Room Temperature Delayed Fluorescence of Gold Nanoclusters in

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Interface-Rich Materials and Assemblies

Room Temperature Delayed Fluorescence of Gold Nanoclusters in Zinc Mediated Two-Dimensional Crystalline Assembly Srestha Basu, and Arun Chattopadhyay Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00149 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 24, 2019

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Room Temperature Delayed Fluorescence of Gold Nanoclusters in Zinc Mediated Two-Dimensional Crystalline Assembly Srestha Basua and Arun Chattopadhyay*a,b a

Department of Chemistry,

Indian Institute of Technology Guwahati, Guwahati, Assam – 781039 b

Centre for Nanotechnology,

Indian Institute of Technology Guwahati, Guwahati, Assam – 781039, India E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract: We report that complexation reaction mediated two dimensional (2D) crystalline assembly of gold (Au14) nanoclusters (NCs) exhibits room temperature delayed fluorescence at 605 nm, with an unprecedented long lifetime of 0.5 ms and an exceptionally high quantum yield of 19.1 ± 0.9 %. Interestingly, the as-synthesized Au NCs had a very weak delayed fluorescence signal. The enhancement in delayed fluorescence of Au NCs upon formation of assembly has been attributed to the crystallization induced structural rigidity, which restricted the non-radiative transitions and enhanced the excited state lifetime. The attainment of crystalline organization was substantiated by electron diffraction analysis. A possible structure was established based on experimental results and computational optimizations. Atomic force microscopy revealed the formation of multilayered two - dimensional nanosheets with thickness of 2.44 ± 0.48 nm. Keywords: Delayed fluorescence, Two dimensional assembly, Gold nanoclusters, crystalline assembly, Complexation reaction


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Introduction Quest for new materials with novel properties has fortunately provided an option in the form of superstructures obtained through hierarchical organization of nanoscale particles.1-4 It can be argued that particles with structural integrity akin to ‘molecular formula’ may provide superior option in generating structures of higher dimensions. In this regard, atomic clusters with well-defined formulas and structures –as guided by the number of constituent atoms provide a nascent opportunity for formation of higher ordered assemblies. An important additional handle is the ligand(s) stabilizing the peripheral atoms of the clusters. They provide the repertoire of chemistry and chemical interactions forming the basis of organizations. It is worth noting here that spatial organization of atomic clusters has recently been pursued using interactions amongst the ligands stabilizing the clusters.5-7 However, superiority of such assembly vis-à-vis the constituent clusters have not been clearly demonstrated. On the other hand, our laboratory has recently introduced the concept of “complexation reaction” based assembly of gold nanoclusters. The idea is to use free groups of the stabilizing ligands - such as amino and carboxylate - for reactions with metal ions (say Zn2+) leading to appropriate complexation. The so-formed assemblies are expected to give rise to new properties owing to interactions involving the clusters, the ligands and the complexes. Thus, zinc mediated assembly of Au14 nanoclusters resulted in crystalline nanoscale particles with improved optical and chemical properties. For example, the threedimensionally assembled particles have been found to be efficient in reversible storage of hydrogen and carbon dioxide and in sensing aided separation of chiral molecules, while the as-synthesized clusters were devoid of those capabilities.8-10 Next, we were interested in pursuing two-dimensional organization of Au nanoclusters. The inspiration comes from the observations that 2D materials such as graphene, MoS2, WS2 and boron nitride provide extraordinary mechanical strength, chemical and electrochemical properties, in addition to novel optical properties.11-13 In this regard, 2D assembly of Au15 nanoclusters was achieved using van der Waals interactions amongst the stabilizing ligands.14 We envision that 2D assemblies of atomic nanoclusters via inorganic complexation reactions would also provide properties considerably different from the clusters. An important choice, in this regard, would be materials with long emission lifetimes.15,16 In addition to applications in electronics, emergency signage and display and other light emitting


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devices,16 phosphorescent materials and materials exhibiting delayed fluorescence, are highly desired in bioimaging, owing to the absence of background phosphorescence. Importantly, development of materials exhibiting delayed fluorescence with emission in and around the biological window is of considerable importance. It is worth mentioning here that phosphorescent materials are common choices with regard to light emitting materials having long emission lifetimes. However, popular rare earth based

materials

need

replacement

by

more

abundant,

cost-effective

and

environmentally friendly materials. In that regard, metal organic frameworks (MOFs), organic crystals and ionic crystals have recently been demonstrated to exhibit phosphorescence.15,16 On the other hand, nanocluster based systems with long emission lifetimes are needed to be developed. To this end, materials exhibiting thermally activated delayed fluorescence are proposed as important alternatives to phosphorescent materials. Thermally activated delayed fluorescence is observed in charge transfer systems where the gap between the triplet and lowest singlet states are thermally accessible. This favors facile reverse intersystem crossing of photo excited electrons.17 In case of phosphorescence an emission - in addition to fluorescence - is observed at higher wavelengths. On the other hand, in case of delayed fluorescence, the emission energy remains the same as that of fluorescence, although the emission lifetime gets enlarged enhanced. The so far developed materials exhibiting delayed fluorescence are generally metal free and have been claimed to be efficient light emitting materials.17There is also a need for developing newer materials especially based on metal clusters with high quantum yield and tuneable emission wavelength. It has recently been reported that solvent mediated aggregation of gold nanoclusters led to appearance of phosphorescence with lifetime of 3.7 µs.18 It is thus plausible that complexation mediated assembly of nanoclusters may lead to superior crystals with high photoluminescence lifetime and quantum yield. However, this is yet to be demonstrated. Herein we report zinc ion coordinated formation of 2D multi-layered nanosheets from gold nanoclusters, with thickness of 2.44 ± 0.48 nm. The crystalline nanosheets evinced room temperature delayed fluorescence with an unprecedented lifetime of 0.5 ms and quantum yield of 19.1 ± 0.9 %. The long-lived delayed fluorescence has been attributed to the structural rigidity acquired by the clusters upon complexation into a crystal. The crystalline nature of the so formed nanosheets was confirmed from


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selected area electron diffraction (SAED) analysis, which revealed hexagonal arrangement of the constituents of the crystalline assembly with regular hexagonal edge length of 4.6±0.1 Å. High resolution transmission electron microscopy (HRTEM) analysis revealed the third dimension of the crystalline assembly to be 3.4 Å. Also, based on experimental observations and computational optimizations, a possible structure of the 2D crystalline assembly of Au NCs has been proposed.

Experimental details Synthesis of Au NCs: 1 mL HAuCl4 (10 mM) was added to 10 mL ethanol. To this, 0.4 mL MPA (0.11 M) was added. Further, ~ 5 mg of MBA (mercaptobenzoic acid) was added to this mixture. This led to formation of a colorless dispersion. The dispersion was centrifuged at a speed of 10,000 rpm for 10 min and the so obtained pellet was redispersed in 1 mL water. Synthesis of Zn Au NCs: To the above-prepared dispersion, ~ 100 mg of Zn-acetate dihydrate was added. Thereafter, the dispersion was centrifuged at a speed of 10,000 rpm for 10 min and the so obtained pellet was redispersed in 1 mL water. The dispersions of Au NCs and Zn Au NCs were centrifuged at a speed of 10,000 rpm for 10 min and the so obtained pellets were redispersed in 1 mL water and were used for further experiments. All luminescence data shown in manuscript and supporting information were acquired using dispersions of the centrifuged samples of Au NCs and Zn Au NCs. Transmission electron microscopic (TEM) analysis and selected area electron diffraction (SAED) analysis: TEM and SAED of Au NCs and Zn Au NC nanosheets were performed in JEOL JEM 2100 and JEOL JEM 2100F, at a maximum accelerating voltage of 200 kV. For preparation of TEM samples, appropriately diluted samples of Au NCs and Zn Au NC nanosheets were drop cast on carbon coated copper grids and left overnight for drying. Computational analysis: Avogadro software was used for theoretical modelling19 of Zn Au NCs. The calculations were done using UFF force field. Atomic force microscopic (AFM) analysis: AFM analysis of Zn Au NC nanosheets were performed Bruker Innova SPM. AFM samples were prepared by spin coating appropriately diluted samples of Zn Au NCs on a glass slide. The samples containing glass slides were dried under ambient conditions.


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Field emission scanning electron microscopic (FESEM) analysis: FESEM analysis of Zn Au NC nanosheets were performed JSM-761OF. Samples for FESEM analysis were prepared by drop casting 50 µL of appropriately diluted Zn Au NCs on a glass slide wrapped with aluminum foil. The samples were dried on a hot plate with the temperature being set at ~ 50 ˚C. Delayed fluorescence spectra acquisition: Delayed fluorescence spectra were acquired in Horiba Jobin Yvon Fluoromax 4P. Parameters for acquisition of delayed fluorescence emission spectra were set as follows: Time per flash = 61 ms; sample window = 500 μs; delay after flash = 50 μs; flash count = 100. Calculation of delayed fluorescence quantum yield: Delayed fluorescence quantum yield of

Zn

Au

NC

nanosheets

was

measured

using

Bis[2-(4,6-

difluorophenylpyridinatoC2,N](picolinato) iridium (III), commonly known as FIRPIC as a standard phosphor with reported quantum yield of 60 %.20 For measuring quantum yield, all samples were appropriately diluted so as to keep their respective absorbance values below 0.1. The quantum yield of Zn Au NCs was calculated using the following formula: Q.Y of Zn Au NCs nanosheets = Q.Yref X ( Area under the emission spectrm of Zn Au NCs / Area under the emission spectrum of FIRPIC ) X

( Absorbance of FIRPIC / Absorbance of Zn Au NCs ) X (Re fractive index of Zn Au NCs / Re fractive index of FIRPIC ) Putting the QY of FIRPIC to be 0.6, the quantum yield of Zn Au NC nanosheets was calculated to be 19.1±0.9 %.

Results and Discussions Characterization of Au NCs. Successive addition of mercaptopropionic acid (MPA) and mercaptobenzoic acid (MBA) to an ethanolic solution of tetrachloroauric acid (HAuCl4) led to the formation of a colourless dispersion. The details of the synthesis and characterization of Au NCs are described in the SI. The dispersion was


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luminescent (λem at 590 nm) upon excitation at 355 nm (Figure S1). This indicated the possible formation of gold nanoclusters (Au NCs), which was substantiated by transmission electron microscopic (TEM) analysis that showed the formation of particles in the size range of 2 nm (Figure S2 A). These particles were observed to be devoid of crystallinity (Figure S2 B). Additionally, electrospray ionization mass spectrometric (ESI-MS) analysis revealed the formation of Au14 NCs. The parent peak in the ESI-MS spectrum at 1522.229 has been ascribed to the formation of Au14 species with the chemical formula of [Au14MPA8MBA6 + 3 Na+]3+ (Figure S3 and Table S1, supporting information). Complexation reaction between Au NCs and Zn2+. Our recent results, on the complexation reactions involving ligand-stabilized Au NCs, suggest options for systematic higher dimensional organization, where interactions between the ligands may provide additional stability and specific structures. Here the presence of MBA ligand stabilizing the Au14 NCs, might have provided an opportunity for using π- π interaction as an additional favourable factor for their organization into specific structure.21Thus, here also, an approach of coordination chemistry was sought for the formation of a few layered two-dimensional assembly of Au14 NCs. Thus, complexation of MBA and MPA stabilized Au14 NCs and zinc ion was pursued at room temperature. The details of the synthesis and characterization of zinc mediated assembly of Au NCs is described in the supporting information. This resulted in formation of a dispersion with enhanced luminescence intensity and lifetime vis-à-vis the constituent Au14 NCs (Figure S4, S5 and Table S2). The retention of increased luminescence intensity was observed following centrifugation and redispersion. This indicated possible coordination of zinc ions with the ligands stabilizing the Au14 NCs leading to the formation of Zn-Au-NCs. The UV-vis absorbance spectrum of Au NCs was hardly altered following complexation with zinc ions (Figure S6). Further, Fourier transformed infrared (FTIR) analysis revealed that the coordination of zinc ions with MBA and MPA might have occurred through carboxylate groups of MBA and MPA moieties (Figure S7, and discussions are given in supporting information). Formation of two dimensional nanosheets of Zn Au NCs. Evidences of formation of two - dimensional assemblies consisting of Au NCs were obtained from TEM and atomic force microscopic (AFM) analyses. Interestingly, TEM analysis of Zn-Au-NCs


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revealed the formation of “sheet-like” structures (Figure 1A-B). The magnified view of these “sheet-like” structures revealed the presence of smaller particles of about 2 nm and hence confirmed the role of Au14 NCs as the constituents of the sheets (Figure S8 A-B).

Further, selected area electron diffraction (SAED) analysis of these

nanosheets confirmed the crystalline nature of the same and indicated hexagonal arrangements of Au NCs constituting the NS (Figure 1 C). The dimension of the hexagonal assembly (analyzed from the diffraction pattern) was calculated to be 4.6±0.1 Å. Further, high resolution transmission electron microscopic (HRTEM) analysis performed on typical crystalline nanosheets, revealed the lattice spacing of 3.4 Å (Figure 1 D). In order to substantiate the concomitant localization of Au, C, O, S and Zn in Zn Au NCs, STEM analysis and elemental mapping were performed (Figure S9). As is evident from the results, the nanosheets of Zn Au NCs were found to comprise of all the aforementioned elements.

Figure 1 (A) TEM image of two - dimensional nanosheet formed as a product of reaction between Au14 NCs and Zn2+. (B) Magnified image of (A). (C) SAED of the nanostructure shown in (A). (D) HRTEM image of a typical Zn-Au-NC nanosheet.


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Moreover, in order to confirm the 2D nature of these NS, AFM analysis was performed, which revealed sheet like structure (Figure 2 A-B) with thickness of 2.44 ± 0.48 nm. This thickness was higher than single nanosheets of CeO2 (0.6 nm),22 boron nitride (2.8 nm)11 and molybdenum sulphide (0.8 nm),23 and hence has been attributed to the formation of “multi-layered” nanosheet. Also, due to fact that the size of a single constituent Au14 NC (without considering the ligands, as obtained from computational optimization discussed in subsequent section) is likely to be around 7 Å. Thus, the presence of several of such Au14 NCs with ligand in the nanosheet, might have led to the observation of such nanosheets with thickness mentioned above. Additional AFM image and corresponding height and size distribution of nanosheets of Zn Au NCs have been shown in Figure S10. The mean height of Zn Au NCs was found to be 2.44 ± 0.48 nm. Also, the mean size of the nanosheets formed out of reaction between Au NCs and zinc ions was found to be 332.72 ± 24.99 nm (Figure. S10). Further, field emission scanning electron microscopic (FESEM) analysis was performed to corroborate the formation of nanosheets (Figure S11 A-D).


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Figure 2 (A) AFM image of 2 dimensional nanosheets formed as a product of reaction between Au14 NCs and Zn2+ (B) Height profile of the Zn Au NCs nanosheets shown in (A).

Plausible chemical interactions leading to formation of two dimensional assemblies. Plausible chemical interactions leading to formation of nanosheet comprising of Au14 NCs has been chiefly attributed to coordination between zinc ions and carboxylate groups of MPA molecules. This can be anticipated to form the network among the Au14 NCs in the XY-plane. Further, coordination between zinc ions and carboxylate groups of MBA molecules might have led to the extension of the network of Au14 NCs in the Z-direction. The extent of elongation of the network of Au14 NCs in z direction might have also been based on the π- π stacking interaction amongst MBA molecules stabilizing the Au14 NCs. This might have served as a key to achieve layered two - dimensional structures of assembled Au14 NCs.21 The role of MBA in favouring the formation of 2 D structures of Zn Au NCs through plausible ππ interactions is supported by the FTIR analysis. The FTIR spectra of Au NCs and Zn Au NCs have been shown in Figure S7 and are shown herein as well. In the FTIR spectrum of Au NCs, the peak due aromatic C=C stretching of MBA molecules was observed at 1588 cm1

. However, upon complexation with zinc ions and formation of 2 D assemblies therein, the

peak due to stretching of aromatic C=C was observed to undergo a downshift to 1541 cm-1. As per literature reports, this is considered to be a conclusive evidence of π- π stacking in aromatic molecules.24Also, as per literature reports25, the conformations of ligands have been extensively studied using gold cluster comprising of 102 gold atoms stabilized by 44 mercaptobenzoic acid, as a model system. Intriguingly, the study revealed ligand to gold and face to face aromatic interactions among MBA molecules as important chemical forces prevalent in the concerned system. In an allied vein, the role of MBA has been verified in several other cases of 2 dimensional supramolecular assemblies mediated by π- π interaction among MBA molecules.26 Plausible modes of chemical interactions leading to the formation of zinc assisted two - dimensional assemblies of Au14 NCs, based on experimental evidences (TEM, HRTEM and SAED) and computational optimization (using Avogadro, Figure S12) are proposed herein (Scheme 1 A). The details of the structural analysis are given in the supporting information. Further, a possible two - dimensional structure of Zn Au NCs is also depicted herein (Scheme 1 B). It is worth mentioning here that related to our work on assembly of nanoclusters9 allied work27 has been done, where, only mercaptopropionic acid (MPA) stabilized Au NCs have been synthesized and were reported to be non-luminescent in


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absence of additional stabilizing ligands (for example MBA as in the current case). Also, addition of zinc ions to such non luminescent clusters has been reported to result in significant enhancement in luminescence. The phenomenon has been attributed to “aggregation induced emission enhancement” of clusters upon interaction with zinc ions, the emission emerging from the formation of “tight aggregates” of gold clusters with no defined geometry and crystallinity.27 This highlights the critical role of the presence of a ligand (such as MBA) - in addition to MPA - in order to imbue the formed nanoclusters with luminescence and formation of ordered assembly having well-defined structures, upon interaction with zinc ions. Further, through control experiments (Figure S13), it was found that in absence of MPA, the mixture of HAuCl4 and MBA was non luminescent indicating the critical role of MPA in formation of Au NCs. Thus from the aforementioned observations and literature reports, it can be conclusively stated that the presence of both MPA and MBA is imperative for synthesis of luminescent clusters and thereby the formation of their assemblies with higher dimensionalities and defined geometries.

A

B


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Scheme 1 (A): Schematic illustration of the mode of chemical bonding between the ligands stabilizing Au NCs and Zn ions. (B) Schematic illustration of the quasi two- dimensional assembly of Au14 NCs following complexation with zinc ions.

Delayed fluorescence of Zn Au NCs. As discussed in former section, our laboratory has reported that the assembly of Au14 NCs exhibit chemical and optical properties, which are significantly different or not present in the constituent clusters. Further, it has recently been reported that an ensemble of nanoscale emitters, when interacts concomitantly with a common light field, might give rise to intriguing quantum phenomenon owing to their collective

coupling.28

Thus,

exploring

the

challenging

and

practically

relevant

phosphorescence/delayed fluorescence of such assemblies of clusters might be worthy of pursuit. Also, the growing importance of nanoclusters in the domains of practical relevance ranging from biosensing to catalysis provides impetus for further studies in this field.29,30Interestingly, upon acquisition of the luminescence emission - beyond the time-scale of the fluorescence signal -, the as prepared nanosheets were found to exhibit delayed fluorescence with an emission maximum at ~605 nm. Also, the delayed fluorescence excitation spectrum of Zn Au NCs acquired upon fixing emission at 605 nm (i.e., the emission maximum of delayed fluorescence), featured the presence of two distinct excitation maxima at 296 nm and 347 nm (Figure S14). On the other hand, the non-assembled Au14 NCs were found to exhibit weak delayed fluorescence with no discernible emission maximum in the visible range (Figure 3 A). Further, the delayed fluorescence lifetime of Zn Au NCs was measured and the decay curve could be fitted with bi-exponential function. The lifetime of the major contributing component was found to be 0.502 ms (Figure 3 B). It is worth mentioning here that the lifetime of 1.3 μs of Zn Au NCs was obtained using time resolved fluorescence spectrometer (Life-Spec-II spectrofluorimeter; Edinburgh Instrument). This instrument allows the measurement of a maximum lifetime of 5 μs. Also, in this method, no “delay in lifetime” could be given during the measurement of lifetime of Zn Au NCs. However, the lifetime of 0.5 ms was measured in an instrument called Fluoromax 4P (Horiba instruments), in phosphorescence mode. In this study, a delay in lifetime of 50 μs was given prior to recording of lifetime. The purpose was to eliminate the interference of any lifetime equal to or below 50 μs in the measurement of lifetime of delayed fluorescence of Zn Au NCs. Thus in this case, excited state lifetimes of Zn Au NCs above 50 μs was recorded only.


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Notably, the nanosheets were found to be highly phosphorescent with a quantum yield of 19.1 ± 0.9 %.

A

B

Figure 3 (A) Delayed fluorescence emission spectra of (a) Zn-Au-NC nanosheets and (b) Au14 NCs, (B) Time resolved delayed fluorescence spectrum of Zn-Au-NC nanosheets.

The delayed fluorescence in Zn-Au-NC nanosheets is proposed to have originated from the structural rigidity attained by the Au14 NCs as a result of complexation with zinc ions. Upon coordination of the ligands stabilizing the Au14 NCs with zinc ions, the restrictions attained in intramolecular motions (rotational and vibrational) of the ligands stabilizing the clusters eventually led to non- radiative transitions being reduced. This led to the enhancement in luminescence intensity and luminescence lifetime of the assembled clusters vis-à-vis the constituent clusters.


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Conclusions In essence, we have introduced a facile and novel strategy of complexation reaction and supramolecular interaction based synthesis of a few layered quasi two dimensional nanosheets of Au14 NCs of thickness of 2.44 ± 0.48 nm. A plausible structure of the nanosheets has been proposed following detailed analytical investigation and computational optimization. Further, the as-prepared nanosheets of Au14 NCs were found to exhibit superior optical properties as compared to the individual clusters. Moreover, the nanosheets comprising of Au14 NCs were found to exhibit room temperature delayed fluorescence, with a lifetime of 0.502 ms and a quantum yield of 19.1±0.9 %, which was otherwise absent in the as-synthesized Au14 NCs. Thus, the study embodied herein is envisioned to open up newer avenues for tailored synthesis of dimensionality specific assemblies of nanoscale particles via controlling the covalent and non-covalent interactions between the ligands stabilizing the nanoscale particles. Additionally, the observation of intriguing optical properties like room temperature phosphorescence/delayed fluorescence following assembly formation of Au14 NCs via complexation reaction reflects the relevance of assembled nanoclusters over constituent clusters for futuristic technological applications.

Associated Content: Supporting information is available free of charge at ACS website.

Author Information: Corresponding Author: [email protected]

Notes: The authors declare no conflict of interest.

Acknowledgements We thank Department of Electronics and Information Technology, government of India, (No. 5(9)/2012-NANO (Vol. II)) for providing financial aid. Assistances from central instruments


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facility of Indian Institute of Technology Guwahati, Gayatri Natu, Jahnu Saikia and Prof. Anumita Paul are acknowledged.

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Room Temperature Delayed Fluorescence of Gold Nanoclusters in

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