Ultraviolet-light-responsive liquid marbles

Article abstract of DOI:10.1246/cl.130119

Photoresponsive liquid marbles were prepared using spiropyran (SP) powder. The liquid marbles were transferred to a Petri dish containing water and remained stable on the water surface for more than a week in the dark. The liquid marbles disintegrated immediately upon irradiation with ultraviolet (UV) light.

Full text of DOI:10.1246/cl.130119

doi:10.1246/cl.130119
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Published on the web May 3, 2013
Ultraviolet-light-responsive Liquid Marbles
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Keita Nakai, Syuji Fujii, Yoshinobu Nakamura, and Shin-ichi Yusa*
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Department of Materials Science and Chemistry, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280
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Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology,
5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585
(
Received February 15, 2013; CL-130119; E-mail: yusa@eng.u-hyogo.ac.jp)
Photoresponsive liquid marbles were prepared using spiro-
pyran (SP) powder. The liquid marbles were transferred to a Petri
dish containing water and remained stable on the water surface
for more than a week in the dark. The liquid marbles disin-
tegrated immediately upon irradiation with ultraviolet (UV) light.
Liquid marbles, which are liquid droplets (typically of
millimeter-sized diameter) encapsulated by relatively hydro-
phobic self-assembled particles at the air­liquid interface,
constitute a novel approach for handling droplets and conducting
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reactions within droplets. Liquid marbles have been shown to
be promising for applications in material-based devices such as
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chemical sensors, micropumps, and microreactors. They are
considered to be a perfect nonwetting system; in addition, they
work as liquid microcarriers, capable of moving quickly without
any leakage, because the hydrophobic particles on the liquid
marble surface form a nonstick interface between the droplet and
the substrate, reducing motion resistance. In order to exploit
liquid marbles as microcarriers, both stability and remote
movement control are highly desirable. Recently, stimulus-
Scheme 1. (a) Light-induced isomerization between spiropyr-
an (SP) and the corresponding merocyanine (MC) by alternating
irradiation with ultraviolet (UV) and visible light. Also shown
are digital photographs of SP and MC powder. (b) Schematic
illustration of a liquid marble prepared using SP powder on the
surface of a pool of water, and subsequent disintegration upon
exposure to UV light.
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responsive liquid marbles that are sensitive to pH, electric
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fields, and magnetic fields have been fabricated. We previously
reported pH-responsive liquid marbles stabilized by polymer
particles carrying pH-responsive hairs, of which the surface
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a­5c
wettability by water could be changed through pH control.
Herein, we report light-responsive liquid marbles prepared
using a spiropyran (SP) powder, of which the wettability by
water changed upon external light stimulus (Scheme 1). These
new liquid marbles were stable on substrates (solid and water) in
the dark but disintegrated under ultraviolet (UV) light.
Here, [MC]t is the amount of the MC isomer at time t, [SP]0 is
the initial amount of the SP isomer, R(t) and R(0) denote the
reflectances at ca. 590 nm at time t and at the beginning of the
coloration process, respectively, and R¨ represents the reflec-
tance at ca. 590 nm after the photostationary state is reached.
The fraction of MC was plotted against the irradiation time
(Figure 1b). About 80% of the SP powder changed to MC
within 10 s of UV irradiation. The first-order kinetic rate
Photoresponsive isomerization of SP was confirmed by
UV­visible absorption spectroscopy in homogeneous solution.
Changes in the UV­vis absorption spectrum of SP in ethyl
acetate upon UV irradiation for 10 min were examined (Sup-
constant (k) for the conversion of SP to MC in the powder
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¹1 ¹1
porting Information (SI), Figure S1). Before UV irradiation,
state was 5.31 © 10
(SI, Figure S2).
s , as estimated from the initial slope
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no absorption peak was observed at 580 nm; however, the
absorbance at 580 nm increased upon UV irradiation; this was
attributed to the formation of the corresponding merocyanine
Figure 2 shows water droplets on the pellet surface before
and after UV irradiation. The contact angle of the SP pellet with
pure water was 85° (Figure 2a), while that of the MC pellet after
UV irradiation was 64° (Figure 2b). The pellet surface became
more hydrophilic than the original SP pellet after UV irradiation
because MC is hydrophilic owing to the presence of charged
groups. This observation suggested that the surface polarity of
the SP powder might be increased by conversion to MC upon
UV irradiation. Rosario et al. reported a similar photoresponsive
change in the water contact angle on SP molecules covalently
(MC). The color of the solution changed from clear to purple
upon UV irradiation.
Figure 1a shows changes in the diffuse reflectance spectrum
of SP powder due to UV irradiation. Upon irradiation with UV
light, the reflectance at ca. 590 nm decreased owing to the
photoisomerization of SP to the MC isomer. The fraction of the
MC isomer (fMC) at irradiation time t can be calculated from the
reflectance by using the following equation:
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ꢀ
ꢁ
bound to a glass surface.
½
MCꢀt
RðtÞ ꢁ R1
Rð0Þ ꢁ R1
fMC ¼
¼ 1 ꢁ
ð1Þ
Liquid marbles were prepared by rolling a droplet of water
over SP powder. The powder coated the water droplet
½
SPꢀ0
Chem. Lett. 2013, 42, 586­588
© 2013 The Chemical Society of Japan
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Figure 3. (a) Digital photograph showing a liquid marble
containing 10 ¯L of water covered with SP powder on a glass
slide. (b) Fluorescence microscopic image of a liquid marble
prepared using SP powder and a 10-¯L aqueous solution
containing gelatin (20 wt %) and fluorescein (0.01 wt %) at
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0 °C. After cooling to room temperature, UV irradiation was
applied for 10 min to transform SP to MC, which emits
fluorescence. The liquid marble was sectioned with a scalpel,
and one of the hemispheres was placed on a glass slide. Note the
yellow region due to fluorescein and the orange powder
aggregates.
Figure 1. (a) Changes in the diffuse reflectance spectrum of
SP powder upon UV irradiation. The irradiation times are
indicated in the figure. (b) Changes in the MC fraction (fMC) of
the powder as a function of irradiation time upon photo-
isomerization of SP to MC.
Figure 2. Water contact angles of the pellet surface of (a) SP
before UV irradiation and (b) MC after UV irradiation.
Figure 4. (a) Experimental setup for measuring light-induced
disintegration of the liquid marble, and photographs of a
floatable liquid marble consisting of 7 ¯L of water covered
with SP powder on the surface of water in a quartz Petri dish
immediately, rendering it both hydrophobic and nonwetting.
These liquid marbles remained intact after transfer onto a glass
slide (Figure 3a) and onto water (Figure 4b). The liquid marbles
clearly had significant surface roughness, which suggested that
they were coated with irregular powder clusters.10 The shape
(
b) before and (c) after UV irradiation for 20 s from below.
of the irregular SP powder clusters could be observed by SEM
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(
SI, Figure S3). These powder clusters may trap air to form the
that the powder acted as a liquid-marble stabilizer and was
present only as a coating on the surface of the water droplet.
Liquid marbles were prepared by rolling a 7-¯L water
droplet over ground SP powder. For the investigation of the
effect of UV irradiation, the liquid marbles were transferred to
the surface of water in a quartz Petri dish, and UV light was
applied from below the dish (Figure 4). Enhanced stability was
achieved in the dark, which indicated that the SP powder was
sufficiently hydrophobic to adsorb strongly at the air­water
interface. The liquid marbles were stable for more than a week
on the water surface, under humid conditions without UV
irradiation. However, the liquid marbles on water were always
unstable in the presence of UV light, disintegrating immediately.
The disintegration time, defined as the time required for the
liquid marble to break after UV irradiation, was measured. After
five measurements, the average disintegration time for a liquid
liquid marble, allowing Cassie­Baxter wetting at the cluster­
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liquid interface.
A liquid marble was examined through fluorescence mi-
croscopy (Figure 3b). The liquid marble was prepared with
fluorescein (0.01 wt %) dissolved in an aqueous solution (10 ¯L)
of gelatin (20 wt %) in combination with SP powder at 60 °C.
After being cooled to room temperature, the liquid marble was
sectioned with a scalpel. This was only possible because of the
nature of gelatin, which is a free-flowing aqueous solution at
0 °C but forms a gel at room temperature. UV irradiation was
applied for 10 min to transform the SP to MC, which emits
fluorescence. The fluorescein within the gel gave rise to a
yellow color, which was surrounded by heterogeneous orange
aggregates ranging from 180 to 5 ¯m; these were clusters of MC
powder. The double-layered fluorescence image clearly showed
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Chem. Lett. 2013, 42, 586­588
© 2013 The Chemical Society of Japan
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marble containing 7 ¯L of water was calculated to be 14.5 s
acknowledged. The authors acknowledge Masato Kamiji
(JASCO) for help with the diffuse reflectance studies.
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(SI, Figure S4). When UV irradiation was applied, the hydro-
phobic SP was converted to ionic MC. Therefore, the MC
powder was wetted spontaneously, and disintegration of the
liquid marble followed. Although liquid marbles containing
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¯L of water remained stable for more than one week on the
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A video of the photoresponsive behavior of the liquid marbles is
8
provided as Supplementary Video S1 in the SI.
Recently, Wang et al.13 reported the preparation of UV-
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(
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In conclusion, we have demonstrated the preparation of
photoresponsive liquid marbles composed of SP powder and
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SP powder were transferred to a water surface, where they
remained stable for more than a week under humid conditions in
the dark. Upon UV irradiation, however, the liquid marbles burst
immediately.
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This work was supported by a Grant-in-Aid (No. 23106717)
for Scientific Research on Innovative Areas, “Molecular Soft-
Interface Science,” from the Ministry of Education, Culture,
Sports, Science and Technology of Japan, which is gratefully
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Chem. Lett. 2013, 42, 586­588
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/