280

Original Research Paper

J. Jpn. Soc. Colour Mater., 93〔9〕，280–287（2020）―Special Edition

Central (Japan) Section : New colorant technologies―

Stabilization of Cu-Chlorophyllin by Incorporation into Hydrotalcite Interlayer
Naoya K atsura＊, Masashi Shibata＊＊, Yoshiumi Kohno＊,†, Yasumasa Tomita＊,
Ryo Watanabe＊ and Choji F ukuhara＊＊ Department of Applied Chemistry and Biochemical Engineering, Shizuoka University,

3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan＊＊ School of Bioscience and Biotechnology, Tokyo University of Technology, 1404 Katakura-machi, Hachioji, Tokyo 192-0982, Japan†Corresponding Author, E-mail: kohno.yoshiumi@shizuoka.ac.jp(Received May 30, 2020; Accepted July 16, 2020 )Abstract
The stability of copper chlorophyllin dye, a derivative of the naturally occurring chlorophyll, was improved by the intercalation into theinterlayer space of a layered inorganic host material. As the layered inorganic compounds, hydrotalcite was used. The copper chlorophyllin
could be inserted between the layers either by the reconstruction method or by the ion exchange method. We found the preparation condition
for the composite materials that successfully included the dye molecules between the layers. The obtained composite samples showed highdurability under visible light irradiation. The intercalation of the copper chlorophyllin into the hydrotalcite layers effectively suppressed
the elution of the dye molecules by water or ethanol. We proposed the potential application of these highly stable composite materials as
environmentally friendly natural dye-based colorants.
Key-words: Copper chlorophyllin, Hydrotalcite, Complexation, Stabilization, Intercalation

1．Introduction
Naturally occurring dyes are more environmentally friendlyand suitable for the human body, compared with synthetic
ones. Natural dyes are used as colorants for cosmetics andfoods due to their safe properties. Chlorophyll is one of themost abundant natural dyes, which generally exists in theleaves of plants 1 ). Chlorophyll exhibits green color, and inaddition, it has a functionality of deodorant effect. However,chlorophyll cannot be easily used as a practical colorant,
because of its quite low stability.
The structure of chlorophyll a is shown in Scheme 1 (a). It
has a porphyrin ring with magnesium ion as a coordination
center. The porphyrin ring has several carboxylate
substituents, one of which is esterified with a long alkyl
chain. Therefore, chlorophyll molecules often exhibit ratherhydrophobic nature.
Scheme 1 (b) shows the structure of copper chlorophyllin( CuChl ) , which is a derivative of natural chlorophyll.
In CuChl, the coordination center is substituted frommagnesium to copper ion, and the long alkyl chains areremoved by hydrolysis. The remaining carboxyl groups are
made sodium salts. Therefore, CuChl shows hydrophilic
property and is highly soluble in water. More importantly,
the stability of CuChl is enhanced compared to theoriginal chlorophyll 2 ). CuChl is also nontoxic and has thefunctionality of deodorant, so that it is used as a functional© 2020 Journal of the Japan Society of Colour Material

food additive 3 ). However, even though the stability isenhanced, the durability of CuChl is still not enough for widepractical use.
Making a composite with an inorganic host is known asa method of improving the stability of natural or syntheticdyes. Itoh et al. reported that natural chlorophyll could bestabilized by incorporation in mesoporous silica 4 ). Besides,
there are many studies that have reported the stability
enhancement of the dye by complexing with an inorganic
host such as clays 5 ) , mesoporous silica 6 ) or zeolite 7 ) .
We have also investigated the stability enhancement－12－(b)-O NaN

N+O-

O

Cu2+N(a)

NN-

ON--

Na

Mg2+
N+O+

NaO

N-OO

HOOO

Scheme 1 Molecular structure of chlorophyll derivativepigment ( a ) chlorophyll a , ( b ) sodium copper

chlorophyllin.