Smart Nanoparticles and Complex Light-Matter Interactions for Single-Molecule Sensing: Investigating Multifunctional Molecular Sensing and Metal-Atom Catalysis on Gold Nanoparticles at Single Molecule Level

Abstract

This thesis investigates the use of Whispering Gallery Mode (WGM) sensors for studying molecular interactions and catalytic processes at the single molecule level, focusing on engineered plasmonic nanoparticles. The research addresses key challenges in developing a real-time single-molecule sensing platform by controlling molecular interactions between analyte molecules and gold nanorod’s surface influenced by small buffer molecules while also advancing catalytic reactions of sub-kD molecules through integrating experimental techniques and theoretical analysis. Molecular dynamics (MD) simulations and Density Functional Theory (DFT) calculations are often used to predict interactions at short timescales and determine binding energies. However, no single-molecule sensor is capable of real-time kinetic validation of these predictions. This gap limits the advancement of nanomaterials in applications such as chem-/bio-sensing and catalysis. Therefore, this thesis presents WGM sensors as a promising platform to bridge this gap by providing real-time, label-free analysis of molecular interactions. The first part of the study probes the multifunctional (phosphate, carboxylate, amine) molecules ( l1kDa), like glyphosate, binding to gold nanoparticles under different buffer conditions. The research focuses on how buffer molecules, acting as ligands, influence binding dynamics and interaction kinetics. The WGM sensor is used to monitor these interactions in real-time, while complementary DFT calculations offer a molecular-level understanding of the observed phenomena. Also, the study explores how small buffer molecules impact molecular binding and stability, investigating how these effects change by changing pH and buffer composition. This approach not only validates DFT predictions but also emphasises the significant role of small molecules, such as buffers or ligands, in influencing the binding kinetics of analytes, which is crucial for designing real-time sensors. The second part of the thesis shifts the focus to catalytic reactions on platinum-modified gold nanorods (Pt@AuNRs), exploring how nanoscale catalyst distribution influences catalytic performance. The WGM sensor is used to monitor the reduction of azobenzene via localised formic acid dehydrogenation at the catalytic site, offering detailed insights into reaction pathways and kinetics at the single-molecule level. The study investigates the impact of Pt loading, from atomic-scale deposition to layered growth, revealing how the architecture of catalytic sites affects performance. Additional experiments involving carbon monoxide poisoning and varying formic acid concentrations confirm the critical role of active Pt sites. Bulk analytical techniques like UV-Vis spectroscopy, Raman spectroscopy, and HPLC-MS further validate the catalytic processes observed in the WGM platform, supported by DFT simulations. In conclusion, this thesis highlights the importance of local environmental changes around plasmonic nanoparticles in influencing both molecular binding and catalytic reactions on engineered smart nanoparticles. The research establishes the WGM sensor platform as a versatile and advanced tool for single-molecule studies, offering a deeper understanding of interaction mechanisms and catalytic efficiency. The findings provide valuable insights into the rational design of nanomaterials, pushing the boundaries of single-molecule science and enabling targeted applications in biosensing, catalysis, and material science.