A Robust Strontium Coordination Polymer with Selective and Sensitive Fluorescence Sensing Ability for Fe3+ Ions

Exploration of sensitive and selective fluorescence sensors towards toxic metal species is of great importance to solve metal pollution issues. In this work, a three-dimensional (3D) strontium coordination polymer of Sr2(tcbpe) (H4tcbpe = 1,1,2,2-tetrakis(4-(4-carboxy-phenyl)phenyl)ethene) has been synthesized and developed as a fluorescent sensor to Fe3+ ions. Sr2(tcbpe) shows a mechanochromic fluorescence with emission shifting from blue of the pristine to green after being ground. Notably, based on a fluorescence quenching mechanism, Sr2(tcbpe) displays a sensitive and selective fluorescent sensing behavior to Fe3+ ions with a detection limit of 0.14 mM. Moreover, Sr2(tcbpe) exhibits high tolerance to water in a wide pH range (pH = 3–13), demonstrating that Sr2(tcbpe) is a potential fluorescent sensor of Fe3+ in water.


Introduction
In the past two decades, great progress has been made in fluorescent coordination polymers (FL-CPs), which have been applied in bioimaging, light emitting, chemical sensing, and so forth [1][2][3][4][5]. Many fluorescence (FL) mechanisms have been developed in FL-CPs, exemplified by linker-or metal-centered emission energy transfer between metals and ligands (LMCT or MLCT) [6][7][8]. Based on the desired features of CPs, therefore, their fluorescence can also be custom-made by careful choice of inorganic metal cation and organic linker. The most important representatives are rare earth (RE)-based CPs, whose fluorescence is derived from RE metal centers. RE-based CPs have received intense attention due to their bright and narrow characteristic emission bands and long emission lifetime [9][10][11][12]. However, the scarcity of RE sources to some extent restricts their future applications. Alkaline earth (AE) metals such as Ca 2+ and Sr 2+ , like RE cations, usually exhibit abundant and flexible coordination modes, making them good candidates as alternative metals to construct FL-CPs [13][14][15]. Because of their d 0 electron configuration, AE 2+ ions are very suitable to build FL-CPs with ligand-centered luminescence by using emissive organic linkers with a unique chromophore. AE metals are also good candidates to build FL-CPs because of their economic and environmental advantages. However, thus far, FL-CPs constructed from AE metals are comparatively rare [13][14][15].
Metal ions play essential roles in biological metabolism. However, metal species usually exist at a trace concentration level in biological systems, while an excess or deficiency of metals would bring great harm to the biological environment and even threaten life. For instance, excess Fe 3+ can cause Alzheimer's disease, while a lack of Fe 3+ can result in hemochromatosis [16,17]. Therefore, many efforts have been devoted to monitoring the Fe 3+ ions. A variety of FL-CP sensors towards Fe 3+ ions have been developed in the Materials 2023, 16, 577 2 of 8 past decade, which are mostly constructed from RE and transition metal (TM) ions [18][19][20]. However, as far as we know, AE-based FL-CPs with sensitive and selective Fe 3+ sensing performance are comparatively rare [13,15].
Bearing this in mind, herein, we report a Sr-based FL-CP of Sr 2 (tcbpe) (H 4 tcbpe = 1,1,2,2tetrakis(4-(4-carboxy-phenyl)phenyl)ethene). Sr 2 (tcbpe) exhibits an interestingly mechanochromic FL inherited from the tetraphenyl ethylene emitting center in H 4 tcbpe with aggregation-induced emission (AIE) characteristics. Sr 2 (tcbpe) shows FL sensing performance towards Fe 3+ with good selectivity and sensitivity, representing the first FL Sr-CP sensor to Fe 3+ ions. It also possesses an excellent tolerance to solvents and water even in acid/base conditions, indicating that Sr 2 (tcbpe) is a promising FL sensor to probe Fe 3+ ions in water.
Physical measurements. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku MiniFlex II diffractometer using CuKα radiation (λ = 1.54178 Å). A graphite monochromator was used and the generator power settings were set at 44 kV and 40 mA ( Figure S3). Data were collected in a 2θ range of 3 and 35 • with a scanning speed of 1.0 • /min. Thermogravimetric (TG) data were collected on a NETZSCH STA449C thermogravimetric analyzer with a temperature ramping rate of 10 • C/min from 30 to 700 • C under nitrogen gas flow ( Figure S4). Elemental analyses for C, H, and O were performed on a German Elementary Vario EL III instrument. Single crystal X-ray diffraction data were collected with graphite-monochromated MoKα (λ = 0.71073 Å) using an XtaLAB Synergy R, HyPix diffractometer at 298(2) K.
FL measurements. The as-made crystalline samples of Sr 2 (tcbpe) were manually ground to obtain a fine powder. A 2 mg powdered sample was dispersed in 2 mL of the given organic solvents or 10 −2 M metal ion solution by ultrasonication to obtain stable FL suspensions. The FL emulsion was then placed in a 1 cm width quartz cell to record the FL spectra using a PerkinElmer LS55 FL spectrometer. The FL detection experiments were carried out by adding varied amounts of 0.5 × 10 −2 M Fe 3+ ions into the prepared suspensions with a pipette. For all FL measurements, the excitation wavelength was monitored at 380 nm and the corresponding emission wavelengths were monitored from 400 nm to 700 nm.
Stability measurements. A 15 mg as-made crystalline sample was immersed in 2 mL of organic solvents or water with a range of pH values over 24 h. Then the immersed samples were collected by filtration and used for further PXRD measurements.
X-ray crystallography. A single crystal suitable for single-crystal X-ray diffraction (SCXRD) was selected under an optical microscope and glued to a thin glass fiber. The structure was solved by direct methods and refined with full-matrix least-squares techniques using the SHELX2018 package [22]. The CCDC number for Sr 2 (tcbpe) is 2224915. The detailed crystallographic data and structure-refinement parameters are summarized in Table 1.

Crystal Structure Analysis
Single-crystal analysis indicates that Sr 2 (tcbpe) displays a three-dimensional (3D) structure. As seen in Figure 1a, there are two crystallographically independent Sr 2+ cations. Sr1 is eight-coordinated by four water molecules (of which two as bridging linkers and two as terminal molecules) and four monodentate coordinating carboxylic groups from four different tabpe 4− ligands ( Figure S5a). Sr2 is nine-coordinated exhibiting similar coordination to RE ions. The Sr2 is connected with three carboxylic groups from three different tcbpe 4− ligands in chelating coordination, one carboxylic group in a monodentate coordinating mode, and two bridging water molecules ( Figure S5b). The stretching vibration peak at 3550 cm −1 for the -OH of the free carboxylic group in H 4 tcbpe has disappeared, which further demonstrated the coordination between the Sr 2+ and tcbpe 4− ligands ( Figure S2). The Sr1 and Sr2 are interconnected by one bridging water and one carboxylic group to form a 1D zigzag chain along the b direction as the secondary building unit (SBU, Figure S5c,d). Such 1D chain-like SBUs are bridged by tcbpe 4− ligand with only one coordination mode to connect the six neighboring 1D chains to generate a 3D nonporous structure (Figure 1b,c).

FL Studies
H 4 tcbpe is a bright emissive ligand with aggregation-induced emission (AIE) [23,24]. Therefore, ligands bearing this AIE center of tetraphenyl ethylene usually show sensitive FL to external stimuli [23][24][25]. Herein, the AIE character is inherited by Sr 2 (tcbpe), which exhibits interestingly mechanochromic FL. As depicted in Figure 2a, the solid-state FL spectrum of the as-made Sr 2 (tcbpe) shows a blue emission maximized at 470 nm when excited by 410 nm light ( Figures S6 and S7). A mechanoresponsive bathochromic shift in FL emission after grinding is observed-that is, a shift from the blue emission centered at 470 nm to a green emission maximized at 485 nm (inset in Figure 2a, Figures S7 and  S8, ESI). Consequently, the chromaticity coordinate for the as-made sample is (0.16, 0.27), while that for the ground sample is (0.21, 0.32) (Figure 2b). When dispersed in various lab solvents (e.g., N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), acetone, and ethanol), the powdered Sr 2 (tcbpe) exhibits no solvent-dependent FL ( Figure S9), which showed, to some extent, a quenching of FL intensity (Figure 3a). The powdered samples of Sr 2 (tcbpe) were dispersed in various 0.5 × 10 −2 M M(AC) n solutions (n = 1-3, M = Na + , K + , Ca 2+ , Zn 2+ , Cu 2+ , Co 2+ , Ni 2+ , Pb 2+ , Fe 3+ , Al 3+ , and Cr 3+ ) to test their FL-sensing selectivity to metal ions. As can be seen in Figure 2b, although different metal ions exhibit mild FL-quenching effects to Sr 2 (tcbpe), only the Fe 3+ ion almost entirely quenches its FL, demonstrating that the material is a selective FL sensor to Fe 3+ ions.

FL Studies
H4tcbpe is a bright emissive ligand with aggregation-induced emission (AIE) [23,24]. Therefore, ligands bearing this AIE center of tetraphenyl ethylene usually show sensitive FL to external stimuli [23][24][25]. Herein, the AIE character is inherited by Sr2(tcbpe), which exhibits interestingly mechanochromic FL. As depicted in Figure 2a, the solid-state FL spectrum of the as-made Sr2(tcbpe) shows a blue emission maximized at 470 nm when excited by 410 nm light ( Figures S6 and S7). A mechanoresponsive bathochromic shift in FL emission after grinding is observed-that is, a shift from the blue emission centered at 470 nm to a green emission maximized at 485 nm (inset in Figures 2a, S7 and S8, ESI). Consequently, the chromaticity coordinate for the as-made sample is (0.16, 0.27), while that for the ground sample is (0.21, 0.32) (Figure 2b). When dispersed in various lab solvents (e.g., N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), acetone, and ethanol), the powdered Sr2(tcbpe) exhibits no solvent-dependent FL ( Figure S9), which showed, to some extent, a quenching of FL intensity (Figure 3a). The powdered samples of Sr2(tcbpe) were dispersed in various 0.5 × 10 −2 M M(AC)n solutions (n = 1-3, M = Na + , K + , Ca 2+ , Zn 2+ , Cu 2+ , Co 2+ , Ni 2+ , Pb 2+ , Fe 3+ , Al 3+ , and Cr 3+ ) to test their FL-sensing selectivity to metal ions. As can be seen in Figure 2b, although different metal ions exhibit mild FL-quenching effects to Sr2(tcbpe), only the Fe 3+ ion almost entirely quenches its FL, demonstrating that the material is a selective FL sensor to Fe 3+ ions.    The FL-quenching percentage was quantitatively monitored by the addition of different amounts of 0.5 × 10 −2 M Fe 3+ ions into the FL emulsion (2 mg Sr2(tcbpe) dispersed in 2 mL H2O). The FL intensity of Sr2(tcbpe) is gradually quenched with increasing Fe 3+ ion content, and the FL was quenched by almost 50% at a concentration of 0.1 mM Fe 3+ ions ( Figure 4a). As shown in Figure 4b 3+ and Ksv is the quenching constant), exhibits good linear behavior. The value of Ksv was found to be 6.73 × 10 3 M −1 . The limit of detection was obtained as 0.14 mM The FL-quenching percentage was quantitatively monitored by the addition of different amounts of 0.5 × 10 −2 M Fe 3+ ions into the FL emulsion (2 mg Sr 2 (tcbpe) dispersed in 2 mL H 2 O). The FL intensity of Sr 2 (tcbpe) is gradually quenched with increasing Fe 3+ ion content, and the FL was quenched by almost 50% at a concentration of 0.1 mM Fe 3+ ions ( Figure 4a). As shown in Figure 4b, the Stern-Volmer equation (I 0 /I = 1 + K sv [M], in which I 0 and I are the FL intensity of Sr 2 (tcbpe) without and with the addition of Fe 3+ , and [M] is the molarity of Fe 3+ and K sv is the quenching constant), exhibits good linear behavior. The value of K sv was found to be 6.73 × 10 3 M −1 . The limit of detection was obtained as 0.14 mM from the ratio of 3δ/slope. The detection sensitivity to Fe 3+ ions is even comparable to that of porous FL-CP sensors [26][27][28][29][30][31][32][33][34][35][36][37][38][39], Table S1. from the ratio of 3δ/slope. The detection sensitivity to Fe 3+ ions is even comparable to that of porous FL-CP sensors [26][27][28][29][30][31][32][33][34][35][36][37][38][39], Table S1. Selectivity is an important parameter for FL sensors. Na + , Ca 2+ , etc. are usually coexisting ions in water in nature. Therefore, the powdered sample of Sr2(tcbpe) was dispersed in separate 0.5 × 10 −2 M Na + and Ca 2+ aqueous solutions to check the sensing selectivity towards Fe 3+ . As depicted in Figure 5, the FL intensity of Sr2(tcbpe) in these interferential metal ions showed a similar quenching response to Fe 3+ as that of the FL emulsion dispersed in water. The decrease in FL intensity also exhibits a good linear relationship with Fe 3+ concentration. The Ksv values are 5.02 × 10 3 and 4.25 × 10 3 M −1 for Fe 3+ ions (Insets of Figure 5), respectively, which are comparable to that in water. The results demonstrate that Sr2(tcbpe) possesses a good sensing selectivity toward Fe 3+ even in water systems with various interferential metal ions. Selectivity is an important parameter for FL sensors. Na + , Ca 2+ , etc. are usually coexisting ions in water in nature. Therefore, the powdered sample of Sr 2 (tcbpe) was dispersed in separate 0.5 × 10 −2 M Na + and Ca 2+ aqueous solutions to check the sensing selectivity towards Fe 3+ . As depicted in Figure 5, the FL intensity of Sr 2 (tcbpe) in these interferential metal ions showed a similar quenching response to Fe 3+ as that of the FL emulsion dispersed in water. The decrease in FL intensity also exhibits a good linear relationship with Fe 3+ concentration. The K sv values are 5.02 × 10 3 and 4.25 × 10 3 M −1 for Fe 3+ ions (Insets of Figure 5), respectively, which are comparable to that in water. The results demonstrate that Sr 2 (tcbpe) possesses a good sensing selectivity toward Fe 3+ even in water systems with various interferential metal ions.
towards Fe 3+ . As depicted in Figure 5, the FL intensity of Sr2(tcbpe) in these interferential metal ions showed a similar quenching response to Fe 3+ as that of the FL emulsion dispersed in water. The decrease in FL intensity also exhibits a good linear relationship with Fe 3+ concentration. The Ksv values are 5.02 × 10 3 and 4.25 × 10 3 M −1 for Fe 3+ ions (Insets of Figure 5), respectively, which are comparable to that in water. The results demonstrate that Sr2(tcbpe) possesses a good sensing selectivity toward Fe 3+ even in water systems with various interferential metal ions. The as-made Sr2(tcbpe) also exhibits excellent solvent-and water-tolerances even in acid/base conditions. The prepared compound has exhibited good anti-solvent stability, as demonstrated by a comparative study of the PXRD patterns for the samples before and The as-made Sr 2 (tcbpe) also exhibits excellent solvent-and water-tolerances even in acid/base conditions. The prepared compound has exhibited good anti-solvent stability, as demonstrated by a comparative study of the PXRD patterns for the samples before and after immersion in common lab solvents ( Figure S10). As shown in Figure S11, the PXRD patterns of the samples immersed in water with pH values of between 3 to 13 remain the same as the simulated pattern, indicating that the skeleton of Sr 2 (tcbpe) is still maintained in acid or base water environments. The good selectivity and chemical stability make Sr 2 (tcbpe) a promising FL sensor for Fe 3+ in water. Fe 3+ ions due to their d 5 configuration possess a strong electron-withdrawing ability. The UV light-excited electrons of Sr 2 (tcbpe) transfer to the Fe 3+ ions, thus resulting in a decrease in the FL intensities of the Sr 2 (tcbpe) [28][29][30][31].

Conclusions
An AIE-emitting ligand of H 4 tcbpe has been coordinated with Sr 2+ to assemble an FL Sr-CP. The obtained compound, Sr 2 (tcbpe), exhibits a mechanoresponsive FL shifting from blue to green emission. Remarkably, the water-stable Sr 2 (tcbpe) represents the first FL Sr-CP sensor for Fe 3+ ions with good sensitivity and selectivity. More AE-based FL-CPs sensors designed to detect toxic species will be created in our lab in the future.