Mussel-Inspired Green Metallization of Silver Nanoparticles on

Article pubs.acs.org/IECR

Mussel-Inspired Green Metallization of Silver Nanoparticles on Cellulose Nanocrystals and Their Enhanced Catalytic Reduction of 4‑Nitrophenol in the Presence of β‑Cyclodextrin Juntao Tang,† Zengqian Shi,† Richard M. Berry,‡ and Kam C. Tam*,† †

Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada ‡ CelluForce, Inc., 625 President-Kennedy Ave., Montreal, Quebec H3A 1K2, Canada S Supporting Information *

ABSTRACT: A green approach to anchor silver nanoparticles (AgNPs) onto the surface of cellulose nanocrystals (CNCs) coated with mussel-inspired polydopamine (PDA) at room temperature in the absence of a stabilizer and a reducing agent is proposed. The resulting nanohybrids possessed a core−shell structure with numerous “satellites” of silver nanoparticles decorating the CNC surface. The nanocatalyst displayed superior dispersibility over pristine AgNPs and was six times more efficient in catalyzing the reduction of 4-nitrophenol. By associating the CNC hybrid with β-cyclodextrin to promote host−guest interactions, the catalytic process was accelerated. The associated physicochemical parameters associated with the catalytic process were investigated and compared.

1. INTRODUCTION Cellulose nanocrystal (CNC), produced via the acid hydrolysis of cotton, wood, or other cellulose sources, is a nanomaterial that offers many new opportunities (for example, specific mechanical properties (strength and stiffness) that are comparable to steel).1−3 Among the sustainable and renewable cellulose-based materials, CNCs possess unique advantages, because of their size and surface functionalities.4−6 The sulfate ester groups render CNCs highly water-dispersible, and the abundant primary hydroxyl groups on the CNC surface provide opportunities for diverse chemical modifications.7,8 Taking full advantage of the surface functionalities and high surface area (∼250 m2/g),9 the deposition of nanoparticles onto the surface of CNC offers a promising approach to fabricate hybrid nanomaterials for use as heterogeneous catalysts in various types of chemical reactions.10,11 CNCs as carriers will improve the colloidal stability of metal nanoparticles and, hence, address issues, such as “catalyst poisoning” and deactivation of nanocatalysts. Several studies have been performed to deposit metal, semiconducting, or magnetic nanoparticles onto the surface of CNCs.11−16 For example, Khaled and co-workers introduced Au nanoparticles onto the CNC surface to act as a support for effective enzyme immobilization.13 Pd and Au nanoparticles were attached to the CNC surface using a hydrothermal method, and the catalytic enhancement of dye degradation was investigated by Wu and co-workers.11,15 Most recently, Marzieh and co-workers used a single step to attach Pd nanoparticles onto the surface of CNCs using supercritical CO2.17 The final product was reported to be an effective catalyst in the Mizoroki−Heck cross-coupling reaction. However, a detailed examination of the preparation techniques reveals some drawbacks, such as the use of reducing agents and/or the use of harsh conditions (hydrothermal treatment). In the present © XXXX American Chemical Society

study, we propose a greener approach to prepare a CNCmetallic nanoparticle hybrid system that possesses properties that are similar or better than those presently available. Inspired by the composition of adhesive proteins in mussels, catechol-based compounds such as 3,4-dihydroxy-L-phenylalanine (dopamine), and their polymeric forms18 have been widely exploited for antifouling,19 antibacterials,20 self-healing hydrogels,21 dye removal,22 and even lithium-ion-battery separators.23,24 Polydopamine (PDA) is the polymeric form of dopamine that contains both catechol and amine functional groups that undergo spontaneous self-polymerization in a defined pH range. The polymerization process is simple, green, and adaptable to virtually all types of material surfaces, offering a biocompatible and highly adhesive platform for further modifications. This process offers two advantages. First, the catechol groups can form strong coordination complexes with metal surfaces through two adjacent hydroxyl groups. Second, the redox-active functional groups can be oxidized under mild solution conditions. Combining these two advantages, this strategy has been applied to many different systems.22,25−30 Although recent reports do not describe or utilize PDA, catechol-based moieties are widely used.31−34 To the best of our knowledge, the approach of using catechol groups to incorporate a nanocatalyst onto a biorenewable nanocarrier (CNC) has not been reported. The strategy described above offers a new approach for developing hybrid nanomaterials with higher stability and adhesion for heterogeneous catalytic reactions. In this study, the PDA-coated CNC (CNC@PDA) was prepared via self-polymerization of Received: January 13, 2015 Revised: March 10, 2015 Accepted: March 16, 2015

A

DOI: 10.1021/acs.iecr.5b00177 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Schematic illustration of the strategy for synthesis of CNC@PDA, and the possible mechanism of metallization of Ag NPs, and binding with β-cyclodextrin.

performed on a TGA Q600 of TA Instruments (New Castle, DE, USA). The sample was heated under dry nitrogen purge at a flow rate of 10 mL/min from room temperature to 800 °C. 2.3. Preparation of CNC@PDA-Ag Nanocatalyst. The hybrid catalyst was prepared via a two-step process adopted from previous studies with slight modifications.20,30 In the first step, 1.0 g of CNC powder was homogeneously dispersed in 500 mL of Milli-Q water and sonicated for 5 min, and Tris (0.6 g) was dissolved in the CNC dispersion by adjusting the pH to 8.0. To this solution, 1.0 g of dopamine hydrochloride was introduced and the coating polymerization was performed over 1 day at room temperature. The products were purified by ultrafiltration using a 0.1-μm membrane filter and washed several times with DI water until the filtrate became colorless. The concentrated CNC@PDA was freeze-dried and stored for subsequent use. A typical protocol for the second step is as follows: 50.0 mg of silver nitrate was introduced to 20 mL of deionized water, and ammonia solution (3.0 wt %) was added slowly until the solution became clear, confirming the formation of diammine silver(I). Then, 0.5 g of CNC@PDA solution (3.0 wt %) was added to the resultant solution and stirred at room temperature for 2 h. The product was purified by centrifugation at 8000 rpm for 10 min, and washed three times with deionized water. The control sample (pure AgNP) was prepared by replacing the CNC@PDA with 4.0 mg of dopamine hydrochloride (in 1.0 mL deionized water), which served as a reducing agent for the Ag ion. The synthesis was performed at 25 and 45 °C in order to evaluate the impact of temperature on the production of nanoparticles. 2.4. The Catalytic Properties of the Nanocomposite Catalyst. The reduction of 4-nitrophenol by NaBH4 was selected as a model reaction for evaluating the catalytic efficiency of the hybrid nanocatalyst. Typically, 12 mM 4-NP (Solution 1) solution was prepared by dissolving 16.7 mg of 4NP powder in 10 mL of water. By diluting Solution 1 and mixing it with freshly prepared NaBH4 solution, another solution (Solution 2) containing 0.12 mM 4-NP and 38 mM NaBH4 was prepared for the experiments described here. Thereafter, 3 mL of Solution 2 was pipetted into a UV cuvette and placed in a thermostated cell at 25 °C. Two hundred microliters (200 μL) of the catalyst dispersion (silver content of 2.0 μg/mL) was added to Solution 2, using an Eppendorf pipet, and mixed for 5 s. The reaction was monitored using UV-vis

dopamine on the surface of CNC nanoparticles. The Ag satellite nanoparticles are generated in situ by reducing Ag+ with catecholamine oxidation. This is followed by immobilizing Ag nanoparticles on the exterior surface of PDA-coated CNCs (see Figure 1). The composite nanoparticles were employed in the catalytic reduction of 4-nitrophenol (4-NP), which is a common organic pollutant in wastewater, in the presence of NaBH4. This reaction converts toxic 4-NP into benign 4aminophenol (4-AP) and addresses an important environmental challenge.12,26,27,35−39 As a further step, β-cyclodextrin (β-CD) was introduced into this biomimetic system to serve as a capping agent that binds to the surface of metal nanoparticles to provide a platform for host−guest interactions.40,41 By combining some ideas reported previously,40,42−45 we developed a novel CNC hybrid system that displays enhanced catalytic properties; its corresponding kinetic and thermodynamic properties were measured to provide the knowledge necessary to design better systems for future applications.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Cellulose nanocrystals with surface charge density of 0.26 mmol/g were provided by Celluforce, Inc. The measured surface area of CNC is ∼250 m2/g. Dopamine hydrochloride tris((hydroxymethyl)aminomethane (Tris), ammonium hydroxide solution, sodium borohydrate, 4-nitrophenol, and silver nitrate were purchased from Sigma−Aldrich. All the chemicals were used as received without further purification. All aqueous solutions were prepared with Milli-Q water. 2.2. Apparatus. Infrared spectra were collected over a range of 400−4000 cm−1 on a Bruker Tensor 27 spectrometer with a resolution of 4 cm−1 and a scanning number of 32. Ultraviolet−visible (UV-vis) spectra were obtained on an Agilent Model 8453 UV-vis spectroscopy system equipped with temperature control. The crystalline phases were determined using an X-ray diffraction (XRD) analyzer (Bruker, Model D8 Discover) equipped with Cu Kα radiation, at a scan speed of 4° min−1 and with a scattering range of 10°−90°. Transmission electron microscopic (TEM) images were obtained using a Philips CM10 TEM microscope at an acceleration voltage of 60 keV. Samples for TEM characterization were prepared by placing a drop of prepared solution on copper grids (200 mesh coated with copper) and allowed to dry with no additional staining. Thermogravimetric analysis (TGA) of samples was B


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Figure 2. TEM images of (A) pristine CNCs, (B) core−shell CNC@PDA, at a weight ratio of 1:1), (C) CNC@PDA-Ag NPs, and (D) pure AgNPs reduced by dopamine. Comparisons of the dispersion stability between CNC@PDA-Ag NPs and pure AgNPs are also shown ((E) side view and (F) view from bottom).

molecules.47 Metallization was achieved by introducing CNC@PDA into a diammine silver(I) solution. The [Ag(NH3)2]+ ions adsorbed onto the surface through interactions between the catechol and nitrogen-containing functional groups. The adsorbed Ag+ ions were reduced to Ag nanoparticles that became immobilized on the CNC surface. Figure 2 shows TEM micrographs comparing the morphologies of CNCs before and after the polydopamine coating. The rodlike CNCs were ∼200−400 nm in length and 10−20 nm in diameter, which is in agreement with previous data.48 When TEM was used to analyze the CNC samples, the images lacked contrast; however, with the addition of the PDA coating, the CNC contrast improved. The diameter of the nanoparticles increased by several nanometers, and the surface roughness increased significantly (see Figure 2B). TEM was also used to observe the morphologies of the CNC@PDA-AgNPs directly. The small Ag nanoparticles (∼10 nm) were densely and uniformly distributed on the CNC@PDA surface. CNC@PDAAgNPs prepared at a higher temperature (45 °C) did not reveal any significant difference in the characteristics of the nanostructures (see Figure S2 in the Supporting Information). Considering this fact, the synthesis of nanoparticles at room temperature would be more beneficial than at higher temperatures (for example, 45 °C). Since it is an aqueousbased reaction, the temperature should be kept low, because, at higher temperature, oxidation of the catechol groups on CNC may occur. The ammonia solution plays two functions in the synthesis process: (1) it acts as a ligand for the Ag ion, forming a more stable diammine silver(I) ([Ag(NH3)2]+) complex; and (2) it controls the reducing kinetics, allowing us to tune the size of produced nanoparticles. We have tried to synthesis the nanoparticles under neutral pH conditions, but we were not able to produce the desired CNC@PDA-AgNPs. As shown in Figure S3 in the Supporting Information, the Ag nanoparticles did not deposit onto the surface of CNC@PDA and the

spectrometry, scanning a range of 250−600 nm, with cycling over an interval of 1 min. For catalyst dosage experiments, catalyst stock solutions with different silver contents were diluted from concentrated CNC@PDA-Ag NPs (20 μg/mL) dispersions. The volume of catalyst introduced into the cuvette was kept constant at 200 μL. 2.5. Catalytic Reduction with Complexes of Nanocatalyst with β-CD. For catalytic experiments with β-CD, stock dispersions with CNC@PDA-Ag NPs (final silver content fixed at 1.5 μg/mL) and different CD concentrations (0, 1 × 10−6, 2 × 10−6, 4 × 10−6 M, respectively) were prepared from a concentrated dispersion solution. Typically, CNC@PDA-Ag NPs (silver content of 2.5 μg/mL, the weight percentage of silver on nanohybrids was determined to be 80 wt % by TGA) dispersion and a β-CD solution with a concentration of 0.01 mM were used as the concentrated stock solution. Then, 0, 0.2, 0.4, 0.8 mL of the β-CD stock solution were adding into 1.2 mL of concentrated CNC@PDA solution, respectively. The final volume of each mixture was fixed at 2.0 mL by adding the appropriate volume of Milli-Q water. The volume of catalyst introduced into the cuvette was kept at 200 μL.

3. RESULTS AND DISCUSSION 3.1. Characterization of CNC@PDA-Ag NPs. The CNC@PDA-AgNPs were prepared via poly(dopamine)assisted reduction of Ag ions at the surface of PDA-CNC, as shown in Figure 1. The PDA coating procedure used the optimized parameters from a previous study,46 where a dopamine:CNC weight ratio of 1:1 was used. During the polymerization process, the color of the dispersion rapidly turned to pink due to the oxidation of catechol groups, which then slowly turned black, indicating the successful polymerization of the monomer. However, the molecular structure of poly(dopamine) is still under debate; it could have either been covalent or noncovalent interactions between dopamine C


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Figure 3. (A) UV-vis spectra, (B) FTIR spectra, (C) XRD patterns, and (D) TGA results of pristine CNC (blue trace), core−shell CNC@PDA nanocomposites (black trace), and CNC@PDA-AgNP nanocomposites (red trace).

diameter of the silver nanoparticles was ∼100 nm in the absence of ammonia solution. UV spectroscopy was used to confirm the presence of PDA and the formation of Ag NPs, and the results are summarized in Figure 3A. A small peak at 281 nm, not seen with pristine CNCs, was observed for PDA-coated CNCs. This absorption band corresponds to the La−Lb coincident transition of dopamine structures.49 Comparing the UV curves of CNCPDA and CNC@PDA AgNPs dispersions, the appearance of a new peak at ∼415 nm in CNC@PDA AgNPs was assigned to the surface plasmon resonance phenomena of electrons in the conduction bands of silver, confirming the formation of nanoscale silver colloids (see Figure 2C). Figure 3B shows the FTIR spectra of CNCs, CNC@PDA, and CNC@PDA-AgNPs. Some of the specific peaks of CNCs are the absorption at 3417 cm−1 for hydroxyl stretching, 2903 cm−1 for symmetric C−H vibrations, and 1644 cm−1 from moisture in CNCs. After coating with PDA, significant changes in the spectra were observed within the region between 1200 and 1600 cm−1. The absorptions at 1512 cm−1 (N−H scissoring), 1350 cm−1 (phenolic O−H bending), and 1284 cm−1 (C−O stretching) are typical peaks of the polymer, indicating the successful coating of PDA on the surface of CNCs.19,26 By further anchoring Ag nanoparticles on CNCs, the intensity of these peaks decreased dramatically together with a blue shift of the peak at ∼3450 cm−1. Thus, we determined that Ag nanoparticles were bonded to the surface of CNC@PDA. The surface functional nanoparticles were characterized using XRD to confirm that only the surface of CNC was modified and the crystal structure remained unchanged (Figure 3C). The spectra of cellulose nanocrystals displayed specific peaks at 2θ = 14.9°, 16.2°, 22.6°, and 34.4°, corresponding to the (110̅ ),

(110), (200), and (004) crystallographic planes, respectively.11,50 The reflections of crystalline structure from CNC@PDA exhibited almost no differences except for a slight reduction in the intensity, confirming that coated PDA did not destroy the crystal structures of cellulose. The hybrid nanomaterial exhibited peaks at 38.1°, 44.3°, 64.4°, 77.4°, and 81.5°, which are respectively assigned to the (111), (200), (220), (311), and (222) phases of the face-centered cubic crystal structure of Ag nanoparticles, which is consistent with the ASTM standard (JCPDS File Card No. 04-0783).51,52 The results confirmed that AgNPs attached to the surface were in the zero valency state. In order to determine the content of Ag nanoparticles in the nanohybrids, TGA tests were conducted; the results are presented in Figure 3D. At 800 °C, the residue was 20.0%, 35.1%, and 51.3% for pristine CNC, CNC@PDA, and pure PDA, respectively. Thus, the content of PDA in CNC@PDA was calculated to be 48.2%, based on the following equations: CCNC + C PDA = 1 0.2CCNC + 0.513C PDA = 0.351

where CCNC is the content of CNC in CNC@PDA, and CPDA is the content of PDA in CNC@PDA. 3.2. Catalytic Properties of CNC@PDA-AgNPs and AgNPs. AgNPs have been extensively studied for the catalytic reduction of nitrophenols and nitroanilines.53−56 However, AgNPs are generally unstable, because of their nanoscopic size and large surface area, which are prone to aggregation.57 Introducing AgNPs onto the surface of CNCan ideal “green” carrierwill not only endow desirable catalytic characteristics to the nanohybrids, but it will also address problems associated with the dispersion stability of AgNPs. Figures 2E and 2F compare the dispersion stability of AgNPs and CNC@PDAD


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Figure 4. Time-dependent UV-vis absorption spectra of 4-nitrophenol reduced by NaBH4 catalyzed by (A) pure AgNPs and (B) CNC@PDAAgNPs. [Conditions: 200 μL of catalyst solution with a silver content of 2.0 μg/mL was introduced to a 3-mL mixture of 4-NP (0.12 mM) and NaBH4 (38 mM), T = 25 °C.] Also shown are plots of (C) absorption at 400 nm versus time t or (D) ln(Ct/C0) versus time t for pure AgNPs (red data points) and CNC@PDA-AgNPs (black data points).

AgNPs; the AgNPs were significantly agglomerated after 1 week, while the CNC@PDA-AgNP dispersion remained stable, with no evidence of sedimentation. The model organic compound, 4-nitrophenol, was introduced to compare the catalytic properties of pristine AgNPs and CNC@PDA supported AgNPs. NaBH4 was used as a reducing agent, and UV-vis was used to monitor the reaction kinetics at 1 min intervals. When 4-NP was exposed to the aqueous solution of NaBH4, a strong absorption peak at 400 nm, corresponding to a yellow-green color, was introduced through the formation of the 4-nitrophenolate ion.51,52,58 Figures 4A and 4B show the comparison between AgNPs and CNC@PDA-AgNPs for catalyzing the reduction of 4-NP. The intensity of the absorption peak of 400 nm decreased, together with the appearance of a new peak at 297 nm, which was caused by the gradual conversion of nitrophenol to 4aminophenol (4-AP) in the presence of AgNPs and CNC@ PDA-AgNPs. There was no reduction in UV absorption after 1 h when the bare substrate (CNC or CNC@PDA) was used, confirming that Ag nanoparticles are needed to catalyze the reduction reaction of 4-NP. Since the reduction occurred under conditions where the concentration of NaBH4 greatly exceeded that of 4-NP, the reaction can be considered independent of the borohydride content. Thus, the kinetic data can be fitted to a Langmuir− Hinshelwood apparent first-order plot.38 The apparent rate constant (kapp) can be calculated using eq 1:

−

dCt = kappCt = k1SCt dt

⎛C ⎞ ⎛A ⎞ ln⎜ t ⎟ = ln⎜ t ⎟ = −kappt ⎝ C0 ⎠ ⎝ A0 ⎠

(1)

where Ct is the concentration of 4-NP at time t, kapp is the apparent rate constant, and k1 is the rate constant normalized to S, the surface area of AgNPs normalized to the unit volume of the system. The rate constant (k) was determined from the linear plot of ln(At/A0) versus time (minutes); its value was determined to be 0.0456 min−1 for AgNPs and 0.2554 min−1 for CNC@PDA-AgNPs (see Figure 4D). The reaction rate for CNC@PDA-AgNPs was 6 times faster than pure AgNPs that contained similar amounts of silver. Two factors for the high catalytic activity of CNC@PDA-AgNPs are proposed: (1) the CNC@PDA hybrid nanomaterials possess a high surface area, which is an important factor in catalytic reactions;38,59,60 and (2) the readily water-dispersible carriers (CNCs) increase the dispersibility and dispersion stability of the AgNPs, to maximize the catalytic activity. PDA-coated CNCs are versatile supporting materials for anchoring and distributing metallic nanoparticles and can be used to address issues, such as catalyst “poisoning” and deactivation in heterogeneous catalytic reactions. To compare the current system with previously reported catalysts, the reaction rate and turnover frequency (TOF, which is defined as the number of moles of 4-NP reduced per mole of catalyst per hour) were evaluated and summarized in Table 1. Although the activity of Ag nanoparticles alone exhibited poorer catalytic E


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Industrial & Engineering Chemistry Research Table 1. Summary of the Properties of Different Catalysts To Reduce 4-NP catalyst supporter

temperature (K)

catalyst type

turnover frequency, TOF (h−1)

CNC CNC CNC CNC CNC@PDA

298 298 298 298 298

Pd Au CuO Cu Ag

879.5 109 885.7 1108.8 1077.3a

ln kapp = −

(3)

where A is the Arrhenius factor, k is the rate constant at temperature T (in Kelvin), and R is the universal gas constant. The experiments were conducted under the reaction conditions: total volume of the system was 3.2 mL, including 3.0 mL of 4-NP stock solution (0.12 mM) mixed with NaBH4 (38 mM) and 0.2 mL catalyst with an Ag content of 2.0 μg/ mL). From the slope of the plot of ln kapp against 1/T, the activation energy Ea for the CNC@PDA-Ag nanocatalyst was determined to be 33.88 kJ/mol, which is in agreement with values reported previously.36,63 For different systems and materials, the experimentally determined Ea can vary significantly.64 For example, Zhu reported an Ea value of 40.9 kJ/mol for Ag nanoparticles immobilized on magnetic Fe3O4@C core− shell nanocomposites.65 An activation energy of Ea = 66.8 kJ/ mol was observed for Ag nanoparticles on a procyanidin-grafted eggshell membrane.34 Usually, for surface-catalyzed reactions, the Ea value lies between 8.4 kJ/mol and 41.9 kJ/mol.66 Thus, the results support the belief that the conversion of 4-NP to 4AP occurred via surface catalysis. 3.4. Cooperation between CNC@PDA-Ag NPs and βCyclodextrin. Cyclodextrins (CDs) are known to readily adsorb onto nanoparticle surfaces such as Au or Ag, through their primary faces, exposing the more-polar, secondary faces. This favors the binding between anchored CD hosts and guests.43,44 The equilibrium constant and standard enthalpy have been measured calorimetrically for the formation of a complex of β-cyclodextrin with 4-nitrophenol in aqueous solution, which is reported to be 350 L/mol and −12 kJ/ mol.67 Thus, in our case, the benzene-structured 4-NP can be an ideal guest for the cavity of cyclodextrin. Plots of ln(kapp) vs 1/T for different concentrations of β-cyclodextrin are shown in Figure 7. As shown in Figure 7a, for the same temperature, the reaction rate increased linearly over the CD concentration range being investigated (see Figures S4−S7 in the Supported Information). The reason for this trend is described below. It has been suggested that the basic mechanism of 4-NP reduction is described by the following reactions:

ref 15 11 12 12 this work

a

Experiments were carried out by mixing 3 mL [0.12 mM] of 4-NP (Solution 1) with 200 μL catalyst (Solution 2) containing 2 μg/m of Ag. The total volume was 3.2 mL. For the calculation, a value of 107.87 g/mol was used as the molecular weight of silver.

performance than other metal nanoparticles,35 the TOF of Ag on PDA-coated CNCs from this study was comparable to that of other types of metal nanoparticles deposited on the surface of pristine CNC. It is anticipated that coating of musselinspired PDA onto CNCs will offer a simple and robust approach to fabricate novel catalyst hybrids for future applications. The effects of CNC@PDA-AgNP catalyst dosing was investigated by varying the concentration of catalyst, while keeping other parameters, such as the concentration of 4-NP and NaBH4, constant. As shown in Figure 5B, the reaction rate generally increased linearly with catalyst concentration, which is consistent with a previous report.61 This observation is consistent with the view that an increase in the total surface area and number of reaction sites enhances the catalytic activity.62 3.3. Temperature Dependence and Activation Energy Calculation. In reaction kinetics, the activation energy (Ea) is an empirical parameter describing the dependency of the rate constant with temperature. A larger Ea value indicates higher sensitivity of the kinetic constant k to reaction temperature. Figure 6B shows the corresponding linear relationship between ln(At/A0) and time t at different temperatures (T). From the linear regression, different rate constants were determined from the slopes of the linear fits shown in Figure 6A. The Ea values were calculated using the Arrhenius equation:

⎛ E ⎞ kapp = A exp⎜ − a ⎟ ⎝ RT ⎠

Ea + ln A RT

2Ag + BH−4 ↔ Ag−BH−3 + Ag−H 6H−Ag + 4‐NP → 4‐AP + 6Ag + 6H+

6Ag−BH−3 + 4‐NP → 4‐AP + 6Ag + 6BH3

(2)

Figure 5. (A) Plot of ln(Ct/C0) versus time t for different silver contents in CNC@PDA-AgNP stock solutions. (B) Plot of rate constant (Ct) versus silver content for 4-NP reduction by NaBH4. [Conditions: 200 μL of catalyst solution was introduced into a 3-mL mixture of 4-NP (0.12 mM) and NaBH4 (38 mM), T = 25 °C.] F


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Figure 6. (A) Plot of ln(Ct/C0) versus time t for different temperatures (T), using CNC@PDA-AgNPs as a catalyst. (B) Arrhenius plot from the slopes of the lines in panel (A). [Conditions: 200 μL of catalyst solution with a silver content of 2.0 μg/mL was introduced to a 3-mL mixture of 4NP (0.12 mM) and NaBH4 (38 mM).]

Figure 7. (A) Plot of rate constant versus concentration of β-CD in catalyst stock solution for 4-NP reduction by NaBH4. (B) Arrhenius plots from the slopes of the lines in panel (A) (the concentrations of β-CD in the catalyst stock solution were 0, 1 × 10−6, 2 × 10−6, 4 × 10−6). [Conditions: 200 μL of catalyst solution with a silver content of 1.5 μg/mL was introduced to a 3-mL mixture of 4-NP (0.12 mM) and NaBH4 (38 mM).]

For the reaction to proceed, 6 mol of electrons are required to transform 4-NP to 4-AP at the metal surface.63 The electron transfer rate is determined by three steps: diffusion of 4-NP to the metal surface, interfacial electron transfer, and the diffusion of 4-AP from the surface. Thus, the observed rate constant can be described by eq 4: 1 kobs

=

⎛ 1 ⎞⎡⎛ 1 ⎞ R⎤ ⎜ ⎟⎢⎜ ⎟+ ⎥ 2 ⎝ 4πR ⎠⎢⎣⎝ ke ⎠ D ⎥⎦

(4)

where ke is the rate constant of electron transfer, D is the diffusion coefficient, and R is the radius of the silver nanoparticles. Introducing CD molecules to the system may increase the diffusion coefficient of 4-NP, and subsequently increase the reaction rate. The possible mechanism is illustrated in Figure 8. The E a values of the systems with different CD concentrations were calculated by fitting eq 3 to the experimental data (Figure 7B); the numerical values are summarized in Table 2. There was a slight decrease in the activation energy when the concentration of β-cyclodextrin was increased. These differences may be attributed to the change of hydration of 4-NP molecules anchored to CD molecules, as well as the change in the mechanism at the surface of CNC-Ag nanohybrids. The Ea data did not display a notable decrease from the results for α-CD reported by Liu and co-workers.45 A similar trend was observed but the magnitude of the activation

Figure 8. Postulated mechanism for the catalytic reduction of 4-NP (a) with or (b) without the incorporation of β-CD.

Table 2. Summary of the Parameters Obtained from Arrhenius Plots with Different Cyclodextrin Concentrations ([CD])

G

CD concentration, [CD] (× 106 M)

slope for Arrhenius plots

Ea (kJ/mol)

R2

0 1 2 4

−4075.2 −3909.6 −3646.3 −3439.1

33.88 32.51 30.32 28.59

0.9907 0.9927 0.9825 0.9874


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energy differed, because of the difference in the binding constant between 4-NP/β-CD and 4-NP/α-CD.67

4. CONCLUSIONS A stable nanocatalyst was successfully synthesized through a mussel-inspired green method by surface coating polydopamine on cellulose nanocrystals (CNCs), followed by in situ reduction of Ag ions. The catalytic properties of the nanohybrids were investigated through a reduction reaction of a model compound (4-nitrophenol) in the presence of NaBH4 as a reducing agent. The reaction rate using CNC@PDA-AgNPs as a catalyst was 6 times faster than that of pristine AgNPs. Using the Arrhenius relationship, the activation energy (Ea) was determined to be 33.88 kJ/mol for the CNC@PDA-Ag nanocatalyst, which is consistent with other systems reported previously. With further incorporation of β-cyclodextrin (CD) into the system, the catalytic process was accelerated due to the host−guest interactions between 4-NP and CD. Therefore, CNC@PDA, which is a facile and green carrier, provides a versatile platform for fabricating stable nanocatalysts for industrial applications.

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ASSOCIATED CONTENT

* Supporting Information S

Size distribution of Ag NPs attached on CNC@PDA and plot of ln(Ct/C0) versus time t for nanohybrid systems incorporated with different concentrations of β-cyclodextrin at various temperature were presented. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to acknowledge Celluforce, Inc., for providing the cellulose nanocrystals. The research funding from CelluForce and AboraNano facilitated the research on CNCs. K.C.T. wishes to acknowledge funding from CFI and NSERC.

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REFERENCES

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