We study gas adsorption and interaction on nanoparticles (NPs) deposited on Au nanodisks acting individually as plasmonic detectors. For this purpose, it is necessary to deposit a thin layer of insulator on the Au nanostructures to make them more stable at high temperatures and to eliminate any (electronic) interactions between the reactive NPs and Au disks. NPs can react with gas molecules that modify the dielectric properties on the surface of the NP and cause a localized surface plasmon resonance (LSPR) shift of the underlying Au detector (Fig. 1). This is called indirect nanoplasmonic sensing (INPS), making it possible to follow, at any pressure, the reactivity with the surrounding gas of small NPs, which by nature do not give measurable LSPR signals. We used INPS to follow, with high sensitivity, the interaction of water vapor on soot particles [1] that are a major contributor to global warming. CO and oxygen adsorption as well as CO oxidation, on Pt NPs, have also been studied by INPS [2]. Due to an optimization of the physical parameters of our plasmonic detector, and coupling with mass spectrometry, we obtain a quantitative and a very sensitive probe that can detect gas adsorption of a few hundredths of a monolayer, even with a very low density of NPs. References: [1] B. Demirdjian, F. Bedu, A. Ranguis, I. Ozerov, A. Karapetyan, and C. R. Henry, Indirect Nanoplasmonic Sensing to Probe with a High Sensitivity the Interaction of Water Vapor with Soot Aerosols, The Journal of Physical Chemistry Letters, 6, 4148-4152 (2015) [2] B. Demirdjian, I. Ozerov, F.Bedu, A. Ranguis and C. R. Henry, CO, and O2 adsorption, CO oxidation on Pt nanoparticles by indirect nanoplasmonic sensing, ACS Omega, 6, 13398-13405 (2021)