<title>Critical review of pH sensing with optical fibers</title>

Abstract

The chemical parameter most investigated with optical fibres is doubtless pH. The first pH optical fibre sensor was described in 1980. Since then, more than one hundred and twenty original papers describing different pH sensors have been published, based on absorption-based indicators or fluorophores. Such interest is perfectly justified, since pH detection is essential in many fields of application, ranging from the environment and medicine to industry and process control. Moreover, pH transduction can be used for measuring different chemical species, such as carbon dioxide, ammonia and pesticides. Notwithstanding the great number ofprototypes realized in different laboratories all over the world, only a few products are available on the market. A critical analysis of the state of art in pH sensing using optical fibres is described, outlining the advantages and disadvantages of an optical approach.

In-vivo continuous measurement of interstitial pH for intensive care applications

“…In fact, there is an intrinsic inaccuracy in any optical measurement of pH, which is based on a measurement of the concentration of hydrogen ions, whereas the pH value is related to their activity. This aspect is very often underestimated, if not completely disregarded, leading to totally wrong accuracies 30 Figure 5 shows the calibration pH curves for the five different ionic strengths. The rather extensive differences clearly show that a knowledge of the ionic strength of the interstitial fluid is extremely important in order to obtain a reliable pH measurement.…”

Optical sensor for interstitial pH measurements

“…This aspect is very often underestimated, if not completely disregarded, leading to totally wrong accuracies. 29 The effect of the ionic strength was carefully investigated and five different series of Britton-Robinson buffers at different ionic strengths were prepared ͑0.005, 0.02, 0.1, 0.5, and 1 M͒. Figure 8 shows the calibration curves for the five different ionic strengths.…”

Novel fiber tip sensor potentially suitable for gastric pH measurement

“…Since then many optical fiber pH sensors were developed [2] mainly for biomedical applications thanks to the potential miniaturization of the optical fiber sensors, capable to assure reliable in vivo measurements. In biomedical field, the main interest was devoted to the measurement of blood pH, but another interesting application is given by the measurement of pH in the stomach and in the esophagus.…”