Synthesis and characterization of gold nanoparticles stabilized by palladium(II) phosphine thiol

Article abstract of DOI:10.1016/j.jorganchem.2007.12.024

This work is focused on the synthesis of innovative hybrids made by linking gold nanoparticles to protected organometallic Pd(II) thiolate. The organometallic protected Pd(II) thiolate, i.e. trans-thioacetate-ethynylphenyl-bis(tributylphosphine)palladium(II) has been synthesized, in situ deprotected and linked to Au nanoparticles. In this way new hybrid, with a direct link between Pd(II) and Au nanoparticles through a single S bridge, has been isolated. The combination of the organometallic Pd(II) thiol with gold nanoparticles allows the enhancement and tailoring of electronic and optical properties of the new organic-inorganic nano-compound. Single-crystal gold nanoparticles, uniform in shape and size were obtained by applying a modified two-phase method (improved Brust-Schiffrin reaction). In addition, the chemical environment of the Au nanoparticles was investigated and a covalent bonding between Au nanoparticles and the organometallic thiols was observed.

Full text of DOI:10.1016/j.jorganchem.2007.12.024

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 1043–1048
www.elsevier.com/locate/jorganchem
Synthesis and characterization of gold nanoparticles stabilized
by palladium(II) phosphine thiol
a,c
a
b
a,
*
Floriana Vitale , Rosa Vitaliano , Chiara Battocchio , Ilaria Fratoddi ,
Emanuela Piscopiello c, Leander Tapfer c, Maria Vittoria Russo a
a Department of Chemistry, University of Roma ‘‘Sapienza” P.le A. Moro, 5-00185 Roma, Italy
b Department of Physics, INFM, INSTM and CISDiC Unit, University ‘‘Roma Tre”, Via della Vasca Navale, 84-00146 Rome, Italy
c ENEA-Brindisi Research Centre, Department of Physics and Technology and New Materials (FIM), S.S. 7 Appia km.713, 72100 Brindisi, Italy
Received 31 October 2007; received in revised form 17 December 2007; accepted 17 December 2007
Available online 28 December 2007
Abstract
This work is focused on the synthesis of innovative hybrids made by linking gold nanoparticles to protected organometallic Pd(II)
thiolate. The organometallic protected Pd(II) thiolate, i.e. trans-thioacetate-ethynylphenyl-bis(tributylphosphine)palladium(II) has been
synthesized, in situ deprotected and linked to Au nanoparticles. In this way new hybrid, with a direct link between Pd(II) and Au nano-
particles through a single S bridge, has been isolated. The combination of the organometallic Pd(II) thiol with gold nanoparticles allows
the enhancement and tailoring of electronic and optical properties of the new organic–inorganic nano-compound. Single-crystal gold
nanoparticles, uniform in shape and size were obtained by applying a modified two-phase method (improved Brust–Schiffrin reaction).
In addition, the chemical environment of the Au nanoparticles was investigated and a covalent bonding between Au nanoparticles and
the organometallic thiols was observed.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Gold nanoparticles; Organometallic thiolates; Pd(II) complexes
1. Introduction
chemically, photophysically and biologically active units
[4]. The tailoring of electronic properties of Au nanoparti-
cles can be obtained by tuning the nature of the thiol group
and the size [5]. Size-control can be achieved by choosing
the suitable Au/S molar ratio [6].
Gold nanoparticles systems stabilized by organothiols
are very interesting for their unique properties and their
usefulness in fabrication of different molecular architec-
tures and nanocomposite materials [1] that have applicative
potentialities in the different fields of optoelectronics, catal-
ysis, chemical sensors and biosensors [2]. The first synthesis
of organothiol monolayer–gold nanoclusters hybrid sys-
tems was carried out by Brust to overcome the instability
of native colloidal gold solutions [3] opening a new field
of self-assembled monolayers. Gold nanoclusters are usu-
ally stabilized by organothiols [2]. The covalently bound
organothiols monolayers improve solubility and stability
and allow them to be functionalized by different electro-
When a capping thiol, covalently bound to the Au nano-
particles, has an additional terminal group, it is possible to
assemble a second functionalized molecule to obtain bi- or
tri-dimensional networks i.e. scaffolds for the development
of nanodevices [7]. Only a few papers deal with aromatic
arenethiols as capping agents for gold nanoclusters [8,9]
which give rise to structural rigidity and compactness to
thiol–gold nanoparticles system [10]. Among metal thio-
carboxylates, palladium(II) based complexes have been
recently prepared [11] with the aim of exploiting their
catalytic properties. In this communication, we propose
the innovative functionalization of gold nanoparticles
upon the in situ deprotection of a Pd(II) thiolate (1,
*
Corresponding author. Tel.: +39 06 49913182; fax +39 06 490324.
E-mail address: ilaria.fratoddi@uniroma1.it (I. Fratoddi).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.12.024

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F. Vitale et al. / Journal of Organometallic Chemistry 693 (2008) 1043–1048
[(CH3COS)Pd(PBu3)2(C„CC6H5)]). The proposed new
Au–Pd organometallic hybrid 2, is expected to show inter-
esting optical properties, considering the high luminescence
quantum yield recently reported for a Pd(II) containing
homologue polymer [12]. In this way it will be possible to
extend the functionalization from model molecules to thi-
ols with extended conjugation of the organic moiety that
is a characteristic of polymetallaynes [13].
Gold nanoparticles can be functionalized with complex
1. In this paper, we report on the synthesis of innovative
organometallic stabilized gold nanoparticles. The gold
nanoparticles were prepared by using a modified Brust’s
two-phase procedure [3]. The synthetic pathway was opti-
mized and a 4.6:1 Au/S molar ratio was adopted for the
preparation of hybrid 2. Briefly, HAuCl4 was transferred
from water into dichloromethane by tetraoctylammonium
bromide (TOAB), a quaternary alkyl-ammonium phase-
transfer agent, which also acted as a de-protecting agent
of the thioacetate Pd(II) complex 1. After addition of
BH4À, the hybrid system 2, was obtained and character-
ized. Infrared spectra of hybrid 2, confirmed the deprotec-
tion of the thiol with the disappearance of the carbonyl
C@O stretching mode.
UV–Vis spectra, collected for complex 1 and hybrid 2,
supported the hybrid formation. In particular, complex 1
showed an absorption band at 280 nm, while hybrid 2
exhibited a shielded plasmon resonance at about 510 nm.
Preliminary photoluminescence measurements showed an
emission band at 337 nm for both complex 1 and hybrid
2. This result proves the possibility for gold nanoclusters
to develop luminescence properties also in UV–Vis range.
The size, shape and crystalline structure of the function-
alized gold nanoparticles are fundamental features for the
future developments in technological applications. X-ray
powder diffraction measurements (XRD) and high-resolu-
tion transmission electron microscopy (HR-TEM) allowed
to characterize hybrid 2.
2. Results and discussion
Our approach was to synthesize an organometallic thio-
late complex which is able to make a direct bond between
Pd(II) and Au nanoparticles through a simple single
S-bridge. The purposes of our work were to give more
structural rigidity and compactness to gold nanoclusters,
due to the aryl moiety. In this way it is possible to explore
the advanced perspectives for a novel organometallic
thiol–gold nanocluster hybrid system and to envisage the
extension to the use of rigid rod thiol derivatized
polymetallaynes.
The complex 1, reported in Scheme 1, was synthesized
on purpose, by ligand exchange reaction between the
square planar monochloride monoacetylide complex
[ClPd(PBu3)2(C„CC6H5)] and KSCOCH3.
The simple reaction conditions and chemical stability
of thiolate organometallic complexes open a new access
to the preparation of Pd(II) containing thiolate. Infrared
spectra of complex 1 confirmed the proposed structure.
The C„C stretching mode was found at 2113 cmÀ1 and
the protecting acetate showed the carbonyl C@O stretch-
ing mode at 1627 cmÀ1. This compound was used for the
preparation of a new gold nanocluster hybrid 2 by a sin-
gle step procedure, in which the deprotection and the
anchoring of the thiol group was achieved by covalent
bonding.
The X-ray diffraction pattern of hybrid 2 shown in
Fig. 1a, exhibits the characteristic Bragg peaks of the gold
nanocrystals. The broadening of the Bragg peaks is due to
the nanometer-size of the crystallites. In Fig. 1b, the simu-
lation of the (111) and (200) peaks by a pseudo-Voigt
function is reported. By using Scherrer’s formula, from
the full width of half-maxima (FWHM) of the most intense
PBu3
Pd
PBu3
S
PBu3
S
O
PBu3
HAuCl4·H2O
S
H3C
S
Pd
Pd
S
Au
S
PBu3
Pd
NaBH
4 , 3h
CH2Cl2/TOAB
PBu3
S
PBu3
S
1
PBu3
2
PBu3
Pd
PBu3
Scheme 1. Chemical structure of Pd(II) organometallic complex, 1, and synthesis of Pd(II) complex thiol–Au nanoparticle hybrid system, 2.

F. Vitale et al. / Journal of Organometallic Chemistry 693 (2008) 1043–1048
1045
Fig. 1. (a) X-ray diffraction pattern of the hybrid 2 exhibiting distinct
Bragg peaks due to the Au nanocrystallites and (b) simulation of the (111)
and (200) peaks by a pseudo-Voigt function. Average Au particle size
(£ ꢀ 3.5 nm).
(111) Bragg peak the average particle diameter can be
determined, which is 3.5 nm.
In Fig. 2a and b, TEM images of sample 2, taken at dif-
ferent magnification are reported. TEM observations show
spherical shaped single-crystal gold particles and confirms
the average diameter of the crystallites determined from
XRD.
Fig. 2. Bright field TEM images of AuNPs of hybrid 2, taken at different
magnifications (a,b). The inset of (b) shows a high-resolution image of a
single Au nanocrystal with well-defined (200) lattice fringes. Mean AuNPs
size 3.2 nm (c).
Fig. 2a gives an overview of the sample’s features. Gold
nanocrystals are distributed in zones, forming high and low
density regions, and in some areas they may also aggregate
in chains. From high-resolution TEM analysis (Fig. 2b),
we can discriminate among elongated and spherical nano-
particles. Elongated nanoparticles are mainly due to a coa-
lescence of two or more spherical nanoparticles which
consist of single crystals. The inset of Fig. 2b shows a
high-magnification image of one single gold nanocrystals.
The spacing of the fringes is d = 0.2 nm and corresponds
to the (200) Au lattice planes. Focusing on spherical nano-
particles, we obtained the size distribution reported in
Fig. 2c, from which we can state that gold nanocrystals
in this sample are uniform in size. The average size is of
about 3–4 nm.
The chemical environment of the functionalized Au
nanoparticles and their interaction with the organothiols
is at present under investigation by X-ray photoelectron
spectroscopy (XPS) [14]. XPS measurements were carried
out on sample 2 with the aim of finding a relationship
between the Au(I)/S atomic ratio in the nanostructured
assembly, which is indicative of the covalent Au–S link.
To this purpose C1s, P2p, Pd4f, Au4f and S2p core level
spectra have been collected and analyzed. The core level
binding energy (BE) and full width at half-maximum
(FWHM) were analyzed with particular attention to

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F. Vitale et al. / Journal of Organometallic Chemistry 693 (2008) 1043–1048
4. Experimental details
4.1. Material and methods
The reactions of complex 1 were performed under an
inert argon atmosphere at room temperature (RT).
Solvents were dried on Na2SO4 before use. Low conduc-
tivity water was obtained from Millipore Milli-Q water
purification system. All chemicals, unless otherwise sta-
ted, were obtained from commercial sources and used
as received. Palladium complex [PdCl2(PBu3)2], i.e.
trans-[dichlorobis(tributylphosphine)palladium(II)]
was
prepared by the following reported methods [16]. Phenyl-
acetylene were purchased from Aldrich and distilled
before use. Potassium thioacetate was purchased from
Aldrich and used without further purifications. Prepara-
tive 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.
Fig. 3. XPS Au4f spectrum of hybrid 2.
Au4f7/2 and S2p3/2 components, which are of main interest
for the assessment of the Au–S bond. Evaluation of the
atomic ratios of all the core spectra normalized to the
S2p3/2 component led to assess that the molecular structure
of complex 1 was clearly maintained in hybrid 2.
4.2. Synthesis of Pd(II) organometallic complex 1
By curve-fitting analysis, the Au4f spectrum, reported in
Fig. 3, results from two pairs of spin–orbit components.
The Au4f7/2 peak found at BE = 83.80 eV is attributed to
metallic gold [15]; the second Au4f7/2 signal at higher BE
values (BE = 84.42 eV) has been associated to Au atoms
that are covalently bound to sulphur containing terminal
groups. Semi-quantitative analysis of the XPS signals,
allowed to estimate an atomic ratio 1:1 between the
Au4f7/2 component at 84.42 eV and the S2p3/2 peak. This
result is consistent with the formation of the hypothesized
S–Au covalent bond, while some gold sites onto the surface
of the nanoparticles seem not to be interacting with the thi-
ols; this experimental result could be explained with the
hindrance of the phosphine ligands linked to Pd(II) square
planar complex.
The organometallic complex [CH3–CO–S–Pd(PBu3)2-
C„CC6H5] was prepared from square planar Pd(II)
complex [Cl–Pd(PBu3)2C„CC6H5] synthesized by the lit-
erature report [17], following a ligand substitution reaction,
in the presence of potassium thioacetate in equimolar
amount [11]. For a typical reaction, 1.5 mmol of [Cl–
Pd(PBu3)2C„CC6H5] were dissolved in CH2Cl2 (50 ml)
and 1.8 mmol of CH3–CO–SK were allowed to react at
room temperature for 6 days.
The obtained solid was filtered and the spectroscopic
characterizations follows: CH3–CO–S–Pd(PBu3)2C„C-
C6H5 (1): IR (film, cmÀ1) m(C„C): 2113; m (C@O): 1627;
1
m (C@C): 1602; UV (CH2Cl2, nm): 272.0 nm; H NMR
(CDCl3ppm): 7.28 (d, 4H, Ar-H), 7.22 (d, 4H, Ar-H),
2.40 (s, CO–CH3), 1.98 (m, 12H P–CH2–), 1.59 (m, 12H,
P–CH2–CH2–), 1.50 (q, 12H, P–CH2–CH2–CH2–), 0.96
(t, 18H, –CH3); 31P NMR (CDCl3ppm): 10.40; Elemental
analysis (%), found (calculated for C34H62P2PdSO):
C = 59.03 (59.42); H = 9.12 (9.09); S = 5.07 (4.67).
3. Conclusions
In conclusion, we synthesized a new organometallic
Pd(II) complex and the corresponding gold nanoparticle
hybrid. XRD, TEM and XPS analyses confirmed the link-
ing between Au and –S–Pd(PBu3)2(C„CC6H5) moiety.
The nanoparticles are homogeneous in size and structure
and are uniformly functionalized by the organometallic
complex.
The obtained hybrid could be a promising candidate
for the achievement of new hybrid systems with extended
electronic delocalization by varying the organic spacer
bonded to Pd(II) centers. In fact, optical spectroscopy
and electronic transport measurements are now under
study in our laboratories in order to evaluate also poten-
tial device applications. These new materials open new
perspectives in the field of thiols stabilized gold
nanoparticles.
4.3. Synthesis of Pd(II) organometallic complex-stabilized
gold nanoparticles 2
The stabilized gold nanoparticles were prepared by
two-phase procedure [10] using a 4.6:1 Au/thiol reactant
molar ratio. A 0.03 M aqueous solution of HAuCl4 Á H2O
(1.12 mmol) was added to a solution of [CH3–CO–
S–Pd(PBu3)2C„CC6H5] (0.243 mmol) in 80 ml of diclo-
romethane (DCM).
A
quaternary alkyl-ammonium
phase-transfer, 1.6 g of tetraoctylammonium bromide
(TOAB), was added to transfer HAuCl4 from water into
organic phase. The gold was reduced with a 0.4 M aque-
ous solution of NaBH4 (30.8 ml) added over a period of

F. Vitale et al. / Journal of Organometallic Chemistry 693 (2008) 1043–1048
1047
1 min. The system reacted at room temperature for 3 h.
Then 50 ml of H2O and 50 ml of CH2Cl2 were added
and the organic phase was separated and dried over
anhydrous Na2SO4. The drying agent and any insoluble
materials were removed by filtration and the solution
was reduced to a solid, in vacuo. The dark brown residue
was resuspended in methanol. The suspension was fil-
tered over celite and subsequently washed with 400 ml
of acetonitrile and 400 ml of hexane to remove any
excess of arenethiolate, TOAB and by-products. The
solid was washed off from the celite with DCM, the sol-
vent was removed in vacuo and the product was dried in
vacuo overnight. The product was highly soluble in
CH2Cl2 and CHCl3.
curve-fitting program for PC. Quantitative evaluation of
the atomic ratios was obtained by analysis of the XPS sig-
nal intensity, employing Scofield’s atomic cross-section
values [19] and experimentally determined sensitivity
factors.
The XRD measurements were performed by using a
high-resolution X-ray diffractometer (HRD3000 Ital Struc-
tures) in parallel beam optical configuration (Max-FluxTM
Optical System). A Copper-target (kCu Ka = 0.154056 nm),
with a X-ray generator setting of 30 mA/40 kV, was
employed as X-ray source (fine focus X-ray tube).
For the structural characterization of the nanoparticles
few drops of the diluted solutions were deposited on pol-
ished (100)-Si surfaces and dried in air. The X-ray diffrac-
tion patterns were recorded in glancing incidence
conditions, i.e. the angle between the incident X-ray beam
and the ‘‘nanoparticle film” surface was kept constant at
x = 1.0°, while the X-ray intensity was measured by 2h-
scans of the detector-system. The average crystallite size
was estimated from the diffraction peak broadening,
pseudo-Voigt function fitting and applying the Scherrer’s
formula.
The samples were studied by HRTEM and diffraction
contrast imaging. All images were recorded with a 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. TEM specimens were prepared by depositing
few drops of the diluted solutions on carbon coated
TEM grids to be directly observed.
4.4. Instrumentation
FTIR spectra were recorded as nujol mulls or as films
deposited from CHCl3 solutions by using CsI cells, on a
Bruker Vertex70 Fourier Transform spectrometer.
1H, 31P NMR spectra were recorded on a Bruker AC
300P spectrometer at 300 and 121 MHz respectively, in
appropriate solvents (CDCl3); the chemical shifts (ppm)
were referenced to TMS for 1H NMR assigning the
residual 1H impurity signal in the solvent at 7.24 ppm
(CDCl3). 31P NMR chemical shifts are relative to H3PO4
(85%). UV–Vis spectra were recorded on a Varian Cary
100 instrument. UV measurements were performed at
room temperature using quantitative solutions of the com-
pounds in CH2Cl2. UV–Vis spectra were recorded on a
Cary 100 Varian instrument. Photoluminescence spectra
were performed on a Perkin–Elmer LS 50 Fluorescence
Spectrometer. All measurements were performed at room
temperature using quantitative solutions in CHCl3 (1 mg/
ml).
Acknowledgements
The authors gratefully acknowledge the financial sup-
port of MIUR (Rome, Italy) and of Regione Puglia (Bari,
Italy) to this research work.
XPS spectra were obtained using a custom designed
spectrometer. A non-monochromatised Mg Ka X-rays
source (1253.6 eV) was used and the pressure in the instru-
ment was maintained at 1 Â 10À9 Torr throughout the
analysis. The experimental apparatus consists of an anal-
ysis chamber and a preparation chamber separated by a
gate valve. An electrostatic hemispherical analyzer (radius
150 mm) operating in the fixed analyzer transmission
(FAT) mode and a 16-channel detector were used. The
film samples were prepared by dissolving our materials
in CHCl3 and spinning the solutions onto polished stain-
less steel substrates. The samples showed good stability
during the XPS analysis, preserving the same spectral fea-
tures and chemical composition. The experimental energy
resolution was 1 eV on the Au 4f7/2 component. The
resolving power DE/E was 0.01. Binding energies (BE)
were corrected by adjusting the position of the C1s peak
to 285.0 eV in those samples containing mainly aliphatic
carbons and to 284.7 eV in those containing more aro-
matic carbon atoms, in agreement with literature data
[18]. The C1s, Pd3d, Pt4f, P2p, Cl2p spectra were decon-
voluted into their individual peaks using the Peak Fit
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