Several reactive or non-reactive plasmas can be created within the glow/discharge apparatus, which modify the surfaces by depositing films having functional groups. These, the so-called plasma-polymerized films covering the crystal surfaces, are extremely thin and homogenous coatings. They adhere strongly to the crystal and are highly resistant to chemical and physical treatments. Moreover, sensors produced using this method are more reproducible from sample to sample and exhibit lower noise than sensors made using conventional immobilization methods.