Green synthesis of Silver Nanoparticle using Porphyran as Nanoreactor
ABSTRACT
Nanotechnology has received great attention from study due to chemical, physical and optical
properties acquired by nanostructured materials. However, methods to synthetize metallic nanoparticles
(NP) described involve the use of toxic reagents and low productivity. In order to minimize the
potential impacts of these systems to human health and the environment and to apply the principles of
green synthesis, the aim of this work was to synthetize silver nanoparticle using a water soluble
polysaccharide as a reductor and stabilizing agent. The soluble polysaccharide was extracted from
NORI and its identity confirmed by NMR analysis which showed to be a porphyran, a sulfated
polygalactans composed of galactose and 3,6-anydrogalactose, sometimes substituted with galactose-6sufate and 6-O-methyl-galactose. The AgNPs were obtained by using AgNO3 as silver ions donor and
porphyran (50 µg/mL) at pH7 and 10, and 70˚C for 60 minutes. The AgNPs prepared showed a
plasmon band centered at ~ 400 nm and a dark-brown color. The nanoparticles showed a diameter size
of 165.16 nm at pH 7.0 and 6.04 nm (13.9%), 41.34 nm (27.6%) e 241.2 nm (58.6%) at pH10 and a
negative zeta potencial value due to de polysaccharide capping. The resultant polyanionic charged
AgNPs showed to be stable after one month, possibly due to its electrostatic stability.
Keywords: Green synthesis; Silver Nanoparticles, Porphyran, Nanoreactor.
1)
INTRODUCTION
The use of nanostructured materials have received great attention of study due to its chemical,
physical and optical properties when the materials acquired a nanoscale (EL-SAYED et al 2001). The
physicochemical properties of nanoparticles-containing materials are quite different to those of the bulk
materials because of the extremely small size and high surface volume ratio of nanoparticles (ELSAYED et al 2011). Due to these properties, nanomaterials have been applied in various fields such as
medicine (FAROKHZAD et al.,2006), textile industries (MATSUSHITA et al., 2012), catalysis
(ZHANG et al., 2012), chemical sensing and imaging, environmental remediation (HUANG et al.
2007), drug delivery (JOSHI et al.,2006), and biological labelling (DANIEL AND ASTRUC 2004).
Over the past few years the use of metal and metal oxide nanoparticles based on silver, gold,
copper, copper oxide, maghemite, magnetite in medicine, dentistry, pharmacy and biology has been
related (MORITZ and GESZE-MORITZ, 2013). Although, most methods to synthetize metallic
nanoparticles described use the reduction of their respective salts with chemical reducing agents, as
citric acid, sodium borohydride and N,N-dimethyl-formamide, which are toxic reagents, hazardous to
the environment and expensive (PLYUTO et al., 1999, WANG et al., 2002, PASTORIZA-SANTOS
and LIZ-MARZAN, 2000 and GUARI et al., 2003). An alternative to minimize the potential impacts of
these systems to human health and to the environment is to apply the principles of green synthesis,
providing the creation of new materials without the use of harsh reducing agents, polymers, solvents,
surfactants and to contribute to reduce the toxic waste generated (ALBRETCH et al.,2006).
The green synthesis of nanomaterial attempt to reduce cost, improve stability and toxicity of the
formed nanoparticles. Several methods are being described to promote these benefits, many of them
includes biological methods, as the use of bacteria (SAIFUDDIN WONG YASUMIRA and NUR,
2009), fungi (EL RAFIE al., 2009), plant extracts (GILAKI, 2010) and polysaccharides (EL - RAFIE et
al., 2013). Sugars, as glucose was also used as a reducing agent to synthesizing gold nanoparticles
entrapped in a thermally evaporated fatty amine film (GOLE et al., 2001). RAVEENDRAN et al.
(2003) also used β-D-glucose as a reducing agent and starch as a capping agent to prepare starch/silver
nanocomposite film. The chitosan and heparin polysaccharides was also used as reducing and
stabilizing agents for the synthesis of Au and Ag nanoparticles, respectively (HUANG et al., 2004).
Porphyran is a water-soluble agaran extracted from the cell wall of red seaweeds belonging to the
genus Porphyra (Rhodophyta) that comprises the species P. yezoensis (susabinori) and P. tenera
(asakusanori) (ANDERSON & REES, 1965). This is one of the most consumed algal polysaccharides,
being commercialized as sheet type of dried food known as NORI, which are extensively used in the
preparation of sushi (NISIZAWA et al., 1987 and FUKUDA et al., 2007). Porphyran are linear sulfated
species constituted by units of (6-O-methyl)-β-D-galactose, 3,6-anhydro-α-L-galactose and α-Lgalactose 6-sulfate. As other sulfated galactans, porphyran may also have some potential
pharmacological applications (POMIN & MOURAO, 2008), hypolipidemic (INOUE et al., 2009) and
anti-allergic properties (ISHIHARA et al., 2005), antioxidant and antitumoral activities (SHAO, CHEN
AND SUN, 2013).
Aiming to obtain stable silver nanoparticles in a low-cost, eco-friendly, non-toxic solvents use,
using biodegradable, biocompatible and bioactive products, in this study a water soluble polysaccharide
fraction was extracted from NORI. The chemical identity and composition of the fraction were
determined. The ability of extracted porphyran to act as a reducing and stabilizing agent of silver
nanoparticles was evaluated.
2)
MATERIALS AND METHODS
Chemicals
Silver Nitrate (AgNO3), sodium borohydride (NaBH4) were purchased from Vetec. Ultrapure
Agarose was obtained from Invitrogen. The Nori (Porphyra genus) was obtained from a local supplier
(Sukina ®).
Extraction of polysaccharides
The dry algal powder (65g) was extracted in distilled water 1.5% w/v under stirring for 18 h. The
slurry was filtered and concentrated by rotary evaporator at temperature below 60 ˚C. The soluble
polysaccharides were precipitated by the addition of 3 fold volume of ethanol 95% (v/v), centrifuged at
12000 rpm for 20 min, dialyzed against water and freeze dried.
Chemical composition of polysaccharides
Protein content was determined by Lowry method (1951). Sugars were determined by the
phenolsulfuric method using glucose as standard (DUBOIS et al., 1956). The sulfate group was
determined using the DOGSON & PRICE method (1962).
13
C NMR spectroscopy
The samples were solubilized in D2O and analyzed on the 400 MHz Bruker NMR spectrometer.
The polysaccharide carbon NMR spectra was recorded at 80 °C using 10240 scans. Chemical shifts
were expressed in ppm in reference to the external standard. The NMR signals of the porphyran
disaccharide were fully assigned.
Synthesis of silver nanoparticles (AgNP) porphyran nanohybrid
28 mg of water soluble polysaccharides were dissolved in distilled water (0.05% w/v) and the pH
adjusted with NaOH solution at 7 or 11. The AgNO3 was slowly added to the system (final volume of
50 mL) to a final concentration of 1µM in the presence or not of NaBH4 (0.10 mg/mL) and the
reaction kept under stirring for 60 minutes at 70˚C.
UV-Visible spectroscopy measurement
The AgNP formation using polysaccharide as nanoreactor was evaluated at times 0, 15, 30, 45,
and 60 minutes of the reaction. Samples collected were diluted 1:20 and the 200-800nm UV-Vis
spectra analyzed in aVarian Cary 50 Bio spectrophotometer.
Characterization of AgNPs
The particle size distribution and zeta potencial of AgNPs nanohybrid and polysaccharides was
determined using Zetasizer Malvern Nano-2590, and Zetasizer software.
3)
RESULTS AND DISCUSSION
In this work, a porphyran was water extracted from red algae type genus Porphyra. The
potential of this polysaccharide to act as a reducing agent and stabilizer in a green synthesis of silver
nanoparticles has been reported.
Chemical analysis
Crude polysaccharides were water extracted from the red algae yielding of 2.5% with respect to
the dry weight of algae. The methodology adopted in this work was eco-friendly and non-toxic when
compared to the methods available in the literature which includes the use of formaline and methanol
(CORRECK et al., 2011, ISHIHARA et al., 2005). Table 1 presents the analysis of the chemical
composition of the obtained fraction. The fraction is mainly composed of carbohydrates (33.8%), and
low protein content (7.8%). As might be expected, the isolated polysaccharide showed the presence of
sulfate groups (11.6%). The zeta potential analysis (Table 1) showed its polyanionic properties,
possibly due to the presence of its sulfate groups. The high zeta potencial value also shows its colloidal
stability (-32.2 mVA) considering that generally when the particles have a large positive or negative Zp
(greater or lower than + 30.0mVA and -30.0 mVA) they will repel each other and stabilize the
dispersion (HADDADA et al., 2014).
Table 1 – Composition of water soluble polysaccharide extracts of red algae genus Porphyra.
Polysaccharide
The
13
Yield (%)
Sugar (%)
Protein (%)
Sulfate (%)
Zeta Potencial (mV)
2,51
33,79
7,80
11,59
-30.8
C NMR spectrum of the isolated fraction (figure 1) confirms that the polysaccharide
isolated is a porphyran, since the presence of the signals at 103.70, 101.41, 102.41 and 98.41 ppm
referring to carbon 1 (anomeric) of the dyads (3 → )-β-D-galactopyranose-(1 → 4)-α-Lgalactopyranose-6-sulfate-(1
→)
and
(→
3)-β-D-galactopyranose-(1
→
4)-α-L-3,6-
anidrogalactopiranose-(1 →), respectively (figure 1,2 and Table 2), as confirmed by the literature
(LAHAYE et al., 1989).
Figure. 1- 13C NMR spectra for the water soluble extracted polysaccharide
Table 2 - 13C NMR assignments of the water soluble polysaccharide isolated
Fraction Analisys
(G-A)
(G-L6S)
C-1
C-2
C-3
C-4
C-5
C-6
G
102.41
70.15
82.25
68.82
75.41
61.44
A
98.41
69.99
80.10
77.38
75.61
69.68
G
103.70
69.81
81.10
69.16
75.94
61.70
L6S
101.41
69.37
71.04
19.07
70.15
67.83
G: (1 → 3) β-D-galactopyranose; A: (1 → 4) 3,6-Anidrogalactopyranose; L6S: (1 → 4) α-Lgalactopyranose 6-sulfate.
Figure 2: Structure of the repetition moieties encountered in porphyran.
The (→3)-β-D-galactopyranose -(1→4)-α-L-galactopyranose-6-sulfate-(1→ ) dyad (G-L6S) and
(→3)-β-D-galactopyranose-(1→4)-α-L-3,6-anidrogalactopyranose-(1) dyad (G-A) are shown. R: H or
CH3;
AgNP-Porphyran synthesis
The synthesis of AgNP-Porphyran nanohybrid (AgNP-Por) were compared in systems using
isolated porphyran as nanoreactor and stabilizer (0.5 mg/ml), AgNO3 (0.10 mg/mL) as ion silver donor,
in the presence or absence of NaBH4 (0.10mg/mL ). After 60 minutes of reaction aliquots of the diluted
samples were analyzed by UV-vis spectroscopy where a band of approximately 400 nm attributed to
the surface plasmon absorption of silver nanoparticles (Figure 3) was observed. Narrow bands of
intense and homogeneous peak was obtained in the presence of sodium borohydride, indicating that a
large amount of spherical AgNPS was formed, with homogeneous size. When the reaction occurred in
the absence of sodium borohydride the formation of AgNPS was evidenced although in lower
concentration. Besides, a higher formation of NPs occurred in pH10 (Figure 3).
Figure 3: UV-visible spectra of porphyran reduced AgNPs.
The UV-visible spectra of the formation of AgNPS a function of time and pH was measured
(Figure 4). An increased intensity of the 400 nm band was observed over time attributed to the greater
number of formed AgNPS (A, B). However, the broad band formed in this region indicates that the
formed particles were polydisperse. The formation of AgNPS was also confirmed by the formation of
brown color solutions and was more evident when the reaction occurred at pH10. After 60 minutes of
reaction, there was a still increase in the plasmon band, indicating that the reaction was not completed.
The presence of reducing sugars and sulfate groups should contribute to the formation of nanoparticles
by biogenic route.
The synthesis of gold nanoparticles using polysaccharides such as chitosan (BHUMKAR et al.,
2007) gellam gum (DHAR et al., 2008) and more recently porphyran (VENKATPURWAR, SHIRAS
AND POKHARKAR, 2013) have been described. In the latter system, the reducing potential of
HAuCl4 by porfirana was assigned to its sulfate group (VENKATPURWAR, SHIRAS AND
POKHARKAR 2013). To assess the contribution of porfirana’s sulfate group in the silver reduction,
the formation of nanoparticles was evaluated by using a non-sulfated polysaccharide, agarose, as a
reductant. There was no evident formation of plasmon band at 400 nm, or brown color development in
the reaction system, indicating that there was no AgNPS formation (Figure 4, C). The result indicates
that the sulfate group participates in the reduction of the silver carbohydrate process.
Figure 4: UV-visible spectra of the reduced AgNPs as function of the reaction time by Porphyran
at pH 10 (A), pH 7(B), and by Agarose at pH 7 (C).
Size distribution of synthetized AgNP-Porphyran nanohybrid
The DLS and zeta potential analysis of isolated porphyran (Figure 5, A) showed that the
polysaccharide was polydisperse as expected. The nanohybrids AgNP-Porphyran formed in the
presence of reducing agent NaBH4 pH 7 (Figure 5, B), there was a formation of samples ranging from
12-30 nm (7.6 %) and 52.85-229.3 nm (55.8 %) and with an increased the zeta potential, indicating an
increase in the instability of the dispersion. The AgNPS formed in the absence of NaBH4 at pH 7 (C),
showed to be homogeneous, with sizes ranging from 29.41 and 307.6 nm with an average radius of
165.16 nm. There was also an increase in the zeta potential, indicating the stability of the solution. The
histogram of the nanohybrid AgNP-Porphyran in the absence of NaBH4 and pH10 showed that there
was formation of AgNPS with no homogeneous size, generating three populations with radius of 6.04
nm in diameter (13.9 % ) , 41.34 nm (27.6 % ) and 241.2 nm ( 58.6 %). The high and negative value of
these systems (above -30.0 mV) indicates that theAgNPS were duly capped by polyanionic porphyran.
The charged particles still have a low probability of aggregating due to its electrostatic repulsion,
remaining stable in solution.
Figure 5: Histogram of Particle Size Distribution of Porphyran (A) and Nanohybrid AgNP-Porphyran
in the presence (B) or absence of NaBH4 at pH 7 (C) or pH 10 (D),
Stability evaluation of Silver Nanoparticles
The stability of AgNPS formed using porphyran as nanoreactor and stabilizing agent was
evaluated 30 days after its synthesis, by monitoring the plasmon band by UV-Vis spectrophotometry.
Figure 6 shows that after 30 days there intensity of plasmon band at approximately 400nm for AgNPS
increased, indicating that not only occurred the stabilization of silver nanoparticles, but also more
nanoparticles were formed during the storage period. A change in the plasmon band would indicate an
increase in the average particle size or aggregation, or both factors (SATO et al., 2003). The results
obtained when AgNPS were synthesized using porphyran as nanoreactor differs from others reported in
the literature, which, over time, tend to occur aggregation and precipitation of the nanoparticles, as
evidenced for gold nanoparticles (VENKATPUWAR et al. 2013)
Figure 6: UV-Visible spectra of AgNP-Porphyran nanohybrid stability study.
4)
CONCLUSION
The green synthesis of silver nanoparticles using water soluble polysaccharide extracted from
NORI algae as reductor and stabilizing agent was reported. All the process, including polysaccharide
extraction and silver nanoparticles synthesis involved a low-cost, eco-friendly and non-toxic method.
The isolated polysaccharide structure was confirmed by NMR and is a porphyran. Its sulfate group and
an alkaline conditions are important to promote silver nitrate reduction, as confirmed by UV-vis
spectra. The silver nanoparticles remains stable in the solution even thirty days after its synthesis. The
nanohybrid possess the biocompatibility, bioactivity and electroactivity of the sulfated polysaccharide
and the antimicrobial and electrochemical properties of the silver nanoparticles and it could be applied
in biomedical and biosensor systems.
Acknowledgment:
To Fundação Araucária, CNPq.for financial support.
5)
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