Microsphere optical ion sensors based on doped silica gel templates

A simple method for doping ion-sensing ingredients into silanized silica particles (∼3.5 μm) was developed to fabricate carrier-based ion-selective microsphere optical ion sensors. These ion sensors operate according to established bulk optode principles. Na+-selective microsphere optical sensors were prepared with the ionophore tert-butylcalix[4]arene tetraacetic acid tetraethylester, the H+-chromoionophore 9-(diethylamino)-5-octadecanoylimino-5H-benzo[a]phenoxazine (ETH 5294), the ion exchanger sodium tetrakis[3,5-bis(tri-fluoromethyl)phenyl]borate (NaTFPB), PVC and the plasticizer bis(2-ethylhexyl)sebacate (DOS). For Ca2+ sensing, the grafted Ca2+ ionophore AU-1 in poly(n-BA) was applied with ETH 5294, NaTFPB and DOS. Fluorescence microscopy and flow cytometry were used for characterization. The resulting microsphere optical ion sensors were found to obey bulk optode theory with reliable response, high selectivities, dynamic ranges suitable for physiological measurements and a shelf time of over 6 months. The measuring ranges (logarithmic molar concentrations) at pH 7.4 were −3.5 to −1.5 for the fabricated Na+-selective optical-sensing microspheres, and −4.5 to −1.5 for Ca2+-selective optical-sensing microspheres. Corresponding thin optode films and regular PVC/DOS-based microspheres were prepared for comparison and showed similar response characteristics. The fabricated microsphere optical ion sensors were also deposited on an optical fiber bundle, and reproducible fluorescence signals were observed.