SYNTHESIS AND ANALYTICAL APPLICATIONS OF POLYDENTATE MACROCYCLES AS CHEMICAL SENSORS

Abstract

Environmental pollution is a major global problem posing serious risk to man and animals. It is defined as the human alteration of chemical or physical characteristics of the environment to a degree that is harmful to living organisms. Rapid pace of urbanization, industrialization and indiscriminate use of chemicals such as pesticides and drugs have resulted in increased contamination and degradation of the environment leading to adverse health effects on living beings and problems associated with residues in food of animal origin. Recent emergences in environmental related toxicity in man and animals and detection of chemical residues in food products at an increasing frequency have underlined the impact of pollution on biotic component. The deleterious health effects may be observed in the form of overt clinical signs and higher morbidity and mortality or as subtle or subclinical effects. Heavy metal toxicity has posed a serious threat to mankind by its incorporation to the food chain resulting in the degradation of ecosystem. Some heavy metals are essential for most of the redox reactions which are fundamental for cellular functions. However, heavy metals become toxic when they are not metabolized by the body and get accumulated in tissues and organs which results in many nutritional *£ deficiencies and can lead to neurological and autoimmune disorders, cancer and other debilitating chronic diseases. Thus, monitoring and evaluation of heavy metal ions in the environment have received increasing attention from both nutritional and toxicological point of view. A number of analytical techniques such as atomic absorption spectroscopy, inductively coupled plasma atomic emission spectroscopy, inductively coupled plasma mass spectrometry, isotopic dilution, radiometric neutron activation analysis, fluorescence, chemiluminescence phosphorescence techniques, high performance liquid (i) chromatography, ion chromatography, voltammetry etc. are available for routine analysis of metal ions in environmental samples. Although, these methods provide accurate and reproducible results, but suffers from high capital cost, large scale infrastructure back up and requires adequate expertise which makes them unsuitable for online analysis and routine analysis of large number of environmental samples. Besides this, the use of these techniques can be problematic and erroneous in coloured and turbid solutions. Thus, there is vital need for the development of selective, portable and inexpensive diagnostic technique for the determination of various metal ions. Ion-selective electrodes (ISEs) have proved promising alternative which overcomes or minimizes the above drawbacks. ISEs find applications in a variety of fields like clinical, environmental, industrial, agricultural and process monitoring, as well as detectors in HPLC and capillary electrophoresis. Ion-selective electrode consists of a semipermeable membrane that separates two different solutions of an appropriate electrolyte and responds selectively to a particular ion in presence of other ions. The membrane constitutes an active ion-exchanger ingredient generally called an ionophore or electroactive material. The design and function of synthetic ionophores is based on diverse parameters viz structure and cavity size of the ligand, stability and selectivity of its metal ion complex, its solubility and the ability to extract the metal ion into membrane phase. Macrocycles are a favoured class in this area as their complexes have high stability constant and sufficient conformation flexibility for rapid ion exchange. The strong and selective interaction of macrocycles with specific metal ions makes these ligands suitable candidate to be used as ionophores in the construction of ion-selective electrodes. The field of ion-selective membrane research is a vigorous and ever expanding one. ISEs have been not only practical solutions to a great variety of analytical problems but also useful tools for probing host-guest chemistry. The aim of the present work is to (ii) synthesize new polydentate macrocyclic ligands and their complexes which are used as electroactive component in the preparation of membranes for determination of various metal ions. The performance of the Polymeric membrane electrode has also been compared with another electrode i.e. Coated graphite electrode having membrane of same ingredients. The electrode which shows better response characteristics in terms of Nernstian response, working concentration range, detection limit, response time and selectivity has been used for real sample analysis. For the sake of convenience, simplicity and clarity, the work embodied in the thesis has been organized as follows: The First Chapter of the thesis is on"General Introduction" and presents a review of all the literature on the previously reported sensors of alkaline, alkali earth, transition, rare earth metal ions and anions. The problem of present research activities have also been posed in the context of the cited work. The Second Chapter on "Theory and Methodology''1 encompasses classification of ISEs, theory of membrane potential and various terms used in the description of ion selective electrodes. The description of the selectivity of sensor membrane and method of its determination has also been described. The Third Chapter of the thesis "Membranes of Macrocyclic ligands as cation selective sensor for transition metal ions" deals with synthesis and characterization of polydentate macrocycles and pendant armed macrocyclic ligands and their analytical application in the preparation of cation-selective electrodes. Polymeric membrane electrode (PME) and coated graphite electrode (CGE) for nickel ion were prepared based on 2,9-(2-methoxyaniline)2-4,ll-Me2-[14]-l,4,8,ll-tetraene-l,5,8,12-N4 (I) as a suitable neutral ionophore. In order to improve the performance characteristics of PVC based membranes, the addition of various plasticizers viz. o-nitrophenyloctylether (o-NPOE), (iii) dioctylphthalate (DOP), dibutylphthalate (DBP), 1-chloronaphthalene (CN) and tri-nbutylphosphate (TBP) and ionic additives sodium tetraphenylborate (NaTPB) was tested for optimization of membrane composition and their potential response was investigated. The best performance was obtained for the membrane sensor having a composition of I:PVC:TBP:NaTPB as 6:90:100:4 (w/w; mg). The electrodes exhibit Nernstian slopes for 94- 7 II Ni ions over wide concentration ranges of 4.6 x 10" to 1.0 x 10" mol L" for PME and 7.7 x 10"8 to 1.0 x 10"' mol L"1 for CGE with limits of detection of 2.7 * 10"7 mol L"1 for O 1 PME and 3.7 x 10" mol L" for CGE. The response time for PME and CGE was found to be 10 s and 8 s respectively. The potentiometric response of both electrodes is independent of the pH of the test solution in the pH range 3.0-8.0. The proposed electrodes revealed a good selectivity over a wide variety of cations including alkali, alkaline earth, transition and heavy metal ions. Since CGE showed better results over PME, it was used as an indicator electrode in the potentiometric titration of nickel ion with EDTA and in direct determination of Ni +ion in different fruit juicesand wine samples. Poly(vinyl chloride) (PVC) based membranes of macrocycles 2,3,4:9,10,11- dipyridine-l,3,5,8,10,12-hexaazacyclotetradeca-2,9-diene [11(a)] and 2,3,4:9,10,11-di pyridine-1,5,8,12-tetramethylacrylate-1,3,5,8,10,12-hexaazacyclotetradeca-2,9-diene [11(b)] were prepared and investigated as Co2+ selective electrodes. The best performance was observed with the membranes having the composition Il(b):PVC:TBP:NaTPB in the ratio of 6:39:53:2 (w/w; mg). The performance of the membrane based on 11(b) was compared for PME and CGE. The PME exhibits detection limit of 4.7 x 10"8 mol L"1 with a Nernstian slope of 29.7 mV decade"1 of activity between pH 2.5-8.5 whereas CGE exhibits the detection limit of 6.8 x 10"9 mol L" with a Nernstian slope of 29.5 mV decade" of activity between pH 2.0-9.0. The response time for PME and CGE was found to be 11 s and 8 s respectively. The CGE has been found to work satisfactorily in partially (iv) non-aqueous media upto 35% (v/v) content ofmethanol, ethanol and 25% (v/v) content of acetonitrile and could be used for a period of4 months. The CGE was successfully applied for the determination of Co2+ in real and pharmaceutical samples and as an indicator electrode in potentiometric titration of cobalt ion. The construction and performance characteristics of polymeric membrane electrodes based on two newly synthesised macrocyclic ligands 6,7:14,15-Bzo2-10,ll-(4- methylbenzene)-[15]-6,8,12,14-tetraene-9,12-N2-l,5-02 111(a) and 6,7:14,15-Bzo2-10,11- (4-methylbenzene)-[151-6,14-diene-9,12-dimethylacrylate-9,12-N2-1,5-02 IH(b) for quantification ofZn2+ ions were investigated and the best response was observed for the membrane having composition III(b):PVC:TBP:NaTPB in the ratio of 4:37:57:2 (w/w; mg). The response characteristics ofPME based on 111(b) was also compared with CGE. The electrode exhibits Nernstian slope for Zn2+ ions with limits of detection of 3.3 x 10"7 mol L"1 for PME and 7.9 x 10"8 mol L"1 for CGE with response time of 12 s and 10 s for PME and CGE respectively. Futhermore, the electrodes generated constant potentials in the pH range of 3.0-8.0 for PME and 2.5-9.0 for CGE. The practical utility of the CGE has been demonstrated by its usage as an indicator electrode in potentiometric titration of EDTA with Zn2+ ion solution. The high selectivity of CGE also permits their use in the determination of Zn2+ ions in water, biological, milk andtea samples. The Fourth Chapter of the thesis "Membranes of Macrocyclic ligands as cation selective sensor for rare earth metal ions" includes the synthesis and characterization of macrocyclic ligands which act as electroactive component in the fabrication of polymeric membrane electrode and coated graphite electrode for the quantification of rare earth metal ions in real samples. The two macrocyclic pendant ligands 3,4,5:12,13,14- dipyridine-2,6,11,15-tetramethyl-1,7,10,16-tetramethylacrylate-1,4,7,10,13,16-hexaazacyclooctadeca- 3,13-diene [IV(a)] and 3,4,5:12,13,14-dipyridine-2,6,ll,15-tetramethyl- (v) 1,7,10,16-tetra(2-cyanoethane)-1,4,7,10,13,16-hexa- azacyclooctadeca-3,13-diene [IV(b)J have been synthesized and explored as neutral ionophores for preparing poly(vinylchloride) (PVC) based membrane sensors selective to Tb3+ ions. The best performance was obtained for the membrane sensor having a composition of IV(a):PVC:l-CN:NaTPB in the ratio of 6:32:58:4 (w/w; mg). The comparison of response characteristics of PME based on IV(a) with CGE revealed the superiority of the latter. The electrodes exhibit Nernstian slope for Tb3+ ions with limits of detection of 3.4 x 10"8 mol L"1 for PME and 5.7 x 10"9 mol L"1 for CGE. The response time for PME and CGE was found to be 10 s and 8 s respectively. The potentiometric responses are independent of the pH of the test solution in the pH range 3.0-7.5 for PME and 2.0-8.5 for CGE. The CGE has found to work satisfactorily in partially non-aqueous media upto 30% (v/v) content of methanol, ethanol and 20% (v/v) content of acetonitrile and could be used for a period of 5 months. The CGE was used as indicator electrode in the potentiometric titration of Tb3+ions with EDTA and in determination of fluoride ions in various samples. It can also be used in direct determination of Tb +ions in tap water and various binary mixtures with quantitative results. The performance characteristics of PME and CGE based on two newly synthesized macrocyclic ligands 2,12-(2-methoxyaniline)2-4,14-Me2-[20]-1,4,11,14-tetraene- 1,5,8,11,15,18-N6 [V(a)J and 2,12-(2-methoxyaniline)2-4,14-Me2-8,18-dimethylacrylate- [20]-l,4,ll,14-tetraene-l,5,8,ll,15,18-N6 [V(b)] have been investigated for the quantification of Yb +ions. The best performance was obtained for the membrane sensor having a composition of V(b):PVC:BA:NaTPB in the ratio of 5:40:52:3 (w/w; mg). The electrodes exhibit Nernstian slope for Yb3+ ions with limits of detection of 4.3 x 10"8 mol L"1 for PME and 5.8 x 10"9 mol L"1 for CGE. The response time for PME and CGE was found to be 10 s and 8 s respectively. The potentiometric responses are (vi) independent ofthe pH ofthe test solution in the pH range 3.0-8.0 for PME and 2.5-8.5 for CGE. The CGEhas found to work satisfactorily in partially non-aqueous media upto 30% (v/v) content ofmethanol, ethanol and 20% (v/v) content ofacetonitrile and could be used for a period of5months. As CGE showed better response characteristics than PME, it was therefore, used as indicator electrode in the potentiometric titration of Yb3+ ions with EDTA and in determination of fluoride ions in mouthwash samples. It can be used for determination of sulfite in red and white wine samples and also in determination of Yb in various binary mixtures with quantitative results. Plasticized membranes using 2,3,4:12,13,14-dipyridine-l,3,5,8,l 1,13,15,18- octaazacycloicosa-2,12-diene [VI(a)] and 2,3,4:12,13,14-dipyridine-l,5,8,l 1,15,18- hexamethylacrylate-l,3,5,8,ll,13,15,18-octaazacycloicosa-2,12-diene [VI(b)] have been prepared and explored as Ce3+ ion selective sensor. Optimum performance was observed for the membrane sensor having a composition of VI(b):PVC:o-NPOE:KTpGPB in the ratio of 6:34:58:2 (w/w; mg). The electrodes exhibit Nernstian slope for Ce34 ions with limits of detection of 8.3 x 10"8 mol L"1 for PME and 7.7 x 10"9 mol I/1 for CGE. The response time for PME and CGE was found to be 12 s and 10 s respectively. The potentiometric responses are independent ofthe pH ofthe test solution in the pH range 3.5-7.5 for PME and 2.5-8.5 for CGE. The CGE could be used for a period of 5 months. The practical utility of the CGE has been demonstrated by its usage as an indicator electrode in potentiometric titration of oxalate and fluoride ions with Ce +ion solution. The proposed electrode was also successfully applied to the determination of fluoride ions in mouthwash solution and oxalate ions in real samples. The Fifth Chapter of the thesis "Membranes of Macrocyclic complexes as anion selective sensor" includes the synthesis and characterization of macrocyclic complexes which were incorporated as ionophore in the fabrication of PME and CGE for the (vii) quantification of anions in water and biological samples. The electrode characteristics and selectivities of PVC-based PME and CGE incorporating the newly synthesized zinc complex of6,7:14,15-Bzo2-10,ll-(4-methylbenzene)-[15]-6,8,12,14-tetraene-9,12-N2-l,5- 02 [VH(a)] and zinc complex of 6,7:14,15-Bzo2-10,ll-(4-methylbenzene)-[15]-6,14- diene-9,12-dimethylacrylate-9,12-N2-l,5-02 [VII(b)] for the determination of thiocyanate ions have been investigated and compared. The best response was observed with the membrane having composition VII(b):PVC:o-NPOE: HTAB in the ratio of 6:33:59:2 (w/w; mg). This electrode exhibited Nernstian slope for thiocyanate ions over working 7 —7 1 concentration range of 4.4 x 10" to 1.0 x 10 mol L" with detection limit of 2.2 x 10"7 mol L" . The performance of this electrode was compared with coated graphite electrode (CGE) which showed better response characteristics with respect to Nernstian slope (59.0 ± 0.2) mV decade" activity, wide concentration range of 8.9 x 10" to —9 I —X 1 1.0 x 10 mol L" and detection limit of 6.7 x 10 mol L" . The response time for CGE and PME was found to be 8 s and 10 s respectively. The proposed electrode (CGE) was successfully applied to direct determination of thiocyanate in biological and environmental samples and also as indicator electrode in potentiometric titration of SCN" ion. The newly synthesized zinc complexes of 6,7:13,14-Dibenzo-2,4,9,ll-tetramethyll, 5,8,12-tetraazacyclotetradecane-l,4,6,8,l 1,13-hexaene [VIII(a)] and 6,7:13,14-Dibenzo- 2,4,9,11 -tetramethyl-1,5,8,12-tetramethylacrylate-1,5,8,12-tetraazacyclotctradecane- 6,13-diene [VIII(b)| were explored as electroactive material in the preparation of PME and CGE and the best response was observed for the membrane having composition VIII(b):PVC:BA:HTAB in the ratio of 7:32:59:2 (w/w; mg). The response characteristics of PME based on the above mentioned membrane was also compared with CGE. The electrode exhibits Nernstian slope for perchlorate ions over wide concentration ranges i.e. 8.3 x 10"7 to 1.0 x 10"2 mol L"1 (with PME) and 1.0 x 10"7 to (viii) 1.0 x 10"2 mol L"1 (with CGE) and response time of 12 s and 9 s for PME and CGE respectively. Futhermore, the electrodes generated constant potentials in the pH range of 3.0-8.0 for PME and 2.5-9.0 for CGE. The high selectivity of CGE for perchlorate ions permits its use in the determination of perchlorate ions in waterand human urine samples. Thus, membranes of the electrodes prepared and studied are an important addition to the existing electrodes as they have shown some better performance characteristics over the existing ISEs.