Optical response of porous titania-silica waveguides to surface charging in electrolyte filled pores

In this work, we present a novel method for hi situ investigation of surface charging and ion transport inside nanopores of titania-silica waveguide by means of the optical-waveguide-lightmode spectroscopy. Porous oxide waveguides show a strong optical response when exposed to electrolyte solutions, and this response is consistent with oxide surface charging due to changes in ionic strength and pH of the solution in contact with the waveguide. The optical response to pH or electrolyte concentration change is stabilized within several minutes when the solution ionic strength is sufficiently high (0.1M),while it takes two orders of magnitude longer-to-reach stable optical response at very low ionic strengths (<0.1mM). The relaxation times at the high ionic strength are still by several orders of magnitude slower than expected from bulk diffusion coefficients of electrolytes in water. Our results indicate that diffusion of electrolytes is severely hindered (and more so with decreasing ionic strength) in charged pores inside waveguides.