Design of a nanoswitch for sequentially multi-species assay based on competitive interaction between DNA-templated fluorescent copper nanoparticles, Cr and pyrophosphate and ALP.

The present work constructs a sequentially triggered nanoswitch (STN) for sequential detection of Cr, PO (PPi) and alkaline phosphatase (ALP) depending on polythymine (T40) templated fluorescent Cu nanoparticles (Cu NPs). A significant phenomenon is that Cr can only causing 5% QE of fluorescent Cu NPs synthesized by lower than 500 μM Cu, but the fluorescence of the Cu NPs synthesized by more than 500 μM Cu can be quenched up to 90% QE by the same concentration of Cr. Then the quenched fluorescence of CuNP-Cr complex provides a sensing platform for PPi due to the strong binding between Cr and PPi, resulting in dissociation of Cr from the surface of Cu NPs and the recovery of fluorescence emission. Further ALP hydrolysis of PPi disrupts Cr-PPi assemble and Cr is released to interact with Cu NPs, which induces fluorescence quenching again. Thus, sequentially detection of Cr (LOD, 0.03 μM), PPi (LOD, 0.005 μM) and ALP (LOD, 0.125 mU/mL) was successfully implemented with high sensitivity and selectivity. The sensor is also successfully used for Cr, PPi and ALP assays in the human serum. Additionally, the sensitive "on-off-on-off" sensing behavior of the Cu NPs allow three chemical inputs (Cr, PPi and ALP) to construct a logic gate.