Fabrication and Electrochemical Investigation of Layer-by-Layer

Apr 18, 2007 - Composite films of titanium phosphate (TiPS)/Prussian blue (PB) were fabricated by the alternative deposition of TiPS layer and PB ...
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6084

Langmuir 2007, 23, 6084-6090

Fabrication and Electrochemical Investigation of Layer-by-Layer Deposited Titanium Phosphate/Prussian Blue Composite Films Qifeng Wang, Ling Zhang, Lingying Qiu, Junqi Sun,* and Jiacong Shen Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun, People’s Republic of China 130012 ReceiVed January 29, 2007. In Final Form: March 6, 2007 Composite films of titanium phosphate (TiPS)/Prussian blue (PB) were fabricated by the alternative deposition of TiPS layer and PB nanocrystals. The layer of TiPS was fabricated by adsorption of hydrated titanium from aqueous Ti(SO4)2 solution and subsequent reaction with phosphate groups. The layer of PB nanocrystals was fabricated by sequential adsorption of FeCl3 solution and K4[Fe(CN)6] solution. Regular deposition of TiPS/PB composite films were verified by UV-vis absorption spectroscopy and quartz crystal microbalance measurements. The successful fabrication of the TiPS/PB composite films was further confirmed by X-ray photoelectron spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. Instead of producing films of TiPS layers alternating with PB nanocrystal layers, the TiPS/PB composite films have a structure in which the interstices of the PB nanocrystal films are filled with TiPS component. TiPS/PB composite films show enhanced electrochemical properties and improved stability in comparison with pure PB films prepared by the multiple sequential adsorption process. TiPS/PB composite films have the capability to catalyze the electrochemical reduction of H2O2 and can be used as a biosensor for detecting H2O2.

Introduction Prussian blue (PB) or ferric hexacyanoferrate is well-known for its interesting electrochemical, electrochromic, photophysical, and magnetic properties.1,2 The preparation of PB and PB composite film materials has attracted much attention of scientists because of their importance in academic research and industrial applications. On one hand, incorporating PB into solid films will facilitate its application as miniaturized devices or functional film materials. On the other hand, the partner components in PB composite films will enhance the properties of PB or produce novel properties that are beyond those of the individual components due to their synergistic effect. There are several ways to prepare PB-containing films. Electrochemical deposition of PB film on conductive substrates was studied by Neff and co-workers and is a simple way to prepare PB films on conductive substrates.2-4 By multiple sequential adsorption of a substrate in aqueous solutions of Fe(CN)63- and Fe2+ or Fe(CN)64- and Fe3+, ultrathin films of PB with tailored film thicknesses can be fabricated.5-7 PB nanotubes were successfully prepared by a modification of this method when porous anodic alumina membrane was used as a template.8 PB film can also be prepared by methods such as dip-coating, casting from colloidal solution, and so forth.9-12 While the above-mentioned methods generally * To whom correspondence should be addressed: phone 0086-43185168723; fax 0086-431-85193421; e-mail [email protected]. (1) Pyrasch, M.; Toutianoush, A.; Jin, W. Q.; Schnepf, J.; Tieke, B. Chem. Mater. 2003, 15, 245-254. (2) Itaya, K.; Uchida, I.; Neff, V. D. Acc. Chem. Res. 1986, 19, 162-168. (3) Neff, V. D. J. Electrochem. Soc. 1978, 125, 886-887. (4) Zhang, D.; Wang, K.; Sun, D. C.; Xia, X. H.; Chen, H. Y. Chem. Mater. 2003, 15, 4163-4165. (5) Pyrasch, M.; Tieke, B. Langmuir 2001, 17, 7706-7709. (6) Millward, R. C.; Madden, C. E.; Sutherland, I.; Mortimer, R. J.; Fletcher, S.; Marken, F. Chem. Commun. 2001, 1994-1995. (7) Moriguchi, I.; Kamogawa, H.; Hagiwara, K.; Teraoka, Y. Chem. Lett. 2002, 31, 310-311. (8) Johansson, A.; Widenkvist, E.; Lu, J.; Boman, M.; Jansson, U. Nano Lett. 2005, 5, 1603-1606. (9) Toshima, N.; Lin, R. J.; Kaneko, M. Chem. Lett. 1990, 19, 485-488. (10) Crumbliss, A. L.; Lugg, P. S.; Morosoff, N. Inorg. Chem. 1984, 23, 4701-4708.

produce single-component PB films, the film preparation methods working in a layer-by-layer (LbL) fashion predominantly make multicomponent PB films because of their alternating preparation process.13-17 Among them, the electrostatic LbL assembly technique, which utilizes oppositely charged species for multilayer preparation, is highly attractive because of its simplicity in film preparation and ease in tailoring film composition and structures.18-20 Multilayer films composed of PB nanoparticles with a size of ∼5 nm and poly(allylamine hydrochloride) (PAH) were prepared by the electrostatic LbL assembly technique and showed high sensitivity when used as a biosensor in detecting H2O2. The sensitivity of the sensor increases with increasing number of deposition cycles because of the increased loading of electroactive PB in the film.14 DeLongchamp and Hammond15 demonstrated that electrostatic LbL assembled multilayer films of PB nanoparticles/poly(aniline) exhibits multiple-hue electrochromic properties because each individual component of PB nanoparticles and poly(aniline) is anodically electrochromic. PB nanoparticles protected by a polycation were alternately assembled with negatively charged glucose oxidase to form multilayer films in which the PB nanoparticles can catalyze the electrochemical reduction of H2O2 produced from enzymatic reaction and, therefore, can be used as a glucose biosensor.16 In general, (11) Hu, Y. L.; Yuan, J. H.; Chen, W.; Wang, K.; Xia, X. H. Electrochem. Commun. 2005, 7, 1252-1256. (12) Zhang, D.; Wang, K.; Sun, D. C.; Xia, X. H.; Chen, H. Y. J. Solid State Electrochem. 2003, 7, 561-566. (13) Ravaine, S.; Lafuente, C.; Mingotaud, C. Langmuir 1998, 14, 63476349. (14) Fiorito, P. A.; Goncales, V. R.; Ponzio, E. A.; Torresi, S. I. C. Chem. Commun. 2005, 366-368. (15) DeLongchamp, D. M.; Hammond, P. T. Chem. Mater. 2004, 16, 47994805. (16) Zhao, W.; Xu, J. J.; Shi, C. G.; Chen, H. Y. Langmuir 2005, 21, 96309634. (17) Zhang, D.; Zhang, K.; Yao, Y. L.; Xia, X. H.; Chen, H. Y. Langmuir 2004, 20, 7303-7307. (18) Decher, G. Fuzzy Nanoassemblies. Science 1997, 277, 1232-1237. (19) Tang, Z. Y.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. AdV. Mater. 2006, 18, 3203-3224. (20) Zhang, X.; Chen, H.; Zhang, H. Y. Chem. Commun. 2007, 1395-1405.

10.1021/la700239r CCC: $37.00 © 2007 American Chemical Society Published on Web 04/18/2007

Layer-by-Layer Deposited TiPS/PB Composite Films

composite PB films are superior to single-component PB films in view of the stability, the abundance in functionalities, and the performance of the resultant film devices. In our previous work, a facile LbL adsorption and reaction method for the preparation of titanium phosphate (TiPS) ultrathin films was developed.21 By repetitive adsorption of hydrated titanium from aqueous Ti(SO4)2 solution and subsequent reaction with phosphate groups, ultrathin films of TiPS can be easily fabricated. This method is characterized by its simplicity in film preparation and its ease in controlling film thickness. The asprepared TiPS film is mechanically stable in solvents such as water, acetone, and carbon tetrachloride under sonication. TiPS has well-known ion-exchange properties and is a good ionic conductor.22-24 We confer that the integration of TiPS and PB into ultrathin films will facilitate charge transfer between PB and substrate surface due to the easy cation diffusion within the film mediated by the TiPS component. Electrochemical and electrochromic properties will benefit from the facilitated electron transfer in PB composite films. Direct alternate deposition of TiPS and PB nanoparticles by electrostatic LbL assembly technique is impossible because they bear the same negative charges. In this work, we combine the LbL adsorption and reaction method to prepare TiPS layers21 and the multiple sequential adsorption to prepare PB layers5-7 together and fabricate successfully TiPS/PB composite ultrathin films. The deposition process, structures of TiPS/PB composite films, and their electrochemical properties are investigated in detail. Experimental Section Materials. Titanium sulfate [Ti(SO4)2], sodium hydrogen phosphate (Na2HPO4‚12H2O), sodium dihydrogen phosphate (NaH2PO4‚ 2H2O), iron chloride (FeCl3‚6H2O), and potassium ferrocyanide (K4[Fe(CN)6]‚3H2O) were of analytical grade and were purchased from Beijing Chemical Reagents Co. Poly(diallyldimethylammonium chloride) (PDDA) aqueous solution with a molecular weight of 100 000-200 000, poly(methacrylic acid) (PMAA) aqueous solution with a molecular weight of ca. 6500, and thioglycolic acid (TGA) were purchased from Sigma-Aldrich. Deionized water was used in all experiments. The phosphate salt (PS) solution comprises 0.1 M phosphate (Na2HPO4 and NaH2PO4) with its pH adjusted by the addition of H2SO4. Preparation of Titanium Phosphate/Prussian Blue Composite Films. Quartz and silicon wafers were immersed in slightly boiled piranha solution (3:1 mixture of 98% H2SO4 and 30% H2O2) for 20 min and rinsed with copious amounts of water. Caution: Piranha solution reacts Violently with organic material and should be handled carefully. The cleaned quartz and silicon wafers were immersed in 1.0 mg/mL PDDA aqueous solution for 20 min to obtain a cationic ammonium-terminated surface and were then ready for TiPS/PB composite film deposition. Subsequent immersion of the PDDAmodified substrate in 1 mg/mL PMAA aqueous solution for 20 min can produce the PMAA-modified substrate for single-component PB multilayer film deposition. In electrochemical experiments, the cleaned gold electrode was immersed in an ethanolic solution of TGA to render the surface carboxylic groups negatively charged because of the self-assembly of TGA on the electrode surface. The TGA-modified gold electrode allows the direct deposition of TiPS layer or PB layer. The procedure for preparation of the TiPS/PB composite films consists of two steps as follows. The first step is preparation of the TiPS layer by the LbL adsorption and reaction method. The (21) Wang, Q. F.; Zhong, L.; Sun, J. Q.; Shen, J. C. Chem. Mater. 2005, 17, 3563-3569. (22) Clearfield, A. Chem. ReV. 1988, 88, 125-148. (23) Pessoa, C. A.; Gushikem, Y.; Kubota, L. T.; Gorton, L. J. Electroanal. Chem. 1997, 431, 23-27. (24) Wang, Q. F.; Yu, H. J.; Zhong, L.; Liu, J. Q.; Sun, J. Q.; Shen, J. C. Chem. Mater. 2006, 18, 1988-1994.

Langmuir, Vol. 23, No. 11, 2007 6085 preparation of the TiPS layer was reported previously21,24 and is described briefly: The PDDA- or TGA-modified substrate was immersed in an aqueous solution of 10 mM Ti(SO4)2 dissolved in 0.1 M H2SO4 for 5 min. Then the substrate was transferred to the first PS solution (denoted as PS-1, pH 4.0) for a few seconds, after which it was immersed in the second PS solution (denoted as PS-2, pH 4.0) for 5 min. Finally, the substrate was rinsed with water and dried in a N2 stream. In this way, one layer of TiPS was deposited on the substrate. The second step is preparation of the PB layer by the multiple sequential adsorption method. The PB layer was prepared following the previously reported multiple sequential adsorption method5-7 with a slight modification. The TiPS-deposited substrate was first immersed in 10 mM FeCl3 solution (pH 3.0, tuned by NaOH) for 5 min, rinsed with water, and dried in a N2 stream. Then the substrate was immersed into 10 mM K4[Fe(CN)6] (pH 3.0, tuned by HCl) solution for 5 min, followed by water rinsing and N2 drying process. In this way, one cycle of TiPS/PB film was prepared. TiPS/PB composite films with increased thickness could be prepared by repeating the two steps in a cyclic fashion. In the following discussion, TiPS/PB films with n cycle deposition is noted as (TiPS/PB)*n. When the TiPS is the outmost layer, it is noted as a half cycle. The successive deposition of single-component PB multilayer films was conducted on a PMAA- or TGA-modified substrate. The process for film preparation is the same as that of TiPS/PB composite films except the steps for the deposition of TiPS layers are omitted. Characterization. UV-vis absorption spectra were recorded on a Perkin-Elmer Lambda 800 spectrophotometer. Fourier transform infrared (FT-IR) spectra were collected on a Bruker IFS 66V instrument. Quartz crystal microbalance (QCM) measurements were taken with a KSV QCM-Z500 using quartz resonators with both sides coated with Ag (F0 ) 9 MHz). Cyclic voltammetry was carried out on a BAS100 (Bioanalytical Systems) electrochemistry work station. The electrochemical cell consisted of three electrodes, where a gold electrode covered with TiPS/PB films acted as the working electrode, a platinum wire as the counterelectrode, and a Ag/AgCl electrode as the reference electrode. Scanning electron microscopy (SEM) observations were carried out on a JEOL FESEM 6700F scanning electron microscope with primary electron energy of 3 kV. X-ray diffraction (XRD) patterns were measured on a Rigaku D/Max 2550 diffractometer with polished single-crystalline silicon (100) as substrate. The positions of the diffractions were calibrated on the basis of those of silicon. 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 energy-dispersive X-ray spectroscopy (EDS) measurement was conducted on an EDAX Genesis 2000 X-ray microanalysis system attached to an XL30 ESEM FEG scanning electron microscope.

Results and Discussion Preparation of TiPS/PB Composite Films. As demonstrated in the Experimental Section, the preparation of TiPS/PB composite films is the result of alternate deposition of TiPS and PB layers on the substrate. As reported previously for the preparation of TiPS multilayers, the deposition of a TiPS layer consists of the deposition of positively hydrated titanium from aqueous Ti(SO4)2 solution and the subsequent reaction with phosphate group to produce a titanium phosphate layer.21 When the TiPS-deposited substrate was immersed into FeCl3 solution, Fe3+ ions were electrostatically adsorbed on the substrate because the phosphate group of TiPS is negatively charged. During the subsequent immersion into aqueous K4[Fe(CN)6] solution, Fe(CN)64- ions interact and react with the previously adsorbed Fe3+ ions to form PB.7 The negatively charged PB layer allows the adsorption of the next layer of positively charged hydrated titanium, and therefore composite films of TiPS/PB with increased thickness can be fabricated by repeating the above processes.

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Figure 2. QCM frequency decrease (-∆F) of alternative deposition of TiPS (O) and PB (b).

Figure 1. UV-vis absorption spectra of (TiPS/PB)*n films with n ranging from 0.5 to 10.5 from bottom to top. Spectra measured following deposition of (---) TiPS layers and (s) PB layers are shown. (Inset) Absorbance at 209 and 733 nm recorded as a function of deposition cycles following deposition of TiPS layer (open symbols) and PB layer (solid symbols).

UV-vis absorption spectroscopy and QCM measurements were employed to monitor the fabrication process of TiPS/PB composite films. UV-vis absorption spectra of different deposition cycles of TiPS/PB films are shown in Figure 1. After the first TiPS layer deposition, a characteristic absorption peaking at ca. 230 nm is noticed, confirming deposition of the TiPS layer.21 After deposition of the PB layer, two absorptions peaking at ca. 209 nm (charge transfer between metal and CN groups) and 733 nm (charge transfer between FeIII and FeII)25 appear, confirming the formation of PB layer on top of the TiPS layer. Linear increase of the absorbance at 209 nm with increasing deposition cycles was observed after two deposition cycles, as shown in the inset of Figure 1. The absorbance at 733 nm shows a zigzag increase with increasing deposition cycles. The deposition of a TiPS layer on top of a PB layer increases the absorbance at 733 nm, while a further PB layer deposition decreases the absorbance a bit. The reason for the increased PB absorbance at 733 nm after deposition of a TiPS layer is not very clear because the TiPS layer does not have absorbance in the visible range. We temporarily assume that Na+ ions presented in the TiPS layer might produce an influence on the electron transfer between FeIII and FeII of PB. As a result, PB absorbance at 733 nm increases after deposition of a TiPS layer. The subsequent deposition of a PB layer replaces the Na+ ions in TiPS layers by K+ ions and decreases the absorbance of previous PB layers again. Incorporation of Na+ ions during the deposition of TiPS layer and the removal of Na+ ion in the subsequent deposition of PB layer was confirmed by XPS measurements, as shown below. The same phenomenon in the UV-vis absorption spectrum was also observed in zirconium phosphate/PB composite films prepared by the same method. As shown in Figure 2, the QCM frequency regularly decreases after three cycles of film deposition because of the successive deposition of TiPS/PB films on the resonator. The frequency decreases for the deposition of one layer of TiPS and PB were 262.2 ( 131.7 and 434.3 ( 54.6 Hz, respectively. Both UV-vis spectroscopy and QCM measurements confirmed the successful deposition of the TiPS/PB composite film in a LbL fashion. Chemical Composition of the As-Prepared TiPS/PB Composite Films. The FT-IR spectrum of an as-prepared (TiP/PB)(25) Robin, M. B. Inorg. Chem. 1962, 1, 337-342.

Figure 3. FT-IR spectrum of an as-prepared (TiPS/PB)*30 film on silicon wafer.

*30 film deposited on a silicon wafer is given in Figure 3. The broad peak at 3400 cm-1 and the one at 1610 cm-1 correspond to hydroxyl groups and adsorbed water in the TiPS/PB composite films. The strong peak located at 2079.1 cm-1 is associated with the cyano stretching mode of the CN groups in FeII-CN-FeIII of PB.26-28 The peak at 497.6 cm-1 is the characteristic bending mode frequency of FeII-CN-FeIII in PB.29-31 A broad peak at 885-1280 cm-1 is related to the TiPS component in the TiPS/ PB composite films, as observed in the single-component TiPS multilayer films. The peaks in this range are often the characteristic frequencies of antisymmetric/symmetric stretching frequencies of P-O and P-OH groups.21 Table 1 presents the XPS data and the atomic ratio of elements for the as-prepared (TiPS/PB)*5 and (TiPS/PB)*5.5 films deposited on silicon wafers. The XPS data show the presence of titanium, phosphorus, oxygen, iron, carbon, and nitrogen in both (TiPS/PB)*5 and (TiPS/PB)*5.5 films. The atomic ratio of Ti:P remained about 1:2 in the (TiPS/PB)*5 and (TiPS/PB)*5.5 films, which is consistent with the atomic ratio of titanium phosphate [Ti(HPO4)2] in the TiPS single-component film.21 In both cases, there is a large deviation between the atomic ratio of Fe:C:N (1:25:6.9) calculated from XPS data and the theoretical value of PB (1:2.57:2.57). The large deviation might originate (26) Jaiswal, A.; Colins, J.; Agricole, B.; Delhaes, P.; Ravaine, S. J. Colloid Interface Sci. 2003, 261, 330-335. (27) Zhao, G.; Feng, J. J.; Zhang, Q. L.; Li, S. P.; Chen, H. Y. Chem. Mater. 2005, 17, 3154-3159. (28) Reguera, E.; Ferna´ndez-Bertra´n, J.; Balmaseda, J. The existence of ferrous ferricyanide. Transition Met. Chem. 1999, 24, 648-654. (29) Guo, Y. Z.; Guadalupe, A. R.; Resto, O.; Fonseca, L. F.; Weisz, S. Z. Chem. Mater. 1999, 11, 135-140. (30) Kulesza, P. J.; Malik, M. A.; Denca, A.; Strojek, J. Anal. Chem. 1996, 68, 2442-2446. (31) Wilde, R. E.; Ghosh, S. N.; Marshall, B. J. Inorg. Chem. 1970, 9, 25122516.

Layer-by-Layer Deposited TiPS/PB Composite Films

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Figure 4. XRD pattern of an as-prepared (TiPS/PB)*30 film on silicon wafer. Table 1. XPS Data and Atomic Ratio of Elements for the As-Prepared (TiPS/PB)*5 and (TiPS/PB)*5.5 Films Deposited on Silicon Wafers bending energy, eV

content, %

(TiPS/PB)*5 (TiPS/PB)*5.5 (TiPS/PB)*5 (TiPS/PB)*5.5 Ti Ti

2p3/2 2p1/2

458.8 464.8

458.9 464.8

1.07

1.71

P 2p O 1s Na 1s

133.39 531.9

133.2 531.1 1071.3

2.90 15.20 0

3.10 18.96 0.70

Fe Fe

2p3/2 2p1/2

708.5 721.4

708.3 721.2

2.29

1.81

C N

1s 1s

284. 7 397.7

284.7 397.6

57.40 15.77

59.94 13.01

K K

2p3/2 2p1/2

293.8 296.6

293.9 296. 6

5.38

0.78

from the adventitious carbon contamination that cannot be avoided during XPS measurement32,33 and the underlying PDDA layer, which is used to render the substrate positively charged. But the atomic ratio of Fe:C:N given by EDS of a (TiPS/PB)*30 film is 1:2.55:2.90, which is closely consistent with the theoretical value of PB. The FT-IR and XPS data further confirm the presence of TiPS and PB components in the asprepared TiPS/PB films. The difference in XPS data between (TiPS/PB)*5 and (TiPS/PB)*5.5 films is the presence/absence of K+ and Na+ signals. For the (TiPS/PB)*5.5 film in which TiPS is the outermost layer, both K+ and Na+ ions are detected, indicating that both K+ and Na+ ions can interpenetrate into the films. For the (TiPS/PB)*5 film in which PB is the outmost layer, only K+ ions are detected, indicating the replacement of Na+ ions by K+ ions during the deposition of the PB layer. It is known that Na+ ions cannot penetrate the PB lattice.1,34,35 Therefore, the Na+ ion detected in the (TiPS/PB)*5.5 film exists in the TiPS layers. The presence of Na+ ions after the TiPS layer deposition might explain the increased absorbance of PB at 733 nm. Structural Characterization of TiPS/PB Composite Films. Figure 4 presents the XRD pattern of the as-prepared TiPS/PB composite films. XRD pattern shows diffraction peaks at 2θ being 17.35° (200), 24.81° (220), 35.27° (400), and 43.49° (422), which are in good agreement with those of the bulk PB with face-centered-cubic phase (JCPDS card 52-1907).36,37 (32) Malik, M. A.; Kulesza, P. J.; Wlodarczyk, R.; Wittstock, G.; Szargan, R.; Bala, H.; Galus, Z. J. Solid State Electrochem. 2005, 9, 403-411. (33) Tosatti, S.; Michel, R.; Textor, M.; Spencer, N. D. Langmuir 2002, 18, 3537-3548. (34) Jin, W. Q.; Toutianoush, A.; Pyrasch, M.; Schnepf, J.; Gottschalk, H.; Rammensee, W.; Tieke, B. J. Phys. Chem. B 2003, 107, 12062-12070. (35) Karyakin, A. A. Electroanalysis 2001, 13, 813-819.

Figure 5. SEM images of (TiPS/PB)*n films with n being 5 (a), 5.5 (b), 15 (c), and 30 (d). The scale bars in all images correspond to 200 nm.

The diffraction peak located at 38.89° belongs to Si (001). These results demonstrate that crystalline PB with facecentered-cubic phase is successfully fabricated in the TiPS/PB films. The surface morphology of the TiPS/PB composite film was investigated by SEM. Figure 5 shows SEM images of the asprepared (TiPS/PB)*n films with n being 5, 5.5, 15, and 30 in panels a-d, respectively. For the (TiPS/PB)*5 film in which PB was the outermost layer, cubelike PB nanocrystals were observed (Figure 5a). The complete PB crystals on the surface have an average size of 67.0 ( 14.0 nm. The smaller PB crystals with incomplete structure are not counted because their size is difficult to measure (therefore, the statistical size of PB crystals obtained from the SEM image is larger than their real size). With a further TiPS layer deposition, all the surface PB crystals were covered by a layer of TiPS, as grains of TiPS was recognized clearly in Figure 5b. The small cubes of PB were no longer distinguished because of the flattening of the surface by the TiPS layer deposition. For the (TiPS/PB)*15 film, the size of the surface complete PB crystals increased to 140.7 ( 26.4 nm, indicating the continuous growth of the individual PB crystals with increasing number of deposition cycles. When the number of deposition cycles reaches 30, complete surface PB crystals as large as 234.3 ( 25.5 nm can be found (Figure 5d). But most PB crystals have an incomplete cubic structure with size around or less than 100 nm because the space hindrance inhibits the further growth of these crystals or they are newly formed. The cross-sectional SEM images of the (TiPS/PB)*30 film is shown in Figure 6a. The PB crystals embedded in the TiPS film matrix can be clearly recognized. The film has a constant thickness of 184.4 ( 31.7 nm. The thickness of the TiPS/PB composite films with deposition cycles of 5, 10, 15, 20, and 30 are shown in Figure 6b. A linear increase of the film thickness with an increase of 5.8 ( 1.2 nm per deposition cycle was observed, with a (TiPS/PB)*5 film having a thickness of 47.3 ( 13.6 nm. Considering the fact that one deposition cycle leads to a film thickness increase of 5.8 ( 1.2 nm, the surface PB crystals in Figure 5a,c are not newly formed in one deposition cycle but represent the further growth of previously formed PB crystals. Meanwhile, a close look at the (36) Song, Y. Y.; Zhang, K.; Xia, X. H. Appl. Phys. Lett. 2006, 88, 053112. (37) Wu, X. L.; Cao, M. H.; Hu, C. W.; He, X. Y. Nanocryst. Growth Des. 2006, 6, 26-28.

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Figure 7. Cyclic voltammograms of a gold electrode deposited with (TiPS/PB)*5 films in 0.1 M KCl electrolyte solution with different scan rates. (Inset) Cathodic peak current as a function of square root of scan rates.

Figure 6. (a) Cross-sectional SEM image of a (TiPS/PB)*30 film and (b) dependence of the thickness of the TiPS/PB films as a function of deposition cycles. The scale bar in panel a corresponds to 200 nm.

SEM images in Figure 5a,c discloses that TiPS/PB composite films with PB as the outermost deposition layer have a very smooth surface. Therefore, partial removal of the TiPS layer that covers the PB crystal in the growth direction takes place during the subsequent deposition of PB. From the surface and cross-sectional SEM images of TiPS/PB composite films, we suppose that, during the deposition of PB crystals, the partial TiPS layer that covers the previous layer of PB crystals dissolved, which allows further growth of the PB crystals. Meanwhile, new PB crystals can also deposit. Therefore, the TiPS/PB film should have a structure in which the interstices of the PB crystal films are filled with TiPS component. Electrochemical Properties of TiPS/PB Composite Films. PB has two sets of peaks with formal potentials occurring at ca. 0.2 and 0.9 V (vs SCE), corresponding to two redox processes, that is, the reduction to Prussian white (PW) and the oxidation to Prussian green (PG).5,35 TiPS is electrochemically inert in a wide range of voltage. Here, the electrochemical signals in a relatively low-potential range corresponding to the redox activity of PB and PW were used to characterize the electrochemical properties of TiPS/PB composite films. Figure 7 shows the cyclic voltammograms (CVs) of a Au electrode deposited with a (TiPS/ PB)*5 film in 0.1 M KCl solution with different scan rates. The peak-to-peak potential separation increases with increasing scan rate. The inset of Figure 7 shows the dependence of cathodic peak current on the square root of scan rate. A linear increase of cathodic peak current with correlation coefficient r ) 0.9997 from 0 to 3 V/s was obtained, indicating that the peak current is controlled by the diffusion process. Figure 8a shows the CVs of TiPS/PB composite films with different deposition cycles deposited on a TGA-modified Au electrode in 0.1 M KCl with a scan rate of 50 mV/s. In the scan range of -0.1 to 0.5 V, the Au electrode after deposition of the first layer of TiPS shows no redox peaks, indicating the inertness of the TGA and TiPS layers in this scan range. Following the first PB layer deposition, a pair of cathodic and anodic peaks located at 163 and 207 mV is observed. After the second layer of TiPS deposition, the cathodic and anodic peaks moved to 158 and 196 mV, respectively,

Figure 8. (a) Cyclic voltammograms of a gold electrode deposited with (TiPS/PB)*n films with n ) 0.5, 1, 1.5, 2, and 2.5. (---) TiPS layers; (s) PB layers. (Inset) Intensity of peak current vs number of deposition cycles following TiPS layer (open symbols) and PB layer (solid symbols) deposition. (b) Cyclic voltammograms of a gold electrode deposited with (PB)*n films with n ) 0-10. (Inset) Intensity of peak current vs number of PB layers. The electrolyte solution used is 0.1 M KCl and the scan rate is 50 mV/s in both cases.

followed by a decrease of the peak currents. The second layer of PB deposition returned the cathodic and anodic peaks back to 162 and 210 mV, respectively and increased the peak currents. Twenty-one deposition cycles of TiPS/PB composite film were deposited on the Au electrode, and the corresponding cathodic and anodic peak currents as a function of the number of deposition cycles are shown in the inset of Figure 8a. In the examined number of deposition cycles, an almost linear increase of the peak currents with increasing number of deposition cycles was

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Figure 9. Cathodic and anodic peak potentials as a function of the number of deposition cycles in TiPS/PB (b) and PB films (9).

observed, indicating that electron transfer from PB crystals to the electrode surface proceeds very easily even in thick TiPS/PB films. The decrease of peak currents after subsequent deposition of the TiPS layer is due to the decreased counterion interpenetration during the electrochemical reaction. The single-component PB multilayer films were deposited on a TGA-modified Au electrode and their CVs are presented in Figure 8b for comparison. The peak currents of the PB films increased with increasing number of deposition cycles with 10 layers, then leveled off and even decreased with further increase in the deposition layers. With the same deposition cycles, the thickness of the TiPS/PB composite film is larger than that of the single-component PB film. For example, with 15 deposition cycles, the TiPS/PB and PB films have respective thicknesses of 111.0 ( 14.2 and 93.5 ( 18.4 nm. On the basis of the above results, one can confirm that TiPS component facilitates electron transfer in TiPS/PB composite films when compared with the single-component PB films. The facilitated electron transfer in TiPS/PB composite films benefits from the easy diffusion of counterions into and out of the TiPS/PB films during the electrochemical reaction of PB, which is contributed by the TiPS component. The cathodic and anodic potentials of TiPS/PB composite films and PB films with different deposition cycles are shown in Figure 9. In both cases, the anodic potentials increase with increasing deposition cycles, and the cathodic potentials decrease with increasing deposition cycles. This result is the indicative that, with increasing film deposition cycles, electron transfer becomes difficult because, on one hand, the electron-transfer rate decreases with increasing distance between the electroactive center and the electrode surface; on the other hand, penetration of the counterions becomes difficult as the film thickness increases. With the same number of film deposition cycles, the TiPS/PB composite film exhibits a smaller peak-to-peak potential separation than the PB film, indicating a faster electron transfer in TiPS/PB composite film than in PB film. This result is consistent with the one obtained in Figure 8. Stability is an important concern that determines the longterm application of the TiPS/PB composite films. The stability of the (TiPS/PB)*5-modified Au electrode was examined by repeated scans between -0.1 and 0.5 V in a 0.1 M phosphatebuffered saline solution (PBS, pH 6.0) containing 0.1 M KCl at a scan rate of 50 mV/s. After 50 cycles of scanning, the Ipa and Ipc decreased ca. 8.06% and 4.14% compared to their original intensity, respectively. Under the same scan conditions, the Ipa and Ipc of a Au electrode deposited with five layers of PB decreased ca. 31.73% and 11.36%, respectively. This result indicates that the TiPS component improved the stability of the PB component in TiPS/PB composite films.

Figure 10. Catalytic reduction of H2O2 on a gold electrode deposited with a (TiPS/PB)*5 film in an aqueous mixture electrolyte solution of 0.1 M KCl and 0.1 M PBS (pH 6.0) with different concentrations of H2O2 added, measured by cyclic voltammograms at a scan rate of 50 mV/s. (Inset) Current at -100 mV as a function of the concentration of H2O2.

As an “artificial peroxidase”, PB shows excellent electron transfer and has the ability to catalyze the reduction of H2O2.17,27,35,38-41 An Au electrode deposited with a (TiPS/PB)*5 film was employed to investigate the catalytic reduction of H2O2 in a deoxidized solution. Figure 10 shows the CV curves of the TiPS/PB-modified Au electrode before and after a successive addition of H2O2 in 0.1 M PBS (pH 6.0) containing 0.1 M KCl. Before the H2O2 addition, a shoulder cathodic peak appears in the lower voltage near the original cathodic peak when compared with the CV curves in Figure 7. The shoulder cathodic peak locates at the lower voltage should be caused by Na+ of PBS, whose hydrate ion size is larger than K+ and could not penetrate the framework of PB.1,34,35 With the successive addition of H2O2, the cathodic current increased rapidly while the anodic current decreased and finally disappeared. This phenomenon provides notable evidence of catalyzed electrochemical reduction of H2O2 by PB. PB acts as an electrontransfer mediator between the electrode and H2O2. The process for the electrochemical reduction of H2O2 by PB is welldocumented in the literature.15,17,39,42 The inset in Figure 10 shows the dependence of the cathodic currents at -100 mV on the concentration of H2O2 added. A good linear calibration ranging up to 29.67 mM H2O2 (r ) 0.9997) is obtained for a (TiPS/ PB)*5-modified Au electrode. The TiPS/PB composite film prepared by the LbL deposition process has the potential to be used in H2O2 biosensing.

Conclusions In the present study, we show that TiPS/PB composite films can be fabricated by the combination of LbL adsorption and reaction method to prepare TiPS layers and the multiple sequential adsorption method to prepare PB nanocrystal films. The resulting TiPS/PB composite films have a structure in which the interstices of the PB nanocrystal films are filled with TiPS component. The presence of the TiPS component makes the interpenetration of (38) Karyakin, A. A.; Karyakina, E. E.; Gorton, L. Anal. Chem. 2000, 72, 1720-1723. (39) Ricci, F.; Palleschi, G. Biosens. Bioelectron. 2005, 21, 389-407. (40) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042-6108. (41) Moscone, D.; D’Ottavi, D.; Compagnone, D.; Palleschi, G.; Amine, A. Anal. Chem. 2001, 73, 2529-2535. (42) Ellis, D.; Eckhoff, M.; Neff, V. D. J. Phys. Chem. 1981, 85, 1225-1231.

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counterions during the electrochemical reaction of PB become easier, and enhanced electrochemical properties in TiPS/PB composite films in comparison with pure PB nanocrystal films is obtained. TiPS/PB composite films can catalyze the electrochemical reduction of H2O2 and are potentially useful as H2O2 biosensors. Potential applications of TiPS/PB composite films in other areas such as electrochromic film materials are highly anticipated.

Wang et al.

Acknowledgment. This work was supported by the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD Grant 200323), National Basic Research Program (2007CB808000), and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT Grant IRT0422). LA700239R