17 Plastic Optical Fiber pH Sensor Using a Sol-Gel Sensing Matrix Luigi Rovati 1 , Paola Fabbri 2 , Luca Ferrari 1 and Francesco Pilati 2 1 Department of Information Engineering, 2 Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia Italy 1. Introduction Because it is the most ubiquitous species encountered in chemical reactions, hydrogen ion occupies a very special place in chemistry and biology, most of the chemical and biological processes being dependent on its activity. From the analytical point of view, the abundance of hydrogen ions is quantified in terms of pH, the negative logarithm of its activity. Its importance is evident by considering that, if the pH of the human blood changes as little as 0.03 pH units or less, the functioning of the body will be greatly impaired; also, brain pH decreases from normal pH of 7.4 to a pH of 6.75 during the brain insult and a continuous monitoring system would be beneficial in the treatment of comatose neurosurgical patients and those who have suffered traumatic brain injury, ischemic brain insult and so forth (Zauner, 1995). Furthermore, the kind of animals and plants living in lakes, rivers and oceans depends on pH values, as well as pH of soil affects the livability of plants. For this reason, the use of pH sensors is widely diffused in various fields to monitor chemical and biological processes and it is finding an increasing number of applications in medicine, biomedicine, industry and environmental monitoring. The earliest methods of pH measurement fall roughly into four categories: indicator reagents, pH test strips, amperometric or potentiometric devices. Since a long time, the glass electrode pH sensor has been the most popular due to its ideal Nernstian response, independent of redox interferences, and because of its high selectivity for hydrogen ions in a solution, short balancing time of electrical potential, reliability and high reproducibility, and long lifetime. For these reasons, it is still considered to be the standard measuring method for the pH measurements. However, glass electrodes have several drawbacks for many applications. Firstly, they exhibit a sluggish response and are difficult to miniaturize. Moreover they can generate electromagnetic interferences with other devices and cannot be used in food or in vivo applications due to their brittle nature. For these reasons, there is increasing need for alternative pH sensors and, in this respect, optical methods offer many advantages over the common glass electrode or other electrochemical devices. Among these, optical fiber colorimetric pH sensors monitoring the change in the optical response of proton-sensitive indicators are the most popular systems. Other pH-sensing devices, based on different