Functionalized carbon nanoparticles: Syntheses and applications in optical bioimaging and energy conversion

• Functionalized carbon nanoparticles or carbon dots are quantum dot-like.
• Carbon dots now represent a rapidly advancing and expanding research field.
• Carbon dots have been investigated extensively for bioimaging and theranostics.
• Carbon dots are semiconductor-equivalent in energy conversion applications.

Semiconductor quantum dots (QDs) are known for their unique optical properties. In recent years, carbon nanomaterials of surface and/or structural defects have been found to exhibit similar properties after functionalization in various schemes. Among these carbon-based QDs are carbon dots, which are generally defined as small carbon nanoparticles with surface passivation. In fact, carbon dots now represent a rapidly advancing and expanding research field. As measured by the optical properties of carbon dots, the most effective passivation has been the surface functionalization of carbon nanoparticles with organic or polymeric molecules, corresponding to much brighter fluorescence emissions across the visible spectrum and extending into the near-IR. Therefore, carbon dots have been pursued extensively for potential bioimaging and other biomedical applications. The mechanistic framework for carbon dots includes photoinduced redox processes, similar to those found in conventional semiconductor QDs. As a result, carbon dots have also been pursued for their photocatalytic functions. In this article on surface-functionalized carbon nanoparticles or carbon dots, their representative syntheses and demonstrated properties and their potential uses as high-performance yet nontoxic fluorescence probes for bioimaging in vitro and in vivo are highlighted, so is their serving as potent photocatalysts in energy conversion applications.