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An immunosensor with low detection limit for detecting human-IgG (IgG, immunoglobulin G) is presented in this work. The sensor was designed using single mode fiber with large core-offset fusion splice based on Mach–Zehnder interferometer (MZI). The refractive index (RI) sensitivity is as high as 13936 nm/RIU in the RI range of 1.3328-1.3398, and the resolution of 1.44 × 10 −6 is obtained. Afterwards, the sensing surface was immobilized with staphylococcal protein A and goat anti-human IgG to detect different concentrations of human-IgG, and a low detection limit of 47 ng/ml was observed. The proposed immunosensor has advantages of far simpler fabrication, high sensitivity, low detection limit, reusability and have promising potential for detecting small refractive index change in biological samples and application in biomedical field. [ABSTRACT FROM AUTHOR]

In this paper, a high sensitivity of splicing regions tapered photonic crystal fiber (PCF) Mach–Zehnder interfero- metric refractive index (RI) sensor is described and experimentally demonstrated. Compared with cascaded bi-tapered single-mode fiber (SMF) Mach–Zehnder interferometer (MZI), the splicing regions tapered PCF MZI have higher sensitivity for the reason that it can better control and excite higher order modes, and has lower energy loss than cascaded bi-tapered PCF in the process of light transmission. Experimental results indicate that the RI sensitivity of splicing regions tapered PCF MZI could be up to 240.16 nm/RIU which is almost four times of cascaded bi-tapered SMF in the RI range of 1.3333–1.3792. Meanwhile, this splicing region tapered PCF Mach–Zehnder interferometric RI sensor has the advantages of higher RI sensitivity, good linearity, simple in making, and more potential practical value in the measurement of external RI. [ABSTRACT FROM PUBLISHER]

• Electric field intensity of multilayer of GNPs coupled with gold film was studied. • The RI sensitivity of the double-layer gold nanospheres sensor was 15747 nm/RIU. • The RI sensitivity of the double-layer gold nanorods sensor was 25642 nm/RIU. • The LOD of double-layer gold nanospheres biosensor is 10 ng/mL. • The LOD of double-layer gold nanorods biosensor is 4.6 ng/mL. The main challenge for all biosensors is how to measure the biological samples at very low concentrations. In this paper, a surface plasmon resonance (SPR) fiber biosensor using multi-layer gold nanoparticles (Au NPs)/Au film coupling to enhance sensitivity was proposed. The coupling effect between such longitudinally aligned multilayer nanoparticles increases the intensity of the local electromagnetic field and the depth of propagation to the outside. We used the finite element method to analyze the local enhancement effect of multi-layer Au nanospheres and Au nanorods, and the fabricated double-layer Au NPs (Au nanospheres and Au nanorods)/Au film coupling enhancement sensors. These two sensors' refractive index sensitivities are 15,747 nm/RIU and 25,642 nm/RIU, respectively. By changing biomolecules immobilized on the sensor's surface, the highly sensitive double-layer Au nanoparticle/Au film fiber sensor enables high-precision measurement of different biomarkers at low concentration. For detection of human IgG, the detection limit of gold nanosphere sensor is 10 ng/mL and gold nanorod sensor is 4.6 ng/mL. [ABSTRACT FROM AUTHOR]