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RSC Adv
RSC Adv
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RSCACL
RSC Advances
2046-2069
The Royal Society of Chemistry

d4ra01445f
10.1039/d4ra01445f
Chemistry
An AIE probe for the fluorescence and colorimetric sensing of La3+†
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra01445f

Lin Wan-Ying de‡
https://orcid.org/0009-0003-9362-0793
Zong Zi-Ao ac‡
Wang Jing c
Huang Qi f
Xing Zhi-Yong c
Fan Chuan-Bin a
Li Na-Na b
a Modern Industrial College of Biomedicine and Great Health, Youjiang Medical University for Nationalities Baise China zongziao@126.com

b Department of Chemistry, Xinzhou Normal University Xinzhou China LMT13643562571@126.com

c Key Laboratory of Research on Environment and Population Health in Aluminium Mining Areas (Youjiang Medical University for Nationalities), Education Department of Guangxi Zhuang Autonomous Region Baise China
d Affiliated Hospital of Youjiang Medical University for Nationalities Baise China
e Guangxi Key Laboratory for Biomedical Material Research Baise China
f Baise Center for Disease Control and Prevention Baise China
‡ These authors contributed equally to this work.

6 9 2024
4 9 2024
6 9 2024
14 39 2846928474
25 2 2024
24 8 2024
This journal is © The Royal Society of Chemistry
2024
The Royal Society of Chemistry
https://creativecommons.org/licenses/by-nc/3.0/ This article is licensed under a Creative Commons Attribution-Non Commercial 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given and it is not used for commercial purposes.
In this work, a fluorescent probe N with aggregation-induced emission effect was synthesized by grafting naphtho[2,3-c]furan-1,3-dione and 2-hydrazinylbenzo[d]thiazole. The probe N could recognize La3+ selectively and sensitively accompanied with an obvious fluorescence and color change from green to blue. Moreover, with the help of AIE properties, probe N achieved the detection of La3+ in the solid state.

A colorimetric fluorescent sensor named N, which exhibited solid state emission, was developed for the fluorescence colorimetric sensing of La3+ in water and in the solid state with high selectivity and sensitivity.

Xinzhou Teachers University 10.13039/501100013809 00001036 pubstatusPaginated Article
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pmc1. Introduction

La3+, a transition metal ion, is useful for radiotherapeutic applications, catalysis, and the synthesis of nanosized materials.1–3 Additionally, La3+ binds strongly to the Ca2+ binding sites in proteins owing to their size equivalence.4–6 Due to the increased utility of lanthanum compounds in the industry, their discharge into the environment poses a greater threat to life.7–10 Therefore, the accurate determination of La3+ is of great significance for human health and environmental monitoring. In recent years, fluorescent probes have been developed as a useful tool for analyte detection because of their low cost, simple operation and potential applications in environmental and biological systems.11–15 Thus, the development of fluorescent probes for sensing La3+ is necessary.

Huang's team synthesized a calix[6]arene-based fluorescent probe for fluorescence enhancement detection of La3+ ions in solution.16 Areti et al. developed a glucopyranosyl conjugate as a fluorescent probe for the fluorescence turn on detection of La3+ in solution and living cells.17

Yang's team reported a coumarin-based fluorescent probe for the fluorescence turn-on detection of La3+ in solution and living cells.18 However, these reported fluorescent probes displayed “turn on” response to La3+, which is not obvious for La3+ detection by the naked eye. To date, AIE-active fluorescent probes have received considerable attention because they can emit strongly in the aggregated or solid state, detecting analytes in the aggregated states via a fluorescence colorimetric method.19–25 Therefore, the development of AIE probes for La3+ is urgent.

In this work, we reported an AIE probe N, which displayed fluorescence colorimetric response towards La3+. Additionally, the sensing mechanism of probe N for La3+ was proved using FT-IR, NMR and HRMS and further demonstrated through DFT calculations. Furthermore, probe N exhibited intense yellow–green emission in the solid state and could detect La3+ in the solid state, resulting in obvious fluorescence and color change. In addition, the comparison of the common methods and probe molecules of probe N with the previously reported La3+ probes is shown in Table S1.†

2. Experimental

2.1. General comments

1H NMR spectra were recorded on a Bruker AV-600 spectrometer in d6-DMSO. IR spectra were recorded with a PerkinElmer IR spectrophotometer. A Pgeneral TU-2550 UV-vis spectrophotometer and a PerkinElmer LS55 fluorescence spectrometer were employed to measure absorption and fluorescence spectra, respectively. Mass spectra were acquired on an Agilent 6220 Quadruple LC/MS (Agilent Co, USA). X-ray diffraction data were collected on a Bruker Smart CCD X-ray single-crystal diffractometer. TEM studies were carried out using a Maerwen zs90 scanning electron microscope at 100 kV. DLS data were obtained using a JEM-2100F nanometer laser particle analyzer.

All the materials for synthesis and spectral analysis are commercially available and used without further purification. The stock solutions (10 mM) of various metal ions were prepared from AgNO3, KCl, MgCl2·6H2O, Ba(ClO4)2, Zn(NO3)2·6H2O, Cu(NO3)2·3H2O, Cd(NO3)2·4H2O, Pb(NO3)CoSO4·7H2O, NiCl2·6H2O, CaCl2, Al(NO3)3·9H2O, MnCl2·4H2O, HgCl2, Na2SO4, FeCl2·4H2O, FeCl3·6H2O, LiNO3, La(NO3)3·6H2O, SrCl2·6H2O, and Bi(NO3)3·5H2O. The pH of L solution was adjusted with HCl and NaOH aqueous solutions. For fluorescence measurements, the excitation wavelength was 380 nm and the excitation and emission wavelength band passes were both set at 5 nm.

2.2. Synthesis

As shown in Scheme 1, naphtho[2,3-c]furan-1,3-dione (198 mg, 1 mmol) and 2-hydrazinylbenzo[d]thiazole (165 mg, 1 mmol) were dissolved in 10 mL glacial acetic acid solution and stirred for 5 h.26 After the completion of the reaction, the solvent was cooled to room temperature and filtered to obtain a pale green compound (169 mg, yield 49%). 1H NMR (Fig. S1†) (600 MHz, DMSO) δ 11.07 (s, 1H), 8.63 (s, 2H), 8.29 (t, 2H), 7.82 (m, 3H), 7.43 (m, 1H), 7.30 (m, 1H), 7.16 (m, 1H). 13C NMR (Fig. S2†) (151 MHz, DMSO-d6) δ (ppm) 191.21, 179.17, 171.63, 162.30, 160.82, 133.05, 131.69, 130.72, 130.25, 127.87, 127.39, 126.69, 124.64, 124.11, 116.54, 112.49, 106.47, 60.97, 55.87. ESI-MS (Fig. S3†): calcd for C22H26N3OS: 346.06502 [N + H]+, found: 346.06381.

Scheme 1 Synthesis route of probe N.

3. Results and discussion

3.1. The optical properties of AIE

As shown in Fig. 1a, probe N displayed obvious green fluorescence with emission peaks at 515 nm and 467 nm. The fluorescence spectra of probe N (10−5 M) in EtOH/H2O mixture with different water fractions (fw) were illustrated in Fig. 1b for studying the AIE properties. The probe N in pure ethanol solution displayed a maximum emission near 475 nm and blue fluorescence was observed. The fluorescence intensity at 475 nm starts to decrease after water fraction higher than 10% and a new shoulder peak appeared at 545 nm. When fw = 90%, the emission peak at 545 nm reached the maximum. In addition, probe N showed average particle diameter with 255 nm in EtOH and 295 nm in 90% water, respectively (Fig. S4†). The result suggested that water molecule induced the aggregation of probe N molecules. The above results indicated that the probe N possessed AIE activity.

Fig. 1 (a) The emission spectra of probe N in the solid state. (b) Fluorescence spectra of probe N (10 μM) in EtOH/H2O with different water fractions.

3.2. Fluorescence spectra studies of probe N

The sensing ability of probe N towards various analytes (Ag+, K+, Na+, Li+, Pb2+, Mg2+, Ca2+, Zn2+, Cd2+, Mn2+, Ba2+, Hg2+, Co2+, Ni2+, Sr2+, Al3+, Fe3+, Cr3+, Bi3+, La3+ and Cu2+) was examined in the EtOH–H2O (v/v = 1/9, pH 7.4) system (Fig. 2a). The emission peak at 545 nm of probe N was blue-shifted to the 460 nm along with obvious fluorescence enhancement and the fluorescence and color changed from green to blue under 365 nm UV lamp after the addition of the La3+ ion. However, upon the addition of the above other metal ions, the fluorescence intensity of probe N displayed insignificant changes. Therefore, the selectivity of probe N towards La3+ was supported by fluorescence spectra or naked eyes.

Fig. 2 (a) Fluorescence spectra of probe N (10 μM) in the presence of various metal ions (50 μM) in EtOH/H2O (9/1, v/v, pH = 7.4) medium. Inset: The color change of probe N (10 μM) before and after the addition of La3+ (50 μM) in the EtOH/H2O (1/9, v/v, pH = 7.4) medium under 365 nm UV light. (b) The fluorescence spectra of probe N (10 μM) upon the addition of different concentrations of La3+ in the EtOH/H2O (1/9, v/v, pH = 7.4) medium.

The competition experiments were carried out in the presence of the above metal ions by measuring the fluorescence intensity at 475 nm under the same condition (Fig. S5†). The probe N (10 μM) was treated with La3+ (50 μM) in the presence of the above metal ions (50 μM) with the same concentration, respectively. N–La3+ still displayed the obvious emission in the presence of other metal ions except for Co2+ and Cu2+, indicating that probe N showed high sensitivity to La3+.

In addition, the fluorescence titrations for La3+ were carried out in EtOH–H2O (v/v = 1/9, pH 7.4) solution. In Fig. 2b, as the concentration of La3+ increased from 10 μM to 10 μM, the fluorescence intensity exhibited a gradual increase and the maximum emission wavelength was blue shifted to 475 nm. Also, the good linear fit range of probe N to La3+ (0–30 μM) was observed. The limit of detection (LOD) of probe N to La3+ was calculated to be 3.6 × 10−7 M based on the 3σ/k criterion (where σ is the standard deviation of the blank measurements, and k is the slope of the intensity ratio versus sample concentration plot). Therefore, the above results indicated that probe N could quantitatively detect La3+.

3.3. Sensing mechanism of probe N for La3+

To explore the sensing mechanism of probe N with La3+, the 1H NMR (Fig. S6†), FT-IR (Fig. S7†) and HRMS spectra (Fig. S8†) of probe N and N + La3+ were measured. In the 1H NMR spectra (Fig. S4†), after the addition of 1 equiv. La3+, the OH proton (Ha) at 11.1 ppm disappeared, indicating the occurrence of the deprotonation process in the presence of La3+ (Scheme 2). Also, the characteristic –OH absorption peak at 3450 cm−1 and the characteristic –C <svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="13.200000pt" height="16.000000pt" viewBox="0 0 13.200000 16.000000" preserveAspectRatio="xMidYMid meet"><metadata> Created by potrace 1.16, written by Peter Selinger 2001-2019 </metadata><g transform="translate(1.000000,15.000000) scale(0.017500,-0.017500)" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg> N absorption peak at 1658 cm−1 disappeared, indicating that the N atom in the –CN group and the O atom in the –CN group might interact with La3+. Further, the HRMS of N + La3+ were measured in the EtOH/H2O (1/9, V/V, pH = 7.4) solution. We found m/z = 544.93903, which could be assigned to [N − H + La3+ + NO3−]+ (m/z, Cal, 544.94354), suggesting that a certain complex was formed between probe N and La3+. Therefore, according to the above analysis, a possible sensing mechanism of probe N for La3+ is proposed in Scheme 2. The stoichiometry of probe N with La3+ was further confirmed by Job's plot analysis (Fig. S7†). The total concentration of probe N and La3+ was kept constant at 10 μM during this experimental process, and the variable molar fraction of La3+ was varied from 0.1 to 0.9. In the Job's plot (Fig. S9†), the fluorescence intensity at 486 nm for N + La3+ showed peaks when the molecular fraction of La3+ was close to 0.5, further suggesting the 1 : 1 binding stoichiometry between probe N and La3+. Based on the above results, the possible sensing mechanisms of probe N with La3+ are proposed in Scheme 2.

Scheme 2 Plausible sensing mechanism of probe N for La3±.

In addition, density functional theory (DFT) calculation was performed to further understand the sensing mechanism of probe N to La3+. First, the structures of probe N and N + La3+ were optimized, and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are shown in Fig. 3. For probe N, the electron cloud distribution in the LUMO (−2.08 eV) was mainly from the naphthalene ring, while the electron cloud distribution in the HOMO (−6.22 eV) was determined by the entire molecular skeleton. For N + La3+, the electron cloud distribution in the LUMO was spread on the partial molecular skeleton, while the electron cloud distribution in the HOMO was mainly localized over the whole conjugated skeleton. The HOMO–LUMO energy gap for probe N and N + La3+ was found to be 4.14 eV and 4.19 eV, respectively.

Fig. 3 The calculated molecular orbitals and the HOMO–LUMO gaps of probe N and N + La3+.

3.4. Application in the solid state

We investigated the solid-state sensing behaviour because of the AIE effect of probe N. We added La3+ to probe N, gently mixed it and recorded the fluorescence spectrum (Fig. 4). After the interaction of probe N with La3+, the emission peak from 530 nm blue shifted to 476 nm, and the obvious fluorescence change from yellow–green to blue emission was observed. These results indicated that probe N could be used to colorimetrically detect La3+ by naked eye in the solid state.

Fig. 4 Fluorescence spectral changes of probe N in the absence and presence of La3+ in the solid state. Inset: the color change in probe N in the absence and presence of La3+ in the solid state under 365 nm UV light.

4. Conclusion

In conclusion, we developed a fluorescent probe N using a one-step reaction and it displayed AIE characteristics. Probe N could serve as a fluorescence colorimetric probe for La3+. Furthermore, probe N exhibited fluorescence and color change from yellow to green for La3+ in the solid state.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Author contributions

Conceptualization, Z. A. Z.; methodology, Z. A. Z. and W. Y. L.; formal analysis, C. B. F.; data curation, Q. H. and J. W.; writing—original draft preparation, Z. Y. X. and N. N. Li.; writing—review and editing, C. B. F. and N. N. Li.

Conflicts of interest

The authors declare no conflict of interest.

Supplementary Material

RA-014-D4RA01445F-s001

This work was supported by Scientific and Technological Inno-vation Programs of Higher Education Institutions in Shanxi (No. 2022L465), Fundamental Research Program of Shanxi Province (No. 202203021222307), Xinzhou Teachers University project fund (No. 00001036), the Scientific Research and Technology Development Program Project of Baise City (No. 20222001), the Science and Technology Baseand the Talent Special Project for Guangxi Province (No. AD22035154 and AD20297056).
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