Medicinal importance and chemosensing applications of Schiff base derivatives for the detection of metal ions: A review

Abstract

Schiff bases, named after Hugo Schiff, are formed when primary amine reacts with carbonyl compounds (aldehyde or ketone) under specific conditions. Schiff bases are economical, simple synthetic routes, and easily accessible in laboratories. They have medicinal and biological applications such as antiviral, antioxidant, antifungal, anticancer, anthelmintic, antibacterial, antimalarial, anti-inflammatory, antiglycation, anti-ulcerogenic, and analgesic potentials. A number of Schiff bases are reported for the detection of various metal ions. They are also used as catalysts, polymer stabilizers, intermediates in organic synthesis, and corrosion inhibitors. In this review, we have highlighted the recent advancements in the development of bioactive Schiff base derivatives and their sensing applications for detecting metal cations. Additionally, various spectroscopic techniques for structural characterization, such as X-ray diffraction analysis (XRD), FT-IR, UV-vis, and NMR spectroscopy were also discussed.

Keywords

Schiff base anticancer biological applications antiglycation antioxidant

List of Abbreviation

FIPV

Punta feline coronavirus

CHEF

Intramolecular charge transfer

GTP

Glutathione peroxidase

SOD

Superoxide dismutase

AChE

Acetylcholinesterase

hCA

Human carbonic anhydrase

PFQ

Paramagnetic fluorescence quenching

ICT

Excimer formation

PET

Chelation enhanced fluorescence quenching

CHEQ

Chelation enhanced fluorescence

HepG2

Human hepatoma G2

Infrared spectroscopy

NMR

Nuclear magnetic resonance spectroscopy

DFT

Density-functional theory

E. coli

Escherichia Coli

HeLa

Human cervical cancer

MLCT

Metal-ligand charge transfer photo induced electron transfer

XRD

X-ray crystallography

1 Introduction

Schiff bases are compounds with the general formula R₁R₂C = NR₃ (Scheme 1). They have outstanding biological applications in several fields such as antiviral, antioxidant, antifungal, anticancer, anthelmintic, antibacterial, antimalarial, anti-inflammatory, antiglycation, anti-ulcerogenic, and analgesic properties [1, 2]. Several Schiff base derivatives shows high efficiency in the vision process [3]. They are also significant in molecular recognition, particularly in the particularly in developing of chemosensors [4 –6]. They also form stable complexes and play an excellent role in biological and medicinal applications [7]. Schiff bases possess catalytic efficiency in different reactions, including Diels–Alder reactions, Henery reactions, ring-opening polymerization, alkene epoxidation, hydroxylation of styrene, aldol reactions, cyclopropanation, desymmetrization and copper-catalyzed azide-alkyne cycloaddition reactions [8 –10]. In this review, we have highlighted the recent advancements in the development of bioactive Schiff base derivatives and their sensing applications for the detection of metal cations, anions, and neutral analytes. Additionally, various spectroscopic techniques for structural characterization, such as X-ray diffraction analysis (XRD), FT-IR, UV-vis, and NMR spectroscopy were also discussed. We hope this article will help researchers to design highly potent Schiff base compounds.

Scheme 1

General synthesis of Schiff base from primary amine and aldehydes/ketones.

2 Characterization of Schiff bases

2.1 Ultraviolet-Visible spectroscopy

Schiff base derivatives show different characteristic bands in the UV-Vis region. The absorption wavelength depends upon the basic structure of the compound. Generally, the band at 300–400 nm in the UV-Vis spectrum indicates the excitation of electrons of the azomethine group (C = N) due to π-π* transition [11 –14].

2.2 FT-IR spectroscopy

FT-IR is used to determine functional groups and bonding in compounds. This technique also gives valuable information about the geometry and secondary interactions among molecules [15]. FT-IR peaks for C = N are typically observed in the range of 1500–1660cm^–1 [16 –26].

2.3 NMR spectroscopy

NMR is one of the most essential and reliable characterization techniques used for the structural elucidation of compounds. NMR deals with magnetically active nuclei of certain atoms, particularly nuclei with nuclear spin 1/2. The NMR technique can obtain detailed information about the reaction state, structure and chemical environment of atoms/nuclei in molecules. Generally, ¹ H NMR of Schiff bases revealed a signal at 7.6–10.24 ppm, which is due to the (CH = N) proton of the azomethine group. In contrast, the signals in the range of 157–169.8 ppm are due to an azomethine carbon atom [18 –26].

2.4 X-ray diffraction analysis

X-ray diffraction analysis is a powerful nondestructive characterization technique for crystalline substances [27]. The diffraction data of Schiff base derivatives show C = N and C-N bond distances in the range of 1.261–1.425 Å and 1.303–1.467 Å, respectively. The C-N-C bond angle covers a range of 117.360–128.1° [18 , 28–34].

2.5 Biological application of Schiff bases

Many organic compounds containing heteroatoms have excellent biological applications [35 –40]. Schiff bases contain heteroatoms have significant biological applications such as antiviral, antioxidant, antifungal, anticancer, anthelminthic, antibacterial, antimalarial, anti-inflammatory, antiglycation, anti-ulcerogenic, and analgesic properties [1 , 41].

2.6 Antibacterial activity

Antibacterial agents kill bacteria or slow their growth rate without being extensively toxic to nearby tissues. Schiff bases have excellent antibacterial potential, i.e., compounds 1 and 2 (Fig. 1) showed good antibacterial activity against M. tuberculosis [42]. Schiff base derivatives 3–7 possessed good activities against Gram-positive and Gram-negative bacterial strains [43, 44]. Schiff base 8 containing isatin moiety showed excellent antibacterial potential against different bacterial strains [45]. Further, Schiff bases 9–11 of morpholine derivatives were screened against various bacterial strains. Among these compounds, compound 9 showed potent antimicrobial activity against S. epidermidis, M. luteus, E. coli, C. albicans, B. cereus, A. nigerand S. aureus, with MIC values of 19, 16, 29, 20, 21, 40, and 25, g/ml, respectively [46]. Schiff base derivatives of 2,4-dichloro-5-fluorophenyl 12–15 have also shown excellent antibacterial activities against four bacterial strains (E. coli, K. pneumonae, P. aeruginosa and S. aureus) [47].

Fig. 1

Schiff bases 1–15 with good antibacterial activities.

2.7 Antifungal activity

Schiff base derivatives have significant antifungal properties. Chitosan-derived Schiff bases 16 and 17 (Fig. 2) have excellent antifungal activity [48]. Schiff base derivatives 18–24 were screened for their antifungal potential against different fungal strains. Among these compounds, compound 18 has shown good inhibition against P. marneffei, A. fumigates, and T. mentagrophytes at 6.25μg/ml concentrations [47].

Fig. 2

Schiff base derivatives 16–43 with excelentantifungal activities.

Antifungal activity of Schiff base derivatives of Piperonyl 25–30 was also screened against Microsporumcanis, Trichophytonrubrum, Epidermophytonfloccosum, and Microsporumgypseum. These compounds were significantly active against Epidermophytonfloccosum at 62.5–250μg ml^-1 concentrations [49]. Isatin derivatives of Schiff bases (31–43) have shown high efficiency in the range of 1.22 to 78.12μg/ml concentrations against various fungal strains like E. floccosum, C. albicans, A. niger, T. mentagrophytes , M. audouinii, H. capsulatum, C. neoformans and M. gypseum [45].

2.8 Antiviral activity

Schiff bases are an excellent class of organic compounds for designing potent antiviral agents [50]. Schiff base of hydroxyl amino guanidines 44 (Fig. 3) was synthesized and tested against coronavirus and mouse hepatitis virus (MHV) for the first time. The synthesized compound was found to be about 376 times more active than hydroxyl guanidine against MHV [51]. Abacavir-derived Schiff bases 45–55 can inhibit reverse transcriptase activity and be extensively effective against HIV-1 [52]. Antiviral activities of Schiff base 56 and 57 were screened against different viruses such as vesicular stomatitis virus, Sindbis virus, herpes simplex virus-2 (G), Coxsackievirus B4, herpes simplex virus-1, reovirus-1, influenza A H3N2 subtype, vaccinia virus, Toro virus, feline herpes virus, influenza A H1N1 subtype, respiratory syncytial virus and Punta feline coronavirus (FIPV). The result has shown that both compounds have significant antiviral activity against all tested viruses [53].

Fig. 3

Schiff base derivatives 44–57 with significant antiviral activities.

2.9 Anticancer activity

A number of Schiff bases are reported for the treatment of different types of cancer. These compounds inhibit cancer propagation and inhibiting certain enzymes responsible for the respective disorders [54 –56]. A new series of Schiff bases 58–62 (Fig. 4) were screened for their anticancer potential against the MCF-7 cancer cell lines. These compounds showed better activity as compared to standard doxorubicin [57]. Schiff base derivatives 63–68 were also screened for their anticancer potential against HeLa, A549 and MCF-7 cancer cell lines. All these compounds were active and have shown good to moderate activities.

Fig. 4

Schiff base derivatives 58–83 with excellent anticancer activities.

Among these compounds, compound 68 is highly active, with IC₅₀ values ranging from 4.8–12.7μM [58]. Schiff bases 69 and 70 containing naphthoquinone were active against Hep-G2, MG-63, and MCF-7 cancer cell lines [59]. Several Schiff base derivatives 71–77 were evaluated for their anticancer potential against the MDA-MB-241cell line. These compounds have shown moderate activities, but compound 76 exhibited the highest anticancer activity with an IC₅₀ value of 7.08μgml^-1 [60].

Schiff base 78 was screened against human cervical cell lines (HeLa), which were shown excellent activity with an IC₅₀ value of 17.84μM [61]. Schiff bases 79–83 derived from benzothiazole were screened against COS-7 and HeLa cell lines. These compounds have exhibited high potency against the Hela cancer cell lines with IC₅₀ values of 2.22–3.06μmol L^-1 [62].

2.10 Antioxidant activity

Free radicals have a more significant impact on humans. About 5% oxygen reduces univalence to free radicals in living organisms during aerobic respiration and other metabolic activities. Various species are responsible for producing free radicals such as organic solvent, tobacco smoke, pesticides and ionizing radiations [63 –66]. These free radicals cause various diseases like cancer, cataract and cardiovascular diseases. Various enzymes, such as glutathione peroxidase (GTP), superoxide dismutase (SOD), etc., control the balance between the production and elimination of these species. However, when these enzymes are disturbed, oxidative stress is generated, which can be slowed down or prevented by antioxidants [67, 68]. Different derivatives of Schiff base are reported to have excellent antioxidant activities. Schiff bases 84–90 (Fig. 5) have been studied for their antioxidant potential using DPPH bioassay. The resultshave demonstrated that compounds 84–87 exhibit better scavenging activity than compounds 89, 90 and standard antioxidant (BTH) [69]. Moreover, Schiff base 91 containing benzophenone moiety has shown excellent antioxidant activity [70].

Fig. 5

Schiff base derivatives 84–91 with good antioxidant activities.

2.11 Antimalarial activity

Malaria is one of the leading causes of death in tropical and sub-tropical countries and kills about one million people worldwide [71]. Medicinal chemists are in search to design new and potent antimalarial agents. Schiff bases have been reported as excellent antimalarial agents. Compound 92 (Fig. 6) is significantly active with an effective ED₅₀ dose of 0.7μg ml^-1 against P. falciparum, a chloroquine-resistant strain [72]. A novel quinolinyl hydrazones containing Schiff base 93 has also exhibited strong antimalarial potential against P. falciparum [73]. Schiff base 94 containing morpholine moiety was evaluated for its antimalarial potential against the P. falciparum K14 strain. This compound has potent antimalarial activity with an IC50 value of 2.28μM and and appears like an excellent biologically active product [74]. A novel series of 9-anilinoacridinyl and 4-aminoquinolinyl Schiff bases 95–101 were evaluated for their antimalarial potential against chloroquine-sensitive 3D7 and chloroquine-resistant K1 strain of P. falciparum. Compounds 95–97 exhibited good activity against 3D7, while compounds 95, 97–101 displayed excellent activities against the K1 strain [75].

Fig. 6

Schiff base derivatives 91–101 with good antimalarial activities.

2.12 Antiglycation activity

The non-enzymatic reaction of protein and sugar is called glycation. Compounds with antiglycation properties can help to reduce glycation-associated diseases [76]. A variety of Schiff bases are reported that show excellent antiglycation properties. A series of acyl hydrazide Schiff bases 102–105 (Fig. 7) were screened for their in vitro antiglycation potentials. These compounds exhibited significant antiglycation potential with an IC₅₀ value in the range of 199–276.2μM as compared to rutin (IC₅₀ = 294.50μM [77]. Bis-Schiff bases 106–109 containing isatin moieties were also evaluated for their antiglycation potency. Among these compounds, compounds106, 107 and 108 have shown better antiglycation potential with IC₅₀ values of 291.14±2.53, 257.61±5.63, and 243.95±4.59μM, respectively, as compared to reference drug rutin (IC₅₀ = 294.46±1.50μM) [78]. Schiff bases 110–113 derived from benzophenone hydrazone have excellent antiglycation potency [79].

Fig. 7

Schiff base derivatives 102–113 with excellent antiglycationactivities.

2.13 Anti-inflammatory activity

Schiff bases have excellent anti-inflammatory activities. Several Schiff bases 114–117 (Fig. 8) containing aminoquinoline moiety were screened for their anti-inflammatory potential. These compounds exhibited excellent anti-inflammatory activities with IC₅₀ values of 1.90±0.90, 2.70±0.50, 1.40±0.10, and 2.40±0.10μgml^-1, respectively [80].

Fig. 8

Schiff base derivatives 114–126 with excellent anti-inflammatory activities.

Schiff base derivatives of 4-aminophenazone 118–121 [81] and 4-nitroaniline 122 [82] have also shown excellent anti-inflammatory activity. Several halogenated Schiff base derivatives 123–125 were screened for their anti-inflammatory potentials using paw edema assay. All these compounds have exhibited excellent anti-inflammatory activities [83]. Besides, Schiff base 126, containing pyrazole moiety, has exhibited excellent activity and can be used as the lead compound in the future [84].

2.14 Analgesic activity

Different sources of analgesic drugs include medicinal plants like like Curcuma alismatifolia, Fumairavaillantii, Elettariacardamomum, AloeveraBarbedensis, Rumexcrispus, Bunts longifolia, AndrographisPaniculata, Cissusquadrangularis, Buxussempervirens, Punicagranatum, MorindaCitrifolia,Phoenix sylvestris etc and synthetic compounds likediclofenac, Ibuprofen,Paracetamol, etc [85]. Some Schiff bases also possessed excellent analgesic activities. For example, Schiff base 127 (Fig. 9) exhibited better analgesic activity than the standard drug pentazocine [86].

Fig. 9

Schiff base derivatives 127–135 with excellent analgesicactivities.

Schiff bases 128–130 containing pyrazolemoiety were tested for their in vivo analgesic potential. These compounds showed better analgesic activity (50 mg/kg, orally) as compared to morphine (1 mg/kg, intraperitoneally) [87]. Schiff base derivatives of fenamateisosteres131–134 were synthesized to locate analgesic potential. These compounds have better analgesic potency as compared to Indomethacin. Among these compounds, compounds 131 and 132 have the most potent activities [88]. Schiff base derivatives 118–121 were investigated for their analgesic activity in mice by the acetic acid-induced writhing (AIW) and formalin-induced paw licking (FIPL) method. These compounds were found to have excellent analgesic activity in the treated mice [83]. The analgesic activity of compound 135 was evaluated by using the Tail flick method. The result exhibited that this compound possesses excellent analgesic activity and might be extended as a novel class of analgesic drugs [89].

2.15 Anthelmintic activity

Schiff base derivatives were reported to exhibit excellent anthelmintic activity. Schiff bases 136–146 (Fig. 10) were synthesized by condensing nalidixic acid hydrazide with 5-(substituted)-2-furfuraldehyde and were tested for their in vitro anthelmintic activity. These compounds have shown moderate to good activity and were the most active against Pheritimaposthuma, and Perionyxexcavatus, compared to the reference drugs Albendazole. The mean paralyzing time of all tested compounds against Pheritima Posthuma and Perionyxexcavatus was 11.23–20.46 and 7.07–16.49 min, respectively.

Fig. 10

Schiff base derivatives 136–146 with excellent anthelmintic activities.

It was observed that the most and the least strong anthelmintic compounds in terms of mean paralyzing time against Perionyxexcavatus were 139 and 145. In contrast, against Pheritima Posthuma, 139 and 140 were similarly active. Compound 139 was more potent than the reference drug in causing the death of worms, which exhibited an average time of 10.69 and 19.32 min against Perionyxexcavatus and Pheritimaposthuma, respectively [90].

2.16 Antitubercular activity

Drug-resistant Mycobacterium tuberculosis is an increasing threat to global health. Therefore, the need for highly effective and potent analogs against drug-resistant Mycobacterium tuberculosis is of significant consideration [91]. Schiff base derivatives can be used as antitubercular drugs as it possesses excellent antitubercular activity. Schiff base of isatin and nalidixic acid carbohydrazide 147 (Fig. 11) were synthesized and structurally characterized by various spectroscopic techniques. The antitubercular activity of the synthesized compound was investigated against Mycobacterium strains such as Smegmatis, Intercellulari and Cheleneo and Xenopi. It was found that compound 147 has excellent antitubercular activity with a MIC value of 0.625μg/ml, almost about twenty times greater than the reference drug isoniazid (MIC = 12.5μg/ml) [92]. Schiff base derivatives 148–151 containing triazole moiety were also synthesized in excellent yield and purity using the microwave technique. These compounds have potent growth-inhibitory activity and completely inhibit the growth of Mycobacteria [93].

Fig. 11

Schiff base derivatives 147–151 with excellent antitubercular activities.

2.17 Enzyme inhibition

Schiff base derivatives 152–167 (Fig. 12) containing chalcone moiety were synthesized, and their in vitro inhibitory effects on the activity of acetylcholinesterase (AChE) and human carbonic anhydrase (hCA) were investigated. All the synthesized compounds showed better activities than standard drugs (Acetazolamide and Tacrine). They could be important in designing and synthesizing new drugs to treat different diseases such as Alzheimer’s and epilepsy. Phenoxyaceto hydrazide Schiff base analogs 168–175 were evaluated for their in vitro activity as β-glucuronidase inhibitors. These compounds possess excellent β-glucuronidase inhibition activity with IC50 value of 9.20±0.3, 15.4±1.56, 9.47±0.16, 12.0±0.16, 13.7±0.40, 19.6±0.62, 22.0±0.14 and 30.7±1.49μM, respectively. Schiff base derivatives 176–182 of tacrine were synthesized and inhibition effects on acetylcholinesterase (AChE) were investigated. These compounds inhibited AChE and havebetter activity than tacrine except for compound 180. The greatest inhibition potency was observed for compound 176 with an IC₅₀ value of 22.1 nM [94].

Fig. 12

Schiff base derivatives 152–167 with excellent enzyme inhibition activities.

2.18 Anti-ulcerogenic activity

Synthetic reagents are one of the best solutions for the treatment of ulcers. Besides other compounds, some Schiff base derivatives also showed potent anti-ulcerogenic activity [95]. Sulphone-based Schiff base 183 (Fig. 13) was synthesized and screened for its anti-ulcerogenic potential. This compound has considerable anti-ulcerogenic activity as compared to cimetidine [96]. Compounds 184 and 185 have also been screened for their anti-ulcerogenic activity. Surprisingly, these compounds have exhibited significant anti-ulcerogenic potential as compared to standard drug indomethacin [88].

Fig. 13

Schiff base derivatives 152–167 with excellent anti-ulcerogenic activities.

2.19 Schiff bases as chemosensors and sensing mechanism

Several Schiff bases have been applied for the sensitive and selective detection of various analytes. Schiff base interacts with these species and gives measurable analytical signals. Some factors affect the sensitivity and selectivity of chemosensors, such as the charge on the metal ion, the structural rigidity and the binding ability of the ligand with the metal ion, and the electron configuration of both the metal ion and ligand [97, 98]. Various mechanisms have been proposed for the interaction between chemosensors and metal ions, including intra ligand charge transfer (ILCT), photoinduced electron transfer (PET), metal-ligand charge transfer (MLCT), excimer/ exciplex formation, paramagnetic fluorescence quenching (PFQ) chelation enhanced fluorescence quenching (CHEQ), fluorescence resonance energy transfer (FRET), intramolecular charge transfer (ICT), excited-state intramolecular proton transfer (ESIPT), bond energy transfer (BET) and chelation enhanced fluorescence (CHEF) mechanism. SeveralSchiff bases being used as chemosensors for the detection of metal ions are briefly discussed [99 –106].

2.20 Schiff bases as chemosensors for detection of metalic cations

Several Schiff bases containing compounds have been synthesized and used for the colorimetric and spectrofluorimetric detection of different cations. For example, Schiff base fluorescent chemosensor SB1 ( Fig. 14) was developed for the colorimetric detection of Ag⁺ and Cu^2 + ions in the aqueous solution. Chemosensor SB1 interacted with Ag⁺ through S- and O- donor atoms and formed a red color complex. Chemosensor SB1 showed a highly sensitive response toward Ag⁺. The selectivity of SB1 was not interfered by other metal ions like Cr^3 +, Zn^2 +,Co^2 +,Cd^2 +, Mn^2 +, Ca^2 +, Na⁺, Mg^2 +, Pb^2 +, Li⁺, Hg^2 +, K⁺, Fe^2 +, Ni^2 +, and Fe^3 + [107]. Another highly selective and sensitive chemosensor SB2 was developed to detect Ag⁺. SB2 showed excellent selectivity for Ag⁺ in the presence of Ca^2 +, Cd^2 +, Fe^3 +, Al^3 +, Zn^2 +, Ni^2 +, Pb^2 +, Mg^2 +, Co^2 +, Cu^2 +, Hg^2 +, and Cr³ ⁺ . SB2 interacts with Ag⁺ and forms complexes that enhance the fluorescent intensity. The complex formation was confirmed by¹ H NMR titration, mass spectrometry, FT-IR spectroscopy, and Job’s plot analysis [98]. A salen-type Schiff base fluorescent chemosensors SB3-SB5 were designed forthe detection of Ag⁺. These chemosensors SB3-SB5 sense selectively Ag⁺ over other cations like Cu^2 +, Co^2 +, Pb^2 +, Ce^3 +, K⁺, Zn^2 +, Cd^2 +, Ba^2 +, Cr^3 +, La^3 +, Mn^2 +, Al^3 +, Hg^2 +, Fe^2 +, Ni^2 +, Ca^2 +, Na⁺, Fe^3 +, and Mg² ⁺ . The binding mechanism SB3-SB5 have been studied by ¹ H NMR, fluorescence spectroscopy, mass spectrometry and theoretical calculation. The results showed that the chemosensor SB3-SB5 forms chelating complexes with Ag⁺, which facilitates fluorescence quenching via heavy atom effects that overruled this C = N isomerization process [108]. Schiff base chemosensor SB6 was designed for the recognition of Ag⁺. The interaction of Ag⁺ with chemosensor SB6 causes a 203-fold enhancement in fluorescence intensity and colorimetric change (yellow to colorless) due to hydrolysis of SB6. The chemosensor SB6 shows excellent selectivity over other cations like Ni^2 +, Zn^2 +, Fe^3 +, Fe^2 +, Cd^2 +, Hg^2 +, Na⁺, Co^2 +, Cr^3 +, Mg^2 +, Pb^2 +, Sn^2 +, Al^3 +, Ba^2 +, Mn^2 +, Ca^2 +, K⁺, and Cu² ⁺ . The chemosensor SB6 was also applied to the ratiometric imaging of Ag⁺ in HeLa and HepG2 cells [109]. A Schiff base chemosensor SB7 contaning thiophene moiety was developed for the determination of Ag⁺. SB7 showed excellent selectivity towards Ag⁺ over other cations like Th^4 +, Ni^2 +, La^3 +, Cu^2 +, K⁺, Cd^2 +, Hg^2 +, Co^2 +, Zn^2 +, Pb^2 +, Mn^2 +, Ca^2 +, Cr^3 +, Fe^3 +, Ba^2 +, Fe^2 +, Li⁺, Zr^2 +, Mg^2 +, Ce^3 +, Na⁺, Sr^2 +, Al^3 +, and Bi² ⁺ . SB7 interacts with Ag⁺ ions and induces a remarkable enhancement in the fluorescence intensity with hypsochromic shift (425 nm to 405 nm) due to the inhibition of the ICT process [110].

Fig. 14

Schiff base chemosensors SB1–SB8 for the detection of Ag⁺.

A Schiff base chemosensor SB8 containing mercapto pyrimidine moiety was designed and synthesized. The chemosensor SB8 exhibits a weak emission at 425 nm when excited at 360 nm. Upon addition of Ag⁺ the fluorescence intensity enhanced due to CHEF, while no significant response was obtained with background cations like Cd^2 +, Mn^2 +, Sr^2 +, Fe^2 +, Cu^2 +, Ba^2 +, Mg^2 +, Ni^2 +, Fe^3 +, Sn^2 +, Cr^3 +, Hg^2 +, Co^2 +, Zn^2 +, and Pb^2 + [111]. Schiff base chemosensors SB9-SB15 (Fig. 15) were developed for the detection and determination of Zn^2 + ions. The Schiff base chemosensor SB9 containing pyrimidine moiety was applied to detects Zn^2 + using the spectrofluorimetric technique. The result showed that chemosensor SB9 formed a chelating complex with Zn^2 + and enhanced the emmision intensity. The chemosensor SB9 was also applied for the detection of free Zn^2 + in real samples e.g.,cabbage, banana stem juice and commercial fruit juice [112]. ASchiff base chemosensor SB10 containing thioether functional group exhibited a “turn-on” response only towards Zn^2 + even in the presence of various cations such as Ca^2 +, Al^3 +, Ni^2 +, Fe^3 +, Cu^2 +, Mg^2 +, Co^2 +, Mn^2 +, Cd^2 +, Cr^3 +, K⁺, Na⁺ and Hg² ⁺ . The chemosensor SB10 also has negligible toxicity in low concentrations and can be efficiently used for live-cell imaging [113]. Schiff base chemosensor SB11 containing 2-imidazole and 8-aminoquinoline moieties was investigated as a highly sensitive luminescent chemosensor for Zn² ⁺ . The fluorescence competition and ratiometric analysis were carried out in the presence of different cations such as Cu^2 +, Fe^2 +, K⁺, Ca^2 +, Na⁺, Mg^2 +, Co^2 +, Ni^2 +, Mn^2 +, Cd^2 +_, and Hg² ⁺ . The chemosensor coordinated with Zn^2 + through nitrogen and formed a complex which showed a ‘turn-on’ fluorescence response due to inhibition of C = N isomerization [114]. A fluorescence enhancement was also observed for Schiff bases chemosensor SB12 upon interacting with Zn^2 + [115]. Schiff basechemosensor SB13 containingimidazole and pyridine moieties was applied for the recognation of Zn² ⁺ . Upon addition of different metal ions like Na⁺, Li⁺, Ca^2 +, K⁺, Mg^2 +, Cd^2 +, Cu^2 +, Ag⁺, Ni^2 +, Hg^2 +, Pb^2 +, Fe^3 +, Mn² ⁺ Cr^3 +, Zn², Co^2 + and Al^3 +, only Zn^2 + could induce a significant fluorescence enhancement and colorimetric change (Colorless to faint yellow). Upon the addition of Zn^2 + ions, the resulting complex could induce bathochromic shifts both in emission and absorption bands due to the ICT process [116]. A new Schiff base fluorescent sensor SB14 having nitrogen and oxygen donor atoms sowed high selectivity towards Zn² ⁺ . SB14 showed weak fluorescent emission and upon the addition of Zn^2 +, the fluorescent enhancement was observed due to inhibitionof PET effect and C = N isomerization [117].

Fig. 15

Schiff base chemosensors SB9–SB15 for the detection of Zn² ⁺ .

Schiff-base fluorescent chemosensor SB15 bearing hydrazino pyridine moiety was applied to detect Zn² ⁺ . SB15 displayed a weak fluorescence intensity, which increases upon the addition of Zn^2 + ions. SB15 selectivity detects Zn^2 + from the mixture guest metal ionssuch as Sr^2 +, Fe^3 +, Li⁺, K⁺, Na⁺, Hg^2 +, Co^2 +, Ag⁺, Ba^2 +, Mn^2 +, Cd^2 +, Pb^2 +, Mg^2 +, Ca^2 + and Cu^2 + [118]. A novel Schiff base chemosensor SB16 (Fig. 16) was designed for the recognition of Mn^2 + ions. SB16 gives a colorimetric (pale yellow to orange) response upon interacting with Mn^2 + ions. The detection limit was calculated to be 0.14μM (Table 1) [119]. Another highly sensitive and selective chemosensor SB17 was developed for the recognition Mn^2 + and Hg² ⁺ . The chemosensor SB17 showed a colorimetric (light-yellow to greenish-yellow) response only towards Mn^2 + and Hg^2 + ions [120]. A Schiff base chemosensor SB18 was synthesized and applied for the recognition of Pd^2 + ions. The chemosensor showed a highly selective response towards Pd^2 + ions even in the presence of different metal ions such as Ag⁺, Mn^2 +, Na⁺, Ni^2 +, K⁺, Pt^2 +, Pb^2 +, Ba^2 +, Hg^2 +, Cu^2 +, Ca^2 +, Fe^2 +, Zn^2 +, Mg^2 +, Co^3 +, Fe^3 +, Al^3 +, and Cd^2 + ions [121]. A salen-type Schiff base chemosensor SB19 was developed for the recognition of Cd^2 + ions. The sensing potency of SB19 was investigated towards different cations; only Cd^2 + enhances the fluorescence intensity due to the formation of a highly fluorescent complex (Fig. 16) [122]. A novel Schiff base chemosensor SB20 was designed and synthesized. The chemosensing properties were investigated towards various metal cations. SB20 displayed a colorimetric and fluorescent response towards Pb^2 + due to inhibition of PET process [123]. Schiff base bearing ditriazole moiety SB21 was developed for the detection of Co² ⁺ . SB21 showed a colorimetric (light yellow to greenish) response towards Co² ⁺ . The sensing mechanism was investigated by mass, NMR, and FT-IR analysis [124]. A new Schiff base chemosensor SB22 ( Fig 17) was designed and synthesized for detecting Ni^2 + ions in an aqueous media. SB22 displayed a colorimetric response (fade yellow to deep orange) only towards Ni^2 + ions [125].

Fig. 16

Schiff base chemosensors SB16–SB20 for the detection of Mn^2 +, Pd^2 +, Cd^2 +, and Pd² ⁺ .

Table 1

Different parameters for the analysis of metal cations using chemosensors (SB1-SB30)

Methods	Sensor	Sensor type	Sensing mechanism	Analytes	Metal: Sensor	LOD μM	λex/λem (nm)	pH	Color change	Matrixes	References
Colorimetric	SB1	–	Chelation	Ag⁺	1:2	0.00086	–	–	Colorless to red	Aqueous medium	[107]
Fluorimetric	SB2	off– on	Chelation	Ag⁺	1:1	0.12	332/448	–	–	–	[98]
Fluorimetric	SB3		C = N isomerization	Ag⁺	1:1	0.28,	388/413	7.4	–	DMSO/H₂O (1:1, (v/v)	[108]
Fluorimetric &Colorimetric	SB4		C = N isomerization	Ag⁺	2:1	17.01	394/411	7.4	Yellow to colourless	DMSO/H₂O (1:1, (v/v)	[108]
Fluorimetric &Colorimetric	SB5		C = N isomerization	Ag⁺	2:1	17.13	394/411	7.4	Yellow to slight brown	DMSO/H₂O (1:1, (v/v)	[108]
Colorimetric &Fluorimetric	SB6	turn-off	Hydrolysis	Ag⁺	1:1	0.061	400/520	7.4	Yellow to colorless	CH₃OH/phosphate buffer (20.0 mM, 1:1, v/v)	[109]
Fluorimetric	SB7	turn-on	ICT	Ag⁺	1:1	0.128	370/425	4– 9	–	MeOH/H₂O, 1:1 (v/v)	[110]
Colorimetric &Fluorimetric	SB8	turn on	CHEF	Ag⁺, CN^-	2:1	1.12	360/425	4– 9	Colorless to yellowish brown	Aqueous DMF (50%)	[111]
Fluorimetric	SB9	turn-on	LMCT	Zn^2 +	1:1	0.69	375/469	7.2		MeOH-H2O,2:8 V/V	[112]
Colorimetric &Fluorimetric	SB10	turn-on	PET &CHEF	Zn^2 +	1:1	0.00173	350/490	6.0– 7.6		DMSO:H2O (1:5, v/v)	[113]
Fluorimetric	SB11	turn-on	C = N isomerization	Zn^2 +	1:1	0.581	420/483	7.4		MeOH/H₂O (4:1)	[114]
Fluorimetric	SB12	turn-on	CHEF	Zn^2 +	1:1	0.250	441/550	7		Living systems	[115]
Colorimetric &Fluorimetric	SB13	turn-on	ICT	Zn^2 +	1:1	0.068	375/462	7.4	Colorless to faint yellow	C₂H₅OH-H₂O (9:1, v:v)	[116]
Fluorimetric	SB14	on-off	PET	Zn^2 +	1:1	0.010	430/498	–	–	Ethanolicmedia	[117]
Fluorimetric	SB15	turn-on	PET	Zn^2 +	1:1	0.0070	402/470	7.4	–	Aqueous media	[118]
Colorimetric	SB16	–	Chelation	Mn^2 +	1:1	7.11	–	–	Pale yellow to orange	Aqueous medium	[119]
Colorimetric	SB17	–	ICT	Mn^2 +	1:1	0.2	–		Light Yellow to greenish-yellow	DMF /Water medium	[120]
Fluorimetric &Colorimetric	SB18	turn-on	Chelation	Pd^2 +	1:1	0.0188	450/598	7-12	Straw to pink	living cells MCF7	[121]
Fluorimetric &Colorimetric	SB19	turn-off	PET &CHEF	Cd^2 +	1:1	0.01296	578 /505	4.8– 8.5	Pale Yellow to green	MCF 7 cells	[122]
Fluorimetric &Colorimetric	SB20	turn-on	CHEF &PET	Pb^2 +	1:2	0.008	340/429	4– 10	Colorless to yellow	Aqueous medium, &Urine	[123]
Fluorimetric	SB21	turn-off	Chelation	Co^2 +	1:1	0.0387	529/622	2– 10	Light yellow to greenish	Aqueous medium	[124]
Colorimetric	SB22	–	Chelation	Ni^2 +	1:1	0.1	–	5.5– 8.0	Faded yellow to deep orange	Aqueous medium	[125]
Fluorimetric &Colorimetric	SB23	turn-on	PET	Cu^2 +	1:1	0.0023	360/475	–	Colorless to greenish-yellow.	RAW 264.7 cells	[126]
Fluorimetric	SB24	turn-on	C = N isomerization inhibition	Al^3 +	1:1		440/458 nm.	–		Acetonitrile &DMSO	[127]
Fluorimetric &Colorimetric	SB25	off-on	Hydrolysis	Au^3 +	1:1	1.51	560/588	7.4	Colorless to pink	acetonitrile aqueous media	[128]
Fluorimetric	SB26	turn-on	Chelation	In^3 +	1:1	9.62	420/470	6– 8		Aqueous medium	[129]
Fluorimetric	SB 27	turn-on	Chelation &PET	Ga^3 +	1:1	0.01	300/434	–	–	Acetonitrile	[130]
Fluorimetric	SB28	turn-on	ESIPT, Inhibition of C = N isomerism	Cr^3 +, Fe^3 +	1:2	2.75	380/502	–	–	CH₃CN	[131]
Fluorimetric &Colorimetric	SB29	turn-on	CHEF	Al^3 +, Cr^3 +	1:2	5.8×10^-5	340./440,512	4.5– 8	Pale yellow to reddish	DMSO	[132]
Fluorimetric	SB30	turn-on	PET, Inhibition of C = N isomerization	Cr^3 +	1:1	0.46	– /498	7– 8	–	Aqueous medium	[133]

Fig. 17

Schiff base chemosensors SB21–SB24 for the detection of Co^2 +, Ni^2 +, Cu^2 +, and Al³ ⁺ .

A Schiff base chemosensor SB23 ( Fig. 17) was developed for the detection of Cu^2 + ions. SB23 showed a significant colororimetric response only towards Cu² ⁺ . SB23 shows a weak fluorescence intensity due to the PET process. However, upon interacting with Cu^2 +, causing the enhancement in fluorescent intensity due to inhibition of PET process [126]. A novel Schiff base chemosensor SB24 ( Fig. 17) bearing thiourea moiety was developed for the detection of Al³ ⁺ .

SB24 displayed a turn-on response towards Al^3 + in the presence of various metal ions such as In^3 +, Zn^2 +, Ag⁺, Cd^2 +, Mg^2 +, Ca^2 +, Cu^2 +, Hg^2 +, Fe^2 +, Ba^2 +, Ni^2 +, Mn^2 +, Li⁺, Co^2 +, K⁺, Sr^2 +, Pb^2 +, and Na⁺. Upon interacting with Al^3 + ions, the inhibition of the C = N isomerization was observed, which enhanced the fluorescence intensity. [127]. A novel chromogenic and fluorogenic and chemosensor SB25 ( Fig. 18) was designed and synthesized for the recognition of Au³ ⁺ . SB25 showed excellent selectivity towards Au³ ⁺ . SB25 also exhibited a colorimetric response upon interacting with Au^3 + ions [128].

Fig. 18

Schiff base chemosensors SB25 for the detection of Au³ ⁺ .

Ahighly selective and sensitive Schiff base chemosensor SB26 ( Fig. 19) was designed for the detecting of In³ ⁺ . The sensing properties of SB26 were investigated towards different metal ions such as Co^2 +, Fe^2 +, Cr^3 +, Ag⁺, Pb^2 +, Cd^2 +, Na⁺, Ca^2 +, Al^3 +, Hg^2 +, Fe^3 +, K⁺, In^3 +, Ag⁺, Cu^2 +, Mg^2 +, Ga^3 +, Zn^2 +, and Ni² ⁺ . SB26 exhibited weak emission with all metal ions except In^3 + ion, which exhibited a significant enhancement in the fluorescence intensity. The chemosensor could sucessfuly detects In^3 + in an aqueous medium and effectively discriminates In^3 + from Al³ ⁺ and Ga³ ⁺ . The sensing mechanism was investigated by FT-IR, ESI-mass, UV-Vis, NMR, and fluorescence measurements [129]. A novel Schiff-base chemosensor SB27, was designed for the recognation of Ga³ ⁺ and Zn² ⁺ . The photophysical properties of SB27 towards different cation such as Mg^2 +, Fe^2 +, Cu^2 +, K⁺, Pb^2 +, Ca^2 +, Cr^3 +, Na⁺, Hg^2 +, Co^2 +, In^3 +, Mn^2 +, Cd^2 +, Ag⁺, Fe^3 +, Ni^2 +, and Al^3 +, Ga^3 + and Zn² ⁺ . SB27 showed high sensitivity towards Ga^3 + and Zn^2 +, with very low detection limit [130]. A novel Schiff base SB28 (Fig. 20) has been developed for the selective recognition of Cr³ ⁺ and Fe³ ⁺ . SB28 showed high selectivity towards Cr^3 + and Fe^3 + over other metal ions, such as Zn^2 +, Ag⁺, Mn^2 +, Cd^2 +, K⁺, Ni^2 +, Ca^2 +, Na⁺, Hg^2 +, Mg^2 +, Co^2 + and Fe² ⁺ . SB28 exhibits a weak fluorescence emission due to ESIPT and C = N isomerization. Upon interacting with Cr^3 + and Fe^3 +, a fluorescence enhancement was observed due to inhibition of C = N isomerization and ESIPT [131].

Fig. 19

Schiff base chemosensors SB26 and SB27 for the detection of In^3 + and Ga³ ⁺ .

Fig. 20

Schiff base chemosensors SB28-SB30 for the detection of Cr³ ⁺ .

Another Schiff base chemosensor SB29 bearing anthracene moiety has been designed for the detectionof Cr^3 + and Al³ ⁺ . SB29 shows a turn-on and colorimetric (blue to green under UV light) responsetowards Cr^3 + and Al^3 + due to CHEF process. For other cations like Fe^2 +, Ni^2 +, Zn^2 +, Co^2 +, Cd^2 +, Pb^2 +, Cu^2 +, Hg^2 +, Mn^2 + and Na⁺, no considerable changes were observed [132]. A Schiff base chemosensor SB30 bearing coumarindesigned and its sensing properties were investigated towards different metal ions like Mg^2 +, Cu^2 +, Li⁺, Fe^3 +, Hg^2 +, Ni^2 +, Al^3 +, Co^2 +, Cr^3 +, Zn^2 +, Pb^2 +, Ca^2 +, Ba^2 +, Cd^2 +, Ag⁺, and Fe³ ⁺ . SB30 displayed a “turn-on” response toward Cr³ ⁺ . The enhancement in fluorescence intensity was attributed to the suppression of PET process and inhibition of C = N isomerization [133].

3 Conclusion

Schiff base derivatives are excellent organic compounds. These compounds are economical, have simple synthetic routes, and are easily accessible in laboratories. They have medicinal and biological applications such as antiviral, antioxidant, antifungal, anticancer, anthelmintic, antibacterial, antimalarial, anti-inflammatory, antiglycation, anti-ulcerogenic, and analgesic potentials. They are also used as catalysts, polymer stabilizers, intermediates in organic synthesis, and corrosion inhibitors. Furthermore, these compounds have excellent chemosensing properties to detect different analytes.

Conflict of interest

Author declare no conflict of interest.

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