1. A method of detecting the presence of a reactive gas in an inert ambient gas comprising (a) exposing part of a film surface to the inert ambient gas; the film being a silicon dioxide derived from the oxidation of a hydrogen silsesquioxane resin; the silicon dioxide film having a jV plot of the current density versus the voltage which includes linear and nonlinear regions, the jV plot of the film including; (i) a first nonlinear region where the current increases with the applied voltage to a maximum; followed by (ii) a second nonlinear region of negative differential resistance leading to a current minimum at and beyond which (iii) a third region is reached from where the rapid decrease of the applied voltage causes the current to decrease linearly with voltage; the jV curve of the silicon dioxide film further including (iv) a fourth region where an increase of the voltage applied to the film across a threshold value causes a rapid transition of the current from the linear region to the first non-linear region (i); the next step being selected from the group consisting of (b) measuring the change in the current flow through the film at an increasing voltage to determine a threshold voltage as a measure of the concentration of the reactive gas in the inert ambient gas, and (c) measuring the change in the current flow through the film at a constant voltage above a reaction barrier voltage as an indication of the presence of the reactive gas in the inert ambient gas.