Interactions between biomolecules and inorganic surfaces play an important role in natural environments and in industry, including a wide variety of conditions: marine environment, ship hulls (fouling), water treatment, heat exchange, membrane separation, soils, mineral particles at the earth's surface, hospitals (hygiene), art and buildings (degradation and biocorrosion), paper industry (fouling) and more. To better control the first steps leading to adsorption of a biomolecule on an inorganic surface, it is mandatory to understand the adsorption mechanisms of biomolecules of several sizes at the atomic scale, that is, the nature of the chemical interaction between the biomolecule and the surface and the resulting biomolecule conformations once adsorbed at the surface. This remains a challenging and unsolved problem. Here, we review the state of art in experimental and theoretical approaches. We focus on metallic biomaterial surfaces such as TiO(2) and stainless steel, mentioning some remarkable results on hydroxyapatite. Experimental techniques include atomic force microscopy, surface plasmon resonance, quartz crystal microbalance, X-ray photoelectron spectroscopy, fluorescence microscopy, polarization modulation infrared reflection absorption spectroscopy, sum frequency generation and time of flight secondary ion mass spectroscopy. Theoretical models range from detailed quantum mechanical representations to classical forcefield-based approaches.

The adsorption of glycine, the building block of amino acids, on hydroxylated (0001)-Cr2O3 model surfaces, representing the stainless steel passive film surface, was modeled by means of the GGA + U method. The roles of glycine coverage and surface termination (hydroxylated Cr- and O-terminated surfaces) on the adsorption mode and self-assembly properties were explored. The hydroxylated Cr-terminated Cr2O3 surface, which presents two types of (H)OH groups exhibiting different acidic character, is more reactive than the hydroxylated O-terminated surface, where one single type of OH group is present, for all adsorption modes and coverages considered. Outer sphere adsorption occurs in the zwitterion form, stabilized at low coverage through H-bond formation with coadsorbed water molecules, and at the monolayer coverage by glycine self-assembling. The OH substitution by glycinate is favored on the hydroxylated Cr-terminated surface and not on the O-terminated one. The inclusion of dispersion forces does not change the observed tendencies. An atomistic thermodynamics approach suggests that outer sphere adsorption is thermodynamically favored over inner sphere adsorption in the whole domain of glycine concentration. The obtained SAM's free energies of formation are rationalized in a model considering the balance between sublimation and solvation free energies, and extrapolated to other amino acids, to predict the SAMs formation above hydroxylated surfaces. It is found that hydrophobic AA tend to self-assemble at the surface, whereas hydrophilic ones do not.

The adsorption of glycine, the smallest amino acid and building block of proteins, on a Cr 2 O 3 surface, taken as a model of stainless steel surfaces, was studied by means of GGA + U (on site Coulomb repulsion). The role of coverage on the adsorption mode and the self-assembly properties were explored. The dispersion contribution to the adsorption energies was also calculated. Glycine adsorbs at low and high coverage in the anionic form. The adsorption is driven by the formation of iono-covalent bonds with the Cr surface atoms. The most stable configurations correspond to maximization of the bonds (Cr-O and Cr-N), where the O or N atoms replace O atoms from the missing anionic plane above the terminal Cr plane. To maximize the bonds formation and minimize lateral repulsion, glycine lies parallel to the surface at low coverage and has a bent orientation at high coverage. The inclusion of dispersion forces strengthens this trend. At mean coverage under the ML, clusterization of high density domains may coexist with domains of lower coverage at the surface. At saturation, a glycinate monolayer with a 1:1 Gly:Cr ratio is formed.