The effect of the ratio of Zol-Ca@DOPA to DSPE-PEG2k (i.e. Zol/DSPE-PEG2k) on the drug (Zol) loading efficiency (DLE) and encapsulation efficiency (EE) of Zol (A), recovery efficiency (RE) of Zol (B), average hydrodynamic size (by number) and polydispersity index (PDI) (C), and zeta potential (D) of the resultant Zol-Ca@bi-lipid NPs. (E) Representative TEM images of Zol-Ca@bi-lipid NPs prepared with different Zol/DSPE-PEG2k ratios (i.e. 15, 20, 60 and 120 µg/1 mg) (bar = 100 nm). In A–D, data shown are mean ± S.D. (n ≥ 3).