A simple colorimetric sensor based on anti-aggregation of gold nanoparticles for Hg2+ detection

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Abstract

A new method has been proposed to realize the visual detection of Hg2+ via anti-aggregation of gold nanoparticles (Au NPs). The positively charged Au NPs were prepared using poly(diallyldimethylammonium) chloride (PDDA) as reducer and stabilizer. In the presence of cysteine, the color of Au NPs solution turned from ruby red to royal purple, indicating the aggregation of Au NPs. Owing to the high stability constant of cysteine with Hg2+, the pre-incubation of Hg2+ with cysteine would form Hg2+–cysteine complexes and significantly reduce the concentration of free cysteine molecules, thus the aggregation of Au NPs was interrupted since there was not enough inducer. With the increase of Hg2+ concentration, the color of the Au NPs solution would progress from purple to red, allowing the visual detection of Hg2+ ranging from 5.0 × 10−8 to 1.0 × 10−5 M, with a detection limit of 2.5 × 10−8M. The proposed method is convenient, low-cost and free of complex equipment, making it possible to analyze Hg2+ in drinking water, rain water or water extracted air samples.

Graphical abstract

In the presence of cysteine, the color of positively charged Au NPs solution turned from ruby red to royal purple, indicating the aggregation of Au NPs. The pre-incubation of Hg2+ with cysteine interrupted the aggregation of Au NPs. With the increase of Hg2+ concentration, the color of the Au NPs solution would progress from purple to red, allowing the visual detection of Hg2+.

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Highlights

► A new method has been proposed to realize the visual detection of Hg2+ via anti-aggregation of gold nanoparticles. ► A detection limit of 2.5 × 10−8 M was obtained. ► The proposed method is used to analyze Hg2+ in drinking water.

Introduction

Mercury, one of the most hazardous environmental toxins, is ubiquitous in the environment. It is generally used in thermometers and blood-pressure cuffs and commercially in batteries, switches, and fluorescent light bulbs [1]. It can contaminate the ecosystems through atmospheric transport and deposition in watersheds. Therefore, the mercury biogeochemical cycle in atmospheric environments and water bodies need to be better constrained. Mercury is unique among the heavy metals in that it can exist in several physical and chemical forms, including metallic, inorganic, and organic forms. Due to the high water solubility, mercury ions (Hg2+) are easily ingested by human beings through water or the food chain. This stable inorganic mercury form passes easily through biological membranes and causes a wide variety of diseases to the brain, nervous system, kidneys, and endocrine system [2], [3].

Currently, several methods and techniques have been established to monitor concentration levels of mercury in water samples. The traditional techniques, such as atomic absorption spectroscopy (AAS) [4], [5], inductively coupled plasma mass spectrometry (ICP-MS) [6], atomic fluorescence spectrometry (AFS) [7], [8] and reversed-phase high-performance liquid chromatography [9], provide limits of detection at ppb (parts-per-billion) level. Although these methods are sensitive, they usually require expensive and sophisticated instrumentation and/or complicated sample preparation processes.

In response to these shortcomings, several techniques, for example, using organic fluorophores [10], [11], [12] or chromophores [13], [14], semiconductor nanocrystals [15], [16], DNAzymes [17], [18], [19], and polymer-oligonucleotide composites [20], have been reported for the detection of Hg2+ in aqueous solution. Unfortunately, these strategies still display drawbacks and have limited practical use owing to the poor aqueous solubility, cross-sensitivity toward other metal ions, high cost and the need for designing molecules such as fluorophores or DNAzymes.

Due to the unique chemical and physical properties, gold nanoparticles (Au NPs) have been used for developing the colorimetric sensors, which can be easily monitored with the naked eye, without any advanced instruments. One of the most interesting properties is their strong surface plasmon resonance (SPR) absorption in the visible wavelength range which depends on their size, shape, interparticle distances, and surrounding medium. These visual methods have been employed for the detection of melamine [21], [22], TNT [23], and heavy metal ions such as Pb2+, and Cu2+ [24], [25], [26], [27], [28], with a high sensitivity. Therefore, Au NPs sensors offer a promising approach for facile tracking of metal ions in aqueous solution. Hg2+ is capable of interacting with thymines and forming stable T–Hg2+–T complexes. Thus, some DNA oligonucleotides bearing T–T mismatches have been designed for the colorimetric detection of mercuric ions [29], [30], [31]. Because of the intrinsic specific interaction between Hg2+ and thymine, this strategy could offer a high selectivity toward Hg2+ over other related environmental heavy metal ions. To overcome the cost issue of using DNA, a cheap ligand, mercaptopropionic acid, was employed for Hg2+ monitoring, based on the coordination between Hg2+ and carboxylic groups of MPA [32], [33], [34]. However, some additional chelating ligand or fluorescent indicator must be used to improve selectivity and/or sensitivity. An alternative strategy has been developed to detect Hg2+ with the use of fluorescent Au nanoclusters (NCs) [35], [36]. Hg2+ could interact with as-prepared protein-modified Au NCs and therefore quench the fluorescence. The high selectivity was attributed to the specific metallophilic interactions between Hg2+ and Au+ [36]. Based on a similar Hg–Au interaction mechanism, citrate-modified gold nanoparticles can be also used for Hg2+ detection. Hg2+ was reduced to Hg(0) by citrate and then directly deposited onto the surface of Au NPs through the formation of Hg–Au alloys [37]. Thus, the stabilizing molecules were desorbed from gold nanoparticles and led to aggregation.

Cysteine is a common amino acid and plays a key role in structure direction and activity of biological processes. Cysteine possesses a terminal thiol moiety, thus it can bind onto the surface of Au NPs through the Ausingle bondS bond and form the cysteine functionalized gold nanoparticles (Cys-Au NPs). Su et al. exploited a simple method based on Cys-Au NPs to detect Hg2+ in aqueous solution [38]. They found that the Cys-Au NPs can be induced agglomeration in the presence of Hg2+. It is well known that mercury has a high thiophilicity [39], thus mercury ions (Hg2+) can combine with cysteine through Hgsingle bondS bond and form Hg–Cys complexes. The binding affinity of the thiol in cysteine for the Hg2+ is substantially greater than that for other metal ions [40]. Taking advantage of this unique attribute of Hg2+, we devised a new, facile and colorimetric sensor to detect Hg2+ based on the anti-aggregation of Au NPs, through the competing combination with cysteine between Hg2+ and Au NPs. We prepared positively charged gold nanoparticles and used cysteine as aggregation agent. Most of previous methods were based on the fact that Au NPs are induced to aggregate by inter-particle crosslinking in the presence of Hg2+, then the color of Au NPs solution changes from red to purple or blue. However, in our method, the detection of Hg2+ was realized through interrupting the aggregation of gold nanoparticles induced by cysteine, which can be observed by the naked eye according to the color changing from purple to red. Up to now there are few reports on the colorimetric detection of Hg2+ based on anti-aggregation of Au NPs. In addition, it is much simpler and more cost-effective than the other existing methods for Hg2+ assay by using a common molecule cysteine in this method, without designing and synthesizing DNA or fluorophore molecules as mentioned before. In this simple way, 2.5 × 10−8 M Hg2+ was detected only by the naked eye.

Section snippets

Chemicals

Poly(diallyldimethylammonium) chloride (PDDA, 25 wt% in water, molecular weight 20,000) was obtained from Sigma–Aldrich (USA). Tetrachloroauric acid (HAuCl4·4H2O) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). l-Cysteine was purchased from North Ringer Biotechnology Co., Ltd. (Beijing, China). HgCl2 and other metal ions were bought from Beijing Chemical Company (Beijing, China). HAc–NaAc buffer solution (pH 4.2) was used to control the acidity of the aqueous

The mechanistic basis for the sensing system

The principle of the Hg2+ colorimetric sensor is illustrated in Fig. 1. The as-prepared Au NPs are stable in the aqueous solution due to the electrostatic repulsion of the positive capping agents, poly(diallyldimethylammonium) chloride (PDDA), against the aggregation between Au NPs even in the presence of a given high concentration of salt. As can be seen in Fig. 1, cysteine could be anchored onto the surface of the positively charged Au NPs through Ausingle bondS covalent bond [42], [43] and cross-link

Conclusions

In this paper, the detection of Hg2+ is realized at room temperature based on anti-aggregation of Au NPs induced by cysteine. That is to say, addition of Hg2+, which is firstly treated with cysteine and leads to the formation of Hg–Cys complex, can deactivate the aggregation reagent cysteine, due to the high affinity between cysteine and Hg2+ through Hgsingle bondS bond. Compared with previous reports, the proposed method needs no complicated pre-modification work, only needing to introduce the simple

Acknowledgments

This work was supported by a grant from the Major National Scientific Research Plan of China (973 Program) (Grant No. 2011CB933202), the Hundred Talents Program of CAS (Y12901FEA2), the National Natural Science Foundation of China (Grant Nos. 20877099 and 20972183), National Sci-Tech Major Special Item for Water Pollution Control and Management (Grant No. 2009ZX07527-007-03), Key Projects in the National Science & Technology Pillar Program during the Eleventh Five-Year Plan Period (Grant No.

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