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Synthesis of gold nanoparticle necklaces using linear–dendritic copolymers
European Polymer Journal 46 (2010) 165–170
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Synthesis of gold nanoparticle necklaces using linear–dendritic copolymers Ashkan Tavakoli Naeini a, Mohsen Adeli b,c,d,*, Manouchehr Vossoughi a,b,* a
Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran Institute for Nanoscience and Technology, Sharif University of Technology, Tehran, Iran c Department of Chemistry, Sharif University of Technology, Tehran, Iran d Department of Chemistry, Faculty of Science, Lorestan University, Khramabad, Iran b
i n f o
Article history: Received 14 April 2009 Received in revised form 11 October 2009 Accepted 18 October 2009 Available online 23 October 2009 Keywords: Linear–dendritic Gold nanoparticles Citric acid Loading capacity Nanoparticle necklaces
a b s t r a c t Linear–dendritic copolymers containing hyperbranched poly(citric acid) and linear poly(ethylene glycol) blocks (PCA–PEG–PCA) were used as reducing and capping agents to synthesize and support gold nanoparticles (AuNPs). PCA–PEG–PCA copolymers with 1758, 1889 and 3446 molecular weights, called A1, A2 and A3 through this work, respectively, were synthesized using 2, 5, and 10 citric acid/PEG molar ratios. The diameter of A1, A2 and A3 in a fresh water solution was investigated using dynamic light scattering (DLS) and it was between 1.8 and 2.8 nm. AuNPs were simply synthesized and supported by addition a boiling aqueous solution of HAuCl4 to aqueous solutions of A1, A2 and A3. Supported AuNPs were stable in water for several months and agglomeration was not occurred. The loading capacity of A1, A2 and A3 and the size of synthesized AuNPs were investigated using UV spectroscopy and transmission electron microscopy (TEM). It was found that the loading capacity of PCA–PEG–PCA copolymers depend on the concentration of copolymers and the size of their poly(citric acid) parts directly. For example average loading capacities for 400 lM concentration of A1, A2 and A3 were 32.24, 37.4 and 41.52 lM, respectively, and average loading capacities for 400, 200 and 100 lM concentration of A1 were 32.24, 20.28 and 9.1 lM, respectively. Interestingly there was a reverse relation between the size of synthesized AuNPs and size of poly(citric acid) parts of PCA–PEG–PCA copolymers. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Nanogold or gold nanoparticles (AuNPs) have been widely studied in the past 10 years because of their unique properties, such as catalysis, quantum size effect, and optical properties [1]. Due to the requirements for control of nanoparticle size and surface functionalization for this broad range of applications, different synthetic methods have been developed to generate monodisperse AuNPs. One of the most widely used methods is the reduction of
* Corresponding authors. Address: Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran. Tel.: +98 21 66165487 (M. Vossoughi); +98 916 3603772 (M. Adeli). E-mail addresses: [email protected], [email protected] (M. Adeli), [email protected] (M. Vossoughi). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.10.017
tetrachloroaurate ions AuCl4 in aqueous medium using sodium citrate to generate particles with diameters typically ranging from 10 to 100 nm [2]. Although this method has good control over producing a particular particle size, it is limited to the synthesis of larger particles. The Brust method and various modifications are useful for the generation of AuNPs having core sizes ranging from 1 to 4 nm [3–5]. In the Brust method, the transfer of AuCl4 into toluene or chloroform is performed using tetraalkylammonium bromide followed by reduction with sodium borohydride in the presence of alkylthiols. Disadvantages of this method include contamination of the synthesized particles with boride [6] and potential presence of impurities introduced by the use of capping ligands which also hinder the surface modification and functionalization of particles for particular applications. A few other single-
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step reduction processes also were developed to generate monodisperse gold nanoparticles, but these are limited to organic media [7–9]. In addition, the reduction of gold salt to form AuNPs using amine-containing organic molecules has been investigated [10–14], but most of the amine compounds used in those AuNPs syntheses are soluble in only organic solvents, and the reduction reactions must take place in an organic solvent or in a biphasic system. As a result, the nanoparticles prepared using those methods are not easily dispersed in aqueous solutions. To use the AuNPs in aqueous-based or biological systems, it is necessary to functionalize the particles with ligands which facilitate phase transfer from an organic to an aqueous medium. Alternative synthetic strategies based on using polymers as both the reducing and stabilizing agent for the generation of stable metal nanoparticles without the use of an additional stabilizing agent have been developed. The resulting nanoparticle-polymer composites have been shown to be useful in catalytic transformations [15] and they could be useful for nanostructured solar cells, photonic band gap materials, storage devices, and drug delivery. Some polymers can fulfill the required dual role as a reducer and stabilizer [16]. Examples include poly(methylhydrosiloxane) [17], poly(N-vinyl-2-pyrrolidone) [18], poly(sodium acrylate) [19], poly(ethylene oxide) [20], poly(vinyl alcohols) [21] and polyethylenimine [22]. However, even in the syntheses using those polymers, either the reduction reaction was carried out in organic solvents or polydisperse nanoparticles were produced. In both situations, the use of the nanoparticles is restricted, or at least complicated for aqueous-based (e.g., biological) applications or for applications that require monodisperse particles (e.g., electronics). The major advantage of using a polymer as a stabilizing agent is that it can be used to tailor the nanocomposite properties and also to provide longterm stability of the nanoparticles by preventing particle agglomeration [23–25]. Recently, the functionalized AuNPs were synthesized through different stabilizing and capping agents and showed the potential in several applications [26]. Although many block copolymers and dendrimers have been used as the stabilizing agents to synthesize AuNPs in the literatures [27], reports of the preparation and characterization of polymer–nanogold nanocomposites are occasional. In the literatures, many kinds of polymers such as poly(methylphenylphosphazene) (PMPP) [28], polyacrylamide [29], acrylic polymers [30–32], polyurethane (PU) [33], polystyrene (PS) [34], poly(ethylene glycol) (PEG) [35], poly(vinylphenol) (PVP) [36], poly(vinyl alcohol) (PVA) [37], poly(o-phenylenediamine) [38], poly(amic acid) (PAA) [39], polyimide (PI) [40], and polyfluorene (PF) [41–43] were used to prepare the polymer– AuNPs nanocomposite. Linear–dendritic macromolecules are hybrid large molecules containing dendrimers and linear polymers. Interesting properties of this type of dendritic macromolecules have stimulated investigation in this area [44–51]. Haba et al. reported preparation and encapsulation of gold nanoparticles using poly(ethylene glycol)-modified poly(amido amine) dendrimers. Encapsulated gold nanoparticles showed a heat-generating ability [52].
Synthesis of PCA–PEG–PCA linear–dendritic copolymers and their ability to encapsulate and release of organic guest molecules was reported previously [53–55]. Citric acid is a unique capping agent to cap and stabilize the metal nanoparticles such as gold nanoparticles, hence the PCA–PEG–PCA copolymers containing dendritic citric acid parts (which contain much more hydroxyl functional groups than citric acid molecule) and PEG as flexible part (which enhances the solubility) could be effective macromolecules to synthesize, protect, encapsulate and also stabilize AuNPs. On the other hand they are biocompatible materials and could be used to transport AuNPs in biological systems. In this work, PCA–PEG–PCA copolymers have been used to synthesize, encapsulate and stabilize AuNPs. The role of dendritic citric acid parts in the case of loading capacity and the size of AuNPs was investigated. 2. Experimental 2.1. Chemicals Citric acid (extra pure anhydrous powder), HAuCl4, 3H2O (99.5%, for analysis) and poly(ethylene glycol) (Mn = 1500) were purchased from Merck. Dialyze bag (a semipermeable membrane cut off 2000) was obtained from Sigma, Aldrich. 2.2. Instrumental measurements and preparation of samples for analysis The zeta potential and dynamic light scattering experiments were done by the commercially available instrument, Zetasizer, Nano series, ZEN 1600, laser 633 nm. Samples were dissolved in distilled water, and measurements were performed at 25 °C and started 10 min after the cuvette was placed in the DLS apparatus to allow the temperature to equilibrate. About 1 ml of the sample was transferred to a special dustfree light-scattering cell and the temperature was controlled within ±0.02 °C. Absorption spectra (300– 800 nm) were recorded using a Perkin–Elmer Lambda 900 UV–vis/NIR spectrophotometer. Samples of linear–dendritic encapsulated nanoparticles (LDENP) were prepared and left at room temperature overnight. Then they were transferred to the spectrophotometer cell and spectra were recorded at 25 °C. Calibration curve was used to determine the loading capacities. TEM photos were recorded using Philips CH 200, LaB6-Cathode 160 kv. 2.3. Synthesis of PCA–PEG–PCA linear–dendritic copolymers PCA–PEG–PCA linear–dendritic copolymers were synthesized according to the reported procedure in literature [56]. 2.4. Preparation of linear–dendritic encapsulated nanoparticles (LDENPs) Typically an aqueous solution of HAuCl4 with a constant concentration (100 lM) was added to aqueous solutions of PCA–PEG–PCA copolymers in different concentrations
(200, 100, 50, 25, 12.5 lM) in the boiling point. After 20 min the color of HAuCl4 solution was changed from clear pale yellow to red concerning the reduction of the gold nanoparticles and trapping by PCA–PEG–PCA copolymers. Finally, the obtained solutions were cooled and used for the UV–visible analysis and TEM. The synthesized LDENPs were also stored in dark glasses wrapped by aluminum foils at room temperature (25 °C) for further UV– visible analyses in a time schedule. 2.5. Determination of the loading capacity of PCA–PEG–PCA copolymers Loading capacity of different PCA–PEG–PCA copolymers was investigated using UV spectroscopy. The UV sepectra of aqueous solutions of AuNPs, prepared using different concentrations of HAuCl4 and citric acid as capping agent, were recorded and a calibration curve consisting concentration of HAuCl4 versus kmax was obtained. This calibration curve was used to determine the highest concentration of HAuCl4 which a certain concentration of PCA–PEG–PCA copolymers was able to change it to AuNPs. This concentration was considered as a loading capacity of copolymer. 3. Results and discussion PCA–PEG–PCA copolymers were synthesized through condensation of citric acid on hydroxyl functional groups of PEG. Degree of polymerization (DP) of poly(citric acid) parts was dominated by citric acid/PEG molar ratio. In this work citric acid/PEG = 2, 5 and 10 were used to synthesize PCA–PEG–PCA copolymers which are called A1, A2 and A3 through this work, respectively. The poly(citric acid) parts of the PCA–PEG–PCA copolymers were used to reduce gold cations and prepare AuNPs. Size of PCA–PEG–PCA copolymers in aqueous solution was determined using DLS experiments. Fig. 1 shows the
Fig. 1. DLS diagrams of A1, A2 and A3 linear–dendritic copolymers.
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DLS diagram of fresh water solution of A1, A2 and A3 copolymers. According to DLS diagrams the diameter of A1, A2 and A3 copolymers were 1.8, 2 and 2.8 nm, respectively. The small size of copolymers in aqueous solutions is assigned to their molecular self-assembly which has been explained previously [56]. Fig. 2 shows the color change of HAuCl4 solution during the reduction of gold salt and preparation of AuNPs in the presence of A1, A2, A3 and copolymers. The clear yellow color of the primary solution turned gradually to red after 20 min. Change in the color of solutions was assigned to the improvement of reaction and the preparation of the AuNPs. Size of metallic nanoparticles and loading capacities of PCA–PEG–PCA linear–dendritic copolymers was depended on the DP of poly(citric acid) parts of linear–dendritic copolymers. Dependence of loading capacities of copolymers on the DP of poly(citric acid) parts was investigated using UV experiments and results are shown in Fig. 3. Due to a direct relationship between the intensity of the maximum absorption of AuNPs and the DP of poly(citric acid) parts of PCA–PEG– PCA copolymers, it could be found that the loading capacity of these PCA–PEG–PCA copolymers is directly depend on the DP of poly(citric acid) parts. Fig. 3 shows the intensity of maximum wavelengths (kmax) of AuNPs versus the concentration of PCA–PEG–PCA copolymers for A1, A2 and A3 (concentration of HAuCl4 was constant in all experiments and equal to 100 lM). As it can be seen the intensity of kmax of synthesized AuNPs increases by increasing the concentration of PCA–PEG–PCA copolymers which proves a direct relationship between the loading capacities of PCA–PEG–PCA copolymers and their concentration. Table 1 shows maximum absorption and loading capacities of A1, A2 and A3 in a 400–25 lM concentration range. In this range, average loading capacities for A1, A2 and A3 were between 0.67–32.24, 1.94–37.4 and 2.14–41.52 lM, respectively. Fig. 4 shows the kmax of AuNPs versus concentration of PCA–PEG–PCA copolymers for A1, A2 and A3. Clearly kmax of the nanoparticles has a blue shift when the DP of
Fig. 2. The color change of the HAuCl4 solution during preparation of AuNPs (1) in absence of copolymers and in the presence of (2) A1, (3) A2 and (4) A3 linear–dendritic copolymers after 20 min.
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Fig. 3. Maximum absorption of encapsulated AuNPs by A1, A2 and A3 in five different concentrations (concentration of HAuCl4 was 100 lM).
Fig. 4. Maximum wavelenght of AuNPs loaded by A1, A2 and A3 in five different concentrations (concentration of HAuCl4 was 100 lM).
Table 1 Maximum absorption of AuNPs loaded by different PCA–PEG–PCA linear– dendritic copolymers (concentration of HAuCl4 was 100 lM) and loading capacities of A1, A2 and A3 in different conditions. AuNPs loaded (lM)
Concentration of linear– dendritic nanocarrier (lM)
Max absorption A1AuNPs
A2AuNPs
A3AuNPs
400 200 100 50 25
0.0541 0.0425 0.0317 0.0262 0.0187
0.0591 0.0457 0.0352 0.0281 0.0207
0.0631 32.247 37.401 41.526 0.0482 20.289 23.588 26.165 0.0381 9.175 12.763 15.752 0.0297 4.959 5.443 7.093 0.0218 0.670 1.948 2.144
A1
A2
A3
poly(citric acid) parts and also the concentration of PCA– PEG–PCA copolymers increase. This diagram shows that the size of AuNPs inversely depends on the DP of poly(citric acid) parts and the concentration of PCA–PEG–PCA copolymers. Size and morphology of AuNPs were investigated using TEM experiments. When low molecular weight PCA–PEG– PCA copolymers such as A1 were used, poly(citric acid) parts were so small that can cap only half of the surface of a AuNP and its other side capped by poly(citric acid) part of another molecule. This capping route led to necklaces AuNPs finally (Fig. 5 and Scheme 1). When two end nanoparticles of a AuNPs necklace are close enough, they can connect together by a PCA–PEG–PCA copolymer to make a AuNPs circle (Fig. 6 and Scheme 2). In the case of PCA–PEG–PCA copolymers with big poly(citric acid) parts such as A3, synthesized nanoparticles have a hyperbranched order. In this case citric acid parts are big enough to interact with two or more nanoparticles and make dendritic structures of nanoparticles (Fig. 7a and b and Scheme 3). Inverse relationship between
Fig. 5. TEM image of AuNPs necklaces synthesized using A1.
+
Scheme 1. Assembly of nanoparticles through linear–dendritic copolymers to make necklaces.
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Fig. 6. TEM image of AuNPs circles synthesized using A1.
+
Moreover solutions of LDENPs were stored in dark and room temperature, for several months and in a time schedule, and their UV–visible spectra were recorded in interval times to compare with previous results and investigate whether the LDENPs were agglomerated or not. Based on these experiments it was found that under the mentioned storing conditions, protected gold nanoparticles by PCA–PEG–PCA copolymers were stable for several months and agglomeration was not occurred. 4. Conclusion
Scheme 2. Assembly of nanoparticles through linear–dendritic copolymers to make a circle shape.
the size of AuNPs and the DP of poly(citric acid) parts of PCA–PEG–PCA copolymers, which was found by UV experiments, could be proved through comparison the TEM images of synthesized AuNPs by A1 and A3 (Figs. 5 and 7). As it can be seen the average sizes of AuNPs synthesized by A1 and A3 are around 10 and 5 nm, respectively.
PCA–PEG–PCA copolymers containing dendritic poly(citric acid) parts were used to synthesize and support AuNPs successfully. Loading capacity of PCA–PEG–PCA copolymers was depended on their concentration and DP of poly(citric acid) parts directly whereas size of nanoparticles depended on these factors inversely. PCA–PEG–PCA copolymers with low molecular weights were led to necklaces of AuNPs, whereas those with high molecular weight were led to dendritic shapes of AuNPs. Supported AuNPs
Fig. 7. TEM image of hyperbranched AuNPs synthesized using A3.
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Scheme 3. Assembly of nanoparticles through linear–dendritic copolymers to make a dendritic shape.
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were stable in water for several months and agglomeration was not occurred.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Synthesis of gold nanoparticle necklaces using linear–dendritic copolymers Ashkan Tavakoli Naeini a, Mohsen Adeli b,c,d,*, Manouchehr Vossoughi a,b,* a
Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran Institute for Nanoscience and Technology, Sharif University of Technology, Tehran, Iran c Department of Chemistry, Sharif University of Technology, Tehran, Iran d Department of Chemistry, Faculty of Science, Lorestan University, Khramabad, Iran b
i n f o
Article history: Received 14 April 2009 Received in revised form 11 October 2009 Accepted 18 October 2009 Available online 23 October 2009 Keywords: Linear–dendritic Gold nanoparticles Citric acid Loading capacity Nanoparticle necklaces
a b s t r a c t Linear–dendritic copolymers containing hyperbranched poly(citric acid) and linear poly(ethylene glycol) blocks (PCA–PEG–PCA) were used as reducing and capping agents to synthesize and support gold nanoparticles (AuNPs). PCA–PEG–PCA copolymers with 1758, 1889 and 3446 molecular weights, called A1, A2 and A3 through this work, respectively, were synthesized using 2, 5, and 10 citric acid/PEG molar ratios. The diameter of A1, A2 and A3 in a fresh water solution was investigated using dynamic light scattering (DLS) and it was between 1.8 and 2.8 nm. AuNPs were simply synthesized and supported by addition a boiling aqueous solution of HAuCl4 to aqueous solutions of A1, A2 and A3. Supported AuNPs were stable in water for several months and agglomeration was not occurred. The loading capacity of A1, A2 and A3 and the size of synthesized AuNPs were investigated using UV spectroscopy and transmission electron microscopy (TEM). It was found that the loading capacity of PCA–PEG–PCA copolymers depend on the concentration of copolymers and the size of their poly(citric acid) parts directly. For example average loading capacities for 400 lM concentration of A1, A2 and A3 were 32.24, 37.4 and 41.52 lM, respectively, and average loading capacities for 400, 200 and 100 lM concentration of A1 were 32.24, 20.28 and 9.1 lM, respectively. Interestingly there was a reverse relation between the size of synthesized AuNPs and size of poly(citric acid) parts of PCA–PEG–PCA copolymers. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Nanogold or gold nanoparticles (AuNPs) have been widely studied in the past 10 years because of their unique properties, such as catalysis, quantum size effect, and optical properties [1]. Due to the requirements for control of nanoparticle size and surface functionalization for this broad range of applications, different synthetic methods have been developed to generate monodisperse AuNPs. One of the most widely used methods is the reduction of
* Corresponding authors. Address: Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran. Tel.: +98 21 66165487 (M. Vossoughi); +98 916 3603772 (M. Adeli). E-mail addresses: [email protected], [email protected] (M. Adeli), [email protected] (M. Vossoughi). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.10.017
tetrachloroaurate ions AuCl4 in aqueous medium using sodium citrate to generate particles with diameters typically ranging from 10 to 100 nm [2]. Although this method has good control over producing a particular particle size, it is limited to the synthesis of larger particles. The Brust method and various modifications are useful for the generation of AuNPs having core sizes ranging from 1 to 4 nm [3–5]. In the Brust method, the transfer of AuCl4 into toluene or chloroform is performed using tetraalkylammonium bromide followed by reduction with sodium borohydride in the presence of alkylthiols. Disadvantages of this method include contamination of the synthesized particles with boride [6] and potential presence of impurities introduced by the use of capping ligands which also hinder the surface modification and functionalization of particles for particular applications. A few other single-
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step reduction processes also were developed to generate monodisperse gold nanoparticles, but these are limited to organic media [7–9]. In addition, the reduction of gold salt to form AuNPs using amine-containing organic molecules has been investigated [10–14], but most of the amine compounds used in those AuNPs syntheses are soluble in only organic solvents, and the reduction reactions must take place in an organic solvent or in a biphasic system. As a result, the nanoparticles prepared using those methods are not easily dispersed in aqueous solutions. To use the AuNPs in aqueous-based or biological systems, it is necessary to functionalize the particles with ligands which facilitate phase transfer from an organic to an aqueous medium. Alternative synthetic strategies based on using polymers as both the reducing and stabilizing agent for the generation of stable metal nanoparticles without the use of an additional stabilizing agent have been developed. The resulting nanoparticle-polymer composites have been shown to be useful in catalytic transformations [15] and they could be useful for nanostructured solar cells, photonic band gap materials, storage devices, and drug delivery. Some polymers can fulfill the required dual role as a reducer and stabilizer [16]. Examples include poly(methylhydrosiloxane) [17], poly(N-vinyl-2-pyrrolidone) [18], poly(sodium acrylate) [19], poly(ethylene oxide) [20], poly(vinyl alcohols) [21] and polyethylenimine [22]. However, even in the syntheses using those polymers, either the reduction reaction was carried out in organic solvents or polydisperse nanoparticles were produced. In both situations, the use of the nanoparticles is restricted, or at least complicated for aqueous-based (e.g., biological) applications or for applications that require monodisperse particles (e.g., electronics). The major advantage of using a polymer as a stabilizing agent is that it can be used to tailor the nanocomposite properties and also to provide longterm stability of the nanoparticles by preventing particle agglomeration [23–25]. Recently, the functionalized AuNPs were synthesized through different stabilizing and capping agents and showed the potential in several applications [26]. Although many block copolymers and dendrimers have been used as the stabilizing agents to synthesize AuNPs in the literatures [27], reports of the preparation and characterization of polymer–nanogold nanocomposites are occasional. In the literatures, many kinds of polymers such as poly(methylphenylphosphazene) (PMPP) [28], polyacrylamide [29], acrylic polymers [30–32], polyurethane (PU) [33], polystyrene (PS) [34], poly(ethylene glycol) (PEG) [35], poly(vinylphenol) (PVP) [36], poly(vinyl alcohol) (PVA) [37], poly(o-phenylenediamine) [38], poly(amic acid) (PAA) [39], polyimide (PI) [40], and polyfluorene (PF) [41–43] were used to prepare the polymer– AuNPs nanocomposite. Linear–dendritic macromolecules are hybrid large molecules containing dendrimers and linear polymers. Interesting properties of this type of dendritic macromolecules have stimulated investigation in this area [44–51]. Haba et al. reported preparation and encapsulation of gold nanoparticles using poly(ethylene glycol)-modified poly(amido amine) dendrimers. Encapsulated gold nanoparticles showed a heat-generating ability [52].
Synthesis of PCA–PEG–PCA linear–dendritic copolymers and their ability to encapsulate and release of organic guest molecules was reported previously [53–55]. Citric acid is a unique capping agent to cap and stabilize the metal nanoparticles such as gold nanoparticles, hence the PCA–PEG–PCA copolymers containing dendritic citric acid parts (which contain much more hydroxyl functional groups than citric acid molecule) and PEG as flexible part (which enhances the solubility) could be effective macromolecules to synthesize, protect, encapsulate and also stabilize AuNPs. On the other hand they are biocompatible materials and could be used to transport AuNPs in biological systems. In this work, PCA–PEG–PCA copolymers have been used to synthesize, encapsulate and stabilize AuNPs. The role of dendritic citric acid parts in the case of loading capacity and the size of AuNPs was investigated. 2. Experimental 2.1. Chemicals Citric acid (extra pure anhydrous powder), HAuCl4, 3H2O (99.5%, for analysis) and poly(ethylene glycol) (Mn = 1500) were purchased from Merck. Dialyze bag (a semipermeable membrane cut off 2000) was obtained from Sigma, Aldrich. 2.2. Instrumental measurements and preparation of samples for analysis The zeta potential and dynamic light scattering experiments were done by the commercially available instrument, Zetasizer, Nano series, ZEN 1600, laser 633 nm. Samples were dissolved in distilled water, and measurements were performed at 25 °C and started 10 min after the cuvette was placed in the DLS apparatus to allow the temperature to equilibrate. About 1 ml of the sample was transferred to a special dustfree light-scattering cell and the temperature was controlled within ±0.02 °C. Absorption spectra (300– 800 nm) were recorded using a Perkin–Elmer Lambda 900 UV–vis/NIR spectrophotometer. Samples of linear–dendritic encapsulated nanoparticles (LDENP) were prepared and left at room temperature overnight. Then they were transferred to the spectrophotometer cell and spectra were recorded at 25 °C. Calibration curve was used to determine the loading capacities. TEM photos were recorded using Philips CH 200, LaB6-Cathode 160 kv. 2.3. Synthesis of PCA–PEG–PCA linear–dendritic copolymers PCA–PEG–PCA linear–dendritic copolymers were synthesized according to the reported procedure in literature [56]. 2.4. Preparation of linear–dendritic encapsulated nanoparticles (LDENPs) Typically an aqueous solution of HAuCl4 with a constant concentration (100 lM) was added to aqueous solutions of PCA–PEG–PCA copolymers in different concentrations
(200, 100, 50, 25, 12.5 lM) in the boiling point. After 20 min the color of HAuCl4 solution was changed from clear pale yellow to red concerning the reduction of the gold nanoparticles and trapping by PCA–PEG–PCA copolymers. Finally, the obtained solutions were cooled and used for the UV–visible analysis and TEM. The synthesized LDENPs were also stored in dark glasses wrapped by aluminum foils at room temperature (25 °C) for further UV– visible analyses in a time schedule. 2.5. Determination of the loading capacity of PCA–PEG–PCA copolymers Loading capacity of different PCA–PEG–PCA copolymers was investigated using UV spectroscopy. The UV sepectra of aqueous solutions of AuNPs, prepared using different concentrations of HAuCl4 and citric acid as capping agent, were recorded and a calibration curve consisting concentration of HAuCl4 versus kmax was obtained. This calibration curve was used to determine the highest concentration of HAuCl4 which a certain concentration of PCA–PEG–PCA copolymers was able to change it to AuNPs. This concentration was considered as a loading capacity of copolymer. 3. Results and discussion PCA–PEG–PCA copolymers were synthesized through condensation of citric acid on hydroxyl functional groups of PEG. Degree of polymerization (DP) of poly(citric acid) parts was dominated by citric acid/PEG molar ratio. In this work citric acid/PEG = 2, 5 and 10 were used to synthesize PCA–PEG–PCA copolymers which are called A1, A2 and A3 through this work, respectively. The poly(citric acid) parts of the PCA–PEG–PCA copolymers were used to reduce gold cations and prepare AuNPs. Size of PCA–PEG–PCA copolymers in aqueous solution was determined using DLS experiments. Fig. 1 shows the
Fig. 1. DLS diagrams of A1, A2 and A3 linear–dendritic copolymers.
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DLS diagram of fresh water solution of A1, A2 and A3 copolymers. According to DLS diagrams the diameter of A1, A2 and A3 copolymers were 1.8, 2 and 2.8 nm, respectively. The small size of copolymers in aqueous solutions is assigned to their molecular self-assembly which has been explained previously [56]. Fig. 2 shows the color change of HAuCl4 solution during the reduction of gold salt and preparation of AuNPs in the presence of A1, A2, A3 and copolymers. The clear yellow color of the primary solution turned gradually to red after 20 min. Change in the color of solutions was assigned to the improvement of reaction and the preparation of the AuNPs. Size of metallic nanoparticles and loading capacities of PCA–PEG–PCA linear–dendritic copolymers was depended on the DP of poly(citric acid) parts of linear–dendritic copolymers. Dependence of loading capacities of copolymers on the DP of poly(citric acid) parts was investigated using UV experiments and results are shown in Fig. 3. Due to a direct relationship between the intensity of the maximum absorption of AuNPs and the DP of poly(citric acid) parts of PCA–PEG– PCA copolymers, it could be found that the loading capacity of these PCA–PEG–PCA copolymers is directly depend on the DP of poly(citric acid) parts. Fig. 3 shows the intensity of maximum wavelengths (kmax) of AuNPs versus the concentration of PCA–PEG–PCA copolymers for A1, A2 and A3 (concentration of HAuCl4 was constant in all experiments and equal to 100 lM). As it can be seen the intensity of kmax of synthesized AuNPs increases by increasing the concentration of PCA–PEG–PCA copolymers which proves a direct relationship between the loading capacities of PCA–PEG–PCA copolymers and their concentration. Table 1 shows maximum absorption and loading capacities of A1, A2 and A3 in a 400–25 lM concentration range. In this range, average loading capacities for A1, A2 and A3 were between 0.67–32.24, 1.94–37.4 and 2.14–41.52 lM, respectively. Fig. 4 shows the kmax of AuNPs versus concentration of PCA–PEG–PCA copolymers for A1, A2 and A3. Clearly kmax of the nanoparticles has a blue shift when the DP of
Fig. 2. The color change of the HAuCl4 solution during preparation of AuNPs (1) in absence of copolymers and in the presence of (2) A1, (3) A2 and (4) A3 linear–dendritic copolymers after 20 min.
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Fig. 3. Maximum absorption of encapsulated AuNPs by A1, A2 and A3 in five different concentrations (concentration of HAuCl4 was 100 lM).
Fig. 4. Maximum wavelenght of AuNPs loaded by A1, A2 and A3 in five different concentrations (concentration of HAuCl4 was 100 lM).
Table 1 Maximum absorption of AuNPs loaded by different PCA–PEG–PCA linear– dendritic copolymers (concentration of HAuCl4 was 100 lM) and loading capacities of A1, A2 and A3 in different conditions. AuNPs loaded (lM)
Concentration of linear– dendritic nanocarrier (lM)
Max absorption A1AuNPs
A2AuNPs
A3AuNPs
400 200 100 50 25
0.0541 0.0425 0.0317 0.0262 0.0187
0.0591 0.0457 0.0352 0.0281 0.0207
0.0631 32.247 37.401 41.526 0.0482 20.289 23.588 26.165 0.0381 9.175 12.763 15.752 0.0297 4.959 5.443 7.093 0.0218 0.670 1.948 2.144
A1
A2
A3
poly(citric acid) parts and also the concentration of PCA– PEG–PCA copolymers increase. This diagram shows that the size of AuNPs inversely depends on the DP of poly(citric acid) parts and the concentration of PCA–PEG–PCA copolymers. Size and morphology of AuNPs were investigated using TEM experiments. When low molecular weight PCA–PEG– PCA copolymers such as A1 were used, poly(citric acid) parts were so small that can cap only half of the surface of a AuNP and its other side capped by poly(citric acid) part of another molecule. This capping route led to necklaces AuNPs finally (Fig. 5 and Scheme 1). When two end nanoparticles of a AuNPs necklace are close enough, they can connect together by a PCA–PEG–PCA copolymer to make a AuNPs circle (Fig. 6 and Scheme 2). In the case of PCA–PEG–PCA copolymers with big poly(citric acid) parts such as A3, synthesized nanoparticles have a hyperbranched order. In this case citric acid parts are big enough to interact with two or more nanoparticles and make dendritic structures of nanoparticles (Fig. 7a and b and Scheme 3). Inverse relationship between
Fig. 5. TEM image of AuNPs necklaces synthesized using A1.
+
Scheme 1. Assembly of nanoparticles through linear–dendritic copolymers to make necklaces.
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Fig. 6. TEM image of AuNPs circles synthesized using A1.
+
Moreover solutions of LDENPs were stored in dark and room temperature, for several months and in a time schedule, and their UV–visible spectra were recorded in interval times to compare with previous results and investigate whether the LDENPs were agglomerated or not. Based on these experiments it was found that under the mentioned storing conditions, protected gold nanoparticles by PCA–PEG–PCA copolymers were stable for several months and agglomeration was not occurred. 4. Conclusion
Scheme 2. Assembly of nanoparticles through linear–dendritic copolymers to make a circle shape.
the size of AuNPs and the DP of poly(citric acid) parts of PCA–PEG–PCA copolymers, which was found by UV experiments, could be proved through comparison the TEM images of synthesized AuNPs by A1 and A3 (Figs. 5 and 7). As it can be seen the average sizes of AuNPs synthesized by A1 and A3 are around 10 and 5 nm, respectively.
PCA–PEG–PCA copolymers containing dendritic poly(citric acid) parts were used to synthesize and support AuNPs successfully. Loading capacity of PCA–PEG–PCA copolymers was depended on their concentration and DP of poly(citric acid) parts directly whereas size of nanoparticles depended on these factors inversely. PCA–PEG–PCA copolymers with low molecular weights were led to necklaces of AuNPs, whereas those with high molecular weight were led to dendritic shapes of AuNPs. Supported AuNPs
Fig. 7. TEM image of hyperbranched AuNPs synthesized using A3.
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Scheme 3. Assembly of nanoparticles through linear–dendritic copolymers to make a dendritic shape.
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were stable in water for several months and agglomeration was not occurred.
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