Voltammetric sensors based on schiff bases for cation/anion recognition

Abstract

Potent electrochemical sensors developed using Schiff base as an additive for the fabrication of screen-printed electrodes (SPE) were explored. A series of Schiff base derivatives having methylene bridge forming a wedge between two Schiff base motifs have been selected as modifier for the fabrication of modified SPEs. Structural variation of Schiff bases depends upon the presence of secondary substituent around each imine bond. Substituent can be either electron withdrawing or electron donating group which ultimately affects the recduction potential of imine double bond. Further, coordination ability of the ionophore was also influenced. The electrochemical and ion-recognition behavior of ionophore molecules namely, SMS-0, SMS-1, SMS-2, SMS-3, SMS-4, SMS-5a and SMS- 5b (used for modification) have been discussed in detail using differential pulse voltammetric (DPV) and cyclic voltammetric (CV) studies. For the modification of working electrodes of the SPEs, ionophores were used as additives and mixed with carbon ink before fabrication. With the inclusion of the ligand in the ink, performance of the SPE enhanced. Scanning electron microscopy (SEM), CV data and Energy-dispersive X-ray spectroscopy (EDX) and was analysed in order to confirm the modification. Differential pulse voltammograms of SMS-0 modified SPEs in the presence of Al (III) showed a characteristic cathodic current at −0.054 V. This characterstic peak was used for quantitative application of ionophore. A linear response of the sensor was found in concentration range of 0.67–4.59 μg L−1 of the analyte. The detection limit was found to be 2.26 ng L−1 for Al (III). The proposed chemosensor selectively detects Al (III) in the presence of some alkali, alkaline earth, transition metal ions like Zn (II), Cd (II), Hg (II) and Pb (II). Complexation behavior of the ionophore for the metal ion has also been supported by theoretical studies. The method has been used for the determination of Al (III) in the real-life samples namely, river water (Ganga), tap water and green tea. The proposed voltammetric sensor can be reused after washing with EDTA (0.1 M) as a cleaning agent. A disposable voltammetric sensor based on SPE modified with SMS-1 has been reported for the detection of ferrous ions in aqueous medium. Detection of Fe (II) is based on change in voltammetric response of the receptor after complexation. Under optimized experimental conditions, peak current of the complex depended linearly on the amount of Fe (II) in the concentration range of 0.6 μM–4.0 μM with the limit of detection (LOD) as 0.54 μM, in an aqueous medium. The proposed sensor can selectively detect Fe (II) in the presence of a number of commonly occurring interfering ions. The method has been used to sense ferrous ions in wheat, lentil and barely seed samples. Moreover, modified voltammetric sensor can be used 3–4 times after washing with EDTA. Receptor–Fe (II) complex formation has also been supported by DFT studies. Further, disposable electrode modified with SMS-2 could selectively detect ferric ions in the aqueous medium. Shifting/quenching of the peak potential of the ionophore on binding with Fe (III) ions was used to quantify the ferric ion concentration. This sensor can detect Fe (III) in the detection range of 0.62 μM to 7.5 μM. The LOD is reported as 0.93 μM. The modified SPE could selectively detect ferric ions in the presence of many other ions. It has been successfully used to find Fe (III) content in blood serum. DFT calculations were used to study energetics of the metal–ionophore interactions. An electrochemical sensor for Zn (II) ions in aqueous medium has been developed using disposable screen printed electrodes in which working electrode has been modified using an ionophore, SMS-3 ligand. The characteristic cathodic peaks at −0.59 V and 0.55 V of the ionophore were quenched in the presence of Zn (II) ions. SMS-3 modified SPE works in the detection range of 0.47 to 5.56 μM with a limit of detection (LOD) 0.92 μM. DFT studies on SMS-3 supported the observations for the modified SPE. Further, sensitivity and selectivity of the electrode were also examined in the presence of various interfering ions which can be present in real-life samples and found that these ions did not interfere in the detection of Zn (II) ions. The electrodes were applied for the detection of Zn (II) in food samples such as bread, rice and juice. Another Schiff base derivative (SMS-4) contains pyridine ring as a substituent adjacent to the imine bond. SMS-4 modified SPE was investigated for its interaction with different metal ions. After testing with alkali, alkaline, transition metals and lanthanides, it was found that modified electrode was selective for Ce (IV) ions and could be used for the detection of Cerium ions with LOD as 0.48 μM. in aqueous medium. Further, this sensor can detect Ce (IV) ions without facing any effect of interference of commonly occurring metal ions and lanthanides and can be used for analysis of Cerium (IV) ions in spiked soil and water samples. Interaction of the SMS-5a and SMS-5b modified SPEs was investigated and analysis showed that prepared probe is selective towards Cu (II) ions and successfully used for the trace level detection of Cu (II) in aqueous medium. The workable concentration range of the electrode was found as 0.6 μM to 3.7 μM. The limit of detection for Cu (II) detection was 0.70 μM calculated using 3σ/m. Practical applicability of the SMS-5 modified SPE was tested by detecting Cu (II) ions present in real-life samples after extracting it in water. In addition, behavior of the SPEs modified using different techniques has also been studied electrochemically. Working electrode of the SPE was modified with SMS-1 ionophore using three different methods. Method 1 was based on drop-coating of the ionophore at bare surface of the working electrode. Second method included drop-casting of the receptor molecule at the silver nanoparticle (AgNPs) modified SPE. While in method 3, electrode surface was modified by mixing the ionophore with the graphite ink (used for printing of WE) during fabrication of the SPE. It was found that method 3 is better than other two techniques, as better response in terms of current magnitude and sharpness of DPV was shown by the method 3. Hence, all the sensing studies were carried out with the SPEs modified with method 3.