Molecular Layer Functionalized Neuroelectronic Interfaces: From Sub-Nanometer Molecular Surface Functionalization to Improved Mechanical and Electronic Cell-Chip Coupling

The interface between electronic components and biological objects plays a crucial role for the success of bioelectronic devices. Since the electronics typically include different elements such as an insulating substrate in combination with conducting electrodes, an important issue of bioelectronics involves tailoring and optimizing the interface for any envisioned application. In this work, we present a method of functionalizing insulating substrates (SiO2) and metallic electrodes (Pt) simultaneously with a stable monolayer of organic molecules ((3-aminopropyl)triethoxysilane (APTES)). This monolayer is characterized by various techniques like atomic force microscope (AFM), ellipsometry, time-of-flight secondary ion mass spectrometry (ToF-SIMS), surface plasmon resonance (SPR), and streaming potential measurements. The molecule layers of APTES on both substrates, Pt and SiO2, show a high molecule density, a coverage of ~ 50 %, a long-term stability (at least one year), a positive surface net charge, and the characteristics of a self-assembled monolayer (SAM). In the electronical characterization of the functionalized Pt electrodes via impedance spectroscopy measurements, the static properties of the electronic double layer could be separated from the diffusive part using a specially developed model. It could be demonstrated that compared to cleaned Pt electrodes the double layer capacitance is increased by an APTES coating and the charge transfer resistance is reduced, which leads to a total increase of the electronic signal transfer of ~13 %. In the final cell culture measurements, it could be shown that an APTES coating facilitates a conversion of bio-unfriendly Pt surfaces into biocompatible surfaces which allows cell growth (neurons) on both functionalized components (SiO2 and Pt) comparable to that of reference samples coated with poly-L-lysine. Furthermore, APTES coating leads to an improved mechanical coupling, which increases the sealing resistance and reduces losses. These increases were finally confirmed by electronic measurements on neurons, which showed action potential signals in the mV regime compared to signals of typically 200 – 400 µV obtained for reference measurements on PLL coated samples. Therefore, the functionalization with APTES molecules seems to be able to greatly improve the electronic cell-chip coupling (here by ~1 500 %). This significant increase of the electronic and mechanical cell-chip coupling might represent an important step for the improvement of neuroelectronic sensor and actuator devices

Platinum substrate for surface plasmon microscopy at small angles

Platinum is reported as the main component of the substrate in surface plasmon microscopy of the metal−dielectric interface for small-angle measurements. In the absence of a narrow dip in the angular spectrum of platinum, the refractive index of the dielectric medium or the thickness of a deposited layer is proven deducible from the observed sharp peak, close to the critical angle. The sensitivities of refractive index and thickness measurements using platinum are compared with that of a gold surface plasmon resonance chip. Furthermore, the thickness of a structured layer of (3-Aminopropyl)triethoxysilane on the platinum substrate is measured to be 0.7 nm, demonstrating the high sensitivity of the technique