Stack of Nano-Films on Optical Fiber End Face for Label-Free Bio-Recognition

This paper presents a study on sensing capabilities of stacks of nano-films deposited on a single-mode optical fiber's end face for application in label-free bio-recognition. The stack consist of five nano-films of periodically interchanging materials characterized by different refractive indices (n at λ = 1550 nm), aluminum Al
_x
O
_y
(n ~ 1.58 RIU) and titanium TiO
_x
(n ~ 2.42 RIU) oxides layers. The stack was deposited with the reactive magnetron sputtering technique. It has been found that the structures show sensitivity to refractive index of surrounding liquid (74.4 nm/RIU and 44.3 dBm/RIU) and negligible sensitivity to variations of temperature. Moreover, we show that these structures when functionalized can be used for selective detection of biological layer formation on their surface. In this study, a biotin-avidin complex was used to show a label-free bio-recognition capability of the approach. Avidin concentration of 1 mg/ml induced shift of the resonance wavelength and change in reflected power by 3 nm and 0.8 dBm, respectively, when BSA, which is nonspecific to biotin, resulted in a negligible change in spectral response. The approach seems to be very interesting, especially when such advantages as batch device fabrication, sensor robustness, and fully automatized stack deposition process are taken into account.