Cell membrane integrity is fundamental to the normal activities of cells and is involved in both acute and chronic pathologies. Here, we report a probe for analyzing cell membrane integrity developed from a 9 nm-sized protein nanocage named Dps via fluorophore conjugation with high spatial precision to avoid self-quenching. The probe cannot enter normal live cells but can accumulate in dead or live cells with damaged membranes, which, interestingly, leads to weak cytoplasmic and strong nuclear staining. This differential staining is found attributed to the high affinity of Dps for histones rather than DNA, providing a staining mechanism different from those of known membrane exclusion probes (MEPs). Moreover, the Dps nanoprobe is larger in size and thus applies a more stringent criterion for identifying severe membrane damage than currently available MEPs. This study shows the potential of Dps as a new bioimaging platform for biological and medical analyses.

Protein nanocages are ideal templates for the bio-inspired fabrication of nanomaterials due to several advantageous properties. During the mineralization of nanoparticles (NPs) inside protein nanocages, most studies have employed a common strategy: seed formation inside protein nanocages followed by seeded NP growth. However, the seed formation step is restricted to gentle reaction conditions to avoid damage to the protein nanocages, which may greatly limit the spectrum of seed materials used for NP growth. We put forward a simple route to circumvent such a limitation: encapsulation of a preformed NP as the seed via self-assembly, followed by the growth of an outer metal layer. Using such a method, we succeeded in mineralizing size-tunable Au NPs and Au@Ag core–shell NPs (< 10 nm in diameter) with narrow size distributions inside the virus-based NPs of simian virus 40. The present route enables the utilization of NPs synthesized under any conditions as the starting seeds for nanomaterial growth inside protein nanocages. Therefore, it potentially leads to novel bioinorganic chimeric nanomaterials with tailorable components and structures.