shows the proposed Hg2+
sensing mechanism of the colorimetric assay. Initially, the
surface of the as-prepared AuNPs is covered with citrate ions,
which are replaced with Hg2+ ions upon the addition of Hg2+
solution (final concentration is 10 μM) to the AuNP solution.
It is well-known that Hg2+ ions can spontaneously react with
the AuNP surface to form a mercury layer.31,39 After addition of
lysine (final concentration is 0.4 mM) to the Hg2+-capped
AuNPs, the color of the AuNP suspension immediately changes
from red to purple or gray due to the aggregation of AuNPs