No abstract

Precious-metal recovery from industrial wastewater has received considerable attention because of rapidly increasing amounts of electronic waste. Existing technologies have yet to be widely applied due to their high cost and low selectivity toward precious-metal ions. Herein, we report a direct Z-scheme tannin–TiO2 heterostructure for selective gold adsorption from electronic waste under solar irradiation. The tannin-coated TiO2 nanoparticles were prepared by a simple dipping method, and under light illumination, both tannin and TiO2 can serve as photosensitive components for the reduction of metal ions, with metal-to-ligand charge transfer from TiO2 to tannin extending the lifetime of the excited electrons. Moreover, no additional electron donors are required because the tannin layer scavenges the reactive oxygen species generated by the holes from the light-activated TiO2 surface. The heterostructure allows for the highly efficient photocatalytic recovery of gold ions, with 11 times higher adsorption capacity in the light compared to the dark. High selectivity toward gold ions was also demonstrated using a metal ion mixture including nine different metal ions that are commonly found in electronic waste. Our findings suggest that the Z-scheme heterostructure of polyphenol and semiconductor provides a promising photochemical pathway for efficient and selective metal ion recovery from electronic waste.

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Rapid, sensitive, specific, and user‐friendly microRNA (miRNA) assays are in high demand for point‐of‐care diagnosis. Target‐catalyzed toehold‐mediated strand displacement (TMSD) has received increasing attention as an enzyme‐free molecular tool for DNA detection. However, the application of TMSD to miRNA targets is challenging because relatively weak DNA/RNA hybridization leads to failure in the subtle kinetic control of multiple hybridization steps. Here, a simple method is presented for miRNA assay based on the one‐pot self‐assembly of Y‐shaped DNAs with streptavidin via an miRNA‐catalyzed TMSD cascade reaction. A single miRNA catalyzes the opening cycle of DNA hairpin loops to generate multiple Y‐shaped DNAs carrying biotin and a quencher at the end of their arms. Introducing a single base‐pair mismatch near the toehold facilitates RNA‐triggered strand displacement while barely disturbing nonspecific reactions. The Y‐shaped DNAs are self‐assembled with fluorescently labeled streptavidin (sAv), which produces nanoscale DNA‐sAv nanogel particles mediating efficient Förster resonance energy transfer in their 3D network. The enhancing effect dramatically reduces the detection limit from the nanomolar level to the picomolar level. This work proves that TMSD‐based DNA nanogel with a base‐pair mismatch incorporated to a hairpin structure is a promising approach towards sensitive and accurate miRNA assay.