**5. Detection of some toxic heavy metal ions**

#### **5.1 Detection of mercury**

Mercury is a toxic heavy metal even at low concentrations and widely dispersed in nature. It can accumulate in the body and is responsible for different diseases including acute kidney failure, prenatal brain damage, and heart diseases. Therefore, the detection of Hg2+ ions in different samples is of great consideration. A novel Schiff

#### *Recent Advancements in Schiff Base Fluorescence Chemosensors for the Detection of Heavy… DOI: http://dx.doi.org/10.5772/intechopen.109022*

base chemosensor 1 containing pyrene-based free thiol derivative was used for the determination of Hg2+ ions in living cells [36]. Therefore, it is quite important to consider finding Hg2+ ions in various samples. For the detection of Hg2+ ions in live cells, a new Schiff base chemosensor 1 with pyrene-based free thiol derivative was employed. Similar to this, a double-naphthalene Schiff base fluorescent chemosensor 2 was used to detect Hg2+ ions at a low detection limit of 0.05595 μM in a DMSO medium (**Figure 2**). In the presence of DMSO, the chemosensor's imine group underwent oxidation to become an amide. The 1:1 metal-ligand complex generated by the N- and O- atoms of amide and Hg2+ ions was confirmed by mass spectrometry, 1 H NMR, and FT-IR spectroscopy [37]. A novel coumarin-based fluorescent chemosensor 3 was employed in another study to find Hg2+ ions. In the presence of secondary metal ions, the chemosensor shows excellent selectivity toward Hg2+. As the amount of Hg2+ gradually increases, the probe's yellow-colored solution becomes colorless. Fluorescence investigations further supported chemosensor 3's selectivity for mercury ions. The bright blue fluorescence at 460 nm gradually decreases in conjunction with a minor red-shift to the cyan channel (470 nm) after the addition of 0.4 M Hg2+ to the probe solution. The lone pair electrons are distributed to Hg2+ by the Schiff base imine nitrogen (CH=N) and phenolic-oxygen atoms, which reduces the emission of acceptor coumarin and prolongs that of the donor 4-(Diethylamino)salicylaldehyde. Therefore, 4-(Diethylamino)salicylaldehyde's distinctive emission at 470 nm is due to the inhibition of the ESIPT and PET-ON processes. The mole fraction at maximum absorbance is 0.3, showing that Cou-S and both Hg2+ form a dimer complex with a binding stoichiometry of 2:1 (probe: metal ion). An 8.3 nM threshold for detection was determined. Density functional theory (DFT) was also used to analyze how the chemosensor and Hg2+ ions interact [38]. Another work used a novel N-salicylidene) benzylamine Schiff base 4 to detect Hg2+ ions via fluorescence ON characteristics. Fluorescence and UV-Vis spectroscopy were used to examine the chemosensor's sensing capabilities. By using fluorescence emission and UV-Vis spectroscopy, it was determined how the chemosensor 4 responded to different cations. Significant fluorescence enhancement and spectral/color alterations supported the sensor's selectivity for Hg2+ [39]. The detection of Hg2+ ions is influenced by the pH of the detection medium. For example, a turn-on fluorescent chemosensor 5 was developed to detect Hg2+ in an aqueous CH3CN medium with a pH range of 5–10 [40]. In the presence of various cations including Hg2+, Pb2+, Ca2+, Ba2+, Cu2+, Cr3+, Mg2+, Zn2+ ions in aqueous media, it was discovered that the Hg2+ ion notably exhibited specificity for the Schiff base chemosensor 6. Additionally, counter anions such as Br<sup>−</sup> , Cl− , F− , I− , S2−, CN− , NO3 − , NO2 − , OH− , CO3 − , SCN− , HSO3 − , H2PO4 − CH3COO− , HPO4 2−, P2O7 4−, and PO4 3− failed to have a discernible impact on the chemosensor's selectivity [41]. The structures of the chemosensors 1–6 and their interaction with Hg2+ ion are shown in **Figure 2**.

## **5.2 Detection of cadmium ion**

Due to the extensive range of uses of cadmium in numerous industries, including mining, smelting, and the combustion of fossil fuels, etc., it is one of the most pervasive health-hazardous contaminants. In light of this, it is crucial to create a reliable and efficient method for detecting Cd2+ ions in a variety of materials. The quinoline moiety-containing Schiff base chemosensor 7 was created as a fluorescence sensor for Cd2+ ions. It's interesting to note that the chemosensor performed superbly when detecting Cd2+ ions in an aqueous media by turn-on fluorescence.

**Figure 2.** *Formation of Hg2+ complexes with Schiff base chemosensors 1–6.*

Very little fluorescence emission was seen from chemosensor 7 when it was unmodified, but a significant turn-on response was seen when Cd2+ was added. The sensor showed excellent selectivity with no interference from any other metal ions. The limit of detection was determined to be 0.0024 μM, and the binding stoichiometry between the Cd2+ and chemosensor 7 was proven to be 1:1 [42]. To identify Cd2+ ions, a pyridine-based Schiff base chemosensor 8 was created. The fluorescence enhancement happened when the Cd2+ ion was applied to the complexion despite the chemosensor not being fluorescent. As a result, the CHEF was stimulated while the PET was blocked, increasing the fluorescence intensity of the chemosensor. Additionally,

*Recent Advancements in Schiff Base Fluorescence Chemosensors for the Detection of Heavy… DOI: http://dx.doi.org/10.5772/intechopen.109022*

chemosensor 8 was successfully used in water sample analysis [43]. With a detection limit of 1.025 × 10−8 M, a Rhodamine-based Schiff-base fluorescence chemosensor 9 was used to find Cd2+ ions. In the presence of cations, the chemosensor 9 shows significant sensitivity and selectivity for Cd2+ ions [44]. An ON-type Schiff base chemosensor 10 was communicated in 2020 and its fluorescence properties against cadmium ions were discussed. According to Job plot, the chemosensor creates a complex with Cd2+ of 1:1 ratio. For an analytical approach based on chemosensor, pH, solvent type, and ligand concentration were tuned for the detection of Cd2+ in aqueous samples. The limit of detection was determined to be 6.0 × 10−7 M. The probe showed a wide range of linearity with Cd2+ [45]. A recent study described fluorescent sensor 11, which is based on rhodamine, and its capacity to find Cd2+ ions. The coordination-induced fluorescence activation (CIFA) mechanism is how the sensor reacts to Cd2+. In the presence of the tested metal ions, chemosensor 11 responds to Cd2+ with a highly quick and reversible fluorescence. The complex stoichiometry between the sensor and Cd2+ was discovered to be 1:1, and the binding constant in acetonitrile (ACN)/HEPES buffer (10 mM, pH, 7.05, v/v 1:1) was calculated to be 2.70 107 M−1. The chemosensor 11's fluorescence detection limit for Cd2+ was discovered to be 0.218 μM, demonstrating a notable sensitivity to Cd2+ [40]. The structures of the chemosensors 7–11 and their interaction with Cd2+ ion are shown in **Figure 3**.

#### **5.3 Detection of chromium ion**

Chromium is a significant transition metal that is used in a variety of industries, including chemical engineering, textile manufacturing, steel production, oil refining, and electroplating. But it raises the risk of lung, sinus, and nasal cancer and results in pulmonary sensitization. A novel Schiff base chemosensor 12 was constructed and subjected to several characterization methods in order to detect Cr3+. Using a spectrophotometric method, the chemosensor was tested against a variety of toxic metal ions, including Mn2+, Ag+ , Cd2+, Co2+, Cu2+, Ni2+, Zn2+, Fe3+, Al3+, and Cr3+. Only Cr3+ ions were found to produce a recognizable hyperchromic shift in the chemosensor's absorbance due to the formation of the stable complex through S and N atom [46].

Additionally, electrochemical methods were employed to investigate the electrochemical properties and reversibility of the chemosensor. To detect Cr3+ ions in the aqueous media, the Schiff base chemosensor 13 was also used. Visual monitoring was done of the chemosensor 13's colorimetric sensitivity to several cations. When chromium ions were added, the chemosensor 13's yellow color turned to colorless. Fluorescence tests were used to investigate the chemosensor 13's sensing capabilities in more detail. The chemosensor 13 displayed less fluorescence intensity at 418 nm, but a significant increase in fluorescence intensity was seen in the presence of Cr3+ due to the imine group of sensor might be converted back to the carbonyl group in the presence of Cr3+ [47]. Recently, a novel Schiff base fluorescent chemosensor 14 for the selective detection of chromium ions was published. The development of a 1:1 stoichiometric sensor-Cr3+ complex, which is supported by Job's plot, has the effect of inhibiting the PET process. The limit of detection was shown to be 1.3 × 10−7 M via fluorescence titration, and the association constant Ka was reported as 2.28 × 105 M−1. Additionally, DFT and TDDFT simulation results were provided to understand the proposed complex's optimal structure as well as its electronic spectra [48]. An innovative thiazole-based fluorescent Schiff base chemosensor 15 for the chemodosimetric method to Cr(III) ion detection was discussed. The structure of the chemosensor 15 was determined using a variety of analytical techniques, including UV-vis, 13C-NMR,

**Figure 3.** *Formation of Cd2+ complexes with Schiff base chemosensors 7–11.*

1 H-NMR, and FT-IR analysis. It's interesting to note that chemosensor 15 responds to different metal cations, such as Ni2+, Na+ , Cd2+, Ag+ , Mn2+, K+ , Zn2+, Cu2+, Hg2+, Co2+, Pb2+, Mg2+, Sn2+, Al3+, and Cr3+, by turning on fluorescently to specific Cr(III) ions [49]. Whereas an inner filter effect-based Schiff base chemosensor 16 was adequately quenched by Cr(VI) through primary and secondary inner filter effects. The addition of L-ascorbic acid in the concentration range of 10 μM to 390 μM with an LOD

*Recent Advancements in Schiff Base Fluorescence Chemosensors for the Detection of Heavy… DOI: http://dx.doi.org/10.5772/intechopen.109022*

of 2.46 μM efficiently turned on the chemosensor-Cr(VI) solution's switched-off fluorescence. The reduction of Cr(VI) to Cr(III) by L-ascorbic acid led to the elimination of both primary and secondary inner filter effects and the recovery of the chemosensor's fluorescence, which is the mechanism proposed for the fluorescence turn-on of the Cr(VI)-chemosensor's quenched fluorescence [50]. The structures of the chemosensors 12-15 and their interaction with Cr3+ ion are shown in **Figure 4**.

#### **5.4 Detection of palladium ion**

A valuable metal, palladium is used in several industrial and electronic devices. However, palladium is extremely carcinogenic and poisonous. The detection of Pd2+ ions in various environmental, biological, and agricultural materials is therefore crucial. For the purpose of detecting Pd2+ ions, the Schiff base fluorescent and colorimetric chemosensor 17 was employed. High selectivity toward Pd2+ ions was demonstrated by this chemosensor in the presence of monovalent, divalent, trivalent, and tetravalent cations such as Ba2+, Cu2+, Zn2+, Fe2+, Tb3+, Hg2+, Eu3+, Mg2+, Gd3+, Mn2+, Sm3+, Cr3+, Ag+ , Ca2+, Sn4+, and Ni2+. Mass, 1 H NMR, 13C NMR, and FT-IR analyses all validated the spirolactam ring opening as the chemosensor's mechanism. Chemosensor 17's Pd2+ detection threshold was established at 0.05 μM [51]. In the presence of the ions Ba2+, Cu2+, Zn2+, Al3+, Fe2+, Hg2+, Mg2+, Co3+, Mn2+, Ag+ , Ca2+, Ni2+, Cd2+, Pb2+, Fe3+, Pt2+, Na+ , and K+ , Coumarinyl-rhodamine Schiff base chemosensor 18 functions as an incredibly selective chemosensor for Pd2+. The spirolactam ring being opened has been suggested as the mechanism for this chemosensor. The Pd2+ ions improve the fluorescence intensity and change the color of the chemosensor from straw color to pink, whereas the free chemosensor was weakly emissive. In the presence of chemosensor 18 and distinct metal cations, this chemosensor was highly selective for the Pd2+ ion [52]. The structures of the chemosensors 17–18 and their interaction with Cr3+ ion are shown in **Figure 5**.

#### **5.5 Detection of Arsenic ion**

Despite being extensively distributed in nature, arsenic is a deadly heavy metal even at low amounts. It builds up in the body and causes a variety of illnesses, such as lung and skin cancers, interference with cellular respiration, and affects the functions of the liver, kidney, bladder, and prostate [53–55]. Therefore, the detection of As3+ ions in different samples is of great consideration. A recent study described the detection of arsenic species found in a real naphtha sample using a fluorescence chemosensor 19 based on excited-state intramolecular proton transfer (ESIPT). The ESIPT process of M-HBT was demonstrated by the pH-related emission migration from 610 nm (pH 14,1) to 510 nm (pH 14,13). The sensing ability of M-HBT was examined in ethanol, and the experimental findings showed that As3+ responded linearly well in the range of 0–32 μM [56]. Similar to this, for the quick detection of As3+/As5+ in mixed aqueous solutions, a novel colorimetric sensor based on a benzothiazole Schiff base chemosensor 20 has been reported. It offers advantages of simplicity, specificity, high sensitivity, selectivity, and economy with a detection limit of 7.0 ppb within 10 s via the naked eye. Moreover, the probe is appropriate for on-site, quick, and convenient detection of As3+/As5+ ions at extremely low concentrations in actual water samples and exhibits remarkable selectivity in the presence of other common ions [57]. A novel coumarin-based fluorescent chemosensor 21 was employed in another study to find As3+ ions. In the presence of secondary metal ions (Na+ , Mg2+, Cu2+, Ni2+,

**Figure 4.** *Formation of Cr3+ complexes with Schiff base chemosensors 12–16.*

Co2+, Zn2+, Pb2+, Fe2+, Fe3+, Hg2+, Ca2+, Cd2+, and Mn2+), the chemosensor shows excellent selectivity toward As3+. The development and evaluation of ArsenoFluor1, the first example of a chemosensor was done for the detection of As3+ in organic solvents at 298 K. At λem = 496 nm in THF, AF1 shows a 25-fold increase in fluorescence that is selective for As3+ over other physiologically relevant ions (such as Na+ , Mg2+, Fe2+, and Zn2+) and has a sub-ppb detection limit. According to AF1's optical characteristics, a strong broad absorption band with a center wavelength of 385 nm in THF may be seen. This band is predominantly dominated by the coumarin chromophore. Due to effective quenching by the thiazoline N lone pair through a photoinduced electron transfer process, the resulting fluorescence emission maximum at 496 nm exhibits an incredibly low quantum yield (f) of 0.004. The nonconjugated AF1 is basically non-fluorescent. When As3+ is added (as AsI3, but AsCl3 also produces comparable effects), AF1's fluorescence intensity increases by around 25 times. This strong turn-ON response is accompanied by red shifts in the absorbance maxima from

*Recent Advancements in Schiff Base Fluorescence Chemosensors for the Detection of Heavy… DOI: http://dx.doi.org/10.5772/intechopen.109022*

**Figure 5.** *Formation of Pd2+ complexes with Schiff base chemosensors 17–18.*

385 to 464 nm, which are indicative of benzothiazole C6-CF3 production. The commercially available coumarin-6 (analogue of C6-CF3 with hydrogen replacing the CF3 group) and other coumarin-benzothiazole compounds typically exhibit fluorescence via an internal-charge-transfer (ICT) mechanism. As a result, at 298 K, AF1 functions as an efficient OFF–ON fluorescence sensor for As3+ in organic media. The sensing process probably entails bis-coordination of the Schiff-base thiolate form of AF1 to the thiolate anion, which then attacks the C-N carbon and loses a proton to produce the benzothiazole [58]. The structures of the chemosensors 19-21 and their interaction with Cr3+ ion are shown in **Figure 6**.

## **6. Detection of other cations**

A fluorogenic and chromogenic chemosensor 22 with great sensitivity was created. The developed chemosensor was distinguished by contemporary analytical methods. The completely characterized chemosensor was utilized to detect the presence of Au3+ ions in acetonitrile-containing aqueous media in the presence of a variety of competing analytes, including cations (chlorides salts of Au3+, Cr3+, Fe3+, Zn2+, Cd2+, Pb2+, Hg2+, Pt2+, Pd2+, perchlorates of Mn2+, Co2+, Ni2+, Cu2+, and nitrate salts of Ag<sup>+</sup> , and Al3+). The chemosensor 22 only demonstrated a selective response to Au3+ ions and revealed a 696-fold fluorescence increase toward Au3+. The structure of the chemosensor 22 and its interaction with Au3+ ion are shown in **Figure 7**. The limit of detection was determined to be 1.51 M. The chemosensor 22 also had a colorimetric response that ranged from colorless to pink. These imine bond (C- -N)

**Figure 6.** *Formation of As3+ complexes with Schiff base chemosensors 19–21.*

hydrolysis-induced fluorometric and colorimetric responses were caused by Au3+ ions. Additionally, chemosensor 22 was successfully used to determine the presence of Au3+ ions in living MC3T3 cells [59]. For the purpose of detecting Y3+, a new tetraphenylethylene decorated with m-aminobenzoic acid (chemosensor 23) was designed and produced with an 89% yield. Based on the AIE effect, it displayed the far-red fluorescence at 550–670 nm in aqueous environments. It demonstrated strong Y3+ selectivity across all types of metal ions and a clear "turn-on" fluorescence, which was seen for the first time for a Y3+ fluorescence sensor. The detection threshold for Y3+ was as low as 2.23 × 10−7 M. Fluorescence titration helped to clarify the sensor mechanism for a 1:1 stoichiometric ratio. The selective sensing of Y3+ was also effectively used to identify Y3+ for tested paper, a simulated water sample, and bio-imaging of live cells, indicating the strong practical application potential of this fluorescent sensor on detecting Y3+ in diverse water media and living body environments [60]. Similarly, a new fluorescent chemosensor 24 was architected for the recognition of Y3+ ions. The probe 2-hydroxy-1-naphtaldehyde salicyloylhydrazone was described in detail. For the determination of yttrium in THF, a very sensitive fluorescent approach and the fluorescence method were developed based on the chelation reaction; they can detect traces of Y3+ with a naked eye in sunlight [61]. The structures of the chemosensors 23–24 and their interactions with Y3+ ion are shown in **Figure 7**. For the detection of Ga3+, a simple Schiff-base based on fluorene and salicylaldehyde was created. Using NMR titration, ESI-mass spectrometry analysis, photophysical experiments, and DFT simulations, the sensing behaviors of chemosensor 25 with Ga3+ were investigated. In the presence of a variety of cations, including K+ , Cu2+,

*Recent Advancements in Schiff Base Fluorescence Chemosensors for the Detection of Heavy… DOI: http://dx.doi.org/10.5772/intechopen.109022*

**Figure 7.** *Formation of Au3+ and Y3+ complexes with Schiff base chemosensors 22 and 23–24 respectively.*

Co2+, Ca2+, Fe3+, Cr3+, Ag+ , Na+ , Ni2+, In3+, Fe2+, Mg2+, Cd2+, Hg2+, Mn2+, Pb2+, and Al3+, the chemosensor 25 produced a highly stable complex and showed good selectivity for Ga3+. In particular, the detection threshold for Ga3+ on chemosensor 25 was as low as 10 nM [62]. Chemosensor 26 and 27 were recently used for the effective detection of Ga3+ ions. The symmetrical Schiff base chemosensor 26 (N′2,N′5-bis((E)-2 hydroxybenzylidene)thiophene-2,5-dicarbohydrazide) and chemosensor 27 (N′5,N′7 bis((E)-2-hydroxybenzylidene)-2,3-dihydrothieno[3,4-b]) are both produced from thiophene. Fluorescence turn-on behavior was observed for Ga3+ detection. After chelation with Ga3+, both of them demonstrated considerable fluorescence amplification in MeCN/H2O buffer solution. The sensors, on the other hand, displayed ultralow detection limits of 3.90 × 10−9 M and 3.97 × 10−9 M, respectively. A 1:2 stoichiometry between the sensors and the Ga3+ was seen in the Job's plots, with association constants of 3.89 × 109 M2 and 8.59 × 109 M2 , respectively. Theoretical investigations into molecule configuration, charge distribution, molecular orbitals, electronic transitions, and energy change results were produced in order to further explain the differences in sensing performance between the two sensors [63]. The structures of the chemosensors 25–27 and their interactions with Ga3+ ion are shown in **Figure 8**.

**Figure 8.** *Formation of Ga3+ complexes with Schiff base chemosensors 25–27.*

## **7. Performance evaluation**

We reviewed spectrofluorometric technique in this chapter for the identification and quantification of various cations in various environmental, biological, and agricultural materials. With a very low detection limit, many of these chemosensors have demonstrated remarkable sensitivity and selectivity for certain heavy metal analytes. The effectiveness of several Schiff base-based chemosensors for the detection of various cations is discussed. The best performing Schiff bases for sensing various cations were suggested after comparing the detection limit values. For the targeted detection of mercury ions, the chemosensors 1–6 were employed. With a detection limit of 8.3 nM, chemosensor 3 was discovered to be the most sensitive of these chemosensors. Cd2+ was detected using chemosensors 7–11. Fluorescence spectroscopy demonstrated the outstanding performance of chemosensor 7 for Cd2+. The calculated detection threshold is 14.8 nM. Additionally, the use of chemosensor 7 for the detection of Cd2+ in real water samples was successful. Similar to this, when compared to chemosensors 12–16, chemosensor 13 had the best performance for Cr3+ detection. With a low detection limit of 0.94 nM, the chemosensor responded to Cr3+ by turning on its fluorescence. For the detection of Pd2+, two different chemosensors using fluorescence (17–18) were employed. One of these chemosensors, 18 (with an outstanding detection limit of 0.0188 μM), was discovered to be very effective and sensitive for Pd2+. Compared to chemosensors 19, 20, and 21 for the detection of As3+, turn-on chemosensor 20 showed great sensitivity with a detection limit of 7.2 ppb in the aqueous medium. The Schiff base fluorescent chemosensor 22 displayed high sensitivity toward Au3+ with a detection limit of 1.51 μM, while the chemosensor 24 displayed good sensitivity toward Y3+ with a detection limit of 0.013 ppb. These findings come from the discussion on the sensing of Au3+, Y3+, and Ga3+. A detection limit of 3.9 nm for the chemosensor 26 indicated that it had good sensitivity to Ga3+ [64].

*Recent Advancements in Schiff Base Fluorescence Chemosensors for the Detection of Heavy… DOI: http://dx.doi.org/10.5772/intechopen.109022*

The different chemosensors and its analytical parameters for the chemosensing analysis of heavy metals are listed in the **Table 1**.
