The possibility to control Rayleigh scattering in glass single-mode optical fibers opens important perspectives in the field of distributed sensing. Optical backscatter reflectometry (OBR), operating in the time domain, makes use of scattering events occurring in optical fibers as sensing methods. The possibility of forming nanoparticles during the fiber drawing process, which results in an increase of the backscattered power even up to 50 dB over standard single-mode fibers, increments the backscattered power and enables multi-fiber sensing architectures, suitable in medical devices. In this contribution, we turn our attention to the possibility of using the augmented elastic scattering in novel systems for biosensing, characterized by a simple fabrication. We present two methods based on reflector-less sensing, and on distributed interferometry, to achieve biosensing and we discuss the application in the detection of cancer biomarkers. Reflector-less biosensors make use of a fiber taper or etching without the need to fabricate any reflector; so far, sensitivity ratings up to ~1 nm/RIU (refractive index units) have been reported; however, detection limits of sub-picomolar levels are reported, due to the precise tracking of the OBR. Distributed interferometers turn a high-scattering fiber into a series of mirrors, which can be simply terminated by a fiber cleave. Even with a simple fabrication, we show the possibility to achieve sensitivity in the order of 60-100 dB/RIU and detection limit as low as few attomolars.
In this work, the investigation of thrombin concentration using microresonator functionalized with thiol-modified thrombin-binding aptamer (TBA) is reported. Resonator is a ball shaped structure on a tip of an optical fiber. Thrombin is a serine protease that plays an important role during blood coagulation; therefore, it is substantial to detect its levels in blood of the patient. The structure has been manufactured using standard single-mode fibers on CO2 laser splicing system. Further, the resonator of diameter 518 µm has been calibrated with sucrose for refractive index changes with optical backscatter reflectometer. Changes in sensitivity and reflectivity in wavelength shift and amplitude fluctuation were measured. This was followed by coating the resonator with gold and functionalizing surface with TBA. Following aptamer immobilization, the performance of 518 µm resonator was examined in different concentrations of thrombin protein from 4.01 nM to 66.84 nM. We report here a sample presenting a sensitivity of (122.65 nm/RIU, RIU = refractive index units), which allows thrombin detection with an average wavelength shift of 1.034 nm.
A reflector-less refractive index fiber sensor, exploiting the high-scattering of a MgO nanoparticle (NP) fiber, is proposed. The sensor is obtained by etching the fiber to expose the core to the surrounding analyte, and the backscattering signal is read by an Optical Backscatter Reflectometer (OBR). This approach permits distributed sensing, offered by OBR detection, and permits spatial-multiplexing of parallel sensors, thanks to the scattering-level multiplexing (SLMux) of the NP-doped fiber. Simultaneous physical properties detection is also possible. Experiments, performed to detect refractive index and temperature, showed a sensitivity of roughly 0.6 nm/RIU, with a simultaneous measurement of 100°C of temperature.
Fiber optic sensors represent an attractive alternative in chemical, bio-chemical and medical applications. Their success can be retrieved in their peculiar properties such as: electromagnetic interference immunity, fast response, high sensitivity, and small size. In this context, Fiber Bragg Gratings (FBGs) play a key role in applications like measurements of temperature and strain. The mechanism of FBGs is related to the dependence between the characteristic wavelength reflected by the FBG and the effective index of the modes propagating inside the fibers. This property can be exploited to engineer a new and inexpensive class of FBG devices for measuring refractive index of solutions. By reducing the cladding thickness of the single mode fiber, where the FBG is inscribed, the structure becomes three layers and the modes guidance properties become more and more dependent on the external environment, including the refractive index of the solution to measure. In this work, an FBG has been etched by a solution of HF acid and immersed in different solutions of water and sucrose. Results show a strong multi-modal behavior induced by the guidance properties of the three-layer system. The reflected spectra, characterized by a large band whose width, are strictly dependent on the sucrose concentration in solution. As the sucrose increases, the refractive index of the solution increases. The bandwidth reduces, showing a wavelength shift toward longer wavelength. While the wavelength shift is not so relevant the variation of the bandwidth is significant, suggesting an effective interrogation method based on wavelet signal processing.
In this work, partially etched chirped fiber Bragg grating (pECFBG) for the real-time multi-parameter measurement of temperature and refractive index is proposed. The sensor is fabricated by wet-etching a portion of a linearly chirped FBG with linear chirp profile. Obtained CFBG has two active areas: the unetched part of the grating that can be used either as a uniform temperature sensor, or to detect thermal gradients experienced through the grating length; the etched part, besides having a similar thermal sensitivity, is exposed to refractive index sensing through the variations of external refractive index. Overall, the pECFBG structure behaves as a compact sensor with multi-parameter capability, that can both measure temperature and refractive index on the same grating, but also spatially resolve temperature detection through the grating section. The results have been validated through both a model and experimental setup, showing that the mutual correlation algorithm applied to different spectral parts of the grating is able to discriminate between uniform and gradient-shaped temperature profiles, and refractive index changes. The reflected spectra showed a clear correlation between the RI change of the surrounding media and spectral shift with temperature variations.
Tilted Fiber Bragg Grating (TFBG) optical fiber sensors can be employed as refractive index sensors, and functionalized as biosensors. The TFBG cladding modes exhibit a small yet detectable sensitivity to the refractive index in the surrounding of the sensing region. One of the weaknesses of TFBG is that classical interrogation methods require narrow wavelength resolution to analyze these spectral features which can be achieved only through a bulky setup. In this work, we propose a demodulation method, namely the short term Karhunen-Loeve Transform (ST-KLT) that can be applied to a low-cost spectrometer that detects the TFBG reflection spectrum. The method is capable of detecting refractive index variations in the order of 10-3 - 10-4 refractive index units (RIU), and is a potential pillar towards low-cost TFBG biosensors.
The response of etched fiber Bragg grating (EFBG) functionalized with 29-mer DNA aptamer to the different concentrations of Thrombin protein has been investigated. Etched FBGs are an efficient technology for detection of refractive index, and have been demonstrated also for biosensors applications. EFBGs have a simpler manufacturing approach comparing to other methodologies and are based on a low-cost device; their fabrication can be achieved by simple chemical etching, without requiring fusion splicing. During the test we assessed its feasibility for small variations of thrombin concentrations (10μg/ml, 20μg/ml, 40μg/ml and 80μg/ml). In particular, we performed experiments of chemical etching with hydrofluoric acid, which progressively depletes the fiber cladding exposing the core to the outer medium. Additionally, unstriped not etched FBGs were also used as a control for temperature pattern compensation. Before functionalization, EFBG was calibrated with different sucrose and ethanol solutions that validated the sensitivity to refractive index change. EFBG was further silanized with 3-Aminopropyl-triethoxysilane (APTES) in order to immobilize Thrombin binding aptamer on the silica surface of the fiber. The change of Bragg wavelength when functionalized EFBG is exposed to different concentrations of Thrombin using Micron Optics Hyperion si255-x55 sensing system was demonstrated. A small yet detectable sensitivity (several tens of nanomolars) even between small protein variations allows hypothesizing a future use of this kind of functionalized fiber for biosensor development.
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