Dip-coated rapeseed meal composite as a green carrier for light-induced controlled release of pesticide

Article abstract of DOI:10.1039/d0nj02771e

In this work, we report the novel fabrication of polymerized octadecylsiloxane (PODS)/4-aminoazobenzene (AAB) coated rapeseed meal (RSM) using a facile one-pot dip-coating method. The as-prepared RSM-PODS/AAB employs RSM as a carrier, the PODS coating endows the material with hydrophobicity, thus improving the loading rate of pinoxaden (Pxd) of RSM-PODS/AAB, and the AAB moieties act as light-driven “stirrers” to stir the release of Pxd. The structure and composition were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, ultraviolet-visible spectroscopy and contact angle measurements. The experimental results indicate that the loading rate of Pxd was increased by 21.38% compared with pristine RSM. More importantly, the Pxd-loaded RSM-PODS/AAB showed perfect UV-visible and sunlight controlled release performance, the excellent biodegradability of RSM effectively supplying the nutrient elements of plant growth in the soil. Overall, this system provides a promising way to improve controlled drug release with UV-visible- and sunlight-responsive behavior.

Full text of DOI:10.1039/d0nj02771e

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DOI: 10.1039/D0NJ02771E
Dip coated rapeseed meal composite as a green carrier for
light induced controlled release of pesticide
Chenxi Yang a, b, c, d, Juan Li a, b, c, d, Jian Wang a, b, c, d, Zhen Guo a, b, c, d, Huanyuan
Wang a, b, c, d, Chendi Shi a, b, c, d, Dongwen Hua e, Ling Li *, f
a. ShaanXi Provincial Land Engineering Construction Group Co., Ltd., Xi'an 710075,
China.
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b. Institute of Land Engineering and Technology, Shaanxi Provincial Land Engineering
Construction Group Co., Ltd., Xi'an 710075, China.
c. Key Laboratory of Degraded and Unused Land Consolidation Engineering, the
Ministry of Natural Resources. Ltd., Xi'an 710075, China.
d. Shaanxi Provincial Land Consolidation Engineering Technology Research Center.
Xi'an 710075, China.
e. Shaanxi Key Laboratory of Land Consolidation. Xi'an 710021, China.
f. College of Land Engineering, Chang'an University. Xi'an 710021, China.
* Corresponding author
Email address: liling_sxdj@163.com
1

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Abstract: In this work, we report the novel fabrication of a polyDmOeI:r1i0z.1e0d39/D0NJ02771E
octadecylsiloxane (PODS)/ 4–aminoazobenzene (AAB) coated rapeseed meal (RSM)
using facile one–pot dip–coating method. The as–prepared RSM–PODS/AAB employs
the RSM as a carrier, PODS coating endows the material with hydrophobicity, thus
improving the loading rate of pinoxaden (Pxd) of the RSM–PODS/AAB and the AAB
moieties act as light–driven “stirrers” to stir the release of Pxd. The structure and
composition was characterized by Fourier transform infrared (FTIR) spectroscopy,
scanning electron microscopy (SEM), X–ray Photoelectron Spectroscopy (XPS), X–
ray diffraction (XRD), ultraviolet–visible spectroscopy and the contact angle measuring
device. The experiment results indicate that the loading rate of Pxd was increased by
21.38% compared with pristine RSM. More importantly, the Pxd-loaded RSM–
PODS/AAB showed perfect UV–vis and sunlight controlled release performance, and
the excellent biodegradability of RSM effectively supplying the nutrient elements of
plant growth in the soil. Overall, this system provides a promising way to improve the
control drug release with UV–vis and sunlight responsive behavior.
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Keywords: Rapeseed meal, Controlled release, Light–responsive, 4–aminoazobenzene
1 Introduction
Pesticides are used to solve the problem of pests, weeds, diseases, and fungus of
crops worldwide, effectively restoring agricultural productivity, which play a critical
role in worldwide agricultural production.1-4 Pinoxaden (Pxd) is one of the most widely
used herbicides globally in agriculture. Its Acetyl-CoA carboxylases inhibitory effect
take place quickly upon contact with green tissue, which causes fatty acid synthesis to
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be blocked and weeds removal. Nevertheless, a great deal of the used Pxd fail DtoOIa: 1p0p.10ly39/D0NJ02771E
the target site because of the traditional release mechanism and practical application of
spraying, resulting in the waste of Pxd. Additionally, in spite of the low environmental
toxicity of Pxd, it still poisons aquatic organisms such as algae. Therefore, with
increasing attention to environmental and agricultural issues, the most indispensable
investigation in Pxd is how to solve agricultural weeds while reducing the
contamination of ecosystems caused by conventional pesticide formulations and the
quantities required in field practical applications.5, 6
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Controlled release systems are designing a platform in which the dosage of
molecule was preseted, promoting site–specific release of pesticide and thus achieving
the desired effect. To our best knowledge, numerous pesticides controlled release
formulations (CRFs) were developed.7-9 These advanced CRFs of pesticides can
respond to external stimulation. The CRFs can release active ingredients to the soil and
crops in a more efficient and targeted method through external stimulation, thus the
CRFs could be a promising strategy for the better concentration distribution of the
pesticides on crop.10, 11 Hereby, the effective behavior greatly conquers the low
utilization rate of traditional formulations of pesticides. Recently, all sorts of CRFs have
been synthesized based on external stimuli–responsive mechanism. Generally, the
advanced CRFs of pesticides can respond to the pH12, 13, NIR14, 15, temperature16, etc.
For instance, Xu, et al15 synthesized a novel core−shell polydopamine@PNIPAm
nanocomposites (PDA@PNIPAm) nanocomposites, this nanocomposites can be
utilized as an NIR–controlled release–targeted carrier for imidacloprid. Chen, et al17
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synthesized a mesoporous silica of Chlorpyrifos/Copper(II) schiff with pH–sensitivity
for pesticide controlled release, which exhibit greatly efficient sustained–release
platform for pesticide release. Nevertheless, it has been proved that these formulations
still have limitation for commercialization due to the high cost in the substrate and
preparation process. In addition, the pH stimuli–responsive CRFs may affect the
activity of the pesticides and the physical and chemical properties of the soil during the
pH change process. Moreover, some substrates of CRFs introduce additional harmful
substances into the soil, which potentially damages the safety of soil and crop. Herein,
we envisage a novel light–controlled–release pesticides using novel carrier with good
biodegradability.
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Rapeseed meal (RSM), a very abundant natural crop waste, are mainly composed
of proteins, lignocellulosic fibers, grease, and minerals.18, 19 On average, the C, H, N
and O contents composed of RSM were 46.9%, 6.3%, 6.4% and 32.2% respectively.19
Consequently, RSM were often used as a protein complement in animal feed rations
and sometimes as fertilizer due to the plentiful sources of nitrogen and carbon nutrients.
20 Herein the RSM, a cheap agricultural waste, was rationally chosen as a substrate to
fabricate advanced biodegradable light–responsive CRFs through attaching the 4–
aminoazobenzene (AAB) particles and hydrophobic coating onto the RSM surface. The
core of such innovation lies in that the UV–vis and sunlight could induce cis and trans
isomerism, which can result in a continuous rotation–inversion of the AAB molecule,
thereby promoting the Pxd release of RSM–PODS/AAB.21 In addition, in order to
increase the loading rate of RSM on Pxd, PODS hydrophobic coating was elaborately
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decorated on the surface of RSM. The AAB particles and PODS hydrophobicDcOoI:a10ti.1n0g39/D0NJ02771E
has furnished an excellent way to endow the RSM with stimuli–responsive release
performance and high load capacity. Overall, we can further devise other advanced
light–responsive agricultural waste CRFs based on the excellent performance of AAB
and agricultural waste.
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2. Experimental section
2.1. Material.
Rapeseed meal (RSM) was obtained from the local farm (Shaanxi, China). Ethanol,
methanol, and methylbenzene were supplied by Tianjin Chemical Reagent Factory
(Tianjin, China). Pinoxaden (Pxd) was purchased from local store (Shaanxi, China). 4–
Aminoazobenzene (AAB) was purchased from Macklin (Shanghai, China).
Octadecyltrichlorosilane (OTS) was furnished by Aladdin (Shanghai, China). The
above-mentioned materials are used directly without purification.
2.2. Pretreatment of RSM
The raw RSM was cleaned ultrasonically (45KHz) in 200 mL 99.5% ethanol and
200 mL deionized water for 1.0 h. Afterwards, the RSM powders were evenly dispersed
into the 200 mL NaOH solution (2 wt%) under stirring for 1.0 h to remove the adhering
substances.22 Ultimately, the obtained RSM was washed with ethanol and deionized
water and dried at room temperature.
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2.3. One–pot dip-coating preparation of RSM–PODS and RSM–
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PODS/AAB
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One pot method is to obtain complex molecules directly without separation of
intermediates. This reaction is obviously more economical and environmentally
friendly. A typical one–pot dip-coating preparation procedure is as follows: First, the
precursor was prepared by magnetic stirring the mixture of the 50 mL of methylbenzene
and 1 mL of OTS at 15°C.23 Afterwards, the pre–RSM was immersed into the OTS
precursor by dip-coating method. The one–pot dip–coating self–assemble modification
was maintained at 15°C for 1.0 h. The RSM–PODS was immersed in methanol for
washing several times to remove the residual chemicals. The washed samples were
dried under ambient condition (temperature: 25.3 ℃; humidity: 60.5%; atmospheric
pressure: 947hPa) to obtain RSM–PODS. Ultimately, the RSM–PODS with high
adsorption capacity were successfully synthesized.
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The RSM–PODS/AAB was prepared by the similar experimental process and the
preparation procedure is as follows: the precursor was prepared by magnetic stirring
the mixture of the 50 mL of methylbenzene, 1.0 g of AAB and 1 mL of OTS at 15 °C.
The subsequent steps are the same as the RSM–PODS synthesis process.
2.4 Characterization of RSM, Pre–RSM, RSM–PODS, and RSM–
PODS/AAB
The RSM, Pre–RSM, RSM–PODS, and RSM–PODS/AAB were ground with a
mortar and taken 2 mg. Subsequently, The Fourier transform infrared (FTIR) spectra of
the as-obtained samples were analyzed in KBr using a PerkinElmer FTIR spectrometer
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in the range of 4000–500 cm–1. The microstructures of RSM, Pre–RSM, RSM–PODS,
and RSM–PODS/AAB were investigated, and 0.2 cm2 was cut. The scanning electron
microscope (SEM) (Hitachi S–4800) was used for observation after gold plating for 2
min. The X–ray Photoelectron Spectroscopy (XPS) were recorded on ESCALAB250XI
spectrometer. The crystalline structure of as–obtained samples were determined by X–
ray diffraction (XRD) using a Rigaku (RINT 2000) diffractometer. The water contact
angle (WCA) and oil contact angle (OCA) were obtained by a Krüss CCA200 contact
angle analyzer at ambient temperature and the size of the liquid droplets was fixed at 2
μL. The UV–vis spectra were obtained by the UV–Vis spectrophotometer (PerkinElmer,
Lambda 750).
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2.5 Measurements of equilibrium adsorption
To investigate the oil adsorption capacity of the hydrophobic/oleophilic RSM–
PODS/AAB, the oil absorbency of the RSM, pre–RSM, RSM–PODS, and RSM–
PODS/AAB were determined by a weighing method (i. e. tea bag method). The oil
absorbency was determined following the procedure as mentioned above and was
calculated according to the following equation:
Q = (Mt – Mi) / Mi
(1)
Where Q (g g–1) is the oil absorbency defined as grams of oil per gram of dried
samples, Mt (g) and Mi (g) are the weights of samples before and after oil adsorption,
respectively. The equilibrium adsorption capacity is the maximum adsorption capacity
of the RSM–PODS/AAB does not increase with time.
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2.6 Pxd loading Experiments
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The Pxd was loaded on RSM–PODS/AAB by simple adsorption experiments.
Briefly, 5.0 g Pxd dissolved in the 100 mL water and the suspension was stirred at room
temperature for 4.0 h. The loading procedure was conducted through simple adsorption
experiment in the dark. The Pxd–loaded RSM–PODS/AAB was determined by weight
method.
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2.7 Controlled Release Experiments
Generally, the soil acts as a medium in the release of pesticides. Therefore, the
release properties of Pxd–loaded RSM–PODS/AAB were measured in beakers
containing moisture soils. Accordingly, 2.0 g of Pxd–loaded RSM–PODS/AAB was
immersed into 250 mL beaker with a 100 g soil with 70% moisture content of moist
soil. Afterwards, the controlled release platform simulates the release behavior at
25.0 °C upon different wavelength light irradiations (UV light, vis light, UV–vis light,
sunlight, and darkness). 2.0ꢀmL of the liquid was gathered and measured
spectrophotometrically at given intervals. Pxd has a large characteristic absorption peak
at 200-230nm and the relative maximum wavelength is 210 nm, thus 210 nm can be
used as the detection wavelength to measure Pxd content. Then, the measured solution
was put back to the beakers to keep constant of the surrounding deliver medium (moist
soils). The results of accumulative release ratio were obtained according to the
following formula 22:
푊푡
Accumulative release (%) = 푊 ×100
(2)
푒
where We and Wt are the masses of the Pxd loaded in RSM–PODS/AAB platform at the
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equilibrium state and released from Pxd–loaded RSM–PODS/AAB at time t during the
release procedure, respectively. The accumulative release ratio differences of Pxd with
UV light, vis light, UV–vis light, daylight, and darkness irradiation were obtained to
evaluate the controlled release effect of the RSM–PODS/AAB with light. The
accumulative release of Pxd with darkness, UV and Vis light irradiation was evaluated
within 72 h, and the liquid were taken every 12 h for testing. The accumulative release
of Pxd from the RSM–PODS/AAB with UV–Vis light and sunlight was evaluated
within 4 and 6 h, respectively, and the liquid were taken every 0.5 h for testing.
2.8 Release kinetics investigation
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To further explore the potential controlled release mechanism of RSM–
PODS/AAB with UV–vis light and sunlight, evaluating the Pxd controlled release
kinetics is especially indispensable. Specifically, the obtained controlled release data
were then expressed to the first–order, Ritger–peppas and Higuchi models in this
section.22
First order model:
ln (Qe–Qt) = k1t + L
(3)
(4)
(5)
Ritger–peppas:
Higuchi model:
푄푡
= k2tn
푄푒
푄푡
= k3t1/2 + b
푄푒
where Qe and Qt refer to the equilibrium loading capacity (mg/g) and accumulative
release amount at any time, respectively. k1, k2 and k3 are the rate constants, t represents
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the release time (h), L is the intercept, n refer to the release exponent of controlled
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release mechanism, b is a constant.
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2.9 Pot experiments
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The soft rock and sand was collected from a Mu Us desert and the pot experiments
located in Weinan, Shaanxi Province, China (108°57´–109°26′E, 34°42′–35°06′N).
Afterwards, the pot experiment was performed with 5 kg soil in 6 L pots, in which the
same amount of gramineous Lolium perenne L was planted.24, 25 The CRFs of RSM–
PODS/AAB with Pxd were evenly sprinkled in soft rock. The number of deaths was
measured every two days after the application of different light, and the weeding rate
(WE) was calculated by equation 5:
WE = Nt/Ne × 100%
(5)
where Nt is the number of dead weeds at time t, and Ne is the initial number of weeds21.
Afterwards, the physical and chemical properties of soil were also tested before and
after the use of RSM–PODS/AAB.
3 Result and discussion
3.1 Formation of RSM–PODS/AAB
The RSM–PODS/AAB was synthesized by the following procedures: the pre–
treatment through alkali solution to remove inborn wax and adhering substances, self–
assemble of PODS onto the RSM surface and mix the photo–responsive regent AAB to
form the RSM–PODS/AAB. The proposed synthesis process and reactions mechanism
were depicted in Scheme 1.
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C18H37
C18H37
OH
OH
OH
OH
OH
Alkali treatment
Si
Si
O
OH
Primitive RSM
O
O
OH
OH
Step 1 Pre-treatment
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O
O
HN
HN
OTS
AAB
N
N
Dip coating
N
N
NH2
Step 2 Dip-coating treatment
N
N
N
N
<365nm
>435nm
UV-Vis
Light
H2N
AAB
stirrers
Pxd
isomerism
Adsorption
Cis-Trans
Step 3 Adsorption and UV-Vis
control release
Scheme 1. Propose formation and light control release mechanism of the RSM–
PODS/AAB.
In step 1, pre–treatment by alkali solution has been conducted to remove the
impurities, making the reactive functional groups of lignin and cellulose fully exposed
for the following modification.26 This is because the surface of RSM is covered with a
mount of inborn wax, which is not conducive to the modification of RSM. Step 2 is the
procedure of hydrophobic/lipophilic modification and endows the substrate with light–
responsive properties at the same time. First, OTS and AAB are dropped into a toluene
to form a precursor.27, 28 Afterwards, the reactive hydroxyl groups of OTS were formed
by in situ precursor hydrolysis. The hydrolysis formed silanol self–assemble into an
unstable head–to–head structure by rapidly diffusing onto RSM surfaces.29 Meanwhile,
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the self–assemble structure can be captured by the surface of RSM and the DreOaI:c10ti.1v0e39/D0NJ02771E
hydroxyl groups generated from OTS hydrolysis can condensation with the hydroxyl
groups of RSM. As a result, the as–formed silanol cross–linked by Si–O–Si
condensation reaction with each other and the longer–chain alkyl hydrophobic coating
of PODS was firmly adhered on the surface of RSM. At the same time of hydrophobic
modification, AAB units with good photo–induced isomerization also decorated with
the surface of RSM. AAB molecules will be evenly dispersed into OTS–toluene
solution when AAB is mixed into toluene. Afterwards, the AAB moieties could be
caught on the surface of RSM through hydrogen bond and electrostatic interaction.
Such reaction process of RSM–PODS/AAB can be obviously observed by a visible
variation of the color of the RSM from brown to tawny. Ultimately, RSM–PODS/AAB
with good hydrophobic/lipophilic properties for photo–stimulated response controlled
release formulations have been synthesized.
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Predicting from the composition and structure of the as–synthesized RSM–
PODS/AAB, this formulation might be very suitable for the development of superior
photo–stimulated response CRFs for oily pesticide (Pxd). First, the RSM pristine
substrate has excellent oil adsorption ability because of the native oil storage
performance of rapeseed.18 Besides, RSM, as a natural polymer waste, will not bring
negative effect on soil and crops when it is used as a pesticide controlled release
formulations carrier. Furthermore, RSM can be utilized as a fertilizer to supplement
sufficient nitrogen and carbon for soil and crops due to its special composition, wide
availability and biodegradability.20 Second, the prominent hydrophobic/lipophilic
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property from the low surface energy PODS coating were rational decorated on the
surface of RSM by hydrophobic/lipophilic modification. The RSM–PODS/AAB have
excellent ability to absorb organic solvents.30 Third, AAB are characteristic
photoisomerized chromophores. This excellent feature has been used in the fabrication
of a photodriven plastic motor31, high–frequency photodriven polymer oscillator32, and
stimuli–responsive CRFs.33 Nevertheless, the traditional AZO is mainly composed of
aromatic rings, and there are few functional groups on the side chain that react with the
substrate. Consequently, AAB can be used to react with the functional group of the
substrate by using the –NH2 on its side chain, making the AAB more firmly attached to
the surface of the substrate. AAB, as a AZO derivative, also has trans and cis isomers,
and they could convert to each other with UV and vis irradiation. Thus, the RSM–
PODS/AAB will arise trans–cis and cis–trans isomer transition of AAB particles with
UV–vis irradiation, which act as light–driven “stirrers” to stir the release of pesticides
from RSM–PODS/AAB system. From these points of view, the RSM–PODS/AAB
composite materials, combined the synergistically hydrophobic/lipophilic property of
PODS coating and light–motivated “stirrers” in RSM, have provided a valuable strategy
for controlled release formulations.
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3.2. Morphology observation
The surface characteristics of samples were further registered in SEM images.
Figure 1 displays the surface morphology of the RSM (a), RSM after pretreatment (b),
RSM–PODS (c) and RSM–PODS/AAB (d), respectively.
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Figure 1a exhibits the original structure of RSM and the surface are quite DsOmI:o10o.1t0h3.9/D0NJ02771E
However, there are many wrinkles in the smooth surface, which offered highly oil–
absorbing ability. Figure 1b reveals the structure of pristine RSMs after alkali–
treatment. Compared the relatively smooth of primitive RSM, the RSM exhibits a
tougher surface after alkali–treatment. Such topography variation provides solid
testimony that the inborn waxes on the surface of RSM are removed and the cellulose
and lignin were also partially destroyed, which are favor of the succedent surface
modification. The surface profiles of the RSM–PODS are demonstrated in Figure 1c,
and obviously, the covered of PODS hydrophobic coating on the surface of pristine
RSM resulted in a rough surface morphological characteristics of RSM–PODS than
RSM. Obviously, there were particles on the surface of the RSM skeleton in Figure 1d.
This novel hybrid structure of the resulted RSM–PODS/AAB has combined the merits
of both AAB moieties and PODS coating, which are conducive to increase the
hydrophobicity/lipophilicity, and endow the RSM–PODS/AAB with feature of photo–
response.
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Figure 1. Comparison SEM micrographs of RSM (a), Pre–RSM (b), RSM–
PODS (c), and RSM–PODS/AAB (d).
3.3 Interaction analyses
The surface of the dip–coating treated RSM was characterized by FTIR to validate
the chemical composition of the RSM, Pre–RSM, RSM–PODS and RSM–PODS/AAB.
In Figure 2, the peak at 3316 cm–1 were ascribed to the stretching vibration of hydroxyl
groups.18 The spectrum of the RSM displayed prominent peaks at 2931 and 1047 cm–1
are attributed to methylene groups and C–O stretching vibration, respectively. What's
more, the absorption peaks at 1529 cm–1 are assigned to the stretching vibration of the
benzene ring of lignin in RSM.34 As shown in Figure 2(d, e), the as–obtained RSM–
PODS and RSM–PODS/AAB displayed stretching adsorption peak of CH2 at 2980 and
2879 cm–1.29 Moreover, adsorption peaks appearing at 1145 cm–1 corresponded to the
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Si–O–Si bond asymmetric stretching mode, demonstrating the formation of long chain
linear polysiloxane.29, 35 Furthermore, compared with the aforementioned spectrums,
the hydrogen bonding of the RSM–PODS/AAB has the effect of shifting the N–H
deformation band to a higher frequencies. In this direction, in–plane N–H deformation
is observed at approximately 1587 cm–1.36 The bands at approximately 1403 cm–1 may
be attributed to the shift of the lower wave number region, which indicates the N=N
group was decorated onto the surface of RSM. On the whole, it can be conclude that
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the RSM–PODS/AAB has been successfully synthesized.
b
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Wavenumber ( cm-1)
Figure 2. FT–IR spectrometry analysis of RSM (a), Pre–RSM(b), RSM–PODS(c) and
RSM–PODS/AAB (d).
To shed light the effect of one–pot dip–coating strategy on the elements and
chemical structure of modified materials, X–ray photoelectron spectroscopy (XPS)
measurements were carried out. The XPS survey spectrums of the RSM, Pre–RSM,
RSM–PODS, and RSM–PODS/AAB are shown in Figure 3. Remarkably, the peak at
102.21 eV was ascribed to Si2p and corresponded to the silicon of RSM–PODS, and
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RSM–PODS/AAB, respectively. This peak is not detected in RSM, Pre–RSM.37
Further, in the high resolution of C1s scans (Fig. 3c and 3e), the binding energy at
284.6, 285.96, and 288.4 were C–C, C–O–C and O–C=O, respectively. The high
proportion of C–C bond in RSM–PODS and RSM–PODS/AAB indicate that PODS
were successfully attached to RSM. Further, the binding energy of RSM–PODS/AAB
at 398.6eV shows a unique peak (see Fig. 3d and 3f). Namely, this nitrogen peak can
be attribute to the –N=N–.38, 39 From this point of view, the long chain siloxane moieties
and AZO–units originating from PODS and AAB have thus been successfully adhered
on the surface of RSM after dip coating modification.
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RSM-PODS
RSM-PODS/AAB
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Binding energy ( eV)
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C-NH2
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7000
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C-NH2
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Binding energy ( eV)
Figure 3. Elemental analysis was performed with XPS of the RSM, Pre–RSM, RSM–
PODS, and RSM–PODS/AAB. (a), (b) Survey spectrum of XPS. (c, d) and (e, f) were
the high–resolution scans of C 1s and N 1s of the RSM and RSM–PODS/AAB,
respectively.
The degree of surface substitution (the number of silyl groups per glucose unit,
DSS) of PODS can be obtained from the surface Si atomic concentrations of RSM–
PODS/AAB. The Si atomic concentration of surface derived from the XPS survey scans
of atomic concentrations (Table S1). The atomic concentrations of Si% is x. MRGU is
the molecular weight of a repeating glucose unit (RGU), MSi is the molecular weight of
silicon and Mgroup grafted is the molar weight of the OTS hydrolysis and grafted through
the silylation. The DSS of the RSM–PODS/AAB is from the equation of DSS =
{MRGU·x/(100·MSi-Mgroup grafted·x)} = {162x/(2800-282x)}.40 Table S2 shows the DSS
results of RSM–PODS/AAB derived from the Si.
To further clarify the RSM–PODS/AAB was synthesized, the XRD profile of RSM
and RSM–PODS/AAB were recorded, respectively. From XRD spectrums profile
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(Figure. 4), the pristine RSM was milled enough, thus resulting in a fairly low
crystallinity.41 Namely, the bread–like diffraction peak states clearly that the original
RSM can be ascribed to amorphous structure.42, 43 Additionally, RSM–PODS/AAB
exhibited 2θ peak at 21.36°. From the law of Bragg, d = λ/(2 sin θ), with λ = 1.5418 Å
and d = 4.16 Å. The value of 4.16 Å is relative close to the chain–chain spacings
observed in dense solid phase of alkyl chains.27, 44 The sharp peak of the crystal
probably came from hydrogen bonds between hydroxyl groups on PODS and RSM
surface as well as the nitrogen on AAB moieties, which represent certain degree of
crystallinity and prominent compatibility of PODS/AAB with the RSM after dip–
coating.45 XRD pattern provided solid evidence that we successfully synthesized the
RSM–PODS/AAB by dip–coating.
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2 
Figure 4. X–ray powder diffraction pattern of RSM and RSM–PODS/AAB.
3.4 The adsorption of oil
RSM, as the waste of oil extraction, can still absorb oil because of its internal pore
structure and oil affinity. However, the inherent low oil adsorption performance is not
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sufficient as a carrier for oil–based pesticide CRFs. Therefore, it is necessary to modify
the RSM to improve its oil affinity. According to the theory of Cassie, an excellent oil
absorbent materials should have a surface composed of low surface energy and high
roughness.46 Based on this theory, for the sake of excellent oil adsorption performance,
we introduced long–chain alkyl hydrophobic coating for the RSM through OTS
modification. As shown in Figure 1 and Figure 2 (SEM and FTIR), the long chain alkyl
and rough coating were decorated on the surface of RSM. The water contact angle
(WCA) and oil contact angle (OCA) are shown in Figure 5a. From Figure 5a, the WCA
and OCA of RSM are 12 and 10°, respectively, which indicate that RSM is an
amphiphilic material. Obviously, the WCAs and OCA of Pre–RSM has not changed
significantly. This phenomenon could be ascribed with no apparent variation in surface
composition. Further, the WCA and OCA of RSM–PODS are 125 and 4°, respectively,
suggesting the hydrophobic coating was introduced to the surface of RSM. The WCA
and OCA of RSM–PODS/AAB are 120 and 5°, respectively. This phenomenon may be
attributed to the fact that the –NH2 functional group on the surface of AAB reduces the
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hydrophobicity
of
the
RSM–PODS.
However,
the
decrease
of
hydrophobicity/lipophilicity does not affect the overall wettability of the RSM–
PODS/AAB due to the low content of AAB, thereby resulting the relatively strong
hydrophobicity and lipophilicity.
In consideration of practical application of loading rate of pesticides, we still need
to evaluate the oil adsorption performance of RSM–PODS/AAB. The oil adsorption
capacities of RSM, Pre–RSM, RSM–PODS, and RSM–PODS/AAB are shown in
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Figure 5b. It is obvious the optimal oil absorbency of RSM, Pre–RSM, RSM–PODS,
and RSM–PODS/AAB were 16.1, 16.4, 20.5, and 19.5 g/g, respectively. To our best
knowledge, the fabrication of rough construction can improve the hydrophobicity of
materials. Sticking to this principle, the treatment of RSM with alkali destroys the
structure of cellulose, lignin and other substances on the surface of the material, which
makes the surface of Pre–RSM rough and improves its adsorption capacity.
Additionally, the facile strategy of dip–coating treatment by OTS precursor solution
has fabricated a rough structure and desirable hydrophobic long alkyl chains, which
improves oil affinity. Considering this excellent property, the oil absorbency of RSM–
PODS and RSM–PODS/AAB is higher than that of unmodified RSM and Pre–RSM.
Furthermore, the optimal adsorption capacity of RSM–PODS/AAB is lower than
RSM–PODS. The phenomenon can be explained by RSM–PODS/AAB composition.
From the composition of RSM–PODS/AAB, there are hydrophilic –NH2 groups on the
surface of RSM–PODS/AAB. The –NH2 functional groups reduce the hydrophobicity
of the RSM–PODS/AAB, thus reducing the ability to load the organic solvent. From
Figure S1, the Pxd loading capacity of as–obtained materials is similar to the oil
adsorption capacity, which indicates that the RSM–PODS/AAB carrier is an excellent
candidate for oil-based pesticides.
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a
WCA
OCA
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Time ( min)
Figure 5. (a) The different contact angle of oil and water on different materials. (b)
The oil loading capacity of RSM, Pre–RSM, RSM–PODS, and RSM–PODS/AAB at
different times by tea bag method.
3.5 Photo–responsive release behavior and mechanism investigation of
Pxd
Benefit from the merit of controlled drug delivery, stimuli–responsive CRFs have
attracted much attention recently. Among numerous stimulus–responsive substance,
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AAB units possessed trans and cis isomers, which could switch to each other under
different illumination.47 This substance with special properties exhibited well–
controlled photo–responsiveness in CRFs. Base on the above consideration of AAB,
we investigated the UV–vis absorption spectra of AAB in ethanol. As shown in Figure
6a, the peak in the UV region decreased when AAB molecule was irradiated by UV
light. Meanwhile, the peaks at vis region increased, this special phenomenon indicate
the UV light could induce trans–cis transition of AAB units.48 Further, the peak in the
UV region increases and the peak in the vis region decreases under vis irradiation.
Namely, the opposite phenomenon occurs with visible light irradiation. As a result, the
trans–cis and cis–trans isomerization induced by different illumination contributed to a
continuous rotation–inversion motion of AAB units which could act as molecular
impellers to facilitate pesticides release from RSM–PODS /AAB.21
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Figure 6. UV–vis spectrum of AAB in ethanol under different light duration (a) UV
(365 nm) irradiation for 0, 30, 60, and 120 min. (b) Vis (435 nm) irradiation for 0, 30,
60, and 120 min.
In order to clarify the release behavior of RSM–PODS/AAB, we further
investigated the light–triggered Pxd controlled release effect of RSM–PODS/AAB. As
can be seen from Figure 7a, the accumulative release rate of Pxd was only 83% within
72 h when the RSM–PODS/AAB was placed in the darkness. Nevertheless, the
accumulative release rate of Pxd was both 90% within 72 h when the RSM–PODS/AAB
was placed in the UV and vis light. Consequently, the result of accumulative release
ratio indicates AAB moieties can only produce one–way isomerization under the
irradiation of UV or vis light, which could not be used as “stirrers” to accelerate Pxd
release. To further validate the “stirrers” performance of AAB molecules in RSM–
PODS/AAB, UV–vis and sunlight were used to stimulate the as–obtained material.
Figure 7b reveals the result of accumulative release ratio with UV–vis and sunlight
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irradiation, respectively.
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The accumulative release ratio can reach 95% in 4 h with UV–vis irradiation. This
phenomenon could be ascribed to the continuously reverse photoisomerization of AAB
moieties driven by UV–vis light, which accelerate the release of Pxd.49 Additionally,
the loaded Pxd was released, and the accumulative release rate reached up to 92% in 6
h with sunlight illumination. These results are well in line with the stimulus release
through UV-vis light. Evidently, this phenomenon might be assigned to the wide
spectrum window (290–3200 nm) of the natural sunlight.50 Compared with UV–vis, the
AAB units exhibited comparably slow photoisomerization with sunlight irradiation,
which is not enough to stimulate AAB units and act as molecular impellers to facilitate
Pxd release from RSM–PODS/AAB. Nonetheless, the increase of temperature can
increase the reaction rate from cis–to–trans from the theory of Neeraj51 and Hideko52.
Namely, high temperature is favorable for AAB to play the role of agitator, thus
speeding up the cumulative release ratio. From this point of view, the increase of
accumulative release ratio may be ascribed to the thermal radiation effect under sunlight
irradiation.
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Figure 7. The accumulative release ratio of Pxd from RSM–PODS/AAB with
different light irradiation. (a) darkness, UV and Vis. (b) UV–Vis, and sunlight. (c)
Release profiles of Pxd from the RSM–PODS/AAB with UV–Vis light and darkness
under constant temperature of 308.15K.
In addition to the UV–vis or sunlight–sensitive property, the thermal cis–to–trans
isomerization feature of AAB units were also harnessed to endow RSM–PODS/AAB
with a little thermal–responsive property.53 To investigate the controlled release
performance of Pxd by thermal stimulation, the temperature of the controlled release
experiment was maintained at 308.15 K and the accumulative release ratio of Pxd was
measured by using UV–vis spectrophotometer, as plotted in Figure 7(c). The
accumulative release of Pxd can reach 90% in 72 h at 308.15 K in the absence of UV–
vis irradiation. This result is similar to the release rate under UV or vis light, indicating
that the Pxd molecules can be released rapidly by heat–stimulation. Additionally, the
accumulative release of Pxd can reach 97% in 4 h under UV–vis radiation at 308.15 K,
this result is higher than that of only UV–vis light irradiation. The thermal–triggered
29