AFM images of FNP solutions after storage at 25°C for 24h.

<p>(a) large-scale image, 963 x 963 nm<sup>2</sup>; (b) small scale image 365 x 203 nm<sup>2</sup>; (c) cross-sections of FNPs in water with sizes of 66 nm, 70 nm and 55 nm. The peak represents particle height. The maximum particle height was 4.7 nm; (d) 3D image of FNPs from a small-scale image on the HOPG surface.</p>