Visible Light-Controlled Inversion of Pickering Emulsions Stabilized by Functional Silica Microspheres

Article abstract of DOI:10.1021/acs.langmuir.7b03822

A new class of donor-acceptor Stenhouse adduct (DASA)-functionalized silica microspheres (SMs) is designed and described to formulate Pickering emulsions with inversion property and large polarity change upon visible light irradiation. By tuning the hydrophilicity of the functional SM particles with visible light, these Pickering emulsions can easily perform inversion from water-in-oil to oil-in-water. The inversion performance of the emulsions is ascribed to DASA photoisomerization from an extended, hydrophobic, and intensely purple-colored triene to a compact, zwitterionic, and colorless cyclopentenone upon irradiation with visible light. This unique inversion behavior has been applied to control encapsulation and the release of fluorescein sodium salt.

Full text of DOI:10.1021/acs.langmuir.7b03822

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Article
Visible light-controlled inversion of Pickering
emulsions stabilized by functional silica microspheres
Yongkui Chen, Zhiyong Li, Huiyong Wang, Yuanchao Pei, Yunlei Shi, and Jianji Wang
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03822 • Publication Date (Web): 30 Jan 2018
Downloaded from http://pubs.acs.org on February 6, 2018
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Visible lightꢀcontrolled inversion of Pickering
emulsions stabilized by functional silica
microspheres
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†
Yongkui Chen, †,‡ Zhiyong Li,† Huiyong Wang,† Yuanchao Pei,† Yunlei Shi† and Jianji Wang*,
†Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine
Chemicals, School of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical
Media and Reactions, Ministry of Education, Henan Normal University, Xinxiang, Henan
453007, P. R. China.
‡School of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang, Henan
453003, P. R. China.
E-mail: Jwang@htu.cn
KEYWORDS: Photoisomerization, Functional particles, Pickering emulsions, Phase inversion,
Visible light response, Stenhouse adducts
ABSTRACT: A new class of donorꢀacceptor Stenhouse adducts (DASAs) functionalized silica
microspheres (SM) is designed and described to formulate Pickering emulsions with inversion
property and large polarity change under visible light irradiation. By tuning the hydrophilicity of
the functional SM particles with visible light, these Pickering emulsions can easily perform
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inversion from w/o to o/w. The inversion performance of the emulsions is ascribed to DASAs
photoisomerization from an extended, hydrophobic, and intensely purple colored triene to a
compact, zwitterionic, and colorless cyclopentenone upon irradiation with visible light. This
unique inversion behavior has been applied to control encapsulation and release of fluorescein
sodium salt.
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INTRODUCTION
Pickering emulsions, which are stabilized solely by colloidal particles, have been known for
more than a century.1,2 Due to the adsorption of colloid particles at the interface of two
immiscible liquids such as oil and water, such emulsions are very stable compared with those
stabilized by surfactants.3 Moreover, after removal of the resolved oil layer, the remnant
emulsion still exhibits good stability without any coalescence.4,5 Studies on Pickering emulsions
are therefore of critical importance in both understanding the fundamentals and developing
potential applications.6,7
In some practical applications, such as biocatalysis, fossil fuel production, oil transport, and
emulsion polymerization, emulsions need to be temporarily stabilized and subsequently
demulsified or inversed. For instance, stimulusꢀresponsive Pickering emulsions stabilized by
switchable surfaceꢀactive colloid particles have shown the ability to allow onꢀdemand
demulsification or phase inversion.8,9 In this case, switchable Pickering emulsions are desirable
and have received considerable attention in recent years.10,11
In this context, the colloid particles used as stabilizers at a range of fluid interfaces should be
convertible between hydrophilicity and hydrophobicity to response environmental triggers.12−14
Nevertheless, the design and synthesis of the functional particles are in general complicated.
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Recently, much effort has been made to develop intelligent Pickering emulsions stabilized by
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surface active particles that respond to a specific trigger such as pH,15−17 temperature,18−20
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magnetic field,21,22 CO2
or specific ion concentration.26,27 In contrast, light is an excellent
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trigger without the need to add chemicals, and the wavelength, polarization direction and
intensity of the light can be easily controlled. For example, Binks et al.28 reported a dual stimuliꢀ
responsive oil in water Pickering emulsion with light and CO2/N2 triggers. Negatively charged
silica nanoparticles in combination with a trace amount of dual switchable surfactant were used
as stabilizers. Qu et al.29 synthesized silica coated upconversion nanoparticles followed by
grafing photochromic spiropyrans on them by amido bond. These nanoparticles were used to
achieve the reversible inversion of a lightꢀtriggered Pickering emulsion for biphasic
enantioselective biocatalysis. Meng et al.30 prepared stable water in oil emulsions by UVꢀinduced
tailored wettability of TiO2 nanoparticles anchoring at the interface. Such emulsions were used
for controlled encapsulation and release of cargos using light trigger. Yang et al.31 reported a
lightꢀtriggered reversible phase transfer of composite colloids. These composite colloids could
transfer reversibly between hydrophobic and hydrophilic phases upon irradiation with different
wavelengths of light. UV responsive Pickering emulsions stabilized by titanium dioxide
nanoparticles for photocatalytic degradation of nitrobenzene32 and Rhodamine B33 were also
developed. Nevertheless, either multistep synthesis or UV light is inevitable. Although some
visible light responsive compounds,34,35 nearꢀInfrared photoswitchable azobenzenes36 and lightꢀ
controlled colloid assembly37,38 were also reported, hydrophilicity/hydrophobicity of the surface
was not suitable to form emulsions. Therefore, developing more convenient approaches to tune
particle surfaces is necessary.39
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Recently, donor−acceptor Stenhouse adducts (DASAs) has attracted a lot of attention.40,41
These compounds can be synthesized easily in two steps from commercially available starting
materials, and undergo reversible isomerization in some organic solvents upon irradiation with
visible light. The primary process is the conversion from an extended, hydrophobic, and
intensely purple colored triene to a compact, zwitterionic, and colorless cyclopentenone upon
irradiation with visible light.42,43 However, to the best of our knowledge, reports on visible light
controlled surface switch of particles modified by DASAs in Pickering emulsions have not been
found in literature so far.
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Scheme 1. Schematic illustration for the synthesis of DASAsꢀfunctionalized SM particles SMꢀ
CXD (x=6, 7 and 9), where x refers to the number of carbons connected to the nitrogen atom
In this work, we propose a conceptually new approach for the first time: a series of new
photochromic DASAs have been used to functionalize silica microspheres (SM) with covalent
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linkage (Scheme 1). Upon irradiation with a visible light, DASAs photoisomerize from a colored
neutral linear form (a) to a colorless zwitterionic cyclic form (b). Such a transformation makes
the hydrophilicity/hydrophobicity of the surface of colloidal emulsifiers switchable, thus leading
to the inversion of Pickering emulsions from w/o to o/w. In particular, this unique inversion
behavior has been applied to control encapsulation and release of fluorescein sodium salt. Thus a
highly efficient process is achieved by using DASAsꢀfunctionalized SM particles as the
emulsifier and visible light as a smart trigger.
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EXPERIMENTAL SECTION
Materials. Tetraethyl orthosilicate (TEOS, 99%), Nꢀethylaminoisobutyltrimethoxysilane
(98%),
Nꢀ(nꢀbutyl)ꢀ3ꢀaminopropyltrimethoxysilane
and
Nꢀcyclohexylꢀ3ꢀaminopropylꢀ
trimethoxysilane were purchased from Aladdin Co. Ltd. Meldrum's acid (98%), ammonia
(NH3·H2O, 25 wt%) and furfural (99%) were obtained from Alfa and used as received. Toluene
(99.5%) was from Aldrich, and trace of water was removed with sodium before use. ethyl
formate (99.5%), methyl acetate (99.5%), ethyl acetate (99.5%), dichloromethane (99.5%),
trichloromethane (99%), tetrachloromethane (99.5%), ethanol (95%), nꢀpentanol (99.5%) and nꢀ
hexanol (99%) were purchased from Alfa and used as received unless stated otherwise.
Deionized water was used in this study.
Synthesis of 5-(Furan-2-ylmethylene)-2,2-dimethyl-1,3-dioxane-4,6-dione. 5ꢀ(Furanꢀ2ꢀ
ylmethylene)ꢀ2,2ꢀdimethylꢀ1,3ꢀdioxaneꢀ4,6ꢀdione was prepared according to the procedures
described in the recent literature.42 The typical procedure was as follows: 2,2ꢀdimethylꢀ1,3ꢀ
dioxaneꢀ4,6ꢀdione (1.51 g, 10.5 mmol) and 2ꢀfuraldehyde (0.83 mL, 10 mmol) were mixed with
water (30 mL) at 30℃ under N2 atmosphere. The obtained mixture was heated to 45 ℃ and
reacted for 2h. After completion of the reaction (as monitored by TLC, hexane : ethyl acetate =
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3:1), the mixture was cooled to room temperature. The precipitated solid was collected by
vacuum filtration and washed twice with 30 mL of cold water. The resulted solid was dissolved
in dichloromethane, washed sequentially with 30 mL of aqueous saturated NaHSO3, 30 mL of
water, 30 mL of aqueous saturated NaHCO3 and 30 mL of brine. The organic layer was dried
over MgSO4, filtered and the solvent was removed by rotary evaporation to give 2.10 g (95%) of
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1
the product as a bright yellow powder. H NMR (600 MHz, CDCl3): δ = 8.47 (d, 1H, CH), 8.36
(s, 1H, CH), 7.85 (d, 1H, CH), 6.76 (ddd, 1H, CH), 1.77 (s, 6H; 2CH3) ppm.
Synthesis of SiO2 microspheres. SiO2 microspheres were synthesized according to the well
established Stöber method.44 For this purpose, 200 mL of 95% ethanol, 50 mL of deionized
water and 11.2 mL of 25 wt% aqueous NH3 were mixed to form a uniform system. After stirring
for 30 min at room temperature, 8.32 g of tetraethyl orthosilicate was added dropwise into this
solution under stirring. After stirring for 12 h, the precipitation was isolated by centrifugation,
washed with deionized water, and then dried under air at 100 ℃ for 2 h to give SiO2
microspheres.
Synthesis of secondary amine-modified silica microspheres. 1.0 g of the synthesized SiO2
microspheres (dried at 125 ℃ for 4 h) was dispersed into 100 mL of toluene, and 1.6 mL of
organosilane was added into this suspension. After refluxing at 60℃for 1 h and then reaction at
110 ℃ for 24 h under N2 atmosphere, the material was isolated by centrifugation, washed five
times with toluene. After being dried at 40 ℃ for 12 h under vacuum, secondary amineꢀ
functionalized SiO2 microsphere was afforded (noted as SMꢀCX particles, x refers to the carbon
number of the secondary amine silane, x=6, 7, 9, respectively).
Synthesis of DASAs-modified silica microspheres. Typically, 5ꢀ(furanꢀ2ꢀylmethylene)ꢀ2,2ꢀ
dimethylꢀ1,3ꢀdioxaneꢀ4,6ꢀdione (0.6 gram, 2.7 mmol) was dissolved in 7 ml of tetrahydrofuran.
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0.61 gram of SMꢀC9 (0.06 mmol of secondary amine was grafted on the surface, as assessed by
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TGA45,46) was added to this solution. After being stirred at 23 ℃ for 10 min, the solution was
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cooled to 0 ℃ and further stirred for 10 min, and then filtered to collect the precipitated solid.
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This solid was washed with cold diethyl ether three times, and dried under vacuum to afford the
DASAsꢀmodified silica microspheres as a pink solid (noted as SMꢀC9D particles). For the
preparation of SMꢀC6D and SMꢀC7D, the procedures were the same as the SMꢀC9D.
Preparation of Pickering emulsion. 2 mL of deionized water was first added into a vial
containing 0.024 g of the functionalized silica microspheres. Then, 2 mL of an organic solvent
(toluene in most of the cases) was added into the vial. After vigorously stirring with a magnetic
bar at 800 rpm for 3 min, SMꢀCXD particlesꢀstabilized emulsion was obtained.
Characterization. Scanning electron microscope (SEM) images were obtained on a HITACHI
UHR FEꢀSEM (SU8010，Japan). Thermogravimetric analysis (TGA) was performed with a
NETZSCHTG analyzer (STA449C, Germany) under nitrogen atmosphere from room
temperature to 900℃with a heating rate of 10 ℃/min. Xꢀray photoelectron spectra (XPS) were
recorded on a Thermo Scientific ESCALAB (250Xi, America). Photographs of each emulsion
were observed on an optical microscope (DYPꢀ990, Shanghai, China). A drop of each emulsion
was mounted on a glass slide and was observed with a digital camera. Water contact angles
(waterꢀsolidꢀair) were measured on a KRUSS Drop Shape Analyzer (DSA25, Germany).
RESULTS AND DISCUSSION
Synthesis and characterization of DASAs-modified silica microspheres. A series of
photochromic DASAsꢀfunctionalized SM particles (Figure. 1a) were designed and synthesized.
The SiꢀOH groups on SiO2 microspheres were reactive and could be selectively modified by
silanes. Thus, the DASAsꢀmodified silica microspheres were obtained by introducing functional
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secondary amine groups on SiO2 microspheres. The successful grafting of DASAs onto SiO2
microspheres was confirmed by FTIR spectroscopy (Figure. 2). The typical carbonꢀnitrogen
bond vibration (at 1190 cm−1), conjugated olefin double bond stretching (at 1600 cm−1), carbonꢀ
hydrogen bond stretching (at 2954 cm−1, 2856 cm−1, 1460 cm−1) and gemꢀdimethyl carbonꢀ
hydrogen bond stretching (at 1383 cm−1, 1365 cm−1) indicate that DASAs were really introduced
on the surface of SiO2 microspheres. To obtain a desired surface chemistry, the carbon number
of secondary amine silane was varied from 6 to 7 and 9. The resultant silica microspheres were
denoted as SMꢀCXD (x=6, 7, 9). It was found that the particles were well dispersed after
modification, and SMꢀCXD was spherical in morphology and its diameter was in the range of
250–350 nm (Figure. 1b). According to the XPS results (Figure 1c and Figures S1ꢀ3), the atom
ratio of carbon to nitrogen gradually increased from SMꢀC6D to SMꢀC9D. Thermogravimetric
analysis results (Figure. S4) indicate that DASAs loadings on these SM particles were,
respectively, 3.84, 2.29 and 3.55 wt%.
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As a representative example, the UVꢀvis absorption spectrum of SMꢀC9D composite colloids
dispersed in toluene–water mixture was determined, and the results were reported in Figure S5. It
can be seen that before irradiation with visible light, there was a strong absorption peak at around
515 nm in the upper toluene phase, which corresponds to the triene form of SMꢀC9D. However,
the characteristic peak became very weak after visible light irradiation by a Crystal Clear 200
bulb (200 watt, 3780 lumens) for 1h. Meanwhile, two absorption peaks at around 515 nm and
268 nm appeared in the bottom water layer. This means that photoisomerization of DASA took
place from triene to cyclopentenone isomers. Because of the stronger hydrophilicity of
cyclopentenone isomer, most of the colloids were transferred into the aqueous phase. These
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results suggest that the surface chemistry can be influenced by the carbon chain length of the
organosilane and polarity change of the functional groups.47
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Figure 1. (a) The structural description for DASAs functionalized silica microspheres; (b) SEM
image of SMꢀC7D, scale bar: 100nm; (c) Three types of functional SM particles with different
loadings of DASAs elements.
Figure 2. FTIR spectra of SM, SMꢀC6D, SMꢀC7D and SMꢀC9D.
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The photoreaction dynamic of the composite colloids was also investigated by UVꢀvis
absorption spectroscopy31 with THF (Tetrahydrofuran) as a dispersion media since both triene
and cyclopentenone forms of SMꢀC9D were dispersible in THF. It is apparent from Figure 3a
that exposure of SMꢀC9D composite colloids under a standard incandescent bulb resulted in a
remarkable decrease in the absorption at around 515 nm over the irradiation course of 1h. A
concomitant increase in the absorption at around 268 nm was also observed for the
cyclopentenone form (Figure 3a and 3b). After irradiation of 1h, saturation absorption
(isomerization equilibrium) was observed.
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Figure 3. (a) Absorption spectra of the SMꢀC9D composite colloids dispersed in THF (0.1
mg/mL) irradiated by visible light as a function of time; (b) Expanded view of absorption spectra
of the SMꢀC9D composite colloids between 250 and 280 nm.
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Characteristics of the Pickering emulsions stabilized by different functionalized silica
microspheres. We examined the interfacial activity and the remote controlled inversion of the
synthesized samples. Typically, after the mixture of the functionalized silica microspheres,
toluene and water was stirred in the dark for 3 min (800 rpm) at ambient conditions, different
phenomena could be observed for these samples. Bare silica microspheres precipitated in the
bottom water layer (Figure. S6) and emulsion droplets were not found under optical microscope.
Different from the bare silica microspheres, SMꢀC6D and SMꢀC6 led to a Pickering emulsion in
the upper toluene layer since droplets were observed (Figure. 4 and Figure. S6). The drop test
confirmed that it was an o/w emulsion. On the contrary, SMꢀC7D, SMꢀC9D, SMꢀC7 or SMꢀC9
was well dispersed in the bottom water layer. Optical microscopy result confirmed that it was
also a Pickering emulsion because spherical droplets were clearly observed (Figure. 4 and
Figure. S6). The drop test indicates that it was a w/o Pickering emulsion. Upon irradiation with
visible light and homogenization, only SMꢀC7D and SMꢀC9D transferred to the upper toluene
layer where o/w emulsions were formed. However, no inversion was found for the systems with
SMꢀC6, SMꢀC7 and SMꢀC9 because of the absence of a visible light sensitive moiety. As
assumed in theory, we did not observe any other changes either (Figure. S6). Interestingly, the
SMꢀC6D stabilized Pickering emulsion was demulsified upon irradiation with visible light, as
observed from the optical micrographs (Figure 4). In addition, we investigated the stability of the
Pickering emulsions before and after visible light irradiation. As a representative example, the
SMꢀC9D stabilized emulsion was found to be well dispersed in the bottom water layer (w/o)
before visible light irradiation. After irradiation with visible light by a Crystal Clear 200 bulb
(200 watt, 3780 lumens) for 50 min followed by agitation, o/w Pickering emulsions were formed
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in the top toluene layer. The two types of emulsions could be stable against sedimentation at
least 12 days (Figure. S7).
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Figure 4. Appearance of emulsions with different functional silica microspheres in tolueneꢀwater
mixture (photographs taken after standing for 30min). Every vial contains 2 mL of toluene, 2 mL
of water, and 0.024 g of functional silica microspheres. State 1: before visible light irradiation;
State 2: after visible light irradiation. Scale bar is 200 ꢁm.
The difference in the emulsion inversion is related to the visible light responsive surface
chemistry. As shown in Figure. 5a, the conductivity variation in the continuous phase of
Pickering emulsions demonstrated the inversion processes. As a representative example, the
conductivity of the emulsions stabilized by SMꢀC9D particles was measured to be approximately
0 before visible light irradiation. After exposed to visible light for 1h, it increased to 1235 µscmꢀ
1, and accompanied by unidirectional conversion of the colored triene to its colorless zwitterionic
cyclopentenone isomer. In contrast, the conductivity always maintained at 1328 µscmꢀ1 for the
bare silica microspheres during irradiation with visible light. The difference between these
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samples could be attributed to the surface modification that alters the surface chemical properties
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Figure 5. a) Conductivity of Pickering emulsions of toluene and 0.01 M aqueous NaCl (1:1)
stabilized by SMꢀCXD particles before and after visible light irradiation; b) Water contact angles
of SMꢀCXD before and after visible light irradiation.
The visible light triggered phase inversion was further supported by water contact angle
measurements (Figure. 5b and Figure. S8). The water contact angle of the freshly prepared SMꢀ
C9D was measured to be 116.8°, which decreased to 58.2° after irradiation with visible light,
confirming the increase of hydrophilic property. Similar trend was observed for the contact angle
of SMꢀC7D. As anticipated, such changes drove the inversion of SMꢀC9D stabilized emulsions
from w/o to o/w. In contrast, the wettability of the bare silica microspheres, SMꢀC6, SMꢀC7 and
SMꢀC9 was almost unaffected by visible light because of the absence of a visible light sensitive
moiety. SMꢀC6D is relatively hydrophilic and its surface is more hydrophilic after visible light
irradiation, resulting in demulsification. The surface silanol group (SiOH) made the silica surface
more hydrophilic, whereas the photochromic DASAs on the silica surface rendered the particles
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surface partially hydrophobic. These changes in the surface wettability of the particles are the
origin of the switching between triene and cyclopentenone isomers.
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Besides water/toluene system, SMꢀC7D and SMꢀC9D were also good emulsifiers for
water/ethyl formate, methyl acetate, ethyl acetate, dichloromethane, trichloromethane,
tetrachloromethane, nꢀpentanol and nꢀhexanol systems (Figures. S9ꢀ11). However, SMꢀC6D
formed w/o emulsions only in water/ dichloromethane and water/trichloromethane systems. In
other systems (not including water/ethyl formate), o/w emulsions were observed. In these
systems, except for water/nꢀpentanol and water/nꢀhexanol, all w/o emulsions could be inverted
easily to o/w emulsions by just exposing to visible light. After exposed to visible light, the
surface of SMꢀC6D became too hydrophilic to stabilize the emulsions, leading to the
demulsification in the case of water/tetrachloromethane, methyl acetate and ethyl acetate
systems.
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Usually, emulsion type (o/w or w/o) is believed to be determined in some way by the particle
wettability, expressed in terms of the contact angle θ. Here θ is the threeꢀphase contact angle
between the colloidal particle, the oil phase, and the water phase at the interface. In fact, the
strength with which a particle is held at an oil/water interface is related not only to θ but also to
the interfacial tension, γow.48ꢀ50 It is well established that a colloidal particle crossing an oil/water
interface needs to overcome an energy obstacle, which can be calculated according to the
equation: 48ꢀ50
(1)
where R is the particle radius. Apparently, the energy obstacle depends on the particle size, the
interfacial tension and the threeꢀphase contact angle. The interfacial tension values of water /
organic solvents used in this work are quite different. The γow values of water / ethyl acetate,
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water / toluene and water / tetrachloromethane are, respectively, 19.8, 36.1 and 43.7 mN mꢀ1 at
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25℃.51,52 From the above equation, it is easy to see that E increased with the increase of γow at a
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given contact angle. For example, if R of 150 nm and γow of 43.7 mN mꢀ1 are considered for SMꢀ
C9D / water / tetrachloromethane system, E is estimated to be as high as 106 kBT (where kB is the
Boltzmann constant and T the temperature) at θ = 90o. This can lead to the greater difficulty for
the particles in crossing the phase interface compared with the other two systems. In the
experiments, we found that although SMꢀC9D particles formed w/o Pickering emulsions in water
/ ethyl acetate, water / toluene and water / tetrachloromethane systems and all the emulsions
inversed from w/o to o/w after irradiation with visible light, the required time of irradiation was
quite different from 30 min, 50 min to 120 min for the three different systems. This is consistent
with the theoretical estimation. All these results indicate that both surface
hydrophilic/hydrophobic transitions and interfacial tension are critically important to this
inversion process.
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Additionally, the emulsion inversion was also observed in the presence of different mass
fractions of SMꢀC7D and SMꢀC9D using toluene as oil phase. Taking SMꢀC9D as an example,
the average droplet diameter decreased from 711 to 150 ꢁm when the content of the colloidal
emulsifier was increased from 0.8 to 1.5 wt% (Figure. S12). Also, the polydispersity of the
emulsion droplets increased simultaneously. More interestingly, after removing the resolved oil
layer or water layer, the remnant emulsion still exhibited good stability without any coalescence,
disproportionation or water drainage.
Encapsulation and release of fluorescein sodium salt. To visually illustrate the emulsion
inversion process and simulate encapsulation and release of cargos, fluorescein sodium salt was
encapsulated into the droplets investigated by fluorescence microscopy. As shown in Figure. 6,
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initially, green emission of fluorescein sodium salt was observed in the droplets of the w/o
Pickering emulsions. Upon irradiation with visible light, the functionalized SM particles, such as
SMꢀC9D, became relatively hydrophilic. Following homogenization, o/w emulsions were formed
and the fluorescein sodium salt was released into the water layer which exhibited green
fluorescence. These results are closely consistent with the isomerization of SMꢀC9D structure
caused by the visible light induced switching of DASAs to a relatively hydrophilic derivative.
This demonstrates that the hydrophilic cargo (fluorescein sodium salt) was encapsulated within
the w/o Pickering emulsions until visible light was irradiated to trigger the Pickering emulsions
inversion. Therefore, this simple but powerful application of the functionalized Pickering
emulsions showed promising potential in visible light controlled cargos release and related areas.
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Figure 6. (a) Photographs of fluorescein sodium salt encapsulation by SMꢀC9D stabilized
Pickering emulsions: w/o (left), o/w (right), where visible light was utilized as stimuli before
homogenization; (b) Fluorescence microscopic images of the corresponding Pickering emulsions
shown in (a). Scale bar: 200 ꢁm.
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CONCLUSIONS
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In summary, by tuning the surface chemistry using photochromic DASAsꢀmodified SM
particles, we have successfully prepared interfacially active functional particles for the
formulation of Pickering emulsions. These functional SM particles show structure isomerization
characteristics upon visible light irradiation. Particularly, DASAs grafing on SMꢀC7D and SMꢀ
C9D can switch from a conjugated, colored form to a ringꢀclosed, colorless structure upon
irradiation with visible light. Such transformation makes the hydrophilicity/hydrophobicity of the
surface of colloidal emulsifiers switchable, thus driving the emulsions inversion from w/o to o/w.
Owing to the appropriate amphiphilic structure, the high yields and the simple synthetic
procedures of the DASAs photoswitches, these functionalized SM particles are important
complementary to previous classes of photoswitches for Pickering emulsions.
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The potential application of these photoswitches was demonstrated through encapsulation of
fluorescein sodium salt in w/o droplets and release in the visible light mediated emulsions
inversion. We anticipate that this remote controlled inversion strategy would provide unique
opportunities for application in areas such as biocatalysis in biphasic systems.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
XPS spectra, TG curves, Water contact angles, UVꢀVis absorption spectra and appearance of
emulsions with different functional silica microspheres.
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AUTHOR INFORMATION
Corresponding Author
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*Tel.: +86-373-3325805; Fax: +86-373-3329030
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E-mail: Jwang@htu.cn
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work is supported by the National Natural Science Foundation of China (No. 21673068)
and the 111 project (No. D17007).
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Pickering emulsions stabilized by functional silica microspheres exhibit behaviors of visible light
responsive inversion, cargos encapsulation and release.
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