Source: https://pubs.rsc.org/en/content/articlehtml/2019/na/c8na00118a
Timestamp: 2019-04-20 04:57:13+00:00

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Upconversion nanothermometry combines the possibility of optically sensing temperatures in very small areas, such as microfluidic channels or on microelectronic chips, with a simple detection setup in the visible spectral range and reduced heat transfer after near-infrared (NIR) excitation. We propose a ratiometric strategy based on Eu3+ ion luminescence activated through upconversion processes. Yb3+ ions act as a sensitizer in the NIR region (980 nm), and energy is transferred to Tm3+ ions that in turn excite Eu3+ ions whose luminescence is shown to be thermally sensitive. Tridoped SrF2:Yb3+,Tm3+,Eu3+ nanoparticles (average size of 17 nm) show a relative thermal sensitivity of 1.1% K−1 at 25.0 °C, in the range of the best ones reported to date for Ln3+-based nanothermometers based on upconversion emission. The present nanoparticle design allows us to exploit upconversion of lanthanide ions that otherwise cannot be directly excited upon NIR excitation and that may provide operational wavelengths with a highly stable read out to fill the spectral gaps currently existing in upconversion-based nanothermometry.
The development of all-optical nanoparticle-based thermometers allows for the measurement of localized temperature with a high spatial resolution in sub-millimeter areas, which can serve as a tool for the characterization of microfluidic channels or electronic microcircuit surfaces.1–3 Particularly interesting are optical luminescent nanothermometers, that, once excited in their absorption region, exhibit a temperature-dependent emission, usually in the ultraviolet (UV), visible or near infrared (NIR) regions. Several parameters related to emission properties can serve as thermal probes, such as intensities, intensity ratios, bandwidths, luminescence lifetimes or band shifts.4–6 However, not all of them are equally advantageous when it comes to real applications. For instance, using the intensity of a single emission band can be misleading if the concentration of nanoparticles in the area under investigation is not well controlled, since it can create intensity fluctuations not related to temperature but to a different number of emitters. For this reason, it is a good option to analyze parameters that are independent of the concentration of nanoparticles, as is the case with luminescence lifetimes, bandwidths, band intensity ratios or peak shifts.3 However, from the implementation point of view, the measurement of light intensity presents the advantage of less complex optical setups than lifetime measurements and often has higher sensitivities than bandwidth or peak shift measurements. In the present investigation, a ratiometric technique was proposed to evaluate the temperature,5 where the intensity of an emission band is used as a reference for a different, separate band, to avoid the mentioned concentration-triggered uncertainty. This luminescence intensity ratio (LIR) offers a further point of reliability also by removing any inaccuracy caused by uncontrolled fluctuations of the excitation light. LIRs are often exploited for lanthanide (Ln3+)-based nanothermometers, where several thermally coupled pairs of states have already been investigated and reported in the literature.7 The emission bands of Ln3+ ions are typically narrow and well defined, and this feature permits us to restrict the range in which emission spectra need to be measured to estimate temperature values. Moreover, narrow emission bands facilitate the option of multiplexing, if more than one probe has to be used.
Tm3+, Yb3+ and Eu3+-tridoped SrF2 upconverting nanoparticles (UCNPs) were synthesized following a hydrothermal method.29 Briefly, SrCl2·6H2O, YbCl3·6H2O, TmCl3·6H2O and EuCl3·6H2O (Aldrich, 99.9%) were used as metal precursors (with Sr2+ : Yb3+ : Eu3+ : Tm3+ = 0.745 : 0.220 : 0.030 : 0.005 as the nominal molar ratio). As a reference sample, Yb3+ and Eu3+-codoped SrF2 nanoparticles (with Sr2+ : Yb3+ : Eu3+ = 0.750 : 0.220 : 0.030 as the nominal molar ratio) were synthesized following the same procedure and are denoted as SrF2:Yb,Eu nanoparticles. Stoichiometric amounts of the metal chlorides (3.5 mmol of total metal ions) were dissolved in 7 mL of deionized water. This solution was then added to 25 mL of a 0.8 M sodium citrate dihydrate solution (Fluka, >99%) and 3.0 mL of a 3.5 M NH4F solution (Aldrich, 99.9%). The obtained solution was heat treated at 190 °C for 6 hours in a stainless-steel Teflon-lined digestion pressure vessel (DAB-2, Berghof). Subsequently, the UCNPs were precipitated with acetone and directly dispersed in deionized water. The colloidal dispersion is stable for at least one month.
2.2.1 Structural and morphological investigation. X-ray powder diffraction (XRPD) measurements were carried out with a Thermo ARL X'TRA powder diffractometer equipped with a Cu-anode X-ray source with a Peltier Si(Li) cooled solid state detector. Before the measurements, the samples were homogenized in a mortar with few drops of ethanol. After evaporation of the ethanol, the sample was deposited on a low background sample stage.
TEM (HRTEM) images were obtained using a JEOL 3010 high resolution electron microscope (0.17 nm point-to-point resolution at the Scherzer defocus), operated at 300 KV, equipped with a Gatan slow-scan CCD camera (model 794) and an Oxford Instruments EDS microanalysis detector (Model 6636). The powder was dispersed in water in order to be deposited on holey-carbon copper grids.
2.2.2 Spectroscopy measurements. Emission spectra (spectral resolution of 5 cm−1) were measured using a 980 nm laser diode (MDLIII980, CNI) as the excitation source and a half meter monochromator (Sr-500i, ANDOR) equipped with a CCD camera (DU420A-BVF, ANDOR) as the recording setup. Emission spectra at different temperatures were recorded by heating the solution with a thermal bath and measuring the temperature with a K-type thermocouple (0.2 °C sensitivity).
The X-ray diffraction pattern (shown in Fig. S1, ESI†) shows that the prepared UCNPs have a cubic fluorite phase, as reported for similar nanoparticles.28 EDX measurements clearly indicate the presence of Yb3+ and Eu3+, while Tm3+ ions are present at a concentration below the limit of detection of the EDX setup (Fig. S2, ESI†). Nonetheless, the presence of Tm3+ ions is clearly demonstrated by the strong UC emission (see below). A representative TEM micrograph of the UCNPs is shown in Fig. 1a, presenting a nice dispersion and average particle size of 16 nm (see Fig. 1b).
Fig. 1 (a) Representative TEM image of the SrF2:Yb3+, Tm3+, Eu3+ UCNPs. Inset: HRTEM image showing the (111) lattice planes. (b) Particle size distribution calculated using Pebbles software and log-normal fit (average particle size: 16 ± 4 nm).
Upon laser excitation at 980 nm, a large number of emission bands in the near UV, blue and red optical regions are observed for the SrF2:Yb3+,Tm3+,Eu3+ UCNPs28,30–32 as shown in Fig. 2. After 980 nm laser excitation, several Tm3+ excited states can be populated following energy transfer processes from Yb3+ to Tm3+ ions, as described by grey dashed arrows in Fig. 3a. Several emission bands are thus related to transitions from different Tm3+ excited states either to the ground state (3H6) or to lower lying excited states (see blue labels in Fig. 2).
Fig. 2 (a) Upconversion emission of a water colloidal dispersion of the SrF2:Yb3+,Tm3+,Eu3+ UCNPs (1 wt%, dopant percentage: Yb3+ 22%, Tm3+ 0.5%, and Eu3+ 3%) after laser excitation at 980 nm (power density of 450 mW mm−2). (b) Same as (a) in the 500–650 nm range. Blue: Tm3+ ion transitions. Orange: Eu3+ ion transitions from the 5D0 level. Green: Eu3+ ion transitions from the 5D1 level. Red: Eu3+ ion transition from the 5D2 level. (c) Picture of the D2O colloidal dispersion of the SrF2:Yb3+,Tm3+,Eu3+ UCNPs (concentration of 1 wt%) after laser excitation at 980 nm (power density of 450 mW mm−2).
Fig. 3 (a) Energy level scheme for Yb3+, Tm3+ and Eu3+ ions, Yb3+ excitation (red arrow) and energy transfer processes (grey dashed arrows). (b) Power study of several Eu3+ upconversion bands of the SrF2:Yb3+,Tm3+,Eu3+ UCNPs upon 980 nm laser excitation at 25 °C.
In addition, a group of less intense bands in the 400–440 nm and 500–630 nm regions are nicely observed, as shown in Fig. 2b and S3 (ESI†), typical of emission of Eu3+ ions, which constitutes clear evidence of the population of the excited energy levels of Eu3+ ions through upconversion processes. The transition assignments for the observed bands have been carried out considering the spectroscopic investigation of Jouart et al.33 and Cortelletti et al.39 for Eu3+ centres in SrF2 using site-selective excitation techniques.
Some weak emission bands observed in the blue region around 415 and 430 nm correspond to emissions from the 5D3 level of the Eu3+ ions, indicating that an energy transfer process from the 1D2 level of Tm3+ to the 5D4, 5GJ or 5LJ levels of Eu3+ is present. A contribution to the population of the Eu3+ excited levels could be in principle also due to an energy transfer process from the 1I6 level of Tm3+, as emission from this level is observed in the UC spectrum (see Fig. 2). Nonetheless, it is reasonable to consider this contribution as much less relevant with respect to those due to energy transfer starting from the lower lying 1D2 and 1G4 excited energy levels of Tm3+ ions. This behavior is due to the much lower population of the 1I6 level with respect to the other two levels, evidenced by the very low relative intensity of the 1I6 → 3F4 band (see Fig. 2a). Moreover, the energy of the 1G4 level of Tm3+ ions is slightly higher than that of the 5D1 level of Eu3+ ions; thus a Tm3+(1G4) → Eu3+(5D1) energy transfer process is reasonably present, with possible phonon emission. A Tm3+(1G4) → Eu3+(5D2) energy transfer process can also be possible considering that the Tm3+(1G4) and Eu3+(5D2) levels are almost resonant, with small phonon absorption assistance.
Fig. 4 (a) Emission spectra of the SrF2:Yb3+,Tm3+,Eu3+ UCNPs at three different temperatures (intensity normalized to the 585 nm band) upon 980 nm excitation. (b) Luminescence intensity ratio, LIR = A(5D0 → 7F1)/A(5D1 → 7F3) vs. T. (c) LIR vs. laser excitation power.
where ΔLIR represents the experimental uncertainty of the LIR. The average value of ΔTmin is evaluated to be 1.9 ± 0.2 °C. It is important to mention that the uncertainty parameter ΔLIR depends on the instrumental setup of the experiments through the signal to noise ratio, and thus it can be improved with a longer integration time, higher laser excitation or better detection equipment.
For this reason, multiphonon relaxation processes for lanthanide ions are much more probable if they are close to H2O molecules than for D2O ones, as the higher the vibrational energy is, the larger is the multiphonon relaxation probability of the Ln3+ excited level. We show in Fig. 5a the comparison between upconversion spectra of the SrF2:Yb3+,Tm3+,Eu3+ UCNPs using H2O and D2O as dispersing solvents, while using identical experimental conditions with respect to the geometrical setup and in particular the same power density of the laser excitation radiation. From Fig. 5a, it can be noted that in the case of D2O dispersions, the Eu3+ upconversion bands corresponding to transitions starting from the 5D1 level are more intense than for those starting from the 5D0 one. The upconversion spectra for D2O dispersions as a function of temperature (20–60 °C) do not change notably on increasing the temperature (see Fig. S7†), suggesting that the populations of the 5D0 and 5D1 levels of the Eu3+ ions do not change significantly with the temperature, at least in the investigated range. The LIR for the D2O dispersed UCNPs shows an almost constant value (around 1.0), within the experimental uncertainties, on increasing the temperature (Fig. 5b). Such a behavior indicates that the relaxation channel for the 5D1 level is much more effective in H2O dispersions than in D2O ones. This behavior is clear evidence that a significant number of Ln3+ ions lie on the nanoparticle surface, close to the solvent molecules, as their emission properties are much influenced by the solvent vibrational energies, inducing non-radiative multiphonon relaxation channels. The depopulation of the 5D1 energy level of Eu3+ is much more influenced by multiphonon relaxations than that of the 5D0 one, due to a much lower energy gap with the lower lying energy level (5D1–5D0, energy gap around 1800 cm−1; 5D0–7F6, energy gap around 12 000 cm−1). Therefore, the multiphonon relaxation probability for the 5D1 level is almost constant in the relatively small investigated temperature range for D2O dispersions, while it is increased for H2O dispersions, due to the much higher vibrational energy of H2O with respect to D2O.
Fig. 5 (a) Eu3+ upconversion for SrF2:Yb3+,Tm3+,Eu3+ UCNPs dispersed in H2O (black line) and in D2O (red line). (b) LIR for H2O (black squares) and D2O (red solid circles)-dispersed UCNPs.
The obtained thermometric values for the SrF2:Yb3+,Tm3+,Eu3+ UCNPs demonstrate that the strategy applied to excite the Eu3+ ions offers good opportunities for thermometry in three aspects. First, a careful selection of Ln3+ dopants allows us to engineer a mechanism that exploits upconversion to excite Eu3+ ions. Second, the different upconversion paths used to excite several Eu3+ states allow the definition of a luminescence intensity ratio that remains unaffected during measurements, also for variations of the laser excitation power. Finally, the thermal sensitivity of such an intensity ratio is on par with the best upconverting nanothermometers reported to date.
In the present study, we investigated colloidal upconverting nanothermometers based on Yb3+, Tm3+ and Eu3+ ions that exploit the matching of the Tm3+ ion energy levels with the ones of Eu3+ ions. This property permits us to transfer the absorbed energy by the antenna Yb3+ ions to the final probe, Eu3+ ions. The developed nanothermometer shows a very good relative sensitivity, around 1% K−1 in the 20–60 °C range, among the highest values shown by the most popular lanthanide-based nanothermometers. Moreover, the relative sensitivity is independent of intensity fluctuations of the excitation radiation owing to the characteristics of the designed upconversion process. Very importantly, this excitation strategy constitutes a new way of engineering upconversion-based nanothermometers that exploit new ions and that are able to operate at different wavelengths.
University of Verona (Italy) is gratefully acknowledged for financial support in the framework of the “Ricerca di base 2015” project. F. V. 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.
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‡ Current Address: BioNanoPlasmonics Laboratory, CIC biomaGUNE, Paseo de Miramón 182, 20014 Donostia – San Sebastián (Spain).

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