The colorimetric Hg2+ sensing principle of our assay
can be explained by a two-step mechanism. In the first step,
Hg2+ was spontaneously reduced on the AuNP surface and
formed a shell around the particles. Then, lysine addition to this
solution induced aggregation of Hg2+-covered AuNPs and
resulted in a rapid color change from red to purple or gray
because the distance between the AuNPs became less than the
average particle diameter.31 The color change of the AuNPs
was investigated using several Hg2+ solutions in distilled or tap
water at different concentrations. The detection limit of this
inexpensive colorimetric assay is 2.9 nM, which makes our assay
one of the most sensitive colorimetric Hg2+ sensors compared
with many metal nanoparticle-based methods (see Supporting
Information, Table S1). To demonstrate the selectivity, several
metal ions were tested with the assay, and it was observed that
the response of the colorimetric assay is highly selective to
Hg2+.