Nicolet is5 ftir

¹H nuclear
magnetic resonance (¹H NMR) spectra were recorded on a
JEOL JNM-ECX400 spectrometer at resonant frequencies of 400 MHz in
deuterated dimethyl sulfoxide (DMSO-d₆) at 50 °C, DMF-d₇ at room temperature,
or on a JEOL JNM-ECZ600/ M1 spectrometer at resonant frequencies of
600 MHz in DMF-d₇ at 90 °C. ¹³C NMR was recorded on a JEOL JNM-ECZ600/M1 spectrometer at
resonant frequencies of 150 MHz in DMF-d₇ at 90 °C. The logarithmic viscosity method was employed to
measure the viscosity using an Ostwald viscometer and a thermostatic
bath (TV-5S, Thomas Kagaku Co., Ltd.). The samples were dissolved
in concentrated H₂SO₄ and adjusted to 0.5 dL/g,
and the flow time was measured at 30 °C. FTIR spectra were measured
using a Thermo Fisher Scientific Nicolet iS5 FTIR with the sample
ground. TG/DTA 6300 and DSC 6200 were used for thermal characterization.
TGA was performed by placing an aluminum pan containing a sample (<10
mg) on a balance in a weighing chamber. The heating rate was set to
10 °C/min; the scan started at room temperature and ended at
550 °C, followed by holding for 10 min. For DSC measurements,
the maximum temperature was set at approximately 20 °C below
the 1% weight loss temperature obtained by TGA of the same sample.
The heating and cooling rates were set to 10 °C/min, and the
1st cooling scan and 2nd heating scan were taken to determine the T_c and T_g/T_m values, respectively.

The size and morphology of GQDs was analyzed by using a JEM 2100F transmission electron microscope at an operating voltage of 200 KV. The height distribution of the GQDs was characterized by atomic force microscope (AFM) (Bruker Multimode, Bruker Corporation, Billerica, MA, USA) operating in tapping mode. The X-ray diffraction (XRD) pattern was measured using a Bruker-D8 Advance (Bruker Corporation) and Cu Kα radiation (λ=1.54051 Å) operating at 1 KV. The Fourier transform infrared spectroscopy (FTIR) spectra were measured by a Nicolet iS5 FT-IR (Thermo Nicolet, Thermo Fisher Scientific) spectrometer with the KBr pellet technique. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific Escalab 250Xi with Al Kα X-ray radiation as the X-ray source for excitation. Binding energies were corrected using the C1s peak at 284.6 eV as the standard. PL spectra of GQDs were recorded using a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). The fluorescence lifetime was measured using a photo-counting HORIBA FL-4 system, with a diode laser emitting at 330 nm as the light source.