Tuning the Size of Elastin-like Polypeptide Nanoparticles

The ability to control the size of biologically-based, environmentally-sensitive colloidal nanoparticles can advance their application in areas such as drug delivery, tissue engineering, and biosensors. Controlling size is a primary task in engineering nanomaterials because many of their properties depend on size. For example, the size of particles carrying drugs affects the rate of delivery. Nanomaterials have extraordinary properties because most of their atoms are at the surface, the interface where reactions take place. With the aim of fine-tuning the size of particles, we characterize mixtures of two elastin-like polypeptide structures: a linear and a trimer configuration. Both constructs undergo phase separations above inverse transition temperatures. However, the aggregation behavior at the transition temperature depends on molecular architecture and chemical polarity. Whereas linear ELP form large aggregates which coalesce into a protein-rich phase, the trimer ELP with polar head groups form stable micelles at low salt concentrations above their transition temperatures. The mixing of these two constructs makes possible a range of sizes of stable particles through the formation of microemulsions.  The linear ELP fills the cores of the micelle aggregates, resulting in larger, stable particles.  We investigate the dependence of particle size on both the salt and linear ELP concentration across a range of temperatures using UV-vis spectroscopy and dynamic light scattering (DLS). We find that a given mixture of linear and trimer constructs has two temperature-based transitions and therefore displays three predominant size regimes. The results help to elucidate the mechanisms of ELP aggregation.