Photoinduced Multiple Spectral Changes of Single Plasmonic Gold Nanospheresby the Aid of Coordination
Ichiro Tanabe and Tetsu Tatsuma*
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505(E-mail: tatsuma@iis.u-tokyo.ac.jp)

One or both of two localized surface plasmon modes (full-surface modeand interface mode) of a commercially available single Au nanosphere on
TiO2 are excited, and the corresponding spectral changes are induced in
the presence of thiocyanate. Similar spectral changes can be simulated
by assuming site-selective oxidation of the Au nanosphere by plasmoninduced charge separation (PICS).

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Vol.43 No.6

2014 p.931–933
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June 5, 2014

The Chemical Society of Japan

Received: February 25, 2014 | Accepted: March 20, 2014 | Web Released: March 28, 2014

CL-140153

Photoinduced Multiple Spectral Changes of Single Plasmonic Gold Nanospheresby the Aid of Coordination
Ichiro Tanabe³ and Tetsu Tatsuma*
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505(E-mail: tatsuma@iis.u-tokyo.ac.jp)
One or both of two localized surface plasmon modes (fullsurface mode and interface mode) of a commercially available
single Au nanosphere on TiO2 are excited, and the corresponding spectral changes are induced in the presence of thiocyanate.
Similar spectral changes can be simulated by assuming siteselective oxidation of the Au nanosphere by plasmon-induced
charge separation (PICS).

Noble metal nanoparticles (NPs) exhibit strong absorption
and scattering of light and generation of optical near ﬁeld atspeciﬁc wavelengths because of localized surface plasmon
resonance (LSPR).1­3 The LSPR wavelength depends largelyon the particle size,4,5 shape,4,6 and dielectric environment.7,8 By
manipulating these factors, the LSPR wavelength can becontrolled. Therefore, plasmonic metal NPs can be used for
optical storage of images or data.9,10 For multiple data storage,
an ensemble of diﬀerent NPs have been used.9­12 We recently
reported storage of 2-bit data in a single Ag NP, which is smaller
than visible light wavelength, on the basis of selective excitationof one or both of the two LSPR modes (full-surface mode andinterface mode).13
The data storage in single Ag NPs is based on plasmoninduced charge separation (PICS)14,15 at a plasmonic NP­
semiconductor interface. PICS is caused by electron transferfrom plasmonic Au,14,16 Ag,9,16 and Cu17 NPs to semiconductor,
possibly due to an external photoelectric eﬀect or hot electron
injection. This phenomenon is applied to a wide range of ﬁelds,
in particular photovoltaic cells and photosensors14,18­21 as well
as photocatalysis.14,22 In the case of Ag NPs, their size and shape
can be controlled by PICS under weak light,11­13 because Ag isoxidized to Ag+ preferentially at the sites where the plasmonicnear ﬁeld is strongly localized.23 An ensemble of Ag NPs withdiﬀerent sizes on the TiO2 semiconductor shows a broad
absorption band over the visible region. If the ensemble isirradiated at a certain wavelength, only Ag NPs resonant at thewavelength are oxidized due to PICS, resulting in a wavelengthselective decrease of optical absorption. It allows multicolor
photochromism and multiwavelength data storage.9,11 By using aAg nanorod ensemble with diﬀerent aspect ratios, spectral dips
can also form in the near-infrared region, leading to nearinfrared photochromism.12 The data storage in single Ag NPs isbased on the site-selective oxidation of Ag NPs by excitation at
two diﬀerent wavelengths.13However, the stored data are volatile, because Ag NPs onTiO2 are gradually oxidized under ambient light unless the NPs
are protected, for instance, with alkanethiol self-assembled
monolayer (SAM).24 This issue can be addressed by using Au,
which is more stable than Ag because of more positive redox
potential. However, Au NPs are not oxidized by PICS becauseof their high stability. In this study, we shift the redox potentialChem. Lett. 2014, 43, 931–933 | doi:10.1246/cl.140153

of Au negatively25 with the aid of coordination26 and demonstrate PICS-based two-wavelength optical data storage in acommercially available single Au nanosphere. The recorded
information is stable after the removal or inactivation of theligands.25
A TiO2 ﬁlm was prepared on a Pyrex glass sheet from aethanolic titanium alkoxide solution (NDH-510C, Nippon Soda)
by a standard dip-coating technique (withdrawal rate was
2 mm s¹1, calcined at 500 °C for 1 h). Its thickness wasca. 150 nm and the surface roughness was ca. 10 nm. Commercially available monodisperse Au nanospheres (diameter:150 nm, Tanaka Kikinzoku Kogyo) or Ag nanospheres(diameter: 100 nm, Sigma-Aldrich) were cast on the TiO2 ﬁlm(50 ¯L cm¹2) and left in dark for 3 h or 5 min, respectively,followed by rinsing with ultrapure water. The optical propertiesof the single metal nanospheres were measured by dark-ﬁeldmicrospectroscopy, in which scattering light from a plasmonresonant NP can be observed.3,13 Figure 1 shows typical darkﬁeld images (obtained using BX-51, Olympus), corresponding
scattering spectra (using C10027, Hamamatsu Photonics), andatomic force microscope (AFM) images (by NanoNavi Station/SPA400, SII Nanotechnology with Morphology Filter for
elimination of the eﬀect of the tip shape) of single Au(Figure 1a) and Ag (Figure 1b) nanospheres on TiO2. The
particle morphology can be precisely measured by AFM,2,11
whereas a scattering light spot observed by dark-ﬁeld microscopy is inevitably larger than the particle size3,13 because of thediﬀraction limit and secondary scattering.

Figure 1. (a, b) Typical dark-ﬁeld image, corresponding
scattering spectra, and AFM images of a single (a) Au and(b) Ag nanosphere on TiO2. (c, d) Scattering spectra anddistributions of electric ﬁeld intensity calculated by a FDTD
method for a (c) Au and (d) Ag NP on TiO2.
© 2014 The Chemical Society of Japan | 931

Au nanospheres exhibit a broad LSPR band from 500 to>700 nm and green scattering light. On the other hand, the Agnanospheres show two LSPR bands in the shorter- and longerwavelength regions than ca. 540 nm and orange scattering light.
We have assigned the two bands of the Ag nanospheres toelectric ﬁeld oscillation distributed over the entire particle(full-surface mode) and that localized at around the Ag­TiO2
interface (interface mode) by a ﬁnite-diﬀerence time-domain(FDTD) simulation on the basis of a model with a 100-nm Ag
sphere whose bottom is cut by 8% on a ﬂat TiO2 (Figure 1d,inset; this model was optimized by spectral ﬁtting).13 In
addition, we selectively excited one or both of the two modes,resulting in the selective suppression of the corresponding
mode(s) and multiwavelength spectral changes.13 In the case of
the Au nanospheres, which exhibit only one broad LSPR band,multiple spectral control is not possible if the band is based on asingle LSPR mode. However, if the band consists of more thanone LSPR mode, the multiple control could be possible.
We, therefore, applied essentially the same model to the Au
nanosphere. To examine the possibility that the single Au
nanosphere has more than one LSPR mode, the scattering
spectrum and electric ﬁeld distribution were simulated based on
the model with a 150-nm Au nanosphere whose bottom is cut by
8% (Figure 1c, inset). As shown in Figure 1c, the calculated
scattering spectrum has two LSPR peaks at about 540 and
700 nm, which are assigned to the full-surface and interface
modes, respectively. In the case of Ag NP, the experimentallymeasured spectrum shows much broader bands than thecalculated ones, likely because the particle shape and contact
with TiO2 are not completely isotropic, the light incident angle isbroad, and the light is not polarized. Because the same things
should also hold for the Au NP and the separation of the two
calculated peaks is smaller than that for the Ag NP, there is apossibility that the experimentally observed broad LSPR band of
the Au NP consists of two broad, poorly separated LSPR modes.
If so, each mode could be excited by irradiating with shorteror longer-wavelength light.
With this in mind, Au nanosphere­TiO2 samples wereirradiated with light at 500­600, 600­700, or 500­700 nm(ca. 5 mW cm¹2, 30 min) to excite one or both the LSPR modesin the presence of SCN¹ (in aqueous 1 M KSCN, pH wasadjusted to ca. 4.0 using a phthalate buﬀer27). The light source
was a Xe lamp (Luminar Ace LA-410UV, Hayashi Watch Works)
equipped with a long-pass ﬁlter (²500 nm, SCF-50S-50Y or²600 nm, SCF-50S-60R, Sigma Koki) and a short-pass ﬁlter(¯600 nm, GRF-530G or ¯700 nm, CLDM-50S, Sigma Koki).
No spectral change was observed for a single Au nanosphere in
the absence of SCN¹ because of highly positive standard electrode potentials of Au (+1.83 and +1.53 V vs. NHE for Au0/+and Au0/3+, respectively). In the presence of SCN¹, however,the potential shifted largely in the negative direction (Au +4SCN¹ ¼ [Au(SCN)4]¹, +0.64 V vs. NHE),28 and the scattering
spectrum of a single Au nanosphere was changed by lightirradiation (Figures 2a­2c). As the corresponding diﬀerence
spectra clearly show, the scattering intensity decreases at aroundthe irradiation wavelengths. When the sample was irradiated with500­600-nm light, scattering intensity in this region decreased,
and that in the longer wavelength region increased (Figure 2d).
In the case of 600­700-nm light, the scattering intensity in thisregion decreased (Figure 2e). In addition, when we irradiated932 | Chem. Lett. 2014, 43, 931–933 | doi:10.1246/cl.140153

Figure 2. (a­c) Typical scattering spectra of a single Au NP(black) before and (colored) after light irradiation. (d­f)
Diﬀerence spectra corresponding to panel a­c. Irradiation
wavelength: (a, d) 500­600, (b, e) 600­700, and (c, f) 500­
700 nm.
Table 1. Changes in the scattering intensity upon lightirradiation
Irradiation
wavelength/nm
500­600
600­700
500­700
aChanges in the scattering intensity/%aAt 550 nm
At 650 nm
79.8 « 5.1
105.5 « 5.1
98.5 « 3.9
79.7 « 9.6
65.3 « 17.4
64.2 « 17.9

Average « standard deviation, n = 5.

the sample with 500­700-nm light, the scattering intensity wassuppressed in the entire visible region (Figure 2f). The changes
in the scattering intensity were reproducible, as summarized inTable 1. Such multiple spectral changes cannot occur if eachsingle Au nanosphere exhibits only one LSPR mode. Therefore,
we conclude that the broad LSPR band of a Au NP includes atleast two LSPR modes, as predicted theoretically.
The spectral changes upon light irradiation should reﬂect
certain morphological changes of the Au nanospheres, whichmust be caused by oxidative dissolution of Au (Au +4SCN¹ ¼ [Au(SCN)4]¹) based on PICS. In the case of AgNPs, it has been reported that the PICS-based oxidative
dissolution of Ag proceeds preferentially at the sites where theplasmonic near ﬁeld is strongly localized.23 Regarding single
Ag nanospheres on TiO2, it has also been conﬁrmed that theoxidative dissolution occurs at the entire particle surface andAg­TiO2 interfacial region, when the full-surface and interface
modes, respectively, are excited.13 Therefore, it is reasonable
to expect that the site-selective oxidation also proceeds for Au
nanospheres. We, therefore, calculated the scattering spectra formodels based on this assumption (Figures 3a and 3b, inset). In
both the models, it is assumed that 5% of Au is dissolved. The
calculation results and the corresponding diﬀerence spectra areshown in Figure 3.

© 2014 The Chemical Society of Japannanospheres (150-nm diameter) on TiO2 based on the excitationof one or both the LSPR modes (full-surface and interfacemodes). Similar spectral changes can be simulated by assuming
PICS-based oxidative dissolution of Au at the sites where theplasmonic near ﬁeld is localized. Such multiple spectral changesof single Au nanospheres would allow application to stable
multiple data storage nanodevices and nanooptical devices. Evensmaller and higher-density devices might be developed by using
smaller Au NPs.
This work was supported in part by JSPS Grant-in-Aid
for Scientiﬁc Research on Priority Area “Coordination Programming” (Area No. 2107) No. 24108708 and Grant-in-Aid
for Challenging Exploratory Research No. 25600002. I.T.
thanks a JSPS Research Fellowship for Young Scientists.
Figure 3. (a, b) Simulated scattering spectra (black) beforeand (colored) after morphological changes (insets). (c, d)Diﬀerence spectra corresponding to panel a and b.
When the oxidative dissolution of Au occurs at the entire
surface and the particle becomes smaller by the full-surface
mode excitation, the scattering intensity due to the full-surface
mode decreases and the peak due to the interface mode slightly
red shifts (Figure 3a). The corresponding diﬀerence spectrum(Figure 3c) is in good qualitative agreement with the experimental result (Figure 2d). On the other hand, in the case of
selective dissolution at the interfacial region by the interface
mode excitation, the peak based on the interface mode blue
shifts (Figure 3b). In addition, the scattering intensity based on
the full-surface mode slightly increases, and the diﬀerence
spectrum (Figure 3d) shows an increase in the intensity around500­650 nm. On the other hand, in the experimentally obtained
diﬀerence spectrum (Figure 2e), no signiﬁcant increase in thescattering intensity was observed even in the shorter-wavelength
region, whereas the intensity decreased in the longer-wavelength
region. This is because the separation between the full-surface
and interface modes is not enough, as described above;
therefore, the oxidative dissolution of Au proceeds not only in
the interfacial region but also in other regions. Even so, weachieved the multiple spectral changes by light irradiation at
diﬀerent wavelengths, as shown in Figure 2. Irradiation at both
wavelengths decreases the scattering intensity in the entre visible
region, as shown in Figure 2f.
On the basis of the present wavelength-selective spectralchanges, a commercially available and simple single Au
nanosphere, which is smaller than the visible light wavelength,
can serve as a two-wavelength optical memory device. Althoughthiocyanate is necessary for data writing, it can be easily
removed by oxidation.29 Therefore, the present system would
allow the development of high-density, read-only opticalmemories with high stability. The present results can besimulated based on the assumption that the PICS-based
oxidative dissolution of Au NPs proceeds preferentially at sites
where the plasmonic near ﬁeld is strongly localized, as is thecase for Ag NPs.
In summary, photoinduced multiwavelength spectralchanges were achieved for commercially available, single Au

Chem. Lett. 2014, 43, 931–933 | doi:10.1246/cl.140153

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