The development of a very sensitive and selective chemical sensor of formaldehyde is presented. We describe the strategies aimed at improving the trapping and detection of formaldehyde. These strategies are based on the use of nanoporous transparent matrices elaborated via the sol-gel process and doped with a colorimetric reagent, and on a fast detection of the absorption or fluorescence of the reaction product. The properties of the sensor are studied as a function of various parameters such as the concentration of the colorimetric agent, the formaldehyde content in the gas mixtures, the presence of other air contaminants and the mixture relative humidity (0-50%). From these studies, calibration curves have been established, which allow the determination of formaldehyde content in air with a fast response time and a sub-ppb sensitivity. We also show a home-made portable and miniaturized detection system developed for this purpose.