The emission intensities elicited by all these analytes were measured. Fig. shows that the decomplexation and resulting ESIPT fluorescence of 6 reached a maximum in presence of polyP8 as compared to other tested anions. Both sensors 6 and 7 had a similar selectivity profile regarding anions. However, the emission intensity was higher for the released ESIPT fluorophore 4, enabling its application at twofold lower concentrations. Fluorescence quantum yields for 4 and 5 were determined to be 0.33 and 0.16 respectively (experimental details in the SI), directly explaining the higher sensitivity of 6 in the assays. The limit of detection (LOD) of polyP8 with 6 and 7 were found to be 97 nM and 110 nM, respectively (details in SI section 9). Both sensors did not respond to phosphate (Pi) and monophosphate esters, such as GMP or AMP. In contrast, both sensors did respond to pyrophosphate (PPi), but the increment in fluorescence gain is small in both cases. Diphosphate esters like e.g. ADP and GDP also elicited a response, but reduced when compared to free PPi. Nucleoside triphosphates like ATP or GTP also released some of the fluorophore, but by far the largest increment of ca. 8-fold was achieved with polyP8 in both cases. Consequently, sensors 6 and 7 represent the first selective short-chain polyP sensors that work in an aqueous environment based on an ESIPT-ON mechanism (Fig. ). Next, we investigated the dependence of the signal of sensors 6 and 7 on polyP chain lengths using several synthetic standards and commercial polydisperse references. Interestingly, we observed that with increasing chain length the signal intensity increased, reaching a plateau around polyP8, when incubated under the same conditions (10 mM HEPES, pH = 7.4). Figure depicts this trend in fluorescence intensity gain from monophosphate to pyrophosphate to polyP8 (10 equivalents of each different polyP with 6 or 7). We also examined the fluorescence response of probes 6 and 7 towards different cyclic metaphosphates like cyP3, cyP4 and cyP8. 29 cyP4 and cyP8 showed a considerable fluorescence response, unlike cyP3. Yet, linear polyP8 still elicited a 1.5 times higher response as compared to its cyclic analogue cyP8 (Fig. ).