Two Birds with One Stone: Fe-DNA nanospheres produced via coordination-propelled self-assembly with excellent peroxidase-like property for versatile ratiometric fluorescent assay and cellular imaging

Exploring novel versatile nanozymes for multi-signal biosensing and cellular application is one of the most promising directions to meet the diversified requirements in this field. Herein, by harnessing coordination-propelled self-assembly between Fe (II) and DNAs, we prepared Fe-DNA nanospheres (Fe-DNA NSs) via a cost-effective one-step hydrothermal method, and pioneered the application of its excellent POD-mimicking property to fluorescent substrates. Initially, we investigated its enzyme-like activity using TMB as canonical colorimetric substrate and screened its catalytic oxidation effects towards different fluorescent substrates, such as T-HCl, AR, OPD and Sc, respectively. Afterwards, by virtue of the contrary fluorescent changes of Sc (decreased FI 465 ) and OPD (increased FI 562 ) and the cooperative effects of FRET/IFE between them, we devised the first universal Fe-DNA nanospheres-based ratiometric fluorescent (RF) platform. Taking H 2 O 2 and glucose as model targets, two RF biosensors based on the alternative direct-nanozyme-catalysis and enzyme/nanozyme-tandem-catalysis were rationally fabricated, respectively. And we further exploited them to evaluate the quality of commercial contact lens care solution, and sensitively determine the blood glucose level of human. Moreover, corresponding cytotoxicity experiments adequately proved the superior biocompatibility of Fe-DNA NSs over most inorganic nanozymes. Furthermore, taking Cy5-labelled A20 strands as templates, we synthesized small-sized (∼60 nm) Fe-DNA fluorescent nanozyme and achieved efficient cellular delivery/imaging. This work not only offered a valid prototype for operating multi-signal-responsive nanozymatic biosensors, but also opened unique avenues for the bio-applications of nucleic acids-originated fluorescent nanozymes in cellular imaging and biotherapy.