volume 9, issue 1, 2020, 880 - 884 issn 2284-6808 letters in … · 2020. 3. 10. · sulfuric acid...
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Synthesis and characterization of methyl acrylamide cellulose nanowhiskers for
environmental applications
Swati Sharma 1
, Ch. Shiney Rashmitha 2
, Lalit M. Pandey 1, * 1Bio-interface & Environmental Engineering Lab Department of Biosciences and Bioengineering, Indian Institute of Technology Guwahati, Assam, 781039, India 2Department of Biotechnology Motilal Nehru National Institute of Technology, Allahabad, Prayagraj, Uttar Pradesh, 211004, India
*corresponding author e-mail address: [email protected], [email protected], [email protected] | Scopus ID 36984082800
ABSTRACT
Cellulose is the most abundant biopolymer on earth with excellent mechanical strength, biocompatibility, and biodegradability. Yet, its
poor chemical reactivity deceives its application in various biotechnological sectors. In this study, the microcrystalline cellulose was
used as low cost substrate for the synthesis of cellulose nanowhiskers (CNWs) using controlled acid hydrolysis. In order to improve their
chemical reactivity, synthesized CNWs were modified with carboxyl functional groups using 2,2,6,6-Tetramethylpiperidinyloxyl
(TEMPO) mediated oxidation reaction. These carboxylated cellulose nanowhiskers (CCNWs) were functionalized with amine functional
groups using carbodiimide reaction. The as-synthesized amine terminated amido-cellulose nanowhiskers (ACNWs) were modified with
methyl acryl functional groups using Michael addition resulting methyl acryl functionalized amido-cellulose nanowhiskers
(MAACNWs). The synthesized MAACNWs were characterized using Fourier-transform infrared spectroscopy, UV-visible spectroscopy
and nuclear magnetic resonance spectroscopy. The physical morphology was studied using Atomic force microscopy, X-Ray Diffraction
and zeta analyser. The modified cellulose nanowhiskers were found to exhibit 4 folds increased surface roughness confirming fabrication
of negatively charged chemical groups on CNWs yielding 7 folds higher surface charge than pristine nanowhiskers. Hence, the modified
CNWs could be acknowledged as potential candidate for effective heavy metal biosorption and remediation studies.
Keywords: Cellulose nanowhiskers; Michael addition; Methyl acrylamide; Carboxylation.
1. INTRODUCTION
Nanomaterials are the major focus of present material
science studies due to their magnificent ability to be custom
tailored as per the synthesis conditions with desired morphological
and chemical characteristics for the diverse applications [1, 2].
They have been explored in various area of research
ranging from drug delivery, drug design to various photocatalytic
and environmental applications [3-7]. Cellulose is the most
abundant renewable biopolymer existing on the earth with distinct
characteristic properties of high mechanical strength,
biocompatibility, biodegradability and economic feasibility.
Owing to such features, makes cellulose a suitable substrate for
the nanomaterial synthesis [8].
Recent studies have explored various forms of cellulose
nanostructures such as cellulose nanocrystals and cellulose
nanotubes for various biotechnological and environmental
applications. In contrast to carbon nanotubes, the presence of
surface active hydroxyl groups on their surfaces boosts cellulose
nano-whiskers exploration in the field of biotechnology due to
their improved solvent dispersity [8, 9]. Apart from surface active
groups, its high surface to volume ratio encourages towards
surface functionalization studies. However, very few reports are
available based on the surface modification of cellulose nano-
whiskers [10].
Branched chain of polymers of ethylene diamine
conjugated methyl meth acrylate have been explored for
improving the surface functionality, overall surface area and
charge. Their chemically reactive structure gains the advantage in
the synthesis of a vigorously branched structure known as
dendrimers [10, 11].
In this study we have fabricated novel acrylamide capping
on the surface of cellulose nano-whiskers using a convergent
approach. The successful functionalization is monitored using
FTIR, UV and NMR spectroscopy. The physical properties of the
modified nano-whiskers like surface charge and surface
morphology are measured. This modified nanomaterial with
featured characteristics could be further explored in the field of
biological and various environmental applications.
2. MATERIALS AND METHODS
2.1. Chemicals and reagents.
Microcrystalline cellulose powder (MCC) (GRM333) and
Sulphuric acid (H2SO4) (RM6244) were used for synthesis of
cellulose nano-whiskers (CNWs). Sodium hypochlorite (NaClO)
(PCT1311-50ML), Sodium bromide (NaBr) (GRM7496), 2,2,6,6-
Tetramethyl-1-piperidinyloxy radical (TEMPO)( RM4807), and
absolute ethanol (MB106) were used for the preparation of
carboxylated CNWs (CCNWs). Ethylenediamine (EDA), 1-Ethyl-
3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC)
(RM1817), N-Hydroxysuccinimide (NHS) (RM1120), Methyl
methacrylate (MMA) (GRM8641), (4-(2-Hydroxyethyl)-1-
piperazineethanesulfonic acid) HEPES buffer (TCL021), and
Dimethyl formamide (DMF) (MB036) were required for the
acrylamide functionalization of CCNWs. Double distilled water
(Mili Q, 18mΩ, Millipore systems) was used in all the
experiments.
Volume 9, Issue 1, 2020, 880 - 884 ISSN 2284-6808
Open Access Journal Received: 19.02.2020 / Revised: 04.03.2020 / Accepted: 06.03.2020 / Published on-line: 10.03.2020
Original Research Article
Letters in Applied NanoBioScience https://nanobioletters.com/
https://doi.org/10.33263/LIANBS91.880884
https://www.scopus.com/authid/detail.uri?authorId=36984082800https://orcid.org/0000-0002-1664-5730https://nanobioletters.com/https://doi.org/10.33263/LIANBS91.880884
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Synthesis and characterization of methyl acrylamide cellulose nanowhiskers for environmental applications
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2.2. Preparation of cellulose nanowhiskers.
In this study we prepared cellulose nanowhiskers using
sulfuric acid hydrolysis as reported elsewhere by our group [9].
Briefly, microcrystalline cellulose powder was treated with 65%
(v/v) sulfuric acid in 1:10 (w/v) ratio at 50°C under constant
stirring for 2 h. The reaction was stopped by adding two folds of
ice cold water to the solution. The obtained solution was further
centrifuged at 10,000 rpm for 20 min at 4°C and washed several
times to neutralize the pH. Finally, the pellet was lyophilized to
obtain CNWs.
2.3. Preparation of carboxylated CNWs.
The hydroxyl (-OH) surface groups of CNWs was initially
oxidized to carboxylic acid (-COOH) functional groups using
TEMPO-mediated oxidation system [9]. For this, 1% (w/v) of
prepared CNWs were mixed with TEMPO (70 mg/g) and NaBr
(180 mg/g) under constant stirring at room temperature. To this
solution, 9% (v/v) of NaClO was added slowly to maintain the pH
of the solution at 10.5 for 10 h. Finally, to arrest the reaction 20
mL of ethanol was added. Further, the obtained mixture washed
and centrifuged at 10000 rpm for 10 min at 4°C till the pH
neutralized and later lyophilized to obtain CCNWs.
2.4. Preparation of acrylamide coated CCNWs.
The acrylamide coated CCNWs were synthesized using
divergent mode of synthesis (Figure 1). In this, the core CNWs
were oxidized to obtain surface active –COOH functional groups
which acted as binder for the acrylamide functionalization. The
prepared CCNWs were used as monomer units for the further
modification. Initially, for the coupling of EDA on the CCNWs,
carbodiimide crosslinking technique was employed [10]. EDC-
NHS were used in HEPES buffer for the formation of amide
bonding between –COOH group of CCNWs and NH2 group of
EDA. To this, methyl methacrylate coating was done using
Michael addition reaction resulting the formation of methyl acryl
amido CCNWs (MAACNWs) [11].
Figure 1. Schematic representation of acrylamide coated cellulose
nanowhiskers.
2.5. Characterization of the functionalized CNWs.
The synthesized modified CNWs were analyzed both
chemically and physically to explore its surface characteristics.
The morphology of MAACNWs was compared to pristine CNWs
using Atomic force microscopy (AFM). Also. X-ray Diffraction
(XRD) was performed to analyse any change in the crystal
structure due to modification conditions. In addition, the prepared
MAACNWs were characterized for the presence of surface active
functional groups using Fourier Transform Infrared spectroscopy
(FTIR). Proton nuclear magnetic resonance (1H NMR) was also
recorded for the prepared material. The surface charge of the
prepared MAACNWs was analysed using zeta potential analyser.
2.5.1. Chemical characterization.
The functional groups analysis of cellulose nano-whiskers
and modified MAACNWs was characterized using FTIR (Perkin
Elmer, Spectrum Two) equipped with ZnSe crystal in attenuated
total reflectance (ATR) mode. Initially, the samples were dried
overnight and later placed directly over the crystal using pressure
clamp. All the spectra were recorded in the range of 400-4000 cm-
1 with an average 32 scans and 4 cm-1 resolution. FTIR spectra
were analysed at each step of modification as qualitative
conformation for proceeding further steps. The synthesized
MAACNWs were also analysed using 400 MHz Nuclear Magnetic
Resonance Spectrometer (Make: VARIAN, Model: MERCURY
PLUS) using D2O as solvent.
The synthesized MAACNWs were also examined using
UV-spectroscopic technique (Carry 100 Bio UV-Vis
spectrophotometer). For this study, 10 mg of synthesized material
was dissolved in MiliQ and scanned in the range of 200-350 nm.
Only cellulose nanowhiskers with no surface modification were
used as control. The presence of additional peak suggested the
change in surface chemistry of material due to modifications.
2.5.2. Physical characterization.
The surface morphology of synthesized CNWs and
MAACNWs were analysed using AFM (Oxford, Model: Cypher)
equipped with silicon nitride tip (Radii
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Swati Sharma, Ch. Shiney Rashmitha, Lalit M. Pandey
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sulfonate groups was seen at 1205 cm-1 confirming the formation
of nano-whiskers due to sulfate esterification. Various literatures
have reported sulfate peak as conformation of hydrolysis of
cellulose [9]. Further, the prepared CNWs were carboxylated
using TEMPO mediated oxidation reaction. In this, the –OH
groups of CNWs were oxidized to –COOH groups which were
observed as a new peak at 1740 cm-1 (Figure 2B) yielding
carboxylated cellulose nanowhiskers (CCNWs) [13].
These CCNWs were further modified using carbodiimide
reaction between EDA and CCNWs. In agreement with that,
appearance of peak at 1545 cm-1 demonstrated the amide II
linkage on the surface of CNWs (Figure 2C) [14, 15].
Furthermore, the MMA coated amido CNWs were explored for
the grafting of esterified amide groups. The presence of peaks at
1450 cm-1 and 1260 cm-1 showed the presence of functional groups
for CH2 groups and CN groups explained the functionalization
MMA on the surface of this synthesized amido-cellulose
nanowhiskers (ACNWs) (Figure 2D) [11, 16]. Hence, FTIR
spectra revealed the successful grafting of methyl acrylate groups
on the surface of CNWs yielding a material with highly enriched
negatively charged functional groups on its surface.
Figure 2. FTIR spectra for (A) CNWs, (B) CCNWs, (C) ACNWs, and
(D) MAACNWs.
Figure 3. UV spectra for pristine CNWs and functionalized MAACNWs.
Thus synthesized material was further explored using UV-
spectroscopy. UV-spectra revealed the presence of a new peak at
255 nm in MAACNWs, which was absent in CNWs (Figure 3).
Literature reports this peak of the presence of amide bonds as in
present in MAACNWs. A similar peak has been reported in
literature for methyl acryl amide surface modifications [17, 18].
NMR spectra of synthesized MACNWs depicted the
presence of peaks at 3.36 and 3.07 ppm signifying the -OCH3
protons and -N-CH3 protons, respectively on the surface of CNWs
after grafting of methyl meth acrylate [19] [20]. The peaks at 4.4
to 4.8 ppm stand for the D2O solvent peaks (Figure 4)
Figure 4. 1H NMR spectra of the synthesized MAACNWs
3.2. Physical characterization.
Due to strong acid based hydrolysis the precursor
microcrystalline cellulose gets deconstructed to smaller fragments
taking various shape based on the reaction conditions used for the
hydrolysis. In the present study, microcrystalline cellulose got
transformed into nanowhiskers, as analysed using AFM. It was
observed that CCNWs were rod shape in morphology with a
length of ~ 144 ± 30 nm and width of about ~25 ± 5 nm (Figure
5A). However, after conjugation with acrylamide, various
extensions in the form of branched projection were observed
(Figure 5B). MAACNWs were found to be ~ 215 ± 20 nm in
length and 30 ± 7 nm in width, deciphering the formation of
branched morphology. Figure 5 showed the length and thickness
of whiskers increased upon modification due to the formation of
branched structures. Prior to functionalization, the surface of
CNWs were found to be comparatively smooth.
Figure 5. AFM images of (A) CNWs and (B) MAACNWs.
The effect of surface functionalization on the morphology
of CNWs was estimated by analyzing the surface roughness in
terms of change in average height (Ra). The Ra value for bare
CNWs was 0.67 nm, which increased to 2.55 nm after surface
functionalization indicated successful modification. Few
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Synthesis and characterization of methyl acrylamide cellulose nanowhiskers for environmental applications
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unbranched whiskers were also present suggesting all the reactive
sites were not occupied. Hence, the present base material can
accommodate more branched structures on its surface and could
be further optimized for effective functionalization.
To see the effect of chemical modification on crystal
structure, XRD was performed for pristine and modified CNWs.
Figure 6A illustrates the diffraction peaks at 15.9, 22.8 and 33.8
angles which corresponded to diffraction planes of Cellulose-Iß
(110), (200) and (211), respectively (JCPDS 00-060-1502). Figure
6(B and C) showed that even after strong acid hydrolysis and
carboxylation reaction, no damage was done to the crystal
structure for prepared pristine and modified CNWs. However
there was a slight blue shift observed in the case of modified
(MAACNWs) (2θ = 22.5) from pristine CNWs (2θ = 23.05) which
could be due to change in chemical composition of modified
CNWs. Yet there was no significant change in crystal size (i.e.
11.02 nm), as calculated from Scherrer equation [4].
The prepared MAACNWs were further studied for their
overall surface charge using zeta potential analysis. It was
observed that obtained pristine CCNWs exhibited a surface
potential of -2.5 ± 1 mV which transformed to a more negative
surface potential of -17.7 ± 1.2 mV after modification. This
explained the presence of a higher amount of acrylate groups on
the surface of functionalized CCNWs. Such improved surface
charge by ~7 folds, signified the successful modification of
cellulose nanowhiskers. This was due to the availability of high
surface area to volume ratio nanowhiskers that encouraged higher
modification sites. Hence, the synthesized methyl acrylamide
cellulose nanowhiskers are found to be highly surface active with
suitable surface area and charge distribution that could be further
optimized for biosorption and other environmental applications.
Figure 6. XRD diffractogram for standard Cellulose 1β (A), modified
CNWs (B) and pristine CNWs.
4. CONCLUSIONS
In this study, cellulose nanowhiskers were prepared and
carboxylated using TEMPO mediated oxidation reaction. These
CCNWs were modified using carbodiimide reaction to obtain
acryl amine coated CNWs with highly improved surface
reactivity. The synthesized nanostructure was found to be whisker
shaped in morphology increased surface roughness after surface
modification indicating the successful functionalization on CNWs.
The crystallinity of cellulose nanowhiskers was retained after
surface modification explaining its high stability. Functional group
analysis showed the presence of acrylate groups on the surface
causing increased negative surface charge by 7 folds after
modification. Conclusively, the synthesized modified
nanowhiskers were found to exhibit excellent surface
functionalization and charge and hence could be further optimized
to improve the overall selectivity and sorption capacity for various
environmental applications.
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6. ACKNOWLEDGEMENTS
The authors would like to express gratitude to the Department of Science and Technology, Government of India for financial
assistance (Sanction Nos: DST/INSPIRE/04/2014/002020, ECR/2016/001027, and DST/INT/UK/P-155/2017) for this work. The authors
would like to acknowledge Central Instrument Facility (CIF), IIT Guwahati for allowing the access of various facilities required for the
present study.
© 2020 by the authors. This article is an open access article distributed under the terms and conditions of the
Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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