Laser-induced tuning of graphene field-effect transistors for pH sensing.

Here we demonstrate, using pulsed femtosecond laser-induced two-photon oxidation (2PO), a novel method of locally tuning the sensitivity of solution gated graphene field-effect transistors (GFETs) without sacrificing the integrity of the carbon network of chemical vapor deposition (CVD) grown graphene. The achieved sensitivity with 2PO was (25 ± 2) mV pH-1 in BIS-TRIS propane HCl (BTPH) buffer solution, when the oxidation level corresponded to the Raman peak intensity ratio I(D)/I(G) of 3.58. Sensitivity of non-oxidized, residual PMMA contaminated GFETs was 20-22 mV pH-1. The sensitivity decreased initially by 2PO to (19 ± 2) mV pH-1 (I(D)/I(G) = 0.64), presumably due to PMMA residue removal by laser irradiation. 2PO results in local control of functionalization of the CVD-grown graphene with oxygen-containing chemical groups enhancing the performance of the GFET devices. The GFET devices were made HDMI compatible to enable easy coupling with external devices for enhancing their applicability.

Tuning protein adsorption on graphene surfaces via laser-induced oxidation.

An approach for controlled protein immobilization on laser-induced two-photon (2P) oxidation patterned graphene oxide (GO) surfaces is described. Selected proteins, horseradish peroxidase (HRP) and biotinylated bovine serum albumin (b-BSA) were successfully immobilized on oxidized graphene surfaces, via non-covalent interactions, by immersion of graphene-coated microchips in the protein solution. The effects of laser pulse energy, irradiation time, protein concentration and duration of incubation on the topography of immobilized proteins and consequent defects upon the lattice of graphene were systemically studied by atomic force microscopy (AFM) and Raman spectroscopy. AFM and fluorescence microscopy confirmed the selective aggregation of protein molecules towards the irradiated areas. In addition, the attachment of b-BSA was detected by a reaction with fluorescently labelled avidin-fluorescein isothiocyanate (Av-FITC). In contrast to chemically oxidized graphene, laser-induced oxidation introduces the capability for localization on oxidized areas and tunability of the levels of oxidation, resulting in controlled guidance of proteins by light over graphene surfaces and progressing towards graphene microchips suitable for biomedical applications.