Electrochemical sensors based on gold nanoparticles modified with rhodamine B hydrazide to sensitively detect Cu(II)
Graphical abstract
Introduction
Among the essential heavy metal ions in the human body, Cu2+ ranks third in abundance among the essential trace metal ions in the human body [1]. It has an important role in various environmental, chemical, and physiological systems. It may cause potential health problems of the level of Cu2+ exceeds the maximum value of 1.3 mg L−1 in drinking water. Hence, the accurate determination of Cu2+ in the real environment is very important. Recently, many advanced analytical methods have been developed to both qualitatively and quantitatively determine Cu2+, such as spectrophotometry, atomic absorption spectroscopy, inductively coupled plasma mass spectroscopy [2], inductively coupled plasma atomic emission spectrometry [3], and electrochemical techniques [4]. Among these methods, electrochemical techniques have received considerable interest because of their easy operation, fast response, low limitation of detection, and high sensitivity [5].
As known for the excellent spectroscopic properties and good water solubility, rhodamine fluorophore is an ideal moiety to construct “turn-on”-type fluorescent chemosensors in terms of metal ions triggering the spirolactam ring-opening mechanism along with changes in chromaticity and fluorescence at the same time. Reports have been made on rhodamine-based derivatives that bear a pyrene group as a chemosensor for Cu2+, in which the rhodamine ring-opening process is introduced to give a colorimetric change and “turn-on” fluorescence signal toward Cu2+ [6], [7], [8]. However, most of the colorimetric sensors are applied in mixed solvents (water/organic solvent), whereas only a few of them are appropriate for neat aqueous solutions, thereby greatly limiting their application. Moreover, the treatment of wastewater containing rhodamine produced from the normal method of determining heavy metal ions is difficult. Consequently, an environmentally friendly and sustainable method to determine heavy metal ions must be explored. Combining the high selectivity of rhodamine-based chemical sensors and the advantages of the electrochemical techniques, it is highly anticipated to fabricate the rhodamine-related electrochemical sensor for detecting the heavy metal ions. Up to now, few investigation was reported in this field. In our previous work, the electrochemical sensor was fabricated based on the fluorescein hydrozine, which was modified by hollow TiO2 nanospheres. Subsequently, it was applied to determine Cu2+ and exhibited highly sensitivity and selectivity [9].
Recently, Au nanoparticles (Au NPs) have also drawn much attention in electrochemical fields because of their excellent characteristics, such as large surface area, high chemical stability, good biocompatibility, and ability to facilitate electron transfer between biomolecules and electrodes [10], [11]. Moreover, functionalized Au NPs have been integrated into electrochemical techniques for the selective and sensitive real-time monitoring of metal ions in environmental samples [12], [13]. Venkataraman et al. reported that a donor-acceptor bond is formed through the delocalization of the lone-pair on amine nitrogen, sulfur, and phosphorus to an undercoordinated gold atom [14], [15]. Organic molecules on the surface of Au NPs normally have a key role in responding to specific guest molecules (analytes) through covalent and non-covalent interactions. These molecules lead to the formation of NPs-metal ion complexes between the electron-rich groups (–OH, –NH2, and –COOH) on the surfaces of NPs and metal ions [16]. For these reasons, organic molecule-modified Au NPs have been successfully used for the signal amplification because of the increased organic molecules loading toward each reaction event, which subsequently enhances the discrimination signal of the detected targets [17], [18].
Inspired by this idea, we aim to fabricate sensing platform to detect Cu2+ based on rhodamine B hydrazide (RBH)-functionalized Au NPs (AuNPs-RBH) (Scheme 1). The amines in the RBH bond to AuNPs as two-electron donors. Electrochemical techniques were applied to evaluate the performance of the developed sensor. In the presence of Cu2+, the pyrene group contained in RBH can coordinate with Cu2+ ions via the rhodamine ring-opening process, resulting in the variation of the electrochemical signals. Therefore, compared with the routine method, the present work exhibits three advantages: (i) the functionality of AuNPs-RBH with the pyrene group provides a binding site with Cu2+; (ii) the presence of Au NPs can enhance the electrochemical performance of the developed sensor; and (iii) the facile preparation of RBH-related electrochemical sensor could broaden its practicability in the field of the ecological environment or biology.
Section snippets
Materials and reagents
Chloroauric acid (HAuCl4), trisodium citrate dihydrate (Na3C6H5O7·2H2O), K3[Fe(CN)6], and K4[Fe(CN)6]·3H2O were purchased from Alfa Aesar (China, www. alfachina. cn). RBH and the other reagents, both of analytical-reagent grade, were purchased from Sigma-Aldrich (China, www.sigmaaldrich.com) and used without further purification. Milli-Q water (≥18.2 MΩ cm) was used throughout the experiments.
Phosphate buffer saline (pH 7.4) was prepared by mixing stock solutions of 0.1 M Na2HPO4 and 0.1 M KH2PO4
Sensor design
Scheme 1 outlines the preparation processes of the AuNPs-RBH-based electrochemical sensor for detecting Cu2+ and the generation of the electrochemical signal. First, Au NPs are coated onto the surface of a bare AE to promote the electron transfer rate at the interface between the electrolyte solution and the electrode. Next, the RBH molecules interact with Au NPs through the delocalization of the lone-pair on amine nitrogen to an undercoordinated gold atom, leading to the formation of an
Conclusion
In summary, an electrochemical sensor based on AuNPs-RBH was fabricated to selectively detect Cu2+ in aqueous solutions, following a simple and feasible method. Most importantly, the prepared electrode exhibits high sensitivity for the detection of Cu2+ with an extremely low LOD of 12.5 fM within the Cu2+ concentration range of 0.1 pM to 1 nM using the EIS approach. The developed electrochemical sensor also shows excellent selectivity toward interfering metal ions, such as Ag+, Mn2+, Pb2+, Na+, K+
Acknowledgements
This work was supported by Programs for the National Natural Science Foundation of China (NSFC: Account No. 51173172) and Innovative Technology Team of Henan Province (2014).
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