Fig. 1

Synthesis and characterizations of Gd@C-dots. a, b Representative TEM images of pPD-Gd@C-dots (a) and CA-Gd@C-dots (b). The scale bars are 100 nm. c, d DLS analysis. pPD-Gd@C-dots (c) and CA-Gd@C-dots (d) bear positive (+ 42.3 mV) and almost neutral (+ 2.84) surfaces, respectively. e, f EDS elemental analysis of pPD-Gd@C-dots (e) and CA-Gd@C-dots (f). g Table showing compositions of pPD-Gd@C-dots and CA-Gd@C-dots, based on EDS results. h Uv–vis absorption (left) and fluorescence (right) spectra of pPD-Gd@C-dots. pPD-Gd@C-dots have absorbance peak at 457 nm, 520 nm, and 570 nm, and an emission peak at ~ 630 nm (excited at 457, 520, or 570 nm). (i) Uv–vis absorption (left) and fluorescence (right) spectra of CA-Gd@C-dots. CA-Gd@C-dots have an absorbance peak at 347 nm and emission peak at 449 nm (excited at 347 nm)