Selectivity in nanoreactors: A hybrid yolk–shell nanostructure that contains gold nanoparticles in the core and thermosensitive microgel poly(N‐isopropylacrylamide) (PNIPA) as shell is presented. The catalytic selectivity of Au‐PNIPA nanoparticles for the reduction of hydrophilic 4‐nitrophenol and more hydrophobic nitrobenzene with NaBH4 can be tuned through the volume transition of PNIPA shell (see picture).

We report on a study of spherical polyelectrolyte brushes that consist of a solid core onto which linear polyelectrolyte chains are chemically grafted. The core particles are made up of solid poly(styrene) and have a radius R of ca. 50 nm. As polyelectrolyte chains the weak polyelectrolyte poly(acrylic acid) or the strong polyelectrolyte poly(styrenesulfonate) was used. These chains were generated directly on the surface of the core particles by a grafting-from technique. Hence, the chains are chemically bound to the surface but can be cleaved off and analyzed separately. The contour length L(c) and the number of grafted chains per unit area sigma can thus be determined accurately. The thickness L of the brush layer on the surface has been determined by dynamic light scattering. It is measured for different L(c)/R as a function of pH, ionic strength, and valency of counterions. Annealed brushes exhibit a transition with increasing pH in which the chains are stretched to nearly full length. This can be traced back to the building up of the osmotic pressure of the counterions. The brush height L decreases considerably with increasing ionic strength, most notably when adding divalent ions. The entire set of L as a function of R, L(c), and sigma can be fully explained in terms of a simple two-parameter theory developed by Hariharan et al. [Macromolecules 31, 7514 (1998)].

The synthesis and characterization of latex particles consisting of a poly(styrene) (PS) core and a shell of linear poly(acrylic acid) (PAA) chains are described. The particles have been synthesized by photoemulsion polymerization of acrylic acid on PS core particles containing a thin shell of the photoinitiator 2-[p-(2-hydroxy-2-methylpropiophenone)]-ethylene glycol-methacrylate (HMEM). The particles thus obtained have a defined core-shell morphology with a narrow size distribution. The dependence of the hydrodynamic radius of the fully dissociated PAA chains on the ionic strength is in qualitative accord with recent theoretical predictions.

The binding of bovine serum albumin (BSA) and β-lactoglobulin (BLG) to TTMA (a cationic gold nanoparticle coupled to 3, 6, 9, 12-Tetraoxatricosan-1-Aminium, 23-mercapto-N, N, N-TriMethyl)- was studied by high-resolution turbidimetry (to observe a critical pH for binding), dynamic light scattering (to monitor particle growth), and isothermal titration calorimetry (to measure binding energetics), all as a function of pH and ionic strength. Distinctively higher affinities observed for BLG vs. BSA, despite the lower pI of the latter, were explained in terms of their different charge anisotropies, namely the negative charge patch of BLG. To confirm this effect, we studied two isoforms of BLG that differ in only two amino acids. Significantly stronger binding to BLGA could be attributed to the presence of the additional aspartates in that negative charge domain for dimer, best portrayed in DelPhi. This selectivity decreases with ionic strength, for which both isoforms bind well below pI, but increases with ionic strength for BLG vs. BSA which binds above pI. This result points to the diminished role of long-range repulsions for binding above pI. Dynamic light scattering reveals a tendency for higher-order aggregation for TTMA–BSA at pH above the pI of BSA, due to its ability to bridge nanoparticles, whereas soluble BLG–TTMA complexes were stable over a range of pH because the charge anisotropy of this protein at pH < pI makes it unable to bridge nanoparticles. Finally, isothermal titration calorimetry shows endoenthalpic binding for all proteins; the higher affinity of TTMA for BLGA vs. BLGB comes from a difference in the dominant entropy term.