Rational Approach to Plasmonic Dimers with Controlled Gap Distance, Symmetry, and Capability of Precisely Hosting Guest Molecules in Hotspot Regions

Molecules in Hotspot Regions

Article

Rational Approach to Plasmonic Dimers with Controlled Gap

Distance, Symmetry, and Capability of Precisely Hosting Guest

Li Tian, Chen Wang, Hongwei Zhao, Fuwei Sun, Hao Dong, Kai Feng, Peng Wang, Guokang He,

and Guangtao Li*

Cite This: J. Am. Chem. Soc. 2021, 143, 8631 −8638

Read Online

ACCESS

Metrics & More

Article Recommendations

sı Supporting Information

ABSTRACT: Plasmonic dimers not only provide a unique platform for studying fundamental

plasmonic behavior and eﬀects but also are functional materials for numerous applications.

The eﬃcient creation of well-deﬁned dimers with ﬂexible control of structure parameters and

thus tunable optical property is the prerequisite for fully exploiting the potential of this

nanostructure. Herein, based on a polymer-assisted self-assembly approach in conjugation with

molecular cage chemistry, a strategy was demonstrated for constructing cage-bridged

plasmonic dimers with controlled sizes, compositions, shape, symmetry, and interparticle

gap separation in a modular and high-yield manner. With a high degree of freedom and

controllability, this strategy allows facilely accessing various symmetrical/asymmetrical dimers

with sub-5 nm gap distance and tailored optical properties. Importantly, as the linkage of the

two constituent elements, the molecular cages embedded in the junction endow the assembled

dimers with the ability to precisely and reversibly host rich guest molecules in hotspot regions,

oﬀering great potential for creating various plasmon-mediated applications.

. INTRODUCTION

plasmonic properties of dimers in a wide range, making

them very promising as versatile functional units for a variety

of applications in enhanced spectroscopies, photothermal

conversion, chemical reactions, biochemical sensing, photo-

Noble metal nanoparticles (NPs) possess unique and

fascinating optoelectronic properties due to light-induced

collective oscillation of conductive electrons, a phenomenon

known as localized surface plasmon resonance (LSPR). The

LSPR properties of individual nanoparticles are determined by

not only the composition, size, and geometry of particles but

also their immediate environment. When these NPs are

brought into proximity to form nanoclusters, their plasmon

oscillations couple to each other, resulting in new or improved

,18−23

active as well as quantum devices, etc.

However, to

realize full exploitation of the big potential of dimer structures,

the eﬃcient creation of diverse plasmonic clusters with highly

ﬂexible control of the above-mentioned structure parameters as

well as the integration of the capability of precisely positioning

rich guest molecules in hotspot regions is the key point and

prerequisite.

−10

plasmonic functionalities,

such as ﬁnely tuned plasmon

resonance, enhanced light−matter interactions, chiral plas-

monics, magnetoplasmonics, and plasmon-induced energy and

charge transfer. In addition, the nanogaps created in plasmonic

clusters form hotspots, in which the electromagnetic ﬁeld is

signiﬁcantly concentrated, and introducing molecules into

these hotspots is the basis of a vast spectrum of plasmon-

Top-down nanofabrication and a bottom-up assembly

approach are currently two main methods developed to

4−30

construct plasmonic dimer structures.

The former

methods, for instance, lithographic techniques, enable precise

control of the size of the nanoparticles, the interparticle gap,

and the orientation but always suﬀer from poor crystal quality

mediated applications. As the simplest form of plasmonic

nanoclusters, dimer structures, especially with gap separations

of a few nanometers, have attracted increasing atten-

(

polycrystallinity), limited material choices, and a larger

interparticle distance (>10 nm). In comparison, the latter

strategies through molecular linkers, such as DNA, origami,

,2,12−15

tion.

Due to a simple and well-deﬁned structure, on

one hand, plasmonic dimers provide an ideal platform

beneﬁcial for both theoretically and experimentally exploring

intriguing plasmon coupling and fundamental plasmonic

Received: December 28, 2020

Published: June 2, 2021

16,17

eﬀects that are absent in individual nanoparticles.

On the

other hand, the variation of the material compositions, sizes,

shapes of constituent components, and interparticle spacing

and symmetry oﬀers a large space for modulating the

2021 American Chemical Society

J. Am. Chem. Soc. 2021, 143, 8631−8638

631

Journal of the American Chemical Society

respectively (see synthesis details of ligands and MOCs in

Article

and thiols, seem more ﬂexible, as they are suitable for a broader

range of materials with high crystal quality and at the same

time eﬀectively achieving very narrow gap distances.

Unfortunately, current assembly approaches aﬀord relatively

poor control over the morphology of the nanoclusters,

particularly in the case of heterostructured clusters. In general,

a mixture of clusters, including monomers, dimers, trimers, and

other multimers, are generated during fabrication, and a

tedious separation process (e.g., electrophoresis, density

gradient centrifugation) is required to obtain high-purity

tunable gap distances of a few nanometers with high accuracy

is easily achievable using molecular cages as linkers. (4)

Unique host−guest chemistry of the molecular cages located in

the junction of dimers facilitates the reversible trapping of rich

guest molecules only in the hotspot region. All results indicate

that the strategy described here holds great promise for the

rational design and synthesis of nanometer-separated NP

dimers with diverse structures.

2. RESULTS AND DISCUSSION

Three metal−organic molecular cages (MOCs) of diﬀerent

sizes were synthesized in this work (Scheme 1b). These cages

32,33

dimers in low yield.

Moreover, the linking molecules

usually occupy the interparticle region, thus preventing target

molecules, especially with weak aﬃnity to a metallic surface,

from entering into hotspots. Therefore, over the past decade,

although great progress has been made in the construction of

dimer structures, a strategy that can simultaneously address all

aforementioned requirements for eﬃciently creating diverse

plasmonic dimers is still needed.

In this work, we report a technique combining polymer-

assisted self-assembly and molecular cage chemistry to facilely

fabricate plasmonic dimers with a high degree of freedom and

controllability (Scheme 1). The prominent features of this

(

MOC1, MOC2, MOC3) were characterized with thioether

moieties at the periphery for controlled assembly of plasmonic

NPs and a pyridine-decorated cavity favorable for capture of

target molecules. Brieﬂy, three ligands, 3,3′-((4-((methylthio)-

methyl)pyridine-2,6-diyl)bis(ethyne-2,1-diyl))dipyridine (L1),

,3′-((4-((methylthio)methyl)pyridine-2,6-diyl)bis(ethyne-

,1-diyl))dipyridine (L2), and 3,3′-((4-((methylthio)methyl)-

pyridine-2,6-diyl)bis(ethyne-2,1-diyl))dipyridine (L3), were

synthesized to construct MOC1, MOC2, and MOC3,

the Supporting Information, Schemes S1 −S3 and Figures S1 −

S6). The molecular cages were obtained as yellow crystals

through vapor diﬀusion of diethyl ether or ethyl acetate into

dimethylformamide (DMF) solution of the corresponding

ligand and Pd ions in high yields of 80−90%, and their

structures were conﬁrmed by single-crystal X-ray diﬀraction.

The obtained MOCs are lantern-type cages (M L ), where two

Pd ions are bridged by four ligands, forming open and

accessible cavities (Scheme 1). The diameters of these cages

are 1.99 (MOC1), 2.59 (MOC2), and 2.94 (MOC3) nm,

respectively, as conﬁrmed from single-crystal structures

Scheme 1. Schematic Illustration of the Strategy for

Construction of Molecular Cage-Bridged Plasmonic Dimer

Structures (a) and the Chemical Structure of the Used

Molecular Cages (b)

(

Tables S1 −S3 ) and di ﬀ usion-ordered NMR (DOSY)

measurements (Figures S1, S3, and S5). Electrospray

ionization mass spectrometry (ESI-MS) further conﬁrmed

the composition of the three synthesized MOCs with several

dominant peaks corresponding to [Pd L (BF ) ]

(n = 2−

4 (4−n)

4) (Figures S2, S4, and S6).

A four-step approach is involved in our strategy for facile and

eﬃcient synthesis of various dimers, as illustrated in Scheme 1.

First, a polystyrene (PS) thin ﬁlm was spin-coated on a glass

slide with a thickness of about 180 nm, and after thermal

annealing at 130 °C for 20 h followed by a short (3 s)

treatment using O₂plasma, the PS-coated substrate was

immersed in an aqueous solution of poly-

(

diallyldimethylammonium chloride) (PDDA, 0.5 wt %, M_w

100−200 K) for 5 min to form a positively charged substrate,

subsequently used for immobilization of the ﬁrst component of

the dimer to be produced. Second, the deposited NPs were

embedded into the PS ﬁlm with a little exposure of the NP

surface (as binding patches) through the control of heating the

substrate at 388 K for a given duration. Third, after a short (3

s) treatment by O plasma, the substrate with partially hidden

−

NPs was immersed into acetonitrile solutions of MOCs (10

strategy are reﬂected in the following aspects: (1) Eﬃcient

fabrication of pure dimers in high yield without puriﬁcation is

possible and in principle is generally applicable using diﬀerent

M) for 2 min, and then the MOCs assembled on the binding

patches were further used as anchor points for integration of

the second particle component by subsequent exposure of the

substrate to a colloidal solution of desired particles. Finally, the

substrate with the created dimer structures distributed over the

PS surface was swollen in DMF, and under sonication the

dimers were freed from PS polymer ﬁlm. This highly modular

protocol enables the formation of various pure plasmonic

(

size, geometry, and material composition) NPs. (2)

Symmetry of the created dimer structures is ﬂexibly regulatable

and controllable in a modular manner, and thus various

homodimers and heterodimers are facilely accessible. (3) The

molecular cages used exhibit suﬃcient rigidity, and creation of

632

J. Am. Chem. Soc. 2021, 143, 8631−8638

Journal of the American Chemical Society

synthesis of MOC1-bridged Au −Au dimers. Figure 1 a shows a

Article

dimers with high variation of sizes, shapes, and gap-spacing as

well as symmetry in remarkably high yields (85−90%). As a

demonstration, gold nanoparticles of 15 nm and MOC1 were

respectively used as building block and molecular linker, and

well-deﬁned homodimers with a sub-2 nm gap distance were

ﬁrst fabricated. Using scanning electron microscopy (SEM)

and transmission electron microscopy (TEM), we monitored

the surface change of the substrate at each stage of the

Figure 2. TEM (a) and HRTEM images (b) of the dimers bridged by

MOC1 (a1, b1), MOC2 (a2, b2), and MOC3 (a3, b3); UV/vis

spectra of the corresponding dimers (c1−3); simulated distribution of

the electric ﬁeld intensity around the dimers (d1−3).

composites of constituents as well as the symmetry and

interparticle spacing of dimers. Figure 2a1−a3 and Figure

b1−b3 show TEM and high-resolution TEM (HRTEM)

images of the Au−Au dimers (MOC1-bridged dimer, MOC2-

bridged dimer, and MOC3-bridged dimer) using MOC-1,

MOC2, and MOC3 as a linker, respectively. From histogram

analysis of gap distances of the formed dimers (Figure S10),

the gaps were measured to be 1.4, 2.1, and 2.6 nm, respectively,

comparable with the geometrical sizes (Scheme S5) of the

used molecular cages (MOC1−3). These results indicate that

the synthesized molecular cages provide a suﬃcient rigidity for

creating gaps of a few nanometers, and the gap distance of

dimers can be precisely tunable dependent on the size of the

used cages. Figure 2c1−c3 display the optical responses of the

created dimers. As expected, due to the plasmon coupling

between two adjacent nanoparticles, all three dimers exhibit

two distinct absorption peaks. The peak at 530 nm is due to a

transverse plasmon coupling mode, while the peak at longer

wavelength is attributed to a longitudinal plasmon coupling

mode along the interparticle axis and is sensitive to gap

distance. When the gap distance decreases from 2.6 nm to 1.4

nm, the interaction between two NPs becomes stronger,

leading to a lower bonding mode in energy and thus a red-shift

of the longitudinal surface plasmon coupling band from 600

nm to 635 nm. The strong near-ﬁeld plasmon coupling of

adjacent nanoparticles gives rise to huge near-ﬁeld enhance-

ment in the junctions of the dimers, as shown by the simulated

results in Figure 2d1−d3. In our case, the molecular cages were

located only at the junctions of the assembled dimers.

Certainly, this unique architecture allows for precisely

positioning rich guest molecules only in the hotspot region

through host−guest chemistry of the integrated cages, thus is

more favorable for numerous plasmon-mediated applications.

Compared to homodimers, due to the introduction of

symmetry breaking, heterodimers oﬀer huge room for tailoring

plasmonic properties and the emergence of new physical

eﬀects, such as Fano resonances, plasmon-induced trans-

parency, directional scattering, induced energy and charge

Figure 1. SEM images of the initially deposited Au NPs (a), Au NPs

partially embedded into PS ﬁlm (b), and the cage-bridged Au−Au

dimers formed on the substrate (c); TEM images of the created

dimers freed from the substrate (d).

SEM image of the PS-coated substrate with deposited Au NPs

after the ﬁrst step. The density of the Au NPs distributed on

the surface can be readily regulated by varying the surface

density of PDDA. After a thermal annealing (the second step),

the deposited Au NPs were embedded in the underlying PS

thin ﬁlm with little exposure to air, as shown in Figure 1b. In

this step, the precise control of the embedding depth of Au

NPs to reach very little exposure is the key point for

guaranteeing the subsequent assembly of dimers in high purity

and yield without complicated separation processes. In our

case, this embedding depth control was realized by

continuously monitoring the vertical sinking of the Au NPs

with the aid of UV/vis spectroscopy and SEM. Indeed, after

the surface modiﬁcation of the partially embedded Au NPs

followed by further binding of a second NP, we observed

massive well-deﬁned dimer structures distributed over the

entire surface area (Figure 1c). It should be noted that some

dots that seem like single NPs in Figure 1c are actually the

dimers with a stacked conﬁguration, which are readily

distinguished from monomers by their stronger contrast

Figure S7a). Upon the removal of the solid substrate, it

reveals that about 85% of the assemblies are dimer structures

Figure 1d, Figure S8 ). Under TEM observation of the dimers

at high ampliﬁcation, it is found that a spacing distance of ca.

.4 nm between two Au particles was formed, which is

comparable with the diameter of used molecular cage MOC1

Figure 2a1,b1). SEM images of dimers bridged by MOC2 and

(

MOC3 are shown in Figure S9 .

,2,34,35

The modular successive assembly approach demonstrated

above provides a high degree of freedom or ﬂexibility to

modulate the structure parameters of the formed dimers,

including the sizes, shapes, geometries, and material

transfer across the heterointerface etc.

In our work, we

found the modular assembly approach described here has

tremendous potential for creating various heterodimers with

diﬀerent sizes, compositions, and shapes of nanoparticles.

633

J. Am. Chem. Soc. 2021, 143, 8631−8638

Journal of the American Chemical Society

geometry were achieved (Figure 3 a1 −a4), and the respective

Article

Instead of using the same Au NPs, when Au NPs with diﬀerent

sizes (7, 10, 20, and 26 nm) were employed as the second

binding NPs at the third step, heterodimers composed of two

particles of varying sizes but with the same composition and

Figure 4. Representative HRTEM image (a), experimental extinction

spectra (b), simulated extinction spectra (c), and distribution of the

electric ﬁeld intensity (d) of the asymmetric dimers made of 15 nm

Au NPs with 12 nm Ag NPs (1), 16 × 12 nm Pt nanocubes (2), 17

nm Pd nanoﬂowers (3), and 12 nm TiO NPs (4), respectively.

Figure 3. Representative HRTEM images (a), experimental extinction

spectra (b), simulated extinction spectra (c), and distribution of the

electric ﬁeld intensity (d) of the asymmetric dimers made of 15 nm

Au NPs with Au NPs of 7 nm (1), 10 nm (2), 20 nm (3), and 26 nm

simulated ones. To verify the correctness of the conclusion

drawn from the current UV−vis data, exempliﬁed UV−vis data

of the three types of created dimers (Au−Au, Au−Ag, and

Au−Pt) at larger wavelengths were collected and compared

with theoretically simulated ones. Figures S18 −S20 show the

comparison between experimental and theoretical data. It is

found that the absorbance zero of three dimer structures is

located in the infrared range and also the experimental data are

consistent with the theoretically simulated ones. It should be

noted that the similar spectral features obtained from the

absorbance measurements are expected if aggregated dimers

were formed instead. The possibility of the existence of

nanoparticle aggregates in the samples during measurements

cannot be excluded. In the future, we will employ advanced

techniques, such as dark-ﬁeld microscopy and microscopy as

well as scanning near-ﬁeld optical microscopy (SNOM), to

study optical and spectral properties of the individual metallic

monomer and dimer structures to get a further understanding

of the created plasmonic structures in our work.

(

4), respectively.

experimental extinction spectra are shown in Figure 3b1−b4.

In Figure 3b1−b4, as the size of the second Au NP increases,

the longitudinal surface plasmon red-shifts, while the trans-

versal mode remains the same. To understand the plasmon

coupling occurring in the dimers with mismatched particle

sizes, the ﬁnite diﬀerence time domain (FDTD) method is

used to simulate the distribution of enhanced electric ﬁelds in

interparticle regions (Figure 3d1−d4) and the optical proper-

ties (Figure 3c1−c4). It is obvious that, compared with the

homodimers shown in Figure 2, the strength of plasmon

coupling between the adjacent particles and the spectral

properties is distinctly improved and modiﬁed through the

introduction of the asymmetry resulting from size mismatch.

As a further demonstration, Ag NPs (d = 12 nm), Pt NPs (a ×

Figure S21 shows the comparison of plasmonic responses

between dimers and their individual nanoparticle components.

Clearly, a single noble metal nanoparticle generally shows only

one plasmonic absorption mode. When two nanoparticles are

assembled to form a dimer structure, the plasmons of

individual nanoparticles can interact with each other and

hybridize to give rise to new hybridized plasmonic modes. The

hybridized plasmon modes obtained are formed in rigorous

analogy with the formation of molecular orbitals from the

hybridization of individual atomic wave functions in molecular

b = 16 × 12 nm), Pd NPs (d = 17 nm), and TiO NPs (d = 12

nm) were used as building blocks, and the heterodimers

diﬀering in shape and composition were also created in a facile

and module manner, as shown in Figure 4a1−a4 and Figure

S11 . Figures S12 −S17 show the wide-ﬁeld SEM images of the

dimers prepared without systematic optimization of prepara-

tion conditions. Clearly, dependent on the type of used metal

nanoparticles, yields of 50−75% were achieved for the dimer

structures, indicating that our approach is robust for eﬃciently

producing dimer structures. The heterogeneities of these

orbital theory. Clearly, it can be seen that the assembly of

dimers (Au−Ag, Au−Pt, Au−Pd, and Au−TiO heterodimers

metal nanoparticles to form dimer structures will bring new

plasmonic modes and show multiple plasmonic responses.

In recent years, numerous research works demonstrated that

such heterodimers combining plasmonic and catalytic metal

nanoparticles can function as a strongly coupled “antenna−

using MOC1 as the linker) led to a dramatic modulation of

plasmonic properties (Figure 4b1−b4, Figure 4c1−c4, and

Figure 4d1−d4). The experimental extinction spectra (Figure

b1−b4) are in good agreement with the simulated spectra

36,37

(

Figure 4c1−c4), revealing a strong near-ﬁeld coupling

reactor” complex

and exhibit very promising and improved

through the cage-bridged nanogap, corroborating the optical

behavior expected from plasmon hybridization. In our work, all

UV−vis data (experimental and simulated) were acquired in

the range from ultraviolet light to visible light. It is found that

the experimental data are consistent with the theoretically

catalytic performance. Over the past decades, great progress

has been made in the synthesis of nanoparticles with controlled

shape, size, and geometry. Thereby, using the modular

assembly strategy described here, in principle, it is possible

to access various well-deﬁned dimer structures on demand

634

J. Am. Chem. Soc. 2021, 143, 8631−8638

Journal of the American Chemical Society

MOC3, respectively, in bulk solution was conﬁrmed by

Article

with ﬁnely tuned plasmon resonance/coupling and enhanced

light−matter interactions and functionalities.

diﬀerent methods, including H NMR, ESI-MS, and UV −vis

titration (see Figures S22 −S26 for L-AA, Figures S27 −S31 for

PBA, Figures S32 −S34 for salen-Ni, Figures S35 and S36 for

Glu, Figures S37 −S39 for coumarin, and the corresponding

discussion in the Supporting Information). As expected, we

found that the host−guest chemistry of the MOCs integrated

remains unchanged, and the chosen guest molecules were

correctly trapped in our created dimers. It should be noted

that, to avoid tedious separation, the substrate-supported

dimer samples were used in our case (see experimental details

in the Supporting Information). Figure 5b presents the

comparison of the SERS spectra of MOC1, L-AA, and the

MOC1-bridged dimer after treatment with an L-AA solution.

Clearly, upon treatment with L-AA followed by thoroughly

rinsing to remove the physically adsorbed L-AA, L-AA was

successfully trapped in the MOC1-bridged dimers and shows

Besides ﬂexible modulation of structural parameters (like

size, shape, geometry, material composition, symmetry, and

interparticle gap spacing) of the fabricated dimers, the

capability of precisely positioning guest molecules in gap

regions of a few nanometers is critical for full exploitation of

the potential of the dimers. In our case, the integrated MOCs

in the resultant dimers serve as a placeholder for creating a

nanogap of a few nanometers. More importantly, the unique

host−guest interactions of the cages located only in the

junction region provide a driving force to precisely and

reversibly place rich molecules in hotspot regions, providing

huge opportunities for envisioning plasmon-mediated applica-

tions. As a demonstration, the host−guest performance of the

molecular cages incorporated in the Au−Au homodimers was

tested through surface-enhanced Raman detection, construc-

tion of plasmonic tuning, and realization of chirality enhance-

ment.

−

the characteristic peaks of L-AA at 1131 and 1322 cm ,

indicating the successful trapping of L-AA in the hotspot region

(Figure 5b and Table S4 ). Interestingly, after immersing and

shaking the dimer sample in methanol and acetonitrile,

respectively, the characteristic peaks assigned to L-AA

molecules disappeared, indicative of the good renewability of

the cage-bridged dimer. To demonstrate the reversible

trapping of L-AA, we measured the SERS response for L -AA

through three cycles, which showed good reversibility (Figure

S40 ). The SERS enhancement factor (EF) of L-AA at 1322

cm calculated from the experimental spectrum is 8.72 × 10

the calculation of the enhancement factor is shown in the

Supporting Information ). As the majority of target molecules

L-AA) were trapped in the hotspot region, the SERS detection

In this work, important biomolecules L-(+)-ascorbic acid (L-

AA), glucose (Glu), and coumarin, the photoresponsive azo-

molecule 4-(phenylazo)benzoic acid (PBA), and chiral metal

complex salen-Ni were selected as the target guest molecules

(

chemical structures in Figure 5 and Scheme S6 ). First, the

formation of host−guest complexes with MOC1, MOC2, and

−

−8

limitation in our case is found to be 1.74 × 10 M (Figure

S41 ), which is 2 orders of magnitude lower than those

reported in the literature (Table S5 ). Remarkably, for the

case of glucose, which is diﬃcult to detect using conventional

SERS, our dimer SERS substrate allows for determination of its

spectroscopic signatures at 1115, 1070, and 767 cm⁻in the

SERS spectrum acquired (Figure 5d), indicative of the

successful trapping of glucose in the plasmonic hotspot.

Similarly, the unique host−guest property of the molecular

cage integrated in the dimer structure also enables easy

detection of the coumarin probe using the SERS technique

(

Figure S42 ). It should be noted that, compared with the

dimer structures composed of smaller AuNPs, the dimer

structures assembled from larger AuNPs show stronger SERS

signals (Figures S42 −S44 ), exhibiting higher SERS sensitivity.

Interestingly, for the case of (R,R- or S,S-)salen Ni (Figure

e), due to the intense electromagnetic ﬁeld, the chirality of

the chiral metal complexes hosted by the cages located in

hotspot regions was signiﬁcantly enhanced. Figure 5f shows a

comparison of circular dichroism (CD) of the MOC3-bridged

Au−Au dimer, the (R,R- or S,S-)salen-Ni in DMSO (10 M),

and the MOC3-bridged dimer treated with (R,R- or S,S-)salen-

Ni (10 M). Obviously, no CD signal in the visible region was

detected for the MOC3-bridged dimer and the DMSO

−

solution of (R,R- or S,S-)salen-Ni (10 M). However, after

being trapped in the junction of the dimer, both R,R-salen-Ni

and S,S-salen-Ni showed a strong plasmon-induced CD signal

in the range of 550−800 nm. More interesting is the case of

the photoresponsive azo-molecule (Figure 5g). It is found that

the cis−trans transition behavior of azo-molecule PBA remains

in a conﬁned space of the MOC2 in dimers, and the refractive

index variation accompanied by the cis−trans transition of the

PBA molecule can give rise to the reversible modulation of the

Figure 5. Placing important molecules in hotspot regions through

host−guest chemistry of the molecular cages integrated in dimer

structures to demonstrate the construction of plasmon-assisted

applications: (a, b) SERS detection of L-AA with very high sensitivity;

(

c, d) SERS detection of glucose with very high sensitivity; (e, f)

encapsulation of a chiral (R,R- or S,S-)salen-Ni complex to induce

enhanced circular dichroism at the plasmon frequency; (g, h) hosting

photoactive azobenzene to create plasmonic tuning.

635

J. Am. Chem. Soc. 2021, 143, 8631−8638

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.0c13377.

Article

dimer structure, leading to a plasmonic tuning nanodevice

applications. However, the droplet-based assembly approach

described produces only a mixture of diﬀerent kinds of Au NP

clusters, and a tedious separation process such as electro-

phoresis or density gradient centrifugation is required to obtain

high-purity dimers in low yield. Moreover, such a preparation

approach lacks the capability for precise control over the

cluster formation, and thus it is nearly impossible for the

rational design and synthesis of geometry or material

composition asymmetric heterodimer structures. Additionally,

in the droplet-based assembly approach the surface attachment

of the cage linker is not regiospeciﬁc; thus the cages are

randomly distributed on the whole surface of the formed

clusters. As a consequence, the precise placement of a targeted

molecule in a hotspot region, which is very critical for most

plasmon-mediated applications, through host−guest chemistry

of the cages integrated in the resulting clusters, is not

realizable. Compared with our previous work, the present

paper addressed the above-mentioned key issues, and the

eﬃcient fabrication of plasmonic dimers has been achieved

with signiﬁcant advances. Over the past years, molecular cages

or macrocycles have been integrated in some plasmonic

(

Figure 5h, Figures S45 and S46). As mentioned in the

Introduction, when incident photons (light) coherently

oscillate with the free electrons in noble metals, metal particles

generate a highly localized and enhanced electromagnetic ﬁeld.

Such so-called LSPR ﬁeld is highly sensitive to its surrounding

environment. When a chiral molecule is placed near a

plasmonic nanoparticle, plasmonic modes of such molecular−

plasmonic assemblies oﬀer another interesting eﬀect: they can

enhance the originally weak CD signal of the chiral molecule,

as well as produce a new prominent feature in the CD

spectrum at the wavelength of the plasmonic resonance of the

metal nanoparticle. The ﬁrst eﬀect is induced by the highly

localized and enhanced electromagnetic ﬁeld (LSPR). The

latter occurs due to induced chiral currents in the nanoparticle,

caused by the interaction with the nearby chiral molecule, and

opens the possibility of using visible and near-infrared light to

detect molecules with intrinsic CD features in the UV region of

the spectrum. Additionally, due to the high sensitivity of a

plasmonic metal particle to its surrounding environment, the

variation of the surrounding refractive index will induce shifts

of the plasmonic absorptions in the spectrum. The theory

mentioned above can be used to interpret the observations in

Figure 5f and h.

42−45

systems.

To the best of our knowledge, however, the

eﬃcient fabrication of plasmonic dimeric structures with

rational control of all critical structural parameters and at the

same time imparting them with the unique capability of

precisely and reversibly hosting guest molecules in hotspot

regions has hardly been achieved in previous reports.

Over the past years, the concept of coupling noble and

catalytic metal nanoparticles has been extensively utilized to

form antenna−reactor catalytic systems, showing great promise

6,37

in catalysis.

In our work, using the synthesized molecular

cages as a bridge, our strategy described above provides the

possibility to couple noble and catalytic metal nanoparticles, as

demonstrated by the fabrication of asymmetric dimer

structures, such as Au−Pd and Au−Pt (Figure 4). Thus, the

cage-bridged Au−Pd dimer structure was exempliﬁed as a

bimetallic nanoreactor for chemical transformation of 4-

nitrophenol (4-NP). The performed experiments show that

3. CONCLUSIONS

In summary, based on the combination of polymer-assisted

assembly and the use of molecular cages, a new strategy is

described to rationally design and synthesize plasmonic dimer

structures in high yields without tedious separation procedures.

With this method all critical parameters of the prepared

dimers, including the sizes, shapes, geometries, materials and

compositions of the individual nanoparticles, interparticle

spacing, and symmetry, could be modulated in a ﬂexible and

modular manner, and thus various symmetrical and asym-

metrical plasmonic dimers with tailored optical properties are

designable and facilely accessible. More importantly, the

molecular cages integrated into the interparticle gaps endow

the created dimers with the unique capability of precisely and

reversibly hosting rich guest molecules in hotspot regions, thus

providing tremendous opportunities for elaborately devising

numerous plasmon-assisted applications. In our present work,

although only a few types of cage-bridged plasmonic dimers are

demonstrated, the high degree of ﬂexibility and freedom as

well as controllability of the method described here

undoubtedly allows eﬃcient access to a huge number of

dimer structures with diﬀerent conﬁgurations, showing great

promising prospects for fully exploiting the potential of this

nanostructure. Thus, we believe that our work represents a

great advance in engineering plasmonic structures and may

open up new horizons and give enormous new opportunities in

plasmonic-assisted chemistry.

-nitrophenol was catalyzed simultaneously to p,p-dihydrox-

yazobisbenzene (DHAB) and 4-aminophenol (4-AP) under

illumination (633 nm laser) at room temperature (Figure S47),

whereas under the same conditions monometallic Pd nano-

particles cannot produce the expected 4-AP as a result of

photocatalytic reduction of 4-NP (Figures S48 and S49). As a

control experiment, it is found that individual gold nano-

particles can only produce DHAB (Figure S50). These results

40,41

are similar to the observation in the literature.

Thus, our

results further conﬁrm that the concept of coupling of

plasmonic and catalytic metals is valid to enhance the catalytic

eﬃciency of catalytic metal components. In our case, it should

be noted that the cage linkers located in the hotspot region of

the created dimer structures should oﬀer the possibility to

regulate the substrate or intermediate selectivity based on

host−guest chemistry and boost the rate and speciﬁcity of the

desired chemical transformation. In the future, we will perform

a systematic work to explore the unique properties of the cage-

bridged dimer structures as a bimetallic reactor for developing

new light-driven catalytic reactions.

In 2018, we reported the assembly of Au NPs in

microﬂuidic droplets in the presence of a thioether-modiﬁed

molecular cage and the preparation of cage-bridged Au NP

clusters. In our previous work, the molecular cage was

employed for creating plasmonic hotspots, and for the ﬁrst

time Au NP clusters with open and accessible hotspot regions

for reversibly and selectively trapping molecules were achieved,

oﬀering huge opportunities for many plasmon-mediated

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

Chemicals; characterization; detailed experimental pro-

cedures; synthesis of organic ligands and cages; single-

636

J. Am. Chem. Soc. 2021, 143, 8631−8638

Journal of the American Chemical Society

(1) Halas, N. J.; Lal, S.; Chang, W.-S.; Link, S.; Nordlander, P.

Article

REFERENCES

crystal X-ray crystallography; host−guest interaction;

UV binding constant determination; near-ﬁeld and

SERS calculation; UV titrations; chiral detection and

light-responsive plasmonic dimer (PDF)

■

Plasmons in Strongly Coupled Metallic Nanostructures. Chem. Rev.

011, 111, 3913−3961.

2) Ha, M.; Kim, J.-H.; You, M.; Li, Q.; Fan, C.; Nam, J.-M.

Multicomponent Plasmonic Nanoparticles: From Heterostructured

Nanoparticles to Colloidal Composite Nanostructures. Chem. Rev.

Accession Codes

CCDC 1951465 and 1975678 contain the supplementary

crystallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif , or by

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

019, 119, 12208−12278.

(3) Grzelczak, M.; Liz-Marzán, L. M.; Klajn, R. Stimuli-responsive

self-assembly of nanoparticles. Chem. Soc. Rev. 2019, 48, 1342−1361.

(

4) Ma, W.; Xu, L. A.; de Moura, F.; Wu, X.; Kuang, H.; Xu, C.;

Kotov, N. A. Chiral Inorganic Nanostructures. Chem. Rev. 2017, 117,

041−8093.

(5) Fan, J. A.; Wu, C.; Bao, K.; Bao, J.; Bardhan, R.; Halas, N. J.;

Manoharan, V. N.; Nordlander, P.; Shvets, G.; Capasso, F. Self-

Assembled Plasmonic Nanoparticle Clusters. Science 2010, 328,

AUTHOR INFORMATION

■

135−1138.

Corresponding Author

(6) Wang, X.; Tang, Z. Circular Dichroism Studies on Plasmonic

Nanostructures. Small 2017, 13, 1601115.

Guangtao Li − Department of Chemistry, Key Lab of Organic

Optoelectronics & Molecular Engineering, Tsinghua

University, Beijing 100084, China; orcid.org/0000-0003-

8) Zhang, Y.; He, S.; Guo, W.; Hu, Y.; Huang, J.; Mulcahy, J. R.;

7) Hentschel, M.; Schäferling, M.; Duan, X.; Giessen, H.; Liu, N.

Chiral plasmonics. Sci. Adv. 2017, 3, e1602735.

(

127-1715 ; Email: lgt@mail.tsinghua.edu.cn

Wei, W. D. Surface-Plasmon-Driven Hot Electron Photochemistry.

Chem. Rev. 2018, 118, 2927−2954.

Authors

9) Brongersma, M. L.; Halas, N. J.; Nordlander, P. Plasmon-

Li Tian − Department of Chemistry, Key Lab of Organic

Optoelectronics & Molecular Engineering, Tsinghua

University, Beijing 100084, China

Chen Wang − Institute of Chemistry, Center for Nanoscience

and Nanotechnology, The Hebrew University of Jerusalem,

Jerusalem 91904, Israel; orcid.org/0000-0003-2555-

induced hot carrier science and technology. Nat. Nanotechnol. 2015,

10, 25−34.

(10) Yoon, J. H.; Selbach, F.; Langolf, L.; Schlucker, S. Ideal Dimers

of Gold Nanospheres for Precision Plasmonics: Synthesis and

Characterization at the Single-Particle Level for Identification of

Higher Order Modes. Small 2018 , 14 , 1702754.

11) Haran, G.; Chuntonov, L. Artificial Plasmonic Molecules and

Their Interaction with Real Molecules. Chem. Rev. 2018, 118, 5539−

580.

12) Liu, N.; Liedl, T. DNA-Assembled Advanced Plasmonic

Architectures. Chem. Rev. 2018 , 118 , 3032 −305.

13) Li, Y.; Deng, Z. Ag Ion Soldering: An Emerging Tool for Sub-

355

Hongwei Zhao − Department of Chemistry, Key Lab of

Organic Optoelectronics & Molecular Engineering, Tsinghua

University, Beijing 100084, China

Fuwei Sun − Department of Chemistry, Key Lab of Organic

Optoelectronics & Molecular Engineering, Tsinghua

University, Beijing 100084, China

Lombardi, A.; Keyser, U. F.; Baumberg, J. J. Gap-Dependent

nanomeric Plasmon Coupling and Beyond. Acc. Chem. Res. 2019, 52,

3442−3454.

Hao Dong − Department of Chemistry, Key Lab of Organic

Optoelectronics & Molecular Engineering, Tsinghua

University, Beijing 100084, China

Kai Feng − Department of Chemistry, Key Lab of Organic

Optoelectronics & Molecular Engineering, Tsinghua

University, Beijing 100084, China

Peng Wang − Department of Chemistry, Key Lab of Organic

Optoelectronics & Molecular Engineering, Tsinghua

University, Beijing 100084, China

Guokang He − Department of Chemistry, Key Lab of Organic

Optoelectronics & Molecular Engineering, Tsinghua

University, Beijing 100084, China

(14) Weller, L.; Thacker, V. V.; Herrmann, L. O.; Hemmig, E. A.;

Coupling of Ag −Au Nanoparticle Heterodimers Using DNA

Origami-Based Self-Assembly. ACS Photonics 2016, 3, 1589−1595.

(15) Bidault, S.; Devilez, A.; Ghenuche, P.; Stout, B.; Bonod, N.;

Wenger, J. Competition between Forster Resonance Energy Transfer

and Donor Photodynamics in Plasmonic Dimer Nanoantennas. ACS

Photonics 2016, 3, 895−903.

(16) Ross, M. B.; Mirkin, C. A.; Schatz, G. C. Optical Properties of

One-, Two-, and Three-Dimensional Arrays of Plasmonic Nanostruc-

tures. J. Phys. Chem. C 2016, 120, 816−830.

(17) Khurgin, J.; Tsai, W.-Y.; Tsai, D. P.; Sun, G. Landau Damping

and Limit to Field Confinement and Enhancement in Plasmonic

Dimers. ACS Photonics 2017, 4, 2871−2880.

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.0c13377

(18) Zrimsek, A. B.; Chiang, N.; Mattei, M.; Zaleski, S.; McAnally,

M. O.; Chapman, C. T.; Henry, A.-I.; Schatz, G. C.; Van Duyne, R. P.

Single-Molecule Chemistry with Surface- and Tip-Enhanced Raman

Spectroscopy. Chem. Rev. 2017, 117, 7583−7613.

Author Contributions

L.T. and C.W. contributed equally.

(19) Kim, S.; Seo, J.; Park, H. H.; Kim, N.; Oh, J.-W.; Nam, J.-M.

Plasmonic Nanoparticle-Interfaced Lipid Bilayer Membranes. Acc.

Chem. Res. 2019, 52, 2793 −2805.

Notes

(20) Sun, M.; Xu, H. A Novel Application of Plasmonics: Plasmon-

The authors declare no competing ﬁnancial interest.

Driven Surface-Catalyzed Reactions. Small 2012 , 8 , 2777 −2786.

21) Jiang, N.; Zhuo, X.; Wang, J. Active Plasmonics: Principles,

Structures, and Applications. Chem. Rev. 2018, 118, 3054−3099.

22) Xu, L.; Gao, Y.; Kuang, H.; Liz-Marzan, L. M.; Xu, C.

ACKNOWLEDGMENTS

■

MicroRNA-Directed Intracellular Self-Assembly of Chiral Nanorod

The ﬁnancial support from the MOST (2017YFA0204501 and

013CB834502), NSFC (21773135, 22032003, and

1821001), and the Deutsche Forschungsgemeinschaft DFG

Dimers. Angew. Chem., Int. Ed. 2018, 57, 10544−10548.

(23) Wang, C.; Tian, L.; Zhu, W.; Wang, S.; Gao, N.; Zhou, K.; Yin,

(

TRR61) are gratefully acknowledged.

X.; Zhang, W.; Zhao, L.; Li, G. Molecular cage-bridged plasmonic

637

J. Am. Chem. Soc. 2021, 143, 8631−8638

Journal of the American Chemical Society

structures with well-defined nanogaps as well as the capability of

Article

(42) Tao, C.-A.; An, Q.; Zhu, W.; Yang, H.; Li, W.; Lin, C.; Xu, D.;

Li, G. cucurbit[n]urils as a SERS hot-spot nanocontainer through

bridging gold nanoparticles. Chem. Commun. 2011, 47, 867−9869.

(43) Taylor, R. W.; Lee, T.-C.; Scherman, O. A.; Esteban, R.;

Aizpurua, J.; Huang, F. M.; Baumberg, J. J.; Mahajan, S. Junctions for

SERS within gold nanoparticle assemblies using cucurbit[n]uril “glue.

ACS Nano 2011, 5, 3878−3887.

reversible and selective guest trapping. Chem. Sci. 2018, 9, 889−895.

(

24) Zhang, M.; Large, N.; Koh, A. L.; Cao, Y.; Manjavacas, A.;

Sinclair, R.; Nordlander, P.; Wang, S. X. High-Density 2D Homo- and

Hetero- Plasmonic Dimers with Universal Sub-10-nm Gaps. ACS

Nano 2015, 9, 9331−9339.

25) Fang, L.; Wang, Y.; Liu, M.; Gong, M.; Xu, A.; Deng, Z. Dry

Sintering Meets Wet Silver-Ion “Soldering ”: Charge-Transfer Plasmon

Engineering of Solution-Assembled Gold Nanodimers From Visible

to Near-Infrared I and II Regions. Angew. Chem. 2016, 128, 14508−

(44) Kim, J.-M.; Kim, J.; Ha, M.; Nam, J.-M. Cyclodextrin-based

synthesis and host-guest chemistry of plasmonic nanogap particles

with strong, quantitative, and highly multiplexable surface-enhanced

raman scattering signals. J. Phys. Chem. Lett. 2020, 11, 8358−8364.

(45) Kim, N. H.; Hwang, W.; Baek, K.; Rohman, M. R.; Kim, J.;

Kim, H. W.; Mun, J.; Lee, S. Y.; Yun, G.; Murray, J.; Ha, J. W.; Rho, J.;

Moskovits, M.; Kim, K. Smart SERS hot spots: single molecules can

be positioned in a plasmonic nanojunction using host-guest chemistry.

J. Am. Chem. Soc. 2018, 140, 4705−4711.

4512.

26) Kneer, L. M.; Roller, E.; Besteiro, L. V.; Schreiber, R.; Govorov,

(

A. O.; Liedl, T. Circular Dichroism of Chiral Molecules in DNA-

Assembled Plasmonic Hotspots. ACS Nano 2018, 12, 9110−9115.

(

27) Jahn, M.; Patze, S.; Hidi, I. J.; Knipper, R.; Radu, A. I.; Muhlig,

A.; Yuksel, S.; Peksa, V.; Weber, K.; Mayerhöfer, T.; Cialla-May, D.;

Poppa, J. Plasmonic nanostructures for surface enhanced spectro-

scopic methods. Analyst 2016, 141, 756−793.

28) Laxminarayana, G. K.; Rozin, M.; Smith, S.; Tao, A. R.

Modular, polymer-directed nanoparticle assembly for fabricating

metamaterials. Faraday Discuss. 2016, 186, 489−502.

(

29) Shegai, T.; Chen, S.; Miljkovic, V. D.; Zengin, G.; Johansson,

P.; Käll, M. A bimetallic nanoantenna for directional colour routing.

Nat. Commun. 2011, 2, 481.

(

30) Pazos-Perez, N.; Wagner, C. S.; Romo-Herrera, J. M.; Liz-

Marzan, L. M.; Garcia de Abajo, F. J.; Wittemann, A.; Fery, A.;

Alvarez-Puebla, R. A. Organized Plasmonic Clusters with High

Coordination Number and Extraordinary Enhancement in Surface-

Enhanced Raman Scattering (SERS). Angew. Chem. 2012, 124,

2860−12865.

31) Ni, S.; Wolf, H.; Isa, L. Programmable Assembly of Hybrid

Nanoclusters. Langmuir 2018, 34, 2481−2488.

32) Liu, M.; Fang, L.; Li, Y.; Gong, M.; Xu, A.; Deng, Z. Flash ”

preparation of strongly coupled metal nanoparticle clusters with sub-

nm gaps by Ag+ soldering: toward effective plasmonic tuning of

solution-assembled nanomaterials. Chem. Sci. 2016, 7, 5435−5440.

(

33) Chen, G.; Wang, Y.; Tan, L. H.; Yang, M.; Tan, L. S.; Chen, Y.;

Chen, H. High-Purity Separation of Gold Nanoparticle Dimers and

Trimers. J. Am. Chem. Soc. 2009, 131, 4218−4219.

34) Qin, F.; Lai, Y.; Yang, J.; Cui, X.; Ma, H.; Wang, J.; Lin, H.-Q.

Deep Fano resonance with strong polarization dependence in gold

nanoplate −nanosphere heterodimers. Nanoscale 2017, 9, 13222−

35) Biswas, S.; Duan, J.; Nepal, D.; Park, K.; Pachter, R.; Vaia, R. A.

3234.

Plasmon-Induced Transparency in the Visible Region via Self-

Assembled Gold Nanorod Heterodimers. Nano Lett. 2013, 13,

(

287−6291.

36) Zhang, C.; Zhao, H.; Zhou, L.; Schlather, A. E.; Dong, L.;

McClain, M. J.; Swearer, D. F.; Nordlander, P.; Halas, N. J. Al −Pd

Nanodisk Heterodimers as Antenna −Reactor Photocatalysts. Nano

Lett. 2016, 16, 6677 −6682.

37) Halas, N. J. Introductory lecture: Hot-electron science and

microscopic processes in plasmonics and catalysis. Faraday Discuss.

019, 214, 13−33.

38) Cholula-Díaz, J. L.; Lomelí-Marroquín, D.; Pramanick, B.;

Nieto-Arguello, A.; Cantu-Castillo, L. A.; Hwang, H. Synthesis of

(

colloidal silver nanoparticle clusters and their application in ascorbic

acid detection by SERS. Colloids Surf., B 2018, 163, 329−335.

39) Kong, X.-T.; Besteiro, L. V.; Wang, Z.; Govorov, A. O.

Plasmonic chirality and circular dichroism in bioassembled and

nonbiological systems: theoretical background and recent progress.

Adv. Mater. 2020, 32, 1801790.

40) Li, Z.; Wang, R.; Kurouski, D. Nanoscale Photocatalytic

Activity of Gold and Gold-Palladium Nanostructures Revealed by

Tip-Enhanced Raman Spectroscopy. J. Phys. Chem. Lett. 2020, 11,

41) Li, Z.; El-Khoury, P. Z.; Kurouski, D. Tip-enhanced Raman

531−5537.

Imaging of photocatalytic reactions on thermally-reshaped gold and

gold-palladium microplates. Chem. Commun. 2021, 57, 891−894.

638