Response time enhancement of pH sensing films by means of hydrophilic nanostructured coatings (original) (raw)
2007, Sensors and Actuators B: Chemical
This work describes the improvement in response time of thin film optical fluorescent sensors for pH detection based on the 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) pH-sensitive molecule. Two different kinds of structures were fabricated to demonstrate these concepts, using the electrostatic layer-by-layer (LbL) assembly method. These structures were built up on both glass slides and tapered ends of standard communications optical fiber substrates. The basic structures are based on a sensitive coating formed by the alternate deposition of the polymer poly(allylamine hydrochloride) (PAH) as the cationic monolayers and the polymer poly(acrylic acid) (PAA) and the fluorescent pH indicator HPTS as the anionic monolayers. These sensitive coatings showed a long response time when they were immersed in different pHs. In order to improve the performance of basic devices one approach was followed, which consisted of the deposition of a highly hydrophilic block before the sensitive coating. These enhanced devices were proved to decrease the response time dramatically. AFM and water contact angle measurements showed that properties of these structures including the hydrophilic block had higher roughness and lower contact angle (around 10 • ) whereas the ones without the hydrophilic block were smoother and had higher contact angles (around 50 • ). The response time of both devices was also measured while the pH was changed several times resulting in a remarkable reduction with the new devices. The most significant reduction was in rise time response between pH 3 and pH 7 which was five times minimized.
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The characterization of nanostructured thin films is critical in the design and fabrication of optical sensors. Particularly, this work is a detailed study of the properties of layer-by-layer electrostatic self-assembled multilayer (LbL) structures fabricated using poly(allylamine hydrochloride) (PAH) and Neutral Red (NR) as cations, and poly(acrylic acid) (PAA) as polyanion. These LbL films, due to the colorimetric properties of the NR, are suitable for sensor applications such as pH sensing in the physiological range. In the (PAH+NR/PAA) LbL structure, it has been observed a very important influence of the pH of the solutions in the properties of the resultant films. Different techniques such as spectroscopy and atomic force microscopy (AFM) are combined to characterize the films, and the results are analyzed showing coherence with previous works. The LbL structure is finally optimized and dramatically improved nanostructured films were fabricated, showing good sensing properties, short response times, and good stability.
Sensors and Actuators B: Chemical, 2014
Stable and reliable operation of an optical sensor for pH monitoring is important for many industrial applications of these types of devices. The layer-by-layer deposition technique is a simple and versatile method used to deposit a sensitive thin film on such an optical fibre-based device but creating a coating which can often be destroyed in use in highly acid or alkali solutions i.e. with very low or very high pH. It is thus important to create stable and durable sensors to meet the needs of users for operation under these extreme environments. The main aim of this study has been to prepare a number of such sensors and compare the performance of three different stabilization approaches used for the development of an effective wavelength-dependent pH-sensitive optical sensor. Techniques such as employing heat treatment, the deposition of two layers of a PAH/SiO 2 thin film and the deposition of two layers of APTMS/SiO 2 as topping layers have been studied to determine the optimum approach to creating a stable and reliable sensor-one yielding the same value of peak wavelength for a measurement of a known value of pH and to do so repeatibly. An improvement in performance and in shelf-life, stability and re-usability of the sensor has been achieved by the addition of two bilayers of APTMS/SiO 2 (3-Aminopropyl-trimethoxy silane/Silica nanoparticle) in the work carried out and the results of the investigation undertaken are reported.
The pH sensor based optical fiber coated with PAH/PAA
Journal of Physics: Conference Series, 2019
pH-sensitive optical fiber sensors were fabricated with layer-by-layer (LbL) method. Multiple layers of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) bilayers were coated on an unclad fiber to create the pH-sensitive region. The coating process was done using a coating system developed in this work. 10 to 40 bilayers of PAH/PAA were coated on the fiber core to evaluate its performance as pH sensor. The existences of coating on the fiber were verified with field emission scanning electron microscopy (FESEM) and energy-dispersive x-ray spectroscopy (EDX) analysis. It was shown that the fabricated sensor is pH sensitive and independent of variation in the refractive index of the surrounding. The best performance was obtained with 30 bilayers fiber sensor. The sensitivity and resolution of the sensor were 0.453 a.u/pH and 0.0022 pH units respectively. With the developed coating system, various physical and chemical sensors can be realized by varying the coating mater...
Langmuir–Blodgett Films as Matrices of Sensitive Elements in Optical Acidity Sensors
Doklady Physical Chemistry - DOKL PHYS CHEM, 2003
A new approach to the design of optical sensors for the determination of solution pH within a wide range of acidity has been suggested. The approach is based on varying the nature of the matrix of Langmuir-Blodgett (LB) films. A technique has been developed for applying such films with a controllable number of monolayers, based on various matrices (polyamide acid salt, arachidic acid) containing Congo Red as the acid-base indicator. Sensor membranes based on LB films containing this indicator enable one to measure pH within an acidity range, for example, from 2.5 M sulfuric acid to pH 5.5.
Design of pH Sensors in Long-Period Fiber Gratings Using Polymeric Nanocoatings
IEEE Sensors Journal, 2000
In this paper, two different pH sensors based on the deposition of nanometric scale polymeric films onto the surface of a long-period fiber grating (LPFG) have been studied and compared. An electrostatic self-assembled (ESA) method has been used to create sensitive films with an optimal overlay thickness. Two types of sensors have been designed: The first one is based on polyallylamine hydrochloride (PAH), polyacrylic acid (PAA), and the second one was done incorporating the pigment Prussian blue (PB) in the PAH/PAA matrix. A theoretical model of multilayer cylindrical waveguides based on coupled-mode theory has been used to predict the position of the attenuation bands as a function of the overlay thickness. Both sensors were tested and compared in terms of sensitivity and response time. A faster response was obtained with the introduction of PB particles in the polymeric matrix. Linear sensors in the pH range 4-7 were obtained, showing good repeatability and high sensitivity.
Review on State-of-the-art in Polymer Based pH Sensors
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This paper reviews current state-of-the-art methods of measuring pH levels that are based on polymer materials. These include polymer-coated fibre optic sensors, devices with electrodes modified with pH-sensitive polymers, fluorescent pH indicators, potentiometric pH sensors as well as sensors that use combinatory approach for ion concentration monitoring.
Procedia Chemistry, 2009
Measuring the change of the conductivity of a Polyaniline (PANI) film while in contact with a solution is considered to be a costeffective tool for pH sensing. The device consists of a PANI film deposited between two identical electrodes, reducing the number of fabrication steps. To ensure the sensors display an optimum response to solutions of different pH values, it is important to understand the effect of the polymer binder, surfactant and film thickness. The effect of varying the amount of Polyvinyl butyral (PVB) and Hypermer (PS3) on the pH sensitivity of screen printed thick film sensors is reported.
Fiber-optic pH sensors fabrication based on selective deposition of Neutral Red
2009 IEEE Sensors, 2009
In this work, a novel application of the electric field directed layer-by-layer self assembly (EFDLA) selective deposition method for the fabrication of optical fiber pH sensors is presented. Here, indium tin oxide (ITO) coated optical fibers have been fabricated via a dip-coating deposition method. These fibers are used as electrodes in the EFDLA protocol in order to deposit selectively the sensitive layer. Neutral Red (NR) colorimetric pH sensitive indicator and the polymers poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) are used in order to obtain a pH sensitive nanostructured coating onto ITO coated optical fibers. The results obtained in this work revealed that the LbL material adsorption on the electrodes can be enhanced or even inhibited when applying a specific direct current voltage between them under some other specific fabrication parameters. Particularly, the response of these sensors to variations of the pH in the surrounding medium was studied when the pH of the solutions used for the fabrication of the films was adjusted to 7.0 and the potential applied between electrodes was set to 2.5 V. These sensors showed fast response time and high repeatability.
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