Carbazole-Based Schiff Bases: Structural Insights and Applications toward Metal Ion Detection

2.1 Detection of Fe³⁺ and Cr³⁺

Fe³⁺ is vital for biological processes like oxygen transport and cellular metabolism, but its imbalance (deficiency or excess) leads to disorders such as anemia, organ damage, and diabetes, highlighting the need for precise detection methods to monitor its levels for human health. Li et al. [19] have synthesized a carbazole-based Schiff base fluorescent probe by reacting f 3-(2-formyl)thienyl-9H-hexylcarbazole (1) with 4-hydroxy-3-nitroaniline (2a) to give probe 1a (P1a) and 3-nitroaniline (2b) to give probe 1b (P1b). The thiophene group was introduced into the 3-position of carbazole to extend the conjugated π-bridge and also to provide a new coordination binding site from the sulfur atom of thiophene, as shown in Figure 1. The conjugation extended from carbazole moiety to thiophene to benzene ring via ‒C=N‒ group. The structures of synthesized probes were confirmed via IR and NMR spectroscopies. Probe 1a showed dramatic enhancement of the fluorescent intensity upon addition of Fe³⁺ or Cr³⁺ in the presence of other metal ions, such as Na⁺, Ni²⁺, K⁺, Ag⁺, Ca²⁺, Cd²⁺, Hg²⁺, Zn²⁺, Co²⁺, Mn²⁺, Mg²⁺, and Fe²⁺, which had no distinct influence on fluoresce intensity. The emission peak was red-shifted from 495 to 502 nm. While probe 1b showed an increase in fluorescent intensity in almost all other ions, suggesting that 1b had no selective recognition for the tested metal ions. This suggested that chemosensors without a phenolic –OH group in the ortho position exhibited weak selective recognition for tested metal ions, indicating that the O atom of –OH is the significant recognition site for metal ions.

The fluorescence enhancement of P1a upon binding with Fe³⁺ or Cr³⁺ was perhaps due to inhibition of -C=N isomerization and ESIPT processes that typically quench fluorescence in the unbound state; the selective coordination with these metal ions stabilized the molecular structure, suppressing non-radiative decay pathways and resulting in increased fluorescence intensity. The authors also investigated the binding stoichiometry and binding mode of P1a for both Fe³⁺ and Cr³⁺. The binding stoichiometry was 2:1 for both ions (P1/M³⁺) as determined by Job’s plot. Binding mode was confirmed by ¹H-NMR, upon coordination with Cr³⁺, the proton signals of P1a broadened and exhibited downfield shift for the hydroxyl, imino, and thienyl protons, indicating that the oxygen atom of the phenolic –OH, the nitrogen atom of the imine group (–N=CH), and the sulfur atom of the thienyl ring served as key binding sites, as presented in Figure 1. The absence of the ortho –OH group in P1b correlates with its diminished metal ion recognition ability.

By increasing the number of nitrogen coordination sites, a much stronger effect can be expected. He et al. [20] conceived this idea and synthesized a dimeric Schiff-base probe 2 (P2) by reacting 9-hexylcarbazole-3-carbaldehyde (3) with 2,3-diaminomaleonitrile in ethanol under reflux conditions as presented in Figure 2. The hexyl chains enhanced the solubility in organic solvents like DMF, while the Schiff base (‒CH=N‒) and nitrile (‒CN) groups provided coordination sites for metal ions. Both absorbance and fluorescence spectra were studied to find the selective response of probe P2 to a variety of 18 metal ions.

P2 exhibited high selectivity for Fe³⁺ over 17 other metal ions, as shown in Figure 3, during fluorescence studies in DMF with 60% quenching at 372 nm exclusively with Fe³⁺, with no significant response to competing ions. Competitive experiments confirmed minimal interference. The detection limit obtained was 3.75 × 10⁻⁸ M, which is well below the EPA’s permissible limit (5.35 μM) for drinking water. Job’s plot and Benesi-Hildebrand analyses established a 1:1 binding stoichiometry between P2 and Fe³⁺. The association constant (K_a = 7.98 × 10⁶ M⁻¹) indicated strong affinity. Binding occurred via coordination of Fe³⁺ to both nitrogen atoms of the imino group (‒C=N‒), forming a rigid 5-membered chelate (Figure 2). This interaction disrupted the ligand’s excited-state processes, leading to fluorescence quenching. In the unbound state, P2 emitted strongly due to restricted C=N isomerization and ICT. Fe³⁺ coordination induced ligand-to-metal charge transfer (LMCT) by dissipating energy non-radiatively and thus quenching fluorescence. Reversibility with EDTA confirmed that the process is dynamic and practical for reuse.

Iron detection by imino nitrogen and phenolic oxygen is highly pragmatic, as in another report by Nandhakumar et al. [21], a carbazole-derived Schiff base fluorescent chemosensor (P3) was synthesized via condensation of 9-ethyl-9H-carbazol-3-amine (4) and salicylaldehyde (5) in ethanol, yielding a 73% of P3 as summarized in Figure 4. The structure was characterized by NMR, IR, and mass spectrometry.

The authors tested P3 for the detection of Fe³⁺ and arginine amino acids. The synthesized probe exhibited a sequential “on-off-on” fluorescence response, where Fe³⁺ induced a turn-off via ICT from the carbazole nitrogen to Fe³⁺, while arginine (Arg) restored fluorescence (turn-on). Job’s plot and Benesi-Hildebrand analysis revealed a 2:1 (P3:Fe³⁺) binding stoichiometry with a binding constant K_a = 2.09 × 10⁴ M⁻¹ and a low detection limit (LOD) of 12.22 nM for Fe³⁺. Coordination involved the carbazole nitrogen, imine nitrogen, and phenolic oxygen, confirmed by FT-IR and ¹H-NMR shifts. The sensor operated in DMF/H₂O (1,1, pH 7.4) with reversibility via EDTA and pH stability (6–8). Authors also investigated the detection ability of P3 in molecular logic gates smartphone-based on-site detection of Fe³⁺ & Arg via RGB colorimetry. The detection of Fe³⁺ ion in A549 cells using live-cell imaging showed quenching of fluorescence upon treating cells with FeCl₃ solution, while a real-sample Fe³⁺ analysis in water with 99–101% recovery was achieved.

2.2 Detection of Al³⁺

Aluminum is the third most abundant element in the Earth’s crust, comprising roughly 8% by weight, and is extensively used across industries such as transportation (aircraft, cars), construction (window frames, roofing), packaging (beverage cans, foil), and electrical applications (power lines, wiring) and in many consumer goods like cookware, appliances, and furniture. Exposure to aluminum is usually not harmful, but excessive contact with Al³⁺ can bioaccumulate in the body and is implicated in neurological disorders (e.g., Alzheimer’s and Parkinson’s disease), kidney damage, chronic renal failure, and bone softening due to its interference with physiological processes [22, 23]. Consequently, developing highly selective and sensitive Al³⁺ detection methods is essential for monitoring environmental and biological systems and mitigating their adverse health and ecological impacts. A challenge in developing chemosensors for Al³⁺ is its lower coordination ability than other transition metals, and it is difficult to identify spectroscopically due to its lack of photophysical characteristics. However, O- and N- being strong hetero donor centers could capture Al³⁺ in soluble form.

Another design similar to P3 was reported by Babu et al. [24] based on Schiff base, derived from 4 and 3,5-diiodo salicylaldehyde (6) in ethanol, as shown in Figure 5. The strong electron donor effect of two iodo groups in P4 could enhance the donor ability of the =OH group.

In 90% aqueous CH₃CN, the probe P4 exhibited a weak emission at 540 nm (λ_ex = 385 nm) due to non-radiative decay via ESIPT and -C=N isomerization. Among various transition metal cations added, such as Sn²⁺, Cd²⁺, Mn²⁺, Al³⁺, Cr³⁺, Co²⁺, Cu²⁺, Hg²⁺, Pb²⁺, Zn²⁺, and Fe³⁺, only Al³⁺ induced a ~ 6-fold “turn-on” fluorescence with a red shift to 559 nm and the appearance of new absorption bands at 428 and 496 nm, which was attributed to ground-state complex formation and Metal to Ligand Charge Transfer (MLCT) transitions. Benesi-Hildebrand and Job’s plot analyses revealed a 1:1 of P4-Al³⁺ stoichiometry with K_a = 5.44 × 10⁴ L mol⁻¹. The LOD was determined as 0.403 mg μL⁻¹. The mechanism of binding involved the coordination of phenolic O- and imine N-donors, conferring high selectivity for Al³⁺ over a panel of competing cations. The authors did not propose a binding mode or bounded structure of complex.

Replacing diiodo salicylaldehyde with naphthaldehyde led to enhanced fluorescence due to extended π-conjugation, as witnessed by a carbazole Schiff-based Al³⁺ detector, reported by Kaya et al. [25] where they condensed 4 and 2-hydroxy-1-naphthaldehyde (7) in ethanol under refluxed conditions to get probe P5 as shown in Figure 6. They tested the probe P5 for both cations (K⁺, Ag⁺, Ba²⁺, Mn²⁺, Mg²⁺, Sn²⁺, Hg²⁺, Ca²⁺, Co²⁺, Zn²⁺, Cu²⁺, Ni²⁺, Pb²⁺, Al³⁺, Fe³⁺, Cr³⁺, and Cr⁶⁺) and anions (F⁻, Cl⁻, Br⁻, I⁻, CO₃²⁻, HCO₃⁻, and HPO₄⁻) in aqueous media. The probe demonstrated high selectivity toward Al³⁺ over 16 competing metal cations and seven anions.

Fluorescence studies revealed an 11-fold emission enhancement at 533 nm upon Al³⁺ addition, with no significant response to other ions. Competitive experiments confirmed that Al³⁺ detection remains unaffected in the presence of interfering species, revealing its specificity. The binding stoichiometry between P5 and Al³⁺ was recognized as 1:1 using Job’s plot and Benesi-Hildebrand analyses. The K_a value was calculated as 5 × 10⁴ M⁻¹, indicating strong affinity. Binding occurred via coordination of Al³⁺ to the imine nitrogen (‒C=N‒) and the deprotonated hydroxyl oxygen (O-), forming a rigid, planar complex as presented in Figure 5. This interaction suppresses excited-state intramolecular proton transfer (ESIPT) and C=N isomerization, which is a key non-radiative decay pathway in the free ligand. The mechanism of detection was the “turn-on” fluorescence response based on chelation-enhanced fluorescence (CHEF) effect. Authors believed that a bathochromic shift in UV-Vis spectra from 462 to 479 nm on binding showed the involvement n → π* electronic transition due to lone pairs of nitrogen and oxygen atoms resulting in a six-membered ring after chelation. The LOD calculated was 2.59 × 10⁻⁷ M, below the WHO-recommended threshold for Al³⁺ in drinking water (2.41 μM), suggesting its utility in practical applications.

In another report by Kaya et al. [26] the selective Al³⁺ detection was achieved by synthesizing carbazole Schiff-base probe 6 (P6) having benzene ring as π-linker between carbazole and imino group while introducing ‒OH and ‒OCH₃ groups on aldehyde partner. The synthesis pathway is shown in Figure 7. The electron-donating ability of methoxy group enhanced the coordination power of the designed probe for low-coordinating metals. The P6 was tested for various metal ions (Co²⁺, Cu²⁺, Ni²⁺, Fe³⁺, Cr³⁺, K⁺, Zn²⁺, Mn²⁺, Al³⁺ and Ag⁺) by fluorescence study, it exhibited weak fluorescence intensity (λ_ex = 320 nm, λ_em = 465 nm) in free form due to ‒C=N isomerization and ESIPT, but the fluorescence intensity increased 108 times with Al³⁺, with no prominent change for other metal ions.

The LOD was 9.29 × 10⁻⁷ M, and K_a value using the modified Benesi-Hildebrand equation was 1.64 × 10⁴ M⁻¹. The binding stoichiometry of the complex between P6 and Al³⁺ was stated at a ratio of 2:1, but in binding mechanism, it showed 1:1. Although this sensor worked well in DMSO/H₂O (1:1) environment, most of such chemosensors suffer from water solubility issues. This limitation makes them impractical for real-world problems. Water is a strong solvent, and its interactions with Al³⁺ (through hydration) can outcompete the interactions with potential ligands in a detector. This usually results in weak, unstable, or even insoluble Al³⁺ complexes, making it difficult to create a reliable detector. Thus, the development of a stable, water-soluble Al³⁺ ion detector remains a challenge due to its lower coordination tendency and leading to weak interactions in water. Chemli et al. [27] has devised a symmetric optical probe (P7) by condensing 2,7-bisformyl-N-pentylcarbazole (8) and 2-aminophenol (9) moieties in methanol at 80°C for 4 h to give P7 as shown in Figure 8. Structure was confirmed by IR, Mass, and NMR spectroscopic methods.

This design led to AIEE utilizing ESIPT in THF/H₂O mixtures, with enhanced fluorescence observed at 480 nm when the water fraction reached 70%. Upon addition of Al³⁺, a distinct color change from yellow to colorless was observed under daylight, and an intense blue emission under UV light, indicating a highly selective “turn-on” fluorescence response. Among a panel of metal ions, only Al³⁺ caused significant spectral changes. Fluorescence and UV-Vis titrations confirmed a 1:1 binding stoichiometry (Job’s plot) and an association constant (K_a) of 1.15 × 10⁴ M⁻¹ (Benesi-Hildebrand method). The LOD was determined as 112 nM (UV-Vis) and 9.4 nM (fluorescence), well below the WHO guideline of 7.4 μM for Al³⁺ in drinking water.

DFT calculations revealed that the regions with high electronegativity, having the strongest attraction to cations binding sites, were observed around the nitrogen atom of the imine groups (‒C=N‒) and the oxygen of the phenol groups (‒OH) (Figure 9). The binding mechanism was attributed to Al³⁺-induced hydrolysis of the imine bond, regenerating 2,7-bisformylcarbazole, as confirmed by ¹H-NMR spectral shifts and fluorescence quantum yield, which increased from 0.5% (Carb-Azo) to 13.7% (product). Furthermore, a smartphone-assisted fluorometric platform using a fiber-optic endoscope was developed for practical Al³⁺ detection in drinking water, achieving a detection limit of 0.4 μM. Recovery experiments in spiked commercial water samples gave values from 94.8 to 116.7% with RSDs <12%, validating the practical applicability of both the spectroscopic method and the portable device for real-time environmental monitoring of Al³⁺ contamination.

Inclusion of benzophenone moiety on carbazole not only can extend conjugation but also enhance the photophysical properties of probe overall, owing to its strong electron-accepting and UV-absorbing abilities. Hu and Tao group [28] recently designed an AIE-active Schiff-base probe, P8, tailored for selective aqueous Al³⁺ detection. The P8 was synthesized by condensing a carbazolyl-benzophenone intermediate (8) with 5 in refluxing toluene. The structure was confirmed by ¹H/¹³C NMR showing imine (‒CH=N‒) CH signal at ~8.7 ppm δ-value and phenolic OH at ~12.9 ppm δ-value (Figure 10).

P8 exhibited classic aggregation-induced emission in THF/H₂O, 1:4. As the water fraction increased from 0 to 95%, its UV–Vis π–π* band red-shifted from 380 to 410 nm and its emission at 545 nm intensified 6.4-fold, driven by aggregation-caused restriction of intramolecular rotations and hydrogen-bond–mediated planarization. Upon addition of Al³⁺, a pronounced “turn-on” fluorescence at 545 nm and a new absorption at 410 nm emerged, while 18 competing metal ions (Li⁺, Na⁺, K⁺, Ag⁺, Cu²⁺, Fe³⁺, Zn²⁺, Mn²⁺, Ca²⁺, Mg²⁺, Cd²⁺, Cr²⁺, Pb²⁺, Ba²⁺, Ga²⁺, La³⁺, Ni²⁺, In³⁺, Ce³⁺) elicited negligible response, owing to chelation-enhanced fluorescence (CHEF) that inhibited C=N isomerization and ESIPT quenching by coordinating Al³⁺ to the imine nitrogen and phenolic oxygen donors. FT-IR confirmed this binding by the disappearance of the 3051 cm⁻¹ OH stretch and shift of the 1600 cm⁻¹ C=N band to 1586 cm⁻¹, and ¹H-NMR titrations revealed loss of the OH singlet and downfield imine CH, verifying O/N chelation. Job’s plot analysis indicated a 1:1 P8-Al³⁺ stoichiometry, and Benesi-Hildebrand fitting gave K_a = 0.477 × 10³ M⁻¹ (Figure 11).

Fluorescence titrations established a linear detection range of 50–500 μM and a low detection limit of 1.486 μM, which is well below the WHO guideline of 7.41 μM. Application to real water samples showed recoveries of 100.72–102.85% with RSD <2.82%, revealing P8’s potential for sensitive, selective, and practical environmental monitoring of Al³⁺.

2.3 Detection of Cu²⁺

Copper is the third most abundant transition metal in the human body and is vital for the development and function of multiple organs such as the heart, central nervous system, immune system, and liver, yet its dysregulation can lead to disorders like Menkes and Wilson’s diseases when deficient or overloaded [29]. Alongside its biological importance, copper has wide utilization in different industries like conducting wires, batteries, switches, transformers, and telecommunications equipment. Therefore, developing highly sensitive and selective chemosensors for trace Cu²⁺ detection is critical. Recent advances in fluorescent probes demonstrating “turn-on” mechanisms and detection limits in the micromolar to sub-micromolar range envisaged the idea of π-bridging between molecules to make their absorption and emission bands red-shifted, and then the color variation can be observed and determined via naked eyes [30, 31]. In view of this concept, Zhu and Fang et al. have disclosed [32] a report where they introduced the 4-aminobenzyl cyanide unit into the 3-position of carbazole to extend the π-conjugated bridge (9), the –NH₂ handle made Schiff base and water solubility boosted via cyano group. The N,N-diethyl-bearing salicylaldehyde (10) was then condensed to yield the Schiff-base probe 9 (P9), enabling metal coordination through the imine (‒C=N‒) nitrogen and phenolic oxygen. The whole design and synthesis are shown in Figure 12. The sensing ability of the final probe was investigated in CH₃CN-Tris (20 mM, v/v = 1:1, pH = 7.2) solution for a range of 13 metal ions (Na⁺, K₊, Ca²⁺, Mg²⁺, Ni²⁺, Fe³⁺, Co²⁺, Cu²⁺, Zn²⁺, Hg²⁺, Ag⁺, Cd²⁺, Pb²⁺, and Al³⁺) via absorption spectra. The UV-Vis spectrum of P9 displayed π–π* and ICT absorption bands at 288 and 430 nm, respectively. On Cu²⁺ addition, the main band blue-shifted to 418 nm with markedly decreased intensity and a color change from bright yellow to near-colorless, whereas other metal ions induced no or minor spectral shifts (Figure 13).

In fluorescence spectroscopy, a bright-yellow fluorescence (λ_ex = 420 nm, λ_em = 520 nm) was observed, which was selectively and dramatically quenched by Cu²⁺ ion, accompanied by a visible color change from yellow-green to colorless, while other metal ions induced negligible spectral changes. UV-Vis titrations and Benesi-Hildebrand analysis revealed an association constant K_a = 1.47 × 10⁵ M⁻¹ and a 1:1 P9-Cu²⁺ stoichiometry (confirmed by Job’s plot), with a fluorescence-based detection limit of 1.14 × 10⁻⁵ M; the rapid (≤30 s) and reversible (via EDTA) response. ¹H-NMR titrations reveal downfield shifts of imine and phenolic protons, pinpointing coordination through the nitrogen of ‒C=N and phenolic oxygen, while ESI-MS and DFT calculations supported the formation of a non-fluorescent ground-state complex that opens a photoinduced electron transfer (PET) quenching pathway. P9 was also deployed on test strips for naked-eye on-site Cu²⁺ detection in aqueous media and demonstrated cellular permeability via Cu²⁺-responsive “turn-off” imaging in HeLa cells, highlighting its dual colorimetric and fluorometric sensing capabilities for environmental and biological applications.

Gosh et al. [33] envisioned another idea to find selectivity for Cu²⁺ by the use of Low Molecular Weight Gelators (LMWGs) as sensory material. For that purpose, they reacted the naphthalimide aldehyde (11) with 4 under reflux in ethanol to give probe 10 (P10) an 85% yield. The P10 formed thermo-irreversible gels with minimum gelation concentration (5 mg mL⁻¹ in DMF/H₂O, DMSO/H₂O, and 1,4-dioxane/H₂O (1:1 v/v)). Gel-sol transition temperatures (T_9el) were 61, 60, and 56°C, respectively. Morphology and rheology revealed fibrous networks whose packing varied with solvent.

Among the different metal ions (Hg²⁺, Ag⁺, Fe²⁺, Cu²⁺, Fe³⁺, Pb²⁺, Cd²⁺, Al³⁺, Ca²⁺, Ni²⁺, and Co²⁺) tested, the gel underwent a gel-to-sol transition upon addition of ≥0.5 equiv. Cu²⁺ or Fe³⁺ in DMF/H₂O as solvent. Subsequent treatment with F⁻ distinguished Fe³⁺ (greenish-yellow) from Cu²⁺. The solution phase sensing showed a π–π* absorption at 455 nm and green emission at 520 nm; both were selectively quenched by Cu²⁺ and Fe³⁺ (other ions inert). Job’s plots indicated a 1:2 (M/P10) stoichiometry, and emission titrations yielded binding constants for Cu²⁺ and Fe³⁺ as K_a = (4.6–6.9) × 10³ M⁻¹, K_a = (5.3–5.8) × 10⁴ M⁻¹ respectively. Detection limits were 2.02 × 10⁻⁷ M (Cu²⁺) and 2.09 × 10⁻⁷ M (Fe³⁺). Single-crystal X-ray of the Cu²⁺-complex revealed a square-planar coordination via two imines (‒C=N‒) N and two phenolate O donors, as shown in Figure 14. DFT/TD-DFT reproduced the disappearance of the 455 nm band upon complexation and mapped HOMO-LUMO transitions consistent with ICT-driven quenching. The extended conjugation and polar imide groups of this design enhanced the fluorescence ability and water solubility, a requirement for practical and industrial applications.

The conjugation can be extended via polyaromatic compounds like naphthalene and incorporating heterocyclic compounds as condensing partners to increase the chances for colorimetric recognition and improved solubility. Zhu et al. [34] synthesized probe 11 (P11) by Suzuki coupling of 3-formyl-6-iodo-N-butylcarbazole (12) with 2-naphthylboronic acid (13) to give 14, which upon condensation with 2-hydrazinylbenzothiazole (15) in ethanol yielded P11 as shown in Figure 15. Structural characterization via¹H-NMR, FT-IR, and ESI-MS confirmed the imine (-CH=N-) linkage and naphthalene extension, enhancing π-conjugation. They tested the sensing ability of designed probe P11 for variety of alkali, alkaline-earth, transition, and heavy metal ions. The compound selectively detected Co²⁺ colorimetrically in DMSO/H₂O (1:1 v/v) via a visible color shift (colorless → pale yellow) with a new absorption band at 436 nm, attributed to LMCT. For Cu²⁺, it functioned as a fluorescent “turn-on” sensor in CH₃CN, showing a 10-fold emission enhancement at 418 nm with a blue shift, driven by suppression of PET and activation of CHEF. The chemical binding ratio of CNS to Co²⁺/Cu²⁺ was 1:1 according to Job’s plot, while the LOD for Co²⁺/Cu²⁺ was 4.31×10⁻⁷ and 1.98×10⁻⁶ molL⁻¹, respectively. ¹H-NMR and DFT studies identified binding sites at the imine nitrogen and thiazole ring nitrogen, with Cu²⁺ coordination stabilizing the complex via LMCT, as shown in Figure 16.

In another report by Mishra et al. [35] a turn off Cu²⁺ sensor based on bis-carbazole and bis-anthracene Schiff base was synthesized by reacting 9-hexylcarbazole-3-carbaldehyde (16) and ethane-1,2-diamine, leading to two imino C=N‒ functionalities in the probe as presented in Figure 17. Due to relevance, only carbazole-based probe (P12) is discussed here. The P12 was tested for nitrate salts of 12 different metals (Fe²⁺, Cr³⁺, Mn²⁺, Cu²⁺, Mg²⁺, Hg²⁺, Al³⁺, Zn²⁺, Cd²⁺, Ni²⁺, Co²⁺, Sn²⁺) and it demonstrated a selective “turn-off” fluorescence quenching for Cu²⁺ ions, attributing to coordination with the imine (C=N-) nitrogen as supported by DFT studies showing a reduced HOMO-LUMO gap (0.03 vs. 0.159 eV for free probe). Job’s plot confirmed a 1:1 binding stoichiometry between P12 and Cu²⁺. The sensor exhibited an LOD of 2.4 × 10⁻⁸ M and a high K_a value (5.1 × 10¹¹ M⁻¹), revealing its sensitivity and strong binding affinity. P12 also displayed an AIEE, with a 7.47-fold fluorescence increase in a 70% water-THF system. However, Cu²⁺ sensing by P12 was effective only in non-aggregated states, as aggregation hindered metal interaction. The authors tested the probe for practical applications using real water samples with 94–102% recovery and reversible sensing using EDTA. TLC-based visualization under UV light enabled on-site detection.

Copper detection has been widely explored using carbazole-based Schiff bases. Another article by Hu and Tao et al. [36] presented the design and synthesis of an AIE-active fluorescent probe (P13) utilizing benzothiazole-Schiff base framework integrated with a carbazole donor group for selective Cu²⁺ detection in aqueous media. P13 was synthesized via a multistep route involving a Schiff base reaction between carbazole-benzothiazole intermediate (17) and a salicylaldehyde-hydrazone (18), yielding a D-π-A structure in the P13 (Figure 18). Characterization by NMR and IR confirmed the structure. The probe demonstrated high selectivity for Cu²⁺ over 20 competing metal ions, with minimal Fe³⁺ interference, and functioned effectively across a broad pH range (4–11). The probe operated via an ESIPT mechanism, exhibiting strong fluorescence at 553 nm in aggregated states (THF/H₂O, 1:9 v/v).

Upon Cu²⁺ binding, chelation-enhanced quenching (CHEQ) occurred due to the paramagnetic nature of Cu²⁺, resulting in a “turn-off” response. Job’s plot analysis revealed a 1:2 binding stoichiometry (P13:Cu²⁺), supported by IR and ¹H-NMR studies showing coordination of one copper ion through deprotonated hydroxyl groups and imine nitrogen of hydrazine, while the other copper bound to oxygen and nitrogen of benzothiazole moiety, as shown in Figure 18. Fluorescence titration established a linear detection range of 0–45 μM, a low detection limit (LOD) of 0.2 μM (3σ/k method), and a binding constant (K_a) of 1.59 × 10⁴ M⁻¹ via the Benesi-Hildebrand equation. Real-sample testing in environmental water (Dongting Lake, tap water) showed recovery up to 99.78–101.28%. Practical applications were demonstrated using probe-loaded filter paper strips and silica gel, enabling rapid visual Cu²⁺ detection under UV light. The probe’s reversibility with EDTA and rapid response time (10 seconds) further underscore its potential for real-time environmental monitoring, meeting WHO guidelines for copper detection in drinking water.

In recent years, large biological molecules, specifically steroids, have attracted great attention in ion recognition, due to their amphiphilic and structurally rigid conformations. Carbazole and cholic acid naturally exhibit compatibility, having a rigid structure. Additionally, the hydroxyl groups present in cholic acid can facilitate subsequent derivatization, and sensors based on cholic acid could be used under physiological conditions due to biocompatibility. In such a report by Mishra et al. [37] an AIEE-active fluorescent probe, based on cholyl hydrazide carbazole-Schiff base (P14), derived from cholic acid and carbazole for selective detection of Cu²⁺ ions, was described. The probe P14 was synthesized by refluxing cholylhydrazide (19) (derived from ethyl cholate and hydrazine) with 16 in ethanol, yielding 70% product (Figure 19). Structural confirmation was achieved via FT-IR, ¹H/¹³C-NMR, and photophysical analyses (Figure 20).

P14 exhibited AIEE properties in a 70% water-THF mixture, showing a 5.93-fold fluorescence enhancement compared to pure THF, attributed to restricted intramolecular rotations in aggregated states. Dynamic light scattering (DLS) and FE-SEM revealed reduced particle size (139 nm) in aggregated states, correlating with emission enhancement. The probe demonstrated selective “turn-off” fluorescence quenching for Cu²⁺via PET by coordinating from the imine nitrogen and carbonyl oxygen with minimal interference from other cations/anions tested. Job’s plot confirmed a 1:1 binding stoichiometry, while Stern-Volmer analysis yielded a high binding constant (K_a = 5.3 × 10⁷ M⁻¹) and a low detection limit (1.59 × 10₋₇ M). DFT studies revealed a narrowed HOMO-LUMO gap (1.42 vs. 4.13 eV free probe) upon Cu²⁺ coordination, validating the quenching mechanism. The practical applicability was demonstrated in tap and river water samples, achieving recoveries of 92.2–107.7%. Additionally, P14 functioned as an IMP logic gate with Cu²⁺ and EDTA as inputs.

3.1 Detection of Hg²⁺

Mercury is a pervasive industrial contaminant used in sectors such as petroleum refining, chemical industry, pharmaceuticals, pulp papermaking, and electronics. It bioaccumulates in brain tissues, leading to neurological damage once critical concentrations are reached. Thiourea moiety is widely used in the field of molecular recognition, especially for heavy metal ions, because of its three coordinating sites, two nitrogen atoms, and one sulfur atom (H₂NCSNH₂). The sulfur group has a special affinity for mercury, as the name mercapto for -SH group comes from the Latin words “mercurium captans,” meaning capturing mercury. Combining carbazole with thiourea could elevate its ability to capture Hg metal ions. Li et al. [38] proposed a carbazole-Schiff base (P15) synthesized from 20 and 4-phenylthiosemicarbazide (21) in ethanol, as shown in Figure 21. The pale-yellow compound obtained was tested by UV-Vis absorption spectroscopy for a variety of metal ions (Na⁺, Ag⁺, Ca2⁺, Mg²⁺, Zn²⁺, Cu²⁺, Fe²⁺, Fe³⁺, K⁺, Cd²⁺, Cr³⁺, Co²⁺, Mn²⁺, Pb²⁺, and Ni²⁺), in DMF. The compound showed two absorption bands in the UV spectrum at 290 and 359 nm. P15 specifically showed a hyperchromic shift for band at 290 nm and a large red shift or bathochromic shift from 359 to 385 nm for Hg²⁺ ion, while the color of the solution turned yellow from colorless with no obvious response for other metals, suggesting it is a colorimetric chemosensor for Hg²⁺. Further fluorescence experiments suggested that the Hg²⁺/P15 ratio was 1:2 during binding. The fluorescence emission from 480 to 490 nm showed a red shift only for Hg²⁺ ion, while for other metal ions, no significant change was observed. Further experiments exhibited an association constant K_a = 2.8 × 10^4, and a detection limit of 6.5 × 10⁻⁶ mol/L as determined by Benesi-Hildebrand, Stern-Volmer, and Job’s plot analyses.

Replacing amino groups with hydrazine and including heterocyclic aromatics not only increases the power of the coordination site but also the extension of π-conjugation. The article by Sekar et al. [39] presented the synthesis and application of a carbazole-hydrazinobenzothiazole-based fluorescent sensor (P16) for selective Hg²⁺ detection. P16 was synthesized via condensation of 9-ethyl-9H-carbazolyl-3-carbaldehyde (22) and 15 in methanol (Figure 22), yielding a 95% product characterized by NMR, IR, and mass spectrometry.

The sensor exhibited a turn-on fluorescence response at 433 nm upon Hg²⁺ binding, driven by an ICT mechanism from the carbazole donor to the benzothiazole acceptor. UV-Vis and fluorescence titration confirmed selectivity for Hg²⁺ over 16 competing metal ions, with a 1:1 binding stoichiometry (Job’s plot) and a binding constant K_a = 1.88 × 105 M⁻¹via the Benesi-Hildebrand equation. The coordination involved the imine nitrogen (‒C=N‒) and benzothiazole sulfur, validated by NMR peak shifts (NH proton disappearance at δ 12.17 ppm upon complexation) and IR spectral changes (shifting of C=N and C‒S stretches). The sensor demonstrated a low LOD of 1.41 × 10⁻⁷ M fluorometrically and 9.2 × 10⁻⁸ M electrochemically, in DMSO/H₂O (1:9) at pH 6–10. Protein binding studies with bovine serum albumin (BSA) revealed strong interaction (K_a = 3.14 × 10⁶ M⁻¹), inducing conformational changes in tryptophan/tyrosine residues, as shown by 3D fluorescence. Cyclic voltammetry highlighted redox potential shifts for Hg²⁺ detection, emphasizing dual optical-electrochemical utility. The sensor’s stability and applicability in biological and environmental monitoring showed its potential for Hg²⁺ sensing in complex matrices (Figure 23).

The thiol (-SH) group can be replaced by pyridine/quinoline nitrogen in the vicinity of methylenehydrazine to serve as a coordinating site for Hg²⁺ ions. The advantage of introducing quinoline moiety is to enhance the fluorescence capacity and quantum yield of designed probe. To this approach, Singh et al. [40] have reported a carbazole-quinoline tagged fluorophore P17, which was synthesized by condensing 22 and 2-hydrazinoquinoline (23) in methanol with a catalytic amount of glacial acetic acid under reflux for 1 h, followed by slow evaporation to yield yellow crystals (Figure 24).

Single-crystal XRD analysis revealed that P17 crystallized in a tetragonal I41/a space group, having an almost planar molecular conformation stabilized by strong intramolecular hydrogen bonding (2.63 Å) and intermolecular N–H···N networks. In CH₃CN/H₂O (1:1, v/v), it exhibited π–π* absorption bands at 330 and 370 nm that diminish markedly upon the addition of Hg²⁺, accompanied by a color change from pale yellow to colorless in comparison with other 15 metals tested, including Pb and Bi. Its bright blue fluorescence emission at 485 nm underwent complete quenching, and a blue shift to 475 nm upon Hg²⁺ coordination was observed, while other competing metal ions elicit a negligible response.

Fluorescence titration and Job’s plot analyses confirmed a 1:1 P17-Hg²⁺ stoichiometry with K_a = 1.01 × 10⁵ M⁻¹ value. The LOD was 2.59 × 10⁻⁸ M. Mechanistic investigations combining FT-IR, ESI-MS, ¹H-NMR titration, and DFT calculations revealed bidentate coordination through azomethine-N and quinoline-N with concomitant –NH deprotonation, leading to a photo-induced electron transfer (PET) pathway (Figure 24). The authors tested probe P17 for practical applicability by using P17-coated paper strips for on-site Hg²⁺ detection and successful analysis of lake and Ganga river water samples with recoveries of 94–99%.

3.2 Detection of Pb²⁺

Recently, Schiff base formation has also become a foundation reaction for making COFs. Carbazole remains at the core of COFs materials, where the Schiff base reaction is being used to construct these 2-D or 3-D materials. The excellent crystallinity, porosity, and tunable pore size of COFs are among the ideal properties for metal ions detection and adsorption.

A carbazole-grafted covalent organic framework (COF-CB) was synthesized by Wang et al. [41] initially by condensation of 1,3,5-tris(4-aminophenyl)benzene and 2,5-dihydroxy-1,4-benzenedicarboxaldehyde to form COF-DhaTab, followed by post-grafting carbazole units onto the framework via Williamson ether reaction as shown in Figure 25. The resulting COF-CB exhibited an eclipsed AA-stacking structure with microporosity (3 nm pore size) and retained crystallinity after functionalization. COF-CB demonstrated a selective “turn-on” fluorescence response to Pb²⁺ ions via ICT upon coordination of Pb²⁺ with nitrogen (imine groups) and oxygen (phenolic hydroxyl) sites within the framework. This coordination suppressed non-radiative decay, enhancing fluorescence emission at 540 nm. The sensor showed a linear detection range of 0–80 μM Pb²⁺ with a low detection limit (LOD) of 1.48 μM (calculated via 3σ/s method) and excellent anti-interference against 15 competing metal ions (Zn²⁺, Cr³⁺, Fe²⁺, Fe³⁺, Hg²⁺, Al³⁺, Cu²⁺, Ni²⁺, Mg²⁺, Ca²⁺, Mn²⁺, Ba²⁺, Co²⁺, Na⁺, and K)⁺. Binding stoichiometry was inferred as 1:1 (Pb²⁺:COF-CB) based on Job’s plot analysis of similar systems, though explicit stoichiometric data were not provided. Reversibility was confirmed using EDTA, which quenched fluorescence by displacing Pb²⁺. The COF-CB’s selectivity stemmed from its tailored pore structure and coordination sites, enabling Pb²⁺ detection in real water samples (tap/drinking water) with 95–98% recovery. While the association constant (K_a) was not explicitly reported, the high sensitivity and selectivity underscored strong binding affinity. This work highlights COF-CB as a robust, reusable chemosensor for environmental Pb²⁺ monitoring, leveraging its structural stability and fluorescence enhancement mechanism.

The above one is just an example of carbazole-based Schiff base COFs materials’ potential toward metal ion detection; there are many exciting carbazole-based Schiff base COFs, for the selective and sensitive detection of metal ions. Since this topic deviates from the title of chapter so COFs are not discussed here.