Label-free aptamer-based colorimetric detection of mercury ions in aqueous media using unmodified gold nanoparticles as colorimetric probe

We report a simple and sensitive aptamer-based colorimetric detection of mercury ions (Hg2+) using unmodified gold nanoparticles as colorimetric probe. It is based on the fact that bare gold nanoparticles interact differently with short single-strand DNA and double-stranded DNA. The anti-Hg2+ aptamer is rich in thymine (T) and readily forms T-Hg2+-T configuration in the presence of Hg2+. By measuring color change or adsorption ratio, the bare gold nanoparticles can effectively differentiate the Hg2+-induced conformational change of the aptamer in the presence of a given salt with high concentration. The assay shows a linear response toward Hg2+ concentration through a five-decade range of 1 x 10(-4) mol L-1 to 1 x 10(-9) mol L-1. Even with the naked eye, we could identify micromolar Hg2+ concentrations within minutes. By using the spectrometric method, the detection limit was improved to the nanomolar range (0.6 nM). The assay shows excellent selectivity for Hg2+ over other metal cations including K+, Ba2+, Ni2+, Pb2+, Cu2+, Cd2+, Mg2+, Ca2+, Zn2+, Al3+, and Fe3+. The major advantages of this Hg2+ assay are its water-solubility, simplicity, low cost, visual colorimetry, and high sensitivity. This method provides a potentially useful tool for the Hg2+ detection.