# Luminescence Lifetime-Based Sensing of Water Turbidity

## TL;DR

A new sensor uses luminescence lifetime measurements to accurately detect water turbidity in a compact and robust design.

## Contribution

A novel dual-lifetime reference scheme for turbidity sensing using luminescence phase shift.

## Key findings

- The sensor detects turbidity up to 1000 NTU with an accuracy of 1 NTU.
- Shorter optical pathlengths allow measurement of higher turbidity levels.
- The sensor is robust against temperature, dissolved oxygen, and chlorophyll interference.

## Abstract

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.