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A new and clean method on synthesis of gold nanoparticles from bulk gold substrates
Materials Chemistry and Physics 125 (2011) 109–112
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A new and clean method on synthesis of gold nanoparticles from bulk gold substrates Chung-Chin Yu a,e , Yu-Chuan Liu b,e,f,∗ , Kuang-Hsuan Yang b,e , Chia-Ching Li c,e , Cheng-Cai Wang d,e a
Department of Environmental Engineering, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan Department of Chemical and Materials Engineering, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan Department of Cosmetic Science, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan d Department of Tourism and Leisure Management, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan e Nano Materials Applications R&D Center, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan f Department of Biochemistry, School of Medicine, Taipei Medical University, Taipei, Taiwan b c
a r t i c l e
i n f o
Article history: Received 10 June 2009 Received in revised form 21 June 2010 Accepted 21 August 2010 Keywords: Gold nanoparticles Chitosan Electrochemical methods
a b s t r a c t In this work, we report a new pathway to prepare clean gold nanoparticles in neutral solutions with aid of natural chitosan. First, an Au substrate was cycled in a deoxygenated aqueous solution containing 0.1N NaCl and 1 g L−1 chitosan from −0.28 to +1.22 V vs. Ag/AgCl at 500 mV s−1 for 200 scans. The durations at the cathodic and anodic vertices are 10 and 5 s, respectively. After this process, positively charged Auand chitosan-containing complexes were produced in the solution. Then the solution was heated from room temperature to boiling at a heating rate of 6 ◦ C min−1 to prepare Au nanoparticles. The particle sizes of prepared Au (1 1 1) nanoparticles are ca. 10 nm. Moreover, the prepared Au nanoparticles in solutions are capable for anti-oxidation and stable in an ambient atmosphere for at least three months. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, noble metal nanoparticles in the most important types of nanomaterials, including gold, silver and platinum, have been extensively investigated [1,2] due to their unusual plasmonic [3], optical [4], chemical [5], photoelectrochemical [6], and electronic [7] properties. Moreover, metal/metal nanocomposites are synthesized to further improve their specific properties [8,9]. The number of potential applications for nanomaterials, especially in the field of proteins detection [10] and catalysts modification [11], is rapidly growing because of their unique electronic structure and extremely large surface areas. As shown in the literature, the developed methods for fabrications of noble metal nanoparticles include chemical reduction [12], sonochemical reduction [13], laser ablation [14], annealing from high-temperature solutions [15], metal evaporation [16], Ar+ ion sputtering [17], sonoelectrochemical reduction [18]. Meanwhile, some stabilizers, like sodium dodecyl sulfate [19], sugar ball [20] and poly(vinylpyrrolidone) [21] were used, and some stabilization technologies of thiol-ligand coatings [22] and polymer capping agents [23] were developed to prevent the prepared nanoparticles from aggregating.
∗ Corresponding author at: Department of Biochemistry, School of Medicine, Taipei Medical University, 250, Wu-Hsing Street, Taipei, Taiwan. Tel.: +886 2 2731661x3155; fax: +886 2 27356689. E-mail address: [email protected] (Y.-C. Liu). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.08.079
Tan et al. [24] reported a synthesis of positively charged silver nanoparticles via photoreduction of AgNO3 in branched polyethyleneimine/HEPES solution. Importantly, these positively charged Ag nanoparticles demonstrate superior surface-enhanced Raman scattering (SERS) activity over negatively charged Ag nanoparticles for the detection of a variety of negatively charged analytes in aqueous solutions. Irzh et al. [25] proposed a method to prepare microwave-assisted coating of poly(methyl methacrylate) PMMA beads by silver nanoparticles. Microwave irradiation was found to be a new technique for coating of Ag nanoparticles onto the surface of PMMA beads. This method was carried out under an argon atmosphere. Bimetallic alloy nanoparticles consisting of two noble metals Pt–Ag supported on carbon with a variable dimension were successfully prepared by ethylene glycol (EG), as reported by Hwang et al. [26]. This work highlighted the viability of EG synthesis methodology. The prepared nanoparticles have particle sizes ranging from 1.2 to 3.1 nm with 1:1 atomic composition but with different alloying extents by a simple control over the solution pH of the preparation medium. Brinas et al. [27] developed a method in preparing size-controllable Au nanoparticles capped with glutathione by varying the pH of the solution before reduction. This method is based on the formation of polymeric nanoparticle precursors, Au(I)–glutathione polymers, which change size and density depending on the pH. In our previous study [18], we developed a whole electrochemical method via aid of an ultrasonic cell disruptor to synthesize Au nanoparticles in solutions from bulk Au substrates without addi-
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tion of any stabilizer and reductant. However, the concentration of Au nanoparticles in solution is only ca. 3 ppm because most Au nanoparticles are redeposited on the working electrode. In this work, we report a new pathway to prepare clean Au nanoparticles with high concentration in solutions from bulk Au substrates via aid of natural chitosan. This methodology contains electrochemical and chemical techniques. 2. Experimental All of the electrochemical experiments were performed in a three-compartment cell at room temperature, 22 ◦ C, and were controlled by a potentiostat (model PGSTAT30, Eco Chemie). First, a sheet of gold with bare surface area of 4 cm2 , a sheet of 2 cm × 4 cm platinum, and a KCl-saturated silver–silver chloride (Ag/AgCl) rod were employed as the working, counter and reference electrodes, respectively. Before the oxidation–reduction cycle (ORC) treatment, the gold electrode was mechanically polished (model Minimet 1000, Buehler) successively with 1 and 0.05 m of alumina slurries to a mirror finish. Then the electrode was cycled in a deoxygenated aqueous solution of 40 mL containing 0.1N NaCl and 1 g L−1 chitosan from −0.28 to +1.22 V vs. Ag/AgCl at 500 mV s−1 for 200 scans under slight stirring. The durations at the cathodic and anodic vertices are 10 and 5 s, respectively. After this roughening procedure, positively charged Au- and chitosan-containing complexes were produced in the aqueous solution at pH 6.5. Immediately, without changing the electrolytes, the solution was heated from room temperature to boiling at a heating rate of 6 ◦ C min−1 in air to synthesize Au nanoparticles. After cooling, the clear Au nanoparticles-containing solution was separated from the settlement of chitosan. Then, this Au-containing solution was placed in an ultrasonic bath for 30 min and was further centrifuged at 3600 rpm for 2 min to remove chitosan for preparing pure Au nanoparticles in solution. In measurements, a single drop of the sample-containing solution was placed on a 300-mesh Cu/carbon film transmission electron microscopy (TEM) sample grid and was allowed to be dried in a vacuum oven. Then the sample was examined using a Philips Tecnai G2 F20 electron microscope with an acceleration voltage of 200 kV. Ultraviolet–visible absorption spectroscopic measurements were carried out on a Perkin-Elmer Lambda 35 spectrophotometer in 1 cm quartz curvettes. For highresolution X-ray photoelectron spectroscopy (HRXPS) measurements, a ULVAC PHI Quantera SXM spectrometer with monochromatized Al K␣ radiation, 15 kV and 25 W, and an energy resolution of 0.1 eV was used. To compensate for surface charging effects, all HRXPS spectra are referred to the C 1s neutral carbon peak at 284.8 eV.
3. Results and discussion As we know, chitosan is a ubiquitous biopolymer. It is a linear polysaccharide obtained by deacetylation of chitin. By replacing the majority of aminoacetyl groups in chitin by the amine moieties chitosan molecules can be dissolved in acidic water. In such solution, chitosan is a positively charged polyelectrolyte due to the protonation of the amine groups. Chitosan has biological characteristics and physicochemical properties. This makes it attractive for many potential applications in food, pharmaceutical and cosmetic industries, and bioengineering [28]. In this work, chitosan was added in a neutral 0.1N NaCl solution to avoid strong attraction between positively charged chitosan and negatively charged AuCl4 − produced in the solution after the ORC treatment. When the chitosan was dissolved in 0.1N HCl solutions, the finally prepared Au nanoparticles in solutions were too dilute and they were difficult to be separated from chitosan, as discussed below. In the ORC treatment, the chloride electrolyte was selected since this facilitates the metal dissolution–deposition process that is known to produce SERS-active roughened surfaces [29]. Fig. 1 shows the results of the 100th scan of cyclic voltammograms for the dissolution and redeposition of Au substrates in 0.1N NaCl solutions with and without the addition of chitosan. Basically, these I–E curves are quite similar, but the area of the cathodic peak is decreased due to the addition of chitosan, as demonstrated in curve b. It indicates that the presence of chitosan is unfavorable for the redeposition of Au nanoparticles on Au substrates. As shown in our previous study [30], complexes of AuCl4 − would be present in the solution after the ORC procedure for roughening the Au substrate. Then AuCl4 − can be adsorbed on the nitrogen of chitosan following the mode
Fig. 1. Cyclic voltammograms at 500 mV s−1 of the 100th scan for electrochemically roughened gold substrates in different solutions: (a) 0.1N NaCl; (b) 0.1N NaCl containing 1 g L−1 chitosan.
of self-assembled monolayers (SAMs) [31,32]. As expected, white chitosan powders changed to yellow ones after the ORC procedure. It reveals that AuCl4 − is readily adsorbed on chitosan as a precursor for the subsequent preparation of red Au nanoparticles in solutions. As shown in spectra (a) and (b) of Fig. 2, the absorbance maxima of AuCl4 − appear approximately at 311 and 313 nm for prepared AuCl4 − in solutions [30] with and without chitosan, which are markedly different from those of zerovalent Au nanoparticles located at ca. 520 nm [33]. These results indicate that chitosan is not a necessity for preparing AuCl4 − in solutions via the ORC procedure. However, it plays an important role in the subsequent preparation of zerovalent Au nanoparticles in solutions. As shown in spectrum (a) of Fig. 3, after heating the AuCl4 − -containing solution without the chitosan for obtaining zerovalent Au nanoparticles, the
Fig. 2. UV–vis spectra of Au-containing complexes in solutions after roughing the Au substrates in different solutions: (a) 0.1N NaCl; (b) 0.1N NaCl containing 1 g L−1 chitosan.
C.-C. Yu et al. / Materials Chemistry and Physics 125 (2011) 109–112
Fig. 3. UV–vis spectra of Au-containing colloidal solutions after reducing the positively charged Au-containing complexes in solutions prepared in different solutions: (a) 0.1N NaCl; (b) 0.1N NaCl containing 1 g L−1 chitosan.
absorbance maximum of the solution still appears approximately at 313 nm. Also, the solution color maintains yellow after heating. These phenomena reveal that zerovalent Au nanoparticles are difficultly obtained in the absence of chitosan. Encouragingly, as shown in spectrum (b) of Fig. 3, after heating the AuCl4 − -containing solution in the presence of chitosan, the absorbance maximum of the solution changes from at 311 to at 522 nm. Also, the solution color changes from yellow to red after heating. These results indicate that zerovalent Au nanoparticles are successfully synthesized [33]. Meanwhile, yellow chitosan powders also change into red ones after heating due to the adsorption of Au nanoparticles. Further inductively coupled plasma-mass spectrometer (ICP-MS) analysis indicates that the concentration of the synthesized zerovalent Au nanoparticles in solutions is ca. 60 ppm. Other metal elements, excepting Na, are not detected. The prepared Au nanoparticles are stable since no aggregation of Au nanoparticles is observed in an ambient atmosphere for at least three months. We also tried to use solutions containing 0.1N HCl and 1 g L−1 chitosan and follow the same procedures discussed above to synthesize Au nanoparticles, but it failed. We think protonated chitosan in acid solutions is unsuitable for the preparation of Au nanoparticles proposed in this work. The dispersion and the particle size of prepared Au nanoparticles in solutions are examined using the TEM image, as shown in Fig. 4. The nanoparticles with a mean diameter of ca. 10 nm demonstrate no aggregation and fairly even dispersion. Fig. 5 demonstrates the high-resolution TEM image to show the moiré patterns of the Au nanoparticles. It exhibits a one-dimensional fringe lattice due to moiré interference, indicating that these nanoparticles are crystalline [34]. Also (1 1 1) lattice fringes with an interplanar spacing of 2.36 A˚ were measured for all lattice planes. This is reasonable since the low index plane (1 1 1) has the lowest surface energy [35]. In this work, we aim to prepare pure and clean Au nanoparticles. Thus the additives of chitosan should be completely separated from the prepared Au nanoparticles. Here we use HRXPS to explicate this issue. Chitosan is composed of H, C, N and O elements. Fig. 6 shows the HRXPS survey spectrum of the prepared Au nanoparticles with the aid of chitosan. There is no N signal (one main component in chitosan). It confirms that the prepared Au
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Fig. 4. TEM micrograph of prepared Au nanoparticles with the aid of chitosan, showing size and dispersion; scale of nano-bar being 20 nm.
nanoparticles are successfully separated from the additives of chitosan. The C and O signals shown in Fig. 6 are unavoidable from contaminant even though samples do not contain components of carbon and oxygen, which always happen in HRXPS experiments. Moreover, the prepared Au nanoparticles in solutions are confirmed to be capable for anti-oxidation in a normal biological test based on the absorption value of prussian blue at 700 nm, which is generally used in the literature [36]. The capability for anti-oxidation is evaluated by comparing the Au nanoparticlescontaining sample solution with a fixed concentration of vitamin C, which can protect prussian blue from oxidation. Experimental results demonstrate that the capability of anti-oxidation for 1 ppm Au nanoparticles is equivalent to that of 0.12 ppm vitamin C. Generally, chitosan is a kind of cationic surfactant in acidic media for
Fig. 5. High-resolution TEM micrograph of prepared Au nanoparticles with the aid ˚ of chitosan, showing the (1 1 1) lattice fringes with an interplanar spacing of 2.36 A; scale of nano-bar being 2 nm.
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Acknowledgement The authors thank the National Science Council of the Republic of China (NSC-97-2622-E-238-008-CC3) for its financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Fig. 6. HRXPS survey spectrum of prepared Au nanoparticles with the aid of chitosan.
biomedical applications. In this work, the new pathway to prepare pure Au nanoparticles with high concentration via the aid of nonprotonated chitosan is interesting. Chitosan play a role of reducing and capping agents. It works like a medium for the adsorbed positively charged Au complexes to further reduce to zerovalent Au nanoparticles in solutions under heating. Detailed mechanism for this new pathway is underway. 4. Conclusions We have developed a new pathway to synthesize pure (1 1 1) Au nanoparticles of ca. 10 nm in diameter in neutral aqueous solutions from bulk Au substrates via the aid of chitosan. The prepared Au nanoparticles in solutions are confirmed to be capable for antioxidation via a biological test. Moreover, they are stable in an ambient atmosphere for at least three months. The study on the potential application of this methodology to other noble metals and the detailed mechanisms are currently underway.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A new and clean method on synthesis of gold nanoparticles from bulk gold substrates Chung-Chin Yu a,e , Yu-Chuan Liu b,e,f,∗ , Kuang-Hsuan Yang b,e , Chia-Ching Li c,e , Cheng-Cai Wang d,e a
Department of Environmental Engineering, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan Department of Chemical and Materials Engineering, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan Department of Cosmetic Science, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan d Department of Tourism and Leisure Management, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan e Nano Materials Applications R&D Center, Vanung University, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan f Department of Biochemistry, School of Medicine, Taipei Medical University, Taipei, Taiwan b c
a r t i c l e
i n f o
Article history: Received 10 June 2009 Received in revised form 21 June 2010 Accepted 21 August 2010 Keywords: Gold nanoparticles Chitosan Electrochemical methods
a b s t r a c t In this work, we report a new pathway to prepare clean gold nanoparticles in neutral solutions with aid of natural chitosan. First, an Au substrate was cycled in a deoxygenated aqueous solution containing 0.1N NaCl and 1 g L−1 chitosan from −0.28 to +1.22 V vs. Ag/AgCl at 500 mV s−1 for 200 scans. The durations at the cathodic and anodic vertices are 10 and 5 s, respectively. After this process, positively charged Auand chitosan-containing complexes were produced in the solution. Then the solution was heated from room temperature to boiling at a heating rate of 6 ◦ C min−1 to prepare Au nanoparticles. The particle sizes of prepared Au (1 1 1) nanoparticles are ca. 10 nm. Moreover, the prepared Au nanoparticles in solutions are capable for anti-oxidation and stable in an ambient atmosphere for at least three months. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, noble metal nanoparticles in the most important types of nanomaterials, including gold, silver and platinum, have been extensively investigated [1,2] due to their unusual plasmonic [3], optical [4], chemical [5], photoelectrochemical [6], and electronic [7] properties. Moreover, metal/metal nanocomposites are synthesized to further improve their specific properties [8,9]. The number of potential applications for nanomaterials, especially in the field of proteins detection [10] and catalysts modification [11], is rapidly growing because of their unique electronic structure and extremely large surface areas. As shown in the literature, the developed methods for fabrications of noble metal nanoparticles include chemical reduction [12], sonochemical reduction [13], laser ablation [14], annealing from high-temperature solutions [15], metal evaporation [16], Ar+ ion sputtering [17], sonoelectrochemical reduction [18]. Meanwhile, some stabilizers, like sodium dodecyl sulfate [19], sugar ball [20] and poly(vinylpyrrolidone) [21] were used, and some stabilization technologies of thiol-ligand coatings [22] and polymer capping agents [23] were developed to prevent the prepared nanoparticles from aggregating.
∗ Corresponding author at: Department of Biochemistry, School of Medicine, Taipei Medical University, 250, Wu-Hsing Street, Taipei, Taiwan. Tel.: +886 2 2731661x3155; fax: +886 2 27356689. E-mail address: [email protected] (Y.-C. Liu). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.08.079
Tan et al. [24] reported a synthesis of positively charged silver nanoparticles via photoreduction of AgNO3 in branched polyethyleneimine/HEPES solution. Importantly, these positively charged Ag nanoparticles demonstrate superior surface-enhanced Raman scattering (SERS) activity over negatively charged Ag nanoparticles for the detection of a variety of negatively charged analytes in aqueous solutions. Irzh et al. [25] proposed a method to prepare microwave-assisted coating of poly(methyl methacrylate) PMMA beads by silver nanoparticles. Microwave irradiation was found to be a new technique for coating of Ag nanoparticles onto the surface of PMMA beads. This method was carried out under an argon atmosphere. Bimetallic alloy nanoparticles consisting of two noble metals Pt–Ag supported on carbon with a variable dimension were successfully prepared by ethylene glycol (EG), as reported by Hwang et al. [26]. This work highlighted the viability of EG synthesis methodology. The prepared nanoparticles have particle sizes ranging from 1.2 to 3.1 nm with 1:1 atomic composition but with different alloying extents by a simple control over the solution pH of the preparation medium. Brinas et al. [27] developed a method in preparing size-controllable Au nanoparticles capped with glutathione by varying the pH of the solution before reduction. This method is based on the formation of polymeric nanoparticle precursors, Au(I)–glutathione polymers, which change size and density depending on the pH. In our previous study [18], we developed a whole electrochemical method via aid of an ultrasonic cell disruptor to synthesize Au nanoparticles in solutions from bulk Au substrates without addi-
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tion of any stabilizer and reductant. However, the concentration of Au nanoparticles in solution is only ca. 3 ppm because most Au nanoparticles are redeposited on the working electrode. In this work, we report a new pathway to prepare clean Au nanoparticles with high concentration in solutions from bulk Au substrates via aid of natural chitosan. This methodology contains electrochemical and chemical techniques. 2. Experimental All of the electrochemical experiments were performed in a three-compartment cell at room temperature, 22 ◦ C, and were controlled by a potentiostat (model PGSTAT30, Eco Chemie). First, a sheet of gold with bare surface area of 4 cm2 , a sheet of 2 cm × 4 cm platinum, and a KCl-saturated silver–silver chloride (Ag/AgCl) rod were employed as the working, counter and reference electrodes, respectively. Before the oxidation–reduction cycle (ORC) treatment, the gold electrode was mechanically polished (model Minimet 1000, Buehler) successively with 1 and 0.05 m of alumina slurries to a mirror finish. Then the electrode was cycled in a deoxygenated aqueous solution of 40 mL containing 0.1N NaCl and 1 g L−1 chitosan from −0.28 to +1.22 V vs. Ag/AgCl at 500 mV s−1 for 200 scans under slight stirring. The durations at the cathodic and anodic vertices are 10 and 5 s, respectively. After this roughening procedure, positively charged Au- and chitosan-containing complexes were produced in the aqueous solution at pH 6.5. Immediately, without changing the electrolytes, the solution was heated from room temperature to boiling at a heating rate of 6 ◦ C min−1 in air to synthesize Au nanoparticles. After cooling, the clear Au nanoparticles-containing solution was separated from the settlement of chitosan. Then, this Au-containing solution was placed in an ultrasonic bath for 30 min and was further centrifuged at 3600 rpm for 2 min to remove chitosan for preparing pure Au nanoparticles in solution. In measurements, a single drop of the sample-containing solution was placed on a 300-mesh Cu/carbon film transmission electron microscopy (TEM) sample grid and was allowed to be dried in a vacuum oven. Then the sample was examined using a Philips Tecnai G2 F20 electron microscope with an acceleration voltage of 200 kV. Ultraviolet–visible absorption spectroscopic measurements were carried out on a Perkin-Elmer Lambda 35 spectrophotometer in 1 cm quartz curvettes. For highresolution X-ray photoelectron spectroscopy (HRXPS) measurements, a ULVAC PHI Quantera SXM spectrometer with monochromatized Al K␣ radiation, 15 kV and 25 W, and an energy resolution of 0.1 eV was used. To compensate for surface charging effects, all HRXPS spectra are referred to the C 1s neutral carbon peak at 284.8 eV.
3. Results and discussion As we know, chitosan is a ubiquitous biopolymer. It is a linear polysaccharide obtained by deacetylation of chitin. By replacing the majority of aminoacetyl groups in chitin by the amine moieties chitosan molecules can be dissolved in acidic water. In such solution, chitosan is a positively charged polyelectrolyte due to the protonation of the amine groups. Chitosan has biological characteristics and physicochemical properties. This makes it attractive for many potential applications in food, pharmaceutical and cosmetic industries, and bioengineering [28]. In this work, chitosan was added in a neutral 0.1N NaCl solution to avoid strong attraction between positively charged chitosan and negatively charged AuCl4 − produced in the solution after the ORC treatment. When the chitosan was dissolved in 0.1N HCl solutions, the finally prepared Au nanoparticles in solutions were too dilute and they were difficult to be separated from chitosan, as discussed below. In the ORC treatment, the chloride electrolyte was selected since this facilitates the metal dissolution–deposition process that is known to produce SERS-active roughened surfaces [29]. Fig. 1 shows the results of the 100th scan of cyclic voltammograms for the dissolution and redeposition of Au substrates in 0.1N NaCl solutions with and without the addition of chitosan. Basically, these I–E curves are quite similar, but the area of the cathodic peak is decreased due to the addition of chitosan, as demonstrated in curve b. It indicates that the presence of chitosan is unfavorable for the redeposition of Au nanoparticles on Au substrates. As shown in our previous study [30], complexes of AuCl4 − would be present in the solution after the ORC procedure for roughening the Au substrate. Then AuCl4 − can be adsorbed on the nitrogen of chitosan following the mode
Fig. 1. Cyclic voltammograms at 500 mV s−1 of the 100th scan for electrochemically roughened gold substrates in different solutions: (a) 0.1N NaCl; (b) 0.1N NaCl containing 1 g L−1 chitosan.
of self-assembled monolayers (SAMs) [31,32]. As expected, white chitosan powders changed to yellow ones after the ORC procedure. It reveals that AuCl4 − is readily adsorbed on chitosan as a precursor for the subsequent preparation of red Au nanoparticles in solutions. As shown in spectra (a) and (b) of Fig. 2, the absorbance maxima of AuCl4 − appear approximately at 311 and 313 nm for prepared AuCl4 − in solutions [30] with and without chitosan, which are markedly different from those of zerovalent Au nanoparticles located at ca. 520 nm [33]. These results indicate that chitosan is not a necessity for preparing AuCl4 − in solutions via the ORC procedure. However, it plays an important role in the subsequent preparation of zerovalent Au nanoparticles in solutions. As shown in spectrum (a) of Fig. 3, after heating the AuCl4 − -containing solution without the chitosan for obtaining zerovalent Au nanoparticles, the
Fig. 2. UV–vis spectra of Au-containing complexes in solutions after roughing the Au substrates in different solutions: (a) 0.1N NaCl; (b) 0.1N NaCl containing 1 g L−1 chitosan.
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Fig. 3. UV–vis spectra of Au-containing colloidal solutions after reducing the positively charged Au-containing complexes in solutions prepared in different solutions: (a) 0.1N NaCl; (b) 0.1N NaCl containing 1 g L−1 chitosan.
absorbance maximum of the solution still appears approximately at 313 nm. Also, the solution color maintains yellow after heating. These phenomena reveal that zerovalent Au nanoparticles are difficultly obtained in the absence of chitosan. Encouragingly, as shown in spectrum (b) of Fig. 3, after heating the AuCl4 − -containing solution in the presence of chitosan, the absorbance maximum of the solution changes from at 311 to at 522 nm. Also, the solution color changes from yellow to red after heating. These results indicate that zerovalent Au nanoparticles are successfully synthesized [33]. Meanwhile, yellow chitosan powders also change into red ones after heating due to the adsorption of Au nanoparticles. Further inductively coupled plasma-mass spectrometer (ICP-MS) analysis indicates that the concentration of the synthesized zerovalent Au nanoparticles in solutions is ca. 60 ppm. Other metal elements, excepting Na, are not detected. The prepared Au nanoparticles are stable since no aggregation of Au nanoparticles is observed in an ambient atmosphere for at least three months. We also tried to use solutions containing 0.1N HCl and 1 g L−1 chitosan and follow the same procedures discussed above to synthesize Au nanoparticles, but it failed. We think protonated chitosan in acid solutions is unsuitable for the preparation of Au nanoparticles proposed in this work. The dispersion and the particle size of prepared Au nanoparticles in solutions are examined using the TEM image, as shown in Fig. 4. The nanoparticles with a mean diameter of ca. 10 nm demonstrate no aggregation and fairly even dispersion. Fig. 5 demonstrates the high-resolution TEM image to show the moiré patterns of the Au nanoparticles. It exhibits a one-dimensional fringe lattice due to moiré interference, indicating that these nanoparticles are crystalline [34]. Also (1 1 1) lattice fringes with an interplanar spacing of 2.36 A˚ were measured for all lattice planes. This is reasonable since the low index plane (1 1 1) has the lowest surface energy [35]. In this work, we aim to prepare pure and clean Au nanoparticles. Thus the additives of chitosan should be completely separated from the prepared Au nanoparticles. Here we use HRXPS to explicate this issue. Chitosan is composed of H, C, N and O elements. Fig. 6 shows the HRXPS survey spectrum of the prepared Au nanoparticles with the aid of chitosan. There is no N signal (one main component in chitosan). It confirms that the prepared Au
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Fig. 4. TEM micrograph of prepared Au nanoparticles with the aid of chitosan, showing size and dispersion; scale of nano-bar being 20 nm.
nanoparticles are successfully separated from the additives of chitosan. The C and O signals shown in Fig. 6 are unavoidable from contaminant even though samples do not contain components of carbon and oxygen, which always happen in HRXPS experiments. Moreover, the prepared Au nanoparticles in solutions are confirmed to be capable for anti-oxidation in a normal biological test based on the absorption value of prussian blue at 700 nm, which is generally used in the literature [36]. The capability for anti-oxidation is evaluated by comparing the Au nanoparticlescontaining sample solution with a fixed concentration of vitamin C, which can protect prussian blue from oxidation. Experimental results demonstrate that the capability of anti-oxidation for 1 ppm Au nanoparticles is equivalent to that of 0.12 ppm vitamin C. Generally, chitosan is a kind of cationic surfactant in acidic media for
Fig. 5. High-resolution TEM micrograph of prepared Au nanoparticles with the aid ˚ of chitosan, showing the (1 1 1) lattice fringes with an interplanar spacing of 2.36 A; scale of nano-bar being 2 nm.
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Acknowledgement The authors thank the National Science Council of the Republic of China (NSC-97-2622-E-238-008-CC3) for its financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Fig. 6. HRXPS survey spectrum of prepared Au nanoparticles with the aid of chitosan.
biomedical applications. In this work, the new pathway to prepare pure Au nanoparticles with high concentration via the aid of nonprotonated chitosan is interesting. Chitosan play a role of reducing and capping agents. It works like a medium for the adsorbed positively charged Au complexes to further reduce to zerovalent Au nanoparticles in solutions under heating. Detailed mechanism for this new pathway is underway. 4. Conclusions We have developed a new pathway to synthesize pure (1 1 1) Au nanoparticles of ca. 10 nm in diameter in neutral aqueous solutions from bulk Au substrates via the aid of chitosan. The prepared Au nanoparticles in solutions are confirmed to be capable for antioxidation via a biological test. Moreover, they are stable in an ambient atmosphere for at least three months. The study on the potential application of this methodology to other noble metals and the detailed mechanisms are currently underway.
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