SYNTHESIS OF CHELATING IONOPHORES AND THEIR ELECTROANALYTICAL STUDIES AS CHEMICAL SENSORS

Abstract

Environmental pollution is defined as the undesirable changes in physical, chemical and biological characteristics of ecosystem. It is therefore important to study the adulteration caused by pollution and its consequences on the ecosystem. Although, increased urbanization and rapid industrialization has improved the standard of life, but associated pollution has adversely affected the quality of life. Some adverse health effects because of metal toxicity are many nutritional deficiencies, neurological disorders, cancer and other debilitating chronic diseases. Monitoring of environmental pollution caused by toxic metals has been one of the primary concern of researches in present time. In view of metal toxicity, attempts have been made to quantify them in environmental samples. Numerous analytical techniques have been developed for quantifying analytes and these includes AAS (atomic absorption spectrometry), ICP-MS (inductive coupled plasma-mass spectrometry), ion chromatography, flame photometry, cyclic voltammetry, isotopic dilution, radiometric activation analysis, fluorescence, chemiluminescence phosphorescence techniques and high performance liquid chromatography etc. Although, these techniques provide accurate results but their widespread use in the analysis of large number of environmental sample is limited due to fact that their operation require expertise and large infrastructure backup. Thus, there is a need to develop a better method which involves simple instrumentation, inexpensive and fast method of analysis with minimum chemical manipulation. Such requirements are generally met with ion-selective electrodes (ISEs), which have emerged as promising tools for direct measurement of various species because of the advantages offered in terms of better selectivity and sensitivity, easy handling and cheap. The technique is generally non-destructive, adaptable to small sample volume with possible application in real-time analysis. Moreover, analysis of colored and viscous samples can also be carried out easily. The ISEs are routinely used not only for the analysis of clinical, industrial, agricultural and environmental samples but are also used as detectors in HPLC and capillary electrophoresis. The development of potentiometric membranebased ion sensor with high selectivity remains a formidable challenge. The present work is aimed to design and synthesize new ligands and their complexes for use as electroactive components (ionophore) in the preparation of membranes for determination of various cations. The work embodied in the thesis has been organized in seven chapters. A brief report on the text of various chapters is described below. (ii) First chapter is on “General introduction to the thesis”. A review of literature on various sensors used for quantification of metal ions and anions is incorporated in this chapter. It also presents briefly classification of ISEs, methods of preparation of membranes and theory of membrane potential. The determination of selectivity coefficient of ISEs is described in this chapter and the significance of selectivity coefficient is also critically discussed. Besides this, the objective of present research activity is also presented at the end of this chapter. The Second Chapter of the thesis entitled “Membranes of Macrocyclic Chelating ligand as Cd2+ Ion-Selective Sensors” deals with the synthesis and characterization of a macrocycle viz. 5,11,17-trithia-1,3,7,9,13,15,19,20,21-nonaazatetracyclo-[14.2.1.14,7.110,13]- henicosa-4(20),10(21),16(19)-triene-6,12,18-trithione (L1) and their use as ionophores application in the preparation of Cd2+ selective ISEs. Preliminary studies on L1 have showed that it has more the affinity towards Cd2+ ion. Thus, L1 was used as an ionophore for the preparation of PVC-membrane sensor for Cd2+ ion. Three electrodes polymeric membrane electrode (PME), coated graphite electrode (CGE) and coated pyrolytic graphite electrode (CPGE) were prepared and investigated as Cd2+ sensor. Their performance characteristics were compared and it was found that of three electrodes CPGE gives the best performance. The best CPGE was found to exhibit quick sensing (10 s) and long durability (4 months), useful pH range of 2.5-8.5. This electrode exhibits low detection limit of 7.58×10-9 mol L-1 and a Nernstian slope of 29.6 mV decade-1 of activity. Applicability of the sensor was evaluated for Cd2+ quantification in numerous samples (water, soil and medicinal plants) and also as a potentiometric indicator electrode. The Third Chapter of the thesis entitled “Diaminopyrimidin Based Chelating Ionophore as Ni2+ Ion-Selective Sensors” deals with the synthesis and characterisation of ligand 5,5'-((3-nitrophenyl)methylene)bis(2,6-diaminopyrimidin-4(3H)-one) (L2). The L2 prepared shows high affinity for Ni2+ ion and thus can be used for the preparation of Ni2+ sensor. CGE and CPGE were prepared using L2 as an ionophore. A number of solvent mediators were used to improve the performance of Ni2+ sensor and it was found that the solvent mediator o-NPOE produces best effect. CPGE with the membrane of optimized composition (L2: PVC: NaTPB : o-NPOE ≡ 7:33:2:58) was found to display linearity (2.04×10-8 - 1.0×10-1 mol L-1), Nernstian slope ( 29.4±0.2 mV decade-1 of activity) and LOD (lower detection limit) of 8.12×10-9 mol L-1and independent of pH (3.0 to 9.0.) This sensor exhibits fast response time of 8 s. The sensor was found to exhibit high selectivity (iii) over a number of metal ions and that`s why quantification of Ni2+ ion in analytical samples (water, soil and plant leaves) reflect the utility of sensor and has also been used as potentiometric indicator electrode in the titration (Ni2+ vs EDTA). The Fourth Chapter entitled “Polydentate Heterocyclic Chelating Ionophores as Cu2+ Ion- Selective Sensors” deals with synthesis and characterization of a number of ligands viz., L3 (1,3-bis[2-(1,3-benzothiazol-2-yl)-phenoxy]propane) and L4 (1,2'-bis[2- (1,3-benzothiazol-2-yl)-phenoxy]-2-ethoxy ethane) and their analytical application as Cu2+ selective electrodes. Preliminary study have shown that two ligands viz., L3 and L4 show high affinity for Cu2+ and hence can be used for the preparation of Cu2+ selective electrode. A number of PVC-based electrodes were prepared. A comparative study of several polymeric membrane electrodes show that the electrodes with the membrane composition (w/w, mg) L4: PVC: NaTPB: 1-CN≡ 6:53:2:39 is found to exhibit detection limit as low as 6.30×10-9 mol L-1, Nernstian slope (29.5 mV decade-1 of activity) and fast response time of 9 s. It also has sufficient life time of 5 months and can be used over a pH range of 2.0- 8.5.The electrode show good selectivity over a number of metal ions and could therefore be employed in quantification of Cu2+ in analytical samples (water and soil). Cu2+ was also estimated in medicinal plant samples and besides this the sensor was also used as potentiometric indicator electrode in the estimation of Cu2+. The Fifth Chapter entitled “N3O2 Chalcone Ligand as Ce3+ Ion-Selective Sensors” deals with the synthesis and characterisation of novel ligand L5 (1,1'-(pyridine-2,6- diyl)bis(3-(1H-pyrrol-2-yl)prop-2-en-1-one)) and explored as ion carrier for the selective monitoring of Ce3+ ion in various samples. Colorimetric and conductometric studies performed on the L5 show that it has high affinity for the Ce3+ ion and therefore, L5 was used in the fabrication of poly (vinyl chloride) based membrane sensors PME, CGE and CPGE were prepared and investigated. Best performance was observed with CPGE having optimized membrane of composition (w/w, mg) L5: PVC: o-NPOE: NaTPB≡7:31:59:3. The CPGE exhibits linearity of 1.9×10-8 mol L-1 and detection limit down to 5.0×10-9 mol L-1 with Nernstian slope of 19.4±0.2 mVdecade-1 of activity. The sensor response is independent of pH in the range of 3.0-8.5 and show quick sensing (9 s). The sensor applicability in partially non-aqueous mixture (water-acetonitrile and water-ethanol) was examined and found that sensor could tolerate about 20% (v/v) of non- aqueous content. The utility of sensor is shown in quantification of Ce3+ in various samples. Further, the sensor was employed as potentiometric indicator electrode (Ce3+ vs F- and C2O4 2- titration). (iv) The Sixth Chapter entitled “Multidentate Schiff Bases of Isonicotinohydrazide as Mn2+ Ion-Selective Sensors” deals with synthesis and characterization of two Schiff bases viz., L6 (N'(N',N'''E,N',N'''E)-N',N'''-((((oxybis(ethane-2,1-diyl))bis(oxy))bis(2,1- phenylene))bis(methanylylidene))di(isonicotinohydrazide)) and L7 ((N',N'''E,N',N'''E)- N',N'''-(((propane-1,3-diylbis-(oxy))bis(2,1-phenylene))bis(methanylylidene))di(isonicotinohydrazide)). Preliminary study on the two Schiff bases L6 and L7 have shown that they have strong affinity for Mn2+. Therefore, two Schiff bases have been selected as ionophores for the fabrication of Mn2+ selective sensors. The electrodes prepared are based on PME of L6 and L7. Three electrodes PME, CGE and CPGE were prepared and investigated. The CPGE with optimized membrane of composition (L7: PVC: o-NPOE: NaTPB≡6:34:58:2) display broad working concentration 1.23×10-8-1.0×10-1 mol L-1, detection limit as low (4.78×10-9 mol L-1) and Nernstian performance (29.5±0.4 mV decade-1 of activity). The useful working pH range has been found 3.5-9.0 and response of the electrode is quite fast (9 s). The sensor can be used for long duration (4 months) without any considerable divergence in its property. In view of good selectivity of the sensor for Mn2+ over large number of metal ions, the electrode could be used to monitor Mn2+ quantitatively in a number of samples and was employed as potentiometric indicator electrode (titration of Mn2+ against EDTA). The Seventh Chapter entitled “Multidentate Schiff Bases of Hydrazinecarbothioamide as Co2+ and Zn2+ Ion-Selective Sensors” deals with synthesis and characterization of a number of Schiff bases viz., L8, L9, L10, L11, L12 and L13. Preliminary study have shown that L8 (2,2'-(((ethane-1,2-diylbis(oxy))bis(3-methoxy-4,1- phenylener))bis(methanylylidene))bis(hydrazinecarbothioamide)), L9 (2,2'-(((propane-1,3- diylbis(oxy))bis(3-methoxy-4,1-phenylene))bis(methanylylidene))bis(hydrazinecarbothioamide)) and L10 (2,2'-((((oxybis(ethane-2,1-diyl))bis(oxy))bis(3-methoxy-4,1- phenylene))bis(methanylylidene)bis(hydrazinecarbothioamide)) show higher affinity for Co2+. Thus, they have been used as ionophores for the preparation of membrane electrodes for Co2+ determination. While preparing the membranes the effects of different plasticizers viz., dioctylphthalate (DOP), dibutylphthalate (DBP), benzylacetate (BA), and 1- choronaphthelene (1-CN) and anion excluders potassium tetrakis-p-(chlorophenyl)borate (KTpClPB) and sodium tetraphenylborate (NaTPB) have also been seen. Polymeric membrane CGE and CPGE were prepared and investigated. A comparative study of all the membrane electrodes have shown that CPGE with membrane composition of L9: PVC: 1- CN: KTpClPB≡6:35:56:3 (w/w, mg) gives the best performance. It shows good linearity (1.4×10-8-1.0×10-1 mol L-1) and detection limit as low (6.1×10-9 mol L-1) with Nernstian (v) compliance (29.4±0.6 mV decade-1 of activity) and fast response time of 7 s. This electrode (CPGE) shows high selectivity over a number of metal ions and could therefore be used successfully for the quantification Co2+ in electroplating waste, medicinal plant and water aliquots and also was employed potentiometric indicator electrode in the titration of Co2+ ion against EDTA. The remaining three Schiff bases L11 (2,2'-(((ethane-1,2-diylbis(oxy))bis(2,1- phenylene))bis(methanylylidene))bis(hydrazinecarbothioamide)), L12 (2,2'-(((propane-1,3- diylbis(oxy))bis(2,1-phenylene))bis(methanylylidene))bis(hydrazinecarbothioamide)) and L13 (2,2'-((((oxybis(ethane-2,1-diyl))bis(oxy))bis(2,1-phenylene))bis(methanylylidene))bishydrazinecarbothioamide)) have been found to show higher affinity for Zn2+ and therefore membranes were fabricated using these ligands as ionophores. Several electrodes of these ionophores were prepared and investigated. A comparison of performance of various electrodes has shown that CPGE with the membrane composition of L13: PVC: DBP: NaTBP≡6:34:58:2 (w/w, mg) exhibit best response of all ISEs performance parameters. For example, this sensor works over range of working concentration (1.1×10-8-1.0×10-1 mol L- 1), detection limit of 8.1×10-9 mol L-1and display Nernstian slope (29.6±0.2 mV decade-1 of activity of Zn2+ ion) with quick sensing (10 s). The electrode exhibited self life time of about 4 months and was found independent of pH (3.0-9.0). The grater selectivity of the sensor for Zn2+ ion in presence of interfering ions permits the use of this electrode to determine Zn2+ in water, medicinal plant and soil samples and as potentiometric indicator electrode. The performance characteristics of the sensor for the determination of Cd2+, Ni2+, Cu2+, Ce3+, Mn2+, Co2+ and Zn2+ ions have been found to be not only comparable but better in some respects over reported sensors. Thus, the present work adds to our knowledge in the field of chemical sensors.