Robust Coatings via CatecholâAmine Codeposition: Mechanism

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Robust Coatings via Catechol - Amine Codeposition: Mechanism, Kinetics and Application Wen-Ze Qiu, Guang-Peng Wu, and Zhi-Kang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18934 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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ACS Applied Materials & Interfaces

[Revised as an article for publication in ACS Applied Materials & Interfaces]

Robust Coatings via Catechol - Amine Co-deposition: Mechanism, Kinetics and Application Wen-Ze Qiu, Guang-Peng Wu*, Zhi-Kang Xu* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, and Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China.

KEYWORDS: Mussel-inspired chemistry, surface coating, catechol−amine reaction, Michael addition, membrane surface.

ABSTRACT Bio-inspired polyphenol/polyamine co-deposition has been demonstrated by the competence for surface modification; however, the reaction processes including mechanism and kinetics remain superficially understood. In this work, the catechol−amine reaction has been thoroughly investigated by using catechol and two amines m-phenylenediamine and piperazine. We verify that both primary and secondary amines are prone to link with catechol through Michael addition to form polyphenol-polyamine oligomers under aerobic and mild alkaline conditions. Molecular simulations indicate that the Michael addition products are dominants for both aromatic and aliphatic amines with *

Corresponding authors: [email protected] (G.-P. Wu); [email protected] (Z.-K. Xu); fax: + 86 571 8795 1773.

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catechol, which supports the durable chem- and phy-stability of the co-deposited coatings. The aggregation kinetics of polyphenol/polyamine is provided for the first time, and the formed aggregates show high adhesive, which can be deposited as the skin layers for high performance nanofiltration membranes.

1. INTRODUCTION Mussel-inspired chemistry has brought revolutionary promotion on the surface modification and functionalization of various materials. Being the most representative example, dopamine (DA), a kind of catecholamine, undergoes spontaneous oxidative polymerization to form polydopamine (PDA) aggregates (Figure 1a), which could be applied as powerful adhesive coatings on virtually all type of substrates.1-4 Despite remarkable progress has been made in the past decade,5 the coexistence of catechol and amine within a single molecule has led to a situation where DA is vulnerable to deterioration in storage.6 Furthermore, DA has also been reported to cause the neurotoxic and cytotoxic effects on human body.7 For importance, the lack of cost efficiency is another limitation for its applicability in industrial settings.8 In this context, it is highly needed to circumvent the inherent challenges of DA and develop convenient bio-inspired coating strategy for the surface modification functional materials including separation membranes. Among those bio-based strategies for the surface modification and functionalization, the polyphenols/polyamine combinations have been enormously valuable in many applications,9-12 owing to their uniform nano-scale adhesive aggregates via robust bonding


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effects (including catechol-amine covalent linking, hydrogen bonding and π-π stacking). Compared with DA, the polyphenols and polyamines are much more stable in air, thus avoiding the storage issue mentioned above. Importantly, the polyphenols/polyamine co-deposited coatings show not only comparable physicochemical properties to the PDA ones, but also better stability and light colors. Furthermore, the strategy displays superiorities in non-toxic or low-cost reactants, controllable process and time-saving deposition.8,13-19 For example, tea catechins can link with chitosan under the biocatalysis of laccase (Figure 1b) and generates compact surface coatings within 30 min, which could be used to fabricate nanofiltration membranes for water treatment.19 The vital practical significance has been proved in these studies, however, a fundamental description for rigorously understanding how the catechol-amine reaction affects the surface coatings remains superficially understood. First, while the catechol-amine reaction mechanism is accepted either through Michael addition or Schiff base process, it is as yet not fully clear which mechanism is dominant in a given case. For examples, Kodadek et al. studied the reaction using two peptide nucleic acids, and confirmed the cross-linked products by Michael addition via MALDI-TOF analysis.20 Burzio and Waite employed natural decapeptides containing both dihydroxyphenylalanine (DOPA) and lysine, observed a satellite peak at ∆m/z = -18, and concluded the existence of Schiff base reaction.21 Feng and other researchers detected an infrared absorption peak at around 1650 cm-1 from the reaction products of PDA and polyethyleneimine (PEI), and reasoned that the signal was attributed to the C=N stretch originating from a Schiff base formation.13, 22, 23 Zhao et al. investigated the fluorescent features of PDA/PEI aggregates, and suggested the bonding



should be Michael addition.24 Second, although there is no doubt that the primary amines are involved into the reactions, whether the secondary amines participate in the process remains a pending issue in current studies.9-12,

22-24

Lastly, the aggregation kinetics,

behaviors and morphologies of polyphenols/polyamine assemblies require detail investigation, owing to their great effects on the adhesive ability, coating formation, physical/chemical properties and potential applications.25 As mentioned above, the difficulty in exploring polyphenols/polyamine reaction and coating behavior may lies in the structural complexity of high molecular weight reactants, such as proteins, peptides and also PEI. Herein, a model compound, catechol (CA) is selected in combination with m-phenylenediamine (MPD) and piperazine (PIP) to illuminate the reaction mechanisms and the aggregation behaviors (Figure 1c). The mass spectrometry study and the Gaussian molecular simulation both demonstrate that Michael addition is the dominant process compared with Schiff base reaction. Furthermore, we indicate that both primary and secondary amines are prone to link with catechol through Michael addition to form polyphenol/polyamine aggregates, which shed light on the durable chem- and phy-stability of the formed coatings because of the absence of instable imine bonds from Schiff base derivatives.26 The aggregating process of polyphenols/ polyamine assembles was studied for the first time, showing that weaker π-π interaction among oligomers may account for the controllable aggregation and the small particle size. Lastly, the CA/MPD coatings were well fabricated as the skin layers of high performance nanofiltration membranes for water treatment.


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(a)

OH

O

OH

OH

O

OH

OH

Tris buffer

OH

NH NH

OH

Oxidation

OH OH

NH NH

NH2

NH2

dopamine

dopaminequinone

OH

polydopamine

dopaminechrome

OH

(b)

OH

HO

O

OH NH2

NH2

O NH2

O

+

O

CH2OH

O

CH2OH O

n

OH

OH

HN

N

HN

NH2

HO

O

O O H2N

N

O

chitosan

catechin

Michael addition

(c)

NH HO

OH

Tris buffer Oxidation

O

O

N

O

PIP

+

N

O

NH

NH

CA

Michael addition NH

O NH2

NH2

MPD

O

Figure 1. Schematic illustration for a) self-oxidative polymerization of dopamine, b) suggested cross-linking process of tea catechin and chitosan. c) reactions of CA/MPD and CA/PIP with the suggested process for the formation of CA/MPD based polyphenol/polyamine aggregates.

2. EXPERIMENTAL SECTION 2.1 Materials. All chemicals were used as received from commercial sources without any further purification. PSf ultrafiltration substrate with a molecular weight cut off (MWCO) of around 50K was purchased from Ande Membrane Separation Technology & Engineering Co., Ltd. Its water permeation flux is about 300 L m−2 h−1 under 0.6 MPa. These substrate samples were cut into rounds with a diameter of 6 cm. Catechol (CA), m-phenylenediamine (MPD) and piperazine (PIP) were purchased from Aladdin (China).



Tris(hydroxymethyl) aminomethane (Tris) and Na2SO4 were procured from Sinopharm Chemical Reagent Co. Ltd, China and used as delivered. 2.2 LC-MS analysis. LC-MS analyses were performed using an Agilent 6460 triple quadrupole mass spectrometer (Agilent Technologies, USA) equipped with an electrospray ionization (ESI) source, operated in both the positive and the negative scan modes, mass range was 0-1500 Da. Agilent Mass Hunter Workstation was used for data acquisition and processing. Nitrogen was used as the sheath gas and drying gas. The nebulizer pressure was set to 45 psi and the flow rate of drying gas was 5 L/min. The flow rate and temperature of the sheath gas were 11 L/min and 350 °C, respectively. Chromatographic separation was carried out on a Zorbax SB C18 column (150 × 2.1 mm, 3.5 µm). The HPLC mobile phases consisted of (A) 25 mM ammonium acetate in distilled water (PH 4.0) and (B) Methanol. The gradient program was as follows: 0–10 min, 45%–55% of B; 10–15 min, 55%–65% of B; 15–20 min, 65%–75% of B; 20–30 min, 75%–95% of B. The flow rate was set at 0.3 mL/min. 2.3 Gaussian calculation. Gaussian molecular calculations were carried out with the software of GaussView 5.0 and Gaussian 09W. The optimization and calculation of all molecular structures were based on DFT method with B3LYP functional and the basis set of 6-311G. 2.4 TEM observation. CA (1 mg/mL) was mixed with MPD (1 mg/mL) and PIP (0.783 mg/mL) in Tris solution (pH = 8.5), respectively, and the solution was incubated at 25 oC for the designed time under mild vibration. Copper meshes for TEM detection were immersed into the solution for 5 s, then the meshes were taken out and blotted up with a


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filter papers. The processes were repeated for 3 times for the preparation of a single sample. Afterwards, the samples were observed under TEM (Hitachi 7650, Japan) with the lattice resolution of 0.2 HM. And average sizes of the nanoparticles in TEM images were calculated via the software of ImageJ. 2.5 Preparation of the polyphenol/polyamine skin layer on polysulfone substrate. CA (1 mg/mL) was mixed with MPD (1 mg/mL) in Tris solution (pH = 8.5). Polysulfone ultrafiltration substrates were washed and pre-wetted thoroughly by ethanol for 12 h and then immersed into the freshly prepared reaction solutions. After incubated at 25oC for the designed time under mild vibration, the prepared composite membranes were rinsed repeatedly with de-ionized water and stored in it for future characterization and performance tests. 2.6 Other Characterization. UV/Vis absorption of the solutions was measured with an ultraviolet spectrophotometer (Shimadzu, UV 2450, Japan) from 700 nm to 250 nm. XPS analyses were performed on an RBD upgraded PHI-5000C ESCA system (Perkin Elmer, USA) with Al Kα radiation (hν = 1486.6 eV). Surface and cross-sectional morphologies of the polysulfone substrate before and CA/MPD deposition were observed by a field-emitting scanning electron microscope (FESEM, Hitachi S4800, Japan) after sputtered with a 10-20 nm gold layer on the samples. Meanwhile, the surface topographies were also measured by atomic force microscopy (AFM, MultiMode, Vecco, USA) in the tipping mode. And the root mean square (RMS) roughness was calculated by three dimensional AFM images from each membrane surface. 2.7 Nanofiltration performance evaluation. Nanofiltration performance was tested with



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a laboratory scale cross-flow flat membrane module under a constant pressure of 0.6 MPa at 25 ± 1 oC. The effective area for each membrane is around 7.07 cm2. Na2SO4 dissolved in DI water (1000 mg/L) was used as the feed solution with a similar pH of 6.0 ± 0.2. The membrane samples were firstly pre-compacted at 0.7 MPa for about 1 h to reach a stable permeation flux. Then, the operation pressure was lowered to 0.6 MPa and data were recorded after their values reach an equilibrium state. Water permeation flux (Jw, L/m2·h) and salt rejection (R, %) were calculated by the following equations:

J𝑤 =

V

A∙t

(1)

where V, A and t represent the filtrate volume, the effective membrane area, and the operation time, respectively. R = �1 −

Cp Cf

� × 100%

(2)

where Cf and Cp represent the concentrations of solute in the feed and the corresponding filtrate, respectively. And the concentrations of salt solutions were average values measured with an electrical conductivity meter (METTLER TOLEDO, FE30, China) for three times. For the evaluation of acid/alkali stability, the membranes were immersed into freshly prepared NaOH (0.1 M) or HCl (0.1 M) solutions under mild vibration. And for every 12 h, the membranes were taken out, rinsed repeatedly with de-ionized water and tested according to the above method.

3. RESULTS AND DISCUSSION Previous studies indicate the bis-phenol structure of phenolics would be oxidized into quinone state under the alkaline and aerobic condition, and then links with


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amine groups through Michael addition or Schiff base reaction. To study the polyphenols/polyamine reaction process, UV/Vis analysis was first performed to monitor the CA/MPD and CA/PIP combinations. MPD and/or PIP was mixed with CA in tris(hydroxymethyl) aminomethane (Tris) buffer solution (10 mmol, pH = 8.5), respectively. It can be clearly seen two broad peaks at 345 nm and 526 nm for CA/MPD solution (see Figure 2a), and two peaks at 325 nm and 515 nm for CA/PIP solution (Figure 2b). The peaks at around 300 nm account for the absorption of quinone structures,27 suggesting the continuous oxidation of CA molecules in the solution, and the peaks at around 500 nm could be attributed to the cross-linked structures between quinone and amine groups.28 On the other hand, no distinct color change and UV/Vis absorption variation can be seen for CA solution within 60 min, indicating the stability of catechol at the absence of amines (Figure S1 in Supporting Information). (a)

(b)

2.0 345 nm

Reaction time:

1.5

Abs.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 min 50 min 40 min 30 min 20 min 10 min 5 min 0 min

1.0

0.5

526 nm

0.0 300

400

500

600

700

Wavelength (nm)

Figure 2. UV/Vis spectra and color change of CA/MPD (a), CA/PIP (b) Tris solutions (pH = 8.5) with different reaction times. The mass ratio of CA (0.3 mg/mL), MPD and PIP is 1:1:0.78.



The

products

of

catechol-amine

reactions

were

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illuminated

by

liquid

chromatography-tandem mass spectrometry (LC-MS). For individual MPD Tris solution, the peaks at 109.1 and 122.1 m/z represent MPD and Tris molecules, respectively (Figure S2 in Supporting Information). After the addition of CA, signals at 215.1, 321.1 and 427.1 m/z arise, and they can be precisely identified as CA-MPD dimer, MPD-CA-MPD trimer and MPD-CA-MPD-CA tetramer, which derived from Michael addition between the two reactants (Figure 3a). Notably, no signals derived from Schiff base reaction were detected from LC-MS spectra according to their mass-charge ratios. For CA/PIP reaction, as illustrated in Figure 3b, the characteristic peaks at 277.2, 299.2, 407.2, 491.2 and 575.3 m/z belong to CA-PIP oligomers from trimer to hexamer via Michael addition (299.2 m/z can also be considered as [PIP-CA-PIP+Na]+ specie). These results clearly indicate that the secondary amines are also reactive in polyphenols/polyamine reaction. This conclusion is also supported by the previous study with mPEG-NH-catechol polymer.29 And the ability for the secondary amines involving in the reaction process could explain, in part, the strong acidic and alkaline stability of PDA/PEI coatings in previous studies.13, 30 It seems that CA/PIP composition has a higher ratio of multimers, we suppose it is due to reason that p-π conjunction would reduce the reactivity of amine groups in MPD. And it should be emphasized that presence of Tris molecules in the solution did not affect the reaction, which was testified by the LC-MS analysis of CA/Tris solution, where only a relatively weak signal of CA dimer was monitored as a result of the slow oxidative self-polymerization of


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polyphenol (Figure S3 in Supporting Information).31 This result also agrees well with the UV/Vis analyses. Overall, LC-MS measurements suggest that catechol groups

covalently

link

to

primary

and

secondary

amines

to

form

polyphenol-polyamine oligomers through Michael addition, rather than Schiff base reaction. This conclusion was further confirmed by X-ray photoelectron spectroscopy (XPS) analyses of CA/MPD coatings on polysulfone substrates, where the appearance of peaks accounting for C-N and C=O bonds were clearly observed and no signals of C=N bond via Schiff base process was detected (Figure S4 in Supporting Information). (b)

(a)

277.2

321.1

m/z = 108.07

m/z = 320.13

Counts

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


109.1

m/z = 276.16

m/z = 406.12

m/z = 298.20

m/z = 214.07

215.1 m/z = 426.13

m/z = 490.19

122.1 427.1 100

200

300

400

m/z (+)

299.2

407.2

300

400

491.2 500

600

200

500

m/z = 574.25

575.3 600

700

800

m/z (+)

Figure 3. LC-MS chromatograms of CA/MPD (a) and CA/PIP (b) Tris solutions (pH =8.5) summed over the 2 to 4 min retention window. The mass ratio of CA (0.3 mg/mL), MPD and PIP is 1:1:0.78. Gaussian molecular calculation was further employed to illuminate the inclination of catechol-amine reactions, as illustrated in Figure 4a. In the case of CA/MPD, Figure 4b indicates the changes of Gibbs free energy (ΔG) for Michael addition products a (4-position, ΔG = -212.04 KJ/mol), b (3-position, ΔG = -182.09 KJ/mol) and the related transition states c (ΔG = -60.62 KJ/mol), d (ΔG = -49.18KJ/mol) are much smaller than those (e, f) for Schiff base reaction (-32.98 KJ/mol, +60.48



KJ/mol), respectively, indicating the former mechanism is much more liable to occur than the latter one in catechol-amine reaction. This simulation is consistent with the above LC-MS analysis. ΔG values of the reactions between CA and other aliphatic amines like ethanediamine, butanediamine and 1,4-cyclohexanediamine were also performed, and have confirmed the universality for the reaction tendency (Table 1, and Figure S5 in Supporting Information). Moreover, the results indicate that CA/PIP bonding is also prone to happen via Michael addition, and the nucleophilic attack by amines is prone to take place at the 4-position of aromatic catechol ring for both CA-MPD and CA-PIP dimers (Figure 4b, and Figure S6 in Supporting Information). In another aspects, as listed in Table 1, the reactivity of different amine species to catechol may obey the following order: aliphatic primary amine > aliphatic secondary amine > aromatic amine, according to the ΔG values for the corresponding Michael addition reactions. As for CA-MPD and CA-PIP trimers, nucleophilic additions at 3,5-position appears the most stable configurations, possibly due to the steric hindrance impacts on the location of the secondary amine (Figure S6 in Supporting Information). With the understanding of the polyphenol-polyamine reaction, we then focus on the reaction kinetics. We monitored the size of the aggregated nanoparticles via transmission electron microscope (TEM) with increasing the reaction time. Figure � ) of CA/MPD particles increases 5a shows that the statistical average diameter (𝐷

from 9.8 nm at 10 min to 97.8 nm at 90 min, while CA/PIP particles grow much slower from 2.6 nm to 27.5 nm. Severe fusion intention among CA/MPD aggregates


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(a)

(b) 100 Transition state (f)

50 0

ΔG (KJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MPD + CA

-50 Transition state (d)

-100 -150

Schiff base product (e) Transition state (c)

-200 -250

Michael addition product (a)

Michael addition product (b)

Figure 4. (a) Illustration of Michael addition and Schiff base reaction between catechol and amine. (b) Schematic energy profile for CA/MPD reaction at the DFT/B3LYP/6-311G level of theory. The energies are shown in KJ/mol and relative to the CA + MPD reactants (zero point).

Table 1. ΔG values towards Michael addition (4-position) and Schiff base reactions between CA and different amine compounds. Amine

ΔG for Michael addition ΔG for Schiff base reaction (KJ/mol)

(KJ/mol)

MPD

-212.04

-32.98

PIP

-215.65

—

ethanediamine

-247.65

-13.38

butanediamine

-252.63

-15.79

1, 4-cyclohexanediamine

-232.42

-17.04



(a) 180 CA/MPD CA/PIP

150

Average diameter (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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120 90 60 30 0 -30 -60 10

30

60

90

Reaction time (min)

(b) 2θ =23.4O

CA/MPD CA/PIP

10

15

20

25

30

35

2θ

Figure 5. (a) Average diameters and TEM images of CA/MPD and CA/PIP aggregates with different reaction time from 10 min to 90 min. The mass ratio of CA (1 mg/mL), MPD and PIP is 1:1:0.78. Scale bar is 100 nm and applied to all images. (b) X-ray diffraction spectra of CA/MPD and CA/PIP powders.

was observed with the dumbbell-like, claviform and botryoidal structures (Figure 5a, Figure S7 in Supporting Information). We speculate that the fusion intention may be due to the strong cohesive interactions among those CA/MPD aggregates or


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oligomers. To rationalize the above results, the CA/MPD and CA/PIP aggregates were collected, dried and analyzed with X-ray diffraction. It was found that only CA/MPD sample possesses a broad peak at 2θ = 23.4o (Figure 5b), which corresponds to the d-spacing in π-π stacking structures.32, 33 Thus, it can be deduced that CA/MPD aggregates would have strong intermolecular π-π interaction because both the reactants are aromatic, and it may explain for their fast growing and more intense cohesion. This conclusion can also be extended to the fact that the cross-linking of polyamine interferes the conjugated structures of PDA oligomers and meanwhile makes them more hydrophilic, thus can suppress the π-π interaction among PDA oligomers and reduce the size of PDA aggregates, giving high uniform surface coatings.13 The polyphenol/polyamine aggregates are versatile building blocks to construct surface coatings for diverse applications. Herein, they were deposited onto the surface of polysulfone ultrafiltration substrates for the preparation of composite nanofiltration membranes, as illustrated in Figure 6a. CA/MPD was employed, and SEM images indicate that the nascent substrate surface becomes relatively rough with the dotted uniform aggregates. The substrate surface is entirely covered after the deposition for 4 h (Figure 6b), giving a compact skin layer of around 182 nm (Figure S8 in Supporting Information). The as-prepared composite membranes exhibit high nanofiltration performance with a 97.89% rejection of Na2SO4 and a water permeation flux of 48.84 L/m2•h under 0.6 MPa (Figure 6c), which is comparable to some commercial nanofiltration membranes34,35 and other high



performance membranes reported in literatures.36,37 Furthermore, Figure 6d shows the Na2SO4 rejection maintains 97% with minor fluctuation of permeation flux during continuous filtration even after 10 day. The endurance of strong acid/alkali treatments for 48 h also indicates the excellent stability of the coatings (Figure S9 in Supporting Information).

80

80

60

60

40

40

20

20

(b) Nascent

0.5 h

0

1h

2h

3h

0 0.5

(d)

4h

Flux (L/m2.h)

100

Rejection (%)

100

1

2

3

4

Co-deposition time (h) 100

60

80

45

60 30 40 15

20 0

0 24 48 72 96 120 144 168 192 216 240

Flux (L/m2.h)

(c)

(a)

Rejection (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

Operation time (h)

Figure 6. (a) Illustration of CA/MPD deposition for the skin layer of nanofiltration membrane. (b) SEM images of the polysulfone ultrafiltration substrate before and after CA/MPD co-deposition with different co-deposition times from 0.5 h to 4 h. Scale bar is 1 µm and applied to all images. (c) Influence of deposition time on the performance of CA/MPD deposited nanofiltration membranes. (d) Performance variation of the CA/MPD coated membrane (deposition for 3 h) over different filtration times. Test conditions: Na2SO4 concentration = 1000 mg/L, pH = 6.0, 25 oC, 0.6 MPa, cross-flow rate = 30 L/h.

4. CONCLUSION In summary, the co-deposition of polyphenol/polyamine for surface coatings was investigated via the reaction between catechol and two model amines MPD and PIP.


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The LC-MS analyses and molecular simulation both reveal that the primary and secondary amines are tended to link with o-quinone structure to form polyphenol/ polyamine oligomers through Michael addition, rather than Schiff base reaction under typical aerobic and alkaline conditions. The kinetics of the polyphenol/ polyamine aggregation process show that the CA/MPD aggregates grows much faster than CA/PIP ones due to stronger intermolecular π-π interaction. These aggregates are efficient adhesives, and can be deposited as the skin layers for high performance nanofiltration membranes. We expect that the insights into the catechol-amine reaction processes and the well performance in nanofiltration membrane provided will facilitate the development of bio-based strategies for surface modification and functionalization, as well as the generation of advanced materials in the future.

ASSOCIATED CONTENT Supporting Information. UV/Vis spectra and LC-MS chromatograms of CA and MPD Tris solutions, XPS spectra of CA/MPD coated surface, Molecular structures and ΔG values towards different catechol-amine reactions, TEM images of CA/MPD nanoparticles, X-ray diffraction spectra of CA/MPD and CA/PIP powders, Cross-sectional SEM images of CA/MPD layer, nanofiltration performance and chemical stability of CA/MPD coated membranes. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mails: [email protected] (G.-P. Wu), [email protected] (Z.-K. Xu)



Acknowledgements Financial support is acknowledged to the National Natural Science Foundation of China (Grant no. 21534009) and the Open Research Fund Program of Collaborative Innovation Center of Membrane Separation and Water Treatment of Zhejiang Province (Grant no. 2016ZD04).

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Table of Contents

The bio-inspired polyphenol/polyamine co-deposition process, including mechanism, kinetics, and application was throughly investigated by using catechol and two model amines MPD and PIP.


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