English

• 0.   Introduction

  Chiral molecules have a close relationship with living organisms and biological compounds, which are of great significance for life on Earth^[1-3]. Propylene glycol (PG) is a transparent, odorless, water-soluble liquid and is non-carcinogenic or non-genotoxic^[4-5]. Based on these aspects, it has remained an important entity for industrial and technological applications^[6-7]. Due to a chiral carbon center, PG forms rectus (R-) enantiomers and sinister (S-) enantiomers. Besides, R/S-PG can serve as precursors for synthesizing chiral compounds and as chiral dopants to trigger the formation of chiral nematic liquid crystals^[8]. Thus, highly enantiosensitive detection of R/S-PG is of considerable importance in the chiral environment and industry. However, scarce studies have been reported on the recognition of chiral PG. The fluorometric method is considered superior among various methods because of its advantages like convenient operation, easy manipulation, and high sensitivity^[9-11]. Consequently, there is an urgent need to develop fluorescent chemosensors that are capable of detecting chiral PG.

  As a class of promising advanced materials with diverse potential functions, coordination polymers (CPs) have raised widespread concerns among scientists in various fields^[12-15]. Chiral CPs (CCPs), a subclass of CPs, have garnered significant attention in frontier research because of their diverse and practical applications in areas such as nonlinear optics, circularly polarized luminescence, asymmetric catalysis, and chiral recognition^[16-19]. Some CCPs have been documented for their ability to recognize enantiomers, such as Yang et al. synthesized a pair of serine-derived homochiral CP (L)-SA-Cd and (D)-SA-Cd, which exhibited high selectivity towards 1-phenylethylamine with an enantioselective factor of 2.89±0.09^[20]. Hua et al. reported that the fluorescent CCP using a dipyridyl ligand with a novel topology, demonstrated efficient chiral sensitivity for Mosher′s acid with an enantioselectivity ratio of 2.61^[21]. CPs have more copious active sites that chelate a series of lanthanide ions^[22]. So, Ln@CPs display fascinating unique features by 4f electrons with narrow luminescent emission and high color purity, which are anticipated to be fluorescence quenching sensors^[23].

  So, [Cd₂(D-cam)₂(2, 2′-bipy)₂]_n (Cd-CP) was synthesized using D-camphoric acid (D-H₂cam) and 2, 2′-bipyridine (2, 2′-bipy). The luminescence center Tb³⁺ ions were further introduced into the reaction system to obtain Tb³⁺@Cd-CP. Tb³⁺@Cd-CP showed more intense luminescence under UV excitation than Cd-CP. Tb³⁺@Cd-CP exhibited efficient detection of enantiomeric PG with different fluorescent turn-off responses.

  1.   Experimental

  1.1   Materials and methods

  All materials and solvents used in this study were of analytical grade and were used as received without any additional purification. Luminescence measurements were carried out using the FS5 fluorescence spectrometer from Edinburgh Instruments. The powder X-ray diffraction (PXRD) patterns were obtained using a Bruker D8 advanced diffractometer, operated at 40 kV and 40 mA, employing Cu Kα radiation (λ=0.154 056 nm) to scan a 2θ range of 5°-50°. The single-crystal data and diffraction-crystal module of the Mercury software were used to generate the simulated PXRD pattern. A Fourier transform infrared spectrometer (FTIR-850) documented infrared (IR) spectra using the KBr pellets method in a wavenumber range of 4 000-500 cm^-1. The thermogravimetric (TG) curve was recorded using a NETZSCH STA449F5 thermal analyzer. The measurements were conducted under N₂ flow, with the temperature increasing gradually from room temperature to 600 ℃ at a heating rate of 10 ℃·min^-1. An EA1110 CHNS-0 CE elemental analyzer was used to perform elemental analysis on Cd, Tb, C, O, and N. Additionally, images of the surface morphology were acquired using a JSM-7500F field emission scanning electron microscope (FESEM). CD spectra were obtained by a JascoJ-810 spectropolarimeter.

  1.2   Synthesis of Cd-CP and Tb³⁺@Cd-CP

  Cd-CP was synthesized according to a modified method reported procedure^[24]. D-H₂cam (0.26 mmol, 0.051 g), 2, 2′-bipy (0.26 mmol, 0.040 g), Cd(NO₃)₂·4H₂O (0.24 mmol, 0.073 g), DMF (2 mL), and ethanol (1 mL) were added into a 10 mL glass vial. The sealed vessel was heated at 95 ℃ for 3 d, and then it was allowed to cool to ambient temperature. The final crystals were obtained through filtering, cleaned using ethanol, and then dried in the air.

  Tb³⁺@Cd-CP was synthesized by an analogous method as that for Cd-CP, but Tb(NO₃)₃·6H₂O (0.002 mmol, 0.001 g) was added to the mixture.

  1.3   Fluorescence quenching experiment

  Initially, the finely ground sample of crystalline Tb³⁺@Cd-CP (30 mg) in 30 mL ethanol was positioned in an ultrasonic bath to form a stable suspension. The suspension of Tb³⁺@Cd-CP was utilized for conducting both the initial luminescence spectra analysis and fluorescence quenching experiments. Then the stock solution of R/S-PG was added to the solution of Tb³⁺@Cd-CP to explore the fluorescence quenching.

  2.   Results and discussion

  2.1   Characterization of Cd-CP and Tb³⁺@Cd-CP

  As depicted in Fig. 1, the dinuclear Cd units are connected by the D-cam^2- ligands into a layer parallel to the ab plane. The FTIR spectrum of Cd-CP in Fig. 2a revealed a strong broad peak at 3 440 cm^-1, belonging to the stretching vibration peak of hydroxyl. The stretching vibrations of the C—H on saturated carbon atoms were at 2 975 and 2 921 cm^-1, respectively. Two bands at 1 662 and 1 569 cm^-1 are attributed to the asymmetric stretching vibrations of the C=O group, while the symmetric stretching vibrations of C—O appeared at 1 440 and 1 386 cm^-1. The band at 1 054 cm^-1 is derived from the stretching vibration of the C—N bond, and 773 and 736 cm^-1 are attributed to the bond stretching and in-plane bending of Cd—O, respectively. The TG curve of Cd-CP indicated an absence of notable weight loss until temperatures reached 300 ℃, with a minor reduction of approximately 3.8% attributed to the presence of physisorbed water molecules from atmospheric humidity. The framework began to decompose and collapse when the temperature was above 300 ℃. This phenomenon aligns with the information provided in the original literature study (Fig. 2b)^[24]. The high phase purity of Cd-CP is indicated by the consistent peak positions observed in the simulated and measured PXRD patterns of the synthesized materials presented in Fig. 2c. Additionally, the variations in peak intensity can be ascribed to the preferential selection process used for the powdered samples. In addition, the enantiomeric nature of Cd-CP and Tb³⁺@Cd-CP originating from D-H₂cam was confirmed by solid-state CD spectra. Fig. 2d illustrates roughly similar two Cotton peaks between 300 and 350 nm of Cd-CP and Tb³⁺@Cd-CP, further confirming the homochirality.

  Figure 1

  

  Figure 1.  Homochiral layer in Cd-CP

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  Figure 2

  

  Figure 2.  (a) IR spectrum of Cd-CP; (b) TG curve of Cd-CP; (c) PXRD patterns and (d) CD spectra of Cd-CP and Tb³⁺@Cd-CP in KBr pellets

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  The surface morphological features of Cd-CP and Tb³⁺@Cd-CP were characterized using SEM images (Fig. 3a and 3b), which revealed that the samples crystallized with a block morphology with flat and smooth surfaces on each branch. The diameter of each block was around 1 μm, exhibiting a typical morphological feature commonly observed in CPs^[3]. The EDS (energy dispersive spectroscopy) elemental mapping of the modified Tb³⁺@Cd-CP indicated the homogeneous distribution of Cd, Tb, C, N, and O (Fig. 3c-3g), and illustrates that Tb³⁺ was successfully introduced^[25-27].

  Figure 3

  

  Figure 3.  SEM images of (a) Cd-CP and (b) Tb³⁺@Cd-CP; (c-g) EDS elemental mappings for Tb³⁺@Cd-CP

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  2.2   Fluorescence characteristics and enantiomer detection

  The room-temperature liquid-state (dispersed in ethanol) photoluminescent (PL) emission spectra of Cd-CP and Tb³⁺@Cd-CP are shown in Fig. 4a. The luminescent spectrum of Tb³⁺@Cd-CP exhibited distinct sharp peaks at 488, 544, 585, and 620 nm, which can be attributed to the transitions of ⁵D₄→⁷F_J (J=6-3) in the Tb³⁺ ion. It is conspicuous that the fluorescent colors shifted from colorless to green with the introduction of Tb³⁺ and can be observed with the naked eye under a UV lamp (inset of Fig. 4a). Therefore, Tb³⁺@Cd-CP can be achieved facilely by doping Tb³⁺ into Cd-CP via the traditional hydrothermal strategy, which in turn enhances the characteristic fluorescence of Tb³⁺. Due to the better fluorescence performance of Tb³⁺@Cd-CP than Cd-CP, the potential application of Tb³⁺@Cd-CP in detecting R/S-PG was investigated. In general, when S- and R-PG are in contact with Tb³⁺@Cd-CP, Tb³⁺@Cd-CP can provide a unique chiral environment for recognition, which will cause the fluorescence intensity of Tb³⁺@Cd-CP to change during PL process. As shown in Fig. 4b, the fluorescence intensity of Tb³⁺@Cd-CP was different from S- and R-PG, in which R-PG has a stronger quenching effect^[28-30].

  Figure 4

  

  Figure 4.  (a) PL spectra of Cd-CP and Tb³⁺@Cd-CP in ethanol; (b) PL spectra of Tb³⁺@Cd-CP during the recognition of PG enantiomers

  Inset: photos of Cd-CP (bottom) and Tb³⁺@Cd-CP (top).

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  2.3   Fluorescence sensing of R/S-PG

  To explore the fluorescence quenching of Tb³⁺@Cd-CP at different concentrations of PG enantiomers, fluorescence quenching titrations were performed with the incremental addition of S- and R-PG to the dispersion of Tb³⁺@Cd-CP in ethanol. As shown in Fig. 5, the fluorescence quenching data of S- and R-PG could be easily analyzed by the linear regression of the Stern-Volmer equation:

  $ \frac{I_0}{I}=1+K_{\mathrm{SV}} c_Q $

  Figure 5

  

  Figure 5.  Emission spectra of Tb³⁺@Cd-CP with varying concentrations of (a) R-PG and (b) S-PG, respectively; the linear relationship between the relative intensity (I₀/I) and the concentrations of (c) R-PG and (d) S-PG, respectively

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  where I₀ and I represent the fluorescence intensities of Tb³⁺@Cd-CP without and with the analyte, respectively. K_SV denotes the quenching constant, while c_Q signifies the analyte concentration^[31-32]. The fluorescence intensity of S- and R-PG had a good linear relationship between 0-0.2 μmol·L^-1. The quenching constants were 5.3×10³ L·mol^-1 for R-PG and 2.0×10³ L·mol^-1 for S-PG. The enantioselectivity factor α (α=K_{SV, R-PG}/K_{SV, S-PG}) was 2.65, revealing that Tb³⁺@Cd-CP could distinguish R-PG better than S-PG. Additionally, in light of the slope k of the linear fitting equation and the standard deviation σ obtained from 15 blank solutions, the limits of detection (LOD=3σ/k) for Tb³⁺@Cd-CP towards R-PG and S-PG were determined to be 9.3 and 19.0 μmol·L^-1, respectively.

  2.4   Recycling performance of Tb³⁺@Cd-CP

  To investigate the practical applicability of Tb³⁺@Cd-CP, the recovery of the powder was determined^[33]. The fluorescence intensity of Tb³⁺@Cd-CP at 544 nm was quenched to varying degrees upon exposure to R/S-PG for 5 min, then Tb³⁺@Cd-CP was immersed in ethanol for 5 min and dried in air, and the fluorescence intensity of Tb³⁺@Cd-CP could be recovered (Fig. 6). The recovered fluorescence intensity of Tb³⁺@Cd-CP experienced minimal decline for three successive reuses. The superior reversible sensing performances demonstrate the feasibility of Tb³⁺@Cd-CP for practical sensing applications.

  Figure 6

  

  Figure 6.  Luminescence intensities of Tb³⁺@Cd-CP at 545 nm before (dark green) and after (light green) the addition of (a) R-PG and (b) S-PG respectively in cyclic tests

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  2.5   Theoretical calculation

  To explore the effect of the chirality from propylene glycol on Cd-CP, a model including two Cd(Ⅱ) ions, two D-cam^2- anions, two 2, 2′-bipy ligands, and one PG molecule was set up. The model systems are shown in Fig. 7, in which yellow, grey, red, and white balls represent Cd, C, O, and H atoms, respectively. All the calculations were performed using the ORCA 5 program package of Neese and co-workers^[34]. The hybrid density functional theory (DFT) method B3LYP, including Grimme′s D3 van der Waals corrections^[35-36], together with the basis set def2-TZVP, were used throughout the calculations. All stationary geometries have been optimized without constraints. The —OH of the PG molecule can form hydrogen bonds with the carboxyl group of D-cam^2- anions in stable configurations (Fig. 7). The configuration containing the R-PG molecule showed a lower energy of 1-4 kJ·mol^-1 than the S-PG molecule, which demonstrates a marked chiral recognition of relative to D-enantiomer by Cd-CP. Accordingly, when Cd-CP becomes Tb³⁺@Cd-CP, it also has a more chiral recognition effect on R-PG than on D-PG.

  Figure 7

  

  Figure 7.  Optimized configurations of the model systems with R-PG (a) and S-PG (b)

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  3.   Conclusions

  In summary, [Cd₂(D-cam)₂(2, 2′-bipy)₂]_n (Cd-CP) and Tb³⁺@Cd-CP was facilely prepared. Tb³⁺@Cd-CP exhibited better fluorescence performance than Cd-CP but maintained the original chirality. Moreover, the luminescence sensing performance demonstrated that Tb³⁺@Cd-CP could distinguish R/S-propylene glycol (R/S-PG) by fluorescence responses, with fluorescence quenching constant of 5.3×10³ and 2.0×10³ L·mol^-1, respectively. This result can be applied to the detection of PG enantiomers by a simple, rapid, and sensitive method.

  
  
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• 

  Figure 1  Homochiral layer in Cd-CP

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  Figure 2  (a) IR spectrum of Cd-CP; (b) TG curve of Cd-CP; (c) PXRD patterns and (d) CD spectra of Cd-CP and Tb³⁺@Cd-CP in KBr pellets

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  Figure 3  SEM images of (a) Cd-CP and (b) Tb³⁺@Cd-CP; (c-g) EDS elemental mappings for Tb³⁺@Cd-CP

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  Figure 4  (a) PL spectra of Cd-CP and Tb³⁺@Cd-CP in ethanol; (b) PL spectra of Tb³⁺@Cd-CP during the recognition of PG enantiomers

  Inset: photos of Cd-CP (bottom) and Tb³⁺@Cd-CP (top).

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  Figure 5  Emission spectra of Tb³⁺@Cd-CP with varying concentrations of (a) R-PG and (b) S-PG, respectively; the linear relationship between the relative intensity (I₀/I) and the concentrations of (c) R-PG and (d) S-PG, respectively

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  Figure 6  Luminescence intensities of Tb³⁺@Cd-CP at 545 nm before (dark green) and after (light green) the addition of (a) R-PG and (b) S-PG respectively in cyclic tests

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  Figure 7  Optimized configurations of the model systems with R-PG (a) and S-PG (b)

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