Anisotropic Fluorescence Emission and Photobleaching at the Surface of One-Dimensional Photonic Crystals Sustaining Bloch Surface Waves. I. Theory

Photonic crystal (PC) enhanced fluorescence has been proposed as a novel tool for early disease detection in a liquid biopsy format. However, photobleaching of the emitters has never been deeply investigated, although its cross section is expected to increase due to the large field intensity enhancement. Herein, we report on a comprehensive theoretical description of the stationary fluorescence emission of molecular emitters bound to the surface of a one-dimensional photonic crystal (1DPC) biosensor. The model considers coupling of the emission to the large local density of the states provided by the 1DPC, in particular to the Bloch surface waves, which can be characterized by different polarization states. The rotational diffusion equation in the presence of photobleaching was solved analytically by a Laplace spherical harmonics analytical approach. The results show that photobleaching can severely affect the fluorescence emission in terms of total intensity and polarization composition, suggesting that a careful analysis of fluorescence anisotropy in biosensing experiments with PC should be carried out. We applied the model to some case of study conditions that take place in the experiments and propose a procedure to rule out the contribution of photobleaching. Last but not least, we propose fluorescence recovery after orientational photobleaching as a new tool to study the rotational diffusion of emitters, or labelled proteins, bound at a surface.