3.2.1. Dynamic Studies of Film *n* and *d* due to Exposure to Water

It can be seen (Figure 3a,d) that when exposed to water, both *n* and *d* of the reference TEOS-only films were relatively constant. This could be explained by the relatively low porosity of the undoped films, which reveals that the amount of water penetrating into the films was insignificant. The change in *<sup>d</sup>* (Figure 3a) was within 0.12% while the change in the films *<sup>n</sup>* was less than 1.3 <sup>×</sup> <sup>10</sup>−<sup>4</sup> (Figure 3d). It is worth noting that no significant disturbance of the samples was observed as a result of the injection of water at 200 s. This confirms that there was no mechanical instability during the injection step causing changes, thus any changes observed in presence of copper in the sample can be attributed solely to the presence of the copper analyte.

The introduction of LTL zeolite nanoparticles in the film increased swelling significantly (up to 1.2% in a 220-nm thick film) and the observed change in *<sup>n</sup>* is 1.1 <sup>×</sup> <sup>10</sup><sup>−</sup>3, revealing that the structure is more flexible (i.e., able to swell) as well as more porous and hydrophilic, which allows for the water molecules to penetrate the film.

**Figure 3.** (Top row) normalized thickness, *d*, as a function of exposure time to 0 (**a**), 2 (**b**) and 4 (**c**) mM Cu2<sup>+</sup> solution; (bottom row) normalized refractive index, *n*, as a function of exposure time to 0 (**d**), 2 (**e**) and 4 (**f**) mM Cu2<sup>+</sup> solution. Black square symbol represents tetraethyl orthosilicate (TEOS)-LTL data, and red spheroidal symbol represents TEOS-only data.
