Regulating Interparticle Interactions to Control the Spatio-Temporal Assembly of Gold Nanoparticles

Abstract

Self-Assembly is nature’s preferred ‘zero-waste’ means of creating animate matter. It typically involves the realization of functional materials from individual building blocks, without the need for any external or human intervention. Researchers are focused on understanding the principles underlying the self-assembly process, so as to form purposeful and useful structures, despite the lack of human intervention. In light of this, the aim of my thesis is to study the effects of finely tuned interparticle interactions in governing the outcomes of both spatial, as well as temporal self-assembly processes. We demonstrate that a control over interparticle interactions can be successfully translated to create systems with fascinating degrees of complexity. This thesis contains a summary of our efforts that show how finely tuned interparticle interactions can (a) improve existing nanoparticle (NP) properties, (b) show the emergence of inherently absent properties, and (c) mimic complex ‘life-like’ behaviour. Here, the property of our choice was aggregation mediated identification of heavy metal ions, while the behaviour of our choice was the formation of transiently stable self-assembled structures.

With these specific goals in mind, in Chapter 2, we designed heterogeneously charged gold nanoparticles ([+/-] AuNPs), where, strengths of different interparticle interactions could be regulated by simply changing the ratio of oppositely charged ligands on the NP surface. We used these NP systems, to balance different attractive and repulsive interparticle interactions and reveal an unprecedented phenomenon of controlled aggregation. These NP systems could reversibly ‘arrest’ toxic ions like lead (Pb2+) and cadmium (Cd2+) through the formation of controlled aggregates, making them a recyclable trapping and scavenging system. A key advantage of the present system is the simplicity with which the mixed-Self-Assembled Monolayer (m-SAM) on the NPs could be tuned to trap and scavenge different triggers of interest (like Pb2+, Cd2+, H+, and citrate). More importantly, we showed that the regulation of interparticle interactions could impart a new function of selectivity towards trapping of toxic ions over biologically relevant ones. These initial signs of selectivity encouraged us to design an identification protocol capable of identifying a specific M2+ ion.

With this challenge in mind, in Chapter 3, we worked towards introducing the notion of selectivity towards strongly binding divalent metal ions (M2+), to inherently nonselective carboxylate functionalized gold nanoparticles ([-] AuNPs). Here, we chose the abilities of M2+ ions to break the interactions between the oppositely charged AuNPs (in a nanoionic precipitate) as the means of identification, rather than the conventional idea of forming an interaction. We observed that out of all the M2+ ions tested, only Pb2+ could break the electrostatic interactions in the nanoionic precipitates, and release [+] AuNPs to the solution (turn-on response). Interestingly, both [+] and [-] AuNPs, despite being ‘blind’ in terms of selectivity toward M2+ ions, gave rise to an assembled state that showed remarkable selectivity towards Pb2+ ions. Furthermore, by tuning the strengths of interparticle interactions, the sensitivity, as well as selectivity of our identification protocol could be improved to ~4 μM. Note that traditionally, similar tasks of selective identification are undertaken with the help of analyte-specific ligands, where the property of selectivity is simply 'added on' to the NPs. This work, therefore, showed a conceptually different strategy of identification, where, the self-assembled state shows the emergence of selectivity.

In Chapters 2, and 3, we demonstrate that to create nanosystems with desirable or improved properties, one need not have to come up with novel materials. A careful understanding, and control over different interparticle interactions can help in not only improving the known properties of NPs (Chapter 2), but can impart inherently absent properties to NPs (Chapter 3). These findings motivated us to use our control over interactions to install life-like properties to a NP system. More specifically, we thought of creating systems that come into existence only for a limited amount of time (transient self-assembly). In order to mimic the formation of such systems, in Chapter 4, we demonstrate a fundamental discovery of creating self-assembled structures that show transient switching/ shuttling between completely precipitated and redispersed stages of nanoparticles (NPs) – a first of its kind in plasmonic NPs. The chemical trigger driven transient self-assembly was accomplished by using the temporal control over electrostatic attractions between positively charged gold nanoparticles ([+] AuNP) and negatively charged EDTA (chemical trigger). Consequently, some of the desirable feats in the field of transient self-assembly were realized such as easy removal of waste, formation of a transiently stable precipitate state and negligible dampness in redispersion. We also reveal the so far unknown ability of atmospheric components to transform a mundane mixture of chemicals into a dynamically active one – a task usually accomplished with a network of chemical reactions.

In summary, my thesis demonstrates the effectiveness of establishing a control over interactions between the building blocks as a potent way of creating intelligent passive as well as active states.