Ytterbium oxide (Yb₂O₃, 99.99%), Europium oxide (Eu₂O₃, 99.99%), Calcium chloride (CaCl₂, AR), Nitric acid (HNO₃, AR), Hydrochloric acid (HCl, AR), Ammonium fluoride (NH₄F, AR) and Ethanol absolute (C₂H₅OH, AR).

The Ca_0.8Yb_(0.2-x)F_2.2:x%Eu³⁺ samples were synthesized hydrothermally. For the representative Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ sample, 2 mmol of CaCl₂ (dissolved in 1 M solution, 2000 µL) was added to 20 mL of deionized water in a 100 mL beaker. Under vigorous stirring, 0.79 mmol of Yb(NO₃)₃ (1 M, 790 µL) and 0.21 mmol of Eu(NO₃)₃ (1 M, 210 µL) were added sequentially. The mixture was stirred for 20 minutes to ensure homogeneity. Subsequently, 7 mmol of NH₄F (4 M, 1750 µL) was added dropwise, and stirring continued for another 30 minutes, resulting in a milky white suspension. This suspension was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 180˚C for 12 hours. After natural cooling to room temperature, the resulting product was collected by centrifugation (5000 rpm), washed thoroughly with deionized water and ethanol, and finally dried at 60˚C in a vacuum oven for 6 hours.

Powder X-ray diffraction (PXRD) patterns were acquired using a MSALNDS diffractometer (Beijing Purkinje General Instrument) with Cu Kα radiation (λ = 0.154 nm, 36 kV, 20 mA), scanning from 10˚ to 90˚ (2θ). Morphological characterization and elemental analysis (EDS and mapping) were performed using a Sigma 500/10102 Field Emission Scanning Electron Microscope (Carl Zeiss, Germany). Photoluminescence spectra (excitation and emission) were recorded on an FLS fluorescence spectrometer (Edinburgh Instruments, UK).

The phase purity and crystal structure of the synthesized Ca_0.8Yb_0.2F_2.2:x%Eu³⁺ powder were confirmed by X-ray diffraction (XRD). As shown in Figure 1, all diffraction peaks align perfectly with the reference pattern for Ca_0.8Yb_0.2F_2.2 (JCPDS 87-0978), indicating the successful formation of the desired crystalline phase without detectable impurities. Elemental composition and distribution were analyzed using Energy Dispersive X-ray Spectroscopy (EDS). The EDS spectrum (Figure 2(a)) confirms the presence of Ca, Yb, F, and Eu elements. Furthermore, elemental mapping (Figures 2(b)-(f)) reveals a homogeneous distribution of all these elements throughout the nanoparticles, providing strong evidence for the successful incorporation of Eu³⁺ ions into the Ca_0.8Yb_0.2F_2.2 host lattice.

To optimize the luminescence output, samples with varying Eu³⁺ doping concentrations were synthesized. Under 392 nm excitation, the emission spectra (Figure 3(a)) are dominated by the characteristic intra-4f transitions of Eu³⁺ (⁵D₀ → ⁷FJ, J = 1 - 4). The dependence of the integrated emission intensity on Eu³⁺ concentration is plotted in Figure 3(b). The intensity initially increases with doping concentration, reaching a maximum at 21%Eu³⁺, beyond which the quenching occurs. This phenomenon is attributed to enhanced non-radiative energy transfer between neighboring Eu³⁺ ions at shorter inter-ionic distances, prevalent at higher doping levels. Therefore, the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ sample, exhibiting the strongest luminescence, was selected for all subsequent sensing experiments.

Figure 1.XRD patterns of Ca_0.8Yb_0.2F_2.2:x%Eu (x = 15, 18, 21, 24, and 28) phosphors.

Figure 2.(a) EDS spectrum and (b-f) elemental mapping of Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ phosphor.

Figure 3.(a) Excitation and emission spectra of Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ and (b) Effect of Eu³⁺ concentration on the emission intensity.

The hydrothermal synthesis method yielded Ca_0.8Yb_0.2F_2.2 material with good aqueous dispersibility, which typically improves with lower crystallinity. Furthermore, as a rare-earth fluoride, Ca_0.8Yb_0.2F_2.2 maintains excellent chemical stability even at relatively low crystallinity levels. Consequently, the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ powder demonstrates both favorable dispersibility and remarkable pH stability in aqueous environments.

To evaluate the luminescence stability of the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ aqueous suspension, emission spectra were systematically recorded at 20-second intervals. The pH-dependent luminescence behavior was investigated by measuring emission spectra from uniformly mixed suspensions adjusted to different pH values. As shown in Figure 4(a), the luminescence intensity of the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ suspension remained essentially unchanged over a 100-second monitoring period. Figure 4(b) demonstrates that the luminescence intensity maintained remarkable consistency across a broad pH range from 3 to 11.

Notably, the crystalline phase of Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ powder remained intact after immersion in aqueous solution. The long-term stability assessment (Figure 5) revealed excellent retention of luminescence properties, with minimal degradation observed even after 50 days of suspension storage. This combination of phase stability and aqueous dispersibility establishes the fundamental basis for employing Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ powder as a reliable sensing platform for analyte detection in aqueous environments [29][30].

The selectivity of the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ probe towards Cr(VI) anions was investigated by monitoring the fluorescence response upon addition of various potential interfering ions, including CO 3 2 − , F − , PO 4 3 − , NO 3 − , SO 4 2 − , I⁻, Br⁻, and ClO 3 − . As depicted in Figure 6(a), the introduction of these ions induced negligible changes in the fluorescence intensity compared to the blank suspension. In stark contrast, the addition of either CrO 4 2 − or Cr 2 O 7 2 − resulted in significant fluorescence quenching. The histogram in Figure 6(b), representing the emission intensity at 592 nm, clearly visualizes this high selectivity, affirming the probe’s specific recognition capability for CrO 4 2 − and Cr 2 O 7 2 − over other common anions.

Figure 4.(a) Emission spectra of the aqueous suspension of Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ powder recorded at different time intervals; (b) Emission spectra of the suspension at different pH values.

Figure 5.Emission spectra recorded on the 1st day and the 50th day after suspension preparation.

The sensitivity of the probe was quantitatively evaluated by titrating the suspension with increasing concentrations of CrO 4 2 − and Cr 2 O 7 2 − ions. As shown in Figure 7(a) and Figure 7(b), the fluorescence intensity progressively decreased with higher analyte concentrations. The quenching data were analyzed using the Stern-Volmer equation: I₀/I = 1 + K_SV [M], where I₀ and I are the fluorescence intensities in the absence and presence of the quencher, respectively, K_SV is the Stern-Volmer quenching constant, and [M] is the molar concentration of the quencher. The plots of I₀/I versus quencher concentration ([ CrO 4 2 − ] or [ Cr 2 O 7 2 − ]) exhibited good linearity (R² = 1 for CrO 4 2 − , R² = 0.996 for Cr 2 O 7 2 − ), as shown in Figure 7(c) and Figure 7(d). The K_SV values were determined to be 0.00867 µM⁻¹ for CrO 4 2 − and 0.0084 µM⁻¹ for Cr 2 O 7 2 − . The limits of detection (LOD), calculated based on LOD = 3σ/S (where σ is the standard deviation of the blank signal and S is the slope of the calibration curve), were found to be 1.45 µM for CrO 4 2 − and 1.50 µM for Cr 2 O 7 2 − . These LODs values demonstrate the high sensitivity of the probe.

Figure 6.(a) Emission spectra of Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ phosphor mixed with different anions (l_ex= 392 nm); (b) Comparison of the emission intensity at 592 nm for solutions containing different ions.

Figure 7.Stern-Volmer plots for the detection of CrO 4 2 − and Cr 2 O 7 2 − ions using the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ suspension; (a) Emission spectra of the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ suspension at different concentrations of CrO 4 2 − ; (b) Emission spectra of the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ suspension at different concentrations of Cr 2 O 7 2 − ; (c) Stern-Volmer plot for CrO 4 2 − ; (d) Stern-Volmer plot for Cr 2 O 7 2 − .

To elucidate the quenching mechanism of Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ luminescence by CrO 4 2 − and Cr 2 O 7 2 − ions, systematic characterization was conducted (Figure 8). Phase stability was first investigated by immersing the phosphor in 0.01 mol/L CrO 4 2 − and Cr 2 O 7 2 − solutions for 24 hours. After centrifugation and drying at 60˚C, XRD patterns revealed that the diffraction peaks of the treated samples remained sharp and closely matched the reference pattern for Ca_0.8Yb_0.2F_2.2. This observation confirmed that the crystal structure of Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ remained unaltered, thereby ruling out structural alteration of the phosphor as the cause of the luminescence quenching by CrO 4 2 − and Cr 2 O 7 2 − ions.

UV-Vis absorption spectroscopy revealed that both CrO 4 2 − and Cr 2 O 7 2 − ions exhibit strong absorption bands in the 200 - 800 nm range (Figure 9). This absorption profile significantly overlaps with the excitation spectrum of the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ probe. This spectral overlap suggests that the primary quenching mechanism is the inner filter effect (IFE). The target oxyanions compete with the fluorophore for the excitation light, effectively reducing the number of photons absorbed by the Eu³⁺ ions in the probe, thereby leading to the observed decrease in fluorescence intensity.

Figure 8.XRD patterns of Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ phosphor after immersion in CrO 4 2 − and Cr 2 O 7 2 − aqueous solutions and subsequent washing/drying.

To assess anti-interference capability, solutions containing CrO 4 2 − or Cr 2 O 7 2 − were mixed with solutions of potential interfering ions ( CO 3 2 − , F − , PO 4 3 − , NO 3 − , SO 4 2 − , I⁻, Br⁻, ClO 3 − ) at a 1:1 concentration ratio. For the fluorescence measurement, 3 mL of the probe suspension (3 mg/mL) was mixed with 100 µL of the mixed ion solution (0.01 M for each ion). The fluorescence emission intensity was then measured and compared to that of the blank suspension and the suspension containing only the target Cr(VI) anion. As shown in Figure 10, the presence of interfering ions did not significantly alter the pronounced quenching effect caused by CrO 4 2 − or Cr 2 O 7 2 − , confirming the high selectivity of the probe for practical applications.

Figure 9.UV-Vis Absorption Spectra of CrO 4 2 − and Cr 2 O 7 2 − Ions and the excitation spectrum of the Ca_0.8Yb_0.2F_2.2:Eu³⁺ phosphor

Figure 10.Relative luminescence intensity of the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ suspension after addition of target and interfering ions.