A simple and effective ratiometric fluorescence nanosensor for the selective detection of Cu²⁺ has been developed by covalently connecting the carboxyl-modified red fluorescent cadmium telluride (CdTe) quantum dots (QDs) to the amino-functionalized blue fluorescent carbon nanodots (CDs). The sensor exhibits the dual-emissions peaked at 437 and 654 nm, under a single excitation wavelength of 340 nm. The red fluorescence can be selectively quenched by Cu²⁺, while the blue fluorescence is a internal reference, resulting in a distinguishable fluorescence color change from pink to blue under a UV lamp. The detection limit of this highly sensitive ratiometric probe is as low as 0.36 nM, which is lower than the U.S. Environmental Protection Agency (EPA) defined limit (20 μM). Moreover, a paper-based sensor has been prepared by printing the hybrid carbon dots–quantum dots probe on a microporous membrane, which provides a convenient and simple approach for the visual detection of Cu²⁺. Therefore, the as-synthesized probe shows great potential application for the determination of Cu²⁺ in real samples.

This review summarizes current advances on the design and the employment of fluorescent carbon dots in sensing applications, especially from the point of analytical view.