Surface Functionalization of Platinum Electrodes with APTES for Bioelectronic Applications

The coating of neuroelectronic surfaces converts bioincompatible electrodes into biocompatible surfaces and permits a perfect electronic cell-chip coupling. Thus, we obtain a surface modification protocol that facilitates the chemical and biocompatible modification of dielectric oxides and metals of bioelectronics devices in one preparation step.

The interface between the usually inorganic electronic components and biological objects such as neurons plays a crucial role for the success of applications ranging from biosensors to bioelectronics. Modifying the surface of substrates in order to tailor their surface properties (ranging from mechanical or electronic properties to bioaffinity) and thus optimizing them for a given application is a well-known strategy. For example, the functionalization of a conventional inorganic substrate such as SiO₂ via silanization is well understood. However, bioelectronic components such as electrophysiological sensors are typically composed of several materials like insulating oxides and metals with very different surface properties. So far, there is no general and simple way to modify these materials. While the modification of SiO₂ is done via silanes, metals are typically treated with thiols. The latter works well for Au but on Pt is more involved. Therefore, the question remains, of how can we functionalize the substrate including its electronic components (e.g. in the form of metallic electrodes) in a similar way, if possible simultaneously and with comparable results.

In this work, we demonstrated that the surface of substrates consisting of an insulating oxide (SiO₂) with a metallic electrodes (Pt) can be functionalized with APTES in one preparation step and that these molecules appear to form a stable SAM layer on SiO₂ and Pt. As a result, a biocompatible surface is generated, which facilitates the growth of neurons on SiO₂ and Pt electrodes and may lead to improved electronic cell-chip coupling between electrodes and neurons.

APTES was deposited via a gas-phase-based MLD which enables the complete preparation consisting of cleaning, activation, deposition and post-deposition treatment to be performed in one recipient without breaking the vacuum. The presence of APTES layers on Pt has been demonstrated by various ex-situ methods such as AFM, ToF-SIMS, ellipsometry, SPR and surface potential experiments. We demonstrated that the APTES layer on Pt grows and behaves similarly to those on SiO₂. The Pt surface can be activated and the resulting difference in the electrokinetic potential is consistent with the calculations and density of possible docking bonds for molecules on the surface. APTES can be deposited on Pt and the resulting layers are stable after the removal of physisorbed molecules. The resulting functionalization of the surface shows a clear impact of the amino group on the surface potential. The coverage of the molecules on SiO₂ and Pt surfaces is similar (~50%). The thickness of the APTES layer on Pt (0.6 to 1nm for SPR and ellipsometry, respectively) is comparable to the values of 0.7 to 0.8nm reported for APTES SAMs on SiO₂. Together with the stability of the APTES layer on Pt (repeated experiments performed after several months) we believe that the gas-phase deposited APTES most likely binds covalently with the silane group on Pt and forms a stable SAM. This coating converts the originally bioincompatible Pt electrodes into biocompatible surfaces, which even seem to allow excellent electronic cell-chip coupling leading to recorded neuron AP signals of a few mV. The general understanding of the functionalization of Pt surfaces by silane molecules allows to tailor the surface properties of complex substrates consisting of insulating oxides and metallic electrodes. The demonstrated deposition of the silane SAMs on Pt and SiO₂ leads to stable electronic interfaces potentially suitable for various biosensor and bioelectronic applications.

Publication: Nikolaus R. Wolf , Xiaobo Yuan, Hossein Hassani, Frano Milos, Dirk Mayer, Uwe Breuer, Andreas Offenhäusser and Roger Wördenweber, Surface Functionalization of Platinum Electrodes with APTES for Bioelectronic Applications, ACS Appl. Bio Mater. 2020, 3, 10, 7113–7121; DOI: 10.1021/acsabm.0c00936

Contact:

Dr. Dirk Mayer

Institute of Biological Information Processing-Bioelectronics (IBI-3)

Tel.: +49 2461 61-4023

E-Mail: dirk.mayer@fz-juelich.de