Effect of two-photon oxidation and calmodulin functionalization on the performance of graphene field-effect transistor biosensors†

Abstract

Solution-gated graphene field-effect transistors (GFETs) were fabricated for Ca²⁺ sensing. The GFETs were functionalized with two-photon oxidation (2PO) and calmodulin (CaM) immobilization, and the effects of these treatments and polymer residues on the sensor performance were systematically studied. Non-oxidized devices having polymer residues from lithographic processing showed initial LoDs of around 10⁻⁹ M and non-oxidized cleaner devices 10⁻⁸ M and the response of the devices was stable and reversible. 2PO showed a positive effect on the sensitivity of the devices, increasing the [Ca²⁺] dependent change in resistance at a constant gate voltage roughly by a factor of two, but at the cost of the LoD as 2PO increased the LoDs to up to 10⁻⁶ M. CaM functionalization was able to improve the LoD in some cases by two to three orders of magnitude, but its effect was limited most likely due to the intrinsic binding constants of the protein. However, CaM did not have a systematic effect on the magnitude of the response of the devices. Post-lithography polymer residues affected the LoD and response magnitude in a similar manner as 2PO, but also caused less reproducible behavior, indicating that a cleaner GFET surface is preferred for sensor applications.