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Gold Nanoparticles with Cyclic Phenylazomethines: One-Pot Synthesis and Metal Ion Sensing Ryo Shomura,†,‡ Keum Jee Chung,§ Hideo Iwai,|| and Masayoshi Higuchi*,†,‡,^ †
Graduate School Pure and Applied Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8577, Japan International Center for Materials Nanoarchitectnics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan § Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea Materials Analysis Station, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan ^ JST-CREST, Japan
)
‡
bS Supporting Information ABSTRACT: New gold nanoparticles covered with cyclic phenylazomethine (CPA) were obtained by a one-pot synthesis. It is confirmed by XPS that imines of CPA in the nanoparticles (Au-CPA) are partially reduced to amines. The amine part of CPA in Au-CPA is attached to the surfaces of gold nanoparticles, and the imine part works as a redox-active site. A glassy carbon electrode modified with Au-CPA was revealed to work as an electrochemical probe for metal ion sensing.
G
old nanoparticles have received much attention for both fundamental research and actual applications. In particular, the surface modification of gold nanoparticles with functional organic molecules expands the possibility of applications as electronic materials, catalysts, and sensors.1 When gold nanoparticles are prepared during the reduction of gold(III) salts, thiols and amines are often added to the reaction solution to stabilize the formed gold nanoparticles.2 However, we noticed that gold(III) ions have a strong ability to coordinate to imine (azomethine) compounds, and the reduction of imines to amine in the presence of reductants is accelerated by the complexation. Therefore, it is expected that the unique, efficient synthesis of gold nanoparticles modified with amines proceeds via the reduction of the complex of gold(III) ions with imine (azomethine) compounds, but to our knowledge such investigations have not been reported. In our previous work we revealed that cyclicphenylazomethines (CPA, Figure 1) are selectively synthesized via the dehydration of diketones with diamines.3 Metal ions such as tin(II) are complexed stepwise to the imine site of CPA according to the different coordination ability among the imines.4 We herein report the one-pot synthesis of gold nanoparticles coated with CPA (Au-CPA) and their metal ion sensing properties. The synthesis of Au-CPA was performed by the two-phase reaction according to Brust’s procedure.5 Because CPA and HAuCl4 are soluble in toluene and water, respectively, the twophase reaction is possible. Au-CPA (the molar ratio of gold ion to CPA is 5:1) is synthesized as follows. HAuCl4 3 3H2O (39.4 mg, 0.1 mmol) and tetraoctylammonium bromide (109.4 mg, 0.2 mmol) were dissolved in a mixture of H2O (60 mL) and toluene (80 mL). CPA (17.9 mg, 0.02 mmol) in toluene was added to the r 2011 American Chemical Society
solution and stirred for 10 min at room temperature. The color of the organic layer turned orange because of the complexation of the gold(III) ion with CPA. Then, a freshly prepared aqueous solution (30 mL) of NaBH4 (37.8 mg, 1.0 mmol) was added to the vigorously stirred solution. After the addition of NaBH4, the color of the reaction mixture became red-violet immediately. The solution was stirred overnight. After the organic phase was extracted and dried in vacuo, Au-CPA (150.3 mg) was obtained. Imines of CPA complexed with a gold(III) ion are easily reduced by NaBH4 to amines. At the same time, gold(III) ions are also reduced by NaBH4 to form gold nanoparticles. A CPA molecule has four imines. When the imine is reduced to an amine, it attaches one gold nanoparticle. Therefore, four gold nanoparticles in the maximum can be combined by one CPA molecule through the AuN bond. The amount (150.3 mg) of Au-CPA was much larger than the sum of gold ions and CPA used, probably because many tetraoctylammonium bromide molecules are included in Au-CPA. Therefore, the gold nanoparticles are stabilized by both CPA and the ammonium salt. It is not clear how strongly the attached ammonium salts prevent the binding of metal ions with CPA, but the lowest concentration limit for the detection may change by using a different length of the ammonium salt. Au-CPAs with different molar ratios of gold ions and CPA (1:1, 10:1, and 50:1) were prepared using a similar synthesis procedure. Received: March 21, 2011 Revised: June 3, 2011 Published: June 06, 2011 7972
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Figure 1. Structure of CPA.
Scheme 1. Synthesis Scheme of CPA-Coated Gold Nanoparticlesa
a
Gold ions were reduced to form gold nanoparticles, and CPA was partially reduced.
The formation of gold nanoparticles was confirmed by UV vis spectral measurement (Supporting Information, Figure S1). Au-CPAs are soluble in toluene. In the spectrum of Au-CPA, a surface plasmon absorption based on the formation of gold nanoparticles and an absorption based on the nπ* transition of imines in CPA appear at 530 and 350 nm, respectively. The particle size of Au-CPAs was characterized by scanning electron microscopy (SEM) and dynamic light scattering (DLS). Spherical particles with 340 nm as the average diameter are confirmed by SEM (Figure 2). The particle size was controlled by changing the molar ratio of gold ions and CPA. Average diameters of the particles in which the molar ratios of gold ions to CPA are 1:1, 5:1, and 50:1 are 146, 371, and 478 nm, respectively, which were determined by DLS (Supporting Information, Figure S2). Using a larger amount of CPA is effective for obtaining smaller gold nanoparticles. This result supports that CPAs bearing an amino group strongly stabilize the gold nanoparticles. Gold nanoparticles without any organic protecting groups or with citrate as a hydrophilic group are soluble in water (Supporting Information, Figure S3). However, Au-CPA is insoluble in water but soluble in organic solvents such as CHCl3 because the surface of gold nanoparticles is covered with hydrophobic CPA molecules. As shown in Figure S4 (Supporting Information), simple addition of CPA to the solution of gold nanoparticles does not change the solubility of the gold nanoparticles because the coordination of the imines of CPA to gold nanoparticles is not strong. This result suggests that the amine part of CPA strongly coordinates to gold nanoparticles. The presence of amines in the CPA part of Au-CPA was confirmed using X-ray photoelectron spectroscopy (XPS) (Supporting Information, Figure S5). Ratios of amine (C-NH) to imine (CdN) are 19.4, 10.0 and 4.2% in AuCPAs with different molar ratios of gold ions and CPA (1:1, 10:1, and 50:1), respectively. The reason that the ratio of amine in AuCPA decreases when the ratio of gold ions for CPA increases is not clear, but the reduction of gold(III) ions may be much faster than the reduction of imines in the CPA complexed with gold(III) ions. Therefore, it is considered that a large amount of NaBH4 is consumed for the reduction of gold(III) ions when the number of gold(III) ions is much larger than that of CPA.
Figure 2. (a, b) SEM images of CPA-coated Au nanoparticles and (c) their particle sizes. (The ratio of gold to CPA is 5:1.)
CPA has two Z- and two E-conformational imines (Figure 1). Because the Z imines have a higher complexation ability to metal ions than the E imines, unique stepwise metal ion assembly is observed during the titration of metal ions to CPA.4 The reduction of imines of CPA by NaBH4 is slow, but the reduction speed is accelerated by complexation with metal ions. The formed amines work as the protecting group of gold nanoparticles and stabilize the nanoparticles.2a,b As a comparison, completely reduced CPA (red-CPA) was synthesized during the reduction by NaBH4 in the presence of trifluoroacetic acid (Supporting Information, Scheme S1). When red-CPA was added to a water solution of gold nanoparticles, a precipitate formed in the organic layer as shown in Figure S6 (Supporting Information). This result indicates that cross-linking among gold nanoparticles by red-CPA with four amines occurs. In our previous paper, we revealed that imines of CPA become redox-active after complexation with metal ions. In Au-CPA, only one or two Z imines of CPA are reduced to amines and the remaining imines are expected to show redox activity in the presence of metal ions. When gold nanoparticles are used as sensing materials, detection is generally done by optical analysis such as surface plasmon resonance (SPR). However, the SPR 7973
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Figure 3. (a) Cyclic voltammograms of Au-CPA (the molar ratio of gold salt to CPA is 1:1 in the synthesis) in the presence of 2 (- 3 3 -), 5 (---), and 10 mM SnCl2 (). (Electrolyte, 0.2 M TBABF4/acetonitrile; scan rate, 100 mV/s; electrode, GC electrode). (b) Nernst plots of SnCl2 3 2H2O (red), Zn(NO3)2 3 6H2O (blue), and CuBr (green). (The redox potentials were determined using differential pulse voltammetry (DPV). Electrolyte, 0.1 M TBABF4/acetonitrile; electrode, GC electrode).
band depends on the shape, the size, and the surrounding medium of the particle.6 In the case of metal ion sensing by Au-CPA, electrochemical detection is possible. The electrochemical sensing of ATP or oxoanions by gold nanoparticles coated by redox-active ligands has been reported by using ferrocene or tetrathiafulvalene.7 Because ferrocene and tetrathiafulvalene are already redox-active, the number of target molecules is estimated by the shift in the redox potential. In the case of Au-CPA, however, CPA itself is not redox-active but shows redox activity only in the presence of metal ions. Therefore, it is expected that a smaller number of metal ions is detectable. Au-CPA was cast on a glassy carbon electrode, and the film was used as a working electrode. The sensing measurement was made in acetonitrile. (CPA is soluble in acetonitrile, but Au-CPA is insoluble in acetonitrile.) We did not succeed in sensing in phosphate-buffered saline solution because tin(II) ions strongly bind the phosphate anions and form a precipitate in the solution. Therefore, we chose acetonitrile as an organic solvent. In the cyclic voltammograms, Au-CPA exhibited reversible redox waves in the presence of tin(II) ions (Figure 3a). In this sensing system, tin(II) ions behave as a Lewis acid and make CPA redox-active. This phenomenon is similar to proton acid doping to polyaniline. Tin(II) ions do not show any redox waves in the potential range of 0 to 0.8 V (Figure S7, Supporting Information). An expected redox mechanism of Au-CPA in the presence of tin(II) ions is shown in Scheme S2 (Supporting Information). We revealed that the relationship between the concentration of metal ions and the redox potential obeys the Nernst equation (Figure 3b). By using this equation, the concentration of the metal ion can be determined. An advantage of this sensing method is to be applied to various metal ions: an electrode modified with Au-CPA was able to detect not only tin(II) ions but also zinc(II) and copper(I) ions. The different metal species give different redox potentials of AuCPA (Figure 3b), probably because of the different binding affinity of metal ions to CPA. However, the Nernst plots have similar slopes, 68, 60, and 55 mV/log[M], for tin(II), zinc(II), and copper(I) ions, respectively, which means that the redox process includes a two-electron and a two-metal ion transfer (theoretical value: 59 mV/log[M]) regardless of the metal species. The lowest limit of the concentration for metal ion detection is 0.5 mM, which is less sensitive than the other reported method (with the lowest sensitivity limit on the order of nanomolar).8 However, this method is more convenient than the
other methods using a SAM film or electropolymerization because the electrode modified with Au-CPA can be easily prepared by simple casting of the Au-CPA solution. Au-CPA also has a potential to be applied to highly sensitive analytical methods such as microelectrodes.9 In conclusion, new gold nanoparticles modified with CPA (Au-CPA) were obtained by a one-pot synthesis. The particle size of Au-CPA was controlled by changing the molar ratio of gold ions and CPA in the synthesis. It is confirmed by XPS et al. that the imines of CPA in Au-CPA are partially reduced to amines. The amine part of CPA in Au-CPA is attached to the surface of a gold nanoparticle, and the imine part works as a redox-active site. A glassy carbon electrode modified with AuCPA was revealed to work as an electrochemical probe for the metal ion sensing of tin(II), zinc(II), and copper(I).
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details, spectral data, and photographic data of the assemblies. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: þ81-29-860-4744. Fax: þ81-29-860-4721.
’ ACKNOWLEDGMENT We thank Dr. Takeuchi and Dr. Sugiyasu for providing an electrochemical instrument. ’ REFERENCES (1) (a) Boettcher, S. W.; Strandwitz, N. C.; Schierhorn, M.; Lock, N.; Lonergan, M. C.; Stucky, G. D. Nat. Mater. 2007, 6, 592. (b) Azuma, Y.; Kanehara, M.; Teranishi, T.; Majima, Y. Phys. Rev. Lett. 2006, 96, 016108. (c) Abad, A.; Corma, A.; Garc’a, H. Chem.—Eur. J. 2008, 14, 212. (d) Kung, M. C.; Davis, R. J.; Kung, H. H. J. Phys. Chem. C 2007, 111, 11767. (e) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362. (f) Hughes, M. D.; Xu, Y. J.; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin, K.; Kiely, C. J.; Hughes, M. D. Nature 2005, 437, 1132. (g) Azuma, Y.; 7974
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