Layered Inorganic Materials as Redox Agents: Ferrocenium

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Layered Inorganic Materials as Redox Agents: Ferrocenium-Intercalated Zirconium Phosphate Mitk’El B. Santiago, Chasterie Declet-Flores, Agustıń Dıáz, Meredith M. Ve´lez, Myrna Zoe´ Bosques, Yannis Sanakis,‡ and Jorge L. Coloń*,† Department of Chemistry, P.O. Box 23346, UniVersity of Puerto Rico, Rıó Piedras, P.R. 00931-3346, and Institute of Materials Science, NCSR “Demokritos”, 15310 Ag. ParaskeVi, Attiki, Greece ReceiVed February 22, 2007. In Final Form: April 17, 2007 The direct intercalation reaction of ferrocene (bis(η5-cyclopentadienyl)iron(II), Fc) with a highly hydrated layered zirconium phosphate (ZrP) resulted in the formation of the ferrocenium ion (Fc+) within the ZrP material. The Fc+-intercalated ZrP material has an interlayer distance of 10.7 Å. The intercalated material was used as an electron acceptor for the oxidation of both ferro-cytochrome c and the excited state of tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy)3]2+). Upon contact of the material with a 1.5 × 10-5 M solution of ferro-cytochrome c, the UV-vis absorption spectrum shows the successful formation of ferri-cytochrome c. Luminescence spectroscopy shows that the Fc+intercalated ZrP material quenches the luminescence of [Ru(bpy)3]2+. The excited-state quenching mechanism of [Ru(bpy)3]2+* by Fc+-intercalated ZrP follows a dynamic plus sphere of action model. The second-order dynamic quenching rate constant kq is 2.2 × 108 M-1 s-1.

Introduction The incorporation of inorganic complexes into rigid-framework matrices has been an intensive area of research.1-4 There is a wide variety of applications for these new modified materials that range from new kinds of biosensors to artificial photosynthesis.2,4-6 Among these rigid-framework matrices are layered zirconium phosphates and their derivatives. Zirconium bis(monohydrogen orthophosphate) monohydrate (Zr(HPO4)2‚H2O, R-ZrP) is the best characterized zirconium phosphate (ZrP), and it has been used for the immobilization of several photo- and redox-active compounds.7-11 Recently, Martı´ and Coloń intercalated tris(2,2′-bipyridyl)ruthenium(II) ([Ru(bpy)3]2+) in ZrP without a preintercalation step by direct ion exchange into a hydrated form of ZrP called the 10.3 Å ZrP phase (10.3 Å-ZrP).12-14 The use of the 10.3 Å ZrP phase is an alternative to the more commonly used preintercalation method in which polar organic molecules, such as alcohols, glycols, and amines, are used as a means of preexpanding the interlayer separation before intercalation of the compound of interest.15,16 The direct ion-exchange method using * To whom correspondence should be addressed. E-mail: jlcolon@ uprrp.edu. † University of Puerto Rico. ‡ NCSR “Demokritos”. (1) Mohanambe, L.; Vasudevan, S. Inorg. Chem. 2005, 44, 2128-2130. (2) Kim, Y.; Das, A.; Zhang, H.; Dutta, P. K. J. Phys. Chem. B 2005, 109, 6929-6932. (3) Kickelbick, G. Prog. Polym. Sci. 2003, 28, 83-114. (4) Rolison, D. R. Chem. ReV. 1990, 90, 867-878. (5) Hashimoto, S. J. Photochem. Photobiol., C 2003, 4, 19-49. (6) Pessoa, C. A.; Gushikem, Y.; Kubota, L. T.; Gorton, L. Electroanal. Chem. 1997, 431, 23-27. (7) Troup, J. M.; Clearfield, A. Inorg. Chem. 1977, 16, 3311-3314. (8) Clearfield, A. Prog. Inorg. Chem. 1998, 47, 371-509. (9) Alberti, G. Acc. Chem. Res. 1978, 11, 163-170. (10) Alberti, G.; Costantino, U. J. Mol. Catal. 1984, 27, 235-250. (11) Kumar, C. V.; Bhambhani, A.; Hnatiuk, N. In Handbook of Layered Materials; Auerback, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker, Inc.: New York, 2004. (12) Martı´, A. A.; Coloń, J. L. Inorg. Chem. 2003, 42, 2830-2832. (13) Clearfield, A.; Nancollas, G. H.; Blessing, R. H. In Ion Exchange and SolVent Extraction; Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1973; Vol. 5. (14) Clearfield, A.; Duax, W. L.; Medina, A. S.; Smith, G. D.; Thomas, J. R. J. Phys. Chem. 1969, 73, 3424-3430.

10.3 Å-ZrP has been used to ion exchange 1-aminomethylpyrene, bis(1,10-phenanthroline-5,6-dione)(2,2′-bipyridine)ruthenium(II), and methyl viologen.17-20 Ferrocene (bis(η5-cyclopentadienyl)iron(II), Fc) has been an intensively studied organometallic compound ever since its first reported synthesis in 1951.21-24 Its low inner-sphere reorganization energy for electron transfer (ET) makes Fc ideal as a redox agent in protein ET studies,25-28 sensing,29 and artificial photosynthesis applications.21,30,31 Fc has been immobilized in zeolites32-34 and into a variety of layered materials such as clays, CdPS3,35 porous silica materials,36 vanadium phosphates,37,38 FeOCl,39,40 and transition metal dichalcogenides,41 among others. (15) Clearfield, A. Chem. ReV. 1988, 88, 125-148. (16) Clearfield, A.; Tindwa, R. M. J. Inorg. Nucl. Chem. 1979, 41, 871-878. (17) Bermu´dez, R. A.; Coloń, Y.; Tejada, G. A.; Coloń, J. L. Langmuir 2005, 21, 890-895. (18) Bermu´dez, R. A.; Arce, R.; Coloń, J. L. J. Photochem. Photobiol., A 2005, 175, 201-206. (19) Santiago, M. B.; Ve´lez, M. M.; Borrero, S.; Dıáz, A.; Casillas, C. A.; Hofmann, C.; Guadalupe, A. R.; Coloń, J. L. Electroanalysis 2006, 18, 559-572. (20) Martı´, A. A. Ph.D. Dissertation, University of Puerto Rico, 2004. (21) Fery-Forgues, S.; Delavaux-Nicot, B. J. Photochem. Photobiol., A 2000, 132, 137-159. (22) Kealy, T. J.; Pauson, P. L. Nature 1951, 168, 1039-1040. (23) Miller, S. A.; Tebboth, J. A.; Tremaine, J. F. J. Chem. Soc. 1952, 632635. (24) Sohn, Y. S.; Hendrickson, D. N.; Gray, H. B. J. Am. Chem. Soc. 1971, 93, 3603-3612. (25) Carney, M. J.; Lesniak, J. S.; Likar, M. D.; Pladziewicz, J. R. J. Am. Chem. Soc. 1984, 106, 2565-2569. (26) Hwang, H. J.; Carey, J. R.; Brower, E. T.; Gengenbach, A. J.; Abramite, J. A.; Lu, Y. J. Am. Chem. Soc. 2005, 127, 15356-15357. (27) Gleria, K. D.; Nickerson, D. P.; Hill, A. O.; Wong, L.-L.; Fu¨lo¨p, V. J. Am. Chem. Soc. 1998, 120, 46-52. (28) van Staveren, D. R.; Metzler-Nolte, N. Chem. ReV. 2004, 104, 59315985. (29) Heller, A. J. Phys. Chem. 1992, 96, 3579-3587. (30) Xia, X.-B.; Ding, Z.-F.; Liu, J.-Z. J. Photochem. Photobiol., A 1995, 86, 85-88. (31) Xia, X.-B.; Ding, Z.-F.; Liu, J.-Z. J. Photochem. Photobiol., A 1995, 88, 81-84. (32) Kaiser, C. T.; Gubbens, P. C. M.; Kemner, E.; Overweg, A. R.; Jayasooriya, U. A.; Cottrell, S. P. Chem. Phys. Lett. 2003, 381, 292-297. (33) Ellison, E. H. J. Phys. Chem. B 1999, 103, 9314-9320. (34) Hashimoto, S.; Hagiri, M.; Barzykin, A. V. J. Phys. Chem. B 2002, 106, 844-852. (35) Bal, B.; Ganguli, S.; Bhattacharya, M. Physica 1985, 133B, 64-70. (36) Halbert, T. R.; Johnston, D. C.; McCandlish, L. E.; Thompson, A. H.; Scanlon, J. C.; Dumesic, J. A. Physica B+C 1980, 99, 128-132.

10.1021/la7005309 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007

Layered Inorganic Materials as Redox Agents

In several of these materials, the intercalation of Fc promotes its oxidation to ferrocenium (Fc+).35,36,39,40 Many of the Fcimmobilized materials have been used as electron donors in intermolecular electron transfer (ET) systems.21,30,31 Xia et al. demonstrated that Fc reductively quenches and Fc+ ion oxidatively quenches the excited states of [Ru(bpy)3]2+ and tris(1,10phenanthroline)ruthenium(II) ([Ru(phen)3]2+) in solution.30,31 Fery-Forgues and Delavaux-Nicot recently published a review of Fc and Fc+ as quenchers in solution as well as immobilized in a variety of environments.21 Rosenthal and Caruso used the preintercalation method with butylamine in an attempt to immobilize Fc into ZrP, but they were unsuccessful.42 In contrast, they were able to exchange Fc+ into butylamine-intercalated ZrP using FeCl3 in the intercalation reaction.43 Rosenthal and Caruso observed that the interlayer distance increased from 7.6 Å in unintercalated R-ZrP to 11.8 Å in Fc+-intercalated ZrP. However, these authors did not study the redox activity of the intercalated organometallic complex. Fc+ is not stable in neutral or basic aqueous solutions,44 undergoing rapid decomposition in these media. Recently, Hwang et al. proposed the incorporation of Fc inside proteins26 as a way to stabilize Fc+ by isolating it from the solvent. These authors covalently attached Fc to the active site of copper-free azurin (Az) and showed that the redox active FcAz protein was stable at different pHs. Furthermore, the Fc+ species was stabilized, and the Fc+-modified protein was able to oxidize cytochrome c.26 We report here that the direct intercalation reaction of Fc with 10.3 Å-ZrP produces Fc+-intercalated ZrP (Fc+/ZrP).12,17-19 The intercalated Fc+ is stabilized within ZrP, remains electroactive, and is capable of oxidizing cytochrome c and quenching the excited state of [Ru(bpy)3]2+ in aqueous solution. Experimental Procedures Reagents. Ferrocene, zirconyl chloride octahydrate (ZrOCl2‚8H2O, 98%), tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate, horseheart cytochrome c, sodium hydrogen phosphate, disodium phosphate, and phosphoric acid were obtained from Sigma-Aldrich. Nanopure water was obtained using a Barnstead purification train (17.5 MΩ/ cm). Capillary tubes were obtained from Kimble products. Procedures. Synthesis of 10.3 Å-ZrP. A volume of 120 mL of 0.05 M ZrOCl2‚8H2O was mixed with 85 mL of 6 M H3PO4 with constant stirring at 94 °C for 48 h. The precipitated solid was filtered and washed three times with nanopure water. The wet precipitate was characterized using X-ray powder diffraction (XRPD); the lowest angle diffraction peak at 2θ ) 8.6° is characteristic for the expected interlayer distance of 10.3 Å in this phase of ZrP. Intercalation Reaction of Fc with ZrP. The intercalation reaction of Fc with ZrP was conducted by suspending ZrP in ethanolic solutions of Fc at 1:20, 1:10, 1:1, and 5:1 Fc/ZrP molar ratios with constant stirring and at ambient temperature, producing materials at different loading levels. Afterward, they were filtered, washed with ethanol until a clear supernatant was obtained, and then dried in air for 48 h. The loading levels of the samples are referred according to the concentration ratio of the mixture used in their preparation. The (37) Davidson, A.; Villeneuve, G.; Fourne´s, L.; Smith, H. Mater. Res. Bull. 1992, 27, 357-366. (38) Matsubayasbi, G.; Ohta, S.; Okuno, S. Inorg. Chim. Acta 1991, 184, 47-52. (39) Palvadeau, P.; Coıć, L.; Rouxel, J.; Meńil, F.; Fourne´s, L. Mater. Res. Bull. 1981, 16, 1055-1065. (40) Schafer-Stahl, H. Mater. Res. Bull. 1980, 15, 1091-1097. (41) Dines, M. B. Science 1975, 188, 1210-1211. (42) Rosenthal, G. L.; Caruso, J. J. Solid State Chem. 1991, 93, 128-133. (43) Rosenthal, G. L.; Caruso, J. Inorg. Chem. 1992, 31, 144-145. (44) Holecek, J.; Handlir, K.; Pavlik, I. Collect. Czech. Chem. Commun. 1972, 37, 1805-1815.

Langmuir, Vol. 23, No. 14, 2007 7811 color of the initial reaction mixtures was orange-yellow, characteristic of Fc. After a period of 5 days, the suspensions were filtered and the solids were dried overnight. The recovered solids had the characteristic blue color of Fc+. Modified Carbon Paste Electrode (CPE) Preparation. Modified CPEs were prepared by thoroughly mixing 20 mg of the Fc+/ZrP material at the specified loading levels, 100 mg of graphite powder, and 75 µL of mineral oil to form a uniform paste suitable for molding the CPEs. Capillary tubes (2 mm diameter) were used to construct the modified CPEs. The electrical contact was done with a copper wire. Oxidation of Ferro-Cytochrome c by Fc+/ZrP. The reaction of cytochrome c with Fc+/ZrP was made as follows. A 1.7 × 10-4 M PBS solution of horse-heart cytochrome c was reduced with dithionite to form ferro-cytochrome c. The ferro-cytochrome c was purified using a PD-10 chromatographic column. The concentration of the ferro-cytochrome c was diluted to a concentration of 1.5 × 10-5 M as determined by UV-vis spectrophotometry (mM (550 nm) ) 29.5). An aqueous suspension of Fc+/ZrP (0.008% w/v) was prepared and then added to the solution of ferro-cytochrome c. The oxidation reaction was monitored using UV-vis spectrophotometry. Luminescence Quenching of Ru(bpy)32+ by Fc+/ZrP. A 4.4 × 10-5 M [Ru(bpy)3]2+ aqueous stock solution was prepared. Different amounts of the 1:1 Fc+/ZrP loading level material were weighed and added to 5 mL volumetric flasks. Aliquots of the stock solution were then added to these flasks to prepare the 5 mL solutions used in the quenching experiments. Luminescence spectra and luminescence lifetime measurements of the samples with different 1:1 Fc+/ ZrP concentrations were obtained. The quenching measurements were performed using front-phase illumination to reduce scattering from the Fc+/ZrP suspensions. The luminescence decay transients were normalized and were successfully fitted to the monoexponential decay law: It ) I∞ + Ate-kt, where It is the intensity of the emission at any time, I∞ is the intensity of the emission at infinite time, At is a preexponential factor, and k is the bimolecular quenching rate constant. Instrumentation. UV-vis absorption spectra were obtained using a HP 8453 diode array spectrophotometer. Diffuse reflectance spectra were obtained using a Cary 1E UV-vis spectrophotometer. XRPD measurements were performed using a Siemens D5000 X-ray diffractometer with Cu KR radiation (λ ) 1.5406 Å) with a filtered flat LiF secondary beam monochromator. IR spectroscopy measurements were performed using a MAGNA-IRTM spectrometer model 750 from Nicolet with a deuterated triglycine sulfate (DTGS) detector, and the spectra were analyzed using OMNIC software (version 4.2). Mo¨ssbauer spectra were recorded at 78 K on a spectrometer operating in constant acceleration mode using an Oxford cryostat and a 57Co source in rhodium. Isomer shifts are reported relative to metallic iron at room temperature. X-ray photoelectron spectroscopy (XPS) analyses were performed at the Materials Characterization Center at the University of Puerto Rico using the Physical Electronics PHI 5600 ESCA system model. Primary excitation was provided by a monochromatic aluminum source biased at 58.7 kV and a power setting of 350 W. The spectra were collected using a fixed analyzer transmission mode on a hemispherical electroanalyzer. The carbon 1s signal at 284.7 eV was used as the internal reference. All electrochemical measurements were performed with a BAS model CV-50W potentiostat in a 5 mL cell containing the modified CPE, a Ag/AgCl (3 M NaCl) reference electrode, and a Pt wire auxiliary electrode. The polyelectrolyte solution was prepared using a phosphate buffer solution (PBS, µ ) 0.1) at pH 7.0. The electrochemical results are reported as the average of three electrodes. Luminescence measurements were performed on a SE-900 spectrofluorometer (Photon Technology International, PTI) using a 150 W xenon lamp as the excitation source and a PTI model 710 photon counting detector with a Hamamatsu R1527P photomultiplier. Time-resolved luminescence measurements were performed at an excitation wavelength of 337 nm using an Oriel lifetime spectrometer with a nitrogen laser (model VSL-337ND-S, full width at half-maximum < 10 ns) as the excitation source. An Oriel 70680

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Figure 1. Diffuse reflectance spectra for the Fc powder and the 1:1 Fc+/ZrP material. photomultiplier was used in the analog detector mode, interphased to a digital oscilloscope (Tektronix TDS3032B model), and the decay transients were analyzed using Origin 7.0. The luminescence decay transients were normalized and were successfully fitted using the monoexponential decay function: It ) I∞ + Ae-kt.

Figure 2. Experimental (circles) 57Fe Mo¨ssbauer spectra of (from top to bottom) ferrocene powder, ferrocenium hexafluorophosphate, and 1:1 Fc/ZrP recorded at 78 K. The solid line in the Fc spectrum is a theoretical spectrum assuming one quadruple doublet with δ ) 0.54(1) mm/s and quadrupole splitting ∆EQ ) 2.36(2) mm/s.

Results and Discussion Figure 1 shows the diffuse reflectance spectrum of Fc powder and of the product of the intercalation reaction of Fc with ZrP at a 1:1 Fc/ZrP molar ratio. The diffuse reflectance spectrum of Fc in powder form shows the characteristic broad band at 450 nm.21,24 The intercalation reaction of Fc with ZrP produced a blue colored solid material with an absorption band at 619 nm. This spectrum is similar to that of Fc+ in aqueous solution, which shows an absorption band at 617 nm,24,45 suggesting that Fc+ was formed between the layers during the intercalation process. To corroborate this hypothesis, we performed 57Fe Mo¨ssbauer experiments on the 1:1 Fc/ZrP material. Figure 2 shows the 57Fe Mo¨ssbauer spectra for Fc, for ferrocinium hexafluorophosphate, and for the 1:1 Fc/ZrP material. The Fc and Fc+ spectra are in agreement with those in the literature.46-48 The Fc spectrum exhibits a quadrupole doublet with parameters (isomer shift δ ) 0.54(1) mm/s and quadruple splitting ∆EQ ) 2.36(2) mm/s) characteristic of the presence of iron(II), while the spectrum of ferrocenium hexafluorophosphate shows a broad singlet centered at ∼0.5 mm/s. The Fc/ZrP sample is noisy due to the low iron content because of the diluted nature of this sample. However, a quadrupole doublet attributable to the Fc species is clearly missing; on the contrary, the Fc/ZrP spectrum is consistent with the Fc+ spectrum. Similar results were reported for ferrocene intercalated in CdPS3, in porous Vycor glass, and in MCM-41.35,49,50 Therefore, the appearance of both the Fc+ characteristic absorption band in the spectrum of the product of the intercalation reaction between Fc and 10.3 Å ZrP and the Mo¨ssbauer spectra indicates that the acidic environment of ZrP induces the formation of Fc+ in the intercalation reaction, as observed for Fc oxidation in acidic solutions.21,43,45,51,52 (45) Sohn, Y. S.; Hendrickson, D. N.; Gray, H. B. J. Am. Chem. Soc. 1970, 92, 3233-3234. (46) Wertheim, G. K.; Herber, R. H. J. Chem. Phys. 1963, 38, 2106-2111. (47) Fluck, E. Chemical Applications of Mo¨ssbauer Spectroscopy; Academic Press: New York, 1968. (48) Collins, R. L. J. Chem. Phys. 1965, 42, 1072-1080. (49) Toda, Y.; Ishimaru, S.; Ikeda, R.; Mitani, T.; Kitao, S.; Seto, M. J. Phys. Chem. Solids 2004, 65, 471-473. (50) Schnitzler, M. C.; Mangrich, A. S.; Macedo, W. A. A.; Ardisson, J. D.; Zarbin, A. J. G. Inorg. Chem. 2006, 45, 10642-10650.

Figure 3. XRPD patterns for the Fc+-intercalated ZrP materials at various molar ratios and the 10.3 Å phase of ZrP. The interlayer distance is indicated next to the first-order diffraction peak.

We performed X-ray powder diffraction (XRPD) experiments to determine if the Fc+ ions produced in the intercalation reaction of Fc with ZrP are intercalated or mainly surface bound. Figure 3 shows the XRPD patterns of dry products of the intercalation reaction of Fc with ZrP at various Fc/ZrP molar ratios. The patterns become sharper as the Fc+/ZrP concentration ratio increases, as expected when a more ordered phase is produced.12 The intercalation reactions with ZrP are known to be topotactic;53 the diffraction peak at the lowest 2θ angle corresponds to the interlayer distance. The XRPD pattern of the sample with the lowest loading level (1:20 Fc+/ZrP) shows the formation of a new phase with an interlayer distance of 9.8 Å. This new phase corresponds to ethanol-intercalated ZrP,54 as expected since the intercalation reaction was performed in Fc ethanolic solutions. If some Fc+ has been intercalated at this low loading level, its amount is not enough to form a new Fc+-intercalated ZrP phase. More likely, any Fc+ present is mainly surface bound. However, (51) Wilkinson, G. J. Organomet. Chem. 1975, 100, 273-278. (52) Honma, I.; Zhou, H. S. Chem. Mater. 1998, 10, 103-108. (53) Backov, R.; Bonnet, B.; Jones, D. J.; Rozie`re, J. Chem. Mater. 1997, 9, 1812-1818. (54) Costantino, U. J. Chem. Soc., Dalton Trans. 1979, 402-405.


Langmuir, Vol. 23, No. 14, 2007 7813 Table 1. Molar Ratios Obtained from the XPS Data Analysis for the Different Fc+/ZrP Materials at Various Loading Levelsa molar ratios of intercalated product Fc+/ZrP loading level

P/Zr ( 0.02

Fe/Zr ( 0.01

5:1 1:1 1:10 1:20

2.23 2.18 2.19 2.25

0.15 0.18 0.04 0.02

a

Figure 4. IR spectra of ferrocinium hexafluorophosphate, ferrocene, ZrP, and Fc/ZrP materials at various loading levels.

as the loading level increases, the patterns show the disappearance of the ethanol-intercalated ZrP phase and the appearance of another phase with an interlayer distance of 10.7 Å, corresponding to the interlayer distance of Fc+-intercalated ZrP. This interlayer distance of the intercalated material is not very different from that of the wet unintercalated starting material (10.3 Å). However, this increase in distance is for the dry intercalated material. Unintercalated wet 10.3 Å-ZrP collapses into R-ZrP (with an interlayer distance of 7.6 Å) upon drying, and therefore, the observed 10.7 Å interlayer distance of the intercalated material indicates that the Fc+ ions have displaced the ethanol and water molecules and expanded the interlayer distance of ZrP by 3.1 Å compared to unintercalated R-ZrP. At these high concentrations, the Fc+ ions can serve as pillars inside the ZrP framework.12 Further confirmation that the 10.7 Å interlayer distance corresponds to that of the Fc+-intercalated material and not to the ZrP material with surface bound Fc+ was obtained from Fourier transform infrared (FTIR) measurements. Figure 4 shows the IR spectra of ferrocinium hexafluorophosphate, Fc, ZrP, and the intercalated materials at various loading levels. The ZrP lattice water vibrational bands that appear at 3580, 3494, and 1610 cm-1 in the unintercalated material diminish in intensity upon intercalation of Fc+, in agreement with previous results by Horsley et al.55 These authors observed that drying R-ZrP removes lattice water, and therefore, the lattice water bands disappear and a broad band at 3400 cm-1 appears.55 Similar results have also been observed by Matsubayasbi et al. in Fc-intercalated R-vanadyl phosphate (another layered phosphate material somewhat similar to ZrP);38,56 intercalation of Fc displaced water, the vanadyl phosphate lattice water vibrational bands disappeared, and a broad band at 3400 cm-1 appeared. Therefore, the results presented in Figure 4 indicate that Fc+ intercalation displaces water from the ZrP lattice. In addition, Figure 4 shows that the band from the vibration of the exchangeable proton of the phosphate group observed as a shoulder at 1074 cm-1 in the IR spectrum of the unintercalated ZrP material is dramatically reduced in the IR spectra of Fc+/ZrP. This reduced intensity is expected since these protons are the ones exchanged upon intercalation with Fc+,55 confirming that intercalation has occurred. Most of the Fc+ vibrational bands in the intercalated ZrP materials are shifted to higher energies compared to the spectra of ferrocinium hexafluorophosphate and Fc.57-59 This shift is (55) Horsley, S. E.; Nowell, D. V.; Stewart, D. T. Spectrochim. Acta 1974, 30A, 535-541. (56) Matsubayasbi, G.; Ohta, S. Chem. Lett. 1990, 787-790.

Values are given as mean ( 1 SD.

particularly evident in the 804 cm-1 band in unintercalated Fc+, from the bending vibrations of the C-H bonds of the cyclopentadienyl (Cp) ligands,57-59 which shifts to 838 cm-1 in the ZrP intercalated material. If the planes of the Cp ligands in Fc+ are perpendicular to the ZrP layers, the layers would restrict the C-H bending motions, and the vibrational energy would increase, as observed in Figure 4. Similar shifts to higher energies in vibrational bands were observed by Mohanambe and Vasudevan when they trapped Fc inside cyclodextrins in a cyclodextrinfunctionalized layered double hydroxide,1 where the inclusion of Fc in the cyclodextrins restricted the Cp vibrational motions. Therefore, the XRPD and IR results are consistent with the formation of Fc+-intercalated material upon reaction of Fc with ZrP. Contrary to the 10.7 Å interlayer distance of the Fc+intercalated ZrP material obtained by us using the direct intercalation method, Rosenthal and Caruso observed an interlayer distance of 11.8 Å for Fc+-intercalated ZrP prepared using the butylamine preintercalation method.43 The difference in interlayer distance is due to the different intercalation procedures used. In the direct intercalation procedure, the resulting intercalated Fc+ ions are only surrounded by the phosphate groups of ZrP and by water, whereas in the butylamine preintercalation procedure some butylamine molecules could remain and increase the interlayer distance. X-ray photoelectron spectroscopy (XPS) data were used to obtain the elemental analyses of the samples. As can be observed in Table 1, the XPS data show that the amount of Fc+ ions inside zirconium phosphate reaches a maximum in the 1:1 Fc+/ZrP molar ratio sample. However, the experimental P/ZrP molar ratios are in disagreement with that expected based on the structure of an R-ZrP-like framework, whose P/ZrP molar ratio should be 2:1.7 This disagreement is due to the synthetic procedure for 10.3 Å ZrP in which a slight excess of phosphoric acid is used, resulting in materials with high P/ZrP ratios. Clearfield et al. also demonstrated that, upon intercalation of cations, although the integrity of the layers is maintained,14,60 the layers tend to move relative to each other to accommodate the cations. As a consequence, different phases of zirconium phosphate are formed relative to each other in a different way than in R-ZrP.14,60 However, if this happens to our materials, this shift of the layers could lead to the formation of new peaks in the XRPD patterns, which was not observed as shown in Figure 3. An estimate of the preferred arrangement of Fc+ ions in ZrP produced by the direct intercalation method can be obtained by considering the XRPD and XPS results, as well as the dimensions of Fc+ (6.8 Å × 5.65 Å × 5.65 Å).41 Previous investigations (57) Lippincott, E. R.; Nelson, R. D. Spectrochim. Acta 1958, 10, 307-329. (58) Wilkinson, G.; Pauson, P. L.; Cotton, F. A. J. Am. Chem. Soc. 1954, 76, 1970-1974. (59) Bodenheimer, J. S.; Low, W. Spectrochim. Acta, Part A 1973, 29, 17331743. (60) Clearfield, A.; Landis, A. L.; Medina, A. S.; Troup, J. M. J. Inorg. Nucl. Chem. 1973, 35, 1099-1108.

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Scheme 1. Schematic Representation of the Ferrocene Reaction with ZrP

have shown that the preferred arrangement of Fc+ ions in layered vanadyl phosphate is with the Cp rings perpendicular to the layers.38,39 In addition, Thompson et al. demonstrated by NMR experiments that the Cp rings of Fc+ in Fc+-intercalated ZrP prepared by the preintercalation method43 are also perpendicular to the ZrP layers.61 Considering that the width of a ZrP layer is 6.6 Å,7,62 the minimum distance that Fc+ would expand the ZrP layers is to an interlayer distance of 12.25 Å (6.6 Å + 5.65 Å) if the Fc+ Cp rings are perpendicular to the ZrP layers. However, in ZrP, the zirconium atoms form near-perfect planes having bridging phosphate groups above and below the plane of the zirconium atoms.63 The phosphorus atom in ZrP is approximately 1.6 Å away from the mean plane of the zirconium phosphate and 5.3 Å away from the neighboring phosphate groups.62 Therefore, nothing precludes the Cp rings from partially penetrating between the phosphate groups reducing the expected interlayer distance in the material, as observed in Scheme 1.19 We can calculate the amount of Fc+ per formula unit of ZrP for the sample with the full loading of Fc+ inside ZrP. The dimensions of the Fc+ ion, which can be considered a cylinder with dimensions of 6.8 Å × 5.65 Å,41 has a cross-sectional area of 38.42 Å2 if the Cp ligands are perpendicular to the layers. Using this cross-sectional area and the area of a ZrP formula unit (24 Å),7,62 a theoretical amount of 0.62 Fc+ ions per ZrP formula unit is obtained. The experimental amount of 0.18 Fc+ ions per ZrP formula unit obtained from the XPS data is in disagreement with this theoretical amount. Similar results have been previously observed for the immobilization of ferrocinium in vanadyl phosphates and in R-ZrP (by the preintercalation method), where the content of ferrocinium inside the layered material was lower than expected.38,64 To determine if the Fc+ ions remain redox active upon intercalation in ZrP, cyclic voltammetry measurements were performed. We prepared separate carbon paste electrodes (CPEs) modified with Fc, ferrocinium hexafluorophosphate, or Fc+/ZrP at different loading levels. Figure 5 shows the cyclic voltammograms of the Fc-, FcPF6-, and Fc+/ZrP-modified CPEs. The cyclic voltammograms observed for the Fc- and Fc+-modified CPEs are the expected characteristic ones for these species.65,66 The Fc-modified CPE shows a formal potential of 335 mV, (61) Lee, C. F.; Myers, L. K.; Valentine, K. G.; Thompson, M. E. J. Chem. Soc., Chem. Commun. 1992, 201-203. (62) Yang, C.-Y.; Clearfield, A. React. Polym., Ion Exch., Sorbents 1987, 5, 13-21. (63) Alberti, G.; Costantino, U.; Gill, J. S. J. Inorg. Nucl. Chem. 1976, 38, 1733-1738. (64) Kalousova, J.; Votinsky, J.; Benes, L.; Melanova, K.; Zima, V. Collect. Czech. Chem. Commun. 1998, 63, 1-19. (65) Page, J. A.; Wilkinson, G. J. Am. Chem. Soc. 1952, 74, 6149-6150. (66) Gale, R. J.; Singh, P.; Job, R. J. Organomet. Chem. 1980, 199, C44C46.

Figure 5. Cyclic voltammograms for the modified CPEs with (top) Fc and Fc(PF6) and with (bottom) Fc+/ZrP at different loading levels in PBS (µ ) 0.1 and pH ) 7.0) at 100 mV/s.

while the Fc+-modified CPE has a formal potential of 359 mV at pH 7.0.66 The cyclic voltammogram of ferrocene is known to depend on the nature of the solvent and the polyelectrolyte solution.67 We observed that the formal potential of the Fc+/Fc couple for the Fc-modified CPE is lower than that for the Fc+modified CPE. Another observed feature is that the shape of the cyclic voltammogram of the Fc+-modified CPE is different compared to that of the Fc-modified CPE. These experiments were performed in aqueous solution, and it is known that in aqueous solution the Fc+ species decomposes. A shoulder at ∼75 mV corresponding to this decomposition product can be observed in the cathodic voltammetric wave of the Fc+-modified CPE.65 Previous reports have shown that when ferrocene is incorporated in a hydrophobic matrix, the formal potential increases, because the ferrocinium ions are destabilized. For example, when Fc is immobilized in β-cyclodextrin or in dendrimers, an increase in the formal potential is observed.68,69 Hwang et al. observed that upon incorporation of ferrocene into the active site of apoazurin (FcAz), the formal potential increases to 107 mV at pH 4.26 However, these authors also observed a decrease in the formal potential with an increase in the pH of the FcAz solution. Increasing the pH deprotonated the amino acid residues surrounding the Fc+ species in the FcAz active site and stabilized the Fc+ species, resulting in a 84 mV decrease in the formal potential.26 (67) Zanello, P. Inorganic Electrochemistry: Theory, Practice and Application; The Royal Society of Chemistry: Cornwall, U.K., 2003. (68) Matsue, T.; Evans, D. H.; Osa, T.; Kobayashi, N. J. Am. Chem. Soc. 1985, 107, 3411-3417. (69) Cardona, C. M.; Kaifer, A. E. J. Am. Chem. Soc. 1998, 120, 4023-4024.


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Figure 6. UV-vis absorption spectra of cytochrome c in the absence and presence of various concentrations of Fc+/ZrP. Table 2. Electrochemical Parameters for CPEs Modified with Fc+/ZrP at Different Loading Levelsa Fc+/ZrP loading level

E°′ (mV)

∆Ep (mV)

ipc/ipa

5:1 1:1 1:10 1:20

318.0 323.0 316.5 276.0

60 56 67 74

1.02 1.05 1.40 1.92

a

Figure 7. Luminescence spectra of a deaerated 5 × 10-5 M [Ru(bpy)3)]2+ aqueous solution in the absence and presence of various concentrations of the 1:1 Fc+-intercalated ZrP material.

Scan rate ) 100 mV/s in PBS (µ ) 0.1 and pH ) 7.0).

For CPEs modified with Fc+/ZrP materials, the cyclic voltammograms showed the expected characteristic voltammetric wave of the Fc+/Fc redox pair, indicating that upon intercalation Fc+ remains redox active. The electrochemical parameters are shown in Table 2. Increasing the amount of Fc+ ions inside the ZrP matrix from 1:20 Fc+/ZrP to 1:1 Fc+/ZrP, the formal potential increases from 276 to 323 mV. However, the formal potential for 1:1 Fc+/ZrP decreases almost 36 mV in comparison with Fc+-modified CPE. The decrease in the formal potential observed in the Fc+/ZrP materials is due to the nature of the ZrP layers. The ZrP layers are negatively charged; for Fc+/ZrP materials with maximum loading, the Fc+ ions are stabilized by the anionic environment of the phosphate groups, resulting in the observed decrease in the formal potential, in a similar fashion to the decrease in the formal potential observed in FcAz at high pHs. The electrochemical reversibility can be evaluated by measuring the peak current ratios. The electron transfer between the electrode and the immobilized Fc+ is maximized as the concentration of Fc+ increases, as can be observed in the peak current ratios that reach a minimum of ∼1.02. Similar behavior was observed in FcAz. Hwang et al. observed that the electrochemical reversibility of Fc+ in FcAz is increased at high pHs, when the amino acids are deprotonated and can compensate the charge to achieve electroneutrality.26 In our materials, the phosphate groups are stabilizing the Fc+ ion. Therefore, Fc+ is stabilized inside the ZrP galleries as demonstrated by the peak current ratios. As a result, the cyclic voltammograms have a nernstian behavior. The small reorganization energy of the Fc+/ Fc pair also promotes fast electron transfer to the electrode surface.70 To elucidate if the Fc+-intercalated ZrP material can serve as a redox agent, we studied its redox reactions with cytochrome c (cyt c) and with the excited state of [Ru(bpy)3]2+. Figure 6 (70) Nielson, R. M.; Golovin, M. N.; McManis, G. E.; Weaver, M. J. J. Am. Chem. Soc. 1988, 110, 1745-1749.

Figure 8. Typical luminescence decay transients for [Ru(bpy)3]2+ quenching by 1:1 Fc+/ZrP at different Fc+/ZrP concentrations. The dashed lines correspond to the monoexponential decay fit to the transients.

shows the UV-vis absorption spectra of 1.5 × 10-5 M cyt c in aqueous solution and in the presence of different concentrations of 1:1 Fc+/ZrP. The absorption spectrum of ferro-cyt c has characteristic absorption bands at 414, 520, and 550 nm.26 The spectra of cyt c in the presence of Fc+/ZrP show the disappearance of the bands that are characteristic of ferro-cyt c (the Soret band at 414 nm and the R and β bands at 520 and 550 nm, respectively) and the appearance of the bands at 409 and 540 nm that are typical of ferri-cyt c. Therefore, the spectra shown in Figure 6 indicate that the presence of Fc+/ZrP oxidizes cyt c. The ability of Fc+/ZrP to participate in redox reactions makes this material suitable to accept electrons for various applications such as biosensors and artificial photosynthesis. Our interest in developing artificial photosynthesis schemes provoked us to study the possibility of oxidative quenching of the excited state of [Ru(bpy)3]2+ by Fc+/ZrP as additional proof of the redox properties of Fc+/ZrP. Figure 7 shows the luminescence spectra of a deaerated 5.0 × 10-5 M [Ru(bpy)3]2+ aqueous solution in the absence and the presence of various concentrations of Fc+-intercalated ZrP (prepared using the 1:1 Fc/ZrP molar ratio sample). The decrease in luminescence intensity upon an increase in the concentration of Fc+/ZrP indicates that the [Ru(bpy)3]2+ luminescence is effectively quenched by Fc+-intercalated ZrP. To elucidate if the quenching

7816 Langmuir, Vol. 23, No. 14, 2007

Santiago et al.

Figure 9. Stern-Volmer plot for the quenching of [Ru(bpy)3]2+ by 1:1 Fc+/ZrP at different Fc+/ZrP concentrations. The errors bars correspond to ( 1 SD.

mechanism is dynamic, static, or a combination of both, we performed luminescence lifetime measurements. Figure 8 shows typical [Ru(bpy)3]2+ luminescence decay transients at different concentrations of Fc+/ZrP. The lifetime transients were successfully modeled using a monoexponential decay model. The monoexponential decay fit to the luminescence transient of [Ru(bpy)3]2+ in deaerated solution in the absence of Fc+/ZrP (Figure 8) gives a luminescence lifetime of 630 ns, in agreement with previous reports in the literature.71-73 Upon an increase in the concentration of Fc+/ZrP, the lifetime of the [Ru(bpy)3]2+ excited state is reduced, indicating that the quenching mechanism has a diffusional component. A Stern-Volmer analysis of the steady-state and lifetime luminescence data was performed to further discern the quenching mechanism. If the quenching mechanism is purely dynamic, both the intensity and lifetime data should be fitted to the following Stern-Volmer equation.74

I0 τ0 ) ) 1 + Kd[Q] I τ

(1)

where I0, I, τ0, and τ are the intensity and lifetimes of the fluorophore in the absence and presence of the quencher Q, respectively, and Kd is the Stern-Volmer constant for a dynamic mechanism. A linear plot with a positive slope is expected. However, if a purely static mechanism is involved, the τ0/τ versus [Q] plot should give a horizontal line, while the I0/I data should have a positive slope. Figure 9 illustrates the SternVolmer plot and shows an upward curvature for the intensity data and a linear relationship for the lifetime data with a small positive slope. The Kd value obtained from the τ0/τ data is 1.84 × 102 M-1. Since both plots are not superimposable, this result suggests that a combination of dynamic and static quenching mechanisms occurs within ZrP for the quenching of [Ru(bpy)3]2+ by Fc+/ZrP. To determine what type of static quenching is contributing to the quenching mechanism, we first assumed the possibility of (71) Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes; Plenum Press: New York, 1994. (72) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: New York, 1992. (73) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277. (74) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/Plenum Publishers: New York, 1999.

Figure 10. Dynamic plus static quenching model fit to the steadystate luminescence quenching data.

Figure 11. Sphere of action model fit to the steady-state emission luminescence quenching data for [Ru(bpy)3]2+ quenching by 1:1 Fc+/ZrP and the luminescence lifetime data. N′ ) 6.022 × 1023/ 1000.

ground-state complex formation, which is the simplest type of static quenching. For a quenching process that has a combination of static quenching by ground-state complex formation and dynamic quenching, the Stern-Volmer equation would become74

I0/I ) (1 + Kd[Fc+/ZrP])(1 + Ks[Fc+/ZrP])

(2)

where Kd is the dynamic Stern-Volmer constant (Kd ) kqτ0, where kq is the bimolecular quenching rate constant) and Ks is the static Stern-Volmer constant, which is the equilibrium constant for complex formation. This equation can be rearranged to allow graphical separation of Ks and Kd74:

I0/I ) 1 + (Kd + Ks)[Fc+/ZrP] + KdKs[Fc+/ZrP]2 (3) A plot of Kapp (Kapp ) (I0/I - 1)/[Fc+/ZrP]) as a function of the concentration of Fc+/ZrP should give a straight line with a slope equal to KdKS and a intercept equal to Kd + Ks.74 The Kd and Ks values obtained from the fit of eq 3 to the steady-state luminescence quenching data (the slope and intercept of Figure 10) are 1.0 and 8.3 × 105 M-1, respectively. Since the value of Kd is already known independently from the luminescence lifetime


Langmuir, Vol. 23, No. 14, 2007 7817

studies (Kd ) 1.84 × 102 M-1), the discrepancy of the Kd values obtained from the plot in Figure 10 and the lifetime data indicates that the model of static quenching by ground-state complex formation is not consistent with our results. In addition, the UVvis absorption spectrum (Figure 1) does not show the appearance of any new bands that could be attributed to the formation of a ground-state complex. We next decided to consider if a different type of static quenching occurs in this system. Perrin proposed a model of static quenching which does not involve complex formation.75,76 This model is termed the “sphere of action” model, where the “sphere of action” is a volume of space that surrounds the excitedstate luminophore. At high quencher concentrations, a fraction of the luminophores are adjacent to a quencher at the moment of excitation and are thus immediately deactivated, and therefore, the probability of quenching is unity. A modified Stern-Volmer equation that takes into account the sphere of action model is74

I0/I ) (1 + Kd[Fc+/ZrP])e[Fc

+/ZrP]VN/1000

(4)

or in linear form

I0 Ie

[Fc+/ZrP]VN/1000

) 1 + Kd[Fc+/ZrP]

(5)

where V is the volume of the sphere of action and N is Avogadro’s number. This equation can be used to correct the intensity data to isolate the dynamic contribution to the quenching mechanism. Therefore, a plot of this linear relationship (eq 5) should be superimposable with the Stern-Volmer plot obtained from the lifetime measurements (τ0/τ versus [Fc+/ZrP] data in Figure 9). Figure 11 shows the quenching data plotted in the form of the dynamic plus sphere of action model. The plot shows that both the lifetime data and the corrected intensity data are colinear, which means that the applied model is consistent with the data. The fit to the sphere of action model gives a radius of the sphere of action of 9.2 Å, which is in agreement with the sum of the radii of Fc+ and [Ru(bpy)3]2+. Our results indicate that the quenching mechanism has a diffusional component and a static component. The diffusional component indicates that diffusion of [Ru(bpy)3]2+* and the microcrystals of Fc+/ZrP occurs in the aqueous suspensions. Meanwhile, the sphere of action model suggests that part of the quenching is occurring statically between the [Ru(bpy)3]2+* species and the Fc+ ions on the surface of the ZrP layers or at the edges of these layers. Using the Kd value from the diffusional component of the quenching mechanism, the bimolecular quenching rate constant kq is 2.2 × 108 M-1 s-1. This kq value is lower than the previously reported values for [Ru(bpy)3]2+ (75) Perrin, F. C. R. Hebd. Seances Acad. Sci. 1924, 178, 1978-1980. (76) Perrin, F. Ann. Chem. Phys. 1932, 17, 283.

quenching by Fc and Fc+ in nonpolar solvents, which are close to the diffusion-controlled limit (1010 M-1 s-1).21,30,31 Coloń et al. have previously obtained similar results in a related system. These authors studied the excited-state quenching reaction of [Ru(bpy)3]2+-intercalated zirconium phosphate sulfophenylphosphonate (ZrPS) by co-intercalated methyl viologen (MV2+) as the electron acceptor. The quenching results were modeled using a combination of dynamic and static quenching through a sphere of action model. Coloń et al. obtained a Kd value of 9.83 M-1 and a kq value of 1.2 × 107 M-1 s-1.77 These authors explained that the kq value in ZrPS was smaller than that obtained in water (3-5 × 108 M-1 s-1)78 because the diffusion of the quencher through the interlayer space of ZrPS is restricted.77 For the present case, [Ru(bpy)3]2+ must diffuse in solution to encounter the Fc+/ZrP microparticles to be quenched by ET upon light excitation. Although there used to be some discrepancy on whether the quenching mechanism in solution between [Ru(bpy)3]2+* and Fc+ is by ET or energy transfer, the currently accepted mechanism is ET.21,30,31,79 For the quenching of [Ru(bpy)3]2+* by Fc+intercalated ZrP, ET is also preferred since the potential difference between Fc+/ZrP (E0′ ) +529 mV versus normal hydrogen electrode (NHE)) and [Ru(bpy)3]2+* (E0′ ) -840 mV versus NHE)71-73 makes electron transfer thermodynamically favorable. Further confirmation awaits future transient absorbance studies.

Conclusions We have directly intercalated Fc+ in ZrP by performing intercalation reactions between Fc and ZrP. The formation of Fc+ upon the intercalation reaction was corroborated using diffuse reflectance spectroscopy, UV-vis spectrophotometry, cyclic voltammetry, Mo¨ssbauer spectroscopy, and IR spectroscopy. The Fc+-intercalated ZrP material can serve as the redox agent of redox proteins and of excited polypyridine metal complexes. For [Ru(bpy)3]2+* quenching, the quenching mechanism follows a dynamic plus sphere of action model. Upon electron transfer, the Fc+ ions are reduced to Fc and [Ru(bpy)3]3+ is immobilized on the surface or at the edges of the ZrP layers to maintain charge compensation. We are in the process of using Fc derivatives to construct a better redox agent system for sensing applications. Electrochemical characterization of these materials is necessary before artificial photosynthesis and biosensor applications can be considered. The results of these investigations will be reported in the future. Acknowledgment. We thank Dr. Antonio Martıńez, Estevao Fachini, Ileana Gonza´lez, and the staff of the UPR’s Material Characterization Center for their help with XRPD, XPS, and IR spectroscopy measurements and Dr. Angel A. Martı´ for helpful discussions. This work was supported by the NIH-RISE (Grant R25GM061151), NIH-SCORE (Grant 5S06GM08102), and PRAGEP (NSF Grant HRD-0302696) programs. LA7005309 (77) Coloń, J. L.; Yang, C.-Y.; Clearfield, A.; Martin, C. R. J. Phys. Chem. 1990, 94, 874-882. (78) Ghosh, P. K.; Bard, A. J. J. Phys. Chem. 1984, 88, 5519-5526. (79) Lee, E. J.; Wrighton, M. S. J. Am. Chem. Soc. 1991, 113, 8562-8564.

Layered Inorganic Materials as Redox Agents: Ferrocenium

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