A first-principle investigation of the effects of multiple Si=O bonds at the surface of silicon-based clusters with different sizes has been carried out. Total-energy pseudopotential calculations withindensity functional theory have been applied varying systematically the number of Si=O bonds at the clusters surface. A nonlinear reduction of the energy gap with the Si=O bond number is found. A sort of saturation limit is displayed, providing a consistent interpretation of the photoluminescence redshift observed in oxidized porous silicon samples. Moreover, our results help to clarify the very recent findings on the single silicon quantum dot photoluminescence bandwidth.