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Highly sensitive photodetectors play significant roles in modern optoelectronic integrated circuits. Constructing p-n junctions has been proven to be a particularly powerful approach to realizing sensitive photodetection due to their efficient carrier separation. Recently, p-n-junction photodetectors based on organic-inorganic hybrid perovskites, which combine favorable optoelectronic performance with facile processability, hold great potential in practical applications. So far, these devices have generally been made of polycrystalline films, which exhibit poor carrier-transport efficiency, impeding the further improvement of their photoresponsivities. Here, a type of ultrasensitive photodetector based on single-crystalline perovskite p-n-junction nanowire arrays is demonstrated. The single-crystalline perovskite p-n-junction nanowire arrays not only possess high crystallinity that enables efficient carrier transport but also form a built-in electric field facilitating effective carrier separation. As a result, the devices show excellent photosensitivity over a wide spectral range from 405 to 635 nm with an outstanding responsivity of 2.65 × 10

Biocompatible Fe3O4 magnetic nanocrystals with reactive moieties on the surface can be prepared via a "one-pot" reaction and used straightforwardly in cancer detection by being coupled with a specific cancer-targeting antibody. It is demonstrated that the as-prepared Fe3O4 nanocrystals can potentially be used as effective magnetic resonance imaging (MRI) contrast agents for cancer diagnosis (see figure).