Low temperature plasmas with their strong non equilibrium character offer unique possibilities for the production of nanoparticles. This contribution deals with size dependent properties of nanoparticles synthesized in a capacitively coupled discharge operated in mixtures of argon and acetylene. X-ray absorption measurements show that the particle properties dramatically change during the growth process. For nanoparticles under 10 nm in diameter, near edge x-ray absorption fine structure spectroscopy shows a sp2 rich graphite-like material. The bonding situation changes with the increasing size of the dust particles, showing the formation of a sp2 poor mantle around the sp2 rich core. This phenomenon can be explained in terms of the nucleation and growth process of nanoparticles, i.e., due to differences in the heating of small nanoparticles (nuclei) and due to differences in the gas phase species involved in the nucleation phase and the surface growth phase.