0.2 μm polyethersulfone membrane

50 nm citrate-stabilized Au NPs were purchased from nanoComposix. Aptamer sequence of 5′ -TTTTTTTTTT-3′ was obtained from Integrated DNA Technologies. All metal salts such as mercury(II) chloride were obtained from Sigma. Stock Au NP solution in 20 mM tris-HCl buffer (TH, pH 8.0) was prepared by centrifugation of raw Au NP-citrate solution, aspiration of the supernatant, and redispersion in TH buffer with 20× dilution to give a working concentration of 0.64 nM. Water samples collected from rivers, lakes, and beaches were filtered by 0.2 μm polyethersulfone membrane (Whatman) to remove sand and other solid particles within the test samples. Tap water samples and calibration solutions containing mercury(II) ions prepared in deionized water were used directly without further purification. In a typical measurement procedure, 4 μL of the sample of interest was mixed with 4 μL of 3 μM aptamer (20 mM TH buffer, pH 8.0), followed by a 5 minute reaction period. Next, 400 μL of Au NPs (0.64 nM) in 20 mM TH buffer solution was added and allowed to react for 5 minutes. Finally, 8 μL of 10 mM NaCl was added and incubated for another 10 minutes before being analyzed by the smart-phone device.