Dual-Emission Fluorescent Silica Nanoparticle-Based Probe for Ultrasensitive Detection of Cu<sup>2+</sup>

An effective dual-emission fluorescent silica nanoparticle-based probe has been constructed for rapid and ultrasensitive detection of Cu<sup>2+</sup>. In this nanoprobe, a dye-doped silica core served as a reference signal, thus providing a built-in correction for environmental effects. A response dye was covalently grafted on the surface of the silica nanoparticles through a chelating reagent for Cu<sup>2+</sup>. The fluorescence of the response dye could be selectively quenched in the presence of Cu<sup>2+</sup>, accompanied by a visual orange-to-green color switch of the nanoprobe. The nanoprobe provided an effective platform for reliable detection of Cu<sup>2+</sup> with a detection limit as low as 10 nM, which is nearly 2 × 10<sup>3</sup> times lower than the maximum level (∼20 μM) of Cu<sup>2+</sup> in drinking water permitted by the U.S. Environmental Protection Agency (EPA). The high sensitivity was attributed to the strong chelation of Cu<sup>2+</sup> with polyethyleneimine (PEI) and a signal amplification effect. The nanoprobe constructed by this method was very stable, enabling the rapid detection of Cu<sup>2+</sup> in real water samples. Good linear correlations were obtained over the concentration range from 1 × 10<sup>−7</sup> to 8 × 10<sup>−7</sup> (<i>R</i><sup>2</sup> = 0.99) with recoveries of 103.8−99.14% and 95.5−95.14% for industrial wastewater and lake water, respectively. Additionally, the long-wavelength emission of the response dye can avoid the interference of the autofluorescence of the biosystems, which facilitated their applications in monitoring Cu<sup>2+</sup> in cells. Furthermore, the nanoprobe showed a good reversibility; the fluorescence can be switched “off” and “on” by an addition of Cu<sup>2+</sup> and EDTA, respectively.