ΦF 3 the other trivalent ions (Cr 3+ , La 3+ ) showed only negligible changes in fluorescence intensity in the presence of 5 (F/F0 = 0.99-1.02). The final trivalent cation, Al 3+ , reproducibly showed a slight increase in intensity (F/F0 = 1.37). These results indicate the possibility of 5 as a selective fluorescence 'turn-off' sensor for Fe 3+ . Due to the ubiquitous nature of Fe 3+ ions in biological and environmental systems, sensitive techniques for Fe 3+ sensing remain highly in demand. The apparent selectivity of 5 for Fe 3+ over Fe 2+ and Cu 2+ is particularly valuable, with these ions the most common competitor ions for Fe 3+ , especially in 'turn-off' fluorescent sensors. The interactions of pyrazolyl-phosphine oxide 3 with most metal ions are very similar to that of 5 (Figure ), including Fe 3+ (F/F0 = 0.03), Fe 2+ (F/F0 = 0.72), and Cu 2+ (F/F0 = 0.66). However, the interaction of 3 with Al 3+ was remarkably more pronounced, with a significant increase in fluorescence intensity observed (F/F0 = 3.71), Despite its toxic nature of in many biological systems, fluorescent sensors for Al 3+ are relatively less common than for other metal cations. The majority of previously reported examples of fluorescent sensors for Al 3+ rely on Schiff bases, with Al 3+ binding to the fluorophore via N and O donor centres and inducing a 'turn-on' response. To further investigate the interactions of 3 and 5 with metal ions, fluorescence titrations of metal ions into solutions of 3 and 5 were performed. We first examined the interactions of Fe 3+ with 3 and 5.