▎ 摘　　要

Carbon based quantum dots find various applications such as imaging, diagnosis and therapeutics due to low toxicity, antibacterial activity, excellent water solubility, and good biocompatibility. However, the UV-blue emission of carbon based quantum dots is unsuited for applications in biomedicine and imaging as the short wavelength lights harm the living cells due to phototoxicity. In the present work, we have synthesised the long wavelength emitting heteroatoms (nitrogen (N) and sulfur (S)) co-doped graphene quantum dots (NS-GQD) via hydrothermal route. These NS-GQD have excitation-wavelength independent photoluminescence with high quantum yield in protonated and neutral forms. The non-covalent interaction of these NS-GQD when studied with a dye rhodamine B (RB) shows evidence of Fo center dot rster resonance energy transfer (FRET). It has been shown here that the NS-GQD-RB composite exhibits prominent two-photon absorption at near infrared wavelength with a capability of deep-tissue imaging upto 175 mu m.