Fig. 2

Characterization of nanoparticles. a–c Representative TEM images of USPIO NPs (a), USPIO-PAA NPs (b) and USPIO-PAA/PEG NPs (c). d Statistical analysis indicated a significant difference among the particle size of USPIO, USPIO-PAA and USPIO-PAA/PEG (n = 20 per group). e Fourier-transform infrared of nanoparticles of USPIO, USPIO-PAA and USPIO-PAA/PEG NPs. The arrows indicate the specific absorption peak at 1728 cm⁻¹ due to the PAA modification, 1595 cm⁻¹ and 3440 cm⁻¹ due to the PEG modification. Numerical data are presented as mean ± SD, **p < 0.01 versus the USPIO NPs, ##p < 0.01 versus the USPIO-PAA/PEG NPs. The bar represents 20 nm