Over the past decades, there has been a dramatic change in the types and amounts of metal ions in the biosphere, since we have introduced huge amount of metal waste into our environment in the course of industrialization of our society. Most of the metal cations persist in ecological systems and in the food chain, exposing top-level predators. Therefore, there is a huge need to develop sensitive, cost-effective, portable, and rapid sensing systems for continuous monitoring of the metal ions in various aquatic environments. The introduction of gold nanoparticles to the field of metal ion sensing has offered extensive opportunities to the design of miniaturized sensors with improved selectivity, limit of detection, signal-to-noise ratio, and response time. The ability to tune nanoparticles’ properties by simple chemical modifications of their core or ligand shell was one of the key for their early successes in this field. In particular, the surface of gold nanoparticles (AuNPs) can be decorated with a wide range of organic ligands, which enables their selective interaction with a specific metal ion. A previous study from our group demonstrated that mixed-ligand coated gold nanoparticles in specific size and ligand compositions ranges were able to capture metal cations selectively. The ultimate goal at this thesis was to develop an understanding of such mixed-ligand coated nanoparticles’ binding to metal cations. To achieve this goal, the ion binding capability of mixed-ligand coated AuNPs were systematically studied for nanoparticles of different ligand shell composition, and core size of the nanoparticles. Our results revealed that the selectivity and the binding ability of the studied particles to the cation of interest (here Cd+2) is strongly influenced by the parameter studies. We also found that the molecular conformation of the ligands for the particles in dry state has an unexpected and significant effect on the binding. In order to summarize the results obtained, a phase diagram was constructed. Understanding the concept behind the selectivity of our AuNPs in terms of ligand-shell conformation, which highly depends on the ligand composition and the size of AuNPs will enable researchers to develop selective sensors for detection of various metal ions.