Visual and fluorescent detection of mercury ions by using a dually emissive ratiometric nanohybrid containing carbon dots and CdTe quantum dots

The authors describe a carbon dot (CD) based dual-emission ratiometric optical probe for the on-site visual and fluorometric determination of mercury(II) ions. The nanoparticle (NP) probe was obtained by covalently linking the blue emissive carbon dots to the surface of silica nanoparticles containing red-emissive quantum dots (QDs). The red emitting QDs in the silica matrix are inert to Hg(II) and provide a reliable and constant reference signal. They also reduce their toxicity and improve the optical and chemical stabilities, while the blue emission CDs are very sensitive to Hg(II). With increasing concentration of Hg(II), a solution containing the NP probe undergoes a continuous color change from light purple to red. This can be seen with bare eyes or detected instrumentally by measurement of fluorescence intensity under excitation/emission wavelengths of 350/453 and 658 nm. The probe exhibits high sensitivity to Hg(II), with a detection limit of 0.47 nM (at an S/N ratio of 3). This is much lower than the allowable level of mercury (10 nM, ~10 ppb) in drinking water set by the U.S. Environmental Protection Agency. For practical use, the probe was used to quantify Hg(II) in (spiked) tap water where it gave recoveries between 95 and 106% and relative standard deviations between 1.9 and 3.2%. The probe can also be applied in filter paper-based assays, and this paves the way to point-of-care pollution control. This ratiometric probe is nontoxic and easily operated, and therefore shows potential applications for rapid and low-cost visual identification of Hg(II).