Upconverting Lanthanide Fluoride Core@Shell Nanorods for

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Functional Inorganic Materials and Devices

Up-Converting Lanthanide Fluoride Core@Shell Nanorods for Luminescent Thermometry in the First and Second Biological Windows - #-NaYF4: Yb3+, Er3+@SiO2 Temperature Sensor Marcin Runowski, Natalia Stopikowska, Daria Szeremeta, Szymon Goderski, Ma#gorzata Skwierczy#ska, and Stefan Lis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00445 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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ACS Applied Materials & Interfaces

Up-Converting Lanthanide Fluoride Core@Shell Nanorods for Luminescent Thermometry in the First and Second Biological Windows β-NaYF4: Yb3+, Er3+@SiO2 Temperature Sensor

Marcin Runowski,* Natalia Stopikowska, Daria Szeremeta, Szymon Goderski, Małgorzata Skwierczyńska, Stefan Lis Adam Mickiewicz University, Faculty of Chemistry, Department of Rare Earths, Umultowska 89b, 61-614 Poznań, Poland KEYWORDS: Energy transfer; Optical thermometer; Up-conversion luminescence; Luminescence intensity ratio (LIR); Functional nanomaterials; Rare earth ions

Abstract Up-converting core@shell type β-NaYF4:Yb3+-Er3+@SiO2 nanorods have been obtained by a two-step synthesis process, which encompasses hydrothermal and microemulsion routes. The synthesized nanomaterial forms stable aqueous colloids and exhibits a bright dual-center emission (λex= 975 nm), i.e. up-conversion luminescence of Er3+ and down-shifting emission of Yb3+, located in the first (I-BW) and the second (II-BW) biological windows of the spectral range. The intensity ratios of the emission bands of Er3+ and Yb3+ observed in the Vis-NIR range

monotonously

change

(2H11/2→4I15/2/4S3/2→4I15/2)

with

and

the

temperature,

i.e.

non-thermally

the

thermalized

coupled

Er3+

Yb3+/Er3+

levels levels

(2F5/2→2F7/2/4I9/2→4I15/2 or 4F9/2→4I15/2). Hence, their thermal evolutions have been correlated with temperature using the Boltzmann type distribution and 2-th order polynomial fits for temperature sensing purposes, i.e. Er3+ 525/545 nm (max Sr = 1.31 %K-1) and Yb3+/Er3+ 1010/810 nm (1.64 %K-1) or 1010/660 nm (0.96 %K-1). Additionally, a fresh chicken breast 1 ACS Paragon Plus Environment

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was used as a tissue imitation in the performed ex vivo experiment, showing the advantage of the use of NIR Yb3+/Er3+ bands, vs. the typically used Er3+ 525/545 nm band ratio, i.e. better penetration of the luminescence signal through the tissue in the I-BW and II-BW. Such nanomaterials can be utilized as accurate and effective, broad-range Vis-NIR optical, contactless sensors of temperature.

INTRODUCTION Temperature is the basic physical parameter influencing the physicochemical properties of compounds. Its precise and accurate determination is important for various industrial and scientific purposes.1–5 Conventional methods of temperature measurement are based on the use of different types of metallic/liquid thermometers, thermocouples, pyrometers, etc.4 However, they require a physical contact with the measured object and cannot be used for the determination of the local temperature gradient in the nano and sub-micro sized areas.6,7 However, luminescent contactless nanothermometers may resolve these problems. Currently, nano-sized multifunctional materials are extensively investigated because of their beneficial magnetic, optoelectronic and structural features.8–14 Especially core/shell nanostructures having an active inner core and external protective and/or functionalized shell (e.g. silica nanoshell) attract the attention of many scientists.11,14–17 This is due to the enhanced biocompatibility, improved stability and reduced cytotoxicity of the internal core particles, after being coated with a silica shell.18–22 The active surface shell enables further modification of their surface and bioconjugation of the particles, increasing the functionality of the material.18–20 The silica shell encompasses the mentioned benefits and additionally increases the temperature stability of the coated core material (up to above ≈900 K),15 as well as makes the particles dispersible in water forming stable aqueous colloids, which is crucial for bioapplications.18–20

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Thanks to the unique optical properties of nanoparticles (NPs) doped with lanthanide ions (Ln3+), their relatively low cytotoxicity, facile synthesis and surface modification, they are very appealing candidates for various scientific and industrial applications.14–22 In particular, the Ln3+-doped materials may exhibit multicolor luminescence (emission of light) under UV and NIR (energy up-conversion) irradiation, long emission lifetimes (in the μs-ms range) as well as narrow absorption/emission bands.8,9,23–28 Such exceptional features are associated with the unique character of 4f-4f electron transitions (forbidden by Laporte selection rules) within Ln3+ ions, the crystal field effects and the shielding of the 4f electrons by the 5s and 5p ones.23,29– 31

One of the most appealing phenomenon occurring in the Ln3+-doped systems is the energy up-conversion (UC) process, i.e. the non-linear anti-Stokes emission, which can be generated by two (or more) lower-energy photons excitation and the subsequent emission of one higher-energy photon.8,9,27,28,32 The UC process may be observed in various host matrices, usually co-doped with Yb3+/Er3+, Yb3+/Ho3+ or Yb3+/Tm3+ ions.27,28 This is because of the large absorption cross-section of Yb3+ in the NIR range (≈950-1050 nm), which acts as a sensitizer and transfers the absorbed energy to emitting ions (activators; e.g. Ho3+, Er3+, Tm3+) having a ladder-like structure of their energy levels.8,27,33–35 In general, Ln3+-doped inorganic particles such as oxides, fluorides, borates, vanadates, phosphates, and so forth, are resistant to photobleaching and high temperature treatment, as well as may exhibit efficient UC luminescence in contrast to organic compounds.8,9,25–28,36,37 Currently, the non-centrosymmetric β-NaYF4 matrix doped with Yb3+/Er3+ is the best examined and the most commonly used up-converting (UC) luminescent material.27,28,38–41 This is due to several factors, i.e. very low phonon energy of the crystal lattice (limited multiphononrelaxation), high quantum yield, the presence of sodium ions in the matrix (enhanced UC intensity), the possibility of synthesis in the form of nano-sized particles (e.g. spheres, rods,



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branched structures, etc.), resistance to high temperature, low cytotoxicity and facile surface modification.38–41 As temperature significantly affects the luminescence properties of Ln3+-doped materials, i.e. their intensity, band ratio, lines broadening, spectral position, lifetimes, etc., a well-defined and monotonic change of the aforementioned parameters can be used for temperature sensing applications in contactless luminescence thermometry.6–9 Currently, the most sensitive and convenient way of optical temperature determination, providing high accuracy/precision and good resolution, is based on the use of fluorescence intensity ratio (FIR) technique, which in the case of 4f-4f transitions of Ln3+-doped materials, should be correctly called luminescence intensity ratio (LIR).42–47 The LIR technique is based on the temperatureinduced changes of the emission bands intensity ratio, associated with the thermally-coupled levels (TCLs) of particular Ln3+ ions (e.g. Nd3+, Pr3+, Er3+, Ho3+, Tm3+).6–8,42,43 The energy difference/separation (∆E) between their thermalized states should be in the range from 200 ≤ ∆E ≤ 2000 cm-1.6–8 A smaller ∆E may result in a strong overlapping of two TCLs, and a larger ∆E may lead to an insufficient population of higher-energy state in a given temperature range. Most reports on lanthanide luminescence thermometry focus on the use of TCLs of the Nd3+ (820/890 nm), Er3+ (525/545 nm) or Tm3+ (700/800 nm) ions, and their emission in the visible and NIR ranges (below ≈950 nm) detected using PMT or silicon CCD camera.6–8,44–48 Among them, the Nd3+-doped NPs seem to be one of the best candidates for temperature sensing in the first biological window (I-BW), due to the presence of intense thermalized emission bands above 800 nm. For example, Carrasco et al.48 used LaF3:Nd3+ 5-25% NPs for thermal sensing using LIR technique, i.e. band ratio of the TCLs, Stark components of the 4F3/2→4I9/2 transition at 865/885 nm (λex =808 nm), resulting in Sr ≈0.26% K−1 at 296 K. They used them also as nanoheaters, for hyperthermia treatment of mice tumours formed by subcutaneous inoculation of cancer cells. Balabhadra et al.45 used Gd2O3:Nd3+ 1-5% nanorods

as


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nanothermometers, using LIR technique, i.e. intensity ratio of two thermalized emission bands at 825/890 nm (4F5/2→4I9/2/4F3/2→4I9/2), resulting in high Sr ≈1.75% K−1 at 288 K (λex ≈580 nm). Hernández-Rodríguez et al.47 used Nd3+-doped nano-perovskites YAlO3:Nd3+ 1% for temperature sensing, using the same LIR technique and TCLs (4F5/2 and 4F3/2), i.e. band ratio at 820/890 nm (λex ≈532 nm), resulting in high Sr ≈1.83% K−1 at 293 K. Besides the mentioned merits of Nd3+-doped NPs, they have some drawbacks such as a necessity of using highresolution detection system (especially in the case of the sensing based on the Stark sub-levels), due to the strong overlapping of the bands used for thermal sensing, which may lead to some difficulties in the precise and accurate temperature determination. Recently, however, there is a growing number of reports concerning the use of band ratios associated with non-thermalized levels and NIR emission of various combinations of Ln3+ ions in the range of ≈1000-1600 nm (single, dual- or multi-center emission), requiring the use of relatively expensive InGaAs detectors.40,42,43,49,50 Such materials are usually very complex (multi-layer structure), and their synthesis is a multi-step process. For example, in the case of Er3+ co-doped nanomaterials, Ximendes et al.49 designed the UC core/shell fluoride NPs (LaF3:Yb3+ 10%, Er3+ 2%/LaF3:Yb3+ 10%, Tm3+ 10%) prepared by co-precipitation and subsequent thermal post-treatment method. The NPs worked as luminescent thermometers (293-323 K) in the NIR range, based on the following band ratios: Yb3+/Tm3+ 1000/1230 nm (2F7/2→2F5/2/3H5→3H6), Yb3+/Er3+ 1000/1550 nm (2F7/2→2F5/2/4I13/2→4I15/2) and Tm3+/Er3+ 1230/1550 nm (3H5→3H6/4I13/2→4I15/2), at λex= 690 nm. Another example is the work of Kamimura et al.40, who reported the use of UC NPs (β-NaYF4:Yb3+ 1%, Ho3+ 1%, Er3+ 1%) synthesized via a thermal decomposition method. The Authors correlated with the temperature (298-323) a linear change of the Ho3+/Er3+ band ratio 1150/1550 nm (5I6→5I8/4I13/2→4I15/2), under the excitation at λex= 980 nm. However, in the case of the Yb3+/Er3+ co-doped materials, i.e. the most commonly utilized UC systems, the typically used Er3+ 525/545 nm band ratio is



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not located in any biological window (BW-I 650-950 nm, BW-II 1000-1350 nm, BW-III 15501870 nm), where the emitted light can penetrate biological species (tissues) more effectively. Here, for the first time, we present the use of the well-separated (non-overlapping) Yb3+/Er3+ 1010/810 and 1010/660 nm emission band ratios of water dispersible, core@shell βNaYF4:Yb3+-Er3+@SiO2 nanorods for the temperature sensing purposes. The synthesized nanomaterials exhibit dual-center emissions, i.e. bright UC luminescence of Er3+ together with a typical down-shifting emission of Yb3+ (λex= 975 nm) located in the I-BW and II-BW spectral range, which were detected by the use of a simple silicon CCD camera. The advantage of using NIR Yb3+/Er3+ band ratios (better light penetration through the tissue) compared to the Er3+ 525/545 nm one was confirmed by a simple ex vivo experiment.

EXPERIMENTAL Materials Rare earth (RE) oxides, i.e. Y2O3, Yb2O3 and Er2O3 (99.99%, Stanford Materials, USA) were separately dissolved in HNO3 (65%, POCh. S.A., Poland) in order to obtain the corresponding nitrates RE(NO3)3 (RE = Y3+, Yb3+, Er3+). Sodium fluoride, NaF (≥99%), tetraethyl orthosilicate (TEOS; reagent grade, ≥98%), IGEPAL® CO-520, cyclohexane (pure p.a., ACS reagent, ≥99.5%) were purchased from Sigma Aldrich. Sodium hydroxide, NaOH (98.8%, pure p.a.) and NH4OH (pure p.a., 25%) were bought from POCh S.A., Poland. Oleic acid (Ph.Eur.) was bought from Fluka. Deionized water was used for all syntheses. Synthesis of β-NaYF4:18% Yb3+-2% Er3+ nanorods In order to synthesize 150 mg of the β-NaYF4:18 mol.% Yb3+, 2 mol.% Er3+ nanorods (NRs), NaOH (1.29 g), oleic acid (30 mL), water (13.5 mL) and ethanol (15 mL) and were mixed together under a continuous vigorous stirring to get a homogeneous solution. Afterwards, aqueous solutions of Yb(NO3)3 (0.438 mL; 0.301 M), Y(NO3)3 (1.652 mL; 0.355 M) and


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Er(NO3)3 (0.293 mL; 0.05M) were mixed together in a separate vessel and added to the previously prepared oleate solution. Subsequently, 0.123 g of NaF (2.932 x 10-3 mol.; RE3+/F= 4/1 molar ratio) dissolved in 5.25 mL of water was added to the as-prepared solution maintaining a vigorous stirring. Afterwards, the whole mixture was shaken for approx. 10 min and transferred to the teflon vessels, which were subsequently placed in a hydrothermal autoclave (Berghof DAB-2), and heated at 200 oC for 24 h. After that, the supernatant was discarded and the deposit (taken from the bottom of the vessel) was dissolved in cyclohexane and subsequently 2-times centrifuged in cyclohexane and 2-times in ethanol, in order to purify the final product. Finally, the product was dried in ambient conditions and then re-dispersed in cyclohexane, forming a colloidal solution. The chemical composition of the synthesized NPs, as determined by ICP-OES analysis, is NaYF4: 2.6 mol.% Er3+-22.4 mol.% Yb3+.

Synthesis of core@shell β-NaYF4:Yb3+-Er3+@SiO2 NRs A certain volume of the colloidal solution containing 50 mg of the NaYF4:Yb3+-Er3+ NRs, was mixed with an additional portion of cyclohexane up to 40 mL. After that, 2 mL of IGEPAL CO520 was added to the colloid and the whole mixture was ultrasonicated for about 10 min. Afterwards, 0.4 mL of concentrated ammonia was added, and the whole mixture was ultrasonicated for a further 10 min. After that, 0.2 mL of TEOS was added to the as-prepared mixture. The microemulsion formed was magnetically stirred for 24 h. After completion of the reaction, the product was precipitated by addition of an excessive amount of acetone (≈30 mL), and centrifuged (twice in ethanol and two-times in water). Afterwards, the as-prepared purified NaYF4:Yb3+-Er3+@SiO2 core@shell NRs were redispersed in water to obtain a colloidal solution.

Characterization



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Powder X-ray diffraction pattern (XRD) was measured with a Bruker AXS D8 Advance diffractometer, working in a Bragg-Brentano geometry, using Cu Kα1 radiation (λ=1.5406 Å) and 0.05ᵒ step scan mode. Transmission electron microscopy (TEM) images were taken using a transmission electron microscope Hitachi HT7700, operating with 100 kV accelerating voltage. The emission spectra were measured using an Andor Shamrock 500i spectrograph with a silicon iDus CCD camera detector. The excitation source was a fiber-coupled solid state diode pumped (SSDP) laser FC-975-2W (CNI) having a spherical beam spot ≈0.2 mm in size. For all measurements the laser power was fixed to ≈0.5 W (no laser-induced heating was observed, i.e. the band ratios remained unchanged turning the laser on/off). FT-IR spectra were collected in a transmission mode (in a transparent KBr pellet) using a FT-IR spectrophotometer JASCO 4200. The energy dispersive X-ray analysis (EDX) was performed with a FEI Quanta 250 FEG Scanning Electron Microscope, having an EDAX detector. Particle size distribution and zeta potentials were measured by the use of Malvern Zetasizer Nano-ZS, using dynamic light scattering (DLS) and electrophoretic light scattering (ELS) methods, respectively. The absorption spectrum was recorded using a JASCO V-770 UV-VIS-NIR spectrophotometer.

RESULTS AND DISCUSSION Structure and morphology The recorded XRD pattern of the synthesized β-NaYF4:Yb3+-Er3+ nanomaterial (Figure 1a) agree with the reference pattern from the ICDD database (International Centre for Diffraction Data), card no 28-1192,51 of hexagonal β-NaYF4 fluoride crystallizing in a P63/mmc space group. Significant broadening of the reflexes results from the nanocrystallinity of the obtained particles. The final product (core) consists of elongated NRs (Figure S1), with a thickness of ≈50 nm and length of ≈300 nm. After their surface coating with an amorphous silica shell


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(thickness of ≈25 nm) they formed core@shell type nanostructures, which are on average 94 ± 9 nm wide and 315 ± 25 nm long (Figures 1b, c and S2). The presence of an external silica shell enables their dispersion in water and the formation of colloidal solutions. The synthesized core@shell NRs form some agglomerates in water, whose hydrodynamic size is around 552 ± 165 nm (Figures S2 and S3). However, their aqueous colloids (Figure 1d) are stable and do not settle down for at least several hours. This is due to their high average surface charge (-27 mV) in the neutral pH range, and the isoelectric point being around pH ≈ 2 (Figure S4), which is typical of the silica shell.52–54 Additional discussion concerning the structure and composition of the synthesized compounds, i.e. FT-IR (Figure S5) and EDX spectra (Figure S6) can be found in the SI data.

Figure 1. (a) Powder XRD pattern of the prepared NaYF4:Yb3+-Er3+ product and the corresponding reference pattern of β-NaYF4; (b) TEM images of the obtained NaYF4:Yb3+-Er3+@SiO2 core@shell NRs; (c) TEM-based size distribution histograms of the core@shell NRs, and (d) photographs of their aqueous colloid luminescence in a cuvette, at λex= 975 nm.

Luminescence properties 9 ACS Paragon Plus Environment


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The synthesized core@shell NaYF4:Yb3+-Er3+ NRs exhibit a bright yellow-green UC luminescence, namely anti-Stokes emission in the range from around 500 to 850 nm (both in solid powder and colloidal form; Figure 1d) at 975 nm NIR laser excitation, in resonance to the Yb3+ 2F7/2→2F5/2 transition. In this system, the Yb3+ ions work as sensitizers, which transfer the absorbed NIR energy to the nearest emitting Er3+ activator ions, and pump their excited states by energy transfer UC processes. Figure 2 schematically presents the observed energy migration paths in the prepared NaYF4:Yb3+-Er3+@SiO2 core@shell NRs, emphasizing the Er3+ UC luminescence and the Yb3+ emission.

Figure 2. Scheme of the major energy migration paths in the obtained core@shell NaYF4:Yb3+Er3+@SiO2 NRs.

Figure 3 presents the emission spectra of the obtained core@shell NRs normalized to the Yb3+ 2F5/2→2F7/2 band, as a function of temperature (≈297-337 K). The spectra consist of several narrow bands characteristic of the Er3+ UC luminescence, associated with its 4f-4f 10 ACS Paragon Plus Environment

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radiative transitions, i.e. 2H11/2→4I15/2 (525 nm), 4S3/2→4I15/2 (545 nm), 4F9/2→4I15/2 (660 nm) and 4I9/2→4I15/2 (810 nm). The observed emission band in the NIR range (above ≈1000 nm) is a classical down-shifting emission originating from Yb3+ ions (2F5/2→2F7/2 transition). It is worth noting that, due to the use of 975 nm laser excitation, we applied a longpass 1000 nm filter to monitor the Yb3+ emission in the II-BW. Obviously, the Yb3+ 2F5/2→2F7/2 band is partially cut off by the filter, but it does not affect the rate of change of the relative intensity ratios of the Yb3+/Er3+ bands as a function of temperature.

Figure 3. Normalized emission spectra of the synthesized core@shell NaYF4:Yb3+-Er3+@SiO2 NRs; λex= 975 nm; for better clarity, the range from 500 to 850 nm was multiplied by 10 for all spectra.

By analyzing the spectra, it is clearly seen that the Er3+ emission bands around 545, 660 and 810 nm decrease significantly with increasing temperature. In contrast, the Er3+ band around 525 nm and the Yb3+ band above 1000 nm are almost unaffected by temperature and their intensities are almost constant (see also the non-normalized spectra in Figure S7). That is why we used them as references for the ratiometric temperature sensing. The relative change of the Er3+ 525/545 nm band ratio is commonly used for temperature sensing purposes because the respective 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions originate from the thermally-coupled levels (TCLs) of Er3+ (2H11/2 and 4S3/2), separated by ΔE ≈800 cm-1. Temperature-induced thermalization processes lead to an increase in the relative intensity of the higher-energy 11 ACS Paragon Plus Environment


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transition (at 525 nm; I2) and a decrease in the intensity of the lower-energy one (at 545 nm; I1), according to the Boltzmann distribution: 𝐿𝐼𝑅 (𝐹𝐼𝑅) ≡

𝐼2

(

∆𝐸

𝐼1 = 𝐵 exp ― 𝑘𝐵𝑇

)

(1)

where LIR (FIR) is the luminescence (fluorescence) intensity ratio of the emission bands, kB is the Boltzmann constant, ΔE is the energy separation between the barycenters of the I2 and I1 bands, T is the absolute temperature and B is a constant dependent on rates of total spontaneous emission, states degeneracies, branching ratio of the transitions in respect to the ground state and transitions angular frequencies.8,42 Using the integrated areas under the 525 and 545 nm bands and applying the Eq. (1), a perfect fit (R2>0.99) to the determined band intensity ratio was obtained (Figure 4; top). The determined energy difference between the mentioned TCLs, i.e. ΔE= 812 cm-1, is very similar to the separation energy between the 2H11/2 and 4S3/2 states calculated from the spectra (≈750 cm-1). Although the typically used Er3+ 525/545 nm band ratio is very useful for accurate, contactless temperature sensing in various scientific and industrial applications, it is not ideal for biological and medical applications requiring subcutaneous temperature determination. This is due to the large absorption cross-section of biological species (skin, internal tissues, fat, blood, etc.) in the visible range up to ≈650 nm, as well as stronger scattering of visible light compared to lower-energy NIR radiation. That is why we have determined the temperature dependences of the spectrally separated Yb3+/Er3+ band ratios, i.e. 1010/810 nm (2F5/2→2F7/2/4I9/2→4I15/2) and 1010/660 nm (2F5/2→2F7/2/4F9/2→4I15/2) located in the II-BW and I-BW ranges. It is worth noting that these transitions are not thermally-coupled in the sense of the Boltzmann type distribution. However, they exhibit different temperature dependences of their quenching and/or energy transfer rates and, hence, their relative intensities, i.e. band ratios, can be correlated with temperature and used for thermal sensing.49 We have successfully correlated the mentioned Yb3+/Er3+ band ratios, applying the 2th-order polynomial fits to their 12 ACS Paragon Plus Environment

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temperature evolutions (Figure 4; bottom). In order to ensure the reversibility of the measured thermometric parameters, after reaching 337 K, the system was cooled down to room temperature, then reheated and cooled to ambient conditions. The determined band ratios at extreme temperature values (Figure 4) are located in the expected positions and match the tendencies in the corresponding series.

Figure 4. (left) Luminescence intensity ratios and (right) corresponding relative sensitivities (Sr) determined for different Er3+ and Yb3+/Er3+ band ratios in the Vis-NIR range, for the synthesized core@shell NaYF4:Yb3+-Er3+@SiO2 NRs; λex= 975 nm; dashed lines indicate Sr at 313 K.

In order to quantitatively compare the performance of various optical thermometers (different dopant ions and host materials), a relative temperature sensitivity (Sr) can be used because it is independent from the sample characteristics and measuring setup. The Sr value is usually expressed in %K-1 and is defined as: 1 𝑑𝑀𝑃

𝑆𝑟 = 100% × 𝑀𝑃

(2)

𝑑𝑇



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where MP is the measured parameter whose change is used for temperature sensing, e.g. band ratio, luminescence lifetime, band width, spectral shift, etc. Sr indicates how MP changes per 1 K. As based on the Eq. (2), the calculated Sr value decreases with the temperature for the 525/545 nm band ratio (see Figure 4; right) from 1.31 %K-1 (299 K) to 1.02 %K-1 (337 K). It is worth noting that this is one of the highest Sr value (around 300 K) for this Er3+ band ratio, especially among nano-sized materials (Table 1).

Table 1. Relative thermal sensitivities (Sr MAX at a given T value) of Er3+-doped nano-sized and bulk materials; spectral range of the used transitions and the studied temperature ranges Host

Dopant ions

Sr MAX (% K-1) 1.31

β-NaYF4/SiO2 core/shell nanorods

Yb3+, Er3+

1.64 0.96

β-NaYF4/SiO2 core/shell SrF2

Yb3+, Er3+

1.02

Yb3+, Er3+

1.20

Gd2O3

Yb3+, Er3+

≈0.85

La2O3, Gd2O3, Y2O3 β-NaYF4

Yb3+, Er3+

≈1.1

Yb3+, Ho3+, Er3+ Yb3+, Er3+, Tm3+

≈0.7

LaF3:Yb,Er/ LaF3:Yb,Tm core/shell SrF2:Yb,Tm/ Y/Yb,Er,Nd/ Nd core/shell

3.9 5.0

Yb3+, Nd3+, Er3+, Tm3+

1.62

T (K)

T-range Transitions (K) Nano-sized particles 299 (Er3+) 2H11/2→4I15/2/ (Er3+) 4S3/2→4I15/2 299-337 337 (Yb3+) 2F5/2→2F7/2/ (Er3+) 4I9/2→4I15/2 337 (Yb3+) 2F5/2→2F7/2/ (Er3+) 4F9/2→4I15/2 2H 4 300 300-900 11/2→ I15/2/ 4S →4I 3/2 15/2 2H 4 298 298-383 11/2→ I15/2/ 4S →4I 3/2 15/2 2 H11/2→4I15/2/ 300 300-900 4S →4I 3/2 15/2 2 H11/2→4I15/2/ 280 280-490 4S →4I 3/2 15/2 298 298-323 (Ho3+) 5I6→5I8/ (Er3+) 4I13/2→4I15/2 293 293-323 (Yb3+) 2F7/2→2F5/2/ (Tm3+) 3H5→3H6 (Yb3+) 2F7/2→2F5/2/ (Er3+) 4I13/2→4I15/2 323 293-333 (Yb3+) 2F5/2→2F7/2/ (Nd3+) 4F3/2→4I11/2

λ (nm)

Ref.

525/545 1010/810 1010/660

this work

520/545

15

525/545

46

523/548

55

525/550

56

1150/ 1550 1000/ 1230 1000/ 1550 980/1060

40

49

50

Bulk materials 0.74

0.34 β-NaLuF4

Yb3+, Er3+, Ho3+

1.73 0.42

298

298-503

503

298-503

293

293-568

293

293-568

2H

4 11/2→ I15/2/ 4S →4I 3/2 15/2 (Er3+) 4F9/2→4I15/2, (Ho3+) 5S/2,5F4→5I7/ (Er3+) 4S3/2→4I15/2, (Ho3+) 5S/2,5F4→5I8 (Ho3+) 5I5→5I8/ (Er3+) 4I9/2→4I15/2 (Ho3+) 5I5→5I8/ (Er3+) 4I13/2→4I15/2

527/547

659/547 42

887/817 887/1545


Page 15 of 24 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


0.21

430

293-568

SrSnO3

Er3+

0.97

294

294-372

Fluorotellurite glass GdOF/SiO2 yolk/shell

Er3+

1.28

293

293-550

Nd3+,Yb3+, Er3+

1.60

260

260/490

(Ho3+) 5I6→5I8/ (Er3+) 4I13/2→4I15/2 2H 4 11/2→ I15/2/ 4S →4I 3/2 15/2 2H 4 11/2→ I15/2/ 4S →4I 3/2 15/2 (Er3+) 2H11/2→4I15/2/ (Er3+) 4S3/2→4I15/2

1177/ 1545 528/549

57

525/550

58

534/543

59

Thus, for the 1010/810 and 1010/660 nm band ratios, Sr increases with temperature from 1.05 to 1.64 %K-1 and from 0.63 to 0.96 %K-1, respectively (Figure 4). However, in the case of bioapplications, the temperature range around 313 K (40 oC) is of crucial importance because the detection of pathologically changed cells is based on their internal temperature elevation from approximately 310 to 315 K, where the most pronounced deviations occur. Hence, we compared Sr values at 313 K in Table 2 together with other fitting parameters and the equations used.

Table 2. Transitions applied in the LIR techniques, wavelengths, equations fitted, coefficients of

correlation, maximum Sr values, corresponding temperature, Sr and δT values at 313 K Transitions

λ (nm)

Equation

R2

SrMAX (%K-1)

T (K)

Sr 313 K (%K-1)

δT 313 K (K)

Er3+:2H11/2→4I15/2/ Er3+:4S3/2→4I15/2

525/545

11.20Exp(-811.9/KBT)

>0.99

1.31

299

1.19

0.59

Yb3+:2F5/2→2F7/2/ Er3+:4I9/2→4I15/2

1010/810

4.41 x 10-3T2 2.46T + 358.9

>0.98

1.64

337

1.44

0.76

Yb3+:2F5/2→2F7/2/ Er3+:4F9/2→4I15/2

1010/660

5.96 x 10-4T2 0.324T + 49.3

>0.97

0.96

337

0.79

1.00

The highest Sr value (1.44 %K-1) at 313 K was determined for the Yb3+/Er3+ 1010/810 nm band ratio. The temperature uncertainties (δT; temperature resolution) were determined based on the Eq. (3),43,46,60 i.e.: 1 𝛿𝑀𝑃

(3)

𝛿𝑇 = 𝑆𝑟 𝑀𝑃



Page 16 of 24

where δMP denotes the uncertainty of the measured parameter determination. The calculated uncertainty of the optical temperature determination at 313 K is about 0.59 K for the Er3+ 525/545 nm band ratio, and about 0.76 K (1010/810 nm) and 1.00 K (1010/660 nm) for the Yb3+/Er3+ band ratios. The calculated δT values for the whole temperature range studied are given in Figure S8 in the SI. In addition to sensitivity and uncertainty, the luminescence signal intensity is also of crucial importance for temperature sensing, especially in biological systems. Even if the value of Sr in the biological range of temperature is high, but the excitation/emission light is strongly absorbed and/or scattered by biological material hampering its penetration through the tissue, accurate optical determination of the temperature is not possible. That is why we conducted a simple ex vivo experiment using a fresh chicken breast (≈1 mm in thickness) as tissue imitation. The experimental setup in schematically presented in Figure 5 (top). Using the same configuration, measuring parameters and laser power, the emission spectra with and without the tissue were recorded (Figure 5; bottom). The NIR emission of Yb3+ decreased slightly after propagation through the tissue (by ≈10%). Due to the strong scattering of the higher-energy photons and their absorption by the tissue, the UC emission intensity decreased significantly (≈100-times) in the recorded spectrum. However, the most pronouncing decrease is observed below ≈650 nm. It is clearly visible that it is hardly possible to determine the Er3+ 525/545 nm band ratio from the recorded spectra with the tissue. However, besides the signal intensity decrease, the Yb3+/Er3+ 1010/810 and 1010/660 nm bands are still clearly observed, and after appropriate recalibration, their intensity ratio could potentially be used for temperature sensing purposes. Taking into account the spectral range, the luminescence signal intensity, thermal sensitivity and resolution, the Yb3+/Er3+ 1010/810 nm band ratio seems to the most suitable for biological temperature sensing, among the parameters studied.


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Figure 5. (top) Scheme of the experimental setup used for the ex vivo optical experiment, i.e. excitation of the sample and its luminescence measurement by the tissue; (bottom) emission spectra of the synthesized core@shell NaYF4:Yb3+-Er3+@SiO2 NRs recorded at ambient conditions, without (upper one; yellow) and with tissue (lower one; pink); λex= 975 nm.

CONCLUSIONS Inorganic luminescent Ln3+-doped nanocrystals have been successfully synthesized in the form of β-NaYF4:Yb3+-Er3+@SiO2 core@shell type nanorods via a combined hydrothermal and microemulsion approaches. The obtained nanomaterial exhibits a bright up-conversion luminescence under NIR 975 nm laser irradiation, accompanied with an intense NIR emission of Yb3+. Thanks to the presence of the silica shell it forms stable aqueous colloids. The 17 ACS Paragon Plus Environment


Page 18 of 24

temperature dependence of its luminescence properties was investigated in the biological range of temperature (299-337 K). Based on the LIR method, the UC emission band ratio of Er3+ 525/545 nm, corresponding to its TCLs 2H11/2→4I15/2/4S3/2→4I15/2 (ΔE= 812 cm-1) was correlated with temperature according to the Boltzmann type distribution, resulting in a high thermal sensitivity (Sr = 1.31 %K-1 at 299 K). Moreover, using the 2th-order polynomial fits, we correlated the emission band ratios of Yb3+/Er3+ related to their non-TCLs, i.e. 1010/810 (2F5/2→2F7/2/4I9/2→4I15/2) and 1010/660 nm (2F5/2→2F7/2/4F9/2→4I15/2) with temperature. It is worth noting that the spectrally separated Yb3+/Er3+ 1010/810 nm band ratio exhibits the highest relative sensitivity values, i.e. Sr = 1.64 %K-1, at 337 K. The performed ex vivo experiment confirmed better tissue penetration using the Yb3+/Er3+ luminescence bands located in II-BW and I-BW, compared to the commonly used in luminescence thermometry Er3+ 525/545 nm band ratio. Considering the spectral position, signal intensity, temperature resolution and thermal sensitivity, it seems that the use of Yb3+/Er3+ 1010/810 nm band ratio is most suitable for the contactless optical temperature sensing applications, especially in the biological systems.

ASSOCIATED CONTENT Supporting Information TEM images; DLS-based size distribution histogram; zeta-potential titration plot; IR and EDX spectra; non-normalized luminescence spectra; determined temperature uncertainties; transmission spectrum of the tissue used.

AUTHOR INFORMATION Corresponding Author M.R.: Tel: 0048618291778


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E-mail: [email protected] Notes The authors declare no conflict of interest. ACKNOWLEDGEMENTS This work was supported by the Polish National Science Centre (grant No. UMO2016/22/E/ST5/00016 and 2016/21/B/ST5/00110) and the Polish Ministry of Science and Higher Education: Iuventus Plus Programme, grant No. IP2014 014573. REFERENCES (1)

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Upconverting Lanthanide Fluoride Core@Shell Nanorods for

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