Leveraging Swelling Polymer Nanoparticle Reversibility for Cargo Loading

pH-responsive polymeric nanoparticles are an exciting class of stimuli-responsive materials that can respond to changes in pH and, as a result, have been developed for numerous applications in biomedicine, such as the loading and delivery of various cargoes. One common transformation is nanoparticle swelling due to the protonation or deprotonation of specific side chain moieties in the polymer structure. When the pH trigger is removed, the swelling can be reversed, and this process can be continually cycled by adjusting the pH. In this work, we are leveraging this swelling-deswelling-reswelling mechanism to develop a simple, fast, and easy loading strategy for a class of cross-linked polymeric nanoparticles, poly-2-(diethylamino) ethyl methacrylate (pDEAEMA), that can reversibly swell below pH 7.3, and a dye, rhodamine B isothiocyanate (RITC), as a proof-of-concept cargo molecule while comparing to poly(methyl methacrylate) (pMMA) nanoparticles as a nonswelling control. A free radical polymerization was used to generate pDEAEMA nanoparticles at three different sizes by varying the synthesis temperature. Their pH-dependent swelling and deswelling were extensively characterized using dynamic light scattering and transmission electron microscopy, which revealed a reversible increase in size for pDEAEMA nanoparticles in acidic media, whereas pMMA nanoparticles remain constant. Following dye loading, pDEAEMA nanoparticles show significant fluorescence intensity when compared to pMMA nanoparticles, suggesting that the reversible swelling is key for successful loading. Upon acidic treatment, there is a significant decrease in the fluorescence intensity when compared to the dye-loaded nanoparticles in basic media, which could be due to dilution of the dye when released in the acidic medium solution. Interestingly, nanoparticle size had no impact on dye loading properties, suggesting that the dye molecules only go so far into the polymer nanoparticle. Additionally, confocal microscopy images reveal pDEAEMA nanoparticles with higher RITC fluorescence intensity in acidic media but a lower RITC fluorescence intensity in basic media, while pMMA nanoparticles show no differences. Together, these results showcase a size reversibility-driven cargo loading mechanism that has the potential to be applied to other beneficial cargoes and for various applications.