Article pubs.acs.org/JPCC
Solution Processed, Versatile Multilayered Structures for the Generation of Metal-Enhanced Fluorescence Eleonora V. Canesi,† Martina Capsoni,‡ Lohith Karanam,†,‡ Andrea Lucotti,‡ Chiara Bertarelli,†,‡ and Mirella Del Zoppo*,‡ †
Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, via Pascoli 70/3, 20133 Milano, Italy Dipartimento di Chimica, Materiali e Ing. Chimica “G. Natta’’, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano, Italy
‡
S Supporting Information *
ABSTRACT: We present an all-solution processed multilayered structure completely obtained via spin-coating, which can be used to study and optimize the phenomenon of metal-enhanced fluorescence. Indeed, the electromagnetic interactions occurring between fluorescent probes and localized surface plasmons typical of metal nanoparticles (NPs), which influence the fluorescence quantum yield, are strongly dependent on the nanoparticle/molecule distance. The platform proposed here offers unique advantages in terms of processability, allowing a fine-tuning of such a distance in a single deposition step. Fluorescence versus fluorophore/AuNP spacing curves are shown for two organic systems, namely, a perylene-based dye dispersed in a polymer matrix and a polyconjugated polymer (poly(3-hexylthiophene)), interacting with a nanostructured gold thin film. In both cases, optimal distances and enhancement factors have been measured.
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INTRODUCTION Metal-enhanced fluorescence (MEF) is the increase of the fluorescence emission of a molecule close to a nanostructured metal. The resonant coupling between the collective metal freeelectron oscillation and the incident light produces a strong, localized electromagnetic field in the proximity of the metal nanoparticle (localized surface plasmon), which can, possibly, enhance fluorescence emission.1,2 In recent years, this unique property of metal nanoparticles (NPs) has generated great interest in many, often very diverse, research fields, such as optical spectroscopy, cell imaging, quantum information processing, biosensors and nanophotonics,3−8 solar cells,9−13 or light-emitting devices.14 The efficiency of MEF depends, in a complex way, on several parameters, such as the excitation rate, and the radiative and nonradiative rates. All of these rates are rapidly decaying functions of the distance so that the actual net effect of the plasmon excitation on the optical properties of an emitter depends on a delicate balance among them.15 The exact decay laws rely on many factors, such as the geometry of the system and the optical properties of the components. Particularly relevant are nature, size, and shape of the metal nanoparticle, surrounding medium, distance and relative orientation of the emitter−plasmon system, and spectral characteristics of the plasmon, that is, energy and line width of plasmon resonance and absorption and emission spectra of the emitter.16 To optimize the enhancement, it is extremely important to control the distance between the metal nanoparticle and the fluorophore in a stable, reliable, and reproducible way. © XXXX American Chemical Society
Several strategies have been proposed to control the nanoparticle/fluorophore distance, mainly involving interlayers that work as spacers. Both inorganic, silica-based coatings17,18 or organic interlayers have been reported. Despite the outstanding versatility that characterizes organic materials, these latter are usually obtained by means of layer-bylayer19−21 or Langmuir−Blodgett22,23 techniques, which require complex, multistep depositions to obtain films of increasing thickness. While solution-based MEF systems are particularly suitable for biological applications, 24,25 MEF-inducing platforms developed on planar glass substrates are required in fields such as sensing or emissive panels for organic electronics, where a high concentration of fluorophores is needed on a large surface. For this reason, the development of fast and repeatable processing is essential to boost the exploitation of MEF in optoelectronic applications. In this work, we present a simple and versatile method to obtain MEF, which satisfies these latter requirements, based on an all-solution processed multilayered structure, where the different layers, even the metal one, are deposited by means of spin-coating. A careful choice of the processing parameters enables the control over the multilayer features and a good reproducibility. Moreover, spin-coating is the processing technique commonly exploited for the deposition of active layers in organic electronics and optoelectronics, so that the Received: May 24, 2013
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dx.doi.org/10.1021/jp4051353 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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After the thermal treatment, the remaining solid was dissolved in 35 mL of toluene and mixed with 400 mL of methanol in order to remove excess alkanethiol and TOAB. The precipitate was filtered off, washed with methanol, and redispersed in toluene (30 mL). Before the spin-coating deposition, the colloidal dispersion was concentrated to 10 mL at reduced pressure. General Procedure for the Preparation of the Metal/ Polymer/Dye Structure. Glass substrates, previously carefully washed by subsequent immersions in distilled water and hydrogen peroxide, were functionalized by immersion in a stirred solution of 3-aminopropyltriethoxysilane (APTES, 1 mL) in ethanol 96% (50 mL) overnight, and subsequently washed with ethanol before use. The first layer was deposited by spin-coating the colloidal dispersion of Au NP onto the functionalized substrate. The interlayer spacer was obtained by spin-coating an aqueous solution of polyvinylalcohol (PVA) with a polymer content ranging from 0.3 to 10% (wt %). In the case of Lumogen F Red 305, the fluorophore was dissolved in polymer solution (5 × 10−3 M in polystyrene) using toluene as solvent. P3HT in chlorobenzene was directly spin-coated onto the second layer without using any supporting polymer matrix. Details on the preparation of the samples are reported in the Supporting Information. Apparatus. The UV−vis absorption spectra were recorded with a UV−visCary 5000. The photoemission spectra were recorded with a JASCO FP 6600 spectrofluorometer. The film deposition was obtained by spin-coating (Laurell WS-4006NPP-LITE). The thickness of each layer has been evaluated by means of a contact profilometer (VEECO DEKTAK 150). Field emission SEM Zeiss SUPRA 40 was used for scanning electron microscopy.
system here proposed can be directly integrated in the processing production of organic devices. Exploitation of spin-coating for MEF observation is not new. Such a technique has been used to process organic dyes dispersed in a polymer matrix deposited onto substrates obtained by means of different methods: for example, a magnetron sputtered silica spacer and drop-cast Au NP colloid,17 or silver and silica layers deposited by vacuum methods.26 Also, organic dyes,27,28 CdTe nanocrystals,8 or quantum dots29 dispersed into a polymer matrix have been directly spun onto metal-containing substrates, without any layer working as a spacer. Finally, spin-coating has been used to deposit single layers comprising blends of phosphor and Ag nanoparticles.30 Incidentally, metal-containing layers deposited by spin-assisted methods have been already reported,31 but a more complex processing requiring several steps was followed. To the best of our knowledge, this is the first study where all the three layers for MEF, namely, metal nanoparticles, spacer, and fluorophore, are deposited through spin-coating, with an independent control over the properties of each layer. Many MEF studies have been conducted with silver-based nanostructures even if the silver plasmon band often does not match the excitation/emission bands of the fluorophores. Gold is rarely used for macroscopic systems because of the difficulty to produce a homogeneous gold NP monolayer with a sufficient surface coverage and because gold is an effective quencher of fluorescence.32 However, we decided to demonstrate our system with gold NPs since its plasmonic band is in the same spectral range of the excitation/emission bands of many widely used fluorophores. The present approach also provides an ideal platform for the evaluation of the enhancement. The three layers are solutionprocessable, and the spacer film thickness is tunable over a three decades range: in this way, the enhancement of the fluorescence emission can be evaluated with respect to the fluorescence measured on a sample with a spacer thick enough to hinder the MEF effect. Thus, the evaluation of the enhancement is disentangled from issues related to the scattering properties of the system structure.
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RESULTS AND DISCUSSION The system herein proposed for the generation of MEF comprises a three-layer-structure: the first layer is the nanostructured metal responsible for the generation of the surface plasmon, the second one acts as a spacer, and, finally, the third layer contains the emitters. For the first layer, a suspension in toluene of gold nanoparticles stabilized with dodecanethiol is used as a precursor. The suspension is obtained following the procedure reported in ref 33. The nanoparticles redispersed in toluene are spread on a substrate, followed by spinning, which causes the solvent evaporation and the subsequent formation of a partially opaque film. Films are deposited on glass substrates and a partial transparency is maintained, to allow the measurement of spectral absorption in transmission. The organic surfactant allows limiting the aggregation of nanoparticles during both the solution concentration and the deposition: this is essential to keep the plasmon band stable. However, a comparison between the UV−visible absorption spectra of the film and the colloidal solution of nanoparticles reveals that, in the film, the plasmon absorption is broader and red shifted, as shown in Figure 1 because of interparticle (dipole−dipole couplings) interactions. Conversely, the presence of a dielectric layer on the NP film has a negligible effect on the plasmonic band (see Figure 1, where the absorption spectrum of a bilayer AuNP/PVA is also reported). The SEM image of the film reported in Figure 2 shows that, on a properly functionalized glass, an ordered arrangement of the metal nanoparticles is obtained. Although the coverage of the substrate is not complete, this does not significantly affect
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EXPERIMENTAL SECTION Materials. Unless otherwise specified, chemicals, solvents, and polymers (PVA: Mw = 89 000−98 000 g mol−1; PS: Mw = 192 000 g mol−1) were all purchased from Sigma Aldrich and used without further purification. Regioregular poly(3-hexylthiophene) (P3HT, MW = 51 000 g mol−1, RR = 95.9%, PD = 2) was purchased from Merck. Lumogen F Red 305 was supplied by BASF. Ethanol (99.9%), toluene (99%), and acetone (99%) were supplied by Carlo Erba. Synthesis of Alkanethiol-Capped Au Nanoparticles. The HAuCl4 (0.6 mmol) aqueous solution (60 mL) was added to a tetraoctylammonium bromide TOAB (1.2 mmol) solution in toluene (160 mL), and the mixture was vigorously stirred. The yellow aqueous solution became colorless, while toluene turned orange as a result of the transfer of [AuCl4]− ions in the organic phase. Under vigorous stirring, this solution was mixed with a solution of dodecanethiol (DT, 0.6 mmol) in toluene (20 mL) for 15 min at room temperature. A 60 mL portion of a 0.1 M NaBH4 aqueous solution was added under vigorous stirring for 4 h. The organic phase was then separated and evaporated under reduced pressure. After complete evaporation, the solid that was obtained was heated to 180 °C with the heating rate of 2 °C min−1 and was held for 30 min in air. B
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Figure 1. Comparison of gold plasmon absorption in colloidal suspension (black line) and in the spin-coated film alone (green line) or covered with 20 nm PVA (blue line).
Figure 3. UV−vis absorption (black curve) and emission (blue curve) spectra of Lumogen F Red 305 in film, compared with the plasmon absorption of a spin-coated film of AuNP (red line). In the inset, chemical structure of Lumogen F Red 305.
matrix at a concentration of 5 × 10−3 M, and dissolved in toluene for its deposition. To ensure solvent orthogonality, we have selected polyvinylalcohol (PVA) as a spacer. PVA is a water-soluble, transparent, high-molecular-weight polymer with excellent film-forming capabilities. Correlations among the processing parameters and the film thickness have been assumed as a calibration reference to finely tune the layer thickness in the range over which the MEF phenomenon takes place, namely, in the order of tens of nanometers. The thickness measurement has been repeated in different points across the sample, and the values thus obtained have been averaged (see the Supporting Information). The measured thicknesses are consistent with those obtained in analogous studies reported in the literature.34 The thickness of the PVA spacer has been varied from 6 to 150 nm, with a larger number of points in the first few tens of nanometers, where the fluorescence enhancement is expected. In addition, a thicker layer of about 1.5 μm, for which interactions between fluorophores and metal layer are negligible, has been deposited. The fluorescence integrated intensity, excited at 550 nm, varies with the PVA thickness, as reported in Figure 4, where we observe the curve of intensity of emission versus interlayer distance.7 The values reported are an average of measurements done in different sample regions.
Figure 2. SEM image of gold nanoparticles spin-coated on a glass substrate.
the measured enhancement, since the values reported correspond to an average obtained on a sampling area wider than 1 mm2, which is larger than the size of the voids shown in the SEM image. This is proven by the repeatability of the measurements acquired in different points across the sample (as shown by the error bars in the graphs hereafter reported). Moreover, we can observe that the nanoparticles form a monolayer and the nanoparticle diameter has been estimated to be around 6 nm by means of high-resolution SEM images (see the Supporting Information). The thickness of the metal layer is thus estimated in the order of a few nanometers. The preparation route of the other layers has been designed in order to prevent any damage to the layers previously deposited, the only requirement being that the polymer to be used as spacer and as fluorophore host matrix be soluble in orthogonal solvents. The thickness of the spacer has been made to vary from a few nanometers to 1.5 μm to span the three regions on the fluorescence/distance curve: quenching, enhancement, and negligible interaction. To modulate the thickness, we acted on the solution concentration and the rotation speed (see the Supporting Information for details). We have applied this protocol for the study of two different systems, which allow us to demonstrate the versatility of the developed platform. In the first attempt, a commercially available dye, LUMOGEN F Red 305 by BASF, was used. It has been chosen because its absorption maximum is resonant with the metal plasmon and has a rather large Stokes’ shift (see Figure 3), which is essential to reduce reabsorption phenomena. Because Lumogen is a small molecule, which provides polycrystalline films by solution processing, the use of a polymer matrix is necessary to assist the formation of a uniform film. The fluorophore was embedded in a polystyrene (PS)
Figure 4. Fluorescence enhancement vs distance curve for Lumogen F Red 305 (the PL spectra are reported in the Supporting Information). C
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The measured variation in the emission is not related to a variation in the absorption (see UV−vis absorption spectra in the Supporting Information). We observe a maximum fluorescence emission already for the first point (corresponding to a 6 nm thick spacer), thus indicating that the maximum of the fluorescence/distance curve is likely to take place at this or at a smaller distance. The enhancement measured at this point is 1.4, and it has been evaluated with respect to the intensity measured for the virtually infinite (>1 μm thick) spacer. In this way, we avoid variations in the reflectivity of the substrate induced by the presence of the metal layer. Such a low enhancement value is consistent both with the fluorescence quantum yield of the chosen chromophore, which is already close to one, and with the small thickness of the AuNP layer.32 In the second set of experiments, we have used poly(3hexylthiophene) (P3HT) as fluorophore. P3HT is a widely used active material in organic electronics and photovoltaics,35,36 and the study here proposed aims at demonstrating the applicability of the developed MEF platform also to polymer emitters. As for the previous case, the spacer is PVA since P3HT has been deposited from chlorobenzene, a solvent that does not affect PVA. The use of a polymer makes the preparation of a blend unnecessary when depositing the third layer of the multilayer platform. Since the fluorophore units are not diluted in an optically inert polymer matrix, energy transfer processes may interfere with the radiative decay processes. From Figure 5, we observe that the resonance between the plasmon and emitter absorptions is similar to that of Lumogen.
Figure 6. Fluorescence enhancement vs distance curve for P3HT (the PL spectra, registered at a fixed excitation wavelength of 540 nm, are reported in the Supporting Information together with the UV−vis absorption spectra).
with the thickest PVA film. Analogous enhancement factors are obtained using the peak intensity at the wavelength of maximum emission; the consistency of these two kinds of determinations depends on the fact that no variation of the spectral pattern is observed increasing the spacer thickness (see the Supporting Information, Figures S03 and S05).
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CONCLUSIONS MEF is an extremely interesting phenomenon that has not been fully understood yet, and it is very difficult to foresee the optimal conditions for its occurrence. This is mainly due to the fact that the delicate balance between competitive radiative and nonradiative decay processes depends on many parameters that are not always easy to control. For this reason, the availability of a user-friendly, fast, and reproducible method to test several samples in order to determine the optimal distance for the observation of MEF is extremely important. We have proposed a versatile, ready to implement, allsolution processed method to obtain MEF. The optimal distance for maximizing the phenomenon has been obtained through the deposition of a spin-coated spacer layer of variable thickness. Also, the nanostructured metal film and the fluorophore layer have been deposited by spin-coating. In summary, this method, despite its simplicity, offers several advantages in that it is applicable to both small fluorophores and polymeric emitters, the only requirement being a solubility of the interlayer polymer and the emitting layer in orthogonal solvents.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 5. UV−vis absorption (black curve) and emission (blue curve) spectra of a thin film of P3HT, compared with the plasmon absorption of a spin-coated film of AuNP (red line). In the inset, chemical structure of P3HT.
Description of layer deposition conditions, high-resolution SEM image, and UV−vis absorption and emission spectra of the multilayer structures. This material is available free of charge via the Internet at http://pubs.acs.org.
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P3HT is known to have a moderate fluorescence quantum yield in the solid state: a larger enhancement, even if still limited by the reduced thickness of the metal layer, should thus be expected. Experimental findings confirm that hypothesis: indeed, an enhancement of 1.6 is obtained (see Figure 6), for a spacer thickness of around 31 nm. For both fluorophore systems, the enhancement factor has been calculated by comparing the integrated area of the most fluorescent sample with respect to that obtained for the sample
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work has been supported by Fondazione CARIPLO 2010 “Nanostrutture organiche ed ibride per la conversione dell’energia solare (SOLCO)”. D
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