Gold nanocomposites with rigid fully conjugated heteroditopic ligands shell as nanobuilding blocks for coordination chemistry

Article abstract of DOI:10.1039/b610631e

Monodisperse and solvent adaptable gold nanoparticles stabilized by rigid and fully conjugated modified neocuproinium and terpyridinium salts have been characterized and further used as nanobuilding blocks for the synthesis of gold nanoparticles functionalized by polypyridyl ruthenium complexes. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b610631e

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Gold nanocomposites with rigid fully conjugated heteroditopic ligands
shell as nanobuilding blocks for coordination chemistry{
Ce´dric R. Mayer,*a Eddy Dumas,a Aude Michelb and Francis Se´cheressea
Received (in Cambridge, UK) 25th July 2006, Accepted 31st August 2006
First published as an Advance Article on the web 21st September 2006
DOI: 10.1039/b610631e
complex was often observed, mainly when using a pyridine linker.
Indeed, such desorption processes have to be efficiently overcome
to maintain the stability of the colloidal solution, but also to avoid
interference of the ‘‘free’’ ligands throughout physical measure-
ments such as luminescence or electrochemical analysis. We
therefore decided to develop a new strategy to graft the ruthenium
complexes onto the Au-NPs to strengthen its interaction with the
metallic surface, while maintaining the fully conjugated nature of
the bridge. Here we report the synthesis of the two heteroditopic
ligands (4-(2-mercaptopyridyl)imidazo[4,5-f])-2,9-dimethyl-1,10-
phenanthroline (NeoSH) and 49-(2-mercaptopyridyl)-2,29:69,20-ter-
pyridine (TerSH), both possessing the 2-mercaptopyridine (2-Mpy)
group that will ensure the connection with the metallic surface and
either a bidentate (NeoSH) or a tridentate (TerSH) pendant group
that will be used to generate the ruthenium complex. Both ligands
have been used to stabilize and functionalize Au-NPs. The Au-
NCs thus obtained have been used as effective platforms to
functionalize Au-NPs by polypyridyl ruthenium complexes.
The design of the two heteroditopic ligands used in this article
had to fulfil three main specifications: (i) the ligand had to
efficiently complex the metallic centre on one side and (ii) strongly
interact with the Au-NP surface on the other side. (iii) In order to
facilitate potential electronic transfer between the ruthenium centre
and the NP, a fully conjugated system was maintained between
them. Concerning the specification (i), we decided to use either a
tridentate terpyridine or a bidentate neocuproine (2,9-dimethyl-
1,10-phenanthroline) to complex the metallic centre. Neocuproine
was chosen instead of phenanthroline since the steric hindrance
introduced by the two methyl groups prevents interaction of the
nitrogen atoms with the Au-NP surface.12 As to the specification
(ii), we used the terminal 2-Mpy which provides two potential
points of attachment to the Au-NP surface and therefore was
expected to limit desorption processes. Interaction of 2-Mpy with
Au-NPs and gold electrodes has already been studied.13,14 The
synthesis of both fully conjugated ligands was achieved using the
2-chloroisonicotinaldehyde (2-ClpyCHO). The aldehyde function
can be used to generate different chelating pendant groups such as
neocuproine and terpyridine, as shown in this communication,
but other potentially interesting functional groups such as
dipyrromethane or porphyrin might also be foreseen.15
NeoSH and TerSH were synthesized in two steps. In the first
step, (4-(2-chloropyridyl)imidazo[4,5-f])-2,9-dimethyl-1,10-phenan-
throline (NeoCl) and 49-(2-chloropyridyl)-2,29:69,20-terpyridine
(TerCl) were obtained by reaction of 2-ClpyCHO with 2,9-
dimethyl-1,10-phenanthroline-5,6-dione (Neodione) or 2-acetylpyr-
idine, respectively. Both intermediates subsequently reacted with
an excess of sodium hydrogensulfide to form NeoSH and TerSH
Monodisperse and solvent adaptable gold nanoparticles
stabilized by rigid and fully conjugated modified neocuproi-
nium and terpyridinium salts have been characterized and
further used as nanobuilding blocks for the synthesis of
gold nanoparticles functionalized by polypyridyl ruthenium
complexes.
Gold nanocomposites (Au-NCs) have been extensively studied
owing to their unique tunable size- and shape-dependent
optoelectronic properties.1 The so-called Brust–Schiffrin method
was decisive in this research area as it offered a straightforward
and adaptable synthesis to obtain stable, rather monodisperse and
easy to handle gold nanoparticles (Au-NPs).2 The initial
alkanethiol used by Brust and co-workers was later replaced by
a variety of functionalizing agents of interest for applications such
as catalysis, bio-sensing and nanotechnology.1,3 Surprisingly, few
examples of metallic complexes used as stabilizing agents have
been described in the literature. Synergistic effects between the
intrinsic properties of both the Au-NP and the metallic complexes
could be expected for such nanocomposites. The metallic
complexes were either interacting with the Au-NPs via electrostatic
interactions,4 or they were grafted onto the metallic surface via a
thiol,5 a dithiol,6 or a dithiocarbamate terminated aliphatic chain.7
Another approach was recently described in which Au-NPs
functionalized by a variable chain length aliphatic thiol terminated
by a terpyridine or a bipyridine group formed 3D aggregates in the
presence of metallic ions.8 To our knowledge, only three examples
of metallic complexes grafted onto the surface of Au-NPs via a
fully conjugated system have been described.9,10
Polypyridyl ruthenium complexes are well-known for their
optical and electronic properties and for their interaction with
DNA.11 We recently reported the functionalization of silver
nanoparticles by polypyridyl ruthenium complexes using a fully
delocalized p-conjugated linker.12 The fully conjugated bridge was
used in order to facilitate potential electronic transfers between the
ruthenium centre and the metallic NP.10 In these systems, the
connection between the ruthenium complex and the metallic
surface was ensured by a pyridine or a phenanthroline group.
While the strength of this connection was sufficient enough to
ensure the stability of the NCs, partial desorption of the ruthenium
aInstitut Lavoisier de Versailles, UMR 8180, Universite´ de Versailles,
78035, Versailles, France. E-mail: cmayer@chimie.uvsq.fr;
Fax: +33 1 39 25 43 81; Tel: +33 1 39 25 43 97
bLaboratoire des liquides Ioniques et Interfaces Charge´es, UMR 7612,
Universite´ Paris 6, 75252, Paris Cedex 05, France
{ Electronic supplementary information (ESI) available: Experimental
details, NMR data, ESI-MS spectra, TEM images, nanoparticle
histograms, UV-vis spectra. See DOI: 10.1039/b610631e
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 4183–4185 | 4183

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that were finally recovered as chloride salts of polypyridinium after
precipitation with an excess of hydrochloric acid (Scheme 1).
Detailed characterizations of NeoCl, NeoSH, TerCl and TerSH are
given in the ESI.{
addition of an excess of NBu4Cl, re-dispersed in water as chloride
salts and so forth. Using these successive solvent transfer processes,
free ligands were efficiently removed while no noticeable change
was detected in the size, shape and dispersity of the Au-NCs. The
final precipitate can then be easily dispersed in a variety of solvents
such as H2O, EtOH, H2O–EtOH (Fig. 1a), acetonitrile, DMF,
DMSO. Concentration in Au-NPs up to 7 6 1026 mol L21 (1.2 6
1022 mol L21 in Au atoms) were obtained in water. A similar
behaviour was observed using NeoSH except that smaller particles
were obtained (diameter of 2.8 ¡ 0.5 nm) and the purification
processes led to few aggregates in aqueous solution (Fig. S4).
The synthesis of Au-TerSH-NCs and the purification processes
were also studied by UV-vis spectroscopy (Fig. 1b). The spectrum
of Au-TerSH-NCs in DMF, before any purification, displays a
surface plasmon resonance (SPR) at 536 nm, characteristic of the
Au-NPs, and bands attributed to TerSH. After purification using
the solvent transfers and the anionic exchanges described above,
the spectrum of Au-TerSH-NCs dispersed in water exhibits a
quasi-identical SPR confirming that the Au-NPs were not
modified by the different treatments. The slight shift of the SPR
from 536 to 531 nm was attributed to the modification of the
solvent refractive index.16 Another important feature of the UV-vis
spectrum after purification concerns the disappearance of the two
bands at 320 (shoulder) and 390 nm characteristic of the thione
tautomer form of the mercaptopyridine group. 2-Mpy moieties are
known to adsorb on Au-NPs in the thiol-like form while the thione
tautomer form is expected for free 2-Mpy in solution.14 Therefore,
the disappearance of these two bands after purification can be used
to confirm the efficiency of the removal of the non-grafted TerSH
ligands. UV-vis spectroscopy was also used to estimate the number
of ligands per particle giving an average composition of
Au1667TerSH90 (see ESI).
The syntheses of Au-NPs coated by NeoSH and TerSH
performed in DMF led to the best results in terms of stability
and monodispersity in size of the final Au-NCs. Syntheses were
also carried out in EtOH–H2O (1 : 1) with TerSH (NeoSH was
poorly soluble in this mixed solvent) but polydisperse colloidal
solutions were generally obtained, except when an excess of TerSH
was used (Fig. S3{). Stable colloidal solutions were obtained by
addition of a DMF solution of HAuCl4 to a mixture of TerSH (or
NeoSH) in DMF and reducing agent (aqueous solution of
NaBH4). In the remainder of this communication, we will mainly
focus on the Au-NPs stabilized by TerSH. The following results
were obtained with an initial ratio [Au3+]/[TerSH] # 2.8 but no
noticeable alteration of the size distribution of the Au-NCs was
observed for ratios between 2.5 and 3. Stable, reproducible and
monodisperse Au-NPs with a diameter of 3.8 ¡ 0.6 nm were
characterized in DMF, without formation of aggregates, as
revealed by TEM (Fig. S4{). The as-synthesized Au-TerSH-NCs
were then efficiently purified. Addition of an excess of acetone to
the DMF colloidal solution led to the flocculation of Au-TerSH-
NCs with the presence of few traces of free TerSH. Free TerSH
was then removed either by successive re-solubilization in DMF
and re-precipitation with an excess of acetone or by successive
aqueous/non-aqueous solvent transfers using anionic exchanges
around the NCs. As mentioned above, TerSH was isolated as
chloride salt of polypyridinium. Au-TerSH-NCs are therefore
positively charged and surrounded by easily exchangeable anions.
The NCs can be dispersed in water, precipitated by addition of an
excess of KPF6, further re-dispersed in acetone (or in various
organic solvents) as hexafluorophosphate salts, re-precipitated by
To illustrate the strength of the interaction of the 2-Mpy group
with the gold surface, we compared the stabilization of Au-NPs
with TerSH and with (49-pyridyl)-2,29:69,20-terpyridine (TerPy).
The synthesis with TerPy was realized in DMF following the
procedure described above with TerSH. After addition of the
DMF solution of HAuCl4 to the mixture of TerPy and NaBH4,
the resulting solution turned blue and a precipitate was observed
after few minutes. The pyridine pendant group ensured a much
weaker interaction with the Au-NPs surface compared with the
2-Mpy pendant group. A stable colloidal DMF solution of Au-
TerPy-NCs was obtained only when a large excess of TerPy was
used to stabilize the Au-NPs.
Fig. 1 (a) TEM image of Au-TerSH-NCs synthesized in DMF, after
purification and re-dispersion in H2O–EtOH (v/v, 1 : 5). Scale: 50 nm. (b)
Absorption spectra of Au-TerSH-NCs in DMF before purification (A)
and after purification and re-dispersion in H2O–EtOH (B). Absorption
spectrum of TerSH in H2O–EtOH (C).
Scheme 1 Reagents and conditions: (i) Neodione, NH4OAc, AcOH,
120 uC, 4 h, then NH3; (ii) 2-acetylpyridine, KOH, MeOH, NH3; (iii)
NaSH, ethylene glycol, 140 uC, 12 h, then acetone; (iv) HCl.
4184 | Chem. Commun., 2006, 4183–4185
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Au-NeoSH-NCs presented in this communication showed an
excellent monodispersity in size, can be easily purified and
reversibly transferred from aqueous to organic medium without
alteration of their size and shape. The strength of the interaction
between the metallic surface and the 2-Mpy pendant group was
exemplified by the grafting of ruthenium complexes with the Au-
NCs in refluxing conditions. Monodisperse and solvent adaptable
Au-NCs functionalized by fully conjugated ruthenium complexes
have been obtained and their electrochemical, luminescence and
NLO properties are currently investigated. This work is a
breakthrough in facile coordination chemistry directly on pre-
functionalized metallic nanoparticles. We currently use the
terpyridine and neocuproine pendant group of the nanocomposites
to generate fully conjugated (hetero)polymetallic complexes.
Moreover the synthetic approach using the 2-chloroisonicotinal-
dehyde to obtain the heteroditopic ligands can be easily modified
to generate a variety of potentially attractive pendant groups
and therefore a large variety of metallic NPs functionalized
by (poly)metallic complexes with tuneable physico-chemical
properties.
Fig. 2 Study of the reactivity of TolylTerpyRuCl3 toward Au-TerSH-
NCs in H2O–EtOH at 80 uC. Absorption spectra after 0 min (bold solid
line), 80 min (dashed line), 240 min (dotted line) and 16 h (solid line).
Notes and references
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Fig. 3 (a) TEM image of the Au-[tolylTerpyRuTerSH]2+-NCs obtained
in H2O–EtOH (v/v, 1 : 5). Scale: 50 nm. (b) Corresponding histogram.
The next step was to show that the monodisperse and purified
Au-NCs characterized in this communication could be used as a
platform for coordination chemistry. We studied the reactivity of
TolylTerpyRuCl3 (TolylTerpy: 49-(4-tolyl)-2,29:69,20-terpyridine)
toward Au-TerSH-NCs. The reaction was performed in a mixed
solvent H2O–EtOH (v/v, 1 : 5) at 80 uC and followed by UV-vis
spectroscopy over a period of 16 h (Fig. 2). In order to understand
this reactivity, we also studied independently the solubilization of
TolylTerpyRuCl3 and the reactivity of TolylTerpyRuCl3 toward
TerCl (instead of TerSH to avoid the formation of disulfide
complexes) using identical synthesis parameters (see ESI). These
studies showed that the complexation of TolylTerpyRuCl3 with
Au-TerSH-NCs proceeds in two steps. TolylTerpyRuCl3 is first
solubilized, as shown by the increase of the intensity of the band
observed at ca. 385 nm, and then reacts with Au-TerSH-NCs. The
formation of the ruthenium complex [TolylTerpyRuTerSH]2+ is
confirmed by the decrease of the band at ca. 385 nm and the
concomitant increase of the bands at 280, 315 and 491 nm,
characteristics of [TolylTerpyRuTerSH]2+ (Fig. 2). TEM analysis
(Fig. 3) showed that the size, shape and dispersity of the Au-NCs
were not noticeably affected by the reaction of complexation which
further substantiates the high stability of the Au-TerSH-NCs even
when treated in refluxing H2O–EtOH mixed solvent. Similar
reactivity and stability were observed during the reaction of
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In conclusion, we have shown that the two heteroditopic ligands
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and functionalize gold nanoparticles. The Au-TerSH-NCs and
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Chem. Commun., 2006, 4183–4185 | 4185