11. Control Over Nanocrystalline Apatite Formation: What Can the X-Ray Total Scattering Approach Tell Us

Living organisms are able to induce and control the nucleation and crystallization of a wide variety of minerals. Vertebrates use calcium phosphates to build their mineral phases in hard tissues (i.e. bone, dentin and tooth enamel) and in pathological deposits (e.g. dental and urinary calculus and stones, atherosclerotic lesions). Understanding how organisms form their extremely specialized mineralized structures and the in vivo mechanisms enabling their control over crystal morphology, size and polymorphism and, ultimately, over functional properties is particularly important. Nevertheless, these systems are usually complex hybrids very difficult to be fully characterized. Bone is one of the most studied mineralized tissues, although many important aspects of its sophisticated mineralization process need further investigations. In this chapter, we highlight the role of citrate in driving the formation of platy-shaped bio-inspired apatite. Recent solid-state nuclear magnetic resonance (NMR) studies evidenced that citrate, which accounts for ~5.5 % wt of the total organic component of bone, is strongly bound to bone apatite platelets. Thus, we also discuss the possible role of citrate in inducing the typical platelike morphology of bone apatite. Furthermore, the chapter aims at highlighting the strength of X-ray total scattering for characterizing nanocrystalline apatites in terms of crystal structure and defects, stoichiometry (i.e. Ca/P ratio), size and morphology. Future perspectives on the use of time-resolved experiments and combination of complementary advanced techniques are briefly outlined aiming to enhance the fundamental knowledge of bone biomineralization at atomic and nanometre length scales.