A strategy of achieving efficient orange emissive carbon nanodots (CNDs) with large sized conjugated sp(2) -domain is achieved in a solvothermal synthetic route using dimethylformamide as solvent, which is the basis of orange bandgap emission; enhanced orange emission with photoluminescent quantum yield of 46% is realized through surface charges engineering by surface metal-cation-functionalization.

Carbon dots (CDs) have significant potential for use in various fields including biomedicine, bioimaging, and optoelectronics. However, inefficient excitation and emission of CDs in both near-infrared (NIR-I and NIR-II) windows remains an issue. Solving this problem would yield significant improvement in the tissue-penetration depth for in vivo bioimaging with CDs. Here, an NIR absorption band and enhanced NIR fluorescence are both realized through the surface engineering of CDs, exploiting electron-acceptor groups, namely molecules or polymers rich in sulfoxide/carbonyl groups. These groups, which are bound to the outer layers and the edges of the CDs, influence the optical bandgap and promote electron transitions under NIR excitation. NIR-imaging information encryption and in vivo NIR fluorescence imaging of the stomach of a living mouse using CDs modified with poly(vinylpyrrolidone) in aqueous solution are demonstrated. In addition, excitation by a 1400 nm femtosecond laser yields simultaneous two-photon-induced NIR emission and three-photon-induced red emission of CDs in dimethyl sulfoxide. This study represents the realization of both NIR-I excitation and emission as well as two-photon- and three-photon-induced fluorescence of CDs excited in an NIR-II window, and provides a rational design approach for construction and clinical applications of CD-based NIR imaging agents.

A novel concept and approach to engineering carbon nanodots (CNDs) were explored to overcome the limited light absorption of CNDs in low-energy spectral regions. In this work, we constructed a novel type of supra-CND by the assembly of surface charge-confined CNDs through possible electrostatic interactions and hydrogen bonding. The resulting supra-CNDs are the first to feature a strong, well-defined absorption band in the visible to near-infrared (NIR) range and to exhibit effective NIR photothermal conversion performance with high photothermal conversion efficiency in excess of 50%.