Optical fiber resonance-based pH sensors using gold nanoparticles into polymeric layer-by-layer coatings

The development of new nanocoatings onto optical fiber core is a hot topic within the optical fiber devices. The possibility of fabricating hybrid nanocoatings based on inorganic (gold nanoparticles, AuNPs) and organic materials (polymeric structure) can be performed using the layer-by-layer embedding deposition technique. The deposition of a nanostructure coating onto an optical fiber core has been performed in order to obtain optical fiber resonance-based pH sensors. The incorporation of gold nanoparticles (AuNPs) into polymeric thin films has been confirmed by atomic force microscopy, scanning electron microscopy and UV–Vis spectroscopy. In addition, two electromagnetic resonances known as localized surface plasmon resonance (LSPR) or lossy mode resonance (LMR), can be generated as a function of the resultant thickness coating. In this work, the fabrication of a dual LSPR-LMR optical fiber pH sensor is presented where the LSPR is used as a reference signal and the LMR is used as a sensing band due to the great difference in their corresponding sensitivities to pH changes of the surrounding medium. It has been demonstrated that LMR improves the sensitivity of the LSPR band in more than one hundred times. The device shows a high sensitivity, fast response time and large dynamical range of 134.7 nm from pH 4.0 to pH 6.0.