Gold nanoparticles containing redox-active supramolecular dendrons that recognize H2PO4-

Article abstract of DOI:10.1039/b106805a

Gold nanoparticles have been functionalized with thiol dendrons containing three redox active amidoferrocenyl or silylferrocenyl units; using cyclic voltammetry, these dendronized gold nanoparticles recognize H₂PO₄^-.

Full text of DOI:10.1039/b106805a

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Gold nanoparticles containing redox-active supramolecular dendrons
2
that recognize H2PO4
Marie-Christine Daniel,a Jaime Ruiz,a Sylvain Nlate,a Jennifer Palumbo,a Jean-Claude Blaisb and
Didier Astruc*a
a Groupe de Chimie Supramoléculaire des Métaux de Transition, LCOO, UMR CNRS, 5802, 33405
Talence Cedex, France. E-mail: d.astruc@lcoo.u-bordeaux.fr
b Laboratoire de Chimie Structurale Organique et Biologique, EP CNRS N° 103, Université Paris VI,
4 Place Jussieu, 75252 Paris, France
Received (in Cambridge, UK) 27th July 2001, Accepted 16th August 2001
First published as an Advance Article on the web 18th September 2001
Gold nanoparticles have been functionalized with thiol
dendrons containing three redox active amidoferrocenyl or
silylferrocenyl units; using cyclic voltammetry, these den-
Our strategy has consisted of performing ligand exchange
reactions between alkylthiol–gold colloids of the Brust type9
and the thiol dendron, a method that keeps the size of the
colloids constant during the ligand exchange reactions, thus
retaining their narrow polydispersity.5b,9 The two syntheses of
the triferrocenyl dendron–gold particle assemblies are repre-
sented in Scheme 1. Both syntheses represent the first routes to
ferrocenyl-containing dendrons, although numerous ferrocenyl-
and amidoferrocenyl-containing dendrimers are known.10–13a
The key to rapid entry into the chemistry of such functionalized
dendrons is the direct hydrosilylation of the three allyl groups of
the dendron 112 using various silanes11–13 without any protec-
tion of the phenol function (Scheme 1).14 The two air-sensitive
thiol dendrons 8 and 10 were fully characterized by elemental
analysis, NMR and the molecular peaks in their MALDI TOF
mass spectra. We synthesized gold colloids with 2.1 nm
2
dronized gold nanoparticles recognize H2PO4
The scope of supramolecular chemistry1 involves nanoscience
including dendrimers2 and nanoparticles3 with applications to
the field of sensors.1,4 Beer et al. have shown various examples
of redox anion recognition by amidoferrocenes bound to
endoreceptors,4 a concept that we have extended to den-
drimers5a and colloids5b as exoreceptors. So far, only a few
other reports of molecular recognition by nanoparticles have
appeared,6 althought ferrocenylalkylthiol ligands bonded to
surfaces in self-assembled monolayers and colloids have been
known for some time.7 Dendrimer-colloid assemblies have also
been successfully designed for the fabrication of a new
generation of catalysts and sensors.3b We now report the
synthesis of dendrons containing silylferrocenyl or amidoferro-
cenyl termini, their functionalization by thiol ligands, fixation
of these dendron-thiol ligands onto gold colloids8 and recogni-
tion of H2PO42 by these thiol-functionalized gold colloids.
diameter core and approximately 108
5 dodecanethiolate
ligands using the method of Brust et al.9 Ligand-exchange
reactions between these gold–dodecanethiolate particles and the
thiol ligands 8 and 10 were carried out under ambient conditions
in CH2Cl2 using 2/3 equiv. functional thiol per dodecane thiol
Scheme 1 Syntheses of the dendronized gold colloids. Reagents and conditions: i, Karstedt cat., HSiMe2Fc (Fc = ferrocenyl), Et2O, reflux, 1 d, 90%; ii,
Karstedt cat., HSiMe2(CH2Cl), Et2O, 1 d at room temp., then 1 d at reflux, 85%; iii, 1,1A-dibromo-p-xylene, K2CO3, MeCN, room temp., 3 d; 7: 76%; 9: 70%;
iv, NaI, NaN3, anhydrous DMF, 80 °C, 1 d, 90%; v, PPh3, H2O, THF, 80 °C, 1 d, 70%; vi, conc. aq. HCl, then NEt3, FcCOCl, CH2Cl2, 16 h, room temp.,
45%; vii, NaSH, THF, 50 °C, 1 d; 8: 76%, 10: 90%; viii, thiol dendron (2/3 equiv./equiv. dodecanethiolate ligand), CH2Cl2, 3 d, room temp.
2000
Chem. Commun., 2001, 2000–2001
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106805a

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ligand. This led to the dendronized particles 11 and 12, and the
excess of functional ligand was removed by washing 11 and 12
with methanol. The percentage of functional thiolate dendrons
introduced as ligands in 11 and 12, determined by combined
nyl-containing receptors are the double hydrogen bonding
between this anion and the amido groups, the enhanced
electrostatic attraction in the oxidized ferrocenium form and the
topographical effect of the receptor.4
1
HRTEM, H NMR spectroscopy and elemental analysis, was
Interestingly, the silylferrocene-containing colloids 11 also
recognize H2PO42. Addition of [n-Bu4N][H2PO4] to an electro-
chemical cell containing 11 provokes the same effect as with 12,
i.e. the appearance of a new wave at less positive potential than
the original one, while the disappearance of the initial wave is
observed for 1 equiv. of anion per silylferrocenyl branch. The
difference of potential between the two waves is 110 10 mV,
which corresponds to a Kapp value 85 30 times larger for the
4.8 and 3%, respectively (a little more than five and three
dendrons per particle for 11 and 12, respectively). TEM images
confirm that the sizes of the gold cores of the particles remain
unchanged after the ligand-substitution reactions.
The cyclovoltammograms of the dendronized colloids 11 and
12 (Pt, CH2Cl2, 0.1 M [n-Bu4N][PF6]) show a chemically (ia/ic
=
1) and electrochemically (DEp 5 50 mV) reversible
ferrocene/ferrocenium wave.14 Thus, all the ferrocenyl units
appear equivalent in each type of particles, which is due, in
particular, to the fact that rotation of the particles is faster than
the electrochemical time scale.15 The separation between the
anodic and cathodic peaks is 50 mV for 11, which almost
corresponds to the value expected at 20 °C for a single-electron
wave (58 mV). In the case of 12, however, this peak separation
is only 20 mV. This indicates some adsorption, although this
phenomenon is not accompanied by an enhanced intensity of
the adsorbed species. The E1/2 value is 0 V vs. Cp2Fe0/+ for 11
and 0.145 V vs. Cp2Fe0/+ for 12.
ferrocenium form than for the ferrocenyl form. So far, only
H2PO42 is recognized, the anions HSO42, Cl2, Br2 and NO3
,
2
for instance, having no significant effect using either 11 or
12.
It is likely that the supramolecular redox properties of
dendronized colloids can be developed for sensoring, catalysis
and molecular electronics in the near future.
Notes and references
1 J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,
VCH, Weinheim, 1995.
We then added [n-Bu4N][H2PO4] to the electrochemical cell
containing 12, which led to a decrease of the intensity of the
amidoferrocene wave of 12 (Fig. 1). The growth of another
wave was then observed at a less positive potential until the
initial wave had disappeared when the amount of [n-
Bu4N][H2PO4] added corresponded to 1 equiv. per amidoferro-
cenyl branch. Thus, the new wave is the signature of a strong
2 G. R. Newkome, C. N. Moorefield and F. Vögtle, Dendritic Molecules:
Concepts, Syntheses and Perspectives, VCH, New York, 1996.
3 (a) J. S. Bradley, in Clusters and Colloïds, ed. G. Schmid, VCH,
Weinheim, 1995, ch. 6 (b) R. M. Crooks, M. Zhao, L. Sun, V. Chechik
and L. K. Yeung, Acc. Chem. Res., 2001, 34, 181.
4 P. D. Beer, Adv. Inorg. Chem., 1992, 39, 79; P. D. Beer, Chem.
Commun., 1996, 689; P. D. Beer, Acc. Chem. Res., 1998, 31, 71; P. D.
Beer, P. A. Gale and Z. Chen, Adv. Phys. Org. Chem., 1998, 31, 1.
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2
amidoferrocenium–H2PO4 interaction. Contrary to the initial
wave, it shows the characteristic of slow electron transfer since
the DEp value is larger than 60 mV and depends on the scan rate,
indicating structural reorganization in the course of the
heterogeneous electron transfer.14 The value of E1/2 for each
wave does not vary during the titration, the difference remaining
equal to 210
10 mV (as for the dendron 8 alone). This
corresponds to an apparent association constant Kapp between
2
the ferrocenium form of 12 and H2PO4 that is 5200 1000
times larger than that between the neutral form of 12 and
2 16
H2PO4
.
This shift is very large compared to monomeric
amidoferrocenes (E1/2(free) 2 E1/2(bound) = 45 mV) and even to
tripodal tris-amidoferrocenes such as PhC{(CH2)3O(CH2)3NH-
COFc}3 (E1/2(free) 2 E1/2(bound) = 110 mV), and is about as
large as with a nona-amidoferrocene dendrimer.5a The known
factors involved in the recognition of H2PO42 by amidoferroce-
9 M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and R. Whyman, J.
Chem. Soc., Chem. Commun., 1994, 801; M. Brust, J. Fink, D. Bethell,
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10 A. E. Kaifer and M. Gomez-Kaifer, Supramolecular Electrochemistry,
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15 C. B. Gorman, Adv. Mater., 1997, 9, 1117; C. B. Gorman, Adv. Mater.,
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Fig. 1 Titration of 12 (1026 M in CH2Cl2) with [n-Bu4N][H2PO4] (1022
M
in CH2Cl2) monitored by CV. Decrease of the intensity of the initial wave
at E1/2 = 0.145 V vs. FeCp2+/0 and increase of the intensity of the new wave
+/0
at E1/2 = –0.065 V vs. FeCp2 vs. the number of equivalents of [n-
Bu4N][H2PO4] added per ferrocenyl branch of 12. [n-Bu4N][PF6] 0.1 M, 20
5
°C; internal reference: [Fe(h -C5Me5)2]; reference electrode: Ag; auxiliary
and working electrodes: Pt; scan rate: 0.2 V s21. A similar response was
16 S. R. Miller, D. A. Gustowski, Z.-H. Chen, G. W. Gokel, L. Echegoyen
and A. E. Kaifer, Anal. Chem., 1988, 60, 2021.
obtained with 11.
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2001