Supporting Information
Plasmonic Colloidal Nanoparticles with Open Eccentric Cavity via
Acid-induced Chemical Transformation
Won Joon Cho1+, Alum Jung2+, Suenghoon Han2, Sung-Min Lee3, Taewook Kang4, KunHong Lee2, Kyung Cheol Choi3 and Jin Kon Kim1
1
National Creative Research Center for Smart Block Copolymers,
1,2
Department of Chemical
Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk
790-784, Korea, 3Department of Electrical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu,
Daejeon 305-701, Republic of Korea,
4
Department of Chemical and Biomolecular
Engineering, Sogang University, Seoul 121-742, Korea.
[+] These authors contributed equally to this work.
* To whom correspondence should be addressed (E-mail: jkkim@postech.ac.kr)
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1. Formation of colloidal gold nanoparticles with open eccentric cavity
Figure S1 shows FE-SEM images at various microwave irradiation times (3 - 20 min). The
sample was prepared by quick quench to liquid nitrogen after being irradiated for a given
time. At a short irradiation time (3 min), thin sheets of copper hydroxyl-nitrate
(Cu2(OH)3(NO3)) crystal are formed whose the size is ~ 500 nm and thickness of ~ 50 nm (a).
At 10 min irradiation, the sheets were assembled irregularly (b), and then they quickly
transformed into spherical shape at 15 min irradiation (c). Interestingly, the sheets do not
have sharp edge; rather, they exhibit round shape. The assembly into spheres is mainly
attributed to the reduction of the high surface energy for the sheets at high temperature of
100 °C. At 20 min of irradiation, Cu2(OH)3(NO3) starts to be transformed into CuO with an
open cavity (d).
Figure S1. The SEM images at various irradiation times (min). (a) 3, (b) 10, (c) 15, and (d)
20 min. The scale bar in each inset is 200 nm.
2
Figure S2 gives XRD profiles at various irradiation times. For irradiation time less than 15
min, the peaks corresponding to copper hydroxyl-nitrate [Cu2(OH)3NO3] crystals are
observed. At 20 min, some of copper hydroxyl-nitrate crystals are converted to copper oxide
[CuO]. At 60 min irradiation, only CuO crystals exist.
Figure S2. (a) XRD profiles at various irradiation times. (b) XRD profile at 3 min. All of the
peaks correspond to copper hydroxyl-nitrate crystals, (c) XRD profile at 60 min. All of the
peaks correspond to copper oxide crystals
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Figure S3 is a schematic of the formation of an eccentric open cavity. During the
microwave irradiation, copper hydroxyl-nitrate crystals located near the outer surface of
spheres are transformed to copper oxide. Since the volume of unit cell of monoclinic crystal
structure of copper hydroxyl-nitrate crystal (235 Å3; a = 5.6005 Å, b = 6.0797 Å, c =
6.9317 Å, β = 94.619°) is larger than that (81 Å3; a = 4.6837 Å, b = 3.4226 Å, c = 5.1288 Å,
β = 99.54°) of copper oxide, volume contraction should be accompanied by transformation to
CuO.R1,2 Due to using microwave irradiation, this transformation is quickly achieved, which
causes cracks to form near the outer surface. This becomes the main source to form an open
mouth. With further microwave irradiation, Cu2(OH)3(NO3) located inside the spheres is
transformed to CuO, which makes an eccentric cavity.
Figure S3. Scheme for the formation of open eccentric cavity in a spherical nanoparticle.
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We found that the irradiation temperature becomes very crucial to forming an open cavity
on CuO nanoparticle. At 95°C, the open eccentric cavity structure was not observed. The
slow heat transfer did not make the crack at the early stage of reaction. On the other hand, at
105°C, totally different particles were synthesized. In this case, there was no chance to make
sheet assembly into spherical form at the early stage of reaction.
Figure S4. The SEM images of nanoparticles synthesized at two different temperatures: (a)
95°C (scale bar in the inset is 100 nm), and (b) 105°C.
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2. Acid-induced replacement reaction from copper oxide (CuO) to gold
We introduce a new acid-induced chemical transformation to prepare plasmonic gold
nanoparticles having unique and very complex structures from CuO nanoparticles. We first
obtained three kinds of CuO nanoparticles with various morphologies (mint candy-like,
walnut-like, and nanoparticles with eccentric cavity) by using microwave irradiation assisted
hydrothermal reaction. Then, by simple addition of an acidic gold precursor (HAuCl4) in the
aqueous phase at room temperature, we obtained gold nanoparticles, while maintaining the
original structure of CuO. Although a direct replacement of CuO by gold is an unfavorable
reaction, an acidic gold precursor such as HAuCl4 could induce this reaction. Figure S5 gives
SEM images and EELS mappings of three different shapes of nanoparticles replaced by gold.
It is evident that all of the original shapes in CuO are well maintained even after the gold
replacement reaction. .
When non-acidic precursors such as AuCl3, and Au(OH)3 were used, the replacement
reaction from CuO to gold did not occur. Figures S6 and S7 give SEM images and energydispersive X-ray spectroscopy (EDX) spectra for nanoparticles with open eccentric cavity
after the addition of three different gold precursors (AuCl3, Au(OH)3, HAuCl4). The gold
peaks observed at 2.12 and 9.71 keV appear only for HAuCl4, not for the first two precursors.
Thus, the replacement reaction did not occur for non-acidic gold precursors
It is very important to control the molar ratio of [HAuCl4]/[Cu(NO3)2] to maintain the
original shape of CuO. At a molar ratio of 0.5, most nanoparticles do not have an open cavity.
When the molar ratios are further increased, the original spherical shape of CuO is no longer
maintained, but becomes irregular agglomerates, as shown in Figure S8.
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Figure S5. SEM images (the top panel) and EELS mappings (the bottom panel) of three
different CuO nanoparticles replaced by gold. The molar ratio of [HAuCl4]/[Cu(NO3)2] in
solution for all three samples is 0.25. Yellow and red regions indicate gold and CuO,
respectively. The scale bar in each inset is 100 nm.
Figure S6. The SEM images of nanoparticles with open eccentric cavity after the addition of
three different gold precursors: (a) AuCl3, (b) Au(OH)3 and (c) HAuCl4.
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Figure S7. EDX spectra of nanoparticles with open eccentric cavity after the addition of three
different gold precursors: (a) AuCl3, (b) Au(OH)3 and (c) HAuCl4.
8
v
Figure S8. The SEM images of nanoparticles with open eccentric cavity (a) before and (b-d)
after replacement at various molar ratios of [HAuCl4]/[Cu(NO3)2] in solution. (b) 0.25, (c) 0.5,
and (d) 0.75.
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3. Morphology of gold nanoparticle with open eccentric cavity
To observe in detail the inner structure of the gold nanoparticle with open eccentric cavity,
we performed focused ion beam (FIB) milling. Before the milling, the gold nanoparticles
with open eccentric cavity in aqueous solution 50 μl were dropped onto a silicon wafer and
slowly dried in an ambient condition, followed by vacuum drying for 12 h to completely
remove water. Figure S9 gives SEM images of gold nanoparticle at various milling times.
Before the milling, a protective layer of carbon was deposited on the original nanoparticles to
prevent beam damage during the milling process (Figure S9a). Then the selected area was
milled by Ga ions gradually. At short times of milling, the cross-section of the eccentric
cavity is seen, but thin mouth part is not clearly observed due to the collapse (Figure S9b). At
the optimum milling where the exact half of nanoparticles was cut, we clearly observed the
open eccentric cavity. The mouth diameter and the depth of the cavity were 60 nm 150 nm,
respectively (Figure S9c). However, at excess milling, only a small part of the cavity was
observed (Figure S9d).
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Figure S9. SEM images of the sample before milling (a) after various degrees of milling (bd). (b) short milling; (c) optimum milling; and (d) excess milling.
Figure S10(b) gives HR-TEM cross-sectional image of gold nanoparticle with eccentric
cavity. Three regions (I, II, III) are chosen and expanded. Region I shows that gold layer is
deposited with a thickness of ~ 50 nm (I-1). Inside this region, tiny gold nanoparticles of ~ 3
nm are shown (I-2). The boundary between Au and CuO (region II) is clearly distinguished
by crystal size. Crystal with big and irregular size in the left image is CuO, while small gold
nanoparticles are seen in the right image. Region III has well defined CuO crystals,
confirmed by the selected area electron diffraction (SEAD).
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Figure S10. (a) cross-sectional HR-TEM image of sliced particle with ~ 50 nm thickness and
three regions (I,II,III) magnified HR-TEM image of blue circled area in (a); (I-1) the
thickness of the gold layer is ~ 50 nm. (I-2) the nanostructure of gold layer consists of tiny
particles. (II) The boundary between Au and CuO. The left side is CuO, and the right side is
gold. (III) Many grains and nanoparticles in the CuO region. Inset shows well developed
SEAD pattern of copper oxide nanoparticles.
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4. N2 absorption-desorption isotherm by BET
The surface areas of the samples were measured by recording the nitrogen adsorption–
desorption isotherms at 77 K with a Micromeritics Tristar II 3020, and the isotherms were
analyzed by the Brunauer–Emmett–Teller (BET) method. The surface area of gold
nanoparticles with eccentric cavity by measured by BET is 71.2 m2/g. On the other hand,
before replacement, the surface area of neat CuO nanoparticles with eccentric cavity was 70.9
m2/g. There was no surface area change before and after replacement reaction; thus this result
shows that the surface morphology is maintained. For the gold nanoparticles without any
cavity prepared microwave irradiation at 95 oC (see Figure S4a), the surface area is 69.1 m2/g.
Additionally, for a smooth gold nanoparticle with eccentric cavity (the diameter of the
particle = 150 nm, the diameter and depth of the cavity are 60 and 150 nm, respectively), the
surface area is calculated to be only 4.0 m2/g.
5. Estimation of SERS enhancement factor (EF)
The enhancement factors were calculated via Eq (1) by measuring the Raman and SERS
signal intensity at the prominent peak of two analytes (737 cm-1 for adenine, and 1619 cm-1
for crystal violet) with different concentrations.
(1)
in which ISERS and IRef are SERS and Raman reference intensities of the analytes, respectively,
and CSERS and CRef are corresponding concentrations used for SERS and Raman analyses.
The concentration of the reference adenine solution was 1.0 × 10-3 M and the reference
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Raman signal intensity was 5156 at 737 cm-1 at a laser excitation of 785 nm. On the other
hand, 10 ml of the colloidal gold nanoparticles with open eccentric cavity was prepared by
dropping 100 μl of adenine solution (1.0 × 10-10 M), where CSERS is calculated to 1.0 × 10-12
M. An incubation time of ~ 30 min after mixing the solutions is sufficiently long for
obtaining stable SERS signal.
We calculated the number of the colloidal gold nanoparticles (n) inside the irradiated laser
volume (Vlaser = πr2h, in which r and h are the radius of laser beam and the penetration depth
of the laser irradiation) in aqueous solution with two assumptions: (1) The conversion of
Cu(NO3)2 to CuO is 100 % and (2) the volume of one colloidal gold nanoparticle with open
eccentric cavity (V) is given by spherical particle (Vs =(4/3)πR3) minus with the cavity (Vc =
(4/3)πrcavity3). Then n is given by
n = (CMole ÷ NCu) Vlaser
where CMole is the concentration of Cu(NO3)2, and NCu is the mole number of Cu atoms in
one particle (NCu = V   ÷ M, in which V is the volume of one particle,  and M are the
density (6.315 g/cm3) and the molecular weight (79.545 g/mol) of CuO).
Since typical r and h are 0.70 µm, and 1.0 cm, n is calculated to be 480 for 0.1 M
Cu(NO3)2 solution. This large number of particles results in larger SERS intensity and better
reliability (see Figure S16 in section 9 of the supporting information).
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6. SERS intensity of crystal violet for colloidal gold nanoparticles with open eccentric
cavity
Figure S11(a) gives SERS spectra of the colloidal gold nanoparticles with open eccentric
cavity and simply spherical (no cavity) gold nanoparticles in aqueous solution with 10-9 M
crystal violet solution at 785 nm excitation wavelength. Gold nanoparticles without any
cavity were prepared by microwave irradiation at 95 oC (see Figure S4a). SERS intensity at
1619 cm-1 for the colloidal gold nanoparticles with open eccentric cavity is 14 times larger
than that for the colloidal gold nanoparticles without cavity. Figure S11(b-d) gives the SERS
spectra and of EF of colloidal gold nanoparticles with open eccentric cavity in aqueous
solution for crystal violet, one of the prominent SERS-active materials. SERS signal was
detected even at an extremely diluted concentration of crystal violet (1.0 pM), which is
similar behavior for adenine. Thus, colloidal gold nanoparticles with open eccentric cavity
fabricated in this study could be used to detect traces of various SERS-active materials.
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Figure S11. (a) SERS spectra of the colloidal gold nanoparticles with open eccentric cavity
and simply spherical (no cavity) gold nanoparticles in aqueous solution with 10-9 M crystal
violet solution at 785 nm excitation wavelength. (b, c) Raman spectra of the colloidal gold
nanoparticles with open eccentric cavity with various concentration of crystal violet and (d)
the SERS enhancement factors.
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7. Effect of the degree of the gold replacement reaction on SERS spectra
We controlled the replacement reaction of gold from CuO by changing the molar ratio of
[HAuCl4]/[Cu(NO3)2] in the solution. Figure S12 gives UV-vis absorption spectra of the
colloidal gold nanoparticles in aqueous solution prepared by using various molar ratios in
solution. Before gold replacement, there is discernible peak because CuO does not show any
LSPR peak in the wavelength range from 400 to 900 nm. With increasing molar ratio, two
peaks at 598 and 779 nm are clearly seen. The intensity of the two peaks becomes the
maximum at molar ratio of 0.25. At a molar ratio of 0.50, the intensity of the two peaks is
reduced significantly. Interestingly, when molar ratios are larger than 0.75, no clear peak is
seen.
The results are explained by the morphology of nanoparticles prepared at various molar
ratios. As shown already in Figure S8, the nanoparticle does not show open cavity but also
starts to collapse at a molar ratio of [HAuCl4]/[Cu(NO3)2] with 0.5. When the molar ratio is
higher than 0.75, the spherical shape of individual nanoparticle was no longer maintained and
becomes big and irregular agglomerates. We found that the best molar ratio for the formation
of open eccentric cavity and the replacement reaction is 0.25. Thus, the appearance of the
second peak at 779 nm is clearly due to the eccentric cavities in the gold colloidal particles,
while the first peak at 598 nm is related to overall spherical shape of gold nanoparticle.
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Figure S12. UV-vis absorption spectra of the colloidal gold nanoparticles in aqueous solution
prepared by using various molar ratios of [HAuCl4]/[Cu(NO3)2].
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We also found that SERS spectra are greatly changed by the molar ratios of HAuCl4, as
shown in Figure S13 for adenine and Figure S14 for crystal violet (CV). The largest signal
intensity of SERS spectra for both adenine and CV is observed for the sample prepared at a
molar ratio of 0.25. Thus, the existence of eccentric cavity contributes greatly SERS intensity
of SERS-active materials.
Figure S13. Adenine SERS spectra and enhancement factors of the colloidal gold
nanoparticles at various molar ratios of [HAuCl4]/[Cu(NO3)2].
Figure S14. Crystal violet SERS spectra and enhancement factors of the colloidal gold
nanoparticles at various molar ratios of [HAuCl4]/[Cu(NO3)2].
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8. Effect of the open eccentric cavity on SERS spectra with different excitation
wavelength
We employed two more different lasers excited at 633 nm and 514 nm, and the results are
shown in Figure S15. At an excitation of 633 nm, the average Raman signal intensity of the
colloidal gold nanoparticles with open eccentric cavity is just 20 % higher than that of gold
nanoparticle without cavity. The small difference is attributed to the fact that the open
eccentric cavity structure of the colloidal gold nanoparticles does not contribute to the Raman
signal at this wavelength. Rather, gold nanoparticles themselves are well matched with the
plasmon resonance wavelength at 633 nm excitation. On the other hand, at the 514 nm laser
excitation, the intensity for both nanoparticles was very small even when higher
concentration of adenine aqueous solution (10-6 M) was used. This is due to the mismatch of
the plasmon resonance wavelength of gold nanoparticles with the excitation wavelength.
Figure S15. Raman signal intensity of adenine at 737 cm-1, the colloidal gold nanoparticle
with open eccentric cavity (red) and simply spherical (no cavity) gold nanoparticle (blue) in
aqueous solution as different excitation wavelengths and concentration: (a) 785 nm and 10-9
M; (b) 633 nm and 10-9 M; and (c) 514 nm with 10-6 M. Error bars are the standard deviation.
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9. The enhancement factors of the colloidal gold nanoparticle with open eccentric cavity
at various concentrations of particles with adenine aqueous solution.
We found that SERS spectra are greatly changed by the number (n) of the colloidal gold
nanoparticle with open eccentric cavity in 10-9 M of adenine aqueous solution, as shown in
Figure S16. With increasing n, EF increases at first and seems to be level-off at higher n. The
largest SERS enhancement and the smallest deviation in the signal is observed for a sample
with n of 480 per laser irradiated volume (1.54 ⅹ 10-11 L). Thus, the number-averaged
ensemble effect of the colloidal gold nanoparticle with open eccentric cavity contributes
greatly SERS intensity and its reproducibility.
Figure S16. Plot of the enhancement factor versus number of colloidal gold nanoparticles in
adenine aqueous solution per laser irradiated volume.
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