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

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 recent years. The determination of the value of pH across a broad range of applications today, important for areas of study such as life sciences, environmental monitoring, manufacturing industry and widely in biological research is now accessible from such optical sensors. The need for such technology arises because familiar, commercial sensors are often limited in terms of their response time and the presence of drift, all of which emphasize the value of newer and rapidly developing technologies such as fiber-optic sensors, to address these wider applications. As a result, a new compact sensor design has been developed, designed around a specially-formed fiber-optic tip, coated with a pH-sensitive dye, and importantly covalently linked to a hydrogel matrix to provide high stability. The sensor developed was designed to have a very fast response time (to 90% of saturation, Δt₉₀) of <5 s and a sensing uncertainty of ∼±0.04 pH units. Given the covalently bonded nature of the dye, the problem of leaching of the indicator dye is reduced, creating a probe which has been shown to be very stable over many days of use. Illustrating this through extended continuous use, over ∼12 h at pH 7, this stability was confirmed showing a drift of <0.05 pH h^-1. In order to give an illustration of the value of the probe in an important biological application, the monitoring of pH levels between pH 7 to pH 8 in an AMES' medium, a substance which is important to maintain the metabolism of retinal cells is shown and the results as well as temperature stability of the probe discussed.