Three Arginine-Rich Cell-Penetrating Peptides Facilitate Cellular Internalization of Red-Emitting Quantum Dots

Nanoparticles, such as semiconductor quantum dots (QDs), have been found increasing use in biomedical diagnosis and therapeutics because of their unique properties, including quantum confinement, surface plasmon resonance, and superparamagnetism. Cell-penetrating peptides (CPPs) represent an efficient mechanism to overcome plasma membrane barriers and deliver biologically active molecules into cells. In this study, we demonstrate that three arginine-rich CPPs (SR9, HR9, and PR9) can noncovalently complex with red light emitting QDs, dramatically increasing their deliv- ery into living cells. Zeta-potential and size analyses highlight the importance of electrostatic interactions between positive-charged CPP/QD complexes and negative-charged plasma membranes indicating the efficiency of transmembrane complex transport. Subcellular colocalization indicates associations of QD with early endosomes and lysosomes following PR9-mediated delivery. Our study demonstrates that nontoxic CPPs of varied composition provide an effective vehicle for the design of optimized drug delivery systems.