Recently, enormous demands on the inorganic-organic hybrid materials are increasing
due to its various new applications. This is because the hybrid materials can
show better properties as compared to their original material. The hybrid material
can be used in various applications. For instant, layered double hydroxide hybrid
material was reported can be used as catalyst for liquid-phase aldol condensation
(Roelofs et al., 2001) and as controlled release
formulation of an herbicide (Hussein et al., 2005).
Layered double hydroxides are layered materials having positively charged layers
and interlayer charge compensating anions. The chemical composition of this
material can be represented by the general formula of [MII(1-x)
MIII (OH)2]x+ Az-x/z
. yH2O or [MI M2III (OH)2]+
Az-1/z . yH2O where MI, MII
and MIII are mono, di- and trivalent cations, respectively, occupying
octahedral positions in hydroxide layers and Az- is an interlayer
charge-compensating anion. These materials have good anion exchange properties
which can be used as fundamental in preparing a controlled release formulation
and biocompatible materials (Constantino et al.,
2008; Tyner et al., 2004; Li
et al., 2004). The formulation prepared from layered double hydroxide
gives some advantages as compared to its counterpart such as prolonged duration
of released, minimize adverse reactions and higher stability of the active agents
in the formulation (Constantino et al., 2008).
The preparation of inorganic-organic hybrid materials of layered double hydroxides
can be carried out by two different methods namely direct or indirect method.
In the direct method, the organic compound can be intercalated into the layers
of layered double hydroxides by co-precipitation reaction (Yasin
et al., 2011). By used of this method, layered double hydroxides
and the organic compound are included in the mother liquor, followed by aging
process to form a well ordered nanolayered structure. By used of indirect method,
the insertion of organic compound can be done by first preparing the layered
double hydroxide followed by modification and finally insertion of the organic
compound into the interlayer (Choy et al., 2001).
Lawsone (2-hydroxy-1,4-naphtaquinone) is an active compound which can be isolated
from the leave of henna. In Asia, human have used henna for centuries
for hair dying and skin painting. Recently, lawsone is becoming more important
due to its profound effect on human health. In the present study, lawsone was
selected as active organic compound species and intercalated into the layers
of Zn and Al layered double hydroxide by both direct and in-direct method. In
our previous paper, we have reported the characterizations of this compound
using Powder X-ray diffraction and Fourier transform infrared (Yasin
et al., 2011). In addition to our previously reported paper, we carried
out further investigations on the characterization using thermo gravimetric
analysis as well as to study and discuss the controlled release property of
the resulting intercalated compound.
MATERIALS AND METHODS
Synthesis of Zn-Al-NO3-layered double hydroxide (ZAN-LDH): All chemicals used in this synthesis were of analytical grade and used without any further purification. Co-precipitation method was adopted to synthesize Zn-Al-NO3 (ZAN-LDH) in this study. In the preparation of Zn-Al-NO3, an aqueous solution of Zn (NO3)2.6H2O (0.1 M) was added to Al (NO3)3.9H2O (0.025 M) to give Zn2+ /Al3+ ratio, R = 4. Aqueous solution of NaOH and Na2CO3 (2.0 M) were then added to the mixture dropwise, with vigorous stirring at room temperature and the pH was adjusted to 10.0±0.2. The precipitate formed was aged at 70°C in an oil bath shaker for 24 h, cooled, centrifuged, washed several times with deionized water and dried in an oven for 48 h. The resulting ZAN-LDH was ground into powder and keep in sample bottles for further used and characterizations.
Synthesis of ZAN-Law (cop) using co-precipitation method: An aqueous solution (100 mL) of sodium hydroxide (NaOH) (1.52 g, 0.003 mol) and lawsone (1.03 g, 0.003 mol) was added drop wise to a solution (250 mL) containing zinc nitrate (Zn(NO3)2) (1.53 g, 0.006 mol) and aluminium nitrate (Al(NO3)3) (0.75 g, 0.002 mol) under nitrogen atmosphere with vigorous stirring until the final pH of 10. The resulting slurry was aged at 28°C for 48 h in an oil batch shaker. The resultant slurry will then filtered, wash with de-ionized water until the final pH of 7 and finally dried at room temperature for 12 h.
Synthesis of ZAN-Law (ie) using ion exchange method: Synthesis of ZAN-Law
using ion exchange method was done under nitrogen atmosphere in which 0.5 g
of ZAN-LDH was added to 50 mL of a 0.06 M aqueous lawsone solution. The pH of
the mixture solution was held constant at 10 by simultaneous addition of 2 mol
L-1 sodium hydroxide solutions. The exchange process was stirred
vigorously for 24 h at room temperature. The precipitate formed was washed several
times with de-ionized water and finally dried at room temperature for 12 h.
Controlled release study: Phosphate buffer solution at pH 7.0, 4.0 and pH 10 were used as aqueous release medium in this study. The release of lawsone from the interlayer of layered double hydroxide was performed by adding 0.025 g of ZAN-Law into 200 mL of aqueous release medium at room temperature. The paddle rotation speed was set at 125 rpm. Two mL of sample was withdrawn at pre-determined intervals and pass through 0.45 μm membrane filter before the concentration of lawsone released into an aqueous release medium was measured using UV-Vis spectrometer which performed at 265 nm wavelength.
Characterizations: The PXRD spectra were recorded on a Xpert PRO PAN powder diffractometer using filtered Cu-KÜ radiation (λ = 1.54Å) at 40 kV and 200 mA and samples scan were done at 5°-90°2θ min-1 at 0.003°steps. All solid samples were mounted on an alumina sample holder. Basal spacing (d-spacing) were determined via powder technique. Fourier Transform Infrared (FTIR) spectra were recorded by a Perkin Elmer 1725X spectrophotometer in the range of 4000- 400 cm-1. Finely ground 1% samples in KBr powder were compressed to obtain a pellet and the pellet was then used to obtain the IR spectrum. Thermo Gravimetric Analysis (TGA) were carried out on a Du Pont Instrument 951 Thermo gravimetric analyzer using a heating rate of 10°C min-1 under nitrogen atmosphere with a flow rate at 50 mL min-1. A temperature range between 50-800°C was used in this study.
RESULTS AND DISCUSSION
Characterizations: Powder X-ray diffraction analysis of original ZAN-LDH
and resulting intercalated material by used of ion-exchange (in-direct method)
and co-precipitation (direct method) intercalated with lawsone are given in
Fig. 1. As shown in the Fig. 1, the PXRD
pattern of original ZAN-LDH layered double hydroxide and its resulting intercalated
material synthesized by used of both methods show high crystallinity recorded
by the narrow width of the reflection peaks especially at 003 and 006 reflections.
The d003 value for original ZAN-LDH was recorded at 8.9 Å which
demonstrated the general feature of layered double hydroxide (Hussein
et al., 2002) while the d003 values recorded for ZAN-Law
(cop) and ZAN-Law (IE) were 8.7 Å and 7.6 Å, respectively. The resulted
d003 values that obtained from both techniques show that the intercalation
process by used of co-precipitation is better as compared to ion-exchange method.
The interlayer spacing occupied by the lawsone between the layers of layered
double hydroxides was found to be 3.9 Å.
||PXRD patterns for ZAN, a: ZAN-Law (ie) b: and c: ZAN-Law (cop)
||FTIR spectra for a: Lawsone, b: ZAN-Law (ie), c: ZAN-Law (cop)
and d: ZAN
The obtained value is based on the standard thickness of the brucite like sheets
that recorded at 4.8 Å and therefore the proposed orientation of the lawsone
between the layers of layered double hydroxide is flat and monolayer since the
thickness of the benzene ring is recorded at 3.7 Å (Hussein
et al., 2009).
Figure 2 shows the Fourier transform infrared (FTIR) spectra for lawsone, ZAN-LDH, ZAN-Law (IE) and ZAN-Law (cop). As shown in Fig. 2a for a lawsone spectrum, a band attributed to the phenolic OH vibrations can be observed around 3200 cm-1. A strong stretching vibration of benzene ring skeleton can be observed around 1600 cm-1 and vibration due to C = O and C = C can be observed around 1510 cm-1. Other bands can be attributed to various functional groups present in lawsone molecule.
Figure 2b shows the FTIR spectrum for ZAN-LDH that prepared
using co-precipitation technique. A broad band centered at around 3400 cm-1
is due to the presence of OH stretching of the hydroxyl group of LDH and physically
absorbed water molecule. The band around 1640 cm-1 is due to H-O-H
bend vibrations. As shown in the figure, the presence of nitrate group can be
observed as the sharp peak at 1385 cm-1. A band around 600 cm-1
can be attributed to the Al-OH and Zn-Al-OH bending vibration (Hussein
et al., 2002).
Figure 2c and 2d show the FTIR spectrum
for ZAN-Law (cop) and ZAN-Law (ie), respectively. As explained earlier, XRD
diffractogram (Fig. 1) shows that lawsone is intercalated
in the layer of LDH and therefore a combination of both lawsone and ZAN-LDH
spectra is expected. The FTIR spectrum of ZAN-Law synthesized using both techniques
show that it resembles a mixture of each spectrum of lawsone and ZAN-LDH. This
indicates that both functional groups of lawsone and ZAN-LDH are present in
ZAN-Law and clearly indicates the intercalation of lawsone inside the layer
of LDH. Also shown in the figure is a band at 1385 cm-1 for ZAN-Law
(cop) became less broad due to intercalation of lawsone which removed some of
nitrates from the layer of LDH. Generally, the band around 600 cm-1
that is attributed to Al-OH and Zn-Al-OH bending vibration became smaller than
that of ZAN-LDH which also confirmed the intercalation of lawsone through an
interaction between the interlayer lawsone and hydroxyl groups of LDH layers.
TG/DSC thermogravimetric analyses profile for lawsone, ZAN-Law (cop) and ZAN-Law (ie) are reported in Fig. 3. For lawsone, thermal studies show that three main thermal events are observed. The first slow event in the temperature range between 90-120°C is attributed to the melting process of lawsone which corresponds to a sharp endothermic peak at ca.198.6°C. The followed mass loss in the temperature range between 150 -250°C is due to the decomposition and subtle combustion of lawsone which correspond to weak endothermic peak at ca. 213.6°C. The last event recorded at temperature between 350-450°C is due to the strong combustion of lawsone corresponding to exothermic peak at ca. 417°C.
For lawsone that intercalated in the layer of layered double hydroxide using
co-precipitation method (ZAN-Law (cop), four thermal events are observed. The
first event recorded at temperature range between 50-150°C which can be
attributed to the loss of surface adsorbed water molecules and interlayer water
that corresponds to DSC curve which shows two endothermic peaks at ca. 182.4
and 264.7°C, respectively. The followed loss of mass at temperature between
150-250°C can be attributed to the loss of residual intercalated water and
trace dehydroxylation of the LDH layer.
||TG-DSC curves for (a) Lawsone, (b) ZAN-Law synthesized by
co-precipitation method and (c) ZAN-Law synthesized by ion-exchange method
The loss of mass that recorded at temperature range between 250-300°C is
due to the dehydroxylation of layered double hydroxides accompanying with the
formation of layer double oxide and the partial decomposition of intercalated
lawsone (Ni et al., 2008). The recorded loss
of mass is corresponds to a sharp exothermic peak recorded at ca.272.5°C
Fig. 3b and 261.9°C Fig. 3c, respectively.
The followed loss of mass at temperature between 400-550°C can be attributed
to the major decomposition of intercalated lawsone that are corresponds to ca.
529°C (Fig. 3b and 509°C Fig. 3c.
TG/DSC thermogravimetric analyses indicate that, the temperature region of Law-LDH
is higher as compared to lawsone. It shows that thermal stability of lawsone
in ZAN-Law (cop) and ZAN-Law (ie) is clearly enhanced due to the host-guest
interaction involving hydrogen bonding (Xia et al.,
Controlled release study: Lawsone can be de-intercalated from the interlayer of LDH through ion-exchange with the surrounding anions, such as phosphate. A series of phosphate buffer solutions with different pH values were used to observe the pH effect on the release rate of lawsone from the interlayer of layered double hydroxide. Figure 4 shows the release profile of lawsone from the interlayer of layered double hydroxide into the release medium at different initial pH values.
As shown in the Fig. 4, the released rate of lawsone into
aqueous release medium increased with increased in contact time between the
intercalated compound and aqueous release medium. The release rate was found
to be rapid for the first one hour for all pH and followed by a more sustained
released thereafter. Equilibrium was achieved after one hour. This is true for
all release medium with initial pH of 4, 7 and 10.
||Release profile of lawsone from ZAN-Law (cop) at different
The release rate was found to be very rapid for the first one hour and recorded
a slower rate thereafter and the release process still continued after 3 h.
A maximum percentage of released are achieved at 90 minutes for pH 10, 480 minutes
for pH 7 and 540 min for pH 4.
The percentage of lawsone released from ZAN-Law into the aqueous release medium
solutions at initial pH 10 was found to be the lowest, as shown in Fig.
4. The highest percentage of lawsone that released was achieved in neutral
and acidic aqueous release medium solution. At equilibrium it was estimated
that 90 and 82% of lawsone could be released from ZAN-Law into the aqueous release
medium solution with initial pH of 7 and 4 respectively. The results was in
agreement with the reported previously study (Constantino
et al., 2008) which explained that the intercalation drug was unionized
in the acidic environment and ionized in neutral and alkaline environment. The
explanation reflected the results that obtained from the released study which
shown that the lowest percentage of released was recorded at pH of 4. This might
be due to the remaining lawsone that is still in the interlayer of layered double
hydroxide which unable to ionized and performed ion exchange while in neutral
and alkaline aqueous release medium the lawsone will be ionized and able to
perform ion exchange which contributed to the high percentage of released.
The drug-inorganic composite, ZAN-Law with Zn/Al = 4 has been successfully synthesized by using co-precipitation and ion exchange method. PXRD analysis of ZAN-Law prepared by both methods show that the basal spacing (d003) shifted to a lower 2θ angles indicating the intercalation of lawsone in the layer of LDH. FTIR study shows that the drug-inorganic composite (ZAN-Law) resemble the spectra of ZAN-LDH and lawsone indicating the presence of both functional groups. The percentage released of lawsone was found to be dependent on the pH of the release medium. The highest percentage of released was achieved in alkaline and neutral release medium. The present study may suggest that the LDH can be used as an alternative medium for a drug delivery system especially in controlling the release rate of lawsone.