Compared with the core particles before coating, the thickness of the PDA layer was estimated to be around 10 nm. FTIR spectra of DFNS-20 and DFNS-20@DA were recorded to provide qualitative information of the PDA coating (Fig. ). Compared to DFNS-20, DFNS-20@DA displayed new characteristic adsorption bands. The band at 1615 cm -1 is attributed to the N-H stretching, and the adsorption bands at 1506 and 1357 cm -1 are assigned to benzene ring C-C vibration . A visible change in colour from white to brown was also observed from the particles after the PDA coating (Fig. ). These results confirmed the presence of the PDA coating on the silica nanoparticles. (Table ). The Zeta potential of these PDA-coated nanoparticles in buffer were measured and found to be -30.47, -19.00 and -18.08 mV, respectively (Fig. ). Since a ζ potential of < -25