NANO LETTERS
Fullerene-Functionalized Gold Nanoparticles. A Self-Assembled Photoactive Antenna-Metal Nanocore Assembly
2002 Vol. 2, No. 1 29-35
P. K. Sudeep, Binil Itty Ipe, K. George Thomas,* and M. V. George† Photochemistry Research Unit, Regional Research Laboratory (CSIR), TriVandrum 695 019, India
Said Barazzouk,‡ Surat Hotchandani,‡ and Prashant V. Kamat*,§ Notre Dame Radiation Laboratory, Notre Dame, Indiana 46556 Received September 27, 2001; Revised Manuscript Received November 1, 2001
ABSTRACT A self-assembled photoactive antenna system containing a gold nanoparticle as the central nanocore and appended fullerene moieties as the photoreceptive hydrophobic shell is designed by functionalizing a gold nanoparticle with a thiol derivative of fullerene. Upon suspension of fullerene-functionalized gold nanoparticles (Au−S−C60) in toluene we observe formation of 5−30 nm diameter clusters. The ease of suspending these nanoassemblies in organic solvents allows us to probe the excited state interactions by spectroscopic methods. The quenching of fluorescence emission as well as decreased yields of triplet excited state suggest the participation of excited singlet in the energy transfer to the gold nanocore. Application of electrophoretically deposited Au−S−C60 nanoassemblies on optically transparent electrodes in the photoelectrochemical conversion of light energy has been demonstrated.
Introduction. Advances in the field of nanotechnology provide an alternate “bottom-up” approach, in which the nanoparticles and bridging molecular units are assembled together as circuits in nanoelectronics.1-3 Such arrangements can lead to the design of optoelectronic nanodevices, which can perform specific functions such as light-induced energy and electron-transfer processes. The metal and semiconductor nanoparticles possess size dependent optical, electronic, and catalytic properties and their synthesis and characterization are well documented.4-10 Recently, attention has been drawn toward capping of metal nanoparticles with photoactive ligands using functional groups such as thiols, amines, and isothiocynates.11-21 Elucidation of photoinduced energy and electron-transfer processes in fluorophore-metal nanoparticles are important in understanding the photochemical behavior of molecules bound to metal nanoparticles. In addition, the construction of two- and three-dimensional nanoassemblies of photoactive molecules with colloidal metal particles are useful for † Also affiliated with Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 012, India and Notre Dame Radiation Laboratory. ‡ Permanent Address: Groupe de Recherche en E Ä nergie et Information Biomole´culaires, Universite´ du Que´bec a` Trois-Rivie`res, Trois Rivie`res, Que´bec, Canada G9A 5H7. § E-mail:
[email protected] or http://www.nd.edu/∼pkamat.
10.1021/nl010073w CCC: $22.00 Published on Web 12/01/2001
© 2002 American Chemical Society
improving photoinduced charge separation and developing sensors for biological applications.22-27 Photoinduced electron transfer and energy transfer in a number of donor-acceptor systems have been extensively studied with an aim to mimic natural photosynthesis, by converting the charge-separated state into chemical or electrical energy.28 Demonstration of such a concept has been shown using self-assembled monolayers (SAMs) of fullerene derivatives on gold electrodes.13,29-33 Of particular interest is the C60 moieties that serve as excellent electron acceptors in molecularly designed donor acceptor diad and triad systems.34,35 Moreover, the charge separation in these systems can be greatly enhanced by forming clusters of fullerenebased diads in mixed solvents.36-38 To the best of our knowledge no effort has been made to probe the excited state interaction between fullerene moieties and gold nanocores in functionalized nanoassemblies. We report herein the functionalization of gold nanoparticles with thiol-derivatized fullerene (Figure 1) and their assembly as nanostructured films on electrode surfaces. Experimental Section. The general method adopted for the synthesis of fullerenethiol 3 is shown in Scheme 1. The conversion to thiol derivative 4 (path b) and the cycloaddition
Figure 1. Fullerenethiol-functionalized gold nanoparticle (AuS-C60). Scheme 1a
a (a) K CO , Br(CH ) Br, acetone; (b) hexamethyldisilathiane, 2 3 2 5 tetrabutylammoniun fluoride; (c) C60, N-methylglycine, toluene.
reactions (path c) were carried out by adopting methods similar to the reported procedures.39-41 Preparation of Compound 2. A suspension of 1,5dibromopentane (9.2 g, 40 mmol), p-hydroxybenzaldehyde (2.44 g, 20 mmol), and potassium carbonate (2.8 g, 20 mmol) was heated under reflux in acetone (30 mL) for 12 h. The reaction mixture was cooled, filtered, and concentrated under reduced pressure. The crude product was chromatographed over neutral alumina and elution with a mixture (1:5) of ethyl acetate and petroleum ether gave 4.1 g (72%) of 2 as a viscous liquid. IR (neat), νmax: 2946, 2873, 2746, 1691, 1604, 1513, 1311, 1260, 1162, 1044, 835, 649 cm-1. 1H NMR (300 30
MHz, CDCl3): δ 1.7-2.02 (6H, m, aliphatic), 3.40-3.45 (2H, t, CH2Br), 4.01-4.05 (2H, t, OCH2), 6.96-6.99 (2H, d, aromatic), 7.79-7.82 (2H, d, aromatic), 9.85 (1H, s, CHO). Preparation of Compound 4. To a stirred solution of compound 2 (2 g, 7.4 mmol) in distilled THF (5 mL) kept at -10 °C was added a mixture of hexamethyldisilathiane (1.59 g, 8.8 mmol) and tetrabutylammonium fluoride (2.31 g, 8.8 mmol). The reaction mixture was stirred for 12 h and then diluted with dichloromethane. The organic layer was washed with saturated ammonium chloride solution. The solvent was distilled off under reduced pressure, and the organic residue was chromatographed over silica gel (100200 mesh) using a mixture (1:5) of ethyl acetate and petroleum ether to give 1.1 g (60%) of compound 4. 1H NMR (300 MHz, CDCl3): δ 1.34-1.83 (6H, m, aliphatic), 2.532.60 (2H, t, CH2SH), 4.02-4.06 (2H, t, OCH2), 6.97-7.00 (2H, d, aromatic), 7.81-7.84 (2H, d, aromatic), 9.87 (1H, s, CHO). 13C NMR (75 MHz, CDCl3): δ 190.59, 163.91, 158.43, 131.79, 129.59, 114.55, 114.20, 67.80, 33.35, 31.89, 28.40, 24.84. Preparation of Fullerenethiol 3. A mixture of C60 (144 mg, 0.2 mmol), compound 4 (45 mg, 0.2 mmol) and N-methylglycine (18 mg, 0.2 mmol) in toluene (144 mL) was refluxed for 30 h under argon atmosphere. The reaction mixture was cooled, and the solvent was removed under reduced pressure. The residue was chromatographed over silica gel (100-200 mesh) using a mixture (1:3) of toluene and hexane to give 60 mg of unchanged fullerene and 25 mg of compound 3; mp > 400 °C. 1H NMR (300 MHz, CDCl3): δ 1.49-1.59 (6 H, m, aliphatic), 2.49-2.56 (2H, q, CH2SH), 2.77 (3H, s, pyrolidine NCH3), 3.91-3.95 (2H, OCH2), 4.21-4.24 (1H, d, pyrolidine ring CH), 4.86 (1H, s, pyrolidine ring CH), 4.93-4.96 (1H, d, pyrolidine ring CH), 6.87-6.90 (2H, d, aromatic), 7.64 (2H, s, aromatic). 13C NMR (75 MHz, CDCl3 + CS2): δ 159.01, 153.54, 147.07, 146.44, 146.19, 146.10, 146.07, 145.92, 145.89, 145.71, 145.57, 145.42, 145.27, 145.12, 145.08, 145.03, 144.95, 144.50, 144.42, 144.15, 142.89, 142.80, 142.48, 142.38, 142.03, 142.01, 141.96, 141.88, 141.84, 141.73, 141.66, 141.470, 141.33, 139.99, 139.75, 139.40, 136.81, 135.52, 130.27, 114.47, 82.97, 76.59, 69.53, 68.51, 67.39, 39.76, 33.82, 29.83, 28.97, 25.00, 24.73. The mass spectrum showed a [M + H]+ peak at 973 (observed by MALDI mass spectrometry). Preparation of Fullerenethiol DeriVatized Gold Nanoparticles. An aqueous solution of hydrogen tetrachloroaurate(III) hydrate (30 mM in 30 mL) was mixed with a solution of tetraoctylammonium bromide (50 mM in 80 mL) in toluene. The biphasic mixture was vigorously stirred until all the tetrachloroaurate was transferred into the organic layer. A solution of dodecanethiol (21 mg) and fullerenethiol (5 mg) in 2 mL toluene was added to 10 mL of the gold solution (Au/S mole ratio 1:1). After stirring for 2-3 min, sodium borohydride (0.1 M) in water (5 mL) was added and stirred for 3 h. The organic layer was separated off, to which ethanol (100 mL) was added and kept at 0 °C for 12 h. The gold nanoparticles formed were precipitated. This was filtered off Nano Lett., Vol. 2, No. 1, 2002
Figure 2. TEM images of (A) dodecanethiol-stabilized and (B) fullerenerthiol/dodecanethiol (1/19 mole fraction) bound gold nanoparticles.
and washed using ethanol (5 × 100 mL) to give a brown powder, redispersible in toluene. Absorption and emission spectra were recorded using a Shimadzu 3101 PC spectrophotometer and SLM S8000 spectrofluorometer, respectively. All experiments were carried out at room temperature (296 K). Transmission electron microscope (TEM) images were taken using a Hitachi H600 transmission electron microscope at a magnification factor of 200 000. One drop of sample was placed on a carboncoated copper grid for imaging and blotted to remove excess liquid. Atomic force microscopy (AFM) was carried out using a digital nanoscope (Nanoscope IIIa) in the tapping mode. An etched silicon tip was used as a probe for imaging the sample surface. Nanosecond laser flash photolysis experiments were performed with a Laser Photonics PRA/ Model UV-24 nitrogen laser system (337 nm, 2 ns pulse width, 2-4 mJ/pulse) with front face excitation geometry. A typical experiment consisted of a series of 3-5 replicate shots per single measurement42 Nanostructured SnO2 films were cast on optically transparent electrodes (OTE) using a colloidal suspension and further annealing at 673 K.43 Electrophoretic deposition of Au-SC60 nanoassemblies on electrode surfaces was carried out by immersing SnO2 film deposited OTE electrode (connected to positive terminal) and another plain OTE electrode in a small glass cell containing Au-S-C60 suspension in toluene. The distance between the two electrodes was maintained at 3 mm. A dc voltage (50 V) was applied to initiate the electrodeposition process.44 The photocurrent measurements were carried out in a three-arm cell that had provision to insert a working electrode (OTE/SnO2 or OTE/SnO2/metal), reference (SCE), and counter (Pt gauze) electrodes. Filtered (400 nm cutoff) light from a xenon lamp was used as the excitation source (Intensity ∼100 mW/cm2). Results and Discussion. Fullerene-functionalized gold clusters can be visualized as self-assembled photoactive antenna systems containing a gold nanoparticle as the central nanocore and appended fullerene moieties as the photoreceptive hydrophobic shell (Figure 1). Such molecular-gold nanoassemblies can serve as important building blocks in the design of light harvesting systems. Figure 2 compares the transmission electron micrograph of dodecanethiol-stabilized gold nanoparticles and fullerenethiol-capped gold nanoparticles. Dodecanethiol-stabilized gold nanoparticles are 2-3 nm in diameter. Because of the smaller Nano Lett., Vol. 2, No. 1, 2002
Figure 3. Absorption spectra of (a) fullerenethiol and (b) a fullerenethiol-functionalized gold nanoparticle (Au-S-C60) in toluene.
size, dodecanethiol-capped gold nanoparticles fail to exhibit the sharp absorption peak corresponding to the surface plasmon band. Only a broad absorption band around 500 nm could be seen. On the other hand the fullerenethiolcapped gold nanoparticles tend to form small clusters. A dilute solution of Au-S-C60 when dried on a carbon grid shows clusters of varying sizes. The TEM image in Figure 2 shows clusters of diameters in the range of 5-30 nm. The presence of large cluster assemblies in the TEM image is an indication that these fullerene-gold nanoassemblies can form larger aggregates even in nonpolar solvents. Absorption and Emission Spectra. The ability of Au-SC60 cluster assemblies to suspend readily in nonpolar solvents allowed us to conduct spectroscopic studies. The absorption spectra of fullerenethiol and gold nanoclusters capped with fullerenethiol in toluene are shown in Figure 3. The broad absorption at 500 nm corresponds to the surface plasmon absorption of the gold nanocore. Because of the small size of the gold nanocore, the plasmon absorption remains rather broad. As compared to fullerenethiol, the absorption spectrum of fullerenethiol-capped gold nanoparticles exhibit absorption extending into the red-near-IR region. The characteristic fullerene absorption peak at 430 nm is also noticeably missing in the Au-S-C60 suspension. This absorption behavior is similar to that we normally observe for clusters of fullerene. The properties of optically transparent clusters of fullerene derivatives in mixed solvents (acetonitriletoluene) can be found elsewhere.36,38,45-47 The stronger affinity of fullerenes toward gold nanoparticles is evident from the aggregation effects.48 For example, in the absence of a thiol linker group, direct interactions between gold nanoparticle and fullerene molecules in toluene lead to precipitation. These precipitation problems in the past have limited spectroscopic study of fullerene-bound gold nanoparticles. Recently, an effort has been made to minimize the aggregation effects by reacting octanethiol-stabilized gold nanoparticles with fullerenethiol.49 These fullerenethiolfunctionalized gold nanoparticles (seven fullerene units per Au particle) suspended well in CHCl3 medium. Surface functionalization of the gold nanoparticle using a thiol derivative of fullerene possessing soluble linker groups 31
Figure 4. Fluorescence emission spectra of the optically matched solutions of the (a) fullerenethiol and (b) Au-S-C60 in toluene (excitation wavelength, 470 nm).
facilitates dispersion of the fullerene units around the gold nanocore quite uniformly. In the present study we have succeeded in loading a relatively high concentration of fullerene moieties on a single gold nanocore (diameter of gold nanoparticle ∼3 nm) so that we can maximize the light harvesting efficiency in these nanoassemblies. By incorporating fullerenethiol during the gold reduction step, it was possible to bind about 90 fullerene moieties per gold nanocore. Since several fullerene moieties are linked to the gold nanocore, we expect a high local concentration of fullerenes around the gold nanoparticle. Under these conditions, intermolecular interactions are expected to dominate on the gold surface. Moreover, interparticle interactions can also play a major role in inducing clustering effects in Au-S-C60 assemblies. Interaction of Excited Fullerene and Gold Nanocore. Fluorophores bound to the bulk gold surface are capable of undergoing excited-state interactions via energy or electrontransfer processes. The small size of the gold nanocore is redox active and hence alters the deactivation pathways of surface-bound fluorophore. In the absence of gold nanoparticles, the emission spectrum of fullerenethiol in toluene shows a maximum at 710 nm, which corresponds to its singlet excited state. These spectral profiles of fullerenethiol emission (trace a Figure 4) were found to be similar to those of the previously reported clusters of functionalized C60 molecules.36,38 The major deactivation pathway for the excited singlet is the intersystem crossing to generate triplet excited state. Interestingly, the fullerenethiol emission is totally quenched when it is anchored on the gold nanocore. This observation shows that the decay of singlet excited fullerene moieties is affected by its binding to the gold nanocore. The quenching of singlet emission would result from either the enhanced intersystem crossing efficiency or by direct energy transfer to the gold nanocore (reactions 1-3). Au-S-C60 + hν f (Au-S-1C60*) 32
(1)
(Au-S-1C60*) f (Au-S-3C60*)
(2)
(Au-S-1C60*) f (Au*-S-C60)
(3)
Moreover, an additional deactivation pathway that involves electron transfer between excited fullerene and gold nanocore could also contribute to the fluorescence quenching of fullerenethiol. In an earlier study of the quenching of excited pyrene bound to the gold nanocore, we have demonstrated the electron-transfer event by characterizing electron-transfer products.50 To establish the excited state deactivation pathways, we recorded transient absorption spectra (Figure 5) following the 337 nm laser pulse as the excitation of fullerenethiol solution. The excited fullerenethiol in toluene shows an absorption band at 700 nm, which is characteristic of a triplet excited fullerene moiety. A majority of excited fullerene triplets studied so far, show a relatively less pronounced band in the near UV region. However, for the fullerenethiol triplet, we observe another prominent UV band at 360 nm with an extinction coefficient similar to that of the 710 nm band. The transient absorption decay at 700 and 360 nm follow the first-order kinetics with a lifetime of 16.7 and 14.3 µs, respectively (Figure 5 inset). The similarity of transient decay lifetimes at these two absorption bands confirms that the triplet excited species is the only long-lived transient formed following the excitation of fullerene thiol. The transient absorption spectrum recorded following the excitation of Au-S-C60 exhibits a relatively weak absorption throughout the UV-visible region. The transient studies do not indicate the formation of any electron-transfer products for the Au-S-C60 system. These results indicate that the electron-transfer process is not a major deactivation pathway for the excited fullerene on gold the surface. Low yields of singlet and triplet of fullerene moiety in excited Au-S-C60 assembly confirm that most of the excited energy of the fullerene moiety is quickly dissipated, following the excitation via the energy-transfer mechanism. Assembling Au-S-C60 as a 3- Dimensional Array. One of the convenient ways to utilize a fluorophore-gold nanoassembly in light harvesting applications is to assemble them in an orderly fashion on an electrode surface. Few attempts have been made in recent years to obtain self-assembled monolayers using a thiol linkage.13,20,30,32,33,51,52 By using dithiol as the surface capping agent, Brust and co-workers12 observed a linear chain of self-assembled gold nanoparticles. These studies have focused on the binding of fullerene derivatives to the gold electrode surface through selfassembled monolayers or by LB film approach. Although such procedures were useful to establish the concept of light energy harvesting using gold-fullerene systems, they failed to deliver high photocurrent efficiencies. We employed an electrophoretic method to deposit AuS-fullerene nanoassemblies on an optically transparent electrode (OTE) surface.37,44,53 Under the application of a dc electric field Au-S-C60 clusters become charged in toluene solution and are deposited on the positively charged electrode surface. The film cast by electrophoretic approach Nano Lett., Vol. 2, No. 1, 2002
Figure 5. Transient absorption spectra recorded 2.5 µs after 337 nm laser pulse excitation of degassed toluene solutions of (a) fullerenethiol and (b) Au-S-C60.
Figure 6. AFM images of Au-S-C60 deposited on an OTE electrode using an electrophoretic approach (applied dc voltage was 50 V).
produces a robust coverage of the fullerene-functionalized gold assemblies on the optically transparent electrode (OTE). The AFM image presented in Figure 6 shows a 3-D assembly of Au-S-C60 clusters, which renders a nanoporous morphology to the film. The grape bunch morphology of the cluster assembly thus provides a high surface area to the electrophoretically deposited film of Au-S-C60 clusters. Upon close examination of the AFM image (Figure 6) it is evident that the nanostructured Au-S-C60 film consists of 50-100 nm diameter clusters and these clusters are larger than those present in solution. Obviously, these clusters have grown in size during the electrodeposition process. As indicated earlier,37 charging of fullerene moieties in the dc electric field plays an important role in the growth and deposition process. These films are quite robust and can be washed with organic solvents to remove any loosely bound Au-S-C60 nanoassemblies. Because of the high coverage, these films are useful for electrocatalytic and sensor applications. Au-S-C60 Film as a PhotosensitiVe Electrode. We further tested the feasibility of employing Au-S-C60 cluster films for light energy harvesting applications by using them as photosensitive electrodes in photoelectrochemical cells. There Nano Lett., Vol. 2, No. 1, 2002
Figure 7. Absorption (s) and photocurrent (- - -) action spectra of (a, b) OTE/SnO2 and (c, d) OTE/SnO2/Au-S-C60 electrodes (electrolyte: LiI (0.5 M) and I2 (0.01 M) in acetonitrile). Incident photon to photocurrent generation efficiency (IPCE) was determined from the expression IPCE (%) ) 100(isc/Iinc)(1240/λ), where isc (mA/ cm2) is short circuit current and Iinc (mW/cm2) is the incident light energy at the excitation wavelength, λ (nm). The inset shows the photocurrent response to ON-OFF cycles of illumination (λ > 400 nm).
have been several attempts to functionalize gold surfaces with fullerenes using a self-assembled monolayer (SAM) approach.31,33,54 The fullerene-functionalized gold systems were photoactive and could serve as photosensitive electrodes in a photoelectrochemical cell. Low coverage of the light harvesting fullerene moieties often limits the absorption of incident light in 2-dimensional arrays. To improve the cross section of light absorption, it is important to assemble the light harvesting nanoassemblies in a 3-dimensional array. The Au-S-C60 film cast on a nanostructured SnO2 electrode (OTE/SnO2) provides an excellent choice to achieve this goal. The absorption spectrum (trace b) recorded in Figure 7 shows significant absorption in the visible region with spectral features similar to those observed for suspensions in toluene. The OTE/SnO2 electrode does not exhibit any absorption in the visible (trace a in Figure 7). We employed OTE/SnO2/Au-S-C60 as the photoanode in a photoelectrochemical cell. I3-/I- dissolved in acetonitrile was used as a regenerative redox couple. Upon illumination of the OTE/SnO2/Au-S-C60 electrode with visible light (λ > 400 nm), a prompt generation of photocurrent could be seen. The photocurrent action spectra of OTE/SnO2 and OTE/SnO2/Au-S-C60 electrodes (recorded as incident photon to photocurrent efficiency or IPCE) are shown in Figure 7 (traces c and d). Only the electrode, OTE/ SnO2/Au-S-C60, shows a prominent response in the visible (400-500 nm), thereby confirming the role of fullerene moieties as the receptor of incident photons. Repeated ON-OFF cycles of illumination show the reproducibility of a steady photovoltage (∼150 mV) and short circuit photocurrent (130 µA/cm2) generation (see, for example, Figure 7 inset). Blank experiments conducted with OTE/SnO2 and OTE/SnO2/Au electrodes did not produce any detectable photocurrents under similar illumination conditions. The observed photocurrents are more than 2 orders of 33
P.K.S., B.I.I., M.V.G.) and the Office of Basic Energy Science of the U.S. Department of the Energy (P.V.K., M.V.G. (in part)) for financial support of this work. This is contribution No. RRLT-PRU 135 from RRL, Trivandrum, and NDRL 4322 from the Notre Dame Radiation Laboratory. M.V.G. thanks the Jawaharlal Nehru Center for Advanced Scientific Research, Bangalore, for financial support. S.B. and S.H. acknowledge the support of Natural Sciences and Engineering Research Council of Canada. References Figure 8. Mechanism of photocurrent generation at a OTE/SnO2/ Au-S-C60 electrode.
magnitude greater than those obtained from fullerene films on gold surfaces using the self-assembled monolayer technique.31 In the present case, the robust coverage and high surface area of the nanostructured Au-S-C60 assembly are the major factors that make the high photocurrent generation possible in these photoelectrochemical cells. The photoelectrochemical properties illustrated above further confirm that the Au-S-C60 nanoassemblies deposited on the OTE/SnO2 electrode absorb the incident visible light, undergo electron transfer with the redox couple, and transfer the charge to the collecting electrode surface. By employing a high concentration of the redox couple, we were able to induce direct interaction with the excited fullerene, which in turn competes with the energy transfer discussed in an earlier section. Figure 8 illustrates the mechanism of photocurrent generation at the OTE/SnO2/Au-S-C60 electrode. In the presence of high iodide concentration, excited fullerenes are reduced to generate C60 anions. As shown earlier,44 these fullerene anions are electroactive and are capable of delivering charges to the collecting electrode surface. It is apparent from these photoelectrochemical studies that the gold nanocores promote the charge separation and facilitate electron transport within the film during the photocurrent generation. The mechanism of charge transport in metal nanostructures has remained an intriguing issue. Brust and co-workers12 have proposed a thermally activated electron hopping from one gold particle to another as a possible way of conducting charge through nanostructured gold films. The ability of gold nanoparticles in storing and shuttling of electrons has also been demonstrated by monitoring molecule like charging effects9,55 and Fermi-level equilibration with semiconductor nanostructures.56-58 The photoelectrochemical measurements presented in this work demonstrate that gold nanocores play an important role of mediating the charge transport process in fullerene-functionalized nanostructured films. Further experiments are underway to design methodologies to assemble the gold nanocore-fluorophore assemblies and employ them to improve light energy conversion efficiencies of photoelectrochemical cells. Acknowledgment. We thank the Council of Scientific and Industrial Research, Government of India (K.G.T., 34
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