Phosphate-Dependent Colloidal Stability Controls Nonendocytic Cell Delivery of Arginine-Terminated Nanoparticles

Although arginine-rich polymers and peptides are extensively used as delivery carriers for drugs/proteins/nanoparticles, their cell delivery mechanism is not clearly understood. Recent studies show that arginine-terminated nanoparticles can enter into a cell via a nonendocytic approach that involves direct membrane penetration. However, poor colloidal stability of arginine-terminated nanoparticles under physiological conditions restricts their application potential. Here, we show that the nonendocytic cell delivery of arginine-terminated nanoparticles is controlled by their colloidal stability in the presence of phosphates. We have designed arginine-terminated quantum dots (QDs) of 10-15 nm hydrodynamic size, which enter into the cell via a nonendocytic approach, provided that they are colloidal and dispersed during cellular uptake. We have demonstrated that arginine-terminated QDs rapidly precipitate in the presence of monophosphates or polyphosphates, and polyphosphates have a stronger effect than monophosphates. Introducing polyethylene glycol at the QD surface can improve the colloidal stability against phosphates. Control experiments show that amine/ammonium-terminated cationic QDs of similar sizes do not have such a type of phosphate-dependent precipitation issue. We propose that arginine-terminated colloidal nanoparticles have a unique advantage over amine/ammonium-terminated nanoparticles as they can bind with the cell membrane phosphate via guanidinium-phosphate salt bridging. Bulk phosphate provides reversibility in this binding interaction so that nonendocytic cell uptake occurs via charge compensation of cationic nanoparticles without membrane damage. The developed surface chemistry approach and the proposed mechanisms can be adapted to other nanoparticles for efficient cell delivery and for designing delivery carriers.