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Evaluation of Controlled Release Property and Phytotoxicity Effect of Insect Pheromone-Zinc Layered Hydroxide Nanohybrid Intercalated with Hexenoic Acid Rozita Ahmad, Mohd Zobir Hussein, Wan Rasidah Wan Abdul Kadir, Siti Halimah Sarijo, and Yun Hin Taufiq-Yap J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b03102 • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on November 18, 2015

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Journal of Agricultural and Food Chemistry

Evaluation of Controlled-Release Property and Phytotoxicity Effect of Insect Pheromone-Zinc Layered Hydroxide Nanohybrid Intercalated with Hexenoic Acid Rozita Ahmad1,2, Mohd Zobir Hussein1*, Wan Rasidah Wan Abdul Kadir 2, Siti Halimah Sarijo3 & Taufiq-Yap Yun Hin4

1

Materials Synthesis and Characterisation Laboratory (MSCL), Institute of Advanced Technology (ITMA), Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia ; E-Mail: [email protected]

2

Forest Biotechnology Division, Forest Research Institute Malaysia (FRIM), 52109 Kepong, Selangor, Malaysia; E-Mail: [email protected] ;[email protected]

3

Faculty of Applied Science, Universiti Teknologi MARA (UiTM), 40540 Shah Alam, Malaysia; E-Mail:[email protected]

4

Chemistry Department, Faculty of Science, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia; E-Mail:[email protected]

*

Corresponding author: E-Mail: [email protected]; Address: Materials Synthesis and Characterisation Laboratory (MSCL), Institute of Advanced Technology (ITMA), Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor Tel:+6-03-8946-8092; Fax:+6-03-8943-5380

1

ACS Paragon Plus Environment


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Abstract

2

A controlled release formulation for insect pheromone, hexenoic acid (HE) was successfully

3

developed using zinc-layered hydroxide (ZLH) as host material through a simple co-precipitation

4

technique, resulted in the formation of inorganic-organic nanolayered material with sustained

5

release properties. The release of HE from its nanohybrid was found to occur in a controlled

6

manner, governed by pseudo-second order kinetics model. The maximum amount of HE released

7

from the nanocomposite into solutions at pH 4, 6.5 and and 8 was found to be 84, 73 and 83%

8

for 1100 mins, respectively. The hexenoate-zinc layered hydroxide nanomaterial, (HEN), was

9

found to be non-toxic for plant when green beans and wheat seeds were successfully germinated

10

in all HEN concentrations tested in the experiment, with higher percentage of seed germination

11

and higher radical seed growth as compared to its counter anion, HE. ZLH can be a promising

12

carrier for insect pheromone towards a new generation of environmentally-safe pesticide

13

nanomaterial for crop protection.

14

Keywords: hexenoic acid, zinc layered hydroxide nanohybrid, insect pheromone, controlled

15

release, phytotoxicity

2


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1. Introduction

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Research on nanotechnology for agricultural applications has gained high interest due to

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major concern on the use of chemical pesticides in crop production. Excessive use of

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agrochemicals are usually not fully utilized by plants as most of them are lost through leaching in

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soil 1-2. Direct application of pesticide onto the plant through spraying will contaminate the crop,

21

leaving harmful chemical residue which enter through direct pathway into human food chain 3.

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It also causes environmental pollution in ecosystem and pose problem to human health. The use

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of pheromone in orchard for insect control has proved successful and could become a potential

24

biopesticide 4-5. However, the application of pheromone is not effective under field condition as

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most of the chemical is volatile and easily affected by environmental factors which include

26

temperature, sunlight, wind and rainfall6. A controlled release formulation hence is required to

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provide precise delivery target and releasing the active chemicals in smaller amounts and thus

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help to reduce the chemical safe level 3.

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A number of controlled released carrier in nanopesticide research have been widely

30

studies to provide an environmentally-safe, non-pesticide product. This include polymer-based

31

nanoformulations comprising of polysaccharide materials such as starch6,7, alginates8,

32

chitosan9,10, polyethylene glycol11 and polyester substance

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from natural materials are also in used which consisted of neem oil13 and garlic essential oil 14.

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The list extended to other forms as well, which govern to nanospheres15, nanogels16 and

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nanofibers17,18.

12

. Plant protection products derived

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Research on inorganic-organic nanomaterial namely layered double hydroxide (LDH)

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and layered hydroxide salt (LHS) nanocomposites has grow rapidly, due to their unique structure

38

as ion exchanger and their tailor-made behavior resulted to various potential applications. Early 3



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work on LDH as catalysts19, catalyst precursor20 then the research proceeded to polymer21,22,

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flame retardant for smoke suppression23,24,corrosion inhibitor25, extended as controlled release

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formulation for drugs26,27, agrochemicals28 and removal of toxic substances in environmental

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applications29,30. Nanolayered structure material of layered inorganic-organic nanocomposite can

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be formed by encapsulation of an organic moiety into an inorganic interlayer spacings of LDH

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and LHS which acted as host. The brucite-like structure of both LDH and LHS provide an ion

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exchange platform for many chemical types: nitrates, sulphates and organic acids. High

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positively charged densities of LDH and LHS can accomodate the anionic organic compound

47

within the interlayer spaces to compensate the positively charged, and consequently form high

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stability material.

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LDH is represented by the general formula, [M2+1-xM3+x(OH)2]x+(Am-)x/m.nH2O, while

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LHS is given by the general formula, M2+(OH)2-x(An-)x/n.mH2O, where M2+ and M3+ is the

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divalent and the trivalent metallic cation, respectively. In both layered hydroxide structure, Am- is

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the interlayered anion that balance the positive charge layers. Thus, LDH and LHS nanomaterial

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are represented by a positively charged brucite-like inorganic layers with anions and water

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molecules existing between the interlayers 31-32.

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ZLH is a type of LHS, comprised of one type of divalent metal cation in which only zinc

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and hydroxyl represent

the inorganic layers. ZLH is comprised of layers of octahedral

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coordinated zinc cations, in which 1/4 of them are displaced leaving an empty octahedral site

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forming cationic centers tetrahedrically coordinated to the top and bottom of the octahedral

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sheet. Water molecules occupy the apexes and the nitrate counter ions are free between the

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interlayers. Due to its higher charge densities and possess larger interspacing than LDH, ZLH

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can become host for higher number of guest anion of different sizes33. This makes it potential to 4


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be use as carrier and slow release delivery system for many chemical agents in various

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applications such as drugs

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absorber 39,40 , catalyst 41, DNA42, food preservatives 43, pharmaceutical and nutracuetical agents

65

44

34,35

, anti-corrosion agent36, herbicides37, dye industry

38

, sunscreen

.

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Meditarranean fruit flies, (Ceratitis capitata) is categorized as agricultural pests as they

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attack both unripe and ripe fruits. This causes problem in the quality of food supply and effect

68

agriculture yield. In this study, hexenoic acid, a pheromone for this pest is being used as guest

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with ZLH as host for the formulation of host-guest nanocomposite with controlled release

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properties. To our knowledge, the use of ZLH as controlled release carrier for insect pheromone

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has not yet been reported in the open literature. Here, we discussed our work on the intercalation

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of trans-2-hexenoic acid (HE), C6H10O6, a short chain carboxylic anion into the interlayer of

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ZLH by simple co-precipitation method. A new organic-inorganic ZLH-HE nanohybrid

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compound was synthesized by direct reaction of ZnO with HE under aqueous environment

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followed by precipitation with alkaline solution, and subsequently evaluated its control released

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behavior and phytotoxicity effect.

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ZLH as the encapsulated host material for active chemicals can provide beneficial input

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to the environment. It can be used as zinc source for soil supplement which is an advantage to

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the soil as most soil around the world is deficient in zinc. It is an essential micronutrient for plant

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development and obtain most of its nutrient from soil in which they grow

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test of ZLH-HE nanohybrid carried out on two types of seed shows higher percentage of

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germination and longer radical seed length compared to its counterpart chemical, HE. This

83

indicates that the synthesized nanolayered structure material is not toxic to plant and intended

45

. The phytotoxicity

5



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towards a new generation of green, environmentally-safe pesticide nanomaterial for insect

85

control and crop protection.

86 87

2. Experimental

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2.1 Synthesis of Materials

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All chemicals used in this experiment were obtained from various chemical supplier and

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applied without any further purification. All solutions were prepared using deionized water.

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Trans-2-hexenoic acid nanohybrid (HEN) was synthesized by co-precipitation method using

92

ZnO as starting material. About 0.20 g of ZnO was firstly suspended into 100 mL deionized

93

water stirred for 15 minutes. Hexenoic acid solution of various concentrations; 0.1M, 0.2M,

94

0.3M and 0.4M were each prepared by dissolving respective amount of HE in 40 mL ethanol and

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adjusted to 100 mL volume in a volumetric flask with deionized water. Each of the HE solution

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was then added dropwise into ZnO suspension with constant stirring, producing a clear mixed

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solution at the end of the process. pH of the solution was adjusted to 7.9 with 0.5 M NaOH

98

aqueous solution to obtain white precipitate. The slurry solution was vigorously stirred via a

99

magnetic stirrer for 3 h and the aging process was further continued in an oil bath shaker for 18 h

100

at 70 oC. The obtained product was centrifuged, thoroughly washed with deionized water to

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wash away any contaminants and then dried in an oven at 70 oC. The obtained material was then

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powdered for further use and characterizations.

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2.2 Characterization

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Powder X-ray diffraction patterns of nanohybrids were obtained at 2–60o on a Shimadzu

105

diffractometer, XRD-6000 using CuKα radiation (λ =1.5418 Å) and dwell time of 4 degrees per

106

minute. Surface characterization of the synthesized product was carried out using a nitrogen gas 6


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adsorption method at 77 K on a Micromeritries ASAP 2000. The sample was degassed in an

108

evacuated heated chamber at 100 oC overnight. Surface morphology of the synthesized

109

nanohybrid was captured on a field emission scanning electron microscope (FESEM), model

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JOEL JSM-7600F. The dried sample was dispersed on a conductive carbon adhesive tape surface

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which was attached to a FESEM stub, and then gold coated. Internal morphology of ZLH-HE

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nanohybrids was observed using a Hitachi H-7100 transmission electron microscope (TEM) at

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magnifications, 30 – 200 k. A drop of the nanohybrid – 20% v/v ethanol dispersion was placed

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onto a 300 mesh Fomvar copper grid and air dried, overnight in a dessicator. The average

115

diameter and particle size distribution (PSD) of the nanohybrid was measured using dynamic

116

light scattering (DLS) photon-cross correlation spectroscopy (PCCS). The sample was dispersed

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in 20% v/v ethanol prior to analysis.

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2.3 Kinetic Release Study

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The release of hexenoic acid from the nanohybrid host into different media were

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accomplished using distilled water at pHs=4, 6.5 and 8 by adding 2 mg of the nanohybrid into

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3.5 mL of the solution. The pH solution in distilled water was obtained by adjusting its pH using

122

HCl or NaOH and pH values were measured using a pH meter. The accumulated amount of

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hexenoic acid released into the solution was measured at preset time at 207 nm using a Perkin

124

Elmer UV-Visible spectrophotometer, Lamda 35. The cumulative release pattern was fitted to

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first, pseudo-second order kinetics and parabolic diffusion models.

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2.4 Phytotoxicity Test

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The pytotoxicity test for ZLH-HE nanohybrid was carried out on two types of seed using

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a method adopted by Keeling 46. The nanohybrid material of different weight, 0.01g, 0.1g and 1g

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and 100 mL distilled water were mixed respectively and shake using an orbital shaker for 1 h. 7



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The mixture was filtered through Whatman filter paper no. 1. 3 mL of the filtered solution was

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pipetted on a petry dish which was placed with a filter paper Whatman no. 1 containing 15 seeds

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of green beans and wheat. The petry dish was covered to avoid loss through evaporation. The

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petry dishes were then incubated at 28 oC in an incubator for 48 h. Various types of controls

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which consisted of distilled water, ZnO and HE were carried out to determine their effect on

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plant. A series of concentration of 0.01g/100 mL, 0.1g/100mL and 1g/100mL of HE and ZnO

136

respectively, were also prepared as in HEN, and then tested on the two seeds.

137 138 139 140

3.

Results and Discussion

3.1 Powder X-ray Diffraction Analysis

141

The PXRD patterns of the unbound chemical, HE and ZLH-HE nanolayered structure

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materials prepared at various concentrations of HE, 0.1 – 0.4 mol/L using 0.2g of ZnO are

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potrayed in Fig. 1. The Powder X-ray diffraction pattern of ZnO reflects five sharp peaks

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between 30 – 60o region, correspond to reflections of 100, 002, 101, 102 and 110 lattice planes

145

which indicate high crystallinity which represents a distinctive pattern of metal oxide. The

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PXRD pattern of the counterpart anion, HE (Fig. 1) shows some reflection peaks, which

147

demonstrates the crystalline nature of this chemical.

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The produced nanolayered structure material synthesized at various concentrations of HE

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(Fig. 1) with fixed amount of 0.2g ZnO, show reflection peaks diffracted at lower 2θ angle with

150

increase in the basal d-spacing, is evidence that inclusion of the anion, HE inside the interlayer

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spacing of ZLH. The resulting nanolayered material prepared from 0.1 M HE, shows the

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presence of ZnO phase, which indicate incomplete reaction, while the nanohybrids synthesized at

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concentrations 0.2 and 0.3 mol/L HE show disappearance of ZnO characteristic peaks. The 8


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expansion basal spacing of nanohybrid material synthesized at 0.2 and 0.3 mol/L HE is 24.6 Å

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and 23.4Å, respectively and disappearance of ZnO peak indicate that successful intercalation of

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the organic anion between the inorganic ZLH interlayer has taken place. However, comparing

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both 0.2 mol/L and 0.3 mol/L HE together, the nanolayered structure material obtained using 0.3

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mol/L HE, shows a symmetric peak with reflection up to three separate harmonics at 2θ angle

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of 3.76o, 7.50o and 12.45o with respective d-basal spacing values of 23.5 Å, 11.8 Å and 7.1 Å

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as shown in the slow scan PXRD patterns (Fig.1b). This indicate the formation of a well ordered

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2D layered structure of the nanocomposite. Hence, the nanolayered structure material obtained

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from 0.3 mol/L HE was selected for further characterizations and labeled as HEN.

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The formation of nanolayered structure material of ZLH-HE from direct reaction of ZnO

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in an hydrated environment of HE, is reported to follow the ‘dissociation-deposition’

165

mecahanism process31,47. The hydrolysis of ZnO in an aqueous environment takes place to form

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Zn(OH)2 on the surface of solid particles. Further reaction lead to the dissociation of Zn(OH)2

167

in the HE solution, releasing Zn2+. The released Zn2+ then reacted with hydroxyls, HE anions

168

and H2O in the solution to obtain nanolayered ZLH-HE material. The process is repeated until all

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the ZnO phase and the Zn(OH)2 phase has completely changed to the layered material. The

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mechanism of the process is given below:

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ZnO + H2O

Zn(OH)2

(1)

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Zn(OH)2

Zn2+ + 2OH-

(2)

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Zn2+ + 2OH- + HE- + H2O

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Zn2+(OH)2-x (HEm-)x/m .nH2O

(3)

3.2 Surface Properties

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The adsorption-desorption isotherms of nitrogen gas on ZnO and its nanolayered material,

176

HEN is depicted in Fig. 2a. Based on the IUPAC classification, both ZnO and HEN potrayed a 9



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similar pattern of adsorption-desorption isotherms with Type IV indicating mesopore and/or

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nonporous types of material. The adsorption for ZnO showed that it started from a slow uptake

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at low relative pressure in the range of 0.0 – 0.9, followed by a rapid increase in adsorption of the

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adsorbent of > 0.9, reaching its optimum uptake of nitrogen gas at 33 cm3/g. Similarly, the

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nanolayered material, HEN gave a slow adsorption increment at low relative pressure in the

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range of 0.0 – 0.8 and then a quick rise of the adsorbate at a relative pressure of more than 0.8,

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approaching saturated uptake of 26 cm3/g. The effect of surface properties of the produced

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nanolayered structure material upon successful intercalation of organic anion, HE into the

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interlayer space of zinc layered hydroxide was evaluated by measuring the surface area and pore

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size distribution and the data are summarized in Table 1.

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The surface area of ZnO and HEN analysed by the Brunauer, Emmet and Teller (BET)

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method shows a reduce in the surface area of HEN with a value of 4 m2/g, compare to ZnO with

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a value of 6 m2/g. The same trend was also observed for the formation of other nanohybrids

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reported for 2,4,5 –trichlorophenoxyacetic acid and ellagic acid as guest anions into LDH and

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ZLH35,48.

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property48. It is believed that the nature of the guest anion used could also contribute to the

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decrease in surface area. As mentioned in the earlier discussion, the counter anion, HE has a

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combination of amorphous and crystallinity structure, which might affect the formation of the

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produced nanolayered material. The intercalation of HE into the ZLH resulting the formation of

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HEN has resulted in the increase in size pore. The BET average diameter pore of HEN was

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found to be higher than ZnO, with values of 297 Å and 64 Å respectively.

The microstructure of the material used was reported to influence the surface

198

The desorption pore size distributon of ZnO and HEN was determined by Barret-Joyner-

199

Halenda (BJH) procedure and the plot is displayed in Fig. 2b. The pore size distribution pattern 10


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of ZnO shows the tallest peak at 18 Å along with two other scattering peaks. While HEN shows

201

trend of pore size distribution adsorption similar to ZnO with the highest peak around 12 Å. The

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intercalation of HE into the interlayer ZLH had changed the pore size and pore volume of the

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obtained nanolayered material HEN higher than its host. The BJH pore diameter and volume

204

was increased from 111 Å (ZnO) to 121 Å (HEN) and 0.01 cm3g-1 (ZnO) and 0.03 (HEN),

205

respectively.

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3.3 Morphology and Particle Size Distribution of Intercalated Compound

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The surface morphology of ZnO and HEN captured on FESEM are presented in Fig. 3a

208

and 3b respectively. The morphology of zinc oxide exhibited non-uniform granular structure

209

with no specific shapes while HEN showed multilayer of non-uniform broad and flat shapes and

210

sizes. This shows that the transformation of ZnO into an intercalated compound resulted changes

211

in the surface morphology.

212

Internal microstructure image of ZnO and HEN examined on TEM are shown in Fig. 4a

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and 4b respectively. It is observed that ZnO has granular shape and this corresponds nicely with

214

FESEM image (Fig. 3a). On the other hand, HEN exhibited non-uniform shape, overlapping

215

each other. The particle size distribution (PSD) of ZnO and HEN on TEM images were measured

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using UTHSCSA image tool software. The PSD was found to be 31 to 157 nm for ZnO and 23 to

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87 nm for HEN. The average PSD of ZnO and HEN was estimated to be 108 + 32 nm and 49 +

218

15 nm respectively. High standard deviation is attributed to the non uniform shapes and irregular

219

sizes for both ZnO and HEN.

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The PSD of ZnO and HEN (Fig. 5a and 5b) were also carried out using DLS (dynamic

221

light scattering) measurement, however the results were not in good agreement with TEM 11



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measurement. The TEM result is more meaningful due to its direct observation. In addition, the

223

tendency of both ZnO and HEN to re-agglomerate during the DLS analysis cannot be ruled out.

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Summary of the results of particle size distribution from TEM and DLS are given in Table 2.

225

3.4 Release Behavior of HE into Distilled Water at Various pHs

226

A series of distilled water at various pH were prepared to study the effect of different

227

pHs on the release behavior of HE from the interlayer spacing of the nanolayered material, HEN.

228

The release profiles of HE from the organic-inorganic layer of its nanomaterial, HEN into pH 4,

229

6.5 and 8 are depicted in Fig. 6. All the release profiles show a broad curve indicating a constant

230

release of the anion from the nanomaterial into the solution at various pHs. Generally, the

231

percentage anion release was observed to increase constantly in the range of 800 mins from the

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starting time of dispersion of the nanomaterial in the pH solution, followed by a slower release

233

before attaining equilibrium rate after 1100 mins.

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A high amount of anion release was observed in the first 200 mins with percentage

235

release recorded to be 34, 26 and 36% in distilled water at pH 4, 6.5 and 8, respectively. This is

236

due to the “burst effect” which is caused by the high release of anions that are weakly adsorbed

237

on the external surface of layered ZLH

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anion release has increased up to 26, 18 and 23% to obtain the accumulated release of 60, 44

239

and 59% for pH 4, 6.5 and 8, respectively. The following 400 to 800 mins, the rate of HE release

240

into pH 4, 6.5 and 8 has increases in the range of 18 to 20% to produce a total amount of anion

241

release of 80, 64 and 77%, respectively. It was observed that the release of HE into pH 4 and 8

242

are faster than at pH 6.5. This is due to the partial dissolution of ZLH occurred with the collapsed

243

of nanolayered structure and the releasing of pheromone anions to the environment48,51-53. This

244

indicate that ZLH is not stable at high and low pHs48,54 which causes the interlamallae layer to

48-50

. Within the ranges of 200 to 400 mins, the rate of

12


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dissolve and affect the composition amount of HE from the nanomaterial. Pan et al55 reported

246

release of drug (doxifluridine) at high pH with release of 60% at pH 6.8 and 72% at pH 7.4 from

247

Mg/AL layered hydroxides.

248

While at pH 6.5, ZLH is more stable and the release of HE is slower as compared to pH 4

249

and 8. The release could be related to the diffusion of anion from the interlayer as suggested by

250

Gao et al.56 that the deintercalation of intercalated anion, vitamin C in deionized water followed

251

the diffusion mechanism whereby the rate of drug diffusion is controlled by the rigidity of the

252

layers and the diffusion pathlength. After diffusion, the release of HE anions were proceeded to

253

anion exchange process. Gao et al.56 reported 36% of vitamin C was released into deionized

254

water from its MgAl intercalated compound. In this study, at pH 6.5 medium, HE could be ion

255

exchange with carbonate anion from the dissolution of carbon dioxide from atmosphere in

256

distilled water

257

chlorine anion from HCl solution for preparation of pH 4 solution51. Excess OH- in pH 8 medium

258

might contribute to the ion exchange process of HE.

57

. The ion exchange process of HE in pH 4 medium could be related to the

259

However, 800 mins and beyond, a slower release process of HE was observed at the rate

260

of 2 to 5%. This could be attributed to the release of HE ions from deeper interlayer sites 52 and

261

ion exchanged with anions in the solution and later proceeded through diffusion within the

262

interlayer space53. The saturated amount of HE released into solutions of pH 4, 6.5 and and 8 was

263

achieved at 84, 73 and 83%, at 1139, 1159 and 1143 mins, respectively. It was found that HE

264

molecules did not be fully released to 100%, could be attributed that the anions were strongly

265

held to positive charge of host resulting the reduced diffusion from the interlayer into medium

266

solution

58

and some remains entrapped inside the inorganic layered host. At equilibrium stage,

13



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the interlayer anions cannot be exchanged completely and the intercalated anions would be

268

released continously in a slow manner51.

269

3.5 Release Kinetics of HE from HEN

270

In order to understand the release mechanism of HE from the nanolayered material, HEN

271

into different pHs solution, the release data were fitted into various kinetic models. The highest

272

correlation coefficient values were used to determine the best kinetic models that governs the

273

kinetic release. The three kinetic equation as given below were used in this work;

274

Pseudo-first order

:

In (qe-qt) = In qe – k1t

(4)

275

Pseudo-second order

:

t/qt = 1/k2qe2 + t/qe

(5)

276

Parabolic diffusion

:

(1-Mt/Mo)/t = k t-0.5 + b

(6)

277

where qe, qt represents the equilibrium release amount and release amount of anion at any time

278

respectively, t and k is the apparent release-rate constant respectively while Mt and Mo

279

correspond to the chemical content remaining in the ZLH at zero time and release time, t,

280

respectively.

281

The plots for all the fitting are given in Fig. 7 and the parameters obtained are

282

summarized in Table 3. It was found that the psuedo-second order model offers a more

283

satisfactory description of the kinetics release of HE from its nanohybrid, HEN compared to

284

other kinetic models use in this work. The highest correlation coeffiecient, R2 were obtained

285

from psuedo-second order model equation with value of 0.9357, 0.9692 and 0.9833 for release of

286

HE into pH 4, 6.5 and 8 solution respectively. The t1/2 value, which represented the time taken

287

for HE concentration to be half of its accumulated release and the release rate values, k of HE

288

into different pH media were calculated and tabulated in Table 3. t1/2 and k values of HE are

289

250 minutes and 1.81x 10-5 g/mg.min (pH 4), 301 minutes and 1.24x 10-5 g/mg.min (pH 6.5), 14


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235 minutes and 2.69 x 10-5 g/mg.min (pH 8). This corresponds to the maximum accumulated

291

values of 84, 73 and 83% HE in pH 4, 6.5 and 8, respectively which strongly indicates that the

292

release of HE from HEN into pH 6.5 is slower compare to pH. 4 and 8. HE release from HEN

293

into pH 6.5 solution has the lowest k rate which delivers small amount of HE at longer duration

294

time. The release of hexenoic acid from its interlamellae of its organic-inorganic nanohybrid at

295

various pHs was found to be in a controlled manner governed by the pseudo-second order kinetic

296

model. The amount of HE released at pH 4 and 8 is higher than pH 6.5 with the order of pH 4 =

297

8 > 6.5.

298

3.5 Phytotoxicity Test

299

In order to investigate the toxicity of HEN to plant, a phytotoxicity test

of

the

300

nanolayered material was tested using two seed types i.e. green beans and wheat seeds

301

germinated on distilled water and series of concentrations of 0.01g/100 mL, 0.1g/100mL and

302

1g/100mL of ZnO, HE and HEN, respectively. The results of seed germination are shown in Fig.

303

8 and Table 4. In Table 4, it can be seen that the percentage seed germination of green beans on

304

untreated sample i.e. distilled water alone was recorded to be the lowest percentage of 91% when

305

compared to the other chemical treatments. The percentage seed germination of green beans

306

exposed to ZnO extract solution increased to 93, 96 and 98% with increment to ZnO

307

concentration of 0.01g/100 mL, 0.1g/100mL and 1g/100mL, respectively. This could be

308

attributed to the availability of Zn from ZnO which is required for plant development. For free

309

anion, HE, at 0.01g/100 mL, 0.1g/100mL concentration the percentage seed germination was 98

310

and 96% respectively. However, at 1g/100mL HE, green beans failed to germinate, which

311

indicate that it is not suitable and toxic for seed germination at high concentration. This is in

312

contrast with nanolayered material, HEN that seed germination of green beans was successful 15



Page 16 of 36

313

in all the concentrations tested. HEN shows high percentage of green beans germination of

314

98% at 0.01g/100 mL, 0.1g/100mL and slightly reduced to 96% at 1g/100 mL concentration. The

315

seed germination result demonstrated that the nanolayered material, HEN is not toxic to plant

316

under our experimental conditions.

317

Wheat seeds gave slightly different response compared to green beans. In distilled water,

318

wheat seeds recorded 82% of germination and the percentage increased to 84 and 87 % when it

319

was treated with 0.1g/100mL and 1g/100mL ZnO, respectively. On the other hand, the

320

germination of wheat seed was found to survive at 0.01g/100 mL HE with 89% but at higher

321

concentration of HE, no wheat seed was found to be germinated. For HEN, wheat seeds were

322

successfully germinated at all concentrations with recorded values of 73% (0.1g/100 mL), 78%

323

(0.1g/100 mL) and 82% (1g/100 mL) for seed germination respectively. This shows that when

324

HE is converted into nanolayered structure material of HEN, the phytotoxicity of HEN on both

325

green beans and wheat seeds was found non-toxic to plant and safe for seed germination. HE

326

from the nanohybrid could be released to surroundings in a sustained manner when in contact

327

with anions of higher affinity such as carbonate in air. The release of small amount of HE from

328

its nanohybrid to surroundings did not toxicate the seed germination as compared to its free

329

anion, HE. Partial dissolution of Zn from the collapsed of ZLH interlayer structure during the

330

anion exchange are used up and increases the seed germination.

331

Radical seed length for both seeds on untreated sample and at different chemical

332

treatments were measured to determine the effect of HE, HEN and ZnO on the radical seed

333

growth and the results are depicted in Fig. 9 and Table 5. Fig. 9 shows that the radical seed

334

length of both green beans and wheat seeds gave higher values for all the chemical treatments

335

when compared to untreated sample, distilled water. The radical seed growth for both seeds 16


Page 17 of 36


336

increases as the amount of ZnO increased. As expected, this is due to the effect of ZnO as zinc

337

source as an essential micronutrient for plant development which has important effect on the

338

radical seed growth.

339

The radical seed length of both seeds germinated on free anion, HE recorded lower

340

values than its nanomaterial, HEN as shown in Table 5. HEN recorded higher radical seed length

341

for green beans measuring at 31.2 mm (0.01g/100 mL) and 27.1mm (0.1g/100 mL) than its

342

counterpart, HE with values of 25.9 mm (0.01g/100 mL) and 20.7 mm (0.1g/100 mL). While

343

radical seed length for wheat seed recorded 10.6 mm (0.01 g/100mL HEN) compared to 7.9 mm

344

(0.01g/100 mL HE). This could be due to the effect of ZLH that enhance the properties of HEN

345

and also contribute Zn source as important micronutrient for plant development. This further

346

indicates that the nanolayered material, HEN influence and improve the radical seed growth

347

compare to its free anion, HE. The controlled release properties of the nanohybrid could

348

contribute to the occurred situation. HE anion is released slowly from its nanohybrid to the

349

surroundings and used by seed for further development. This study suggests the possibility of

350

zinc layered hydroxide as a promising encapsulate material for insect pheromone with controlled

351

release capabilities for the generation of environmentally-safe pesticide for crop protection.

352

Abbreviations

353

ZLH = zinc layered hydroxide ; ZnO = zinc oxide; LHS = layered hydroxide salt;

354

HE = hexenoic acid (C6H10O6); HEN = hexenoic acid nanohybrid; NaOH = sodium hydroxide;

355

FESEM = field emission scanning electron microscope; TEM = transmission electron

356

microscope; PSD = particle size distribution ; DLS = dynamic light scattering (DLS); PCCS =

357

photon-cross correlation spectroscopy.

358 17



Page 18 of 36

359

Acknowledgement

360

Funding for this research was provided by the Ministry of Higher Education (MOHE) under the

361

Grant No. FRGS/1/11/SG/UPM/01/2 (vot. No. 5524165) is greatly appreciated. The authors

362

gratefully acknowledged FRIM for sponsor for RA in her MSC program.

363 364

References

365

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25



0.4 M HEN

Page 26 of 36

20000 23.5 Å

23.47 Å 0.3 M HEN

24.64 Å

15000

0.2 M HEN 0.1 M HEN

10000 0.3M HEN

5000

ZNO

11.8 Å

HE 7.1 Å 20

40

60

Fig.1a PXRD patterns of HE, ZnO and HEN nanohybrids prepared at different concentrations of HE.

0

5

10

15

20

Fig.1b Slow scan of PXRD patterns of HEN nanohybrid prepared at 0.3 mol/L HE.

Abbreviations: ZnO, zinc oxide; HE, hexenoic acd; HEN, hexenoic acid nanohybrid (o = ZnO phase, peak from left to right is for 100, 002, 101, 101 reflections)

Trans-2-hexenoic acid

26


Page 27 of 36


40

1.8 1.6

2a

2b

ZnO

ZnO

1.4

30

P o r e V o lu m e (c m 3 /g )

V o lu m e a b s o r b e d a t S T P ( c m 3 /g )

35

25 20

HEN 15 10

1.2 1.0 0.8 0.6

HEN 0.4

5

0.2 0 0.0

0.2

0.4

0.6

0.8

Relative pressure (P/Po)

1.0

0.0 10

100

1000

Pore diameter (Å)

520 and HEN (a) and Barret-Joyner-Halenda method Fig.2 Adsorption-desorption isotherms for zinc oxide pore size distribution for zinc oxide and HEN (b). Abbreviations: ZnO, zinc oxide;HEN, hexenoic acid nanohybrid

27



3a

Page 28 of 36

3b

Fig. 3 FESEM micrograph of (a) ZnO and (b) HEN at 50,000 x magnification

4a

4b

Fig. 4 TEM micrograph of (a) ZnO and (b) HEN

28


Page 29 of 36


15

10

1.0

1.0

5a

0.8

8

5b 0.8

10

6 0.6

0.6 4

5

0.4

0.4 0.2

2 0.2

0 0.0

0 0

1000

2000

3000

0.0

4000

0

1000

2000

3000

Fig. 5 Particle size distribution studied using DLS for (a) ZnO and (b) HEN

100

pH 8 pH 6.5 pH 4

% Anion Release

80

60

40

20

0 0

200

400

600

800

1000

1200

1400

Time(min) Fig. 6 Release profiles of hexenoic acid (HE) from its nanohybrid (HEN) interlayers into distilled water at pH=4, 6.5 and 8.


29


A (Pseudo-first order)

Page 30 of 36

B (Pseudo-second order)

C (Parabolic diffusion)

18

1.0

pH 4 2 R =0.9130

12

t/qt

1.0

10 8

pH 4 R =0.9357

6 0.5

2

4

0.8

1-(M t/M o)

14

1-(Mt – Mo)

1.5

16

t/qt

Log(q –qt) lo g(q e-qet)

2.0

0.6

pH 4 2 R =0.8856

0.4

0.2

2 0.0

0 200

400

600

800

1000

1200

1400

0.0 0

200

400

Time (min)

1000

1200

1400

0

5

10

15

20

t

E (Pseudo-second order) 1.0

Equation

y = a + b*x

Weight

No Weighting

t/q t

t/qt

1 -(M t/M o )

15

10

0.5

5

0.0

0

40

0.96844

Adj. R-Square

1.0

35

6.09721

Residual Sum of Squares

1.5

30

pH 6.5 2 R =0.9692

0.5

F (Parabolic diffusion)

20

pH 6.5 2 R =0.9536

25

t^0.5

Time (min)

D (Pseudo-first order)

Log(q e–qt) lo g (q e -q t)

800

Time(min)

Time (min)

2.0

600

1-(Mt – Mo)

0

0.8

Value

Standard Error

1-(Mt/Mo)

Intercept

-0.05864

0.00302

1-(Mt/Mo)

Slope

0.03081

1.11508E-4

0.6

0.4

pH 6.5 R =0.9684 2

0.2

0.0

0

200

400

600

800

1000

1200

1400

0

0

200

Time(min)

800

1000

1200

pH 8 2 R =0.9411

1-(Mt – Mo)

10

0.8

8

0.6

6

0.4

4

0.2

1 -(M t/M o )

t/qt

t/q t

1.0

pH 8 2 R =0.9833

2 800

Time(min)

1000

Time (min)

1200

35

40

1400

0.6

0.4

pH 8 R =0.9111 2

0.2

0.0 600

30

0.8

12

400

25

1.0

14

1.2

200

20

16

1.4

0

15

I (Parabolic diffusion)

18

1.6

10

t^0.5 t0.5

H (Pseudo-second order)

2.0 1.8

5

1400

Time (min)

G (Pseudo-first order)

lo g (q e -q t)

600

Time(min)

Time (min)

Log(qe–qt)

400

0.0

0 0

200

400

600

800

1000

1200

1400

TimeTime(min) (min)

0

5

10

15

20

t

25

30

0.5 t^0.5

30 Fig. 7 Fitting the data of hexenoic acid released from its nanohybrid into distilled water at different pHs using first, pseudosecond order and parabolic diffusion models at pH4 (A-C), 6.5 (D-E) and 8 (G-I).


35

40

Page 31 of 36


Fig. 8 Phytotoxicity of green beans and wheat seeds in distilled water (water), zinc oxide (ZnO), hexenoic acid nanohybrid (HEN) and hexenoic acid (HE) at 0.01, 0.1 and 1.0 g/100mL concentration. The data were presented as mean + S.D.(n=90).

31



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Fig. 9 Radical seed growth of green beans and wheat seeds in distilled water (water), zinc oxide (ZnO), hexenoic acid nanohybrid (HEN) and hexenoic acid (HE) at 0.01, 0.1 and 1.0 g/100mL concentration . The results are given as mean + S.D. (n = 90)

32


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Table 1. Surface properties of ZnO and HEN Material

BET surface area (m2g-1)

BET BJH average pore desorption pore volume (cm3g-1) diameter (Å)

BJH average pore diameter (Å)

ZNO

6

0.01

64

111

HEN

4

0.03

297

121

Table 2 Particle size distribution (PSD) of ZnO and HEN analysed from DLS and TEM Mean diameter (nm)

Method

Sample

TEM

DLS

ZnO

108 + 32

579 + 12

HEN

49 + 15

257 + 3

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Table 3. Parameters derived from the fitting of the data obtained from the release of HE from HEN into distilled water at pH 4, 6.5 and 8. pH

Saturation

Correlation coefficient ,R

Release (%)

Pseudofirst order

Pseudosecond order

2

Pseudo-second order

Parabolic diffusion

Rate constant, k (g/mg.h) x 10-5

t1/2 (min)

4

84

0.9130

0.9357

0.8856

1.81

250

6.5

73

0.9536

0.9692

0.9684

1.24

301

8

83

0.9411

0.9833

0.9111

2.69

235

Table 4. Mean values of percentage seed germination of green beans and wheat seeds in distilled water (water), zinc oxide (ZnO), hexenoic acid nanohybrid (HEN) and hexenoic acid (HE) at different concentrations. Seed germination (%) Conc. g/100mL

Green beans Water

0

ZnO

HEN

Wheat seed HE

Water

ZnO

HEN

HE

91.11+0.47

-

-

-

82.22+0.55

-

-

-

0.01

-

93.33+0.82

97.78+0.47

97.78+0.77

-

80.00+0.82

73.33+0.94

88.89+0.85

0.10

-

95.56+0.47

97.78+0.65

95.56+0.47

-

84.44+0.65

77.78+0.45

0

1.00

-

97.78+0.55

95.56+0.72

0

-

86.67+0.75

82.22+0.65

0

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Table 5. Mean value of radical seed length of green beans and wheat seeds in distilled water (water), zinc oxide (ZnO), hexenoic acid nanohybrid (HEN) and hexenoic acid (HE) at different levels of concentration solution. (n=90). Radical seed length (mm) Conc. g/100mL

Green beans

Water 0

ZnO

Wheat seed

HEN

HE

Water

ZnO

HEN

HE

13.33+0.47

-

-

-

6.63+0.87

-

-

-

0.01

-

28.65+0.88

31.22+0.56

25.9

-

9.34+0.69

10.65+0.49

7.93+0.75

0.10

-

33.55+0.54

27.35+0.78

20.7

-

10.22+0.78

15.67+0.85

0

1.00

-

35.25+0.79

22.22+0.85

0

-

13.15+0.94

12.28+0.77

0

35



Page 36 of 36

TOC GRAPHIC

ZLH (host) ZnO

Release media

Co-precipitation

+

Dissolution of host and diffusion of anion

ZLH (host)

Hexenoic Acid

Release of guest hexenoate from ZLH host into release media

ZLH (host)

100

Green beans seeds

) 35 m m ( 30 h t 25 g n el 20 d ee s 15 la ci d10 a R5

HEN

ZnO

HE

pH 8 pH 6.5 pH 4

80

% Anion Release

40

Water

Wheat seeds

60

40

20

0 0

0 0

0.01

0.1

1

0

0.01

0.1

200

400

600

800

1000

1200

Time(min)

1

Concentration (g/100 mL)

14 12

t/q t

10 8 6 4 2

40

40

35

35

ZnO 30 25 20

HEN 15 10 5

1.8 1.6

25 20 15

200

400

600

800

1000

1200

Time(min)

1.0 0.8

HEN

0.6

HEN 0.2

5 0.0

1400 0

10

0 0.0

0.2

0.4

0.6

0.8

1.0


Pseudo-second kinetic model

1.2

0.4

10

0 0

ZnO

ZnO

1.4

30

P o r e V o lu m e (c m 3 /g )

V olum e a bs orbe d at S T P (c m 3 /g)

16

V olum e a bs orbe d at S T P (c m 3 /g)

18

0.0

0.2

100

0.4

0.6 Pore diameter 0.8 (Å)

Adsorption-desorption Isotherm (BET) & BJH for ZnO and HEN


1000

1.0


36

1400

Evaluation of Controlled-Release Property and ... - ACS Publications

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