Formaldehyde sensing with plasmonic near-infrared optical fiber grating sensors

摘要

A tilted fiber Bragg grating is photo-inscribed in the core of a single-mode optical fiber, leading to the coupling of cladding mode resonances all along a wide region of the near-infrared spectrum. The grating is then coated with a thin film of gold in order to create a metal-dielectric interface. This way, light propagating through the cladding of the optical fiber is able to excite a surface plasmon wave on the outer interface. As sensitive element, a molecularly imprinted polymer is deposited by electropolymerization as a thin film around the previous gold coating. The thickness of the polymer is controlled by means of the surface plasmon resonance signature in order to preserve a correct surrounding refractive index sensitivity when used in a gaseous environment. The chosen polymer has an affinity to formaldehyde, which is a volatile organic compound worth to detect, especially because of its toxicity for the human being. We report a global wavelength shift of the grating cladding mode resonances in the presence of formaldehyde in gaseous state. This shift is due to a change in the refractive index of the polymer when it bounds to the target molecules. The sensor exhibits a linear response, together with a low limit of detection.