We also profiled longer polyphosphate chains, which are commercially available. These are polydisperse mixtures of different chain length and so it is difficult to obtain their real concentration. Usually, the phosphate content in thus indicated in terms of Pi monomers after digestion with exopolyphosphatase (PPX). 1 After increasing the concentration of the longer chain polyPs (polyP14 -polyP700) to 125 µM for 6 and 250 µM for 7 (in terms of Pi residues), we observed a trend of reduced response from both sensors 6 and 7 with increasing chain length from polyP14 (7-fold increase) to polyP130 and polyP700 (4-fold increase), indicating that our sensors are most responsive to short-chain polyP with an average chain length of around 8. (Fig. ).