We employ fluorescence correlation spectroscopy (FCS) and coarse-grained molecular dynamics simulations to study the mobility of tracers in polymer solutions. Excluded volume interactions result in crowding-induced slowdown, depending only on the polymer concentration. With specific tracer-polymer attractions, the tracer is slowed down at much lower concentrations, and a second diffusion component appears that is sensitive to the polymer chain length. The two components can be resolved by FCS, only if the distance traveled by the tracer in the polymer-bound state is greater than the FCS focal spot size. The tracer dynamics can be used as a sensitive probe of the nature and strength of interactions, which-despite their local character-emphasize the role of chain connectivity.

Experimental sectionFigure.S1. Normalized fluorescence intensity profile, I F (z)/I F,max , for A488 in HG-6 (white) and in the supernatant solution (grey) vs. distance z normal to the substrate (grey), at different ionic strength (I) values of a monovalent salt (KNO 3 ) , at 25°C. The grey highlighted regions denote the bounds of the fully swollen HG thickness at each I value.Interpretation of the z-scans: For the fitting procedure, we consider a gel grafted on a glass slide such that the glass-gel interface spans the xy plane and z is the direction normal to the xy plane. We further assume that the interface between the gel and the glass slide is located at z = A and the interface between the gel and the supernatant solution is located at z = B. We further assume that the tracer concentration

The permeation and translational diffusion of antibodies through the porous matrix of hydrogel materials is of fundamental relevance for many biological systems in living nature, but equally important in medical and technological applications, such as implanted drug release systems and biosensors. In this respect the diffusion of fluorophore-labeled protein immunoglobulin G (IgG) in micrometer thick, grafted hydrogel layers based on thermoresponsive poly­(N-isopropyl­acrylamide) (pNiPAAm) is studied here by fluorescence correlation spectroscopy (FCS). The pore size of the gel gradually changes with its swelling state, which is controlled by the cross-link density of the network, temperature, and pH value of the surrounding medium. Notably, IgG permeation in these hydrogel layers exhibits a much more complex dependence on these factors. This rich variability of IgG permeation is attributed to the varying balance of protein interactions with the polymer network through electrostatics, controlled pH-dependent protein ionization, excluded volume repulsion, and hydrophobic attraction. A combined analysis of the fluorescence intensity profiles and the dynamics monitored by FCS allows us to quantify the thermodynamically controlled partitioning of IgG as well as the slowdown of its diffusion. Contrary to the complex behavior of the permeation, the diffusion slowdown seems to be a universal function of polymer volume fraction, which is rather robust with respect to temperature or pH changes. The presented findings suggest a model approach to explore the synergy between crowding and thermodynamics with respect to the controlled protein transport in pNiPAAm-based hydrogels.

The water vapor-induced swelling, as well as subsequent phase-transition kinetics, of thin films of a diblock copolymer (DBC) loaded with different amounts of the salt NaBr, is investigated in situ. In dilute aqueous solution, the DBC features an orthogonally thermoresponsive behavior. It consists of a zwitterionic poly(sulfobetaine) block, namely, poly(4-(N-(3′methacrylamidopropyl)-N,N-dimethylammonio) butane-1-sulfonate) (PSBP), showing an upper critical solution temperature, and a nonionic block, namely, poly(N-isopropylmethacrylamide) (PNIPMAM), exhibiting a lower critical solution temperature. The swelling kinetics in D 2 O vapor at 15 °C and the phase transition kinetics upon heating the swollen film to 60 °C and cooling back to 15 °C are followed with simultaneous time-of-flight neutron reflectometry and spectral reflectance measurements. These are complemented by Fourier transform infrared spectroscopy. The collapse temperature of PNIPMAM and the swelling temperature of PSBP are found at lower temperatures than in aqueous solution, which is attributed to the high polymer concentration in the thin-film geometry. Upon inclusion of sub-stoichiometric amounts (relative to the monomer units) of NaBr in the films, the water incorporation is significantly increased. This increase is mainly attributed to a salting-in effect on the zwitterionic PSBP block. Whereas the addition of NaBr notably shifts the swelling temperature of PSBP to lower temperatures, the collapse temperature of PNIPMAM remains unaffected by the presence of salt in the films.