A fluorescence lifetime-based solid sensor for water

A fluorescence lifetime-based water sensor was developed, based on a solvent-polarity-sensitive fluorescent metal-ligand compound, dipyridol[3,2-a:2″,3″-c]phenazine, di[cis-l,2-bis(diphenylphosphino)-ethylene] osmium(II) hexafluorophosphate, [Os(dppz)(dppe)2](PF6)2. When excited in acetone solution, the compound emitted orange-red fluorescence with a peak wavelength of 610 nm. Fluorescence quenching was observed from both intensity and lifetime measurements when water was presented in the acetone. To fabricate a water sensor, the compound was immobilized by ionic bonding onto an ion-exchange resin, carboxymethyl cellulose, and then sandwiched between a thin sol-gel layer and a glass substrate. This formed a water-sensitive solid film sensor that, when re-inserted from a water-free into a water-containing organic solvent, displayed a lifetime decrease. The lifetime change could be measured in the frequency domain using phase-modulation fluorometry. Because of the long decay time of this compound the phase-modulation could be performed using an amplitude-modulated blue LED with a low modulation frequency near 2 MHz. For a change in the water content of an acetone solution from 0% to 20%, 39.6 degrees of phase angle decrease was observed. The degree of the change in phase angle varied from solvent to solvent. The typical response and recovery time for a 90% total signal change was a few seconds. The detection limit was solvent-dependent. When ethyl acetate was used as the solvent, the detection limit could be as low as 0.02% (vv) of water. The sensor also displayed very good long term stability, as little change in performance was discoverable after two months.