Despite the variety of phosphorus-appended pyrazoles discussed above, there are no examples of metal sensing using these systems. In this manuscript, we report the synthesis and characterisation of a new range of pyrazolylphosphine oxides via the reaction of five-membered phosphorus-based heterocycles with H2O. Our method allows one-pot access to the unexplored 3hydroxy-1H-pyrazol-4-yl)phosphine oxides (Figure ), henceforth simplified to pyrazolyl-phosphine oxides. The mechanism of this transformation has been probed both computationally and experimentally. The pyrazolyl-phosphine oxides are highly fluorescent, which allows these molecules to act as metal sensors. These air-and water-stable sensors are selective for different metals, affording either a 'turn-off' dynamic quenching mechanism for Fe 3+ , or a 'turn-on' binding mechanism for Al 3+ , representing the first example of a phosphorusappended pyrazole as a fluorescent metal sensor.