![Figure 4. a) Representation of the reversible activation of Au NP 1 driven by the electrochemically controlled release and deposition of Cu2+-ions on the electrode. b) Rate of HPNPP transphosphorylation after two cycles of activation/deactivation of Au NP 1. The control experiment indicates the observed rate when Au NP 1 was just activated once by the electrochemical release of Cu2+ at the start of the experiment. Experimental conditions: [TACN]=20 mm, [HPNPP]=1 mm, [HEPES]=10 mm, pH 7.0, 25 8C. Error bars: 1 s.d. calculated from three independent measurements. Electrochemical release of Cu2+: ramp of potential from 0.3 to 0.5 V vs. Ag/AgCl for 60 s. Electrochemical deposition of Cu2+: fixed potential of 1.0 V vs. Ag/AgCl for 10 min.](/figures/figure-4-a-representation-of-the-reversible-activation-of-au-3qy0nm27.webp)
![Figure 3. a) Representation of the electrochemical upregulation of catalytic activity. Exposure of Au NP 1 to a Cd0-coated electrode results in a weak increase in catalytic activity. Application of an electrochemical trigger to the same solution with a Cu0-coated electrode causes the release of Cu2+. The released Cu2+ displaces Cd2+ from the TACNhead groups. b) Representative example of a displacement experiment (all experiments were performed in triplicate; see the Supporting Information, Figure S9). Trace I : initial exposure of Au NP 1 to a Cdcoated electrode followed by exposure (after 30 min) to a Cu-coated electrode to which an oxidative potential was applied. Trace II : initial exposure of Au NP 1 to a Cu-coated electrode with application of an oxidative potential and subsequent exposure (after 30 min) to a Cdcoated carbon chip. PNP-concentration was measured by UV/Vis spectroscopy at 405 nm. Electrochemical release of Cu2+: ramp of potential from 0.3 to 0.5 V vs. Ag/AgCl for 60 s. c) Averaged rates of HPNPP transphosphorylation after exposure of Au NP 1 to the different carbon electrodes described in (b). Experimental conditions: [TACN]=20 mm, [HPNPP]=1 mm, [HEPES]=10 mm, pH 7.0, 25 8C. Error bars: 1 s.d. calculated from three independent measurements.](/figures/figure-3-a-representation-of-the-electrochemical-3obay30t.webp)
![Figure 2. a) Comparison of the efficacy of electrochemical delivery of metal ions as compared to manual addition. b) Example of PNPformation upon the electrochemical (red) or manual (blue) delivery of Cu2+ metal ions to a solution of Au NP 1 ([TACN]=20 mm) and HPNPP (1 mm). For comparison also the traces for only Au NP 1 and Cu2+ are given. c) Averaged initial rates (vinit, three independent measurements for each trace) for the HPNPP transphosphorylation obtained from the experiments shown in (a). Error bars: 1 s.d. Experimental conditions: [HEPES]=10 mm, pH 7.0, 25 8C. Electrochemical release of Cu2+: ramp of potential from 0.3 to 0.5 V vs. Ag/ AgCl for 60 s.](/figures/figure-2-a-comparison-of-the-efficacy-of-electrochemical-1yenyx23.webp)
![Figure 1. a) Representation of the activation of Au NP 1 through the electrochemically activated release of metal ions from an electrode. b) Initial rate (vinit) of HPNPP transphosphorylation as a function of the concentration of Cu2+ (red) or Cd2+ (green). The gray bar marks the difference in reactivity of Au NP 1 (vinit=8.6( 1.9) 10 10ms 1, background), Au NP 1·Cu2+ (vinit=7.7( 0.9) 10 9ms 1), Au NP 1·Cd2+ (vinit=2.0( 0.3) 10 9ms 1) when [TACN]/[M2+] ratio is 1:1. Conditions: [TACN]=20 mm, [HPNPP]=1 mm, [HEPES]=10 mm, pH 7.0, 25 8C.](/figures/figure-1-a-representation-of-the-activation-of-au-np-1-bj6xim5q.webp)
German Edition: DOI: 10.1002/ange.201605309
Supramolecular Catalysis
International Edition: DOI: 10.1002/anie.201605309
Reversible Electrochemical Modulation of a Catalytic Nanosystem
Flavio della Sala, Jack L.-Y. Chen, Simona Ranallo, Denis Badocco, Paolo Pastore,
Francesco Ricci,* and Leonard J. Prins*
Abstract: A catalytic system based on monolayer-functional-
ized gold nanoparticles (Au NPs) that can be electrochemically
modulated and reversibly activated is reported. The catalytic
activity relies on the presence of metal ions (Cd
2+
and Cu
2+
),
which can be complexed by the nanoparticle-bound mono-
layer. This activates the system towards the catalytic cleavage of
2-hydroxypropyl-p-nitrophenyl phosphate (HPNPP), which
can be monitored by UV/Vis spectroscopy. It is shown that
Cu
2+
metal ions can be delivered to the system by applying an
oxidative potential to an electrode on which Cu
0
was deposited.
By exploiting the different affinity of Cd
2+
and Cu
2+
ions for
the monolayer, it was also possible to upregulate the catalytic
activity after releasing Cu
2+
from an electrode into a solution
containing Cd
2+
. Finally, it is shown that the activity of this
supramolecular nanosystem can be reversibly switched on or
off by oxidizing/reducing Cu/Cu
2+
ions under controlled
conditions.
Complexity is emerging as a major theme in chemistry.
[1]
Not
only does it mark a shift from the study of relatively simple
molecules to complex molecular structures similar to those
found in nature, but it also marks a shift from the study of
single molecules to networks of molecules.
[2–4]
Within the
subfield of catalysis,
[5]
the emergence of nanozymes, defined
as nanomaterials with enzyme-like activity, nicely illustrates
this development.
[6]
Nanozymes are prepared following
a bottom-up strategy relying on the use of simple synthetic
components for the formation of structures with a size and
structural complexity similar to that of enzymes.
[7,8]
Their high
stability, uniformity, and ease of modification is favoring
applications in the fields of sensing,
[9]
materials science,
[10]
and
systems chemistry.
[11]
The observation that nanozymes can
exhibit an emerging property such as cooperativity is indeed
a sign that significant progress has been made in the design of
functional complex systems.
[12]
However, whereas nature has
gained exquisite control over the complex biological machi-
nery by using specific triggers to up- and down-regulate
catalytic processes, similar regulatory pathways are still
largely inexistent for nanozymes. Herein, we present
a simple setup for the reversible activation and modulation
of nanozymes using an electronic input. The electrochemical
activation is highly attractive because of its ease of imple-
mentation, cleanliness, precision, and rapid response.
[13–16]
The availability of this kind of regulatory mechanism will
strongly determine the success of chemists in constructing
synthetic networks for studying complexity on the systems
level.
The main component of our system is Au NP 1, which are
gold nanoparticles (d = 1.5 0.3 nm) covered with C9-thiols
terminating with a 1,4,7-triazacyclononane (TACN) head
group (Figure 1 a).
[11]
Such nanoparticle-bound TACN moi-
eties are able to coordinate Zn
2+
-metal ions forming a supra-
Figure 1. a) Representation of the activation of Au NP 1 through the
electrochemically activated release of metal ions from an electrode.
b) Initial rate (v
init
) of HPNPP transphosphorylation as a function of
the concentration of Cu
2+
(red) or Cd
2+
(green). The gray bar marks
the difference in reactivity of Au NP 1 (v
init
= 8.6(1.9)10
10
ms
1
,
background), Au NP 1·Cu
2+
(v
init
= 7.7(0.9)10
9
ms
1
), Au NP
1·Cd
2+
(v
init
= 2.0(0.3)10
9
ms
1
) when [TACN]/[M
2+
] ratio is 1:1.
Conditions: [TACN]= 20 m m, [HPNPP]= 1mm, [HEPES] = 10 mm,
pH 7.0, 25
8
C.
[*] Dr. F. della Sala, Dr. J. L.-Y. Chen, Dr. D. Badocco, Prof. Dr. P. Pastore,
Prof. Dr. L. J. Prins
Department of Chemical Sciences, University of Padova
Via Marzolo 1, 35131 Padova (Italy)
E-mail: leonard.prins@unipd.it
S. Ranallo, Prof. Dr. F. Ricci
Chemistry Department, University of Rome Tor Vergata
Via della Ricerca Scientifica, 00133 Rome (Italy)
E-mail: francesco.ricci@uniroma2.it
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201605309.
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molecular catalyst that very efficiently promotes the trans-
phosphorylation of 2-hydroxypropyl-p-nitrophenyl phos-
phate (HPNPP), a common substrate used for mimicking
RNA-hydrolysis.
[12]
The role of the nanoparticle support is critical, as the
observed rate acceleration is due to the cooperative effect
between TACN head groups in close proximity. Limited
activity is observed for unbound TACN analogues.
[12]
Exten-
sive previous studies have shown that the system displays
Michaelis–Menten-like catalytic activity, resulting from the
formation of catalytic pockets between two neighboring NP-
bound TACN·Zn
2+
-complexes.
[17]
For this reason, these sys-
tems have also been referred to as nanozymes.
[12]
The
numerous analogous multivalent systems (both molecular
and NP-based) that have been reported require the presence
of Zn
2+
or other metal ions (for example, Cu
2+
,Co
3+
,Ce
4+
,
Fe
3+
) for catalytic activity.
[18]
The observation that Au NP
1 and related systems are poorly active in the absence of metal
ions implies that the controlled association and dissociation of
metal ions can be used as a regulatory mechanism for
catalysis. The low residual activity of non-metalated Au NP
1 presumably originates from the presence of protonated
TACN-ligands, at neutral or acidic pH.
[19]
In this study, Cu
2+
and Cd
2+
metal ions were deliberately
chosen for a number of reasons. Firstly, measurement of the
catalytic activity as a function of the amount of Cu
2+
and Cd
2+
added to Au NP 1 showed that both metal ions activated the
system for catalysis, reaching a maximum when a stoichiom-
etry of 1:1 between the TACN head group and the metal ion
was obtained ([TACN] = [Cu
2+
or Cd
2+
] = 20 mm, Figure 1b).
The sharp transition to the plateau-level at this ratio indicates
that metal binding occurs under saturation conditions.
Importantly, for a 1:1 ratio of [TACN]:[M
2+
], the rate
acceleration was significantly higher for Au NP 1·Cu
2+
(9
times, red squares) than for Au NP 1·Cd
2+
(2.5 times, green
squares) when compared to the background (Figure 1b,
dashed line). It is pointed out that the absolute activities of
the Cu
2+
and Cd
2+
systems are much lower compared to that
of the analogous Zn
2+
system that we have studied previ-
ously.
[12,17,18]
Yet, the more accessible redox-chemistry of Cu
and Cd made us focus on these metals, despite the lower
catalytic activity. Second, the difference in terms of binding
affinity for the TACN group is about six orders of magnitude
in favor of Cu
2+
(logK
TACNCu
2þ
= 15.4 and logK
TACNCd
2þ
=
9.3),
[20]
resulting in complete displacement of Cd
2+
ions from
the NP-bound TACN head groups upon the addition of just an
equimolar amount of Cu
2+
ions. The difference in the activity
of the Cu
2+
and Cd
2+
systems provides a means to upregulate
the catalysis by exchanging Cd
2+
for Cu
2+
.
[21]
Finally, Cu
2+
and
Cd
2+
metal ions were chosen for their different standard
reduction potentials (respectively 0.40 and + 0.34 V for
Cd
2+
and Cu
2+
, relative to the standard hydrogen elec-
trode).
[22]
We focused our initial investigations on the electrochem-
ical generation of metal ions from a source in order to switch
on catalytic activity. To that purpose, we used a carbon chip
[23]
previously coated with a film of Cu
0
. The deposition was
carried out by dipping the electrode into an aqueous solution
of Cu(NO
3
)
2
and by applying a fixed reductive potential of
1.0 V vs. Ag/AgCl.
[22]
Cu
2+
ions were released from the
electrode into a buffered solution containing Au NP 1 and
HPNPP by applying a potential oxidative ramp from 0.3 to
0.5 V vs. Ag/AgCl for 60 seconds. We employed square wave
voltammetry (SWV; Supporting Information, Figure S4),
which quantitatively suggested that about 20 mm of Cu
2+
was
electrochemically released from the electrode.
[22]
The absence
of NP precipitation and/or aggregation was proven by
a combination of UV/Vis spectroscopy, DLS and TEM
analyses, confirming that Au NP 1 is stable under the release
conditions (Supporting Information, Figures S6–S8). Kinetic
measurements (Figure 2) of the initial rate of HPNPP trans-
phosphorylation confirmed that the catalytic system achieved
the same activity after the electrochemical release of Cu
2+
(v
init
= 7.4(1.2) 10
9
m s
1
) as compared to manual addition
(v
init
= 7.7(1.2) 10
9
m s
1
). Control experiments confirmed
that, without application of the potential, the HPNPP trans-
phosphorylation occurred at a low rate, which is comparable
to Au NP 1 in the absence of metal ions (Supporting
Information, Figure S5). Quantitative ICP-MS analysis con-
firmed that the concentration of electrochemically released
Cu
2+
was consistently above the concentration of the NP-
bound TACN head groups (20 mm; Supporting Information,
Table S1), confirming the results initially suggested by SWV
(Supporting Information, Figure S4). It is noted that an excess
of Cu
2+
, with regards to the concentration of head groups,
does not affect the catalytic activity of the system, as the metal
Figure 2. a) Comparison of the efficacy of electrochemical delivery of
metal ions as compared to manual addition. b) Example of PNP-
formation upon the electrochemical (red) or manual (blue) delivery of
Cu
2+
metal ions to a solution of Au NP 1 ([TACN]= 20 mm) and
HPNPP (1 mm). For comparison also the traces for only Au NP 1 and
Cu
2+
are given. c) Averaged initial rates (v
init
, three independent
measurements for each trace) for the HPNPP transphosphorylation
obtained from the experiments shown in (a). Error bars: 1 s.d.
Experimental conditions: [HEPES]= 10 mm, pH 7.0, 25
8
C. Electro-
chemical release of Cu
2+
: ramp of potential from 0.3 to 0.5 V vs. Ag/
AgCl for 60 s.
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ion by itself is not active at these concentrations (see also
Figure 1b). The study was then repeated using Cd instead of
Cu. In this case, because of the low oxidation potential of Cd
0
,
controlled release of Cd
2+
from the electrode was not
possible. ICP-MS (Supporting Information, Table S1) and
kinetic experiments confirmed that Cd
2+
is spontaneously
released from the electrode upon exposure to the solution
containing Au NP 1 and HPNPP (Supporting Information,
Figure S5). For this reason, the electrode initially covered
with Cd was used as a spatially confined metal source in
a displacement experiment (Figure 3a). More specifically,
a solution containing Au NP 1 (20 mm) and HPNPP (1 mm)
was deposited onto the Cd-loaded electrode, which allowed
spontaneous Cd
2+
release. HPNPP transphosphorylation was
then monitored for 30 min by UV/Vis spectroscopy (Fig-
ure 3b, trace I, green). The efficient delivery of Cd
2+
was
evidenced by the observed rate (v
init
= 2.1 10
9
m s
1
), which
corresponded to that of Au NP 1·Cd
2+
(v
init
= 2.0(0.3)
10
9
m s
1
upon manual addition of Cd
2+
). Next, the same
solution was re-deposited on a Cu-loaded electrode and
a potential ramp from 0.3 to 0.5 V vs. Ag/AgCl was applied,
after which the kinetic measurements were continued for an
additional 30 minutes. A clear increase in the rate of PNP-
formation was observed (v
init
from 2.1 10
9
to 1.1 10
8
m s
1
,
Figure 3b, trace I, red), which is in full agreement with the
value obtained when Cu
2+
was manually added or electro-
chemically released in the presence of Au NP 1. This
demonstrates the effective displacement of Cd
2+
by Cu
2+
from the monolayer resulting in upregulation of catalytic
activity. It was important to observe that the rate of PNP-
formation did not change at all when Cd
2+
was released in
a solution containing Au NP 1·Cu
2+
and HPNPP, confirming
that Cd
2+
does not displace Cu
2+
from the monolayer
(Figure 3b, trace II, empty squares).
The described experiments demonstrate that Cu
2+
ions
can be efficiently delivered through an electrochemical input
for the activation of a pre-catalyst. However, full electro-
chemical control over catalytic activity is achieved only when
catalyst activation is a reversible process.
[24,25]
The reversi-
bility of the electrochemical activation of the system was
investigated using Cu
2+
(Figure 4a). As before, Cu
2+
was
released from a prepared electrode into a buffered solution
containing Au NP 1 and HPNPP after which the rate of PNP-
Figure 3. a) Representation of the electrochemical upregulation of
catalytic activity. Exposure of Au NP 1 to a Cd
0
-coated electrode results
in a weak increase in catalytic activity. Application of an electrochem-
ical trigger to the same solution with a Cu
0
-coated electrode causes
the release of Cu
2+
. The released Cu
2+
displaces Cd
2+
from the TACN-
head groups. b) Representative example of a displacement experiment
(all experiments were performed in triplicate; see the Supporting
Information, Figure S9). Trace I: initial exposure of Au NP 1 to a Cd-
coated electrode followed by exposure (after 30 min) to a Cu-coated
electrode to which an oxidative potential was applied. Trace II: initial
exposure of Au NP 1 to a Cu-coated electrode with application of an
oxidative potential and subsequent exposure (after 30 min) to a Cd-
coated carbon chip. PNP-concentration was measured by UV/Vis
spectroscopy at 405 nm. Electrochemical release of Cu
2+
: ramp of
potential from 0.3 to 0.5 V vs. Ag/AgCl for 60 s. c) Averaged rates of
HPNPP transphosphorylation after exposure of Au NP 1 to the differ-
ent carbon electrodes described in (b). Experimental conditions:
[TACN]= 20 mm, [HPNPP] = 1mm, [HEPES]= 10 mm, pH 7.0, 25
8
C.
Error bars: 1 s.d. calculated from three independent measurements.
Figure 4. a) Representation of the reversible activation of Au NP
1 driven by the electrochemically controlled release and deposition of
Cu
2+
-ions on the electrode. b) Rate of HPNPP transphosphorylation
after two cycles of activation/deactivation of Au NP 1. The control
experiment indicates the observed rate when Au NP 1 was just
activated once by the electrochemical release of Cu
2+
at the start of
the experiment. Experimental conditions: [TACN]= 20 mm,
[HPNPP]= 1mm, [HEPES] = 10 mm, pH 7.0, 25
8
C. Error bars: 1 s.d.
calculated from three independent measurements. Electrochemical
release of Cu
2+
: ramp of potential from 0.3 to 0.5 V vs. Ag/AgCl for
60 s. Electrochemical deposition of Cu
2+
: fixed potential of 1.0 V vs.
Ag/AgCl for 10 min.
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formation was measured for 30 minutes. Next, the same
solution was deposited on a clean electrode and a fixed
reductive potential of 1.0 V vs. Ag/AgCl was applied for
10 min to re-deposit Cu
2+
as Cu
0
onto the electrode surface.
Control experiments performed by UV/Vis spectroscopy,
DLS and TEM analyses confirmed that the monolayer-
protected Au nanoparticles were stable under these condi-
tions (Supporting Information, Figures S10–S12). Further-
more, UV/Vis titration experiments of Cu
2+
to a solution of
Au NP 1 after application of the reducing conditions con-
firmed no changes in the ability of TACN to complex Cu
2+
(Supporting Information, Figure S13).
[17]
Importantly, contin-
uation of the reaction monitoring by UV/Vis revealed that the
rate of PNP-formation (v
init
= 1.2(0.4) 10
9
m s
1
) had
dropped to the background rate observed for Au NP 1,
demonstrating that Cu
2+
was sufficiently depleted from the
system to inhibit catalytic activity. To fully demonstrate
reversibility, a second cycle of activation (using a new
electrode with freshly deposited Cu) and deactivation was
performed with the same solution. Notably, when the system
was electrochemically re-activated (from Au NP 1 to Au NP
1·Cu
2+
), the PNP rate of formation was strictly comparable,
within experimental error, to that measured for a control
experiment monitored for the same length of time where the
oxidative potential was applied only at the beginning of the
analysis (Figure 4, dashed line).
In conclusion, we have shown the reversible electro-
chemical regulation of a supramolecular nanocatalyst. Metal
ions have been electrochemically released from a pre-coated
electrode and complexed by the pre-catalytic system resulting
in its activation. This mechanism of catalyst activation
permitted the upregulation of catalytic activity through the
electrochemical release of Cu
2+
-metal ions possessing
a higher affinity for the macrocyclic ligands compared to
Cd
2+
. Finally, the reversibility of the process was demon-
strated by showing that the re-deposition of Cu
2+
on the
electrode resulted in a loss of catalytic activity. These results
show that electrochemistry is a very effective and clean tool
for the regulation of the catalytic activity of a supramolecular
catalyst. The release of metal ions from a well-defined solid
support also opens up the possibility for spatial and temporal
control over catalyst activation in systems of higher complex-
ity.
Acknowledgements
Financial support from the University of Padova
(CPDA138148) is acknowledged.
Keywords: gold nanoparticles · nanocatalysis · nanozymes ·
supramolecular catalysis · supramolecular chemistry
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Received: May 31, 2016
Published online: && &&, &&&&
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Communications
Supramolecular Catalysis
F. della Sala, J. L.-Y. Chen, S. Ranallo,
D. Badocco, P. Pastore, F. Ricci,*
L. J. Prins*
&&&& — &&&&
Reversible Electrochemical Modulation of
a Catalytic Nanosystem
Hop on, hop off: Metal ions are reversibly
released from an electrode to regulate the
activity of a gold nanoparticle catalyst
(see picture). The system catalyzes the
cleavage of 2-hydroxypropyl-p-nitrophenyl
phosphate and the formation of p-nitro-
phenolate can be monitored by UV/Vis
spectroscopy.
A
ngewandte
Chemi
e
Communications
5Angew. Chem. Int. Ed. 2016, 55, 1 – 5 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org
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