Light-triggered self-assembly of gold nanoparticles based on photoisomerization of spirothiopyran

Article abstract of DOI:10.1002/anie.201302430

Give it some stick: UV irradiation of spirothiopyran in an aqueous solution of gold nanoparticles (AuNPs) gives AuNP aggregates. The aggregate size is tuned by the photoirradiation time and the aggregation is facilitated by the covalent binding of photois

Full text of DOI:10.1002/anie.201302430

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Communications
Nanoparticles
Y. Shiraishi,* K. Tanaka, E. Shirakawa,
Y. Sugano, S. Ichikawa, S. Tanaka,
T. Hirai
&&&& — &&&&
Light-Triggered Self-Assembly of Gold
Nanoparticles Based on
Photoisomerization of Spirothiopyran
Give it some stick: UV irradiation of
spirothiopyran in an aqueous solution of
gold nanoparticles (AuNPs) gives AuNP
aggregates. The aggregate size is tuned
by the photoirradiation time and the
aggregation is facilitated by the covalent
binding of photoisomerized spirothio-
pyran to the AuNPs surface. This binding
decreases the electrostatic repulsion
between the AuNPs.
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DOI: 10.1002/anie.201302430
Nanoparticles
Light-Triggered Self-Assembly of Gold Nanoparticles Based on
Photoisomerization of Spirothiopyran**
Yasuhiro Shiraishi,* Kazuya Tanaka, Eri Shirakawa, Yoshitsune Sugano, Satoshi Ichikawa,
Shunsuke Tanaka, and Takayuki Hirai
Gold nanoparticles (AuNPs) are unique building blocks with
sophisticated optical,[1] electronic,[2] and chemical[3] properties
that differ dramatically from those of the bulk metal. They
have potential for cancer diagnosis and therapy on account of
their surface plasmon resonance (SPR) enhanced light
scattering and absorption properties.[4] Owing to their high
surface-to-volume ratio and high surface energy, AuNPs
exhibit unusual catalytic[5] and photocatalytic activities[6,7] as
well as electronic properties.[8] These properties are strongly
size-dependent;[9] therefore, accurate size control of AuNPs is
a challenge for advanced processing of AuNPs.
molecules to the surface for aggregation, is necessary for
rapid and simple processing of AuNPs.
Herein we report a simple method for the light-triggered
self-assembly of AuNPs, which does not require the prepa-
ration of AuNPs modified with photoresponsive molecules
and promotes aggregation with only the minimum amount of
the molecules. We use spirothiopyran (1), a sulfur-containing
spiropyran dye which has photochromic properties, as an
initiator for aggregation (Scheme 1a). As shown in Sche-
me 1b, 1, when dissolved in aqueous solution with AuNPs,
Self-assembly of AuNPs is one powerful method for the
size control, a simple and low-cost method to produce
ensembles of AuNPs in a controllable manner.[10–12] Light-
triggered methods have been studied extensively[13–24] because
light is non-invasive and can be delivered instantaneously to
a precise location. All of the early reported methods employ
AuNPs modified with covalently bound photoresponsive
molecules. These AuNPs, when left in the dark, are well
dispersed in solvents because of the electrostatic repulsion
between AuNPs. Photoactivation of the surface molecules
promotes several reactions, such as isomerization of azoben-
zenes,[13–21] dimerization of thymine,[22,23] and coupling of
aldehyde with amine moieties.[24] These reactions change the
surface polarity, surface electronic charge, or particle geom-
etry, promoting aggregation of the AuNPs. All of these
methods, however, suffer from a fundamental problem:
AuNPs need to be modified with an excess amount of
photoresponsive molecules in advance, and many of these
molecules are left unused for particle aggregation. The design
of a new method, which avoids the preparation of surface-
modified AuNPs and only attaches the minimum amount of
Scheme 1. a) Photoisomerization of spirothiopyran and b) mechanism
for light-induced aggregation of AuNPs.
[*] Dr. Y. Shiraishi, K. Tanaka, E. Shirakawa, Dr. Y. Sugano, Prof. T. Hirai
Research Center for Solar Energy Chemistry and Division of
Chemical Engineering, Graduate School of Engineering Science,
Osaka University
exists as a spirocyclic (SP) form and scarcely associates with
AuNPs. UV irradiation promotes photoisomerization of 1 and
produces spirocycle-opened merocyanine (MC) form. Its
thiolate moiety becomes bonded to the AuNPs surface. This
changes the electronic charge of AuNPs surface and triggers
aggregation. This simple system successfully produces aggre-
gates of AuNPs with tunable sizes and narrow size distribu-
tions.
Toyonaka 560-8531 (Japan)
E-mail: shiraish@cheng.es.osaka-u.ac.jp
Dr. S. Ichikawa
Institute for NanoScience Design, Osaka University (Japan)
Dr. S. Tanaka
Department of Chemical, Energy and Environmental Engineering,
Kansai University (Japan)
Compound 1 was obtained, by condensation of 5-nitro-
salicylaldehyde with N,N-dimethylthiocarbamoyl chloride
and subsequent condensation with 1,3,3-trimethyl-2-methyl-
eneindoline, as a yellow solid (overall yield: 63%).[25,26] The
purity of 1 was confirmed by 1H, 13C NMR spectroscopy and
[**] This work was supported by the Grant-in-Aid for Scientific Research
(No. 23656504) from the Ministry of Education, Culture, Sports,
Science and Technology (Japan) (MEXT).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302430.
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EI-MS analysis (Figure S1–S3, Supporting Information).
Figure 1 shows the absorption spectra of 1 (20 mm). In the
dark, 1 shows almost no absorption at over 450 nm, and the
spectrum does not change even when the sample is left for
SPR band, although a band assigned to the SP form of
1 appears at 369 nm. As shown in Figure 3a, the transmission
electron microscopy (TEM) image of the sample shows highly
Figure 1. a) Absorption spectra of 1 (20 mm) measured in a buffered
water/MeCN mixture (7/3 v/v; HEPES 10 mm, pH 7.0) at 258C, under
irradiation of 280 nm light. b) Change in 531 nm absorbance by
sequential UV–visible light irradiation.
12 h, indicating that 1 exists as a SP form (Scheme 1a) and
scarcely undergo thermal isomerization.[27] Irradiation of the
solution with 280 nm light, however, creates a distinctive
absorption at 531 nm, assigned to the spirocycle-opened MC
form. As shown in Figure S4 (Supporting Information),
irradiation of the solution with 530 nm light reverts the
spectrum back to that for SP form. As shown in Figure 1b,
sequential UV/Vis light irradiation of the solution promotes
repeated spectral change. These data suggest that 1 undergoes
reversible SPQMC isomerization by UV/Vis irradiation
(Scheme 1a).
AuNPs were simply prepared according to a literature
procedure,[28] by the reduction of HAuCl4 with trisodium
citrate in water and g-aminobutyric acid (GABA), a surface
protecting agent. Figure 2 shows the absorption spectra of
AuNPs stabilized with GABA. As shown by the green line,
the solution exhibits a strong SPR band arising from the
AuNPs at 527 nm. As shown by the red line, stirring the
solution with 1 (5 mm) in the dark for 12 h scarcely changes the
Figure 3. TEM images of AuNPs (0.12 nm) stabilized with 4.8 mm
GABA in a buffered water/MeCN mixture (7/3 v/v; pH 7.0), measured
a) after stirring with 1 (5 mm) in the dark for 60 min, and after stirring
with 1 (5 mm) under 280 nm irradiation for b) 15, c) 20 and d) 30 min.
TEM observations were carried out after the sample grid was dipped
into the respective solutions and dried in vacuo.
dispersed AuNPs with an average diameter of approximately
30 nm. In contrast, as shown by the black line in Figure 2,
irradiation of the solution with 280 nm light decreases the
SPR band and creates red-shifted band at over 600 nm,
assigned to the interparticle-coupled plasmon excitons of
aggregated AuNPs.[29] TEM observation (Figure 3b–d)
indeed shows the formation of aggregated AuNPs and the
size of the aggregates increases with photo-irradiation time.
As shown in Figure S5 (Supporting Information), UV irradi-
ation of an AuNPs solution without 1 shows almost no
spectral change. In addition, the solution containing aggre-
gated AuNPs obtained after UV irradiation in the presence of
1, when left in the dark for 12 h, scarcely promotes further
aggregation. These findings suggest that photoactivation of
1 triggers aggregation of AuNPs.
Figure 4 shows the change in hydrodynamic diameter of
AuNPs with photoirradiation time, determined by a dynamic
laser scattering analysis. As shown in Figure 4b (black), the
size of aggregates increases from 30 to 500 nm with photo-
irradiation time, and the aggregates maintain narrow size
distribution with the standard deviation being less than 25%.
This result suggests that the method successfully creates
aggregates with tunable sizes and narrow size distributions. It
is also noted that the aggregation can be precisely controlled
by on–off switching of UV irradiation. Figure 5 shows the
change in the 740 nm absorbance of the AuNPs solution
during the repeated on–off sequence of UV irradiation.
Turning off the light completely suppresses the absorbance
Figure 2. Absorption spectra of AuNPs (0.12 nm) stabilized with
4.8 mm GABA in a buffered water/MeCN mixture (2 mL; 7/3 v/v;
HEPES 10 mm, pH 7.0), green: after stirring without 1 in the dark for
1 h, red: after stirring with 1 (5 mm) in the dark for 12 h, and black:
after stirring with 1 (5 mm) under irradiation of 280 nm light. The
numbers denote the irradiation time (min). Inset: photos of the
solution at the indicated times.
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the electrostatic repulsion between AuNPs and triggers their
aggregation.
The adsorption of the MC form of 1 onto the AuNPs
ꢀ
surface by the covalent Au S bonding is confirmed by X-ray
photoelectron spectroscopy (XPS). Figure 6 shows the XPS
Figure 6. XPS spectrum (S 2p region) of AuNPs (0.12 nm) stabilized
with 4.8 mm GABA, recovered by centrifugation a) after stirring with
1 (5 mm) in the dark for 60 min and b) after stirring with 1 (5 mm)
under 280 nm irradiation for 30 min; gray: theoretical curve, broken
lines: deconvoluted spectra.
Figure 4. a) Time-dependent change in hydrodynamic diameter of
aggregates for AuNPs stabilized with 4.8 mm GABA, during 280 nm
light irradiation in the presence of 1 (5 mm: 10 nmol). b) Change in
average diameter (black) of aggregates and amount of 1 (white)
adsorbed on AuNPs. The adsorbed amount of 1 was determined by
absorption spectra of the supernatant obtained after centrifugation of
respective samples.
results for S 2p region of AuNPs. The AuNPs recovered after
stirring with 1 in the dark do not show any signals (Figure 6a).
In contrast, the AuNPs recovered after UV irradiation with
1 show characteristic S 2p1/2 and 2p3/2 peaks at 162.9 and
164.1 eV, respectively, with the ratio of their peak areas being
2:1 (Figure 6b), indicative of a thiolate moiety bound to
AuNPs surface.[31] These suggest that the photoformed MC
form of 1 is adsorbed onto the AuNPs surface by covalent
ꢀ
Au S bonding, whereas the SP form of 1 is not adsorbed. The
ꢀ
covalent Au S bonding is further confirmed by visible light
irradiation of aggregates. As shown in Figure S6 (Supporting
Information), irradiation of the aggregates with 530 nm light
does not promote any change in the absorption spectra,
although the free MC form of 1 undergoes reversion to the SP
form (Figure S4, Supporting Information). This result sug-
gests that the thiolate moiety of the MC form of 1 is stabilized
ꢀ
by the Au S bonding and does not undergo reversion to the
Figure 5. Change in 740 nm absorbance of the solution containing
AuNPs (0.12 nm) stabilized with 4.8 mm GABA, during repeated on-off
sequence of UV irradiation in the presence of 1 (5 mm).
SP form even under visible light irradiation. As shown in
Figure 4b (white), the amount of 1 adsorbed onto the AuNPs
surface increases with irradiation time, which is consistent
with the increase in the size of aggregate. This finding clearly
indicates that adsorption of MC form of 1 onto the AuNPs
surface indeed triggers aggregation.
increase, indicating that on–off switching of UV light
precisely controls aggregation.
The adsorption of the MC form of 1 onto the AuNPs
surface leaves a positively charged indole moiety (Sche-
me 1b). These moieties neutralize the negative charge of
carboxylic acid groups on the AuNPs surface, promoting
aggregation. This situation is confirmed by the potential of
zero charge (PZC)[32] for AuNPs. Figure 7 shows the acid–
base titration curves for AuNPs measured in a water/MeCN
mixture by the addition of HCl, where the titration curve for
the blank solution is depicted by the dotted line. As shown by
curve (a), the PZC of AuNPs stabilized with GABA is 6.0,
indicating that the AuNPs surface is indeed charged neg-
atively due to the carboxylic acid groups of GABA. As shown
by curve (b), the PZC of AuNPs obtained after stirring with
The mechanism for the light-induced aggregation of
AuNPs with 1 is explained in Scheme 1. In the dark, AuNPs
are well dispersed in solution owing to the electrostatic
repulsion of AuNPs by negatively charged carboxylic acid
groups (-COOꢀ) of the surface-attached GABA molecules.
Thus, the SP form of 1 scarcely associates with the AuNPs
surface. In contrast, UV irradiation promotes SP!MC
isomerization of 1. The thiolate moiety (-Sꢀ) of the MC
form is strongly adsorbed onto the AuNPs surface by the
covalent Au–S binding.[30] The positive charge on the indole
moiety of the adsorbed MC form neutralizes the negative
charge of the carboxylic acid groups of GABA. This decreases
4
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under 25%). The basic concept presented herein, based on
the photoisomerization of spirothiopyran, may contribute to
the design of a more efficient method for light-induced self-
assembly of metal nanoparticles and may open a new strategy
for the rapid and simple processing of AuNPs.
Received: March 23, 2013
Revised: May 18, 2013
Published online: && &&, &&&&
Keywords: aggregation · gold nanoparticles ·
.
photoisomerization · spiropyran · spirothiopyran
Figure 7. Acid–base titration curves for AuNPs (0.12 nm) stabilized
with 4.8 mm GABA, obtained a) after stirring without 1 in the dark for
60 min, b) after stirring with 1 (5 mm) in the dark for 60 min, and
c) after stirring with 1 (5 mm) under UV irradiation for 30 min.
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1 in the dark is similar (6.0). In contrast, as shown by curve (c)
in Figure 7, the AuNPs obtained after stirring with 1 under
UV irradiation show a higher PZC (8.1). This result indicates
that the negative charge on the AuNPs surface is neutralized
by the positive charge of the indole moiety on the adsorbed
MC form of 1. This decreases the electrostatic repulsion
between AuNPs[33] and triggers the aggregation of AuNPs.
It is noted that the aggregation rate of AuNPs strongly
depends on several factors, such as 1) the amount of 1, 2) the
intensity of incident UV light, 3) the amount of GABA, and
4) the ionic strength of the solution. As shown in Figure S7
(Supporting Information), UV irradiation of an AuNPs
solution containing lower amounts of 1 shows a decreased
aggregation rate. In addition, as shown in Figure S8, the
decrease in incident light intensity also decreases the aggre-
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intensity produces lower amounts of the MC form. This
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in ionic strength of the solution accelerates the aggregation
rate of AuNPs. This is because the increased ionic strength
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triggered self-assembly of AuNPs based on the photoisome-
rization of spirothiopyran (1). UV irradiation of 1 dissolved in
an aqueous solution containing AuNPs stabilized with
GABA, a common surface protecting agent, successfully
promotes aggregation of the AuNPs. Photoisomerization of
1 promotes covalent binding of its thiolate moiety onto the
AuNPs surface. This binding neutralizes the surface negative
charge of the AuNPs and promotes aggregation. This simple
method avoids the tedious preparation of surface-modified
AuNPs and creates AuNPs aggregate with tunable size (30–
500 nm) and narrow size distribution (standard deviation
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