Fig. 1. (a) Representative transmission electron microscopy (TEM) and (b) high angle annular dark-field-scanning transmission electron microscopy (HADDF-STEM) images of the AuNP/GQDs. (c) High resolution transmission electron microscopy (HRTEM) image of the AuNP/GQDs; the red circle inside is GQD, inset is a scaled up view of GQD with high resolution, lattice spacing is 0.21 nm, and crystal plane is (100). (d) Selected area electron diffraction (SAED) pattern of AuNP/GQDs.

Fig. 2. (a) Representative transmission electron microscopy (TEM) and (b) high resolution transmission electron microscopy (HRTEM) images in the red region of AuNPs. (c) Statistical diameter of AuNPs.

Fig. 3. (a) Binding energies correspond to the C 1s of GQDs and (b) O 1s of AuNP/GQDs. (c) Ratio of different groups in GQDs and AuNP/GQDs. (d) 4f electron of Au. (e) Raman spectrum of AuNP/GQDs and GQDs. (f) Fourier transform infrared (FTIR) spectra of AuNP/GQDs and GQDs.

Fig. 4. (a) Ultraviolet-visible (UV-Vis) absorption spectra. (b) Excitation wavelength-dependent photoluminescence spectra with monitoring wavelength of 550 nm. Photoluminescence spectra of GQDs, GQDs:AuNP = 400:1, 400:2, 400:3, 400:4 excited by (c) 300 nm, (d) 325 nm, (e) 368 nm, and (f) 421 nm.

Fig. 5. (a) Photoluminescence lifetime of GQDs, GQDs:AuNP = 400:1 and 400:3. (b) Extracted photoluminescence lifetime parameters of GQDs and AuNP/GQDs.

Fig. 6. (a), (b) TEM images of AuNPs coated with silicon oxide. (c) Excitation wavelength-dependent photoluminescence spectra and (d) photoluminescence spectra of GQDs and AuNP/GQDs coated with silicon oxide of different thicknesses, with excitation wavelength of 325 nm. Calculated electric field of (e) AuNP, (f) AuNP@SiO2-10  nm, (g) AuNP@SiO2-16  nm, and (h) AuNP@SiO2-20  nm at electric dipole resonance (325 nm). The excitation light is along the z axis and polarized along the y axis in the calculations.

Fig. 7. (a) Calculated electronic band structure and (b) projected density of states (PDOS) of Au. (c) Band diagram of AuNP/GQDs.

Fig. 8. (a) Fluorescence spectra of AuNP/GQDs for different heavy metal ions. (b) Different heavy metal ion detection effect statistics of AuNP/GQDs at a concentration of 50 μM. (c) Fluorescence spectra for detection of different concentrations of chromium ions. (d) Correlation coefficient of AuNP/GQDs and different concentrations of chromium ions; blue area corresponds to poison concentrations.