This invention belongs to the field of optical chemical sensors. Specifically, it relates to sensors based on the absorbance and emission of light by an indicator molecule where the optical properties of the indicator molecule change in response to a particular analyte. These indicator molecules are immobilized in a transparent substance that is exposed to light, where the substance is typically a solid such as a sol-gel or a polymer. More specifically, it is a new process for manufacturing a material (a medium or matrix) to hold or encapsulate sensing molecules. This new material has an improved resistance to exposure to hydrocarbons. These materials are used to immobilize colorimetric and/or fluorescence indicators in a matrix that repels hydrocarbons in general and non-polar hydrocarbons (i.e. aromatics hydrocarbons) in particular.