Optical fibre sensors based on long period gratings (LPG) modified with functional coatings that can change their optical properties in response to the presence of the particular analyte have attracted much research attention. Ultimately such sensors have the capability to become highly sensitive and selective bio- and chemical sensors. In addition to the advantages of all optical fibre sensors such as miniature size, immunity to electromagnetic interference and low power consumption, LPGs offer wavelengths encoded information which eliminates the need for a reference and are mechanically robust which makes them attractive for practical application. One of the key components in LPG bio- and chemical optical fibre sensors is a capture layer that provides chemical selectivity to the target analyte. In this work, an overview of the different LPG bio- and chemical optical fibre sensors functionalised with various capture layers such as mesoporous thin films, molecularly imprinted polymers (MIPs) and metal-organic frameworks (MOFs) will be presented. Particular focus will be given to potential applications of the novel optical fibre LPG sensors in healthcare with examples of measuring biomarkers (protein M-immunoglobulin) and drug delivery (anaesthetics) in intensive care.
A tip-based fiber-optic localized surface plasmon resonance (LSPR) sensor is reported for sensing of acetone. It is designed by coating the tip of multi-mode optical fiber with gold nanoparticles (size: ~ 40 nm) via a chemisorption process and further functionalization with a metal-organic framework (MOF) HKUST-1 via a layer-by-layer process. Two sensors with a different number of layers (80 and 120) corresponding to different thicknesses are reported. Both sensors show a redshift of resonance wavelength to acetone as a result of an increase in local refractive index induced by acetone adsorption into the HKUST-1 thin film. Sensors gradually saturate as acetone concentration increases and are fully reversible when the concentration decreases. The sensor with a thicker film exhibits slightly higher sensitivity to acetone than the thinner film with a wavelength shift of 5.27 nm for the concentration of 3.4 %.
An optical fibre temperature sensor modified with coating a thermochromic liquid crystal (TLC) film on the tip of fibre is reported. The interrogation is conducted in the wavelength domain using a CCD spectrometer and halogen light source. The TLC sensor shows a reversible wavelength shift of the reflective light peak in the detected temperature range (28 – 46°C). The response of the sensor to temperature was linear with a sensitivity of -4.52 nm/°C for temperature increasing and -4.60 nm/°C for decreasing. There is no measurable hysteresis in the temperature calibration experiment for TLC sensor.
Optical fibre sensing is a platform technology for applications in biomedical and environmental monitoring. Fibre Bragg Gratings can be used to monitor parameters such as pressure and temperature. Alternatively, coating the fibres with functional layers, either at the tip of the fibre, on a U-shaped fibre, or a long period grating enables sensing of analytes in liquids and gases. This paper describes the application of optical fibre sensing techniques during mechanical ventilation via an endotracheal tube in critical care. Functional coatings on the fibres are used to monitor humidity of inspired air and can be used to monitor other analytes.
Contact between a medical device and tissue accounts for approximately a third of damage to soft tissue in hospitals. The use of sensors at the interface between the device and tissue can be used to maintain the optimum pressure and reduce such injuries. A sensorised endotracheal tube has been developed and is proposed as a method of reducing soft tissue damage and improving design of future devices. Optical fibre sensing is used to monitor both cuff contact pressure (via a fibre Bragg grating) and tissue perfusion (via reflectance photoplethysmography). The monitoring system developed has been used in an animal study and demonstrates reliable measurement of contact pressure and perfusion.
An array of three long period gratings (LPGs) fabricated in a single optical fibre and multiplexed in the wavelength domain was used to measure simultaneously temperature, relative humidity (RH) and volatile organic compounds (VOCs). Each LPG sensor was designed to optimize its response to a desired measurand. The LPGs were fabricated with periods such that they operated at or near the phase matching turning point. The sensors were calibrated in the laboratory and the simultaneous measurement of the key indoor air quality parameters was undertaken in laboratory and office environments. It was demonstrated successfully that the data produced by the LPG sensor array under real conditions was in a good agreement with that produced by commercially available sensors. Further, the potential application of fibre optic sensors for VOCs detection at high levels has been demonstrated.
An optical fibre long period grating (LPG) modified with a thin film of HKUST-1, a material from metal organic framework (MOF) family, was employed for the detection of carbon dioxide. The sensing mechanism is based on the measurement of the change of the refractive index (RI) of the coating that is induced by the penetration of CO2 molecules into the HKUST-1 pores. The responses of the resonance bands in the transmission spectrum of an LPG modified with 40 layers of HKUST-1 upon exposure to carbon dioxide in mixture with nitrogen were investigated.
Optical fibres functionalised with chemically sensitive layers offer a powerful platform for the development of sensing systems with a wide range of potential applications, ranging from the monitoring of industrial processes to healthcare. Sensors based upon optical fibres to probe the optical characteristics of nanomaterials that exhibit changes in their optical properties upon exposure to targeted chemical species are particularly attractive, due to their potential high sensitivity, selectivity, the ready ability to multiplex arrays of sensors, and the prospect for remote sensing. This paper provides examples of the application of optical fibres sensors to improve the functionality of the medical devices, for biomarker detection and drug monitoring, and draws upon work that has been conducted as collaboration between teams at the Universities of Kitakyushu, Cranfield and Nottingham.
The development of an ammonia sensor, formed by the deposition of a functionalised titanium dioxide film onto a tapered optical fibre is presented. The titanium dioxide coating allows the coupling of light from the fundamental core mode to a lossy mode supported by the coating, thus creating lossy mode resonance (LMR) in the transmission spectrum. The porphyrin compound that was used to functionalise the coating was removed from the titanium dioxide coating upon exposure to ammonia, causing a change in the refractive index of the coating and a concomitant shift in the central wavelength of the lossy mode resonance. Concentrations of ammonia as small as 1ppm was detected with a response time of less than 1min.
The monitoring of an array of fibre Bragg gratings (FBGs) strain sensors was performed through a single channel, single mode fibre optic rotary joint (FORJ) mounted on a geotechnical centrifuge. The array of three FBGs was attached to an aluminum plate that was anchored at the ends and placed on the model platform of the centrifuge. Acceleration forces of up to 50g were applied and the reflection signal of the monitored FBGs recorded dynamically using a 2.5kHz FBG interrogator placed outside the centrifuge. The use of a FORJ allowed the monitoring of the FBGs without submitting the FBG interrogator to the high g-forces experienced in the centrifuge.
An optical fibre sensor for simultaneous temperature and humidity measurements consisting of one fibre Bragg grating (FBG) to measure temperature and a mesoporous film of bilayers of Poly(allylamine hydrochloride)(PAH) and silica (SiO2) nanoparticles deposited onto the tip of the same fibre to measure humidity is reported. The hygroscopic film was created using the layer-by-layer (LbL) method and the optical reflection spectra were measured up to a maximum of 23 bilayers. The temperature sensitivity of the FBG was 10 pm/°C while the sensitivity to humidity was (-1.4x10-12 W / %RH) using 23 bilayers. The developed sensor was tested in the mechanical ventilator and temperature and humidity of the delivered artificial air was simultaneously measured. Once calibrated, the optical fibre sensor has the potential to control the absolute humidity as an essential part of critical respiratory care.
Optical fibre sensors have the potential to be incorporated into wound dressings to monitor moisture and predict healing
without the need to remove the dressing. A low cost polymeric optical fibre humidity sensor based on evanescent wave
absorption is demonstrated for skin humidity measurement. The sensor is fabricated by coating the fibre with a hydrophilic
film based on bilayers of Poly(allylamine hydrochloride) (PAH) and SiO2 mesoporous nanoparticles. The Layer-by-Layer
method was used for the deposition of the layers. Multimode polymeric optical fibre with a cladding diameter of 250μm
was covered by 7 layers of PAH/SiO2 film on the central region of an unclad fibre with a diameter of 190μm. The length
of the sensitive region is 30mm. Experiment results show a decrease in light intensity when relative humidity increases
due to refractive index changes of the fibre coating. The sensitivity obtained was 200mV/%RH and the sensor was
demonstrated to provide a faster response to changes in the humidity of the skin microenvironment than a commercial
sensor.
A method for measuring the contact pressure between an endotracheal tube cuff and the trachea was designed and
developed by using a fibre Bragg grating (FBG) based optical fibre sensor. The FBG sensor is encased in an epoxy based
UV-cured cuboid patch and transduces the transversely loaded pressure into an axial strain that induces wavelength shift
of the Bragg reflection. The polymer patch was created by using a PTFE based mould and increases tensile strength and
sensitivity of the bare fibre FBG to pressure to 2.10×10-2 nm/kPa. The characteristics of the FBG patch allow for
continuous measurement of contact pressure. The measurement of contact pressure was demonstrated by the use of a 3D
printed model of a human trachea. The influence of temperature on the measurements is reduced significantly by the use
of a second FBG sensor patch that is not in contact with the trachea. Intracuff pressure measurements performed using a
commercial manometer agreed well with the FBG contact pressure measurements.
A fast method for the fabrication of the long period gratings (LPG) optical fibres operating at or near the phase matching turning point (PMTP) with the period of 109.0, 109.5 and 110.0 μm based on an amplitude mask writing system is described. The proposed system allows fabricating 3 cm long LPG sensors operating at PMPT within 20 min that is approximately 8 times faster than point-by-point approach. The reproducibility of the fabrication process was thoroughly studied. The response of the fabricated LPGs to the external change of the refractive index was investigated using water and methanol.
An optical fibre long period grating (LPG) modified with a mesoporous film infused with a calixarene as a functional compound was employed for the detection of a mixture of volatile organic compounds (VOCs). The sensing mechanism is based on the transduction of the refractive index change induced by the complexion of the VOCs with calixarene into a change in the form of the transmission spectrum of the LPG. An LPG, modified with a calixarene-infused coating comprising 5 cycles of silica nanoparticles/poly(allylamine hydrochloride) polycation (SiO2/PAH), was exposed to mixture of VOCs emitted from paint at conditions simulating ISO standards test (16000-10).
An optical fibre long period grating (LPG) modified with a thin film of ZIF-8, a zeolitic immidazol framework (ZIF) material, a subgroup of the metal organic framework (MOF) family, was employed for the detection of organic vapours. ZIF-8 film was deposited onto the surface of the LPG using an in-situ crystallization technique. The sensing mechanism is based on the measurement of the refractive index (RI) change induced by the penetration of the chemical molecules into the ZIF-8 pores. An LPG modified with 5 growth cycles of ZIF-8 responded to exposure to methanol and ethanol vapours.
An optical fibre long period grating (LPG) coated with an inorganic molecularly imprinted thin film based on TiO2 was used to measure in real time the photodecomposition of an organic target compound. The TiO2 film was deposited onto the LPG using the aqueous liquid phase deposition method and its photo-catalytic properties were employed to decompose a porphyrin. The principle of operation is based on the measurements of the refractive index (RI) change of the TiO2 thin film at the binding and removal of the imprinted organic compound.
An optical fibre Bragg grating (FBG) was used to measure local strain (due to contact pressure) at the interface of a cuffed endotracheal tube (ETT) tested in a tracheal model. The tracheal model consisted of a corrugated tube. Two FBG sensors written in a single optical fibre were attached to the outside wall of the cuff of the ETT. Intracuff endotracheal pressure was measured using a digital manometer, while the contact pressure between the model trachea and the ETT was measured using Flexiforce sensors. Changes in the Bragg wavelengths in response to the inflation of the cuff of the ETT, and concomitant pressure increase, were observed to be dependent on the location of the FBGs at the corrugations, i.e., the annular peaks and troughs of the corrugated tube. The performance of both contact pressure sensors FBG and Flexiforce suggests that FBG technology is better suited to this application as it allows the measurement of contact pressures on non-uniform surfaces such as in the tracheal model.
A Long period grating (LPG) with a period of 111 μm was fabricated in the highly birefringent (Hi-Bi) optical fibre with the aim of developing a sensor for chirality measurements. The LPG sensor was exposed to different concentrations of glucose D(+) and fructose D(-) in water, which have similar structures but exhibit opposite optical rotations, i.e. chirality. The behaviour of the resonance bands of the submodes corresponding to the two orthogonal polarization states was different depending on the chirality of the compound, thus allowing discrimination between two compounds.
A layer-by-layer (LbL) approach was used for the deposition of coatings with a nano-meter thickness onto a multimode optical fibre that was bent into a U-shape with the aim of demonstrating a fibre-optic ammonia sensor. The film was composed of alternate layers of tetrakis(4-sulfophenyl)porphine, TSPP, and poly(diallyldimethylammonium chloride), PDDA, deposited using the LbL electrostatic self-assembly process. Exposure of the assembled film to ammonia induced a change in its absorption spectrum, which could be observed in the transmission spectrum of the coated U-shaped optical fibre. The sensor showed a linear sensitivity (77.7 mV/ppm) to ammonia in the concentration range 1–100 ppm.
An evanescent-wave optical fibre sensor modified with tetrakis(4-sulfophenyl)porphine (TSPP) and poly(allylamine hydrochloride) (PAH) bilayers using an layer-by-layer (LbL) approach was tested to measure the gas emitted from human skin. Optical intensity changes at different wavelengths in the transmission spectrum of the porphyrin-based film were induced by the human skin gas and measured as sensor response. Influence of relative humidity, which can be a major interference to sensor response, was significantly different when compared to the influence of skin emanations. Responses of the current optical sensor system could be considered as composite sensor array, where different optical wavelengths act as channels that have selective response to specific volatile compounds. Data obtained from the sensor system was analyzed through principal component analysis (PCA). This approach enabled to distinguish skin odors of different people and their altered physiological conditions after alcohol consumption.
The use of bacteriorhodopsin (Br) coatings to create photosensitive optical fibre long period gratings (LPGs) is described. The response of the coated LPGs both sustained and pulsed illumination at a wavelength of 532 nm is monitored. The results show a clear response to the illumination and full recovery of the optical properties of the coating. This technique could allow the use of LPG for typifying photosensitive compounds and to develop optically controlled chemical sensors.
Two highly sensitive ammonia sensors, formed by depositing coatings composed of titanium dioxide (TiO2) onto the cladding of an optical fibre sensing platform, are evaluated. A long period grating (LPG) of period 111 μm was fabricated in the core of an optical fibre so that the LPG operates at or near the phase matching turning point (PMTP). The first coating that was investigated was composed of TiO2 nanoparticles deposited by liquid phase deposition. The sensor showed high sensitivity and allowed low concentrations of ammonia in water (0.01 ppm) to be detected with a response time of less than 60 sec. The second coating was composed of TiO2 with subsequent layers of poly (allyamine hydrochloride) (PAH), and SiO2 nanospheres infused with a sensitive element composed of porphine. The ammonia adsorption to the porphine compound led to the changes in the LPG’s transmission spectrum and allowed 0.1 ppm of ammonia in water to be detected with a response time of less than 60 sec.
The response to temperature and refractive index (RI) of cascaded long period gratings operating at or near the phase matching turning point was studied. Temperature- and RI-induced changes of the phase of the channelled spectrum and wavelength shift of the long period grating’s resonance band envelope were observed for two conditions; when the length of fibre separating the long period gratings was perturbed and when the whole length of the device was perturbed. It is shown that when the central section of the device is perturbed the phase of the channelled spectrum changes and the envelope remains fixed, while when the entire device is perturbed the phase and central wavelength of the envelope shift at the same rate. Thus measurement of changes in the channelled spectrum with respect to the centre of the resonance band envelope offers a means for developing sensors whose response is compensated for temperature and RI.
An optical fibre long period grating (LPG), modified with a coating of silica gold (SiO2:Au) core/shell nanoparticles (NPs) deposited using the layer-by-layer (LbL) method, was employed for the development of a bio-sensor. The SiO2:Au NPs were electrostatically assembled onto the LPG with the aid of a poly(hydrochloride ammonium) (PAH) polycation layer. The LPG sensor operates at the phase matching turning point to provide the highest sensitivity. The SiO2:Au NPs were modified with biotin, which was used as a ligand for streptavidin (SV) detection. The sensing mechanism is based on the measurement of the refractive index change induced by the binding of the SV to the biotin. The lowest detected concentration of SV was 19 nM using an LPG modified with a 3 layer (PAH/SiO2:Au) thin film.
A long period grating (LPG) modified with a mesoporous film infused with a functional compound, calix[4]arene, was employed for the detection of volatile organic compounds (VOCs). The mesoporous film consisted of an inorganic part, of SiO2 nanoparticles (NPs) along with an organic moiety of poly(allylamine hydrochloride) polycation PAH, which was finally infused with functional compound, p-sulphanatocalix[4]arene (CA[4]). The LPG sensor was designed to operate at the phase matching turning point to provide the highest sensitivity. The sensing mechanism is based on the measurement of the refractive index (RI) change induced by the complexion of the VOCs with calix[4]arene (CA). The LPG modified with 5 cycles of (SiO2 NPs/PAH)5PAA responded to exposure to chloroform and benzene vapours. The sensitivity to humidity as an interfering parameter was also investigated.
The response of optical fibre long period gratings (LPGs), fabricated with precise control of the grating period to operate at or near the phase matching turning point (PMTP), to temperature was studied. The effect of the grating period on sensor performance was studied. The sensitivity of the LPG operating at the phase matching turning point to temperature was 0.99 nm/°C for 111.5 μm grating period that is ca. 7 times higher than that of an LPG operating far from turning point. The possibility of using LPG sensors to determine the thermo-optic coefficient of the surrounding material was explored.
A fibre optic long period grating (LPG) with an nano-assembled mesoporous coating of alternate layers of poly
(allylamine hydrochloride) (PAH) and SiO2 nanospheres was used for the development of a fibre-optic refractometer.
PAH/SiO2 films of different thickness have been deposited onto an LPG in order to study the effect of the film thickness
on sensor performance. The device showed a sensitivity of 1927 nm/RIU over a RI range of 1.3233-1.4906.
The transmission spectrum of a fibre optic long period grating (LPG) coated with a porous multilayer coating of
thickness of order 400 nm formed from silica nanospheres is shown to exhibit a strong sensitivity to the infusion of a
functional, chemically sensitive material into the coating. Subsequently, the transmission spectrum of the LPG
shows sensitivity to changes in the properties of the functional material when exposed to a particular chemical
species in an aqueous solution. The operation of such a device as an ammonia sensor is demonstrated, exhibiting 1
ppm sensitivity. The sensing mechanisms are discussed.
The bacteriorhodopsin film in gelatin matrixes which are used as sensitive elements of integrated optic and fibre-optic
sensors of various vapor and gases components will not be able to carry out the chemical control of aqueous solutions. In
the given paper the results of technological development of obtaining the bacteriorhodopsin (bR) films in a sol-gel matrix
are represented. The films are obtained in a broad thickness range (from 0.5 to 20 microns) with various bR
concentrations and photosensitize additives. The optimal technological conditions of obtaining of uniform films with
given optical parameters are defined. The surface morphology and cross section of the obtained films was studied using
an AFM and SEM. The films have a reasonable surface roughness (~ 100 nm) and a uniform distribution of the purple
membrane fragments in the nanostructured sol-gel glass matrix along the films surface and thickness. The transmission
spectrums have the characteristic for bR the absorption band, the value of which depends on bR concentration and
technological features of the films deposition. The investigated photosensitive properties of the obtained films and
influence on them of chemical components of aqueous solutions, allow recommending the thin bR films in sol-gel
matrixes for creation of planar waveguides in the role of components of the chemical sensors of liquid solutions.
The sensitive thin film elements based on bacteriorhodopsin in polymeric matrixes for the multichannel fiber-optical sensor system of measurement of chemical composition of gas mixtures are offered. It is shown, that modification of films compositions by the photosensitizing chemical additives changes the photocycle parameters and exhibits selectivity to the effect of various gases.
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