Passive tumor-targeting kinetics of PEG-AuNPs in MCF-7 tumor bearing nude mice. a) In vivo NIR fluorescence images of the mouse IV injected with PEG-AuNPs at 5, 12, 18, 24, and 48 h p.i. (arrow: tumor location). b) Time-dependent contrast index (CI) of tumor area after IV injection of PEG-AuNPs and GS-AuNPs, respectively. c) Accumulation and retention kinetics of PEG-AuNPs in tumor and normal tissues.