Elsevier

Tetrahedron

Volume 70, Issue 41, 14 October 2014, Pages 7429-7438
Tetrahedron

Solvent-dependent chromogenic sensing for Cu2+ and fluorogenic sensing for Zn2+ and Al3+: a multifunctional chemosensor with dual-mode

https://doi.org/10.1016/j.tet.2014.08.026Get rights and content

Abstract

A new multifunctional chemosensor 1 was synthesized and characterized by spectroscopic tools along with a single crystal X-ray crystallography. It can exhibit selective recognition responses toward Cu2+, Zn2+ and Al3+ in different solvent systems with bimodal methods (colorimetric and fluorescence). This sensor 1 detected Cu2+ ions through a distinct color change from colorless to yellow in aqueous solution. Interestingly, the receptor 1 was found to be reversible by EDTA. The detection limit (11 μM) of 1 for Cu2+ is much lower than WHO guideline (30 μM) in drinking water. In addition, the sensor 1 showed significant fluorescence enhancements in the presence of Zn2+ ion and Al3+ ion in two different organic solvents (DMF and MeCN), respectively. The binding modes of the three complexes were determined to be a 1:1 complexation stoichiometry through Job plot, ESI-mass spectrometry analysis, and 1H NMR titration.

Introduction

Unlike other traditional methods (e.g., inductively coupled plasma atomic emission spectrometry,1 atomic absorption spectroscopy,2 and electrochemical methods3), chemosensors are highly estimable means for the selective recognition of chemical and biological species in environmental chemistry and biology.4 The design and construction of colorimetric and fluorescent chemosensors with high selectivity and sensitivity for trace metal ions, such as copper, zinc, and aluminum, are currently of great importance as they allow nondestructive and prompt detection of metal ions by a simple absorbance and fluorescence enhancement (turn-on) response.5 Among the various types of chemosensors, the chromogenic chemosensors are also widely used owing to the inexpensive equipment required or no equipment at all.6 Furthermore, fluorescence sensors are excellent tools for detecting metal ions due to their high sensitivity, good selectivity, high response speed and simple operation.6(a), 6(b)

Copper, as the third most abundant essential trace element in the human body, performs an important role in many fundamental physiological processes in organisms.7 However, with excessive loading, copper ion can cause extremely negative health effects such as gastrointestinal disturbance and liver or kidney damage.8 Zinc, the second most abundant transition metal, is fundamental in biology systems.9 In a human body, it is well known that many catalytic centers and structural cofactors are composed of several Zn2+-containing enzymes and DNA-binding proteins.10 Excess zinc, however, can cause various intoxications and a number of severe neurological diseases (e.g., Alzheimer's disease, cerebral ischemia, and epilepsy), developmental defects, and malfunctions.11 Aluminum has been identified as a neurotoxin over one hundred years, and can cause health hazards like Alzheimer's disease and osteomalcia, and even contribute to the risk of cancer of the breast.12 Therefore, the development of new analytical methods for the selective determination of Cu2+, Zn2+, and Al3+ is highly desirable. Up to now, a number of colorimetric and fluorescent chemosensors for Cu2+, Zn2+ or Al3+ have been reported.13 However, most of the reported fluorescent chemosensors can selectively sense only one of them. Therefore, single chemosensors for multiple analytes have recently become very popular among the analysts, because of their fast detection time and cost reduction.14 For example, they include Cr3+/Al3+,15a Cu2+/Hg2+,15b Zn2+/Cd2+,15c and Al3+/Fe3+.14a

The julolidine group is a well-known chromophore and chemosensors with the julolidine moiety are usually water-soluble.16 In addition, the antipyrine moiety is one of the biologically important moieties with good optical properties.17 We expected that the combination of the julolidine and the antipyrine groups might show good chromogenic or fluorogenic responses to metal ions. Moreover, the combination form of the julolidine and the antipyrine moieties has three potential binding sites (a nitrogen atom of imine and two oxygen atoms of julolidine and antipyrine), which might act as a strong coordination donor toward metal ions. Therefore, we combined these two moieties to make a new chemosensor, and tested its sensing properties. Importantly, receptor 1 exhibited selective recognition responses toward Cu2+, Zn2+ and Al3+ in different solvent systems with bimodal methods (colorimetric and fluorescence).

Herein, we report a new Schiff base with both a julolidine group and an antipyridine one, which acts as a colorimetric and fluorescent chemosensor for detection of Cu2+, Zn2+ and Al3+ in a dual-channel mode (naked-eye and fluorescence emission). In the presence of Cu2+, the chemosensor 1 showed an instant color change from colorless to yellow in the DMSO-buffer solution. Moreover, this sensor 1 displayed fluorescence enhancements in the presence of Zn2+ and Al3+ in DMF and MeCN, respectively.

Section snippets

Synthesis, characterization and solvent-dependent sensing properties of 1

A new chemosensor 1 was obtained by the condensation reaction of 8-hydroxyjulolidine-9-carboxaldehyde with 4-aminoantipyrine in ethanol at room temperature (Scheme 1), and characterized by 1H NMR, ESI-mass spectrometry analysis, elemental analysis, and X-ray crystallography.

Crystals of 1 were obtained by slow evaporation in methanol and its structure is shown in Fig. 1. We tested sensing abilities of 1 towards various metal ions in several solvent systems such as MeOH, EtOH, DMSO, DMF, MeCN and

Conclusion

We have developed a new chemosensor 1 based on Schiff base with julolidine and antipyrine moieties. The formulation and detailed structural characterizations of 1 have been established by using spectroscopic and single crystal X-ray crystallographic tools. Compound 1 can be used as a multifunctional chemosensor for highly selective detection of Cu2+, Al3+ and Zn2+ depending on solvents. It distinguished Cu2+ ion from other metal ions by color change in aqueous media without expensive equipment.

Materials and instrumentation

All the solvents and reagents (analytical grade and spectroscopic grade) were obtained from Sigma–Aldrich and used as received. NMR spectra were recorded on a Varian 400 spectrometer. Chemical shifts (δ) are reported in parts per million, relative to tetramethylsilane Si(CH3)4. Absorption spectra were recorded at room temperature using a Perkin Elmer model Lambda 2S UV/Vis spectrometer. Electrospray ionization mass spectra (ESI-mass) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQTM

Acknowledgements

Financial support from Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012001725 and 2012008875) are gratefully acknowledged.

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