Recently, Wang et al. [77] studied the interactions between 8-nm QDs coated with the small, zwitterionic amino acid ligand D-penicillamine (DPA) and RBCs. At neutral pH, the charges on the amino and carboxylic acid groups of the surface ligands are balanced. After incubation with 10 nM DPA-QDs in PBS solution for different time periods and separation of free DPA-QDs by centrifugation, the RBC cells were transferred to a microscope sample cell and imaged using confocal fluorescence microscopy. The data clearly showed that the DPA-QDs adhered to the RBC membranes, and the number of fluorescence spots, either close to the cell membranes or inside the cells, increased with exposure time (Figure 3). Moreover, the adsorbed DPA-QDs did not induce strong local membrane deformations. In fact, the RBC membranes remained largely intact during NP penetration of the bilayer, as evidenced by confocal microscopy images taken in the presence of calcein violet AM. This cell membrane-permeant dye becomes impermeant after entering the cell because of hydrolysis by intracellular esterases [81]. Surface-enhanced infrared absorption spectroscopy (SEIRAS) measurements carried out on model membrane preparations resembling RBC membranes revealed that the bilayer structure was softened in the presence of DPA-QDs, which may facilitate penetration of DPA-QDs into the lipid bilayer without causing poration.