In conclusion, we have reported the synthesis of two new 3-hydroxy-1H-pyrazol-4-yl)phosphine oxides via the hydrolysis of appropriately substituted phosphorus-based heterocycles. A one-pot methodology was developed to allow higher yields of the target products directly from air-stable azophosphine-boranes. The proposed mechanism was supported by computational and experimental studies. These pyrazolyl-phosphine oxides are an interesting new class of dual-responsive fluorescent sensor for metal ions. Both sensors demonstrate significant 'turn-off' responses to Fe 3+ with good selectivity, large Stokes shifts, and submicromolar limits of detection. The fluorescence quenching with Fe 3+ is via a dynamic quenching mechanism, and does not require complex formation. In contrast, a 'turn-on' response in the presence of Al 3+ was observed, with pyrazolyl-phosphine oxide 3 exhibiting a particularly significant enhancement in fluorescence, and this was shown to occur via a complexation process. We anticipate that this framework will open new avenues in the exploration of novel phosphorus-based fluorophores, and their applications in metal sensing.