A facile strategy for synthesis of nearly white light emitting mesoporous silica nanoparticles

Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 565–571

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

A facile strategy for synthesis of nearly white light emitting mesoporous silica nanoparticles Ying Qu a , Lijuan Feng b,∗ , Bingxin Liu a , Cuiyan Tong a , Changli Lü a,∗ a b

Institute of Chemistry, Northeast Normal University, Changchun 130024, PR China Centre of Analytical and Test, Beihua University, Jilin 132013, PR China

h i g h l i g h t s

g r a p h i c a l

• (BTZ)x Zn@MSN with strong blue-

A series of novel fluorescent mesoporous silica nanoparticles (MSNs) was fabricated by introducing bluelight emitting Zn complex of benzothiazole (BTZ) and orange-red light dye (Rhodamine B). The near white-light MSNs can be easily obtained by controlling the ratio of RhB to BTZ–Zn complex.

emission was successfully synthesized. • Different fluorescent MSNs were fabricated by incorporating RhB molecules into the blue emission (BTZ)x Zn@MSNs. • Near white-light MSNs were obtained by controlling the ratio of RhB to (BTZ)x Zn@MSN.

a r t i c l e

i n f o

Article history: Received 26 July 2013 Received in revised form 18 September 2013 Accepted 2 October 2013 Available online 10 October 2013 Keywords: Mesoporous silica nanoparticles Fluorescence Near-white light-emission BTZ Rhodamine B

a b s t r a c t

a b s t r a c t A series of novel fluorescent mesoporous silica nanoparticles (MSNs) with a diameter of about 50 nm was fabricated by introducing a blue-light emitting Zn complex of 2-(2-hydroxyphenyl)benzothiazole (BTZ) and an orange-red light dye (Rhodamine B). The bright nearly white emitting MSNs (RhB0.65 –(BTZ)x Zn@MSN) can be obtained by tuning the concentration and ratio of different fluorescent emitting centers (RhB and (BTZ)x Zn). The sample of RhB0.65 –(BTZ)x Zn@MSN had a chromaticity coordinate of (0.33, 0.30) which is well within the white region of the chromaticity diagram. The obtained fluorescent MSNs still remained the good mesoporous structure and highly dispersibility in ethanol solution after functionalized with dyes. The prepared different fluorescent MSNs have a potential application feature in many field of biomedical drug delivery, cell labeling and optoelectronic devices. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Fluorescent nanoparticles are the important and promising materials used in biomaterial detections, optical images, proteinsensors, intracellular delivers and photodynamic therapy of cancers [1,2]. The fluorescent nanocomposites can be typically fabricated by

∗ Corresponding authors. Tel.: +86 431 85099236. E-mail addresses: [email protected] (L. Feng), [email protected] (C. Lü). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.10.017

incorporating either inorganic or organic fluorescent dyes into the nanomaterials [3]. However, there are different advantages and disadvantages between the organic and inorganic dyes. The inorganic dyes are typically more stable, while their limited variety, relatively low quantum yield and compatibility are the prominent issues restricting their broad applications [4]. The organic dyes have a high fluorescence quantum yield and photostability, but greater toxicity [5]. Recently, the composite luminescent materials have received much attention based on their extensive application prospects in different fields. The growing interest in large varieties of fluorescent

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dye doped nanomaterials comes from their excellent fluorescence characteristics [6–8]. There are generally two ways to introduce fluorescent molecules into mesoporous silica nanoparticles (MSNs). One method is in situ doping fluorophores into MSNs during the growth of nanoparticles using alkoxysilane-functionalized dyes and the sol–gel processes [9]. Another approach is the direct impregnation of functional fluorophores into MSNs [10]. Among a large number of light-emitting materials, much attention has been focused on the fabrication of white-light-emitting materials [11]. In the most light-emitting materials, the white fluorescence arises from the mixing of various dyes with blue, green and orange fluorescence simultaneously [12]. For examples, Gai et al. successfully obtained the white light emission via singlet excitation by doping three dyes encapsulated inside SCMNPs and by using radiative and nonradiative energy transfer processes between the HCE–SCMNPs and free TPP [13]. Zhang et al. introduced the lanthanide(III)–dpa complexes to the colloidal mesoporous silica nanoparticles through covalent bonds to get hybrid nanomaterials, and the functionalized MSNs can also be turned to white light materials [14]. In these strategies, it requires a careful control of each color contribution and of the energy transfer between the different dyes to obtain the pure white color according to the CIE standards [15]. As a versatile material with a unique nature, MSNs have been the research hotspots since the first report of the colloidal synthesis and functionalization of fluorescent molecules, as well as the photophysical properties of such substances [16–19]. In this area, the most fascinating discovery is that to fabricate a super-bright nanomaterial, it is required to coordinated control the different amount of dye molecules in each nanoparticles. That means from the accumulation of different light-sensitive molecules inside the silica core, the complex photo-physical events can arise [20–22]. The new composite multiple color fluorescence emission of nanosubstance can also be obtained in a single excitation wavelength, and these nanomaterials have the potential applications in many fields [23,24]. In this paper, we aimed to integrate different fluorescence chromophores into the channels of MSNs to fabricate a series of hybrid fluorescent nanoparticles, especially with white light emission. Firstly, the mercapto-functionalized MSNs were synthesized, and then the Zn2+ ions and 2-(2-hydroxyphenyl)benzothiazole (BTZ) were introduced into MSNs respectively to in situ form a blue emission of Zn–BTZ complex. Finally, a series of different fluorescent MSNs were fabricated by incorporating RhB molecules into the above blue light emitting MSNs, and the near white-light MSNs can be easily obtained by controlling the ratio of RhB to BTZ–Zn2+

complex. Here, the interactions between the dye molecules and the MSNs are mainly from two aspects: one is the coordination interaction between the Zn–BTZ complex and the mercapto groups in the channels of MSNs. Another is the adsorption interaction between the orange-red light dye (Rhodamine B) and MSNs. These functionalized MSNs with perfect morphology and mesostructure as well as excellent fluorescence features can be promising potential for the optoelectronic devices and bio-applications, especially in encapsulation, controlled release, labeling or imaging [6,8]. Scheme 1 shows the synthetic process of white light emitting MSNs containing RhB and Zn–BTZ complex. 2. Experimental 2.1. Chemicals and reagents Tetraethyl orthosilicate (>98%, TEOS) and 3mercaptopropyltrimethoxysilane (>98%, KH-590) were purchased from Shanghai Chemical Company. The aqueous solution of hexadecyl trimethylammoniumchloride (25 wt%, CTACl) and triethanolamine (TEA) were purchased from Beijing Chemical Works. 2-(2-Hydroxyphenyl)benzothiazole (>98%, BTZ) was obtained from Aldrich. Rhodamine B (>99%, RhB) was purchased from Acros. All materials were of analytical grade and used as received without any further purification. 2.2. Synthesis of mercapto-functionalized MSNs (MSN-SH) MSN-SH was synthesized according to a previous procedure [25,26]. 2.52 g CTACl was dissolved in a solution of 10.4 mL distilled water and 10.5 mL ethanol, and then 64 mL water was added under vigorous stirring for 10 min at room temperature. After the solution became clear, 4.1 mL TEA was added quickly and further stirred until all TEA were dissolved. Usually, 40 mL of the mixed solution was heated in an oil bath to 60 ◦ C, then 2.9 mL TEOS (6.5 mmol) and 0.5 mL KH-590 was added under constant stirring. The vigorous stirring was continued for 2 h, and then milk-white as-synthesized samples were collected by centrifugation at 9500 rpm for 35 min, washed with deionized water for three times. Template extraction was performed with 15 mL conc. HCl in 120 mL ethanol from the above-obtained samples. Usually, 1 g of sample was treated two to three times with 100 mL extraction solution by sonication in an ice bath for 30 min. The mercapto-functionalized MSNs were immediately taken up in distilled water, sonicated, and washed for several times.

Scheme 1. Synthetic route of RhB–(BTZ)x Zn@MSNs.

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Table 1 Recipes for preparation of different RhB–(BTZ)x Zn@MSN hybrid nanoparticles. Samplesa

(BTZ)x Zn@MSN (g)

0.1 × 10−4 g/mL RhB solvent (mL)

Ethanol solvent (mL)

CIE values

RhB0.6 –(BTZ)x Zn@MSN RhB0.65 –(BTZ)x Zn@MSN RhB0.7 –(BTZ)x Zn@MSN RhB0.8 –(BTZ)x Zn@MSN RhB0.9 –(BTZ)x Zn@MSN

0.1 0.1 0.1 0.1 0.1

0.6 0.65 0.7 0.8 0.9

20 20 20 20 20

(0.28, 0.31) (0.33, 0.30) (0.25, 0.22) (0.24, 0.22) (0.29, 0.26)

a

RhBX–(BTZ)x Zn@MSN digital X represents the volume of RhB, (BTZ)x Zn@MSN quality are 0.1 g.

2.3. Synthesis of zinc(II)–BTZ complex functionalized MSNs((BTZ)x Zn@MSNs) The above-obtained MSN-SH (1.05 g) were dispersed in aqueous solution of zinc acetate dihydrate (0.05 mol L−1 ) and stirred at room temperature for 12 h. The resulting solids of Zn2+ -containing MSNs (MSN-SH-Zn2+ ) were collected by centrifugation and washed several times with ethanol. Then, the obtained MSN-SH-Zn2+ were dispersed in 50 mL methanol, and 200 mL of BTZ (2.315 g, 10.16 mmol) in methanol was added simultaneously. After stirring for 6 h at 50 ◦ C, the green precipitate was separated by centrifugation process for several times, and dried at room temperature under vacuum. Finally, the zinc(II)–BTZ complex functionalized MSNs ((BTZ)x Zn@MSN) were obtained. 2.4. Synthesis of near-white light emitting MSNs

groups and adsorbed water molecules [27]. From the sample of (BTZ)x Zn@MSN, the weak new bands at 1466, 1376 and 721 cm−1 can be observed and they are assigned to the characteristic absorptions of (BTZ)x Zn complex. For RhB0.65 –(BTZ)x Zn@MSN, the bands of 1560 (C H stretching), 1523 (C N stretching), 1419 (CH3 symmetrical bending), 754 and 690 cm−1 (N H bending) are originated from the characteristic vibrations of the absorbed RhB molecules into MSNs [27,28]. XPS in Fig. 2 shows that the element components of (BTZ)x Zn@MSN and RhB0.65 –(BTZ)x Zn@MSN include Zn and N except Si, S, C and O of MSN. The above results conform the successful incorporation of (BTZ)x Zn metal complex and RhB molecules into MSNs. The morphological and structural features of the obtained samples were examined by TEM. Fig. 3 shows the TEM images of the as-synthesized MSN-SH and RhB0.65 –(BTZ)x Zn@MSN. The mesoporous nanoparticles with narrow size distribution are well dispersed without obvious aggregation, and the single particle

0.1 g dried (BTZ)x Zn@MSN was dispersed in 20 mL of ethanol and placed in a conical flask, while adding different volumes of ethanol solutions of RhB (0.1 × 10−4 g/mL). After the solution was stirred vigorously at 30 ◦ C for 6 h, the resulting pink solid precipitates were collected with a centrifuge (9500 rpm), washed with methanol, and dried at room temperature under vacuum. In order to regulate the luminous color, we have prepared a series of RhB–(BTZ)x Zn@MSN hybrid nanoparticles, and the synthetic recipes are shown in Table 1.

MSN-SH

(BTZ)xZn@MSN

T%

721

1466

RhB-(BTZ)xZn@MSN

3500

3000

2500

690 754

1560~1523

1419

Fourier transform infrared (FTIR) spectra were recorded on a Magna 560 FTIR spectrometer. XPS spectra were obtained from the surfaces with a diameter of 500 ␮m in area by means of a Quantum 2000 spectrometer using a non-monochromatized Al K␣ excitation radiation. X-ray powder diffraction (XRD) analysis of the samples was carried out on a Rigaku D/max-IIB with Cu K␣ radiation ˚ Transmission electron microscopy (TEM) was obtained ( = 1.5 A). from a JEOL-2021 microscope that was operated at 200 kV. Thermogravimetric analysis (TGA) was performed on a PerkineElmer TGA-2 thermogravimetric analyzer under air atmosphere at a heating rate of 10 ◦ C min−1 . Nitrogen adsorption/desoprtion isotherms were obtained on an ASAP 2020 analyzer. The specific surface area was determined by the Brunauer–Emmett–Teller (BET) method and pore sizes by the Barrett–Joyner–Halenda (BJH) methods. The content of zinc ions in MSNs was detected by inductively coupled plasma-atomic emission spectrometry (ICPAES) method on a LEEMAN Prodigy High Dispersion ICP. Photoluminescence spectra were recorded on a Cary Eclipse fluorescence spectrometer.

1376

2.5. Characterization

1500

1000

Fig. 1. FTIR spectra of MSN-SH, (BTZ)x Zn@MSN and RhB0.65 –(BTZ)x Zn@MSN.

3. Results and discussion The FTIR spectra of different samples are shown in Fig. 1. The MSNs show the typical Si O Si bands around 1080–1200 and 460 cm−1 associated with the formation of a condensed silica network. The broad bands centered at 3440 cm−1 the strong peaks around 1637 cm−1 can be assigned to O H bonds in silanol

500

Wavenumbers (cm-1)

Fig. 2. XPS of (BTZ)x Zn@MSN and RhB0.65 –(BTZ)x Zn@MSN.

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Fig. 3. TEM images of (a) MSN-SH and (b) RhB0.65 –(BTZ)x Zn@MSN.

(inset in Fig. 3b) shows clearly wormhole mesoporous structure of MSNs. The mercapto-functionalized mesoporous silica nanoparticles (MSN-SH) have a relatively smooth surface and an average diameter of about 50 ± 5 nm (Fig. 3a). After the RhB and BTZ are introduced into the channels of MSNs, the obtained RhB0.65 –(BTZ)2 Zn@MSN particles also remain a relatively smooth surface and uniform size (Fig. 3b). Powder XRD patterns of MSN-SH, (BTZ)x Zn@MSN and RhB0.65 –(BTZ)x Zn@MSN are shown in Fig. 4. The XRD patterns of these samples exhibit a broad peak at small angles for the first reflection with a maximum between 2 = 1.5◦ and 2.0◦ . The results indicate that all of the samples have the mesostructure and this mesoporous feature is still well preserved after the introduction of RhB and BTZ complex. The broadening of reflection peak is commonly caused by the structure of samples with a wormhole arrangement and the small size of the nanoscale particles. In addition, it can also be seen that the peak intensities of the samples decrease with the embedding of fluorescent substance. The Zn–BTZ complex is formed in the core, the diffraction peak intensity of (BTZ)x Zn@MSN is decreased as compared with that of MSN-SH. When re-embedding RhB molecules into mesoporous particles, the diffraction peak intensity of RhB0.65 –(BTZ)x Zn@MSN further declines, indicating that more molecules are successfully introduced into MSNs. Actually, the presence of dyes and metal complex inside the channels of MSNs can impact the scattering power. We can also see that the diffraction peak of

RhB0.65 –(BTZ)x Zn@MSN shifts toward smaller angles slightly, indicating a deviation in packing of the pores with respect to the ordered MSN-SH. So the above results have proved that the loading of RhB and BTZ complex into the channels of MSNs has been carried out [29–31]. Fig. 5 shows the TGA curves of MSN-SH, MSN-SH-Zn2+ , (BTZ)x Zn@MSN and RhB0.65 –(BTZ)x Zn@MSN. All the samples have the initial decomposition temperature at about 200 ◦ C, and the weight loss of the samples increases significantly with the introduction of the fluorescent substance. A largest weight loss of 32% between 200 and 600 ◦ C is observed in RhB0.65 –(BTZ)x Zn@MSN, which is mainly attributed to the decomposition of zinc complexes of BTZ and RhB molecules. The calculation result shows that the doped weight percents of BTZ and RhB are about 6.3% and 25%, respectively. The content of zinc ions in MSN-SH-Zn2+ was determined to be 2.39 wt% by ICPAES measurement. So, the molar ratio of Zn ions to BTZ molecules was calculated to be 1.13, indicating that the (BTZ)x Zn complex formed in the MSNs is primarily a single coordination structure (x = 1). Fig. 6 displays the N2 adsorption/desorption isotherms of MSNSH, MSN-SH-Zn2+ , (BTZ)x Zn@MSN and RhB0.65 –(BTZ)x Zn@MSN respectively. Evidently, these isotherms are classified as the type IV characteristic of the mesoporous materials, indicating the existence of mesopores in four samples. This offers the possibility for the introduction of other functional groups or as a biomedical carrier. Table 2 summarizes the structural properties of these samples.

Fig. 4. X-ray powder diffraction patterns of MSN-SH, (BTZ)x Zn@MSN and RhB0.65 –(BTZ)x Zn@MSN.

Fig. 5. TGA curves of MSN-SH, MSN-SH-Zn2+ , (BTZ)x Zn@MSN and RhB0.65 –(BTZ)x Zn @MSN.

Quantity Adsorbed (cm /g STP)

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3

MSN-SH 2+

MSN-SH-Zn

(BTZ)xZn@MSN RhB0.65-(BTZ)xZn@MSN

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0) Fig. 6. Nitrogen adsorption–desorption isotherms of MSN-SH-Zn2+ , (BTZ)x Zn@MSN, RhB0.65 –(BTZ)x Zn@MSN and MSN-SH.

With the introduction of the zinc complexes of BTZ and RhB, all the MSN samples filled with fluorescence molecules exhibit the decreasing BET surface area and total pore volume as compared to that of bare MSNs. After the functionalization of (BTZ)x Zn, the specific surface area of MSNs decreases to 752 cm3 /g, especially that of RhB0.65 –(BTZ)x Zn@MSN sharply reduces to 312 cm3 /g as compared with that (1139 cm3 /g) of MSN-SH. Above results also further testify that we have succeeded in the integration of the different luminescent molecules into the mesoporous channels of MSNs. Fig. 7 shows the PL spectra of (BTZ)x Zn@MSN, (BTZ)2 Zn, RhB@MSN, RhB and different RhB–(BTZ)x Zn@MSNs under 420 nm excitation. The (BTZ)2 Zn has a blue emission at 467 nm, while (BTZ)x Zn@MSN exhibits a broad and red-shifted emission peak centered at 479 nm when (BTZ)2 Zn is introduced into the MSNs. The Table 2 Some properties of different samples for N2 adsorption–desorption isotherms. Sample

SBET a (m2 g−1 )

Vt b (cm3 /g)

DBJH c (nm)

MSN-SH MSN-SH-Zn2+ (BTZ)x Zn@MSN RhB0.65 –(BTZ)x Zn@MSN

1139.4 1034.2 752.2 311.8

0.6620 0.6317 0.813 0.455

4.17 3.87 3.25 2.09

a b c

BET surface area. Total pore volume. Average pore diameter calculated using BJH method.

Fig. 7. PL spectra of (BTZ)x Zn@MSN, (BTZ)2 Zn, (BTZ)Zn, RhB@MSN, RhB at 420 nm (a) and PL spectra of RhBx –(BTZ)2 Zn@MSN under excitation at 420 nm (b).

characteristic emission of RhB can be observed at 578 nm. However, when RhB is incorporated into MSNs, the obtained RhB@MSN also has the multi-wavelength emission peaks at 470–550 nm except the primary emission at 578 nm. This result may be affected by the mesoporous environment. RhB is a molecule with conjugated aromatic rings, and different environments can produce a strong influence on the fluorescence emission. When a fluophor is excited from ␲ to ␲*, the excited state usually shows bigger polarity than the ground state and the increasing environment polarity will stabilize the excited state. Therefore, the decreasing polarity

Fig. 8. Representation of CIE chromaticity diagram for (a) RhB0.6 –(BTZ)x Zn@MSN, (b) RhB0.65 –(BTZ)x Zn@MSN and (c) RhB0.9 –(BTZ)x Zn@MSN. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)

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CIE chromaticity coordinates of RhB–(BTZ)x Zn@MSN with different ratios of RhB molecules and BTZ zinc complex are shown in Table 1. Fig. 8 presents the CIE color coordinate diagrams of RhB0.6 –(BTZ)x Zn@MSN, RhB0.65 –(BTZ)x Zn@MSN and RhB0.9 –(BTZ)x Zn@MSN. It is can be seen that the sample of RhB0.65 –(BTZ)x Zn@MSN shows a near white emission at 420 nm excitation (see Fig. 8b), and the chromaticity coordinate (0.33, 0.30) falls in the white region which is very close to the one for pure white light (0.33, 0.33). Fig. 9 is the optical photographs of different samples in their solid state (upper) and in ethanol solution (down) under daylight (upper) and under UV light excitation (down). All the hybrid mesoporous nanoparticles have good dispersion and fluorescence characteristics, and the emission colors are coincident with above results. Especially, the emission color of RhB0.65 –(BTZ)x Zn@MSN is very close to the white-light sample. The fluorescence confocal testing is also coincident with the above results. Fig. 10 is the confocal fluorescence images of (BTZ)x Zn@MSN and RhB0.65 –(BTZ)x Zn@MSN. The (BTZ)x Zn@MSN particles exhibit a blue-light color with 488 nm excitation on a confocal laser scanning microscope, while RhB0.65 –(BTZ)x Zn@MSN show the nearly white light colors. 4. Conclusions Fig. 9. Optical photographs of MSN-SH (a), (BTZ)x Zn@MSN (b), RhB0.6 –(BTZ)x Zn@MSN (c), RhB0.65 –(BTZ)x Zn@MSN (d), RhB0.9 –(BTZ)x Zn@MSN (e) in their solid state (upper) and in ethanol solutions (down) under daylight (upper) and under 365 nm UV light excitation (down). (For interpretation of the references to color in text, the reader is referred to the web version of this article.)

of the environment will result in the blue shift of the fluorescence spectrum. The surface of mesoporous silica channels is full of hydroxyl (OH− ) and oxygen ions (O2− ). When RhB molecules inserted into the channels, the hydroxyl and oxygen ions will generate the hydrogen bonding and static interactions with the partial RhB molecules, respectively. These interactions will bind RhB molecules in the channels of mesoporous silica and the electronic cloud of ␲-electron on chromophore groups will be restricted around oxygen ions, resulting in a rise of electron excitation energy in RhB molecules and the blue-shift of the spectrum [21,32,33]. But, the detailed formation mechanism of the complicated multiwavelength emissions at 470–550 nm is not clear at present. As shown in Fig. 7b, the RhB–(BTZ)x Zn@MSN exhibits two main PL emission bands at 467 and 578 nm with the same multi-wavelength emissions at 470–550 nm, and the fluorescence emission intensity of 580 nm increases with the increasing doped concentration of RhB on the whole. The

We have successfully synthesized the mercapto-functionalized mesoporous silica nanoparticles by an in situ method. Then the blue fluorescent nanoparticles (BTZ)x Zn@MSN with a emission peak at 470 nm was obtained by introducing (BTZ)x Zn complex. The orange light emission fluorophores (RhB) were also incorporated into the blue fluorescent mesoporous nanoparticles of (BTZ)x Zn@MSN to achieve a series of different light-emitting mesoporous nanomaterials through a simple physical adsorption and the regulation of the proportion of the two fluorescent substances. The efficient near white light emitting RhB0.65 –(BTZ)x Zn@MSN with a color coordinate (0.33, 0.30) at 420 nm excitation can be obtained by changing the feed ratio of RhB to (BTZ)x Zn@MSN to 0.65:0.1. The white light may originate from the cooperative interaction of the bluelight (BTZ)x Zn complex and the multi-wavelength emissions at 500–600 nm for RhB molecules in the mesoporous channels of MSNs. The obtained novel fluorescent MSNs have the well mesoporous structure and good dispersibility, and these nanoparticles may be loaded with different drugs and tracked simultaneously in living systems. We also expect this study to open up a new perspective in the design of multifunctional mesoporous nanoparticles which may possess potential applications in optoelectronic devices and fluorescent probes.

Fig. 10. Confocal fluorescence images of (a) (BTZ)x Zn@MSN and (b) RhB0.65 –(BTZ)x Zn@MSN. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)

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