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Quenching and enhancement of fluorescence of fullerene molecules on gold particle
Chemical Physics 323 (2006) 169–172 www.elsevier.com/locate/chemphys
Quenching and enhancement of fluorescence of fullerene molecules on gold particle Yan Zhao b
a,b
, Yijian Jiang
a,*
, Yan Fang
b
a National Center of Laser Technology, Beijing University of Technology, Beijing 100022, PR China Beijing Key Lab for Nano-Photonics and Nano-Structure, Department of Physics, Capital Normal University, Beijing 100037, PR China
Received 12 June 2005; accepted 10 September 2005 Available online 9 March 2006
Abstract The fluorescence spectra of fullerene were first observed to change greatly in the system of C60/C70-pyridine-gold hydrosol, due to the adsorption of the fullerene molecules on the gold nanoparticles. The energy transfers between C60 molecules and gold nanoparticles, leading to the quenching and enhancement of fluorescence bands centered at 450 and 700 nm, respectively. The fluorescence bands in the range from 550 to 800 nm of C70 are enhanced, arising from the increased local field intensity through the excitation of surface plasmon resonance of gold nanoparticles. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Quench; Enhancement; Fluorescence; Fullerene
1. Introduction One of the most important phenomenon accompanied surface-enhanced Raman scattering (SERS) is the quenching and enhancement of fluorescence of adsorbed molecule, for it can directly reveal the physical and chemical profound significance of the interface, and it is also important for understanding the adsorption and SERS enhancement mechanism of the molecule. However, till now, the quenching and enhancement of fluorescence are usually studied in the different molecule systems due to the adsorption substrate, which results in the difficulty for the further theoretical research [1]. Therefore, it is necessary to develop a method for the research in the same molecule system to understand the energy transfer and interface property. Obviously, the key to the observation of fluorescence quenching and enhancement in the same one molecule system is that this molecule has rich fluorescence peaks. It is well known that fullerene has the potential of being a new photosensitive and luminescent material [2–9]. We have *
Corresponding author. Tel.: +86 10 6739 2756; fax: +86 10 6739 1421. E-mail addresses: [email protected], [email protected] (Y. Jiang).
0301-0104/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.09.034
reported well-resolved abundant fluorescence peaks for C60 dissolved in pyridine in the range from 399 to 750 nm at room temperature, resulting from the reduction in high symmetry of C60 molecules [10]. This provides the promise and possibility for observation of fluorescence quenching and enhancement in the same one molecule system. In this paper, the gold nanoparticle is chosen as the SERS substrate. The C60 fluorescence bands centered at 450 nm and at 700 nm were quenched and enhanced, respectively, in C60-pyridine solution. The whole fluorescence peaks of C70 molecule in the range from 550 to 800 nm were enhanced in C70-pyridine solution. Combined with the SERS of fullerene in gold hydrosol, the tentative explanation is given to the changes of fluorescence bands. 2. Experimental C60 (99.9%), C70 (99%) were dissolved in pyridine (HPLC grade), forming the brown solution and amaranth solution, respectively. Preparation of gold colloid: 75 mg KAuCl4 was dissolved in 550 ml deionized water and the solution was heated to boiling. Then 9 ml of a 1% aqueous solution of
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Y. Zhao et al. / Chemical Physics 323 (2006) 169–172
trisodium citrate was added into the boiling solution, accompanied with vigorous stirring. The mixed solution was kept boiling till it reached 200 ml. The fluorescence spectra were recorded on Fluorolog-3 spectrometer. The SERS spectra were obtained using the FT–IR spectrometer (RFS 100/s, Bruker) and UV–vis spectrometer (RM-2000, Renishaw). All the measurements were carried out at room temperature. 3. Results and discussion Fig. 1(a) exhibits the fluorescence spectrum of C60 in C60-pyridine solution, excited at 380 nm. There are three broad regions of fluorescence bands, centered at 425, 575 and 700 nm, respectively. Fig. 1(b) shows the fluorescence spectrum of C60 in C60-pyridine solution mixed with gold hydrosol. Compared with Fig. 1(a), it can be observed from Fig. 1(b) that the intensity of fluorescence band centered at 450 nm reduced greatly and almost was unobservable, on the contrary, the fluorescence band centered at 700 nm is enhanced. In C60-pyridine solution, interaction between pyridine and C60 distorts the icosahedral symmetry of C60. The relaxation of selection rules due to the reduced symmetry of C60 molecule induces the observed rich fluorescence peaks [10]. As far as the assignment of the fluorescence peaks are concerned, after repetitious experiments, we inferred that the fluorescence band centered at 450 nm was assigned to completely soluble C60 molecules in the C60pyridine solution, and fluorescence band 700 nm is attributed to the undissolved C60 crystal particle. In the experiment, from the clear C60-pyridine solution (C60 molecules are dissolved in pyridine), only the strong fluorescence band centered at 450 nm can be observed, and the fluorescence band centered at 700 nm is so much weak as unobservable. Then, C60 is ultrasonically dissolved in
pyridine to increase the concentration of C60 until the solution is in supersaturation state. During this process, the intensity of 450 nm band reduced and the 700 nm band increased accompanied the observation of fine structure, which suggests that the intensity of fluorescence bands are sensitive to the concentration of C60 molecule. The intensity difference between 450 and 700 nm fluorescence bands is dependent on the concentration of C60 molecules, and these two bands intensity have opposite changes when the concentration varies. For comparison, Fig. 1(a) and (b) are recorded at the same concentration of C60 molecules, which indicates that concentration should not account for the fluorescence changes of C60 molecules. It seemed that gold nanoparticles in the solution should be responsible for the change. It is well known that C60 is hydrophobic molecules and has poor solubility in hydrosol. How can the gold hydrosol have influence on the fluorescence of C60 molecules? Is there an interaction between C60 molecule and gold nanoparticles? Surface-enhanced Raman scattering (SERS) spectrum of C60 in C60-pyridine gold hydrosol system indicated the adsorption interaction between C60 molecules and gold nanoparticles. Fig. 2 displays the FT-NIR SERS spectrum of C60-pyridine. It contains abundant information of C60, which indicates a host of C60 molecules have successfully adsorbed on the surface of gold particles with the help of pyridine [11]. The enhancement is 104, arising from symmetry lowering and selection rule relaxing of C60 due to adsorption on the gold surface. Table 1 shows attribution of the observed bands in Fig. 2 [12,13]. Therefore, the changes of C60 fluorescence in C60-pyridine solution should be attributed to the adsorption of C60 molecules on gold particles. The system is simplified as the interaction between a single molecule and a spherical gold nanoparticles. Drawn by pyridine molecules owing to the formation of adduct between C60 and pyridine, the dis-
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wavelength (nm) Fig. 1. Fluorescence spectra of C60 in solution at room temperature with the excitation of 380 nm: (a) in C60-pyridine solution; (b) in C60-pyridine solution mixed with Au colloid.
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Raman Shift (cm-1)
Fig. 2. SRES spectra of C60-pyridine solution mixed with Au colloid of treble volume excited at 1064 nm.
Y. Zhao et al. / Chemical Physics 323 (2006) 169–172
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Peaks (cm1)
Assignment
271 342 429 493 524 579 707 772 1008 1035 1100 1217 1251 1426 1468 1573
C60Hg(1) C60Hu(1) C60Hg(2) C60Ag(1) C60T1u(1) C60T1u(2) C60Hg(3) C60Hg(4) Pyridine symmetric ring breathing Pyridine symmetric ring breathing C60Hg(5) Pyridine in-plane C-H bending C60Hg(6) C60Hg(7) C60Ag(2) C60Hg(8)
Absorbance
Table 1 Band assignment of Fig. 2
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. Adsorption spectrum of the gold colloid.
solved C60 molecules in the solution could be adsorbed onto the surface of gold nanoparticles [10]. Weitz et al. reported there were two major effects that SERS substrate has on the molecular light scattering process [1]. First, the strength of the local excitation fields are increased by the morphology and dielectric properties of the substrates through the excitation of surface plasmon resonances. The increased local field intensity results in the enhanced radiation rate. The second effect is that any of the molecular excited states will also have an additional nonradiation decay channel when the molecule is near the surface. This demonstrates that the two effects are converse and competitive processes, namely, compared with the enhanced local field effect, if the nonradiation decay channel effect plays the key role, the fluorescence will be quenched and on the contrary, the fluorescence is enhanced. The excitation wavelength of Fig. 1 is 380 nm. The gold nanoparticles have an intense surface plasmon peak centered at 550 nm, as shown in Fig. 3, which is far from the excitation wavelength. Therefore, the enhanced local field effect is weak and the energy transfer from C60 molecules to the gold nanoparticles is the key factor, as shown in Fig. 4(a). As a result, the intensity of fluorescence band centered at 450 nm reduced sharply. The energy transfer from molecules to the gold nanoparticles could increase the surface energy of Au, one part of which has been released in the form of Joule heat. As is mentioned before that there are C60 crystal particles in the solution, these relatively neutral molecules can collide with the gold nanoparticles, which may led to another part of the energy transfer to the first excited state of the C60 crystal molecules in the form of nonradiation scattering. However, the excited state is unstable, and the molecules tends to have a transition to the ground state, resulting in the enhanced fluorescence band centered at 700 nm, as shown in Fig. 4(c). Similar to that of C60, in C70-pyridine-gold hydrosol system, the fluorescence spectrum also changed greatly, result-
a
1 > of dissolved C60 molecule Au
450nm
|1′ > of C60 crystals
b 700nm
c
|g> of C60 molecule Fig. 4. The fluorescence transition of C60 molecules.
ing from the adsorption between C70 molecules and gold particles, which is also illustrated by SERS of C70. Fig. 5 is the SERS spectrum of C70 in C70-pyridine solution mixed with gold hydrosol with the excitation of 1064 nm. It gives rich information concerning with the C70 molecules as well as the adsorption pattern on the metal surface. The high quality SERS spectrum suggests a great deal of C70 molecules have adsorbed on the surface of gold nanoparticles. Fig. 6(a) shows the fluorescence of C70 in C70-pyridine solution, and (b) is the fluorescence spectrum of C70 in C70-pyridine solution mixed with gold hydrosol, excited at 520 nm. Compared with a, the intensity of fluorescence b is enhanced greatly. According to the theory of Weitz, the scattering intensity ratio for fluorescence of adsorption molecules and neutral ones is as follows [1]: RF1 ¼ jAðxL Þj2 jEL j2
C0 ; C0 þ Cs0
ð1Þ
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Y. Zhao et al. / Chemical Physics 323 (2006) 169–172
Raman Intensity
from 50 to 80 nm. After add the gold hydrosol into C70pyridine solution, C70 molecules can be adsorbed onto the gold nanoparticles. With the excitation of 520 nm, the local field intensity is increased through the excitation of surface plasmon resonances. As is shown in Eq. (1), jA(xL)j2jELj2i1, leading to the enhanced scattering intensity of fluorescence. 4. Conclusion
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Fig. 5. SRES spectra of C70-pyridine solution mixed with Au colloid with the excitation of 1064 nm.
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In C60/C70-pyridine-gold hydrosol system, the fluorescence spectra for fullerene have changed greatly due to the adsorption between fullerene molecules and gold nanoparticles. There are two possible existing states in C60-pyridine solution: dissolved single C60 molecules and C60 crystal particles, in which gold nanoparticles served as the energy transfer channel. The excitation wavelength for C60 is 380 nm, which is far from resonance, and the nonradiation decay from C60 molecules to the surface of gold particles is the key factor, leading to the quenching of fluorescence band centered at 450 nm. One part of surface energy of gold particles is transferred to the first excited state of C60 crystal particles, and then the molecules have the transition to the initial vibronic state, resulting in the enhanced fluorescence band centered at 700 nm. The fluorescence bands in the range from 550 to 800 nm of C70 are enhanced, arising from the increased local field intensity through the excitation of surface plasmon resonance of gold nanoparticles.
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Acknowledgments
a
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0 550
600
650
700
750
800
The authors are grateful for the support of this research by the National Natural Science Foundation of China and Natural Science Foundation of Beijing.
Wavelength (nm)
Fig. 6. Fluorescence spectra of C70 in solution at room temperature with the excitation of 520 nm: (a) in C70-pyridine solution; (b) in C70-pyridine solution mixed with Au colloid.
where EL is local electric field at the molecule due to the laser. The amplification factor A(xL) is increased local field intensity by the morphology and dielectric properties of the gold nanoparticles through the excitation of electronic plasmon resonances. C0 denotes the radiation and nonradiation transition rate from excited state to the ground state for neutral molecules. Cs0 is taken to represent an average value of the surface-induced decay rate for molecules on the gold nanoparticles. Fig. 3 displays the typical optical adsorption spectrum of gold nanoparticles prepared by citrate reduction. The gold nanoparticles have an intense surface plasmon peak centered at 550 nm, and the size of which is in the range
References [1] D.A. Weitz, S. Garoff, J. Chem. Phys. 78 (1983) 5324. [2] J.X. Cheng, Y. Fang, Q.J. Huang, Y.J. Yan, Chem. Phys. Lett. 330 (2000) 262. [3] D.K. Pailt, A.V. Spare, J.P. Mittal, Chem. Phys. Lett. 195 (1992) 1. [4] J.W. Arbogast, A.P. Darmanyan, C.S. Foote, Y. Rubin, F.N. Diederich, M.M. Alvarez, S.J. Anz, R.L. Whetten, J. Phys. Chem. 95 (1991) 11. [5] D. Kim, M. Lee, J. Am. Chem. Soc. 114 (1992) 4429. [6] Y.P. Sun, P. Wang, N.B. Hamilton, J. Am. Chem. Soc. 115 (1993) 6378. [7] Y. Wang, J. Phys. Chem. 96 (1992) 764. [8] D.J. Heuvel, I.Y. Chan, M. Matsushita, J. Schmidt, G. Meijer, Chem. Phys. Lett. 233 (1995) 284. [9] J.H. Rice, R. Aures, J.P. Galaup, S. Leach, Chem. Phys. Lett. 263 (2001) 401. [10] Y. Zhao, Y. Fang, J. Phys. Chem. 108 (2004) 13586. [11] Y. Fang, Q.J. Huang, P.J. Wang, Chem. Phys. Lett. 381 (2003) 255. [12] W.H. Li, Chem. Phys. Lett. 305 (1999) 303. [13] R.A. Gu, J. Electroanal. Chem. 505 (2001) 95.
Quenching and enhancement of fluorescence of fullerene molecules on gold particle Yan Zhao b
a,b
, Yijian Jiang
a,*
, Yan Fang
b
a National Center of Laser Technology, Beijing University of Technology, Beijing 100022, PR China Beijing Key Lab for Nano-Photonics and Nano-Structure, Department of Physics, Capital Normal University, Beijing 100037, PR China
Received 12 June 2005; accepted 10 September 2005 Available online 9 March 2006
Abstract The fluorescence spectra of fullerene were first observed to change greatly in the system of C60/C70-pyridine-gold hydrosol, due to the adsorption of the fullerene molecules on the gold nanoparticles. The energy transfers between C60 molecules and gold nanoparticles, leading to the quenching and enhancement of fluorescence bands centered at 450 and 700 nm, respectively. The fluorescence bands in the range from 550 to 800 nm of C70 are enhanced, arising from the increased local field intensity through the excitation of surface plasmon resonance of gold nanoparticles. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Quench; Enhancement; Fluorescence; Fullerene
1. Introduction One of the most important phenomenon accompanied surface-enhanced Raman scattering (SERS) is the quenching and enhancement of fluorescence of adsorbed molecule, for it can directly reveal the physical and chemical profound significance of the interface, and it is also important for understanding the adsorption and SERS enhancement mechanism of the molecule. However, till now, the quenching and enhancement of fluorescence are usually studied in the different molecule systems due to the adsorption substrate, which results in the difficulty for the further theoretical research [1]. Therefore, it is necessary to develop a method for the research in the same molecule system to understand the energy transfer and interface property. Obviously, the key to the observation of fluorescence quenching and enhancement in the same one molecule system is that this molecule has rich fluorescence peaks. It is well known that fullerene has the potential of being a new photosensitive and luminescent material [2–9]. We have *
Corresponding author. Tel.: +86 10 6739 2756; fax: +86 10 6739 1421. E-mail addresses: [email protected], [email protected] (Y. Jiang).
0301-0104/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.09.034
reported well-resolved abundant fluorescence peaks for C60 dissolved in pyridine in the range from 399 to 750 nm at room temperature, resulting from the reduction in high symmetry of C60 molecules [10]. This provides the promise and possibility for observation of fluorescence quenching and enhancement in the same one molecule system. In this paper, the gold nanoparticle is chosen as the SERS substrate. The C60 fluorescence bands centered at 450 nm and at 700 nm were quenched and enhanced, respectively, in C60-pyridine solution. The whole fluorescence peaks of C70 molecule in the range from 550 to 800 nm were enhanced in C70-pyridine solution. Combined with the SERS of fullerene in gold hydrosol, the tentative explanation is given to the changes of fluorescence bands. 2. Experimental C60 (99.9%), C70 (99%) were dissolved in pyridine (HPLC grade), forming the brown solution and amaranth solution, respectively. Preparation of gold colloid: 75 mg KAuCl4 was dissolved in 550 ml deionized water and the solution was heated to boiling. Then 9 ml of a 1% aqueous solution of
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trisodium citrate was added into the boiling solution, accompanied with vigorous stirring. The mixed solution was kept boiling till it reached 200 ml. The fluorescence spectra were recorded on Fluorolog-3 spectrometer. The SERS spectra were obtained using the FT–IR spectrometer (RFS 100/s, Bruker) and UV–vis spectrometer (RM-2000, Renishaw). All the measurements were carried out at room temperature. 3. Results and discussion Fig. 1(a) exhibits the fluorescence spectrum of C60 in C60-pyridine solution, excited at 380 nm. There are three broad regions of fluorescence bands, centered at 425, 575 and 700 nm, respectively. Fig. 1(b) shows the fluorescence spectrum of C60 in C60-pyridine solution mixed with gold hydrosol. Compared with Fig. 1(a), it can be observed from Fig. 1(b) that the intensity of fluorescence band centered at 450 nm reduced greatly and almost was unobservable, on the contrary, the fluorescence band centered at 700 nm is enhanced. In C60-pyridine solution, interaction between pyridine and C60 distorts the icosahedral symmetry of C60. The relaxation of selection rules due to the reduced symmetry of C60 molecule induces the observed rich fluorescence peaks [10]. As far as the assignment of the fluorescence peaks are concerned, after repetitious experiments, we inferred that the fluorescence band centered at 450 nm was assigned to completely soluble C60 molecules in the C60pyridine solution, and fluorescence band 700 nm is attributed to the undissolved C60 crystal particle. In the experiment, from the clear C60-pyridine solution (C60 molecules are dissolved in pyridine), only the strong fluorescence band centered at 450 nm can be observed, and the fluorescence band centered at 700 nm is so much weak as unobservable. Then, C60 is ultrasonically dissolved in
pyridine to increase the concentration of C60 until the solution is in supersaturation state. During this process, the intensity of 450 nm band reduced and the 700 nm band increased accompanied the observation of fine structure, which suggests that the intensity of fluorescence bands are sensitive to the concentration of C60 molecule. The intensity difference between 450 and 700 nm fluorescence bands is dependent on the concentration of C60 molecules, and these two bands intensity have opposite changes when the concentration varies. For comparison, Fig. 1(a) and (b) are recorded at the same concentration of C60 molecules, which indicates that concentration should not account for the fluorescence changes of C60 molecules. It seemed that gold nanoparticles in the solution should be responsible for the change. It is well known that C60 is hydrophobic molecules and has poor solubility in hydrosol. How can the gold hydrosol have influence on the fluorescence of C60 molecules? Is there an interaction between C60 molecule and gold nanoparticles? Surface-enhanced Raman scattering (SERS) spectrum of C60 in C60-pyridine gold hydrosol system indicated the adsorption interaction between C60 molecules and gold nanoparticles. Fig. 2 displays the FT-NIR SERS spectrum of C60-pyridine. It contains abundant information of C60, which indicates a host of C60 molecules have successfully adsorbed on the surface of gold particles with the help of pyridine [11]. The enhancement is 104, arising from symmetry lowering and selection rule relaxing of C60 due to adsorption on the gold surface. Table 1 shows attribution of the observed bands in Fig. 2 [12,13]. Therefore, the changes of C60 fluorescence in C60-pyridine solution should be attributed to the adsorption of C60 molecules on gold particles. The system is simplified as the interaction between a single molecule and a spherical gold nanoparticles. Drawn by pyridine molecules owing to the formation of adduct between C60 and pyridine, the dis-
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intensity
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0 400
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429 524
579
772
707
878
1008
1100
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1217
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1573
10000
750
wavelength (nm) Fig. 1. Fluorescence spectra of C60 in solution at room temperature with the excitation of 380 nm: (a) in C60-pyridine solution; (b) in C60-pyridine solution mixed with Au colloid.
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Raman Shift (cm-1)
Fig. 2. SRES spectra of C60-pyridine solution mixed with Au colloid of treble volume excited at 1064 nm.
Y. Zhao et al. / Chemical Physics 323 (2006) 169–172
171
Peaks (cm1)
Assignment
271 342 429 493 524 579 707 772 1008 1035 1100 1217 1251 1426 1468 1573
C60Hg(1) C60Hu(1) C60Hg(2) C60Ag(1) C60T1u(1) C60T1u(2) C60Hg(3) C60Hg(4) Pyridine symmetric ring breathing Pyridine symmetric ring breathing C60Hg(5) Pyridine in-plane C-H bending C60Hg(6) C60Hg(7) C60Ag(2) C60Hg(8)
Absorbance
Table 1 Band assignment of Fig. 2
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. Adsorption spectrum of the gold colloid.
solved C60 molecules in the solution could be adsorbed onto the surface of gold nanoparticles [10]. Weitz et al. reported there were two major effects that SERS substrate has on the molecular light scattering process [1]. First, the strength of the local excitation fields are increased by the morphology and dielectric properties of the substrates through the excitation of surface plasmon resonances. The increased local field intensity results in the enhanced radiation rate. The second effect is that any of the molecular excited states will also have an additional nonradiation decay channel when the molecule is near the surface. This demonstrates that the two effects are converse and competitive processes, namely, compared with the enhanced local field effect, if the nonradiation decay channel effect plays the key role, the fluorescence will be quenched and on the contrary, the fluorescence is enhanced. The excitation wavelength of Fig. 1 is 380 nm. The gold nanoparticles have an intense surface plasmon peak centered at 550 nm, as shown in Fig. 3, which is far from the excitation wavelength. Therefore, the enhanced local field effect is weak and the energy transfer from C60 molecules to the gold nanoparticles is the key factor, as shown in Fig. 4(a). As a result, the intensity of fluorescence band centered at 450 nm reduced sharply. The energy transfer from molecules to the gold nanoparticles could increase the surface energy of Au, one part of which has been released in the form of Joule heat. As is mentioned before that there are C60 crystal particles in the solution, these relatively neutral molecules can collide with the gold nanoparticles, which may led to another part of the energy transfer to the first excited state of the C60 crystal molecules in the form of nonradiation scattering. However, the excited state is unstable, and the molecules tends to have a transition to the ground state, resulting in the enhanced fluorescence band centered at 700 nm, as shown in Fig. 4(c). Similar to that of C60, in C70-pyridine-gold hydrosol system, the fluorescence spectrum also changed greatly, result-
a
1 > of dissolved C60 molecule Au
450nm
|1′ > of C60 crystals
b 700nm
c
|g> of C60 molecule Fig. 4. The fluorescence transition of C60 molecules.
ing from the adsorption between C70 molecules and gold particles, which is also illustrated by SERS of C70. Fig. 5 is the SERS spectrum of C70 in C70-pyridine solution mixed with gold hydrosol with the excitation of 1064 nm. It gives rich information concerning with the C70 molecules as well as the adsorption pattern on the metal surface. The high quality SERS spectrum suggests a great deal of C70 molecules have adsorbed on the surface of gold nanoparticles. Fig. 6(a) shows the fluorescence of C70 in C70-pyridine solution, and (b) is the fluorescence spectrum of C70 in C70-pyridine solution mixed with gold hydrosol, excited at 520 nm. Compared with a, the intensity of fluorescence b is enhanced greatly. According to the theory of Weitz, the scattering intensity ratio for fluorescence of adsorption molecules and neutral ones is as follows [1]: RF1 ¼ jAðxL Þj2 jEL j2
C0 ; C0 þ Cs0
ð1Þ
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Raman Intensity
from 50 to 80 nm. After add the gold hydrosol into C70pyridine solution, C70 molecules can be adsorbed onto the gold nanoparticles. With the excitation of 520 nm, the local field intensity is increased through the excitation of surface plasmon resonances. As is shown in Eq. (1), jA(xL)j2jELj2i1, leading to the enhanced scattering intensity of fluorescence. 4. Conclusion
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Fig. 5. SRES spectra of C70-pyridine solution mixed with Au colloid with the excitation of 1064 nm.
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b
Intensity
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15000
In C60/C70-pyridine-gold hydrosol system, the fluorescence spectra for fullerene have changed greatly due to the adsorption between fullerene molecules and gold nanoparticles. There are two possible existing states in C60-pyridine solution: dissolved single C60 molecules and C60 crystal particles, in which gold nanoparticles served as the energy transfer channel. The excitation wavelength for C60 is 380 nm, which is far from resonance, and the nonradiation decay from C60 molecules to the surface of gold particles is the key factor, leading to the quenching of fluorescence band centered at 450 nm. One part of surface energy of gold particles is transferred to the first excited state of C60 crystal particles, and then the molecules have the transition to the initial vibronic state, resulting in the enhanced fluorescence band centered at 700 nm. The fluorescence bands in the range from 550 to 800 nm of C70 are enhanced, arising from the increased local field intensity through the excitation of surface plasmon resonance of gold nanoparticles.
10000
Acknowledgments
a
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0 550
600
650
700
750
800
The authors are grateful for the support of this research by the National Natural Science Foundation of China and Natural Science Foundation of Beijing.
Wavelength (nm)
Fig. 6. Fluorescence spectra of C70 in solution at room temperature with the excitation of 520 nm: (a) in C70-pyridine solution; (b) in C70-pyridine solution mixed with Au colloid.
where EL is local electric field at the molecule due to the laser. The amplification factor A(xL) is increased local field intensity by the morphology and dielectric properties of the gold nanoparticles through the excitation of electronic plasmon resonances. C0 denotes the radiation and nonradiation transition rate from excited state to the ground state for neutral molecules. Cs0 is taken to represent an average value of the surface-induced decay rate for molecules on the gold nanoparticles. Fig. 3 displays the typical optical adsorption spectrum of gold nanoparticles prepared by citrate reduction. The gold nanoparticles have an intense surface plasmon peak centered at 550 nm, and the size of which is in the range
References [1] D.A. Weitz, S. Garoff, J. Chem. Phys. 78 (1983) 5324. [2] J.X. Cheng, Y. Fang, Q.J. Huang, Y.J. Yan, Chem. Phys. Lett. 330 (2000) 262. [3] D.K. Pailt, A.V. Spare, J.P. Mittal, Chem. Phys. Lett. 195 (1992) 1. [4] J.W. Arbogast, A.P. Darmanyan, C.S. Foote, Y. Rubin, F.N. Diederich, M.M. Alvarez, S.J. Anz, R.L. Whetten, J. Phys. Chem. 95 (1991) 11. [5] D. Kim, M. Lee, J. Am. Chem. Soc. 114 (1992) 4429. [6] Y.P. Sun, P. Wang, N.B. Hamilton, J. Am. Chem. Soc. 115 (1993) 6378. [7] Y. Wang, J. Phys. Chem. 96 (1992) 764. [8] D.J. Heuvel, I.Y. Chan, M. Matsushita, J. Schmidt, G. Meijer, Chem. Phys. Lett. 233 (1995) 284. [9] J.H. Rice, R. Aures, J.P. Galaup, S. Leach, Chem. Phys. Lett. 263 (2001) 401. [10] Y. Zhao, Y. Fang, J. Phys. Chem. 108 (2004) 13586. [11] Y. Fang, Q.J. Huang, P.J. Wang, Chem. Phys. Lett. 381 (2003) 255. [12] W.H. Li, Chem. Phys. Lett. 305 (1999) 303. [13] R.A. Gu, J. Electroanal. Chem. 505 (2001) 95.