Control the Entire Journey of Pesticide Application on Superhydrophobic Plant Surface by Dynamic Covalent Trimeric Surfactant Coacervation

Article abstract of DOI:10.1002/adfm.202006606

Vast wastage of pesticides has caused significant environmental pollution and economic loss, which occurs in any step during the entire process of pesticide application. However, the existing strategies for controlling pesticide losses are step specific.

Full text of DOI:10.1002/adfm.202006606

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Control the Entire Journey of Pesticide Application on
Superhydrophobic Plant Surface by Dynamic Covalent
Trimeric Surfactant Coacervation
Bin Liu, Yaxun Fan,* Haofei Li, Weiwei Zhao, Siqi Luo, Hua Wang, Bo Guan,
Qiaolian Li, Jiling Yue, Zhichao Dong,* Yilin Wang,* and Lei Jiang*
of pesticides by biological targets is less
than 1.0%.[6–8] To achieve insecticidal effect,
pesticides have to be seriously overused
and have reached 4.1 million tons per year
worldwide. The lost pesticides and large
amounts of organic solvents in pesticide
formulas have caused a series of ecological
and environmental problems and have even
harmed human health.[7,9–11] Therefore, sus-
tainable agriculture demands the elimina-
tion of organic solvents and improvement
of pesticide efficiency. Desired pesticide for-
mulas are mainly aimed at taking water as
solvent and require that pesticides must be
1) well encapsulated, 2) completely depos-
ited after spray, 3) firmly remain under
wind and rain erosion, and 4) precisely
released so as to reduce use amount and
application frequency.[4–6,12–14]
Vast wastage of pesticides has caused significant environmental pollution and
economic loss, which occurs in any step during the entire process of pesticide
application. However, the existing strategies for controlling pesticide losses
are step specific. Here, a comprehensive strategy to substantively improve
pesticide efficiency on the basis of precise designs from beginning to end is
developed. A water-based coacervate with synthesized imine-based dynamic
covalent trimeric surfactants to synergistically control encapsulation, depo-
sition, retention, and release of pesticides on water-repellent plants is con-
structed. The coacervate consists of nanosized networks and abundant tightly
bonded water, leading to effective encapsulation of hydrophilic/hydrophobic
pesticides. Meanwhile, the network-like microstructure entangles with the
micro/nanostructures of superhydrophobic surface, ensuring complete deposi-
tion on superhydrophobic plant surface after high-speed impact and inhibition
of wind/rainwater erosion. Moreover, the CO2-induced degradative surfactant
coacervate determines the precise pesticide release. The dynamic coacervate as
an innovative pesticide formula provides a prospective way for pesticide appli-
cation, and is expected to promote productive and sustainable agriculture.
During water-based pesticide applica-
tions, the spray process is responsible
for 50% of pesticide loss.[4,8] The very
short contact time of spray droplets on
a water-repellent leaf surface results in unavoidable bouncing
and splashing.[15–17] The contact time is defined as the time
the sprayed droplets take from contacting the solid surface to
bouncing off the surface, generally including a spread stage and
extraction stage. The characteristic time scale of water droplets
is low to several milliseconds on superhydrophobic surface. To
enhance pesticide utilization efficiency, mainstream researches
have therefore established sub-micrometer-scaled assem-
blies and capsules to maximize the specific area for inhibiting
rebound,[18–20] enhancing retention,[21,22] and controlling pesticide
1. Introduction
Pesticides are indispensable for modern agriculture, save 30–40%
crops, and will continue to increase the overall agricultural pro-
duction to meet 9.7 billion people in 2050.[1–3] Conventional pesti-
cide formulas, mainly emulsifiable concentrate (EC) and wettable
powder (WP), easily drift, runoff, and scour into the environ-
ment, and loaded active ingredients are largely decomposed and
leached due to poor pesticide encapsulation.[4,5] The actual uptake
B. Liu, Prof. Y. Fan, H. Li, W. Zhao, S. Luo, H. Wang, B. Guan, Q. Li,
J. Yue, Prof. Y. Wang
Prof. Z. Dong, Prof. L. Jiang
CAS Key Laboratory of Bio-inspired Materials and Interfacial Sciences
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
CAS Key Laboratory of Colloid
Interface and Chemical Thermodynamics
CAS Research/Education Center for Excellence in Molecular Sciences
and Beijing National Laboratory for Molecular Science
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, P. R. China
E-mail: yxfan@iccas.ac.cn; yilinwang@iccas.ac.cn
Beijing 100190, P. R. China
E-mail: dongzhichao@iccas.ac.cn; jianglei@iccas.ac.cn
Prof. Z. Dong, Prof. L. Jiang
University of Chinese Academy of Sciences
Beijing 100049, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202006606.
DOI: 10.1002/adfm.202006606
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Scheme 1. Schematic of the strategy for controlling the entire journey of pesticide application on a superhydrophobic plant surface. Dynamic covalent
surfactant self-assembles into coacervate, with intermediate assembly of wormlike micelle. Then, pesticide is easily and completely encapsulated by
coacervate. The pesticide encapsulated coacervate fully realizes the impact deposition on super-hydrophobic leaf surface, excellent anti-rainwater ero-
sion performance, and controlled release of pesticide, which inhibit pesticide loss covering multiple aspects and provides a new and feasible strategy
for the design of pesticide formulation with comprehensive abilities.
release.[5,23] Unfortunately, the assemblies applied to inhibiting
the rebound have not considered the encapsulation ability and
control release. On the contrary, the capsules specially targeted
for active ingredient encapsulations[24–27] are unable to enhance
droplets deposition or long-term adhesion on the superhydro-
phobic surface due to rigid structure and low permeability.[28,29]
Moreover, the capsules applied in encapsulating pesticide
active ingredients still suffer from complex manufacture, poor
encapsulate efficiency, incompletely release, and unavoidable
organic solvents usage. As a result, existing strategies for control-
ling pesticide loss and environmental pollution still fail to work
at two or more aspects in all the pesticide application stages.
To address these problems, herein we design and synthe-
size imine-based dynamic covalent surfactants, and develop a
comprehensive pesticide formulation to improve the pesticide
utilization efficiency from all the aspects, including encapsu-
lation, deposition, anti-wind/rainwater erosion, and release.
As Scheme 1 depicts, the synthesized imine-based dynamic
covalent trimeric surfactants show self-coacervation property
on the basis of the strong aggregation ability of oligomeric sur-
factants. The surfactant-based coacervate does not need any
organic solvents. Moreover, because of high surface activity and
nanosized micelle network, the surfactant coacervate facilely
encapsulates hydrophilic/hydrophobic pesticides and exhibits
superior affinity to the epicuticular wax and micro/nanostruc-
tures of superhydrophobic leaf surface. The latter endows the
coacervate with complete deposition on superhydrophobic
surface after high-speed impact and the wind/rainwater
resistance during the subsequent application stages. Due to the
dynamic imine bonds in the trimeric surfactant, the coacervate
can also precisely control the pesticide release with the act of
CO2. Our method provides a new and feasible strategy for the
pesticide formula with comprehensive abilities.
Active ingredients of pesticides are high melting-point
hydrophobic solid or hydrophilic compounds. Normally coacer-
vate is formed by the association of amphiphilic colloidal com-
ponents in water,[30] and is supposed to show functional loading
and releasing capacity for both hydrophilic and hydrophobic
substances,[31–34] and superior adhesive ability to a wet solid
surface.[35,36] Herein, the surfactant coacervate was designed
with three goals in mind: first, to synthesize an aqueous cap-
sule with high loading efficiency and capacity to both hydro-
phobic and hydrophilic pesticides, second, to make the capsule
with high affinity to superhydrophobic surface after high-speed
impact, and third, to make the capsule with environment-con-
trolled dissociation and release property.
2. Results and Discussion
2.1. Synthesis of Dynamic Covalent Trimeric Surfactants
and Coacervate Preparation
Traditional surfactants rarely generate coacervate by them-
selves and often demand severe conditions. By comparison,
oligomeric surfactants[37,38] with three or more amphiphilic
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Figure 1. Surfactants characterization and pesticide coacervate preparation. a) Synthetic route of imine-based covalent trimeric surfactants with dif-
1
ferent hydrophobic chain length; b) Imine conversion rate characterized by H NMR increases with increasing the surfactant concentration and the
hydrophobic chain length for the three surfactants, in which TIS8 and TIS10 have high conversion rate even at a concentration lower than 0.1 mM in
water of pH 12.5; c) Turbidity graph of TIS10 solution shows it experiences micellization, coacervation, and precipitation with increasing the concentra-
tion, and the critical coacervate concentration is 4.8 mM at pH 12.5; the inserted image is of 5.0 mM TIS10 coacervate dispersion; d) Cryo-TEM image
of 2.5 mM TIS10 shows wormlike micelles; e) Schematic illustration for TIS10 coacervate solution encapsulating pesticides; f) Optical microscopy;
g) CLSM images show a great number of coacervate microdroplets, and h,i) Cryo-SEM images show nanosized network structure of the coacervate of
5.0 mM TIS10 with or without the pesticide being encapsulated. All experiments are completed at 25.0 0.5 °C.
moieties show much stronger self-assembling ability and more
acting sites, facilitating the formation of coacervate. Meanwhile,
dynamic imine bonds[39–41] may render the coacervate with
environment-responsive dissociation and pesticide release. On
the basis of these principles, we designed and synthesized the
novel dynamic covalent trimeric surfactants through connecting
three hydrophobic alkyl chains with cationic headgroups by
imine bonds (Figure 1a). Surface inactive cationic trimeric
headgroup TDA-(PhC = O)3 was synthesized (Figures S1
and S2, Supporting Information) from 1-bromo-3-phenyl-
propane (compound 1), by acylation and quaternization with
1,1-dichlorodimethyl ether and tris[2-(dimethylamino) ethyl]
amine (compound 3) under catalyzation, respectively. Then, a
series of imine-based trimeric surfactants, TISn (n = 7, 8, and
10, standing for the length of hydrophobic chain), are sepa-
rately obtained by mixing TDA-(PhC = O)3 with heptylamine,
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octylamine, and decylamine under alkaline condition
(Figure S3, Supporting Information).
2.2. Coacervate Droplet Deposition on Superhydrophobic Surface
The surfactant TIS10 with the longest hydrophobic chains
shows the highest transformation rate (Figure 1b), and the
lowest critical micelle concentration (CMC, ≈1.5 µM) (Figures S4
and S5, Supporting Information). Upon increasing the sur-
factant concentration, the size of surfactant micelles increases
(Figure S6, Supporting Information) and the solution changes
from transparent to turbid (Figure 1c, Figure S7, Supporting
Information). The cryogenic transmission electron microscope
(Cryo-TEM) image (Figure 1d) shows that 2.5 mM TIS10 forms
a number of wormlike micelles with a size of about 2–3 ×
100 nm. The extremely small zeta potential (ζ) value (0.834
0.062 mV) indicates the wormlike micelles tend to crosslink
with each other. As expected, the further association of them
takes place and coacervate is formed at a higher concentration
(Figure 1c inset), which is evidenced by a lot of 1–2 µm-sized
spherical microdroplets under light microscopy (5.0 mM TIS10,
Figure 1f). The cryogenic scanning electron microscope (Cryo-
SEM) image presents a randomly entangled network structure
with nanosized fibers in coacervate droplets (Figure 1h), which
is rich in both hydrophobic microdomain and hydrophilic inter-
face. Oligomeric surfactant has shown many unique aggrega-
tion behaviors, but to the best of our knowledge, there are no
known examples of oligomeric surfactant with self-coacervation
property. Moreover, TIS10 displays a superior self-coacervation
ability compared with the other two, of which the critical coac-
ervate concentration is the lowest (15.8, 8.4, 4.8 mM for TIS7,
TIS8, and TIS10, Figure S7, Supporting Information). There-
fore, TIS10 is chosen for all the following studies.
The nanosized networks within the coacervates facilitate the
uptake and concentration of both hydrophobic and hydrophilic
guest molecules simultaneously. Buprofezin is a representative
hydrophobic pesticide with high melting-point, and has been
widely used for pest control in rice, fruit, tea, vegetables, and
other crops.[42] Its solubility in water, only 9.0 mg L−1, limits the
use for sustainable agricultural spray. By adding buprofezin
solid powder directly to the TIS10 coacervate dispersion, the
buprofezin-encapsulated coacervate can be prepared (Figure 1e,
Figure S8, Supporting Information). Without organic solvent,
5.0 mM TIS10 dramatically enhances the solubility of bupro-
fezin up to 916.3 mg L−1, which is one hundred times larger
than pure water. The encapsulation efficiency of the pesticide
is quantified by inductively coupled plasma-mass spectrom-
etry (ICP-MS) spectroscopy in the two phases, indicating that
buprofezin is almost sequestered into the coacervates as long
as the loading amount is without exceeding its maximum solu-
bility. The Cryo-SEM image reveals that the buprofezin-encap-
sulated coacervate maintains the nanosized network structure
(Figure 1i). In addition, hydrophilic pesticides are more and
more desired, but it still suffers from low encapsulate effi-
ciency and/or release lack controlled.[4,5] Herein, Fluorescein as
a model hydrophilic pesticide also can be preferentially parti-
tion into the coacervates of TIS10 (Figure 1g). Its encapsulation
efficiency is estimated by the ratios of the fluorescence inten-
sities inside and out of droplets; high by up to 93%≈98% as
the loading amount, and lower than 498.4 mg L−1 (Figure S9,
Supporting Information). In brief, pesticide-encapsulated coac-
ervate can be easily prepared and exhibits a high encapsulation
efficiency to both hydrophilic and hydrophobic pesticides.
Enhancing deposition of high-speed impacting water droplets
on water-repellent, especially superhydrophobic surface, is
extremely difficult because of only several milliseconds con-
tact time.[16,43–45] Several new approaches have been developed
to improve the deposition efficiency by forming precipitate
defecting,[18] transforming surface wettability,[19] and jamming
the surface micro/nano structures.[20] However, the deposi-
tion and long-term performance is still a challenge for the real
water-based pesticide formula. Figure 2 shows the high-speed
drop deposition of the TIS10/pesticide coacervate (Figure 2a)
on superhydrophobic cabbage leaves (Figure 2b–d, Movie S1,
Supporting Information).
The cabbage leaf characterized by micro/nano hierarchical
structure and strong water-proof epicuticular wax layer shows
the water contact angle of 156.4 3.1° (Figure 2b). All the impact
droplets are controlled at the diameter of 2.0 mm and the
impact velocity of 2.42 m s−1. While impacting the droplets of
traditional suspending concentrated buprofezin formula (SC-B)
of practical application concentration (0.03%), the droplets
splash and bounce on the leaf surface prominently (Figure 2e).
Strikingly, no splashing, bouncing, and cracking behavior takes
place for the droplets of 5.0 mM TIS10 coacervate dispersion
(Figure 2f) and the coacervate with encapsulated Fluorescein
(Figure 2g) and buprofezin (Figure 2h). The impact outcomes
show effective and complete deposition of the droplets on the
superhydrophobic leaf surface. The same impact experiments
are also performed on the artificial superhydrophobic surface
(Figure S10, Supporting Information) and achieve similar
deposition results, in which the average diameter ratio of
a final spreading liquid to the initial droplet (Df/Do) is up to
2.8 (Figure S11, Supporting Information). The controlled
liquid deposition by the act of TIS10 is capable of significantly
reducing the off-target pollution of pesticides in spraying.
Why is the trimeric surfactant capable of realizing the liquid
deposition and inhibiting pesticide loss from splashing and
rebounding? It is noted that the dynamic surface tension of TIS10
only slightly changes within 0–0.1 s (Figure 3a). The surface ten-
sion can be reduced by the TIS10 molecules diffused onto the sur-
face. At the TIS10 concentration of 5 mM, which is much higher
than its CMC (≈1.5 µM), most of the surfactant molecules in the
bulk exist as micelles, and the diffusion of the surfactant mole-
cules as monomers and in micelles affects the final situations of
the surfactant molecules onto the surface. This suggests that the
big molecule of TIS10 and its aggregates limits the molecular dif-
fusion from the bulk solution to the newly created interface, and
thus could not lower the surface energy of a superhydrophobic
substrate. Thereby, the surface tension effect of TIS10 does not
play an important role, that is, the mechanism for the present
trimeric surfactant in inhibiting the splashing and retracting of
droplets is different from that for other surfactants.[19,20,46]
Then we turn to check the rheological property of TIS10. While
impacting the TIS10 coacervate dispersion, the initial Weber
number (We) is about 390 and Reynolds number (Re) is about
6 ≈ 30, showing that the inertial force drives the expansion stage
(We > 100). The TIS10 coacervate dispersion displays larger vis-
cosity and shear thinning property (Figure 3b), and the loss
modulus (G″) is higher than storage modulus (G′) for angular fre-
quency from 0.1 to 33.0 rad s−1 (Figure 3c). This suggests that the
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Figure 2. Drop of coacervate dispersion impacts on superhydrophobic surface as a model of the pesticide spraying process. a) Chemical structures of
buprofezin and TIS10, b) water contact angle reveals the superhydrophobic property of the leaf surface, c) optical image of cabbage used, and d) SEM
image of surface morphology of superhydrophobic cabbage leaf. Impact process on the cabbage leaf surface of e) traditional pesticide formula SC-B as
a control shows prominent bounding and splashing. As to the coacervate dispersion of 5.0 mM TIS10 f) without encapsulation, and with encapsulated
g) Fluorescein of 1.0 mM and h) buprofezin of 1.0 mM, respectively, all achieve complete deposition on the superhydrophobic cabbage leaf surface.
All impact experiments with an impact velocity of 2.42 m s−1.
impact kinetic energy might be dissipated during the expansion
and retraction stage,[47] and thus viscous effect is possible to play
a role in expanding the drop area and attenuating drop retraction.
Nevertheless, we found even if the TIS10 wormlike micelle solu-
tion (4.0 mM) own a very similar rheological property to the TIS10
coacervate dispersion (Figure 3b,c), the micelle droplets rebound
prominently from the surface (Figure S12, Supporting Informa-
tion). Therefore, the viscosity is also not the determinative role to
the complete deposition of the TIS10 coacervate on the surface.
We thus infer that the complete deposition should stem from
the special dense nanosized network structure of the coacervate
and the resultant certain firm pinning force between the coac-
ervate and superhydrophobic surface. This is approved by the
Cryo-SEM images (Figure 3d). The images reveal that after the
droplet impact, the epicuticular nanopillars of the cabbage leaf
are fully immersed by the TIS10 coacervate dispersion, clearly
showing the contact line. Some of the nanopillars impale the
thin liquid layer on the cabbage leaf surface, and in particular,
the freeze fractured and sublimed images disclose that the
coacervate spherical microdroplets firmly stick to the nanopillar
of the leaf surface, demonstrating the wetting transformation
from the Cassie state to the Wenzel state.
In addition, considering that the coacervate is rich in both
hydrophobic microdomain and hydrophilic interface, and it is
separated from the dilute equilibrium phase but still maintains
liquid state, the interaction of coacervate with water is supposed
to be very different from micelles. The water relaxation spectra
(Figure 3e) by low-field nuclear magnetic resonance (LF-NMR)
indicate that the amount of tightly bound water (corresponding
to the left peaks around 1 ms) by TIS10 coacervate, no matter
with or without pesticides (16.8%, 13.0%, 11.7%), is much larger
than that by SC-B (3.2%) or TIS10 wormlike micelles (5.6%).
The stronger ability of binding water should be beneficial for
the coacervate to bind with various specific polar microregions
at the superhydrophobic leaf surface and helpful for the surface
wettability transition from superhydrophobic to hydrophilic.
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Figure 3. Mechanism of the deposition of high-speed impact coacervate drop on the superhydrophobic surface. a) Dynamic surface tension curve of
TIS10 coacervate dispersion at 5.0 mM shows slow dynamic of TIS10, b) Steadily-shear rheological data of 5.0 mM TIS10 coacervate with comparison
to wormlike micelle solution of 4.0 mM TIS10 and 0.03% SC-B, showing the moderate viscous and shear thinning property of TIS10 coacervate but not
viscous of SC-B. c) The dynamic rheological test indicates viscous behavior (G″) dominate for both of 5.0 mM TIS10 coacervate dispersion and worm-
like micelle solution of 4.0 mM TIS10. d) Cryo-SEM images demonstrate a coacervate droplet entangled with the micro/nano- hierarchical structure of
the Cabbage leaf surface. The two images at the bottom are freeze fractured states of the upper left image. The upper right image shows its model.
e) Relaxation time spectra of water for the TIS10 coacervate dispersion with and without encapsulation characterized by LF-NMR, with comparison to
the TIS10 micelle solution and suspending concentrated buprofezin (SC-B). Peaks of short relaxation time in left correspond to the bound water, while
peaks of long relaxation time in right correspond to the free water. All the above experiments are completed at 25.0 0.5 °C.
2.3. CO2-Induced Coacervate Dissociation and Controlled
Pesticide Release
is also dissociated. On the basis of this property, more than
80% hydrophilic Fluorescein and hydrophobic pesticide bupro-
fezin can be released in 24 and 40 h (Figure 4c), respectively.
While utilizing the CO2 in air, the same release process was
completed in three months. The images captured by confocal
laser scanning microscopy (Figure 4d) present the kinetic
release process and the coacervate dissociation: Fluorescein is
initially enriched in the coacervate microdroplets; when the pH
changes to 10.5, the coacervate microdroplets are partially dis-
sociated; further decreasing pH down to 9.5, most Fluorescein
is released into the solution but some Fluorescein is still kept
in the smaller coacervate microdroplets; and finally when the
pH decreases to 5.8, the TIS10 coacervate microdroplets are
dissociated and Fluorescein is released completely. Therefore,
After pesticides are effectively deposited on crop surfaces,
the control release of pesticides becomes a key step to ensure
the long-term activity of pesticides. The longer release time
means more of a tendency to keep pesticides on a leaf surface.
CO2 is ubiquitous in a natural environment and can acidify
the TIS10 solution and in turn dissociate the imine groups
(Figure 4a). With the injecting of CO2 into the TIS10 solution,
the pH declines from 12.5 to 8.4 and further down to 5.8, cor-
respondingly, the imine groups are partially hydrolyzed and
completely dissociated in the final state(Figure 4b, Figures S3
and S4, Supporting Information). As a result, the coacervate
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Figure 4. Characterization of surfactant dissociation and pesticide release by TIS10 coacervate. a) Schematic of TIS10 dissociation under CO2. b) H
NMR spectra for TIS10 of 3.0 mM at pH 12.5, 8.4, and 5.8 respectively, demonstrating imine is gradually dissociated into aldehyde by CO2 admission
and the resultant pH declines, and when pH down to 5.8, imine is completely decomposed. c) Release profile of Fluorescein and buprofezin encap-
sulated by the TIS10 coacervate of 3 mL in 60 mL releasing medium with increasing CO2 input time at a flow rate of 6 mL min−1. d) CLSM images of
Fluorescein-encapsulated TIS10 coacervate with the pH decrease by inputting CO2, showing gradual coacervate dissociation and Fluorescein release.
All the above experiments are completed at 25.0 0.5 °C.
the precisely controlled and complete release of pesticides is
achieved by controlling pH with applying CO2. Besides in air,
CO2 can be also generated by plant respiration and water tran-
spiration, and the nanosized micelle network microstructure
of coacervate has strong hydration ability, so this CO2-induced
degradative surfactant-based coacervate should be favorable for
retaining water and continuously releasing pesticides. The pes-
ticide dose released by CO2 at crop surface may adapt to the dif-
ferent growth periods of various crops, and thus be the smart
doses.
To evaluate the wind influence, we assess the adhesive ability
of the TIS10 coacervate on the cabbage leaf surface by spinning
under the centrifugal force of 16.4 folds of mass, with compar-
ison to SC-B and TIS10 micellar solution. As revealed by the
time-sequence images (Figure 5a), all the drops tend to lean for-
ward at the beginning, then gradually elongate, and finally throw
out the front part. But the retention time of the TIS10 coacervate
droplet is nearly five-times to that of SC-B and the TIS10 micelle
solution (Figure 5b). We have also measured the advancing
and receding contact angles of TIS10 coacervate dispersion
on superhydrophobic surface. However, there is a motionless-
ness of the droplet when it contacts the superhydrophobic sub-
strate and whatever the tilt angle of the substrate (Movie S2,
Figure S13, Supporting Information). That is to say, the coacer-
vate possesses the superior pinning ability on the leaf surface.
Then we tend to evaluate the rainfastness of the TIS10 coac-
ervate droplet by testing the scour performance. Samples are
naturally dried on cabbage leaf surface to imitate the real state
of pesticide mostly in a natural condition. Fluorescein is washed
away and almost bare in SC-B just as it contacts rainwater. In
2.4. Adhesion and Rainfastness
Pesticide deposited on crop leaves often loss from wind or rain
under the natural environment.[4,48] The extended-release time of
pesticides requires the sufficient adhesion of the coacervate on
the superhydrophobic leaf surface. The network structure of the
coacervate could be a robust design to overcome this problem
due to its strong binding with the superhydrophobic surface.
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Figure 5. Adhesion ability and rainfastness characterization. a) Images of drop adhesion state on cabbage leaf surface vary with the spinning time,
the arrows indicating spinning direction, the rotating speed is 500 rpm, and the initial position distance of drop sample from the center is 6.0 mm.
b) Drop length of 5.0 mM TIS10 coacervate dispersion varies with centrifuge rotation time, compared with 0.03% SC-B solution and 4.0 mM TIS10
wormlike micelle solution. The coacervate droplet shows a sixfold time to the other two in resisting wind on the cabbage leaf surface. c) Rain erosion
test of the TIS10 coacervate with encapsulated 0.03% buprofezin and 0.1 mM Fluorescein on cabbage leaf surface with comparison to SC-B. The yellow
dash circles indicate the sample positions. The disappearance of green fluorescence in the circle of SC-B at 10 s represents that SC-B is washed away.
The long-time existence of the green fluorescence indicates that the TIS10 coacervate owns superior adhesive ability on the superhydrophobic cabbage
leaf surface, but it can be washed away after CO2 treatment for a relatively long time. All the above experiments are completed at room temperature.
contrast, Fluorescein-encapsulated TIS10 coacervate remains
intact and emits a strong green fluorescence under 70 s wash
(Figure 5c, Movie S3, Supporting Information). This means
that the coacervate has strong anti-rain ability on the superhy-
drophobic cabbage leaf surface, ensuring the long-term validity
of the pesticides.
inside (Figure S14, Supporting Information). This structure is
favorable for retaining water, continuously releasing pesticides
as well as bioavailability. Therefore, the coacervate formulation
can keep its entire functions under a natural condition.
Even if treating the coacervate with CO2 for 20 min makes
it dissociate partly, the drop still withstands a short period of
pour rain erosion, but it is thoroughly washed off after 30 s
(Figure 5c, Movie S4, Supporting Information), which ensures
the sustained release for the later application period of pesticides
and no pesticide residue on crops finally. Moreover, owing to the
very low critical micelle concentration of the trimeric surfactant
and the large size of the formed coacervate, the surfactant pene-
tration into the plant tissue can be greatly inhibited according to
the principle of surfactant penetration.[49] Undoubtedly, all these
greatly contribute to the safety of final crop products.
3. Conclusion
Taken together, our research provides a sustainable and inno-
vative pesticide formulation relying on dynamic oligomeric
surfactant coacervate to enhance the efficiencies by control-
ling the entire journey of pesticide application. The nano-
sized network microstructure in the coacervate enriches
both hydrophilic and hydrophobic domains and strongly
entangles with the superhydrophobic surface, making the
surfactant coacervate efficiently encapsulate hydrophilic/
hydrophobic pesticides by direct mixing, inhibit drop splash
and rebound from the surface after high-speed impact, and
can keep long-term performances by anti-wind/rain erosion
ability. Moreover, the dynamic covalent bonds in the surfactant
In addition, when the coacervate is naturally dried on the leaf
surface, the SEM images suggest that a continuous film exists on
the external surface but maintains a loosely nanosized network
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Olympus), and Cryo-SEM (S-4300, HITACHI) equipped with cryo-
preparation chamber (EM ACE600, LEICA) at −137 °C. Steady and dynamic
rheological experiments were performed on an MCR302 stress-controlled
rheometer (Anton Paar OptoTec, Germany).
molecule endow the coacervate with environment-responsive
dissociation and CO2-controlled complete release. We believe
that this comprehensive strategy would address the challenge
from the inefficient use of pesticides, which has caused severely
negative influences on biodiversity and current unsustainable
agricultural practices in the face of a growing population. Fur-
thermore, we envision that the innovative pesticide formulation
would promote scientific fundamentals and technological appli-
cations in other fields, such as active substance immobilization
and an intelligent biological nanodevice adhesive on both artifi-
cial and natural surfaces.
Pesticide Encapsulation: Pesticide coacervate was prepared by mixing
hydrophobic pesticide buprofezin or hydrophilic model pesticide
Fluorescein with the surfactant coacervate dispersion. Buprofezin was
completely encapsulated by TIS10 coacervate below the loading capacity.
The efficiency of Fluorescein encapsulated by the coacervate was
tested by determining the unencapsulated Fluorescein concentration
with the UV–vis absorption of the upper solution after centrifugation.
The encapsulation efficiency of buprofezin is quantified by ICP-MS
spectroscopy in the two phases. The encapsulation efficiency and
loading capacity were separately determined as the percentage of the
weight of encapsulated pesticide to the weight of the initial added
pesticide, or to the weight of surfactant used.
Fabrication of Superhydrophobic Surfaces: A dip-coating method via a
literature procedure[20]was employed to fabricate a superhydrophobic
surface on a glass substrate. The contact angle of water was tested about
160.4 2.3°. The contact angle was measured using a contact angle
measurement device (DSA100, KRUSS). The CA values were obtained by
measuring more than three different positions on the superhydrophobic
surface.
Drop Impacting on Superhydrophobic Surface: Drop impact velocity
was controlled at 2.42 m s−1 by free falling the droplet from a stainless-
steel syringe needle with a settled height. The diameter of the liquid
drop was controlled at 2.0 mm by adjusting the inner diameter of the
needle. The droplet impacting dynamics on a superhydrophobic surface
was captured by a high-speed camera (FASTCAM Mini UX100 Photron)
at a frame rate of 2000 fps with a shutter speed 1/20000 s. The PPI of the
impact test images are 1000 × 682.
CO2-Induced Fluorescein and Buprofezin Release: The controlled-release
property of Fluorescein and buprofezin encapsulated in coacervate was
evaluated by dynamic dialysis method. The TIS10/Fluorescein coacervate
dispersion was added into a dialysis membrane (with a molecular weight
cutoff of 3500 Da), and the end-capped dialysis membrane was put
into a beaker with pure water as the release medium, stirred at 150 rpm
and 25 °C. At various time intervals, the solution outside the dialysis
membrane was withdrawn, mixed with Na2HPO4 to reach pH 9.2, and
the released Fluorescein concentration was measured by a UV–vis
spectrophotometer to determine the kinetic profile of release (Figure S17,
Supporting Information). The release procedure of buprofezin was the
same as that of Fluorescein, but taking 30% ethanol-water as a release
medium. The buprofezin concentration was quantified by detecting 32S16O
with field inductively coupled plasma mass spectrometer (ICP-MS) by
S standard addition method (Figure S18, Supporting Information). The
final accumulated release data was calculated by summing up the released
buprofezin or Fluorescein mass divided by the mass of feed. Micro state
of coacervate during the release was visualized by CLSM (FV1000-IX81,
Olympus). The pH of each sample was adjusted by CO2 injection at
different times and ensured equilibrium before imaging.
4. Experimental Section
General Information: 1-Bromo-3-phenylpropane (98%), tris[2-
(dimethylamino) ethyl] amine (99%), octylamine (99%), and hydrogen
bromide were purchased from Alfa Aesar. Heptyl amine (99%),
decylamine (99%), Nile red (99%), sodium deuteroxide (NaOD, 99%),
and deuteroxide chloride (DCl, 99%) were purchased from Acros
Organics. 1,1-dichlorodimethyl ether (Cl2CHOCH3, >97%) was purchased
from Adamas, titanium tetrachloride (TiCl4, 99.9%), and buprofezin
(99.5%) was purchased from Aladdin. Fluorescein was purchased from
J&K Chemical. Deuteroxide was from Innochem. A sodium hydroxide
standard solution was purchased from TCI. All organic solvents used in
the experiments were purchased from Beijing Chemical Works. All the
materials and solvents were used as commercial suppliers without further
purification. Deionized water (18.2 MΩ cm) from Milli-Q equipment was
used in all experiments. The cabbage leaf of 70 days age was obtained
from the Institute of Vegetables and Flowers, Chinese Academy of
1
Agricultural Sciences. The H NMR and 13C NMR of all the substances
were recorded on a Bruker Avance 400 MHz spectrometer. Mass spectra
were recorded on a Thermo Exactive spectrometer for ESI and Waters GCT
for EI. UV–vis spectra were recorded in quartz cuvettes (light path 10 mm)
with a Hitachi U-3900 spectrophotometer. Fluorescence spectra were
measured on a Hitachi F-4600 spectrometer in a quartz cell with 10 mm
path length. Transmission electron microscopy (TEM) was captured by
JEM-2011 with accelerating voltage 120 kV.
Synthesis of TISn: p-(bromopropyl) benzaldehyde was synthesized
(Figures S15 and S16, Supporting Information) via
a literature
procedure.[41] TDA-(PhC = O)3 was synthesized from the quaternization
reaction of p-(bromopropyl) benzaldehyde and tris[2-(dimethylamino)
ethyl] amine with 5:1 ratio in acetone/ethanol (10:1, v/v%) under nitrogen
atmosphere and reflux. After the solution was stirred and refluxed
for 56 h, the solution was cooled to room temperature, added excess
acetone and filtered to obtain yellow precipitates. With recrystallized by
isopropanol/dichloromethane for three times and then recrystallized by
acetone/ethanol for twice, TDA-(PhC = O)3 was obtained as light-yellow
1
crystal with yield of 40%. H NMR (D2O, 400 MHz, ppm): δ 9.84(s, 3H,
CHO), 7.87 (d, 6H, CH), 7.46 (d, 6H, CH), 3.38 (m, 12H, N+-CH2-N,
N+-CH2-Ar), 3.09 (s, 18H, N+-CH3), 3.00. 13C NMR (125 MHz, D2O): δ
195.84, 148.27, 134.16, 130.52, 129.26, 64.23, 60.48, 51.09, 46.62, 31.60,
23.44. HRMS-ESI (m/z): calculated for C42H63N4O3Br3, 911.7; found 223.8
[M-3Br−]3+/3); 376.2 [M-2Br−]2+/2); 831.7 [M-Br−]+).
Characterization of Adhesion Ability and Rainfastness: Droplet
adhesion was characterized by a spin coater to imitate wind erosion.
The controlled drop volume was 8.0 µL, the initial position distance
of a drop from the center was 6.0 mm, and the rotating speed was
500 rpm. The adhesion state was recorded by a high-speed camera. Rain
erosion test was characterized by pouring down water to the samples
from a garden watering can with a spray head at a set height from the
substrate. The samples of 40 µL coacervate dispersion with 5.0 mM
TIS10, 0.03% buprofezin and 0.1 mM Fluorescein, and 40 µL SC-B with
0.03% buprofezin and 0.1 mM Fluorescein were parallelly titrated to
the cabbage leaf surface and naturally dried for 4.0 h. The 700 µL water
sprayed onto the leaf surface within ≈100 s, and each experiment was
performed six times in order to confirm the observations. The process
was recorded by a video camera under violet ray.
Characterization of Surfactant and Coacervate: Surface tension was
measured by DCAT21 tensiometer (DataPhysics Co., Germany). The critical
micelle concentrations of surfactants were determined from the break of
surface tension curves. The surfactants aggregates were measured by a
DLS spectrometer (ALV/SP-125) and visualized by Cryo-TEM (JEM-2011,
JEOL) at about –174 °C. Zeta potentials were determined on a Malvern
Zetasizer Nano ZS. Turbidity curves of the surfactant solutions were
measured at 450 nm using a Brinkman PC950 probe colorimeter. The
maximum Nile Red emission wavelength (λmax) at different pH was
recorded by a fluorescence spectrophotometer (F-4600, Hitachi). The
coacervate morphology was characterized by light microscope (IX83,
OLYMPUS), confocal laser scanning microscopy (CLSM, FV1000-IX81,
Cryogenic Transmission Electron Microscopy (Cryo-TEM): Samples for
Cryo-TEM were prepared as follows: 5 µL of each sample was loaded
onto a carbon-coated holey TEM grid and blotted with a filter paper
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to obtain a thin liquid film on the grid. After a few seconds, the grid
was quickly plunged into a reservoir of liquid ethane (cooled by
liquid nitrogen) at −165 °C. The vitrified sample was then stored in
liquid nitrogen until it was transferred to a cryogenic sample holder
and examined with a JEOL JEM-2011 TEM (120 kV) at about −174 °C.
Digital images were recorded in the minimal electron dose mode by a
Gatan multi-scan cooled charge-coupled device (CCD) camera.
Cryogenic Scanning Electron Microscopy (Cryo-SEM): a) The sample
solution was “sandwiched” between two gold planchettes and then
plunged into liquid nitrogen slush. After that, the samples were
Conflict of Interest
The authors declare no conflict of interest.
Keywords
coacervation, deposition, dynamic covalent, superhydrophobic surfaces,
sustainable agriculture
transferred into
a cryo-preparation chamber (LEICA EM ACE600)
Received: August 6, 2020
Revised: September 30, 2020
Published online:
under vacuum and sublimed by cooling to −100 °C for about 15 min.
The frozen surface of the samples was coated with Pt to make it
conductive under an argon environment (15 mA for 200 s). Then, the
samples were transferred to the cryostage of −137 °C in the microscope
(S-4300, HITACHI, Ltd, Japan). Finally, imaging was performed using
a 3.0 kV landing energy and 10 µA current. b) The microstates of the
interface between the deposited buprofezin/TIS10 coacervate solution
and the cabbage leaf surface was characterized by a scanning electron
microscope (FEI Helios NanoLab G3 UC, Thermo Scientific). The
coacervate solution was dripped to the cabbage leaf from a certain
height, then the leaf was cut to a suitable size and glued to the sample
stage by conductive tape. The leaf with coacervate solution sample was
frozen in subcooled liquid nitrogen (−210 °C) and transferred in vacuum
to the cold stage of the chamber, where the cryogenic sample was
fractured with a cold knife and then sublimation (−90 °C, 90 min) and
sputter coating (10 mA, 60 s) with platinum were conducted. Finally, the
samples were transferred to another cold stage in the scanning electron
microscope and imaged. The image was recorded using the electron
beam at 2 kV and 0.2 nA with 15° tilt degree and a working distance of
4 mm. The resolution of the final data was 3072 × 2048.
Low Field NMR Spin-Spin Relaxation (T2) Measurements (LF-NMR):
The LF-NMR analysis was performed at room temperature by means of a
Niumag Minispec NMI20 (0.47 T, 20 MHz, China). The determination of
the average water protons transverse (spin-spin) relaxation time (T2) inside
the samples was performed according to the Carr–Purcell–Meiboom–
Gill (CPMG) sequence[50] {90°[-τ-180°-τ(echo)]n-TR} with a 20.52 µs wide
90° pulse, echo time τ = 1.5 ms, and TR (sequences repetition rate) equal
to 27 s. The T2 relaxation curve was fitted to a multi-exponential curve with
the MultiExp Inv Analysis software. All the measurements were conducted
six times under the same experimental condition.
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