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 (Cu 2+) and mercuric ions (Hg 2+ ) in sequence. AuNPLs were found to be etched efficiently when producing triiodide ions (I 3 − ) by a redox reaction between Cu 2+ 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 Hg 2+ the etching of AuNPLs was suppressed due to the consumption of I − by the formation of HgI 2 . With an increase of the concentration of the Hg 2+ 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 Cu 2+ and Hg 2+ was accompanied with color variations and band shifts of localized surface plasmon resonance (LSPR), allowing for visual and spectroscopic determination of Cu 2+ and Hg 2+ successively within 15 min. In the range 0.01–1.5 μM for Cu 2+ and 0.02–3.0 μM for Hg 2+ , the linear relationship between the band shift values and the target ions concentration was found good ( R 2  > 0.996). The limit of detections (3S/k) was 19 nM for Cu 2+ and 9 nM for Hg 2+ , respectively. The lowest visual estimation concentration was 80 nM for both Cu 2+ and Hg 2+ through the distinguishable color changes. This system exhibited desirable selectivity for Cu 2+ and Hg 2+ over other common ions tested. The method has been successfully applied to sequential determination of Cu 2+ and Hg 2+ in real water and food samples. Scheme 1 Schematic illustration for sequential detection of Cu 2+ and Hg 2+ based on etching of AuNPLs