3.2.2. Dynamics Studies of Film *n* and *d* due to Exposure to Copper Ions in Water

As seen from Figure 3b,c, the introduction of copper in the solution did not lead to a significant dimensional change in the TEOS-only films (within 0.13% for both 2 and 4 mM), while, in the TEOS-LTL zeolite-doped films, the swelling was similar to water solutions and within 1.2%. Nevertheless, a close examination revealed that the dynamics of the swelling is different, with a small jump at 200 s for the 4 mM concentration. One can conclude from these results that the swelling of the zeolite doped films is mainly due to the film being immersed in water and it is within 1.2% of the original thickness of the film for the first 700 s.

The examination of the dynamics of the refractive index of TEOS-LTL films under exposure to copper ions in water revealed that the rate of initial change of the refractive index after introduction of the copper ions strongly depends on their concentration (Figure 3d–f). The absolute refractive index change measured 500 s after the injection of copper ions was 7 <sup>×</sup> 10−4, 7 <sup>×</sup> 10−<sup>4</sup> and 11 <sup>×</sup> 10−<sup>4</sup> for samples with a concentration of 0, 2 and 4 mM correspondingly. This can be explained by the high ion exchange capacity of the LTL zeolite nanocrystals. The higher amount of Cu cations replacing the original K cations in the LTL zeolite nanocrystals leads to a higher decrease in the absolute refractive index (Figure 3).

As *d* increases due to swelling, the average *n* of the film will decrease as the TEOS-LTL matrix molecules become more separated in space. While one would expect that water replacing air will increase *n* of the film voids, the net effect of swelling of the TEOS-LTL matrix, which has an initially higher refractive index, likely overcomes this effect, causing an overall decrease in the *n* of the film with increasing water/Cu2<sup>+</sup> exposure. This effect is not observed for the TEOS-only films, which are highly rigid. Understanding these effects is important for sensor design.
