An rhodamine-based fluorescence probe for iron(III) ion determination in aqueous solution
Introduction
The development of fluorescent chemical devices is a promising field [1], [2], [3]. An important area within this field is the design of fluorescent probes for various metal ions [4], [5], [6], [7], [8]. Ions play a fundamental role in a wide range of chemical and biological processes, and numerous efforts have been made to the development of effective fluorescent probes. Sensors based on ion-induced changes in fluorescence appear to be particularly attractive due to their simplicity, high sensitivity and instantaneous response [1], [2], [3], [4], [5], [6], [7], [8]. The trivalent form of iron is an essential element in man. It provides the oxygen-carrying capacity of heme and acts as a cofactor in many enzymatic reactions involved in the mitochondrial respiratory chain. On the other hand, iron deficiency chlorosis is a widespread agricultural problem that affects the development and decreases the yield of many susceptible crops growing on calcareous and alkaline soils [9]. Thus, there is an urgent need to develop chemical sensors that are capable of detecting the presence of iron ions in environmental and biological samples at physiological pH value. Considerable efforts have been devoted to developing fluorescent probes for various metal ions over the last few decades [10], [11], [12], [13]. To date, some Fe(III)-selective fluorescent probes have been achieved [14], [15], [16], [17], since paramagnetic Fe(III) is described as one of the most efficient fluorescence quenchers among the transition metal ions, the signal transduction occurrence via chelation enhanced fluorescence (CHEF) with these inherent quenching metal ions is a challenging task. The rhodamine framework is an ideal mode to construct CHEF OFF–ON fluorescent probes due to its particular structural property, i.e. it undergoes equilibrium between non-fluorescent spirocyclic (“off” signal) and strongly fluorescent ring-open (“on” signal) forms and the two forms always behave with completely different fluorescent properties [18]. Heretofore, some studies on rhodamine-based probes have been reported [13], [19], [20], [21], [22], [23], [24]. We synthesized a rhodamine-based probe (RC) able to recognize and determine Fe(III) at biological pH value in aqueous solution, depending on a fluorescence “off–on” mode.
Furthermore, the real samples contain lots of impurities, which influence the determination. Hence, an appropriate sample pretreatment method is necessary before the determination. Solid-phase extraction (SPE) techniques have recently been among the most popular separation methods for the enrichment of metal ions prior to their determination. The basic principle of SPE is the concentration and purification of analytes from solution by sorption on a solid sorbent [25]. This has several advantages over other techniques, namely conservation of species, good enrichment factors, and ease of automation [25], [26]. Accordingly, the choice of the solid sorbent is the most crucial factor. According to previous research, activated carbon-bound ethylenediamin (AC-EDA) showed an excellent adsorption capacity toward Fe(III). And, the application of AC-EDA for the separation, preconcentration and elution of trace Fe(III) from real samples was performed with satisfactory results [27].
In this study, firstly, we applied AC-EDA to pretreat aqueous samples for the purification and Fe(III) enrichment. Secondly, based on the high selective response to Fe(III) of the fluorescent probe RC, a fluorescence method was establish to determine Fe(III) in aqueous solution with satisfactory result.
Section snippets
Apparatus
Fluorescence spectra measurements were performed on an F-4500 spectrofluorimeter (Hitachi, Japan). The absorption spectra were observed by a UV-Cary100 spectrophotometer (Varian, Australia). Infrared spectra (4000–400 cm−1) in KBr were recorded on a Nicolet NEXUS 670 FT-IR spectrometer, Nicolet (Madison, WI, USA). A VarioEL element analyzer (Elementar Analysensysteme, Hanau, Germany) was used for elemental analysis. An YL-110 peristaltic pump (General Research Institute for Non-ferrous Metals,
Optical characteristics of the probe RC
As depicted in Fig. 2, compounds RC was facilely synthesized from the reaction of rhodamine 6G with triethylenetetriamine. Although RC is a derivative of rhodamine 6G, it form nearly colorless solutions in HEPES aqueous buffer solution (pH = 7.2), indicating that the spirocyclic forms exist predominantly. The characteristic peak near 64.7 ppm (9-carbon) in the 13C NMR spectra of RC also supports this consideration [31].
When no metal ion was added to the solution of RC, almost no fluorescence
Conclusions
In conclusion, we have developed a novel fluorescent probe RC for Fe(III) based on the enhancement of fluorescence intensity of RC after adding Fe(III). Under the optimum conditions, the relative fluorescence intensity increase was linearly proportional to the concentration of Fe(III) in a wide range, and the limit of detection was 14 nmol L−1. The probe RC had remarkably high selectivity and sensitivity. Moreover, the analytical results of real samples were satisfactory. Fluorescence enhance
Acknowledgements
This study was supported by the research fund of Anhui Science and Technology University and the NSFC (Grants 20431010, 20621091 and J0630962).
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