Lexiang Zhou, Lu Lei, Xiao Qu, Shanshan Hu^*
School of Chemistry and Chemical Engineering, Southwest University, Chongqing, China.
DOI: 10.4236/msce.2026.141002   PDF    HTML   XML   62 Downloads   316 Views   Citations

Abstract

The accurate detection of hexavalent chromium (Cr(VI)) is crucial for environmental protection and public health. Rare earth fluorides have attracted considerable attention as fluorescent sensing materials due to their distinctive properties, including low lattice phonon energy, exceptional chemical stability and long fluorescence lifetime. This study reports the successful one-step hydrothermal synthesis of a Eu³⁺-doped Ca_0.8Yb_0.2F_2.2 fluorescent probe with excellent aqueous dispersibility. The obtained probe showed high sensitivity towards ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ ions, achieving detection limits of 1.45 µM and 1.50 µM. The obtained probe exhibited remarkable stability across a wide pH range (3 - 11) and strong anti-interference capability, confirming its suitability for analyzing real water samples. Owing to its robust performance and straightforward operation, this fluorescent probe presents a promising approach for efficient monitoring of trace Cr(VI) contaminants in aqueous environments.

Keywords

Fluorescent Probe, Rare Earth Fluorides, Cr(VI) Detection

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1. Introduction

Chromium (Cr) is a pervasive heavy metal pollutant posing significant risks to ecosystems and human health. In environmental and industrial wastewater contexts, chromium primarily exists in two stable oxidation states: the less toxic trivalent form (Cr(III)) and the highly toxic hexavalent form (Cr(VI)) [1]-[3]. Cr(VI) species, predominantly occurring as oxyanions such as chromate (${\text{CrO}}_4^{2-}$ ), hydrogen chromate (${\text{HCrO}}_4^{-}$ ), and dichromate (${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ ) depending on pH, are notably soluble and mobile in water systems [4]-[8]. Recognized for its potent toxicity and carcinogenicity, Cr(VI) is classified as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC) [9] [10]. Human exposure, often through industrial discharges, can lead to severe health issues including dermatological irritation, respiratory impairments, ocular damage, and gastrointestinal disorders. At the molecular level, Cr(VI) induces oxidative stress by interfering with cellular redox processes, promotes DNA adduct formation, and can cause chromosomal aberrations, ultimately increasing cancer risk [11] [12].

Consequently, the effective monitoring and removal of Cr(VI) from drinking water and groundwater sources is imperative, particularly its anionic forms (${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ ). Regulatory bodies worldwide have established stringent limits; for instance, the U.S. Environmental Protection Agency (EPA) sets a maximum contaminant level (MCL) of 0.1 mg/L for total chromium [13], while Chinese standards (GB 5749-2006) impose a stricter limit of 0.05 mg/L specifically for Cr(VI) [14]. This underscores the urgent need for developing reliable, sensitive, and selective methods for Cr(VI) detection in water.

Conventional techniques for chromium analysis, such as Flame Atomic Absorption Spectrometry (FAAS), Graphite Furnace Atomic Absorption Spectrometry (GFAAS), Inductively Coupled Plasma-based methods (ICP-OES, ICP-MS), and X-ray Fluorescence (XRF), offer precision but often require sophisticated instrumentation, extensive sample preparation, and skilled operation, limiting their field applicability [15]-[17]. In contrast, fluorescence-based sensing has emerged as a powerful alternative due to its inherent advantages of high sensitivity, selectivity, rapid response, and operational simplicity. While numerous fluorescent probes for chromium detection have been explored, and most of them are based on organic fluorophores. Recent efforts have focused on designing novel probes for enhanced Cr(VI) selectivity and sensitivity. For example, Subash et al. developed a water-compatible N-aryl carbamate-substituted quinoxaline sensor for Cr(VI) with a quantum yield of 0.26, emitting blue-green fluorescence at 498 nm [18]. Wang et al. fabricated a composite film incorporating copper nanoclusters for visual and ratiometric fluorescence detection of Cr(VI) [19]. Narasimhappa et al. have reported aggregation-induced emission (AIE)-active probes acting as “turn-off” sensors for Cr(VI) and Fe(III), and naphthalene-functionalized metal-organic frameworks (MOFs) for sensing these ions [20].

However, organic probes frequently suffer from limitations such as potential toxicity, poor photostability, or inadequate water solubility, hindering their practical deployment. Inorganic nanomaterials, particularly those doped with lanthanide ions, offer a compelling solution due to their superior photophysical properties—high quantum yield, long luminescence lifetime, excellent photostability and comparatively low toxicity [21]-[24]. Among these, rare earth fluorides stand out as promising host matrices for fluorescent sensors, benefiting from low phonon energies that minimize non-radiative decay, high chemical stability [25] [26] and relatively straightforward synthesis routes like hydrothermal method that often yields products with good aqueous dispersibility [27] [28].

Herein, we present the hydrothermal synthesis of a Eu³⁺-doped Ca_0.8Yb_0.2F_2.2 fluorescent probe exhibiting excellent water dispersibility. The Ca_0.8Yb_0.2F_2.2:21% Eu³⁺ material shown stable luminescence and phase integrity in aqueous solutions from pH = 3 to pH = 11. The probe serves as a highly selective and sensitive fluorescent sensor for ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ ions via a quenching mechanism.

2. Experimental Section

2.1. Materials and Synthesis

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.

2.2. Characterization

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).

3. Results and Discussion

3.1. Morphology, Phase and Luminescence

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.

3.2. Aqueous Stability and Dispersion

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].

3.3. Selectivity and Sensitivity

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 ${\text{CO}}_3^{2-}$ , ${\text{F}}^{-}$ , ${\text{PO}}_4^{3-}$ , ${\text{NO}}_3^{-}$ , ${\text{SO}}_4^{2-}$ , I⁻, Br⁻, and${\text{ClO}}_3^{-}$ . 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 ${\text{CrO}}_4^{2-}$ or ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ 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 ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ 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 ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ 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 ([${\text{CrO}}_4^{2-}$ ] or [${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ ]) exhibited good linearity (R² = 1 for ${\text{CrO}}_4^{2-}$ , R² = 0.996 for ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ ), as shown in Figure 7(c) and Figure 7(d). The K_SV values were determined to be 0.00867 µM⁻¹ for ${\text{CrO}}_4^{2-}$ and 0.0084 µM⁻¹ for${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ . 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 ${\text{CrO}}_4^{2-}$ and 1.50 µM for${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ . 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 ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ 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 ${\text{CrO}}_4^{2-}$ ; (b) Emission spectra of the Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ suspension at different concentrations of ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ ; (c) Stern-Volmer plot for${\text{CrO}}_4^{2-}$ ; (d) Stern-Volmer plot for ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ .

3.4. Quenching Mechanism

To elucidate the quenching mechanism of Ca_0.8Yb_0.2F_2.2:21%Eu³⁺ luminescence by ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ ions, systematic characterization was conducted (Figure 8). Phase stability was first investigated by immersing the phosphor in 0.01 mol/L ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ 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 ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ ions.

UV-Vis absorption spectroscopy revealed that both ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ 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 ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ aqueous solutions and subsequent washing/drying.

3.5. Anti-Interference

To assess anti-interference capability, solutions containing ${\text{CrO}}_4^{2-}$ or ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ were mixed with solutions of potential interfering ions (${\text{CO}}_3^{2-}$ , ${\text{F}}^{-}$ , ${\text{PO}}_4^{3-}$ , ${\text{NO}}_3^{-}$ , ${\text{SO}}_4^{2-}$ , I⁻, Br⁻, ${\text{ClO}}_3^{-}$ ) 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 ${\text{CrO}}_4^{2-}$ or${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ , confirming the high selectivity of the probe for practical applications.

Figure 9. UV-Vis Absorption Spectra of ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ 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.

4. Conclusion

In summary, a highly water-dispersible Eu³⁺-doped Ca_0.8Yb_0.2F_2.2 fluorescent probe was successfully synthesized via a facile one-step hydrothermal method. The optimal luminescence intensity was achieved with a Eu³⁺ doping concentration of 21%. The probe serves as a selective sensor for toxic Cr(VI) oxyanions (${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ ) in water, operating primarily through an inner filter effect mechanism where the analytes compete for excitation energy. It demonstrates high sensitivity, with detection limits of 1.45 µM and 1.50 µM for ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$ , respectively, values that are significantly lower than the regulatory limit for drinking water. The material exhibits exceptional long-term stability in water and maintains its performance over a wide pH range (3 - 11) with excellent selectivity against common interfering ions. Combined with its simple synthesis and straightforward operation, the Ca_0.8Yb_0.2F_2.2:21% Eu³⁺ fluorescent probe represents a reliable and effective tool for the on-site monitoring of trace-level Cr(VI) contamination in real water samples. Furthermore, the design strategy presented here offers valuable insights for developing advanced sensing materials targeting other hazardous heavy metal ions.

Fund

This project was financially sponsored the Natural Science Foundation of Chongqing (cstc2020jcyj-msxmX0332).

Conflicts of Interest

The authors assert that there are no conflicts of interest about the publishing of this work.

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