STUDIES ON SOME CHEMICAL SENSORS FOR BIOMOLECULES AND TOXIC METALS

Abstract

Analytical chemistry has developed tools and methods for qualitative and quantitative analysis of species in a various variety of samples. In these days, growing industrialization has exerted substantial pressure on the environment. The global emission of raw materials, intermediates and final products from industries have become a severe problem to living organisms. Because of the negative influences of these contaminants, a number of analytical methods have been applied for their removal from water and soil samples. However, many of those methods are expensive as they require specialized reagents and apparatus, and they may also produce a large quantity of waste. Currently an overview of analytical chemistry expansion reveals that amongst the wide variety of sensors, spectrophotometric and electrochemical sensors are preferred choice of analytical chemists as they provide convenient, fast and low cost analysis over a wide measuring concentration range. However, sometimes the application of sensors is limited by poor selectivity and sensitivity. Thus, there is a need for preparing sensors of higher sensitivity and selectivity with wide working concentration range. Keeping this in view, a number of sensors have been prepared and investigated for the determination of some metals and biomolecules. The work carried out is incorporated in the present thesis which consists of five chapters. A brief abstract of the subject matter presented in various chapters is discussed here. The First Chapter is a general introduction about the subject and summarizes important literature on sensors dealing with determination of biomolecules, metals and anions. The chapter ends up with an outline of the objective of the present work. The first chapter is followed by the Second Chapter which mainly deals with principles, theory and practice of voltammetric, ion selective electrodes and spectrophotometric sensors. Furthermore, the methodology and experimentation has also been detailed in this chapter. The Third Chapter deals with the simultaneous determination of ascorbic acid (AA) and caffeine (CAF) by a voltammetric sensor using a glassy carbon electrode (GCE). The glassy carbon electrode was further modified with a multiwall carbon nanotube (MWCNT) to improve the performance of the electrode. It was found that the oxidation of AA and CAF ii occurred at 202 mV and 1402 mV with bare GCE whereas the same process occurred at –10 mV and 1103 mV respectively for MGCE, a much lower oxidation potential. Further mechanistic investigation of the oxidation process has shown that the equal number of electrons and protons are involved in the oxidation of both the drugs. The peak current was found to be proportional to concentration of drugs and could therefore be used for their determination. The electrodes could thus be used for the determination of AA and CAF in a wide working concentration range 10–500 μM, with a detection limit of 1.0 × 10–2 μM and 3.52 × 10–3 μM for MWCNT modified GCE, whereas for bare GCE are 5.29 × 10–1 μM and 9.41 × 10–2 μM respectively. The lower value shows that the modified glassy carbon electrode is superior to bare glassy carbon electrode. Further, the alternative approach of determining AA and CAF by square wave voltammetry is convenient, faster and accurate. In view of high sensitivity for the detection of the drugs, the technique has been used for the reliable determination of AA and CAF in tea leaves, coffee, cold drink (mountain dew), pharmaceutical preparations and urine samples. Thus, it can be said that this biosensor is a useful addition in the field of analytical chemistry for the determination of biomolecules in environmental as well as medicinal samples. The Fourth Chapter deals with the preparation and investigation of a cadamium selective sensor. The sensor makes use of poly(vinyl chloride) (PVC) based membranes of ptert- butylcalix[6]arene as an ionophore (I). The preliminary investigations revealed that these PVC membranes developed shows potential response to Cd2+ ions, hence can be used for its determination. The performance of the membrane was improved by the addition of plasticizer and anion excluder sodium tetraphenylborate (NaTPB). The plasticizer di-octyl phthalate (DOP) was found to improve the performance to the maximum extent. By varying the amounts of various ingredients of the membrane, the composition of the membranes was optimized. It was found that the best performance of the membrane is obtained when its composition is I-PVC-NaTPB-DOP in the ratio 1:33:1:65 (w/w). The electrode gives linear potential response to Cd2+ ions over the concentration range 9.7 × 10–5 to 1.0 × 10–1 mol dm–3 with a Nernstian slope of 29.0 ± 1 mV decade–1 of activity. Hence, it could be used for Cd2+ determination in this concentration range. The sensor was found to be work satisfactorily in non-aqueous medium (water-ethanol (20%) and water-methanol (20%) mixtures). Further, the response of the electrode is fast with a response time of 35 seconds. The sensor exhibited a iii shelf life time of about 4 months. The selectivity studies show that the electrode is selective to Cd2+ over many alkali, alkaline earth and heavy metals. Thus, the Cd2+ selective electrode developed is sufficiently selective and sensitive and can be considered a good addition to the family reported of Cd2+ selective sensors. The Fifth Chapter deals with determination of aluminum by fluorescent sensors based on Schiff bases and an azo compound. A new azo compound, 1-(2-pyridylazo)-2-naphthol (R1) has been prepared and characterized by various analytical techniques such as elemental analysis, FT–IR, 1H–NMR, 13C–NMR and HRMS. Preliminary studies revealed that this azo compound shows strong interaction with Al3+ metal ions with the emission of fluorescence. The fluorescence developed is proportional to concentration of Al3+ and can be used for its determination. Further, this azo compound shows less fluorescence emission with other metal ions (Ba2+, Cs+, Ca2+, Cr3+, Fe2+, Fe3+, Gd3+, Hg2+, Li+, Na+, K+, Mg2+, Mn2+, Nd3+, Pb2+, Co2+, Cd2+, Cu2+, Zn2+, Ni2+ and Sr2+) indicating that the fluorescence response of the azo compound is selective to Al3+ ions with high sensitivity (detection limit 1.81 × 10−8 M). Thus, this fluorescence chemosensor could be used for the determination of Al3+ and may be a useful tool for quantification of Al3+ in various environmental and biological samples. Two new Schiff bases N,N'-bis(salicylidene)-m-phenylenediamine (R2) and N,N'- bis(salicylidene)-o-phenylenediamine (R3) have been synthesized and characterized by HRMS, FT–IR, 1H–NMR and 13C–NMR spectroscopic techniques. The Schiff bases were found to strongly interact with Al3+ causing emission of sharp bright blue fluorescence on exposure to UV light owing to chelation enhanced fluorescence (CHEF) effect. Thus, the Schiff bases formed complexes with Al3+ and act as receptors for it. The fluorescence intensity was found to be proportional to concentration of Al3+ and can be used for its determination. The stability constants of Al3+ and receptor complexes were determined to be 1.41 × 104 M–1 and 1.59 × 104 M–1, respectively. Both the receptors were used to determine Al3+ with the detection limit of 4.79 × 10−8 M and 8.28 × 10−8 M for receptors R2 and R3, respectively. Moreover, the reported receptors work glowing in the physiological pH spectrum. The spectroscopic studies showed that the response of receptors to Al3+ is selective over a number of metals (Ba2+, Ca2+, Co2+, Cd2+, Cs+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Na+, Mg2+, Mn2+, Gd3+, Nd3+, Pb2+, Sr2+, Zn2+ and Ni2+). Finally, the voltammetric studies (decrease in HOMO– LUMO band gap energy) coupled with spectroscopic studies showed the higher binding ability iv of receptors to Al3+. Hence, the fluorescence sensors developed using R2 and R3 can be used for quantification of Al3+ in various samples. In addition to the above reported Schiff bases, two more Schiff bases N,N'-bis(ohydroxyacetophenone)- m-phenylenediamine (R4) and N-(o-hydroxyacetophenone)-ophenylenediamine (R5) were synthesized and characterized by a number of analytical techniques viz. elemental analysis, FT–IR, HRMS, 1H–NMR and 13C–NMR. Both the Schiff bases act as receptors for Al3+ due to complex formation. The complex formed emits strong bright blue fluorescence on exposure to the UV radiation. The intensity of fluorescence was found to be directly proportional to Al3+ concentration, hence could be used for its determination. Studies revealed that the stability constant of the complexes were found to be 6.64 × 103 M–1 and 7.29 × 103 M–1 for receptors R4 and R5, respectively. These receptors do not show significant fluorescence emission on the addition of other metal ions. Hence, the response is selective and the fluorescence sensor developed can be used for Al3+ determination in various samples.