Core-shell 2 mesoporous nanocarriers for metal-enhanced fluorescence

Transcription

1 Core-shell 2 mesoporous nanocarriers for metal-enhanced fluorescence Jianping Yang a, Fan Zhang a *, Yiran Chen a, Sheng Qian a, Pan Hu a, Wei Li a, Yonghui Deng a, Yin Fang a, Lu Han a, Mohammad Luqman b, Dongyuan Zhao a * a Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Key Laboratory of Molecular Engineering of Polymers of the Chinese Ministry of Education, Laboratory of Advanced Materials, Fudan University, Shanghai , P. R. China b Chemical Engineering Department, College of Engineering, King Saud University, Kingdom of Saudi Arabia dyzhao@fudan.edu.cn, zhang_fan@fudan.edu.cn Tel: ; Fax:

2 Experimental Section Chemicals. All chemicals were of analytical grade and used without further purification. AgNO 3, ethylene glycol, ammonia aqueous solution (28 wt %), NaCl, acetone, NH 4 NO 3, tetraethyl orthosilicate (TEOS) and hexadecyltrimethylammonium bromide (CTAB) were purchased from Sinopharm Chemical Reagent Co. (China). Polyvinylpyrrolidone (PVP, Mw = 55000), Eosin isothiocyanate (EiTC), Fluorescein isothiocyanate (FiTC), Rhodamine B (Rh B) and poly(allylamine hydrochloride) (PAH, Mw = 56000) were obtained from Sigma Aldrich. Deionized water was used in all experiments. Synthesis of Ag nanoparticles. Ag nanoparticles with diameter about 50 nm were synthesized in large scale via a modified method reported by Xia et al g of PVP (Mw = 55000) was dissolved in 200 ml of ethylene glycol before 0.5 g of AgNO 3 was added. After the three-neck flask was settled in an oil bath, the mixture was then heated to ~ 130 C within 25 min under vigorous stirring and maintained at 130 C for 1 h to obtain the Ag nanoparticles. The nanoparticles were isolated by precipitating the solution with acetone (800 ml), followed by centrifugation at rpm for 3 min, and re-dispersed in 4 ml of ethanol to obtain the 0.05 g (Ag nanoparticles)/ml solution. Synthesis of Ag@SiO 2 core-shell particles. The Ag@SiO 2 particles with different silica thickness were prepared according to Stöber method. In a typical procedure for the silica layer coating with the thickness of ~ 3 nm, 2 ml of Ag nanoparticle/ethanol solution (0.05 g/ml) obtained above was dispersed in the mixture of ethanol (80 ml) and water (20 ml) and 1 ml of ammonia aqueous solution (28 wt%) under stirring, then 15 μl of TEOS was added slowly with continuous stirring for 5 sec. The reaction was continued for 6 h. The Ag@SiO 2 particles was separated by centrifugation and washed by ethanol and water for several times. The thickness of the silica coating layer could be increased gradually with increasing the TEOS concentration. For example, silica coating layer with the thickness of ~ 8 nm was obtained using the procedure similar to the above increasing the TEOS concentration to 60 μl.

3 Synthesis of 2 core-shell particles. The Ag@SiO 2 core-shell particles were prepared through a surfactant-templating sol-gel approach by using CTAB as a template. In brief, the above synthesized Ag@SiO 2 particles were added into the solution containing 25 ml of water, 15 ml of ethanol, 75 mg of CTAB and 0.25 ml of ammonia aqueous solution (28 wt %). The mixture was turned to homo-dispersed solution after being agitated ultrasonically and mechanically for 30 min each. It was followed by the addition of 120 μl of TEOS was added dropwisely with continuous stirring for about 10 seconds and the reaction was continued for 6 h. The particles were collected by centrifugation and washed with ethanol and water, respectively. The CTAB surfactant was removed by solvent extraction method using 60 ml of NH 4 NO 3 /ethanol solution (6 g/l) and refluxed at 60 C for 1 h. This extraction process was repeated twice. After centrifugating and washing with ethanol and water, the Ag@SiO 2 core-shell nanocarriers were obtained. Loading of fluorophores in mesoporous silica shells. The dyes including EiTC, FiTC and Rh B were loaded in the mesoporous silica shells via an impregnation method. For example, 12 mg of Ag@SiO 2 core-shell particles with SiO 2 spacer of 8 nm were re-dispersed in 6 ml of ethanol. Brown vials were loaded with 500 μl of Ag@SiO 2 /ethanol solution and 500 μl of ethanol, followed by addition of 5, 10, 15, 20 μl of EiTC/ethanol solution (0.5 mg/ml), respectively. After being stirred for 24 h, 500 μl PAH (2 mg/ml) ethanol solution was injected and kept stirring for 3 h. The products were collected by centrifugation and washed with ethanol for 3 times. Finally, the products were re-dispersed in 4 ml ethanol to obtain the 0.25 mg/ml solution. FiTC and Rh B were loaded in the mesopore channels of the Ag@SiO 2 nanocarrier using the same procedure as that of EiTC. Loading of FiTC-EiTC in the mesoporous silica shells. 500 μl of the Ag@SiO 2 ethanol solution (2 mg/ml) with SiO 2 spacer of 8 nm was diluted with 500 μl of ethanol. 15 μl of EiTC/ethanol solution (0.5 mg/ml) was injected and stirred for 24 h, followed by adding different amount of 3, 6, 9, 12 and 15 μl of FiTC (0.5 mg/ml) was added and continuous stirring for another 6 h, respectively. 500 μl of PAH (2 mg/ml) ethanol solution was injected and kept stirring for 3 h. The

4 particles were separated by centrifugation and washed with ethanol, and then diluted to 4 ml with ethanol (0.25 mg/ml) for use. Dissolving Ag core from Ag@SiO 2 (control sample). The Ag cores in the core-shell structured Ag@SiO 2 particles could be removed by NaCl solution. 2-3 For example, 200 μl of the above-mentioned EiTC-, FiTC-, Rh B- and EiTC/FiTC-loaded Ag@SiO 2 composite particles in ethanol solution (0.25 mg/ml) were dispersed into 3.8 ml of NaCl solution (250 mm) and kept stirring for one day, respectively. Thus the hollow particles without Ag cores (control sample) were obtained without washing and centrifugation. The corresponding compared Ag@SiO 2 nanocarrier was also diluted with 3.8 ml water to ensure concentration of dyes in the Ag@SiO 2 nanocarrier and control sample are the same. Characterization. Transmission electron microscopy (TEM) measurements were carried out on a JEOL 2011 microscope (Japan) operated at 200 kv. All samples were first dispersed in ethanol and then collected using copper grids covered with carbon films for measurements. Scanning electron microscopic (SEM) images were obtained on a Philip XL30 microscope (Germany). A thin film of gold was sprayed on the sample before this characterization. Field-emission scanning electron microscopy (FESEM) images were obtained on a Hitachi S-4800 microscope (Japan). Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D4 X-ray diffractometer (Germany) with Ni-filtered Cu Kα radiation (40 kv, 40 ma). Nitrogen sorption isotherms were measured at 77 K with a Micromeritcs Tristar 3000 analyzer (USA). Before measurements, the samples were degassed under vacuum at 180 C for at least 6 h. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas (S BET ), using adsorption data in a relative pressure range from 0.04 to 0.2. The pore volume and pore size distributions were derived from the adsorption branches of isotherms using Barrett-Joyner-Halenda (BJH) model. The total pore volume, V t, was estimated from the amount adsorbed at a relative pressure P/P 0 of Fluorescence spectra were recorded on an F-4500 spectrofluorometer (Hitachi High-Technologies). FiTC, EiTC and Rh B were excited at 485, 520 and 540 nm,

5 respectively. The bandpass was set at 5 nm both excitation and emission, scan speed at 2400 nm/min and PMT voltage at 700 V for all measurements. UV-Vis absorption spectra were measured on a Jasco spectrophotometer (V-550) (Japan). Confocal luminescence images were made with an Olympus FV1000 (Japan), with λ ex = 515 nm as the excitation source and emissions were collected in the range of λ = nm. All the measurements were carried our in the same condition. References 1. Y. G. Sun and Y. N. Xia, J. Am. Chem. Soc., 2004, 126, M. L.-Viger, M. Rioux, L. Rainville and D. Boudreau, Nano Lett., 2009, 9, M. L.-Viger, D. Brouard and D. Boudreau, J. Phys. Chem. C, 2011, 115, 2974.

6 Electronic Supplementary Material (ESI) for Chemical Communications Fig. S1 SEM images (a, c, e) and FESEM images (b, d, f) of the Ag nanoparticles (a, b), the core-shell nanocarrier with the silica spacer in the thickness of 3 nm (c, d) and 8 nm (e, f).

7 Electronic Supplementary Material (ESI) for Chemical Communications Fig. S2 TEM image of the core-shell nanocarrier with the SiO2 spacer in the thickness of 3 nm (left). Powder XRD patterns (right) of the Ag@SiO2@mSiO2 nanocarrier with the silica spacer in the thickness of 3 nm (a) and 8 nm (b).

8 Fig. S3 Absorption spectra of (a) EiTC-loaded 2 nanocarrier and (b) after dissolving the silver nanoparticles (inset). The thickness of the silica spacer is 8 nm and the concentration of EiTC is 10.5 x 10-6 mol/l.

9 Fig. S4 Fluorescence emission spectra of the FiTC-loaded 2 nanocarrier with the silica-spacer in the thickness of 3 nm (a) and 8 nm (b), in which the FiTC concentration increases from 6.4 x 10-6 to 19.2 x 10-6 mol/l. Fluorescence spectra of the FiTC-loaded Ag@SiO 2 nanocarrier with the silica spacer thickness of 3 nm (c) and 8 nm (d) and after dissolving the silver nanoparticles. The excited wavelength is 485 nm.

10 Fig. S5 Fluorescence emission spectra of the RhB-loaded 2 with the silica spacer in the thickness of 3 nm (a) and 8 nm (b), in which the RhB concentration increases from 5.2 x 10-6 to 20.8 x 10-6 mol/l. Fluorescence spectra of the RhB-loaded Ag@SiO 2 nanocarrier with the silica spacer thickness of 3 nm (c) and 8 nm (d) and after dissolving the silver nanoparticles. The excited wavelength is 540 nm.

11 Fig. S6 Fluorescence spectra of (a) FiTC (donor)/eitc (acceptor) (red line) and EiTC (acceptor)-only (black line) loaded in 2 nanocarrier, (b) FiTC (donor)/eitc (acceptor) loaded Ag@SiO 2 nanocarrier before (red line) and after (black line) dissolving the silver nanoparticles. The loading amounts of EiTC and FiTC are 10.5 x 10-6 and 19.2 x 10-6 mol/l, respectively. Fluorescence spectra of FiTC (donor)/eitc (acceptor) loaded Ag@SiO 2 nanocarrier before (c) and after (d) dissolving the silver nanoparticles, while the concentration of EiTC (acceptor) is kept constant at 10.5 x 10-6 mol/l. The thickness of the silica-spacer layer is 8 nm and the excitation wavelength is 485 nm for (a-d).