Dual Visible and Near-Infrared Luminescent Silica Nanoparticles

Mar 12, 2010 - E-mail: [email protected]., † ... on a Graphite Surface Using Cetyltrimethylammonium Bromide Hemi- and Precylindrical Micelle Templat...
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J. Phys. Chem. C 2010, 114, 6350–6355

Dual Visible and Near-Infrared Luminescent Silica Nanoparticles. Synthesis and Aggregation Stability Svetlana V. Fedorenko,† Olga D. Bochkova,† Asiya R. Mustafina,*,† Vladimir A. Burilov,† Marcel K. Kadirov,† Cyril V. Holin,† Irek R. Nizameev,† Viktoriya V. Skripacheva,† Anastasiya Yu. Menshikova,‡ Igor S. Antipin,§ and Alexander I. Konovalov† A.E. ArbuzoV Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, ArbuzoV Str., 8, 420088, Kazan, Russia, Institute of Macromolecular Compounds, RAS, Bolshoi 31, 199004, St. Petersburg, Russia, and A.M. ButleroV Chemistry Institute, Kazan State UniVersity, KremlyoVskaya Str., 18, 420008, Kazan, Russia ReceiVed: December 29, 2009; ReVised Manuscript ReceiVed: January 28, 2010

Novel silica nanoparticles exhibiting near-infrared (NIR) and dual NIR-visible emission were synthesized according to reverse microemulsion procedure through the encapsulation of Yb(III) complex with p-sulfonatothiacalix[4]arene (Yb) and [Ru(dipy)3]Cl2 (Ru) as NIR- and visible-emitting luminophores into silica matrix. The synthesis was carried out at various Yb:Ru molar ratio. The numbers of Ru and Yb complexes per one nanoparticle were calculated from both fluorimetric and inductively coupled plasma atomic emission spectroscopy data. The dynamic light scattering measurements of aqueous dispersions of Yb, Ru, and Ru-Yb nanoparticles elucidate the relationship between the complexes inserted into nanoparticles and their aqueous dispersity. The transmission electron microscopy images were used to measure the size of the nanoparticles. The atomic force microscopy images reveal the different aggregation morphology of Yb- and Ru-doped nanoparticles. Introduction Multifunctional nanoparticles are of enhanced interest during recent decades due to their increasing use as biosensors and biomarkers.1-3 Such nanomaterials possess a combination of properties that do not exist in single-phase materials. Luminescent and electrochemiluminescent properties play the key role in the application of nanoparticles in biodetection. Lanthanidebased markers and sensors have gained a great deal of attention in recent years2,4 owing to their unique spectroscopic characteristics, including long fluorescence lifetime, large Stokes shift, and sharp line-like emission bands.5 These properties are particularly attractive because they enable temporal and spectral discrimination against background fluorescence often associated with commonly used fluorophores in chemical biology, leading to excellent detection sensitivity. The near-infrared (NIR)emitting lanthanide complexes are of particular interest because biological tissues are transparent in this spectral range. Ruthenium tris-dipyridyl is another well-known luminescent and redox-active complex, which is widely used in detection of DNA, viruses, and other biorelevant substrates.3,6 The combination of these two luminescent centers within one molecule enables dual visible and NIR-emitting probes for targeting DNA to be obtained.7 The design of heterometallic Ru-Yb(Nd) complexes is of particular interest due to the possibility of Ru-Yb(Nd) energy transfer, evidenced from an efficient NIR luminescence upon the excitation of the exited triplet metalto-ligand charge transfer (MLCT) band.8 Thus the doping of two Ru-Yb luminescent complexes into a silica matrix of nanoparticles is a way to obtain dual NIR-visible nanoparticles * To whom correspondence should be addressed. E-mail: asiyamust@ mail.ru. † Russian Academy of Sciences. ‡ Institute of Macromolecular Compounds. § Kazan State University.

of low toxicity. It is also worth noting that the doping of luminophores into silica matrix keeps the silica surface unchanged, thus providing the opportunities for its decoration by anchor groups required for further binding with receptor groups. Though there are some fine examples of doping lanthanide complexes into silica nanoparticles,4,9 examples of the doping of NIR emitting lanthanides are rather rare.9m The analysis of literature data indicates that organic chromophores are more commonly used for development of NIR emitting nanoparticles.10 The present report introduces heterometallic silica nanoparticles with dual NIR-visible luminescence. Complexes [Ru(dipy)3]Cl2 and [YbHL]Na4 (H4LNa4 is p-sulfonatothiacalix[4]arene) serve as visible and NIR luminescent centers. Though the insertion of [Ru(dipy)3]Cl2 into silica nanoparticles is well documented in the literature,3,6 there are a lack of examples of the synthesis of silica nanoparticles with both [Ru(dipy)3]Cl2 and NIR luminescent complexes inserted into the silica matrix. The applicability of nanoparticles in nanobiotechnology is greatly dependent on their colloidal stability, which in turn is conditioned by the hydrophilicity of the silica/ water interface. The dissociation of both luminophoric complexes leads to contrary charged ions [Ru(dipy)3]2+ and [YbHL]4-, thus the colloidal stability of nanoparticles should be dependent on their molar ratio. Therefore the self-aggregation behavior of silica nanoparticles synthesized at various [YbHL]Na4: [Ru(dipy)3]Cl2 (Yb:Ru) molar ratios has been studied and compared with the aggregation behavior of both [YbHL]Na4and [Ru(dipy)3]Cl2-doped nanoparticles. Experimental Section 1. Materials. Tetraethyl orthosilicate (TEOS, 98%), ammonium hydroxide (28-30%), n-heptanol (98%), and cyclohexane (99%) were from Acros; terbium(III) nitrate hexahydrate (99.9%) was from Alfa Aesar; and Triton X-100 was from

10.1021/jp912225u  2010 American Chemical Society Published on Web 03/12/2010

Dual NIR-Visible Luminescent Silica Nanoparticles TABLE 1: Synthetic Conditions of Yb-, Ru-, and Yb-Ru-Doped Silica Nanoparticles SiO2 Ru:Yb

CRu complexes, mM

CYb complexes, mM

0:1 1:0 2:1 1:1 0.9:1 0.5:1

0 7.8 15.6 7.8 7.02 3.9

7.8 0 7.8 7.8 7.8 7.8

Sigma-Aldrich are used. The synthesis of p-sulfonatothiacalix[4]arene tetrasodium salt ([H4L]Na4) was carried out according to the known procedure.11 [Ru(dipy)3]Cl2 · 6H2O and Yb(NO3)3 · 6H2O were commercially available from Acros Organics. The synthesis of silica-coated Yb(III) and Yb(III)-Ru(II) nanoparticles at various Yb:Ru molar ratios has been performed according to reverse microemulsion procedure presented previously12 with the addition to the reaction mixture of 7.8 mM [YbHL]Na4 and concentrations of [Ru(dipy)3]Cl2 varying from 3.9 to 15.6 mM (Table 1). 2. Methods. The transmission electron microscopy (TEM) images were obtained with JEOL JEM 100 S microscope, Japan. The dynamic light scattering (DLS) measurements were performed by means of the Malvern Mastersize 2000 particle analyzer, as well as PhotoCor Complex dynamic light scattering equipment consisting of a goniometer and a digital correlator. A He-Ne laser operating at 633 nm wavelength and emitting vertically polarized light was used as a light source. The secondorder cumulant expansion and the DynaLS methods were used to analyze the measured autocorrelation functions, using the software provided by the company. The effective hydrodynamic radius (RH) was calculated by the Einstein-Stokes relation from the first cumulant: D ) kBT/6πηRH, where D is the diffusion coefficient, kB is the Boltzmann constant, T is the absolute temperature, and η is the viscosity. Though the samples may be polydispersed the apparent hydrodynamic radius was used in all cases for the characteristics of the particle size. The diffusion coefficient was measured at least three times for each sample. The average error in these experiments was approximately 4%. All samples were prepared from the bidistilled water with prior filtering through the PVDF membrane, using the Syringe Filter (0.45 µm). ξ-potential “Nano-ZS” (MALVERN) using laser Doppler velocimetry and phase analysis light scattering was used for ξ-potential measurement. The steady-state emission spectra were recorded on a spectrofluorometer FL3-221-NIR (Jobin Yvon) under 320 and 450 nm excitation for recording Yb-centered and MLCT emission of Yb and Ru complexes correspondingly. UV-vis spectra were recorded on a Lambda 35 spectrophotometer (Perkin-Elmer). All samples were ultrasonicated within 30 min before measurements. Determination of metal ions was performed with a simultaneous inductively coupled plasma atomic emission spectrometry (ICP-AES) model iCAP 6300 DUO from Varian Thermo Scientific Company equipped with CID detector. The spectrometer provides the simultaneous measurement of the peak heights within the range from 166 to 867 nm. The optical resolution is less than 0.007 at 200 nm. The operating frequency is 27.12 MHz. Both radial and axial view configuration provide the optimal peak height measurements with suppressed spectral noises.

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6351 The samples were prepared in accordance with the following procedure: The 0.5 mL aliquot of the nanoparticles aqueous dispersions (4-5 g L-1) was diluted in 50 mL by deionized water. After 10 min of ultrasonification the samples were ready for analysis. The spectral lines at 240.272 and 211.667 nm were chosen for the analysis of Ru and Yb correspondingly. All measurements were performed three times with further averaging. The calibration was carried out with the use of multielement standards from Perkin-Elmer at various concentrations in the range of 0.04-2 mg L-1. The axial view configuration was used for the analysis. The number of Ru and Yb complexes per one nanoparticle was calculated from ICP-AES and fluorimetry data. The calculation was based on the assumption that the density of the nanoparticles is equal to that of pure silica (1.96 g cm-3).13 Therefore the weight of one nanoparticle (r ) 16 nm for Yb and Yb-Ru nanoparticles and 20.5 nm for Ru-doped ones) was calculated according to eq 114 with further evaluation of molar concentration of nanoparticles in their aqueous dispersions:

m ) 4/3Fπr3

(1)

The number of Ru and Yb complexes within nanoparticles is evaluated as the ratio of their concentration to concentration of nanoparticles multiplied by Avogadro constant. An atomic force microscope (MultiMode V, USA) was used to reveal the morphology of the aggregated nanoparticles. The 250-350 kHz canterlivers (Veeco, USA) with silicone tips (tip curvature radius is of 10-13 nm) were used in all measurements. The microscopic images were obtained by means of 8279JV scanner with 256 × 256 resolution. The scanning rate was 1 Hz. The antivibrational system (SG0508) was used to eliminate external distortions. The aqueous dispersions of nanoparticles (about 0.4-0.5 g L-1) were ultrasonificated within 10 min and then the droplet of the sample was placed on mica surface with the roughness no more than 1-5 nm. The AFM imaging was performed after water evaporation. Results and Discussion Synthesis of Silica Nanoparticles Doped with [Ru(dipy)3]Cl2 and [YbHL]Na4 Complexes. The emission properties of [Ru(dipy)3]Cl2 are well-known. The formation of [TbHL]Na4 and its luminescent properties are well-documented in literature,15 therefore it is natural to await the formation of [YbHL]Na4 in similar conditions. The emission of Yb3+ in an aqueous solution of Yb(NO3)3 is insignificant. The increase in emission intensity at 976 nm is observed when p-sulfonatothiacalix[4]arene is added to Yb(NO3)3 in aqueous solution at definite pH conditions (Figure 1). This emission increase is conditioned by the pH-dependent formation of [YbHL]4(Scheme 1A). The excitation spectrum reveals the band at 320 nm, which is very close to the absorbance of [YbHL]4- (316 nm). This indicates that p-sulfonatothiacalix[4]arene provides an antenna effect on the Yb-centered luminescence (2F5/2 f 2F7/ 4upon excitation at 320 nm. 2) of [YbHL] Silica nanoparticles doped with [YbHL]Na4 (Yb), [Ru(dipy)3]Cl2 (Ru), as well as [Ru(dipy)3]Cl2-[YbHL]Na4 at various Yb:Ru molar ratios were synthesized according to a well-known reverse microemulsion procedure, which was previously used for the synthesis of silica-coated [TbHL]Na4 nanoparticles.12 The TEM images reveal 41 ( 5 nm for [Ru(dipy)3]Cl2doped nanoparticles and 32 ( 5 nm for Yb(III)- and Yb(III)-Ru(II)-doped nanoparticles (Figure 2). It is well-known

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Figure 1. Emission intensity at 976 nm (λex ) 320 nm) of aqueous solutions of Yb(III)-p-sulfonatothiacalix[4]arene (C ) 0.1 mM) vs pH. The required pH values were adjusted by various buffer systems: acetic-acetate buffer at pH 3.8-6.3; TRIS buffer at pH 7.1-8; and CHES buffer at pH 8 - 10.

SCHEME 1: Schematic Representation of Complex [YbHL]4- (A) and the Ion Pair between [YbHL]4- and [Ru(dipy)3]2+ (B)

that the water:oil and Triton X100:water ratios are the key factors affecting the size of reverse micelles in water-in-oil macroemulsions and thus the size of nanoparticles.6j Indeed, no effect of dopants is revealed for Ru-doped and previously reported12 Tb-doped nanoparticles. Though Tb and Yb complexes are of the same stoichiometry and structure, the latter is much more soluble in alkaline aqueous solutions. This fact should be noted as the difference between synthetic conditions for Tb- and Yb-doped nanoparticles and as the probable reason of the various sizes of Tb- and Yb-doped nanoparticles. The UV-vis spectral features of the aqueous dispersions of Ybdoped nanoparticles are very similar with those of Tb-doped ones represented in our previous article.12 The UV-vis spectra of Yb-Ru-doped nanoparticles possess the bands typical for Yb (316 nm) and Ru (455 nm) complexes. Luminescent Properties of Nanoparticles. The fluorimetric measurements indicate Yb(III)-centered NIR luminescence at 968 nm upon the 320 nm excitation for both Yb- and Yb-Rudoped nanoparticles at pH 7 (Figure 3B). The MLCT emission of [Ru(dipy)3]Cl2 is also evident for Ru and Yb-Ru nanoparticles from the spectra represented in Figure 3A and the photo of the luminous dispersion of Yb-Ru nanoparticles (Figure 4). The fluorometric measurements have been performed for single components [Ru(dipy)3]Cl2 and [YbHL]Na4, as well as their mixture at various molar ratios in aqueous solutions, in order to compare the luminescent properties of luminophores in aqueous solutions with those within nanoparticles. Both luminescent complexes are in the form of complex ions

Fedorenko et al. ([Ru(dipy)3]2+ and [YbHL]4-) due to the dissociation of the counterions in aqueous solutions. As was mentioned above the complex [YbHL]4- is formed at definite pH conditions (Figure 1). Therefore pH 8 was maintained by Tris buffer in all solutions for emission spectra recording. The Yb-centered luminescence in aqueous solution is quenched with the increase of [Ru(dipy)3]2+ concentration (Figure 5B). Taking into account that [Ru(dipy)3]2+ and [YbHL]4- are counterions, the ion pairing between them according to Scheme 1B is the most probable reason of the spectral behavior of [YbHL]4- in the presence of [Ru(dipy)3]2+. According to literature data the joining of Ru (MLCT) and Yb luminescent centers can lead to Ru-Yb energy transfer, which can be evidenced from the NIR Yb-centered luminescence upon the excitation of MLCT transition of [Ru(dipy)3]2+, as well as from the decreased emission intensity of [Ru(dipy)3]2+.8 The analysis of Yb-centered luminescence does not reveal any detectable emission at 976 nm upon the excitation at 450 nm for the mixtures of [Ru(dipy)3]2+ and [YbHL]4- in aqueous solution at pH 8. The emission of [Ru(dipy)3]2+ exhibits a insignificant decrease at various amounts of [YbHL]4- (Figure 5A). This decrease does not result from Yb(III), because p-sulfonatothiacalix[4]arene itself provides the same quenching. The quenching of [Ru(dipy)3]2+ emission by p-sulfonatocalix[4]arene and p-sulfonatothiacalix[4]arene was discussed in more detail in ref 16. The obtained data indicate the lack of Ru-Yb energy transfer and the quenching effect of [Ru(dipy)3]2+ on Yb(III)-centered luminescence of [YbHL]4- in aqueous solutions resulting from their ion pairing. The averaged quantity of luminophores within each nanoparaticle can be calculated from the fluorimetric data assuming that the luminescent intensity of the [YbHL]Na4 complex is the same in aqueous solution and inside the silica nanoparticle (the calculation procedure is given in the Experimental Section). The comparative analysis of Yb-centered emission within silica nanoparticles and in aqueous solutions reveals that each nanoparticle possesses about 10 000 luminescent at 968 nm complexes, which are blue-shifted in comparison with complexes in aqueous solutions (976 nm) (Table 2). The comparison of MLCT emission of [Ru(dipy)3]2+ in silica nanoparticles and in aqueous solutions elucidates the embedding of about 11 000 of Ru complexes into each nanoparticle without detectable shift of the emission band. The MLCT emission intensity falls below with the decrease of quantity of Ru(II) complexes doped within the silica matrix, when the Yb:Ru ratio is varied from 1:2 to 1:0.5 (Figure 3A). The analysis of Yb-centered emission intensity of nanoparticles indicates no correlation between the intensity of Yb-centered luminescence and the Yb:Ru molar ratio (Figure 3 B). Moreover the deviation of the intensity of Ybcentered luminescence for nanoparticles with the above mentioned Yb:Ru molar ratio is not dramatic and does not exceed the experimentally observed deviation for Yb nanoparticles (Figure 3B). It should be concluded that Yb-centered emission in silica nanoparticles is not quenched by [Ru(dipy)3]Cl2 in these cases (Figure 3B). Keeping in mind the quenching of Ybcentered luminescence in aqueous solutions due to the ion pairing between [YbHL]4- and [Ru(dipy)3]2+, the obtained data indicate that Ru(II) and Yb(III) complexes within nanoparticles are not in close proximity to each other. It is worth noting that the formation of silica nanoparticles occurs within aqueous nanodroplets of reversed micelles. The conditions of tetraethyl orthosilicate hydrolysis should be taken into account to explain the restricted ion pairing between [YbHL]4- and [Ru(dipy)3]2+. The excess amounts of NH4+ and OH- ions, which are

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Figure 2. TEM images of silica nanoparticles doped with [Ru(dipy)3]Cl2 (A), [YbHL]Na4 (B), and [Ru(dipy)3]Cl2 and [YbHL]Na4 at 1:1 molar ratio (C).

Figure 3. Emission spectra of silica nanoparticles (0.23 g L-1) doped with Ru (curve 1), Yb-Ru at 1:2 (curve 2), 1:1 (curve 3), 1:0.9 (curve 4), 1:0.5 (curve 5) molar ratio and Yb (curve 6) in aqueous dispersions at pH 7 adjusted by Tris buffer (C ) 2.5 mM). Ru-centered emission was recorded at λex ) 450 nm, slit 2 (A); Yb-centered emission was recorded at λex ) 320 nm, slit 14 (B).

weakening the electrostatic attraction between [Ru(dipy)3]2+ and [YbHL]4-, are the most probable reason of their doping as [Ru(dipy)3]X2 and [YbHL]Y4, where X ) Cl or OH and Y ) Na or NH4. The Content of Ru and Yb in Nanoparticles Obtained from AES Data. As has been mentioned above the calculation of the number of Ru and Yb luminophores from the fluorimetric data is based on the assumption that the luminescent intensities of luminophores are not affected by the silica shell. This assumption disregards that the shielding by silica shell, as well as the concentration of luminophores within nanoparticle, should affect their luminescent properties.9 Therefore the atomic emission spectroscopy was applied to evaluate the numbers of Ru and Yb per each nanoparticle. The obtained data are represented in the Table 2 with the number of Ru and Yb complexes calculated from the fluorimetric data for comparison (the calculation procedure is given in the Experimental Section). The numbers of complex species determined from the luminescent data for single Ru and Yb and Yb-Ru embedded nanoparticles are more than the corresponding values determined from ICPAES data. The confirmation between the numbers of complexes calculated from the data of these two methods is somewhat better for Yb-Ru than for single Ru and Yb nanoparticles. Nevertheless the obtained results reveal no concentration quenching of luminophores within nanoparticles. Moreover the shielding of luminophores by silica shell, which restricts the quenching by aqueous environment, can be regarded as one of the probable reasons of more efficient

Figure 4. The photographs of the aqueous dispersion of Ru-Yb (0.9: 1) nanoparticles before (lower) and after (upper) light irridiation. The irridiation has been carried out by means of ultraviolet lamp VL-6.LC (6W-365 nm tube).

Figure 5. Emission spectra of [Ru(bipy)3]Cl2 (0.1 mM) (curve 1); in the presence of equimolar amounts of [H4L]Na4 (curve 2) and [YbHL]Na4 (curve 3) (A, λex ) 450 nm, slit 2). Yb-centered luminescence of [YbHL]Na4 (curve 4); in the presence of [Ru(bipy)3]Cl2 in 1:0.5 (curve 5) and 1:1 (curve 6) concentration ratio (Yb:Ru) (B, λex ) 320 nm, slit 14) in aqueous solutions at pH 8 (Tris buffer, 0.1 M).

luminescent properties of luminophores within nanoparticles in comparison with corresponding complexes in aqueous solutions.

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TABLE 2: The Number of Ru(II) and Yb(III) Complexes Calculated from Atomic Emission Spectroscopy and Steady-State Emission Spectroscopy Data atomic emission spectroscopy SiO2Ru: Yb

CRu, g L-1, ((1%)

no. of Ru(II) complexes

0:1 1:0 0.9:1 0.5:1

0.0598 0.0542 0.0269

4768 1800 1438

no. of Yb(III) complexes

0.1148

3084

0.2329 0.2115

TABLE 3: The Averaged Hydrodynamic Diameters (d), ξ-Potential Values ((10%), and Polydispersity Index (PDI) Dependening on Type and Concentration of Silica Nanoparticles in Water SiO2Ru:Yb 0:1 1:0 2:1 0.9:1 0.5:1

CSiO2, g L-1

ξ, mV

d, nm

PDI

0.0175 0.07 0.67 0.0175 0.07 0.67 0.07 0.67 0.0175 0.07 0.67 0.0175 0.07 0.67

-28 -28 -28 -0.3

249 ( 4 235 ( 3 233 ( 3 712 ( 69 709 ( 41 861 ( 28 2285 ( 114 2731 ( 125 254 ( 6 434 ( 3 896 ( 30 228 ( 30 286 ( 5 268 ( 2

0.139 0.157 0.174 >0.4 >0.4 >0.4 >0.4 0.126 >0.4 >0.4 >0.4 >0.4 >0.4 >0.4

-18 -18 -20 -16 -19 -21

emission spectroscopy

CYb, g L-1 ((1%)

The Aggregation of Ru, Yb, and Yb-Ru Nanoparticles in Aqueous Dispersions. The aggregation behavior of nanoparticles is of particular importance for their application in biodetection. Therefore DLS, ξ-potential, and AFM measurements have been performed for silica-coated Yb, Ru, and Ru-Yb nanoparticles. These methods are mutually complementary. In particular, DLS and electrophoretic methods highlight the relationship between the hydrophilicity of the silica interface and the colloidal stability of nanoparticles in aqueous dispersions, while AFM images, obtained through gravitational settling of nanoparticles on the mica surface, can indicate the

Figure 6. AFM images obtained from aqueous dispersions of silica nanoparticles doped with [Ru(dipy)3]Cl2 (A) and [YbHL]Na4 (B).

4500 6594

ratio Ru::Yb

no. of Ru(II) complexes

0.40:1 0.22:1

11700 2900 2400

no. of Yb(III) complexes

ratio Ru:Yb

10000 8800 7550

0.33:1 0.32:1

morphology of these aggregates. The obtained data are presented in Table 3 and Figure 6. The size distributions by intensity obtained from DLS data for silica-coated Yb, Ru, and Ru-Yb nanoparticles are represented in Figure 1S in the Supporting Information. The colloidal stability of Yb-doped nanoparticles is much more than that of Ru-doped ones, which is in good agreement with their ξ-potential values. The ξ-potential value of Yb nanoparticles (about -30 mV) is rather close to the value for empty silica nanoparticles and enough to provide good aqueous dispersity (Table 3).17 The nanoparticles doped with [Ru(dipy)3]Cl2 tend to aggregate in aqueous dispersions at pH 7, which is in good agreement with the ξ potential being close to zero (Table 3). It is natural to assume that the experimentally observed low colloidal stability of Ru nanoparticles arises from [Ru(dipy)3]2+ cations, which result from the dissociation of [Ru(dipy)3]Cl2 embedded into silica nanoparticles. The AFM images for Ru nanoparticles indicate that their sedimentation results in the formation of island-like aggregates, indicating that the aggregation of nanoparticles is more favorable than their interaction with the mica surface. This is in good confirmation with their enhanced aggregation, revealed from the DLS data. The morphology of the gravitational settling of Yb-doped nanoparticles is quite different from that of Ru nanoparticles. The Yb(III)-doped nanoparticles provide more uniform covering of the mica surface, which is in good agreement with their better colloidal stability in aqueous dispersions at pH 7. It is worth noting that the DLS data reveal 230-250 nm particles in aqueous dispersions of Yb-doped nanoparticles, which are staying unchanged within wide concentration range (0.0175-0.67 g L-1) (Table 3). Since the size of nanoparticles elucidated from the TEM images is about 30 nm, the values of 230-250 nm should be attributed to the nanoparticle aggregates. Taking into account the presence of both Si-OH and Si-O- groups on the silica interface in neutral aqueous solutions18 the interparticle hydrogen bonding is one of the most probable driving forces of such aggregation. The more detailed analysis of the aggregation morphology reveals rod-like architectures, being 30-50 nm in width and 130-200 nm in length, indicating rather specific mode of Yb-doped nanoparticles aggregation (Figure 6). Thus it is very probable that 230-250 nm aggregates revealed from the DLS data are conditioned by such rod-like aggregates. According to the data presented in Table 3 the ξ potential of Yb-Ru nanoparticles is becoming less negative and their colloidal stability is decreasing with the increase of Ru(II) content, while the doping of Yb(III) does not affect the colloidal stability of Yb-Ru nanoparticles. Moreover both DLS and AFM data indicate the enhancement of the aggregation due to the insertion of [Ru(dipy)3]Cl2 even in the case when it is inserted in significant deficiency to [YbHL]Na4. In other words the negatively charged complex [YbHL]4-, derived from [YbHL]Na4, does not neutralize the effect of [Ru(dipy)3]2+. This fact confirms the lack of ion pairing between these luminophoric counterions within nanoparticles.

Dual NIR-Visible Luminescent Silica Nanoparticles Conclusion In conclusion it should be pointed out that the use of the reverse microemulsion procedure enables dual visible and NIR emitting monodisperse nanoparticles to be obtained through the doping of [Ru(dipy)3]Cl2 and [YbHL]Na4 into silica matrix at various Yb:Ru molar ratios. Though both luminophoric complexes exist as counterions ([YbHL]4- and [Ru(dipy)3]2+) in the aqueous nanodroplet of reverse micelles, no ion pairing is revealed within silica nanoparticles. The correlation of the aggregation stability of Yb-Ru nanoparticles with the Yb:Ru molar ratio confirms the lack of ion pairing between [Ru(dipy)3]2+ and [YbHL]4- within nanoparticles and reveals the predominant effect of [Ru(dipy)3]2+ ions on the colloidal stability of the nanoparticles. The obtained results reveal the optimal Yb:Ru molar ratio for the synthesis of dual luminescent nanoparticles with satisfactory colloidal stability. Acknowledgment. We thank RFBR (projects 07-03-00282, 09-03-12260 Ofi_M) and BRHE REC Y5-C07-03 for financial support. Supporting Information Available: The size distribution by intensity for Yb, Ru, Yb-Ru (1:1), (0.9:1), (0.5:1) doped silica nanoparticles in aqueous solutions. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Ren, C.; Li, J.; Chen, X.; Hu, Z.; Xue, D. Nanotechnology 2007, 18, 345604 (6 pp). (b) Wu, S.-H.; Lin, Y.-Sh.; Hung, Y.; Chou, Y.-H.; Hsu, Y.-H.; Chang, Ch.; Mou, Ch.-Y. ChemBioChem. 2008, 9, 53–57. (c) Liu, Zh.; Yi, G.; Zhang, H.; Ding, J.; Zhang, Y.; Xue, J. Chem. Commun. 2008, 694–696. (d) Law, W.-Ch.; Yong, K.-T.; Roy, I.; Ding, H.; Bergey, E. J.; Zeng, H.; Prasad, P. N. J. Phys. Chem. C 2008, 112, 7972–7977. (e) Wu, J.; Ye, Zh.; Wang, G.; Yuan, J. Talanta 2007, 72, 1693–1697. (f) Tallury, P.; Payton, K.; Santra, S. Nanomedicine 2008, 3, 579–592. (g) Yong, K.-T.; Roy, I.; Swihart, M. T.; Prasad, P. N. J. Mater. Chem. 2009, 19, 4655–4672. (h) Lin, Y.-Sh.; Wu, S.-H.; Hung, Y.; Chou, Y.-H.; Chang, Ch.; Lin, M. L.; Tsai, Ch.-P.; Mou, Ch.-Y. Chem. Mater. 2006, 18, 5170– 5172. (i) Banerjee, Sh. S.; Chen, D.-H. Nanotechnology 2008, 19, 505104 (8 pp). (j) Wong, H.-T.; Chan, H. L. W.; Hao, J. H. Appl. Phys. Lett. 2009, 95, 0225121. (k) Kim, J.; Kim, H. S.; Lee, N.; Kim, T.; Kim, H.; Yu, T.; Song, I. Ch.; Moon, W. K.; Hyeon, T. Angew. Chem., Int. Ed. 2008, 47, 8438–8441. (l) Ma, Z.; Dosev, D.; Nichkova, M.; Dumas, R. K.; Gee, S. J.; Hammock, B. D.; Liu, K.; Kennedy, I. M. J. Magn. Magn. Mater. 2009, 321, 1368–1371. (2) (a) Wu, Ch.; Hong, J.; Guo, X.; Huang, Ch.; Lai, J.; Zheng, J.; Chen, J.; Mu, X.; Zhao, Y. Chem. Commun. 2008, 750–752. (b) DelgadoPinar, E.; Frı´as, J. C.; Jime´nez-Borreguero, L. J.; Albelda, M. T.; Alarco´n, J.; Garsı´a-Espan˜a, E. Chem. Commun. 2007, 3392–3394. (3) (a) Li, M.-J.; Chen, Z.; Yam, V.W-W.; Zu, Ya. ACS Nano 2008, 2, 905–912. (b) Santra, S.; Bagwe, R. P.; Dutta, D.; Stanley, J. T.; Walter, G. A.; Tan, W.; Moudgil, B. M.; Mericle, R. A. AdV. Mater. 2005, 17, 2165–2169. (4) (a) Hanaoka, K.; Kikuchi, K.; Kojima, H.; Urano, Y.; Nagano, T. J. Am. Chem. Soc. 2004, 126, 12470–12476. (b) Montalti, M.; Prodi, L.; Zaccheroni, N.; Charbonnie`re, L.; Douce, L.; Ziessel, R. J. Am. Chem. Soc. 2001, 123, 12694–12695. (c) Charbonnie`re, L.; Ziessel, R.; Montalti, M.; Prodi, L.; Zaccheroni, N.; Boehme, C.; Wippf, G. J. Am. Chem. Soc. 2002, 124, 7779–7788. (d) Jenkins, A. L.; Uy, O. M.; Murray, G. M. Anal. Chem. 1999, 71, 373–378. (e) Li, S.-H.; Yu, C.-W.; Yuan, W.-T.; Xu, J.-G. Anal. Sci. 2004, 20, 1375–1377. (f) Plush, S.; Gunlaugsson, T. Dalton Trans. 2008, 3801–3804. (g) Mizukami, S.; Tonai, K.; Kaneko, M.; Kikuchi, K. J. Am. Chem. Soc. 2008, 130, 14376–14377. (h) Hanaoka, K.; Kikuchi, K.; Kojima, H.; Urano, Y.; Nagano, T. Angew. Chem., Int. Ed. 2003, 42, 2996– 2999. (i) Leonard, J. P.; dos Santos, C. M. G.; Plush, S. E.; McCabe, T.; Gunnlaugson, T. Chem. Commun. 2007, 129–131. (j) Barja, B. C.;

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6355 Aramendia, P. F. Photochem. Photobiol. Sci. 2008, 7, 1391–1399. (k) de Silva, A. P.; Fox, D. B.; Moody, T. S.; Weir, S. M. Pure Appl. Chem. 2001, 73, 503–511. (l) Wang, D.-Y.; Zhao, Y.-B.; Xu, J.-G.; Guo, X.-G. Fresenius’ J. Anal. Chem. 1997, 358, 514–518. (m) Martı´nez, E. J. L.; Reyes, J. F. G.; Barrales, P. O.; Dı´az, A. M. Anal. Chim. Acta 2005, 532, 159–164. (5) (a) Bunzli, J. C. G.; Piguet, C. Chem. Soc. ReV. 2005, 34, 1048– 1077. (b) Thibon, A.; Pierre, V. C. Anal. Bioanal. Chem. 2009, 394, 107– 120. (c) dos Santos, C. M. G.; Harte, A. J.; Quinn, S. J.; Gunnlaugsson, T. Coord. Chem. ReV. 2008, 252, 2512–2527. (d) de Silva, A. P.; Fox, D. B.; Moody, T. S.; Weir, S. M. Pure Appl. Chem. 2001, 73, 503–511. (6) (a) Wang, L.; Wang, K.; Santra, S.; Zhao, X.; Hilliard, L. R.; Smith, J. E.; Wu, Y.; Tan, W. Anal. Chem. 2006, 647–654. (b) Hun, X.; Zhang, Z. Biosens. Bioelectron. 2007, 22, 2743–2748. (c) Hun, X.; Zhang, Z. Talanta 2007, 73, 366–371. (d) Lian, W.; Litherland, S. A.; Badrane, H.; Tan, W.; Wu, D.; Baker, H. V.; Gulig, P. A.; Lim, D. V.; Jin, Sh. Anal. Biochem. 2004, 334, 135–144. (e) Legrand, S.; Catheline, A.; Kind, L.; Constable, E. C.; Housecroft, C. E.; Landmann, L.; Banse, P.; Pielesa, U.; WirthHeller, A. New J. Chem. 2008, 32, 588–593. (f) Yan, J.; Este´vez, M. C.; Smith, J. E.; Wang, K.; He, X.; Wang, L.; Tan, W. Nano Today 2007, 2, 44–50. (g) Wu, H.; Huo, Q.; Varnum, S.; Wang, J.; Liu, G.; Nie, Z.; Liu, J.; Lin, Y. Analyst 2008, 133, 1550–1555. (h) Tansil, N. C.; Gao, Zh. Nano Today 2006, 1, 28–37. (i) Qhobosheane, M.; Santra, S.; Zhang, P.; Tan, W. Analyst 2001, 126, 1274–1278. (j) Bagwe, R. P.; Yang, C.; Hillard, L. R.; Tan, W. Langmuir 2004, 20, 8336–8342. (7) Nonat, A. M.; Quinn, S. J.; Gunnlaugsson, Th. Inorg. Chem. 2009, 48, 4646–4648. (8) Ward, M. D. Coord. Chem. ReV. 2007, 251, 1663–1677. (9) (a) Yuan, J.; Wang, G. Trends Anal. Chem. 2006, 25, 490–500. (b) Ai, K.; Zhang, B.; Lu, L. Angew. Chem., Int. Ed. 2009, 48, 304–308. (c) Chen, Y.; Lu, Z. Anal. Chim. Acta 2007, 587, 180–186. (d) Ha¨rma¨, H.; Kera¨nen, A.-M.; Lo¨vgren, T. Nanotechnology 2007, 18, 075604 (7 pp). (e) Ye, Zh.; Tan, M.; Wang, G.; Yuan, J. Talanta 2005, 65, 206–210. (f) Piao, Y.; Burns, A.; Kim, J.; Wiesner, U.; Hyeon, T. AdV. Funct. Mater. 2008, 18, 3745–3758. (g) Diamente, P. R.; Raudsepp, M.; van Veggel, F. C. J. M. AdV. Funct. Mater. 2007, 17, 363–368. (h) Tan, M.; Wang, G.; Hai, X.; Ye, Z.; Yuan, J. J. Mater. Chem. 2004, 14, 2896–2901. (i) Iwu, K. O.; Soares-Santos, P. C. R.; Nogueira, H. I. S.; Carlos, L. D.; Trindade, T. J. Phys. Chem. C 2009, 113, 7567–7573. (j) Ye, Z.; Tan, M.; Wang, G.; Yuan, J. Anal. Chem. 2004, 76, 513–518. (k) Ye, Z.; Tan, M.; Wang, G.; Yuan, J. J. Mater. Chem. 2004, 14, 851–856. (l) Ye, Z.; Tan, M.; Wang, G.; Yuan, J. Talanta 2005, 65, 206–210. (m) Tong, B.; Wang, S.; Meng, Yu.; Wang, B. Photochem. Photobiol. Sci. 2007, 6, 519–520. (10) (a) Nooney, R. I.; McCahey, C. M. N.; Stranik, O.; Le Guevel, X.; McDonagh, C.; MacCraith, B. D. Anal. Bioanal. Chem. 2009, 393, 1143– 1149. (b) He, X.; Chen, J.; Wang, K.; Qin, D.; Tan, W. Talanta 2007, 72, 1519–1526. (11) Iki, N.; Fujimoto, T.; Miyano, S. Chem. Lett. 1998, 27, 625–626. (12) Mustafina, A. R.; Fedorenko, S. V.; Konovalova, O. D.; Menshikova, A.Yu.; Shevchenko, N. N.; Soloveva, S. E.; Konovalov, A. I.; Antipin, I. S. Langmuir 2009, 25, 3146–3151. (13) Santra, S.; Zhang, P.; Wang, K.; Tapec, R.; Tan, W. Anal. Chem. 2001, 73, 4988–4993. (14) Ye, Z.; Tan, M.; Wang, G.; Yuan, J. Anal. Chem. 2004, 76, 513. (15) (a) Iki, N.; Horiuchi, T.; Koyama, K.; Morohashi, N.; Kabuto, Ch.; Miyano, S. J. Chem. Soc., Perkin Trans. 2 2001, 2219–2225. (b) 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. (16) (a) Chiba, M.; Kim, H.-B.; Kitamura, N. J. Photochem. Photobiol. A 2002, 151, 67–74. (b) Mustafina, A. R.; Skripacheva, V. V.; Burilov, V. A.; Gubaydullin, A. T.; Nastapova, N. V.; Yanilkin, V. V.; Solovieva, S. E.; Antipin, I. S.; Konovalov, A. I. Russ. Chem. Bull., Int. Ed. (Engl. Transl.) 2008, 57, 1897–1904. ¨ st, A.-B.; Duchanoy, A.; (17) (a) Bergman, L.; Rosenholm, J.; O Kankaanpaa, P.; Heino, J.; Linden, M. J. Nanomater. 2008, DOI: 10.1155/ 2008/712514, (9 pp). (b) Bagwe, R. P.; Hilliard, L. R.; Tan, W. Langmuir 2006, 22, 4357–4362. (c) Bryleva, E. Yu.; Vodolazkaya, N. A.; MchedlovPetrossyan, N. O.; Samokhina, L. V.; Matveevskaya, N. A.; Tolmachev, A. V. J. Colloid Interface Sci. 2007, 316, 712–722. (18) Rimer, J. D.; Lobo, R. F.; Vlachos, D. G. Langmuir 2005, 21, 8960–8971.

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