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Chitosan Confinement Enhances Hydrogen Photogeneration ARTICLE Received 7 Jun 2013 | Accepted 1 Oct 2013 | Published 25 Oct 2013 DOI: 10.1038/ncomms3695 Chitosan confinement enhances hydrogen photogeneration from a mimic of the diiron subsite of [FeFe]-hydrogenase Jing-Xin Jian1,*, Qiang Liu1,2,*, Zhi-Jun Li1, Feng Wang1, Xu-Bing Li1, Cheng-Bo Li1, Bin Liu1, Qing-Yuan Meng1, Bin Chen1, Ke Feng1, Chen-Ho Tung1 & Li-Zhu Wu1 Nature has created [FeFe]-hydrogenase enzyme as a hydrogen-forming catalyst with a high turnover rate. However, it does not meet the demands of economically usable catalytic agents because of its limited stability and the cost of its production and purification. Synthetic chemistry has allowed the preparation of remarkably close mimics of [FeFe]-hydrogenase but so far failed to reproduce its catalytic activity. Most models of the active site represent mimics of the inorganic cofactor only, and the enzyme-like reaction that proceeds within restricted environments is less well understood. Here we report that chitosan, a natural polysaccharide, improves the efficiency and durability of a typical mimic of the diiron subsite of [FeFe]-hydrogenase for photocatalytic hydrogen evolution. The turnover number of the self-assembling system increases B4,000-fold compared with the same system in the absence of chitosan. Such significant improvements to the activity and stability of artificial [FeFe]-hydrogenase-like systems have, to our knowledge, not been reported to date. 1 Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & University of Chinese Academy of Sciences, the Chinese Academy of Sciences, Beijing 100190, China. 2 State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to L.-Z.W, (email: [email protected]). NATURE COMMUNICATIONS | 4:2695 | DOI: 10.1038/ncomms3695 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3695 nzymes may bind substrates through multiple interactions – h e in elaborate pockets to direct a specific reaction pathway under mild conditions1–4. [FeFe]-hydrogenase ([FeFe]- E – 5,6 e N H2ase) , a natural enzyme for hydrogen (H2) evolution, is O H2 O + OH OH CdTe S deeply embedded within the protein matrix to enable the O NH3 S OC CO reversible reduction of protons to H with low overpotential QDs Fe Fe + 2 H N À 1 h+ OC CO 3 O and high turnover frequencies (TOF 6,000–9,000 s per OH OC CO catalytic site). The high-resolution X-ray crystallographic OHO + OHO NH3 – OH structures establish that [FeFe]-H2ase, isolated from e + O – + H N Desulfovibrio desulfuricans5 and Clostridium pasteurianum6, HA + H 3 O OH + OH features a butterfly [Fe2S2] subunit coordinated by a cysteine- O H N + O OH 3 H A + 2 H3N OH linked [Fe4S4] cluster, carbon monoxide and cyanide ligands, and + O OHH3N H3N OH O by a dithiolate bridging the two iron centres. The diiron [Fe2S2] O OH O OH subunit serves as the catalytic centre for proton reduction, and the OH O O O OH [Fe4S4] cluster mediates transfer electron to and from the active site of the H-cluster. The astonishing rates of H production from 2 Figure 1 | Chitosan-confined H2 photogeneration. A schematic describing the non-precious diiron catalysts via a group of enzymes under the H2 photogeneration of a chitosan-confined mimic of the diiron mild conditions can exceed those of platinum. However, the subsite of [FeFe]-H2ase in the presence of CdTe quantum dots and H2A. large-scale isolation of the enzyme from natural systems is rather difficult, hence the development of artificial [FeFe]-H2ase analogues capable of reproducing the enzymic activity has amines of chitosan and the catalytic intermediate of photo- spurred considerable interest in both the scientific and reduced mimic of the diiron subsite of [FeFe]-H2ase but also as a industrial communities7–25. Over the past decade, a variety of sacrificial electron donor to regenerate MPA-CdTe QDs for mimics of the diiron subsite of [FeFe]-H2ase have been shown to photocatalytic H2 production. Significantly, the self-assembled 26–33 function as catalysts for chemical reduction of protons . It has system that comprises chitosan, [Fe2(CO)6(m-adt)CH2C6H5], been clear that electron transfer, either electrochemical or MPA-CdTe QDs and H2A is capable of producing H2 with 4 photochemical, to a mimic of the active site of [FeFe]-H2ase is TON of up to (5.28±0.17) Â 10 and initial TOF of 1.40± 10–14,22 À 1 a prerequisite for H2 evolution . From a photochemical 0.22 s with respect to [Fe2(CO)6(m-adt)CH2C6H5] catalyst point of view, the electron transfer is triggered by the absorption under visible light irradiation (l4400 nm). The catalytic stability of a photon by a photosensitizer13–25. Since the first attempt by is enhanced from 8 to 60 h and the catalytic activity is over Sun and Åkermark34 to construct an artificial photocatalytic 4.16 Â 103-fold higher than that of the same system without system for H2 evolution in 2003, a large number of synthetic chitosan. The activity and stability are, to the best of our model complexes have been pursued to mimic the structure and knowledge, the highest to date for light-driven catalytic H2 functionality of the diiron subunit of the natural [FeFe]-H2ase evolution from mimics of the diiron subsite of [FeFe]-H2ase. H-cluster35–51. It is encouraging to see that the catalytic efficiency for H2 evolution from artificial photocatalytic systems using mimics of the diiron subsite of [FeFe]-H2ase as catalysts has been Results increased from null to more than hundreds or thousands of The photocatalytic activity of H2 evolution. An initial photo- turnover numbers (TON) under different irradiation conditions. catalytic experiment of [Fe2(CO)6(m-adt)CH2C6H5]catalystwith In comparison to the efficient diiron active site of [FeFe]-H2ase in MPA-CdTe QDs was evaluated in the absence of chitosan. To keep nature, however, no [FeFe]-H2ase mimic has been able to the solubility of [Fe2(CO)6(m-adt)CH2C6H5]catalystthrough- duplicate the high level of reactivity of natural [FeFe]-H2ase. out the experiment, we carried out the reaction in a mixture Review of the literature indicates that the synthetic mimics of of methanol and water. The anaerobic solution, containing À 5 À 1 [FeFe]-H2ase reported thus far are mainly focused on the [Fe2(CO)6(m-adt)CH2C6H5]catalyst(1.00Â 10 mol l ), MPA- À 6 À 1 À 1 inorganic cofactor only, and the enzyme-like reaction that CdTe QDs (0.86 Â 10 mol l ), along with H2A(0.10moll ), proceeds within restricted environments is to date poorly was irradiated by light-emitting diodes (l ¼ 410 nm) at room understood. temperature, where the best ratio of methanol to water was found With this in mind, we initiated the study of a chitosan- to be 1:3 (v-v) (Supplementary Fig. S1). The photoproduct of H2 confined mimic of the diiron subsite of [FeFe]-H2ase for H2 was characterized by gas chromatography (GC) analysis with production. Chitosan is a naturally occurring polysaccharide methane as the internal standard. The time course showed that the containing a significant content of primary amines and hydroxyl amount of H2 increased in the first 4 h and then leveled off, 52–54 groups . When the amines are protonated by acids, chitosan yielding a TON of only 1.74±0.06 based on [Fe2(CO)6(m-adt) bears a polycationic character. In view of the chelation and CH2C6H5] catalyst (Fig. 2a, line A). In sharp contrast, the electrostatic interactions, we envision that chitosan may catalytic performance of the same solution was improved sig- À 1 incorporate mimics of the diiron subsite of [FeFe]-H2ases nificantly in the presence of 1.0 g l of chitosan. Line B in Fig. 2a intimately, as is the case of [FeFe]-H2ase, which is buried shows the H2 production over time from the mixture under deeply within the protein matrix in nature. To avoid side-chain visible light irradiation. The amount of H2 reached 1.27±0.01 ml effects, the simplest mimic of the diiron subsite of [FeFe]-H2ases, (TON ¼ 569±2) within 10 h of irradiation, and the rate of H2 27,37 [Fe2(CO)6(m-adt)CH2C6H5][m-adt ¼ N(CH2S)2] , is selected evolution was almost linear even after 10 h of irradiation. Control as a catalyst (Fig. 1). The 3-mercaptopropionic acid (MPA)- experiments further proved that the components in the system, capped CdTe quantum dots (MPA-CdTe QDs), promising for H2 [Fe2(CO)6(m-adt)CH2C6H5] catalyst, MPA-CdTe QDs, H2A, evolution in combination with a mimic of the diiron subsite of chitosan or light are all essential for efficient H2 generation. [FeFe]-H2ase (ref. 45), are used as the photosensitizer. Herein, The absence of [Fe2(CO)6(m-adt)CH2C6H5] catalyst led to the CdTe QDs are stabilized by MPA and their negatively charged rate of H2 evolution dropping dramatically and no H2 could 55 surfaces preferably interact with cationic chitosan. Ascorbic be detected when either MPA-CdTe QDs or H2Awasabsent acid (H2A) serves as not only a proton source to protonate the from the reaction system with chitosan (Supplementary Fig. S2). 2 NATURE COMMUNICATIONS | 4:2695 | DOI: 10.1038/ncomms3695 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3695 ARTICLE a 1.5 b 1.5 2 h 600 4 h 600 6 h 8 h 1.0 1.0 10 h 400 TON 400 TON (ml) (ml) B 2 2 H H V V 0.5 200 0.5 200 A(× 50) 0 0 0 0 0246810 00.51.0 2.0 Irradiation time (h) Chitosan concentration (g l–1) c 800 d 60,000 2 h 12.5 1.5 4 h 6 h 600 10.0 8 h 40,000 10 h TON TON 7.5 1.0 (ml) (ml) 400 2 2 H H 5.0 V V B 20,000 0.5 200 2.5 A(× 200) 0 0 0 0 3.5 4.0 4.5 5.0 6.0 10 0 102030405060 pH value Irradiation time (h) À 1 Figure 2 | H2 evolution under visible light irradiation.
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