Open Access
Add to BibSonomy
Abstract
The feasibility of using CaF2:Eu3+ powders as a luminescent probe to detect Cr2O72- ions was studied in this article. A series of CaF2:Eu3+ powders with different Eu3+ substitution rates was prepared by a modified sol-gel method, ending with a sintering procedure of the xerogel. The luminescent probing ability of CaF2:Eu3+ powders on Cr2O72- ions was studied for the first time. It is found that the CaF2:10%Eu3+ powder exhibits a sensitive response to Cr2O72- ions, with an obvious quenching of the Eu3+ luminescence with increasing concentration of Cr2O72- ions in the aqueous solution. The investigation of the selective luminescence quenching mechanism of CaF2:10%Eu3+ powder on Cr2O72- ion reveals that the adsorbed Cr2O72- ions on the CaF2:10%Eu3+ powder surface screen the latter from the excitation source, since there is a competitive absorption of excitation light between Eu3+ and adsorbed Cr2O72-. This study expands the application of old and low cost CaF2: Eu3+ powder into a new area of luminescent probe, especially for Cr2O72- ion detection.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
3yuyan@fzu.edu.cn
1. Introduction
Chromium is a common pollutant in natural environment as a result of its widespread use in leather tanning, metal electroplating, dyestuff industries, etc [1]. Hexavalent chromium, Cr (VI), is a carcinogen and a mutagen because of their high oxidation potential and the possibility to penetrate biological membranes [2]. Consequently, the detection of Cr (VI) in aqueous solution becomes one of the most important works of the study on Cr (VI) elimination. Numerous approaches have been developed for monitoring of Cr (VI) anions, such as atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectroscopy (ICP-MS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-Ray fluorescence spectroscopy (XRF), Ultraviolet-visible spectrophotometry (UV-VIS), Raman spectroscopy [3–5] and luminescent probe [6–9]. All the methods mentioned above have successful application on the detection of Cr2O72-. Some of them are characteristic of ultrahigh detection limitation, e.g. UV-VIS, ICP-AES and ICP-MS. However, equipment for these methods are usually expensive, which limits their popular application. The study of Raman spectroscopy-based detection technique usually focuses on the design of optical fibers and transmission and collection of light signals, which are crucial components of Raman probe. Although it could sense target with a very high sensitivity at very low concentration, the selective detection for a specific anion is difficult for Raman spectroscopy method. Among these detection methods, luminescent probe is an attractive technology due to its high sensitivity, simple operation and relatively low cost.
As one of the most famous inorganic luminescent materials, CaF2 based optical material has been widely investigated in the form of crystal, glass or nano-particles since early 1900s. CaF2 based single crystals or transparent ceramics are considered to be excellent laser gain media because of its relative small phonon energy and its high thermal stability. CaF2 single crystals that are doped with Yb3+ ions (CaF2:Yb3+) are optically active in the near infrared wavelength range, have been grown by Siebold M., Petit V. and their associates [10,11]. Furthermore, these crystal shows excellent thermal stability during lasing. Using the SESAM mode-locked technology, CaF2:Yb3+ single crystals could also be used in the ultra-fast pulse laser area [12,13]. A CaF2:Tm3+, Y3+ single crystal was grown in 2017 by Liu X. et al., who found that the obtained single crystal could be used as a tunable laser material [14]. In 2009, Grudinin I. S. et al. was the first to report a stimulated Brillouin scattering in high-Q whispering gallery mode resonators, which makes their use as ultra-narrow linewidth Brillouin lasers possible [15]. Besides single crystals, CaF2 based transparent ceramics are also explored as laser gain media and white light-emitting diode materials [16,17]. CaF2:Re3+ (Re = Yb, Er, Tm, Ce or Tb) nanoparticles, especially those with size smaller than 20 nm and could be dispersed in solvent, have very important application in the biological medicine field since they could be used as bio-imaging and photodynamic therapy material and smart drug carrier [18,19]. It could be found that CaF2 based optical materials really have wide potential applications in the field of modern technology and our daily life. However, as far as we know, the application of CaF2:Re3+ (Re = rare earth ion) powders as luminescent probes to detect Cr2O72- ions in aqueous solution has yet to be explored.
In this work, we prepared a series of CaF2:Eu3+ powders via sol-gel procedure combined with sintering of the xerogel. Ammonium hydrogen fluoride was used as the resource of fluoride ion. The local structure of CaF2:Eu3+ crystals was confirmed by analyses of powder X-ray diffraction (XRD) patterns. Meanwhile, the ability of CaF2:Eu3+ powders in probing of Cr2O72- ions was also investigated, as well as the luminescent probing mechanism.
2. Experimental section
2.1 Reagents and materials
Calcium acetate monohydrate (Ca(Ac)2·H2O, ≥ 98.0%), isopropyl alcohol (99.7%), and europium oxide (Eu2O3, 99.99%) were all purchased from Sinopharm Chemical Reagent Co. China. Ammonium hydroxide (25~28%), ethyl alcohol (≥ 99.7%), and acetic acid (≥ 99.5%) were purchased from Xilong Chemical Co., Ltd. The fluorinating reagent, ammonium hydrogen fluoride (NH4HF2, ≥ 98.0%), was obtained from Tianjin Fuchen chemical reagents factory. All of the chemicals were used as received without any purification. A batch of rare-earth acetate solid powders, Eu(Ac)3·4H2O, were prepared through the following procedure: First, the corresponding metal oxides are dissolved in nitric acid at 80°C with agitation, then dehydrated to get europium nitrate (Eu(NO3)3·4H2O). Then, the europium nitrate is dissolved in deionized water to get a transparent aqueous solution. A certain amount of ammonium hydroxide was then slowly added into the aqueous solution obtained above until the corresponding metal hydroxide flocculates. After centrifugation, the precipitation was collected and re-dissolved in acetic acid. The europium acetate powders could be obtained after vaporizing of the excess of acetic acid.
2.2 Synthesis of CaF2 and CaF2:Eu3+ powders
The CaF2 powders were prepared through a modified sol-gel method, the typical procedure of which is described as follow: Ca2+ ion source (Ca(Ac)2·H2O, 1 mmol) and F- ion source chemicals (NH4HF2, 1 mmol) were mixed into the aqueous mixtures containing deionized water (3 ml), ethyl alcohol (3 ml) and isopropanol (4 ml) under vigorous stirring. The mixture was stirred for 1 h to get a transparent solution. Afterward, the gel, which was obtained by dehydrating the mixed solution first at 100°C for 4 h, was heated to 300°C at the heating speed of 4°C/min. Once kept at 300°C for 1 h, the sample was cooled down to room temperature in the oven. The sintered sample was immersed in water for ultrasonic treatment and then dried at 60°C for several hour. After drying, the powders were grinded and collected for further study. CaF2:Eu3+ powders with Eu3+ substituting rate from 1 at% to 16 at% with interval of 1 percent were prepared through a similar way described above, at the beginning of which Eu(Ac)3·4H2O was (total amount of Ca2+ and Eu3+ was kept as 1 mmol) used as the source of substitutes.
2.3 Adsorption and sensing experiments for Cr2O72- ions
A 5 mL amount of 10 mg/L KnX (X = Cl−, Br−, I−, CO32−, NO3−, S2O82−, H2PO4−, PO43−, Cr2O72−, n = 1, 2, 3) solution was loaded into a centrifuge tube for sensing experiments. Several servings of 10 mg CaF2:Eu3+ were dispersed into the different anions solution listed above. After addition of CaF2:Eu3+ powders, the suspension in centrifuge tubes was thoroughly dispersed by shaking at room temperature for 1 h. Then, the powders were separated from the aqueous solution by centrifugation and dried under vacuum for the following study. Before the adsorption and sensing study, we investigated the adsorption kinetic between CaF2:10%Eu3+ and Cr2O72−. The result shows that adsorption between CaF2:10%Eu3+ powder and Cr2O72- ions reaches equilibrium within 10 minutes, which means that the reaction time of 1 h used here is enough long to reach the adsorption equilibrium. To investigate the influence of other anions for the selectively quenching response, CaF2:Eu3+ were also dispersed in the mixture solution containing above anions with and without Cr2O72− anions. The equilibrium capacity (qe, mg·g−1) and the removal efficiency (R, %) were calculated according to:
where C0 and Ce are the initial and the equilibrium concentration (mg·L−1) of the contaminant ions, respectively, V is the volume of solution (L), and m is the weight of the adsorbent (g).2.4 Characterization
XRD patterns of polycrystalline powders were recorded using a Rigaku MiniFlex 600 X-ray diffractometer operated at 40 kV and 15 mA with Cu Ka radiation. The powder XRD data was collected in the step scanning mode in the range of 10° - 75° with a step of 0.02° and an exposure of 7 s at each point. Optical patterns of the excitation and emission spectra were recorded through a FluoroMax-4 fluorescence spectrophotometer operated at room temperature (HORIBA Jobin Yvon Co.). The concentration of the contaminants remaining in the solution was determined by flame atomic absorption spectrometry methods (FAAS, AA-6880, Shimadzu Co.). Zeta potential (ζ) measurements were performed by dynamic light scattering analysis (ZetasizerNano ZS-90).
3. Results and discussion
3.1 Characterization of the as-prepared sintering products
As displayed in Fig. 1, all the diffraction patterns are consistent with the standard one of cubic CaF2 with space group Fmm [20]. It could be found that the diffraction peaks slightly shift towards the lower 2θ side when the Eu3+ concentration increases. According to Vegard's law [21], the lattice cell of CaF2:Eu3+ crystals is predicted to shrink slightly at higher Eu3+ concentration due to the smaller ionic radius of eight-coordinated Eu3+ ions compared to that of Ca2+ ions (0.107 nm vs. 0.112 nm) [22]. However, the shifting of diffraction peaks in Fig. 1 reveals that the CaF2:Eu3+ lattice cell expands with increasing Eu3+ concentration, which is opposite to the Vegard's law. It has been pointed by Corish J. et al. that the interstitial F- ion will be introduced into CaF2 lattice when Ca2+ ions are replaced with trivalent Re3+ ions such as Yb3+, Nd3+ and others, to balance the charge [10,23–25]. Consequently, the lattice cell of CaF2 expands slightly and the diffraction peaks shift to the lower 2θ side. On the other hand, the introduction of oxygen ions is also possible since the sintering of xerogel was performed in air, without any protection of reducing atmosphere. To investigate the influence of Eu3+ substitution on the microstructure and phase composition of the sintered product, patterns displayed in Fig. 1 were refined by Rietveld refinement using GSAS program [26]. The structure data of cubic CaF2 with space group Fmm was applied during the fitting procedure. As a representative, the refined profile of the CaF2:Eu3+ samples with substitution rate 5%, 9%, 13% and 16% Eu3+ are displayed in Fig. 2. Values of the fitting parameters such as Rp, Rwp and χ2 shown in this figure demonstrate there are good refinements for the patterns. The calculated cell parameters are a = 5.4661 Å, 5.4789 Å, 5.4887 Å, and 5.4981 Å for the above four samples.
Fig. 2 Rietveld refinement result of XRD patterns of samples of CaF2:5%Eu3+ (a), CaF2:9%Eu3+ (b), CaF2:13%Eu3+ (c) and CaF2:16%Eu3+ (d).
3.2 Optical spectroscopic properties and electronic transitions of CaF2: Eu3+ powders
Excitation spectra for all of the sol-gel products were measured by monitoring 616 nm emission. As a representative, the excitation and emission spectra of CaF2:10%Eu3+ is displayed in Fig. 3(a). The peaks emerged in the excitation spectrum could be attributed to the 7F0→5D4, 7F0→5GJ, 7F0→5L7, 7F0→5L6, 7F0→5D3, 7F0→5D2 and 7F0→5D1 electronic transitions of Eu3+ ion, respectively. The typical emission spectrum of CaF2:10%Eu3+, as shown in Fig. 3(a), is comprised of peaks assigned to electronic transitions 5D0→7F0 (579 nm), 5D0→7F1 (591 nm), 5D0→7F2 (616 nm), 5D0→7F3 (648 nm) and 5D0→7F4 (694 nm) of Eu3+ ion. When pumped from ground 7F0 state by 393 nm light, Eu3+ ions will be excited to 5L6 energy level and then non-radiatively relaxed to the metastable 5D0 level through 5D3, 5D2 and 5D1. Luminescence happens when Eu3+ ions radiatively decay from 5D0 state to the ground sub-levels (Fig. 3(b)) in accordance with the selection rule of quantum transition. The optical emission pattern is dominated by strong emission bands of 5D0→7F1, 5D0→7F2 and 5D0→7F4 transitions with 5D0→7F1 as the most intensive one. Two weak peaks assigned to 5D0→7F0 and 5D0→7F3 transitions also appear. According to the J-O theory, transitions from 5D0 to low-lying levels 7FJ (J = 0, 3, 5) of Eu3+ are both magnetic and electric forbidden. The appearance of these weak peaks is the result of crystal field induced J-mixing effect, which makes the electronic wave functions of 5D0 overlap slightly with that of 7FJ (J = 0, 3, 5) [27–30]. Similar phenomenon could also be found in some oxide luminescent materials such as molybdates [31], tungstates [32–34], borosilicates [35] or rare earth silicates [36], in which Eu3+ ions locate at lattice site with relatively low site symmetry. The integrated luminescence intensity of overall and every single transition from 5D0 to 7FJ all enhances monotonously when the Eu3+ concentration increases, as shown in Figs. 3(c) and 3(d). When observe closely at the evolution of integrated intensity, one can find that the value of R (asymmetry radio, the intensity ratio of 5D0→7F1 emission to 5D0→7F2 emission) is nearly unchanged with the increase of Eu3+ concentration, falling into the range of 0.78 - 1.02 (the dash dot in Fig. 3(d)).
Fig. 3 The excitation and emission spectrum of CaF2: 10%Eu3+, monitored at 616 nm emission(a), the energy level diagram of Eu3+ (b), the evolution of luminescence spectra (c), integrated luminescence intensity and R value (d) of CaF2:Eu3+ powders as a function of Eu3+ concentration.
3.3 Fluorescence response of CaF2:Eu3+ powder to Cr2O72- ions
To understand how selective the sintered products are towards Cr2O72- ions, CaF2:10%Eu3+ powder was treated with aqueous solutions with different anion, for example, I-, Br-, Cl-, CO32-, NO3-, PO43-, H2PO4-, S2O72- and Cr2O72-. Influence of these anions on the luminescence of CaF2:10%Eu3+ powder is shown in Fig. 4(a). Only Cr2O72- ions leads to dramatic luminescence quenching of the powder while other ions hardly affect the luminescence of CaF2:10%Eu3+. In addition, the influence of other anions for the luminescence intensity in CaF2:10%Eu3+/Cr2O72- system was also investigated by comparing the intensity of mixture anions solution with and without Cr2O72-. As observed in Fig. 4(b), the luminescence intensities quenching of CaF2:10%Eu3+/Cr2O72- system is rarely influenced by other anions. These results demonstrate the good sensitivity of CaF2:10%Eu3+ powder on Cr2O72−, implying that the CaF2:10%Eu3+ powders are a candidate to luminescent detect Cr2O72- ion.
Fig. 4 The emission spectra of CaF2:10%Eu3+ powder after mixing with different anions (a); The bar diagram of overall luminescence intensity change of CaF2:10%Eu3+ powder after treating with aqueous solution containing different anions and mixed anions (Cl−, Br−, I−, CO32−, NO3−, S2O82−, H2PO4−, PO43−, 10 mg/L for each) with/without Cr2O72− (b); Response of optical emission of CaF2:10%Eu3+ powder on the Cr2O72- ion: the evolution of emission spectrum on the Cr2O72- ion concentration (c) and the fitted relationship between the integrated emission intensity (range from 500 nm to 750 nm) of CaF2:10%Eu3+ and Cr2O72- ion concentration (d). The powders were excited by light with wavelength of 393 nm.
To verify the sensitivity degree towards Cr2O72- ions, the CaF2:10%Eu3+ powder was used to perform the luminescence probing experiment. The response of optical emission to Cr2O72- ions was demonstrated by Figs. 4(c) and 4(d). For each concentration in Fig. 4(d), three CaF2:10%Eu3+ powder samples were used to perform the adsorption experiment, after which their luminescence spectrum was recorded. Figure 5 exhibits the reproducibility of the luminescence intensity at a given concentration for three separate samples, taking 2 mg/L, 4 mg/L, 6 mg/L, 14 mg/L, 16 mg/L and 20 mg/L as representative. In general, the reproducibility of luminescence for separate samples is pretty good, especially for these at lower concentration of Cr2O72-. Therefore, we took the average luminescence intensity of every three samples as value of I to plot the line shown in Fig. 4(d). With the initial concentration of Cr2O72- increasing from 0.4 to 20 mg/L, the luminescence intensity of CaF2:10%Eu3+ powder quenches gradually. The fitted line of I0/I versus Cr2O72- concentration at the range of 0-20 mg/L shown in Fig. 4(d) can be described by the Stern−Volmer equation:
where I0 and I are the luminescence intensity of CaF2:10%Eu3+ powders in the absence and presence of Cr2O72- ions, respectively; KSV the Stern–Volmer’s quenching constant; [Q] the concentration of Cr2O72- ions. As shown in this figure, the Stern-Volmer quenching curve for Cr2O72- ion is nearly linear with a fit correlation coefficient of 0.998 and a Stern-Volmer value of 0.943. Obviously, the quantitative detection of Cr2O72- ions in an aqueous system can be realized by measuring the relative luminescence intensity (I/I0) of CaF2:10%Eu3+/Cr2O72- systems with different anion content level. The limit of detection (LOD) is calculated by 3δ/S, where δ (4.64 × 10−2) represents the standard deviation of the blank and S is the slope of the calibration curve. Here, the standard deviation of the blank was determined by recording luminescence spectra of 10 different CaF2:10%Eu3+ powder samples. Therefore, the calculated detection limit of Cr2O72- is 0.148 mg/L. Meanwhile, we compared the LOD value of CaF2:10%Eu3+ powders with that of other methods or materials, as listed in Tab. 1. It could be found that the LOD of our powder is not the best one, when compared to other luminescent probes [6–9, 37] or some common optical sensors [38, 39]. Besides, the LOD of CaF2:10%Eu3+ powder is also a little larger than the highest concentration of chromium in drinking water recommended in National Primary Drinking Water Regulations (United States Environmental Protection Agency), 0.1mg/L. Whereas, the low cost and high yield of this powder still make it competitive in the field of dichromate sensing in industrial waste water, i.e. leather tanning, metal electroplating, dyestuff industries.Fig. 5 The reproducibility of the luminescence intensity at a given Cr2O72- concentration for separate samples: (a) the luminescence spectra of three CaF2:10%Eu3+ powder samples recorded at Cr2O72- concentration of 20 mg/L; (b) the integrated intensities of separate CaF2:10%Eu3+ powders at different concentration of Cr2O72- in aqueous solution.
Table 1. LOD of different materials applied in detection of Cr(VI) in aqueous solution.
Adsorption process between CaF2:10%Eu3+ and Cr2O72- ions is confirmed by measuring Cr2O72- concentration and XPS spectra. As shown in Fig. 6(a), Cr2O72- ions are almost completely adsorbed by CaF2:10%Eu3+ powder when the initial Cr2O72- concentration lower than 4 mg/L. In addition, the adsorption capacity enlarges gradually with increase of the initial Cr2O72- concentration, which in turn lead to the luminescence reduction of CaF2:10%Eu3+/Cr2O72- system. The nearly invariable Ca:F:Eu atomic ratio calculated from XPS spectra (Fig. 6(b)) also demonstrates that adsorption of Cr2O72- happens when CaF2:10%Eu3+ powders immersed into aqueous solution containing Cr2O72-. The adsorption mentioned above could be ascribed to -OH2+ or -NH3+ groups on the surface of CaF2:10%Eu3+ powders.
Fig. 6 (a) The adsorption rate and capability of CaF2:10% Eu3+ in 2-20 mg/L Cr2O72- solution. (b) The XPS pattern of CaF2: 10%Eu3+ powders before and after immersed in Cr2O72- solution. Isotherm modeling of (c) Langmuir isotherm and (d) Freundlich isotherm for Cr2O72- ions adsorption on the CaF2:10%Eu3+ powder.
Using initial concentration of Cr (VI) between 2 mg/L and 20 mg/L at an adsorbent dosage level of 2 g/L, the adsorption isotherms for CaF2:10%Eu3+/Cr2O72- system were studied according to Langmuir and Freundlich isotherm models, which are usually used to describe and understand the adsorption process between adsorbent and target ions. The Langmuir isotherm model is often applied for monolayer adsorption assuming a homogeneous surface containing certain number of identical sites and no interaction between the adsorbate molecules, while the Freundlich isotherm model is valid for multilayer adsorption onto a heterogeneous surface. The linear forms of the Langmuir [40] and Freundlich isotherm model [41] can be expressed as:
where qe is the equilibrium Cr2O72- ions concentration on the adsorbent (mg/g), Ce is the equilibrium Cr (VI) concentration in solution (mg/L), qmax is the maximum monolayer Cr(VI) ions uptake capacity (mg/g) and KL is the Langmuir constant (L/mg) and associated to the adsorption energy. KF is the Freundlich constant (L/mg) and n is the heterogeneity factor. The results fitted by Langmuir and Freundlich isotherms are shown in Figs. 6(c) and 6(d), respectively. As compared to the value of correlation coefficients R2 (0.996 vs. 0.878) of two models, Langmuir isotherm model fits the Cr2O72- ions adsorption process much better, which indicates the Cr2O72- ions mainly adsorbed on the surface of CaF2:10%Eu3+ adsorbents by monolayer adsorption process and there are fixed number of active sites at the surface. Furthermore, the asymmetry ratio R of CaF2:10%Eu3+ powders that adsorb different content of Cr2O72- were also calculated. The value of R is almost constant and independent on the presence of Cr2O72-, indicating that the symmetrical environment of Eu3+ ion in CaF2 host lattice is not influenced by the adsorption of Cr2O72- ions, i. e., there is weak bonding between adsorbent and Cr2O72- ions on the surface of CaF2:Eu3+ powder. The zeta potentials (ζ) of the CaF2:10%Eu3+ solution under varying pH conditions i.e. pH = 3, 5, 7, 9, 11 were determined to be 37.56, 35.14, 34.68, 34.62 and 31.82 mV, respectively. The value of ζ for CaF2:10%Eu3+ powder is larger than 30 mV even when the pH value of aqueous solution equals 11, as shown in Fig. 7(a). This result clearly demonstrates that the CaF2:10%Eu3+ surface is positively charged in aqueous phase, which mainly contributed to the electrostatic bonding with Cr2O72- ions. Furthermore, the effect of solution pH on the adsorption capacity (qe) and luminescence intensity of CaF2:10%Eu3+ powders were also investigated. As shown in Fig. 7(b), both adsorption capacity and luminescence intensity are nearly invariant during the pH range of 5 – 9, which means that CaF2:10%Eu3+ powders work stably in aqueous solution with pH from 5 to 9.Fig. 7 (a) Zeta potentials (ζ) of CaF2:10%Eu3+ aqueous solutions as a function of pH value. (b) The adsorption capability and luminescence intensity of CaF2:10% Eu3+ in 20 mg/L Cr2O72- solution with different pH value.
As for the common inorganic luminescent probe materials, it is generally accepted that fluorescence quenching results from energy and electron transfer between the probe materials and the sensing targets [42, 43]. However, the nearly unchanged R value mentioned above indicates that there is rare energy transfer between Eu3+ and adsorbed Cr2O72- ions in CaF2:10%Eu3+/Cr2O72- system. To investigate the possible mechanism for the quenching effect of Cr2O72- anions, the UV−vis absorption spectra of every anion in solid state were recorded and displayed in Fig. 8. It is obvious that the excitation spectrum of the CaF2: Eu3+ is completely overlapped by the absorption spectra Cr2O72− anions, especially at 393 nm, whereas no other anions have absorption spectrum overlapped the excitation spectrum of the CaF2:Eu3+. This fact demonstrates that Cr2O72− anions can shield the excitation light for CaF2:Eu3+ powders, since there is a competitive absorption of the exciting light (393 nm) between Cr2O72− and Eu3+. Therefore, the adsorbing enhancement of Cr2O72− anions could be successfully converted to luminescence quenching of CaF2:Eu3+ powders. The quenching mechanism in CaF2:10%Eu3+/Cr2O72- detection system is illustrated in Fig. 9.
Fig. 8 Ultraviolet absorption spectra of different anions in solid state and excitation and emssion spectra of CaF2: Eu3+ particle.
4. Conclusion
Serial CaF2:Eu3+ powders have been synthesized via a modified sol-gel method. Phase investigation result indicates the crystal lattice cell of these samples enlarges with the increase of Eu3+ content, which is mainly caused by the interstitial F- ions. The optical spectroscopic properties demonstrate that Eu3+ ions are accommodated in CaF2 lattice sites without inversion symmetry. The asymmetry radio (R) value, which is calculated from intensity ratio of 5D0→7F1 to 5D0→7F2 under excitation at 393 nm, is almost unchanged with the increase of Eu3+ content, which indicates the local symmetry of Eu3+ ions in the CaF2 lattice is stable. It is found that CaF2:10%Eu3+ powder exhibit selective luminescence quenching on Cr2O72- ions, while the presence of other anions rarely affects the luminescence quenching of CaF2:10%Eu3+/Cr2O72- system. This fact tells us that the CaF2:10%Eu3+ powder is a candidate luminescent probe to sense Cr2O72- ions in aqueous solution. Investigation on the quantitative detection ability of CaF2:10%Eu3+ on Cr2O72- ions shows that the former is sensitive to Cr2O72- ions with a detection limit of 0.148 mg/L. Furthermore, the mechanism analysis on luminescence quenching of CaF2:10%Eu3+/Cr2O72- system reveals that the adsorbed Cr2O72- ions at CaF2:10%Eu3+ powder surface screen it from exciting light, because there is a competitive absorption of the exciting light between Cr2O72− and Eu3+. Benefitting from the easy available laser diode (LD) with 393 nm light emission and photoelectric transducer that could collect and transfer the luminescent light emitted from test powders to electric signal, a simple sensor based on the technique described in this contribution could be realised with a mature package technique. Furthermore, it could also be expected that a colorimetric type sensor would be developed when we use CaF2:Tm3+, Yb3+ or CaF2:Er3+, Yb3+ powders as probing materials and infrared light as excitation light, since the optical absorption of Cr2O72- ion at 400 nm - 500 nm range overlaps with the green emission branch of the upconversion luminescence spectra of these powders.
In a word, the feasibility of using CaF2:Eu3+ powders as a luminescent probe was studied, the result demonstrates that low cost and high yield CaF2:Eu3+ powder that prepared in this work is a potential probing material to detect Cr2O72- ions in aqueous solution. Above all, this study opens a new field of application of CaF2 based optical material to environmental conservation.
Funding
National Natural Science Foundation of China (Grant No. 61308085, 51672046, 51472050, 61775217); the Natural Science Foundation of Fujian Province, China (2017J01746); Fujian Provincial Department of Science and Technology (2017H6006); Fujian Provincial Department of Education (JK15001).
Acknowledgement
The authors would like to thank Prof. Liuyi Li, Prof. Zanyong Zhuang and Dr. Michael T. Yeung for the kind and useful discussion.
References and links
1. D. E. Kimbrough, Y. Cohen, A. M. Winer, L. Creelman, and C. Mabuni, “A critical assessment of chromium in the environment,” Crit. Rev. Environ. Sci. Technol. 29(1), 1–46 (1999). [CrossRef]
2. X. Shi, A. Chiu, C. T. Chen, B. Halliwell, V. Castranova, and V. Vallyathan, “Reduction of chromium(VI) and its relationship to carcinogenesis,” J. Toxicol. Environ. Health B Crit. Rev. 2(1), 87–104 (1999). [CrossRef] [PubMed]
3. Z. Li, M. J. Deen, S. Kumar, and P. R. Selvaganapathy, “Raman Spectroscopy for in-line water quality monitoring - instrumentation and potential,” Sensors (Basel) 14(9), 17275–17303 (2014). [CrossRef] [PubMed]
4. C. Song, B. Yang, Y. Zhu, Y. Yang, and L. Wang, “Ultrasensitive sliver nanorods array SERS sensor for mercury ions,” Biosens. Bioelectron. 87, 59–65 (2017). [CrossRef] [PubMed]
5. C. Y. Liu, S. Y. Wang, G. Chen, S. P. Xu, Q. Jia, J. Zhou, and W. Q. Xu, “A surface-enhanced Raman scattering (SERS)-active optical fiber sensor based on a three-dimensional sensing layer,” Sensing and Bio-Sensing Research 1, 8–14 (2014). [CrossRef]
6. W. Liu, Y. Wang, Z. Bai, Y. Li, Y. Wang, L. Chen, L. Xu, J. Diwu, Z. Chai, and S. Wang, “Hydrolytically Stable Luminescent Cationic Metal Organic Framework for Highly Sensitive and Selective Sensing of Chromate Anions in Natural Water Systems,” ACS Appl. Mater. Interfaces 9(19), 16448–16457 (2017). [CrossRef] [PubMed]
7. J. Liu, G. Ji, J. Xiao, and Z. Liu, “Ultrastable 1D europium complex for simultaneous and quantitative sensing of Cr(III) and Cr(VI) ions in aqueous solution with high selectivity and sensitivity,” Inorg. Chem. 56(7), 4197–4205 (2017). [CrossRef] [PubMed]
8. B. Ding, C. Guo, S. X. Liu, Y. Cheng, X. X. Wu, X. M. Su, Y. Y. Liu, and Y. Li, “A unique multi-functional cationic luminescent metal–organic nanotube for highly sensitive detection of dichromate and selective high capacity adsorption of Congo red,” RSC Advances 6(40), 33888–33900 (2016). [CrossRef]
9. M. Karimi, A. Badiei, and G. Mohammadi Ziarani, “SBA-15 functionalized with naphthalene derivative for selective optical sensing of Cr2O7(2-) in Water,” Anal. Sci. 32(5), 511–516 (2016). [CrossRef] [PubMed]
10. M. Siebold, S. Bock, U. Schramm, B. Xu, J. L. Doualan, P. Camy, and R. Moncorgé, “Yb:CaF2-a new old laser crystal,” Appl. Phys. B-Lasers O. 97(2), 327–338 (2009). [CrossRef]
11. V. Petit, J. L. Doualan, P. Camy, V. Ménard, and R. Moncorgé, “CW and tunable laser operation of Yb3+ doped CaF2,” App.l Phys. B-Lasers O. 78(6), 681–684 (2000). [CrossRef]
12. A. Lucca, G. Debourg, M. Jacquemet, F. Druon, F. Balembois, P. Georges, P. Camy, J. L. Doualan, and R. Moncorgé, “High-power diode-pumped Yb3+:CaF2 femtosecond laser,” Opt. Lett. 29(23), 2767–2769 (2000). [CrossRef] [PubMed]
13. M. Kowalczyk, A. Major, and J. Sotor, “High peak power ultrafast Yb:CaF2 oscillator pumped by a single-mode fiber-coupled laser diode,” Opt. Express 25(21), 26289–26295 (2017). [CrossRef] [PubMed]
14. X. Liu, K. Yang, S. Zhao, T. Li, C. Luan, X. Guo, B. Zhao, L. Zheng, L. Su, J. Xu, and J. Bian, “Growth and Lasing Performance of a Tm,Y:CaF2 crystal,” Opt. Lett. 42(13), 2567–2570 (2017). [CrossRef] [PubMed]
15. I. S. Grudinin, A. B. Matsko, and L. Maleki, “Brillouin Lasing with a CaF2 whispering gallery mode resonator,” Phys. Rev. Lett. 102(4), 043902 (2009). [CrossRef] [PubMed]
16. X. S. Qiao, X. P. Fan, J. Wang, and M. Q. Wang, “Luminescence behavior of Er3+ ions in glass-ceramics containing CaF2 nanocrystals,” J. Non-Cryst. Solids 351(5), 357–363 (2005). [CrossRef]
17. P. Babu, K. H. Jang, ChS. Rao, L. Shi, C. K. Jayasankar, V. Lavín, and H. J. Seo, “White light generation in Dy3+-doped oxyfluoride glass and transparent glass-ceramics containing CaF2 nanocrystals,” Opt. Express 19(3), 1836–1841 (2011). [CrossRef] [PubMed]
18. G. Wang, Q. Peng, and Y. Li, “Upconversion luminescence of monodisperse CaF2:Yb3+/Er3+ Nanocrystals,” J. Am. Chem. Soc. 131(40), 14200–14201 (2009). [CrossRef] [PubMed]
19. C. Zhang, C. Li, C. Peng, R. Chai, S. Huang, D. Yang, Z. Cheng, and J. Lin, “Facile and controllable synthesis of monodisperse CaF2 and CaF2:Ce3+/Tb3+ hollow spheres as efficient luminescent materials and smart drug carriers,” Chemistry 16(19), 5672–5680 (2010). [CrossRef] [PubMed]
20. W. L. Bragg, “The analysis of crystals by the X-ray spectrometer,” Roy. Soc. Proc. 89(613), 468–489 (1914). [CrossRef]
21. A. R. Denton and N. W. Ashcroft, “Vegard’s law,” Phys. Rev. A 43(6), 3161–3164 (1991). [CrossRef] [PubMed]
22. R. D. Shannon, “Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallogr. 32(5), 751–767 (1976). [CrossRef]
23. J. Corish, C. R. A. Catlow, P. W. M. Jacobs, and S. H. Ong, “Defect aggregation in anion-excess fluorites. dopant monomers and dimers,” Phys. Rev. B 25(10), 6425–6438 (1982). [CrossRef]
24. M. L. Falin, K. I. Gerasimov, V. A. Latypov, A. M. Leushin, H. Bill, and D. Lovy, “EPR and optical spectroscopy of Yb3+ ions in CaF2, and SrF2,” J. Lumin. 102–103, 239–242 (2003). [CrossRef]
25. S. A. Payne, J. A. Caird, L. L. Chase, L. K. Smith, N. D. Nielsen, and W. F. Krupke, “Spectroscopy and gain measurements of Nd3+ in SrF2 and other fluorite-structure hosts,” J. Opt. Soc. Am. B 8(4), 726–740 (1991). [CrossRef]
26. A. C. Larson, and R. B. Von Dreele, General Structure Analysis System (GSAS) (Los Alamos National Laboratory Report LAUR 86–748, (2004).
27. F. Auzel, “A fundamental self-generated quenching center for lanthanide-doped high-purity solids,” J. Lumin. 100(1–4), 125–130 (2002). [CrossRef]
28. B. R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127(3), 750–761 (1962). [CrossRef]
29. X. Y. Chen, W. Zhao, R. E. Cook, and G. K. Liu, “Anomalous luminescence dynamics of Eu3+, in BaFCl microcrystals,” Phys. Rev. B 70(20), 205122 (2004). [CrossRef]
30. Y. Wang, Y. Liu, Q. Xiao, H. Zhu, R. Li, and X. Chen, “Eu3+ doped KYF4 nanocrystals: synthesis, electronic structure, and optical properties,” Nanoscale 3(8), 3164–3169 (2011). [CrossRef] [PubMed]
31. Y. Tian, B. J. Chen, R. N. Hua, J. S. Sun, L. H. Cheng, H. Y. Zhong, X. P. Li, J. S. Zhang, Y. F. Zheng, T. T. Yu, L. B. Huang, and H. Q. Yu, “Optical transition, electron-phonon coupling and fluorescent quenching of La2(MoO4)3:Eu3+ phosphor,” J. Appl. Phys. 109(5), 053511 (2011). [CrossRef]
32. Y. Tian, B. J. Chen, R. N. Hua, N. S. Yu, B. Q. Liu, J. S. Sun, L. H. Cheng, H. Y. Zhong, X. P. Li, J. S. Zhang, B. N. Tian, and H. Zhong, “Self-Assembled 3D flower-shaped NaY(WO4)2:Eu3+ microarchitectures: microwave-assisted hydrothermal synthesis, growth mechanism and luminescent properties,” CrystEngComm 14(5), 1760–1769 (2012). [CrossRef]
33. K. K. Rasu, D. Balaji, and S. M. Babu, “Spectroscopic properties of Eu3+: KLa(WO4)2 novel red phosphors,” J. Lumin. 170, 547–555 (2016). [CrossRef]
34. X. Y. Huang, B. Li, and H. Guo, “Highly efficient Eu3+-activated K2Gd(WO4)(PO4) red-emitting phosphors with superior thermal stability for solid-state lighting,” Ceram. Int. 43(13), 10566–10571 (2017). [CrossRef]
35. L. Y. Li, Y. P. Lin, L. Z. Zhang, Q. C. Cai, and Y. Yu, “Flux exploration, growth, and optical spectroscopic properties of large size LaBSiO5 and Eu3+-substituted LaBSiO5 crystals,” Cryst. Growth Des. 17(12), 6541–6549 (2017). [CrossRef]
36. G. Ramakrishna, H. Nagabhushana, B. Daruka Prasad, Y. S. Vidya, S. C. Sharma, K. S. Anantharaju, S. C. Prashantha, and N. Choudhary, “Spectroscopic properties of red emitting Eu3+ Doped Y2SiO5 nanophosphors for WLED׳s on the basis of Judd-Ofelt analysis: calotropis gigantea, latex mediated synthesis,” J. Lumin. 181, 153–163 (2017). [CrossRef]
37. X. X. Wu, H. R. Fu, M. L. Han, Z. Zhou, and L. F. Ma, “Tetraphenylethylene Immobilized Metal−Organic Frameworks: Highly Sensitive Fluorescent Sensor for the Detection of Cr2O72− and Nitroaromatic Explosives,” Cryst. Growth Des. 17(11), 6041–6048 (2017). [CrossRef]
38. H. Vaskova, “Raman microscopic detection of chromium compounds,” MATEC Web of Conferences 76, 05012 (2016). [CrossRef]
39. W. Ji, Y. Wang, I. Tanabe, X. Han, B. Zhao, and Y. Ozaki, “Semiconductor-driven “turn-off” surface-enhanced Raman scattering spectroscopy: application in selective determination of chromium(vi) in water,” Chem. Sci. (Camb.) 6(1), 342–348 (2015). [CrossRef] [PubMed]
40. I. Langmuir, “The adsorption of gases on plane surfaces of glass, mica and platinum,” J. Chem. Phys. 40(12), 1361–1403 (1981).
41. H. M. F. Freundlich, “Over the adsorption in solution,” J. Chem. Phys. 57, 385–470 (1906).
42. D. Ghosh and M. N. Luwang, “Selective Detection of Fe3+, Cr3+ and Cu2+ in Water using Highly Luminescent Gd2O3:Eu3+ Nanoparticles,” J. Lumin. 171, 1–8 (2016). [CrossRef]
43. S. Sarkar, M. Chatti, and V. Mahalingam, “Highly Luminescent Colloidal Eu3+-Doped KZnF3 Nanoparticles for the Selective and Sensitive Detection of Cu II Ions,” Chemistry 20(12), 3311–3316 (2014). [CrossRef] [PubMed]
