Polyelectrolyte brush-grafted polydopamine-based catalysts with

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Polyelectrolyte brush-grafted polydopamine-based catalysts with enhanced catalytic activity and stability Byung Kwon Kaang, Nara Han, Ha-Jin Lee, and Won San Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15489 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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ACS Applied Materials & Interfaces

Polyelectrolyte brush-grafted polydopamine-based catalysts with enhanced catalytic activity and stability

Byung Kwon Kaang,a Nara Han,a Ha-Jin Lee,*,b and Won San Choi*,a

a

Department of Chemical and Biological Engineering, Hanbat National University, 125

Dongseodaero,

Yuseong-gu,

Daejeon

305-719,

Republic

of

Korea,

E-mail:

[email protected], bWestern Seoul Center, Korea Basic Science Institute, 150 Bugahyunro, Seoudaemun-gu, Seoul, 120-140, Republic of Korea, E-mail: [email protected]

Keywords: polyelectrolyte brush, polydopamine, nanocatalyst, catalytic activity, dye.

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Abstract Three types of surface treatments, namely, polyethyleneimine (PEI) coating, short PEI grafting, and long PEI grafting, were performed on polydopamine (Pdop)-based catalysts to enhance their catalytic activity and stability. Brush-grafted catalysts were prepared by the stepwise synthesis of Au and short (or long) PEI brushes on Pdop particles (PdopP/Au/S- or L-PEI grafting). PEI-coated Pdop-based catalysts (PdopP/Au/PEI coating) were also prepared as non-brush-grafted catalysts. Among the surface-treated PdopP/Au catalysts, the brushgrafted catalysts (S-PEI and L-PEI grafting) exhibited excellent and stable catalytic performance because the brush grafting enabled the protection of the catalysts against harsh conditions, effective transfer of reactants to the catalysts, and confinement of reactants around the catalysts. The brush-grafted catalysts could also more effectively decompose larger dyes than the non-brush-grafted catalysts. The process-to-effectiveness of PEI coating is the best because the release of Pdop from PdopP/Au was moderately inhibited by the presence of only 1 layer of PEI coating on the PdopP/Au. Thus, this approach could be an alternative method to enhance the stability of PdopP/Au catalysts.


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1. Introduction Polydopamine (Pdop) has attracted considerable interest because of its ability to be readily coated onto various types of organics and inorganics with controllable film thickness.1,2 Pdop can also be used as both the reaction site and the anchor for the loading of metal ions because Pdop possesses many functional groups, such as amines, catechols, and imines.1,2 Thus, many research groups have studied the use of Pdop-based nanomaterials.3-12 Nanomaterials have been considered important tools for environmental remediation, which can be ascribed to their large specific surface areas and high reactivities. The high surface area-to-mass ratio of nanomaterials can remarkably enhance their adsorption or catalytic performance. Although the stability of nanomaterials is a critical issue for their practical application, the use of Pdopbased nanomaterials has also been limited by poor alkali resistance,2,8 which arises from the decomposition and destruction of noncovalent interactions in Pdop under strongly alkaline conditions.2,8 Shao et al. demonstrated that the incorporation of polyethyleneimine (PEI) during the polymerization of dopamine can enhance the alkaline resistance of Pdop.13 However, there are other factors that can chemically damage Pdop.2,8,14 In addition to poor alkali resistance, Pdop was verified to have poor chemical and acid resistance in our current study. For instance, the dissolution of the Pdop was observed in the presence of NaBH4 when a Pdop-based catalyst was used for a catalytic test. This drawback considerably limits the catalytic applications of Pdop-based materials. Developing Pdop-based nanomaterials with chemical stability under various conditions is highly desirable for practical applications, including catalysts and adsorbents. A polyelectrolyte (PE) brush is a layer of polymer chains attached to a surface via the end of the chains.15 Various types of PE brushes can be grafted onto a substrate by the socalled “grafting-from” and “grafting-to” polymerizations.16-22 The “grafting-from” method involves immobilizing an initiator system on the surface of a solid substrate and then polymerizing the PE layer in situ at the initiator sites on the substrate.15 In the “grafting-to” 3 ACS Paragon Plus Environment


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approach, end-functionalized PEs are attached to the appropriate functional groups on a solid surface.15 A common feature of two different approaches is increasing the density of surface functional groups because PE brushes possess numerous repeating units and therefore numerous functional groups. Thus, nanomaterials grafted with PE brushes show excellent adsorption performance compared to other materials. For example, the Yameen group designed cationic or anionic PE brush-grafted magnetic nanoparticles (NPs) for highly efficient water remediation.23 The Tang group also reported amine-based PE brush-grafted magnetic NPs for highly efficient adsorption of heavy metal ions.24 Moreover, the Guan group demonstrated Cu2+-modified PE brush-grafted magnetite NPs for protein adsorption.25 However, to the best of our knowledge, PE brush-grafted nanocatalysts with enhanced catalytic activity and stability have not yet been reported. The abovementioned challenges inspired us to develop PE brush-grafted Pdop-based catalysts that are protected from external stimuli, are endowed with new functionalities, and have controllable surface properties. Herein, we report brush-grafted Pdop-based catalysts in which the catalysts are protected against harsh conditions and the reactants are effectively transferred to and confined around the catalysts. We also found that the brush-grafted catalysts can more effectively decompose large dyes than non-brush-grafted catalysts. We expect that because of the enhanced chemical stability conferred on the catalysts, this brush-grafting technique for Pdopbased materials can be applied to catalysts or adsorbents that must be used under harsh conditions.

2. Experimental section 2.1 Materials Ethanol (EtOH, 96.5%), ammonium hydroxide solution (NH4OH, 28-30%), dopamine hydrochloride

(100%),

gold(III)

chloride

trihydrate

(HAuCl4·H2O

≥49.0%),

tris(hydroxymethyl)aminomethane (≥99.8%), PEI solution [(L-PEI, MW=750,000, 50 wt% in 4 ACS Paragon Plus Environment

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H2O) and (S-PEI, MW=25,000, ≤1% in H2O)], polystyrene sulfonate (PSS, MW=70,000), sodium borohydride (NaBH4, 98%), methyl orange (MO), congo red (CR), and direct red 80 (DR) were purchased from Sigma-Aldrich. All chemicals were used without further purification. Deionized (DI) water with a resistance of 18.2 MΩ cm-1 was obtained from a Millipore Simplicity 185 system. 2.2 Synthesis of PdopP A mixture of EtOH (4 mL), DI water (9 mL), and NH4OH (40 µL) was prepared and stirred at room temperature for 30 min.26 An aqueous solution (1 mL) of dopamine hydrochloride (50 mg/mL) was slowly added dropwise to the abovementioned mixture. The resulting solution was reacted for 30 h. The pH of the resulting solution almost maintained during the reaction (pH=8.68 - 8.46). The resulting precipitate was washed three times with DI water. 2.3 Synthesis of PdopP/Au Pdop particles (PdopP, 20 mg) were added to 17 mL of HAuCl4 solution (10 mM). The dispersion was agitated vigorously on a shaking apparatus for 4 h to allow the adsorption of the Au ions onto the PdopP. The product was washed three times with ethanol and dried in an oven at 50 °C. 2.4 Synthesis of PdopP/Au/PEI, PEI/PSS/PEI, or (PEI/PSS)2/PEI coating A 0.5 M aqueous NaCl solution (2 mL) containing PEI (2 mg/mL) was first added to an aqueous suspension of PdopP/Au (10 mg). The dispersion was agitated vigorously on a shaking apparatus for 15 min, and the resulting particles were collected after rinsing three times with water. The resulting particles (PdopP/Au/PEI coating) were redispersed in 2 mL of NaCl solution (0.5 M) containing PSS (2 mg/mL) for 15 min to form PdopP/Au/PEI/PSS coating. The rinsing step was repeated three times with water. The adsorption and rinsing steps were repeated until the desired number of layers was obtained. 2.5 Synthesis of PdopP/Au/S- and L-PEI grafting



PdopP/Au (10 mg) was added into 30 mL of a solution containing S-PEI (0.3 mg/mL, MW=25,000)/Tris-HCl (10 mM, pH=8.5) or L-PEI (0.3 mg/mL, MW=750,000)/Tris-HCl (10 mM, pH=8.5). The resulting solution was reacted under reflux at 60 °C for 3 h to graft the PEI brushes onto PdopP/Au via the “grafting-to” method. After 3 h, the solution was further shaken for 16 h at room temperature. The resulting mixture was washed three times with DI water and dried in an oven at 50 °C. 2.6 Stability tests of three types of catalysts The acid, alkali, and NaBH4 resistance of three types of catalysts was evaluated. To test their stability to each condition, the three types of catalysts (10 mg) were dispersed in acidic (pH=2), basic (pH=12), or NaBH4 (100 mM) solution and shaken for 3 h. After incubation for 3 h, the supernatant was separated from the catalyst and analyzed by a UV-vis spectrophotometer to monitor the Pdop released from each catalyst. 2.7 Catalytic tests for the decomposition of dyes by three types of catalysts To investigate the catalytic decomposition of dyes (MO, CR and DR), 15 mL of freshly prepared aqueous dye solution (0.08 mM) and 7 mL of aqueous NaBH4 solution (100 mM) were mixed together. The pH values of the mixture solutions including each dye and NaBH4 were almost analogous and the average value was 10.01. The three catalysts (PdopP/Au/PEI coating, PdopP/Au/S-PEI grafting, or PdopP/Au/L-PEI grafting) (20 mg) were added separately to the resulting solutions. After the addition of each catalyst, the UV-vis spectrum of the mixture was monitored at regular intervals to observe the reaction progress. 2.8 Characterization Field-emission transmission electron microscopy (FE-TEM) and scanning electron microscopy (SEM)/energy-dispersive X-ray (EDX) analyses were performed using a JEOL JEM 2100F and Hitachi S-4800, respectively. The UV-vis absorption spectra were recorded on a UV-vis spectrophotometer (Sinco Evolution 201). FT-IR spectra were obtained using a Sinco Nicolet IS5 instrument. X-ray diffraction (XRD) patterns were obtained on a Rigaku X6 ACS Paragon Plus Environment

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ray diffractometer equipped with a Cu Kα source. The zeta potential measurements were performed on a Malvern Nano-ZS Zetasizer at room temperature. The average values of three measurements were used. Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (Sinco TGA N-1500) over a temperature range of 25-800 °C at a heating rate of 5 °C/min under air (flow rate, 60 cm2/min). The average particle size was measured by a particle size analyzer (PSA, UPA-150).

3. Results and discussion Figure 1 shows a schematic illustration of the synthesis of PEI-grafted composites of Pdop particles (PdopP/Au/PEI grafting), which were prepared by the stepwise synthesis of AuNPs and PEI brushes on the PdopP. Two types of PdopP/Au composites grafted with short and long PEI (S-PEI and L-PEI) were prepared by employing the “grafting-to” method and varying the PEI chain lengths (PdopP/Au/S-PEI and L-PEI grafting) and then were used as brush-grafted catalysts (Figures 1a and 1b). The “grafting-to” process allows for precise control of the molecular mass distribution of the surface-attached polymers.27 Initially free PEI chains diffuse to the surface of the PdopP/Au and react with the appropriate surface sites of the PdopP/Au. The PEI can be grafted onto the PdopP via a Michael-addition reaction between the ortho position of the catechol group on the Pdop and the amine group on the PEI.28,29 After the surface is significantly covered with the PEI, the grafting is terminated. PEI-coated PdopP (PdopP/Au/PEI coating) were also prepared as non-brush-grafted catalysts (Figure 1c) and compared with the brush-grafted catalysts. The brush-grafted catalysts (S-PEI and L-PEI grafting) exhibited excellent and stable catalytic performance in decomposing organic pollutants with various sizes because of the chemical buffer, mobility, and cage characteristics of the PEI brush (lower panel). The rate and efficiency of pollutant decomposition could be controlled by varying the PEI chain lengths. The brush-grafted



catalysts could also more effectively decompose large dyes than the non-brush-grafted catalysts. Figure 2 shows scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning TEM (STEM) images of each step of the formation of the PdopP/Au/PEI grafting. The PdopP showed a relatively smooth surface morphology, whereas the surface morphology of the PdopP/Au exhibited the formation of tiny particles on the PdopP, indicating the successful synthesis of the AuNPs on the PdopP (Figures 2a and 2b). The ability to AuNPs to be synthesized on Pdop has been shown.2,8 The catechol groups of the PdopP show highly efficient reactive adsorption and excellent selectivity for Au ions, compared to other metal ions.2,8 The Au ions can also interact with the amine groups of the PdopP.2,8 These Au ions interacted with each group can be reduced into the AuNPs by reduction treatment. The PdopP/Au/PEI coating was prepared by PEI coating on the PdopP/Au and showed analogous surface morphology with the PdopP/Au (Figure 2c). However, after S- and L-PEI grafting, tiny particles gradually disappeared from the surface of the PdopP/Au, suggesting that the AuNPs were hidden by grafting the S- and L-PEI brush chains onto the PdopP/Au (Figures 2d and 2e). The PdopP/Au/L-PEI grafting was used as the representative PdopP/Au/PEI grafting for TEM analyses because no significant differences from the PdopP/Au/S-PEI grafting were observed. The corresponding STEM image revealed that AuNPs with an average size of 25 nm were mainly concentrated in the center of the PdopP (Figure 2f). An enlarged image of the PdopP/Au/L-PEI grafting exhibited several nanometer-sized AuNPs in the shell region of the PdopP/Au/L-PEI grafting (Figure 2g). Energy-dispersive X-ray (EDX) and X-ray diffraction (XRD) analyses confirmed the presence of Au (as AuNPs) (Figures 2h and i). Fourier transform infrared (FT-IR) spectroscopy was performed to further confirm the formation of the PdopP/Au/S- and L-PEI grafting (Figure 3a). The absorption peaks at approximately 1582 cm-1 (-NH2 bending), 1504 cm-1 (-NH- scissoring), and 1292 cm-1 (C-O 8 ACS Paragon Plus Environment

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stretching) corroborated the synthesis of Pdop (black line).2,8 The peak intensities at 1582 cm1

(-NH2 bending) and 1504 cm-1 (-NH- scissoring) reversed for the PdopP/Au, suggesting the

attachment of the AuNPs to the amine groups (black and red lines).2,8,12 For the PdopP/Au/Sand L-PEI grafting, the successful grafting of PEI onto the PdopP/Au via a Michael addition reaction between the ortho-catechol group on Pdop and the amine group on PEI was confirmed by the changed -NH2/-NH- ratio at 1582 cm-1 (-NH2 bending) and 1504 cm-1 (-NHscissoring) (red, pink, and greenish blue lines).2,8,12 The absorption peaks at 1047 cm-1 (-N= scissoring) further demonstrate the grafting of S-PEI and L-PEI onto PdopP/Au (red, pink, and greenish blue lines). Zeta potential, dynamic light scattering (DLS),

and

thermogravimetric analysis (TGA) measurements were performed to investigate the characteristics of the PdopP/Au/S-PEI and L-PEI grafting. The zeta potential data confirmed the grafting of the S- and L-PEI brushes onto the PdopP/Au. The surface charge of the PdopP/Au before the PEI grafting was -29.13 mV, which was due to the catechol groups on Pdop (Figure 3b). After the PEI coating, the surface charges of the resulting particles slightly increased to -22.37 mV. However, after the S- and L-PEI grafting, the surface charges of the resulting particles remarkably increased to -13.63 mV and -6.73 mV, respectively, because of the positive charges on PEI, indicating the successful grafting of both the short and long PEI brushes. The hydrodynamic radius of the PdopP/Au/PEI coating was 991 nm in aqueous solution (Figures 3c and S1). After the S- and L-PEI grafting, the sizes of the resulting particles increased to 1089 nm and 1129 nm, respectively, suggesting that the S- and L-PEI brushes with lengths of 98 nm and 138 nm, respectively, were successfully grafted onto the PdopP/Au. The TGA data revealed that the PdopP/Au composites grafted with S- and L-PEI possessed 13.69% and 14.87% more organic mass, respectively, than the PdopP/Au (Figure 3d, blue and greenish blue lines), further confirming that S- and L-PEI were grafted onto the PdopP/Au. To investigate the degree of grafting, the grafting density of each PEI was



calculated.30 The grafting density (σ) of the PEI brush film (chains/nm2) can be calculated from the surface concentration using Eq. (1).

σ=

Eq. (1)

where NA is the Avogadro’s number and MW is the molecular weight of PEI. Surface concentration (Γ) of the PEI brush film can be calculated using the following equation. Γ =dρo

Eq. (2)

where d is the thickness and ρo is the density of the PEI brush film (assumed to be 1.03 g/cm3). The grafting density of L-PEI (3.05 chains/nm2) was lower than that of S-PEI (15.6 chains/nm2) because of the steric hindrance effect (Figure 3e). Overall, the results mentioned above indicate that the S- and L-PEI brushes were successfully grafted onto the PdopP/Au. When the PdopP was dispersed into acidic or basic solution (pH=2 or 12), a browncolored solution stemming from the particles slowly appeared in both cases (Figure S2). A darker brown-colored solution was released from the PdopP in basic solution than in acidic solution, suggesting that Pdop can be seriously damaged by basic conditions. Figure S2 shows the time-dependent UV-vis spectra of the brown solution released from the PdopP. A strong absorbance peak near 285 nm appeared, confirming the degradation or destruction of the PdopP. The decomposition/destruction of noncovalent interactions in Pdop can be occurred under strongly basic conditions because the PdopP consist of covalent and noncovalent interactions of Pdop.2,8 The resistance of catalyst-loaded PdopP was also investigated. Interestingly, after the synthesis of AuNPs on the PdopP, the amount of Pdop released from the PdopP/Au decreased under both acidic and basic conditions (Figures 4a and b). However, since the amount of Pdop released from the PdopP/Au was still substantial, several comparative experiments were performed to investigate the influence of surface treatments on the resistance of the composite. PEI coating, S-PEI grafting, and L-PEI grafting were performed on the PdopP/Au, and the resulting species were exposed to acidic conditions, 10 ACS Paragon Plus Environment

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basic conditions, and the presence of chemicals. We observed very interesting phenomenon owing to the surface treatments of the PdopP/Au. The amount of Pdop released from the PdopP/Au under acidic and basic conditions further decreased with each surface treatment (Figures 4a and b). Among the surface treatments, the grafting of L-PEI onto the PdopP/Au exhibited the lowest amount of Pdop released, i.e., the highest inhibition rate of Pdop release (insets of Figures 4a and b), indicating that grafting L-PEI onto the PdopP/Au maximizes its acid and alkali resistance. The inhibition rates of Pdop release by the Pdop/Au/L-PEI grafting were 67% and 94% under acidic and basic conditions, respectively. The inhibition rate of Pdop release was remarkably higher under basic conditions than under acidic conditions, which will be discussed in depth in the section on the chemical buffer characteristics of the PEI brush. Recently, the Shao group reported Pdop-based adsorbents with crosslinking between Pdop and PEI exhibiting excellent alkali resistance.13 Thus, we prepared a Pdop-PEI layer as a shell on the PdopP/Au (PdopP/Au/Pdop-PEI) according to the Shao group’s method. The chemical stability of the PdopP/Au/Pdop-PEI was compared with that of the PEI coating and S- and L-PEI grafting on the PdopP/Au. The PdopP/Au/Pdop-PEI showed inhibition rates of 46% and 85% against Pdop release under acidic and basic conditions, respectively. The alkali resistance of PdopP/Au/Pdop-PEI was excellent, whereas its acid resistance was somewhat low, which was analogous with our previous results. The Shao group reported that the poor alkaline resistance of the Pdop is due to the decomposition/destruction of noncovalent interactions in Pdop, and the introduction of covalent interactions (crosslinking) between Pdop and PEI will effectively and significantly improve the alkaline resistance of Pdop-based materials.13 Thus, we guess that the acid and alkali resistances of PdopP/Au/Pdop-PEI can be further enhanced by increasing the crosslinking density. Nevertheless, the above results indicate that the PdopP/Au/L-PEI grafting prepared by the method that we proposed here has a comparative advantage regarding its chemical stability under both acidic and basic conditions. 11 ACS Paragon Plus Environment


To further demonstrate this prominent feature, both the PdopP/Au and PdopP/Au/LPEI were dispersed in acidic and basic solution for 3 h. The PdopP/Au/L-PEI was used as the representative surface-treated PdopP for analysis because among the various catalysts, it showed the best resistance. Obvious weight loss (6.64%) from PdopP/Au was observed before and after the acid treatment (pH=2) (red and black lines, Figure 4c). However, the TGA curves of the PdopP/Au/L-PEI before and after the acid treatment were almost the same (green and blue lines, Figure 4c). Moreover, only 0.69% weight loss was observed after the acid treatment. These results indicate that the grafting of L-PEI onto the PdopP/Au prevents the degradation or destruction of Pdop during the acid treatment. Next, we examined whether this approach had the same effect on the resistance of Pdop to base treatment. Although remarkable weight loss (11.5%) of the PdopP/Au was observed after the base treatment (pH=12) (red and black lines, Figure 4d), only 0.89% weight loss was observed for PdopP/Au/L-PEI after undergoing the same treatment (green and blue lines, Figure 4d). The above results demonstrate that surface treatments (S- and L-PEI grafting) of PdopP/Au can enhance the resistance of the composite to acidic and basic conditions. To investigate why the PEI-coated and PEI-grafted PdopP/Au show good acid and alkali resistance, each sample was dispersed in acidic or basic solution (pH=2 or 12) for 3 h and separated from the solution, which was then monitored for pH changes. Under low pH conditions (pH=2), no remarkable pH change in the acidic solution was observed after removing the PdopP/Au/PEI coating (Figure 5a, red line). However, the pH of acidic solution increased to 2.92 for the PdopP/Au/L-PEI (green line). When the same experiments were performed using each sample under high pH conditions (pH=12), no remarkable pH change in the basic solution was observed after separating the PdopP/Au/PEI coating (Figure 5b, red line). However, a remarkable pH change in the basic solution from 12 to 9.08, which was much lower than the pH in the other experiments, was observed for the PdopP/Au/L-PEI grafting (green line). These results suggest that the PdopP/Au/L-PEI grafting more effectively 12 ACS Paragon Plus Environment

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neutralizes acidic (pH=22.9) and basic (pH=129.08) solutions than the others. The degree of neutralization of the basic solution was more remarkable than that of the acid solution, which explains why the inhibition rate of Pdop release was remarkably higher under basic conditions than under acidic conditions, as shown in insets of Figures 4a and b. The PdopP/Au/L-PEI grafting possesses numerous amine groups compared to the PdopP/Au/PEI coating and PdopP/Au/S-PEI grafting. These amine groups can be protonated by capturing protons from the acidic solution, thereby increasing the pH of the acidic solution (pH=22.9). Protonated amine groups can also be deprotonated to neutralize hydroxyl ions in basic solution, leading to the observed decrease in the pH of the basic solution (pH=129.08). In summary, the brush layer of the PdopP/Au/L-PEI has an important function as a chemical buffer layer in changing harsh conditions to milder conditions. The PdopP/Au/PEI coating showed fairly good acid and alkali resistance, even though its ability to tune the pH of the acidic or basic solution was not remarkable (Figures 4ab and 5ab). We speculated that the PEI coating on the surface of the PdopP/Au also prevents decomposition and destruction of noncovalent interactions of the Pdop chains in the PdopP. Thus, to investigate the effect of the PE coating on enhancing the resistance of Pdop, 3 and 5 layers of PEs were coated onto the PdopP/Au (PdopP/Au/(PEI/PSS)/PEI coating and PdopP/Au/(PEI/PSS)2/PEI coating, respectively). Electrostatic layer-by-layer (LbL) selfassembly methods involving PEs have been widely used in the preparation of polyelectrolyte multilayers (PEMs) onto planar or colloidal surface by electrostatic interactions between cation and anion PEs.31-33 Thus, (PEI/PSS)2/PEI can be fabricated by the stepwise adsorption of PEs with charges that are opposite from their aqueous media onto the PdopP/Au. Figure S3 summarizes

the

zeta-potential

recorded

with

layer

deposition

for

the

PdopP/Au/(PEI/PSS)2/PEI coating system. The zeta-potential alternates between negative and positive values, thereby indicating the successful coating of the particles with each layer. These composites were prepared and exposed to the acidic and basic solutions. As the number 13 ACS Paragon Plus Environment


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of PE coating layers increased, the amount of Pdop released from the PdopP/Au remarkably decreased

under

acidic

and

basic

conditions

(Figures

6a

and

6b).

For

the

PdopP/Au/(PEI/PSS)2/PEI coating, its resistance was comparable or superior to that of the PdopP/Au/L-PEI. Nevertheless, no remarkable pH changes in the acidic or basic solution were observed in either case, indicating that neither composite could tune the pH of a low or high pH (2 or 12) solution (Figures 5a and 5b). These results suggest that grafted PEI brushes acting a chemical buffer layer and coated PEI acting as a shield layer help ensure the chemical stability of Pdop. Potentially, the PE coating tightly binds the Pdop chains that are loosely bound by noncovalent interactions, and the multilayer coating ((PEI/PSS)2/PEI) prepared via the layer-by-layer self-assembly technique further enhances the chemical stability of the loosely bound Pdop. Thus, PEI coating, a simple and convenient approach, could be an alternative method to enhance the stability of the PdopP/Au because the release of Pdop from the PdopP/Au was moderately inhibited by the presence of only 1 layer of PEI coating on the PdopP/Au. The release of Pdop was also observed in the presence of NaBH4 when the PdopP/Au catalyst was used for a catalytic test. This drawback considerably limits the catalytic applications of Pdop-based materials. Upon the dissolution of NaBH4 in water, NaBH4 reacts with water to produce hydrogen gas and generates basic conditions.34 The pH of 100 mM NaBH4 solution was 10.4 in our study. To investigate the possibility of protecting the catalysts via the abovementioned surface treatments, the chemical stability of various surfacetreated PdopP/Au catalysts were tested in the presence of NaBH4. In the absence of surface treatments, the amount of Pdop released from the PdopP/Au increased as the reaction time increased (Figure 7a, black line). By contrast, the amount of Pdop released from surfacetreated PdopP/Au considerably decreased (red, blue, and green lines). The inhibition rates of Pdop release by the PEI coating, S-PEI grafting, and L-PEI grafting treatments were 79%, 83%, and 85%, respectively (Figure 7b). The Pdop/Au/L-PEI grafting also exhibited the 14 ACS Paragon Plus Environment

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lowest amount of Pdop released, i.e., the highest inhibition rate of Pdop release, indicating that L-PEI grafting maximizes the resistance of PdopP/Au to NaBH4, as shown in the Figures 4a and b. We expect that because of the ability of these treatments to protect Pdop-based materials against variable pH conditions and NaBH4, the PEI grafting and PEI coating methods proposed here will be applied to Pdop-based adsorbents or catalysts that need to be used under harsh conditions. To explore the potential applications of the surface-treated PdopP/Au as catalysts, three types of catalysts, namely, the PdopP/Au/PEI coating, PdopP/Au/S-PEI grafting, and PdopP/Au/L-PEI grafting, were employed in the purification of aqueous pollutants. A representative organic dye with mutagenic properties, methyl orange (MO), was chosen for the catalytic test.35 Its chemical structure is shown in Figure 1. For comparison, the catalytic activity of the PdopP/Au as no surface-treated catalysts was also evaluated (Figure S4). However, the PdopP/Au could not perform the catalytic reaction because a darker browncolored solution was significantly released from the PdopP/Au upon catalytic tests. The insets of Figure 8a, 8b, and 8c show the time-dependent UV-vis spectra of MO degradation catalyzed by the three types of catalysts. The absorption peak of MO at 460 nm quickly decreased and reached a steady state after 0.5-2 min with the simultaneous appearance of a new peak at approximately 250 nm, reflecting the degradation of MO. A new peak can be explained as resulting from cleavage of the azo bond (-N=N-) of MO possessing an azobenzene group (460 nm) and formation of new degradation products. It is believed that possible degradation products include sulfanilic acid and N, N-Dimethyl-p-phenylenediamine, which have UV absorption at wavelength of 240-250 nm.36,37 The initial catalytic performances of S- and L-PEI grafting species were relatively well maintained up to the 10th cycle in recycling tests (Figures 8b, 8c, and S5-S7). We expected the PdopP/Au/PEI coating to show the best catalytic performance because of the absence of bulky shielding layers such as PEI brushes to prevent the approach of dyes to Au catalysts on PdopP. However, among 15 ACS Paragon Plus Environment


the surface-treated PdopP/Au catalysts, the PdopP/Au/S-PEI and L-PEI grafting composites exhibited the fastest reaction time, reaching a steady state in 0.9 min, and the best catalytic efficiency (93%), respectively (Figures 8a-c, 9a, and 9d). These results suggest that the presence/absence and the chain length of the PEI brushes affect the catalytic performance of the PdopP/Au. To further confirm this important feature, other dyes, such as Congo red (CR) and direct red (DR), were also employed as pollutants. The same reactions were performed using each surface-treated PdopP/Au catalyst for CR. An analogous tendency to the one shown in the MO case was observed (Figures 8d-f, 9b, 9e, and S8-S10). This trend was also observed for DR (Figures 8g-i, 9c, 9f, and S11-S13). Overall, the PdopP/Au/S-PEI and L-PEI grafting composites showed the fastest reaction time (S-PEI grafting>L-PEI grafting>PEI coating) (Figures 9a-c) and the best catalytic efficiency (L-PEI grafting>S-PEI grafting>PEI coating) (Figures 9d-f), respectively, for all three types of dyes. Although the PdopP/Au/Sand L-PEI grafting cases featured S- and L-PEI brushes acting as bulky shielding layers, they showed faster reaction times than the PdopP/Au/PEI coating (Figures 9a-c), suggesting that the movement of the PEI brushes assists approach of dyes to the Au catalysts on the PdopP. Furthermore, the S-PEI brushes resulted in a faster reaction time than the L-PEI brushes (green and blue bars). We speculate that the difference in performance can be ascribed to the different chain mobilities of the PEI brushes, which are related to the MW. The low MW PEI (MW=25,000) possesses a higher chain mobility than the high MW PEI (MW=750,000) because of its greater free volume, which facilitates the more rapid and easier approach of dyes to the catalysts. In other words, the brushes at the surface of catalysts could still be used for the effective transfer of reactants to the reaction sites. A plausible mechanism for the reduction of organic dyes, such as MO, CR, and DR, can be described as follows: the Au catalysts on the PdopP can act as a medium of electron transfer in the entire catalytic process.38,39 BH4−1 quickly approaches and donates electrons to the Au catalyst surface on the support like PdopP. Then, the Au catalysts on the PdopP pass electrons to each dye, leading to 16 ACS Paragon Plus Environment

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the reduction of each dye into single phenyl ring compounds with amine group, which are much less toxic than the starting chemicals.38,39 Reductive decomposition using Au catalysts, an effective pre-treatment process, can reduce the azo bond, cleaving dye molecules into products that are more amenable to mineralization in biological treatment processes.40 Furthermore, the PdopP/Au/S- and L-PEI grafting cases showed better catalytic efficiency than the PdopP/Au/PEI coating case (Figures 9d-f), which can be ascribed to the PEI brush layers acting as a cage by keeping the dyes around the Au catalysts. Compared to the PEI coating case without brush layers, the dyes have a higher probability to contact or react with Au catalysts because the dyes are captured within the PEI brush layers. This phenomenon is more substantial in the L-PEI grafting case with the longer PEI chains than in the S-PEI grafting case (Figures 9d-f, green and blue bars). To confirm a cage effect of the PEI brushes, each dye was mixed with the different surface-treated PdopP/Au catalysts for 5 min, separated, and analyzed to measure the final amounts. Subsequently, the difference between the initial and final amounts of dye was used to calculate the amount of dye captured by each surface-treated PdopP/Au catalyst. Among the surface-treated PdopP/Au catalysts, the L- and S-PEI grafting cases exhibited the 1st and 2nd highest capture rates, respectively, for all types of dyes (Figures 10 and S14), suggesting that the brush-grafted catalyst are better suited to keeping dyes confined in a cage, similar to a brush forest. We speculate that another role of the brushes is maintaining the dyes within the brush layers, thereby increasing the probability of the dyes reacting with the Au catalysts. The sizes of the dyes used here were approximately 1, 2, and 4 nm for MO, CR, and DR, respectively. Thus, the effect of dye size on catalytic properties was also evaluated. As the size of the dyes increased, the non-brushgrafted catalysts (PEI coating case) showed poor catalytic performance, e.g., longer reaction times and lower catalytic efficiency (Figure 9, red bars), which is a general feature of most catalysts. However, the brush-grafted catalysts (the S- and L-PEI grafting cases) exhibited enhanced catalytic performance, including shorter reaction time and higher catalytic 17 ACS Paragon Plus Environment


efficiency, compared to the non-brush-grafted catalysts (the PEI coating case) (Figures 9c and 9f). These results suggest that the brush-grafted catalysts can more effectively decompose large dyes than the non-brush-grafted catalysts. To investigate catalytic activity of the brush-grafted catalysts under harsh conditions, catalytic tests of the PdopP/Au/L-PEI as the representative catalyst were performed at the low pH (2.8) and high pH (12) conditions. For low pH, stable catalytic activity of the PdopP/Au/L-PEI was confirmed when the PdopP/Au/L-PEI was tested at the low pH (2.0) (Figure S15). However, quantitative analysis of catalytic activity of the PdopP/Au/L-PEI was difficult because adsorption was also observed with catalytic activity. Thus, catalytic activity of the PdopP/Au/L-PEI was evaluated at the pH=2.8 which can decrease the adsorption effect. Figure S16-S21 show the time-dependent UV/Vis absorption spectra of degradation of three types of dyes. It was observed that the dye degradation occurred immediately upon the addition of the PdopP/Au/L-PEI into each dye solution. At t = 0 min, the curves represent the absorption spectra of each dye solution without the catalysts. After the addition of the PdopP/Au/L-PEI at pH = 2.8 and pH=12, the decreases in the intensities of the absorption bands at a certain wavelength (460-580 nm) were observed. Simultaneously, the formation of aromatic products was indicated by the appearance of a new peak (240-260 nm) and the intensities of these absorption peaks increased over reaction time. This suggests that each dye can be successfully decomposed by the brush-grafted catalyst (PdopP/Au/L-PEI) even under harsh conditions such as low and high pH conditions. The initial catalytic performances of the PdopP/Au/L-PEI under harsh conditions were relatively well maintained up to the 8th cycle in recycling tests (Figure S16-21). Catalytic activities of the PdopP/Au/L-PEI obtained at the harsh conditions (pH=2.8 and pH=12) were compared with those of the PdopP/Au/L-PEI obtained at the pH=10 (Figure 11). An analogous tendency to the one shown in the pH=10 case was observed at the low pH (2.8) (Figures 9 and 11). This trend was also observed at the high pH (12). Overall, as the size of the dyes increased (MO

Polyelectrolyte brush-grafted polydopamine-based catalysts with

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