Luminescence of Eu3+ Activated CaF2 and SrF2 Nanoparticles: Effect

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Luminescence of Eu3+ activated CaF2 and SrF2 nanoparticles: effect of the particle size and co-doping with alkaline ions Paolo Cortelletti, Marco Pedroni, Federico Boschi, Sonia Pin, Paolo Ghigna, Patrizia Canton, Fiorenzo Vetrone, and Adolfo Speghini Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01050 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Crystal Growth & Design

Luminescence of Eu3+ activated CaF2 and SrF2 nanoparticles: effect of the particle size and codoping with alkaline ions Paolo Cortelletti,a Marco Pedroni,a Federico Boschi,b Sonia Pin,c Paolo Ghigna,c Patrizia Canton,d Fiorenzo Vetrone,e,f Adolfo Speghinia,* a

Nanomaterials Research Group, Dipartimento di Biotecnologie, Università di Verona and

INSTM, UdR Verona, Strada Le Grazie 15, I-37134 Verona, Italy b

Dipartimento di Informatica, Università di Verona, Strada Le Grazie 15, I-37134 Verona,

Italy c

Dipartimento di Chimica, Università di Pavia and INSTM, UdR Pavia, V.le Taramelli 16, I-

27100, Pavia, Italy d

Centro di Microscopia Elettronica "Giovanno Stevanato", Dipartimento di Scienze

Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Via Torino 155/B, VeneziaMestre, Italy e

Institut National de la Recherche Scientifique, Centre Énergie, Matériaux et

Télécommunications (INRS - EMT), Université du Québec, 1650 Boul. Lionel-Boulet, Varennes, QC, J3X 1S2 (Canada)

ACS Paragon Plus Environment

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f

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Centre for Self-Assembled Chemical Structures (CSACS), McGill University, Montréal,

H3A 2K6, Québec, Canada *Corresponding Author: [email protected]


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ABSTRACT Eu3+ doped CaF2 and SrF2 nanoparticles were synthesized through a facile hydrothermal technique, using citrate ions as capping agents and Na+ or K+ as charge compensator ions. A proper tuning of the reaction time can modulate the nanoparticle size, from few to several tens of nanometers. Analysis of EXAFS spectra indicate that the Eu3+ ions enter into the fluorite CaF2 and SrF2 structure as substitutional defects on the metal site. Laser site selective spectroscopy demonstrates that the Eu3+ ions are mainly accommodated in two sites with different symmetries. The relative site distribution for lanthanide ions depends on the nanoparticle size, and higher symmetry Eu3+ sites are prevalent for bigger nanoparticles. Eu3+ ions in high symmetry sites present lifetimes of the 5D0 level longer than 20 ms, amongst the longest lifetimes found in the literature for Eu3+ activated materials. As a proof of concept of possible use of the Eu3+ activated alkaline-earth fluoride nanoparticles in nanomedicine, the red luminescence generated by two-photon absorption using pulsed laser excitation at 790 nm (in the first biological window) has been detected. The long Eu3+ lifetimes suggest that the present nanomaterials can be interesting as luminescent probes in time resolved fluorescence techniques in biomedical imaging (e.g. FLIM) where fast autofluorescence is a drawback to avoid.


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INTRODUCTION The importance of trivalent lanthanide (Ln3+) doped luminescent inorganic nanoparticles (NPs) has continuously grown in the past years as their luminescence and paramagnetic features can be exploited in several technological applications. Ln3+ doped luminescent NPs are useful as optical labels and probes in biomedical applications as well as in photovoltaic devices.1-6 Upconverting NPs, that present emissions at higher energies with respect to the excitation radiation, are interesting in nanomedicine, where near-infrared (NIR) excitation radiation is required7, 8. Fluoride based hosts are very suitable for hosting luminescent ions, and their low phonon energies reduce non-radiative relaxation of the excited states and therefore increase the radiative emission9-11. In particular, Ln3+ doped alkaline-earth fluorides, such as CaF2 and SrF2, show bright emissions in the visible and NIR regions, also in nanosized form12, 13. Moreover, they can be prepared by facile and environmental friendly methods.12, 14-16 The local environments of Ln3+ ions as dopants in single crystals of alkalineearth fluorides have been previously investigated 17-19. However, a detailed study of the local environment for Ln3+ ions in nanosized alkaline-earth fluorides is still lacking. To this purpose, in the present investigation, the Eu3+ ion is used as a luminescent probe to investigate the site symmetry in which the Ln3+ ions are accommodated20-22. Although some reports have appeared in the literature on the spectroscopic properties of Eu3+ doped CaF2 and SrF2 nanoparticles12, 23, 24, a detailed investigation of the sites occupied by the Ln3+ ions in the crystal structure for nanocrystalline hosts deserves to be investigated. Moreover, understanding the effect of co-doping of Ln3+ doped alkaline-earth NPs with alkali metal ions as charge compensators25 is essential for possible optical applications, as it can lead to further modulation of their luminescence properties. Some charge compensating mechanisms have been proposed in CaF2 and SrF2 single crystals, leading to the formation of Ln3+ sites with C3v or C4v symmetry in the case of local charge compensation, and to sites with Oh symmetry


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in the case of non-local charge compensation. The presence of interstitial fluoride ions also induces a distortion in the lattice structure due to the repulsion between them.25-28 The NPs under investigation were prepared with a hydrothermal method using citrate moieties as hydrophilic capping agents, ensuring excellent dispersion in polar solvents such as water. Moreover, Na+ or K+ ions were added as counter ions of the citrate anion to provide charge compensation29-31. The local chemical environment of Eu3+ was investigated by optical as well as Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies. The excited state dynamics for the energy levels of the Eu3+ ions were also investigated, by monitoring the emission decay of the luminescence. Since the Eu3+ luminescence can be very useful for bioimaging purposes, we also investigated the emission in the red region of colloidal dispersions of the present NPs by two-photon excitation using a pulsed laser in the first biological window (around 790 nm).

EXPERIMENTAL SECTION

2.1 Synthesis Eu3+ doped MF2 (M=Ca2+, Sr2+) NPs were prepared using a hydrothermal synthesis using a nominal M:Eu3+ molar ratio of 0.99:0.01. Stoichiometric quantities of CaCl2·2H2O (Baker, >99%) or SrCl2·6H2O (Carlo Erba, >99%) and EuCl3·6H2O (Aldrich, 99.99%), with 3.5·10-3 mol of total metal amount, were dissolved in 5.0 ml of deionized water in a Teflon vessel. A volume of 20.0 ml of a 1.00 M sodium citrate (Na-cit, Fluka, ≥ 99%) or potassium citrate (Kcit, Fluka, ≥ 99%) solution was added under vigorous stirring for 5 minutes. A 3.5 M aqueous solution of NH4F (Baker, 99%) was added to the previous solution, in order to have a slight excess of fluoride ions, (M+Eu):F=1:2.5 molar ratios. The obtained clear solution was heat treated at 190 °C under autogenous pressure in an autoclave (DAB-2, Berghof). Several


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reaction times (10, 35, 360 and 480 minutes) were used to prepare samples with increasing particle size. The autoclave was then cooled to room temperature and after centrifugation (7000 rpm, 10 min), the NPs were collected and stored in acetone. The NPs are easily dispersible in water as well as in saline solution to form transparent colloids. To obtain the NPs in powder form, the gel pellets are dried at room temperature for 24 h. Samples will be hereafter denoted as CN(time, min) for CaF2 NPs or SN(time, min) for SrF2 NPs prepared with Na-cit as starting reagent. Moreover, CaF2 NPs prepared with K-cit as starting reagent are denoted as CK(time, min).

2.2 X-Ray powder diffraction measurements X-Ray powder diffraction (XRPD) patterns were recorded with a Thermo ARL X´TRA powder diffractometer in Bragg-Brentano geometry, equipped with a Cu-anode X-ray source (Kα, λ=1.5418 Å) and a Si(Li) solid state detector. The patterns were collected with a scan rate of 2.5°/s and 2θ range of 20°-90°. The phase identification was carried out with the PDF4+ 2009 database (International Centre for Diffraction Data, ICDD). The instrumental X-Ray peak broadening was determined using LaB6 standard reference material (SRM 660a) provided by NIST and the cell parameters were determined using an α-SiO2 as an internal standard. The samples were carefully homogenized in a mortar, and deposited in a lowbackground sample stage.

2.3 Transmission Electron Microscopy Transmission Electron Microscopy (TEM) and high resolution TEM (HRTEM) images were measured using a JEOL 3010 high resolution electron microscope (0.17 nm point-topoint resolution at Scherzer defocus), operating at 300 KV, equipped with a Gatan slow-scan CCD camera (model 794) and an Oxford Instrument EDS microanalysis detector (Model


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6636). The powders were dispersed in water in order to be deposited on Holey-Carbon Copper grids.

2.4 Spectroscopy measurements Luminescence spectra were measured using a Nd:YAG pumped tunable dye laser (Quanta System) as the excitation source. The emission signal was analyzed by a half-meter monochromator (HR460, Jobin Yvon) equipped with a 1200 lines/mm grating and detected with a CCD detector (Spectrum One, Jobin Yvon) with a spectral resolution of 0.15 nm. Emission decays were measured using a GaAs photomultiplier and a 500 MHz digital oscilloscope. 77 K emission spectra were acquired with an L-N2 immersion cold finger cryostat. Excitation spectra were measured using a Fluorolog-3 (Horiba-Jobin Yvon), spectrophotometer, with spectral resolution of 0.2 nm.

2.5 ICP-MS Elemental analysis was carried out using a Thermo Fisher Scientific X Series II ICP-MS equipped with a collision/reaction cell (CCT). The citrate groups and alkaline ions were completely removed from the NPs surface by washing the powder samples several times with a 0.5% HNO3 solution. The obtained NPs were then dissolved in a 5% nitric acid solution.

2.6 EXAFS spectra The EXAFS spectra at the Ln–K edges were measured as a function of the temperature in the range 300-20 K at the BM23 beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, Experiment CH-2330), in fluorescence mode, using a 13 element Ge detector for the fluorescence signal, and a double crystal Si(511) monochromator. For the measurements, a proper amount of sample (ca 0.3 g) intended to give the maximum contrast


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at the Ln-K edge was thoroughly mixed with cellulose in an agate mortar and then pressed to a pellet. For the EXAFS data analysis, at all the investigated edges, the EXAFS were extracted using the ATHENA code. The data analysis was performed using the EXCURVE program. Phases and amplitudes were calculated using the muffin-tin approximation, in the framework of the Hedin–Lundquist and Von Bart approximations for the exchange and ground state potentials, respectively: this includes the effects of inelastic losses due to the electron inelastic scattering (photoelectron mean free path). The fittings were done in the k space, using a k2 weighting scheme, and full multiple scattering calculations within the cluster used in the fits (see below). The stability of the fits was verified by testing different weighting schemes and validating that the fitting parameters were recovered within the errors. The ܵ଴ଶ parameter, which provides a measure of events such as two-electron transitions (where the energy difference between the photon and the photoelectron is so large that they are not seen in the spectrum), was always found to be unity within the error: this was expected, as multi-channel events are included in the Hedin–Lundquist approximation. The accuracy of the phase shifts and amplitudes were tested by fitting the CeO2 Ce–K edge EXAFS spectra, and recovering the crystallographic distances within 0.01 Å.

2.7 Two Photon microscope setup A two-photon microscope DM-6000 CS (Leica) was used to perform optical imaging of the NPs. The samples were excited by a Chameleon ULTRA II laser (Coherent) at 790 nm and visualized with a 20x objective (water immersed, 1.0 NA) in the range 570 – 650 nm. Acquisition and analysis of the images were done by using the LAS AF (Leica Application Suite Advance Fluorescence, Leica) software.


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RESULTS AND DISCUSSION

3.1 Elemental analysis, structural analysis and morphology The ICP-MS elemental analysis, phase and structural analysis (from XRPD data) are described in Table S1 and Figures S1-S3, respectively (Supporting Information). All the samples are single phase Fm-3m cubic fluorite structure. The NPs size clearly increases on increasing the reaction time, as observed in TEM images (Figure 1). HRTEM images confirm the crystallinity of the NPs.

Figure 1 around here

3.2 EXAFS analysis EXAFS spectra at the Eu-K edge at 20 K are shown in Figures 2 and 3, respectively, for the CN360 and SN360 samples. Visible EXAFS oscillations are present, above the noise level, up to k=14 Å-1. Both spectra are better discussed by looking at the Fourier Transform (panel b).


The first peak (ca. 2.5 Å) is attributed to the 8 fluoride ions around the Ca, Sr site of the fluorite structure and the second peak is attributed to Ca (Sr) atoms that are the next nearest neighbours. The spectra can then be fit using a fluorite structural model, in which the Nearest Neighbour (denoted as NN) shell is made up of 8 fluoride ions, and the Next Nearest Neighbour (denoted as NNN) shell is comprised of twelve Ca or Sr atoms.


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The results of the fit for the spectrum shown in Figure 3 are reported in Table 1. Concerning the Eu3+ doped CaF2 sample, the EXAFS spectra have been measured as a function of temperature (T). The Eu-F and Eu-Ca distances are shown as a function of T in Figure 4. It is clearly observed that Eu3+ doping induces a small structural disorder for what concerns both the NN (fluoride) shell and the NNN (calcium), the fitted distances being not dependent of T.

Table 1. EXAFS results for the spectrum in Fig. 3: n: coordination number, r: distance; σ2: EXAFS Debye-Waller factor; r0: crystallographic distance in SrF2. R=11.6 %. Shell

Atom

n

r (Å)

σ2 (Å2)

r0 (Å)

1

F

8

2.41(1)

4(2)⋅10-3

2.4738

2

Sr

12

4.12(1)

3.3(7)⋅10-3

4.0397

The Eu-F distance is slightly contracted compared to the Ca-F distance, as for the Eu3+ doped SrF2 sample, in agreement with the ionic radii (1.07 Å for 8-fold coordinated Eu3+)32. In addition, the Eu-M distance is slightly increased both for the CaF2 and for the SrF2 samples. This can be related to the increased charge on Eu3+ compared to Ca2+ or Sr2+. Finally, the trends of the EXAFS Debye-Waller (DW) factors as a function of T, are also shown in Figure 4. The upper panel shows the DW factor for the NN (fluoride), the lower panel for the next nearest neighbours (Ca). The continuous lines are fits using the Einstein model. A small amount of static disorder is found in both cases (5⋅10-4 and 1⋅10-3 Å2,


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respectively). The Einstein temperatures are 433 K and 200 K, while the reduced masses agree quite well with those for the Eu-F and Eu-Ca couples.


3.3 Spectroscopic investigation 3.3.1 Non-selective spectroscopy As demonstrated by the EXAFS results and corroborated by XRPD analysis, the Eu3+ ions substitute the Ca2+ or Sr2+ ions in the crystal structure and therefore a charge compensation mechanism is involved 28, 33. It was observed that the Na+ and K+ alkaline ions, present in the NPs preparation procedure, can provide charge compensation29,

34

. Due to this charge

compensation, the Eu3+ ions can occupy sites with different symmetries and therefore, they are subjected to different crystal fields, that strongly influence their emission features. In fact, the Eu3+emission is strongly dependent on the local symmetry20,

28, 35

, and this behavior

makes the Eu3+ ion an excellent structural probe. As an example, the emission spectrum of the CN10 sample upon 465 nm laser excitation is shown in Figure 5.


The emission spectra for the CK10 and SN10 samples are very similar to the CN10 one (see Figure S4, Supporting Information Section). As shown in Figure 5, the emission spectrum presents the typical Eu3+ emission bands due to 4f-4f transitions. The strongest features are due to the 5D0→7F1 and 5D0→7F2 transitions, around 590 nm and 615 nm, respectively. The 5D0→7F1 transition is a magnetic dipole transition and it is independent from the Eu3+ local environment, while the 5D0 → 7F2 transition is an electric dipole (ED)


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transition, a so-called “hypersensitive transition”, and its intensity strongly depends on the U(2) and U(4) reduced matrix elements35. Room temperature excitation spectra for the CN10, CK10 and SN10 samples have been measured considering emissions at 589 nm and 611 nm, corresponding to the 5D0 → 7F1 and 5

D0 → 7F2 transitions, respectively, and they are shown in Figure 6. The excitation spectra

show typical bands due to Eu3+ ions, corresponding to transitions starting from the 7F0 or 7F1 levels to higher excited states. The strongest band centered at 395 nm is due to the 7F0 → 5L6 transition.


The excitation spectra for the two different emission wavelengths present some differences. As a general feature, the excitation band around 525 nm due to the 7F0 → 5D1 transition is stronger for the 589 nm emission than for the 611 nm one. Moreover, the excitation bands at the two different emission wavelengths due to the 7F0 → 5D2 transition are different (inset, Figure 6). In fact, the excitation spectrum shows five Stark transitions, in the 455 to 475 nm range, for the emission at 589 nm, while for the excitation spectrum at 611 nm only one band located around 465 nm is clearly observed. This behavior is similar for all the CN10, CK10 and SN10 samples (see Figure 6). These behaviors evidence the presence of at least two sites with different symmetries accommodating the Eu3+ ions, corresponding to different local crystal fields, generating different splitting of the 5D2 Stark levels. As indicated in Figure 6, we indicate as site 1 the one responsible for the excitation spectrum represented by the blue line, and site 2 the one represented by the red line. Site 1 can be associated to the so-called P site indicated by Hamers et al.19 for an Eu3+ doped CaF2 single crystal, since the two main features at 464 nm


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and 466.5 nm assigned to its 7F0 → 5D2 transition perfectly match the two excitation bands reported by Hamers et al. for this site. Moreover, Hamers et al. also report on two excitation bands for the 7F0 → 5D1 transition around 525.5 nm, that resulted to be separated by 0.2 nm. These bands are probably too close in energy to be resolved with our instrumental setup. On the other hand, the site 2 can be attributed to the so-called Q or R sites, shown by Hamers et al., consisting of Eu3+ dimers that present multiple Stark transitions around 525.5 nm and 465 nm. Nonetheless, the spectral resolution of our instrumental setup does not permit to assign precisely the site 2 to a single Q or R site, because the bands due to these sites are most probably strongly overlapped.

3.3.2 Site selective spectroscopy Considering the features observed in the excitation spectra, we carried out site selection spectroscopy measurements using a tunable dye laser in the 455-475 nm range as the excitation source. The site selection emissions are shown in Figure 7.


The excitation spectrum (see inset of Figure 6) indicates that it is possible to select almost completely site 1 by exciting with a laser radiation at 461 or 472 nm. On the other hand, site 2 can be only partly selected, because its excitation band, peaked at 465 nm, partially overlaps with the site 1 excitation band (see inset of Figure 6). By tuning the laser wavelength at 461.0 nm, the site 1 emission shows a dominant 5D0 → 7F1 band for all the samples under investigation (Figures 7 and S5, blue lines), although features due to the other transitions are also observed. On the other hand, upon laser excitation at 464.5 nm, site 2 is mainly excited, and the intensity of the 5D0 → 7F2 transition is comparable to the intensity of


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the 5D0 → 7F1 transition, although contributions from both sites are present (Figure 7, red lines). It is worth noting that upon 461.0 nm excitation, a small contribution from site 2 is also present in the emission spectra, although this is very small for the CN480 sample (see Figure 7). In fact, the 7F0 → 5D2 excitation band for site 2 is extremely weak around 461 nm (see inset in Figure 6) and therefore site 2 can also be weakly excited with laser radiation at 461.0 nm. The almost complete absence of the site 2 emission for the CN480 sample can be due to the fact that almost all the Eu3+ ions are located in site 1. This hypothesis is also corroborated by the extremely low site 2 emission, upon 464.5 laser excitation, with respect to the overall site 1 emission for the CN480 sample. From the measured emission spectra, it is possible to make some inferences about the Eu3+ site symmetry. First, it can be noticed that for site 1 the 5D0 → 7F0 transition is weak or almost absent, while for site 2 it is clearly observable around 580 nm (see Figures 7 and S5). The presence of this band indicates that the possible local symmetries for the Eu3+ ions are Cs, C1, C2, C3, C4, C6, C2v, C3v, C4v, and C6v20, 35. Since the 5D0 → 7F0 transition for site 1 is absent or very weak (probably because of contributions from site 2), it can be inferred that its symmetry is higher than for site 2. Another clear feature of the emission spectra of all samples is that the relative intensity between the emission around 612 nm (5D0 → 7F2 transition) and 590 nm (5D0 → 7F1 transition) is different for excitation in the two different sites and it changes for samples prepared with longer reaction times. The ratio between the 5D0 → 7F2 integrated intensity transition and the magnetic dipole 5D0→7F1 one is called the asymmetry ratio, which is an indicator of the local Eu3+ symmetry 20, 35, 36, defined as

ܴ=

஺లభమ ஺ఱవబ

(1)


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where A612 is the integrated area of the 5D0 → 7F2 emission band and A590 is the integrated area of the 5D0 → 7F1 emission band. An increase of the asymmetry ratio points to a decrease of symmetry and vice-versa. The asymmetry ratio for site 1 was calculated for all the samples synthesized with different reaction times, and it is shown in Figure 8. Since site selection for site 2 was not possible because of the overlapping of its excitation spectra with the one of site 1, its asymmetry ratio was not calculated.


For all samples under investigation, a general R decreasing trend is observed when the reaction time increases. It is important to note that the behaviour of CaF2 co-doped with Eu3+ and Na+ is different from CaF2 co-doped with Eu3+ and K+ and from SrF2 co-doped with Eu3+ and Na+. In fact, R for the CN samples is always lower than R for the other samples. The CN10 and SN10 samples have very similar R values, around 0.75, meaning that the Eu3+ ions have a similar average symmetry, while the R value for the CK10 sample is around 0.9, suggesting a lower symmetry of the Eu3+ environment. On increasing of the reaction time, the behavior of the R values for the CN, CK and SN samples are different. In particular, a decrease of the R value from 0.75 to 0.4 for the CN10 and CN480 samples, respectively, is observed. A decrease of R from 0.75 to 0.6 is also observed for the SN10 and SN480 samples. On the other hand, the CK samples show a small variation of R, ranging from 0.92 to 0.78. Therefore, it is possible to infer that by increasing the size of the nanoparticle, the symmetry of site 1 increases. A relevant difference in the R behaviour is between CN and the CK samples. In fact, for the CN samples R undergoes a large decrease on increasing the particle size while for the CK samples, the R decrease is much less pronounced. This behavior can be due to the different co-doping ions, Na+ for the CN samples and K+ for the


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CK samples, that are involved in the charge compensation when the Eu3+ ions substitute Ca2+ ones in the crystal structure. In order to investigate the contribution of the different Stark sub-levels to the emission spectrum, emission measurements at 77 K were also carried out. The local symmetry of the Eu3+ ions can be investigated considering the observed number of bands due to transitions between Stark levels28, 35, since the number of transitions depends on the crystal field splitting due to the local lanthanide symmetry. In Figure 9, the 77 K emission spectra for the CN480 sample under site selection are shown. The spectra for the other samples are shown in Figure S6 (Supporting Information). The emission of the CN480 sample is shown since it is the only sample where site 1 can be perfectly selected by exciting at 461.0 nm. As can be observed in Figure 9, all the bands show a narrowing behavior with respect to the ones at room temperature (see Figure 7). As found also at room temperature, the 5D0 → 7F0 transition for site 1 is not present, and therefore the one observed for excitation at 464.5 can be attributed only to site 2.


This behavior confirms that the Eu3+ ions accommodated in site 1 have a higher symmetry than the ones accommodated in site 2. Other well-defined narrow bands are observed for the 5

D0 → 7F1 and 5D0 → 7F2 transitions in the 580 – 630 nm range. In Figure 10, a detailed

section of the emission spectra is shown, presenting the emission bands of the 5D0 → 7F0, 5D0 → 7F1 and 5D0 → 7F2 transitions. All the 77 K emission spectra, shown Figure S6 (Supporting Information), present the same spectral behavior as the CN480 sample, although multiple site emission is present at some extent. Moreover, from these spectra it can clearly noticed that the emission bands are broader for smaller NPs (see Figure S6). As it can be seen in Figure


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10, both sites give rise to three emission bands for the 5D0 → 7F1 transition around 590 nm. For site 2 the 5D0 → 7F0 transition is clearly present, therefore its possible symmetry is restricted to C2v, C2, Cs or C135. It is difficult to perfectly assign the symmetry of site 2 since we observe multiple site emission when the samples are excited at 464.5 nm, and the emission bands are quite overlapped even at 77 K.


Like the room temperature emission spectra, site 1 presents no 5D0 → 7F0 transition (see inset of Figure 10) and three sharp bands due to 5D0 → 7F2 transitions, one located at 607.5 nm, and the others at 627 nm and 627.5 nm. These emission bands of the 5D0 → 7F2 transitions perfectly match those found by Hamers et al. in Eu3+ doped CaF2 single crystals33 for the P site. It is worth noting that Hamers et al. do not assign a symmetry for the P site. Since the 5D0 → 7F0 transition is absent, and both the 5D0 → 7F1 and 5D0 → 7F2 transitions show three bands due to Stark splitting, it is possible to assign a D2 symmetry to site 120, 35. To the best of our knowledge, this is the first time that a site with D2 symmetry has been found and assigned for Eu3+ ions in CaF2 or SrF2. Moreover, the emission decays of the 5D0 level exciting at 461.0 for all the samples were acquired and they are shown in Figure 11.


Since the emission decays present a non-exponential behaviour (except for the CN480 sample) the effective lifetimes have been calculated using the formula:


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೅

߬௘௙௙ =

‫׬‬బ ೘ೌೣ ௧ூሺ௧ሻௗ௧ ೅

‫׬‬బ ೘ೌೣ ூሺ௧ሻௗ௧

(2)

where I(t) is the measured decay curve and Tmax >> τeff, and they are reported in Figure 11. The uncertainties on the decay times are 0.1 ms. The effective decay times of the 5D0 level range from 9 to 27 ms, similar to that found by Hamers et al. for the P site (16 ms)33. From Figure 11, it can be noted that the 5D0 state effective decay time increases by increasing the NPs size. For CK and SN (see Figures 11b and 11c), a lower variation on passing from smaller to larger NPs size is present, since it varies from 9.0 ms for CK10 to 12.7 ms for CK480 and from 9.9 ms for SN10 to 13.4 ms for SN480. For the CK and SN samples the emission decay curves do not present a single exponential behaviour, suggesting that contributions from both sites could be presents. Conversely, CN samples (see Figure 11a) show a large difference among samples with different NPs size. The difference between the effective decay times for the CN10, CN35, CN360 and CN480 samples is much relevant, ranging from 10.4 ms for CN10 to 27.2 ms for CN480. To the best of our knowledge, 27.2 ms is among the longest decay times ever observed for the 5D0 level of Eu3+ ions. Of remarkable interest, the CN480 sample exhibits a single exponential behavior, corroborating the evidence from the site selection spectroscopy showing that the emission is due only to Eu3+ ions that are accommodated in the site 1, with D2 symmetry. The rise in the emission observed in the decay curves of the CN360 and CN480 samples can be ascribed to a slow 5D1 → 5 D0 feeding37. It is worth to remind that the amount of Na+ ions in the crystal lattice for the CN samples is higher than the amount of K+ ions in CK samples, as evidenced by the ICP-MS results (see Table S1, Supporting Information). Moreover, it is known that charge compensation by Na+ ions in case of Ln3+ doped CaF2 single crystals favors the presence of sites with high symmetry38 . Therefore, the long decay time for Eu3+ in the CN samples, in particular for


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CN480 and CN360 samples, can be accounted for by the relatively high symmetry (D2) of the Ln3+ environment, although no inversion center belongs to this point group.

3.4 Multiphoton spectroscopy As a proof of concept, the possibility of using the NPs as optical probes in multiphoton spectroscopy was tested. In particular, the Eu3+ emission of the CN10 NPs deposited on a microscope glass slide was measured after two-photon excitation using a microscope. The emission is shown in Figure 12, where the orange-red emission in the 570-650 nm range can be easily observed upon 790 nm pulsed laser excitation.


As it can be evidenced in Figure 12, the emission appears to be strong and quite homogeneous at a 20x magnification in the regions covered by the NPs and still uniform up to an ulterior 5% of electronic zooming. Moreover, the observed optical contrast is suitable for possible applications of the present NPs in nanomedicine, in particular for imaging of histological specimens or for in-vivo intravital microscopy.

CONCLUSIONS Eu3+ doped CaF2 and SrF2 NPs in single phase were prepared by a one-step, simple and environmental friendly hydrothermal technique. The NPs are easily dispersible in water solutions, without the need of any post-synthesis reaction. The size of the NPs can be properly tuned by changing the reaction time. Site selective laser spectroscopy revealed that the Eu3+ ions are accommodated in at least two sites of different symmetry, and one site has been identified to be of D2 symmetry. Moreover, the relative populations of the different sites


19


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changes by varying the size of the NPs. As a general behavior, the asymmetry ratio decreases and the effective decay time increases on increasing the NPs size. The 5D0 level effective decay time measured for the CN480 sample (27.2 ms) is among the longest ever measured for Eu3+ ions. This long decay time paves the way to the use of the present NPs as nanomaterials in time resolved imaging spectroscopy (e.g. Fluorescence Lifetime Imaging Microscopy, FLIM39),

to avoid the very fast autofluorescence of biological tissues in

fluorescence microscopes. Preliminary two-photon excitation images prove that the NPs under investigation can be useful for potential in-vitro as well in-vivo applications using NIR excitation sources in the first biological window.

Acknowledgements University of Verona, in the framework of the “Ricerca di base 2015” project is gratefully acknowledged for financial support. Prof. L. Perbellini, University of Verona, Italy, is gratefully acknowledged for ICP-MS elemental analysis measurements. FV is grateful for financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Fonds de recherche du Québec – Nature et technologies (FRQNT). P. Canton is grateful for financial support from Ca’ Foscari University of Venice ADIR-2016.

ASSOCIATED CONTENT Supporting Information. Information on ICP-MS elemental analysis, X-ray diffraction patterns and Rietveld analysis, excitation spectra and site selective spectroscopy at 77 K and room temperature.

AUTHOR INFORMATION


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Corresponding Author * Adolfo Speghini, Nanomaterials Research Group, Dipartimento di Biotecnologie, Università di Verona and INSTM, UdR Verona, Strada Le Grazie 15, I-37134 Verona, Italy

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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Figure 1. TEM images of a) CN10, b) CN480, c) CK10, d) CK480, e) SN10 and f) SN480 samples. To guide the eye, nanoparticles are delimited by yellow circles.


22

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k2χ(k) (arb. un.)

1.5 a

1.0 0.5 0.0 -0.5 -1.0 -1.5 2

4

6

8

10

12

14

-1

k (Å )

|FT[k2χ(k)]| (arb. un.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18 16 14 12 10 8 6 4 2 0

b

0

2

4

6

8

r (Å) Figure 2. Eu-K edge spectrum of the CaF2 doped sample at 20 K (a), and the corresponding Fourier transform (b). Black line: experimental results. Red line: fit according to the model described in text.


23


k 2 χ(k) (arb. un.)

1.5 a

1.0 0.5 0.0 -0.5 -1.0 -1.5 2

4

6

8

10

12

14

-1

k (Å )

|FT[k 2 χ(k)]| (arb. un.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 36

18 16 14 12 10 8 6 4 2 0

b

0

2

4

6

8

r (Å) Figure 3. Eu-K edge spectrum of the SrF2 doped sample at 20 K (a), and the corresponding Fourier transform. Black line: experimental results. Red line: fit according to the model described in text.


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3.95

r (Å)

3.90 3.85 2.40 2.35 2.30 2.25 0

50

100

150

200

250

300

200

250

300

200

250

300

T (K) 6x10-3 6x10-3

σ2 (Å 2)

5x10-3 5x10-3 4x10-3 4x10-3 3x10-3 3x10-3 0

50

100

150

T (K) 10-2 8x10-3

σ2 (Å 2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6x10-3 4x10-3 2x10-3 0 0

50

100

150

T (K) Figure 4. EXAFS fitting results for the Eu3+ doped CaF2 sample as a function of T. Upper panel: distances; the horizontal lines correspond to the crystallographic distances in the undoped CaF2. Middle and lower panel: EXAFS Debye-Waller factors for the first and second shell, respectively. The continuous lines are fits according to the Einstein model.


25


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Figure 5. Emission spectrum of the CN10 sample (λexc = 465 nm) (left). Energy level scheme for Eu3+ ions (right).


26

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a)

site 1 site 2 450

460

470

480

b)

Intensity (arb.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c) 7

5

L6

F0 → 5

5 5

5

F2 Hj

D1

5

G2-6 D4

5 5

D3

D2

275 300 325 350 375 400 425 450 475 500 525 550

Wavelength (nm) Figure 6. Room temperature excitation spectra for a) CN10, b) CK10, c) SN10 samples. Blue lines: 589 nm emission. Red lines: 611 nm emission. Inset: 7F0 → 5D2 transition for the CN10 sample. The spectra were acquired under the same experimental conditions.


27


CN10

CN480

x5

Intensity (arb. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

560

580

600

CK10

CK480

SN10

SN480

620

640

660

680

700

560

580

Wavelength (nm)

600

620

640

660

680

700

Wavelength (nm)

Figure 7. Room temperature site selection emission for samples prepared with 10 min reaction time (left) and 8 hours reaction time (right). Blue lines: λexc = 461.0 nm. Red lines: λexc = 464.5 nm.


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1.0 0.9 0.8

Asymmetry Ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.7 0.6 0.5 0.4 0.3 0

10

20

30

40

300

350

400

450

500

550

Reaction Time (min)

Figure 8. Asymmetry ratios for the CN samples (blue dots), CK samples (green stars), and SN samples (red triangles) synthesized at different reaction times (λexc = 461.0 nm). Uncertainties are indicated as error bars.


29


40000

Intensity (arb.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

30000

x20 20000

10000

0 550 560 570 580 590 600 610 620 630 640 650 660 670 680 690 700 710

Wavelength (nm)

Figure 9. 77 K emission spectra for the CN480 sample. Blue line: λexc = 461.0 nm; Red line: λexc = 464.5 nm.


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5

4000

Intensity (arab.u.)

40000

Intensity (arb.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30000

D0 → 7F0

3000

2000

1000

0 578.0

578.5

579.0

20000

579.5

580.0

580.5

581.0

Wavelength (nm)

x20 10000

0 580

585

590

595

600

605

610

615

620

625

Wavelength (nm)

Figure 10. Emissions bands due to the 5D0 → 7F0 (around 580 nm), 5D0 → 7F1 (around 590 nm) and 5D0 → 7F2 (605-630 nm range) transitions for the CN480 sample at 77 K. Blue line: λexc = 461.0 nm; Red line: λexc = 464.5 nm. Inset: emission band due to the 5D0 → 7F0 transition.


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Figure 11. Room temperature emission decays of the 5D0 level for a) CN samples, b) CK samples, c) SN samples (λexc=461.0 nm, λem=591.0 nm). Effective decay times are also reported.


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Figure 12. Room temperature two-photon excitation image for the CN10 sample, deposited on microscope glass slide (λexc = 790 nm, emission in the 570-650 nm range).


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REFERENCES (1) Hillhouse, H. W.; Beard, M. C. Current Opinion in Colloid & Interface Science 2009, 14, 245-259. (2) Di, W. H.; Li, J.; Shirahata, N.; Sakka, Y.; Willinger, M. G.; Pinna, N. Nanoscale 2011, 3, 1263-1269. (3) Bunzli, J. C. Acc. Chem. Res. 2006, 39, 53-61. (4) Ma, Z. Y.; Dosev, D.; Nichkova, M.; Gee, S. J.; Hammock, B. D.; Kennedy, I. M. J. Mater. Chem. 2009, 19, 4695-4700. (5) Dong, N. N.; Pedroni, M.; Piccinelli, F.; Conti, G.; Sbarbati, A.; Ramirez-Hernandez, J. E.; Maestro, L. M.; Iglesias-de la Cruz, M. C.; Sanz-Rodriguez, F.; Juarranz, A.; Chen, F.; Vetrone, F.; Capobianco, J. A.; Sole, J. G.; Bettinelli, M.; Jaque, D.; Speghini, A. ACS Nano 2011, 5, 8665-71. (6) Reisfeld, R. Opt. Mater. 2010, 32, 850-856. (7) Dong, H.; Sun, L. D.; Yan, C. H. Chem. Soc. Rev. 2015, 44, 1608-1634. (8) Naccache, R.; Yu, Q.; Capobianco, J. A. Adv. Opt. Mater. 2015, 3, 482-509. (9) Lorbeer, C.; Cybinska, J.; Mudring, A.-V. J. Mater. Chem. C 2014, 2, 1862-1868. (10) Sharma, R. K.; Mudring, A.-V.; Ghosh, P. J. Lumin. 2017, 189, 44-63. (11) Sarkar, S.; Chatti, M.; Adusumalli, V. N.; Mahalingam, V. ACS Appl. Mater. Interfaces 2015, 7, 25702-8. (12) Ritter, B.; Krahl, T.; Rurack, K.; Kemnitz, E. J. Mater. Chem. C 2014, 2, 8607-8613. (13) Lorbeer, C.; Behrends, F.; Cybinska, J.; Eckert, H.; Mudring, A. V. J. Mater. Chem. C 2014, 2, 9439-9450. (14) van der Ende, B. M.; Aarts, L.; Meijerink, A. Phys. Chem. Chem. Phys. 2009, 11, 11081-11095. (15) Chatterjee, D. K.; Rufaihah, A. J.; Zhang, Y. Biomaterials 2008, 29, 937-43. (16) Dolcet, P.; Mambrini, A.; Pedroni, M.; Speghini, A.; Gialanella, S.; Casarin, M.; Gross, S. RSC Adv. 2015, 5, 16302-16310. (17) Cirillo-Penn, K. M.; Wright, J. C. Phys. Rev. B 1990, 41, 10799. (18) Laval, J. P.; Mikou, A.; Frit, B.; Roult, G. Solid State Ionics 1988, 28-30, 1300-1304. (19) Hamers, R.; Wietfeldt, J.; Wright, J. J. Chem. Phys. 1982, 77, 1-10. (20) Ju, Q.; Liu, Y. S.; Li, R. F.; Liu, L. Q.; Luo, W. Q.; Chen, X. Y. J. Phys. Chem. C 2009, 113, 2309-2315. (21) Jouart, J. P.; Bissieux, C.; Mary, G.; Egee, M. J. Phys. C: Solid State Phys. 1985, 18, 1539-1551. (22) Werts, M. H. V.; Jukes, R. T. F.; Verhoeven, J. W. Phys. Chem. Chem. Phys. 2002, 4, 1542-1548. (23) Ritter, B.; Haida, P.; Fink, F.; Krahl, T.; Gawlitza, K.; Rurack, K.; Scholz, G.; Kemnitz, E. Dalton Trans 2017, 46, 2925-2936. (24) Sarkar, S.; Hazra, C.; Mahalingam, V. Chem. Eur. J. 2012, 18, 7050-7054. (25) Pedroni, M.; Piccinelli, F.; Passuello, T.; Polizzi, S.; Ueda, J.; Haro-Gonzalez, P.; Maestro, L. M.; Jaque, D.; Garcia-Sole, J.; Bettinelli, M.; Speghini, A. Cryst. Growth Des. 2013, 13, 4906-4913. (26) Wells, J.-P.; Reeves, R. Phys. Rev. B 2001, 64, 1-10. (27) Wells, J.; Reeves, R. J. Phys. Rev. B 2000, 61, 13593-13608. (28) Gastev, S. V.; Choi, J. K.; Reeves, R. J. Physics of the Solid State 2009, 51, 44-49. (29) Jones, G. D.; Reeves, R. J. J. Lumin. 2000, 87-89, 1108-1111.


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For Table of Contents Use only Luminescence of Eu3+ activated CaF2 and SrF2 nanoparticles: effect of the particle size and co-doping with alkaline ions

Paolo Cortelletti, Marco Pedroni, Federico Boschi, Sonia Pin, Paolo Ghigna, Patrizia Canton, Fiorenzo Vetrone, Adolfo Speghini

CaF2 Eu3+ 1.5

k2χ(k) (arb. un.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 36

a

1.0 0.5 0.0

SrF2

-0.5 -1.0 -1.5 2

4

6

8

10

12

14

k (Å-1)

Eu3+ doped CaF2, SrF2 nanoparticles are prepared, adding Na+ or K+ ions as charge compensators. Site selective spectroscopy reveals at least two Eu3+ sites with different symmetry. The D2 site shows an emission decay time of 27.2 ms. Two-photon excitation of the CaF2:Eu3+ nanoparticles generates a strong emission in the orange-red region, useful for nanomedicine.


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Luminescence of Eu3+ Activated CaF2 and SrF2 Nanoparticles: Effect

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