Morphological and electrochemical properties of different PNA-based sensing platforms – Impact of the receptor-surface binding modes

By using self-assembled monolayers of phosphonic acids (SAMPs) on silicon native oxide surfaces as anchor platforms, two distinct organic interfaces with a high density of PNA bioreceptors are prepared. The impact of the PNA-bioreceptor orientation on the surface properties of the sensing platform is characterized in detail by water contact angle (CA) measurements, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Our results suggest that multidentate binding of PNA bioreceptor via attachment groups at the γ-points along the PNA backbone produces an extended, protruding and netlike 3D-metastructure with no preferential spatial direction. Contrary, a spatially more localized and cylindrical metastructure is realized by the monodentate binding. Furthermore, cyclic voltammetry measurements performed in a redox buffer solution, which is containing a small and highly mobile Ru-based redox active complex, reveal strikingly different insulating properties (diffusion kinetics) of these two PNA layers. Finally, investigation by electrochemical impedance spectroscopy confirms that the binding mode has a significant impact on the electrochemical properties of the functional PNA sensing surface. Here, we could observe changes of the conductance and capacitance of the underlying silicon-based semiconducting substrate in the range of 30-50 % which are strongly depending on the surface organization of the bioreceptors at different bias potential regimes. Consequently, a well-chosen modification of the PNA backbone is a valid approach to influence the sensing properties of surface-immobilized PNA bioreceptors, which might provide an additional parameter to further tune and tailor the sensing capabilities of PNA-based biosensing devices.