Nanoporous TiO2/polyion thin-film-coated long-period grating sensors for the direct measurement of low-molecular-weight analytes

Highly sensitive detection of low-concentration sodium chloride solutions based on polymeric nanofilms coated long period fiber grating

Long period fiber gratings (LPFGs) have special advantages in the detection of salt concentrations due to small volume, corrosion resistance and immunity to electromagnetic interference. However, it is very difficult to distinguish low-concentration salt solutions with usual LPFGs owing to the poor sensitivity. In this paper, the detection capability of the LPFG to low-concentration salt solutions was significantly improved by assembling salt-containing poly (diallyldimethylammonium chloride) (PDDA) and salt-containing poly (sodium-p-styrenesulfonate) (PSS). Experimental results showed that, the responsive wavelength range of the LPFG was remarkably broadened in low-concentration salt solutions after assembling nanofilms. The suitable detection range of the PDDA/PSS films coated LPFG for salt concentrations was 0-3%. In such a range, the average refractive index sensitivity and the average salinity sensitivity of the LPFG was as high as 29545.9 nm/RIU and 52.2 nm/% respectively. Compared with the LPFG without nanofilms, the discrimination ability of the PDDA/PSS films coated LPFG to 0-3% salt solutions increased by 568 times. The analysis demonstrated that PDDA and salt in the assembly solutions played a pivotal role in the above effects. The proposed sensor has extensive application prospects in the monitoring of salt concentration in many fields such as seawater, food processing, fermentation process, etc.

High-Sensitivity Optical Fiber-Based Glucose Sensor Using Helical Intermediate-Period Fiber Grating

An all-fiber glucose sensor is proposed and demonstrated based on a helical intermediate-period fiber grating (HIPFG) produced by using a hydrogen/oxygen flame heating method. The HIPFG, with a grating length of 1.7 cm and a period of 35 μm, presents four sets of double dips with low insertion losses and strong coupling strengths in the transmission spectrum. The HIPFG possesses an averaged refractive index (RI) sensitivity of 213.6 nm/RIU nm/RIU in the RI range of 1.33-1.36 and a highest RI sensitivity of 472 nm/RIU at RI of 1.395. In addition, the HIPFG is demonstrated with a low-temperature sensitivity of 3.67 pm/°C, which promises a self-temperature compensation in glucose detection. In the glucose-sensing test, the HIPFG sensor manifests a detection sensitivity of 0.026 nm/(mg/mL) and a limit of detection (LOD) of 1 mg/mL. Moreover, the HIPFG sensor exhibits good stability in 2 h, indicating its capacity for long-time detection. The properties of easy fabrication, high flexibility, insensitivity to temperature, and good stability of the proposed HIPFG endow it with a promising potential for long-term and compact biosensors.

Polarization Maintaining Fiber Temperature and Stress Gradient Sensitization Sensor Based on Semiconductor-Metal-Polymer Three-Layer Film Coating

Increasing sensitivity, measuring points, and stability have always been the pursuit of sensors. ZnSe₉:CO₁ and Ag composite nano films were coated on polarization maintaining fiber (PMF). Then, the coated PMF was nested in capillary and hose which was encapsulated with polydimethylsiloxane (PDMS) and epoxy resin. The integrated capillary sensor and thermoplastic hose sensor were prepared. The gradient sensitization of various measurement parameters such as temperature, stress, and micro bending is realized. The temperature sensitivity is 1.49 nm/°C, the micro bending sensitivity is 1.72 nm/10² g, and the stress sensitivity is 6.27 nm/mε. The sensors maintain good linearity and instantaneous response while having high sensitivity. By adjusting the length of PMF, the number of troughs is increased in the same band range, and different troughs have different sensitivities, which solves the inherent problem of cross sensitivity and realizes multiparameter measurement. Capillary sensors are used for remote safe real-time monitoring of mechanical overheating, and hose sensors are used for real-time monitoring of bridge load and human joint bending. This work is of great significance to the extension of the application range of optical fiber sensor.

Electrochemical Long Period Fiber Grating Sensing for Electroactive Species

We present an electrochemical long period fiber grating (LPFG) sensor for electroactive species with an optically transparent electrode. The sensor was fabricated by coating indium tin oxide onto the surface of LPFG using a polygonal barrel-sputtering method. LPFG was produced by an electric arc-induced technique. The sensing is based on change in the detection of electron density on the electrode surface during potential application and its reduction by electrochemical redox of analytes. Four typical electroactive species of methylene blue, hexaammineruthenium(III), ferrocyanide, and ferrocenedimethanol were used to investigate the sensor performance. The concentrations of analytes were determined by the modulation of the potential as the change in transmittance around the resonance band of LPFG. The sensitivity of the sensor, particularly to methylene blue, was high, and the sensor responded to a wide concentration range of 0.001 mM to 1 mM.

Graphene-Based Long-Period Fiber Grating Surface Plasmon Resonance Sensor for High-Sensitivity Gas Sensing

A graphene-based long-period fiber grating (LPFG) surface plasmon resonance (SPR) sensor is proposed. A monolayer of graphene is coated onto the Ag film surface of the LPFG SPR sensor, which increases the intensity of the evanescent field on the surface of the fiber and thereby enhances the interaction between the SPR wave and molecules. Such features significantly improve the sensitivity of the sensor. The experimental results demonstrate that the sensitivity of the graphene-based LPFG SPR sensor can reach 0.344 nm%⁻¹ for methane, which is improved 2.96 and 1.31 times with respect to the traditional LPFG sensor and Ag-coated LPFG SPR sensor, respectively. Meanwhile, the graphene-based LPFG SPR sensor exhibits excellent response characteristics and repeatability. Such a SPR sensing scheme offers a promising platform to achieve high sensitivity for gas-sensing applications.

Optical fiber LPG biosensor integrated microfluidic chip for ultrasensitive glucose detection

An optical fiber sensor integrated microfluidic chip is presented for ultrasensitive detection of glucose. A long-period grating (LPG) inscribed in a small-diameter single-mode fiber (SDSMF) is employed as an optical refractive-index (RI) sensor. With the layer-by-layer (LbL) self-assembly technique, poly (ethylenimine) (PEI) and poly (acrylic acid) (PAA) multilayer film is deposited on the SDSMF-LPG sensor for both supporting and signal enhancement, and then a glucose oxidase (GOD) layer is immobilized on the outer layer for glucose sensing. A microfluidic chip for glucose detection is fabricated after embedding the SDSMF-LPG biosensor into the microchannel of the chip. Experimental results reveal that the SDSMF-LPG biosensor based on such a hybrid sensing film can ultrasensitively detect glucose concentration as low as 1 nM. After integration into the microfluidic chip, the detection range of the sensor is extended from 2 µM to 10 µM, and the response time is remarkablely shortened from 6 minutes to 70 seconds.

Sol-Gel-Based Titania-Silica Thin Film Overlay for Long Period Fiber Grating-Based Biosensors

An evanescent wave optical fiber biosensor based on titania-silica-coated long period grating (LPG) is presented. The chemical overlay, which increases the refractive index (RI) sensitivity of the sensor, consists of a sol-gel-based titania-silica thin film, deposited along the sensing portion of the fiber by means of the dip-coating technique. Changing both the sol viscosity and the withdrawal speed during the dip-coating made it possible to adjust the thickness of the film overlay, which is a crucial parameter for the sensor performance. After the functionalization of the fiber surface using a methacrylic acid/methacrylate copolymer, an antibody/antigen (IgG/anti-IgG) assay was carried out to assess the performance of sol-gel based titania-silica-coated LPGs as biosensors. The analyte concentration was determined from the wavelength shift at the end of the binding process and from the initial binding rate. This is the first time that a sol-gel based titania-silica-coated LPG is proposed as an effective and feasible label-free biosensor. The specificity of the sensor was validated by performing the same model assay after spiking anti-IgG into human serum. With this structured LPG, detection limits of the order of tens of micrograms per liter (10(-11) M) are attained.

Lab-on-fiber optofluidic platform for in situ monitoring of drug release from therapeutic eluting polyelectrolyte multilayers

A lab-on-fiber (LOF) optofluidic platform that provides physiologically relevant microenvironment was developed by integrating a long period grating (LPG) coupled with high order cladding mode to achieve high index sensitivity and a liquid-tight capillary tube assembly as a microfluidic chamber for LPG to mimic physiologically relevant microenvironment. We demonstrate the utility of LOF for in situ monitoring the construction of the [chitosan (CHI)/poly (acrylic acid) (PAA)/gentamicin sulfate (GS)/PAA]n multilayers at monolayer resolution as well as evaluating the rate of GS release at a flow rate of 0.127 mL/min at 37 °C in real time. We reveal that GS is released at a faster rate under the dynamic flow condition than in a static medium. Our findings underscore the importance of conducting drug release studies in physiologically relevant conditions.

Label-free sensitivity of long-period gratings enhanced by atomic layer deposited TiO(2) nano-overlays

In this paper, we discuss an impact of thin titanium dioxide (TiO(2)) coatings on refractive index (RI) sensitivity and biofunctionalization of long-period gratings (LPGs). The TiO(2) overlays on the LPG surfaces have been obtained using atomic layer deposition (ALD) method. This method allows for a deposition of conformal, thickness-controlled, with well-defined optical properties, and high-RI thin films which are highly desired for optical fiber sensors. It has been found that for LPGs working at a dispersion turning point of higher order cladding modes only tens of nanometers of TiO(2) overlay thickness allow to obtain cladding mode transition effect, and thus significant improvement of RI sensitivity. When the TiO(2) overlay thickness reaches 70 nm, it is possible to obtain RI sensitivity exceeding 6200 nm/RIU in RI range where label-free sensors operate. Moreover, LPGs with TiO(2)-enhanced RI sensitivity have shown improved sensitivity to bacteria endotoxin (E. coli B lipopolysaccharide) detection, when TiO(2) surface is functionalized with endotoxin binding protein (adhesin) of T4 bacteriophage.

A review of refractometric sensors based on long period fibre gratings

In the last decade refractometric sensors have attracted an increasing interest by the scientific community due to their ability to perform ambient monitoring, to assess food quality and safety, and also to the fact that they enable the development of label free sensors in the biomedical area. These advances result, namely, from the use of long period fibre gratings in the turning points and/or with thin films in the transition region that allows resolutions of 10⁻⁶ to changes in the refractive index of the surrounding medium. Resolutions exceeding 10⁻⁸ can also be achieved when long period fibre gratings are combined with evanescent field based devices. This paper reviews the recent path towards the development of ultrahigh sensitive optical fibre refractometric sensors.