An efficient 2-aminothiazolesalicylaldehyde fluorescent chemosensor for Fe2+ ion detection and a potential inhibitor of NUDT5 signaling hormone for breast cancer cell and molecular keypad lock application.

A novel thiazole phenol conjugate, 2-aminothiazolesalicylaldehyde (receptor1) was designed and synthesized for the first time through a single step process via Schiff base condensation reaction. The formation of receptor1 was confirmed by FTIR, 13C NMR, and 1H NMR. The IR spectra confirmed the presence of the aldimine formation. It is further supported by the proton NMR, showing the disappearance of aldehyde peaks and the formation of a new imine peak. This is further corroborated by the 13C NMR. The receptor1 complexing with various metal ions were studied through fluorescence spectroscopy showed its selectivity toward Fe2+ ion following a reverse photoinduced electron transfer (PET) process compared to all other potentially competing ions. The receptor1 was applied as a sensor to sense Fe2+ ion in water samples. The detection limit for Fe2+ ion in drinking water was substantially lower (0.003 µM) than the EPA (environmental protection agency) recommendation (5.37 M). The capability of receptor1 in recovering Fe2+ ion in bore water, tap water, and drinking water was up to 99.5%. The receptor1 was also used as a chelating ligand (receptor1) in molecular docking and it was assessed as a potential inhibitor of NUDT5, a silence hormone signaling for breast cancer. The test compound (PDB: 5NWH) showed good affinity toward the target receptor1 with the binding energy of - 5.23 kcal mol-1. Furthermore, the receptor1 showed excellent reversibility property on adding EDTA solution. Due to the marvelous reversible property, a molecular-scale sequential information processing circuit is designed for the multi-task behavior such as 'Writing-Reading-Erasing-Reading' in the form of binary logic gate. The consecutive addition of Fe2+ ion and EDTA solution to receptor1 paves a way for the construction of INHIBIT logic gate. Additionally, the receptor1 showed the mimicking behavior of molecular keypad lock. Supplementary Information: The online version contains supplementary material available at 10.1007/s11696-022-02373-z. © Institute of Chemistry, Slovak Academy of Sciences 2022, Springer Nature or its licensor holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Introduction

The fluorometric studies have great impact on the detection of metal ion in the biological and environmental system in recent years (Khairnar et al. 2015). Iron is the most abundant metal found in the earth’s crust next to aluminum. It is a major mineral required for the human body for the development of the immune system, synthesis of hemoglobin, and transfer of electrons in the photophysical process (Jo 2017; Tamil Selvan 2018; Kim 2013; Jo 2013; Bhuvanesh 2017). Due to its significant catalytic property, iron is used as a catalyst in numerous reactions such as DNA and RNA synthesis, enzyme catalysis, and cellular metabolism (Lee et al. 2012). Intake of excess iron may cause neurodegeneration such as Parkinson’s disease, Alzheimer’s, Huntington’s, and Menke’s diseases (Li et al. 2016). Iron deficiency weakens the immune system, allowing lethal viruses such as pneumonia, typhoid, COVID-19, and others to enter the body. Liver damage, diabetes, anemia, kidney damage and heart disease are all possible side effects of iron deficiency. (Chereddy 2013; Xu 2017; Murugan 2018; Joseph 2011; Gong 2019). In the biological system, the distinction between ferric and ferrous ion is rather difficult in the detection process of iron (Zhan 2012; Yoldas 2015; Farhi 2018; Wang 2016; Yin 2011; Sen 2012; Moon 2010; Dong 2010; Asiri 2019), the Schiff-based compound like 2-aminothiazolesalicylaldehyde plays a major role in detecting the ferric ion in particular. In the Schiff-based compound, the presence of a π-conjugated aromatic chromogenic group helps to sense cation and anion molecules (Zhang et al. 2020). Among the entire heterocyclic group, the sulfur-containing thiazole group has the greatest impact in the medicinal field due to its anti-cancer, anti-tubercular, and antimicrobial properties (Balasubramanian et al. 2020). For the past few years, biosensor, chromogenic sensors, mass sensors, and the electrochemical sensor were reported for sensing toxic metals in the environment (Gholami et al. 2015). Among these techniques, the fluorescent chemosensor method is highly efficient due to its specific selectivity of ion, responds for both solid and liquid phases, responds for a short time and is cost-efficient (Huang et al. 2012). As per literature review,  Golbedaghi et al. have reported 81% yield of salicylaldehyde (2-[3-(2-formyl phenoxy)propoxy]benzaldehyde-2-aminobenzenethiol) Schiff-based compound. The compound sensed Fe3+ ion in H2O-DMF solution (Golbedaghi et al. 2018). Li et al. reported salicylaldehyde (4-N,N-diethylaminosalicylaldehyde hydrazone) Schiff-based chemosensor with the yield of 57% for specific detection of Zn2+ ion (Li et al. 2010). Chao Gou et al. have reported 75% yield of salicylaldehyde (4- hydroxysalicylaldhyde-hydrazine) bis Schiff-based chemosensor for selective detection of Cu2+ and Al3+ ion (Chao Gou et al. 2011). In this article, 2-aminothiazolesalicylaldehyde (receptor1) was synthesized by conjugating thiazole with salicylcaldehyde to form C = N imine based Schiff-based compound with the yield of 88%, which showed fluorescence quenching with Fe2+ ion through a probable reverse photoinduced electron transfer (PET) process. Various studies conducted on Schiff-based compound showed the limit of detection of Fe2+ ion to be 0.32 µM (Tae Geun Jo), 1.09 µM (Atikal Farhi), 1.3 µM (Kyung Beom Kim), 0.0048 µM (Bhuvanesh), and so on. This is the first time that the Schiff-based compound detected the Fe2+ metal ion at a very low concentration of 0.0033 µM. The 2-aminothiazolesalicylaldehyde showed an excellent reversibility property (turn on–off–on–off) with EDTA. The reversible property of Schiff base is applied for the establishment of INHIBIT molecular logic gate and molecular keypad lock. The receptor1 was subjected to the detection of Fe2+ ion in real water sample analysis using fluorescence technique. The synthesized receptor1 was assessed for its ability to interact with protein receptor through molecular docking, which is an important tool for drug discovery. The receptor1 was docked with NUDT5 (PDB ID: 5NWH) which is a hormone, signaling for breast cancer cells.

Experimental

Materials and instruments

Grade quality solvents and reagents were purchased commercially. Highly purified compound such as 2-aminothiazole and salicylaldehyde was purchased from Hi-media. The 1H and 13C NMR spectra were recorded from Bruker 400  and 100 MHz spectrometer using DMSO-d6 solvent and Trimethylsilane (TMS) as an internal standard. Absorption studies were carried out using Shimadzu UV-240 spectrophotometer. The fluorescence study was recorded on Jasco FP-8200 spectrofluorometer. The absorption and emission spectra were recorded at 24 ± 1 °C.

Synthesis of 2-aminothiazolesalicylaldehyde

Salicylaldehyde (0.5 mL, 1 mmol) was dissolved in 30 mL of methanol, and then 2-aminothiazole (200 mg, 1 mmol) was added. About 3–4 mL glacial acetic acid was added to this mixture and stirred constantly for 17 h at room temperature (Zhou et al. 2011). Thin-layer chromatography was used to examine the product formation as a preliminary test. The solvent evaporation was detected after a few days, along with an oily brownish-yellow coating at the top of the reaction mixture, and the reaction mixture eventually transformed into a brownish-yellow colored chemical. FTIR, 13CNMR, and 1H NMR were used to validate the synthesis of 2-aminothiazolesalicylaldehyde (Scheme 1), which was given the name receptor1. The yield of the synthesized product is 88%.

Scheme 1

Synthesis of receptor1 (2-aminothiazolesalicylaldehyde)

UV–Vis absorption studies

UV–Vis absorption tests were conducted to determine the excitation wavelength for the synthesized receptor1 and, as a result, to analyze the photophysical characteristic by fluorescence investigations. Receptor1 (2 mg) was dissolved in 5 mL of DMF (dimethylformamide) at a concentration of 2 × 10−3 M to make the stock solution. To keep the pH constant, 50 mM of HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) buffer was dissolved in DMF and H2O at a ratio of 1:1 (Saravanan et al. 2019). To observe the change in absorbance, a concentrated solution of 2 × 10−5 M and 2 × 10−6 was produced from the stock solution. The experiment was carried out at a pH of 7.4 (neutral).

Fluorescence titration

To conduct the fluorescence titration, a stock solution of receptor1 at a concentration of 4 × 10−6 M was produced (2 mg, 0.01 mmol). Metal nitrate salts of Ag+, Zn2+, Fe2+, Fe3+,K+, Li+, Na+ (1 mg, 0.01 mmol)) Ba2+, Ca2+, Co2+, Cu2+,Ni2+, Sr2+ (2 mg,0.01 mmol) Al3+,Pb2+, Hg2+,Zr2+, Mg2+, Mn2+ (3 mg, 0.01 mmol) Bi3+, Cr3+ (4 mg, 0.01 mol), and Cd2+ (7 mg, 0.01 mol) were weighed and dissolved in 25 mL DMF–H2O, 1:1 v/v, HEPES 50 mM buffer solution (Prabhu et al. 2015a, b). To make 100 equivalents, 2 mL of metal solution was mixed to 2 mL of receptor1. At an excitation wavelength of (λex) 275 nm, the fluorescence intensity of each metal bound to receptor1 was measured.

Competition with other metal ions

The interference of other metal ions during the sensing of Fe2+using receptor1 was investigated by including 100 equivalents of each metal salt (0.01 mmol) (M = Ag+, Al3+, Ba2+, Bi3+, Ca2+, Cd2+, Co2+, Cr3+,Cu2+, Fe3+, Hg2+, K+, Li+,Mg2+, Mn2+, Na+, Ni2+, Pb2+, Sr2+, Zr2+, and Zn2+) (Nagarajan et al. 2021) dissolving in buffer solution. At the excited wavelength (λex) 275 nm, the fluorescence intensity was measured. Two mL of receptor1 and 2 mL of each metallic solution were added to each vial, and fluorescence was measured. To test the interference behavior, 2 mL of Fe2+ ion solution was added to each vial.

Job’s plot measurements.

To assess the stoichiometry of the receptor1–Fe2+ complex equal concentration of receptor1 (host) and Fe2+ (guest) of strength 1 × 10–6 M was prepared. The receptor1 and the metal ion solution were mixed at the ratio of 0:4, 0.1:3.9, 0.2:3.8, 0.3:3.7, 0.4:3.6, 0.5:3.5, 0.6:3.4, 0.7:3.3, 0.8:3.2, 0.9:3.1, and 1:3 mL. The fluorescence analysis was carried out by transferring 4 mL of the above solution to the cuvette. The binding stoichiometric was evaluated using the Jobs plot (Suresh et al. 2018) and the nonlinear fitting curve.

Quantum yield measurement

The quantum yield (ϕ) was calculated using quinine sulfate (ϕ = 0.54) as a standard, the equation for calculating quantum yield (Kumar 2020; Chae 2019) iswhere ϕR is a standard value (Quinine Sulfate), FU and FS are the integrated fluorescence intensity of unknown and standard sample, AS and AU are the absorbance of the standard and unknown sample, ηU2 is the refractive index of solvent in which unknown sample was dissolved (1.4305), and ηS2 is the refractive index of solvent in which standard was dissolved (1.33) (Khan et al. 2021). The quantum yield of receptor1 (ϕU) is 0.0126 and for receptor1–Fe2+ is 0.0036 at 275 nm.

Docking parameters

Molecular docking plays a vital role in drug discovery. It is a useful platform for predicting the protein ligand interaction through binding mechanism. The structure of the protein–ligand was accomplished using Schrodinger maestro software. Structure of the Schiff bases was sketched using 2D chemsketch. The active site residue information was collected from PDB (Protein Data Base) sum entry. In receptor-grid generation, a 10 × 10 x 10 Å receptor grid box was generated around the active site residues of target protein. The docking study exposes the most suitable binding sites in the target protein for the ligand to bind.

Results and discussion

Design and synthesis of 2-aminothiazolesalicylaldehyde

Receptor1 was synthesized by condensation of 2-Hydroxy benzaldehyde and 2-amino thiazole in methanol with 88% yield as shown in Scheme 1 and characterized by 1H-NMR and 13 C-NMR. In receptor1 benzaldehyde, the group acts as a fluorophore, the imine group acts as a spacer, and the heterocyclic thiazole group containing N and S atom acts as a binder. The imine group links the fluorophore and binding unit. From FTIR analysis, the presence of skeletal stretching of the aromatic group is confirmed from the peak at 1600.92 cm−1, the peak at 1192.21 cm−1 showed C–O stretching vibration of alcohol, the peak at 1492.00 cm−1 confirms C–C stretching vibration, the peak at 756.00 cm−1 confirms C–S stretching vibration, and the peak at 2742.78 cm−1 confirms C-H stretching vibration. The peak at 1658.78 cm−1 confirms the presence of the C = N stretching band (Fig. S1) (Ahmed et al. 2015). The total number of carbon was confirmed by 13C NMR. The chemical shift was noticed at (100 MHz, DMSO: d6, ppm) 130.14, 122.72, 128.32, 117.70, 154.62, 128.53, 140.48, 139.36, 128.96, 129.07. The total number of hydrogen was confirmed by 1H NMR. The chemical shift of H1 NMR (400 MHz, DMSO: d6, δ, ppm) (J, Hz): 7.01, 7.12, 7.15 and 7.17 (d, 4H, Ar–H), 6.94 (s, 1H, CH = N), 6.85 and 6.94 (d, 2H, CH2), and 9.23 (s, 1H, 1-OH) (Fig. S2 and S3) (Kim et al. 2020) was observed. Analytically calculated mass value for C10H8N2SO (204.23): C, 120.10; H, 8.06; N, 28.01.

Selectivity studies

The selectivity studies are performed to assess the sensing behavior of receptor1 for selective detection of the metal ion. The metal ions such as Ag+, Al3+, Ba2+, Bi3+, Ca2+, Cd2+, Co2+, Cr3+,Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Sr2+, Zr2+, and Zn2+ are taken in the form of nitrate salts and added to receptor1 and buffer solution. The excitation wavelength is fixed as 275 nm from UV–Vis spectroscopy. The receptor1 responded to a drastic quenching effect with Fe2+ and Ag+ ions as shown in (Fig. 1). However, other metal ions showed no significant changes during the identical condition. As a result, the receptor1 showed a potential response for Fe2+ ion with a turn on–off fluorescence through a possible reverse PET mechanism.

Fig. 1

Fluorescence spectra to assess the response of various metal ion (100 equivalents. of each, λex = 275 nm) toward receptor1 of concentration 4 × 10.−6 M solution (DMF–H2O, 1:1 v/v, HEPES 50 mM, pH = 7.4)

Proposed binding mechanism

In PET-based sensors, a fluorophore is usually connected via a spacer to a binding unit containing a relatively high energy non-bonding electron pair found in nitrogen donors, capable of transferring an electron to the excited fluorophore to quench its fluorescence (off–on) (Golbedaghi et al. 2018). The quenching takes place because of the deactivation of the excited state of the fluorophore by the addition of an electron to its excited state frontier orbitals from the functional group of the fluorophore that quenches the fluorescence intramolecularly (Li et al. 2009). This leads to the non-emissive state of a fluorophore. Here, in receptor1 the free electron pair of nitrogen present in thiazole and C = N group attached to the fluorophore has strong fluorescence emission, and binding of cation results in weak fluorescence emission because the Fe2+ ion bound to receptor1 is paramagnetic with unfilled d-subshell that strongly quench the fluorescence intensity of the fluorophore through electron transfer between receptor1 and Fe2+ ion (Gou et al. 2011). The Fe2+ ion bound to the fluorophore acts as a signal transduction unit and is also bound with a thiazole moiety and spacer (Das et al. 2013). There is no possibility of Fe2+ ion binding with sulfur because of the weak interaction between Fe2+ and sulfur (Fan et al. 2014). Therefore, the quenching response of receptor1 toward Fe2+ ascribed to electrons of fluorophore transferring to the binding unit by metal ion complexation (reverse-PET process), resulting in fluorescence quenching (Scheme 2) (Luo et al. 2016). Additionally, the ESIPT mechanism takes place between the proton donating hydroxyl group and withdrawing imine group upon excitation of a molecule via intramolecular hydrogen bond formation.

Scheme 2

Proposed binding mechanism

Interference study

An interference study was carried out to predict if any competitive metal ion interferes during the complexation of receptor1 with Fe2+. Figure 2 shows a change in the fluorescence intensity for the addition of metal ions to receptor1. The emission intensity of receptor1 with various metal ion such as Al3+, Ba2+, Bi3+, Ca2+, Cd2+, Co2+, Cr3+,Cu2+, Fe3+, Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Sr2+, Zr2+, and Zn2+ is on par to the emission intensity of receptor1 alone, whereas the emission intensity of receptor1–Ag+ is low compared to the emission intensity of other receptor1–metal ion complex. This is due to the fact that the electronic configuration of the Ag+ ion in the 4d subshell is filled, resulting in diamagnetic property, which causes a drop in emission intensity since no electron transfer occurs between Ag+ and receptor1. The HSAB (Hard-Soft and Acid–Base) theory predicts that the borderline base (C = N) group can coordinate with a borderline acid Fe2+ ion in receptor1's response to Fe2+ (Ahmed Nuri Kursunlu 2015; Li 2018). The reverse PET mechanism caused the fluorescence intensity of receptor1 to decrease when it was complexed with Fe2+. When receptor1 is complexed with Fe2+ ion, none of the interfering metal ions examined showed any evident interference (Fig. 2). This result implies that receptor1 could be used for the selective detection of Fe2+ ion in various applications.

Fig. 2

Metal ions competition analysis of receptor1 (4 × 10−6 M) solution (DMF–H2O, 1:1 v/v, HEPES = 50 mM, pH = 7.4) in the presence of various metal ions (100 equivalents. of each, λex = 275 nm). The green bar represents the receptor1 + metal ion and the red bar represents receptor1 + metal ion + Fe2+ ion that shows the quenching fluorescence intensity

Binding and stoichiometry studies

To find the binding ability of receptor1 with Fe2+ ion, the fluorescence study was carried out by gently increasing the concentration of Fe2+ ion from 0 to 85 equivalents. A decrease in fluorescence intensity was observed with an increase in Fe2+ ion concentration (Fig. 3). The fluorescence intensity was almost saturated upon the addition of 85 equivalents of Fe2+. The binding ratio of ligand and complex was predicted by Job’s plot. The Job’s plot confirms the binding mode of the host–guest complex, the maximum mole fraction of the complex was 0.5 at excitation wavelength 275 nm (Fig. 4). Job’s plot concludes that the receptor1 interacts with Fe2+ at 1:1 stoichiometry. The stoichiometry binding of receptor and metal was predicted theoretically with the help of the Benesi–Hildebrand nonlinear fitting curve (Fig. 5) (Velmurugan 2014; Prabhu 2015a; Velmurgan 2015). The association constant (Ka) of receptor1–Fe2+ ion is 2.22 × 102 M−1. The limit of detection (LOD) calculated using 3δ/S is 33.7 × 10–9 M, where δ denotes the standard deviation of the free probe, and S denotes the slope of the linear calibration plot indicating correlation coefficient (R2) value of 0.9863 derived from the linear graph plot (Fig. 5). Imidazole, tripodal, tris-salicylidene (2-aminoethyl) amine, naphthalene, oxadiazole-based chemosensor results in detection of Fe2+ ion at the concentration of 0.32, 1.09, 1.3, 0.0048, 7.78 µM, respectively (Table 1), but in the present study receptor1 was able to detect the Fe2+ ion at the concentration of 0.003 µM which is very low compared to other Schiff-based compound.

Fig. 3

Fluorescence titration spectrum of receptor1 (4 × 10−6 M) solution (DMF–H2O, 1:1 v/v, HEPES 50 mM, pH = 7.4) upon addition of different concentration of Fe.2+ (0–85 equivalents. λex = 275 nm)

Fig. 4

Job’s plot of receptor1 and Fe.2+ in DMF–H2O solution (1:1 v/v, HEPES 50 mM, pH = 7.4)

Fig. 5

Benesi–Hildebrand plot of receptor1 (4 µM) with Fe.2+ in DMF–H2O solution (1:1 v/v, HEPES 50 mM, pH = 7.4) (λex = 275 nm)

Table 1

Literature reports of other Schiff’s-based compound sensing Fe2+ ion

Receptor Type	Mode of detection	Testing media	Binding constant Ka(M−1)	Limit of detection	References
Imidazole-based	Turn-Off	Bis–tris buffer	1.4 × 104	0.32 µM	Tae Geun Jo et al.
Tripodal-based	Turn-Off	Methanol: Water	2.49 × 104	1.09 µM	Atikal Farhi et al.
Tris salicylidene(2-aminoethyl)amine based	Turn-On	HEPES/Methanol	4.30	1.3 µM	Kyung Beom Kim et al.
Naphthalene based	Turn-Off	HEPES/Water	1.91 × 105	0.0048 µM	Bhuvanesh et al.
Oxadiazole based	Turn-Off	THF/Water(1:1)	4859.54	7.78 µM	Xue Gong et al.

Reversibility studies

The reversible characteristic of the synthesized chemosensor makes it suitable for a variety of applications. Using the chelating chemical EDTA, the reversible property of receptor1 is investigated (ethylenediaminetetraacetic acid). Figure 6a represents receptor1’s reversible property. For receptor1, the fluorescence intensity is enhanced. The fluorescence intensity is lowered due to receptor1 complexation with the Fe2+ ion, by the addition of Fe2+ ion to the receptor1. The fluorescence intensity is also increased when EDTA is added to receptor1 and Fe2+ ion solution. This research demonstrates receptor1’s excellent reversibility. The repeatability of the reversible property is further demonstrated by the addition of Fe2+ ion and EDTA multiple times. The complexation of EDTA and Fe2+ ions with receptor1 causes the intensity of Fe2+ and EDTA to vary. The cycle was repeated a total of ten times (Fig. 6b).

Fig. 6

a Reversibility spectra of receptor1 in the presence of Fe2+ ion and EDTA in DMF–H2O solution (1:1 v/v, HEPES 50 mM, pH = 7.4) (λex = 275 nm). b Sequential addition of Fe2+ and EDTA showing reversibility behavior

Effect of pH and time

The pH investigation is conducted to determine the synthesized receptor1’s ability to function under various environmental conditions. Adjusting the pH from acidic to basic (2–11) conditions has a consequence on the receptor1 and receptor1–Fe2+ complex. In acidic pH, the fluorescence intensity of the receptor1–Fe2+ complex falls, increases at neutral pH, and then decreases again at basic pH (Fig. 7). The decrease in fluorescence intensity at acidic pH is because the imine (C=N) group of receptor1 gets protonated due to the increase in H+ ion concentration and cleavage may take place between the fluorophore and the heterocyclic group hindering the binding of Fe2+ ion with receptor1 (Li et al. 2006). On the other hand, even under the basic condition, the fluorescence intensity decreases. This is because of the deprotonation of H+ ion from a phenolic group of fluorophore due to the increase in the concentration of –OH ion. After deprotonation, the Na+ ion occupies the phenoxide group and results in O-Na+ which hinders the binding of Fe2+ ion with receptor1 leading to a decrease in fluorescence intensity. The change in fluorescence intensity at acidic and basic conditions is due to the presence of the fluorophore group in receptor1. The increased fluorescence intensity at 7.4 indicates that the receptor1 works at physiological pH of 7.4 which is suitable for biological and environmental application. Hence, all the other experiments are carried out at pH 7.4.

Fig. 7

Change in fluorescence intensity of A) Receptor1 B) Receptor1–Fe2+ with respect to pH

A study was carried out further to assess the time taken for the complex formation between receptor1 and Fe2+ (Kumar et al. 2018). The decrease in fluorescence intensity from 0 to 4th minute showed that the complexation between receptor1 and Fe2+ ion was unstable. After the 4th minute, the saturation in the graph indicates the stable complexation between receptor1 and Fe2+ ion. The solution was kept further for 60 minutes and the fluorescence intensity was noted. There were no desirable changes after four minutes (Fig. 8). Hence, the result shows that receptor1 favors for detection of Fe2+ ion in a short interval of time.

Fig. 8

Time taken for the complexation of receptor1–Fe.2+ in DMF–H2O solution (1:1 v/v, HEPES 50 mM, pH = 7.4) (λex = 275 nm)

Application studies

Determination of Feion in water sample

The synthesized receptor1 was subjected to the detection of Fe2+ ion in the water sample. Three different water samples (Bore well water (B1–3 , B2–6 µM), tap water (T1–3 , T2–6 µM), and RO drinking water (D1–3 , D2–6 µM)) are collected from Coimbatore Institute of Technology. The sample water is spiked with Fe2+ ion concentrations of 3 and 6 µM. About 2 mL of 4 × 10–6 M concentration of receptor1 was added to the spiked water sample. The recovery percentage of Fe2+ ion from spiked water sample after adding receptor1 is calculated from fluorescence spectroscopy (Fig. S4). The receptor1 showed the enhanced fluorescence intensity, but on adding the spiked water sample with receptor1, the quenching in fluorescence intensity is noticed. The difference in the fluorescence intensity is due to the complexation of Fe2+ ion with receptor1. The quenching in intensity proves that the receptor1 can attract Fe2+ ion from the spiked water sample. The recovery percentage of Fe2+ ion for B1, B2 is 96, 99.3%, for T1, T2 is 100.7, 98.8%, and for D1, D2 is 101.3, 99.5%, respectively (Table 2). The results proved that receptor1 could be practically applicable for selective detection of Fe2+ ion in real water sample analysis.

Table 2

Fe2+ ion recovered from real water sample spiked with Fe2+ ion using receptor1

CIT Water Sample	Fe2+ added (µM)	Fe2+ analyzed from fluorescence spectra (µM) mean[a] ± SD[b]	Recovery (%)	Relative error (%)
Borewell water 1	3.0	2.88 ± 0.15	96.0	– 4.0
Borewell water 2	6.0	5.96 ± 0.05	99.3	– 0.7
Tap water 1	3.0	3.02 ± 0.05	100.7	0.7
Tap water 2	6.0	5.93 ± 0.03	98.8	– 1.2
RO drinking water 1	3.0	3.04 ± 0.14	101.3	1.3
RO drinking water 2	6.0	5.97 ± 0.06	99.5	– 0.5

aMean of three measurement

bStandard deviation

Molecular logic gate

Simple molecular structures combinational molecular gates can be created by combining Boolean logic gates, allowing a single molecule to conduct complex processes. Combinational molecular logic gates shown in the literature are used as a molecular calculator that performs distinct arithmetic operations such as addition and subtraction; as a molecular keypad lock for information security; and as a fluorescence sensor for the detection of multiple metal ions. We presented an easy sequential molecular logic circuit in the form of a binary logic circuit for molecular level information processing that demonstrates the ‘Writing-Reading-Erasing-Reading’ way (Cakmak 2018; Zong 2009). The excellent reversible property of receptor1 helped to construct a molecular logic gate using two binary inputs. The absence and presence of Fe2+ and EDTA were represented by ‘0’ and ‘1’ (i.e., presence of Fe2+ and EDTA as 1 and the absence of Fe2+ and EDTA as 0). The output is monitored at 420 nm. Based on the change in fluorescence intensity, the truth table was constructed. The output ‘0’ denotes the quenching intensity (fluorescence-off) and ‘1’ denotes the enhanced intensity (fluorescence-on). Based on the truth table, the INHIBIT logic gate was constructed as shown in Fig. 9. Thus, the addition of Fe2+ to receptor1 shows the weak fluorescence and on adding EDTA to receptor1–Fe2+ complex the fluorescence intensity is enhanced (Table 3).

Fig. 9

INHIBIT molecular logic gate of 2-aminothiazolesalicylaldehyde (receptor1)

Table 3

Truth table of INHIBIT logic gate (λem- emission wavelength) 

Input 1 (Fe2+)	Input 2 (EDTA)	Output (λem 420 nm)
0	0	1
1	0	0
0	1	1
1	1	1

Molecular keypad lock

At the molecular level, the combinational logic gate is used for the imitation of variety of electronic devices. The main goal is to create new information processing pattern, by pushing over silicon-based technology which could result in electronic devices with compactness, power usage and efficiency. It is currently possible to integrate various imitating electrical components, such as comparators, memory units, and DE multiplexers. The recent application of molecular logic gate showed the mimicking behavior. A major electrical logic device, the keypad lock, has recently been molecularly replicated. This technology can be used in a variety of circumstances wherein admission to an object or information is limited to a small group of persons who know the valid password to open the keypad lock. The fundamental advantage of a keypad lock system over a simple logic gate is that its output signals are dependent not only on the right element of a sequence but also on the order in which they are delivered. As a result, a molecular device construction authorizing password entry is an extremely intriguing research goal because it represents a novel approach of safeguarding information at the molecular level (Wang et al. 2011). The excellent reversible property helped us to propose a sequence of code for the molecular keypad to lock. Three different chemical input was given as receptor1 ‘R,’ Fe2+ metal ion ‘F,’ and EDTA ‘E.’ The six feasible input fusions are RFE, REF, FRE, FER, ERF, and EFR. In these combinations, RFE resulted in enhanced fluorogenic output, whereas the other combination such as REF, FRE, FER, ERF, and EFR results in quenching intensity as output as shown in Fig. 10. The keypad contains the alphabet from A to Z, and the password RFE unlocks the keypad. At the same time, the other codes result in the wrong password. Due to this reason, this type of sensing will safeguard molecular-level information.

Fig. 10

Output for recepor1, corresponding to six possible input combinations of receptor1 series at 420 nm. Inset: a molecular keypad lock generating emission at 420 nm at correct password RFE entered. The keys R, F, and E hold the relevant input receptor1, Fe2+, and EDTA, respectively

Molecular docking studies

Molecular docking has been shown in several studies to be a valuable tool for analyzing the interaction of a protein receptor with its ligand and disclosing its binding mechanism. In this study, flexible docking techniques were used to find the best ligand. Binding sites have been found in the target proteins (Subhasri et al. 2014). Here, the structure-based receptor1 bound with NUDT5 could inhibit the activity of NUDT5 and thereby inhibit the proliferation of breast cancer cells. NUDT5 is a nucleotide metabolizing enzyme linked with ADP ribose and 8-oxoguanine metabolism. NUDT5 influences the hormones that regulate the gene pool and increases the breast cancer cells. The synthesized receptor1 acts as a ligand and it is subjected to molecular docking using Schrodinger Maestro software to assess its interactions with the protein. The performance of receptor1 was high when it is docked with NUDT5, a macromolecule that acts as a silence hormone signaling in breast cancer (PDB IB: 5NWH) (Sultana et al. 2019). In NUDT5, the aminoacid residues implicated in binding pockets are so predicted with the ligand molecule. Nine potential binding sites of NUDT5 involved in interaction with receptor1 are ILE 141, TRP 28, ARG 51, ALA 96, VAL 29, GLY 97, LEU 98, MET 132, and CYS 139 (Fig. S4). As a result, all of these binding pockets were confirmed to be significant and were used to generate the docking grid. The receptor1 binds with ASP 274 through hydrogen bonding at the ortho-position of the receptor1 moiety. The receptor1 has electrostatic interaction with LEU 178, VAL 174, LEU 169, LEU 164, PHE 144, VAL 147, LEU 210, LEU 208, and ALA 273, as well as it has a polar contact with ASP 274. The phenyl group of receptor1 appears to create a parallel, π–π stacked configuration, according to the molecular docking model (Fig. 11). The docking score of electrostatic interaction between receptor1 and protein is -5.23 kcal mol−1. The docking score of net negative energy represents a stable complex. The binding phenomenon is due to existence of heteroatoms, asymmetric charge distribution over the receptor and electrostatic interactions. The docking result indicates that the receptor1 has shown good affinity toward NUDT5 target. Thus, receptor1 may act as an inhibitor for blocking NUDT5 activity.

Fig. 11

2D Binding activity of receptor1 with NUDT5

Advantages over other methods

The adopted fluorescence method for the determination of iron is compared with the other reported methods (Table 4). From the table, it is inferred that the LOD and time for detection are superior in the proposed work compared to other reported methods (Andrea 2012, Alikhani et al. 2017, Xin 2009, Taeuk 2017, Ugo 2002). The accessibility, quickness, inexpensive, and specific detection are the other advantages of the proposed method.

Table 4

Comparison of the current work for determination of Fe2+ with other reported methods

ReferenceS	Detection of metal	Detection method	Solvent	Limit of detection	Time (min)
Andrea Spoalaor et al.	Fe2+	ICP-MS	–	0.01 ng mL−1	shorter
Amench AliKhani et al.	Fe2+	Flame atomic absorption	Methanol:CHCl3	5.6 µg L−1	< 1
Xin Jin et al.	Fe2+	Electrochemical	–	0.02 mM	–
Taeuk An et al.	Fe2+	Magnetic resonance imaging	Distilled water	–	4.26
Ugo et al.	Fe2+	Potentiometry	–	–	10
Present work	Fe2+	Fluorometric	DMF–H2O	0.0033 µM	< 4

Conclusion

A Schiff base receptor1 is synthesized using salicylaldehyde and thiazole moieties. The receptor1 is highly selective toward Fe2+ ion showing quenching with turn off–on process following reverse PET mechanism. The receptor1 selectively and preferentially binds with Fe2+ ion among other competitive metal ion. The association constant of receptor1–Fe2+ ion is 2.22 × 102 M−1 and limit of detection (LOD) is 33.7 × 10–9 M. The receptor1 detect Fe2+ ion at a very low concentration of 0.003 µM, which is lower than the EPA’s drinking water limits of 5.37 µM. The binding ratio of receptor1–Fe2+ is proposed to be 1:1 by Job’s plot. The receptor1–Fe2+ ion is found to be reversible with EDTA solution. Based on this property, a molecular logic gate is constructed with two inputs and one output and this type of receptor1 can be successfully applied for the mimicking behavior of molecular keypad lock which is highly utilized in security devices. The receptor1 is applied in real time for detecting Fe2+ ion in water samples and in molecular docking study. The Fe2+ ion recovery percentages for (Bore Water) B1, B2 are 96 and 99.3%, respectively, for (tap water) T1, T2 is 100.7 and 98.8% and for (drinking water) D1, D2 is 101% and 99.5%. Receptor1 docks with NUDT5, a hormone signaling a breast cancer cell, show the good docking score of – 5.23 kcal mol−1. To conclude, a new selective fluorescent receptor1 (2-aminothiazolesalicylaldehyde) is a good candidate which could be used for both biological and environmental system.

Below is the link to the electronic supplementary material.

Supplementary file1 (DOCX 3265 kb)