The use of main-group elements in sensing applications has expanded greatly in recent years. The Lewis acidic character of many main-group elements has allowed for the development of sensors for an array of substrates, including halide ions, and small molecules such as nitrous oxide and carbon monoxide. Another key class of substrates for such sensors are metal ions. The sensing of metal ions used in biological processes, such as iron and copper, is vital for medical contexts whereby either an excess or deficiency can cause health issues. The ability to sense metal ions not directly involved in biological processes, such as aluminium, also retains importance due to their toxicity at even low concentrations. Therefore, sensors that can detect these metals quickly and selectively is of widespread interest. Various analytical methods can be employed to sense such ions, such as mass spectrometry, atomic absorption spectroscopy, or electrochemical techniques. However, sensing materials based on fluorescence remain particularly popular owing to their high sensitivity, relatively lower cost, and rapid analysis. This has allowed for the development of a variety of molecular systems able to sense metal ions in solution with fluorescence spectroscopy, via either a 'turn-off' or 'turn-on' response. While numerous systems have been developed as fluorescent metal sensors, there are far fewer examples of fluorescent metal sensors that feature main-group elements, such as phosphorus. Such systems typically rely on a phosphorus-based functional group that is responsible for interacting or binding with the metal, taking advantage of the wellestablished donor abilities of trivalent phosphines or pentavalent P=O containing moieties, such as phosphine oxides or phosphoric acid derivatives. This is tethered to a fluorescent backbone, allowing for a 'turn-off' or 'turn-on' response in the presence of metal ions (Figure ). In 2016, Christianson and Gabbaï reported a phosphine-decorated fluorescein, in which an arylphosphine was linked to the fluorescein backbone via an o-phenylene group. Coordination of the phosphine to Au 3+ yielded a 'turn-on' fluorescence response, and use of the system as a sensor for Au 3+ in water. In 2020, Gates and coworkers reported a polymeric phosphine sensor which also exhibited 'turn-on' fluorescence in the presence of Au + or Au 3+ . This effect was also attributed to coordination of the P(III) centre to the metal ions, in conjunction with a fluorene-based backbone. Other phosphorus-based moieties have also been reported in the context of metal sensing: in 2017, Niemeyer and co-workers developed a BINOL (1,1′-binaphthyl-2,2′diol) derived bis-phosphoric acid framework which exhibited a selective 'turn-off' response in the presence of Fe 3+ . Other examples based on phosphine sulfides, phosphonates, and phospholes have also been reported. These systems benefit as metal sensors from the donor abilities of the phosphorus moieties, in combination with a robust fluorescent backbone.