Monitoring of TiO$_{\bf 2}$ and ZnO Nanoparticle Penetration Into Enamel and Dentine of Human Tooth IN VITRO and Assessment of Their Photocatalytic Ability

Penetration of nanoparticles into tooth enamel and dentine is of significant interest upon solving problems related to reduction of tooth sensitivity, enamel strengthening, disinfection, restoration as well as cosmetic bleaching. This paper aims at studying the process of nanoparticle penetration into tooth enamel and dentine samples using nonlinear optical microscopy and at investigating the influence of the same nanoparticles on the generation of free radicals using the electronic paramagnetic resonance technique. We presented in vitro measurements demonstrating that nonlinear optical microscopy, namely, two-photon-excited autofluorescence, second harmonic generation, and hyper-Rayleigh scattering-based microscopy can be used for monitoring and imaging TiO
$_{2}$
and ZnO nanoparticle penetration into tooth tissues. The results indicate that ZnO nanoparticles penetrated into the human tooth enamel and dentine up to a depth of 12 and 45 μm, respectively, and TiO
$_{2}$
nanoparticles penetrated into dentine to a depth of 5 μm. The penetration mainly occurs along either enamel rods or dentinal tubules. Permeability of the dentine was found to be higher than that of enamel (for ZnO particles) by one order of magnitude and the diffusion rate was affected by the particle size being higher for smaller, submicron particles (ZnO) than for micron-sized aggregates (TiO
$_{2}$
). Nitrogen-doped TiO
$_{2}$
nanoparticles generate more radicals in the UV–VIS spectral range in comparison to pristine TiO
$_{2}$
(anatase) and ZnO nanoparticles, therefore, they can potentially be used for disinfection purposes of superficial tooth areas (up to 5-μm deep).