Paper-based colorimetric sensor for mercury ion detection using smartphone digital imaging

In this research, we have developed a colorimetric paper-based sensing platform for the determination of mercury ion (Hg2+) using gold nanoparticles (AuNPs). To evaluate the selectivity in the response to a special target, urea is used as recognizing ligands on AuNPs for selective sensing Hg2+. Colorimetric readings in the presence of various environmentally relevant metal ions, such as Na+, K+, Zn2+, Pb2+, and Hg2+, were also studied. The results show that amongst all the tested metal ions, the SPR peaks are located at 525 nm except in the case of the present of Hg2+, in which is red shifted to 537 nm. Furthermore, a new broad peak is observed at the wavelength of 695 nm corresponding to the color change from red to purple. The relations of the ratios of maximum absorbance at 695 nm and 537 nm, and the concentration of Hg2+ illustrate the linear behavior with the sensitivity of 2.0 × 10-5 nM-1 and the detection limit of 30 nM. The modified AuNPs with urea are applied on the paper-based device for quantitative Hg2+ detection with smartphone digital imaging. The intensity ratios between blue and red colors are linear-dependence relations which can be used to determine the concentration of Hg2+.