j. Cosmet. Sci., 56, 121-134 (March/April 2005) Reaction characteristics of a tooth-bleaching agent containing H 2 02 and NaF: In vitro study of crystal structure change in treated hydroxyapatite and chemical states of incorporated fluorine Y. TANIZA WA, Analytical Research Center, Lion Corporation, 13-12, Hirai 7 -chome, Edogawa-ku, Tokyo, 13 2-00 3 5 Japan. Accepted for publication February 16, 2005. Synopsis This in vitro study was performed to elucidate the reaction mechanism of sodium fluoride (NaF), which is added to tooth-bleaching agents to lessen the adverse effect of hydrogen peroxide (H2O2) on teeth. Both hydroxyapatite (HAP) and dihydrated dicalcium phosphate (DCPD), model substances for dental hard tissues, dissolved easily in a simple H2O2 solution. In the H2O2/NaF solutions, however, fluorine com­ pounds that could not be identified by X-ray diffraction (XRD) due to the smallness of the products were formed on the surface of the HAP. X-ray photoelectron spectroscopy (XPS) studies demonstrated that fluoridated hydroxyapatite (FHAP) was formed on HAP, and that calcium fluoride (CaF2) formation was accelerated by increasing the concentrations of fluorine and H2O2 along with the partial dissolution of HAP. In H2O)NaF solution, DCPD also transformed easily to FHAP and CaF2 , which are favorable to the remineralization process on the tooth surface. Thus, the mechanism of NaF was elucidated, and its use together with H2O2 for tooth bleaching was proved to be effective. Methodologically, the XPS two­ dimensional plot made it possible for the first time to directly estimate the ratio of FHAP and CaF2 in the reaction products, in contrast to the conventional wet-analytical method, which is simply based on the difference in solubility of the two components. INTRODUCTION White teeth are preferable from a cosmetic point of view, and tooth whitening is of interest to dentists and their patients. Toothpastes remove external discoloration, which is localized mainly in the pellicle on the surface of teeth, commonly through the polishing effect of abrasives ( 1-3 ). In our previous work ( 4), attention was directed to the mechanism of pellicle formation. Internal discoloration, however, is a result of defects in the tooth development process, and is also acquired through the use of medicines like tetracycline. Those kinds of discolorations cannot be removed by toothpaste, and there­ fore dental bleaching has been used to reach this objective. The development of bleach­ ing agents and their formulation have been important goals for oral care researchers and dental product manufacturers. To attain these goals, an understanding of the reaction characteristics between the tooth surface and the bleaching agents is required. 121

122 JOURNAL OF COSMETIC SCIENCE Tooth bleaching became an accepted routine treatment in dental offices in the 1970s (5). The use of liquid hydrogen peroxide (H2O 2 ) in dentistry, described as early as 1884, is currently the most common procedure (6-13). Several dentifrices containing H 2 O 2 are also currently being marketed (14). Generally, tooth bleaching has been found to be effective on discolored teeth. On the other hand, the use of such strong oxidizing agents has raised questions as to adverse side effects on tooth structure and restorative materials. Tooth sensitivity is a potential side effect of dentist-dispensed home tooth-whitening systems (15 ). In the past decade, numerous studies have evaluated the effects of peroxide­ containing bleaching agents on tooth hard tissue. Most of the studies found insignificant alterations of the enamel surface (16-24). Some researchers, however, actually observed enamel surface alterations by surface analysis techniques (25,26). Especially when pa­ tients had enamel cracks or other damage, careful treatment has been proposed to be necessary, and fluoride treatment has been recommended to lessen discomfort (27). The manufacturers of the in-home bleaching systems generally provide these products with various extras such as fluoride rinses and pre- and post-bleaching treatments. The mechanism of H202 or fluoride has been individually investigated and described as follows: H 2 Orcontaining bleaching agents remove tooth discolorations, where H202 acts as an oxygenator and an oxidant and then affects the organic phases in the dental enamel (11,28). The caries-preventive effect of fluoride has been well known for many years, and the mechanism has also been proposed (29-3 7). According to the current concepts on the action mechanism of fluoride (38-40), the caries-preventive effect is mainly attributed to the enhancement of remineralization at the interface of the tooth and oral fluids. Although the individual mechanisms of H 2 O 2 and fluoride are fairly well known, the combined use of both H 2 O 2 and fluoride complicates the action mechanism, and the problem of how these two components react all at once with the enamel surface has not been fully understood. The use of fluoride for reducing the discomfort caused by H 2 O 2 has been recommended without an understanding of the detailed mechanism. Thus, this study was undertaken to examine the mechanism of NaF added to lessen the adverse side effects of H2O2 using hydroxyapatite (HAP) as a model material of tooth enamel. X-ray photoelectron spectroscopy (XPS) studies have made it possible for the first time to observe directly the successive change in the ratio of fluoridated hydroxy­ apatite (FHAP) and calcium fluoride (CaF 2 ) formed on the surface of the HAP. We also have clarified the reaction mechanism, discovering that FHAP or CaF 2 is produced not simply by depending on the fluoride concentration as shown elsewhere (3 5 ), but also by depending on the H2O2 concentration. The information about the mechanism and the methodology should be useful in developing bleaching agents. METHODS MATERIALS Hydroxyapatite (HAP Ca/P molar ratio = 1.65 surface area = 8 m2g- 1 ), dehydrated dicalcium phosphate (DCPD), sodium fluoride (NaF), and hydrogen peroxide (H 2 O 2 , 30%) were purchased from Wako Pure Chemicals Co., Japan. Fluorapatite (FAP) crystal from Mexico was powdered, and used as XPS standard material. Calcium fluoride (CaF 2 ), purchased from Kan to Chemicals Co., Japan, was also used as XPS standard material. NaF and H 2 O 2 were diluted with distilled water to given concentrations. The powder