Introduction

Sensing of cations is gaing more attention by many scientists, including chemists, biologists, and environmentalists. Metals are involved in many vital biological course of action such as transmission, muscle contraction, cell activity, etc. [1,2,3,4,5]. Metals play a crucial role in cell functions. It is involved in electron transfer processes in DNA and RNA synthesis by facile redox chemistry and its high affinity for oxygen [6,7,8,9,10,11,12,13].

Several methods such as F-AAS, AAS, ICP emission spectrometry, spectrophotometry, voltammetry and electrochemical stripping analysis have been developed for detecting metal ions [14,15,16,17,18,19,20]. Conventional process has good accuracy but needed expensive instrument and tedious process for detection, and very less applicability in situ analysis [21,22,23,24,25,26]. UV–visible and fluorescence spectroscopy is most favorable modes over other common technique for the detection of environmental significant samples because its concentration is very less in nature [27,28,29,30,31].

Among various photophysical pathways like Charge Transfer (CT), Electron Transfer (ET), Förster Resonance Energy Transfer (FRET), are generally utilized for reporting the binding induced phenomena [32,33,34,35,36,37,38,39]. In last few decates, many authors sythesise the sensor for the sensing of metal ions and susesfully reported its biological application.

Need for Chemosensors

In the last few decades, the awareness regarding the significance and toxic effect of cation is well understood by the scientist in the field of chemistry, biology and environment [40,41,42]. Various heavy metal ions uses are banned by various international agencies, because of its toxic nature, nonbiodegradable and hence can accumulate in the environment and foodchain [43,44,45,46,47,48,49].

Today, scientist and chemist are trying to develop a cation chemical sensor which can be used for analysis of environmental sample and industrial sample. On the other hand the biosensor is used in medicine and biological application.

Chemosensors for Cations

1 was developed by pyridine-2-hydrozine which shows 7.63-fold fluorescent enhancement for Zn2+ in CH3CN/HEPES(v/v 1:1) [50]. 1:1 complexation formation supported using HRMS. The association constant 1.83 × 105M−1 and LOD for Zn2+ ion 2.2 × 10− 6M and effectively useful in the cell-imaging of Zn2+. 2-((benzylimino)-methyl)-naphthalen-1-ol (2) shows discraminating emmision for Cu2+ and Zn2+ under ACN [51]. The binding constant for Cu2+ and Zn2+ (6.55 ± 0.8) × 103 M− 1 and (4.32 ± 0.8) × 104M−1 respectively and free energy change calculated − 26.44 kJmol− 1 for Zn2+ and − 21.77 kJmol− 1 for Cu2+, negative free energy indictates the thermodynamic feasibility. Probe 3 [52] selective for Au3+ by discriminant colour transform from yellow-to-pink in PBS 4% ethanol pH = 7.4 (Fig. 1). The LOD was determined to be 8.44 × 10− 6M and cell imaging response rates of 3 to Au3+ in differentiated adipocytes are greater than HeLa cells, because lipid droplets in adipocytes act as surfactants.

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Fig. 1
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Left: Au3+ ion-induced (0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0 equiv) changes in the fluorescence spectrum of 3 (20 µM) in PBS buffer containing 4% ethanol at pH 7.4 in the presence of CTAC (50 µM). Right: Fluorescence response of 3 (20 µM) to various metal ions (5 equiv) in PBS buffer containing 4% ethanol at pH 7.4 in the presence of CTAC (50 µM). All the fluorescence data were obtained after 60 min incubation of 3 with metal ions (λex = 364 nm, slit: 3 nm/3 nm). ‘Reprinted from reference number 52 with permission of Elsevier publication’

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4a and 4b [53] absorption data shows a yellow solution to orange-red solution on Hg2+ (λabs.max = 506→532 nm) addition with “naked eye” detection at concentrations 10− 6M. Cellular neurobiological studies were then undertaken extremely low cell toxicity is also evident with this probe. 5a and 5b detect Hg2+ with change of colour from colourless to pink, visible to the naked eye [54]. The quantum yield for 5a and 5b was (Φ = 0.004 and 0.0034), 5a.Hg2+ and 5b.Hg2+ was (Φ = 0.68 and 0.46) and LOD of 5a and 5b 50 × 10− 9M and 10 × 10− 8M respectively. Rhodamine-naphthalimide derivative sensor 6 was used for effective sensing of Cu2+ with large red shift (115 nm) [55]. The detection limit was 3.88 × 10− 7M for Cu2+ and Cytotoxicity testing in MCF-7 cells shows that 6 is almost nontoxic to living cells (Fig. 2).

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Sensing mechanism of chemosensor 6 with Cu2+ ion. ‘Reprinted from reference number 55 with permission of ACS publication’

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A cyano-rhodamine moiety 7 easily undergoes selective Pd2+ induced ‘C–CN’ bond breaking to produce the pink coloration of rhodamine with an 88-fold enhancement of quantum yield [56]. Sensor 7 biological applicability shown by the cell imaging of Pd2+ on HeLa-cells and LOD of the Pd2+ is found to be 0.57 µM. Cyclen based receptor 8 was developed for the recoginition of Zn(II) with enhancement in fluoresecnec spectra [57]. In MeOH, 8 is poorly fluorescent (ϕ ≈ 0.4%) with addition of Zn2+, the fluorescence increases. Stiochiometric was 1:1 and 2:1 for Zn:8 complexes. Sikdar and its coauthors [58] synthesised a rhodamine-B-based 9 for detection of Cu2+ ion aqueous media and inside living cells. The 1:1 complex is confirmed, bindng constant is found at 2.5 × 104 and LOD at 1.22 µM. Pyrene-based 10 (ϕ ≈ 0.001) was synthesized as Fe3+ sensor in biological environment (ϕ ≈ 0.0.041) [59]. The 1:1 stiochiometry 10.Fe3+ is estimated by Job’s method and ESI-MS and association constant found to be 1.27 × 104M−1.

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Wang et al. [60] synthesized 11 for Zn2+ sensing, with red fluorescence in methanol (λem ≈ 630 nm, Фfl.≈0.8). A probe 12 based on 2,2′,6′,2′′-terpyridine for the detection of Zn2+ (green) and Cd2+ (blue) [61]. The association constants (log Ka) of the probe 12 for Zn2+ and Cd2+ are found to be 3.75 and 3.62 respectively. The detection limit of 12 for Zn2+ and Cd2+ found to be 1.63 and 2.81 ppb respectively. Probe 13 shows colorimetric colour [62] and significant enhancment in absorbance (Fig. 3) change on the micromolar level addition of Cu2+ and the LOD at 7.27 × 10− 7M− 1.

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Change in color of 10 µM 13 in CH3OH/HEPES (20 mM, pH7.2, 1:1,v/v) semiaqueous solution with 5-time cation from left to right: Cu2+, Hg2+, Ag+, Al3+, Fe3+, Cr3+, Co2+, Ni2+, Pb2+, Na+, K+, Ba2+, ca.2+, Cd2+, Zn2+, Mg2+ and only 13. ‘Reprinted from reference number 62 with permission of Elsevier publication’

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Chawla et al.. synthesized 14 a calix[4]arene derivatives used for the detection of Cu2+ [63]. Analysis of the data obtained from Job’s-plot revealed the 1:1 molecular complexation. A peak at m/z 1170.5413 in ESI-MS data showed the formation of 1:1 complex [8 + Cu2++ClO4]+. The detection limit was found to be 6.02 × 10− 6M and association constant, value 1.655 × 106M − 1. Kuwar et al.. synthesized sensor 15 characterized by X-ray crystallography and explore its sensing ability towards Cu2+ [64]. 1:1 stoichiometry formed between 15 and Cu2+ and association constant calculated by B-H and S-P at (43000 ± 11)M− 1. Probe 16 based on 1,8-naphthalimide and rhodamine shows green-to-orange transformation with addition of Cr3+ in CH3CN–HEPES buffer solution. The LOD found to be at 0.14 nM and 1:2 complexation. 16 is successfully applied to detect Cr3+ in cell lysate and blood serum [65]. Chereddy et al.. synthesized the naphthalimide based probe (17) which demonstrated high emission intensity upon Fe3+ addition [66]. The binding constants and LOD were found to be 1.04 × 105M−1 and 3.0 × 10− 8M, respectively. The 17 is stable, nontoxic and ‘turn-on’ for the imaging of intracellular Fe3+ ions.

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A TACN (1,4,7-triazacyclononane) derivatives of 18a and 18b which has ability to recognize the zinc ion [67]. The fluorescence quantum yield of 18a-Zn (ϕ = 0.070) is 2.6-fold greater than free 5 (ϕ = 0.027), and more than twice that of 18b-Zn (ϕ = 0.032) with 1:1 complexation. A 5-diethylamino-2-(quinolin-8-yliminomethyl)-Phenol probe (19) which is capable of detecting multiple ions, Mg2+ and Zn2+, fluorescence and Co2+ by UV/Vis [68]. The LOD for Mg2+ and Zn2+ was 70 nM and1.85 µM. The binding constant for the complex of Mg2+, Zn2+ and Co2+ was calculated to be 8.17 × 106M−2, 1.03 × 106M−2 and 1.78 × 1022M−2 respectively. Hu et al.. [69] developed a sensor 20 for the effective sensing of Hg2+ with limit of detection very low at 9.56 × 10− 9M and furthermore it is used as test kit for Hg2+ sensing. In 1H-NMR titration experiments chemical shifts of NH appearing at a low-field of the probe 20 at 13.21 ppm, and led to the sensor 20 of the conjugate rigid plane structure.

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Fegade et al. developed probe 21 for selective detection of Cu2+ [70]. DFT indicates 21:Cu2+ showed the ICT, lesser the band-gap between the HOMO-LUMO of 21 which due to change in spectra (Fig. 4). The LOD was calculated at 50 nM and Kb at 11667M− 1. Rhodamine-B derivative probe 22 is colorimetric and fluorescent naked eye sensor for Hg2+ [71]. Quantum yield of 22 was < 0.05%, quantum yield of 22-Hg2+ was found to be 14.6%. The stoichiometric ratio is 1:3 and LOD found to be 0.75 ppb. A PET fluorescence chemodosimeter 23 was designed for Cu2+ detection [72]. Upon addition of Cu2+, 23 solution changed from pink to green and acts as a colorimetric chemodosimeter. The 1:1 stoichiometry estimated by Job’s plot and the LOD at 2.3 × 10− 7M. Roy et al.. [73] synthesized probe N,N/-bis(salicylidene)trans1,2–diaminocyclohexane (24) has chemoselective Zn2+ sensor. The quantum yield increases drastically from 3.6 × 10− 3 for 24 compared to 1.8 × 10− 1 for 24–Zn2+ complex. 1:1 complexation estimated by continous variation method and the Ka was at 3.7 × 104M−1. Rhodamine-B derivative sensor 25 was developed, which exhibits effective sensing for Hg2+ in ethanol solution [74]. 1:1 stoichiometry estimated by Job’s plot and LOD at 3.1 × 10− 6M. The filter paper in the present Cu2+ or Hg2+ showed a pink color change, only Hg2+ induced a strong yellow color change under UV lamp.

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a Receptor 21 and 21.Cu2+, b LUMO, c HOMO of 21 and 21 − Cu2+ complex. ‘Reprinted from reference number 70 with permission of Elsevier publication’

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Probe 26, can selectively detect Co2+ in CH3OH/H2O (70:30,v/v) solution [75]. DFT study shows the tautomeric form of 26 found to be less stable by 11.37kcalmol− 1 and 26.Co2+ lowering of energy by -358.88kcalmol− 1 which indicates stable Complex. 1:1 complexation was determined using continous variation method and Ks at 50000M− 1. 27 exhibited a remarkable selectivity for Zn2+ and its absorbance at 256 nm gradually increased with addition of Zn2+. Fluorescence emission of 27 exhibits at 408 nm with Φ = 0.069 and with the addition of Zn2+ the 81 nm red-shift from 408 to 489 nm with Φ = 0.138. DFT shows the HOMO–LUMO energy gap for the 27–Zn complex (3.34 eV) is smaller than 27 (4.20 eV) [76]. Zhang et al. [77] developed chemosensor (28) for selective detection of Hg2+. In the UV-vis spectrum of 28 shows broad absorption from 405 to 490 nm vanished when Hg2+ were added. 28 emitted very weakly (λem = 428 nm; λex = 345 nm), demonstrating that the predictable fluorescence of the naphthalene units was quenched via transformation in ICT state. A probe, (E)-3-(3-(4-([2,2’:6’,2’’-terpyridin]-4’-yl)phenyl)acryloyl)-7-(diethylamino)-2H-chromen-2-one (29), used for sensing of Zn2+ with low LOD at 10 nM and also shows imaging of Zn2+ in cells as applications in cell-imaging [78]. DFT shows when Zn2+ coordinated with the 29, the electrons are located on the coumarin part in the ground state, but are rearranged to the part from carbonyl to the terpyridine group when excited, showing ICT effect. 30 had a high affinity and selectivity towards Zn2+ and shows detection of Zn2+ in intracellular environment of HeLa cell [79]. The detection limit for Zn2 at 35 nM and binding constant Ka was about (2.1 ± 0.3) × 105M−1 (Fig. 5).

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Fig. 5
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A Control cells devoid of receptor 30 and Zn2+, B HeLa cells with receptor 30, C Fluorescence image with only receptor 30, D Fluorescence image with receptor 30 and Zn2+, E In higher magnification uptake of Zn2+ is visible [arrow head]. F Fluorescence image with only Zn. ‘Reprinted from reference number 79 with permission of Elsevier publication’

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Mian Wang et al. synthesized “turn-on” chemosensor (31) for Pd2+ [80] with LOD calculated at 2.4 nM. 1:1. Job-plot and HRMS provided evidence for the formation of a 1:1complex of 31-Pd2+ (Figs. 6 and 7). Probe 32 demonstrated high selectivity by discriminating Al3+ over other metal ions and LOD at 21.6 nM [81]. Furthermore, to check its biocompatibility sensing of Al3+ has been done in live cells. Azodye–rhodamine based probe (33) has been designed [82] for selective detection of Pd2+ with 40-times increses in fluorescence by red-shift. The 1:1 stoichiometry determined by Job’s plot and LOD 0.45 µM at pH 7.4. Furthermore, the probe 33 used to image Pd2+ in living cells. A reversible fluorescent-colorimetric imino-pyridyl bis-Schiff base receptor 34 (N1E,N4E)-N1,N4-bis(pyridine-4-ylmethylene)benzene-1,4-diamine for the detection of Al3+ in aqueous medium [83]. The LOD at 0.903 µM for Al3+ is very low than the limit recommended by WHO (7.41 µM). 1H-NMR titration and MASS shows that the formation of 1:2 complexation. Zhiyuan Zhang et al. synthesised probe 35 and used for the effective detection of Cr4+[84]. The sensing mechanism was explored by reversibility and LC/MS, and the results suggested that the recognition was based on the oxidation of the primary alcohol in the structure of the sensor by the Cr4+ sources.

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Proposed Mechanism for the Identification of Pd2+ by 31. ‘Reprinted from reference number 80 with permission of ACS publication’

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Colorimetric detection of Pd2+ by 31 test paper. ‘Reprinted from reference number 80 with permission of ACS publication’

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Yan-Cheng Wu et al. synthesized three bisbenzimidazole derivatives 36 (Fig. 8) as dual-functional fluorescent and visual sensors for effective sensing of Ag(I) and Fe(III) with a response time of 10 s and steadily work in wide pH 4–13 range. [85]. Jitendra Bhosale et al. explored the selctive sensing of Zn2+ cation by pyrrole-based derivative 37 [86]. The sensing behaviour has been supported by UV-vis absorption and DFT calculations indicating the formation of a 1:1 complex between the pyrrole based receptor 37 and Zn2+. The asprepared 37 was further used for cell-imaging. A phthalazine based sensor 38 was designed for sensitive detection of Co2+ in CH3CN–H2O with LOD at 25 nM. Sensor 38 show that change in colour from yellow to green with red-shifted from 383 nm to 435 nm in the presence of Co2+ [87]. The sensor 38 has capability to monitoring Co2+ in cell-imaging. Rahul Patil et al. developed probe 3-((2-(1H-benzo[d]imidazol-2-yl)phenylimino)methyl)benzene-1,2-diol (39) for selective sensing of Hg2+ with LOD low at 0.20 mM [88]. The emission spectrum of probe 39 was quenched on complexation with Hg2+ ion. Chemosensor, 2-((E)-(-2-aminophenylimino)methyl)-6-isopropyl-3-methylphenol (40), has been designed and confirmed by the single crystal X-ray [89]. The binding constant values for Ni2+ and Cu2+ were calculated to be 25000 and 30000M− 1 and detection limits of Ni2+ and Cu2+ were 100 and 50 nM.

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a Ultra violet – visible spectra of 36 adding up of sixteen time of Fe(III) and hundred time of further ions. b Emmision of 36 existence of hundred time ions

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Fluorescent receptor (41) was designed for the detection of Cu2+ and Zn2+ with 1:1 complexation and LOD at 5 nM and 15 nM for Cu2+ and Zn2+ ions, respectively. [90]. Furthermore, it is used as INHIBIT type logic gate at molecular level (Fig. 9). Di Zhou et al. synthesized highly selective fluorescent chemosensor for aluminum(III) ions in DMF [91]. The outcome of 1H-NMR titration, HRMS and density functional theory shows that 42-Al3+ form a 1:1 complexation. The Ka was calculated at 2.67 × 106 and the LOD was found at 10− 6M. Jie Cui et al. designed a fluorescent probe 43 which has selective recoginition ability for Pd2+ which have 21.3 nM LOD [92]. A selective fluorescent sensor 44 based on a pyrazoline derivate was synthesized and applied for the detection of Al3+ ion with 1:1 complexation via fluorescent quenching [93]. The Ka value obtained at 1.75 × 105M−1 and LOD found to be 2.27 × 10− 7M.

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A The changes in the fluorescence spectrum of receptor 41 and B the fluorescence intensity at 450 nm in the absence and presence of Zn(II) and Cu(II) for the fabrication of inhibit gate. ‘Reprinted from reference number 90 with permission of Elsevier publication’

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Lianqing Li et al. successfully developed a rhodamine derivative chemodosimeter which shows naked-eye fluorescent in the presence of Pd2+ [94] (Fig. 10). The detection limit of 45 at 10− 7M level and H-G ratio was found at 1:2 according via continuous variation jobs method. Rhodamine functionalized fluorogenic Schiff base 46 was synthesized [95] It exhibited highly selective colorimetric and “off-on” fluorescence response towards Al3+. Kb and LOD of Al3+ to 46 are calculated at 1.0 × 104M−1 and 1.4 × 10− 7M, respectively. Berberine (47), an important medicinal herb which effectively utilized as a sensing probe for silver ion [96]. The 47 is found to be selective towards Ag + with a detection limit of 0.1 × 10− 4 molL− 1. The effective quenching of 47 uponbinding with Ag+ ion is attributed to suppression of intramolecular charge transfer (ICT). Quinoline-base sensor 48 showed a highly selective fluorescent enhancement towards Mg2+[97]. The association constant of a 1:1 complex was determined as 1.91 × 107M−1 and the detection limit was determined as 19.1 ppb. Ahmadreza Bekhradnia et al. reported nitro-3-carboxamide coumarin derivatives (49), fopr selective sensing of Cu2+ in (HEPES:DMSO) 9:1,v/v) solution [98]. The emmision of 6-nitro-N-[2-(dimethylamino)ethyl]-2-oxo-2H-chromene-3-carboxamide enlarged on the adding up Cu2+ with stronger excitation at k = 320 nm than for the other cations tested.

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Fig. 10
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UV–vis changes of 45 (10 lM) with 5 equiv. of Pd2 + in CH3CN–H2O. ‘Reprinted from reference number 95 with permission of Elsevier publication’

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A dye 50 (Fig. 11) is found to be highly specific in detecting of Al3 + ion with sensitivity of 0.19 mM [99]. Moreover, the dye shows moderately cytotoxicity and can be employed for the detection of intracellular concentration of Al3+ ions in living cells. The energy gap between HOMO and LUMO in the probe 50 and 50–Al3+ complexes are 2.6691 eV and 2.1089 eV respectively (Fig. 12). Xu Zheng et al. synthesized a bis(pyridine-2-ylmethyl)amine derivative 51 displays significant colorimetric and fluorescent changes upon binding of Cu2+ [100]. 51 has potential candidate for the Cu2+ sensing in aqueous solution and mammalian cells. Qi Huang et al. developed a ‘‘off–on’’ fluorescent sensor for the detection of Al3+ with a high sensitivity and detection limits of 0.23 and 1.90 lM [101]. The sensor 52 was used for the effective sensing of Al(III) furthermore it can be used as a bioimaging reagent for imaging of Al3+ in living cells.

A dicyanoisophorone-based turn-on chemodosimeter 53 has been synthesized to detect Cu2+ with significant color change [102]. The LOD of chemodosimeter 53 was calculated as low as 0.2 µM for Cu2+. The probe 53 was also successfully applied to fluorescence imaging of Cu2+ in HeLa cells. Enze Wang et al. synthesized 54 by condensation of 5-Hydroxymethylfurfural and rhodamine B hydrazide which indicate high selective and reversible colorimetric chemosensor for Cu2+ [103]. The high absorbance at 565 nm, molecular fraction close to 0.33, which exhibit the 1:2 complexation (Fig. 13).

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Chemical structure of Al3+ ion selective receptor 50

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π-MO’s distribution and energy gap between HOMO and LUMO of 50 and 50–Al3 + complex. ‘Reprinted from reference number 99 with permission of Elsevier publication’

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Proposed mechanism of 54 for H + and Cu2 + respectively. ‘Reprinted from reference number 103 with permission of Elsevier publication’

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Jing-Can Qin et al. synthesized 55a and 55b fluorescent probes for Al3+, upon addition of Al3+, they exhibit a large fluorescence enhancement by PET process [104]. More importantly, the lowest detection limits of the sensors for Al3+ were determined as 4 × 10− 8M and 8 × 10− 8M (Fig. 14). Vinod Kumar Gupta et al. synthesized probes 56a and 56b which displayed excellent “off-on” fluoregenic selectivity with Zn(II) [105]. The results revealed that the sensors provided colorimetric and fluoregenic sensing excellent response with low limit of detection, under neutral conditions. Hak-Soo Kim et al. designed 57 which displayed “OFF–ON–OFF” dual responce for the sensing of Cu2+ and Al3+ [106]. LOD of the 57 for Cu2+ and Al3+ were 4.726 × 10− 7 and 4.43 × 10− 7M, respectively. The 1:1 complexation was anticipated between 57and Cu2+/Al3+ by the 1H NMR binding studies.

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Proposed mechanisms for detection of Al3 + by receptor 55a and 55b. ‘Reprinted from reference number 104 with permission of Elsevier publication’

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Kandasamy Ponnuvel et al. designed chemosensor 58 which shows extremely good sensing ability towards Zn2+ ions (Fig. 15) [107]. 1:1 complexation formation is supported by Job’s plot and its can be employed for fluorescent imaging of Zn2+ (Fig. 16). Jiao Geng et al. developed 4,4,-n-butyl-5,5-(pyridin-4-yl)-2,2,-bithiazol (59) and it exhibits high senstivity toward Fe3+ with LOD at 0.6 µM. The Ka of [Fe592] is determined at 2.76 × 103M−2[108]. The living cell and zebrafish imaging experiments demonstrated its applicability in biological systems. Fluorenyl-diformyl phenol Schiff (60) base shows maximum intensity upon addition of Al3+ at 600 nm and the LOD is 6.22 × 10− 9M. [109]. Applicability for DSSC device fabrication of probe shows photovoltaic efficiency 0.021%. Probe N,N–bis((2–hydroxynaphthalen–1–yl)methylene)malonohydrazide (61), exhibited selective detection of Al3+ ions with a binding constant KB=5.74 × 109M−1 and detection limit 5.78 × 10− 8M [110]. The (612Al3+) complex mechanism was studied by DFT and cell-imaging study shows that the aluminium ion in cells can be detected by 61.

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Proposed mechanism of 58 with Zn2+. ‘Reprinted from reference number 107 with permission of Elsevier publication’

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a Image of cells with QT-1, b bright-field image of 58 treated MCF7 cells, c Image of MCF7 cells with 58 and ZnCl2 (λem = 460 nm). ‘Reprinted from reference number 107 with permission of Elsevier publication’

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Chemosensor 62 displayed 16-time increases in fluorescence for selective detection of Mg2+ with limit of detections for Mg2+ at 10− 8M in DMSO:H2O (1:5v/v) medium [111]. Sensor 62 has ability to detect Mg2+ in cells (Fig. 17). The thioethers 63a and 63b have shown excellent selective recognition toward Hg2+ with detection limit 6.93 × 10− 7M and 4.79 × 10− 7M respectively [112]. Moreover, ferrocenyl-based sulphone 63c and 63d which exhibited more selective recognition toward Cu2+ and the detection limit values can reach 5.22 × 10− 7M and 4.97 × 10− 7M (Fig. 18). A pyridylvinyl-rhodamine-naphthalimide fluorescent probe 64, was synthesized which shows recognition ability towards Fe3+ and Hg2+ [113]. The 64 represented dual-channel behavior, with detection limit at 2.72 × 10− 8M and 9.08 × 10− 8M, and the Kd were calculated to be 4.95 × 10− 7M3/2 and 6.68 × 10− 8M3/2, respectively. The probe 65 only showed an enhancement and quenching (naked-eye colour change) in fluorescence for Hg2+ and Cu2+ respectively [114]. Additionally, the probe was effectively used in cell imaging, indicating its promising application in living cells. A rhodamine derivative (66) designed for the detection of Cd2+ with LOD of 1.025 × 10− 8M [115]. New emmision peak produce at 590 nm due to recognition of Cd2+ ions with 66 in a 1:1 cmplexation with a Kb of 4.2524 × 104M−1. 66 has exhibited extremely superior results in Cells imaging.

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PET mechanism of 62 with Mg2+ ions. ‘Reprinted from reference number 111 with permission of Elsevier publication’

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Chemical structure and synthesis of receptors 63a to 63d. ‘Reprinted from reference number 112 with permission of Elsevier publication’

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A Schiff-base 67 was synthesized and used for detection of Zn2+ with on the CHEF and PET mechanisms [116]. The UV–vis spectra of the Zn2+ complex exhibit four clear isosbestic points which may be assigned to the ONON moieties binding to Zn2+ and complex crystal characterized by X-ray crystallography (Fig. 19). Ujjal Ghosh et al. synthesized meta-di-4-methylpyridyl benzene 68 probe for Hg2+ detection [117]. The probe shows a dual fluorescence emission at long wavelength region (350, 425 nm) and theoretical calculation and NMR titration suggest that the probe binds Hg2+ through the coordination with two pyridyl nitrogens. A schiff base (69) based on 4,5- diazafluorene used for selective sensing of Al3+ ions and it shows 1312-time fluorescence enhancement when the Al3+ added in the 69 [118]. The LOD found to be at 3.7 × 10− 8M. A sensor (70) designed for the effective detection of Cu2+ through the PET mechanism [119]. The sensor showed “off–on” fluorescence response with a 120-fold increase toward Cu2+, and its limits of detection were 0.26 mM and 0.17 mM for UV-vis and fluorescence measurements, respectively.

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Chemical structure of sensor 67 and the proposed receptor-metal chelation mechanism. Two metal ions and two ligand molecules are used to express 1:1 stoichiometry of complexation for clarity purpose. ‘Reprinted from reference number 116 with permission of Elsevier publication’

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Chemosensors (71) which shows excellent sensitivity for Hg(II) with LOD at 1.06µmolL− 1 [120]. 1:1 complexation is confirmed by 1H-NMR titration (Fig. 20) and interference study (Figs. 21) shows that the good capability of chemosensor for detction of Hg2+ in presence of other ions. Rui Yan et al. fabricated a biomimetic chemosensor, 72 used for the detection of zinc(II) and cell imaging [121]. Moreover, cytotoxicity and bio-imaging tests were conducted to study the potential bio-application of the chemosensor. Xianjiao Meng et al. synthesized chemosensor, ethyl(E)-2-((2-((2-(7-(diethylamino)- 2-oxo-2H-chromene-3-carbonyl)hydrazono)methyl)quinolin-8-yl)oxy)acetate (73), which showed an “on–off” fluorescence response to Pb2+ with a 1:1 complexation and LOD determined to be 0.5µmM [122]. A “off-on-off” probe (74) displayed the selective detection towards Hg2+ with Ka estimated to be 4.66 × 106 and LOD calculated as 2.64 × 10− 8M [123]. Furthermore, the HOMO-LUMO energy gaps of the complex are lower than the free probe (Figs. 22 and 23).

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A Mechanism for 71 and 71.Hg2+. B 1H-NMR of 71 in the presence of different cncentration of Hg2+. ‘Reprinted from reference number 120 with permission of Elsevier publication’

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A Image of 71 and with the additin of metals under irradiation of 365 nm UV light. B Interference study. ‘Reprinted from reference number 120 with permission of Elsevier publication’

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Proposed sensing mechanism of 74 with Hg2+ ion. ‘Reprinted from reference number 123 with permission of Elsevier publication’

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Optimized structural geometry of probe and its complexes (74 and 74-Hg2+ from left to right). ‘Reprinted from reference number 123 with permission of Elsevier publication’

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Somnath Khanra et al. synthesized imine and azine derivatives 75a, 75b, 75c and 75d, shows fluorescence turns ON for Zn2+ detection at nano-molar level. The LOD of 75a, 75b, 75c and 75d for Zn2+ are 32.66 nM, 36.16 nM, 15.20 nM and 33.50 nM respectively [124]. Pravat Ghorai et al. represented economic probe (76) for efficient detection of both Zn(II) and Cu (II) with the formation 1:1 complexation confirmed by Job’s Plot. The LOD values of both the ions are 2.29 × 10− 9M and 3.67 × 10− 9M, respectively. The probe used for Candida albicans cell fluorescence imaging [125]. Yingying Zhang et al. reported water-soluble probe 77 for the detection of Hg2+ in real water samples and showing changed from pale yellow to pink. Probe was confirmed to have low cytotoxicity and excellent cell membrane permeability [126]. Serkan Erdemir et al. developed Triphenylamine appended rhodamine (78) (Fig. 24) was built as a selective fluorescent probe for Al3+ and Hg2+ ions with 1:1 complexation confirmed by by job plot analysis. The LOD of 78 for sensing Al3+ and Hg2+ are down to 71.8 nM and 0.48 µM, respectively [127].

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Proposed mechanism for Al3+ and Hg2+ detection by probe 78. (Reprinted from reference number 127 with permission of Elsevier publication)

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Kalyani Rout et al. fabricated a triazole-based probe (79) which have good sensing ability for Cu2+ and Pb2+ ions with the naked eye colour change. The 79 shows its potential application in real samples, living cells and building of molecular logic gate [128]. Yunfan Yang et al. developed fluorescence probe 80 for detecting Hg2+ and OCl− ions. The FMO and overlap between hole and electron analyses confirmed that the interaction between 80 and Hg2+ impeded the ESICT behavior [129]. Vishaka V. H et al. demonstrated a biocompatible fluorescent receptor (81) for detection of Fe3+ upto 8.2 nM LOD. Receptor imaging Fe3+ in cells is a significant increase towards biosensing and cytotoxicity studies also proved the nontoxic nature of this receptor [130]. Jae Min Jung et al. synthesized naphthol-based chemosensor 82 for the Zn2+ sensing through π→π* transition with a unique fluorescence enhancement (Fig. 25). We confirmed the sensing properties of 82 toward CN − and Zn2+ with theoretical calculation [131].

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Sensing mechanism of Zn2+ by 82. (Reprinted from reference number 131 with permission of Elsevier publication)

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Yaping Zhang et al. synthesized a fluorescent receptor 83 by attaching a diarylethene molecule to a functional group for the detection of Al3+ and Zn2+ at detection limit is very low. Moreover, based on the properties of 83, we designed a logic circuit, and that also can be used for water sample testing [132]. Jeya Shree Ganesan et al. synthesized pyrazole bearing imidazole derivative 84 for the selective detection of Al3+/Fe3 + ions with 1:1 binding stoichiometry confirmed by Job’s plot. The LOD of 84 with Al3+/Fe3 + was calculated as 2.12 × 10− 7M and 1.73 × 10− 6M, respectively [133]. Pinkesh G. Sutariya et al. reported calix[4]arene conjugate bearing 1-aminoanthraquinone with amide linkage (85) recognize three metals La3+, Cu2+ and Br− with detection limit 0.88 nM for La3+, 0.19 nM for Cu2+ and 0.15 nM for Br− (Fig. 26) [134]. Akshay Krishna T G et al. developed isatin appended Schiff’s base probes (86a-86c) and characterized by spectroscopic techniques. The sensing ability of probe towards Hg2+ ions was established through UV-Visible techniques and achieved detection limit at ppm levels [135].

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Structure of Probe 85. (Reprinted from reference number 134 with permission of Elsevier publication)

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Suman Srivastava et al. synthesized a fluorescent probe 87 for the selective sensing for Fe3+ and Hg2+. “The LOD of 87 toward Fe3+ and Hg2+ has been found at 4.0 ppb and 1.0 ppb, respectively and also shows fluorescence signalling both in vitro and in vivo” [136]. Awad I. Said et al. developed rhodamine-pyrazole based probe 88 which has ability to detect the Cu2+, Fe3+, Al3+, Hg2+ and Ni2+ discriminately. Furthermore, the probe exhibited a high potential for logical operations and INHIBIT logic gates [137]. Jessica C. Berrones-Reyes et al. developed (S,E)-11-amino-8-((8-hydroxybenzylidene)amino)-11-oxopentanoic acid (89) receptor for the Zn2+ ions detection with LOD of 1.20 mM [138]. Barbara Panunzi et al. reported a complex of pyridyl/phenolic/benzothiazole functionalized ligand (90) with Zn(CH3COO). The structural and photoluminescence properties of the complex were investigated by X-ray diffraction and DFT study and LOD at 375 nM (Fig. 27) [139].

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Frontier orbitals representation of Zn-90(H2O). (Reprinted from reference number 139 with permission of Elsevier publication)

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Shengling Li et al. reported highly selective probe 91 for the recognition of Cu2+ and HSO3− with detection limit of the sensor 91 was 0.36 mM to Cu2+ and 1.4 mM to HSO3− (Fig. 28) [140]. Yun-Qiong Gu et al. developed sensor 92, based on pyrazolopyrimidine for the simultaneous detection of Ni2+ and Cu2+ ions with LOD at 8.9 nM for Ni2+ and 8.7 nM for Cu2+ and fluorescence imaging in T-24 cells was investigated because of the low cytotoxicity of 92 [141]. Jianwei Xu et al. synthesized Ferrocene–based naphthalene receptors 93a and 93b and behaved as naked-eye receptor for Cu2+ and with low detection limit. Furthermore, 93a and 93b were nontoxicity and receptor 93a exhibited certain antibacterial activity (Fig. 29) [142]. Qiang Zhang et al. fabricated multi-response probe 94 for the detecting Al3+, Cu2+ and Mg2+ in ethanol. The optimized structure of the sensor 94 and its sensing mechanism for Al3+, Cu2+ and Mg2+ were confirmed by the calculations of TD-DFT methods Fig. 30 [143].

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The possible mechanism of 91-Cu2 + with HSO3. (Reprinted from reference number 140 with permission of Elsevier publication)

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Proposed binding mode for 93a and 93b with Cu2+. (Reprinted from reference number 142 under Creative Commons Attribution-NonCommercial 3.0 Unported Licence of RSC publication)

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Proposed binding modes of 94 for Al3+, Cu2 + and Mg2+.(Reprinted from reference number 143 with permission of Elsevier publication)

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Xiaowei Mao et al. reported 1,2-bis-(2-pyren-1-ylmethylamino-ethoxy)ethane (95) brought to the surface of GNs via π − π stacking, which shows Mn2+ sensing with 641 LOD of 4.6 × 10− 5M (Fig. 31). These sensing capabilities of 95 in living cell make it applicable in intracellular tracking, intracellular imaging, etc. [144]. Jianping Guan et al. developed a luminophor, 96, which showed a sensitive fluorescence response to Fe2+ with low detection limits of 115.2 nM. Sensing mechanism indicates that fluorescence-quenched due to Fe2+ chelate with the oxygen and nitrogen of 96 (Figs. 32) [145]. Gyeong Jin Park et al. synthesized a colorimetric probe 97 for Co2+ detection with changing its color from yellow to orange. Furthermore 97 could be used as a practical, visible colorimetric test kit for Co2+ [146]. R. P. Cox et al. developed Crown ethers (98a and 98b) for sensing of cations, via changes in absorption/emission and with a 1:1 addition of Na+ or K+, providing clear colourimetric readout [147].

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Synthetic Route for 95.(Reprinted from reference number 144 with permission of ACS publication)

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a Sensing mechanism of the 96 probe for Fe2+; b Benesi-Hildebrand plot of 96 for Fe2+; c Job’s plot of 96 with Fe2+. (Reprinted from reference number 145 with permission of Elsevier publication)

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Minoo Bagheri et al. developed fluorescent MOF sensor, 99 for the ca.2+ sensing concentrations similar to that of blood plasma. The two dimensional signal transduction produce by 99 can reduce interfering responses from the environment and thus generate outstanding sensitivity (Fig. 33) [148]. Jing-Ru Zhou et al. synthesized tripodal amide based probe (100), for the colorimetric sensing for cobalt(II) ions by an obvious color change from colorless to yellow [149]. Xiaopeng Yang et al. reported turn on NIR-fluorescent probe 101 based ICT for detection of Fe2+ with excellent sensitivity (DL = 4.5 µM) (Fig. 34), rapid response (15 min) and “naked-eye colorimetric sensor” [150]. Diana Pendin et al. synthesized a fluorescent ca2+ sensor 102, shows ratiometric ca2+ indicator. 102 binds ca2+ with a dissociation constant of &1.5 mm in vitro [151].

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Structure of 99. (Reprinted from reference number 148 with permission of ACS publication)

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The UV–vis absorption spectrum of probe 101-Fe response to Fe2+ and other various metal ions. (Reprinted from reference number 150 with permission of Elsevier publication)

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Minuk Yang et al. synthesized sensor with pyridyl and carbohydrazide (103) gives visibly blue colour in the presence of Fe2+ and yellow when exposed to Co2+ and Cu2+. The binding constants of the sensor are: Fe2+: 1.0 × 109M−2, Co2+: 2 × 109M−2, and Cu2+: 3.0 × 109M−2 [152]. Meng-Xia Huang et al. synthesized fluorescence probe (104) showed a outstanding fluorescence enhancement toward Cd2+ with a LOD of 29.3 nM. The binding stoichiometry between 104 and Cd2+ was 2:1 as confirmed by the Job’s Plot and 1H NMR titration experiment (Fig. 35) [153]. Suman Swami et al. reported sensor (105a to 105c) for excellent selectivity and sensitivity towards detection of Mn2+ and Zn2+ ions using UV-visible titration, fluorescent titration. Job’s plot methods revealed that sensor interact with Mn2+ and Zn2+ ion in 1:1 and 1:2 binding stoichiometry respectively [154].

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1H NMR titration of 104 and 104 with Cd2+ (DMSO-d6) for (a) 104 only, (b) ADMPA with 0.25 equiv. of Cd2+, (c) 104 with 0.5 equiv. of Cd2 + and (d) 104 with 1.0 equiv. of Cd2+. (Reprinted from reference number 153 with creative common licence 3.0 RSC publication)

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Azzurra Sargenti et al. presented fluorescent sensor, 106b for the quantitative assessment of total intracellular Mg content. The 106b accurately quantify the intracellular total Mg in much smaller samples than 106a, also displaying an increased stable intracellular staining [155]. Bing Zhao et al. developed probe containing 1H-phenanthro[9,10-d]imidazole (107) moieties linked to double ethylenediamino units for the detection of Ag+ with LOD calculated at1.01 × 107M. The 1:1 binding stoichiometry of L–Ag + complex was confirmed by Job’s plot and ESI-MS (Fig. 36) [156]. Juhye Kang et al. reported sensor of bispicolylamine (108) covalently attached to coumarin for the sensing of Mg2+. The formation of a 1:2 complex between the sensor and Mg2+ ions was confirmed based on NMR as well as Job’s plot [157].

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Possible recognition pattern between compound 107 and Ag+. (Reprinted from reference number 156 with permission of Elsevier publication)

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Concluding Remarks

The significance and allied adverse consequences of the cationic presence in biological and environmental systems have recognized. Therefore, attention has been given towards the developing of cationic sensors. The sensors were classified into several categories according to their main moiety, such as rhodamine, anthracene, pyrene, imine, quinoline, benzimidazole, BODIPY and nanoparticles based sensing systems. In this review, a huge number of diverse approaches for the development of sensors for cationic ions have been discussed and successfully applied to monitor environmental metal concentrations with high accuracy, precision, reproducibility with a low detection limit is a challenging area from a present day perspective.