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Highlights • We propose the Vernier effect induced by the birefringence in fiber-optic modal interferometers for sensitivity enhancement. • We experimentally demonstrate an ultrahigh RI sensitivity of 35,823.3 nm/RIU at low RI of 1.333 using a fiber coupler with the Vernier effect. • We apply this sensor for human cardiac troponin T detection, and a LOD of 1 ng/mL is achieved. Abstract We report a simple and effective method to improve the sensitivities of fiber-optic modal interferometers with birefringence induced Vernier effect. Taking optical microfiber coupler as an example, we study the sensitivity enhancement of a microfiber coupler for refractive index (RI) sensing both theoretically and experimentally in the RI range of 1.33–1.35 where bioassays are typically carried out. Numerical results show that by tracing the wavelength shifts of dips in the envelope formed by the Vernier effect, RI sensitivities can be improved by almost one order of magnitude compared to the sensors without the Vernier effect. Then we experimentally achieve an ultra-high sensitivity of 35,823.3 nm/RIU using a microfiber coupler with a width of 3.2 μm. More importantly, we apply this ultra-sensitive sensor to detect human cardiac troponin, and a limit of detection of 1 ng/mL is achieved. This sensor is simple in configuration andcan serve as a bio-photonic platform for clinical diagnostics environmental monitoring and food safety. [ABSTRACT FROM AUTHOR]

The recognition of abrin is of great civil and military importance. As an alternative to general detecting techniques, such as enzyme-linked immunosorbent assay (ELISA) and electrochemiluminescence (ECL)-based assay, micro/nano optical fibers (MNFs) for the label-free detection of abrin have been investigated for the first time in this paper. The MNFs fabricated through chemical etching are robust and have excellent mechanical strength for practical applications. This MNF-based biosensors feature fast response, high sensitivity, and low cost. A limit of detection of 10 pg/mL was obtained using a MNF with a waist diameter of circa 1.0 μm, and the typical measurement time was less than 10 min. By taking advantage of the label-free detection, Langmuir kinetics fitting for the dynamic reaction curve suggested a large affinity constant between the antibody and abrin. Regeneration ability of the sensor was also demonstrated by the maintenance of high sensitivity and selectivity after releasing experiments. [ABSTRACT FROM AUTHOR]

Highlights • An eco-friendly one-step synthesis of β-CD-capped AuNPs is demonstrated. β-CD acts as both reducing and stabilizing agent. • The systematic characterization and study on the formation mechanism, size control and quality of the β-CD-capped AuNPs. • Integration of β-CD-capped AuNPs and microfiber to realize a biosensor with high-performance LSPR behavior, flexibility and small footprint. • Ultralow LOD of cholesterol of 5 aM is achieved, providing extremely low consumption of clinical samples. • Selective detection of cholesterol molecules in human serum is demonstrated, taking a step towards the practical applications. Abstract Plasmonics in conjunction with supramolecular chemistry brings new opportunities for biosensing. Modified with macrocyclic supramolecules, noble metal nanoparticles possess improved biocompatibility and highly efficient biomolecule immobilization and recognition via host-guest interaction. In this work, we synthesize high-quality and monodisperse β-cyclodextrin-capped gold nanoparticles (β-CD-capped AuNPs) in one-step facile and eco-friendly process, where β-CD functions as both reducing and capping agent. Integrated with a microfiber, the β-CD-capped AuNPs deliver promising localized surface plasmon resonance properties and ultralow detection limit of cholesterol molecules of 5 aM. The selective recognition of our proposed biosensor to cholesterol is further verified by interference study and analysis of real human serum samples. This study shows the bright prospects of supramolecular chemistry assisted plasmonics in achieving highly integrated and highly sensitive biosensing with extremely low consumption of clinical samples and small footprint. [ABSTRACT FROM AUTHOR]