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Photoelectric Performance of Bacteria Photosynthetic Proteins Entrapped on Tailored Mesoporous WO3-TiO2 Films Yidong Lu,† Minjia Yuan,† Yuan Liu,‡ Bo Tu,† Chunhe Xu,‡ Baohong Liu,† Dongyuan Zhao,† and Jilie Kong*,† Chemistry Department, Fudan University, Shanghai 200433, China, and Shanghai Institute of Plant Physiology, Chinese Academy of Sciences, Shanghai 200003, China Received December 4, 2004. In Final Form: March 3, 2005 Novel three-dimensional wormlike mesoporous WO3-TiO2 films with tailored pore size (∼7.1 nm) were applied to prepare the bio-photoelectrodes (Bio-PEs) through direct entrapping the bacteria photosynthetic reaction center (RC) proteins. These mesoporous WO3-TiO2 films exhibited unique characteristics in the specific loading of RC with high activity retained. Moreover, well-matched energy levels of WO3-TiO2 and RC contributed to the photoelectric performance, especially in the red to near-infrared (NIR) region, of the derived Bio-PEs. Such strategy of manipulating the Bio-PEs based on well-designed mesoporous metal oxides and RC provides an alternative system to probe the photoinduced multiple-pathway electron transfer of photosensitive chromophores, which may open a new perspective to develop versatile bio-photoelectric devices.
Introduction Construction and fabrication of functionalized films based on photosensitive chromophores1-4 or proteins5-12 provide an alternative concept to develop various bioelectronic and bio-photoelectric devices. Much effort has been devoted to employing the bacteria photosynthetic reaction center (RC), a transmembrane pigment-protein complex that primarily performs a light-driven charge separation across the bacteria photosynthetic membrane,13-16 as a promising candidate for the main element of such apparatus.5-12 The advantages of using RC lie not only in its robust characters but also in its high quantum yield of the near-infrared (NIR) photoinduced charge separation in native state.13-16 * To whom correspondence may be addressed. Tel: +86-2165642405. Fax: +86-21-65641740. E-mail:
[email protected]. † Fudan University. ‡ Shanghai Institute of Plant Physiology. (1) Pan, J. X.; Xu, Y. H.; Sun, L. C.; Sundstrom, V.; Polivka, T. J. Am. Chem. Soc. 2004, 126, 3066. (2) Pan, J.; Benko, G.; Xu, Y. H.; Pascher, T.; Sun, L. C.; Sundstrom, V.; Polivka, T. J. Am. Chem. Soc. 2002, 124, 13949. (3) Schaetzel, M. L.; Bhise, A. D.; Gliemann, H.; Koch, T.; Schimmel, T.; Balaban, T. S. Thin Solid Films 2004, 451-452, 16. (4) Amao, Y.; Komori, T. Biosens. Bioelectron. 2004, 19, 843. (5) Katz, E. J. Electoanal. Chem. 1994, 365, 158. (6) Yasuda, Y.; Sugino, H.; Toyotama, H.; Hirata, Y.; Hara, M.; Miyake, J. Bioelectrochem. Bioenerg. 1994, 34, 135. (7) Trammell, S. A.; Wang, L.; Zullo, J. M.; Shzshidhar, R.; Lebedev, N. Biosens. Bioelectron. 2004, 19, 1649. (8) Das, R.; Kiley, P. J.; Segal, M.; Norville, J.; Yu, A. A.; Wang, L.; Trammel, S. A.; Reddick, L. E.; Kumar, R.; Stellacci, F.; Lebedev, N.; Schnur, J.; Bruce, B. D.; Zhang, S.; Baldo, M. Nano. Lett. 2004, 4, 1079. (9) Kong, J. L.; Lu, Z. Q.; Lvov, Y. M.; Desamero, R. Z. B.; Frank, H. A.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 7371. (10) Zhao, J. Q.; Liu, B. H.; Zhou, Y. L.; Xu, C. H.; Kong, J. L. Electrochim. Acta 2002, 47, 2014. (11) Zhao, J. Q.; Ma, N.; Liu, B. H.; Zhou, Y. L.; Xu, C. H.; Kong, J. L. J. Photochem. Photobiol., A: Chem. 2002, 152, 54. (12) Zhao, J. Q.; Zhou, Y. L.; Liu, B. H.; Xu, C. H.; Kong, J. L.Biosens. Bioelectron. 2002, 17, 711. (13) Feher, G.; Allen, J. P.; Okamura, M. Y.; Rees, D. C. Nature 1989, 339, 111. (14) Deisenhofer, J.; Michel, H. Science 1989, 245, 1463. (15) Blankenship, R. E., Madigan, M. T., Bauer, C. E., Eds. Anoxygenic Photosynthetic Bacteria: Kluwa: Dordrecht, The Netherlands, 1995. (16) Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1119.
RC (from purple non-sulfur bacteria, Rb. sphaeroides), which is embedded in the bacterial cytoplasmic membrane, is composed of three subunits labeled L, M, and H. The cofactors including one bacteriochlorophyll dimer (P) known as the primary donor, two monomer bacteriochlorophylls (BchlA and BchlB), two bacteriopheophytins (BpheA and BpheB), two quinones (QA and QB), and one non-heme iron are 2-fold symmetrically arranged in the L and M subunits.17-19 Upon direct excitation of P or by excitation energy transferring from the antenna, the excited state of P (P*) forms immediately. Then, a sequence of photoinduced electron transfer quickly occurs to reach the long-lived final charge separation state of P+QB-, which contributes to the proton uptake, the driving force for ATP formation.20 Our previous attention has focused on preparing the RC-modified bio-photoelectrodes (Bio-PEs) in which multiself-assembled monolayers (SAMs)10 and low-temperature sol-gel approaches11 were employed. As expected, obvious photoelectric responses in the NIR region were detected in both cases. However, two factors considered may greatly hamper the photoelectric conversion of such RC derived Bio-PEs. First, a great deal of energy diminishes to form the final charge separation state of RC.13,14 Second, inevitable charge recombination during the sequence of photoinduced electron transfer for isolated RC partly quenches the separated electron-hole pairs.16 Recently, TiO2-doped amorphous WO321 and nanocluster WO3TiO222 have been reported showing higher capability of splitting photoinduced electron hole pairs than single TiO2 or WO3. Considering the matched energy levels of RC and WO3-TiO2, the latter may afford potential role to promote (17) Allen, J. P.; Feher, G.; Yeates, T. O.; Rees, D. C.; Deinsenhoffer, J.; Michel, H.; Huber, R. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 8589. (18) Richradson, J.; Richardson, D. Science 1988, 240, 1648. (19) Van Brederode, M. E.; Van Grondelle, R. FEBS Lett. 455, 1999, 1. (20) Okamura, M. Y.; Feher, G. Annu. Rev. Biochem. 1992, 61, 861. (21) (a) Shiyanovskaya, I.; Hepel, M. J. Electrochem. Soc. 1998, 145, 3981. (b) Shiyanovskaya, I.; Hepel, M. J. Electrochem. Soc. 1999, 146, 243. (22) He, Y.; Wu, Z.; Fu, L.; Li, C.; Miao, Y.; Cao, L.; Fan, H.; Zou, B. Chem. Mater. 2003, 15, 4039.
10.1021/la0470129 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/31/2005
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Scheme 1. Experimental Setup for Preparation of a Bio-PE Based on the Tailored Mesoporous WO3-TiO2 Film and RC
the photoinduced charge separation of RC. However, the existing WO3-TiO2 reported lacks tailored nanopores, which may dampen the possibility of selective immobilization of proteins with a certain specific dimensional size.23 Therefore, preparation of a novel matrix for both entrapping RC and promoting its photoelectric conversion is strongly anticipated. Here, a new kind of tailored threedimensional (3D) wormlike mesoporous WO3-TiO2 based RC bio-photoelectrode (Bio-PE) was designed and applied to favor the photoinduced electron injection from RC to the conduction band (Cb) of WO3-TiO2, which both promoted the charge splitting and retained the original charge separation energy of RC substantially. Such strategy of manipulating the Bio-PEs based on welldesigned mesoporous metal oxides and RC provides an alternative system to probe the photoinduced multiplepathway electron transfer of photosensitive chromophores, which may open a new perspective to develop versatile bio-photoelectric devices. Experimental Section Preparation of the ITO/WO3-TiO2/RC Bio-PEs. Synthesis of the tailored 3D wormlike mesoporous WO3-TiO2 films (pore size of 7.1 nm) was inspired by the concept of “acid-base pairs” reported recently.24 Therein, 1 g of triblock copolymer P123 (EO20PO70EO20) was dissolved in 10 g of ethanol, then 0.8 g of WCl6 and 2.4 g of Ti(OBu)4 were added into the solution and the mixture was further stirred for 2 h at room temperature. The target film was achieved by spin-coating of the mother solution on indium tin oxide (ITO) grass with the thickness of ca. 150 nm. The solvent was fully evaporated in air (20-30% relative humidity). After gelation at 45 °C for 1 day, the inorganic framework was obtained via calcination at 350 °C in air. Another kind of 3D wormlike mesoporous WO3-TiO2 film (pore size of 3.4 nm) and the 2D hexagonal mesoporous WO3-TiO2 films (pore size of 9.8 nm) were prepared similarly for comparison, except the amphiphiles used were substituted with P85 (EO26PO39EO26) and F127 (EO106PO70EO106), respectively. RC from the photosynthetic bacterium RS601 (one of Rb. sphaeroides strain) was separated and purified (23) Xu, X.; Tian, B. Z.; Kong, J. L.; Zhang, S.; Liu, B. H.; Zhao, D. Y. Adv. Mater. 2003, 15, 1932. (24) Tian, B. Z.; Liu, X. Y.; Tu, B.; Yu, C. Z.; Fan, J.; Wang, L. M.; Xie, S. H.; Stucky G. D.; Zhao, D. Y. Nat. Mater. 2003, 2, 160.
as described previously.25 Protein immobilization was achieved by immersing the freshly prepared WO3-TiO2 films (∼1.5 cm2) in the pH 8.0 Tris-HCl buffer solution of RC (at 4 °C) for 2-3 days. Prior to all measurements, the films were rinsed and kept in buffer solution. The whole experimental setup for a Bio-PE preparation through entrapping the RC on the tailored mesoporous WO3-TiO2 film is presented in Scheme 1. Characterization of the WO3-TiO2 Film and ITO/WO3TiO2/RC Bio-PEs. The structural quality of the WO3-TiO2 films was characterized by X-ray diffraction (XRD), nitrogen sorption measurement, and transmission electron microscopy (TEM). XRD patterns were recorded with a Bruker D4 diffractometer with Cu KR radiation; N2 sorption measurements were performed at 77 K using a Micrometities Tri-star 3000 analyzer; TEM experiments were conducted on a JEOL 2011 microscope operated at 200 kV. All data for contact angle of the WO3-TiO2 or TiO2 films were measured with a Phoenix-300 analyzer at 298 K in air. Thickness of the matrix was determined with a SEA 5120 element monitor MX instrument. Ultraviolet (UV)-visible-NIR absorption spectra and fluorescence emission spectra for probing the ITO/WO3-TiO2 composite films and ITO/WO3-TiO2/RC BioPEs were obtained at room temperature by using a SM-240 CCD spectrophotometer (CVI spectral instruments, Putnam, CT) and a SM-300 luminescence spectrometer (CVI spectral instruments, Putnam, CT), respectively. Photoelectric Measurements. Photoelectric responses were measured in a self-made quartz cell filled with pH 8.0 Tris-HCl buffer solution containing 8 mM sodium dithionite. A platinum wire was used as the counter electrode and a Ag/AgCl electrode as the reference. A 20 W incandescent lamp with incident light intensity (Iinc) of 5 mW cm-2 was employed to illuminate the Bio-PEs. A filter (λ > 600 nm) was applied or not, and the intensity of the output through the filter was measured to be 0.1 mW cm-2. Photocurrent action spectrum in the NIR region of the Bio-PEs was determined using a Ti-sapphire laser (Tsunami, Spectra Physics, USA) with wavelength of the output tunable from 700 to 900 nm (10 nm bandwidth). Intensity of the output was measured with a photometer (Spectra Physics, 407A TC). The photoelectric signals were recorded by a CHI-660A electrochemical workstation (CHI Instrument Co., USA). Without extra illustration, the electrode potential in the photoelectric measurements was set at the open-circuit voltage. (25) Zeng, X. H.; Yu, H.; Wu, Y. Q.; Wu, M. J.; Wei, J. M.; Shong, H. X.; Xu, C. H. Acta Biochim. Biophys. Sin. 1997, 29, 46.
Photoelectric Performance of Photosynthetic Proteins
Figure 1. XRD pattern of calcined mesostructured WO3-TiO2. Inset shows the TEM image of the 3D wormlike mesoporous WO3-TiO2.
Figure 2. Nitrogen sorption isotherms and pore-size distribution plots (inset) for calcined 3D wormlike mesoporous WO3TiO2 film.
Results and Discussion Structural Characterization of the Tailored WO3TiO2 Films. Powder XRD pattern of calcined WO3-TiO2 films (W/Ti ) 1:3, molar ratio) prepared by using triblock copolymer Pluronic P123 as a template (Figure 1) exhibits an intense reflection at low 2θ value of 0.8°, suggesting the mesoporous structure of the products. Nitrogen sorption data (Figure 2) show that the calcined mesoporous WO3-TiO2 products have uniform pore size centered at 7.1 nm and relatively high BET surface area of 150 m2/g, which greatly enhances the availability for protein molecules immobilization. The TEM image shown in Figure 1 (inset) displays wormlike mesostructured patterns, further confirming that the WO3-TiO2 products have 3D disordered and opened mesostructure. There are many microcracks on the outer surface of the mesoporous WO3-TiO2 film modified ITO electrode, allowing for the efficient diffusion of biomolecules within the pore channels. These structural features make the WO3-TiO2 films be a novel inorganic “host” for entrapping the “guest” biomolecules. Spectral Study of the Tailor-Made RC Bio-PEs. The successful entrapment of RC on the tailored mesoporous WO3-TiO2 films is proved by the steady NIRvisible absorption spectra presented in Figure 3. For comparison, the UV-visible-NIR absorption spectrum of the tailored WO3-TiO2 films is provided as inset. RC (RS601) shows three major absorption peaks at 760, 802,
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Figure 3. NIR-visible absorption spectra of (a) RC (2 µM) in pH 8.0 Tris-HCl buffer and (b) the tailor-made ITO/WO3TiO2/RC film at 293 K. Absorption of bare ITO/WO3-TiO2 film is subtracted as background. Inset reveals the UV-visibleNIR absorption spectrum of the tailor-made ITO/WO3-TiO2 films recorded using blank ITO as background.
and 870 nm, which correspond predominantly to the Qy transition for Bphe, Bchl, and P, respectively.26 No distinct differences are found between the two spectra (RC in solution and on WO3-TiO2 films), which displays that the high-efficient NIR light-harvesting capability of RC remains unaltered on the WO3-TiO2 matrix. Three absorption bands including a main peak centered at 333 nm, a shoulder at around 385 nm, and a small peak at around 520 nm are observed for the tailored WO3-TiO2 films shown in Figure 3 (inset). No obvious absorption band is found at wavelength longer than 600 nm due to the wide band-gap of the semiconductors. The features at 385 and 520 nm could be attributed to the interference effect of thin film, as reported similarly for nanosized TiO2.27,28 The maximum loading of RC on the tailored mesoporous WO3-TiO2 films calculated from the differential absorption spectra reaches 0.63 µmol/g. For a full understanding of the relationship between RC loading with pore size, structural topology, and surface hydrophilicity of the matrix, 3D wormlike mesoporous WO3TiO2 films with smaller pore size (∼3.4 nm) and 2D hexagonal mesoporous WO3-TiO2 films (pore size of ∼9.8 nm) were prepared similarly according to the “acid-base pairs” concept.29 The results from nanocrystalline TiO2 films prepared by the anodic electrodeposition28 are also given, as shown in Table 1. Obviously, the tailored 3D wormlike mesoporous WO3-TiO2 films allow the maximum protein loading among the entire matrix listed. The favored RC entrapment on such mesoporous WO3-TiO2 films could be explained in two aspects. First, the tailormade mesoporous WO3-TiO2 has opened mesostructure and narrow-distributed pore size (pore diameter of ∼7.1 nm, pore area of ∼39.6 nm2) well-matching one 2D dimension of RC (∼21 nm2). Furthermore, the contact angle measurements show better hydrophilicity for the mesoporous WO3-TiO2 films (∼24°) than for the TiO2 films (>33°), which promotes the adsorption of the hydrophilic (26) Arnett, D. C.; Moser, C. C.; Dutton, P. L.; Scherer, N. F. J. Phys. Chem. B 1999, 103, 2014. (27) Sakai, N.; Ebina, Y.; Kazunori, T.; Sasaki, Takayoshi. J. Am. Chem. Soc. 2004, 126, 5851. (28) Kavan, L.; Regan, B. O.; Kay, A.; Gratzel, M. J. Electroanal. Chem. 1993, 346, 291. (29) Structural characterization of the 3D wormlike mesoporous WO3-TiO2 films (pore size of 3.4 nm) and the 2D hexagonal mesoporous WO3-TiO2 films (pore size of 9.8 nm) is displayed in Supporting Information.
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Table 1. Comparison of Matrix Property with Quantity of RC Adsorbed matrix of RC Bio-PEs TiO2 WO3-TiO2 WO3-TiO2 WO3-TiO2
pore structure
pore size (nm)
thickness (nm)a
contact angle (deg)b
M of RC immobilized (µmol/g)c
intercrystalline voids 2D hexagonal 3D wormlike 3D wormlike
4-200 9.8 ( 0.8 3.4 ( 0.3 7.1 ( 0.6
∼500 ∼150 ∼150 ∼150
38.6 23.4 24.6 24.2
0.18 0.29 0.31 0.63
a Thickness of WO -TiO and TiO films was determined with an average of five measurements. b All data for contact angle were 3 2 2 measured with an average of four measurements. c Molar amount (M) presented here were calculated from the differential absorption spectra of RC solution before and after immobilization (molar extinction coefficient of RC at 802 nm is ca. 2.88 × 105 M-1 cm-1) with an average of three measurements.
Figure 4. Steady fluorescence emission spectra of (a) bare ITO/WO3-TiO2 film, (b) ITO/WO3-TiO2/RC film, and (c) ITO/ TiO2/RC film at 293 K, excited at 800 nm. Inset displays steady fluorescence emission spectra of 2 µM RC in pH 8.0 Tris-HCl buffer.
RC (treated with detergents). The prior possible orientation that the more hydrophilic H-subunit domains6 of RC face to the matrix should be expected. By contrast, the 3D wormlike mesoporous WO3-TiO2 with smaller pore size of ∼3.4 nm (mismatching any 2D dimension of RC) yields less protein adsorption of 0.31 µmol/g. Correspondingly, the 2D hexagonal mesoporous WO3-TiO2 with larger pore size of ∼9.8 nm also leads to less protein adsorption of only 0.29 µmol/g, which may attribute to the reason that the distribution of all the channels paralleled with the substrate greatly restricts the RC molecule to diffuse into the matrix. The existence of photoexcited electron injection (via P* or P+Bphe- f WO3-TiO2) is proved by the fluorescence
emission spectra, which gives a useful functional and conformational probe of RC. As shown in Figure 4, the fluorescence intensity of RC on the tailored mesoporous WO3-TiO2 films is only half of that on the nanocrystalline TiO2 films mentioned above, while the protein loading on the former is far more than that on the latter. The steady fluorescence peak centered at ∼875 nm can be definitely attributed to the emission of energy transferring from Bchl to P.30 The increased emission quenching is mainly due to the facile electron injection from the excited singlet state of P or the charge separation state of P+Bphe- to the Cb of the mesoporous WO3-TiO2 particles, which may benefit from the well-matching energy level of RC and WO3-TiO2 as well as the ideal orientation of RC on such a mesoporous matrix. Photoelectrochemical Study of the Tailor-Made RC Bio-PEs. Photoelectric performances of the tailormade RC Bio-PEs are displayed in Figure 5. No obvious photocurrent ( 600 nm, Iinc ) 0.1 mW cm-2) or not (B, Iinc ) 5 mW cm-2). The electrode bias is set at the open-circuit voltage (a, ∼-0.15 V vs SHE; b, ∼-0.1 V vs SHE).
Photoelectric Performance of Photosynthetic Proteins
Figure 6. Short-circuit photocurrent responses of the ITO/ WO3-TiO2/RC film upon 1 h of continuous illumination. Other conditions are the same as in Figure 5.
Figure 7. Photocurrent action spectra of the tailor-made ITO/ WO3-TiO2/RC film (a, circle) and the Al2O3/RC film (b, square) in the NIR region excited using a Ti-sapphire laser with tunable output wavelength from 700 to 900 nm.
applications, it may be of potential use for constructing bioelectronic devices. Photocurrent of the ITO/WO3-TiO2/ RC Bio-PEs as a function of the NIR illumination light wavelength is displayed in Figure 7. Data plots from the RC/Al2O3-based Bio-PEs reported in our previous work11 are also shown as a comparison. The photocurrent action spectrum of the ITO/WO3-TiO2/RC Bio-PEs almost overlays the absorption spectrum of RC both in solution and on matrix, which further confirms that the photocurrents measured in the NIR region are indeed generated by the entrapped proteins and the high-efficient NIR light-
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harvesting capability of RC is well remained. Remarkably enhanced photoelectric performance of the tailor-made RC Bio-PEs is also observed from the action spectrum compared with that of the RC/Al2O3 Bio-PEs. Despite the loading of RC on the tailor-made ITO/WO3-TiO2/RC BioPEs is far less ( 600 nm apparently exists in the neutral region, as shown in Figure 8A. The intrinsic effect of pH on the photocurrent can be interpreted in two aspects. On one hand, proton uptake upon combination of the photoreductive QB with H+ features an irreversible process in isolated RC,20 which might somehow hamper the light-induced charge separation of the proteins. On the other hand, the net surface charge of RC changes from the positive to the negative as pH of the electrolytes become higher than the isoelectric point of the proteins (∼7.2), which may destroy the stability of the ITO/WO3-TiO2/RC films since the WO3-TiO2 matrix surface is also negatively charged in this pH region. Figure 8B depicts the relationship between the photocurrent measured at λ > 600 nm with the electrode potential applied. For RC, the applied bias deeply influences the sequence of photoinduced electron transfer between the electron donors and acceptors that are mostly redox-active inside the proteins12 and thus the photoelectric responses of the Bio-PEs. The maximal photocurrent was obtained at about -0.3 V, which is similar to the results for the RC multi-SAMs10 and RC/Al2O311 based Bio-PEs reported previously. The reasonable operation mechanism for partly explaining the enhancement of photoelectric conversion as described above is presented in Scheme 2. For RC alone, upon direct excitation of P or by excitation energy transferring from the antenna, electron transfer is triggered from P* to Bphe in about 3-4 ps at room temperature and, subsequently, to QA with a time constant of ∼200 ps.16,31 Fluorescence emission acts as a secondary way to quench the P*. A third electron-transfer pathway, that is,
Figure 8. Effects of pH (A) and electrode bias (B) on the short-circuit photocurrent responses for the tailor-made ITO/WO3TiO2/RC films at the wavelength of incident light λ > 600 nm (Iinc ) 0.1 mW cm-2).
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a The photoanode is ITO/WO3-TiO2/RC film and the counter electrode is Pt. The electrolyte is pH 8.0 Tris-HCl buffer containing 8 mM sodium dithionite. a1, Vb of WO3; a2, Cb of WO3; b1, Vb of TiO2; b2, Cb of TiO2; c1, PBpheQAFe2+QB; c2, P*BpheQAFe2+QB; c3, P+Bphe-QAFe2+QB; c4, P+BpheQA-Fe2+QB; c5, P+BpheQAFe2+QB-; P, bacteriochlorophyll dimer, the primary donor; Bphe, bacteriopheophytin; QA, the primary quinone; QB, the secondary quinone.
the photoinduced electron injection (via P* or P+Bphe- f TiO2 f WO3), may occur on the basis of RC being entrapped effectively on the well-arranged mesoporous matrix and compete with the fluorescence emission of P* as well as the electron transfer in RC itself in a certain extent. The mutual position of energy levels for P* (∼-0.7 V vs standard hydrogen electrode, SHE)32 or P+Bphe- (∼-0.5 V vs SHE),33,34 TiO2 (Cb ∼-0.2 V vs SHE),35 and WO3 (Cb (30) Wright, C. A.; Clayton, R. K. Biochim. Biophys. Acta 1973, 333, 246. (31) Van Brederode, M. E.; Van Grondelle. R. FEBS. Lett. 1999, 455, 1. (32) (a) Moser, C. C.; Sension, R. J.; Szarka, A. Z.; Repinec, S. T.; Hochstrasser, R. M.; Dutton, P. L. Chem. Phys. 1995, 197, 343. (b) Lao, K. Q.; Franzen, S.; Steffen, M.; Lambright, D.; Stanley, R.; Boxer, S. G. Chem. Phys. 1995, 197, 259. (33) Stocker, J. W.; Taguchi, A. K. W.; Murchison, H. A.; Woodbury. N. W.; Boxer, S. G. Biochemistry 1992, 31, 10356. (34) Nagarajan, V.; Parson, W. W.; Gaul, D.; Schenck, C.; Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 7888. (35) Gratzel, M. Nature 2001, 414, 338. (36) Andersson, Mikael.; Linke, Myriam.; Chambron, J.-C.; Davidsson, J.; Heitz, Valerie.; Hammarstrom, L.; Sauvage, J.-P. J. Am. Chem. Soc. 2002, 124, 4347. (37) Van der Boom, T.; Hayes, R. T.; Zhao, Y.; Bushard, P. J.; Weiss, E. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2002, 124, 9582.
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> -0.2 V vs SHE)35 may promote the electron injection from RC to the WO3-TiO2. Similar ultrafast long-range electron transfer in some artificial photosynthetic systems could also be achieved.36,37 On the other hand, the helical peptides in RC might play an important role for attaining such efficient long-range electron transfer as reported for those biomimetic helical peptides.38,39 Although it seems impossible for the excitation energy of RC to release completely through the photoinduced electron injection, and it is still ambiguous how much proportion the photoinduced electron injection accounts for the total charge separation of RC, the photocurrent generation is promoted for isolated RC and the original photoinduced separation energy of RC is partially retained due to the existence of the photoinduced electron injection. Conclusion Bacterial photosynthetic reaction center (RC) from the Rb. Sphaeroides strain RS601 is specifically entrapped on the tailored 3D wormlike mesoporous WO3-TiO2 films prepared by the self-adjusted sol-gel syntheses. Structure and activity of the immobilized RC remain unaltered. Enhanced photoelectric responses, especially in the red to NIR region, are observed for RC-derived Bio-PEs. Existence of partial photoinduced electron injection directly from RC to the matrix could be suggested. Despite the still relative low energy conversion efficiency compared with that of the synthetic dye-sensitized cells, the attractive features of such mesoporous metal oxides modified with the bacterial photosynthetic proteins may provide an alternative way to probe the photoinduced multiplepathway electron transfer of photosensitive chromophores and create a new perspective to develop versatile biophotoelectric devices. Acknowledgment. This work was supported by NSFC (20335040, 20173012, 20373013, 20475012), Shanghai Nano-project (0452nm003). Supporting Information Available: Structural characterization of the 3D wormlike mesoporous WO3-TiO2 films (pore size of 3.4 nm) and the 2D hexagonal mesoporous WO3TiO2 films (pore size of 9.8 nm). This material is available free of charge via the Internet at http://pubs.acs.org. LA0470129 (38) Yanagisawa, K.; Morita, T.; Kimura, S. J. Am. Chem. Soc. 2004, 126, 12780. (39) Morita, T.; Kimura, S.; Kobayashi, S.; Imanishi, Y. J. Am. Chem. Soc. 2000, 122, 2850.
