Hydrophobic Monomer Incorporation Tunes the pH Responsivity of Polycationic Brushes

Abstract

We tune the pH-responsive swelling response of polycationic brushes via the incorporation of hydrophobic repeat units. Careful compositional tuning realizes apparent brush pK_a values, which we term pK^*_a, between 5.0 and 5.7. Further, we observed monotonic decreases in pK^*_a with an increase in hydrophobic monomer incorporation. Although brush pK^*_a was readily tuned with composition, the cooperativity of deprotonation was influenced only by polycation identity and not hydrophobic monomer incorporation. Interestingly, hydrophobicity-driven pK^*_a decreases were observed exclusively for brushes and not untethered, compositionally matched polymers, suggesting that confining polymers in the form of nanoparticle-tethered brushes might grant more fine-tuned control over pH-responsivity than the tunability offered by free polymers alone. We readily reproduced pK^*_a trends from planar substrates to nanoparticle-tethered brushes, potentiating applications in intracellular pH sensing and nucleic acid delivery that demand precise pK_a tuning. We will apply pK^*_a-tuned brush-grafted nanoparticles to overcome intracellular barriers that have previously hindered free polymers.