A multifunctional [CdI₄]²⁻·[H₂Bimb]²⁺ complex to probe high performance photocatalytic degradation of methyl violet and fluorescent detection of Cr₂O₇²⁻ and Hg²⁺ ions

Abstract

A new organic–inorganic complex, [CdI₄]²⁻·[H₂Bimb]²⁺ (KA@S) (Bimb = 1,4-bis[(1H-imidazol-1-yl)methyl]benzene), has been synthesized at ambient temperature and characterized using single-crystal X-ray diffraction (SC-XRD), elemental analysis, variable temperature powder X-ray diffraction (VT-PXRD), Fourier transform infrared (FT-IR) spectroscopy and thermal stability analysis. SC-XRD analyses reveal that KA@S has anionic and cationic moieties, [CdI₄]²⁻ and [H₂Bimb]²⁺, respectively. C–H⋯I and N–H⋯I interactions, in addition to strong N–H⋯N and N–H⋯O hydrogen bonds and parallel displaced π⋯π stacking interactions, are instrumental towards the extended 1D chain, 2D layer and 3D supramolecular network of KA@S. Promising photocatalytic degradation efficiencies for the dyes MV (96.5%), Rh6G (94.4%), RB (93.5%), CR (91.2%) and CV (89.2%) under sunlight irradiation in 80 minutes were observed among the 13 screened harmful dyes. The detection limits for dyes were found to be 0.39 ppm (MV), 0.57 ppm (Rh6G), 0.58 ppm (RB), 0.88 ppm (CR) and 1.02 ppm (CV). It was found that KA@S is an excellent luminescent sensor for Cr₂O₇²⁻ anions and Hg²⁺ metal ions in aqueous solution at the ppm level among various probed cations and anions. The minimum detection limit (LOD) of 2.62 × 10⁻⁶ M (0.77 ppm) for Cr₂O₇²⁻ and 4.81 × 10⁻⁷ M (0.096 ppm) for Hg²⁺ metal ions were observed.

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1. Introduction

Non-covalent interactions are useful in crystal engineering, molecular identification, catalysis, drug designing, and self-assembly because they play an important role in cooperative effects in several macromolecular systems.^1–4 Non-covalent interactions in inorganic–organic hybrids disclose ferroelectricity, magnetism, optical properties, heterogeneous catalysis^5–7 and other traits due to the design ability and controllable characteristics of the inorganic and organic constituents.^8–10 Physical, chemical, and biological systems can all benefit from these non-covalent interactions. These interactions are critical for maintaining the structure of essential bio-macromolecules including DNA and proteins.^11–16 When using hydrogen bonds in molecular recognition, whether for constructing supramolecular assemblies or developing receptors, it is necessary to have a good sense of desired geometries and energetics to help with the design process. The potential of halogens to act as hydrogen bond accepters has attracted a lot of attention recently. Halide ligands with metals (M–X) have been demonstrated to be excellent hydrogen bond acceptors. Supramolecular synthons that take advantage of the directed capabilities of metal halides to generate hydrogen bonds can be utilized to modulate molecular building blocks.¹⁷ Flexible bridging ligands with alkyl spacers are good candidates as N-donor ligands because the flexible spacers allow the ligands to rotate and bend while coordinating, allowing structural variations. Furthermore, nitrogen-containing heterocycles generate hydrogen bonds easily and support the self-assembly of supramolecular structures. Non-covalent interactions including π–π stacking, hydrogen bonding, and van der Waals contacts can also have an impact on the final network structure and stability.¹⁸

One of the biggest issues faced by human society today is environmental degradation, which puts our ecological and physical well-being in grave peril. The textile, printing, paper, food, cosmetic, and pharmaceutical industries all use carcinogenic organic dyes, which are highly widespread organic contaminants. Additionally, although organic dyes by themselves are safe when released into water from various sources, they typically deplete the dissolved oxygen and result in a wide range of additional environmental problems.^19–22 Photocatalytic degradation is therefore thought to be a potential method for removing dangerous impurities, non-biodegradable toxins, environmental and agricultural pollutants, and pollutants from wastewater.^23–25

The global situation caused by heavy metal ion contamination has gotten worse in recent years. In particular, the quick detection of organic small molecules and heavy metal ions has garnered a lot of interest. Heavy metal ions are not easily broken down like organic contaminants; therefore, they build up in the environment and in people. Toxic ions can affect the biological environment and the human body even at low quantities.^26,27 As is well known, the Pb²⁺ ion is one of the most harmful heavy metal ions. It is frequently found in batteries, fuel, and pigments, all of which are used in our daily lives. The kidneys, neurological system, reproductive system and brain cells will all suffer damage after it enters the human body.²⁸ The Environmental Protection Agency (EPA) recommends Pb²⁺ ion concentrations in drinking water not to exceed 0.015 ppm due to its widespread concern as a prevalent contaminant.^29,30 Arsenic (As) intoxication is a serious global environmental stress, according to perceptible study. Arsenic typically coexists in a number of oxidation forms, with As³⁺ being the most toxic to humans in an aerobic environment.^31–33 Human exposure to As³⁺ increases the risk of bladder, liver and lung cancer and is primarily brought on through contaminated food and drinking water. In addition, prolonged As³⁺ exposure results in arsenicosis, a skin disorder.^34–38 The maximum contamination level (MCL) for As in drinking water was reduced by the U.S. EPA and the WHO from 50 to 10 ppb in 2001 due to concerns around As exposure.³⁹ The research to create novel techniques for monitoring As was also motivated by the decreased MCL. Due to their potential risk to human health and the environment, highly sensitive and selective determination of heavy metal ions like Cd²⁺, As³⁺, Pb²⁺, and Hg²⁺ has drawn a lot of attention in recent years. Hg²⁺, one of the most pervasive hazardous substances in the environment, is thought to pose a major threat to human health. Cells in the brain, kidney, and central nervous system become dysfunctional due to the high affinity of Hg²⁺ for thiol groups in proteins and enzymes.^40–42 Cr₂O₇²⁻ ions are a common oxidant in industry and agriculture. As a result of their extensive use in industrial processes and exceptional solubility in water, these oxo-anions are severely contaminating the environment and water sources. We should pay close attention to the harm Cr₂O₇²⁻ ions do to the environment and to people's health.^43–45 Therefore, selective, time-saving and high sensitivity Pb²⁺ and Cr₂O₇²⁻ ion detection is crucial for human health and environmental science.⁴⁶ Additionally, procaterol and sodium bicarbonate (NaHCO₃) have an impact on myoclonus and metabolic alkalosis development as well as the management of hypertension. The human body's serum pH can be easily adjusted by HCO₃⁻, and procaterol combined with HCO₃⁻ will cause metabolic alkalosis and myoclonus by sharply raising the pH.^47,48

With Bimb and orotic acid potassium salt (OAK), we attempted to build mixed ligand cadmium coordination polymers, but instead produced an inorganic–organic hybrid. In this situation, the OAK utilised did not engage in the reactions. Bimb was unable to coordinate with the metal centre due to protonation of two imidazolyl nitrogen atoms in Bimb, which may have occurred in an acidic environment. The same hydrogen attached to imidazolyl nitrogen atoms participated in non-covalent interactions with coordinated iodides. The Bimb ligand can exist in both gauche and anti-periplanar confirmations and act as a highly flexible ligand because of the presence of active methylene (–CH₂–) groups. This work looked into the possibility of self-assembling inorganic–organic hybrids and the weak intermolecular interactions of Cd₄²⁻ with protonated Bimb cations derived from Bimb since these complexes can form stable supramolecular structures. We employ transition metal Cd(II) iodide salt and a highly flexible aromatic imidazole-based N-donor ligand 1,4-bis[(1H-imidazol-1-yl)methyl]benzene (Bimb) and orotic acid potassium salt (OAK) as the ligand to assemble a new non-covalent complex, [CdI₄]²⁻·[H₂Bimb]²⁺ (KA@S), and report the results of its study by VT-PXRD, FT-IR spectroscopy and variable temperature luminescence. Photocatalytic effectiveness of KA@S toward the degradation of organic dyes such as Methyl Violet (MV), Rhodamine 6G (Rh6G), Rose Bengal (RB), Congo Red (CR) and Crystal Violet (CV) in contaminated water has also been explored in detail. Additionally, the fluorescence sensing capabilities of KA@S for the detection of hazardous anions (HCO₃⁻, CO₃²⁻, SO₄²⁻ and Cr₂O₇²⁻) as well as heavy metal ions (As³⁺, Pb²⁺, and Hg²⁺) were thoroughly studied along with other cations and anions.

2. Experimental section

2.1 Materials and physical measurement

All reagent grade chemicals, solvents, orotic acid potassium salt, 1,4-bis[(1H-imidazol-1-yl)methyl]benzene, methanol, H₂SO₄ and CdI₂ were commercially available and used with no further purification. All chemicals utilized in this work are more than 99% pure. Single crystal X-ray diffraction was conducted using an XtaLAB Synergy, Dualflex, HyPix3000 diffractometer, fitted with a graphite monochromator and Mo-Kα radiation (λ = 0.71073 Å). A PerkinElmer 2400 Series II element analyzer was used for C, H and N analysis of KA@S. Powder X-ray diffraction (PXRD) experiments were performed on a PANalytical X’Pert PRO instrument with Bragg–Brentano geometry. FT-IR spectra of the prepared pure solid sample were recorded in the range of 4000–400 cm⁻¹ with an FT-IR Spectrometer (Bruker Optics, GmBH, Germany). To evaluate the thermal stability of the material, a thermal analyzer TG-DTA 7200 (Hitachi, Japan) was used. Solid state luminescence was recorded using an RF-5301PC spectrofluorophotometer (Shimadzu). Photocatalytic degradation was performed using a UV-1780 UV-vis spectrophotometer (Shimadzu) with a 190–1100 nm wavelength range and a photometric range of −0.5–3.0 A. Fluorescence emission spectra were recorded on an Agilent Cary Eclipse Fluorescence spectrophotometer.

2.2 Photocatalytic degradation activity

Photocatalytic degradation of MV, Rh6G, RB, CR and CV was studied using KA@S in aqueous solution under sunlight. The dyes were dissolved in distilled water and the concentration of dyes used was 10 mg L⁻¹. For the effective photocatalytic degradation, 6 mg of KA@S was dispersed into 8 mL dye aqueous solution (10 mg L⁻¹), separately. Before the light irradiation, the resultant solution was equilibrated between KA@S and dye molecules by stirring in the dark for 30 minutes. To evaluate the photodegradation performance, the solution was irradiated under sunlight and taken out in a certain time interval (10 minutes) within 80 minutes. Then the solutions were transferred to a 1 cm quartz cell to record UV-Vis spectra on a UV-1780 UV-vis spectrophotometer (Shimadzu). In the end, the photocatalytic degradation efficiency of the solutions was evaluating using a UV-Vis spectrophotometer.

2.3 Fluorescence sensing experiments

For the fluorescence measurements, a finely ground sample of KA@S (3 mg) was immersed in 4 mL of diverse solvents (water (H₂O), dimethylformamide (DMF), dichloromethane (DCM), ethanol (EtOH) and methanol (MeOH)), treated by ultrasonication for 25 minutes and aged for 24 hours to provide stable suspensions. All emission spectra of the suspensions were recorded at room temperature in the range 200–600 nm upon excitation at 340 nm. The stable suspension of KA@S displayed diverse fluorescence intensities depending on solvent molecules. The water suspension of KA@S displayed high emission intensity at 376 nm upon excitation at 340 nm. Further the fluorescence experiments were carried out in aqueous solution of different anions and metal ions. The finely ground sample of KA@S (3 mg) was added into a glass vial containing 4 mL of urea and different K_nX (X = MnO₄⁻, Cl⁻, Br⁻, I⁻, CO₃²⁻, and Cr₂O₇²⁻) and Na_nX (F⁻, N₃⁻, ClO₄⁻, NO₂²⁻, HCO₃⁻, and SO₄²⁻ anions) aqueous solutions with a concentration of 5 × 10⁻⁴ M. The powder of KA@S (3 mg) was immersed in 4 mL of different M(NO₃)_x aqueous solutions with a concentration of 5 × 10⁻⁴ M (M = Co²⁺, Cu²⁺, Cd²⁺, Zn²⁺, Fe³⁺, Ni²⁺, La³⁺, Gd³⁺, As³⁺, Pb²⁺, and Hg²⁺, respectively). Further, the mixed solutions were treated through ultrasonication for 25 minutes, and used for the measurements of fluorescence recognition. Eqn (1) is applied to calculate the fluorescence quenching efficiency (Q). Moreover, the Stern–Volmer (S–V) eqn (2) is employed to estimate the process of fluorescence quenching.

Q (%) = {(I₀ − I)/I₀} × 100 (1)

I₀/I = 1 + K_SV[M] (2)

where I₀ and I are the fluorescence emission intensities before and after the addition of anions or metal ions, respectively, K_SV is the quenching constant and [M] is the molar concentration of anions or metal ions.^49,50

2.4 Synthesis of [CdI₄]²⁻·[H₂Bimb]²⁺ (KA@S)

In a conical flask CdI₂ (0.082 mmol, 30 mg) was dissolved in 1 mL of H₂O and the pH was adjusted to 2 with 0.2 mL of 1 M H₂SO₄. Over this, 4 mL of a hot H₂O/CH₃OH (v/v, 3 : 1) solvent mixture containing orotic acid potassium salt (0.077 mmol, 15 mg) and 1,4-bis[(1H-imidazol-1-yl)methyl]benzene (0.063 mmol, 15 mg) were added without stirring. The reaction mixture was retained for slow evaporation at ambient temperature for crystallization. One month later, colourless block-like crystals of KA@S suitable for X-ray diffraction were obtained in 54% yield based on Cd. Anal. calcd (%) for C₁₄H₁₆I₄N₄Cd: C, 19.54; H, 1.87; N, 6.51. Found (%): C, 19.35; H, 1.86; N, 6.48. FT-IR (cm⁻¹, KBr): 3436 w, 3134 m, 1568 s, 1440 s, 1273 m, 1146 m, 1085 s, 746 m, 715 m, 615 m, 475 m, 469 w, 459 w, 446 w, 440 w, 432 w, 425 w, 415 w, 405 w.

2.5 Crystal structure determination

Single crystals of KA@S were placed in a nylon loop. A suitable crystal was selected and measurements were carried out using a XtaLAB Synergy, Dualflex, HyPix3000 diffractometer at ambient temperature. During the data collection process, the crystal was kept at 100(2) K. Using Olex2,⁵¹ the structure was solved with the olex2.solve⁵² structure solution program with charge flipping and refined with the olex2.refine⁵² refinement package using Gauss–Newton minimization. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon atoms were placed in calculated positions and refined isotropically with a riding model. The detailed crystal data and structure refinement result of KA@S are given in Table 1. All selected bond lengths and angles are summarised in Table S1 (ESI†).

Table 1 Crystallographic and structural refinement parameters for KA@S

Empirical formula	C14H16CdI4N4
Formula weight	860.34
Temperature/K	100(2)
Crystal system	Triclinic
Space group	P1̄
a/Å	9.827(2)
b/Å	10.722(3)
c/Å	10.954(3)
α/°	86.422(12)
β/°	80.972(9)
γ/°	73.540(12)
Volume/Å^3	1093.0(5)
Z	2
ρ calc/g cm^−3	2.6139
μ/mm^−1	6.651
F(000)	771.4
Crystal size/mm^3	0.49 × 0.31 × 0.17
Radiation	Mo Kα (λ = 0.71073)
2θ range for data collection/°	5.34 to 50.1
Index ranges	−13 ≤ h ≤ 13, −14 ≤ k ≤ 14, −14 ≤ l ≤ 14
Reflections collected	16 942
Independent reflections	3853 [Rint = 0.0574, Rsigma = 0.0537]
Data/restraints/parameters	3853/0/202
Goodness-of-fit on F^2	1.042
Final R indexes [I >= 2σ(I)]	R 1 = 0.0381, wR2 = 0.0990
Final R indexes [all data]	R 1 = 0.0400, wR2 = 0.1008
Largest diff. peak/hole/e Å^−3	1.27/−2.33
CCDC number	2 072 960

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3. Results and discussion

3.1 Crystal structure analysis

Single-crystal X-ray structure analysis showed that the asymmetric unit consists of a distorted tetrahedral geometry of a Cd(II) ion and a bi-protonated Bimb molecular ion of complex [CdI₄]²⁻·[H₂Bimb]²⁺ (KA@S). The KA@S crystallized in the triclinic space group P1̄, with one Cd1 ion surrounded by four iodine atoms with the bond distance (Cd1–I) ranging from 2.7730(8) to 2.8032(10) Å, which is comparable to that previously reported in the literature.⁵³ The cadmium center is in a somewhat distorted tetrahedral geometry with bond angles of I–Cd–I in the range 105.09(2) to 113.37(2)°, which are similar to those in an ideal tetrahedral geometry, Bimb molecules get protonated with two protons situated in gauche configurations (Fig. 1). A polyhedral view of the crystal lattice displays 3D packing of KA@S (Fig. 2). The following is a description of KA@S: halide enhances supramolecular stability through hydrogen bond interactions.^18,54 In KA@S, N–H groups of the protonated Bimb cations engage with tetraiodocadmate(II) anions through hydrogen bonding interactions. As a result of the availability of hydrogen bond donors in the form of N–H and C–H the existence of four iodine atoms in each Cd₄²⁻, bi-acceptors and tri-accepters (I) is very likely to be found in these complexes. In the absence of additional molecules, such as solvent molecules, the crystalline cell interacts with the organic component through hydrogen bonds. This fact suggests that supramolecular chains run along the crystallographic “a” axis in KA@S. The dimensionality of the KA@S is extended through the excess of hydrogen bond acceptor iodine atoms of Cd₄²⁻ and excess of hydrogen donor atoms of Bimb ligand via C–H⋯I and N–H⋯I interactions. The adjacent Cd₄²⁻ are bridged by the Bimb ligand to form 1D supramolecular chains running along a direction through C11–H11b⋯I4; 116.6(7) Å, C12–H12⋯I1; 144.1(7) Å, with graph set R₂² (8) and N4–H4⋯I2; 161.9(7) Å, C13–H13⋯I4; 122.2(7) Å, interactions with graph set R₂² (7) interactions (Fig. 3). These supramolecular chains are further linked by tri-acceptors I4 with the Bimb ligand through C14–H14⋯I4; 157.8(8) Å, interactions that furnish a graph set R₄² (12) to form a 2D supramolecular layer structure; here, I4 acts as a tri-acceptor (Fig. 4). In the 3D supramolecular network, adjacent supramolecular synthons are connected with the Bimb ligand via three C1–H1a⋯I3; 150.8(8) Å, N1–H1⋯I3; 156.2(6) Å, with graph set R₂² (10) and C6–H6⋯I1; 79.0(6) Å, hydrogen bond generating the centrosymmetric motif; here, I3 acts as a bi-acceptor (Fig. 5). All hydrogen bond parameters for KA@S are given in Table S2 (ESI†).

Fig. 1 Assymetric unit and coordination environments of Cd ions in KA@S.

Fig. 2 A polyhedral view of the crystal lattice displaying 3D packing of KA@S (hydrogen atoms are omitted for clarity).

Fig. 3 View of the supramolecular 1D chain via tri-acceptors I4 with C–H⋯I, (8) and N–H⋯I, R₂² (7) in KA@S along with the crystallographic “a” axis.

Fig. 4 View of the supramolecular 2D structure via tri-acceptors I4 with C14–H14⋯I4, R₄² (12) in KA@S along with the crystallographic “a” axis.

Fig. 5 View of the supramolecular 3D network via bi-acceptors I3 with C–H⋯I and N–H⋯I, R₄² (10) of KA@S along with the crystallographic “a” axis.

3.2 FT-IR spectra analysis

FT-IR spectra of the prepared pure solid sample in the range of 4000–400 cm⁻¹ have been recorded with an FT-IR spectrometer (Bruker Optics, GmBH, Germany). The FT-IR spectrum of KA@S is shown in Fig. 6; the broad, medium bands observed at ∼3436 cm⁻¹ are assigned to the ν(N–H) stretching band of the protonated Bimb molecule. Strong bands appearing at ∼3134 cm⁻¹ are assigned to aromatic ν(C–H) stretching vibrations. At ∼1568, ∼1273 and ∼1440 cm⁻¹, stretching vibrations can be assigned to ν(C=C) and ν(C–N) and bending vibrations of the (CH) group of organic compounds H₂Bimb²⁺ are seen. The IR spectra of the compounds are different in the 700–800 cm⁻¹ region, in which the distinctive bands of out-of-plane bending vibrations of the unsubstituted protons of the ring occur.⁵⁵ The spectrum of KA@S shows two bands at 746 and 715 cm⁻¹.

Fig. 6 FT-IR spectrum for as-synthesized KA@S.

3.3 Thermogravimetry analysis

The thermal stability and behaviours of the prepared [CdI₄]²⁻·[H₂Bimb]²⁺ (KA@S) are investigated under a nitrogen (N₂) atmosphere in the temperature range 40–800 °C with a heating rate of 10 °C min⁻¹ by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and derivative thermogravimetry (DTG). The thermal behaviour of the KA@S is represented graphically in Fig. 7. The thermogravimetric study of KA@S starts with chemical decomposition at around 263 °C, and ends at about 384 °C with the weight loss of 27% (calcd: 27.69%), which shows that it corresponds to the removal of the Bimb molecule. Thereafter, the weight loss occurs continuously with another weight loss of approximately 58% (calcd: 58.99%) from 385 to 600 °C, corresponding to the complete removal of iodide anions, and the decomposition of the whole complex occurs at about 800 °C. The DTA curve of KA@S reveals two endothermic peaks at 209 and 223 °C attributed to the decomposition of H₂ attached with the Bimb molecule as well as two distinct exothermic peaks at 338 and 411 °C, which correspond to the major decomposition of the complex.

Fig. 7 TG-DTA-DTG curves of KA@S.

3.4 VT-PXRD spectra

Variable temperature powder X-ray diffraction (VT-PXRD) experiments were performed on a PANalytical X’Pert PRO instrument with Bragg–Brentano geometry. Intensity data were collected using an X’Celerator detector and 2θ scans were performed in the range of 5–50°. During the experiment, the powdered sample was subjected to Cu-Kα (λ = 1.5418 Å) radiation.

The PXRD patterns of the as-synthesized crystalline product KA@S were recorded in order to examine its phase purity by comparing them to the corresponding simulated spectra measured from the single-crystal X-ray diffraction data. The close agreement of the different peak positions between the simulated and as-synthesized (27 °C) PXRD patterns indicates the high phase purity of the compounds. When heating the complex at 90 °C, there is no reasonable change in spectra observed but when heating the complex at higher temperature about 180 °C and 300 °C, changes in PXRD patterns are observed. These peaks disappear when the temperature is raised over 90 °C. The variation noticed in PXRD patterns at higher temperature could be owing to the deformation of the complex from its original structure. The overall VT-PXRD patterns of KA@S recorded from 90 to 300 °C along with experimental and simulated patterns are shown in Fig. 8.

Fig. 8 Powder X-ray diffraction patterns for KA@S before and after heating at variable temperature.

3.5 VT-photoluminescence properties

The photoluminescence properties of KA@S were investigated as-synthesized (27 °C) and after variable heating of the KA@S at different temperatures (90 °C, 180 °C and 300 °C) in the solid-state at ambient temperature. Fig. 9 shows that the as-synthesized form KA@S shows luminescent emission at 444 nm when excited at 380 nm and KA@S exhibits luminescence emission at 443 nm, 450 nm and 444 nm when excited at 380 nm after heating of the crystal at 90, 180 and 300 °C, respectively. The emission wavelengths of all are close to each other but their intensity decreases continuously and very low intensity occurs in (III) after heating at 300 °C. This may be due to the quenching of luminescence intensities at the higher temperature. Furthermore, the emission characteristics of the free ligand Bimb were investigated in the solid-state at ambient temperature. As shown in Fig. 9, the Bimb ligand exhibits luminescence with maximum emission at 438 nm when excited at 360 nm. Notably, the emission spectrum of KA@S is close to the emission spectrum of the free Bimb ligand. The emission bands of this hydrogen-bonded complex are currently neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT), as the Cd(II) ions with the d¹⁰ configuration are hard to oxidize or reduce. Regarding the luminescence activity of the hydrogen-bonded complex, there is the possibility of intraligand or ligand-to-ligand charge transfer (LLCT) or n → π* & π → π* emissions^56–59 resulting in red shift as compared to the free Bimb ligand.

Fig. 9 (a) The luminescence emission spectra of Bimb ligand and (b) KA@S in the solid-state at ambient temperature (as-synthesized (27 °C) and after heating at 90, 180 and 300 °C).

4. Photocatalytic activity

4.1 Photocatalytic degradation of dyes

To study the photocatalytic properties of the as-prepared pure KA@S samples, the degradation of Methyl Violet (MV), Rhodamine 6G (Rh6G), Rose Bengal (RB), Congo Red (CR), Crystal Violet (CV), Neutral Red (NR), Rhodamine B (RhB), Methylene Blue (MB), Eriochrome Black T (EBT), Safranin O (SO), Fluorescein (FS), Bromophenol Blue (BPB) and Methyl Orange (MO) as model dyes was studied by irradiation with visible sunlight. In particular, important factors including catalyst size, crystal structure, morphology and composition concentration affected the activity of photocatalysts. By monitoring the changes in their characteristic UV-Vis absorption spectra upon exposure to sunlight, the photocatalytic degradation efficiency assessed by the decolourisation of model organic contaminants was determined. Time-dependent UV-Vis absorption spectra of dyes MV, Rh6G, RB, CR and CV solution with the KA@S catalyst under sunlight irradiation are demonstrated in Fig. 11 and UV-Vis absorption spectra of the remaining dyes are shown in Fig. S2 and S3 (ESI†). The chemical structures of the highest degraded dyes and remaining dyes are shown in Fig. 10 and Fig. S1 (ESI†).

Fig. 10 Chemical structures of organic dyes used which shows the highest degradation efficiency.

Fig. 11 Time-dependent UV-Vis absorption spectra of dyes (a) MV, (b) Rh6G, (c) RB, (d) CR and (e) CV solution with the KA@S catalyst under sunlight irradiation, respectively.

The degradation efficiency was calculated by using eqn (3):

Degradation % = {(C₀ − C_t)/C₀} × 100 (3)

where C₀ and C_t are the initial concentration and concentration at time t of aqueous dye solution. The degradation efficiency was determined using eqn (1). The obtained photocatalytic degradation efficiencies for MV, Rh6G, RB, CR and CV are shown in Fig. 13a and remaining dye are shown in Fig. S4c (ESI†).

The catalyst KA@S achieved the degradation efficiency of about 96.5% for MV, 94.4% for Rh6G, 93.5% for RB, 91.2% for CR and 89.2% for CV under irradiation with visible sunlight for 80 minutes. The photocatalytic degradation kinetic plot of ln(C₀/C_t) versus time interval and C_t/C₀versus time interval plot are presented in Fig. 12 (Fig. 12a–f). The photocatalytic degradation kinetic plot of C_t/C₀versus time interval plot and ln(C₀/C_t) versus time interval are presented in Fig. S4a and b (ESI†).

Fig. 12 Photocatalytic degradation kinetic plot for dyes: (a–e) plot of ln(C₀/C_t) versus time interval and (f) C_t/C₀versus time interval plot.

For the degradation of organic dyes in aqueous solution, the pseudo-first-order kinetic rate equation is widely utilized.⁶⁰ The pseudo-first order kinetic model can be described by eqn (4):

ln(C₀/C_t) = kt (4)

where C₀ and C_t are the initial concentration and concentration at time t of aqueous dye solution, and k is the first-order rate constant of reaction. A straight line is produced by plotting ln(C₀/C_t) vs. time, and its slope is equal to the first-order rate constant (k). The computed values of rate constant were highest for MV (0.0404 min⁻¹) and lowest for CV (0.0284 min⁻¹). Eqn (5) can be used to get the half-life time (t_1/2) of the reaction:

t_1/2 = ln 2/k (5)

It appears that MV degrades more quickly under the same photocatalytic circumstances because the MV degradation reaction had a significantly lower half-life than the other dyes used in the reaction process. The kinetic parameters for the photocatalytic degradation of dyes on the catalyst KA@S are given in Table 2.

Table 2 Kinetic parameters for photocatalytic degradation of dyes on KA@S under sunlight irradiation

Catalyst	Dye	Degradation (%)	R ^2	Rate constant (k, min^−1)	Half-life (t1/2, min)
KA@S	MV	96.5	0.8769	0.04045	17.13
	Rh6G	94.4	0.9191	0.03715	18.4
	RB	93.5	0.6561	0.03647	19.0
	CR	91.2	0.9106	0.03029	22.87
	CV	89.2	0.9159	0.02846	24.34

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4.2 Catalyst reusability and stability

The recyclability of the catalyst KA@S was tested after the related dyes had completely deteriorated. The catalyst KA@S was recovered by filtration, washed several times, dried, and then reused again. This was done five times and the result of the KA@S reusability in terms of degradation efficiency (%) for each cycle is shown in Fig. 13b. The results demonstrated a mild drop in degradation efficiency over the five cycles. Additionally, after five cycles (Fig. 14) and after being immersed into the solution with a different pH for over 24 hours (Fig. S5, ESI†), the PXRD pattern shows no changes in the stability and structure of reusable KA@S.

Fig. 13 (a) Degradation efficiency of the dyes with catalysts and (b) reusability of KA@S-catalyzed dyes under visible sunlight irradiation for 80 minutes.

Fig. 14 PXRD patterns of KA@S before and after photocatalytic reaction with dyes (MV, Rh6G, RB, CR and CV).

4.3 Photocatalytic mechanism for the degradation of dyes

Prior to the catalysis experiments, we used a UV-Vis spectrophotometer at room temperature to determine the band gap energy for the compound KA@S. To determine the band gap energy (E_g) for the catalyst KA@S, the Kubelka-Munk method and Tauc plot (Fig. 15b) and UV-Vis absorption spectra were used (Fig. 15a).⁶¹ Band gap was found to be 3.39 eV for KA@S. The band gap energy implied that the compound KA@S might be a photocatalytically active material. In general, a smaller E_g value is better for catalysis since it is more conducive to electron transition.^62,63

Fig. 15 (a) UV-Vis absorption spectra of compound KA@S and (b) Tauc plot ((αhν)²vs. energy (eV)) for band gap study of KA@S.

A plausible KA@S catalyzed photodegradation pathway under exposure to visible sunlight is shown in Scheme 1. The electrons (e⁻) in the valence band (VB) were stimulated to a higher-energy conduction band (CB) along with the holes (h⁺) of the corresponding catalysts. The catalyst was treated under visible light; e⁻ was transferred from the VB to the CB via an appropriate band gap energy (E_g), resulting in the generation of electron-deficient h⁺. The surface of the active catalyst included the e⁻–h⁺ pair, which was captured by oxygen and water molecules, along with the corresponding dye molecules.^64,65 H₂O molecules in the VB section were attached and tricked into becoming highly reactive hydroxyl (OH˙) radicals. Superoxide (O2˙⁻) radical anions were produced by the simultaneous interaction of dissolved molecular oxygen present in H₂O molecules with the electrons (e⁻) in the CB of the corresponding catalyst. These radicals were then permitted to interact with H₂O molecules to produce OH˙ radicals, which have potent oxidizing properties. The presence of such a strong oxidizing agent caused the associated dyes to oxidise and turn colorless.^66,67

Scheme 1 Plausible mechanism of the photocatalytic degradation of dyes using the KA@S catalyst.

4.4 Capture study

Capture study was carried out by utilizing a UV-Vis spectrophotometer. According to the UV-Vis results, KA@S swiftly eliminated various dyes from the polluted water samples, including MV (0.39 ppm), Rh6G (0.57 ppm), RB (0.58 ppm), CR (0.88 ppm) and CV (1.02 ppm). From 0.39 ppm to 1.02 ppm concentration of detection limit, clearance efficiencies of more than 90% of the dyes indicated were achieved in 80 minutes. Fig. 16 depicts the capture study for KA@S with an 80 minutes contact time. Despite this, all of the colours selected are regarded as among the most dangerous and frightening water toxins due to their negative effects on both human health and the ecosystem at large (Table 3).

Fig. 16 The degradation efficiency/concentration of KA@S with dyes (a) MV, (b) Rh6G, (c) CR, (d) RB and (e) CV as a function of contact time (80 minutes).

Table 3 Comparison of photocatalytic degradation of dyes by KA@S and other known photocatalysts

Catalyst	Dye	Condition	Time	DE (%)	Ref.
[Cu2(L1)·5DMF],	MV	UV, 500 ppm	45 min	70.0	68
{[Co3(BTC)2(Bimb)2.5]·2H2O}n	CV	UV, 10 ppm	135 min	53.9	69
	MV			68.7
[Zn(bpe)(fdc)]·2DMF	CV	UV-Hg, 5 ppm	120 min	92.5	70
{[Zn(PMBD)(DPB)]·DPB}n	Rh6G	Hg, 10 ppm	90 min	75.8	71
{[Cu(L-NO2)(H2O)]·0.25(CH3CN)}n	R6G	UV, 20 μM	40 min	92.1	72
Zn-U3	RB	UV, 5 ppm	4 h	93.0	73
[Cd5L2(CH3COO)6]	CR	Visible, 25 ppm	150 min	90.0	74
KA@S	MV	Sun light, 10 ppm	80 min	96.5	This work
	Rh6G			94.4
	RB			93.5
	CR			91.2
	CV			89.2

---

5. Fluorescence behavior and sensing properties

5.1 Detection of anions

The relative fluorescence emission intensities of KA@S in the presence of different organic solvents are shown in Fig. 17. To study the potential of KA@S for detection of urea and anions, aqueous solutions containing 5 × 10⁻⁴ M of different K_nX (X = MnO₄⁻, Cl⁻, Br⁻, I⁻, CO₃²⁻, and Cr₂O₇²⁻) and Na_nX (F⁻, N₃⁻, ClO₄⁻, NO₂²⁻, HCO₃⁻, and SO₄²⁻ anions) were taken. The relative fluorescent emission intensities of KA@S in aqueous solutions (5 × 10⁻⁴ M) of different inorganic anions are demonstrated in Fig. 18. The quenching efficiencies of anions were calculated to be 11.5% (urea), 31.7% (F⁻), 35.4% (N₃⁻), 36.5% (MnO₄⁻), 59.8% (ClO₄⁻), 72.7% (NO₂²⁻), 72.7% (I⁻), 74.6% (Cl⁻), 74.9% (Br⁻), 79.7% (HCO₃⁻), 80.4% (CO₃²⁻), 91.6% (SO₄²⁻) and 99.4% (Cr₂O₇²⁻) (Fig. 19). It is interesting that HCO₃⁻ (79.7%), CO₃²⁻ (80.4%), SO₄²⁻ (91.6%) and Cr₂O₇²⁻ (99.4%) cause the maximum quenching for the fluorescence intensity compared to other anions, indicating a high selectivity for detecting HCO₃⁻, CO₃²⁻, SO₄²⁻ and Cr₂O₇²⁻ ions among the all anions screened. A quantitative fluorescent titration experiment was performed to evaluate the sensing sensitivity for Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻ in an aqueous medium. Aqueous solutions (1 mM) of Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻ were added dropwise to a stable KA@S dispersed solution, and the fluorescence intensity was measured after stabilization, separately. The reduced fluorescence intensity could be significantly noticed when the concentration of anions was gradually decreased (Fig. 20). Furthermore, the quenching constant was calculated according to the Stern–Volmer (S–V) equation: I₀/I = 1 + K_SV [M]. On the basis of the experimental data, linear correlation coefficient for Cr₂O₇²⁻ is R² = 0.9923, for SO₄²⁻R² = 0.983, for CO₃²⁻R² = 0.9558 and for HCO₃⁻R² = 0.9909 and the calculated values of K_SV at low concentration are 4.22 × 10⁴ M⁻¹ for Cr₂O₇²⁻, 2.56 × 10⁴ M⁻¹ for SO₄²⁻, 1.98 × 10⁴ M⁻¹ for CO₃²⁻ and 1.63 × 10⁴ M⁻¹ for HCO₃⁻. The quenching plot (Fig. 21–24) indicates that the quenching rate shows a linear relationship at low concentrations. The minimum detection limits are 2.62 × 10⁻⁶ M (0.77 ppm), 3.83 × 10⁻⁶ M (0.37 ppm), 6.47 × 10⁻⁶ M (0.39 ppm) and 2.91 × 10⁻⁶ M (0.18 ppm) for Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻, respectively, calculated by the formula LOD = 3σ/m (σ: standard error; m: slope).⁷⁵ Thus, KA@S can be used as a highly sensitive fluorescence sensing material for the quantitative detection of Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻ anions.

Fig. 17 Fluorescence emission intensities of KA@S in the presence of different organic solvents.

Fig. 18 Fluorescence emission intensities of KA@S in aqueous solutions (5 × 10⁻⁴ M) of different inorganic anions.

Fig. 19 Relative fluorescence quenching efficiencies of KA@S dispersed in different inorganic anion aqueous solutions.

Fig. 20 Fluorescence emission intensities when (a) Cr₂O₇²⁻, (b) SO₄²⁻, (c) CO₃²⁻ and (d) HCO₃⁻ are added dropwise to the aqueous solution of KA@S separately.

Fig. 21 S–V plot of KA@S dispersed in water after a gradual addition of Cr₂O₇²⁻ (1 mM). The linear S–V curve of KA@S at low concentration of Cr₂O₇²⁻ (inset).

Fig. 22 S–V plot of KA@S dispersed in water after a gradual addition of SO₄²⁻ (1 mM). The linear S–V curve of KA@S at low concentration of SO₄²⁻ (inset).

Fig. 23 S–V plot of KA@S dispersed in water after a gradual addition of CO₃²⁻ (1 mM). The linear S–V curve of KA@S at low concentration of CO₃²⁻ (inset).

Fig. 24 S–V plot of KA@S dispersed in water after a gradual addition of HCO₃⁻ (1 mM). The linear S–V curve of KA@S at low concentration of HCO₃⁻ (inset).

5.2 Recyclability and reusability

Fluorescence emission spectra of KA@S in aqueous solution were recorded to assess the accuracy of KA@S as a fluorescent sensor for Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻ anions. After being washed with water, the recyclable nature of the KA@S for Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻ ions was determined. The outcomes of the experiment validate our measurements and demonstrate that the KA@S is stable in water (Fig. S8, ESI†). As demonstrated in Fig. S7a (ESI†), luminescence intensity of KA@S may be reused for at least four cycles. The PXRD patterns of this sample after four recycling tests are still in close agreement with those of the as-synthesized KA@S, implying that the KA@S is pure and retains its original structure.

5.3 The possible mechanism of fluorescence quenching

According to prior investigations, the processes of sensing anions and metal ions are typically attributed to the collapse of the framework, competitive absorption/interaction, and ion exchange between the complex and different ions.^76–78 The potential sensing mechanism for fluorescence quenching by Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻ ions was further studied. PXRD was performed and the results showed that the structure of KA@S could still be considered intact after sensing of Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻ ions (Figs. S8, ESI†). Consequently, the fluorescence quenching is not caused by the breakdown of the structure of KA@S.^79,80 The decrease in energy transfer between the π and π* orbitals of the N-containing ligands as a result of the electron-transfer transitions of the Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻ ions may also contribute to the shift in fluorescence intensity. Alternatively, the energy-transfer quenching mechanism was proposed. The probability of resonance energy transfer is influenced by the degree of spectral overlap between the absorption bands of analytes (anions) and excitation bands of the fluorescence sensors (KA@S). Fig. S6a (ESI†) demonstrates that the absorption spectra of Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻ exhibit significant overlaps with the excitation spectra of KA@S, respectively. However, there was little to no overlap observed for the other analytes.⁸¹ Cr₂O₇²⁻ has a greater quenching property because it exhibits the highest degree of spectral overlapping with the excitation spectra of KA@S. Based on these findings, the overlaps with the UV-Vis spectra of KA@S indicate that Cr₂O₇²⁻, SO₄²⁻, CO₃²⁻ and HCO₃⁻ compete with the organic ligands for the obvious competitive absorption of excitation wavelength energy, which causes the quenching effect.^82,83

5.4 Detection of metal ions

To study the potential of KA@S for detection of metal ions, aqueous solutions containing 5 × 10⁻⁴ M of different M(NO₃)_x (M = Co²⁺, Cu²⁺, Cd²⁺, Zn²⁺, Fe³⁺, Ni²⁺, La³⁺, Gd³⁺, As³⁺, Pb²⁺ and Hg²⁺) were taken. The relative fluorescent emission intensities of KA@S in aqueous solutions (5 × 10⁻⁴ M) of different inorganic metal ions are demonstrated in Fig. 25. The Equenching efficiencies of metal ions were calculated to be 6.5% (Co²⁺), 7.7% (Cu²⁺), 10.2% (Cd²⁺), 11.0% (Zn²⁺), 11.8% (Fe³⁺), 12.1% (Ni²⁺), 32.7% (La³⁺), 40.3% (Gd³⁺), 76.8% (As³⁺), 82.7% (Pb²⁺) and 90.1% (Hg²⁺) (Fig. 26). It is interesting that As³⁺ (76.8%), Pb²⁺ (82.7%) and Hg²⁺ (90.1%) cause the maximum quenching for the fluorescence intensity compared to the other metal ions (Fig. 26), indicating a high selectivity for detecting As³⁺, Pb²⁺ and Hg²⁺ ions. A quantitative fluorescent titration experiment was conducted to assess the sensing sensitivity of Hg²⁺ in an aqueous medium. Aqueous solutions (1 mM) of Hg²⁺, Pb²⁺ and As³⁺ ions were added dropwise to a stable KA@S dispersed solution separately, and the fluorescence intensity was evaluated after stabilization. The reduced fluorescence intensity could be significantly noticed when the concentration of Hg²⁺, Pb²⁺ and As³⁺ ions was steadily reduced (Fig. 27). On the basis of the experimental data, the linear correlation coefficient for Hg²⁺ is R² = 0.9996, for Pb²⁺R² = 0.9886 and for As³⁺R² = 0.9903 and the calculated value of K_SV at low concentration for Hg²⁺ is 3.0 × 10⁴ M⁻¹, for Pb²⁺ 2.3 × 10⁴ M⁻¹ and for As³⁺ 2.6 × 10⁴ M⁻¹. The quenching plot indicates that the quenching rate has a linear relationship at low concentrations (Fig. 28–30). The minimum detection limits for Hg²⁺, Pb²⁺ and As³⁺ ions are 4.81 × 10⁻⁷ M (0.096 ppm), 2.65 × 10⁻⁶ M (0.55 ppm) and 2.45 × 10⁻⁶ M (0.18 ppm), respectively. Thus, KA@S can be used as a highly sensitive fluorescence sensing material for the quantitative detection of Hg²⁺, Pb²⁺ and As³⁺ ions.

Fig. 25 Fluorescence emission intensities of KA@S in different metal ion aqueous solutions (5 × 10⁻⁴ M).

Fig. 26 Relative fluorescence quenching efficiencies of KA@S dispersed in different metal ion aqueous solutions.

Fig. 27 Fluorescence emission intensities when (a) Hg²⁺, (b) Pb²⁺ and (c) As³⁺ are added dropwise to the aqueous solutions of KA@S separately.

Fig. 28 S–V plot of KA@S dispersed in water after a gradual addition of Hg²⁺ (1 mM). The linear S–V curve of KA@S at low concentration of Hg²⁺ (inset).

Fig. 29 S–V plot of KA@S dispersed in water after a gradual addition of Pb²⁺ (1 mM). The linear S–V curve of KA@S at low concentration of Pb²⁺ (inset).

Fig. 30 S–V plot of KA@S dispersed in water after a gradual addition of As³⁺ (1 mM). The linear S–V curve of KA@S at low concentration of As³⁺ (inset).

5.5 Recyclability and reusability

Fluorescence emission spectra of KA@S in aqueous solution were recorded in order to assess the efficacy of KA@S as a fluorescent sensor for Hg²⁺, Pb²⁺ and As³⁺ metal ion anions. The outcomes of the experiment demonstrate the stability of the KA@S in water and the reliability of our measurements (Fig. S9, ESI†). As demonstrated in Fig. S7b (ESI†), fluorescence intensity of KA@S may be reused for at least four cycles. The PXRD patterns of this sample after four recycling tests are still in great agreement with those of the as-synthesized KA@S, implying that the synthesized KA@S is pure and retains its original structure.

5.6 The possible mechanism of fluorescence quenching

The following studies were carried out to clarify the mechanism of KA@S for sensing of Hg²⁺, Pb²⁺ and As³⁺ ions through quenching effects. The original and added analytes (Hg²⁺, Pb²⁺ and As³⁺ metal ions) of KA@S structures were investigated using PXRD analysis. The PXRD patterns of KA@S soaked in solutions containing Hg²⁺, Pb²⁺ and As³⁺ ions are comparable to those of KA@S, indicating that the integrity of the structure of KA@S could be maintained after sensing of Hg²⁺, Pb²⁺ and As³⁺ ions (Fig. S9, ESI†). As a result, the fluorescence quenching is not led by the collapse of the structure of KA@S.^46,84 The decreasing energy transfer between the π and π* orbitals of the N-containing ligands may account for the change in fluorescence intensity caused by the electron-transfer transitions of the Hg²⁺, Pb²⁺ and As³⁺ ions.^85,86 The extent of spectral overlap between the absorption bands of analytes (metal ions) and excitation bands of the fluorescence sensors (KA@S) determines the probability of resonance energy transfer. Fig. S6b (ESI†) shows that the absorption spectra of Hg²⁺, Pb²⁺ and As³⁺ have significant overlaps with the excitation spectra of KA@S, respectively. The mechanism was consistent with those that other groups had previously proposed.^87,88 The UV-Vis absorption spectra of Hg²⁺ have greater overlap with the excitation spectra of KA@S, which gives it the ability to quench more than any other metal ions. Based on these considerations, the overlaps between the UV-Vis spectra of KA@S and Hg²⁺, Pb²⁺ and As³⁺ suggest that these metal ions compete with organic ligands for the obvious competitive absorption of excitation wavelength energy, which causes the quenching effect.

A comparison of sensing of ions by KA@S and other known compounds is given in Table 4.

Table 4 Comparison of sensing of ions by KA@S and other known compounds

Material	Analyte	K SV (M^−1)	LOD (M)	Ref.
[Co(TBTA)(L3)1.5]n	Cr2O7^2−	1.59 × 10^4	6.70 × 10^−7	79
{[Cd(L14)(BPDC)]·2H2O}n	Cr2O7^2−	6.40 × 10^3	3.76 × 10^−5	89
AASH-CdSe QDs	CO3^2−	—	2.30 × 10^−8	90
Binol–pyrene conjugate (BPC)	CO3^2−	4.33 × 10^4	14.0 × 10^−6	91
CaF2:Tb^3+ NCs	CO3^2−	2.22 × 10^4	9.90 × 10^−7	92
	HCO3^−	1.01 × 10^4	2.15 × 10^−6
[Tb(ppda)(npdc)0.5(H2O)2]n	Cr2O7^2−	4.97 × 10^6	6.00 × 10^−5	46
	Pb^2+	1.05 × 10^5	9.44 × 10^−5
[Zn(HL)(bipy)0.5(H2O)]·2H2O	Pb^2+	1.18 × 10^4	8.00 × 10^−7	93
[Ag4(BPY)4](IPA-NH2)2·10H2O	Pb^2+	2.00 × 10^4	2.10 × 10^−6	94
[Cd(TPA)(BIYB)]n	Hg^2+	4.60 × 10^4	1.90 × 10^−7	95
[Zn(μ2-1Had)(μ2-SO4)]n	Hg^2+	7.70 × 10^3	7.00 × 10^−8	96
[Cd(4-tkpvb)(5-tert-BIPA)]n	Hg^2+	1.94 × 10^4	1.50 × 10^−7	85
CdTe-Cys	As^3+	1.17 × 10^8	2.00 × 10^−9	97
KA@S	Cr2O7^2−	4.22 × 10^4	2.62 × 10^−6	This work
	SO4^2−	2.56 × 10^4	3.83 × 10^−6
	CO3^2−	1.98 × 10^4	6.47 × 10^−6
	HCO3^−	1.63 × 10^4	2.91 × 10^−6
	Hg^2+	3.0 × 10^4	4.81 × 10^−7
	Pb^2+	2.3 × 10^4	2.65 × 10^−6
	As^3+	2.6 × 10^4	2.45 × 10^−6

---

6. Conclusions

In summary, using an aromatic carboxylate ligand and heterocyclic bimb, we have assembled a new hydrogen bonded supramolecular complex. The structure of KA@S is extended to form a 1D chain, 2D layer and 3D supramolecular network via H-bonding interactions. The photocatalytic activity of KA@S was shown to degrade organic dyes with degradation efficiencies of around 96.5% for MV, 94.4% for Rh6G, 93.5% for RB, 91.2% for CR and 89.2% for CV under the influence of sunlight irradiation. KA@S can be an efficient fluorescent probe for the real-time detection of hazardous anions (HCO₃⁻, CO₃²⁻, SO₄²⁻ and Cr₂O₇²⁻) as well as heavy metal ions (As³⁺, Pb²⁺, and Hg²⁺) under aqueous conditions. The minimum detection limit (LOD) for Cr₂O₇²⁻ is 2.62 × 10⁻⁶ M (0.77 ppm) and for Hg²⁺ metal ions 4.81 × 10⁻⁷ M (0.096 ppm). Additionally, the fluorescence sensing mechanism of KA@S for various analytes (anions/metal ions) was elucidated.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

One of the authors, Somnath, would like to convey his heartfelt appreciation to the National Institute of Technology Raipur for providing him with an Institutional Research Fellowship. The authors are also grateful for the research facilities provided by the Department of Chemistry. Dr Kafeel Ahmad Siddiqui wishes to express his gratitude to the Director, National Institute of Technology Raipur, India, for his cooperation with the “Crystal Engineering” research.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2072960. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2nj04957k

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