Full-Color Light-Emitting Carbon Dots with a Surface-State-Controlled Luminescence Mechanism

TLDR

Although the carbon cores had similar size and graphite structure, the surface state varied gradually, especially in oxidation degree. Carbon dots were hydrothermally synthesized in a one‑pot reaction and purified by silica column chromatography, yielding tunable photoluminescence with a quantum yield up to 35 % in water. The CDs exhibited bright, stable, gradient blue‑to‑red emission with a single, excitation‑independent PL peak and monoexponential lifetimes; the red shift from 440 to 625 nm was attributed to increasing surface oxidation that narrows the band gap, enabling color‑imaging of cells and live mice under a single‑wavelength UV source.

Abstract

Carbon dots (CDs) with tunable photoluminescence (PL) and a quantum yield of up to 35% in water were hydrothermally synthesized in one pot and separated via silica column chromatography. These separated CDs emitted bright and stable luminescence in gradient colors from blue to red under a single-wavelength UV light. They exhibited high optical uniformity; that is, every sample showed only one peak in the PL excitation spectrum, only one peak in the excitation-independent PL emission spectrum, and similar monoexponential fluorescence lifetimes. Although these samples had similar distributions of particle size and graphite structure in their carbon cores, the surface state gradually varied among the samples, especially the degree of oxidation. Therefore, the observed red shift in their emission peaks from 440 to 625 nm was ascribed to a gradual reduction in their band gaps with the increasing incorporation of oxygen species into their surface structures. These energy bands were found to depend on the surface groups and structures but not on the particle size, not as in traditional semiconductor quantum dots. In addition, because of their excellent PL properties and low cytotoxicity, these CDs could be used to image cells in different colors under a single-wavelength light source, and the red-emitting CDs could be used to image live mice because of the strong penetration capability of their fluorescence.