**1. Introduction**

Inorganic nanocrystals, due to their high mechanical, thermal and chemical stability, have garnered an immense interest from the point of view of their potential implementation in biomedical application, i.e., optical and magnetic resonance imaging, drug delivery, light-induced hyperthermia generation etc. [1–4]. Their optical properties may be in a facile way modified by the introduction of the appropriate optically active ions like lanthanide (Ln<sup>3</sup>+) and/or transition metals (TM) ions [5–13] to the host material. Besides unique chemical and physical features, they reveal size- and shape-dependent spectroscopic properties, which are not observed for organic-based nanomaterials [1]. Due to the fact that the optical properties of such nanoparticles are strongly affected by the temperature, their luminescence may be employed to non-contact temperature sensing (luminescent thermometry, LT). In LT, temperature readout relies on the analysis of thermally-affected spectroscopic parameters like emission intensity, luminescence lifetime, peak position, band shape and polarization anisotropy [14–16]. One of the most important advantages of LT in respect to other temperature measurement techniques is the fact that it provides a real-time temperature readout with unprecedented spatial and thermal resolution [15,17,18]. Additionally, temperature readout is provided in an electrically passive mode what enables to achieve the information about, i.e., the condition of living organisms where even small temperature fluctuations are usually accompanied by serious health diseases and improper cellular biochemical

processes [16,19–22]. The use of the nanosized LTs enables the improvement of the spatial resolution of temperature readout. However, in order to obtain high thermal resolution of temperature measurement, different approaches, which enable to increase the relative sensitivity of LT to temperature changes, were proposed up to date. As was recently demonstrated, the utilization of transition metal ions luminescence with lanthanide co-dopant as a luminescent reference enables the enhancement of temperature sensing sensitivity, luminescence brightness and the broadening of usable temperature range in which LT operates [23–25]. For this purpose, optical properties of different TM were investigated, such as V<sup>3</sup>+/V<sup>4</sup>+/V5<sup>+</sup> [23,26], Co2+ [27], Ti<sup>3</sup>+/Ti4<sup>+</sup> [28], Cr3+ [24,25], Mn<sup>3</sup>+/Mn4<sup>+</sup> [29] and Ni2<sup>+</sup> [30]. Another advantage of using TM is the susceptibility of their optical properties to the modification of the crystal field strength via host stoichiometry due to the fact that *d* electrons, located on the valence shell, are exposed to the local environment and crystal field changes. This phenomenon was investigated in detailed in case of temperature sensing performance of Cr3+ ions where the structure of host materials were varying from Gd3Al5O12 (GAG) to Gd3Ga5O12 (GGG), and from Y3Al5O12 (YAG) to Y3Ga5O12 (YGG) via changing the Al3+ to Ga3+ ratio [24,25]. As was recently shown for Cr3+ ions, such modification enables not only enhancement of the sensitivity of LT but also tuning of the spectral position of emission band [25]. These kinds of studies have not ye<sup>t</sup> been conducted for V-based luminescent thermometers.

Therefore, in this work, we present for the first time a strategy that enables the improvement of temperature-sensing properties of V-based luminescent nanothermometers via modification of the host material composition. This approach bases on the gradual substitution of Al3+ ions by Ga3+ ions into YAG nanocrystals. The introduction of gallium ions, which possess larger ionic radii in respect to Al3+ ones leads to the lowering of crystal field (CF) strength. This arises from the elongation of the metal-oxygen (M-O) distance along with the enhancement of the contribution of Ga3+ ions. The modification of the crystal field strength should strongly influence the temperature-dependent luminescent properties of V ions of different oxidation state (V<sup>5</sup>+, <sup>V</sup>4+, V3<sup>+</sup>). Moreover, the introduction of the gallium ions facilitates the stabilization of V4<sup>+</sup> oxidation state that possesses favorable performance for luminescent thermometry. However, these expectations have not ye<sup>t</sup> been experimentally verified. Therefore, the aim of this work is to study the influence of the Ga3+ ions concentration of the temperature dependent luminescent properties of vanadium ions in Y3Al5−<sup>x</sup>GaxO12:V nanocrystals, with the special emphasize put on their application in luminescent thermometry.

#### **2. Materials and Methods**

#### *2.1. Synthesis of V-doped Y3Al5*−*<sup>x</sup>GaxO12*

The Y3Al5−<sup>x</sup>GaxO12 nanocrystals doped with 0.1% concentration of V ions were synthesized via a modified Pechini method, where the Ga3+ amount was set to x = 1, 2, 3, 4 and 5. The amount of V ions was set to 0.1% due to the fact that this V concentration provides the most significant temperature sensing properties of YAG:V, Ln3<sup>+</sup> luminescent nanothermometers [23]. The first step was the creation of yttrium nitrate from yttrium oxide (Y2O3, 99.995% purity from Stanford Materials Corporation, Lake Forest, CA, USA) using the recrystallization process, including the dissolution in distillated water and ultrapure nitric acid (65%). All nitrates, namely appropriate amounts of Ga(NO3)3·9H2O (Puratronic 99.999% purity from Alfa Aesar, Kandel, GERMANY), Al(NO3)3·9H2O (Puratronic 99.999% purity from Alfa Aesar, Kandel, GERMANY) and Y(NO3)3 were dissolved in water and mixed together. After that, NH4VO3 (99% purity from Alfa Aesar, Kandel, GERMANY) were added to the solution. To enable the dissolution of ammonium metavanadate and the complexation of each metal, calculated quantity of citric acid (CA, C6H8O7 with 99.5+% purity from Alfa Aesar, Kandel, GERMANY), used in six-fold excess in respect to the total amount of metal ions, was mixed with all reagents and heated up to 90 ◦C for 1 h. Next, PEG-200 (poly(ethylene glycol), from Alfa Aesar, Kandel, GERMANY) was added dropwise to the CA-metal complex and stirred for 2 h at 90 ◦C (CA: PEG-200 was 1:1) to conduct the polyestrification reaction. Then, the resin was obtained by heating at 90 ◦C for 1 week. In turn, the nanopowders were received via annealing of resin at 1100 ◦C for 16 h in air atmosphere.
