Plasmonic optical fiber gratings are usually relying on centimeter-long sections of fibers locally modified with a thin metal film to enhance their sensitivity to the surrounding refractive index through surface plasmonic resonance (SPR). They are considered as a transposition of the Kretschmann prism configuration (cf. commercial SPR devices) and enable the development of lab-on-fiber tools for versatile applications, from biomedical diagnosis to environmental sensing where they can bring unique features. In this paper, we provide an overview of the main achievements in label-free biosensing with gold-coated tilted fiber Bragg gratings (TFBGs), from in vitro bioassays to the ex vivo detection of biomarkers expressed at the surface of cancer tissues. We discuss their performances and draw the lines for the next improvements and their implementation in the real world.
The inscription of a tilted Bragg grating in the core of a standard telecommunication-grade fiber grants the latter a sensitivity to the refractive index (RI) of the fluid in which it is immersed. A gold-coated tilted fiber Bragg grating (Au-TFBG) is a transposition of the Kretschmann prism configuration onto a cylindrical structure. Taking advantage of the surface plasmon resonance (SPR), the RI sensitivity is further increased. In this work, we extract relative phase difference spectra from two orthogonal polarization states using the Jones formalism and use them to experimentally detect the insulin hormone at concentrations ranging from 0.1 ng/mL to 100 ng/mL.
Graphene oxide (GO) thin films fabricated by the vacuum filtration method were deposited on bare tilted fiber Bragg grating for refractometry measurements. Two different layer thicknesses (100 nm and 200nm) were used to prepare the samples. The amplitude spectra of the GO-coated TFBGs (GO-TFBGs) were measured with linearly polarized light for different refractive index values of LiCl solutions (1.3333-1.3342). We show that when polarized light is used, the 200 nm GO-TFBGs achieve similar behavior as plasmonic gold-coated TFBGs (Au-TFBGs). This latter exhibits a characteristic attenuation in the amplitude spectrum when P-polarized light excites a surface plasmon resonance (SPR). This behavior suggests that GO is present as a mix of discontinuous and stratified flakes favorable for plasmon-plasmon hybridization, which can be generated for both P and/or S-polarization of the light.
Gold-coated tilted fiber Bragg gratings (Au-TFBG) are promising platforms leading to the development of highly sensitive biosensors. The spectral resonance of the surface plasmon polariton carried by the gold layer interface depends on the refractive index of the surrounding medium. Once covered with bioreceptors, the p-polarized mode spectrum can be used to detect the change in refractive index induced by the analyte-bioreceptor interactions. In practice, a polarization controller is used to extract its insertion loss spectrum. Usual demodulation techniques track the evolution of the insertion loss spectral attenuation instead of the phase evolution. Indeed, to extract the phase, the polarization controller needs to be removed, and the p-polarized mode must be retrieved by other means. In this paper, a new demodulation technique based on phase evolution with a sensitivity of several thousands of degrees/RIU is presented. Using the complete transfer matrix function (Jones’s matrix), the amplitudes and phases for both p and s polarized modes are mathematically exact for each wavelength considered. Thanks to this procedure, every characteristic like phase and amplitude field evolution can be retrieved. The physical platform sensitivity is measured using LiCl solutions and the fiber is functionalized with anti-HER2 (Human Epidermal Growth Factor Receptor-2,a breast cancer marker) aptamers. Phase demodulation is utilized to extract the refractive index modification induced by all the process. Biosensing experiments on specific antiaptamer/HER2 interactions are also tested for concentrations of HER2 proteins at 1μg/mL.
Gold-coated tilted fiber Bragg gratings (TFBGs) have been extensively studied over the past years, particularly for biosensing purposes. Surface Plasmon Resonance (SPR) is generated through the deposition of a gold layer of appropriate thickness onto the grating region. The combination of SPR and TFBG permits to create a comb-like spectrum of narrow-band cladding mode resonances, which is usually demodulated by tracking the change of optical features of a selected peak. Here, for the first time to the best of our knowledge, a twenty-fold more sensitive demodulation technique based on the intersection of the upper and lower envelopes of gold-coated TFBG spectra is presented. This method has been successfully applied in biosensing for the detection of HER2 (Human Epidermal Growth Factor Receptor-2) proteins, a crucial breast cancer biomarker. Some practical improvements have also been proposed and assessed: first, a uniform FBG has been superimposed on the TFBG to reduce the read-out wavelength span to 10 nm instead of 70 nm, while keeping the temperature-compensated measurements; second, a microfluidic system has been designed and integrated to inject the samples towards the sensor at controlled flow rates. All these novelties make this sensing platform even more attractive and promising for use in practical applications.
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