Synthesis of nickel (Ni) nanoparticles (NPs) suspensions was performed using a 120 femtosecond (fs) pulse laser to ablate a Ni solid target in n-heptane and water. Analysis of structure, configuration and sizing was carried out using different independent techniques such as Optical Extinction Spectroscopy (OES), Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and Electron Diffraction (ED), which yield interrelated information. AFM microscopy allows determining the spherical shape and size distribution of the NPs in the obtained colloids while TEM provides knowledge about shape, structure and size distribution. ED allows identification of the different metal and metal oxide composition as well as their crystallographic phase. On the other hand, OES gives information related to size distribution, structure, configuration and composition. Interpretation of these spectra is based on Mie theory which, in turn, depends on Ni dielectric function. For NPs radii smaller than 3 nm, size-dependent free and bound electron contributions to the dielectric function must be considered. To account for the full size span, complete Mie expansion was used for optical extinction cross-section calculations. A theoretical analysis of the dependence of plasmon resonance of bare core and core-shell Ni NPs with core size and shell thickness provide insight about their spectroscopic features. For n-heptane, species like bare core Ni and hollow Ni NPs are found in the colloid, being the latter reported for the first time in this work. Instead, for water, the colloid contains hollow nickel NPs and nickel oxide in different core-shell configurations: Ni-NiO and NiO-Ni, being the latter also reported for the first time in this paper. In both cases, size distribution agrees with that derived from TEM and AFM analysis. The formation of the oxide species is discussed in terms of oxidation-reduction processes during ablation. Possible mechanisms for the formation of hollow species are proposed.Centro de Investigaciones ÓpticasInstituto de Física La Plata