Reversible Self-Assembly of Gold Nanoparticles Based on Co-Functionalization with Zwitterionic and Cationic Binding Motifs*

Hierarchically Responsive Alternating Nano-Copolymers with Tailored Interparticle Bonds

Self-assembly of inorganic nanoparticles (NPs) is an essential tool for constructing structured materials with a wide range of applications. However, achieving ordered assembly structures with externally programmable properties in binary NP systems remains challenging. In this work, we assemble binary inorganic NPs into hierarchically pH-responsive alternating copolymer-like nanostructures in an aqueous medium by engineering the interparticle electrostatic interactions. The polymer-grafted NPs bearing opposite charges are viewed as nanoscale monomers ("nanomers"), and copolymerized into alternating nano-copolymers (ANCPs) driven by the formation of interparticle "bonds" between nanomers. The resulting ANCPs exhibit reversibly responsive "bond" length (i.e., the distance between nanomers) in response to the variation of pH in a range of ~7-10, allowing precise control over the surface plasmon resonance of ANCPs. Moreover, specific interparticle "bonds" can break up at pH≥11, leading to the dis-assembly of ANCPs into molecule-like dimers and trimers. These dimeric and trimeric structures can reassemble to form ANCPs owing to the resuming of interparticle "bonds", when the pH value of the solution changes from 11 to 7. The hierarchically responsive nanostructures may find applications in such as biosensing, optical waveguide, and electronic devices.

Examination of Oil Structural Motifs and Temperature in Promoting Reversible Self-Assembly of Gold Nanoparticle Langmuir Films

The interaction between gold nanoparticles (AuNPs) with other components and phases has important consequences on their use in materials and devices as well as their fate in the environment or at biological interfaces. Previously we determined that long oil chain lengths and lower temperatures optimized the mixing of n-alkanes with alkanethiol-capped AuNPs which improved nanoparticle self-assembly into superlattices at aqueous interfaces. In this study, a variety of liquid phase hydrocarbon oils with structural and functional variations were surveyed for their mixing efficacy and propensity to enable reversible self-assembly of nanoparticle domains. Transmission electron microscopy (TEM) images and pressure vs area isotherms across this series reveal isotherm features that distinguish between the mixing and inclusion of the oil at the interface and that which enables reversible self-assembly. Structural and functional characteristics of the oil for promoting reversible self-assembly are identified which surpass the importance of chain length previously described. Temperatures below the ligand order-disorder transition were found to improve the reversibility of AuNP domains and are understood by application of a reparametrized x-DLVO model.

Reversible Self-Assembly of Gold Nanoparticles Based on Co-Functionalization with Zwitterionic and Cationic Binding Motifs*

We report a pH‐ and temperature‐controlled reversible self‐assembly of Au‐nanoparticles (AuNPs) in water, based on their surface modification with cationic guanidiniocarbonyl pyrrole (GCP) and zwitterionic guanidiniocarbonyl pyrrole carboxylate (GCPZ) binding motifs. When both binding motifs are installed in a carefully balanced ratio, the resulting functionalized AuNPs self‐assemble at pH 1, pH 7 and pH 13, whereas they disassemble at pH 3 and pH 11. Further disassembly can be achieved at elevated temperatures at pH 1 and pH 13. Thus, we were able to prepare functionalized nanoparticles that can be assembled/disassembled in seven alternating regimes, simply controlled by pH and temperature.