Bioinspired Polydopamine (PDA) Chemistry Meets Ordered

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Bioinspired Polydopamine (PDA) Chemistry Meets Ordered Mesoporous Carbons (OMCs): A Benign Surface Modification Strategy for Versatile Functionalization Yang Song,†,∥ Gang Ye,*,†,‡ Fengcheng Wu,† Zhe Wang,† Siyuan Liu,§ Maciej Kopeć,∥ Zongyu Wang,∥ Jing Chen,†,‡ Jianchen Wang,*,†,‡ and Krzysztof Matyjaszewski*,∥

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†

Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China ‡ Beijing Key Lab of Radioactive Waste Treatment, Tsinghua University, Beijing 100084, China § Department of Materials Science and Engineering, ∥Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *

ABSTRACT: Mussel-inspired polydopamine (PDA) chemistry was employed for the surface modification of ordered mesoporous carbons (OMCs), improving the hydrophilicity, binding ability toward uranium ions, as well as enriching chemical reactivity for diverse postfunctionalization by either surface grafting or surface-initiated polymerization. Uniform PDA coating was deposited on the surface of CMK-3 type OMCs via self-polymerization of dopamine under mild conditions. Surface properties and morphology of the PDA-coated CMK-3 can be tailored by adjusting the dopamine concentration and coating time, without compromising the meso-structural regularity and the accessibility of the mesopores. Due to high density of −NH groups (4.7 μmol/m2 or 2.8 group/nm2) and −OH groups (9.3 μmol/m2 or 5.6 group/nm2) of the PDA coating, the modified CMK-3 showed improved hydrophilicity and superior adsorption ability toward uranyl ions (93.6 mg/g) in aqueous solution. Moreover, with the introduction of α-bromoisobutyryl bromide (BiBB) initiator to the PDA-coated CMK-3, we demonstrated for the first time that activators regenerated by electron transfer for atom transfer radical polymerization (ARGET ATRP) can be conducted for controlled growth of polymer brushes from the surface of OMCs. Thus, PDA chemistry paves a new way for surface modification of OMCs to create a versatile, multifunctional nanoplatform, capable of further modifications toward various applications, such as environmental decontamination, catalysis, and other areas.

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INTRODUCTION Ordered mesoporous carbons (OMCs) have spurred great research interest1−5 since the first report by Ryoo in 1999.6 Due to large specific surface area, high meso-structured regularity, uniform and tunable pore size, OMCs possess extraordinary potential in various fields, such as surpercapacitors, 7,8 catalysis,9−11 adsorption,12,13 and so on. A robust carbon framework endows OMCs with favorable thermal conductivity, chemical and mechanical stability. However, their hydrophobic nature and chemically inert surface hinder more extensive applications, especially in the areas requiring wettability and specific functionalities on the surface. Over the past few years, tremendous efforts have been directed to the surface modification of OMCs,14 aiming at improving their hydrophilicity15 and chemical reactivity16 for more advanced applications.17−19 Oxidative treatment is the most frequently used method to alter the surface chemistry of OMCs by generating large amounts of oxygen-containing groups.20−24 The presence of © 2016 American Chemical Society

abundant carboxyl groups after oxidative treatment enables further modification of OMCs by secondary reactions,25,26 leading to more specific functionalization of OMCs. However, oxidants usually compromise the regularity of the mesochannels, or even cause structural collapse of OMCs. This is especially severe for the CMK-5- and CMK-3-type OMCs with less stable three-dimensional frameworks,21,27 which consist of either carbon tubes with very thin walls or carbon nanoarrays connected by narrow nanopillars. Diazonium chemistry18,28,29 and 1,3-dipolar cycloaddition30 are established techniques for surface modification of OMCs. High grafting densities of functional groups (∼0.9−1.5 μmol/m2 or ∼0.5−0.9 group/ nm2) were obtained by chemical reduction of aryl diazonium salts under solvent-free conditions,28,29 while relatively lower grafting density was reached by 1,3-dipolar cycloaddition.30 Received: April 28, 2016 Revised: July 6, 2016 Published: July 7, 2016 5013

DOI: 10.1021/acs.chemmater.6b01729 Chem. Mater. 2016, 28, 5013−5021

Article

Chemistry of Materials Compared to oxidative processes, diazonium chemistry and 1,3dipolar cycloaddition are capable of not only adjusting the hydrophobic and hydrophilic nature of OMCs but also more readily controlling the grafting densities of functional groups.14 Unfortunately, serious blockage of mesopores may limit accessibility of active sites. Moreover, surface coverage of functional groups is still not comparable to the silanol groups on silica surface (∼6−9 μmol/m2 or ∼3.6−5.4 group/nm2).31 Polymer surface coating is another effective approach to engineer carbonaceous surfaces by introducing functional groups with increased grafting density. However, complicated procedures are often required to retain the accessibility of mesopores.32−34 In addition, the polymer/nanocarbon composites usually have weak bonding between the filled polymers and chemically inert carbon surfaces.35 Therefore, the development of a benign and versatile strategy for surface modification of OMCs with significantly improved hydrophilicity, wellpreserved meso-structure, and high grafting density remains challenging. Polydopamine (PDA) chemistry, inspired by mussel adhesive proteins, has attracted extensive attention since Messersmith’s report in 2007.36 Dopamine molecules can easily selfpolymerize under weak alkaline conditions, leading to a facile deposition of PDA coatings on various surfaces.37−39 This provides a convenient method to modify the surface chemistry of materials for improving their hydrophilicity,40 biocompatibility,41 adsorption ability,42 and so forth. More importantly, the PDA coatings, which contain a high density of catechol and imine functional groups,43 can serve as a versatile platform for secondary reactions, stimulating further modification for specific applications.36,44,45 For instance, amine- and thiolterminated molecules can be easily grafted to the PDA-coated surface by Schiff base reaction or Michael addition,46,47 whereas acyl bromide- and acyl chloride- containing molecules can be introduced via nucleophilic substitution reactions.48−50 Nevertheless, the self-polymerization of dopamine for surface engineering of OMCs has been rarely reported. Herein, we report a strategy to impart multiple functional moieties to OMCs by using PDA chemistry under different scenarios. PDA coatings could be readily generated on the surface of OMCs with tunable thickness in ethanol/water system without compromising the mesostructured regularity and accessibility of the mesopores. The modified OMCs contained a high concentration of catechol and imine groups on the surface, resulting in the materials with significantly enhanced hydrophilicity, excellent dispersibility, and adsorption capacity toward actinide ions like U(VI). Moreover, αbromoisobutyryl bromide (BiBB) moieties were introduced, and surface-initiated atom transfer radical polymerization (SIATRP) of methyl methacrylate (MMA) was conducted to demonstrate the versatility of the PDA-modified OMCs platform. (Scheme 1) CMK-3, as a typical OMC material, was chosen to conduct surface modification by the deposition of PDA. The coating process was comprehensively studied by varying dopamine concentration and coating time. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to examine the texture and morphology of PDAcoated CMK-3. Small-angle X-ray diffraction (SAXRD) and N2 adsorption−desorption measurement were conducted to characterize structural properties. Elemental analysis was carried out to determine the elemental composition for evaluating the densities of the imine and hydroxyl groups.

Scheme 1. Illustration of Surface Modification of CMK-3 OMC by PDA Chemistry Leading to Improved Water Dispersibility, Uranium Binding Ability, and Potential for Post-Functionalization through Secondary Reactions

Additionally, the surface hydrophilic properties and the adsorption behavior of U(VI) were studied. The effectiveness for postfunctionalization by introducing organic functionalities and by SI-ATRP of MMA was proved.

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EXPERIMENTAL SECTION

Materials. Ordered mesoporous carbon (CMK-3) was purchased from Nanjing Jicang Nanotechnology Co. LLC. Dopamine hydrochloride (∼98%) and tris(hydroxymethyl)aminomethane (Tris base, 99.5%) were supplied by J&K Scientific. Methyl methacrylate (MMA, 99%, Aldrich) was purified by passing through a basic alumina column to remove the inhibitor before use. Triethylamine (TEA, > 99%, Aldrich), α-bromoisobutyryl bromide (BiBB, 98%, Aldrich), ethyl 2bromoisobutyrate (EBiB, 98%, Acros), copper(II) bromide (CuBr2, > 99%, Acros Organics), tin(II) 2-ethylhexanoate (Sn(EH)2, ∼ 95%, Aldrich), anisole (99%, Aldrich Reagent Plus), and N,N-dimethylformamide (DMF, > 99.8%, Fisher) were used as received. Tetrahydrofuran (THF) was purified and dried with a solvent purification system provided by JC Meyer Solvent Systems. Tris(2-pyridylmethyl)amine (TPMA) was synthesized referring to a published work.51 Preparation of PDA-Coated CMK-3. 100 mg of CMK-3 was dispersed in a 70 mL mixed solution of ethanol and water with the volume ratio of 4:3. After ultrasonic treatment for 10 min, a determined amount of dopamine hydrochloride (50, 100, 200, or 400 mg) was added under magnetic stirring. Five min later, 20 mL of 25 mM Tris buffer solution was added dropwise, followed by adjusting of the pH value to 8.5 using HCl and NaOH solutions. The coating process was maintained at 25 °C for a specific time (5, 10, or 24 h). Then, PDA-coated CMK-3 was separated by centrifugation and washed with ethanol several times. The resulting product was placed in a vacuum oven at 60 °C for 24 h. In this paper, the PDA-coated CMK3 is named as CMK-3-PDA-x-y, where “x” and “y” represent the dopamine concentration (0.6, 1.1, 2.2, and 4.4 g/L) and coating time (5, 10, and 24 h), respectively. Uranium Adsorption. U(VI) solution with the initial concentration of 50 mg/L was prepared. The pH value was adjusted to 5 by the addition of HCl or NaOH solution and calibrated by a PHS-3C model meter. Adsorption experiments were performed at 301 K with 0.25 g/L phase ratio and contact time of 41 h. After the adsorption process, 0.45 μm micropore filters were used to separate the PDAcoated CMK-3 adsorbents from the aqueous phase, and the residual concentration of U(VI) was determined by the arsenazo III method.52 Adsorption capacity Q and distribution coefficient Kd are defined as Q= 5014

(C0 − C t) × V M

(1) DOI: 10.1021/acs.chemmater.6b01729 Chem. Mater. 2016, 28, 5013−5021

Chemistry of Materials Kd =

C0 − Ct V × Ct M

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RESULTS AND DISCUSSION A series of PDA-coated CMK-3 type OMCs were synthesized by self-polymerization of dopamine in Tris buffer solutions (pH = 8.5). The conversion of dopamine into the corresponding quinone under alkaline conditions is a radical process and a rate-determining step of self-polymerization.43 Thus, in order to realize the controllable deposition of PDA coatings, ethanol was added to the reaction system as a radical-trapping agent to slow down the growth rate of PDA. The volume ratio of ethanol and water was kept at 1:1.25, which is appropriate for effectively controlling the formation of PDA coatings on the surface of carbon nanotubes (CNTs).54,55 The effects of dopamine concentration and coating time were studied. Figure 1a,b show the TEM images of pristine CMK-3 and the PDA-coated product CMK-3-PDA-4.4-10. Apparently, the

(2)

where C0 and Ct are the initial and residual U(VI) concentrations in aqueous solutions, respectively. V is the volume of the initial solution and M is the dosage of the adsorbent. To evaluate the selectivity of the PDA-coated CMK-3 adsorbents toward U(VI) in a multicomponent solution, a simulated nuclear industry effluent containing a series of different metal ions was prepared (Table S1).53 Competitive adsorption experiments were then conducted with the concentration of the metal ions determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Besides, the reusability of the PDA-coated CMK-3 adsorbents was studied by three adsorption−desorption cycles. Ten milligrams of CMK-3-PDA-4.4-24 was mixed with 20 mL of U(VI) solution with an initial concentration of 50 mg/L. After a contact time of 24 h, the supernatant was separated, and 0.1 mol/L HCl solution was used for stripping the adsorbed U(VI). Both the U(VI) concentrations in the supernatant and the eluent were measured for determining the adsorption and desorption efficiency during the cycle use. Immobilization of the ATRP Initiator. CMK-3-PDA-1.1-24 (10 mg) was added to 20 mL of dry THF in a Schlenk flask, followed by ultrasonic dispersion for 10 min. Then, 1 mL of TEA (7.2 mmol) was slowly added to the flask under N2 protection and magnetic stirring. After 5 min, a solution, containing 0.9 mL of BiBB (7.2 mmol) and 10 mL of dry THF, was added dropwise. The reaction was kept for 24 h at 25 °C. The product was washed several times with acetone, methanol, and water. Then, the obtained CMK-3-PDA-BiBB was placed in a vacuum oven at 30 °C for 24 h. SI-ATRP Grafting of PMMA. First, 10 mL of purified MMA was added into a 25 mL Schlenk flask, followed by the addition of 10 mg of CMK-3-PDA-BiBB under magnetic stirring. After 10 min, 4 mL of anisole, 1.5 mL of DMF, 196 μL of EBiB, 200 μL of 5 g/L CuBr2/ anisole solution, and 18.9 mg of TPMA were added sequentially. Then, the Schlenk flask was sealed and degassed by N2 purging for 30 min. Subsequently, 1.8 mL of 5 g/L Sn(EH)2/anisole solution was added via a syringe, and the reaction was kept for 24 h at 25 °C. The molar ratio of the reagents was the following: MMA/EBiB/CuBr2/ TPMA/Sn(EH)2 = 70/1/0.0035/0.035/0.0175. Lastly, CMK-3-PDAPMMA was separated from the free poly(methyl methacrylate) (PMMA) by centrifugation and then washed with THF three times, accompanied by ultrasonic treatment for 10 min each time. Characterizations. Transmission electron microscopy (TEM) (HT-7700, Hitachi Ltd. Tokyo, Japan) was conducted at an accelerating voltage of 120 kV. Surface morphology of the OMCs was characterized by Quanta 600 FEG scanning electron microscopy (SEM). The mesostructured regularity was measured by small-angle Xray diffraction (SAXRD) (Rigaku D/max-2400 X-ray powder diffractometer) using Cu Kα radiation. N2 adsorption measurements were conducted by using Surface Area and Porosity Analyzer (Nava 3200e), and all samples were treated at 80 °C under vacuum for 3 h before the measurement. Specific surface area was calculated by the Brunauer−Emmett−Teller (BET) method, and pore size distribution was acquired by the Density Functional Theory (DFT) method. Elemental analysis was performed on Elementar Vario EL III. U(VI) concentration was measured by the arsenazo III method with a 721 type spectrophotometer at 650 nm. Thermogravimetric analysis (TGA) was carried out on a TA Instrument TGA 2950, with the heating from 25 to 600 °C at a rate of 10 °C/min under nitrogen atmosphere. Number-average molecular weight (Mn), weight-average molecular weight (Mw), and molecular weight distribution of PMMA were determined by size exclusion chromatography (SEC). SEC was operated with a Waters 515 pump and Waters 410 differential refractometer using PSS columns (Styrogel 105, 103, 102 Å) in THF as an eluent at 35 °C and at a flow rate of 1 mL/min. Linear PMMA standards were used for calibration.

Figure 1. TEM and SEM images of CMK-3 (a,c) and CMK-3-PDA4.4-10 (b,d).

ordered pore channels were well-maintained after the deposition of PDA. No self-polymerized PDA microspheres were observed in the TEM images of all PDA-coated CMK-3, suggesting that the nucleation and growth rates of PDA were well-controlled with the addition of ethanol. In addition, the absence of large-sized PDA microspheres by self-nucleation implies that most of the dopamine molecules underwent a heterogeneous nucleation at the pore surfaces of the CMK-3 and thereby developed to be a coating layer. This can be explained by the strong π−π interaction between the dopamine molecules and the resulting PDA aggregates containing aromatic rings and the carbonaceous surface of the CMK-3. Previous research studies have revealed that both dopamine and PDA exhibited strong affinity to the sidewalls of CNTs through π−π stacking.56−58 Due to the similar carbonaceous surface structure, OMCs are expected to interact strongly with aromatic rings containing molecules.59,60 This would facilitate the formation of a robust PDA coating on the surfaces and pore walls of the CMK-3. The morphology of the PDA-coated CMK-3 as well as the pristine counterpart were characterized by SEM. Compared with CMK-3 (Figure 1c), a uniform layer of PDA was observed on the surface of modified product (Figure 1d). In different dopamine solutions with the concentration varying from 0.6 to 4.4 g/L, uniform PDA layers 5015

DOI: 10.1021/acs.chemmater.6b01729 Chem. Mater. 2016, 28, 5013−5021

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Figure 2. SAXRD patterns (A,C), N2 adsorption−desorption isotherms (B,D) and the corresponding pore size distribution curves (insets) of (a) CMK-3; (b) CMK-3-PDA-0.6-10; (c) CMK-3-PDA-1.1-10; (d) CMK-3-PDA-2.2-10; (e) CMK-3-PDA-4.4-10; (f) CMK-3-PDA-1.1-5; (g) CMK-3PDA-1.1-24.

PDA-0.6-10 was unchanged, implying that the majority of dopamine polymerized inside the micropores. With the enhancement of dopamine concentration, a thin PDA coating was gradually formed on the wall of the mesopores, leading to the decrease of both diffraction intensities (Figure 2A (c−e)) and pore sizes (Table S2). The pore size distribution curves (Figure 2B (inset)) show parallel shifts to smaller pore sizes, suggesting uniform growth of PDA coatings. Figure 2B (a) shows a typical N2 adsorption−desorption isotherm of mesoporous materials with a type IV curve and a pronounced hysteresis loop. Deposition of the PDA coatings caused the decreases of surface area and pore volume (Table S2), but the hysteresis loops were maintained for CMK-3-PDA0.6-10 and CMK-3-PDA-1.1-10.This demonstrates that the deterioration of structural properties is mainly attributed to the blocking of micropores while the mesostructure can be retained. When the dopamine concentration reached 4.4 g/L, both surface area and pore volume experienced a significant decrease, suggesting that high dopamine concentration dramatically deteriorated the structure of CMK-3. Because the structural properties of CMK-3-PDA-1.1-10 were relatively well-retained, the effect of coating time at 1.1 g/ L of dopamine concentration was further studied. The coating times were 5, 10, and 24 h. From SAXRD patterns (Figure 2C), all PDA-coated CMK-3 retained strong 100 diffraction peaks, indicating prolonged coating time did not dramatically destroy the mesostructured regularity of CMK-3. When the coating time was only 5 h, the obtained CMK-3-PDA-1.1-5 showed the

could be readily deposited on the surface of the CMK-3, suggesting the robustness of the PDA chemistry for surface modification of OMC materials. SAXRD and N2 adsorption−desorption measurement were employed to determine the structural property of CMK-3 and PDA-coated CMK-3. A strong (100) reflection peak and weak (110), (200) signals are visible in the SAXRD pattern of CMK3 (Figure 2A (a)), corresponding to the typical hexagonally ordered mesostructure. After PDA coating under different conditions, the mesostructured regularity of CMK-3 underwent interesting evolution. A remarkably stronger (100) peak was obtained for CMK-3-PDA-0.6-10, while its intensity weakened with the increase of dopamine concentration. Because CMK-3 is a replica of SBA-15, the etching leads to the formation of three-dimensional interconnected nanorods with significant microporosity in the mesopore walls. The preferential accumulation of organic molecules inside micropores results in an increase of apparent density of the mesopore wall, which is beneficial to improve the mesostructured regularity.32,61 For CMK-3-PDA-0.6-10, the increased diffraction intensity of the (100) peak can be attributed to the low dopamine concentration, leading to the formation of small PDA aggregates which mainly filled the micropores rather than the mesopores, because of the reinforced π−π interaction by the enhancement of adsorption potential occurring between the narrower parallel pore walls of the CMK-3. 59,60 This phenomenon was further verified by N2 adsorption−desorption measurement. Compared to CMK-3, the pore size of CMK5016

DOI: 10.1021/acs.chemmater.6b01729 Chem. Mater. 2016, 28, 5013−5021

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Figure 3. Evolution of grafting densities and U(VI) adsorption properties under different coating conditions: (A) dopamine concentration; (B) coating time.

same pore size as that of CMK-3-PDA-0.6-10, implying that the initial polymerization of dopamine took place inside the micropores, whereas a longer coating time led to the formation of uniform PDA coatings on the wall of the mesopores and slightly reduced the pore size (Table S2 and Figure 2D (inset)). Similarly, the structure of CMK-3 was deteriorated with the increase of coating time (Table S2), but all N2 adsorption− desorption isotherms of PDA-coated CMK-3 in Figure 2D present type IV curves with clear hysteresis loops ranging from 0.35 to 0.95 of relative pressure. Overall, by simply adjusting the dopamine concentration and coating time, we could realize a controlled growth of PDA coatings on the surface of CMK-3. Thus, PDA-coated CMK-3 with well-maintained mesoporous structure were acquired. The elemental composition and grafting densities of functional groups were characterized by elemental analysis. With the increase of dopamine concentration or coating time, the nitrogen content correspondingly increased, suggesting that higher dopamine concentrations or prolonged coating time resulted in more PDA deposited to the surface of CMK-3 (Table S3). In addition, grafting densities of functional groups showed a similar evolution with the change of coating conditions (Figure 3). Grafting densities of both −NH and −OH groups with the values of 4.7 μmol/m2 or 2.8 group/nm2 and 9.3 μmol/m2 or 5.6 group/nm2 were obtained when 4.4 g/ L of dopamine concentration and 10 h of coating time were used. They are significantly higher than the OMCs modified by diazonium chemistry28 and comparable to the concentration of silanol groups on silica surface (∼6−9 μmol/m2 or ∼3.6−5.4 group/nm2).31 More importantly, grating density can be easily controlled by simply adjusting the coating conditions. As mentioned above, the catechol and imine can easily react with acyl bromide- or acyl chloride-terminated molecules. And, amine- and thiol-terminated molecules can be readily attached to the PDA-coated surfaces via Schiff base reaction or Michael addition reaction. Hence, the highly reactive PDA coating provides massive opportunities for further surface modification of OMCs. High grafting densities of −NH and −OH groups can dramatically alter the hydrophobic nature of CMK-3. Pristine and PDA-coated CMK-3 were dispersed in water, resulting in a significantly improved dispersibility for all PDA-coated samples (Figure S1). The enhancement of hydrophilicity could further broaden the application range of CMK-3.

Batch adsorption experiments were conducted to evaluate the potential of PDA-coated CMK-3 for enrichment of uranyl ions. As a valuable source of nuclear power plants and a hazardous radionuclide, the removal and recycling of uranium have attracted increasing attention, and therefore, potential adsorbents for U(VI) are continuously explored.62−65 Due to the high surface area and chemical stability in weakly acidic solutions, OMC materials modified with organic functional groups have been already utilized for the removal of U(VI) from aqueous solutions. Previously, complicated processes were required to realize surface modification of OMCs in order to improve their adsorption capacities.53,66 In this study, by facile deposition of PDA coating on the surface of CMK-3, high densities of −NH and −OH functional groups were introduced, which imparted the CMK-3 with significantly increased U(VI) adsorption capacities (Figure 3). The largest adsorption capacity of U(VI) was obtained by CMK-3-PDA-4.4-10, with the Q value of 93.6 mg U(VI)/g adsorbent and the Kd value of 3286 cm3/g (Table S3), which were significantly larger than that of the typical oxime-grafted CMK-5 adsorbent.53 The blue curve in Figure 3 shows that the U(VI) adsorption capacity of the PDA-coated CMK-3 well correlates with the dopamine concentration and coating time. Competitive adsorptions were performed in a simulated nuclear industry effluent (Table S1) to evaluate the selectivity of the PDA-coated CMK-3 toward U(VI).53 It can be seen in Figure S2 that all the PDA-coated CMK-3 samples show selective binding ability to U(VI) against most of the coexistent metal ions, such as La(III), Ce(III), Sr(II), Zn(II), Mn(II), Ni(II), Co(II), K(I), and so on. It is noteworthy to mention that Cr(III) with a moderate adsorption capacity may be a potential interfering ion in the effluent. Reusability of the PDAcoated CMK-3 was studied in three adsorption−desorption cycles. Figure S3 shows that the recycled adsorbent still possess effective adsorption ability to U(VI). However, due to the incomplete desorption of the adsorbed U(VI) by using 0.1 mol/L HCl for elution, the adsorption efficiency shows about 10% decrease after each cycle. Overall, it can be concluded that the PDA-coated OMCs have potential in removing U(VI) from aqueous solutions, but efforts are still needed to seek out a more efficient desorption method for cycle use. Furthermore, the PDA-modified OMCs with abundant−NH and −OH groups on the surface can also become good candidates for enrichment of other metal ions, which may further elevate the 5017

DOI: 10.1021/acs.chemmater.6b01729 Chem. Mater. 2016, 28, 5013−5021

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Chemistry of Materials applicability of OMCs in various fields, for example, as an advanced catalyst support. To verify the effectiveness of PDA-coated CMK-3 as a versatile platform for secondary reactions, SI-ATRP was employed for the surface modification of OMCs, which could alter the physical and chemical properties to an even larger extent. ATRP is a controlled/living radical polymerization (CRP) technique, used to synthesize polymers with predetermined molecular weight and low dispersities.67−74 SI-ATRP has been broadly applied to functionalize various nanomaterials, by efficiently controlling the growth of polymer chains from surfaces75−87 In this study, activators regenerated by electron transfer ATRP (ARGET ATRP) technique was used to graft PMMA brushes from the surface of CMK-3. Compared to traditional ATRP, ARGET ATRP technique requires more relaxed oxygen-free conditions and lower concentration of Cu catalyst.88−90 Before grafting, ATRP initiator (BiBB) was immobilized to the CMK-3 through the nucleophilic substitution reaction with the catechol and/or the amine groups in the PDA coating. The TGA measurement (Figure 4(a,b)) showed 12.0 wt % of weight loss for CMK-3-PDA-1.1-

Figure 5. (a) TEM and (b) SEM images of PMMA-grafted CMK-3.

uniform PMMA layer with the thickness of ∼9 nm was formed on the surface of PDA-coated CMK-3, while increased claddings were observed for the nanoarrays of CMK-3 by SEM, suggesting efficient grafting of PMMA brushes from the initiator-immobilized CMK-3. Furthermore, the molecular weight of the free PMMA obtained from the sacrificial initiator was Mn = 11 000 with narrow molecular weight distribution (Mw/Mn = 1.14) (Figure S4). This confirms that the surfaceinitiated polymerization of MMA from the surface of the CMK3 was well-controlled.90 Furthermore, the grafting density of PMMA was calculated as 0.02 group/nm2, which is reasonable for large surface area and limited pore channels of PDA-coated CMK-3. Overall, the PDA-coated CMK-3 can act as an effective platform to accommodate other functional molecules, and to conduct secondary reactions or surface-initiated polymer grafting, which opens new possibilities for surface modification of OMCs.

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CONCLUSION

The mussel-inspired PDA chemistry is a new tool for the surface modification of OMCs, which could activate the inert surfaces and mesoporous channels of OMCs with improved hydrophilicity, enriched functional groups, adsorption ability toward metal ions, as well as chemical reactivity for diverse post functionalization. Uniform PDA coating was first deposited on the surface of CMK-3 under mild conditions without compromising the meso-structural regularity while maintaining the accessibility of mesopores. The surface properties and morphology of the PDA-coated CMK-3 could be tailored by adjusting the dopamine concentration and coating time. Grafting densities of −NH groups (4.7 μmol/m2 or 2.8 group/nm2) and −OH groups (9.3 μmol/m2 or 5.6 group/ nm2) generated on the surface of CMK-3 were significantly higher than previously reported. The enhanced hydrophilicity of the PDA-coated CMK-3 and the superior adsorption capacity of U(VI) showed great promise in the field of environmental decontamination and even as a potential catalyst support. Moreover, the immobilization of BiBB initiators on the PDA coating demonstrated, for the first time, that SI-ATRP could be employed for further functionalization of OMCs. Controlled growth of PMMA brushes (Mn = 11 000, Mw/Mn = 1.14) from the initiator-modified surface of CMK-3 was realized. In conclusion, the mussel-inspired PDA chemistry is expected to open new avenues for surface modification and/or functionalization of carbon materials.

Figure 4. TGA curves of (a) CMK-3; (b) CMK-3-PDA-1.1-24; (c) CMK-3-PDA-BiBB; (d) CMK-3-PDA-PMMA.

24 in the temperature range from 120 to 600 °C. This is 10.4 wt % higher than that of pristine CMK-3. Moreover, the TGA curve of CMK-3-PDA-1.1-24 still has a decreasing tendency over 600 °C. This suggests that at least 10.4 wt % of PDA was coated on the surface of CMK-3. Compared to the TGA curve of PDA-coated CMK-3 in the temperature range of 150−350 °C, 10.1 wt % more weight loss was found for CMK-3-PDABiBB (Figure 4(c)), indicating that initiator was successfully attached on the surface of a PDA-coated CMK-3. Subsequently, grafting of PMMA was conducted via ARGET ATRP using 50 ppm of CuBr2/TPMA as catalyst in the presence of sacrificial initiators. The resulting product, CMK-3PDA-PMMA, showed a more significant drop (43.1 wt %) in the TGA curve from 150 to 400 °C (Figure 4(d)) due to the decomposition of PMMA brushes grafted from the surface of CMK-3. The successful grafting of PMMA brushes proved the accessibility of active sites distributed in the mesopores even after immobilization of BiBB. Additional evidence for successful grafting of PMMA was obtained by TEM and SEM characterization (Figure 5). The TEM image of PMMA grafted CMK-3 shows that a highly 5018

DOI: 10.1021/acs.chemmater.6b01729 Chem. Mater. 2016, 28, 5013−5021

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01729. Data with respect to structural parameters, elemental composition, functional group densities, U(VI) adsorption capacity, selectivity and reusability of the PDAcoated CMK-3. Digital image showing the water dispersibility of the PDA-coated CMK-3. Molecular weight distribution of PMMA (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for G.Y.: [email protected]. *E-mail for J.W.: [email protected]. *E-mail for K.M.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was supported by the Changjiang Scholars and Innovative Research Team in University (IRT13026), the National Science Fund for Distinguished Young Scholars (51425403), National Natural Science Foundation of China under Project 51473087, 91226110, 51425403, and U1430234 as well as the National Science Foundation (DMR 1501324). M.K. thanks Polish Ministry of Science and Higher Education (“Mobilnosc Plus” grant no. 1055/MOB/2013/0) for financial support.

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