Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.)
_______________________________________________________________________________________________
Natural polymers used as matrices for metallic nanoparticle synthesis by
ionizing irradiation
M. Brașoveanu and M. R. Nemțanu
National Institute for Lasers, Plasma and Radiation Physics, Electron Accelerator Laboratory, 409 Atomiștilor St., P.O.
Box MG-36, 077125 Bucharest-Măgurele, Romania
Polymers, natural or synthetic, are used as dispersion media as well as stabilizers for metallic nanoparticles to avoid their
aggregation, resulting in clusters. The chapter intends to be an overview of the published literature concerning the natural
polymers that are used as stabilizing matrices for the synthesis of noble metallic nanoparticles by irradiation techniques.
Therefore, aspects regarding types of polymeric stabilizers, reaction mechanism, characterization methods, and
applications are approached.
Keywords: polymeric matrix; stabilizing agent; nanoparticle stabilization
1. General remarks
Metal nanoparticles (NPs) have received increasing attention during the last decades because of their original and size
dependent properties, exhibiting physical and chemical properties of their respective metal [1].
Metal nanoparticles can generally be synthesized and stabilized by chemical (chemical reduction, photochemical
reduction, sonochemical reduction, electrochemical techniques, pyrolysis), physical (arc-discharge method, physical
vapour condensation, irradiation) and biological (use of bacteria, fungi, yeast, plants) methods [2-4]. Radiation-based
method is a green synthesis tool for synthesis of metallic nanoparticles, having advantages over conventional methods
that involve chemical agents associated with environmental toxicity [4]. Radiation-induced synthesis of metallic
colloids involves a simple, eco-friendly and fast process that has harmless feature and provides nanoparticles in fully
reduced, highly pure and highly stable state without disturbing impurities [5,6]. Radiolytic methods can use both
ionizing (gamma rays, X-ray, electron beam) and non-ionizing (ultraviolet light, microwave) radiation. Further, the
approach will be made in the context of the synthesis procedures using ionizing radiation.
Stability is one of the great challenges of nanoparticles. Stabilization of nanoparticle dispersion is difficult due to the
high surface area to volume ratio and the high surface energy of the nanoparticles [7]. In order to overcome this
limitation, different polymers have been receiving attention considering that a polymer is a good choice as a matrix
material since it enables an easy processing of the required product into technologically useful forms. Furthermore,
depending on the preparation procedures, polymer matrices can be used to control the size and shape of the NPs [8].
2. Types of polymeric stabilizers
Polymers are used as dispersion media as well as stabilizers for metallic nanoparticles to avoid their aggregation,
resulting in clusters. Scheme 1 shows concisely types of polymers used as dispersion media and stabilizing agents for
metal nanoparticles.
Synthetic
(PVA, PVP, PMMA)
Proteins
Polymers
as matrices
for NPs
Peptides
Natural
DNA
Polysaccharides
(chitosan, starch,
alginates)
348
Scheme 1
Types of polymers used as
matrices for NPs.
Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.)
_______________________________________________________________________________________________
2.1 Synthetic polymers
Several synthetic polymers such as poly(vinyl alcohol) (PVA) [9-12], poly(vinyl pyrrolidone) (PVP)[5,13], poly(methyl
methacrylate) (PMMA)[14], and poly(ethylene glycol) (PEG) [15] provided good results by using them as matrices for
metal nanoparticle synthesis in order to control the size by protecting from agglomeration among NPs.
Synthetic polymers are associated with the environmental toxicity, biological hazards and lack of biodegradability.
Therefore, the searching for new substitutes to mitigate these drawbacks is a continuous concern of researchers.
2.2 Natural polymers
On the other hand, studies that employed natural polymers as stabilizers or both as capping/reducing agents and
stabilizers were performed. Organic macromolecules such as proteins, peptides, deoxyribonucleic acid (DNA), and
carbohydrates have been of great interest due to their applications in biomedicine [16]. Natural polysaccharides used as
anti-aggregation and stabilizing agents in metal nanoparticle synthesis by irradiation will be further discussed.
Lately, the use of polysaccharides is of interest due to the increasing progress particularly in biomedical applications
because polysaccharides are biological polymers generated by natural sources [17], considered to be safe and
biocompatible for application in biomedical, cosmetic and pharmaceutical fields [18]. Moreover, the natural
polysaccharides are highly abundant, biodegradable and low cost. Generally, polysaccharides are produced and
extracted from various renewable biological sources such as vegetal sources (e.g. starch and cellulose), animal sources
(e.g. chitosan and gelatin) and microbial sources (e.g. dextran, glucan, and alginate).
Some examples of various polysaccharides that have been studied as a stabilizer for capping of NPs instead of
synthetic polymers are displayed in Table 1.
Table 1 Polysaccharides used as matrices for NPs.
Biopolymer
Chitosan
Oligochitosan
Hyaluronan
Starch
Sodium alginate
Gum Arabic (gum acacia)
NPs type
Gold
Silver
Silver
Gold
Silver
Silver
Gold
Silver
Silver
Type of ionizing radiation
Electron beam
Gamma radiation
Gamma radiation
Gamma radiation
Gamma radiation
Electron beam
Gamma radiation
Gamma radiation
Gamma radiation
Gamma radiation
References
[19]
[20-22]
[23]
[18]
[24]
[25]
[26]
[27]
[28]
The polymer architecture influences the size and size distribution, shape and stability of synthesized NPs by
irradiation method. Biopolymers have a bulky structure and many functional groups (carbonyl, alcohol, aldehyde,
amine and carboxylate) that have also reducing capability [7]. The particle size is related to the amount of the polymer
chains that increases at high irradiation dose, and the more polymer chains occur, the more they inhibit the aggregation
of NPs [27].
The most common polysaccharides used as reducing and stabilizing agents for NPs are succinctly presented further.
2.2.1 Chitosan
Chitosan is a natural polysaccharide polymer with abundant reactive amino and hydroxyl functional groups, being
soluble in aqueous acidic media and showing good biocompatibility and degradability [19].
The morphology (size and shape) of radiation synthesized NPs and their stability over time can be controlled by
using chitosan that acts as a reducing agent and a stabilizer, depending strongly on polymer concentration, metal salt
precursor concentration and irradiation conditions [19-21].
2.2.2 Starch
Starch is a macromolecular complex of at least two polymeric components: amylose, a linear (1,4) linked-α-D-glucan
and amylopectin, a highly branched molecule that consists of short chains of (1,4) linked-α-D-glucose with (1,6) -αlinked branches [30,31].
Starch as a natural polysaccharide, highly abundant, low cost and biodegradable can be successfully used as organic
dispersion media with stabilizing role for synthesis of NPs. The use of starch avoids the demand of relatively toxic
organic solvents, and the binding interactions between starch and NPs are weak and can be reversible at higher
temperatures, allowing separation of the synthesized particles [31,32]. Also, the quantity and dimensions of the NPs
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Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.)
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synthesized by ionizing irradiation using starch can be controlled by varying the irradiation parameters (irradiation dose
and dose rate), metal ions and starch concentrations [24,25].
2.2.3 Alginate
Alginates are generally referred to as a family of natural polysaccharides that can be also used as stabilizing agents for
synthesizing NPs. Alginates consist of two basic building blocks, α-L-guluronic acid (G) and β-D-mannuronic acid (M)
residues, linearly linked together by 1-4 linkages [33].
NPs prepared by ionizing irradiation using sodium alginate have a very narrow size distribution and can be stable
over several months at room temperature without aggregation in correlation with alginate and salt precursor
concentrations as well as irradiation parameters [26,27].
3. Mechanism
The metallic nanoparticles can be synthesized in situ in an aqueous solution in the presence of natural polymer used as a
stabilizer by ionizing irradiation. The formation of NPs occurs by several steps [5,22,23,25,27,34], which are further
described.
In aqueous solutions most part of the energy of the radiation is absorbed in water during radiolysis and generates the
primary products of radiolysis as follows:
H2O
Radiation
H2O eaq, H3O+ , HO•, H•, H2O2, H2, ….
(1)
Hydrated electrons (eaq) and hydrogen atoms (H•) are strong reducing agents, being able to reduce some metal (M)
ions to metal atoms. Formed metal atoms associate with other ions, leading to clusters.





eaq + M+ 


H• + M+ 


+ M+  M2+
(4)
eaq + M2+

....................
eaq + Mn+n
For free clusters such as nanocolloids in solution, the coalescence is limited by polymer matrix (P) that has the role
of cluster stabilizer.
(P) + M+  (P)M+
(7)
eaq + (P)M+ P

P+ M+  PM2+
(9)
eaq + PM2+P
....................

(P) + Mn+  (P)Mn+
(11)
eaq + PMn+Pn
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On the other hand, the hydroxyl radicals (HO•) produced in aqueous medium by irradiation induce the crosslinking
of polymer molecules.

HO• + (P) P•H2O
2(P)• P-(P) cross-linked polymer
Moreover, the hydroxyl radicals are able to oxidize the ions or atoms into a higher oxidation state. To avoid the metal
oxidation by oxidant radicals as a result of radiolysis, a radical scavenger must be added to the reaction mixture before
irradiation. The most common scavenger is 2-propanol used to convert efficiently HO• and H• radicals to 2-propanol
radical as follows:
HO• (CH3)2CH OH)  (CH3)2C•OH + H2O
(15)
H• (CH3)2CH OH)  (CH3)2C•OH + H2
(16)
At the same time, the isopropyl radical acts as a reducing agent, being able of reducing metal ions.
(CH3)2 C•OH + Mn+ (CH3)2CO n H
4. Characterization methods
Characterization of metallic nanoparticles is the key for understanding and controlling both nanoparticle application and
synthesis. Concerning NPs, parameters such as particle size, shape, crystallinity, fractal dimensions, pore size and
surface area are of interest for different applications. In this direction, various different techniques are suitable to
identify and characterize NPs depending on the desired property to be investigated. The most used characterization
techniques for NPs synthesized by ionizing irradiation and stabilized by using polysaccharides are: imaging techniques
based on different types of microscopies such as transmission and scanning electron microscopy (TEM/SEM), atomic
force microscopy (AFM); scattering techniques such as dynamic light scattering (DLS), X-ray powder diffraction
(XRD); and spectroscopic techniques such as Fourier transform infrared spectroscopy (FTIR), and UV-Vis
spectroscopy.
A brief overview of methods for characterization of NPs in different natural polymer matrices is given in Table 2.
Table 2
Characterization methods of polysaccharide stabilized NPs.
Biopolymer
Chitosan
Oligochitosan
Hyaluronan
Starch
Sodium alginate
Gum Arabic (gum acacia)
NPs type
Gold
Silver
Silver
Gold
Silver
Gold
Silver
Silver
Method of characterization
DLS, UV-Vis spectroscopy
TEM, XRD, FTIR and UV-Vis spectroscopies
TEM, XRD, UV-Vis spectroscopy
TEM, XRD, UV-Vis spectroscopy
TEM, XRD, FTIR and UV-Vis spectroscopies
TEM, UV-Vis spectroscopy
TEM, XRD, UV-Vis spectroscopy
TEM, DLS, XRD, FTIR and UV-Vis spectroscopies
References
[19]
[21,22]
[23]
[18]
[24,25]
[26]
[27]
[28]
Microscopy techniques are suitable for the characterization of NPs, providing information about morphology, size
distribution and degree of aggregation [32,35].
Electron microscopy techniques such as TEM and SEM are based on the incidence of an electron beam on probe,
being a well-known option in visualization and characterization of NPs. This kind of techniques provides the direct
observation of the morphology (size and shape) and the aggregation state of NPs. However, their major disadvantage is
the requirement of numerous visualizations of particles to get a representative result [36].
TEM images provide the size and the shape as well as the aggregation state of the particles. For high-resolution
TEM, even the layers of atoms of crystalline samples can be clearly demonstrated. However, TEM investigation
requires preparation of proper sample in order to be dry and at most hundreds of nm thick [36].
SEM produces images of the surface or near surface of a sample by scanning it a focused electron beam and
detecting various signals that are produced when the electron beam interacts with electrons in the sample [35]. Coating
the sample with a layer of conductive material could be necessary to avoid the accumulation of static electric charge on
the sample during electron irradiation [35,36].
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AFM is an ideal characterization tool for determining particle size distributions having the advantage over traditional
microscopies that it measures three-dimensional images so that the particle height and volume can be calculated
[32,37].
Scattering techniques such as DLS and XRD are used to determine the size distribution profile and NPs aggregation
in suspension [35]. DLS is a rapid and simple method that is widely applied for the determination of average size, size
distribution and polydispersity of NPs in colloidal suspensions [35]. DLS technique should be used in combination with
another technique such as SEM or TEM [38] because of aggregates or dust that can lead to underestimation or
overestimation of results, involving a limitation in the interpretation, (especially for polydisperse systems) [39].
Moreover, the XRD technique allows structural phase identification of the NPs crystallinity, giving also the average
particle size [23,35].
Elemental composition, structural information, and concentration are typically provided by spectroscopic techniques
[35]. FTIR spectroscopy is a valuable and ease of handling technique that confirm NPs formation and gives information
about the local molecular environment of the organic molecule on the NP surface. UV-Vis spectroscopy is a fast and
cheap method used to confirm NPs formation by showing the plasmon resonance band typical for the investigated NPs.
Information on the average particle size can be obtained from the absorption maximum of the measured UV-Vis
spectrum of the sample, whereas its full width at half maximum (FWHM) can be used to estimate particle dispersion
[40]. On the other hand, the intensity of the absorption spectrum is proportional to the number of nanoparticles formed
[41].
5. Applications
Generally, metallic nanoparticles have received considerable attention due to their versatile applications in the fields of
medicine, textiles, agriculture, electronics, etc. as a result of their unique properties. Gold and silver have been used
mostly for synthesis of stable dispersion of nanoparticle that are useful in areas such as pharmaceutics, cosmetics,
agriculture, wastewater treatment, etc.. Polysaccharide stabilized NPs are of interest, especially for applications in
biomedical, cosmetic, pharmaceutical, environment fields.
NPs synthesized in a polysaccharide matrix by using ionizing radiation, with controllable size and high purity, can
potentially be applied mainly in biological applications [18,20,26-28]. However, thoroughgoing studies on specific
application are required further.
Acknowledgements This work was supported by a grant of the Romanian National Authority for Scientific Research, CNDI–
UEFISCDI, project number 64/2012.
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