Cyclic voltammetry and linear sweep voltammetry were performed using a potentiostat (Autolab PGSTAT128N, Eco Chemie, The Netherlands). With its control software (NOVA Eco Chemie), a three-electrode cell was used, which consisted of a carbon screen-printed electrode (area of 2 × 3 mm2, Quasense. Co., Ltd., Bangkok, Thailand) performing as a working electrode, a platinum wire used as a counter electrode (length of 5.7 cm), and an Ag/AgCl electrode in saturated KCl (NF343 RF-1B, ALS Co., Ltd., Tokyo, Japan), used as a reference electrode. Cyclic voltammetry of MB/GP film was carried out using a phosphate-buffered solution containing no hemeproteins. Each of the electrodes was kept in the desiccator until being used for evaluation in the next experiment. The buffer solution was purged with high-purity nitrogen for 30 min before each experiment. X-ray photoelectron spectroscopy (XPS, PHI VersaProbe II) was carried out at the SUT-NANOTEC-SLRI Joint Facility, BL5.2: SUT-NANOTEC-SLRI, Synchrotron Light Research Institute. The excitation energy was 1486 eV using an Al Kα source. The morphology of MB/GP nanohybrids was characterized by using a scanning electron microscope (FE-SEM) (NovaTM NanoSEM 450), (JEM-1011, JEOL, Tokyo, Japan) operated at 10.0 kV, with a magnification of 2500× and 10,000×. Ultraviolet–visible (UV-Vis) absorption spectra of MB in aqueous solutions were detected with a UV-Vis spectrophotometer (CYT5FV, Agilent BioTek, San Diego, CA, USA). The characteristic functional groups of MB/GP nanohybrids were examined using Fourier transform infrared spectroscopy (INVENIO, Bruker, Germany). The zeta potentials of MB/GP nanohybrids were measured using electrophoretic light scattering (ELS) spectrophotometer (Dynamic Light scattering model Zetasizer Nano ZS, Malvern, England).
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