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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.