Chemical modification of monolayer-protected gold nanoparticles using hyperbaric conditions

Article abstract of DOI:10.1021/ja070828m

Sluggish or impractical Diels-Alder reactions of a maleimide modified-MPN with a series of dienes of varying electronic and steric demands at ambient pressure and temperature proceed under hyperbaric conditions in less than 10 min without altering the gold core of the nanoparticle or its inherent physical properties. Copyright

Full text of DOI:10.1021/ja070828m

Published on Web 03/31/2007
Chemical Modification of Monolayer-Protected Gold Nanoparticles Using
Hyperbaric Conditions
Jun Zhu, Michael D. Ganton, Michael A. Kerr, and Mark S. Workentin*
Department of Chemistry, The UniVersity of Western Ontario, London, Ontario, Canada N6A 5B7
Received February 5, 2007; E-mail: mworkent@uwo.ca
Scheme 1. Preparation of the Maleimide Modified MPN via the
Retro-Diels-Alder Reaction of the Furan-Protected Maleimide
The ability to perform interfacial reactions on monolayer-
protected gold nanoparticles (MPNs) to introduce new functionality
is a key step for their use in applications. Such reactions may be
important for altering the physical or chemical properties of the
MPN or be the chemical recognition event itself.1-7 Interfacial
organic reactions in the MPN environment are often substantially
different than the corresponding solution-phase reactions and for
this reason cannot necessarily be used effectively to modify the
MPN without first examining and understanding the factors that
control the reaction.
The Diels-Alder reaction has been used effectively for the
chemical modification of two-dimensional self-assembled mono-
layer (SAM) surfaces, especially on gold. The reaction has been
used to explore fundamental physical organic interfacial reaction
phenomena8 and used practically for the immobilization of biologi-
cal substrates onto the surfaces.9,10 The use of the Diels-Alder
reactions for modification of MPNs has not yet been examined in
as much detail.11,12 Recently we published a method to prepare a
mixed maleimide-dodecanethiolate modified monolayer-protected
gold nanoparticle (2-C12MPN) and showed that it could serve as a
dienophile and react with furan via a Diels-Alder reaction. Further,
we demonstrated that 2-C12MPN reacts with a corresponding furan
modified-MPN for the reVersible assembly of a 3D network of
MPNs.12 This study relied on the well-known Diels-Alder reaction
of maleimide with furan, which is reversible at slightly elevated
temperatures.13 While the reverse reaction on the MPN is rapid, a
major drawback for the interfacial Diels-Alder reaction is that
the forward process is very slow; in the case of the reaction of
2-C12MPN with a large excess of furan it took over 5 days at
ambient conditions.
Recognizing that the Diels-Alder reaction is subject to rate
acceleration under high pressure because of its negative volume of
activation,14 we report the interfacial Diels-Alder reaction of a
2-C12MPN with a series of dienes of varying steric and electronic
demand under hyperbaric conditions. Reactions that did not pro-
ceed or proceeded only after days on the MPN at ambient temper-
ature and pressure were completed in less than 10 min at 25 °C at
11000 atm, with no change in the gold core size. As far as we are
aware the use of high pressure to accelerate reactions on metal
surfaces is unprecedented; recently high-pressure was used to
activate Diels-Alder reactions for the functionalization of single-
wall carbon nanotubes.15 The results here suggest that hyperbaric
conditions offer a powerful route for interfacial chemical modifica-
tion of organic-modified metal surfaces with appropriate reactions
that are otherwise sluggish or impractical.
(1) was incorporated onto the C12-MPN using the standard place-
exchange method,17 specifically using C12-MPN/1 in a 1:1.5 ratio
(Scheme 1). Because maleimide reacts rapidly with thiols in a
Michael-type reaction a 12-maleimide-dodecanethiol cannot be
place-exchanged directly. Further, the mixed ligand MPN is
prepared because the MPN containing only maleimide ligands has
limited solubility in the solvents used. The maleimide containing
MPN (2-C12MPN) is liberated by heating the 1-C12MPN in toluene
to initiate a retro-Diels-Alder reaction and releasing furan into
solution. The furan was then washed away with 95% ethanol to
leave 2-C12MPN. This protocol is used to prepare 2-C12MPN as
needed, because 1-C12MPN is stable over long periods. The
2-C12MPN was characterized using 1H NMR spectroscopy, which
showed the absence of free furan, TEM (2.5 ( 0.5 nm), TGA (23%
1
organic content), and IR spectroscopy. The H NMR spectrum of
2-C12MPN, provided in the Supporting Information, shows the
characteristic broad NMR signals observed for MPNs. Notable in
the spectrum of 2-C12MPN are the olefinic protons of the maleimide
moiety at 5.80 ppm and the -CH2- methylene protons R to the N
at 3.40 ppm. The intense broad signal from ca. 1.1-2.0 ppm is
mostly due to CH2 resonances in the alkyl chains, and the smaller
broad signal at 0.9-1.0 ppm is due to the terminal CH3 group of
the C12 ligand. The ratio of maleimide/C12 is estimated to be 1:2.2
on the particle, with slight variation from batch to batch depending
on place-exchange conditions.12
The prepared 2-C12MPN was then mixed with a 15 times molar
excess of the appropriate diene (a-m) and dissolved in methylene
chloride as the solvent. This mixture was then separated into two.
One portion was purged with argon and left at ambient conditions
(1 atm, 25 °C) and the other was placed in an argon purged Teflon
tube and placed in a high-pressure reactor and subjected to pressures
of ca. 162 Kpsi, which is equivalent to 1120 MPa, or 11000 atm.
The minimum reaction time of the latter is ca. 10 min because of
the time restrictions imposed by pressurization and depressurization.
The 2-C12MPN was prepared using the method we recently
reported.12 Briefly, dodecanethiolate modified nanoparticles
(C12-MPN) were prepared using the Brust-Schiffrin method by
mixing an organic solution of a 1:1 ratio of dodecanethiol/hydro-
gen tetrachloroaurate with TOAB and 10 fold excess of NaBH4.16
Next, the Diels-Alder protected furan-maleimide dodecyl thiol
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J. AM. CHEM. SOC. 2007, 129, 4904-4905
10.1021/ja070828m CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Times for Completion for the Diels-Alder Reactions of
2-C12MPN and N-dodecylmaleimide with Dienes a-m under
Ambient and Hyperbaric Conditionsa
to be the kinetic endo product; this is typical under high-pressure
conditions. In the case of furan, the corresponding exo-endo isomers
are easily identified and their ratio established. Noteworthy is that
the extreme pressure conditions do not affect the core size of the
MPN or otherwise alter its properties. The former was demonstrated
in part by comparing TEM of the samples before and after being
exposed to high-pressure conditions for over 24 h: the average
core sizes did not change from the 2.5 ( 0.5 nm determined prior
to exposure to hyperbaric conditions.
In summary, we demonstrate that hyperbaric pressure conditions
can be used to facilitate interfacial Diels-Alder reactions in gold
MPN environments that do not proceed efficiently at ambient pres-
sure. Because the dramatic reaction acceleration using high pressure
is unprecedented on metal nanoparticle surfaces the results suggest
that hyperbaric conditions can be used to facilitate other reactions
subject to hyperbaric acceleration (or retardation) in the MPN
environment and be exploited to covalently link carbohydrates,
proteins, nucleic acids, and other biomolecules using a Diels-Alder
cycloaddition reaction. We are currently pursuing these ideas.
diene
conditions a
conditions b
conditions c,d
a
b
c
d
e
f
g
h
i
70% in 7 d
1 h (0 °C)
35 h
14 d
3 d
3 d
NR
40 h
10% in 7 d
24 h
7 db
3 d (0 °C)
6 d
NR 21 d
70% in 7 d
60% in 7 d
NR
6 d
NR in 7 d
6 d
2 d
NR in 7 d
NR in 7 d
10 min
10 min
10 min
10 min
10 min
10 min
NR 24 h
10 min
10 min
10 min
10 min
10 min
10 min
j
k
l
10 h
7 d
50% in 21 d
m
a Times indicate to reaction completion (or as indicated as determined
Acknowledgment. The authors thank the Natural Science and
Engineering Research Council (NSERC) and The University of
Western Ontario (ADF) for support of this work.
1
by H NMR spectroscopy; NR ) no reaction. b Reaction run using 3000
times excess furan.
Supporting Information Available: General experimental details,
details of the synthesis and characterization of 1-C12MPN, 2-C12MPN
and dodecylmaleimide; 1H NMR spectra of 3-C12MPN and character-
ization details of all Diels-Alder cycloadducts from dodecylmaleimide
with the corresponding dienes. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
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It should be noted that the reactions were not carried out at elevated
temperature because high temperatures can lead to the decomposi-
tion of the MPN or stripping of ligands. For comparison, and to
provide NMR spectra of the Diels-Alder adducts to compare to
those obtained with the reactions carried out on the MPN, each
diene was also mixed with N-dodecylmaleimide in CH2Cl2 and
subjected to ambient and high-pressure conditions.
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The progress of the reactions was monitored by 1H NMR
spectroscopy by the appearance of signals from the cycloadduct
products and the disappearance of the signals due to the maleimide
moiety. Absence of the latter was taken as completion of the
reaction.18 The results are summarized in Table 1. All dienes react
much more readily with the model N-dodecylmaleimide (conditions
a) than toward 2-C12MPN at ambient conditions (conditions b). In
some cases there was no reaction toward 2-C12MPN after more
than a week. Even the electron rich Danishefsky’s diene (k) took
2 days to react with 2-C12MPN, whereas reaction with N-
dodecylmaleimide was complete in about 10 h at 25 °C. The
environment of the MPN must impose unique constraints on the
orientation required for reaction. As expected, electron deficient
or more sterically hindered dienes, such as d, i, l, m, reacted more
sluggishly. Reactions of N-dodecylmaleimide with the same dienes
at 11000 atm (conditions c) were completed in 10 min or less.
Incredibly, the hyperbaric reactions of 2-C12MPN were also
completed in 10 min or less (conditions d). Of the dienes
investigated only 2,4-dimethyl-2,4-hexadiene (g) did not react at
high pressures, presumably because of the high steric demands.
1H NMR spectra of each of the 3-C12MPN adducts obtained are
provided in the Supporting Information along with the spectral
details of the corresponding reaction with the model dodecyl-
maleimide. The main and often exclusive stereoisomer was found
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(18) The extent of reaction was followed by 1H NMR spectroscopy. Reactions
were stopped at various times, the excess diene was removed by washing
with 95% ethanol and acetonitrile and the NMR spectrum of the MPNs
were measured. Reactions were deemed complete when the signals from
the maleimide were absent.
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