Evanescent optical waves for label-free monitoring of live cell status and behavior

Traditional evanescent wave optical biosensors generate an electromagnetic wave at the sensor surface that penetrates 100–200 nm into the analyzed aqueous medium [1]. This has proven to be a highly sensitive tool to monitor refractive index changes in the close vicinity of the sensor probe and well suited to monitor thin films of biomolecules (proteins, DNA etc.). However, in several recent applications a larger penetration depth is needed or the probing volume must be precisely controlled to monitor refractive index variation in living cells cultured on the sensor surface. Embedding low refractive index materials (such as nanoporous silica with refractive index 1.2) or nanometer scale metal films (with negative real part of their complex dielectric constants) in the sensor structures the penetration depth can be significantly extended and fine tuned (Fig. 2a) [1–6]. Using several modes even the optical sectioning of the cell sample is feasible (Fig. 2bc) [6].