Fluorescent Gold Nanoprobes for the Sensitive and Selective Detection for Hg2+
© The Author(s) 2010
Received: 25 May 2010
Accepted: 2 August 2010
Published: 2 September 2010
A simple, cost-effective yet rapid and sensitive sensor for on-site and real-time Hg2+ detection based on bovine serum albumin functionalized fluorescent gold nanoparticles as novel and environmentally friendly fluorescent probes was developed. Using this probe, aqueous Hg2+ can be detected at 0.1 nM in a facile way based on fluorescence quenching. This probe was also applied to determine the Hg2+ in the lake samples, and the results demonstrate low interference and high sensitivity.
Among heavy metal ions, mercury is considered highly toxic and widespread pollutant, and its damage to the brain, nervous system, endocrine system and even the kidneys is well known . Water-soluble divalent mercuric ion (Hg2+) is one of the most usual and stable form of mercury pollution, which provides a pathway for contaminating vast amounts of water and soil. Ionic mercury can be converted into methyl mercury by bacteria in the environment, which enters the food chain and accumulates in higher organisms . Therefore, environmental monitoring of aqueous Hg2+ becomes an increasing demand. Traditional methods such as inductively coupled plasma mass spectrometry (ICPMS) to detect mercury ions often are costly, nonportable and require sophisticated instruments for analysis . Thus, the development of new, practical assays for Hg2+ detection remains a challenge. Toward this goal, a number of the sensitive and selective sensors for the detection of Hg2+ have been developed, based on gold nanoparticles [4–7], fluorophores [8–11], DNA or DNAzymes [12–16], polymer materials [17, 18] and proteins [19, 20]. Among these sensors, many are constructed with oligonucleotides containing thymine (T) as the sensing elements. But using nucleic acids or enzyme as sensing element makes it too costly and time-consuming for real life. Moreover, the US Environmental Protection Agency (EPA) standard for the maximum allowable level of inorganic mercury in drinking water is 2 ppb (10 nM) . This concentration is much lower than the detection limit of most available sensors. There is a need for more sensitive, convenient and economic methods to detect Hg2+ in environment.
Recently, highly fluorescent gold nanoparticles were used as probes to detect Hg2+ based on fluorescence quenching through Hg2+-induced aggregation of AuNPs . The limit of detection was 5 nM, which was a lower result than EPA standard, and confirmed that the fluorescent gold nanoparticles were sensitive sensors. And Ying et al. have reported a red-emission high-fluorescent bovine serum albumin (BSA) functionalized gold nanoparticles (BSA-GNPs) by a simple, one-pot, “green” synthetic route . Considering the BSA molecular surface is rich in amino, carboxyl groups that can readily coordinate with heavy metal ions, maybe resulting in the fluorescence quenching of BSA-GNPs, we employed BSA-GNPs to detect Hg2+ by the fluorescence quenching in this work. In addition, a practical environmental assay was carried out.
Chemicals and Materials
All chemicals used were of analytical grade or of the highest purity available. Chloroauric acid (HAuCl4) was obtained from Sigma–Aldrich (USA) and used as received. BSA was obtained from Genview. The used metal salts Pb(NO3)2, Ni(NO3)2·6H2O, Cd(NO3)·4H2O, FeCl2·4H2O, Mg(NO3)2·6H2O, CaCl2·2H2O, Co(Ac)2·6H2O, Zn(Ac)2·2H2O, BaCl2·2H2O, CuSO4·5H2O, Hg(NO3)2·2H2O, Cr(NO3)3·9H2O, Mn(Ac)2·4H2O and NaOH were purchased from Beijing Chemical Reagent Company (Beijing, China). The stock solution of Hg2+ was 5 mM. All glassware was thoroughly cleaned with freshly prepared 3:1 HCl/HNO3 (aqua regia) and rinsed thoroughly with Mill-Q (18 MΩ cm−1 resistance) water prior to use.
The fluorescence spectra were measured using a Cary Eclipse Fluorescence spectrophotometer (USA Varian). Both the excitation and emission slit widths were set to 20 nm, and the measurement was done at 20°C. The excited wavelength was 470 nm.
Preparation of BSA-GNPs
Procedures for Hg2+ Detection
The as-prepared BSA-GNPs were diluted to 40 times, the resulting BSA-GNPs solution was used as the detection probe. The samples were added into the BSA-GNPs probe with a volume ratio (V/V) 4:1, which was the mixture of the 400 μL Hg2+ solution and 100 μL BSA-GNPs probe. In the experiments of sensitivity of the detection, the solution of Hg2+ was prepared from 0.1 nM to 4 μM. To confirm the practical application of the probes, a water sample from a lake was filtered through a 0.2-μm membrane. A 100 μL of 100 μM Hg2+ solution was added into 900 μL lake water, the resulting solution was 10 μM, which was further diluted a series of different concentrations of samples with lake water.
Result and Discussion
Fluorescent Detection of Hg2+
The overall detection strategy is shown in Fig. 1a. BSA is a large globular protein with a good essential amino acid profile and bear fluorescence emission groups (tryptophan, tyrosine and phenylalanine). So, there are lots of amino acid residues on the surfaces of BSA-GNPs, which can be proved by the FTIR spectra (Fig. S2) .
To evaluate the attachment of BSA onto the surface of Au, the FTIR spectra of BSA and BSA-GNPs were shown in Fig. S2. Compared to the spectrum of pure BSA, the characteristic peaks of BSA-GNPs such as at 1,654, 2,958 and 3,421 cm−1 can be clearly seen, which indicating the modification of BSA on the Au nanoparticles. Compared with the spectrum of pure BSA (Fig. S2a), the characteristic peaks of BSA-GNPs such as at 1,654, 1,546 cm−1 can be clearly seen (Fig. S2b), which are attributed to the amide I and amide II vibrations of BSA on the Au nanoparticles [24, 25]. The FTIR spectra indicated that the basic structure of BSA still maintained after modified the gold nanoparticles. In addition, the residues of amino acid can readily coordinate with heavy metal ions [26, 27], when Hg2+ coordinated with the BSA-GNPs, the fluorescence emission of BSA-GNPs may be quenched.
The fluorescence emission of the BSA-GNPs was shown in Fig. 1b; the maximum emission and excitation wavelengths were at 640 and 470 nm, respectively. The fluorescence emission was readily quenched in the presence of Hg2+. As illustrated in Fig. 1b, the BSA-GNPs solution showed strong emission at 640 nm (curve a), when added the solution of 4 μM Hg2+ (curve b), the fluorescence emission was quenched immediately. The inset is the corresponding photograph, the BSA-GNPs solution showed strong red emission under UV light and the red emission of BSA-GNP-probes was quenched by the addition of Hg2+. The corresponding TEM images of original and the aggregated BSA-GNPs were shown in Fig. S3. After the addition of Hg2+, the dispersed BSA-GNPs with diameter in ca. 2 nm (Fig. S3a) aggregated seriously (Fig. S3b). Therefore, Hg2+ detection could be easily realized via monitoring the fluorescence quenching of the BSA-GNPs under the UV light.
Selectivity of the Sensing System
To further evaluate the selectivity, the Hg2+ detection in the presence of other metal ions was investigated. The fluorescence responses of BSA-GNPs to different concentrations of Hg2+ in mixture solutions were obtained in Fig. S5. The results indicated that the fluorescent intensity of BSA-GNPs still showed a gradual decrease with the concentration of Hg2+ increasing coexisting with other metal ions in solution 1. Compared with mixture solution 1 (Fig. S5a), the fluorescent intensity of BSA-GNPs decreased a lot coexisting with Cu2+, Pb2+ and Ni2+ (Fig. S5b), but it did not affect the detection of Hg2+. Though other metal ions coexisted, the experimental results show that Hg2+ can be detected against other heavy metals.
In summary, a simple, portable detection method based on fluorescent BSA-GNPs probe that allows rapid, on-site, real-time detection of Hg2+ has been developed. The method would also address many of the challenges of the instrument-intensive methods of analysis. The experimental results show that Hg2+ can be detected quickly and accurately with very low detection limit (0.1 nM) and excellent discrimination against other heavy metals. We believe this method may act as a platform for the rapid detection of Hg2+ in aqueous biological and environmental samples with high selectivity and sensitivity.
Electronic supplementary material
(See supplementary material 1)
The authors gratefully acknowledge the financial support from the National High-tech Research and Development Program of China (863 Program 2007AA03Z354), the National Natural Science Foundation of China (Project No. 20703009, 20971020).
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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