Optical Fiber-Based Sensor for Calcium Using Hydrophobically Associated Calcein and Laser-Induced Fluorescence Detection
by Wang, Y. L.; Baten, J. M.; McMaughan, S. P.; Bobbitt, D. R.
An optical fiber-based sensor for Ca2+ has been developed using hydrophobically associated calcein. In this measurement protocol, the optical fiber surface is derivatized with a C-18 moiety thereby allowing specific reagents to be reproducibly associated with the surface through a hydrophobic process. The C-18 surface is shown to provide significant immunity from chemical degradation at the extremes of the pH scale, at elevated temperatures, and in the presence of specific organic solvents. This capability was exploited in the development of an optical fiber sensor for Ca2+ using C-12-derivatized calcein as the sensing reagent. The reagent can be reproducibly associated with the optical fiber surface and used to quantitate Ca2+ at pH 12. The derivatized calcein exhibited enhanced fluorescent properties at pH 12 in the absence of Ca2+ which limited the quantitative capabilities of the sensor. As now configured, the optical fiber-based sensor has a demonstrated limit-of-detection for Ca2+ of 20 ppb (0.5 mu M) (SNR = 2) in a probe volume of 20 mu l, or less.