A phosphorous compound such as STMP is used as a cross-linking agent while making a starch nanoparticle in an emulsion process. Negative charge of the nanoparticle is reduced or reversed by adding cations and/or cationizing the starch optionally while forming the nanoparticles. Anionic active agents, such as fluoride or fluorescein, are optionally incorporated into the nanoparticle during the formation process. For example, a fluoride salt can also be used, which promotes the crosslinking reaction while also providing fluoride in the nanoparticle. The retention of both calcium and fluoride in the nanoparticle is improved when both salts are used. Alternatively, the nanoparticle may be used without added calcium and/or fluoride. The nanoparticles may be useful for tooth remineralization, the treatment of dentinal hypersensitivity, to treat caries, or as a diagnostic agent to locate carious lesions.