Radiation synthesis and characterization of chitosan stabilized gold nanoparticles and catalytic activity study

Journal of Science and Technology 55 (1B) (2017) 13–23 RADIATION SYNTHESIS AND CHARACTERIZATION OF CHITOSAN STABILIZED GOLD NANOPARTICLES AND CATALYTIC ACTIVITY STUDY Vo K. D. N.1, 2, *, Vu Q. A.3 1Institute of Applied Materials Science, Vietnam Academy of Science and Technology 01A TL29 Street, Thanh Loc Ward, District 12, Ho Chi Minh City, Vietnam 2Graduate University of Science and Technology, Vietnam Academy of Science and Technology 18 Hoang Quoc Viet Street, Nghia Do Ward, Cau Giay District, Ha Noi, Vietnam 3Department of Energy Materials, Faculty of Materials Technology, HCMUT–VNUHCM 268 Ly Thuong Kiet Street, Ward 14, District 10, Ho Chi Minh City, Vietnam *Email: vndkhoafr@gmail.com Received: 30 December 2016; Accepted for publication: 26 February 2017 ABSTRACT In this paper, gold nanoparticles (AuNPs) were synthesized in a single and efficient procedure by e–beam and γ–irradiation using chitosan as a stabilizing agent. The investigations on synthesis of AuNPs under ionizing radiation by studying the influence of initial conditions of the preparation of Au(III)–chitosan solutions prior to irradiation on the nucleation process and on the morphological characteristic of the formed nanoparticles. The results of UV–vis absorption spectroscopy, transmission electron microscopy indicated that spherical well– dispersed gold nanoparticles ranging from 5 to 10 nm were elaborated, depending on the irradiation dose, the dose rate and the [GLA]/[Au(III)] ratio (GLA: glucosamine units). Furthermore, we also reported the application of the synthesized gold nanoparticles as catalyst in the reduction of 4–nitrophenol (4–NP) to 4–aminophenol (4–AP) by excess sodium borohydride. Keywords: gold nanoparticles, chitosan, irradiation, reduction, 4–nitrophenol. 1. INTRODUCTION Discoveries in the past decade have shown that once materials are synthesized in the form of nanoparticles, they were influenced significantly by their physical and chemical properties. In several cases, new phenomena are established. Gold nanoparticles have received considerable attention to potential applications in the fields of physics, chemistry, biology, optics, electronics and materials science as well due to their unique physical, chemical, optical, electrical and catalytic properties. Up to now, a variety of methods or techniques have been reported and reviewed for the preparation of AuNPs. Thermolysis [1], microwave irradiation [2] as well as more conventional methods involving the reduction of gold salts by various reducing agents, such as sodium borohydride [3], sodium citrate [4] have been successfully developed. Radiation synthesis and characterization of chitosan stabilized gold nanoparticles and catalytic 14 The use of ionizing radiation for the synthesis of metal nanoparticles appears as a promising alternative since reactive species with high reduction potential are generated in situ, which is hard to achieve by other methods. The equation for radiolysis of water is H2O •H, eaq–, •OH, H2, H2O2, H3O+ [5–7]. Water radiolysis generates hydrated electron (eaq−) and hydrogen atom (H.) which can easily reduce metal ions, including Au(III) ions, down to zero–valent Au(0). Moreover, the reducing species can be uniformly distributed in the solution yielding metal nanoparticle evenly dispersed with possible size control by varying the irradiation dose and dose rate. Among the species generated by water radiolysis, hydrated electron (eaq−) and hydrogen atom (H.) exhibit a strong reducing power which can easily convert gold ions to zero–valent metal clusters. Whatever the synthetic route, AuNPs tend to aggregate during their synthesis. Therefore, the stabilizer (e.g., surfactants and polymers) is often added to avoid uncontrolled growth on aggregation in the preparation of nanoparticles. To ensure sufficient stability over time, protective or stabilizing agents such as proteins [8], surfactants [9] and various types of coordinating natural or synthetic polymers [10], have been used. Among them, chitosan (CTS) which is obtained by N–deacetylation product of chitin, has been reported as a dispersant preventing metal particles from agglomeration. It has found application in the preparation of precious metal nanoparticles (Pd, Ag, Pt and Au) nanocomposites [1–3]. Chitosan is a natural cationic biopolymer constituted of D–glucosamine units with abundant reactive amino and hydroxyl functional groups, it is soluble in aqueous acidic media (pH < 6.5) and shows good biocompatibility and degradability. It has been known as an efficient stabilizer for numerous metal nanoparticles and has been extensively used for silver and gold nanoparticles synthesis [2, 3]. Nitrophenols are among the most common and versatile organic pollutants in industrial and agricultural waste waters. The 4–NP is highly stable in water and takes a long time to degrade and also causing environmental risk by exhibiting carcinogenic activities [11, 12]. Many methods have been developed for the removal of nitrophenols including adsorption, microbial degradation, photocatalytic degradation, electro–Fenton method, microwave assisted degradation and so on [13]. Moreover, 4–AP is an intermediate for the manufacture of analgesic and antipyretic [13, 14]. Therefore, it is very interesting to develop aqueous phase conversion of 4–NP to 4–AP under mild conditions. Our investigations of the synthesis of AuNPs by irradiation of Au(III) solutions in the presence of chitosan via γ–irradiation and the main process parameters and synthesis factors (effect of irradiation dose, dose rate effect, [GLA]/[Au(III)] ratio) on the characteristic properties of gold nanoparticles solutions was studied. In addition, the conversion of 4–nitrophenol to 4–aminophenol with the presence of chitosan–stabilized gold nanoparticles catalyst synthesized by γ–irradiation is considered. 2. MATERIALS AND METHODS 2.1. Reagents HAuCl4.3H2O (99.9 %) and low molecular weight chitosan (poly–(1,4–D– glucopyranosamine) were purchased from Aldrich. The chitosan was purified by successive dissolutions in acetic acid and precipitations in alkaline media. Its degree of deacetylation determined by 1H NMR was 82%. The average molecular weight, determined by viscosimetry, following the methodology described by Roziak et al. [11, 15], was Mv= 56,000 g.mol–1. Viscosity measurements were performed with an Ubbelohde viscosimeter 531 10/I of Schott ins we 2.2 do (G 3.5 wa var dis Vi do Af 2.3 spe res tra elo nan Ep 2.4 F 4– truments. A re of high pu . Synthesis Before th uble distilled LA)) was pr ). Due to th s obtained. iable volum tilled water etnam) on a simetry syst ter irradiatio . Character Gold nan ctrophotom pectively. T nsmission el ngated cell oparticles idemiology . Catalytic r igure 1. UV– To study nitrophenol ll starting so rity grade. of gold nano e experimen water. 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All other red by disso in glucosam % acetic ac until a clear uCl4 soluti 5 mL by a (Ho Chi M hanol–chloro kGy and 2 re analysis. using a Var ize, zeta p by TEM ima ound shape nts needed e of Hygi les ([Au(0)] es, the redu he methodol , Vu Q.A. 15 reagents lution in ine units id (pH = solution on and a dding of inh City, benzene 3.4 kGy. ian 5000 otential), ges on a d versus for gold ene and = 1 mM) ction of ogy is as Ra 16 fol aqu ho pre dem reg to 3 c 3.1 23 spe sur spe wa 28 gly 26 bo go rad the kG red diation synth lows: 0.9 m eous soluti mogenizatio sence of bo onstrated b ular time in 800 nm at ro m3. . Synthesis AuNPs w .4 kGy. Aft ctra exhibit face plasm ctrum of th s observed 5 nm could b cosidic bon 0 nm may be th reduction ld nanoparti iolytic yield ory for redu y. The radia uced to Au( Figure 2. UV 15.6 kGy, 23 esis and ch L of an aqu on of 4–nit n of the solu th the react y the UV– terval on a V om tempera of gold nano ere prepared er irradiation ed an absor on resonanc e unirradiate at 260 and 2 e attributed ds by H–ab due to C=O and defragm cles are capp for gamma cing 1 mm tion dose of 0). –vis spectra .4 kGy. [HAu aracterizatio eous solutio rophenol an tion was ad ant 4–nitrop vis absorptio arian Cary ture (Figure 3. RESU particles by by γ–irradi , the initial ption band e phenome d solution, t 85 nm, grad to a termina straction rea in COOH c entation of ed/stabilize radiation G ol.L–1 Ag(I) 7.8 kGy is t of gold nanop Cl4] = 1 mM analy n of chitosa n of 15 mM d 1.35 mL ded 15 μL o henolate an n spectrosc 50 spectrop 1). The opti LTS AND gamma ir ation with a ly colorless around 520 non shown he appearan ually increa l carbonyl g ction under arboxyl gro chitosan ch d in the pre red = 0.6 μm would be a o ensure tha article prepar , [CTS] = 0.4 sis with optic n stabilized g NaBH4 ar of distilled f gold nano ion and the opy. The ab hotometer a cal path has DISCUSSIO radiation total absorb solutions tu nm. This ab by gold n ce of new ba sing for hig roups on C1 irradiation ups. This m ains happen sence of frag ol.J–1 [6]. bout 2 kGy t Au(III) of ed by γ–irrad 8 %. All sam al path 1 cm. old nanopar e added to a water in th particles ([A product 4– sorption sp t a waveleng a width of 1 N ed dose rang rned into pi sorption is anoparticles nds labellin her radiatio and C4 resul [16, 17]. Th eans that dur simultaneo ments of ch On this basi and for 1 m 0.5 – 1.0 mm iation at dose ples were dilu ticles and c 0.75 mL ( e quartz ce u(0)] = 1 m aminopheno ectra are rec th ranging cm and a v ing between nk, their UV characterist . Compared g a strong ab n doses. Th ting from sc e absorption ing the γ–irr usly and the itosan. The s, the radiat mol.L–1 Au ol.L–1 is co 7.8 kGy, 11.7 ted 10 times atalytic 0.2 mM) ll. After M). The l can be orded at from 200 olume of 7.8 and –visible ic of the to the sorption e peak at ission of band at adiation, reduced effective ion dose (III) is 6 mpletely kGy, before no con to the sug ph use dif no [G nar ob of sli pre At mo Mo fra nm 0.9 lik irr Th 10 the Figure 2 effect was ditions, the 2,400 dm3.m samples w gesting tha enomenon o of this type Figure 3. UV Gold nan ferent conce observable LA]/[Au] ra row range. This obse servation sh chitosan. Fr ghtly on the pared from chitosan co lecule is hi reover, the gments prod . Indeed, th 6 % it is 1,7 e characteri adiation and erefore, AuN .5 ± 4.4 nm size distribu showed that observed o molar extin ol–1.cm–1. I ere recorded t the AuNP ver such a t of nanopart –vis spectra [GLA]/ oparticles pr ntrations of effect on the tio ranging rvation is ows that the om the TEM diameter di chitosan sol ncentration gher than t stabilization uced by the e reduced v 60 cps. As p stics reduce promotes Ps prepare and show a b tion histogr all the Au(I n the absor ction coeffic n order to e after 45 da s were sta imeframe co icles for var of gold nanop [Au] ratios: 1 epared by γ chitosan sol absorption from 10 to confirmed b gold nanopa micrograp stribution o ution at 0.48 of 0.16 %, th hat at 0.48 of nanopar irradiation p iscosity of a reviously ob the mobil the agglom d from solu roader size am (Figure 3 II) ions pres bance of pl ient per Au( valuate the ys. No sign ble over a uld be an im ious applica article prepa 0, 20, 30 and –irradiation utions are sh spectra with 60. This m y TEM an rticles produ hs, it is obs f gold nanop % have th e mean num % leading ticle is less rocess. In t 0.48 % chi served by H ity of radi eration, in tion with 0 distribution ). ent were red asmon peak 0) atom was stability of t ificant chan long perio portant asse tions. red by γ–irrad 60 with optic with a total own in Figu the increase eans that th alysis. Inde ced are sph erved that c articles. It i e smallest m ber of Au( to the form efficient du his condition tosan soluti uang et al. cals and go ducing the .96 % of ch than 0.48 an uced to a d with highe determined he AuNPs, ge was obs d. The abs t when con iation at dose al path 1 mm absorbed do re 3. It is no of chitosan e size of na ed, TEM (F erical whate hitosan con s interesting ean diamete III) ion coor ation of lar e to a lower , the mean on is 1,364 [1] with silve ld clusters formation itosan have d 0.16 wt% Vo K.D.N. ose of 7.8 k r dose. Und approximat absorption s erved in the ence of agg sidering the 7.8 kGy at v . se of 7.8 k ticeable tha concentratio noparticles igure 4a, b ver the conc centration in to note tha r as of 6.7 ± dinated to a ge Au(0) p amount of diameter is at 20 °C, w r nanopartic generated of larger a mean dia solution as , Vu Q.A. 17 Gy since er these ely equal pectra of spectra, regation potential arious Gy using t there is n within lies in a and c) entration fluences t AuNPs 2.0 nm. chitosan recursor. chitosan 7.4 ± 2.5 hereas at les, gel– after the particles. meter of shown in Radiation synthesis and characterization of chitosan stabilized gold nanoparticles and catalytic 18 Figure 4. Gold nanoparticles with ratio of [GLA]/[Au]: 10 (a), 30 (b), 60 (c) prepared by γ–irradiation at dose 7.8 kGy. Size distribution of gold nanoparticles is shown on the right–hand side of the TEM micrographs. Complementary experiments were conducted in order to study the dependence of AuNPs size on Au(III) concentration. AuNPs solutions were prepared with a concentration of chitosan of 0.48 % and Au(III) concentrations ranging from 5 × 10–4 to 10–2 mol.dm–3. In these conditions, the [GLA]/[Au(III)] ratio ranges from 300 to 15. In our conditions, the influence of Au(III) concentration on the nanoparticle dimensions is negligible. The mean size diameters of AuNPs determined by TEM with a chitosan solution at 0.48 % are 4.8 ± 0.9 nm; 6.7 ± 2.0 nm; 6.6 ± 2.7; 6.0 ± 3.0 and 5.1 ± 2.4 nm for [GLA]/[Au] ratio of 15; 30; 60; 120; 300, respectively (Figure 5). It is suggested that when the ratio of [GLA]/[Au(III)] exceed a value of 15, the concentration of chitosan is sufficient to prevent aggregation. Compared to the preceding experiments, the increase of Au(III) concentration lead to the decrease in the viscosity of Au–CTS solutions in some preceding case. A smaller influence on the mobility of gold clusters is observed. This trend has been already observed by Hien et al. [7] with AuNPs capped with hyaluronan and has been related to the competition between the adsorption of Au(III) onto the resultant gold cluster and the reduction reaction of Au(III) to Au(0) to form new cluster. At high dose rate, the reduction reaction is predominant. Therefore, there are many new clusters allowing smaller AuNPs to be formed. In contrast, at low dose rate which the adsorption of Au(III) onto cluster is predominant, therefore AuNPs will be larger. Other experiments were undertaken, while varying the dose rate for a total dose of 7.8 kGy. The results showed a decrease of the mean average diameter of AuNPs when the dose rate increased. Indeed, the mean average diameters of AuNPs determined by TEM are of 8.8 ± 4.6; 5.8 ± 2.4; 4.7 ± 1.3 and 4.8 ± 1.6 nm for dose rate of 0.3; 1; 2 and 4 kGy.h–1, respectively (Table 1). F T D Pe rc en ta ge (% ) igure 5. Gold 60 (c), [GLA able 1. Param ose rate (kG 0.3 1.0 2.0 4.0 0 15 30 45 60 5 nanoparticles ]/[Au] = 120 eters of gold y.h–1) 10 Diameter (nm) 0 15 30 45 60 Pe rc en ta ge (% ) (a with chitosan (d) and [GLA nanoparticles Absorban 0.23 0.22 0.19 0.16 15 0 15 30 45 60 Pe rc en ta ge (% ) 5 10 Diameter (nm) ) of [GLA]/[A ]/[Au] = 300 with differen [CTS] = 0.4 ce 5 10 Diameter Pe rc en ta ge (% ) 15 (d) u] = 15 (a), [ (e) prepared t dose rates ( 8 %. λmax (nm 524 525 524 522 15 20 (nm) 0 15 30 45 60 5 D (b) GLA]/[Au] = by γ–irradiati dose: 7.8 kGy ) 0 15 30 45 60 Pe rc en ta ge (% ) 10 iameter (nm) (e Vo K.D.N. 30 (a), [GLA on at dose 7.8 ). [HAuCl4] = d (nm 8.8±4 5.8±2 4.7±1 4.8±1 5 1 Diameter (nm) 15 ) , Vu Q.A. 19 ]/[Au] = kGy. 1 mM, ) .6 .4 .3 .6 0 15 (c) Radiation synthesis and characterization of chitosan stabilized gold nanoparticles and catalytic 20 In some experiments, HAuCl4 solutions (1 mM) with chitosan 0.48 % were irradiated with accelerated electrons. The samples were electron beam processed with a pulsed accelerator, irradiation doses vary between 5 to 50 kGy followed by 5 kGy or 25 kGy per pass. The time interval between two successive irradiation cycles was equal to five minutes. NaCl was added progressively to Au(III)/chitosan solution irradiated at 10 kGy in order to increase the ionic strength of the medium up to 0.1 mol.dm–3. Upon addition of NaCl the colour of the gold nanoparticles solutions evolved progressively from brown to pink, corresponding to an increase in absorbance of the plasmon peak as depicted in Figure 6. According to the DLVO theory, the increase of ionic strength leads to decrease in the repulsive interactions between gold nanoparticles thus promoting aggregation of Au cluster and the increase of the nanoparticle sizes. Increasing the NaCl concentration beyond 0.1mol.dm–3 did not induce significant change in the UV–visible spectrum of gold nanoparticle solutions, which means that aggregation process was limited. It should be noted in this case, the UV–visible spectrum showed similar features than this of solutions irradiated at 5 kGy. The efficiency of e–beam irradiation on the reduction of gold nanoparticles was assessed by determining the concentration of residual Au(III) after separation of the nanoparticles of the solutions by ultracentrifugation. Prior to ultracentrifugation, NaCl 1 mol.dm–3 was added to solutions irradiated at 10 and 15 kGy, to induce partial aggregation of the nanoparticles in order to improve the efficiency of separation. Experiments were conducted on suspension after 24 h irradiation. Whatever the irradiation dose, the concentration of the residual gold ions determined by ICP–OES was found negligible (less than 5 %) compared to initial gold concentration. This means that reduction is complete, even at 5 kGy which is theoretically not sufficient to ensure the direct reduction of 1 mmol of Au(III). In this case, the reduction is achieved by chemical reaction between the remaining Au(III) ions adsorbed onto preformed cluster and the Au(0) clusters. Figure 6. UV–vis spectra of gold nanoparticle prepared by e–beam irradiation at dose 10 kGy: (a) without NaCl addition and (b), (c) upon addition NaCl 1.0 mol.L–1 (V = 50 and 100 μL, respectively). Samples were conditioned under air before irradiation. [HAuCl4] = 1 mM, [CTS] = 0.48 %. Optical path 1 mm. All the solutions turned pink after 14 days. The absorbance of plasmon resonance peak increased with times, whereas the background flattened off. After 30 days, the analysis of UV– Vo K.D.N., Vu Q.A. 21 visible spectra showed that the influence of the dose on the size of AuNPs diminished sharply for all solutions. This seems to indicate that the final size of AuNPs does not depend on the initial dose rate and that scission chain does not alter the stabilizing effect of chitosan [20]. Whatever the experimental conditions, zeta potential values for AuNPs were positive and range between 35 and 45 mV. 3.2. Catalytic activity of gold nanoparticles In the absence of gold nanoparticles, the aqueous mixture of 4–nitrophenol and NaBH4 shows an absorption maximum at 400 nm which is characteristic of the 4–nitrophenolate in alkaline conditions. There is no observed change in absorbance with time after the addition of NaBH4, suggesting that the reduction does not occur in the absence of catalyst. The adding of the gold nanoparticles causes visually progressive fading of the reaction medium which results in a decrease in the absorbance peak at 400 nm and the appearance of two new peaks at 277 nm and 310 nm characteristic of the presence of 4–aminophenol, and the absorbance increases with time [12, 13, 18, 19]. The reaction is considered complete when the absorbance of the characteristic peak of 4–nitrophenol and is constant near 0. Each manipulation lasts between 10 and 20 min. Figure 1 shows an example of changes in UV–visible absorption spectra of this reaction. In this example, gold nanoparticles ([Au(0)] = 1 mM) were synthesized from a solution of chitosan and HAuCl4 packaged in air and irradiated with γ–irradiation at dose 7.8 kGy. It is well–known that the size of metal nanoparticles influences the catalytic reduction. In the present study, it is found that the smaller size AuNPs function as more effective catalyst than the larger particles in the reduction 4–NP. The size of AuNPs decreases from 10.5 ± 4.4 nm to 6.7 ± 2.0 nm, the smaller particles show faster activity. When the size of gold nanoparticles decreases, there is an increase in the number of low–coordinated Au atoms which promote the adsorption of the reactants (4–nitrophenolate ions and BH4–) on the catalyst surface and facilitates the reduction. On the contrary, the larger particles have relatively high–coordinated Au atoms related to lower surface roughness and this is unfavorable for the adsorption of reactants and does not facilitates the reduction [21, 22]. Generally, the gold nanoparticles synthesized by γ–irradiation exhibit interesting catalytic activity vis–à–vis the toxic pollutant reduction of 4–NP to 4–AP. This catalytic activity should be studied other reactions to judge potential applications of these systems 4.CONCLUSIONS A simple convenient and “green” method for the synthesis of stable gold nanoparticles in chitosan aqueous solution has been investigated. Gold nanoparticles were successfully synthesized in the presence of chitosan under e–beam and γ–irradiation. Our results show that the size of the preformed Au cluster prior to aggregation and the nucleation process is controlled by the dose rate and the ratio between glucosamine units and Au(III). Whatever the irradiation process, chitosan act as an efficient stabilizing agent when its concentration ranges between 0.24% and 0.48%. Lower chitosan concentrations do not provide sufficient adsorption of GLA units to avoid aggregation of AuNPs. At higher chitosan concentration the high viscosity of solutions reduces the mobility of reducing species and provokes local aggregation leading to the formation of larger nanoparticles. The synthesized chitosan–stabilized gold nanoparticles exhibited excellent catalytic property in the reduction of toxic pollutant 4–nitrophenol to 4– aminophenol. 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