A novel fluorescent probe for Cr3 + based on rhodamine–crown ether conjugate and its application to drinking water examination and bioimaging

https://doi.org/10.1016/j.saa.2015.11.025Get rights and content

Highlights

  • A novel rhodamine derivative (RhC) was prepared for Cr3 + detection.

  • Sensing mechanism was a complexing one with binding stoichiometry of 2:1 for RhC–Cr3 + complex.

  • The probe was successfully applied to drinking water examination and living cell imaging.

Abstract

A trivalent chromium (Cr3 +) fluorescence probe (RhC) was designed and synthesized via Schiff base reaction based on rhodamine–crown ether conjugate. This probe displayed a favorable selectivity for Cr3 + over a range of other common metal ions in DMF/H2O (3:7, v/v; PBS buffer 50 mmol L 1; pH = 6.8) solution, leading to prominent fluorescence “OFF–ON” switching of the rhodamine fluorophore. The limit of detection was calculated to be 1.5 μmol L 1 (S/N = 3). The binding ratio of RhC–Cr3 + complex was determined to be 1:2 according to the Job's plot and HR-MS. The probe was successfully applied to examination of Cr3 + in drinking water spiked samples. The average recoveries ranged from 104.9% to 106.9% at spiked concentration level of 10.00 μmol L 1, and the obtained results were consistent with those obtained using atomic absorption spectrometry (AAS). Moreover, bioimaging experiments showed that RhC can sense the Cr3 + in living cells with a fluorescence enhancement signal.

Introduction

Trivalent chromium (Cr3 +) is an important micronutrient in human nutrition, and plays an important role in the metabolism of fats, nucleic acids, carbohydrates and proteins [1]. A normal person needs to absorb 25–35 μg of Cr3 + per day to improve the metabolism of glucose and lipids [2]. A deficiency of Cr3 + in the human body could increase the risk for cardiovascular diseases and diabetes [3], while excessive intake of Cr3 + has an adverse effect on cellular structure [4]. Also, chromium is an environmental contaminant due to various industrial and agricultural activities [5]. As an environmental contaminant, chromium is found mostly in CrO42  and Cr2O72  form and its reduction to Cr3 + is considered [6]. Therefore, it is essential to explore effective methods for detecting the presence of Cr3 + in environmental and biological samples.

In recent years, common methods for determining Cr3 + include spectrophotometry [7], [8], electrochemical methods [9], [10], atomic absorption spectrometry (AAS) [11], [12], [13], inductively coupled plasma atomic emission spectroscopy (ICP-MS) [14], [15], [16] and fluorimetry [17], [18], [19], [20], [21], [22]. Although AAS and ICP-MS are fast and accurate, the apparatuses of which are expensive. Electrochemical sensors require longtime preparation, frequent calibration and are unsuitable for long-term operations. The limit of detection of spectrophotometry is poor due to the low sensitivity of UV–Vis absorption analysis. Because of the simplicity, high sensitivity and instantaneous response of fluorimetry, fluorescent sensors have attracted widespread attention, and a rapidly increasing number of metal responsive fluorescent probe have been developed in chemistry, biology and environmental science field [23], [24], [25]. However, due to the paramagnetic nature of Cr3 +, its presence is mostly signaled by fluorescence quenching. The signal transduction occurrence via chelation enhanced fluorescence with these inherent quenching metal ions is a challenging task [26].

Rhodamine based fluorescent probes are popular for their excellent photophysical properties, such as low excitation energy, high molar extinction coefficient, high fluorescence quantum yield and high stability against light [27]. It is an ideal framework to construct chelation enhanced fluorescence “OFF–ON” fluorescent probe due to its particular chemical structural property, which can produce a strong fluorescence emission and a visible color change upon selective opening of rhodamine spirolactam ring in the presence of specific metal ions [28], [29]. Recently, some spirolactam forms of rhodamine derivatives have been utilized for the detection of metal ions such as Hg2 + [30] and Cu2 + [31], [32], [33], [34] in aqueous solutions via ring-opening processes of spirolactam amides or hydrazides. In spite of this, high sensitivity and selectivity fluorescent probe for determination of Cr3 + in aqueous solution and living cell remain extremely scarce compared with other metal ions.

In this paper, a novel fluorescent probe (RhC, Fig. 1) was designed and synthesized. Without any metal ions, RhC is non-fluorescent as rhodamine moieties, which is in ring closed spirolactam form. However, upon addition of Cr3 + ions, ring-opening reaction of the rhodamine becomes operative responsible for the absorbance and emission enhancement at 532 nm and 556 nm, respectively along with fluorescence and visible color changes. Furthermore, the probe was successfully applied to drinking water and living cells samples.

Section snippets

Chemicals and Reagents

Rhodamine 6G (98%), dibenzo-18-crown-6 (98%) and trifluoroacetic acid (CF3COOH, 99%) were purchased from J&K Chemical Ltd. Ethylenediamine (absolute), hexamethylenetetramine, toluene, CH2Cl2, CH3CN, absolute ethanol (EtOH), sodium hydroxide (NaOH, 98%), H2SO4, petroleum ether (60–90 °C), NaCl, KCl, CH3COOLi, AgNO3, CuCl2·5H2O, ZnCl2, MgCl2·6H2O, MnSO4, FeCl2·4H2O, BaCl2, CaCl2, Pb(CH3COO)2·3H2O, Co(NO3)2·6H2O, Hg(NO3)2, CdSO4·8/3H2O, NiCl2·6H2O, FeCl3·6H2O, Cr(NO3)3, KAl(SO4)2·12H2O and Bi(NO3)2

Fluorescence and Absorbance Spectra

The fluorescence emission spectrum of RhC in a DMF/H2O mixture (3:7, v/v; PBS buffer 50 mmol L 1; pH = 6.8) solution are shown in Fig. 2 (Blank). RhC exhibits very weak fluorescence emission at the excitation wavelength of 520 nm. Due to the spirolactam ring form of its molecular structure, RhC did not demonstrate the typical rhodamine framework emission peaks in the wavelength range from 525 to 660 nm. Nineteen types of common metal ions at the concentration level of 2 × 10 4 mol L 1, including Na+, K+,

Conclusions

In the current work, the organic dye RhC was used as novel fluorescent probe for Cr3 + detection. This fluorescent probe showed excellent selectivity and sensitivity toward Cr3 +. The detection limit of Cr3 + was 1.5 μmol L 1. The detection limit using RhC was lower than the previously reported data by fluorescence method directly in the literatures [17], [18], [19], [20], [21], [22]. Highly selective and sensitive detection method for Cr3 + was successfully applied to drinking water samples analysis

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21405057 and 21207047) and the Science and Technology Developing Foundation of Jilin Province (Nos. 20121808 and 20125006).

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