Detection of mercury ions in water using a membrane-based colorimetric sensor

A basic requirement of colorimetric sensors is that they must allow the analyte to access the chromophores. This study elucidates the fundamental sensitivity of membrane morphology (membrane pore sizes between 0.32 and 1.30 mu m) and its influence on the loading of chromophores (dithizone, DTZ, in present work), to detect mercury (Hg2+) at concentrations ranging from 3 ppm to 10 ppm. The flat sheet cellulose acetate (CA) membrane acts as the platform for dithizone loading through non-covalent hydrogen bonding of DTZ to the CA membrane. The membranes were prepared by using different polymer-dissolving solvents (formic acid and acetic acid) with the addition of water as the non-solvent to improve the membrane porosity. A significant positive correlation between the membrane porosity and the sensor's sensitivity was observed through the visible color changes of the colorimetric membrane strip from green (color intensity at R-axis of 70) to magenta (color intensity at R-axis of 187) when it was in contact with 3 ppm Hg2+ solution. A colorimetric strip allowing naked eye detection at 3 ppm Hg2+ was achieved. This work demonstrates that the physical and chemical characteristics of a membrane are important factors that determine the performance and sensitivity of a colorimetric sensor.