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Label-free detection of charged biomolecules, such as DNA, has experienced an increase in research activity in recent years, mainly to obviate the need for elaborate and expensive pretreatments for labeling target biomolecules. A promising label-free approach is based on the detection of changes in the electrical surface potential on biofunctionalized silicon field-effect devices. These devices require a reliable and selective immobilization of charged biomolecules on the device surface. In this work, self-assembled monolayers of phosphonic acids are used to prepare organic interfaces with a high density of peptide nucleic acid (PNA) bioreceptors, which are a synthetic analogue to DNA, covalently bound either in a multidentate (∥PNA) or monodentate (⊥PNA) fashion to the underlying silicon native oxide surface. The impact of the PNA bioreceptor orientation on the sensing platform’s surface properties is characterized in detail by water contact angle measurements, atomic force microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. Our results suggest that the multidentate binding of the bioreceptor via attachment groups at the γ-points along the PNA backbone leads to the formation of an extended, protruding, and netlike three-dimensional metastructure. Typical “mesh” sizes are on the order of 8 ± 2.5 nm in diameter, with no preferential spatial orientation relative to the underlying surface. Contrarily, the monodentate binding provides a spatially more oriented metastructure comprising cylindrical features, of a typical size of 62 ± 23 × 12 ± 2 nm2. Additional cyclic voltammetry measurements in a redox buffer solution containing a small and highly mobile Ru-based complex reveal strikingly different insulating properties (ion diffusion kinetics) of these two PNA systems. Investigation by electrochemical impedance spectroscopy confirms that the binding mode has a significant impact on the electrochemical properties of the functional PNA layers represented by detectable changes of the conductance and capacitance of the underlying silicon substrate in the range of 30–50% depending on the surface organization of the bioreceptors in different bias potential regimes.

Field-effect transistors (FETs) form an established technology for sensing applications. However, recent advancements and use of high-performance multigate metal–oxide semiconductor FETs (double-gate, FinFET, trigate, gate-all-around) in computing technology, instead of bulk MOSFETs, raise new opportunities and questions about the most suitable device architectures for sensing integrated circuits. In this work, we propose pH and ion sensors exploiting FinFETs fabricated on bulk silicon by a fully CMOS compatible approach, as an alternative to the widely investigated silicon nanowires on silicon-on-insulator substrates. We also provide an analytical insight of the concept of sensitivity for the electronic integration of sensors. N-channel fully depleted FinFETs with critical dimensions on the order of 20 nm and HfO2as a high-k gate insulator have been developed and characterized, showing excellent electrical properties, subthreshold swing, SS ∼ 70 mV/dec, and on-to-off current ratio, Ion/Ioff∼ 106, at room temperature. The same FinFET architecture is validated as a highly sensitive, stable, and reproducible pH sensor. An intrinsic sensitivity close to the Nernst limit, S= 57 mV/pH, is achieved. The pH response in terms of output current reaches Sout= 60%. Long-term measurements have been performed over 4.5 days with a resulting drift in time δVth/δt= 0.10 mV/h. Finally, we show the capability to reproduce experimental data with an extended three-dimensional commercial finite element analysis simulator, in both dry and wet environments, which is useful for future advanced sensor design and optimization.