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Langmuir 2008, 24, 11684-11690
Incorporation of Water-Soluble and Water-Insoluble Ruthenium Complexes into Zirconium Phosphate Films Fabricated by the Layer-by-Layer Adsorption and Reaction Method Qifeng Wang, Dingyi Yu, Yue Wang, Junqi Sun,* and Jiacong Shen State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, China ReceiVed July 23, 2008. ReVised Manuscript ReceiVed August 11, 2008 Water-soluble tris(2,2′-bipyridine)ruthenium(II) complex (Rubpy) and water-insoluble tris(4,7-diphenyl-1,10phenanthroline)ruthenium(II) complex (Rudpp) were successfully incorporated into zirconium phosphate (ZrPS) films fabricated by a facile layer-by-layer (LbL) adsorption and reaction method. Rubpy-incorporated ZrPS films were fabricated by the LbL adsorption and reaction method employing a mixture solution of Rubpy and phosphate salt. Rubpy was incorporated into ZrPS films during the formation of ZrPS layers because of the electrostatic interaction between them. Rudpp was dissolved in the mixture solution of aqueous phosphate salt and ethanol and was incorporated into ZrPS films through the formation of ethanol preintercalated ZrPS layers. The successful incorporation of Rubpy and Rudpp into ZrPS films was confirmed by UV-vis absorption spectroscopy. Rubpy and Rudpp have concentrations of 0.915 × 1014 and 3.23 × 1014 molecules/cm2 in each ZrPS layer, respectively. Scanning electron microscopy measurements indicate that the as-prepared Rubpy-incorporated ZrPS films are smooth while the as-prepared Rudppincorporated ZrPS films are porous. The porous Rudpp-incorporated ZrPS films are suitable to use as oxygen sensors because the luminescence of Rudpp incorporated into the ZrPS films can be quenched by oxygen and the porous film structure facilitates the permeation of oxygen into and out of the film.
Introduction Inorganic phosphorus-containing materials have received much attention during the past decade.1,2 Among these materials, zirconium and titanium phosphates have been given special attention because they have been extensively studied with respect to their potential applications in areas such as ion exchange, intercalation, proton conduction, catalysis, electronics, optoelectronics, and so forth. Compared with organic materials, these inorganic materials are chemically stable and mechanically robust and could endure high temperature,3 irradiation, scratching, erosive solvents, and other harsh conditions. These characteristics make them very attractive to be employed as matrixes for immobilization of functional guest materials. Recently, we developed a facile layer-by-layer (LbL) adsorption and reaction method for the preparation of titanium phosphate (TiPS) and zirconium phosphate (ZrPS) film materials.4 By repetitive adsorption of hydrated titanium from aqueous Ti(SO4)2 solution and subsequent reaction with phosphate groups, TiPS films with easily tailored film thickness and structures can be fabricated. ZrPS films can also be prepared with this method by replacing Ti(SO4)2 with Zr(SO4)2. The LbL adsorption and reaction method is characterized by its simplicity in film preparation and its ease in controlling film thickness, structures, and surface morphologies. Meanwhile, TiPS and ZrPS films fabricated by the LbL adsorption and reaction method preserve well the free phosphate groups, which are the basis for their functionalities such as ion exchange, intercalation, proton conduction, catalysis, and so forth. Our previous studies showed that silver ions could be easily incorporated into TiPS films by an ion-exchange process to * To whom correspondence should be addressed. Fax: 0086-431-85193421. E-mail:
[email protected]. (1) Clearfield, A. Annu. ReV. Mater. Sci. 1984, 14, 205–229. (2) Clearfield, A. CRC Press: Boca Raton, FL, 1982. (3) Clearfield, A. Chem. ReV. 1988, 88, 125–148. (4) Wang, Q.; Zhong, L.; Sun, J.; Shen, J. Chem. Mater. 2005, 17, 3563–3569.
prepare highly efficient and transparent antibacterial coatings.5 In composite films of TiPS/Prussian blue (PB) fabricated by the alternative deposition of TiPS layers and PB nanocrystals, TiPS could facilitate the electron transfer from PB to the electrode surface and improve the stability of PB embedded in TiPS matrixes.6 The incorporation of metal ions into films and powders of TiPS and ZrPS can be realized by a direct ion-exchange process, but the incorporation of guest materials with large dimensions by a direct ion-exchange process is unsuccessful because of the limited interlayer distance. Large guest materials were incorporated into TiPS or ZrPS by first enlarging their interlayer distance with preintercalation of alkylamines, aromatic amines, alcohols, amino acids, and so forth, followed by an intercalation process.7,8 Another practical way to incorporate large guest materials is the synthesis of zirconium phosphate organic derivatives (zirconium phosphonates) with expanded interlayer distances.8-10 Although the preintercalation and phosphate organic derivative methods enable the incorporation of large guest materials into the powder of zirconium phosphates, a facile method to incorporate large guest materials into ZrPS or TiPS films fabricated by the LbL adsorption and reaction method remains a challenge. In the present work, we chose water-soluble tris(2,2′bipyridine)ruthenium(II) complex (Rubpy) and water-insoluble tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) complex (Rud(5) Wang, Q.; Yu, H.; Zhong, L.; Liu, J.; Sun, J.; Shen, J. Chem. Mater. 2006, 18, 1988–1994. (6) Wang, Q.; Zhang, L.; Qiu, L.; Sun, J.; Shen, J. Langmuir 2007, 23, 6084– 6090. (7) Alberti, G.; Marmottini, F.; Cavalaglio, S.; Severi, D. Langmuir 2000, 16, 4165–4170. (8) Clearfield, A.; Wang, Z. J. Chem. Soc., Dalton Trans. 2002, 2937–2947. (9) Colon, J. L.; Yang, C. Y.; Clearfield, A.; Martin, C. R. J. Phys. Chem. 1988, 92, 5777–5781. (10) Colon, J. L.; Yang, C. Y.; Clearfield, A.; Martin, C. R. J. Phys. Chem. 1990, 94, 874–882.
10.1021/la802364z CCC: $40.75 2008 American Chemical Society Published on Web 09/12/2008
Ru Complex Incorporation into ZrPS Films Scheme 1. Chemical Structures of Rubpy and Rudpp
pp) as model materials to demonstrate the strategies to incorporate guest materials with large dimensions into ZrPS films fabricated by the LbL adsorption and reaction method. Ruthenium complexes have widespread applications in oxygen and glucose sensors, electrochemiluminescent immunoassays, photocatalysts, lightemitting devices, and so forth because of their excellent photophysical and electrochemical functionalities.11-15 The incorporation of ruthenium complexes in thin host films can provide a way to tailor their properties via controlling their interaction with the surrounding media and to integrate them with other functionalities, and finally advanced film materials of ruthenium complexes with facilitated applications will be produced.
Experimental Section Materials. Zirconium sulfate [Zr(SO4)2], sodium hydrogen phosphate (Na2HPO4 · 12H2O), and sodium dihydrogen phosphate (NaH2PO4 · 2H2O) were of analytical grade and were purchased from Beijing Chemical Reagents Co. Poly(diallyldimethylammonium chloride) (PDDA) aqueous solution with a molecular weight of 100000-200000 was purchased from Sigma-Aldrich. Tris(2,2′bipyridyl)ruthenium(II) chloride hexahydrate (abbreviated as Rubpy) and tris(4,7-diphenyl-1,10- phenanthroline)ruthenium(II) chloride (abbreviated as Rudpp) were synthesized following the references.16,17 Their chemical structures are shown in Scheme 1. The phosphate salt (PS) solution comprised 0.1 M phosphate (Na2HPO4 and NaH2PO4) with a pH of 4.0 adjusted by the addition of H2SO4. The dipping solutions used in all experiments were prepared as follows: aqueous Zr(SO4)2 solution was obtained by dissolving Zr(SO4)2 in 0.1 M H2SO4 with a final concentration of 10 mM Zr(SO4)2. Zr(SO4)2 and Rubpy mixed solution (Zr-Rubpy) was prepared by adding 500 µL of 10 mM Rubpy aqueous solution to 5 mL of Zr(SO4)2 solution. PS and Rubpy mixed solution (PS-Rubpy) with a molar ratio of PS to Rubpy of 100:1 or 1000:1 was prepared by adding 500 or 50 µL of 10 mM Rubpy aqueous solution to 5 mL of PS solution, respectively. To dissolve Rudpp, a PS and ethanol mixture solution (abbreviated as PS-EtOH) composed of 4 mL of PS solution and 1 mL of ethanol was employed. A 250 µL portion of 10 mM Rudpp was added to 5 mL of PS-EtOH solution, resulting in a PS-EtOH-Rudpp mixture solution with a molar ratio of PS to Rudpp of 160:1. In the control experiments, a PS-EtOH-Rubpy mixture solution with a molar ratio of PS to Rubpy of 80:1 was prepared. A mixture solution of water and ethanol with a volume ratio of 4:1 (abbreviated as H2O-EtOH) was used to rinse the substrates during the fabrication of Rudpp-incorporated ZrPS films. Treatment of the Substrate. Quartz slides and silicon wafers were immersed in slightly boiled piranha solution (3:1 mixture of (11) Whitten, D. G. Acc. Chem. Res. 1980, 13, 83–90. (12) Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F.; Gleria, M. Science 1975, 189, 852–856. (13) Gra¨tzel, M. Pure Appl. Chem. 1982, 54, 2369–2382. (14) Bacon, J. R.; Demas, J. N. Anal. Chem. 1987, 59, 2780–2785. (15) Chang-Yen, D. A.; Lvov, Y; McShane, M. J.; Gale, B. K. Sens. Actuators, B 2002, 87, 336–345. (16) Crosby, G. A.; Watts, R. J. J. Am. Chem. Soc. 1971, 93, 3184–3188. (17) Zhang, P.; Guo, J.; Wang, Y.; Pang, W. Mater. Lett. 2002, 53, 400–405.
Langmuir, Vol. 24, No. 20, 2008 11685 98% H2SO4 and 30% H2O2) for 20 min and rinsed by a copious amount of water. Caution: Piranha solution reacts Violently with organic material and should be handled carefully! The cleaned quartz slides and silicon wafers were immersed in PDDA aqueous solution (1.0 mg/mL) for 20 min to obtain a cationic ammonium-terminated surface and were ready for film deposition. Preparation of Rubpy- and Rudpp-Incorporated ZrPS Films. The preparative procedure for the preparation of Rupby-incorporated ZrPS (abbreviated as Rubpy@ZrPS) films employs a modified LbL adsorption and reaction method for the fabrication of pure ZrPS films and is described as follows: The PDDA-modified substrate was immersed in an aqueous Zr(SO4)2 solution for 5 min. Then the substrate was blown dry with compressed N2 to remove the redundant Zr(SO4)2. Next the substrate was transferred to the PS-Rubpy mixture solution for 5 min. Finally, the substrate was rinsed with water and dried with a N2 flow. In this way, a one-layer ZrPS film containing Rubpy was deposited on the substrate. Multilayer films of Rubpy@ZrPS could be prepared by repeating the above processes. The host ZrPS films without Rubpy incorporation were fabricated by replacing the mixture solution of PS-Rubpy with PS solution. Multilayer films of Rubpy@ZrPS/ZrPS could be fabricated by alternate deposition of Rubpy@ZrPS and pure ZrPS layers. The procedure for the fabrication of Rudpp-incorporated ZrPS (abbreviated as Rudpp@ZrPS) films was different from that of Rubpy@ZrPS films because Rudpp was water insoluble. First, the PDDA-modified substrate was immersed in an aqueous Zr(SO4)2 solution for 5 min. Then the substrate was transferred to the PS-EtOH mixture solution forafewseconds,afterwhichitwasimmersedinthePS-EtOH-Rudpp solution for 5 min. Finally, the substrate was rinsed with the mixture solution of H2O-EtOH and dried in a N2 stream. In this way, one layer of Rudpp@ZrPS was deposited, and thick Rudpp@ZrPS films could be prepared by repeating the above procedures in a cyclic fashion. Measurements of Oxygen-Sensing Behavior. A quartz slide with both sides coated with multilayer films of Rudpp@ZrPS was fixed inside a laboratory-made cell that was equipped with two quartz windows and an airtight stopper which had inlet and outlet lines to allow gas flow. To investigate the sensing properties of Rudpp@ZrPS films under different oxygen concentrations, a gas mixture of drying nitrogen and oxygen with variable oxygen concentrations was obtained by controlling the relative flow rates of nitrogen and oxygen by two separate gas flow meters. All emission spectra were recorded with a photoluminescence emission spectrophotometer at ambient laboratory temperature in the dark. Instruments and Measurements. UV-vis absorption spectra were recorded with a Shimadzu UV-2550 spectrophotometer. Fourier transform infrared (FT-IR) spectra were collected on a Bruker IFS 66V instrument equipped with an MCT detector. Atomic force microscopy (AFM) images were taken with a Nanoscope IIIa AFM MultiMode instrument (Digital Instruments, Santa Barbara, CA) under ambient conditions. The AFM instrument was operated in the tapping mode with an optical readout using Si cantilevers. Scanning electron microscopy (SEM) observations were carried out on a JEOL FESEM 6700F scanning electron microscope with a primary electron energy of 3 kV. For cross-sectional SEM imaging, the substrate with the deposited films was broken off and glued perpendicularly to the sample holder. All samples were sputtered with a layer of gold prior to imaging. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB250 (VG Microtech) photoelectron spectrometer with a monochromatic Al KR X-ray source. The luminescence spectra were recorded with a Shimadzu RF5301PC emission spectrophotometer.
Results and Discussion Incorporation of Rubpy into ZrPS Films. Rubpy has characteristic absorptions of electronic π-π* transition near 290 nm and metal-to-ligand charge transfer (MLCT) near 450 nm. ZrPS has no characteristic absorption in the whole UV-vis range except a gradual absorption elevation in the UV region. Therefore, the incorporation of Rubpy into ZrPS films could be easily
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examined by UV-vis absorption spectroscopy.9 It is well-known that metal complexes such as Rubpy and Rudpp are too large to be directly incorporated into ZrPS materials by an ion-exchange process.10,18,19 This is also the case for ZrPS films fabricated by the LbL adsorption and reaction method. As shown in Figure 1a, after immersion of a 10-layer ZrPS film in 1 mM aqueous Rubpy solution, only a slightly increased absorption of electronic π-π* transition at 286 nm was detected, corresponding to the surfacebound Rubpy. This absorbance value did not increase with further elongation of the duration of the immersion of the film in the aqueous Rubpy solution. Rubpy was mixed with PS solution, and the LbL adsorption and reaction processes were performed to fabricate Rubpy@ZrPS films, as described in the Experimental Section. Figure 1b shows the UV-vis absorption spectra of Rubpy@ZrPS films with different layers of film deposition using the PS-Rubpy mixture solution with a molar ratio of PS to Rubpy of 100:1. All spectra show absorption peaks at 290 and 453 nm, which are the typical spectral characteristics of electronic π-π* transition and MLCT absorption of Rubpy, respectively. The absorbance at 290 and 453 nm increases almost linearly with increasing number of film deposition cycles (inset of Figure 1b), indicating that a satisfactory deposition process with an almost equal amount of Rubpy incorporated in each deposition cycle was obtained. Each deposition layer leads to an increase of absorbance at 290 and 453 nm, being 0.0082 and 0.0024, respectively. The amount of Rupby incorporated in each ZrPS layer depends on the concentration of Rubpy in the PS-Rubpy mixture solution. The amount of Rupby incorporated decreased to ca. 1/7 in Figure 1b when the PS-Rubpy mixture solution with a molar ratio of PS to Rubpy of 1000:1 was used. Rubpy was also mixed with aqueous Zr(SO4)2 solution to fabricate Rubpy@ZrPS films. Figure 1c shows the UV-vis absorption spectra of the Rubpy@ZrPS films fabricated by using the Zr-Rubpy mixture solution. Although the absorbance of electronic π-π* transition at 286 nm was observable for different deposition cycles, its intensity did not increase obviously with the number of film deposition cycles when the absorbance contributed by the successive deposition of ZrPS layers in this region was considered. Meanwhile, the MLCT absorbance at 453 nm was quite low and did not increase with the number of film deposition cycles. Deep investigation disclosed that Rubpy could be codeposited with zirconium ions during immersion of the substrate in the mixture Zr-Rubpy solution. When the substrate was transferred to PS solution, phosphate groups reacted with zirconium ions to produce a ZrPS layer. Most of the entrapped Rubpy desorbed from the substrate to the PS solution before a dense layer of ZrPS formed because, on one hand, there exists repulsion between the same positively charged zirconium ions and Rubpy and, on the other hand, Rubpy has a high solubility in PS solution. Therefore, mixing Rubpy with Zr(SO4)2 solution does not provide an effective way to incorporate Rubpy into ZrPS films. It should be noted that, in our previous study,4 the fabrication of ZrPS films by the LbL adsorption and reaction method employed two baths of PS solution. The substrate adsorbed with zirconium ions was immersed in the first PS solution for a few seconds to remove the excess zirconium ions and then in the second PS solution for complete reaction with phosphate groups. For incorporation of Rubpy into ZrPS films, it is important to replace the step of immersion in the first PS solution with N2 drying because the instantly formed ZrPS layer in the first PS bath prevents the diffusion of Rubpy into the inner layer of ZrPS, (18) Marti, A. A.; Colon, J. L. Inorg. Chem. 2003, 42, 2830–2832. (19) Yeates, R. C.; Kuznicki, S. M.; Lloyd, L. B.; Eyring, E. M. J. Inorg. Nucl. Chem. 1981, 43, 2355–2358.
Wang et al.
Figure 1. UV-vis absorption spectra of various Rubpy@ZrPS films with different deposition layers. (a) A 10-layer ZrPS film after immersion in 1 mM aqueous Rubpy solution for 30 min. The inset shows the absorbance at 286 nm vs the immersion time. (b) Rubpy@ZrPS films fabricated using a mixture solution of PS-Rubpy with a molar ratio of 100:1. (c) Rubpy@ZrPS films fabricated using a mixture solution of Zr-Rubpy. (d) Rubpy@ZrPS films fabricated using a two-bath PS reaction process. The insets in (b)-(d) show the absorbance of π-π* transition (9) and MLCT (b) of Rubpy vs the number of deposition layers. The number of layers is 1-10 from the bottom to the top.
Ru Complex Incorporation into ZrPS Films
Figure 2. (a) UV-vis absorption spectra of Rubpy@ZrPS/ZrPS films with different deposition layers. The number of layers is 13, 14, and 15, where the odd numbers and even number correspond to the deposition of ZrPS and Rubpy@ZrPS layers, respectively. The inset shows the absorbance at 289 nm (9 and 0 represent the Rubpy@ZrPS layer and ZrPS layer, respectively) and 457 nm (b and O represent the Rubpy@ZrPS layer and ZrPS layer, respectively) vs the number of deposition layers. (b) UV-vis absorption spectra of a (ZrPS/ Rubpy@ZrPS)7/ZrPS film immersed in deionized water for 0 and 4 h from top to bottom. The inset shows the absorbance change at 289 nm (9) and 457 nm (b) along with the immersion time.
leading to a low-efficiency incorporation of Rubpy into ZrPS films (Figure 1d). From Figure 1, it is evident that Rubpy mixed with PS solution provides an efficient way to incorporate Rubpy into ZrPS films by the LbL adsorption and reaction method. When the substrate absorbed with zirconium ions was immersed into the Rubpy-PS mixture solution, zirconium ions reacted with phosphate groups to produce a ZrPS layer. The adsorption of Rubpy took place immediately after the formation of ZrPS because of the strong electrostatic interaction between the negatively charged ZrPS and positively charged Rubpy. The adsorption of Rubpy continued until all adsorbed zirconium ions completely reacted with phosphate ions. Meanwhile, the high concentration of Rupby in the Rubpy-PS mixture solution prevented the desorption of Rupby during the formation of the Rubpy@ZrPS layer. Starting from the second layer for the preparation of Rubpy@ZrPS films, the Zr(SO4)2 solution turned a bit orange after the substrate covered with Rubpy@ZrPS films was immersed in Zr(SO4)2 solution. This means that the Rubpy incorporated in the previous layers partially dissolved in the next Zr(SO4)2 solution. Meanwhile, the Rubpy@ZrPS films were not stable when immersed in deionized water. The absorbance of a 13-layer Rubpy@ZrPS film at 290 and 453 nm decreased about 60% after immersion in deionized water for 4 h. To stabilize the entrapped Rupby in ZrPS films, multilayer films of Rubpy@ZrPS/ZrPS were fabricated by alternate deposition of Rubpy@ZrPS and pure ZrPS layers. Figure 2a shows the
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Figure 3. UV-vis absorption spectra of Rudpp@ZrPS films with different deposition layers: (a) Rudpp@ZrPS films fabricated using PS-EtOH-Rudpp solution with the process in Figure 1b, (b) Rudpp@ZrPS films fabricated using a two-bath reaction process. The insets show the absorbance at π-π* transition (9) and MLCT (b) vs the number of deposition layers. The number of layers is 1-10 from the bottom to the top.
representative UV-vis absorption spectra of Rubpy@ZrPS/ZrPS films with successive deposition of Rubpy@ZrPS and ZrPS layers. After a Rubpy@ZrPS layer was deposited, an obvious increase of absorbance at 289 and 457 nm was observed. The subsequent deposition of a ZrPS layer led to a decrease of absorbance at 289 and 457 nm. The absorbance at 289 and 457 nm shows a zigzag behavior with increasing number of film deposition layers (the inset of Figure 2a). After three bilayers of film deposition, the absorbance at 289 and 457 nm of the Rubpy@ZrPS/ZrPS films increases linearly with the number of film deposition cycles, with an average absorbance increase of 0.024 and 0.008 per deposition cycle at 289 and 457 nm, respectively. The amount of Rubpy incorporated in a one-bilayer Rubpy@ZrPS/ZrPS film is 2 times larger than that in a one-layer Rubpy@ZrPS film. Most importantly, the stability of Rubpy incorporated in Rubpy@ZrPS/ZrPS films was improved greatly. The absorbance of a (ZrPS/Rubpy@ZrPS)7/ZrPS film at 289 and 457 nm decreased about 10% after immersion of the film in deionized water for 4 h (Figure 2b). The additional ZrPS layer can stabilize the Rubpy incorporated in the previous ZrPS layer. Ethanol-Assisted Incorporation of Rudpp into ZrPS Films. Rudpp is water insoluble but can be dissolved in a mixture of PS and ethanol (4:1, v/v). The process employed in Figure 1b for the preparation of Rubpy@ZrPS films was used to fabricate Rudpp-incorporated ZrPS films by simply replacing the PS-Rubpy solution with PS-EtOH-Rudpp solution and the water rinsing step with rinsing with a mixed solvent of H2O-EtOH. As shown in Figure 3a, the absorbance at 281 and 449 nm, which corresponds to the electronic π-π* transition
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and MLCT of Rudpp, increases slowly with the number of films deposited. An alternative method was applied by replacing the N2 drying step after absorption of zirconium ions with immersion of the substrate in the mixture PS-EtOH solution for a few seconds. Successful fabrication of Rudpp@ZrPS films with a high loading amount of Rudpp was realized. UV-vis absorption spectra in Figure 3b indicate that the absorbance at 280 and 457 nm increases linearly with the number of film deposition layers, confirming the successful incorporation of Rudpp in ZrPS films. One Rudpp@ZrPS layer leads to an absorbance increase of 0.048 and 0.018 at 280 and 457 nm, respectively. It is possible that a thick ZrPS layer with intercalated EtOH could be obtained by first rinsing the substrate with PS-EtOH mixed solvent and subsequently immersing it in PS-EtOH-Rudpp solution. The intercalated EtOH increased the interlayer distance of the ZrPS layer and simultaneously provided a hydrophobic environment which favored the incorporation of Rudpp, while for the process employed in Figure 3a a thin ZrPS film was obtained because the N2 flow removed most of the adsorbed zirconium ions. X-ray photoelectron spectroscopy (XPS) measurements and FT-IR spectra detected no ethanol in the Rudpp@ZrPS films fabricated in Figure 3b. The absence of ethanol means that ethanol evaporates easily during the drying step, without an influence on the properties of the resultant Rudpp@ZrPS films. The disappearance of ethanol in Rudpp@ZrPS films was consistent with previous studies about the preparation of Re complex-incorporated ZrPS powder employing Re(phen)(CO)3Cl dissolved in water-acetonitrile solution where the acetonitrile evaporates from ZrPS during the drying process.20 However, the method for the preparation of Rudpp@ZrPS films was not satisfactory for the incorporation of Rubpy into ZrPS films when PS-EtOH-Rudpp solution was replaced with PS-EtOH-Rubpy (see Supporting Information Figure S1). This is because the ZrPS layers intercalated with ethanol formed in the first PS-EtOH bath become hydrophobic and prevent the diffusion of water-soluble Rubpy into the ZrPS layers. The Rudpp@ZrPS films are stable in water without dissolution of the Rudpp from the films. Structure of Rubpy- and Rudpp-Incorporated ZrPS Films. The structure of Rubpy- and Rudpp-incorporated ZrPS films was investigated by SEM and AFM. Figure 4 shows the AFM and cross-sectional SEM images of 20-layer ZrPS (fabricated with one PS bath method), (Rubpy@ZrPS)20 (the film in Figure 1b), and (ZrPS/Rubpy@ZrPS)10/ZrPS (the film in Figure 2a) films. The surface of these films is quite smooth, with rootmean-square (rms) roughness of 1.4, 2.1, and 2.1 nm for a 20layer ZrPS, (Rubpy@ZrPS)20, and (Rubpy@ZrPS/ZrPS)10/ZrPS film, respectively. The film thickness was determined from their cross-sectional SEM images. The 20-layer ZrPS film has a constant thickness of 29.6 ( 1.7 nm, corresponding to a thickness of 1.5 nm for one ZrPS layer. The (Rubpy@ZrPS)20 film has a thickness of 39.4 ( 2.3 nm. Therefore, the incorporation of Rubpy increased the average thickness of one layer of ZrPS from 1.5 to 2.0 nm. This does not mean that the increased thickness of 0.5 nm is absolutely contributed by the incorporated Rubpy because the amounts of ZrPS deposited in each Rubpy@ZrPS layer and the pure ZrPS layer might be different. The (ZrPS/ Rubpy@ZrPS)10/ZrPS film has an intermediate thickness of 34.1 ( 2.9 nm, which is close to the total thickness of 11-layer ZrPS and 10-layer Rubpy@ZrPS. The Rudpp@ZrPS film has a rougher surface and thicker film thickness than the Rubpy@ZrPS film when both films have the same number of deposition cycles. As shown in Figure 5a, the (20) Marti, A. A.; Rivera, N.; Soto, K.; Maldonado, L.; Colon, J. L. Dalton Trans. 2007, 1713–1718.
Wang et al.
Figure 4. AFM and SEM images of 20-layer ZrPS (a, b), (Rubpy@ZrPS)20 (c, d), and (ZrPS/Rubpy@ZrPS)10/ZrPS (e, f) films. The scale of the AFM images is 3.0 µm × 3.0 µm.
Figure 5. Top-down and cross-sectional SEM images of a (Rudpp@ZrPS)15 (a, b) and 15-layer ZrPS-EtOH (c, d) films. The scale bars in all images correspond to 200 nm.
surface of a (Rudpp@ZrPS)15 film is rather rough, with tiny particles of 15-23 nm being observable. The (Rudpp@ZrPS)15 film has a thickness of 164.6 ( 11.6 nm, as determined from its cross-sectional SEM image (Figure 5b). One pure ZrPS layer has a thickness of 4.3 ( 0.9 nm,4 which was prepared by first deposition of a layer of zirconium ions and then immersion of the substrate into the first phosphate solution for several seconds followed by reaction in the second phosphate solution for 5 min. The average thickness of about 11.0 nm for one Rudpp@ZrPS layer is much larger than that of one layer of pure ZrPS prepared with phosphate solution without the addition of ethanol. Therefore, the large thickness increase of Rudpp@ZrPS films compared
Ru Complex Incorporation into ZrPS Films
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with that of pure ZrPS films is caused not only by the incorporation of Rudpp, but also by the participation of ethanol in the ZrPS film fabrication. In the control experiment, a 15-layer ZrPS-EtOH (to differentiate ZrPS films prepared from aqueous phosphate solution, the ZrPS films prepared using the PS-EtOH mixture solution was abbreViated as ZrPS-EtOH) film was prepared by a film preparative process similar with that of Rudpp@ZrPS films except that no Rudpp was added in the PS-EtOH mixture solution. The surface and cross-sectional SEM images of a 15layer ZrPS-EtOH film are presented in parts c and d, respectively, of Figure 5. The surface contains tiny particles with an average size of 22-39 nm and looks rougher than that of the 15-layer ZrPS-EtOH film. The 15-layer ZrPS-EtOH film has a thickness of 175.5 ( 33.3 nm, which corresponds to a thickness of 11.7 nm for one ZrPS-EtOH layer. Zr(SO4)2 is insoluble in ethanol, and the addition of EtOH to the PS solution prevents the dissolution of the adsorbed zirconium ions in the first PS-EtOH solution and, therefore, increases the thickness of the resultant ZrPS-EtOH layer per deposition cycle. Chemical Composition of Rubpy- and Rudpp-Incorporated ZrPS Films. The amount of Rubpy and Rudpp incorporated into ZrPS films was determined from their UV-vis absorption spectra using the Beer-Lambert law on the assumption of the same molar absorption coefficients of the Ru-containing complexes in both the solution and films. The surface concentration (Γ) is defined as
Γ ) cl ) A/ε
(1)
where c, l, A, and ε correspond to the concentration, film thickness, absorbance, and molar absorption coefficient of the film, respectively. Because the MLCT band of Rubpy in Rubpy@ZrPS films and Rudpp in Rudpp@ZrPS films shows no shift compared with that in their corresponding diluted aqueous and ethanol solution and ZrPS matrix films show no absorbance in the MLCT absorbance range of Rubpy and Rudpp, the MLCT absorbance band at 453 and 463 nm was used for concentration calculation of Rubpy in Rubpy@ZrPS and Rudpp in Rudpp@ZrPS films, respectively. The molar absorption coefficient for the MLCT band of Rubpy in aqueous solution and of Rudpp in ethanol solution was calculated to be εRubpy ) 1.58 × 107 cm2 · mol-1 and εRudpp ) 3.36 × 107 cm2 · mol-1, respectively. The average absorbance of each layer of Rubpy@ZrPS at 453 nm was 0.0024, corresponding to a Rupby concentration of ΓRubpy ) 0.92 × 1014 molecules/cm2 per layer. The average absorbance of each layer of Rudpp@ZrPS at 463 nm was 0.0184, corresponding to a Rudpp concentration of ΓRudpp ) 3.23 × 1014 molecules/cm2 per layer. This value is comparable with the concentration of 1.52 × 1014 and 2.01 × 1014 molecules/cm2 reported by Yam and co-workers21 where polypyridylruthenium(II) complexes were covalently attached or deposited as a an LB monolayer on a glass surface. The larger amount of Rudpp in Rudpp@ZrPS films per deposition cycle than that of Rubpy in the Rupby@ZrPS layer originated from the thicker layers in the former case. XPS spectra show that the atomic ratios of P to Ru in Rubpy@ZrPS and Rudpp@ZrPS films are 94:1 and 71:1, respectively, which are higher than the corresponding ratios of 100:1 and 160:1 in solution. The amount of Rubpy incorporated in each ZrPS/Rubpy@ZrPS bilayer is about 2 times higher than that in each Rubpy@ZrPS layer on the basis of their corresponding UV-vis absorption spectra (Figures 1b and 2a). The greater amount of Rubpy does not increase the thickness of Rubpy@ZrPS layers in ZrPS/Rubpy@ZrPS films (Figures 2d,f and 4b), indicating that the final quantity of the incorporated Rubpy is determined by to what extent the dissolution (21) Chu, B. W. K.; Yam, V. W. W. Langmuir 2006, 22, 7437–7443.
Figure 6. (a) Emission spectra of a (Rudpp@ZrPs)20 film on exposure to 100% oxygen and 100% nitrogen. (b) Intensity Stern-Volmer plots for a (Rudpp@ZrPs)20 film (9) and Rudpp casting film (b) on quartz slides at different concentrations of oxygen. Ex ) 468 nm.
of the Rubpy from the ZrPS matrix films can be prevented. Obviously, ZrPS/Rubpy@ZrPS films are more efficient in incorporation of Rubpy than Rubpy@ZrPS films. Application of the Rudpp@ZrPS Films as Oxygen Sensors. Oxygen sensors are of importance in medicinal, industrial, and environmental applications. Ru(II) complexes have been investigated as oxygen sensors for a long period of time. Ru(II) complexes are usually incorporated into a solid matrix to facilitate their applications as oxygen sensors. The matrix has to exhibit particular properties such as oxygen permeability and chemical and mechanical stability. Herein, a (Rudpp@ZrPS)20 film deposited on a quartz slide was selected for investigation of its oxygen-sensing properties because Rudpp exhibits better oxygensensing capability than Rubpy.22 Meanwhile, the Rudpp@ZrPS film shows a more porous film structure than the Rubpy@ZrPS film, which can facilitate the permeation of oxygen into and out of the film. Figure 6a shows the emission spectra of a (Rudpp@ZrPS)20 film under 100% nitrogen and 100% oxygen atmospheres. Under a 100% nitrogen atmosphere, Rudpp showed strong emission in the (Rudpp@ZrPS)20 film, while, under a 100% oxygen atmosphere, the emission decreased obviously, indicating that oxygen acted as a quencher for Rudpp emission in Rudpp@ZrPS films. A casting Rudpp film was prepared for control experiments. The response time (the time required for the luminescent intensity to decrease by 95% on changing from 100% nitrogen to 100% oxygen) and recovery time (the time required for the luminescent intensity to reach 95% of the initial value recorded in 100% nitrogen on changing from 100% oxygen to 100% nitrogen)23 of the (Rudpp@ZrPS)20 film are 2.8 and 4.7 s, respectively. As a comparison, the response time and recovery time for a casting Rudpp film are 5.3 and 7.8 s, respectively. Therefore, the Rudpp@ZrPS film shows faster response and recovery times to oxygen than the casting Rudpp film. The Stern-Volmer plots for the (Rudpp@ZrPS)20 film and casting Rudpp film are presented in Figure 6b. From their corresponding Stern-Volmer plot, it can be seen clearly that, at the same oxygen concentration, the (Rudpp@ZrPS)20 film has a higher sensitivity than the casting Rudpp film. The sensitivity (I0/I100, where I0 and I100 represent the luminescent intensities in 100% nitrogen and 100% oxygen, respectively) is ca. 2.76 for the (Rudpp@ZrPS)20 film and ca. 1.41 for the casting Rudpp film. The nonlinear response of the (Rudpp@ZrPS)20 film in a high oxygen concentration originates mainly from the hetero(22) Han, B. H.; Manners, I.; Winnik, M. A. Chem. Mater. 2005, 17, 3160– 3171. (23) Kumar, C. V.; Williams, Z. J. J. Phys. Chem. 1995, 99, 17632–17639.
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geneous film structure of the Rudpp@ZrPS films. This kind of phenomenon was well investigated in luminescence-based sensors which employed matrix films to accommodate the sensing lumophore molecules.21,24,25 The enhanced performance of the Rudpp@ZrPS film over the casting Rudpp film originates from the porous structure of Rudpp@ZrPS films, which facilitates the permeation of oxygen into and out of the films. The anticipated advantage of LbL-deposited porous Rudpp@ZrPS films for oxygen sensor applications lies in its convenience in depositing the films on nonflat surfaces such as in the inner wall of optical fibers, which will largely widen the application of the resultant sensors.
Conclusions In the present study, we show that water-soluble Rubpy and water-insoluble Rudpp can be successfully incorporated into ZrPS films during the LbL adsorption and reaction process for the fabrication of ZrPS films. Because of the large dimension of Rubpy and Rudpp, the direct incorporation of them into ZrPS films through the ion-exchange process was unsuccessful. By mixing Rubpy with phosphate solution, the Rubpy was incorporated into ZrPS films during the formation of ZrPS films. By adding ethanol to the phosphate solution, Rudpp becomes soluble and can be incorporated into ZrPS films through the formation of ethanol-preintercalated ZrPS layers. While the incorporated Rubpy can be stabilized in the Rubpy@ZrPS/ZrPS alternate films, the Rudpp@ZrPS films are stable in water because of the waterinsoluble property of the Rudpp. The Rudpp@ZrPS films are particularly suitable for the fabrication of oxygen sensors of (24) Demas, J. N.; DeGraff, B. A.; Xu, W. Anal. Chem. 1995, 67, 1377–1380. (25) Li, X.-M.; Wong, K.-Y. Anal. Chim. Acta 1992, 262, 27–32.
Wang et al.
high sensitivity because the porous structure of the Rudpp@ZrPS films enables the quick permeation of oxygen into and out of the film. The present method for the incorporation of guest organic molecules into ZrPS films is characterized by its simplicity and effectiveness in controlling the amount of Rubpy or Rudpp incorporated by simply tailoring the concentration of the mixture ratio of Rubpy or Rudpp to phosphate salt. Compared with organic or polymeric matrix films, ZrPS films fabricated by the LbL adsorption and reaction method present excellent mechanical and chemical stability and can widen the application window of the ZrPS matrix films functionalized with guest materials. We believe that other cationic organic molecules, both water-soluble and water-insoluble, could also be incorporated into ZrPS or TiPS films by the present method to prepare various kinds of functionalized film materials. Acknowledgment. This work is supported by the National Natural Science Foundation of China (NSFC Grant 20304004), the Foundation for the Author of National Excellent Doctoral Dissertation of the People’s Republic of China (FANEDD Grant 200323),theNationalBasicResearchProgram(Grant2007CB808003), the Program for New Century Excellent Talents in University (NCET), and the Jilin Provincial Science and Technology Bureau of Jilin Province (Grant 20070104). Supporting Information Available: Characterization of Rubpy and Rudpp, UV-vis absorption spectra of Rubpy@ZrPS films fabricated with a process similar to that for Rudpp@ZrPS films, and the response time and recovery time curves of the (Rudpp@ZrPS)20 film and casting Rudpp film. This material is available free of charge via the Internet at http://pubs.acs.org. LA802364Z
