Study of layer-by-layer films on thermoresponsive nanogels using temperature-controlled dual-focus fluorescence correlation spectroscopy.

While a few studies have reported on the layer-by-layer (LbL) assembly of polyelectrolytes on soft and porous templates, none have really demonstrated direct proof that the layers are actually on the template. Thermoresponsive nanogels present challenges that render a quantitative proof of successful polyelectrolyte deposition extremely difficult. Additionally, the fate of the polyelectrolyte has never been investigated during the phase transition of the coated nanogel. Here, the auto- and cross-correlation functions of a labeled polyelectrolyte assembled via the LbL technique onto soft and porous thermoresponsive labeled nanogels using dual-focus fluorescence correlation spectroscopy (2f-FCS) are presented. Performing 2f-FCS as a function of temperature, hydrodynamic radii of nanogels coated with various numbers of layers are determined, which are found to be in excellent agreement with values obtained from dynamic light scattering. This study presents irrefutable quantitative evidence of successful LbL assembly on thermoresponsive nanogels and demonstrates that the layers are not stripped off during the phase transition of the nanogels. Forster Resonance Energy Transfer (FRET) detection also supports our findings.