Hydroxyapatite Thin Films Synthesis By Electrodeposition
Authors: Garcia Estefania , Poyraz Altug Suleyman , Abbott-Lyon Heather 
Published: February 2020

One of the greatest challenges within origins of life research is known as the “phosphorus problem.” The most abundant sources of phosphorus on the early Earth were phosphate minerals such as hydroxyapatite (HA) or Ca10(PO4)6(OH)2. Although HA is insoluble and does not react readily in most aqueous solutions, HA has been shown to phosphorylate nucleobases such as adenosine and uracil in solutions containing urea, ammonium formate and water. However, the mechanism for the dissolution of phosphorus from the surface of HA is unknown within these prebiotic solutions is unknown. Because HA is an insulator, it is difficult to study the mineral surface in situ using traditional surface-sensitive methods. Therefore, we are developing methods to grow thin hydroxyapatite films on a copper substrate.

Polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) is a technique that allows the structure of gas-solid and liquid-solid interfaces to be probed in situ. In order to utilize this analysis method, HA thin films must meet several criteria. The films must be deposited onto a substrate that has high reflectivity in the infrared region of the electromagnetic spectrum. The choice of metal and the number of defects on the underlying substrate influence the infrared reflectivity. The underlying substrate should also provide easy temperature control. The films must be thinner than the wavelength of the light being reflected (in this case less than 2.5 microns) and they must be relatively uniform in structure and thickness.

Electrochemical deposition of the HA films has been chosen for several reasons. First, this method allows for growth on mirror- finished samples, which results in higher signal intensity for the infrared measurements. Electrochemical deposition produces films with a homogeneous composition and customizable thickness. Additionally, this technique is cost efficient compared to synthesis using vacuum deposition methods (e.g., plasma enhanced chemical vapor deposition or physical vapor deposition). Parameters, such as the scan rate and potential can be optimized to produce thin films conducive to PM-IRRAS experiments. The electrochemical deposition method used for the growth of HA thin films used cyclic voltammetry with a potential from 0 to -1.6 V, an electrolytic solution composed of 0.0168 M Ca(NO3)2·4H2O and 0.0100 M NH4H2PO4 (due to the 1.67 stoichiometric ratio of Ca/P in HA), an Ag/AgCl reference electrode, and a Pt counter electrode. Results thus far indicate that optimal films are achieved using elevated temperatures (preferably 60°C), scan rate of at least 25 mV/s, and an electric potential of 0 to -1.6 V. These conditions produce a more uniform distribution of HA on the substrate. In the most recent experiments, the number of cycles was decreased from 5 to 3 and the temperature was lowered with a constant range to provide for a thinner film. Films were initially characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive x-ray (EDX). Films with favorable characteristics were then further analyzed by PM-IRRAS, to determine their reflectivity. Future work includes optimizing this electrodeposition method for PM-IRRAS analysis, with the aim of conducting reactivity studies on the surface of HA.