Fig. 3

a The image of UV-vis spectrum showing a characteristic absorption peak of AuNPs at 521 nm; b FT-IR spectrum showing the absorption peak of the infrared spectrum at 1635 cm^− 1 is significantly enhanced; c Value changes of Au, Au @PDA, Au @PDA-PEG and Au @PDA-PEG-MTX zeta potential; d Size distribution of Au, Au @PDA, Au @PDA-PEG and Au @PDA-PEG-MTX