Immunosensing Based on Optical Fiber Technology: Recent Advances

Abstract

The evolution of optical fiber technology has revolutionized a variety of fields, from optical transmission to environmental monitoring and biomedicine, given their unique properties and versatility. For biosensing purposes, the light guided in the fiber core is exposed to the surrounding media where the analytes of interest are detected by different techniques, according to the optical fiber configuration and biofunctionalization strategy employed. These configurations differ in manufacturing complexity, cost and overall performance. The biofunctionalization strategies can be carried out directly on bare fibers or on coated fibers. The former relies on interactions between the evanescent wave (EW) of the fiber and the analyte of interest, whereas the latter can comprise plasmonic methods such as surface plasmon resonance (SPR) and localized SPR (LSPR), both originating from the interaction between light and metal surface electrons. This review presents the basics of optical fiber immunosensors for a broad audience as well as the more recent research trends on the topic. Several optical fiber configurations used for biosensing applications are highlighted, namely uncladded, U-shape, D-shape, tapered, end-face reflected, fiber gratings and special optical fibers, alongside practical application examples. Furthermore, EW, SPR, LSPR and biofunctionalization strategies, as well as the most recent advances and applications of immunosensors, are also covered. Finally, the main challenges and an outlook over the future direction of the field is presented.

Relevance of the Spectral Analysis Method of Tilted Fiber Bragg Grating-Based Biosensors: A Case-Study for Heart Failure Monitoring

“…The performance of the immunosensors and of the demodulation methods were assessed and compared based on the LOD obtained for each case. This figure of merit was determined through the following expression [ 2 ]: in which σ is the standard deviation of the y-intercept and S corresponding to the sensitivity, i.e., the slope of the linear regression obtained for the wavelength or amplitude variations as a function of NT-proBNP concentration, in the linear region of the response.…”

“…The performance of the immunosensors and of the demodulation methods were assessed and compared based on the LOD obtained for each case. This figure of merit was determined through the following expression [2]:…”

“…The overlapped spectra of the bare TFBG, each one acquired with the fiber immersed in PBS upon incubation in each concentration of NT-proBNP, are displayed in Figure 5, The performance of the immunosensors and of the demodulation methods were assessed and compared based on the LOD obtained for each case. This figure of merit was determined through the following expression [2]:…”

“…Biosensors consist of analytical devices capable of recognizing a specific analyte and converting an event into a measurable signal using a proper transducer. Such devices have been widely explored in a broad number of fields such as medical diagnosis, the food industry, drug delivery and environmental monitoring [ 1 , 2 ]. The biorecognition molecules employed depend on the intended application and are generally enzymes, antibodies/antigens or nucleic acids, being responsible for the biochemical interactions with the target that lead to physicochemical changes detectable by the transducer [ 3 ].…”

“…This can be achieved through different sensing structures such as unclad [ 8 ], D-shaped [ 9 ], tapered [ 10 ], end-face reflected [ 11 ] fibers and fiber gratings (namely, etched fiber Bragg gratings) [ 12 ], long period gratings [ 13 ] and tilted fiber Bragg gratings (TFBGs) [ 14 ]. TFBGs have proved to be a great biosensing tool, garnering high refractometric sensitivity and a low limit of detection (LOD), while providing benefits such as easy light injection, self-compensation of temperature oscillations and not requiring cladding removal, hence maintaining its mechanical strength [ 2 , 5 , 15 ]. The sensitivity can be enhanced by combining the TFBG with the surface plasmon resonance (SPR) effect, which is achieved through coating the fiber surface with a thin metallic layer.…”