An imidazole-based fluorescent sensor for selective detection of Cu²⁺ and BF₃ with environmental applications

Abstract

The specific detection of Cu²⁺ and BF₃ provided the basis for the design of the distinctive dual-sensing chemosensor, 2-(benzo[d]thiazol-2-yl)-6-(1,4,5-triphenyl-1H-imidazol-2-yl) phenol (SP26). SP26 was synthesized successfully using a multi-step process, with its identity confirmed by NMR spectroscopy and HR-MS analysis. The studies were conducted in an 8 : 2 THF/water mixture. The ligand was solubilized in THF/water, whereas the cation salts were dissolved in water. The absorption measurements indicated no detection of cations other than Cu²⁺. The emission experiments revealed that the optical selectivity for the Cu²⁺ ion leads to a reduction in emission intensity. Likewise, with BF₃, the emission intensity diminishes with the bathochromic shift. The limit of detection (LoD) for Cu²⁺ is 381 pM, and for BF₃ it is 307 pM. After adding BF₃ and Cu²⁺ to SP26, the complex formation was so quick that it happened within a fraction of a second. Triethylamine (TEA) was used for BF₃, and ethylenediamine tetraacetic acid (EDTA) for Cu²⁺ to determine the reversibility. FT-IR, HR-MS, Job's plot, DFT, and ¹H NMR titration analyses confirmed that chemosensor SP26 was bound to Cu²⁺ and BF₃. Paper test strips showed the potential of the chemosensor SP26 for the environmental detection of Cu²⁺ and BF₃. The quantitative analysis of Cu²⁺ was examined with environmental water samples.

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1. Introduction

Copper is one of the most prevalent transition metal ions in the human body due to its important applications in biology, chemistry, and the environment, and it has garnered much attention. It is essential for haemopoiesis and other processes catalyzed by proteins and enzymes. While Cu²⁺ is necessary for human health, an excess of it can tilt the delicate balance within cellular functions and leads to severe neurodegenerative illnesses, including Parkinson's, Wilson's, and Alzheimer's diseases, and similar conditions.^1,2 Furthermore, Cu²⁺ is widely used in industry and daily life, even though it is considered a significant metal pollutant. Thus, creating straightforward, effective, sensitive, and precise techniques for Cu²⁺ detection is imperative.

Numerous methods have been developed in the last few decades for the detection of Cu²⁺, including electrochemical methods,^3,4 atomic absorption spectrometry (AAS),^5,6 inductively coupled plasma-atomic emission spectrometry (ICP-AES),^7,8 inductively coupled plasma mass spectroscopy (ICPMS),⁹ and emission methods.^10–15 Fluorescent probes have emerged as one of the most effective tools for detecting metal ions and other pollutants because of their many exceptional benefits, including simplicity, ease of manipulation, great sensitivity and selectivity, and real-time detection. Notwithstanding, most documented emission probes for Cu²⁺ detection utilize emission quenching or augmentation,^16–19 in which the instrument, auto-emission, manipulation errors, and surroundings can be readily impacted.

They are unable to offer measures that are quantitatively accurate enough. The current probe is capable of detecting a very low level of Cu²⁺ (381 pM). However, ratiometric chemosensor probes for Cu²⁺ detection are still uncommon as of right now. Cu²⁺ ions are also harmful to marine plants such as algae.²⁰ These things make the detection of Cu²⁺ ions indispensable and immediately warranted. The synthesis and application of a good fluorescent sensor should be straightforward, as on-site chemical information can be easily obtained. The design of a fluorescent sensor is the art of clubbing the points of attachment with the fluorophore. In the case of Cu²⁺ ion detection sensors containing quinoline,^21–23 naphthalimide,^24,25 rhodamine,^26,27 and pyrene,²⁸ development has been achieved. However, the stability and ease of synthesis are the major stumbling blocks for these sensors.

The important inorganic compound boron trifluoride (BF₃) is frequently employed as a catalyst in various organic synthesis processes,²⁹ including condensation, ionic polymerization, and isomerization.³⁰ However, BF₃ is extremely poisonous and corrosive, and even small leaks can result in biological dangers and other environmental problems. Moreover, BF₃ has a very high reactivity and can react violently with metals, organic materials, etc. BF₃ will produce a powerful explosion and break down into HF, which can irritate the nose, eyes, skin, respiratory tract, or even cause death, especially when it comes into contact with water or even humid air.³¹ In summary, BF₃ is a hazardous gas that must be handled carefully during application, transportation, production, and disposal activities. Dependable gas leak detection systems for BF₃ are now vitally needed to provide a safe working environment and reduce production loss.³²

The chemisorption of BF₃ and its monohydrate with a gas-reactive thin film placed on a slice of quartz served as the basis for the construction of the quartz microbalance (QMB) sensor technology. They have the following shortcomings, which should be noted: complicated instruments, lengthy response times, high detection limits, and operating procedures. Thus, the benefits of the fluorescence sensing approach, which include easy operation, high sensitivity, fast response, and field test availability, have led to its widespread application in recent years.³³ T.-H. Tran-Thi and colleagues³³ in 2008 demonstrated a hybrid mesoporous organo-silica functionalized with grafted dibenzoylmethane (DBM) exhibiting fluorescence that can be enhanced in the presence of BF₃ over a few hours at a low sensitivity concentration (<1 ppm) to break the Et₂O–BF₃ complex and permit BF₃ to diffuse and react with DBM. A novel molecular design for sensing BF₃ was used by Eric T. Kool et al. in 2011. DNA-polyfluorophores may produce distinct selective responses at concentrations as low as 20 ppm when seen under an epifluorescence microscope.³⁴ This system offers straightforward operation, fast response time, and in situ detection, and it does not require large-scale sensors as previous approaches did. However, the sensitivity of this approach exceeds the permissible exposure limit (PEL) and threshold limit value (TLV) of 1 ppm set by the Occupational Safety and Health Administration (OSHA) and the American Conference of Governmental Industrial Hygienists (ACGIH).

Imidazole-based fluorescent sensors were studied. However, they have some disadvantages, like some imidazole sensors' limited solubility. Imidazole possesses a nitrogen donor capable of coordinating with several metal ions (e.g., Cu²⁺, Zn²⁺, Fe³⁺), making the differentiation between analogous ions challenging.

An imidazole sensor ensures precision and dependability in selective detection. Certain imidazole-based sensors demonstrate sluggish reaction times or irreversible interactions with the target, rendering them inadequate for real-time or dynamic monitoring.

For example, a highly fluorescent imidazole-based diboron complex has been synthesized, and only photophysical studies were carried out.³⁵ Similarly, imidazoyl-phenol-based boron complexes have been studied extensively.³⁶ Cu²⁺ sensing and electrochemical removal were done using triphenyl-imidazole-based sensors.³⁷ A novel imidazole-derived chemosensor was developed for the detection of Cu²⁺ and sulphide ions.³⁸ Similarly, a bifunctional chemosensor for the detection of Cu²⁺ and Fe²⁺ was developed using push–pull imidazole–triazole.³⁹ However, an imidazole-derived Schiff base was used for the identification of Cu²⁺ and applied in fingerprint images from our laboratory.⁴⁰ Similarly, from our laboratory, an imidazole-tethered benzothiazole sensor was developed for the detection of picric acid and latent fingerprint images.⁴¹

To effectively synthesize fluorescent chemosensors for the recognition of both Cu²⁺ and BF₃, we have established a new chemical technique for the synthesis through the cyclocondensation reaction yielding 2-(benzo[d]thiazol-2-yl)-6-(1,4,5-triphenyl-1H-imidazol-2-yl) phenol (SP26). To address this, we have created the small-molecule chemosensor SP26, which interacts with Cu²⁺ and BF₃ by deprotonation. SP26 has outstanding selectivity, sensitivity, and optical responsiveness due to multiple points of attachment (N, O, N) to the sensor.

Furthermore, the chemosensor SP26 has robust electron-donating characteristics that facilitate energy induction and electron transfer and facilitate fluorescence quenching through fluorophore–analyte interactions. For Cu²⁺ and BF₃ in test strips, SP26 was used to demonstrate practical applicability. Therefore, the information from this work will be useful in designing more sophisticated fluorescent chemosensors for precise and targeted detection at very low concentrations.

1.1 General

All the chemicals used in this investigation, including solvents like ethanol and tetrahydrofuran (THF), were of analytical reagent (AR) grade. B(OMe)₃, borax, BPh₃, H₃BO₃, BCl₃, NaBF₄, H₂S, HClO₄, NaF, HCl, NaCN, N₂H₄, NH₃, and NH₄Cl, and the metal salts containing Ag⁺, Al³⁺, Ba²⁺, Ca²⁺, Cr³⁺, Cd²⁺, Cu²⁺, Co²⁺, Hg²⁺, Ni²⁺, Zn²⁺, Zr²⁺, and Th⁴⁺, were obtained from Sigma Aldrich and Spectrochem and were used without any additional purification. The supplier of boron trifluoride diethyl etherate (C₄H₁₀BF₃O) was Avra Chemicals. Nuclear Magnetic Resonance (NMR) spectra, comprising ¹H and ¹³C, were acquired using Bruker Avance III 400 MHz and 100 MHz. A JASCO FP-8655 fluorophotometer was used for fluorescence spectroscopy. The instrument utilized for UV-visible spectroscopy was a JV-750. Column chromatography, which combines stationary (silica gel) and mobile (ethyl acetate-hexane) phases, was used to purify reaction mixtures.

1.2 Synthesis and characterisation

A solution of salicylaldehyde (1.5 g, 12.3 mmol) and 2-aminothiophenol (1.25 mL, 11.9 mmol) in EtOH (25 mL) was mixed dropwise with aq. HCl (37%, 30 mmol) and aq. H₂O₂ (30%, 48.0 mmol). We refluxed the mixture for one hour. After the reaction mixture attained room temperature, we filtered it and was given an ethanol wash followed by air drying. We obtained 1.4 g of pure white solid 2-(2-hydroxyphenyl) benzothiazole (HBA) with a yield of 82%. Hexamethylenetetramine (HMTA) (0.926 g, 6.6 mmol) was mixed into a solution of HBA (1 g, 5 mmol) in CH₃COOH (20 mL) and refluxed for 3 hours. The reaction mixture was cooled to room temperature. Then, the reaction mixture was quenched in cold water, pH-neutralized, filtered, and dried. Then, the precipitate was purified by column chromatography using silica gel 100–200 mesh and using ethyl acetate : n-hexane (1 : 9) as a solvent system, yielding 420 mg (45%). The solid of 3-(benzothiazole-2-yl)-2-hydroxybenzaldehyde (HBAA) is light-yellow colored.⁴²

Synthesis of 2-(benzo[d]thiazol-2-yl)-6-(1,4,5-triphenyl-1H-imidazol-2-yl) phenol (SP26) through cyclocondensation. 3-(Benzo[d]thiazol-2-yl)-2-hydroxybenzaldehyde 0.300 g (1 equivalent), benzil 0.240 g (1 equivalent), and 0.1 mL (1 equivalent) of aniline were all dissolved in glacial acetic acid⁴³ and refluxed for 3 hours. The reaction's progress was seen using TLC with ethyl acetate : n-hexane (3 : 7). After the reaction's completion, the reaction mixture was cooled to room temperature, and cold water was added. After that, sodium hydroxide solution was added to neutralize the reaction mixture. The solid was separated and allowed to air dry. For purification, the reaction mixture was separated and purified using column chromatography with a silica gel mesh size of 100–200 and ethyl acetate : n-hexane as the eluent. In the end, a solid light-yellow compound with a yield of 76% was achieved. The compound purity was 99.20% (Fig. S3). High-resolution mass spectrometry and NMR spectroscopy were used to analyze the molecule (M. P.: 237 °C). The estimated mass of the compound was 521.1628, while its observed mass with the proton adduct was 522.1636 (Fig. S4). ¹H NMR (400 MHz, DMSO) δ: 11.835 (s) (1H), 8.305–8.299 (d) 2.4 Hz (1H), 8.143–8.123 (d) 8 Hz (1H), 8.060–8.041 (d) 7.6 Hz (1H), 7.196–7.551 (m) (18H), 6.988–6.966 (d) 8.8 Hz (1H) SI (Fig. S1). ¹³C NMR (100 MHz, DMSO) δ: 164.81, 156.73, 156.65, 151.86, 156.81, 145.87, 134.90, 134.77, 132.42, 131.78, 131.60, 131.50, 130.90, 130.13, 129.79, 129.34, 129.24, 129.15, 129.09, 129.05, 128.94, 128.85, 128.66, 126.87, 126.57, 125.68, 122.78, 122.69, 121.51, 122.42, 118.62, 117.22 SI (Fig. S2) (Scheme 1).

Scheme 1 The synthesis of 2-(benzo[d]thiazol-2-yl)-6-(1,4,5-triphenyl-1H-imidazol-2-yl) phenol (SP26).

1.3 Stock solution preparation

Metal nitrate salts were dissolved in double-distilled water to yield solutions with a concentration of 1 × 10⁻³ M. Meanwhile, a 2 × 10⁻⁵ M stock solution of the chemosensor (SP26) was prepared using tetrahydrofuran–water (8 : 2-THF : H₂O) as the solvent before conducting every photophysical study, and the excitation wavelength is 360 nm for all photophysical studies.

2. Results and discussion

2.1 Absorption studies at different H₂O–THF combinations

SP26 produces a green color in solution when dissolved in THF. After that, several combinations of THF/water fraction of 0–90% were created; the absorption spectrum was recorded with each solution; there are two peaks at 300 and 360 nm, and there is a slight decrease at 300 nm in the absorption up to 70% of water. At 80% and 90% of water, there is a sudden increase in absorption (Fig. 1a).

Fig. 1 (a) Absorption spectra of SP26 in various combinations of THF/water. (b) Emission spectra of SP26 in various combinations of THF/water; the excitation wavelength is at 360 nm and the emission wavelength is at 500 nm, and the scanned wavelength is 400 to 650 nm at room temperature. (c) The bar graph of SP26 at various combinations of THF/water under 365 nm UV light.

2.2 Aggregation caused quenching (ACQ) of emission

Similar to the absorption spectrum, the emission spectrum was recorded, and there is a considerable reduction in the emission intensity as we increase the water content of the solvent system. The FL spectrum shows that emission intensity is gradually decreasing up to 50% of water. After 60% of water, the emission disappeared as the water was added, due to the aggregation-caused quenching (ACQ) effect (Fig. 1b). It is shown in the form of a bar chart with different water fractions, under 365 nm UV light, as seen in Fig. 1c. In this particular study, the ACQ may help the detection of Cu²⁺, since the addition of Cu²⁺ leads to quenching.

2.3 Solvatochromism properties

The absorption and emission spectra of compound SP26 in different solvents, such as n-hexane (HEX), dichloromethane (DCM), tetrahydrofuran (THF), ethyl acetate (EA), 1,4-dioxane, methanol (MeOH), acetonitrile (ACN) and dimethyl sulfoxide (DMSO), show a broad signal as shown in Fig. 2(a) and (b). The photophysical properties of the compound are summarized in Table 1 in various solvents. The sensitivity of the absorption spectrum is weak because the local environment of the molecules in the ground and excited states remains unchanged during the fast absorption process, and there is not much difference. On the other hand, a blue shift to 30 nm was observed in the emission spectra in different solvents when the solvent system's polarity was increased from hexane to DMSO. As a result, it displays negative solvatochromism, which shifts from the longer wavelength to the shorter wavelength as polarity increases. This shows that the ground state is more stabilised than the excited state due to a large energy gap.^44,45 The interaction between the dipoles of the solvent and ligand makes the ligand unstable in the excited state, leading to a very high energy state. This was evident from the difference between the HOMO and LUMO of the ligand from theoretical calculations.

Fig. 2 (a) Absorption spectra of SP26 in various solvents (solvatochromism). (b) Emission spectra of SP26 in various solvents (solvatochromism); the excitation wavelength is at 360 nm and the emission wavelength is at 500 nm, and the scanned wavelength is 400 to 650 nm at room temperature. (c) Colours of SP26 in various solvents under 365 nm UV light.

Table 1 Photophysical properties of SP26 in different solvents at room temperature

Probe	Solvent^a	λ max (nm)	log ε^c (10^4 M^−1 L^−1 cm^−1)	FWHM^d (nm)	λ em (nm)	λ s (cm^−1)	ϕ
a Polar to non-polar solvents. b Maximum absorbance in UV-vis spectra. c Extinction coefficient. d Full-width at half maximum. e Wavelength of the maximum emission intensity. f Stokes shifts. g Quantum yield.
SP26	HEX	355	1.2	134.6	503	8289	0.0221
SP26	DCM	354	1.25	125	485	7630	0.0198
SP26	THF	351	1.35	126.4	495	8288	0.0181
SP26	EA	351	1.3	130.5	487	7957	0.0202
SP26	1,4-Dioxane	353	1.25	137.7	496	8167	0.0200
SP26	MeOH	348	1.3	132.4	486	8159	0.0252
SP26	ACN	351	1.2	132.4	479	7614	0.0220
SP26	DMSO	354	1.2	130.9	494	8006	0.0174

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2.4 Selectivity studies

In the presence of several boron species and metal ions (BF₃, B(OMe)₃, borax, BPh₃, H₃BO₃, BCl₃, NaBF₄, H₂S, HClO₄, NaF, HCl, NaCN, N₂H₄, NH₃, and NH₄Cl, Ag⁺, Al³⁺, Ba²⁺, Ca²⁺, Cr³⁺, Cd²⁺, Cu²⁺, Co²⁺, Hg²⁺, Ni²⁺, Zn²⁺, Zr²⁺, and Th⁴⁺), the UV-vis absorption and emission studies were conducted on SP26 in 2 × 10⁻⁵ M concentration in a solvent ratio of 8 : 2 tetrahydrofuran : water (THF : H₂O). Two absorption bands at 300 nm and 360 nm in the absorption spectrum of SP26 are due to the π–π* and n–π* transitions, respectively. New absorption bands appeared after complexation with the addition of BF₃ and Cu²⁺. When BF₃ was added gradually, the absorption band that had first emerged at 360 nm moved hypsochromically to 340 nm. The band at 360 nm disappears when we add Cu²⁺, and a second, smaller absorption band at 400 nm appears, as seen in Fig. 3a. Thus, for fluorescence spectroscopic examination of the sensor SP26, the excitation wavelength was set at 360 nm. Similarly, the sensor gave an emission band at 500 nm. The sensor SP26's emission band was measured in the presence of Cu²⁺, the emission band was quenched at 500 nm, and for BF₃, the emission band was quenched and gradually redshifted to 550 nm.

Fig. 3 (a) Absorption spectra of chemosensor SP26 with different metal ions and different boron species in 8 : 2 THF : H₂O solution. (b) Emission spectra of chemosensor SP26 with different metal ions and different boron species in 8 : 2 THF : H₂O solution; the excitation wavelength is at 360 nm and the emission wavelength is at 500 nm, and the scanned wavelength is 400 to 650 nm at room temperature. (c) Colours of SP26 with different metal ions and boron species upon adding 1 × 10⁻³ M metal ions into 2 × 10⁻⁵ M concentrations of SP26 in 8 : 2 THF : H₂O solution under 365 nm UV light.

2.5 Absorption and emission titration studies

The π–π* transition of sensor SP26 appears at 300 nm for the absorption; the n–π* transition is exhibited at 360 nm. Fig. 4a shows that the intensity of the absorption peak at 300 nm increases and a new peak at 400 nm increases progressively, and the peak at 360 nm decreases, when 0 to 3 equivalents of Cu²⁺ are added (Fig. 4a). Hence, we obtained an isosbestic point at 372 nm and there is a marked red shift in the absorption spectra. However, upon adding 0 to 2 equivalents of BF₃ to SP26, a new peak at 260 nm emerges at the expense of the peak at 300 nm. Unlike copper, in the case of BF₃, the absorption increases as the BF₃ concentration is increased, and also there is a clear blue shift. Another new absorption peak at 310 nm decreases till 320 nm, again it increases up to 362 nm, then it decreases. A regular blue shift was seen, unlike copper. As a result, we obtained three isosbestic points at 300, 320, and 362 nm upon adding BF₃. However, in the case of copper, we obtained only one isosbestic point. As a result, the shapes of the absorption spectra of Cu²⁺ and BF₃ are different. The complex formation between SP26 and BF₃ was verified (Fig. 4b). Similarly, the fluorescence titration of SP26 was performed by gradually adding Cu²⁺ (0 to 3 equivalents) (Fig. 4c). When the concentration of Cu²⁺ increases, the SP26 fluorescence band at 500 nm was quenched. Similarly, adding 0 to 2 equivalents of BF₃ resulted in a redshift towards 550 nm and diminished with an increase in BF₃ concentration. However, we also detected a second emission peak at 680 nm, which slightly increases as the BF₃ concentration rises. Hence, we obtained the isoemissive point at 600 nm (Fig. 4d). It is observed that fluorescence is quenched in the case of Cu²⁺ without any shift in the emission, and the fluorescence is red-shifted (cyan to yellow) in the case of BF₃.

Fig. 4 (a) Absorption spectra of chemosensor SP26 (2 × 10⁻⁵ M) in 8 : 2 THF : H₂O solution with 0–3 equivalents of Cu²⁺. (b) Absorption spectra of chemosensor SP26 (2 × 10⁻⁵ M) in 8 : 2 THF : H₂O solution with 0–2 equivalents of BF₃ analytes. (c) Emission spectra of chemosensor SP26 (2 × 10⁻⁵ M) in 8 : 2 THF : H₂O solution with 0–3 equivalents of Cu²⁺. (d) Emission spectra of chemosensor SP26 (2 × 10⁻⁵ M) in 8 : 2 THF : H₂O solution with 0–2 equivalents of BF₃ analytes, and for both, the excitation wavelength is at 360 nm and the emission wavelength is at 500 nm, and the scanned wavelength is 400 to 650 nm at room temperature.

Absorption coefficients were found to be 11 000 cm⁻¹ M⁻¹ for SP26, and for SP26 with Cu²⁺ and BF₃, they were found to be 6500 and 14 500 cm⁻¹ M⁻¹, respectively. The limit of quantification was determined to be as low as 1.27 nM for Cu²⁺ and 1.025 nM for BF₃. The receptor's quantum yield (ϕ_fl) was determined to be 0.94, using quinine sulfate as a standard reference (ϕ_fl = 0.54). Upon addition of Cu²⁺ and BF₃ to the chemosensor SP26, the fluorescence intensity decreases and increases, respectively, with a quantum yield of 0.98 for SP26 with the BF₃ complex and 0.53 for SP26 with the Cu²⁺ complex.

2.6 Binding constant and limit of detection

After FL titration, the limit of detection (LoD) of SP26 towards Cu²⁺ ions and BF₃ was computed utilizing the formula 3σ/slope from the analyte concentration of Cu²⁺ and BF₃ against emission intensity, where σ represents the standard deviation and the slope value is obtained from the linear graph. The results were 381 pM for Cu²⁺ and 307 pM for BF₃ respectively [Fig. 5(a) and (b)]. The Benesi–Hildebrand graph was obtained, illustrating the connection between the reciprocal of concentrations of Cu²⁺ and BF₃ against 1/[F − F₀] (Fig. 4b). The binding association constants (K_a) for the SP26+ Cu²⁺ and SP26+ BF₃ complexes were calculated to be 7.9 × 10⁴ M⁻¹ and 1.27 × 10⁴ M⁻¹ [Fig. 5(c) and (d)].

Fig. 5 (a) Calibration plot between the concentration of Cu²⁺ and measured emission intensity. (b) Calibration plot between the concentration of BF₃ and measured emission intensity. (c) Benesi–Hildebrand plot for chemosensor SP26 with 1/concentration of Cu²⁺ with emission intensity. (d) Benesi–Hildebrand plot for chemosensor SP26 with 1/concentration of BF₃ with emission intensity.

2.7 Job's plot, response time studies, and pH studies

In Job's plot analysis (Fig. 6a and b), it is possible to determine the binding ratio of the analyte to chemosensor SP26. The 0.5-mole fraction suggests that the binding ratio is 1 : 1 for both SP26 : Cu²⁺ and SP26 : BF₃. Additionally, as shown in Fig. 6c and d, the fluorescence intensity is drastically reduced within a fraction of a second when Cu²⁺ and BF₃ are added to SP26. This indicates that the emission intensity decreased immediately, since a complex was generated by SP26+ Cu²⁺ and SP26+ BF₃ as soon as analytes are added. The emission spectrum of the chemosensor SP26 revealed an observable shift in the pH range of 5.0 to 12.0; below 5.0 pH, the emission intensity was high as a result of an increase in the supply of H⁺, as seen in Fig. 6e and f. The protonation of nitrogen causes a small increase in emission intensity. Then, the emission intensity varies randomly with pH, influencing the chemosensor SP26 with Cu²⁺. The stability at pH 8 up to 12 is observed to some extent. The stability in emission intensity is observed in SP26 with BF₃ in the pH range of 8.0 up to 12.0. As in the previous case, nitrogen is protonated at lower pH values between 2.0 and 7.0, resulting in the poor complexation of SP26 with BF₃ lacking a single pair of electrons. Hence, emission intensity changes with different pH on treating SP26 with BF₃ (Fig. 6f).

Fig. 6 (a) Job's plot of SP26 with various mole fractions of Cu²⁺ and emission intensities. (b) Job's plot of SP26 with various mole fractions of BF₃ and emission intensities. (c) Response time for change in emission intensity after adding Cu²⁺ to SP26 in 8 : 2 THF : H₂O (2 × 10⁻⁵ M). (d) Response time for change in emission intensity after adding BF₃ to SP26 in 8 : 2 THF : H₂O (2 × 10⁻⁵ M); for both, the excitation wavelength is at 360 nm and the emission wavelength is at 500 nm, and the scanned wavelength is 400 to 650 nm at room temperature. (e) The pH effect on the emission intensity of chemosensor SP26, and the emission intensity of chemosensor SP26 with Cu²⁺ pH (2.0–12.0). (f) The pH effect on the emission intensity of chemosensor SP26, and the emission intensity of chemosensor SP26 with BF₃ pH (2.0–12.0).

2.8 Interference studies and reversibility studies

The interference experiments were carried out for chemosensor SP26. Along with Cu²⁺ and BF₃, different metal ions and boron species were added separately to chemosensor SP26, and there were no changes except for Cu²⁺ and BF₃.

In the case of Cu²⁺ and BF₃, the emission intensity decreases even in the presence of other analytes. This shows the selectivity of chemosensor SP26 towards Cu²⁺ and BF₃ (Fig. 7(a) and (b)). Additionally, the reversibility of sensor SP26 was evaluated using EDTA for Cu²⁺ ions. When Cu²⁺ was added to SP26, the colour changed from cyan to light blue, and the emission intensity falls (Fig. 7c). The emission intensity increased, and the colour of the SP26 fluorescence changed from light blue to cyan upon the addition of EDTA. This phenomenon was repeatedly evaluated three times. Similarly, when BF₃ was added to SP26, the colour was shifted from cyan to light yellow, and the emission intensity diminished. However, the emission intensity increases and the color changes back to cyan upon adding triethylamine (TEA) (Fig. 7d). This is due to the formation of a stronger complex between EDTA and Cu²⁺ than between SP26 and Cu²⁺. Hence, EDTA replaces SP26, and similarly a stronger complex is formed between BF₃ and TEA than between SP26 and BF₃. Hence, TEA replaces SP26 (Scheme 2). After this, the chemosensor SP26 was recovered, and this phenomenon was regularly examined for reproducibility over three cycles.

Fig. 7 (a) Interference of other metal ions and boron species (1 × 10⁻³ M) along with Cu²⁺ on SP26 in an 8 : 2 THF : H₂O solution (2 × 10⁻⁵ M); the excitation wavelength is at 360 nm and the emission wavelength is at 500 nm, and the scanned wavelength is 400 to 650 nm at room temperature. (b) Interference of other metal ions and boron species (1 × 10⁻³ M) along with BF₃ on SP26 in an 8 : 2 THF : H₂O (2 × 10⁻⁵ M) solution. (c) The reversibility studies of SP26 in 8 : 2 THF : H₂O (2 × 10⁻⁵ M) solution with Cu²⁺ by adding EDTA. (d) The reversibility studies of SP26 in 8 : 2 THF : H₂O (2 × 10⁻⁵ M) solution with BF₃ by adding TEA.

Scheme 2 The mechanism of the reversibility of the chemosensor SP26 with the addition of Cu²⁺ + EDTA and SP26 with the addition of BF₃ + TEA.

2.9 Study of the binding mechanism

We have performed ¹H NMR for the sensor before and after adding BF₃. Between BF₃ and Cu²⁺, Cu²⁺ is paramagnetic and hence NMR cannot be recorded. Hence, we have recorded the FT-IR spectrum for the complex, and it has been compared with that of the ligand. By comparing these two, in the spectrum of the complex, the –OH stretching peak at 3400 cm⁻¹ completely disappears, and there is a change in the shape of the –C=N stretching peaks at 1300 and 1600 cm⁻¹, indicating that copper forms a complex through oxygen and nitrogen (Fig. 8a).

Fig. 8 (a) IR spectra of SP26 and SP26 with Cu²⁺. (b) IR spectra of SP26 and SP26 with BF₃. (c) ¹H NMR spectra of SP26 and SP26 with BF₃ in DMSO-d₆. (d) The optimized structures of the HOMO and LUMO of SP26 and SP26 with Cu²⁺ and BF₃ complexes at the DFT/B3LYP analysis.

Binding interactions between SP26 and BF₃ were studied using ¹H NMR titration experiment in DMSO-d₆ and FT-IR. BF₃ binds with the phenolic oxygen when SP26 is added. In ¹H NMR, the –OH proton disappears completely since boron is getting attached to oxygen after removing H⁺. Similarly, electrons from the carbon in the benzene ring, next to the nitrogen of benzothiazole, flow to the nitrogen due to the attachment of BF₃. As a result, the peak of the particular proton (H-1) becomes more pronounced and shifted downfield (8.692 ppm). However, all other protons also shifted downfield due to the attachment of BF₃ (8.053, 8.131, 8.169, 8.237 ppm) (Fig. 8c). Also in FT-IR, the –OH peak disappears completely, and there is a change in the shape of the –C=N stretching peaks at 1321 and 1614 cm⁻¹, indicating that boron forms a complex through oxygen and nitrogen (Fig. 8b).

The HR-MS spectrum of SP26+ Cu²⁺ shows a peak at 583.1092 m/z (Fig. S5), and SP26+ BF₃ shows a peak at 570.1635 m/z (Fig. S6). Utilizing the Gaussian 16W package and theoretical calculations at the B3LYP level with the 6-31'G(d,p) level of theoretical analysis,⁴⁶ density functional theory (DFT) was employed to determine the energy of the optimized structures of SP26 and SP26 with Cu²⁺ and BF₃ complexes. In SP26, the electron density is concentrated in two phenyl groups, one of which is connected to nitrogen in the HOMO. Upon transitioning to a linear molecular orbital LUMO, the electron density disperses across the molecule, sparing all but one phenyl group; as a result, ΔE is 4.08 eV. The HOMO and LUMO of the SP26+ Cu²⁺ combination do not differ significantly due to electron spreading. However, the band gap decreases from 4.08 to 3.1781 eV. In the case of the SP26+ BF₃ complex, the difference in electron spreading further decreases (ΔE). As a result, the band gap is further closed to 2.9171 eV. The energy gap suggests that the SP26+ Cu²⁺ and BF₃ complexes stabilize the system (Fig. 8c). The chemosensor demonstrates that the interaction of SP26 with Cu²⁺ and BF₃ affects both twisted intramolecular charge transfer (TICT) and intramolecular charge transfer (ICT).

3. Application in real water samples

Human health suffers significant harm when copper sources contaminate water. The identification of Cu²⁺ ions in environmental samples is therefore essential. Cu²⁺ ion detection was carried out utilizing water samples from several locations across Vellore Institute of Technology, including tap, drinking, and lake water, to track the practical application of sensing probe SP26. Chemosensor SP26 shows high sensitivity and selectivity towards Cu²⁺. To test how well it can detect Cu²⁺ in real water, we performed a spike and recovery test by checking the brightness of chemosensor SP26 in tap water, lake water, and drinking water with the addition of Cu²⁺. All the water samples were centrifuged and filtered before use. The water samples were spiked with a set amount of Cu²⁺ (25 μM). As shown in Table 2, Cu²⁺ was tested in each sample with satisfactory accuracy, which means chemosensor SP26 could be used for Cu²⁺ detection in water samples. It was discovered that the Cu²⁺ ion concentrations in Cu²⁺ spiked samples agreed fairly well. Further, over 98% of Cu²⁺ was recovered, demonstrating the developed probe's ability to detect Cu²⁺ ions in practice (Table 2).

Table 2 Determination of Cu²⁺ ions in environmental samples

Various water samples	SD (%)	Con. of spiked Cu^2+ ions (μM)	Con. of the found Cu^2+ ions (μM)	Recovery of Cu^2+ added (%)
Tap water	0.458	25	24.85	99.4
Lake water	0.503	25	24.7	98.8
Drinking water	0.529	25	24.85	99.4

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3.1 Application of chemosensor SP26 in the paper strip test

We created paper strips to detect Cu²⁺ and BF₃ levels, which were simple to use and convenient. The paper strips were created by dipping Whatman filter paper into a solution of 2 × 10⁻⁵ M concentration of SP26, followed by air drying. Three drops of analyte at a concentration of 1 × 10⁻³ M were applied to paper strips. We illuminated the strips under a 365 nm UV lamp after allowing them to air dry. Except for Cu²⁺ and BF₃, there was no change in colour. Cu²⁺ produces a colour shift from cyan to light blue. In contrast, BF₃ causes a color change from cyan to yellow (Fig. 9a). The emission color was then gradually altered for both Cu²⁺ and BF₃ (Fig. 9b), as the gradual addition of Cu²⁺ and BF₃ to the paper strips increased from 0 to 3 equivalents and 0 to 2 equivalents, respectively. The results revealed the qualitative detection of Cu²⁺ and BF₃ by the chemosensor SP26.

Fig. 9 (a) The photographs exhibiting the emission color change of SP26 on paper strips, on the addition of different analytes under UV light at 365 nm, and (b) the gradual addition of different equivalents of Cu²⁺ and BF₃ of SP26.

Conclusion

We have synthesized and presented SP26, a new imidazole chemosensor characterized by NMR and HRMS analytical techniques. Cu²⁺ quenches emission intensity at 500 nm, and BF₃ reduces emission intensity at 550 nm with a corresponding red shift. The chemosensor SP26 can be made reversible using EDTA; emission intensity was first quenched by Cu²⁺, and then it was restored by adding EDTA to SP26. Similarly, adding BF₃ to SP26 caused the emission to move to 550 nm with a reduction in intensity; however, adding TEA to the same solution caused the emission intensity to reversibly change back to SP26 emission. After that, we studied the binding mechanism using Job's plot, mass spectroscopy, DFT investigations, NMR titration, and FT-IR spectroscopy. For SP26 with Cu²⁺ and BF₃, a 1 : 1 complex was produced with coordinated imidazole nitrogen and phenolic oxygen atoms. The limit of detection was 381 pM for Cu²⁺ and 307 pM for BF₃, and the response time was within a second. The rapid qualitative application of paper strip studies and quantitative application in environmental water samples were carried out with our synthesized chemosensor SP26.

Conflicts of interest

There are no conflicts to declare.

Data availability

Supplementary information: Copies of ¹H/¹³C {¹H} NMR and HRMS spectra for the sensing receptor SP26 and the SP26 + Cu and BF₃ complexes, along with HRMS spectra and a comparison of results with previous work reported in the literature. See DOI: https://doi.org/10.1039/D5SD00021A.

The data supporting this article have been included as part of the SI.

Acknowledgements

Prakash Seenu is grateful to the Vellore Institute of Technology for the Research Associateship, which allows him to receive financial assistance. The DST-FIST NMR facility at VIT is appropriately acknowledged. Dr. R. Srinivasan from SSL, VIT, is acknowledged for language editing.

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