All-Aqueous Nanoprecipitation: Spontaneous Formation of Hydrogen-Bonded Nanoparticles and Nanocapsules Mediated by Phase Separation of Poly(N-Isopropylacrylamide)

Spontaneous formation of polymer nanoparticles of well-defined, <100 nm sizes with controlled solid/hollow morphology and fluorescent properties is reported. The nanoparticle formation is assisted by temperature-triggered nucleation of an amphiphilic polymer—poly(N-isopropylacrylamide) (PNIPAM)—and mediated by hydrogen bonding of the emerged nuclei with tannic acid (TA). The pH of solution and TA/PNIPAM ratios are explored as parameters that define TA/PNIPAM assembly. Well-defined nanoparticles are formed in a wide range of neutral pH when the TA/PNIPAM ratio exceeds its critical, pH-dependent value. Dynamic light scattering and zeta potential measurements as well as atomic force microscopy and electron energy loss spectroscopy indicate that solid nanoparticles or nanocapsules are formed depending on the solution pH and that enhanced ionization of TA favors hollow morphology. Nanocapsules exhibit label-free fluorescence at neutral pH values and therefore can be useful in imaging applications.

Solid and hollow polymer nanoparticles that exhibit label-free fluorescence are self-assembled in aqueous solutions at neutral pH. The assembly is templated by nuclei of a temperature-responsive polymer and induced by formation of hydrogen-bonded complexes with a polyphenol. Both morphology and fluorescence of the assembled particles can be controlled by solution pH.

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