Bull. Soc. Photogr. Imag. Japan. (2021) Vol. 31 No. 1: 1–4

LetterPreparation of Poly(Cyclodextrin)-Protected Platinum Colloids and Application
as Catalysts for Visible-Light-Induced Hydrogen Generation
Yukihide SHIRAISHI a *, Yoshimi NAKANISHI a, Tomoko KOTANI a, Shinichi HATA a, Yukou DU b and Naoki
TOSHIMA c

Abstract:

 etal colloids are attracting significant attention because of their uniform size, shape, size distribution in nanometers, and easy preparation
M
process; their impact is evident in the recent developments in nanoscience and nanotechnology. Although protective colloids are often employed to prevent the aggregation of metal colloids, they also promote some of the special properties of metal colloids. Cyclodextrin (CyD) is
known to form inclusion complexes with organic compounds in solution. However, platinum colloids are properly stabilized by poly(cyclodextrin) (PCyD) not CyD. Platinum colloids protected by PCyD were prepared by the photoreduction of hexachloroplatinic(IV) acid in thepresence of PCyD. The PCyD-protected platinum colloids exhibited higher catalytic activities than the dextran-protected platinum colloids
for visible-light-induced hydrogen generation.

Key words:

Metal Colloids, Platinum, Poly(cyclodextrin), Visible-Light-Induced Hydrogen Generation, Catalyst

The fundamental materials required for nanoscience and nanotechnology research are expected to be afforded by organic-inorganic hybrid complexes. Metal colloids stabilized by organic molecules
constitute a new class of materials that are different from conventional bulk materials or atoms, affording one of the smallest building
blocks of matter.1-3) Most of the remarkable properties of metal colloids are based on their high surface areas and quantum size effect.
Conventionally, metal colloids are prepared chemically by the reduction of the corresponding metal salts4) and physically by the pulverization of the metallic mass.5) The chemical method, wherein metal
ions are reduced by various kinds of reductants to produce metal
atoms and subsequently metal colloids by their aggregation, is advantageous for mass production. Silver and/or platinum colloids
prepared by chemical methods are of interest in the photographic
industry. 6,7)
The conversion of solar photoenergy is an important step in realizing the sustainable development goals (SDGs). One of the mostimportant and simplest routes is the visible-light-induced decomposition of water to produce oxygen and hydrogen.8) To realize thisprocess, several approaches have been proposed, including the construction of heterogeneous photocatalytic systems composed ofsemiconductors, such as TiO2,9,10) and the use of homogenous aqueous solutions composed of photosensitizers, electron mediators, andoxidative/reductive catalysts. Dispersions of noble metals, particularly platinum colloids, are well known as reductive catalysts inaqueous solutions, producing molecular hydrogen from protons andelectrons. Since platinum colloids are transparent in the visible light

range, they are effective as catalysts for photocatalytic reactions.
Many studies on visible-light-induced hydrogen generation using
platinum colloids have been reported.11-13) Metal colloids are protected by citrate ions, surfactants, polymers, and organic ligands.14)
Protective colloids are often used to prevent the aggregation of metal colloids and promote some of their special functions.
Alpha-, β- and γ- cyclodextrins (αCyD, βCyD, and γCyD) are
cyclic oligomers of 6, 7, and 8 glucose units, respectively, linked byα(1-4) bonds with a central cavity. The external diameter at the secondary hydroxyl side of CyD is larger than that at the primary hydroxyl side of CyD. The cavity diameters of αCyD, βCyD, and
γCyD are 4.7~5.2, 6.0~6.4, and 7.9~8.0 Å, respectively. The outsideof the CyD molecule is hydrophilic, while the inside cavity contains
a hydrophobic medium, such as diethyl ether; various hydrophobic
substances can be included in the aqueous medium as "guests," byspatially fitting them in the cavity.15,16) The formation of inclusion
complexes in solution is a dynamic process of equilibrium between
the inclusion state and the free state of the substrate. Herein, we
report the synthesis of poly(cyclodextrin)-protected platinum(PCyD-Pt) colloids by a chemical method and their application as
novel catalysts for visible-light-induced hydrogen generation. The
visible-light-induced electron-transfer system was constructed using tris(bipyridine)ruthenium(III), methyl viologen, and ethylenediaminetetraacetic acid as the photosensitizer, electron relay, and sacrificial electron donor, respectively.17,18)
Hexachloroplatinic(IV) acid was obtained from FUJIFILM
Wako Pure Chemical Corporation, Ltd. Poly(α-cyclodextrin) (Pα−Received 17th April, 2021; Accepted 22nd May, 2021 ; Published 27th May, 2021
a Department of Applied Chemistry, Sanyo-Onoda City University, Daigakudori, Sanyo-Onoda, Yamaguchi 756-0884, Japan.
b College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China.
c Professor Emeritus, Tokyo University of Science Yamaguchi, Japan.
* shiraishi@rs.socu.ac.jp

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Bull. Soc. Photogr. Imag. Japan. Vol. 31 No. 1 (2021)

tubes were filled with nitrogen at 1 atm. Photoirradiation was carried out with an Ushio 500 W super-high-pressure mercury lamp
through a UV-39 cut filter in a water bath maintained at 30 °C. The
products in the gas phase were analyzed with a Shimadzu model
GC-14A gas chromatography (GC) apparatus using an MS13X
column at 60 °C.
Dispersions of platinum colloids protected by PCyD were prepared by the photoreduction of a mixed solution of hexachloroplatinic(IV) acid and PCyD in ethanol/water, which had a dark browncolor and was stable for months at room temperature. Figure 1
shows the UV-Vis absorption spectra of the dispersions of the Pα−
CyD-Pt, PβCyD-Pt, and PγCyD-Pt colloids. The dispersions of the
PCyD-Pt colloids obtained after photoreduction show broad absorption spectra without a structure from the visible to the near-UV
regions. The absorption peak due to PtCl62− completely disappeared
upon photoreduction, indicating the completion of the reduction of
the platinum ions.13)

CyD), poly (β-cyclodextrin) (PβCyD), and poly (γ-cyclodextrin)(PγCyD) (CycloLab R&D. Ltd.) were used without further purification as the protective polymers for the platinum colloids. The other reagents employed in this study were of commercial general reagent grade. The PCyD-Pt colloids were prepared using aphotoreduction method. PCyD (0.528 mmol in monomeric units,
40 times the total amount of metal ions) and hexachloroplatinic(IV)acid (0.0132 mmol) were mixed in ethanol/water (1/1, v/v) to form
a 20 cm3 solution. The mixed solutions in a quartz vessel were degassed by three freeze-thaw cycles, filled with pure nitrogen, andsubsequently exposed to light supplied by an Ushio 500 W super-high-pressure mercury lamp for 1 h in a water bath maintained
at 30 °C. The dextran-protected Pt colloids were prepared by thesame photoreduction method.
Ultraviolet-visible (UV-Vis) spectra were obtained at room temperature using a Shimadzu 2500PC recording spectrophotometerequipped with a 10 mm quartz cell. The PCyD-Pt colloids employed
here were characterized by transmission electron microscopy (TEM)
at 80 kV on a JEOL JEM-1230 electron microscope. The samples
for TEM were prepared by placing a drop of the PCyD-Pt colloid
dispersion onto a carbon-coated copper microgrid for high-resolution TEM, followed by natural evaporation of the solvent. The mean
diameter and standard deviation were calculated from the TEM
photograph by measuring the diameters of 200 particles 10 times
with a magnifier at 100,000 magnification.
The typical visible-light-induced hydrogen generation was performed as follows: a 20 cm3 Pyrex Schlenk tube was charged with 10
cm3 of an aqueous solution containing 0.1 mol dm-3 of an ethylenediaminetetraacetic acid disodium salt (Na2EDTA, the sacrificial
electron donor), 1.4×10-4 mol dm-3 of tris(bipyridine)ruthenium dichloride ([Ru(bpy)3]Cl2, the photosensitizer), and 1.4×10-3 mol dm-3
of 1,1'-dimethyl-4,4'-bipyridium dichloride (methyl viologen,MVCl2, the electron relay). The PCyD-Pt colloids were added last to
obtain the above-mixed solution at the designed concentrations. The
mixtures were degassed by three freeze-thaw cycles, after which the
100

80

PαCyD-Pt60

dav = 1.2 nm
σ = 0.3 nm

Fraction / %Fraction / %

100

40

80

PβCyD-Pt60

dav = 1.2 nm
σ = 0.4 nm

40
20

20
0

Figure 1. UV-Vis absorption spectra of an aqueous solution of hexachloroplatinic(IV) acid and PCyD-Pt colloids.

00

11

22

33

44

55

6
6

Diameter / nm

0

00

11

22

33

44

5
5

66

Diameter / nm

Fraction / %100
80

PγCyD-Pt60

dav = 1.2 nm
σ = 0.4 nm

40
20
0

00

11

22

33

44

55

66

Diameter / nm

Figure 2. Transmission electron micrographs and particle size distribution histograms of PCyD-Pt colloids. dav=average diameter, σ=standard deviation.

Yukihide Shiraishi, Yoshimi Nakanishi, Tomoko Kotani, Shinichi Hata, Yukou Du and Naoki Toshima

Preparation of Poly(Cyclodextrin)-Protected Platinum Colloids and Application as Catalysts for Visible-Light-Induced Hydrogen Generation

CyD-Pt and PβCyD-Pt colloids. The solubilities of αCyD, βCyD,
and γCyD in water are 14.5, 1.85, and 23.2 g/100 mL, respectively.21)
The high hydrogen-generation activity of the PγCyD-Pt colloids
may be due to the high solubility of PγCyD in water. On the other
hand, Osa et al. reported that the formation constants of the
CyD-methyl viologen cation radical were 10, 30, and 40 M-1 for
αCyD, βCyD, and γCyD, respectively.22) Methyl viologen ion, as anelectron relay, transformed into the methyl viologen cation radical,
as represented by Eq. 2. The PγCyD-Pt colloids may include moremethyl viologen cation radicals than the PαCyD-Pt and PβCyD-Pt
colloids.

(2)To examine the electron transfer of methyl viologen cation radicals, viologen molecules with several alkyl chain lengths (ethyl viologen, propyl viologen, butyl viologen, pentyl viologen, hexyl viologen, and octyl viologen) were synthesized. 4,4'-Bipyridine (21.2mmol) and 42.4 mmol of the corresponding alkyl bromides (ethyl
bromide, propyl bromide, butyl bromide, pentyl bromide, hexyl bromide, and octyl bromide) were mixed in acetonitrile to form a 100
cm3 solution. The mixed solutions were stirred and heated to reflux
at 90 °C under a nitrogen atmosphere for 5 h. The various viologen
molecules synthesized were purified by recrystallization. Hydrogenproduction was also observed by GC upon irradiation with a visible-light-induced electron transfer system of EDTA/Ru(bpy)32+/viologen molecules/PβCyD-Pt colloids. Figure 5 exhibits the relationship between the hydrogen-generation rate coefficient and theviologen molecules with varying alkyl chain lengths by PβCyD-Pt
colloids (■). The hydrogen-generation rate coefficients for the ethyl
viologen (1.17 s-1), propyl viologen (1.61 s-1), and butyl viologen(1.21 s-1) were higher than that of methyl viologen (1.04 s-1). Furthermore, as the alkyl chain length increased, the hydrogen-genera6
Hydrogen generation rate / μmol dm -3 s-1

Figure 2 depicts the transmission electron micrographs and thecorresponding histograms, indicating the particle-diameter distributions of the colloids. Most of the PCyD-Pt colloids were distributed in the diameter range of 1-3 nm, suggesting that they are homogeneous. The average diameters of the PαCyD-Pt, PβCyD-Pt,
and PγCyD-Pt colloids are 1.2±0.3, 1.2±0.4, and 1.2±0.4 nm, respectively. When βCyD was used instead of PCyD, rapid precipitation occurred and the colloidal dispersion was not obtained. WhenαCyD and γCyD were used instead of PCyD, some aggregates, several times larger in diameter than the single particle, were observed.
The polymer molecule could coordinate to the metal particle at multiple sites.19) Even though each coordination bond was weak, themulticoordination resulted in the strong chemical adsorption ofpolymer molecules on the metal colloid surface. The coordination to
the platinum surface at multiple sites of PCyD may account for thestability of the PCyD-Pt colloids.
The PCyD-Pt colloids were used as catalysts for visible-light-induced hydrogen generation. The reductive production of hydrogen
was observed by GC upon irradiation with visible light in the electron relay system of EDTA/Ru(bpy)32+/MV2+/Pt colloids. In thesteady state of the reaction, the hydrogen-generation amount increased linearly in proportion to the irradiation time of visible light.
A typical hydrogen-generation curve for the catalyzed PβCyD-Pt
colloids is shown in Figure 3. The total amount of hydrogen generation is limited, probably because of the degradation of methyl viologen by the reduction.20) The hydrogen-generation rate (rH2), measured from the initial slope for 20 min of the hydrogen-generation
curve in Figure 3, is proportional to the metal concentration. Figure
4 exhibits the relationship between the hydrogen-generation rate
and the metal concentration. The hydrogen-generation rate can be
expressed by Eq. 1 using the hydrogen-generation rate coefficients(kH2).
rH2=kH2[Metal](1)The hydrogen-generation rate coefficients of the PαCyD-Pt, Pβ−
CyD-Pt, and PγCyD-Pt colloids were 1.22, 1.04, and 1.31 s-1, respectively. The hydrogen-generation rate coefficient of commercial
platinum black (0.0008 s-1) was considerably lower than those of the
PCyD-Pt colloids. The commercial platinum black (< 10 nm) is
composed of relatively large particles and their aggregates. The Pγ−
CyD-Pt colloids exhibited a higher catalytic activity than the Pα−

3

5

4

● PγCyD-Pt●▲■▲ PαCyD-Pt●■ PβCyD-Pt

▲
■
●
▲

3

■
2

1
0●
0

1

2

3

4

5

[Metal]/ μmol dm -3

Figure 3. The typical hydrogen generation curve catalyzed by PβCyD-Pt colloids.

Figure 4. Hydrogen generation rate (rH2) vs. [Metal] in the presence of
PCyD-Pt colloids.

4
Hydrogen generation rate coefficients / s-1

Bull. Soc. Photogr. Imag. Japan. Vol. 31 No. 1 (2021)

1.8
1.6

PβCyD-Pt
PCyD-Pt

1.4

Dextran-Pt
Dextran-Pt

1.2
1

Acknowledgements

This study was partially supported in part by the projects KAKENHI (Nos. 18K14017 to S.H. and 19K05633 to Y.S.) from theJapan Society for the Promotion of Science, Japan.

0.8
0.6
0.4

References

0.2
0

MV

EV

PrV

BV

PeV

HV

OV

Figure 5. Relationship between hydrogen generation rate coefficient (rH2)
and viologen molecules with varying alkyl chain length (ethyl viologen; EV, propyl viologen; PrV, butyl viologen; BV, pentyl viologen;
PeV, hexyl viologen; HV, and octyl viologen; OV).

tion rate coefficients decreased. In contrast, the dextran-protected Pt
colloids (4.4±1.1 nm) with a linearly bound glucose frame did notshow a significant difference regarding the alkyl chain length. The
PβCyD-Pt colloids exhibited a higher catalytic activity than the
dextran-protected Pt colloids (□). CyD exhibits remarkable catalytic activities owing to the following effects:23) (1) the solubilizationof insoluble compounds, (2) protection of intermediates, (3) conformation effect, (4) microscopic solvent effect, and (5) control of molecular size. Due to the CyD moieties of PCyD-Pt, the viologen
cation radical is placed near the platinum colloids and it readilytransfers the electron from the viologen cation radical to the platinum colloids. Thus, the high catalytic activity of the PβCyD-Pt colloids can be explained by the protective stability of the methyl viologen cation radical as intermediates by the formation of
PβCyD-methyl viologen cation radical inclusion complex.
PCyD-Pt colloids were prepared by the photoreduction of hexachloroplatinic(IV) acid in the presence of PCyD. These colloids exhibited excellent monodispersion and high stability at room temperature for several months. The catalytic properties of the PCyD-Pt
colloids were studied for visible-light-induced hydrogen generation.
The PCyD-Pt colloids are more active than the dextran-protected
Pt colloids. The unprecedented PCyD-protected Pt colloids withclathration ability are expected to be applied to the development of
new functional materials.

1)
2)

G. Schmid, Chem. Rev., 92, 1709 (1992).
J. H. Fendler, in Nanoparticles and Nanostructured Films, WILEY-VCH,
Weinheim (1998).
3) H. Bönnemann, W. Brijoux, in Advanced Catalysts and Nanostructured
Materials, edited by W. Moser, Academic Press, New York (1996).
4) N. Toshima and Y. Shiraishi, in Encyclopedia of Surface and Colloid Science, edited by A. T. Hubbard, Marcel Dekker, New York (2002).
5) B.-J. Tan, K. J. Klabunde, P. M. A. Sherwood, J. Am. Chem. Soc., 113,855 (1991).
6) T. Tani, in Silver Nanoparticles from Silver Halide Photography to Plassmonics, Oxford University Press, Oxford (2015).
7) Y. Teshima, K. Ohzeki, J. Soc. Photogr. Imag. Jpn., 76(6), 487 (2013).
8) N. Toshima, K. Hirakawa, Polym. J., 31(11-12) 1127 (1999).
9) A. Fujishima, K. Honda, Nature, 238, 37 (1972).
10) A. Fujishima, T. N. Rao, D. A. Tryk, J. Photochem. Photobio. C, 1(1) 1(2000).
11) J. Kiwi, M. Grätzel, Nature, 281, 657 (1979).
12) A. J. Frank, K. L. Stevenson, J. Chem. Soc., Chem. Commun, 593(1981).
13) N. Toshima, M. Kuriyama, Y. Yamada, H. Hirai, Chem. Lett., 793(1981).
14) N. Toshima, H. Yan, Y. Shiraishi, in Metal Nanoclusters in Catalysis andMaterials Science: The Issue of Size Control, edited by B. Corain, G.
Schmid, N. Toshima, Amsterdam, Elsevier (2007).
15) M. L. Bender, M. Komiyama, in Cyclodextrin Chemistry, Springer-Verlag, Berlin-Heidelberg- New York, 1 (1978).
16) Y. Shiraishi, Kagaku To Kyoiku (Chemistry and Education) 68, 224(2020).
17) K. Kalyanasundaram, O. Micic, E. Pramauro and M. Gratzel, Hely.
Chim. Acta, 62, 2432 (1979).
18) K. Yamada, M. Shakuto, K. Teshima, N. Kobayashi, K. Ikeda, R. Hirohashi, J. Soc. Photogr. Imag. Jpn., 57(6), 445 (1994).
19) Y. Shiraishi, M. Hayashi, and N. Toshima, Jpn. J. Polym. Sci. Tech., 64, 74(2007).
20) P. Keller, A. Moradpour, E. Amouyal, H. Kagan, J. Mol. Catal., 7, 539(1980).
21) W. Saenger, Angew. Chem. Int. Ed. Engl., 19, 344 (1980).
22) T. Matsue, T. Kato, U. Akiba, T. Osa, Chem. Lett., 1825 (1981).
23) F. Toda, A. Ueno, Cyclodextrin, Sangyotosho, Tokyo, 92 (1995).