Characterization of a fast response fiber-optic pH sensor and measurements in a biological application

Abstract

Optical, and especially fiber-optic techniques for the sensing of pH have become very attractive and considerable research progress in this field has been made over a number of years. The determination of the pH level across a broad range of applications today, e.g. in life sciences, environmental monitoring, industry and widely in biologically research is now accessible from such optical sensors. This arises because familiar sensors are often limited in terms of their response time and drift, which reduces the use of the current group of such fiber-optic sensors in wider applications. A new compact sensor design has been developed in this work, based on a specially-formed fiber-optic tip that was coated with a pH-sensitive dye, covalently linked to a hydrogel matrix to provide high stability. The sensor developed has a very fast response time (to 90% of saturation, Δt₉₀) of < 5 seconds, a sensing uncertainty of about ± 0.04 pH units and given the covalently bonded nature of the dye, leeching is reduced and the probe is very stable over many days of use. During extended continuous use over ~12h in pH 7, this stability was confirmed, with drift of < 0.05 pH/h. Preliminary experiments in an important biological application, monitoring over pH levels from pH 5 to pH 8.5, are shown and discussed.