Shell thickness dependent non-monotonic plasmon shift of gold-protein core-shell nanoparticle thin films

Based on Maxwell-Garnett effective medium theory, the surface plasmon resonance (SPR) absorption of gold-protein core-shell nanostructure embedded thin films is analyzed as a function of protein shell thickness and core volume fraction in total particle. The increases of both shell thickness and volume fraction create the red shift of SPR. When the shell thickness and the core volume fraction take effect at the same time, there is a competition. When the gold core radius is a fixed value and the total particle radius increases, in other words, the shell thickness increases but the core volume fraction decreases, the absorption peak blue shifts firstly and then red shifts. That means the core volume fraction plays the major role for thinner shell. Whereas when the shell is thick, the shell thickness plays the major role. There is linearity between the core radius and the total radius in determining the critical point of the plasmon shift. By this linear relation, we can control the shift direction of the SPR absorption. This work shows a potential bio-sensing method based on detecting the shift direction of the SPR absorption.