Nanomaterials have become more relevant in several sectors (environmental, energy storage, nanomedicine, etc) over recent years, where their unique properties compared to bulk are exploited. The difference in surface area is one of the reasons nanomaterials display tailorable chemical, mechanical, and physical properties due to extremely small grain size. Unfortunately, the properties of most of the nanomaterials are still to be characterized due to the need of precise instrumentation, preventing from making them convenient for consumer/industry applications. This thesis addresses the study and understanding of the relationship of nanomaterials’ optical properties and their physical structure, as well as the creation of protocols to facilitate characterization. By understanding this relationship, nanomaterials can be tailored to exploit their properties for many applications in many sectors. Based on this premise, two different systems were studied and correlated. First, the sensitivity-structure correlation of gold nanoparticles (NPs) for biosensing applications is studied. In such study, a single-nanoparticle approach was used, and statistical models were applied to correlate the refractive index sensitivity of gold NPs with their shape and size. Based on such correlation, rounder NP resulted to be more sensitive to surface events, providing insights on particle selection for biosensors. Furthermore, the nanoparticles were functionalized as DNA sensors as a proof of concept. Next, the Micro Extinction Spectroscopy development is described, as well its use as an analytical tool to study the influence of defects in 2D semiconductors. With such tool, a monolayer transition metal dichalcogenides library was create by detecting excitons with their local absorption. Finally, monolayer MoS2 was treated by both oxygen plasma exposure and hydrogen treatment to create defects and alter its band gap. The material was characterized before and after the treatment using optical spectroscopy, and information about its composition, bandgap, and optical response was revealed. In sum, the projects presented here are unified by the need for structure-function correlations, which are achieved with optical spectroscopy and electron microscopy.