Network assembly of gold nanoparticles linked through fluorenyl dithiol bridges

Article abstract of DOI:10.1039/c3tc32567a

Gold nanoparticles stabilized by two novel bifunctional fluorenyl thiols, generated in situ from 9,9-didodecyl-2,7-bis(acetylthio)fluorene (1) and 9,9-didodecyl-2,7-bis(acetylthiophenylethynyl)fluorene (2), exhibit bridged structures which self-assemble in parallel lines. The size, shape and structure of the AuNPs have been determined by means of dynamic light scattering (DLS), scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). AuNPs modified with fluorenyl thiol derivatives show diameters in the range of 3-7 nm. The linkage between the nanoparticles can be envisaged with the formation of dyads supported by TEM analysis and XPS measurements. Remarkably, investigation by scanning electron microscopy of the AuNP films revealed an ordered distribution of well-separated individual nanoparticles to form a 2D network. The formation of interconnected networks between AuNPs with different distances, depending on the nature of the thiol linkers (1) or (2), and the photoluminescence properties open perspectives for applications in optical devices and electronics.

Full text of DOI:10.1039/c3tc32567a

Journal of
Materials Chemistry C
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PAPER
Network assembly of gold nanoparticles linked
through fluorenyl dithiol bridges†
Cite this: J. Mater. Chem. C, 2014, 2,
2517
Maurizio Quintiliani,a Mauro Bassetti,b Chiara Pasquini,b Chiara Battocchio,c
Marco Rossi,de Francesco Mura,d Roberto Matassa,e Laura Fontana,a
a
ad
*
Maria Vittoria Russo and Ilaria Fratoddi
Gold nanoparticles stabilized by two novel bifunctional fluorenyl thiols, generated in situ from 9,9-
didodecyl-2,7-bis(acetylthio)fluorene (1) and 9,9-didodecyl-2,7-bis(acetylthiophenylethynyl)fluorene (2),
exhibit bridged structures which self-assemble in parallel lines. The size, shape and structure of the
AuNPs have been determined by means of dynamic light scattering (DLS), scanning electron microscopy
(FE-SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). AuNPs
modified with fluorenyl thiol derivatives show diameters in the range of 3–7 nm. The linkage between
the nanoparticles can be envisaged with the formation of dyads supported by TEM analysis and XPS
measurements. Remarkably, investigation by scanning electron microscopy of the AuNP films revealed
an ordered distribution of well-separated individual nanoparticles to form a 2D network. The formation
of interconnected networks between AuNPs with different distances, depending on the nature of the
thiol linkers (1) or (2), and the photoluminescence properties open perspectives for applications in optical
devices and electronics.
Received 26th December 2013
Accepted 15th January 2014
DOI: 10.1039/c3tc32567a
www.rsc.org/MaterialsC
properties of AuNPs and to improve their stability. In this
context, the research in our group has been recently centered in
Introduction
the stabilization of gold nanoparticles by mono and bi-func-
tional arenethiols,9 porphyrin-bridged Pd complexes,10 organo-
metallic complexes containing Pt(II) or Pd(II) centers and thiol
capped Ag nanoparticles.11–13
Gold nanoparticles (AuNPs) elicit an intense research activity
because of their peculiar chemical and physical properties. They
have been extensively studied for a wide range of applications,
ranging from catalysis,1 chemical and biosensing,2 nano-
electronics,3 nonlinear optics,4 magnetism,5 biolabelling6 and
cancer nanotheranostics for the combined diagnosis and
therapy in medicine.7
The control of size, shape and assembly of nanoparticles, on
which their optical, electrical and catalytic properties depend, is
a challenge in current nanoscience. A convenient approach that
allows control of particle composition, shape and narrow size-
distribution consists of functionalization of AuNPs with thiol
ligands.8 Thus, by varying the nature, size of the thiol group and
Au/S molar ratio it is possible to tailor the optical and electronic
Fluorene, oligouorene, and polyuorene derivatives14–16
have attracted attention because of their pure blue and efficient
electroluminescence that is effective in polymer-based emissive
displays, organic light-emitting devices, and solar cells. The
chemical structure modication of the uorene backbone
allows tuning of their properties.17 To the best of our knowl-
edge, only few examples of gold nanoparticles stabilized by
uorenyl-alkane-1-thiolates, i.e. (9-methyl-9-(8-thiooctyl)-uo-
rene, 9-(9-uorenyl)-nonane-1-thiolate and 12-(9-uorenyl)-
dodecane-1-thiolate), have been reported.18,19
Moreover, self-assembled monolayers (SAM) starting from
2,7-bis(acetylthio)uorene have been described.20
aDepartment of Chemistry University of Rome Sapienza P.le A. Moro 5, 00185, Rome,
Italy. E-mail: ilaria.fratoddi@uniroma1.it
bCNR, Istituto di Metodologie Chimiche, Sezione Meccanismi di Reazione, P.le A. Moro
In the present work, 9,9-didodecyl-2,7-bis(acetylthio)uo-
rene (1) and 9,9-didodecyl-2,7-bis(acetylthiophenylethynyl)uo-
rene (2) have been synthesized for the rst time and used to
stabilize gold nanoparticles. We have varied the conjugation
length between the sulphur donor atoms in order to achieve
interconnected networks with different delocalization degrees,
with the aim to modulate the optical properties. In Fig. 1, the
chemical structure of AuNP-1 and AuNP-2 is reported, together
with thiolates 1 and 2. The size, shape, and structure of these
functionalized nanoparticles have been determined by FTIR,
5, 00185 Rome, Italy
c
`
Department of Physics, Unita INSTM and CISDiC University Roma Tre, Via della
Vasca Navale 85, 00146 Rome, Italy
dCenter for Nanotechnology for Engineering (CNIS) University of Rome Sapienza P.le A.
Moro 5, 00185 Rome, Italy
eDepartment Fundamental and Applied Sciences for Engineering, University of Rome
Sapienza Via Antonio Scarpa 14, 00161 Roma, Italy
† Electronic supplementary information (ESI) available: Reaction schemes, main
spectroscopic characterizations and tables. See DOI: 10.1039/c3tc32567a
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with acetyl chloride as reported in the literature.29,30 Finally, Pd-
mediated coupling of the diyne (1b) with 4-iodo-1-acetylth-
iobenzene gave thiolester 2 as a yellow oil in 33% yield.
The new compounds 1 and 2 have been characterized by
1H-NMR, 13C-NMR, FTIR, and UV-vis spectroscopy. Elemental
analyses are in agreement with the proposed structures,
although evidenced the presence of some residual solvent
molecules due to the oily compounds. In the 1H-NMR spectra it
was possible to observe the peaks corresponding to the acetyl
groups (2.4 ppm), the aliphatic chains (0.97–1.84 ppm), the
aromatic protons (7.45–7.74), and, for compound 2, the peaks
relative to the phenyl groups (7.6 and 7.4 ppm). Similarly, the
13C-NMR spectra presented the peaks corresponding to
the acetyl groups (30.2 and 194.2 ppm), and, for compound 2,
the peaks relative to the triple bonds (89.0 and 92.1 ppm) and to
the phenyl groups. The formation of the nal compounds was
conrmed also by FTIR spectroscopy, as we observed the pres-
ence of the carbonyl band (n ¼ 1712 cmꢀ1), and, for compound
2, the peak assigned to the C^C triple bond stretching mode
(2199 cmꢀ1).
Fig. 1 Chemical structure of acetyl protected thiol compounds 1 and
2 together with AuNP-1 and AuNP-2 nanoparticles.
Synthesis and characterization of gold nanoparticles
The gold nanoparticles AuNP-1, stabilized with the uorenyl
bis-thiol, in situ obtained from compound 1, were prepared with
Au/S molar ratios 1.00/1, 0.70/1, 0.50/1 and 0.25/1 by using a
modied Brust's two-phase procedure,31–33 consisting of a
chemical reduction of HAuCl4 in the presence of the thiol
ligand, NaBH4 as a reducing agent and tetraoctylammonium
bromide (TOAB) as a phase transfer agent (Fig. 2). The thio-
acetyl deprotection occurs and the bifunctional uorene based
thiol is generated during the synthesis.34
UV-vis, NMR, dynamic light scattering (DLS), scanning electron
microscopy (FE-SEM), transmission electron microscopy (TEM)
and X-ray photoelectron spectroscopy (XPS).
A
photo-
luminescence study was carried out to compare the emission
properties of the thioesters with those of the corresponding
nanoparticles.
Results and discussion
Synthesis and characterization of ligands
The preparation of AuNP-2, stabilized with thiolate 2,
appeared to be very challenging. Following the modied Brust's
two-phase procedure (with the Au/S molar ratio 1.00/1)
according to the standard conditions, we found that gold
nanoparticles formed aggregates very quickly and consequently
precipitated. For this reason, it was necessary to carry on the
Fluorenyl bis-thiolacetate (1) has been prepared starting from
commercially available 9,9-didodecyl-2,7-dibromo-uorene
using a three-step one-pot protocol reported in the literature for
the functionalization of 2,7-dibromouorene and other aromatic
dibromides (see ESI†),20 in analogy to a dimethyl analog of
compound 1, reported in the literature.21 The nucleophilic
aromatic substitution reaction of the two bromine atoms with
methylthiolate and in situ transprotection of the resulting
methylsulfanyl functional groups provided the terminally thio-
acetyl-functionalised uorene (1) as a yellow oil in 48% yield.
The synthesis of uorenyl thiolate (2) bearing two ethynyl-
phenyl spacers between uorene and the terminal thioacetyl
groups has been carried out (see ESI†). The rst step consisted in
a Sonogashira coupling reaction of 9,9-didodecyl-2,7-dibromo-
uorene with 2 equivalents of trimethylsilylacetylene (TMSA) to
give 1a in 73% yield, according to literature reports for similar
compounds,22–28 followed by deprotection of the triple bonds to
give compound 1b in 90% yield. The last step in the synthesis of
the target compound 2 required the use of 1-(S-acetylthio)-4-
iodobenzene, since all attempts to couple 1b with commercially
available 4-bromophenylthioacetate failed. 4-Iodo-1-acetylth-
iobenzene has been obtained by reduction of pipsyl chloride to
ꢁ
reaction at a controlled low temperature (19 C) and to reduce
drastically the reaction time to 10 min instead of 2 hours (Fig. 3,
route a). Alternatively, we decided to use a ligand exchange
reaction,35–37 starting from
a toluene solution of freshly
prepared AuNPs stabilized with tetraoctylammonium bromide
(AuNP–TOAB), according to a procedure reported by M. Brust
et al.38 The formation of AuNPs stabilized with TOAB was fol-
lowed by UV-vis spectroscopy in toluene, which showed the
plasmon absorption peak at 525 nm, characteristic of gold
nanoparticles. Subsequent addition of the deacetylated deriva-
tive of 2, prepared in situ by treating 2 with NH4OH,35 afforded
1-iodo-4-mercaptobenzene and subsequent treatment in situ Fig. 2 Reaction scheme for the synthesis of AuNP-1.
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Fig. 3 Reaction scheme for AuNP-2. Reagents and conditions: (i)
HAuCl4$3H2O, TOAB, NaBH4, toluene, H2O, 19 ꢁC, 10 min; (ii) NH4OH
30%, ethanol and (iii) toluene, 25 ꢁC, 2 h.
AuNP-2 (Fig. 3, route b). Comparing the two synthetic pathways,
route b resulted to be more advantageous than route a, as it was
more effective in the stabilization of the gold nanoparticles and
afforded AuNP-2 in a higher yield. On the other hand, thiolate 1
allowed obtaining more stable gold nanoparticles, while thio-
late 2 showed an enhanced tendency towards aggregation
during the formation of nanoparticles, and this can be attrib-
uted to the less distance between the two thiolic moieties in
compound 1.
Fig. 4 (a) UV-vis spectra in CH2Cl2 of thiolester 1 (blue line) and AuNP-
1 (red line) with Au/S ratio 1 : 1; (b) DLS data: size distribution in CH2Cl2
for AuNP-1 with Au/S ratio 1 : 1.
plasmon resonance at about 530 nm conrms the formation
of gold nanoparticles. The formation of nanoparticles
induces a broadening and a bathochromic shi of 13 nm of
the band attributed to the ligand chromophore, while the
maximum at about 530 nm corresponds to the plasmon
absorption resonance band and conrms the formation of
small-sized nanoparticles. The different thiolic ligand
content obtained by varying the Au/S molar ratio does not
seem to determine any signicant inuence on the absorp-
tion spectra, since the optical features are overlapping in the
different samples.
The size and size-distribution of AuNP-1 in organic solutions
(CH2Cl2) have been investigated by means of DLS measure-
ments. The hydrodynamic data and the average size expressed
as h2RHi gave a size dispersion in the range of 10–18 nm, with a
lower diameter obtained by increasing the thiolic ligand
content, as expected. In particular, for the 1/1 AuNP-1 sample,
the mean size was 10 nm. The size distribution consists of a
single peak with a narrow size distribution, as shown in Fig. 4b
for the 1 : 1 AuNP-1 sample.
AuNP-1 nanoparticles were soluble in CH2Cl2, CHCl3,
toluene and DMF; AuNP-2 nanoparticles were soluble only in
DMF. All compounds were characterized by spectroscopic
techniques (FTIR, UV-vis and NMR). In the FTIR spectra of
AuNP-1 and AuNP-2, we observed the bands relative to the
stretching mode of CH in aliphatic chains (2958, 2933 cmꢀ1
)
and to the aromatic system CC (1100–600 cmꢀ1), while a weak
peak at 1712 cmꢀ1, characteristic of the stretching of the
carbonyl group, indicates that the deacetylation was not
complete and a small percentage of nanoparticles are linked
only on one side of the bifunctional thiolate. We could exclude
the presence of free thiolates as we accurately washed the solids
with ethanol until we could not observe the peaks of the starting
thiolates in the UV-vis spectra. The FIR spectra showed a band
at 226 cmꢀ1, which can be attributed to the stretching Au–S,
conrming the formation of gold nanoparticles.39
1
In the H-NMR spectra, broad signals corresponding to the
aromatic protons (7.7–7.3 ppm) and aliphatic chains (2.1–0.5)
were observed, as a consequence of the quadrupole moment of
gold;40 a small peak at 2.44 nm corresponding to the acetyl
groups indicated that a small number of nanoparticles are
linked only on one side of the bifunctional thiolate, as observed
in the FTIR spectra, and this percentage could be calculated to
be 14% from the ratio between the integrals of this peak and
those of the terminal alkyl chain. As a consequence, 86% of the
uorene spacer is covalently bound to the gold surface span-
ning two nanoparticles.
The electronic absorption spectra of thiolester 2 and nano-
particles AuNP-2 are depicted in Fig. 5. Compound 2 exhibits a
maximum at 360 nm, which is red-shied compared to the
absorption of thiolester 1, and this bathochromic shi can be
attributed to the increase of the p-conjugation. Gold nano-
particles AuNP-2 show two peaks at 365 and 593 nm. As in the
case of AuNP-1, we observe an enlargement and bathochromic
shi of the band attributed to the ligand chromophore.
However, the plasmon resonance band is centered at ꢂ593 nm,
and we can attribute this value to the cross-linking of particles
into network agglomerates.
Optical properties
The hydrodynamic radius of AuNP-2 supported the strong
Fig. 4 shows the electronic absorption spectra of thiolester 1 tendency to aggregate even in dilute solution, with a mean
and nanoparticles AuNP-1. Compound 1 presents two narrow particle size of tens of nm for samples obtained with either
peaks at 293 and 317 nm, while AuNPs show a broad peak at route a or route b. The nanosize was however evidenced by
330 with a shoulder at about 300 nm. The presence of the microscopy studies (see the next paragraph).
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Fig. 5 (a) UV-vis spectra of thiolester 2 (blue line) in CH2Cl2 and AuNP-
2 (purple line) obtained by route b, with Au/S ratio 1/1 in DMF; inset (b)
enlargement of the plasmon band of AuNP-2 obtained by route b (red
line) compared with AuNP–TOAB (black line).
The uorescence of metal nanoparticles has been the topic
of dramatic research efforts.41 Photoluminescence is an
important property and a study was carried out at different
excitation wavelengths in order to obtain information on the
different contributions to uorescence that arises from the
organic chromophore and chromophore stabilized metal
nanoparticles. Then, a comparison of the emission properties
of the thioesters 1 and 2 with those of the corresponding
nanoparticles AuNP-1 and AuNP-2 has been carried out.
Fig. 6a shows the uorescence spectra of thiolester 1 and
nanoparticles AuNP-1. Compound 1 shows one peak at 355 nm
and a shoulder at 390 nm, while AuNP-1 nanoparticles show two
peaks at 387 and 409 nm. The formation of nanoparticles
induces a bathochromic shi of about 30 nm of the emission
band compared to the free ligand.
As reported in the literature, metal nanoparticles have an
effect on the uorescence of chromophores: if the chromophore
is in close proximity to the metal surface, its strong electromag-
netic eld induces a quenching, whereas if the distance
increases, typically with long linkers, the uorescence is
enhanced.42 However, in the case of our thioesters 1 and 2, this
phenomenon was not evident. The red-shied emission observed
for AuNP-1, with respect to thiolate 1, can be explained consid-
ering that the covalent bond formation is accompanied by an
electron density ow to the gold center through the S-bridge. As
Fig. 6 (a) Fluorescence spectra of thiolester 1 (blue line) in CHCl3 and
AuNP-1 (red line) with Au/S ratio 0.25/1 in CH2Cl2 at lexc 330 nm. The
inset shows the emission spectrum of AuNP-1 at lexc 530 nm; (b)
fluorescence spectra of thiolester 2 (blue line) in CHCl3 and AuNP-2
(purple line) with Au/S ratio 1.00/1 in DMF at lexc 360 nm. The inset
shows the emission spectrum of AuNP-2 at lexc 580 nm.
gold nanoparticles was also studied, by exciting the samples at the
plasmon resonance maxima, respectively at 530 nm for AuNP-1
and 580 nm for AuNP-2. As can be observed in insets of Fig. 6a and
b, emission at about 790 nm is observed for AuNP-1 and two peaks
are visible for AuNP-2 (765 and 820 nm). Considering that as the
dimension of the nanoparticle increases the emission wavelength
redshis,44 in our AuNP-2 sample different size populations are
probably present.
Chemical structure study
reported in the literature, the charge transfer has an effect on the XPS measurements were performed on thiolate 1, thiolate 2,
relative arrangement of energy levels in the AuNP–thiol systems,43 AuNP-1 (with Au/S ¼ 1/1, 0.25/1 and 0.7/1 stoichiometric ratios)
mainly due to the chemical structure of the thiolate ligand.
and AuNP-2 (with Au/S ¼ 1/1 stoichiometric ratio). These
In the case of the uorescence of thiolester 2 and nanoparticles measurements aim to detect the degree of linkage between gold
AuNP-2, no bathochromic shi was observed moving from the nanoparticles and uorenyl dithiols, i.e. whether the ligands
ligand to AuNPs, and both spectra showed two emission bands at always act as bridges between gold nanoparticles or some are
around 385 and 405 nm (Fig. 6b). This result can be explained attached on one side leaving the thiolate end group free. For the
considering that as the linker length increases, the distance from thiolates the measured C 1s, S 2p, and O 1s core level spectra
the gold core increases and the interaction becomes less effec- conrm the hypothesized molecular structure (BE values,
tive.43 The distance between the spacer and nanoparticles is not FWHM and atomic ratios estimated for all thiolates and AuNPs
the only parameter that affects the emission properties, the are reported in Table 1 in the ESI†). C 1s signals are composites
conjugation effect should also be taken into account. Emission of for all samples, and by applying a peak-tting procedure at least
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three components can be individuated, corresponding respec-
tively to C–C (285.0 eV), C atoms of the thioacetyl group (C–S,
COC*H3) (about 286.5 eV) and carbonyl groups (C*OCH3) (288.0
eV nearly); a fourth component can be observed at higher BE
values for uorenyl-thiolate samples, and is associated with the
shake-up peak of the aromatic rings (due to the p–p* transition,
at about 292 eV). S 2p spectra of thiolates show a single pair of
spin-orbit components with the main S 2p3/2 feature at 163.5–
164.0 eV BE, as expected for S atoms in –SCOCH3 groups.9
S 2p and Au 4f signals of AuNP samples provide useful
information about the interaction between the organic thiolate
and the metallic cluster. Au 4f spectra of AuNP-1 and AuNP-2 are
shown in Fig. 7; both spectra appear structured, and by
following a peak-tting procedure two pairs of spin–orbit
components can be individuated. The rst feature (Au 4f7/2 BE ¼
84.0 eV) is associated with metallic gold, and is due to gold
atoms in the bulk of nanoparticles.
The spin–orbit pair of lower intensity at higher BE values
(Au 4f7/2 component at nearly 85.0 eV BE) is indicative for
positively charged gold atoms, as expected for surface atoms
bonded to sulphur. S 2p spectra provide complementary infor-
mation about the chemical bond between thiolates and surface
gold atoms of AuNPs. As shown in Fig. 8, in AuNP S 2p spectra a
new pair of spin-orbit components appears at lower BE values (S
2p3/2 BE ¼ 162 eV) (blue line), as expected for sulfur atoms
covalently bonded to metals.12
Fig. 8 S 2p spectra of AuNP-1 and AuNP-2 compared with thiolates 1
and 2.
In all AuNP-1 samples the S 2p feature at higher BE values
due to free thiolate end-groups is still observable, while in
AuNP-2 only the signal associated with bonded sulfur atoms can
be detected. The atomic ratio between sulfur atoms bonded to nanoparticles topologically connected by two novel bifunctional
gold and free thiolate terminal groups is usually correlated with uorenyl thiolates is essential for assessing their properties. To
the mean size of functionalized nanoparticles.13 In AuNP-2, the obtain insight into the evidence of the local nano-structure of
presence of a single S 2p spin–orbit pair attributed to sulfur the assembly of heterobridged gold nanoparticles, a combina-
atoms covalently bonded to gold suggests that all thiolate tion of electron microscopy measurements has been applied.
terminal groups are involved in AuNP functionalization,
resulting in a high degree of linkage between nanoparticles.
Investigation by scanning electron microscopy at high
magnication of the surface of AuNP-1 lms revealed an array
of interacting nanoparticles i.e. a network (see Fig. 9a–c),
showing a uniform distribution of the well-separated func-
tionalized nanoparticles. Accurate observations of SEM images
show the presence of NPs morphologically distributed in
randomly oriented parallel chains, both straight and curve (see
Fig. 9b and c), with a larger Au nanoparticle at the centre that
seems to connect two parallel chains. Based on these experi-
mental results, a series of transmission electron microscopy
measurements have been performed to study the local organi-
zation of functionalized AuNP network. An image analysis
technique has been applied to quantify the dimension of the
AuNPs (diameter) and the distance among nearest neighbours
of the self-assembled AuNPs. Fig. 9d shows representative
bright-eld micrographs of AuNP-1 exhibiting a two-dimen-
sional network and details are shown in Fig. 9e and f.
Morphology and self-assembly
The ability to control the uniformity of the size, shape,
composition, and semi-crystalline structure properties of gold
The AuNP distributions of a given micrograph have been
quantied on a probed area of 152 nm by 152 nm, showing a
contour map of the AuNP distribution (see in Fig. 9g). The
dispersity of the particle size of the AuNP-1 compound is clearly
visible in the TEM image (see in Fig. 9h). AuNP-1 nanoparticles
have an approximately spherical shape and an average diameter
Fig. 7 Au 4f spectra of AuNP-1 and AuNP-2.
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is centered around 1.5 ꢃ 0.3 nm corresponding roughly to the
length of the uorenyl linker,45 suggesting that about 50% of the
NPs are bridged through the organic spacer, in good agreement
with XPS results.
AuNP-2 samples obtained by route a and route b have been
compared by SEM and TEM analyses. In Fig. 10 the TEM image
together with the quantitative analysis on the distribution of
size and distance between AuNP-2 obtained with route a is
shown.
The histogram in Fig. 10c shows an average diameter of the
nanoparticles equal to 4.9 ꢃ 1.2 nm with a polydispersity of
25%. The value of the intense peak at about 2.7 ꢃ 0.3 nm
(Fig. 10d) is in agreement with the calculated length of the
uorenyl spacer (2). The next main counts are distributed
around 5.5 ꢃ 0.4 nm and 8.2 ꢃ 0.4 nm, which can be related to
the parallel non-covalent interactions among uorenyl bridges
of two nearest stabilized AuNPs. These ds values can be
consistent with the distance between the head–tail of sulphur
atoms of two aligned separated complexes.12
SEM and TEM images of AuNP-2 obtained by route b are
shown in Fig. 11. High magnication study of AuNP-2 revealed
also in this case a network of interconnected nanoparticles (see
Fig. 11a–c), showing well-separated nanoparticles that self-
assemble in almost parallel chains, more distanced with respect
to AuNP-1 (see SEM images in Fig. 9). A TEM bright eld image of
AuNP-2 obtained by route b is shown in Fig. 11e. As can be
observed, aggregates in a 3D network are present, probably due to
the high degree of bridging between NPs, as XPS data suggested.
This avoided the visualization of well isolated NPs and it did
not allow us to rule out the statistical image of TEM analysis.
Fig. 9 (a) High magnification SEM image illustrating the effective order
of AuNP-1; (b and c) enlarged images of figure (a), evidencing the
AuNP-1 arranged in a network; (d) low-resolution TEM image of
AuNP-1; (e and f) details of figure (d); (g) processed image of figure (d),
showing the distribution of 403 gold nanoparticles; (h) plot showing
the frequency distribution of the diameters of AuNP-1; (i) distance
distribution histogram of AuNP-1 nearest neighbours.
of 5.2 ꢃ 2.0 nm with a polydispersity of 38%. Given the presence
of spatially well-separated nanoparticles without overlapping in
the bi-dimensional view, it was possible to use a simple method
to quantify the distance ds among the AuNPs.
To simplify the quantication description, the di–j distances
have been measured between the center of mass of the nearest
neighbour nano-objects minus the corresponding ri,j radius of
each AuNP associated with the distance di–j (ds ¼ di–j ꢀ (ri + rj)).12
In Fig. 9i the histogram of the distribution of the calculated
Fig. 10 (a) Low-resolution TEM image of AuNP-2 monolayers sepa-
rated by thiolate 2 obtained by route a; (b) processed image showing
the distribution of 107 gold nanoparticles, corresponding to the bright
field micrograph. (c) Plot showing the frequency distribution of the
diameters of AuNP-2. (d) Distance distribution histogram of AuNP
distances ds between AuNP-1 is shown. The higher-count value nearest neighbours.
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formation of aggregated structures. The band attributed to the
ligand chromophore exhibited a bathochromic shi with
respect to that of the free thiolate ligands and was observed at
330 and 365 nm for AuNP-1 and AuNP-2, respectively. Emission
spectroscopy evidenced a red shi of about 30 nm of the
emission band for nanoparticles AuNP-1, which show two peaks
at 387 and 409 nm, compared to the free thioester 1, suggesting
an electron density ow to the gold center through the S-bridge.
This effect was quenched in AuNP-2, which shows two emission
bands at around 385 and 405 nm, probably due to the increased
distance of the uorenyl chromophore from the gold core. S 2p
and Au 4f XPS signals of AuNP samples provide useful infor-
mation about the interaction between the organic thiolate and
the nanoparticle. Au 4f spectra allowed isolation of the
components arising from metallic bulk gold atoms from those
due to surface atoms bonded to sulphur. S 2p spectra provide
complementary information about the chemical bond; sulfur
atoms are mainly covalently bonded to gold surfaces. In AuNP-1
samples few free thiolate end-groups are still observable, while
in AuNP-2 only the signal associated with bonded sulfur atoms
can be detected. In AuNP-2 all thiolate terminal groups are
involved in the covalent bonding, resulting in a high degree of
linkage between nanoparticles. Investigation by FESEM and
TEM of the surface of AuNP-1 lms revealed an array of inter-
acting nanoparticles i.e. a network showing a uniform distri-
bution of the well-separated functionalized nanoparticles with
approximately spherical shapes and an average diameter of
5.2 ꢃ 2.0 nm with a polydispersity of 38%. The distance between
neighbour nanoparticles of 1.5 ꢃ 0.3 nm is in agreement with
the formation of NPs bridged through the organic uorenyl
spacer. AuNP-2 nanoparticles showed an average diameter
equal to 4.9 ꢃ 1.2 nm with a polydispersity of 25%. A network of
interconnected, well-separated nanoparticles more distanced
with respect to the AuNP-1 was observed. The bifunctional u-
orenyl thiolates induce the self-assembly of a network archi-
tecture with a uniform distribution, which makes these
materials promising candidates in the eld of optical devices
and electronics.
Fig. 11 (a) High magnification SEM image illustrating the effective
order of AuNP-2 obtained by route b; (b and c) enlarged images of
figure (a) evidence the AuNP-2 arranged in a network. (e) Low-reso-
lution TEM image of poly-dispersed AuNP aggregation separated by a
complex bridge. Inset: the corresponding diffraction pattern of Fig. 3a
shows distinct diffraction spots or rings for each of the face-centered-
cubic Au phases. (f) High magnification image showing the multilayers
of AuNP-2, the inset showing the gold lattice fringes.
The corresponding high magnication shown in Fig. 11f
evidences an overlapping of the quasi-spherical shaped gold
nanoparticles with a diameter of about 5 nm.
The inset is a representative enlarged image of a white circle
area showing the lattice fringes of the Au nanoparticles having
interplanar spacing of d111 ¼ 0.238 nm. In order to conrm the
crystallographic ngerprint of the atomic species, the electron
diffraction pattern (EDP) (inset of Fig. 11e) has been performed
showing the typical pattern of the face-centered-cubic (fcc) gold
ꢀ
Experimental section
crystal of space group Fm3m (yellow arcs). EDP analysis reveals
the presence of concentric diffraction rings generated by the
different crystallographic orientations of the NPs with respect to
the electron beam direction. The experimental intensive inner
ring corresponds to the typical diffraction ring of Au with
interplanar spacing of d111 ¼ 0.236 nm.
1H-NMR and 13C-NMR spectra were recorded on a Bruker
Avance II 300 instrument with CDCl3 solvent peaks as reference.
FTIR spectra have been recorded as lms deposited by casting
from CH2Cl2 solutions using KRS-5 cells, with a Bruker Vertex
70 spectrophotometer. UV-vis spectra were run in CHCl3,
CH2Cl2 or DMF solution by using quartz cells with a Varian Cary
100 Scan UV-vis spectrophotometer. The molar extinction
coefficients (3, Mꢀ1 cmꢀ1) were measured in the concentration
Conclusion
In conclusion, two series of gold nanoparticles stabilized by two range of 5.0 ꢄ 10ꢀ6 to 1.8 ꢄ 10ꢀ4 M, and the values are given
novel bifunctional thiol derivatives of 9,9-didodecyl-2,7-dibro- with ꢃ5% experimental error. Photoluminescence measure-
mouorene with a different degree of conjunctional length, ments were carried out on a Horiba Jobin Yvon Fluoromax-4
AuNP-1 and AuNP-2, have been synthesised. The size control of instrument, on freshly prepared solutions in CHCl3, CH2Cl2 or
the Au nanoparticles was achieved by careful control of the DMF solvent, in the concentration range of 10ꢀ6 M. Elemental
synthesis parameters. UV-vis spectroscopy evidenced the plas- analyses have been carried out on an EA 1110 CHNS-O instru-
mon resonance for AuNP-1 at about 530 nm, whereas in the case ment. The size and size distribution of AuNPs in CH2Cl2 or DMF
of AuNP-2 it was found at about 593 nm, suggesting the solution have been investigated by means of the dynamic light
This journal is © The Royal Society of Chemistry 2014
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Journal of Materials Chemistry C
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scattering (DLS) technique by using a Brookhaven instrument product was puried by column chromatography (silica gel,
(Brookhaven, NY) equipped with a 10 mW HeNe laser at a 632.8 petroleum ether/CH2Cl2 with gradient from 3/1 to 2/1) affording
ꢁ
nm wavelength at a temperature of 25.0 ꢃ 0.2 C. Correlation 1 as a yellow oil (0.470 g, 48%), Rf ¼ 0.14 (petroleum ether/
data have been acquired and tted in analogy to our previous CH2Cl2 3/1). 1H-NMR (CDCl3, 300 MHz): d 7.74 (dd, J ¼ 7.5, 1.0
work.46 X-ray photoelectron (XPS) spectra were recorded using a Hz, 2H, H-4, H-5), 7.45–7.35 (m, 4H, H-3, H-6, H-1, H-8), 2.44 (s,
custom designed spectrometer, described in a previous study13 6H, COCH3), 2.07–1.84 (m, 4H, CH2(CH2)9), 1.41–0.97 (m, 36H,
and equipped with a non-monochromatized Mg Ka X-ray source (CH2)9), 0.97–0.80 (m, 6H, CH2CH3), 0.80–0.53 (m, 4H,
(1253.6 eV pass energy ¼ 25 eV, step 0.1 eV). The spectra have CH2CH3). 13C-NMR (CDCl3, 75.5 MHz): d 14.11 (CH3), 22.67,
been acquired on lms cast or spin deposited from CHCl3, 23.75, 29.20, 29.32, 29.53, 29.55, 29.60, 29.94 (CH2), 30.19
CH2Cl2 or DMF solvents on TiO2/Si (111) wafers. The spectra (COCH3), 31.89, 39.98 (CH2), 55.49 (C-9), 120.68, 127.01, 129.03,
were energy referenced to the C 1s signal of aliphatic C atoms 133.11, 141.32, 151.89 (Ar), 194.16 (COCH3). FT-IR (lm, n,
having a binding energy BE ¼ 285.00 eV. Atomic ratios were cmꢀ1): 2926, 2854, 1712 (C]O), 1462, 1454, 1403, 1352, 1259,
calculated from peak intensities using Scoeld's cross-section 1123, 1006, 948, 886, 815, 754, 723, 614. UV-vis (CHCl3), lmax
values and l factors were used.47 Curve-tting analysis of C 1s, S (nm) (log 3o): 317 (4.4), 293 (4.3). Elemental analysis calcd (%)
2p and Au 4f spectra was performed using Gaussian proles as for C41H62O2S2 (651.06): C 75.64, H 9.60, S 9.85; found: C 74.44,
tting functions, aer subtraction of a Shirley-type background. H 9.41, S 10.06.
For quantitative data, the BE values were referred to NIST data.48
Transmission electron microscopy measurements were carried
out using a FEI-TITAN TEM operating at 80 kV. Measurements
were carried out using the same experimental setup of the
previously reported structural studies12 on cast deposited
samples obtained from CH2Cl2 or DMF solutions on Cu grids
coated with amorphous carbon. FE-SEM images have been
acquired with the Auriga Zeiss instrument (resolution 1 nm,
applied voltage 6–12 kV) on freshly prepared lms drop cast
from CH2Cl2 or DMF solution on a metallic sample holder.
9,9-Didodecyl-2,7-bis(trimethylsilylethynyl)uorene (1a)
Air was removed from a solution of 9,9-didodecyl-2,7-dibromo-
uorene (2.094 g, 3.17 mmol), CuI (0.082 g, 0.43 mmol), and
[Pd(PPh3)Cl2] (0.223 g, 0.3177 mmol) in anhydrous diisopro-
pylamine (30 mL) by blowing Ar. Then trimethylsilylacetylene
(1.35 mL, 0.938 g, 9.55 mmol) was slowly added at room
temperature. The solution turned red at once and then changed
to deep brown. The mixture was stirred at 75 ꢁC overnight. The
solvent was evaporated to dryness, the residue was redissolved
in Et2O (50 mL) and the solution was washed with dilute HCl
(2 ꢄ 50 mL) followed by water (2 ꢄ 50 mL). Upon ltration
Materials and methods
9,9-Didodecyl-2,7-dibromouorene, 1,3-dimethyl-2-imidazoli- through a pad of celite, the ltrate was treated with saturated
dinone (DMI), sodium methanethiolate, acetyl chloride, CuI, Zn NaHCO3 (2 ꢄ 50 mL) and water (2 ꢄ 50 mL) and the organic
powder, trimethylsilylacetylene, K2CO3, Pd(dba)2, PPh3, phase dried over Na2SO4. The dark oily product was puried by
[Pd(PPh3)2Cl2],
tetrachloroauric(III)
acid
triidrate column chromatography (silica gel, petroleum ether) affording
(HAuCl4$3H2O) tetraoctylammonium bromide (TOAB), and 1a as a yellow oil that solidied at ꢀ20 ꢁC (1.621 g, 73%),
1
sodium borohydride (NaBH4) have been used as received Rf ¼ 0.18 (petroleum ether). H-NMR (CDCl3, 300 MHz): d 7.60
(Aldrich reagent grade). Anhydrous solvents: toluene, THF, (dd, J ¼ 7.8, 0.6 Hz, 2H, H-4, H-5), 7.47 (dd, J ¼ 7.8, 1.4 Hz, 2H,
CH2Cl2, DMF, MeOH, CHCl3, diisopropylamine, and diisopro- H-3, H-6), 7.43 (dd, J ¼ 1.3, 0.6 Hz, 2H, H-1, H-8), 2.02–1.88 (m,
pylethylamine (DIEA) (Aldrich reagent grade) have been used as 4H, CH2(CH2)9), 1.36–0.95 (m, 36H, (CH2)9), 0.95–0.81 (m, 6H,
received. Column chromatography was carried out on silica gel CH2CH3), 0.61–0.46 (m, 4H, CH2CH3), 0.30 (s, 18H, Si(CH3)3).
(Merck 70–230 mesh), and thin-layer chromatography (TLC) on 13C-NMR (CDCl3, 75.5 MHz): d 0.04 (Si(CH3)3), 14.12 (CH2CH3),
aluminium sheets precoated with silica gel 60 F254 (Merck). 22.68, 23.66, 29.32, 29.55, 29.59, 30.03, 31.90, 40.37 (CH2), 55.21
Deionized water, obtained from a Zeener Power I Scholar-UV (C-9), 94.22 (C^CSi), 106.09 (C^CAr), 119.80, 121.75, 126.20,
(18.2 MU), was degassed for 30 minutes with Argon, before use. 131.21, 140.83, 150.91 (Ar). FT-IR (lm, n, cmꢀ1): 2957, 2926,
4-Iodo-1-acetylthiobenzene was prepared according to proce- 2854, 2154 (C^C), 1464, 1421, 1377, 1331, 1249, 1216, 1130,
dures reported in the literature.29,30
1104, 844, 759, 721, 699, 647. UV-vis (CHCl3), lmax (nm) (log 3o):
341 (5.0), 326 (4.8), 308 (4.7).
9,9-Didodecyl-2,7-bis(acetylthio)uorene (1)
9,9-Didodecyl-2,7-diethynyluorene (1b)
9,9-Didodecyl-2,7-dibromo-uorene (1.001 g, 1.51 mmol) was
dissolved in dry, Ar-saturated DMI (30 mL) and the mixture was 9,9-Didodecyl-2,7-bis(trimethylsilylethynyl)uorene (1a) (0.502
heated to 120 ꢁC. Sodium methanethiolate (1.015 g, 14.49 g, 0.72 mmol) was dissolved in Et2O (13 mL) to give a colorless
mmol) was added at once. The reaction was kept at this solution and a catalytic amount of K2CO3 (0.105 g, 0.76 mmol)
temperature for 18 h and then cooled to room temperature. in MeOH (13 mL) was added. A milky solution was obtained
Aer addition of acetyl chloride (1 mL, 1.104 g, 14.06 mmol) the aer 24 h at room temperature. The solvent was removed and
solution was stirred at room temperature for 3 h and then the crude product was puried by column chromatography
poured on ice/water. The aqueous phase was extracted with (silica gel, petroleum ether) to afford a yellow solid (1b) (0.360 g,
toluene, the organic extracts were dried over Na2SO4 and the 90%), Rf ¼ 0.34 (petroleum ether). 1H-NMR (CDCl3, 300 MHz): d
solvent was removed by rotary evaporation. The dark oily 7.64 (dd, J ¼ 7.7, 0.9 Hz, 2H, H-4, H-5), 7.53–7.46 (m, 4H, H-3,
2524 | J. Mater. Chem. C, 2014, 2, 2517–2527
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Journal of Materials Chemistry C
ꢁ
H-6, H-1, H-8), 3.16 (s, C^CH, 2H), 2.02–1.87 (m, 4H, mixture was kept overnight at ꢀ18 C and then centrifuged at
CH2(CH2)9), 1.36–0.97 (m, 36H, (CH2)9), 0.95–0.81 (m, 6H, 1500 rpm for 15 min. The supernatant, containing excess thiol
CH2CH3), 0.68–0.49 (m, 4H, CH2CH3). 13C-NMR (CDCl3, 75.5 and TOAB, was separated and the precipitate was washed by
MHz): d 14.11 (CH3), 22.67, 23.66, 29.24, 29.32, 29.54, 29.59, centrifugation with ethanol in the same way for 10 more times.
29.94, 31.89, 40.20 (CH2), 55.19 (C-9), 77.32 (C^CH), 84.51 Aer removal of the supernatant, a red ruby solution of AuNP-1
(HC^CAr), 119.94, 120.84, 126.53, 131.23, 140.96, 151.03 (Ar). nanoparticles was then prepared with 10 mL of toluene; the
FT-IR (lm, n, cmꢀ1): 3311 (CCH), 2926, 2854, 2107 (C^C), yield was 40 wt%. Elemental analysis (%): C 39.97, H 5.18, S
1464, 1415, 1377, 1200, 891, 823, 755, 721, 648, 605. UV-vis 5.50. UV-vis (CHCl3), lmax (nm): 330, 530.
(CHCl3), lmax (nm) (log 3o): 330 (4.8), 316 (4.5), 302 (4.6).
Elemental analysis calcd (%) for C41H58 (550.90): C 89.39, H
10.61; found: C 89.94, H 10.68.
Synthesis of gold nanoparticles stabilized with the thiol
ligands generated in situ from the acetyl derivative (2) (AuNP-2)
AuNP-2 nanoparticles have been prepared by means of the
9,9-Didodecyl-2,7-bis(acetylthio phenyl ethynyl)uorene (2)
above described method (route a) and by means of the ligand
Air was removed from a solution of 9,9-didodecyl-2,7-diethynyl-
uorene (1b) (0.559 g, 1.015 mmol), 4-iodo-1-thioacetylbenzene
(0.847 g, 3.047 mmol), Pd(dba)2 (0.042 g, 0.0725 mmol), CuI (0.027
g, 0.1428 mmol), and PPh3 (0.076 g, 0.2904 mmol) in anhydrous
tetrahydrofuran (16 mL) and anhydrous diisopropylethylamine
(1.5 mL) by blowing Ar. The mixture was stirred at 50 ꢁC for 24 h.
Upon ltration through a pad of celite, the solvent was evaporated
to dryness. The dark oily product was puried by column chro-
matography (silica gel, petroleum ether/CH2Cl2 with gradient from
3/1 to 2/1) affording 2 as a yellow oil (0.269 g, 33%), Rf ¼ 0.34
exchange procedure (route b). In route a, AuNP-2 nanoparticles
stabilized with the thiolate derived from 2 were prepared by
reduction of HAuCl4$3H2O with NaBH4 in the presence of
thiolester 2, as described above, with Au/S molar ratios 1.00/1.
However, to prevent aggregation, it was necessary to carry out
ꢁ
the reaction at 19 C and to stop stirring just 10 minutes aer
the addition of sodium borohydride.
By using route b, the preparation of AuNP-2 has been achieved
in a two step way. In the rst step, the preparation of a toluene
solution of gold nanoparticles stabilized with TOAB was carried
out as follows. An aqueous solution of HAuCl4$3H2O (0.046 g,
0.117 mmol) in deionized water (3.9 mL) was mixed with a
solution of TOAB (0.284 g, 0.52 mmol) in toluene (10.4 mL) and
vigorously stirred until all the tetrachloroaurate was transferred
into the organic layer. A freshly prepared aqueous solution of
sodium borohydride (0.055 g, 1.46 mmol) in deionized water (3
mL) was rapidly added with vigorous stirring. Aer further stir-
ring for 2 h the ruby-coloured organic phase was separated and
washed once with aqueous sulfuric acid (0.1 M) to remove excess
NaBH4, washed twice with aqueous sodium carbonate (1 M), and
washed 5 times with water. The yield was 18 wt%.
In the latter step, the freshly prepared AuNPs in toluene
solution have been stabilized in a ligand exchange procedure
with 2, as follows. To a solution of compound 2 (0.050 g, 0.058
mmol) in ethanol (10 mL), NH4OH 30% (150 mL) was added and
the resulting mixture was stirred vigorously for 10 min. This
mixture was added to the toluene solution of gold nanoparticles
stabilized with TOAB previously prepared. Aer further stirring
for 2 h, the volume of the solvent was reduced to 2 mL in a rotary
evaporator, and 40 mL of ethanol were added. The mixture was
kept overnight at ꢀ18 ꢁC and then centrifuged at 1500 rpm for
15 min to remove the excess of thiol and TOAB. The supernatant
was separated and the precipitate was washed by centrifugation
with ethanol, and then with dichloromethane for 10 more
times. Aer the removal of the supernatant, a dark solution of
AuNPs was then prepared by taking up with 10 mL of DMF. The
yield was 20 wt%. Elemental analysis (%): C 27.40, H 2.73, S
2.73. UV-vis (CHCl3), lmax (nm): 365, 593.
1
(petroleum ether/CH2Cl2 2/1). H-NMR (CDCl3, 300 MHz): d 7.69
(d, J ¼ 7.7 Hz, 2H, H-4, H-5), 7.60 (m, JAX ¼ 8.4 Hz, 4H, AA0XX0 Ar),
7.56–7.50 (m, 4H, H-3, H-6, H-1, H-8), 7.42 (m, JAX ¼ 8.3 Hz, 4H,
AA0XX0 Ar), 2.45 (s, 6H, COCH3), 2.07–1.92 (m, 4H, CH2(CH2)9),
1.38–0.97 (m, 36H, (CH2)9), 0.94–0.79 (m, 6H, CH2CH3), 0.71–0.52
(m, 4H, CH2CH3). 13C-NMR (CDCl3, 75.5 MHz): d 14.09 (CH3),
22.65, 23.72, 29.29, 29.54, 29.58, 29.68 (CH2), 30.00, 30.27 (COCH3),
31.87, 40.32 (CH2), 55.28 (C-9), 89.01 (C^C), 92.09 (C^C), 120.03,
121.62, 124.61, 126.01, 127.96, 130.87, 132.06, 132.11, 132.13,
134.16, 134.21, 134.23, 140.87, 151.16 (Ar), 193.48 (COCH3). FT-IR
(lm, n, cmꢀ1): 2925, 2853, 2216, 2199 (C^C), 1905, 1714 (C]O),
1591, 1490, 1464, 1417, 1397, 1377, 1352, 1289, 1264, 1122, 1016,
949, 890, 826, 739, 623, 605, 544. UV-vis (CHCl3), lmax (nm) (log 3o):
360 (4.9). Elemental analysis calcd (%) for C57H70O2S2 (851.29): C
80.42, H 8.29, S 7.53; found: C 78.92, H 8.36, S 7.82.
Synthesis of gold nanoparticles stabilized with the thiol
ligands generated in situ from the acetyl derivative (1) (AuNP-1)
Gold nanoparticles were prepared by mixing HAuCl4$3H2O and
compound 1 with Au/S molar ratios 1.00/1, 0.70/1, 0.50/1 and
0.25/1. One of these syntheses (Au/S ¼ 1.00/1) is reported as a
typical procedure: an aqueous solution of HAuCl4$3H2O (0.050
g, 0.128 mmol) in deionized water (4 mL) was mixed with a
solution of tetraoctylammonium bromide (TOAB) (0.083 g,
0.152 mmol) in toluene (9 mL). The two-phase mixture was
vigorously stirred until all the tetrachloroaurate was transferred
into the organic layer and a solution of thiolester 1 (0.040 g,
0.062 mmol) in toluene (9 mL) was then added. A freshly
prepared aqueous solution of sodium borohydride (0.047 g,
1.271 mmol) in deionized water (4 mL) was rapidly added with
vigorous stirring. Aer further stirring for 3 h, the organic phase
Acknowledgements
was separated, washed with water, and then reduced to 2 mL in The authors acknowledge for nancial support Ateneo Sapienza
a rotary evaporator. Aer addition of 40 mL of ethanol, the 2011/C26A11PKS2 and 2013/C26A13HRZ4. This work was
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 2517–2527 | 2525

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Journal of Materials Chemistry C
Paper
¨
partially supported by the Dipartimento di Chimica, Sapienza 20 A. Shaporenko, M. Elbing, A. Blaszczyk, C. von Hanisch,
`
Universita di Roma through the Supporting Reseach Initiative
M. Mayor and M. Zharnikov, J. Phys. Chem. B, 2006, 110,
4307–4317.
2013. This part of the work was carried out with the support of
´
the European Community. We appreciate the support of the 21 W. Haiss, C. Wang, R. Jitchati, I. Grace, S. Martın,
European Research Infrastructure EUMINAfab (funded under
the FP7 specic programme Capacities, Grant Agreement
Number 226460) and the Advanced Microscopy Laboratory
A. S. Batsanov, S. J. Higgins, M. R. Bryce, C. J. Lambert,
P. S. Jensen and R. J. Nichols, J. Phys.: Condens. Matter,
2008, 20, 374119.
´
´
(AML) in CRANN for the provision of their facility and expertise. 22 M. Quintiliani, E. M. Garcıa-Frutos, P. Vazquez and
Financial support from MIUR, PRIN project no. 2010JMAZML –
T. Torres, J. Inorg. Biochem., 2008, 102, 388–394.
´
´
MultiNanoIta is acknowledged.
23 M. V. Martınez-Dıaz, M. Quintiliani and T. Torres, Synlett,
2008, 1–20.
24 D. L. Pearson and J. M. Tour, J. Org. Chem., 1997, 62, 1376–
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