Surface functionalization of graphene quantum dots with small organic molecules from photoluminescence modulation to bioimaging applications: an experimental and theoretical investigation

Surface chemistry provides an alternative approach to modulate the emission colour and efficiency of graphene quantum dots. We systematically investigated the surface chemistry of graphene quantum dots functionalized with a series of small organic molecules combining experimental and theoretical approaches. Experimental results indicated that surface functionalization with functional groups such as alcohol, amine and thiol can effectively tune the fluorescence of graphene quantum dots, and proved that amino groups can highly elevate the quantum yields of modified graphene quantum dots. The emission efficiency of 1,2-ethylenediamine functionalized graphene quantum dots reached up to 17.6% due to specific proton transfer to the conjugated fluorophore-like structure from ammonium formed by protonation. The polyaromatic structure within the graphene quantum dots was proposed to explain the fluorescence enhancement mechanism of graphene quantum dots functionalized by diamines. The computational results suggested that not only the size of the polyaromatic structures within graphene quantum dots can change their emissions, but surface functionalization can also tune their photoluminescence through modulating their band gaps. Toxicity experiments indicated that diamine-functionalized graphene quantum dots showed low cell toxicity similar to that of pristine graphene quantum dots. Moreover, the bioimaging experiments suggested that functionalized graphene quantum dots had identical abilities to label cells at a lower concentration than pristine graphene quantum dots owing to their higher quantum yields.