A highly selective colorimetric and fluorescent probe for quantitative detection of Cu2+/Co2+: The unique ON-OFF-ON fluorimetric detection strategy and applications in living cells/zebrafish.

ABSTRACT

Identifying and detecting similar target cations through combining "turn on" and "turn off" fluorescence mechanism is effective and challenging. Now a new colorimetric and ON-OFF-ON fluorescent probe N'-((7-(diethylamino)-2-oxo-2H-chromen-3-yl)methylene)-3-hydroxy-2-naphthohydrazide (L) was reported, which could detect Cu2+ and Co2+ in phosphate buffered CH3CH2OH-H2O solvent system. With the assistance of glutathione and pH adjustment, a unique ON-OFF-ON fluorescence detection strategy could be achieved for distinguishing Cu2+ and Co2+. The emission of probe could recover from the L-Cu2+ and L-Co2+ system by addition of GSH or adjusting pH value to 4, respectively, which is due to the abolishment of paramagnetic Cu2+/Co2+. Based on fluorescence titration experiments, the limit of detection was determined as 3.84 × 10-9 M and 4.55 × 10-9 M for Cu2+ and Co2+, respectively. Meanwhile, the detection limit reached 6.21 × 10-8 M for Cu2+ and 6.96 × 10-8 M for Co2+ according to absorbance signal output. Fast recognition of Cu2+/Co2+ can be achieved by obvious color changes from green to colorless under UV light, as well as from yellow to orange-red in room light. The binding mode of L toward Cu2+ and Co2+ have been systematically studied by Job's plot analysis, ESI-MS, IR and density functional theory calculations. Most strikingly, further practical applications of the probe L in fluorescence imaging were investigated in MCF-7 cells and zebrafish due to its low cytotoxicity and good optical properties, suggesting that L could serve as a fluorescent sensor for tracking Cu2+ and Co2+in vivo.