Introduction

Heavy metals are a group of naturally occurring elements characterized by their high atomic weight and density [1]. While some of these metals play very important roles in different natural processes and human activities, their accumulation in the environment has raised major concerns due to their adverse effects on both human health and ecosystems [1,2,3,4]. The introduction of these metals into the environment occurs through both natural processes, such as weathering of rocks and volcanic activity, as well as anthropogenic activities, such as industrial processes, mining, waste disposal, agriculture processes and many others [5,6,7,8]. These metals can persist in the environment for extended periods due to their nondegradable nature, leading to their accumulation in water, soil, and air [9,10,11]. As a consequence, these metals cause serious environmental problems and have the potential to disrupt ecosystems and threaten human health, even at a trace level. The detrimental impact of heavy metals on the environment and living organisms is well-documented [3, 11,12,13,14]. Heavy metals can enter the food chain through processes such as bioaccumulation and biomagnification, which occur as metals are ingested by lower organisms and subsequently transferred to higher trophic levels. This accumulation can lead to significant health risks for humans and wildlife. In humans, exposure to heavy metals can have profound and deleterious effects on various organ systems. Acute heavy metal intoxication can lead to severe damage to vital organs such as the cardiovascular, gastrointestinal, and central nervous systems. Prolonged exposure to heavy metals, even in trace amounts, can result in chronic health issues, affecting various organs such as liver, kidneys, lungs, bones and endocrine glands [1, 4, 15,16,17,18]. Among these metals, Cu2+ has attracted considerable attention due to their diverse roles in chemical, biological and industrial processes. Its distinctive properties, including its excellent malleability, conductivity, and corrosion resistance, have led to its extensive use in diverse applications ranging from electrical wiring and plumbing to electronics [4, 19,20,21,22]. In biological systems, Cu2+ ions are essential to the proper functioning of proteins and enzymes involved in cellular processes. Enzymes such as superoxide dismutase, tyrosinase and cytochrome c oxidase depend on Cu2+ ions for their catalytic activities. Additionally, Cu2+ ions are essential for the development and maintenance of immune response modulation, iron metabolism regulation and connective tissues [23,24,25,26]. However, despite its essentiality, an excess amount of Cu2+ ions can lead to toxic effects. Excessive Cu2+ ions exposure can lead to cellular damage, oxidative stress, neurological disorders, and many other health issues [27,28,29]. On the other hand silver (Ag) has garnered substantial attention among various metals over recent decades. Renowned for its relatively soft nature, exceptional ductility, and malleability, silver stands as a premier transition metal that shows supreme thermal and electrical conductivity. This adaptability extends to its capacity to be molded into sheets or drawn into wires, rendering it invaluable across an extensive spectrum of applications. It has a wide range of applications in the electronics industry, catalysis, jewelry, long-life batteries, pharmaceuticals, mirror manufacturing, and aerospace applications [30, 31]. However, this widespread utility has many implications. Effluents containing Ag+ ions, stemming from diverse applications, have found their way into the environment over the years, inducing various health concerns. Chronic exposure to Ag+ ions has been linked to liver and kidney issues, argyrosis, skin and eye irritation, respiratory and intestinal tract effects, and alterations in blood cells. Enzymes can fall victim to their inactivation, as Ag+ ions displace essential metal ions or bind to key functional groups, throwing biological systems into disarray [32,33,34,35]. Therefore, it is very important to develop highly sensitive and selective methods for the detection of these metal ions. Several techniques such as atomic absorption and emission spectroscopy, high-performance liquid chromatography, inductively coupled plasma spectrometry, anodic stripping voltammetry, capillary electrophoresis and many other techniques have been reported for the detection of these metal ions [36,37,38,39,40,41,42,43,44]. These methods have excellent selectivity and sensitivity but still suffer from certain limitations such as complicated operations, excessive sample pretreatment and expensive [45, 46]. Hence, the development of a sensitive, straightforward, cost-effective and selective methods to detect Cu2+ and Ag+ ions in diverse samples is very important. Among the various analytical techniques available, colorimetric and spectrofluorimetric methods stand out for their inherent simplicity, exceptional sensitivity, and remarkable selectivity. These techniques are instrumental in both quantitative and qualitative assessment of various metal ions and other pollutants without the need for expensive or sophisticated equipment. Over the years, numerous colorimetric and fluorescent chemosensors have been engineered to detect various metal ions, utilizing diverse sensing mechanisms. However, an ideal chemosensor must exhibit high selectivity and sensitivity towards the target analyte, water solubility, visible color change, and intense fluorescence. Schiff bases are indeed known for being relatively inexpensive, easy to synthesize and have excellent photophysical properties. These compounds containing N, O, and S atoms are particularly effective due to their strong coordinating abilities with various metal ions and give fluorescence signals, which could be used for the detection of various metal ions [47, 48]. In this study, we present the synthesis and characterization of a novel Schiff base-centered chemosensor ((E)-1-(((6-methylbenzo[d]thiazol-2-yl)imino)methyl)naphthalen-2-ol), and its application as a versatile tool for the colorimetric and fluorescence detection of Cu2+ and Ag+ ions in aqueous solutions. Through comprehensive spectroscopic analysis and mechanistic investigations, we demonstrate the remarkable selectivity and sensitivity of chemosensor towards these target metal ions. The visual color changes observed upon metal ion binding, combined with the precise analysis of absorption spectra, enable the quantification of these metal ions at trace levels. Our study highlights the potential of Schiff base chemosensors as valuable assets in the field of environmental monitoring and analytical chemistry, offering a versatile approach to addressing the challenges posed by heavy metal contamination.

Experimental

Materials and Methods

Methanol, ethanol, acetic acid, and distilled water were consistently employed throughout the experimental work. 2-amino-6-methyl benzothiazole, 2-hydroxy-1-nepthaldehyde, metal salts such as Li+, CuCl2, MgCl2, NiCl2, MnCl2, CoCl2, AlCl3, ZnCl2, HgCl2, NaCl, FeCl3, CdCl2, CrCl3, AgNO3 and Pb(acetate)2 were obtained from Sigma Aldrich. The FT-IR data was recorded using FT IR spectrophotometer (ABB MB3000) Japan. 1H NMR spectra were obtained using a Varian INOVA 300 MHz spectrometer. For UV-Vis spectra analysis a Shimadzu UV/Visible spectrophotometer (UV-3600 plus) was used. Emission spectra of the chemosensor were recorded using the F-7000: Hitachi fluorescence spectrophotometer.

Synthesis of SB

SB was synthesized following the procedure described in the literature [49] as depicted in Scheme 1. 2-hydroxy-1-nepthaldehyde (1.0 mmol) was dissolved in 20 mL of methanol and a few drops of acetic acid were subsequently added, and the mixture was stirred vigorously for 20 minutes. A methanolic solution containing 2-amino-6-methyl benzothiazole (1.0 mmol) was then introduced, and the reaction was refluxed for 4 hours. The reaction progress was monitored with the help of thin-layer chromatography (TLC). Upon the completion of the reaction, the mixture was subsequently cooled to room temperature, resulting in the formation of yellow precipitates. These precipitates were isolated by filtration and washed several times with methanol. The resultant product was subjected to comprehensive characterization using modern spectroscopic techniques, including FT-IR, 1H-NMR, and UV-Vis spectroscopy (Figs. 1, 2 and 3). Yield: 92%; IR (KBr, cm-1): 3400 (phenolic OH), 1617 (C=N imine group), 1549 (C=N thiazole group), 1315 (phenolic C-O), 818 (C-S-C); 1H-NMR (300 MHz, CDCl3, δ, ppm): 2.482 (s, 3H, Methyl), 7.159 (d,1H, Hb, j = 9.604), 7.243 (s, 1H, Hi), 7.314(d, 1H, Hj, j = 8.644 Hz), 7.404 (t, 1H, He, j = 8.164 Hz), 7.623 (t, 1H, Hf, j = 7.683 Hz), 7.750 (d, 1H, Hd, j = 7.683 Hz), 7.859 (d, 1H, Hg, j = 8.644 Hz), 7.922 (d, 1H, Hi, j = 9.124 Hz), 8.316 (d, 1H, Hc, j = 9.604 Hz), 10.135 (s, 1H, CH=N), 14.423 (s, 1H, OH).

Scheme 1
scheme 1

Synthesis of ((E)-1-(((6-methylbenzo[d]thiazol-2-yl)imino)methyl)naphthalen-2-ol)

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General Procedure for Fluorescence  Measurements

The sensing properties of SB were studied toward various cations such as Mg2+, Ni2+, Co2+, Ag+, Al3+, Zn2+, Hg2+, Na+, Fe3+, Cd2+, Cr3+, Pb2+, Mn2+ and Cu2+ ions. A 100 ppm stock solution of SB was prepared in methanol, while the above metal salts were dissolved in double-distilled water to prepare a 100 ppm solution. The stock solution of SB was subsequently diluted to prepare the working solution (10 ppm) in methanol: water (98:2 v/v) system. Different metal ion solutions were combined with SB in a 1:1 ratio within separate vials. The fluorescence titration experiments were conducted by transferring 2ml solution from each one stock solution into a quartz cuvette and the fluorescence spectra were recorded after a 10-minute interval in each instance. The chemosensor SB distinct response to Cu2+ and Ag+ ions was subjected to in-depth exploration encompassing aspects like pH effect, LOD,LOQ, stoichiometric ratio, selectivity and the effect of time. Furthermore, the developed chemosensor was applied for detecting Cu2+ and Ag+ invarious water samples, including tap, pond and river water. The titration experiments were carried out at room temperature.

Samples Analysis

In order to assess the practical utility of the newly synthesized chemosensor SB, environmental samples (river water, tap water, pond water and soil samples) were subjected to analysis. These samples were spiked with a known quantity of Cu2+ or Ag+ ions and were subjected to triplicate experiments. The quantification of Cu2+ and Ag+ ions in these samples was conducted using the previously described procedure, and the percent recoveries were thoroughly investigated to gauge the accuracy of the method.

Results and Discussion

Characterization of SB

The chemosensor SB was subjected to comprehensive characterization using modern spectroscopic techniques, including UV-Vis, fluorescence, 1H-NMR and FT-IR (Figs. 1, 2 and 3). The UV–Vis absorption spectra (λmax) for the free chemosensor SB was recorded in methanol:water (98:2 v/v) system at room temperature, with a concentration of 10 ppm. The spectra were recorded across a wavelength range from 200 to 600 nm. In the UV-Vis absorption spectrum of SB, two distinct absorption peaks are prominently observed at 216 nm and 428nm, as depicted in Fig. 1a. These absorption bands hold significant implications as they are indicative of specific electronic transitions occurring within the molecule. The absorption peak centered at 216 nm is attributed to the π–π* transition. This transition involves the movement of an electron from a lower-energy π orbital to a higher-energy π* antibonding orbital. Such transitions typically occur in systems containing conjugated double bonds or aromatic rings, suggesting the presence of such structural motifs within SB. Conversely, the absorption peak at 428 nm is associated with the n–π* transition.

Fig. 1
figure 1

a UV-Vis spectra of SB (10 ppm). b Fluorescence spectra of SB (10 ppm)

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This transition involves the excitation of an electron from a non-bonding (n) orbital to a π* antibonding orbital. It is commonly observed in compounds containing electronegative atoms or groups, indicating the presence of imines and hydroxyl groups within the molecule [50]. The fluorescence spectra of the chemosensor was recorded at 523 nm when excited at 262 nm (Fig. 1b). The free chemosensor exhibited a relatively weak emission band at 523 nm. This weak emission photo-induced electron transfer (PET) from the N- atom imines group to naphthalene moiety. The 1H NMR spectra, highlighting distinct proton signals corresponding to specific parts of the molecule. Notably, a singlet peak at 2.482 ppm, signifies the presence of a methyl substituent. In the aromatic region, the spectrum reveals distinct peaks at 7.159 ppm, 7.243 ppm, 7.314 ppm, 7.404 ppm, 7.623 ppm, 7.750 ppm, 7.859 ppm, and 7.922 ppm, each representing different proton environments within the aromatic rings. Additionally, a singlet at 10.135 ppm confirms the presence of a CH=N group, while another singlet at 14.423 ppm indicates the presence of an -OH group (Fig. 2). Overall, the 1H-NMR spectrum provides precise information about the chemical shifts of protons in SB, facilitating the identification of the formation of the product.

Fig. 2
figure 2

1H NMR spectrum of ((E)-1-(((6-methylbenzo[d]thiazol-2-yl)imino)methyl)naphthalen-2-ol)

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FT-IR of SB and SB-Cu2+ and SB-Ag+

The FT-IR peaks of SB and its Cu2+ and Ag+ complexes are presented in Fig. 3. Notably, the disappearance of specific FT-IR peaks associated with amino and carbonyl functional group provides compelling evidence for the formation of SB. Additionally, the emergence of new absorption peaks, particularly at 1617 cm-1, is of significant importance. This specific band corresponds to the characteristic azomethine stretching vibration, confirming the condensation reaction between 2-hydroxy-1-nepthaldehyde and 2-amino-6-methyl benzothiazole. Additionally, the peak at 1549 cm-1 is associated with the C=N stretching vibration of the thiazole ring, confirming the presence of this specific motif. The absorption peak at 1315 cm-1 is indicative of the phenolic C-O stretching vibration. The appearance of absorption bands at 818 cm-1, corresponding to ν(C-S-C). Furthermore, the absorption peak at 3400 cm-1 is attributed to the phenolic hydroxyl (OH) group, indicating the presence of this moiety within the compound. In the complexes, a notable shift of the azomethine C=N band to lower frequencies by approximately 16 cm-1 for Cu2+ and 7 cm-1 for Ag+ was observed. This shift can be attributed to the withdrawal of electron density from the nitrogen atom. This change signifies the coordination of the C=N group to the metal ions. This phenomenon is indicative of the structural alteration upon complexation. Simultaneously, the peak observed at 1315 cm-1 of SB due to C-O phenolic group, exhibited a shift towards lower frequencies by approximately 12 cm-1 for Cu2+ and 5 cm-1 for Ag+. This shift is consistent with the coordination of the phenolic oxygen to the metal ions within the complexes. The distinctive band attributed to –OH stretching vibrations at 3400 cm-1 in the Schiff base was conspicuously absent in the complexes. This absence can be attributed to the deprotonation of the –OH group upon complexation, indicating the interaction of the phenolic oxygen with the metal ions. In the low-frequency region of the spectra, the complexes displayed new bands at 560 cm-1 and 459 cm-1 for the Cu2+ complex, 564 cm-1 and 466 cm-1 for the Ag+ complex. These bands were assigned to ν(M-O) and ν(M-N) stretching vibrations, respectively. This observation provides evidence for the presence of specific metal-oxygen and metal-nitrogen bonding interactions within the complexes. The ν(C-S-C) stretching vibration, which occurs at 818 cm-1, and the ν(-C=N-) stretching vibration at 1549 cm-1, remained practically unchanged in the complexes. This consistency supports the notion that the nitrogen and sulfur of the thiazole moiety are not directly involved with the metal ions [51,52,53].

Fig. 3
figure 3

FT-IR spectraof SB, SB-Cu2+ and SB-Ag+

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Naked Eye Colorimetric Detection of Cu2+and Ag+ Ions

The naked eye colorimetric studies were performed toward various cations such as Li+, Mg2+, Ni2+, Co2+, Ag+, Al3+, Zn2+, Hg2+, Na+, Fe3+, Cd2+, Cr3+, Pb2+, Mn2+ and Cu2+ ions in a methanol:water (98:2 v/v) system at room temperature. Equimolar amounts of the aforementioned metal ions were separately introduced into the chemosensor solution. Significantly, the color of chemosensor solutions changes within a remarkably short reaction time of 1 minute upon the addition of respective metal ions. Notably, the color changes from yellow to yellowish orange for Cu2+ and yellow to radish for Ag+ ions (Fig. 4). This discernible alteration can be readily observed by the naked eye, exemplifying the simplicity and efficiency of the method. In Fig. 4, a digital photograph illustrates the evident color change in a solution containing chemosensor SB when introduced to different metal ions. This straightforward visual distinction shows the method potential for sensitive and selective multi ions detection in aqueous environments.

Fig. 4
figure 4

Colorimetric changes of chemosensor SB(10 ppm) in methanol: water (98:2 v/v) in the presence of 1equivalent of different metal ions

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Sensing Properties of SB

To explore the sensing capabilities of chemosensor SB, absorption spectra were recorded for the chemosensor SB (10 ppm) in the presence of various metal ions at a concentration of 1 ppm. Various metal ions such as Li+, Mg2+, Ni2+, Co2+, Ag+, Al3+, Zn2+, Hg2+, Na+, Fe3+, Cd2+, Cr3+, Pb2+, Mn2+ and Cu2+ ions were added to the chemosensor solution (methanol: water (98:2 v/v)) at room temperature to understand their impact on the absorption properties of chemosensor SB. The free chemosensor exhibited two distinct absorption peaks prominently observed at 216 nm and 428 nm, as depicted in Fig. 5. The absorption peak centered at 216 nm is attributed to the π–π* transition. Conversely, the absorption peak at 428 nm is associated with the n–π* transition. Remarkably, substantial changes were observed in the absorption properties of chemosensor SB upon interaction with Cu2+ or Ag+ ions. The addition of Cu2+ and Ag+ ions induced noticeable changes, causing absorbance values to alter along with distinctive bathochromic spectral shifts of 44 and 60 nm, respectively, in the absorption spectrum of the chemosensor. This distinct bathochromic spectral shift in the absorption spectrum, specific to Cu2+ and Ag+ ions, not only differentiated them from other metal ions but also set them apart from each other. The changes in absorption properties, coupled with the observed color changes, strongly suggested significant interactions between the respective metal ions (Cu2+ and Ag+ ions) and functional groups, such as sulfur (C=N) and hydroxyl (-OH) groups, present in chemosensor.

Fig. 5
figure 5

UV-Vis spectra of SB and its complexes with Cu2+and Ag+

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The fluorescence characteristics of SB were also investigated toward Li+, Mg2+, Ni2+, Co2+, Ag+, Al3+, Zn2+, Hg2+, Na+, Fe3+, Cd2+, Cr3+, Pb2+, Mn2+ and Cu2+ in methanol: water (98:2 v/v) system. The free chemosensor exhibited a relatively weak emission band at 523 nm when excited at 262 nm. This weak emission is due to PET from N- atom of imine group to naphthalene moiety. Upon the introduction of the above metal ions, a significant fluorescence enhancement was observed at 523 nm only for Cu2+ and Ag+ ions (Fig. 6). This fluorescence enhancement is due to the formation of complexes of SB with Cu2+ and Ag+ ions, which inhibit the PET effect. To gain further insights into Cu2+ and Ag+ ions sensing capabilities of SB, we conducted a titration experiment, gradually increasing the concentration of Cu2+ and Ag+ ions, (ranging from 0.005 to 1.00 ppm for Cu2+ and 0.004 to 1.2 ppm for Ag+ ) in a methanol: water (98:2 v/v) medium (Figs. 7 and 8).

Fig. 6
figure 6

Fluorescence spectra of SB (10 ppm) in the presence of various metal ions (1 ppm)

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Fig. 7
figure 7

Fluorescence spectra of SB (10 ppm) with various concentration of Ag+ (0.004-1.2 ppm)

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Fig. 8
figure 8

Fluorescence spectra of SB (10 ppm) with various concentration of Cu2+ (0.005-1ppm)

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The limit of detection (LOD) for Cu2+ and Ag+ ions were calculated as 0.0016 ppm (0.0251 µM) and 0.00389 ppm (0.0360 µM) respectively by using the formula: LOD = 3.3σ/S, and the limit of quantification (LOQ) was calculated as 0.00495 and 0.00389 µM, respectively by using the formula: LOD = 10/S, with the equation 10σ/S. Here, "σ" denotes the standard deviation, and "S" represents the slope of the curve obtained by plotting F/Fo against the concentration of metal ions (Figs. 9 and 10).

Fig. 9
figure 9

The plot of fluorescence spectra of SB (10 ppm) with various concentration of Ag+ (0.004-1.2 ppm)

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Fig. 10
figure 10

The plot of fluorescence of SB (10 ppm) with various concentration of Cu2+ (0.005-1ppm)

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pH Effect

The pH effect on the sensing applications of SB was studied with pH values ranging from 2 to 12. In separate test tubes, solutions at varying pH levels were prepared, each containing chemosensor SB, and their fluorescence intensity was measured. As depicted in Figs. 11 and 12, an interesting pH-dependent behavior was observed for the SB-Cu2+ and SB-Ag+ complexes. Notably, within the pH range of 2 to 6, very low enhancement was detected. This observation suggests that the SB-Cu2+ and SB-Ag+ complexes do not form during this pH range. This phenomenon can be attributed to the protonation of -OH and -C=N- functional groups of SB, resulting in a reduction of its electron donation capability [54]. Consequently, the complexation between these metal ions and SB is hindered in this acidic pH range. However, as the pH increased from 7.0 to 11.0, a considerable increase in the fluorescence emission was observed at 523 nm, indicative of the formation of the SB-Cu2+ and SB-Ag+ complexes. This spectral change suggests that the interaction between the cations and SB becomes favorable as the solution becomes more neutral and basic. In this pH range, the deprotonation of relevant functional groups occurs, enhancing the electron donation ability and consequently leading to a stronger binding affinity between chemosensor and Cu2+ and Ag+ ions. The results conclusively indicate that the chemosensor SB can effectively serve as a detector for Cu2+ and Ag+ ions within the pH range of 7 to 11.

Fig. 11
figure 11

The effect of pH on the sensing response of SB toward Ag+ ions

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Fig. 12
figure 12

The effect of pH on the sensing response of SB toward Cu2+ ions

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Time Effect

Time-dependent studies were conducted to investigate the impact of time on the stability of the SB-Cu2+ and SB-Ag+ complexes, with observations extending up to 50 minutes. The results revealed that the reaction between SB and metal ions (Cu2+ and Ag+) is remarkably fast. After dilution, consistent maximum fluorescence emission at 523 nm was rapidly achieved within just 10 minutes. This maximum absorbance remained constant and stable for the entire 50-minute duration of the experiment, as illustrated in Figs. 13 and 14. This observation indicates that the SB-Cu2+ and SB-Ag+ complexes form rapidly and exhibit excellent stability over an extended period, making them suitable for practical applications requiring prolonged detection or monitoring of Cu2+ and Ag+.

Fig. 13
figure 13

The effect of time on the stability of SB-Ag+ complex

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Fig. 14
figure 14

The effect of time on the stability of SB-Cu2+ complex

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Interference Experiments

In order to assess the practical utility of SB as an effective chemosensor for Cu2+ and Ag+ ions, interference experiments were conducted in the presence of other competitive coexisting metal ions. These experiments were carried out with 1 equivalent of various metal ions, such as Li+, Mg2+, Ni2+, Co2+, Al3+, Zn2+, Hg2+, Na+, Fe3+, Cd2+, Cr3+, Pb2+ and Mn2+ ions. The results of these experiments revealed that the fluorescence intensity of SB by Cu2+ and Ag+ ions (1 equivalent each) remained unaffected in the presence of 1 equivalent of the other metal ions (Figs. 15 and 16). This observation suggests a strong interaction between chemosensor SB and Cu2+ or Ag+ ions in comparison to its interaction with other metal ions.

Fig. 15
figure 15

The effect various metal ions on the fluorescence intensity of SB-Ag+ complex

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Fig. 16
figure 16

The effect various metal ions on the fluorescence intensity of SB-Cu2+ complex

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Job’s Plot Analysis

The stoichiometric ration between chemosensor SB and Cu2+ or Ag+ ions investigated by Job’s plot analysis using various volume ratios of chemosensor SB and Cu2+ or Ag+ ions (Figs. 17 and 18). The fluorescence intensity of SB at 523 nm was recorded against the molar fraction of Ag+ or Cu2+. Remarkably, in each case, the maximum fluorescence intensity was consistently reached at a molar fraction of 0.3. These findings provide compelling evidence supporting a 2:1 stoichiometric ratio between SB and Ag+ or Cu2+ ions. Scheme 2 presents the proposed mechanism depicting the formation of the SB-Cu2+ and SB-Ag+ complexes.

Fig. 17
figure 17

Job’s plot of SB and Ag+ in methanol: water (98:2 v/v) system

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Fig. 18
figure 18

Job’s plot of SB and Cu2+ in methanol: water (98:2 v/v) system

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Scheme 2
scheme 2

Possible sensing mechanism of the designed chemosensor SB for Cu2+ or Ag+

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Sensing Mechanism

The development of effective sensing mechanisms is very important for the detection and analysis of specific ions or molecules in various applications, ranging from environmental monitoring to biomedical diagnostics. The chemosensor SB has demonstrated remarkable capabilities in detecting and interacting with Cu2+ and Ag+. When these cations were introduced into the chemosensor solution, they formed chelating complexes (SB-Cu2+ and SB-Ag+). The binding events primarily occur through the involvement of the phenolic oxygen and the nitrogen atom of the azomethine group (-CH=N-) (Scheme 2). These interactions represent pivotal steps in the sensing mechanism, leading to distinctive changes in the properties of SB. One of the notable alterations observed in the presence of Cu2+ or Ag+ is the pronounced color changes exhibited by SB in the presence of Cu2+ or Ag+ ions. The color of the chemosensor SB changes from yellow to yellowish orange for Cu2+ and yellow to radish for Ag+ ions. These color changes are reflective of the specific interactions occurring between SB and these metal ions.

Another intriguing aspect of the sensing mechanism is the appearance of red shifts of 44 nm for Cu2+ and 60 nm for Ag+ ions, which is a key indicator of the complex formation. This red shift can be attributed to two significant phenomena: ligand-to-metal charge transfer (LMCT) and the intramolecular charge transfer (ICT) effect. As these metal ions bind to the electron-donating groups of SB (O and N atoms), there is a reduction in the energy gap associated with the ICT band. This reduction results in the observed red shift in the absorption spectrum, providing spectroscopic evidence of complex formation. The fluorescence studies showed that the free chemosensor exhibited a relatively weak emission band at 523 nm due to the PET effect from the N-atom of imine group to the naphthalene moiety. Upon the introduction of Cu2+ and Ag+ ions, a significant fluorescence enhancement was observed at 523 nm. This fluorescence enhancement is due to the formation of complexes of SB with Cu2+ and Ag+ ions, which inhibit the PET effect.

To delve deeper into the binding stoichiometry between SB and Cu2+ or Ag+ ions, Job plot analysis was employed, which provides compelling evidence supporting a 2:1 stoichiometric ratio between SB and Ag+ or Cu2+ ions. This finding elucidates the precise nature of the interactions, where two molecules of chemosensor SB coordinate with one ion of the respective metal. Spectroscopic evidence provides additional insights into the structural alterations induced by complex formation. Notably, a conspicuous IR shift in the azomethine C=N band to lower frequencies was observed in the complexes. This shift signifies the withdrawal of electron density from the nitrogen atom, highlighting the coordination of the C=N group with the metal ions. Simultaneously, the peak observed at 1315 cm-1 of SB due to C-O phenolic group, exhibited a shift towards lower frequencies, also exhibited a shift towards lower frequencies. This shift aligns with the coordination of the phenolic oxygen to the metal ions within the complexes. Furthermore, the absence of the distinctive band attributed to -OH stretching vibrations at 3400 cm-1 in the Schiff base of the complexes is indicative of the deprotonation of the -OH group upon complexation. This absence shows the interaction of the phenolic oxygen with the metal ions, revealing a significant structural change during complex formation. In the low-frequency region of the spectra, the complexes exhibited new bands at 564-560 cm-1 and 466-459 cm-1. These bands were assigned to ν(M-O) and ν(M-N) stretching vibrations, respectively. This spectral evidence offers concrete proof of the presence of metal-oxygen and metal-nitrogen bonding interactions within the complexes. Remarkably, certain key vibrational modes, such as the stretching vibration of (C-S-C) and (-C=N-) at 818 and 1549 cm-1, respectively remained practically unchanged in the complexes. This consistency confirmed that nitrogen and sulfur of thiazole moiety are not directly involvedwith the metal ions. In conclusion, the sensing mechanism of chemosensor SB is a multifaceted process involving complexation with Cu2+ and Ag+ ions, resulting in distinct color, absorbance, and spectroscopic changes. These changes are supported by specific coordination interactions between SB and the metal ions, as evidenced by spectroscopic analysis and Job plot studies. This mechanistic understanding enhances the appreciation of SB as a versatile chemosensor for these metal ions.

Comparison the Developed Chemosensor With Previously Reported Work

The sensing performance of the developed chemosensor SB was compared with reported chemosensors. It was observed that the chemosensor SB demonstrates notable advantages in terms of several parameters. One of the key strengths of the chemosensor SB is its remarkable sensitivity. When compared with similar chemosensors in the literature, the SB consistently exhibits higher sensitivity, allowing for the detection of even trace amounts of Cu2+ and Ag+ (Table. 1). Additionally, the chemosensor SB exceptional performance in an aqueous medium deserves special recognition. Aqueous environments are frequently encountered in various real-world applications, such as environmental monitoring and biomedical diagnostics, where maintaining chemosensor sensitivity and stability is challenging. The chemosensor SB, however, excels in such conditions, making it a highly promising choice for applications where the presence of water is a significant factor.

Table 1 Comparison of the performance of SB with various reported work for the detection of Cu2+ and Ag+
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Sensing Applications of SB

The chemosensor SB was applied for detecting Cu2+ and Ag+ in r river water, tap water, pond water and soil samples. The results, as summarized in Table 2, showed excellent recoveries ranging from 90% to 105%. These findings highlight the reliability and versatility of chemosensor SB in environmental and analytical chemistry, for the sensitive detection of Cu2+and Ag+ ions. This suggests that SB holds promise as a valuable tool for monitoring and assessing these ions in real-world samples.

Table 2 % Recovery of Cu2+ and Ag+ form river, tap and pond water samples
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Conclusion

In this work we synthesized a novel Schiff base chemosensor SB and characterized by various spectroscopic techniques, including FT-IR, 1H NMR, and UV-Vis spectroscopy, to verify its structural properties. The sensing properties of chemosensor were investigated toward different metal ions. Among the tested metal ions, SB demonstrated exceptional selectivity and sensitivity toward Cu2+ and Ag+. Notably, this chemosensor facilitated rapid, observable color changes visible to the naked eye upon interaction with these target metal ions. One of the most striking features of SB was its impressively low detection limit, highlighting its potential to detect trace levels of Cu2+ and Ag+ ions. The mechanism underlying the change in color and fluorescence emission of SB involved the coordination of metal ions with the nitrogen and oxygen atoms within the chemosensor. This coordination blocks the PET process which changes its photophysical properties and serves as the basis for detection. Our approach offers quantitative determination of these metal ions in river water, tap water, pond water and soil samples. The results showed excellent recoveries ranging from 90% to 105%. These findings highlight the reliability and versatility of chemosensor SB in environmental and analytical chemistry, for the sensitive detection of Cu2+ and Ag+ ions.