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pH sensing using whispering gallery modes of a silica hollow bottle resonator

Stoian, Razvan-Ionut, Lavine, Barry K., Rosenberger, A.T.
Talanta 2019 v.194 pp. 585-590
diodes, electrochemistry, pH, polymers, refractive index, silica, titration, turbidity
The limitations of electrochemical pH sensors have stimulated the development of optical pH sensing methods. In the method reported here, swellable pH-sensitive polymer particles are deposited on the interior surface of a silica hollow bottle resonator. As the pH of the buffer solution in contact with the particles increases, the refractive index of the particles decreases. As a result, whispering gallery modes with internal evanescent components shift in frequency as a function of pH. This shift is monitored by the throughput of tunable diode laser light coupled into the whispering gallery modes using a tapered fiber. Plots of selected mode frequencies vs. pH yielded sigmoid shaped titration curves similar to those obtained using turbidity to monitor refractive index changes of the particles as a function of pH. The response time of 10–15 s and best resolution of 0.06 pH unit represent improvements over previous optical pH sensing methods.