ASSESSMENT OF METAL SENSING ABILITIES OF SOME OPTICAL CHEMOSENSORS

Abstract

Transition and post-transition metal ions such as Al3+, Hg2+, Cr3+, Ni2+, Zn2+, Pb2+, As3+ etc. are presented in many industrial effluents. Many of these metal ions have toxic pollutant nature and are responsible for several diseases in human. In this Context, investigation of environmental pollutants is a major challenge to analytical chemistry. Various instrumental techniques such as atomic absorption spectroscopy (AAS), flame photometry, inductively coupled plasma-atomic emission spectrometry (ICP–AES), neutron activation analysis (NAA), ion chromatography, high performance liquid chromatography (HPLC) have been applied for the quantitative analysis of pollutants. All of these listed techniques are highly sophisticated, time consuming and require huge infrastructure and expert handling. In comparison of above mentioned techniques optical chemosensors have gained significant attention, owing to its simplicity, high selectivity and sensitivity towards metals and anions. In context, the design and exploration of new optical chemosensors for the investigation of metal ions is of continuing interest. Previously, a number of optical chemosensors for metal ions have been designed based on various moieties such as rhodamine, fluorescein, DPA, quinoline, and bipyridine moiety, etc. However, the availability of new optical chemosensors moiety based on thiazole, antipyrine, furan, phthalocyanine and pyridine derivative is rather limited. This prompt us to initiate a work in this direction. We designed and synthesized some thiazole, antipyrine, furan, phthalocyanine and pyridine based chemosensors and characterized their metal sensing abilities with the help of absorbance and photo-fluorescence techniques. The results of these investigations are incorporated in the present thesis, which is divided into six chapters and discussed briefly here. First chapter presents a “General Introduction” which describes a general overview of molecular sensors and a brief introduction about fluorescent sensors with a basic principle. The chapter also summarized a review of optical chemosensor for metals and anions based on a different fluorophore sensing moiety. Second Chapter presents; “Highly Selective Turn-on Chemosensors for Al3+ based on Thiazole Schiff base”. iii S N N OH S N N (L1) (L2) OH Thiazole Schiff bases 2-(4-phenyl-1,3-thiazol-2-yliminomethyl)phenol (L1) and 1-(4-phenyl- 1,3-thiazol-2-yliminomethyl)naphthalen-2-ol (L2) was synthesized by simple condensation of thiazole with salicylaldehyde or naphthaldehyde. Sensing abilities of Schiff bases with various metal ions has been studied with UV–vis absorption and photofluorescent spectroscopy. The results indicated that Schiff base L1 and L2 exhibited turn-on fluorescent behavior with Al3+ ions in methanol, which could be directly detected by the naked-eye under the UV-lamp. The limits of detection was calculated for L1 and L2 since 1.0 × 10−6 M and 7.5 × 10−7 M by the titration method. Both sensors exhibited excellent fluorescent behavior in 5.0–13.5 pH range. The fluorescent behavior of L1+Al3+ and L2+Al3+ complex solution goes to turn-off in the presence of Ni2+ and EDTA. Third Chapter presents; “Highly Selective Turn-on Chemosensors for Zn2+, Cr3+ and Al3+ based on Antipyrine Schiff base”. Antipyrine Schiff bases 4-(4-hydroxy-3- methoxybenzylideneamino)-2,3-dimethyl-1-phenyl-1,2-dihydropyrazol-5-one (L3) 2,3- dimethyl-4-(2-oxo-1,2-diphenylethylideneamino)-1-phenyl-1,2-dihydropyrazol-5-one (L4) and 2,3-dimethyl-4-(3-oxo-1,3-diphenylpropylidene-amino)-1-phenyl-1,2-dihydropyrazol-5-one (L5) were successfully synthesized and characterized by NMR and HRMS. N N N O OH O (L3) N N N O N N N O O O (L4) (L5) The complexation activities of chemosensors (L3, L4 and L5) with various metal ions were investigated by absorption and photofluorescent spectroscopy. Results showed that the chemosensor (L3) exhibited turn-on fluorescence sensing for Zn2+ ions with 55 fold iv fluorescence enhancements at 533 nm in 5.0–13.5 pH range. In other hand, chemosensors (L4 and L5) exhibited more than 10-fold fluorescence enhancement towards Al3+ and Cr3+ ions in acidic and neutral medium. Such fluorescent responses could be directly detected by naked eye under UV-lamp. The limit of detection of chemosensors was calculated to be 10−7 M by titration method and Job plot titration confirmed 1:1 stoichiometry. Fourth Chapter presents; “Phthalocyanine tetrasulfonic acid based chemosensor for Cu2+”. Phthalocyanine tetrasulfonic acid (PCTS) used as an optical probe for selective detection of Cu2+ in DMSO solution over other metal ions used. The complexation behavior of chemodosimeter toward various metal ions was studied via UV–vis absorption and fluorescence emission spectra. Induced changes in the presence of selective metal ions could be observed by naked eye detection. NH N NH N N N N N S S S S O O OH O OH O O O HO O O HO (L6) The association constant (Ka) and low limit of detection were found to be 8.80 × 105 M−1 and 5.53 × 10−7 M, respectively. The optode showed excellent selectivity behavior in the environmental pH range. Fifth Chapter presents; “Turn-off Cu2+ selective chemosensors based on Furo[3,4-c]quiniolin- 4-ol Scaffold” Furo[3,4-c]quiniolin-4-ol Scaffold were synthesized and characterized with NMR and HRMS techniques. Optical sensing properties of L7-L9 were investigated in the presence of various metal ions in methanol. Chemosensors exhibited significant fluorescence quenching in the presence of Cu2+ ions. The sensitivity of chemosensors towards Cu2+ was not interfered by various other metal ions namely Li+, K+, Na+, Ca2+, Mg2+, Mn2+, Co2+, Fe2+, Al3+, Cr3+, Ni2+, v Zn2+, Pb2+, In3+, Gd3+ and Fe3+ ions. The synthesized probes displayed higher affinity towards Cu2+ ions which verified by high binding constant 1.89 × 104, 2.11 × 104 and 1.87 × 104 M−1, respectively for L7, L8 and L9, and greater selectivity with micromolar detection (10−7 M order) in organic and semi-aqueous solvents. N O NH Cl OH N O NH N Cl OH O N O NH N Br OH O (L7) (L8) (L9) Sixth Chapter presents; “Highly selective chemosensor for Zn2+ based on 5-methyl-2- aminopyridine Schiff base”. N N HO N N HO (L10) (L11) Two new highly selective chemosensors (L10 and L11) were synthesized by refluxing of 5- methyl-2-aminopyridine with 2-hydroxy-1-naphthaldehyde or salicylaldehyde. The resultant imines were characterized via standard NMR and HRMS techniques. Studies based on Absorbance and fluorescence spectroscopy demonstrated their metal sensing ability with higher selectivity towards Ni2+, Zn2+, Fe3+ and UO2 2+ ions. The different aspect of this sensing behavior such as stoichiometry, association constant, pH range, sensitivity, reversibility and interference effect were studied extensively. Both sensors displayed excellent selectivity with low LOD, under neutral conditions.