Reversible meso-scale smart polymer--protein particles of controlled sizes.

Functionalized beads and particles in the size range of tens to hundreds of nanometers (nano- to meso-scale) are finding increased applications in the bioanalytical field. We show here that conjugates of streptavidin and the temperature-responsive polymer poly(N-isopropylacrylamide) (PNIPAAm), synthesized with low polydispersities by reversible addition--fragmentation chain transfer (RAFT) polymerization, rapidly formed mesoscale polymer--protein particles above the lower critical solution temperature (LCST). The average hydrodynamic diameters of these particles could be controlled between 250 nm to 900 nm by the choice of conjugate concentration and polymer molecular weight, and/or through control of the rate of temperature change. Once formed, the biohybrid particles were found to be stable for >16 h at the controlled size, unlike the free PNIPAAm which continued to aggregate and grow over time into very large and polydisperse aggregates. The reversibility between the smart polymer--protein particles and the free polymer--protein conjugates opens potential uses in traditional diagnostic formats and in microfluidic formats where the differential diffusive and physical properties might be exploited for separations, analyte concentration, and signal generation.