**4. Conclusions**

The use of ellipsometry as a characterization tool to provide further insight into optical sensor operation and zeolite–analyte interactions is presented. The suitability of both single wavelength and spectroscopic ellipsometry for the evaluation of changes in the refractive index, *n*, and thickness, *d*, of zeolite-TEOS films during exposure to copper ion solutions has been investigated. Additionally, the influence of the initial film thermal treatment conditions on the response of both doped and undoped films has been studied.

Due to the short acquisition time involved, single wavelength ellipsometry offers excellent temporal resolution and thus is suitable for the real-time study of optical changes in thin films. This approach was used to study the dynamic changes in the *n* and *d* of TEOS-only and TEOS-LTL films immersed in water and copper ion solutions. Due to the competing effects of film swelling and refractive index change due to water and copper ion adsorption, no significant difference in the absolute values of *n* and *d* were observed between the different concentrations of Cu2<sup>+</sup> ions. However, close examination reveals that the dynamics of change in *n* and *d* are different for the different Cu2<sup>+</sup> concentrations, as well as between the TEOS-only and TEOS-LTL films. This highly sensitive measurement technique offers a unique insight into the sensor response mechanism over short timescales.

Spectroscopic ellipsometry was used as a second method for the study of the changes in TEOS-LTL film optical properties. While the temporal resolution of the spectroscopic system in this instance was limited by the necessity to acquire data over a larger wavelength range, the dispersion model used for data analysis in spectroscopic ellipsometry facilitates a more accurate estimation of the absolute values of the optical constants. Using this approach, detailed analysis of changes in *n* and *d* for the undoped and LTL zeolite doped TEOS films as a result of water and copper ion exposure was conducted. It was observed that the amount of film swelling can be controlled by the temperature at which the samples are treated before exposure to the analyte. The higher the temperature, the smaller the swelling of the TEOS films during immersion. Additionally, it was observed that the inclusion of LTL zeolites in the TEOS increases the rigidity and mechanical stability of the film under prolonged exposure to aqueous solutions.

Both the single wavelength and spectroscopic measurements of films pre-treated at 170 ◦C show a decrease in *n* after the sample is exposed to water or Cu2+-containing water solution. Samples pre-treated at 320 ◦C and doped with LTL zeolites show no swelling, and *n* slightly increases when immersed in water and decreases after exposure to copper ions. This is explained by (i) the lower refractive index of copper water solution as compared to pure water and (ii) the decrease in the hydrophilicity of zeolites with the addition of copper.

The suitability of ellipsometry for the study of optical material and sensor–analyte interactions, including the differentiation of the influence of simultaneous and competing processes, has been demonstrated. Future work will study a wider range of zeolite–analyte combinations over a wider range of annealing temperatures and conditions.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-6412/10/4/423/s1, Figure S1: Contact angle results for uncoated and coated films: SRG-surface relief grating; SRG-TEOS-sol-gel coated surface relief grating; SRG-TEOS-LTL-zeolite-doped sol-gel coated surface relief grating; SRG-TEOS-LTL-Cu II zeolite-doped sol-gel coated surface relief grating exposed to 4 mM Cu2<sup>+</sup> solution, Table S1: Extinction coefficients at wavelength of 633 nm.

**Author Contributions:** Conceptualization, D.C. and I.N.; methodology, D.C., T.B., V.M., S.M., S.-e.-G., A.K., C.J.B. and I.N.; formal analysis, D.C., T.B., V.M. and I.N.; resources, D.C., T.B., V.M., S.M., C.J.B. and I.N.; writing—original draft preparation, D.C., I.N. and T.B.; writing—review and editing, D.C., T.B., V.M., S.M., S.-e.G., A.K., C.J.B. and I.N.; visualization, D.C., T.B. and V.M.; supervision, I.N. and C.J.B.; funding acquisition, I.N. and D.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Arnold F Graves Postdoctoral Scholarship from Technological University Dublin, and a Flaherty Research Scholarship from the Ireland Canada University Foundation.

**Acknowledgments:** The authors would like to thank Oleg I. Lebedev from CRISMAT, CNRS-ENSICAEN, France, for preparation of the TEM images. The authors gratefully acknowledge the financial support provided for this work by the Arnold F. Graves Postdoctoral Scholarship from Technological University Dublin, and a Flaherty Research Scholarship from the Ireland Canada University Foundation. TB and VM acknowledge the support of the European Regional Development Fund within the Operational Programme "Science and Education for Smart Growth 2014–2020" under the Project CoE "National center of mechatronics and clean technologies" BG05M2OP001-1.001-0008-C01.

**Conflicts of Interest:** The authors declare no conflict of interest.
