In situ observation of the dynamic self-assembly process was carried out using a BX-51 polarizing microscope (Olympus) under identical light intensity. The process was simultaneously recorded using a high-speed camera (M320S, Phantom Miro) at a frame rate of 3000 frames s−1 with an exposure time of 5.5 μs. The birefringence intensity in the POM images in fig. S5 was quantified using the ImageJ freeware. The birefringence intensity profiles of the two images along the lines inside the glass nozzle were similar (fig. S5C). Compression tests were carried out with an Instron 5966 tester (Instron Engineering Corporation) with a load cell of 10 N. A cylindrical GO hydrogel (diameter, 14 mm; height, 13 mm) was placed between two parallel plates and compressed at a strain rate of 10% min−1. Tensile tests of single dried GO fibers were also carried out at a strain rate of 10% min−1. The GO fibers were fixed onto a rectangle frame with 25 mm of gauge length using epoxy glue to prevent the slip of fibers. The mechanical strengths of fibers were calculated by the force divided by the cross-sectional area. The mechanical properties of GO fibers were averaged over at least 10 samples. The linear densities of dried GO fibers were measured with a FAVIMAT sing fiber tester (Textechno H. Stein) using a vibroscope method. XRD patterns of GO powder and freeze-dried bulk gels were obtained with an x-ray diffractometer (Rigaku, SmartLab) using a Cu Kα radiation source (λ = 1.5418 Å). XPS analysis was carried out with an XPS spectrometer (Theta Probe, Thermo Fisher Scientific) using monochromatic Al Kα radiation. The zeta potential of aqueous GO dispersion with a concentration of 0.05 mg ml−1 was measured using a Zetasizer Nano ZS90 (Malvern Instruments). The pH of GO dispersion was adjusted with dilute NH4OH solution. Raman analysis of GO gels was performed on a NRS-3100 (JASCO Inc.) with 532-nm laser excitation. Dynamic rheological properties of GO hydrogels were measured with the Advanced Rheometric Expansion System (AR50, TA Instruments). A parallel-plate geometry with a gap of 8 mm was adopted. Dynamic frequency sweep experiments were carried out over the frequency sweep range of 0.1 to 100 rad s−1 at a fixed oscillatory strain of 1%. Synchrotron SAXS measurements were carried out at the 4C SAXS II beamline of the Pohang Accelerator Laboratory (Pohang, Republic of Korea) with an x-ray beam wavelength of 0.675 Å at a sample-to-detector distance of 4 m. A single GO gel fiber was loaded on an open-circle holder, and both sides of the holder were sealed with polyimide tape to prevent drying of the gel fiber during SAXS measurements for an exposure time of 30 s. SEM images of the freeze-dried gel fibers and dried fibers were obtained using a Hitachi S-4800 microscope (Hitachi High-Technologies Co.). The topography of the single-layered GO sheet was characterized using an atomic force microscope (XE-70, Park Systems).

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