Adsorption Property of UV-cured Films Containing
Disulfide Bonds in Water

Masahiro FURUTANI1,*, Keiji WASHIDA1, Tomoki HATA1, Daichi UCHIDA1, Chisato SAKAMOTO1

Department of Chemistry and Biology, National Institute of Technology, Fukui College（Geshi-cho, Sabae,
Fukui, Japan 916-8507）

Abstract
Disulfide bond-containing polymer films were fabricated in a radical UV curing process to adsorb metal
cations or proteins. Bi-functional methacryl monomers having a disulfide bond were designed and used
for the UV curing. Resulting films showed practical pencil-hardness, H to 3H, with 1 J/cm2 of irradiation
at a wavelength of 365 nm. It was confirmed that disulfide bond-containing films adsorbed colored metal
cations such as gold(III) and copper(II) cations. During adsorption experiments, disulfide bond-containing
polymer films were not detached from a glass substrate, probably due to relaxation of internal stress bybond exchange reactions between disulfide bonds. Bovine serum albumin seemed to be also adsorbed
toward films only in the case that the film had ionic moieties.
Keywords：Adsorption, UV-cured film, Disulfide bond, Radical UV curing, Methacryl monomer

Received July 11, 2023; accepted July 29, 2023

1) Department of Chemistry and Biology, National Institute of Technology, Fukui College, Geshi-cho,
Sabae, Fukui 916-8507, Japan*Corresponding author: Masahiro Furutani, E-mail: furutani@fukui-nct.ac.jp42

1. Introduction
Separation technology plays quite importantrole for sustainable chemical industries and ourhealthful daily lives. Wastewater must undergoremoval treatments of various kinds of pollutants,
before streaming into rivers and seas. Pesticides
are one of major pollutants, and organic dye has
been used as a model compound in researches ofadsorbents. It was reported that such compounds
were adsorbed by clays1) and metal-organic
frameworks (MOFs),2) while these materials could
not adsorb high-molecular-weight molecules due
to adsorption sites in angstrom scale. Adsorbents
for proteins (up to several dozen nm) and viruses(several dozen to hundred nm) would be desired.
Heavy metal cations are also representative as
inorganic pollutants, and their adsorbents havebeen developed by using inorganic carriers suchas mesoporous silica,3) iron tailings,4) graphene
oxide,5) and MOFs.6) Furthermore, abundantorganic carrier materials have been used, including
pectinbased7)
andpolysaccharide-based
8)
adsorbents. Inherent carboxy and hydroxy groups
in the materials would capture metal cations.
Nitrogen- containing moieties would also contributeto the entrapment, while Sulfur-containing ones
were not in the limelight. Sulfur moieties are known
to work as soft base, and they interact strongly with
soft acids including several heavy metal cations.
Nakayama reported that mercury(II) and silver(I)cations were adsorbed by interaction with disulfide(SS) bonds in the interlayer space of magnesium–
aluminum layered double hydroxide materials.9) In
this case, however, there is a limit of retention ratioof SS bonds to the carrier. On the other hand, organic
polymer materials containing SS bonds have beendeveloped in our group as photo- adhesivematerials.10-12) It was found that SS bonds would
contribute to strong adhesion toward metal
substrates. In this article, we have tried to fabricate
organic polymer UV-cured films which contain SS
bonds at high density (Fig 1), for using as an

SS

SS

SS

SS

SS

pollutant

SS

SS

SS

SS
SS

Fig 1. Adsorption of pollutants such as heavymetal cations and proteins by interaction withdisulfide (SS) bonds in a UV-cured film.

Fig 2. Chemical structures of bi-functional
methacryl monomers having a disulfide bond (1,3), and a control monomer 2.

43

2.2 Monomers
Bi-functional methacryl monomer 1 wassynthesized from 3,3’-dithiopropionic acid and
HEMA as starting compounds in a 21% yield. The
synthetic procedure was described elsewhere.15)
Monomer 3 was prepared as follows.
tBuAEMA (0.89 g, 4.8 mmol) in DCM (10 mL)
was dropped to 3,3’-dithiopropionic acid (0.50 g, 2.4mmol) in DCM (10 mL), and the solution was stirred
under nitrogen atmosphere at room temperature for
178 h. After evaporating, 3 was obtained in a 95%
yield as a white solid. 1H-NMR (400 MHz, CDCl3):
 1.29 (18H, s, -C(CH3)3), 1.96 (6H, s, -CH3), 2.64
(4H, m, -CH2CH2-O-), 2.95 (4H, m, -CH2CH2-O-),
3.09 (4H, m, -CH2CH2-S-), 4.41 (4H, m, -CH2CH2S-), 5.62 (2H, m, =CH2), 6.17 (2H, m, =CH2), 7.96
(4H, br, -N+H2-). 13C-NMR (100 MHz, CDCl3):  18,
27, 35, 37, 40, 54, 62, 126, 136, 167, 177.

adsorbent of heavy metal cations and proteins.
Gold(III) and copper(II) cations were chosen in ourstudy, because visual observation

could beconducted easily. Furthermore, adsorption of aprotein was examined with our UV-cured films. SS
bonds in a protein work for structural stability of its
tertiary structure.13) It was expected that proteins
were fixed through exchange reactions between SS
bonds14) on the film surface “open space”. Bifunctional methacryl monomers (monomers 1 and 3,
Fig 2) having a SS bond were prepared and used inradical UV curing.

2. Experimental
2.1 Materials and Equipment
3,3’-Dithiopropionic acid, 2-hydroxyethyl
methacylate (HEMA), 2-(tert-butylamino)ethyl
methacrylate (tBuAEMA), 1,12-dodecanediol
dimethacrylate (monomer 2), and Omnirad819(phenylbis(2,4,6-trimethylbenzoyl)
phosphineoxide) were purchased from Tokyo Chemical
Industry Co., Ltd. (Tokyo, Japan). Chloroform(CHCl3) and acetone were purchased from
NACALAI TESQUE, Inc. (Kyoto, Japan).
Dichloromethane (DCM), sodium tetrachloroaurate (NaAuCl4, dihydrate), copper(II) chloride(CuCl2, dihydrate), and chloroform-d (CDCl3) werepurchased from FUJIFILM Wako Pure Chemical
Corporation (Osaka, Japan). Bovine serum albumin(BSA, IgG-free, protease-free) was purchased from
Jackson ImmunoResearch Laboratories, Inc.
(Pennsylvania, USA). All chemicals were usedwithout further purification.
1H- and 13C-NMR spectra were recorded using a
Bruker AVANCE III. ATR-IR spectral measurements
were performed using a Perkin Elmer Spectrum100.
Radical UV curing was performed with 3UVTM36UVLamp (Analytik Jena AG) and UIT-250/UVDC365 ultraviolet radiometer (Ushio Inc.). Pencilhardness of the UV-cured films was evaluated with
MJ-PHT tester (SATO SHO-JI Corporation). Static
water contact angle of films was measured using an
SImage Entry 6 (Excimer Inc.).

2.3 Fabrication of UV-cured films
Glass substrates were ultrasonicated with acetone
and then CHCl3. Films were fabricated by following
two methods.
Bar-coat method was used to prepare film samples
for adsorption of heavy metal cations. To bifunctional methacryl monomers 1-3 were added
HEMA (300-600 mol%), Omnirad819 (4-13 wt%
toward monomers), and at least amount of solvent(THF for 1 and 2, CHCl3 for 3). Each solution was
bar-coated on a glass substrate (4 mil = 101.6 m inthickness). Films were subjected to UV irradiation(1.5 mW/cm2, 1 J/cm2) at a wavelength of 365 nm.
On the other hand, drop-cast method was used toprepare film samples for adsorption of BSA protein.
To bi-functional methacryl monomers 1-3 wereadded HEMA (ca. 2500 mol%), Omnirad819 (7 wt%
toward monomers), and at least amount of CHCl3.
Each solution was drop-cast on a glass substrate.
Films were subjected to UV irradiation (1.5 mW/cm2,
1 J/cm2) at a wavelength of 365 nm.

44

2.4 Adsorption of heavy metal cations
Following the procedure of bar-coat method in
section 2.3, UV-cured films were fabricated withmonomer 1 and 2. Resulting films were named as
Film-1 and Film-2. These film samples were
immersed into 30 mM of NaAuCl4 or CuCl2 aqueous
solution at room temperature. After 1, 3, 5, and 60
min, films were taken out and washed with distilledwater. Air dried film samples were then subjected tovisual observation (for Au3+ and Cu2+ cations), or
color image analysis (for Au3+ cation).
Color image analysis was conducted by using
arbitrary nine pixels for each film image (0, 1, 3, 5
min for Film-1, and 0, 1, 3, 5 min for Film-2). One
pixel was at center of film in the image, and others
were within the radius of 60 px from the center pixel.
A film in an image was a square, ca. 150-210 px eachside. Each pixel had a set of standard RGB decimalvalues, e.g. yellow: Y (R, G, B) = (255, 255, 0).
These values were converted to their ratios r, g andb by equations below,
𝑟=

𝑅
𝑅+𝐺+𝐵

𝑔=

𝐺
𝑅+𝐺+𝐵

0.343

g
G

Y

Green (0,1)
Yellow (0.5,0.5)

g 0.333

WG
B

0.323
0.323

R
0.333
r

0.343

White-to-Gray(0.333,0.333)
Red (1,0)r
Blue (0,0)

Fig 3. Five representative colors; yellow: Y, red:R, green: G, blue: B, and white-to-gray: WG in rg plane that is for color image analysis in this study.

section 2.3, UV-cured films were fabricated withmonomers 1, 2 and 3. Resulting films were named
as Film-I, Film-II and Film-III. These film samples
were immersed into 7.9 M of BSA aqueous
solution at room temperature. After 60 min, films
were taken out and washed with distilled water. Air
dried film samples were then subjected to ATR-IR
spectral measurements.

3. Results and Discussion
3.1 Fabrication of UV-cured films
Table 1 and Table 2 show the values of pencilhardness of UV-cured films. In bar-coat method, theweight ratio of monomer/HEMA was fixed to 1/1
Table 1. Pencil-hardness of UV-cured films
fabricated in bar-coat method using monomers 13. Film-1 and Film-2 were used for adsorption ofheavy metal cations.

𝐵
𝑏=
𝑅+𝐺+𝐵
Values (r, g, b) of representative five colors were
calculated as follows, yellow: Y (0.5, 0.5, 0), red: R(1, 0, 0), green: G (0, 1, 0), blue: B (0, 0, 1), andwhite-to-gray: WG (0.333, 0.333, 0.333). These fiveaBi-functional methacryl monomer/HEMA = 1/1, w/w.
b2.9 mW/cm2, 15 J/cm2.

points were plotted in an r-g plane as a projection
view (Fig 3). Total nine sets of (r, g) were also

Table 2. Pencil-hardness of UV-cured films
fabricated in drop-cast method using monomers 13. Film-I, Film-II and Film-III were used foradsorption of BSA protein.

plotted in a plane for each film image, to show
graphically time course of color of each film sample
during immersion in Au3+ aqueous solution. All sets
were plotted in the range of 0.323 < r < 0.343 and0.323 < g < 0.343.

2.5 Adsorption of BSA protein
Following the procedure of drop-cast method in45

(a) G

(b) G

Y

0 min

WG

B

R

G

Y

r

WG
B

R

G

Y

1 min

WG

WG

B

R

B

R

G

Y

G

Y

3 min

WG

Fig 4. Photographs of UV-cured films (a) Film1 and (b) Film-2, before immersion (upper),after immersion into 30 mM of Au3+ (middle) or
Cu2+ (bottom) aqueous solution at room
temperature for 60 min.

g

Y

WG

B

R

B

R

G

Y

G

Y

5 min

WG
B

R

WG
B

R

Fig 5. Time course of color of film samples (a)Film-1 and (b) Film-2 during immersion into
30 mM of Au3+ aqueous solution at room
temperature. Longitudinal axis: g, 0.323 g
0.343, horizontal axis: r, 0.323 r 0.343. Nine
sets of (r, g) are plotted in each r-g plane.

(see annotation below the Table). In the evaluation

and Film-III were fabricated successfully to obtainof physical property of films, the hardness of pencil

H to 3H of pencil-hardness.

is arranged as follows: 6B (softest), 5B, 4B, 3B, 2B,

3.2 Adsorption of heavy metal cations
Both films were transparent before immersion
into aqueous solution of heavy metal cations. As
shown in Fig 4(a), color of Film-1 became yellow
after immersion in Au3+ aqueous solution,comparing with color of Film-2 (Fig 4(b)). This
indicated adsorption of Au3+ cations throughinteraction with SS bonds. Although such color
change was not observed in the case of Cu2+ aqueous
solution, there was a clear difference between two
films where Film-1 became opaque.
Fig 5(a) showed that color of Film-1 was changed
from cool color to warm one, like pale yellow, within
1 min. Color change was observed in the case ofFilm-2, while the variation range was small (Fig5(b)). Film-2 was detached from the glass substrateduring immersion experiments. Furthermore, it
seemed that color dispersion of Film-1 was larger

B, HB, F, H, 2H, 3H, 4H, 5H, 6H (hardest). Practical
pencil-hardness was recorded in cases of Film-1(2H) and Film-2 (H). Static water contact angles ofFilm-1 and Film-2 were 83° and 85°, respectively.
Films were more hydrophobic than a glass substrate(32°).12) Films was not cured with monomer 3 under
the same condition. Even increasing both intensity(2.9 mW/cm2) and irradiation energy (up to 15
J/cm2), the pencil-hardness was less than 6B.
Monomer 3 had ionic bonds which could beexchanged readily even in a UV-cured film if thedensity was high, resulting in softening the film
surface.
In drop-cast method, the molar ratio ofmonomer/HEMA was fixed to ca. 1/25, decreasing
the density of monomers 1-3 (monomer/HEMA=1/6 to 1/10, w/w). UV-cured films Film-I, Film-II
46

than of Film-2. This could indicate non-uniformityof the surface in microscopic level. Indeed, Film-1

Internal stress

S S
S S

after immersion into Au3+ and Cu2+ aqueous solution,
ruggedness was observed visually in Fig 4.
Adsorption of heavy metal cations would change
film surface profiles dynamically, probably due to

S

S

S

S

cross-linking between cations and SS bonds, andswelling with water.

(a)

(b)

Relaxation

Detachment

Fig 6. A possible mechanism of (a) filmremaining in cases of Film-1, and (b) filmdetaching in cases of Film-2, during immersion.

Films were fabricated in a radical process, which
was accompanied with film-shrinking.16) Resulting
UV-cured films on a substrate would hold somewhat(a)internal stress (Fig 6). In the case of Film-1 having
SS bonds, exchange reactions between SS bonds
would be induced by internal stress (Fig 6(a)).17)
Consequently, internal stress was relaxed, resulting

(b)
Abs.

in remaining cross-linked polymer networks on asubstrate. On the other hand, in the case of Film-2
having no SS bonds, internal stress was not relaxed(Fig 6(b)). Stress concentration at the film-substrate(c)interface would result in film-detachment. It wasconsidered that SS bonds in UV-cured films would
work for not only adsorption but also fixation of afilm on a substrate.

2000

1800

1600

1400

1200

1000

Wavenumber [cm-1]

3.3 Adsorption of BSA protein
Film-detachment from glass substrate was notobserved in all film samples, even in the case ofFilm-II, after immersion for 60 min. It would result
from low-crosslinking density, comparing with thecase of Film-2. In ATR-IR spectra, intensity of two
peaks around 1640 cm-1 and 1550 cm-1 seemed toincrease after the immersion, only in case of FilmIII (Fig 7). These peaks could be assigned to amide I
and II bands of secondary acyclic amide
compounds.18) This could be an evidence that BSA
proteins were adsorbed on Film-III. Such spectralchange was not observed in cases of Film-I and
Film-II. These results indicate that BSA proteinswould interact electrostatically with ionic moieties
rather than SS bonds.

Fig 7. ATR-IR spectral changes of (a) Film-I,(b) Film-II and (c) Film-III. Upper and lower
spectra are recorded after and before immersion
into 7.9 M of BSA protein aqueous solution,
respectively.

4. Conclusion
UV-cured films were fabricated successfully byusing bi-functional methacryl monomers having a
SS bond. In adsorption experiments of heavy metal
cations, it was found that SS bonds would interact
with such cations and that SS bonds could dissipate
internal stress through exchange reactions among
themselves. Furthermore, UV-cured films having
ionic moieties could adsorb proteins probably due toelectrostatic interaction. Our SS bond-containing
organic polymer material will be a powerful
candidate of absorbents. UV coating toward various
kinds of substrates and adsorbing ability of thematerials are now under investigation.
47

Acknowledgement
This work was supported by a research grant from
Advanced Research Center for Regional
Cooperation, National Institute of Technology,
Fukui College.

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619-622.
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