Biomimetic nanohydroxyapatite (nanoHA) plays a prominent role among of calcium-phosphate biomaterials. An important aspect of biomimetism is the plate morphology of nanoparticles, resulting in the overwhelming exposure of (01-10) type surfaces. Noteworthy, the atomic structure of these surfaces can range from stoichiometric, Ca-rich and P-rich . Despite the advantage of direct imaging, HRTEM allows the inspection of a limited number of nano-HA particles , while the method developed by some of us exploiting IR spectroscopy of adsorbed CO as molecule probing the local structure of surface Ca<sup>2+</sup> sites allows a definitely easier and more extended assessment of the relative amount of the above three type of (01-10) terminations. 
We applied this method to a series of nanoHA materials, produced by changing the source of Ca<sup>2+</sup>, temperature, presence/absence of carboxylates. In all cases, the (01-10) terminations were only of the Ca-rich and P-rich types, in a ratio ranging from 1:2 to 2:1.
Following the Kasemo causal sequence among interfacial molecular events ruling the impact of a biomaterials on the host tissue,, we investigated the interaction with water and a protein (a model one, namely BSA), by in situ molecular spectroscopies (FT-MIR, NIR, CD-UV), microgravimetry, DLS, Z-potential.
The (01-10)_Ca-rich and (01-10)_P-rich ratio was found to impact on the structure of water multilayers along the normal to the surfaces. Moreover, the ratio determined also the conformation of adsorbed proteins, resulting in the exposure toward the exterior of a different protein surface. This finding offers a new view on factor possibly ruling the “biological identity” of nanoHA.