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Multifunctional Fe3O4-Au/Porous Silica@Fluorescein Core/Shell Nanoparticles with Enhanced Fluorescence Quantum Yield X. F. Zhang,†,‡ L. Clime,‡ H. Q. Ly,| M. Trudeau,§ and T. Veres*,†,‡ Industrial Materials Institute, National Research Council, 75 BouleVard de Mortagne, BoucherVille, Canada J4B 6Y4, INRS E´nergie, Mate´riaux et Te´le´communications, 1650 BouleVard Lionel-Boulet, Varennes, Que´bec, Canada, J3X 1S2, Hydro-Que´bec Research Institute, 1800 BouleVard Lionel-Boulet, Varennes, Quebec, Canada, J3X 1S1, and Department of CardioVascular Medicine, Montreal Heart Institute, UniVersity of Montreal School of Medicine, Montreal, Quebec, Canada ReceiVed: June 3, 2010; ReVised Manuscript ReceiVed: September 10, 2010
Core/shell Fe3O4-Au/silica nanoparticles with controllable fluorescein doped into nanoporous silica shells are synthesized. The quantum yield of fluorescein significantly increases through a synthetic electromagnetic field coupling of the dumbbell-like Fe3O4-Au cores. Theoretical simulation further proves that the anomalously plasmonic oscillation of Fe3O4-Au nanoparticles is a dominant contributor to the fluorescence enhancement. 1. Introduction
2. Experimental Section
Superparamagnetic nanoparticles have exhibited considerable potential in the fields of biomedical applications and have attracted more and more research fervor. Most of the assynthesized products need to be further functionalized for stability and biocompatibility and for the incorporation of optically active components for multimodal bioimaging. Such multifunctional nanoparticles with both magnetic and optical properties have been widely exploited, such as dumbbell,1-5 core/shell,6-10 and alloyed11,12 nanoparticles. Among them, it is recognizably difficult to synthesize plasmonic-fluorescent composite nanoparticles with good fluorescence quantum yields (QYs)because the combination of these two effects often gives rise to a strong fluorescence quenching.13 Considering this issue, a common solution is to construct a separation layer between the plasmonic particle and the fluorophores/quantum dots in order to suppress the strong plasmonic field.14 Despite these successes, integrating magnetic, plasmonic, and fluorescent components into a biocompatible silica shell by assembly of magnetic and plasmonic particles without significant quenching of the fluorescence is still an important challenge. Inspired by the dualfunctions of dumbbell-like oxide-metal nanoparticles,1-5 herein we present the synthesis of fluoresceindoped nanoporous silica nanoparticles with dumbbell-like Fe3O4-Au cores (denoted as Fe3O4-Au/porous silica@F). By doping the fluorescein molecules inside nanoporous shells, Fe3O4-Au/porous silica@F nanoparticles exhibit enhanced fluorescence QYs compared to that of Fe3O4/porous silica@F nanoparticles; moreover, the enhancement is dependent on the concentration of doped fluorescein molecules. Compared to conventional fluorescence enhanced systems such Au nanoparticles15 and Au flat surfaces,16 Fe3O4-Au/porous silica@F nanoparticles showing both magnetic and plasmonic properties provide advantages for a large area of applications including multimodal imaging and magnetically targeted drug delivery.
Materials. Oleic acid (90%), oleylamine(90%), 1-hexanol anhydrous (99%), octyl ether (98%), ammonia solution (NH4OH, 28-30 wt % in water), Triton X-100, hexane (95%), cylcohexane (99.5%), tetraethoxysilane (TEOS, 99.999%), 1-octadecene (99.5%), 1,2-hexadecandiol (90%), and gold(III) chloride trihydrate (99.9%) were all purchased from SigmaAldrich, Inc. Iron pentacarbonyl (99.999%) was purchased from Strem Chemicals, Inc., and N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAP3, 90%) was purchased from Gelest (Tullytown, PA). Synthesis. The Fe3O4-Au and Fe3O4 nanoparticles were synthesized based on well-known methods, and the details were described in refs 1 and 17, respectively. The silica-coated nanoparticles were synthesized by hydrolyzing TEOS in a water/ oil (W/O) microemulsion that contains the as-synthesized nanoparticles as seeds. The as-synthesized nanoparticles were first dispersed in cyclohexane with a concentration of 1 mg/ mL, and then 0.5 mL was rapidly injected into a mixture of 1.77 g of triton X-100, 1.6 mL of 1-hexanol anhydrous, and 7 mL of cyclohexane under a strong vortex for about 1 h. Subsequently, 0.5 mL of ∼6% ammonia solution, without and with various concentrations of fluorecein molecules (0.005, 0.02, 0.05, and 0.1 mmol/mL), was added in the above solution for another 1 h. The various concentrations fluorecein molecules in ammonia solutions result in different doping concentrations of as-made silica shells, exhibiting the adjustable fluorescence QY. As we expected, the experimental results show that the quantity of doped fluorescein molecules in silica shells increases as the the initial concentrations of fluorescein solutions increase. Finally, 25 µL of TEOS was added, and the mixture was allowed to react for 24 h. To make preliminary silica shells nanoporous and amino-functionalized, 25 µL of AEAP3 was added and carried out for another 24 h. The resultant products were separated by adding excess ethanol and centrifuged at 9000 rpm for 30 min, and washing was repeated at least three times. The resultant product was dried under vacuum or directly dispersed in deionized water for characterization. Characterizations. The structures of nanoparticles were characterized by transmission electron microscopy (TEM) operated at a voltage of 30 kV and a high-resolution TEM
* Corresponding author. E-mail:
[email protected]; fax: +450 641-5105. † National Research Council. ‡ INRS E´nergie. § Hydro-Que´bec Research Institute. | University of Montreal School of Medicine.
10.1021/jp1051112 2010 American Chemical Society Published on Web 10/11/2010
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Figure 1. TEM images of (a) Fe3O4-Au nanoparticles, (b) Fe3O4-Au/silica, and (c) Fe3O4-Au/porous silica nanoparticle. (d) Statistic distribution of the number of Fe3O4-Au nanoparticle cores per particle and the corresponding Z-contrast mode TEM images.
microscope (HRTEM) coupled to an energy dispersive X-ray spectroscope (EDS) operated at 200 kV (Hitachi HD-2700 Scan transmission electron microscope). Fourier transmission infrared (FTIR) spectra were collected with a Nicolet Fourier spectrophotometer at wavenumbers between 600 and 4000 cm-1. The phases of the nanoparticles was studied by an X-ray diffractometer with Cu KR (λ ) 0.154 nm) radiation at a voltage of 30 kV and current of 30 mA. UV-visible (UV-vis) spectra were collected on a Perkin-Elmer Lamda 950 spectrometer with a dual sampling compartment. Fluorescence measurements were performed with a Thermo Scientific NanoDrop 3300 fluorospectrometer and an Eclipse TE2000-U inverted fluorescence microscope from Nikon (Melville, NY) equipped with an electron multiplying charge-coupled device (EMCCD) camera from Hamamatsu (Bridgewater, NJ). The fluorescence imaging specimen was prepared by dropping a droplet of dispersion on a nonfluorescent glass slide. 3. Results and Discussion The Fe3O4-Au nanoparticles were synthesized based on a well-known method.1 Figure 1a shows a typical TEM image of as-synthesized Fe3O4-Au nanoparticles with a dumbbell-like structure consisting of a Au particle component ∼5 nm in diameter and an Fe3O4 particle component ∼15 nm in diameter with a narrow size distribution. Statistical analyses show up to 96% of the dumbbell-like Fe3O4-Au nanoparticles and only a small fraction of independent Au and Fe3O4 nanoparticles, which is in agreement with the results reported previously.1 Fifteennanometer Fe3O4 nanoparticles (Figure S2a) were also synthesized by a thermal decomposition method17 and served as cores for the synthesis of the reference system Fe3O4/porous silica nanoparticle in the study. The silica-coated nanoparticles were synthesized through a two-step procedure in a W/O microemulsion that contains the as-synthesized Fe3O4-Au (or Fe3O4) nanoparticles as seeds. In a first reaction step, the Fe3O4-Au/silica nanoparticles are created by the hydrolysis of the TEOS molecules. Figure 1b
shows that Fe3O4-Au cores are completely coated by dense silica shells with an average shell thickness of ∼18 nm and relatively good monodispersity. In the next step, AEAP3 molecules were injected into the reaction mixture for a subsequent co-condensation reaction that leads to the formation of nanoporous silica shells (Figure 1c). The process was repeated with Fe3O4 nanoparticles as seeds and led to very similar nanoporous silica shells. Figure 1d shows the atomic-number contrast images obtained by scanning transmission electron microscopy (Z-STEM) of individual Fe3O4-Au/porous silica and silica nanoparticles, indicating the presence of several populations of Fe3O4-Au/porous silica and a number of Fe3O4-Au cores ranging from 0 to about 3 per particle. Due to the stronger electrons scattering on the Au compared with Fe3O4, the Au nanoparticles of the dumbblell-like Fe3O4-Au cores appear as white spots on the Z-STEM images. Statistic analysis of these images reveals that the fraction of Fe3O4-Au/porous silica nanoparticles with Fe3O4-Au core numbers of 0, 1, 2, and more than 2, are 3%, 54%, 38%, and 5%, respectively. The magnified TEM figures of nanoporous and solid silica shell structures with the same Fe3O4-Au cores are as shown in Figure 2a,b. The silica shell of Fe3O4-Au/silica nanoparticles syntheiszed by two-step hydrolysis of the TEOS (25 µL + 25 µL) is the solid structure (Figure 2a). In comparison, it is worth noting that all the Fe3O4-Au/porous silica nanoparticles present nanoporous with sponge-like ultrathin pores extending to the particle surface, as shown in Figure 2b. The similar microstructures observed by Z-STEM are inset in Figure 1d. The pore sizes measured by TEM observation are on the scale of subnanometers. Such nanoporous structure possesses a distinct advantage for drug delivery, and will be reported in a separate paper. EDS analyses shown in Figure 3 correspond to the elemental distribution of Fe, O, Si, N, and Au. One can observe an obvious core/shell feature, indicating that the core is rich in Fe, O, and Au, while the shell is mainly made of O, Si and N. This result confirms that the dumbblell-like Fe3O4-Au cores are encap-
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Figure 2. Magnified TEM images of (a) Fe3O4-Au/solid silica and (b) Fe3O4-Au/porous silica nanoparticles.
Figure 4. TEM images of Fe3O4-Au/porous silica@F nanoparticles with doping concentrations of fluorescein molecules of (a) 0.005, (b) 0.02, and (c) 0.1 mmol/mL. (d) Optical images of nanoparticles dispersed in distilled water: (i), (ii), and (iii) correspond to panels a, b, and c, respectively; (iv) Fe3O4-Au/porous silica nanoparticles.
Figure 3. A series of EDS mapping images of a single Fe3O4-Au/ porous silica nanoparticle, of which the core contains several Fe3O4-Au nanoparticles, revealing the elemental distributions of Fe, O, Si, N, and Au.
sulated into nanoporous silica shells, and are unchanged during the reaction process. The X-ray diffraction (XRD) pattern (Figure S1) further confirms the phase composition of Fe3O4-Au and Fe3O4 nanoparticles. The detailed formation mechanism of the nanoporous silica shells proposed in this paper is under investigation; however, we believe that the triton X-100 surfactant molecules innitially adsorbed on the surface of Fe3O4 nanoparticles play a key role. The triton X-100 surfactant molecules were hybridized into initial silica shells that were formed from simultaneous cocondenstaion of TEOS/AEAP3 molecules. Subsequently, they can be removed by high-velocity centrifugation and ethanol washing, resulting in the appearance of nanopores, which are preserved in this process due to the steric hindrance of the long chains of -O2Si(OH)R and -O3SiR (R represents an aminoethylaminopropyl group). While this process is similar to the thermal annealling process used to remove the organic groups, which leads to similar porous silica shells,18 in our approcah
the formation of nanoporous silica shells is due to a combined effect of both surfactant and the steric hindrance of molecular backbones. While many kinds of organic dyes can be incorporated in the silica nanoporous shell with the process described, in this study, we used fluorescein as the dye based on the strong overlap between Fe3O4-Au plasmonic resonance (∼530 nm, in Figure S4) and fluorescein emission band (∼520 nm). Figure 4a-c shows TEM images of Fe3O4-Au/porous silica@F nanoparticles doped with increased fluorescein concentrations. It is wellknown that doping the fluorescein inside can result in a reduction of pore sizes, even leading to a dense shell once the concentration reaches a saturated value, whereas the coupling with only external surface of shells should not change the pore sizes. As expected, in our experiments, nanoporous silica shells were gradually transformed into a dense hybrid shell (silica-dyes) structure when the fluorescein concentration reached 0.1 mmol/ mL (Figure 4c) (also see Fe3O4/silica@F in Figure S3). This shows that the fluorescein molecules can be covalently bound to silica shells, not only to the outer silica sufaces but also inside the nanopores. This is indeed shown by the TEM analyses, indicating the shell structure changes by the controlled doping in the process. Figure 4d shows the optical images of Fe3O4-Au/ porous silica and Fe3O4-Au/porous silica@F nanoparticles dispersed in distilled water, indicating a good stability and dispersibility. In order to eliminate the background interference (gold absorption and particle scattering), a dual-sampling-compartment measurement was employed (Figure S5), which allows one to ascribe the absorbance in ultraviolet-visible (UV-vis) spectra only to the extinction of dyes. Figure 5a shows a series of UV-vis and fluorescence spectra from Fe3O4-Au/porous silica@F nanoparticles in aqueous solution that are doped with various concentrations of fluorescein molecules. For all the concentrations of fluorescein used herein, the UV-vis showed an expected strong monomer absorption peak at ∼490 nm. For the largest concentration of 0.1 mmol/mL, an extra absorption peak appeared at ∼466 nm due to the dimerization of fluorescein. By decreasing the fluorescein concentrations, the intensity of the absorption peaks decreases as shown in Figure 5b, which
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Figure 6. QY values of Fe3O4-Au/porous silica@F and Fe3O4/porous silica@F nanoparticles with various concentrations of fluorescein molecules.
Figure 5. (a) UV-vis and fluorescence spectra; (b,c) UV-vis absorbance and fluorescence intensity of Fe3O4/porous silica@fluorescein and Fe3O4-Au/porous silica@fluorescein nanoparticles.
is basically the same for both Fe3O4/silica@F and Fe3O4-Au/ silica@F nanoparticles systems. However, the decrease in the fluorescence intensity (Figure 5c) is slower than expected for the case of the dumbbell-like Fe3O4-Au/silica@F nanoparticles. By comparing their UV-vis absorption and fluorescence intensity, we demonstrate that the apparent QY (Figure 6) of the fluorescein doped into the silica shell of the Fe3O4-Au/ silica@F system gradually increases to a maximum of ∼0.89 as the concentration decreases, which is very close to ∼0.92, the value for free fluorescein molecules. The origin of this enhancement is then attributed to the interactions between fluorophores and the induced plasmonic oscillations in Fe3O4-Au nanoparticles. It was demonstrated that the enhancement of luminescence intensity of fluorophores can successfully be engineered through coupling with surface plasmons in metallic nanostructures.19 This phenomenon can be explained by the electromagnetic enhancement in the near field of the gold nanoparticles and the
interactions of the fluorophores with the metallic plasmons.20 A metallic structure (eventually a nanoparticle) placed near a fluorophore will induce both absorption and scattering of the electromagnetic energy radiated by that fluorophore, both terms being dependent on their separation distance.21,22 If the fluorophore is too close to the metal surface, the absorption term is dominant and fluorophore quenching is expected to occur due to the mutual nonradiative energy transfer. This quenching can be avoided if the fluorophore is far enough from the metal surface such that quenching does not occur, but at the same time relatively close in order to be able to still interact with the induced metal plasmons (by scattering). The enhanced fluorescence intensity then primarily originates in the match of the wavevectors between the fluorophore emission and the induced metallic plasmons.23 Moreover, the weak coupling between Fe3O4 and Au nanoparticles can further increase the electromagnetic near field. The observed increase of the apparent QY is due to the fluorophores that are close enough to the metallic surface so that their energy is rapidly transferred to the metallic plasmons, which then radiate it into the far-field.24 The effective QY may approach unity in this case, and the overall QY will be given by the QY for plasmonic scattering. Herein the Fe3O4-Au/porous silica nanoparticles and integrated Au surface plasmonic resonance in each particle are in fact expected to play the role of energy acceptor in the interacting process between the fluorophore and plasmonic field. Aiming to theoretically prove the contribution of the Fe3O4-Au cores to the enhancement process, the discrete dipole aproximation (DDA) theory was employed to calculate the electromagnetic near field around these dimers.25,26 We performed fully three-dimensional (3D) intensive numerical simulations of the electromagnetic field scattered by isolated Au, Fe3O4, as well as Fe3O4-Au nanoparticle dimers. The propagation direction of the incident electromagnetic waves was chosen along the Z axis, and two polarizations along the symmetry axis and perpendicular to it have been considered. The dielectric properties of the particles were simulated by using the dispersion relation for bulk Fe3O427 and the corrected size-dependence of the dielectric constants of Au for a 5-nm diameter nanoparticle.28 In order to reduce the computational effort, the interface between silica and aqueous solution has not been considered in simulation. However, the influence of silica on the surface plasmonic response has been taken into account by considering the Fe3O4-Au nanoparticle embedded in a homogeneous medium whose optical constants are identical to those of silica at 490 nm. Figure 7a shows the model of an Fe3O4-Au nanoparticle, in which about ∼1/3 of Au nanoparticle is embedded in the
Fe3O4-Au/Silica@Fluorescein Nanoparticles
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18317 ing potential in practical applications such as multimodal imaging, biosensing, or localized drug delivery. Acknowledgment. The work was jointly supported by the A grant and the NSERC-CRD grant, the Canadian Institutes of Health Research and the National Research Council of Canada, Industrial Materials Institute (IMI-NRC). The authors would also like acknowledge the support of the The´Cell Network of the Fonds de la Recherche en Sante´ du Que´bec.
Figure 7. (a) The model used for electromagnetic DDA calculations and the electric field distribution of Fe3O4 and Au nanoparticles (insets), of which the diameters are 15 and 5 nm, respectively. (b) The electric field distribution of Fe3O4-Au nanoparticle embeded in silica medium when the incident electromagnetic wave along the z direction are polarized along the symmetry axis and perpendicular to it (inset).
Fe3O4 nanoparticle. The cut plot of near field electromagnetic enhancement factors (f ) |E/E0|2) of individual Fe3O4 and Au nanoparticles are shown in the inset of Figure 7a for comparison (E and E0 stand for the electric field intensities of total and incident electromagnetic waves, respectively). For individual Fe3O4 nanoparticles, there is no obvious enhancement with respect to the incident wave, while the enhancement of plasmonic field appears in both isolated Au and dumbbell-like Fe3O4-Au nanoparticles (Figure 7b). It should be noted that the distribution of the electromagnetic field for the Fe3O4-Au nanoparticle is slightly different from that of isolated Au nanoparticles, due to the eventual electromagnetic coupling between these two particles. This coupling is dependent on the polarization of the incident light and is obviously less pronounced when the electric field vector oscillates perpendicularly to the symmetry axis of Fe3O4-Au nanoparticle. Consequently, the most important contribution to the enhancement of the field originates from longitudinal plasmonic excitations of the Fe3O4-Au nanoparticles. Combining the theoretical simulation together with what we experimentally observed, it is reasonable to conclude that the special “dumbbell-like” architecture of Fe3O4-Au nanoparticles supports much stronger near-field enhancement than that of individual Fe3O4 and Au nanoparticles, with an obvious beneficial influence for the metal-enhanced fluorescence phenomenon. 4. Conclusion In summary, we reported a facile strategy for the synthesis of Fe3O4-Au/porous silica@F nanoparticles showing magnetic, plasmonic, and fluorescent properties. We demonstrate that the QY of the fluorescein doped into the nanoporous silica shells, used here as a proof of concept, is increased due to the electromagnetic field coupling of plasmonic dumbbell-like Fe3O4-Au cores. Theoretical simulation further proves that the observed behavior originates in the distribution of the near field enhancement around the gold nanoparticles. Monodisperse magnetic-plasmonic Fe3O4-Au/porous silica core/shell nanoparticles, with nanoporous silica shells containing both primary and secondary amine groups, exhibit a significant interest for coupling biomolecules. The multifunctional nanoarchitectures presented herein could also offer an ideal platform to study the molecules with low fluorescence efficacies, and exhibit promis-
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