Fluorescent bio-nanocomposites based on chitosan reinforced hemicyanine dye-modified montmorillonite

Article abstract of DOI:10.1039/c6ra23320a

The present investigation describes the synthesis and detailed characterization of novel fluorescent bio-nanocomposite films of chitosan reinforced by hemicyanine dye-modified montmorillonite (MMT-HD) using a solvent-casting method. A series of hemicyanine dyes (HD) have been synthesized by catalyst-free and solvent-free conditions in high yields. These organic compounds were intercalated within the interlayer gallery of montmorillonite providing an increase in the basal spacing and affording inorganic-organic host-guest materials. The fundamental properties of these dyes as well as their intercalated MMT were studied by X-ray, FTIR, and thermal analysis, completed by photophysical properties studies. The results showed that the guest species (HD) are successfully intercalated and stacked as J-aggregates (head-to-tail aggregates) into the host layer of MMT, which is confirmed by spectral characterization. Improvement in thermal stability of the intercalated MMT was indicated by results from thermal analysis compared to the pure dye. Next, the resulting organo-modified clays were dispersed within the chitosan biopolymer. The obtained bio-films were investigated and characterized with varying content of clay (1, 2, 5 and 10 wt%). The structural, hygroscopic (hydrophobicity and water solubility) and fluorescence properties of the resulting materials were evaluated. Results demonstrate that the incorporation of MMT-HD into chitosan improves the hygroscopic properties of the chitosan film, thus showing potential for active food packaging materials. In addition, the solid state fluorescence spectra of the bio-films have shown a significant fluorescence emission at room temperature with increasing clay content.

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Fluorescent Bio-nanocomposites Based on Chitosan Reinforced
Hemicyanine Dye-Modified Montmorillonite
M. E. M. Mekhzoum,a,b E. M. Essassi,b A. Qaiss,a R. Bouhfid*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The present investigation describes the synthesis and detailed characterization of novel fluorescent bio-nanocomposite
films of chitosan reinforced by hemicyanine dye-modified montmorillonite (MMT-HD) using a solvent-casting method. A
series of hemicyanine dyes (HD) have been synthesized by catalyst-free and solvent-free conditions in high yields. These
organic compounds were intercalated within the interlayer gallery of montmorillonite providing an increase in the basal
spacing and affording inorganic–organic host-guest materials. The fundamental properties of these dyes as well as their
intercalates MMT were studied by X-ray, FTIR, thermal analysis and were completed by photophysical properties. The
results showed that the guest species (HD) are successfully intercalated and stacked as J-aggregation (head-to-tail
aggregates) into the host layer of MMT, which is confirmed by spectral characterization. Improvement in thermal stability
of the intercalated MMT was indicated by results from thermal analysis compared to the pure dye. Next, the resulting
organo-modified clays have been dispersed within chitosan biopolymer. The obtained bio-films were investigated and
characterized with varying content of clay (1, 2, 5 and 10wt%). The structural, hygroscopic (hydrophobicity and water
solubility) and fluorescence properties of the resulting materials were evaluated. Results demonstrate that the
incorporating of MMT-HD into chitosan improves the hygroscopic properties of chitosan film, thus showing potential for
active food packaging materials. In addition, the solid state fluorescence spectra of the bio-films have shown a significant
fluorescence emission at room temperature with increasing of the clay content.
www.rsc.org/
Among all the natural polymers, chitosan has increased much
attention recent years for packaging applications due to its
Introduction
intrinsic properties that depend on environmental variables.8
In the last few decades, science and technology has started to
move in the direction of renewable materials that are
environmentally friendly and sustainable.1 This evolution
motivates academic and industrial research to develop novel
materials from alternative resources, with lower energy
consumption, biodegradable and non-toxic to the
environment.2,3 These materials are often formulated with
natural biopolymer sources, such as starch, cellulose, chitin or
biodegradable synthetic polymers namely, polycaprolactone
and polylactic acid, capable of forming a cohesive and
continuous matrix.4,5 Nowadays, biopolymers became the
subject of intensive studies for the development of
biodegradable films.6,7 Unfortunately, the usage of
biopolymers as food packaging materials has drawbacks such
as poorer mechanical, thermal, and barrier properties as
compared to the conventional non-biodegradable materials
made from petroleum.
Chitosan,
a natural linear polysaccharide obtained from
crustaceans consisting mainly of β-(1,4)-linked 2-deoxy-2-
amino-D-glucopyranose units and partially of β-(1,4)-linked 2-
deoxy-2-acetamido-D-glucopyranose, is the deacetylated
derivative of chitin, which is the second most abundant natural
polymer after cellulose.9 Chitosan has some unique properties
including biocompatibility, biodegradability, hydrophilicity,
non-toxicity, non-antigenicity, bio-adherence and cell affinity.
It is the only positively charged naturally occurring
polysaccharide which strongly interacts with negatively
charged entities.10 Owing to its low cost, large-scale availability
and antimicrobial activity, chitosan has been widely used in
food and pharmaceutical industry.11 Additionally, this
polysaccharide has been extensively studied in the field of
biomaterials; particularly on chitosan based functional
nanomaterials.12 Nevertheless, poor mechanical and gas
a.Moroccan Foundation for Advanced Science, Innovation and Research (MAScIR),
Institute of Nanomaterial and Nanotechnology (NANOTECH). Rabat Design, Rue
Mohamed El Jazouli, Madinat Al Irfane, 10100 Rabat, Morocco.
Email: r.bouhfid @mascir.com
b.Laboratoire de Chimie Organique Hétérocyclique, Université Mohammed V, Faculté
des Sciences, Av. Ibn Battouta, BP 1014, Rabat, Morocco.
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization data and the relevant results. CCDC1512675, 1512676.
DOI: 10.1039/x0xx00000x
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barrier properties, and weak water resistance limits its dye, by spectroscopic analysis. However, the incorporation of
application particularly in the presence of water and organic dyes into chitosan/clay nanocomposites, is neither
humidity.13 In an attempt to obtain chitosan with an ultimate contemplated nor disclosed. Historically, a lot of classes of
property, different nano-sized reinforcements were fluorescent organic dyes have found their successful
incorporated.14,15 The presences of nanosized filler result in a application in science and technology.38 As a popular example,
very promising materials since they exhibit enhanced hemicyanine dye and its derivatives having donor-π-acceptor
properties with preservation of the material biodegradability structure are widely applied in different areas of technology
without eco-toxicity.16 At present, the addition of layered due to their diversified properties.39 They are commonly
silicates in particular montmorillonite (MMT) in chitosan has applied to lasers, electronics, and nonlinear optics.40 Several
been extensively studied.17,18 Unfortunately, the incompatible such systems containing benzothiazole-derived dyes with a d-
nature of the two components discounts the desirable π-a setup have already been synthesized and studied
properties of the composite. For this reason, clay must be intensively for several decades due to their potential
treated before used and organomodifying the surface applications in molecular electronics and biological
properties of the clay with surfactants maybe a highly effective fluorescence probing.41 Taking into account these antecedents,
method. In this respect, the compatibility between the in this study, we herein reported a facile and efficient
inorganic clay and organic chitosan is enhanced. Recent synthesis of series of 2-styrylbenzothiazolium hemicyanine
research has indicated that incorporation of organically dyes and employed as an intercalating agent for the
modified MMT shows much promise for chitosan-based preparation of organophilic clay through cation exchange
polymer nanocomposites in terms of improvements their reaction. Afterward, the prepared MMT-HD have been mixed
performance in various properties.19 There are several reports with chitosan (CS) via solvent-casting method to produce
about the nanocomposites based on chitosan. In light of these CS/MMT-HD bio-films. The incorporation of MMT-HD into
facts. Günister et al.20 found that both the structure of chitosan enhances both the fluorescence and hygroscopic
chitosan/MMT nanocomposites and its thermal stability are properties of the bio-films. By combining the advantages of
strongly affected by the solvent-casting procedure used. On the good properties of chitosan, montmorillonite and
the other hand. Darder et al.21 employed cationic biopolymer hemicyanine dyes, consequently, the bio-film can be
+
chitosan into MMT-Na+, providing a -NH3 functional group for developed to fulfill the properties needed as fluorescence and
the anion exchange site as an anion sensor. The d spacing of packaging materials.
pristine clay increased from 1.20 nm to 2.09 nm at the 10:1
chitosan-clay ratio. In another recent study, Sahoo et al.22
Results and discussion
prepared a blend of chitosan, polycaprolactone (80:20 ratio),
and Cloisite 30B by solution mixing. The results indicated that
the interaction of Cloisite 30B occurs together with exfoliation
and the extent of interaction increases with an increase in clay
concentration from 1 to 5wt%. However, the above interesting
finding was attributed and limited to the evolved structure.
Hitherto, the design of materials with fluorescent properties
has raised huge attention due to their potential applications in
Synthetic procedure and structure details of hemicyanine dye.
Our target structures can be synthesized into two steps: (i)
preparation of 3-ethyl-2-methylbenzothiazolium salt
preparation of 3-ethyl-2-(p/o-substitutedstyryl)benzothiazol-3-
ium iodide HDa-d where the corresponding dyes are
2 and (ii)
,
connected via an ethenylene spacer. The structures of the 4
styryl benzothiazolium dyes synthesized have one type of
numerous advanced technologies.23 While,
a number of
fluorescent nanomaterials, such as quantum dots (QDs),24
upconversion nanoparticles (UPNPs),25 polymer-based
fluorescent nanoparticles and dye-doped silica nanoparticles
(DDSNs), have already been reported in the literature.26,27
Further, the intercalation of organic guest species into
smectites is a way of constructing ordered inorganic–organic
assembly with unique microstructures controlled by host–
guest and guest–guest interactions. Molecules with
appropriate structural features upon intercalation can
preserve/exhibit a number of specific properties such as
photoluminescence,28 photochromism,29 optical nonlinearity
(NLO)30 and among others.31 Along these lines, several reports
have been published on the photophysical properties of MMT-
based dye nanocomposites with cationic azobenzene,32
pyrene,33 β-carotene,34 rhodamine B35 and coumarin36 types of
dye chromophores. Arbeloa et al.37 have studied the
aggregation of several Rhodamine dyes adsorbed into different
smectite-type clays in dilute aqueous suspensions, depending
on the nature of the clay and the hydrophobic character of the
electron acceptor group (benzothiazole) but differ in
a
structure of phenyl moiety substituent (scheme 1). As
indicated in scheme 2, the first step concerns the formation of
the benzothiazolium salt by alkylation with the corresponding
iodoethane, readily available using DMF as solvent to afford
the compound close to 92% yield. The second one was carried
out under catalyst-free and solvent-free conditions between 3-
ethyl-2-methylbenzothiazolium iodide and series of p/o-
substitued benzaldehydes. The hemicyanine dye structures
were obtained in good to high yields independent of
substituents on the phenyl rings. Furthermore, product
isolation does not require purification by column
chromatography.
2 | J. Name., 2012, 00, 1-3
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"Push"
S
π
R
σ
σ
C
C
H
H
N
I
"Pull"
Scheme 1. Structure of push–pull 3-ethyl-benzothiazolium salts with an ethylene π-
bridge: Quaternary nitrogen atom as electron acceptor group in the benzothiazole,
Electron donor group R in the p/o-position of the phenyl ring, Ethyl group at the
heterocyclic nitrogen.
A general route for the synthesis of the prepared dyes is
shown in scheme 2.
R
b
S
S
N
S
N
i)
ii)
R
a
O
N
I
I
1
2
HDa-b
Comp.
HDa
R
Yield%
94
4-OCH3
4-OH
HDb
84
HDc
HDd
4-Cl
80
83
2-OCH3
Scheme 2. Synthetic pathway for preparation of 3-ethyl-2-(p/o-substituted
styryl)benzothiazolium iodides HDa-d. Reagents and conditions: (i) C2H5I, DMF, reflux;
(ii) Catalytic free and solvent-free.
Figure 1. Single crystal structures of HDa and HDb.
The successful modification of MMT-Na has been evidenced by
FTIR analysis. Figure 2 shows example of the spectra of HDa,
MMT-HDa and MMT-Na+. For the spectrum of the parent
sodium-montmorillonite. Three bands absorption exist in the
region 4000-1500 cm-1. The band at 3625 cm-1 is assigned to
the stretching vibration of structural (octahedral) hydroxyl (O-
H) groups linked either to Al3+ or Mg2+. The peak at 1635 cm-1
and the broad band at 3440 cm-1 were assigned to –OH
bending and stretching vibration modes of adsorbed water
respectively. The strong absorption band at 1035 is attributed
to Si‒O stretching vibrations. The FT-IR spectrum of the MMT-
The prepared final compounds HDa-d have “push-pull”
structure characterized by the electron-donor and electron-
acceptor groups interacting through π-conjugated spacer, The
benzothiazole based styryl dyes studied show several specific
properties that are similar to other styryl dyes reported in
literature.42 The structures of dyes were unambiguously
confirmed by 1H and 13C NMR, mass spectrometry and were
assigned on the basis of their spectral data in comparison to
those reported in other literature. The most characteristic
signals in the 1H NMR spectra of this family of compounds
were those corresponding to the CH=CH ethene protons at
7,86–8,24 ppm with coupling constants ≈ 15 Hz indicating their Na+ also shows bands in the 400–600 cm-1 area that are
ascribed to Mg–O and Al–O bending vibration.43
E
conformation, and the N–CH2 protons of the
benzothiazolium ring which resonate between 4.90 and 4.98
ppm. These data confirms the push-pull character of these
hemicyanine dyes derivatives with a significant intramolecular
charge transfer from the functionalized donor group in the
phenyl moiety to the methylbenzothiazolium acceptor group.
The structure of compounds HDa and HDb has been identified
by single crystal X-ray diffraction (see Figure 1).
In the spectra of HDa dye, the peaks for aliphatic and aromatic
groups confirm the structure of the compound (figure 2c).
When the corresponding dye was inserted into the MMT-Na
layered the FT-IR spectra of the hybrid material show the
bands of the MMT-Na+ as well as cationic dye with slight shift
(figure 2b). Besides, the broad signal of the Si–O–Si at 1047 cm-
1. Three bands were observed in the MMT-HDa at 1585, 1515
and 1443 cm-1 can be attributed to the stretching’s vibrations
of C=C and C=N aromatic rings in the structure of dye.
Whereas, a small absorption bands with low-intensity are
corresponding to asymmetrical and symmetrical C–H
stretching modes of methyl (CH3) and methylene (CH2) groups
in the structure are unreadable due to the short-chain of ethyl
group. These data, in sum, confirm the presence of the organic
guest intercalation agent without altering its chemical
structure.
Characterization of the layered hybrid material.
Hemicyanine-modified montmorillonites (MMT-HDa-d) were
prepared via cationic exchange procedure using the above-
mentioned dyes. The reaction between HD+ I- and Na-
montmorillonite provided color solids. Once the organo-clays
were prepared, the isolated solid materials have been
characterized by a set of analysis including FTIR, XRD, TGA,
solid state UV-vis/fluorescence and fluorescence microscopy.
The IR spectra are used to highlight the presence of some
characteristic vibration bands of functions specific to clays, as
well as that of the organic matter by the appearance of
different absorption bands corresponding to the cationic dye.
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montmorillonite, the basal spacing increased from 1.17 nm to
1.63 nm due to the insertion/organization of dyes into the
sheets of MMT as well as the amount of the exchanged
organic dyes. Further, the obtained d-spacing value was direct
proof of the negligible effect of the substituents in the phenyl
44
ring of the corresponding dyes. According to Yariv a basal
spacing of less than 1.4 nm does not permit any kind of
aggregation of the dye cation to take place inside the
interlayer space. The adsorbed dye cations probably form
monolayers of monomeric species in the interlayers, with
aromatic rings lying parallel to the silicate layer or almost
parallel with the very small tilting. Parallel orientation should
be facilitated by π interactions between the organic dyes and
O-planes of the smectites. In this study, the interlayer
expansion (gallery height) was obtained by subtracting the
thickness of an individual layer of montmorillonite (0.96 nm)
from the observed basal spacing; the gallery distance of the
entire intercalated series was calculated to be 0.67 nm.
Judging from the gallery height and the size of dyes, the
orientation of the intercalated cation was discussed. The
molecular length and breadth of four dyes determined from
the software ChemBio3D after energy minimization were 1,28-
1,49 nm and ≈0.65 nm, respectively.
As previously discussed, the gallery space was estimated to be
about 0.67 nm. This value was lower than the vertical length
and much higher than the thickness of the dye molecules.
Therefore, the MMT-HD suggests an interdigited monolayer of
the dye oriented flat with the molecular long axis inclined to
the silicate layer by
a A flat
title angle (θ<54,7°).45-47
arrangement of these molecules with conjugated D-π-A system
within the silicate layers after dye modification was assumed.
In fact, the dye can interact with the anionic sites of the clay
surface at the terminal benzothiazolium nitrogen atom; this is
called ‘head-on type” adsorption.48 The results for basal
spacing of pure and the intercalated MMT are shown in table
1.
Figure 2. FTIR spectra of (a) MMT-Na, (b) MMT-HDa and (c) HDa.
Additional insight concerning the MMT intercalation by the
cationic dyes has been gained by XRD analysis. This allows to
evaluate different periodicities and particularly in our case, the
d001 frequency and provides the exact value of the basal
distance between lamellar MMT layers. In order to avoid the
effect of the adsorbed water molecules in the interlamellar
space, the samples used for this study were all dried at 80 °C
for 1 night. The XRD patterns in the range of 2-10° are shown
in figure 3 for the samples of MMT-Na+ with adsorbed dye. The
clear distinction of MMT-Na+ and MMT dye complex is the
diffraction peak corresponding to the (001) plane. Peak for
MMT-Na+ was observed at 2θ= 7.26° while the diffraction
peaks of MMT-HDa-d were found approximately at ≈ 5.45°.
The comparison of MMT-Na+ with MMT dye complex shows a
shifting of the reflection band to a shorter value. This shifting
of the diffraction peaks of MMT dyes complexes to lower angle
indicated that intercalations structures have been formed. This
operation is named “organophilization of layered silicates”
scheme 3. If this process is incomplete, it is possible to find
from the XRD pattern the interlayer distances of both the
intercalated and original phases. In the present case, MMT dye
gives rise to unique single phase with specific interlayer
distance. As compared with that of pristine sodium-
Figure 3. XRD patterns of MMT-Na, and MMT-HDa-d clays.
4 | J. Name., 2012, 00, 1-3
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water
to the
J
-type aggregation of hemicyanine dye units into the
MMT - Na
interlayer position of MMT.
To demonstrate the -aggregation in MMT intercalated layer,
ds=1.17nm
J
Na+
Na+
Na+
Gallery spacing =0.21 nm
we have studied the solid-state fluorescence spectra of the
dye-intercalated MMT and pure dyes (figure 5). The dyes
exhibit significant fluorescence with intense emission at room
temperature when intercalated into clay. These significant red
shifting to higher wavelength varied depending on the type of
substituent on the phenyl ring. These results indicate strong
π−π interaction between successive dye moieties and
interactions of cationic dyes with MMT nanogalleries.50
R
Organophilization
of montmorillonite
1.27-1.49 nm
S
N
R
R
R
N
N
S
S
S
S
ds= 1.62 nm
0.67 nm
Therefore, the MMT-HD clearly suggests J-type aggregation of
dye into the interlayer position of MMT, which is also
supported by UV-vis and XRD analysis.
N
N
θ
R
MMT - HD
Scheme 3. Representation of MMT-Na clay sheet, dye molecule (HDa-d) before and
after organophilization process.
Table 1. Main peaks (2θ), basal spacing (ds) of the MMT-Na, MMT-HDa-d calculated
from XRD analyses.
Samples
2θ (°)
7.26
5.45
5.43
5.45
5.44
ds (nm)
1.17
MMT-Na
MMT-HDa
MMT-HDb
MMT-HDc
MMT-HDd
1.63
Figure 4. Solid state UV-visible spectra of (a) HDa-d and (b) MMT-HDa-d
1.62
1.63
1.62
The possible arrangement of cationic molecules in MMT layers
has been suggested from XRD data in which their optical
behavior would be strongly affected. These changes should get
reflected in UV-vis and photoluminescence spectroscopy.
Generally speaking, the physical attachment of dye to MMT
affects its absorption or emission spectra. However, if the
absorption or emission spectra of intercalates are shifted to
higher wavelength (red-shifted band) as compared to that of
Figure 5. Solid state Fluorescence spectra of (a) HDa-d and (b) MMT-HDa-d
pure dyes, there is
region, whereas
J
-type aggregation of dyes in the interlayer
The study of photophysical properties and orientation of dye
molecules at clay mineral interfaces has only recently been
clearly identified. Numerous research articles have been
published in this respect and valuable results reported. As can
be depicted from table 2, both the orientation/arrangement
and type-aggregation of dye into clay are strongly depending
on many parameters. The structure and the amount of loaded
dye as well as the interlayer distances of the dye/clay are
among key factors which affect the photophysical properties
of the guest-host hybrid materials.
H
-type aggregation refers to the aggregation
showing a blue-shifted band (hypsochromic band or H-band).
The emission spectra will express the information about dye-
dye as well as dye-clay interactions.49
Solid state UV−vis spectra of enꢀre caꢀonic compounds and
their intercalated are presented in Figure 4. Each dye has a
specific wavelength around (423-482 nm). In the host-guest
hybrid materials, the absorption band gets some red shifting to
480, 500, 445 and 477 nm in MMT-HDa, MMT-HDb, MMT-HDc
and MMT-HDd, respectively. This red shifting of ≈16 nm is due
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Table 2. Summary of recent works on photophysical properties and orientation of dye at clay mineral.
Dye-clay
Orientationa
Interlayer
Aggregation
type
Photophysical properties
of Dye-clay
Ref.
distances (nm)
λex
λem
1
2
Perylenediimide-MMT
Azobenzene-MMT
Interdigitated monolayer
0.74-1,24
0.37-0.9
J-type
J-type
482
675-695
51
52
Interdigitated monolayer
or monolayer coverage
345-353
-
3
4
Azobenzene- magadiite
interdigitated monolayer
1.57
-
H-type
J-type
335-340
250-371
-
53
54
or bilayer coverage
-
Rhodamine B-MMT
Cyanine -MMT
420-622
5
6
-
-
J-type
J-type
525
445-500b
-
55
Styrylbenzothiazolium-
MMT
Interdigitated monolayer
0.67
519-575b
This work
adepend on the amount of loaded dye and the interlayer distances of dye-clay; bdepend on the R substituted in the phenyl ring
In order to have a deeper insight on the arrangement of the major weight loss was occurred between 220-400 °C
inorganic matrix in the nanocomposite materials, fluorescence (presented by two characteristic peaks) corresponded to the
confocal images have been recorded. Figure 6 shows a series degradation first of the benzothiazolium salt moiety and then
of fluorescence images recorded on dyes in a scan range the styryl ring. The last degradation step between 450-730 °C
where the size particles can be observed. These particles are can be represent the oxidation of the organic cation which
due to the clay aggregates which are fluorescent (bright results in the formation of H2O, CO2 and NO2.56 It can be noted
colour) and observable; the emission from the host-guest that only one of the samples (HDd) exhibit a weight loss
material is due to the cationic dye, which was intercalated in between 30 and 145°C which may be due to the evaporation
the surface of MMT-Na+ sheets. In addition, the particles of residual solvents recrystallization or the presence of some
present a homogeneous fluorescence distribution in terms of bound moisture. In terms of thermal properties, the
intensity and spectral properties since the particle features decomposition temperatures revealed that HDb had the
have all the same colours.
highest thermal stability comparing to other dyes, starting to
decompose at 259 °C. Therefore, the presence of hydroxyl
group in the styryl ring efficiently enhanced its thermal
stability. These results indicated that the thermal properties of
these dyes varied depending on the types and the position of
the substituents within the styryl ring.
Figure 8 shows the TGA results performed on the entire
intercalated MMT during heating under air. As can be seen
from this figure. All samples had low amount of moisture; they
lost about ≈2% of water as moisture content, as can be seen by
drop in its mass below 100 °C. While for the untreated MMT,
the water uptake was approximately 6.62%. This initial step of
weight loss is due to physically adsorbed water molecules.
These results suggest a change from a hydrophilic to a
hydrophobic nature due to the incorporation of the dye
molecules. Similar results were obtained about the
dehydration peaks in the case of azo dye intercalated MMT-
Na+.57 Briefly, after the initial moisture, the figure 9 shows two
major distinct regions of weight loss on heating. Organic dye
substance evolves from 285 °C to 550 °C. Dehydroxylation of
the aluminosilicate occurs from 600°C to 700°C, observed also
in the parent MMT-Na. The content of intercalated
hemicyanine dye could be determined by TGA. Thus, the styryl
Figure 6. Fluorescence confocal images before and after exitation of a) MMT-HDa, b)
MMT-HDd.
Thermogravimetric analysis was done to evaluate the thermal
stability of the cationic dyes as well as for MMT dyes. TGA and
DTG curves of different benzothiazolium dyes are shown in
figure 7. The thermal studies reveal important information
about the stability and thermal behavior of these compounds.
dye content was estimated to be 11.72 wt
% of the
intercalation complex. We can see that the thermal stability of
the intercalated dye is significantly enhanced. The onset of
weight loss for the intercalated dye did not occur until 290°C,
while that for the pure dye began at 219 °C. In other words,
the onset of decomposition of MMT-HDa shifted to higher
temperatures compared to pure HDa. The enhanced stability
Both decomposition temperatures (Tonset
,
Tmax
)
were
determined by DTG and reported in table 3. The
thermogravimetric analysis and derivative curves show that
the decomposition/degradation occurs in two steps. A first
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of cationic dye in MMT is attributed to limited mobility of the
intercalated dye and the special confinement within the
silicate galleries.58,59 Considering the results of XRD, FTIR
analysis, it is implied that the intercalation of HD molecules
has occurred in interlayer spaces of MMT-Na. Therefore, its
thermal stability has improved accordingly.
Table 3. Decomposition temperatures of hemicyanine benzothiazolium dye
HD
R
Tonset.1(°C) Tmax.1(°C)
Tonset.2a(°C)
248
Tmax.2b(°C)
289
HDa 4-OCH3
HDb -OH
219
259
224
214
241
271
240
230
281
329
HDc -Cl
249
291
HDd 2-OCH3
242
304
aTonset,2 refers to peak valley temperature between 1st peak and 2nd peak in DTG.bTmax,2
refers to peak maximum temperature of 2nd DTG peak. Onset: decomposition starting
temperature.
Figure 8. TGA curves of hemicyanine-modified montmorillonite
Figure 9. TGA of a) MMT-Na, b) MMT-HDa, and c) HDa. The samples were heated to
800 at 10°C/min in air atmosphere.
Characterization of the bio-film
FTIR analysis attempted to characterize the effect of
organoclay incorporation on the chitosan films and to
determine the infrared bands and shifts related to MMT-
HD/chitosan interactions. The position of the peaks of chitosan
film spectrum is similar to those described by different
authors. Figure 10a show FT-IR spectrum of neat chitosan and,
CS/MMT-HDa nanocomposites films at different weight
percentage (1, 2, 5, 10wt%). The absorption peaks of the pure
chitosan films CS are mainly assignable to the stretching intra-
and intermolecular O–H vibrations at 3500–3000 cm-1,
overlapped with amine and secondary amide stretching mode.
Symmetric and asymmetric C–H vibrations at 2920–2870 cm-1.
Amide I vibrational mode (acetamido group) at 1635 cm-1, and
Amide II (amino group) at 1542 cm-1.60 Indeed, due to the
incorporation of organo-modified montmorillonite to the
chitosan matrix, some differences can be observed in the FTIR
Figure 7. TGA and DTG curves of benzothiazolium hemicyanine Dye
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Figure 10. FT-IR spectra of chitosan and chitosan/MMT-HDa bio-nanocomposites (a)
and enlargement in the 3100-2700 cm-1 frequency range
spectra of the bio-nanocomposites films. The intensity of the
bands in the range of 1600–1300 cm−1 increases comparing to
the bands of the pure chitosan. Interestingly, the band at 1026
cm−1 corresponding to asymmetric Si-O-Si stretching had its
intensity increased after organoclay addition. It is also
interesting to note that a modification resulted in stretching
C–H vibrations at 2920–2870 cm-1, with an inverted intensity
of methylene compared to methyl bands after treatment with
organoclay (figure 10b). In fact, the introduction of MMT-HDa
into the chitosan leads to an increase at 2877 cm-1
corresponding to methyl groups from methoxy branch of
phenyl ring and a decrease at 2843 cm-1 related to methylene
groups from alkyl chain of benzothiazolium dye. Further
introduction of various kinds of modified clays into chitosan
did not show much difference in light of a related chemical
structure found in chitosan (NH and CH) and organoclays.
Accordingly, there is a difficulty in identifying the interaction
through the ammonium site on the modifier to interact with
chitosan, although some literature observed widening of FTIR
bands or multiplicity that may be attached to the electrostatics
interactions between clay and other bio-based polymers such
as soy protein or gelatin.61
Contact angle is defined as the angle between the substrate
surface and the tangent line at the point of contact of the
liquid droplet with the substrate.62 The contact angle (CA) of
water is one of the basic wetting properties of packaging
materials and is an indicator of the hydrophilic/hydrophobic
properties of the material. Usually, the more hydrophilic a
material is, the lower the contact angle value it has. According
to Araújo et al.63 angles greater than 50° indicate
a
hydrophobic surface. The surface hydrophobicity of the bio-
nanocomposite film was evaluated by means of contact angle
determination. Table
4 shows contact angles of bio-
nanocomposite film, exhibiting a value of 85° for CS control
film. Almeida and coworkers64 reported a contact angle of 95°,
attributing this result to the hydrophobicity of chitosan chains.
For the bio-nanocomposites. The results indicate that the
organoclay content addition affect film surface hydrophobicity,
it can be clearly seen that an increase in organoclay content
brought about an increase in contact angle values of the films.
This observation can be explained by the fact that interaction
between chitosan and modified montmorillonite reduced the
availability of hydrophilic groups, thus decreasing the
hydrophilicity. This interaction increased the distance between
the clay layers, increasing hydrophobicity of films as the
organoclay concentration increased. Furthermore, the effect
of chain length corresponding to the hydrophobic groups is
particularly important to interfacial properties and
technologies of films. However, it should be noted that the
slight increase of CA is due to the short chain containing two
carbons of the dye. In this case, For CS/MMT-HDa, the contact
angles of films increase with increase in the organoclay
content from 1, 2, 5 and 10%, respectively. Considering all
these aspects, the obtained results revealed that the
wettability of the bio-nanocomposite films decreased with an
increase in the organoclay content owing to the replacement
of Na+ in the interlayer region of MMT with hydrophobic
benzothiazolium dyes.
The water solubility of the bio-nanocomposite films as a
function of MMT dye content is also shown in Table 4.
Increasing organoclay concentration in film matrix caused to
decrease in water solubility of the films. In the case of
CS/MMT-HDa. The WS was 12.3% for the sample without clay
and decreased to 10.1, 9.8 and 9.2% for the films containing 1,
2, 5 wt% MMT, respectively. Though, significant decreases in
solubility observed in high levels of MMT. At the level of 10%
wt% MMT, CS/MMT-HDa bio-nanocomposite showed the
lowest WS values at 8.7%. It should be noted that the same
behavior have been observed in all films CS/MMT-HDa-d
comparing with the neat one. As a result, the reduction of the
film solubility is indicative of a better interaction between
chitosan chains and clay in the film, which is facilitated by a
better dispersion of the nanoclay into the polymer matrix.
Some authors observed similar behavior when MMT added to
films composed of agar and soy-protein.65,66 In our case, the
hydroxyl groups of organoclay can form strong hydrogen
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bonds with the hydroxyl groups on chitosan, thus improving distribution is spectrally homogeneous since the bright areas
the interactions between the molecules, improving the in the image appear of same colors. This observation suggests
cohesiveness of biopolymer matrix and decreasing the water that the hybrid host-guest materials are uniformly dispersed in
sensitivity. Solubility could be an important characteristic for the chitosan matrix with a small amount of agglomerates.
biodegradable films because it can affect resistance of film to
water. For example, films on high-moisture foods must be
insoluble, while films for water-soluble pouches must be
readily soluble.
Table 4. Effect of organoclay content on hydrophobicity properties (CA) and water
solubility (WS) of chitosan-based nanocomposite films.
MMT-HDa content (%)
Neat CS
CA (°)
WS (%)
12.3±0.9
10.1±1.2
9.8±1.4
9.2±0.6
8.7±0.2
85.0±0.5
87.9±1.5
90.7±1.6
94.6±0.8
98.0±1.7
Cs/MMT-HDa1%
Cs/MMT-HDa2%
Cs/MMT-HDa5%
Cs/MMT-HDa10%
Each value is the mean of 3 replicates with the standard deviation; CA, contact
angle of water; WS, water solubility
The bio-films have been prepared and its fluorescence
properties were analyzed at the optimum excitation
wavelengths. As we know, Chitosan is transparent in the UV-
vis visible region and shows poor optical properties.
Commercial chitosan has a very weak absorption and emission
band on the contrary; the CS/MMT-HDa-d bio-nanocomposites
films have shown excellent optical properties. All film exhibit
an increase in the emission wavelengths with the increasing of
the clay content starting from 1 to 10% (figure 11). In the same
point of view, the intensity of fluorescence increases almost
gradually. As stated before, there is an electrostatic attraction
between the cationic dyes and negative charge on the clay
particles. These interactions of the cationic dyes with the clay
have several unique advantages in their photophysical
properties such as restriction of molecular, internal-torsional
motions, reduction of self-aggregation, increase of rigidity,
reduction of strong excitation coupling, protection the
fluorophore dye from the quenching action of dioxygen, and
decrease of vibration relaxation pathways.67 In addition, the
dispersion of photoactive or photoresponsive clays is generally
performed to simply transfer the photophysical properties of
the hybrid host-guest material to a polymer matrix.68 In our
case, the incorporation of the organoclay into chitosan matrix
enhance the fluorescence emission for the organoclays,
suggesting the presence of both dye into the clay and dye
dispersed in the chitosan matrix. To reach a direct visualization
of the labeled clays into the polymer, the bio-nanocomposite
materials have been investigated by fluorescence imaging
through a confocal microscope, providing pictures with bright
particles accounting the dispersion level of the platelets. The
fluorescence images recorded on the bio-nanocomposite
materials are shown in figure 12. The bio-film samples
presented small fluorescent particles. These fluorescent
particles are due to the clay aggregates labeled by the dye
inside the chitosan matrix. This grain analysis indicates that the
inorganic matrix is well dispersed in the polymer matrix with
no space between clay and polymer. The fluorescence
Figure 11. Emission wavelengths of bio-films in terms of the clay content %; Excitation
wavelengths: Cs/MMT-HDa (λex= 412), Cs/MMT-HDb (λex= 435), Cs/MMT-HDc (λex
395), Cs/MMT-HDd (λex= 410).
=
Figure 12. Fluorescence confocal images of Cs/MMT-HDa, Cs/MMT-HDb, Cs/MMT-HDc
and Cs/MMT-HDd.
Conclusions
Several hemicyanine dyes with donor–acceptor group have
been successfully intercalated into the interlayer space of
montmorillonite by the conventional ion exchange method, as
indicated by FTIR spectra. As compared with that of pure dye,
the thermal stability of the intercalated dye was greatly
enhanced, as indicated by TGA curve, and the emission band
corresponding to hemicyanine group in intercalated HD shifted
towards longer wavelength. This could be ascribed to J-type
aggregate HD in the interlayer space which exhibited the
presence of an interdigited monolayer of dye into the host-
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Journal Name
Carbohydrate Polymers, 2007, 67, 358–365.
21 M. Darder, M. Colilla and E. Ruiz-Hitzky, Chemistry of Materials,
2003, 15, 3774–3780.
22 D. S. and P. N. S. Sahoo, A. Sasmal, Journal of Applied Polymer
Science, 2010, 118, 3167–3175.
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guest material. The intercalation of the dye in the clay layers
may find application in melt processing of thermoplastic
polymers. In addition, the dried organophilic clays were then
mixed with chitosan to obtain the corresponding bio-films.
Both fluorescence and hygroscopic properties were reported.
The results show enhancement in the hydrophobicity of the
hybrid materials, whereas the fluorescent emission
wavelength were also increase with increasing the clay
content. According to confocal fluorescence image, the
homogeneous fluorescence distribution suggests that between
the filler and the polymer there is no space occupied by the
dye. These results show an improvement of the interfacial
adhesion and random dispersion/distribution of clay nano-
particles which can afford a fluorescent bio-nanocomposite
film with potential applications in packaging and biosensors.
29 H. Tomioka and T. Itoh, Journal of the Chemical Society,
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