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The effect of nanometer size of porous anodic aluminum oxide on adsorption and fluorescence of tetrahydroxyflavanol
Spectrochimica Acta Part A 59 (2003) 1139 /1144 www.elsevier.com/locate/saa
The effect of nanometer size of porous anodic aluminum oxide on adsorption and fluorescence of tetrahydroxyflavanol Sui Wang, Haiqing Luo, Yongzhen Wang, Guoquan Gong * College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China Received 10 July 2002; accepted 26 July 2002
Abstract Nanoporous anodic aluminum oxide, which was obtained by two-step electrochemical anodization aluminum process, showed strong physical adsorption capability of tetrahydroxyflavanol (THOF). The fluorescence peak of THOF was also dependent on its environment because the surrounding electron field affected the molecule luminescence in nanoporous alumina. The effect of nanometer size on adsorption and fluorescence of THOF is observed. The mechanism is primarily discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Porous anodic aluminum oxide; Tetrahydroxyflavanol; Adsorption; Fluorescence
1. Introduction Nanometer science and technology are developing rapidly recently. Owing to the surprising special performances of nanometer materials, it has focused attention upon the entire world. There were many nanometer materials fabricated by scientists and engineers, and their properties in mechanics, electromagnetism, optics and other physical and chemical characters were probed in detail [1 /3]. The size of nanoholes can be controlled by adjusting the height of voltage in types of anodizing solutions. Because of nanometer size effect, the nanohole arrays show especial characters compared with general materials. At present * Corresponding author. Fax: /86-931-8912582. E-mail address: [email protected] (G. Gong).
much work has focused on the magnetic and mechanical properties and the fabricating conditions; only are there a few studies that have been made towards the effect of fluorescence and adsorption on fluorescence molecules [4 /7]. It is an important aspect in the study of molecular luminescence and adsorption. There are many kinds of fluorescence molecules in nature and synthetic substance. Tetrahydroxyflavanol (THOF) is widely applied in many fields as dyes, medicament for remedying tumors, sensitive fluorescent probes, and analytical reagents for metal ions, etc. There are two theses on nanoparticles as fluorescent biological labels for ultra sensitive unconventional fluorescent analytical method [9,10]. In this work, THOF as a fluorescent adsorbate in nanoporous anodic aluminum oxide (P-Al) was
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 3 1 4 - 1
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tested for effect of nanometer size of porous anodic aluminum oxide on adsorption and fluorescence of THOF. The results show a strong effect of nanometer size. It provided with some referenced value for expanding THOF’s usage, exploring new fluorescence probe basic on unconventional analysis method and obtaining more luminescence materials.
2.2. Reagents
steps: (1) anodized the polished Al sheet for 30 min; (2) dissolved away the oxide film in a mixed solution of 0.2 M H2CrO4 and 0.4 M H3PO4 at 60 8C for 5 min; (3) rinsed the Al sheet with deionized water, then anodized it for 1/3 h; (4) removed oxide film, and anodized the Al sheet for 1 h again. Consequently, the Al2O3 template with highly ordered pores was formed. The P-Al with different size ranges from several tens to hundreds was achieved by adjusting the height of voltage in types of anodizing solution such as sulfuric, oxalic and phosphoric acid. The nanoporous structure and appearance had been characterized by XRD and SEAM at Key Laboratory for Magnetism and Magnetic Materials of MOE of Lanzhou University. The average diameters of porous anodic alumina used in the present work were 40 and 100 nm, respectively. Pure aluminum in the following indicates high purity slick aluminum foils. Pure aluminum foil was exposed to dried air for several days and one thin alumina film formed spontaneously. By this means, we got the natural alumina.
A solution of THOF (1.0 /10 3 mol l 1) was prepared by dissolving commercial sample (THOF, Shanghai Biochemistry Reagent Limited Company, China) in 40% C2H5OH. All of the reagents used in this experiment were analytical pure grade and were used without further purification. Distilled de-ionized water was used throughout.
2.3.2. Adsorption and luminescence of THOF in samples The P-Al dealt with above and other samples to be tested were soaked in THOF solution. Some time later, they were taken out and flushed slowly by 40% C2H5OH to get rid of adhesive molecules. Then we scanned their fluorescence spectra by spectrofluorimeter.
2. Experimental section 2.1. Apparatus A Shimadzu RF-540 spectrofluorimeter (Kyoto, Japan), equipped with a 150 W xenon lamp, a 1 / 1 cm2 cell and a function recorder, was employed to scan the fluorescence spectra and to measure the fluorescence intensities. The slit-width for both the excitation and emission monochromators was set at 5 nm.
2.3. General procedure 3. Results and discussions 2.3.1. Fabrication of P-Al We have fabricated P-Al template by the following process according to the two-step electrochemical anodization [11 /13]. First, high purity (99.99%) aluminum foils were annealed at 600 8C for 6 h. Subsequently, the sample was ultrasonically degreased in acetone, and cleaned in 5% NaOH at 15 8C for 20 min, and a smooth surface could be obtained. Then, the aluminum was electropolished in a 25:75 volume mixture of HClO4 and C2H5OH. The polished Al samples were anodized at 40 V dc in 2.7 wt.% (0.3 M) H2C2O4 at 10 8C, which contained the following 4
3.1. Adsorption capacity The entire experiments included three parts. We experimented the effect of soak time, dealt methods and different molecules on adsorption capacity, respectively (Table 1). Relativity intensity was judged according to chroma by human eyes. Table 1 provides us with information on adsorption capacity and mechanism. The effect of nanometer size on adsorption is remarkable. The adsorptions are strengthened by reducing the nanoholes. Furthermore, we speculate that the mechanism is
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Table 1 Results of different samples’ adsorption Molecule
Time (min)
Dealt method
Pure aluminum
Natural alumina
100 nm P-Al
40 nm P-Al
THOF THOF THOF THOF THOF THOF Currcium Rhodamine B
10 30 60 60 60 120 60 60
Flushed Flushed Soaked Flushed Re-flushed Flushed Flushed Flushed
/ / / / / / / /
/ / /// // / // / /
// // //// /// / /// // /
/// //// ///// //// // //// /// //
Relativity intensity: /, none; /, very weak; //, weak; ///, middle; ////, strong; /////, very strong. Re-flushed means that the sample was soaked and flushed repetitiously.
primary physical adsorption. On the other hand, different molecules have different adsorption and natural alumina has some adsorption because of coarse surface. The phenomena indicate that chemical adsorption or the effects of molecule structure and size effect exist partly. Three molecular structures are displayed in Fig. 3.
3.2. Spectral characteristics of THOF solution
3.4. Emission spectra of THOF in samples Emission spectra of THOF in samples and their maximum wavelengths are displayed in Fig. 2. The spectra show that blue shift is more prominent when the fluorescence molecule is more restricted within surroundings limits. We assume the process may be the following. The excited molecules, which restricted weakly, take place easily intersystem crossing and reduce the energy interval from activation state to ground
THOF in solution has fluorescence around 540 nm when excited at 420 nm (Fig. 1). There are slight change on peaks’ positions in different surroundings and testing conditions.
3.3. Excitation spectra of THOF in samples We scanned the excitation spectra of THOF in samples. The spectra does not present character peak of THOF owing to self-luminescence of P-Al. Therefore, we did not provide with the excitation spectra. One of the principles of molecule fluorescence is that maximum peak’s position is invariable with the variably excitation wavelength [8]. The fluorescence of THOF was hereby confirmed. One possible explanation is that the large overlaps of excitation spectrum of THOF and P-Al cause the former covered. A favorable proof is that blank P-Al and the sample have the same excitation spectra.
Fig. 1. Fluorescence excitation and emission spectra of THOF in solution (1.0 /10 4mol l 1, 40% C2H5OH): (a) excitation spectrum; (b) emission spectrum (excited at 420 nm).
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Fig. 2. Emission spectra of THOF in samples (excited at 420 nm): (a) THOF-Al3 complex solution (1.0 /10 4 mol l 1, 40% C2H5OH), maximum emission wavelength (Em) 495 nm; (b) THOF solution (1.0 /10 4mol l 1, 40% C2H5OH), Em 545 nm; (c) on 100 nm P-Al, Em 520 nm; (d) on 40 nm P-Al, Em 515 nm; (e) 40 nm blank P-Al, Em 470 nm; (f) in flushed process.
state. Hence, red shift is observed. If the molecules are restricted strongly and immovably within other molecules, the character of molecule fluorescence is closed to single molecule luminescence due to subdued interaction of luminescence molecules. Thereby blue shift presents. The curve e in Fig. 2 shows the wider emission spectrum of blank 40 nm P-Al and the peak at 470 nm was maximum. Although the luminescence is blazing, the peak at 470 nm is faint when the nanoholes in P-Al are filled with THOF. It is estimated that an energy transfer takes place between P-Al luminescence and THOF absorbance. The fact that wider band of P-Al luminescence overlays the absorbance spectrum of THOF results in that luminescence of P-Al are absorbed by THOF and only presents fluorescence of THOF. Meanwhile enhanced luminescence efficiency may explain why a few molecules can send out strong fluorescence.
The curve f in Fig. 2 shows spectrum of THOF on 40 nm P-Al in soaked and flushed process. With molecule removed in nano-porous alumina, the behavior of luminescence changes gradually into self-luminescence of P-Al from THOF. The curve a in Fig. 2 shows the peak of THOF / Al3. The peak’s position is about 495 nm, which is 20 nm less than that of the curve d. It can be concluded that the interaction of P-Al and THOF is different from that of Al3 and THOF in the solution.
4. Conclusion We found a significant variation in the peak intensity and peak position in the fluorescence of THOF when different adsorption carriers were used. Strong effect of nanometer size on adsorption and fluorescence was observed. The mechan-
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Fig. 3. Molecular structures of three adsorbates: (a) rhodamine B; (b) THOF; (c) diferuloylmethane.
ism of variation of fluorescence intensity and position was primarily discussed. It was a significative work for searching more organic adsorption carriers and nonometer fluorescence analysis probes.
Acknowledgements Key Laboratory for Magnetism and Magnetic Materials of MOE of Lanzhou University in China supported this work. They offered all the
nano-porous anodic aluminum oxide in this experiment and tested relational data of the material.
References [1] G.L. Hornyak, C.J. Patrissi, C.R. Martin, J. Phys. Chem. B 101 (1997) 1548. [2] A. Ortiz, J.C. Alonso, V. Pankov, D. Albarran, J. Lum. 81 (1999) 45. [3] A.P. Li, F. Mueller, A. Birner, K. Nielsch, U. Go¨sele, J. Appl. Phys. 84 (1998) 6023. [4] C.X. Xun, Q.H. Xue, L. Ba, B. Zhao, N. Gu, Y.P. Cui, J. Chin. Sci. Bull. 46 (2001) 984. [5] Y. Engelborghs, Spectrochim. Acta 57A (2001) 2255.
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[6] J. Hofkens, W. Schroeyers, D. Loos, M. Cotlet, F. Kohn, T. Vosch, M. Maus, A. Herrmann, K. Mu¨llen, T. Gensch, F.C. De Schryver, Spectrochim. Acta 57A (2001) 2093. [7] Y.L. Shi, X.G. Zhang, H.L. Li, Chem. J. Chin. Univ. 22 (2001) 821. [8] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, New York, 1983.
[9] M. Bruchez, M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013. [10] W.C. Chan, S.M. Nie, Science 281 (1998) 2016. [11] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [12] H. Masuda, F. Hasegawa, S. Ono, J. Electrochem. Soc. 144 (1997) L127. [13] F.Y. Li, L. Zhang, R.M. Metzger, J. Chem. Mater. 10 (1998) 2473.
The effect of nanometer size of porous anodic aluminum oxide on adsorption and fluorescence of tetrahydroxyflavanol Sui Wang, Haiqing Luo, Yongzhen Wang, Guoquan Gong * College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China Received 10 July 2002; accepted 26 July 2002
Abstract Nanoporous anodic aluminum oxide, which was obtained by two-step electrochemical anodization aluminum process, showed strong physical adsorption capability of tetrahydroxyflavanol (THOF). The fluorescence peak of THOF was also dependent on its environment because the surrounding electron field affected the molecule luminescence in nanoporous alumina. The effect of nanometer size on adsorption and fluorescence of THOF is observed. The mechanism is primarily discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Porous anodic aluminum oxide; Tetrahydroxyflavanol; Adsorption; Fluorescence
1. Introduction Nanometer science and technology are developing rapidly recently. Owing to the surprising special performances of nanometer materials, it has focused attention upon the entire world. There were many nanometer materials fabricated by scientists and engineers, and their properties in mechanics, electromagnetism, optics and other physical and chemical characters were probed in detail [1 /3]. The size of nanoholes can be controlled by adjusting the height of voltage in types of anodizing solutions. Because of nanometer size effect, the nanohole arrays show especial characters compared with general materials. At present * Corresponding author. Fax: /86-931-8912582. E-mail address: [email protected] (G. Gong).
much work has focused on the magnetic and mechanical properties and the fabricating conditions; only are there a few studies that have been made towards the effect of fluorescence and adsorption on fluorescence molecules [4 /7]. It is an important aspect in the study of molecular luminescence and adsorption. There are many kinds of fluorescence molecules in nature and synthetic substance. Tetrahydroxyflavanol (THOF) is widely applied in many fields as dyes, medicament for remedying tumors, sensitive fluorescent probes, and analytical reagents for metal ions, etc. There are two theses on nanoparticles as fluorescent biological labels for ultra sensitive unconventional fluorescent analytical method [9,10]. In this work, THOF as a fluorescent adsorbate in nanoporous anodic aluminum oxide (P-Al) was
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 3 1 4 - 1
1140
S. Wang et al. / Spectrochimica Acta Part A 59 (2003) 1139 /1144
tested for effect of nanometer size of porous anodic aluminum oxide on adsorption and fluorescence of THOF. The results show a strong effect of nanometer size. It provided with some referenced value for expanding THOF’s usage, exploring new fluorescence probe basic on unconventional analysis method and obtaining more luminescence materials.
2.2. Reagents
steps: (1) anodized the polished Al sheet for 30 min; (2) dissolved away the oxide film in a mixed solution of 0.2 M H2CrO4 and 0.4 M H3PO4 at 60 8C for 5 min; (3) rinsed the Al sheet with deionized water, then anodized it for 1/3 h; (4) removed oxide film, and anodized the Al sheet for 1 h again. Consequently, the Al2O3 template with highly ordered pores was formed. The P-Al with different size ranges from several tens to hundreds was achieved by adjusting the height of voltage in types of anodizing solution such as sulfuric, oxalic and phosphoric acid. The nanoporous structure and appearance had been characterized by XRD and SEAM at Key Laboratory for Magnetism and Magnetic Materials of MOE of Lanzhou University. The average diameters of porous anodic alumina used in the present work were 40 and 100 nm, respectively. Pure aluminum in the following indicates high purity slick aluminum foils. Pure aluminum foil was exposed to dried air for several days and one thin alumina film formed spontaneously. By this means, we got the natural alumina.
A solution of THOF (1.0 /10 3 mol l 1) was prepared by dissolving commercial sample (THOF, Shanghai Biochemistry Reagent Limited Company, China) in 40% C2H5OH. All of the reagents used in this experiment were analytical pure grade and were used without further purification. Distilled de-ionized water was used throughout.
2.3.2. Adsorption and luminescence of THOF in samples The P-Al dealt with above and other samples to be tested were soaked in THOF solution. Some time later, they were taken out and flushed slowly by 40% C2H5OH to get rid of adhesive molecules. Then we scanned their fluorescence spectra by spectrofluorimeter.
2. Experimental section 2.1. Apparatus A Shimadzu RF-540 spectrofluorimeter (Kyoto, Japan), equipped with a 150 W xenon lamp, a 1 / 1 cm2 cell and a function recorder, was employed to scan the fluorescence spectra and to measure the fluorescence intensities. The slit-width for both the excitation and emission monochromators was set at 5 nm.
2.3. General procedure 3. Results and discussions 2.3.1. Fabrication of P-Al We have fabricated P-Al template by the following process according to the two-step electrochemical anodization [11 /13]. First, high purity (99.99%) aluminum foils were annealed at 600 8C for 6 h. Subsequently, the sample was ultrasonically degreased in acetone, and cleaned in 5% NaOH at 15 8C for 20 min, and a smooth surface could be obtained. Then, the aluminum was electropolished in a 25:75 volume mixture of HClO4 and C2H5OH. The polished Al samples were anodized at 40 V dc in 2.7 wt.% (0.3 M) H2C2O4 at 10 8C, which contained the following 4
3.1. Adsorption capacity The entire experiments included three parts. We experimented the effect of soak time, dealt methods and different molecules on adsorption capacity, respectively (Table 1). Relativity intensity was judged according to chroma by human eyes. Table 1 provides us with information on adsorption capacity and mechanism. The effect of nanometer size on adsorption is remarkable. The adsorptions are strengthened by reducing the nanoholes. Furthermore, we speculate that the mechanism is
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Table 1 Results of different samples’ adsorption Molecule
Time (min)
Dealt method
Pure aluminum
Natural alumina
100 nm P-Al
40 nm P-Al
THOF THOF THOF THOF THOF THOF Currcium Rhodamine B
10 30 60 60 60 120 60 60
Flushed Flushed Soaked Flushed Re-flushed Flushed Flushed Flushed
/ / / / / / / /
/ / /// // / // / /
// // //// /// / /// // /
/// //// ///// //// // //// /// //
Relativity intensity: /, none; /, very weak; //, weak; ///, middle; ////, strong; /////, very strong. Re-flushed means that the sample was soaked and flushed repetitiously.
primary physical adsorption. On the other hand, different molecules have different adsorption and natural alumina has some adsorption because of coarse surface. The phenomena indicate that chemical adsorption or the effects of molecule structure and size effect exist partly. Three molecular structures are displayed in Fig. 3.
3.2. Spectral characteristics of THOF solution
3.4. Emission spectra of THOF in samples Emission spectra of THOF in samples and their maximum wavelengths are displayed in Fig. 2. The spectra show that blue shift is more prominent when the fluorescence molecule is more restricted within surroundings limits. We assume the process may be the following. The excited molecules, which restricted weakly, take place easily intersystem crossing and reduce the energy interval from activation state to ground
THOF in solution has fluorescence around 540 nm when excited at 420 nm (Fig. 1). There are slight change on peaks’ positions in different surroundings and testing conditions.
3.3. Excitation spectra of THOF in samples We scanned the excitation spectra of THOF in samples. The spectra does not present character peak of THOF owing to self-luminescence of P-Al. Therefore, we did not provide with the excitation spectra. One of the principles of molecule fluorescence is that maximum peak’s position is invariable with the variably excitation wavelength [8]. The fluorescence of THOF was hereby confirmed. One possible explanation is that the large overlaps of excitation spectrum of THOF and P-Al cause the former covered. A favorable proof is that blank P-Al and the sample have the same excitation spectra.
Fig. 1. Fluorescence excitation and emission spectra of THOF in solution (1.0 /10 4mol l 1, 40% C2H5OH): (a) excitation spectrum; (b) emission spectrum (excited at 420 nm).
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Fig. 2. Emission spectra of THOF in samples (excited at 420 nm): (a) THOF-Al3 complex solution (1.0 /10 4 mol l 1, 40% C2H5OH), maximum emission wavelength (Em) 495 nm; (b) THOF solution (1.0 /10 4mol l 1, 40% C2H5OH), Em 545 nm; (c) on 100 nm P-Al, Em 520 nm; (d) on 40 nm P-Al, Em 515 nm; (e) 40 nm blank P-Al, Em 470 nm; (f) in flushed process.
state. Hence, red shift is observed. If the molecules are restricted strongly and immovably within other molecules, the character of molecule fluorescence is closed to single molecule luminescence due to subdued interaction of luminescence molecules. Thereby blue shift presents. The curve e in Fig. 2 shows the wider emission spectrum of blank 40 nm P-Al and the peak at 470 nm was maximum. Although the luminescence is blazing, the peak at 470 nm is faint when the nanoholes in P-Al are filled with THOF. It is estimated that an energy transfer takes place between P-Al luminescence and THOF absorbance. The fact that wider band of P-Al luminescence overlays the absorbance spectrum of THOF results in that luminescence of P-Al are absorbed by THOF and only presents fluorescence of THOF. Meanwhile enhanced luminescence efficiency may explain why a few molecules can send out strong fluorescence.
The curve f in Fig. 2 shows spectrum of THOF on 40 nm P-Al in soaked and flushed process. With molecule removed in nano-porous alumina, the behavior of luminescence changes gradually into self-luminescence of P-Al from THOF. The curve a in Fig. 2 shows the peak of THOF / Al3. The peak’s position is about 495 nm, which is 20 nm less than that of the curve d. It can be concluded that the interaction of P-Al and THOF is different from that of Al3 and THOF in the solution.
4. Conclusion We found a significant variation in the peak intensity and peak position in the fluorescence of THOF when different adsorption carriers were used. Strong effect of nanometer size on adsorption and fluorescence was observed. The mechan-
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Fig. 3. Molecular structures of three adsorbates: (a) rhodamine B; (b) THOF; (c) diferuloylmethane.
ism of variation of fluorescence intensity and position was primarily discussed. It was a significative work for searching more organic adsorption carriers and nonometer fluorescence analysis probes.
Acknowledgements Key Laboratory for Magnetism and Magnetic Materials of MOE of Lanzhou University in China supported this work. They offered all the
nano-porous anodic aluminum oxide in this experiment and tested relational data of the material.
References [1] G.L. Hornyak, C.J. Patrissi, C.R. Martin, J. Phys. Chem. B 101 (1997) 1548. [2] A. Ortiz, J.C. Alonso, V. Pankov, D. Albarran, J. Lum. 81 (1999) 45. [3] A.P. Li, F. Mueller, A. Birner, K. Nielsch, U. Go¨sele, J. Appl. Phys. 84 (1998) 6023. [4] C.X. Xun, Q.H. Xue, L. Ba, B. Zhao, N. Gu, Y.P. Cui, J. Chin. Sci. Bull. 46 (2001) 984. [5] Y. Engelborghs, Spectrochim. Acta 57A (2001) 2255.
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[6] J. Hofkens, W. Schroeyers, D. Loos, M. Cotlet, F. Kohn, T. Vosch, M. Maus, A. Herrmann, K. Mu¨llen, T. Gensch, F.C. De Schryver, Spectrochim. Acta 57A (2001) 2093. [7] Y.L. Shi, X.G. Zhang, H.L. Li, Chem. J. Chin. Univ. 22 (2001) 821. [8] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, New York, 1983.
[9] M. Bruchez, M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013. [10] W.C. Chan, S.M. Nie, Science 281 (1998) 2016. [11] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [12] H. Masuda, F. Hasegawa, S. Ono, J. Electrochem. Soc. 144 (1997) L127. [13] F.Y. Li, L. Zhang, R.M. Metzger, J. Chem. Mater. 10 (1998) 2473.