Fig. 2. In vitro DOX uptake and retention analyses. (a) Illustration of the effect of NDs on inhibiting ABC transporter protein mediated drug efflux. (b) Representative CLSM images of HepG2 cells incubated with different formulations for 4 h before efflux and after 12 h efflux (blue fluorescence represented Hoechst and red represented DOX). (c) Flow cytometry analysis of DOX in HepG2 cells treated with different formulations for 4 h before efflux and after 12 h efflux (DOX = 10 mg L⁻¹). (d) Mean fluorescence intensity of DOX in HepG2 cells from the flow cytometric result (**P < 0.01, n = 3).