Formation of stable hydrophilic C60 nanoparticles by 2-hydroxypropyl-β-cyclodextrin

A number of papers have reported that the large cavity of γ-CyD is favorable for inclusion of C(60) and forms a 1:2 (C(60):γ-CyD) complex, whereas it is thought to be difficult for β-CyD to form a complex at the molecular level. This is because the cavity size of β-CyD (0.78 nm) is smaller than the van der Waals diameter of C(60) (1.0 nm). In this paper, we will report on the formation of the stable C(60) nanoparticles by the hydrophilic 2-hydroxypropyl-β-cyclodextrin (HP-β-CyD) layer through weak interaction on the surface of the nanoparticles. C(60) was coground with β-CyD, γ-CyD or HP-β-CyD mainly in a 1:2 molar ratio by an automatic magnetic agitating mortar, the coground powders were dispersed in water, and the resulting solutions were filtered through a pore size of 0.8 μm filter. The γ-CyD and HP-β-CyD systems gave transparent colloidal solutions consisting of C(60)/CyD nanoparticles with the size lower than 100 nm, with high yields (about 100%). The C(60)/HP-β-CyD nanoparticles are physically stable, keeping a small size for more than 28 days, whereas the γ-CyD nanoparticles are readily aggregated to form large particles (>800 nm). Solid and liquid NMR spectroscopic studies including measurements of spin-lattice relaxation times indicated that C(60) interacted with γ-CyD and HP-β-CyD in the solid and colloidal solutions. When compared with the γ-CyD nanoparticles, adsorption studies of a hydrophobic dye on the surface of C(60)/CyD nanoparticles indicated that the surface of the HP-β-CyD nanoparticles is largely covered by HP-β-CyD molecules forming hydrophilic hydration layers. The present results suggest that HP-β-CyD is useful for the preparation of C(60) nanoparticles and medical applications such as photodynamic therapy, in spite of having a cavity size smaller than that of γ-CyD.