Regulation of Interparticle Interactions Enables the Formation of Functional Nanoparticle Assemblies

Abstract

Self-assembly is undoubtedly one of the most efficient ways to create complex materials with distinct functions. The ability to control the interparticle interactions between the building blocks is the key to achieving the desired outcomes from a self-assembly process. This requires the smart choice of building blocks that respond to stimuli in a precise fashion. Nanoparticles (NPs) are a unique class of building blocks, as they offer both their surface and core to install the desired interactions and functions. In the proposed thesis work, the surface of plasmonic NPs has been appropriately functionalized with the ‘ligand of choice’ to regulate their interaction with biomolecules, leading to the formation of ‘functional’ bio-nano hybrids. Further, the impact of such finely tuned interactions in NP self-assembly processes under equilibrium as well as non-equilibrium domains was studied. First, we have prepared a multifunctional bioplasmonic network via a long-range electrostatically directed self-assembly process between positively charged NPs and negatively charged ATP molecules in the presence of CaCl2. The screening of charges by Ca2+ ions and the coordination between ATP- Ca2+ helped in regulating the strength of electrostatic attraction between oppositely charged ATP molecules and AuNPs. This turned out to be decisive in transforming an uncontrolled aggregation (instant precipitation) into a kinetically controlled self-assembly process (bioplasmonic network). ATP and AuNP retained their inherent properties in the bioplasmonic network, thereby enabling their use for various catalytic, photocatalytic, and SERS-based applications. In another study, we observed that the ‘dead’ ATP-AuNP precipitated state slowly transformed into the ‘live’ bioplasmonic network. Detailed microscopic and spectroscopic studies revealed mechanistic insight into the role of the self-association property of ATP molecules in transforming the kinetically trapped precipitate into an active bioplasmonic network. This motivated us to explore the dynamic nature of the ATP-AuNP precipitate to construct non-equilibrium self-assembled systems through the sequential and tandem action of two competing phosphoryl transferase enzymes (hexokinase and creatine phosphokinase that consume and generate the aggregating trigger ATP, respectively). The enzymatically driven dynamic conversion between ATP and ADP molecules helped in regulating the electrostatic interactions between AuNP and aggregating trigger. After establishing the decisive role of ligand chemistry and interplay of forces, our next focus was to incorporate the thermoplasmonic property of core-NPs in achieving multi-responsive dynamic self-assembled system. The assembly step was triggered by the autonomous dissolution of moisture in the hygroscopic mixture of thymine-capped AuNPs in DMSO medium, leading to the formation of controlled aggregates. The sunlight-triggered plasmonic heat generated from the thymine-AuNPs in the controlled aggregates was used as the thermal energy source for the evaporation of water from the solvent mixture, thereby triggering the disassembly process. The use of thermoplasmonic property enabled the coupling of light into the solvent-mediated dynamic self-assembly process. Thus, the use of both surface and core properties of NPs helped in imparting desirable functions of dynamicity, adaptability, multi-stimuli-responsiveness, and multifunctionality in the NP self-assembly process. In short, fine control over the assembly pathways enabled us to achieve various static and dynamic self-assembled structures from a limited set of NP building blocks.