A Chinese research team developed an effective colorimetric fluorescent detection of lead ions in water, and achieved visual, real-time and semi-quantitative detection by printing the paper strip with fluorescence probe solution and a smartphone application (Color recognizer, Xiyi Technology).

This work was done by JIANG Changlong and his colleagues with Institute of Intelligent Machines (IIM), Hefei Institutes of Physical Science. It was published in Analytical Chemistry.

Among the metal ions, lead is considered as one of the most toxic elements because it damages human health and pollutes the environment. Traditional methods of detecting lead ions can complete the sensitive and selective analysis of the amount of Pb²⁺ in water sample. However, the application of these technologies is limited by the high cost of instruments, the need for professional operation and the long testing period.

In order to achieve semi-quantitative visual detection of lead ions, the team prepared colorimetric fluorescent probe by the mixture of blue carbon dots (BCDs,) and red carbon dots (RCDs) with a fixed fluorescence intensity ratio.

As the amount of Pb²⁺ increases, the decrease of the blue fluorescence intensity made the luminescence color obvious from blue to red, which could be conveniently distinguished using a smartphone under a UV lamp.

By combining the smartphone and fluorescent paper strips, the instantaneous semi-quantitative detection of metal ions could be realized.

According to the study, the detection limits of the fluorescent probe solution were 2.89 nM and 35.26 nM respectively by using luminescence spectrometer and the smartphone application, which were much lower than permissible limit of Pb²⁺ (10 mg·L-1) for drinking water (WHO).

Using a smartphone identified the RGB values of fluorescent paper strips, on-site and semi-quantitative detection of lead ions in water could be realized in 5 minutes.

This work is financially supported by National Natural Science Foundation of China, the National Basic Research Program of China and the Key Research and Development Program of Anhui Province.

Figure: a) Schematic illustration of visual detection of lead ions in water; b) The selectivity of colorimetric fluorescent probes to detect lead ions; c) The fluorescent images corresponding to the selectivity of detect lead ions (photographed under 365 nm UV light); d) Schematic drawing for the detection of Pb²⁺ using a smartphone; e) The plot of fluorescence probe solution color change (blue channel/red channel) versus concentration of Pb²⁺ (insets show that photos were taken with a smartphone under a 365 nm UV lamp) (Image by WANG Haiqian)