Positive-Charge Functionalized Mesoporous Silica Nanoparticles as

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Positive-Charge Functionalized Mesoporous Silica Nanoparticles as Nanocarriers for Controlled 2, 4Dichlorophenoxy Acetic Acid Sodium Salt Release Lidong Cao, Zhaolu Zhou, Shujun Niu, Chong Cao, Xiuhuan Li, Yongpan Shan, and Qiliang Huang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b01957 • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on June 25, 2017

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Journal of Agricultural and Food Chemistry

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Positive-Charge Functionalized Mesoporous Silica Nanoparticles as Nanocarriers for Controlled 2, 4-Dichlorophenoxy Acetic Acid Sodium Salt Release

5

Lidong Cao,†,ǁ Zhaolu Zhou,†,ǁ Shujun Niu,‡ Chong Cao,† Xiuhuan Li,† Yongpan

6

Shan,† and Qiliang Huang*,†

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†

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Institute of Plant Protection, Chinese Academy of Agricultural Sciences, No. 2

9

Yuanmingyuan West Road, Haidian District, Beijing 100193, P. R. China.

1 2 3

Key Laboratory of Integrated Pest Management in Crops, Ministry of Agriculture,

10

‡

11

Nongkeyuan New Village, An'ning District, Lanzhou 730070, P. R. China.

Institute of Plant Protection, Gansu Academy of Agricultural Sciences, No. 1

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*Correspondence author: [email protected]; tel./fax: +86-10-6281-6909

14 15 16 17 18 19 20

ACS Paragon Plus Environment


Due

to its relatively high

water solubility and

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mobility,

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ABSTRACT:

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2,4-dichlorophenoxy acetic acid (2,4-D) has a high leaching potential threatening the

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surface water and groundwater. Controlled release formulations of 2,4-D could

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alleviate the adverse effects on the environment. In the present study, positive-charge

25

functionalized mesoporous silica nanoparticles (MSNs) were facilely synthesized by

26

incorporating trimethylammonium (TA) groups onto MSNs via a post-grafting

27

method. 2,4-D sodium salt, the anionic form of 2,4-D, was effectively loaded into

28

these positively charged MSN-TA nanoparticles. The loading content can be greatly

29

improved to 21.7% compared to using bare MSNs as a single encapsulant (1.5%).

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Pesticide loading and release patterns were pH, ionic strength and temperature

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responsive, which were mainly dominated by the electrostatic interactions. Soil

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column experiments clearly demonstrated that MSN–TA can decrease the soil

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leaching of 2, 4-D sodium salt. Moreover, this novel nano-formulation showed good

34

bioactivity on target plant without adverse effects on the growth of non-target plant.

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This strategy based on electrostatic interactions could be widely applied to charge

36

carrying agrochemicals using carriers bearing opposite charges to alleviate the

37

potential adverse effects on the environment.

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KEYWORDS: Mesoporous silica nanoparticles, 2, 4-D sodium salt, electrostatic

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interaction, controlled release, soil leaching, bioactivity

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INTRODUCTION

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The extensive use of pesticides in agriculture has contributed significantly to

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farmers’ income and food productivity. However, depending on the mode of

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application and environmental conditions, more than 90% of the applied pesticides are

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either lost in the environment or unable to reach the target area required for pest

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control, which not only increases the cost but also brings about adverse impacts on the

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environment.1 The 2,4-dichlorophenoxy acetic acid (2,4-D) is one of the most

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commonly used herbicides worldwide for post-emergence control of broad-leaf weeds

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due to its low cost and good selectivity.2 Due to its relatively high water solubility,

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2,4-D exists predominantly in anionic form and is weakly retained by soil particles.3

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Therefore, it has a high leaching potential threatening surface water and groundwater

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particularly if heavy rains occur shortly after herbicide application.4 To overcome this

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concern, the development of controlled release formulations (CRFs) of 2,4-D,

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especially with nanomaterials as carriers, could be advantageous because CRFs allow

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the use of minimal amounts of herbicide for the same period of activity, which will

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reduce the leaching potential and environmental pollution.5

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The performance of CRFs on controlling the release of herbicide is closely

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related to the carrier materials. Many natural inorganic and organic polymers have

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been used to prepare 2,4-D CRFs. Clay minerals including layered double

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hydroxides,3 cationic surfactant-modified Arizona montmorillonites6 and bentonites,7,

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8

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and controlled release of 2,4-D from granule matrix formulations based on lignins,10

and organo-palygorskite9 are the most extensively investigated materials. Adsorption



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activated carbon,11 and biochars12 have also been studied. Ethylcellulose as

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microsphere matrix and Gelatin–Arabic gum complex as an envelope for CRFs of

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2,4-D was elaborated by double encapsulation using a solvent evaporation technique

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followed by the complex coacervation method.13 In addition to physical encapsulation

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or absorption in polymeric matrix, chemical combination of 2,4-D with polymers

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through covalent bonding provides another useful strategy for controlled release of

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active ingredient. The 2,4-D was chemically caged by coupling with photoremovable

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protecting groups of coumarin or perylene-3-ylmethanol derivatives, and controlled

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release of 2,4-D was achieved by irradiating the caged compounds using UV-vis

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light.14-16

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Since Mobil’s discovery of MCM-41,17 research on mesoporous silica

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nanoparticles (MSNs) has gained worldwide interest due to MSNs’ remarkable

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properties, such as low cost, facile preparation, biocompatibility, large specific surface

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area, tunable pore size for high loading capacity, and ability for targeted and

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controlled release of cargo molecules with surface functionalization and polymer

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coatings.18-20 Taking advantage of these unique properties, MSNs have attracted

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widespread interest and are an ideal scaffolding for delivery systems. A slow release

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formula of biological pesticide pyoluteorin was prepared using mesoporous silica as

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carriers, giving an example of putting an unstable compound inside the pores to avoid

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its fast degradation.21 The insecticide imidacloprid was effectively loaded into

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unmodified MSNs for termite control, and the effect of pore size, specific surface area

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and mesoporous structure on uptake and release of biocide was systematically


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studied.22 Prado reported nanosized silica modified with carboxylic acid as a support

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for the controlled release of the herbicides 2,4-D and picloram.23 For controlled

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release in response to external stimuli, a novel redox-responsive decanethiol

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gatekeeper was grafted onto MSNs to mediate the delivery of salicylic acid with

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glutathione reducing agent.24 Other pesticides, such as metalaxyl,25 tebuconazole,26

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vancomycin,27 and essential oil components28 were physically or chemically

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combined with MSNs for controlled release. Recently, we have developed quaternized

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chitosan-capped mesoporous silica nanoparticles as nanocarriers for controlled

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pyraclostrobin release.29

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Non-covalent (e.g. hydrophobic, hydrogen bonding, and ionic) interaction

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between cargo molecules and carrier material mainly affect the loading capacity and

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release profile.30, 31 Although research on the controlled release of pesticides using

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MSNs as carriers has had some progress, the driving forces for pesticide loading and

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release in most studies are hydrogen bonding and hydrophobic interactions. Ionic

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interactions are the long-range interactions that involve the electrostatic attraction and

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repulsion between oppositely-charged ions. These are studied less and therefore are

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poorly exploited as tool for achieving satisfactory loading and controlled release of

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pesticides. Taking full advantages of ionic interactions, positive charge functionalized

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MSNs have been used for drug and gene delivery.32, 33 For charge carrying pesticide

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molecules, the loading content should be expected to increase by strengthening the

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electrostatic attraction through a modification of the surface of MSNs to bear more

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opposite charges.



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During the pursuit of ideal carrier for controllable loading and release of 2,4-D to

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alleviate the adverse effects on the environment, positive charge functionalized MSNs

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have to be ideal carriers for 2,4-D sodium salt, the anionic form of 2,4-D. In the

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present study, we report the synthesis of positively charged MSN (MSN-TA) by

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incorporating trimethylammonium (TA) functional groups onto the pristine MSN

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(P-MSN), and the adsorption of 2,4-D sodium salt into MSN-TA samples. The

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parameters that impact loading content such as the ratio of carrier to pesticide,

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temperature, pH value, various anions, as well as the release profiles were studied.

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Furthermore, the ability to decrease soil leaching was studied in comparison to the

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free pesticide 2,4-D sodium salt. Finally, we investigated the bioactivity of this

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nano-sized CRFs of 2,4-D sodium salt against one dicot target plant cucumber

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(Cucumis sativus L.) and one monocot non-target plant wheat (Triticum aestivum L.).

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MATERIALS AND METHODS

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Materials. N-Trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (50%

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in methanol), cetyltrimethylammonium bromide (CTAB, 99%), and 2,4-D-sodium salt

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monohydrate (98%) were purchased from J&K Scientific Ltd., Beijing, China.

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Tetraethyl orthosilicate (TEOS, 99%) was purchased from Fluorochem Ltd., Hadfield,

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UK. All other chemicals and reagents were commercially available and used without

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further processing. Deionized water was obtained from a Milli-Q water system

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(Millipore Corporation, Bedford, MA, USA) and was utilized for all reactions and

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treatment processes.


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Synthesis of Positive-Charge Functionalized MSN (MSN-TA). The MSN-TA

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sample with a hexagonal well-ordered pore structure was synthesized from pristine

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MSN (P-MSN) by post-grafting during a two-step preparation. The first step of

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P-MSN synthesis used a sol-gel method reported by Radu with minor modifications.34

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Briefly, 3.0 g of CTAB was dissolved in 2000 mL of water, and then 10.5 mL of 2.0

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M sodium hydroxide was slowly introduced into the CTAB solution at room

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temperature under constant stirring with the stirring rate of 800 r/min. The mixture

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was heated to 80°C in an oil bath, and then 15.0 mL of TEOS was added dropwise.

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The solution was stirred vigorously for 6 h at 80 °C. The white solid that formed

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during the process was collected, washed several times with ethanol and water, and

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dried at 80°C overnight in an oven. To remove the surfactant, the as-synthesized white

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powder was calcined at 550 °C for 5 h.

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Positive-charge functionalization used a post-grafting synthesis according to

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amino-functionalized mesoporous silica with a little modification.35 Specifically, 0.5 g

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of P-MSN was re-suspended in 20 mL of anhydrous toluene solution. After vigorous

146

stirring

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N,N,N-trimethylammonium chloride solution (50% in methanol) was added. The

148

resulting mixture was refluxed for 4 h under vigorous stirring. Samples were collected

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by centrifuging at 10,000 rpm for 10 min, washed, and re-dispersed with deionized

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water and ethanol several times. The nanoparticles were dried at 80°C overnight in an

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oven.

152

for

30

min,

the

2.0

mL

of

N-trimethoxysilylpropyl-

Characterization. Fourier transform infrared (FTIR) spectroscopy was



153

conducted on a spectrometer (NICOLET 6700, Thermo Fisher Scientific, Waltham,

154

MA, USA) with a potassium bromide pellet and recorded over the spectral region of

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400 to 4000 cm−1 at a spectral resolution of 4 cm-1. Thermogravimetric analysis (TGA)

156

was performed with a Perkin Elmer Pyris Diamond (Woodland, CA, USA) from 30 to

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550 °C at 20 °C/min under a N2 atmosphere.

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The specific surface area and pore characteristics of the samples were studied by

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determining the nitrogen adsorption using a specific surface area and pore size

160

analyzer (TriStarII 3020, Micromeritics Instruments Corp, Norcross, GA, USA) at

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−196 °C. The sample was outgassed at 10-3 Torr and 120 °C for about 6 h prior to the

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adsorption experiment. From the adsorption data, the Brunauer-Emmett-Teller (BET)

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equation36 was used to calculate the specific surface area at a relative pressure (P/P0)

164

of 0.06−0.22; the Barrett-Joyner-Halenda (BJH) model37 was used to estimate the

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pore size distribution from the desorption branches of the isotherms.

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The morphology and particle size of the prepared samples were characterized

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using scanning electron microscopy (SEM, SU8000, Hitachi Ltd., Tokyo, Japan,

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operated at 10 kV) and transmission electron microscopy (TEM, Tecnai G2, F20

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S-TWIN, FEI, Oregon, USA, with an accelerating voltage of 200 kV). For SEM

170

observations, the samples were gold-plated and dried under vacuum prior to imaging.

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The average particle size was determined by statistical analysis of the SEM images of

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more than 200 nanoparticles. For TEM analysis, specimens were prepared by

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dispersing the as-obtained powder in water and then placing a drop of the suspension

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onto carbon-coated copper grids with air drying.


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The zeta potential measurements were performed using distilled water as a

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solvent on a ZetaSizer Nano ZS Analyzer (Zetasizer Nano ZS, Malvern Instruments

177

Ltd., Malvern, UK). The samples were prepared at 1 mg mL-1 to make intensities in

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the range suitable for scattering. Different pH values were adjusted by the addition of

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0.01 M HCl or NaOH. Before measurement, each sample was ultrasonicated for 5 min

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to prevent any aggregation.

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X-Ray photoelectron spectroscopy (XPS) was conducted on a photoelectron

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spectrometer (ESCALab 250Xi, Thermo Fisher Scientific, USA) using 150 W

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monochromatic Al Kα radiation (1486.6 eV, 500 µm spot size) as the excitation source;

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all binding energies were calibrated by the C1s peak of the surface adventitious

185

carbon at 284.8 eV.

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Loading of 2,4-D Sodium Salt into MSN-TA Samples. A typical procedure for

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loading 2,4-D sodium salt into MSN-TA samples followed the procedure reported by

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Lee et al with minor modification.32 Specifically, about 30 mg of MSN-TA were

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dispersed in aqueous solution of 2,4-D sodium salt with different concentrations (1.0

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mL) followed by another 4.0 mL of water. The suspension was stirred at room

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temperature for 4 h and centrifuged at 10,000 rpm for 10 min. The nanoparticles were

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collected and washed one time with 1 mL of water, and were freeze-dried with

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vacuum freeze dryer under -40°C. The 2,4-D sodium salt-loaded MSN-TA sample

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was denoted as 2,4-D sodium salt@MSN-TA. The amount of unloaded 2,4-D sodium

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salt in the supernatant and washes were determined using high performance liquid

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chromatography (HPLC, 1200-DAD (Diode Array Detector), Agilent, Santa Clara,



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CA, USA). The loading content (LC) of 2,4-D sodium salt was determined by an

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indirect method. The difference between the amount of 2,4-D sodium salt initially

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employed and its content in the combined supernatant and washes was defined as the

200

amount of pesticide entrapped. The LC (%) of 2,4-D sodium salt was calculated as

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follows: LC (%) = (weight of pesticide entrapped in nanoparticles/weight of

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nanoparticles) × 100%. The encapsulation efficiency (EE) of 2,4-D sodium salt was

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calculated as follows: EE(%) = (weight of pesticide entrapped in nanoparticles/initial

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weight of pesticide) × 100%.

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The HPLC operating parameters were as follows: Eclipse Plus C18

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reversed-phase column (5 µm × 4.6 mm × 150 mm); column temperature: 30°C;

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mobile phase: (acetonitrile: 0.2% formic acid aqueous solution (V/V) = 75:25); flow

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rate: 1.0 mL/min; and DAD signals: 284 nm.

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In vitro Release of 2,4-D Sodium Salt. About 20 mg of 2,4-D sodium

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salt@MSN-TA nanoparticles were dispersed in 2.0 mL of release medium in dialysis

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bags (MWCO: 2,000 g/mol). The dialysis bag was placed into 200 mL of release

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medium in a D-800LS dissolution tester (Tianjin University, Tianjin, China) at a

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stirring speed of 100 rpm. The release medium was water at different pH values via

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diluted HCl and NaOH. To study the effects of ionic strength on the release profiles, a

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NaCl aqueous solution (0.1 M with pH of 6.8) was used as a release medium. To

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study the temperature effect, pure water at different temperatures ( 20, 30 and 40°C)

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was adopted as release medium. The accumulative release profile of 2,4-D sodium

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salt was calculated by measuring the concentrations of 2,4-D sodium salt dissolved in


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the release medium at different times. To measure the concentration, 1.0 mL of release

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medium was withdrawn at a given time intervals for HPLC analysis followed by

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supplying the same volume of fresh release medium to ensure the same total solution

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volume. The accumulative 2,4-D sodium salt released was calculated according to the

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following equation: n-1

Ve ∑ Ci +V0Cn Er =

224

i= 0

mpesticide

× 100%

225

where Er is the accumulative 2,4-D sodium salt released (%) in regard to loaded

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pesticide; Ve is the sampled volume taken at a predetermined time interval (Ve = 1.0

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mL); Cn (mg/mL) is the 2,4-D sodium salt concentration in release medium at time n;

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V0 is the volume of release solution (200 mL); The mpesticide (mg) is the total amount of

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pesticide entrapped in the nanoparticles. The measurements were performed in

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triplicate.

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Soli Leaching Experiment. The performance of nano-carrier of MSN-TA in

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reducing the leaching of 2,4-D sodium salt was evaluated with leaching experiments

233

through a soil column. The soil was collected locally from a depth of 0 to 15 cm, dried

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in the shade, ground to pass through a 2-mm sieve, and stored in polythene bags at

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room temperature. It has a pH of 7.2 and organic matter content of 4.0%. The

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composition was sand (85.3%), clay (1.7%), and silt (13%). Glass columns (20 cm

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length, 2.5 cm diameter) were packaged uniformly with air-dried soil, which occupied

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about 12 cm of the column.38 The top 2 cm were filled with quartz sand, and the

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bottom 2 cm with other quartz sand plus glass wool to minimize losses of soil and

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contamination of leachates with soil particles. Before pesticide application, columns



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were conditioned by passing them with water using a flow rate of 1 mL min-1

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controlled by peristaltic pump. This was followed by pesticide soil leaching

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experiments. The 2,4-D sodium salt@MSN-TA or 2,4-D sodium salt were evenly

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dispersed in 500 mg of silica gel (corresponding to ~20 mg active ingredient), and

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then were placed on the top of the columns. Another 1 cm of quartz sand was

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uniformly placed on top of the pesticide. Every 10 mL of column leachate in each

247

experiment was collected, and 2,4-D sodium salt was determined using HPLC. The

248

accumulative 2,4-D sodium salt leached was calculated according to the following

249

equation: n

Ve∑ Ci 250

El =

i=1

mpesticide

× 100%

251

where El is the accumulative 2,4-D sodium salt leached (%) in regard to pesticide

252

applied; Ve is the leached volume at every sample time (Ve = 10 mL); Cn (mg/mL) is

253

the 2,4-D sodium salt concentration in leachate at sample time n; The mpesticide (mg) is

254

the total amount of pesticide applied. The measurements were performed in triplicate.

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Bioactivity of 2,4-D Sodium Salt@MSN-TA Nanoparticles. The bioactivity of

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2,4-D sodium salt@MSN-TA nanoparticles were assessed by means of a laboratory

257

bioassay with one target dicot plant cucumber (Cucumis sativus L.) and one monocot

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non-target plant wheat (Triticum aestivum L.). For cucumber bioassay experiments, 9

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cm Petri dishes with a filter paper were used, according to the laboratory bioassay of

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fluorescent 2,4-D derivatives against Vigna radiate reported by Atta et al.14 Fifteen

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similarly sized germinated seeds were placed in each Petri dish moistened with 10 mL

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of 2,4-D sodium salt@MSN-TA samples at a concentration equal to the field


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application rate (0.6 kg/ha). Control was similarly performed with the same amount of

264

2,4-D sodium salt free technical and pure distilled water. Each treatment was

265

performed in triplicate. The Petri dishes were incubated in a light growth chamber

266

with a 12 h photoperiod with a light intensity of 80 µmol photon m-2s-1 provided by

267

fluorescent lamps. Day and night temperatures were set at 26 and 15°C, respectively,

268

and the humidity was kept at 80%. Each Petri dish was moistened with an equal

269

volume of distilled water for daily watering. After 10 days of incubation, the root

270

length and fresh weight were recorded to evaluate the bioactivity on the target plant.

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For the non-target plant wheat, the bioactivity was tested on pots (10 cm high

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with diameter of 9.0 cm) filled with 240 g of soil, according to the bioassay of 2,4-D

273

nanoformulations against Zea mays reported by Abigail et al.39 Six seeds of wheat were

274

sown in each pot and grown in the greenhouse. The pots were watered daily with 10

275

mL of distilled water. One week after germination, the solution of 2,4-D sodium

276

salt@MSN-TA samples was applied post-emergence at an application rate of 2.5

277

kg/ha corresponding to the maximum application dose recommended for field

278

application of 2,4-D.6 The same amount of free 2,4-D sodium salt technical and

279

treatment without herbicide were used as controls. Each treatment was replicated

280

three times. One week after herbicide application, the plant height and fresh weight of

281

the aerial part of the wheat were determined to monitor non-target plant responses to

282

the nano-formulation.

283

Statistical Analysis. Statistical analysis of the values was conducted using SPSS

284

10.0 (SPSS Inc., Chicago, IL, USA) software. Statistical analysis was performed



285

using one way analysis of variance (ANOVA) followed by Duncan’s multiple range

286

test (DMRT). The values are mean ± SD for three determinations in each group. P

287

values ≤0.05 were considered as significant.

288 289

RESULTS AND DISCUSSION

290

Preparation and Characterization of MSN-TA Nanoparticles. P-MSN was

291

synthesized via a liquid crystal templating mechanism using CTAB as the

292

structure-directing agent and TEOS as the silica source under basic conditions.

293

Positive-charge functionalized MSNs (MSN-TA) were fabricated through a

294

post-grafting strategy by incorporating TA group onto P-MSN. Figure 1 shows the

295

schematic illustration of the synthesis of MSN-TA nanoparticles. The morphology of

296

MSN-TA nanoparticles was observed using SEM and TEM (Figure 2). The SEM and

297

TEM micrographs show that the as-synthesized MSN-TA nanoparticles exhibited

298

spherical morphology with a relatively smooth surface and an average particle size of

299

about 423 nm. Particle diameters were estimated by statistical analysis of the SEM

300

images of randomly selected 300 nanoparticles. The histograms of particle size

301

distributions of MSN-TA are shown in Supporting Information (Figures S1). Well

302

ordered mesoporous structures with hexagonal arrays (Figure 2D) and straight lattice

303

fringes (Figure 2E) can be seen when the electron beam is parallel and perpendicular

304

to the pore axis, which is the characteristic of MCM-41-type MSN.40 Good

305

monodispersity was achieved due to the electrostatic repulsion between positively

306

charged particles from the introduction of quaternary ammonium groups.


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307

Incorporation of TA functional groups onto the P-MSN samples did not obviously

308

affect the P-MSN’s morphology and size (data not shown).

309

The FTIR spectra of P-MSN and MSN-TA are shown in Figure 3. The 1087 cm−1

310

broad absorption band found in P-MSN and MSN-TA was attributed to characteristic

311

Si–O–Si (siloxane) stretching vibrations. The absorption band at 1478 cm-1 in

312

MSN-TA could be assigned to the C–H bending vibration of –N(CH3)3+, confirming

313

the conjugation of TA groups on P-MSN.

314

TGA is frequently used to study the thermal stability and decomposition pattern

315

of chemicals and materials. Figure 4 displays TG analysis of P-MSN and MSN-TA.

316

Calcined P-MSN is thermo-stable and maintains a constant weight in the temperature

317

ranges studied here. About 15% weight loss is clearly seen in the TGA curve of

318

MSN-TA, and this is mainly due to the decomposition of organic TA groups, which is

319

further evidence that P-MSN was successfully grafted with positively charged TA

320

groups.

321

XPS provides valuable information about the elements on the MSN surface. The

322

XPS spectra given in Figure 5 display bands assigned to elements in P-MSN and

323

MSN-TA. In Figure 5A, binding energies of about 104.1 and 533.4 eV are assigned to

324

the Si2p and O1s in P-MSN, respectively. The weak signal at 284.8 eV corresponds to

325

the C1s originated from the residual carbon after calcination for removing the

326

template. Figure 5B shows the signals at 402.7, 286.2, and 197.6 eV corresponding to

327

the N1s, C1s and Cl2p, respectively, which confirmed successful incorporation of TA

328

functional groups onto P-MSN. The XPS results show that the atomic percentage of N



329

Page 16 of 39

is 3.27%.

330

BET specific surface area and BJH pore size and volume analysis were used to

331

explore the nanoparticles’ mesoporous structure. The values for the BET specific

332

surface area (SBET), the total pore volume (Vt), and the BJH pore diameter (DBJH) are

333

summarized in Table 1. Nitrogen adsorption–desorption isotherms and pore-size

334

distribution of P-MSN and MSN-TA are shown in Figure 6. The P-MSN samples

335

display a type IV isotherm curve with a sharp increase in volume adsorbed between

336

0.3 and 0.4 of P/P0 shown in Figure 6A, which is characteristic of a well-defined

337

mesoporous structure. The BET specific surface area reduced from 1356.0 to 956.4

338

m2/g after the TA groups were grafted, while the pore volume decreased from 1.65 to

339

0.59 cm3/g suggesting that part of the pores were blocked with the TA groups.

340

However, the MSN-TA nanoparticles also have a narrow pore-size distribution with

341

an average pore diameter of 2.54 nm according to Figure 6B. The adsorption of 2,4-D

342

sodium salt into MSN-TA samples was related to extreme decreases in pore volume

343

(0.59 to 0.03 cm3/g). This decrease occurred because the nanochannels were almost

344

fully occupied by the pesticide molecules, which left little space for nitrogen

345

adsorption.

346

The zeta potential measures residual charges on the surface of nanoparticles. It is

347

an indicator for the quaternary ammonium groups on the surface of MSN. The

348

magnitude of the zeta potential is very important in determining the stability of

349

nanoparticle systems. Generally, nanoparticles having the zeta potential values higher

350

than +30 mV or lower than -30 mV are considered as stable systems.41,


42

The

Page 17 of 39


351

presence of quaternary ammonium may produce higher zeta potential values. The zeta

352

potential of P-MSN and MSN-TA samples in solutions at pH 3, 7, and 9 was

353

measured. At pH 3.0, both the P-MSN and MSN-TA showed the positive zeta

354

potential values of +7.2 and +67.8 mV, respectively. At pH 7.0, the zeta potential

355

values for P-MSN and MSN-TA were –22.9 and +19.6 mV, respectively. At this

356

neutral solution condition, the silanol groups (Si-OH) on the surface of MSN became

357

deprotonated, and thus the P-MSN exhibited negative zeta potential. The MSN-TA

358

retained their positive zeta potential due to the high density of positively charged TA

359

groups. At pH 9.0, all the samples showed negative zeta potential: –42.4 mV for

360

P-MSN and –23.1 mV for MSN-TA. This pH-dependent zeta potential could explain

361

the pH-responsive controlled release.

362

Loading of 2,4-D Sodium Salt into MSN-TA Nanoparticles. With this

363

MSN-TA carrier in hand, the LC and EE of 2,4-D sodium salt were next optimized

364

including the solvent and ratio of carrier-to-pesticide. The LC and EE results of 2,4-D

365

sodium salt under various conditions are presented in Table 2. The LC increased with

366

increasing 2,4-D sodium salt. This is possibly due to the higher concentrations of

367

2,4-D sodium salt, which generated a strong gradient to facilitate the diffusion of

368

cargo molecules into the MSN pores. The amount of 2,4-D sodium salt adsorbed onto

369

the MSN-TA nanoparticles plateaued (up to 21.7%) when the mass ratio of pesticide

370

to carrier reached 1:0, which implied a saturation absorption of 2,4-D sodium salt

371

(entry 4, Table 2). The EE decreased with the increase of the mass ratio of cargo to

372

carrier. When methanol was used as the solvent, the LC decreased from 21.7% to 14.5%



373

(entry 6, Table 2) due to the higher solubility of 2,4-D sodium salt in water. It is very

374

interesting that when P-MSN instead of MSN-TA was used, the LC decreased sharply

375

to 1.5% (entry 7, Table 2). At this neutral solution condition, the determined zeta

376

potential value for P-MSN was –22.9 mV. As a consequence, the electrostatic

377

repulsions between the negative charges and 2,4-D sodium salt lead to the prominent

378

decrease of loading content. On the other hand, these results clearly demonstrated the

379

vital importance of electrostatic attractions for enhanced loading content. Considering

380

the LC and EE together, the conditions of the cargo/carrier ratio of 1:1, pure water as

381

solvent and room temperature (entry 4, Table 2) were adopted for scale preparation of

382

2,4-D sodium salt-loaded MSN-TA samples, which were used for the in vitro release,

383

soil leaching and bioactivity assay. The successful loading of 2,4-D sodium salt into

384

MSN-TA was also confirmed by FTIR and TGA analysis (Figures 3 and 4).

385

The effects of solution pH, ionic strength, and temperature on the LC were also

386

studied. When inorganic salt NaCl or Na2SO4 was added to the solution, the LC of

387

2,4-D sodium salt decreased with increasing ionic strength, pH, and temperature

388

(Figures S2-S4). The electrostatic interactions dominated the loading of 2,4-D sodium

389

salt into MSN-TA nanoparticles under various conditions. Loading and release are

390

opposite process—higher LC means lower release. Thus, we next discussed the

391

possible reasons underlying the stimuli-responsive loading and release patterns.

392

Controlled Release of 2,4-D Sodium Salt. Pesticide carriers with controllable

393

release in response to environmental stimuli are highly desirable for better efficacy

394

and fewer side effects. The pesticide release profiles of the as-prepared positively


Page 18 of 39

Page 19 of 39


395

charged MSNs were studied to reveal their potential in pesticide delivery system. The

396

release profiles of 2,4-D sodium salt@MSN-TA samples under three different pH

397

values of 3, 7 and 10 were investigated. Figure 7A shows that the release behaviors of

398

2,4-D sodium salt were obviously pH-sensitive. The samples showed the lowest initial

399

release in the first 2 h (16%) under pH 3.0, while the corresponding release rate was

400

43% under pH 10.0. The electrostatic interactions dominated the release profiles when

401

the pH of the environment changed. The schematic illustration of the

402

controlled-release mechanism of an anionic pesticide 2,4-D sodium salt is shown in

403

Figure 8. In a weak acid solution, the MSN-TA nanoparticles carry more positive

404

charge (zeta potential: +67.8 mV at pH 3.0), and a strong electrostatic attraction

405

impeded the release of negatively charged 2,4-D sodium salt. At higher pH values, the

406

silanol groups (Si-OH) in the MSN-TA nanoparticles are deprotonated, and a strong

407

electrostatic repulsion between the negative charges of SiO– groups and negative

408

charges of 2,4-D sodium salt would increase the release rate (Mechanism A, Figure

409

8).

410

The effect of ionic strength on the release profiles was also studied. Figure 7A

411

shows that when a NaCl solution (0.1 M) was used as a release medium, the release

412

rate was faster than in pure water (red line in Figure 7A). The release profile has an

413

obvious burst release. About 90% of the 2,4-D sodium salt was released after only 5 h;

414

the release was only 40% in pure water at the same time interval. This ionic

415

strength-triggering release is mainly due to the ion-exchange mechanism. The

416

competitive electrostatic attraction between negatively charged chloride ions and



417

positively charged TA groups would compel 2,4-D sodium salt to be far away from

418

the TA group centers (Mechanism B, Figure 8). This definitely facilitated payload

419

release.

420

We also studied thermo-responsive cargo molecule delivery because it is one of

421

the most common stimuli-responsive strategies. Figure 7B shows that the release of

422

2,4-D sodium salt was temperature-dependent. More 2,4-D sodium salt was released

423

at higher temperatures. At 40, 30 and 20°C, the accumulative releases of 2,4-D

424

sodium salt were 96, 75 and 52%, respectively, after 900 min. These

425

temperature-controllable release patterns could possibly occur via the well-known

426

temperature-dependent, diffusion-controlled process. High temperature may facilitate

427

diffusion of payloads from the pores of MSN to the release medium.

428

Retarded 2,4-D Sodium Salt Leaching in Soil. The results of the soil column

429

experiments are seen in breakthrough curves (BTCs) in which the amount of 2,4-D

430

sodium salt leached (mg) in each collected fraction is shown as the ordinate in relation

431

to the cumulative volume of eluent applied presented as the abscissa. The BTCs of the

432

2,4-D sodium salt@MSN-TA samples and free 2,4-D sodium salt technical as control

433

are shown in Figure 9A. Although the breakthrough of 2,4-D sodium salt occurs under

434

the same cumulative volume of eluent for 2,4-D CRFs and free technical, the

435

maximum leaching amount was greatly reduced from 3.7 to 1.7 mg. When the

436

cumulative volume of 340 mL eluent was applied, the total amount of 2,4-D sodium

437

salt leached was clearly lower in the CRFs (48.4%) than in the free system (97.3%)

438

(Figure 9B). The soil column leaching test confirmed controlled release of 2,4-D


Page 20 of 39

Page 21 of 39


439

sodium salt from formulations based on MSN-TA nanoparticles. This retarded the

440

vertical movement of the herbicide through soil and reduced the leaching potential.

441

Bioactivity of 2,4-D sodium salt@MSN-TA samples. The bioactivity of the

442

nano-formulations was compared to free herbicide and pure water using one dicot

443

target plant cucumber (Cucumis sativus L.) and one monocot non-target plant wheat

444

(Triticum aestivum L.). The 2,4-D sodium salt nano-formulations were statistically as

445

effective as the free herbicide in controlling of the test plant cucumber (Figure 10),

446

when 2,4-D sodium salt at a concentration equal to the field application rate (0.6

447

kg/ha) was applied. There was obvious root length inhibition. Ten days after

448

application, the fresh weight was reduced to about 50% compared to treatment

449

without herbicide, demonstrating good herbicidal bioactivity. Slight lower inhibition

450

effect of 2,4-D sodium salt nano-formulation than free herbicide indicated the

451

controlled release at the first stage.

452

2,4-D is a selective, systemic herbicide used for control of broad-leaved weeds.

453

It is safe for monocot plant under recommended dosage. In the present study, wheat

454

was selected as model plant to evaluate the safety of 2,4-D sodium salt@MSN-TA

455

samples toward monocot plant. At an application rate of 2.5 kg/ha corresponding to

456

the maximum application dose recommended for field application of 2,4-D, both the

457

nano-formulation and free 2,4-D sodium salt applied post-emergency did not affect

458

the plant height and free weight (Figure 11). Abigail et al also reported that the

459

nano-formulation of 2,4-D based on rice husk nanosorbents as carriers does not affect

460

the development of non-target plant (Zea mays).39 Therefore, the prepared CRFs of



461

2,4-D sodium salt can be satisfactorily used to control dicot plant without injuring the

462

monocot plant.

463

In summary, a stimuli-responsive controllable anionic pesticide release system

464

has been designed by incorporating positive charges on the surface of MSN.

465

Electrostatic interactions are the driving forces that facilitate pesticide loading. This

466

regulates release and decreases soil leaching potential. Good bioactivity was seen on

467

the target plant with no impact on the non-target plant. Hence, positively charged

468

MSNs used as nanocarriers for 2,4-D sodium salt could reduce environmental

469

pollution without affecting bioactivity. The strategy based on electrostatic interactions

470

could be widely applied to charge-carrying agrochemicals using carriers bearing

471

opposite charges to alleviate the potential adverse effects on the environment.

472

473

ASSOCIATED CONTENT

474

Supporting Information. This material is available free of charge via the Internet at

475

http://pubs.acs.org.

476

The distribution of particle size of MSN-TA nanoparticles (Figure S1); The effects of

477

solution pH, ionic strength and temperature on the loading content of 2,4-D sodium

478

salt into MSN-TA nanoparticles (Figures S2-S4).

479 480

AUTHOR INFORMATION

481

Corresponding Author

482

Qiliang Huang, [email protected]; Tel./Fax: +86-10-6281-6909.


Page 22 of 39

Page 23 of 39


483

ORCID

484

Lidong Cao: 0000-0001-7217-7102

485

Qiliang Huang: 0000-0001-9820-7218

486

Author Contributions

487

ǁ

488

Funding

489

This work was supported by the State Key Development Program for Basic Research

490

of China (No. 2014CB932204) and the National Natural Science Foundation of China

491

(NSFC) (No. 31471805).

492

Notes

493

The authors declare no competing financial interest.

L.C. and Z.Z. contributed equally to this work.

494 495

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496

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polyamidoamine dendrimer-capped mesoporous silica nanosphere-based gene

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nanosorbents containing 2,4-dichlorophenoxyacetic acid herbicide to control weeds

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[40] Chenite, A.; Le Page, Y.; Sayari, A. Direct TEM imaging of tubules in calcined

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Aspects, 2012, 398, 37–47.

625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641



642

FIGURES CAPTIONS

643

Figure 1. Schematic illustration of the synthesis of MSN-TA nanoparticles.

644

Figure 2. SEM (A, B) and TEM (C, D, E) images of the TA groups functionalized

645

MSN (MSN-TA). TEM images D and E were viewed along and perpendicular to the

646

channel direction. Scale bars: (A) 5.0 µm; (B) 1.0 µm; (C) 0.5 µm; and (D, E) 50 nm.

647

Figure 3. FTIR spectra of P-MSN, MSN-TA, 2,4-D sodium salt and 2,4-D sodium

648

salt@MSN-TA.

649

Figure 4. TGA of P-MSN, MSN-TA, 2,4-D sodium salt and 2,4-D sodium

650

salt@MSN-TA.

651

Figure 5. XPS spectra of P-MSN (A) and MSN-TA (B).

652

Figure 6. Nitrogen adsorption–desorption isotherms of P-MSN, MSN-TA and 2,4-D

653

sodium salt@MSN-TA (A), and pore size distributions of P-MSN and MSN-TA (B).

654

Figure 7. 2,4-D sodium salt released at different pH value and ionic strength (A) and

655

temperature (B). Error bars correspond to standard errors of triplicate measurements.

656

Figure 8. Schematic illustration of the controlled-release mechanism of an anionic

657

pesticide 2,4-D sodium salt loaded in positive-charge functionalized MSNs.

658

Mechanism A: pesticide released by electrostatic repulsion under neutral or basic

659

solution; Mechanism B: pesticide released through ion-exchange by increasing the

660

ionic strength.

661

Figure 9. Breakthrough curves (A) and accumulative leaching profiles (B) for 2,4-D

662

sodium salt applied for soil columns as MSN-TA formulations and free technical.

663

Error bars correspond to standard errors of triplicate measurements.

664

Figure 10. Bioactivity for target plant cucumber (Cucumis sativus L.) determined in

665

terms of root length and fresh weight. (A) Control without treatment; (B) Free 2,4-D


Page 30 of 39

Page 31 of 39


666

sodium salt technical; (C) 2,4-D sodium salt@MSN-TA samples. Bars marked with

667

different letters are statistically different at P ≤0.05 as determined by Duncan’s

668

multiple range test.

669

Figure 11. Bioactivity for non-target plant wheat (Triticum aestivum L.) determined in

670

terms of plant height and fresh weight. (A) Control without treatment; (B) Free 2,4-D

671


672


673


674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690



Page 32 of 39

Table 1. Mesoporous structure characterization of nanoparticles.a

691

a

692

Sample

SBET (m2/g)

Vt (cm3/g)

DBJH (nm)

P-MSN

1356.0

1.65

3.75

MSN-TA

956.4

0.59

2.31

2,4-D sodium salt@MSN-TA

454.8

0.03

–

SBET: BET specific surface area; Vt: total pore volume; DBJH: BJH pore diameter.

693 694

Table 2. The loading content (LC) and encapsulation efficiency (EE) of

695

2,4-D sodium salt into MSN-TA nanoparticles. a Entry

Solvent

Mass ratiob

LC (%)

EE (%)

1

H2O

0.4

15.0±0.3d

44.0±1.2a

2

H2O

0.6

17.5±0.3c

35.3±0.3b

3

H2O

0.8

19.2±0.2b

29.7±0.4c

4

H2O

1.0

21.7±0.3a

27.7±0.4d

5

H2O

1.2

22.0±0.4a

23.5±0.6e

6

MeOH

1.0

14.5±0.3

17.0±0.4

7c

H2O

1.0

1.5±0.1

1.6±0.1

696

a

MSN-TA (30 mg), H2O (5.0 mL), room temperature; b Mass ratio of 2,4-D sodium

697

salt to MSN-TA;

698

three replicates. Values in each column followed by different letters are statistically

699

different at P ≤0.05 as determined by Duncan’s multiple range test.

c

P-MSN instead of MSN-TA was used. Values are mean±SD of

700 701 702


Page 33 of 39


703 704

Figure 1. Schematic illustration of the synthesis of MSN-TA nanoparticles.

705 706

707 708

Figure 2. SEM (A, B) and TEM (C, D, E) images of the TA groups functionalized

709

MSN (MSN-TA). TEM images D and E were viewed along and perpendicular to the

710

channel direction. Scale bars: (A) 5.0 µm; (B) 1.0 µm; (C) 0.5 µm; and (D, E) 50 nm.

711 712



713 714

Figure 3. FTIR spectra of P-MSN, MSN-TA, 2,4-D sodium salt and 2,4-D sodium

715

salt@MSN-TA.

716

717 718

Figure 4. TGA of P-MSN, MSN-TA, 2,4-D sodium salt and 2,4-D sodium

719

salt@MSN-TA.

720 721


Page 34 of 39

Page 35 of 39


722 723

Figure 5. XPS spectra of P-MSN (A) and MSN-TA (B).

724

725 726 727

Figure 6. Nitrogen adsorption–desorption isotherms of P-MSN, MSN-TA and 2,4-D

728

sodium salt@MSN-TA (A), and pore size distributions of P-MSN and MSN-TA (B).

729



730 731

Figure 7. 2,4-D sodium salt released at different pH value and ionic strength (A) and

732

temperature (B). Error bars correspond to standard errors of triplicate measurements.

733 734

Figure 8. Schematic illustration of the controlled-release mechanism of an anionic

735

pesticide 2,4-D sodium salt loaded in positive-charge functionalized MSNs.

736

Mechanism A: pesticide released by electrostatic repulsion under neutral or basic

737

solution; Mechanism B: pesticide released through ion-exchange by increasing the

738

ionic strength.

739 740 741


Page 36 of 39

Page 37 of 39


742

743

Figure 9. Breakthrough curves (A) and accumulative leaching profiles (B) for 2,4-D

744

sodium salt applied for soil columns as MSN-TA formulations and free technical.

745

Error bars correspond to standard errors of triplicate measurements.

746

747 748

Figure 10. Bioactivity for target plant cucumber (Cucumis sativus L.) determined in

749

terms of root length and fresh weight. (A) Control without treatment; (B) Free 2,4-D

750


751


752




753 754

Figure 11. Bioactivity for non-target plant wheat (Triticum aestivum L.) determined in

755

terms of plant height and fresh weight. (A) Control without treatment; (B) Free 2,4-D

756


757


758


759 760

761

762

763

764

765

766

767

768


Page 38 of 39

Page 39 of 39


769

770

For Table of Contents use only

771

772


Positive-Charge Functionalized Mesoporous Silica Nanoparticles as

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