**Dervil Cody 1,\*, Tsvetanka Babeva 2, Violeta Madjarova 2, Anastasia Kharchenko 3, Sabad-e-Gul 1, Svetlana Mintova 3, Christopher J. Barrett <sup>4</sup> and Izabela Naydenova 1,\***


Received: 13 March 2020; Accepted: 21 April 2020; Published: 24 April 2020

**Abstract:** Optical sensors fabricated in zeolite nanoparticle composite films rely on changes in their optical properties (refractive index, *n*, and thickness, *d*) to produce a measurable response in the presence of a target analyte. Here, ellipsometry is used to characterize the changes in optical properties of Linde Type L (LTL) zeolite thin films in the presence of Cu2<sup>+</sup> ions in solution, with a view to improving the design of optical sensors that involve the change of *n* and/or *d* due to the adsorption of Cu2<sup>+</sup> ions. The suitability of two different ellipsometry techniques (single wavelength and spectroscopic) for the evaluation of changes in *n* and *d* of both undoped and zeolite-doped films during exposure to water and Cu2<sup>+</sup>-containing solutions was investigated. The influence of pre-immersion thermal treatment conditions on sensor response was also studied. Due to the high temporal resolution, single wavelength ellipsometry facilitated the identification of a Cu2<sup>+</sup> concentration response immediately after Cu2<sup>+</sup> introduction, indicating that the single wavelength technique is suitable for dynamic studies of sensor–analyte interactions over short time scales. In comparison, spectroscopic ellipsometry produced a robust analysis of absolute changes in film *n* and *d*, as well as yielding insight into the net influence of competing and simultaneous changes in *n* and *d* inside the zeolite-doped films arising due to water adsorption and the ion exchange of potassium (K<sup>+</sup>) cations by copper (Cu2<sup>+</sup>).

**Keywords:** optical sensors; optical materials; zeolites; ellipsometry; single wavelength ellipsometry; spectroscopic ellipsometry
