Tuning asymmetric electronic structure endows carbon dots with unexpected huge stokes shift for high contrast in vivo imaging

Abstract

In vivo imaging has become a key tool in biomedical studies because it provides a large amount of information about biological stimuli. Owing to the excellent biocompatibility and optical properties, carbon dots (CDs) have been widely used for fluorescence bioimaging. Whereas strong background autofluorescence from tissues is one of the largest concerns with CDs-based in vivo imaging, the development of CDs with large Stokes shifts is becoming critical for reducing the background autofluorescence. Herein, a novel design strategy to increase the Stokes shift of CDs was developed. By breaking the symmetry of the electronic structure of the precursor, the Stokes shift was significantly increased from 75 nm in p-phenylenediamine-derived CDs to 161 nm in 2-nitro-4-aminodiphenylamine (NAP)-derived CDs. In addition, the asymmetric electronic structure also enhanced the intramolecular charge transfer effect, which endowed NAP-CDs with a sensitive response to micro-environment polarity. As cancer cells possess lower polarity than normal cells, NAP-CDs would be effectively applied for cancer imaging with remarkably reduced background fluorescence from cell, tissue, organ to in vivo levels. The asymmetric electronic structure strategy reported here provides an important theoretical basis for the rational design and effective synthesis of CDs with large Stokes shift for practical biomedical applications.