Label-free mapping of osteopontin adsorption to calcium oxalate monohydrate crystals by tip-enhanced Raman spectroscopy.

In the ectopic biomineralization of calcium oxalate kidney stones, the competition between calcium oxalate monohydrate (COM) formation and its inhibition by the phosphoprotein osteopontin (OPN) plays a key role in COM stone-forming processes. To get more insights into these processes, tip-enhanced Raman spectroscopy (TERS) was used to provide surface-specific information about the adsorption of OPN to faces of COM crystals. In TERS, the surface plasmon resonance of a metallic AFM tip is locally excited when the tip is placed in the optical near-field of a laser focused on the crystal surface. Excitation of this localized surface plasmon resonance allows the enhancement of the Raman signal as well as the improvement of the spatial resolution beyond the diffraction limit of the light. As TERS works label free and noninvasively, it is an excellent technique to study the distribution of adsorbed proteins on crystal faces at the submicrometer scale. In the present work, we generated Raman intensity maps indicating high spatial resolution and a distinct variation in relative peak intensities. The collected TERS spectra show that the OPN preferentially adsorbs to edges and faces at the ends of COM crystals (order: {100}/{121} edge > {100} face > {100}/{010} edge ≈ {121}/{010} edge > {010} face) providing also relevant information on the inhibition of crystal growth. This study demonstrates that TERS is an excellent technique for detailed investigations of biomolecules adsorbed, layered, or assembled to a large variety of surfaces and interfaces.