Surface Plasmon Resonance Sensing Characteristics of Thin Copper and Gold Films in Aqueous and Gaseous Interfaces

The sensing properties of thin copper (Cu) films near the surface plasmon resonance (SPR), were re-evaluated for the metal-air and metal-water interfaces and compared with gold (Au) films. For this, we used three different SPR sensing designs, i.e., a prism-based set-up, an optical fiber-based system, and a grating coupler-based solution. Optimum SPR-excitation conditions are restricted to a Cu-metal film thickness of 45 +/- 5 nm for the ATR set-up in aqueous solution. The impacts of the surface Cu-oxide layer were discussed and prevented employing an Au ultra-thin protective overlayer. This strategy proved to be efficient for non-reversible binding. Cu-SPR responsivities are higher for the prism-based set-up (quite similar to Au-SPR ones) than the ones for the optical fiber-based system and a grating coupler-based solution. In contrast to thin Au films, bovine serum albumin (BSA) and other proteins do not bind irreversibly onto thin Cu-films. Our findings confirm the suitability of thin copper films for selected SPR sensing applications.