Langmuir 2001, 17, 6393-6395
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Fullerenethiolate-Functionalized Gold Nanoparticles: A New Class of Surface-Confined Metal-C60 Nanocomposites Hisashi Fujihara* and Hidetaka Nakai Department of Applied Chemistry, Kinki University, Kowakae, Higashi-Osaka 577-8502, Japan Received July 18, 2001 This paper reports the preparation and characteristic property of a new nanocomposite gold cluster with fullerene as a carbon nanocluster, C60-Au nanoparticles, together with the first account of electrochemical comparison of the C60-Au nanoparticle films and the self-assembled monolayers of C60-thiol 1 on gold electrodes. Octanethiol-stabilized gold nanoparticles were treated with a fullerene C60-terminated alkanethiol (1) to give new fullerenethiol-functionalized gold nanoparticles (C60-Au nanoparticles) of 2 nm diameter with a very narrow size distribution. Immersion of a gold electrode in a solution of C60-Au nanoparticles deposited the colloid films of them on the electrode surface which showed two distinct reversible reduction/ oxidation waves and significant electrochemical stability. The C60-Au nanoparticle films were firmly immobilized on the planar electrode by a simple self-assembly method without use of terminal ligand like bifunctional aminosiloxane or mercaptosiloxane. The electrochemical responses for the C60-Au nanoparticle films and self-assembled monolayers of C60-thiol 1 adsorbed on gold electrodes are significantly different.
There has been much recent interest in the synthesis and property of nanoscale materials such as functional fullerenes and metal nanoparticles.1,2 Fullerene-terminated alkanethiolates adsorb onto planar gold surfaces to form the self-assembled monolayers (SAMs) of fullerene C60.3 Most types of metal nanoparticles can be stabilized by a variety of organic materials; e.g., gold nanoparticles are stabilized by monolayers of alkanethiolate ligands.4,5 As such examples, we previously reported the remarkably stable self-assembled monolayers of a tetrathiol on a flat gold surface and the tetrathiafulvalenylthiol-stabilized gold nanoparticles as a metal cluster with an electron donor group.6 Recently, Brust and co-workers reported that an unsubstituted-fullerene mediates the aggregation of tetraoctylammonium bromide-stabilized gold particles.7 In contrast, surprisingly, we have now found that octanethiol-stabilized gold nanoparticles were treated with a fullerene C60-terminated alkanethiol (1) to give new fullerenethiol-functionalized gold nanoparticles (C60-Au nanoparticles, Chart 1) of 2 nm diameter with a very narrow size distribution which were strongly adsorbed onto a planar gold surface to form nanocomposite cluster films by a simple self-assembly method. The octanethiolstabilized gold nanoparticles did not aggregate upon addition of the fullerene C60-thiol 1. The C60-Au nanoparticle films adsorbed on a gold electrode show two * To whom correspondence may be addressed. FAX: +81-6-67274301. E-mail:
[email protected]. (1) (a) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1996. (b) For a review of fullerene films, see: Mirkin, C. A.; Caldwell, W. B. Tetrahedron 1996, 52, 5113. (c) Diederich, F.; Gomez-Lopez, M. Chem. Soc. Rev. 1999, 28, 263. (2) Schmid, G., Ed. Clusters and Colloids; VCH: Weinheim 1994. (3) (a) Shi, X.; Caldwell, W. B.; Chen, K.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 11598. (b) Imahori, H.; Azuma, T.; Ozawa, S.; Yamada, H.; Ushida, K.; Ajavakom, A.; Norieda, H.; Sakata, Y. J. Chem. Soc., Chem. Commun. 1999, 557. (4) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (5) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (6) (a) Fujihara, H.; Nakai, H.; Yoshihara, M.; Maeshima, T. J. Chem. Soc., Chem. Commun. 1999, 737. (b) Nakai, H.; Yoshihara, M.; Fujihara, H. Langmuir 1999, 15, 8574. (7) Brust, M.; Kiely, C. J.; Bethell, D.; Schiffrin. D. J. J. Am. Chem. Soc. 1998, 120, 12367.
Chart 1
distinct reversible reduction/oxidation peaks of C60 units and the significant electrochemical stability. Generally, the modification of flat solid surfaces with colloidal gold nanoparticles has been accomplished by the adsorption of the metal nanoparticles onto the substrates coated with bifunctional aminosiloxane or mercaptosiloxane as terminal ligand.8 To our knowledge, far less is known about the modification of electrode surfaces by colloidal gold nanoparticles without use of terminal ligands such as aminosiloxane. This paper presents the synthesis and characteristic property of a new nanocomposite metal cluster with fullerenethiolate ligand as a carbon nanocluster, C60-Au nanoparticles, together with the first account of electrochemical comparison of the SAMs of C60thiol 1 and the C60-Au nanoparticle films on gold electrodes. Fullerene C60-thiol 1 was prepared by the condensation of sarcosine (CH3NHCH2CO2H), 10-mercaptodecyl aldehyde (HS-(CH2)10-CHO),9 and C60.3,10 In a typical experiment, C60 (300 mg, 0.41 mmol), sarcosine (185 mg, 2.05 mmol), and 10-mercaptodecyl aldehyde (83 mg, 0.41 mmol) were dissolved in 900 mL of toluene and refluxed for 24 h under an Ar atmosphere. After usual workup, the residue was purified by preparative liquid chromatography to give the thiol 1. The structure of 1 was confirmed by high-resolution fast atom bombardment mass spectrom(8) The modification of flat solid surfaces with metal nanoparticles has received considerable current interest: (a) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (b) Musick, M. D.; Pena, D. J.; Botsko, S. L.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Langmuir 1999, 15, 844 and references therein. (9) 10-Mercaptodecyl aldehyde was prepared from the deprotection of the thioester group of 10-thioacetoxydecyl aldehyde which was obtained by the reaction of undecylenic aldehyde with thioacetic acid in the presence of AIBN under UV irradiation. (10) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115, 9798.
10.1021/la0111250 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/13/2001
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Figure 1. TEM micrograph of C60-Au nanoparticles.
etry (calcd for C73H28NS (MH+), 950.1942; found, 950.1974) and 1H NMR spectroscopy.11 New C60-Au nanoparticles were synthesized by the exchange reaction5 of octanethiol-derivatized gold nanoparticles (2.9 ( 0.9 nm)12 with C60-thiol 1. A solution of C60-thiol 1 (16.3 mg, 0.043 mmol) and octanethiolderivatized gold nanoparticles (100 mg) in CHCl3 (50 mL) was stirred for 48 h at room temperature. After the solution was stirred, the organic phase was evaporated to 10 mL in vacuo and mixed with EtOH (50 mL). The resulting precipitate was collected by filtration and washed serially with EtOH (200 mL) and acetone (100 mL). C60-Au nanoparticles obtained were very stable and soluble in CH2Cl2 and CHCl3. C60-Au nanoparticles were characterized by spectroscopic means. The UV-vis spectrum of the C60-Au nanoparticles solution in CHCl3 exhibited a plasmon resonance at 505 nm and characteristic peaks due to C60pyrrolidines at 435, 325, and 255 nm. Figure 1 shows the transmission electron microscopy (TEM) micrograph of C60-Au nanoparticles. The size range for C60-Au nanoparticles is found to be 2.0 ( 0.2 nm. The number of fullerenes attached per gold nanoparticle is seven units, which is determined from the 1H NMR spectral data. In contrast, tetraoctylammonium bromide-stabilized gold particles were aggregated by addition of fullerene.7 The electrochemical responses for C60-thiol 1, SAMs of 1, and C60-Au nanoparticles and their nanoparticle films (vide infra) on metal electrodes are significantly different as follows. The cyclic voltammetry of 1 in CH2Cl2-0.1 M Bu4NClO4 at a glassy carbon electrode exhibited two reversible reduction/oxidation peaks corresponding to C60/ C60•-/C602- system at E1/2 ) -0.87 and -1.26 V (vs Ag/0.1 M AgNO3) (Figure 2a).13 The C60-thiol 1 spontaneously self-assembles into monolayer films on a flat gold surface,14 in which the cyclic voltammogram for the SAMs of 1 showed one peak at E1/2 ) -0.97 V and a second peak at (11) 1: 1H NMR (CDCl3) δ 1.23-1.30 (m, 10H, CH2CH2CH2), 1.32 (t, 1H, J ) 7.6 Hz, SH), 1.47 (q, 2H, J ) 7.0 Hz, NCHCH2CH2CH2), 1.59 (q, 2H, J ) 7.0 Hz, CH2CH2SH), 1.90 (m, 2H, NCHCH2CH2), 2.37 and 2.52 (m, m, 1H, 1H, NCHCH2, diastereotopic), 2.51 (dt, 2H, J ) 7.6, 7.0 Hz, CH2SH), 2.98 (s, 3H, NCH3), 3.89 (t, 1H, J ) 5.5 Hz, NCH C60), 4.16 and 4.82 (d, d, 1H, 1H, J ) 9.5 Hz, NCH2C60, diastereotopic). (12) Octanethiol-derivatized gold nanoparticles were prepared according to the established method.4 (13) A third reversible electrochemical reduction/oxidation is accessible for 1 (E1/2 ) -1.84 V) and for C60-Au nanoparticles (E1/2 ) -1.80 V). The third peak is smaller than the first and the second peaks. The third peak in Figures 2 and 3 is omitted for clarity. (14) Immersion of a gold electrode in a solution of thiol 1 for 24 h followed by copious rinsing in CH2Cl2 led to the monolayers on gold electrode.
Letters
Figure 2. Cyclic voltammograms of (a) 1 at GC electrode in 0.1 M Bu4NClO4-CH2Cl2 (dotted curve) and (b) SAMs of 1 on Au electrode in 0.1 M Bu4NClO4-CH2Cl2 (solid curve). Scan rate was 100 mV s-1.
Figure 3. Cyclic voltammograms for the C60-Au nanoparticle films adsorbed onto a gold electrode in 0.1 M Bu4NClO4-CH2Cl2: (a) first scan; (b) multiple scans. Scan rate was 100 mV s-1.
E1/2 ) -1.28 V (vs Ag/0.1 M AgNO3), respectively (Figure 2b). The second peak of the SAMs of 1 is smaller than that of the first one, which is consistent with the previous reported data of C60-thiol monolayer modified gold electrodes.3 The cyclic voltammogram of C60-Au nanoparticles in CH2Cl2-0.1 M Bu4NClO4 at a glassy carbon electrode showed reversible redox peaks corresponding to a reduction/oxidation system at E1/2 ) -0.93 and -1.27 V (vs Ag/0.1 M AgNO3).13 Interestingly, when a clean gold electrode was soaked for 24 h in a solution of C60-Au nanoparticles in CHCl3,15 rinsed repeatedly with CHCl3, and inserted into a CH2Cl2-0.1 M Bu4NClO4 solution, two distinct reversible waves associated with fullerene reduction/oxidation at E1/2 ) -0.94 and -1.29 V (vs Ag/0.1 M AgNO3) were observed by cyclic voltammetry (Figure 3a). The potential difference between the anodic and cathodic peaks (∆E) was 15 mV at v ) 100 mV s-1, and the peak current increased linearly (15) C60-Au nanoparticles (5 mg) were completely dissolved in CHCl3 (10 mL).
Letters
with the scan rate. These two voltammetric features unequivocally indicate the surface-confined nature of the electroactive C60 groups. This contrasts with SAMs of a 1-modified gold electrode which showed the broad weak reduction/oxidation waves with large peak splittings (∆E ) 130 mV) in the cyclic voltammogram (Figure 2b).16 Significantly, the repeated electrochemical cycling of the C60-Au nanoparticle films adsorbed on gold electrode showed no diminution of the peak currents due to the C60 units, indicating that the C60-Au nanoparticle films adsorbed on the gold electrode were remarkably stable (Figure 3b). Thus, C60-Au nanoparticles are strongly immobilized on the electrode surface by the simple immersion technique without use of terminal ligands such as aminosiloxane and mercaptosiloxane.17 (16) This large peak splitting is also observed.3b (17) Fullerene itself did not immobilize onto a gold electrode by the immersion of the electrode in a fullerene solution as evidenced from the electrochemical study. (18) The SAMs of C60 onto aminopropylsilanized indium-tin-oxide surfaces and of C60 onto cysteamine-modified gold electrodes exhibit weak electrochemical responses: (a) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193. (b) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945.
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In summary, the ligand exchange reaction of octanethiol-derivatized gold nanoparticles with fullerene C60-thiol 1 gave new redox-active C60-Au nanoparticles of 2 nm diameter with a very narrow size distribution. The interfacial self-assembly of the C60-Au nanoparticles onto flat gold surface provides a new modified electrode with the nanocomposite metal cluster bearing an electronacceptor carbon nanocluster. The C60-Au nanoparticle films adsorbed on a gold electrode show two distinct reversible redox peaks and are considerably stable in organic solvent under repeated potential cycling. The electrochemical response of C60-Au nanoparticle films is incontrast to that of the SAMs of C60-thiol 1 which show the broad redox peaks.18 Experiments are underway to study the immobilization of C60-Au nanoparticles onto an electrode and to characterize these fascinating new nanocomposite materials. Acknowledgment. This work was supported in part by the Grant-in-Aid for Scientific Research Nos. 12042279 and 12640530 from the Ministry of Education, Science and Culture, Japan. LA0111250
