Graphene quantum dot (GQD) is a derivative of 2D material graphene. It normally refers to small fragments of graphene having lateral size below 100nm. Not only do GQDs inherit some of the wonder properties of bulk graphene, but they possess properties unique from bulk graphene due to the quantum confinement and edge effects. As an emerging material, GQDs presents a new open field for broad investigations, from synthesis, explanation of properties to promising applications including sensing, bio-imaging, nanomedicine (e. g. drug delivery), energy conversion (e. g. photovoltaic devices and photocatalyst) optoelectronics, spintronics etc. This PhD project is dedicated to three correlated aspects of GQDs: 1) development of new methods for synthesis of GQDs; 2) mechanistic studies of the photoluminescence (PL) possessed by GQDs, and; 3) the applications of GQDs in sensing and light harvesting.