DNA-Directed Assembly of Gold Nanohalo for Quantitative Plasmonic Imaging of Single-Particle Catalysis

Article abstract of DOI:10.1021/jacs.5b00324

Plasmonic imaging under a dark-field microscope (DFM) holds great promise for single-particle analysis in bioimaging, nanophotonics, and nanocatalysis. Here, we designed a DNA-directed programmable assembly strategy to fabricate a halo-like Au nanostructure (nanohalo) that couples plasmonic large gold nanoparticles (L-AuNPs) with catalytically active small AuNPs (S-AuNPs) in a single nanoarchitecture. Catalytic reaction occurring on S-AuNPs changes its permittivity, which results in a significant variation of the plasmonic resonance of the nanohalo. Hence, we can indirectly monitor catalytic reactions on a single nanohalo under DFM, on the basis of which we have obtained quantitative information on both nanocatalysis and catalyst poisoning. Our study thus provides a cost-effective means to quantitatively study metal NP-based catalysis at single-particle level.

Full text of DOI:10.1021/jacs.5b00324

Communication
pubs.acs.org/JACS
DNA-Directed Assembly of Gold Nanohalo for Quantitative
Plasmonic Imaging of Single-Particle Catalysis
Kun Li,† Kun Wang,† Weiwei Qin,† Suhui Deng,† Di Li,*,† Jiye Shi,†,‡ Qing Huang,† and Chunhai Fan†
†Division of Physical Biology & Bioimaging Center, Shanghai Synchrotron Radiation Facility, CAS Key Laboratory of Interfacial
Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
‡UCB Pharma, Slough, SL1 14EN Berkshire, U.K.
S
* Supporting Information
comprise the two properties of AuNPs, the plasmonic property
for L-AuNPs, and the catalytic property for small, catalytically
active AuNPs (S-AuNPs, d < 15 nm).
ABSTRACT: Plasmonic imaging under a dark-field
microscope (DFM) holds great promise for single-particle
analysis in bioimaging, nanophotonics, and nanocatalysis.
Here, we designed a DNA-directed programmable
assembly strategy to fabricate a halo-like Au nanostructure
(nanohalo) that couples plasmonic large gold nano-
particles (L-AuNPs) with catalytically active small AuNPs
(S-AuNPs) in a single nanoarchitecture. Catalytic reaction
occurring on S-AuNPs changes its permittivity, which
results in a significant variation of the plasmonic resonance
of the nanohalo. Hence, we can indirectly monitor catalytic
reactions on a single nanohalo under DFM, on the basis of
which we have obtained quantitative information on both
nanocatalysis and catalyst poisoning. Our study thus
provides a cost-effective means to quantitatively study
metal NP-based catalysis at single-particle level.
One possible solution is to assemble plasmonic NPs and
nanocatalysts into a multicomponent nanoarchitecture, dubbed
nanoantenna, for indirect monitoring of catalytic processes.9
For example, Larsson10 and Alivisatos11 fabricated nanoantenna
composed of Au plasmonic NPs and Pd nanocatalysts by using
nanolithography, on the basis of which they plasmonically
studied gas sensing and gas-phase catalysis. Despite the
elegance of their work, the use of top-down nanolithography
is cost-intensive and limits its application in studying NP-based
catalysis in solution.12 In this work, we report on a DNA-based
bottom-up approach to programmable construct a nanohalo
architecture that can indirectly monitor solution-phase catalysis
at single-particle level. Specially, the plasmonic L-AuNPs and
catalytic S-AuNPs are brought to the close proximity by DNA
self-assembly. The locally enhanced electromagnetic field in the
gap between L- and S-AuNPs allows indirect and sensitive
monitoring of the catalytic reaction, which provides ready
extraction of physicochemical parameters for quantitative
understanding of AuNP-based catalysis.
The Au nanohalo was prepared by the DNA-directed self-
assembly of AuNPs of different diameters (Figure 1A). Briefly,
AuNPs with diameters of 50 and 13 nm were modified with
thiolated oligonucleotides with complementary sequences. Of
note, the choice of AuNPs (13 nm) as S-NPs is a compromise
between catalytic activity and plasmon-induced electromagnetic
field enhancement (see part 3 and Figure S1, Supporting
Information). DNA plays two critical roles in the hierarchical
assembly: controlling the distance and modulating the catalytic
activities. The distance (∼9 nm) between L- and S-AuNPs was
controlled by the length of hybridized oligonucleotides (20 bp),
a short distance that favors strong plasmon coupling (Figure
1B). The catalytic activity of AuNPs in nanohalo was also
modulated by rationally tuning the surface density of modified
oligonucleotides on L- and S-NPs (Figure 1C). The L-NPs with
dense modification are catalytically inert, while S-NPs with
loose modification are still active. As a result, the catalytic
activity of Au nanohalo is exclusively contributed by the loosely
modified S-NPs (Figure 1C, inset). We carried out finite-
difference time-domain (FDTD) simulations to calculate the
enhanced electromagnetic field in Au nanohalo. The electric
he ability to assemble different functional building blocks
Tto a hierarchical nanoarchitecture is highly desirable for
nanoelectronics, nanophotonics, and intelligent sensing. Be-
sides the aesthetic value of well-defined ordered structure,
nanoarchitectures with multiple functions provide new tools to
explore physical and chemical properties at the nanoscale.1
Gold nanoparticles (AuNPs) have garnered considerable
attention in the last decades due to their excellent
biocompatibility and prominent plasmonic and catalytic
properties.2 In particular, local surface plasmon resonance
(LSPR) of AuNPs involves the collective oscillation of
dielectrically confined conduction electrons, which enables
the confinement of light to regions with subwavelength
dimensions. Therefore, Au-based hierarchical nanoarchitectures
have been popularly explored for studying new plasmonic
phenomenon3 and for applications such as bioimaging,4
targeted cargo delivery,5 and photothermal therapy.6
LSPR of plasmonic NPs is highly sensitive to the refractive
index or dielectric constant of surrounding media, thereby
holding great promise for monitoring the states of plasmonic
nanocatalysts during catalysis.7 For example, Mulvaney
demonstrated the possibility of direct observing of a redox
reaction on single gold nanorod using DFM.8 However, owing
to the limit of CCD sensitivity, DFM could only capture the
scatter light from large AuNPs (L-AuNPs) with diameters over
50 nm (d > 50 nm), which is often catalytically inactive.
Therefore, it is highly desirable to develop a new way to
Received: January 12, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b00324
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Journal of the American Chemical Society
Communication
volume ratio and high chemical potentials. However, owing to
the complexity of heterogeneous reactions, detailed physico-
chemical information obtained on heterogeneous catalysts is
scarce. Typically, heterogeneous catalytic processes involve the
contact of gas or liquid reactant with nanocatalysts. During
catalysis, several physicochemical processes occurred succes-
sively on nanocatalysts including adsorption of reactants on
catalysts, electron transfer between donor and acceptor
molecules, and detachment of products from catalysts to
eventually catalyst poisoning that ends the reaction.13 All these
processes are associated with the permittivity changes of the
plasmonic nanocatalysts; therefore, we could correlate the
plasmonic band shifts of the nanohalo with the state of AuNPs
during catalysis and derive quantitative information.
We thereby employed this nanohalo as a plasmonic probe to
monitor a catalytic reaction. Because of the difficulty of
synthesis of monovalent-modified L-NPs, currently we are
unable to obtain Au nanohalo with only one S-NP to observe
single NP catalytic dynamics. In addition, DNA functionaliza-
tion intrinsically influenced the catalytic properties of nano-
catalysts, which compromises the value of the present
method.14 Future development in DNA nanotechnology, for
example, addressable DNA origami,15 may help to synthesis
nanohalo with distinct one catalytic NP, however, may still not
to exclude the influence of DNA modifications to the catalytic
actives.
Figure 1. (A) Synthetic scheme for the preparation of Au nanohalo
through the self-assembly of AuNPs of different diameters modified
with complementary oligonucleotides. (B) Schematic demonstration
of the distance between L- and S-AuNPs, which is determined by the
rationally designed length of oligonucleotides. (C) Number of
oligonucleotides per particle of different kinds of L- and S-AuNPs.
Inset shows the colored product that is catalyzed by L- and S-AuNPs
that loaded different amounts of oligonucleotides, respectively, as
tested by a HRP-cascaded colorimetric reaction. (D) Electromagnetic
finite-difference time-domain (FDTD) simulation of the local electric
fields enhancement (Log |E|) of the Au nanohalo (left) and L-AuNPs
(right) at the X−Y plane. |E| = |Elocal/Ein|, where Elocal and Ein represent
the amplitude of the local and incident electric fields, respectively.
We chose an AuNPs-catalyzed redox reaction between
glucose and O2 (eq 1) as a model system (for reasons for
choosing this reaction, see part 5, Supporting Information).
AuNPs
glucose + O2 ⎯⎯⎯⎯⎯⎯→ gluconic acid + H2O2
(1)
The LSPR spectrum of the Au nanohalo was monitored
during the catalytic process. At t = 0 min, glucose solution was
injected as an electron source to a final concentration of 0.5 M,
and LSPR spectrum of this nanohalo was measured (Figure
2A). Upon the addition of glucose, we observed an immediate
red-shift of 2.52 nm, followed by a rapid blue-shift of 6.88 nm
field intensity (|E|) in the gap between L- and S-AuNPs was
found to have ∼15-fold enhancement as compared with that on
the surface of L-AuNPs (Figure 1D). The strongly enhanced
plasmon near-field in the gap can sense the change in the
dielectric function of the proximal S-NPs. Therefore, small
change in the permittivity of S-AuNPs should bring about
significant change of plasmonic resonance of L-AuNPs, which
makes it possible to monitor catalytic reactions occurring on S-
AuNPs.
The structure and its LSPR response of the as-prepared
nanohalo were characterized by TEM and DFM, respectively
(see part 4, Figure S2, Supporting Information). Of note, 3D
Au nanohalo was rearranged to 2D-packed structures owing to
the capillary forces during the drying process on TEM grid
(Figure S2A). The ratio of S-/L-NPs ranged from 11−14 in
200 randomly counted particles (Figure S2B). The as-prepared
Au nanohalo was then attached on ITO for DFM measure-
ments. Figure S2, panels C and D of the Supporting
Information show representative high-resolution SEM and
DFM images of an individual Au nanohalo with an S-/L- ratio
of 12 (colocalization of this Au nanohalo in SEM and DFM
images, see Figure S3, Supporting Information). The Au
nanohalo scattered yellow light with plasmon band at 616.45
nm (Figure S2E), while L-NPs scattered green light with the
plasmon band at 557.78 nm (Figure S4).
Figure 2. (A) LSPR spectra and (B) the corresponding plasmonic
band peak shifts of one Au nanohalo during the catalytic reaction. (C)
LSPR spectra and (D) the corresponding plasmon band peak shifts of
one L-NPs along the catalytic process under same experimental
condition. LSPR spectra were collected by DFM with a time interval of
1 min and then fitted by Lorentz curve to determine the peak position.
Error bars in panels B and D represented the deviations coming from
the Lorentzian fitting procedure.
Metal NPs are important catalysts for many chemical
transformations due to their size-induced high surface-to-
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in about 2 min, then another red-shift of 3.53 nm in about 40
min (Figure 2B). In a control experiment, we also analyzed the
plasmonic band shifts of L-NPs under the same experimental
conditions (Figure 2C) and observed only an initial red-shift of
1.16 nm (Figure 2D).
In catalytic reactions, interactions of reactant with the surface
solid catalyst are often decisive. Chemisorption of reactants on
nanocatalysts plays a critical role for catalytic performance.16 It
is still a challenge to obtain quantitative results of
chemisorption under real reaction conditions.
We speculate that the discontinuous up and down in
plasmonic band (Δλ) of Au nanohalo in Figure 2, panel B
reflects the different physicochemical process of nanocatalysts
in catalysis. Heterogeneous catalysis reaction is activated by the
contact of reactants with the surface of catalyst; we thereby
assume that the initial 2.52 nm red-shift is attributed to
adsorption of glucose on S-NPs. To confirm that, we
investigated the relationship between the refractive index of
surrounding medium (n), the plasmon bandshift (Δλ), and the
change of glucose concentration (Δcglucose). The sensitivity
factors, Δλ/Δn, were estimated from the slope of plasmonic
response in four different solvents and estimated to be 106.77
nm RIU−1 for Au nanohalo (Figure 3A) and 88.26 nm RIU−1
initial 2.52 nm red-shift in Figure 2, panel B to the change of
the refractive index of surroundings from pure water to 0.97 M
of glucose or the adsorption of 0.97 M glucose on Au nanohalo.
The calculated concentration of surface adsorbed glucose is,
however, almost twice concentrated than that in bulk solution
(0.5 M), which suggests an enrichment of glucose on Au
nanohalo. We thus defined the concentration of surface
adsorbed glucose at catalytic sites as [S]; the relationship
between Δλ and surface adsorbed reactant was changed to Δλ
= 2.60839[S]. However, for the L-NPs, the 1.16 nm of red-shift
in Figure 2, panel D corresponds to 0.54 M of glucose
adsorption, only slightly higher than in bulk solution (0.5 M),
which further confirms that L-NPs are catalytically inert.
The [S] dependence of Δλ enables us to quantitatively
analyze the chemisorption of glucose on Au nanohalo. By
employing Langmuir−Hinshelwood mechanism to simulate
this catalysis, we obtained [Sm] = 1.06 0.05 M, Kads = 20.90
3.93 M−1 (Figure 3, panel D and part 7, Supporting
Information). We calculated the number of glucose adsorbed
on each S-NPs (Nads) as 302 (see part 7, Supporting
Information). FDTD simulation suggested that the adsorption
of 302 glucose molecules would lead to a 2.14 nm of red-shift
(Figure S6), similar to but slightly lesser than the
experimentally obtained 2.52 nm, which indicates that glucose
in bulk solution also contributes to the plasmonic of Au
nanohalo to a lesser extent. Of note, the temporal resolution of
present DFM setup is 1 min, thereby only few plasmonic band
shift were recorded in the fast adsorption process. Introducing a
supercontinuum laser as light source could improve the
temporal resolution down to 10 ms,17 which might provide
sufficient adsorption information. Despite the presence of these
constraints, the present experimental results fit well with
numerical stimulation, which confirms the validity of our
method.
In heterogeneous redox reactions, following chemisorption
of reactant on catalyst, donor and acceptor molecules should
transfer electrons on the surface of catalysts. According to
microelectrode model,18 the metal nanocatalyst behaves as an
electron reservoir that couples two redox reactions. Hence, we
assigned the rapid blue-shift of 6.88 nm and slow red-shift of
3.53 nm to the charging and discharging of S-NPs, respectively.
We estimated that ∼11 833 electrons are injected in 3 min, or
66 electrons per second (see part 8, Supporting Informa-
tion).8,19 Interestingly, the discharging process (red-shift) is
much slower than the rapid charging process (blue-shift).
Previous work, however, suggested that rate-determining step
of this reaction is the oxidation of adsorbed glucose, while the
reduction of O2 is fast.20 In our condition, the concentration of
glucose (0.5 M) is much higher than that of dissolved oxygen.
Therefore, we suggest that the initial dissolved oxygen is
quickly reduced by glucose and O2 in air is then redissolved to
the reaction solution and diffuses to Au nanocatalysts to accept
the excess of stored electrons, which will result in the retarded
discharging.8
Catalyst deactivation, or the loss of catalytic activity, is a
general problem in heterogeneous catalysis. Catalyst poisoning,
the chemisorption of reactants, products, or impurities on
active sites, has a large contribution.21 Indeed, our previous
work suggested that catalytic product gluconic acid was
adsorbed on Au nanocatalyst surfaces, resulting in catalyst
poisoning.22 Significantly, the plasmonic band of Au nanohalo
in Figure 2, panel B does not shift back to its original position
after 40 min of reaction, instead, another 3.53 nm of red-shift.
Figure 3. Plots of plasmonic band peak of individual (A) Au nanohalo
and (B) L-AuNPs versus refractive index (n) of four different solvents
(air (n = 1), water (n = 1.333), ethylene glycol (n = 1.438), and
immersion oil (n = 1.518)). Error bars are the standard deviations of
12 signals. (C) Plot of refractive index (n) versus bulk concentration of
glucose. The refractive index was obtained from Abbe refractometer.
Error bars are the standard deviations of three signals. (D) Langmuir
adsorption isotherm of glucose on Au nanohalo. cglucose is the
concentration of glucose in bulk solution; [S] is the concentration
of glucose molecules that adsorbed on single Au nanoparticle surface.
The error bars represent the error in determining of [S] by plasmon
band shifts.
for L-NPs (Figure 3B), respectively. Although the signals in
Figure 3, panels A and B obtained from 12 individual NPs show
different plasmonic response, the slopes remained constant,
which indicate that the sensitivity factor is an intrinsic character
of their structure (see part 6 and Figure S5, Supporting
Information). The concentration factor, Δn/Δcglucose, was
calculated from Abbe refractometer by measuring the refractive
index at different glucose concentrations and estimated to be
0.02443 (Figure 3C). Therefore, the relationship between Δλ
and Δcglucose follows Δλ = 2.60839Δcglucose for Au nanohalo and
Δλ = 2.15619Δcglucose for L-AuNPs. We thereby assigned the
C
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Accordingly, the catalytic activity of Au nanohalo (in solution)
is also diminished after 40 min (Figure S7). We thus consider
that the 3.53 nm of red-shift should come from the surface-
adsorbed gluconic acid (see part 9 and Figure S8, Supporting
Information). Quantitative analysis also revealed that 97.5% of
gluconic acid is detached from the AuNPs’ surface in each
turnover and the remaining 2.5% molecules passivate the
catalyst (see part 9, Supporting Information), which eventually
results in the poisoning.
Having established single-particle analysis for glucose
catalysis using Au nanohalo, we further demonstrate that this
is a generic approach to study catalysis on nanocatalysts (see
part 10 and Figure S9, Supporting Information). Since the
principle of indirect monitoring is the sensing of LSPR
response induced by changes of surface coverage and charging
state of catalysts, there is no limitation to the other catalysts and
catalytic reaction.
In summary, we have constructed a halo-like Au nano-
architecture that integrates plasmonic and catalytic properties
of AuNPs. The close proximity of large and small NPs in the Au
nanohalo results in a confinement and enhancement of local
electromagnetic field. Hence, catalytic reactions occurring on S-
AuNPs changes its permittivity, which leads to significant
changes of the plasmonic resonance of the Au nanohalo. Our
work extends the concept of plasmonic nanoantenna to
monitor liquid-phase catalytic reactions and provides a cost-
effective alternative to study a wide range of catalytic reactions,
especially in homogeneous solution.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and supplementary discussion and figures.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*lidi@sinap.ac.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Basic Research
Program of China (973 program, 2013CB932803,
2012CB825800, and 2013CB933800), NSFC (21222508,
3108JC1422600, 21329501, 61378062, 21390414, 91313302),
the Shanghai Municipal Commission for Science and
Technology (13QH1402300), and Youth Innovation Promo-
tion Association, CAS. We would like to dedicate this paper to
Professor Qing Huang, who unfortunately passed away during
the reviewing process. Qing Huang played an essential role in
the research described here and is greatly missed.
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