X uelin T ian ,H ao B ai ,Y ongmei Z heng ,* and L ei J iang 1. Introduction
H eterostructured materials are greatly promising for the achievement of exceptional properties and multiple function-alities through combining different chemical components. Especially the design and synthesis of one-dimensional (1D) heterostructured materials with modulated architectures and compositions are important factors because of their enhanced performance in various ? elds, such as, the miniaturization of
electrical/optical devices, [ 1,2 ] sensitive biosensors, [ 3,4 ] and highly
active catalyst. [ 5,6 ] Various methods, including vapor-phase synthesis, [ 7,8 ] solution-phase deposition, [ 9–11 ] and template-based synthesis, [ 12,13 ] have been tried to fabricate 1D micro/
nanoscale heterostructured materials with different composi-tions and con? gurations. However, most of these methods are
comparatively complex and usually need
multi-step operations. In the vapor-phase or solution-phase approaches, different gas ? ow or solution precursors need to be carefully introduced in an alternating
fashion. [ 7–11 ] For template-based syn-thesis, different components are deposited sequentially into the pores of membranes,
and the subsequent removal of this tem-plate further complicates the fabrication procedure. [ 12,13 ] Therefore a simple and
versatile approach toward developing 1D
heterostructured materials is very signi? -cant and desirable.
S piders can smartly spin silk with ordered hierarchical structures through multi-microchannels that supply different chemical components to the spinning
process. [ 14–16 ] For example, ecribellate spi-ders can spin capture silk with a uniform
coating of glycoprotein glue that sponta-neously breaks up into droplets on the centric silk (observed by environmental scanning electron microscopy (ESEM), F igure 1 a ,b). Inspired by the spinning behavior of an ecribel-late spider and the structural characteristics of its capture silk,
a novel electrohydrodynamic approach, combining electrospin-ning and electrospraying in electri?
ed coaxial jets is proposed for the fabrication of bead-on-string heterostructured ?
bers. 2. Fabrication of Heterostructured Fibers T he experimental setup is sketched in Figure 1 c . Two blunt metallic needles were arranged concentrically. The conductive
inner needle was connected to the cathode of a high-voltage
generator, and a square piece of aluminum foil was connected to the anode as a collecting substrate. The spinnable inner ? uid with high viscosity and sprayable outer ? uid with low viscosity were fed through the inner and outer needles, respectively. Dif-ferent from conventional coaxial electrospraying [ 17 ] or electros-pinning, [ 18,19 ] in our approach the electrospraying of an outer ? uid and electrospinning of an inner ? uid were combined for the ? rst time. Our strategy consisted of using a sprayable outer ? uid to imprint a series of heterogeneous beads on the centric ? ber formed by the spinnable inner ? uid. Through controlling the components of the inner and outer ? uids, bead-on-string heterostructured ? bers (BSHFs) with tunable compositions can be prepared. For this work, the outer ? uid was a 20% (w/v)
poly(ethylene glycol) (PEG, M = 20 000) solution in a mixed
B
io-inspired Heterostructured Bead-on-String Fibers That Respond to Environmental Wetting
I nspired by the geometric structure of ecribellate spider capture silk and its spinning characteristics, we propose a one-step electrohydrodynamic method
to fabricate bead-on-string heterostructured ? bers (BSHFs). By combining
electrospinning and electrospraying strategies using a sprayable outer ? uid with low viscosity and a spinnable inner ? uid with high viscosity in a coaxial jetting process, hydrophilic poly(ethylene glycol) beads are successfully imprinted on a hydrophobic polystyrene string. It is demonstrated that the
BSHFs are capable of intelligently responding to environmental change. With a change in relative humidity, the ? bers show a segmented swelling and
shrinking behavior in the “bead” parts whereas the “string” parts remain the same. The elastic BSHFs with alternating hydrophilic and hydrophobic surface characteristics represent a type of mesoscale analogues that block
copolymers and may bring about new properties and applications. Moreover,
the combined electrohydrodynamic approach developed herein should open
new routes to multifunctional one-dimensional heterostructured materials. D OI: 10.1002/adfm.201002061 P rof. Y . Zheng
School of Chemistry and Environment Beihang University Beijing 100191, P.R. China E-mail: zhengym@https://www.doczj.com/doc/ed5107407.html,
D r. X. Tian, Prof. L. Jiang Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, P.R. China D r. H. Bai National Center for Nanoscience and Technology
Beijing 100190, P.R. China
elements and only PEG possesses an oxygen
element, it can be concluded that the compo-
nent of the “string” part is pure PS and the
PEG component only appears on the “bead”
part. A quantitative element analysis is offered
in the Supporting Information (see T able S1),
which corresponds to the EDS results shown
in Figure 2c and d. For the “bead” part, the
atomic percentage of carbon and oxygen is
67.93% and 27.45%, respectively, and the molar
ratio between them is about 2.47. This value is
close to that of plain PEG, which has a carbon
to oxygen ratio of 2. The discrepancy of the
carbon to oxygen atomic ratio between our EDS
results and that of PEG may be attributed to the
interference of the centric PS ? ber.
I t is indispensable to employ miscible
inner/outer ? uids to facilitate the prepara-
tion of these BSHFs. Conventionally, for
electri?ed coaxial jets, the two ? uids may
or may not be miscible, [17–19]as the short-
time duration of the electrohydrodynamic
process can prevent the two ? uids from
mixing signi? cantly. Different from coaxial
electrospinning, which uses a spinnable
outer ? uid with high viscosity to envelope
the inner ? uid, [18]in our strategy the outer
? uid has a low viscosity so as to be easily
sprayable. If an immiscible solvent system
is used, the low viscous outer ? uid can
hardly envelope the highly viscous inner
one in a stable manner, thus both ? uids
tend to separate easily during the jetting process and can
not result in BSHFs. We have tested two immiscible inner/
outer solvent systems: butanone/water and toluene/water. In
both cases, the steady compound cone (namely Taylor cone)
below the concentric needles was hard to form and the prod-
ucts were mainly made of uniform ?bers originating from
the inner spinnable ?uid (see Figure S1 in the Supporting
Information).
T he ? ow rate of the inner ? uid also had to be tuned subtly to
favor the formation of BSHFs. If the ? ow rate was too low, the solvent of N,N-dimethyl formamide (DMF) and methylene
chloride (MC) (v/v =1:1), and the inner ? uid was a 35% (w/v)
polystyrene (PS, M w=350 000) solution in DMF. The distance
between the needles and the aluminum foil was about 22 cm,
and the applied voltage was set to 19 kV. After a coaxial elec-
trohydrodynamic jetting process, the samples were collected on
the aluminum foil.
3. Results and Discussions
Figure 1d shows the typical morphology of
the sample ?bers that exhibit unique bead-
on-string pro?les. The “string” parts have
diameters ranging from a few micrometers to
10 μm, and the “bead” parts ranged from a few
micrometers to more than 20 μm in diameter. T o
verify whether the PEG beads were successfully
imprinted on the PS ? ber, energy-dispersive
X-ray spectroscopy (EDS) was carried out.
As shown in Figure2a, carbon is the main
element in both the “bead” and “string” parts of
the sample, and oxygen is only presented in the
“bead” part. The gold element present in the
EDS comes from the coating used for sample
observation in scanning electron microscopy.
As PS is composed of carbon and hydrogen
F igure 1.a) ESEM image of capture silk of an ecribellate spider with regularly distributed glue-
droplet on it. b) Magni? ed ESEM image of a glue-droplet on the capture silk. c) Illustration of
combined electrospinning/electrospraying approach. A spinnable inner ? uid (red color) with
high viscosity and sprayable outer ? uid (green color) with low viscosity were fed through the
inner and outer needle, respectively. An electrohydrodynamic approach combining electrospin-
ning and electrospraying in electri? ed coaxial jets was carried out. The bead (green color)-on-
string (red color) heterostructured ? bers can be collected on an aluminum sheet (the collector,
see inset). d) SEM image of the BSHFs, accompanied by a magni? ed image (inset).
F igure 2.a–c) EDS microanalysis on different parts of a single BSHF. The carbon element is
the main component in both the “bead” and “string” parts of the sample (b), the oxygen is only
present in the “bead” part (c).
compound ? uid tended to drip from the nozzle. Whereas if it was too high, the beaded structure decreased evidently or even disappeared (see Figure S2 in the Supporting Information). The increased supply of inner ? uid might result in a decrease of the relative content of outer ? uid, and thereby inhibit the forma-tion of “beads”. In our fabrication the ? ow rate was tuned from
3 to 6 mL h ? 1 for the formation of the desired BSHFs.
T he achievement of BSHFs can be further rationalized by considering the surface energy difference between the core and shell components. Generally, in a coaxial electrohydrody-namic approach, the spinnable inner ? uid exerts a viscous drag force on the outer ? uid because of the shearing action at the liquid-liquid interface, and thus may inhibit the break-up of the outer ? uid into “beads”. [ 20 ] In our design, poly(ethylene glycol) with high surface energy and polystyrene with low surface energy were chosen as the components of the outer and inner ? uid, respectively. Although the two ? uids should be miscible as described above, the interface action can also be mani-pulated through an appropriate choice of solute components. Employing a shell polymer with a higher surface energy and an inner polymer with a lower surface energy can mitigate the viscous drag force exerted by the inner ? uid; the higher surface
energy of the shell polymer favors the spontaneous Rayleigh break-up of the outer ? uid during the electrohydrodynamic jet-ting process. [ 21 ] These effects bene? t the imprinting of beads on the ? ber. Furthermore, a higher surface energy of the shell component can prevent it from wetting and adhesion on the lower energy core ? ber during the evaporation of the solvent, and thus favor the formation of perfect bead-on-string heterostructures. T hrough selectively introducing a ? uo-rescent component into speci? c parts of the BSHFs, a patterned emission from the ? ber could be obtained. We chose to label the PEG mole-cules with ? uorescein isothiocy-anate (FITC) as FITC could be grafted onto the PEG via a reaction between the isothio-cyanate and the PEG hydroxyl groups, [ 22 ]
and the beads on the BSHFs could thus be visualized ( F igure 3 ). The patterned emis-sion obtained supports the use of these 1D BSHFs for the fabrication of optical microdevices. 4. Environmental Wetting Properties T he BSHFs show an interesting response to environmental change because of their alternating hydrophilic and hydrophobic surface characteristics. This segmented swelling behavior was
observed in situ inside the ESEM, via water condensation that was realized by a tunable vapor pressure at a given tempera-ture.
[ 23,24 ] The beads of the BSHFs had a diameter of 7.2 μm under a relative humidity (RH) of about 55% (vapor pressure
of ca. 3.0 torr at a temperature of 2
° C , corresponding to the atmosphere and room environment) and they swelled up to
9.5
μ m when the RH increased to more than 100% (vapor pres-sure of about 5.6 torr at the same temperature), whereas the
diameter of the string remained at 2.2 μ m during the whole process (
F igure 4 ). The swelling behavior of the beads is quite similar to that of the capture silk of ecribellate spiders in a
humid environment.
[ 14 ] T he environmental responsive property of the BSHFs is reversible, and the swollen beads (indicated with numbers
from 1 to 5 in F igure 5 a –c) shrink with decreasing RH from 90% to 70% and 35% (Figure
5 a to c), which was observed in F igure 3. a ) Optical image of a BSHF. b) The corresponding ? uorescent image of the ? ber with emitting beads. Scale bar = 10 μm . F igure 4. S egmented swelling behavior of a BSHF observed inside the
ESEM. a) Initial bead on the BSHF under the experimental condition of ca. 55% humidity with a vapor pressure of ca. 3.0 torr at a temperature of 2 ° C . b) Swollen bead on the BSHF under relatively humid experimental conditions of > 100% humidity with a vapor pressure of ca. 5.6 torr at the same temperature. Scale bar = 10 μm . F igure 5. a –c) In situ observation of the shrinkage of the “beads” indicated by numbers from 1 to 5 with decreasing RH: the RH decreases from a) ca. 90%, to b) ca. 70%, and c) ca. 35% realized by tuning the vapor pressure from about 4.7, to 3.7, and 1.8 torr at a temperature of 2 ° C inside the ESEM. Scale bar = 10 μm .
F iber Products Characterization : Scanning electron microscopy (SEM)
images were obtained using a Hitachi S-4300 SEM. To prepare the SEM sample, a thin layer of Au was coated onto the ? brous products. Fluorescence imaging of the labeled ? ber samples was carried out in the confocal mode of a Witec-Alpha scanning near-? eld optical microscope (SNOM) at 442 nm excitation. Photoluminescence was separated from the excitation light by a colored-glass long pass ? lter (OG570, Schott). To observe the environmentally responsive property of the heterostructured ? bers in situ, environmental scanning electron microscopy (ESEM, Quanta FEG 250, FEI) was employed. The segmented swelling and shrinkage behavior of BSHF can be investigated through changing the relative humidity via adjusting the water vapor pressure inside the ESEM at a temperature of 2 °C . S upporting Information
S
upporting Information is available from the Wiley Online Library or from the author.
A cknowledgements W
e thank Prof. Zhiyong Tang for fruitful discussions. We also thank Prof. Chuanyi Wang for internal reviewing o
f the manuscript. Mr. Xiangmin Men
g is acknowledged for the SEM and EDS characterization. This work is supported by the State Basic Researc
h Program of China (2010CB934700, 2007CB936403 and 2009CB930400), the National Natural Science Foundation of China (20974113, 20601005, 20973018) and the Fundamental Research Funds for the Central Universities (YWF-10–01-C10, YWF-10–01-B16). Note: This section has been amended on April 22, 2011 to add a funding agency that was omitted in the version originally published online.
R
eceived: September 29, 2010 Published online: February 15, 2011
situ inside the ESEM by tuning the vapor pressure from about 4.7 to 3.7 and 1.8 torr at a temperature of 2 °C . [ 24 ] Such a smart ? ber with segmented wetting behavior has potential uses as a micro-reactor in the biological medicine realm through the heterostructured beads on the ? ber. Moreover, the elastic BSHFs, which have alternate hydrophilic and hydrophobic sur-face characteristics, can be viewed as a type of mesoscale ana-logues to block copolymers. As block copolymers have exhib-ited many unique characteristics, such as phase separation and
the ability to assemble into nanoscale ordered structures, [ 25 ]the
BSHFs are expected to be potential building blocks for the self-assembly of superstructures with higher levels of complexity and functionality. 5. Conclusions
W
e proposed a facile and powerful method to prepare novel BSHFs, which opens the ? eld o
f electrohydrodynamic jet-tin
g to a muc
h wider range of surface chemistries and appli-cations. In addition to its simplicity, the method also opens the possibility to quickly fabricate low-dimensional hetero-structured materials on a large scale. Owing to the versa-tility of the electrohydrodynamic technique, our method will open the potential to construct heterostructured materials with alternating surface con? gurations using a wide range of combinable component materials, including polymers, inorganic materials, and hybrid materials with various func-tional groups such as optical, electronic, hydrophobic, and hydrophilic chemical components. Such 1D BSHFs are of great signi? cance in highly integrated functional devices, and are anticipated to bring about many important properties and
applications. [ 26,27 ]
6. Experimental Section
C ombined Electrospinning/Electrospraying Fabrication : PS ( M w = 350 000) was purchased from Aldrich. PEG ( M = 20 000) was purchased from Beijing Chemical Reagents Company. In a standard fabrication, the inner ? uid was a 35% (w/v) PS solution in DMF, and the outer ? uid was a 20% (w/v) PEG solution in a DMF/MC (v/v = 1:1) mixed solvent. For constructing the coaxial needles, the inner diameter of the outer needle was 1.4 mm, and the inner and outer diameter of the inner needle were 0.7 mm and 0.9 mm, respectively. The outer ? uid was loaded into the outer needle through a 5 mL syringe, and the inner ? uid was fed
at a ? ow rate of 4 mL h ? 1
. The distance between the needles and the aluminum foil was about 22 cm. The electric potential was controlled by a Spellman SL50P60 high-voltage generator (USA). The voltage was set at 19 kV.
L abeling of PEG with FITC : FITC was purchased from Beijing Chemical Reagents Company. The labeling was performed according
to the literature. [22]
At room temperature, the FITC reacted with a 5% (w/v) PEG solution in water under stirring for 72 h. The whole reaction was conducted in the dark. The product (PEO-FITC) was dialyzed for several days to remove any residual FITC, and the ? nal product was obtained by freeze-drying. The reaction between PEG and FITC is as follows:
(P E G)?OH +S =C =N ?(F )→(P E G)?O ?CS ?NH ?(F )
(1)
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