Yes there is difference between optical properties of undoped GQD and N-doped GQD. The undoped GQD has a very high fluorescence while doped GQD have no detectable photo-luminescence. Please refer to paper by Q Li published in JACS. The following explanation is taken from the same.

This is consistent with the absence of n→π* , the lowest-energy transitions, in

the absorption spectra of doped GQD. The n→π* transitions are expected for the N-substituted aromatic heterocycles. The large size of doped GQD explains the diminishing oscillator strengths of the n→π* transitions, because of the decreasing orbital overlap between the increasingly delocalized π* electrons and the nonbonding electrons localized on the nitrogen atoms. In contrast, the lowest-energy transition in undoped GQD is π→π*, which is responsible for the emission and has a less size-dependent oscillator strength.

Recent studies shows after doping nitrogen fluorescence of graphene quantum dots is enhanced. see this paper

http://www.nature.com/articles/srep05294