The surface chemistry of nanoparticles: towards biological and engineering applications

Nanomaterials are gaining large amounts of traction due to the enhancement of certain properties that materials exhibit when they exist on the nanoscale. In particular, gold nanoparticles (Au NPs) ranging from 1 – 100 nm in size show great promise in fields ranging from sensing to cancer photothermal therapy due to its interactions with light. These interactions can be tuned such that Au NP solutions will undergo colorimetric changes when exposed to certain materials, or will radiate heat intense enough to induce cell death. All of these applications are only possible through fine-tuning of the surface chemistry of the nanoparticle and precise control over the chemical functionalities displayed. However, the interactions between these surface chemistries and biological systems are not fully quantified; therefore it remains difficult to predict the behavior of cells or tissues in response to exposure to NPs, as well as the response of NPs to a new environment. In order for these applications to be realized, the interface between biology and nanotechnology must be better understood. When nanomaterials enter biological systems, they are immediately surrounded by proteins, which form a ‘protein corona’. Past research has determined that how NPs are processed by the body (and how the body responds to the presence of NPs) is largely dependent on the composition of the protein corona. Furthermore, proteins adsorbed to the surface of NPs can have their functions altered, upsetting cellular homeostasis. Thus, understanding the changes that proteins undergo upon adsorption to NPs is crucial to understanding the driving factors behind the formation of a protein corona, and can lead to better predictability and control over the formation of the corona. This dissertation focuses on the effects of surface chemistry of Au NPs on protein adsorption and conformation, as well as on NP deposition behavior. Chapter 1 will introduce the element gold, its evolution throughout history, and the ways in which it’s currently being used in industry and biomedical science. This chapter will also introduce the concept of nanomaterials, and cover the basis on which many applications are developed. Au NPs exhibit strong tunable absorbance of visible and infrared light called plasmon resonances. Synthesis and surface chemistry modification techniques that allow for creation of precisely defined NPs are also introduced. Challenges facing the application of NP technologies are discussed, as well as an overview of this thesis. Le Chatelier’s Principle, which describes the behavior of systems whose equilibrium are disturbed, is applied in Chapter 2 to model protein adsorption to gold nanorods (Au NRs). In this study, it is found that the wrapping of polyelectrolyte polymers around highly curved nanomaterials does not change its thermodynamic interactions with certain proteins. Two protein/polyelectrolyte pairs were selected for their thermodynamic behavior to demonstrate Le Chatelier’s Principle applied on the nanoscale. Protein adsorption and desorption are observed upon plasmonic heating by laser excitation of the Au NRs; these behaviors match well with predictions made through Le Chatelier’s Principle. Furthermore, it is shown that this simple model can be used to describe complex systems, such as NRs in fetal bovine serum. In Chapter 3, we focus on the interactions between the protein α-synuclein (α-syn) and spherical Au NPs. α-Syn is a protein of great interest not only because of its ability to take different structural conformations in different environments, but also because of its strong link to Parkinson’s Disease. By using nuclear magnetic resonance (NMR) techniques, the binding orientation of α-syn is deciphered on the amino acid level. Additionally, we show that the binding orientation of α-syn can be reversed by changing the surface chemistry of the Au NP, which was orthogonally corroborated by molecular dynamics simulation. This study provides the grounds for using Au NPs as a platform to study protein adsorption and behavior. The binding of α-syn to lipid vesicles mimics is discussed in Chapter 4. One important trait of this protein that has been previously identified is its strong interaction to anionic vesicles. SDS coated Au NPs of different sizes displaying an architecture similar to anionic vesicles were created to model vesicles with variable radii of curvature. Creation of such NPs provides a guarantee that vesicle size and shape does not change upon protein binding, which cannot be said about vesicles created with phospholipid molecules. Furthermore, two α-syn mutants are included in this study to highlight the differences in protein binding as a function of NP size and protein mutation. Using optical methods in conjunction with protein digestion techniques, a general picture of this system is built, and effects of protein mutations highlighted. Chapter 5 turns towards materials chemistry, and explores the role that surface chemistry plays when NPs are deposited onto textured substrates. Textured substrates can be used to wrinkle 2D materials such as graphene or MoS2, imparting new properties that are able to drive new technologies. By adjusting the surface chemistry, as well as a number of other parameters, the deposition behavior of NPs can be precisely controlled. Deposited NPs are analyzed using optical and electron microscopy, as well as various forms of spectroscopy. Until now, achieving structures such as the ones demonstrated in this Chapter are most commonly done using lithographic techniques, which is time-consuming and expensive. Deposition and alignment of NPs using this method offers the ability to create substrates decorated with NPs on a large scale with ease