Cation Sensing of Pyridoxal Derived Sensors Towards Fe ( II ) Ion in Pure Aqueous Solution

A novel sensor (L) was synthesized using Schiff base reaction of pyridoxal with orthophenylene diamine and was characterized by various spectroscopic techniques such as FTIR, 1H NMR and Mass spectrometry. The cation recognition ability of the synthesized sensor was investigated by experimental (UV-VIS, IR and 1H NMR) and theoretical (B3LYP/6-31G**) methods. Among the tested anions, the developed sensor showed a naked-eye detectable color change from colorless to yellow and spectral changes in the presence of Fe(II) due to the formation of hydrogen bonded complexes with these cations followed by the partial deprotonation of sensor. With a micromolar detection limit, the developed sensor proved highly efficient and can be utilized for the colorimetric detection of Fe(II) ions.


Introduction
Designing of chemosensors which are capable of recognizing and sensing cations is one of the most bourgeoning areas in research, because of their important role in different biology, industrial and environmental processes .Among the various chemosensing systems, colorimetric sensors have great advantages due to its ability to offer a simple, sensitive, selective, accurate and economical method for the detection of a target analyte without the role of sophisticated instrumentation [25][26][27][28].The criteria for good sensors are stability, metal selectivity, metal affinity, signal transduction, kinetically rapid sensitization, ease of delivery to target systems and availability.More importantly, the sensor should work in pure aqueous medium.Among the transition metals, iron in the form of Fe 2+ /Fe 3+ , is the most important bioactive metal as it plays a crucial function in the transport and storage of oxygen and also in electron transfer in diverse metalloenzymes.It is truly ubiquitous in living systems and tracking its homeostasis using a suitable technique is of great signi icance to clarify its biological effects [29,30].However, both its de iciency (hypoferremia) and excess (hyperferremia) can induce a variety of diseases.Several serious diseases such as Alzheimer, Parkinson's and Huntington's disease etc. known to occur due to the cellular toxicity caused by iron ions [31][32][33][34][35].The biological importance as well as the adverse effects of the metal ions Fe 2+ /Fe 3+ encourage us to develop some novel chromogenic sensors using the vitamin B6 cofactor.In this work, the cation sensing of the receptors L was investigated.The molecular structure of the receptor L was shown in Figure 1.The cation sensors developed showed striking color change in pure aqueous medium for the selective detection of cations Fe 2+ .Different analytical parameters of the receptors such as the binding constant for the interaction of cations with the receptors, binding stoichiometry, detection limit, effect of pH, interference of other metal ions etc. were studied in this chapter.Also, the experimental outcomes were complemented with theoretical results.

Materials and Methods
The reagents and chemicals were purchased commercially either from SpectroChem Pvt.Ltd or Sigma-Aldrich.Solvents such as DMSO and methanol were used without any further purification.The cations used for the sensing studies were in the form of their sulphates/ chloride salts, and were purchased from Rankem Pvt. Ltd., India.
All the experiments were carried out at 298 K, unless otherwise mentioned. 1H NMR spectra were recorded on a BRUKER AVANCE II 400 MHz NMR in DMSO-d6 using Tetramethylsilane (TMS) as an internal standard.The IR spectra were recorded on a Perkin-Elmer IR Spectrophotometer using KBr pellet.The mass spectra were recorded on a Waters, Q-TOF micromass (LC-MS).Melting point were recorded with a digital melting point apparatus VMP-DS "VEEGO" and is uncorrected.UV-Vis spectra were recorded on a VARIAN CARY 50 Spectrophotometer in the wavelength range of 200-700 nm with a quartz cuvette (path length=1 cm at 25°C) and width of the slits used is 2.5/5 nm.For all the spectroscopic experiments, stock solutions of the receptors (1.0 × 10 -3 M and 1.0 × 10 -4 M) and the metal ions (1.0 × 10 -3 M and 1.0 × 10 -4 M) were prepared in water and stored under a dry atmosphere.These solutions were used for various experiments after appropriate dilutions.For spectroscopic titrations, required concentration of the receptor (2 ml) was taken directly into the cuvette and the spectra were recorded after successive addition of selective metal ions by using micropipette.

Synthesis of Fe 2+ L complex
Receptor L was synthesized according to our reported method [33].Hot ethanolic solution of receptor L (0.306 g, 0.0011 mol) and to it corresponding FeSO 4 (0.305 g, 0.0011 mol) were added together with constant stirring.The mixture was refluxed for 3 h at 70-80°C.On cooling, reddish brown colored solid metal complex of Fe 2+ .L were precipitated out.The precipitated metal complex was filtered, washed with cold ethanol and dried under vacuum.Yield: 70%; M.P.=280°C; FT-IR (KBr disc, υ max /cm

Results and Discussion
Receptor L was synthesized by following the reported method [36][37][38], and its sensing ability was tested towards various metal ions.The colorimetric sensing ability of receptor L was investigated in aqueous solution upon addition of various metal ions such as Al 3+ , Cd 2+ , Co 2+ , Cr 3+ , Cu 2+ , Fe 3+ , Mn 2+ , Ni 2+ , Ba 2+ , Fe 2+ , Fe 3+ , Hg 2+ and Zn 2+ .The receptor L alone gave absorbance peak at 250 nm and 350 nm.Upon addition of equivalent amount of each cation, receptor L showed almost no change in absorption peak in the presence of Al 3+ , Cd 2+ , Co 2+ , Cr 3+ , Fe 3+ , Mn 2+ , Ni 2+ , Ba 2+ , Hg 2 , Cu 2+ and Zn 2+ , while the presence of Fe 2+ ion led to a red shift and gave rise to new broad absorption band between 350 nm to 650 nm (Figure 2).Also, selective sensing of Fe 2+ ion was done in an aqueous system through a color change from colorless to yellow.No detectable color change of L was observed in the presence of other tested metal ions.These results clearly delineated the selective Fe 2+ sensing ability of the receptor L in pure aqueous medium.
The binding ability of receptor L with Fe 2+ was further studied by UV-V is titration experiment.Upon successive addition of Fe 2+ to the solution of receptor L, the new charge transfer absorption band in the visible region was observed (Figure 3).From the UV-Vis absorption titration, the association constant (K) of receptor L with the Fe 2+ was calculated using B-H equation (inset Figure 3) and the K value was found to 9.81 × 10 -4 M. Good linearity was observed in the 45 μM to 95 μM range for Fe 2+ ion with the detection limit down to 8.1 × 10 -6 M (Figure 4).The Job plot analysis showed a 1:1 stoichiometry for the Fe 2+ and L complex (Figure 5).Accordingly, the proposed structure of 1:1 complex formation of receptor L with Fe 2+ was shown in Figure 6.The molecular structure of L and its L.Fe 2+ complex was theoretically optimized in the gas phase by applying the DFT method (B3LYP/6-31G**/LANL2DZ) coded in the computational program Gaussian 09W (Figure 7), and the changes in the electronic properties were compared with the experimental results.The basis set LANL2DZ was considered only for Fe atom whereas 6-31G** for remaining atoms (C, H, N and O).On complexation of 3 with Fe 2+ resulted in the lowering in the interaction energy by -199.80 kcal/mol, which indicates the formation of a stable complex.The Frontier Molecular Orbitals (FMOs) plots (Figure 8) of L and its L.Fe 2+ complexes was next analyzed which indicate the intramolecular charge transfer occurred between the receptor and Fe 2+ .In addition, it was observed that the band gap between the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of L was significantly lowered on complexation with Cu 2+ .These theoretical results corroborate well with the red-shift in the absorption band of L on complexation with Fe 2+ due to ICT process.The preferential selectivity of receptor L as colorimetric chemosensor for the detection of Fe 2+ ion in the presence of various competing metal ions was carried out.For competition studies, receptor L was treated with 0.5 equivalent of Fe 2+ in the presence of 0.5 equivalent of other metal ions (Figure 9).No significant interference was observed in the detection of Fe 2+ ion in presence of Al 3+ , Cd 2+ , Co 2+ , Cr 3+ , Fe 3+ , Mg 2+ , Ni 2+ , Ba 2+ , Cu 2+ , Hg 2+ and Zn 2+ ion.Thus, receptor L could be used as a selective colorimetric sensor for Fe 2+ ion in the presence of most competing metal ions including Fe 3+ .Further, the effect of pH on the absorption response of Fe 2+ .L complex was carried out with different pH values ranging from 4 to 12 (Figure 10).The color of the Fe 2+ .L complexes remained in the yellow region between pH 6 and 12.These results indicate that the detection Fe 2+ ion using receptor L over a wider pH range and importantly in the physiological pH.In addition, to investigate the practical application of receptor L, a test paper kit was prepared by immersing filter paper in an aqueous solution of receptor L (5.0 × 10 -5 M) and then dried in the air.The dried test paper kit was treated with the Fe 2+ ion solution at 1.0 × 10 -3 M. As shown in the Figure 11, the detectable color change of test kit coated with the receptor L solution would be convenient for detecting Fe 2+ metal ions.

Conclusion
Receptor L showed micromolar recognition of Fe 2+ ion in a fully aqueous solution.Selective sensing of Fe 2+ ion was done in aqueous system through a colour change from colourless to yellow brown.The Job plot analysis showed a 1:1 stoichiometry for the Fe 2+ .L complex.The K value was found to be 9.81 × 10 4 M.The linearity was observed in the range of 45 μM to 95 μM for Fe 2+ ion with the detection limit down to 8.1 × 10 -6 M.

Figure 2 :
Figure 2: UV-Vis absorption spectral changes of receptor L (5.0 × 10 -5 M) upon addition of equivalent amount of different cations in aqueous solvent.

Figure 3 :
Figure 3: Changes in the absorption spectrum of receptor L (1.0 × 10 -5 M) upon addition of incremental amounts of Fe 2+ ion (1.0 × 10 -3 M) in water.Inset showing the BH plot.

Figure 4 :
Figure 4: Calibration curve of receptor L.

Figure 10 :
Figure 10: Effect of pH on the UV-Vis absorption spectra of L (5 × 10 -5 M) containing equimolar amount of Fe 2+ .Inset showing the color change.

Figure 11 :
Figure 11: Test paper kit of receptor L in presence of Fe2+ ion.