Evaluation of Controlled-Release Property and Phytotoxicity Effect of ...

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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 Taufiq-Yap Yun Hin∥ †

Material Synthesis and Characterization Laboratory (MSCL), Institute of Advanced Technology (ITMA), Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia ‡ Forest Biotechnology Division, Forest Research Institute Malaysia (FRIM), 52109 Kepong, Selangor, Malaysia § Faculty of Applied Science, Universiti Teknologi MARA (UiTM), 40540 Shah Alam, Selangor, Malaysia ∥ Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia ABSTRACT: A controlled release formulation for the insect pheromone hexenoic acid (HE) was successfully developed using zinc-layered hydroxide (ZLH) as host material through a simple coprecipitation technique, resulting in the formation of inorganic−organic nanolayered material with sustained release properties. The release of HE from its nanohybrid was found to occur in a controlled manner, governed by a pseudo-second order kinetics model. The maximum amount of HE released from the nanocomposite into solutions at pH 4, 6.5, and 8 was found to be 84, 73, and 83% for 1100 min, respectively. The hexenoate zinc-layered hydroxide nanomaterial (HEN) was found to be nontoxic for plants when green beans and wheat seeds were successfully germinated in all HEN concentrations tested in the experiment, with higher percentage of seed germination and higher radical seed growth as compared to its counter anion, HE. ZLH can be a promising carrier for insect pheromone toward a new generation of environmentally safe pesticide nanomaterial for crop protection. KEYWORDS: hexenoic acid, zinc-layered hydroxide nanohybrid, insect pheromone, controlled release, phytotoxicity well, which include nanospheres,15 nanogels16 and nanofibers.17,18 Research on inorganic−organic nanomaterial, namely, layered double hydroxide (LDH) and layered hydroxide salt (LHS) nanocomposites, has grown rapidly, due to their unique structure as ion exchanger and their tailor-made behavior resulting in various potential applications. After early work on LDH as catalysts19 and catalyst precursor,20 the research proceeded to polymer, 21,22 flame retardant for smoke suppression,23,24and corrosion inhibitor25 and was extended as controlled release formulation for drugs,26,27 agrochemicals,28 and removal of toxic substances in environmental applications.29,30 Nanolayered structure material of layered inorganic−organic nanocomposite can be formed by encapsulation of an organic moiety into inorganic interlayer spacings of LDH and LHS which act as host. The brucite-like structure of both LDH and LHS provides an ion exchange platform for many chemical types: nitrates, sulfates and organic acids. High positive charge densities of LDH and LHS can accommodate the anionic organic compound within the interlayer spaces to compensate the positive charge, and consequently form high stability material.

1. INTRODUCTION Research on nanotechnology for agricultural applications has gained high interest due to major concern on the use of chemical pesticides in crop production. Excessive use of agrochemicals is usually not fully utilized by plants as most of them are lost through leaching in soil.1,2 Direct application of pesticide onto the plant through spraying will contaminate the crop, leaving harmful chemical residue, which enters through direct pathway into the human food chain.3 It also causes environmental pollution in the ecosystem and poses a problem to human health. The use of a pheromone in the orchard for insect control has proved successful and could become a potential biopesticide.4,5 However, the application of pheromone is not effective under field condition as most of the chemical is volatile and easily affected by environmental factors which include temperature, sunlight, wind and rainfall.6 A controlled release formulation hence is required to provide precise delivery target and release the active chemicals in smaller amounts and thus help to reduce the amount of chemical to a safe level.3 A number of controlled released carriers in nanopesticide research have been widely studied to provide an environmentally safe, non pesticide product. This includes polymerbased nanoformulations comprising polysaccharide materials such as starch,6,7 alginates,8 chitosan,9,10 polyethylene glycol,11 and polyester substance.12 Plant protection products derived from natural materials are also in used which consist of neem oil13 and garlic essential oil.14 The list extends to other forms as © 2015 American Chemical Society

Received: Revised: Accepted: Published: 10893

June 26, 2015 October 11, 2015 October 26, 2015 October 26, 2015 DOI: 10.1021/acs.jafc.5b03102 J. Agric. Food Chem. 2015, 63, 10893−10902

Article

Journal of Agricultural and Food Chemistry

for 15 min. Hexenoic acid solution of various concentrations, 0.1, 0.2, 0.3, and 0.4 M, were each prepared by dissolution of the respective amount of HE in 40 mL of ethanol and adjusted to 100 mL volume in a volumetric flask with deionized water. Each of the HE solutions was then added dropwise into a ZnO suspension with constant stirring, producing a clear mixed solution at the end of the process. The pH of the solution was adjusted to 7.9 with 0.5 M NaOH aqueous solution to obtain white precipitate. The slurry solution was vigorously stirred via a magnetic stirrer for 3 h, and the aging process was further continued in an oil bath shaker for 18 h at 70 °C. The obtained product was centrifuged, thoroughly washed with deionized water to wash away any contaminants, and then dried in an oven at 70 °C. The obtained material was then powdered for further use and characterizations. 2.2. Characterization. Powder X-ray diffraction patterns of nanohybrids were obtained at 2−60° on a Shimadzu diffractometer, XRD-6000 using Cu Kα radiation (λ = 1.5418 Å) and dwell time of 4 degrees per minute. Surface characterization of the synthesized product was carried out using a nitrogen gas adsorption method at 77 K on a Micromeritics ASAP 2000. The sample was degassed in an evacuated heated chamber at 100 °C overnight. Surface morphology of the synthesized nanohybrid was captured on a field emission scanning electron microscope (FESEM), model JEOL JSM-7600F. The dried sample was dispersed on a conductive carbon adhesive tape surface which was attached to a FESEM stub, and then gold coated. Internal morphology of ZLH-HE nanohybrids was observed using a Hitachi H7100 transmission electron microscope (TEM) at magnifications 30− 200K. A drop of the nanohybrid−20% v/v ethanol dispersion was placed onto a 300 mesh Formvar copper grid and air-dried overnight in a desiccator. The average diameter and particle size distribution (PSD) of the nanohybrid were measured using dynamic light scattering (DLS) photon cross correlation spectroscopy (PCCS). The sample was dispersed in 20% v/v ethanol prior to analysis. 2.3. Kinetic Release Study. The release of hexenoic acid from the nanohybrid host into different media was accomplished using distilled water at pH = 4, 6.5, and 8 by adding 2 mg of the nanohybrid into 3.5 mL of the solution. The pH solution in distilled water was obtained by adjusting its pH using HCl or NaOH, and pH values were measured using a pH meter. The accumulated amount of hexenoic acid released into the solution was measured at a preset time at 207 nm using a Perkin Elmer UV−visible spectrophotometer, Lambda 35. The cumulative release pattern was fitted to first and pseudo-second order kinetics and parabolic diffusion models. 2.4. Phytotoxicity Test. The pytotoxicity test for ZLH-HE nanohybrid was carried out on two types of seed using a method adopted by Keeling.46 The nanohybrid material of different weights, 0.01, 0.1, and 1 g, and 100 mL of distilled water were mixed respectively and shaken using an orbital shaker for 1 h. The mixture was filtered through Whatman filter paper no. 1. 3 mL of the filtered solution was pipetted on a Petri dish which was placed with a filter paper Whatman no. 1 containing 15 seeds of green beans and wheat. The Petri dish was covered to avoid loss through evaporation. The Petri dishes were then incubated at 28 °C in an incubator for 48 h. Various types of controls which consisted of distilled water, ZnO, and HE were carried out to determine their effect on plants. A series of concentrations of 0.01 g/100 mL, 0.1 g/100 mL, and 1 g/100 mL of HE and ZnO, respectively, were also prepared as in HEN, and then tested on the two seeds.

LDH is represented by the general formula [M2+1−xM3+x(OH)2]x+(Am−)x/m·nH2O, while LHS is given by the general formula M2+(OH)2−x(An−)x/n·mH2O, where M2+ and M3+ are the divalent and trivalent metallic cations, respectively. In both layered hydroxide structures, Am− is the interlayered anion that balances the positive charge layers. Thus, LDH and LHS nanomaterial are represented by positively charged brucite-like inorganic layers with anions and water molecules existing between the interlayers.31,32 ZLH is a type of LHS, composed of one type of divalent metal cation in which only zinc and hydroxyl represent the inorganic layers. ZLH is composed of layers of octahedral coordinated zinc cations, in which 1/4 of them are displaced leaving an empty octahedral site forming cationic centers tetrahedrally coordinated to the top and bottom of the octahedral sheet. Water molecules occupy the apexes, and the nitrate counterions are free between the interlayers. Due to its higher charge densities and possession of larger interspacing than LDH, ZLH can become host for a higher number of guest anions of different sizes.33 This makes it potential to be used as a carrier and slow release delivery system for many chemical agents in various applications such as drugs,34,35 anticorrosion agent,36 herbicides,37 dye industry,38 sunscreen absorber,39,40 catalyst,41 DNA,42 food preservatives,43 and pharmaceutical and nutracuetical agents.44 Meditarranean fruit flies (Ceratitis capitata) are categorized as agricultural pests as they attack both unripe and ripe fruits. This causes a problem in the quality of the food supply and affects agriculture yield. In this study, hexenoic acid, a pheromone for this pest, is being used as guest with ZLH as host for the formulation of a host−guest nanocomposite with controlled release properties. To our knowledge, the use of ZLH as a controlled release carrier for insect pheromone has not yet been reported in the open literature. Here, we discussed our work on the intercalation of trans-2-hexenoic acid (HE), C6H10O2, a short chain carboxylic anion, into the interlayer of ZLH by a simple coprecipitation method. A new organic−inorganic ZLHHE nanohybrid compound was synthesized by direct reaction of ZnO with HE under aqueous environment followed by precipitation with alkaline solution, and subsequently its control release behavior and phytotoxicity effect were evaluated. ZLH as the encapsulated host material for active chemicals can provide beneficial input to the environment. It can be used as a zinc source for soil supplement, which is an advantage to the soil as most soil around the world is deficient in zinc. It is an essential micronutrient for plant development, and plants obtain most of their nutrients from the soil in which they grow.45 The phytotoxicity test of ZLH-HE nanohybrid carried out on two types of seed shows higher percentage of germination and longer radical seed length compared to its counterpart chemical, HE. This indicates that the synthesized nanolayered structure material is not toxic to plants and is intended toward a new generation of green, environmentally safe pesticide nanomaterial for insect control and crop protection.

3. RESULTS AND DISCUSSION 3.1. Powder X-ray Diffraction Analysis. The PXRD patterns of the unbound chemical, HE, and ZLH-HE nanolayered structure materials prepared at various concentrations of HE, 0.1−0.4 mol/L using 0.2 g of ZnO are portrayed in Figure 1. The powder X-ray diffraction pattern of ZnO reflects five sharp peaks in the 30−60° region, corresponding to reflections of 100, 002, 101, 102, and 110 lattice planes, which indicate high crystallinity, which represents a distinctive pattern of metal oxide. The PXRD pattern of the counterpart anion,

2. EXPERIMENTAL PROCEDURES 2.1. Synthesis of Materials. All chemicals used in this experiment were obtained from various chemical suppliers and applied without any further purification. All solutions were prepared using deionized water. trans-2-Hexenoic acid nanohybrid (HEN) was synthesized by coprecipitation method using ZnO as starting material. About 0.20 g of ZnO was first suspended into 100 mL of deionized water and stirred 10894

DOI: 10.1021/acs.jafc.5b03102 J. Agric. Food Chem. 2015, 63, 10893−10902

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Journal of Agricultural and Food Chemistry Table 1. Surface Properties of ZnO and HEN

av pore diameter (Å) material

BET surface area (m2 g−1)

BJH desorption pore vol (cm3 g−1)

BET

BJH

ZnO HEN

6 4

0.01 0.03

64 297

111 121

Figure 3. FESEM micrograph of (a) ZnO and (b) HEN at 50000× magnification.

Figure 1. (a) PXRD patterns of HE, ZnO, and HEN nanohybrids prepared at different concentrations of HE. (○ = ZnO phase, peak from left to right is for 100, 002, 101, 102, 110 reflections). (b) Slow scan of PXRD patterns of HEN nanohybrid prepared at 0.3 mol/L HE.

HE (Figure 1), shows some reflection peaks, which demonstrates the crystalline nature of this chemical. The produced nanolayered structure material synthesized at various concentrations of HE ( Figure 1) with fixed amount of 0.2 g of ZnO, showing reflection peaks diffracted at lower 2θ angle with increase in the basal d-spacing, is evidence of inclusion of the anion, HE, inside the interlayer spacing of ZLH. The resulting nanolayered material prepared from 0.1 M HE shows the presence of ZnO phase, which indicates incomplete reaction, while the nanohybrids synthesized at concentrations 0.2 and 0.3 mol/L HE show disappearance of ZnO characteristic peaks. The expansion basal spacing of nanohybrid material synthesized at 0.2 and 0.3 mol/L HE is 24.6 and 23.5 Å, respectively, and disappearance of ZnO peak indicates that successful intercalation of the organic anion

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

between the inorganic ZLH interlayer has taken place. However, comparing both 0.2 and 0.3 mol/L HE together, the nanolayered structure material obtained using 0.3 mol/L HE shows a symmetric peak with reflection up to three separate harmonics at 2θ angle of 3.76°, 7.50°, and 12.45° with

Figure 2. Adsorption−desorption isotherms for zinc oxide and HEN (a) and Barret−Joyner−Halenda method pore size distribution for zinc oxide and HEN (b). 10895

DOI: 10.1021/acs.jafc.5b03102 J. Agric. Food Chem. 2015, 63, 10893−10902

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Journal of Agricultural and Food Chemistry

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

changed to the layered material. The mechanism of the process is given in eqs 1−3.

Table 2. Particle Size Distribution (PSD) of ZnO and HEN Analyzed from DLS and TEM mean diam (nm) sample

TEM

DLS

ZnO HEN

108 ± 32 49 ± 15

579 ± 12 257 ± 3

ZnO + H 2O → Zn(OH)2

(1)

Zn(OH)2 → Zn 2 + + 2OH−

(2)

Zn 2 + + 2OH− + HE− + H 2O → Zn 2 +(OH)2 − x (HEm −)x / m . nH 2O

(3)

3.2. Surface Properties. The adsorption−desorption isotherms of nitrogen gas on ZnO and its nanolayered material, HEN, are depicted in Figure 2a. Based on the IUPAC classification, both ZnO and HEN portrayed a similar pattern of adsorption−desorption isotherms with type IV indicating mesopore and/or nonporous types of material. The adsorption for ZnO showed that it started from a slow uptake at low relative pressure in the range of 0.0−0.9, followed by a rapid increase in adsorption of the adsorbent of >0.9, reaching its optimum uptake of nitrogen gas at 33 cm3/g. Similarly, the nanolayered material, HEN, gave a slow adsorption increment at low relative pressure in the range of 0.0−0.8 and then a quick rise of the adsorbate at a relative pressure of more than 0.8, approaching saturated uptake of 26 cm3/g. The effect of surface properties of the produced nanolayered structure material upon successful intercalation of organic anion, HE, into the interlayer space of zinc-layered hydroxide was evaluated by measuring the surface area and pore size distribution, and the data are summarized in Table 1. The surface area of ZnO and HEN analyzed by the Brunauer, Emmet, and Teller (BET) method shows a reduction in the surface area of HEN with a value of 4 m2/g, compared to ZnO with a value of 6 m2/g. The same trend was also observed for the formation of other nanohybrids reported for 2,4,5trichlorophenoxyacetic acid and ellagic acid as guest anions into LDH and ZLH.35,48 The microstructure of the material used was reported to influence the surface property.48 It is believed that the nature of the guest anion used could also contribute to the decrease in surface area. As mentioned in the earlier discussion, the counter anion, HE, has a combination of amorphous and crystallinity structure, which might affect the formation of the produced nanolayered material. The intercalation of HE into the ZLH resulting the formation of HEN has resulted in the increase in pore size. The BET average

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

respective d-basal spacing values of 23.5, 11.8, and 7.1 Å as shown in the slow scan PXRD patterns (Figure 1b). This indicates the formation of a well ordered 2D layered structure of the nanocomposite. Hence, the nanolayered structure material obtained from 0.3 mol/L HE was selected for further characterizations and labeled as HEN. The formation of nanolayered structure material of ZLH-HE from direct reaction of ZnO in a hydrated environment of HE is reported to follow the “dissociation−deposition” mecahanism process.31,47 The hydrolysis of ZnO in an aqueous environment takes place to form Zn(OH)2 on the surface of solid particles. Further reaction led to the dissociation of Zn(OH)2 in the HE solution, releasing Zn2+. The released Zn2+ then reacted with hydroxyls, HE anions, and H2O in the solution to obtain nanolayered ZLH-HE material. The process is repeated until all the ZnO phase and the Zn(OH)2 phase has completely 10896

DOI: 10.1021/acs.jafc.5b03102 J. Agric. Food Chem. 2015, 63, 10893−10902

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Journal of Agricultural and Food Chemistry

Figure 7. Fitting the data of hexenoic acid released from its nanohybrid into distilled water at different pHs using first-order, pseudo-second order, and parabolic diffusion models at pH 4 (A−C), 6.5 (D−F), and 8 (G−I).

distribution adsorption similar to that of ZnO with the highest peak around 12 Å. The intercalation of HE into the interlayer ZLH had changed the pore size and pore volume of the obtained nanolayered material HEN higher than its host. The BJH pore diameter and volume were increased from 111 Å (ZnO) to 121 Å (HEN) and 0.01 cm3 g−1 (ZnO) to 0.03 cm3 g−1 (HEN), respectively. 3.3. Morphology and Particle Size Distribution of Intercalated Compound. The surface morphology of ZnO and HEN captured on FESEM are presented in Figures 3a and 3b, respectively. The morphology of zinc oxide exhibited nonuniform granular structure with no specific shapes while HEN showed multilayer of nonuniform broad and flat shapes and sizes. This shows that the transformation of ZnO into an intercalated compound resulted in changes in the surface morphology. Internal microstructure images of ZnO and HEN examined on TEM are shown in Figures 4a and 4b, respectively. It is

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 correlation coeff, R2

pH

saturation release (%)

pseudofirst order

pseudosecond order

4 6.5 8

84 73 83

0.9130 0.9536 0.9411

0.9357 0.9692 0.9833

pseudo-second order

parabolic diffusion

rate constant, k (mg/min) × 10−5

t1/2 (min)

0.8856 0.9684 0.9111

1.81 1.24 2.69

250 301 235

pore diameter of HEN was found to be higher than ZnO, with values of 297 and 64 Å respectively. The desorption pore size distributon of ZnO and HEN was determined by Barret−Joyner−Halenda (BJH) procedure, and the plot is displayed in Figure 2b. The pore size distribution pattern of ZnO shows the tallest peak at 18 Å along with two other scattering peaks, while HEN shows a trend of pore size 10897

DOI: 10.1021/acs.jafc.5b03102 J. Agric. Food Chem. 2015, 63, 10893−10902

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Journal of Agricultural and Food Chemistry

Figure 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/100 mL concentration. The data are presented as mean ± SD (n = 90).

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 (%) green beans seeds concn g/100 mL

water

0 0.01 0.10 1.00

91.11 ± 0.47

ZnO

wheat seeds

HEN

HE

water

ZnO

HEN

HE

80.00 ± 0.82 84.44 ± 0.65 86.67 ± 0.75

73.33 ± 0.94 77.78 ± 0.45 82.22 ± 0.65

88.89 ± 0.85 0 0

82.22 ± 0.55 93.33 ± 0.82 95.56 ± 0.47 97.78 ± 0.55

97.78 ± 0.47 97.78 ± 0.65 95.56 ± 0.72

97.78 ± 0.77 95.56 ± 0.47 0

particle size distribution (PSD) of ZnO and HEN on TEM images were measured using UTHSCSA image tool software. The PSD was found to be 31 to 157 nm for ZnO and 23 to 87 nm for HEN. The average PSD of ZnO and HEN was estimated to be 108 ± 32 nm and 49 ± 15 nm, respectively. High standard deviation is attributed to the nonuniform shapes and irregular sizes for both ZnO and HEN. The PSD of ZnO and HEN (Figures 5a and 5b) was also carried out using DLS (dynamic light scattering) measurement, however the results were not in good agreement with TEM measurement. The TEM result is more meaningful due to its direct observation. In addition, the tendency of both ZnO and HEN to reagglomerate during the DLS analysis cannot be ruled out. A summary of the results of particle size distribution from TEM and DLS is given in Table 2. 3.4. Release Behavior of HE into Distilled Water at Various pHs. A series of distilled water at various pHs was prepared to study the effect of different pHs on the release behavior of HE from the interlayer spacing of the nanolayered material, HEN. The release profiles of HE from the organic− inorganic layer of its nanomaterial, HEN, into pH 4, 6.5, and 8 are depicted in Figure 6. All the release profiles show a broad

Figure 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/100 mL concentration. The results are given as mean ± SD (n = 90).

observed that ZnO has granular shape, and this corresponds nicely with the FESEM image (Figure 3a). On the other hand, HEN exhibited nonuniform shape, overlapping each other. The

Table 5. Mean Values 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 radical seed length (mm) green beans concn g/100 mL

water

0 0.01 0.10 1.00

13.33 ± 0.77

wheat seed

ZnO

HEN

HE

28.65 ± 0.88 33.55 ± 0.54 35.25 ± 0.79

31.22 ± 0.56 27.35 ± 0.78 22.22 ± 0.85

20.9 + 0.67 25.7 + 0.84 0

water

ZnO

HEN

HE

9.34 ± 0.69 10.22 ± 0.78 13.15 ± 0.94

10.65 ± 0.49 15.67 ± 0.85 12.28 ± 0.77

7.93 ± 0.75 0 0

6.63 ± 0.87

10898

DOI: 10.1021/acs.jafc.5b03102 J. Agric. Food Chem. 2015, 63, 10893−10902

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coefficient values were used to determine the best kinetic models that govern the kinetic release. The three kinetic equations given below were used in this work:

curve, indicating a constant release of the anion from the nanomaterial into the solution at various pHs. Generally, the percentage anion release was observed to increase constantly in the range of 800 min from the starting time of dispersion of the nanomaterial in the pH solution, followed by a slower release before attaining equilibrium rate after 1100 min. A high amount of anion release was observed in the first 200 min with percentage release recorded to be 34, 26, and 36% in distilled water at pH 4, 6.5, and 8, respectively. This is due to the “burst effect” which is caused by the high release of anions that are weakly adsorbed on the external surface of layered ZLH.48−50 Within the range of 200 to 400 min, the rate of anion release has increased up to 26, 18, and 23% to obtain the accumulated release of 60, 44, and 59% for pH 4, 6.5, and 8, respectively. The following 400 to 800 min, the rate of HE release into pH 4, 6.5, and 8 has increased in the range of 18 to 20% to produce a total amount of anion release of 80, 64, and 77%, respectively. It was observed that the release of HE into pH 4 and 8 is faster than at pH 6.5. This is due to the partial dissolution of ZLH occurring with the collapse of nanolayered structure and the releasing of pheromone anions to the environment.48,51−53 This indicates that ZLH is not stable at high and low pHs,48,54 which causes the interlamellar layer to dissolve and affect the composition amount of HE from the nanomaterial. Pan et al.55 reported release of drug (doxifluridine) at high pH with release of 60% at pH 6.8 and 72% at pH 7.4 from Mg/Al layered hydroxides. At pH 6.5, ZLH is more stable and the release of HE is slower as compared to pH 4 and 8. The release could be related to the diffusion of anion from the interlayer as suggested by Gao et al.56 such that the deintercalation of intercalated anion vitamin C in deionized water followed the diffusion mechanism, whereby the rate of drug diffusion is controlled by the rigidity of the layers and the diffusion path length. After diffusion, the release of HE anions proceeded via anion exchange process. Gao et al.56 reported that 36% of vitamin C was released into deionized water from its MgAl intercalated compound. In this study, at pH 6.5 medium, HE could be ion exchanged with carbonate anion from the dissolution of carbon dioxide from the atmosphere in distilled water.57 The ion exchange process of HE in pH 4 medium could be related to the chlorine anion from HCl solution for preparation of pH 4 solution.51 Excess OH− in pH 8 medium might contribute to the ion exchange process of HE. However, at 800 min and beyond, a slower release process of HE was observed at the rate of 2 to 5%. This could be attributed to the release of HE ions from deeper interlayer sites52 and ion exchanged with anions in the solution and later proceeding through diffusion within the interlayer space.53 The saturated amount of HE released into solutions of pH 4, 6.5, and 8 was achieved at 84, 73, and 83%, at 1139, 1159, and 1143 min, respectively. It was found that HE molecules not fully releasing to 100% could be attributed to the anions being strongly held to positive charge of host, resulting in reduced diffusion from the interlayer into the medium solution,58 and some remains entrapped inside the inorganic layered host. At equilibrium stage, the interlayer anions cannot be exchanged completely and the intercalated anions would be released continuously in a slow manner.51 3.5. Release Kinetics of HE from HEN. In order to understand the release mechanism of HE from the nanolayered material, HEN, into solutions of different pHs, the release data were fitted into various kinetic models. The highest correlation

pseudo‐first order: ln(qe − qt ) = ln qe − k1t

(4)

pseudo‐second order: t /qt = 1/k 2qe 2 + t /qe

(5)

parabolic diffusion: (1 − M t /M 0)/t = kt −0.5 + b

(6)

where qe and qt represent the equilibrium release amount and release amount of anion at any time t, respectively, and k is the apparent release-rate constant respectively while M0 and Mt correspond to the chemical content remaining in the ZLH at zero time and release time, t, respectively. The plots for all the fitting are given in Figure 7, and the parameters obtained are summarized in Table 3. It was found that the pseudo-second order model offers a more satisfactory description of the kinetics release of HE from its nanohybrid, HEN, compared to other kinetic models use in this work. The highest correlation coeffiecients, R2, were obtained from the pseudo-second order model equation with value of 0.9357, 0.9692, and 0.9833 for release of HE into pH 4, 6.5, and 8 solution, respectively. The t1/2 value, which represented the time taken for HE concentration to be half of its accumulated release, and the release rate values, k, of HE into different pH media were calculated and tabulated in Table 3. t1/2 and k values of HE are 250 min and 1.81 × 10−5 mg/min (pH 4), 301 min and 1.24 × 10−5 mg/min (pH 6.5), and 235 min and 2.69 × 10−5 mg/min (pH 8). This corresponds to the maximum accumulated values of 84, 73, and 83% HE in pH 4, 6.5, and 8, respectively, which strongly indicates that the release of HE from HEN into pH 6.5 is slower compared to pH 4 and 8. HE release from HEN into pH 6.5 solution has the lowest k rate which delivers small amount of HE at longer duration time. The release of hexenoic acid from its interlamellae of its organic−inorganic nanohybrid at various pHs was found to be in a controlled manner governed by the pseudo-second order kinetic model. The amount of HE released at pH 4 and 8 is higher than at pH 6.5 with the order of pH 4 = 8 > 6.5. 3.6. Phytotoxicity Test. In order to investigate the toxicity of HEN to plants, a phytotoxicity test of the nanolayered material was tested using two seed types, i.e., green beans and wheat seeds germinated on distilled water and a series of concentrations of 0.01 g/100 mL, 0.1 g/100 mL, and 1 g/100 mL of ZnO, HE, and HEN, respectively. The results of seed germination are shown in Figure 8 and Table 4. In Table 4, it can be seen that the percentage seed germination of green beans on untreated sample, i.e., distilled water alone, was recorded to be the lowest percentage of 91% when compared to the other chemical treatments. The percentage seed germination of green beans exposed to ZnO extract solution increased to 93, 96, and 98% with increment to ZnO concentration of 0.01 g/100 mL, 0.1 g/100 mL, and 1 g/100 mL, respectively. This could be attributed to the availability of Zn from ZnO, which is required for plant development. For free anion, HE, at 0.01 g/100 mL and 0.1 g/100 mL concentration, the percentage seed germination was 98 and 96% respectively. However, at 1 g/100 mL HE, green beans failed to germinate, which indicates that it is unsuitable and toxic for seed germination at high concentration. This is in contrast with nanolayered material, HEN, for which seed germination of green beans was successful in all the concentrations tested. HEN shows a high percentage of green 10899

DOI: 10.1021/acs.jafc.5b03102 J. Agric. Food Chem. 2015, 63, 10893−10902

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bean germination of 98% at 0.01 g/100 mL and 0.1 g/100 mL, and slightly reduced to 96% at 1 g/100 mL concentration. The seed germination result demonstrated that the nanolayered material, HEN, is not toxic to plants under our experimental conditions. Wheat seeds gave a slightly different response compared to green beans. In distilled water, wheat seeds recorded 82% of germination and the percentage increased to 84 and 87% when it was treated with 0.1 g/100 mL and 1 g/100 mL ZnO, respectively. On the other hand, the germination of wheat seed was found to survive at 0.01 g/100 mL HE with 89%, but at higher concentration of HE, no wheat seed was found to be germinated. For HEN, wheat seeds were successfully germinated at all concentrations with recorded values of 73% (0.1 g/100 mL), 78% (0.1 g/100 mL), and 82% (1 g/100 mL) for seed germination, respectively. This shows that when HE is converted into nanolayered structure material of HEN, the phytotoxicity of HEN on both green beans and wheat seeds was found nontoxic to plant and safe for seed germination. HE from the nanohybrid could be released to surroundings in a sustained manner when in contact with anions of higher affinity such as carbonate in air. The release of a small amount of HE from its nanohybrid to surroundings did not toxicate the seed germination as compared to its free anion, HE. Partially dissolved Zn from the collapse of ZLH interlayer structure during the anion exchange is used up and increases the seed germination. Radical seed length for both seeds on untreated sample and at different chemical treatments was measured to determine the effect of HE, HEN, and ZnO on the radical seed growth, and the results are depicted in Figure 9 and Table 5. Figure 9 shows that the radical seed length of both green beans and wheat seeds gave higher values for all the chemical treatments when compared to untreated sample, distilled water. The radical seed growth for both seeds increased as the amount of ZnO increased. As expected, this is due to the effect of ZnO as zinc source as an essential micronutrient for plant development, which has important effect on the radical seed growth. The radical seed length of both seeds germinated on free anion, HE, recorded lower values than its nanomaterial, HEN, as shown in Table 5. HEN recorded higher radical seed length for green beans, measuring 31.2 mm (0.01 g/100 mL) and 27.1 mm (0.1 g/100 mL), than its counterpart, HE, with values of 25.9 mm (0.01 g/100 mL) and 20.7 mm (0.1 g/100 mL), while radical seed length for wheat seed was recorded as 10.6 mm (0.01 g/100 mL HEN) compared to 7.9 mm (0.01 g/100 mL HE). This could be due to the effect of ZLH that enhances the properties of HEN and also contributes Zn source as an important micronutrient for plant development. This further indicates that the nanolayered material, HEN, influences and improves the radical seed growth compared to its free anion, HE. The controlled release properties of the nanohybrid could contribute to the situation that occurred. HE anion is released slowly from its nanohybrid to the surroundings and used by seed for further development. This study suggests the possibility of zinc-layered hydroxide as a promising encapsulate material for insect pheromone with controlled release capabilities for the generation of environmentally safe pesticide for crop protection.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel:+6-03-8946-8092. Fax:+603-8943-5380. Funding

Funding for this research was provided by the Ministry of Higher Education (MOHE) under Grant No. FRGS/1/11/ SG/UPM/01/2 (vot. No. 5524165) and is greatly appreciated. The authors gratefully acknowledge FRIM for sponsoring R.A. in her MSC program. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS USED ZLH, zinc-layered hydroxide; ZnO, zinc oxide; LHS, layered hydroxide salt; HE, hexenoic acid (C6H10O2); HEN, hexenoic acid nanohybrid; NaOH, sodium hydroxide; FESEM, field emission scanning electron microscope; TEM, transmission electron microscope; PSD, particle size distribution; DLS, dynamic light scattering; PCCS, photon cross correlation spectroscopy

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