English

• Aluminum and its main group congeners gallium and indium have found widespread applications in modern industries. However, the extensive utilization of these elements has led to their progressive environmental accumulation and bioenrichment, posing potential risks to human health through long-term exposure. Compelling evidence indicates that excessive intake of Al³⁺ or prolonged contact with Ga³⁺/In³⁺ may induce detrimental effects on multiple physiological systems, including the central nervous system, digestive tract, and immune function. These findings underscore the critical need for developing convenient detection strategies that target these metal ions^[1]. Although conventional analytical techniques, such as inductively coupled plasma (ICP) spectroscopy and atomic absorption spectrometry, offer reliable quantification, their practical implementation is often hindered by complex instrumentation and time-consuming procedures. In contrast, luminescent detection methods have emerged as promising alternatives due to their rapid response, simplified sample pretreatment, and superior sensitivity^[2].

  Metal-organic frameworks (MOFs), a class of crystalline materials constructed from metal nodes and organic linkers, have recently garnered considerable interest as versatile luminescent probes^[3-4]. Their unique advantages, including tunable energy levels, structural modularity, and analyte pre-concentration capabilities, enable tailored design for specific sensing applications^[4-5]. Despite numerous reports on MOF-based sensors for diverse analytes^[5-6], a significant limitation persists: many MOF probes exhibit poor aqueous stability, severely restricting their utility in environmental water monitoring^[7]. In addition, this type of probe is currently also used for the detection of Al³⁺, Ga³⁺, and In³⁺, but basically one probe can only detect one or two of these metal ions^[8-9]. The detection of these three metal ions using the same MOF probe can greatly save the preparation cost of the probe. However, there are very few related reports, and they all need to work in an organic solvent-containing system^[10-11]. These challenges highlight the urgent demand for developing water-stable MOF probes with broad-spectrum detection capability for Al³⁺, Ga³⁺, and In³⁺ ions.

  To address these limitations, we present a Cd-MOF probe, [Cd(L)(H₂O)₃]·H₂O (1, H₂L=5-[(naphthalen-1‐methylene)‐amino]‐isophthalic acid), engineered with excellent aqueous stability and selective responsiveness toward Al³⁺, Ga³⁺, and In³⁺ ions. Compound 1 demonstrated remarkable luminescent "turn-on" behavior upon exposure to these trivalent cations, achieving detection limits of 2.31 μmol·L^-1 (Al³⁺), 3.06 μmol·L^-1 (Ga³⁺), and 2.78 μmol·L^-1 (In³⁺), respectively. Mechanistic investigations revealed that the enhanced emission originates from structural transformation triggered by metal coordination.

  1.   Experimental

  1.1   Materials and methods

  All chemicals used in this study were commercially available reagents of analytical grade and were utilized as received. Luminescence spectra were obtained using a Hitachi F-7000 FL spectrophotometer. Powder X-ray diffraction (PXRD) patterns were collected on a D/MAX-2400 X-ray diffractometer employing Cu Kα radiation (λ=0.154 060 nm) at a scan rate of 10 (°)·min^-1 (voltage: 40 kV, current: 25 mA, scan range: 5°-50°). Infrared spectra were recorded in a range of 650-4 000 cm^-1 using a Nicolet-iS50 spectrometer via the KBr pellet pressing method. The synthesis of compound 1 was carried out according to the established literature method^[12].

  1.2   Luminescence detection of metal ions in aqueous solution

  Each luminescence detection experiment was repeated at least three times to ensure the reliability and accuracy of the experimental data. Each time, 1.0 mg finely-ground powders of compound 1 were dispersed in 2 mL deionized water, ultrasonicated for 15 min to form a stable emulsion for the luminescent studies. Then 0.1 mol·L^-1 aqueous solutions of M³⁺ (M=Al, Ga, and In) were prepared for the detection experiments. After adding the aqueous solution of metal ions to the obtained suspension to a specific concentration, luminescence detection was carried out.

  2.   Results and discussion

  2.1   Synthesis and characterization of compound 1

  Compound 1 was synthesized following a reported method^[10]. As illustrated in Fig. 1a, the compound adopts a 1D chain structure. Adjacent chains assemble into a 3D supramolecular structure via π‐π stacking and hydrogen‐bonding interactions (Fig. 1b). The phase purity of the synthesized sample was confirmed by PXRD. Fig. 1c demonstrates that the experimental PXRD pattern closely matched the simulated pattern derived from single-crystal data^[12], confirming the successful synthesis and high purity of 1.

  Figure 1

  

  Figure 1.  Structure and characterization of compound 1: (a) 1D chain structure along the a-axis; (b) packing of the chains viewed along the a-axis (π-π stacking and hydrogen-bonding interactions are presented as green and blue dotted lines, respectively); (c) experimental and simulated PXRD patterns; (d) normalized solid-state excitation and emission spectra

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  The photoluminescent properties of compound 1 and its ligand (H₂L) were investigated. Upon excitation at 273 nm, the free ligand exhibited an emission peak at 454 nm. In contrast, 1 displayed a redshifted emission peak at 460 nm (Fig. 1d), which is redshifted by 6 nm compared with that of the ligand and can be classified as a ligand-related emission. Notably, the aqueous suspension of 1 showed a slightly blueshifted emission at 450 nm.

  The stability of compound 1 in water was evaluated. After being immersed in water for one month, the PXRD pattern of the resulting sample was basically the same as that before immersion, indicating that long-term water immersion did not cause structural transformation or collapse. The luminescence spectrum of the suspension of 1 after one month of storage was basically the same as that before storage. These results indicate that 1 has good structural and luminescent stability in water and may be a promising candidate for luminescence-based sensing in aqueous systems.

  2.2   Detection of Al³⁺, Ga³⁺, and In³⁺ ions

  The potential of compound 1 as a luminescent probe for metal ion detection was explored. Water suspensions of 1 were treated with nitrate salts of 21 metal ions [M(NO₃)_x] (M^x+=Na⁺, K⁺, Li⁺, Ag⁺, Ca²⁺, Mg²⁺, Sr²⁺, Cd²⁺, Ba²⁺, Cu²⁺, Co²⁺, Zn²⁺, Ni²⁺, Mn²⁺, Pb²⁺, Fe³⁺, Al³⁺, Ga³⁺, In³⁺, V³⁺, Cr³⁺), and AuCl₃. Then the color changes before and after adding metal ions under the irradiation of sunlight and a 254 nm ultraviolet lamp were observed. The results showed that there was no observable color change in the suspensions upon adding metal ions when irradiated with sunlight. However, under the irradiation of a 254 nm ultraviolet lamp, distinct differences in emission were evident among the suspensions.

  With the exception of the Fe³⁺ ion, the addition of all other trivalent metal ions (Al³⁺, Ga³⁺, In³⁺, V³⁺, Cr³⁺, and Au³⁺) and Pb²⁺, Cu²⁺ could lead to varying degrees of enhancement in the luminescence of the suspensions of compound 1. Notably, following the addition of Al³⁺, Ga³⁺, and In³⁺ ions, the suspensions exhibited not only a significant increase in luminescence intensity but also an apparent color change‒transitioning rapidly from blue to blue-green‒which distinctly sets them apart from other tested suspensions.

  The luminescence spectra of the suspensions of compound 1 after the addition of different metal ions were further analyzed (Fig. 2b). Upon introducing Al³⁺, Ga³⁺, and In³⁺ ions, a significant enhancement in luminescence was observed, with increases to 11.6, 8.0, and 12.0 times that of the initial state, respectively. Additionally, all these enhancements were accompanied by a redshift of approximately 35 nm, which aligned with the changes in luminescence color noted visually. In contrast, when Au³⁺, V³⁺, Cr³⁺, Cu²⁺, and Pb²⁺ ions interacted with compound 1, the corresponding luminescence enhancement factors for the suspension were only 3.5, 2.4, 2.4, 1.2, and 2.4 times that of the initial state, respectively; notably, no shift in emission peak occurred under these conditions. The remaining metal ions did not induce any significant changes in the luminescent properties of suspensions of 1. These findings suggest that compound 1 serves as an effective luminescent probe capable of selectively detecting Al³⁺, Ga³⁺, and In³⁺ ions through both increased luminescent intensity and observable shifts in color.

  Figure 2

  

  Figure 2.  Luminescence responses of water suspensions of compound 1 to different metal ions: (a) photographs under sunlight (upper) and 254 nm UV light (lower); (b) corresponding emission spectra; (c) corresponding changes in luminescence intensity (I₀ and I are the luminescence intensities of the suspensions before and after adding different metal ions, respectively.)

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  To thoroughly investigate the detection performance of this probe for Al³⁺, Ga³⁺, and In³⁺ ions, luminescence titration experiments were conducted. As illustrated in Fig. 3, varying amounts of aqueous solutions containing Al³⁺, Ga³⁺, and In³⁺ ions were added to the suspensions of compound 1, respectively. Subsequently, the emission spectra of these systems were analyzed. With an increase in the concentration of metal ions, the intensity of the emission peak at 447 nm gradually increased while exhibiting a redshift in peak position. The luminescence intensity demonstrated a linear relationship with concentrations of Al³⁺ (0-0.45 mmol·L^-1), Ga³⁺ (0-0.44 mmol·L^-1), and In³⁺ (0-0.46 mmol·L^-1), respectively. The linear equations derived from fitting are presented as Eq.1, 2, and 3, where I₁, I₂, and I₃ correspond to the luminescence intensities after interaction with Al³⁺, Ga³⁺, and In³⁺ ions:

  $ I_1=2.30 \times 10^5 c_{\mathrm{Al}^{3+}}+6\;597.90 $

  (1)

  $ I_2=1.41 \times 10^5 c_{\mathrm{Ga}^{3+}}+6\;906.02 $

  (2)

  $ I_3=2.32 \times 10^5 c_{\mathrm{In}^{3+}}+5\;464.02 $

  (3)

  Figure 3

  

  Figure 3.  Emission spectra of the suspensions of compound 1 after the addition of different amounts of Al³⁺ (a), Ga³⁺ (b), and In³⁺ (c) ions; Corresponding linear relationships between luminescence intensity and the concentration of Al³⁺ (d), Ga³⁺ (e), and In³⁺ (f) ions

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  The corresponding correlation coefficients (R²) were found to be 0.997, 0.992, and 0.992.

  Based on the signal-noise ratio of being 3σ/k (σ represents the relative standard deviation of emission intensity of blank samples across ten tests; k is the slope), the detection limits of compound 1 to Al³⁺, Ga³⁺, and In³⁺ ions were calculated to be 2.31, 3.06, and 2.78 μmol·L^-1, respectively. Notably, the detection limit of Al³⁺ was lower than the maximum allowable concentration of Al³⁺ in drinking water (7.41 μmol·L^-1) as stipulated by the World Health Organization (WHO). All three detection limits are remarkably low and comparable to those reported for other probes targeting these three metal ions^[8-11], indicating that 1 serves as an excellent probe for their efficient detection.

  In the actual detection process, interference problems from other metal ions often occur. Therefore, an anti-interference study was carried out. In the experiments, both the measured ion concentration and the interfering ion concentration were 0.5 mmol·L^-1. Fig. 4a shows the interference of other metal ions on the luminescence of the "1+Al³⁺" system. When Al³⁺, Ga³⁺, and/or In³⁺ ions coexist in the system simultaneously, strong interference will occur, resulting in enhanced luminescence. Unfortunately, there is no linear relationship between the luminescence intensity and the total concentration at this time, indicating that this probe cannot be used for the detection of the total concentration of the mixed system of the three metal ions. In addition, the presence of Fe³⁺ and Au³⁺ ions could cause a certain degree of quenching of the luminescence in the detection system, which may be caused by the paramagnetic characteristics of the ions. For the "1+Ga³⁺" and "1+In³⁺" systems, Fe³⁺, as well as Cr³⁺ and Au³⁺, would respectively cause a certain degree of quenching of their luminescence. Overall, the vast majority of metal ions do not interfere with the detection.

  Figure 4

  

  Figure 4.  Evaluation of the detection performance of compound 1: (a) interference of other metal cations on the detection of Al³⁺ by 1; (b) PXRD patterns of the samples of 1 after being immersed in solutions of different pH values for one day; (c) luminescence intensity variations of 1 and "1+Al³⁺" suspensions under different pH conditions; (d) PXRD patterns of the solid samples obtained after adding Al³⁺, Ga³⁺ and In³⁺ ions to the suspensions of 1 respectively

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  The emission spectra that changed over time after adding 0.5 mmol·L^-1 of Al³⁺, Ga³⁺, and In³⁺ ions, respectively, to the suspension of compound 1 were measured. The results showed that after the addition of Al³⁺, the luminescence intensity of the suspension increased rapidly and reached equilibrium within 120 s. After adding Ga³⁺ or In³⁺, the response time was even shorter, and the luminescence intensity reached equilibrium within 25 s. In the subsequent half an hour, the luminescence basically remained unchanged. The above results indicate that compound 1 has the advantage of rapid response as a probe for Al³⁺, Ga³⁺, and In³⁺ ions.

  The luminescence stability of the probe under different pH environments was studied. Firstly, the PXRD patterns of the samples after being soaked for one day under different pH conditions (pH=1-11) were tested (Fig. 4b). The results show that the structure was stable under the condition of pH 1-10. However, when pH=11, the structure collapsed and transformed into an amorphous phase. Further research found that the luminescence of the suspension in the pH range of 3-10 was not affected by pH (Fig. 4c), and the detection of Al³⁺, Ga³⁺, and In³⁺ ions by compound 1 in this pH range was not interfered with by pH. These results indicate that 1 has a good pH working window.

  2.3   Sensing mechanism

  The investigation into the widespread detection capability of compound 1 for three metal ions within the same group has been conducted. Initially, we examined the influence of absorbance caused enhancement (ACE) on detection efficacy^[10]. The UV-Vis absorption spectra of the suspensions of 1 were measured before and after the addition of 0.5 mmol·L^-1 Al³⁺, Ga³⁺, and In³⁺, respectively. The results indicated that there was no significant difference in absorbance between the suspensions prior to and following the introduction of these metal ions. This suggests that the luminescence enhancement observed in 1 during detection is not attributable to changes in absorbance; thus, its interaction with these metal ions does not involve an ACE mechanism.

  To further ascertain whether any structural alterations occurred in compound 1 before and after detection, Al³⁺, Ga³⁺, and In³⁺ were added separately to suspensions of 1. The resulting solid samples were then collected for PXRD analysis. As illustrated in Fig. 4d, the PXRD patterns obtained from these samples exhibited significant differences compared to that of 1 itself, indicating that a structural transformation induced by metal ion interaction may have transpired upon contact with these ions‒ultimately leading to enhanced luminescence from the probe. Notably, distinct PXRD patterns were observed for "1+Al³⁺", "1+Ga³⁺", and "1+In³⁺" samples; this variability may explain why this probe cannot be employed for detecting total concentrations of all three ions simultaneously. Our previous research has established that generating products with identical structures is a prerequisite condition for MOF-based probes when detecting total analyte content through structural transformation mechanisms^[13]. Regrettably, despite numerous attempts, we have yet to determine the specific structure of the probe post‐ detection.

  Control experiments were conducted by introducing metal ions into a ligand solution that had been neutralized with sodium hydroxide (with a molar ratio of NaOH to H₂L of 2.1∶1), followed by the assessment of the emission spectra of the resulting mixtures. The results indicated that as the concentrations of Al³⁺, Ga³⁺, and In³⁺ ions increased, there was a gradual enhancement in the luminescence intensity of the solutions. This finding substantiates the validity of our inference.

  To date, several MOF-based probes for detecting Al³⁺, Ga³⁺, and In³⁺ ions have been reported^[8-11]. However, most existing probes are limited to detecting only one or two of these metal ions. Notably, only two instances have demonstrated the capability to detect all three metal ions simultaneously^[10-11]. In comparison with these two probes, the probe presented in this study exhibits several distinctive features: (1) the detection occurs within an all-aqueous system with a relatively broad working pH range; (2) it demonstrates excellent detection specificity for only three ions (Al³⁺, Ga³⁺, and In³⁺ ions), and other ions, particularly trivalent metal ions, do not exhibit significant interference; (3) the probe displays a "turn on" luminescence response upon interaction with these metal ions, accompanied by a color change from blue to blue-green‒facilitating easy observation without specialized equipment; (4) its detection limit is comparable to those reported for other probes^[8-11,14]. Collectively, these characteristics suggest that this probe possesses notable advantages in detecting Al³⁺, Ga³⁺, and In³⁺ ions.

  3.   Conclusions

  In summary, we have successfully developed a "turn on" Cd-based MOF luminescent probe capable of specifically detecting Al³⁺, Ga³⁺, and In³⁺ among 22 metal ions in an all-aqueous medium; its respective detection limits were 2.31 μmol·L^-1 for Al³⁺, 3.06 μmol·L^-1 for Ga³⁺, and 2.78 μmol·L^-1 for In³⁺. Furthermore, both its structure and luminescent properties demonstrate exceptional stability against water as well as acid/alkali conditions. During detection processes, the luminescence color transitions from blue to blue-green, further enhancing visibility through naked-eye observation. The recognition mechanism is the structural transformation induced by the three metal ions, resulting in an enhanced luminescence of the probe.

  
  Acknowledgements: This research is supported by the National Natural Science Foundation of China (Grant No.21871038).
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• 

  Figure 1  Structure and characterization of compound 1: (a) 1D chain structure along the a-axis; (b) packing of the chains viewed along the a-axis (π-π stacking and hydrogen-bonding interactions are presented as green and blue dotted lines, respectively); (c) experimental and simulated PXRD patterns; (d) normalized solid-state excitation and emission spectra

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  Figure 2  Luminescence responses of water suspensions of compound 1 to different metal ions: (a) photographs under sunlight (upper) and 254 nm UV light (lower); (b) corresponding emission spectra; (c) corresponding changes in luminescence intensity (I₀ and I are the luminescence intensities of the suspensions before and after adding different metal ions, respectively.)

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  Figure 3  Emission spectra of the suspensions of compound 1 after the addition of different amounts of Al³⁺ (a), Ga³⁺ (b), and In³⁺ (c) ions; Corresponding linear relationships between luminescence intensity and the concentration of Al³⁺ (d), Ga³⁺ (e), and In³⁺ (f) ions

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  Figure 4  Evaluation of the detection performance of compound 1: (a) interference of other metal cations on the detection of Al³⁺ by 1; (b) PXRD patterns of the samples of 1 after being immersed in solutions of different pH values for one day; (c) luminescence intensity variations of 1 and "1+Al³⁺" suspensions under different pH conditions; (d) PXRD patterns of the solid samples obtained after adding Al³⁺, Ga³⁺ and In³⁺ ions to the suspensions of 1 respectively

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