This review provides an overview on modern developments in optical sensing of mercuric ion with a focus on the chemical design of the optical probes. According to the mechanisms of mercury recognition, the sensing strategies are categorized into several types, namely Hg2+ participated reaction, coordination and chelation with Hg2+, “T-Hg-T” pair forming between Hg2+ and DNA, interaction between Hg2+ and other metal nanoparticles, and quantum dots-based sensors. Compared with the traditional instrumental methods, these novel optical strategies have advantages such as no requirement of expensive instrumentation, ease in sample preparation, high sensitivity, and better compatibility with in situ and remote sensing. For solution-based analysis, further research needs to put more emphasis on signal stability and reliability in complex samples to develop nano-optical sensors into practical devices, while for solid-state device integration, immobilization strategy, and stability in various sensing environments require attention.