A Brief Review of Schiff Bases of Pyridine Derivatives as Chemosensors

Citation

Khairnar, D., & Patil, V. (2026). A Brief Review of Schiff Bases of Pyridine Derivatives as Chemosensors. International Journal of Research, 13(13), 74–91. https://doi.org/10.26643/ijr/2026/s13/8

A Brief Review of Schiff Bases of Pyridine Derivatives as Chemosensors

Dinesh Khairnar^{1, 2,*}, Dr. Vikas Patil^1,*

¹University Institute of Chemical Technology, Kavyitri Bahinabai Chaudhari North Maharashtra University, Jalgaon, M.S., India

²Department of Chemistry, VVM’s S. G. Patil Arts, Science and Commerce College, Sakri, Dhule, M.S., India

¹viaksudct@gmail.com

Abstract

There is a growing need to accurately detect pollutants like toxins and metal ions, especially in health and environmental fields. Current detection methods, such as flame atomic absorption spectroscopy and inductively coupled plasma mass spectrometry, are effective but often expensive, time-consuming, and not very sensitive.To address these issues, researchers are exploring optical chemosensors, particularly those based on Schiff bases, for detecting metal ions. Schiff bases are useful in chemistry, especially for binding and detecting metal ions. Schiff base complexes with transition metals exhibit properties like catalytic activity, fluorescence, and magnetic features.Pyridine-based Schiff bases, formed from pyridine derivatives, are especially notable for their strong binding abilities and bioactivity. These Schiff bases are valuable in medicinal and analytical chemistry due to their ability to selectively detect metal ions. This review focuses on the development of fluorescence probes using pyridine-based Schiff bases over the last decade, highlighting their usefulness in detecting specific ions across environmental, biological, and industrial fields.

Keywords: Schiff Base, Pyridine, Chemosensors, Detection.

1. Introduction

The demand for precise and highly sensitive identification of pollutant species such as toxins and metal ions is on the rise, particularly in fields related to health and the environment. Industrial and agricultural activities have led to an increase in the release of cations and anionic pollutants, posing significant threats to human health and ecological balance. Presently, various methods including flame atomic absorption spectroscopy, inductively coupled plasma optical emission spectroscopy, stripping voltammetry, X-ray fluorescence spectrometry, and inductively coupled plasma mass spectrometry are utilized for metal ion detection.^1,2 However, many of these techniques are expensive, time-consuming (especially during sample preparation), and exhibit limited sensitivity.In response to these challenges, researchers have explored different optical chemosensors for the detection of metal ions, aiming to overcome the drawbacks associated with conventional methods.³

Among these alternatives, Schiff base-based structures have shown remarkable potential for metal ion determination. Schiff base ligands have attracted considerable attention from researchers due to their facile synthesis and their ability to form complexes with a wide range of metals.⁴ Schiff bases represent a category of organic compounds distinguished by the presence of an imine (-C=N-) functional group formed through the reaction between an amine amino group and either an aldehyde or a ketone carbonyl group.⁵ This imine functionality bestows Schiff bases with significant chemical and biological characteristics, facilitating their wide-ranging applications across various branches of chemistry, particularly in coordinating and complexing with metal ions. By virtue of the imine group, Schiff base compounds can serve as ligands, forming complexes with transition metals known as transition metal Schiff base complexes.^6,7 These complexes often possess desirable traits such as catalytic activity, fluorescence, and magnetic properties.^8,9

Pyridine derivatives containing formyl or amino groups readily undergo Schiff base condensation reactions with suitable substrates under optimal conditions. Schiff bases derived from pyridine are considered superior ligands compared to pyridine itself due to their stronger binding capabilities, structural flexibility, and enhanced bioactivity. These Schiff bases, originating from pyridine derivatives, hold significant interest in medicinal chemistry for their role as bioactive ligands, demonstrating physiological effects akin to pyridoxal-amino acid systems crucial in numerous metabolic reactions. Moreover, pyridine-based Schiff bases play a vital role in analytical chemistry. Given their robust binding affinities toward various cations and anions, along with their structural adaptability and distinctive photophysical properties, they find utility in ion recognition. Consequently, they are extensively employed in the development of diverse chemosensors tailored for the selective detection of specific ions across environmental, biological, industrial, and agricultural domains.This review outlines the straightforward fluorescence probes based on Pyridine-based Schiff bases created over the past decade.¹

• Fluorescent probes based on Pyridine-based Schiff bases for diverse metal ions, anions, and small chemical entities are discuss below.

Bawa et al.synthesized a new pyridine-dicarboxylate based hydrazone Schiff base probe 1 (referred to as DAS) and demonstrated to function as a colorimetric chemosensor for detecting Ni²⁺ ions. It exhibited rapid and specific detection of Ni²⁺ ions even in the presence of other coexisting metal ions in MeOH/PBS (5/1, v/v) solution at pH 7.4. The formation of a 2:1 complex between Ni²⁺ions and DAS, with a binding constant (Ka) of 3.07×10³M⁻², was confirmed through Job’s plot and Benesi–Hildebrand plot analysis. Additionally, single crystal X-ray diffraction further supported the formation of the 2:1 complex between DAS and Ni²⁺. The detection limit of DAS for Ni²⁺ions was determined to be 0.14×10^-6M. Furthermore, the DAS- Ni²⁺ ensemble exhibited selective detection of pyrophosphate with a binding constant of 8.86 × 10³ M⁻¹ and a detection limit of 0.33×10^-6M.¹¹

Fig. 1: Structure of Probe 1

Mohanasundaram et al.tested how well the Schiff base receptor probe 2 (at a concentration of 5×10^-5 M) can detect different metal ions in a mixture of CH₃CN/H₂O (7:3, v/v). When Cu²⁺ions were present, the color of the receptor solution changed from colorless to yellow and cyan under visible and UV light. This receptor probe 2 also offers an easy way to detect copper using just the naked eye, without needing any special equipment. UV–visible and fluorescence spectra show that the receptor probe 2 binds to copper in a 1:1 ratio, with a binding strength of 7.59×10⁴ M^-1. The lowest amount of copper that the receptor can detect is as small as 0.25×10^-6M, and it doesn’t get interfere by other metal ions.¹²

Fig. 2: Structure of Probe 2

Yan et al.synthesised the fluorescence probe 3 that can detect Ce³⁺ and F⁻ in a recyclable manner, switching “ON-OFF-ON” in phosphate buffered saline (PBS) buffer having concentration 10×10⁻³ M and pH 7.4. The detection limits for Ce³⁺ and F⁻ were found to be 4.48×10⁻⁶ M and 11.58×10⁻⁶M, respectively, within concentration ranges of 0-50 μM and 0-150 μM. UV–visible and fluorescence spectra show that the receptor probe 3 binds to Ce³⁺ in a 1:1 ratio, with a binding strength of 1.78×10⁴ M^-1. Using DFT, the spatial structure, electron density distributions, binding mode, and sensing mechanism of probe 3 with Ce3+ were investigated. Probe 3 was tested for real-time qualitative detection of Ce³⁺ and F⁻ in actual water samples and Poly vinylidene fluoride (PVDF) membrane. This probe 3is highly soluble in water, biocompatible, and suitable for bioimaging in Vascular Mesenchymal Stem Cells (VSMCs).¹³

Fig. 3: Structure of Probe 3

Xu et al.developed Schiff base chemosensors derived from 2,2’:6’,2”-terpyridines, named 2,2’:6’,2”-terpyridine salicylidene Schiff bases (TPySSB) and 2,2’:6’,2”-terpyridine Schiff bases (TPySB) probe 4, were investigated for their ability to selectively detect Al³⁺ ions in ethanol(1×10⁻⁵ M). The sensing capabilities of TPySSB and TPySB were examined using UV-Vis, fluorescence, FTIR, and 1H NMR experiments. Upon the introduction of metal ions, TPySSB exhibited significant fluorescence enhancement specifically for Al³⁺ ions. Furthermore, it demonstrated exceptional selectivity towards Al³⁺ ions with a 1:2 binding mode as indicated by Job’s plot analysis and confirmed through ¹H NMR analysis. The binding constant of TPySSB with Al³⁺ ions is 6.8×10⁵ M⁻¹.These findings suggest that the combination of the 2,2’:6’,2”-terpyridine unit and salicylidene unit holds promise for the development of highly selective chemosensors.¹⁴

Fig. 4: Structure of Probe 4

Hossain et al.synthesized a novel fluorescent chemosensor probe 5 that was extensively studied for its ability to detect Cu²⁺ ions. This chemosensor demonstrated efficient functioning in aqueous solution of H₂O/MeCN (8/2, v/v) at neutral pH levels, and its low toxicity was confirmed by a high IC50 value of approximately 35 mM. Furthermore, it demonstrated exceptional selectivity towards Cu²⁺ ions with a 1:1 binding mode as indicated by Job’s plot analysis and confirmed through 1H NMR analysis. The detection limit of probe 5 for Cu²⁺ ions was found to be 0.66×10⁻⁶ M. Encouraged by these findings, researchers conducted further experiments using confocal fluorescence microscopy for bioimaging, which produced a green fluorescent image in Vero cell line tests. Detailed analysis of the X-ray structure of the hexanuclear Cu²⁺: probe 5 complex, known as metal–organic macrocycle, provided valuable information about the sensor’s precise mechanism of interacting with the metal ion.¹⁵

Fig. 5: Structure of Probe 5

Sahu et. al.synthesised a chemosensor probe 6, which is based on thiosemicarbazide and can detect Cu²⁺ ions through a color change and Ag⁺ ions through both color change and fluorescence in MeOH/H₂O solvent mixture (1:1 v/v). The sensor is highly efficient at identifying these ions even when they are mixed with other ions in water. Studies have shown that probe 6 binds with Cu²⁺ ions in a 2:1 ratio and with Ag⁺ ions in a 1:2 ratio, which was confirmed through tests like absorption titration and mass spectrometry. The sensor is highly sensitive, capable of detecting concentrations as low as 1.7×10⁻⁶M for Cu²⁺ ions and 2.2×10⁻⁶M for Ag⁺ ions through color change and 1.6×10⁻⁶M for Ag⁺ ions through fluorescence. It functions effectively in wide range of pH levels and can be used to test water samples for Cu²⁺and Ag⁺ ions in the environment. Based on these findings, probe6 could be a significant step in the development of a single sensor that can detect multiple substances.¹⁶

Fig. 6: Structure of Probe 6

Mukherjee et. al.synthesised pyridine based novel luminescent compoundprobe 7and studied as a sensor that detect both Cr³⁺and Al³⁺ in DMSO solvent. The metal salts were prepared in DMSO/H₂O mixture (2:1). The binding stoichiometry of probe 7 with both Al³⁺ and Cr³⁺ ions is in 2:1 ratio which is determined by Job’s plot. The values of limit of detection and association constant for both Cr³⁺ and Al³⁺ are in range of 10⁻¹¹ M and 10⁵ M⁻¹ respectively.  They also used a technique called first derivative synchronous fluorescence spectroscopy to measure the amounts of Al³⁺ and Cr³⁺ in a mixture without having to separate them first, which turned out to be more effective than traditional methods like liquid-liquid extraction.¹⁷

Fig. 7: Structure of Probe 7

Singh et. al.synthesised two receptors, R₁ and R₂ which are denoted as probe 8. The ability to detect anions was investigated using various methods including visual observation, UV-vis spectroscopy, 1H-NMR titration, and electrochemical and computational analyses. R₁ was found to be highly selective for fluoride ions (F^–), while R₂ could effectively distinguish between fluoride and acetate ions (AcO^–) by changing color from pale yellow to aqua and green in the presence of different competitive anions in DMSO. UV-vis titration studies revealed strong binding of fluoride ions with receptors R₁ and R₂, with binding constants of 2.3×10⁴ M⁻² and 8.57×10⁴ M⁻², respectively. Additionally, 1H-NMR titration and mass spectral data indicated a 1:2 binding ratio between receptors R₁ and R₂ and fluoride ions, confirming the involvement of a deprotonation process in the binding mechanism. Both receptors bound to carbonate ions () in a 2:1 stoichiometric ratio, leading to a rapid color change from yellow to aqua, with significant shifts in absorbance spectra. The receptors R₁ and R₂ offered several advantages for carbonate ion detection, including simple synthesis via Schiff base condensation reaction, high selectivity over other competing ions in aqueous DMSO:H₂O (9:1 v/v), and practical application using test strips. Therefore, receptors R₁ and R₂ served as simple and cost-effective chemosensors for detecting carbonate ions in aqueous DMSO: H₂O (9:1 v/v). Additionally, density functional theory (DFT) and time-dependent DFT (TD-DFT) studies supported the experimental data and proposed sensing mechanism.¹⁸

Fig. 8: Structure of Probe 8

Peng et. al.synthesized two pyridine-based Schiff-bases, HL₁ and HL₂ which are refered as probe 9, which act as sensors for detecting aluminum ions (Al³⁺) using a mechanism involving photoinduced electron transfer (PET) and excited-state intramolecular proton transfer (ESIPT). Both HL₁ and HL₂ quickly emitted fluorescence when exposed to Al³⁺ions in a solution of DMF/H₂O (1/9 v/v). The binding stoichiometry of both HL₁ and HL₂ with Al³⁺ ions was found to be 1:1. The binding constant of HL₁ and HL₂ with Al³⁺ ion was 3.38×10³ and 2.07×10³ respectively which determined from Bensi-Hildebrand equation. The detection limit of HL₁ and HL₂ for Al³⁺ ion was 3.2×10⁻⁹ M and 2.9×10⁻⁸ M respectively. These results showed good selectivity and sensitivity to Al³⁺, changing color from clear to aquamarine even in the presence of other metal ions. Furthermore, HL₁ and HL₂ effectively detected Al³⁺ ions in real water samples.¹⁹

Fig. 9: Structure of Probe 9 HL₁ and HL₂

Kumar et. al.synthesised pyridine dicarbohydrazide based chemosensor that can detect both positive and negative ions by changing color. In tests with positive ions, probe 10 senses specifically to Cu²⁺, showing a strong color change. It was very sensitive to Cu²⁺, detecting concentrations as low as 0.12×10⁻⁶M. When tested with negative ions, probe 10 could also bind to Adenosine monophosphate ion (AMP²⁻), F⁻, and AcO⁻. However, it showed the strongest binding to AMP²⁻among all other negative ions, with a binding strength measured by the association constant (K_a) value of 1.47×10⁵ M⁻¹ and a detection limit of 0.08×10⁻⁶ M. Using computer simulations, they found that Cu²⁺ bind to specific sites of probe 10, while negative ions like F⁻, and AcO⁻ bind to different sites through hydrogen bonding. Probe 10 was able to distinguish between AMP²⁻, ADP²⁻, and ATP²⁻ by color changes. They also tested the practical use of probe 10 by detecting fluoride ions in commercially available toothpaste. Probe 10 has the potential to be used to detect both the metal ion Cu²⁺ and important biological ions like AMP²⁻, F⁻, and AcO⁻.²⁰

Fig. 10: Structure of Probe 10

Wang et. al. synthesized Schiff base chemosensors which can act as a fluorescent switch for Zn²⁺ion and a color-changing indicator for Cu²⁺ ion at the same time. They found that probe 11 could detect concentrations as low as 0.35×10⁻⁶ M and 0.18×10⁻⁶M for Cu²⁺ and Zn²⁺ respectively. Probe 11forms stable complex with Zn²⁺ and Cu²⁺. Probe 11 forms a 1:1 binding stoichiometry when it complexes with Cu²⁺/Zn²⁺. The association constants (K_a) for Cu²⁺ and Zn²⁺ were approximately 9.67×10⁴ M⁻¹ and 1.25×10⁴ M⁻¹ respectively. This indicated that probe 11 has a higher coordination affinity for Cu²⁺ than for Zn²⁺. The Cu²⁺ complex that was formed subsequently functioned as a colorimetric sensor for PPi by disrupting the 1+Cu²⁺ complex.In addition, the utilization of fluorescent probe 11 for biological imaging was exhibited.²¹

Fig. 11: Structure of Probe 11

Gao et. al. synthesised Schiff base photochromic fluorescent probe 12 for Cu²⁺ion based on the diarylethene combined with a benzo[1,2,5]oxadiazol-4-ylamine. The probe 12 has been thoroughly examined for its photochromic and fluorescent behaviors through the use of light, acid, base and metal ion solution in acetonitrile.The fluorescence changed from dark red to bright red when Cu²⁺ ions were added. The intensity of light released increased by 90 times, and the emission wavelength was shifted 56 nm toward the blue end of the spectrum. This indicates that probe 12 acts as good chemosensors for Cu²⁺ even in low concentration solution. The complex ratio between probe 12 and Cu²⁺ was 1:2 in acetonitrile. For Cu²⁺, the association constant (K_a) was determined to be 4×10⁴ M⁻¹. The detection limit of probe 12 for Cu²⁺ was determined as 1.49×10⁻⁶M. Moreover, probe 12 exhibited varying responses to light, acidity or alkalinity, and Cu²⁺ ions, enabling the design and construction of two logic circuits.²²

Fig. 12: Structure of Probe 12

Maity et. al.synthesized a new fluorescent sensor probe 13 based on 2H-pyrrolo[3,4-c]pyridine-1,3,6(5H)-trione. Compared to other common ions, probe 13 is exceptionally good at detecting iron ions (Fe³⁺/Fe²⁺) in DMSO/H₂O (1:9, v/v) solution. The Job’s plot, ESI-mass spectroscopy, and the Benesi Hildebrand equation demonstrated that probe 13 forms a 1:1 complex with the iron metal ion.The probe 13 has binding constant in the range of 10⁵ M⁻¹ and detection limit in the range 10⁻⁷ M. They have used EDTA as a coordinating agent to release the ligand from its complex form, which then binds with the metal ion in a 1:1 ratio. Additionally, they conducted experiments with fluorescent cell imaging and found that this sensor is biocompatible and has low toxicity, making it suitable for detecting Fe³⁺ ions in biological samples.²³

Fig. 13: Structure of Probe 13

Yu et. al.have synthesised fluorescent chemosensor probe 14 to detect biological thiols. The probe 14 is capable of fast, sensitive, and selective ratiometric fluorescence detection for GSH. Its copper complex can identify Cys in a mildly acidic PBS buffer solution (pH 7.4) for a range of analytes, including homocysteine (Hcy) and glutathione (GSH). It is also possible to effectively use probe 14 and its copper complex (probe 14:Cu²⁺) for GSH and Cys fluorescence imaging in HeLa cells, respectively.For Cys, the probe 14:Cu²⁺ complex detection limits in PBS buffer solution are 0.3×10⁻⁶ M for absorbance and 6.4×10⁻⁴ µM for fluorescence, respectively.²⁴

Fig. 14: Structure of Probe 14

Wang et. al. have synthesised fluorescent sensors probe 15 using 5,5’-methylenebis(salicylaldehyde) for detecting Al³⁺ ionswith high selectivity and sensitivity. In a H₂O/DMSO (19:1, v/v) solution, the sensitivity of the Al³⁺and probe 15 complexes at various pH values were investigated. The stoichiometry of Al³⁺ and probe 15 complex is 1:1, as established by Job’s plot analysis, LC–MS data, and 1H NMR study.The binding constants of sensors R₁ and R₂ were determined as 2.01×10⁴ and 5.46×10⁵ respectively from Benesi-Hildebrand equation. The detection limits for Al³⁺ are as low as 10⁻⁸M, significantly below the World Health Organization’s guideline for drinking water (7.4×10⁻⁶M).²⁵

Fig. 15: Structure of Probe 15

Ghorai et. al. developed and synthesized a fluorescent colorimetric chemosensor probe 16, capable of detectingAl³⁺ ions in MeOH:H₂O solution with a ratio of 2:1 (v/v).The Job plot analysis indicated that the probe 16 and Al³⁺ formed 1:2 stoichiometric complexes.By using a Hill plot, the association constant was found to be 1.26×10⁵ M⁻¹ based on the fluorescence titration profiles.Probe 16 demonstrates outstanding selectivity and sensitivity towards Al³⁺, manifesting as enhanced fluorescent intensity and a rapid color change from yellow to colorless in the presence of HSO₃⁻. The detection limit of probe 16 for Al³⁺ was 0.903×10⁻⁶ M significantly surpasses the WHO guidelines (7.41×10⁻⁶ M). Furthermore, probe 16 operates effectively across a wide pH range and can be successfully utilized in biological samples for Al³⁺ detection and bisulfite measurement in food samples. ²⁶

Fig. 16: Structure of Probe 16

Annaraj et. al.development and synthesised water-soluble pyridine-based chemosensor probe 17 designed for the visual detection of Ag⁺ions in a fully aqueous environment (pH 7.3). This sensor exhibits selectivity for detecting Ag+ ions in aqueous solutions containing various metal ions. The detection limit for Ag⁺ ions is remarkably low at 4.18×10⁻⁶M in aqueous solution, without any interference from other metal ions. A 1:1 complex was formed between probe 17 and Ag⁺ ions, according to the results of the ESI-MS spectra and the Job plot analysis. Using the Benesi–Hildebrand plot, the binding constant of probe 17 with Ag⁺ was determined to be 4.95×10⁴ M⁻¹. The predicted binding mode between probe 17 and the Ag⁺ ion, as well as the probe 17 fluorescence behaviours with Ag⁺ ion, are validated by computational calculations.²⁷

Fig. 17: Structure of Probe 17

Tayade et. al., synthesized fluorescent receptor probe 18, which exhibits selectivity and sensitivity towards the Pb²⁺ ion in DMF/H₂O (9:1, v/v) medium. The presence of Pb²⁺ ions leads to a distinctive enhancement in fluorescence and induces a color change easily observable by the naked eye under UV light. Moreover, interference from other ions in the detection of Pb²⁺ with probe 18 was found to be negligible. From Job’s plot, the stoichiometry between probe 18 and the Pb²⁺ ion is found to be 1:1. The association constant (K_a) values obtained from fluorescence and UV titration data using the Benesi-Hildebrand plot were found to be in agreement, with K_a values of 5.142×10³ M⁻¹ and 5.213×10³ M⁻¹, respectively.²⁸

Fig. 18: Structure of Probe 18

Zhang et. al., synthesised a highly efficient “off-on” chemosensor probe 19, for detecting Cu²⁺. This investigation demonstrated that probe 19 exhibits exceptional selectivity and sensitivity towards Cu²⁺ in EtOH/H₂O (3:2, v/v) solution having pH 7.4 and offering significant promise for environmental sensing applications. From Job’s plot, the stoichiometry between probe 19 and the Cu²⁺ ion is found to be 1:1. For Cu²⁺, the association constant (K_a) was determined to be 6.2×10⁵ M⁻¹. These findings present novel opportunities for developing similar “off-on” probes targeting other metal ions.²⁹

Fig. 19: Structure of Probe 19

Conclusion

Pyridine-derived Schiff bases have emerged as a powerful class of fluorescent chemosensors owing to their structural simplicity, strong coordination ability, and tunable photophysical behavior. The cooperative interaction between the pyridine nitrogen and the azomethine (–C=N–) unit provides an efficient binding framework, while strategic substitution enables precise modulation of fluorescence responses. As discussed in this review, these systems operate through diverse mechanisms including photoinduced electron transfer (PET), intramolecular charge transfer (ICT), excited-state intramolecular proton transfer (ESIPT), chelation-enhanced fluorescence (CHEF), chelation-enhanced quenching (CHEQ), and aggregation-induced emission (AIE). Such versatility has enabled the sensitive and selective detection of environmentally and biologically relevant metal ions, often with detection limits in the micromolar to nanomolar range.Despite significant progress, several challenges limit broader applicability. Many reported fluorescent probes exhibit reduced performance in aqueous media, interference from competing ions, or insufficient validation in real samples and live-cell systems. Additionally, quantitative structure–fluorescence relationships remain underexplored, and mechanistic interpretations are sometimes inferred without comprehensive spectroscopic or theoretical support.

Future developments should prioritize the design of water-compatible, ratiometric, and near-infrared (NIR) emissive probes to enable real-time imaging and in vivo applications. Integration into solid-state platforms, test strips, and portable fluorescence devices will further enhance practical utility. Coupling rational molecular engineering with computational modeling and time-resolved spectroscopic studies is expected to accelerate the development of highly efficient and application-oriented fluorescent sensors. Overall, pyridine-based Schiff base frameworks continue to offer a promising and adaptable foundation for next-generation fluorescence sensing technologies.

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