Nanoparticles functionalization is a modern strategy in nanotechnology to build up devices for several applications. Modelling fully decorated metal oxide nanoparticles of realistic size (few nanometers) in an aqueous environment is a challenging task. In this work we present a case study, relevant for solar-light exploitation and for biomedical applications, i.e. a dopamine-functionalized TiO2 nanoparticle (1700 atoms) in bulk water, for which we have performed an extensive comparative investigation with both MM and QM/MM approaches of the structural properties and of the conformational dynamics. We have used a combined multi-scale protocol for a more efficient exploration of the complex conformational space. On the basis of the results of this study and of some QM and experimental data, we have defined strengths and limitations of the existing force field parameters. Our findings will be useful for animproved modeling and simulation of many other similar hybrid bioinorganic nanosystems in an aqueous environment that are pivotal in a broad range of nanotechnological applications.