Metal oxide nanoparticles having at least some surface acid sites functionalized
with an adhesion promoter and at least some surface acid sites functionalized with
a coupling agent. The nanoparticles are useful in forming composites comprising
photopolymerizable matrix monomers, and are primarily suitable for dental and medical
restoration. In a preferred embodiment, the metal oxide comprises zirconium, and
the coupling agent is a zirconate.