Functionalised MCM-41 mesoporous silica nanoparticles were used as carriers of Gd(III) complexes for the development of nanosized magnetic resonance imaging contrast agents. Three Gd(III) complexes based on the 1,4,7,10-tetraazacyclododecane scaffold (DOTA; monoamide-, DOTA- and DO3A-like complexes) with distinct structural and magnetic properties were anchored on the silica nanoparticles functionalised with NH(2) groups. The interaction between Gd(III) chelates and surface functional groups markedly influenced the relaxometric properties of the hybrid materials, and were deeply modified passing from ionic -NH(3)(+) to neutral amides. A complete study of the structural, textural and surface properties together with a full (1)H relaxometric characterisation of these hybrid materials before and after surface modification was carried out. Particularly for the anionic complex 2 attached to MCM-41, an impressive increase in relaxivity (r(1p)) was observed (from 20.3 to 37.8 mM(-1) s(-1), 86.2% enhancement at 20 MHz and 310 K), mainly due to a threefold faster water exchange rate after acetylation of the surface -NH(3)(+) ions. This high r(1p) value, coupled with the large molar amount of grafted 2 onto the silica nanoparticles gives rise to a value of relaxivity per particle of 29,500 mM(-1) s(-1), which possibly allows it to be used in molecular imaging procedures. Smaller changes were observed for the hybrid materials based on neutral 1 and 3 complexes. In fact, whereas 1 shows a weak interaction with the surface and acetylation induced only some decrease of the local rotation, complex 3 appears to be involved in a strong interaction with surface silanols. This results in the displacement of a coordinated water molecule and in a decrease of the accessibility of the solvent to the metal centre, which is unaffected by the modification of ammonium ions to neutral amides.