Polydopamine-based strategy towards multifunctional heterogeneous nanostructures
Date of Issue2016-03-01
School of Chemical and Biomedical Engineering
The work in this thesis takes advantage of mussel-inspired polydopamine (PDA) to develop heterogeneous nanostructures with complement and/or synergistic properties that are not available in individual building blocks. PDA, which is self-polymerized by dopamine under alkaline conditions, exhibits a unique set of physicochemical properties such as universal adhesion on virtually any solid organic and inorganic substructures, versatile chemical reactivity such as reducing activity and ready coupling with nucleophilic amine and thiol groups via Michael addition and/or Schiff base reactions, strong metal chelating capability, and highly crosslinked hydrogel structures. In three individual systems summarized in this thesis, I have demonstrated that these properties collectively offered new possibilities for preparing heterogeneous nanostructures with unique catalytic, optical, and structural properties arising from the structural integration of individual components. In particular, the universal adhesion allows PDA forming a conformal layer of coating on the nanostructures of interest, which opens the avenue for exploring the other properties towards desired functionalization; the versatile chemical reactivity can be used for reducing metal precursors and introducing surface functionalities by spontaneous chemical conjugation of amine or thiol groups. In the first system, I discovered the ability of polydopamine to direct the interfacial self-assembly of colloidal nanoparticles to form closely packed two-dimensional (2D) assembly. On the basis of this finding, recyclable heterogeneous catalysts of core–shell bimetallic nanocrystals were developed through PDA coating-directed one-step seeded growth, interfacial assembly, and substrate-immobilization of Au@Ag core–shell nanocrystals. In addition, the hydrogel structure provides excellent permeability for chemical substrate to access the coated nanocrystals for catalytic conversion. The Au@Ag@PDA nanocatalysts capitalize on a bimetallic synergistic effect, showing better catalytic activity than PDA-coated monometallic Au and Ag nanocrystals. This strategy provides new opportunities to design and optimize heterogeneous nanocatalysts with tailored size, morphology, chemical configuration, and supporting substrates for metal-catalyzed reactions. In the second system, I employed PDA for fixing the nanochains of aligned magnetic nanoparticles in an external magnetic field, which led to a new type of multifunctional 1D magnetic nanochains. The key finding is that self-polymerization of PDA around magnetically aligned nanoparticles affords robust rigid magnetic nanochains with versatile reactivity imparted by PDA. Loading of metal nanoparticles on the nanochains via localized reduction by PDA gave rise to magnetically recyclable, self-mixing nanocatalysts. Surface coupling of PDA with nucleophilic thiol and amine groups through Michael addition and/or Schiff base reactions, on the other hand, enabled easy bioconjugation of targeting ligands such as DNA aptamer for specific recognition of the nanochains to cancer cells, which led to magnetolysis of the cancer cells in a spinning magnetic field. Finally, I used PDA for constructing single-nanoparticle@metal‒organic framework (MOF) core–shell nanohybrids. In this case, the capability of polydopamine to form a robust conformal coating on colloidal substrates of any composition and to direct the heterogeneous nucleation and growth of MOFs provides new opportunities for customized structural integration of a broad range of inorganic/organic nanoparticles and functional MOFs. Furthermore, the redox activity of polydopamine adds additional possibilities to tailor the functionalities of the nanohybrids by sandwiching plasmonic/catalytic metal nanostructures between the core and shell via localized reduction. By combining different functional parts, I successfully obtained MagNP@PDA@Au@MOF nanohybrid, which can serve as a recyclable nanocatalyst with molecular sieving property. In summary, this PDA-based strategy allows us to fabricate 0D, 1D, and 2D heterogeneous nanostructures easily via epitaxial growth or self-assembly. The obtained multifunctional heterogeneous nanostructures then have been explored for many applications, involving catalysis and biomedicine.