J. Phys. Chem. C 2009, 113, 17669–17677
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How Gold Particles Suppress Concentration Quenching of Fluorophores Encapsulated in Silica Beads M. Martini,†,‡ P. Perriat,*,† M. Montagna,‡ R. Pansu,§ C. Julien,§ O. Tillement,| and S. Roux| MATEIS, INSA-Lyon, UniVersite´ de Lyon, 69621 Villeurbanne Cedex, France, Dipartimento di Fisica, UniVersita` di Trento, Trento, Italy, Laboratoire de Photophysique et Photochimie Supramole´culaires et Macromole´culaires, ENS Cachan, 94235 Cachan Cedex, France, and Laboratoire de Physico-Chimie des Mate´riaux Luminescents, UniVersite´ Claude Bernard Lyon 1, 69622 Villeurbanne Cedex, France ReceiVed: May 13, 2009; ReVised Manuscript ReceiVed: September 1, 2009
Increasing the luminescence of silica particles encapsulating fluorescein by increasing fluorescein concentration is a promising strategy which is unfortunately limited by the phenomenon of self-quenching. In this paper, we demonstrate that this quenching can be almost entirely suppressed by the presence of gold inclusions within silica. Precisely, in core(gold)/shell(polysiloxane) particles incorporating fluorescein molecules, the quantum yield attains 80% of that of isolated fluorescein for an interdye distance of 3 nm whereas it is less than 15% in the absence of gold. From systematic measurements of quantum yield and lifetime, we proved that, contrary to all literature expectations, the reduction of self-quenching does not originate from the enhancement of the radiative decay rate but from a dramatic decrease of the nonradiative one: the presence of metal inclusions impeding energy transfers to fluorescein H-type dimers that act as traps for light. 1. Introduction The wide diffusion of luminescent nanometric hybrid objects for biological applications1-5 requires their size miniaturization and the continuous increase in their optical efficiency.6,7 Due to their high brightness and photostability,8 quantum dots9,10 and organic-fluorophore-tagged silica nanobeads11 constitute promising extrinsic probes.12 Indeed, each fluorescent nano-object may emit a thousand times more than single dye molecules and is also less affected by the external environment.13 Although dyedoped silica particles offer several advantages, their interest is limited by the fact that increasing the concentration of the encapsulated fluorophore leads to the undesired phenomenon of luminescence self-quenching.14-18 Precisely, when the interdye distance becomes comparable to the Fo¨rster distance, energy homotransfer takes place and the excitation propagates through the sample with the possibility to vanish upon nonfluorescent species that act as traps for light. This transfer results in a strong decrease of the quantum yield, Φ, defined as the fraction of the excited fluorophores that decay through radiative emission. Φ is given by the following formula:
Φ )
Γr Γr + knr
(1)
in which Γr and knr are the radiative and nonradiative decay rates, respectively. To overcome the limitation due to selfquenching, several solutions were proposed in literature. First, the replacement of organic dyes by lanthanide chelates19-21 with a larger Stokes shift that reduces the overlap between absorption and emission bands significantly decreases the rate of energy * Corresponding author. E-mail:
[email protected]. † Universite´ de Lyon. ‡ Universita` di Trento. § ENS Cachan. | Universite´ Claude Bernard Lyon 1.
transfer and, consequently, that of nonradiative decay (knr). The second and more unusual solution lies in the direct modification of the emission properties of the fluorophores by the presence of metallic interfaces.22-25 This possibility, mentioned in the pioneering work of Purcell26 that showed that the lifetime of an excited atomic state depends on the environment of the atom, waswidelyinvestigatedfromatheoretical27-30 andexperimental31-37 point of view. It appears that metal interfaces can differently modify dyes’ optical properties (from a complete quenching to a significant enhancement38) according to the distance, the geometry, and the relative orientation of the fluorophore dipole and the metal plasmon. The metal/dye interaction results from at least two interconnected processes: the modification of the electromagnetic field and a change in the photonic mode density (PMD) in the vicinity of the dyes.39-41 For small metal/dye distances (inferior to the Fo¨rster distance ∼3 nm), the damping of dipole oscillation coupled to surface plasmon modes leads to a strong quenching of luminescence.42-44 For larger distances, the system “organic dye-metallic interface” was extensively studied by the group of Lakowicz in terms of dipole-dipole interactions and resumed in the so-called radiative decay engineering (RDE).45,46 For intermediate distances (between the Fo¨rster distance and a distance of the order of magnitude of the metal size), there is an increase in the emission intensity due to an enhancement of the local field, particularly for metal surfaces with high curvatures like particles.47,48 For larger distances, there is still an increase in the quantum yield of the dyes. To explain this phenomenon, Lakowicz et al. proposed that a nearby metal interface increases the radiative rate of dyes by addition of a new radiative path, Γm, without significant changes in nonradiative rates. According to the authors themselves, this assumption is unfamiliar to fluorescence spectroscopy because the quantum yield of a fluorophore is in general only altered by changes in the nonradiative pathways. Because to our best knowledge direct measurements of the manner how knr and Γr are influenced by metal proximity have never been performed, we propose to measure them in this paper. We
10.1021/jp9044572 CCC: $40.75 2009 American Chemical Society Published on Web 09/18/2009
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Figure 1. Schematic structure of (a) gold/polysiloxane particles and (b) polysiloxane particles. TEM images of (c) gold/polysiloxane particles containing 5500 dyes and (d) polysiloxane particles containing 4800 dyes with in inset size distributions obtained by TEM and PCS. Schematic steps for formation of (e) gold/polysiloxane particles and (f) polysiloxane particles.
perform these measurements in core/multishell particles (Figure 1a) made of a gold core with a size of ∼5 nm, a first silica shell that does not contain fluorophores and has a thickness of ∼3 nm (in this shell, there is a complete quenching of luminescence) and a second silica shell containing fluorescein with a thickness of ∼25 nm. The choice of gold for metal core and fluorescein for dyes molecules is motivated by the strong overlap between gold plasmon (∼536 nm) and fluorescein emission band (∼515 nm). To evaluate the enhancement of electromagnetic field and the modification of the radiative and nonradiative decays induced by the presence of metal, reference
samples consisting of silica particles with the same size and the same concentration of dyes were also elaborated (Figure 1b). Similar structures have already been synthesized with silver cores and different silica thicknesses by Viger et al.24 The presence of silver core (with a plasmon peaking at ∼420 nm) permits a reduced lifetime and a greater emission intensity of the fluorescein encapsulated in the silica shell. However, the decay rates knr and Γr were not determined so that the reduction of self-quenching remains in a large extent unexplained. knr and Γr can be obtained from measurements of lifetime and emission intensity after a pulse.32,49 In the present paper, these rates are
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derived from direct measurements of quantum yield and lifetime (the average time spent in the excited state) whose expression is given by the following formula:50
τ )
1 Γr + knr
(2)
Indeed, from relations 1 and 2 giving Φ and τ as functions of Γr and knr, one can easily derive the expressions of Γr and knr as functions of Φ and τ:
Φ τ
(3)
1 (1 - Φ) τ
(4)
Γr )
knr )
If it has been possible to determine precisely these decays, it is because we succeeded in overcoming the difficulty of measure the quantum yield. When samples present significant scattering which is the case here since particles have a size comparable to the wavelength of the incoming light,51,52 the easiest method to measure Φ (the method of William53,54 that compares the absorbance and the emission of the dye to test with those of a reference) is not applicable.55-58 The reason is that the photons scattered by the silica particles are absorbed by the dyes and generate an additional light emission which leads to an overestimation of the quantum yield. We showed in a previous paper how William’s method could be adapted to be still valid in the case of scattering media.59 By applying systematically this modified method to the architectures studied, we provided evidence that the presence of metal cores allows us to reduce the self-quenching of fluorescein and maintain their quantum yield close to that of isolated molecules. However, we demonstrate that, contrary to all literature expectations, this suppression of quenching is not due to an increase of the radiative decay rate but to a strong decrease of the nonradiative one: the presence of metal inclusions impeding energy transfers to fluorescein molecules that act as traps for light. 2. Experimental Section Synthesis. Triton X-100, n-hexanol, cyclohexane, HAuCl4 · 3H2O, sodium 2-mercaptoethanesulfonate (MES), NaBH4, fluorescein-isothiocyanate isomer I 90% (FITC), tetraethoxysilane (TEOS), and (3-aminopropyl)triethoxysilane (APTES) were purchased from Aldrich. NH4OH was purchased from Chimie Plus, and 3-(trihydroxysilyl)propylmethylphosphonate (THPMP) (42% in water) was purchased from ABCR. Only Milli-Q water (F > 18 MΩ) was used for the preparation of the aqueous solution. The elaboration of the two families of samples (with and without gold cores) is described in detail in a previous paper and in the Supporting Information. TEM Characterization. TEM was used to obtain detailed structural and morphological information about the samples and was carried out using a JEOL 2010F microscope operating at 200 kV. The samples were prepared by depositing a drop of a diluted colloidal solution onto a carbon grid (200 mesh) and allowing the solvent to evaporate in air at room temperature. Size Measurements. Direct measurement of the size distribution of the particles was performed using a Zetasizer NanoS PCS (photon correlation spectroscopy) from Malvern Instrument.
ζ- Potential Measurements. The ζ-potential of the silica nanoparticles containing fluorescein molecules was directly determined using a Zetasizer NanoS from Malvern Instruments. Prior to experiment, the solution was diluted in an acqueous solution containing 0.01 M NaCl. UV-Vis Spectroscopy Studies. UV-vis extinction spectra were recorded at room temperature using a WPA 800 diode array spectrometer. The extinction band was measured in the 330-800 nm range. Quartz cells with an optical length of 10 nm were used for all optical measurements. Luminescence Spectra. The emission spectra were measured at room temperature using a Hitachi F-2500 spectrophotometer. The excitation wavelength was adjusted to 493 nm that corresponds to the absorption maximum for fluorescein dyes in aqueous solutions (0.1 N NaOH). Samples were diluted to have an optical density τ2) are two decay times, the ratio (R1)/(R2) gives immediate information upon the deviation from ideal monoexponential behavior. Concerning polysiloxane samples (Figure 5a), the ratio R1/ R2 is close to 50/50 which shows that, in accordance with literature,14,16 a monoexponential fit is effectively not adequate to describe correctly the decay curves. The lifetime obtained from eq 7 shows that it does not depend on the dye concentration (Figure 6) and is comprised, in all samples, between 3.5 and 4.1 ns, a value very close to that of isolated fluorescein (4.0 ( 0.2 ns). Logically, the parameter γ is found proportional to the dye concentration (inset in Figure 6), indicating that the ratios between the different prototropic forms of the fluorescein molecules do not depend on the concentration.
Figure 6. Fluorescein lifetime, τ, as a function of interdye distance for polysiloxane (SiOx) and gold/polysiloxane (Au@SiOx) particles. In the inset is shown the parameter γ, related to energy transfers toward quenchers, as a function of the dye concentration.
Concerning gold/polysiloxane particles, Figure 5b shows a completely different trend. Surprisingly, all the intensity decay curves almost perfectly coincide regardless of the interdye distance and have almost perfect monoexponential behavior. Also, the lifetime is quite similar to that found in polysiloxane particles (between 3.6 ( 0.1 and 4.1 ( 0.1 ns) and is also consistent with the lifetime value of isolated fluorescein 4 ( 0.2 ns. This result was unexpected because the presence of metal was found to decrease the lifetime in similar architectures already studied in the literature (silver/silica particles). Only a minority of the studies devoted to the behavior of dyes in the vicinity of metal (gold or silver) surfaces mentioned the possibility of an absence of modification or an increase in jτ. Moreover, a lot of studies indicated that the lifetime of a dye was expected to vary with the distance to the metal.45 In the particles studied here, dyes are different distances from the gold core (from 3 to 26.5 nm) so that it is very surprising that the fluorescein molecules do not present a distribution of lifetime (characterized by a multiexponential time evolution in the decay curves) but all possess the same lifetime. That would indicate that a system consisting of a metal particle and a collection of dyes should behave differently from the usual system composed of metal (surface or particle) and a single molecule. Here all the dyes behave collectively as a result of the multiple dipole-dipole interactions between, on one hand, each couple of dyes and, on the other hand, each particle/dye subsystem. Finally, completely consistent with the monoexponential form of the decay cure in the presence of gold, the parameter γ is found to e equal to zero for all samples containing gold (inset of Figure 6). According to the wavelength dependence of
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Figure 7. Fluorescein (a) radiative and (b) nonradiative decay rates as a function of interdye distance for polysiloxane (SiOx) and gold/polysiloxane (Au@SiOx) particles.
absorbance of both series (Figure 2a for polysiloxane samples, Figure 3 of the Supporting Information for gold/polysiloxane samples), the proportion of H-type dimers is the same in both kinds of materials. That γ be equal to zero is then the first indication that the presence of gold suppresses all energy transfers from monomers to quenchers (dimers). In other words, gold preferentially enhances all excitation jumps from monomers to the same forms of fluorescein molecule. 3.8. Influence of a Gold Core upon Radiative and Nonradiative Decay Rates. From both values of Φ and τ, it is finally possible to extract the nonradiative and radiative decay rates, knr and Γr, from eqs 3 and 4. Figure 7a reports the values of Γr for both series of particles. First, regardless of the presence of gold, the radiative decay decreases when the dye concentration increases (i.e., the interdye distance decreases). That means that when the interdye distance becomes comparable to the Fo¨rster distance, the photonic mode density in the vicinity of the molecules is modified in a way that decreases the radiative decay rate. In polysiloxane particles, the radiative rate asymptotically tends to the value of 2.3 × 108 s-1 for large interdye distance, a value that corresponds to the radiative lifetime of isolated fluorescein. Such a modification of Γr with dye concentration was to our best knowledge never reported in the literature. Indeed, the quantum yield was rarely measured and all lifetime modifications were generally assigned to a change in nonradiative decays. In the presence of gold, the radiative decay rate is increased in proportions between 10% at low concentrations and 40% for higher ones. This result is consistent with literature expectations except that the enhancement found here is smaller than that generally reported. Again, this can be explained by the fact that all literature makes the assumption that the suppression of self-quenching brought by the presence of metal entirely arises from a change in Γr (knr remaining constant). By comparing parts a and b of Figure 7, we find evidence that this usual assumption is not fully justified. Indeed, the presence of a gold core leads to a diminution of knr of almost an order of magnitude in the range of high concentrations: for an interdye distance of 5 nm, k is equal to 0.3 × 108 s-1 in the presence of gold instead of 1.9 × 108 s-1 in its absence. According to the wavelength dependence of absorbance of both series (Figure 2a for polysiloxane samples, Figure 3 of Supporting Information for gold/polysiloxane samples), the proportion of H-type dimers is the same in both kinds of materials. The dramatic decrease of knr means then that energy transfers to dimers (Figure 2b) are almost completely forbidden in the presence of gold. By additional anisotropy measurements (not shown in this paper) we proved that the energy transfers between organic molecules are accelerated by a factor >10 in gold/
polysiloxane particles. Therefore, we have provided evidence in the present paper that the acceleration of energy transfers brought by the presence of gold cores is effective only between molecules that have the same density of states and reduces significantly the probability of excitation transfer from a monomer to dimers acting as traps for light. 4. Conclusions In this paper, we have demonstrated that increasing the luminescence of silica particles encapsulating fluorescein by increasing the dye concentration is a promising strategy since self-quenching can be almost entirely suppressed by the presence of gold inclusions within silica. From independent measurements of quantum yield and lifetime, we have determined the radiative and nonradiative decay rates for two series of samples (gold/ polysiloxane and polysiloxane particles) having different concentrations of fluorescein. We have pointed out five main items: (i) an increase of the fluorescein quantum yield in core(gold)/ shell(polysiloxane) up to values close to that of isolated fluorescein until concentrations corresponding to interdye distance of 3 nm, (ii) a monoexponential trend of fluorescein decay curves in the presence of gold, (iii) a collective behavior of the fluorophores whose optical properties are surprisingly not dependent on the metal distance but all equal to those of isolated molecules, and (iv) an increase in the radiative rate less important than that predicted in the literature, accompanied by (v) a large and unpredicted decrease in the nonradiative rate. Contrary to all expectations, this decrease of knr is the major explanation in the increase of the quantum yield and thus the reduction of self-quenching. It is related to the suppression of energy transfers toward H-type dimers that act as traps for light so that the nonradiative rate becomes independent of dye concentration. To better understand this last phenomenon, anisotropy measurements are currently being performed to determine the modification brought by the presence of a gold core upon the energy transfer rate between fluorescein molecules. Supporting Information Available: More details about the synthesis procedure, the lifetime experiments, and the method for estimating correctly the quantum yield of FITC molecules in a colloidal system. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chen, W. J. Nanosci. Nanotechnol. 2008, 8, 1019. (2) Wang, F.; Tan, W. B.; Zhang, Y.; Fan, X.; Wang, M. Nanotechnology 2006, 17, R1.
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