The use of energy looping between Tm³⁺ and Er³⁺ ions to obtain an intense upconversion under the 1208 nm radiation and its use in temperature sensing

Abstract

The upconversion phenomenon allows for the emission of nanoparticles (NPs) under excitation with near-infrared (NIR) light. Such property is demanded in biology and medicine to detect or treat diseases such as tumours. The transparency of biological systems for NIR light is limited to three spectral ranges, called biological windows. However, the most frequently used excitation laser to obtain upconversion is out of these ranges, with a wavelength of around 975 nm. In this article, we show an alternative – Tm³⁺/Er³⁺-doped NPs that can convert 1208 nm excitation radiation, which is in the range of the 2^nd biological window, to visible light within the 1^st biological window. The spectroscopic properties of the core@shell NaYF₄:Tm³⁺@NaYF₄ and NaYF₄:Er³⁺,Tm³⁺@NaYF₄ NPs revealed a complex mechanism responsible for the observed upconversion. To explain emission in the studied NPs, we propose an energy looping mechanism: a sequence of ground state absorption, energy transfers and cross-relaxation (CR) processes between Tm³⁺ ions. Next, the excited Tm³⁺ ions transfer the absorbed energy to Er³⁺ ions, which results in green, red and NIR emission at 526, 546, 660, 698, 802 and 982 nm. The ratio between these bands is temperature-dependent and can be used in remote optical thermometers with high relative temperature sensitivity, up to 2.37%/°C at 57 °C. The excitation and emission properties of the studied NPs fall within 1^st and 2^nd biological windows, making them promising candidates for studies in biological systems.

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Introduction

The photon upconversion process in nanoparticles (NPs) is one of the most intensively investigated phenomena in recent years. The conversion of low energetic photons, usually from the near-infrared range (NIR), into photons with higher energy can be applied in medicine for photodynamic therapy,^1,2 in the detection of cancer markers,^3–5 in bioimaging,⁶ or drug delivery.⁷ Recently, remote temperature sensing is one of the most important areas where the upconversion process finds applications.^8–10 Monitoring temperature is essential for understanding biological processes and diagnosing or treating various diseases.¹¹

The diversity of applications arises from the distinctive properties of upconversion (UC). NPs showing this process are usually inorganic structures with high chemical and thermal stability. The nature of the UC process in the NPs gives such important features as stability of emission, lack of photobleaching and significant shift between excitation and emission wavelengths.¹² The lack of autofluorescence when UC is applied is also important for biological applications. These features result from the properties of lanthanide ions (Ln³⁺), whose electronic structure, low sensitivity to the local environment and multiple long-living energy states allow for the conversion of NIR photons to higher energetic ones.

UC is possible due to the sequential absorption of multiple photons in a single Ln³⁺ ion or energy transfer (ET) between two different ions, e.g. Yb³⁺ and Er³⁺.¹³ There are many examples of efficient UC systems with multicolour emissions, various types of Ln³⁺ ions and their concentrations, and based on different host compounds forming NPs.^14–18 Among various types of upconverting NPs, the most popular are those doped with Yb³⁺ ions as sensitisers for NIR light with a wavelength around 975 nm and co-doped with Er³⁺, Tm³⁺ or Ho³⁺ used as emitters.^{1,3,6,7,19,20} It is caused by the high effectivity of this system due to the intense absorption of Yb³⁺ ions and the possibility of energy migration between these ions in the structure of NPs.^21,22 Yb³⁺ ions have a unique electronic structure and only one excited state (²F_5/2), limiting the possible quenching processes and energy loss.²²

UC based on the properties of the Yb³⁺/Ln³⁺ system has its consequences, such as high efficiency²³ or the possibility of NIR conversion up to ultraviolet,^4,24 but also some drawbacks, e.g. low penetration of the 975 nm wavelength into tissue and high absorption of water in this range causing heating of the biological medium.^25–27 The laser wavelength used for excitation of UCNPs doped with Yb³⁺ ions falls between the 1^st and 2^nd biological windows, where tissue shows lower transparency for radiation.^26–28 Better for medical applications are NPs excitable within biological windows: 1^st (650 to 950 nm), 2^nd (1100 to 1350 nm) or 3^rd (1600 to 1870 nm). Also optimal is the emission of such NPs within these windows, most favourably within the 1^st optical window, as many popular optical detectors work within this range.

There are only a few types of Ln³⁺ ions capable of absorption within the ranges mentioned above, and one of the most promising are Tm³⁺ ions as they absorb radiation at around 1208 nm, and their emission occurs within the 1^st biological window. However, not too many examples of Tm³⁺-based UC in NPs exist.^29–33 Other possibilities are Nd³⁺ ions used for 808 nm excitation,^13,28 Ho³⁺ ions capable of absorption of 1156 nm,^34,35 and Er³⁺ ions, which show emission under 1490–1580 nm excitation, which is on the edge of the 3^rd biological window.^27,35,36

UC based on the ions mentioned above can also be efficient and intense.^32,37 Such processes as energy looping, photon avalanche (PA) or self-sensitisation can significantly enhance the UC in NPs without Yb³⁺ in their structure.^32,38–40 These processes are based on a cycle or loop of GSA, ESA and/or energy transfers followed by CR, leading eventually to a substantial population of the reservoir level and, therefore, to intense UC emission.^38,41

The UC-based optical nanothermometers excitable via 1208 nm radiation have not been reported yet. The most frequently used NIR excitation wavelength for nanothermometry is 975 nm,^9,42,43 whereas for biological applications, 808 nm.⁸ The first excitation wavelength falls outside the biological windows, and the obtained emission is usually based on the properties of Er³⁺ ions and their emission at around 520 and 545 nm. The application of such thermometers in biology and medicine is therefore limited due to the absorption of green light by, e.g. red blood cells and lowered transparency of water in the 950–1050 nm range.^26,27 The conception of using 1208 nm excitation radiation and emission bands in the red to NIR range is new and promising for applications in biology, where lowering interactions of biological environment with excitation beam is of high importance.¹¹

This article reports the spectroscopic properties of Tm³⁺ and Er³⁺/Tm³⁺ doped core@shell NPs based on the NaYF₄ host compound and their emission under 1208 nm laser excitation. We selected core@shell structures to obtain better emission intensity, a common practice for upconverting NPs studies.⁴⁴ We also discuss the mechanism of the observed UC process. The literature reports show that Tm³⁺ under 1208 nm laser radiation undergo ground state absorption (GSA) and excited state absorption (ESA) to populate ³F₃ and ³H₄ excited states.^29,30,40 Also, ET between Tm³⁺ ions can be involved in their excitation.^29,40 These few reports do not explain all of the processes taking place in Tm³⁺ and Er³⁺/Tm³⁺-doped NPs studied by us, e.g. why number of photons necessary to populate the emitting levels highly exceeds the theoretical values and how exactly Er³⁺ are being excited. The UC under 1208 nm by using Tm³⁺ ions seems to be still not sufficiently studied and understood. Additionally, we present here a proof of concept of how the properties of Tm³⁺-sensitized NPs can be used to detect temperature changes.

Experimental

Chemicals

All reagents were used as follows, without further purification. Commercial rare earth oxides Er₂O₃ (99.99%), Tm₂O₃ (99.99%), Y₂O₃ (99.99%), from Stanford Materials (United States), acetic acid (≥99%), ammonium fluoride (99%, ACS Reagent), and n-hexane (≥99%) from Honeywell (Poland), 1-octadecene (ODE, 90%) and oleic acid (OA, 90%) from Alfa Aesar (Germany), sodium oleate (82%) from Sigma-Aldrich (Poland), ethanol (99.8%) from Avantor (Poland), aqueous solutions of acetic acid (40% and 80%) were prepared by dissolution in de-ionised water of acetic acid (≥99%), and nitrogen (99.99%) from Linde (Poland) were used as received.

Synthesis of core@shell NPs

To synthesise NPs, we applied the procedure described by Homann et al.²³ The core and core@shell NPs were obtained in the high-boiling point solvents, i.e. OA and ODE and from the rare earth acetate salts. Ammonium fluoride was a source of fluorine ions during the synthesis.

Synthesis of α-NaYF₄ and α-NaYF₄:Ln³⁺ NPs

Initially, the OA and 1-ODE (1 : 1) mixture was put into a three-neck flask connected to a reflux condenser and outgassed using a Schlenk line at 100 °C under low pressure. After 1 h, the mixture was cooled to room temperature, and the selected powder rare earth acetates (see ESI for the synthesis procedure†) were added. The solution of acetates was heated to 100 °C under low pressure, and the mixture was kept at this temperature for 1 h. In the next step, the powder of sodium oleate was put into the solution at 100 °C. Then, the solution was outgassed for 20 min until the dissolution of sodium oleate and the powder of ammonium fluoride was added to the mixture. The solution was again outgassed in three cycles and heated to 200 °C under the nitrogen flow. After heating for 1 h, the synthesis mixture was cooled to room temperature and centrifuged. The collected supernatant was applied to the isolation of fluoride NPs. The addition of ethanol to the supernatant led to the precipitation of NPs. The colloidal solution of NPs was centrifuged, and then the NPs were dissolved in n-hexane (1 mmol NPs : 5 mL of n-hexane). The NPs were precipitated again after the addition of ethanol and centrifuged. The following materials were obtained: α-NaYF₄; α-NaYF₄:2%Tm³⁺; α-NaYF₄:2%Er³⁺,2%Tm³⁺; α-NaYF₄:2%Er³⁺,5%Tm³⁺ and α-NaYF₄:5%Er³⁺,2%Tm³⁺. The obtained materials were applied to the synthesis of β and core@shell NPs.

Synthesis of core β-NaYF₄:Ln³⁺ NPs

The core β-NaYF₄:Ln³⁺ NPs were obtained by recrystallising the α-core precursor at 300 °C. The purity of the α-core precursor before synthesis was analysed using thermogravimetric analysis (TGA). In the beginning, the α-core precursor was dissolved in the mixture of organic solutions (1 mmol of the α-core precursor was dissolved in 3.72 mL of ODE and 3.72 mL of OA). This mixture was outgassed at 100 °C for 2 h under low pressure. In the next step, the mixture was heated to 300 °C under the flow of nitrogen, and after 55 min, the synthesis mixture was cooled to room temperature. The NPs from the solution were precipitated by adding ethanol and centrifugation. In the next step, the NPs were dissolved again in n-hexane, precipitated one more time after adding ethanol and centrifuged. The following materials were prepared: β-NaYF₄:2%Tm³⁺; β-NaYF₄:5%Er³⁺,2%Tm³⁺; β-NaYF₄:2%Er³⁺,2%Tm³⁺; and β-NaYF₄:2%Er³⁺,5%Tm³⁺.

Synthesis of β-NaYF₄:Ln³⁺@β-NaYF₄ core@shell NPs

The β-NaYF₄:Ln³⁺ and α-NaYF₄ NPs were mixed in a 1 : 7 ratio and dissolved in the ODE/OA solution, keeping the ratio: 1 mmol of NPs : 2 mL of ODE : 2 mL of OA. In the beginning, precursors were outgassed at 100 °C for 2 h under low pressure and heated to 300 °C under nitrogen flow. After 135 min, the mixture was cooled to room temperature. The NPs from the solution were precipitated by the addition of ethanol and centrifuged, followed by the dissolution of NPs in n-hexane. The NPs were precipitated again after the addition of ethanol and centrifuged. The following materials were obtained: β-NaYF₄:2%Tm³⁺@β-NaYF₄; β-NaYF₄:5%Er³⁺,2%Tm³⁺@β-NaYF₄; β-NaYF₄:2%Er³⁺,2%Tm³⁺@β-NaYF₄; and β-NaYF₄:2%Er³⁺,5%Tm³⁺@β-NaYF₄.

Characterisation of materials

The materials were characterised using selected analytical techniques. TGA was carried out in the flow of pure nitrogen using a Thermogravimetric Analyzer TGA 4000 (PerkinElmer) in the temperature range of 30–600 °C with a temperature ramp of 10 °C min⁻¹.

X-ray diffraction (XRD) measurements were performed for fresh powder samples using a Bruker AXS D8 Advance diffractometer equipment with Cu K_α radiation (λ = 0.154 nm), with a step size of 0.05° in the angle range (2θ = 10–80°). The XRD results were used to estimate the size of NPs using the Scherrer formula. The diffraction patterns were compared with those reported in the Inorganic Crystal Structure Database (ICSD).

Before measurements using transmission electron microscopy (TEM), the samples in the form of colloid solution of fluorides in n-hexane were deposited on a copper grid covered with a holey carbon film. TEM images of samples were carried out using a JEOL 1400 electron microscope or Hitachi HT7700 electron microscope operating at 80–120 kV, and the size of particles was estimated using ImageJ™ software.

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with energy-dispersive X-ray spectroscopy (EDXS) was used to investigate the chemical composition of core@shell NPs. The experiments were performed on an FEI Osiris ChemiSTEM microscope at 200 keV electron energy, which is equipped with a Bruker Quantax system (XFlash detector) for EDXS. EDXS spectra are quantified with the FEI software package “TEM imaging and analysis” (TIA) version 4.7 SP3. Using TIA, element concentrations were calculated based on a refined Kramers’ law model, which includes corrections for detector absorption and background subtraction. EDXS spectra obtained while scanning a rectangular area inside an NPs ensemble were used to determine their average chemical composition. The concentration profiles of different chemical elements within a single nanoparticle were determined from EDXS spectra measured along a line scan that passes through its centre.

EDXS line profiles were recorded by applying a drift-correction routine via cross-correlation of several images, which yields a local precision better than 1 nm. The drift-corrected EDXS line profiles were taken with a probe diameter of 0.5 nm and a distance of about 1 nm between two measuring points. The quantification of Na-, F-, Y-, Tm- and Er-content from EDXS line or area scans was performed by using the Na-K, Y-L, Tm-L and Er-L series, as well as the F-K_α line.

The crystal structure of selected NPs was investigated by high-resolution transmission electron microscopy (HR-TEM) performed on an aberration-corrected FEI Titan³ 80–300 microscope at 300 kV acceleration tension. HR-TEM images were evaluated by calculating the two-dimensional Fourier transform (FT), which yields information on the crystal structure (lattice parameters and crystal symmetry) of single NPs. The analysis was performed by comparing the experimental FT and the calculated diffraction patterns with Miller indices, where the latter were obtained by using the Jems software. The zero-order beam (ZB) is indicated by using a white circle.

Photoluminescence properties were studied at room temperature using a PIXIS:256E Digital CCD Camera equipped with an SP-2156 Imaging Spectrograph (Princeton Instruments) and QuantaMaster™ 40 spectrophotometer (Photon Technology International), equipped with a H10330C-75 photomultiplier (Hamamatsu). A continuous multiwavelength, 2 W diode laser (from CNI) was used as the excitation source at 1208 nm, coupled to a 200 μm optical fibre and collimator (from Thorlabs). For the luminescence risetimes measurements the laser wave was triggered by arbitrary function generator GW Instek AFG-2005. All spectra were corrected for the spectral response of the equipment. A 10A-PPS power meter (Ophir Photonics) determined the beam size and laser powers.

Temperature-dependent emissions were measured by exciting samples with a 1208 nm laser placed in a tubular electric furnace (Gero RES-E 230/3), where the temperature of the sample was controlled via a type K thermocouple in contact with it. A 10 ns pulsed optical parametric oscillator OPO (EKSPLA/NT342/3/UVE) was used as a laser source. Emissions from the oven were focused on the entrance slit of a spectrograph (Andor SR-303i-A) equipped with a cooled CCD camera (Andor Newton). All spectra were corrected from the spectral response of the equipment.

Results and discussion

Structure and morphology

The obtained XRD patterns of the products (Fig. S1†) show the presence of the crystal structure typical for cubic phase (ICSD no. 77099) for samples obtained at a lower temperature (200 °C). At higher temperature (300 °C), cubic NPs transformed to the hexagonal crystal phase (ICSD no. 51916). Also, core@shells crystallised as hexagonal NPs (Fig. 1a).

Fig. 1 (a) XRD patterns performed for core@shell nanomaterials, (b) TEM image and particle size distribution of NaYF₄:5%Er³⁺,2%Tm³⁺@NaYF₄ sample, (c) left panel: HR-TEM image of the NaYF₄:5%Er³⁺,2%Tm³⁺@NaYF₄ nanoparticle, and right panel: its FT pattern, which agrees with the calculated diffraction pattern of bulk hexagonal NaYF₄ (space group P6̄, No. 174) in the [215]-zone axis (orange circles); (d) left panel: HAADF STEM image with the EDXS line scan direction, which was applied to calculate a chemical composition of a NaYF₄:5%Er³⁺,2%Tm³⁺@NaYF₄ nanoparticle; right panel: concentration profiles determined after the quantification of the EDXS line scan recorded from the core@shell NP on the left panel.

TEM measurements (Fig. 1, S2 and S3†) revealed that all β-cores precipitated as small NPs with average 8–14 nm sizes. The shell deposition on their surface increased the size of NPs to around 10–22 nm depending on the size of source core NPs. TEM images in Fig. S2 and S3† reveal that the synthesis procedure successfully generated homogeneous NPs. The composition of NPs influenced their shape. In the case of β-core doped only with Tm³⁺ ions (NaYF₄:2%Tm³⁺), NPs are hexagonal rounded. The NaYF₄:2%Tm³⁺@NaYF₄ core@shell NPs also had a hexagonal structure (Fig. 1b). The addition of a higher concentration of Er³⁺ ions to the structure of NaYF₄ led to the change in the shape of NPs to more elongated, rod-like (Fig. S2†). It is caused by the anisotropic growth of NPs in the presence of Er³⁺ ions, which have different sizes than Y³⁺ ions.⁴⁵ HR-TEM and FT presented in Fig. 1c also confirm the hexagonal structure of prepared NPs similar to pure NaYF₄ without significant changes after doping with Er³⁺ and Tm³⁺ ions.

The differences in NPs sizes may result from the nature of the synthesis method, which is quite sensitive to changes in conditions and has limited repeatability, mainly affecting small NPs. As evident from the comparison of Fig. S2 and S3,† the final NPs sizes were significantly influenced by the coating with shell applied to the obtained cores. Controlling this process is particularly challenging, and size variations may result from fluctuations in the synthesis conditions.

To investigate the average chemical composition of NaYF₄:2%Tm³⁺,5%Er³⁺@NaYF₄ NPs, we used EDXS spectra obtained by scanning a rectangular area, which includes inside an ensemble of NPs. Fig. 1d shows the distribution profiles of Na, F, Y, Tm and Er elements in the single nanoparticle determined from the EDXS line scan, which was recorded along a line that passes through the object's centre (arrow in Fig. 1d, left panel). The quantification of EDXS line scans performed using the X-ray characteristic lines of the corresponding elements, i.e. lines of the Na-K, Y-L, Tm-L and Er-L series, as well as of the F-K line, from the core@shell NPs gave information about the chemical composition. The chemical composition of NaYF₄: 2%Tm³⁺,5%Er³⁺@NaYF₄ was calculated from the EDXS line scan shown in Fig. 1d, and the obtained results Na_16.0±0.5Y_16.0±1.3Er_1.5±0.5Tm_0.3±0.1F_66.2±1.4 correspond with the assumed composition.

Spectroscopic properties

NPs doped with Tm³⁺ can absorb 1208 nm NIR light due to the ³H₆ → ³H₅ electronic transition. Fig. S4† demonstrates the absorption spectra of NaYF₄:2%Tm³⁺,5%Er³⁺@NaYF₄ NPs, where between 1150 and 1250 nm, a broad and intense absorption band is visible. This fact makes it possible to use 1208 nm to excite Tm³⁺ ions to higher energy levels due to the sequential absorption of several NIR photons.²⁹ The effect of this process is the UC of Tm³⁺ ions at 698 and 802 nm. Fig. 2a shows the emission of NaYF₄:2%Tm³⁺@NaYF₄ NPs (red line). The main emission occurs at 802 nm due to the ³H₄ → ³H₆ transition. Under 1208 nm excitation, some of the Tm³⁺ ions are excited even up to the ¹G₄ excited state, but emission connected with ¹G₄ → ³H₆ transition is relatively weak (see Fig. 2b). It means a very low probability of the ¹G₄ state population. NaYF₄:2%Tm³⁺@NaYF₄ NPs also show down-shifted emission in the NIR range with the maximum at 1625 nm connected with ³F₄ → ³H₆ transition (Fig. 2c).

Fig. 2 (a) emission spectra of the NPs obtained under 1208 nm laser excitation, (b) emission in the blue region of spectra indicating transitions from the higher excited states of Er³⁺ and Tm³⁺ ions and (c) emission in the NIR range of the sample containing the highest concentration of Er³⁺ ions showing down-shifted emission of NPs under 1208 m laser excitation.

The emission properties change dramatically with co-doping with Er³⁺ ions. Under 1208 nm excitation, all of the NaYF₄:Er³⁺,Tm³⁺@NaYF₄ NPs show red emission with a maximum at 660 nm, related to the ⁴F_9/2 → ⁴I_15/2 transition. Tm³⁺ ions transfer the absorbed energy to Er³⁺ ions, which results in their population up to the ²H_9/2 excited state (see Fig. 2b). Emission of Er³⁺ ions under 1208 nm also occurs at around 526 and 546 nm due to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions and at 982 nm as the result of the ⁴I_11/2 → ⁴I_15/2 transition. Second, the most intense emission band has its maximum at around 802 nm, which is a result of the ³H₄ → ³H₆ transition of Tm³⁺ ions. However, it is also possible that in the case of Er³⁺ and Tm³⁺ doped NPs, this band also corresponds to the ⁴I_9/2 → ⁴I_15/2 transition of Er³⁺ ions in some part. Interesting is the enhancement of Tm³⁺ ions UC at 475 nm and new emission band at around 455 nm when Er³⁺ ions are co-dopants (Fig. 2b). It means that Er³⁺ take part in the population of higher excited states of Tm³⁺ ions.⁴⁶ Down-shifted emission of NaYF₄:Er³⁺,Tm³⁺@NaYF₄ NPs is visible in Fig. 2c as two broad bands with the maximum at around 1525 nm, which is the result of ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions and at 1625 nm which is connected with ³F₄ → ³H₆ transition of Tm³⁺ ions.

The dependence of UC intensity on laser power densities presented in Fig. 3 shows the complex nature of the studied system. Three to four photons are required to observe the emission of NaYF₄:2%Tm³⁺@NaYF₄ NPs at 698 and 802 nm (see ESI for the methodology of calculations†). Above power density of 70 W cm⁻², slope values increase from 2.1 to above 3.3. In previous reports, usually, two photons were required to populate the ³F₃ and ³H₄ excited states of Tm³⁺ ions under similar excitation wavelength but with lower power densities (<30 W cm⁻²).^29,30

Fig. 3 Upconversion intensity dependencies on laser power density measured for (a) NaYF₄:2%Tm³⁺@NaYF₄, (b) NaYF₄:5%Er³⁺,2%Tm³⁺@NaYF₄, (c) NaYF₄:2%Er³⁺,2%Tm³⁺@NaYF₄ and (d) NaYF₄:2%Er³⁺,5%Tm³⁺@NaYF₄ NPs.

Five 1208 nm photons are required to excite Er³⁺ to the ²H_11/2 and ⁴S_3/2 green-emitting levels (see Fig. 3b–d). The slope values determined for emission bands at 526 and 546 nm of NaYF₄:Er³⁺,Tm³⁺@NaYF₄ NPs vary between 4.2 and 4.8 in the 70–130 W cm⁻² power density range. The green emission is not even visible below the power density threshold mentioned above. In the case of red emission of Er³⁺ ions from the ⁴F_9/2 excited state, the slope values vary between 3.9 and 4.4 in the 70–130 W cm⁻² power density range. Emission at 982 nm from the ⁴I_11/2 energy level requires fewer photons than from the ⁴F_9/2 excited state as slope values oscillate between 3.2 and 3.6.

Below 70 and above around 130 W cm⁻² slopes of presented curves are lower than in the mentioned 70–130 W cm⁻² range. Lowering the slope values at high power densities results from saturation effects where intermediate excited states undergo depletion by different nonradiative mechanisms.⁴⁷ Similar power dependencies were found for Gd₂O₃ NPs doped with Er³⁺ ions under 970 nm laser excitation.⁴⁸ Such anomalous behaviour can be explained by a positive feedback looping upconversion mechanism.^41,48 An extreme form of looping is known as photon avalanche.³⁸ Avalanche processes can be seen in many papers,^49,50 recently also in core@shell NPs.⁵¹ Furthermore, it should also be taken into account that in the case of a typical photon avalanche process, slope values above the critical excitation power density do not represent the number of photons. The high number is associated with the nonlinear effect of photon avalanche. However, in the case of the samples under investigation, we are not dealing with a typical photon avalanche process.

Upconversion mechanism

The mechanism of observed UC is complex and not obvious. Under excitation with 1208 nm laser light, Tm³⁺ ions undergo GSA to the ³H₅, the second excited state. The excitation to higher excited states is possible via the ESA of the second 1208 nm photon.^30,40 However, here we postulate different mechanism of excitation of Tm³⁺ ions. Based on luminescence decays (Fig. 4) and the rate equation model (see ESI Fig. S6†), the ³F₃ excited state of Tm³⁺ is populated mainly by ET1 (Fig. 5) from Tm³⁺ ions in their ³F₄ excited state. This is also consistent with the slope values calculated for the 698 nm emission band at low power densities <70 W cm⁻² (Fig. 3a).

Fig. 4 Luminescence rises and decays measured under laser λ_ex = 1208 nm excitation for (a) NaYF₄:2%Tm³⁺@NaYF₄ and (b) NaYF₄:5%Er³⁺,2%Tm³⁺@NaYF₄ NPs.

Fig. 5 Mechanism of up-conversion in (a) NaYF₄:Tm³⁺@NaYF₄ NPs and (b) NaYF₄:Er³⁺,Tm³⁺@NaYF₄ NPs, where ET – energy transfer, CR – cross-relaxation process. The Er³⁺ ions are equivalent; the processes involving them were separated only for the sake of clarity.

Interesting is the origin of emission from the ³H₄ excited state of Tm³⁺ ions at 802 nm, which dominates the emission spectra of NaYF₄:2%Tm³⁺@NaYF₄ (Fig. 2a). There are different possibilities to populate the ³H₄ level: it can be populated by relaxation from ³F₃ excited state or other energy transfer processes from lower levels. However, as evident from Fig. S7 in ESI,† when considering relaxation from the ³F₃ level, the temporal evolution of the UC intensity at 802 nm aligns remarkably well with the theoretical pattern.

Once the ions are excited to the ³F₃ and ³H₄ levels, they can relax through different processes, including CR channels that repopulate the ³F₄ and ³H₅ levels. From these levels, the upconversion process ET1 could start again. This dynamic can potentially initiate a looping process or an avalanche effect that gains efficiency with higher pumping power. This phenomenon can elucidate the observed slope alterations, as illustrated in the curves of Fig. 3. A change in the rise time can also characterise these processes. Therefore, the rise times of the UC emissions, defined by the Kohlrausch function, have been calculated. As shown in Fig. S5,† about 80 W cm⁻², the rise time reaches a maximum that is a characteristic of the avalanche process.^49,50 Moreover, this maximum at about 80 W cm⁻² coincides with the slope change shown in Fig. 3.

The excitation mechanism of Er³⁺ ions under 1208 nm laser light is based on the ET process originating from Tm³⁺ ions. Due to the similar energies between ³H₄ of Tm³⁺ and ⁴I_9/2 state of Er³⁺ ions, we postulate ET between these two electronic states as the primary population pathway of Er³⁺ ions (ET2 in Fig. 5). Emission lifetime of Tm³⁺ ions connected with ³H₄ → ³H₆ transition is five times shorter when Er³⁺ ions are present in the structure (see Fig. 4) which confirms our conclusion. Next, Er³⁺ ions can be excited to the ²H_11/2 or ⁴S_3/2 energy states by another ET from Tm³⁺ ions in their ³F₄ excited state (ET3). A higher concentration of Tm³⁺ ions result in a more intense emission of Er³⁺ ions. However, the probability of ET3 from the ⁴I_9/2 excited state of Er³⁺ ions is relatively low as Er³⁺ ions undergo multiphonon relaxation from the ⁴I_9/2 to the ⁴I_11/2 excited state. Fig. 3b–d shows that up to five photons are necessary to achieve green emitting levels of Er³⁺ ions. Four of them are consumed to populate Tm³⁺ to the ³H₄ excited state and another one to excite Er³⁺ to ²H_11/2 or ⁴S_3/2 energy states. The role of Tm³⁺ ions in the population of Er³⁺ is also visible in Fig. 2b, where the increase of Tm³⁺ concentration from 2% to 5% allowed for another ET and excitation of Er³⁺ ions up to the ²H_9/2 excited state and their blue emission at around 410 nm.

Increasing the concentration of Er³⁺ from 2% to 5% quenched green emission. Er³⁺ ions undergo CR with Er³⁺ ions in their ⁴I_15/2 ground state (see Fig. 5b). There are many possible CR pathways between Er³⁺ ions,^52,53 but in the studied system, the CR2 presented in Fig. 5 seems to be the most probable. CR process may also precede the population of Er³⁺ ions to their ⁴F_9/2 energy level but this population pathway is rather negligible. The excitation of Er³⁺ ions from ⁴I_11/2 to ⁴F_9/2 excited state occurs via ET from Tm³⁺ ions. Tm³⁺ to Er³⁺ ET is a subject of many reports and explains the shift from green to red upconversion in materials co-doped with Tm³⁺.^54–57 Here, we postulate that the ET3 from the ⁴I_11/2 excited state of Er³⁺ ions is more probable than from the ⁴I_9/2 state. Red luminescence does not result from multiphonon relaxation from the ⁴S_3/2 excited state.⁵⁸ Our conclusion also confirms the decrease of the Er³⁺ emission band at 982 nm with an increased concentration of Tm³⁺ ions. A higher concentration of Tm³⁺ ions means depopulation to the ⁴I_11/2 excited state, leading to the excitation of Er³⁺ to the ⁴F_9/2via ET3. Furthermore, we believe Er³⁺ ions also participate in energy looping by populating Tm³⁺ ions to the ³F₄ level through ET4. This is also reflected in the prolonged luminescence lifetime from the ³F₄ level (1660 nm) in the sample doped with both Er³⁺ and Tm³⁺ ions compared to the sample containing only Tm³⁺ ions (Fig. 4).

Temperature sensing properties

NaYF₄:Er³⁺,Tm³⁺@NaYF₄ NPs are ideal candidates for temperature sensing applications in biological medium. They can be excited within the 2^nd biological window, whereas their luminescence is in the 1^st biological window. Fig. 6 presents the spectroscopic properties of the NaYF₄:2%Er³⁺,5%Tm³⁺@NaYF₄ NPs measured in the 21–115 °C range. The intensity of emission bands changes with increasing temperature. The dependence of luminescence intensity ratio (LIR) between two luminescence bands on temperature is a perfect tool for remote temperature determination.^43,59,60Fig. 6b shows that the ⁴I_11/2 → ⁴I_15/2 transition band of Er³⁺ ions with a maximum of around 982 nm is the most sensitive to the temperature change (due to the multiphonon relaxation from ⁴I_9/2 state, see Fig. 2), while the ³F₃ → ³H₆ transition band of Tm³⁺ ions, with the maximum at 698 nm, shows relatively low sensitivity. These two bands can be used for ratiometric temperature sensing with the best effect. The 982/698 LIR dependence on temperature depicted in Fig. 6c confirms that assumption. Other promising bands for temperature sensing are connected with the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ ions (660 nm) and ³H₄ → ³H₆ transition of Tm³⁺ ions (802 nm). Therefore, the 660/698 and 982/802 LIRs are also relatively sensitive to temperature change, but less than in the case of 982/698 LIR.

Fig. 6 (a) Temperature-dependent upconversion spectra of NaYF₄:2%Er³⁺,5%Tm³⁺@NaYF₄ NPs under λ_ex = 1208 nm excitation, (b) integrated intensity of Tm³⁺ and Er³⁺ emission bands in function of temperature, (c) luminescence intensity ratios and (d) relative temperature sensitivities determined from the measured spectra.

The dependencies of measured LIRs on temperature were fitted with Boltzmann-like or cubic functions to calculate relative temperature sensitivities, S_r (see ESI for more details†). The determined S_r values as the dependencies on temperature are presented in Fig. 6d. The 982/698 LIR shows the best sensitivity in the whole temperature range with a maximum value of 2.37%/°C at 57 °C. This value determines that the obtained nanothermometer is very good for temperature-sensing applications. Notably, in the physiological range, 22–50 °C, the obtained nanothermometer offers a relatively high sensitivity between 1.6 and 2.3%/°C. The sensitivity of thermometers close to 3%/°C is recently considered high.⁶¹ The relative sensitivities of 660/698 nm and 982/802 nm LIRs are lower than the 982/698 nm LIR but still at a high level, i.e. 1.69%/°C at 77 °C and 1.58%/°C at 67 °C respectively.

The green ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 luminescence bands of Er³⁺ ions are the most frequently studied for applications in optical thermometry.^9,42,43,62 The ²H_11/2 and ⁴S_3/2 energy states of Er³⁺ ions are thermally coupled as their energy difference is around 625 cm⁻¹, considering the recorded emission spectrum (the Boltzmann function fit yields the value of 655 cm⁻¹). Thermalisation processes lead to an increase in the intensity of the higher-energy transition (at 526 nm) and a decrease in the intensity of the lower-energy one (at 546 nm), according to the Boltzmann distribution. In many cases, the relative temperature sensitivity of green-emitting bands is satisfactory and can be used for temperature determination. However, in the case of NaYF₄:2%Er³⁺,5%Tm³⁺@NaYF₄ NPs, the calculated S_r values indicate much lower sensitivity of 526/546 LIR (maximum 1.08%/°C at 22 °C) than those determined for 982/698 and 982/802 or 660/698 LIRs, but similar to those reported in the literature.^9,42,63 Additionally, biological materials are usually less transparent for green luminescence, which makes the use of green bands of Er³⁺ ions less beneficial than those in the red and NIR range.²⁷

Conclusions

In summary, our study shows that Tm³⁺ ions can be used as sensitisers in upconverting core@shell NPs. Under 1208 nm NIR radiation, NaYF₄:Tm³⁺@NaYF₄ and NaYF₄:Er³⁺,Tm³⁺@NaYF₄ NPs showed intense emission. The Tm³⁺-doped only NPs are able of UC to 698 and 802 nm via the ground state absorption followed by energy transfers and CR between Tm³⁺ ions. Adding Er³⁺ as a second co-dopant results in higher luminescence intensity and red luminescence of NPs. The emission from Er³⁺ ions is possible due to the ET from Tm³⁺ ions.

The studied system revealed the complex mechanism of UC based on energy looping. Such a process generates multiple Tm³⁺ ions in their ³F₄ excited state by a sequence of GSA, ET and CR processes. The proposed mechanism explains high slope values determined from emission intensities’ dependencies on laser powers. The observed UC resulted from 4 photon excitation mechanism in the case of Tm³⁺ emissions at 698 and 802 nm or 5 photons for Er³⁺ emissions at 526, 546 and 660 nm.

The Er³⁺/Tm³⁺ system is promising for biological applications because of excitation possibility within the 2^nd biological window at 1208 nm and emission within the 1^st biological window, with major bands at around 660 and 802 nm. The emission at 982 nm is also intense. Additionally, we presented that luminescence of NaYF₄:Er³⁺,Tm³⁺@NaYF₄ NPs is temperature dependent, and the 982/698 nm, 660/698 nm and 982/802 nm band ratios show high relative temperature sensitivities. The emission bands used for temperature-sensing applications must be close to each other, which lowers possible effects and distortions caused by the properties of the biological environment. Our NPs show such property.

The presented article proves that the UC phenomenon is possible in other than Yb³⁺-based systems, the most frequently used. However, the obtained results indicate that the Er³⁺/Tm³⁺ system still requires further research, mainly to optimise its emission and fully understand the mechanism of UC at different concentrations of dopants. It is also essential to test the proposed NPs as nanothermometers in real biological systems where the potential of excitation within 2^nd biological system can be fully exploited.

Author contributions

T. G.: conceptualisation, data curation, formal analysis, funding acquisition, investigation, methodology, visualisation, writing – original draft, writing – review & editing. I. R. M.: conceptualisation, data curation, methodology supervision writing – original draft, writing – review & editing, formal analysis. R. P.: formal analysis, methodology, resources, visualisation, writing – original draft.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The National Science Centre, Poland, provided funding for this research under grant no. 2016/22/E/ST5/00016. We want to express our sincere gratitude to Dr Piotr Kamiński for his help with core/shell nanoparticles synthesis and Dr Artur Tymiński for measurements of the power dependencies.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis procedures, structure and morphology studies, absorption spectra and luminescence decays, methodology of calculations. See DOI: https://doi.org/10.1039/d3nr04418a

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