Z. also thanks the financial support from the Program of International Cooperative Research and Communication for PhD student in Zhejiang University.

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A Cost-effective Strategy for Surface Modification via Complexation of

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