The simple manner for detecting a chemical/biological interaction is undoubtedly the measurement of a change in refractive index (RI) without resorting to luminescence- and absorption-based measurements. Among all the existing optical methodologies, which are used to measure the surrounding RI (SRI), surface plasmon resonance (SPR) is until now the most popular and widespread. However, during the years, different optical technologies have been developed, such as optical resonators, interferometric configurations, and, very recently, optical fiber gratings (OFGs). In particular, OFGs have been effectively proposed as optical tools in the field of biochemical sensing [1], especially long-period gratings (LPGs), thanks to their high RI sensitivity [2]. This kind of devices takes also the typical advantages of optical fibers, such as compactness, lightweight, multiplexing, and remote measurement capabilities. Broadly speaking, a biochemical interaction along the grating portion induces a change of the RI and thickness of the selective biolayer deposited onto the fiber. Consequently, a change in the transmission spectrum of the fiber (i.e., a change in the spectral position of dips or resonance wavelengths) occurs. Starting from this principle and using the previously published compensated RI sensing system [3], a label-free biosensor based on a copolymer-functionalized optical fiber LPG has been developed and characterized.
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