Letter pubs.acs.org/JPCL
Surface Plasmon-Coupled Emission of Rhodamine 110 Aggregates in a Silica Nanolayer Simeonika Rangełowa-Jankowska,† Dawid Jankowski,† Robert Bogdanowicz,‡ Beata Grobelna,§ and Piotr Bojarski*,† †
Department of Mathematics, Physics and Informatics, Institute of Experimental Physics, University of Gdańsk, Wita Stwosza 57, 80-952 Gdańsk, Poland ‡ Department of Optoelectronics & Electronic Systems, Faculty of Electronics, Telecommunication and Informatics, Gdańsk University of Technology, Narutowicza 11/12, 80-233 Gdańsk, Poland § Faculty of Chemistry, University of Gdańsk, Sobieskiego 18/19, 80-952 Gdańsk, Poland ABSTRACT: First analysis of strong directional surface plasmon-coupled emission (SPCE) of ground-state formed intermolecular aggregates of Rhodamine 110 (R110) in silica nanofilms deposited on silver nanolayers is reported. Until now, the processes of energy transport and its trapping due to aggregate formation have not been studied in the presence of SPCE. A new approach to multicomponent systems with weakly and strongly fluorescent centers making use of fluorophore−surface plasmon interaction is presented. The analysis is based on comparison of experimental free-space emission spectra (F-SE), experimental SPCE with theoretical surface plasmon resonance spectra (SPR). It is shown that, due to the dispersion of SPCE, the detection of weak aggregate emission is straightforward if only the monomers and aggregates fluorescence spectra are somewhat spectrally shifted. SPCE studies confirmed the formation of weakly fluorescent higher order aggregates of R110 in silica films. The results indicate that the increase of energy transfer from monomers to aggregates is due to fluorophore−plasmon interaction. SECTION: Plasmonics, Optical Materials, and Hard Matter
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These facts can be important in the analysis of other biologically active complex systems with weakly and strongly fluorescent components. Similarly, the unique features of SPCE lead to various important applications in biosensing, studying antigen−antibody reactions, or energy transfer in special systems.4,5,7,8 Comparison of experimental F-SE spectra, experimental SPCE spectra, and theoretical SPR spectra is, in our opinion, a base of valuable analysis of such complex objects. Silica xerogel with incorporated dyes as an organic−inorganic material can play an important role as efficient optical material, in sensing technology, lasing medium or a part of drug delivery system.1,2,8−11 Hybrid organic−inorganic materials can be obtained based on the sol−gel method. The final product of the sol−gel process is obtained as a result of hydrolysis of the alkoxy groups and condensation of precursors, which are metal alkoxides M(OR)n. Colloidal particles dispersed in liquid (sol) systematically grow as a result of condensation reaction; they aggregate and form three-dimensional network over the whole volume of solution, which is called gel in the final stage of reaction. Previously, it has been found that hybrid matrix forms a friendly environment for effective SPCE observation.6 This
unctional materials obtained by sol−gel process are the subject of many papers.1−3 Similar interest has been observed recently in enhanced fluorophore emission induced by the interaction of surface plasmons at metallic nanofilms with vicinal fluorophores located closer than 200 nm to the metal surface.4,5 Surface plasmon-coupled emission (SPCE) can be obtained by illuminating the sample through the glass hemispherical prism and a metal film (the so-called Kretschmann configuration) or through direct excitation of hybrid thin film (reverse Kretschmann configuration (Figure 1a−c)). In both configurations, intensive angle resolved emission passing through the metallic layer and back into the prism can be recorded. The use of a hemispherical prism results in the formation of SPCE sharp color rings observed in the detector plane (Figure 1b). This directional character and total linear polarization (p and s type6) of SPCE results from a nearfield interaction between the excited fluorophore and surface plasmons. It should be stressed that SPCE contains usually identical spectral information as the free-space emission (F-SE), but unlike normal fluorescence, it can be much more effectively collected. However, if we deal with monomer−dimer, monomer−excimer, or other multicomponent system, the FSE and SPCE spectra can differ significantly. We show in this paper that the difference of SPCE and F-SE can be a consequence of the selectively enhanced emission of the individual components in the aggregated system and the surface plasmon mediated excitation energy transfer (SPMEET7). © XXXX American Chemical Society
Received: October 25, 2012 Accepted: November 23, 2012
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the goal of the paper, strongly fluorescent Rhodamine 110 (R110) was chosen (fluorescence quantum yield η = 0.99 in silica matrix was determined by us with standard comparative method). The host matrix was formed by a silica nanolayer with refractive index n = 1.67 for 473 nm (Figures 1c,d) spun on a thin (50 nm) silver film. Figure 2 shows the results obtained for layers containing R110 in SiO2 at different dye concentrations (in sol): (a,d) CR110 = 0.002 M; (b) CR110 = 0.007 M, and (c) CR110 = 0.01 M. Figure 2a−c shows in the left panel the absorption spectra of R110 in SiO2 (obtained for R110 in SiO2 on glass substrates). The change in the profile of absorption spectrum evidence the aggregation process developing with the increase in dye concentration. In the case of less concentrated samples (CR110 ≤ 0.002 M) the presence of higher order aggregates in silica nanolayers can be neglected, and it can be assumed that the analyzed system consists only of monomers and dimers. On the basis of the previously described procedures,13−15 the absorption spectrum of R110 dimers in silica nanolayer was obtained (Figure 2d). For higher concentrations of R110 (CR110 = 0.007 M and CR110 = 0.01 M), nonproportional intensity increase in longwave dimer absorption band (J band) was observed, which strongly indicates the presence of higher order aggregates. Excitation at 473 nm takes place in the region of strong overlap between monomer and aggregate absorption bands of R110 closely to the maximum of dimer H band (shortwave band), which results in the excitation of different luminescent centers. In all three cases shown in Figure 2a−c, the pictures show characteristic rings of directional SPCE (panel on the right). Higher number of rings results from larger thickness of the hybrid layer and accompanying waveguide phenomenon. The conditions for plasmonic resonance change depending on the thickness of the dielectric layer. Hence, the number of rings and their angular locations are different for particular
Figure 1. (a−c) Experimental setup for measurements of angular distribution of SPCE in the reverse Kretschmann configuration from R110 in silica nanolayers distributed on the surface of thin silver films. Directional emission coupled with surface plasmon propagates in a small spacial angle of the cone, forming characteristic rings on the surface of the observation screen.12 The system consists of the hemispherical glass prism, glass substrate coated with 50 nm silver layer and silica nanolayer with incorporated R110 fluorophores. T − rotatable table with stepper motors (SM) and controller (C); P − fiber; M − monochromator; PMT − photomultiplier tube; DMM − digital multimeter; PC − personal computer with LabVIEW package; L − DPSS CW laser; F − fluorophores in dielectric thin film; θF − the angle of SPCE propagation; n1, n2 − refractive index of light for air and silica tetramethoxysilane (TMOS) based, respectively. (d) Dependence of refractive index and extinction coefficient in silica on wavelength.
paper presents first observations of directional aggregate emission enhanced by the silver surface plasmons. To realize
Figure 2. (a−c) Absorption spectra of R110 in silica nanolayers (left panel) and images of the directional emission on the screen in the form of characteristic SPCE rings (right panel) for three dye concentrations in SiO2: (a) CR110= 0.002 M; (b) CR110= 0.007 M and (c) CR110 = 0.01 M. (d) The result of numerical separation of the measured absorption spectrum of R110 in silica for CR110 = 0.002 M: monomer, dimer, and fitted absorption for R110 in SiO2. Calculations were performed with our software using procedures described previously.15 3627
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Figure 3. The results obtained for particular dye concentrations in silica nanolayers: (a) CR110 =0.002 M; (b) CR110 = 0.007 M, and (c) CR110 = 0.01 M. (Left panel) Calculated reflectance as a function of angle (SPR) for wavelength 530 and 570 nm (pp − polarization p; ps − polarization s) for the system consisting of glass substrate (BK7), Cr layer (2 nm), Ag layer (50 nm), SiO2 protective layer (5 nm), and hybrid silica film. (Right panel) Angular distribution of R110 emission in silica nanolayers for 530, 570, 600, 630, and 660 nm. The range between 35° and 90° corresponds to the angular distribution of SPCE, and that from 90° to 135° corresponds to F-SE.
is necessary to create conditions for radiation, for example, to increase thickness of the dielectric layer. The conditions for plasmonic resonance depend on the dielectric properties of the medium: refractive index (n) and extinction coefficient (k), the thickness of the layers and frequency of radiation. The electric properties of metals and glasses are well-known,17 however, data on dielectric properties of matrices with incorporated molecules are usually unknown, and in such cases separate measurements, for example, the ellipsometric ones are required. Figure 1d shows the dependence of refractive index of the medium and extinction coefficient on wavelength for silica matrices for which tetramethoxysilane (TMOS) plays a role of a precursor. The obtained data were used to calculate conditions for plasmonic resonance (performed with TFCalc software, Spectra, USA). From these calculations, thickness of the layers can also be determined. This is an example of a semiempirical simulation performed for known parameters describing particular media (including thickness), where the real conditions for resonance are found. The parameters are introduced independently for each layer of the whole system (Figure 3, left panel). On the basis of the calculations, the resonance conditions for the
systems. At higher concentrations of R110, it is better to perform SPCE studies for thicker dielectric layers because of two reasons. First, during gelation of the solution significant changes in the structure of silica matrix occur, which strongly affects aggregation of the dye. The best material for concentration studies is a layer obtained from the solution at the advanced stage of gelation, for which a three-dimensional net has almost been created. The thickness of the layers obtained this way is significantly larger. The second reason originates from the conditions for plasmonic resonance. The angular location of plasmonic resonance in SPR spectrum depends, among other things, on wavelength and thickness of a dielectric layer.16 For certain small thicknesses of dielectric films, the conditions for plasmonic resonance are not fulfilled at lower frequencies in view of the value of the angle for total internal reflection (longer wavelengths are not emitted). This fact leads to an additional restriction in SPCE studies of strongly aggregated systems in which they emit light at longer wavelengths (this is the case of R110 for which the maximum of fluorescence of higher order aggregates is above 600 nm, cf. Figure 4b). In such a case, to enable the observation of SPCE, it 3628
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Figure 4. (a) F-SE of R110 SiO2 nanolayers with concentration CR110 = 0.002 M, CR110= 0.007 M, and CR110 = 0.01 M (non-normalized emission spectra are presented in the right corner of the figure). The F-SE measurement was carried out close to the normal to the surface of the layer. (b) The F-SE spectrum of R110 in SiO2 for CR110 = 0.01 M as well as its components (monomer, dimer, and higher order aggregate).
to λ = 550 nm, which corresponds to the J dimer emission maximum (Figure 4b). Table 1 shows relative emission intensities for several wavelengths in SPCE and F-SE. The obtained values evidence
system are determined. The results of calculations are compared with those of experiment, i.e., SPR and/or SPCE. (Figure 3, right panel). By fitting the conditions of resonance (location, depth of the resonance in the case of SPR, polarization), the thickness of a given layer is obtained. From TFCalc calculations, the following values of thickness for silica layers were obtained: 590 nm for CR110 = 0.002 M, 640 nm for CR110= 0.007 M, and 560 nm for CR110 = 0.01 M. The dependence of plasmonic resonance on wavelength and the thickness of dielectric layer for the system studied is presented in Figure 3 (left panel). Figure 3 shows (right panel) the angular distributions of emission intensity for particular wavelengths: 530, 570, 600, 630, and 660 nm for R110 layers at the following concentrations: (a) 0,002 M, (b) 0,007 M, (c) 0,01 M. The measurement was carried out from the prism side (between 35° and 90° degrees), where the angular distribution of SPCE intensity was studied as well as from the sample side (from 90° to 135°), which corresponds to the angular distribution of FSE. From the figure (similarly to pictures from Figure 2a−c), it can be seen that with the increase in R110 concentration the monomer emission of R110 in rings is strongly quenched, but the intensity of longwave aggregate emission increases strongly. This evident difference in the intensity of directional emission of monomers and aggregates observed for three concentrations of R110 in silica does not appear in the case of emission measured from the sample side. Figure 4a presents the results obtained for three concentrations of R110 in silica for the emission irradiated in free space (F-SE). Figure 4b shows decomposition of total F-SE spectrum obtained for the highest concentration of R110 in silica into monomer emission, H and J dimer emission, and higher order aggregate emission. The total F-SE spectrum for CR110 = 0.01 M was numerically decomposed making use of the pure monomer spectrum, i.e., the spectrum separately measured at low R110 concentration (C = 10−6 M in gel), at which the presence of dimers in the system can be neglected. Emission of aggregates was approximated by the Gauss profiles. The maximum of R110 emission spectrum in SiO2 nanolayers in the presence of metal measured from the sample side (F-SE) for CR110 = 0.002 M and CR110 = 0.007 M is at about λ = 520 nm, i.e., it is the same as monomer band maximum, whereas for CR110 = 0.01 M it shifts
Table 1. Relative Intensity of F-SE and SPCE Spectra for the Following Observation Wavelengths, λobs: 520, 530, 550, 570, 600, 630, and 660 nm, for Three Concentrations of R110 in SiO2: CR110= 0.002 M; CR110= 0.007 M, and CR110= 0.01 Ma relative intensity of F-SE IF‑SE
relative intensity of SPCE ISPCE
λobs (nm)
0.002 M
0.007 M
0.01 M
0.002 M
0.007 M
0.01 M
520 530 550 570 600 630 660
1 0.89 0.55 0.44 0.17 0.10 0.04
1 0.94 0.67 0.56 0.24 0.14 0.07
0.74 0.85 1 0.91 0.40 0.27 0.17
0.73 1 0.95 0.90 0.44 0.19 0.07
0.17 0.22 0.75 1 0.81 0.61 0.33
0.06 0.13 0.31 0.78 1 0.83 0.53
a F-SE and SPCE were carried out upon the same sample configuration and identical excitation and observation conditions, with the only change of angular location of fiber entrance.
that the changes in relative emission intensity (F-SE) with the increase of R110 concentration from CR110 = 0.002 M to CR110 = 0.007 M are small or moderate at the highest concentration. However, due to the coupling between fluorophores and surface plasmons of the metal, the maximum of directional emission shifts with the R110 concentration increase from λ = 530 nm for CR110 = 0.002 M to λ = 570 nm for CR110 = 0.007 M and finally to λ = 600 nm for CR110= 0.01 M. For the highest concentration of R110 in silica strong increase of directional emission is observed in the higher order aggregate band above 600 nm. This effect indicates the enhancement of directional aggregate emission. Recently also the effect of enhancement of energy transfer efficiency from donors to acceptors in the presence of surface plasmons (SPMEET) has been reported.7,18 Therefore, the presented effect seems also to follow from the increase of excitation energy transfer from monomers to aggregates as the directional emission of monomers is almost totally quenched, but as seen from Figure 4a (upper right corner) and Figure 4b F-SE of monomers remains still strong 3629
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even at the highest concentration of R110. Also no significant differences in the surface plasmon resonance (SPR) intensity at 530 and 570 nm were observed (SPR spectra in Figure 3, left panel), which additionally confirms that the changes in SPCE spectra (strong monomer emission quenching at λ = 530 nm in Figure 3, right panel) do not follow only from the plasmonic emission enhancement, but also from the increase of energy transfer efficiency from monomers to aggregates. For the highest concentration of R110 (CR110 = 0.01 M) the maximal increase in SPCE intensity compared to that of F-SE can be found between 630 nm and 660 nm, which corresponds to the higher order aggregates (the extracted higher order aggregate emission is presented in Figure 4b). This evidences the emission enhancement of weakly fluorescent higher order aggregate. We found that ISPCE/IF‑SE = 2.5 for λobs = 600 nm; ISPCE/IF‑SE = 3.07 for λobs = 630 nm, and ISPCE/IF‑SE = 3.11 for λobs = 660 nm. The obtained results confirmed the appearance of higher order aggregate emission, which is difficult to observe in typical fluorescence measurements of R110 in SiO2. In this work first observations of directional emission of aggregates enhanced by surface plasmons were presented. SPCE spectra reveal a strong shift of emission maximum for R110 in silica toward longer wavelengths, especially for the highest dye concentration (about 50 nm). This fact indicates strong plasmonic amplification of directional emission of higher order aggregates. This effect is accompanied by almost total quenching of monomer emission. On the basis of the SPR and SPCE spectra it was found that this effect originates from the enhancement of energy transfer from monomer to dimer and further to larger aggregates (excitation sink toward energetic levels of lower energies). The obtained results may be useful in identification and analysis of other weakly emitting species, the emission of which is hard to be detected by other methods. Shortly, we would like to present a detailed study of R110 aggregates structure in a silica nanolayer and the effect of nonlocalized surface plasmons on energy transfer in donor− acceptor ultrathin films.
camera. Figure 1c shows in turn zoom presenting the sequence of layers forming the system studied (pink square). The pictures were taken with the exposition time t = 10 s, at distance e = 2 cm between the plane of the sample and the screen using the GG495 long-pass filter (Edmund Optics, USA). Ellipsometric measurements were carried out using the phase-modulated ellipsometer Jobin-Yvon UVISEL (HORIBA Jobin-Yvon Inc., Edison, USA) equipped with data analysis software (DeltaPsi v. 2.4.3). The angles Δ and Ψ were acquired in a spectrum ranging from 350 to 800 nm with intervals of about 7.5 nm. Data were recorded at room temperature using an angle of incidence fixed at 60°, and the compensator was set at 45°. DeltaPsi software was employed to determine the spectral distributions of refractive index n(λ) and the extinction coefficient k(λ) of silica nanolayers. Fitting calculations were based on a four-phase optical model (ambient/silica nanolayer/ Ag/glass). Silica nanolayer was assumed as an isotropic, homogeneous material, and its dispersion was simulated by the Cauchy equation.19 Moreover, Ag and glass substrate dispersions were taken from a database.17 The assumed optical model was fitted to experimental data by nonlinear Levenberg− Marquardt regression20 using mean-square error minimization. Obtained optical parameters of silica nanolayers (n(λ) and k(λ) in Figure 1d) were used in the fitting procedure of theoretical SPR results to the experimental SPCE spectra. The chemical formula of R110 was obtained using ChemSketch (Advanced Chemistry Development, Inc., Canada).
EXPERIMENTAL METHODS Silica nanolayers with incorporated R110 were synthesized by the sol−gel technique and prepared with the spin-coating method. The starting compounds were of analytical grade. TMOS Si(OCH3)4 was purchased from Aldrich, methanol from POCh (Polish Chemical Reagents, Poland) and spectroscopically pure R110 from Aldrich. The molar ratio TMOS/ methanol/water was 0.13:0.5:0.55. The final concentrations of the dye in sol were 0.002, 0.007, and 0.01 M. R110-doped sol was deposited on the clean substrates with silver (EMF, USA) or reference glass substrates using the spin-coating technique. This procedure has been described in detail.6 SPCE and F-SE were measured with the SPCE spectrofluorometer designed and constructed in our laboratory (Figure 1a). Similar setup was presented in other papers.6,7 DPSS lasers (λexc= 473 nm, beam output power set to 4 mW, CNI, China) were used to excite R110 molecules in a silica matrix deposited on a 50 nm silver thin film. Emission of the photons was collected by the fiber with the core diameter r = 0.55 mm and NA = 0.22 (model M26L, Thorlabs) and directed to photomultiplier tube H10721P-01 (Hamamatsu, Japan) through the monochromator (Princeton Instruments, SP2500i). More detailed description was presented earlier.6 Figure 1b shows the zoom of detection part of the setup (blue square) employed to record images using the Nikon D80
Notes
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AUTHOR INFORMATION
Corresponding Author
*E-mail address: fi
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by the Polish Ministry of Science and Higher Education with Grant N202 286538 (S.R.J.) and by NCN 2011/03/B/ST5/03094 (P.B. and B.G.). D.J. research work was supported by the European Union in the framework of the European Social Fund, the system project of the Pomorskie Voivodeship ‘‘InnoDoktorant” − Scholarships for PhD students, II edition.
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ABBREVIATIONS R110, Rhodamine 110; TMOS, tetramethoxysilane; SPCE, surface plasmon-coupled emission; F-SE, free-space emission; SPR, surface plasmon resonance; SPMEET, surface plasmon mediated excitation energy transfer
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REFERENCES
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