point assuming that the decrease from the Au NP bleaching at
35 nm was proportional to the decrease at 400 nm using the
reported cooperative effects displayed on Au NP surfaces. In
addition, we can accelerate the catalytic rate of the hydrolysis
of MeP by a factor of B2 by irradiating the plasmon absorp-
tion band of the gold NP with a low power green laser. While
we have only focused on 10 nm AuNPs, in the scientific
literature a large effort has been place on optimizing plasmonic
heating by changing the size, shape and morphology of gold
5
ratio of A400 nm/A535 nm at time = 0. In the absence of MeP,
this ratio changes very little with time using laser excitation.
In Fig. 2C, the initial rates of product formation normalized to
the Cu concentration are shown for a number of experiments.
There are several observations that stand out. First, the initial rate
of hydrolysis of MeP normalized to [Cu(bpy)] increases by
B2 fold compared to the catalyst, 4, in solution when not attached
to Au NPs. One explanation, which is consistent with recent
reports of multiple-metal centers on Au NP surfaces would
suggest that the enhancement in catalysis rate originates from
the cooperativity of two metal centers acting on a single
2
3
NPs which should add an extra dimension to exploring new
catalysts driving by plasmonic properties of nanoparticles.
For future work, we will be looking at, (i) substrate concen-
tration effects focusing on Michaelis–Menten kinetics,
(ii) cooperative effects by varying the ratio of the bpy ligand
to the PEG ligand, (iii) bulk temperature effects and (iv) gold
NP size effects on plasmon-assisted catalysis.
2
0
substrate. However, 4 is a poorer catalyst in water compared
21
to smaller Cu bipyridine complexes for hydrolysis of MeP which
may be due to the steric effect of a long alkane chain tail on the
bpy ligand. None-the-less, the use of gold nanoparticle supports
should allow us to explore both multi-metallic and multi-
component cooperative effects on catalysis.
Notes and references
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Secondly, the rate is accelerated by another factor of B2 by
irradiating the plasmon absorption band of the gold NP with a
low power green laser at 532 nm. This acceleration does not
occur if the Cu(bpy) catalyst is not attached to the Au NPs
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(
Fig. 2C). We also compared the reaction rates when changing
the sequence of laser excitation. As shown in Fig. 2C, the rates
with laser excitation or rates without excitation are similar
regardless of their order suggesting that bleaching of the Au
NP plasmon band does not affect the reactivity properties of
the attached Cu(bpy) catalyst. This is further supported by the
fact that with laser excitation, the initial rate of product
formation is linear over the same time course as the bleaching
occurrence.
6
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The phenomenon of the acceleration for the initial rate of
hydrolysis of MeP by excitation of the plasmon absorption
band of the AuNPs could have several mechanistic explana-
tions. For example, energy transfer sensitizing the Cu metal
center could create an excited d–d state which might accelerate
the rate of hydrolysis if a dissociative step at the metal center is
rate limiting. Another explanation maybe the reduction of
Cu(II) to Cu(I) by the AuNPs creating a more active catalyst.
However, the simplest explanation is the creation of localized
heating of microenvironment near the Au NP surface. Since
we see an increase in rate by a factor of B2, an increase by
330–2333.
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0 degrees of the microenvironment would be a common
prediction by the Arrhenius equation for this type of reaction.
Au nanoparticles are good light-to-heat converters with
excitation of their plasmon absorption bands dissipating
2
2
energy in the microenvironment.
In conclusion, we have been able to attach Cu catalysts
based on bipyridine ligands to 10 nm gold nanoparticles using
alkane thiol chemistry and have demonstrated that the catalysis
activity of the Cu complexes remain intact with a possible
enhancement in rate for the hydrolysis of MeP consistent with
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This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4121–4123 4123