Mercury
is a ubiquitous heavy-metal pollutant and poses serious
ecological and human-health risks. There is an ever-growing demand
for rapid, sensitive, and selective detection of mercury in natural
waters, particularly for regions lacking infrastructure specialized
for mercury analysis. Here, we show that a sensor based on multi-emission
carbon dots (M-CDs) exhibits ultrahigh sensing selectivity toward
Hg(II) in complex environmental matrices, tested in the presence of
a range of environmentally relevant metal/metalloid ions as well as
natural and artificial ligands, using various real water samples.
By incorporating structural features of calcein and folic acid that
enable tunable emissions, the M-CDs couple an emission enhancement
at 432 nm and a simultaneous reduction at 521 nm, with the intensity
ratio linearly related to the Hg(II) concentration up to 1200 μg/L,
independent of matrix compositions. The M-CDs have a detection limit
of 5.6 μg/L, a response time of 1 min, and a spike recovery
of 94 ± 3.7%. The intensified emission is attributed to proton
transfer and aggregation-induced emission enhancement, whereas the
quenching is due to proton and electron transfer. These findings also
have important implications for mercury identification in other complex
matrices for routine, screening-level food safety and health management
practices.