ARTICLE pubs.acs.org/JPCC
Photophysical and Lasing Properties of Rhodamine 6G Confined in Polymeric Nanoparticles ngel Costela,† and Virginia Martín,† Jorge Ba~nuelos,‡ Eduardo Enciso,*,§ I~nigo Lopez Arbeloa,‡ A † Inmaculada García-Moreno †
Departamento de Sistemas de Baja Dimensionalidad, Superficies y Materia Condensada, Instituto de Química-Física “Rocasolano” (CSIC), Serrano 119, 28006 Madrid, Spain ‡ Departamento de Química Física, Universidad del País Vasco-EHU, Aptdo. 644, 48080 Bilbao, Spain § Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain ABSTRACT: The photophysics and laser action of rhodamine 6G (Rh6G) confined in polymeric nanoparticles, based on copolymers of methyl methacrylate and 2-hydroxyethyl methacrylate, are analyzed as a function of the dye concentration, as well as the composition and size of the latexes. The optical properties of the final system are found to be dependent on the nanoparticle size: for diameters higher than 70 nm, increased light scattering in the samples results in extinguished laser action and prevents the adequate recording of the photophysics. The encapsulation of Rh6G into small nanoparticles leads to homogeneously dispersed suspensions characterized by a high fluorescence capacity, regardless of both dye content and latex composition. Rh6G exhibits a low tendency to self-associate in the synthesized polymeric nanoparticles, even at concentrations as high as 10-2 M. These qualities ensure that these dye-sensitized latexes are adequate active media for tunable lasers, with improved lasing performance by nonresonant feedback of the emission. The evaluation of the influence of the dye concentration and size and composition of the nanoparticles on both laser action and scattering phenomena allows prediction of the optical behavior of the final system, opening novel routes to active disorder-based photonic devices.
’ INTRODUCTION At the core of fluorescence signaling, imaging, and sensing are fluorescent dyes, which are easily accessible, excitable, and detectable with conventional methods and instrumentation, bright, sufficiently stable under experimental conditions, extremely small in size (making them compatible on the scale of, e.g., molecular biology), soluble in application-relevant media, and equipped with functional groups for site-specific labeling. Unfortunately, many potential applications subject these dyes to harsh environments, including acidic or basic extremes, enzymatic activity, and interactions with ions.1 These environments may quench the dye emission through chemical interactions with its structure, self-quenching due to its insolubility, or cause aggregation, which is one of the most important effects present in organic dyes in solution, affecting their color, solubility, and photophysical properties.2 The incorporation of fluorophores into nano- and micrometer-sized organic and inorganic particles is a strategy of increasing importance3 to circumvent the often undesired sensitivity and interactions of most chromophores to their local environment, i.e., allowing the use of hydrophobic dyes in a biologically relevant aqueous environment. In addition, fluorophore encapsulation often results in an increased fluorescence quantum yield and photostability, providing an attractive materials platform for a wide array of biophotonic applications.4 r 2011 American Chemical Society
Our group has recently developed new dye-sensitized organic nanoparticles based on rhodamine 6G (Rh6G) confined in latexes.5 The particle synthesis approach, using a modified Nagao et al. recipe,6 provides photonic systems with important advantages over existing particles fluorescing in the visible spectral region, including (i) small particle sizes and size distributions, (ii) improved chemical compatibility with organic dyes, assuring their potential for generalization, (iii) control over the particle architecture and composition, which gives much desired versatility to the system, (iv) high stability in water, with no leaching of the dye observed, (v) the capability to modify the polarity and viscoelasticity of the nanoparticles similarly to conventional solvents, and (vi) enhanced laser efficiency and photostability as compared to the parent dye in water. In fact, we demonstrated, for the first time up to now, that if the media comprise dye-doped particles that are both small enough and randomly enough distributed, then the transparency of the system is not affected and the coherent laser action from the dye is actually enhanced by nonresonant feedback of the emission by multiple scattering.7 The presence of dye confined Received: November 13, 2010 Revised: January 15, 2011 Published: February 23, 2011 3926
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Table 1. Set D Dispersions: Rh6G Dye Effects on the Synthesis of Poly(MMA-HEMA-GMA) Particles at 65°C in the Presence of 3.3 10-3 M SDS sample
a
[Rh6G]a (wt %)
[Rh6G]suspension (10-4 M)
[Rh6G]particle (10-3 M)
particle diameter (nm)
ζ (mV)
polymerization yield (%)
D1
0.066
0.7
1.6
46
-33
89
D2 D3
0.13 0.22
1.5 2.5
3.2 5.5
53 54
-29
86 92
D4
0.28
3.2
6.9
54
D5
0.4
4.6
9.9
55
-34
94
82
Based on the concentration (wt %) of the monomer feed mixture.
into polymeric nanoparticles acting simultaneously as scattering centers and gain media, instead of being detrimental to conventional laser emission as thought so far, results in an improvement of the laser emission. The unique properties of this dye-sensitized system render a straightforward procedure to build nanoparticlebased fluorescence media where multiple dye molecules can be loaded within a single particle and their photophysical properties can be controlled independent of the particle size. The aim of the present work is to investigate in depth the influence of the dye concentration and the composition, size, and morphology of the polymeric nanoparticles on both the laser action and photophysical properties, as well as on the scattering phenomena, to forecast the optical properties of the final systems, opening new perspectives to active disorder-based photonic devices.
’ EXPERIMENTAL SECTION Materials. Methyl methacrylate (MMA) (Aldrich, 99%) was purified with a 0.1 M sodium hydroxide solution to remove inhibitor. 2-Hydroxylethyl methacrylate (HEMA) (Aldrich, 97%), glycidyl methacrylate (GMA) (Fluka, 97%), potassium persulfate (KPS) (Sigma, 99%), rhodamine 6G (Rh6G) (Fluka), and sodium dodecyl sulfate (SDS) (Sigma, 99%) were used without further purification. Deionized water was obtained from a Direct Q 5 Millipore system. Synthesis of Nanoparticles. Rh6G-doped polymeric nanoparticles were prepared from a batch emulsion polymerization of MMA by modifying the Nagao et al. recipe.6 Taking into account that the laser action of xanthene dyes is enhanced in protic but not very polar media,8 the original recipe was modified by copolymerizing the hydrophobic monomer MMA with more hydrophilic monomers, such as HEMA and GMA, introducing into the monomer mixture feed a weight ratio of those monomers of 70:20:10. SDS was added into the feed mixture as a colloidal surfactant stabilizer9 to stabilize the large number of particle nuclei and, as consequence, to control the final particle diameter. Since the employed concentration of SDS is always below the critical micelle concentration (cmc) in water (around 8 10-3 M), the monomer polymerization occurs basically in the aqueous phase, until the oligomer concentration reaches the precipitation threshold, as is claimed in soap-free emulsion polymerization. The free radical polymerization was carried out at 65 °C with a monomer content in the feed of 7 wt % related to the total mass in the suspension. The reaction was initiated by adding a 1 wt % concentration, with respect to the monomer present in the feed, of a water-soluble initiator such as KPS. In a standard preparation, surfactant and water (106 mL) were added to a five-necked round-bottom flask equipped with a condenser and a gas inlet. The dye dissolved in the monomer
feed mixture was added to the reaction flask with stirring (300 rpm), and the mixture was heated at 65 °C and degassed by nitrogen for 30 min. The reaction was started by introducing the KPS solution, and the polymerization was performed for 3 h. The reaction was terminated by introducing air inside the reactor and cooling the reaction flask in ice. The monomer conversion was determined gravimetrically by removing aliquots after polymerization and drying them in the oven. The obtained dispersions, with a latex content around 5.5 wt %, were not cleaned, since the centrifugation led to aggregation and dialysis produced a gel after several water changes. In terms of evaluating the effect of the dye content (set D), the size of the nanoparticles (set S), and the latex composition (set C) on the photophysical and laser properties of the dye-doped emulsions, different samples have been synthesized. Modified parameters in each sample are collected in Tables 1-3. Particle Characterization. Transmission electron microscopy (TEM) was carried out with a JEOL 2000 FX electron microscope operating at 200 kV by placing drops of the latex suspensions over copper grids covered with holey carbon support films. The morphology and microscopic structures of the latex monoliths were observed using field emission scanning electron microscopy (FESEM; JEOL, JSM-6700F) with an acceleration voltage of 10 kV and an emission current of 10 mA. The samples were fragments of particle monoliths obtained after the suspension was dried in an oven at 40 °C. The diameters of the nanoparticles were measured by dynamic light scattering (DLS) with Nanosizer ZetaSizer equipment (Malvern Instruments, United Kingdom), being in good agreement with those observed by FESEM and TEM. Electrophoretic mobilities were determined (ZetaSizer Nano ZS, Malvern) using in each case three dilute samples (volume fraction 2.4 10-4) at a temperature of 25 °C and an ionic strength of 1 mM KNO3. The electrophoretic mobility of spheres was related to the ζ potential according to the Helmholtz-Smoluchowski relation. Photophysical Properties. The photophysical properties were registered in diluted aqueous suspensions of latex nanoparticles doped with Rh6G to avoid aggregation and reabsorption/reemission phenomena. These suspensions are obtained by diluting the original ones to a latex content of 0.03-0.04 wt %, with the Rh6G concentration in the suspension thus being reduced from 3 10-4 to 2 10-6 M. UV-Vis absorption and fluorescence spectra were recorded on a Cary 4E spectrophotometer and on a SPEX Fluorolog 3-22 spectrofluorimeter, respectively. The absorption spectra were recorded using a blank suspension of latex in water as a reference. However, in most cases the baseline was subsequently corrected manually to appropriately adjust the zero of absorbance. Fluorescence quantum yields (φ) were evaluated from corrected spectra using a diluted solution (10-6 M) of Rh6G dye in water (φ = 0.59) as a 3927
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Table 2. Set S Dispersions: Surfactant Effects on the Synthesis of Poly(MMA-HEMA-GMA) Particles at 65°C with 0.28 wt % (with Respect to the Monomer Feed Mixture) Rh6G sample a
a
[SDS]
particle
(10-3 M)
diametera (nm)
polymerization ζ (mV)
Table 3. Set C Dispersions: Monomer Feed Composition Effects on the Synthesis of Poly(MMA-HEMA) Particles at 65°C with Introduction of Rh6G (0.28 wt % Based on the Amount of Monomer) and SDS (6.7 10-3 M) sample
[HEMA]a (wt %)
particle diameter (nm)
polymerization yield (%)
yield (%)
S1
7.2
20
S2
32.2
27
S3
7.2
39
S4 S5
4.9 3.3
44 55
S6
2.4
59
78
S7
1.7
70
64
S8
1.3
103
81
S9
1.0
166
75
85 86
C1
10
38
75
-35
89
C2
20
28
81
C3
30
24
64
-37
87 82
C4
40
23
78
Sample prepared by reducing the monomer feed to 4 wt %.
reference.10 Radiative decay curves were registered by the timecorrelated single-photon-counting technique (Edinburgh Instruments, model FL920). Fluorescence emission was monitored at the maximum emission wavelength after excitation at 470 nm by means of a diode laser (PicoQuant, model LDH470) with 150 ps fwhm (full width at half-maximum) pulses. The fluorescence lifetime (τ) was obtained from the slope after the deconvolution of the instrumental response signal from the recorded decay curves by means of an iterative method. The goodness of the exponential fit was controlled by statistical parameters (χ2, Durbin-Watson, and analysis of the residuals). The rate constants of radiative (kfl) and nonradiative (knr) deactivation pathways were calculated by means of kfl = kfl = φ/τ and knr = (1 φ)/τ. Lasing Properties. Original suspensions, without dilution, were contained in 1 cm optical path quartz cells carefully sealed to avoid solvent evaporation. The samples were transversely pumped at 532 nm with 5.5 mJ, 6 ns fwhm pulses from a frequency-doubled Q-switched Nd:YAG laser (Monocrom STR2þ) at a 10 Hz repetition rate. The exciting pulses were linefocused onto the front face of the cell, providing pump fluences on the active medium of 180 J/cm2. The oscillation cavity (2 cm length) consisted of a 90% reflectivity aluminum mirror, with the end lateral face of the cell as the output coupler. The photostability of the gain medium was evaluated by irradiating under lasing conditions 10 μL of a water solution of the dye in the presence and absence of latex nanoparticles, as well as with the dye inside the nanoparticles. The solutions were contained in cylindrical Pyrex tubes (1 cm height, 1 mm inner diameter) carefully sealed. Monitoring of sample photolysis was carried out by recording the laser-induced fluorescence emission from the dye solutions in the capillary, which were placed horizontally and excited along the axis with the same pump pulses from the Nd:YAG laser used for producing dye laser emission, as a function of the pump pulses at a 10 Hz repetition rate. The fluorescence emission was monitored perpendicular to the exciting beam, collected by an optical fiber, imaged onto the input slit of a monochromator (Acton Research Corp.), and detected with a charge-coupled device (CCD) (SpectruMM: GS128B). The fluorescence emission was recorded by feeding the signal to a boxcar (Stanford Research, model 250) to be integrated before being digitized and processed by a computer. Each experience was repeated at least three times. The estimated error of the energy measurements was 10%, and the experimental
a
Based on the concentration (wt %) of the monomer feed mixture.
error in the photostability measurements was estimated to be on the order of 7%. Details of the experimental setup can be found elsewhere.11
’ RESULTS AND DISCUSSION Particle Preparation. As is shown in Tables 1-3, high polymerization yields are obtained after 3 h of reaction at 65 °C. The incorporation of Rh6G slightly modifies the particle size (Table 1), which is mainly tuned by the SDS concentration (Table 2). At low surfactant concentrations, the particles are mostly monodispersed (polydispersity index (PDI) of 0.70), and the need to consider a second exponential for a more accurate adjustment of the fluorescence decay curves. However, for larger particles the suspensions are no longer transparent and become translucent, which increases the light scattering in the samples, hampering the adequate recording of the photophysics, mainly of the absorption spectra. These spectra are also recorded with a decrease of the optical pathway of the light through the sample (cuvettes of 1 and 0.5 mm path lengths), but it is still difficult to obtain adequate baselines. Since the incidence of the optical path length makes sense only when compared with the mean free path of light in the sample, trying to estimate this parameter, we analyze the transmission of the samples without dye as a function of the optical path length of the cells under laser irradiation. However, the lack of adequate equipment (cells of very short path length and a highly sensitive detection system) hinders accurate measurements. Thus, we proceeded to estimate the order of magnitude of the mean free path using the interactive Mie scattering calculator (http://omlc. ogi.edu/calc/mie_calc.html). Taking into account that for samples with large nanoparticles (diameter >70 nm) the “mean free path” is ca. 100 μm and the difficulty to obtain adequate absorption spectra, the corresponding photophysical properties should be considered with care. Regardless of the experimental conditions, in these large nanoparticles (diameter >100 nm), both the absorption probability and fluorescence capacity decrease with regard to smaller ones, as is reflected in Figure 4 and Table 6. Moreover, the biexponential character of the decay curve is slightly more pronounced, and the lifetimes decrease. Such behavior is not attributed to dye aggregation since the absorption spectrum of Rh6G in the biggest latexes matches that registered in the smaller particles and the excitation spectra perfectly match the typical absorption band of Rh6G, indicating that the emission comes
Figure 3. Fluorescence decay curve of a sample with a high content of Rh6G in the latex (sample D5) together with the residuals of the corresponding mono- and biexponential fits.
Table 5. Lasing Properties Registered for Rh6G Encapsulated in the Latex as a Function of the Dye Content (Weight Proportion of Latex Nanoparticles) in the Aqueous Suspensions [latex] (wt %)
eff (%)
λla (nm)
6.5 5.5
21 21
579 577
4.5
23
576
3.8
27
574
3.5
30
574
Table 6. Photophysicala and Lasingb Properties of Rh6G Trapped in Latex Nanoparticles of Different Diameters (D)
a
sample
D (nm)
λabs (nm)
εmax (104 M-1 cm-1)
λflu (nm)
Φ
τ (ns)
S1
20
533.0
9.6
559.0
0.75
2.75 (15%), 4.54 (85%)
S2
27
S3
39
S4
44
533.5
9.7
559.5
0.77
2.74 (17%), 4.54 (83%)
S5
55
533.5
9.9
559.5
0.74
S6
59
533.0
8.5
559.0
S7 S8
70 103
533.0 533.0
7.9 4.3
S9
166
534.0
3.9
eff (%)
λla (nm)
33
578
21
579
19
578
10
575
2.70 (19%), 4.50 (81%)
6
573
0.71
2.50 (17%), 4.38 (83%)
3
570
559.0 558.5
0.68 0.58
2.50 (18%), 4.35 (82%) 2.80 (20%), 4.25 (80%)
2 569 no laser emission
560.0
0.50
1.47 (24%), 3.61 (76%)
no laser emission
Content of latex nanoparticles in aqueous suspensions, 0.03-0.04 wt %. b Content of latex nanoparticles in aqueous suspensions, ∼5.5 wt %. 3931
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losses result in overall loss of laser capability due to the decrease of the effective gain as seen by the active medium. When the latex particles are small enough and distributed randomly enough, the light emitted by the dye is weakly scattered by the nanoparticles, the transparency of the composites is not affected, and laser emission is reinforced by the phenomenon known as “incoherent random lasing”,7,21 where weak multiple scattering of the emission enhances lasing by elongating the light path inside the gain media, providing extra feedback. As has been previously demonstrated,7 under carefully prepared conditions, “light scattering” and “optical performance” do not head in opposite directions, but walk hand-in-hand. Composition of the Latex. Finally, the photophysical properties of Rh6G encapsulated in nanoparticles with different polymeric compositions, sized within the range of 23-38 nm and dispersed in water (dye concentration in the latex around 5 10-3 M and latex content in the suspension around 0.035 wt %) are analyzed. A general overview of the data (Table 7) reveals that the proportion of the monomers has a minor effect on the photophysical properties of Rh6G in good agreement with the low sensibility of this dye to the solvent nature.10,22 Thus, the photophysics of Rh6G in latexes with different proportions of monomers is similar to that observed in the above studied samples, with the exception of the larger particles (Tables 4 and 6). However, the lasing properties of Rh6G-doped polymeric nanoparticles are very dependent on the polarity of the matrix, which can be modulated by the appropriate copolymerization of MMA with different proportions of a monomer functionalized with polar groups, such as HEMA. Thus, the absence of HEMA induces the disappearance of the laser action probably due to the low solubility of Rh6G in MMA, especially at the high concentration selected in the present work, which could lead to an actual concentration of the dye in the latexes lower than the selected one. An increase of the HEMA concentration in the nanoparticle composition improves the laser efficiency since the presence of this monomer increases the polarity of the system, assuring the
exclusively from dye monomers (Figure 4). Thus, the loss of absorption and fluorescence capacity recorded in the largest particles can be related to effects induced by the high light scattering (i.e., a significant decrease of both the light arriving to the dye and the light leaving from it). Then, from a photophysical point of view, the encapsulation of Rh6G in latexes with diameters higher than ca. 70 nm should have a deleterious effect on the laser action of the system. Indeed, this assumption is corroborated by the recorded lasing efficiencies, which exhibit a more remarked dependence on the size of the nanoparticles than the photophysical properties (see Table 6). In fact, and in good agreement with the photophysics, laser action from Rh6G inside latexes is only induced for nanoparticles of diameter
