The present application relates sensing reactive components in fluids by monitoring band gap changes to a material having interacted with the reactive components via physisorption and/or chemisorption. In some embodiments, the sensors of the present disclosure include the material as a reactive surface on a substrate. The band gap changes may be detected by measuring conductance changes and/or spectroscopic changes. In some instances, the sensing may occur downhole during one or more wellbore operations like drilling, hydraulic fracturing, and producing hydrocarbons.