In medicine knowledge of pH values can provide us with information not only about the patients' status but also about physiological processes in the patient's body. Measurements of pH in small-sample volumes and online pH monitoring in vivo can be employed to obtain such information. For such measurements we have developed and investigated U-shaped fiber-optic probes with immobilized pH indicators in this paper. U-shaped probes with a diameter of about 2 mm were prepared. Three different pH indicators, methyl red, methyl orange, and bromothymol blue, were immobilized in two types of matrices, namely, porous silica (PS) and ethylcellulose (EC), and applied on the U-shaped probes. Changes in spectra of transmitted power were measured and calibration curves were determined from these spectra. It has been found that a working pH range of prepared probes was from 3.1 to 7.6. The maximum sensitivity was about 0.1 1/pH unit. Effects of structural relaxations of detection layers and indicator leaching observed in experiments are discussed.
Measurements of pH are important in industry, agriculture, medicine, etc. Two main principles, namely electrochemical and optical ones, have been employed in pH meters and sensors. This paper is aimed at the development of extrinsic fiber-optic sensors of saliva pH. Such sensors have already been used for monitoring pH changes caused by biological processes. In this paper, fiber-optic sensors consisted of inlet and outlet silica fibers transmitting light from a halogen lamp to a sample cell and then to a diode-array spectrometer. Two types of sample cells were used; namely a silica cell with a measurement path of 10 mm, and a special silica capillary cell with a hole diameter of 0.07 mm and measurement path up to 40 mm. This capillary, produced at the Institute, consists of a Bragg mirror applied onto the inner silica capillary wall. Both the sensors were calibrated by using Sorensen buffers with bromothymol blue and a commercial pH meter. The calibration curves were used for the determination of pH of saliva samples collected from one healthy person at different times. It has been found that the fiber-optic sensors provide us with lower pH values than the pH meter. This result can be explain by effects of saliva components on bromothymol blue spectra. By using a correlation line between pH values measured by the sensors and those from the pH meter the sensor reliability of about 0.3 pH units can be estimated.Keywords Fiber-optic sensor · Rectangular cell · Bragg capillary cell · Sorensen buffer · Bromothymol blue · Saliva pH Paper based on an Invited Lecture presented at MADICA
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