Nanoscopic assemblies between supramolecular redox active metallodendrons and gold nanoparticles: Synthesis, characterization, and selective recognition of H2PO4-, HSO4-, and adenosine-5′-triphosphate (ATP2-) anions

Article abstract of DOI:10.1021/ja021325d

Tri- and nonaferrocenyl thiol dendrons have been synthesized and used to assemble dendronized gold nanoparticles either by the ligand-substitution method from dodecanethiolate - gold nanoparticles (AB₃ units) or Brust-type direct synthesis from a 1:1 mixture of dodecanethiol and dendronized thiol (AB₉ units). The dendronized colloids are a new type of dendrimers with a gold colloidal core. Two colloids containing a nonasilylferrocenyl dendron have been made; they bear respectively 180 and 360 ferrocenyl units at the periphery. These colloids selectively recognize the anions H₂PO₄^- and adenosine-5′-triphosphate (ATP^2-) with a positive dendritic effect and can be used to titrate these anions because of the shift of the CV wave even in the presence of other anions such as Cl^- and HSO₄^-. Recognition is monitored by the appearance of a new wave at a less positive potential in cyclic voltammetry (CV). The anion HSO₄^- is also recognized and titrated by the dendronized colloid containing the tris-amidoferrocenyl units, because of the progressive shift of the CV wave until the equivalence point. These dendronized colloids can form robust modified electrodes by dipping the naked Pt electrode into a CH₂Cl₂ solution containing the colloids. The robustness is all the better as the dendron is larger. These modified electrodes can recognize H₂PO₄^-, ATP^2- and HSO₄^-, be washed with minimal loss of adsorbed colloid, and be reused.

Full text of DOI:10.1021/ja021325d

Published on Web 02/04/2003
Nanoscopic Assemblies between Supramolecular Redox
Active Metallodendrons and Gold Nanoparticles: Synthesis,
-
-
Characterization, and Selective Recognition of H2PO4 , HSO4 ,
and Adenosine-5′-Triphosphate (ATP2-) Anions
Marie-Christine Daniel,† Jaime Ruiz,† Sylvain Nlate,† Jean-Claude Blais,‡ and
Didier Astruc*,†
Contribution from the Groupe Nanoscience et Catalyse, LCOO, UMR CNRS Nï. 5802,
UniVersite´ Bordeaux I, 33405 Talence Cedex, France and LCSOB, UMR CNRS Nï. 7613,
UniVersite´ Paris VI, 75252 Paris, France
Received November 1, 2002 ; E-mail: d.astruc@lcoo.u-bordeaux.fr
Abstract: Tri- and nonaferrocenyl thiol dendrons have been synthesized and used to assemble dendronized
gold nanoparticles either by the ligand-substitution method from dodecanethiolate-gold nanoparticles (AB3
units) or Brust-type direct synthesis from a 1:1 mixture of dodecanethiol and dendronized thiol (AB9 units).
The dendronized colloids are a new type of dendrimers with a gold colloidal core. Two colloids containing
a nonasilylferrocenyl dendron have been made; they bear respectively 180 and 360 ferrocenyl units at the
periphery. These colloids selectively recognize the anions H2PO4- and adenosine-5′-triphosphate (ATP2-
)
with a positive dendritic effect and can be used to titrate these anions because of the shift of the CV wave
even in the presence of other anions such as Cl- and HSO4-. Recognition is monitored by the appearance
of a new wave at a less positive potential in cyclic voltammetry (CV). The anion HSO4- is also recognized
and titrated by the dendronized colloid containing the tris-amidoferrocenyl units, because of the progressive
shift of the CV wave until the equivalence point. These dendronized colloids can form robust modified
electrodes by dipping the naked Pt electrode into a CH2Cl2 solution containing the colloids. The robustness
is all the better as the dendron is larger. These modified electrodes can recognize H2PO4-, ATP2- and
HSO4-, be washed with minimal loss of adsorbed colloid, and be reused.
catalysts,1,3,6 and components for molecular electronics.1,7 So
far, only very few examples of assemblies between dendrimers
Introduction
Nanoscopic supramolecular assemblies1 between dendrimers2
and colloids3 should be fruitful to provide a new generation of
materials that are likely to give applications as sensors,1,4,5
or dendrons8 and colloids are known.1,9 We have been interested
in such assemblies disclosing supramolecular properties in order
to provide means to approach new sensors. The recognition of
anions has indeed been the subject of special scrutiny, given
their role in biology.10 In particular, Beer has shown various
examples of redox anion recognition by amidoferrocenes bound
to endoreceptors.11 We have addressed the use of redox-active
† University Bordeaux I. This article is part of the Ph.D. thesis of M.-
C.D.
‡ University Paris VI (MALDI TOF mass spectroscopy).
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Y.; Yeung, L. K.; Crooks, R. M. J. Am. Chem. Soc 2001, 123, 6840. (d)
Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem.
Res. 2001, 34, 181.
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Commun. 2001, 2000.
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J. AM. CHEM. SOC. 2003, 125, 2617-2628
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A R T I C L E S
Daniel et al.
metallodendrimers12a,b and colloids5,12c,d as exo-receptors for the
recognition of anions. Dendrimers show positive dendritic
effects, that is, the recognition improves as the generation
number increases, because the redox centers become close to
one another at the periphery for high generations.12a,b On the
other hand, colloids are also relatively good sensors that present
the advantage over dendrimers that they are rapidly prepared,
although the magnitude of the recognition phenomena only
reaches that of low-generation dendrimers.5,12c,d Thus, the novel
assemblies between supramolecular dendrons containing redox-
active groups and colloids should disclose new features involv-
ing hopefully good recognition properties. We already know
that AB3 units bearing thiol functions and three amido- or
silylferrocenyl groups can be introduced to a certain extent onto
gold colloids by the classic ligand-substitution procedure that
leaves the core intact.5 These colloids do recognize H2PO4- as
well as or slightly better than gold colloids containing thiol
ligands bearing a single amidoferrocenyl unit.12c We now report
the extension of these studies to direct synthetic methods of
assemblies between gold colloids and two real dendrons (AB9
units) containing nine ferrocenyl groups. This allows us to
compare assemblies containing colloid-AB3 units and colloid-
AB9 dendrons for the recognition of H2PO4-. We have also
now applied these principles to the recognition of the biologi-
cally important adenosine-5′-triphosphate anion (ATP2-). Fi-
nally, we also present here successful attempts specific to these
assemblies to prepare stable derivatized electrodes that also
recognize these anions. So far, only very few other examples
of molecular recognition by nanoparticles have been reported,13-17
although ferrocenyl-alkylthiol ligands bonded to surfaces in self-
assembled monolayers and particles have been known for some
time.18 Fitzmaurice and Stoddart et al. have shown the recogni-
tion of dibenzylammonium cation using crown ethers located
at the periphery of nanoparticles.13 Rotello et al. have investi-
gated supramolecular recognition between flavin and the diacetyl
derivative of diamidopyridine.15 Thiol-modified oligonucleotides
have been fixed onto gold nanoclusters. In particular, Mirkin
and Letsinger have used such modified nanoparticles for several
studies involving DNA analysis,16 and Mann et al have built
three-dimensional networks by antigen-antibody associations.17
Recently, Nishihara has reported a series of studies on gold
nanoparticles bonded to thiolate ligands bearing mixed-valent
biferrocenyl (AB2) units.19
Results and Discussions
We have used two synthetic strategies to introduce the AB3
and AB9 units onto the colloids. The first one is the exchange
of a limited proportion of alkylthiol ligands in Brust-type
alkylthiol-gold colloids.18c,20 This method was already involved
to introduce amidoferrocenylalkylthiols in our preliminary
work.12c It has the advantage of keeping the core size unchanged
during the ligand-substitution reaction, but the proportion of
substituted ligands can be small with dendrons that are much
bulkier than linear thiols. The other one is new, exploratory,
and consists of a direct synthesis,20 derived from Brust’s method,
for which both dodecanelthiol and dendronized alkylthiols are
allowed to competitively react during the direct colloid synthesis.
These two synthetic approaches of the supramolecular colloids
have been applied to the AB3 and AB9 units and compared.
Synthesis of the Dendronized Thiol Ligands. (a) AB3 Units.
The synthesis starts with the phenoltriallyl derivative 1. This
compound is an AB3 unit that is now easily available in good
yield from [FeCp(η6-p.MeC6H4OEt)][PF6], itself synthesized in
large scale from [FeCp(η6-p.MeC6H4Cl)][PF6], ethanol, and
potassium carbonate.21 A convenient entry into functionalized
dendrons derived from 1 is the direct hydrosilylation with
various silanes22,23 of the three allyl groups of 1 without any
protection of the phenol function, as exemplified in Scheme 1
for the synthesis of 2 and 3 (top). The following organic
reactions also proceed in good yields to provide the phenol
compound 6 derived with three amidoferrocenyl groups. The
introduction of the thiol group into these dendrons 2 and 6 was
achieved by selective reaction with p.di(bromomethyl)benzene
(in excess) giving 7 and 9 followed by reaction with NaSH
which finally yielded 8 and 10. These new functional thiols are
very air sensitive and are quickly oxidized to the corresponding
disulfide in air, possibly because the ferrocenyl groups act as
redox catalysts for this aerobic oxidation process.
However, the two thiol dendrons 8 and 10 were fully
characterized by analytical and spectroscopic techniques includ-
ing the molecular peaks in their MALDI TOF mass spectra (see
the experimental procedures in the Supporting Information).
B. AB9 Dendrons. From the AB3 unit 1, two phenolic nona-
allyl AB9 dendrons were synthesized using convergent strategies
(Scheme 2). The allyl branches of 1 were either hydroborated,
and then oxidized to alcohol and transformed into iodo termini,
or catalytically hydrosilylated by dimethylchloromethylsilane
followed by halogen exchange for iodo. These two triodo
derivatives were protected at the phenol focal point by reaction
with propionyl iodide, which gave the protected triodo deriva-
tives 13 and 14. These two compounds reacted with 1 to give
nona-allyl AB9 units 15 and 16 after deprotection. These two
phenol nona-allyl derivatives were hydrosilylated using ferro-
cenyldimethylsilane yielding 17 and 18, then reaction of the
phenol group with p.dibromoxylene gave the bromomethyl
(12) (a) Vale´rio, C.; Fillaut, J.-L.; Ruiz, J.; Guittard, J.; Blais, J.-C.; Astruc, D.
J. Am. Chem. Soc. 1997, 119, 2588. (b) Vale´rio, C.; Alonso, E.; Ruiz, J.;
Blais, J.-C.; Astruc, D. Angew. Chem., Int. Ed. 1999, 38, 1747. (c) Labande,
A.; Astruc, D. Chem. Commun. 2000, 1007. (d) Nlate, S.; Ruiz, J.; Sartor,
V.; Navarro, R.; Blais, J.-C.; Astruc, D. Chem.sEur. J. 2000, 6, 2544. (e)
Labande, A.; Ruiz, J.; Astruc, D. J. Am. Chem. Soc. 2002, 124, 1782.
(13) Fitzmaurice, D.; Rao, S. N.; Preece, J.; Stoddart, J. F.; Wenger, S.;
Zaccheroni, N. Angew. Chem., Int. Ed. 1999, 38, 1147.
(14) Sampath, S.; Lev, O. AdV. Mater. 1997, 9, 410.
(15) (a) Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 1999, 121, 4914. (b)
Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2000, 122, 734. (c) Niemz,
A.; Rotello, V. M. Acc. Chem. Res. 1999, 32, 44.
(16) (a) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger,
R. L. J. Am. Chem. Soc. 1998, 120, 1959. (b) Mucic, R. C.; Storhoff, J. J.;
Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 12674. (c)
Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A.
Science, 1997, 277, 1078.
(17) Shenton, W.; Davis, D. A.; Mann, S. AdV. Mater. 1999, 119, 11132.
(18) (a) Weber, K.; Creager, S. E. Anal. Chem. 1994, 66, 3164. (b) Weber, K.;
Hockett, L.; Creager, S. E. J. Phys. Chem. B. 1997, 101, 8286. (c) Hosteler,
M. J.; Green, S. J.; Stockes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996,
118, 4212.
(20) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J.
Chem. Soc., Chem. Commun. 1994, 801. (b) Brust, M.; Fink, J.; Bethell,
D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655.
(21) Sartor, V.; Djakovitch, L.; Fillaut, J.-L.; Moulines, F.; Neveu, F.; Marvaud,
V.; Guittard, J.; Blais, J.-C.; Astruc, D. J. Am. Chem. Soc. 1999, 121, 2929.
Sartor, V.; Nlate, S.; Djakovitch, L.; Fillaut, J.-L.; Moulines, F.; Neveu,
F.; Marvaud, V.; Guittard, J.; Blais, J.-C.; Astruc, D. New J. Chem. 2000,
24, 351. Nlate, S.; Blais, J.-C.; Astruc, D. New J. Chem. 2003, 27, 178.
(22) Jutzi, P.; Batz, C.; Neumann, B.; Stammler, H. G. Angew. Chem., Int. Ed.
Engl. 1996, 35, 2118.
(19) (a) Horikoshi, T.; Itoh, M.; Kurihara, M.; Kubo, K.; Nishihara, H. J.
Electroanal. Chem. 1999, 473, 113. (b) Yamada, M.; Tadera, T.; Kubo,
K.; Nishihara, H. Langmuir, 2001, 17, 2263.
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9
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Scheme 1. Synthesis of the AB3 Thiol Ligands 8 and 10 Containing Ferrocenyl Groups
derivatives 19 and 20, and finally substitution of Br- by SH-
yielded the desired thiol dendrons 21 and 23.
by combined HRTEM (see the TEM pictures and histograms
in the Supporting Information, Figures SI 1 to SI 4), 1H NMR,
and elemental analysis, were 4.8% and 3%, respectively. This
corresponds, in average, to a little less than seven dendrons for
11 and five dendrons for 12. The TEM pictures confirm that
the sizes of the gold cores of the particles remain unchanged
after the ligand-substitution reactions.
The limit of the ligand-substitution reaction was noted with
larger dendrons. For instance, attempts to synthesize dendron-
colloid assemblies using the nona-allyl dendron HSCH2p.C6H4-
CH2p.OC6H4C[C(CH2)3OC6H4C(CH2CHCH2)3]3 resulted in the
incorporation of very little dendron, that is, less than one per
colloid particle. Under these conditions, reactions with even
larger nonametallic dendrons containing nine ferrocenyl groups
for recognition purpose appeared hopeless. Therefore, we
embarked into a different strategy that consisted in carrying out
direct Brust-type nanoparticle syntheses using mixtures of linear
dodecanethiols and dendronized thiols containing nine ferrocenyl
groups. The direct synthesis was first successfully attempted
using the trisilylferrocenyl thiol which gave 11a, whose
characteristics turned out to be similar to those of the colloid
11 obtained by the previous ligand-substitution procedure. It
was expected that linear dodecanethiol molecules would become
thiolate ligands of gold particles more easily than the den-
The AB9 thiols 21 and 23 were fully characterized including
by the prominent molecular peak in their MALDI TOF mass
spectra. The thiol dendrons are air sensitive and show a strong
tendency to oxidize to disulfide (see Scheme 3 where 21 is
oxidized to 22). Thus, reduction of the eventually partly oxidized
thiols was systematically carried out just before dendron-colloid
assembly in order to optimize the colloid yields whatever the
type of colloid synthesis.
Colloid-Dendron Assemblies. Brust’s method reproducibly
leads to the synthesis of gold-dodecanethiolate particles of
various sizes with narrow polydispersities,20 and we chose to
synthesize such colloids with a 2.3-nm diameter core and
approximately 150 thiol ligands, which was checked by
combined HRTEM and elemental analysis. Ligand-exchange
reactions between these gold-dodecanethiolate particles and the
thiol ligands 8 and 10 were carried out under ambient conditions
in dichloromethane (Scheme 4). The functional thiol dendrons
were used in excess (1 equiv of functional thiol per dode-
canethiol ligand) leading to the dendron-functionalized particles
11 and 12, and the excess of functional ligand was removed by
washing 11 and 12 with methanol. The percentages of functional
thiol dendrons introduced as ligands in 11 and 12, determined
9
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Scheme 2. Synthesis of the Two Nonasilylferrocenyl Thiol Dendrons (AB9 Units) 21 and 23 Starting from the Triallylphenol Molecular Brick
1
Scheme 3. Reversible Dimerization of the Nonaferrocenyl Thiol Dendrons upon Exposure to Air
dronized thiols, especially with the bulky nonaferrocenylthiol
dendrons. Indeed, their proportion found in the nanoparticles
obtained was higher than that in the reaction mixture that
systematically contained an equimolar ratio of both dode-
canethiol and dendronized thiol. Nevertheless, this technique
is very successful and allows efficient synthesis of colloids 24
and 25 with good amounts of nonaferrocenyl dendronized thiols
(Schemes 5 and 6). Adjusting the proportion of ligands and gold
source can control the size of the particles, although the particles
made in this way are larger (2.9 nm diameter) than those
synthesized by the substitution method. We first attempted it
using the trisilyl dendron that could be introduced in a proportion
of 25%. Then, the two nonaferrocenylthiol dendrons could form
dendronized nanoparticles containing, respectively, 10 and 20%
of dendrons among the thiol ligands. This means that, for a
colloid of a 2.9-nm diameter bearing around 200 thiolate ligands,
there are about 20 and 40, respectively, dendronized thiolates
bearing around 180 and 360, respectively, ferrocenyl units linked
to the colloidal core, which makes a rather bulky periphery.
The cavities near the core, located between the particle surface
and the ferrocenyl layer at the periphery, are filled with about
180 and 160, respectively, linear dodecanethiolate ligands, as
schematically represented on Schemes 5 and 6. The proportions
of dendronized and linear ligands in 24 and 25 were determined
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Assemblies between Metallodendrons and Gold Nanoparticles
A R T I C L E S
Scheme 4. Synthesis of the Dendronized Gold Colloids 11 and 12 Using the Thiol-Ligand Substitution Procedure
Scheme 5. Direct Synthesis of the Dendronized Gold Colloid 24 Containing the Nonaferrocenyl Thiol Dendron 21 (About 180 Ferrocenyl
Groups)
by integration of the respective 1H NMR signals of these ligands
and by electrochemical titration (vide infra).
Cyclic Voltammetry Studies of the Metallodendron-
Colloid Assemblies. The cyclovoltammograms (CV) of the
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Scheme 6. Direct Synthesis of Dendronized Gold Colloid 25 Containing the Nonaferrocenyl Thiol Dendron 23 (About 360 Ferrocenyl
Groups)
dendronized gold nanoparticles 11, 12, 24, and 25 (Pt, CH2Cl2,
0.1 M [NBu4][PF6]) show a chemically (ia/ic ) 1) and
electrochemically (∆Ep e 50 mV) reversible ferrocene/ferro-
cenium wave.24-26 Although these waves look like single-
electron waves, the total number of electrons exchanged per
particles corresponds to the total number of ferrocenyl units
per particle, that is, for instance about 30 for 11 and 18 for 12.
These numbers cannot be determined using the Bard-Anson
formula,24b which is working well with polymers including
dendrimers, however, contrary to what could be achieved with
metallodendrimers,12 because the gold core is too large and
heavy. All the ferrocenyl units look equivalent, in each type of
particle, which is due, in particular, to the fact that rotation of
the particles is faster than the electrochemical time scale.27 The
separation between the anodic and cathodic peaks is 50 mV
for 11 and 12, which almost corresponds to the 58-mV value
expected at 20 °C for a single-electron wave. Values lower than
58 mV indicate that some adsorption27b,c occurs, although this
phenomenon is weak and not accompanied by an enhanced
intensity of the adsorbed dendronized colloids alone (see Table
1).
The E1/2 value is 0 V versus Cp2Fe0/+ for 11 and 0.145 V
versus Cp2Fe0/+ for 12.25b The dendronized colloids 24 and 25
containing the nonaferrocenyl dendrons adsorb more strongly
than the colloids 11 and 12 that contain AB3 units when the
CV are recorded using CH2Cl2 solutions. This latter parameter
can eventually influence the choices of conditions for the
recognition experiments in solution or at modified electrodes
(vide infra).
Recognition of H2PO4-, HSO4-, and ATP2-. (a) H2PO4
.
-
We have examined the interaction between H2PO4- and the new
dendronized colloids by adding [n-Bu4N][H2PO4] to the elec-
trochemical cell containing one of the dendronized colloids. This
led to the decrease of the intensity of the ferrocenyl wave and
to the concomitant appearance and growth of another wave at
a less positive potential until the initial wave disappeared when
the amount of [n-Bu4N][H2PO4] added corresponded to 1 equiv
per amidoferrocenyl branch. Thus, the new wave corresponds
(24) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New
York, 1980. (b) Flanagan, J. B; Margel, S.; Bard, A. J.; Anson, F. C. J.
Am. Chem. Soc. 1978, 100, 4248.
-
to the interaction between H2PO4 and the branch containing
(25) (a) Astruc, D. Electron-Transfer and Radical Processes in Transition-Metal
Chemistry; VCH: New York, 1995; Chapters 2 and 7. (b) the internal
reference was decamethylferrocene, [FeCp*2], a much better reference than
ferrocene (the E° values are then converted vs [FeCp2], however). See:
Ruiz, J.; Astruc, D. C. R. Acad. Sci., Ser. IIc: Chim. 1998, 21. Astruc, D.
In Electron Transfer in Chemistry; Balzani, V., Mattay, J., Astruc, D., Eds.;
Vol. 2. Organic, Inorganic and Organometallic Molecules; Wiley-VCH:
New York, 2001; section 2, Chapter 4, p 728.
the ferrocenyl group, and this appearance of a new wave is the
signature of a strong interaction. This new wave, contrary to
the initial one, shows the characteristic of a slow electron
transfer, since the ∆Ep value is more or less larger than 60 mV
and depends on the scan rate, meaning that some structural
reorganization intervenes in the course of the heterogeneous
electron transfer.24,25 The value of E1/2 for each wave does not
vary during the titration. For instance, with the dendronized
colloid 12 containing AB3 units that bear three amidoferrocenyl
groups, the difference remains equal to 210 ( 10 mV. Yet, in
this particular case, the recognition is marred by the partial
chemical irreversibility of the system in solution, contrary to
(26) Kaifer, A E.; Gomez-Kaifer, M. Supramolecular Electrochemistry; Wiley-
VCH: Weinheim, Germany, 1999; Chapter 16, p 207.
(27) (a) Gorman, C. B.; Smith, J. C.; Hager, M. W.; Parkhurst, B. L.;
Sierzputowsska-Gracz, H.; Haney, C. A. J. Am. Chem. Soc. 1999, 121,
9958. (b) For the adsorption of organothiol dendrons on a gold surface,
see: Gorman, C. B.; Miller, R. L.; Chen, K. Y.; Bishop, A. R.; Haasch, R.
T.; Nuzzo, R. G. Langmuir, 1998, 14, 3312. (c) For a review on dendritic
thin films and monolayers, see: Schenning, A. P. H. J.; Weener, J. W.;
Meijers, E. W. In Conjugated Polymers and Molecular Interfaces; Salaneck,
W. R. R., Seki, K., Kahn, A., Pireaux, J.-J., Eds.; Marcel Dekker: New
York, 2001, p 1.
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2622 J. AM. CHEM. SOC. VOL. 125, NO. 9, 2003

Assemblies between Metallodendrons and Gold Nanoparticles
A R T I C L E S
-
Table 1. Electrochemical Characteristics of the Dendronized Gold Colloids 11, 12, 24, and 25 before and after Titration of the H2PO4 and
ATP2- Anions and of the Stable Modified Electrodes Obtained with Colloids 12, 24, and 25
a
c
c
E
E
1/2initial − E
E1/2initial − E
with ATP
1/2
1/2new
1/2new
2-
b
d
e
(Epa − E )
ipc/ipa
with H PO -
K /K
K /K
(+) (0)
pc
2
4
(+) (0)
colloid-3-amido-Fc
12
colloid-3-silyl-Fc
0.680 (0.050)
0.545 (0.050)
0.545 (0.025)
0.545 (0.025)
1.0
1.0
1.2
1.2
1.0
0.200 (0.130)
0.115 (0.070)
0.135 (0.060)
0.125 (0.050)
0.160 (0.070)
2800 ( 600
96 ( 20
0.170 (0.170)
0.080 (0.070)
0.100 (0.070)
0.090 (0.050)
850 ( 200
24 ( 5
11
colloid-3-silyl-Fc
24
colloid-9-silyl-silyl-Fc
210 ( 40
140 ( 30
600 ( 100
53 ( 10
36 ( 7
25
modified Pt electrode
with the
0.680 (0.0)
∆EFWHM ) 0.100f
0.175 (0.110)
∆EFWHM ) 0.150
1040 ( 200
∆EFWHM ) 0.150
colloid-3-amido-Fc 12
modified Pt electrode
with the
colloid-9-silyl-Fc 24
modified Pt electrode
with the
0.530 (0.0)
∆EFWHM ) 0.060
1.0
1.0
0.120 (0.070)
∆EFWHM ) 0.100
120 ( 30
170 ( 40
0.090 (0.050)
∆EFWHM ) 0.100
36 ( 7
36 ( 7
0.540 (0.0)
∆EFWHM ) 0.060
0.130 (0.040)
∆EFWHM ) 0.100
0.090 (0.060)
∆EFWHM ) 0.100
colloid-9-silyl-silyl-Fc 25
a E1/2 vs FeCp*2; electrolyte, [n-Bu4N][PF6 ]; working electrode, Pt. b Intensity ratio ipc /ipa, whose unity value shows the chemical reversibility and lack
of adsorption. c Difference of E1/2 value between the initial wave and the new wave at half titration in order to observe and compare both waves (see Figures
-
1-3). d Ratio between the apparent association constants K(+)/K(0) of the cationic and neutral forms with the H2PO4 anion. e Ibid with the ATP2- anion.
f Full width of potential at half-maximum.
the other dendronized colloids that bear silylferrocenyl groups.
This corresponds to an apparent association constant K between
the ferrocenium form of 12 and H2PO4- that is 4200 times larger
ibility (ic/ia < 1) or adsorption (ic/ia > 1). The comparison
between the colloid dendronized with the AB3 units and those
dendronized with the AB9 units shows the advantage of the AB9
dendronized colloid with silylferrocenyl groups disclosing a
larger potential shift due to the positive dendritic effect (Figure
SI 5). These two dendrons show identical characteristics for
the recognition of these anions. Given these satisfactory redox
recognition features, titration of [n-Bu4N][H2PO4] can be carried
out. The equivalence points of these titrations can be determined
either from the decrease of intensity of the initial ferrocenyl
than the same constant between the neutral form of 11 and
- 28
H2PO4
.
This ratio is very large compared to that with
monomeric amidoferrocenes (E1/2free - E1/2bound ) 45 mV) and
even that with tripodal tris-amidoferrocenes (E1/2 free - E1/2bound
) 110 mV) and is about as large as that with a nona-
amidoferrocene dendrimer.12a The recognition of H2PO4- is very
-
selective. We have tested other anions (HSO4-, Cl-, Br-, NO3
)
-
which did not provoke the appearance of a new wave or a
significant shift of the ferrocenyl wave. In particular, it is
wave or increase of intensity of the new ferrocenyl-H2PO4
wave at less positive potential. Both variations yield close
results, and the data obtained also corresponded with 5-10%
errors to the amount of redox active species. Indeed, this amount
-
noteworthy that the other oxo-anion HSO4 did not interact
significantly, whereas it is recognized by amidoferrocene
dendrimers and colloid 12.12a
1
was also determined by integration of the H NMR signals, in
With the dendronized colloids 11 containing AB3 units
bearing three silylferrocenyl groups, the difference between the
potentials E1/2 of the initial and new waves is smaller (110 mV)
than that with 12. The chemical reversibility obtained under
ambient condition, however, was encouraging to investigate
further such silylferrocenyl systems with AB9 dendrons and
especially with electrodes modified with dendronized colloids
bearing such large dendrons (vide infra). Gratifyingly, this
difference of potentials appears to be somewhat larger with the
two dendronized colloids bearing these AB9 silylferrocenyl units,
meaning that the recognition is subjected to a positive dendritic
effect. The characteristics of the electrochemical recognition
features by all the dendronized colloids are gathered in Table
1. The potential differences between the two waves show the
magnitude of the recognition. The differences ∆Ep between the
anodic and cathodic peaks of each wave indicate whether the
heterogeneous electron transfer is fast (∆Ep ) 60 mV) or slowed
by the structural reorganization (∆Ep > 60 mV), especially
during the increase of interaction with the anion upon oxidation
of the ferrocenyl unit to ferrocenium. The intensity ic/ia ratio
shows cases for which one is dealing with chemical irrevers-
the dendrons coordinated to the colloidal core (assuming a one-
to-one interaction of the ferrocenyl group with H2PO4- and two-
to-one for ATP2-). All the dendronized colloids gave satisfactory
titration graphs (see all the titration graphs in the Supporting
Information, Figures SI 7 to SI 15), since the separation between
the initial and new wave is relatively large and the intensities
are not much marred by adsorption in the beginning or during
the titration.
(b) Adenosine-5′-triphosphate, ATP2-. The addition of
[n-Bu4N]2[ATP] to the electrochemical cell containing the
dendronized colloid provokes the apparition of a new wave,
just as with the [n-Bu4N][H2PO4] salt. The potential difference
between the initial and new wave is of the same order of
magnitude with those of [n-Bu4N]2[ATP] and [n-Bu4N][H2PO4]
(slightly smaller for [Bu4N]2[ATP] than for [n-Bu4N][H2PO4])
with all the dendronized colloids studied here. Table 1 also
gathers the data of this recognition of [n-Bu4N] [ATP] in the
2
same fashion as for [n-Bu4N][H2PO4]. Figure 1 shows the CVs
in the course of the titrations; that is, both the initial and new
waves are visible at this stage.
Thus, these waves can be compared for the tris-amidoferro-
cenyl-dendronized colloid 12 (Figure SI 6) and a nonaferrocenyl-
dendronized colloid (good wave separation and chemical
(28) Reynes, O.; Moutet, J.-C.; Pecaut, J.; Royal, G.; Saint-Aman E. Chem.s
Eur. J. 2000, 6, 2544.
9
J. AM. CHEM. SOC. VOL. 125, NO. 9, 2003 2623

A R T I C L E S
Daniel et al.
Figure 1. Titration of ATP2- with 24 (colloid 9-silyl-Fc) in CH2Cl2. Cyclic
voltammograms: electrolyte support, 0.1 M [n-Bu4N][PF6]; reference
electrode, Ag; auxiliary and working electrodes, Pt; scan rate, 0.2 V/s;
solution of [n-Bu4N]2[ATP], 5 × 10-3 M; internal reference, FeCp2*. (a)
nanoparticle alone; (b) in the course of the titration (note the two close
waves on the cathodic side); (c) with an excess of [n-Bu4N]2[ATP].
Figure 3. Titration of ATP2- with 24 (colloid-9-silyl-Fc) in the presence
of [n-Bu4N][Cl] and [n-Bu4N][HSO4] (both anions, 5 × 10-2 M, 0.5 equiv
per ferrocenyl branch): cyclic voltammograms (see Figure 1 for the
experimental conditions). (a) nanoparticle 24 alone; (b) in the course of
the titration; (c) with an excess of [n-Bu4N]2[ATP].
Figure 2. Titration of ATP2- with 25 (colloid-9-silyl-silyl-Fc). Decrease
of the intensity of the initial CV wave ([) and increase of the intensity of
the new CV wave (9) vs the number of equiv of [n-Bu4N]2[ATP] added
per ferrocenyl branch. Nanoparticles: 3.8 × 10-6 M in CH2Cl2. See the
caption to Figure 1 for the conditions.
Figure 4. Titration of ATP2- with 24 (colloid-9-silyl-Fc) in the presence
of [n-Bu4N][Cl] and [n-Bu4N][HSO4] (both anions, 0.5 equiv per ferrocenyl
branch). Decrease of the intensity of the initial CV wave ([) and increase
of the intensity of the new CV wave (9) vs the number of equiv of
[n-Bu4N]2[ATP] added per ferrocenyl branch. Nanoparticles: 2 × 10-6
M
in CH2Cl2. See also Figure 1 for the conditions.
reversibility, Figure 1). A key difference between H2PO4- and
ATP2- resides in the stoichiometry obtained in the titration of
ATP2- that is half that of [n-Bu4N][H2PO4] because of the
double anionic charge of ATP2-. This finding is not trivial,
however, since recent ATP2- titration with monoferrocenyl
derivatives led to very different ATP2- stoichiometries.28 An
example of CV obtained during ATP2- titration with the colloid
24 dendronized with the AB9 unit 21 is shown in Figure 1,
whereas the corresponding titration graph is shown in Figure 2
for colloid 25.
The selective recognition and titration of ATP2- can also be
carried out in the presence of [n-Bu4N][Cl] and [n-Bu4N][HSO4]
using colloid 24. The addition of these latter salts to 24 does
not provoke the appearance of a new CV wave or a shift of the
initial CV wave. A new wave appears, upon addition of
[n-Bu4N]2[ ATP], only on the cathodic side at a potential 100
mV less positive than that of the initial wave. On the anodic
side, the initial wave is progressively shifted until the equivalent
point is reached, the total shift along the titration being 50 mV.
These features are original. Figure 3 shows the CV before,
during, and after titration, and Figure 4 shows the titration graph
using the changes of intensities of the initial and new wave.
(c) HSO4-. The interaction of the silylferrocenyl-containing
colloids 11, 24, and 25 with HSO4- is negligible, but recognition
and titration can be carried out using the colloid 12 that bears
the tris-amidoferrocenyl units. The latter provides a stronger
interaction than those of the other colloids with HSO4-. The
9
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Assemblies between Metallodendrons and Gold Nanoparticles
A R T I C L E S
More recently, Cuadrado et al. have extensively studied the
derivatization of silylferrocenyl-terminated and other ferrocenyl
dendrimers.30,31 The only report of modified electrodes with
thiol-gold colloid assemblies functionalized by ferrocenyl
dendrimers, however, is that of Nishihara’s group.32 To test the
possible applications of dendronized colloids, we attempted to
modify platinum electrodes by depositing these polyferrocenyl
dendronized colloids. Previous attempts to do so with amido-
ferrocenylalkylthiol-gold colloids that were recently reported
met with failure, because they resulted in unstable modified
electrodes. Nishihara has prepared modified electrodes with
alkylthiol-gold colloids terminated by biferrocenyl units that
may be considered as AB2 units.32 The seminal works of Crooks
and Tomalia, who prepared dendrimer-colloid assemblies,
showed that such assemblies are stable.1 In the previous
electrochemical study of the polyferrocenyl dendronized gold
colloids, we could note the tendency of these nanoscopic
assemblies to adsorb on electrodes. Indeed, we found that the
adsorption of the dendronized gold colloids was all the better,
as the thiols contained a larger number of ferrocenyl groups.
The dendronized colloids bearing three ferrocenyl groups in AB3
units adsorb better than monoferrocenylalkylthiols and allow
preparing modestly stable electrodes. In this respect, the tris-
amidoferrocenyl branching was found to give better results than
the tri-silylferrocenyl one. The most stable modified electrodes
were those prepared with the dendronized gold colloids contain-
ing either of the nonaferrocenyl dendrons. Indeed, the den-
dronized colloids containing the nonasilylferrocenylthiolate
dendron give excellent modified electrodes, whereas the intensi-
ties and stabilities observed with those containing the trisilyl-
ferrocenylthiolate dendron are much weaker. These modified
electrodes were prepared by simply dipping a platinum electrode
into a CH2Cl2 solution of the dendronized colloid and scanning
the potential back and forth around the ferrocenyl wave. This
scanning provoked the appearance of the classic symmetric CV
wave with the same anodic and cathodic potential disclosing
an increase of its current intensity until saturation was reached
after about fifty scans. These modified electrodes were perfectly
stable (including in air), and they also showed a remarkable
change when the H2PO4- or ATP2- anion was introduced into
the CH2Cl2 solution. Figure 6 shows the progress of the CV
waves from the initial one to the new one.
-
Figure 5. Titration of HSO4 with 12 (colloid-3-amido-Fc): shift of Epa
toward positive potentials recorded by CV as a function of the number of
equiv [n-Bu4N][HSO4] added per amidoferrocenyl branch of the colloid.
Nanoparticles: 4 × 10-5 M in CH2Cl2. See also Figure 1 for experimental
conditions.
addition of [n-Bu4N][HSO4] to the electrochemical cell contain-
ing 12 does not provoke the apparition of a new CV wave,
however, and only a shift of the CV wave is observed. The
maximum shift of 60 mV for the anodic wave potential and 25
mV for the cathodic one are reached around the equivalent point,
which allows titrating [n-Bu4N][HSO4], as shown in Figure 5.
The absolute apparent association constant K+ between 12 and
the anion HSO4 is then defined by log K+c ) ∆E1/2/0.05833
-
at 20 °C, giving K+) (18 ( 4) × 103 L mol-1. The weaker
interaction of the amidoferrocenyl group with HSO4- than that
with H2PO4- has already been discussed.12e It is due to the fact
that the negative charge density on the oxygen atoms is weaker
in HSO4 than in H2PO4-. The H bonding between these O
-
atoms and the positively polarized nitrogen atom of the amido
group dominates the overall H-bonding interaction. The silicon
atom plays this role of the positively polarized nitrogen atom
in the silylferrocenyl-branched dendrons because of the stabi-
lization of a positive charge in the â position relative to the
metal. This difference in H-bonding abilities of the functional
ferrocenyl branch with HSO4- and H2PO4 is seemingly respon-
sible for the observed selectivity.
Modified Electrodes. Electrodes modified by polymers
containing ferrocenyl units have been known for a long time.29
The new wave seen in the presence of one of these anions is
largely shifted to a less positive potential as noted for the
dendronized colloids in solution (see the potential shifts in Table
1 for the modified electrodes with the various dendronized
colloids). The anodic and cathodic peak potentials are no longer
identical; this indicates electrochemical irreversibility, that is,
strong structural rearrangement due to the supramolecular
interactions (hydrogen bonding and especially electrostatic
interaction) in the course of electron transfer. Remarkable
features were the following:
(29) (a) Brown, A. P.; Anson, F. C. Anal. Chem. 1977, 49, 1589. (b) Peerce, P.
J.; Bard, A. J. Electroanal. Chem. 1980, 114, 89. (c) Laviron, E. J.
Electroanal. Chem. 1981, 122, 37. (d) Abruna, H. D. In ElectroresponsiVe
Molecular and Polymeric Systems; Stotheim, T. A., Ed.; Dekker: New York,
1988; Vol. 1, p 97. (e) Murray, R. W. In Molecular Design of Electrode
Surfaces; Murray, R. W., Ed.; Techniques of Chemistry XXII; Wiley: New
York, 1992; p 1. (f) Mora´n, M.; Casado, C. M.; Cuadrado, I. Organo-
metallics 1993, 12, 4327. (g) Audebert, P.; Cerveau, G.; Corriu, R. J. P.;
Costa, N. J. Electroanal. Chem. 1996, 413, 89.
(30) (a) Alonso, B.; Mora´n, M.; Casado, C.; Lobete, F.; Losada, J.; Cuadrado,
I. Chem. Mat. 1995, 7, 1440. (b) Cuadrado, I.; Mora´n, M.; Losada, J.;
Casado, M.; Pascual, C.; Alonso, B.; Lobete, F. In AdVances in Dendritic
Macromolecules; Newkome, G., Ed.; Jai Press: Stanford, CT, 1996; Vol.
3, p 151. (c) Cuadrado, I.; Casado, M.; Alonso, B.; Mora´n, M.; Losada, J.;
Belsky, V. J. Am. Chem. Soc. 1997, 119, 7673.
(31) Reviews on ferrocenyl dendrimers and their electrochemistry:(a) Cuadrado,
I.; Mora´n, M.; Casado, C. M.; Alonso, B.; Losada, J. Coord. Chem. ReV.
1999, 193-195, 395-445. (b) Casado, C. M.; Cuadrado, I.; Mora´n, M.;
Alonso, B.; Garcia, B.; Gonzales, B.; Losada, J. Coord. Chem. ReV. 1999,
185-186, 53.
(32) (a) Biferrocenyl-substituted thiol-nanoparticles deposited on metal sur-
faces: Yamada, M.; Quiros, I.; Mizutani, J.; Kubo, K.; Nishihara, I. Phys.
Chem. Chem. Phys. 2001, 3, 3377. (b) Yamada, M.; Kuzume, A.; Kurihara,
M.; Kubo, K.; Nishihara, H. Chem. Commun. 2001, 2476. (c) Yamada,
M.; Nishihara, H. Chem. Commun. 2002, 2578.
(33) Miller, S. R.; Gustowski, D. A.; Chen, Z.-H.; Gokel, G. W.; Echegoyen,
L.; Kaifer, A. E. Anal. Chem. 1988, 60, 2021.
(i) The stability of these CV waves upon multiple scanning
the potentials around the waves.
-
(ii) The selective recognition of H2PO4 or ATP2- in the
-
presence of other anions, such as HSO4 and Cl-.
(iii) The possibility to selectively wash the salts from these
modified electrodes, leaving only the dendronized colloid on
the electrode surface (Figure 6d). This allowed sensing this anion
again in another solution or another anion. These experiments
9
J. AM. CHEM. SOC. VOL. 125, NO. 9, 2003 2625

A R T I C L E S
Daniel et al.
anions even in the presence of other anions. Their recognition
properties are the subject of dendritic effect; that is, the shift of
the ferrocenyl redox potential observed upon introduction of
the anion is larger as the generation number increases. The
assembly around the core provoking the clusterification of a
large number of peripheral ferrocenyl groups induces the
formation of narrow channels that facilitate the appearance of
microcavities at the dendritic surface for a tighter supramolecular
interaction with the anion. The silylferrocenyl system presents
the advantage, over the amidoferrocenyl one, that its oxidation
is fully reversible, although the provoked shift by addition of
an anion is slightly larger with the amido group than with the
silyl group. Although structural and electronic variations were
designed concerning the size of the tethers and the number of
Lewis-acidic silicon atoms in the dendrons 21 and 23, the
colloids 24 and 25 obtained with these two dendrons give similar
recognition properties.
(c) The fabrication of modified electrodes is very successful
only with the dendronized nanoparticles that contain the
triamidoferrocenyl and nonaferrocenyl dendrons. This specific
property cannot be obtained with the linear thiolate ligands and
only begins to appear more or less weakly with the nanoparticles
dendronized by the thiolate ligand containing the trisilylferro-
Figure 6. Recognition of ATP2- with a Pt electrode modified: with 24
(colloid 9-silyl-Fc). Cyclic voltammograms: (a) 24-modified electrode alone;
(b) in the course of the titration; (c) with an excess of [n-Bu4N]2[ATP]; (d)
after removing ATP2- by washing the 24-modified electrode with CH2Cl2.
See experimental conditions in the caption to Figure 1.
-
cenyl units. These modified electrodes recognize the H2PO4
and ATP2- anions even in the presence of other anions such as
HSO4- and Cl-. HSO4- can also be recognized. Moreover, the
salt of these anions can be easily removed after rinsing simply
by dipping the used electrode a few minutes in CH2Cl2. In this
way, the modified electrode with either of the dendronized
nanoparticles can be reused many times.
show that adsorption is very strong upon scanning around the
ferrocenyl potential area with such dendronized colloids loaded
with a large number of ferrocenyl units, a feature that is very
profitable for practical use of such modified electrodes.
Finally, the Pt electrode modified with 12 is also rather stable
with a larger full width at half-maximun (100 mV) larger than
the 90 mV value above which the adsorbed sites exert repulsive
interactions among one another. This modified electrode also
recognizes HSO4-, the potential being also shifted by 60 mV
at the anode and by 25 mV at the cathode upon addition of this
anion in solution (vide supra), which makes the signal slightly
Experimental Section
The synthesis of the AB3 units from 1 is described in the Supporting
Information. The preparation of the organic intermediates 13-17 was
carried out as reported in ref 21.
Dendron 18. Karstedt catalyst (250 µL) was added to a mixture of
nona-allyl dendron 16 (0.5 g, 0.44 mmol) and ferrocenyldimethylsilane
(1.5 g, 6 mmol) in Et2O, and this reaction mixture was stirred for 4
days at ambient temperature. The solution was filtered on Celite, and
the solvent was removed under vacuum. After chromatographic
separation on silica gel using a pentane/Et2O (95:5) mixture as eluent,
dendron 18 was obtained as an orange oil (1.2 g, 83%). Elemental
analysis calcd for C181H248Si12Fe9O4: H, 7.51; C, 65.33. Found: H, 7.59;
C, 65.11. MALDI TOF mass spectrum, m/z: 3327.08 [M]+ (calcd
-
disymmetrical. As with H2PO4 and ATP2, the modified
-
electrode can be rinsed with CH2Cl2, after recognition of HSO4
,
with minimal loss of adsorbed colloid for reuse. After using
and washing 5 times, half the adsorbed redox active species is
left, but further cycling and washing does not modify this
amount any longer; the modified electrode is then stable.
1
3327.60). H NMR (CDCl3): δppm 7.20 (d, C6H4, 6H); 7.09 (d, C6H4,
2H); 6.87 (d, C6H4, 6H); 6.67 (d, C6H4, 2H); 4.30 (s, C5H4, 2H); 4.09
(s, C5H5, 5H); 4.02 (s, C5H4, 2H); 3.51 (s, CH2O, 6H); 1.61 (m broad,
CH2, 18H); 1.14 (m broad, CH2, 18H); 0.62 (m broad, CH2, 18H);
0.17 (s, SiMe, 54H); 0.08 (s, SiMe, 18H). 13C NMR (CDCl3): δppm
159.38 (Cq, ArO); 152.84 (Cq, ArO); 139.77 (Cq, Ar); 137.17 (Cq, Ar);
127.51 (CHAr); 127.40 (CHAr); 114.65 (CHAr); 113.52 (CHAr); 73.04
(C5H4); 72.91 (Cq, C5H4); 70.69 (C5H4); 68.17 (C5H5); 60.09 (CH2O);
43.13 (Cq-CH2); 41.96 (CH2); 17.90 (CH2); 17.40 (CH2Si); 14.58
(CH2); -2.05 (SiMe); - 4.61 (SiMe).
Dendron 20. A mixture of 18 (0.600 g, 0.180 mmol), K2CO3 (0.076
g, 0.540 mmol), and 4, 4′-dibromomethylbenzene (0.476 g, 0.900 mmol)
in CH3CN/THF (50:50) was stirred for 4 days at 50 °C. After removal
of the solvent under vacuum, the product was extracted with 3 × 30
mL of Et2O, and the solvent was removed under vacuum. The residue
was chromatographed on silica gel using a pentane/ether mixture (20:
80) as eluent, and 20 was obtained as a yellow oil (0.600 g, 95%).
Elemental analysis calcd for C189H255O4Si12BrFe9: H, 7.32; C, 64.66.
Found: H, 7.50; C, 64.33. MALDI TOF mass spectrum, m/z: 3510.07
[M+] (calcd 3510.65). 1H NMR (CDCl3): δppm 7.38 (s, C6H4, 4H); 7.18
Concluding Remarks
(a) Gold nanoparticles have been synthesized with both
dodecanethiolate and the dendronized thiolate that contained
triferrocenyl or nonaferrocenyl units either by the ligand-
substitution procedure (AB3 units) or by direct synthesis from
mixtures of functional and nonfunctional thiols (AB9 dendrons).
The latter route is remarkably efficient for the nanoparticle-
nonaferrocenyl dendronized assemblies when the substitution
procedure is marred by the steric inhibition with large dendrons.
With a gold nanoparticle-nonaferrocenylthiolate assembly, the
colloidal gold core is surrounded by 180 or 360 silylferrocenyl
units, so that these assemblies closely resemble large metallo-
dendrimers in which the core is the gold nanoparticle.
(b) These dendronized gold nanoparticles combine the
advantages of dendrimers and gold nanoparticles as sensors for
the selective recognition and titration of H2PO4 and ATP2-
-
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Assemblies between Metallodendrons and Gold Nanoparticles
A R T I C L E S
(d, C6H4, 10H); 6.86 (d, C6H4, 10H); 5.04 (s, OCH2, 2H); 4.51 (s,
BrCH2, 2H); 4.31 (s, C5H4, 18H); 4.18 (s, C5H5, 45H); 4.02 (s, C5H4,
18H); 1.61 (m broad, CH2, 18H); 1.14 (m broad, CH2, 18H); 0.62 (m
broad, CH2, 18H); 0.17 (s, SiMe, 54H); 0.08 (s, SiMe, 18H). 13C NMR
(CDCl3): δppm 159.30 (Cq, ArO); 156.84 (Cq, ArO); 139. 70 (Cq, Ar);
137. 17 (Cq, Ar); 130.60 (Cq, Ar); 129.10 (Cq, Ar); 128.60 (CHAr);
127.40 (CHAr); 127.10 (CHAr); 114.65 (CHAr); 113.50 (CHAr); 72.97
(C5H4); 71.44 (Cq, C5H4); 70.62 (C5H4); 68.13 (C5H5); 66.10 (CH2O);
60.09 (CH2O); 43.13 (Cq-CH2); 42.22 (CH2); 33.09 (CH2Br); 18.11
(CH2); 17.59 (CH2Si); 15.18 (CH2); -1.85 (SiMe); -4.50 (SiMe).
Dendron 19. A mixture of 17 (0.500 g, 0.160 mmol), K2CO3 (0.045
g, 0.320 mmol), and 4,4′ -dibromomethylbenzene (0.211 g, 0.800 mmol)
in a CH3CN was stirred for 4 days at 50 °C. After removal of the solvent
under vacuum, the product was extracted with 2 × 20 mL of Et2O.
The solvent was removed in a vacuum. After chromatographic
separation on silica gel using a pentane/ether mixture (20:80) as eluent,
the bromobenzyl derivative 19 was obtained as a yellow oil (0.406 g,
77%). Elemental analysis calcd for C180H231O4Si9BrFe9 : H, 7.07; C,
65.63. Found: H, 7.51; C, 66.32. MALDI TOF mass spectrum, m/z:
3294.64 [M+] (calcd 3294.11). 1H NMR (CDCl3): δppm 7.42 (s, C6H4,
4H); 7.15 (d, C6H4, 10H); 6.80 (d, C6H4, 10H); 5.04 (s, OCH2, 2H);
4.51 (s, BrCH2, 2H); 4.30 (s, C5H4, 18H); 4.08 (s, C5H5, 45H); 4.01(s,
C5H4, 18H); 3.87 (s, OCH2, 6H); 1.61 (m broad, CH2, 18H); 1.11 (m
broad, CH2, 18H); 0.59 (m broad, CH2, 18H); 0.15 (s, SiMe, 54H).
13C NMR (CDCl3): δppm 156.50 (Cq, ArO); 156.42 (Cq, ArO); 139.74
(Cq, Ar); 138.90 (Cq, Ar); 130.82 (Cq, Ar); 129. 19 (Cq, Ar); 128.73
(CHAr); 127.82 (CHAr); 127.28 (CHAr); 114.27 (CHAr); 113.55 (CHAr);
72.92 (C5H4); 71.40 (Cq, C5H4); 70.57 (C5H4); 68.20 (C5H5); 68.09
(CH2O); 60.10 (CH2O); 43.07 (Cq-CH2); 42.04 (CH2); 33.13 (CH2-
Br); 29.60 (CH2); 17.95 (CH2); 17.41 (CH2Si); -2.03 (SiMe).
Dendron 21. A mixture of 19 (0.396 g, 0.120 mmol) and NaSH
(0.068 g, 1.200 mmol) in THF was stirred for 24 h at 50 °C. After
removal of the solvent under vacuum, the reaction product was extracted
with 2 × 20 mL of Et2O and chromatographed on a silica column using
Et2O, providing 21 as a yellow-orange oil (0.370 mg, 0.114 mmol,
95%). Elemental analysis calcd for C180H232O4Si9Fe9S: H, 7.20; C,
66.58. Found: H, 7.61; C, 66.91. MALDI TOF mass spectrum, m/z:
3247.27 [M+] (calcd 3445.60). 1H NMR (CDCl3): δppm 7.38 (s, C6H4,
4H); 7.15 (d, C6H4, 8H); 6.69 (d, C6H4, 8H); 5.01 (s, OCH2, 2H); 4.29
(s, C5H4, 2H); 4.08 (s, C5H5, 5H); 4.01 (s, C5H4, 2H); 3.87 (s, OCH2,
6H); 3.60 (d, HSCH2, 2H); 1.60 (m, CH2, 18H); 1.13 (m, CH2, 18H);
0.60 (m, CH2, 18H); 0.16 (s, SiCH3, 54H). 13C NMR (CDCl3): δppm
156.70 (Cq, ArO); 156.42 (Cq, ArO); 139.74 (Cq, Ar); 138.90 (Cq, Ar);
130.80 (Cq, Ar); 129. 20 (Cq, Ar); 128.70 (CHAr); 127.82 (CHAr); 127.28
(CHAr); 114.26 (CHAr); 113.55 (CHAr); 73.04 (C5H4); 71.43 (Cq, C5H4);
70.70 (C5H4); 68.21 (C5H5); 66.01 (CH2O); 60.10 (CH2O); 43.05 (Cq-
CH2); 42.00 (CH2); 35.50 (HSCH2); 29.60 (CH2); 17.95 (CH2); 17.41
(CH2Si); -1.88 (SiMe).
(0.277 mg, 0.080 mmol, 94%). Elemental analysis calcd for C189H256O4-
Si12Fe9S: H, 7.45; C, 65.54. Found: H, 7.88; C, 65.90. MALDI TOF
mass spectrum, m/z: 3463.66 [M+] (calcd 3463.81); 6927.21 [2M+]
1
(calcd 3927.62). H NMR (CDCl3): δppm 7.38 (s, C6H4, 4H); 7.18 (d,
C6H4, 8H); 6.87 (d, C6H4, 8H); 5.01 (s, OCH2, 2H); 4.29 (s, C5H4,
2H); 4.08 (s, C5H5, 5H); 4.01 (s, C5H4, 2H); 3.60 (d, HSCH2, 2H);
3.51 (s, OCH2, 6H); 1.59 (m, CH2, 18H); 1.13 (m, CH2, 18H); 0.61
(m, CH2, 18H); 0.16 (s, SiCH3, 54H); 0.07 (s, SiCH3, 18H). 13C NMR
(CDCl3): δppm 158.86 (Cq, ArO); 156.60 (Cq, ArO); 140.30 (Cq, Ar);
138.10 (Cq, Ar); 136.53 (Cq, Ar); 129.23 (Cq, Ar); 127.75 (CHAr); 127.10
(CHAr); 113.29 (CHAr); 72.95 (C5H4); 71.43 (Cq, C5H4); 70.60 (C5H4);
68.11 (C5H5); 66.11 (CH2O); 60.10 (CH2O); 43.04 (Cq-CH2); 42.07
(CH2); 35.50 (HSCH2); 17.96 (CH2); 15.18 (CH2); -2.02 (SiMe); -
4.64 (SiMe).
Ligand Substitution in Alkylthiol-Gold Nanoparticles. A CH2-
Cl2 (20 mL) solution of alkylthiol-gold nanoparticles (0.080 g, 10-6
mmol) and tris-ferrocenyl thiol dendron (see amounts later) was stirred
under positive nitrogen pressure at room temperature. After 3 days, the
solvent was evaporated under reduced pressure. The dark brown product
was washed 3 times with 10 mL of methanol and then 3 times with 10
mL of acetone in order to remove the noncoordinated thiols, the desired
colloids being not soluble in these two solvents (the washing solvents
were finally colorless). The black solid was dried under vacuum.
(a) Dendron 11 (0.080 g, 0.073 mmol) gives 0.065 g of 11 (85%
1
yield) and 4.8% of substitution in alkylthiol-gold nanoparticles. H
NMR (250 MHz, CDCl3) δppm: 7.33 (CH(C6H4CH2S)); 7.21 (CH-
(C6H4O)); 6.91 (CH(C6H4O)); 4.94 (CH2O); 4.31 (CH(C5H4Si)); 4.10
(Cp); 4.03 (CH(C5H4Si); 3.51 (SCH2-arom.); 1.27 (CH2 alkylthiol);
0.89 (CH3 alkylthiol); 0.62 (CH2Si); 0.08 (CH3Si). 13C NMR (62.9 MHz,
CDCl3) δppm: 156.41 (C(C6H4O)); 142.13 (C(SCH2(C6H4)CH2); 133.01
(CH(C6H4CH2S)); 132.3 (CH(C6H4O)); 112.4 (CH(C6H4O)); 72.5 (CH-
(C5H4Si)); 70.5 (CH(C5H4Si)); 69.6 (CH(Cp)); 63.8 (CH2O); 31.90-
29.4 (CH2 alkylthiol); 28.82 (SCH-arom.); 22.56 (CH2 alkylthiol);
17.92 (CH2C-arom.); 15.23 (CH2CH2Si); 14.1 (CH3 alkylthiol); 3.11
(CH2CH2Si); -4.0 (CH3Si). E1/2 (V vs Fc; CH2Cl2; 20 °C) 0.00 (r)
(see text).
(b) Dendron 12. (0.080 g, 0.063 mmol) gives 0.073 g of 12 (90%
yield) and 3% of ligand substitution in alkylthiol-gold nanoparticles.
1H NMR (250 MHz, CDCl3) δ (ppm): 7.37 (CH(C6H4CH2S)); 7.15
(CH(C6H4O)); 6.91 (CH(C6H4O)); 4.64 (CH2(C5H4CO)); 4.31 (CH-
(C5H4CO)); 4.19 (Cp); 3.62 (SCH2-arom.); 2.86 (SiCH2N); 1.27 (CH2
alkylthiol); 0.89 (CH3 alkylthiol); 0.08 (CH3Si). 13C NMR (62.9 MHz,
CDCl3) δ(ppm): 170.00 (CONH); 156.41(C(C6H4O)); 139.13 (C(SCH2-
(C6H4)CH2)); 129.71 (CH(C6H4CH2S)); 127.3 (CH(C6H4O)); 114.4
(CH(C6H4C)); 70.0 (CH(C5H4CO)); 69.6 (CH(Cp)); 67.9 (CH(C5H4-
CO)); 42.0 (SiCH2N); 32.0-29.4 (CH2 alkylthiol); 28.7 (SCH-arom.);
22.6 (CH2 alkylthiol); 17.7 (CH2C-arom.); 15.2 (CH2CH2Si); 14.1 (CH3
alkylthiol); -4.0 (CH3Si). IR (KBr, cm-1): νCON 1625.3, 1542.5. E1/2
(V vs Fc; CH2Cl2; 20 °C) 0.145 (r) (see text and Scheme 4).
Reduction of Disulfides to Thiols. The dendron 21 oxidized by air
to disulfide 22 (250 mg, 0.038 mmol) was dissolved with 10 mL of
DMF in a Schlenk flask. Water (70 µL, 100 equiv) was added, then
the reaction mixture was degassed, tris-n-butylphosphine (100 µL, 10
equiv) was introduced, and the mixture was stirred at room temperature
for 3 h. Then, 50 mL of ethyl acetate was added, and the mixture was
washed with 1 N HCl. The organic layer was separated, degassed, dried
under Na2SO4, and filtered. The solvent was then removed under
vacuum, and the solid residue was rinsed under positive nitrogen
pressure using degassed petroleum ether. The orange solid thiol 21 (220
mg, 0.067 mmol, 88%) was dried under vacuum. The same procedure
was applied to all the thiol dendrons just before the synthesis of
dendronized nanoparticles.
Direct Brust Colloid Synthesis. General Method. (a). A colorless
solution of N(n-C8H17)4Br (0.524 g, 0.959 mmol) in 10 mL of toluene
was added to a yellow water solution (10 mL) of HAuCl4 (0.093 g,
0.274 mmol). The mixture was stirred under positive nitrogen pressure,
and separation between the red organic phase (top) and colorless
aqueous phase (bottom) resulted. A mixture of dodecanethiol C12H25-
SH (0.028 g, 0.137 mmol) and dendronized-thiol containing the tri-
silyl ferrocenyl unit (0.150 g, 0.137 mmol) in 10 mL of toluene was
added to the organic phase. Then, NaBH4 (0.114 g, 3.04 mmol) in 10
mL of water was slowly added to the stirred reaction mixture. The red
color turned to black brown, and the reaction mixture was vigorously
stirred for 3 h. The organic phase was separated from the aqueous phase,
its volume was reduced to 3 mL, and 100 mL of ethanol was added.
The mixture was kept at -20 °C for 12 h. The resulting dark brown-
black precipitate was filtered on Celite and then washed twice with
ethanol and twice with acetone to remove excess thiol. The crude
product was dissolved in CH2Cl2 and precipitated again with methanol.
Dendron 23. A mixture of 20 (0.300 g, 0.085 mmol) and NaSH
(0.048 g, 0.854 mmol) in THF was stirred for 24 h at 50 °C. After
removal of the solvent under vacuum, the reaction product was extracted
with 2 × 20 mL Et2O and chromatographed on a silica column using
a pentane/Et2O (90:10) mixture, providing 23 as a yellow-orange oil
9
J. AM. CHEM. SOC. VOL. 125, NO. 9, 2003 2627

A R T I C L E S
Daniel et al.
The dark black colloid containing the mixture of ligands C12H25S/
dendron-thiol tri-silyl ferrocene ) 75:25 (ratio determined by 1H NMR)
was then dried under vacuum, which gave 0.085 g. 1H NMR (250 MHz,
Elemental analysis found for 24: S, 2.25; Au, 42.80. Atomic ratio
Au/S ) 3.1. HRTEM: average diameter: 2.8 ( 0.5 nm. Number of
gold atoms per core: 676. ns ) 218. Proportion of dendron 21 ) 20%.
The average number of dendron 21 per particle is ndendron21 ) 43.6;
nalkylthiolate )174.4. MW ) 309 806 g/mol.
CDCl3) δppm
: 7.33 (CH(C6H4CH2S)); 7.21 (CH(C6H4O)); 6.91
(CH(C6H4O)); 4.94 (CH2O); 4.31 (CH(C5H4Si)); 4.10 (Cp); 4.03
(CH(C5H4Si)); 3.51 (SCH2-arom.); 1.27 (CH2 alkylthiol); 0.89 (CH3
alkylthiol); 0.62 (CH2Si); 0.08 (CH3Si). 13C NMR (62.9 MHz, CDCl3)
General Method for the Titration of H2PO4 or ATP2-: First,
-
[n-Bu4N][PF6] was introduced in the electrochemical cell (that contained
the working electrode, the reference electrode, and the counter electrode)
and dissolved in freshly distillated dichloromethane. A blank voltam-
mogram was recorded without colloid in order to check the working
electrode. Then, the colloid was solubilized in a minimum of dichlo-
romethane and added into the cell. About 1 mg (3 × 10-6 mol) of
decamethylferrocene was also added. After the solution was degassed
by dinitrogen flushing, the CV of the nanoparticle alone was recorded.
Then, the anion H2PO4- or ATP2- was added by small quantities using
a microsyringe. After each addition, the solution was degassed, and a
CV was recorded. The appearance and progressive increase of a new
wave was observed while the initial wave decreased and finally
disappeared (see all the titration graphs in the Supporting Information).
When the initial wave had completely disappeared, addition of the salt
of the anion was continued until reaching twice the volume already
δppm
: 156.41 (C(C6H4O)); 142.13 (C(SCH2(C6H4)CH2)); 133.01
(CH(C6H4CH2S)); 132.3 (CH(C6H4O)); 112.4 (CH(C6H4O)); 72.5(CH-
(C5H4Si)); 70.5 (CH(C5H4Si)); 69.6 (CH (Cp)); 63.8 (CH2O); 31.90-
29.4 (CH2 alkylthiol); 28.82 (SCH-arom.); 22.56 (CH2 alkylthiol);
17.92 (CH2C-arom.); 15.23 (CH2CH2Si); 14.1 (CH3 alkylthiol); 3.11
(CH2CH2Si); -4.0 (CH3Si).
(b). A reaction between HAuCl4 (0.031 g, 0.092 mmol), N(n-C8H17)4-
Br (0.176 g, 0.322 mmol), C12H25SH (0.0094 g 0.046 mmol) dendron
21 (0.150 g, 0.046 mmol), and NaBH4 (aq) solution (0.038 g, 1.00
mmol) was carried out as in procedure (a) which gave nanoparticle
1
24. H NMR integration indicated that the ratio of ligands C12H25S
and 21 was 80:20. 1H NMR (250 MHz, CDCl3) δppm: 7.33 (CH(C6H4-
CH2S)); 7.21 (CH(C6H4O)); 7.01 (CH(C6H4CH2S)); 6.91 (CH(C6H4O));
5.05 (CH2O); 4.29 (CH(C5H4Si)); 4.10 (Cp); 4.01 (CH(C5H4Si)); 3.87
(O-CH2); 1.90 (CH2); 1.27 (CH2 alkylthiol); 0.89 (CH3 alkylthiol);
0.62 (CH2Si); 0.16 (CH3Si). 13C NMR (62.9 MHz, CDCl3) δppm: 155.91
(Cq ); 139.58 (Cq ); 127.15 (CHAr); 113.40 (CHAr); 72.68 (CH (C5H4-
-
introduced. The titration of ATP2- in the presence of Cl- and HSO4
was carried out similarly, the salts [n-Bu4N][Cl] and [n-Bu4N][HSO4]
being added before [n-Bu4N]2[ATP].
Ar
Ar
Si)); 70.5 (CH (C5H4Si)); 69.6 (CH (Cp)); 65.52 (CH2O); 42.94 (Cq-
CH2); 41.89 (CH2); 29.4 (CH2 alkylthiol); 17.82; 17.28; 15.03 (CH2);
14.30 (CH3 alkylthiol); -2.14.0 (CH3Si).
(c). Procedure (a) applied to the mixture of HAuCl4 (0.029 g, 0.086
mmol), N(n-C8H17)4Br (0.165 g, 0.301 mmol), C12H25SH (0.009 g 0.043
mmol), dendron 23 (0.150 g, 0.043 mmol), and aq NaBH4 solution
(0.036 g, 0.95 mmol) gave nanoparticle 25. The proportion of ligands
C12H25S and 23 in the mixture was 90:10. 1H NMR (250 MHz, CDCl3)
Modification of Electrodes with the Dendronized Gold Nano-
particles. A platinum electrode (Sodimel, Pt 30) was dipped into 10%
aq HNO3 for 3 h, then rinsed with distilled water, dried in air, and
polished using cerium oxide pulver (5 MU). The nanoparticles were
electrodeposited onto such platinum-disk electrodes (A ) 0.078 cm2)
from degassed CH2Cl2 solutions of metallodendron gold nanoparticles
(10-6 M) and [n-Bu4N][PF6] (0.1 M) by continuous scanning (0.10 V
s-1) up to 50 cycles between 0.0 and 0.80 V vs FeCp*2. The coated
electrode was washed with CH2Cl2 in order to remove the solution of
material and dried in air. This modified electrode was characterized
by CV in freshly distilled CH2Cl2 as containing only the supporting
electrolyte. It showed a single symmetrical CV wave, and the linear
relationship of the peak current with potential sweep rate was verified.
The surface coverage Γ (mol cm-2) by the dendronized gold nanopar-
ticles was determined from the integrated charge of the CV wave. Γ )
Q/nFA, where Q is the charge, n is the number of electrons transferred,
F is the Faraday constant, and A is the area. Thus, the surface coverage
for the electrode modified with 12 was 2.3 × 10-10 mol cm-2
(ferrocenyl sites), corresponding to 1.7 × 10-11 mol cm-2 of colloid-
3-amido-Fc. The coverage surface Γ for the electrode modified with
24 was 5.6 × 10-10 mol cm-2 (ferrocenyl sites) or 1.55 × 10-12 mol
cm-2 of colloid-9-silyl-Fc 24. The nanoparticle 25 electrodeposited onto
the Pt-disk electrode showed a surface coverage of Γ ) 1× 10-10 mol
cm-2, corresponding to a number of colloid-9-silyl-silyl-Fc 25 of 5.5
δppm: 7.33 (CH(C6H4CH2S)); 7.15 (CH(C6H4O)); 6.55 (CH(C6H4O));
5.05 (CH2O); 4.29 (CH(C5H4Si)); 4.08 (Cp); 4.01 (CH(C5H4Si)); 3.52
(OCH2); 1.58 (CH2); 1.27 (CH2 alkylthiol); 1.14 (CH2); 0.89 (CH3
alkylthiol); 0.62 (CH2Si); 0.16 (CH3Si). 13C NMR (62.9 MHz, CDCl3)
δppm: 158.96 (C(C6H4O)); 139.45 (Cq(SCH2(C6H4)CH2); 127.19 (CHAr);
113.39 (CH (C6H4O)); 72.90 (CH(C5H4Si)); 70.57 (CH (C5H4Si)); 68.07
(CH (Cp)); 60.15 (CH2O); 43.12 (Cq-CH2); 42.16 (CH2); 29.93 (CH2
alkylthiol); 18.04, 17.52 (CH2); 14.21 (CH3 alkylthiol); 3.11 (CH2CH2-
Si); -1.19 (CH3Si).
Determination of the Number of Ligands in the Dendronized
Gold Nanoparticles. Elemental analysis found for 11: S, 3.41; Au,
54.68. Atomic ratio Au/S ) 2.6. HRTEM: average diameter ) 2.3 (
0.4 nm. Number of gold atoms per core: 375. ns ) 144. Proportion of
dendron 8 ) 25%. Average number of dendron 8 per particle: ndendron8
) 36; nalkylthiolate ) 108. MW ) 135 072 g/mol.
Elemental analysis found for 12: S, 4.39; Au, 67.52. Atomic ratio
Au/S ) 2.5. HRTEM: average diameter ) 2.3 ( 0.7 nm. Number of
gold atoms per core: 375. ns ) 150. Proportion of dendrons 10 ) 3%.
Average number of dendron 10 per particle: ndendron10 ) 4.5; nalkylthiolate
) 145.5. MW ) 108 868 g/mol.
× 10-13 mol cm-2
.
Acknowledgment. We are grateful to Prof. J.-C. Moutet
(University of Grenoble) for helpful discussions and the Institut
Universitaire de France (IUF, grant to D. A.), the Centre
National de la Recherche Scientifique (CNRS), the Ministe`re
de la Recherche et de la Technologie (MRT, Ph. D. grant to
M.-C. D.), and the Universities Bordeaux I and Paris VI for
financial support.
Supporting Information Available: Syntheses of the AB3
derivatives leading to thiols containing three ferrocenyl groups;
histograms and TEM pictures of the dendronized gold nano-
particles (Figures SI 1 to SI 4) and CVs (Figures SI 5 and SI 6)
and graphs of current variations (Figures SI 7 to SI 13) for the
anion titrations by the dendronized gold nanoparticles. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Elemental analysis found for 25: S, 2.53; Au, 57.68. This elemental
analysis provides the atomic ratio Au/S ) 3.7 ) X. HRTEM: average
diameter D (gold core): 2.9 ( 0.5 nm. Number of gold atoms per
core:34 NAu ) 4πR3/3νg ) 4π(D/2)3/51 ) 751 Αu atoms per particle
in average (νg ) 17 Å3 for a gold atom). The number ns of thiolate
ligand per particle can then be deduced: ns ) NAu/X ) 751/3.7 ) 203.
1
The proportion of dendron 23 is given by the H NMR spectrum of
25: 10%. The average number of dendron 23 per particle is ndendron23
)
10/100 × 203 ) 20.3 The average number of remaining dodecanethi-
olate ligands is nalkylthiolate ) 203 - 20.3 ) 182.7. The average molecular
weight per dendronized particle is MW ) NAu × 196.97 +
ndendronMWdendron + nalkylthiolateMWalkylthiolate ) 254 157 g/mol.
(34) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem.
1995, 99, 7036.
JA021325D
9
2628 J. AM. CHEM. SOC. VOL. 125, NO. 9, 2003