Abreu et al. Nanoscale Research Letters 2011, 6:482 http://www.nanoscalereslett.com/content/6/1/482 Table 2 Hydrodynamic diameter, polydispersity, zeta potential, and encapsulation efficiency of several drug-loaded liposomes Hydrodynamic diameter (nm) (mean ± SD) Polydispersity (mean ± Zeta potential (mV) (mean SD) ± SD) Encapsulation efficiency 115.4 ± 0.5 116 ± 2 116.0 ± 0.8 120 ± 2 104.3 ± 0.6 79.3 ± 0.8 103.5 ± 0.9 95.4 ± 0.5 104 ± 3 0.15 ± 0.01 0.15 ± 0.01 0.15 ± 0.01 0.19 ± 0.01 0.25 ± 0.01 0.37 ± 0.01 0.12 ± 0.01 0.14 ± 0.01 0.27 ± 0.01 -30 ± 1 -27 ± 4 -19 ± 2 -39 ± 3 -52 ± 6 -43 ± 3 Page 4 of 6 97% 96% 99% 98% 98% 99% Drug-loaded liposomes DPPC/Ch/DSPE-PEG (5:5:1) 1 week after 2 weeks after DSPC/Ch/DSPE-PEG (5:5:1) Egg-PC/Ch/DSPE-PEG (5:5:1) Egg-PC/DCP/Ch (7:2:1) Egg-PC/Ch/DPPG (6.25:3:0.75) 2 weeks after Egg-PC/DPPG/DSPE-PEG (5:5:1) Standard deviations were calculated from the mean of the data of a series of five experiments conducted using the same parameters. widely used fluorescence probe nitrobenzoxadiazole, NBD. The FRET mechanism involves a donor fluoro- phore in an excited electronic state (here compound 1), which may transfer its excitation energy to a nearby acceptor chromophore (NBD) in a nonradiative way through long-range dipole-dipole interactions. Because the range over which the energy transfer can occur is limited to approximately 100 Å and the efficiency of transfer is extremely sensitive to the donor-acceptor separation distance, resonance energy transfer measure- ments can be a valuable tool for probing molecular interactions [11]. Taking advantage of the possibility of FRET from the excited compound 1 (donor) to the nitrobenzoxadiazole moiety, the diffusion of compound 1 in dialysis experi- ments was monitored using this photophysical process. Two different dialysis membranes (6 to 8 KDa or 12 to 14 KDa) were tested. The experiments were carried out at 25° C for 36 h and are schematically illustrated in Figure 2. DPPC/DPPG (1:1) liposomes with encapsulated com- pound 1 (donor liposomes) were placed at one side of the dialysis membrane (Figure 2, left), while NBD-labelled lipid/DPPC/DPPG liposomes without compound (accep- tor liposomes) are placed at the other side (Figure 2, right). After the experiment (36 h), the occurrence of energy transfer (FRET) from compound 1 to NBD, detected in the solution located at the right side, is a proof of compound diffusion from the donor liposomes, passing across the dialysis membrane and incorporation in the membrane of the acceptor liposomes. The phospholipids DPPC and DPPG are the main components of biological membranes and are both in the gel phase at room tem- perature. This fact is expected to restrain the diffusion of compound 1 and, therefore, if the compound diffuses Figure 2 Schematic dialysis experiment from DPPC/DPPG liposomes to NBD-labelled lipid/DPPC/DPPG liposomes.