Vol. 57 No. 3

Material Science and Technology of Japan, 57（2020）109～113

29Regular Paper
A Colorimetric Indicator Based on Methylene Blue-Dyed Pullulan Thin
Films for the Detection of Hydroxyl Radicals
Saranya YENCHIT1,*, Hiromi YAMANAKA2, Pasika TEMEEPRASERTKIJ1, Yoshiki
ODA3, Yosuke OKAMURA1, Toshiyuki INAZU1 and Satoru IWAMORI1
1Graduate School of Science and Technology, Tokai University, 4-1-1 Kitakaname, Hiratsuka-shi, Kanagawa

259-1292, Japan
2Graduate School Engineering, Tokai University, 4-1-1 Kitakaname, Hiratsuka-shi, Kanagawa 259-1292,

Japan
3Research Promotion Division, Tokai University, 4-1-1 Kitakaname, Hiratsuka-shi, Kanagawa 259-1292,

Japan
Received Feb. 25, 2020; accepted Apr. 17, 2020
Abstract
Methylene blue (hereinafter MB)-dyed pullulan thin ﬁlms are applied as hydroxyl radical (OH*) indicators. In thepresent study, the chemical reaction and decolorization mechanism of MB-dyed pullulan thin films upon exposure toactive oxygen species (hereinafter AOS) were elucidated under low and high humidity conditions. Importantly,decolorization was observed only under high humidity. The 1H-nuclear magnetic resonance (NMR) and 13C-NMR
analyses revealed the presence of the interaction between MB and pullulan. It was also determined that thedecolorization of the ﬁlm was caused by the decomposition of MB upon exposure to OH*.
Keywords: Methylene blue dye, Hydroxyl radicals (OH*), Pullulan film

1. Introduction

form MB/pullulan composite films. Pullulan is a natural andwater-soluble polysaccharide having excellent film-forming

Owing to their remarkably strong oxidative ability, AOSability.

generated in the atmosphere can be applied in various industrial
processes. It is well known that excited singlet oxygen atoms [OHumidity affects the type of the generated AOS. Under low(1D)], ozone (O3), and OH* are valuable in the field of surface

humidity conditions, O (1D) and O3 are typically produced, while

modiﬁcation. AOS can also be generated by ultraviolet (UV)

under high humidity conditions, the most oxidative species

lamps with wavelengths of 185 and 254 nm [1]. Although“OH*”numerous different AOS are known, which have long lifetimes,

decolorization mechanism of the composite film has beenor high oxidization abilities, such as

O(1D),isgenerated.

Basedonthisobservation,

theO3, OH*, andpreviously investigated, and it was found that decolorization

and OH* are of particularoccurred only at high humidity, presumably caused by theinterest [2-10]. We previously reported that MB can bepresence of the OH* species [12]. Moreover, the mechanism ofdecomposed by O3. MB is the convenient indicator for thethe specific decolorization of the MB-dyed Nafion® films wasdetection of AOS, as following exposure to AOS, decomposing

reported [11]. However, the described indicator film was not athe dye, and decolorization [11]. Notably, O3 has a lower

uniform thin film.

hydrogen peroxide (H2O2),O(1D)

oxidative ability compared with other AOS. To detect AOS with

Consequently, we aimed to elucidate their chemical reactions

higher oxidative ability, it is necessary to stabilize MB by mixing

as well as the decolorization mechanism upon exposure to OH*

it with pullulan, which has a high affinity for the dye, in order toby using the uniform thin film indicator based on MB andpullulan.

*Corresponding author: (7ltad003@cc.u-tokai.ac.jp)(109)