Introduction
Zinc is a crucial trace element for human health; it is essential for maintaining
our physical and mental well-being as well as for the growth of our immunity.[1]
[2]
[3]
[4] Intake of either an excess or deficiency of Zn2+ ions can cause several diseases.[5–12] Deficiency of Zn2+ ions in the human body is known to be associated with the onset of Alzheimer’s disease.[13]
[14]
[15] While, its excess can hamper the bacteriostatic power of scavenger cells, which
reduces the immune functions of the body, resulting in decreased capability to fight
diseases.[16]
[17]
[18] In addition, Zn2+ is crucial to plant metabolism because it influences the growth and development of
plants.[19] However, an excess of zinc in the environment can cause serious harm. Given these
difficulties, it is essential to discover and develop sensitive and specific ways
to identify Zn2+ ions that are simple to use and provide a swift response.[20]
[21]
[22] Currently, several techniques are available for metal ion recognition, such as electrochemical,[23] neutron activation analysis,[24] high-performance liquid chromatography (HPLC),[25] atomic absorption and emission spectroscopy,[26]
[27] mass spectrometry,[28] anodic stripping voltammetry,[29] electrothermal atomic absorption spectrometry,[30] capillary electrophoresis,[31] and inductively coupled plasma mass spectrometry.[32] Although these techniques have good selectivity, sensitivity, and rapid response,
they also have some limitations or drawbacks because these techniques can be expensive,
require complex instrumentation handling, and require significant sample pre-treatment.[33]
[34] Therefore, highly sensitive, selective, and efficient methods are still needed for
the detection of Zn2+ ions at trace levels. Two types of promising chemosensors have been attracting significant
attention; namely, colorimetric and fluorometric chemosensors. Both approaches are
targets of vigorous research for the detection of Zn2+ ions, the majority of which are based on Schiff bases.
Schiff bases have a spectacular coordinating affinity with zinc ions and are recognized
as exceptional chelating ligands.[35]
[36] Given their extraordinary properties, Schiff bases are currently employed as efficient
chemosensors for the identification of various cations and anions and electron-deficient
substances such as nitro-aromatics.[37,38]
Colorimetric sensors are a type of chemical sensor that detects analytes by producing
visible color change. Colorimetric sensing[39] offers selective and sensitive detection of various kinds of species such as metal
cations, anions, drugs, toxic waste, biomolecules, and organic dyes. Without the use
of sophisticated devices, the colorimetric chemosensors provide reproducible, low-cost,
effective, fast, and appropriate detection of environmental and biological toxicities
through indicators that are visible to the unaided eye.[40] Internal charge transfer (ICT), which is brought on by interactions between the
chromophore and the hydrogen atoms of the analyte, leads to a considerable shift in
the ability of the chromophore unit to absorb light,[41] which means that such systems have great potential for analyte detection methods.
Table 1 Schiff Base Probes for the Recognition of Zn2+ Ions with the Lowest Detection Limit
|
Entry
|
Chemical structure
|
Ions detected
|
Detection limit (μM)
|
Ref.
|
|
5
|
|
Zn2+
|
0.00223
|
[51]
|
|
9
|
|
Zn2+
|
0.00421
|
[55]
|
|
14
|
|
Zn2+
|
0.00343
|
[60]
|
|
16
|
|
Zn2+
|
0.00272
|
[62]
|
|
18
|
|
Zn2+
|
0.00129
|
[64]
|
|
30
|
|
Zn2+
|
0.00560
|
[76]
|
|
34
|
|
Zn2+
|
0.00028
|
[80]
|
|
44
|
|
Zn2+
|
0.0012
|
[90]
|
|
49
|
|
Zn2+
|
0.00675
|
[95]
|
The fluorometric chemosensors are a chemical family that can be used to easily determine
various types of analytes. When interacting with different species of analytes,[42] such as biological scaffolds, neutral molecules, anions, and cations, fluorometric
chemosensors can detect a specific substance based on its ability to emit fluorescence
because its fluorescent characteristics can undergo photophysical changes. Fluorescent
sensors are frequently utilized as analyte-determining probes for an extensive variety
of biological applications because they exhibit a change in emission spectra, wavelength,
and lifetime decay during molecule recognition.[43] While increasing the wavelength, a probe is deactivated from the excited singlet
state to the ground state, to produce fluorescence with a typical lifespan decay of
10–9 seconds.[44] There are two types of phenomena involved in fluorescence: quenching and enhancement.
Fluorescence enhancement is the process of increasing the intensity of fluorescence
emitted by the fluorescent molecules. Whereas, fluorescence quenching is the process
that decreases the emission intensity of the fluorescent probes.[45] Most of the reported fluorometric and colorimetric probes are based on organic skeletons
that include Schiff base moieties, because of their simple synthetic accessibility
and because of the extraordinary coordination of the binding sites with the ions.[46]
In this article, Schiff base chemosensors that have been synthesized by different
research groups since 2018 to encapsulate zinc metal ions selectively and specifically
using various spectrophotometric techniques are summarized.
It is believed that these Schiff base probes will provide crucial information for
the further development of chemosensors that are more efficient. Table [1] compares the ability of Schiff base probes to recognize Zn2+ ions and summarizes the current lowest detection limits.
Schiff Base Probes for Zn2+ Recognition
For the determination of Zn2+ ions, a fluorescent Schiff base sensor 1 (Figure [1]) was synthesized by Gau Xu and his research group.[47] Xu et al. reported a hydrazone Schiff base based chemosensor that was further characterized
by various spectroscopic techniques. The sensor showed weak fluorescence alone, but
with the incorporation of Zn2+ ions, the intensity of fluorescence enhanced rapidly and exhibited an intense yellow-green
emission. The binding stoichiometry ratio of sensor 1 was observed to be 1:1, as confirmed by the Job plot, and the binding constant was
found to be 184 M–1. The limit of determination was found to be 112 μM and the authors also designed
a test paper strip for rapid detection of Zn2+ ions.
Figure 1 Structures of Zn2+ ion sensing probes 1–4
A responsive multi-stimulated organic fluorescent compound 2 (Figure [1]) was synthesized by Francis Joy and her research group[48] for the detection of Zn2+ ions in a water-based solution. This hydrazone-based Schiff base turn-on chemosensor
was highly sensitive and selective in nature towards the Zn2+ ion. The chemosensor was investigated and showed a remarkable increase in emission
intensity towards the Zn2+ cation in a water-based solution, whereas the intensity drastically decreased for
other metal cations. The stability constant was found to be 4.587 × 103 and the detection limit was 9.315 μM. The researchers also made a film composite
of the synthetic compound to sense Zn2+ ions in the solid state. In addition, the determination of Zn2+ ions from images of nervous cells of rats using this fluorescent sensor 2 was demonstrated.
Li et al.
[49] reported, a novel naphthalene-based Schiff base probe 3 (Figure [1]) for the detection of Zn2+ ions. This synthesized probe showed a turn-on fluorescent nature towards Zn2+ ions. The probe shows good sensitivity and selectivity towards Zn2+ ions in DMF/H2O solution. The sensing was ascribed to a chelation-enhanced fluorescence mechanism
(CHEF) and the probe showed absorbance bands at 280 and 380 nm after adding Zn2+ ions into the solution. When Zn2+ was introduced, the absorption peak of L shifted towards the blue spectrum, shifting
from 312 nm to 308 nm. At 357 nm, a new absorption band was identified. This finding
supports the theory that Zn2+ coordination increased conjugation within the L-Zn2+ system, resulting in the formation of a unique absorption peak. The emission intensity
shown by the probe - Zn2+ ions at 425 nm. The coordination ratio was observed to be 3:1 between the probe and
Zn2+ ion, which was determined with a Job’s plot. The lower limit of detection was 8.94
× 10–8 M and the response time was just a few seconds.
Yang and co-workers[50] synthesized a dicyanisophorone-based fluorescent probe 4 (Figure [1]) for the detection of Zn2+ ions. This synthesized probe was rapidly combined with Zn2+ ions and shows an absorbance band at 500 nm. During the combination of the probe
and Zn2+ ions, color changes were observed from light-yellow to orange. Meanwhile, the intensity
of fluorescence shows a large Stokes shift of 179 nm. The response time was found
to be less than 30 seconds. The limit of detection was calculated to be 4.8 nM, which
was suitable for the detection of Zn2+ ions in real water samples. In addition, a sharp and clear image was obtained in
A549 cells.
To detect the Zn2+ ions, a Schiff base scaffold chemosensor 5 (Figure [2]) was developed by Malini Nelson and her research group.[51] This synthesized chemosensor was characterized with the help of different spectroscopic
techniques and showed high selectivity and responsiveness toward Zn2+ ions in DMSO/H2O solution. The sensor showed a fluorogenic response with Zn2+ ions and weak emission of bluish-green fluorescence accompanied by turn-on and -off
processes. The limit of detection and binding constant for the Zn2+ ions were calculated to be 2.23 nM and 2.3115 × 103 M–1, respectively. In addition, the quantum yield calculated for the sensor and complex
was determined to be 0.008 and 0.8, respectively. Water samples, swab tests, and test
strips were also used to demonstrate the determination of Zn2+ ions. This sensor was also compatible with the detection of Zn2+ ions in A549 cell imaging.
Fluorescent probe 6 (Figure [2]) was designed by Yu and his group[52] for the determination of Zn2+ ions in an aqueous solution. A rapid increase in the fluorescent emission intensity
was observed with Zn2+ ions in the solution that was very specific towards the Zn2+ ions. The molar binding ratio was 2:1 between the probe 6 and the Zn2+ ion. The limit of detection was calculated to be 2.43 × 10–5 M. No cytotoxicity towards HeLa cells was observed.
A benzothiazole-based fluorescent chemosensor 7 (Figure [2]) was reported by Li et al.
[53] The sensing capacity was identified using fluorescence spectroscopy and there was
a high degree of selectivity of the probe towards Zn2+ ions. This sensor works well within a wide pH range of 7–10. The sensor had a stoichiometry
ratio of 1:1, with a detection limit of 0.68 μM. The response time for the detection
of Zn2+ ions was just 30 seconds.
Figure 2 Structures of Zn2+ ion sensing probes 5–7
A bidentate Schiff base ligand 8 (Figure [3]) was synthesized for the determination of Zn2+ ion in water by Viviana et al.
[54] The structure of the synthesized ligand 8 was characterized with the help of FTIR and NMR analyses, and its optical characteristics
were established by UV/Vis absorbance spectroscopy and photoluminescence measurements.
In the UV/Vis spectra, the synthesized probe shows two intense absorption bands that
were observed at 318 nm and 380 nm due to the π-π* electronic transition of the conjugated
aromatic ring and n-π* electronic transition of the amine group, respectively. Fluorescence
studies revealed that luminescent quenching of the ligand occurred upon the addition
of Zn2+ ions to the ligand. Furthermore, a simple modified screen-print carbon electrode
(SPCE) was used to determine the micro detection limit of the synthesized ligand with
Zn2+ ion. For the detection of Zn2+ ion, good linearity, high sensitivity, and selectivity of 50.1 mV/dec were established.
The response time was calculated to be 10 seconds.
The successful synthesis of a phenolphthalein derivative chemosensor 9 (Figure [3]) for the detection of Zn2+ ions was reported by Aydin and Alici.[55] This sensor 9 could be used to selectively monitor Zn2+ ions easily and impart color change in EtOH/HEPES buffer solution. On the addition
of Zn2+ ions into the solution, sensor 9 showed a high enhancement in the fluorescence intensity. The stoichiometry ratio
was found to be 1:2 between the sensor 9 and Zn2+ ions. The detection limit was calculated to be 4.21 nm, determined from a Job’s plot.
This chemosensor also detects Al3+ ions.
Figure 3 Structures of Zn2+ ion sensing probes 8–10
Sahu et al.
[56] synthesized a turn-on fluorescence moiety 10 (Figure [3]) based on the isoindole-imidazole scaffold for the detection of Zn2+ ions. This Schiff base chemosensor was synthesized and characterized with the help
of various spectroscopic techniques such as HRMS, HMR, and FTIR spectroscopy. The
optical properties were investigated using UV/Vis and fluorescence spectroscopic techniques.
From the UV/Vis studies, the absorption peaks of the moiety were identified at 405
nm and 480 nm. This moiety changed color from colorless to pale yellow in the presence
of Zn2+ ions. Gradual increase in the concentration of Zn2+ ions in the solution increased the absorbance peak at 419 nm, which showed the interaction
of Zn2+ ions with probe 10. The moiety shows weak fluorescence intensity with a quantum yield of 0.036, while
the addition of Zn2+ ions to the solution resulted in a 19-fold fluorescence intensity enhancement with
a quantum yield of 0.69. The detection limit was found to be 0.073 μM. The titration
and Job’s plot indicate the formation of 1:1 of moiety/Zn2+ bindings. The binding constant was found to be 3 × 104 M–1.
Han and his research group[57] developed a quinoline-based supramolecular gel 11 (Figure [4]) for the selective detection of Zn2+ ions. This Schiff base probe 11 forms organogels with organic solvents such as acetone, DMSO, DMF, ACN, EtOH, and
1,4-dioxane. This assembly was analyzed by field emission scanning electron microscopy
(FESEM), UV/Vis, fluorescence, and FTIR spectroscopy, and by XRD and water content
angles. Under noncovalent interactions, self-assembly takes place to form nanofiber
structures. Weak fluorescence intensity was shown by the gelator group on the addition
of Zn2+ ions into the solution. The sensitivity and selectivity were also relatively high
towards Zn2+ ions. This shows an enhancement in the fluorescence intensity. The emission intensity
increased from 423 nm to 545 nm, and the limit of detection was found to be 5.5 μM.
Figure 4 Structures of Zn2+ ion sensing probes 11–14
A turn-on fluorescence Schiff base probe 12 (Figure [4]) based on 2-hydroxy-1-naphthylaldehyde was developed for the recognition of Zn2+ ions in real water samples by Mu et al.
[58] The probe was analyzed by absorption spectroscopy and showed three absorption bands
at 262, 325, and 362 nm. A gradual decrease in the absorbance band was observed with
the addition of Zn2+ in the solution. New bands appeared at 337 nm and 428 nm, and a color change was
observed from colorless to yellow. Four isosbestic points were observed at 284, 333,
339, and 387 nm, which indicate the formation of a new complex between the probe and
the Zn2+ ion. The emission intensity was enhanced by the addition of Zn2+ ions to the solution. The complexation constant was found to be 1.13 × 105 M–1. The HOMO-LUMO energy gap between the probe and Zn2+ was 1.49 eV. The stoichiometry ratio was 1:1 between the probe and Zn2+. Moreover, cell imaging fluorescence experiments demonstrated that trace Zn2+ could be observed in living cells with low cytotoxicity.
A visible colorimetric probe 13 (Figure [4]) was designed and synthesized by Venkatesen et al.
[59] Schiff base condensation reaction was performed using this diaminomaleonitrile derivative
to sense Zn2+ ions selectively. The sensing ability of this probe was successfully investigated
with the help of visual and UV/Vis techniques. The absorption band appeared at 595
nm with molar absorptivity 14578 LM–1cm–1. This arises due to the metal-to-ligand charge-transfer phenomenon (MLCT). The response
of the ligand was dependent on the pH of the solution; complex formation easily takes
place within the range of pH 8–11. The stoichiometry and binding constant calculated
with the help of B-H plots were 1:1 and 2.63 × 103M–1. The HOMO-LUMO energy gap between the probe and complex was found to be 2.23 and
2.63 eV, respectively. The points of reference for coordination were (N,N,O,O). The
detection level was found to be 5.8 × 10–7 M. The employment of 13 in logic gates is one of its additional beneficial applications.
A new innovative coumarin-based fluorescent probe 14 (Figure [4]) was synthesized and characterized. The fluorescent probe was shown by Arvas et al.
[60] to exhibit significant fluorescent quenching in the presence of Zn2+ ions. The limit of detection was found to be 3.43 × 10–9 M for the detection of Zn2+ ion. The stoichiometry ratio of the probe and Zn2+ ion, as determined by the Job’s plot linear function was 1:1, and the association
constant was calculated to be 2.39 × 104 M–1. No toxic effect was observed on L929 cells by the MMT method.
He, Cheng, and Zheng[61] synthesized two fluorescent probes 15 (Figure [5]) for the detection of Zn2+ ions in real water. These two probes were synthesized from salicylaldehyde benzoyl
hydrazine using the Schiff base condensation reaction pathway and were characterized
by various spectroscopic techniques. Probe 15 showed weak fluorescence intensity but, on the addition of Zn2+ ion in solution, a very high enhancement in the intensity of fluorescence was observed
even in the presence of many other metal ions. This shows the selectivity and sensitivity
of the designed probe 15 towards Zn2+ metal ions. The calculated molar binding ratio of the probe and Zn2+ ion was 1:1, and the coordinating sites were O and N of the fluorophore. The probe
was also used for bio-imaging after incubation of ECV304 with the probe for 30 min
at 37 °C. A red color was emitted from the intracellular area, which indicates that
15 could be used for dynamic imaging and for tracing Zn2+ ions in living cells.
Figure 5 Structures of Zn2+ ion sensing probes 15–18
A coumarin-tagged diprotic scaffold fluorophore 16 (Figure [5]) was derived by Bhasin and co-workers[62] for the detection of Zn2+ ions in semi-aqueous medium. This multidentate ligand works well with Zn2+ ion, with which it can easily bind, and shows turn-on fluorescence with 0.272 nM
concentration in a semi-aqueous medium. The multidentate ligand has a triclinic crystal
system and P-1 space group with a three-coordinating site. The molar ratio of the
probe and Zn2+ ion binding was 1:1, and the detection limit was calculated to be 0.272 nM. Another
potential application of 16 is in the field of medicine, as Zn2+ ion-binding moieties play vital roles as pharmacophores in certain drugs.
To sense metal ions such as Zn2+ ions, Juan Li and his group[63] synthesized probe 17 (Figure [5]) based on oxime and salicylaldehyde. This chemosensor performed well in the case
of an aqueous medium (DMSO/H2O, 9:1 v/v). The selectivity and sensitivity of probe 17 towards Zn2+ ions were determined using fluorescence and UV/Vis spectroscopic techniques. A weak
absorption band was shown by the ligand at 425 nm, but after the addition of Zn2+ ions to the solution, the absorbance band shifted towards a higher range around 458
nm; i.e, a redshift was observed with a color change of the solution from colorless
to yellow. The UV/Vis characteristics show that the molar ratio of metal and ligand
was 1:2, and the association constant was 5.6 × 104 M–1. For the fluorescence recognition of Zn2+ ions using probe 17, the emission peak of the probe was observed at 437 nm, and, after the addition of
Zn2+ ions, the emission peak moved to higher wavelength at 502 nm. When Zn2+ ions were introduced to the solution the intensity increased by 16 times. It was
determined that the detection threshold was 2.53 × 10–8 M. The HOMO-LUMO energy gap between the probe and complex, based on DFT calculations,
was found to be 3.44 eV. This was utilized in various water samples, including drinking
water, tap water, and water from the Yellow River.
For the detection of Zn2+ ions in water, food supplements, and bio-imaging applications, Aydin et al.[64] designed a new ultra-sensitive sensor using a thiazole derivative. This novel fluorescent
probe 18 (Figure [5]) was excited at 420 nm, and its emission peak was observed at 503 nm in HEPES/MeCN
buffer (5:95 v/v, pH 7.4). The quantum yield of the probe increased from 0.073 to
0.78 with Zn2+ ions. The detection limit and binding constant were calculated to be 1.29 nM and
1.102 × 106 M–1, respectively. The stoichiometric ratio between the probe and Zn2+ ion was 1:1 as determined by a Job plot. The mechanism of fluorogenic response was
explained by photoelectron transfer, and further theoretical DFT studies were also
performed.
A solvent-dependent fluorescent probe 19 (Figure [6]) for the detection of Zn2+ ions was developed by Qin et al.
[65] by using the Schiff base condensation reaction pathway. This chemosensor was synthesized
with the help of the 8-hydroxyquinoline group and used to determine the Zn2+ ions in an EtOH solvent with high sensitivity and selectivity. The stoichiometric
molar ratio of this sensor was determined to be 1:1. It was found to inhibit the PET
process due to the presence of lone pairs on the nitrogen atom of the C=N group, resulting
in an enhancement of fluorescence intensity. The limit of detection was calculated
to be 2.36 × 10–7 M for Zn2+ ion. DFT calculations were performed that verified the binding ratio and HOMO-LUMO
energy gap between the sensor and complex.
Figure 6 Structures of Zn2+ ion sensing probes 19–22
Zhang et al.
[66] designed a fluorescent organic nano-material having well-defined AIE characteristics
for a colorimetric chemosensor. This organic fluorescent coumarin-based material has
been synthesized for the determination of Zn2+ ions. Probe 20 (Figure [6]) shows AIE aggregation-induced luminescence enhancement properties under various
aqueous conditions. The determination of Zn2+ ion concentration was carried out using UV/Vis and fluorescent multi-mode analysis
of the given probe and showed high sensitivity and selectivity or specificity. The
response time of the probe was significantly reduced because of the high instability
of the complex. The limit of detection was observed to be 0.026 μM in an aqueous solution.
The probe displayed green fluorescence in the solid powder phase, which can be used
to sense ZnCl2 powder. The material was also combined with a paper strip and polymer thin-film to
indicate Zn2+ ions with a color change. The probe was also used to detect Zn2+ ions in vivo in zebrafish cell imaging.
Mondal and her research group[67] developed a colorimetric and fluorometric triazine-based sensor 21 (Figure [6]) for the recognition of Zn+2 ions. This new probe has three different binding sites for three metal ions. It is
selective for detecting Zn2+ ions at all the sites of the molecule; i.e., all the three cavities of the probe
21. The probe was present in two forms, one was the enol-form, and the second was the
keto-form. The selectivity of this probe was observed with the help of UV/Vis and
emission spectroscopy. The absorbance of this probe was observed at 330 nm and 338
nm, and after the addition of Zn2+ ions, a new peak appeared at 385 nm with a sharp isosbestic point at 355 nm that
indicates the formation of a new complex. The emission peak was observed at 430 nm.
After the addition of Zn2+ ions, a new peak was observed in the emission spectra also at 468 nm with fluorescence
intensity enhancement of up to five times. The coordination ratio of the probe and
Zn2+ ion was 3:1. The limit of detection was calculated to be 1.22 × 10–7 and 1.13 × 10–7 M for R1 and R2, respectively.
Liu et al.
[68] synthesized and designed a novel diaryl-ethene derivative for the detection of Zn2+ ions. The structure of the synthesized probe 22 (Figure [6]) was characterized with various spectroscopic techniques. The emission spectra displayed
excellent sensitivity and selectivity towards Zn2+ ions, with emission intensity enhancement of 154-fold with bright-blue fluorescence.
The stoichiometric ratio of the probe and Zn2+ ion was 1:1, as confirmed by a Job’s plot. The probe can also detect phosphate and
sulfate via the quenching effect, and another application is that it could also be
used as a logic gate.
A new fluorescent moiety was synthesized by Mathew and Sreekanth.[69] The thiophene-dicarbohydrazide based moiety works well as a colorimetric sensor
for the detection of Zn2+ ions. The chemosensor 23 (Figure [7]) was observed to be highly emissive in nature when it binds with the Zn2+ ions and forms the complex in DMSO/water (6:4, v/v) medium. The binding constant
was calculated to be 1.15 × 104 M–1. The detection limit was found to be 1.51 × 10–7 M, which is well below the permissible level of Zn2+ ion (70 μM) in drinking water listed by the WHO. The binding molar ratio was 1:1
between Zn2+ ions and probe 23. The reversibility and INHIBIT/IMPLICATION logic gate behavior were also shown for
this chemosensor.
Figure 7 Structures of Zn2+ ion sensing probes 23–26
An important fluorescent probe 24 (Figure [7]) was designed by Wang and his research group[70] for the detection of Zn2+ ions in a real water sample. This synthesized probe was shown to have very high selectivity
and sensitivity towards Zn2+ ions. Emission spectroscopy techniques were used to show a large Stokes shift of
110 nM with a change in the concentration of solution of Zn2+ ions from 0 to 20 μM. The limit of detection was 19.1349 nM and the binding constant
was calculated to be 3.24 × 104 M–1 and the stoichiometric binding ratio was 1:1. This probe was sensitive to a wide
pH range. All the interactions of ligand and Zn2+ ions were verified by density function theory studies.
Two similar kinds of Zn2+ ion sensors 25 (Figure [7]) were developed based on a quinoline derivative by Fan et al.
[71] This fluorescent probe was synthesized with the help of a Schiff base condensation
pathway. These two sensors were investigated using UV/Vis and fluorescence spectroscopy.
In the UV/Vis spectrum, these sensors showed peaks at 321 nm and 295 nm, respectively.
The intensity of these bands decreased gradually upon the addition of Zn2+ ions to the probe solution, with the appearance of a new band at 306 nm. Two isosbestic
points were also observed at 341nm and 267 nm, which indicates that the stable complex
was formed between the sensor and the Zn2+ ion. The emission band appeared at 450 nm in an ethanolic solution. Significant enhancement
in emission intensity was observed with the addition of Zn+2 ions. The molar stoichiometric ratio was 1:1 for the sensor and Zn2+ ions. The lower value of detection was found to be 3.5 × 10–7 M.
A simple synthesis of probe 26 (Figure [7]) for the detection of Zn+2 ion was developed by Jiao et al.
[72] This synthesized fluorescent probe was highly sensitive and selective towards Zn2+ ions and was successfully investigated by emission spectroscopy. The Job’s plot and
HRMS experiments showed that the probe 26 and Zn2+ ion binding ratio was 1:1, and that the binding constant was 7.998 × 104 M–1. The detection limit was observed to be 46 μM. The probe could also be applied for
the determination of Zn2+ ions in living C6 cells.
A fluorometric-based chemosensor 27 (Figure [8]) that can easily sense Zn2+ ions in an aqueous medium was established by Kim et al.
[73] This sensor was designed with the help of salicylaldehyde and benzyl carbonate in
the ethanol solvent using Schiff base condensation methods. Emission spectroscopy
was used to investigate its sensitivity and selectivity. The fluorescence emission
of the sensor was observed at around 452 nm, but after the addition of Zn2+ ions into the solution, a significant enhancement of fluorescence intensity was observed.
The binding molar ratio of the sensor and Zn2+ ion was 1:1, as determined by analyzing B-H and the Job’s plot. The association constant
was calculated to be 2.1 × 103 M–1. The LOD was found to be 1.12 μM, which is lower than the permissible value given
by the WHO. The binding interaction was also explained with the help of DFT calculations.
The probe was also used for zebrafish cell imaging.
Figure 8 Structures of Zn2+ ion sensing probes 27–29
Coumarin-based Schiff base fluorescent chemosensor 28 (Figure [8]) was designed for the detection of Zn2+ ions by Fu et al.
[74] The probe displayed high selectivity and sensitivity for Zn2+ ions in a buffered medium (DMF/HEPES) solution. The sensor showed a good fluorescent
response on the addition of Zn2+ ions into the solution, resulting in an enhancement of fluorescence intensity that
can be seen in the visible range by the naked eye. The detection limit of probe 28A and 28B towards Zn2+ ion was separately calculated as 1.03 × 10–7 M and 1.87 × 10–7 M, respectively. These two probes can also be used to detect phosphate anions. Furthermore,
the cell imaging demonstrated that these probes could detect Zn2+ ions in cells in vitro.
A Schiff base probe 29 (Figure [8]) was designed for the determination of Zn2+ ions in acetonitrile/HEPES solution at pH 7.0 by Chang et al.
[75] The probe showed high enhancement in fluorescence intensity towards Zn2+ ions, with a color change from faint yellow to blue. The molar stoichiometric ratio
between the probe and Zn2+ ion was 1:2, which resulted in the inhibition of intramolecular charge transfer and
chelation-enhanced fluorescence process. The mechanism of bonding was also confirmed
by using UV/Vis, fluorescence, HRMS, and NMR techniques, and theoretical calculations
were performed using DFT computational studies. The probe can be used efficiently
to detect Zn2+ ions in living cells. The limit of detection was calculated to be 8.19 × 10–8 M, and the binding constant was found to be 1.75 × 104 M–2.
Lu and his research group[76] successfully designed and synthesized a novel kind of fluorescent moiety 30 (Figure [9]), based on the diarylethene and benzohydrazide unit, to detect Zn2+ ions. The fluorescent and photochromic switching properties have been studied in
detail by UV/Vis spectroscopy. The moiety shows very high sensitivity and selectivity
towards the Zn2+ ion in tetrahydrofuran solvent. The fluorescence intensity was enhanced by 84 times
on the addition of Zn2+ ions in solution and a 44 nm blueshift was also observed after the addition of Zn2+ ions. The molar ratio of sensor 30 and Zn2+ ion was 1:1 and the limit of detection was found to be 5.60 × 10–9 M.
Figure 9 Structures of Zn2+ ion sensing probes 30–32
Ahmed and his research group[77] developed probe 31 (Figure [9]) with the help of malonitrile for the detection of Zn2+ ions. To investigate the sensitivity and selectivity of this probe, UV/Vis spectroscopic
analysis was used, which showed an absorbance band at 487 nm. Further, with the addition
of Zn2+ ions, a light-brown color was observed in the solution, even in the presence of other
metal ions and amino acids. In the case of fluorescence emission, the enhancement
of fluorescence intensity was observed after the addition of Zn2+ ions into the solution. The response time was just a few minutes. This probe works
well with a wide range of pH 3–13. Cytotoxicity and cell imaging of living cells were
also studied with this probe.
For the detection of Zn2+ ions, Karuppian and her group[78] synthesized an indole-derived colorimetric and fluorometric chemosensor 32 (Figure [9]). This probe underwent a tremendous fluorescent enhancement towards Zn2+ ions in DMSO/H2O (9:1 v/v) medium. This shows metal-to-ligand charge transfer operated with the synthesized
probe, which was clearly visible to the naked eye. The binding ratio of the probe
32 and Zn2+ was 1:1, which was confirmed by Job’s plot analysis. DFT computational calculations
were also done successfully for this probe and its complex with Zn2+ ions. This probe could also sense F– ions. One of the probe’s essential applications is in cell bio-imaging to monitor
Zn2+ ions in HeLa cells and living cell imaging in zebrafish embryos.
Acridine-based chemosensor 33 (Figure [10]) was developed to sense Zn2+ ions in aqueous medium by Nunes.[79] It has been found that this sensor is extremely selective for Zn2+ ions when used as a chemosensor for metal ions. With a concentration range of 17.8–600
M, a linear fluorescence enhancement of approximately 230% was attained. Understanding
how the stable complex was formed was made easier by B-H and Job’s plots, and the
stoichiometry ratio of the sensor 33 and Zn2+ ion was found to be 1:2. The lowest level of detection was 5.36 μM. The association
constant was found to be 2.43 × 1014L2mol–2.
Figure 10 Structures of Zn2+ ion sensing probes 33–36
A turn-on fluorescent-based chemosensor 34 (Figure [10]) was synthesized by Rajasekaran and his research group[80] for the determination of Zn2+ ions. The chemosensing behavior of this probe was established with the help of absorption,
fluorescence, NMR, HRMS, and FTIR spectroscopic methods. This probe selectively detects
Zn2+ ions even in the presence of other metal ions. The probe shows absorbance peaks at
335 and 370 nm, but upon the addition of Zn2+ ions into the solution, a slight redshift appeared to 338 and 385 nm. The probe shows
weak emission at 515 nm but enhanced fluorescence intensity was observed on the addition
of Zn2+ ions. The binding constant was calculated to be 9.3 × 107M–1. The quantum yield was 0.016 and 0.173 for the probe 34 and complex, respectively. The limit of detection was found to be 0.28 nM. This probe
could also detect phosphate anions.
For the detection of Zn2+ ions in real water samples, Aziz and his research group[81] synthesized a fluorometric probe 35 and characterized it with the help of different spectroscopic techniques (Figure
[10]). Weak emission intensity was shown by the probe at 586 nm (quantum yield was 0.026)
and further excitation upon the wavelength 360 nm showed enhancement of emission intensity.
A 1:2 stoichiometric ratio was found between the probe 35 and the Zn2+ ion. The probe has a rapid response time, and its limit of detection was found to
be 0.0311 μM. The fluorescence cell imaging demonstration reveals that the probe has
good cell-membrane permeability and can be used for Zn2+ ion detection in living cells.
Yan et al.[82] synthesized a fluorescent probe 36 to detect Zn2+ ions based on aggregation-induced emission using picolyl-amine (Figure [10]). This probe showed very high selectivity towards Zn2+ ions and displayed good Stokes shift (100 nm) after the addition of Zn2+ ions to the probe solution. The color change was visible to the naked eye. The detection
limit was observed to be 1.3 × 10–7 M. Low toxicity and excellent permeability of this probe were beneficial for its
use in living cell imaging.
The development of a Schiff base probe 37 for the determination of Zn2+ ions based on the naphthyl group was established by Li et al. (Figure [11]).[83] This probe displayed absorbance bands at 353 nm and 367 nm in ethanol. Upon the
addition of Zn2+ ion to the solution, its absorbance decreased; i.e., a blueshift was observed with
an isosbestic point at 376 nm that tells us about the formation of new interacted
species. In emission intensity, almost no fluorescence emission was observed, whereas
adding Zn2+ ions to the solution resulted in a sharp and clear enhancement in fluorescence intensity
(63-fold). The stoichiometric ratio of the given complex was 2:1, as determined by
a Job’s plot. The limit of detection was found to be 7.52 M.
Figure 11 Structures of Zn2+ ion sensing probes 37–38
Yun et al.
[84] reported a novel benzimidazole-based fluorescence moiety 38 for the recognition of Zn2+ ions in aqueous medium (Figure [11]). This moiety shows a turn-on fluorescence effect for the detection of Zn2+ ions. The sensor showed weak fluorescence, but upon the addition of Zn2+ ions into the solution, a remarkable emission intensity shift was observed. The quantum
yield was calculated to be 0.0012 and 0.0122 for the moiety and the complex, respectively.
The limit of detection was calculated to be 2.26 μM, which was below the WHO permissible
limits. This probe could be recycled with EDTA. This moiety was also used in the case
of cell imaging to detect Zn2+ ions in zebrafish embryos.
A fluorescent chemosensor 39 for the detection of Zn2+ ions was designed by Yang Li and his research group[85] based on diarylethene hybrids (Figure [12]). The probe acts as a turn-on fluorescence chemosensor to detect Zn2+ ions in MeOH. The chemosensor is highly selective and sensitive in nature over the
other metal ions. This chemosensor shows weak fluorescence emission alone, but after
the addition of Zn2+ ions into the solution, a new strong fluorescence peak was observed with high intensity
(105-fold) and showed a color change in solution from dark- to bright-yellow. The
detection limit was 13.4 nM. This sensor was used to design a logic gate and could
also be used in the case of real water analysis to detect Zn2+ ions.
Figure 12 Structures of Zn2+ ion sensing probes 39–41
Chae et al.
[86] developed a fluorometric sensor 40 for the determination of Zn2+ ions in aqueous solutions (Figure [12]). Fluorescent titration was done to investigate the sensitivity and selectivity
of the sensor. On the addition of Zn2+ ions into the solution, there was a rapid increase in the fluorescence intensity.
The quantum yield was calculated to be 0.0083 and 0.0387 for the Sensor and complex,
respectively. The interaction behavior between the sensor and Zn2+ ion was established with the help of UV/Vis titration, which showed absorbance bands
at 300 nm and 350 nm with two isosbestic points at 277 and 330 nm, respectively. The
limit of detection was calculated to be 0.27 μM. The probe could also apply to live-cell
imaging for HeLa cell imaging in zebrafish.
A water-soluble fluorescence sensor 41 was synthesized based on the benzoimidazolyl-pyridine by Jiang and research group[87] for the detection of Zn2+ ions and picric acid using a cascade mechanism (Figure [12]). To investigate the sensing ability of sensor 41, UV/Vis and fluorometric analyses were performed. When Zn2+ ions were added to the solution, the fluorescence intensity increased rapidly to
415 nm and the color changed from colorless to blue. The emission band of the sensor
alone had a wavelength of 383 nm. A 1.83 × 10–7 M limit of detection was discovered.
Wyss and his group[88] developed a pyridine-containing Schiff base probe 42 for the detection of Zn2+ ions (Figure [13]). This multidentate ligand was examined with the help of absorbance and emission
spectroscopy, and was found to be highly sensitive and selective in nature with a
rapid response time. The quantum yield was calculated to be 1.60% for the complex.
The stoichiometric ratio of the ligand 42 and complex was 1:1, and the lower limit of detection value was 7.2 μM.
Figure 13 Structures of Zn2+ ion sensing probes 42–45
A chemosensor 43 was synthesized by Gao et al. based on a 3-aminobenzofuran carboxamide Schiff base reaction pathway (Figure [13]).[89] The chemosensor has good sensing ability towards Zn2+ ions, which was established by using several spectroscopic tools. This chemosensor
binds with the Zn2+ ion selectively and shows enhancement in fluorescence emission intensity. A color
change was observed from dark- to bright-orange and could be seen by the naked eye.
The binding stoichiometry was observed as 1:1, and the association constant was calculated
to be 1.77 × 105 ML–1. The limit of detection was calculated to be 3.2 × 10–8 M. A logic gate circuit was also developed with the help of fluorescence intensity
optical signals.
With the help of a simple process, a Schiff base probe 44 was developed by Xu and his research group[90] to detect Zn2+ ions based on imidazo[2,1-b]thiazole-6-carboxylic acid and ethoxy hydroxybenzaldehyde (Figure [13]). This probe showed binding capability towards Zn2+ ions and was investigated by using various spectroscopic techniques. This probe shows
excellent fluorescence enhancement with Zn2+ ions in the presence of other metal ions, and shows an enrichment in sensitivity
and selectivity towards Zn2+ ions. The binding stoichiometry was observed to be 1:1 between the probe 44 and Zn2+ ion, and was confirmed by a Job’s plot. The association constant was calculated to
be 2.22 × 105 ML–1. The detection limit was 1.2 × 10–9 M and the fluorescent signals were utilized in the formation of logic gates.
A turn-on fluorescent probe 45 was synthesized for the detection of Zn2+ ions by Mondal and her group (Figure [13]).[91] The fluorescent probe was based on the sulfaguadine Schiff base. This probe has
high selectivity and sensitivity, which was investigated with various photo-optical
tools and several spectroscopic techniques. The limit of detection was observed to
be 37.2 nM, which is much lower than the WHO guidelines. The stoichiometry ratio was
2:1 for the probe and Zn2+ ions in the complex. The complex shows antibacterial activity against gram-positive
and gram-negative bacteria.
Based on acylpyrazolones and benzhydrazide, the novel ligand 46 was found to work well for the detection of Zn2+ ions by Shaikh et al. (Figure [14]).[92] The working of the ligands was investigated with various spectroscopic techniques.
This ligand also showed anti-malarial activity.
Figure 14 Structures of Zn2+ ion sensing probes 46–49
A new Schiff base probe 47 (Figure [14]) was developed by using a condensation reaction path by Chen and research group.[93] This probe shows a rapid response towards the Zn2+ ion. The probe shows an enhancement of fluorescence intensity while detecting Zn2+ ion in an aqueous media; a color change was also observed from yellow-green to intense-green.
The limit of detection of the probe was 1.9 × 10–8 M. Furthermore, a filter paper-based strip-indicator with a response that could be
observed with the naked eye also demonstrated good ability to bind Zn2+ ion. The binding ratio was 1:1.
A simple naphthylamide-based fluorescent sensor 48 was developed for the determination of Zn2+ ion via a Schiff base condensation reaction pathway by Xiang et al. (Figure [14]).[94] This probe shows a very rapid response time (30 s) and high selectivity towards
the Zn2+ ions. The probe also showed a remarkable enhancement in fluorescence intensity. The
binding ratio of the probe and Zn2+ ion was found to be 1:1 based on a Job’s plot, and the association constant was calculated
to be 1.18 × 105 M. The limit of detection was found to be 39 μM, which is lower than the permissible
limit set by the WHO. A plausible mode of binding of the complex was verified by DFT
computational studies.
To remove Zn2+ ions from water, Wang and his research group[95] reported a turn-on fluorescent ligand 49 for the recognition of Zn2+ ions (Figure [14]). The synthesized ligand was investigated using several spectroscopic techniques.
The sensitivity and selectivity of the ligand towards the Zn2+ ion were analyzed by emission spectroscopy. The intensity of fluorescence increased
with the addition of Zn2+ ions to the solution. The stoichiometric ratio was found to be 1:1 and the detection
limit was 6.75 × 10–9 M. This probe was also utilized to detect Zn2+ ions in living tumor cells.
