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Tuning the Emission Properties of Ru(phen)32þ Doped Silica Nanoparticles by Changing the Addition Time of the Dye during the St€ober Process Dawei Zhang, Zhenzhu Wu, Jianquan Xu, Jinglun Liang, Jun Li, and Wensheng Yang* State Key Laboratory for Supramolecular Structures and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China Received October 21, 2009. Revised Manuscript Received December 30, 2009 A series of tris(1,10-phenanthroline)ruthenium ion (Ru(phen)32þ) doped silica nanoparticles were prepared by introducing the dye at different stages of the St€ober process. The emission properties of the doped silica particles were found to be dependent on the time (0-8 h) of the dye introduced into the reaction system. A turnover of the emission properties was identified for the doped silica particles by introducing the dye before and after 3 h of the reaction. Compared to the particles prepared by adding the dye at the beginning of the reaction (0 h doping), the particles prepared by introducing the dye before 3 h of the reaction (3 h doping) showed enhanced emission intensity and blueshifted emission with the delayed addition time. The particles prepared by introducing the dye during the period of 3-8 h of the reaction showed decreased emission intensity and red-shifted emission with the delayed addition time compared to those prepared by introducing the dye at 3 h of the reaction. The emission intensity of the 3 h doping silica particles was about 3.3 times that of the 0 h doping particles, and the emission maximum shifted from 592 to 575 nm correspondingly. The 8 h doping particles showed emission maximum at 581 nm, and their emission intensity was only 15% of that of the 3 h doping particles. However, both the emission intensity and maximum of the 8 h doping particles would be similar to those of the 3 h doping particles after further deposition of silica protection layer. The switching of the emission properties of the doped silica particles prepared by introducing the dye before and after 3 h is attributed to the suppressed aggregation of the dye molecules and decreased thickness of the silica protection layer, respectively.
1. Introduction Luminescent dye doped silica nanoparticles have been considered as promising candidates as labels in bioassays due to their improved photostability and intensified luminescence compared to free dyes.1-3 In the past decades, a variety of strategies have been developed to incorporate dye molecules into silica nanoparticles. For example, Van Blaaderen and Vrij reported the incorporation of fluorescein isothiocyanate (FITC) with isothiocyanate group into silica nanoparticles through covalent coupling.4 Arriagada and Osseo-Asare realized the preparation of metal-organic dyes or nonpolar dye molecule doped silica nanoparticles by the reverse microemulsion method.5 Recently, Rosenzweig et al. proved that a positively charged dye, such as Ru(II) complexes, could be readily incorporated into silica nanoparticles by electrostatic interactions.6 It has been documented that the emission properties of the dye doped silica nanoparticles are affected by the exterior environment, mainly the oxygen in air and the solvent, *To whom correspondence should be addressed. E-mail:
[email protected]. (1) Basabe-Desmonts, L.; Reinhoudt, D. N.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 993–1017. (2) Burns, A.; Ow, H.; Wiesner, U. Chem. Soc. Rev. 2006, 35, 1028–1042. (3) Wang, L.; Wang, K. M.; Santra, S.; Zhao, X. J.; Hilliard, L. R.; Smith, J. E.; Wu, J. R.; Tan, W. H. Anal. Chem. 2006, 78, 646–654. (4) Van Blaaderen, A.; Vrij, A. Langmuir 1992, 8, 2921–2931. (5) Arriagada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1995, 170, 8–17. (6) Rossi, L. M.; Shi, L. F.; Quina, F. H.; Rosenzweig, Z. Langmuir 2005, 21, 4277–4280. (7) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85–277. (8) Wheeler, J.; Thomas, J. K. J. Phys. Chem. 1982, 86, 4540–4544. (9) Innocenzi, P.; Kozuka, H.; Yoko, T. J. Phys. Chem. B 1997, 101, 2285–2291. (10) Ogawa, M.; Nakamura, T.; Mori, J.; Kuroda, K. J. Phys. Chem. B 2000, 104, 8554–8556. (11) Rampazzo, E.; Bonacchi, S.; Montalti, M.; Prodi, L.; Zaccheroni, N. J. Am. Chem. Soc. 2007, 129, 14251–14256. (12) Imhof, A.; Megens, M.; Engelberts, J. J.; De Lang, D. T. N.; Sprik, R.; Vos, W. L. J. Phys. Chem. B 1999, 103, 1408–1415.
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and dispersed state of the dye molecules.2,3,7-12 The effect of the oxygen and solvent can be eliminated effectively by subsequent growth of the silica protection layer or closing the pores13-15 of the porous silica structure. However, there is still no effective way to control over the dispersed state of the dye molecules since they are ready to go spontaneous accumulation while being incorporated into the silica nanoparticles.11,12 The St€ober method,16 in which tetraethoxysilane (TEOS) was used as silica source and ammonia as catalyst, is the most widely adopted reaction for the synthesis of dye doped silica nanoparticles. It is well-known that the reaction involves the hydrolysis of TEOS and the occurring of oligomers and primary particles at the initial stage of the reaction, then stable particles (secondary particles) from aggregation of the primary particles and oligomers, and finally the growth of the secondary particles by monomer deposition.17-26 To prepare dye doped silica particles, generally the dye was added into the reaction system at the (13) Sanchez, C.; Soler-Illia, G. J.; de, A. A.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061–3083. (14) Liu, Q. X.; Xu, Z. H.; Finch, J. A.; Egerton, R. Chem. Mater. 1998, 10, 3936–3940. (15) Lu, Z. Y.; Xu, J. Q.; Han, Y. D.; Song, Z. Q.; Li, J.; Yang, W. S. Colloids Surf., A 2007, 303, 207–210. (16) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69. (17) Matsoukas, T.; Gulari, E. J. Colloid Interface Sci. 1988, 124, 252–261. (18) Matsoukas, T.; Gulari, E. J. Colloid Interface Sci. 1989, 132, 13–21. (19) Matsoukas, T.; Gulari, E. J. Colloid Interface Sci. 1991, 145, 557–562. (20) Kim, S.; Zukoski, C. F. J. Colloid Interface Sci. 1990, 139, 198–212. (21) Bogush, G. H.; Zukoski, C. F., IV J. Colloid Interface Sci. 1991, 142, 1–18. (22) Bogush, G. H.; Zukoski, C. F., IV J. Colloid Interface Sci. 1991, 142, 19–34. (23) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: Boston, 1990. (24) Van Blaaderen, A.; Van Geest, J.; Vrij, A. J. Colloid Interface Sci. 1992, 154, 481–501. (25) Green, D. L.; Lin, J. S.; Lam, Y.-F.; Hu, M. Z.-C.; Schaefer, D. W.; Harris, M. T. J. Colloid Interface Sci. 2003, 266, 246–358. (26) Green, D. L.; Jayasundara, S.; Lam, Y.-C.; Harris, M. T. J. Non-Cryst. Solids 2003, 315, 166–179.
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beginning of the reaction.6,11,12 The dye molecules will interact with the oligomers and primary particles; thus, they have a great opportunity to go aggregation upon the aggregation of the oligomers and primary particles during the St€ober process. It is supposed that the aggregation of the dye molecules may be suppressed to some extent if they were introduced into the reaction system after the consumption of the oligomers and primary particles. In this work, a series of Ru(phen)32þ doped silica nanoparticles were prepared by introducing the dye into the St€ober process at different stages of the reaction. It was found the emission properties of the doped silica particles are dependent on the time of the dye added into the reaction. It was identified that the aggregation of the dye molecules Ru(phen)32þ in silica particles can be suppressed greatly if they were added around 3 h of the reaction, when the silica oligomers and primary particles were almost consumed completely and the secondary particles became dominant in the reaction system. As a result, the doped silica particles prepared by introducing the dye before 3 h presented blue-shifted and intensified emission, and those prepared after 3 h showed redshifted and weakened emission with delayed addition time attributed to the suppressed aggregation and decreased thickness of the silica protection layer, respectively.
2. Experimental Section Materials. Dichlorotris(1,10-phenanthroline)ruthenium(II) hydrate was purchased from Aldrich Chemicals Inc. Tetraethoxysilane (TEOS, Tiantai Chemical Int.) was distilled under reduced pressure before use. Ethanol and ammonia hydroxide (25%) were purchased from Beijing Chemical Int. (analytical grade) and used without further purification. High-purity water with a resistivity of 18.2 MΩ 3 cm (Pall Purelab Plus) was used in all experiments. 2þ Preparation of Ru(phen)3
Doped Silica Nanoparticles.
Ru(phen)32þ doped silica nanoparticles were prepared by the St€ ober method. In a typical reaction, 2.4 mL of TEOS was added to premixed ethanol solution (30 mL) containing ammonia (0.93 M) and water (2.9 M). The reaction mixture was kept at 25 C for 12 h under stirring (150 rpm), and 3 mL of Ru(phen)32þ solution (0.1 mg/mL) was added at different reaction times (0 h, 13 min, 1 h, 3 h, 6 h, 8 h); the resulting particles were isolated by centrifugation (12 000 rpm, 15 min) and dispersed in ethanol by ultrasonics. After three cycles of the centrifugation-dispersion procedures, the doped silica nanoparticles were finally dispersed in 30 mL of ethanol. The particles obtained by adding the dye at 0 h, 13 min, 1 h, 3 h, 6 h, and 8 h were named as 0 h doping, 13 min doping, 1 h doping, 3 h doping, 6 h doping, and 8 h doping particles, respectively. Conductivity Measurements. Conductivity measurements were performed on an Ec215 conductivity meter of a HANNA instrument. The reaction mixtures were kept in a 25 C water bath during the measurements. Transmission Electron Microscopy. Transmission electron microscopic (TEM) observations were carried out on a JEOL2010 electron microscope operating at 200 kV for determining the size of silica nanoparticles. All the characterizations were done at room temperature (25 C). The samples used for detection of the particles growth was obtained as the following process: 0.5 mL of reaction mixture was mixed with 0.5 mL of -20 C ethanol to terminate the reaction process; the mixture was centrifuged (15 000 rpm, -20 C) and then redispersed with ethanol at room temperature (25 C). Photophysical Measurements. Emission spectra were measured on an Edinburgh FS900 steady-state fluorescence spectrometer with a 450 W xenon lamp as excitation source. The excitation wavelength was set at 450 nm, and the scan range was from 500 to 800 nm. All the experiments were carried out in the atmosphere except where stated otherwise. The real-time emission spectra measurements were also carried out on the 6658 DOI: 10.1021/la903995r
Figure 1. Emission spectra of the Ru(phen)32þ doped silica nanoparticles prepared at the different addition time (0 h, 13 min, 1 h, 3 h, 6 h, 8 h) and photo of samples irradiated by 450 nm LED (inset).
FS900 steady-state fluorescence spectrometer. The parameter was set as the same as above. The quartz curet was used as a reaction vessel, and the spectra were collected at appropriate time points through the whole reaction. A water bath was connected with the fluorescence spectrometer by a circulating pump to keep the temperature of reaction mixture at 25 C in the quartz curet. Time-resolved luminescence experiments were carried out at room temperature (25 C) using the time-correlated single photon counting (TCSPC) with a Spex 1702 fluorescence lifetime spectrometer. EKSPLA NT340 ns tunable laser system was used, the pulse duration of which was 3-5 ns. The suspensions were diluted to the appropriate concentrations and the measurements were carried out under atmospheric conditions. Quantitative Analysis of TEOS Consumption. The supernatant of the reaction mixture collected at different times was performed for measuring unreacted TEOS by the molybdosilicate blue spectrophotometric method.27 The supernatant (20 μL), H2O (2 mL), ethanol (0.4 mL), and (NH4)Mo2O7 solution (0.3 mL, 5 wt %) and HCl (2.5 M) were mixed and set on the rocking device for 40 min. Then ascorbic acid solution (0.2 mL, 5 wt %) and HCl solution (0.6 mL, 12 M) were added to the mixture and kept rocking for 3 h. The absorbance at 810 nm was used to express the amount of the molybdosilicate in the mixture. Absorption spectra were collected with a Varian Cary-100 spectrometer. Quartz curets with optical path length of 1 cm were used.
3. Results and Discussion Figure 1 shows the emission spectra of the resulting doped silica particles prepared by introducing the dye into the reaction system of the St€ober process at different times. After the centrifugation treatments, no absorption and emission of the dye could be detected in the supernatants of all the samples, indicating that the dye added be incorporated completely into the silica particles. When the dye was introduced before 3 h of the reaction, the emission intensity of the resulting silica particles increased with the delayed addition time compared with 0 h doping particles. Emission intensity of the particles prepared by introducing the dye at 3 h (3 h doping particles) was about 3.3 times that of the particles prepared by introducing the dye at the beginning of the reaction (0 h doping particles). At the same time, the emission maximum of the particles underwent a blue shift with the delayed addition time. The 0 h doping, 13 min doping, 1 h doping, and 3 h doping particles showed emission maxima at 592, 580, 577, and 575 nm, respectively. Compared to the emission maximum of the dye in ethanol (588 nm, see Figure S1 in the Supporting Information), it is obvious that the dye molecules underwent serious (27) Coradin, T.; Livage, J. Colloids Surf., B 2001, 21, 329–336.
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Figure 2. TEM pictures of the Ru(phen)32þ doped silica nanoparticles prepared under the different addition times (0 h doping particles, 3 h doping particles, and 8 h doping particles).
aggregation in the 0 h doping particles, and the degree of aggregation was suppressed gradually with the delayed addition time. If the dye was added after 3 h of the reaction, the emission properties of the particles showed a reverse tendency with the delayed addition time. The emission intensity of the 6 h doping and 8 h doping particles is only 36% and 15% of that of the 3 h doping particles, and their emission maximum shifted to 577 nm (6 h doping) and 581 nm (8 h doping), respectively. Such redshifted emission may be induced by either the aggregation of the dye molecules or the effect of the oxygen and solvent, which will be further addressed in the following part of this work. The inset of Figure 1 gives the photos of the nanoparticle dispersions under irradiation of 450 nm LED; their difference in brightness could be readily distinguished by the naked eye. TEM observations showed that all the particles have almost the same average size and similar spherical morphology. Figure 2 gives the TEM images of the 0 h doping, 3 h doping, and 8 h doping particles. The average size of all the three samples was about 103 nm with a relative standard deviation of ∼0.2, which was almost the same as that of the pure silica particles prepared by the St€ober method (see Figure S2 in the Supporting Information). These results indicated that the dye molecules introduced at each addition time has little effect on the St€ober process because of the very low ionic strength brought out by the dissolved Ru(phen)32þ and Cl- ions.24 It is reasonable to conclude that the difference in the emission properties of the particles is not related to the particle size. Conductivity measurements as well the consumption of TEOS were taken to identify each stages of the St€ober process in our experiments. Figure 3 shows the variation of the conductivity and the consumption of TEOS in the ethanol-ammonia mixture as a function of time. The conductivity increased rapidly at the initial stage of the reaction (0-10 min), and nearly 10% of TEOS was consumed at this period, which should correspond to the fast hydrolysis of TEOS and the condensation of the hydrolyzed TEOS.21 After 10 min, the conductivity increased slowly and reached a maximum at 28 min. During this period, the condensation rate increased gradually and reached a balance with the consumption rate of TEOS at 28 min of the reaction.21 After 28 min, the conductivity decreased gradually, indicating the condensation of TEOS became dominant in the reaction although there was 80% of TEOS remained in the solution. Temporal evolution of the size and shape of silica particles in the reaction was followed by TEM (Figure 4). In order to suspend the possible reaction during preparation of the TEM samples,28 the solution was cooled down to -20 C before preparing the samples. After 13 min of the reaction, small silica particles became observable, which were considered as aggregates of the oligomers or the primary particles since the hydrolysis of TEOS was still faster than the condensation of hydrolyzed TEOS at this time. Between 1 and 2 h of the reaction, there should be continuous formation of the oligomers and primary particles since the (28) Bailey, J. K.; Mecartney, M. L. Colloids Surf. 1992, 63, 151–161.
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Figure 3. Conductivity measurements (9, right axis) and the TEOS consumption rate (2, left axis) of the as-prepared St€ ober condition without dye doping. The inset is the conductivity measurement data from 0 to 80 min.
consumption of TEOS is still pretty fast. TEM observations showed that the particles became larger attributed to the aggregation of the oligomers and primary particles. Around 3 h of the reaction, the hydrolysis of TEOS became slow, and there was only ∼25% of TEOS remaining in the reaction, indicating the formation process of the oligomers and primary particles almost came to an end at this time. TEM observations showed that the particles became more and more uniform in size in the following stages. Thus, it is reasonable to consider the particles existed at 3 h of the reaction as the secondary ones and the subsequent growth of the particles was dominated by the monomer attachment fashion.12,24 From the above experimental results, it is speculated that the difference in emission properties of the silica particles prepared by introducing the dye at different stages of the St€ober process is related to the different kinds of silica species presented in the solution. To illustrate this assumption, variations in emission intensity and maximum of the dye in the St€ober process were followed as shown in Figure 5. If the dye was introduced at the initial stage of the reaction (0 h), the maximum of the emission peak experienced a rapid red shift from 588 to 600 nm in 10 min and then experienced a rapid blue shift from 600 to 593 nm after 20 min. In the following stages of the reaction, there is almost no shift of the emission maximum. The positively charged dye molecules can interact with Si-O- groups of hydrolyzed TEOS, the oligomers, and primary particles through electrostatic interactions. Upon the formation of the oligomers and primary particles, the dye molecules have a great opportunity to approach each other and thus showed red-shifted emission. The increased emission intensity at this time should be attributed to the protection effect of the matrix.8,9,29,30 As the reaction proceeded, the further condensation of the hydrolyzed TEOS and oligomers would further prohibit the contact of the dye molecules from the (29) Maruszewski, K.; Strommen, D. P.; Kincaid, J. R. J. Am. Chem. Soc. 1993, 115, 8345–8350. (30) Castellano, F. N.; Heimer, T. A.; Tandhasetii, M. T.; Meyer, G. J. Chem. Mater. 1994, 6, 1041–1048.
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Figure 4. TEM pictures of the silica species existed at different times (13 min, 1 h, 2 h, 3 h, 6 h, and 8 h) of the particle formation and growth. The scale bar is 200 nm.
Figure 5. Temporal evolutions of the emission maximum (top) and the emission intensity (bottom) of the dye added at different times during the doping reaction (9, 0 h doping process; f, 3 h doping process; 2, 8 h doping process; 0, 8 h-doping process with additional TEOS).
oxygen and solvent; thus, the dye presented blue-shifted and intensified emission until 20 min. After that, the maximum of the emission peak almost remained unchanged, implying that the effect of the oxygen and solvent is neglectable until the end of the reaction. It is noted that the emission intensity decreased about 46% during this period, indicating that the dye molecules still have the possibility to approach each other during the following stage,31 especially before the disappearance of the oligomers and primary particles (3 h). When the dye was introduced at 3 h of the reaction, the maximum of the emission peak experienced a gradual blue shift from 594 to 578 nm, and no shift of the emission maximum was observed after 8 h. As shown by TEM observations, the average diameter of the particles was about 89 nm at 3 h of the reaction and increased to about 99 nm at 8 h of the reaction, corresponding to the growth of silica layer with a thickness of about 5 nm on the (31) Costa, C. A. R.; Leite, C. A. P.; Galembeck, F. J. Phys. Chem. B 2003, 107, 4747–4755.
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particle surface. After addition of the dye, the reaction system was cooled down to -20 C immediately and centrifuged at 15 000 rpm for 15 min. There was no emission of the dye detectable in the supernatants, indicating the dye molecules added were absorbed completely on the particle surface. Therefore, the blue-shifted emission of the dye in the period of 3-8 h of the reaction was attributed to the growth of the silica layer which shielded the dye from the oxygen and solvent. Along with the deposition of the silica protection layer, the emission intensity of the dye also increased gradually and kept almost unchanged after 8 h. These results mean that the silica layer with a thickness of about 5 nm is enough to shield the dye from the oxygen and solvent during the synthesis process. When the dye was introduced at 8 h, the maximum of the emission peak experienced a gradual blue shift from 594 to 581 nm until the end of the reaction. Similar to the situation of 3 h doping, no emission of the dye was detectable in the supernatants after the centrifugation treatment, indicating the dye molecules added were absorbed completely on the particle surface. The blue-shifted emission was likely to be attributed to the protection effect of the silica layer deposited but not the suppressed aggregation of the dye molecules during the subsequent reaction. At 3 h of the reaction, there was 25% of TEOS remaining in the reaction system, which was enough to allow the growth of silica layer with a thickness of 5 nm at the end of the reaction. At 8 h of the reaction, there was only 5% of TEOS remaining in the solution, which can only supply the growth of a silica layer with a thickness of 2 nm until the end of the reaction. To further illustrate the protection effect of the silica matrix, 0.6 mL of TEOS was added into the reaction system immediately after the addition of the dye; it was observed that the emission intensity and maximum of the resulting dye doped particles were the same as those of the 3 h doping ones. However, if additional TEOS was supplied for the 0 h doping particles and 3 h doping particles, there was almost no change in their emission intensity and maximum (see Figure S3 in the Supporting Information). These results indicated that once the dye was introduced after 3 h of the reaction, the decrease in emission intensity and red-shifted emission maximum with the delayed addition time is attributed to the decreased thickness of the silica protection layer but not the change in degree of aggregation of the dye molecules. For the dye doped particles prepared by adding the dye before 3 h of the reaction, the increased emission intensity and blue-shifted Langmuir 2010, 26(9), 6657–6662
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emission should be attributed to the suppressed aggregation of the dye molecules with delayed addition time but not the protection effect of the silica layer. Time-resolved luminescence measurements were carried out to further understand the difference in emission properties of the dye doped silica nanoparticles prepared by introducing the dye at different stages of the St€ober process. The decay curves (Figure 6) could be fitted well with a double-exponential function, and the data of the time-resolved luminescence measurements and the fitting results are illustrated in Table 1. The percentage of the long
lifetime increased from 14% to 56% and the average lifetime from 0.53 to 2.53 μs when addition time of the dye was delayed from 0 to 3 h, which is consistent with the suppressed aggregation of the dye molecules with delayed addition time.7,10,12 When the addition time was delayed from 3 to 8 h, the percentage of the long lifetime decreased from 56% to 6%, and the average lifetime decreased from 2.53 to 0.23 μs. The average lifetime of the 8 h doping nanoparticles was 0.23 μs, which was close to that of the dye in ethanol solution (∼0.14 μs). Figure 7 shows the variation in the average lifetime and emission intensity of the dye doped particles prepared by introducing the dye at different stages of the St€ober process. The changes in the average lifetime and emission intensity showed the same tendency with the delayed addition time, which further supported the conclusion that both the suppressed aggregation and protection effect of silica matrix are vital important to improve the emission efficiency of the dye doped silica particles. Based on the above experimental results, the formation and growth of the dye doped silica particles are proposed as shown in Scheme 1. If the dye was introduced at the beginning of the reaction, the hydrolysis and condensation of TEOS go rapidly under the catalysis of the ammonia. The dye molecules have a
Figure 6. Normalized emission decay curves of the Ru(phen)32þ doped silica nanoparticles prepared by introducing the dye at different time of the reaction. Table 1. Spectroscopic Properties and Fitting Results of the Emission Decay Curves of the Ru(phen)32þ Doped Silica Nanoparticles sample
λabs (nm)
λem (nm)
τ1 (μs)
τ1 (%)
τ2 (μs)
τ2 (%)
Æτæ (μs)
0 h doping 13 min doping 1 h doping 3 h doping 6 h doping 8 h doping
450 450 450 450 450 450
592 580 577 575 577 582
0.26 0.32 0.32 0.42 0.12 0.07
86 76 51 44 75 94
0.97 1.81 2.40 2.79 1.30 0.55
14 24 49 56 24 6
0.53 1.26 2.15 2.53 1.04 0.23
Figure 7. Comparison of the emission intensity (9) and the average lifetime (0) of the doped silica nanoparticles prepared by introducing the dye at different time of the reaction.
Scheme 1. Schematic Illustration of the Silica Species at Different Stage of the St€ ober Reaction Corresponding to Different Reaction Time and the Final Situation of the Dye Molecules Added at Certain Time in the Particles
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great opportunity to approach each others since they can interact with the hydrolyzed TEOS through electrostatic interactions. If the dye molecules were introduced at later stage of the reaction, for example, 1 h, they still have the opportunity to approach each other upon the condensation and aggregation of the oligomers and primary particles. As a result, the emission intensity of the particles will be suppressed attributed to the promoted electron tunneling among the dye molecules.32,33 However, the aggregation of dye molecules was suppressed to some extent due to the existence of the oligomers and primary particles. As the reaction proceeded to 3 h, the oligomers and primary particles are consumed and the secondary particles become dominant in the solution. If the dye molecules were added at this stage, they will be absorbed on surface of the secondary silica particles and silica protection layer will be grown gradually on the particle surface upon the deposition of the remaining monomers. The silica protection layer will be not thick enough to shield the doped dye molecules from the oxygen and solvent if they were added too late, for example, the 8 h doping particles. However, the emission properties of the doped particles prepared by introducing the dye after 3 h can be recovered by supplying additional TEOS. As a result, the emission properties of the dye doped silica particles can be manipulated by changing the aggregation degree and the thickness of the silica protection layer, which are related to the addition time of the dye during the St€ober process. To elucidate the effect of the oxygen and solvent on the emission properties of the particles, the emission spectra were also collected with oxygenated and oxygen-free suspensions (see Figure S4 in the Supporting Information). It is found for the 8 h doping particles the emission intensity of the oxygenated suspension was about 40% of the aerated one, and the intensity of the oxygen-free suspension was about 3.5 times that of the aerated one. These results indicate that the oxygen dissolved in the solvent is an effective quencher of the dye.7 For the 3 h doping particles, (32) Miyashita, T.; Hasegawa, Y.; Matsuda, M. J. Phys. Chem. 1991, 95, 9403– 9405. (33) Zou, G.; Fang, K.; He, P. S.; Lu, W. X. Thin Solid Films 2004, 457, 365–371. (34) Glomm, W. R.; Volden, S.; Sj€oblom, J.; Lindgren, M. Chem. Mater. 2005, 17, 5512–5520.
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the emission intensity kept almost unchanged whether the oxygenated or oxygen-free suspension was used, indicating the emission properties of the 3 h doping particles are less sensitive to the dissolved oxygen than the 8 h doping ones due to the thicker silica shell deposited. It is noted that the emission intensity of the oxygen-free suspension of the 8 h doping particles is only 58% of that of the 3 h doping ones, suggesting that besides the oxygen the solvent also affects the emission properties of the doped silica particles via the nonradiative thermal relaxation.34
4. Conclusion A series of Ru(phen)32þ doped silica nanoparticles were prepared by introducing the dye at different stages of the St€ober process. The emission properties of the as-prepared dye doped particles are dependent on the time of the dye introduced into the reaction. The silica particles prepared by adding the dye before 3 h of the reaction showed increased emission intensity and blueshifted emission maximum with the delayed addition time compared to those of the 0 h doping particles due to the suppressed aggregation of the dye molecules in the silica matrix, whereas the silica particles prepared by adding the dye during the period of 3-8 h of the reaction showed decreased emission intensity and red-shifted emission maximum with the delayed addition time compared to those of the 3 h doping particles due to the increased effect of the solvent and the dissolved oxygen. This work demonstrated that the emission properties of dye doped silica particles prepared by employing the conventional St€ober process are tunable by simply changing the addition time of the luminescent dye molecules. Acknowledgment. This work was supported by the National Nature Science Foundation of China (20773053, 50825202), the National Research Fund for Fundamental Key Project (2009CB939701), and the Program for NCET in University of Chinese Ministry of Education. Supporting Information Available: Figures S1-S4. This material is available free of charge via the Internet at http:// pubs.acs.org.
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