We report on the photoluminescence (PL) of GaAs-Al0.32Ga0.68As core-shell nanowires grown by MOVPE, and their dependence on the precursors V:III molar ratio utilised in the vapor during growth. It is shown that the PL emission of the GaAs nanowire core red-shifts with decreasing the V:III ratio from 30:1 to 4:1, an effect tentatively ascribed to the build-up of a space-charge region at the core-shell hetero-interface, the latter associated to the unintentional incorporation of impurities, namely C in GaAs and Si in AlGaAs.