Humidity sensing with a luminescent Ru(II) complex and phase-sensitive detection

A simple, inexpensive optode for relative humidity (RH) monitoring in air has been fabricated using the water-sensitive luminescent dye [Ru(phen)2(dppz)]2+ (where phen and dppz stand for 1,10-phenanthroline and dipyrido[3,2-a:2′,3′-c]phenazine, respectively) immobilized on poly(tetrafluoroethylene). When interrogated with a tailored emission lifetime-based instrument, the novel device outperforms humidity optosensors reported so far for environmental monitoring. The operational range of the optode is from 4 to 100% RH at 20 °C. Its response time (t90) is shorter than 1.4 min (recovery time < 1.2 min), and it shows repeatability and reproducibility of the RH measurements better than 1 and 4%, respectively. Furthermore, the robust optode can even be dipped in water with full recovery of its analytical features and its stability has been verified for more than 2.5 years (discontinuous measurement). The sensor has been validated in the 10–30 °C temperature range using a NIST-traceable humidity generator. It has been applied successfully to water activity measurements in food and relative humidity monitoring in a weather station using a ruggedized fiber-optic phase-sensitive fluorometer.