Materials fiber-optic chemical sensors

The sPS has been exploited as sensitive material for fiber optic chemical sensors based on reflectance measurements and aimed to detection of chloroform and toluene in water and air environments48 50. The refractive index of sPS thin films is estimated to be about 1.578. The effect of the analyte sorption in the crystalline domain was modeled as an increase in the material density, which in turn leads to an increase in the refractive index according to the Lorentz-Lorenz law ... 

By incorporating these three material phenomena for chemical measurements with fiber optic structures creates a basis for a fiber optical chemical sensor or FOCS. Two classes of FOCS have been reported in the literature ... 

For the evaluation of the SIM technique in chemical sensors application, an experimental set-up was developed as shown in Fig. 31. A fiber-optic chemical sensor was prepared, using polyaniline as the modified cladding for a short MM sensing fiber. The spin-casting method was used for coating a thin layer of polyaniline material on the fiber core surface. Then, the modified fiber was tested for the detection of HCl vapor and NH3 gas. The sensor was tested by both the total intensity modulation and the SIM techniques. 

Yuan J, El-Sherif MA (2003) Fiber-optic chemical sensor using polyaniline as modified cladding material. IEEE Sens J 3 5-12... 

Wolfbeis OS (1991) Fiber optic chemical sensors and biosensors, vol 1. CRC Press, Boca Raton, FL Wolfbeis OS (ed) (1992) Fiber optic chemical sensors and biosensors, vol 2. CRC Press, Boca Raton, FL Wolfbeis OS (2005) Materials for fluorescence-based optical chemical sensors. J Mater Chem 15 2657-2669 Wolfbeis OS, Posch HE (1986) Fiber-optic fluorescing sensor for ammonia. Anal Chim Acta 185 321-327 Wolfbeis OS, Weis LJ, Leiner MJP, Ziegler WE (1988) Fiber-optic fluorosensor for oxygen and carbon dioxide. Anal Chem 60 2028-2030... 

Successful development of fiber optic chemical sensors requires co-operation of many specialists in various fields of science. Scientists in analytical chemistry, polymer science, material science, optoelectronics and electronics etc. can be involved in this multidisciplinary task. In dependence on the application of the sensor biologists, medical doctors or environmentalists can be incorporated to the working group. Although, the contribution of all specialists cannot be classified by the importance, analytical chemistry and material science seem to be the key to the success. 

Fiber-optic chemical sensors are composed of a sensing material and a transducer. The transducer converts the recognition and sensing events obtained by the sensing materials into a response such as an optical signal. Optical measurements can provide rapid, sensitive, and nondestructive analysis of many important compounds. In fiberoptic chemical sensors, optical fibers are used to transmit the optical signal to the measurement device, enabling a remote detection of the analyte in the sample. The use... 

The basic design of a fiber-optic chemical sensor system is shown in Fig. 1. The fiber-optic chemical sensor s main components are (a) a light source, (b) optical fibers fo both transmit the light and act as the substrate for (c) the sensing material, and (d) a detector to measure the output light signal. Usually computers or microprocessors control the fiber-optic chemical sensor instrumentation and are employed to analyze the output signals. 

Fiber-optic chemical sensors can be divided into two categories based on their structure (a) Intrinsic sensors are based on the analyte s intrinsic optical properties, and (b) extrinsic sensors are based on sensing materials (chemical or biological) immobilized to the fiber surface, with... 

Extrinsic fiber-optic chemical sensors are constructed by immobilizing indicator chemistries on the fiber tip or on the annulus of the fiber. In this section, the mechanism of the chemical and biological reagents employed for sensing are described as well as methods for immobilizing sensing materials. 

Fiber-optic biosensors are a subtype of fiber-optic chemical sensors that rely on sensing materials of biological origin. In fiber-optic biosensors, biological recognition components such as enzymes, antibodies, DNA, or cells are attached to the optical fiber sensing layer in order to alter the specificity of the sensor. 

Most commercially available environmental fiber-optic chemical sensors are for pH and oxygen monitoring in water and wastewater. The monitoring of unusual changes in oxygen and pH can be used as an indirect indication of the presence of pollutants. These sensors are based on fluorescent dyes as the sensing material. The optical fiber and the sensing elements are covered with a metal jacket that provides immunity from harsh environmental conditions. 

The FPI principle can also be used to develop thin-film-coating-based chemical sensors. For example, a thin layer of zeolite film has been coated to a cleaved endface of a single-mode fiber to form a low-finesse FPI sensor for chemical detection. Zeolite presents a group of crystalline aluminosilicate materials with uniform subnanometer or nanometer scale pores. Traditionally, porous zeolite materials have been used as adsorbents, catalysts, and molecular sieves for molecular or ionic separation, electrode modification, and selectivity enhancement for chemical sensors. Recently, it has been revealed that zeolites possess a unique combination of chemical and optical properties. When properly integrated with a photonic device, these unique properties may be fully utilized to develop miniaturized optical chemical sensors with high sensitivity and potentially high selectivity for various in situ monitoring applications. 

It was mentioned before that fiber-optic chemical and biosensors are broadly classified into two categories extrinsic- and intrinsic-type sensors. In the extrinsic-type sensors, the fiber is acting as a link connecting optical signals to (and from) the active material (medium) positioned at the end of the fiber, such as the Optode case. In the intrinsic-type sensors, the fiber is modified in different ways, through construction of the sensing component, which will be explained next. 

Specialized microwave reactors for chemical synthesis are commercially available from such companies as CEM [20], Lambda Technologies [21], Microwave Materials Technologies (MMT), Milestone [22], PersonalChemistry [23], and Plazmatronika [24] which are mostly adjusted from microwave systems for digestion and ashing of analytical samples [25]. They are equipped with built-in magnetic stirrers and direct temperature control by means of an IR pyrometer, shielded thermocouple or fiber-optical temperature sensor, and continuous power feedback control, which enable one to heat reaction mixture to a desired temperature without thermal runaways. In some cases, it is possible to work under reduced pressure or in pressurized conditions within cavity or reaction vessels. 

Silicones are extremely versatile materials that have found manifold apphcations in optical sensing of chemical species. Their high gas permeability, processability, optical transparency, composition tunabihty, resihency to biofilm adhesion and low cost, together with good chemical and thermal stability, have made silicones one of the polymer materials of choice to immobilize optical indicator dyes at the sensitive layer of fiber-optic gas sensors (mostly for monitoring). Silicones can also be used to manufacture optosensors for nonvolatile species, as long as they are modified with other materials to confer the required permeability to the indicator films. 

Nieuwenhuizen and Harteveld [92] have realized a nerve agent dosimeter gas sensor based on the strong interaction between certain metal ions and organophosphorus compounds. In this case, the sensor material contains La(III) 2-bis(carboxymethyl)amino hexadecanoic acid and different factors such as humidity, concentration and layer thickness have been studied and optimized. Using a combination of a metallic complex with a molecular-imprinted polymer, a very sensitive sensor was developed for the detection of soman, a chemical warfare agent (the detection limit was 7 ppt) [93]. The biosensing material is based on a polymer coated onto a fiber-optic probe modified with a luminescent europium detection complex. This complex was... 

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