Luminescence Lifetime-Based Sensing of Water Turbidity

Current commercial turbidity sensors may be costly, bulky, or fragile. As such, many research groups have investigated alternative methods for the accurate determination of this essential water quality parameter. This work describes a new sensor based on luminescence measurements under a dual-lifetime reference scheme. By letting the turbid water pass between two dye layers with similar absorption and emission features but widely different emission lifetimes, an overall luminescence phase shift is measured, the magnitude of which depends on the turbidity. Dimethyl 2,5-bis(cyclohexylamino)terephthalate (BCT) is the reference fluorophore placed on the optical fiber tip after immobilization in a thin poly(vinyl chloride) (PVC) layer. The indicator luminophore, tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) (RD3), embedded into poly(ethyl 2-cyanoacrylate) (PCA), is separated from the reference layer by a user-selectable distance (1-2 cm). With increasing turbidity, the emission intensity of the indicator dye layer decreases, while the fluorescence intensity of the reference layer remains constant. In this way, the ratio between the two emission intensities is translated into changes in the lifetime (and phase shift) of the composite emission. The sensor's working range depends on the distance between the two dye layers. The sensor is capable of detecting turbidity levels in the range of 0-1000 NTU, 0-500 NTU, and 0-300 NTU for 1, 1.5, and 2 cm optical pathlengths, respectively, with an accuracy of 1 NTU (0.3 NTU between 0 and 10 NTU), limited by the accuracy of the turbidity standards. Shorter pathlengths allow the measurement of higher turbidity. The temperature-dependent response is instantaneous and devoid of dissolved O₂ and chlorophyll interferences. The sensor has been tested in a real-world environment for 11 days with good performance.