The concentration of mercury in a sample is measured by a reader secured to a camera-containing mobile electronic device. The reader has holders for sample and control solutions. First and second light sources emitting light at different colors illuminate the sample and control holders. Each holder contains gold nanoparticles, thymine-rich aptamers, and sodium chloride. The light sources illuminate the sample and control holders. An image is captured of the transmitted light through the sample and control holders, wherein the image comprises two control regions of interest and two sample regions of interest. The device calculates the intensity of the two control regions of interest and the two sample regions of interest and generates intensity ratios for the sample and control, respectively, at each color. The device calculates a normalized color ratio based on the intensity ratios and outputs a concentration of mercury based on the normalized color ratio.