Optical and structural properties of radiolytically in situ synthesizedsilver nanoparticles stabilized by chitosan/poly(vinyl alcohol) blends

Jelena Krstić, Jelena Spasojević, Aleksandra Radosavljević n, Milorad Šiljegovć,Zorica Kačarević-PopovićVinča Institute of Nuclear Sciences, University of Belgrade, 11001 Belgrade, Serbia

H I G H L I G H T S

� Ag NPs were synthesized by γ-irradiation and stabilized by CS/PVA blends.� Composition of CS/PVA blends has influence on the size of spherical Ag NPs.� simulation based on Mie theory was used to calculate the parameters of Ag NPs.� Ag NPs are stabilized through interactions with -OH and -NH2 groups of polymers.� Optical band gap energy was calculated from UV–vis spectra by Tauc's expression.

a r t i c l e i n f o

Article history:Received 20 September 2012Accepted 26 September 2013Available online 5 October 2013

Keywords:Gamma irradiationAg-nanoparticlesAg–CS/PVA nanocompositesOptical propertiesMie theoryOptical band gap energy

a b s t r a c t

In this study, the potential of chitosan/poly(vinyl alcohol) (CS/PVA) blends as capping agent forstabilization of Ag-nanoparticles (Ag NPs) during their in situ gamma irradiation induced synthesiswas investigated. The UV–vis absorption spectra show the surface plasmon absorption band around410 nm, which confirms the formation of Ag-nanoparticles. It was found that the composition of CS/PVAblend affected the size of the obtained Ag-nanoparticles, as well as the parameters such as density, molarconcentration and effective surface area, calculated from the experimentally obtained UV–vis absorptionspectra and spectra obtained by simulation according to the Mie theory. SEM micrograph andXRD measurement indicated a spherical morphology and face centered cubic crystal structure ofAg-nanoparticles, with diameter around 12 nm. The values of optical band gap energy between valenceand conduction bands (Eg), calculated from the UV–vis absorption spectra, also show dependence on theblend composition for Ag–CS/PVA colloids as well as for Ag–CS/PVA nanocomposites.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

One of the major challenges in designing of polymer nano-composites is the ability to control the size and the morphologyof nanoparticles, as well as to achieve their homogeneousdistribution through the polymer. Methods for the preparationof polymer nanocomposites are mostly based on in situ poly-merization in the presence of nanoparticles (Džunuzović et al.,2009a, 2009b) or on incorporation of previously synthesizednanoparticles in the polymer (Pandey et al., 2011; Vodnik et al.,2012). However, by these methods, sometimes is too difficult toachieve homogeneous distribution of nanoparticles and preventtheir agglomeration in the polymer. Therefore, the method ofin situ synthesis of nanoparticles within polymer matrix was

investigated in many studies (Agnihotri et al., 2012; Chahal et al.,2011; Krklješ et al., 2007a; Luo et al., 2009; Mohan et al., 2007;Radosavljević et al., 2012).

The gamma irradiation induced reduction of metal ions inpolymer solutions i.e., radiolytic in situ synthesis of metal nano-particles in the presence of polymers such as chitosan (CS) andpoly(vinyl alcohol) (PVA) or other bio- and synthetic polymers, isparticularly suitable for preparation of nanocomposites based onAg-nanoparticles (Ag NPs) incorporated in polymer matrix(Gerasimov, 2011; Huang et al., 2009; Kumar et al., 2005;Naghavi et al., 2010; Phu et al., 2010, Rao et al., 2010; Temgireand Joshi, 2004; Zhou et al., 2012). The advantage of in situradiolytic method, over the other methods, is possibility to obtaina homogeneous distribution of synthesized Ag-nanoparticleswithin the polymer matrix as well as to control their size bychanging the experimental conditions. The obtained material isclean and sterilized at the same time, which is very important inthe case of their potential biomedical applications. The presence of

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0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.radphyschem.2013.09.013

n Corresponding author. Tel.: þ381 11 8066428; fax: þ381 11 3408607.E-mail address: krkljes@vinca.rs (A. Radosavljević).

Radiation Physics and Chemistry 96 (2014) 158–166

polymer molecules during the radiolytic in situ synthesis ofAg-nanoparticles significantly suppresses the process of theiragglomeration and further growth, by interaction of polymer'sfunctional groups with a high affinity for the metal with atoms onthe surface of metal clusters. In the previous research, the radioly-tically synthesized Ag-nanoparticles were successfully stabilized byusing poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), poly(N-isopropylacrylamide) (PNiPAAm) and poly(bis-co-2-hydroxyethylmethacrylate-co-itaconic acid) poly(bis-co-HEMA-co-IA) (Cvetićaninet al., 2010; Jovanović et al., 2011, 2012; Kačarević-Popović et al.,2007, 2010; Krklješ et al., 2007a, 2007b).

On the other hand, polymer blending is simple and convenientprocedure to obtain new material with required properties espe-cially in the bioapplication when blend components are syntheticand natural polymers. In particular, blending the CS with PVAimproves tensile strength, flexibility, bulk and surface hydrophili-city of the blended films. Moreover, water uptake in CS/PVA blendfilms as well as the selective protein immobilization can becontrolled by variation of their contents and the pH of solution.Therefore, combination of CS with PVA as blend stabilizer of Ag-nanoparticles creates materials which will be useful in the range ofapplications from electronics to chemo/bio-sensing platforms,tissue engineering systems or antibacterial materials (Bahramiet al., 2003).

In this study, the chitosan/poly(vinyl alcohol) (CS/PVA) blendswere used as capping agent for stabilization of Ag-nanoparticles,during their in situ gamma irradiation induced synthesis. Chitosanis a linear polysaccharide primarily composed of β-(1,4)-linked2-deoxy-2-amino-D-glucopyranose units and partially of b-(1,4)-linked 2-deoxy-2-acetamido-D-glucopyranose units. It is preparedby the N-deacetylation of chitin, the most abundant naturalpolymer after cellulose. Chitin is not easily soluble in any solvent.Nevertheless, unlike chitin, chitosan is dissolved in aqueoussolutions of some organic and inorganic acids and becomescationic polymer because of protonation of amino groups on theC-2 position of the pyranose ring. Chitosan consists of a largenumber of functional amino groups and hydroxyl groups. Sincechitosan is non-toxic and biocompatible with the human physio-logical system, it has been investigated as biomaterial in the fieldssuch as biomedicine, pharmacology and biotechnology. Chitin andchitosan have already been used in agricultural, food, industrialand medical fields (Tuhin et al., 2012). In addition, the antibacterialproperties of chitosan are suitable for use as wound dressings, andfurther improvement of wound healing is achieved by incorpora-tion of Ag-nanoparticles, which also exhibit antibacterial proper-ties. Chitosan also can be applied in the process of reducingradiation damage to the radiation workers or radiation curedpatients as well as in other areas of oncology (Chmielewski,2010). Poly(vinyl alcohol) is a vinyl type polymer, produced byfree radical polymerization of vinyl acetate monomers. The degreeof hydrolysis, i.e. the content of acetate groups in the polymer, hasa comprehensive impact on its physico-chemical properties. Poly(vinyl alcohol) is non-toxic, water-soluble, biocompatible andbiodegradable polymer that is widely used in biochemical andbiomedical applications (Bahrami et al., 2003; Finch, 1973; Yanget al., 2010).

On the other hand, many nanomaterials such as titaniumdioxide, zinc oxide, magnesium oxide or copper exhibit significantantibacterial properties, but nanocrystalline silver has proved asmost effective antimicrobial agent. Ag-nanoparticles exhibitsstrong antimicrobial activity and a wide biocide inhibitory spec-trum against microbes, both bacteria and viruses, and eveneukaryotic microorganisms, in vitro and in vivo (Kemp et al.,2009; Pattabi et al., 2010; Rujitanaroj et al., 2008; Secinti et al.,2008). In addition, Ag-nanoparticles have high optical absorptionefficiency, which is very suitable for biomedical diagnostics,

biosensors and heat absorption in special devices (Jovanovićet al., 2012).

Ag-nanoparticles in CS/PVA blends as a stabilizer can beobtained by reduction of Agþ ions by using sodium borohydrideor by using the functional groups of polymers themselves(�COOH, �NH2, �OH) (Agnihotri et al., 2012; Vimala et al.,2011). In this study, Ag-nanoparticles were in situ synthesized bygamma irradiation, using CS/PVA blends as a capping agent.Reduction of Agþ ions was performed by radiolytically formedreduction species. The influence of composition of CS/PVA blendson optical and structural properties of obtained Ag–CS/PVA nano-composite systems was investigated using UV–vis spectroscopy,scanning electron microscopy (SEM) and X-ray diffraction (XRD).Finally, the optical properties of synthesized Ag–CS/PVA nanosys-tems were analyzed by the optical band gap energy (Eg), calculatedfrom the experimentally obtained UV–vis absorption spectra. Ingeneral, the band gap of a material is defined as the energydistance between the valence and conduction bands (Gasaymehet al., 2010), and the most of a material's behaviors, such asintrinsic conductivity, optical transitions, or electronic transitions,depend on it. These properties of materials are important para-meters in applied science.

2. Experimental

2.1. Materials

Poly(vinyl alcohol) (PVA) with molecular weight of 72 kDa and99% of minimal degree of hydrolysis, silver nitrate (AgNO3) and 2-propanol ((CH3)2CHOH) were products of Merck. A mediummolecular weight chitosan (CS) with 200–800 cP viscosity (1%solution in 1% acetic acid) and 75–85% degree of deacetylation wasobtained from Sigma-Aldrich, while acetic acid (CH3COOH) wasproduct of Zorka Pharma. All chemicals were commercial productsof analytical grade and were used without additional purification.Water from Millipore Milli-Q system was used in all experiments,while the high purity argon gas (99.5%) fromMesser Tehnogas wasused for removing the oxygen from solutions.

2.2. Synthesis of Ag–CS/PVA nanocomposites

The solution of CS (2.5% (w/w)) was prepared by dissolving theCS in aqueous solution of CH3COOH (5% (v/v)), at room tempera-ture under the constant stirring for 3 h, while the aqueous solutionof PVA (5% (w/w)) was prepared by dissolving PVA at 90 1C underthe constant stirring for 6 h. Obtained solutions of CS and PVAwere used to prepare solutions in which the mass ratios of CS/PVAwere 100/0, 80/20, 60/40, 40/60, 20/80 and 0/100, while the totalweight of polymers in the systems was constant. Then, in allprepared solutions, AgNO3 and (CH3)2CHOH were added up to theconcentration of 5 mM and 0.2 M, respectively. In order to removeoxygen, solutions were bubbled with Ar for 30 min, and thenexposed to gamma irradiation (60Co radiation facility), at roomtemperature to the absorbed dose of 8.5 kGy for reduction of 5 mMAgþ ions, at a dose rate of 6 kGy/h. Ag–CS/PVA nanocomposites (10–30 mm thick films) were obtained after solvent evaporation fromsynthesized Ag–CS/PVA colloids, at room temperature.

2.3. Methods of characterization

The optical properties of synthesized Ag–CS/PVA systems,colloids and nanocomposites, were investigated by UV–vis absorp-tion spectroscopy. Absorption spectra were recorded usingThermo Fisher Scientific Evolution 600 UV–vis spectrophotometer,in the wavelength range 300–800 nm. Theoretical calculations of

J. Krstić et al. / Radiation Physics and Chemistry 96 (2014) 158–166 159

size of Ag-nanoparticles were made using software “MiePlot V.3.4”.Scanning electron microscopy (SEM) analysis of Ag–CS/PVA nanocom-posites was performed on JEOL JSM-6610LV instrument, operated atan accelerating voltage of 20 kV. Prior to the analysis of surfacemorphology, the samples were coated with thin layer of gold (around15 nm). Microstructural properties of Ag–CS/PVA nanocompositeswere investigated by X-ray diffraction (XRD) measurements per-formed on Bruker D8 Advance Diffractometer (Cu Kα1 radiation,λ¼0.1541 nm).

3. Results and discussion

The ions of noble metals, as well as of many electronegativemetals, can be reduced by exposing their aqueous solutions togamma irradiation. Due to the much higher concentration of thesolvent compared to the solute, the absorption of radiation energymainly occurs in the solvent, and thus induces the radiolysis ofwater and formation of primary species (Draganić and Draganić,1971)

H2O-γe�aq; OH

d; Hd; H3Oþ ; H2; H2O2 ð1Þ

The solvated electrons (e�aq) and hydrogen atoms (Hd) are strong

reducing agents while the hydroxyl radicals (OHd) are able tooxidize the ions or the atoms into a higher oxidation state and thusto counterbalance the reduction reactions. For this reason, an OHd

radical scavenger is added in the solution. In the present study,2-propanol was used as a scavenger to convert OHd radicals to2-propanol radicals ((CH3)2CdOH) (Belloni, 2006)

ðCH3Þ2CHOHþOHdðCH3Þ2CdOHþH2O ð2Þ

In general, when chitosan is irradiated in solid state it undergoesdegradation mainly through the glycosidic bond breaking accom-panied by the formation of carbonyl groups, the elimination ofamine group and the release hydrogen and ammonia. On the otherhand, in the case of diluted aqueous solution of CS and PVA, majorpart of the radiation energy is absorbed by the solvent andproducts of water radiolysis are formed (Eq. (1)). As alreadymentioned, the OHd radicals react with 2-propanol, but they arealso responsible for abstraction of hydrogen atoms from thepolymers and formation of CSd and PVAd macroradicals (Eqs. (3–4), respectively) (Gachard et al., 1998; Huang et al., 2009; Kumaret al., 2005; Long et al., 2007)

C6H11O4N½ �nþOHd-½C6H11O4N�n�1½C6H10O4N�dþH2O ð3Þ

PVA Hð ÞþOHd-PVAdþH2O ð4Þ

Rate of reaction of OHd radicals with CS and PVA have similarbimolecular rate constant of the order of 109 dm3/mol s (Tahtatet al., 2011; Varshney, 2007). Subsequent reactions of macroradi-cals in the case of CS can be: chain scission, hydrogen transfer,inter- and intramolecular recombination and finally disproportio-nation of macroradicals. When macroradicals are localized atvarious carbon atoms (C1–C6) within the glucosamine unit, onlyradicals formed at C1 and C4 atoms can undergo rearrangementinvolving breakage of 1–4 glycosidic bonds and formation of C¼Odouble bond (Long et al., 2007; Wasikiewicz et al., 2005; Yoksanand Chirachanchai, 2009). However, in the case of experimentalconditions specified in the paper, radiation induced degradationof CS is unlikely to a considerable extent due to the presence of2-propanol in reaction solution as OHd radical scavenger.

The process of Ag-nanoparticles formation by the gammaradiolytic method can be divided into two main steps. First occursthe formation of atoms by nucleation processes, which is followedby the formation of nanoparticles by aggregation processes.

The nucleation process can be described by following reactions:

Agþ þe�aq-Ag0 ð5Þ

Agþ þ ðCH3Þ2CdOH - Ag0 þ ðCH3Þ2CO þ Hþ ð6Þ

Agþ þPVAd-Ag0þPVAþHþ ð7Þ

Agþ þ C6H11O4N½ �n- C6H11O4N½ �nAgþ ð8Þ

C6H11O4N½ �nAgþ þe�aq- C6H11O4N½ �nAg0 ð9Þ

The solvated electrons (e�aq), 2-propanol ((CH3)2CdOH) and PVAd

radicals are a strong reducing species and can reduced Ag ions(Agþ) into zero-valent Ag atoms (Ag0) (Eqs. (5–7)) (Gachard et al.,1998; Krklješ et al., 2007b; Saion et al., 2013). Moreover, due to thepresence of high number of hydroxyl groups in CS, Agþ ions formscomplexes with CS giving the C6H11O4N½ �nAgþ ions (Eq. (8)).These ions are also reduced by e�

aq, resulting in formation ofC6H11O4N½ �nAg0 species (Eq. (9)) (Huang et al., 2009).

Because the reducing species are as randomly distributed as theAgþ ions in the solution, the Ag0 atoms are formed with a homo-geneous distribution throughout the solution. The binding energybetween two metal atoms is stronger than the atom-solvent or atom-ligand bond energy. Therefore, the produced Ag0 atoms dimerizewhen they encounter (Eqs. (10 and 11)) and/or associate with anexcess of ions (Eqs. (12–15)), and by cascade of coalescence processesprogressively grow, yielding the formation of metal clusters withhigher nuclearities (Belloni, 2006; Huang et al., 2009)

Ag0þAg0-Ag2 ð10Þ

C6H11O4N½ �nAg0þAg0- C6H11O4N½ �nAg02 ð11Þ

Ag0þAgþ-Agþ2 ð12Þ

C6H11O4N½ �nAg0þAgþ- C6H11O4N½ �nAgþ2 ð13Þ

Ag0nþAgþ-Agþnþ1 ð14Þ

C6H11O4N½ �nAgþ2 þAg0- C6H11O4N½ �nAgþ

m ð15ÞThe fast collision reactions of ions with atoms or clusters play veryimportant role in the mechanism of clusters growth. Reductionprocesses of free and adsorbed Agþ ions are competitive and theyare controlled by formation rate of reduction radicals. Accordingly tothat, formation of clusters by direct reduction, accompanying withcollision, is dominant at higher dose rate (applied in this work), whenthe nanoparticles with smaller dimension were obtained (Belloni andMostafavi, 2001).

At any stage of the coalescence, the ions adsorbed on theclusters may be reduced by e�

aq, (CH3)2CdOH, PVAd and CSd radicals(Eqs. (16–20)) (Belloni, 2006; Gachard et al., 1998; Huang et al.,2009; Krklješ et al., 2007b; Long et al., 2007; Phu et al., 2010)

Agþm þe�

aq-Ag0m ð16Þ

Agþm þðCH3Þ2CdOH-Ag0mþðCH3Þ2COþHþ ð17Þ

Agþm þPVAd-Ag0mþPVAþHþ ð18Þ

C6H11O4N½ �nAgþm þe�

aq- C6H11O4N½ �nAg0m ð19Þ

C6H11O4N½ �nAgþm þ C6H11O4N½ �n�1 C6H10O4N½ �d- C6H11O4N½ �nAg0m

þ C6H11O4N½ �n�1 C6H9O4N½ �þHþ ð20ÞTo prevent clusters collision and their growth into bigger nano-particles, the polymer molecules with functional groups that have

J. Krstić et al. / Radiation Physics and Chemistry 96 (2014) 158–166160

a great affinity for metals were added. In the case of CS andPVA, the amino (�NH2) and hydroxyl (�OH) groups interactwith the atoms on the surface of metal nanoparticles and thusstabilize them, preventing their agglomeration and furthergrowth (Fig. 1a).

After the gamma irradiation, the yellow colored Ag–CS/PVAcolloids were obtained (Fig. 1b), which is characteristic of Ag-nanoparticles. These colloids are transparent and stable for a longperiod of time. By absorption spectroscopy it was confirmed thatthe optical properties of the synthesized Ag–CS/PVA colloids doesnot change for 9 months (results not shown), which clearlyindicates that there is no agglomeration of Ag-nanoparticles.Moreover, even after evaporation of solvent from the Ag–CS/PVAcolloids and formation of transparent Ag–CS/PVA nanocompositesthe agglomeration of Ag-nanoparticles was not observed, and thenanocomposites retains the same yellow color as starting colloids(Fig. 1c).

The formation of Ag-nanoparticles in CS/PVA solutions wasconfirmed by UV–vis spectral studies. Fig. 2 depicts the absorptionspectra of synthesized Ag–CS/PVA colloids, which clearly indicatesthe presence of surface plasmon resonance (SPR) bands, with thecharacteristic maximum of absorption at about 410 nm. Suchintensive absorption occurs as consequences of collective oscilla-tions of conductive electrons, caused by interaction of metalnanoparticles with electromagnetic radiation. Noble metal nano-particles exhibit characteristic optical properties in the visibleregion (Kelly et al., 2003; Mulvaney, 1996). The precise position,intensity and width of the SPR band depends on a number of

compositional attributes, including nanoparticle size, shape andsurface structure and medium dielectric constant, refractive indexand temperature, within three discrete size domains, namelyquantum (o2 nm), intrinsic (2–20 nm) and extrinsic (420 nm)regimes. For nanoparticles in the extrinsic regime, informationconcerning the precise nanostructure can be inferred from therelative position of the SPR band, with larger nanoparticlesresulting in a red shift to lower energies (shift to higher λmax inabsorption spectra), while for particles smaller than 20 nm thesituation becomes more complicated (Rance et al., 2008). In thiscase, the average radius of the nanoparticles can be determined bythe width of their corresponding SPR band. Their symmetry can beestimated by fitting the experimental absorption spectrum to asimulated one calculated using a theoretical model consideringdifferent shapes for the nanoparticles. The simplest exact theore-tical model is given by Gustav Mie, and it describes the effect oflight on a spherical metallic nanoparticle embedded in a dielectricmedium. This model is important because it is considered as thefirst approximation to describe the interaction of one electromag-netic wave with small particles (Peña et al., 2007). The classicaltheory developed by Mie (Mie, 1908) predicts that the SPRwavelength should be independent of the particle size when themean particle diameter is smaller than the incident wavelength.For Ag-nanoparticles with radii (r) in the range 1–10 nm, the 1/rsize dependence of the FWHM (full width at half maximum) of theSPR band was observed, and the quasi-static approximation of theMie theory can be applied. In this size regime, the relationrexp ¼ vf =Δω 1=2 can be used, where rexp is the particle radius, vfis the Fermi velocity of the metal, and Δω1/2 is the FWHM for theSPR band in units of angular frequency (Veenas et al., 2009). Thecalculated mean particle radii of Ag-nanoparticles by this methodare given in Table 1.

Fig. 1. Mechanism of stabilization of Ag-nanoparticles (a) and photograps of Ag–CS/PVA: colloid (b) and nanocomposite (c).

Fig. 2. UV–vis absorption spectra of Ag–CS/PVA colloids.

Table 1The obtained values for the parameters of Ag-nanoparticles stabilized by CS/PVAblends: the radius (rexp), the average number of atoms (Nav), the density in colloids(D), the molar concentration in colloids (C) and the theoretical effective surfacearea (S.A.).

CS/PVA FWHM (nm) rexp (nm) Nav D (NPs/cm3) C (mol/dm3) S.A.(m2/g)

100/0 91.3 6.74 74961 4.02�1012 6.67�10�9 42.3980/20 91.2 6.73 74666 4.03�1012 6.69�10�9 42.4560/40 90.8 6.70 73858 4.08�1012 6.77�10�9 42.6440/60 83.8 6.18 58024 5.19�1012 8.61�10�9 46.2320/80 80.1 5.91 50694 5.94�1012 9.86�10�9 48.340/100 74.8 5.52 41203 7.31�1012 12.10�10�9 51.76

J. Krstić et al. / Radiation Physics and Chemistry 96 (2014) 158–166 161

Assuming that the silver bulk density is 5.86�1022 atoms/cm3

it is possible to determine the average number of atoms (Nav)belonging to spherical nanoparticles in metallic phase and thenanoparticles density (D) in the sample as a first approximation(Peña et al., 2007). The average number of atoms in Ag-nanoparticles was calculated using equation Nav ¼ ρ Agð Þ Vsf , wereVsf is volume of the nanosphere. Dividing the total amount of Agatoms (Ntot, equivalent to the initial amount of silver salt added tothe reaction solution) by the average number of atoms in eachnanoparticles, a density for the Ag-nanoparticles in the colloid canbe estimated. In addition, the molar concentration of the Ag-nanoparticles in colloids were calculated by C ¼Ntot=ðNav V NAÞ,were V is the volume of the reaction solution and NA is theAvogadro's constant (Liu et al., 2007). The theoretical effectivesurface area (S.A.) of Ag-nanoparticles was also calculated byS:A:¼ 6=Dsf ρ, were Dsf is the diameter of Ag-nanoparticles and ρis the theoretical density of silver (10.5 g/cm3) (Wani et al., 2011).The obtained values for the parameters of Ag-nanoparticles: theaverage number of atoms (Nav), the density in colloids (D), themolar concentration in colloids (C) and the theoretical effectivesurface area (S.A.) are listed in Table 1.

Taking into account the calculated values for the Ag-nanoparticles radii (Table 1), the corresponding theoretical opticalextinction spectra are simulated using the software “MiePlot v.3.4”(http://www.philiplaven.com/mieplot.htm), which algorithm isbased on Mie's theory i.e. on scattering and absorption crosssections of isolated spherical nanoparticles in solution. Program“MiePlot v.3.4” calculates efficiencies (Qext – extinction, Qabs –

absorption, Qsca – scattering) as functions of wavelength. Basically,light absorption dominates in the extinction (extinc-tion¼absorptionþscattering) spectrum for particles relativelysmall radius (o20 nm), and light scattering becomes the domi-nant process for large particles (Xia and Halas, 2005). Fig. 3 showsthe optical extinction spectra of Ag–CS/PVA colloids which areexperimentally obtained (solid line) and obtained by simulationaccording to the Mie theory (dashed line). The values of averageAg-nanoparticles radii obtained by the fitting to experimentalspectra are also given in Fig. 3. In general, a good agreementbetween experimentally and theoretically obtained optical extinc-tion spectra was observed, with a better adjustment for largerAg-nanoparticles. For all samples, the experimental spectra pre-sent a shift from 7 nm to 15 nm in the position of SPRband towards longer wavelengths with respect to the simulation.

These red shifts have been observed previously (Peña et al., 2007),and they are probably result of the increasing density of nano-particles (D) with decreasing of particle size. The density ofAg-nanoparticles increase from 4.02�1012 NPs/cm3 in the sampleCh/PVA (100/0) (rexp¼6.74 nm) up to 7.31�1012 NPs/cm3 in thesample Ch/PVA (0/100) (rexp¼5.52 nm) (Table 1). Also, the shiftson the SPR band can be affected by different atomic parameters invery small particles compared to those used in the calculations,corresponding to the Ag bulk material. Furthermore, the calcula-tions based on the Mie theory depend on the radius of thenanosphere and on the refraction indexes of the matrix and themetal, and is the reason why these shifts also can be relatedto differences in the dielectric constants (Peña et al., 2007).As already mentioned, the absorption of light is dominant overthe scattering for very small nanoparticles, as confirmed bysimulation (Fig. 4). The simulation results also show that con-tribution of scattering in extinction spectra increases with increas-ing the size of Ag-nanoparticles from 6.51% for sample Ag–CS/PVA (0/100) (rsim¼6.75 nm) up to 11.94% for sample Ag–CS/PVA(80/20) (rsim¼8.46 nm).

According to the values of Ag-nanoparticles radii, calculatedfrom experimentally obtained UV–vis absorption spectra (Table 1)as well as from Mie's simulation (Fig. 3), it can be observed thatsmaller nanoparticles were produced when the PVA content in theblend is greater than 50%. Theoretical studies have shown that thecompatibility of CS and PVA in the blend is greater in the systemswith the PVA content less than 50% (Jawalkar et al., 2007; NaveenKumara et al., 2010). In the case of greater compatibility of CS andPVA, probably the better interactions of blend components cancause the weaker stabilization during synthesis i.e., the formationof larger Ag-nanoparticles as a consequence. However, the mostlikely reason for increase in particle size with the increase of CScontent might be the decrease in pH of reaction solution. Namely,acidic medium with higher Hþ concentration would be unfavor-able for the formation of small Ag-nanoparticles (Phu et al., 2009;Phu et al., 2010).

The UV–vis absorption spectra of synthesized Ag–CS/PVA(40/60) colloid, corresponding nanocomposite and colloid obtainedby dissolving the nanocomposite are presented in Fig. 5. For allinvestigated systems, the red shift in the position of SPR band ofnanocomposite (red (2) curve), in comparison with starting colloid(black (1) curve), was observed. This red shift can be mainlyexplained by the change of the dielectric constant of surrounding

Fig. 3. Extinction spectra of Ag–CS/PVA colloids: experimentally obtained (solid line) and obtained by simulation according to the Mie theory (dashed line).

J. Krstić et al. / Radiation Physics and Chemistry 96 (2014) 158–166162

medium in which particles are dispersed. Namely, in the case ofnanocomposite Ag-nanoparticles are dispersed in the polymer(refractive indices are nPVA¼1.53 and nCS¼1.55), whereas in thecase of colloid they are dispersed in water (nH2O ¼ 1:33). Theincreasing of refractive index of surrounding medium causes theshift of SPR band towards longer wavelengths. In addition, thelong wavelength shift of SPR band of nanocomposite compared tostarting colloid can be consequence of charge particle redistribu-tion at the particle-matrix interface (Stepanov, 2004). This can beadditional reason why experimental spectra present a shift ofposition of SPR band towards longer wavelengths with respect tothe Mie simulation, described in the previous section. To excludeany possibility that observed changes in position of SPR band areconsequence of agglomeration of Ag-nanoparticles upon forma-tion of nanocomposite, the Ag–CS/PVA nanocomposite was dis-solved in water and absorption spectrum (green (3) curve) wascompared with absorption spectrum of starting colloid (black (1)curve). Absorption spectrum of the dissolved nanocomposite hasSPR band at 409 nm, and it is almost identical to the absorption

spectrum of starting colloid (407 nm) indicating that observedoptical changes are not the consequence of agglomeration of Ag-nanoparticles. These small differences of SPR band position arecaused by the formation of a thin polymer layer-shell on

Fig. 4. Graphs obtained by Mie's model simulation for Ag-nanospheres in CS/PVA blends. The black (1) curve is the extinction, the blue (2) is the absorption, and the red (3)is the scattering. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. UV–vis absorption spectra of Ag–CS/PVA (40/60) colloid, correspondingnanocomposite and colloid obtained by dissolving of nanocomposite.

Fig. 6. SEM micrograph of Ag–CS/PVA (40/60) nanocomposite (a) and size distribu-tion of Ag-nanoparticles (b).

J. Krstić et al. / Radiation Physics and Chemistry 96 (2014) 158–166 163

nanoparticles surface (Krklješ et al., 2007a; Radosavljević et al.,2012). Additionally, in Fig. 5 is noticeable broadening of SPR bandfor nanocomposite (red (2) curve) and colloid obtained by dissol-ving the nanocomposite (green (3) curve) with respect to thestarting colloid (black (1) curve). In the case of nanocomposite, theinterparticle interactions became significant due to reduced dis-tance between Ag-nanoparticles, and the spectral broadeninggoverned by the plasmonic coupling effect (Dutta et al., 2013).On the other hand, broadening of SPR band of colloid obtained bydissolving the nanocomposite could be assigned to the incompletedissolution of polymer (especially CS) and remnant of polymerlayer on nanoparticle surface. Namely, according to the literature,the adsorbed molecules on the nanoparticle surface can cause adamping and broadening of the plasmon band (Henglein, 1993;Link and El-Sayed, 1999; Linnert et al., 1993).

Morphological properties of Ag–CS/PVA nanocomposites wereexamined by scanning electron microscopy (SEM). The obtainedmicrograph of Ag–CS/PVA (60/40) nanocomposite and particle sizedistribution of synthesized Ag-nanoparticles are shown in Fig. 6aand b, respectively. The average size of Ag-nanospheres was foundto be 12.7 nm, estimated from the histogram by subtracting thesize of the gold layer of 15 nm.

The X-ray diffraction pattern of Ag–CS/PVA (60/40) nanocom-posite is shown Fig. 7. Diffraction maxima at 2θ angle values of38.21, 44.21, 64.41 and 77.61 correspond to Bragg's reflections fromthe crystal planes (111), (200), (220) and (311), which are char-acteristics of the face centered cubic (FCC) crystal structure ofAg-nanoparticles (JCPDS File no. 04-0783). The absence of silveroxide peaks indicates that the prepared nanoparticles are puresilver. The Scherrer diffraction formula was used to calculate thecrystalline domain size, D¼ kλ=β cos θ where k is a constant forcubic structure (0.9), λ is the X-ray wavelength (0.1541 nm), β is thefull width at half maximum (FWHM), and θ is the diffraction angle.Generally, the most intense and well defined peak for FCC materialsis (111) reflection. In the case of investigated Ag–CS/PVA (40/60)nanocomposite, the diffraction peak is broad (FWHM¼0.721) whichindicating that the crystallite size is very small. The crystallinedomain size was found to be 11.7 nm, which is in a good agreementwith average diameter of Ag-nanoparticles estimated by SEM andUV–vis spectra. Additionally, the lattice constant (a) was calculated asa¼ dhkl

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffih2þk2þ l2

p(dhkl is space between the planes in the atomic

lattice, while h, k and l are Miller indices) and obtained value of0.4079 nm for spacing d111¼0.2355 nm is with accordance withliterature value of 0.4086 (Huang et al., 2009).

The results of optical (UV–vis), morphological (SEM) andstructural (XRD) analysis show that the obtained Ag-nano-particles are less than 20 nm in diameter, i.e., they are withinthe intrinsic regime (Rance et al., 2008).

The optical properties of synthesized Ag–CS/PVA nanosystemswere analyzed by the value of optical band gap energy (Eg)between valence and conduction bands, calculated from UV–visspectra. The shift in the fundamental absorption edge of UV–visspectra can be correlated with the Eg by Tauc's expressionα h νð Þ1=m ¼ B h ν�Eg

� �, where α is the absorption coefficient

corresponding to the fundamental absorption edge, hν is thephoton energy, B is the constant known as the disorder parameterwhich is nearly independent of the photon energy, and m is theparameter measuring type of transition. Parameter m may havedifferent values, such as 1/2, 2, 3/2 or 3 for allowed direct, allowedindirect, forbidden direct and forbidden indirect transitions,respectively (Chahal et al., 2011; Jabbar et al., 2010). In order toget the property of the band it is needed to test the transition to fitwith the experimental results. The optical band gap energy isdetermined by plotting (αhν)1/m as a function of photon energyhν. In this study, the values of Eg for allowed direct transitions(m¼1/2) were obtained from extrapolation of the straight lineFig. 7. XRD patern of Ag–CS/PVA (40/60) nanocomposite.

Fig. 8. Plots of (αhν)2 vs. hν for Ag–CS/PVA (40/60) colloid (a), corresponding nanocomposite (b) and colloid obtained by dissolving of nanocomposite (c).

J. Krstić et al. / Radiation Physics and Chemistry 96 (2014) 158–166164

portion of (αhν)2 vs. hν curves to x-axis in which (αhν)2¼0 (Fig. 8).This gave the optical band gap energies of Ag–CS/PVA in colloidand nanocomposite systems, as shown in Fig. 9. In addition, thevalues of Eg were determined for pure components: 5.08 eV for CSin solution, 5.39 eV for CS in solid state, 5.03 eV for PVA in solutionand 5.81 eV for PVA in solid state.

Fig. 9 clearly indicates that the values of optical band gapenergy of investigated Ag–CS/PVA nanosystems (colloids andnanocomposites) depend on the blend composition. In the caseof starting Ag–CS/PVA colloids, the value of Eg continuouslydecreases with addition of PVA as blend component (black curve).On the other hand, for Ag–CS/PVA nanocomposites the value of Egincrease up to 40% of PVA in the blend, and with further increaseof PVA content Eg decreases (red curve). The similar behavior, butwith the highest optical band gap energies, was observed for thecolloids obtained by dissolving of nanocomposites (green curve).According to previous investigation, after dissolution of thenanocomposite, nanoparticles remained coated with thin polymerlayer-shell (Radosavljević et al., 2012). When the metal nanopar-ticle is capped with a thin dielectric layer, it induces perturbationwhich can results in a change of optical band gap energy. Inaddition, surface effects on the crystal lattice of metal, such aslattice contraction, atomic relaxation, surface reconstruction, sur-face passivation, or strain induced by a host material, also canchange the values of optical band gap energy (Sattler, 2002).

4. Conclusion

In this work, Ag-nanoparticles were successfully obtained bygamma irradiation induced in situ synthesis within the CS/PVAblends, with different ratio of polymers, as a capping agent. It wasfound that the composition of CS/PVA blend affects the size of theobtained Ag-nanoparticles, as well as the parameters such asdensity, molar concentration and the effective surface area, calcu-lated from the experimentally obtained UV–vis absorption spectraand spectra obtained by simulation according to the Mie theory.Spherical morphology of Ag-nanoparticles was confirmed bySEM micrograph, while the microstructural analysis of synthesizedAg–CS/PVA nanocomposite, performed by XRD measurement,indicates that Ag-nanoparticles have face centered cubic structure,with the crystalline domains around 12 nm. The optical band gapenergy (Eg), calculated from the experimentally obtained UV–visabsorption spectra using Tauc's expression, show nonlinear depen-dence of blend composition, both for colloids as well as fornanocomposites. According to the fitting procedure it wasobserved that all Ag–CS/PVA nanosystems exhibit allowed direct

(m¼1/2) transitions. Optical properties of investigated Ag–CS/PVAnanosystems, as measured by the values of Eg, were the result ofthe simultaneous effect of several parameters: the interparticledistance, the concentration and the structure of surroundingmedium as well as the charge particle redistribution at theparticle-matrix interface.

Acknowledgments

The authors are grateful to Dr. Miodrag Mitrić for help inperforming the XRD measurements. This work is financed by theMinistry of Education and Science of the Republic of Serbia, projectIII 45005, and by International Atomic Energy Agency (IAEA),project CRP: F22051, contract No. 16733.

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