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J. Phys. Chem. B 2004, 108, 19368-19372
Surface-Enhanced Fluorescence and Reverse Saturable Absorption on Silver Nanoparticles I-Yin Sandy Lee,* Honoh Suzuki, Kanako Ito, and Yusuke Yasuda Department of Chemistry, Toyama UniVersity, 3190 Gofuku, Toyama 930-8555, Japan ReceiVed: June 29, 2004; In Final Form: September 7, 2004
Langmuir isotherm is applied to analyze the laser-induced fluorescence from solutions containing silver nanoparticles and tris(2,2′-bipyridyl)ruthenium(II) chloride. Fluorescence enhancement factors based on the simple adsorption model are found to be 47-90, depending on the nanoparticle concentration. The enhancement is attributed to the laser-induced free-carrier absorption on the silver surfaces, which is a reverse saturable absorption process, at the excitation wavelength of 532 nm.
1. Introduction Surface-enhanced phenomena in Raman scattering (SERS), fluorescent emission, and harmonic generation have been known for decades.1-3 They provide high sensitivity and near-surface specificity that are useful in analytical applications in chemistry, biology, medicine, pharmacology, and environmental science; surface-enhanced one- and two-photon fluorescence also offers a selective and rapid tool for the detection of a single molecule.4-7 Experiments have shown an 80-fold enhancement of two-photon fluorescence from Rhodamine B near silver surfaces.2 By applying LB films of arachidic acid as spacers for fluorophore layers near rough silver surfaces, Aroca and co-workers found the enhancement by a factor of 400 for P-polarized excitation.8 Electromagnetic resonances (EM) and charge transfer (CT) are the two major mechanisms accepted for SERS.9,10 On the other hand, mechanisms for surface-enhanced fluorescence still seem to be under discussion. It is suggested that amplification of the electric field on fluorophores near the metal surface could result in the fluorescence enhancement;11 however, the adsorption and resonance interactions between the fluorophore and the surface also change the radiative and nonradiative decay rates, so that the net effect will depend on the fluorophoresurface distance and the intrinsic quantum yield of the fluorophore.12 Qualitatively speaking, an increase in the radiative decay rate decreases the fluorescence lifetime, whereas a change in the electric field of excitation does not.13 Therefore, to elucidate which enhancement mechanism is operating, it is important to measure the lifetime. In this study, we have examined fluorescence enhancement due to the adsorption of fluorophores at nanoparticle surfaces. To clarify the mechanism, we have performed static and timeresolved measurements on absorption and laser-induced fluorescence. The silver nanoparticles have been prepared from starter sols, where the sol particles serve as the nucleation centers.14 The nanoparticle size can be controlled by adjusting the mixing ratio, and a diameter of 40 nm has been chosen because the resulting plasmon resonance overlaps the excitation wavelength of 532 nm (Figure 1). The tris(2,2′-bipyridyl)ruthenium(II) complex, [Ru(bpy)3]2+, has been used as the fluorophore. It is widely known as an excellent charge injector and is of great interest for potential applications in nanophotonic * Corresponding author. E-mail:
[email protected].
Figure 1. Absorption spectra of silver nanoparticles (silver concentration 13.2 µM).
technology and solar energy utilization.15-19 The adsorption equilibrium has been analyzed by using the Langmuir isotherm,20 which is found to be consistent with the experimental result of the laser-induced reverse saturable absorption (RSA). 2. Experimental Section The starter sol was prepared by mixing a silver nitrate solution (0.6 mM) with a sodium tetrahydroborate solution (0.1 mM). Further addition of silver nitrate (0.3 mM) and ascorbic acid (0.4 mM) gave a solution of silver nanoparticles of 40 nm in diameter.14 Stock solutions containing a known concentration of silver (13.2 µM) were prepared. They were usually stable at 10 °C for a few days. Dilute solutions (4.95-9.90 µM) were prepared 1 day before the experiment and allowed to settle overnight. For each measurement of fluorescence, tris(2,2′bipyridyl)ruthenium(II) chloride was freshly added, and the solution was purged in argon for 5 min to avoid quenching by oxygen. All the solutions were prepared with deionized water. Absorption spectra were measured with a Shimadzu UV-160 spectrophotometer. A centrifuge (Millipore, 6400 rpm, 2000g) was used to separate particles from the bulk solution to determine the surface coverage, which is a parameter needed for the Langmuir isotherm analysis. The excitation light source was a frequency-doubled, Qswitched Nd:yttrium-aluminum-garnet (YAG) laser (λ ) 532 nm) with a pulse width of 5 ns (Continuum Surelite I-10). Laser pulses were focused at the center of a 1-cm sample cell to a spot size of 8 µm. Time-integrated fluorescent emission was collected into an optical fiber connected to a CCD spectrometer
10.1021/jp0471554 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/09/2004
Enhanced Fluorescence and RSA on Ag Nanoparticles
J. Phys. Chem. B, Vol. 108, No. 50, 2004 19369
Figure 2. The experimental setup. Laser pulses are directed through an attenuator composed of a half waveplate (H) and a polarizer (P), reflected by a 45° IR mirror (M), and frequency-doubled with a KDP crystal. After passing through an IR filter (F), it is focused by a lens (L1) at the sample cell (S). Fluorescence is collected by a lens (L2) into an optical fiber bundle connected to the detector (D), which is either a CCD spectrometer for time-integrated measurements or a combination of a monochromator, a photomultiplier, and an oscilloscope for time-resolved measurements. The data are transferred to a computer (PC) for further processing. Single-shot timings are controlled by a delay generator.
(Ocean Optics, CHEM2000), which was triggered by a delay generator (SRS DG535) synchronized with the laser. Timeresolved emission profiles were obtained using a monochromator (Optometrics DMC1-03), coupled with a photomultiplier tube (Hamamatsu R1417-06) and an oscilloscope (Tektronix TDS3032). The laser-induced absorption measurement for RSA was also performed with the same instruments. In this case, the optical fiber was used to collect the transmitted light of laser pulses into the detector. For each data point, results of four single shots were averaged. The experimental setup is shown in Figure 2. 3. Results and Discussion Figure 3a shows the laser-induced fluorescence from silver nanoparticle solutions (13.2 µM as silver) with varied concentrations of the ruthenium dye, whereas Figure 3b shows the corresponding spectra without silver nanoparticles. The peak at 532 nm is the scattering signal from the excitation source. The fluorescence peaks at 608 nm. Enhancement of fluorescence by the nanoparticles is obvious. To obtain the enhancement factor quantitatively, we analyzed the data in terms of the adsorption equilibrium of the dye to the surface of the silver nanoparticles. A simple model based on the Langmuir isotherm was applied to determine adsorption parameters, where the monolayer surface coverage was assumed. Let σ denote the surface coverage, i.e., the net molar concentration of the adsorbed dye complexes per unit volume of the solution, and σo the full coverage, i.e., that for the completely covered surface (adsorption-saturated solution). Note that σo and σ both have the unit of mol/L. The concentration of available adsorption sites is then σo - σ. With the total incident dye concentration of [Ru]T, σ can be expressed as21
σ)
σoKeq[Ru]T 1 + Keq[Ru]T
(1)
Keq is the equilibrium constant defined as the ratio of kf/kb, where kf and kb denote the rate constants of adsorption (forward) and desorption (backward), respectively. If the total fluorescence intensity I is assumed to be the sum of those from the adsorbed dyes and from the “free” dyes in the bulk, I can be written as
I ) σQad + ([Ru]T - σ)Qfree I - Io ) σ(Qad - Qfree)
(2)
Figure 3. Fluorescent emission spectra of solutions containing (a) Ag (13.2 µM) and [Ru(bpy)3]2+ and (b) [Ru(bpy)3]2+ only, at the dye concentrations of 4.99, 9.95, 14.9, 19.8, 24.7, 48.8, 72.3, 95.2, and 117.6 µM (from bottom to top).
where Qad is the emission efficiency for the adsorbed dye and Qfree is that for the free dye. Io denotes the fluorescence intensity from the dye-only solution without nanoparticles and is equal to the product Qfree[Ru]T. Io was measured separately following the same procedure as for the nanoparticle solutions. Substituting σ gives the intensity difference as
I - Io )
σoKeq[Ru]T 1 + Keq[Ru]T
(Qad - Qfree)
(3)
Emission at 650 nm was employed for the analysis to avoid errors from the scattered excitation light. In addition, σo for each silver concentration was estimated from independent centrifugation experiments as follows. A solution containing a high concentration of the dye (0.4 mM) and the silver nanoparticles was prepared, and the nanoparticles were removed by centrifugation. The absorbance of the residual solution was then measured (Acen). If the full coverage is assumed for the solution even after the centrifuge-induced aggregation, σo can be estimated as σo ) (1/d) (Afree - Acen), where is the molar extinction coefficient of the dye at 450 nm, d is the path length, and Afree is the absorbance of the dye-only solution (0.4 mM). Figure 4 shows the full surface coverage as a function of the silver concentration. It indicates an increasing tendency as the silver concentration increases, but strongly deviates from the expected linear dependence; it seems to saturate or even decrease at the high silver concentrations. As will be discussed below, we observed aggregation of silver nanoparticles at higher concentrations, which might lead to the decrease in the effective surface area for adsorption and cause the observed tendency. By using these σo values as the known constants, curve fitting was performed on the basis of the Langmuir model of eq 3.
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Figure 4. The full surface coverage, σo, as a function of the silver concentration, estimated from the centrifuge method.
Figure 5. Fluorescence enhancement by the nanoparticles as a function of the dye concentration. Difference in fluorescence intensities between the solutions with the nanoparticles (13.2 µM as silver) and without the nanoparticles is plotted. Experimental results (O) are fitted using the Langmuir isotherm model (solid line).
Figure 5 shows the typical results, in which the intensity difference I - Io was obtained from Figure 3. The curve indicates that, with the increasing dye concentration, the enhancement increases with a saturating tendency at the higher concentrations, in accordance with the expected behavior of the Langmuir model. The least-squares fit successfully reproduced the experimental data points, and the enhancement factor Qad/ Qfree, together with Keq, was obtained for each silver concentration (Figure 6). The enhancement factor is found to be larger than 40 for all the samples. The origin of the fluorescence enhancement near the metal surface is in general complicated, and the lifetime measurement is useful in revealing the mechanism and the nature of the dyesurface interactions. To examine the effect of the nanoparticles on the fluorescence lifetime, we fitted the time-resolved emission profiles to a single-exponential decay. It turns out that all the solutions (with and without nanoparticles) essentially have the same lifetime of 0.55 µs ((0.025 µs), which is also close to the literature values reported for [Ru(bpy)3]2+ in water.22,23 As the time resolution of our instruments is determined by the photomultiplier response time (2.2ns) and is much shorter than the lifetime, we conclude that the nanoparticles have little effect on the lifetime. Fluorescence near the metallic surface is often hindered by surface damping, which arises from nonradiative energy transfer. The nonradiative decay rate decreases as d-3, where d is the distance between the excited dye and the metal surface.24 Thus, at a small d, the fluorescence is dominated by the nonradiative decay, whereas at a large d, the radiative decay is predominant.25 In view of this, the ruthenium dye complexes are probably adsorbed rather loosely to the nanoparticle surfaces, with the distance d beyond the nonradiative zone; the surface damping is negligible. In addition, the identical lifetime implies that the free and the adsorbed dyes have the same decay rates, i.e., the nanoparticles do not change the intrinsic properties of
Figure 6. (a) Adsorption constants and (b) fluorescence enhancement factors as a function of the silver concentration.
the fluorophores. Accordingly, the fluorescence enhancement may be caused by the amplified energy transfer from the nanoparticle surface to the dye. Metallic nanoparticles interact with the radiation through surface plasmon resonances, which are also sensitive to the environment. Dye adsorption often induces aggregation and formation of chemisorption complexes in nanoparticles, nanodots, and nanorods,26-29 and such aggregates and complexes show new absorption bands.30-33 In the study of SERS on DNAadsorbed gold nanoparticles, Murphy reported that the SERS intensity strongly depends on the DNA-induced aggregation, i.e., the excitation at the aggregation-modified plasmon resonance band causes a greater enhancement.34 To see if there is any effect such as aggregation or chemisorption in our system, we closely examined the absorption at 532 nm. The absorption change was, however, too small to account for the enhancement factors over 40. Thus, the enhancement in absorption due to the aggregation is minor. On the other hand, metals may react nonlinearly to the intense radiation. Figure 7 shows the absorption of the dye-nanoparticle solutions at 532 nm as a function of the incident pulse energy. For the solutions containing nanoparticles, nonlinear absorption indeed takes place and leads to the high absorbance (0.9-1.2), in contrast to the dye-only solutions, where the absorbance remains small throughout the measured energy region. Figure 8 plots the laser-induced absorbance change against the dye concentration at the pulse energy of 30 mJ. The curves indicate that the absorption change increases with the increasing silver concentration, whereas the dye concentration has little effect. Clearly, the nonlinear absorption originates from the silver surfaces, in agreement with the lifetime measurements. The observed fluorescence enhancement can be explained in terms of the laser-induced absorbance as follows. Assuming that the nonlinearly absorbed energy at the nanoparticle surface is evenly and completely transferred to the adsorbed dye complexes, we may evaluate the effective absorption coefficient, ad ) ∆A/dσo, where ∆A is the laser-induced absorbance change
Enhanced Fluorescence and RSA on Ag Nanoparticles
Figure 7. The nonlinear absorbance versus the incident pulse energy. The solutions containing silver nanoparticles (13.2 µM): Ag only (b), Ag/[Ru(bpy)3]2+ (4.98 µM, 9), and Ag/[Ru(bpy)3]2+ (181.8 µM, 2). The dye-only solutions: 4.98 µM (O) and 181.8 µM (0). The solid lines are a guide to the eyes.
Figure 8. Laser-induced absorbance change of the dye-nanoparticle solutions at the pulse energy of 30 mJ. Silver concentration: 13.2 µM (b), 9.90 µM (9), 6.60 µM (2), and 4.95 µM (O).
TABLE 1: Enhancement Factors for the Fluorescent Emissiona sample (µM)
ad (M-1 cm-1)
θexp
θNL
13.2 9.90 6.60 4.95
7.90 × 5.63 × 104 3.99 × 104 7.47 × 104
75.6 75.0 47.2 89.0
109 77.9 55.2 103
104
a ad (effective absorption coefficient) denotes the nonlinearly enhanced absorbance of the adsorbed dye at 532 nm. θexp is the enhancement factor obtained by the Langmuir analysis. θNL is the nonlinearly induced enhancement factor estimated from the RSA measurements.
of the full-coverage (adsorption-saturated) solution and d is the path length. The nonlinear enhancement factor may be defined as θNL ) ad/free, where free is the linear absorption coefficient of the dye at 532 nm (722.22 M-1 cm-1). These enhancement factors, along with those obtained from the Langmuir analysis (Figure 6b), are summarized in Table 1. Considering the assumptions involved in the analysis, we may say that the two independent determinations of the enhancement factors are in fair agreement and that the fluorescence enhancement is mainly attributable to the laser-induced nonlinear absorption at the nanoparticle surface, followed by the efficient energy transfer to the adsorbed dyes. Table 1 also shows that the solutions with the highest and the lowest silver concentrations have greater enhancement factors. Further studies will be necessary to verify and explain this tendency. Nonlinear absorption of metals and semiconductors is commonly attributed to the photon-induced free-carrier absorption.35,36 Free carriers originate from the intraband conduction electrons, the surface plasmon resonance, or through the
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Figure 9. Nonlinear scattering from the solutions containing silver nanoparticles (13.2 µM) with various dye concentrations. Dye concentration: 0 µM (b), 19.8 µM (9), and 181.0 µM (2).
interband transition between the valence and the conduction bands.37 Similarly to the excited-state absorption in molecules, the free-carrier absorption will result in a reverse saturable absorption (RSA) process under intense pump fields. RSA materials have been extensively studied due to their applications as photonic limiters.38 In particular, colloidal silver is among the most studied for metallic limiters. For silver particles, nonlinear phenomena of different characters have been observed, depending on the excitation time scale: RSA is responsible for the nonlinear responses to nanosecond pulses, whereas twophoton absorption is the mechanism for the picosecond excitation.39 Our results agree with their observation in that nanosecond pulses have induced RSA on the silver nanoparticle surfaces. In his studies on carbon black suspensions, Mansour observed the optical-limiting phenomena as the result of nonlinear surface scattering.40 We also examined the possibility of such nonlinear scattering in our system, by measuring the scattering intensity as a function of the incident pulse energy (Figure 9). The observed scattering intensity, however, decreases with the increasing pulse energy. Thus, nonlinear scattering is unlikely to contribute to the enhancement. In summary, we have obtained dye-adsorption parameters using the Langmuir isotherm and monitored changes in the absorption bands. That the fluorescent lifetime and emission profile of adsorbed dyes remain unaltered suggests a loose contact between the dye and the nanoparticle surface. Although the aggregation causes slight changes in the absorption, absorbance gains generated through photon-induced free-carrier absorption are by far more significant. The observed nonlinear absorption resembles an RSA process in optical limiters under intense laser pulses. The enhancement factors based on the Langmuir model are found to be highly correlated with the results from the nonlinear absorption. We have demonstrated that the free-carrier absorption, an RSA process, plays a key role in the fluorescence enhancement for the adsorbed ruthenium dyes on silver nanoparticles. Acknowledgment. This work is supported by the Japanese Ministry of Education (Grant No. 13740391) and the Iketani Science and Technology Foundation (Grant No. 0131023-A). References and Notes (1) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435. (2) Gryczynski, I.; Malicka, J.; Shen, Y.; Gryczynski, Z.; Lakowicz, J. R. J. Phys. Chem. B 2002, 106, 2191. (3) Tessier, G.; Malouin, C.; Georges, P.; Brun, A.; Renard, D.; Pavlov, V. V.; Meyer, P.; Ferre, J.; Beauvillain, P. Appl. Phys. B 1999, 68, 545. (4) Nie, S.; Chiu, D. T.; Zare, R. N. Science 1994, 266, 1018.
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