Quantitative colorimetric sensing of heavy metal ions via analyte-promoted growth of Au nanoparticles with timer or smartphone readout

This work describes two new colorimetric nanosensors for label-free, equipment-free quantitative detection of nanomolar copper (II) (Cu2+) and mercury (II) (Hg2+) ions. Both are based on the analyte-promoted growth of Au nanoparticles (AuNPs) from the reduction of chloroauric acid by 4-morpholineethanesulfonic acid. For the Cu2+ nanosensor, the analyte can accelerate such a redox system to rapidly form a red solution containing dispersed, uniform, spherical AuNPs that is related to these particles' surface plasmon resonance property. For the Hg2+ nanosensor, on the other hand, a blue mixture consisting of aggregated, ill-defined AuNPs with various sizes can be created, showing a significantly enhanced Tyndall effect (TE) signal (in comparison with that produced in the red solution of AuNPs). By using a timer and a smartphone to quantitatively measure the time of producing the red solution and the TE intensity (i.e., the average gray value of the corresponding image) of the blue mixture, respectively, the developed nanosensors are well demonstrated to achieve linear ranges of 6.4 nM to 100 mu M and 6.1 nM to 1.56 mu M for Cu2+ and Hg2+, respectively, with detection limits down to 3.5 and 0.1 nM, respectively. The acceptable recovery results obtained from the analysis of the two analytes in the complex real water samples including drinking water, tap water, and pond water ranged from 90.43 to 111.56%.

Automated summary

This work presents two new colorimetric nanosensors for label-free and equipment-free quantitative detection of nanomolar concentrations of copper (II) (Cu2+) and mercury (II) (Hg2+) ions. The Cu2+ nanosensor utilizes the analyte to accelerate the formation of dispersed, uniform, spherical gold nanoparticles (AuNPs), resulting in a red solution. On the other hand, the Hg2+ nanosensor produces a blue solution consisting of aggregated AuNPs with various sizes and an enhanced Tyndall effect signal. By measuring the time of producing the red solution and the Tyndall effect intensity of the blue solution using a timer and a smartphone, respectively, the nanosensors achieve linear ranges and detection limits for Cu2+ and Hg2+.