Optical fibre plasmonic biosensors

Rapid, accurate detection of analytes (proteins, DNA strands, tumor markers, etc.) in low concentrations is crucial in many fields, such as medical diagnostics, environmental monitoring and quality control in the food chain.

Fiber optic sensors provide an elegant solution to this problem, since they provide a sensitive, compact platform in the hundred-micrometer range, and offer the possibility of in-situ measurements with remote instrumentation that can be located several meters or even several hundred meters from the measurement site.

In addition to these highly valued advantages, this type of sensor has all the benefits inherent in the use of fiber optics. Among other things, they are insensitive to electromagnetic interference, resistant to high temperatures and chemical corrosion, and can address several measurement points simultaneously.

However, an optical fiber is not intrinsically sensitive and selective to biochemical or biological compounds. In practice, it must first be made locally sensitive to a change in the refractive index of the external medium. This ability can result from various configurations, such as reducing the thickness of the optical cladding or photo-inscribing a periodic radiative structure into the core of the fiber. This sensitive part is then surface-treated to give it both affinity and selectivity with the biochemical species to be detected. In the case of plasmonic sensors, the surface of the optical fiber is first coated with a nanometric layer of gold, to generate a surface plasmon resonance. Bioreceptors are then grafted onto the gold, which have an affinity for the chemical species to be detected, also known as "ligands", according to an antibody-antigen model. Thus, by adsorption of the ligands, the bioreceptor layer undergoes a modification of its refractive index, which impacts the surface plasmon resonance and in turn affects the light wave propagated within the optical fiber.

The aim of this dossier is to present the physical principle underlying the generation of surface plasmon resonances in optical fibers. It then looks at the main configurations of fiber-optic plasmonic biosensors. Finally, concrete examples are discussed.