Nano particles have a positively charged outer segment that binds negatively charged sites on plaque biofilms and tooth enamel. It anchors particles in place.

Clinical trials done to test the effectiveness of the nanoparticles showed that head-to-head in a topical application, the drug-loaded nanoparticles were four times more powerful in destroying the bugs (Streptococcus mutans, in this study) than the free drug alone.
The nanoparticles were able to adhere and deliver the drug in a controlled-release fashion, targeting sites of bacterial growth (biofilms). Once the safety of the technology is proved, these particles can be used in things like toothpaste or mouthwash to keep the teeth free of dental caries and prevent a painful trip to the dentist’s office.
Source-Medindia