Non-covalent adsorption of neurotransmission-relevant proteins on locally laser-oxidized and pristine graphene

Femtosecond pulsed laser two-photon oxidation (2PO) was used to modulate protein adsorption on graphene surfaces on a Si/SiO2 substrate. The adsorption behavior of calmodulin (CaM) and a muscarinic acetylcholine receptor (mAchR) fragment on pristine (Pr) and 2PO-treated graphene were studied, utilizing atomic force microscopy and infrared scattering-type scanning near-field optical microscopy for characterization. The results showed that proteins predominantly bound as a (sub-)monolayer, and selective adsorption could be achieved by carefully varying graphene oxidation level, pH during functionalization, and protein concentration. The most pronounced selectivity was observed at low 2PO levels, where predominantly only point-like oxidized defects are generated. Preferential binding on either Pr or oxidized graphene could be achieved depending on the 2PO and adsorption conditions used. Based on the incubation conditions, the surface area covered by mAchR on single-layer graphene varied from 29% (Pr) vs. 91% (2PO) to 48% (Pr) vs. 13% (2PO). For CaM, the coverage varied from 53% (Pr) vs. 95% (2PO) to 71% (Pr) vs. 52% (2PO). These results can be exploited in graphene biosensor applications via selective non-covalent functionalization of sensors with receptor proteins.