A ratiometric fluorescent probe for bioimaging and biosensing of HBrO in mitochondria upon oxidative stress

Article abstract of DOI:10.1039/C8CC07125J

A novel ratiometric fluorescent probe with high selectivity and sensitivity was designed and developed for bioimaging and biosensing of HBrO in mitochondria. Using this useful tool with low cytotoxicity and good biocompatibility, it was found that O₂^?-induced oxidative stress triggered the burst of HBrO in mitochondria.

Full text of DOI:10.1039/C8CC07125J

Nanoscale
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The wettability of gas bubbles: from macro
behavior to nano structures to applications
Cite this: Nanoscale, 2018, 10, 19659
a,b
Can Huanga,b and Zhiguang Guo
*
In recent years, various interfaces related to bubble wettability have been fabricated, which have already
been widely applied in various disciplines and fields. Therefore, to better research and understand the
wettability of gas bubbles, recent progress with interfaces and wettability of bubbles in aqueous media,
including superaerophilicity and superaerophobicity, is summarized. Many biological interfaces which
exhibit marvelous characteristics are discussed for reference. Because of the similar behavior between
gas bubbles in aqueous media and droplets in air, the two wetting conditions are compared together to
better illustrate theories of gas bubble wettability. Based on these theories, effective and available
manipulation of gas bubbles’ wettability provides a novel idea and method to solve practical problems in
various aspects, i.e., superaerophobic electrodes for gas evolution reactions, superaerophilic electrodes
for gas compensation reactions, superaerophilic interfaces for directional collection and transportation of
gas bubbles, and so on.
Received 8th September 2018,
Accepted 7th October 2018
DOI: 10.1039/c8nr07315e
rsc.li/nanoscale
1. Introduction
aHubei Collaborative Innovation Centre for Advanced Organic Chemical Materials
and Ministry of Education Key Laboratory for the Green Preparation and Application
of Functional Materials, Hubei University, Wuhan 430062, People’s Republic of
China. E-mail: zguo@licp.cas.cn; Fax: +86-931-8277088; Tel: +86-931-4968105
bState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China
After billions of years, natural creatures have formed reason-
ably optimized composite structures and excellent wetting
characteristics because of their continuous development and
evolution on the earth. Composite structures with a macro-
scopic and microscopic combination show a stable compre-
Mr Can Huang joined Professor
Guo’s Biomimetic Materials of
Tribology (BMT) group at Hubei
University in 2017 to pursue his
PhD degree. His current scientific
interests are focused on surface
Professor Zhiguang Guo received
his PhD from the Lanzhou
Institute of Chemical Physics
(LICP), Chinese Academy of
Sciences (CAS), in 2007. After
that, he joined Hubei University.
From October 2007 to August
2008, he conducted his post-
doctoral studies at the University
of Namur (FUNDP), Belgium.
From September 2008 to March
2011, he worked in the Funds for
wettability
and
researching
potential applications.
Mr Can Huang
Professor Zhiguang Guo
National
Research
Science
(FNRS), Belgium, as a “Charge
de Researcher”. From February 2009 to February 2010, he worked
in the Department of Physics, University of Oxford, UK, as a visit-
ing scholar. Currently, he is a full professor at the LICP and is
financed by the “Top Hundred Talents” program of the CAS. He
has published more than 200 papers about the interfaces of
materials.
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hensive performance and unusual environmental adaptability. pinning effect of air bubbles” in nature set an example for the
For example, the gliding of water striders without falling into problems to be solved. Furthermore, an abundance of crea-
the water,1 the sucking up of water by beetles in a dry desert,2 tures show the behaviour of using bubble wettability in nature.
the persistent flying of termites in the rain,3 the breathing of In 2009, Jiang et al. discovered air bubbles quickly bursting
water spiders under water4–6 and the “self-cleaning effect” of within several milliseconds on a “self-cleaning” lotus leaf.43
the lotus leaf surfaces,7,8 and the formation of these amazing The phenomenon of the steady pinning effect of air bubbles
functions is related to the micro/nano hierarchical structure of on a rose petal with a combination of hierarchical rough struc-
legs, tails, wings, abdomens and leaves. Almost all the excep- tures was described in 2011.25 Cerman et al. reported that
tional properties previously mentioned that are shown on the Salvinia has a unique property drying in the water in 2009.44
macro scale have microstructure support. In particularly, the Therefore, some research and explorations of wetting inter-
“self-cleaning effect” of the lotus leaf surfaces is also known as faces have been developed and the various bio-inspired wett-
lotus leaf effect or superhydrophobicity because of the nano- ability surfaces have been prepared using this special property
structures on top of the micro papillae.9 Ultimately, the con- of gas bubbles. As described previously, the numerous reports
cepts of wettability derived from it are widely accepted.10–21 In of theoretical research and preparation methods of bubbles’
generally, the wetting in air is behavior that a solid interface is wettability surfaces and their practical applications need a sys-
completely covered by another liquid.8,22 In brief, the process tematic and complete review. For interfaces in nature,
of spreading of a liquid on a substrate is called wetting on the especially, it is necessary and far-reaching to recognize and
macroscopic scale. On the nanoscopic scale, when a solid sub- analyse them attentively from macro behaviours to nano struc-
strate is wetted by a liquid, the liquid always spreads quickly tures and to imitate them.
on its surfaces. There is only has one contact point, solid/
The various interfaces in nature illustrate how the wettabil-
liquid, between them and some hydrophobic/superhydropho- ity of bubbles is used perfectly. The complementary roles of
bic surfaces have three contact points solid/liquid, solid/gas chemical component and interfacial structures on multiple
and liquid/gas. It is worth mentioning that air bubbles have scales are essential to this wettability. However, a few of these
been found by researchers to have similar wettability under natural examples are presented separately in this review to
water. Another property of these hydrophobic/superhydro- illustrate and compare in detail different types of bubble wett-
phobic surfaces is that they carry a gaseous plastron while ability. In this review, some natural examples about the wett-
being immersed in liquid. The gaseous plastron which is also ability of bubbles are summarized in detail to explain how
called an air cushion is a thin air film between the substrates organisms tactically use them to survive and adapt to the
and the liquid. During the formation of the air film, the suba- environment from the macro to the nano scales. At the same
queous surfaces seem to be wetted by air bubbles.23 In fact, time, the wettability of bubbles under water is very similar, to
bubbles spreading on a superhydrophobic surface on a macro- a large extent, to the wettability of water drops in the air.
scopic scale can be seen as the complementary problem of Therefore, the theories relating to them in this review are put
wetting drops, where the two fluid phases have been together for comparison and to better explain the wettability of
exchanged.24 The contact state between the substrates and bubbles. A numbers of fundamental and essential theories
liquid, which is the solid/liquid/gas three-phase contact state about the wettability of bubbles are explained. Furthermore,
or solid/liquid two-phase contact state, is crucial to the recent advances in the research on the wettability of bubbles,
behaviour of bubbles on substrates in liquid media on a nano- including how to design air bubble wettability by combining
scopic scale.25 Therefore, wettability/superwettability of gas microstructures and chemical compositions of artificial inter-
bubbles is proposed, including aerophilicity, superaero- faces and applications are discussed.
philicity, aerophobicity, and superaerophobicity.26 The wett-
ability of gas bubbles has attracted great attention because of
its many potential applications and extensive fundamental
research. Subsequently, more recently the behaviours of
2. The wettability of bubbles from
bubbles on substrates have taken on an important role in a macro behaviour to nano structures in
wide range of scientific research and industrial applications
natural creatures
especially in liquid media.27–33 For example, the amount of air
bubbles is a critical factor in electrochemical reduction. If too Abundant surfaces in nature, such as that of the lotus leaf, the
many bubbles are attached to the electrodes, which block the wings of a butterfly, the legs of a water strider, and the com-
contact between the electrodes and the electrolyte, resulting in pound eyes of mosquito, have attracted worldwide attention in
a remarkable reduction of the gas-evolution reaction (GER) recent decades in both academia and industry because of their
efficiency.22,34,35 However, too little bubble adhesion makes extraordinary characteristics. In particular, the nano structures
the gas-consumption reactions (GCRs) impossible. How to dis- and macro behaviour of some plants and animals perfectly
tribute the moderate bubbles evenly on the electrode interfaces explain the wettability/superwettability of air bubbles. These
is a critical step in determining the performances of various examples have helped investigators better understand the
electrochemical reduction reactions of gas bubbles.36–42 mechanisms and processes of gas bubbles. Inspired by this,
Fortunately, the “bubble bursting effect” and “the steady biomimetic material interfaces can be designed to meet
19660 | Nanoscale, 2018, 10, 19659–19672
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different needs. Therefore, it is essential that some examples induces the bubbles to merge into one, allowing the bubbles
are described in this review.
to be pinned on the petal surfaces. The macroscopic manifes-
tation of this phenomenon is the CA hysteresis of the bubble,
which makes it stick to the surface of the petals. It was found
that petals stick to bubbles when they follow the Cassie model.
Furthermore, the size of the micron scale protrusions and the
interval between them are the main reason for determining
the steady pinning effect of the air bubbles. Therefore, this
bionic mechanism is widely used in the manufacture of
various special surfaces. Thus, microstructures mainly affect
the superhydrophobicity of rose petals, whereas the nano-
structures are the key factor for the high adhesion to the rose
petals.
2.1 Rose petals
Rose petals have typical superhydrophobic surfaces which
have a high water contact angle (WCA) of >150° and a low
rolling angle of <10°, and this is well known by researchers.
However, unlike ordinary superhydrophobic surfaces, rose
petals also have a high adhesion to water droplets in the air,
which is attributed to a combination of hierarchical rough
structures (i.e., micro-papillae and nano-folds). This phenom-
enon is defined as the “petal effect” (Fig. 1a).49 In research,
rose petals have not only been found to have an intriguing
“petal effect”, but also exhibit unique characteristics when
immersed in an aqueous medium. Rose petals have an ability
2.2 Water spiders
to trap air bubbles and rapidly stabilize them on its surfaces. The water spider (Argyroneta aquatica), is a unique spider that
This phenomenon is described as “the steady pinning effect of lives virtually its entire life under water.4 Levi5 found that the
air bubbles”.25 Numerous studies have revealed that the rough way they build underwater dwelling places, which are called
micro/nano multi-layered composite structures of rose petals diving bells, is amazing. The tough silk the water spiders spit
are crucial to this interesting property. Micro-sized rough pro- out is not used to make a net, but is used to form a single, bell
tuberances and nano-papillae on the protuberance form an air shaped dwelling in the water. In order to make the bell more
cushion layer in the water environment. When bubbles solid and full, they use a special way to fill the diving bell full
approach the petal surfaces underwater, the air cushion of air: they capture air at the surface of the water using the
Fig. 1 The wettability of bubbles in natural creatures. (a) Based on hierarchical micro- and nanostructures, roses have a high adhesion and aeropho-
bicity property, which is called steady pinning effect of air bubbles. Reproduced from ref. 43 with permission from Thin Solid Films, Copyright 2011.
(b) The phenomenon of the earth’s physical lung on water spiders with its superaerophilicity hairs of abdomen and legs. Reproduced from ref. 44
and 45 with permission from SpringerPlus and Journal of Experimental Biology. Copyright 2013. (c) Capturing the air by forming a shiny silvery film
using multiple micro and nano superaerophilicity hairs of a water boatman. Reproduced from ref. 46 with permission from Journal of Morphology.
Copyright 2011. (d) The phenomenon of lotus effect (i.e., bubble-bursting performance) on Superaerophilic lotus leaves with micro-papillae and
hydrophobic waxy nano-peaks. Reproduced from ref. 47 with permission from ACS Applied Materials & Interfaces. Copyright 2017. (e) A stable plas-
tron is trapped on Salvinia under water because of its closed claw-like structures on the surfaces. Reproduced from ref. 48 with permission from
Advanced Materials. Copyright 2010. (f) The scales of carp show good superaerophobicity in water. Reproduced from ref. 47 with permission from
ACS Applied Materials & Interfaces. Copyright 2017.
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hydrophobic hair on their abdomen and legs as shown in bursting effect”. Similar to the wetting phenomena of water
Fig. 1b and then climb back under the water to inject the air droplets in an air environment, the bubbles quickly spread
in the body of the diving bell. They repeat this behaviour more and wetting its surfaces.23 In fact, the spreading of air bubbles
than once to make sure that they can survive safely and for a on lotus leaf surfaces can be seen as the wetting of droplets on
prolonged time underwater. This discovery was reported by its surfaces and the difference between them is that the gas
Messner and Adis in 1995.6 It is worth mentioning that the phase and the liquid phase are exchanged.24
hairs, which have many nanometre-sized vertical dendritic
2.5 Salvinia
structured layers, and the regular arrangement on the
abdomen of water spiders are superaerophilic, which is the key The floating water fern, Salvinia molesta has been so called
factor that makes them able to easily collect air.44 When a because of its pinnate leaves. When immersed in water, its sur-
water spider is immersed in water, this special micro/nano faces maintain a stable layer of air. Unlike other organisms
multi-layered structure forms a thin film of air on the surfaces that only maintain the air layers temporarily, S. molesta offers
of its abdomen, which looks like a wrapping around the a novel mechanism for long-term air retention. Researchers
surface, giving it the function of transporting and storing air. found that the hierarchical architecture of the leaf surface is
This behaviour perfectly utilizes and interprets the wettability dominated by complex elastic eggbeater shaped hairs coated
of bubbles under water.
with nanoscopic wax crystals as shown in Fig. 1e. These
special structures are the key factor for supporting the air–
water interfaces to keep the air layers stable. This magical
2.3 Water boatman
The water boatman (Notonecta glauca) has two pairs of wings, phenomenon is known as the “Salvinia effect”. The unique
anterior wings and posterior wings. The posterior wings which combination of hydrophilic patches on superhydrophobic sur-
are membranous hide under the anterior wings. Sometimes faces provides a promising concept for the development of a
the water boatman rises to the water surfaces to breathe, some- coating with long-term air retention properties.48 This special
times it sinks to the bottom of the water to avoid predators, structure of the natural interface has the characteristics of
and it can move very quickly. When the water boatman dives being superaerophilic, and is able to capture a certain amount
down to the bottom, there is a silvery shiny air film on the of air by utilizing the wettability of the bubbles in the water.
edge of its wings as shown in Fig. 1c. The existence of the air
2.6 Carp
film is because of the excellent superhydrophobic properties
of the surfaces of the posterior wings and the presence of Most fish have fish scales, and carp, as a species of fish, has a
numerous curving hairs. The formation of the superaerophili- very smooth and layered small scale, as shown in Fig. 1f. These
city properties is related to the micro/nano composite struc- symmetrical and ordered scales not only enable the carp to
tures of the wings’ surfaces. These multilayer composite struc- move rapidly under water, but also to have excellent antifoul-
tures enable the boatman to capture large volumes of air in its ing properties because of the superoleophobic interface with
wings and numerous curving micro-sized hairs. It is able to the water.52 In reality, the surfaces of the fish scales are super-
exhibit these properties under water because superhydropho- hydrophilic, which is attributed to its micro/nanoscale hier-
bic surfaces become superaerophilic after being immersed in archical rough structures, as shown in Fig. 1f′. It is very clear
water. However, unfortunately the silvery shine of the air film from the image that the protuberances, which are like small
is not stable for very long.45,46,50
peaks are spaced apart from each other. When wet with water,
the peak shaped protuberances increase the CA between the
water and scales, which gives superhydrophilic properties.
2.4 Lotus leaf
The self-cleaning effects of plants have intrigued researchers Furthermore, these special hierarchical structures make it
over the past few decades. The lotus leaf is an outstanding appear to be superaerophobic in aqueous media.47 This dis-
example of this type of characteristic and has good develop- covery further developed the applications of fish scales. The
ment potential and prospects. In 1997, Barthlott7 and biological phenomenon which was mentioned previously is
Neinhuis51 proposed the “lotus effect” and in their continued highly significant and can be used to provide guidance to
research concluded that its self-cleaning properties were the researchers and engineers to help them obtain excellent
result of its microstructures and waxy materials as shown in control of bubble behaviour and gas bubble wettability on a
Fig. 1d. Subsequently, the combination of micro/nano hier- solid surface in a water medium.
archical structures on the lotus leaf surfaces and nano-
structures on top of micro papillae were discovered by Feng
et al. and they finally discovered that the hierarchical micro/ 3. Fundamental understanding of gas
nanostructures considerably increased the CA and decreased
the sliding angle, which are the crucial factors in giving
bubbles wettability in aqueous media
the lotus leaf, low adhesion, superhydrophobicity, and self- When a solid surface is immersed in a liquid, the behaviour of
cleaning properties.8 Nevertheless, the phenomenon of an air gas bubbles in aqueous media is largely similar to the behav-
bubble bursting on a lotus leaf surface was first observed in iour of droplets on solid surfaces in air. The in air superhydro-
2009 by Jiang et al.,43 which was described as the “bubble philic surface generally shows superaerophobicity in aqueous
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Fig. 2 Schematic diagrams showing the relationship between wettability of solids in air and water. (a),( b) and (c) show the comparison between
wettability of drops in air and wettability of bubbles in water. (d) shows the similarity of CAH in different environments.
media and the in air superhydrophobic surface generally when the wetting process reaches equilibrium on an ideal
shows submerged superaerophilicity, as shown in Fig. 2a and solid surface, the interface tension forces of each phase satisfy
c.47 Thus, to better illustrate the wettability of gas bubbles, the Young’s equation as follows:
two wetting conditions are compared together in the following
section.
γSV ꢀ γSL
cos θw
¼
ð1Þ
γLV
where γSV, γSL, and γLV represent the surface tension forces
between solid-vapor, solid–liquid, and liquid–vapor, respect-
ively. θw is the static WCA, as shown in Fig. 2b. Because of its
generally similar nature, the concept of a bubble contact angle
(BCA; θb) is also proposed:54
3.1 Contact angle and Young’s equation in water
The wetting of solid surfaces by droplets is essentially a
process of a gas–solid phase being replaced by a liquid–solid
phase in air. The water molecules are spread out and as close
as possible to the solid substrate. Some of the gas molecules
are squeezed out by water molecules and the rest of them
remain on the surface of the solid, keeping the three phase
(solid/liquid/gas) balance. Therefore, the behaviour of bubbles
is largely determined by the molecular forces between the
three phases. Similarly, when a solid surface is immersed in
an aqueous environment, the water molecules are in close
proximity to the solid substrate, and the gas bubble molecules
attempting to adhere to the substrate have to compete with the
liquid film over the surface.53 The competitive relationship
between the gas phase and the liquid phase is the key factor
determining the behaviour of underwater bubbles. Thus,
wetting of an air bubble on an ideal solid surface immersed in
water depends on the CA of a sessile droplet (θw) on the same
θb ¼ 180° ꢀ θw
ð2Þ
Subsequently, Young’s equation can be rewritten in terms
of θb as:
γSV ꢀ γSL
cosð180° ꢀ θbÞ ¼
ð3Þ
ð4Þ
γLV
γSL ꢀ γSV
cos θb ¼
γLV
Another way to derive this is based on the conservation of
energy as follows:55
δw ¼ γdALV þ γSLdASV þ ΔPdV
ð5Þ
substrate in air.53 In order to measure the wettability of dro- where γLV, γSL, and γSV represent liquid–vapor, solid–liquid,
plets, the concept of a CA is proposed. The CA of liquid dro- and solid–vapor surface tensions, respectively. The dALV, dASL
,
plets on solid surfaces is created by the surface tension of the dASV, are liquid–vapor, solid–liquid, and solid–vapor area
interface between solid, liquid and gas. In an air environment, changes, respectively, and ΔP is the Laplace pressure change
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ΔP and dV is the volume change dV. If the conditions of δw = 0, on the solid surface in aqueous media, the bubble contact
ΔV = 0, dASL = dASV, and dALV/dASL = cos θ for a semi-spherical angle hysteresis (CAHb).
shape are enforced with surface tension dominating over buoy-
A superhydrophobic surface has a low WCA (CAH, less than
ancy, eqn (5) simplifies to the easily recognizable Young’s 5°) and also a high CA (more than 150°). Similarly, a superaer-
equation of:
ophobic interface defined as surfaces with a BCA of θb > 150°
and negligible hysteresis. However, surfaces with a BCA of θb ≈
0°and appreciable hysteresis are defined as superaerophilic
interfaces.
γLV cos θ ¼ γSV ꢀ γSL
ð6Þ
where, θ is the complement of θb. Eqn (6) can be written as:
γLV cosð180° ꢀ θbÞ ¼ γSV ꢀ γSL
ð7Þ
ð8Þ
3.3 Two states in water
γLV cos θb ¼ γSL ꢀ γSV
Analogous to the wetting behavior of droplets on solid sur-
faces, there are two possible different states when a super-
hydrophobic or superhydrophilic surface is immersed in
water: (1) like the Cassie–Baxter model in air, the rough
solid surfaces are completely filled and covered by liquid so
that the bubbles are fully lifted, showing a high BCA (θb)
and negligible hysteresis as shown in Fig. 2c. (2) Because of
the superhydrophobicity of solid surfaces, the gas bubbles
are trapped and left behind to form a layer of air cushion,
exhibiting the affinity of the gas and appreciable hysteresis
as shown in Fig. 2a.56 Importantly, it does not matter what
type of state the bubbles are in underwater, they always
follow the lowest energy principle and maintain the most
stable state.
eqn (8) further explains that gas bubble wettability in aqueous
media can be approximately calculated as the complementary
process of liquid wettability in air.
According to eqn (2) and (4), the value of the underwater
BCA can be roughly calculated using the CA of droplets under
the same conditions in air. Furthermore, the behaviour and
existence of bubbles can be satisfactorily explained and pre-
dicted in different states. As mentioned previously, a super-
hydrophilic surface is superaerophobic in water. This is
because of the complementary relationship between the BCA
(θb) and the WCA (θw). When the WCA decreases, the bubble
contact angle increases continuously, reaching superaeropho-
bicity as shown in Fig. 2c.
It is worth noting that the relationship between the BCA
(θb) and the WCA (θw) does not always satisfy the complemen-
tary angle. The most typical example of this is the phenom-
enon of the steady pinning effect of an air bubble on a rose
petal. Rose petals are superhydrophobic surfaces, and the
WCA is 152.4°, whereas its BCA reaches an amazing 126.9°.25
Rose petals have the ability to capture bubbles and fix them on
the surface because of their special hierarchical rough struc-
tures. These special structures endow the surfaces with a high
contact angle hysteresis (CAH), which is the main cause of this
phenomenon. CAH is described in detail in the following
sections.
3.4 Contact force of bubbles
In an aqueous environment, the behavior of bubbles is mainly
affected by adhesive force (Fadhesion). The value of this critical
force is determined using the volume of bubbles and CAHb.
It can be calculated with the following formula:57
Fadhesion ¼ kdγLVðcos θR ꢀ cos θAÞ
ð9Þ
where γLV, k, d are the retentive force factor, surface tension of
the liquid and diminished contact width, respectively.
Undoubtedly, eqn (9) illustrates the fact that the wettability of
bubbles is closely related to the contact width (d), which in
turn affects the adhesive force. Furthermore, buoyancy as the
main force affecting the contact width also plays a dominant
role in surface tension.
3.2 Bubble contact angle hysteresis (CAHb)
The static CA of droplets on a smooth, solid, substrate surface
The wetting of bubbles is essentially the process of repla-
have been shown to be inextricably linked to the CA of bubbles cing liquid molecules on the surface of a solid substrate with
under water. When the droplet just rolls on the smooth the molecules of a vapor. In this process, the vapor molecules
surface because of gravity, a new concept CAH occurs. If the are often covered by a thin layer of liquid film. Therefore, to
CA is measured while the volume of the drop is increasing, better understand the wettability of bubbles, it is necessary to
practically this is done just before the wetting line starts to study the liquid film coated on the surface of bubbles.
advance, and it is called the advancing contact angle (ACA) θA. Fortunately, the bubble probe technique which is combination
If afterwards the volume of the drop decreases and the CA is of surface force apparatus or atomic force microscopy (AFM)
determined just before the wetting line is receding, the angle and reflection interference contrast microscopy (RICM) is
of this measurement is called the receding contact θR. The ACA found to measure the interaction contact force and spatial-
is always larger than or equal to the RCA (θA < θR). The differ- temporal visualization of the thin-film drainage process.56,58–62
ence between the cosines of the two is called CAH as shown in
Reynolds lubrication theory and the augmented Young–
Fig. 2d. For the wetting of the bubbles in water, the behavior Laplace equation are used to determine the interaction force
of the bubbles on an ideal inclined solid surface is largely and the spatiotemporal evolution of the confined thin water
determined by buoyancy, because the bubbles are surrounded film.59,63–66 In a liquid medium, the mechanism of the drai-
by water. Accordingly, bubbles also have the same properties nage dynamics of a thin liquid film in narrow gaps between
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solids and bubbles is revealed using the Reynolds lubrication Subsequently, integrating P and Π can be used to derive the
theory:
overall interaction force F(t) of the air bubble on the solid
surface:
ꢀ
ꢁ
@h
1
@
@P
@r
¼
rh3
ð10Þ
ð1
@t 12μ @r
FðtÞ ¼ 2π
½Pðr; tÞ þ Πðhðr; tÞÞꢁrdr
ð12Þ
0
where μ is the viscosity of the liquid and r is the radial distance
from the center of the film, h is the film thickness and t is
time. However, when the liquid thin-film reaches a certain
thickness, the influence comes mainly from hydrodynamic
pressure, Laplace pressure and the disjoining force. Thus, the
augmented Young-Laplace equation illustrates the relationship
between them:
Zeng and co-workers have made good progress in this
respect.67–69 They measured forces and spatiotemporal evolution
of thin water films between an air bubble and mica surfaces of
different hydrophobicity in a comprehensive and multidirec-
tional way. The interaction forces were measured using AFM by
driving a cantilever anchored air bubble towards the mica surface
in 500 mM of sodium chloride (NaCl) solution at a velocity of
1 μm s−1. The fringe patterns that arose from interference
between light reflected from the air/water interface of the bubble
ꢀ
ꢁ
γ @
@h
@r
2γ
R0
r
¼
ꢀ P ꢀ Π
ð11Þ
2r @r
where, R0 is the radius of the bubble and γ is the surface and the mica/water surface were obtained using RICM and ana-
tension of the liquid, P is the excess hydrodynamic pressure in lysed using eqn (10) and (11) as shown in Fig. 3a.61
the liquid film (between the air bubble and the solid) relative
For hydrophilic mica surfaces, it is worth knowing that a
to the bulk liquid and Π is the disjoining pressure arising high concentration of NaCl made the contribution (ΠEDL(h(r,
from van der Waals (vdW) interactions, electrical double layer t))) of the EDL interaction between the bubble and the mica
(EDL)
interactions,
and
hydrophobic
interaction. surfaces to the whole disjoining pressure (Π(h(r,t))) insignifi-
Fig. 3 Measuring the interaction forces between an air bubble and surfaces of different hydrophobicity by using AFM in combination with RICM. (a)
and (c) The device for studying bubble wettability which integrate AFM and RICM. (b) The interaction force of bubble on the mica surfaces changing
with time, on which the “jump in” phenomenon can be clearly seen. (d) The interaction force of bubble on the soot-templated superhydrophobic
surfaces changing with piezo-displacement. (a) and (b) reproduced from ref. 61 with permission from ACS Nano. Copyright 2015. (b) and (c)
Reproduced from ref. 56 with permission from Langmuir. Copyright 2015.
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cant. Thus, the main disjoining pressure is derived entirely 4.1 Superaerophilic electrodes for GCRs
from vdW interactions. Furthermore, based on the Lifshitz
The GCRs including the oxygen reduction reaction (ORR) is a
crucial step for determining the performances of various
power storage and release equipment in electrochemical reac-
tions involving gases. One of the main limiting factors is the
inadequate reactive gases.71 It is very difficult, and uneconomi-
cal, to enhance their performance by improving the catalyst. In
fact, the solubility of gas is extremely low and its diffusion rate
is very low in an aqueous electrolyte. The shortage of gas in an
ORR on an electrode often limits its further reaction rate. Not
getting an oxygen (O2) supplement in time makes the next
reaction impossible. Thus, to remove this dilemma a desirable
architecture was designed to facilitate electron transport from
the current collector to the catalyst layer and provide an unob-
structed gas diffusion pathway to continually supply enough
gas reactant to the reactive catalytic sites. The novel structures
which use the new three-phase contact point (TPCP) instead of
the original two-phase contact allow gas to participate more
easily in the reaction. Because of its superaerophilic pro-
perties, a stable gas layer is formed on the electrode surfaces
instead of being wetted so that a lot of TPCPs greatly enhanced
the gas diffusion rate and it also guarantees the electron trans-
mission efficiency.
Sun and co-workers achieved this concept by fabricating
porous cobalt incorporated nitrogen-doped carbon nanotube
(CoNCNT) arrays on carbon-fibre paper (CFP) as shown in
Fig. 4a.40 They used direct natural growth to ensure the com-
pactness and high conductivity of the nanoarrays, and modi-
fied the highly roughened surfaces using poly(tetrafluoroethyl-
ene) (PTFE) to give them superaerophilic properties with a
stable oxygen gas layer under aqueous media. Because of the
unique architecture, the TPCP, the electron transmission
efficiency and enough gas reactant was kept at a relatively
balanced level, it gave an excellent ORR performance in both
base and acid media.
theory of vdW force, the vdW interaction of bubbles and mica
surfaces is repulsive which maintained a thin water film
between the air bubble and the hydrophilic mica surfaces.61
For hydrophobic mica surfaces, a “jump-in” phenomenon
was observed when the bubble was close to the hydrophobic
surfaces as shown in Fig. 3b. According to the classical wetting
model, the sudden decrease of the force caused the thin water
film under the bubble to be completely removed and the
bubble contacted the gas layers on the hydrophobic surfaces.61
When the negative disjoining pressure just exceeds the Laplace
pressure of the bubble, the bubble attaches to the surface, as
shown in eqn (11).
Shi et al. continued the research and studied the interaction
between air bubbles and superhydrophobic surfaces in almost
the same way as shown in Fig. 3e.56 They found that once the
EDL force and a hydrodynamic repulsion were overcome,
bubbles jumped onto the surface and fully merged with the
entrapped air. Furthermore, the same “jump in” behavior was
observed on the soot templated surfaces, which indicated that
the superhydrophobic surface is superaerophilic under water as
shown in Fig. 3f. At the same time, Cao et al. also studied and
discussed this problem.70 They argue that the two wetting states
could not be completely equivalent, and they also designed
experiments to use superhydrophobic surface defects to achieve
controlled directional bubble transport in aqueous media.
These theories and experiments opened up a new field in the
research and application of underwater bubble wettability.
To better illustrate the wettability of gas bubbles, the differ-
ence and similarity between the wettability of underwater
bubbles and the wettability of water drops in the air are sum-
marized in Table 1.
Similarly, Feng and co-workers immobilized glucose
oxidase (GOx) on platinum (Pt) catalyst modified conducting
superaerophilic carbon fiber mesh, which is used as an
electrochemical STP-biosensor as shown in Fig. 4b.72 A stable
and adequate layer of oxygen was formed on the surfaces to
solve the problem of insufficient oxygen supply when the
superaerophilic multi-layer micro/nano structure electrode was
immersed in the analyte solution. This ingenious design
replaced two-phase contact with TPCP, in which oxygen
4. Functions and applications
With further study of bubble wettability, various theories and
fabrication methods have been improved. At the same time,
superaerophobicity and superaerophobicity surfaces have been
shown to have special applications in many other fields. For
example, superaerophilic electrodes for GCRs, superaeropho-
bic electrodes for GERs, gas-bubble transport on interfaces
with superwettability of bubbles and so on.
Table 1 A comparison between the wettability of underwater bubbles and the wettability of the water drops in the air
Contact angle
Young’s equation
γSL ꢀ γSV
Contact angle hysteresis
Wetting states
The wettability of gas
θb = 180° − θw
cos θb
¼
CAHb = cos Rb − cos Ab
Superaerophobic
Aerophobic
γLV
bubbles in aqueous media
Aerophilic
Superaerophilic
Superhydrophobic
Hydrophobic
Hydrophilic
γSV ꢀ γSL
The wettability of the
drops in The air
θw
cos θw
¼
CAHw = cos Rw − cos Aw
γLV
Superhydrophilic
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Fig. 4 Superaerophilic and superaerophobic electrodes used in GARs and GERs. (a) The superaerophilic electrode is covered by directly growing
CoNCNTs on CFP associated with subsequent PTFE-modification. The intimate connection of the catalyst to the substrate offers an accelerated
electron-transport process, and the highly porous structure provides abundant nanoscale TPCP for ORR. Both electron-transport and oxygen-
diffusion processes are accelerated, leading to a superior ORR performance. Reproduced from ref. 40 with permission from Advanced Materials.
Copyright 2016. (b) It is fabricated by superaerophilic electrode, which is constructed using Pt-catalyst-modified carbon-fibre mesh. Reproduced
from ref. 72 with permission from Advanced Materials. Copyright 2016. (c) Superaerophobic electrode interface constructed by vertically aligned
MoS2 nanoplatelets (typically 200 nm in size and less than 5 nm thick). Schematic of nanostructured MoS2 electrode. The active Mo edge for HER is
marked in the magnified image of the crystal structure. The rapid removal of small gas bubbles can provide a constant working area for the HER
reaction. Blue sphere, Mo; yellow sphere, S. Reproduced from ref. 27 with permission from Advanced Materials. Copyright 2014.
diffused directly from the air phase to the oxidase active sites. application of superaerophilic surfaces in electrodes.37
The successful manufacture of the STP-biosensor indicates summary of these electrodes is given in Table 2.
that the superhydrophilic surfaces provide a unique way to
A
4.2 Superaerophobic electrodes for GERs
solve the gas shortage problem in many reaction systems. In
addition, Liu et al. highly boosted ORR activity by tuning the The GERs generally refers to the process of producing a gas
underwater wetting state of the superaerophilic electrode, phase on electrodes in a liquid phase reactant in an electro-
which achieved satisfactory results and further developed the chemical reaction. For example, water splitting generates a
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Table 2 The application of electrodes for different electrochemical reactions and examples
Electrodes
wettability
Reaction
types
Application
Examples
Superaerophilic
electrodes
GCRs
Forming gas layer to build more TPCPs to
provide a stable and sufficient gas volume to
participate in the reaction.
Superaerophilic carbon nanotube array electrode,40 oxygen-
rich enzyme biosensor,72 superaerophilic pillar structure
substrate electrode,37 a novel method for producing gas
diffusion layers with patterned wettability,73 a vapor phase–
polymerized PEDOT Electrode74
Superaerophobic
electrodes
HER
Preventing gas adhesion from stacking
electrode surfaces
MoS2 thin film nanosheet arrays,27 pine-shaped Pt nanoar-
ray electrode,75 Pt nanosheet boron-doped diamond elec-
trode,76 Ni2P nanoarray electrode,77 SiO2–polypyrrole
hybrid nanotubes supported on carbon fibers78
Cu3P microsheets,79 NiFe-LDH nanoplates,80 plasma-
engraved Co3O4 Nanosheets,81 Ni3S2 nanosheet arrays,82
ZnxCo3−xO4 nanoarrays83
OER
Reduce the size of the bubble
HzOR
CIER
Increase the escape velocity of the bubble
3D nanostructured Cu nanoarrays,84 superaerophobic gra-
phene nano-hills,85 single-crystalline ultrathin nickel
nanosheet array86
RuO2@TiO2 nanosheet array36
hydrogen (H2) and O2 gas phase on the electrodes in the liquid used for the absorption and transport of gases. Inspired by
phase. In GERs, a three-phase interface consisting of gas this, Su and co-workers constructed a novel underwater super-
phase, liquid phase and solid phase is formed on the electrode aerophilic sponge for selective collection, stable storage and
surfaces, and the gas phase usually separates and escapes continuous transport of gas.87 The sponge modified with a
directly. However, in some industrial reactions a large number porous structure and superaerophilic poly(urethane) (PU) has
of bubbles often accumulate on the electrode surfaces because a certain selectivity to gas under water as shown in Fig. 5a. The
of the excessive reaction rate, which blocks the generation of bubbles quickly burst and spread on its surfaces, and are col-
the reaction and reduces the reaction rate. Thus, superaero- lected continuously by direct transport. On the contrary, water
phobic electrodes are widely used in GERs including the is blocked by superaerophilic sponge.
hydrogen evolution reaction (HER), the oxygen evolution reac-
Subsequently, they improved the superaerophilic sponge to
tion (OER), the hydrazine oxidation reaction (HzOR), the and successfully absorb methane gas, which had great significance
chlorine evolution reaction (ClER), to solve this problem. Lu for reducing greenhouse gases and reducing the greenhouse
and co-workers fabricated a molybdenum disulfide (MoS2) effect as shown in Fig. 5a.
nanostructured film, which contained vertically aligned MoS2
In nature, many organisms have the ability to capture and
nanoplatelets (typically 200 nm in size and less than 5 nm in transport water directly from the environment, using special-
thickness), as superaerophobic electrodes used for HER as ized materials such as spider silk and cactus spines.90,91
shown in Fig. 4c.28 Because of the low adhesion of the Researchers discovered that this ability is because of the
bubbles, superaerophobic electrodes prevent gas bubbles from unique surfaces of the multilevel micro/nano structures, which
accumulating on the electrode surfaces to form a gas layer that yield to the gradient of the Laplace pressure and surface free
isolates the reactants. Thus, the bubbles detach from the elec- energy to serve as the cooperative driving forces to transport
trode surfaces while they are a small size and the two-phase water droplets directionally.92–94 Based on this behaviour,
contact between the liquid phase and the solid phase is con- Wang and co-workers manufactured a superaerophilic copper
stantly sustained, which achieves an efficient HER. wire with a type of 3D gradient porous interconnected
Furthermore, in view of the various GERs, more superaeropho- network.95 Because of the combination of the prepared unique
bic electrodes with different materials, different preparation surfaces and a simple to control electrochemical method, the
methods and different structures were fabricated. The appli- copper wires can maintain a perfectly constant gas layer in an
cation of electrodes for different electrochemical reactions and aqueous medium. At the same time, gas bubbles can be con-
examples are given in Table 2.
tinuously and directionally transported through the film on
the surfaces by the difference of the Laplace pressure.
Ultimately, Li and co-workers fabricated a helical superaero-
philic copper wire and found that the bubble tends to stay on
4.3 Air bubble transport on interfaces with superwettability
of bubbles
The wettability of bubbles is not only widely used in electro- the summit of the helix structure and moves along with the
chemical reactions, but also plays an important role in the helix rotation in an aqueous environment.89 The buoyancy force
transport of gases. According to the “bubble burst effect” on of the bubbles in water and the adhesion force of the bubbles
the lotus leaf, when the superaerophilic interface is immersed to the copper wire are simultaneous. The direction of the
in a water environment, the bubbles burst and spread rapidly adhesion force is always perpendicular to the surface of the
on its surface to form a gas layer. This special property can be copper wires, but the direction of the buoyancy force is always
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Fig. 5 (a) The sponge modified by PU; continuous collection and transport bubbles in water. Reproduced from ref. 87 with permission from
Langmuir. Copyright 2010. (b) The microstructure of the superhydrophilic surface and the transport of bubbles on its surface. Reproduced from ref.
88 with permission from Angewandte Chemie. Copyright 2017. (c) and (d) The structure and transport of bubbles schematic diagram of a helical
superaerophilic copper wire. Reproduced from ref. 89 with permission from Journal of Materials Chemistry A. Copyright 2016.
vertical as shown in Fig. 5d. When the angle of the copper wire
changes, the adhesive force of the bubble can be offset by the
component of the buoyancy force, while the remaining com-
ponent of the buoyancy force drives the bubble to the top of
the helix, and at this point the buoyancy force and the
adhesion force are balanced. Therefore, the copper wire is con-
tinuously rotated in one direction, while the bubble will always
move afterwards as shown in Fig. 5c. In this way, the direction
and rate of the bubbles’ transport can be precisely controlled
by adjusting the length, direction and rotation speed of the
copper wire. Based on the multiple control of the bubble,
more and more superaerophilic surfaces are produced.
Fig. 6 On the interfaces with gradient curvature, the bubbles exhibit
different behavior on the interface with different bubble wettabilities. (a)
and (b) The bubbles encounter a superaerophobic or aerophobic cone,
and the bubble bounces away. (c) An aerophilic cone can easily capture
bubbles, but can only transport bubbles for a short distance. (d) A super-
aerophilic cone can easily capture bubbles and transport bubbles for a
long distance. Reproduced from ref. 100 with permission from
Advanced Functional Materials. Copyright 2016.
However, the most reliable and effective way to manipulate
bubbles occurs on the interfaces with simple shapes, i.e.,
cylindrical and cone shapes in the water medium. Wang and
co-workers carried out further discussion and research on this
question with a type of three-dimensional (3D) gradient
porous network which was constructed on copper wire inter-
faces, with rectangle, wave, and helix shapes as shown in
Fig. 5b.88 It was eventually found that the average velocity of
the bubble was related to the shape of the copper wire, and
the small gas bubble spontaneously and directionally moves to side of the bubble by introducing bubbles into the superaero-
the larger bubbles. Based on these research theories, more philic interface with gradient curvature. Only in the case of a
and more wettabilities of bubble surfaces have been designed Laplacian pressure gradient can the bubbles be continuously
to control bubbles in aqueous environments.96–99
and steadily transported directly to the root of the cones
Yu and co-workers illustrated that on the interfaces with a regardless of the direction as shown in Fig. 6d. In addition,
gradient curvature, the bubbles exhibit different behaviours this cone-shaped superaerophilic surface can also continu-
on the interface with different bubble wettabilities, which ously collect and directionally transport carbon dioxide (CO2)
include aerophilicity, superaerophilicity, aerophobicity and microbubbles in CO2 supersaturated solutions101 or H2 micro-
superaerophobicity.100 When a bubble encounters a superaero- bubbles for HER,102 and control the size of the bubbles.105
phobic or aerophobic interface, the bubble bounces away
In the superwettability of surfaces, there are two different
because of small adhesive forces, as shown in Fig. 6a and b. wettability interfaces called Janus. In the same way, the
This phenomenon indicates that the superaerophobic or aero- manipulation of bubbles could be achieved by using bubbles
phobic cones cannot capture or transport bubbles. Conversely, with different wettabilities on the Janus interfaces. A super-
an aerophilic cone can easily capture bubbles, but can only hydrophobic/hydrophilic cooperative Janus mesh has very inter-
transport bubbles for a short distance as shown in Fig. 6c. In esting properties. In an aqueous environment, when the super-
fact, this transient transport process is achieved through con- aerophilic side is placed on top, bubbles under the membrane
tinuous aggregation of bubbles, but as buoyancy continues to form a larger bubble through its mesh.103,104 When the super-
increase, the bubbles will eventually escape into the water. The aerophobicity side is facing upwards, bubbles are blocked by
Laplacian pressure gradient can only be produced on the other the membrane and form a large air bubble. This kind of intel-
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ligent Janus membrane with one-way air bubble transport can
• By simulating the macroscopic behavior of some organ-
realize intelligent control of air bubbles just like the “diode” of isms, useful artificial interfaces which have micro/nanohier-
an underwater air bubble, which has a potential application in archical structures and chemical compositions can be manu-
the development and design of functional devices.
factured to manipulate the gas-bubbles superwettability. The
Yang and co-workers reported a superaerophobic-hydro- effective and available manipulation of gas bubble superwett-
phobic Janus membrane with asymmetric wettability for fine ability is a novel idea and a method to solve various practical
bubble aeration.103 The hydrophobic side prevents water from problems in aspects, e.g., superaerophobic electrodes for
penetrating through the membrane holes and draws in the gas GERs, superaerophilic electrodes for GCRs, superaerophilic
to reduce the pressure, while the superaerophobic side, which interfaces for directional collection and transportation of gas
is achieved by adjusting the surface wettability and hierarchi- bubbles, and so on. However, problems and challenges still
cal structures, makes the bubbles detach rapidly and decreases exist. In practical applications, the complex environment
the volume of the bubbles. Therefore, the goals of fine bubble including different physical and chemical conditions, the flow
aeration are realized.
rate of the solution, changes in buoyancy and adhesion, pH
value and immersion depth, and so on, will have an unpredict-
able impact on surfaces of bubble wettability. Thus, more
5. Conclusions and prospects for the attempts should be expended to prepare the interfaces in
different media, in different ways and using different
structures.
future
In this review, the recent progress in wettability of bubbles in
In summary, the field of bubble wettability research is still
aqueous media has been briefly summarized, including in the early stages. The ultimate goal of artificial superaero-
natural species with superwettability of bubbles, theoretical phobic/superaerophilic surfaces is the perfect combination of
background, functions and applications, using an appropriate preparation methods, structural and functional characteristics
classification. Subsequently, issues and challenges were dis- and practical applications. Therefore, enormous efforts are
cussed objectively and some prospects for the future develop- required to achieve this goal.
ment of the bubble superwettability interfaces are presented
next:
• Nature is a treasure trove for scientific research, in which Conflicts of interest
many biological surfaces exhibit marvellous characteristics.
There are no conflicts to declare.
Numerous natural interface structures remain to be explored
and developed as more is learned from nature.
• At present, the theories of bubble wettability remain to
complement and improve. For example, a more scientific and
Acknowledgements
concise definition of bubble superwettability (superaerophobic
This work is supported by the National Nature Science
and superaerophilic) needs to be considered. Whether the
Foundation of China (No 51522510, 51675513 and 51735013).
superaerophilicity in water can be equated with the superhy-
drophobicity in air, a complete and systematic theory for the
relationship between the wettability of bubbles underwater
and the wettability of droplets in the air remains to be deter-
Notes and references
mined. Although AFM in combination with RICM can be used
to measure the interaction forces between an air bubble and
surfaces of different hydrophobicity, the operation is compli-
cated and there are many other limiting factors for it to be
used in practical applications. Thus, a new method for
measuring bubble wettability which is simpler and more com-
prehensive needs to be developed.
• Even though various bionic and artificial interfaces for
bubble superwettability have been created, this field is still in
its infancy. As is known, the superaerophobic and superaero-
philic surfaces need to be immersed in the water environ-
ment for them to show their special properties. However,
only in Cassie–Baxter models, can the interface have the
characteristics of superaerophilicity. Thus, the longevity of
artificial interfaces is a serious problem. A long-life and
durable interfacial material urgently needs to be designed
and fabricated, which can maintain wettability under water
for a long time.
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