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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Colloidal Rare Earth Vanadate Single Crystalline Particles as Ratiometric Luminescent Thermometers Paulo Cesar de Sousa Filho, Juliette Alain, Godefroy Leménager, Eric Larquet, Jochen Fick, Osvaldo Antonio Serra, and Thierry Gacoin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12251 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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The Journal of Physical Chemistry

Colloidal Rare Earth Vanadate Single Crystalline Particles as Ratiometric Luminescent Thermometers Paulo C. de Sousa Filho*†, Juliette Alain‡, Godefroy Leménager‡, Eric Larquet‡, Jochen Fick§┴, Osvaldo A. Serra∥, Thierry Gacoin‡ † Department

of Inorganic Chemistry; Institute of Chemistry; University of Campinas (Unicamp). R. Monteiro Lobato, 270, 13083-970. Campinas, SP, Brazil. ‡ Solid State Chemistry Group, Laboratoire de Physique de la Matière Condensée; Ecole Polytechnique, CNRS, Université-Paris Saclay. 91128 Palaiseau cedex, France. § Université

Grenoble Alpes; Institute Néel. 38042 Grenoble, France.

┴CNRS,

Institut Néel, 38042 Grenoble, France. ∥ Department of Chemistry; Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto; University of São Paulo. Av. Bandeirantes, 3900, 14040-901, Ribeirão Preto, SP, Brazil. KEYWORDS. Thermometry; Nanoparticles; Vanadates; Rare Earth; Luminescence; Thermal Sensors; Optical Trapping.

ABSTRACT: Thulium/ytterbium-doped yttrium vanadate particles provide ratiometric thermal response as both colloids and powders via downshift or upconversion emissions. Here, we synthesized yttrium vanadates by controlled colloidal conversion of hydroxycarbonate precursors. A protected annealing process yielded single crystalline and readily dispersible particles that were manipulated individually by optical tweezers in water. Because individual particles displayed detectable emissions, this system has potential applications as a single-particle luminescent temperature sensor. Excitation on Yb3+ sensitizers (exc=980 nm) or at vanadate groups (exc=300 nm) resulted in Tm3+ emissions that effectively correlated with the temperature of the sample from 288 to 473 K with high relative thermal sensitivity (0.8-2.2% K-1), one of the highest reported for vanadate nanocrystals so far. Different pairs of Tm3+ transitions afford a ratiometric thermal response, which fitted common sensing requirements such as large [3F2,3→3H6 (=700 nm)/1G4→3H6 (=475 nm)] or small [3F2,3→3H6 (=700 nm)/1G4→3F4 (=650 nm)] spectral gaps, and emission wavelengths at the first near infrared biological window [3F2,3→3F4 (=700 nm)/3H4→3H6 (=800 nm)]. Our findings open new perspectives for the use of luminescent nanothermometers with controllable spatial localization, which is a remarkably interesting prospect to investigate microscopically-localized events related to changes in temperature. Rare earth-based luminescent nanomaterials are applicable in several optical detection systems1-4 due to the unique properties associated to this group of elements.5,6 Temperature sensing is a remarkably important subject in the chemistry of nanosized phosphors,1,7-9 especially where contact thermometry is not suitable.10-12 Hence, nanothermometers have highly promising applications in electronics,13 optics,14 and fluidics15 at micro- and nanoscales, not to mention fundamental studies of Brownian motion of colloidal particles in the ballistic regime.16 This interest is notably greater for biological applications because numerous physiological events involve highly localized fluctuations in the intracellular temperature.12,17,18 Improved description of the cellular activity thereby depends on new non-invasive thermometers with space and time resolution.10 In this sense, nanoparticles with tunable size, shape and interactions can be specifically employed to trace intracellular chemical reactions involving absorption or release of heat. Combining thermal sensing activity with other functionalities of inorganic nanoparticles is a very interesting prospect towards multimodality3 and theranostics.19-21

If emitting levels of single trivalent lanthanoid centers can be considered to be thermally coupled, transition probabilities will change according to the temperature of the sample.1-3,10,12 This effect can be pronounced not only at ordinary temperatures (273-400 K), but also under cryogenic22 and pyrogenic23 conditions. However, the concentration of the sensor, its interaction with the dispersing medium, the inhomogeneity of the sample, and instrumental parameters strongly affect the absolute intensities of the emissions, which makes accurate evaluation of the changes in intensity difficult.10 In this context, it is better to apply ratiometric sensors in nanothermometry because they dismiss repeated calibration procedures.24 Whilst many rare earth ions have been reported to be photostable and little toxic25,26 ratiometric temperature sensors,3-6,10,12 trivalent erbium has by far been the most investigated emitter for this purpose. Indeed, NaYF4:Yb3+,Er3+ is an archetypical luminescent material for thermographic phosphor thermometry.8,10,12 Nonetheless, lanthanoid-based nanothermometers can still be significantly improved for biological applications. For instance, the Er3+ emissions

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usually considered in ratiometric thermometry occur at 525 nm (2H11/2→4I15/2) and 545 nm (4S3/2→4I15/2), which do not lie in the so-called first near infrared (NIR) biological window (700980 nm). Thulium ions overcome this problem,23,27-29 since they can afford upconversion luminescence in the red and infrared regions, so that both the excitation (980 or 808 nm) and the emission lines lie at the NIR window for ratiometry. In addition, even though low phonon energy fluoride hosts are well known for providing efficient upconversion luminescence, fluoride nanoparticles are often stabilized with capping agents,30 which affects particle stability in intracellular conditions and hinders further functionalization processes. Consequently, modification of fluoride surfaces is a complex issue and commonly requires intermediate layers.30,31 On the other hand, oxides are frequently more compatible with aqueous medium, and direct functionalization of watersynthesized oxide surfaces for biological tracking has been well described in the literature.32 Besides such advantages of oxide-based hosts, rare earth orthovanadates present efficient upconversion luminescence33-35 and can be synthesized as highly crystalline, non-aggregated and readily dispersible particles34 excitable both by NIR and UV excitation.35 Consequently, several works report on different strategies for the use of rare earth vanadates in ratiometric thermometry in a wide temperature range using, for example, Tm3+,28,29 Nd+3,36 Sm3+,37,38 Eu3+,39 Dy3+,40 Ho3+,41 or Er3+,42 as emitting centers. However, the rare earth vanadate-based thermometers currently described generally consist in highly aggregated particles and bulk solids,29,36-42 which usually do not afford stable colloidal suspensions. In turn, this precludes the use of such compounds as single particle luminescent thermometers. Even though particle aggregation can be overcome by coating vanadate nanoparticles with SiO2,28 there is still a lack of examples of readily dispersible vanadate nanoparticles acting as luminescent thermometers. In addition, there are only few examples41 exploiting the potentiality of dual-mode thermometry in rare earth vanadates under NIR and UV excitation, which can expand the application range of these solids concerning temperature sensing. So, here we propose that YVO4 nanoparticles doped with Yb3+/Tm3+ synthesized via colloidal conversion of hydroxycarbonate precursors be used for luminescent thermometry. We prepared single crystalline and readily dispersible particles which afforded thermometric response as powders and colloids by luminescence both under UV (downshift) and NIR (upconversion) excitation. Besides high thermal sensitivities that favorably compare to other ratiometric thermometers, the proposed system displays the additional advantage of emissions located on the near infrared biologic window. Hence, our results allowed us to develop new colloidal thermal detection systems based on upconversion and downshift luminescence, which can be detected as isolated particle by optical tweezers.

EXPERIMENTAL SECTION

Chemicals. Rare earth nitrate hydrates [Y(NO3)3.6H2O, Yb(NO3)3.5H2O, and Tm(NO3)3.5H2O, 99.99% purity], ammonium metavanadate (NH4VO3, 99%), tetraethyl orthosilicate (TEOS, Si(OCH2CH3)4, 98%), and hydrofluoric acid (HF, 40%) were used as received from Aldrich. Urea (CO(NH2)2, 99% Merck), ethylene glycol (EG, Fischer) and ethanol (EtOH, Fischer) were also used without further purification. Ammonia (NH3(conc.), 25% m/v) and the polymers

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PE6800 [block-poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide), (C2H4O)x-(C3H6O)yC2H4O)z-, MM~8080 g mol-1, BASF] and PAA [poly(ammonium acrylate), 80 mmol L-1 in water, prepared by dissolution of poly(acrylic acid), (CH2CHCOOH)n, MW 1800 g mol-1, with concentrated ammonia, pH=8.5] were also employed to prepare the final colloids. Hydroxycarbonate Precursors. Rare earth hydroxycarbonates [RECO3OH.xH2O, where RE= (Y0.78Yb0.20Tm0.02)] were prepared by adapting the protocol described by Gaspar et al.43 In this case, the preparation of ~100 nm RE hydroxycarbonate particles involved the use of a water/ethylene glycol mixture as solvent. Typically, 16.7 mmol L-1 rare earth nitrate solutions were prepared by dissolving solid rare earth nitrates in deionized water. EG was added to these aqueous solutions to give a final H2O/EG ratio of 3/2 (v/v). After homogenization, urea was added to a final concentration of 75 g L-1, and the mixture was left under stirring at room temperature for 30 min. The solution was heated up to 368 K (95 °C) and kept under stirring for 2 h. The particles were collected by centrifugation (15000 rpm, 25 min) and washed with deionized water until conductivity of ~100 mS cm-1 was achieved. REVO4 Particles and Protected Annealing. The hydroxycarbonate precursors were converted into elongated REVO4 [where RE= (Y0.78Yb0.20Tm0.02)] particles via colloidal treatment in water. To this end, suspensions of RECO3OH.xH2O (0.1 mol L-1) were ultrasonicated and heated at 373 K. After 30 min, a 0.1 mol L-1 NH4VO3 solution containing NH3 at a NH3/VO3- ratio of 2/1 (mol/mol) was added to the colloid under stirring. The system was kept at 373 K for 3 h and the particles were collected by centrifugation and washed with deionized water. A protected annealing step was conducted to improve the crystallization of the particles. This approach has been described elsewhere.35 Briefly, REVO4 nanoparticles stabilized with PAA were dispersed in silica gel obtained by acid hydrolysis of TEOS in EtOH in the presence of the polymer PE6800. The particles dispersed in silica were calcined in air at 1273 K for 4 h after an annealing step at 773 K (4 h). Silica was then removed by dissolution with 5% hydrofluoric acid (Si/F=1/10) for 6 h, which was followed by centrifugation and washing with deionized water. Characterizations. Infrared (FTIR) spectra were measured on a Bruker Equinox 55 spectrometer. Thermogravimetry and differential thermal analysis of the precursors and particles dispersed in silica were carried out on a Netzsch Luxx STA 409 PC analyzer. Structural characterization by X-ray diffractometry was performed on a PANalytical (Philips) X’Pert diffractometer using Cu-K radiation (1.5406+1.5444 Å). Transmission electron microscopy (TEM) images were acquired on a JEOL 2010F microscope operating at 200 kV. Size distributions were evaluated by dynamic light scattering (DLS) on a Malvern Zetazizer Nano ZS equipment. The mean particle sizes and standard deviations were computed by fitting DLS data for log-normal distributions. Luminescence spectra were measured in a Horiba Fluorolog 3 spectrometer equipped with a double-grating (F180D) emission monochromator and a Hamamatsu R928P photomultiplier as detector. Upconversion luminescence measurements were performed using a CrystaLaser diode laser at 980 nm (345 W cm-2) and down-shift emissions were measured under excitation of a Xenon arc lamp (450 W) using a double grating F180D excitation monochromator (Horiba).


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Figure 1. TEM images and size distributions of (a-d) precursor hydroxycarbonate particles, (e-h) pristine (Y0.78Yb0.20Tm0.02)VO4 particles, and (i-l) final (Y0.78Yb0.20Tm0.02)VO4 nanoparticles after the protected annealing procedure. (d), (h) and (l) show DLS number profiles of colloidal particles in water (~ 1 mmol L-1, black histograms) and the respective log-normal fits of experimental data (red dotted lines). [Inset values in (d), (h) and (l) correspond to fitting parameters of size distributions, namely mean diameters (dm), size standard deviations (g) and correlation coefficients (r2). dm and g are not shown in (h) due to the nonspherical shape of the particles]. _______________________________________________________________________________________________________________ phosphates, vanadates and fluorides), so they act as sacrificial Powder samples were placed in platinum crucibles in a templates in the synthesis of complex inorganic particles.47-49 Linkam Scientific temperature-controlled stage (TS1200), using Horiba optic fibers for excitation and signal acquisition. Whilst the colloidal conversion of submicron-sized (300Luminescence spectra of colloidal samples were measured in 500 nm) hydroxycarbonate particles into orthovanadates quartz cuvettes (1 cm, 4 mL) placed on a F3004 Peltierusually requires hydrothermal treatments, here smaller and controlled sample holder (Horiba) with magnetic stirring (800 more reactive precursors afforded REVO4 structures in milder rpm). Additional experimental details about data acquisition conditions. It is possible to obtain RE hydroxycarbonates with and processing are provided in the Supporting Information small particle sizes by enhancing the nucleation rates and (SI). The fiber optical tweezers setup is described in detail avoiding the re-dissolution of the particles. EG has been elsewhere.44-46 Briefly, a 808 nm trapping laser was divided reported to reduce the solubility of particles and to stabilize between two chemically etched optic fiber tips, which were their surface via coordination.43 In this work, we have applied introduced in a drop of the colloid. Particle trapping was similar conditions to obtain colloidally stable ~95 nm RE visualized by a CMOS camera (Hamamatsu Orca FLASH hydroxycarbonate non-crystalline nanoparticles (Figure 1a-d) 4.0LT) coupled to a long working distance 50x objective (N.A. with homogeneous spherical shape [SI: =0:55) perpendicular to the trapping beams. A 980 nm pump (Y0.78Yb0.20Tm0.02)CO3OH.xH2O, x≈1 (thermal analysis, Figure laser was injected into the objective to excite nanoparticles S1); mean diameter (dm): 95 nm, size standard deviation (g): upconversion. The particle emission was captured by one 0.24, polydispersity index (PDI): 0.24 (DLS, Fig 1d and Figure trapping fiber tip and measured by spectrometer coupled to S3); FTIR: 3700-2700 cm-1 (OH), 1650+1520+1395 cm-1 an EM-CCD camera (Princeton Instruments SP2150 & ProEM). (HOH+ CO), 1083 cm-1 ( CO), 846 cm-1 ( OCO) (Figures as

RESULTS AND DISCUSSION

S4 and S5)].

s

s

Structural and Morphologic Characterization. Rare earth hydroxycarbonate particles are chemically suitable towards colloidal conversion to more insoluble phases (e.g.


3


Figure 2. (a) X-ray diffraction patterns of the (Y0.78Yb0.20Tm0.02)VO4 particles compared to the (i) yttrium vanadate JCPDS standard (card nº 00-017-0341, green histogram): (ii) as-prepared particles (black), (iii) particles after dispersion in SiO2, and (iv) after protected annealing and dissolution of SiO2 (blue). (b) Williamson-Hall plots obtained from XRD results (using K=0.9) of as-prepared particles (black squares), particles dispersed in SiO2 (red circles) and final particles (blue triangles). [In (b), T denotes average crystallite sizes (intercepts) and  denotes strain contribution to peak broadening (slopes)].

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The colloidal conversion of the precursors yielded crystalline REVO4 particles with characteristic elongated shape (Figures 1e-h and S6) [SI, FTIR: 3600-2700 and 1640 cm-1 (OH+HOH, adsorbed water), 1399 cm-1 (asNO, adsorbed nitrates), and ~860 cm-1 (asVO, B2+E), Figure S4].The particle widths lie around 105±37 nm and the lengths lie around 176±67 nm (Figure S6), as determined from TEM images [in this case, size values obtained from DLS distributions (PDI=0.40, Figure S3) are not reliable due to the elongated shape of the particles]. The precursors lost their spherical morphology suggesting that the REVO4 phase originated via an intermediate mechanism between particle re-dissolution and Kirkendall conversion.49 X-ray diffractograms (Figure 2a) confirmed the formation of the tetragonal I41/amd xenotime-type phase, whereas the occurrence of a diffraction halo denotes that the particles also display amorphous domains. The WilliamsomHall (WH) plot (Figure 2b) revealed a crystalline coherence length of ~55 nm, which attested to the polycrystalline character in the REVO4 particles obtained herein. Because upconversion luminescent sensing usually requires highly crystalline solids, we employed a protected annealing approach to improve the crystallinity of the particles.34,35 After dispersion in a SiO2 matrix (Figure S7), the REVO4 particles kept the same structural features of the initially prepared solids (Figure 2), which indicated that the initial low pH conditions applied did not affect the state of the particles. The heat treatment eliminated water and organic compounds at ~873 K, and complete crystallization of the tetragonal REVO4 phase occurred up to 1073 K (Figure S2). After annealing and dissolution of SiO2, the REVO4 particles exhibited a nearlyspherical shape, with diameters around ~140 nm, as shown by TEM and DLS results (Figures 1i-l and S8) [dm=147 nm, g=0.25, PDI=0.27 (DLS); S.I.: FTIR: ~3500 and 1640 cm-1 (OH+HOH, adsorbed water), and 918+822 cm-1 (asVO, B2+E), Figure S4;]. In addition, the final particles became clearly faceted, with the presence of voids of variable sizes (1040 nm). This probably resulted from atom diffusion at high temperatures, which prompted the vacancies to aggregate into voids. This agreed with the increased slopes in the WH plot (Figure 2b), which also presented an improved linear correlation in comparison to as-prepared particles. Moreover, the intercept of the WH plot of the final particles also indicated a crystalline coherence length of ~127 nm. This is roughly in agreement with particle sizes observed in TEM and DLS, thus suggesting that final particles are nearly single crystals well dispersed in suspension.

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Figure 3. (a) Upconversion spectra of final (Y0.78Yb0.20Tm0.02)VO4 particles under exc=980 nm excitation at different powers. (b) Bi-logarithmic plots of integrated intensities versus excitation powers (exc=980 nm) for different Tm3+ transitions, namely 3H →3H (black rhombi, slope: 1.64), 1G →3H (blue squares, slope: 2.34), 1G →3F (red circles, slope: 1.74), 3F →3H (dark red 4 6 4 6 4 4 2,3 6 triangles, slope: 1.87). (c) Energy level diagrams illustrating the upconversion mechanism of the Yb3+/Tm3+ couple.


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Figure 4. Upconversion spectra of final (Y0.78Yb0.20Tm0.02)VO4 powder particles under exc=980 nm (20 W cm-2) at different temperatures ranging from 300 K (violet) to 473 K (red). (Inset: amplification of the 450-750 nm range showing 1G →3H , 1G →3F and 3F →3H transitions). 4 6 4 4 2,3 6 _____________________________________________________ Upconversion Luminescence. Improved crystallinity of the final REVO4 particles allowed upconversion under excitation at =980 nm. The light emitted by the particles is observable by the naked eye at pump powers higher than ~5 W cm-2, appearing as bluish-pink light at powers higher than 40 W cm2 (chromaticity diagram, Figure S9). Figure 3a depicts power dependence of upconversion spectra, showing the Tm3+ emissions in the blue (475 nm, 1G4→3H6), red (650 nm, 1G →3F , 700 nm 3F →3H ) and near-infrared (800 nm, 4 4 2,3 6 3H →3H ) regions. Figure 3b illustrates the bi-logarithmic 4 6 relation between the integrated intensities and power densities from 3 to 50 W cm-2. In this configuration, only the 3H4→3H6 and 1G →3H transitions presented suitable signal-to-noise ratio at 4 6 powers lower than ~15 W cm-2 (Figure 3a; data with low signalto-noise ratio were not included in the plot of Figure 3b). Also, the increase of the excitation intensity results in the change of the emission color from red to bluish-pink because of the progressively higher contribution of the blue 1G4→3H6 emission at increasing power densities (Figure S10). The integrated intensities increased bi-exponentially with pump powers with slopes smaller than the theoretical values expected for the population of 1G4, 3F2,3 and 3H4 emitting states (2.34, 1.74, 1.87, and 1.64 for the 1G4→3H6, 1G4→3F4, 3F2,3→3H6, and 3H4→3H6 transitions, respectively). This resulted from a competition between upconversion and non-radiative decay processes to deplete the intermediate states within the complex level structure of Tm3+ (Figure 3c). Normalized upconversion spectra collected at different laser powers (Figure S11(a), SI) revealed that the ratios between the intensities of the different Tm3+ transitions varied with the pump power in the evaluated range (3-45 W cm-2), as a consequence of the different energy migration pathways within the several electronic levels of Tm3+.50 The largest variation of intensity ratios with the pump power occurs for the ratio between the 3H4→3H6 and the 1G4→3H6 transitions (Figure S11(b), SI). This is because the low lying 3H4 level (~12500 cm-1) is more easily populated at than the other Tm3+ emitting states, resulting in higher relative contributions of the 3H4→3H6 transition at low powers.

Figure 5. Luminescence spectra (normalized intensities) of final (Y0.78Yb0.20Tm0.02)VO4 colloidal particles (5 mmol L-1 in water) under (a) exc=980 nm (40 W cm-2, upconversion) and (b) exc=300 nm (downshift) at temperatures ranging from 283 K (violet) to 333 K (red). Insets in (a) and (b) show amplifications of the 450-550 nm and 620-850 nm regions, respectively). _____________________________________________________ The values of intensity ratios became practically constant at higher excitation powers (up to 45 W cm-2), and an excitation pump power of 20 W cm-2 was chosen to perform temperature dependent measurements of powder samples. A similar behavior was observed for particles as colloids under exc=980 nm and exc=808 nm (Figures S11-S13, SI). Luminescence Thermometry. Figures 4 and S15 (SI) corresponds to the temperature-dependent upconversion spectra of the final REVO4 powder particles (300-473 K). The intensity of the different Tm3+ transitions did not change homogeneously, which allowed us to correlate the intensity ratios with the temperature of the sample. The alteration of relative intensities with the temperature resulted in the change of the emission color from magenta (273 K) to red (473 K), as shown in the chromaticity diagram of Figure S16 (SI). In addition, because the particles were highly crystalline and stable from a colloidal standpoint, we were able to verify that the upconversion spectra of the colloidal particles in water also depended on the temperature (Figure 5a). In this case, however, only intensities of the 1G4→3H6, and the 3H4→3H6 transitions could be effectively compared due to the limited signal-to-noise ratio.


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Figure 6. Dependence of the intensity ratios [(IM/IN)] on the absolute temperature for the different emissions in final (Y0.78Yb0.20Tm0.02)VO4 particles as (a) powders and (b)-(c) colloids under excitation at (a)-(b) exc=980 nm (upconversion) or (c) exc=300 nm (downshift). Dotted lines correspond to exponential fits of experimental data with the form (IM/IN)=C + ae(-b/T), where IM/IN refers to the ratio between integrated intensities of the Tm3+ transitions, namely 3H4→3H6 and 3F →3H (I 3 3 1 3 3 3 1 3 2,3 6 800/I700, purple squares), H4→ H6 and G4→ H6 (I800/I475, dark yellow circles), F2,3→ H6 and G4→ H6 (I700/I475, cyan triangles), and 3F2,3→3H6 and 1G4→3F4 (I700/I650, orange pentagons). Fitting parameters are presented in Table S1 (SI). _______________________________________________________________________________________________________________ Vanadate nanoparticles also display a broad absorption in the Boltzmann-type factor (E/kB) related to the energy UV region due to the parity allowed O2-→V5+ charge transfer separation between the two emitting states,52 but it may also (CT) transition, which usually gives rise to energy transfer to include the vibrational influence of the host lattice phonons lanthanoid ions.35,49 Indeed, the final (Y0.78Yb0.20Tm0.02)VO4 on the excited states in the case of high energy oscillators (e.g.: V-O: 900 cm-1).53 In addition, the term b may also contain the particles displayed blue luminescence under exc=300 nm 3+ effects introduced by intermediate levels populated by nonexcitation (Figure S9, SI), confirming that the Tm emitting radiative transitions.28 For that reason, we do not discuss the levels could also be populated by energy transfer from vanadate groups. Figure 5b shows the downshift emissions of b parameter in terms of E/kB in the present text. the final REVO4 particles as colloids, where the four The intensity ratio profiles (Figure 6a) revealed very good characteristic Tm3+ transitions were also observed. In both correlations with the exponential function of Equation (1) for cases involving colloidal particle (i.e., upconversion and the ratios between the intensities and the temperature from downshift), absolute intensities could not be considered for 300 and 473 K. Considering the four detectable upconversion comparison because of inherent intensity fluctuations due to emissions of Tm3+ in the 300-900 nm range, the ratio between convection and decantation. Hence, only intensitythe 3H4→3H6 and 1G4→3F4 transitions did not yield any normalized spectra were considered for computation of correlation with the applied temperature. No correlation was intensity ratios of colloidal particles. possible using the ratio between the 1G →3F and 1G →3H The non-homogeneous alterations in the relative intensities of the Tm3+ transitions allowed us to make ratiometric correlations between emissions of (Y0.78Yb0.20Tm0.02)VO4 particles with the temperature of the sample both via upconversion and downshift luminescence. Figure 6 and Table S1 (SI) summarize the correlations between the intensity ratios of the different Tm3+ transitions and the temperature in powders and colloidal particles as obtained from Figures 4 and 5. The results were fitted by means of an exponential relation represented by Equation (1):

( ) = 𝐶 + 𝑎𝑒 𝐼𝑀 𝐼𝑁

―𝑏 𝑇

,

(1)

where, IM and IN are the integrated intensities of the two transitions considered for the correlation with the temperature (T) and C is a constant. As inclusion of the additive constant C provided better fitting of experimental data, Equation (1) was used instead a regular exponential relation. This is justified because some intensity ratios involve emitting levels with populations that are not defined by the Boltzmann distribution only.24 Also, the additive constant was included because some intensity ratios involve overlapped transitions, and because the vanadate host lattice plays and important role on energy transfer processes in emitting levels,10 thus deviating the results from the conventional exponential behavior. The pre-exponential factor a depends on the multiplicity, radiative decay rates and frequency of the involved transitions.10,51 The term b is usually described as a

4

4

4

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transitions neither, as both have the same emitting state. However, the possibility of using the other four intensity ratios between the Tm3+ transitions in luminescence thermometry is one of the main advantages of the (Y0.78Yb0.20Tm0.02)VO4 particles. Figure 6a also confirms that prepared solids provide thermal sensing where both excitation and emission wavelengths fell within the NIR biological window. This is a crucial parameter for thermometry in biological media and is potential a drawback of green emitting Er3+-based thermographic phosphors.54,55 In addition, the I800/I700 (3H4→3H6/2F2,3→3H6) and the I700/I650 (2F2,3→3H6 / 1G →3F ) ratios involve transitions with low spectral 4 4 separations (

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