Open Access Article
This Open Access Article is licensed under a
Creative Commons Attribution 3.0 Unported Licence

Multistate luminescent probe: ICT-driven dual ESIPT-AIE for selective fluoride and cyanide ion recognition

Aastha Palta a, Gulshan Kumar b, Kamaldeep Paul a and Vijay Luxami *a
aDepartment of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala, 147004, India. E-mail: vluxami@thapar.edu
bDepartment of Chemistry, Banasthali University, Banasthali Newai, 304022, Rajasthan, India

Received 2nd January 2025 , Accepted 9th June 2025

First published on 10th June 2025


Abstract

A highly selective and sensitive fluorescent chemosensor, HNBZT, has been designed and developed for the detection of F and CN ions. HNBZT exhibits excited-state intramolecular proton transfer (ESIPT), intramolecular charge transfer (ICT), and aggregation-induced emission (AIE) phenomena on a single molecular platform. Its photophysical and sensing behaviour has been evaluated using absorption and fluorescence spectroscopy. Notably, HNBZT displays a ratiometric fluorescence “turn-on” response to both anions in a CH3CN solvent system. The binding of F and CN ions to HNBZT results in distinct absorption and emission colour changes, attributed to the unique electronic and structural perturbations induced by their respective interactions, facilitating visual detection. The lowest detection limits for F and CN ions are 7.6 × 10−8 M and 1.1 × 10−7 M, respectively. The coherence between the DLS and FESEM results underscores the consistency and reliability of the observed aggregation behaviour of HNBZT. 1H NMR titrations of HNBZT with F and CN ions reveal distinct binding mechanisms, with F ions forming hydrogen bonds, while CN ions induce nucleophilic addition, which is also supported by theoretical studies. The binding stoichiometry of HNBZT with F and CN ions is determined to be 1[thin space (1/6-em)]:[thin space (1/6-em)]1 using Job's plot analysis.


1. Introduction

In the field of molecular design, there is a growing interest in developing compounds with multiple functionalities, capable of exhibiting diverse photophysical phenomena simultaneously. Among these phenomena, excited-state intramolecular proton transfer (ESIPT), intramolecular charge transfer (ICT), and aggregation-induced emission (AIE) stand out as key areas of exploration, offering flexible platforms for various applications ranging from sensing to optoelectronic devices.1–6 Integrating these phenomena within a single molecular framework not only enhances their individual capabilities, but also reveals emergent properties to opening up new possibilities for advanced molecular sensing.

ESIPT, ICT, and AIE are three key phenomena shaping the development of fluorescent probes and materials.7–10 ESIPT facilitates the transfer of a proton between two molecular sites upon photoexcitation, inducing significant structural and electronic changes in excited states, thereby altering their photophysical properties like emission wavelength and intensity.11–15 Similarly, ICT involves the redistribution of electron density upon photoexcitation, offering a versatile platform for controlling charge transfer processes within molecules and enabling substantial changes in absorption and emission spectra.16–19 Furthermore, the AIE phenomenon enhances light emission intensity upon aggregation, diverging from conventional fluorophores that often undergo fluorescence quenching in aggregated states. This transformative property of AIE-active molecules makes them highly attractive for diverse applications including biological imaging, chemical sensing, and optoelectronic devices.20–25

The emergence of molecules with double ESIPT, ICT, and AIE properties presents an exciting opportunity to combine these phenomena and unlock novel functionalities. Such molecules hold great promise for applications requiring multi-functionality, including simultaneous detection of ions.26,27 Detecting different ions is crucial for environmental monitoring, biomedical diagnostics, and industrial safety due to their significance as indicators of water quality, environmental contamination, and chemical toxicity.28–30

The selective detection of anions, such as fluoride (F) and cyanide (CN), is of significant interest due to their unique chemical properties and widespread relevance in environmental, biological, and industrial processes.31–33 For example, fluoride ions play a critical role in dental care, such as water fluoridation, and are used in various industrial applications, while cyanide is known for its toxicity, yet it is widely utilized in gold mining and chemical synthesis.34–36 As a result, the development of sensitive and selective methods for detecting these anions is essential for both environmental monitoring and safety.37,38

The challenge lies in designing a sensor that can selectively interact with both anions despite their distinct sizes, geometries, and chemical behaviors. Fluoride is a small, highly electronegative ion, whereas cyanide is larger, less electronegative, and can engage in both hydrogen bonding and covalent interactions. A well-designed chemosensor must not only recognize these differences but also exhibit distinct, measurable responses to each anion.

This study introduces a novel fluorescent chemosensor capable of differential sensing for F and CN ions with exceptional selectivity and sensitivity. HNBZT showed ESIPT, ICT, and AIE phenomena within a single molecular framework. Furthermore, HNBZT exhibited a ratiometric fluorescence “turn-on” response to both anions when dissolved in CH3CN solvent. Moreover, we elucidated HNBZT's stoichiometry in interaction with F and CN ions through Job's plot analysis. This comprehensive investigation illuminates the multifaceted behavior of HNBZT.

2. Experimental section

2.1. Materials and instruments

All chemicals and solvents used in the experimental work were of analytical grade and were procured from Sigma-Aldrich, Spectrochem, and Rankem Ltd, depending on availability. These were used without further purification. The progress of the chemical reactions was monitored using thin layer chromatography (TLC). Melting points were recorded using the open capillary method and were uncorrected. The synthesized compounds were characterized using NMR spectroscopy on a JEOL ECS-400 MHz instrument in DMSO-d6 with TMS as the internal standard. HRMS spectra were obtained using an XEVO G2-XS QTOF of WATERS. Absorption studies were performed with a SHIMADZU UV-2600 spectrophotometer, employing quartz cells with a 1 cm path length. Fluorescence studies were conducted using an Agilent Technologies Cary Eclipse Spectrophotometer, with excitation and emission slit widths set to 5 nm each. Absorption and emission scans were saved as ACS files and subsequently processed in Excel™ to generate the presented graphs. The stoichiometry of the complexes was analyzed using Job's plot analysis. Time-resolved fluorescence studies were carried out using a DeltaFlex Modular Fluorescence Lifetime Spectrofluorimeter from HORIBA Scientific. The FE-SEM analysis was carried out using a ZEISS MERLIN Compact field emission-scanning electron microscope.

2.2. Solutions for absorption and emission studies

The stock solution of HNBZT was prepared in CH3CN[thin space (1/6-em)]:[thin space (1/6-em)]DMSO (9.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5) at 10−3 M concentration. Tetrabutyl ammonium salts of F, Cl, CN, Br, I, SCN, AcO, NO3, P2O74−, H2PO4, and HSO4 were used for anions. The stock solutions of the anions were prepared in acetonitrile at 10−1 M concentration. As needed, the stock solutions were further diluted to the desired concentration.

2.3. Calculation of binding constants and detection limits

The binding constants were calculated using the Benesi–Hildebrand method using eqn (1) where I0, I, and Imax are the absorption/emission intensities of the compound in the absence of analyte, at an intermediate, and at complete titration with the analyte, respectively. Ka is the binding constant; C is the concentration of analyte and n is the number of analytes bound per molecule.
 
image file: d5nj00019j-t1.tif(1)
Limit of detection (LOD) was determined from the following equation:
 
image file: d5nj00019j-t2.tif(2)

2.4. Synthetic procedure for HNBZT

Compound 1a39 (200 mg, 0.8 mmol) and 2-hydroxy-1-naphthaldehyde (156 mg, 0.9 mmol) were stirred at 70 °C in ethanol for 3 h (Scheme 1). On completion, the reaction mixture was cooled at room temperature, and the crude solid was filtered and washed with ethanol to obtain an orange coloured solid of HNBZT in 85% yield. M.p. 270–275 °C; 1H NMR (DMSO-d6, 400 MHz): δ (ppm) 9.63 (d, 1H, J = 8.0 Hz, –CH[double bond, length as m-dash]N), 8.48 (d, 1H, J = 8.0 Hz, ArH), 8.22 (d, 1H, J = 7.8 Hz, ArH), 8.13 (d, 1H, J = 8.0 Hz, ArH), 8.05 (d, 1H, J = 8.0 Hz, ArH), 7.92 (d, 1H, J = 12.0 Hz, ArH), 7.76 (dd, 1H, 1J = 3.4 Hz, 2J = 8.0 Hz, ArH), 7.54–7.50 (m, 2H, ArH), 7.44–7.42 (m, 1H, ArH), 7.34–7.30 (m, 2H, ArH), 7.24 (d, 1H, J = 4.0 Hz, ArH), 6.95 (d, 1H, J = 7.4 Hz, ArH) (Fig. S1, ESI). 13C NMR (DMSO-d6, 100 MHz): δ (ppm) 173.1, 165.4, 157.9, 155.4, 151.9, 147.1, 138.4, 134.5, 133.7, 130.3, 129.6, 128.8, 127.1, 125.6, 124.3, 123.2, 122.5, 121.0, 116.9, 112.5, 109.1, 108.6 (Fig. S2, ESI). HRMS (ESI-TOF): (m/z) [M + H]+ calcd for C24H17N2O2S: 397.1007, found: 397.1015 (Fig. S3, ESI). FT-IR (cm−1): 3462 (νO–H), 3204 (νC–H[thin space (1/6-em)]str., aromatic), 2969 (νC–H[thin space (1/6-em)]str., aromatic), 1573 (νC[double bond, length as m-dash]NH), 1343 (νC–C), (Fig. S4, ESI).
image file: d5nj00019j-s1.tif
Scheme 1 Synthesis of HNBZT.

2.5. Real sample analysis

For the practical application of HNBZT for detecting F and CN ions, water samples were collected from different sources. The qualitative applications of F and CN ions were estimated through a calibration curve. All these samples were further spiked with different concentrations of F and CN ions (20 μM and 40 μM). HNBZT (20 μM) was added to these solutions (3 mL) having different F and CN ion concentrations. The spiked samples were estimated using the calibration curve.

3. Results and discussion

3.1. Photophysical properties of HNBZT

The photophysical properties of HNBZT were investigated using absorption and emission spectroscopy. The absorption spectrum of HNBZT (20 μM) in CH3CN displayed a high-energy transition band at 400 nm and two low-energy transition bands at 445 nm and 475 nm. Upon excitation at 400 nm, HNBZT (20 μM) exhibited an emission band at 505 nm, corresponding to a Stokes shift of 105 nm. Due to the presence of electron donor and acceptor units, an intramolecular charge transfer (ICT) process is likely enabled in HNBZT. To explore the influence of ICT on steady-state absorption and emission spectra, various solvents with differing polarities were employed. In non-polar solvents, the absorption peak was observed at 400 nm. However, as solvent polarity increased, the absorption maximum red-shifted to 475 nm, accompanied by an enhancement of a shoulder band at 535 nm (Fig. 1a). This red shift in absorption maxima is attributed to the ICT process, involving electron transfer from the hydroxy naphthalene unit to the benzothiazole moiety. Similarly, while the emission maxima were predominantly at 505 nm in most solvents, they shifted to 535 nm in CH3OH and 560 nm in H2O as the solvent polarity increased (Fig. 1b, Table 1). These shifts in emission maxima further support the presence of ICT in the excited state. Additionally, HNBZT exhibited noticeable color changes from light yellow to orange with increasing solvent polarity (Fig. 1c), providing a visual indication of the ICT process.
image file: d5nj00019j-f1.tif
Fig. 1 (a) Absorption and (b) emission spectra of HNBZT (20 μM) in solvents of different polarity, and (c) colorimetric response of HNBZT (20 μM) upon increasing polarity of the solvents.
Table 1 Photophysical properties of HNBZT
S. no. Solvent λ max (nm) Molar absorptivity ε (M−1 cm−1) λ em (nm) Stokes shift Δν (cm−1) Quantum yield (Φ)
Stokes shift = 1/(λemλmax), reference for quantum yield is quinine sulphate.
1 Hexane 405 6050 505 4890 0.49
2 Cyclohexane 405 16[thin space (1/6-em)]300 505 4890 0.45
3 Toluene 405 19[thin space (1/6-em)]500 509 4890 0.51
4 Chloroform 405 14[thin space (1/6-em)]900 509 4890 0.59
5 Tetrahydrofuran 405 5300 510 4890 0.53
6 Acetone 405 15[thin space (1/6-em)]550 507 4890 0.55
7 Dioxane 405 15[thin space (1/6-em)]700 508 4890 0.55
8 Ethyl acetate 405 19[thin space (1/6-em)]550 511 4890 0.56
9 Dimethylsulphoxide 455 17[thin space (1/6-em)]550 513 2180 0.48
10 N,N-Dimethylformamide 455 15[thin space (1/6-em)]300 515 2180 0.54
11 Acetonitrile 405 16[thin space (1/6-em)]900 506 4890 0.72
12 Isopropylalcohol 475 15[thin space (1/6-em)]400 507 1250 0.75
13 Ethanol 455 18[thin space (1/6-em)]350 515 2180 0.58
14 Methanol 475 16[thin space (1/6-em)]100 535 2360 0.65
15 Water 475 5300 560 3190 0.69


HNBZT exhibited torsional flexibility concerning single bonds, specifically C—N rotation at the imine center connection and C—C rotation at the benzothiazole unit connection, which resulted in different conformations. The theoretical calculations indicate that the plausible configurations, which were optimized at the S0 state, have a significant population of 55% and 45%, respectively, and are relatively stable due to the formation of strong intramolecular hydrogen bonding in the HNBZT-I and HNBZT-II conformers. In the S0 state with no imaginary frequency, HNBZT's geometry was slightly twisted in both of these configurations.

image file: d5nj00019j-u1.tif

Three low-lying excitations of HNBZT-I and HNBZT-II were calculated in accordance with S0 geometry in order to investigate the photoexcitation process. Table 2 contains a tabulation of the computed excitations. With an oscillation strength of 1.2213 at 419 nm for S0 → S1 excitation, the TDDFT calculation for HNBZT-I showed an orbital transition contribution of 98.5% from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), which is close to the experimental absorption peak at 405 nm. In contrast, S0 → S1 excitation for HNBZT-II was calculated at 416 nm, with an orbital contribution of 98.5% from the HOMO to LUMO transition and oscillation strength of 1.2079. We have only taken into consideration the S0 → S1 excitation for further analysis because the calculated S0 → S2 and S0 → S3 excitations have low oscillation strengths for both configurations.

Table 2 Summary of excitation spectra of HNBZT-I and HNBZT-II
HNBZT-I HNBZT-II
Excitation λ (nm) MO Excitation λ (nm) MO
λ = calculated excitation wavelength (nm); MO = molecular orbitals contributing to excitations; H = HOMO; L = LUMO.
S0 → S1 419.72 (f = 1.2213) H → L (98.5%) S0 → S1 416.66 (f = 1.2079) H → L (98.5%)
S0 → S2 363.29 (f = 0.0216) H-1 → L (95%) S0 → S2 360.66 (f = 0.0261) H-1 → L (96%)
S0 → S3 332.23 (f = 0.0757) H-2 → L (89.6%) S0 → S3 333.26 (f = 0.0760) H-2 → L (59%), H-4 → L (15%), H → L + 1 (14%)


Furthermore, both qualitative and quantitative assessments were made for the change in electronic distribution. For both configurations, HNBZT-I and HNBZT-II, the involved molecular orbitals showed a shift in electron density from the benzothiazole unit (in the HOMO) to the Schiff base unit (in the LUMO) (Fig. 2). The ensuing hole–electron analysis revealed a large distance of electron/hole centroid (D = 7.953 Å for HNBZT-I and 7.863 Å for HNBZT-II) and a low overlap integral (Sr = 0.42174 for HNBZT-I and Sr = 0.42741 for HNBZT-II) of hole/electron distribution, establishing an intramolecular charge transfer process in HNBZT for both configurations.


image file: d5nj00019j-f2.tif
Fig. 2 Optimized structures of HNBZT along with their frontier molecular orbitals.

3.2. Geometrical parameters and tautomeric conversion

HNBZT contained two kinds of asymmetrical intramolecular hydrogen bonding (IraHB) labeled as A and B with short interaction distance (less than the sum of van der Waals radii of hydrogen and nitrogen) and angles. For HNBZT-I, on photoexcitation to S1 state, the intramolecular distances for A type interaction were decreased from 1.686 Å to 1.635 Å, while the bond angle increased from 146.80° to 149.23°. On the other hand, B type interaction showed a decrease in distance from 1.759 Å to 1.745 Å, while the bond angle increased from 145.87° to 146.86°. A similar trend was observed for the HNBZT-II conformer too. It was also noted that a significant molecular planarity was achieved with a decrease of dihedral angle of the Schiff base unit for both conformers in the S1 state. These alterations in bond distance and angle (on photoexcitation) established strengthening of intramolecular hydrogen bonding, which could prompt the proton transfer process in the excited state (Fig. 3). Therefore, the tautomeric conversion through the proton transfer process in HNBZT could result in four different tautomeric forms (EE, EK1, EK2, KK). The relative free energy (ΔG) profile of the tautomeric forms for HNBZT-I established a relationship of EE (0 kcal mol−1) > EK1 (−3.33 kcal mol−1) < EK2 (6.02 kcal mol−1) > KK (2.21 kcal mol−1) in the S0 state and EE (57.78 kcal mol−1) > EK1 (53.42 kcal mol−1) < EK2 (60.43 kcal mol−1) > KK (56.95 kcal mol−1) in the S1 state. A similar trend has been observed for HNBZT-II.
image file: d5nj00019j-f3.tif
Fig. 3 (i) Optimized structures of HNBZT-I and HNBZT-II along with significant geometrical parameters and (ii) energy profile of the tautomeric forms at S0 and S1.

3.3. Aggregation induced emission (AIE) studies

Aggregation-induced emission (AIE) characteristics are commonly observed in organic fluorophores containing rotor structures. We investigated the AIE effect of the naphthalene rotor in HNBZT by conducting experiments in CH3CN/H2O mixtures with varying H2O content. The absorption spectra of HNBZT exhibited a blue shift from 400 nm to 355 nm as the H2O content increased from 0% to 50%. However, as the H2O fraction increased from 60% to 100%, the absorption intensity progressively increased, accompanied by a noticeable red shift from 355 nm to 480 nm and the appearance of a levelling-off tail in the absorption band (Fig. 4a). The bathochromic shift observed is associated with the formation of J-type aggregates. Further variations in the emission spectra of HNBZT were observed with changing the H2O percentages in the CH3CN solutions (0–100%). As the H2O fraction increased, the emission intensity also increased significantly (Fig. 4b). This enhancement in emission intensity is attributed to the aggregation-induced emission (AIE) process in HNBZT. The visible color change from dark yellow to pale of the HNBZT solution upon increasing the H2O ratio was observed, which is attributed to Mie scattering and the aggregation of HNBZT with increasing H2O content (Fig. 4c). Thus, the observed tailing between 450–550 nm in Fig. 4a is characteristic of aggregate formation, and this aggregate-induced emission enhancement is supported by emission spectra, DLS, and FESEM results. Regarding the emission spectra, intermolecular hydrogen bonding with water would typically shift the parent emission of HNBZT to a shorter wavelength, as seen in its interactions with F and CN ions. The characteristic keto emission at 515 nm arises from intact intramolecular hydrogen bonding in HNBZT, whereas disruption of this bonding results in enol-type emission around 450 nm. This behaviour is further supported by theoretical calculations.
image file: d5nj00019j-f4.tif
Fig. 4 (a) Absorption and (b) emission spectra of HNBZT with increasing H2O ratio in CH3CN. (c) Colorimetric and (d) emission response of HNBZT upon increasing H2O content in CH3CN.

To investigate the aggregation behaviour of HNBZT, dynamic light scattering (DLS) experiments were conducted. The results confirmed the aggregation behaviour of HNBZT and provided insights into the size of aggregates under varying CH3CN:H2O ratios. In pure CH3CN, the aggregate size ranged from 20–100 nm, with an average size of 50 nm and a polydispersity index (PDI) of 0.3. In a 50% CH3CN–H2O mixture, the aggregate size increased to 100–500 nm, with an average size of 250 nm and a PDI of 0.6. This hydrodynamic diameter further increased significantly to 1250 nm in pure H2O, with a PDI value of 0.8 (Fig. 5). Field emission scanning electron microscopy (FESEM) analysis supported these findings, showing particle sizes of 20 nm in pure CH3CN, 150 nm in 50% CH3CN–H2O, and 993 nm in pure H2O (Fig. 6). The coherence between the DLS and FESEM results underscores the consistency and reliability of the observed aggregation behaviour of HNBZT.


image file: d5nj00019j-f5.tif
Fig. 5 DLS of HNBZT in (a) CH3CN, (b) 50% CH3CN:H2O and (c) H2O.

image file: d5nj00019j-f6.tif
Fig. 6 FESEM images of HNBZT in (a) CH3CN, (b) 50% CH3CN:H2O and (c) H2O.

3.4. Anion binding behavior of HNBZT

The binding behavior of HNBZT (20 μM) toward various anions was investigated in CH3CN. As previously discussed, HNBZT exhibited a high-energy transition band at 400 nm and two low-energy transition bands at 445 nm and 475 nm in the absorption spectrum. A weak emission band was observed at 505 nm upon excitation at 400 nm. The anion-binding affinity of HNBZT was examined in the presence of different anions, including F, Cl, Br, I, H2PO4, NO3, AcO, HSO4, P2O74−, CN, and SCN using absorption and emission spectroscopy where HNBZT demonstrated a strong binding affinity for F and CN anions. For both F and CN, the absorption band at 475 nm intensified with a quantum yield of 0.81 and 0.79, respectively, while the bands at 400 nm and 445 nm disappeared (Fig. 7a). In the fluorescence spectra, the emission maximum of HNBZT at 505 nm underwent a blue shift to 450 nm upon interaction with F and CN anions (Fig. 7b). Additionally, HNBZT exhibited a notable colorimetric response, changing from light yellow to orange with F and to dark yellow with a CN anion (Fig. 7c) while HNBZT under UV light showed emission colour change from mild green fluorescence to purplish blue fluorescence in the presence of F ions (Fig. 7d). These results highlighted the strong binding and distinct optical responses of HNBZT to F and CN anions.
image file: d5nj00019j-f7.tif
Fig. 7 (a) Absorption and (b) emission spectra of HNBZT (20 μM) upon interaction of different anions (1000 μM) in CH3CN. (c) Colorimetric and (d) fluorogenic responses of HNBZT (20 μM) upon interaction with different anions in CH3CN.

Titration assays were presented to investigate the binding affinities of HNBZT for F and CN ions. In the absorption spectra, the band centered at 405 nm decreased with a slight blue shift to 390 nm, while the band at 475 nm intensified upon incremental addition of F (0–100 μM). Additionally, the absorption band at 330 nm showed a slight increase in intensity, whereas the band at 295 nm decreased. The formation of two isosbestic points at 435 nm and 355 nm indicated the equilibrium between multiple species (Fig. 8a). Similarly, emission titration experiments with sequential addition of F ions (0–360 μM) revealed an enhancement of the emission band at 450 nm, while the band at 515 nm disappeared (Fig. 8b). To determine the reaction stoichiometry between HNBZT and F ions, emission spectra were analyzed at different mole fractions of F. The emission maximum observed at a mole fraction of 0.5 indicated a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometry, determined with Job's plot analysis (Fig. S5a, ESI).


image file: d5nj00019j-f8.tif
Fig. 8 (a) Absorption spectra of HNBZT upon incremental addition of 0–100 μM of F ions and (b) emission spectra of HNBZT upon incremental addition of 0–360 μM of F ions.

Using the Benesi–Hildebrand equation, the binding constant for HNBZT and F was calculated to be 2.9 × 105 M−1. The lowest detection limit for F ions was determined to be 7.6 × 10−8 M, highlighting the high sensitivity of HNBZT for F detection.

Similar results were observed for the CN ion. During absorption titration, the band at 475 nm was increased, while the band at 405 nm decreased with a slight blue shift to 390 nm. Moreover, the absorption band at 340 nm revealed a slight increase in intensity, while the band at 290 nm decreased (Fig. 9a). The presence of two isosbestic points indicated an equilibrium between multiple species. In the emission titration, the emission band at 505 nm decreased, accompanied by an increase in emission intensity at 450 nm (Fig. 9b). The binding constant for CN was calculated to be 7.5 × 103 M−1, with the lowest detection limit of 1.1 × 10−7 M (Table 3). Job's plot analysis confirmed a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometry for the interaction between HNBZT and CN ions (Fig. S5b, ESI).


image file: d5nj00019j-f9.tif
Fig. 9 (a) Absorption spectra of HNBZT upon incremental addition of 0–50 μM of CN ions and (b) emission spectra of HNBZT upon sequential addition of 0–120 μM of CN ions.
Table 3 Binding constants and detection limits of HNBZT for F and CN ions
Binding constant LOD
HNBZT + F 2.9 × 105 M−1 7.6 × 10−8 M
HNBZT + CN 7.5 × 103 M−1 1.1 × 10−7 M


An interference study was also conducted to evaluate the selectivity of HNBZT toward F and CN ions. No significant changes were observed in the emission spectra of HNBZT + F and HNBZT + CN upon the addition of excess competitive anions (1000 μM), including F, CN, Br, SCN, ClO, AcO, NO3, P2O74−, H2PO4, and HSO4 (Fig. 10). This demonstrated that HNBZT exhibits high selectivity for both F and CN anions.


image file: d5nj00019j-f10.tif
Fig. 10 Relative emission intensity of HNBZT (20 μM) in CH3CN (λex = 390 nm) with different competing anions in the absence and presence of F and CN ions at λem = 450 nm, where blue bars represent the emission intensity change of HNBZT with different anions (1000 μM), red bars represent HNBZT + CN in the presence of different relevant competing anions (1000 μM) and green bars represent HNBZT + F.

3.5. Time-correlated single photon counting (TCSPC) study

The fluorescence enhancement behaviour of HNBZT upon binding with F and CN ions was further corroborated by time-correlated single photon counting (TCSPC) studies. The fluorescence decay profiles of HNBZT and its complexes were best fitted using a three-exponential function (Fig. 11). For HNBZT, three lifetime components were observed: 1.12 ns, 6.90 ns, and 0.07 ns, with population percentages of 7.32%, 8.45%, and 84.23%, respectively. The average lifetime of HNBZT was calculated to be 0.09 ns. Upon addition of F ions to HNBZT, the fluorescence decay revealed three components with lifetimes of 1.43 ns, 1.53 ns, and 2.44 ns, and corresponding population percentages of 27.85%, 39.02%, and 33.14%. The average lifetime was increased to 1.71 ns. Similarly, after the addition of CN ions to HNBZT, the average lifetime was increased from 0.09 ns to 2.12 ns (Table 4). The observed increase in average lifetime upon interaction with F and CN ions supports the fluorescence enhancement of HNBZT in the presence of these anions.
image file: d5nj00019j-f11.tif
Fig. 11 Time resolved fluorescence decay of HNBZT and HNBZT with F and CN ions.
Table 4 Fluorescence lifetime measurements for HNBZT and its complexes with F and CN ions in CH3CN
CH3CN τ 1 (ns) τ 2 (ns) τ 3 (ns) α 1 α 2 α 3 χ 2 τ av (ns)
HNBZT 1.12 6.90 0.07 7.32 8.45 84.23 1.12 0.09
HNBZT + F 1.43 1.53 2.44 27.85 39.02 33.14 1.18 1.71
HNBZT + CN 1.76 9.52 2.36 35.42 0.94 63.64 1.07 2.12


3.6. 1H NMR of HNBZT with F and CN ions

To investigate the binding mechanisms of F and CN ions with HNBZT, 1H NMR titrations were conducted in CD3CN-d3. In the 1H NMR spectrum of HNBZT, an imine proton was observed at δ 8.98 ppm. No significant change was detected at this position upon the addition of 1.0 equivalent of F ions (Fig. 12). Additionally, the disappearance of the hydroxyl proton indicated hydrogen bonding between the F ion and the naphtholic hydrogen. Further changes were observed in the aromatic region upon the addition of F ions. The peak corresponding to the Ha proton at δ 8.28 ppm was shifted, and the peak for the Hd proton at δ 7.95 ppm shifted to δ 8.05 ppm. Similarly, the peak at δ 7.88 ppm due to the He proton shifted to δ 7.95 ppm. These shifts suggested charge delocalization and redistribution within the HNBZT molecule after binding with F ions, likely as a result of deprotonation. In contrast, the protons in the phenyl ring, appearing at δ 7.49–7.36 ppm and benzothiazole ring at δ 8.82 ppm and δ 8.68 ppm, showed negligible changes, indicating that the phenyl and benzothiazole moieties were not significantly involved in the interaction (Fig. S6, ESI). These findings provided insights into the binding mechanism of F ions with HNBZT (Scheme 2).
image file: d5nj00019j-f12.tif
Fig. 12 1H NMR spectra of HNBZT with F ions in CD3CN-d3.

image file: d5nj00019j-s2.tif
Scheme 2 Possible binding mechanisms of HNBZT with F and CN ions.

Similarly, the addition of 1.0 equivalent of CN ions to HNBZT caused significant changes in the proton signals. The imine proton at δ 8.98 ppm disappeared completely upon interaction with CN ions (Fig. 13). Additionally, the phenyl ring protons at δ 7.49 and 7.38 ppm were shifted upfield, and the naphthalene ring protons merged, indicating charge propagation across the molecule. In contrast, the protons in the benzothiazole ring, appearing at δ 8.82 and 8.71, showed negligible changes, thus not involved in the interaction. These observations suggested that the binding mechanism involved nucleophilic addition of CN ions to HNBZT (Scheme 2). 13C NMR was also performed to confirm the binding behaviour of HNBZT with CN ions. With the addition of 1.0 equivalent CN ions, a new peak appeared at 50.40 ppm indicating the nucleophilic addition of CN to HNBZT (Fig. S7, ESI). The FTIR spectrum of HNBZT before and after the addition of CN also confirmed the binding mechanism (Fig. S8, ESI).


image file: d5nj00019j-f13.tif
Fig. 13 1H NMR spectra of HNBZT with CN ions in CD3CN-d3.

The binding interactions of F and CN ions with HNBZT were further corroborated through theoretical calculations. The optimized structures of the HNBZT·F and HNBZT·CN complexes are shown in Fig. 14. The predicted absorption spectra based on these optimized structures were in good agreement with the experimental results. For HNBZT·F, three low-lying vertical excitations were identified: the HOMO → LUMO transition (99%; f = 0.9663) at 473 nm, the HOMO−1 → LUMO transition (77%; f = 0.2191) at 396 nm, and the HOMO → LUMO+1 transition (82%; f = 0.0373) at 362 nm. Similarly, for HNBZT·CN, three low-lying vertical excitations were observed: the HOMO → LUMO transition (99.5%; f = 0.0299) at 455 nm, the HOMO → LUMO+1 transition (89%; f = 0.2896) at 368 nm, and the HOMO−1 → LUMO transition (82%; f = 1.0059) at 358 nm. In both complexes, the HOMO → LUMO transitions exhibited a significant electron density shift from the benzothiazole unit to the Schiff base unit, indicating intramolecular charge transfer (ICT). This ICT is responsible for the observed colour changes in the presence of F and CN ions.


image file: d5nj00019j-f14.tif
Fig. 14 Optimized structures of HNBZT.F and HNBZT.CN along with their frontier molecular orbitals.

3.7. pH titration

We also checked the pH stability of HNBZT and its complex with the CN ions. To check the pH stability, we performed acid–base titration. From the acid–base titration, it is clear that our compound was stable in the pH range from 2–12 (Fig. 15). Similarly, the HNBZT and CN complex was also stable in the pH range of 2–12. The stability of HNBZT and its complex in the pH range 2–12 established its benefit for rapid monitoring in environmental and biological samples. The complex of HNBZT and F ions was also stable in the pH range from 4–10, so the detection of these ions using HNBZT could be well performed in a given pH range.
image file: d5nj00019j-f15.tif
Fig. 15 Effect of pH on emission spectra of HNBZT (20 μM) and its complexes with ions in CH3CN.

3.8. Detection of F/CN ions in real samples

HNBZT was used to detect F and CN in real water samples in order to investigate the practical applicability of the chemosensor where satisfactory results were achieved. F and CN sample solutions were prepared using tap water from the laboratory. In this instance, known F and CN ion concentrations were directly spiked into tap water. The emission intensity was then measured at 450 nm after the samples were subjected to 20 μM of HNBZT in CH3CN (Fig. 16). The favorable recoveries from tap water with varying F and CN ion concentration show practical applicability of HNBZT in everyday situations (Table 5). HNBZT can detect F and CN ions with high efficiency as compared to the previous reports (Table S1, ESI). Similarly, good recoveries were obtained from toothpaste samples (Table 6).
image file: d5nj00019j-f16.tif
Fig. 16 Emission spectra of HNBZT in the presence of different concentrations of spiked (a) F and (b) CN from 0 to 100 μM.
Table 5 Determination of F and CN ions in tap water
Ion Sample Ion added (μM) Ion recovered (μM) Recovery (%) RSD
F 1 20 20.5 102.5 0.31
2 40 41.1 102.75 0.35
CN 1 20 18.1 90.5 0.42
2 40 40.1 100.25 0.55


Table 6 Determination of F ions in toothpaste samples
Sample F found Added (μM) Recovered (μM) Recovery (%) RSD
Toothpaste 1 1 10 9.5 95 0.71
2 20 20.2 101 0.80
Toothpaste 2 1 10 9.9 99 0.78
2 20 19.7 98.5 0.74


4. Conclusion

In conclusion, an efficient chemosensor, HNBZT was synthesized, and its photophysical properties were thoroughly investigated in various solvent systems. HNBZT demonstrated aggregation-induced emission (AIE) behaviour and was successfully employed for the selective and sensitive detection of F and CN ions in a CH3CN solvent system. Detection of F and CN ions by HNBZT was achievable with the naked eye. The detection limits for F and CN ions were as low as 7.6 × 10−8 M and 1.1 × 10−7 M, respectively. 1H NMR titration of HNBZT with F and CN ions revealed distinct binding mechanisms, with F ions forming hydrogen bonds, while CN ions induced nucleophilic addition which is also supported by theoretical studies. Notably, F and CN ions interacted with HNBZTvia distinct binding mechanisms.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and/or its ESI.

Acknowledgements

The authors would like to acknowledge CEEMS, TIET (TIET/CEEMS/Regular/2023/044) for financial support and DST-FIST (SR/FST/CS-II/2018/69) for HRMS analysis.

References

  1. A. Mondal, P. Mondal and P. Chattopadhyay, Regulatory principles, parameters and probe design to ascertain the interconnection between ICT, ESIPT and PET mechanisms, Inorg. Chim. Acta, 2024, 122194 CrossRef CAS.
  2. R. M. Duke, E. B. Veale, F. M. Pfeffer, P. E. Kruger and T. Gunnlaugsson, Colorimetric and fluorescent anion sensors: an overview of recent developments in the use of 1,8-naphthalimide-based chemosensors, Chem. Soc. Rev., 2010, 39, 3936–3953 RSC.
  3. P. Zhou and K. Han, ESIPT-based AIE luminogens: Design strategies, applications, and mechanisms, Aggregate, 2022, 3, e160 CrossRef CAS.
  4. P. R. Dongare and A. H. Gore, Recent advances in colorimetric and fluorescent chemosensors for ionic species: Design, principle and optical signalling mechanism, ChemistrySelect, 2021, 6, 5657–5669 CrossRef CAS.
  5. H.-W. Zheng, Y. Kang, M. Wu, Q.-F. Liang, J.-Q. Zheng, X.-J. Zheng and L.-P. Jin, ESIPT-AIE active Schiff base based on 2-(2′-hydroxyphenyl) benzo-thiazole applied as multi-functional fluorescent chemosensors, Dalton Trans., 2021, 50, 3916–3922 RSC.
  6. M. Chen, Y. Chen, M. Zhong, D. Xie, C. Wang, X. Ren, S. Huang, J. Xu and M. Zhu, The Synergistic Mechanisms of AIE, ESIPT and ICT in the α-cyanostilbene-based Derivative: A Red-fluorescence Probe With a Large Stokes’ Shift for Copper(II) Ion Determination and Reversible Response to Amine/acid Vapor, J. Fluoresc., 2024, 34, 1075–1090 CrossRef CAS.
  7. A. Mondal, E. Ahmmed, B. Ball and P. Chattopadhyay, Rational Design of a New AIE-Coupled ESIPT-Based Multi-chromic State Depended Organo-luminophore With Turn-on Emissive Response to Zn(II) in Aqueous and Solid-state, ChemistrySelect, 2022, 7, e202103857 CrossRef CAS.
  8. J. Qin, B. Wang, Z. Yang and K. Yu, A ratiometric fluorescent chemosensor for Zn2+ in aqueous solution through an ESIPT coupled AIE process, Sens. Actuators B, 2016, 224, 892–898 CrossRef CAS.
  9. J. Y. Shang, Y. Li, K. Chen and H. Li, Synthesis and properties of an AIE fluorescent probe for Cu2+ detection based on ESIPT system, Chem. Pap., 2021, 75, 1851–1859 CrossRef CAS.
  10. A. Jain, S. De, P. Saraswat, J. Haribabu, J. F. Santibanez and P. Barman, An ESIPT active coumarin-diphenyl azine-based AIEgen: Nanomolar Cu2+ ion sensing, Latent Fingerprinting, live-cell imaging, and real sample analysis, J. Mol. Struct., 2024, 1310, 138383 CrossRef CAS.
  11. S. Suresh, G. Prabakaran, J. Prabhu, P. Vijayanand, R. S. Kumar, R. Karthick, G. Velraj and R. Nandhakumar, A pyridine naphthalene conjugate: ESIPT based molecular chemosensor for Al3+ ions and sequential detection of H2PO4-and applications in milk, water samples and bio-imaging, J. Food Compos. Anal., 2024, 132, 106364 CrossRef CAS.
  12. U. Duraisamy, P. Jerome, N. Vijay and T. H. Oh, ESIPT: An approach and future perspective for the detection of biologically important analytes, J. Lumin., 2023, 120350 Search PubMed.
  13. S. Paul, A. Ray Choudhury and N. Dey, Dual-mode multiple ion sensing via analyte-specific modulation of keto–enol tautomerization of an ESIPT active pyrene derivative: experimental findings and computational rationalization, ACS Omega, 2023, 8, 6349–6360 CrossRef CAS.
  14. T. Jiang, J.-H. Lu, C. Huang, D.-M. Chen and B.-X. Zhu, Two helical Schiff bases with “AIE+ ESIPT” characteristics exhibiting selective ion recognition properties, Dyes Pigm., 2024, 223, 111972 CrossRef CAS.
  15. A. I. Said, N. I. Georgiev and V. B. Bojinov, Simple excited state intramolecular proton transfer (ESIPT) based probe for pH and selective detection of copper(II) ion in aqueous alkaline environment: Sensitivity, selectivity and logic behavior, J. Photochem. Photobiol., A, 2024, 446, 115176 CrossRef CAS.
  16. F. Y. Wu, S. G. Cao and C. X. Xie, A highly selective chemosensor for copper ion based on ICT fluorescence, Chin. Chem. Lett., 2012, 23, 607–610 CrossRef CAS.
  17. C. I. David, P. Movuleeshwaran, H. Jayaraj, G. Prabakaran, M. S. Kumar, A. Abiram, T. S. Babu, J. Prabhu and R. Nandhakumar, Highly selective, reversible and ICT-based fluorescent chemosensor for bismuth ions: Applications in bacterial imaging, logic gate and food sample analysis, J. Photochem. Photobiol., A, 2022, 422, 113558 CrossRef.
  18. Pooja, H. Pandey, S. Aggarwal, M. Vats, V. Rawat and S. R. Pathak, Coumarin-based Chemosensors for Metal Ions Detection, Asian J. Org. Chem., 2022, 11, e202200455 CrossRef CAS.
  19. N. Kaur, Anthraquinone appended chemosensors for fluorescence monitoring of anions and/or metal ions, Inorg. Chim. Acta, 2022, 536, 120917 CrossRef CAS.
  20. F.-Y. Ye, M. Hu and Y.-S. Zheng, Advances and challenges of metal ions sensors based on AIE effect, Coord. Chem. Rev., 2023, 493, 215328 CrossRef CAS.
  21. K. S. Jagadhane, S. R. Bhosale, D. B. Gunjal, O. S. Nille, G. B. Kolekar, S. S. Kolekar, T. D. Dongale and P. V. Anbhule, Tetraphenylethene-based fluorescent chemosensor with mechanochromic and aggregation-induced emission (AIE) properties for the selective and sensitive detection of Hg2+ and Ag+ Ions in aqueous media: application to environmental analysis, ACS Omega, 2022, 7, 34888–34900 CrossRef CAS PubMed.
  22. M. H. Chua, B. Y. K. Hui, K. L. O. Chin, Q. Zhu, X. Liu and J. Xu, Recent advances in aggregation-induced emission (AIE)-based chemosensors for the detection of organic small molecules, Mater. Chem. Front., 2023, 7, 5561–5660 RSC.
  23. M. D. Pandey, S. Asthana, M. V. Mouli, A. Tamrakar, M. A. Wani, A. K. Mishra and R. Pandey, Recent Advances in AIEgen-based Chemosensors for Small Molecule Detection, with a Focus on Ion Sensing, Anal. Methods, 2024, 16, 4431–4484 RSC.
  24. K. Krishnaveni, S. Gurusamy, K. Rajakumar, V. Sathish, P. Thanasekaran and A. Mathavan, Aggregation induced emission (AIE), selective fluoride ion sensing and lysozyme interaction properties of Julolidinesulphonyl derived Schiff base, J. Photochem. Photobiol., A, 2022, 427, 113822 CrossRef CAS.
  25. L. Chen, H. Jiang, N. Li, Q. Meng, Z. Li, Q. Han and X. Liu, A Schiff-based AIE fluorescent probe for Zn2+ detection and its application as “fluorescence paper-based indicator”, Spectrochim. Acta, Part A, 2022, 268, 120704 CrossRef CAS.
  26. S. Zhang, D. Wu, X. Jiang, F. Xie, X. Jia, X. Song and Y. Yuan, A novel fluorescent probe with one-excitation and dual-emission for selective and simultaneous detection of Glutathione and Arginine in NIR and blue regions, Sens. Actuators, B, 2019, 290, 691–697 CrossRef CAS.
  27. S. Ahamed, M. Mahato, R. Sahoo, N. Tohora, T. Sultana, A. Maiti and S. K. Das, Decoding the ICT-PET-ESIPT Liaison Mechanism in a Phthalimide-based Trivalent Transition Metal Ions Specific Chromo-fluorogenic Probe, New J. Chem., 2024, 48, 13131–13143 RSC.
  28. A. Pramanik, R. Das, P. J. Boruah, S. Majumder and S. Mohanta, A very rare fluorescent chemosensor of zinc(II) exhibiting AIEE, ESIPT and TICT: Spectroscopic, crystallographic and theoretical exploration, Spectrochim. Acta, Part A, 2024, 308, 123780 CrossRef CAS.
  29. X. Zhang, X. Weng, Z. Yang, P. Zhao, W. Chen, Z. Wu, X. Zhuang and A. Chalcone-based Fluorescence Probe, for H2S Detecting Utilizing ESIPT Coupled ICT Mechanism, J. Fluoresc., 2024, 34, 821–828 CrossRef CAS.
  30. S. Enbanathan, S. Munusamy, D. Jothi, S. M. Kumar, P. Seenu, M. F. Noor and S. K. Iyer, An AIE dynamic a highly selective and expeditious benzothiazole-pyrazine based colorimetric chemosensor for Ni2+ and fluorogenic chemosensor for Cu2+ and Al3+ detection, J. Mol. Liq., 2024, 404, 124949 CrossRef CAS.
  31. B. Devi, A. K. Guha and A. Devi, Fluoride ion detection in aqueous medium: Colorimetric and turn-off fluorescent Schiff base chemosensor, Spectrochim. Acta, Part A, 2024, 305, 123448 CrossRef CAS.
  32. H. M. Al-Saidi and S. Khan, Recent advances in thiourea based colorimetric and fluorescent chemosensors for detection of anions and neutral analytes: a review, Crit. Rev. Anal. Chem., 2024, 54, 93–109 CrossRef CAS PubMed.
  33. A. Palta, G. Kumar, K. Paul and V. Luxami, Highly selective colorimetric and fluorescent probe for F and P2O74− based on AIEE and dual ESIPT, J. Mol. Struct., 2024, 138880 CrossRef CAS.
  34. M. Ilakiyalakshmi, A. A. Napoleon, Phenothiazine-derived fluorescent chemosensor: a versatile platform enabling swift cyanide ion detection and its multifaceted utility in paper strips, environmental water, food samples and living cells, J. Photochem. Photobiol., A, 2024, 447, 115213 CrossRef CAS.
  35. C. I. David and H.-i Lee, Cutting-edge advances in colorimetric and fluorescent chemosensors for detecting lethal cyanide ion: A comprehensive review, Microchem. J., 2024, 110359 CrossRef.
  36. W. Bouali, M. Yaman, N. Seferoğlu and Z. Seferoğlu, Colorimetric and fluorimetric detection of CN ion using a highly selective and sensitive chemosensor derived from coumarin-hydrazone, J. Photochem. Photobiol., A, 2024, 448, 115227 CrossRef CAS.
  37. C. K. Maurya and P. K. Gupta, Discriminative Chromogenic Detection of Fluoride and Cyanide Ions, ChemistrySelect, 2024, 9, e202304016 CrossRef CAS.
  38. B. S. Cugnasca, F. Duarte, J. L. P. de Albuquerque, H. M. Santos, J. L. Capelo-Martínez and C. Lodeiro, A. A. Dos Santos, Precision detection of cyanide, fluoride, and hydroxide ions using a new tetraseleno-BODIPY fluorescent sensor, J. Photochem. Photobiol., A, 2024, 457, 115881 CrossRef CAS.
  39. A. Palta, S. Sharma, G. Kumar, D. Choudhary, K. Paul and V. Luxami, A highly specific benzothiazole-based Schiff base for the ratiometric detection of hypochlorite (ClO) ions in aqueous systems: a real application in biological imaging, New J. Chem., 2024, 48, 15402–15413 RSC.

Footnotes

Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nj00019j
Current address: University Centre for Research and Development, Chandigarh University, Mohali, 140413, India.

This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2025
Click here to see how this site uses Cookies. View our privacy policy here.