Surface Plasmon Resonance Sensing Characteristics of Thin Aluminum Films in Aqueous Solution

Surface plasmon resonance (SPR) sensors, utilizing thin aluminum (Al) films, are reported. The sensor designs are based in the novel optical trapezoidal prism chip, made from either polymer or BK7 glass. Optimum Al-film thickness was determined to 20 +/- 5 nm, and deposited by thermal e-beam evaporation and rf-magnetron sputtering. Upon contact to air, and in de-ionized aqueous solution, a self-limited surface oxide layer forms. Determined by ellipsometric recordings, its thickness grows from 4.6 +/- 0.6 nm in air to 12.3 +/- 2 nm in the wet interface. A single SP-resonance applies to the dry interface. Unusual spectral broadening, due to the presence of multiple resonances, is predicted and experimentally verified at the wet interface. The Al-oxide adlayer is beneficial in the angular interrogation, where the instrumental response approaches the one exhibited by the noble metals. Under wavelength interrogation conditions, the presence of the Al-oxide adlayer causes severe degradation of the sensor response. Both, angular and wavelength interrogation at the metal-aqueous solution interface were exploited experimentally and compared with theoretical predictions regarding the sensing features of gold (Au), copper (Cu), and silver (Ag) metal films as well. Limitations, cost aspects and alternative routes to overcome degradation and the loss of SPR-activity in the presence of ionic phosphate buffered saline aqueous solutions are outlined for Al-films.