Design of a nanoswitch for sequentially multi-species assay based on competitive interaction between DNA-templated fluorescent copper nanoparticles, Cr³⁺ and pyrophosphate and ALP

The present work constructs a sequentially triggered nanoswitch (STN) for sequential detection of Cr³⁺, P₂O₇ ⁴⁻ (PPi) and alkaline phosphatase (ALP) depending on polythymine (T40) templated fluorescent Cu nanoparticles (Cu NPs). A significant phenomenon is that Cr³⁺ can only causing 5% QE of fluorescent Cu NPs synthesized by lower than 500 μM Cu²⁺, but the fluorescence of the Cu NPs synthesized by more than 500 μM Cu ²⁺ can be quenched up to 90% QE by the same concentration of Cr³⁺. Then the quenched fluorescence of CuNP-Cr³⁺ complex provides a sensing platform for PPi due to the strong binding between Cr³⁺ and PPi, resulting in dissociation of Cr³⁺ from the surface of Cu NPs and the recovery of fluorescence emission. Further ALP hydrolysis of PPi disrupts Cr³⁺-PPi assemble and Cr³⁺ is released to interact with Cu NPs, which induces fluorescence quenching again. Thus, sequentially detection of Cr³⁺ (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 Cr³⁺, 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 (Cr³⁺, PPi and ALP) to construct a logic gate.