pH-Responsive Polymer Nanoparticles Containing Pendant Carboxy and Sulfonate Groups

Herein, pH-responsive nanoparticles (P(AaHn/AMPS)NPs; n = 89, 79, and 59 mol %) were prepared via soap-free emulsion polymerization using pH-responsive 6-acrylamidohexanoic acid (AaH), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), and a cross-linker. Incorporating the hydrophilic AMPS comonomer improved the colloidal stability of P(AaHn/AMPS)NPs over a wide pH range. At pH ≤ 6.4, protonation of the pendant carboxy groups of the AaH units enhanced hydrophobic interactions, thereby decreasing particle size and enabling encapsulation of hydrophobic guest molecules, such as N-phenyl-1-naphthylamine. At pH > 6.4, deprotonation of the carboxy groups increased the osmotic pressure, causing particle swelling and concomitant release of the guest molecules. Notably, P(AaH89/AMPS)NPs exhibited pronounced swelling, with their hydrodynamic radius (R_h) increasing from 119 to 613 nm as the pH changed from 3 to 12, corresponding to an approximately 137-fold increase in volume. Over the same pH change, the zeta potential of P(AaH89/AMPS)NPs became more negative, changing from -45 to -55 mV. A higher AaH content led to considerable changes in both swelling degree and zeta potential. Furthermore, pH-response kinetics were evaluated using a continuous-flow mixing method, which reveal that P(AaHn/AMPS)NPs completed their response within 1.16-2.37 s, with acidification (from pH 12 to pH 3) occurring faster than basification (from pH 3 to pH 12).