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Article

An Acylhydrazone Fluorescent Sensor: Bifunctional Detection of Thorium (IV) and Vanadyl Ions over Uranyl and Lanthanide Ions

by
Xin Lin
1,
Hua Liang
1,*,
Ke Dai
1,
Jing Zhou
1,
Qiang Tian
1,
Yuge Xiang
2,
Zhicheng Guo
2 and
László Almásy
1,3,*
1
State Key Laboratory of Environment-Friendly Energy Materials, School of Materials and Chemistry, Southwest University of Science and Technology, Mianyang 621010, China
2
School of National Defense, Southwest University of Science and Technology, Mianyang 621010, China
3
Institute for Energy Security and Environmental Safety, HUN-REN Centre for Energy Research, Konkoly-Thege Miklós út 29-33, 1121 Budapest, Hungary
*
Authors to whom correspondence should be addressed.
Int. J. Mol. Sci. 2025, 26(7), 3231; https://doi.org/10.3390/ijms26073231
Submission received: 11 March 2025 / Revised: 28 March 2025 / Accepted: 28 March 2025 / Published: 31 March 2025
(This article belongs to the Special Issue Analytical Methods in Biomedical and Environmental Applications: A Molecular Approach (2nd Edition))
Browse Figures
Versions Notes

Abstract

Thorium is a notable candidate for resolving uranium shortage caused by the global application of nuclear power generation. Uranium extraction from seawater is another attempt to handle its source deficiency, however, vanadium is one of the main competitive elements in that process. Exploration of probes which can discriminatively detect thorium and vanadium from uranium has primary significance for their further separation and for environmental protection. Herein, N′-(2,4-dihydroxybenzylidene)-4-hydroxylphenylhydrazide, AOH, is used as sensor for Th4+ and vanadyl (VO2+) determination. AOH demonstrates a specific “turn-on” fluorescence selectivity towards Th4+ over f-block and other foreign metal ions, with a detection limit (LOD) of 7.19 nM in acidic solution and a binding constant of 9.97 × 109 M−2. Meanwhile, it shows a “turn-off” fluorescence response towards VO2+ over other metal ions at the coexistence of Th4+, with a LOD of 0.386 μM in the same media and a binding constant of 4.54 × 104 M−1. The recognition mechanism, based on HRMS, 1H NMR, and FT-IR results, demonstrates that VO2+ causes the fluorescence quenching by replacing Th4+ to coordinate with AOH. In real water detection tests, Th4+ and VO2+ exhibited satisfying recoveries. These findings expand the application of sensors in nuclide pollution control.

1. Introduction

Thorium is a natural radioactive element and it has a quantity roughly 3 to 4 fold greater than that of uranium in the Earth’s crust, which makes it an alternative nuclear energy source owing to the insufficiency of uranium [1,2,3]. Meanwhile, thorium is known as a strategic element in a variety of industries. However, thorium contributes to radioactive contamination, mainly because of the processing and mining activity of rare earth elements (or lanthanide elements, Ln), owing to the wide range of applications of Ln in many fields, such as magnetic and optical materials [4,5]. Ln are typically found in conjunction with uranium and particularly thorium due to their similar ionic radii [6]. The detection and separation of lanthanide and actinide elements in nuclear waste are one of the most challenging tasks, due to a great similarity in their physical and chemical properties [7]. Th(IV) is the most stable form of thorium in nature and solution. Effective determination of Th4+ in coexistence with uranyl and Ln3+ ions is important in nuclear waste liquid control and in their further separation and recycling. Instrumental analysis methods such as X-ray fluorescence spectrometry (XRF) [8], gamma spectrometry, inductively coupled plasma mass spectrometry (ICP-MS) [9], and neutron activation analysis (NAA) [10,11,12], have high precision and sensitivity in nuclide ions analysis, yet shortcomings like laborious sample preparation and expensive instruments make them unsuitable for on-site analysis. Fluorescence spectrometry is an exception though it is an instrumental analysis method. It has significant advantages like fast response time, cost-effectiveness, low detection limits with high sensitivity, and selectivity, provided that a suitable fluorescent sensor is chosen [13,14]. Fluorescence sensors have been proven to be excellent luminescent probes toward Th4+ [15,16,17,18]. Nevertheless, the issues of anti-interference, water compatibility, and simple sensors preparation are yet to be improved [19,20,21].
In addition to exploring the substitution of uranium, its extraction from seawater is another possibility to solve its shortage, as for the amount of uranium, though with a very low concentration of 3–9 μg/L, it is thousands of times greater than that in the Earth’s crust [22,23]. However, in that process, vanadium is an important competitive element [24,25]. Vanadium exists usually in the form of trace elements and is not essential for most living organisms. In aqueous solution, vanadium mainly exists in +4 and +5 valence states. Its effect on organisms is still controversial yet with potential hazards [26]. Compared with other harmful heavy metals, the detection methods for vanadium are very limited, in spite of its importance in the steel industry, medical treatment, batteries, and other fields [27,28]. A few pioneering works have been done on vanadyl sensing [29,30]; however, its distinguishing detection from uranium are still to be explored [31]. From the studies on uranium pollution control and recovery reported, thorium and vanadium are two interfering elements [25,32], but the determinations of them from uranium at the same time, to our knowledge, have not been considered.
In this work, a simple acylhydrazone, AOH, studied earlier as a pharmacophore [33,34], has been investigated for its detection abilities toward Th4+ and VO2+ ions. Here, AOH shows a fluorescence “turn-on” to Th4+, and “turn-off” for VO2+ with the help of Th4+ in acidic aqueous solution even at the co-existence of UO22+ and rare earth metal ions. The detection limit and detection mechanism were also investigated. In addition, the detections of Th4+ and VO2+ in real water were investigated to check the sensor practicability.

2. Results and Discussion

2.1. Selectivity of AOH Towards Th4+

The selectivity response of AOH to Ln3+ and the nuclides Th4+ and UO22+ were tested preliminarily in pure ethanol. In that medium, it exhibited blue luminescence to La3+, Lu3+, and Th4+ (Figure 1a). When changing the medium from pure ethanol to EtOH/H2O (1/1, v/v, pH = 2.0), the detection selectivity was improved and AOH only exhibited strong response to Th4+ with no sensitivity to La3+ and Lu3+ (Figure 1b).
The pH effect investigations show that at pH = 2.0 (50% EtOH and 50% H2O in volume, Figure S3), AOH had the most sensitive “turn-on” fluorescence response intensity toward Th4+. As pH 2.0 is a very acidic condition that may affect the stability of the sensor, the change of the UV-vis spectra of AOH with time in EtOH/H2O (1/1, v/v, pH = 2.0) was analyzed to check its stability. (Figure S4). The absorbance of AOH at 326 nm decreased slightly and there was nearly no change between 350 nm and 600 nm in one hour (1 h), indicating the good stability of AOH in such media at the tested time. Since all the fluorescence testing was performed in less than 1 h with the excitation wavelength at 374 nm, under conditions where AOH was stable, the subsequent experiments were conducted under such media unless otherwise noted.
The fluorescence spectra of AOH to Th4+, UO22+, and lanthanide ions (Ln3+) with some alkaline earth metal ions show the sensor response uniqueness toward Th4+ (Figure 2). That specific selectivity is an improvement over reported Th4+ sensors, which are susceptible to the interference from UO22+ and some Ln3+ [35,36,37]. When common metal ions (Na+, Fe3+, Ni2+, Zn2+, Cu2+, Al3+) were added in the AOH solution, no luminescence except that of Th4+ was observed (Figure S5), reproving the specific selectivity to Th4+.

2.2. Interfering Ions Effect on Th4+ Response

Real Th4+-containing wastes (nuclear waste) contain a large amount of actinide and lanthanide ions in highly acidic environments [38,39]. The detection of Th4+ thus requires sensors exhibiting good anti-interference properties under acidic conditions. Here, the detection abilities of AOH toward Th4+ in the presence of competing metal ions were investigated (Figure 3). AOH shows excellent anti-interference ability toward Th4+ in the coexistence of tested metal ions except VO2+ (10 equiv.). Even though AOH presents distinguishable emission when VO2+ (1 equiv.) coexisted with Th4+ (10 equiv.) (Figure S6). Interestingly, AOH has an extreme resistance ability to f-block elements, especially to uranyl ions. It shows nearly no interference when 10 folds of UO22+ and Th4+ are mixed with 1 equivalent of AOH. In addition, there was no substantial decline of the fluorescence intensity even when the concentration of UO22+ enlarged to 100 folds over that of AOH (Figure 3b), superior to reported probes [40,41,42]. That will provide the possibility for further separation of the two nuclides using AOH.

2.3. Limit of Detection (LOD) on Th4+ Response

The LOD for Th4+ was determined by titration experiments. An increase of the fluorescence emission with a maximum at 455 nm was observed during the gradual addition of Th4+. A linear relationship with the amount of added thorium was seen in the range of 0–5 μM (Figure 4).
AOH has a LOD of 7.19 nM for Th4+ in EtOH/H2O (1/1, v/v, pH = 2.0), calculated by the formula 3α/k (α is the standard deviation of AOH fluorescent intensity at 455 nm, k is the slope value in Figure 4b) [19,41]. Its limit of quantitation (LOQ, calculated by the formula 10α/k) for Th4+ is 24.1 nm. Both the LOD and LOQ are much lower than the World Health Organization (WHO) limits for thorium in drinking water (246 μg/L; 1.06 μM) [43]. Considering the tests were conducted in a benign aqueous solution, it is an improvement for practical Th4+ detection [16,44,45].
Meanwhile, the LOD of AOH to Th4+ with better water tolerance is lower than that of a similar sensor, N′-(2,4-dihydroxybenzylidene)-4-fluorophenyl hydrazide (AF, where the hydroxyl in AOH changed into fluorine in AF), we reported previously [46]. Under the same test condition for Th4+ response, AOH is superior to AF with over 2 folds of fluorescence enhancement (Figure S7). That is probably caused by the hydroxyl group in the structure of AOH which may not only take part in Th4+ detection but also may improve its solubility in water.
The Job’s plot (Figure 5a) shows a 2:1 binding ratio of AOH: Th4+, in accordance with the HRMS result (ESI+, [2AOH + Th4+ + NO3− − O − H+]: experimental m/z 819.1680, calculated m/z 819.1789) (Figure S8). Based on this ratio, a binding constant value of 9.97 × 109 M−2 was determined [41,47].

2.4. Selectivity on VO2+ Response

Compared to other metal ions in the above anti-interference tests shown in Figure 3, only VO2+ caused a decrease in fluorescence when it coexisted with Th4+ and AOH, and even the week fluorescence of AOH itself was quenched by VO2+ alone (Figure S9). Based on these results, the selective response of AOH to VO2+ with the help of Th4+ was tested among 21 metal ions including f-block ones (Figure 6). The results showed that the fluorescence was quenched especially when VO2+ coexisted with AOH and Th4+, whereas other metal ions cannot quench the luminescence of the AOH-Th4+ system. This indicates that AOH-Th4+ can be used as a selective sensor for the targeted detection of VO2+.

2.5. Interfering Ions Effect on VO2+ Response

The AOH-Th4+ system showed specific fluorescence “turn-off” towards VO2+. In anti-jamming effect tests, when other metal ions were mixed with VO2+ respectively, the luminescence was still not recovered (Figure 7). It is worth mentioning that Cu2+ [48,49], which has been reported to interfere with VO2+ detection, shows no obvious interference effect on the detection of VO2+ by AOH-Th4+. The system also shows different response to VO2+ and Fe3+ [50] (Figure S10), which demonstrates that AOH-Th4+ can be used as a new type of VO2+ fluorescence sensor with good anti-interference ability.

2.6. LOD of VO2+

Since VOSO4 is the only salt of VO2+ we could obtain from chemical commercial companies, to prevent the effect of SO42−, 10 folds of BaCl2 which has no effect on the fluorescence intensity of AOH-Th4+ (Figure S11), was added in the process during the titration tests to determine the LOD of the AOH-Th4+ system to VO2+. As shown in Figure 8a, the fluorescence intensity decreases gradually as the concentration of VO2+ increases from 0 to 4 equiv. of AOH. The change of fluorescence intensity at 455 nm is linearly related to the VO2+ concentration in the range of 4 to 24 µM (Figure 8b). The calculated detection limit of AOH-Th4+ to VO2+ is 0.386 µM (31 µg/L), which is much lower than the reference human risk dose of 350 µg/d for vanadium for an adult with a weight of 50 kg (7 µg/kg/d, USEPA. 2021) [51].
The Job’s plot shows a 1:1 binding ratio of AOH-Th4+: VO2+ (Figure S12a). Based on that, the binding constant is calculated to be 4.54 × 104 M−1 (Figure S12b) [19].

2.7. Recognition Mechanism of AOH on Th4+ and VO2+ Response

In order to understand the detection mechanisms, the 1H NMR spectra of AOH before and after Th4+ and VO2+ blending were compared (Figure 9). The results show that there was no change in the proton hydrogen signal (δ 11.71 ppm) of -NH-, either on addition of Th4+ or VO2+, indicating that the nitrogen of the amide group in AOH was not involved in the detection. The addition of Th4+ caused the hydrogen signals (δ 11.60, 10.13, 9.93 ppm) of -OH groups in AOH to move to lower field or damped significantly (Figure 9b). Meanwhile, the hydrogen of -CH=N- shifted from 8.45 ppm to 8.38 ppm with different extent of shifts of the hydrogens in the aromatic rings of the sensor, indicating their involvements in the binding with Th4+. The FT-IR spectra changes of C=O (1602 cm−1 to 1614 cm−1) and OH (1359 cm−1 to 1384 cm−1) demonstrate that the oxygens are involved in the reaction of AOH and Th4+ (Figure S13). The broad or disappearing signals of all the hydrogens in -OH groups of AOH after VO2+ addition (Figure 9c) illustrates the oxygen participation in the complexing reaction. The minor shifts of the residual hydrogens in AOH demonstrate the different combination modes of VO2+ to Th4+ with AOH.
When AOH was mixed with Th4+ and VO2+, the corresponding HRMS spectrum shows the main m/z at 369.0299 (Figure 10a), attributed to the signal of AOH-VO2+ with a 1:1 ratio ([AOH + VO2+ + CH3O− − H+], calculated: m/z 369.0291), indicating that Th4+ is not involved in the binding and that the nuclide ion coordinated to AOH was replaced by VO2+. This speculation was verified by the FT-IR results when AOH was mixed with Th4+ or VO2+ or their mixture (Figure 10b). The signals at 1542 cm−1 and 1456 cm−1 appeared clearly in the mixed sample of AOH with Th4+ and VO2+, which correspond to the characteristic absorption peaks in AOH-VO2+. In addition, the peak at 1335 cm−1 in AOH-Th4+ was shifted to 1329 cm−1 of AOH-VO2+, and the signal at 1308 cm−1 corresponding to the absorption band of AOH-Th4+ disappeared when VO2+ was added in the sample. These changes partly explain the reason for fluorescence quenching of AOH-Th4+ when VO2+ coexisted with it.
Based on the above test analyses, the sensing mechanism of AOH to Th4+ and VO2+ is proposed as shown in Figure 11.
To further understand the response mechanism, the optimized structure of AOH and AOH-metal ion complexes were calculated by density functional theory (DFT) at B3LYP/6-311G (d, p) level using Gaussian 09 software. In this process, the metal atoms used are described using Stuttgart small-core pseudopotentials and basis sets [52]. Subsequently, based on the wave function information, Multiwfn 3.8(dev) software [53,54] was used to calculate the HOMO and LUMO orbitals. Their frontier molecular orbitals are shown in Figure 12.
In the free ligand, the HOMO is located at the imine groups and dihydroxybenzene ring respectively, while the LUMO is distributed between the imine groups and dihydroxybenzene ring, indicating the intramolecular charge transfer (ICT) effect in AOH. After its combination with Th4+, the LUMO is moved to the hydrazone bond with the benzamide ring that belongs to one of the two AOHs, suggesting the block of ICT process and the enhancement of fluorescence [55]. The calculated HOMO-LUMO energy gap in AOH-Th4+ (1.76 eV) is much lower than that in free ligand (4.21 eV), indicating that AOH is reactive to Th4+. On the other hand, although the energy gap in AOH-UO22+ (3.31 eV) is a bit lower than that of the free ligand, it is still much higher than 1.76 eV in AOH-Th4+, demonstrating reaction precedence of AOH with Th4+ over UO22+. That explains the excellent anti-uranyl ability of AOH in Th4+ detection when it coexists with UO22+. The energy gap in AOH-VO2+ (1.63 eV) is even a bit lower than that in AOH-Th4+, indicating that AOH will react with VO2+ if its amount is over than that of Th4+, which is consistent with the experimental results. The HOMO of AOH-Th4+ is located on the nitrate whereas it is located on VO2+ of AOH-VO2+, which in part explains the experimentally observed phenomena of fluorescence “turn-on” in the former and “turn-off” in the latter.

2.8. Detection Comparison to Reported Sensors on Th4+ or VO2+

As we have not found other probes that work on Th4+ and VO2+ detection, here, we list in Table 1 some related chemical sensors reported on Th4+ or VO2+ for comparison. Considering various factors such as testing media, binding constant, detection limit, and anti-interference abilities, AOH, with its easy accessibility, can be a suitable candidate for Th4+ and VO2+ analysis.

2.9. Th4+ and VO2+ Determination in Real Water

The detection applications of AOH towards Th4+ and VO2+ in real water samples were tested and the results are listed in Table 2 and Table 3. Either in tap water or in pool water, Th4+ and VO2+ had good recovery with less than 1.5% mean relative standard deviation (RSD), indicating good real water application prospects of AOH in Th4+ and VO2+ detection. Anion influence tests showed negligible effect on Th4+ and VO2+ detection (Figure S14), supporting its application ability in real conditions.

3. Materials and Methods

3.1. Materials and Reagents

All analytical grade reagents were purchased from the Aladdin-Reagent or Tansoole company (Shanghai, China) and used directly without further purification. The corresponding metal ion stock solutions were prepared by dissolving LaCl3, CeCl3, Pr(NO3)3, Nd(NO3)3, LuCl3, TbCl3, EuCl3, Er(NO3)3, Yb(NO3)3, Tm(NO3)3, Gd(NO3)3, Sm(NO3)3, Dy(NO3)3, Ho(NO3)3, UO2(NO3)2, VOSO4, CuCl2, BaCl2, and FeCl3, respectively in ultrapure water. Thorium standard solution was purchased from Weiye Measurement Co. (Xinyang city, Henan, China) at a concentration of 1000 μg/mL (5% of HNO3 in volume). NaF, NaCl, KNO3, K2SO4, and KBr were used for anions effect tests. The desired pH solutions were obtained by adding dilute HNO3 or NaOH to pure water. The water was deionized to a specific resistivity of 18.2 MΩ cm using the ULUPURE Water Purification System.
CAUTION: Uranyl and thorium ions have a certain level of radioactivity. Appropriate protective equipment should be worn during operation, such as laboratory work clothes, gloves, and goggles, to reduce the risk of direct contact and inhalation.

3.2. Synthesis of AOH

AOH (Scheme 1) was synthesized based on modified processes of our former work [58]. Typically, 1 mmol (0.156 g) of 4-Hydroxybenzhydrazide was dissolved in 15 mL of ethanol. Then 1 mmol (0.140 g) of 2,4-dihydroxybenzaldehyde in 10 mL of ethanol was added into the above solution. The reaction mixture was stirred at 80 °C for 6 h and the reaction process was monitored by TLC. After the reaction, a large amount of white solid precipitated from the solution. It was collected by filtration and purified by re-crystallization from 40 mL of hot ethanol to obtain the target product: white solid, 0.202 g, yield: 74%. The synthesis route is shown in Scheme 1. 1H NMR, UV-vis, HRMS, and IR spectra (Figures S1 and S2) verified its excellent purity and its structure [34].
1H NMR of AOH (600 MHz, DMSO-d6, Figure S1): δ 11.71 (s, 1H, -NH), δ 11.60 (s, 1H, -OH), δ 10.13 (s, 1H, -OH), δ 9.93 (s, 1H, -OH), δ 8.46 (s, 1H, -CH=N), δ 7.81 (d, 2H, ArH), δ 7.27 (d, 1H, ArH), δ 6.86 (d, 2H, ArH), δ 6.35 (q, 1H, ArH), δ 6.31 (d, 1H, ArH).
IR of AOH (ν, cm−1, Figure S2b): 3345 (-OH), 3123 (N-H), 1602 (C=O), 1542 (C=N).

3.3. Apparatus

1H NMR measurements were performed on a Bruker Avance III 600 MHz system (Billerica, MA, US), using DMSO-d6 as the solvent with TMS as the internal standard. The pH adjustments were performed by a Thermo Fisher Scientific (model XL200, Shanghai, China) digital pH meter equipped with a combination glass electrode. Fluorescence spectra were recorded on a F-4600 spectrophotometer (Tokyo, Japan) with a quartz cuvette (path length = 1 cm) with an excitation of 374 nm. The excitation and emission bandwidths were both 5.0 nm unless otherwise noted. FT-IR spectra of the samples (KBr disk) were recorded on a Bruker (Billerica, MA, USA) FT-IR spectrometer in the range of 4000~400 cm−1. ESI-HRMS spectra were recorded on an Agilent 6224 (Waldbronn, Germany) TOF MS spectrometer or Waters (Milford, MA, USA) HClass MS spectrometer.

3.4. Solution Preparation and Testing Process

AOH was dissolved in ethanol to obtain a stock probe solution (1 mM). The corresponding metal salts were dissolved in pure water to obtain the corresponding metal ion stock solutions (10 mM). The standard solution of thorium nitrate was diluted with pure water to obtain a stock solution of Th4+ (4 mM), and a stock solution of UO22+ (10 mM) was prepared by dissolving uranyl nitrate hexahydrate in pure water. All of the stock solutions were used within one month.
Analysis procedures: The selectivity and anti-interference abilities to metal ions were tested in the media of EtOH/H2O (1/1, v/v, pH = 2.0). The probe stock solution (1 mM) was mixed with metal ion salt (10 equiv.) solution and diluted with ethanol and water to 5 μM. In anti-interference tests, Th4+ ions (10 equiv.) and interfering metal ions (10 equiv.) were added to AOH (1 mM) and diluted to 5 μM of AOH (EtOH/H2O, 1/1, v/v, pH = 2.0). In evaluating the anti-interference performance of VO2+, the probe (5 μM), Th4+ ions (10 equiv.), interfering ions (50 equiv.), and VO2+ (50 equiv.) were mixed and then diluted to the target concentrations for tests. The samples were placed at room temperature for 30 min before fluorescence testing.
In real water tests, the lake water was taken from the lake of Southwest University of Science and Technology, Mianyang, China. The lake water sample was filtered by a 0.45 μm membrane before use. Tap water in the lab was used directly. The water pH values were adjusted to 2.0 with HCl and mixed with ethanol in an equal volume.

4. Conclusions

Acylhydrazone, AOH, a small drug molecule, was first developed for sequential Th4+ and VO2+ monitoring in an acidic aqueous solution (EtOH/H2O, 1:1, v/v, pH = 2.0) without changing the medium conditions. Its specific luminescence enhancement to Th4+ and subsequent fluorescence quenching to VO2+ (more notable at the coexistence of Th4+), demonstrate its value as a chemosensor towards the two ions responses. In the presence of disturbing lanthanide ions and the most competing uranyl ions, AOH has excellent anti-interference abilities during Th4+ and VO2+ detection. The presented results provide the possibility of the nuclide ions detection and future separation in a simple way.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ijms26073231/s1.

Author Contributions

Conceptualization, H.L., K.D. and Q.T.; Data curation, X.L., H.L., J.Z. and Z.G.; Formal analysis, X.L. and J.Z.; Investigation, X.L., H.L., J.Z., Y.X., Z.G. and L.A.; Methodology, H.L., K.D. and Q.T.; Resources, H.L.; Software, Y.X. and Z.G.; Supervision, H.L. and L.A.; Validation, X.L. and J.Z.; Visualization, L.A.; Writing—original draft, X.L., H.L., K.D. and L.A.; Writing—review & editing, H.L., Q.T. and L.A. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Sichuan Science and Technology Program for Youth innovation (No. 2022JDTD0017) and the award to Hua Liang from the China Scholarship Council (No. 202109390004).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Experimental data are available from the authors upon reasonable request.

Acknowledgments

We appreciate Guy Royal and Eder Tomás-Mendivil from Département de Chimie Moléculaire, Université Grenoble Alpes for their kind suggestions on our work.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
AOHAcylhydrazone, N’-(2,4-dihydroxybenzylidene)-4-hydroxylphenylhydrazide
ICTIntramolecular charge transfer
LODLimit of detection

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Ijms 26 03231 g001
Figure 1. Images of AOH before and after the addition of metal ions under 365 nm UV light in (a) EtOH and in (b) EtOH/H2O (1/1, v/v, pH = 2.0).
Figure 1. Images of AOH before and after the addition of metal ions under 365 nm UV light in (a) EtOH and in (b) EtOH/H2O (1/1, v/v, pH = 2.0).
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Figure 2. Fluorescence spectra of AOH (5 μM) before and after metal ions (50 μM) addition in EtOH/H2O (1/1, v/v, pH = 2.0).
Figure 2. Fluorescence spectra of AOH (5 μM) before and after metal ions (50 μM) addition in EtOH/H2O (1/1, v/v, pH = 2.0).
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Figure 3. Fluorescence spectra of AOH (5 μM) at the coexistence of Th4+ and other metal ions in EtOH/H2O (1/1, v/v, pH = 2.0). (a) Th4+ (10 equiv., 50 μM) coexisted with other metal ions (10 equiv., 50 μM). (b) Effect of coexisted UO22+ at different concentrations (10 equiv., 50 μM, and 100 equiv., 500 μM) with Th4+ (10 equiv., 50 μM).
Figure 3. Fluorescence spectra of AOH (5 μM) at the coexistence of Th4+ and other metal ions in EtOH/H2O (1/1, v/v, pH = 2.0). (a) Th4+ (10 equiv., 50 μM) coexisted with other metal ions (10 equiv., 50 μM). (b) Effect of coexisted UO22+ at different concentrations (10 equiv., 50 μM, and 100 equiv., 500 μM) with Th4+ (10 equiv., 50 μM).
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Figure 4. (a) Fluorescence spectra of AOH (10 μM) with gradual addition of Th4+ (0–10 μM) in EtOH/H2O (1/1, v/v, pH = 2.0). Inset is the corresponding fluorescence intensity at 455 nm with the concentration change of Th4+. (b) Calibration curve of the titration experiments.
Figure 4. (a) Fluorescence spectra of AOH (10 μM) with gradual addition of Th4+ (0–10 μM) in EtOH/H2O (1/1, v/v, pH = 2.0). Inset is the corresponding fluorescence intensity at 455 nm with the concentration change of Th4+. (b) Calibration curve of the titration experiments.
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Figure 5. (a) The Job’s plot of AOH–Th4+. (b) Binding constant (Ka) determination of AOH toward Th4+.
Figure 5. (a) The Job’s plot of AOH–Th4+. (b) Binding constant (Ka) determination of AOH toward Th4+.
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Figure 6. (a) Fluorescence spectra of AOH (5 μM) at the coexistence of Th4+ (10 equiv., 50 μM) and other metal ions (100 equiv., 500 μM) in EtOH/H2O (1/1, v/v, pH = 2.0). (b) The corresponding fluorescence response at 455 nm.
Figure 6. (a) Fluorescence spectra of AOH (5 μM) at the coexistence of Th4+ (10 equiv., 50 μM) and other metal ions (100 equiv., 500 μM) in EtOH/H2O (1/1, v/v, pH = 2.0). (b) The corresponding fluorescence response at 455 nm.
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Figure 7. (a) Fluorescence spectra of AOH (5 μM)-Th4+ (10 equiv., 50 μM) in EtOH/H2O (1/1, v/v, pH = 2.0) coexists with VO2+ (50 equiv., 250 μM) and other metal ions (50 equiv., 250 μM). (b) Corresponding fluorescence intensities at 455 nm.
Figure 7. (a) Fluorescence spectra of AOH (5 μM)-Th4+ (10 equiv., 50 μM) in EtOH/H2O (1/1, v/v, pH = 2.0) coexists with VO2+ (50 equiv., 250 μM) and other metal ions (50 equiv., 250 μM). (b) Corresponding fluorescence intensities at 455 nm.
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Figure 8. (a) Fluorescence spectra AOH (10 μM) -Th4+ (2 equiv., 20 μM) with gradual addition of VO2+ into EtOH/H2O (1/1, v/v, pH = 2.0). Inset is a scatter plot of the corresponding fluorescence intensity at 455 nm with the concentration increase of VO2+. (b) Linear relationship between VO2+ concentration and fluorescence intensity.
Figure 8. (a) Fluorescence spectra AOH (10 μM) -Th4+ (2 equiv., 20 μM) with gradual addition of VO2+ into EtOH/H2O (1/1, v/v, pH = 2.0). Inset is a scatter plot of the corresponding fluorescence intensity at 455 nm with the concentration increase of VO2+. (b) Linear relationship between VO2+ concentration and fluorescence intensity.
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Figure 9. 1H NMR spectra of AOH before (a) and after (b) Th4+ and (c) VO2+ addition. The asterisks stand for the solvent signals.
Figure 9. 1H NMR spectra of AOH before (a) and after (b) Th4+ and (c) VO2+ addition. The asterisks stand for the solvent signals.
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Figure 10. (a) HRMS spectrum of AOH mixed with Th4+ and VO2+. Insets are the full FT-IR spectra of AOH before and after mixing with Th4+ and VO2+ respectively. (b) The enlarged FT-IR spectra of AOH before and after it was mixed with Th4+ and VO2+ at 1800–850 cm−1.
Figure 10. (a) HRMS spectrum of AOH mixed with Th4+ and VO2+. Insets are the full FT-IR spectra of AOH before and after mixing with Th4+ and VO2+ respectively. (b) The enlarged FT-IR spectra of AOH before and after it was mixed with Th4+ and VO2+ at 1800–850 cm−1.
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Figure 11. Proposed complexing mechanism of AOH with Th4+ and VO2+.
Figure 11. Proposed complexing mechanism of AOH with Th4+ and VO2+.
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Figure 12. The frontier molecular orbitals of AOH and AOH-Th4+, AOH-VO2+ and AOH-UO22+ complexes.
Figure 12. The frontier molecular orbitals of AOH and AOH-Th4+, AOH-VO2+ and AOH-UO22+ complexes.
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Scheme 1. The synthesis route of AOH.
Scheme 1. The synthesis route of AOH.
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Table 1. Fluorescent recognition comparison of AOH towards Th4+ or VO2+ with some reported sensors in aqueous solutions.
Table 1. Fluorescent recognition comparison of AOH towards Th4+ or VO2+ with some reported sensors in aqueous solutions.
Sensor
(Ref.)		Detecting ObjectMedia (v/v)KaLODInterfering Ions
[19]Th4+MeOH/H2O (7:3)2.03 × 104 M−120.0 nMUO22+, Cu2+
[41]Th4+MeOH/H2O (7:3)2.46 × 104 M−134.4 nMUO22+
[56]Th4+CH3CN/H2O (9:1)5.67 × 103 M−12.10 nMFe3+
[57]Th4+MeOH/H2O (1:1)NR0.60 μMCu2+, Fe3+, UO22+, Lu3+
[46]Th4+EtOH/H2O
(7:3, pH 2.0)		6.64 × 109 M−229.2 nMNO
[48]VO2+DMSO-Tris-HCl
(7:3, pH 7.4)		NR3.65 nMCu2+
[50]VO2+H2O4.40 × 103 M−10.34 μMFe3+
This workTh4+,
VO2+		EtOH/H2O
(1:1, pH 2.0)		9.97 × 109 M−2
4.54 × 104 M−1		7.19 nM
0.386 μM		NO
NO
NR: not reported; NO: No interference.
Table 2. Determination of Th4+ by AOH (20 μM) in different water samples.
Table 2. Determination of Th4+ by AOH (20 μM) in different water samples.
SampleTh4+ Added (μM)Th4+ Found (μM)Recovery (%)RSD (%)
Tap water25.025.0100.00.9
25.025.3101.2
25.024.899.3
Pool water25.024.999.50.9
25.025.3101.3
25.025.1100.3
Table 3. Determination of VO2+ by AOH (20 μM) -Th4+ (20 μM) in different water samples.
Table 3. Determination of VO2+ by AOH (20 μM) -Th4+ (20 μM) in different water samples.
SampleTh4+ Added (μM)Th4+ Found (μM)Recovery (%)RSD (%)
Tap water25.024.999.80.6
25.025.1100.3
25.025.3101.0
Pool water25.025.1100.41.2
25.025.7102.7
25.025.3101.0
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Lin, X.; Liang, H.; Dai, K.; Zhou, J.; Tian, Q.; Xiang, Y.; Guo, Z.; Almásy, L. An Acylhydrazone Fluorescent Sensor: Bifunctional Detection of Thorium (IV) and Vanadyl Ions over Uranyl and Lanthanide Ions. Int. J. Mol. Sci. 2025, 26, 3231. https://doi.org/10.3390/ijms26073231

AMA Style

Lin X, Liang H, Dai K, Zhou J, Tian Q, Xiang Y, Guo Z, Almásy L. An Acylhydrazone Fluorescent Sensor: Bifunctional Detection of Thorium (IV) and Vanadyl Ions over Uranyl and Lanthanide Ions. International Journal of Molecular Sciences. 2025; 26(7):3231. https://doi.org/10.3390/ijms26073231

Chicago/Turabian Style

Lin, Xin, Hua Liang, Ke Dai, Jing Zhou, Qiang Tian, Yuge Xiang, Zhicheng Guo, and László Almásy. 2025. "An Acylhydrazone Fluorescent Sensor: Bifunctional Detection of Thorium (IV) and Vanadyl Ions over Uranyl and Lanthanide Ions" International Journal of Molecular Sciences 26, no. 7: 3231. https://doi.org/10.3390/ijms26073231

APA Style

Lin, X., Liang, H., Dai, K., Zhou, J., Tian, Q., Xiang, Y., Guo, Z., & Almásy, L. (2025). An Acylhydrazone Fluorescent Sensor: Bifunctional Detection of Thorium (IV) and Vanadyl Ions over Uranyl and Lanthanide Ions. International Journal of Molecular Sciences, 26(7), 3231. https://doi.org/10.3390/ijms26073231

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