## 7 Conclusions and Outlook  
In this article, a review is presented on gas sensors based on the localized surface plasmon resonance (LSPR) phenomenon, focused on earliest and modern applications of plasmonic nanoparticles. Furthermore, the synthesis of nanoparticle-embedded nanostructured oxide/nitride thin films and experimental strategies to enhance their sensitivity are discussed, complemented by simulation and modelling results.  
According to the historical and technological overview on conductometric and optical gas sensors (electronic-nose systems), it is foreseen that in the forthcoming years nanomaterial-based sensors will play an important role in several industrial sectors, leading to growth in a worldwide market. Among several applications that plasmonic nanoparticles can offer, LSPR sensing platforms have been the focus of much research due to the straightforward detection mechanism and experimental setups needed.  
We show that metal oxides can be robust materials to host nanoparticles and boost their sensing capabilities. The preparation method of nanocomposite plasmonic thin films highlighted in this paper consists of reactive magnetron sputtering deposition, followed by thermal annealing, while glancing angle deposition (GLAD) and low-pressure plasma treatment are presented as important techniques to tailor the film nanostructures and hence to improve their optical sensing responses. Of course, it does not mean that chemically synthesized nanoparticles are not promising for sensing applications [82; 103].  
Finally, several simulation methods are described, and the optical LSPR sensitivity of gold nanoparticles with different shapes and sensing volumes estimated using discrete dipole approximation (DDA) models. The simulations confirmed that the enhanced sensitivity in spheroidal nanoparticles comes from the longitudinal vibration modes of the localized surface plasmons that arise when their aspect ratio increases. When Au nanoparticles are embedded in an oxide matrix, their sensitivity is reduced. From these simulations, it becomes clear that an oxide matrix can be used to mechanically protect the nanoparticles, and still allowing them to maintain their sensing capabilities. Furthermore, the sensing capabilities of these embedded nanoparticles can be optimized by adjusting their aspect ratios, and physically erode the excess oxide layer around them. To further investigate the sensitivity of nanoparticles in real systems, simulation models using thetheory of disordered systems, with a self-consistent approach or a kind of perturbation theory must be used, since even the most powerful modern computers do not allow for modelling large systems that include many nanoparticles by direct solution of discretized Maxwell equations or even with approximations such as DDA.  
Conceptualization, M.S.R., J.B., and F.V.; methodology, M.S.R., J.B., C.L., and R.M.S.P.; software, M.S.R. and R.M.S.P.; validation, J.B., M.I.V., and F.V.; formal analysis, M.S.R., J.B., and R.M.S.P.; investigation, M.S.R., J.B., C.L., and R.M.S.P.; resources, J.B. and F.V.; data curation, M.S.R., R.M.S.P., and J.B.; writing--original draft preparation, M.S.R., J.B., and C.L.; writing--review and editing, M.S.R., J.B., C.L., R.M.S.P., M.I.V., and F.V.; visualization, M.S.R., J.B., C.L., M.I.V., and F.V.; supervision, J.B. and F.V.; project administration, J.B. and F.V.; funding acquisition, J.B. and F.V. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2020; and by the project NANO4 BIO POCI-01-0145-FEDER-032299, with FCT reference PTDC/FISMAC/32299/2017. [PERSON] acknowledges FCT for his PhD Scholarship, SFRH/BD/118684/2016.  
The authors declare no conflict of interest.