Sequential colorimetric sensing of cupric and mercuric ions by regulating the etching process of triangular gold nanoplates.

A triangular gold nanoplate (AuNPL)-based colorimetric assay is presented for ultrasensitive determination of cupric ions (Cu2+) and mercuric ions (Hg2+) in sequence. AuNPLs were found to be etched efficiently when producing triiodide ions (I3-) by a redox reaction between Cu2+ and iodide ions (I-), leading to a change of the shape of AuNPLs from triangular to sphere along with a color change from blue to pink. In the presence of Hg2+ the etching of AuNPLs was suppressed due to the consumption of I- by the formation of HgI2. With an increase of the concentration of the Hg2+ a transformation from sphere to triangular in the shape of AuNPLs occurred with a color change from pink to blue. The evolution of AuNPLs from etching to anti-etching state by sequential addition of Cu2+ and Hg2+ was accompanied with color variations and band shifts of localized surface plasmon resonance (LSPR), allowing for visual and spectroscopic determination of Cu2+ and Hg2+ successively within 15 min. In the range 0.01-1.5 μM for Cu2+ and 0.02-3.0 μM for Hg2+, the linear relationship between the band shift values and the target ions concentration was found good (R2 > 0.996). The limit of detections (3S/k) was 19 nM for Cu2+ and 9 nM for Hg2+, respectively. The lowest visual estimation concentration was 80 nM for both Cu2+ and Hg2+ through the distinguishable color changes. This system exhibited desirable selectivity for Cu2+ and Hg2+ over other common ions tested. The method has been successfully applied to sequential determination of Cu2+ and Hg2+ in real water and food samples. Graphical abstract Scheme 1 Schematic illustration for sequential detection of Cu2+ and Hg2+ based on etching of AuNPLs.