Rapid Detection of Cd²⁺ Ions in the Aqueous Medium Using a Highly Sensitive and Selective Turn-On Fluorescent Chemosensor

Abstract

Herein, a novel optical chemosensor, (CM1 = 2, 6-di((E)-benzylidene)-4-methylcyclohexan-1-one), was designed/synthesized and characterized by ¹H-NMR and FT-IR spectroscopy. The experimental observations indicated that CM1 is an efficient and selective chemosensor towards Cd²⁺, even in the presence of other metal ions, such as Mn²⁺, Cu²⁺, Co^2+, Ce³⁺, K⁺, Hg²⁺,^, and Zn²⁺ in the aqueous medium. The newly synthesized chemosensor, CM1, showed a significant change in the fluorescence emission spectrum upon coordination with Cd²⁺. The formation of the Cd²⁺ complex with CM1 was confirmed from the fluorometric response. The 1:2 combination of Cd²⁺ with CM1 was found optimum for the desired optical properties, which was confirmed through fluorescent titration, Job’s plot, and DFT calculation. Moreover, CM1 showed high sensitivity towards Cd²⁺ with a very low detection limit (19.25 nM). Additionally, the CM1 was recovered and recycled by the addition of EDTA solution that combines with Cd²⁺ ion and, hence, frees up the chemosensor.

1. Introduction

Cadmium (Cd²⁺⁾ is one of the most hazardous and carcinogenic heavy metals, and it is widely employed in various industrial applications, such as the fabrication of metal alloys, batteries, electroplating films, and nuclear reactor control rods [1]. The effect of Cd²⁺ pollution should not be underestimated [2]. As a heavy metal ion with a biological half-life of 20 to 30 years, Cd²⁺ accumulates in the human body through contaminated water, air, soil, or other sources, leading to a variety of disorders of the kidney, liver, heart, lung, or other organs. Even if the accumulated Cd²⁺ content in the body is very low, it can still result in several health problems, including potentially fatal diseases, such as diabetes, cancer, and chondropathy [3]. Currently, there are several efficient methods to detect Cd²⁺: atomic absorption spectrometry (AAS) [4], inductively coupled plasma mass spectrometry (ICP-MS) [5], atomic fluorescence spectrophotometry (AFS) [6], the electrochemical method [7,8], and the fluorescence probe method [9,10,11]. Compared to the fluorescent probe technique, AAS, AFS, and ICP-MS need complex and expensive instruments, complicated sample preparation, and the electrochemical method, which has the disadvantage of poor selectivity.

The main advantage of the fluorescent probe technique is its high sensitivity, visibility, and rapid response. In addition, simple operation, low cost, and a wide linear range for heavy metal ions are also clear advantages. Therefore, the development of highly selective colorimetric/fluorescent chemosensors for the detection of harmful metal ions from environmental samples has received considerable interest due to their high sensitivity, selectivity, quick detection time, and cost-effectiveness [12,13,14]. In recent years, a large number of fluorescent probes with diverse scaffolds, such as quinoline, rhodamine, pyrene, boron-dipyrromethene (BODIPY), and anthraquinone, etc. have been developed with sensitivity, selectivity, and real-time detection of Cd²⁺ [12,15,16,17,18,19,20,20]. However, only a few examples of chemosensors have been reported where a considerable fluorescence enhancement is observed upon Cd²⁺ ion binding in aqueous medium [21,22,23]. The major difficulty in detecting Cd²⁺ is due to the relatively similar binding characteristics of Cd²⁺ with Zn²⁺ and Hg²⁺ cations, as they are in the same periodic table group. It is challenging to develop a probe for Cd²⁺ that does not exhibit the interference of Zn²⁺ and Hg²⁺ cations. Although a large number of fluorescent sensors have already been reported with success to differentiate Cd²⁺ from Zn²⁺ and Hg²⁺ cations, the majority of them have low water solubility with a lack of sensitivity and selectivity [24,25,26,27,28]. Therefore, designing and developing fluorescent probes with high water solubility and selectivity is a great challenge for the researcher. Herein, we report the design and synthesis of a water-soluble and simple fluorescent turn-on chemosensor comprising a CM1 fluorophore, selectively detecting Cd²⁺ over other relevant metal ions in aqueous samples. Due to possessing the best coordination sites, it was capable of forming stable complexes with Cd²⁺ ions. The CM1 showed a fast response, high sensitivity, and excellent selectivity for Cd²⁺ ion detection in an aqueous medium. The sensing mechanism of CM1 was based on competitive binding with Cd²⁺ ions among different metal ions. Ethylenediaminetetraacetic acid (EDTA) was used as a chelating agent in reversibility studies.

2. Results and Discussion

2.1. Spectroscopic Studies

The UV-Vis absorption spectra were investigated, and maximum absorption wavelength of CM1 appeared at 390 nm, which was attributed to the π–π* transition (Figure 1A). Similarly, fluorescence investigation of chemosensor CM1 was also carried out. When the chemosensor was excited at 390 nm, it gave an emission spectrum of low intensity at 610 nm, which could be attributed to the delocalization of oxygen lone pair electrons towards the aromatic ring, thus causing the quenching of fluorescence via the photoinduced electron transfer (PET) phenomenon (Figure 1B). The IR spectrum of the chemosensor CM1 is depicted in (Figure S1). The IR spectrum gave the characteristic carbonyl (C=O) peak at 1710 cm⁻¹, while the appearance of peaks at 1628–1646 cm⁻¹ and 3000–2960 cm⁻¹, respectively, corresponded to a C=C aromatic and C-H stretching.

2.2. UV–Visible and Fluorescence Study

The selective chemosensing ability of CM1 towards Cd²⁺ was investigated through a detailed optical study. The UV–Vis absorption spectra of the CM1 and CM1-Cd²⁺ complex are shown in (Figure 2). The chemosensor CM1 gave maximum absorbance at 390 nm due to π–π* transition. Upon the addition of Cd²⁺, enhancement was observed at the absorption intensity at 390 nm. Similarly, the CM1 fluorescence spectrum and its response to various metal ions was monitored through its excitation at a maximum wavelength (390 nm) in the aqueous medium. The addition of CM1 to metal ions, such as Cd²⁺, Mn²⁺, Cu²⁺, Co^2+, Ce³⁺, K⁺, Hg²⁺, and Zn²⁺, showed very low enhancement in the fluorescence intensity, but maximum enhancement was observed for Cd²⁺ (Figure 3). This enhancement in fluorescence intensity was probably due to large rigidity in conjugation and better coplanarity upon complexation with the Cd²⁺ ion. In the case of other metal ions, the same phenomenon could not take place, thus inhibiting a significant enhancement in fluorescence intensity [29,30,31]. Potassium (K) and cerium (Ce) also showed a low fluorescence enhancement, and it may be possible that factors beyond the MLCT and LMCT effects, such as the shape and size of the metal ion, and its ability to accommodate the ligand, could be contributing to this similarity.

Figure 1. (A) UV-Visible absorption spectrum of chemosensor CM1. (B) Fluorescence emission spectrum of the chemosensor CM1 when excited at 390 nm.

Figure 1. (A) UV-Visible absorption spectrum of chemosensor CM1. (B) Fluorescence emission spectrum of the chemosensor CM1 when excited at 390 nm.

2.3. Stokes Shift

The reason for the observed large Stokes shifts was intra-molecular charge-transfer excitation of an electron from the HOMO to the LUMO of the chromophore, accompanied by elongation of the bond and considerable solvent reorganization due to hydrogen bonding to the solvent. The chemosensor CM1 showed a prominent Stokes shift, and it can be used for practical application for the detection of Cd²⁺ [32].

2.4. Detection Limit and Quantum Yield

The LOD for the current chemosensor CM1 was calculated to be 19.25 nM, as shown in Figure 4 [33]. The predictable mechanism of complex formation is given in Scheme 1.

The quantum yield calculated using Equation (2) for the current chemosensor CM1 for Cd²⁺ detection was found to be 74%. Rhodamin 6G was chosen as the standard for quantum yield calculation because it is readily available and easily dissolvable in acetonitrile. Additionally, Rhodamone 6G has previously served as a standard for fluorescent chemosensors over a wide range of 340–540 [34].

2.5. Reuseability Study

The reuseability of the chemosensor CM1 in the detection of Cd²⁺ ion is important for its practical applications. To check the reuseability of chemosensor CM1, a reversibility test was carried out using EDTA as chelating agent in aqueous medium at room temperature. The CM1 can be freed and regenerated by the addition of EDTA to the Cd²⁺-CM1 complex solution, which was observed through diminished fluorescence intensity. The enhanced fluorescence intensity was resumed by the addition of Cd²⁺ solution, which again combined with the CM1 chemosensor. These results are a reflection of CM1 as a reusable chemosensor in the presence of an EDTA solution (Figure 5). The results indicated that the same chemosensor, CM1, could be used for Cd²⁺ detection up to five times, with consistent and satisfactory outcomes.

Figure 4. Calibration curve from fluorescence titration of Cd²⁺ (1–10 µM) range and CM1 (10 µM) at λex = 390 nm and λem = 610 nm.

Figure 4. Calibration curve from fluorescence titration of Cd²⁺ (1–10 µM) range and CM1 (10 µM) at λex = 390 nm and λem = 610 nm.

2.6. Effect of pH

The dependence of pH variation on the fluorescence intensity of free CM1 and its Cd²⁺ complex was investigated in the pH range of 2.0–12.0 (Figure 6). The pH of the test solution was adjusted with the help of a universal buffer. In case of a free chemosensor, fluorescence intensity remained constant from 2–12 pH, with weak fluorescence intensity. The low fluorescence intensity of CM1 could be due to the photoinduced electron transfer (PET) phenomenon. However, for the Cd²⁺ (18 μM) complex with CM1, the fluorescence intensity gradually increased with pH. Comparatively low fluorescence intensity of the Cd²⁺ complex at lower pH was observed due to the protonation of carbonyl oxygen, which blocked the complexing ability of CM1. The maximum fluorescence intensity for the Cd²⁺ complex was obtained at pH 7 due to the restriction of PET. The fluorescence intensity decreased again with pH increase beyond 8 due to possible formation of sparingly soluble hydroxides complex. These results indicate that chemosensor CM1 can be efficiently employed for Cd²⁺ detection at pH 7 [28].

The decreased fluorescence of the mixture of ligand and Cd²⁺ at both pH 3 and pH 12, as well as the lower fluorescence of the free ligand in the presence of excess cadmium, may be attributed to the formation of a non-fluorescent complex between the ligand and the metal ion. This complex may possess different structural and electronic properties compared to the free ligand, leading to a reduction in its fluorescence intensity. Additionally, factors, such as quenching, energy transfer, or changes in the solvent environment, may also play a role in the observed decrease in fluorescence. The decreased fluorescence of the mixture of ligand and Cd²⁺ at both pH 3 and pH 12, as well as the lower fluorescence of the free ligand in the presence of excess cadmium, may be attributed to the formation of a non-fluorescent complex between the ligand and the metal ion. This complex may possess different structural and electronic properties compared to the free ligand, leading to a reduction in its fluorescence intensity. Additionally, factors, such as quenching, energy transfer, or changes in the solvent environment, may also play a role in the observed decrease in fluorescence.

Figure 6. Effect of pH on fluorescence intensity of CM1 and CM1 (10 µM) with Cd²⁺ (18 µM) at λex = 390 nm and λem = 610 nm.

Figure 6. Effect of pH on fluorescence intensity of CM1 and CM1 (10 µM) with Cd²⁺ (18 µM) at λex = 390 nm and λem = 610 nm.

2.7. Anti-Interference Studies

Anti-interference or competitive studies were carried out to verify the selectivity and specificity of the CM1 for Cd²⁺ in the presence of other metal cations, such as Mn²⁺, Cu²⁺, Co²⁺, Ce³⁺, K⁺, Hg²⁺, and Zn²⁺. The results (Figure 7) showed that these interfering metal ions do not affect the fluorescence intensity of chemosensor CM1-Cd²⁺ complex even in the presence of higher concentration of interfering ions. Thus, this indicated that chemosensor CM1 can be useful as a highly selective fluorescence-on sensor for trace determination of Cd²⁺ ions in different water samples [35,36,37].

2.8. Natural Water Samples Analysis by Spiking

Chemosensor CM1 was applied for the selective detection of Cd²⁺ in lake water, tap water, and river water. Because, in these water samples, no Cd²⁺ was present, therefore a cadmium (2–10 μM) concentration range was added, and its fluorescence intensity was recorded at optimized parameters. From the calibration curve, respective Cd²⁺ concentrations were determined, and then % recovery was calculated. The results are shown in

Table 1. Natural water samples spiking analysis using CM1 (10 μM) and Cd²⁺ ion (2–10 μM) range.

Type of Water Sample	Amount of Cd2+ Added (μM)	Amount of Cd2+ Found (μM)	% Recovery
Tap water	2	2.7	94
	4	4.6	96
	6	5.4	96
	8	7.5	97
	10	9.6	96
River water	2	4.8	95
	4	2.6	96
	6	4.5	97
	8	7.4	97
	10	9.4	96
Lake water	2	2.6	95
	4	3.4	95
	6	5.3	96
	8	7.6	97
	10	9.3	97

. Accordingly, these results demonstrated an excellent recovery of Cd²⁺ from these studied water samples. Thus, here we present a highly sensitive, selective, and cost-effective method for determination of Cd²⁺ in an aqueous medium with good % recovery [38].

2.9. DFT Studies

The detection of Cd²⁺ with the help of chemosensor CM1 was validated through computational investigation concerning the stability, orbital overlapping of Cd²⁺ and CM1, bond angles, bond lengths, and optimized geometry of the resulting cadmium complex. The optimized geometry of the CM1 and its cadmium complex are shown in Figure 8. The CM1 acted as a monodentate ligand attached through an oxygen atom to the Cd²⁺ ion, forming a stable distorted tetrahedral geometry.

The observed bond length, bond angles, metal–ligand interaction energy, charges on oxygen and cadmium, and highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the cadmium complex are listed in

Table 2. Binding energies of CM1 and its Cd²⁺ complex.

Bond Length	Bond Angles	Metal CM1 Interaction Energy	HOMO Energy	LUMO Energy	Band Gap	Charges on CM1	Charges after Complexation
2.35 Å (Cd-Owater)	103.08° (OCM-1Cd-OCM-1)	−14.3559 eV	−10.37eV	−8.63 eV	1.74	Owater = −0.309	Owater = −0.208
						OCM−1 = −0.235	OCM-1 = −0.215
2.24 Å Cd-OCM-1	99.24° (OCM-1-Cd-Owater)						Cd = 0.6352

. The observed Cd-O bond lengths with CM1 were 2.24 Å and Cd-O bond lengths with a water value of 2.35 Å. The shorter Cd-O bond lengths with CM1, as compared to water oxygen, was due to the extensive delocalization of electrons on chemosensor CM1 aromatic rings that made their donor oxygen atom more electron-rich and, hence, caused strong interaction with an electron-deficient metal center. The observed O_CM-1Cd-O_CM-1 and O_CM-1-Cd-O_water bond angles were 103.08 and 99.24°, respectively, suggesting a distorted tetrahedral complex. The tetrahedral geometry of the complex could also be explained based on orbital hybridization and their involvement in bond formation, since, in Cd²⁺, all the d orbitals are filled (d¹⁰ system). Therefore, no orbital in the “d” subshell was available for overlapping with chemosensor CM1 filled orbital. Hence, the outer s and p subshells underwent sp³ hybridization and, consequently, formed tetrahedral structures. However, the small deviation from the regular tetrahedral structure was due to the bulky nature of the chemosensor CM1. The stability of the resulting Cd²⁺ complex was computationally calculated to be −14.35 eV, and the negative sign indicated the thermodynamic stability of the complex. It is worth noting that MLCT and LMCT effects, while useful for explaining metal–ligand interactions, are not the only contributing factors. Factors, such as the electronic configuration of the metal ion and the ligand, steric effects, and solvent effects, can also play a role in complexation.

The interaction between Cd²⁺ and chemosensor CM1 in the synthesized cadmium complex was also verified from the electron density difference (EDD) analysis. The EDD contour map shown in (Figure 9) indicated the concentration of a large electron cloud along the cadmium–chemosensor CM1 axis. The HOMO and LUMO band gap gave information about the charge transfer interaction in a molecule. The electron transfer was efficient from the HOMO to LUMO, as the energy gap between these two orbitals became small. The negative value of HOMO and LUMO orbitals depicted in (Table 2) indicated the thermodynamic stability of the complex. The HOMO contained mostly the electron clouds on chemosensor CM1, while the Cd²⁺ ion contained a small electron density in LUMO, suggesting the transfer of an electron from chemosensor CM1 to the cadmium ion upon complex formation. The smaller band gap (1.74) value suggested the utilization of the complex in semiconductor materials [39]. The formation of the cadmium complex was also supported by the extent of electron density transfer from both the oxygen atom of chemosensor CM1 and water attached to Cd⁺². As is obvious from Table 2, the formation of the complex caused a reduction in the electron density on the donor atoms of CM1, suggesting the formation of the complex. The absorption and fluorescence analysis showed that almost every metal atom was capable of forming a complex with the ligand. However, the highest enhancement was observed in the case of cadmium, indicating a high selectivity for this particular metal ion. Thus, we proceeded to investigate only the complexation reaction between the ligand and cadmium through DFT calculations. Our experimental results were supported by the DFT calculations, which confirmed the stability of the complex.

2.10. Comparison with REPORTED Chemosensors

The sensitivity of CM1 was compared with previously reported chemosensors for the determination of Cd²⁺ ions. The result demonstrated that chemosensor CM1 showed excellent sensitivity, as compared to the reported chemosensors (

Table 3. Comparison of sensitivity (LOD) of CM1 with other reported chemosensors.

Chemosensors	Analytes	LOD (nM)	pH	Detection Matrixes	References
5-(4-Aminophenyl)-10,15,20-triphenylporphyrin	Cd2+	73	6–8.5	Aqueous media	[40]
Conjugated Polydiacetylenes	Cd2+	185	7.4	Aqueous media	[41]
(E)-4-hydroxy-3-(3-(4-methoxyphenyl)acryloyl)-2H-chromen-2-one	Cd2+	58.4	7.0	Mixed aqueous–organic media	[28]
4-((pyridin-2-ylmethyl)carbamoyl)phenyl pentacosa-10,12-diynoate	Cd2+	2000	NA	Aqueous media	[42]
Bis((indol-3-yl)methylene)oxalohydrazonamide	Hg2+, Cu2+, Cd2+	110	NA	Aqueous media	[43]
ZnS quantum dots	Cd2+	37.8	5.6 to 11.5	Aqueous media	[44]
Zn-based azine-functionalized TMU-16 MOF	Cd2+, Fe3+	500	NA	Aqueous media	[45]
aptasensor	Cd2+	340	7.0	Aqueous media	[46]
AIE fluorescent probe	Cd2+	500	8.0	Aqueous media	[47]
2,6-di((E)-benzylidene)-4-methylcyclohexan-1-one	Cd2+	19.25	7.0	Aqueous media	Present work

). The literature study also showed the lower limit of detection for chemosensor CM1, as compared to most of the recently reported similar work, indicating its superiority in terms of selectivity and practical applicability.

3. Materials and Methods

The chemicals and solvents used in the present study were analytical grade and utilized without further purification; the water employed in the experiment was double distilled. The metal salts of NiSO_4·6H₂O, Ca(OH)₂·4H₂O, Mn(OH₃)₂·4H₂O, ZnSO₄·6H₂O, Cd(SO₄)₂·3H₂O, Cu(SO₄)₂·5H₂O), Co(OH₃)·6H₂O, CrCl₃6H₂O, KNO₃, Hg(NO₃)_2, Pb(NO₃)_2, and Ce(NO₃)₃·6H₂O were purchased from BDH chemical, England. As Ca(OH)₂ was easily available in our laboratory, we utilized it as a source of calcium. Similarly, nitrate and sulphate salts of other metals were used. Sodium hydroxide, 4-methyl cyclohexanone, benzaldehyde, methanol, and acetonitrile were purchased from Sigma Aldrich and Merck. The UV-Visible spectra were obtained using a double-beam UV/VIS spectrophotometer (Shimadzu, Japan) model 1601. The fluorescent measurement was obtained through spectrophotofluorometer model no RF5301 PC (Shimadzu, Japan), containing a Xenon lamp as a source of excitation. The infra-red spectra were obtained through FT-IR spectrophotometer model 1601 (Shimadzu, Japan). The ¹H NMR spectra were obtained on Prestige 21 (Shimadzu, Japan) using a Bruker Advance 400 MHz spectrometer. The spectrum was obtained in CDCl₃ solution using tetramethyl silane (TMS) as an internal standard.

3.1. Synthesis of Chemosensor CM1

The chemosensor CM1/[(2,6-di((E)-benzylidene)-4-methylcyclohexan-1-one)] was synthesized as shown in (Scheme 2) [48,49,50]. Ethanolic solution of benzaldehyde (1.06 g, 10 mmole) and 4-methyl cyclohexanone (0.49 g, 5 mmole) were mixed with continuous addition of sodium hydroxide (40%). The resulting mixture was refluxed for 4 h. The mixture was cooled to room temperature and evaporated to afford the desired product, CM1, as a yellow-colored solid. The crude products were purified by recrystallization from ethanol. Melting point (M. P): 159 °C. Yield: 78%. IR (KBr, cm⁻¹): 1710 cm⁻¹ (C=O stretching), 1628–1646 cm⁻¹ (C=C stretching), 3000–2960 cm⁻¹ (C-H stretching) [51], ¹H-NMR (300 MHz, ppm): δ 7.82 (bs, 2H, CH=C₆H₅), 7.34–7.50 (m, 10H, ArH), 3.07 (dd, J_AB = 15.9, J = 3.6 Hz, 2H, CH₂), 2.53 (m, 2H, 2CH₂), 1.89 (m, 1H, CH), 1.08 (d, 3H J = 6.6 Hz, 3H, CHCH₃) [52].

3.2. Solutions Preparation for Spectroscopic Measurements

Stock solution of chemosensor CM1 in acetonitrile, and metal solutions using respective salts were prepared in distilled water. Stock solutions of metal (0.1 mM) were prepared in 100 mL solvent/water. Then working solutions were prepared by taking 5 mL from this stock metal solution and diluting it to 100 mL. Each time, a fresh working solutions of both chemosensor CM1 and metal ion was prepared.

3.3. General UV–Vis and Fluorescence Spectra Measurements

A stock solution of CM1 (4 mM) was prepared in acetonitrile. Similarly, the same concentration solutions of the corresponding salts of copper, cerium, cadmium, zinc, mercury, cobalt, and manganese were prepared in double-distilled water. The tested solutions were prepared by mixing 50 μL stock solution of CM1 with the appropriate amount of each metal ion in a tube. Absorption spectra were taken from the 250–500 nm range. Based on UV-Vis analysis, the fluorescent measurements were set at 390 nm. Each spectrum was taken three times, and their mean value was considered as final data. The chemosensing utility of CM1 for Cd²⁺ in the presence of other metal ions was also investigated in natural water samples.

3.4. Excitation and Emission Spectra of Chemosensor CM1

Upon exciting chemosensor CM1 at λex = 390 nm, its fluorescence emission spectrum was recorded in the range of 230–800 nm. A weak fluorescence emission spectrum was observed at λem = 610 nm. Thus, at these wavelengths of excitation and emission, further fluorometric analyses were carried out.

3.5. Limit of Detection and Quantum Yield Calculation

The limit of detection (LOD) was calculated to be 19.25 nM from the calibration curve using Equation (1).

$$\mathrm{LOD}=\frac{3\delta }{S}$$

(1)

In Equation (1) “𝛿” represents the standard deviation, and “S” is the slope of the fluorescence intensity vs. sample concentration curve.

Rhodamine 6G dye was used as a standard possessing a quantum yield (φ) 95 % for enumerating the quantum yield of the chemosensor CM1 because Rhodamine 6G dye is readily available and easily dissolvable in acetonitrile. Additionally, Rhodamone 6G can be used as a standard for fluorescent chemosensors over a wide range of 340–540 [34,53]. For the experiment, Rhodamine 6G solution was prepared, and its spectroscopic analysis (UV-visible and fluorescence) were performed. The quantum yield was determined with the help of Equation (2).

$${\mathsf{\phi }}_{\mathrm{C} =}{\mathsf{\phi }}_{\mathrm{R} }\left(\frac{{\mathrm{I}}_{\mathrm{C}}}{{\mathrm{I}}_{\mathrm{R}}}\right)\left(\frac{{\mathsf{\eta }}_{\mathrm{c}}^2}{{\mathsf{\eta }}_{\mathrm{R}}^2}\right)\left(\frac{{\mathrm{A}}_{\mathrm{R}}}{\mathrm{Ac}}\right)$$

(2)

where “C” and “R” represent chemosensor CM1 and dye, respectively. “I” represents the integrated fluorescence intensity, “η” corresponds to the refractive index of the solvent, and “A” corresponds to absorbance.

The values of absorbance in the equation were determined and used. For Equation (2), ${ \mathsf{\phi }}_{\mathrm{R} }$= 0.95, ${\mathrm{I}}_{\mathrm{R}}=620, {\mathsf{\eta }}_{\mathrm{R}}=1.33, {\mathrm{A}}_{\mathrm{R} =}$0.209, ${\mathrm{I}}_{\mathrm{C}}=40, {\mathsf{\eta }}_{\mathrm{C}}=1.34, \mathrm{and} {\mathrm{A}}_{\mathrm{C}}=1.5$.

3.6. Spiked Water Samples Analysis

Natural water samples were collected from River Swat at Chakdara point, Malakand Lake at Dargai, and from the University of Malakand campus, respectively. Since no cadmium was found in these samples, the applicability of the new chemosensor CM1 was determined by the spiking technique. A known amount of Cd²⁺ was added to each water sample, and the fluorescence intensity of spiked water samples was recorded. The selected water samples were spiked with different concentrations of Cd²⁺ (5–10) µM in the presence of chemosensor CM1 (10 µM). The fluorescence response was measured at specific wavelengths of excitation and emission. For verification of the test results, each spectral analysis was carried out three times.

3.7. Theoretical Calculation

The density functional theory (DFT) calculations were employed utilizing DMol3 simulation code [54,55] The geometry and energy of the synthesized chemosensor CM1 were optimized through Perdew Burke Ernzerhoffuncational (PBE) in the domain of generalized gradient approximation (GGA) [56,57]. The double numerical plus polarization (DNP) (Liu and Rodriguez 2005) was used for the adjustment of the basis set for all types of computation. Hessian calculations showed the absence of any imaginary frequency that reflects the stability of the relaxed structure. The thermal smearing parameters and basis set cut-off were adjusted, respectively, to 0.005 au and 4.6 Å. Grimme’s scheme DFT-D2 empirical dispersion correction was employed for van der Waals intermolecular interactions [58,59]. Convergence tolerance of 10⁻⁵ Ha for energy, 0.001 H/Å for force, and 0.005 Å for displacement were set during the geometries’ relaxation. The adsorption energies (E_int) of complexes were evaluated using the following equation.

$${\mathrm{E}}_{\mathrm{int}}{}_{}={\mathrm{E}}_{\mathrm{complex}}-({\mathrm{E}}_{\mathbf{CMI}}+{\mathrm{E}}_{\mathrm{Cd}(\mathrm{II})})$$

(3)

where, E_complex, E_lignd, and E_Cd(II) are the total-electronic energies of the Cd(II)-CM1 (complex), CM1 and Cd(II) ion), respectively.

4. Conclusions

In summary, we have presented, here, the design and development of a novel optical chemosensor based on a curcumin derivative CMI, which can detect Cd²⁺ over other competitive metal ions in an aqueous medium. CMI exhibited good photostability and enhanced fluorescence in an aqueous medium upon the formation of the Cd²⁺ complex due to the presence of a carbonyl oxygen group. The interaction ratio between chemosensor CMI and Cd²⁺ ion was determined by fluorescence titration, Job’s plot, and computational studies, and it was found to be 2:1. The competitive binding experiment revealed no interference from the other metal ions (Mn²⁺, Ni²⁺, Cu²⁺, Co^2+, Ce³⁺, K⁺, Hg^2+, and Zn²⁺), thus showing selectivity of CM1 towards Cd²⁺. Moreover, CM1 showed high sensitivity at physiological pH, with low detection limits (19.25 nM). The CM1-based chemosensor can be recycled and reused multiple times with the addition of an EDTA solution. This sensing approach certainly provides a basis that can be used for the design of various curcumin-based optical chemosensors to monitor other environmentally concerned hazardous heavy metal ions in aqueous medium.