Phytochemical from Zingiberaceae as a sustainable optical probe for heavy metal determination

Abstract

Pinostrobin (PS) extracted from fingerroot was successfully applied as a dual signalling optical sensor that selectively detected Fe3+ and Al3+ via colorimetric and fluorometric methods, respectively. The selectivity of detection was pH-dependent and was optimal at pH 7. Chelation between PS and Fe3+ resulted in the formation of a brown complex with a maximum absorbance of 335.0 nm, while the formation of a colorless complex with Al3+ caused an enhancement of fluorescent intensity of PS at the wavelength of 500.0 nm. The linearity range for Fe3+ were 1–150 ppm and 0.05–100 ppm for Al3+. The limits of detection were 0.27 ppm for Fe3+ and 0.02 ppm for Al3+. Binding stoichiometry for both metal ions to ligand was the same at 1:2 as Fe3+-(PS)2 and Al3+- (PS)2. The brown color of Fe3+- (PS)2 complex can be explained using the crystal field and charge transfer theory. For the Al3+- (PS)2 complex, the fluorescent intensity was considerably enhanced compared to that of free PS because the proton transfer process was inhibited by the complex formation. Theoretical calculation was performed to confirm the complex formation between PS and the metal ions. Based on the proposed complex structure, the binding energies of Fe3+ and Al3+ complexes were calculated, and the values were found to be −11.45 eV and −19.37 eV, respectively, indicating high stability of the structure and geometry of the metal complexes.