A New Coumarin-Based Colorimetric and Fluorometric Sensor for Cu2+

Abstract

Chemosensors, small chemical compounds that sense the presence of analytes or energy, typically consist of two components: a receptor moiety that interacts with the target analytes and a read-out system that signals binding. And one of the most utilized research tool for the study of chemosensors employs a colorimetric and fluorometric spectroscopic techniques. So far successful reports on metal ion sensors have been documented including our recent result. Many different kinds of optical or fluorescent sensors have several advantages (such as high sensitivity and selectivity, non-destructive analysis, low cost and real-time monitoring), which allow naked-eye detection of color and fluorescent emission change upon metal ion binding without the use of any expensive spectroscopic equipment. Copper, the third most abundant transition metal following zinc and iron in human body, exerts essential functions in many cellular enzymes and proteins such as Cu/Zn superoxide dismutase, dopamine monooxygenase, cytochrome C oxidase and ceruloplasmin. However, it becomes toxic when excessive amounts of copper ion were accumulated. It is believed that the disruption of copper homeostasis is associated with neurodegenerative illnesses such as Parkinson’s, Wilson’s, Alzheimer’s and Menke’s diseases. Coumarin skeleton is often utilized as fluorescent sensor due to its excellent photophysical properties like great fluorescent intensity, high quantum yield, high photostability, biological stability, nontoxicity and derivatizable backbone. There have been many excellent coumarin-based fluorescent probes reported for not only anions but also cations such as Fe, Ag, Al, Ni and Hg during the last decade. Herein, we report a new colorimetric and fluorescent “turn-off” sensor that responds to Cu through coumarin derivative. The binding properties of sensor A ward various metal ions were investigated by UV-Vis absorption and fluorescence spectroscopy. Sensor A was synthesized as shown in Scheme 1. Compound 1 and 2 were prepared by the known procedures. Investigations on the photophysical properties revealed that sensor A showed high selectivity to Cu in 10 mM trisHCl buffer solution (acetonitrile/water = 9:1, pH = 7.01). Sensor A showed an absorption band at 375 nm in 10 mM tris-HCl buffer solution. In the presence of Cu (20 μM), there appeared a new red-shifted absorption band at 425 nm at the expense of peak at 375 nm (Figure 1(a) in red). The absorption bands at 375 and 425 nm linearly decreased and increased, respectively, by the increasing concentration of Cu (Figure 1(b)). However, none of the other metal ions (Ag, K, Li, Na, Ca, Cd, Co, Ni, Zn and Fe (20 μM, 2.0 equivalent, nitrate salt)) showed such a red-shift in their absorption spectra (Figure 1(a)). The red-shift in the absorption spectra upon addition of the Cu can be rationalized by intramolecular charge transfer (ICT). It is well known that an electron-donating group at 7position and an electron-withdrawing group at 3-position in coumarin skeleton induce absorption band into visible region as a result of effective ICT through the electron push– pull system. The complexation of a Cu in sensor A would increase electron-withdrawing character of 3-position, resulting a stronger ICT. To investigate the binding mode of sensor A to Cu, Job’s method was carried out. A 1:2 stoichiometry was determined by Job’s plot. The maximum absorption change was