Simple, universal colorimetric strategy for amplified detection of nanomolar toxic heavy metal ions based on multi-component complexation and Tyndall effect

This work describes a new class of universal nanoprobe-free colorimetric assay with Tyndall effect (TE) signaling for amplified detection of nanomolar heavy metal ions based on analyte-triggered multi-component complexation. In the presence of NH2OH·HCl, Cu2+ (model analyte) is reduced to Cu+ which in turn reacts with KSCN and CHClN2O3 to form colloidal three-component complexation particles unexpectedly producing an enhanced TE signal with the aid of a 635 nm laser pointer. The results show that the developed method could detect Cu2+ in a linear concentration range of 156 nM ∼ 120 µM with a limit of detection (LOD) of ∼38 nM (3σ) using a smartphone for quantitative readout. The generality of the new equipment-free strategy was demonstrated well with selective detection of Cd2+ as the second model analyte with a LOD down to ∼1.27 nM where KI and CHClN2O3 were adopted as the three-component complexation reagents. In addition, satisfying recovery results obtained from analysis of Cu2+ and Cd2+ in several polluted real water samples ranged from 93.1% to 104.67% and 93.21–101.85%, respectively. To the best of our knowledge, this may be the first report of nanoprobe-free TE-based colorimetric assay based on multi-component complexation.