A novel approach to detect type G immunoglobulins (IgGs) has been developed, based on the combination of two ways of improving sensitivity applied to a simple optical structure. The first one is the fact of tapering the optical fiber, which permits to access the evanescent field of the light propagating through the waveguide and increases the detection surface. The second one is the use of a novel technology known as lossy mode resonances (LMRs), which have proved to present higher versatility in some characteristics than surface plasmon resonances (SPRs). Due to this, although SPRs are normally used to detect biological reactions, LMRs can also address this topic.
To this purpose, a 30 mm uncladded segment of a 200/225 m core/cladding diameter optical fiber (FT200EMT, Thorlabs Inc.) was tapered by a system designed by Nadetech Innovations S.L. until a waist diameter of 100 m and a waist length of 10 mm were reached. This tapered uncladded multimode fiber (T-UMF) was subjected to a sputtering process (Quorum Technologies Inc.), in order to deposit a silver mirror on its tip, so that a simple reflective set-up could be prepared. The materials used for the thin-film fabrication were poly(allylamine hydrochloride) (PAH) and the polyanions poly(acrylic acid) (PAA) and poly(styrene sulfonate) (PSS), all from Sigma-Aldrich. These three substances were chosen since they have reported to generate LMRs by using the layer-by-layer electrostatic selfassembly technique (LbL-ESA), which is described elsewhere. Apart from that, both anti-IgGs and IgGs, extracted from goat serum, were also obtained from Sigma-Aldrich.
Keywords: Lossy mode resonance; Optical fiber; Sensor