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Gold Nanoparticle Dyads Stabilized with Binuclear Pt(II) Dithiol Bridges Ilaria Fratoddi,† Iole Venditti,† Chiara Battocchio,*,‡ Giovanni Polzonetti,‡ Federica Bondino,§ Marco Malvestuto,^ Emanuela Piscopiello,z Leander Tapfer,z and Maria Vittoria Russo† †
Department of Chemistry, Sapienza University of Rome, P.le A. Moro 5, 00185 Rome, Italy Department of Physics, unita INSTM and CISDiC University Roma Tre, Via della Vasca Navale 85, 00146 Rome, Italy § Laboratorio Nazionale TASC, INFM-CNR, S.S. 14, km 163.5, Area Science Park, 34012 Basovizza Trieste, Italy ^ Sincrotrone Trieste, S.S. 14 Km 163.5 in Area Science Park, I-34012 Trieste, Italy z ENEA, UTTMATB, Centro Ricerche Brindisi, Strada Statale 7 “Appia” km.703, 72100 Brindisi, Italy ‡
bS Supporting Information ABSTRACT: The synthesis and characterization of gold nanoparticles (AuNPs-1) stabilized by a novel bifunctional thiolate organometallic complex containing Pt(II) centers, that is, trans,trans-[(CH3COS)(PBu3)2PtCtCC6H4C6H4Ct CPt(PBu3)2(SCOCH3)] (complex 1), has been carried out. As a comparison, gold nanoparticles stabilized with an organic thiol, allylmercaptane, that is, AuNPs-2, and self-assembled monolayers (SAMs) of both thiols were also prepared and investigated. The AuNPs-1 show a direct link between Pt(II) and Au nanoparticles through a single S bridge and are candidates for the achievement of 2D or 3D networks. The size control of the Au nanoparticles was achieved by careful control of synthesis parameters, and the hybrids were characterized by means of high-resolution transmission electron microscopy (HR-TEM) and synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS). SR-XPS measurements allowed the assessment of the anchoring of the organic or organometallic thiols onto gold substrates as well as onto gold nanoparticles. AuNPs-2 with diameters in the range from 1.6 to 3.9 nm were obtained. AuNPs-1 with an average diameter in the range of 4.53.6 nm were obtained, and linkage between the nanoparticles can be envisaged with the formation of dyads supported by SR-XPS measurements. In fact, S2p core-level data indicate that both sulfur atoms of the organometallic thiol chemically interact with gold grafting vicinal nanoparticles.
1. INTRODUCTION Nanoscience has attracted continuous research interest over recent years. The perspectives of nanomaterials for research and technology development are widely addressed in the literature1 because of their properties and surface functionalities. Nanomaterials of different natures and shapes find promising applications in nanoelectronics, biomedicine, and catalysis.2 Among the variety of materials, interest in metal nanoparticles has risen exponentially in the past decade because of the unique optical properties of metals in nanostructured dimensions.3 Stabilized gold nanoparticles have emerged as a broad new research field in the domain of colloids and surfaces on the basis of not only their peculiar optical characteristics4 but also their high chemical stability and catalytic and size-dependent electrochemical properties recently reviewed.5,6 For example, the catalytic properties of 24 nm in diameter AuNPs have been successfully tested for the heterogeneous low-temperature oxidation of CO.7 Chromophore-functionalized gold nanoparticles have been synthesized for a variety of applications, such as photovoltaics, fluorescent r 2011 American Chemical Society
display devices, and light-mediated binding and release of biologic molecules.8 Thiol-protected gold nanoparticles have also recently found relevance in the development of gas sensors; as an example of biomedical application, highly sensitive and fastresponse arrays of sensors based on gold nanoparticles smell volatile organic compounds in exhaled breath for lung cancer screening and diagnosis.9 When properly stabilized by a shell of ligands such as thiols, amines, ammonium salts, and polymers, AuNPs display excellent stability toward aggregation, which enables attempts to achieve nanoparticles with different sizes and shapes.10 The fabrication of Au nanoparticles has been greatly facilitated by the two-phase method proposed by Brust et al. in a pioneering work.11 In this approach, chemical reduction of HAuCl4 is carried out in the presence of thiol ligands and strong reducing agents such as Received: March 23, 2011 Revised: July 7, 2011 Published: July 12, 2011 15198
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The Journal of Physical Chemistry C NaBH4. The size of nanoparticles can be controlled through the control of the stoichiometry of the reactants providing nanoparticles ranging in overall diameters of 115 nm. The synthesis in general produces nanoparticles with a certain degree of polydispersion. However, well-defined clusters, that is, p-mercaptobenzoic acid (p-MBA) protected gold nanoparticle, which comprises 102 gold atoms and 44 p-MBA structures, could be achieved and fully assessed by X-ray crystallographic determinations showing that the ligands are bonded in bridging coordination and that many of the Au atoms are not part of the core but exist with the ligands in semiring or staple structures.12 Our group has recently developed the research on the stabilization of gold nanoparticles with Pd(II) containing organometallic thiols13,14 and on the self-assembled monolayers (SAMs) of Pd(II) based thiolates on gold surface.15,16 In this paper, the one-pot synthesis of gold nanoparticles (AuNPs-1) stabilized with the organometallic bifunctional thiol trans,trans-[dithiodibis(tributylphosphine)diplatinum(II)-4,40 diethynylbiphenyl] (complex 1) is reported. The same reaction was accomplished with an organic thiol, that is, allylmercaptane (AM) as capping agent, yielding AuNPs-2 nanoparticles. The research aimed to compare the link of the bifunctional organometallic thiolate complex, which is able to directly link Pt(II) to Au nanoparticles through a trinuclear AuSPt bridge, with that of an organic monofunctional ligand. The size, shape, and crystalline structure of these functionalized nanoparticles were determined by a full-pattern X-ray powder diffraction, X-ray diffraction (XRD) analysis, high-resolution transmission electron microscopy (TEM), and synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS). The structure of the synthesized nanoparticles has been investigated and discussed in comparison with that of self-assembled monolayers of complex 1 and AM on gold surfaces (SAM-1 and SAM-2).
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4.3H2O, Aldrich, 99.9+%), potassium tetrachloroplatinate (K2PtCl4, Aldrich, 99.9+%), tributylphosphine (PBu3, Aldrich, 98%), allylmercaptane (AM, Aldrich 98% pure), potassium thioacetate (CH3COSK, Aldrich, 98%), tetraoctylammonium bromide (TOAB, Aldrich, 98%), sodium borohydride (NaBH4, Aldrich, 99%), and organic solvents (Aldrich reagent grade) were used as received. Platinum complex trans- [dichlorobis(tributylphosphine)platinum(II)] and 4,40 -diethynylbiphenyl (HCtCC6H4C6H4CtCH, DEBP) were prepared by reported methods.17,18 Preparative thin-layer chromatography (TLC) separation was performed on 0.7 mm silica plates (Merck Kieselgel 60 GF254), and chromatographic separations were obtained with 70-230 mesh silica (Merck) by using n-hexane/dichloromethane mixtures. Deionized water, obtained from Millipore-SIMPAKOR1 (Simplicity185), was degassed for 30 min with Argon before use. Other solvents and materials were reagent grade (Aldrich). 2.2. Synthesis of Functionalized Nanoparticles and SAMs. 2.2.1. Synthesis of trans, trans-[(CH3COS)Pt(PBu3)2 CtCC6H4C6H4CtCPt(PBu3)2(SCOCH3)]. The organometallic complex 1, trans,trans-[(CH3COS)Pt(PBu3)2CtCC6H4 C6H4CtCPt(PBu3)2(SCOCH3)], was prepared from the square planar Pt(II) complex trans,trans-[ClPt(PBu3)2CtCC6H4 C6H4CtCPt(PBu3)2Cl]19 by using the ligand substitution reaction in the presence of potassium thioacetate in equimolar
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amount. For a typical reaction, 0.200 g, 0.129 mmol of trans, trans-[ClPt(PBu3)2CtCC6H4C6H4CtCPt(PBu3)2Cl], dissolved in CH2Cl2 (20 mL), and 0.299 mmol of CH3COSK, dissolved in 10 mL EtOH, were allowed to react at ambient temperature for 6 days. Complex 1 was recovered from the reaction solution by precipitation with methanol (yield 90%). Spectroscopic characterization of complex 1: 1 H NMR (300 MHz, CDCl3, δ): 7.44 (d, Ar H), 7.30 (d, Ar H), 2.34 (s, CH3CO), 2.03 (m, PCH2), 1.53 (m, CH2),1.41 (m, CH2), 0.92 (t, CH3); 13C NMR (77 MHz, CDCl3, δ): 127.55, 126.23,130.92, 137.53 (Ar), 204.30 (CdO), 105.22 (PtCt), 98.21 (tCAr), 22.64 (PCH2), 24.35 (CH2), 26.22 (CH2), 13.72 (CH3), 35.07 (CH3COS); 31P NMR (121 MHz, CDCl3, δ): 4.58 (coupling constant JPt-P 2394 Hz); IR (film, cm1): 2117 (CtC), 1622 (CdO), 1260 (SCdO); 1622 (phenyl). UVvis (CH2Cl2): λmax = 347 nm; Mp 7274 C. 2.2.2. Synthesis of Gold Nanoparticles Stabilized with Complex 1 (AuNPs-1) and AM (AuNPs-2). Gold nanoparticles stabilized with complex 1 were prepared with Au/S molar ratios 0.70/1 and 0.25/1. The 0.70/1 synthesis is reported as a typical procedure: 48.8 mg (0.124 mmol) of HAuCl4 3 3H2O in 4.45 mL of deionized water was poured into a solution of complex 1 (68.6 mg, 0.040 mmol) in 10 mL of toluene. Then, 86.5 mg of tetraoctylammonium bromide in 10 mL of toluene was added together with 62.4 mg of NaBH4 dissolved in 4 mL of deionized water. The reaction mixture was allowed to react for 2 h at room temperature. Extraction with H2O/toluene followed, and the obtained brown solid was isolated by evaporation of the organic layer. The solid was resuspended in ethanol, was centrifuged five times at 5000 rpm with water, and was recovered with a total yield 32%. Gold nanoparticles stabilized with AM were prepared with the Au/S molar ratio 0.10/1, 0.25/1, 0.50/1, 0.70/1, 1.00/1; a typical procedure for the 0.10/1 is reported: 0.508 mmol of HAuCl4 3 3H2O aqueous solution (0.025 M) was added to a solution of TOAB (0.816 mmol) in 35 mL of toluene together with AM 5.08 mmol dissolved in 40 mL toluene (0.127 M). A 0.46 M aqueous solution of NaBH4 (258.3 mg in 14.7 mL of H2O) was added drop to drop, and the reaction mixture was allowed to react for 3 h at room temperature. Then, extraction with H2O/ toluene followed, and the obtained brown solid was isolated by evaporation of the organic layer. The solid was resuspended in ethanol, was washed several times, and was recovered from CHCl3 or hexane; the yield was 38%. 2.2.3. Preparation of SAMs of Complex 1 and AM on Gold Surface: SAM-1 and SAM-2. Preparation of self-assembled monolayers: terminal thiol compound trans,trans-[(HS) Pt(PBu3)2CtCC6H4C6H4CtCPt(PBu3)2(SH)] was in situ obtained by a deacylation procedure carried out from the precursor thiolate complex 1 and was allowed to selfassemble on gold surfaces, namely, SAM-1. In a typical procedure, complex 1 (30 mg, 0.019 mmol) was dissolved in 20 mL of tetrahydrofuran (THF), and 260 μL of NH4OH (30%) was added. Gold-coated silica wafers prepared by growing Au film 4000 Å thick onto Si(111) substrates were cut into slides (ca. 1 cm2) and were washed with several organic solvents, that is, acetone, ethanol chloroform, and were blown dry with nitrogen. The thiol solution was stirred at 30 C for 2 h and was filtered on Celite, and freshly washed gold substrates were dipped into the solution for 4 h to achieve the anchoring. The obtained multilayer was rinsed with different solvents (ethanol, THF, and acetone) in order to achieve the formation of an SAM-1 film in 15199
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the monolayer thickness regime. The same procedure was applied to obtain the SAM-2 sample from AM. The correlation between the signal intensity and the degree of coverage (or molecule density) at the surface allows the evaluation of the film thickness by X-ray photoelectron spectroscopy measurements; the measured attenuation of the substrate signals can be used to calculate the coverage thickness according to eq 1: I ¼ I 0 expð d=λÞ
ð1Þ
where I is the integrated intensity of the substrate Au4f7/2 signal in the presence of the overlayer, I0 is the intensity of the same signal for the clean surface, d is the overlayer thickness, and λ is the attenuation length for the Au4f photoelectron. The value of λ was estimated using the expression λ ¼ BðKEÞ1=2 where B = 0.087 nm (eV)1/2 for organic materials.20 The organometallic SAM thickness can be easily evaluated by highresolution (HR)-XPS measurements by following this procedure as discussed in detail in ref 15. At that time, a degree of coverage of no more than one monolayer on the gold substrate was ascertained for SAMs of trans,trans-[(CH3COS)Pd(PBu3)2 CtCC6H4C6H4CtCPd(PBu3)2(SCOCH3)] complexes anchored as dithiols on gold by comparing the obtained thickness value with the molecular lengths estimated by near-edge X-ray absorption fine structure (NEXAFS) and XRD data.15 The same calculation has been applied to SAM-1 obtaining an estimated film thickness value of 1.2 nm that is consistent with a degree of coverage of about one monolayer if we suppose that the Pt-containing molecule is oriented on the gold surface with the same geometry as the analogous Pd-containing dithiol (as suggested by NEXAFS data collected on thin films of the trans,trans-[ClPt(PBu3)2CtCC6H4C6H4CtC Pt(PBu3)2Cl] and trans,trans-[ClPd(PBu3)2CtCC6H4 C6H4CtCPd(PBu3)2Cl] precursors deposited onto gold substrates21). 2.3. Instruments and Methods. Fourier transform infrared (FTIR) spectra have been recorded as nujol mulls or as films deposited by casting from CHCl3 solutions using ZSM5 cells with a Bruker Vertex 70 spectrophotometer. UVvis spectra were run in CHCl3 with a Varian Cary 100 scan UVvisible spectrophotometer. The 1H and 13C NMR spectra were recorded on a Bruker AvanceII 300 spectrometer operating in the FT mode at 300.13 MHz (1H) and 75.5 MHz (13C), respectively. The 1H chemical shifts are referenced to the residual proton peaks of CDCl3 at 7.24 ppm versus Tetramethylsilane (TMS). The 13C resonances are referenced to the central peak of CDCl3 at 77.0 ppm. The high-resolution electron microscopy (HREM) observations and the diffraction contrast imaging were achieved with an FEI TECNAI G2 F30 Supertwin field-emission gun scanning transmission electron microscope (FEG STEM) operating at 300 kV and with a point-to-point resolution of 0.205 nm. The TEM specimens were prepared by depositing a few drops of the diluted solutions on carbon-coated TEM grids to be directly observed in the instrument. XRD measurements were carried out by using an X-ray diffractometer in parallel beam geometry (Philips MPD PW1880). For all the measurements, Cu K radiation (λCu KR = 0.154186 nm) was employed. The measurements were performed in glancing-incidence diffraction geometry, that is, by keeping the incident angle ωi (angle between incident beam and sample surface) fixed at 0.5
Figure 1. Bright-field TEM image that gives an overview of sample AuNPs-1 (Au/S 0.70/1) deposited on the amorphous carbon film (bright contrast in the image) of a TEM grid. Au nanoclusters (dark spots) are well visible. The chemical draw of AuNPs-1 is depicted in the inset.
while recording the scattered X-ray beam by moving the detector along the goniometer circle in the 2Θ range between 10 and 100. Synchrotron radiation induced X-ray photoelectron spectroscopy experiments were performed at ELETTRA storage ring using the BACH (beamline for advanced dichroism) beamline and relative experimental station, which is connected to an undulator front-end. Photoelectron spectroscopy was performed in the fixed analyzer transmission mode with the pass energy set to 50 eV. Photons of 380 eV energy have been used for C1s, S2p, P2p, Pt4f, and the Au4f spectral regions with the monochromator entrance and exit slits fixed at 30 μm. Calibration of the energy scale was made by referencing all the spectra to the gold Fermi edge, and the Au 4f7/2 signal was always found at 83.80 eV. The achieved resolving power was of 0.5 eV. Curve-fitting analysis of the C1s, P2p, Pt4f, S2p, and Au4f spectra was performed using Voigt curves as fitting functions. XPS measurements were carried out on AuNPs-1 and AuNPs-2 samples prepared with different Au/thiol ratios.
3. RESULTS AND DISCUSSION Gold nanoparticles were prepared with a modified two-phase procedure with different Au/S molar ratios leading to AuNPs-1 in analogy to previously reported studies.13,14 UVvis spectra, which show the plasmon absorption band at about 530 nm, supported the nanoparticle formation. The shape and structure of AuNPs-1 were investigated by TEM analysis, and Figure 1 shows a low-resolution bright-field (BF) TEM image of AuNP-1 sample with Au/S 0.70/1 together with the molecular structure of the hybrid system. The linkage between the nanoparticles can be envisaged with the formation of dyads, which is the first step for the achievement of more complex networks. Au nanoparticles of spherical shape and of an average size of about 4.5 ( 1.2 nm were obtained. The 15200
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Figure 2. HRTEM image of Au nanoparticles AuNPs-1 with molar ratio Au/S 0.70/1 having a round shape and an averaged size of (4.5 ( 1.2) nm. The insets A and B show two different single gold particles with well-defined lattice fringes. The particles are single crystal, and dyads can be envisaged (A, B). Lattice fringes are well visible. The dimension distribution is reported as inset C.
Figure 3. Bright-field TEM image of AuNPs-1 with Au/S ratio 0.25/1 having a round shape and an averaged size of (3.6 ( 0.8) nm. The dimension distribution is reported as inset.
TEM pictures evidence the existence of nanocrystals (Figure 2), and the insets A and B of Figure 2 show the gold particles linked to each other with well-pronounced lattice fringes. By studying the FTIR spectra (see Supporting Information), it can be observed that the CtC stretching mode is not detected supporting the formation of symmetric dyads. The smaller particles are single crystals, while larger particles are constituted by polycrystals (typically a larger particle is
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Figure 4. Bright-field TEM image of sample AuNPs-2 with molar ratio Au/S = 0.10/1. The dimension distribution reported as the inset shows an average size of (7.1 ( 1.3) nm.
formed by 2, 3, or 4 crystallites). In fact, grain boundaries between single crystallites of the polycrystalline particles are well observed. By varying the Au/S molar ratio in sample AuNPs-1 to 0.25/1, Au nanoparticles with a spherical shape and an average size of (3.6 ( 0.8) nm have been observed (see Figure 3) with a lower diameter by increasing the thiolic ligand content as expected. Compared with the Au nanoparticles obtained with Au/S 0.70/1 (average size (4.5 ( 1.2 nm)), these ones are slightly smaller with a narrower dimensions distribution. In the UVvis spectra of these samples (see Supporting Information), the plasmon resonance has been observed at about 533 nm in the sample with Au/S ratio 0.70/1 and 525 nm in the AuNPs-1 with Au/S ratio 0.25/1. The Brust procedure was also applied to obtain AM stabilized AuNPs. By varying the Au/S molar ratio, the mean diameter of the particles was tuned. In Figure 4, HR-TEM of AuNP-2 with molar ratio Au/S = 0.10/1 is reported. To investigate the crystallographic structure of the clusters more accurately, we performed X-ray powder diffraction experiments. The nanocrystalline particle size (average diameter Ø) was calculated from the full width at half-maximum (fwhm) of the Bragg peaks by using the Scherrer’s formula. In particular, the fwhm was obtained by fitting the (111), (200), and (220) Bragg peaks using pseudoVoigt functions. UVvis spectra (see Supporting Information) show an absorption feature because of the plasmon resonance, and the position of the maximum wavelength increases by increasing the mean diameter of the nanoparticles from about 520 to 530 nm. Figure 5 shows the experimental X-ray diffraction patterns (Figure 5a) of different AuNPs-2 samples: by increasing the Au/S ratio from 0.10:1 to 1:1, the mean diameter of the nanoparticles increases linearly from 1.6 to 3.9 nm (Figure 5b). Indeed, the Bragg peaks become narrower with increasing the Au/S ratio, that is, the particle size increases, and in addition, a more pronounced separation between (111) and (200) peaks occurs. 15201
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Table 1. XPS Data (BE, fwhm, Atomic Ratios) Collected on AuNPs-1, AuNPs-2, SAM-1, and SAM-2 sample
signal
SAM-1
83.80
0.89
1
161.96 163.37
1.72 1.72
56% 44%
Au4f7/2
83.80
1.05
66%
85.30
1.05
34%
S2p3/2
162.22
1.64
1
Au4f7/2
83.80
1.05
73%
85.08
1.05
28%
S2p3/2
162.28
1.66
1
SAM-2
Au4f7/2 S2p3/2
83.80 162.00
0.85 1.49
1 1
AuNP-2 (Au/S = 0.5/1)
Au4f7/2
83.80
0.80
88%
84.73
0.80
12%
S2p3/2
161.80
1.60
1
Au4f7/2
83.80
0.83
87%
84.79
0.83
13%
S2p3/2
162.35
1.27
1
AuNP-2 (Au/S = 0.1/1)
Au4f7/2
83.80 84.58
0.83 0.83
88% 12%
S2p3/2
162.43
1.60
1
AuNP-2 (Au/S = 1/1)
Au4f7/2
83.80
0.83
89%
84.79
0.83
11%
162.30
1.51
1
AuNP-2 (Au/S = 0.25/1)
S2p3/2 a
Iratiosa
Au4f7/2
AuNP-1 (Au/S = 0.25/1)
The particle size determined from X-ray diffraction measurements refers to the crystal domain size, that is, if the particles are polycrystals, XRD will provide information on the average single crystallite size only and not on the polycrystal size and will yield smaller particle size values. This fact explains the apparent discrepancy between TEM and XRD data on the particle size determination. The investigations on the morphology of AuNPs-1 and AuNPs-2 highlight that the presence of two terminal thiol groups in the stabilizing ligand leads to interparticle bonds and that the dimensions of the nanoparticles with a ligand with one thiol functionality can be finely tuned by the Au/S molar ratio. 3.1. SR-Induced X-ray Photoemission Spectroscopy Investigation. SR-XPS measurements of C1s, P2p, Cl2p, S2p, Pt4f, and Au4f core levels were carried out for all the samples considered here. XPS data (BE, fwhm, atomic ratios), collected on AuNPs-1 and AuNPs-2 nanoparticles with different Au/S ratios, are reported in Table 1. Data collected on SAMs of the same molecules anchored on gold (SAM-1 and SAM-2) are also reported for comparison considering that analogous organometallic SAMs based on Pd-containing thiols were extensively discussed in a previous work and that the results concerning mainly the SR-XPS S2p signal analysis will be used as reference for the interpretation of the data discussed here.15 The analysis of the Au4f and S2p signals sheds some light on the chemical interaction arising between SH terminal groups and gold surface atoms either on the flat gold substrates or on the nanoparticles. For AuNPs-1 and SAM-1 samples, Pt4f, C1s, S2p, and P2p signals were also collected and analyzed evidencing that all the organometallic molecules are surface-associated and that PtS bond cleavage is not occurring (see the extended Table 2 in the
fwhm (eV)
S2p3/2 AuNP-1 (Au/S = 0.7/1)
Figure 5. (a) X-ray diffraction patterns of AuNPs-2 samples at increasing Au/S ratios from 0.10:1 to 1:1. (b) Mean diameter Ø of AuNPs-2 samples at increasing Au/S ratios.
BE (eV)
I ratios = Ipeak/Itotal signal for a selected element.
Figure 6. Au4f spectra of (a) SAM-1 anchored on gold; (b) AuNP-1 with Au/S ratio 0.25/1; (c) AuNP-1 with Au/S ratio 0.70/1.
Supporting Information). Au4f spectra of AuNPs-1 samples with Au/S molar ratio 0.25/1and 0.70/1 are, respectively, reported in Figure 6 b and c; Au4f spectrum of SAM-1 is reported for comparison in Figure 6 a. For SAM-1, a single pair of spinorbit components is detected with the main Au4f7/2 component at 83.80 eV BE as expected for metallic gold atoms. In the case of AuNPs-1 nanoparticles, XPS Au4f spectra show a broadening and an asymmetry at higher binding energy that, on the basis of a simple charge potential model which neglects changes in the final state, has been attributed to a component associated with Au centers showing 15202
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Figure 7. XPS Au4f spectra collected on (a) SAM-2 and (b) AuNP-2 (Au/S = 0.10/1).
an increase in the electron density. Considering the data analysis of the Au4f spectrum, the final state effects could be also responsible for the spectral broadening after Au-SH moiety interaction, but these might be accounted for only by appropriate calculations. However, as will be shown in the following discussion about XPS data analysis, the conclusion that Au(0) signal, after gold linkage to thiolic end groups, undergoes chemical shift toward higher binding energy is supported by literature reports for AuS bonds formed upon thiol anchorage.22 The Au4f spectral broadening suggests the presence of chemically different Au sites. Curve fitting of Au4f spectra was performed by using four peaks generated by the two spinorbit components 4f7/2 and 4f5/2 separated by a spinorbit coupling constant equal to 3.7 eV (as reported for metallic gold in the literature) and with the expected area ratios 4f7/2:4f5/2 = 4:3. By means of the curvefitting analysis, two spinorbit pairs can be observed as associated to two Au atoms involved in different chemical environments; the first Au4f7/2 component is found at 83.80 eV for all samples, and the latter one at about 85.0 eV is close to the BE value reported for Au(I) compounds.23 This signal was assigned to Au atoms bonding thiol terminal groups. The signal at lower BE, taken from the Voigt profiles used for peak fitting, occurs at the same energy value as the Au4f7/2 spinorbit components observed for the SAM-1 sample, that is, metallic gold (the signal associated to surface gold atoms binding SAM end groups is not detectable in SAM-1 sample because of the very intense signal arising by the bulk metallic gold atoms). On the basis of the above-reported discussion, we assign the Au signal at 83.80 eV to the unperturbed bulk gold atoms in the nanoparticles that are not involved in the interaction with thiol end groups. Conversely, the Au4f7/2 spectral component that occurs at a higher BE value (nearly 85.0 eV) can be associated with Au metal sites coordinating the electron acceptor thiol ligands. In the case of AM-based systems, nanoparticles AuNPs-2 (four samples of different Au/AM molar ratios) and SAM-2 were also investigated by XPS, and the collected spectra were compared to the measurements performed on the more complicated AuNP-1 and SAM-1 materials. In Table 1, the data collected on AuNPs-2 and SAM-2 are reported. Au4f spectra are reported in Figure 7 for a selected AuNP-2 sample (Au/S = 0.10/1) taken as an example of the different Au/AM molar ratios as well as for SAM-2. A strong AuS interaction is assessed also for AuNPs-2 similarly to what already was discussed for AuNPs-1. In fact, Au4f spectra of
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Figure 8. XPS S2p spectra of (a) AuNP-2 (Au/S 0.1/1) and (b) of AuNP-1 (Au/S 0.25/1).
Figure 9. Schematic presentation of AuNPs-1, AuNPs-2, SAM-1, and SAM-2.
AM-based systems show complex signals, and two pairs of spin orbit components were individuated by peak fitting. As extensively discussed for AuNPs-1 samples, the first Au4f7/2 component is due to metallic gold atoms of NPs bulk, while the Au4f7/2 peak at higher BE values (about 85.0 eV) was assigned to gold atoms bonded to sulfur atoms of the thiol ending groups. In Figure 7, the similarity between the Au4f7/2 component at lower BE values (83.80 eV) of AuNP-2 and the signal associated to metallic gold of the corresponding SAM-2 sample is evidenced. As for XPS S2p core levels, only one spinorbit pair was observed in all AuNPs-1 and AuNPs-2 spectra; the observed signal was assigned to sulfur atoms bonded to Au; no signal arising from unbound SH was detected. In Figure 8, S2p spectra collected on AuNP-1 (with Au/S molar ratio 0.25/1) together with AuNP-2 (Au/S = 0.10/1) are displayed. The S2p3/2 component, as measured for all the AuNPs-2 hybrid samples, occurs at nearly 162.2 eV (Figure 8a), a BE value that is significantly different from the one expected for the unbound thiols and that is consistent with a model in which the sulfur species are bonded to the surface of gold nanoparticles largely as RS-Au.24,25 The S2p spectrum of AuNPs-2 shows no evidence of S-acetyl or oxidized sulfur, which normally appear at 163.5 and 167 eV, respectively.25 The investigation performed on the simple AM-based hybrid systems provided a useful reference for the study of the more complicated organometallic AuNPs-1. 15203
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The Journal of Physical Chemistry C S2p spectrum of the AuNP-1 sample is reported in Figure 8 b; as for the previously discussed AuNP-2, a single pair of spinorbit components is observed with the S2p3/2 component at nearly 162.2 eV and is attributed to sulfur atoms bonded to gold surface atoms consistently with the already discussed organometallic thiol SAMs. As reported in Table 1, two S2p spinorbit doublets are observed for the SAM-1 S2p spectrum with the 2p3/2 components occurring at 162.0 and 163.4 eV, respectively; the low-energy S2p3/2 peak is ascribed to sulfur bonded to gold subsequent to the anchoring of the thiol on the metal surface while the high-energy S2p3/2 signal is assigned to sulfur in the free thiol terminal group. Since the relative contributions of the different sulfur-containing species can be estimated from the ratio between the respective signal intensities (peak areas), the calculated intensity ratios reported in Table 1 indicate that about 50% of the S-terminal group in SAM-1 is covalently bonded to the gold surface and, therefore, only one of the two thiol-ending groups appears involved in the interaction. Conversely, only one S2p spinorbit doublet can be observed in AuNPs-1 spectra with an S2p3/2 BE value of about 162.2 eV thus suggesting that the organometallic thiol grafts vicinal nanoparticles with both terminal groups in dyads in agreement with TEM observation. In Figure 9, a scheme of AuNP-1 and AuNP-2 is depicted together with SAM-1 and SAM-2.
’ CONCLUSIONS The synthesis and characterization of gold nanoparticles stabilized by a novel bifunctional thiolate organometallic complex containing Pt(II) centers (AuNPs-1) and allylmercaptane (AuNPs-2) with different thiol/Au ratios is reported. Selfassembled monolayers (SAMs) of the organometallic complex and allylmercaptane onto gold surfaces were prepared and investigated as a reference. The size control of the Au nanoparticles was achieved by direct control of synthesis parameters, and the morphology of the nanoparticles was characterized by means of high-resolution transmission electron microscopy (HRTEM). AuNPs-1 with an average diameter 4.5 ( 1.5 nm were obtained, and linkage between the nanoparticles can be envisaged with the formation of dyads. In the case of AuNPs-2, nanoparticles with diameters in the range from 1.6 to 3.9 nm were obtained. Synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS) measurements allowed the assessment of the covalent attachment of the sulfur atoms as thiolates to the surface of gold. S2p spectra of AuNPs-1 showed a single signal suggesting that the organometallic thiol can graft vicinal nanoparticles with both terminal groups in dyads supporting TEM observations. The new hybrids show a direct link between Pt(II) and Au nanoparticles through a single S bridge and are candidates for the achievement of 2D or 3D networks. ’ ASSOCIATED CONTENT
bS
Supporting Information. Table 2. Pt4f, C1s, S2p, and P2p XPS data (BE, fwhm, atomic ratios) collected on AuNPs-1 and SAMs-1. UVvis and FTIR spectra of selected samples are also reported. This material is available free of charge via the Internet at http://pubs.acs.org.
ARTICLE
’ ACKNOWLEDGMENT The authors acknowledge for the financial support of this research MAE-MIUR Progetti di Ricerca Scientifica e Tecnologica Bilaterale 2008-2010 and Ateneo Federato AST 2008 (26F09MA27). ’ REFERENCES (1) Ozin, G. A.; Arsenault, A. C.; Cademartori, L. Nanochemistry, 2nd ed.; RSC Publishing: Cambridge, 2009. (2) Das, I.; Ansari, S. A. J. Sci. Ind. Res. 2009, 68, 657–667. (3) Mulvaney, P. Langmuir 1996, 12, 788–800. (4) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Wachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17–30. (5) Murray, R. W. Chem. Rev. 2008, 108, 2688–2720. ZabetKhosousi, A.; Dhirani, A. Chem. Rev. 2008, 108, 4072–4124. (6) Hutchings, G. J.; Brust, M.; Schmidbaur, H. Chem. Soc. Rev. 2008, 37, 1759–1765. (7) Torres Sanchez, R. M.; Ueda, A.; Tanaka, K.; Haruta, M. J. J. Catal. 1997, 168, 125–127. (8) Thomas, K. G.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888–898. (9) Barash, O.; Peled, N.; Hirsch, F. R.; Haick, H. Small 2009, 5, 2618–2624. (10) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (11) Brust, M.; Walker, M; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801–802. (12) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430–433. (13) Vitale, F.; Vitaliano, R.; Battocchio, C.; Fratoddi, I.; Giannini, C.; Piscopiello, E.; Guagliardi, A.; Cervellino, A.; Polzonetti, G.; Russo, M. V.; Tapfer, L. Nanoscale Res. Lett. 2008, 3, 461–467. (14) Vitale, F.; Vitaliano, R.; Battocchio, C.; Fratoddi, I.; Piscopiello, E.; Tapfer, L.; Russo, M. V. J. Organomet. Chem. 2008, 693, 1043–1048. (15) Vitaliano, R.; Fratoddi, I.; Venditti, I.; Roviello, G.; Battocchio, C.; Polzonetti, G.; Russo, M. V. J. Phys. Chem. A 2009, 113, 14730– 14740. (16) Battocchio, C.; Fratoddi, I.; Venditti, I.; Yarzhemsky, V. G.; Norov, Y. V.; Russo, M. V.; Polzonetti, G. Chem. Phys. 2011, 379, 92–98. (17) Kauffman, G. B.; Teter, L. A. Inorg. Synth. 1963, 7, 248–252. (18) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980, 627–630. (19) Battocchio, C.; D’Acapito, F.; Fratoddi, I.; La Groia, A.; Polzonetti, G.; Roviello, G.; Russo, M. V. Chem. Phys. 2006, 328, 269– 274. (20) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1 (1), 2–11. (21) Battocchio, C.; Fratoddi, I.; Russo, M. V.; Carravetta, V.; Monti, S.; Iucci, G.; Borgatti, F.; Polzonetti, G. Surf. Sci. 2007, 601, 3943–3947. (22) Gentilini, C.; Evangelista, F.; Rudolf, P.; Franchi, P.; Lucarini, M.; Pasquato, L. J. Am. Chem. Soc. 2008, 130, 15678–15682. (23) Tang, Z.; Xu, B.; Wu, B.; Germann, M. W.; Wang, G. J. Am. Chem. Soc. 2010, 132, 3367–3374. (24) Bourg, M.-C.; Badia, A.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 6562–6567. (25) Zhang, S.; Leem, G.; Lee, T. R. Langmuir 2009, 25, 13855– 13860.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: battocchio@fis.uniroma3.it; phone: 0039 06 57333388; fax: 0039 06 57333390. 15204
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