Dielectric coated plasmonic interfaces: Their interest for sensitive sensing of analyte-ligand interactions

Surface plasmon resonance (SPR) sensors have matured over the last 2 decades into very powerful tools for the study of biomolecular interactions, chemical detection and immunoassays. The performance of the sensor depends on several parameters, such as the choice of the metal thin fi lm where the plasmonic wave propagates, the excitation wavelength and the refractive index (RI) of the glass prism. Next to these physical parameters, the strategy selected to bind the desired receptors to the SPR chip, has a strong infl uence on the overall sensitivity and selectivity of the device. This review focuses on the advancement made using lamellar SPR structures, where a thin dielectric layer is deposited onto the surface plasmon active metal thin fi lm. Silver-based SPR interfaces can be developed using this approach, as these overlayers allow an effi cient protection of the underlying silver fi lm. At the same time, these interfaces open the scope for new surface functionalization schemes, which can be employed for anchoring ligands to the SPR sensor chip. While self-assembled monolayers (SAMs) are widely used, due to the possibility of easily incorporating carboxylate, amine or hydroxyl groups, the drawbacks of such fi lms include limited chemical and electrochemical stability. Moreover, a poor orientation and potential problems of protein adsorption and fouling, is often encountered if no synthetic effort in the synthesis of more sophisticated thiols is made. In addition, while the surface chemistry developed on gold has been of great value, the limitations of working on gold are becoming more noticeable, with increasingly complex fabrication requirements for biometric systems and arrays. Lamellar SPR interfaces represent an alternative route. Finally, the contribution of the thin dielectric top layer to the sensitivity of SPR sensors will be discussed.