A New Model for Nanoscale Enamel Dissolution Lijun Wang, ² Ruikang Tang, ² Tammy Bonstein, ‡ Christine A. Orme, § Peter J. Bush, ‡ and George H. Nancollas* ,² Department of Chemistry, School of Dental Medicine, UniVersity at Buffalo, The State UniVersity of New York, Buffalo, New York 14260, and Lawrence LiVermore National Laboratory, LiVermore, California 94550 ReceiVed: August 6, 2004; In Final Form: October 14, 2004 The dissolution kinetics of human tooth enamel surfaces was investigated using nanomolar-sensitive constant composition (CC) and in situ atomic force microscopy (AFM) under simulated caries formation conditions (relative undersaturation with respect to hydroxyapatite ) 0.902, pH ) 4.5). Scanning electron microscopic (SEM) examination of the resulting etched enamel surfaces showed that deminerzalization, initiated at core/ wall interfaces of rods, developed anisotropically along the c-axes. After an initial rapid removal of surface polishing artifacts, the dissolution rate decreased as the reaction proceeded in accordance with our recently proposed crystal dissolution model, resulting in hollow enamel cores and nanosized remaining crystallites, resistant to further dissolution. Generally, dissolution of minerals is regarded as a spontaneous reaction in which all the solid phase can be dissolved in undersaturated solutions. However, the dissolution of some biominerals may be suppressed when the crystallites approach nanometer size. This study shows that CC demineralization of enamel in acidic medium follows this new model that can be used to mimic carious lesion formation. In dissolution studies, nanosized enamel crystallites exhibit a remarkable degree of self- preservation in the fluctuating physiological milieu. I. Introduction Tooth enamel, the hardest human mineralized tissue, is composed almost exclusively (above 95 wt %) of apatite-like crystallites with highly organized hierarchical structures. Scan- ning electron microscopy (SEM) of enamel surfaces shows well- organized rodlike structures, with the apatite crystals elongated in their c-axis directions, which lie predominantly parallel to the rod axes. 1 Despite these complex hierarchical structures, the basic building blocks for mineralized tissues are of nanoscale dimensions. In the process of enamel crystal formation, thin ribbonlike crystals that initially deposit in the enamel matrix eventually grow into flat hexagonal prisms. 2 High-resolution transmission electron microscopy (TEM) has revealed the role of nanometer-sized particles which serve as precursors in the formation of the elongated ribbonlike crystals. 3,4 Amelogenin proteins constitute the primary structural entity of the extracellular protein framework of the developing enamel matrix. Recent data show that the dominant protein of enamel, amelogenin, self-assembles by protein-to-protein interactions to form nanospheres (18-20 nm diameter). 5,6 Hydrophobic nano- spheres further assemble to form larger nanospheres with diameters of 20-200 nm, thereby stabilizing the matrix contain- ing the initial enamel crystallites. 7 Nanospheres can interact with specific surfaces of growing hydroxyapatite crystallites and limit crystal growth to certain kinetically preferred orientations. 8-10 Following secretion, the amelogenins undergo a proteolytic degradation, 11 and crystallites thicken and eventually may fuse to generate the mature enamel. Previous studies and our current results of in situ electron dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and infrared (IR) indicate identical chemical and crystallographic properties of the minerals in the cores and on the walls of enamel rods after complete maturation. 12 Gao et al. have shown that the size of the mineral particles (typically tens to hundreds of nanometers) is not arbitrary. 13 Rather, it seems to give biominerals such as bone and tooth remarkable characteristics: As the mineral size falls below a critical length scale (around 30 nm), the strength of a perfect mineral crystal is maintained despite defects. It seems clear that the weakening effect of structural flaws vanishes specifically at the nanoscale. 13 Recent dissolution studies of synthetic enamel-like hydroxyapatite have revealed an interesting and unusual behavior in that dissolution rates decreased, eventually resulting in effective suppression, when their sizes fell into a critical length scale, always at the nanoscale. 14,15 The important questions are why the nanoscale is so important for biomaterials and how this phenomenon of self-inhibiting dissolution can be modeled. In this paper, we combine experimental observations of enamel dissolution kinetics and propose a model for nanoscale enamel dissolution. II. Materials and Methods Twenty freshly extracted caries-free and filling-free human molars, stored for less than two weeks in 0.5% Chloramine-T, were used in this study. Each tooth was cut horizontally at the cemento enamel junction (CEJ) to remove the root portion. The crown was cut vertically (parallel to the tooth axis). Vertical cuts from the buccal and lingual sides of the crown produced two enamel disks about 2 mm thick from each tooth (40 disks in all). All disks were made using a slow-speed diamond saw (Buehler Isomet 1000 precision saw) with water irrigation. * Corresponding author: Prof. George H. Nancollas, 756 Natural Sciences Complex, University at Buffalo, The State University of New York, Buffalo, NY 14260 U.S.A. Email: ghn@buffalo.edu. Phone: +1-716-645- 6800 ext. 2210. Fax: +1-716-645-6947. ² Department of Chemistry. ‡ School of Dental Medicine. § Lawrence Livermore National Laboratory. 999 J. Phys. Chem. B 2005, 109, 999-1005 10.1021/jp046451d CCC: $30.25 © 2005 American Chemical Society Published on Web 12/24/2004