Polystyrene nanoparticles doped with a luminescent europium complex

Journal of Colloid and Interface Science 376 (2012) 12–19

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Polystyrene nanoparticles doped with a luminescent europium complex Jessie Desbiens, Benjamin Bergeron, Maxime Patry, Anna M. Ritcey ⇑ Département de chimie and CERMA, Université Laval, Pavillon Alexandre-Vachon, 1045, avenue de la Médecine, Québec (Québec), Canada G1V 0A6

a r t i c l e

i n f o

Article history: Received 22 December 2011 Accepted 11 February 2012 Available online 22 February 2012 Keywords: Miniemulsion Polystyrene nanoparticles Luminescent nanoparticles Europium

a b s t r a c t Polystyrene nanoparticles doped with a luminescent europium complex, Eu(tta)3phen, are prepared by miniemulsion polymerization. The influence of the complex on the miniemulsion polymerization is investigated by the systematic variation of the initial concentration of Eu(tta)3phen from 2 to 7 wt% relatively to styrene. A maximum doping level of about 2% by weight in the final particles can be achieved. At higher doping levels, destabilization of the miniemulsion leads to a loss of reproducibility with respect to both the degree of conversion and the final Eu content of the particles. Doped nanoparticles of varying diameter, ranging from 19 to 94 nm, are successfully prepared. Steady-state and time-resolved luminescence measurements indicate that the luminescence properties of Eu(tta)3phen in the doped latexes are unchanged from those found in THF solution. Aqueous dispersions of the doped particles exhibit characteristic red emission under UV light irradiation. The luminescence intensity increases linearly with Eu(tta)3phen content, indicating the absence of self-quenching despite the relatively high local concentrations within the particles. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Luminescent nanoparticles are used in many fields of application, including bioimaging [1–3] and thermal sensing [4], also as nanosensors [5–7] and in light emitting diodes [8]. The preparation of fluorescent nanoparticles often involves the doping of a host material, and doped particles composed of silica [9,10], inorganic crystals [11,12] or polymers [13,14] have been reported. The use of polymers as host matrices is particularly attractive because of the versatility of such materials [15–17]. The large inventory of existing polymers means that a number of properties can be adjusted to meet specific requirements. For example, particle polarity, biocompatibility, mechanical properties and refractive index can all be modified by the choice of polymer. Furthermore, the surface chemistry of many polymeric nanoparticles is well known and can be easily modified. Polymeric latex particles can be accessed by a number of different routes [15]. The starting material can be either polymeric or monomeric. So-called artificial latexes are prepared from preformed polymer that is dissolved in the dispersed phase of the emulsion and forms particles as the solvent is evaporated. In the case of the preparation of particles from monomers, available techniques include dispersion, suspension, emulsion, microemulsion and miniemulsion polymerization. Many considerations influence the exact choice of method, but if nano-sized particles are required, only emulsion, miniemulsion or microemulsion polymerization ⇑ Corresponding author. Fax: +1 418 656 7916. E-mail address: [email protected] (A.M. Ritcey). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2012.02.020

can be used, the last less frequently because of the limited quantity of material obtained. Furthermore, for the preparation of doped nanoparticles, miniemulsion polymerization is often the method of choice. This is because of a fundamental feature related to the way in which the monomer is supplied to the growing particles. In contrast to emulsion polymerization, in which monomer must diffuse through the continuous phase to feed particle growth, in the miniemulsion process, all of the monomer is pre-dispersed as nano-droplets prior to the initiation step. Thus, in miniemulsion polymerizations, each droplet acts as an independent preformed nanoreactor, and dopants can be dissolved in the monomer at the desired concentration before dispersion [18]. This is particularly important for the incorporation of hydrophobic dopants, for which transport through the aqueous phase, as required in emulsion polymerization, is impossible. In the present work, miniemulsion polymerization is employed to prepare luminescent polystyrene nanoparticles. Fluorescent polymer nanoparticles prepared in this way have been previously reported, with dopants that include organic dyes [19], some of which are polymerizable [20,21], quantum dots [22] and metal chelates [23]. In this paper, we report the preparation of polystyrene nanospheres doped with a luminescent organo-soluble Eu3+ complex. Luminescent metal complexes have received considerable recent attention because of their numerous potential applications in, for example, electroluminescent devices [24], fiber lasers and amplifiers [25,26] and bioanalytical assays [9,27]. Organic lanthanide complexes in particular often exhibit strong luminescence that can be further enhanced through the appropriate choice of ligand [28–30].

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J. Desbiens et al. / Journal of Colloid and Interface Science 376 (2012) 12–19

A few examples of polystyrene particles doped with europium complexes can be found in the literature. Ando and Kawaguchi [23] prepared particles doped with (tris-4,4,4,-trifluoro-1-(2-naphtyl-1,3-butanedionato))europium(III) (Eu(NTFA)3) and (tris-4,4,4,trifluoro-1-(2-naphtyl-1,3-butanedionato)) bis(trioctylphosphine oxide)europium(III) (Eu(NTFA)3(TOPO)2). Their work focused primarily on the optical properties of the particles, and the concentration of complex included in polystyrene was not investigated. Vancaeyzeele et al. [31] prepared polystyrene particles doped with a number of metal complexes, including (tris-4,4,4-trifluoro-1-(2naphthyl-1,3-butadione))europium(III). In this case, a maximum doping level of 7 mg of Eu complex/g of polystyrene was determined, but the luminescence properties of the resulting particles were not discussed. Finally, Tamaki et al. [32] employed the precipitation method to prepare micron size polystyrene particles doped with the same Eu complex as the present study, but did not determine the Eu content of the final particles. In the present paper, we report the preparation and characterization of polystyrene nanoparticles doped with(1,10-phenanthroline)tris(4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato)europium (III) (Eu(tta)3phen). The concentration of both surfactant and dopant is varied in order to establish the influence of these parameters on the polymerization reaction and the final particles. The maximum loading of luminophore in the particles is also determined, and the luminescence properties of the doped nanoparticles are studied. The present report thus differs from previous literature both in the specific choice of ligands in the Eu complex, and in the breadth of the study. 2. Experimental section 2.1. Materials All starting materials were commercially obtained from Aldrich. Europium (III) chloride hexahydrate was purchased at the highest available purity (99.99%). Sodium dodecylsulfate (SDS, 99%), potassium persulfate (KPS, 99%), 1,10-phenanthroline (phen, 99%), thenoyltrifluoroacetone (tta, 99%), ethanol, methanol were of the best available reagent grade and were used as received. Styrene (99%) and divinylbenzene (80%, mixture of isomers) were purified on a basic aluminium oxide column before use to remove the inhibitor.

F3C O

N

Eu(III)

HC O S

N 3

Fig. 1. Structure of the luminescent complex Eu(tta)3phen.

the aqueous phase and stirred with magnetic agitation for 1 h at 70 °C. For some samples, hexadecane is added in styrene as a hydrophobic agent instead of or in addition to Eu(tta)3phen. The mixture is then treated with ultrasound to obtain the miniemulsion. Ultrasonic pulses were generated with a Fisher Scientific Model 500 Dismembrator, operated at 80% amplitude, with 1 s pulse on, 1 s pulse off over a 6 min period. The mixture was cooled in an ice-bath during ultrasonication to avoid heating. 2.4. Miniemulsion polymerization The resulting miniemulsion is placed in a three-neck round-bottom flask equipped with a condenser and a gas inlet. The miniemulsion is heated to 70 °C and magnetically stirred under nitrogen for 30 min in a paraffin bath equipped with a thermostat. The initiator, persulfate potassium (KPS) (0.3 g), dissolved in nanopure water (10 mL), is then added to the reaction mixture. Heating and stirring are maintained, under nitrogen, for 3 h. Final polymerization yields vary between 50% and 90%. The solid content of the latex is determined gravimetrically with a Mettler Toledo HB43-S Moisture Analyzer. The yield of the polymerization is calculated from the solid content of the sample. A table summarizing the exact composition of all miniemulsions is provided as Supporting Information. 2.5. Miniemulsion polymerization with crosslinking The same polymerization procedure is followed, with the exception that 2 g of divinylbenzene is added dropwise to the miniemulsion, 30 min after the initiation with KPS.

2.2. Synthesis of Eu(tta)3phen

2.6. Washing of the latex

The europium complex, Eu(tta)3phen, was synthesized according to a previously reported procedure [30]. Thenoyltrifluoroacetone (6 mmol) and 1,10-phenanthroline (2 mmol) are first dissolved in ethanol (30 mL), and NaOH (2 M) is then added to adjust the pH to 7. A second solution is prepared from EuCl3 6H2O (2 mmol) and water (20 mL). The first solution is heated to 60 °C under magnetic stirring, and the second solution is then added. A precipitate immediately forms, and agitation is continued for 1 h at 60 °C. Finally, the Eu complex is isolated by filtration, washed with water and dried in an in oven at 70 °C overnight. The chemical structure of the complex is provided in Fig. 1. Eu (tta)3phen is a pale yellow solid that is soluble in THF at room temperature and in styrene upon heating. It is partially soluble in methanol and insoluble in water.

Latexes were treated with ion exchange resins or by dialysis in order to remove any free Eu that was not incorporated in the polymer particles. Two ion exchange resins, one cationic (Amberlite IR-120) and one anionic (Amberlite IRA-67), were used as a 1:1 mixture. The mixed resins (2 g) were added directly to 30 mL of the latex and stirred for 30 min. The resin was removed by sedimentation. Dialysis of latexes samples was carried out against deionized water (Nano Pure II, 18.2 MX cm1) using a membrane tubing (Spectrum labs) with a molecular weight cut-off of 3500. Samples were treated for 3 consecutive periods of 8 h, with fresh water being used for each washing.

2.3. Miniemulsion preparation

Elemental analysis was performed on a LECO, model CHNS-932 analyzer. Post-combustion analysis of carbon and sulfur was carried out by infrared absorption whereas nitrogen was measured by thermal conductivity. A small quantity of V2O5 was added to the samples as a combustion aid.

Sodium dodecylsulfate (SDS) is dissolved in 62 mL of nanopure water and Eu(tta)3phen is dispersed in 15 g of styrene and heated until complete dissolution. The resulting organic phase is added to

2.7. Characterization methods

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J. Desbiens et al. / Journal of Colloid and Interface Science 376 (2012) 12–19

Transmission electron microscopy (TEM) was performed with a JEOL 1230 electron microscope operated at an acceleration voltage of 80 V. Samples were prepared by evaporating a drop of diluted aqueous latex directly on a 200 mesh carbon-coated nickel grid. Dynamic light scattering (DLS) measurements were carried out on the latexes with a Malvern Zetasizer Nano ZS instrument, equipped with a He–Ne 633 nm laser. Reported particle sizes correspond to the Z-average diameter (ZD). The width of the particle size distribution is described by the polydispersity index (PDI), defined as

PDI ¼

r2 Z 2D

ð1Þ

;

where r is the standard deviation of the Z-average diameter. The Eu content of the polystyrene particles was determined by inductively coupled plasma atomic emission at 381.967 nm with a Perkin Elmer model Optima 3000 spectrometer. Doped polystyrene particles were digested before analysis with a mixture of H2SO4:H2O2 30% (2:1 by volume). Steady-state fluorescence spectra were recorded with a Varian Cary Eclipse spectrophotometer with a 450 W xenon lamp as excitation source. Measurements were performed in fused silica cells with an optical path length of 1.0 cm and at an excitation wavelength of 343 nm. The luminescence of the europium complex was studied both in THF solution and in aqueous suspensions of doped polystyrene particles. Reported luminescence intensities were determined by area under the emission peak at 612 nm for samples at a fixed solid content of 0.05%. Spectrally- and time-resolved luminescence experiments based on analog recording in the time domain were performed using a custom-made system described in the Supporting Information. 3. Results and discussion 3.1. Properties of Eu(tta)3phen The structure of the complex was confirmed by melting point measurements and elemental analysis, performed after drying under vacuum for 1 day. A melting point of 247 °C was obtained for Eu(tta)3phen, in accordance with the literature value [33]. Elemental analysis confirmed the chemical composition of the synthesized complex as C36H20O6F9S3N2Eu (Eu(tta)3phen) Calcd: C, 43.4%; N, 2.8%; S, 9.7%. Found: C, 45.2%; N, 2.9%; S, 9.4%. 3.2. Characteristics of the polymerization reaction Miniemulsions are typically protected against Ostwald ripening by the addition of a small amount of a hydrophobic substance such as hexadecane. Eu(tta)3phen is essentially insoluble in water and

can thus serve as the stabilizing hydrophobic agent in miniemulsions employed for the preparation of doped particles. Miniemulsions containing Eu(tta)3phen are, however, found to be less stable than those stabilized with hexadecane and must be polymerized within 1–2 h, whereas standard miniemulsions are stable for many days. Furthermore, in some cases, a yellowish precipitate is observed to form on the magnetic stirrer during polymerization, leading to a decrease in the latex particle yield. This destabilization is presumably due to the limited solubility of Eu(tta)3phen in styrene. At the highest doping level, the styrene nano-droplets are initially saturated with the complex, and dopant crystallization may therefore occur as monomer is consumed during polymerization. Since an unknown fraction of the Eu complex is lost in the residual polystyrene that precipitates from the miniemulsion, the Eu content of final latexes must be measured independently.

3.3. Properties of the doped particles 3.3.1. Doped vs undoped particles Polymer particles with and without Eu(tta)3phen were prepared under identical conditions in order to verify the influence of the complex on the polymerization process. TEM images of doped and undoped polystyrene particles are presented in Fig. 2. In both cases, the particles are spherical and relatively monodisperse in size. From these images, average particle diameters can be estimated as 65 nm for the undoped sample (PS_1) and 55 nm for the doped sample (PS_Eu1). It should be noted that the apparent particle fusion observed in TEM images results from interaction with the electron beam during observation and therefore does not indicate particle aggregation within the latex. Particle size was also determined by dynamic light scattering, and the results are provided in Table 1, along with other sample characteristics. Z-average diameters of 90 and 71 nm are obtained for PS_1 and PS_Eu1, respectively. The difference between diameters measured by TEM and DLS can be explained by the fact that the z-average is an intensity weighted average and, since the scattering intensity varies with the square of the particle size, the z-average will be skewed to larger diameters. Furthermore, DLS measurements yield the hydrodynamic diameter, which includes the solvation layer surrounding the polar head groups of the surfactant at the particle surface. However, since the hydration layer can be estimated to be less than 1 nm in thickness, and since the particles do not swell in water, the difference between the particle sizes evaluated from TEM and those obtained by DLS can be primarily attributed to the z-average being more heavily weighted toward larger particles. At first view, the results obtained for PS_1 and PS_Eu1 may suggest that incorporation of the complex leads to a reduction in particle size. Results obtained for the third entry in Table 1, PS_Eu2, however, indicate that this is not the case. Although this sample

Fig. 2. TEM images of: (a) undoped polystyrene particles (PS_1) and (b) doped polystyrene particles (PS_Eu1).

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J. Desbiens et al. / Journal of Colloid and Interface Science 376 (2012) 12–19 Table 1 Miniemulsion polymerization of styrene in presence of Eu(tta)3 phen.

Table 2 Particle characteristics as a function of surfactant concentration.

Sample

Surfactant/ monomer ratio (wt)

Eu(tta)3phen contentb (wt%)

Polymerization yield (%)

Particle diameterc (nm)

PDIc

Sample

Surfactant/ monomer ratio (wt)

Eu(tta)3phen contentb (wt%)

Yield (%)

Particle sizec (nm)

PDIc

PS_1a PS_Eu1 PS_Eu2

0.015 0.015 0.015

0 2 2

76 37 84

90 71 94

0.008 0.086 0.049

PS_Eu2 PS_Eu3 PS_Eu4 PS_Eu5 PS_Eu6 PS_Eu12a

0.015 0.016 0.039 0.066 0.068 0.302

2 3 2 2 3 0.6

84 87 95 88 78 80

94 83 63 52 52 19

0.049 0.053 0.102 0.083 0.062 0.079

a Prepared from a miniemulsion containing 0.515 g of hexadecane as a hydrophobic agent. b Initial content relative to styrene. c Determined by DLS.

was prepared in a manner identical to that employed for PS_Eu1, significantly larger particles are obtained. Normally, in a miniemulsion polymerization, particle size is controlled by the surfactant to monomer ratio, S. In the specific case of the polymerization of styrene in the presence of SDS, previous studies [34] have indicated that an S ratio of 0.015 should lead to particle diameters of about 90 nm, as observed for samples PS_1 and PS_Eu2. In the case of PS_Eu1, the size is smaller than expected. The explanation for the smaller particle size probably resides in the low conversion found for this reaction. Fig. 3 highlights the relationship between polymerization yield and the size of the resulting nanoparticles. Comparison of samples PS_Eu1 and PS_Eu2 thus illustrates two important facts; that yield must be taken into account when comparing particle size and that a significant variation in conversion is found, even for samples prepared under seemingly identical conditions. The polydispersity index (PDI), as measured by DLS, is reported for each of the samples in the final column of Table 1. A significant difference is observed between the doped and undoped particles with the former exhibiting a broader size distribution. As noted above, miniemulsions containing Eu(tta)3phen are less stable than the corresponding undoped formulations. For the majority of the doped samples reported here, Eu(tta)3phen replaced hexadecane as the stabilizing hydrophobic additive. In order to test whether the absence of hexadecane is responsible for the increase in polydispersity, miniemulsion polymerizations were also carried out in the presence of both Eu(tta)3phen and hexadecane. As illustrated by the last entry in Table 2, for example, the presence of hexadecane does not reduce the width of the size distribution of the doped particle population. The polydispersity index is, however, found to depend on the yield, as illustrated by Fig. 4.

a Prepared from a miniemulsion containing 0.492 g of hexadecane as a hydrophobic agent. b Initial content relative to styrene. c Determined by DLS.

The outlier at a PDI of 0.008 corresponds to the single undoped sample included in the plot. These results illustrate the important point that destabilization of the miniemulsion by the europium complex leads to decreased conversion accompanied by an increase in the width of size distribution of the resulting particles. 3.3.2. Doped particles of various sizes In a miniemulsion polymerization, each droplet acts as an isolated nanoreactor. The final size of the polymer particles thus corresponds to the initial size of the styrene droplets, which is, in turn, determined by the surfactant to monomer ratio. Polystyrene particles doped with Eu(tta)3phen of varying diameter, ranging from 94 to 19 nm, were successfully prepared under the conditions summarized in Table 2. As expected, particle size decreases as the surfactant ratio S is increased. The range of accessible sizes is limited by S values that correspond to the region of miniemulsion stability (0.015 < S < 0.302). Landfester et al. [34] have shown that below an S ratio of 0.01, there is insufficient surfactant to coat the polystyrene nanoparticles, whereas at S ratios above 0.5, excess surfactant can lead to the possible formation of free micelles. The results presented in Table 2 indicate that the concentration of Eu(tta)3phen does not significantly affect particle size. Samples with 2% or 3% of the complex have approximately the same size for a given S ratio. 3.3.3. Eu(tta)3phen concentration The primary aim of this study is to prepare doped polystyrene particles with a lanthanide based luminophore. It is, therefore, important to determine the maximum concentration of the Eu

100

Polydispersity index

NP size by DLS (nm)

0.09

90

80

0.06

0.03

70

40

60

80

100

Polymerization yield (%) Fig. 3. Variation in nanoparticle size as a function of polymerization yield for a series of samples with varying concentrations of Eu(tta)3phen but an identical surfactant/monomer ratio of 0.015.

0.00 40

60

80

100

Polymerization yield (%) Fig. 4. Variation in the polydispersity index, as measured by DLS, as a function of the polymerization yield for a series of samples with a constant surfactant/ monomer ratio of 0.015 but varying concentrations of Eu(tta)3phen.

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J. Desbiens et al. / Journal of Colloid and Interface Science 376 (2012) 12–19

Table 3 Particle characteristics as a function of initial Eu(tta)3phen content.

a b

Surfactant/ monomer ratio (wt)

Eu(tta)3phen contentb (wt%)

yield (%)

Particle diameterc (nm)

PDIc

Doping Level (±0.1 g Cpx/100 g solid material)

PS_1 PSR_Eu1a PSR_Eu2 PS_Eu12 PSR_Eu3 PSR_Eu4 PSR_Eu5 PS_Eu2 PS_Eu3 PS_Eu7 PS_Eu8 PS_Eu9 PS_Eu10 PS_Eu11

0.015 0.015 0.015 0.302 0.015 0.015 0.015 0.015 0.016 0.019 0.016 0.016 0.016 0.016

0 0.5 0.6 0.6 0.7 0.8 0.9 2 3 4 5 6 7 7

76 93 82 80 98 90 93 84 87 53 52 62 63 91

90 – – 19 43 50 60 94 83 68 74 77 84 86

0.008 – – 0.079 0.031 0.080 0.074 0.049 0.053 0.093 0.090 0.076 0.060 0.047

0 0.6 0.6 0.7 0.7 0.7 0.7 2.0 1.2 2.1 2.2 3.2 1.0 1.6

The notation PSR is used for crosslinked particles. Initial content relative to styrene. Determined by DLS.

Final Eu(tta)3 phen concentration (wt%)

c

Sample

4.0

3.0

2.0

1.0

0.0 0.0

2.0

4.0

6.0

8.0

Initial Eu(tta) 3 phen concentration (wt%) Fig. 5. Eu(tta)3phen content of the final latex particles as a function the initial concentration of the complex in styrene.

complex that can be incorporated within the nanoparticles. A series of miniemulsion polymerizations were thus carried out with the initial concentration of Eu(tta)3phen systematically varied from 2% to 7%, relatively to styrene. The results are summarized in Table 3. The concentration of Eu complex added to styrene before polymerization is indicated in the third column of the table, whereas the doping level of the final particles, as determined by ICP atomic emission, is provided in the final column.

As shown in Table 3, the final doping level of the nanoparticles is not equal to the concentration of Eu(tta)3phen added to styrene prior to polymerization. Although a maximum doping level of 3.2% is observed for sample PS_Eu9, large variations are observed for initial complex concentrations exceeding 2%. Despite these variations, Fig. 5 illustrates that a general correlation is found between final Eu content and initial doping, except at the highest doping level. The Eu complex not included in the particles is presumably lost in the aggregate that forms around the magnetic stirrer in certain reactions. The solid data points in Fig. 5, corresponding to the highest doping level, emphasize that this effect can be attributed to destabilization of the miniemulsion by the limited solubility of the complex. Comparison of the last two entries in Table 3 indicates that the destabilization process is more complex than the simple aggregation of styrene droplets, which would lead to loss of polymer and complex in a fixed ratio. In the case of sample PS_Eu11, the final Eu content of the particles is much lower than the initial doping level, even though this reaction is characterized by a relatively high conversion. Miniemulsion destabilization at high complex contents thus must involve the exclusion of complex from monomer droplets during polymerization. 3.3.4. Crosslinked particles The Eu complex is presumably embedded within the polystyrene matrix during polymerization, but since it is not covalently linked to the polymer, it may be free to migrate to the surface. For this reason, divinylbenzene was added to the polymerization

Fig. 6. TEM images of crosslinked polystyrene particles doped with Eu(tta)3phen (sample PSR_Eu8) (a) before and (b) after a single treatment with ion exchange resins.

17

Eu conc. (g Eu/100 g solid material)

J. Desbiens et al. / Journal of Colloid and Interface Science 376 (2012) 12–19

3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

1

2

3

4

5

Number of washings Fig. 7. Eu(tta)3phen content of the polystyrene latex particles as a function of the number of successive treatments with ion exchange resins. Open and filled symbols correspond to crosslinked and non-crosslinked particles, respectively.

curious feature of Fig. 7 is the inefficiency of the first washing for the most highly doped samples. In fact, the three samples with the highest initial Eu content show an apparent increase in Eu concentration after a single treatment with the resins. These observations may be related to the removal of the surfactant layer during the first treatment or to the selective retention of material with a lower than average Eu content in a heterogeneous sample. These results, combined with the destabilization of the miniemulsion discussed above, establish an upper doping limit of about 2% for the reproducible preparation of luminescent nanoparticles in the present system. It is important to note that treatment with ion exchange resins is relatively harsh. When the latex is washed by dialysis, no loss of europium is observed, even over a 24 h period. The high hydrophobicity of Eu complex prevents leaching into the aqueous phase, and doped latexes are stable for many months. No flocculation is observed, and the luminescent properties are maintained. 3.4. Luminescence properties

mixture in order to obtain crosslinked particles. Crosslinking should reduce the diffusion of the complex to the particle surface and also result in particles that remain intact in organic solvents such as acetone and chloroform. The TEM images of Fig. 6 indicate that crosslinking does not modify the morphology of the particles. Because of the variability found in the final Eu concentrations, the robustness of the doped particles was tested by treatment with ion exchange resins. The Eu content of the particles is plotted in Fig. 7 as a function of the number of successive washings for a series of samples differing in their initial doping levels. Both crosslinked (open symbols) and non-crosslinked (filled symbols) samples are included in the plot. Although crosslinking appears to somewhat inhibit the loss of Eu(tta)3phen, it does not eliminate it completely. Fig. 7 exhibits two unexpected features. Firstly, the fraction of complex removed, particularly during the initial exposures to the ion exchange resin, is a function of the Eu(tta)3phen content of the particles, with the more highly doped particles showing more significant losses. This observation suggests that at high doping levels, excess complex tends to migrate to the particle surface and is readily removed upon contact with the ion exchange resins. The first washing, however, does not remove all of the Eu(tta)3phen, indicating that the complex is not completely located at the surface. The results suggest, rather, that once the complex at the interface is removed by the resin, the dopant diffuses to re-equilibrate the distribution throughout the particle. A second

Excitation and emission spectra of Eu(tta)3phen dissolved in THF, as well as the emission spectrum of an aqueous suspension of doped polystyrene nanoparticles, are presented in Fig. 8. A photograph of a doped latex under UV light is included in the inset. The luminescence properties of b-diketonates complexes of Eu3+ are known to involve absorption by the organic ligand followed by energy transfer to the lanthanide ions [35]. The observed emission maximum at 612 nm is in agreement with literature values for the 5D0 ? 7F2 transition of Eu3+ [36,37]. The identical emission spectrum found for the latex indicates that the complex remains intact in the polystyrene matrix. Luminescence is observed for all of the latex samples listed in Table 3. The luminescence of the Eu complex can be further characterized by the intensity ratio of the 5D0 ? 7F2 and 5D0 ? 7F1 transitions. The 5D0 ? 7F1 is a magnetic dipole transition and, as such, has an intensity that is essentially independent of the surrounding environment. The intensity of the 5D0 ? 7F1 transition can thus be employed as an internal references [37,38]. The 5D0 ? 7F2 transition, on the other hand, is an electric dipole transition with an intensity that is influenced by the local environment of the europium ion. The steady-state intensity ratio I(5D0–7F2)/I(5D0–7F1) for Eu(tta)3phen, both in THF solution and in doped polystyrene nanoparticles, is reported in Table 4. Similar values are obtained, indicating the europium complex remains intact during polymerization since the loss of a ligand would result in an important

(a)

(b) 1.0

Relative intensity

Relative intensity

1.0

0.5

0.5

0.0

0.0 300

400

500

Wavelength (nm)

600

700

500

550

600

650

Wavelength (nm)

Fig. 8. (a) Excitation (blue, kem = 613 nm) and emission (pink, kex = 343 nm) spectra of Eu(tta)3phen dissolved in THF. (b) Emission spectrum (kex = 343 nm) of a latex doped with Eu(tta)3phen. The inset shows a photograph of a test tube containing the doped latex under UV light. (For the interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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J. Desbiens et al. / Journal of Colloid and Interface Science 376 (2012) 12–19

Table 4 Luminescence properties of Eu(tta)3phen in THF solution and in polystyrene NPs at room temperature. Intensity ratios (I(5D0–7F2)/I(5D0–7F1)) were evaluated from steady-state measurements, luminescence lifetimes (s) and coefficients of determination (R2) from exponential fits of time-resolved decay curves.

I(5D0–7F2)/I(5D0–7F1) s (ls) R2

Eu(tta)3phen in THF

Eu(tta)3phen in PS NPs

15.6 707 ± 1 0.99

12.3 720 ± 2 0.99

of the miniemulsion leads to a loss of reproducibility with respect to both the degree of conversion and the final Eu content of the particles. Treatment of doped latexes with ion exchange resins indicates that Eu(tta)3phen is free to migrate within the particles and can be removed at the surface. Importantly, no leaching of the dopant into the aqueous phase is observed in the absence of ion exchange resins. The luminescence properties of the lanthanide complex are unchanged by incorporation in the polymer matrix. Aqueous dispersions of polystyrene nanoparticles doped with Eu(tta)3phen exhibit characteristic red emission under UV light irradiation and no self-quenching is observed within the particles.

Luminescence Intensity

2500

Acknowledgments 2000

The authors would like to acknowledge NanoQuébec, le Fonds Québécois de la recheche sur la nature et les technologies (FQRNT) and the National Sciences and Engineering Research Council of Canada (NSERC) for their financial support. J.-F. Gravel and D. Boudreau are gratefully acknowledged for their assistance with the time-resolved luminescence measurements.

1500

1000

500

Appendix A. Supplementary data 0 -6 2.0x10

-6

3.0x10

-6

4.0x10

-6

5.0x10

-6

6.0x10

Eu cpx concentration (g Eu cpx/100 ml latex) Fig. 9. Luminescence intensity at 612 nm as a function of Eu(tta)3phen content for latexes with a fixed solid content of 0.05%.

reduction in the intensity ratio. For comparison, Wang et al. [39] reported that the intensity ratio of EuCl3 in solution is 0.92. Time-resolved luminescence experiments were also carried out on the doped latexes and Eu(tta)3phen dissolved in THF. Intensity decay curves could be fitted to single exponentials (Supporting Information), yielding the luminescence lifetimes reported in Table 4. Relatively long lifetimes (hundreds of microseconds) are obtained as is typical for Eu(III) complexes [37]. Taniguchi et al. [40] reported an increase in the fluorescence lifetime of pyrene in polystyrene particles as compared with THF solution, which they attributed to the suppression of thermal deactivation of excited state fluorophores due to restriction of molecular motion in the rigid polymer matrix. In the present case, this effect seems to be minor. In fact, there is only a small increase in lifetime for the Eu complex embedded in polystyrene (720 ls), and the difference is not significant enough to imply the presence of a different deactivation pathway. Finally, the steady-state luminescence intensity of the doped latexes was investigated as a function of doping levels. Results are shown in Fig. 9 for Eu(tta)3phen contents ranging from 0.5% to 1.1%. The observed linearity indicates that there is no self-quenching interaction between the europium complexes within the polystyrene matrix, as this would lead to a decrease in luminescence intensity at higher concentrations. A complex content of 1.1% corresponds to a doping level of about 3000 molecules per particle. The ability to achieve these relatively high doping levels without loss in luminescence intensity through self-quenching can be attributed to the large Stokes shift of the Eu(tta)3phen complex. 4. Conclusions Polystyrene nanoparticles prepared by miniemulsion polymerization can be doped with Eu(tta)3phen introduced into the polymerization medium. A maximum doping level of about 2% by weight can be achieved. At higher doping levels, destabilization

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