Journal of Colloid and Interface Science 418 (2014) 234–239

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Synthesis, characterization and optical properties of an amino-functionalized gold thiolate cluster: Au10(SPh-pNH2)10 Christophe Lavenn a, Florian Albrieux b, Alain Tuel a, Aude Demessence a,⇑ a b

Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR 5256 CNRS/Lyon 1 University, 2 Avenue Albert Einstein, 69626 Villeurbanne, France Centre Commun de Spectromètrie de Masse, UMR 5246 CNRS/Lyon 1 University, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne, France

a r t i c l e

i n f o

Article history: Received 24 October 2013 Accepted 9 December 2013 Available online 14 December 2013 Keywords: Gold cluster Aminothiophenol Mass spectrometry Optical properties

a b s t r a c t Research interest in ultra small gold thiolate clusters has been rising in recent years for the challenges they offer to bring together properties of nanoscience and well-defined materials from molecular chemistry. Here, a new atomically well-defined Au10 gold nanocluster surrounded by ten 4-aminothiophenolate ligands is reported. Its synthesis followed the similar conditions reported for the elaboration of Au144(SR)60, but because the reactivity of thiophenol ligands is different from alkanethiol derivates, smaller Au10 clusters were formed. Different techniques, such as ESI-MS, elemental analysis, XRD, TGA, XPS and UV–vis-NIR experiments, have been carried out to determine the Au10(SPh-pNH2)10 formula. Photoemission experiment has been done and reveals that the Au10 clusters are weakly luminescent as opposed to the amino-based ultra-small gold clusters. This observation points out that the emission of gold thiolate clusters is highly dependent on both the structure of the gold core and the type of the ligands at the surface. In addition, ultra-small amino-functionalized clusters offer the opportunity for extended work on self-assembling networks or deposition on substrates for nanotechnologies or catalytic applications. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Sub-2 nanometer materials have recently attracted much attention because of their unique physical and chemical properties, such as strong quantum confinement, enhanced catalytic activity, intense fluorescence and ferromagnetism in typically nonmagnetic materials [1]. However, compared to the well-studied, larger-sized nanostructures, the fundamental understanding of these sub-2 nm particles, also called clusters, is still in its infancy regarding synthesis, characterizations, structure–property relationships, and potential applications. Gold clusters Aun(SR)m, made of n gold atoms and stabilized by m organic thiolate linkers, hold great promises in term of new nanomaterials, because they are atomically well-defined and for some of them their structure has been resolved by single-crystal X-ray crystallography. Since the development of thiolate gold nanoclusters, a large number of clusters have been isolated and characterized with different numbers of gold atoms and ligands. The largely studied ones are Au25(SR)18, Au38(SR)24, Au102(SR)44 and Au144(SR)60 [2–5]. With the size of clusters becoming smaller and smaller, the geometric and electronic structures influence dramatically their properties at the borderline of metallic nanoparticles and molecular complexes. Thus, gold clusters formed ⇑ Corresponding author. E-mail address: [email protected] (A. Demessence). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.12.021

between 5 and 23 gold atoms and encapsulated in poly(amidoamine) are highly fluorescent quantum dots with size dependency [6]. In addition, very small gold clusters of only 3–10 atoms can efficiently catalyze the addition of water to alkynes with a reaction turnover number of 107 at room temperature [7]. However apart from those heterogeneous ultra-small clusters, the development and characterizations of clusters made of few tens of atoms remain little explored due to their challenging synthesis. Ultra small Au10 clusters have been mainly reported with amino acids (histidine His: Au10(His)n [8] and cysteine Cys: Au10(Cys)10) [9], peptidic molecules (glutathione SG: Au10(SG)10) [10], proteins (bovine serum albumin BSA: Au10(BSA)n) [11] and also thiophenol derivates and mixture of phosphine, amino, thiols or halide-based ligands [12]. With the biological molecules used as ligands, two clusters, Au10(Cys)10 and Au10(SG)10, have well-defined formula, however other Au10 clusters stabilized by histidine or Bovine Serum Albumin molecules have a number of surrounding organic molecules in excess, making difficult to evaluate the influence of the ligands on the structure and the properties. From all the Au10 clusters reported, three structures have been resolved by single-crystal Xray diffraction. The two first one are made of phosphine-based ligands: [Au10Cl3(PCy2Ph)6](NO3) [13] (Cy = cyclohexyl) and [Au10(PPh3)7{S2C2(CN)2}2] [14]. The two Au10 clusters are geometrically identical; the gold atoms define a core with an idealized D3h symmetry where a central atom is surrounded by 9 gold atoms in a

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circular fashion. Finally, Au10(SPhp-CMe3)10 (with SPhp-CMe3 = 4tert-butylthiophenol) is the only Au10 clusters stabilized by thiolate ligands that has been characterized by single-crystal X-ray diffraction [15]. It is a catenane structure with two interpenetrated pentagonal rings of Au5(SR)5, each one having five sulfur and gold atoms. Among the gold atoms of the rings, two are in the middle of the cycle forming an Au2 gold core. For small gold clusters made of only thiolate ligands (AuSR)n with n = 10–12, it has been shown experimentally [15,16] and theoretically [17,18] that the catenane structure is the most stable one. However, even if the structure of this Au10(SPh-pCMe3)10 cluster has been reported, as well as calculations, there is no extended work on its structural characterization and its optical properties. Here we present a one step synthesis of a new Au10 cluster, stabilized by ten 4-aminothiophenolates (SPhpNH2) in order to bring an external function. Its synthesis leads to a relatively pure and high yield compound without extended purification. Characterizations have been done by ESI-MS, XPS, DRX, TGA and FT-IR and its optical properties (absorption and emission) are reported.

2. Experimental section 2.1. Materials and equipments 4-Aminothiophenol (HSPh-pNH2) was purchased from TCI. Tetrachloroauric acid trihydrate (HAuCl43H2O) was supplied from Alfa Aesar. Tetraoctylammonium bromide (TOABr), sodium borohydride (NaBH4), methanol (MeOH, puriss p.a.) and N,N-dimethylformamide (DMF) were purchased from Sigma–Aldrich. All reagent and solvents were of commercial quality and used without further purification. The glassware used in the synthesis was cleaned with aqua regia (aqua regia is a very corrosive product and should be handled with extreme care), then rinsed with copious amount of distilled water and dried overnight prior to the use. All reactions were carried out in atmospheric conditions. The mass spectra were recorded in a positive ion mode on a hybrid quadrupole time-of-flight mass spectrometer (MicroTOFQ-II, Bruker Daltonics, Bremen) with an Electrospray Ionization (ESI) ion source. The gas flow of spray gas is 0.6 bar and the capillary voltage is 4.5 kV. Mild conditions were used for the ions transfer to keep intact the functionalized nanoclusters. Prior to the experiments, the Au10(SPh-pNH2)10 clusters are dissolved in DMF (1 mg/ml). The solutions are infused at 180 lL/h in a mixture of solvents (methanol/dichloromethane/water 45/40/15). The mass range of the analysis is 800–5000 m/z and the calibration was done with cesium iodide and tune mix (Agilent). Thermogravimetric analysis (TGA) was performed with a TGA/ DSC 1 STARe System from Mettler Toledo. Around 2 mg of sample was heated at a rate of 10 °C min1 from 25 to 900 °C, in a 70 lL alumina crucible, under air atmosphere (20 mL min1). Shining droplets were observed at the end of experiment with the clusters, which corresponds to bulk gold. Gold loading is determined by inductively coupled plasma optical emission spectroscopy (HORIBA Jobin Yvon Activa ICPOES) and focused at 242.79 and 208.21 nm onto the detector. The clusters are decomposed in aqua regia solution + HF and then in HCl. Sulfur percentage is determined by full combustion at 1320–1360 °C under O2 stream and analysis of SO2 and is titrated in a coulometric–acidimetric cell. Carbon and hydrogen percentage is determined by full combustion at 1030–1070 °C under O2 stream and transformed into CO2 and H2O and is titrated on a coulometric detector. Nitrogen percentage is determined by full combustion under He:O2 stream (3% in O2) and is titrated on a katharometer. Analysis precision is 0.3% absolute for carbon, sulfur, nitrogen and hydrogen.

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X-ray diffraction (XRD) is carried out on a Bruker D8 Advance A25 diffractometer using Cu Ka radiation equipped with a 1dimensional position-sensitive detector (Bruker LynxEye). X-Ray scattering was recorded between 4° and 80° (2h) with 0.02° steps and 0.5 s per step. Divergence slit was fixed to 0.2° and the detector aperture to 189 channels (2.9°). X-ray photoelectron spectroscopy (XPS) experiments are carried out on a Kratos Axis Ultra DLD spectrometer using monochromated Al Ka X-rays (1486.6 eV, 150 W), a pass energy of 20 eV, a hybrid lens mode and an indium sample holder in ultra-high vacuum (P < 109 mbar). The analyzed surface area is 700 lm  300 lm. Charge neutralization is required for all samples. The peaks (Au 4f, S 2p, N 1s) are referenced to the C–(C, H) components of the C 1s band at 284.6 eV. Shirley background subtraction and peak decomposition using Gaussian–Lorentzian products are performed with the Vision 2.2.6 Kratos processing program. The infrared spectra were obtained on a Bruker Vector 22 FT-IR spectrometer with KBr pellets at room temperature and registered from 4000 to 400 cm1. Transmission electron microscopy (TEM) was carried out on a JEOL 2010 LaB6 microscope operating at 200 kV. The sample was dispersed in DMF with an ultrasonic bath and then a drop was deposited on a copper grid and let to dry under ambient conditions of atmosphere and temperature. The UV–vis-NIR absorption spectra of solutions were carried out on an Agilent 8453 diode-array spectrophotometer and registered from 270 nm to 1000 nm. The spectrophotometric interval was 1.0 nm and integration time was 0.5 s. Emission spectrum was performed on a home-made setup. The excitation was provided by a 20 mW laser diode emitting at 405 nm. The emission from the sample was collected by an optical fiber connected to a Jobin Yvon TRIAX320 monochromator equipped with a Peltier cooled CCD camera. The resolution of the system was 2 nm and the integrating time 10 s. The emission from the laser diode was filtered by an edge filtrate 500 nm FELH0500 from Thorlabs. 2.2. Synthesis of Au10(SPh-pNH2)10 The synthesis of Au10(SPh-pNH2)10 nanoclusters is a modified Brust-Schiffrin method [4]. Tetrachloroauric acid trihydrate (200 mg, 1 eq., 508 lmol) was dissolved in 30 mL of methanol in a 200 mL Erlenmeyer flask. To this solution, TOABr (253 mg, 0.91 eq., 462 lmol) in 30 mL methanol was added in one time. After 15 min under vigorous stirring, the solution color changed from yellow to dark red. At room temperature, the solution containing the 4-aminothiophenol (700 mg, 11 eq., 5.59 mmol in 15 mL methanol) was quickly added in one portion. The solution color changed immediately from dark-red to dark-green. After 15 min of vigorous stirring, a freshly prepared solution of NaBH4 (250 mg, 13 eq., 6.60 mmol in 12 mL of cold distilled water) was quickly added to the stirred solution. After the addition of the aqueous NaBH4, the solution turned immediately dark brown and a strong H2 gas releasing was observed. Stirring was maintained 30 min in the stoppered flask. The black precipitate was collected by centrifugation (15 min, 11 000 rpm). The product was then washed with methanol and collected via centrifugation again (15 min, 11 000 rpm). The last step was done three times to completely remove the unreacted species, thiol excess and salts. The washed black precipitate was dried at air under atmospheric conditions and stored under these conditions. The so-obtained product (110 mg) is poorly dispersible in almost all solvents except N,N-dimethylformamide. The average yield obtained is 65% based on gold. No further purifications (such as size exclusion chromatography, recrystallisation and

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precipitation) were done before any of the characterizations. Elemental analysis calculated for Au10(SPH-pNH2)101.2H2O (%): C 21.02, H 2.47, N 4.09, S 9.36, Au 57.46; found: C 22.13, H 1.71, N 4.10, S 9.77, Au 57.62. 3. Results and discussions 3.1. Synthesis of Au10(SPh-pNH2)10 The chemistry of atomically well-defined thiolate gold nanoclusters started about ten years ago and it is still difficult to rationalize their synthesis. One can observe that the synthesis is highly dependent on the thiolate ligand. Among all the thiol ligands used to get gold nanoclusters, most are alkanethiol-based derivatives (HSCH2-R) like phenylethanethiol, alkanethiol chains or glutathione [19]. But with thiophenol derivatives (HSPhX, X = H, NO2, Br, CH3, OCH3, CO2H), most of the syntheses are achieved by exchange reaction starting from Aun(SC2H4Ph)m clusters with (n, m) = (25, 18) and (38, 24) [3,20,21]. However ligand exchange reaction from well defined clusters may not lead to the formation of the same cluster. Indeed Jin et al. reported an exchange reaction from Au38(SC2H4Ph)24 with 4-tert-butylthiophenol and obtained a cluster with a totally different cluster structures that is: Au36(SPh)24 [21]. Only three examples have been reported for the direct synthesis of gold nanoclusters surrounded by thiophenol derivatives, which are Aun(SPh)m, [22] Au102(SPh-pCO2H)44 [5] and Au25(SPhpNH2)17 [23]. Additionally, in order to get Au144(SR)60 with SR = phenylethanethiolate and alkanthiolates, Jin et al. reported two synthetic methods. The first one is a two-step synthesis starting with a modified Brust–Schriffin method and followed by a size focusing reaction [24]. The second synthesis is a simple one-pot method in methanol, at room temperature, with HAuCl4/TOABr/ HSR/NaBH4 in the ratio 1/1.16/5.3/10 [4]. In order to synthesize amino-functionalized clusters with the 4-aminothiophenol ligand for further applications, we followed the experimental conditions described above to get Au144, by mixing HAuCl4/TOABr/HSR/NaBH4 in the ratio 1/0.9/11/13. However instead of Au144, we got Au10 clusters. So, similar synthetic conditions used for different thiol ligands do not always give the same compound. This shows that the synthesis of gold clusters is really sensitive to the type of thiolate ligand. The difference between alkanethiol and thiophenol derivatives seems to be the reason of this different reactivity [22]. Consequently, the formation of Au102(SPhCO2H)44, Au36(SPh)24, Au25(SPh-pNH2)17 and Au10(SPh-pNH2)10 clusters suggests that the synthesis of gold nanoclusters with thiophenol-based ligands is different from clusters with alkanethiol-based molecules and may be explained by the rigidity and steric hindrance of those ligands. The one-step synthesis of this Au10 cluster gives a high yield of product and is much simple that the two-step synthesis reported for the analog Au10(SPh-pCMe3)10 requiring [AuCl(C2H5SC4H4OH)] as a precursor [15]. 3.2. Characterizations Multiple experiments were conducted to determine the chemical formula Aun(SR)m and the purity of the clusters both in the solution and in the solid state. Electro-spray ionization mass spectrometry (ESI-MS) has been primary employed, because, firstly it is a soft technique that prevents fragmentation and secondly, it is a powerful tool to establish the formula, the purity and also to get an idea of the structure of thiolate gold clusters [10,25]. We carried out analysis of the sample dissolved in DMF; experiments are reported in Fig. 1. The positive-ion ESI-MS spectrum of the clusters (Fig. 1a and Table 1), shows two groups of peaks for +1 and +2 charge states attributed to Au10 clusters. An extended spectrum

from m/z = 800 to 10 000, reported in Fig. S1, confirms that no larger or smaller clusters are detected, pointing out the purity of the sample. In z = +1 charge state domain (Fig. 1a and a zoom Fig. S2), four species [Aun(SPh-pNH2)m], are observed with a decreasing intensity as follows: (n, m) = (10, 9), (10, 10), (10, 11) and (10, 12). Considering from the literature that the Au10(SR)10 clusters are the more stable clusters, the presence in the gas phase of the (10, 9), (10, 11) and (10, 12) species may be generated from the fragmentation or a recombination of (10, 10) clusters. This proposition is also justified by ESI-MS experiments carried out at lower temperature (20 °C instead of 180 °C) where the predominant peak is attributed to (10, 10) species (Fig. S3). The peak at m/ z = 3208.8877 can be assigned to [Au10(SPh-pNH2)10-H]+, which matches perfectly with the simulated isotope pattern (peak at m/ z = 3208.8832) (Fig. 1b). Other peaks can be assigned as [Au10(SR)9]+, [Au10(SR)11]+ and Au10(SR)12-H]+ (Figs. S4–S6 and Table 1 for detailed assignments). In the z = +2 charge state area of [Au10(SR)m] (Fig. 1c and Table 1), the more intense peak at m/ z = 1606.1073 corresponds to [Au10(SR)10 + 2H]2+ and is also present with other Au10 clusters of different numbers of ligands from 8 to 12. The protonation or deprotonation of the clusters observed in the gas phase, and their positive net charge still remains unclear. The presence of the amino groups surrounding the clusters seems to be responsible for this complex ionization and the disparity in the presence or absence of proton [26]. Indeed, we also carried out positive-ion ESI mass spectrometry on Au10(SCH2CH2Ph)10 clusters that we got as a side product from a reported reaction (Figs. S8–S11) [27], but did not observed (de)protonation of the species. In case of glutathione (SG)-based clusters Au10(SG)10, the net charge of the clusters is also neutral [28]. Collisional activation has been applied on Au10(SPh-pNH2)10 cluster and leads to its dissociation into mainly Au5(SR)5 motifs. The formation of these smaller clusters is in good accordance with a catenane structure with two interpenetrated Au5(SPh-pNH2)5 rings. Fig. 1c and a zoom in Fig. S7, show the smaller clusters singly charged Au5 ðSRÞþ 5 that are superimposed with [Au10(SR)10 + 2H]2+. Other small clusters Aun(SR)m with z = +1 are also observed with less intensity: (n, m) = (4, 4), (4, 5), (5, 4), (5, 6), (6, 5) and finally (6, 6). Those small clusters are originating from different fragmentation pathways of Au10(SR)10 and form three pairs [(4, 4)(6, 6)], [(4, 5)(6, 5)] and [(5, 4)(5, 6)] coming from the fragmentation of the structure of Au10(SR)10 clusters. In case of glutatione-based clusters, the collisional activation of Au10(SG)10 corresponds to the formation of Au4(SG)4 and Au2(SG)2 motifs, suggesting that Au10(SG)10 clusters have a different structure and may not form interpenetrated Au5(SG)5 rings but Au4(SG)4-based rings, which are more favorable with voluminous thiolate ligands, like the glutathione [28,29]. Consequently, positive ESI-MS experiments confirmed the identity of Au10(SPh-pNH2)10 clusters, their purity and support the formation of a structure made of Au5(SPh-pNH2)5 rings like in the catenane one. Other characterization techniques also support this conclusion. C, H, N, S and Au elemental analyses of the clusters match perfectly with the ratio established in the proposed formula (S/Au and N/Au expected: 1, found: 1.04 and 1.00, respectively). Infrared spectroscopy of the Au10 clusters confirms that all thiolate ligands are coordinated to the gold atoms by the disappearance of the S–H stretching band at 2550 cm1 (Fig. S12). Thermal gravimetric analysis has also been performed under air flow to determine the stability of the cluster and the ratio between organic and inorganic parts. Fig. S13 in the Supplementary information shows that the thermal decomposition starts at 150 °C and the loss of the ligand is 40.0% (expected 38.7%). Powder X-ray diffraction of the compound is reported in Fig. 2. The diagram of Au10(SPh-pNH2)10 is constituted of an intense and thin peak at low angle (5.27°) and a large and weak one at higher

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Fig. 1. (a) Positive-ion ESI mass spectrum of as-synthesized Au10 clusters with (n, m)z corresponding to the formula [Aun(SPh-pNH2)m] and z the charge state, (b) experimental (black) and simulated (gray) isotopic patterns of [Au10(SPh-pNH2)10-H]+ (zoom 1) and (c) assignments of the different [Aun(SPh-pNH2)m] species present from m/z = 1250– 2000 (zoom 2) and their charge state.

Table 1 Experimental isotopic distribution of the different Au10 clusters observed in the positiveion ESI mass spectrum for the charge state (z). Au10 clusters

Position (m/z)

z = +1 [Au10(SR)9]+ [Au10(SR)10-H]+ [Au10(SR)11]+ [Au10(SR)12-H]+

3085.8773 3208.8877 3333.8603 3456.8361

z = +2 [Au10(SR)9 + H]2+ [Au10(SR)10 + 2H]2+ [Au10(SR)11+H]2+ [Au10(SR)12]2+

1543.5878 1606.1073 1667.6037 1729.1160 Fig. 3. Au4f XPS spectra of Au10(SPh-pNH2)10 nanoclusters (black) and Au(I)-(SPhpNH2) polymers (gray).

Fig. 2. Experimental powder X-ray diffraction pattern of Au10(SPh-pNH2)10.

angle (36.0°). The broad peak centered at 2h = 36.0° is the characteristics of gold atoms stacking and their inter-metallic distances. The FWHM of this peak is around 12°, which is much wider than Au25 clusters, in accordance with the smaller size of the clusters (Fig. S14). Compared to Face-Centered Cubic phase of bulk metallic gold, where the (1 1 1) peak is at 38.18°, this shift to lower angles is due to longer Au–Au distances, as observed in catenane Au10(SPhpCMe3)10. The peak at low angle X-ray scattering is at 2h = 5.27° which corresponds to d = 1.68 nm by applying the Bragg law. The presence of this peak reveals a short ordering distance of clusters packing and is a mark of homogeneous clusters composition and structure. The position of the peak at 1.68 nm may also be viewed as the average diameter of a functionalized cluster, surrounded by ligands of 0.6 nm length. Consequently, powder X-ray diffraction

remains an efficient tool to get the fingerprint of ultra-small Aun clusters [4,30]. TEM experiment of the clusters has been also carried out (Fig. S15) and picture confirms the presence of particles of less than one nanometer. However the instability of the clusters under the electron beam coupled to the limit of the microscope resolution does not lead to evaluate accurately the particles diameter. Fig. 3 shows the high resolution photoelectron spectra of Au4f binding energy of Au10(SPh-pNH2)10. The 4f7/2 and 4f5/2 binding energies of gold are at 84.9 and 88.6 eV, respectively with a FWHM of 1.08 eV, suggesting that there is only one gold electronic state in the clusters. The position of Au 4f7/2 matches with the Au 4f7/2 peak of a white Au(I)-(SPh-pNH2) polymer [31] (84.9 eV), which means that all the gold atoms are in the +I oxidation state and that they are all coordinated to thiolate ligands like in the catenane structure. By comparing different size particles functionalized by SPhpNH2 ligands, the shift of the Au4f binding energies through the high energy follows the core size reduction due to the increasing of gold surface interacting with the thiolate ligands [32]. Thus, the Au4f7/2 binding energy is at 84.2 eV for particles of ca. 3 nm [33] and at 84.7 eV for Au25(SPh-pNH2)17 [23]. Additionally N1s and S2p photo-emission spectra have been reported in Figs. S16 and S17. The presence of a single peak of N1s at 399.6 eV is assigned to free amino groups as opposed to Au25(SPh-pNH2)17 where goldamine interactions have been reported. The S2p binging energy at 163.4 eV is typical of thiolate ligands and more importantly shows that the sulfur atoms are not oxidized. The absence of halides or other elements as counter ions is also confirmed by XPS analysis (survey Fig. S18). Elemental quantification with XPS gives an atomic ratio S/Au and N/Au of 1.01 and 0.99, in good accordance with the proposed formula Au10(SPh-pNH2)10. To conclude, all those

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described characterization techniques reveal that the ratio of Au/ SPhp-NH2 is one (elemental analysis, TGA and XPS), that there is no bigger clusters than Au10 observed either by PXRD or ESI-MS, that the only oxidation state of gold atoms is +1 (XPS) and so that the formula is Au10(SPh-pNH2)10.

3.3. Optical properties of Au10(SPh-pNH2)10 clusters Absorption experiments achieved in DMF solution are reported in Fig. 4. No surface Plasmon resonance peak of gold nanoparticles is observed. There is only a continuous increase on the spectral range from 700 to 300 nm. This featureless band absorption has already been observed for Au10 clusters encapsulated in an excess of histidine and bovine serum albumin (BSA) molecules, and is due to the molecular state of clusters [8,11,34]. This absorption curve is also consistent with the broad one that has been calculated for the catenane Au10(SMe)10 structure that presents a HOMO–LUMO gap evaluated at 2.5 eV [17,35]. The weak peak structure has been attributed to the low symmetry of this system. For Au10(SPh-pNH2)10 clusters, there is no fluorescence under UV light in solution and experiments performed in solid-state show a weak emission band centered at 560 nm when excited at 405 nm (Fig. 5). The weakness of this solid-state photoluminescence spectrum is consistent with the dark brown color of the powder. Only Au10@histine clusters have been stated as fluorescent (green under 365 nm UV light) and show a maximum of emission at 490 nm in the solution and at 520 nm for the film [8,34]. Optical properties of ultra-small glutathione Au10–12(SG)10–12 clusters have also been reported and the resultant mixture of clusters presents two distinct peaks of absorption at 330 and 375 nm, and even if they are not luminescent under UV light [36], they present two emission bands at 830 and 410 nm [10]. By comparing the ultrasmall clusters Au10@histidine, Au10@BSA, Au10–12(SG)10–12 and Au10(SPh-pNH2)10 clusters, the difference in the emission peak position and intensity may be due to the type of ligands and their interaction with gold atoms: strong gold-sulfur bonds versus weaker bonds (imidazole, amine or carboxylic acid) [37]. Despite considerable theoretical and experimental efforts on sub-2 nm gold clusters [38], fundamental understanding of the effect of the structure (composition, shape and size of the gold core and the type of organic layers) on their luminescence properties, as it has been observed for up-2 nm gold nanoparticles [39], remains far from being complete. This is the reason why more optical properties of pure and well-characterized thiolate gold clusters have to be reported.

Fig. 4. UV–vis spectra of Au10(SPh-pNH2)10 (black) and HSPh-pNH2 (gray) measured in DMF at room temperature. Inset shows a photograph of the Au10 clusters in DMF at different concentrations.

Fig. 5. Photoemission spectrum of Au10(SPh-pNH2)10 in solid state at room temperature with kex = 405 nm.

4. Conclusions A new ultra small amino functionalized gold cluster has been synthesized in large scale. Its formula, Au10(SPh-pNH2)10, has been established by high resolution ESI-MS and is also confirmed by powder X-ray diffraction and UV–vis spectroscopy. Synthesis and characterization of atomically well-defined clusters still remain challenging, their characterizations have to be carefully completed to establish a database of model thiolate gold nanoclusters and fully understand the rule of structure of the gold core and the organization of the thiolate molecules at the surface on the properties. In addition, due to the presence of the amino group at the surface of this ultra small Au10 clusters, they can be used as building blocks for coupling reactions or deposition on surfaces for further applications in optics or catalysis. Acknowledgments C.L. thanks the French ministry for his PhD grant. Authors wish to thank G. Bergeret for fruitful discussions, G. Ledoux for emission spectroscopy experiments, P. Delichère for XPS measurements and N. Cristin and P. Mascunan for elemental analyses. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.12.021. References [1] H.F. Qian, M.Z. Zhu, Z.K. Wu, R.C. Jin, Acc. Chem. Res. 45 (2012) 1470; S. Choi, R.M. Dickson, J.H. Yu, Chem. Soc. Rev. 41 (2012) 1867; G. Li, R. Jin, Acc. Chem. Res. 46 (2013) 1749; Y. Lu, W. Chen, Chem. Soc. Rev. 41 (2012) 3594; T. Tsukuda, Bull. Chem. Soc. Jpn 85 (2012) 151. [2] H. Qian, Y. Zhu, R. Jin, ACS Nano 3 (2009) 3795. [3] J.F. Parker, C.A. Fields-Zinna, R.W. Murray, Acc. Chem. Res. 43 (2010) 1289. [4] H. Qian, R. Jin, Chem. Mater. 23 (2011) 2209. [5] Y. Levi-Kalisman, P.D. Jadzinsky, N. Kalisman, H. Tsunoyama, T. Tsukuda, D.A. Bushnell, R.D. Kornberg, J. Am. Chem. Soc. 133 (2011) 2976. [6] J. Zheng, C.W. Zhang, R.M. Dickson, Phys. Rev. Lett. 93 (2004) 077402. [7] J. Oliver-Meseguer, J.R. Cabrero-Antonino, I. Dominguez, A. Leyva-Perez, A. Corma, Science 338 (2012) 1452. [8] X. Yang, M. Shi, R. Zhou, X. Chen, H. Chen, Nanoscale 3 (2011) 2596. [9] Y. Zhang, S. Shuang, C. Dong, C.K. Lo, M.C. Paau, M.M.F. Choi, Anal. Chem. 81 (2009) 1676. [10] Y. Negishi, K. Nobusada, T. Tsukuda, J. Am. Chem. Soc. 127 (2005) 5261. [11] Y. Yu, Z. Luo, C.S. Teo, Y.N. Tan, J. Xie, Chem. Commun. 49 (2013) 9740. [12] M.F. Bertino, Z.M. Sun, R. Zhang, L.S. Wang, J. Phys. Chem. B 110 (2006) 21416. [13] C.E. Briant, K.P. Hall, A.C. Wheeler, D.M.P. Mingos, J. Chem. Soc., Chem. Commun. (1984) 248. [14] G.M.T. Cheetham, M.M. Harding, J.L. Haggitt, D.M.P. Mingos, H.R. Powell, J. Chem. Soc., Chem. Commun. (1993) 1000.

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Synthesis, characterization and optical properties of an amino-functionalized gold thiolate cluster: Au10(SPh-pNH2)10.

Research interest in ultra small gold thiolate clusters has been rising in recent years for the challenges they offer to bring together properties of ...
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