The state of nanoparticle-based nanoscience and biotechnology: progress, promises, and challenges.

Colloidal nanoparticles (NPs) have become versatile building blocks in a wide variety of fields. Here, we discuss the state-of-the-art, current hot topics, and future directions based on the following aspects: narrow size-distribution NPs can exhibit protein-like properties; monodispersity of NPs is not always required; assembled NPs can exhibit collective behavior; NPs can be assembled one by one; there is more to be connected with NPs; NPs can be designed to be smart; surface-modified NPs can directly reach the cytosols of living cells.