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In this work, a rhodamine derivative was developed as a colorimetric and ratiometric fluorescent probe for Hg 2+ . It exhibited a highly sensitive fluorescence response toward Hg 2+ . Importantly, studies revealed that the probe could be used for ratiometric detection of Hg 2+ , with a low detection limit of 0.679 μM. The mechanism of Hg 2+ detection using compound 1 was confirmed by ESI-MS, 1 H NMR, and HPLC. Upon the addition of Hg 2+ , the rhodamine receptor was induced to be in the ring-opening form via an Hg 2+ -promoted hydrolysis of rhodamine hydrazide to rhodamine acid. In addition to Hg 2+ detection, the naphthalimide-rhodamine compound was proven to be effective in cell imaging., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

In this work, a colorimetric and ratiometric fluorescent sensor based on a coumarin-rhodamine B hybrid for the sequential recognition of Cu 2+ and arginine (Arg) via the FRET mechanism was designed and synthesized. With the addition of Cu 2+ , the solution displayed a colorimetric change from pale yellow to pink which is discernible by the naked eye. Additionally, the fluorescence intensities of the sensor exhibited ratiometric changes for the detection of Cu 2+ at 490 and 615 nm under a single excitation wavelength of 350 nm, which corresponded to the emissions of coumarin and rhodamine B moieties, respectively. The fluorescence color change could be visualized from blue to pink. The limits of detection were determined to be as low as 0.50 and 0.47 μM for UV-vis and fluorescence measurements, respectively. More importantly, the sensor not only can recognize Cu 2+ and form a sensor-Cu 2+ complex but can also sequentially detect Arg with the resulting complex. The detection limits for Arg were as low as 0.60 μM (UV-vis measurement) and 0.33 μM (fluorescence measurement), respectively. A fluorescence imaging experiment in living cells demonstrated that the fabricated sensor could be utilized in ratiometric fluorescence imaging towards intracellular Cu 2+ , which is promising for the detection of low-level Cu 2+ and Arg with potentially practical significance., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A novel colorimetric and fluorometric sensor with unique dual-channel emission to sequentially detect Cu 2+ and hydrogen sulfide (H 2 S) was synthesized from naphthalimide-rhodamine B through the PET and FRET mechanism. The sensor showed a selective "off-on" fluorescence response with a 120-fold increase toward Cu 2+ , and its limits of detection were 0.26 μM and 0.17 μM for UV-vis and fluorescence measurements, respectively. In addition, 1-Cu 2+ was an efficient "on-off" sensor to detect H 2 S with detection limits of 0.40 μM (UV-vis measurement) and 0.23 μM (fluorescence measurement), respectively. Furthermore, the sensor can also be used for biological imaging of intracellular staining in living cells. Therefore, the sensor should be highly promising for the detection of low level Cu 2+ and H 2 S with great potential in many practical applications., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)