This work aims at optimizing a dielectric barrier discharge (DBD) process in a reactor designed for coating nanopowders. A copolymerization involving maleic anhydride and vinyltrimethoxysilane has been chosen to functionalize samples with carboxylic groups. Prior to any deposition on powder, an optimization of the process on flat substrates is required to allow a carboxyl functionalization independent of the position in the reactor. Accordingly, flat substrates have been placed along the reactor and subsequently analysed by XPS, SEM and Auger. From the results obtained, experimental conditions allowing batches of functionalized powder are deduced.