DOI: 10.1002/chem.201406285
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A Cascade-Reaction Nanoreactor Composed of a Bifunctional Molecularly Imprinted Polymer that Contains Pt Nanoparticles Jiao Wang, Maiyong Zhu, Xiaojuan Shen, and Songjun Li*[a]
To that end, the unique imprinted polymer was fabricated by using two well-coupled templates, that is, 4-nitrophenyl acetate and 4-nitrophenol. The catalytic hydrolysis of the former compound at the acidic catalytic sites led to the formation of the latter compound, which was further reduced by the encapsulated Pt nanoparticles to 4-aminophenol. Therefore, this nanoreactor demonstrated a catalytic-cascade ability. This protocol opens up the opportunity to develop functional catalysts for complicated chemical processes.
Abstract: This study was aimed at addressing the present challenge of cascade reactions, namely, how to furnish the catalysts with desired and hierarchical catalytic ability. This issue was addressed by constructing a cascade-reaction nanoreactor made of a bifunctional molecularly imprinted polymer containing acidic catalytic sites and Pt nanoparticles. The acidic catalytic sites within the imprinted polymer allowed one specified reaction, whereas the encapsulated Pt nanoparticles were responsible for another coupled reaction.
Introduction
way to acquire molecular-recognition and selective catalytic abilities.[6, 7] Specifically, this technology can “imprint” a substrate within a synthetic polymer, recording the stereochemical image of the substrate in terms of the shape and the position of the functional groups. For the fabrication of MIPs, the template and functional monomers are first allowed to form a selfassembled architecture in which the functional monomers are regularly positioned around the template. Polymerization is then performed to fix the self-organized architecture, followed by removal of the imprinted template from the polymeric matrix, thereby leaving binding sites that are stereochemically complementary to the template. The match between the template and the binding sites constitutes an induced molecular memory, which endowed the prepared MIPs with molecularrecognition and specific catalytic abilities. Therefore, MIP catalysts allow desired reactions and avoid unexpected reactions.[8] Enlightened by these intricate works, we aimed to address the present challenge of cascade reactions by constructing a unique cascade-reaction nanoreactor made of a bifunctional molecularly imprinted polymer containing acidic catalytic sites and Pt nanoparticles. The acidic catalytic sites within the imprinted polymer allow one specified reaction, whereas the Pt nanoparticles take responsibility for a coupled reaction (Scheme 1). Therefore, this nanoreactor demonstrates a catalytic-cascade ability. To that end, 4-nitrophenyl acetate (NPA) and 4-nitrophenol (NP) were selected as the tentative substrate and templates, as both compounds are a well-coupled pair for catalytic-cascade testing.[9, 10] The catalytic hydrolysis of NPA results in the formation of NP, which can be further reduced by the encapsulated metal nanoparticles to 4-aminophenol. Furthermore, both the catalytic hydrolysis of NPA and the reduction of NP can be performed under mild conditions (i.e., room temperature),[10] thus greatly facilitating the experimental studies. Therefore, the novel imprinted polymer nanoreactor was
Despite the tantalizing prospect in chemical synthesis, catalysis in cascade reactions remains still a significant challenge. At the forefront of this field is the use of polymer nanoreactors to achieve hierarchical catalytic ability. From the earliest endeavors, exemplified by acidic and basic-site-containing architectures,[1, 2] polymer nanoreactors have been shown to demonstrate hierarchical catalytic ability. This effect arises from bifunctional catalytic properties in which the acidic sites are responsible for one reaction and the basic sites are responsible for another coupled reaction. Therefore, catalysis by the polymer nanoreactors demonstrates a catalytic-cascade ability. Nonetheless, the practical applications of these polymer nanoreactors are limited, mainly because most of the cascade reactions involve many active intermediates together with the initial reactants,[3–5] which probably react with each other and lead to unexpected reactions. Thus, the cascade reactions in practical applications often require the polymer nanoreactors to have the characteristics that allow specified reactions and avoid unexpected reactions. Unfortunately, it is difficult to directly acquire such polymer nanoreactors based on the currently available results. For centuries, mankind has learnt and gained knowledge from nature. Scientists have learnt to create and resolve complicated problems based on inspiration sought from nature. One of these solutions has been the development of molecularly imprinted polymers (MIPs), which provide a promising [a] J. Wang, Dr. M. Zhu, Dr. X. Shen, Prof. S. Li School of Materials Science and Engineering Jiangsu University Zhenjiang 212013 (P.R. China) E-mail: [email protected] Homepage: http://material.ujs.edu.cn/en/people-view.asp?id = 15 Chem. Eur. J. 2015, 21, 1 – 9
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Scheme 1. Proposed mechanism for the cascade reactions catalyzed by the nanoreactor.
prepared (named as “MIP-NPA-PtNP”; Scheme 2). Considering the whole catalytic process in which Pt is necessary for the cascade reactions, two Pt-containing control moieties and one Ptfree reference moiety were also prepared under comparable conditions (i.e., “MIP-PtNP”, “NIP-Pt”, and “NIP”, respectively). MIP-PtNP was the nanoreactor prepared without NPA and NIPPt was the nanoreactor prepared without NPA and NP. NIP was the nonimprinted (blank) polymer and was prepared in the absence of Pt, NPA, and NP. For the catalytic test, we also prepared a nonacidic MIP-NPA-PtNP, in which the acidic monomer was replaced with acrylamide. Given that a few studies have shown the effectiveness of metal-nanoparticle-containing MIP catalysts in individual reactions,[11, 12] the focal point of this study was on the overall performance of this cascade-reaction nanoreactor to highlight the synergy between the imprinted
acidic sites and the Pt nanoparticles. For convenient discussion, all of the prepared polymer nanoreactors and the blank polymer are mentioned henceforth as the conceptual nanoreactors. The objective of this study is to demonstrate that the desired cascade reactions can be realized by using this novel protocol, which opens up the opportunity to develop functional catalysts for complicated chemical processes.
Results and Discussion FTIR, SEM, TEM, and BET analysis The polymer nanoreactor MIP-NPA-PtNP was prepared by using NPA and NP as templates. The encapsulated Pt ions were reduced with an excess of sodium borohydride. The im-
Scheme 2. Technical outline for the preparation of the MIP-NPA-PtNP system. AIBN = azobisisobutyronitrile.
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Full Paper printed NPA and NP moieties were removed from the polymeric matrix, thus resulting in the formation of imprinted species in the prepared nanoreactor (Scheme 2). FTIR spectroscopic analysis was first used to monitor the imprinting process (Figure 1). Four major bands (n˜ = 2900–3700, 1600–1800, 1450–
Figure 2. FTIR spectra of the template molecules NPA and NP.
Figure 1. FTIR spectra of the prepared polymer nanoreactors.
1600, and 1000–1400 cm1) appeared in the spectrum of this polymer nanoreactor. These bands were complex due to the complicated composition of the nanoreactor, which may be associated with the stretching of the OH/NH, C=O, S=O, and CN/CC bonds.[13] For a comparative study, we included the three control moieties (i.e., MIP-PtNP, NIP-Pt, and NIP) and the MIP-NPA-PtNP precursor (in which the imprinted NPA and NP had not been removed from the polymeric matrix) in Figure 1, together with the templates NPA and NP in Figure 2. The MIPNPA-PtNP precursor included the major bands of NPA and NP in the FTIR spectrum at approximately n˜ = 3350 and 1800 cm1, respectively (Figure 2). After removing NPA and NP from the precursor, the spectrum of the resulted nanoreactor (i.e., MIPNPA-PtNP) became comparable to the spectra of NIP, NIP-Pt, and MIP-PtNP. In conjunction with the preparation process (Scheme 2), this outcome indicates that the imprinting behavior had occurred in the imprinted MIP-NPA-PtNP system (further discussion on the specific interaction is presented in below). Figures 3 and 4 display the SEM and TEM images of the morphology of the polymer carriers and the metal nanoparticles encapsulated in the polymer nanoreactors, respectively. Pt nanoparticles of 20–30 nm were encapsulated in the polymeric building blocks. Relative to the blank NIP, the other nanoreactors have a larger surface area and higher pore volume (Table 1). These larger values may be related to the presence of both the templates and Pt species during the preparation process, which normally affect the resulting polymers. Relative to NPA and NP, the Pt species in the prepared nanoreactors led to a greater increase in the pore volume than in the surface Chem. Eur. J. 2015, 21, 1 – 9
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Figure 3. SEM images of the surface morphology of the prepared polymers: a) MIP-NPA-PtNP, b) MIP-PtNP, c) NIP-Pt, d) NIP.
Table 1. BET surface area and pore volume of the prepared nanoreactors. Nanoreactor[a]
Surface area [m2 g1]
Pore volume [mL g1; 103]
MIP-NPA-PtNP MIP-PtNP NIP-Pt
138.3 104.9 30.4
59.7 56.5 49.6
[a] Relative to NIP.
area, which can be due to the formation of the Pt nanoparticles which have a larger steric size than the molecular templates. Thus, these nanoreactors were prepared in the desired form, thereby making a further and comparative study feasible. 3
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Full Paper consequence of molecular imprinting. Because molecular recognition by the imprinted polymers is essentially a result of the induced molecular memory, the stronger interactions offered by the imprinted nanoreactors are to be expected.
Catalysis and cascade processes The catalytic hydrolysis of NPA promoted by these nanoreactors is presented in Figure 6. For a contrastive study, we have also included the catalytic hydrolysis of NPA promoted by the
Figure 4. TEM images of the metal nanoparticles contained in the prepared polymer nanoreactors: a) MIP-NPA-PtNP, b) MIP-PtNP, c) NIP-Pt, d) NIP.
Specific interaction between nanoreactors and substrate Temperature-programmed desorption (TPD) experiments were further conducted to verify the interaction between the prepared nanoreactors and substrate.[14] The template molecule NP, which desorbed from MIP-NPA-PtNP, MIP-PtNP, NIP-Pt, and NIP, appeared at about 270, 272, 200, and 203 8C, respectively (Figure 5). The NP-imprinted MIP-NPA-PtNP and MIP-PtNP systems demonstrated stronger interactions with NP, in contrast to the nonimprinted NIP-Pt and NIP system. The result was similar for NPA. The NPA-imprinted MIP-NPA-PtNP system exhibited the strongest interaction with NPA relative to the other systems prepared without NPA. In conjunction with the characterization discussed above, the TPD profiles reflect again the
Figure 6. Catalytic hydrolysis of NPA by the prepared nanoreactors.
nonacidic MIP-NPA-PtNP system. All the acidic-site-containing nanoreactors showed higher activities than the nonacidic MIPNPA-PtNP system. The presence of acidic sites in these nanoreactors acted as a catalyst for the hydrolysis of NPA. It is noted that the NPA-imprinted MIP-NPA-PtNP system demonstrated much higher catalytic activities relative to other nanoreactors prepared without NPA. The NPA-imprinted species in MIP-NPA-PtNP clearly played a role in boosting the hydrolysis of NPA. After the catalytic activity of MIP-NPA-PtNP was deduced from MIP-PtNP, the contribution of the NPA-imprinted species to the hydrolysis was consequently exposed (Figure 7). For evaluation of the catalytic specificity, 2-methyl-5-nitrophenyl acetate (MNPA) was further selected as the control. MIPNPA-PtNP demonstrated a catalytic preference for NPA in contrast to MNPA. This effect remained consistent with the time (Figure 7). This result, as often described, indicates that catalysis by the imprinted nanoreactor was a substrate-selective process. As such, one can expect that the cascade reactions of NPA in MIP-NPA-PtNP would become feasible in the presence of borohydride compounds. The imprinted acidic catalytic sites would allow the specified hydrolysis and the Pt nanoparticles would take responsibility for the following reduction. In this way, this nanoreactor demonstrates the catalytic-cascade ability. To meet the expectation, UV spectroscopic analysis was used to monitor the catalytic hydrolysis of NPA in the presence
Figure 5. TPD profiles of the desorption of NPA and NP from the prepared nanoreactors.
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Figure 7. Contribution of the imprinted species in MIP-NPA-PtNP to the selective catalysis.
of sodium borohydride (Figure 8). The catalytic hydrolysis of NPA in the NIP system led to a decreasing peak for NPA and an increasing peak for NP (l = 271 and 400 nm, respectively; Figure 8 a), thus exhibiting the conversion from NPA into NP. In contrast, the catalytic hydrolysis of NPA in the MIP-NPA-PtNP system was more complicated, in which the hydrolysis reaction led to a decreasing peak for NPA and nevertheless an increasing peak for 4-aminophenol (AP; l = 295 nm; Figure 8 b). Therefore, the NP intermediate formed in the MIP-NPA-PtNP system can be further reduced to AP, and the desired cascade reactions did occur in the MIP-NPA-PtNP system as expected. The result was similar in the MIP-PtNP system, in which the AP peak was smaller than that achieved in the MIP-NPA-PtNP system because of the lower amount of NP released from the hydrolysis of NPA. It is worth noting that NIP-Pt further led to the formation of 4-aminophenyl acetate (AA; Figure 8 c), which relates to the straightforward reaction of NPA hydrogenating to AA. Unlike MIP-NPA-PtNP, NIP-Pt did not appear to allow the consecutive hydrolysis and reduction reactions. In conjunction with the selective properties of the imprinted nanoreactors, this outcome suggests that the catalytic-cascade reactions became feasible only under the prerequisite that the molecular-recognition properties have been incorporated into the catalytic sites. The molecular-recognition properties allowed the specified reaction, whereas the catalytic sites took responsibility for the catalytic processes. Further tests on the catalytic reproducibility indicate that the reactivity of MIP-NPA-PtNP would not be subject to a significant decrease after a series of reaction cycles; therefore, this nanoreactor is relatively stable. This phenomenon, as generally known, can be ascribed to the highly crosslinked structures in the prepared nanoreactor, which offer the advantage of stabilizing the imprinted conformations. As such, this suggested protocol opens up the potential opportunity to develop functional catalysts for complicated chemical processes.
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Figure 8. The change of the UV spectrum of NPA induced by sodium borohydride in the presence of the nanoreactors: a) NIP, b) MIP-NPA-PtNP/MIPPtNP, c) MIP-Pt.
Dynamic-binding behavior The electrochemical test was further employed to investigate the interaction between the prepared nanoreactors and substrate.[15] It is known that the potential to reduce/oxidize a binding molecule depends upon the binding constant. Stronger binding needs more energy to overcome the binding. Thus, the electrochemical studies may provide valuable information on the binding mechanism between these nanoreactors and 5
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Full Paper Consecutive scanning with cyclic voltammetry up to a stable desorption/redox profile gives Equation (6): Scheme 3. Schematic presentation of the electrochemical process with the binding molecule B.
ln K ¼ aE þ b
The interaction of the substrate with different nanoreactors further leads to the expression:
substrate. The theory and detail, as outlined in Scheme 3, have been discussed elsewhere.[15, 16] Substrate B in this electrochemical system would normally involve desorption, diffusion to the surface of the electrode, and a terminal reaction. Therefore, the overall reaction rate in the system is determined by the slowest step, that is, the rate-determining step. In the event that the diffusion is eliminated with sonication, the overall reaction rate will be directly associated with the terminal reaction. By using thermodynamic theories, the chemical potential of the substrate in the bulk solution can be expressed as: mb ¼ mf þ RT ln
C1 Cf
ð6Þ
D ln K ¼ aDE
ð7Þ
The binding constant shows direct dependence upon the redox potential. A larger binding constant would cause a higher redox potential. As such, the electrochemical study with substrate desorbing was performed in accordance with this paradigm. NPA attached onto NIP exhibited a desorption/ reduction peak at 107 mV (Figure 9 a). In contrast, the reduction peak in MIP-NPA-PtNP shifted to 149 mV (Figure 9 b). The NPA-imprinted MIP-NPA-PtNP system demonstrated a stronger interaction with NPA than the nonimprinted NIP nanoreactor. The outcome was similar for NP, for which MIPNPA-PtNP also demonstrated a stronger interaction with NP, in
ð1Þ
where mf and Cf are the standard chemical potential and the standard concentration of the substrate, respectively; C1 is the practical concentration; R is the gas constant (8.314 J mol1 K1); and T is the reaction temperature. The combination of Equation (1) with the absorption/desorption equilibrium of the substrate gives Equation (2): mb ¼ mn ¼ mn þ RT ln
C1 C ¼ RT ln K þ RT ln 1n Cn C
ð2Þ
Herein, the superscript and subscript ‘n’ represents the nanoreactor involved in the desorption process and indicates the interaction between the nanoreactor and the substrate, and K is the equilibrium constant of the substrate that absorbs onto the nanoreactor and indicates the affinity of the nanoreactor for the substrate. Similarly, the use of thermodynamic theories on the surface of the electrode gives Equation (3): me ¼ me þ RT ln
C2 C ¼ NEF þ RT ln 2e Ce C
ð3Þ
where E is the redox potential of the substrate, N is the molar number of the electrons transferred during the redox process, and F is the Faraday constant (96 485 C mol1). The combination of Equation (2) with Equation (3) gives Equation (4):
ln K ¼
NF m mn CnC Eþ e ln e 2 RT C C1 RT
ð4Þ
Once the concentration gradient of the substrate is eliminated from the bulk solution by using sonication, Equation (4) can be converted into Equation (5):
ln K ¼ &
&
NF Dm Cn Eþ ln e RT RT C
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Figure 9. Reduction profiles of the desorption of NPA from a) NIP and b) MIP-NPA-PtNP.
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Full Paper tion/reduction potentials for NP. This outcome strongly suggests that the stronger interactions with NPA and NP offered by MIP-NPA-PtNP are the result of the imprinted polymer carrier, which allowed access for the specific substrate to be achieved. In conjunction with the catalytic study discussed above, this result further suggests that the catalytic-cascade reactions occurred in MIP-NPA-PtNP were due to the synergy between the recognition properties and the catalytic sites. The molecular-recognition properties allowed for the desired hydrolysis, whereas the catalytic sites took responsibility for the catalytic processes.
Conclusion This study was aimed at addressing the present challenge of cascade reactions by constructing a cascade-reaction nanoreactor made of a bifunctional molecularly imprinted polymer that contained acidic catalytic sites and Pt nanoparticles. Differing from conventional imprinted catalysts and polymer nanoreactors, this novel polymer nanoreactor incorporated molecular-recognition properties into the catalytic sites. These molecular-recognition properties allowed for a specified reaction, whereas the catalytic sites took responsibility for the catalytic processes. As such, this nanoreactor has shown the catalyticcascade ability. Therefore, it has been confirmed that desired cascade reactions can be realized by using this unique polymer nanoreactor made of a multifunctional molecularly imprinted polymer that contains metal nanoparticles, which opens up the opportunity to develop functional catalysts for complicated chemical processes. Future development in this field will significantly increase the potential for applications and lead to the appearance of novel catalytic materials and functional catalysts.
Figure 10. Reduction profiles of the desorption of NP from a) NIP and b) MIP-NPA-PtNP.
Table 2. Reduction potentials of the desorption of NPA and NP from the prepared nanoreactors. Nanoreactor MIP-NPA-PtNP MIP-PtNP NIP-Pt NIP
Experimental Section
Substrate [mV] NPA
NP
149 111 108 107
120 122 94 92
Preparation of nanoreactors The chemicals used were purchased from Sigma–Aldrich and used as received. The polymer nanoreactor MIP-NPA-PtNP was prepared by using classic molecular imprinting technology[17] (Scheme 2). The templates used for the imprinting process were NPA (36.2 mg, 0.2 mmol) and a complex of NP and Pt4 + ions, which was formed by the addition of chloroplatinic acid hexahydrate (51.8 mg, 0.1 mmol) to a solution of NP in DMSO (0.04 mmol mL1, 5 mL).[18] The 2-acrylamido-2-methylpropanesulfonic acid monomer (0.21 g, 1.0 mmol), N,N’-methylenebisacrylamide crosslinker (0.54 g, 3.5 mmol), and the AIBN initiator (0.2 g) were added to the template in solution. After being dispersed and deoxygenated with sonication and nitrogen, the mixture system was irradiated with UV light (l = 365 nm) overnight. The encapsulated Pt ions were reduced with an excess of sodium borohydride. The resulting nanoreactor precursor was crushed and washed with ethanol containing 10 % acetic acid to remove the imprinted NPA and NP. The prepared nanoreactor was dried in flowing nitrogen.
contrast to NIP (Figure 10). The imprinted species in MIP-NPAPtNP clearly played a role in steering the stronger interactions. To address the interaction further, Table 2 shows the reduction potentials at which both NPA and NP desorbed from all of the prepared nanoreactors. The NPA-imprinted MIP-NPA-PtNP system exhibited the highest desorption/reduction potential for NPA relative to the other systems prepared without NPA. Despite the encapsulated Pt nanoparticles, NIP-Pt exhibited almost the same potentials as NIP. The NP-imprinted MIP-NPAPtNP and MIP-PtNP systems also revealed comparable desorpChem. Eur. J. 2015, 21, 1 – 9
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Characterization TEM images of the prepared nanoreactors were obtained on a transmission electron microscope (JEM-2100; Japan). The surface morphology was observed on a scanning electron microscope (Hitachi S-450; Japan). The BET surface area and porosity were investigated by means of nitrogen adsorption/desorption analysis with a nitrogen surface-area analyzer (SA3100; USA). The IR spectra were recorded on a FTIR apparatus (Nicolet MX-1E; USA).
The authors want to express their gratitude to the National Science Foundation of China (Nos. 51473070, 21403091, 51402128). Thanks also should be expressed to Jiangsu Province and Jiangsu University for support under the distinguished professorship program (Sujiaoshi [2012]34, No. 12JDG001) and the innovation/entrepreneurship program (Suzutong [2012]19). The authors also appreciate project support from the Science and Technology Agency of Jiangsu Province (BK20130486). Keywords: cascade reactions · catalysis · nanoreactors · polymers
TPD By using a device made of a gas chromatography furnished with a thermal conductivity detector and a data-processing system, the prepared nanoreactors (20 mg) were placed into an online Ushaped quartz tube (internal diameter (I.D.) = 4 mm). After the substrate (0.06 mmol mL1, 10 mL) in acetonitrile had adsorbed on the nanoreactors, the quartz tube was heated in flowing nitrogen (55 mL min1, 0.4 MPa) at a rate of 10 8C min1 from room temperature up to the temperature where the absorbed substrate desorbed. The desorbing signal was recorded with the data-processing system.
[1] Y. Yang, X. Liu, X. Li, J. Zhao, S. Bai, J. Liu, Q. Yang, Angew. Chem. Int. Ed. 2012, 51, 9164 – 9168; Angew. Chem. 2012, 124, 9298 – 9302. [2] T. Yang, J. Liu, Y. Zheng, M. J. Monteiro, S. Z. Qiao, Chem. Eur. J. 2013, 19, 6942 – 6945. [3] M. Filice, J. M. Palomo, ACS Catal. 2014, 4, 1588 – 1598. [4] A.-M. L. Hogan, D. F. O’Shea, J. Am. Chem. Soc. 2006, 128, 10360 – 10361. [5] R. S. Paton, S. E. Steinhardt, C. D. Vanderwal, K. N. Houk, J. Am. Chem. Soc. 2011, 133, 3895 – 3905. [6] Y. Hoshino, T. Kodama, Y. Okahata, K. J. Shea, J. Am. Chem. Soc. 2008, 130, 15242 – 15243. [7] B. C. G. Karlsson, J. O’Mahony, J. G. Karlsson, H. Bengtsson, L. A. Eriksson, I. A. Nicholls, J. Am. Chem. Soc. 2009, 131, 13297 – 13304. [8] S. Li, Y. Ge, A. Tiwari, S. Wang, A. P. F. Turner, S. A. Piletsky, J. Catal. 2011, 278, 173 – 180. [9] T. G. O’Lenick, X. Jiang, B. Zhao, Polymer 2009, 50, 4363 – 4371. [10] K. Ohkubo, Y. Urata, K.-I. Seri, H. Ishida, T. Sagawa, T. Nakashima, Y. Imagawa, J. Mol. Catal. 1994, 90, 355 – 365. [11] H. Zhang, T. Piacham, M. Drew, M. Patek, K. Mosbach, L. Ye, J. Am. Chem. Soc. 2006, 128, 4178 – 4179. [12] X. Zhang, M. Zhu, S. Li, J. Inorg. Organomet. Polym. Mater. 2014, 24, 890 – 897. [13] S. Li, Y. Ge, S. A. Piletsky, A. P. F. Turner, Adv. Funct. Mater. 2011, 21, 3344 – 3349. [14] D. Zhang, S. Li, W. Li, Y. Chen, Catal. Lett. 2007, 115, 169 – 175. [15] B. Peng, X. Yuan, M. Zhu, S. Li, Polym. Chem. 2014, 5, 562 – 566. [16] S. Li, Y. Luo, M. Whitcombe, S. A. Piletsky, J. Mater. Chem. A 2013, 1, 15102 – 15109. [17] T. Takeuchi, T. Mori, A. Kuwahara, T. Ohta, A. Oshita, H. Sunayama, Y. Kitayama, T. Ooya, Angew. Chem. Int. Ed. 2014, 53, 12765 – 12770; Angew. Chem. 2014, 126, 12979 – 12984. [18] S. Li, S. Gong, Adv. Funct. Mater. 2009, 19, 2601 – 2606. [19] Y. Kawanami, T. Yunoki, A. Nakamura, K. Fujii, K. Umano, H. Yamauchi, K. Masuda, J. Mol. Catal. A 1999, 145, 107 – 110.
Catalytic testing: Catalytic hydrolysis by using these nanoreactors was evaluated in a batch format.[19] The initial concentration of the substrate NPA was 0.2 mmol mL1 (10 mL). The solid content of the nanoreactors was 1.0 mg mL1. The formation of NP was spectrophotometrically detected. The catalytic activity of the nanoreactors was obtained from the average value of three runs. Considering the effect of self-hydrolysis on the catalytic process, the hydrolysis of NPA in the absence of the nanoreactors was also performed under comparable conditions; accordingly, this value was deducted from the overall activity of these nanoreactors. Experiments for the catalytic reduction reactions were further conducted as for the hydrolytic experiments, but with the addition of an excess of sodium borohydride (4fold relative to NPA). Electrochemical testing: An electrochemical workstation equipped with a conventional three-electrode configuration of a Au-plate working electrode, Pt-wire counterelectrode, and a Ag/AgCl reference electrode (CHI 760E, China) was used. The nanoreactors (10 mg) were preabsorbed with the template (ca. 2 mmol) and then placed into a cuvette encircled by a diffusion-eliminating sonication apparatus (supporting electrolyte: 0.01 mmol mL1 KCl, 10 mL). The transient desorption process of the substrate was measured by consecutive scanning with cyclic voltammetry until a stable desorption/reduction profile was reached (scanning range: ca. + 0.6– 0.4 V, scanning rate: 1 mV s1).
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FULL PAPER & Nanotechnology J. Wang, M. Zhu, X. Shen, S. Li* && – && A Cascade-Reaction Nanoreactor Composed of a Bifunctional Molecularly Imprinted Polymer that Contains Pt Nanoparticles
Making a good impression: The construction of a cascade-reaction nanoreactor composed of a bifunctional molecularly imprinted polymer (MIP) containing acidic catalytic sites and Pt nanoparticles (PtNPs) is presented. The
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acidic catalytic sites within the imprinted polymer allow one reaction and the encapsulated Pt nanoparticles allow a coupled reaction (see picture; NPA = 4-nitrophenyl acetate), thus, demonstrating a catalytic-cascade ability.
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& Nanotechnology
A Cascade-Reaction Nanoreactor Composed of a Bifunctional Molecularly Imprinted Polymer that Contains Pt Nanoparticles Jiao Wang, Maiyong Zhu, Xiaojuan Shen, and Songjun Li*[a]
To that end, the unique imprinted polymer was fabricated by using two well-coupled templates, that is, 4-nitrophenyl acetate and 4-nitrophenol. The catalytic hydrolysis of the former compound at the acidic catalytic sites led to the formation of the latter compound, which was further reduced by the encapsulated Pt nanoparticles to 4-aminophenol. Therefore, this nanoreactor demonstrated a catalytic-cascade ability. This protocol opens up the opportunity to develop functional catalysts for complicated chemical processes.
Abstract: This study was aimed at addressing the present challenge of cascade reactions, namely, how to furnish the catalysts with desired and hierarchical catalytic ability. This issue was addressed by constructing a cascade-reaction nanoreactor made of a bifunctional molecularly imprinted polymer containing acidic catalytic sites and Pt nanoparticles. The acidic catalytic sites within the imprinted polymer allowed one specified reaction, whereas the encapsulated Pt nanoparticles were responsible for another coupled reaction.
Introduction
way to acquire molecular-recognition and selective catalytic abilities.[6, 7] Specifically, this technology can “imprint” a substrate within a synthetic polymer, recording the stereochemical image of the substrate in terms of the shape and the position of the functional groups. For the fabrication of MIPs, the template and functional monomers are first allowed to form a selfassembled architecture in which the functional monomers are regularly positioned around the template. Polymerization is then performed to fix the self-organized architecture, followed by removal of the imprinted template from the polymeric matrix, thereby leaving binding sites that are stereochemically complementary to the template. The match between the template and the binding sites constitutes an induced molecular memory, which endowed the prepared MIPs with molecularrecognition and specific catalytic abilities. Therefore, MIP catalysts allow desired reactions and avoid unexpected reactions.[8] Enlightened by these intricate works, we aimed to address the present challenge of cascade reactions by constructing a unique cascade-reaction nanoreactor made of a bifunctional molecularly imprinted polymer containing acidic catalytic sites and Pt nanoparticles. The acidic catalytic sites within the imprinted polymer allow one specified reaction, whereas the Pt nanoparticles take responsibility for a coupled reaction (Scheme 1). Therefore, this nanoreactor demonstrates a catalytic-cascade ability. To that end, 4-nitrophenyl acetate (NPA) and 4-nitrophenol (NP) were selected as the tentative substrate and templates, as both compounds are a well-coupled pair for catalytic-cascade testing.[9, 10] The catalytic hydrolysis of NPA results in the formation of NP, which can be further reduced by the encapsulated metal nanoparticles to 4-aminophenol. Furthermore, both the catalytic hydrolysis of NPA and the reduction of NP can be performed under mild conditions (i.e., room temperature),[10] thus greatly facilitating the experimental studies. Therefore, the novel imprinted polymer nanoreactor was
Despite the tantalizing prospect in chemical synthesis, catalysis in cascade reactions remains still a significant challenge. At the forefront of this field is the use of polymer nanoreactors to achieve hierarchical catalytic ability. From the earliest endeavors, exemplified by acidic and basic-site-containing architectures,[1, 2] polymer nanoreactors have been shown to demonstrate hierarchical catalytic ability. This effect arises from bifunctional catalytic properties in which the acidic sites are responsible for one reaction and the basic sites are responsible for another coupled reaction. Therefore, catalysis by the polymer nanoreactors demonstrates a catalytic-cascade ability. Nonetheless, the practical applications of these polymer nanoreactors are limited, mainly because most of the cascade reactions involve many active intermediates together with the initial reactants,[3–5] which probably react with each other and lead to unexpected reactions. Thus, the cascade reactions in practical applications often require the polymer nanoreactors to have the characteristics that allow specified reactions and avoid unexpected reactions. Unfortunately, it is difficult to directly acquire such polymer nanoreactors based on the currently available results. For centuries, mankind has learnt and gained knowledge from nature. Scientists have learnt to create and resolve complicated problems based on inspiration sought from nature. One of these solutions has been the development of molecularly imprinted polymers (MIPs), which provide a promising [a] J. Wang, Dr. M. Zhu, Dr. X. Shen, Prof. S. Li School of Materials Science and Engineering Jiangsu University Zhenjiang 212013 (P.R. China) E-mail: [email protected] Homepage: http://material.ujs.edu.cn/en/people-view.asp?id = 15 Chem. Eur. J. 2015, 21, 1 – 9
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Scheme 1. Proposed mechanism for the cascade reactions catalyzed by the nanoreactor.
prepared (named as “MIP-NPA-PtNP”; Scheme 2). Considering the whole catalytic process in which Pt is necessary for the cascade reactions, two Pt-containing control moieties and one Ptfree reference moiety were also prepared under comparable conditions (i.e., “MIP-PtNP”, “NIP-Pt”, and “NIP”, respectively). MIP-PtNP was the nanoreactor prepared without NPA and NIPPt was the nanoreactor prepared without NPA and NP. NIP was the nonimprinted (blank) polymer and was prepared in the absence of Pt, NPA, and NP. For the catalytic test, we also prepared a nonacidic MIP-NPA-PtNP, in which the acidic monomer was replaced with acrylamide. Given that a few studies have shown the effectiveness of metal-nanoparticle-containing MIP catalysts in individual reactions,[11, 12] the focal point of this study was on the overall performance of this cascade-reaction nanoreactor to highlight the synergy between the imprinted
acidic sites and the Pt nanoparticles. For convenient discussion, all of the prepared polymer nanoreactors and the blank polymer are mentioned henceforth as the conceptual nanoreactors. The objective of this study is to demonstrate that the desired cascade reactions can be realized by using this novel protocol, which opens up the opportunity to develop functional catalysts for complicated chemical processes.
Results and Discussion FTIR, SEM, TEM, and BET analysis The polymer nanoreactor MIP-NPA-PtNP was prepared by using NPA and NP as templates. The encapsulated Pt ions were reduced with an excess of sodium borohydride. The im-
Scheme 2. Technical outline for the preparation of the MIP-NPA-PtNP system. AIBN = azobisisobutyronitrile.
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Full Paper printed NPA and NP moieties were removed from the polymeric matrix, thus resulting in the formation of imprinted species in the prepared nanoreactor (Scheme 2). FTIR spectroscopic analysis was first used to monitor the imprinting process (Figure 1). Four major bands (n˜ = 2900–3700, 1600–1800, 1450–
Figure 2. FTIR spectra of the template molecules NPA and NP.
Figure 1. FTIR spectra of the prepared polymer nanoreactors.
1600, and 1000–1400 cm1) appeared in the spectrum of this polymer nanoreactor. These bands were complex due to the complicated composition of the nanoreactor, which may be associated with the stretching of the OH/NH, C=O, S=O, and CN/CC bonds.[13] For a comparative study, we included the three control moieties (i.e., MIP-PtNP, NIP-Pt, and NIP) and the MIP-NPA-PtNP precursor (in which the imprinted NPA and NP had not been removed from the polymeric matrix) in Figure 1, together with the templates NPA and NP in Figure 2. The MIPNPA-PtNP precursor included the major bands of NPA and NP in the FTIR spectrum at approximately n˜ = 3350 and 1800 cm1, respectively (Figure 2). After removing NPA and NP from the precursor, the spectrum of the resulted nanoreactor (i.e., MIPNPA-PtNP) became comparable to the spectra of NIP, NIP-Pt, and MIP-PtNP. In conjunction with the preparation process (Scheme 2), this outcome indicates that the imprinting behavior had occurred in the imprinted MIP-NPA-PtNP system (further discussion on the specific interaction is presented in below). Figures 3 and 4 display the SEM and TEM images of the morphology of the polymer carriers and the metal nanoparticles encapsulated in the polymer nanoreactors, respectively. Pt nanoparticles of 20–30 nm were encapsulated in the polymeric building blocks. Relative to the blank NIP, the other nanoreactors have a larger surface area and higher pore volume (Table 1). These larger values may be related to the presence of both the templates and Pt species during the preparation process, which normally affect the resulting polymers. Relative to NPA and NP, the Pt species in the prepared nanoreactors led to a greater increase in the pore volume than in the surface Chem. Eur. J. 2015, 21, 1 – 9
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Figure 3. SEM images of the surface morphology of the prepared polymers: a) MIP-NPA-PtNP, b) MIP-PtNP, c) NIP-Pt, d) NIP.
Table 1. BET surface area and pore volume of the prepared nanoreactors. Nanoreactor[a]
Surface area [m2 g1]
Pore volume [mL g1; 103]
MIP-NPA-PtNP MIP-PtNP NIP-Pt
138.3 104.9 30.4
59.7 56.5 49.6
[a] Relative to NIP.
area, which can be due to the formation of the Pt nanoparticles which have a larger steric size than the molecular templates. Thus, these nanoreactors were prepared in the desired form, thereby making a further and comparative study feasible. 3
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Full Paper consequence of molecular imprinting. Because molecular recognition by the imprinted polymers is essentially a result of the induced molecular memory, the stronger interactions offered by the imprinted nanoreactors are to be expected.
Catalysis and cascade processes The catalytic hydrolysis of NPA promoted by these nanoreactors is presented in Figure 6. For a contrastive study, we have also included the catalytic hydrolysis of NPA promoted by the
Figure 4. TEM images of the metal nanoparticles contained in the prepared polymer nanoreactors: a) MIP-NPA-PtNP, b) MIP-PtNP, c) NIP-Pt, d) NIP.
Specific interaction between nanoreactors and substrate Temperature-programmed desorption (TPD) experiments were further conducted to verify the interaction between the prepared nanoreactors and substrate.[14] The template molecule NP, which desorbed from MIP-NPA-PtNP, MIP-PtNP, NIP-Pt, and NIP, appeared at about 270, 272, 200, and 203 8C, respectively (Figure 5). The NP-imprinted MIP-NPA-PtNP and MIP-PtNP systems demonstrated stronger interactions with NP, in contrast to the nonimprinted NIP-Pt and NIP system. The result was similar for NPA. The NPA-imprinted MIP-NPA-PtNP system exhibited the strongest interaction with NPA relative to the other systems prepared without NPA. In conjunction with the characterization discussed above, the TPD profiles reflect again the
Figure 6. Catalytic hydrolysis of NPA by the prepared nanoreactors.
nonacidic MIP-NPA-PtNP system. All the acidic-site-containing nanoreactors showed higher activities than the nonacidic MIPNPA-PtNP system. The presence of acidic sites in these nanoreactors acted as a catalyst for the hydrolysis of NPA. It is noted that the NPA-imprinted MIP-NPA-PtNP system demonstrated much higher catalytic activities relative to other nanoreactors prepared without NPA. The NPA-imprinted species in MIP-NPA-PtNP clearly played a role in boosting the hydrolysis of NPA. After the catalytic activity of MIP-NPA-PtNP was deduced from MIP-PtNP, the contribution of the NPA-imprinted species to the hydrolysis was consequently exposed (Figure 7). For evaluation of the catalytic specificity, 2-methyl-5-nitrophenyl acetate (MNPA) was further selected as the control. MIPNPA-PtNP demonstrated a catalytic preference for NPA in contrast to MNPA. This effect remained consistent with the time (Figure 7). This result, as often described, indicates that catalysis by the imprinted nanoreactor was a substrate-selective process. As such, one can expect that the cascade reactions of NPA in MIP-NPA-PtNP would become feasible in the presence of borohydride compounds. The imprinted acidic catalytic sites would allow the specified hydrolysis and the Pt nanoparticles would take responsibility for the following reduction. In this way, this nanoreactor demonstrates the catalytic-cascade ability. To meet the expectation, UV spectroscopic analysis was used to monitor the catalytic hydrolysis of NPA in the presence
Figure 5. TPD profiles of the desorption of NPA and NP from the prepared nanoreactors.
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Figure 7. Contribution of the imprinted species in MIP-NPA-PtNP to the selective catalysis.
of sodium borohydride (Figure 8). The catalytic hydrolysis of NPA in the NIP system led to a decreasing peak for NPA and an increasing peak for NP (l = 271 and 400 nm, respectively; Figure 8 a), thus exhibiting the conversion from NPA into NP. In contrast, the catalytic hydrolysis of NPA in the MIP-NPA-PtNP system was more complicated, in which the hydrolysis reaction led to a decreasing peak for NPA and nevertheless an increasing peak for 4-aminophenol (AP; l = 295 nm; Figure 8 b). Therefore, the NP intermediate formed in the MIP-NPA-PtNP system can be further reduced to AP, and the desired cascade reactions did occur in the MIP-NPA-PtNP system as expected. The result was similar in the MIP-PtNP system, in which the AP peak was smaller than that achieved in the MIP-NPA-PtNP system because of the lower amount of NP released from the hydrolysis of NPA. It is worth noting that NIP-Pt further led to the formation of 4-aminophenyl acetate (AA; Figure 8 c), which relates to the straightforward reaction of NPA hydrogenating to AA. Unlike MIP-NPA-PtNP, NIP-Pt did not appear to allow the consecutive hydrolysis and reduction reactions. In conjunction with the selective properties of the imprinted nanoreactors, this outcome suggests that the catalytic-cascade reactions became feasible only under the prerequisite that the molecular-recognition properties have been incorporated into the catalytic sites. The molecular-recognition properties allowed the specified reaction, whereas the catalytic sites took responsibility for the catalytic processes. Further tests on the catalytic reproducibility indicate that the reactivity of MIP-NPA-PtNP would not be subject to a significant decrease after a series of reaction cycles; therefore, this nanoreactor is relatively stable. This phenomenon, as generally known, can be ascribed to the highly crosslinked structures in the prepared nanoreactor, which offer the advantage of stabilizing the imprinted conformations. As such, this suggested protocol opens up the potential opportunity to develop functional catalysts for complicated chemical processes.
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Figure 8. The change of the UV spectrum of NPA induced by sodium borohydride in the presence of the nanoreactors: a) NIP, b) MIP-NPA-PtNP/MIPPtNP, c) MIP-Pt.
Dynamic-binding behavior The electrochemical test was further employed to investigate the interaction between the prepared nanoreactors and substrate.[15] It is known that the potential to reduce/oxidize a binding molecule depends upon the binding constant. Stronger binding needs more energy to overcome the binding. Thus, the electrochemical studies may provide valuable information on the binding mechanism between these nanoreactors and 5
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Full Paper Consecutive scanning with cyclic voltammetry up to a stable desorption/redox profile gives Equation (6): Scheme 3. Schematic presentation of the electrochemical process with the binding molecule B.
ln K ¼ aE þ b
The interaction of the substrate with different nanoreactors further leads to the expression:
substrate. The theory and detail, as outlined in Scheme 3, have been discussed elsewhere.[15, 16] Substrate B in this electrochemical system would normally involve desorption, diffusion to the surface of the electrode, and a terminal reaction. Therefore, the overall reaction rate in the system is determined by the slowest step, that is, the rate-determining step. In the event that the diffusion is eliminated with sonication, the overall reaction rate will be directly associated with the terminal reaction. By using thermodynamic theories, the chemical potential of the substrate in the bulk solution can be expressed as: mb ¼ mf þ RT ln
C1 Cf
ð6Þ
D ln K ¼ aDE
ð7Þ
The binding constant shows direct dependence upon the redox potential. A larger binding constant would cause a higher redox potential. As such, the electrochemical study with substrate desorbing was performed in accordance with this paradigm. NPA attached onto NIP exhibited a desorption/ reduction peak at 107 mV (Figure 9 a). In contrast, the reduction peak in MIP-NPA-PtNP shifted to 149 mV (Figure 9 b). The NPA-imprinted MIP-NPA-PtNP system demonstrated a stronger interaction with NPA than the nonimprinted NIP nanoreactor. The outcome was similar for NP, for which MIPNPA-PtNP also demonstrated a stronger interaction with NP, in
ð1Þ
where mf and Cf are the standard chemical potential and the standard concentration of the substrate, respectively; C1 is the practical concentration; R is the gas constant (8.314 J mol1 K1); and T is the reaction temperature. The combination of Equation (1) with the absorption/desorption equilibrium of the substrate gives Equation (2): mb ¼ mn ¼ mn þ RT ln
C1 C ¼ RT ln K þ RT ln 1n Cn C
ð2Þ
Herein, the superscript and subscript ‘n’ represents the nanoreactor involved in the desorption process and indicates the interaction between the nanoreactor and the substrate, and K is the equilibrium constant of the substrate that absorbs onto the nanoreactor and indicates the affinity of the nanoreactor for the substrate. Similarly, the use of thermodynamic theories on the surface of the electrode gives Equation (3): me ¼ me þ RT ln
C2 C ¼ NEF þ RT ln 2e Ce C
ð3Þ
where E is the redox potential of the substrate, N is the molar number of the electrons transferred during the redox process, and F is the Faraday constant (96 485 C mol1). The combination of Equation (2) with Equation (3) gives Equation (4):
ln K ¼
NF m mn CnC Eþ e ln e 2 RT C C1 RT
ð4Þ
Once the concentration gradient of the substrate is eliminated from the bulk solution by using sonication, Equation (4) can be converted into Equation (5):
ln K ¼ &
&
NF Dm Cn Eþ ln e RT RT C
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Figure 9. Reduction profiles of the desorption of NPA from a) NIP and b) MIP-NPA-PtNP.
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Full Paper tion/reduction potentials for NP. This outcome strongly suggests that the stronger interactions with NPA and NP offered by MIP-NPA-PtNP are the result of the imprinted polymer carrier, which allowed access for the specific substrate to be achieved. In conjunction with the catalytic study discussed above, this result further suggests that the catalytic-cascade reactions occurred in MIP-NPA-PtNP were due to the synergy between the recognition properties and the catalytic sites. The molecular-recognition properties allowed for the desired hydrolysis, whereas the catalytic sites took responsibility for the catalytic processes.
Conclusion This study was aimed at addressing the present challenge of cascade reactions by constructing a cascade-reaction nanoreactor made of a bifunctional molecularly imprinted polymer that contained acidic catalytic sites and Pt nanoparticles. Differing from conventional imprinted catalysts and polymer nanoreactors, this novel polymer nanoreactor incorporated molecular-recognition properties into the catalytic sites. These molecular-recognition properties allowed for a specified reaction, whereas the catalytic sites took responsibility for the catalytic processes. As such, this nanoreactor has shown the catalyticcascade ability. Therefore, it has been confirmed that desired cascade reactions can be realized by using this unique polymer nanoreactor made of a multifunctional molecularly imprinted polymer that contains metal nanoparticles, which opens up the opportunity to develop functional catalysts for complicated chemical processes. Future development in this field will significantly increase the potential for applications and lead to the appearance of novel catalytic materials and functional catalysts.
Figure 10. Reduction profiles of the desorption of NP from a) NIP and b) MIP-NPA-PtNP.
Table 2. Reduction potentials of the desorption of NPA and NP from the prepared nanoreactors. Nanoreactor MIP-NPA-PtNP MIP-PtNP NIP-Pt NIP
Experimental Section
Substrate [mV] NPA
NP
149 111 108 107
120 122 94 92
Preparation of nanoreactors The chemicals used were purchased from Sigma–Aldrich and used as received. The polymer nanoreactor MIP-NPA-PtNP was prepared by using classic molecular imprinting technology[17] (Scheme 2). The templates used for the imprinting process were NPA (36.2 mg, 0.2 mmol) and a complex of NP and Pt4 + ions, which was formed by the addition of chloroplatinic acid hexahydrate (51.8 mg, 0.1 mmol) to a solution of NP in DMSO (0.04 mmol mL1, 5 mL).[18] The 2-acrylamido-2-methylpropanesulfonic acid monomer (0.21 g, 1.0 mmol), N,N’-methylenebisacrylamide crosslinker (0.54 g, 3.5 mmol), and the AIBN initiator (0.2 g) were added to the template in solution. After being dispersed and deoxygenated with sonication and nitrogen, the mixture system was irradiated with UV light (l = 365 nm) overnight. The encapsulated Pt ions were reduced with an excess of sodium borohydride. The resulting nanoreactor precursor was crushed and washed with ethanol containing 10 % acetic acid to remove the imprinted NPA and NP. The prepared nanoreactor was dried in flowing nitrogen.
contrast to NIP (Figure 10). The imprinted species in MIP-NPAPtNP clearly played a role in steering the stronger interactions. To address the interaction further, Table 2 shows the reduction potentials at which both NPA and NP desorbed from all of the prepared nanoreactors. The NPA-imprinted MIP-NPA-PtNP system exhibited the highest desorption/reduction potential for NPA relative to the other systems prepared without NPA. Despite the encapsulated Pt nanoparticles, NIP-Pt exhibited almost the same potentials as NIP. The NP-imprinted MIP-NPAPtNP and MIP-PtNP systems also revealed comparable desorpChem. Eur. J. 2015, 21, 1 – 9
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Characterization TEM images of the prepared nanoreactors were obtained on a transmission electron microscope (JEM-2100; Japan). The surface morphology was observed on a scanning electron microscope (Hitachi S-450; Japan). The BET surface area and porosity were investigated by means of nitrogen adsorption/desorption analysis with a nitrogen surface-area analyzer (SA3100; USA). The IR spectra were recorded on a FTIR apparatus (Nicolet MX-1E; USA).
The authors want to express their gratitude to the National Science Foundation of China (Nos. 51473070, 21403091, 51402128). Thanks also should be expressed to Jiangsu Province and Jiangsu University for support under the distinguished professorship program (Sujiaoshi [2012]34, No. 12JDG001) and the innovation/entrepreneurship program (Suzutong [2012]19). The authors also appreciate project support from the Science and Technology Agency of Jiangsu Province (BK20130486). Keywords: cascade reactions · catalysis · nanoreactors · polymers
TPD By using a device made of a gas chromatography furnished with a thermal conductivity detector and a data-processing system, the prepared nanoreactors (20 mg) were placed into an online Ushaped quartz tube (internal diameter (I.D.) = 4 mm). After the substrate (0.06 mmol mL1, 10 mL) in acetonitrile had adsorbed on the nanoreactors, the quartz tube was heated in flowing nitrogen (55 mL min1, 0.4 MPa) at a rate of 10 8C min1 from room temperature up to the temperature where the absorbed substrate desorbed. The desorbing signal was recorded with the data-processing system.
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Catalytic testing: Catalytic hydrolysis by using these nanoreactors was evaluated in a batch format.[19] The initial concentration of the substrate NPA was 0.2 mmol mL1 (10 mL). The solid content of the nanoreactors was 1.0 mg mL1. The formation of NP was spectrophotometrically detected. The catalytic activity of the nanoreactors was obtained from the average value of three runs. Considering the effect of self-hydrolysis on the catalytic process, the hydrolysis of NPA in the absence of the nanoreactors was also performed under comparable conditions; accordingly, this value was deducted from the overall activity of these nanoreactors. Experiments for the catalytic reduction reactions were further conducted as for the hydrolytic experiments, but with the addition of an excess of sodium borohydride (4fold relative to NPA). Electrochemical testing: An electrochemical workstation equipped with a conventional three-electrode configuration of a Au-plate working electrode, Pt-wire counterelectrode, and a Ag/AgCl reference electrode (CHI 760E, China) was used. The nanoreactors (10 mg) were preabsorbed with the template (ca. 2 mmol) and then placed into a cuvette encircled by a diffusion-eliminating sonication apparatus (supporting electrolyte: 0.01 mmol mL1 KCl, 10 mL). The transient desorption process of the substrate was measured by consecutive scanning with cyclic voltammetry until a stable desorption/reduction profile was reached (scanning range: ca. + 0.6– 0.4 V, scanning rate: 1 mV s1).
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FULL PAPER & Nanotechnology J. Wang, M. Zhu, X. Shen, S. Li* && – && A Cascade-Reaction Nanoreactor Composed of a Bifunctional Molecularly Imprinted Polymer that Contains Pt Nanoparticles
Making a good impression: The construction of a cascade-reaction nanoreactor composed of a bifunctional molecularly imprinted polymer (MIP) containing acidic catalytic sites and Pt nanoparticles (PtNPs) is presented. The
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acidic catalytic sites within the imprinted polymer allow one reaction and the encapsulated Pt nanoparticles allow a coupled reaction (see picture; NPA = 4-nitrophenyl acetate), thus, demonstrating a catalytic-cascade ability.
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