▎ 摘　　要

The direct production of macromolecular scale (sub-5 nm) porous nanocrystals with high surface area has been a considerable challenge over the past two decades. Here we report an interfacial site-directed capping agent-free growth method to directly produce porous ultrasmall (sub-5 nm), fully crystalline, macromolecular scale nanocrystals. The porous sub-5 nm Prussian blue nanocrystals exhibit uniform sizes (similar to 4 + 1 nm), high surface area (similar to 855 m(2) g(-1)), fast electron transfer (rate constant of similar to 9.73 s(-1)), and outstanding sustained catalytic activity (more than 450 days). The nanocrystal-based biointerfaces enable unprecedented sub-nanomolar level recognition of hydrogen peroxide (similar to 0.5 nM limit of detection). This method also paves the way towards the creation of ultrasmall porous nanocrystals for efficient biocatalysis.