Ionization-Induced Reversible Aggregation of Self-Assembled Polycarbonyl Hydrazide Nanoparticles: A Potential Candidate for Turn-On Base Sensor and pH-Switchable Materials

Hierarchical assembly of nanostructures remains one of the desirable targets in nanoscience. Herewith, we report a hydrogen-bond-promoted polymeric nanoparticle (NP) system that reversibly aggregates into different microstructures upon variation of the concentration of the base in the medium. Polycarbonyl hydrazide, a polyaza-Michael adduct, formed uniform spherical NPs in solution owing to the presence of inherent CO---HNCO hydrogen-bond-based physical cross-links in the system. In the presence of the base, the CONH groups ionized to form the corresponding nitranions, and the resulting anion−π interaction between the ionic polymer NPs promoted the secondary aggregation to different shapes and sizes in the microdomain. The shape of the aggregated microparticles gradually changed from spherical to fiber through flakes upon a gradual increase in the base concentration in the medium. The modulus of these superstructures notably decreased compared to that of the original un-ionized NPs, suggesting the involvement of anion−π interaction and loss of hydrogen bonding in the system. Importantly, these dynamic shape changes in the submicron range were reversible, and the addition of a protic solvent or acid recovered the original shape and size. PBTH in sufficiently low concentration (40 μg/mL) is capable of detecting various organic and inorganic bases in the ppm level and pH values between 8.4 and 11.4 with 1.0 precision. The polymer is also a promising candidate for pH-switchable applications.