Simultaneous Intercalation of 1-Naphthylacetic Acid and Indole-3-butyric Acid into Layered Double Hydroxides and Controlled Release Properties

Controlled release formulations have been shown to have potential in overcoming the drawbacks of conventional plant growth regulators formulations. A controlled-release formulation of 1-naphthylacetic acid (NAA) and indole-3-butyric acid (IBA) simultaneous intercalated MgAl-layered double hydroxides (LDHs) was prepared. The synthetic nanohybrid material was characterized by various techniques, and release kinetics was studied. NAA and IBA anions located in the gallery of MgAl-LDHs with bilayer arrangement, and the nanohybrids particles were of typical plate-like shape with the lateral size of 50–100 nm. The results revealed that NAA and IBA have been intercalated into the interlayer spaces of MgAl-LDHs. The release of NAA and IBA fits pseudo-second-order model and is dependent on temperature, pH value, and release medium. The nanohybrids of NAA and IBA simultaneously intercalated in LDHs possessed good controlled release properties.


Introduction
In recent years, plant growth regulators (PGRs) including indole-3-butyric acid (IBA) and 1-naphthalene acetic acid (-NAA) are widely used in agriculture to increase plant growth and reproduction [1,2].Although PGRs found their way into wide applications and have played a significant part in constantly boosting agricultural production, the hazards they have brought along with them to food safety and human health have increasingly become the focus of world attention [3][4][5].Controlled release formulations (CRFs) have been shown to have potential in overcoming the drawbacks of conventional PGRs formulations, since they allow usage of minimum amount of PGRs for the same activity.Many research efforts have been devoted to developing controlled release formulations for PGRs, mainly using microencapsulation and inorganic material technology [6][7][8][9][10][11][12].Recently, layered double hydroxides (LDHs) have attracted intensive attentions because of their potential in functional materials [13][14][15][16][17][18][19][20][21][22].The layered double hydroxides (LDHs), also known as hydrotalcite-like compounds, are a class of anionic clays whose structures are based on brucite-like layers.LDHs have the general formula [M II  1− M III  (OH) 2 ] + (A − ) / ]⋅H 2 O, whereby M II are divalent, M III are trivalent metal cations, and A represents the intercalated anion [23].The application of plant growth regulators intercalated LDHs has been reported.Hussein et al. [8][9][10] first reported the synthesis of naphthaleneacetate, indole-2-carboxylate intercalated LDHs and interpreted the release mechanism.Most recently, Qiu and Hou [12] prepared the indole-3-butyric acid intercalated LDHs and studied the release kinetics.However, in several cases a concurrent controlled release of these two herbicides is necessary [24,25].To our knowledge, the preparation and simultaneous controlled release of NAA and IBA anions from LDHs interlayer have not been reported in the literature.Therefore, in the present work, NAA and IBA cointercalation into the interlayer of MgAl-LDHs is prepared.The

Measurements of Total Release Amount of NAA and IBA in
MgAl-NAA/IBA-LDHs Nanohybrids.To measure the release performances of NAA and IBA from MgAl-NAA/IBA-LDHs, 0.05 g of NAA and IBA cointercalation LDHs nanohybrid powder was dispersed in 500 mL water/ethanol solution with volume ratio 9 : 1 under magnetic stirring [12].At specified time intervals, 2 mL of solution was removed and filtered through a 0.45 m microfiltration membrane and the total contents of NAA and IBA were determined by monitoring the absorbance at 280 nm with UV-vis spectroscopy to obtain the total release amounts (  ) of NAA and IBA from LDHs nanohybrids and in turn to calculate the accumulated percent releases (  ) of NAA and IBA from the nanohybrids.
Release tests were performed in triplicate and the results were recorded as an average.

Results and Discussion
The molecular structures of NAA and IBA are given in Figures 1(a) and 1(b), respectively.The formation of carboxylate anion was done by dissolving the acid in NaOH solution and the anion was intercalated between inorganic lamella of the MgAl-LDHs as evidence from powder X-ray and FT-IR studies and will be discussed later.Based on the basal spacing  003 of 2.00 nm for MgAl-NAA/IBA-LDH observed by XRD and subtracting the thickness of brucite layer (0.48 nm) the gallery height is calculated to be 1.52 nm, which is bigger than that of NAA (0.70 nm) and IBA anions (0.85 nm).However, according to the size of IBA and NAA, a probable morphology of IBA and NAA molecules in the gallery of NAA-LDHs, IBA-LDHs, and cointercalation LDHs was proposed, as illustrated in Figures S1, S2, and S3, available online at http://dx.doi.org/10.1155/2014/862491.The gallery height of NAA/IBA cointercalation LDHs is smaller than the double sizes of the IBA anions and little bigger than the double sizes of the NAA anions.So, as shown in Figure S3, it was suspected that NAA and IBA anions located in the form of bilayer arrangement by turning the functional group on the contrary and opposing the fields of aromatic ring mutually by - interaction [28].

Fourier Transform Infrared Spectroscopy.
Figure 3 shows the FT-IR spectra of MgAl-CO 3 -LDHs, MgAl-NAA-LDHs, MgAl-IBA-LDHs, and MgAl-NAA/IBA-LDHs.In the spectrum of the MgAl-CO 3 -LDHs precursor, shown in Figure 3(a), the absorption band around 3450 and 1360 cm −1 can be ascribed to the characteristic peaks of the MgAl-CO 3 -LDHs [28].The FT-IR spectrum characteristic absorption peaks of NAA and IBA are shown in Figures S4 and S5, respectively.As shown in Figure 3, the asymmetric stretching band of -COOH in NAA and IBA at about 1695 cm −1 moves markedly toward low wavenumber (about 1553 cm −1 ) in the   nanohybrids, which can be ascribed to the ionization of NAA and IBA [12].The SEM images of NAA/IBA-LDHs nanohybrid samples are shown in Figure 5.As can be seen, the NAA/IBA-LDHs particles are of typical plate-like shape with the lateral size of 50-100 nm.

Elemental Analysis.
The results of elemental analysis and the calculated structural formula of MgAl-NAA/IBA-LDHs are listed in Table 1.As listed in Table 1, the composition and general formula of MgAl-NAA/IBA-LDHs were determined from the ICP, CHON elemental analysis, and TG-DTA analysis.The molar ratio of Mg to Al is close to the expected value (theoretical of 2.0), indicating that the reaction was complete during the reaction of coprecipitation.temperature, an exothermic peak around 380 ∘ C in DTA curve was observed, showing the decomposition of NAA and IBA.
Three different temperatures at 25, 37, and 44 ∘ C were selected to observe the effect on the total release of NAA and IBA from MgAl-NAA/IBA-LDHs nanohybrid.For temperature of 25 ∘ C, the total release of NAA and IBA was initially rapid in the first 100 minutes and then followed by a more gradual release shown in Figure 7.In addition, an increase of temperature induces the increase of NAA and IBA total release extent, indicating that the release process was an endothermic reaction [12].

Effect of Solution pH Value on NAA and IBA Total
Release. Figure 8 shows the release profile of NAA and IBA from MgAl-NAA/IBA-LDHs nanohybrid at various initial pH values.The total amounts of NAA and IBA release at pH 4 and 12 were found to be higher than that at pH 7.

Effect of Electrolyte Type on NAA and IBA Total Release.
The release profiles of NAA and IBA from MgAl-NAA/IBA-LDHs nanohybrids into the aqueous solutions of Na 2 CO 3 , Na 2 SO 4 , and NaCl (0.01 M) are shown in Figure 9.As we can see, the presence of salts may increase the release rate and accumulate release amount of NAA and IBA.The release of NAA and IBA into salt aqueous solution was found to be dependent on the anion in the aqueous solution in the order of CO 3 2− ≈ SO 4 2− > Cl − with the percentage release of 52, 51, and 47%, which is because the exchange ability of CO 3 2− for NAA and IBA is higher than those of SO 4 2− and Cl − [30].The total release of NAA and IBA from nanohybrids   should involve dissolution of nanohybrids as well as ion exchange between the intercalated NAA and IBA anions and salt anions.

3.7.
Release Kinetics of NAA and IBA from NAA/IBA-LDHs Nanohybrids.Release kinetics of NAA and IBA has been evaluated with pseudo-first-order and pseudo-second-order models.Pseudo-first-order kinetic equation [12] may be represented in the linear form where  1 is the rate constant of pseudo-first-order release kinetics.If the pseudo-first-order kinetics is applicable, the plot of − ln(1 −   ) versus  will give a straight line, and the  1 value can be obtained from the slope of the linear plot.Pseudo-second-order kinetic equation may be represented in the linear form where   is equilibrium release amount,  2 is the rate constant of pseudo-second-order release kinetics.If the pseudosecond-order kinetics is applicable, the plot of /  versus  will give a straight line, which allows computation of  2 .As shown in Figure 10, the  2 values of release data fitting pseudo-second order model are in the range of 0.9995-1, while those with pseudo-first-order model are in the range of 0.4745-0.8303.The rate constant ( 2 ) and correlation coefficient ( 2 ) values obtained from straight lines are listed in Table 2.It can be seen that the pseudo-second-order model is better satisfaction for describing the release kinetic processes of NAA and IBA than the MgAl-NAA/IBA-LDHs nanohybrids.synthesized by coprecipitation method.After intercalation of NAA and IBA, the interlayer distance of the nanohybrids is 2.00 nm and NAA and IBA anions located in the gallery of MgAl-LDHs with bilayer arrangement.The result shows that the release of NAA and IBA is dependent on temperature, pH value, and release medium.The nanohybrids possessed good controlled-release properties and the release of intercalated NAA and IBA from the nanohybrids fitted pseudo-secondorder model.

Table 2 :
The rate constant of fitting the release data to pseudo-second-order kinetics model.