Hydrogen Treatment of SPR Film Sensors: Experiments and Theoretical Modeling

The present paper deals with the experimental and theoretical studies of changes of optical properties of SPR sensors under the influence of hydrogen. Theoretical modeling of the SPR sensor treated with hydrogen was performed using the transfer matrix method and effective medium approaches. Different possibilities of hydrogen influence were considered during the modeling. It was established that the accumulation of hydrogen at the glass-chromium interface could not change the SPR spectrum. The experimentally observed shifts in the SPR spectrum were mainly related to the accumulation of hydrogen in all thicknesses of the gold layer by means of the formation of cavities filled with hydrogen. The accumulation of hydrogen in the bulk of the gold film also led to an increase in the SPR resonance value. On the contrary, the theoretical modeling of the increase in the surface roughness of gold predicted a decrease in the amplitude of the resonance. As a result, a complex theoretical description of the processes, which took place in SPR sensors during hydrogen treatment, was proposed. The predictions were as follows. The oscillations of the surface level before hydrogen treatment were 2 nm, and due to the possibility of surface destruction, the oscillations of the surface level increased to 3 nm. The layer of "solid" gold before hydrogen treatment was 48.5 nm, and after hydrogen it increased to 53.35 nm. The average volume concentration of cavities in this layer was about 10 %. The cavities with hydrogen were homogeneously distributed over the volume. The thickness of the chromium layer before hydrogen treatment was 5 nm, and after hydrogen treatment the thickness of this layer increased to 6 nm. The volume of cavities with hydrogen in chromium was 20 %.