| The nucleation and growth of calcium phosphate is of great importance to the formation of mammalian hard tissue structures such as bone and teeth and for unwanted, ectopic calcium phosphate deposition on arteries and implants. In spite of its importance, the mechanisms of nucleation and growth of calcium phosphate are not well known, but are believed to involve an organic template. The nucleation and growth of calcium phosphate was studied onto model nucleation templates composed of alkanethiol self-assembled monolayers on gold that were developed and tailored to have various surface functionalities, various surface site densities composed of mixtures of two thiols, and various degrees of conformational disorder composed of mixtures of SAMs of various chain lengths. The quartz crystal microbalance was developed as an in-situ technique to study the nucleation and growth kinetics and ex-situ techniques such as X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy were used to assess adsorbate molecular chemistry in the initial stages of deposition. Significant nucleation and growth of calcium phosphate onto SAMs involved the adsorption of solution-formed critical nuclei. This mechanism is in contrast to heterogeneous nucleation and may have important implications for bone formation. An initial slow growth region occurred which involved the adsorption and assembly of solution-formed nanometer-sized particles. A second fast growth period occurred which involved the adsorption and growth of solution-formed critical nuclei or the assembly of supercritical particles. There was evidence for the heterogeneous nucleation of a very low density of crystals at low solution supersaturation. Heterogeneous nucleation may be limited due to the use of planar surfaces and to limits on phosphate adsorption due to electrostatic double layer anisotropy at the charged interfaces. Surface selective deposition was found in the initial slow growth region with growth promoted onto charged SAMs such as carboxylic acid in contrast to hydroxyl, methyl ester, and methyl. The nanoparticle assembly growth mechanism was used to form ultrathin films of calcium phosphate which have not been formed previously. The micron-sized and ultrathin films are of interest as biosensors, bioelectronic devices, and bioactive coatings on implants. |
