VIETNAM JOURNAL OF CHEMISTRY DOL 10 15625/0866-7144 2014-0035
VOL, 52(5) 584-589 OCTOBER 2014
PREPARATION AND CHARACTERIZATION OF CHITOSAN-STABILIZED GOLD NANOPARTICLES INDUCED IN AQUEOUS SOLUTIONS BY
Y-IRRADIATION AND ITS CATALYTIC APPLICATION
Bui Duy Du'*, Lai Thi Kim Dung', Le Nghiem Anh Tuan', Nguyen Quoc Hien\ Vo Nguyen Dang Khoa
'institute of Applied Materials Science, Vietnam Academy of Science and Technology, 01 Mac Dinh Chi Street, f District, Ho Chi Minh City, Vietnam
^Research and Development Center for Radiation Technology, Vietnam Atomic Energy Institute.
202A, 11 Street, Linh Xuan Ward. Thu Due District, Ho Chi Minh City. Vietnam Received22 July 2014; Accepted for Publication 15 October 2014
Abstract
Gold nanoparticles were synthesized in a single procedure by y-irradiation using chitosan as a stabilizing agent. The influence of the main process parameters and synthesis factors ((GLA: glucosamine units) [GLA]/[Au(Ill)] ratio, effect of irradiation dose, dose rate effect) on the characteristic properties of gold nanoparticles solutions was studied. The results of UV-vis absorption spectroscopy and transmission electron microscopy indicated that spherical well-dispersed gold nanoparticles ranging from 5 to 10 nm, depending on the [GLA]/[Au(III)] ratio, the irradiation dose and the dose rate.
Keywords: Gold nanoparticles, chitosan, irradiation, 4-nitrophenol.
1. INTRODUCTION
Gold nanoparticles (AuNPs) have received considerable attention for potential applications in the fields of physics, chemistry, optics, biology, 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.
For the last decade, the increased awareness towards the environment has encouraged nanomaterial scientists to look for greener methods.
The use of non-toxic chemicals, environmental- friendly solvents and renewable resources are important issues in green synthesis strategies.
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 [5-7]. Water radiolysis
generates hydrated electron (eaq ) and hydrogen atom (H) which can easily reduce metal ions down to zero-valent state. 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 (Caq") and hydrogen atom (H) exhibit a strong reducing power which can easily convert gold ions into zero-valent metal clusters. Whatever the synthetic route, AuNPs tend to aggregate during their synthesis. In order 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 to be used. Among them, chitosan (CTS) which is obtained by N- deacetylation product of chitin, has been reported as a dispersant preventing of 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
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biocompatibility and degradability. Although y- irradiation was currently used as reducing agent to synthesize metal nanoparticle, to our knowledge there has not been a detail report on the synthesis of AuNPs in the presence of chitosan via the radiolytic route.
Our first results on the synthesis of Au nanoparticles by irradiation of Au(n!) solutions in the presence of chitosan via e-beam and y-irradiation show that the nucleation and the growth of Au nanoparticles are controlled by the dose rate of ionizing radiation and are somewhat different during e-beam and y-irradiation [11]. In this paper we complete our investigations on synthesis of Au nanoparticles 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 characteristics of the formed nanoparticles exhibite.
2, EXPERIMENTAL 2.1. Reagents
HAUCI4.3H2O (99.9%) and low molecular weight chitosan (poly-( 1,4-D-glucopyranosamine) were purchased from Aldrich. The chitosan was purified by successive dissolution in acetic acid and precipitation in alkaline media. Its degree of deacetylation determined by ' H NMR was 82%.
The average molecular weight, determined by viscosimetry, following the methodology described by Roziak et al. [12], was Mv ^ 56000 g moi"' Viscosi^ measurements were performed with a Ubbelohde viscosimeter 531 lO/I of Schott instruments. All starting solutions were prepared with double distilled water. The other reagents were of high purity grade.
2.2. Synthesis of gold nanoparticles
Before the experiments, a stock solution of 10 mM HAUCI4 was prepared by dissolution in double distilled water. A stock solution of 3 g L"' chitosan (0.015 moi L"' in glucosamine units (GLA)) was prepared by dissolving the required amount of polymer in 1% acetic acid (pH = 3.5). Due to the poor solubility of chitosan, the mixture was kept overnight until a clear solution was obtained.
Studied samples were prepared by mixing 0.5 mL of HAuCLt solution and a variable volume of chitosan solution. The final volume was adjusted to 5ml by addition of distilled water. 7-irradiations were performed at VI>.AGAMMA Center (Ho Chi Minh City, Vietnam) on a ^°Co source with dose rate of 1.1 kGy/h measured by the ethanol-chlorobenzene
Bui Duy Du, et al.
dosimetry system (ISO/ASTM 51538-2002(E)).
Doses vary between 7.8 kGy and 23.4 kGy. After irradiation, the samples were stored at room temperature for 24 hrs before analysis.
2.3 Characterization of gold nanoparticles Gold nanoparticles were characterized by UV- visible spectroscopy using a Varian 5000 spectrophotometer. The size and size distribution of the AuNPs were characterized by TEM images on a transmission electron microscope (TEM) model JEMIOIO (JEOL, Japan). Round shaped versus elongated cells were counted by using ImageJ software.
3. RESULTS AND DISCUSSION
3.1 Synthesis of gold nanoparticles by gamma irradiation
Au nanoparticles were prepared by y-irradiation with a total absorbed dose ranging between 7.8 and 23.4 kGy. After irradiation, the initially colorless solutions turn pink, their UV-visible spectra an absorption band around 520 nm. This absorption is characteristic of the surface plasmon resonance phenomenon shown by gold nanoparticles (Fig. 1).
Compared to the spectrum of the unirradiated solution, the appearance of new bands labelling a strong absorption is observed at 260 and 285 nm, gradually increasing for higher radiation doses. The peak at 285 nm could be attributed to a terminal carbonyl groups on Cj and C4 resulting from scission of glycosidic bonds by H-abstraction reaction under irradiation [13, 14]. The absorption band at 260 nm may be due to C=0 in COOH carboxyl groups. This means that during the y-irradiation, both reduction and defragmentation of chitosan chains happen simultaneously and the reduced gold nanoparticles are capped/stabilized by the presence of fragments of chitosan. The effective radiolytic yield for gamma radiation Gred= 0.6 pmol J"' [6]. On this basis, the radiation dose theory for reducing 1 mmol L"' Ag(I) would be about 2 kGy and for 1 mmol L"' Au(lII) is 6 kGy. The radiation dose of 7.8 kGy is to ensure that Au(III) of 0.5-1.0 mmol.L'' is completely reduced to Au(0).
Figure 1 shows that all the Au(lll) ions present are reduced for a dose of 7.8 kGy since no effect is observed on the absorbance of plasmon peak with higher dose. Under these conditions, the molar extinction coefficient per Au(0) atom was determined approximately equal to 2400 dm^mo^'.cm"•. In order to evaluate the stability of the AuNPs, absorption spectra of the samples were
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recorded after 45 days. No significant change was observed in the spectra, suggesting that the AuNPs are stable over a long period. The absence of aggregation phenomenon over such a timeframe could be an important asset when considering the potential use of this type of nanoparticles for various applications.
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Fig. I: UV-vis spectra of gold nanoparticle prepared by y-irradiation at dose 7.8 kGy, 11.7kGy, 15.6 kGy, 23.4 kGy. [HAuC^] = 1 mM, [CTS] = 0.48 %.
All samples were diluted 10 times before analysis with optical path 1 cm
-e- [QLA]/lAu]=10 --[OLA],TAU]=20
- B - E O L A ] / I A Q > 6 0
Fig. 2: UV-vis specfra of gold nanoparticle prepared by y-irradiation at dose 7.8 kGy at various [GLA]/[Au] ratios: 10, 20,30 and 60 with optical
path 1 mm
Gold nanoparticles prepared by y-irradiation with a total absorbed dose of 7.8 kGy using different concenfrations of chitosan solutions are shown in Fig.2. It is noticeable, that there is no observable effect on the absorption spectra with the increase of chitosan concentration within [GLA]/[Au] ratio ranging from 10 to 60, meaning that the size of nanoparticles lies in a narrow range. This
Preparation and characterization of chitosan...
observation is confirmed by TEM analysis. Indeed TEM micrographs (Fig.3a, b and c) observation shows that the gold nanoparticles produced are spherical whatever the concenfration of chitosan.
From the TEM micrographs, it is observed that chitosan concentration influences slightly on the diameter disfribution of gold nanoparticles. It is interesting to note that AuNPs prepared from chitosan solution at 0.48 % have the smallest mean diameter which is 6.7±2 nm. At chitosan concentration of 0.16 %, the mean number of Au(IlI) ion coordinated to a chitosan molecule is higher than with 0.48 % leading to the formation of Au(0) precursor of large. Moreover, the stabilisation of nanoparticle is less efficient due to a lower amount of chitosan Iragments produced by the irradiation process. In this condition, the mean diameter is 7.4±2.5 nm. Whereas at concenfration higher than 0.48 % the increase in viscosity of the chitosan solutions gives rise to gel-like characteristics. Indeed the reduced viscosity of a 0.48 % chitosan solution is 1364 at 20 °C, whereas at 0.96 % it is 1760 cps. As previously observed by Huang et al. [1] with silver nanoparticles, gel-like characteristics reduce the mobility of radicals and gold clusters generated after the irradiation and promotes the agglomeration, inducing the formation of larger particles. Therefore, AuNPs prepared from solution with 0.96 % of chitosan have a mean diameter of 10.5±4.4 nm and show a broader size distribution than 0.48 and 0.16 wt% solution as shown in the size disfribution histogram (Fig. 3).
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) concenfrations ranging from SxlO"* to 10" mole.dm". In these conditions, the [GLA]/[Au(III)] ratio ranges from 300 to 15. As seen in Fig. 3, in our conditions, the influence of Au(III) concenfration on the nanoparticle dimensions is negligible. The mean size diameters of AuNPs detennined 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 (Fig. 4) for [GLA]/[Aul ratio of 15; 30; 60; 120; 300 respectively. This suggests that when the ratio of [GLA]/[Au(III)]
exceed a value of 15, the concenfration of chitosan is sufficient to prevent aggregation. Compared to the preceding experiments, the increase of Au(Iir) concenfration decreases the viscosity of Au-CTS solutions but to a lesser extent than in the preceding case. A smaller infiuence on the mobility of gold clusters is observed.
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Fig. 3: Gold nanoparticles with ratio of [GLA]/[Au]:
10 (a), 30 (b), 60 (c) prepared by y-irradiation at dose 7.8 kGy. Size distribution of gold nanoparticles
is shown on the right-hand side of the TEM micrographs
Other experiments were undertaken, while varying the dose rate for a total dose of 7.8 kGy. The results show a decrease of the mean average diameter of AuNPs when the dose rate increases Indeed, the mean average diameters of AuNPs detennined by TEM are of 8.8±4.6; 5.8±2.4; 4.7±1.3 and 4.8±1.6 tun for dose rate of 0.3; 1; 2 and 4 kGy/h, respectively (table I). This frend has been already observed by Hien et aL [7] with AuNPs capped with hyaluronan and is related to the competition between the adsorption of Au(III) onto the resultant gold cluster and the reduction reaction of Au(IIl) 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 confrast at low dose rate the adsorption of Au(III) onto cluster is predominant, therefore AuNPs will be larger.
Fig. 4: Gold nanoparticles with ratio of [GLA]/[Au]- 15 (a), 30 (b), 60 (c), 120 (d) and 300 (e) prepared ' by y-irradiation at dose 7.8 kGy. Size disfribution of gold nanoparticles is shown on the right-hand side of
the TEM micrographs
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Table I: Parameters of gold nanoparticles with different d o s e rates
(Dose: 7.8 kGy). [HAuCU] = I m M , [ C T S ] = 0.48 %
Dose rate, kGy/h 0.3 1.0 2.0 4.0
Absorbance 0.23 0.22 0.19 0.16
^TTiax, n m 524 525 524 522
d, nm
8.8±4.6 5.8±2.4 4.7±1.3 4.8±1.6
Preparation and characterization of chitosan...
lasts between 10 and 20 min. Figure 5 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 HAuCU packaged in air and irradiated with y-irradiation at dose 7.8 kGy.
3.2. Application of gold nanoparticles in catalysis Gold has fraditionally been considered inactive as a catalyst. However, the gold nanoparticles show surprisingly high catalytic activity towards many chemical reactions. In this study, the conversion of 4- nitrophenol to 4-aminophenol with the presence of gold nanoparticles catalyst synthesized by y- irradiation is considered. This reaction was followed by UV-vis spectrophotometer. More specifically, the appearance of 4-nitrophenol can be seen a characteristic band at 400 run [15, 16]. The catalytic reduction reaction is carried out directly in the quartz cuvette of the spectrophotometer. The optical path has a width of 1 cm and a volume of 3 cm^.
The methodology is as follows: 0.9 mL of an aqueous solution of 15 mM NaBH4 are added to a 0.75 mL (0.2 mM) aqueous solution of 4-nitrophenol and 1.35 mL of distilled water in the quartz cell.
After homogenization of the solution was added 15pL of gold nanoparticles ([Au(0)] = 1 mM). The absorption specfra are recorded at regular time interval on a Varian Cary 50 specfrophotometer at a wavelength range of 200 to 800 nm at room temperature.
In the absence of gold nanoparticles, the aqueous mixture of 4-nitrophenol and NaBHj shows an absorption maximum at 400 nm, characteristic of the 4-nifrophenolate in alkaline conditions. There is no observed change in absorbance with time after the addition of NaBH,, 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 [16]. The reaction is considered complete when the absorbance of the characteristic peak of 4- nifrophenol is constant near 0. Each manipulation
Fig. 5: UV-vis specfra of the reduction of 4-nitrophenol by the gold nanoparticles ([Au (0)] = 1 mM) by y-irradiation in air Generally, the gold nanoparticles synthesized by Y-irradiation exhibit interesting catalytic activity vis- a-vis the toxic pollutant reduction of 4-nifrophenol to 4-aminophenol. This catalytic activity should be studied for other reactions to judge potential applications of these systems.
4. CONCLUSION
This work provides a simple convenient and
"green" method for the synthesis of stable gold nanoparticles in chitosan aqueous solution. Gold nanoparticles were successfully synthesized in the presence of chitosan under y-irradiation. The particles are characterized by UV-visible spectroscopy and TEM. Our results show that the size of the preformed Au cluster prior to aggregation and the nucleation process are confrolled by the dose rate and the ratio between glucosamine units and Au(IlI). 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. Our results underline the potential of
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this green m e t h o d to p r o d u c e size controlled nanoparticles for various fields o f application.
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Corresponding author: B u i D u y D u
Institute o f Applied Materials Science - V A S T 01 M a c Dinh Chi St., 1" Dist., H o Chi M i n h City, Viettiam E-mail: v i n a 9 8 0 2 ( g g m a i l . c o m ; p h o n e : 0 9 64 7 7 15 2 3 .
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