Functionalized gold electroless-plated optical fiber gratings for reliable surface biosensing

In the context of plasmonic sensing using optical fibers, the controlled deposition of thin metal films remains a challenge, especially because of their cylindrical shape. In this work, we study the gold electroless plating (ELP), which is a direct and cost-effective technique to improve the quality and reproducibility of the metal deposition. In this paper, we use optical fibers to monitor the deposition process in real time. Our analysis is conducted on gratings photo-imprinted in the fiber core. Using polarized light, we show how the comb-like amplitude spectra of tilted fiber Bragg gratings (TFBGs) evolve depending on the ELP duration. This spectral evolution linked to a thorough study of the metal surface allows us to define the optimized process for improved surface refractometry. We experimentally show that this happens for a particular arrangement of gold nanoparticles yielding spectral features comparable to those obtained for localized surface plasmon resonance (LSPR) excitation. Biosensing experiments were performed and a LOD of 1 ng/mL (14 pM) is reported. This comes along with an enhanced specificity, contrasting with a poor sensitivity to bulk refractive index. All this is paving the way to a fast online coating procedure for nano-scaled and label-free biosensing using optical fibers