4.4. Quantification of Available Binding Sites

XPS integrated peak areas of the O 1s binding energy core level were used to quantify the relative number of available binding sites for material in the extracellular matrix to interact with on the NiO and Ni(OH)₂ nanoparticle surfaces. Quantification was achieved by noting the physisorbed versus chemisorbed oxygen on the NP surfaces as described in our previous study [20]. In the case of NiO NPs, the deconvoluted O 1s peak denoting the metal oxide represents the underlying substrate while those peak areas of those oxidation states not from the metal oxide denote physisorbed O. The number of available binding sites were quantified by dividing the XPS peak area of the physisorbed O by that of chemisorbed O.

The O 1s core levels of the NiO NPs were observed at 529.0 eV and 531.0 eV, denoting the metal oxide [41,42,43] and physisorbed O on the NP surfaces, respectively. For the Ni(OH)₂ NP surfaces, the binding energy centers of the O 1s orbitals appear at 531.2 and 532.9 eV, denoting chemisorbed OH groups and physisorbed OH/H₂O [41,44] on the NP surfaces, respectively. While binding energies for adsorbed OH/H₂O are well established, they are not for chemisorbed OH on Ni(OH)₂; this information was obtained via XPS scans on the dried Ni(OH)₂ NPs performed in our laboratory as a reference standard. The binding energy peak center for this oxidation state was found to be 531.2 eV, having a full width at half maximum (FWHM) of 3.4 km/sec, the parameters of which were used in peak area deconvolution of the O 1s envelopes.