Bis-Chalcones as Efficient Optical Chemosensors for Rapid and Visual Detection of Toxic Metal Ions

Abstract

In continuation of our ongoing exploration of electronically delocalized Bis-chalcone scaffolds as versatile molecular platforms, the previously reported chemosensors 3-(4-(dimethylamino)benzylidene)pentane-2,4-dione (DBPD) and 3-(3-(4-dimethylamino)phenyl)allylidene)pentane-2,4-dione (DPAPD) were revisited to evaluate their metal-responsive properties beyond the initially characterized photophysical behavior. Although their synthesis and fundamental optical characteristics have been well established, systematic investigations into their metal-ion sensing capabilities have remained limited. Given the strong electron-donating nature of the dimethylamino groups conjugated with β-diketone moieties, which facilitate intramolecular charge transfer (ICT) and metal coordination, the present study examines the optical properties of DBPD and DPAPD in response to environmentally relevant metal ions, Cu²⁺, Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cd²⁺, and Fe³⁺. Distinct visual color changes were observed for Co²⁺, Cu²⁺, Ni²⁺, and Pb²⁺, whereas Zn²⁺, Cd²⁺, and Fe³⁺ elicited negligible optical response, highlighting the selective interactions of the Bis-chalcones with specific metal ions. The absorption measurements revealed characteristic bands at 455 nm (DBPD) and 474 nm (DPAPD). Upon metal ion addition, Co²⁺ enhanced the absorbance and induced bathochromic shifts, while Cu²⁺ and Ni²⁺ caused decreased absorbance and the emergence of new spectral features, indicative of complex formation and perturbation of the ICT process. Fluorescence studies further corroborated these interactions, showing metal-dependent quenching or enhancement, with DPAPD exhibiting a pronounced hypsochromic shift upon Ni²⁺ binding. Quantitative analyses using the Benesi–Hildebrand method and Job’s plot confirmed 1:1 stoichiometry for the metal–chemosensor complexes and demonstrated the preferential binding of DBPD to Co²⁺ and DPAPD to Cu²⁺. To further probe the structural characteristics, solid-state complexes of DBPD-Co²⁺ and DPAPD-Cu²⁺ were isolated and characterized via molar conductance, Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), and X-ray diffraction (XRD), confirming coordination interactions and complex stability. Reversibility studies revealed that the original optical properties of the chemosensors were fully restored upon addition of the chelating agent ethylenediaminetetraacetic acid disodium salt (EDTA), establishing the dynamic and non-permanent nature of the metal-ligand interactions suitable for reversible sensing applications. These findings establish DBPD and DPAPD as robust, selective, and reversible fluorescent chemosensors, capable of sensitive and cost-effective detection of specific metal ions in environmental and real-sample contexts, while their ICT-active frameworks provide a tunable platform for multifunctional optical applications.

Graphical abstract

This is a visual representation of the abstract.

Keywords

Bis-chalcones optical sensing fluorescent probes metal ions

Introduction

Heavy metal ions are widely recognized as hazardous environmental contaminants that can pose significant threats to human health, even when present in minute concentrations. The profound toxicity and enduring nature of heavy metal ions have prompted considerable attention from both the general populace and the scholarly community. Co²⁺, Ni²⁺, and Cu²⁺ ions stand out as highly impactful heavy metal ions exhibit significant toxicity toward environmental and biological systems.^1–4 While Fe³⁺ and Zn²⁺ ions are not profoundly toxic, they still possess potential dangers at elevated concentrations.⁵ In contrast, Cd²⁺ and Pb²⁺ ions exhibit substantial toxicity even at low levels,^6,7 thereby establishing a clear association with neurological and malignant disorders.^8,9 Various analytical techniques, such as surface-enhanced Raman spectroscopy, inductively coupled plasma, inductively coupled plasma mass spectrometry, and electrochemical sensors, enable the identification and quantification of diverse metal elements. However, these methodologies demand considerable time, financial resources, and advanced technology to achieve accurate results. Hence, there is a pressing need for direct, intuitive, and focused approaches to detect toxic metal ions.^10,11 Fluorescent probes have emerged as a valuable solution due to their capacity for precise detection, heightened sensitivity, selectivity,^12,13 and visual identification with the naked eye. These probes offer simplicity, affordability, superior specificity, low detection thresholds, rapid response times, and technical straightforwardness. Consequently, their popularity has surged in recent times,^14,15 as they obviate the necessity for specialized equipment. Furthermore, fluorescent probes serve as effective tools for investigating micropolarity, hydrogen-bonding interactions, and the microenvironmental attributes of biological systems,^16,17 owing to their solvatochromic and fluorosolvatochromic attributes. Beyond their academic significance, these probes find practical applications in photopolymer imaging systems and even as laser dyes.¹⁸

Within the realm of fluorescent probes, chalcones have garnered significant attention due to their diverse utility in a range of optical applications. These applications encompass the realms of non-linear optics,¹⁹ wherein they serve as second harmonic generation materials, as well as in photorefractive polymers and holographic recording materials.²⁰ Moreover, chalcones have found purpose as fluorescent probes for the detection of metal ions^21–25 and biological macromolecules, along with their role in exploring the microenvironment within micelles.^26–28

In this context, the present study explores the optical behaviors of Bis-chalcones 3-(4-(dimethylamino)benzylidene)pentane-2,4-dione (DBPD) and 3-(3-(4-dimethylamino)phenyl)allylidene)pentane-2,4-dione (DPAPD). Their ability to detect and differentiate heavy metal ions including Cu²⁺, Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cd²⁺, and Fe³⁺ was systematically evaluated through both colorimetric observations and spectroscopic analyses. The study emphasizes the remarkable sensitivity and selectivity of these Bis-chalcones, enabling naked-eye detection of heavy metals even at low concentrations. Additionally, solid metal complexes of DBPD-Co²⁺ and DPAP-Cu²⁺ were synthesized and thoroughly characterized using molar conductance, FT-IR, TGA, and XRD techniques.

Experimental

Materials and Methods

All chemical reagents and solvents were procured from commercial suppliers and employed in their original state without undergoing additional purification steps. Acetylacetone, 4-(N,N-dimethylamino) benzaldehyde, 4-(N,N-dimethylamino) cinnamaldehyde, piperidine, were obtained from Sigma-Aldrich Chemical Co. (USA). The metal salts under investigation encompass cobalt(II) chloride hexahydrate (CoCl₂.6H₂O), nickel(II) chloride hexahydrate (NiCl₂.6H₂O), lead(II) acetate trihydrate (Pb(CH₃COO)₂.3H₂O), cadmium chloride (CdCl₂), zinc chloride (ZnCl₂), ferric chloride (FeCl₃.6H₂O), and copper chloride dihydrate (CuCl₂.2H₂O).

Fourier transform infrared (FT-IR) spectra were obtained using a Jasco FT-IR 4100 spectrophotometer with KBr pellets in the range of 4000 to 400 cm⁻¹, and the acquired spectra were processed employing the KBr disc technique. Thermogravimetric (TGA) analysis was executed employing a Shimadzu TGA-50H thermal analyzer, spanning a thermal range of 20 to 800 °C, under a uniform heating regime of 10 °C/min in an atmosphere of nitrogen sustained at a flow rate of 30 mL/min. Small-angle powder X-ray diffraction (XRD) measurements were recorded at room temperature using XRD equipment model a GNR, APD 2000 PRO step scans X-ray diffractometer, with Cu Ka radiation (40 kV, 30 mA), and a scanning range of 5–80 (2θ), with a step of 0.02.

Electronic absorption spectra were measured with an Agilent Cary Eclipse ultraviolet–visible (UV–Vis) scanning spectrophotometer, while emission spectra were recorded using an Agilent Cary Eclipse fluorescence spectrophotometer.

Synthesis of Bis-Chalcones, DBPD and DPAPD

The Bis-chalcones 3-(4-(dimethylamino)benzylidene)pentane-2,4-dione (DBPD) and 3-(3-(4-dimethylamino)phenyl)allylidene)pentane-2,4-dione (DPAPD) were synthesized following the previously reported method,²⁹ see Scheme S1 (Supplemental Material), involving the condensation of acetylacetone with the corresponding aromatic aldehydes in methanol under piperidine/glacial acetic acid catalysis. The reactions were monitored by thin-layer chromatography (TLC), and the mixtures were poured into ice-cold water, resulting in the formation of a precipitate, which was subsequently filtered and dried to achieve the following: (i) DBPD: reddish brown crystals; yield 85%. Characterization was confirmed by ¹H- and ¹³C-nuclear magnetic resonance, IR spectroscopy, mass spectrometry, and elemental analysis. (ii) DPAPD: Orange-red crystals; yield 89%. Characterization was similarly verified. This approach provided high-yielding, pure Bis-chalcone chemosensors suitable for subsequent optical and metal-sensing studies. These well-characterized Bis-chalcones were thus prepared for systematic evaluation of their optical responses and selective metal-ion sensing capabilities.

Synthesis of the Solid Metal Complexes

The solid complexes were synthesized by refluxing 30 mL of ethanolic DBPD or DPAPD solution (0.001 mol) with 30 mL of hot ethanolic CoCl₂.6H₂O or CuCl₂.2H₂O solution (0.002 mol) for 24 h under constant stirring. The precipitate was isolated, washed with distilled water, ethanol, and diethyl ether, and then dried in a vacuum over anhydrous CaCl₂.

Colorimetric Detection of Toxic Metal Ions

In ethanol, stock solutions containing the investigated Bis-chalcones at a concentration of 1 × 10^–3 M have been prepared. Additionally, solutions containing metal ions, Cu²⁺, Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cd²⁺, and Fe³⁺ were prepared at a concentration of 1 × 10^–3 M. The solution of the chemosensor was prepared at a concentration of 2 × 10^–4 M and placed within a quartz optical cell with a length of 1 cm. This cell was maintained at room temperature, and both absorption and fluorescence spectra were acquired from it.

To ensure homogeneity and equilibrium before each experimental cycle, a waiting period of 10 minutes was observed following the mixing of solutions. Fluorescent spectra were meticulously collected within the wavelength ranges of 330–800 nm for DBPD and DPAPD. The titration procedure was employed, where in the concentrations of DBPD and DPAPD were held constant while systematically varying the concentration of metal ions.

To determine the stoichiometric ratio, Job’s method was employed. Briefly, a series of solutions was prepared with a constant total concentration of metal ions and chemosensor, while systematically varying their mole fractions. The absorbance at the selected wavelength was plotted against the chemosensor mole fraction (X = C_L/(C_L] + C_M)), and the maximum of the resulting curve was used to determine the stoichiometry, where C_L and C_M represent the concentrations of the chemosensor (chemosensors) and metal ions, respectively.

Results and Discussion

Response of Metal Ion Sensing

The optical responses of the chemosensors DBPD and DPAPD to metal ions Cu²⁺, Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cd²⁺, and Fe³⁺ were examined both qualitatively (via naked-eye and colorimetric detection) and quantitatively using steady-state absorption and emission spectroscopy. Each metal ion was added in equimolar concentrations (1 × 10^–3 M) to ethanol solutions of the chemosensors. These ions were chosen due to their recognized environmental toxicity.

For DBPD, the addition of Co²⁺, Cu²⁺, Ni²⁺, and Pb²⁺ caused visible color changes from yellow to green, yellow, colorless, and faint yellow, respectively (Scheme 1). No visible change was observed with Zn²⁺, Cd²⁺, or Fe³⁺. Similarly, DPAPD exhibited color shifts from orange to pink, colorless, faint grey, and faint pink upon addition of Co²⁺, Cu²⁺, Ni²⁺, and Pb²⁺, respectively, while Zn²⁺, Cd²⁺, and Fe³⁺ induced no noticeable color change.

Scheme 1.

Color changes upon addition of various metal ions to the investigated Bis-chalcones.

Further titration experiments with increasing concentrations of Co²⁺ (from 1 × 10⁻⁵ to 9 × 10⁻⁵ M) resulted in significant, progressive color changes in both chemosensors, as depicted in Scheme 2. These visual observations were corroborated by corresponding changes in the absorption and emission spectra.

Scheme 2.

Color changes upon addition of different concentrations from Co²⁺ ions (1 × 10^–5–9 × 10^–5 M) to the investigated Bis-chalcones.

This study investigated the effect of various metal ions on the absorption behavior of DBPD and DPAPD in ethanol (Figures 1 and 2). The free Bis-chalcones exhibited characteristic absorption peaks at 455 nm and 474 nm, respectively. Upon gradual addition of Co²⁺ ions to DBPD, a progressive increase in absorbance was observed along with the appearance of a new band at 655 nm and an isosbestic point at 594 nm, indicating equilibrium between two species (Figure 1). Similarly, Co²⁺ addition to DPAPD resulted in enhanced absorption and a bathochromic shift to 502 nm (Figure 2).

Figure 1.

Absorption of spectral changes of DBPD upon addition of different concentrations of metal ions in ethanolic solution.

Figure 2.

Absorption spectral changes of DPAPD upon addition of different concentrations of metal ions in ethanolic solution.

Furthermore, Cu²⁺ caused a decrease in absorbance in both chemosensors, accompanied by new band at 275 nm for DBPD and DPAPD, with corresponding isosbestic point at 337 nm and 330 nm, respectively.

These spectral changes indicate ground-state complex formation and enhanced intramolecular charge transfer (ICT) from the dimethylamino group (electron donor) to the carbonyl groups (electron acceptors) through the π-system upon metal binding.

Incremental addition of Ni²⁺ led to a gradual decrease in absorbance and the emergence of new bands at (402, 457 nm) (DBPD) and (400, 473 nm) (DPAPD) (Figures 1 and 2). Meanwhile, the introduction of Pb²⁺, Cd²⁺, Zn²⁺, and Fe³⁺ resulted in increased absorbance without a shift in λ_max, suggesting complex formation without significant electronic transition alterations. These findings confirm the strong metal–chemosensor interactions in the presence of selected ions, highlighting DBPD and DPAPD as promising candidates for simple, cost-effective, and rapid colorimetric detection of heavy metal ions in environmental or analytical applications.

As a representative example, the stoichiometry of the complexes formed between DBPD and Co²⁺, and between DPAPD and Cu²⁺ was determined using continuous variation method. The absorbance versus mole fraction plots (C_L / [C_L + C_M]) shown in Figure 3 display maxima at 0.51 and 0.53, respectively, confirming the formation of 1:1 metal–chemosensor complexes.

Figure 3.

Job's plot DBPD and DPAPD with Co²⁺ and Cu²⁺ ions, respectively.

The fluorescence behavior of DBPD and DPAPD in the presence of various metal ions Cu²⁺, Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cd²⁺, and Fe³⁺was systematically investigated, as illustrated in Figures 4 and 5. In an ethanolic medium, DBPD displayed a characteristic fluorescence emission peak at 603 nm. Upon incremental addition of Cu²⁺, Co²⁺, Ni²⁺, and Pb²⁺ ions to a 2 × 10⁻⁵ M solution of DBPD, a gradual quenching of fluorescence intensity was observed. Notably, this quenching effect occurred without any discernible shift in the maximum emission, indicating a non-radiative interaction mechanism between DBPD and these metal ions.

Figure 4.

Emission spectra of DBPD upon addition of different concentrations of some selected metal ions in ethanolic solution.

Figure 5.

Emission spectra of DPAPD upon addition of different concentrations of some selected metal ions in ethanolic solution.

In contrast, the introduction of Zn²⁺, Cd²⁺, and Fe³⁺ ions into the DBPD solution produced a steady enhancement in fluorescence intensity, again with no significant change in the emission wavelength. This suggests that these metal ions may stabilize the excited state of DBPD, thereby promoting radiative decay pathways (Figure 4).

Similarly, the fluorescence spectrum of DPAPD in ethanol exhibited a well-defined emission band centered at 657 nm. When varying concentrations of Cu²⁺ and Co²⁺ions were added to a 2 × 10⁻⁵ M DPAPD solution, a consistent quenching of fluorescence intensity was observed, with the emission maximum remaining unchanged (Figure 5). However, the interaction of DPAPD with Zn²⁺, Fe³⁺, Cd²⁺, Pb²⁺ and Ni²⁺ ions resulted in both a gradual decrease in fluorescence intensity and a pronounced hypsochromic shift of 4, 5,6, 11, 41 nm, indicating a change in the electronic environment of the fluorophore upon complex formation.

Collectively, these results demonstrate distinct fluorescence response patterns of DBPD and DPAPD toward different metal ions, indicating their potential utility as selective fluorescent probes for metal ion detection based on quenching or enhancement mechanisms.

The complexation behavior of the Bis-chalcone systems (DBPD and DPAPD) toward the investigated metal ions was systematically quantified through steady-state fluorescence titration. The association constants were derived using a modified Benesi–Hildebrand formalism, which enables reliable estimation of binding parameters from emission intensity variations.³⁰

\frac{1}{Δ F} = \frac{1}{Δ F_{\max}} + (\frac{1}{K_{b} {[M]}^{n +}}) (\frac{1}{Δ F_{\max}})

(1)

The fluorescence response was expressed in terms of the differential signal (ΔF = F_x − F_o), while the limiting value (ΔF_max = F_∞ − F_o) corresponds to the fully saturated coordination state. In this framework, F_o denotes the intrinsic emission of the unbound ligand, F_x represents the emission at a given metal ion concentration, and F∞ reflects the emission upon complete formation of the metal–ligand complex.

Linear correlations obtained from plotting the reciprocal fluorescence change (1/ΔF) against the inverse metal ion concentration (1/[M]ⁿ⁺) (Figure S1, Supplemental Material) provided access to the binding constants (K_b), which were extracted from the slope to intercept ratios. The resulting K_b values (L·mol⁻¹), summarized in Tables I and II, reveal a pronounced dependence of the binding affinity on both the electronic structure of the ligand and the physicochemical characteristics of the metal ions.

Table I.

Spectral data, ionic radius, maximum absorption and fluorescence wavelengths, binding constants in the excited states K^f_b and LOD parameters between DBPD and the metal ions in EtOH.

Metal ions	λ^a_max(nm)	λ^f_max(nm)	K^f_b × 10³(M⁻¹)	K_SV (M⁻¹)	LOD (μM)
Fe³⁺	455 (455)*	603 (603)*	19.254	–	18.1
Co²⁺	475,655	612	41.970	85714	11.8
Ni²⁺	402,457	611	29.103	78000	0.67
Cu²⁺	275	608	16.339	9822	0.4
Zn²⁺	455	603	18.486	–	12.8
Pb²⁺	455	615	2.150	7055	3.86
Cd²⁺	455	603	4.652	–	3.61

*The asterisks represent the wavelength of free DPAPD measured in EtOH only

Table II.

Spectral data, ionic radius, maximum absorption and fluorescence wavelengths, binding constants in the excited state K_b and LOD parameters between DPAPD and the metal ions in EtOH.

Metal ions	λ^a_max(nm)	λ^f_max(nm)	K^f_b × 10³(M⁻¹)	K_SV(M⁻¹)	LOD(μM)
Fe³⁺	474 (474)*	652 (657)*	4.271	–	0.78
Co²⁺	502	657	20.501	55403	1.57
Ni²⁺	400	616,652	21.067	4640	1.28
Cu²⁺	275	657	29.074	82174	0.046
Zn²⁺	474	653	2.092	7112	0.94
Pb²⁺	474	646	5.254	6259	1.84
Cd ²⁺	474	652	5.682	5179	1.33

*The asterisks represent the wavelength of free DPAPD measured in EtOH only.

Notably, DBPD exhibits a preferential affinity toward Co²⁺, whereas DPAPD demonstrates enhanced selectivity for Cu²⁺ ions. The superior binding performance of Co²⁺ and Cu²⁺ relative to the other examined ions can be ascribed to their favorable coordination geometries and strong electronic coupling with the donor-acceptor framework of the Bis-chalcone systems. This interaction is expected to facilitate efficient perturbation of the intramolecular charge-transfer (ICT) process, thereby stabilizing the resulting complexes and amplifying the observed fluorescence response.

The fluorescence quenching behavior of the investigated Bis-chalcone chemosensors in the presence of various metal ions was systematically analyzed using the Stern–Volmer formalism. The relationship between the relative fluorescence intensity and quencher concentration is described by³¹

F / F_{o} = 1 + K_{SV} [M^{n +}]

(2)

where F₀ and F represent the emission intensities in the absence and presence of the quencher (metal ions), respectively, and K_SV is the Stern–Volmer quenching constant.

The obtained Stern–Volmer plots (Figure S2, Supplemental Material) exhibit noticeable upward curvature, particularly at elevated metal ion concentrations. Such positive deviations from linearity indicate that the quenching process cannot be adequately described by a purely dynamic (collisional) mechanism. Instead, the observed behavior strongly suggests the coexistence of static quenching, arising from ground-state complex formation between the chemosensors and the metal ions. This implies the formation of non-fluorescent or weakly emissive complexes prior to excitation.

To further elucidate the quenching mechanism, the nonlinear Stern–Volmer data were treated using the modified double-reciprocal model³¹

F / F_{o} - F = 1 / f_{o} + 1 / f_{o} k_{sv} 1 / [Q]

(3)

where F_o denotes the fraction of fluorophores accessible to the quencher. The corresponding modified Stern–Volmer plots (Figure S3, Supplemental Material) display excellent linearity, with correlation coefficients (R²) in the range of 0.97–0.99, confirming the validity of this model for describing the system.

The Stern–Volmer constants were extracted from the intercepts and slopes of these linear plots, and the calculated values are compiled in Tables I and II. The results demonstrate that the studied chemosensors exhibit pronounced sensitivity and selectivity toward specific metal ions, with DBPD showing a preferential response to Co²⁺ ions, while DPAPD displays enhanced selectivity toward Cu²⁺ ions. This selective quenching behavior can be attributed to the stronger binding affinity and more efficient electronic interaction of these ions with the donor–acceptor framework of the Bis-chalcone systems, leading to more effective fluorescence suppression.

The reliability of an analytical method is continually assessed based on a range of parameters, prioritized in the following order: recovery, sensitivity, selectivity, analyte stability, analytical simplicity, required expertise, time efficiency, and cost. Among these, a key criterion during method validation is the optical limit of detection (LOD), which represents the lowest concentration of an analyte that can be reliably detected, though not necessarily quantified, by the analytical technique. In this study, the LOD values for the synthesized Bis-chalcones in response to the target metal ions were determined. The calculation of the LOD for the free Bis-chalcones was performed using the following formula³²

LOD = 3 S / b

(4)

where S is the standard deviation of the blank,

S = \sqrt{\frac{(F - \bar{F})^{2}}{N - 1}}

, F is the fluorescence intensity of the Bis-chalcone with metal ion,

\bar{F}

is the average blank intensity, and N is the number of measurements. The parameter b corresponds to the slope of the calibration curve (Figure S4). The calculated limits of detection (LOD) for the metal ions were compiled and are presented in Tables I and II. The determined LOD for the studied metal ions using the synthesized probes were below the maximum allowable concentrations specified by the World Health Organization (WHO) for drinking water.³³ Although these detection limits were slightly higher than those reported for some advanced sensing platforms, they were still significantly lower than many previously published values for other organic sensor systems,^34–40 indicating good sensitivity of the probes.

The comparative data presented in Table III clearly demonstrate the analytical performance of the previously reported probes in terms of their LOD toward various metal ions. Most of the reported systems exhibit LOD values generally in the micromolar range depending on the metal ion and detection method. Although some systems show relatively improved sensitivity for specific ions (e.g., Co²⁺ or Fe³⁺), their performance is often limited either by moderate sensitivity, lack of selectivity, or dependence on dual-mode detection techniques.

Table III.

Comparative features of selected recently reported metal ion probes.

Probe	Measured signal	Metal ions	LOD	Reference
Substituted 1,2,3-triazole-indolin-2-one siloxy framework derivatives	UV–Vis	Cu²⁺	3.21 µM	³⁴
Bis thiophene chalcone	UV–Vis/fluorescence	Fe³⁺	11.703 μM	³⁵
(E)-1-(2-(bis((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)amino)phenyl)-3-(4-chlorophenyl)prop-2-en-1-one	UV–Vis/ fluorescence	Co²⁺ Cu²⁺	1.64 μM 3.19 μM	³⁶
(E)-1-(3-(bis((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)amino)phenyl)-3-(4-chlorophenyl)prop-2-en-1-one	UV–Vis/ fluorescence	Co²⁺ Cu²⁺	2.08 μM 2.30 μM	³⁷
(E)-1-(4-(bis((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)amino)phenyl)-3-(4-chlorophenyl)prop-2-en-1-one	UV–Vis/ fluorescence	Co²⁺ Cu²⁺	1.81 μM 1.17 μM	³⁸
Triethoxysilane tethered ferrocene–chalcone–triazole triad	Colorimetric, UV–Vis/ fluorescence	Cu²⁺	9.42 μM	³⁷
2E, 2Eʹ)-1,1ʹ- (2,6-dimethyl-1,4-dihydropyridine-3,5-diyl)bis(3-(4-(dimethylamino)phenyl)prop-2-en-1-one)	Colorimetric, UV–Vis/ fluorescence	Fe³⁺ Co²⁺ Ni²⁺ Cu²⁺ Zn²⁺	4.69 × 10^–4M 4.70 × 10^–4M 0.14 × 10^–4M 1.64 × 10^–4M 3.38 × 10^–4M	³⁸
(2E,2E’,4E,4E’)-1,1ʹ -(2,6-dimethyl-1,4-dihydropyridine-3,5-diyl)bis(5-(4-(dimethylamino)phenyl) penta-2,4-dien-1-one)	Colorimetric, UV–Vis/fluorescence	Fe³⁺ Co²⁺ Ni²⁺ Cu²⁺ Zn²⁺	6.73 × 10^–4M 4.10 × 10^–4M 4.46 × 10^–4M 3.62 × 10^–4M 0.96 × 10^–4M	³⁸
(2E,4E)-5-(4- (dimethylylamino)phenyl)-1-(thiazol-2-yl)penta-2,4-dien-1-one	Colorimetric, UV–Vis/fluorescence	Fe³⁺ Co²⁺ Ni²⁺ Cu²⁺ Zn²⁺	3.20 × 10^–3M 0.153 × 10^–3M 0.762 × 10^–3M 1.33 × 10^–3M 0.175 × 10^–3M	³⁹
Chalcone-based fluorescent chemosensor	UV–Vis/fluorescence	Fe³⁺	3.49 μM	⁴⁰
3-(3-(4-dimethylamino)phenyl)allylidene)pentane-2,4-dione (DPAPD)	Colorimetric, UV–Vis, fluorescence, reversibility, selectivity	Fe³⁺ Co²⁺ Ni²⁺ Cu²⁺ Zn²⁺ Pb²⁺ Cd ²⁺	0.78 μM 1.57 μM 1.28 μM 0.046 μM 0.94 μM 1.84 μM 1.33 μM	^{This work}
3-(4-(dimethylamino)benzylidene)pentane-2,4-dione (DBPD)	Colorimetric, UV–Vis, fluorescence, reversibility, selectivity	Fe³⁺ Co²⁺ Ni²⁺ Cu²⁺ Zn²⁺ Pb²⁺ Cd ²⁺	18.1 μM 11.8 μM 0.67 μM 0.4 μM 12.8 μM 3.86 μM 3.61 μM	^{This work}

In contrast, our synthesized compounds (DPAPD and DBPD) display significantly enhanced analytical performance, particularly in terms of sensitivity and selectivity. Notably, the LOD values of our probes toward key metal ions such as Fe³⁺, Co²⁺, Ni²⁺, and Cu²⁺ fall within a markedly lower range, outperforming most of the previously reported systems listed in the table. For example, DPAPD exhibits superior detection limits compared to several literature probes that require higher concentrations for effective sensing. Similarly, DBPD demonstrates remarkable sensitivity, especially toward Fe³⁺ and Cu²⁺ ions, with improved reversibility and selectivity.

The enhanced performance of our compounds can be attributed to the strong donor–acceptor electronic framework and efficient charge-transfer interactions, which facilitate more pronounced optical responses upon metal binding. Additionally, the ability of these probes to operate effectively using simple colorimetric and fluorescence techniques further highlights their practical applicability.

This comparison underscores the superiority of our designed sensors over previously reported systems, making them promising candidates for sensitive, selective, and cost-effective detection of metal ions in environmental and analytical applications.

The solid metal complexes formed between the Bis-chalcone chemosensors DBPD and DPAPD with their most strongly binding metal ions, Co²⁺ and Cu²⁺ respectively, have been successfully synthesized and characterized using molar conductance, FT-IR, TGA, and XRD techniques.

The molar conductance values of DBPD–Co²⁺ and DPAPD–Cu²⁺ complexes determined in DMF (1 mM) solution at room temperature were found to be Λ_M = 33.31 and 41.31 Ω⁻¹ cm² mol⁻¹, respectively. These numbers confirmed the non-electrolytic behavior of DBPD–Co²⁺ and DPAPD-Cu²⁺ complexes, proving that all chloride ions were present inside the coordination sphere of the complexes.⁴¹

Figure 6a displays the FT-IR spectra of the free chemosensors DBPD and DPAPD, as well as their corresponding metal complexes. The FT-IR spectrum of free DBPD shows characteristic absorption bands at 1162, 1620, 2906, and 3435 cm⁻¹, corresponding to the stretching vibrations of the C–N bond, carbonyl (C=O) group, aliphatic C–H, and aromatic C–H groups, respectively. Upon coordination with Co²⁺ ions, the carbonyl stretching band shifts to a higher wavenumber by 36 cm⁻¹, appearing at 1656 cm⁻¹, indicating involvement of the carbonyl oxygen in coordination. Additionally, a broad, intense absorption band appears at 3451 cm⁻¹, attributed to the O–H stretching vibrations of coordinated and hydrated water molecules in the complex. A new absorption band appeared in the infrared spectra of DBPD-Co²⁺ complex at 529 cm⁻¹ assigned to (Co–O) bond which is absent in the infrared spectrum of free chemosensor, DBPD.

Figure 6.

(a) FT-IR, (b) TGA, and (c) XRD of the free Bis-chalcone DBPD and DPAPD and their metal complex with Co²⁺ and Cu²⁺, respectively.

Similarly, the FT-IR spectrum of free DPAPD displays absorption bands at 1166, 1583, 2898, and 3442 cm⁻¹, corresponding to C–N, C=O, aliphatic C–H, and aromatic C–H stretching vibrations, respectively. In the FT-IR spectrum of the DPAPD–Cu²⁺ complex, the carbonyl band shifts to a lower wavenumber by 16 cm⁻¹, appearing at 1611 cm⁻¹, suggesting coordination through the carbonyl oxygen atom. A broad band at 3422 cm⁻¹ is also observed, consistent with the presence of coordinated and lattice water molecules. Additionally, a new absorption band appeared in the infrared spectra of DPAPD–Cu²⁺ complex at 522 cm⁻¹ assigned to (Cu–O) bond which is absent in the infrared spectrum of free chemosensor, DPAPD.

These spectroscopic shifts confirm that both DBPD and DPAPD chelate Co²⁺ and Cu²⁺ ions, respectively, through the oxygen atoms of their carbonyl groups, forming stable metal-chemosensor complexes.

Under specific experimental conditions, heat induces both physical and chemical changes in most substances, including organometallic complexes. These changes, which reflect the intrinsic properties of the substances, can be examined both qualitatively and quantitatively. Thermal analysis is employed to assess the thermal stability of metal complexes, determine whether solvent molecules are located within or outside the coordination sphere, calculate the metal ion content, and outline the general decomposition pathway of the complexes.

The TG and DTG curves of the DBPD-Co²⁺ and DPAPD-Cu²⁺ complexes are shown in Figure 6b. The decomposition stages, temperature ranges, DTG peak temperatures, and both experimental and calculated mass loss values are summarized in Table IV.

Table IV.

Thermoanalytical results (TG-DTG) of investigated of DBPD–Co²⁺ and DPAPD–Cu²⁺ complexes.

Complex (Mol. wt.)	Temp. range (°C)	Mass loss %		DTG (°C)	Assignment
Complex (Mol. wt.)	Temp. range (°C)	Calculated	Found	DTG (°C)	Assignment
DBPD-Co²⁺ complex (430.2) [Co(DBPD Cl₂(H₂O)]3 H₂O	28.22–110	11.55	11.51	51.94	Loss of 3H₂O (Lattice)
	110–175.89	4.2	4.28	132.5	Loss of 1H₂O (coordination)
	175.89–281.97	8.28	8.32	241.14	Loss of Cl
	281.97–363.08	8.28	8.32	317.15	Loss of Cl
	363.08–801.74	33.41	33.14		Loss of C₈H₁₉NO
	801.74	34.28	34.33	–	CoO+6C residue
Cu²⁺-DPAPD complex (383.76)[CuLCl₂] H₂O	30.41–103.9	4.39	4.35	55.78	Loss of H₂O (Lattice)
	103.9–205.79	8.66	9.70	154	Loss of Cl^– ion
	217.81–345	8.66	8.67	303	Loss of Cl^– ion
	345–812.71	47.15	47.18		Decomposition of part of the organic compoundC₁₂H₁₉NO^•
		31.14	31.10		CuO +4C

The TG curve of the DBPD-Co²⁺ complex reveals four distinct thermal decomposition steps: The first step shows a DTG peak at 51.94 °C and a mass loss of 11.51% (calculated: 11.55%), corresponding to the loss of three moles of crystallization water. The second step occurs within the temperature range of 110–175.9 °C, with a DTG peak at 132.5 °C and a mass loss of 4.28% (calculated: 4.2%), attributed to the removal of one mole of coordinated water. The third decomposition stage, from 175.9 °C to 363.1 °C, involves the release of two moles of chloride ions, with a DTG peak at 317.2 °C. The fourth step takes place between 363.1 °C and 801.74 °C, corresponding to the partial decomposition of the organic moiety (C₈H₁₉NO), resulting in a mass loss of 33.14% (calculated: 33.41%). Beyond this temperature range, a gradual mass loss continues, reaching a total of 34.33%, which is consistent with the formation of a CoO + 6C residue.

In contrast, the DPAPD-Cu²⁺ complex undergoes three primary decomposition steps: The first step occurs between 30.41–103.9 °C, with a DTG peak at 55.78 °C and a mass loss of 4.35% (calculated: 4.39%), corresponding to the loss of one lattice water molecule. The second step, between 103.9 °C and 345 °C, is associated with the removal of two moles of chloride ions and features a DTG peak at 303 °C. The third decomposition stage, from 345 °C to 812.71 °C, corresponds to the breakdown of the organic moiety (C₁₂H₁₉NO), with a mass loss of 47.18% (calculated as 47.15%). Following this stage, a gradual further mass loss occurs, leading to a final residue corresponding to CuO + 4C.

Figure 6c illustrates the X-ray diffraction (XRD) patterns of DBPD, DPAPD, and their corresponding metal complexes, DBPD-Co²⁺ and DPAPD-Cu²⁺. The XRD pattern of pure DBPD exhibited a broad hump rather than sharp diffraction peaks. This broad scattering feature is characteristic of an amorphous structure, where there is a lack of long-range atomic order. In amorphous materials, the atoms or molecules are arranged in a disordered fashion, leading to diffuse X-ray scattering in all directions rather than discrete Bragg reflection.

In contrast to the free chemosensor DBPD, the XRD pattern of the DBPD-Co²⁺ complex exhibits well defined crystalline peaks at 2θ values of 15.67°, 31.78°, and 45.75°, confirming its crystalline nature. This crystallinity is attributed to the intrinsic ordered structure imparted by the cobalt(II) metal center. The significant difference in the XRD patterns between the free chemosensor and its metal complex highlights the structural transformation upon complexation. The crystallite size (d_XRD) was calculated using Scherrer’s equation^42,43

d_{XRD} = (0.9 λ) / (β \cos θ)

(5)

where λ is the X-ray wavelength, β is the full width at half-maximum (FWHM) of the diffraction peak, and θ is the Bragg angle. The resulting average crystallite size for the DBPD-Co²⁺ complex was found to be 15.53 nm.

The DPAPD chemosensor, on the other hand, showed a distinctly different XRD profile compared to DBPD. Its pattern exhibited sharp peaks at 6.41°, 12.7°, 24.81°, 28.16°, and 31.64°, suggesting that DPAPD is inherently more crystalline than DBPD. The presence of these defined peaks indicates that DPAPD molecules adopt a more ordered structure even in their uncoordinated state.

Upon complexation with Cu²⁺, the DPAPD-Cu²⁺ complex displayed even more intense and numerous diffraction peaks at 16.47°, 22.11°, 26.67°, 29.10°, 34.34°, 41.19°, 44.8°, 49.37°, 51.49°, and 57.57°. This increase in crystallinity is attributed to the formation of a metal–chemosensor coordination framework, which further stabilizes the molecular packing and enhances the long-range order within the material.

These findings confirm that coordination with transition metal ions (Co²⁺ and Cu²⁺) significantly enhances the crystallinity of both DBPD and DPAPD chemosensors. While DBPD transitions from an amorphous to a crystalline state upon metal binding, DPAPD already exhibits further structural refinement and increased crystallinity upon complexation.

The optical sensing behavior of DBPD toward Co²⁺ and DPAPD toward Cu²⁺ was further examined in terms of reversibility using ethylenediaminetetraacetic acid (EDTA) as a competitive chelating agent. Upon complexation with the target metal ions, the absorption spectra of both systems underwent significant changes; however, the subsequent introduction of EDTA resulted in the complete restoration of the original spectral profiles corresponding to the free ligands (Figure S5, Supplemental Material). This observation confirms the dissociation of the metal–ligand complexes and the regeneration of the pristine chemosensors.

Such reversible behavior indicates that the interaction between the Bis-chalcone frameworks and the metal ions is governed by dynamic, non-covalent coordination rather than irreversible binding. Consequently, the system exhibits the essential characteristics of a reusable optical sensing platform.

In parallel, fluorescence measurements revealed a pronounced recovery of the quenched emission upon EDTA addition to the pre-formed metal–chemosensor complexes (DBPD + 4.0 × 10⁻⁵ M Co²⁺ and DPAPD + 5.0 × 10⁻⁵ M Cu²⁺), effectively switching the emission state from “off” back to “on” (Figure S5, Supplemental Material). This fluorescence restoration is attributed to the superior binding affinity of EDTA toward the metal ions, which facilitates competitive sequestration and disrupts the initially formed complexes, thereby releasing the free fluorophore.

Collectively, these results establish a well-defined “on–off–on” fluorescence switching mechanism for both DBPD and DPAPD in response to Co²⁺ and Cu²⁺ ions, respectively. The efficient reversibility, coupled with high selectivity, highlights the robustness of these systems as switchable chemosensors. Furthermore, the distinct ion-specific responses exhibited by DBPD and DPAPD emphasize their potential utility in the development of selective sensing platforms capable of discriminating between closely related metal ions in both visual and spectroscopic detection schemes.

The present study delves into the impact of coexisting metal ions on the efficacy of Bis-chalcones, specifically DBPD and DPAPD, in the detection of Co²⁺ and Cu²⁺ ions, respectively. This investigation employs competitive complexometric fluorometric titration as a paradigm, considering that selectivity constitutes a pivotal attribute in optical sensor applications. The alteration in fluorescence intensity of DBPD and DPAPD is gauged in the presence of Co²⁺ and Cu²⁺ ions, each at a concentration of 4 × 10⁻⁴ M, alongside equivalent concentrations of interfering metal ions. The outcomes demonstrate that the examined interfering metal ions exert a marginal influence on the emission intensity during the Co²⁺ and Cu²⁺ ion detection process for DBPD and DPAPD, respectively, as depicted in Figures 7 and 8. These discernments collectively underscore the potential of both DBPD and DPAPD as promising chemosensors, manifesting high selectivity and proficiency in the accurate identification and quantification of Co²⁺ and Cu² ⁺ ions in real samples.

Figure 7.

Selectivity of DBPD to Co²⁺ ions with the different metal ions in ethanolic solution at the same concentration (4 × 10⁻⁴ M).

Figure 8.

Selectivity of DPAPD to Cu²⁺ ions with the different metal ions in ethanolic solution at the same concentration (4 × 10⁻⁴ M).

Conclusion

In summary, the previously synthesized Bis-chalcones, DBPD and DPAPD, have been successfully revisited to uncover their unexplored metal-responsive properties. Both chemosensors exhibited distinct and selective optical responses toward Co²⁺, Cu²⁺, Ni²⁺, and Pb²⁺, while remaining unresponsive to Zn²⁺, Cd²⁺, and Fe³⁺. Spectroscopic analyses revealed clear evidence of complexation and intramolecular charge transfer mechanisms, with 1:1 chemosensor–metal stoichiometry and strong binding affinities for Co²⁺ and Cu²⁺. The formation of solid metal complexes, along with reversible optical behavior upon EDTA treatment, demonstrated the dynamic and reusable nature of these interactions. Competitive studies further confirmed minimal interference from other metal ions, highlighting the high selectivity and sensitivity of DBPD and DPAPD. These Bis-chalcones represent versatile, cost-effective, and reliable fluorescent chemosensors with significant potential for environmental monitoring and selective detection of heavy metal ions in practical applications.

Supplemental Material

sj-docx-1-asp-10.1177_00037028261459472 - Supplemental material for Bis-Chalcones as Efficient Optical Chemosensors for Rapid and Visual Detection of Toxic Metal Ions

Supplemental material, sj-docx-1-asp-10.1177_00037028261459472 for Bis-Chalcones as Efficient Optical Chemosensors for Rapid and Visual Detection of Toxic Metal Ions by Basma Hussein, Ahmed A. Noser and Marwa N. El-Nahass in Applied Spectroscopy

Footnotes

Acknowledgments

The authors are grateful to the Scientific Research Fund at Tanta University, Egypt, for funding this work through Research Project Code TU:02-19-03.

Consent for Publication

Not applicable.

CRediT Author Statement

B.H.: methodology, investigation, resources, writing-original draft. A.A.N.: Resources, Supervision M.N.E.: conceptualization, investigation, methodology, writing-original draft, writing-review & editing draft, supervision.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Marwa N. El-Nahass

Supplemental Material

All supplemental material mentioned in the text accompanies this paper online.

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