Mercury is a global pollutant that is easily absorbed into the atmosphere, long-lasting in the environment and damaging to human health and the ecosystem. In the environment mercury circulates through a biogeochemical cycle, first by evaporation into the atmosphere, and further into the hydrosphere and the biosphere. The monitoring of the concentration of mercury in the environment is extremely important because it is a highly toxic substance that adversely affects the central nervous system. With the advancement of technology, chemical sensors are designed to fit to the size of the micrometer and give a quick response to the concentration of mercury ions in a sample. The chemical sensor consists of the receptor and the conversion part, and the task is to turn the "chemical signal" into an analytically useful signal. The optical sensor is a subtype of a chemical sensor based on an optical change visible to the naked eye. Most optical sensors are based on fluorescence, luminence, light disperse, absorbanation, etc. The paper describes methods for determining small concentrations of mercury in the sample. Methods are: Colorimetric detection of mercuric ion in aqueous media using DNA-functionalized gold nanoparticles, Optical sensor for the visual detection of mercury using mesoporous silica anchoring porphyrin moiety, A highly selective and simple fluorescent sensor for mercury (II) ion detection based on cysteamine-capped CdTe quantum dots synthesized by the reflux method. The most recent research has been described in order to detect the mercury ions in the living organism.