A fibre‐optic calcium ion sensor using a calcein derivative

This paper describes the development of a new fibre-optic calcium ion sensor based on the immobilization of the calcein derivative, calcein acrylamide (CA) onto an optical fibre. The method involves the covalent attachment of CA in a polymer which itself is covalently bonded to an optical fibre. The fluorescence of CA is pH-sensitive in the region 4--12 and also increases as a function of Ca(2+) concentration at pH > 12. Typically, the indicator is sensitive to Ca(2+) in the concentration range 0--20 micromol/L, with a linear region occurring when the concentration of calcium is equal to or below that of CA. The binding ratio of Ca(2+) to CA in the linear calibration region is 1:1 and beyond the linear region is 2:1. The calcium-sensitive polymer is prepared using photo-initiated polymerization of a stock solution containing CA, monomer, a crosslinker and a photo-initiator. The monomer units used were acrylamide and acrylamide doped with MAPTAC. The fluorescence characteristics of bound CA in acrylamide are similar to that of CA in solution. However, the cationic nature of the polymer doped with MAPTAC allows analysis of Ca(2+) to be carried out in pH 8.5 solution. The Ca(2+) sensing range of the sensor is typically 0--12 micromol/L.