Nano Res.
Electronic Supplementary Material
Matryoshka-caged gold nanorods: Synthesis, plasmonic
properties, and catalytic activity
Wei Xiong1,2,3, Debabrata Sikdar4, Lim Wei Yap1,2, Pengzhen Guo1,2, Malin Premaratne4, Xinyong Li3,
and Wenlong Cheng1,2 ()
1
Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia
The Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, VIC 3168, Australia
3
Key Laboratory of Industrial Ecology and Environmental Engineering and State Key Laboratory of Fine Chemicals, School of
Environmental Sciences and Technology, Dalian University of Technology, Dalian 116024, China
4
Advanced Computing and Simulation Laboratory (AL), Department of Electrical and Computer Systems Engineering, Monash
University, Clayton, VIC 3800, Australia
2
Supporting information to DOI 10.1007/s12274-015-0922-8
Experimental section
Chemicals: Gold (III) chloride trihydrate (HAuCl4), hexadecyltrimethylammonium bromide (CTAB), silver nitrate
(AgNO3), sodium borohydride (NaBH4), L-ascorbic acid (AA), cetyltrimethylammonium chloride (CTAC, 25 wt.%
in H2O), polyvinylpyrrolidone (PVP), and 4-nitrophenol (4-NTP) were obtained from Sigma Aldrich. All glassware
and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by thorough rinsing with deionized
water before drying in an oven at 80 °C.
Synthesis of gold nanorods: The gold nanorods (GNRs) were prepared according to the well-established
seed-mediated growth method [S1]. A brownish-yellow seed solution was prepared by mixing CTAB (5.0 mL,
0.2 M) and HAuCl4 (5.0 mL, 0.5 mM) followed by adding ice-cold NaBH4 (0.6 mL, 0.01 M). The seed solution
was aged at 27 °C before seeding to growth solution. Then, CTAB (25 mL, 0.20 M) and HAuCl4 (25 mL, 1.0 mM)
were added to AgNO3 solution (1 mL, 4 mM) in sequence and with the addition of AA (0.4 mL, 0.08 M), the
yellowish mixture became colorless and the growth solution was obtained. To grow NRs, 60 μL of premade
seed was added into the growth solution and aged at 30 °C for two hours. The CTAB capped GNRs were
collected by centrifugation (7,000 rpm for 30 min), washed with water twice and redispersed in CTAC solution
(5 mL, 80 mM).
Synthesis of AuNR@AgNCs: The AuNR@AgNCs were prepared according to the procedure reported previously
[S2]. The 1 mL GNRs solution was dispersed in 9 mL CTAC solution and stirred at 65 °C in water bath. Then
AgNO3 (1 mL, 10 mM) and AA (0.5 mL, 100 mM) were added in sequence. The resultant solution was continually
stirred at 65 °C for 3 h. The AuNR@AgNCs were collected by centrifugation (7,000 rpm for 10 min) and washed
with water twice.
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Nano Res.
Synthesis of caged gold nanorods: The CGNRs were prepared via the galvanic replacement reaction [S3].
The AuNR@AgNCs prepared were centrifuged and redispersed in 2.5 mL deionized water, followed by the
addition of CTAB (5 mL, 0.2 M) solution and PVP (2.5 mL, 2 wt.%) solution sequentially. The resultant solution
was heated at 90 °C for 2 minutes, following by mixing with HAuCl4 (4 mL, 0.5 mM) for another 10 minutes.
The CGNRs were collected by centrifugation (7,500 rpm for 10 min) and washed with water twice.
Numerical modeling: The numerical simulations were performed using the commercial package CST Microwave
StudioTM Suite. The frequency-domain finite element method (FEM) solver was used to obtain the light absorption
spectra of the matryoshka-caged gold nanorods (mCGNRs), as a function of wavelength of the incident light.
Open boundaries were used so that incident waves can pass those boundaries with minimal reflections, thus
essentially emulating a perfectly matched layer (PML) condition. Tetrahedral mesh, which is more accurate at
metallic material interfaces, was used in the frequency-domain simulations with automatic mesh refinement to
study the optical response over the wavelength window of interest.
At first, a cylindrical nanorod of gold was modeled with hemispherical caps at both ends. Then the simulation
models (see Fig. S2) for mono-CGNR, bi-CGNR, tri-CGNR, tetra-CGNR, penta-CGNR, and hexa-CGNR were
developed considering the length, width, and cage thickness of each structure, as given in Table S1. Note that, the
cage walls were considered to be solid (with no holes) and the hollow spaces in each structure were considered
to be filled with aqueous solution (with relative permittivity of 1.7689). Moreover, for all the simulations we
considered the target mCGNR structure to be immersed in aqueous solution. The permittivity of the gold
material for the nanorod was obtained from the bulk gold permittivity values [S4], whereas for gold in cage
walls additional size-dependent corrections [S5, S6] to bulk permittivity values were incorporated. This considers
the effect of enhanced electron surface scattering losses in the thin metallic cage walls. In order to obtain the
optical absorbance spectra of such mCGNR nanoparticles in solution (where the interparticle spacing are large
enough to ignore interparticle coupling), we calculated the absorbance spectra of a single-particle for unpolarized
light by obtaining weighted average of the spectra for light polarizations along longitudinal and transverse edges
of the nanoparticle. The absorption spectra (see Fig. 3) clearly show the trends of red-shift, spectral broadening,
and strengthening of plasmon resonance peak with increase in number of matryoshka layers on the mCGNR
structure.
Figure S1 Transmission electron microscopy (TEM) image of bi-CGNRs.
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Nano Res.
Figure S2 Simulation models used for calculation of the absorbance spectrum in Fig. 3(b) of (a) mono-, (b) bi-, (c) tri-, (d) tetra-,
(e) penta-, and (f) hexa-CGNRs, having the physical dimensions given in Table S2. The aqueous background has relative permittivity of
1.7689.
Table S1
Physical dimensions of the mCGNRs as measured from scanning electron microscope (SEM) images
Length (nm)
Width (nm)
Thickness (nm)
NR
46.1
19.5
CGNR
61.6
37.5
3.2
bi-CGNR
72
47.6
3
tri-CGNR
80
55
2.7
tetra-CGNR
87
63
2.6
penta-CGNR
94
71
2.4
hexa-CGNR
99
77
2.3
Figure S3 UV-Vis spectra of 4-NTP at different reaction time with bi-CGNRs, indicating the disappearance of the peak for 4-NTP due to
the reduction of –NO2 group into –NH2 group.
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Nano Res.
Figure S4 (a) UV-Vis spectra of bi-CGNRs prepared by increasing amount of HAuCl4 and related TEM images: (b) bi-CGNR0.1,
(c) bi-CGNR0.4, and (d) bi-CGNR0.7.
Figure S5 Photograph of 4-NTP reduction by NaBH4 catalysized by polyurethane foam supported hexa-CGNRs for 7 cycles.
References
[S1]
Nikoobakht, B.; El-Sayed, M. A. Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method.
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[S2] Jiang, R. B.; Chen, H. J.; Shao, L.; Li, Q.; Wang, J. F. Unraveling the evolution and nature of the plasmons in (Au core)–(Ag shell)
nanorods. Adv. Mater. 2012, 24, OP200–OP207.
[S3] Xiong, W.; Sikdar, D.; Walsh, M.; Si, K. J.; Tang, Y.; Chen, Y.; Mazid, R.; Weyland, M.; Rukhlenko, I. D.; Etheridge, J. et al. Singlecrystal caged gold nanorods with tunable broadband plasmon resonances. Chem. Commun. 2013, 49, 9630–9632.
[S4] Johnson, P. B.; Christy, R. W. Optical constants of the noble metals. Phys. Rev. B 1972, 6, 4370–4379.
[S5] Sikdar, D.; Rukhlenko, I. D.; Cheng, W. L.; Premaratne, M. Effect of number density on optimal design of gold nanoshells for
plasmonic photothermal therapy. Biomed. Opt. Express 2013, 4, 15–31.
[S6] Sikdar, D.; Rukhlenko, I. D.; Cheng, W. L.; Premaratne, M. Tunable broadband optical responses of substrate-supported metal/
dielectric/metal nanospheres. Plasmonics 2014, 9, 659–672.
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