Pt Nanoparticles Supported on TiO2 Colloidal Spheres with

Jul 2, 2009 - great promise for sensitive and fast detection of glucose. 1. Introduction ..... the free GOD in solution (18 mM).29 In addition, the Km...
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J. Phys. Chem. C 2009, 113, 13023–13028

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Pt Nanoparticles Supported on TiO2 Colloidal Spheres with Nanoporous Surface: Preparation and Use as an Enhancing Material for Biosensing Applications Dan Wen, Shaojun Guo, Junfeng Zhai, Liu Deng, Wen Ren, and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academy of Sciences, Beijing, 100039, P.R. China ReceiVed: January 14, 2009; ReVised Manuscript ReceiVed: June 2, 2009

An easy surface-modified method has been developed to link -NH2 groups to the TiO2 colloidal spheres with nanoporous surface (f-TiO2). It was found that the as-prepared f-TiO2 is positively charged in neutral conditions and could act as an electrostatic anchor for nanosructures with opposite charge. Furthermore, platinum nanoparticles (Pt NPs) are successfully assembled on the f-TiO2 mainly via electrostatic interaction to fabricate a new kind of Pt NPs/TiO2 hybrid nanomaterial (f-TiO2-Pt NPs). The morphology, structure, and composition of the hybrids were characterized by the means of diverse techniques such as transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction, and Raman spectra. Electrochemical experiments indicate the electrode modified with f-TiO2-Pt NPs shows prominent electrocatalytic activity toward the oxidation of hydrogen peroxide. In particular, as an example, a glucose biosensor fabricated by casting on additional glucose oxidase containing a biocompatible polymer, chitosan on the f-TiO2-Pt NPs film exhibits great promise for sensitive and fast detection of glucose. 1. Introduction TiO2 has been widely used in many different applications, including photocatalysts, solar cells, biomaterials, and environmental catalysts.1 During the past decade, the research on the synthesis of nanosized porous TiO2 materials and their application in the catalytic industry and photocells has been rather intense. Because of the important optical and electronic properties, it is useful for catalyst support,2 sensors,3 and so on. Also, it has been widely used as additives in paint, toothpaste, and cosmetics because of its great biocompatibility. However, metal nanoparticles (MNPs) (e.g., gold, silver, platinum, and palladium) have received a great deal of attention because of their unique physical and chemical properties as well as their enormous potential applications such as catalysts, nanoelectronic devices, sensors, and surface-enhanced Raman scattering. Thus, functionalizing nanoporous TiO2 supports with MNPs combined with the properties of two functional nanomaterials to achieve a wider range of applications will probably play an important role in the development of nanoscience and nanotechnology.4 Several techniques have been devised to synthesize MNPs/ TiO2 support composites like impregnation,5 sol-gel-based processes,6 flame spray synthesis,7 electrodeposition,8 laser pyrolysis,9 sonochemistry,10 and UV irradiation.11 Despite these successes in preparing MNPs/TiO2 hybrid nanostructures, most of the sequential methods involve complicated procedures or rigorous conditions. And also, MNPs loaded on TiO2 supports with nanoporous surface are often attractive for catalyzing reactions to energy generation and environmental preservation (e. g., photocatalytic generation of hydrogen from water,12 carbon monoxide oxidation,13 and photodegradation of organic pollutant14). However, to the best of our knowledge, relatively few studies have probed on its biosensing application. * To whom correspondence should be addressed. E-mail: dongsj@ ciac.jl.cn.

Platinum (Pt), as one of the most researched noble metals, has extensive research in the fields of sensors, fuel cells, and catalysts in the reduction of pollutant gases emitted from automobiles.15 By taking advantage of biocompatibility, huge surfaces, and good electrocatalytic activity toward hydrogen peroxide (H2O2), Pt NPs have been reported to be very efficient as a matrix of enzyme sensors. As we know, the immobilization of MNPs on solid supports emerges as an efficient procedure to retain their catalytic activities. For instance, carbon nanotubes (CNTs) have often been viewed as an excellent support of Pt NPs in sensing applications.16 TiO2 nanostructures were also used as the support for Pt NPs to fabricate TiO2/Pt NPs nanocomposites.17 Here, we present a simple strategy for surface modification of TiO2 colloidal spheres with nanoporous surface by 3-aminopropyltrimethoxysilane to prepare a functionalized TiO2 nanomaterial (f-TiO2), which is positively charged in neutral conditions and offers a versatile solid support for oppositely charged nanostructures. Citrate-stabilized Pt NPs are easily assembled on the f-TiO2 to fabricate a nanostructure (noted as f-TiO2-Pt NPs) mainly via electrostatic interaction. Our work presented here has two main important benefits. First, the assembly of the f-TiO2 and Pt NPs is simply achieved by mixture at room temperature; the f-TiO2 can be used as a versatile solid support for the fabrication of other NPs/TiO2 nanohybrids. Second, this hybrid nanomaterial retains the catalytic activity of Pt NPs and also gains advantages over its parent materials (i.e., good solubility and dispersibility in water). Because of these benefits, H2O2 and glucose were selected as probe molecules to investigate the catalytic property of f-TiO2-Pt NPs. It was found that the present hybrid nanomaterial possesses enhanced electrochemical sensing ability toward H2O2 and a high-sensitivity glucose sensor was obtained on the hybrid material/glucose oxidase (GOD) composite film, which compares favorably with Pt NPs supported on other kinds of nanomaterials, such as CNTs which were recently reported.

10.1021/jp9003714 CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

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2. Experimental Section Chemicals. GOD (EC 1.1.3.4, from Aspergillus niger) and chitosan (CS) were obtained from Sigma Chemical Co., and the later dissolved in 2% acetic acid solution. Sodium citrate, H2PtCl6 · 6H2O, ammonium hydroxide and ethanol were purchased from the Shanghai Chemical Factory and used as received without further purification. 3-Aminopropyltrimethoxysilane (APTMS) and NaBH4 were obtained from Acros. A 30% H2O2 solution was purchased from Beijing Chemical Reagent and a fresh solution of H2O2 was prepared daily. β-D-(+)glucose was also purchased from Beijing Chemical Reagent and the stock solution was allowed to mutorotate at room temperature overnight before use. Other chemicals were of analytical grade. Doubly distilled water was used throughout. A 0.1 M phosphate buffer solution (PBS) (pH 7.1) consisting of KH2PO4 and Na2HPO4 was employed as the supporting electrolyte. Preparation of f-TiO2-Pt NPs. TiO2 nanospheres with nanoporous surface (about 100 mg) were prepared according to the reported literature18 and dispersed in 40 mL of ethanol. The resulting product was functionalized with APTMS. Briefly, 400 µL of APTMS was added to the above solution, followed by the addition of 2 mL of water and 2 mL of ammonia. After the solution was continuously stirred for 10 h, it was centrifuged and then washed with ethanol and water three times, respectively. Finally, the collected white precipitation was redispersed into 5 mL of water. Pt NPs were prepared according to the previous literature with slight modification.19 f-TiO2-Pt NPs hybrid nanomaterial was synthesized in a 5 mL vial by adding excess Pt NPs colloid to 0.5 mL of surface-modified TiO2 nanospheres. The mixture was sonicated for 30 min and then kept overnight at room temperature. The above solution was centrifuged and washed with water three times. Finally, the purified f-TiO2-Pt NPs were dispersed in 2 mL of water, which formed a gray, stable, and homogeneous suspension. Preparation of Modified Electrodes. Glassy carbon (GC, 2.6 mm in diameter) electrode was polished with 1.0 and 0.3 µm alumina slurry sequentially and then washed ultrasonically in water and ethanol for a few minutes, respectively. The cleaned GC electrode was dried with a high-purity nitrogen steam for the next modification. Five microliters of as-prepared f-TiO2-Pt NPs suspension was deposited on the GC electrode. It was then left to dry at room temperature, after which 5 µL of 0.5% CS solution was spread onto the electrode surface to form a film (noted as CS/f-TiO2-Pt NPs/GC). For the immobilization of enzyme, 5 µL of GOD and CS mixture (v:v ) 2:1) was dropped onto the f-TiO2-Pt NPs-modified electrode and dried at 4 °C (noted as CS/GOD/f-TiO2-Pt NPs/GC). For comparison, two other types of enzyme electrodes were formed: CS/GOD/GC and CS/GOD/f-TiO2/GC. The enzyme electrodes were stored at 4 °C in a refrigerator if not used. Apparatus. The morphologies of f-TiO2 colloidal spheres with nanoporous surface and f-TiO2-Pt NPs hybrid nanomaterial were characterized by scanning electron microscopy (SEM, PHILIPS XL-30) and transmission electron microscopy (TEM, Hitachi H-8100 EM). An energy-dispersive X-ray (EDX) spectrometer fitted to the SEM was used for elemental analysis of the sample. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB-MKII spectrometer (VG Co.) with Al KR X-ray radiation as the X-ray source for excitation. X-ray powder diffraction (XRD) analysis was performed with a D/Max 2500 V/PC X-ray diffractometer using Cu (40 kV, 30 mA) radiation. The Raman spectra were obtained with a Jobin-Yvon Model T64000 Laser Raman Spectrometer, using the 514.5 nm line of an argon ion laser (Spectra Physics Stabliti Model 2017)

Wen et al. as the excitation source. All electrochemical measurements were carried out at a CHI 832 electrochemical workstation (Shanghai) at room temperature. ξ-Potential value measurement was performed on a Zeta PALS, zeta potential analyzer (Brookhaven Instruments Corporation) with a 2 mg/mL f-TiO2 aqueous suspension as a sample. The modified electrode was used as the working electrode. Platinum flat and Ag/AgCl (saturated KCl) were used as the counter electrode and the reference electrode, respectively. Magnetic stirring was maintained mildly throughout amperometric measurements. 3. Results and Discussion A two-step process was employed to prepare f-TiO2-Pt NPs nanomaterial. The first step was to prepare the surface-modified TiO2 colloidal spheres with nanoporous surface. In the second step, the NH2-functionalized TiO2 spheres were mixed with Pt NPs to form the f-TiO2-Pt NPs hybrid nanomaterial (as shown in Figure 1A). The assembly of oppositely charged nanostructures has been widely viewed as an efficient route to fabricate multifunctional hybrid nanomaterials.20 Herein, Pt NPs were anchored to the surface of f-TiO2 by a simple and versatile scheme of electrostatic adsorption. The f-TiO2 in aqueous solution was determined to have a positive ξ-potential of +23.7 mV as a result of the hydrolyzation of APTMS. Thus, the NH2functional TiO2 prepared could act as an electrostatic anchor for absorption of negatively charged citrate-stabilized Pt NPs. When dispersed in water, f-TiO2-Pt NPs formed a gray homogeneous suspension that could stay stable for at least 2 months. Photographs can be found in Figure 1B. The TiO2 colloidal spheres with nanoporous surface were synthesized based on in situ hydrolysis of titanium glycolate. The XRD pattern (Figure 2A) reveals that the as-obtained sample is made of pure anatase (JCPDS Card No. 21-1272), which is in accordance with the reference reported.18 The peaks are broad, suggesting that the crystallite size in the colloidal spheres is small. Raman spectra further illuminate the changes in phase transition during the preparation of f-TiO2 (Figure 2B). Before hydrolysis, titanium glycolate spheres are amorphous, as is expected.18 It can be seen that the emergence of the peaks after mixing with water at refluxing temperature suggests the crystallinity of TiO2 formed; Raman peaks at 148, 200, 401, 521, and 642 cm-1, which can be attributed to the five Ramanactive modes of anatase phase with the symmetries of Eg, Eg, B1g, A1g, and Eg, respectively, indicates nanpoporous TiO2 crystallizes as anatase.21 It is noticed that the surface-modified process with the NH2 group did not change the crystal structure of TiO2 since there are no differences in peaks of f-TiO2 compared to those of TiO2 colloidal spheres with nanoporous surface. Figure 3A shows the typical SEM image of the f-TiO2-Pt NPs hybrid nanospheres. The as-prepared hybrid nanomaterial possesses uniform size with an average diameter of 250 nm. TEM was employed to further reveal the detailed structure of f-TiO2-Pt NPs. It can be seen from Figure 3B that the rough surface features and the presence of channels in the f-TiO2 nanospheres offer a great opportunity for Pt NPs to load on, resulting in hybrid nanostructures. Figure 3C shows the typical TEM image of f-TiO2-Pt NPs. Compared with the f-TiO2 nanospheres, the hybrid nanomaterial has hierarchical structures, consisting of interconnected small NPs with a pore size of several nanometers. The numerous individual dark nanodots spread along the gray nanoporous spheres in Figure 3 C are Pt NPs, which indicates that the well-dispersed Pt NPs decorate the f-TiO2 quite uniformly. This reveals that the NH2 group on

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Figure 1. (A) Assembly procedure for the surface-modified TiO2 nanospheres with nanoporous surface-supported Pt NPs and (B) photographs of f-TiO2 (left) and f-TiO2-Pt NPs (right).

Figure 2. (A) XRD pattern of the TiO2 colloidal spheres with nanoporous surface and (B) Raman spectra of TiO2 nanospheres during the surfacemodified process.

the surface of the TiO2 plays a key role in the attachment of Pt NPs. The chemical composition of f-TiO2-Pt NPs was determined by EDX, as shown in Figure 3D. The main peaks of Ti, Pt, and O are noticed. Obviously, the strong Ti and O peaks should be ascribed to TiO2 nanospheres and Pt peak is attributed to Pt NPs decorated on TiO2 supports. XPS was employed for the surface analysis of the f-TiO2-Pt NPs sample for further investigation of Pt NPs supported on TiO2. Figure 4 depicts the Pt signal in the XPS spectra of the f-TiO2-Pt NPs. The peaks at 70.5 and 73.8 eV were 4f1/2 and 4f5/2 of the Pt0 metallic state reported on titanium oxide, respectively,22 which further confirmed the existence of Pt NPs on f-TiO2. However, compared to free citrate-stabilized Pt NPs, the binding energy of Pt on the surface is higher (∼0.2 eV). Considering that the reference level has no change in measurements, the change in binding energy (BE) is probably due to the electron transfer from f-TiO2 to Pt NPs and could also be

explained by the electrostatic interaction between the COO- of citrate-stabilized Pt NPs and the NH2 group of the f-TiO2. Additionally, with the choice of APTMS, the surface of the TiO2 colloidal spheres with nanoporous surface was tailored to be positively charged. Other NPs such as semiconductor nanocrystals and magnetic NPs can be selectively attached to f-TiO2. That is, this f-TiO2 would act as a versatile matrix and the NPs-decorated TiO2 could be used in catalytic, electronic, optical, and magnetic applications. Considering the excellent catalytic activity and biocompatibility of the hybrid nanomaterial, it would be a useful enhancing material for sensing application. And H2O2 and glucose were selected as probe molecules to demonstrate the potential application in biosensing of f-TiO2-Pt NPs. On one hand, H2O2, as a product of enzymatic reaction, is recognized as one of the major factors in the progression of important diseases. On the other hand, the electrocatalytic effect

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Figure 3. SEM image of f-TiO2-Pt NPs (A), TEM image of f-TiO2 (B), and TEM (C) and EDX (D) images of f-TiO2-Pt NPs.

Figure 4. XPS pattern of Pt NPs supported on f-TiO2 (a) and free Pt NPs (b).

of nanomaterials (individual component and hybrids) toward H2O2 is well-documented in the literature.23 To evaluate the catalytic activity of the f-TiO2-Pt NPs film-modified electrode for the oxidation of H2O2, the f-TiO2-Pt NPs-modified electrode was characterized by cyclic voltammetry in the presence of H2O2. The cyclic voltammograms (CVs) measured using the f-TiO2-Pt NPs-coated electrode in the absence (a) and presence (b) of H2O2 in 0.1 M PBS (pH 7.1) are shown in Figure 5A. It is observed that the CS/f-TiO2-Pt NPs/GC electrode exhibits strong oxidation current starting at around +260 mV. This substantially lowered the detection potential for H2O2 compared to the bulk GC electrode, which shows an oxidation of H2O2 starting at +800 mV (data not shown). Meanwhile, the current plateau appeared at +650 mV. These results indicate that the f-TiO2-Pt NPs dramatically enhanced the oxidation of H2O2. The steady-state currents increase with the increasing scan rate. And the former are linearly related to the square root of the

scan in the range from 10 to 200 mV/s, suggesting a diffusioncontrolled process (data not shown). The prominent performance of f-TiO2-Pt NPs toward H2O2 is attributed to the high catalytic effect of Pt NPs supported on f-TiO2. This also represents a great advantage in electrochemistry because the possibility to determine the H2O2 at low potential allows enhancement of its sensitivity and stability. The excellent performance of the f-TiO2-Pt NPs-modified electrode toward the detection of H2O2 makes it attractive for the fabrication of oxidase-based biosensor. GOD was selected as a model enzyme and a glucose biosensor was constructed when GOD mixed with CS, a biocompatible polymer with excellent film-formation ability, was immobilized on the f-TiO2-Pt NPs/GC electrode surface. CVs of CS/GOD/fTiO2-Pt NPs/GC electrode in 0.1 M PBS (pH 7.1) with (b) or without (a) 5 mM glucose are displayed in Figure 5B. Obviously, the oxidation current in the glucose solution is much higher than that in a buffer solution, which corresponds to the electrooxidation of H2O2 generated from the enzymatic reaction of glucose with dissolved oxygen, indicating that GOD in CS film on the f-TiO2-Pt NPs matrix retains high bioactivity. The current plateau obtained from the cyclic voltammogram reveals that amperometric determination of glucose at the CS/GOD/fTiO2-Pt NPs/GC electrode can be performed at a potential of +650 mV. Figure 6 shows the comparison amperometric response to the glucose on three types of electrodes, respectively, modified with CS/GOD, CS/GOD/f-TiO2, and CS/GOD/f-TiO2Pt NPs. Clearly, the CS/GOD/f-TiO2-Pt NPs/GC electrode is sensitive toward glucose, whereas the other electrodes exhibits negligible responses even at high glucose concentration, which is attributed to the high catalytic activity of f-TiO2-Pt NPs. To evaluate the performance of the designed glucose biosensor, CS/GOD/f-TiO2-Pt NPs/GC electrode, more experiments in glucose detection were studied in detail. Figure 7 shows the typical amperometric responses of the biosensor to the

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Figure 5. (A) CVs of CS/f-TiO2-Pt NPs/GC electrode in the absence (a) and presence (b) of 5 mM H2O2 in 0.1 M PBS (pH 7.1). (B) CVs of CS/GOD/f-TiO2-Pt NPs/GC electrode in the absence (a) and presence (b) of 5 mM glucose in 0.1 M PBS (pH 7.1). The scan rate is 50 mV/s.

Figure 6. Amperometric response to the glucose on three types of electrodes, respectively, modified with CS/GOD (2), CS/GOD/f-TiO2 (0), and CS/GOD/f-TiO2-Pt NPs (9) at the applied potential of +650 mV in the stirred PBS (pH 7.1). The inset shows a magnification of CS/GOD (2) and CS/GOD/f-TiO2 (0).

Figure 7. Current response of CS/GOD/f-TiO2-Pt NPs/GC electrode to the successive addition of 0.5 mM glucose in stirred PBS (pH 7.1) at an applied potential of +650 mV. The inset (left) shows the current response caused by 1.25 µM glucose. The inset (right) shows the calibration curve for glucose concentration between 1.25 µM and 3 mM.

successive addition of 0.5 mM glucose at +650 mV in the stirred PBS. The calibration curve (inset of Figure 7) for the sensor shows a linear response to glucose concentration in the range of 1.25 µM to 3 mM with a detection limit of 0.25 µM based on S/N ) 3. The detection limit compares favorably to recently reported Pt NPs supported on other kinds of nanomaterials. For example, when Pt NPs were deposited on functionalized MWNTs, a glucose biosensor possessed a detection limit of 30 µM.24 A glucose biosensor based on electrodeposition of Pt NPs onto CNTs and immobilizing GOD with CS-SiO2 sol-gel obtained a detection limit of 1 µM.25 In the case of dendrimerencapsulated Pt NPs on polyaniline nanofibers and Pt nanoclusters embedded in the polypyrrole nanofibers as electrode materials resulted in detection limits of 0.5 µM26 and 0.45 µM,27 respectively. Additionally, a glucose biosensor employed with exfoliated graphite nanoplatelets, as an inexpensive alternative to CNTs used in biosensors, decorated with Pt NPs obtained the result of 1 µM.28 The improved detection limit in glucose detection based on the f-TiO2-Pt NPs hybrid nanomaterial can be ascribed to the fact that the f-TiO2-Pt NPs composite can effectively decrease the detection noise and in the meantime be used as an enhanced

material for improving the electron transfer between detection molecule and electrode. That is, (1) the f-TiO2-Pt NPs processes the advantage of the f-TiO2 with nanoporous surface, resulting in increased surface reactivity, improved mass transport, and good catalytic property of Pt NPs. (2) The introduction of Pt NPs could increase surface area together with surface roughness of the nanohybrids, and also improve the conductivity of the anatase f-TiO2, which facilitates the electron transfer in the catalytic process. (3) Nanosized f-TiO2 as a support can maintain well the bioactivity of GOD because of their great biocompatibility. app ) of the CS/GOD/fThe apparent Michaelis-Menten (Km TiO2-Pt NPs/GC electrode for glucose was estimated in terms of the Lineweaver-Burk equation. Figure 8 shows the Lineweaver-Burk plots based on the data in Figure 7. The Kapp m value was estimated as 18.2 mM, which is similar to those of app in the free GOD in solution (18 mM).29 In addition, the Km the present work is smaller than that of GOD in silica matrix (20.3 mM)30 and polypyrrole (23.9 mM).27 It is clear the present biosensor processes higher biological affinity for glucose. The great biocompatibility of CS and TiO2 makes it favorable to maintain the activity of enzyme.

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Figure 8. Lineweaver-Burk plots based on the data in Figure 7.

The reproducibility was examined with the addition of 0.5 mM glucose using the same electrode, and the relative standard deviation was 3.9% (n ) 9). After the electrode was stored at 4 °C under dry conditions for 1 month, the steady-state response current remained at 83%. Such results demonstrate that the biosensor processed good stability, which can be attributed to the case that GOD is a very stable enzyme and CS and TiO2 nanospheres offered a great microenvironment for GOD. 4. Conclusions Functionalized TiO2 nanospheres with nanoporous surface have been prepared by a simple surface-modified method with APTMS. The as-formed f-TiO2 with functional groups has reactive surfaces, which facilitate the loading of small NPs on the surface by a self-assembly technology. Pt NPs have been selected to decorate the surface of f-TiO2 to prepare multifunctional TiO2/Pt NPs hybrids. And the method is versatile and easy for constructing TiO2-based nanohybrids. Considering the great biocompatibility of TiO2 and the excellent catalytic activity of Pt NPs, f-TiO2-Pt NPs have been used as enhancing materials for biosensing application. GOD, as a model enzyme, was employed to fabricate a glucose biosensor based on f-TiO2-Pt NPs, which exhibits the sensitive detection of glucose with a low detection limit of 0.25 µM. Our work not only provides a facile and effective route for the preparation of MNPs-loaded nanoporous TiO2 colloidal spheres but also sets one of few examples of using MNPs/TiO2 hybrid nanomaterial in biosensing. Acknowledgment. The work was supported by the National Natural Science Foundation of China (Nos. 20675076 and 20820102037). References and Notes (1) (a) Ryu, J. H.; Choi, W. Y. EnViron. Sci. Technol. 2006, 40, 7034. (b) Chen, X. B.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (c) Brammer, K. S.; Oh, S.; Gallagher, O. J.; Jin, S. H. Nano Lett. 2008, 8, 786. (d)

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