Novel Highly Charged Silica-Coated Tb(III) Nanoparticles with

Jan 30, 2009 - Asiya R. Mustafina*, Svetlana V. Fedorenko, Olga D. Konovalova, Anastasiya Yu. ... Chemistry, Arbuzov Street, 8, 420088, Kazan, Russia,...
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3146

Langmuir 2009, 25, 3146-3151

Novel Highly Charged Silica-Coated Tb(III) Nanoparticles with Fluorescent Properties Sensitive to Ion Exchange and Energy Transfer Processes in Aqueous Dispersions Asiya R. Mustafina,*,† Svetlana V. Fedorenko,† Olga D. Konovalova,† Anastasiya Yu. Menshikova,‡ Nataliya N. Shevchenko,‡ Svetlana E. Soloveva,† Alexander I. Konovalov,† and Igor S. Antipin§ A. E. ArbuzoV Institute of Organic & Physical Chemistry, ArbuzoV Street, 8, 420088, Kazan, Russia, Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoi 31, 199004, St. Petersburg, Russia, and A. M. ButleroV Chemistry Institute, Kazan State UniVersity, KremlyoVskaya Street, 18, 420008, Kazan, Russia ReceiVed October 3, 2008. ReVised Manuscript ReceiVed December 17, 2008 Novel silica-coated Tb(III) nanoparticles with high luminecsence were synthesized using the reverse microemulsion procedure. The quenching of luminescent properties of these nanoparticles can be achieved by ion exchange and energy transfer mechanisms. The quenching through the ion exchange of Tb(III) by H+ or La(III) is time dependent, indicating that the ion exchange is probably diffusion controlled. The quenching by Co(III) complex cations is achieved by the energy transfer mechanism and thus is not time dependent. The analysis of quenching data in Stern-Volmer cooordinates reveal the negative charge of the silica-coated Tb(III)-TCAS nanoparticles and several types of luminophoric species, located within the core and close to the surface of silica nanoparticles.

Introduction The development of nanoparticles functionalized by metal complexes is on the top of current interest today. This interest is determined by their applicability in bioanalysis.1-18 The attractivity of silica nanoparticles results from the well-developed procedures of their synthesis with controlled size and uniformity, as well as nontoxicity.19,20 From the viewpoint of the application in bioanalysis, nanoparticles are most commonly functionalized by luminescent,1-8 redox active complexes9-19 or complexes acting as MRI agents.18 There are two main strategies to insert metal complexes into silica nanoparticles: doping inside the * Corresponding author. E-mail: [email protected]. † A. E. Arbuzov Institute of Organic & Physical Chemistry. ‡ Institute of Macromolecular Compounds. § Kazan State University.

(1) Schmidt, H. Appl. Organometal. Chem. 2001, 15, 331. (2) Tansil, N. C.; Gao, Z. Nano Today 2006, 1, 28. (3) Yan, J.; Este´vez, M. C.; Smith, J. E.; Wang, K.; He, X.; Wang, L.; Tan, W. Nano Today 2007, 2, 44. (4) Tan, M.; Wang, G.; Hai, X.; Ye, Z.; Yuan, J. J. Mater. Chem. 2004, 14, 2896. (5) Ye, Z.; Tan, M.; Wang, G.; Yuan, J. Anal. Chem. 2004, 76, 513. (6) Ye, Z.; Tan, M.; Wang, G.; Yuan, J. J. Mater. Chem. 2004, 14, 851. (7) Ye, Z.; Tan, M.; Wang, G.; Yuan, J. Talanta 2005, 65, 206. (8) Yuan, J.; Wang, G. Trends Anal. Chem. 2006, 25, 490. (9) Zhu, N.; Cai, H.; He, P.; Fang, Yu. Anal. Chim. Acta. 2003, 481, 181. (10) Lian, W.; Litherland, S. A.; Badrane, H.; Tan, W.; Wu, D.; Baker, H. V.; Gulig, P. A.; Lim, D. V.; Jin, S. Anal. Biochem. 2004, 334, 135. (11) Chang, Z.; Zhou, J.; Zhao, K.; Zhu, N.; He, P.; Fang, Yu. Electrochim. Acta 2006, 52, 575. (12) Hun, X.; Zhang, Zh. Talanta 2007, 73, 366. (13) Wei, H.; Zhou, L.; Li, J.; Liu, J.; Wang, E. J. Colloid Interface Sci. 2008, 321, 310. (14) Wang, X.-Y.; Yun, W.; Zhou, J.-M.; Dong, P.; He, P.-G.; Fang, Y.-Z. Chin. J. Chem. 2008, 26, 315. (15) Wei, H.; Liu, J.; Zhou, L.; Li, J.; Jiang, X.; Kang, J.; Yang, X.; Dong, S.; Wang, E. Chem.;Eur. J. 2008, 14, 3687. (16) Zhang, L.; Liu, B.; Dong, S. J. Phys. Chem. B 2007, 111, 10448. (17) Plumere, N.; Speiser, B. Electrochim. Acta 2007, 53, 1244. (18) Wu, C.; Hong, J.; Guo, X.; Huang, C.; Lai, J.; Zheng, J.; Chen, J.; Mu, X.; Zhao, Yi. Chem. Comm. 2008, 750. (19) Eastoe, J.; Hollamby, M. J.; Hudson, L. AdV. Colloid Interface Sci. 2006, 128-130, 5. (20) Yao, L.; Xu, G.; Dou, W.; Bai, Y. Colloids Surf., A: Physicochem. Eng. Aspects 2008, 316, 8.

core5-16 and functionalization of the surface.17,18 The metal complex-doped nanoparticles have an advantage over those functionalized through the surface, since the silica shell protects the metal complex from photodegradation, while the surface of the nanoparticle can be functionalized by various anchoring groups. Two main procedures are usually used to synthesize luminophore-doped silica nanoparticles. The first is the Sto¨ber method, which is more convenient for the incorporation of hydrophobic luminophores.21 The second is the reverse microemulsion strategy, which requires hydrophilic species for insertion inside the silica shell.22 The second procedure is more preferable to obtain uniform monodisperse nanoparticles.23 Luminescent lanthanide complexes3-8 and redox-active Co(III) and Ru(II) tris-dipyridyls9-16 are most commonly inserted inside silica nanoparticles. Metal complexes of calixarenes are well-known as luminophores,24,25 MRI agents,26 and redox-active species.27 Calixarenes have several advantages, which make them very promising as ligands for metal complexes with high luminescence or relaxivity. The first advantage is the ease of calix[n]arene functionalization, resulting in the wide variation of ligands with various complexabilities and water solubilities.28 The second advantage is derived from the unique three-dimensional bowllike structure of calix[n]arenes.28 The three-dimensional cavity (21) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (22) Arriagada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1999, 211, 210. (23) Bagwe, R. P.; Yang, C.; Hilliard, L. R.; Tan, W. H. Langmuir 2004, 20, 8336. (24) de Sa, G. F.; Malta, O. L.; Donega, C.; de, M.; Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; da Silva, E. F., Jr Coord. Chem. ReV. 2000, 196, 615. (25) (a) Sabbatini, N.; Casnati, A.; Fischer, C.; Guardigli, Manet, I.; Sarti, G.; Ungaro, R. Inorg. Chim. Acta 1996, 252, 19–24. (b) Kajiwara, T.; Katagiri, K.; Hasegawa, M.; Ishii, A.; Ferbinteanu, M.; Takaishi, S.; Ito, T.; Yamashita, M.; Iki, N. Inorg. Chem. 2006, 45, 4880–4882. (26) (a) Bryant, L. H., Jr.; Yordanov, A. T.; Linnoila, J. J.; Brechbiel, M. W.; Frank, J. A. Angew. Chem., Int. Ed. 2000, 39, 1641. (b) Aime, S.; Barge, A.; Botta, M.; Casnati, A.; Fragai, M.; Luchinat, C.; Ungaro, R. A. Angew. Chem., Int. Ed. 2001, 40, 4737. (27) Guillemot, G.; Castellano, B.; Prange, T.; Solari, E.; Floriani, C. Inorg. Chem., 2007, 46, 5152. (28) Mandolini, L., Ungaro, R., Eds. Calixarenes in Action; Imperial College Press: London, 2000; p 270.

10.1021/la8032572 CCC: $40.75  2009 American Chemical Society Published on Web 01/30/2009

Tb-TCAS Silica Nanoparticles Scheme 1. Schematic Representation of the Tb-TCAS Complex

results in preorganized donor groups on the upper and lower rims of calix[4]arene, which enables obtaining very stable complexes due to multicentered coordination of metal ions.28 For example, the water-soluble macrocycle-p-sulfonatothiacalix[4]arene (TCAS) shown on the Scheme 1 possesses donor groups for metal ion binding on both the upper and lower rims.29,30 Moreover, TCAS, its sulfoxide, and sulfonic derivatives are especially important from the viewpoint of luminophoric properties and stability of their lanthanide complexes.30 Taking into account the above-mentioned reasons, it is of a great interest to obtain new luminophore silica nanoparticles by insertion of TCAS metal complexes inside their core. We have chosen Tb(III) ion as a metal ion since its complex with TCAS generated in neutral and weakly alkaline media is rather hydrophilic, very stable, and exhibits efficient luminescence.30 The reverse microemulsion procedure was chosen to get Tb(III)-TCASdoped silica nanoparticles. The present report outlines the data concerning the synthesis of a new type of luminophoric nanoparticle and its luminophoric properties in aqueous dispersions, including the dependence of its luminescent properties on the concentration and pH conditions of the aqueous dispersion.

Experimental Section Materials. Tetraethyl orthosilicate (TEOS) 98%, ammonium hydroxide (28-30%), n-heptanol 98%, cyclohexane 99%, [Co(NH3)6]Cl3 99,999%, K3[Co(CN)6] 95% were obtained from Acros, terbium(III) nitrate hexahydrate (99.9%) was purchased from Alfa Aesar, and Triton X-100 was obtained from Sigma-Aldrich. The synthesis of TCAS and complexes [Co(dipy)3](ClO4)3 and [Co(en)2ox]Cl were carried out according to the known procedures.31a-c Safety Note. Perchlorate salts of metal complexes are potentially explosiVe and should be handled with care. In particular, they should neVer be heated as solids.31d K3[Co(CN)6] is hazardous. Synthesis of TCAS-Tb(III) Complex and Its Nanoparticles. The precipitate obtained by the addition of Tb(NO3)3 to the aqueous solution of TCAS (CTb ) CTCAS ) 7.8 mM) was dispersed by ultrasonication for 10 min. The obtained suspension (CTb-TCAS ) 7.8 mM) was used in the synthesis of luminophoric silica nanoparticles according to the reverse microemulsion procedure.5 A mixture of Triton X-100 (2.38 g), n-heptanol (2.29 mL), cyclohexane (9.32 mL), TEOS (0.2 mL), and a 1.1 mL aqueous suspension of Tb-TCAS complex (CTb-TCAS ) 7.8 mM) was prepared and stirred for 30 min. The obtained water-in-oil (W/O) microemulsion was mixed with a W/O microemulsion containing Triton X-100 (2.38 g), n-heptanol (2.29 mL), cyclohexane (9.32 mL), and aqueous solutions of NH3 (28-30%) with stirring. After 24 h of stirring, dye-doped silica nanoparticles were precipitated from the microemulsion by adding acetone, centrifuging, washing by solutions of ethanol-acetone (1:1), ethanol (two times), and water (several times). We used ultrasonication while washing the silica nanoparticles to remove surfactant and physically absorbed Tb-TCAS complex from the particles’ surfaces. (29) Liu, Y.; Wang, H.; Wang, L.-H.; Zhang, H.-Y. Thermochim. Acta 2004, 414, 65–70. (30) Iki, N.; Horiuchi, T.; Koyama, K.; Morohashi, N.; Kabuto, Ch.; Miyano, S. J. Chem. Soc., Perkin Trans. 2 2001, 2219.

Langmuir, Vol. 25, No. 5, 2009 3147 Silica-coated Tb-TCAS nanoparticles are stable enough within a year of their storage at room temperature and in refrigerated conditions in aqueous dispersions (6.3 g/L), which is evident from their steady luminescent properties. Spectroscopic Measurements. The steady-state emission spectra were recorded on a spectrofluorometer FL3-221-NIR (Jobin Yvon) under 330 nm excitation. All samples of definite concentrations of nanoparticles (C ) 0.0189 g · L-1), admixtures and pH-values were ultrasonicated before measurements. The pH range of 6.5-7.0 of aqueous dispersions was adjusted by Tris buffer (2.5 mM), while the required pH values in the range 2-4 were adjusted by the addition of HCl in the required amounts. UV-vis spectra were recorded on a Lambda 35 spectrophotometer (Perkin-Elmer). Aqueous solutions of Tb(III)-TCAS (C ) 5 × 10-2 mM) for spectrophotometric measurements were prepared by mixing definite aliquots of stock solutions of Tb(NO3)3 (C ) 0.1 mM) and TCAS (C ) 0.1 mM) with further adjusting the pH of 7.18 by aqueous solutions of NH3. The concentration of the Tb(NO3)3 stock solution was evaluated by standardization with EDTA at pH 6, using xylenol orange as the indicator.31e Conductivity Measurements. Conductivity measurements were performed on an inoLab Cond Level 1 in a thermostatted bath at t ) 20 ( 0.1 °C at a constant concentration of silica nanoparticles (C ) 1.1025 g · lL-1) and concentration of [Co(dipy)3](ClO4)3 varied within 0-0.4 mM. All samples were made from the stock solutions of [Co(dipy)3](ClO4)3 (5 mM), silica nanoparticles (18.0 g · lL-1), and silica-coated Tb(III)-TCAS nanoparticles (6.3 g · L-1). All measurements were at least duplicated. All aqueous solutions were prepared from doubly distilled water (2-3 µSm/cm).

Results and Discussion SynthesisandCharacterizationofSilica-CoatedTb(III)-TCAS Nanoparticles. According to literature data, Tb(III) exhibits great enhancement of its luminescence when it is coordinated by phenolate groups of the lower rim of TCAS (Scheme 1). Definite pH conditions (pH > 8) are required to generate Tb(III)-TCAS complex with high luminescence.30,32 Though this coordination mode does not provide efficient dehydration of the inner sphere of Tb(III), Tb(III)-TCAS complex (Scheme 2) exhibits very efficient luminescent properties in aqueous solutions with quantum yield (φ ) 0.141) and fluorescence lifetime (τ ) 0.726 ms) due to efficient antennae effect, provided by TCAS.30,33 The nanoparticles were prepaired according to the known procedure by hydrolysis of TEOS with ammonium hydroxide in a W/O microemulsion,5 containing Tb(III)-TCAS complex. In order to optimize the procedure of Tb(III)-TCAS encapsulation into silica nanoparticles, the synthesis was performed at various concentrations of Tb(III)-TCAS. The silica-coated Tb(III)-TCAS nanoparticles were characterized by transmission electron microscopy (TEM) using a JEOL JEM 100 S microscope, Japan. As shown in Figure 1, the nanoparticles are sperical and uniform in size (40 ( 5 nm). The aqueous dispersions of nanoparticles were analyzed by absorption and emission spectra. The absorption spectra of Tb(III)-TCAS-doped nanoparticles possesses bands at 245 and 316 nm, which are inherent to the Tb(III)-TCAS absorption spectrum in aqueous solution (Figure 2). (31) (a) Iki, N.; Fujimoto, T.; Miyano, S. Chem. Lett. 1998, 27, 625. (b) Ferguson, J.; Hawkins, C. J.; Kane-Maguire, N. A. P.; Lip, H. Inorg. Chem. 1969, 8, 771. (c) Dwyer, F. P.; Reid, I. K.; Garvan, F. L. J. Am. Chem. Soc. 1961, 83, 1285. (d) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335. (e) Merck, E. Methodes d’Analyses Complexometriques par les Titriplex; Merck: Darmstadt, Germany, 1964. (32) Amirov, R.; McMillan, Z.; Mustafina, A.; Chukurova, I.; Solovieva, S.; Antipin, I.; Konovalov, A. Inorg. Chem. Commun. 2005, 8, 821. (33) Skripacheva, V. V.; Mustafina, A. R.; Rusakova, N. V.; Yanilkin, V.V.; Nastapova, N.V.; Amirov, R. R.; Burilov, V. A.; Zairov, R. R.; Kost, S. S.; Solovieva, S. E.; Korovin, Yu. V.; Antipin, I. S.; Konovalov, A. I. Eur. J. Inorg. Chem. 2008, 3957–3963.

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Scheme 2. Schematic Representation of the Ion Exchange of the Tb-TCAS Complex

The aqueous dispersions of nanoparticles obtained exhibit very efficient luminescence, which confirms the formation of Tb(III)-TCAS silica-coated nanoparticles (Figure 3). The comparison of emission spectra of Tb(III)-TCAS complex in aqueous solutions and within the silica core reveals that Tb(III)-TCAS in both above-mentioned cases show the same spectrum patterns [5D4f7F6(489nm), 5D4f7F5(543nm), 5 D4f7F4(582nm), 5D4f7F3(620nm)] without any emission band shift (Figure 3). Tb(III)-TCAS complex exhibits restricted solubility in aqueous solutions, being in the form of precipitate at concentrations of more than 2.5 mM. Thus, Tb(III)-TCAS species can be inserted into the W/O microemulsion in the form of true solution or aqueous dispersion. These two procedures were used to obtain Tb(III)-TCAS-doped nanoparticles. The nanoparticles obtained according to both procedures at the same concentration of Tb(III)-TCAS (2.5 mM) exhibit similar emission properties. The increase of Tb(III)-TCAS concentration in the W/O microemulsion from 2.5 to 7.8 mM results in nanoparticles with high Tb(III)-centered luminescence. The quantitative analysis of luminescence data was performed to obtain information on the number of Tb-TCAS complexes doped within the silica network at varied Tb(III)-TCAS concentrations in the W/O microemulsion. Assuming the density of the nanoparticles is equal to that of pure silica (1.96 g/cm3),34 the weight of one nanoparticle (r ) 20 nm) calculated according to eq 15 is 6.6 × 10-17 g, the concentration of nanoparticles in their aqueous dispersions can be obtained.

m ) 4 ⁄ 3 · F · πr3

(1)

The luminescent spectra of silica-coated Tb(III)-TCAS nanoparticles were performed for aqueous dispersions with 0.0189 g · L-1 content of nanoparticles, which correspond to 4.77 × (34) Santra, S.; Zhang, P.; Wang, K.; Tapec, R.; Tan, W. Anal. Chem. 2001, 73, 4988.

10-10 M. The evaluation of the quantities of luminescent species doped within a silica network of silica-coated Tb(III)-TCAS nanoparticles (n) is based on the assumption that the luminescent intensity of Tb(III)-TCAS complex is the same in aqueous solution and inside silica nanoparticles. Thus the fluorescence intensities of silica-coated Tb(III)-TCAS nanoparticles synthesized at various Tb(III)-TCAS concentrations coincide with various amounts of luminescent species inside each nanoparticle (Table 1). The highest level is 4900 luminescent complexes doped within a silica network, which is much more than that for silicacoated BPTA-Tb(III) nanoparticles (1500).5 According to literature data, the luminescence intensity of luminophores inside a silica network can be increased as well as decreased in comparison with the intensity in aqueous solutions as a result of restricted quenching by oxygen and water molecules or concentration-induced self-quenching.35 The concentration of Tb(III)-TCAS complex can also be evaluated from the absorption spectra of Tb(III)-TCAS-doped nanoparticles using the assumption that molar absorption of Tb(III)-TCAS complex is the same in aqueous solutions and within nanoparticles. The comparison of fluorescence intensities of nanoparticles and Tb(III)-TCAS complex in aqueous solutions reveals the fluorescence intensity of Tb(III)-TCAS complex inside a silica core being nearly 6 times more than the intensity of a similar complex in aqueous solution at the same concentrations of Tb(III)-TCAS (Figure 3). Taking into account that the abovementionedevaluationsareapproximate,silica-coatedTb(III)-TCAS complex exhibits enhanced luminescence in comparison with Tb(III)-TCAS complex in aqueous solution, thus indicating the lack of quenching of Tb-centered luminescence by a silica network, as it was revealed in a previous article.7 Such behavior may be the result of two opposite processes. The first is a selfquenching induced by the close proximity of luminescent complexes inside a silica core. The second is an isolation of luminescent complexes from the quenching by the aqueous environment. The data obtained indicate that the self-quenching is less pronounced than the effect of the protection from the quenching by environment. According to literature data, the selforganization of luminophoric species within a silica network36 also can affect their luminescent properties inside a silica core, but at the moment there is no data for a more detailed discussion. Effect of Additives (H+, La3+, Complexes of Co3+) on Fluorescent Properties of Silica-Coated Tb(III)-TCAS Nanoparticles in Aqueous Dispersions. The most significant application of luminiphoric nanoparticles is their use as bioassays after surface functionalization with further bioconjugation to biomolecules.37 The Tb(III)-TCAS silica-coated nanoparticles provide an opportunity for further surface modification, which we are going to perform in the near future. Another strategy is application of nanoparticles as nanosensors, which is attained by the encapsulation of pH- or substrate-sensitive dyes inside nanoparticles. As it was mentioned above, Tb(III)-TCAS complex is also pH dependent.30,32 Moreover, Tb(III)-TCAS complex exhibits rather efficient complex ability toward organic molecules and metal complexes (Scheme 2) due to the presence of the cavity, surrounded by sulfo-groups on the upper rim.38-42 Such ternary complex formation can be detected through the (35) Lis, S. J. Alloys Compd. 2002, 341, 45. (36) Prodi, L. New J. Chem. 2005, 29, 20. (37) Burns, A.; Sengupta, P.; Zedayko, T.; Baird, B.; Weisner, U. Small 2006, 2, 723. (38) Gao, F.; Tang, L.; Dai, L.; Wang, L. Spectrochim. Acta, Part A 2007, 67, 517. (39) Guo, Q.; Zhu, W.; Ma, S.; Dong, S.; Xu, M. Polyhedron 2004, 23, 1461. (40) Liu, Y.; Guo, D.-S.; Zhang, H.-Y.; Ma, Y.-H.; Yang, E.-C. J. Phys. Chem. B 2006, 110, 3248.

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Figure 1. TEM image of silica-coated Tb(III)-TCAS fluorescent nanoparticles.

Figure 2. UV-vis spectra of Tb(III)--TCAS (1), (aqueous solution, C ) 5 × 10-2 mM, pH ) 7.18) and an aqueous dispersion of Tb(III)-TCAS silica-coated nanoparticles (2) (C ) 0.84 g · l-1) in comparison with net silica nanoparticles (3).

Figure 3. Corrected emission spectra of aqueous solutions of Tb(III)-TCAS (1, 1:1, C ) 0.5 µM) and Tb(III)-TCAS-doped silica nanoparticles (2, C ) 0.0189 g · L-1), in Tris buffer (C ) 2.5 mM) at pH ) 6.89. Table 1. The Quantity of Tb(III)-TCAS Species (n) within Silica Nanoparticles Synthesized at Various Concentrations of Tb(III)-TCAS (CTb-TCAS) in W/O Microemulsion CTb-TCAS, mM

n

2.5 5.0 7.8

1140 1550 4900

change of luminophoric properties, if additional substrate serves as the antennae or quencher of Tb-centered luminescence.33,42 As we previously showed, the binding of some Co(III) complexes with Tb(III)-TCAS quenchs Tb(III)-centered luminescence in (41) Mustafina, A.; Skripacheva, V.; Gubaidullin, A.; Latipov, Sh.; Toropchina, A.; Yanilkin, V.; Solovieva, S.; Antipin, I.; Konovalov, A. Inorg. Chem. 2005, 44, 4017. (42) Mustafina, A.; Skripacheva, V.; Gubskaya, V.; Gruner, I.; Solovieva, S.; Antipin, I.; Konovalov, A.; Habicher, W. D. Russ. Chem. Bull. 2004, 53, 1511.

Figure 4. Corrected emission spectra of Tb(III)-TCAS-doped silica nanoparticles (C ) 0.0189 g · L-1) at various pHs, where 1 represents that in pure water (pH ) 6.78) and 2-4 denote that at the various pH-values (2: pH ) 4; 3: pH ) 3; 4: pH ) 2), after 3 days of stirring.

aqueous media as a result of energy transfer processes.33 Thus the goal of the present work is to examine the effect of cations, which can quench the Tb-centered luminescence ([Co(dipy)3]3+, [Co(NH3)6]3+, [Co(CN)6]3-, [Co(en)2ox]+) of Tb(III)-TCASdoped silica nanoparticles. It is also worth noting that Co(III) complexes can be regarded as models of Co(III)-containing biomolecules such as vitamin B12.43 Since luminescent properties of Tb(III)-TCAS complex in aqueous media can be decreased through the ion exchange processes the effect of H+ or La3+ on the luminescence of silica-coated Tb(III)-TCAS nanoparticles will be discussed first of all. The acidification results in the decay of Tb(III)-TCAS complex as a result of substitution of Tb(III) ions by protons, which is evident from the dramatic decrease of Tb-centered luminescence intensity.30 Silica-coated Tb(III)-TCAS nanoparticles are also pH dependent, as is obvious from Figure 4. However, the effect of pH on the emission spectra of silicacoated Tb(III)-TCAS nanoparticles is time dependent, which is unlike Tb(III)-TCAS complex in aqueous solutions. The adjustment to pH 2 should result in the whole decay of Tb(III)-TCAS, while the admixture of HCl in required amounts (to achieve pH 2) results in the partial decay, which is evident from the emission intensities (Figure 5). The data presented in Figure 6 indicate that the whole decay of the complex at pH 2 is gained only after 3 days of stirring of the nanoparticles at pH 2. It is also worth noting that 3 days of stirring of an aqueous dispersion of Tb(III)-TCAS nanoparticles (C ) 0.0189 g · L-1) provides no more than 10% decrease of Tb(III)-TCAS luminescence, while 2-fold decrease results from 7 days stirring of aqueous dispersions. This decrease indicates the self-leakage of Tb(III)-TCAS complexes, which are rather unsteady in aqueous solutions. (43) Meskers, S.; Dekkers, H. P. J. M. J. Phys. Chem. A 2001, 105, 4589– 4599.

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Figure 5. Corrected emission spectra of Tb(III)-TCAS-doped silica nanoparticles (C ) 0.0189 g · L-1) in dependence of stirring time (1: in the absence of HCl; 2-4: in the presence of HCl, pH ) 2: 2: immediately after the addition of HCl, 3: after 1 h of stirring, 4: after 3 days of stirring).

Figure 6. Corrected emission spectra of Tb(III)-TCAS-doped silica nanoparticles (C ) 0.0189 g · L-1) at various time of stirring in the presence of La(NO3)3 (C ) 0.5 mM, pH ) 6.78) (1) immediately after the addition of La(NO3)3, (2) after 1 h of stirring, and (3) after 3 days of stirring.

The ion exchange effect (Scheme 2) on silica-coated Tb(III)-TCAS nanoparticles can be revealed from the intensity decrease of Tb(III)-centered luminescence in the presence of La(III). Indeed, the presence of 0.5 mM La(III) in aqueous dispersions of silica nanoparticles has an insignificant effect on Tb-centered luminescence just after the addition of required amounts of LaCl3 to an aqueous dispersion of Tb(III)-TCAS silica-coated nanoparticles. The luminescence intensity sufficiently decreases after 1 h, becoming negligible after 3 days of stirring of Tb(III)-TCAS silica-coated nanoparticles in the presence of LaCl3. According to our recent published results,33 positively charged Co(III) complexes efficiently quench the luminescence of Tb(III)-TCAS complex (Scheme 1) because of their binding with the upper rim, which is mainly driven by electrostatic attraction between the four negatively charged sulfonate groups and the positively charged complex. The encapsulated Tb(III)-TCAS complex is also sensitive to the admixture of 0.05 µM of [Co(dipy)3]3+. The quenching effect exhibits less pronounced dependence on the time of stirring (Figure 7) than occurs for the above-mentioned ion exchange processes. The data presented in Figure 7 reveals that 1 h of stirring is enough to obtain equilibrium conditions, since further prolongation of stirring time (up to 3 days) produces a rather small decrease of emission intensity (no more than 10%), which can be attributed to the self-leakage of Tb(III)-TCAS. Thus we have undertaken

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Figure 7. Corrected emission spectra of Tb(III)-TCAS-doped silica nanoparticles (C ) 0.0189 g · L-1) at various times of stirring in the presence of [Co(dipy)3]3+ (C ) 0.05 µM, pH ) 6.78), where 1 represents Tb(III)-TCAS silica nanoparticles without any admixture, 2 denotes immediately after the addition of [Co(dipy)3]3+, 3 is after 1 h of stirring, and 4 is after 3 days of stirring.

Figure 8. Stern-Volmer plot of the quenching of silica-coated Tb(III)-TCAS nanoparticles (C ) 0.0189 g · L-1) by Co(III) complexes (1: [Co(dipy)3]3+, 2: [Co(NH3)6]3+, 3: [Co(en)2C2O4]+, and 4: [Co(CN)6]3-).

the effort to analyze the quenching effect at various concentrations of Co(III) complexes by the simplified Stern-Volmer equation (eq 2).35,43

I0 ⁄ I ) 1 + kq[Co(III)],

(2)

where I0/I is the ratio of the emission intensities without and with the presence of various concentrations of CCo(III), kq is the decay constant. The graphical analysis of I0/I versus CCo(III) reveals (Figure 8) that the quenching effect of [Co(dipy)3]3+ on Tb(III)-TCAS complex inside nanoparticles is quite different from that in aqueous solutions of Tb(III)-TCAS complex. First of all, it should be mentioned that even excess amounts of Co(III) complex species provide only partial quenching, indicating that some of the Tb(III)-TCAS complex species are inaccessible for efficient quenching because of their disposal inside the silica core. The slope of I0/I versus CCo(III) profile is greatly dependent on the charge of Co(III) complex. In particular, it is reduced upon going from triply scharged [Co(dipy)3]3+ and [Co(NH3)6]3+ to [Co(en)2ox]+, while no quenching occurs in the same concentration range by negatively charged [Co(CN)6]3-. Roughly, the nonlinear I0/I versus CCo(III) profile can be regarded as consisting of two linear regions with their own decay constants (kq). It is worth noting that the quenching of Tb(III)-TCAS luminescence by Co(III) complexes is the result of the energy transfer according to the Fo¨rster mechanism.44,45 The Fo¨rster energy transfer (44) Bu¨nzli, J.-C., G.; Piguet, C. Chem. Soc. ReV. 2005, 34, 1048–1077.

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Langmuir, Vol. 25, No. 5, 2009 3151

Analyzing the data presented in Figure 9, it should be mentioned that the electrostatic attraction of [Co(dipy)3]3+ ions by negatively charged surface of nanoparticles should be accompanied by the elimination of sodium ions (for Tb(III)-TCAS-doped nanoparticles) or H+ (for empty ones). Assuming that the deviation of χ versus CCo(III) is caused by the ion exchange process, in the first approximation the deviation of the χ value of aqueous dispersion of Tb(III)-TCAS-doped nanoparticles from the similar value of aqueous solution at the definite concentration of [Co(dipy)3](ClO4)3 can be represented as eq 4.

∆x ) xobs - xnp ) ΛCo[Co3+] + ΛClO4[ClO4-] + ΛNa[Na+] (4) Figure 9. The dependence of ∆χ versus concentration of [Co(dipy)3](ClO4)3 (CCo(III)) in aqueous solutions, ∆χ ) χobs - χnp, where χobs is the conductivity of [Co(dipy)3](ClO4)3 in the presence silica nanoparticles, and χnp is the conductivity of silica nanoparticles in aqueous solutions: without nanoparticles, ∆χ ) χobs (1); empty nanoparticles (C ) 1.1025 g · L-1) (2); and Tb(III)-TCAS-doped nanoparticles (C)1.1025 g · L-1) (3).

is still detectable even when the distance between the emission center and the quencher (r) is 100 nm, but the efficiency of quenching decreases following r-6 with the r increase.46 Thus the nonlinear profile of I0/I versus concentration of quencher reveals the nonequivalence of emitting species within nanoparticles due to their location close to the surface or inside the silica core. The kq value for quenching of lanthanide complexes by Co(III) complexes correlates with the binding constant of lanthanide complex with a quencher (K) and energy transfer constant (ket) by eq 3.43

kq)Kket

(3)

Both static (due to formation of dark complex) and dynamic (resulting from collision of luminescent and quenching species) quenching is typical for lanthanide complexes.43 The enhanced dependence of decay constant on the charge of Co(III)-containing quenchers reveals the particular importance of static quenching of silica-coated Tb-TCAS nanoparticles through their ion-pairing with Co(III) cationic complexes. The ion-pairing in turn reveals that silica-coated Tb(III)-TCAS nanoparticles possess high negative charge. Silica nanoparticles themselves are known to possess some negative charge due to the dissociation of SiOH groups, which is probably enhanced by Tb(III)-TCAS complexes doped within the silica network. To distiguish the negative charge provided by the silica network and that derived from Tb(III)-TCAS complexes, we performed conductometric measurements in aqueous dispersions of empty and Tb(III)-TCAS-doped silica nanoparticles. According to Figure 9, the χ versus CCo(III) profile is linear within the concentration range 0.16-0.40 mM, with a small deviation at Co(III) concentrations less than 0.16 mM in the presence of “empty” silica nanoparticles. It should be mentioned that silica nanoparticles provide an insignificant contribution to the conductivity of aqueous solution (14.7 µSm/cm). The silicacoated Tb(III)-TCAS nanoparticles themselves provide rather high conductivity (46.8 µSm/cm) and the deviation from χ versus CCo(III) linear dependence in aqueous solutions occurs in the wider concentration range 0.05-0.28 mM. (45) Piguet, C.; Rivara-Minten, E.; Bernardinelli, G.; Bunzli, J.-C.; Hopgartner, G. Dalton Trans. 1997, 421. (46) Van Der Meer, B. W.; Coker, G. I.; Chen S.-Y. Resonance Energy Transfer: Theory and Data; VCH Publishers: New York, 1994.

where ΛCo, ΛClO4, and ΛNa are molar conductivities of [Co(dipy)3]3+, ClO4-, and sodium ions, respectively, and [Co3+], [ClO4-], and [Na+] are their equilibrium concentrations. It is worth noting that eq 4 takes into account those sodium ions that are derived from the ion exchange process. Thus the deviation observed at higher concentration range for Tb(III)-TCAS-doped nanoparticles indicates that more [Co(dipy)3]3+ ions can be bound through the ion exchange on the surface of Tb(III)-TCAS-doped nanoparticles in comparison with the empty ones. So, the comparison of conductivity versus [Co(dipy)3](ClO4)3 concentration in the presence of empty and Tb(III)-TCAS-doped silica nanoparticles (Figure 9) confirms the above-mentioned difference in their ion-pairing efficiency with Co(III)-containing counterions, which in turn is derived from the different charge of the empty and Tb(III)-TCAS-doped silica nanoparticles. Thus the analysis of the quenching data through Stern-Volmer coordinatesrevealstwotypesofluminophoricspecies:Tb(III)-TCAS complexes located within ordered silica cores and those located closer to the surface within so-called disordered silica cores.1 Tb(III)-TCAS complexes (Scheme 1) possess four SO3Na groups, which are completely dissociated in aqueous solutions, thus providing high negative charge of these complexes. Taking into account different hydration efficiency of ordered and disordered layers of silica core, it can be assumed that the negative charge of silica-coated Tb(III)-TCAS nanoparticles is derived from the partial dissociation of Tb(III)-TCAS complexes located within the disorded layer of the silica network.

Conclusions Hence, the synthesis of silica-coated Tb(III)-TCAS nanoparticles exhibiting efficient luminescence was successfully fulfilled using reverse microemulsion procedure. The luminescent properties of nanoparticles stay unchanged for 3 days in aqueous dispersions (C ) 0.0189 g · L-1). Only beyond this time does the detectable self-leakage of Tb(III)-TCAS begin. Thus Tb(III)-TCAS silicacoated nanoparticles provide a suitable choice for covalent linkage of luminophoric species to a silica network. The decay of luminescent properties of silica-coated Tb(III)-TCAS nanoparticles resulting from the ion exchange of Tb3+ on La3+ or H+ is slowed down in comparison with that of Tb(III)-TCAS complexes in an aqueous environment. It is also worth noting that the quenching effect of positively charged Co(III) complexes is revealed immediately after the addition of the quencher. The I0/I versus concentration profiles for Co(III) complexes with various charges reveal the negative charge of silica-coated Tb(III)-TCAS nanoparticles and the presence of several types of luminophoric species located close to the surface and within the silica core. Acknowledgment. We thank RFBR (grant N 07-03-00282) and BRHE REC 007 for financial support. LA8032572