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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]

In situ and highly sensitive monitoring of tumor cells provides a fundamental understanding of the valuation of cancer diagnosis. However, existing techniques are largely unable to rapidly capture and precisely quantify the tumor cells. To address this, we propose a novel approach based on a nanotube-functionalized & microfluidic-controlling multiresonance optical fiber sensor. The sensor consists of a tilted fiber Bragg grating (TFBG) inscribed in the fiber core and specially designed halloysite nanotubes (HNTs) coated over the fiber surface. In particular, the HNTs are arranged in an orderly manner along the fiber surface to form slit-like patterned layers for enhanced the interaction with tumor cells, resulting in more effective capture of tumor cells. Such a sensor provides a powerful light scattering sensing ability. Based on the spectral area interrogation method, normal cells and tumor cells can be unambiguously discriminated within a few minutes, providing high sensitivity (limit of detection of 10 cells/mL), and a linear response for a large cell concentration range (10 ∼ 105 cells/mL). Meanwhile, by integrating the fiber sensor with a well-designed microfluidic chip, rapid and precise measurement of different tumor cell samples with low consumption (sub-microliter volumes) can be achieved, which provides a potential tool for cancer diagnosis and treatment. • A novel fiber sensor for tumor cells at very low concentrations is proposed. • A new demodulation method based on the light scattering effect is proposed. • The sensor could rapidly capture of tumor cells through HNTs. • The sensor is promising for the in situ initial screening of tumor cells. [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]

A side-channel photonic crystal fiber (SC-PCF) is proposed and fabricated to serve as a high sensitivity surface enhanced Raman scattering (SERS) sensing platform. The SC-PCF has a solid core with triangular lattice cladding structure, and one third of the cladding is removed intentionally to enable fast liquid infiltration and enlarge effective interaction area between liquid samples and the fiber core guided lightwave. The geometry of the SC-PCF is designed to obtain high evanescent field power in cladding side-channel, high free space coupling efficiency and low transmission loss at the excitation wavelength of 632.8 nm. Rhodamine 6G (R6G) solutions mixed with gold nanoparticles (AuNPs) are used to test the SERS performance of the proposed platform. A low detection limit of 50 fM R6G solution is achieved. Moreover, an accumulative effect of SERS signal along the SERS-active fiber length is demonstrated experimentally, and a numerical–analytical model for backward scattering scheme is developed to investigate the dependence of Raman intensity on fiber length. [ABSTRACT FROM AUTHOR]