Tailoring the Charge and pH Responsiveness of Nanoparticles by Combining LCST Smart Polymers and Polyelectrolytes on...

The modification of nanoparticles with combinations of low critical solution temperature (LCST) polymers and polyelectrolyte (PE) layers was studied. Three n-isopropylacrylamide-co-acrylamide polymers were synthesized, in which the charges could be varied from neutral to negative or positive by incorporating polyacrylic acid (pAAc) or poly-4-vinylpyridine (p4VP) units. Polymerization was centered around a disulfide initiator, which, when reduced, produced two polymer chains and a convenient linkage to gold NPs (NPs). The resulting LCST-NP conjugates possessed sharp phase changes at critical temperatures (T_C), while the incorporation of pAAc or p4VP allowed for tunable charge and pH sensitivity. ¹H NMR and FTIR spectroscopy confirmed the polymer synthesis and NP functionalization, while UV-vis and DLS were used to monitor pH and temperature sensitivity by observing changes in the surface plasmon resonance (SPR), hydrodynamic diameter (D_h), and zeta potential (ζ-Pot). The isoelectric point of each system was determined, as was the relative buffering capacity of the LCST-NP, which indicated only a few LCST polymers per NP, which limited stability and charge. To overcome this, PE layers were deposited in a layer-by-layer (LbL) manner, forming PE/LCST-NP conjugates, which had layer-dependent charge and improved stability, while retaining the LCST properties of the base layer.