o -Phenylenediamine as a reaction-based probe for the fluorescent detection of Ce(IV) ions

Introduction

Cerium (Ce), the most abundant rare-earth element, has been widely used in industry as a polishing powder, as a phosphor agent, as magnets, as a catalyst, and as a ceramic colorant.^1–3 Although Ce(III) exists naturally in cerium fluorocarbonate minerals, during the process of mineral extraction and metallurgy, Ce(III) is usually converted into Ce(IV) for easier extraction in the post-treatment stage.^4–6 This process further increases the exposure of Ce(IV) to the community. High concentrations of Ce ions are known to be harmful to the environment and human health.^7,8 For instance, long-term exposure to Ce(IV) ions would probably lead to dysfunction in circulatory systems, the immune system, and the central nervous system in humans. ⁹ Therefore, accuracy, precision, and rapid determination of Ce(IV) ions are very important for the environment and in clinical applications.

Various instruments are commonly used for the detection of Ce ions, such as X-ray fluorescence, ¹⁰ inductively coupled plasma atomic emission spectrometry (ICP-AES), ¹¹ spectrophotometry, ¹² neutron activation analysis (NAA), ¹³ inductively coupled plasma mass spectrometry (ICP-MS), and electrochemical methods.^14,15 However, these methods possess several shortcomings, such as high cost, long processing times, and complex sample preparation. Recently, fluorescence sensors have become a powerful tool for the detection of important analytes in the environment and in organisms because of their simplicity and high sensitivity.^16–20 Up to now, several fluorescence sensors have been exploited to detect the Ce ions.^21–25 However, some of these methods are restricted by toxic inorganic nanomaterials, low sensitivity, or low selectivity. Recently, a rhodamine-based probe ²⁶ and a Schiff base probe ²⁷ have been reported for the recognition of Ce(IV) ions, respectively. The two probes both show high selectivity and sensitivity, but with relatively complex molecular structures. We also reported a cascade-reaction-based fluorescence probe for the detection of Ce(IV) ions. ²⁸ Ce(IV) ions can oxidize 2-naphthol, which subsequently reacts with o-phenylenediamine (OPD) to produce fluorescent benzo[a]phenazine. Although this method is efficient for the fluorescence detection of Ce(IV) ions, multistep reactions are inconvenient for detecting ions. Therefore, it is necessary to develop a rapid, sensitive, and efficient Ce(IV)-ion-sensing platform.

In recent years, the luminescent properties of 2,3-diaminophenazine (OPDox) have received extensive attention in the fields of analytical chemistry, organic synthesis, and materials chemistry.^29–32 Notably, the Ce(IV) ions have been generally studied as a single-electron oxidant in various reactions.^33,34 As an efficient single-electron oxidizer, Ce(IV) can efficiently promote the oxidative cyclization of OPD to generate OPDox.^35–37 In this work, as a continuation of our ongoing studies on reaction-based fluorescent probes,^38–41 OPD is used as a reaction-based fluorescent probe for the detection of Ce(IV) ions. In this detection process, OPD can be rapidly oxidized by Ce(IV) ions to an imine, which then undergoes as a further intermolecular cycloaddition to form the product OPDox which possesses yellow fluorescence, thus realizing the fluorescent detection of Ce(IV) ions (Scheme 1).

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Scheme 1.

Reaction principle for the detection of Ce(IV) ions.

Results and discussion

The solvent for the reaction of Ce(IV) ions and OPD was first investigated, including MeCN, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), EtOH, CH₂Cl₂, and H₂O. In the experiment, OPD (40 μM) was mixed with Ce(IV) ions (0.4 μM) for 1 h at room temperature. It was observed that the effect of the solvent was significant, and the maximum fluorescence enhancement was observed in MeCN (Figure 1(a)). Control experiments were carried out in order to investigate the effect of the various components in the sensing system. It was thus shown that enhancement in fluorescence only occurred when both Ce(IV) and OPD were present in the system (Figure 1(b) and (c)).

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Figure 1.

Fluorescence emission spectra (λ_ex = 436 nm) of the sensing system containing 40 μM OPD, (a) with or without Ce(IV) ions (0.4 μM) in the indicated solvents and (b) the emission spectra of the control experiments in MeCN. (c) Photographs of 1 mM of Ce(IV) (I), 1 mM of OPD (II), and 1 mM of Ce(IV) with 1 mM of OPD(III) under 365 nm UV illumination.

Furthermore, only when OPD and Ce(IV) ions were present, a strong absorption band between 350 and 550 nm could be found in the nature of UV-Vis absorption spectrum (Figure S1). To confirm the product, the detection reaction was performed at a millimolar level, and the OPDox product was obtained with the structure being confirmed by ¹H/¹³C NMR spectroscopy (Figure S2 and S3). At the same time, bright yellow fluorescence could be observed with the naked eye via irradiation with an ultraviolet lamp at 365 nm (Figure 1(b)) when OPD (1 mM) and Ce(IV) ions (1 mM) were mixed at room temperature. Hence, OPD as a new and practical probe for the sensing Ce(IV) ions was studied in further detail.

In order to ascertain the optimal sensing system, varying MeCN/H₂O ratios were investigated for the luminescence response of the detection reaction. The highest luminescence enhancement was obtained with a 4:1 MeCN/H₂O ratio (Figure 2). However, the luminescence rate started to decrease slowly thereafter. The incubation time was then investigated. It can be found that the fluorescence intensity of the system containing Ce(IV) ions increases with an extended incubation time and reaches a steady state at 3 h (Figure 3). However, no obvious fluorescence enhancement was measured without the Ce(IV) ions. We therefore chose a 4:1 MeCN/H₂O as the best mixed solvent and 3 h as the optimal incubation time.

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Figure 2.

Fluorescence enhancement of OPD (40 μM) with 0.4 μM of Ce(IV) ions in various percentages of water in MeCN.

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Figure 3.

The fluorescence intensity of the system in the presence and absence of Ce(IV) ions with the indicated time. The concentration of Ce(IV) ions is 0.4 and 40 μM for OPD in MeCN containing 20% H₂O. The fluorescence was monitored at 15 min intervals and collected at a wavelength of 526 nm.

Sensitivity is of importance for an analytical method. Hence, the fluorescence responses of the detection method to various concentrations of Ce(IV) ions were examined in detail. At room temperature, a system containing OPD (40 μM) was uniformly mixed with different concentrations of Ce(IV) ions (Figure 4). A highest 150-fold increase in fluorescence was obtained on increasing the concentration of Ce(IV) ions (0–88 μM). Two linear relationships were established at ranges of 0.4–4 μM (R² = 0.9979) and 5–36 μM (R² = 0.9988), respectively. At low Ce(IV) concentrations, OPD was converted into fluorescent OPDox and could be rapidly detected, resulting in a high sensitivity of the fluorescence response. At higher Ce(IV) concentrations, more fluorescent product was formed and the reaction proceeded over a larger time window. This, together with the higher fluorescence intensity, resulted in a lower slope. Therefore, the probe exhibited different linear correlations at different Ce(IV) concentration ranges. According to the limit of detection (LOD) = 3σ/s, 26 nM of the LOD was calculated. Compared to our previous work, ²⁶ this method shows a wider detection range and a lower LOD.

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Figure 4.

(a) Fluorescence responses of the system to various concentrations of Ce(IV) ions. The concentration of OPD is 40 μM and the concentration of Ce(IV) ions is in the range of 0–88 μM. (b) Fluorescence responses at 526 nm of the system at various concentrations of Ce(IV) ions (1–120 µM). The linear correlation between fluorescence intensity and the concentration of Ce(IV) ions (c) 0.4–4 µM and (d) 5–36 µM. λ_ex = 436 nm, λ_em = 526 nm.

The method was also applied to solid-phase conditions. The letters of “Ce” were written on a thin-layer chromatography (TLC) aluminum sheet using the reaction solution of OPD and Ce(IV) ions as an “ink” (Figure 5). When the solvent was dried, we observed the TLC sheet under daylight and UV 254 and 365 nm irradiation. Compared with the reaction solution that did not contain any Ce(IV) ions, the “Ce” containing both OPD and Ce(IV) ions emits bright yellow fluorescence under 365 nm UV irradiation. Therefore, this method serves as a visual fluorescence probe for Ce(IV) ions.

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Figure 5.

Photographs of TLC sheets with letters written using OPD (top row) or OPD-Ce(IV) (bottom row) as “ink” recorded under (a) daylight and UV light at (b) 254 nm and (c) 365 nm.

To show the excellent selectivity of this sensing method, 19 common cations, 11 common anions, and 15 rare-earth cations were all investigated (Figure 6). It can be found that only Ce(IV) ions (0.4 μM) gave an obvious enhancement in the fluorescence. As for other ions, only a slight fluorescence enhancement was obtained, even at a concentration of 20 μM. The specificity of the probe for Ce(IV) ions was also excellently observed from a competition experiment conducted by mixing Ce(IV) ions (0.4 μM) in a system with another ion (20 μM). Comparable fluorescence enhancements indicated that this organic reaction–based probe possesses high selectivity toward Ce(IV) ions.

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Figure 6.

Fluorescence intensity upon addition of 19 common cations (a) (Cu²⁺, Mg²⁺, K⁺, Zn²⁺, Ag⁺, Co²⁺, Al³⁺, Fe³⁺, Na⁺, Ca²⁺, Li²⁺, Mn²⁺, Cr³⁺, Fe²⁺, Ba²⁺, Sn²⁺, Ni²⁺, Hg²⁺, and Pt²⁺), (b) 11 common anions (I⁻, CO 3 2 − , Br⁻, Cl⁻, SO 3 2 − , SO 4 2 − , HSO₄⁻, HCO³⁻, S²⁻, S 2 O 3 2 − , NO³⁻), and (c) 15 rare-earth cations (Ce³⁺, Nd³⁺, Gd³⁺, Dy³⁺, Tb³⁺, Bi³⁺, Y³⁺, La³⁺, Lu³⁺, Eu³⁺, Pr³⁺, Pm³⁺, Yb³⁺, Sc³⁺, and Er³⁺). The red bars represent the addition of 20 μM of different ions to the sensor system. The black bars represent the subsequent addition of 0.4 μM Ce(IV) ions to the system. λ_ex = 436 nm, λ_em = 526 nm.

A Job’s plot experiment was carried out for the determination of the stoichiometric ratio of the interaction between Ce(IV) ions and OPD (Figure 7). The 2:1 ratio obtained from the Job’s plot experiment is consistent with the reaction mechanism.

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Figure 7.

Job’s plot for determining the stoichiometry of Ce(IV) ions and OPD; the total concentration of Ce(IV) ions and OPD was 100 μM. Molar fraction is given by [Ce(IV)]/([Ce(IV)] + [OPD]).

Finally, the detection of Ce(IV) ions was studied in real samples using the fluorescent probe. Samples of Ganjiang River water, tap water, rainwater, and lateritic soils were collected and pretreated according to previous methods.^42,43 When exogenous Ce(IV) ions (19.2 μM) were added to the samples, significant luminescence intensities were observed. As shown in Table 1, recovery rates of 94.78% and 93.68% could be obtained in Ganjiang River water and tap water samples, respectively. Higher recovery rates of 97.39% and 98.77% were obtained for rainwater and lateritic soils samples, respectively. These results show that our fluorescent detection method can be applied for Ce(IV) ions sensing in real samples with good sensitivity.

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Table 1.

Investigations on the detection of Ce(IV) ions in water samples and lateritic soil samples. ^a

Samples	pH	Added amount (μM)	Found amount (μM)	Standard deviation (μM)	RSD (%, n = 3)	Recovery (%, n = 3)
Ganjiang River water	8.07	19.20	18.20	0.65	1.91	94.78
Rain water	6.21	19.20	17.99	1.22	2.31	93.68
Tap water	5.49	19.20	18.70	0.43	0.95	97.37
Lateritic soils	5.40	19.20	18.96	0.23	0.46	98.77

RSD: relative standard deviation.

a

Results for the determination of Ce(IV) ions in real samples. Experimental conditions: MeCN/H₂O (v/v = 4:1); OPD (40 μM), Ce(IV) (19.2 μM).

Conclusion

In this work, a reaction-based switch-on fluorescent probe for Ce(IV) ions has been reported. The reaction is based on a tandem oxidation–cyclization reaction between the OPD and Ce(IV) ions, forming fluorescent OPDox. A maximum 150-fold fluorescence enhancement is observed in response to Ce(IV) ions, with good selectivity and sensitivity. Its applicability can provide a platform for the development of the next-generation Ce(IV)-ion probes for use in real-world scenarios.

Experimental

Supplemental Material