Optical sensors based on absorption

The immobilization of phenol red (phenolsulfonphthalein) in microspheres of polyacrylamide and polystyrene produces an optical sensor that is sensitive to 0.01 pH units in the pH range 7.0 to 7.4 [197]. Phenol red exists in two tautomeric forms, one acidic and one basic, which have different absorption spectra. Two light emitting diodes (LED) are used a reference diode, emitting at 810 nm, and a working diode, emitting at 565 nm which corresponds to the wavelength of the maximal absorption of the basic form. This sensor can follow variations in pH between 6.5 and 9.0 [198]. Bromothymol blue has a maximal absorption at 593 nm and can be used instead of phenol red [199]. 

The basic form of p-nitrophenol has a maximal absorption at 404 nm, and can detect pH variations in the internal electrolyte solution of an optical ammonia sensor. The optical sensor is equipped with a hydrophobic teflon membrane to keep a solution of p-nitrophenol and ammonium chloride at the tip of the flber the sensor responds linearly to ammonia in the concentration range of 0.25 to 1 mM [2(X)]. A colored indicator (oxazine perchlorate) can also be deposited on the wall of the optical fiber, in the form of a solid thin film. Using an LED that emits at 560 nm and a phototransistor, ammonia vapors can be detected down to 60 ppm, in the presence of 40 % humidity [201]. This low detection limit is possible due to multiple reflections (about 600) along the capillary tube. 

Optical sensors that are sensitive to pH and ammonia can save as a basis for the production of optical enzyme sensors [8]. Certain enzymes catalyse reactions with a variation in pH (penicillinase, cholinesterase, glucose oxidase or urease, etc.), or with liberation of ammonia (urease, glutaminase, amino acid oxidase) and are simply deposited onto the optical sensors. 

Optical enzyme sensors do not necessarily require a colored immobilized reagent. Sometimes the product of the reaction is q tically absorbent. One example is the simple optical sensor that uses immobilized alkaline phosphatase to catalyse the following reaction [9]  

The product p-nitrophenoxide has a maximal absorption at 404 nm, and a selective filter for this wavelength is placed in front of the photomultiplier in order to detomine p-nitrophenylphosphate. 

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The reflecting surface can be a mirror or a membrane with a light-scattering surface. In any case, the sensor has the appearance of a monolithic probe (i.e., a dip-stick probe). Optical sensors based on absorption, fluorescence, phosphorescence, and luminescence can employ such a configuration. Various highly optimized fiberoptic probes for UV-Vis, NIR, and IR ranges are now commercially available, and their designs are shown in Fig. 9.23. 

Weisser M, Menges B, Mittler-Neher S (1998) Multimode integrated optical sensor based on absorption due to resonantly coupled surface plasmons. Proc SPIE 3414 250-256... 

The field of electrode materials has been one of the earliest areas of applications of PTs. Initially envisioned in the perspective of energy storage, the field has heen progressively reoriented toward applications such as (bio)electrochemical sensors and electrochromic devices. Recently, the field of PT-based sensors has grown considerably and progressively evolved fi-om purely electrochemical systems to optical sensors based on absorption or luminescence processes. 

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