J. Phys. Chem. 1993,97, 7264-7271
7264
Photoinduced Electron-Transfer Processes Using Organized Redox-Functionalized
Bipyridinium-Polyethylenimine-Ti02 Colloids and Particulate Assemblies Itamar Willner’ and Yoav Eichen Institute of Chemistry and Farkas Center for Light Induced Processes, The Hebrew University of Jerusalem, Jerusalem 91 904. Israel
Arthur J. Frank National Renewable Energy Laboratory, 161 7 Cole Boulevard, Golden, Colorado 80401
Marye Anne Fox Department of Chemistry, University of Texas at Austin, Austin, Texas 78712 Received: November 10, 1992; In Final Form: April 6, 1993
Polyethylenimine, PEI, acts as an effective supporting polymer for the stabilization of semiconductor colloids of TiO2. Chemical modification of PEI by N-(2-carboxyethyl)-N’-methyl-4,4’-bipyridinium, MVP2+ (2), generates a redox-functionalized polymer, PEI-MVP2+, that also stabilizes Ti02 colloids. Photoreduction of bipyridinium units, which are covalently linked to the polymer backbone within the Ti02-PEI-MVP2+ assembly, proceeds effectively upon excitation of the semiconductor colloid. At pH = 8.9, photoreduction of bipyridinium units of the Ti02-PEI-MVP2+ assembly is ca. 54-fold and 13-fold faster than the reduction of N,N’-dimethyl4,4’-bipyridinium, MV2+,or N,N’-bis( 3-sulfonatopropyl)-4,4’-bipyridinium,PVSO, by T i O r P E I , respectively. At pH = 5.1, photoreduction of PEI-MVP2+ is ca. 92-fold and 12-fold faster than that of MV2+ and PVSO by TiO2-PEI, respectively. The enhanced yield of reduction of the bipyridinium relay units of the TiO2PEI-MVP2+ assembly is attributed to the control of electron-transfer reactions a t the semiconductor-solution interface. The redox polymer stabilized colloid, Ti0rPEI-MVP2+, concentrates the electron relay units at the semiconductor interface. Consequently, conduction band electrons formed upon photoexcitation of the Ti02 are effectively trapped by the redox relay units on the polymer. This electron trapping competes with the degradative “electron-hole” recombination. Time-resolved laser photolysis studies reveal that the interfacial electron transfer in the Ti02-PEI-MVP2+ assembly proceeds within the laser pulse time constant ( 300 nm, results in the formation of MVP'+-PEL The efficiencies of MVP'+-PEI formation in aqueous solution were followed at pH = 8.9 and 5.1 and are shown in parts A and B of Figure 3, respectively. At these pH values, different protonation states of the polymer backbone are obtained. At pH = 8.9,only one-quarter of the amino groups are protonated, whereas at pH = 5.1, three-quarters of the amines are protonated.2z Evidently, the rate of MVP'+-PEI formation is pH dependent and is faster in basic solutions,although in both systems formation of MVP'+-
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It should be noted that illumination of the MVP2+-PEI-Ti02 colloid or the photosystemsconsistingof PEI-Ti02 and the various bipyridinium relays in the absence of the sacrificialelectrondonor, EDTA, results in only trace amounts of the respective reduced bipyridinium radical cations. This indicates that PEI acts as a poor hole scavenger when compared to EDTA. It also suggests that in the presence of EDTA the holes are reduced by the latter electron donor, and PEI is not affected. Nevertheless, the validity of the above-outlinedcomparison of the rates of MVP2+-PEI reduction relative to those of MV2+and BV2+ might be questioned. As polyethylenimine is protonated, it generates a positively charged interface that electrostatically repels MV2+ and BV2+. Thus, the control systems comprising Ti02-PEI and MVZ+ or BV2+do not necessarily represent pure diffusional electron relay systems but, rather, assemblies where the electron traps are repelled from the polymer interface. To overcome these difficulties, a further control system has been applied, where N,N'-bis(3-sulfonatopropyl)-4,4'-bipyridinium, PVSO (4). has been used as a diffusional electron acceptor. The zwitterionicstructure of PVSO (4) ensures that this electron relay will attain very little electrostatic interaction with the protonated PEI interface." The rate of PVS'- formation upon illumination of the colloid system T i O l P E I is summarized in Table I and displayed in Figure 3A,B (curves d). Evidently, the rate of MVP'+-PEI formation is substantially higher than that of PVS*-. At pH = 8.9, MVP'+-PEI formation is ca. 13-fold faster than that of PVS'- production, while at pH = 5.1 MVP'+ formation is 12-fold faster than that of PVS*-. Thus, the control system that includes PVSO as a diffusional electron trap of conduction band electrons clearlyreveals the superior activityof the organized MVP2+-PEI-Ti02 colloid assembly as compared to diffusional electron trapping. Furthermore, it is evident that at pH = 8.9 the rates of M V + and PVS'- formation are almost identical. At pH = 5.1, the rate of PVS'- formation is, however, ca. 5.5-fold faster than that of MV+. These results reveal the effects of electrostatic interactions exerted by the charged Ti02-PEI interface on electron-transfer processes at the semiconductorsolution interface utilizing diffusional electron acceptors. At pH = 8.9, only ca. one-quarter of the polyethylenimineamino groups are present in the protonated state, and the charge density on the polymer backbone is low. As a result, electrostatic repulsive interactions between the polymer interface and MV2+ are weak, and consequently the electron-transfer rates from the semiconductor to MV2+or to PVSO are almost identical. At pH = 5.1, the electrical repulsion of MV2+by the highly protonated surface affects its rate of reduction at the interface. At pH = 5.1, ca. three-quarters of the amino groups of the polymer backbone are protonated, and the Ti02 surface is also positively charged.23 Consequently, the semiconductor-solutioninterface exhibits high positive charge density, and the free MV2+ is electrostatically repelled from the surface. Electron trapping of a photoexcited conduction band electron is therefore less effective. Improved kinetics for electron trapping at the semiconductorsolution interface in the organized Ti0z-MVP2+-PEI assembly
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is revealed by time-resolved laser photolysis experiments. Laser pulse irradiation of Ti02-MVPZ+-PEI, Figure 5a, results in the instantaneous formation of MVP'+-PEI within the pulse width of the laser (ca. 0.5 ns). Laser flash excitation of T i O l P E I in the presence of MV2+, at a bulk concentration similar to that of polymer-anchored MVP2+, yields an extremely weak trace of MV*+. When the concentration of solubilized MV2+ is 10-fold increased as compared to the bulk concentration of MVP2+ anchored to PEI, laser illumination of Ti02-PEI leads to the formation of M V + , Figure 5b. In this latter experiment, MV'+ is formed by two pathways: one part of MV*+is instantaneously formed (within the laser pulse) and the second fraction is formed over a time scale of ca. 1800 ps. While the fraction of M V + formed instantaneously within the laser pulse is attributed to TiOz-adsorbed MV2+,the time-controlledaccumulation of MV*+ representsthe fraction of MV'+ formed by electron uptake through diffusion of MV2+to the semiconductor interface. The derived net electron-transfer rate constant of a conduction-band electron to the diffusional electron relay, MV2+,is k,, = 750 f 80 s-l. Nevertheless, it should be noted that not only is the kinetics of reduction of the bipyridinium units different in the organized TiOz-MVP2+-PEI, as compared to the separated diffusional system comprising Ti02-PEI and MVZ+,but also the efficiency of electron trapping is influenced. From Figure 5a,b, it can be seen that even though theconcentration of MV2+in thediffusional system is 10-fold higher than the bulk concentration of MVP2+PEI, the amount of MVP'+-PEI formed within the time duration of the laser pulse is ca. 5-fold higher than that of MV'+ within this time scale. It is also 3.5-fold higher than the overall amount of accumulated MV'+ by the two pathways at longer time scales of ca. 1800 p s . We thus conclude that the time-resolved and steady-state illumination studies reveal that electron trapping at the semiconductor-solutioninterface is improved by theadsorption of the redox-functionalized polymer MVP2+-PEI at the TiOz surface. Concentration of the electron relay at the colloid surface yields effective trapping of photogenerated conduction-band electrons, and electron transfer competes effectively with energy wasting through electron-hole recombination. A further aspect to discuss involves the improved rate of reduction of MVP2+-PEI at pH = 8.9 as compared to that at pH = 5.1. As mentioned above, the rate of MVP2+-PEI reduction at pH = 8.9 is ca. 2-fold enhanced as compared to the system at pH = 5.1. Theimprovedelectron-transferratesin the photosystem TiOrMVP+-PEI at pH = 8.9 might be attributed to the thermodynamic driving force as well as to electrostatic efforts associated with the microheterogeneous SC-polymer interface. The reduction potential of the Ti02 conduction band is pHdependent,z4eq 1. Eo,,(TiOz) = (-0.1 1) - 0.06 pH
(1) Thus, at pH = 8.9, the reduction potential of the conductionband electrons is shifted negatively by ca. 0.22 V from that at pH = 5.1. As a result, electron transfer to the relay is energetically
Photoinduced Electron-Transfer Processes
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7269
E
t 0
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Figure 6. Formation of nitrite as a function of illumination time of a polyethylenimine-modified Ti02 colloid: X > 300 nm. All systems are composed of Tris buffer (pH = 7.2) and include [EDTA] = 1 X lo-’ M, [KNOs] = 0.01 M, and nitrate reductase = 2 units. (a) TiOtMVP*+PEI, [MVP2+]= 1 X l o ‘ M; (b) TiOrPEI with no relay; (c) TiOrPEI and MV2+, [MV2+] = 1 X l o ‘ M.
more favored at pH = 8.9 as compared to that at pH = 5.1. Alternatively, the difference in the reduction rates of MVP2+PEI at pH = 5.1 and 8.9 can be attributed to the different protonation states of the polymer and their effects on the redox polymer backbone configurations relative to the semiconductor surface. At pH = 5.1, both the Ti02 surface and the polymer are highly positively charged.23 As a result, segments of the polymer are repelled from the Ti02 surface. Consequently, a substantial fraction of the MVP2+ units are fixed in a remote position from the colloid surface. As a result, the local concentration of the relay units at the Ti02 surface is lower than in the system where such electrostatic interactions are not operative. In contrast, at pH = 8.9, the polymer exhibits low positive charge density, and the Ti02 surface is negatively charged.23 Under these conditions, MVP2+-PEI is strongly adsorbed to the semiconductor surface, and the high relay concentration at the semiconductor-solution interface results in effective trapping of conduction-band electrons. Photoinduced Biocatalyzed Reduction of Nitrate (NOS-) to Nitrite (NO23 Using the Organized Ti02-MVP2+-PEI Assembly The efficient photoinduced electron trapping at the semiconductor-solution interface in the organized TiOrMVP2+-PEI assembly can be coupled to a subsequentchemical transformation. Previous studies have revealed that photogenerated N,N’dimethyl-4,4’-bipyridiniumradical cations, MV*+,act as electron mediators and electrically communicate with the enzyme nitrate reducta~e.2~Thus, photogenerated M V + mediates the biocatalyzedreductionof nitrate (NOS-)to nitrite (NO2-) in the presence of nitrate reductase. As the pK, of the enzyme nitrate reductase is 6. 12,26the negatively charged protein is electrostatically associated with the positively charged TiOlMVP2+-PEI assembly (at pH = 7.2the polymer is protonated and the relay units contribute to the overall positive charge of the polymer backbone). Thus, electron transfer from MVP’+-PEI to the enzyme by primary photoexcitation of the semiconductor is expected to proceed effectively in the assembly containing nitrate reductase associated to TiOZ-MVPz+-PEI. A photosystem consisting of Ti02-MVP2+-PEI (1.8X 10-4M MVP2+-PEI with a loading degree of 0.75% w/w), the enzyme nitrate reductase (E.C. 1.6.6.2, 2 units), nitrate (0.01M), and EDTA (1 X 10-3 M) at pH = 7.2was illuminated, X > 300 nm. Figure 6 (curve a) shows the rate of nitrite (NO23 formation upon illumination of the system. Nitrite is formed effectively in the photosystem. The turnover numbers of the bipyridinium units linked to the polymer and of the enzyme correspond to T N = 25 and 3100, respectively. Illumination of the photosystem in the absence of
any electron mediator yields only traceamounts of nitrite (Figure 6, curve b). Also, illumination of TiOrPEI in the presence of the diffusional electron acceptor MV2+ and nitrate reductase leads to an inefficient production of NO2-, Figure 6, curve c. The rate of nitrite formation in the photosystem composed of T i O r MVP2+-PEI is ca. 8.5-fold higher than that in the photosystem that includes the diffusional electron mediator MV2+. Thus, the interfacial electron-transfer reaction at the semiconductor interface controls the biocatalyzed transformation. As MVP‘+PEI is effectively formed, the subsequent reduction of NO,-is also enhanced as compared to the TiOrPEI system that includes the diffusional electron mediator MV2+. The superior performance of the TiOrMVP2+-PEI colloid in driving the photoinduced reduction of nitrate using nitrate reductase as biocatalysthas encouragedus to design a macroscopic organized ensemble in which Ti02 powders are modified by the redox polymer MVP2+-PEI and the enzyme nitrate reductase is covalently linked to the photocatalyst. The organization of the electrically communicated photo-biocatalytic semiconductor assembly is schematically displayed in Scheme 11. PEI has been chemisorbed to the Ti02 powder. The resulting polymer-coated powder was cross-linked with glutaric dialdehyde to generate solid beads where Ti02 is encapsulated in the nondetachable polymer matrix. The PEI-modified particles were chemically modified by MVP2+(2), using 1-(3-(dimethylamino)propyl)-3ethylcdrbodiimide as coupling agent. The Ti02 particles generated in this way are coated by the redox-functionalized crosslinked polymer, MVP2+-PEL The enzyme was then coupled to the redox-functionalized polymer beads, using a bifunctional spacer, 4,4’-diisothiocyanatostilbene-2,2’-disulfonate(5), by a sequential process whereby the spacer units are linked to vacant polymer amino groups and protein lysine residues, respectively.27 The polymer-modified beads TiOrMVP2+-PEI covalently linked with nitrate reductase were illuminated (A > 300 nm) in the presence of nitrate. The rate of nitrite (NO*-) formation as a function of illumination time is displayed in Figure 7. When PEI cross-linked beads, without MVP2+ functionalization but with covalently-linked nitrate reductase, are used as a photocatalyst in the presence of thediffusional electron mediator MV2+, only trace amounts of NO2- were detected. Thus, organization of the biocatalyst-modified redox polymer around the Ti02 particles is crucial as a means to control the interfacial electron transfer and the subsequent biocatalyzed transformation. The chemically modified Ti02beads that include the covalently linked enzyme can be precipitatedand reused as a photoactive biocatalyst. Thus, these tailored redox-functionalized beads (which include the semiconductor particles and a linked biocatalyst, nitrate reductase) can be consideredas a model system for the construction of recyclable rigid photoactive semiconductor-biocatalytic assemblies that can be used for continuous light-driven biotransformations. Application of such functionalized beads as column supportsand in the production of valuablechemicalsin illuminated flow systems seems feasible. Conclusions Polyethylenimine is strongly associated with the Ti02 surface of colloids and powders. The polymer acts as a powerful surfaceactive agent for the stabilization of the Ti02 colloids. Chemical modification of the polymer by bipyridinium redox groups has allowed us to produce stable Ti02 colloids modified by the redox bipyridinium polymer MVPZ+-PEI. The redox-polymer-functionalized semiconductor colloid provides an organized microheterogeneous assembly where electron trapping across the semiconductor-solution interface by the polymer-anchored bipyridinium relay proceeds efficiently and competes effectively with degradative pathways involving electron-hole pair r e a m bination. The effective capture of a conduction-band electron is due to the high local cancentration of the electron acceptor MVP2+ onto the Ti02 surface by means of the polymer backbone.
Willner et al.
7270 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993
SCHEME II: Modification of a Ti02 Powder by the Cross-Linked Redox-Functionalized MVP2+-PEI Polymer and the Covalently-Linked Nitrate Reductase To Form Photoactive Semiconductor Biocatalytic Beads
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matrix. The cross-linked TiOrMVPz+-PEI particles, bearing covalently-linked nitrate reductase, constitute a photoactive materialwhich exhibitsbiocatalytic activity. Further experiments exploiting these unique properties of redox-functionalized PEI semiconductors are under way in our laboratory.
c 0
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Figure 7. Formation of nitrite (NO29 as a function of illumination time, X > 300 nm, of a system a t pH = 7.5 that includes 10 mg of Ti02 beads composed of cross-linked MVPZ+-PEI, [EDTA] = 5 X 10-3 M, and [KNO3] = 5 X le3M.
The polyethylenimine polymer, when associated with the Ti02 colloids or particles, provides a versatile interface for the developmentof novel photocatalysts: (i) It allowsthemodification of the polymer backbone by different redox components and thereby controls electron-transfer efficiencies at the semiconductor-solution interface. (ii) It provides chemical functionalities for cross-linking the polymer and allows the encapsulation of semiconductor particles within a rigid redox-functionalized polymer matrix. (iii) Because polyethylenimine is a polydentate polymer capable of complexing with metal ions, it can provide a general vehicle for reagent immobilization onto the semiconductor surface. Such catalyst-modified semiconductors should enhance the efficiencies of novel photocatalytic processes. (iv) Because polyethylenimineis positively charged in aqueous media at low pH, it can attract negatively charged molecules or surfaces. Of particular interest is the association of enzymes to redoxmodified PEI semiconductor colloids or particles as a means of generating novel photo-biocatalytic assemblies. In the present study, we have used polyethylenimine to tailor an organized photo-biocatalytic ensemble incorporating nitrate reductase. This assembly was constructed at the colloid level by electrostatic attraction of the enzyme to the electron conductive matrix Ti0rMVP2+-PEI and at the macroscopic particulate level by covalent attachment of the enzyme to the SC-polymer
Acknowledgment. This research was supported by a grant from the United States-Israel Binational Science Foundation (BSF) (I.W. and M.A.F.), and by the Office of Basic Energy Sciences, Division of Chemical Sciences,U.S. Departmentof Energy, under Contract DE-AC02-83CH 10093 (A.J.F.). References and Notes (1) (a) Homogeneous and HeterogeneousPhotocatalysis; Pelizzetti,E., Serpone, N., Eds.; D. Reidel Publishing: Dordrecht, 1986. (b) Fox, M. A. In Topics in Organic Electrochemistry;Fry, A. J., Britton, W. E.,Eds.; Plenum Press: New York, 1985; Vol. 4, p 177. (c) Memming, R. Top. Curr. Chem. 1988,143,79. (2) (a) Heller, A. Science 1984,223, 1141. (b) Wrighton, M. S . Pure Appl. Chem. 1985,57,57. (c) Pleskov, V. Prog. Surf.Sci. 1984,15,401. (d) Bard, A. J. Science 1980, 207, 139. (3) (a) Serpone, N.; Pelizzetti, E. Photocatalysis: Fundamentals and Applications; Wiley: New York, 1989. (b) Henglein, A. Top. Curr. Chem. 1988,143, 113. (4) (a) Bard, A. J. Ber. Bunsen-Ges. Phys. Chem. 1988.92, 1187. (b) Kalyanasundaram, K.; Griitzel, M.; Pelizzetti, E. Coord. Chem. Reu. 1986, 69, 57. (5) (a) Nakabayashi, S.;Kawai, T. In Photoinduced Electron Transfec Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Vol. 2, p 599. (b)
Griitzel, M. Heterogeneous Photochemical Electron Transfec CRC Press: Boca Raton, FL, 1989. (c) Energy Resources through Photochemistry and Catalysis; Grltzel, M., Ed.; Academic Press: New York, 1986. (6) (a) Griitzel, M. In Photoinduced Electron Transfer, Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Vol. 4, p 394. (b) Photogeneration of Hydrogen; Harriman, A., West, M. E., Eds.; Academic Press: London, 1983. (7) (a) Aurian-Blajeni, B.; Halmann, M.; Manassen, J. Sol. Energy 1980, 25, 165. (b) Halmann, M.; Ullman, M.; Aurian-Blajeni,B.So1. Energy 1983, 31,429. (c) Eggins, B. R.; Imine, J. T. S.;Murphy, E. J. Chem. Soc., Chem. Commun. 1988, 1123. (d) Henglein, A. Pure Appl. Chem. 1984,56, 1215. ( e ) Goren, Z.; Willner, I.; Nelson, A. J.; Frank, A. J. J. Phys. Chem. 1990, 94, 3784. (8) Willner, I.; Eichen, Y . J. Am. Chem. Soc. 1987, 109,6862. (9) Nosaka, Y.; Yamaguchi, A.; Kuwabara, A.; Miyama, H.; Baba, R.; Fujishima, A. J. Photochem. Photobiol., A: Chem. 1992,64, 375. (10) Nakahira, T.; Grltzel, M. J. Phys. Chem. 1984,88,4006. (1 1) (a) Nosaka, Y.;Fox, M. A. Lungmuir 1987,3, 1147. (b) Rajh, T.; Rabani, J. Lungmuir 1991,7,2057. (c) Frank, A. J.; Willner, I.; Goren, Z.; Degani, Y. J. Am. Chem. Soc. 1987,109, 3568. (1 2) (a) Willner, I.; Mandler, D.; Maidan, R. New J. Chem. 1987, 12, 109. (b) Mandler, D.; Willner, I. J. Chem. Soc., Perkin Trans. 2 1988,977.
Photoinduced Electron-Transfer Processes (13) (a) Willner, I.; Mandler, D. EnzymeMicrob. Technol. 1989,II,467. Willner, I.; Lapidot, N.; Rubin, S.;Riklin, A,; Willner, B. In Biotechnology: Bridging Research and Applications; Kamely, D., Chakrabarty, A. M., Kornguth, S. E., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; p 341. (14) (a) Mandler, D.; Willner, I. J . Chem. Soc., Perkin Trans. 2 1986, 805. (b) Maidan, R.; Willner, I. J . Am. Chem. SOC.1986,108, 1080. (15) (a) Willner, I.; Lapidot, N.; Riklin, A. J. Am. Chem. Soc. 1990, I 1 2, 6438. (b) Willner. I.: Laoidot. N. J. Am. Chem. Soc. 1991. 113. 3625. (c) Willnet. I.: Katz. E.:’Rikiin. A.: Kasher. R. J . Am. Chem. Soc. 1992. Ilk. 10965. ’(d) Willner,’I.; Kasher,’R.; Zahavy, E.; Lapidot, N. J. Am. Chem: SOC.1992,114, 10963. (16) Pileni, M. P.; Braun, A. M.; GrBtzel, M. Photochem. Photobiol. 1980,31, 423. (17) Willner, I.; Ford, W. E. J . Heterocycl. Chem. 1983, 20, 1113. (18) Meyers, W. E.; Royer, G. P. J . Am. Chem. Soc. 1977,99,6141.
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7271 (19) Trudinger, P. A. Anal. Biochem. 1970,36,222. (20) Snel, F. D.; Snel, C. T. Colorimetric Methods ofdnalysis; D. Van Nostrand New York, 1949; p 804. (21) Langmuir, I. J. Am. Chem. SOC.1918,40, 1361. (22) At pH = 5.1, onequarter of PEI amino groups are nonprotonated. These groups still enable the association of the polymer to Ti02 despite the electrostatic repulsive interactions between the positively charged semiconductor surface and the polymer backbone. (23) Barringer, E. A,; Bowen, H. K. Lungmuir 1985, I , 420. (24) Duonghong, D.; Ramsden, J.; GrHtzel, M. J. Am. Chem. Soc. 1982, 104,2911. (25) Willner, I.; Lapidot, N.; Riklin, A. J . Am. Chem. Soc. 1989,I l l , 1883. (26) Steiner, S.X.;Downey, R. J. Biochim. Biophys. Acta 1982,706,203. (27) For a similar procedure for covalent linkage of enzymes to electrodes, see: Katz, E.; Riklin, A.; Willner, I. J . Electroanal. Chem., in press.
