A Water-Stable 2-Fold Interpenetrating cds Net as a Bifunctional Fluorescence-Responsive Sensor for Selective Detection of Cr(III) and Cr(VI) Ions.

Reactions of ZnSO4∙7H2O, N-(pyridin-3-ylmethyl)-4-(pyridin-4-yl)-1,8-naphthalimide (NI-mbpy-34), and 5-bromobenzene-1,3-dicarboxylic acid (Br-1,3-H2bdc) afforded a luminescent coordination polymer, [Zn(Br-1,3-bdc)(NI-mbpy-34)]n (1), under hydro(solvo)thermal conditions. Single-crystal X-ray structure analysis revealed that 1 features a three-dimensional (3-D) 2-fold interpenetrating cds (or CdSO4) net topology with the point symbol of (65·8), where the Zn(II) centers are considered as 4-connected square-planar nodes. X-ray powder diffraction (XRPD) patterns and thermogravimetric (TG) analysis confirmed that 1 shows high chemical and thermal stabilities. Notably, 1 displayed solvent dependent photoluminescence properties; the fluorescence intensity and emission maximum of 1 in different solvent suspensions varied when a solvent was changed. Furthermore, the H2O suspension of 1 exhibited blue fluorescence emission and thus can be treated as a selective and sensitive fluorescent probe for turn-on detection of Cr3+ cations through absorbance caused enhancement (ACE) mechanism and turn-off detection of Cr2O72-/CrO42- anions through collaboration of the absorption competition and energy transfer process, with limit of detection (LOD) as low as μM scale.

1. Introduction

The monitoring and detection of chemical pollutants and/or controlled chemicals in complicated samples are very important tasks in managing the environment, water resources, and the food industry. Among various conventional instrumental techniques, fluorescence sensing responding to fluorescence turn on, turn off, or ratiometric signal, has attracted immense attention in recent years because of its particular aspects such as economics, user-friendliness, short response time, visualization, monitoring in real-time, excellent sensitivity, and high selectivity [1,2,3,4]. Various advanced fluorophore materials, including organic dyes [5,6], porous organic polymers [7], quantum dots (QDs) [8,9], carbon dots (CDs) [1,2], nanoparticles (NPs) [3,10], lanthanide organic/inorganic hybrid materials (LHMs) [11], and metal–organic frameworks/coordination polymers (MOFs/CPs) [12,13,14] have emerged.

Chromium existing as Cr(III) and Cr(VI) oxidation states in the aquatic environments can directly contaminate the soil and aquatic systems. As an essential trace biological element in humans, Cr(III) is considered to be harmless and safe. However, excessive Cr(III) may combine with DNA to cause mutations and malignant cells [10,15,16,17]. Cr(VI) shows high carcinogenicity and mutagenicity and can cause allergic reactions, hereditary genetic defects and various types of cancers that adversely affect human health [17,18,19]. The World Health Organization (WHO) has claimed a permissible limit of 50 μg/L for Cr(VI) in drinking water [20]. Lately, MOF/CP-based, fluorescence-sensory materials have been actively pursued as excellent platforms for the flourishing utilization in detection of Cr(III) and Cr(VI) ions though fluorescence quenching (turn off) effect [15,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45]. However, there are still rare examples to achieve the detection of Cr(III) via the fluorescence enhancement (turn on) response [15,43,44,45,46,47,48] and fluorescence shift (ratiometric) effect [41,42,43].

As part of our ongoing work in fluorescence detection of hazardous chemical contaminants [39,40,41,42,43,44,49,50,51,52], we acquired, herein, a Zn(II)-based luminescent coordination polymer, namely [Zn(Br-1,3-bdc)(NI-mbpy-34)] (1, Br-1,3-bdc = 5-bromobenzene-1,3-dicarboxylate; NI-mbpy-34 = N-(pyridin-3-ylmethyl)-4-(pyridin-4-yl)-1,8-naphthalimide), featuring a three-dimensional (3-D) 2-fold interpenetrating cds net. Of note, coordination polymer 1 exhibited fluorescence emissions in solid-state and solvent suspensions, being a bifunctional fluorescence sensor for sensitively and selectively detecting chromium(III) cations and chromium(VI) oxyanions.

2. Experimental Section

2.1. Materials and Methods

All of the chemicals and solvents were acquired from market sources and used without further processing. Ligand NI-mbpy-34 was synthesized according to the previously reported literature [44]. The thermal analysis was conducted by a Thermo Cahn VersaTherm HS TG analyzer (Thermo, Newington, NH, USA) from 25 to 900 °C at a heating rate of 5 °C/min under a flow of nitrogen. The X-ray powder diffraction (XRPD) patterns were measured in the 2θ range of 5–50° by a Shimadzu XRD-7000 diffractometer (Shimadzu, Kyoto, Japan) using Cu Kα radiation (λ = 1.5406 Å) operating at 30 kV and 30 mA. Infrared (IR) spectroscopy was tested in a Perkin-Elmer Frontier Fourier transform infrared spectrometer (Perkin-Elmer, Taipei, Taiwan), and the region 4000–500 cm−1 was recorded with attenuated total reflection (ATR) technique. UV-Vis absorption spectra were obtained on a JASCO V-750 UV/VIS spectrophotometer (JASCO, Tokyo, Japan) at room temperature. The solid-state and solution fluorescence spectra were measured on a Hitachi F7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) at room temperature, with the excitation and emission slits of 5 nm × 5 nm and a scan rate of 1200 nm/min. A 150 W xenon arc lamp was used as an exciting light source. Elemental analyses of C, H, and N were performed on a Vario EL III elemental analyzer (Elementar, Langenselbold, Germany). X-ray photoelectron spectroscopy (XPS) was measured by an ULVAC-PHI PHI 5000 VersaProbe/Scanning ESCA Microprobe instrument (ULVACPHI Inc., Kanagawa, Japan).

2.2. Synthesis of [Zn(Br-1,3-bdc)(NI-mbpy-34)]n ()

NI-mbpy-34 (9.1 mg, 0.025 mmol) was dissolved in 2 mL of N,N′-dimethylformamide (DMF); ZnSO4∙7H2O (14.3 mg, 0.050 mmol) was dissolved in 2 mL of H2O; Br-1,3-H2bdc (12.3 mg, 0.050 mmol) was dissolved in 1 mL of DMF. The above-mentioned solutions were sequentially added to a 23 mL Teflon-lined stainless steel reactor placed in an autoclave. This was sealed and then heated to 80 °C for 6 h and kept at 80 °C for 48 h. After slowly cooling to 30 °C for 36 h, the mixture was washed with distilled water and ethanol, and yellowish crystals were filtered off and dried. The yield based on NI-mbpy-34 was about 60%. IR (ATR, cm−1): 3071, 1617, 1322, 1462, 990, 884, 723. Anal. Calcd for C31H18BrN3O6Zn: C, 55.21; H, 2.67; N, 6.23%. Found: C, 54.90; H, 2.65; N, 6.20%.

2.3. Single-Crystal X-ray Structure Determinations

The single-crystal data taken at 150(2) K for 1 were collected on a Bruker D8 Venture diffractometer with a graphite monochromated Mo Kα radiation (λ = 0.71073 Å) and a PHOTO100 CMOS detector. The structures were solved by direct methods using SHELXTL [53] and refined on F2 by the full-matrix least-squares using the SHELXL-2014/7 [54] and WINGX [55]. Non-hydrogen atoms were confirmed by successive difference Fourier syntheses and were refined with anisotropic displacement parameters. The hydrogen atoms were produced theoretically on their calculated positions and refined with isotropic displacement parameters set to 1.2U of the attached atom. The single-crystal data and refinement parameters of 1 are summarized in Table 1. CCDC 1991626 (1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (23 December 2022).

Table 1

Crystallographic data for 1.

	1
Empirical formula	C31H18BrN3O6Zn
M w	673.76
Crystal system	Monoclinic
Space group	C2/c
a, Å	14.254 (2)
b, Å	12.566 (2)
c, Å	29.985 (5)
β, °	102.648 (8)
V, Å3	5240.2 (15)
Z	8
T, K	150 (2)
λ, Å	0.71073
Dcalc, g cm−3	1.708
F 000	2704
μ, mm−1	2.516
Reflns collected	43704
Unique reflns (Rint)	5360 (0.0751)
Obsd reflns (I > 2σ (I))	4534
Params	379
R1a, wR2b (I > 2σ (I))	0.0633, 0.1296
R1a, wR2b (all data)	0.0778, 0.1353
GOF on F2	1.114
Δρmax, Δρmin, e Å−3	1.312, −0.916

R1 = . wR2 = .

2.4. Fluorescence Measurements

Finely ground powders of 1 (1 mg) were suspended in various solvents (3 mL) including dichloromethane (CH2Cl2), N,N′-dimethylacetamide (DMAc), N,N′-dimethylformamide (DMF), H2O, methanol (CH3OH), and toluene. The prepared suspensions were ultrasonicated via pulsed ultrasound for 10 min and then agitated for further 30 min to yield more stable suspensions.

The H2O suspensions of 1 were utilized to conduct fluorescence sensing experiments. Aqueous solutions of metal ions, including AgNO3, Al(NO3)3, Mg(NO3)2, Ca(NO3)2, Co(NO3)2, Cr(NO3)3, Cu(NO3)2, Fe(NO3)3, NaNO3, KNO3, Mn(NO3)2, Ni(NO3)2, and Pb(NO3)2, and anions, including NaF, KCl, KBr, KI, KClO4, K2CO3, K2Cr2O7, K2CrO4, KNO3, and K3PO4 were prepared with concentration of 0.10 M for fluorescence sensing studies.

Qualitative studies were carried out by adding 0.10 M analyte (30 μL) into the well-prepared H2O suspensions of 1; then, the fluorescence spectra were recorded after waiting for 3 min. Anti-interference studies were conducted on a series of competition experiments with addition of the solution of different perturbed analytes (0.10 M, 30 μL) followed by the targeted analyte (0.10 M, 30 μL) into the H2O suspensions. In each step, the fluorescence spectra were recorded.

The fluorescence quantitative titration experiments were performed with the gradual addition of analytes in aqueous solutions (0.10 M), and then the fluorescence spectra were monitored. The Stern–Volmer equation: I0/I = 1 + Ksv[Q], where I0 and I denote the fluorescence intensities before and after the addition of analytes, respectively, Ksv is the Stern–Volmer quenching constant (M−1), and [Q] is the concentration of analyte (mM), was applied to quantitatively analyze the fluorescence quenching effect.

Limit of detection (LOD) determinations were performed at low concentrations of analyte. Prior to the fluorescence titration, five blank measurements of fluorescence for the H2O suspensions of 1 were carried out for determining the standard deviation (σ). LODs were calculated using the equation: LOD = 3σ/k, where k represents the absolute value of the slope of the calibration curve.

3. Results and Discussion

3.1. Crystal Structure of [Zn(Br-1,3-bdc)(NI-mbpy-34)]n ()

Single-crystal X-ray structure analysis reveals that the crystal structure of 1 belongs to the monoclinic space group C2/c. There is one cationic Zn(II) center, one fully-deprotonated Br-1,3-bdc2− anion, and one neutral NI-mbpy-34 ligand in the asymmetric unit. The Zn(II) center is surrounded by two oxygen atoms of two carboxylate groups from two distinct Br-1,3-bdc2− ligands and two nitrogen atoms of one 3-pyridyl (imide end) and one 4-pyridyl (naphthalene end) groups from two distinct NI-mbpy-34 ligands to adopt a {ZnO2N2} tetrahedral geometry (Figure 1a). The anionic Br-1,3-bdc2− ligand has a μ2-Br-1,3-bdc-κO:κO mode to bridge two Zn(II) centers; each of the two carboxylate groups is in a monodentate-κO coordination mode (Figure 1b). The Zn(II) centers are connected by the anionic Br-1,3-bdc2− and the neutral NI-mbpy-34 ligands to form a three-dimensional (3-D) porous framework (Figure 1c). If the Zn(II) centers are considered as 4-connected square-planar nodes and both the Br-1,3-bdc2− and NI-mbpy-34 ligands are considered as linear linkers (Figure 1a), the 3-D framework of 1 can be simplified as a 4-connected cds (or CdSO4) net topology with the point symbol of (65·8) (Figure 1d). The potential voids of the single cds network are occupied by the other independent identical framework via interpenetration in opposite orientation to generate a 2-fold interpenetrating net (Figure 1e), leaving insufficient solvent accessible voids. Notably, two neighboring naphthalimide skeletons in the two independent equivalent cds frameworks are nearly parallel in a head-to-tail manner and the distance between them is about 3.50 Å (Figure S1), suggesting significant π–π interactions.

Figure 1

Crystal structure of 1: (a) the coordination environment around the Zn(II) center and schematic representation of the 4-connected node; (b) the coordination mode of Br-1,3-bdc2− dianion; (c) a single 3-D framework; (d) schematic representation of the 4-connected cds network with the point symbol of (65·8); (e) 2-fold interpenetrating cds networks.

3.2. X-ray Powder Diffraction (XRPD) Patterns and Chemical Stability

X-ray powder diffraction (XRPD) patterns of as-synthesized 1 are in agreement with the simulated patterns calculated from single-crystal X-ray diffraction data (Figure 2), confirming the phase purity of bulky samples. Further, the chemical stability of 1 in different solvents was checked. After immersing in dichloromethane (CH2Cl2), N,N′-dimethylacetamide (DMAc), N,N′-dimethylformamide (DMF), H2O, methanol (CH3OH), and toluene for 24 h, the XRPD patterns of the solvent-treated samples showed that the characteristic peaks match well with those of the XRPD pattern of as-synthesized 1 and that simulated from the single crystal data, although the peak intensities are somewhat different (Figure 2). This demonstrates that the original framework of 1 can retain a high crystallinity after immersion in solvents, confirming its high stability.

Figure 2

Simulated XRPD pattern of 1 and XRPD patterns of as-synthesized 1 and 1 immersed in different solvents for 24 h.

3.3. Thermal Properties

The thermal properties of 1 were evaluated from the thermogravimetric (TG) analysis. As a representative, the TG analysis plot of 1 shows no weight loss before 378 °C (Figure S2), indicating high thermal stability. Then a two-step decomposition of the framework occurred, which was ended upon heating to ca. 640 °C. During the decomposition, bromide might react with divalent zinc to generate ZnBr2 (b.p. = 697 °C), which escaped at higher temperature. The remaining residue of 6.2% was reasonably assigned to the ZnO component (calcd 6.0%).

3.4. Photoluminescence Properties

Previous research has shown that NI-mbpy-34 is highly emissive and can be a luminescence source for coordination polymers due to its highly conjugated π-electron system [44]. In solid-state, NI-mbpy-34 showed emission band(s) in the region of 400–600 nm with maximum at 462 nm upon excitation at λex = 370 nm, while Br-1,3-H2bdc displayed only an extremely weak emission band upon excitation at λex = 360 nm (Figure S3). When excited at λex = 306 nm, 1 exhibited solid-state fluorescence with two emission peaks centered at 444 nm and 504 nm. From the band position and shape, the emissions were tentatively attributed to the ligand-centered emission of NI-mbpy-34 perturbed by metal coordination.

Subsequently, the fluorescence properties of 1 in different solvent suspensions, such as CH2Cl2, DMAc, DMF, H2O, CH3OH, and toluene were also investigated (Figure 3). We observed that the fluorescence intensity and emission maximum of 1 in different solvent suspensions varied as the solvent was changed, implying solvent-dependent photoluminescence properties. Upon excitation, 1 emitted strong fluorescence emissions in CH3OH and DMF suspensions, moderate emissions in H2O and DMAc suspensions, and weak emissions in CH2Cl2 and toluene suspensions. In addition, the emission maxima of these suspensions varied from 384 nm to 432 nm, showing remarkable blue shift compared to the solid-state fluorescence. The phenomena can most likely be attributed to the different collision interactions rather than crystal structure change [56,57], since that 1 is highly stable in all chosen solvents. Additionally, it is noted that the fluorescence emission intensities are nearly directly proportional to the concentrations of 1 in H2O suspensions (Figure S4).

Figure 3

Fluorescence emission spectra of 1 in suspension-phase of different solvents.

3.5. Fluorescence Sensing of Metal Ions

The fluorescence sensing properties of 1 toward metal ions have been explored, and the fluorescence sensing measurements were carried out in water. Aqueous solutions of nitrate salt of thirteen different metal ions, including Ag+, Al3+, Mg2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+, Na+, K+, Mn2+, Ni2+, and Pb2+, were separately added into the H2O suspensions of 1 in a quartz cuvette with the concentration at 1.0 mM. The photoluminescence measurements were obtained at an excitation wavelength of 306 nm before and after addition of metal ions under the same experimental conditions (Figure 4a). Upon addition of the different metal ions, the mono- and divalent metal ions exerted a relatively weak effect (intensity change ≤ 10%) on the emission of 1, and the Fe3+ ion addition led to a weak enhancement effect with ca. 20-nm blue shift. Interestingly, the trivalent metal ions of Cr3+ and Al3+ resulted in a remarkable fluorescence enhancement by 8.7 and 3.3 times, respectively, along with ca. 20-nm blue shift. The results demonstrate that 1 may be an excellent fluorescence sensor for Cr3+ detection with efficient selectivity. To confirm our assumption, interference experiments were carried out to examine the ability of 1 to selectively detect Cr3+ ions in the co-existence of interfering metal ions with equal concentrations of 1.0 mM. Experimental results clearly indicated that in sensing Cr3+ by 1, Al3+ displayed strong competitive effect while other selected perturbed metal ions showed insignificant interference (Figure 5), suggesting that 1 has good selectivity along with anti-interference ability for Cr3+ sensing in water. Briefly stated, 1 is highly selective for Cr3+ detection over other perturbed metal ions with the exception of Al3+. Further studies on Cr3+ detection by varying the concentrations of 1 in H2O suspensions showed almost unchanged fluorescence enhancement ratios (Figure S5), suggesting specific Cr3+ sensing performances in water.

Figure 4

(a) Fluorescence emission spectra, and; (b) fluorescence relative ratio responses of 1 in H2O suspensions containing various metal ions at 1.0 mM.

Figure 5

Fluorescence relative ratio responses of 1 in H2O suspensions containing various metal ions before and after addition of Cr3+ ions with equal concentrations at 1.0 mM.

To further investigate the sensitivity of 1 toward Cr3+ ions, the fluorescence titration experiments were executed. As expected, gradually increasing fluorescence emission intensities were observed at around 420 nm with increasing concentrations of Cr3+ ions. As shown in Figure 6b, there exists a nonlinear relationship between the fluorescence intensity and the Cr3+ ion concentration, with the formula of I = −2823.98 × exp(−[Cr3+]/0.83) + 2906.06 (R2 = 0.9929), suggesting a saturation behavior at high concentrations. On the basis of quantitative titrations (Figure S6), the LOD for Cr3+ was determined to be 3.13 μM (corresponding to 162.9 ppb). This proves that 1 can effectively detect Cr3+ ions with remarkable sensitivity.

Figure 6

(a) Concentration-dependent fluorescence emission spectra of 1 in H2O suspensions upon incremental addition of Cr3+ ions when excited at λex = 306 nm, and; (b) Plot of fluorescence intensity versus Cr3+ ion concentration for 1 in H2O suspensions.

The possible fluorescence sensing mechanism toward Cr3+ was investigated. The XRPD patterns of 1 recovered from Cr3+ aqueous solutions showed high consistency with the XRPD patterns of as-synthesized 1 in peak positions (Figure S7), which suggested that the framework of 1 keeps its integrity after Cr3+ detection. Thus, the turn-on sensing mechanism can exclude the possibility of framework collapse. However, small but appreciable changes in the relative intensity of the XRPD peaks were observed, so it seems that some changes in the crystal structure occurred. Indeed, X-ray photoelectron spectroscopy (XPS) analysis on 1 indicated the existence of Cr3+ cation in the framework of 1 after immersion as the observation of the Cr 2p3/2 and Cr 2p1/2 peaks at around 577.1 and 586.6 eV, respectively (Figure S8a). This might alter the intensity of the XRPD peaks. Notably, the O 1s peak in the XPS spectra did not shift after Cr3+ immersion (Figure S8b), and also the IR spectra did not change significantly (Figure S9). These phenomena imply that the influence of Cr3+ is not through bonding or there might be extremely weak interactions only between Cr3+ and the framework of 1 instead of the ligand-containing system [47]. Furthermore, the UV−vis absorption spectra of 1 were further checked, which demonstrated that 1 has an absorption band at around 350 nm corresponded to the excitation wavelength applied. Obviously, the absorbance increased remarkably after the addition of Cr3+ but exhibited no significant change after the addition of other different metal ions, such as Al3+ and Fe3+ (Figure S10), which implied that the turn-on effect of 1 toward Cr3+ can be properly explained by the absorbance caused enhancement (ACE) mechanism [46,58].

3.6. Fluorescence Sensing of Anions

The fluorescence sensing properties of 1 toward anions were also explored, and ten different anions, including F−, Cl−, Br−, I−, ClO4−, CO32−, Cr2O72−, CrO42−, NO3−, and PO43−, were chosen. Similar to the procedures used for metal ion sensing, the fluorescence sensing measurements were carried out in water; each individual aqueous solution of anion was added to the well-prepared H2O suspension of 1, and the photoluminescence measurements were obtained at an excitation wavelength of 306 nm before and after addition of anion. As can be seen, most of the chosen anions exerted a relatively weak effect (intensity change ≤ 10%) on the emission of 1 (Figure 7). The strongest fluorescence quenching effect was observed in the cases of the two chromium(VI) oxyanions, Cr2O72− and CrO42−, which showed quenching efficiencies of about 90% and 74%, respectively (quenching efficiency (%) = (I0 − I)/I0 × 100%, where I0 and I are the maximum fluorescence intensity of 1 before and after addition of analytes). Notably, when different concentrations of 1 in H2O suspensions were utilized, the high fluorescence quenching efficiencies are almost retained (Figure S5). Hence, the concentration of 1 in H2O suspension has no significant effect on the detection performances toward Cr2O72− and CrO42−. Furthermore, interference experiments have shown that the quenching efficiencies of 1 toward Cr2O72− and CrO42− anions are hardly affected by other competitive anions (Figure 8), confirming the excellent anti-interference ability and thus the high selectivity of 1 as a fluorescence probe for detection of Cr2O72− and CrO42− anions in water.

Figure 7

(a) Fluorescence emission spectra and (b) fluorescence relative ratio responses of 1 in H2O suspensions containing various anions at 1.0 mM.

Figure 8

Fluorescence relative ratio responses of 1 in H2O suspensions containing various anions before and after addition of Cr2O72−/CrO42− ions with equal concentrations at 1.0 mM.

Since Cr3+ enhances fluorescence of 1 in H2O suspension and Cr(VI) anions quench it, and both species can coexist in environmental conditions, it is of interest to study the influence of Cr3+ detection in the coexistence of Cr(VI) anions and vice versa. Experimental results clearly indicate that Cr(VI) anions strongly interfere with Cr3+ detection while Cr3+ ions cause no interference on the detection of Cr2O72− and CrO42− anions (Figure S11). Again, this confirms that 1 is highly selective for Cr2O72−/CrO42− detection.

The detection sensitivity can be determined by quantitative analysis and LOD. Hence, fluorescent titration experiments were performed. As expected, the recorded fluorescence intensities gradually decreased with the gradual increase in the volume concentrations of Cr2O72− and CrO42− in the H2O suspensions of 1 (Figure 9a,b). Furthermore, the dependence of the fluorescence intensity on Cr2O72− or CrO42− ion concentration was investigated, which can be well fitted to I = 180.62 × exp(−[Cr2O72−]/0.56) − 2.86 (R2 = 0.99717) for Cr2O72− and I = 154.46 × exp(−[CrO42−]/1.06) + 15.61 (R2 = 0.99789) for Cr2O72− (Figure S12). The quantification of fluorescence quenching effect was further examined through the Stern–Volmer equation. As observed, the Stern–Volmer plots for sensing Cr2O72− and CrO42− analytes by 1 both exhibited upward curves of I0/I against the analyte concentration over the titration concentrations (Figure 9c,d), implying the cooperation of dynamic and static quenching processes [34,59,60]. On the basis of quantitative titrations, the good linear regression analyses on Stern–Volmer plots gave the Ksv value of 2.52 × 103 M−1 (R2 = 0.99259) in the range of 0–0.5 mM for sensing Cr2O72− and 1.42 × 103 M−1 (R2 = 0.99672) in the range of 0–2.0 mM for sensing CrO42− (inset in Figure 9c,d). The LOD was determined to be 43.36 μM (corresponding to 9.36 ppm) for Cr2O72− and 25.57 μM (corresponding to 2.97 ppm) for CrO42− (Figure S13).

Figure 9

Concentration-dependent fluorescence spectra of 1 in H2O suspensions by incremental addition of (a) Cr2O72−, and; (b) CrO42− upon excitation at λex = 306 nm, and Stern–Volmer plot of I0/I versus concentration of; (c) Cr2O72−, and; (d) CrO42− for 1 in H2O suspensions (inset: linear Stern–Volmer plot).

The plausible fluorescence-quenching mechanisms have been investigated. The XRPD patterns of 1 before and after treatment of Cr2O72− and CrO42− showed a high degree of similarity (Figure S7), suggesting the maintenance of framework integrity, thus ruling out framework collapse as being the fluorescence quenching mechanism. However, the excitation wavelength to irradiate 1 was greatly overlapped with the absorbance band of Cr2O72− and CrO42−, implying that the competitive absorption of excitation energy might serve dominant influence on the fluorescence quenching detection of 1 toward Cr2O72− and CrO42−. Further, energy transfer process might also contribute efforts in quenching the fluorescence of 1 because the fluorescence-emission band of 1 in H2O suspension was partially overlapped and the absorbance band of Cr2O72− and CrO42− in aqueous solutions (Figure S14).

4. Conclusions

In this research, we have successfully synthesized a 2-fold interpenetrated coordination polymer 1 featuring a 4-connected cds network topology with the point symbol of (65·8). Coordination polymer 1 emits fluorescence in both solid-state and suspension-phase of different solvents, making it a potential candidate to be employed in detection of Cr(III) cations via remarkable fluorescence enhancement response due to ACE mechanism, and in sensing of Cr(VI) oxyanions (Cr2O72− and CrO42−) via fluorescence-quenching effect due to collaboration of absorption competition and energy transfer process, with high sensitivity and selectivity.