S 4800

High-resolution scanning electron microscopy (HR-SEM) images of the conductive hydrogels were obtained using a Hitachi S-4800 scanning electron microscope (Hitachi S-4800). Before imaging, each hydrogel sample was frozen at 0 °C for 24 h and then freeze-dried (SCIENTZ-10N) at − 20 °C for 24 h. A small amount of freeze-dried hydrogel was placed on a jig or sample holder with conductive adhesive. The samples were subsequently coated with a 5 nm thick Pt layer using metal sputtering (K575X sputter coater, Quorum Emitech, UK) to prevent charging. Infrared spectra of the conductive hydrogels were recorded using Fourier-transform infrared (FTIR) spectroscopy (Nicolet iS10, Thermo Fisher Scientific, USA) in the range of 400–4000 cm^–1, employing the potassium bromide (KBr) tablet method. X-ray diffraction (XRD; Rigaku Ultima(III) analysis was conducted on both GO and rGO samples using an instrument with CuK α radiation (λ = 0.154 nm). The 2θ range was set to 5–60° with a scan speed of 2°/min and a step rate of 0.02 per second. To characterize the chemical composition of GO and rGO-PDA samples, X-ray photoelectron spectroscopy (XPS) analysis was performed using a photoelectron spectrometer (Thermo Scientific, USA).

Microsphere morphology was investigated by SEM (S-4800; Hitachi Ltd., Tokyo, Japan). First, the microspheres were affixed on metallic mount for coating and were coated with platinum using sputter coater (K575K; Emitech Ltd., Ashford, UK). After coating with platinum, the morphology of microspheres was observed at different zoom magnifications by SEM (S-4800; Hitachi Ltd., Tokyo, Japan). SEM images of microspheres after 1 and 28 days in release buffer were also measured.

The materials obtained were characterized by using various techniques, including scanning electron microscopy (SEM, operating at 10 kV, Hitachi S-4800, Tokyo, Japan), scanning transmission electron microscopy (STEM, operating at 30 kV, Hitachi S-4800, Tokyo, Japan), powder X-ray diffraction (operated at 40 kV, Cu-Kα radiation (=0.1541 nm) RINT2000 diffractometer, Rigaku, Tokyo, Japan), Fourier transform infrared (FT–IR) spectroscopy (ATR–FTIR, Nexus 670, Tokyo, Japan), Raman scattering (NRS-3100 Raman spectrometer, JASCO, Tokyo, Japan), and X-ray photoelectron spectroscopy (XPS; Thermo Electron Co. Karlsruhe, Germany, a monochromatic Al-Kα radiation of photon energy 15 keV). The electron microscopy samples were prepared on carbon-coated copper grids by dropping C₆₀ self-assembled crystals (the selected ones) suspension in isopropyl alcohol (3 μL) and drying under vacuum at 70 °C.

The morphology, composition and structure were investigated by field emission-scanning electron microscopy (FESEM, S–4800, Hitachi, Tokyo, Japan), mapping (Mapping, S–4800, Hitachi, Tokyo, Japan) and X-ray diffraction (XRD, Bruker D8 Advance, Karlsruhe, Germany). Functional groups of the samples were obtained by Fourier transform infrared spectroscopy (FTIR, Bruker Vertex 70, Karlsruhe, Germany). Raman spectra were recorded using a Raman spectrometer (RAMAN, iHR550, Shanghai, China) at a wavelength of 532 nm.

The sizes of NiO_x NPs were estimated using transmission electron microscopy (TEM, JEOL JEM-2100F, Tokyo, Japan) images. The surface morphology of the sensor electrodes was characterized by scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan) and atomic force microscopy (AFM, Park System XE100, Suwon, Korea). Energy-dispersive X-ray spectrometry (EDS, Hitachi S-4800, Tokyo, Japan) analysis was conducted to investigate the elemental composition of the electrode. X-ray diffraction (XRD, Bruker D8 Advance, Billerica, MA, USA) patterns were recorded for phase identification. The resistance was measured using a multimeter (Agilent U1251B, Santa Clara, CA, USA) while the temperature was precisely controlled by a hot plate (Fisher Scientific, Hampton, NH, USA). The temperature of the hot plate also was monitored by a commercial thermocouple (Type K, EA11A). The resistivity (ρ) of the electrode was calculated using the equation: ρ = R·(A/l), where R, A, and l are the resistance, cross-sectional area, and length of the electrode, respectively.