I. M. El Nahhal, S. M. Zourab, F. S. Kodeh, and A. I. Qudaih, “Thin film optical BTB pH sensors using sol–gel method in presence of surfactants”, Int. Nano Lett., vol. 2, no. 1 (December 2012), Jul. 2012, doi: 10.1186/2228-5326-2-16.