2025

Lampinen, Aku

This dissertation investigates the effects of two-photon oxidation (2PO) and non-covalent adsorption of proteins on chemical vapor deposition grown single-layer graphene (SLG) in tuning of the performance of graphene field-effect transistors (GFETs) for measuring pH and calcium ion concentration ([Ca2+]). The aim was to learn how 2PO could be utilized in the development of graphene-based devices and biosensors for advanced neural interface applications, which are crucial for next-generation neuroprosthetics. Based on the findings of this work, 2PO was found to be an extremely powerful method of controlling SLG functionalization and modification. 2PO was successfully used to achieve area-selective non-covalent adsorption of different proteins, including those closely related to the function of the nervous system. The adsorption was controlled by varying the oxidation level achieved with 2PO, the pH or protein concentration of the protein incubation solution, and/or the number of incubation cycles performed. Results showed controlled binding ranging from monolayer to multilayer deposition. Additionally, 2PO was used to increase and control the sensitivity of GFETs to pH and [Ca2+]. However, polymer residues from the lithographic fabrication processes were found to affect the GFET response in a similar way as 2PO, but less systematically. Bare GFETs achieved limits of detection (LoD) for [Ca2+] down to 10 nM and performing non-covalent functionalization of the GFETs with calmodulin (CaM), a protein that selectively binds to Ca2+, the LoD was improved for worse-performing and/or polymer-contaminated devices.