Study of the penetration depth of synthesized copper nanoparticles into cut samples of human dentin was conducted. The scanning electron microscopy was used to determine the elemental composition of fresh transverse cleavage of the dentin cut for determination of the copper nanoparticles penetration with an effective antiseptic effect. The morphology of the cut surface of the dentin of a human tooth was studied and the lower limit of the diffusion boundary was determined. It was found that copper nanoparticles penetrate into the dentin cut to a depth of ~ 1.8 μm with the diffusion coefficient of 1.8×10<sup>–11</sup> cm<sup>2</sup>/s. Despite the rather small size of the synthesized copper nanoparticles (20-80 nm), a rather small penetration depth can be explained by the high aggregation ability of copper nanoparticles, as well as the ability of a micellar solution of sodium dodecyl sulfate, in which nanoparticles were stabilized, to form conglomerates in micelles of much larger sizes.