High reducibility of gallic acid allows synthesis of small sized monodisperse gold nanoparticles (GNPs) at ambient temperature (25 C-o). Mg2+ rapidly interacts with the gallic acid ligands and suppresses the dispersion of GNPs therefore, causing a decrease in UV-vis absorbance intensity, and color change from red to blue. Thus, the colorimetric response of GNPs with Mg2+ was investigated by observing temporal quenching of UV-vis absorbance and precise tuning of fractal growth of GNP aggregates. Moreover, Mg2+ at concentrations as low as 200 ppb can be detected using gallic acid ligand-mediated coordination chemistry which results quenching in UV-vis absorbance proportional to the exposure time. This gallic acid -based colorimetric sensor shown a great potential for the selective detection of pathologically important electrolyte Me' without any interference from other cations Ca2+ and K+.(C) 2017 Elsevier Inc. All rights reserved.