On the role of Gd3+ ions in enhancement of UV emission from Yb3+–Tm3+ up-converting LiYF4 nanocrystals

Materials capable of emitting ultraviolet (UV) radiation are sought for applications ranging from theranostics or photodynamic therapy to specific photocatalysis. The nanometer size of these materials, as well as excitation with near-infrared (NIR) light, is essential for many applications. Tetragonal tetrafluoride LiY(Gd)F4 nanocrystalline host for up-converting Tm3+–Yb3+ activator-sensitizer pair is a promising candidate to achieve UV–vis up-converted radiation under NIR excitation, important for numerous photo-chemical and bio-medical applications. Here, we provide insights into the structure, morphology, size and optical properties of up-converting LiYF4:25%Yb3+0.5%Tm3+ colloidal nanocrystals, where 1, 5, 10, 20, 30 and 40% of Y3+ ions were substituted with Gd3+ ions. Low gadolinium dopant concentrations modify the size and up-conversion luminescence, while the Gd3+ doping that is exceeding the structure resistance limit of the tetragonal LiYF4 results in appearance of foreign phase and significant decrease of luminescence intensity. The intensity and kinetic behavior of Gd3+ up-converted UV emission are also analyzed for various gadolinium ions concentrations. The obtained results form a background for further optimized materials and applications based on LiYF4 nanocrystals.


Introduction
Up-converting lanthanide doped colloidal luminescent nanoparticles capable to emit light from ultraviolet (UV) to near infrared (NIR) region under near-infrared photoexcitation are important class of materials utilized for fluorescent imaging labels [1,2], multifunctional phototherapy agents [3][4][5], fluorescent sensors [6][7][8] or in super-resolution imaging as well as in efficient reactive oxygen species generation [9][10][11]. They also gain attention as mediators for light activated photo-switching [12], photopolymerization [13][14][15] or photodynamic therapy [16]. Usually, modification of luminescent and up-converting properties of these materials is achieved by selecting various activators (e.g. Tm 3+ , Er 3+ , Pr 3+ , Ho 3+ etc) and sensitizer (Yb 3+ , Nd 3+ ) ions, or involvement of core-shell designs with core and shell doped differently [17,18]. Alternatively, selection of appropriate host matrix may determine the luminescent properties. This is because crystal phase, determining the ion-ion distance, their coordination, phonon energy of the lattice and thus probabilities of energy transfer and non-radiative processes, is crucial parameter for the up-conversion luminescence [19]. In yet another strategy to augment up-converting properties of lanthanide doped nanoparticles, structural ions are substituted with the other one (e.g. A = K/Na/Li or B = Y/Lu/Gd in ABF 4 ) to directly modify the crystallographic properties of the host and thus impact the properties of luminescent ions. Tetragonal LiYF 4 is a known host material, but far less studied than NaYF 4 matrix in terms of up-conversion. LiYF 4 has excellent properties e.g. low phonon energy, high stability and optical transparency, what makes it an ideal host material for incorporation of lanthanide ions. Out of many up-converting materials, tetragonal LiYF 4 nanocrystals doped with Yb 3+ and Tm 3+ ions are known for their efficient energy conversion to the higher thulium ions energy levels i.e. 1 D 2 and 1 I 6 [20][21][22], which outperform conventional NaYF 4 nanoparticles (NPs). Yb and Tm co-doped LiYF 4 -based nanocrystals were also one of the first reported nanoparticles emitting efficient UV light under NIR light excitation [20,23]. Emission in the UV region is an important property of nanocrystals, which extends the possibility of their application in optoelectronics, sensors, biological imaging, photodynamic therapy and photopolymerization. For this reason, it is important to research nanomaterials to optimize their emission in the ultraviolet range.
Substituting Y 3+ structural ions in tetragonal tetrafluorides crystal structure with Gd 3+ ions is promising from the point of view of further applications, but has not been extensively studied. In particular, combining Gd 3+ and efficiently UV up-converting Yb-Tm compositions offers many interesting new properties for further applications. Despite the fact Gd 3+ ions do not participate nor modulate energy transfer up-conversion and are not luminescent in the visible spectral range, these ions were demonstrated to play a role of migration centers in energy migration enhanced luminescence of e.g. Eu 3+ , Tb 3+ ions [24][25][26][27]. Energy migration through the Gd 3+ ions network is important especially in terms of applications requiring control of energy transfer within a core and shell or surface of nanocrystals [24,28,29]. Förster resonance energy transfer (FRET) based applications such as biological sensing and imaging at single molecule level can benefit from properly chosen nanocrystals architecture and Gd 3+ ions are known to enable energy migration at distances longer than nanocrystals dimensions [30]. Moreover, doping with Gd 3+ often results in crystal structure stabilization [31][32][33], but also in variation of morphology and size of nanocrystals [34,35], providing an additional factor used for tuning the luminescence. Owing to the magnetic moment of the gadolinium ions incorporated into the matrix, dual-mode luminescent and magnetic properties can also be observed [36][37][38][39]. NPs co-doped with Gd 3+ ions can be used as diagnostics materials, allowing to obtain higher contrast in magnetic resonance imaging (MRI) [40].
There are many literature reports utilizing Gd 3+ doping in up-converting nanocrystals, but most often hexagonal NaYF 4 is reported [41][42][43][44][45]. Although literature reports on doping of LiYF 4 with Gd 3+ ions exist, considered dopant concentrations were low or the articles were focused on LiGdF 4 synthesis itself, and thus high Gd 3+ concentrations were tested [23,35,46]. Here, LiYF 4 :25%Yb 3+ 0.5%Tm 3+ nanocrystals were used, despite the report that this dopants concentration is not necessary the most efficient [22]. However, because of different synthesis method utilized in different laboratories (hot-injection and trifluoroacetates as precursors, acetates with methanolic solution of LiOH and NH 4 F) we decided to use 25% Yb 3+ and 0.5% Tm 3+ utilized earlier as a kind of standard [47]. These nanocrystals were codoped with a wide (up to 40%) concentration range of gadolinium ions, and investigated spectroscopically to understand relationship between luminescent or magnetic properties and the composition, morphology and size of the nanoparticles. Up-converted Gd 3+ UV luminescence intensity and kinetics were characterized due to utilization of several milliseconds long excitation pulses that enabled saturation of Gd 3+ excited state.

Chemicals
Nanocrystals were prepared using commercially available reagents without any further purification. Yttrium oxide Y 2  The synthesis was based on the procedure described in [48] with some modifications. 30 ml of oleic acid and 75 ml of 1-octadecene were added to the round-bottom flask containing dry precursor. The solution was heated to 140°C and stirred under vacuum for 30 min to remove water and oxygen, and to form lanthanide oleates. Subsequently, the temperature was decreased to ∼50°C and the flask was placed under nitrogen flow. Then the solution of 12.5 mmol LiOH·H 2 O and 20 mmol NH 4 F in methanol was added to the reaction. The solution was stirred under nitrogen at 50°C for 15 min, then the flask was opened, temperature was slowly elevated up to 110°C and methanol was evaporated from the reaction mixture. The solution then was kept under vacuum for 15 min at 120°C. Afterwards, the temperature was increased to 300°C and the reaction was kept under nitrogen for 90 min. The mixture was then cooled to the temperature around 70°C and ethanol was added to precipitate the synthesized nanocrystals. Next, the nanocrystals were collected by centrifugation at 10 000 rpm for 10 min, washed with n-hexane and ethanol, centrifuged and dispersed in 10 ml of cyclohexane. The clear colloidal solution, taken from above white precipitate containing unreacted LiF, was used for further experiments.

Methods
Powder diffraction data were collected on an X'Pert PRO x-ray diffractometer equipped with PIXcel ultrafast line detector, focusing mirror and Soller slits for Cu Kα radiation. Morphology of the NPs was investigated by transmission electron microscope (TEM) (Philips CM20 SuperTwin at 160 kV). Magnetization measurements were carried out employing a quantum design MPMS-XL magnetometer. The photo-luminescence spectra were collected with a FLS1000 (Edinburgh Instruments) under 980 nm continuous wave fiber laser diode (P max =10 W) photoexcitation with utilization of additional external self-built system for measurements. All the photo-luminescence spectra were automatically corrected for R928P photomultiplier tube detector sensitivity in FLS1000 software. The spectra were collected from a colloidal solution of nanocrystals in cyclohexane. Upon high power density excitation at 980 nm the temperature can be locally higher and luminescent properties of materials often strongly depend on temperature [49,50]. However, in our measurements the diameter of the beam going through the sample cuvette (516 μm) was significantly smaller than cuvette diameter (5 mm) and thus, due to convection and mixing of the heated part with rest of the solution being at room temperature average temperature was unchanged. Moreover, no changes in the peaks intensity ratio were observed when measurements were collected for a longer time or repeated several times. Luminescence risetimes and lifetimes of Gd 3+ ions were measured using FLS 1000 with a R928P photomultiplier tube detector from Hamamatsu under excitation with TTL modulated 980 nm CW laser diode (pulse duration set to 15 ms). Luminescence lifetimes of Tm 3+ were recorder similar way using 20 μs pulse duration with 100 Hz repetition rate.

Structural and morphological characterization
In order to investigate the influence of various concentrations of Gd 3+ dopant on the crystallographic structure of the synthesized materials, powder x-ray diffraction analysis, shown in the figure 1, was carried out. The results indicate disturbance in crystal structure purity in the case of the samples doped with 40% Gd 3+ where another phase of GdF 3 appears. The diffraction peak in the 2θ range of 18°to 20°s hifts to smaller diffraction angles upon doping with gadolinium ions up to 20%. This indicates incorporation of Gd 3+ into LiYF 4 matrix [51]. Further increase in gadolinium concentration in the reaction solution results in shift back to position of Gd 3+ -undoped LiYF 4 , indicating no gadolinium incorporated in LiYF 4 tetragonal structure. This coincides with observation of the dominant GdF 3 foreign phase in the TEM images for Gd 3+ concentration >20% (figure 2). Despite the appearance of a foreign phase at 20% doping concentration, all the obtained results are analyzed because it allowed to show a more complete picture of the observed effects, including a gradual phase transition. It is known that obtaining pure LiGdF 4 phase nanocrystals is difficult and GdF 3 nanoparticles appear often instead of LiGdF 4 , therefore seed mediated synthesis or doping approaches are required [25,46,[52][53][54]. Here, in case of doping LiYF 4 matrix with more than 20% of gadolinium, Gd 3+ ions takes part in the formation of GdF 3 NPs instead of LiY(Gd)F 4 , what is accompanied with significant change in nanocrystals size and morphology (figures 1 and 2). Additional analysis of the precipitate containing excessive amount of LiF used in the synthesis reveals the presence of tetragonal phase nanocrystals that are aggregated with LiF [55]. The details can be found in the supporting information (figures S1 and S2). The NPs morphology variations in response to increased gadolinium dopant concentration are presented in the TEM images together with the statistical analysis of nanocrystals sizes (figure 2). The analysis of nanocrystals size and morphology clearly indicates that doping up to 10% of Gd 3+ ions does not significantly affect the morphology of nanoparticles, but increases their size. For higher Gd 3+ concentrations the changes in nanocrystals homogeneity and morphology can be clearly observed. It is caused by the formation of GdF 3 nanocrystals with rice-like and rhombic plates morphologies (figure 2).
In figure 2, it is shown that when the doping concentration of Gd 3+ is 20%, NPs with two different morphologies and sizes appear (table 1). Smaller rice-shaped particles can be assigned to GdF 3 nanocrystals. It can be assumed that GdF 3 nanoparticles are formed from excessive Gd 3+ ions that were not deposited in LiYF 4 structure. Moreover, it should be considered that some Yb 3+ and Tm 3+ ions can be deposited in the GdF 3 nanoparticles resulting in a lack of the control over the exact amount of co-dopants incorporated in LiYF 4 structure. In addition to the appearance of a foreign phase, replacing of larger amount of Y 3+ by Gd 3+ ions leads to an increase in the size of LiYF 4 nanoparticles and eventually to their precipitation and sticking with excess of LiF deposited at the bottom of a vial with colloidal solution of nanoparticles. Such fractionation of the samples may be confirmed by the presence of tetragonal LiYF 4 phase in the precipitate originating from the samples doped with 30% and 40% of gadolinium ions (figures S1 and S2 supporting information).

Luminescent properties
In order to investigate the impact of gadolinium ions doping on optical properties of LiYF 4 :YbTm nanocrystals, luminescence spectra and luminescence decay times were taken and analyzed in details. Up-conversion luminescence spectra of the samples under excitation with 980 nm laser diode, measured for the colloidal samples, show the increasing UC luminescence intensity with increasing dopant up to 10% of Gd 3+ (figure 3(a)). Further increase in Gd 3+ ions doping results in sharp decrease in luminescence intensity. To better illustrate the observed trends, integrated intensity changes of up-conversion UV Gd 3+ , UV-vis and NIR luminescence are shown in figures 3(b)-(d), respectively. The peak at 311 nm comes from 6 P 7/2 → 8 S 7/2 emission of Gd 3+ ions and its intensity is first increasing due to the increasing amount of emitting gadolinium ions in the nanocrystals. Above 20% of Gd 3+ doping the intensity is decreased significantly due to the change of structure and morphology of nanocrystals. Further increase of Gd 3+ ions is not able to compensate huge increase of surface quenching. In the case of the samples doped with 30 and 40% of gadolinium, the luminescence intensity of each transition is very low, which can be attributed to disturbance in crystal phase and much higher surface quenching resulting from surface-to-volume ratio increase due to the smaller size and flattened shape of these nanoparticles.
The mechanism of up-conversion in the Yb-Tm-Gd system is based on the sequential transfer of the absorbed energy from Yb 3+ ions 2 F 5/2 energy level to Tm 3+ 3 H 5 , 3 F 2,3 , 1 G 4 , 1 D 2 and then 3 P 2 energy level. Gd 3+ ions in such system do not participate in emptying energy levels of thulium up to excitation of thulium ion to its 1 D 2 energy level because no matching energy level is available in gadolinium ions. However, when the Tm 3+ is excited to 3 P 2 level, non-radiative relaxation though 3 P 0,1 to 1 I 6 energy level occurs and energy transfer between Tm 3+ and Gd 3+ becomes possible ( figure 4(c)). The excited gadolinium ion may then emit the energy or migrate energy to the neighboring Gd 3+ ions and transfer this energy through the network of Gd 3+ up to the surface of nanocrystals. For the lower energy levels of thulium, gadolinium ions can affect the luminescence from these levels mainly by disturbing crystal phase or neighboring crystal environment of emitting thulium ions. The observed luminescence changes in the emission spectra after normalization to the 3 H 4 → 3 H 6 emission are shown in the figure 4. The intensity of Tm 3+ 1 G 4 → 3 H 6 and 1 G 4 → 3 F 4 emission increases with increasing concentration of gadolinium ions and reaches its maximum for the sample doped with 10% Gd 3+ . Further increase in Gd 3+ ions doping results in decrease of the 1 G 4 → 3 H 6 and 1 G 4 → 3 F 4 emission intensity. In the case of emission from 1 D 2 and 1 I 6 , low concentration of Gd 3+ (1%) results in slight decrease in intensity, but for higher Gd 3+ content increase of emission intensity can be observed. This is contrary to the assumptions according to which Gd should only affect 1 I 6 thulium level emission and thus decreased emission intensity upon doping with gadolinium should be observed while emission from 1 D 2 energy level should be disturbed to a lesser extent. This observation shows that the main reason of the observed trends are not the effect of energy transfer processes between thulium and gadolinium ions, which, however, cannot be ignored, but rather crystal structure and morphology changes caused by  doping with gadolinium ions. The best relative intensity of Tm 3+ ions UV emission is found for the samples doped with 10 and 20% Gd 3+ that corresponds to the lowest surface-tovolume ratio of LiYF 4 nanocrystals (largest particles). The intensity ratios of selected luminescent bands as a function of Gd 3+ concentration are shown in the figure 4(b). The ratio of 6 P 7/2 → 8 S 7/2 (Gd 3+ ) to the 1 D 2 → 3 H 6 (Tm 3+ ) and 1 I 6 → 3 F 4 (Tm 3+ ) emission increases with increasing gadolinium ions concentration. In the case of Tm 3+ luminescence intensity ratios (LIR), defined as the intensity ratio between the two 1 D 2 → 3 H 6 and 1 G 4 → 3 H 6 transitions, first, low doping level of Gd 3+ (1 and 5%) results in LIR decrease what is followed by the LIR increase with further increasing gadolinium ions concentration up to 30%. Interestingly, doping with 40% Gd 3+ leads to further decrease of this value. These variations can be due to the changes observed in size, morphology and crystal structure of the samples. The schematic illustration of energy transfer and emission occurring in the discussed system is presented in the figure 4(c). The normalized spectra of the samples taken in the UV region clearly show increase of Gd 3+ luminescence intensity with increasing Gd 3+ concentration upon indirect excitation through Yb 3+ ions (figure 5). The excitation of Yb 3+ with 980 nm laser results in energy transfer and excitation of thulium ions up to their 3 P J , 1 D 2 and 1 I 6 energy levels ( figure 4(c)). Then, the energy can be transferred to 6 P J energy levels of gadolinium ions leading to 6 P J → 8 S 7/2 Gd 3+ emission.
Under continuous wave laser excitation (∼890 mW), 1 I 6 → 3 F 4 emission intensity is lower than 1 D 2 → 3 H 6 emission for all the samples, due to the higher amount of photons needed for excitation of higher lying 1 I 6 energy level. While 6 P 7/2 → 8 S 7/2 Gd 3+ emission intensity increases with increasing gadolinium concentration, the 1 I 6 → 3 F 4 / 1 D 2 → 3 H 6 emission ratio shows more complex behavior. Following slight decrease for small gadolinium dopant concentration, an increase can be observed for 10% and 20% Gd 3+ , while for higher dopant amount the ratio gradually decreases again (figures 5(a) and (b)).
One should consider possible energy transfer between Gd 3+ and Tm 3+ ions that can be responsible for the observed changes. First, with small amount of gadolinium, which however exceeds the amount of thulium in the matrix, the 1 I 6 energy level of Tm 3+ can be effectively emptied by energy transfer to 6 P J levels of gadolinium ( figure 4(c)). This transfer is confirmed by appearance of gadolinium 6 P 7/2 → 8 S 7/2 emission under excitation of Yb 3+ ions. Simultaneously, other factors, such as nanocrystals size and phase purity also influence this ratio and thus further increase in gadolinium concentration, resulting in disturbance in crystal lattice and nanocrystals size, can be responsible for the increased share of emission from 1 I 6 energy level of thulium ions. When the doping concentration reaches 30 and 40% and smaller GdF 3 nanocrystals fraction becomes dominant, the share of emission from 1 I 6 level decreases again.
Due to the large energy gap between Gd 3+ 6 P 7/2 and 8 S 7/2 energy levels, gadolinium excited state lifetime is much longer  (figures 6 and 7) than in case of other lanthanide ions. The 6 P 7/2 → 8 S 7/2 emission of gadolinium ions kinetic behavior is shown in the figure 6. The luminescence risetime is growing with increasing concentration of Gd 3+ and reaches maximum of almost 8 ms for 40% Gd 3+ . On the other hand, the luminescence lifetime is first slightly growing when gadolinium concentration changes from 1% to 5% and then decrease with increasing amount of Gd 3+ is observed (figures 6(b) and (d)). We suspect, the increase in risetime and simultaneous decrease in lifetime may be a result of energy migration between Gd 3+ ions. This migration enables to spread the energy over the nanoparticle volume and thus excite more Gd 3+ ions, not only those in the close proximity of up-converting Tm 3+ ions. Due to smaller amount of thulium ions that are responsible for energy transfer to gadolinium and long lifetime of gadolinium excited states, one thulium ion can be responsible for pumping more than one gadolinium. Such an assumption can be supported by a very long Gd 3+ luminescence risetime, i.e. >2 ms exceeding Tm 3+ 1 I 6 lifetime (<150 μs). This excitation of Gd 3+ 6 P 7/2 requires successive sequences of 5-photon excitation of Tm 3+ by Yb 3+ which is followed by energy transfer to Gd 3+ . Owing to energy migration from one excited gadolinium to neighbor gadolinium ions the luminescence risetime becomes elongated with increasing Gd 3+ /Tm 3+ ratio [56].
Increasing probability of energy transfer from thulium to gadolinium ions should result in respective decrease in luminescence lifetime of Tm 3+ ( 1 I 6 ). However, in case of our studies the trend of changes in Tm 3+ 1 I 6 lifetime upon doping with Gd 3+ is the same as for 1 D 2 energy level and moreover corresponds to the changes of nanocrystals size (figures 7(b) and (c)). The surface-to-volume ratio greatly impact luminescence intensity through energy migration to surface quenchers. This effect is so large that it obscures other more subtle effects such as the effect of increased energy transfer and emptying of the excited 1 I 6 level of the thulium ions. These effects should be obviously more visible when size and morphology of nanocrystals is not affected by dopant introduction. Here, in the studied LiYF 4 nanocrystals the energy migration through the network of Gd 3+ ions can be also responsible for higher quenching rate due to the surface or lattice defects, that together with increase of surface-to-volume ratio may be responsible for shortening of gadolinium luminescence lifetime.
The decrease of luminescence lifetime with increasing surface-to-volume ratio, structural and morphology changes can be also observed for 3 H 4 Tm 3+ emitting level for emission at 800 nm ( figure 7(a)). The luminescence lifetime increases up to 10% Gd 3+ , due to nanocrystals size, and is followed by the decrease caused by foreign phase appearance and significant increase in surface-to-volume ratio.

Magnetic properties
Besides luminescent properties, inorganic nanoparticles doped with Ln 3+ ions are promising candidates for the multimodal bio-applications due to the coexistence of unusual optical and magnetic properties [57]. These bio-applications are diverse, starting from bioimaging and MRI, through bioseparation and drug delivery, to theranostics [58][59][60]. The presence of Gd 3+ ions in the studied materials makes them especially interesting from the point of view of their possible application as MRI contrast agent, because seven unpaired 4f electrons of Gd can efficiently alter the relaxation time of surrounding water protons [61,62]. Therefore, the magnetic properties of investigated materials deserve to be studied. The results of magnetic properties measurements performed at T = 300 K are summarized in figure 8. Magnetic field dependent magnetization, σ(H), is perfectly linear for all samples, indicating paramagnetic behavior. This is consistent with the results obtained for similar materials [63]. The slope of the σ(H) curves increases with the increasing concentration of Gd 3+ ions, indicating that magnetic properties can be easily tuned by Gd 3+ doping, for example the magnetization of LiYF 4 :YbTm can be tuned from the 0.16 emu g −1 for parent compound to 0.76 emu g −1 for sample with Gd 3+ concentration 40%. The last value of σ is close to those reported previously for materials which can be used as MRI contrast agents [64]. To see clearly the evolution of magnetic properties with Gd 3+ doping concentration, the magnetic mass susceptibility (χ m ) calculated for H = 20 kOe is shown as a function of the Gd 3+ concentration in the inset to figure 8. χ m gradually increases with the increasing Gd 3+ concentration.
Increasing Gd 3+ content together with increasing surface-to-volume ratio in highly gadolinium-doped materials makes them the most appropriate candidates for exploring as a bright longitudinal relaxation (T1) for MRI. However, the structural and morphological changes resulting in low luminescence efficiency show that the balance between the optimal concentration of Gd 3+ for magnetic properties and optimal Gd 3+ content for optical properties must be obtained.

Conclusions
In this work, a series of LiYF 4 :25% Yb 3+ 0.5% Tm 3+ nanocrystals doped with Gd 3+ ions (1%-40%) have been Figure 7. Luminescence lifetime of Tm 3+ 3 H 4 (a), 1 I 6 and 1 D 2 (b) energy levels changes upon doping with Gd 3+ ions and the changes in nanocrystal size (here length of nanocrystals was taken into account) (c). The luminescence lifetimes for Tm 3+ 3 H 4 energy level were taken from luminescence decay measured at 800 nm (supporting information figure S3). Tm 3+ 1 I 6 and Tm 3+ 1 D 2 were calculated from luminescence decay measurements at 347 and 361 nm respectively. Dashed lines are guides for the eye. synthesized in oleic acid and 1-octadecene high boiling solvents. The highest amount of gadolinium ions that can be doped into LiYF 4 differs significantly from the one reported in the literature (up to 60% [46]) which indicates that nuances of synthesis conditions have significant impact on the doping possibilities. The Gd 3+ doping up to 10% under used synthetic conditions leads to obtaining pure phase tetragonal nanocrystals with size increasing with increase of gadolinium concentration. At higher gadolinium dopant concentrations, the appearance of foreign phase has been observed in the transmission electron microscope images and XRD diffraction pattern. The observed morphological and structural changes are accompanied by corresponding variations in overall up-conversion luminescence with the highest intensity observed for the sample doped with 10% Gd 3+ . The comprehensive analysis of luminescent properties of the samples shows increasing relative intensity of 6 P 7/2 → 8 S 7/2 Gd 3+ ions emission with increasing concentration of gadolinium ions. This observation can be rationalized by growing rate of Tm 3+ →Gd 3+ energy transfer. The relatively long, compared to other lanthanide ions, luminescent risetime of gadolinium 6 P 7/2 level results from sequences of energy transfer upconversion between Yb 3+ and Tm 3+ required to reach 3 P 2 level of Tm 3+ and subsequent energy transfer to upper level of gadolinium ions. The thulium luminescence lifetime variations under doping with Gd 3+ were compatible with the changes observed in size and phase purity. Magnetic mass susceptibility gradually increases with the increasing Gd 3+ concentration confirming growing number of Gd 3+ ions incorporated into the nanoparticles. This dependence is not consistent with the dependence of luminescence efficiency on the Gd 3+ concentration, therefore the balance between the optimal concentration of Gd 3+ for magnetic properties and optimal Gd 3+ content for optical properties must be obtained. Systematic optimization of UV performance of wide concentrations range of Gd 3+ co-doped LiYF 4 is important in terms of use in Gd 3+ supported migration of energy to Eu 3+ /Tb 3+ ions in order to enhance their up-conversion and to allow utilization of these ions in multiplexed imaging. Gd 3+ based energy migration may also increase the applicability of up-converting nanoparticles in FRET based applications such as highly sensitive analyte sensing and imaging below diffraction limit.

Acknowledgments
The work was financially supported with the statutory funds of the Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).
CRediT authorship contribution statement M Misiak conceptualization; supervision; synthesis; measurements; data analysis; writing-original draft; writingreview and editing. O Pavlosiuk methodology; measurements; writing-review and editing. M Szalkowski methodology; discussion; writing-review and editing. A Kotulska measurements; writing-review and editing. K Ledwa measurements, writing-review and editing. A Bednarkiewicz resources; supervision; writing-review and editing.