Controlled release of imidacloprid from biodegradable grafted starch matrix
Abstract
In
present
work
attempts
were
made
to
prepare
starch
graft
poly(methylacrylate) through the advanced microwave irradiation technique
obtaining very high yield in aqueous medium at atmospheric conditions and to
characterize the St-g-PMA by FTIR, SEM, DTA, TGA and DSC. Imidacloprid of
technical grade was individually encapsulated within the potato starch matrix
modified by poly(methylacrylate) through chemical grafting and in vitro release
study of encapsulated St-g-PMA-97.5%,St-g-PMA-50%,and pure potato starch
were compared. The kinetic experiment of imidacloprid release in water have
shown us that the encapsulation of imidacloprid with high grafting efficiency had
the slower release of the active ingredient from several hours to one day and the
release data were fitted to empirical equation m t/mo=ktn where mt/mo is the
fraction of insecticide released at time t, k & n are the constant and n indicates
the mechanism of release which is diffusion controlled along with this release
behaviour, insecticides content and swell ability were also investigated. Scanning
electron microscopy revealed that the insecticides encapsulated within the St-gPMA matrix were dispersed in the form of tiny cells.
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Controlled release of imidacloprid from biodegradable grafted starch matrix
1. Introduction
Many water soluble agrochemicals need controlled release systems for
steady or pulsed release of intact agrochemical for humanizing the consumption
of nutrients and minimizing a variety of adverse health hazards of these
agrochemicals to the general population [1] and other serious ecological
problems [2,3] associated with leaching [4] surface run-off, degradation [5] and
volatilization [6] of integrated agrochemicals. The purpose behind controlling
drug delivery is to achieve more effective therapies while eliminating the potential
for both under- and overdosing.
A successful controlled release device depends on technological factors
such as entrapment efficiency, integrity and desired release characteristics. In
recent years modified natural polymers are used as controlled release device in
agro industries and emerged as a novel technique with enhanced commercial
viability than the use of conventional synthetic polymers [7-10].
Natural
starch
is
notorious,
adaptable
inexpensive
and
entirely
biodegradable agricultural material used for a verity of industrial applications
[11]. Although natural starch is effectively used as agrochemical encapsulating
material [12-13] but the hydrophilic nature of starch showing enormous swelling,
results in premature release of pesticides from matrix which reduces the survival
life in field uses especially in high water environment. Therefore low water
resistance and week bioadhesion of starch improved through modification
including cross linking [14], addition of protective coating or grafting. Among
various methods microwave radiation provides a highly advantageous means of
grafting due to production of large concentration of free radical in irradiated
material in absence or low concentration of initiator [15-16].
Grafted starch matrix have achieved immense victory due to their wide
spread
application
in
pharmaceutical,
biomedical,
biotechnological
and
environmental fields [17-19]. Owing to their low toxicity and high enzymatic
degradation at desired sites, Potato starch graft copolymers have been
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Controlled release of imidacloprid from biodegradable grafted starch matrix
frequently considered as a potential matrix system for controlled release of
agrochemicals.
Encapsulated starch graft copolymers of hydrophobic characteristics has
yielded some promising result in to agrochemical release to enhance agriculture
production and responsive to minimize the environmental pollution [20]. A
hydrophobic behavior can be obtained if starch is grafted with hydrophobic
monomers [21-22]. Encapsulation of highly water soluble pesticides with in
hydrophobic starch matrix results the low swellings of matrix. For this reason the
release rate of highly water soluble insecticides from the matrix in wet
environments would be reduced.
Imidacloprid [1 - (6 - chloro - 3 - pyridinylmethyl) - N - nitroimidazolidinylideneamine] used for the controlled release device is a systemic Insecticide
with novel modes of action [23-24]. This insecticide is effective for controlling,
whiteflies, thrips, aphids, scales, psyllids, plant bugs, and other various harmful
pest species including resistant strains.
The aim of
the present study is to
synthesize potato starch
Poly(methylacrylate) graft copolymer by microwave irradiation technique using
Thiourea- K2S2O8 redox pair and evaluate the potential of controlled-release
formulations for lower application rates with reduce leaching of imidacloprid.
Therefore, a biodegradable controlled-release formulation was prepared by
integration of insecticide with grafted potato starch and the release rate of the
active ingredient from CR granules was studied in aqueous medium.
2. Materials and methods
Domestic microwave oven model no LG Intellocook TH MS-1947 was
used for the all synthesis. Distilled water was used for the whole study. Technical
grade Imidacloprid (99%) kindly provided by National fertilizer chemicals, India
was chosen as module insecticide. All other reagents were of analytical grade.
Pure starch used in this study was a commercially available potato starch from
Sd Fine chem. Ltd., India.
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Controlled release of imidacloprid from biodegradable grafted starch matrix
Encapsulating matrices starch and St-g-PMA were prepared and fully
characterized during previous study as describes in chapter second. The starch
samples
were
modified
to
obtain
resistant
and
mechanical
strength.
Polymethylacrylate was used to modify starch surface in presence of thiourea
and potassium persulfate under treatment of microwave irradiation. The optimum
concentration of thiourea and potassium persulfate and monomer was observed
respectively 0.01M, 0.02M and 0.11M.
The grafting parameters such as percentage grafting (%G) and efficiency
(%E) were calculated in weight in the following manner [25-26].
%G= (Wt of PMA grafted /Wt of Starch) x100
%E= (Wt of PMA grafted / Wt of MA charged) x100
Maximum grafting efficiency (97.5%) and a medium efficiency (50%) grafted
samples were chosen as encapsulating matrix and compared with pure potato
starch.
The IR spectra of the imidacloprid encapsulated St-g-PMA, St-g-PMA
matrix and pure starch were recorded as KBr pellets on a FTIR-Perkin Elmer
Spectrometer. The spectra were taken from 4000 to 400 cm-1 with resolution
4cm-1.Thermal stability studies of St-g-PMA dry samples were performed on
thermo gravimetric analyzer (EXSTAR TG/DTA 6300), with a temperature range
of 25–800 0C at a heating rate of 20 0C/min under atmospheric conditions,
surface morphology examined by scanning electron micrograph (LEO 430) .
2.1.
Swelling studies
The swelling equilibrium was measured according to the conventional ‘‘tea
bag’’ method. The completely dried weighed quantity of graft copolymer was
immersed in 200 ml of distilled water at 25 0C. After 8 hours the tea bag was
taken out, wiped superficially with filter paper to eradicate surface water and
then, weighed. The percentage mass swelling was determined using the
following expression [27-28]:
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Controlled release of imidacloprid from biodegradable grafted starch matrix
% SM =Mt –M0 x100
M0
Where, M0 and Mt are the initial mass and mass at different time intervals,
respectively.
2.2.
Encapsulation
545mg (dry base) of the St-g-PMA and 30 ml of distilled water were
placed in a glass beaker and stirred mechanically for 15–25 min to form
dispersion. The dispersion was heated to gelatinize the starch and kept at about
90 0C for 20–30 min under stirring. Then, the pesticides (110mg) were thoroughly
mixed with the gelatinized starch paste with a glass rod. The mixture was kept at
room temperature for 24 h to obtain a solid gel. Then, the gel was dried at about
90 0C. Finally, the samples were crushed into pieces and sieved for the collection
of the 10–40-mesh fraction for analysis.
2.3.
Encapsulation efficiency (%)
The Encapsulation efficiency is the weight percentage of the imidacloprid
actually encapsulated within the matrix. The granular sample with dry weight
(100 mg) was washed with 20 ml of water to remove the pesticides exposed. The
pesticides content in water was determined through spectrophotometric analysis.
The encapsulation efficiency was calculated with the following equation:
Encapsulation efficiency (%)
= [W1/ (W0XC)] X 100%
Where W 1 and W 0 denote the weight of the pesticides washed away in the
distilled water and the weight of the granular sample, respectively, and C is the
pesticide content of the sample.
2.4.
Pesticide release studies
In vitro release of the imidacloprid has been carried out by keeping dried
and loaded samples of each formulation (100mg) in 250ml water at room temp.
About 2ml sample was withdrawn on specified time intervals and then release
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Controlled release of imidacloprid from biodegradable grafted starch matrix
was measured spectrophotometrically. All the experiments were carried out in
duplicates.
3. Results and discussion
3.1. Effect of graft modification
Natural polymers, especially polysaccharides, have been used recently as
controlled release systems because of their unique advantages thus under
influence of microwave irradiation, grafting of poly(methylacrylate) onto potato
starch was found to take place in presence of very little concentration of thiourea
and potassium per sulfate redox pair obtaining very high grafting yield 390% of
97.5% efficiency within only 3 min at 320 W MW power.
Microwave wave irradiation technique offers a number of advantages
over conventional method such as low creation of homopolymer, short reaction
time, instantaneous and rapid heating. Thus in grafting under MW, low
concentration of redox pair is needed and grafting takes place in an eco-friendly
respect in aqueous medium.
3.2. FTIR Spectroscopy
IR spectra of potato starch and St-g-PMA are shown in Figure 1. The IR
spectrum of Starch showed absorption bands at 3362.4 (-OH stretching) and
1021.1 cm-1 (skeletal vibration of C-O-C). IR spectra of St-g-PMA show peaks at
3450.6 and 1047.8 cm-1, which may be ascribed to the -OH stretching and
skeletal (C-O-C) vibration of starch in addition to the bands at 1746.1 cm-1 due to
the carboxyl groups (>C=O stretching) of PMA, indicating that MA has been
successfully grafted onto potato starch.
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Controlled release of imidacloprid from biodegradable grafted starch matrix
A
B
Figure 1: FTIR Spectra of Pure Starch (A) and St-g-PMA (B)
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Controlled release of imidacloprid from biodegradable grafted starch matrix
3.3. Thermal analysis
The thermogravimetic analysis (TGA) of potato starch and starch-graftpoly(methylacrylate) copolymer were also carried out (fig 2). Potato starch
showed a very small weight loss below 100 0C, implying a loss of moisture.
Starch and St-g-PMA copolymer had significant weight loss of 8.5% and 12.3%
at 250 and 200 0C, respectively. The major weight loss of starch started at 289
0C
(44.0%), whereas for St-g-PMA composite the major weight loss started at
300 0C (41.8%). Therefore, starch and St-g-PMA composite had a decomposition
temperature of 289 and 300 0C, respectively. The results indicated that the
introduction of PMA to polymer network resulted in an increase in thermal
stability
The DSC plots (Fig 3) show the respective exothermic and endothermic
peaks for all the weight losses which have occurred confirming the corresponding
decomposition temperatures and also the decomposition processes.
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Controlled release of imidacloprid from biodegradable grafted starch matrix
120
Starch
Starch-g-PMA
100
TGA %
80
60
40
20
0
-20
100
200
300
400
500
600
700
800
900
Temp cel
Figure 2: TGA of starch and St-g-PMA Samples
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Controlled release of imidacloprid from biodegradable grafted starch matrix
Starch
St-g-PMA
200
180
160
140
DSC mw
120
100
80
60
40
20
0
-20
0
100
200
300
400
500
600
700
800
900
Temp cel
Figure 3: DSC Curves of Starch and St-g-PMA samples
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Controlled release of imidacloprid from biodegradable grafted starch matrix
3.4. Surface morphology
Surface morphology of the potato starch and grafted starch was evaluated with
the help of scanning electron micrographs. SEM images of starch and grafted
starch are shown in fig. 4(a, b). The morphology of the potato starch surface, that
is granular quite clear in fig 2a. Starch granules were partially destroyed during
the graft copolymerization process and attested to the very good interfacial
adhesion between the starch and the poly(methylacrylate) chains( fig 4b). Thus
comparison of these figures reveals that grafting has taken place.
The SEM image of the imidacloprid encapsulated St-g-PMA matrix (fig 4c)
revealed that the imidacloprid particles were regularly dispersed in continuous Stg-PMA matrix phase. It is assumed that it forms a reservoir type structure, similar
to the matrix structure of St-g-PLA [20]. The amphiphlic St-g-PMA had a core
shell structure with a hydrophilic starch core and a hydrophobic grafted PMA
shell. The imidacloprid was surrounded by St-g-PMA wall and released through
diffusion.
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Controlled release of imidacloprid from biodegradable grafted starch matrix
Figure 4a: SEM Images of Pure Starch, (4b) St-g-PMA, (4c) Imidacloprid
encapsulated St-g-PMA matrix
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Controlled release of imidacloprid from biodegradable grafted starch matrix
3.5.
Swelling studies
Table1 shows the dynamic uptake of water at room temperature where
extent of swelling was seen to be 150% and 350% in 8 hours respectively for Stg-PMA-97% and St-g-PMA -50%. These results show that the swelling of the
graft
copolymers
decreased
poly(methylacrylate).
The
strong
with
increasing
intermolecular
graft
interaction
efficiency
of
between
the
component polymers of the graft copolymers renders the polymeric segments
rigid thereby hindering the water uptake and lowering the extent of swelling.
Thus, St-g-PMA showed improved resistance compared to pure potato starch to
water due to significant improvement of the matrix hydrophobicity.
Table-1
S No
1
properties
St-g-PMA-97%
St-g-PMA-50%
Swelling
equilibrium
(in
1.5
3.5
aqueous
medium)
3.6.
Encapsulation Efficiency
The percentage of entrapment data shows that imidacloprid is being
encapsulated efficiently in the entire matrix by both the grafted starch and pure
starch matrix. After water washing, the pesticide exposed on the surface matrix,
encapsulated imidacloprid could be determined. The entire matrix contains
almost same concentration of imidacloprid. Encapsulated imidacloprid dispersed
uniformly in form of minute cells as clearly seen in SEM pictures. This is further
informative that the grafted starch matrix become more rigid when compared to
the starch matrix after grafting. This suggests that the grafted PMA polymeric
chains tend to reflux much slower than the diffusion rates of the water molecules.
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Controlled release of imidacloprid from biodegradable grafted starch matrix
3.7.
Pesticide release studies:
Fig.5 shows the release profiles of imidacloprid from the graft copolymer
containing different amount of PMA. It is well known that imidacloprid is highly
water soluble pesticides dissolves in water within few minutes. Our experimental
data shows that the release of imidacloprid has been evidently slowed down by
both the grafted starch matrix of different graft efficiencies.
Almost complete release was observed in eight hours for starch matrices
while grafted starch matrix show extended release where it was about 42% for
St-g-PMA-97% and 75% for St-g-PMA-50% within ten hours. The St-g-PMA
copolymers reveal decrease in the pesticide release with increasing the PMA %
add on. Also it was seen that in both matrices of grafted starch initial release of
pesticide is very slow that in, up to 2-3 hour and then after release increased.
This indicated that the most imidacloprid released after reaching the St-g-PMA in
swollen state.
Thus all the results suggest the St-g-PMA as a more compact and useful
matrix than either of the polymer alone to protect the pesticide encapsulated with
matrices in heavy water environment. In this way the major amount of the
pesticide loaded can reach the fields in wet environment without disintegration.
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Controlled release of imidacloprid from biodegradable grafted starch matrix
st-97%
st-50%
st
110
100
90
Imidacloprid release %
80
70
60
50
40
30
20
10
0
0
100
200
300
400
500
600
700
Time (min)
Figure 5: The Release Curve of Imidacloprid from Starch and the
encapsulated St-g-PMA matrix
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Controlled release of imidacloprid from biodegradable grafted starch matrix
3.8.
Diffusion mechanism
The following equation was used to determine the nature of diffusion of
imidacloprid into hydrogel matrix [29]:
F = Mt/M∞ = ktn
Where Mt/M∞ is the fraction of imidacloprid diffused into the gel at time t,
and infinite time (at equilibrium), respectively. K is the constant related to the
structure of the network and the exponent n is a numerical value which
determines the type of diffusion. For normal fickian diffusion the value of n= 0.5,
case II diffusion n=1.0, and non fickian n=0.5 to 1.0. [30].
Table 2
Samples (%)
n
K
St-g-PMA-97
1.143
-7.249
St-g-PMA-50
1.0419
-6.384
Starch
1.009
-5.798
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Controlled release of imidacloprid from biodegradable grafted starch matrix
St-g-PMA-97 %
St-g-PMA-50 %
St pure
1
0
ln F
-1
-2
-3
-4
2
3
4
5
6
7
ln t
Figure 6: Kinetic study of imidacloprid release from starch, St-g-PMA-97%
and St-g-PMA-50% matrix
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Controlled release of imidacloprid from biodegradable grafted starch matrix
Plots of ln F against ln t yielded straight line from which the exponent n
and K were calculated from the slope and intercept of the line. It is clearly seen
from the table 2 the values of the diffusion exponent range lies within 1.009 to
1.143. For St-g-PMA matrix was taken as case II character.
Chemically modified starch graft poly(methylacrylate) copolymer which shows
ensured biodegradation provided a more effective matrix for controlled release of
agrochemicals, especially highly water soluble pesticides. The release of
imidacloprid from the various formulations of modified starch into aqueous
medium has been shown to be diffusion controlled.
4. Conclusion
The modified starch matrix improve the water resistance, thermal stability
lesser swell ability when compared to pure starch which may be attributed to the
presence of PMA grafted polymer chains resulting in slower release of the loaded
pesticides. Release rate of imidacloprid from controlled release system is not
affected only by the swell ability but also, and to greater extent by graft efficiency
of PMA. The hydrophobilized modification of starch would exhibit potential
application in starch encapsulation for the controlled release of agrochemicals
which reduces the toxic effects associated with the leaching and surface runoff of
chemicals and to reach better delivery. Thus St-g-PMA shows improved
characteristics in respect of slow release of pesticides as well as biodegradable
behavious.
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Controlled release of imidacloprid from biodegradable grafted starch matrix
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