Novel enhancing materials for biosensor design: The case studies of erbium-, gadolinium- and strontium-doped Ca10(PO4)6(OH)2 hydroxyapatite

Hydroxyapatite (HAp), ideal formula Ca₁₀(PO₄)₆(OH)₂, has unique physicochemical properties, including an excellent adsorption ability for functional biomolecules (e.g. nucleic acids, proteins) thanks to its specific large crystal surface. This property can be further improved with cationic and anionic replacements within the HAp framework. The adsorption of such biomolecules, indeed, can cause changes in the electric properties of the HAp surface in terms of resistivity and capacitance, generating the conditions for an improvement of the materials targeted for sensor applications. This work relates to the multiple routes for the synthesis of HAp materials, their electrochemical and structural investigations, and a short overview on the most well-known applications in sensor design. Moreover, with the aim of finding new promising HAp-based materials tailored for bioreceptor immobilization in biosensing, we underwent some doped-hydroxyapatite materials, specifically Sr-HAp, Gd-HAp, and Er-HAp, to a complete characterization. Electrochemical analyses, based on differential pulse voltammetry and cyclic voltammetry, evidenced improved analytical performances of HAp in terms of signal enhancement, repeatability, reproducibility, and reusability, in particular concerning the Er-HAp phase. A multi-methodological structural study, based on powder X-ray diffraction analysis, microscopy techniques (optical, electron, and fluorescence), energy dispersive X-ray spectroscopy (for chemical analyses), Fourier transform infrared spectroscopy, and absorption/fluorescence spectroscopies, showed the mechanism of doping replacement in HAp crystallographic sites, owing to the results of the Rietveld refinement from powder X-ray data, and a strong fluorescence for Sr-HAp.