Development of colorimetric chemosensors for metal ions recognition in aqueous environment using statistical approach

Abstract

Three thiosemicarbazone derivatives named 2-acetylpyrazine thiosemicarbazone (P1), N(1)-(2-acetylpyrazine)-N(4)-(2-hydroxyphenyl)-thiosemicarbazone (P2) and the novel N(1)-(4-acetylpyidine)-N(4)-(2-hydroxyphenyl)-thiosemicarbazone (P3) were synthesized through the proposed methods. The characterization through melting point analysis, Fourier transform infrared (FTIR) spectroscopy, CHNS elemental analysis and nuclear magnetic resonance (NMR) spectroscopy showed high purity of the products. The synthesized thiosemicarbazones were optimized as selective and sensitive chemosensors for cobalt (Co2+) and mercury (Hg2+) ions through spectroscopic study using ultraviolet-visible (UV-Vis) spectroscopy. Colorimetric response was observed where P1, P2 and P3 changed their color from colorless to orange, orange and yellow respectively after being added with their respective metal ions. Response surface methodology (RSM) was used to study the effects of interactive variables and identify the most optimum condition for each chemosensor. The absorbance values of P1-Co2+, P2-Co2+ and P3-Hg2+ at 450, 423 and 414 nm respectively were collected as responses. Results showed that metal ion concentration, pH and time were significant to the model. The optimum condition for 100 μM P1 was found with 70 μM Co2+ in 8:2 v/v DMSO/Tris-HCl at pH 5.3 after 15 minutes reaction, P2 at 100 μM was optimum with 80 μM Co2+ in 8:2 v/v DMSO/Tris-HCl at pH 7.5 after 10 minutes reaction whereas P3 at 20 μM was ideal with 60 μM Hg2+ in 8:2 v/v DMSO/citrate-phosphate at pH 7.8 after 18 minutes reaction. These chemosensors were highly sensitive as the limit of detection (LOD) of P1, P2 and P3 towards selected metal ions was found to be low at 1.64, 1.52 and 3.56 μM respectively. The binding stoichiometry of P1-Co2+, P2-Co2+ and P3-Hg2+ were 2:1, 2:1 and 1:1 molar ratio respectively. The interference analysis showed that the presence of 1 molar equivalent of other metal ions did not interfere the interactions of P1-Co2+, P2-Co2+ and P3-Hg2+. Computational study through COSMO-RS showed the compatibility of chemosensors to selected solvent DMSO. Calculation of molecular electron potential (MEP) and Fukui function in density functional theory (DFT) suggested that nitrogen, sulphur and oxygen atoms of the chemosensors involved in the interaction with metal ions. Calculation of highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap and formation energy showed that the formation of P1-Co2+, P2-Co2+ and P3-Hg2+ were preferred over non-interacted chemosensors in the presence of the metal ions. The evaluation on several water samples showed that they were efficient to be used for environmental water samples. The optimized P1, P2 and P3 in this study promised three new selective and sensitive chemosensors from thiosemicarbazone derivatives for Co2+ and Hg2+ detection.