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.

In this work, we report fiber Bragg gratings (FBGs) embedded in silicone tubes for joint movement monitoring. We integrate the sensors on finger guard bands and worn on a mechanical finger setup for finger joint bending monitoring. We demonstrate the shift of the central wavelength of the FBG of about 0.45 nm for 80° bending, with a sensitivity of 5.6±0.5pm/°. By integrating the sensor on the finger, arm, and knee with guard bands in sinuous-shape, we monitor the bending of the joint, with the ability to distinguish the joint motion. The proposed sensor has a potential for application in Stroke rehabilitation.

In this study, a localized surface plasmon resonance (LSPR)-based S-flex fiber optic (SFFO) sensor is designed and developed for the qualitative as well as quantitative measurement of cortisol. The SFFO configuration allows the generation of highly effective evanescent fields, which are used to excite LSPR phenomena of precious metal nanoparticles on the surface of the probe. In this experiment, gold nanoparticles (AuNPs) are utilized as precious metal nanoparticles owing to their favorable attributes of facile manufacturing and enduring physical and chemical properties. In order to enhance the sensing performance of the sensing probe, MoS ₂ -NPs are utilized to modify the proposed sensor. The optimization and enhancement of sensor performance by MoS ₂ -NPs are verified through comparative experiments. Different levels of cortisol solutions are detected using (i) probe 1 (AuNPs) and (ii) probe 2 (AuNPs/MoS ₂ -NPs) based on different nanoparticle modifications. The logarithmic sensitivity and detection limits of the sensing probes are probe 1 (1.34 nm/log(ng/mL) and 147.9 pg/mL) and probe 2 (3.07 nm/log(ng/mL) and 148.5 pg/mL), respectively. The experimental results reveal the significant contribution of MoS ₂ -NPs to the performance of the sensor. Furthermore, performance metrics such as reproducibility, reusability, selectivity, and stability of probe 2 are also investigated and found satisfactory. The sensors presented in this study exhibit significant potential for utilization within various application scenarios, such as, aerospace, clinical medicine and aquaculture.

Optical fiber technology has rapidly progressed over the years, providing valuable benefits for biosensing purposes such as sensor miniaturization and the possibility for remote and real-time monitoring. In particular, tilted fiber Bragg gratings (TFBGs) are extremely sensitive to refractive index variations taking place on their surface. The present work comprises a case-study on the impact of different methods of analysis applied to decode spectral variations of bare and plasmonic TFBGs during the detection of N-terminal B-type natriuretic peptide (NT-proBNP), a heart failure biomarker, namely by following the most sensitive mode, peaks of the spectral envelopes, and the envelopes’ crossing point and area. Tracking the lower envelope resulted in the lowest limits of detection (LOD) for bare and plasmonic TFBGs, namely, 0.75 ng/mL and 0.19 ng/mL, respectively. This work demonstrates the importance of the analysis method on the outcome results, which is crucial to attain the most reliable and sensitive method with lower LOD sensors. Furthermore, it makes the scientific community aware to take careful attention when comparing the performance of different biosensors in which different analysis methods were used.

The fast and specific detection of the commensal and pathogenic bacteria Escherichia coli is an essential challenge in the aquaculture industry. Herein, we developed and validated a liquid crystal-based optical biosensor that exhibited the capacity to detect whole-cell E. coli with a detection limit of 2.8 CFUs/mL. The inner surfaces of the device were treated so that the molecules of nematic liquid crystal 4-Cyano-4’-pentylbiphenyl were perfectly aligned along a direction normal to the confinement surfaces. Anti-E. coli specific antibodies were immobilized on the inner surfaces of the device, to function as biorecognition molecules. The size of the bacterial cells is over one thousand times higher the size of the liquid crystal molecules so, when in contact, they efficiently distort the mean orientation of the liquid crystal molecules, which is the principle behind the detection method. Our whole-cell biosensor presents several advantages; it is a low-cost device, of simple optical reading and interpretation, that relies in portable technology. This immunosensor presents a faster detection time and a lower LOD than most LC-based sensors described to detect bacteria. This study is an important contribution for a fast diagnose which allows to address a specific threat, with immediate application to the aquaculture field, an increasingly important industry.

We report a label‐free impedimetric immunosensor electrode for prostate‐specific antigen (PSA) detection, using laser‐induced graphene (LIG) coated with ZnO nanorods via pulsed electrodeposition. Two immobilization methods for anti‐PSA antibodies were evaluated: covalent bonding and direct adsorption. Only adsorption enabled clinically relevant detection within 0.1–10 ng/mL in PBS buffer, achieving a detection limit of 81 pg/mL and strong specificity against non‐target glycoproteins. Hence, this work demonstrates the potential of LIG/ZnO composites for low‐cost, scalable sensors suitable for decentralized diagnostics, particularly in low‐resource settings. Unlike conventional impedimetric immunosensors, the LIG/ZnO/adsorbed anti‐PSA sensor exhibited a “signaling‐off” behavior, with charge transfer resistance decreasing upon PSA binding—attributed to antibody conformational changes or desorption (as suggested by XPS measurements), possibly coupled to ZnO surface charge effects. This counterintuitive behavior challenges conventional models of impedimetric immunosensing, highlighting the need to reconsider design assumptions and to account for the unique properties of the transducing material and its biofunctionalization.