Introduction

Fluorescent chemosensors attracted a huge deal of interest due to their high simplicity, sensitivity, versatility, selectivity, and capabilities through the sensing of various chemical, biological, and environmental significance species, including metal ions and anions, between in vitro and also in vivo (Yiran et al. 2014), and, in addition, portability and the availability of a broad variety as indicator dyes. There is nothing to doubt fluorescent chemosensors with their different responsibilities against several analytes for cost-effective and instrumental practical applications (Sun et al. 2017). In earlier studies, small molecular-structured fluorescent chemosensors are used as a monitoring agent over the analyte in a living system. These contained high selectivity and sensitivity with uncomplicated recognition procedures. Therefore, this decade has seen various experiments paying attention to the construction of sensitive and selective chemosensors using spectral techniques with both spectrofluorometric and spectrophotometric techniques for the recognition (Vijay et al. 2019). In general, chemosensors either enhance or reduce the emission and absorption band due to binding with metal ions (Nayab and Shkir 2017). The most benefit of fluorescence “turn-on” chemosensors related to “turn-off” chemosensors is for their low concentration compared to a “dark” background, which decreases the opportunity of wrong positive signals and enhances the sensitivity, as established by several studies. Moreover, colorimetric chemosensors got involved in much attention for allowed “naked-eye” recognition in a difficult and inexpensive method, to contribute qualitative and quantitative information (Li et al. 2014). Detection and sense of different metal ions have to befall a dynamic field of analysis due to its potential application in various fields including such as biomedicine, chemistry, and finally environmental studies (Hyvönen and Aksela 2010). The high content of transition and heavy metals in soil, air, drinking water, and plants is responsible for their accumulation in the human body, and this occurrence has been coupled to the tumors, insurgence of allergies, and other serious diseases above the genetic pathologies, particularly in children (Lvova et al. 2016). Among the different transition metals, Ni2+ ion is an essential nutrient for living organisms incorporating into the biological process lists such as biosynthesis, metabolism, and respiration (Prabhu et al. 2017). The Ni2+ ion, in particular at big doses, results in genotoxic and also mutagenic activity, and its genotoxicity might be aggravated by the generation of DNA-damaged ROS (reactive oxygen species) and inhibited of DNA (deoxyribonucleic acid) repairs (Inoue et al. 1997). The small amount of Ni2+ examined is very often investigated through diverse methods such as inductively coupled plasma-mass spectroscopy (Fujishiro et al. 1994) and atomic absorption spectroscopy (Miller et al. 2001; Lidén et al. 2008). Among different metal ions over Fe3+ ions being a vital element with the human body, it plays a much important role factor in the growth and improvement of living systems (Madhu and Sivakumar 2019a, b). Recent research shows that the Fe3+ participates in a few neurodegenerative diseases both of Parkinson’s and Alzheimer’s disease (Madhu and Sivakumar 2019a, b). And, either it is a deficiency or an excess in the body which had generally persuaded several disorders with iron smuggling and storage, and the balance being tightly uncomfortable in an organism (Zhang et al. 2018). Provided the paramagnetic nature of Fe3+, fluorescent chemosensors were based on the fluorescence quenching mechanism (Wu et al. 2018). And also the most important is to maintain the redox balance of labile iron species which can generate iron catalysis reaction of oxygen species (ROS) (D’Autréaux et al. 2005; Galaris et al. 2008; Natarajan et al. 2019). Between inadequacy and excess of iron absorption in the human body cause cellular homeostasis and also cause different biological diseases (Haas and Brownlie 2001; Thum and Anker 2007; Zheng et al. 2005; Bonda et al. 2011; Pithadia and Lim 2012). Mg2+ ions are among the majority abundant divalent metal cations present in cells and play crucial roles in numerous cellular processes such as the proliferation of cells, stabilization of DNA conformation, and enzyme-driven biochemical reactions (Hariharan and Anthony 2014). A deficient and excessive amount of metal ions leads to several diseases. The main source is drinking water which gives magnesium in fair quantities. The standard technique for the determination of Mg2+ ions in drinking water is with EDTA complexometric titration (Li et al. 2016). A drug designed and synthesized as a chemosensor for Mg2+ recognition in an aprotic solvent with far above the ground selectivity is desired (Marimuthu and Ramu 2018). Schiff bases are unique classes among discovered and had drawn special attention by tempting to the chemists for their manufacture access, efficient chemical and physical properties, and easy isolation, and, in addition, applications as the synthesis of coordination ligand, catalysis, molecular ion recognition, magnetic properties, and optical properties (Chowdhury et al. 2018) Schiff base compounds are called “privileged compounds” because they are shortly synthesized with the condensation of both aldehydes and amines, a well-known one. The strong ability of Schiff base–metal complexes enables their use across a different range of fields, to catalysis, from medicine, fluorescent sensors, and chemosensors (Abdel-Rahman et al. 2013, 2014, 2016, 2017, 2018 ; Abu-Dief and Nassr 2015, Abdel-Rahman et al. 2019a, b, 2020; Al-Saeedi et al. 2018; Gao et al. 2018). There are different processes answerable for the photophysical reaction of these fluorophores for detection of Ni2+, Fe3+ and Mg2+ ion, such as photoinduced electron transfer (PET) (Banerjee et al. 2013; Sahana et al. 2013; Zhou et al. 2015), excited-state ESIPT (intramolecular proton transfer) (Dhineshkumar et al. 2018; Qin et al. 2015a, b, c; Hossain et al. 2015; Qin et al. 2015a, b, c), CHEF (chelation-enhanced fluorescence) (Ta et al. 2019; Dhineshkumar et al. 2020), ICT (internal charge transfer) (Dhineshkumar et al. 2019; Maity and Govindaraju 2012), FRET (fluorescence resonance energy transfer) (Qin et al. 2015a, b, c; Kim et al. 2008), and C=N isomerization (Kim et al. 2014). Water pollution is the contamination of bodies of water (such as groundwater, aquifers, rivers, lakes, seas, and oceans) typically caused by human activities. Water pollution is any change in water’s physical, biological, or chemical properties that would be harmful to any living organism (Ali et al. 2005, 2013, 2016, 2017, 2018; Alharbi et al. 2018). Drinking water, also known as potable water, is the water considered safe enough for both animal and human use. This is water, usually used to wash, drink, eat, irrigate crops, etc. Chemicals, bacteria, and other pollutants affect even our drinking water these days ( Basheer 2018a, b; Basheer and Ali 2018; Burakova et al. 2018).

In the part of this research, a Schiff base chemosensor has been synthesized and characterized by 1H&13C-NMR, FT-IR, and ESI–MS spectroscopy. The chemosensor is studied by UV–visible and fluorescence measurement and found to recognize three metal ions such as Ni2+, Fe3+, and Mg2+ using 10 mM HEPES buffer in EtOH/H2O (1:4, v/v, pH = 7.0) medium. Competitive metal ion studies and associated constant value are also predicted. In chemosensor sensing achieved by PET mechanism due to linked nitrogen atom by forming the metal complex, it is followed by a 1:1 ratio of stoichiometry.

Experimental

Chemicals and reagents

The essential materials used were high purity of Analar grade indole-3-carbaldehyde and substituted aldehydes purchased from (India) Sigma–Aldrich and used without further purification. To prepare the solutions of different metal ions, the nitrate or chlorate salts of Cu2+, Fe3+, Mg2+, Mn2+, Ni2+, Zn2+, Pb2+, Cd2+, Al3+, Hg2+, Ca2+, Sn2+, Sr2+, V3+, and Ti3+ were used in distilled water for this experiment. The compounds were confirmed and investigated by 1H &13C-NMR, FT-IR, and ESI–MS spectroscopy. Interestingly, the 1H NMR spectrum was recorded on a Bruker-400 MHz and 13C NMR 100 MHz enhancement of the employed with TMS as an internal standard in DMSO-d6, respectively. Infrared measurement was calculated at 4000–400 cm−1 region on an Agilent Carry 630 FT-IR spectrometer. UV and fluorescence measurements were conducted on a PerkinElmer LS45 fluorescence spectrophotometer with a scan rate at 1200 nm at room temperature. For fluorescence measurements, the emission and excitation are at distanced enhancement value, respectively.

Synthesis of the chemosensor ICPA

(4)-4-(phenyldiazenyl)aniline (0.34 g, 1 mmol) and indole-3-carbaldehyde (0.29 g, 1 mmol) were added with 20 mL absolute CH3CH2OH in a RB (round-bottom) flask and refluxed for 12 h. The solution was continuously cooled at room temperature and added with absolute distill water to form a yellow precipitate solution. The solution was filtered and washed with H2O and then recrystallized from ethanol. As shown in Scheme 1 Yield: 0.5 g, 55%. M.p = 197 °C; FT-IR (KBr, cm−1): 3429 (νN–H), 2979 (νAr–C–H), 2927 (νAli.C–H), 1628, 1605 (νC=N), 1576, 1503, 1438 (νC=C), 1389–1194 (βC–H), 1120 (βN–H), 1082–691 (ΓC–H) shown in Fig S1; 400 MHz DMSO-d6 (1H NMR) 9.9 (s, NH), 8.31–8.28 (t, 3H J = 1.2), 8.09–8.07 (d, 2H J = 7.6), 7.954–7.932 (d, 2H J = 8.8), 7.70 (s. 1H), 7.52–7.50 (d, 2H J = 8.0), 7.28–7.21 (7H J = 2.6), 7.73 (s, –ArH), 6.60–6.58 (d, –ArH J = 8.8) shown in Fig S2; 100 MHz, DMSO-d6, (13C NMR) ppm: 155.66, 153.91, 149.18, 138.43, 136.99, 135.57, 128.33, 126.36, 125.42, 124.06, 123.42, 122.09, 120.77, 118.11, 112.37, 112.33 shown in Fig S3: Anal. Calcd for C13 H16 N4: 365.00 shown in Fig S4.

Scheme 1
scheme 1

Synthesis of compound ICPA

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UV–visible and fluorescence titration measurements

In this experiment, chemosensor ICPA (0.0032 mg, 0.001) was dissolved in absolute ethanol (10 mL) and different metal ions such as Fe3+, Co2+, Ni2+, Cu2+, Pb2+, Mn2+, Cd2+, Zn2+, Al3+, Hg2+, Sn2+, Ca2+, V3+ Sr2+, and Ti3+ (50 µM) were also dissolved in absolute distil water. The stock solutions were diluted with 10 mM HEPES buffer (pH = 7.0) to make the final concentration (50 µM). A variety of stock solutions (0.5 equiv) with the solution of metal ions including 0.5 equiv were makeup in CH3CH2OH/H2O (1:4, v/v, pH = 7.0). Fluorescence and UV–Vis spectra were taken at room temperature.

Job’s plot studies

The stock solutions of ICPA (0.0036 mg, 0.002 mmol) were diluted with 10 mM HEPES buffer in EtOH/H2O (1:4, v/v) medium to make a concluding concentration of 5.0, 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, and 0.5 µM and transferred to quartz cells. For metal solutions, 0.5 µM of the Ni2+, Fe3+, and Mg2+ solution (10 mM) were diluted to 10 mM HEPES buffer in EtOH/H2O (1:4, v/v) medium with 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 µM with ICPA and fluorescence spectra were taken at room temperature.

Cell culture and cytotoxicity assays

HeLa cells were grown in DMEM (Dulbecco’s modified Eagle’s medium) supplemented with 10% FBS. All cells were supplemented with an (0.1 mg mL−1 streptomycin, 0.25 mg mL−1 amphotericin B and 100 units mL penicillin) antibiotic antimycotic solution and grown-up at 37 °C in standard (5% CO2, 95% humidity) cell culture conditions. HeLa cells were cultured in culture media (DMEMC, High Glucose) in an atmosphere of 95% air and 5% CO2 at 37 °C. The cells were seeded into 96-well plates at a density of 4 × 103 cells per well in culture media, and then (1) 0, (2) 7.8, (3) 15.6, (4) 31.2, (5) 62.5, (6) 125, (7) 250, (8) 500, (9) 1000, and (10) 2000 μM (final concentration) compounds were added. The cells were an atmosphere of 95% air and 5% CO2 for 24 h then incubated at 37 °C. The absorbance of the cells was measured by ELISA.

Results and discussion

The chemosensor’s response is often described by spectroscopic methods (Fluorescence and UV–Vis spectroscopy) with various cations such as detectable silent substances. The chemosensor ICPA shows that it is sensitive to certain metal cations. Addition of 1.0 equiv. of Cu2+, Mn2+, Ni2+, Fe3+, Co2+, Zn2+, Pb2+, Hg2+, Al3+, Cd2+, Ca2+, Sn2+, Sr2+, V3+, and Ti3+ cations into the ICPA leads to the change of absorption and fluorescence spectra.

UV–visible studies

Absorption spectrum behavior of chemosensor ICPA (10 μM) was investigated in EtOH/H2O medium (1:4, v/v, and pH 7.0 and 10 mM buffer solution HEPES) at room temperature. The selectivity of different heavy metal ions such as Ni2+, Fe3+, Co2+, Zn2+, Cu2+, Pb2+, Hg2+, Al3+, Ca2+, Cd2+, Sn2+, Sr2+, Mn2+, V3+, and Ti3+ (50 μM) was carried out by spectroscopic studies as shown in Fig. 1a.

Fig. 1
figure 1

a Absorption spectrum of ICPA (10 μM) with 0.5 equiv of various metal ions used in EtOH/H2O (1:4, v/v, pH 7.0) 10 mM HEPES buffer solution, b upon addition of Ni2+, c upon addition of Fe3+, d upon addition of Mg2+

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When added 0.5 equiv concentration of chemosensor ICPA in various metal ion (0.5 equiv) concentrations' absorption of three bands at 243, 260, and 299 nm appeared, while the mentioned peaks are for Ni2+ ion, also for the all transition metal ions. Chemosensor ICPA responsibility for another two metal ions Fe3+ and Mg2+ shows a slight increase from other metal ions, as two absorption band peaks at 260 nm and 299 nm appeared while initial absorption peak at 243 nm disappears. However, 0.5 equiv of Ni2+ ion addition increases gradually the binding of chemosensor ICPA to provide the three absorption bands one at 243 nm, while new absorption band peaks at 260 and 299 nm normally decreased with concentration (Fig. 1b). Both binding formations of Fe3+ and Mg2+ (0.5 equiv) used with chemosensors ICPA show the first absorption band at 260 nm slightly decrease while concomitantly at 299 nm decreased (Fig. 1c and d).

Fluorescence studies

Fluorescence studies for chemosensor ICPA (20 μM) participated with different heavy metal ions such as Ni2+, Fe3+, Co2+, Pb2+, Cu2+, Mn2+, Cd2+, Hg2+, Al3+, Ca2+, Zn2+, Sn2+, Sr2+, V3+, and Ti3+ (50 μM) carried out in EtOH/H2O medium(1:4, v/v, and pH 7.0 and 10 mM HEPES buffer solution) at room temperature. Upon 0.5 equiv addition of different metal ions react with 0.5 equiv of chemosensor ICPA, the fluorescence intensity around at 487 nm appeared while excitation wavelength at 299 nm as shown in Fig. 2a. Therefore, fluorescence “turn-on” for selectively of three metal ions Ni2+, Fe3+,, and Mg2+, which lead to a slight decrease or increase to all other the transition heavy metal ions. The chemosensor ICPA is completely involved downfield below all metal ions. The presence of three metal ions Ni2+, Fe3+, and Mg2+ could be displayed in “Off–On” fluorescence behavior on the spectroscopic measurement experiment. Also the sensing of chemosensor ICPA behavior in different concentrations in EtOH/H2O medium (1:4, v/v, pH 7.0) is provided by Ni2+, Fe3+, and Mg2+. Upon the addition of Ni2+ (0.5 equiv) increase, the (fluorescence) intensity at 487 gradually increased in Fig. 2b, and it was great that the linear relationship of emission intensity at 487 nm in that concentration Ni2+ (0–5.0 equiv) (R2 = 0.9960) could be seen in Fig. 2b. The intensity was evaluated through the limit of detection calculated at 6.82 × 10−7M. Furthermore, with the gradual increase in Fe3+ (0.5 equiv), the 487 nm fluorescence intensity gradually increased in Fig. 2c.

Fig. 2
figure 2

a Fluorescence spectra of ICPA (20 μM) in the presence 0.5 equiv of various metal ions in EtOH/H2O (1:4, v/v, pH 7.0) 10 mM HEPES buffer solution (λex = 299 nm). b Fluorescence titration spectra of receptor ICPA (10 μM) used in EtOH/H2O (1:4, v/v, pH 7.0) 10 mM HEPES buffer solution upon incremental addition of Ni2+ (0–5.0 equiv.). c Fluorescence titration spectra of receptor ICPA (20 μM) in EtOH/H2O (1:4, v/v, pH 7.0) 10 mM HEPES buffer solution upon incremental addition of Fe3+ (0–2.4 equiv.). d Fluorescence titration spectra of receptor ICPA (20 μM) in EtOH/H2O (1:4, v/v, pH 7.0) 10 mM HEPES buffer solution upon incremental addition of Mg2+ (0–2.4 equiv.). Inset: (λex = 299 nm, λem = 487 nm)

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The good linear relationship of the emission intensity at 487 nm predicted with the concentration of Fe3+ (0–20 μM) (R2 = 0.9970) could be seen (inset in Fig. 2c), and the result was evaluated by the limit of detection 5.27 × 10−7 M. Finally, with 0.5 equiv of Mg2+ ions gradually increasing with the fluorescence intensity at 487 nm (Fig. 2d), a linear relationship was assumed that the fluorescence intensity with different concentrations of selectivity for Mg2+ (0–20 μM) (R2 = 0.9912) could be seen (inset in Fig. 2d, with the detection limit calculated 5.96 × 10−7M. Chemosensor ICPA binding affinity with the selectivity of Ni2+, Fe3+, and Mg2+ was examined by fluorescence experiments. A Job plot indicated that 1:1 stoichiometric complex of ICPA + Ni2+, ICPA + Fe3+, and ICPA + Mg2+ has been predicted different concentrations by each 0.5 equiv of Ni2+, Fe3+, and Mg2+.

These results confirmed by Job’s plot with 1:1 stoichiometric complexes with chemosensor ICPA were detected to maximum at 0.5 μM mole fraction determined through the binding addition of [Ni2+]/{[Ni2+] + [ICPA]}, [Fe3+]/{[Fe3+] + [ICPA]} and [Mg2+]/{[Mg2+] + [ICPA]}, respectively (Fig. 3a–c). In combination with fluorescence titration in EtOH/H2O (1:4, v/v, pH 7.0) 20 mM HEPES buffer solution, the association constants (Ka) for ICPA + Ni2+, ICPA + Fe3+ and ICPA + Mg2+ have been determined by Benesi–Hildebrand method and calculated as 3.45 × 108 M−2 (Fig. 4a), 5.63 × 104 M−2 (Fig. 4b) and 8.63 × 103 M−2 (Fig. 4c), respectively.

Fig. 3
figure 3

Job’s plot of the ICPA-Ni2+ complexes in EtOH/H2O (1:4, v/v, pH 7.0) 10 mM HEPES buffer solution, keeping the total concentration of ICPA (20 μM) and a Ni2+, b Fe3+, and c Mg2+ (0–5.0 equiv.) indicating 1:1 stoichiometry at intensity = 487 nm

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Fig. 4
figure 4

a Benesi–Hildebrand plot of ICPA using 1:1 stoichiometry for the association between ICPA and Ni2+. b Benesi–Hildebrand plot of ICPA using 1:1 stoichiometry for the association between ICPA and Fe3+. c Benesi–Hildebrand plot of ICPA using 1:1 stoichiometry for the association between ICPA and Mg2+

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Proposing the sensing mechanism

The FT-IR spectra recognized the binding mechanism of chemosensor ICPA-Ni2+ ICPA-Fe3+ and ICPA-Mg2+ together analyzed in this experiment (Fig. S5). Chemosensor ICPA showed FT-IR peaks at (C=N) group in 1628 cm−1 and NH-group in 3429 cm−1 obtained. Meantime, 0.5 equiv of Ni2+ addition results in the functional group decrease from 1628 cm−1 shifted to 1613 cm−1 peaks as well as NH-group stretching vibration decrease from 3429 cm−1 shifted to 3422 cm−1 was obtained (Fig. S5).

Furthermore, 0.5 equiv of Fe3+ addition which results in a change from (C=N) 1628 to 1617 cm−1, while NH-group stretching vibration shifted to change from 3429 to 3391 cm−1 (Fig. S6). Finally, 0.5 equiv of Mg2+ addition with 1628 cm−1 shifted to 1620 cm−1 gradually decrease peaks observed; however, NH-group stretching vibration shifted to change from 3429 cm−1 to 3401 cm−1 (Fig. S7). As shown in Scheme 2, the fluorescence “turn-on” in the presence of Ni2+, Fe3+, and Mg2+ metal ions, by enhancing emission intensity of the excited state with intersystem crossing to electron and energy transfer mechanism in proposed for chemosensor ICPA, emission intensity increases around 487 nm selectively of three metal ions due to inhibition of PET (photoinduced electron transfer) and also >C=N isomerization methods. PET mechanism showed C=N isomerization of imine group “N” to the excited fluorescence indole-dye moiety with more profit of non-radiative decay with excited energy. In Chemosensor ICPA (fluorescence turn-on) binding modes with Ni2+, Fe3+, and Mg2+ (fluorescence turn-off), these PET and >C=N isomerization method inhibitions caused effective complexation formation.

Scheme 2
scheme 2

Proposed binding mechanism of ICPA-Ni2+, ICPA-Fe3+, and ICPA-Mg2+

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Competitive metal studies

In this work, the competitive metal ions such as (1 × 106 M−1) Ni2+, Fe3+, Co2+, Hg2+, Al3+, Pb2+, Mn2+, Cd2+, Ca2+, Cu2+, Sn2+, Zn2+, Sr2+, V3+, and Ti3+ (50 μM) at concentrations mentioned at experimental section were used with the binding of chemosensor ICPA (20 μM) and the fluorescence spectra shown in Fig. 3. Addition of 0.5 equiv different metal ions and 0.5 equiv of Ni2+ shows an increase in fluorescence intensity at 487 nm while excitation wavelength at 299 nm for Ni2+ only. The spectrum revealed that Cu2+ and Co2+ decreased 100% and 90% by 0.5 equiv of Fe3+ ion above the others transition metal ions; therefore, no change for other metal ions due to well-known paramagnetism is shown in Fig. 5a. Similarly, 0.5 equiv of Fe3+ with chemosensor ICPA binding and 0.5 equiv various metal ions contain best binding interaction with Al3+, Zn2+, and Mn2+ gradually decreased from 100%, 90%, and 80% except all metal ions due to which not only paramagnetism, as a result, enormous three metal ions in coordinated 2 + Fe3+ compared with highly paramagnetic for ICPA +Ni2+ as shown in Fig. 5b. Finally, 0.5 equiv of Mg2+ concentrating all metal ions contained to bind with chemosensor ICPA caused the interaction of transition metal ions Cu2+, Al3+ and Ca2+ having been decreased level of 100%, 70%, and 50%; consequently, in Cu2+ paramagnetic, remaining Al3+ and Ca2+ are not paramagnetic, while it was great coordinated both sides binding of ICPA +Ni2+ complexes formation as shown in Fig. 5c. Competitive metal ions completely involved or participated in surroundings.

Fig. 5
figure 5

Fluorescence intensity at 487 nm of ICPA (20 μM) upon addition of various metal ions in EtOH/H2O (1:4, v/v, pH 7.0) 10 mM HEPES buffer solution. Brown bars represent the fluorescence intensity of ICPA in the presence of metal ion (20 μM). Green bars represent the fluorescence intensity in the presence of a Ni2+, b Fe3+, and c Mg2+ (50 μM) after the addition of various metal ions

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Reversibility test

This experiment proved that Schiff base chemosensor has been confirmed for practical application with reversibility. The studies of the reversibility experiment of ICPA with Ni2+, Fe3+, and Mg2+ were carried out compared with EDTA solution. The addition of EDTA solution to ICPA-Ni2+ complex resulted in the complete quenching its fluorescence intensity; however, ICPA solution just showed over EDTA solution; afterward, the solution of ICPA-Ni2+ complex increases the original fluorescence intensity again; and therefore, result was chemosensor ICPA completely reversible sensing for Ni2+as shown in Fig. 6a. Simultaneously, another ICPA-Fe3+ and ICPA + Mg2+ complex must be recognized by the same fluorescence intensity at 487 nm between up (Fe3+) and down (Mg2+) as a result of reversible chemosensor, although EDTA solution of both ICPA-Fe3+ and ICPA + Mg2+ complexes it is completely quenching fluorescence intensity as shown in Fig. 6b and c. These synthesized chemosensors have been compared with the EDTA compound with this experiment.

Fig. 6
figure 6

Fluorescence spectrum of ICPA (20 μM) with compared EDTA solution in the presence of a Ni2+, b Fe3+ and c Mg2+ used in EtOH/H2O (1:4, v/v, pH 7.0) 10 mM HEPES buffer solution

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The cytotoxicity effects

Cancer is a disease caused by an abnormal cell growth that leads to disturbance to other parts of the body, and it is now one of the most dangerous health issues. The combined number of deaths from AIDS, malaria, and tuberculosis is far smaller than those who die from cancer (Rain 2009; Reinberg 2008). The anticancer drugs available on the market have suffered from certain limitations, so the development of new drugs is essential for society (Singh et al. 2009). The indole and its analogs demonstrated a wide spectrum of anticancer activity. We synthesized 4E,10E-4-(2-(4-nitrophenyl)-N-((1H-indol-3-yl)methylene)benzenamine and investigated their anticancer activity across the human cancer cell line of HeLa. The cytotoxicity activity of probe ligand was investigated in different cell lines. HeLa cell lines grown at 37 °C by the culture of different concentrations were prepared in an atmosphere of CO2-5% and air-95% for 24 h. Furthermore, the Hela cell lines were applied in different concentrations of ligand and have been treated for many hours; it caused much time to kill cancer cells by ligand as exposed in Fig. 7a, in which the first empty plate of HeLa cells before the addition of ligand, the second plate is applied with ligand and cell lines for the time needed to cell death for half of the concentration shown in Fig. 7b.

Fig. 7
figure 7

HeLa cells treatment with ligand ICPA in different concentrations a empty cell lines, b ligand added with cell lines inhibited for a long time, c ligand added with whole-cell line killed

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Finally, the third plate shows the spread of ligand and cancer cells where the complete death of cancer cells within 2 h is exposed in Fig. 7c. The cell viability with an increased order of concentration with the ligand was calculated and detected at 4.7 μM as shown in Fig. 8. As well as, the IC50 value also established at different concentrations of ligand shows decreased value due to applied various concentrations of cancer cell lines such as (1) 0, (2) 0.9, (3) 1.95, (4) 3.9, (5) 7.8, (6) 15.6, (7) 31.2, (8) 62.5, (9) 125, (10) 250, and (11) 500 μM in Table 1.

Fig. 8
figure 8

The cells viability of HeLa cells different concentrations used with ligand ICPA

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Table 1 IC50 values calculated different concentrations of HeLa cells
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Conclusion

In this work, we have designed and synthesized a new Schiff base of 4E,10E-4-(2-(4-nitrophenyl)-N-((1H-indol-3-yl)methylene)benzenamine ICPA fluorescent probe which has been evaluated as a sensor for Ni2+, Fe3+, and Mg2+ ions by fluorescence “turn-on” response using steady-state absorption and fluorescence spectroscopy. Based on Job’s plot, ESI-mass spectral analysis, and Benesi–Hildebrand relation, we propose the structure of ligand and its metal ion as 1:1 stoichiometry complex. This chemosensor exhibits a very good fluorescence sensing ability to Ni2+, Fe3+, and Mg2+ ions over are wide pH range. An efficient way for the regeneration of free ligand from the complex probe was achieved using EDTA as a coordinating ligand Ni2+, Fe3+, and Mg2+ ions sensing. The MTT assay experiments further demonstrated the cytotoxic in vitro against the HeLa cell line. Thus, the chemosensor meets all the requirements to be an excellent fluorescent probe for wide applications in the field of biolabeling, biosensing, imaging, and so on. Moreover, this work provides a new approach with the economically cheap and less complicated synthetic route for selective, sensitive, and quantitative detection of these three most abundant and essential traces elements in the human body.