Dynamic Self-Assembly Adhesion of a Paraquat Droplet on a Pillar[5]arene Surface

Article abstract of DOI:10.1002/anie.201603906

The adhesion of herbicide droplets on leaf surfaces plays an important role in the herbicide's adsorption by crops. How to control the adhesive binding which occurs through dynamic self-assembly between the macroscopic droplet and the surface is a challenging task. We introduce a host onto surfaces that controls the binding of guests in the paraquat droplets. The pillar[5]arene-functional surface showed the selective binding of paraquat droplets via the host–guest interaction. The work is promising for improving the efficiency of herbicides.

Full text of DOI:10.1002/anie.201603906

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201603906
German Edition: DOI: 10.1002/ange.201603906
Surface Adhesion
Dynamic Self-Assembly Adhesion of a Paraquat Droplet on a
Pillar[5]arene Surface
Li Luo, Guanrong Nie, Demei Tian, Hongtao Deng, Lei Jiang, and Haibing Li*
1
Abstract: The adhesion of herbicide droplets on leaf surfaces
plays an important role in the herbicideꢀs adsorption by crops.
How to control the adhesive binding which occurs through
dynamic self-assembly between the macroscopic droplet and
the surface is a challenging task. We introduce a host onto
surfaces that controls the binding of guests in the paraquat
droplets. The pillar[5]arene-functional surface showed the
selective binding of paraquat droplets via the host–guest
interaction. The work is promising for improving the efficiency
of herbicides.
Supporting Information. The H NMR, 13C NMR, and mass
spectra (Figure S2,S3,S4) confirmed the structure of P5A.
The alkynyl group at lower rim of P5A was used to link with
the silicon surface and the electron-rich cavity of P5A served
as the recognition unit. Then, the P5A was bonded onto the
silicon surface by the click reaction.[25] The P5A-functional
surface can induce the dynamic self-assembly binding of
specific paraquat droplet (Scheme 1).
T
he self-assembly of macroscopic droplets is of long-stand-
ing and continuing interest owing to its fundamental and
technological relevance.[1–3] For example, the attachment of
herbicide droplets on leaf surfaces by self-assembly plays an
important role in the adsorption of the herbicide by crops, as
it can reduce the amount of herbicide[4–6] required. So far, the
study of the self-assembly processes mainly focused on static
structures,[7–10] but dynamic systems are largely unexplored
and pose a great challenge. For instance, how to control the
dynamic self-assembly of specific herbicide droplet on the
surface?
Paraquat is a widely used herbicide.[11–14] To address the
challenging problem, we introduce host–guest chemistry onto
surfaces to control the dynamic self-assembly binding of the
paraquat droplet to the surface. The pillar[n]arenes,[15–19]
composed of hydroquinone unit linked by methylene bridges
at para positions, are rigid as well as easily functionalized with
various substituents.[20,21] The unique cavity structure and easy
modification of pillar[n]arenes endow them with an out-
standing ability to selectively bind different kinds of
guests,[22–24] which provides a useful platform for the con-
struction of the functional surface.
Herein, an inclinable pillar[5]arene-modified silicon sur-
face was fabricated to accomplish the selective dynamic self-
assembly adhesion of paraquat droplets to the surface via the
host–guest interaction. The pillar[5]arene (P5A) was synthe-
sized by the synthetic route shown in Figure S1 of the
Scheme 1. Schematic illustration of the dynamic self-assembly of para-
quat-droplet–surface complex.
The interaction between P5A and methyl paraquat (G2)
was investigated in solution phase by UV and 1H NMR
spectroscopy and mass spectrometry (MS). The UV analysis
of the P5A and different paraquat derivatives was carried out.
As shown in Figure 1b, after adding the G2 into P5A solution,
the UV spectra presented a new peak at 243 nm. However,
there were no changes at 243 nm for other guests. Further-
more, the association constant of P5A and G2 determined by
[*] L. Luo, G. R. Nie, Dr. D. M. Tian, Dr. H. T. Deng, Prof. H. B. Li
Key Laboratory of Pesticide and Chemical Biology (CCNU), Ministry
of Education, College of Chemistry, Central China Normal University
Wuhan 430079 (P.R. China)
UV continuous titration is (1.32 Æ 0.08) ꢀ 103 mÀ1 which is
,
bigger than that of P5A and G1, G3, G4, G5 (( 3.73 Æ 0.03) ꢀ
102, (1.35 Æ 0.05) ꢀ 102, (2.67 Æ 0.02) ꢀ 102, (2.46 Æ 0.03) ꢀ
102 mÀ1 respectively)as shown in Figure S5. Further, the
complex ratio of P5A and G2 was determined to be 1:1 by
E-mail: lhbing@mail.ccnu.edu.cn
Prof. L. Jiang, Prof. H. B. Li
Beijing National Laboratory for Molecular Sciences (BNLMS), Key
Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Sciences
1
Jobꢁs plot (Figure S6). H NMR experiments are shown in
Figure 2. The G2a, G2b and G2c protons of G2 underwent
upfield shifts of 0.08 ppm and 0.05 ppm, respectively, mean-
while the H1 protons underwent upfield shift of 0.12 ppm and
the H2, H3 protons of P5A shifted downfield by 0.03 ppm,
Beijing, 100190 (P.R. China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201603906.
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also examined by Gaussian03 calculations (Figure S9). All the
energy were obtained by the equation of DE(binding
=
energy)
E(host–guest)À(Ehost + Eguest). The binding energy of P5A and
G2 (À210.04 kJmolÀ1) is smaller than the binding energy of
P5A and other guests, indicating the complex of P5A with G2
is more stable. These results indicate the host P5A selectively
binds to G2.
The micro-structured silicon surfaces, which mimic the
surfaces of leaves,[4–6] have been selected as a substrate. The
silicon surface topography was observed by SEM (Fig-
ure S10). The P5A-functionalized silicon surface was con-
structed by click reaction. After the bare silicon surface was
deposited, as can be seen in Figure S11, the hydrophilic
hydroxy groups are exposed on surface where the measured
contact angle (CA) was 8.2 Æ 3.08. Then, the surface was
modified with azide and the azide groups (-N3) were exposed
on the surface, giving rise to the increase of hydrophobicity so
that the contact angle increased to 77.3 Æ 3.08. Finally, the
host P5A was added to the surface by the click reaction
ꢀ
between the -N3 and the alkynyl group (-C CH) of P5A. The
introduction of the hydrophobic P5A onto the surface caused
the contact angle to further increase to 142.7 Æ 3.08, which
suggested a super-hydrophobic surface was successfully
fabricated. To further confirm the P5A was successfully
attached to the surface, additional evidence was provided by
XPS (Figure S12) and FTIR (Figure S13). As show in Fig-
ure S12, the XPS results showed that the content of carbon,
oxygen, and nitrogen on surface changed before and after
modification, which indicated the P5Awas successfully added
to the surface. As can be seen in Figure S13, the -N3 peak at
2100 cmÀ1 observed suggests that the Si-N3 was formed
successfully. After the P5A was added, the -N3 peak at
2100 cmÀ1 distinctly weakened, while the peak of the benzene
skeleton vibration near 1500 cmÀ1 was observed, which
indicated the P5A was modified on the surface.
Figure 1. a) The chemical structures of the tested host and guests;
b) the UV spectra of P5A after adding different guests. The difference
value DA ((A243/A217
)P5A+GuestÀ(A243/A217
)P5A) was shown in the column
diagram after the different guest was added into P5A solution showing
P5A is selective to G2. The dotted circle highlights the new signal at
243 nm.
Subsequently, we investigated the self-assembly of the
paraquat-derivative droplets with the concentration of 10À3
m
on the flat P5A-modified surface. As shown in Figure S14,
there are no distinctive difference by the measurement of
static contact angles after the guests and pure water droplet
was added to the P5A surface. However, the contact angle
hysteresis (CAH) in Figure S15 is much different. The
measured CAH were 8.48, 9.98, 10.48, 9.48, 11.18, respectively
after adding pure water, G1, G3, G4, and G5 droplets to the
P5A-modified surface, while the CAH was 45.38 after adding
G2 droplets, which is much higher than that of the pure water
droplet and the other guests. According to the formula:[27]
sina = wgLA (cosqrecÀcosqadv)/mg, where a means the sliding
angle, w is the width of the droplet, gLA is surface tension of
the droplet, qrec (qadv) is the receding (advancing) contact
angle, m is the mass of the droplet and g is the gravity
acceleration, the theoretical sliding angle of the different
droplets on the P5A surface was calculated. As shown in
Figure S16, the sliding angle of pure water droplet, G1, G3,
G4 and G5 droplet was about 78, 138, 108, 128 and 108,
respectively, on the P5A surface, while the sliding angle of G2
droplet was about 568. Therefore, we tilted the P5A-modified
to 458 to observe the dynamic assembly behavior of guest
droplet. As shown in Figure 3a, the pure water, G1, G3, G4,
Figure 2. The 1H NMR spectra of P5A (black spectrum; 30 mm,
CD3CN:D2O=50:1, 400 MHz, 298 K), G2 (blue spectrum; 30 mm,
CD3CN:D2O=50:1, 400 MHz, 298 K). And a mixture of P5A and
G2 (red).
which are in accord with the literature.[26] It was deduced that
G2 entered into the cavity of P5A. The MS (Figure S7) and
1H NMR titration test (Figure S8) of P5A and G2 were
conducted to further confirm the interaction and 1:1 complex
stoichiometry. Furthermore, the binding of P5A with G2 was
2
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Figure 4. Schematic representation of the possible mechanism for the
binding of the G2 droplet to the P5A-modified surface.
surface. However, the other droplets can slide off the surface
because their binding strength on P5A surface is too low to
resist the droplet gravity.
Meanwhile, the sliding angle of the G2 droplet increased
with the increasing concentration of guest in the droplets,
which is a result of more G2 molecules being able to bind with
the P5A-functional surface thereby increasing the binding
strength to prevent the droplet from sliding.
To verify the binding strength of the pesticide guests on
the P5A surface, the quartz crystal microbalance (QCM) was
used to study the behavior of paraquat pesticides on pillar-
[5]arene-modified surfaces. In this experiment, pillar[5]arene
coated quartz crystals were obtained by the reaction between
the terminal alkyne groups of pillar[5]arene and the gold layer
on crystals.[28] As shown in Figure S18, the G2 exhibited
distinctly stronger adsorption on pillar[5]arene surface with
the biggest binding constant 145.4mÀ1 than other paraquat
derivates.
The surface can be reused to adhere to G2 droplets after
washing the surface by water. Therefore, the repeatability of
this phenomenon was investigated as shown in Figure S19.
Upon alternately adding pure water and G2 droplets, the
P5A-functional surface, tilt angle of 458, can still adhere to G2
droplets after seven cycles, which suggests that the P5A
constructed surface is stable.
In conclusion, P5A was synthesized and the P5A-func-
tional surface was constructed by the click reaction to
investigate the dynamic self-assembly adhesion of paraquat
droplets. The P5A-functional surface exhibited the dynamic
self-assembly adhesion of specific methyl paraquat droplets
G2 because of the matched host–guest interaction between
P5A and G2. The fabricated surface is promising in the
application of the rapid detection of paraquat for environ-
mental monitoring and provides an approach for improving
the utilization efficiency of herbicide.
Figure 3. The selectivity of the P5A-functional surface to G2 droplets:
a) the G1, G2, G3, G4, G5 behavior on the P5A-modified surface tilted
to 458; b) The sliding-angle change of the different guest droplets with
varying concentrations (1.0ꢀ10À8–1.0ꢀ10À3 m) on the P5A-modified
surface showing the selective adhesion of the G2 droplet.
G5 droplets slide quickly off the surface. However, the G2
droplet stayed on the surface by the assemble adhesion. The
sliding and assemble adhesion process of the droplets with
time are shown in Figure S17. Moreover, it is found that the
sliding angle of the droplet changes with the increasing
concentration of the guest in the droplets. As can be seen in
Figure 3b, the sliding angles of the G1, G3, G4, G5 droplets
did not change significantly whereas the sliding angle of the
G2 droplet increased with the increasing guest concentration
of the droplets(10À8 m–10À3 m).
The possible mechanism is shown in Figure 4. The para-
quat molecules in droplet bind with the functional surface by
the host–guest interaction to form the stable complex with the
system energy reducing. Because the binding strength of G2
with P5A is stronger than the other guests, the G2 molecules
in droplet is easier to assemble with the P5A modified on the
surface to generate the enough binding force to resist the
droplet gravity. Hence, the G2 droplet can stay on the inclined
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21572076, 21372092), Natural
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Published online: && &&, &&&&
4
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Communications
Surface Adhesion
Stick around: The adhesion of herbicide
droplets to a leaf surface plays an
important role in the adsorption of the
herbicide by crops. A pillar[5]arene-func-
tional surface was constructed to control
the adhesion of paraquat droplets to the
surface by the dynamic self-assembly of
host–guest interactions. The ability to
attach the droplet to the surface is
promising for improving the utilization
efficiency of herbicides.
L. Luo, G. R. Nie, D. M. Tian, H. T. Deng,
L. Jiang, H. B. Li*
&&&& — &&&&
Dynamic Self-Assembly Adhesion of
a Paraquat Droplet on a Pillar[5]arene
Surface
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!