Spray-deposited carboxylated carbon nanotube (CNT) layers were characterized using AFM, Raman, and spectroscopic ellipsometry. The layers’ thickness, diameters and band gap of CNTs, as well as the changes in the CNT layer  refractive index at 1319 nm and 2010 nm after H2O and NH3 adsorption in air and H2O in N2 were analyzed. Refractive index changes and modeling the necessary length of the modified interferometer arm for a π/2 phase shift allow  us to propose the use of such CNT layers for integrated interferometric sensors and gas recognition.