A Cd(II) Luminescent Coordination Grid as a Multiresponsive Fluorescence Sensor for Cr(VI) Oxyanions and Cr(III), Fe(III), and Al(III) in Aqueous Medium

Abstract

Hydro(solvo)thermal reactions of Cd(NO₃)₂, 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, {[Cd(Br-1,3-bdc)(NI-mbpy-34)(H₂O)]∙2H₂O}_n (1). Single-crystal X-ray diffraction analysis showed that 1 features a two-dimensional (2-D) gridlike sql layer with the point symbol of (4⁴·6²), where the Cd(II) center adopts a {CdO₅N₂} pentagonal bipyramidal geometry. Thermogravimetric (TG) analysis confirmed the thermal stability of 1 up to about 340 °C, whereas XRPD patterns proved the maintenance of crystallinity and framework integrity of 1 in CH₂Cl₂, H₂O, CH₃OH, and toluene. Photoluminescence studies indicated that 1 displayed intense blue fluorescence emissions in both solid-state and H₂O suspension-phase. Owing to the good fluorescent properties, 1 could serve as an excellent turn-off fluorescence sensor for selective and sensitive Cr(VI) detection in water, with LOD = 15.15 μM for CrO₄²⁻ and 14.91 μM for Cr₂O₇²⁻, through energy competition absorption mechanism. In addition, 1 could also sensitively detect Cr³⁺, Fe³⁺, and Al³⁺ ions in aqueous medium via fluorescence-enhancement responses, with LOD = 2.81 μM for Cr³⁺, 3.82 μM for Fe³⁺, and 3.37 μM for Al³⁺, mainly through an absorbance-caused enhancement (ACE) mechanism.

1. Introduction

With the advanced development of modern society, rapid industrial and agricultural productions and rich human activities have increasingly brought about severe chemical pollution. Among various chemical pollutants, heavy metal ion based inorganic contaminates are of higher concern compared with other contaminants such as organic pollutants due to their nondegradability and bioaccumulation [1]. Moreover, heavy metal ions are well-known poisonous contaminants in water due to their high toxicity, which could cause serious environmental and ecological harm and cause a detrimental effect on human health [2]. For example, chromium exists in aquatic environments usually in the forms of Cr(VI) oxyanions, i.e., dichromate (Cr₂O₇²⁻) and chromate (CrO₄²⁻) ions, and/or free cation, i.e., trivalent Cr(III) ion. The Cr(VI) oxyanions are highly carcinogenic and mutagenic, causing hereditary genetic defects and various types of cancers [3,4,5]. While the Cr(III) ion is essentially harmless due to its low toxicity, it may, however, cause mutations and malignant cells when excessive accumulation occurs [5,6,7]. The permissible limit for Cr(VI) in drinking water is set as 50 μg/L by the World Health Organization (WHO) [8]. Iron and aluminum are two ubiquitous metals widely used in daily applications around human living environments [9]. Trivalent Fe(III) and Al(III) cations are the forms of iron and aluminum that can enter human body. As one of the most important elements for living organisms, Fe(III) ion influences a variety of vital bioprocesses such as electron transfer, oxygen storage, oxygen metabolism, among others [10,11,12]. The deficiency or excess of Fe(III) ion is harmful to human health, resulting in some diseases [13,14,15]. Additionally, Al(III) ion in body fluids is toxic to humans and will induce harmful effects that cause diseases such as Alzheimer’s disease and Parkinson’s disease when its content in the body is over an acceptable standard [9,12,16]. The tolerable daily ingestion of Al³⁺ for the human body is about 3–10 mg/day [7] and the permissible level in drinking water is set as 7.41 μM by the WHO [17].

In recent years, the sensing and detection of chemical pollutants has attracted tremendous interest from many researchers. Benefitting from their unique merits, low-cost, easy manipulation, instant, visual identification, excellent sensitivity, and high selectivity [7,12], fluorescence based detection has gained considerable attention among various conventional instrumental techniques. Nowadays, various advanced fluorescence sensory materials have emerged, such as organic dyes [18], quantum dots (QDs) [19,20], and carbon dots (CDs) [21,22], among others. Of particular note is one kind of organic–inorganic hybrid material called coordination polymers (CPs), fabricated by metal ions or clusters as nodes and bridging organic ligands as linkers through the connection of coordination bonds. As a matter of fact, luminescent CP-based chemo/biosensors have been actively used to detect small molecules, explosives, ions, gas, and pH, among others, and several reviews have been devoted to the sensory properties [23,24,25,26,27,28,29]. Recently, we have made advances in fluorescence detection of hazardous chemical contaminants by using luminescent organic–inorganic hybrid materials as sensory platforms [30,31,32,33,34,35,36,37,38,39]. Herein, we report a new Cd(II) based luminescent CP, {[Cd(Br-1,3-bdc)(NI-mbpy-34)(H₂O)]∙2H₂O}_n (1), where NI-mbpy-34 = N-(pyridin-3-ylmethyl)-4-(pyridin-4-yl)-1,8-naphthalimide, Br-1,3-H₂bdc = 5-bromobenzene-1,3-dicarboxylic acid. The solid-state structure of 1 has a simple two-dimensional (2-D) layer structure adopting a gridlike net with the point symbol of (4⁴·6²), which is highly stable in water. The remarkable emission properties make 1 a functional multiresponsive fluorescence sensor for Cr(VI) oxyanions detection via a quenching effect, and Cr(III), Fe(III), and Al(III) sensing via an enhancement response in aqueous medium, with high sensitivity and remarkable selectivity.

2. Experimental Section

2.1. Materials and Characterization

All of the chemicals and solvents were acquired from market sources (MATRIX, ULTRA, ACROS, PanReac Applichem, SHOWA, Fluka, VETEC, ALFA, MACRON, J.T. Baker, SIGMA ALDRICH), and used without further processing. Ligand NI-mbpy-34 was synthesized according to the previously reported literature [40]. Thermogravimetric (TG) analyses were performed using a Thermo Cahn VersaTherm HS TG analyzer (Thermo, Newington, NH, USA) under flow nitrogen with a heating rate of 5 °C/min. X-ray powder diffraction (XRPD) patterns were measured using a Shimadzu XRD-7000 diffractometer (Shimadzu, Kyoto, Japan) with a graphite monochromatized Cu Kα radiation (λ = 1.5406 Å) at 30 kV and 30 mA. Infrared (IR) spectroscopic measurements were performed on a Perkin-Elmer Frontier Fourier transform infrared spectrometer (Perkin-Elmer, Taipei, Taiwan) using attenuated total reflection (ATR) technique. Fluorescence spectroscopic measurements were performed at room temperature using a Hitachi F7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) equipped with a 150 W xenon lamp as an excitation source. UV-Vis absorption spectra were recorded at room temperature using a JASCO V-750 UV/VIS spectrophotometer (JASCO, Tokyo, Japan). Elemental microanalyses (C, H, N) were performed on an Elementar Vario EL III analytical instrument (Elementar, Langenselbold, Germany). Ultrasonic agitation of suspensions was conducted using a Qsonica Q125 instrument. X-ray photoelectron spectroscopy (XPS) analyses were performed on an ULVAC-PHI PHI 5000 VersaProbe/Scanning ESCA Microprobe instrument (ULVACPHI Inc., Kanagawa, Japan).

2.2. Synthesis of {[Cd(Br-1,3-bdc)(NI-mbpy-34)(H₂O)]∙2H₂O}_n (1)

A DMF solution (2 mL) of NI-mbpy-34 (9.1 mg, 0.025 mmol), an aqueous solution (2 mL) of Cd(NO₃)₂∙4H₂O (15.4 mg, 0.050 mmol), and a DMF solution of Br-1,3-H₂bdc (12.3 mg, 0.050 mmol) were sequentially added into an acid digestion bomb placed in a Teflon-lined stainless steel autoclave. The mixture was kept inside a furnace at 80 °C for 48 h and then cooled to ambient temperature. Yellowish crystals suitable for X-ray diffraction were collected after washing with distilled water and ethanol, and dried at room temperature. Yield: 60% based on NI-mbpy-34 (12.1 mg, 0.015 mmol). IR (ATR, cm⁻¹): 3072, 1590, 1460, 1333, 992, 889, 723. Anal. Calcd. for C₃₁H₂₄BrCdN₃O₉: C, 48.01; H, 3.09; N, 5.42%. Found: C, 48.24; H, 2.94; N, 5.41%. The phase purity of the bulky sample was confirmed by the closely matched XRPD patterns between the simulated pattern, calculated from single-crystal diffraction data and the experimental pattern of as-synthesized 1 without grinding (Figure S1). Of particular note, the XRPD patterns of the same microcrystalline sample after grinding showed alternations in intensity, and in some peak positions, compared to the simulated XRPD patterns. This is tentatively attributed to the influences of either the variation in preferred orientation of the powdered sample [30] or the partial crystal structure distortion caused by grinding [41] or both.

2.3. Single-Crystal X-ray Structure Determinations

The diffraction data were collected using a Bruker D8 Venture diffractometer configured with a PHOTO100 CMOS detector at 150(2) K, equipped with a graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods with the SHELXTL program [42] and refined by full-matrix least-squares methods on F² using the SHELXL-2014/7 [43], incorporated in WINGX [44] crystallographic collective package. Non-hydrogen atoms were refined with anisotropic displacement parameters, except where noted. Carbon-bound hydrogen atoms were calculated in ideal positions and refined as riding mode. Oxygen-bound hydrogen atoms were structurally evident in the difference Fourier map. All of the hydrogen atoms were refined with isotropic displacement parameters, U_iso, constrained to be 1.2 or 1.5 times U_eq of the carrier atom. Experimental details for X-ray data collection and the refinements are summarized in

Table 1. X-ray data collection and structure refinement for 1.

Empirical Formula	C31H24BrCdN3O9
Mw	774.84
Crystal system	Triclinic
Space group	P$\overline{1}$
a, Å	10.169(3)
b, Å	11.285(4)
c, Å	14.502(4)
α, °	97.526(10)
β, °	107.465(10)
γ, °	108.503(10)
V, Å3	1457.8(8)
Z	2
T, K	150(2)
λ, Å	0.71073
Dcalc, g cm−3	1.765
F000	772
μ, mm−1	2.182
Reflns collected	44,573
Unique reflns (Rint)	5934(0.0424)
Obsd reflns (I > 2σ (I))	5201
Params	406
R1a (I > 2σ (I))	0.0289
wR2b (I > 2σ (I))	0.0601
R1a (all data)	0.0368
wR2b (all data)	0.0646
GOF on F2	1.152
Δρmax, Δρmin, e Å−3	1.015, −0.561

${}^a R_1=\sum \left|\left|F_o\right|-\left|F_c\right|\right|/{\displaystyle \sum \left|F_o\right|}$; ${}^b w{R}_2=\{\sum [w{(F_o{}^2-F_c{}^2)}^2]/\sum [w{(F_o{}^2)}^2]{\}}^{1/2}$.

. Hydrogen-bonding parameters are shown in Table S1. CCDC 1991627 (1) contain 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 (accessed on 19 March 2020).

3. Results and Discussion

3.1. Synthsis and Crystal Structure of {[Cd(Br-1,3-bdc)(NI-mbpy-34)(H₂O)]∙2H₂O}_n (1)

Hydro(solvo) thermal reactions of Cd(NO₃)₂∙4H₂O, Br-1,3-H₂bdc, and NI-mbpy-34 in DMF/H₂O media afforded 1 as yellowish crystals (Scheme 1). Single-crystal X-ray structure analysis reveals that the crystal structure of 1 belongs to the triclinic space group P$\overline{1}$. The asymmetric unit contains one Cd(II) center, one Br-1,3-bdc²⁻ dianion, one NI-mbpy-34 ligand, and one coordination and two lattice water molecules (Figure 1a). The Cd(II) center adopts a {CdO₅N₂} pentagonal bipyramidal geometry, where the equatorial plane is made up of two carboxylate groups of two distinct Br-1,3-bdc²⁻ dianions in one asymmetric (Cd1–O3 = 2.254(2) Å, Cd1–O4 = 2.753 Å) and one symmetric (Cd1–O5#2 = 2.393(2) Å, Cd1–O6#2 = 2.389(2) Å, #2, x − 1, y, z) chelating modes and one 4-pyridyl nitrogen atom (naphthalene end) from one NI-mbpy-34 ligand (Cd1–N3 = 2.301(2) Å), while the two apical positions are located by one coordination water molecule (Cd1–O7 = 2.348(2) Å) and one 3-pyridyl nitrogen atom (imide end) from the other NI-mbpy-34 ligand (Cd1–N2#1 = 2.354(3) Å, #1, x + 1, y + 1, z + 1). Each Br-1,3-bdc²⁻ dianion adopts a μ₂-Br-1,3-bdc-κO,Oʹ:κO,Oʹ mode to bridge two Cd(II) centers, where the two carboxylate groups suit an asymmetric and a symmetric chelating coordination mode (Figure 1a). Connection of Cd(II) centers by Br-1,3-bdc²⁻ dianions and NI-mbpy-34 ligands simultaneously forms a two-dimensional (2-D) gridlike layer (Figure 1b), which can be simplified as a 4-connected sql net with the point symbol of (4⁴·6²) (Figure 1c). Two such gridlike layers with the coordination water molecules oriented face-to-face are linked together in pair through O–H∙∙∙O hydrogen bonding interactions (O∙∙∙O, 2.723(3) and 2.932(3) Å, Table S1) formed between the coordinated water molecules and the carboxylate oxygen atoms of the Br-1,3-bdc²⁻ ligands (Figure 1d), generating a 2-D hydrogen-bonded bilayer (Figure 1e). When viewed down the crystallographic [100] direction, there are small pores with potential sufficient solvent accessible voids of only about 9.8% of the unit cell volume [45], accompanied by lattice water molecules (Figure 1e). These lattice water molecules form hydrogen-bonding interactions with each other (O∙∙∙O, 3.035(5) Å) and, importantly, with the framework (O∙∙∙O, 2.839(5) and 2.855(4) Å) to expand the 2-D hydrogen-bonded bilayers to be the three-dimensional (3-D) hydrogen-bonded network (Figure 1e,f).

3.2. Chemical Stability and Thermal Properties

The chemical stability of 1 in different solvents including dichloromethane (CH₂Cl₂), N,N′-dimethylacetamide (DMAc), N,N′-dimethylformamide (DMF), H₂O, methanol (CH₃OH), and toluene was checked. After separately immersing 1 in CH₂Cl₂, H₂O, CH₃OH, and toluene for 24 h, the checked XRPD patterns of so-obtained powdered samples were very similar to the patterns of as-synthesized 1 after grinding, with slight differences in peak intensity (Figure 2). This might suggest the preferred orientation effect or the minor extent of partial distortion of the long range order in 1. However, the checked XRPD patterns still imply the maintenance of framework integrity and crystallinity, confirming the high stability of 1. In contrast, 1 displayed low stability after immersing in DMAc and DMF due to the poorly matched XRPD profiles.

Thermogravimetric (TG) analysis was performed under a nitrogen atmosphere to examine the thermal stability of 1 (Figure S2). The TG trace of 1 exhibited a weight loss of 5.4% from room temperature to 69 °C, corresponding to the escape of lattice water molecules (calcd. 4.6%). A gradual weight loss of 1.6% corresponding to the removal of coordinated water molecules (calcd. 2.3%) followed when the temperature was raised to approaching 186 °C. Then, the TG trace showed the existence of a stable plateau before the framework began to process a two-step collapse from ca. 340 °C to ca. 633 °C. During the decomposition, bromide might react with divalent cadmium to generate CdBr₂ (b.p. = 844 °C), which would escape at higher temperature. The final residue of 8.4% was reasonably assigned to the CdO component (calcd. 8.3%).

3.3. Gas Adsorption Properties of Activated 1

In the crystal structure of 1, there are free void spaces of about 9.8% of the unit cell volume, hence, the porous properties of activated 1 were investigated by gas adsorption studies. Prior to gas adsorption experiments, as-synthesized 1 (about 100 mg) was thermally treated at 100 °C under a reduced pressure for 24 h to remove solvent molecules and thus to give activated 1. For activated 1, N₂ adsorption isotherms exhibited no appreciable uptakes of 5.92 cm³ g⁻¹ STP at P/P₀ = 1 and 77 K, whereas CO₂ adsorption isotherms showed negligible uptakes of 11.19 cm³ g⁻¹ STP at P/P₀ = 1 and 195 K (Figure S3). The low N₂ and CO₂ uptakes of thermally activated 1 might be attributed to the small sufficient solvent-accessible voids and framework distortion induced pore-reduction, which resulted in surface adsorption. The latter assumption was supported by the checked XRPD patterns, which showed obvious differences with the experimental profiles of as-synthesized sample of 1 (Figure S4).

3.4. Photoluminescence Properties

When excited at λ_ex = 370 nm, the solid-state emission spectrum of NI-mbpy-34 showed an intense emission band centered at 462 nm, which was overlapped with two further bands as shoulders at around 433 and 480 nm (Figure S5). After irradiation at λ_ex = 360 nm, Br-1,3-H₂bdc emitted an extremely weak solid-state emission centered at around 468 nm. Comparably, 1 emitted intense blue fluorescence with an emission band centered at 436 nm in solid-state and 422 nm in H₂O suspension-phase upon excitation at λ_ex = 365 nm. From the band position and shape, the emissions were tentatively assigned to the intraligand charge transfer of the NI-mbpy-34 ligand perturbed by metal coordination; this is further supported by the high resemblance in excitation spectra between NI-mbpy-34 and 1.

3.5. Detection of Anions

In view of the high water stability and excellent fluorescence properties of 1 in H₂O suspension-phase, its potential ability to detect anions was explored in water. Anion detection studies were carried out by separately adding aqueous solutions of NaF and K_mX (X^m− = Cl⁻, Br⁻, I⁻, ClO₄⁻, CO₃²⁻, Cr₂O₇²⁻, CrO₄²⁻, NO₃⁻, and PO₄³⁻, m = 1, 2, 3) into the well-prepared H₂O suspensions of 1, with a concentration of 1 mM. Upon excitation at λ_ex = 365 nm, the fluorescence detection results showed that most anions had an inconspicuous fluorescence intensity change effect (<10% change) on 1, except for the CO₃²⁻, Cr₂O₇²⁻, CrO₄²⁻, and PO₄³⁻ ions (Figure 3). It is noted that CO₃²⁻ and PO₄³⁻ caused significant fluorescence enhancement by 63% and 44%, respectively. However, the two Cr(VI) oxyanions, Cr₂O₇²⁻ and CrO₄²⁻, showed obviously high quenching effects with an efficiency up to 97% and 99%, respectively, (quenching efficiency (%) = (I₀ − I)/I₀ × 100%, where I₀ and I are the maximum fluorescence intensities of 1 before and after addition of a quencher). Therefore, it is suggested to use the fluorescence quenching of 1 to detect trace amounts of Cr₂O₇²⁻ and CrO₄²⁻ ions in water media. The anti-interference ability of 1 toward Cr₂O₇²⁻ and CrO₄²⁻ was examined with the existence of different competitive anions in equal concentration. The competition experiments clearly indicated that Cr₂O₇²⁻ and CrO₄²⁻ both retained high quenching ability to almost completely turn off the fluorescence of 1 when the other interference anions are present (Figure 4), implying high selectivity of 1 toward Cr₂O₇²⁻ and CrO₄²⁻ over others perturbed anions in water media.

The sensitivity of Cr(VI) oxyanions detection in water media can be evaluated by quantitative analysis and limit of detection (LOD) values. The concentration-dependent fluorescence intensity of 1 was determined by gradually adding different concentrations of Cr(VI) oxyanion into well-dispersed H₂O suspensions of 1. As observations, the fluorescence intensity of 1 gradually decreased with increasing concentration of Cr₂O₇²⁻ and CrO₄²⁻ (Figure 5a,b), indicating that the fluorescence quenching of 1 caused by the introduction of Cr₂O₇²⁻ and CrO₄²⁻ ions can be quantified. It is also noted that the fluorescence intensity (λ_em = 414 nm) versus the concentration of Cr₂O₇²⁻ and CrO₄²⁻ can be well fitted to the first-order exponential decay (Figure S6), suggesting a diffusion-controlled fluorescence quenching. The fluorescence quenching efficiencies were further analyzed by a Stern−Volmer analysis based on the equation: I₀/I = 1 + K_SV[Q], where I₀ and I are the fluorescence intensities before and after the addition of analytes, respectively, [Q] is the molar concentration of the analyte (mM), and K_SV is the Stern−Volmer quenching constant (M⁻¹). The Stern–Volmer plots exhibited good linear correlations at low concentrations (inset in Figure 5c,d), which gave the K_SV values of 5.56 × 10³ M⁻¹ for Cr₂O₇²⁻ and 1.32 × 10⁴ M⁻¹ for CrO₄²⁻. Furthermore, the Stern–Volmer plots displayed upward deviation from linearity at high concentrations (Figure 5c,d), as a result of the combination of a dynamic energy transfer mechanism and a static self-absorption mechanism [46,47]. The LOD values for Cr₂O₇²⁻ and CrO₄²⁻ were determined using the equation: LOD = 3σ/k, where σ is the standard deviation of five blank measurements of fluorescence for the H₂O suspensions of 1 and k is the absolute value of the slope of the calibration curve at low concentrations. The LOD values for Cr₂O₇²⁻ and CrO₄²⁻ were determined to be 14.91 (3.22) and 15.15 (1.76) μM (ppm), respectively, (Figure S7). Owing to the excellent anti-interference ability and high K_SV and lower LOD values, 1 can be an effective fluorescence sensor displaying high detection selectivity and sensitivity toward Cr₂O₇²⁻ and CrO₄²⁻ in water media.

3.6. Detection of Metal Ions

The influence of different metal ions on the fluorescent properties of 1 in H₂O suspension-phase was also investigated through similar procedures by adding well-prepared aqueous solutions of metal ions, including M(NO₃)_n (Mⁿ⁺ = Ag⁺, Al³⁺, Mg²⁺, Ca²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Na⁺, K⁺, Mn²⁺, Ni²⁺, and Pb²⁺, n = 1, 2, 3), into the H₂O suspension of 1, with a concentration of 1 mM. Upon excitation at λ_ex = 365 nm, interestingly, trivalent metal ions of Cr³⁺, Fe³⁺, and Al³⁺ have striking enhancement responses of 10.9, 5.4, and 5.4 times on the fluorescence intensity of 1 in H₂O suspension-phase, while other mono- and divalent metal ions have no or only minor effects (intensity change ≤ 10%) on the fluorescence intensity (Figure 6). It is also noteworthy that the addition of metal ions did not cause significant wavelength shift. To examine the selectivity of 1 toward Cr³⁺, Fe³⁺, and Al³⁺, the interference experiments were carried out. After adding Cr³⁺, Fe³⁺, and Al³⁺ ions into the H₂O suspension of 1 containing other competitive metal ions in equal concentration, the fluorescence intensity of 1 in H₂O suspension-phase increased immediately but showed a certain degree of reduction in some cases, with a maximum of about 43% reduction compared to that without the co-existence of competitive metal ions (Figure 7). The observations indicated that changes in intensity for sensing Cr³⁺, Fe³⁺, and Al³⁺ were somewhat negatively affected, but still within acceptable limits, by the competitive metal ions. As a result, 1 displayed accessible anti-interference ability and detection selectivity for recognizing Cr³⁺, Fe³⁺, and Al³⁺ over other competitive metal ions.

To further quantitatively evaluate the fluorescence response of 1 in H₂O suspension-phase toward Cr³⁺, Fe³⁺, and Al³⁺, fluorescence titration for the above three metal ions was also investigated. It was observed that the fluorescence intensity of 1 in H₂O suspension-phase increased gradually upon incremental addition of the aqueous solutions of Cr³⁺, Fe³⁺, and Al³⁺ ions (Figure 8a–c). The fluorescence intensity (λ_em = 414 nm) obeyed the first-order exponential decay relationship with the concentration of Cr³⁺, Fe³⁺, and Al³⁺ (Figure 8d–f), suggesting diffusion-controlled fluorescence enhancement. The curvilinear dependence between I and concentration of metal ion conforms to the equation I = −2611.22 × exp(−[Cr³⁺]/0.31) + 2845.97 (R² = 0.9956) for Cr³⁺ detection, I = −2671.16 × exp(−[Fe³⁺]/0.90) + 2895.00 (R² = 0.9909) for Fe³⁺ detection, and I = −2414.09 × exp(−[Al³⁺]/0.85) + 2590.77 (R² = 0.9972) for Al³⁺ detection. Moreover, there is a linear relationship between the fluorescence intensity of 1 in H₂O suspension-phase and the Cr³⁺ concentration in the range of 0−0.4 mM, the Fe³⁺ concentration in the range of 0−0.6 mM, and the Al³⁺ concentration in the range of 0−0.7 mM (Figure S8), suggesting that 1 could potentially quantitatively determine Cr³⁺, Fe³⁺, and Al³⁺ ions. Accordingly, the LOD is calculated to be 2.81 (146.2) μM (ppb) for Cr³⁺ detection, 3.82 (198.7) μM (ppb) for Fe³⁺ detection, and 3.37 (90.9) μM (ppb) for Al³⁺ detection, through the equation of 3σ/k.

3.7. Fluorescence-Responsive Sensing Mechanisms

Herein, the possible fluorescence-responsive sensing mechanisms are elucidated. For Cr(VI) oxyanions detection, the XRPD patterns of 1 treated with Cr₂O₇²⁻ and CrO₄²⁻ in water were closely matched to that of as-synthesized 1 (Figure S9), implying that the framework integrity of 1 was retained. Thus, the turn-off sensing mechanism could not be due to the collapse of the framework. Ultimately, the mechanism of the quenching effect could be interpreted by UV−vis absorption spectroscopy. The UV−vis absorption band of Cr(VI) oxyanions and the fluorescence emission band of 1 show a small degree of overlap, suggesting that fluorescence quenching was not caused by energy transfer mechanism (Figure S10). However, the large extent of overlap between the UV−vis absorption spectra of Cr₂O₇²⁻/CrO₄²⁻ and the fluorescence excitation spectrum of 1 demonstrates that the competition of absorption of irradiated light source energy between the Cr₂O₇²⁻ and CrO₄²⁻ ions and 1 leads to the high fluorescence quenching efficiencies.

The XRPD patterns of 1 after being treated with Cr³⁺, Fe³⁺, and Al³⁺ in water were very similar to that of as-synthesized 1 (Figure S11), suggesting that 1 kept the framework integrity and crystallinity. This excluded the possibility of framework collapse caused fluorescence enhancement. Literature has shown that framework–analyte interactions usually demonstrated one of the most possible mechanisms for fluorescence turn-on detection toward metal ions [12,13,31,40,48,49]. To verify this assumption, IR and X-ray photoelectron spectroscopy (XPS) spectra were measured. Unfortunately, the IR spectra of 1 did not show significant changes after immersion (Figure S12). Meanwhile, the O 1s and N 1s peaks in the XPS spectra were almost unchanged (Figure S13). These findings indicate that there might be no framework–analyte interactions or that such interactions are too weak to cause spectra change. Thus, the turn-on detection mechanism is not due to the framework–analyte interactions. Interestingly, the UV–vis absorption spectra of 1 after addition of Cr³⁺, Fe³⁺, and Al³⁺ exhibited significant increase in the absorbance within the sphere of 300–400 nm (Figure S14), corresponding to the excitation energy used. The results illustrated that the turn-on detection of 1 toward Cr³⁺, Fe³⁺, and Al³⁺ can be explained by the absorbance caused enhancement (ACE) mechanism [12,50], meaning that the M³⁺-treated 1 would release more energy than pristine 1 to demonstrate turn-on effect in the emission process.

4. Conclusions

In summary, a luminescent Cd(II) coordination polymer 1 adopting a 2-D gridlike sql layer has been successfully synthesized. Coordination polymer 1 shows strong fluorescence emissions in solvent suspension-phase, making them potential candidates to be employed in sensing Cr₂O₇²⁻ and CrO₄²⁻ ions via fluorescence quenching effect and in detecting Cr³⁺, Fe³⁺, and Al³⁺ ions via fluorescence enhancement response, with high sensitivity and selectivity. From the fluorescence-sensing mechanism studies, absorption energy competition and absorbance caused enhancement can, respectively, interpret the quenching effect toward Cr(VI) oxyanions and the enhancement effect toward Cr³⁺, Fe³⁺, and Al³⁺ metal ions.