Charged colloids and oppositely charged polyelectrolytes react spontaneously slef-assembly by electrostatic interactions with formation of complexes, which may change considerably when different parameters, such as composition or ionic strength are modified. The comprehension of the underlying mechanism requires an extensive investigation of the interaction between the components, which represents a fundamental problem in soft matter self-assembly. Polyelectrolyte adsorption onto oppositely charged surfaces represents the core of the problem since it drives overcharging and charge inversion phenomena . Although the relevance of polyelectrolyte-colloid complexation has been established in a variety of systems undergoing self-assembly, its features have been always discussed in presence of colloids with fixed charge density.
Charged thermoresponsive particles with volume phase transition (VPT) give the opportunity to finely tune physical adsorption of polyelectrolytes simply by changing temperature, since VPT affects dramatically the charge density and polymer adsorption as well. Poly(N-isopropylacrylamide) (PNIPAM) is a well-known thermosensitive microgel system. It exhibits a significant VPT above the lower critical solution temperature (LCST), around 33°C in aqueous media . Many studies have shown that the VPT can be modified by addition of inorganic salt, and other macromolecules. Moreover, both VPT and swelling/deswelling behavior are also controlled by the introduction of charged groups to microgel networks, thus offering the opportunity to tune charge density just by changing temperature.
In this work we exploit the unique features of PNIPAM microgels to study their complexation with e-polylysine (e-PLL) a short biocompatible polycation. We employ a combination of light scattering, TEM-AFM microscopy, electrophoretic and dielectric spectroscopy measurements to characterize e-PLL/PniPam complexes. We show that complexation is driven by the microgels VPT and a large overcharging occurs only for T > TLCST where bare microgels collapse and a negative electrophoretic mobility is built. Polyelectrolyte adsorption gives rise to a reentrant condensation of microgels for T > TLCST, as opposed to a continuous enhancement of particle condensation observed for monovalent salt. This peculiar electrostatically-driven controlled aggregation is here tuned by the VPT-transition of PNIPAM and opens new intriguing scenarios for the controlled self-assembly of soft colloids.
 F Bordi et al J. of Physics: Cond. Matter, 2009 21:203102 (Topical Review); S. Sennato et al Coll Surf B 2016 137,109.
 A. Fernandez-Nieves et al Microgel Suspensions John Wiley & Sons (2011)
 D Truzzolillo, S. Sennato et al (in preparation)
Acknowledgements S. S. thanks E. Zaccarelli for scientific discussions and funding from ERC-MIMIC project.