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

Abstract

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.

1. 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 Ca²⁺ and Sr²⁺, 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 d⁰ electron configuration, AE²⁺ 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 Fe³⁺ can cause Alzheimer’s disease, while a lack of Fe³⁺ can result in hemochromatosis [16,17]. Therefore, many efforts have been devoted to monitoring the Fe³⁺ ions. A variety of FL-CP sensors towards Fe³⁺ ions have been developed in the 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 Fe³⁺ sensing performance are comparatively rare [13,15].

Bearing this in mind, herein, we report a Sr-based FL-CP of Sr₂(tcbpe) (H₄tcbpe = 1,1,2,2-tetrakis(4-(4-carboxy-phenyl)phenyl)ethene). Sr₂(tcbpe) exhibits an interestingly mechanochromic FL inherited from the tetraphenyl ethylene emitting center in H₄tcbpe with aggregation-induced emission (AIE) characteristics. Sr₂(tcbpe) shows FL sensing performance towards Fe³⁺ with good selectivity and sensitivity, representing the first FL Sr-CP sensor to Fe³⁺ ions. It also possesses an excellent tolerance to solvents and water even in acid/base conditions, indicating that Sr₂(tcbpe) is a promising FL sensor to probe Fe³⁺ ions in water.

2. Materials and Methods

Reagents. SrCl₂ (≥99%, Adamas-beta, Shanghai Titan Chemical Co., Ltd., Shanghai, China); DMF (N,N-dimethyl-Formamide, AR, Greagent, Shanghai Titan Chemical Co., Ltd.); formic acid (≥98%, Adamas-beta, Shanghai Titan Chemical Co., Ltd.). H₄tcbpe has been synthesized using a previously reported method [21].

Preparation of Sr₂(tcbpe). A mixture of strontium chloride (50 mg) and H₄tcbpe (20 mg) in 5 mL DMF, 2 mL H₂O, and 500 μL formic acid was sealed into a 20 mL glass vial and heated at 120 °C for 2 days. Light-yellow block-like crystals were obtained by filtration after the vial was cooled to room temperature (50 mg, 30% yield based on strontium, Figures S1 and S2). Elemental analyses calculated for Sr₂(tcbpe): C 61.76%, H 3.26%. Found: C61.20%, H 3.83%.

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 Sr₂(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 Fe³⁺ 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 Sr₂(tcbpe) is 2224915. The detailed crystallographic data and structure-refinement parameters are summarized in

Table 1. Crystallographic data and structural refinement details for Sr₂(tcbpe).

Empirical formula	Sr2C54H40O12
Formula weight	1056.10
Crystal system	Monoclinic
Space group	P21
T/K	298(2)
λ/Å	0.71073
a/Å	10.0613(2)
b/Å	9.8710(2)
c/Å	22.7537(3)
β/º	90.0016(11)
V/Å3	2259.79(7)
Z	2
Dc/Mg·m−3	1.552
μ/mm−1	2.428
F(000)	1072
Measured refls.	50,879
Independent refls.	11,257
Rint	0.0504
No. of parameters	638
GOF	1.038
Flack parameter	0.401(10)
aR1, bwR2 [I > 2σ(I)]	0.0490, 0.1226
aR1, bwR2 (all data)	0.0516, 0.1238

^aR₁ = ∑║F_o│ − │F_c║/∑│F_o│. ^b wR₂ = {∑w[(F_o)² − (F_c²)]²/∑w(F_o²)²}^1/2.

.

3. Results and Discussion

3.1. Crystal Structure Analysis

Single-crystal analysis indicates that Sr₂(tcbpe) displays a three-dimensional (3D) structure. As seen in Figure 1a, there are two crystallographically independent Sr²⁺ 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 tabpe⁴⁻ ligands (Figure S5a). Sr2 is nine-coordinated exhibiting similar coordination to RE ions. The Sr2 is connected with three carboxylic groups from three different tcbpe⁴⁻ 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 H₄tcbpe has disappeared, which further demonstrated the coordination between the Sr²⁺ and tcbpe⁴⁻ 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 tcbpe⁴⁻ ligand with only one coordination mode to connect the six neighboring 1D chains to generate a 3D nonporous structure (Figure 1b,c).

3.2. 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 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 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 0.5󠆼 × 10⁻² M M(AC)_n solutions (n = 1–3, M = Na⁺, K⁺, Ca²⁺, Zn²⁺, Cu²⁺, Co²⁺, Ni²⁺, Pb²⁺, Fe³⁺, Al³⁺, and Cr³⁺) 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 Fe³⁺ ion almost entirely quenches its FL, demonstrating that the material is a selective FL sensor to Fe³⁺ ions.

The FL-quenching percentage was quantitatively monitored by the addition of different amounts of 0.5󠆼 × 10⁻² M Fe³⁺ 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 Fe³⁺ ion content, and the FL was quenched by almost 50% at a concentration of 0.1 mM Fe³⁺ ions (Figure 4a). As shown in Figure 4b, the Stern–Volmer equation (I₀/I = 1 + K_sv[M], in which I₀ and I are the FL intensity of Sr₂(tcbpe) without and with the addition of Fe³⁺, and [M] is the molarity of Fe³⁺ and K_sv is the quenching constant), exhibits good linear behavior. The value of K_sv was found to be 6.73 × 10³ M⁻¹. The limit of detection was obtained as 0.14 mM from the ratio of 3δ/slope. The detection sensitivity to Fe³⁺ 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⁺, Ca²⁺, etc. are usually coexisting ions in water in nature. Therefore, the powdered sample of Sr₂(tcbpe) was dispersed in separate 0.5󠆼 × 10⁻² M Na⁺ and Ca²⁺ aqueous solutions to check the sensing selectivity towards Fe³⁺. As depicted in Figure 5, the FL intensity of Sr₂(tcbpe) in these interferential metal ions showed a similar quenching response to Fe³⁺ as that of the FL emulsion dispersed in water. The decrease in FL intensity also exhibits a good linear relationship with Fe³⁺ concentration. The K_sv values are 5.02 × 10³ and 4.25 × 10³ M⁻¹ for Fe³⁺ 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 Fe³⁺ 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 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 Sr₂(tcbpe) is still maintained in acid or base water environments. The good selectivity and chemical stability make Sr₂(tcbpe) a promising FL sensor for Fe³⁺ in water. Fe³⁺ ions due to their d⁵ configuration possess a strong electron-withdrawing ability. The UV light-excited electrons of Sr₂(tcbpe) transfer to the Fe³⁺ ions, thus resulting in a decrease in the FL intensities of the Sr₂(tcbpe) [28,29,30,31].

4. Conclusions

An AIE-emitting ligand of H₄tcbpe has been coordinated with Sr²⁺ to assemble an FL Sr-CP. The obtained compound, Sr₂(tcbpe), exhibits a mechanoresponsive FL shifting from blue to green emission. Remarkably, the water-stable Sr₂(tcbpe) represents the first FL Sr-CP sensor for Fe³⁺ 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.