Introduction

Mercury is one of the most toxic elements and highly toxic because it has serious medical effects on human beings and ecological safety, such as brain damage, kidney failure, and various cognitive and motion disorders [1, 2]. Thus, a rapid and efficient method for determination of Hg2+ is important. In general, absorption spectrometry is one of the most popular methods to detect heavy metals. The absorption efficiency, selectivity, equilibrium time, regeneration, and stability usually depend on the material characteristics of the absorbents [3, 4]. Inductively coupled plasma mass spectrometry (ICPMS) is a powerful technique for the determination of heavy metal ions. However, ICPMS is rather expensive, complex, and not suitable for on-site analysis [5]. Several sensors have been applied for determination of mercury in environmental samples. Because the absorptivity of Au NPs in visible-region are often higher than those of organic dyes [6, 7], Au NPs-based methods for visual or colorimetric sensing of Hg2+ have become of great interest. For example, Chansuvarn [8] used gold nanoparticles stabilized with a dithia-diaza ligand for detection of mercury(II) ion and Tian group [9] used chemodosimeter-functionalized Au NPs for detection of Hg2+. The colorimetric methods are simple. The complex and expensive instruments are not required. But most of these methods involve a color change, which results in the decrease of the sensitivity due to the interference from background colors [10]. The preparation of these colorimetric sensors involves several steps, which is complex and relatively expensive. Fluorescence spectrophotometry can be applied for the determination of the analytes at low concentration. Some new methods based on fluorescence-based probes and sensors for determination of Hg2+ have been reported, such as DNAzymes [11], oligonucleotide platforms [12], CdTe quantum dots [13]. However, nucleic acid or enzyme sensors are rather costly. Most of the quantum dots trend to have narrow excitation spectra and often exhibit broad emission band with red tailing. Thus, it is necessary to develop a new method to meet the requirement for the determination of Hg2+ from wastewater.

It is well known that Raman spectroscopy emerged as fast and non-invasive determination of adulterants in many fields [14–17]. Signals in conventional Raman spectroscopy are very weak and great progress has been made in the development of surface-enhanced Raman scattering (SERS) spectroscopy. SERS is an exceptionally analytical technique for its high sensitivity, fluorescence-quenching property and fingerprint information about the chemical structures. Recently, SERS spectrometry has been developed as a potential trace method and is widely applied for detecting single molecule in chemistry, biology and medicine. When Raman scattering takes place on molecules at the surface or in the close vicinity of noble metal surfaces, such as gold and silver, a very high enhancement (up to 1014–1015) is required [18–22]. Theoretically, the huge local electromagnetic field is attributed to the so-called hot spots, which may be junctions between nanostructures. SERS substrates consist of various metallic nanostructures, including roughened gold and silver electrodes, evaporated thin films, Au or Ag nanoparticles and Au/ Ag core-shell nanostructures [23]. The noble-metal SERS substrate not only provides enhanced Raman signals but also quenches the fluorescence generated by many organic colorants. The Ag NPs as SERS-active sites have been used due to the unique electronic, optical, and chemical properties. Ag NPs are simple to prepare, with good control of size and shape and widely used in electronics [24], sensing [25], biology [26] and SERS [27, 28].

In this work, a method for determination of Hg2+ with mesna modified Ag NPs was described. When mesna molecules are adsorbed on Ag NPs by Ag-S covalent bond, the SERS signals increased significantly due to electromagnetic and chemical enhancement factors [29, 30]. However, after the addition of Hg2+, the mesna was released from the surface of Ag NPs, which caused the decrease of SERS intensity. Hg2+ can be directly determined by monitoring the shift of SERS intensity.

Materials and methods

Reagents and instruments

Sodium 2-mercaptoethanesulfonate (mesna), silver nitrate (AgNO3, 99%) and mercury(II) chloride were purchased from Beijing Ding Guo Biotech. Co. Ltd., China (http://www.dingguo.com). The stock solution of Hg(II) was prepared by dissolving HgCl2 in ultrapure water containing several drops of HCl. The working solution of Hg2+ was prepared by diluting the stock solution with ultrapure water. The metal salts KCl, NaBr, BaCl2·2H2O, NiCl2·6H2O, Na2HPO4·12H2O, MgCl2·6H2O, CaCl2·2H2O, Na2SO4, Na2CO3, Al2(SO4)3·18H2O, FeCl2·4H2O, CuCl2·2H2O, Pb(CH3COO)2·3H2O, MnSO4·H2O, COCl2·6H2O, FeCl3·6H2O, CdSO4·8/3H2O, CrK(SO4)2·12H2O, AgNO3, and SnCl2·2H2O were purchased from Beijing Chemical Reagent Company (http://www.18show.cn/zt340222/Intro.html). Britton–Robinson buffer containing 0.04 mol L−1 H3BO3, 0.04 mol L−1 H3PO4 and 0.04 mol L−1 CH3COOH was adjusted to the desired pH with 0.2 mol L−1 NaOH. The buffer was used to control the acidity of the sample solution. All glassware was cleaned with freshly prepared aqua regia (HCl/HNO3, 3/1, v/v) and rinsed thoroughly with ultrapure water prior to use. Other reagents used here were of analytical reagent grade and used without further purification or treatment.

The transmission electronmicroscope (TEM) image was obtained with a Hitachi H 800 transmission electron microscope (http://www.labbase.net/) operated at an accelerating voltage of 200 kV. The samples for TEM images were prepared by dropping the solutions containing Ag NPs on the carbon-coated copper grid and drying at room temperature. Absorption spectra were recorded on an Astralian GBC Cintra 10e UV–vis–NIR spectrometer within the wavelength range from 300 to 800 nm. Raman spectra were obtained using a BTR 111 MiniRam B&W Tek, Inc. (http://www.shtqsw.com) with a 1 cm quartz cell and the wavelength of the emission line for the laser is 785 nm. The laser power was chosen as 70 mW. Exposure time used for data collection was typically 10 s. The datum processing was operated by using Origin 6.0 software.

Preparation of Ag NPs

The Ag NPs were prepared according to reported method [31]. First, 90 mg of AgNO3 was dissolved in 500 mL of ultrapure water and heated to boiling. Then 10 mL of 1% sodium citrate solution was added into the boiling AgNO3 solution with vigorous stirring. The mixed solution was kept at boiling for 1 h. The green-gray Ag NPs were obtained and the average diameter of the Ag NPs is about 50 nm. The transmission electron microscopy (TEM) image of the Ag NPs is shown in Fig. 1. The resulting colloid was yellowish gray with an absorption maximum at 420 nm and pH of 7.8. Finally, the Ag NPs solution was allowed to cool down to room temperature for further experiments.

Fig. 1
figure 1

TEM image of Ag NPs

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Determination of Hg2+

2 mL of sample solution, 50 uL of 6.4 × 10−5 mol L−1 mesna solution and 800 uL of Ag NPs solution were added into the 5 mL colorimetric tube. Then, 35 uL of 2 mol L−1 NaCl solution and 115 uL of BR buffer solution were added into the mixed solution. The resulting solution was kept at room temperature for 6 min. A quantitative analysis was performed based on the measured peak area at 795 cm−1 in SERS spectrum. The SERS intensity was expressed in the peak area. All the experiments were performed in triplicate.

Results and discussions

Mechanistic basis for the system

Figure 2 shows the schematic illustration for determining mercury. Mesna molecules can adsorb on the surface of Ag NPs by –SH group, which can result in the enhancement of SERS signals. The mesna molecules are released from the surface of Ag NPs in the presence of mercury(II), which caused a decrease in the observed SERS signal. The experimental results indicated that SERS signal decreased with the increase of Hg2+ concentration. Thus, a quantitative determination for Hg2+ was developed.

Fig. 2
figure 2

Schematic illustration of the determination process

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SERS spectra of mesna modified Ag NPs

A typical SERS spectrum of mesna modified Ag NPs is shown in Fig. 3. The band in SERS spectrum located at 795 cm−1 is attributed to the C-S stretching vibration and sulfur is from the sulfonic group. The other major bands in the SERS spectrum are located at 540, 591, 704, 1,058 and 1,295 cm−1. All of the bands are characteristic for chemisorbed mesna molecules. The bands at 540 and 591 cm−1 are attributed to the C-C stretching vibrations. The band at 540 cm−1 is characteristic of the gauche (G) conformer of the Ag-S-C-C chain, whereas the band at 591 cm−1 is typical of the trans (T) conformer. The band at 704 cm−1 is also due to the C-S stretching vibration but sulfur is from the thiol group. The 704 cm−1 band is characteristic of chemisorbed mesna molecules having a trans conformation of the Ag-S-C-C and the bands at 1,058 and 1,295 cm−1 are due to the symmetric and anti symmetric (SO 2−3 ) stretch, respectively [32–35].

Fig. 3
figure 3

SERS spectrum of mesna-Ag NPs system

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There is not any observable S-H stretching vibration band in the range of 2,500–2,600 cm−1. It suggests that the S-H bond is cleaved and adsorbed onto the surface of Ag NPs. In some complexes the Ag-S vibration was reported at a frequency as high as 363 cm−1 [36]. So, in the present study the 371 cm−1 band may be assigned to the Ag–S stretching mode.

The optimization of experimental conditions

Effect of mesna concentration

The effect of mesna concentration on the SERS intensity was investigated and is shown in Fig. 4. When the concentrations of the mesna vary from 0.4 × 10−5 to 2.4 × 10−5 mol L−1, the SERS intensities increase with the increase of the concentrations of mesna and change insignificantly until 2.8 × 10−5 mol L−1. The shift of SERS intensity is expressed as ΔI SERS = I 0SERS − I SERS. Here I 0SERS and I SERS are the SERS intensities of the system in the absence and presence of Hg2+, respectively. The ΔI SERS intensity reaches the maximum when the concentration of mesna is 1.6 × 10−5 mol L−1. Therefore, the 1.6 × 10−5 mol L−1 mesna was selected for subsequent work.

Fig. 4
figure 4

Effect of mesna concentration on the SERS intensity of mesna-Ag NPs system in the absence (a) and presence (b) of Hg2+. Concentration of NaCl, 0.035 mol L−1. Concentration of Hg2+, 0.8 μmol L−1. pH, 7

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Effect of pH value

The effect of pH on the SERS intensity was investigated in the pH value range from 4 to 10. As shown in Fig. S1 (Electronic Supplementary Materials, ESM) the ΔI SERS values in the presence and the absence of Hg2+ change slightly with the change of pH value. So pH value 7 was selected for the determination of Hg2+.

Effect of incubation time

The influence of incubation time of mesna modified Ag NPs was also investigated. As shown in Fig. S2 (ESM), the reaction between mesna modified Ag NPs and Hg2+ occurs rapidly at room temperature. The ΔI SERS values in the presence and the absence of Hg2+ change slightly within 9 min. The incubation time of 6 min was selected to achieve stability SERS signal in the experiment.

Effect of sodium chloride concentration

The concentration of sodium chloride plays an important role in the experiment. Fig. S3 (ESM) shows the dependence of chemical enhancement on the sodium chloride concentration. The SERS peak at 795 cm−1 was chosen. It can be seen from Fig. S3 that the SERS intensity increases with the increase of NaCl concentration and reaches a maximum at 0.035 mol L−1. When the concentration of NaCl is higher than 0.035 mol L−1, the SERS intensity decreases slightly. It is well known that NaCl is used as an aggregating agent for Ag NPs in the enhancement process, and the concentration of NaCl can influence the aggregation degree of Ag NPs.

It is generally agreed that two fundamentally different mechanisms dominated in the SERS phenomenon, one is electromagnetic (EM) enhancement and the other one is the resonance-like charge-transfer (CT) enhancement. The salts can improve electromagnetic enhancement caused by particle aggregation, anion-induced adsorption/reorientation of analyte and an charge transfer contribution. First, the aggregated particle widens the plasmon band so that a wide range of excitation laser wavelength can be used for electromagnetic enhancement. Aggregation processes can produce junctions between particles, and these junctions act as electromagnetic hot spots. The analyte can easily adsorb onto those hot spots during the aggregation processes. Thus, the analyte has to be present during the particle aggregation process so that the analyte molecules have a higher chance of adsorbing on those hot spots [18]. Second, Cl- is chemisorbed onto the colloidal surface owing to their high affinity for silver and can change the surface morphology of the SERS-active substrate [37]. These latter are active sites for the formation of molecule/metal complexes by charge-transfer effect that leads to an increase in the interaction between mesna and the Ag NPs [38].

In order to understand the effect of NaCl, the absorption spectra of Ag NPs were measured in the absence and presence of NaCl. The absorption spectra can provide the aggregation degree of Ag NPs. As shown in Fig. 5, with the increase of NaCl concentration, the intensity of the surface plasmon absorption band decreases and the absorption peaks of Ag NPs are broadened. The surface plasmon band is close to the laser excitation wavelength of 785 nm, which can increase the electromagnetic enhancement effect.

Fig. 5
figure 5

Absorption spectra of the Ag NPs in the presence of different concentrations of NaCl. Concentrations of NaCl: (1) 0.0, (2) 0.014, (3) 0.027, (4) 0.036, (5) 0.042 mol L−1

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Selectivity of the sensing system

To investigate the selectivity of the present method, several metal ions were tested under the same experimental conditions when the concentration of Hg2+ was 4 × 10−7 mol L−1. The experimental results are listed in Table 1. Most of the metal ions at high concentrations have a little influence on the SERS intensity of the system. Mesna modified Ag NPs exhibit high selectivity for the determination of Hg2+ in the presence of a variety of competitive ions. The affinity between Hg2+ and –S is stronger than those between other ions and –S. Hg2+ ions are expected to bind with mesna modified Ag NPs through cooperative metal–ligand interaction to form a stable complex. It is can be concluded that mesna modified Ag NPs can be used to selectively determine Hg2+ in practical samples.

Table 1 Influence of potentially interfering ions
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Determination of Hg2+

Under the optimal conditions, different concentrations of Hg2+ were determined. Figure 6 shows the representative SERS spectra of mesna and Ag NPs system in the presence of Hg2+. It is can be seen from Fig. 6 that the intensity of SERS decreases with the increase of Hg2+ concentration. The intensity of the Raman peak at 795 cm−1 was used for the quantification. Inset shows the SERS responses in the presence of Hg2+ at different concentrations. ΔI SERS is directly proportional to the concentration of Hg2+ in the range of 1–200 × 10−8 mol L−1 with a correlation coefficient (R2) of 0.996. The regression equation is ΔI SERS = 1,767 × 108 C + 1,964 (C: mol L−1). The detection limit, which corresponds to signal-to-noise ratio 3, is 0.0024 μmol L−1. For comparation, the experimental results for the determination of Hg2+ obtained by some other methods are listed in Table 2. Compared with other methods when this method is applied the operation is simpler and the selectivity is superior.

Fig. 6
figure 6

SERS spectra of mesna-Ag NPs system in the presence of Hg2+. Inset shows the relationship between ΔI SERS and the concentration of Hg2+. Concentrations of Hg2+: (1) 0.1 × 10−7, (2) 1 × 10−7, (3) 3.5 × 10−7, (4) 5 × 10−7, (5) 12 × 10−7, (6) 20 × 10−7 mol L−1. Concentration of mesna, 1.6 × 10−5 mol L−1. Concentration of NaCl, 0.035 mol L−1. pH, 7

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Table 2 Comparison of previously reported method with the present method for the detection of Hg2+
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Analysis of environmental water samples

The application of the present method was evaluated for determination of Hg2+ in three water samples. Accuracy of this method was evaluated by determining the recoveries of Hg2+ in the water samples. Three water samples were collected from South Lake, Groundwater and Jingyue Lake in Changchun respectively. The samples were spiked with Hg2+ at two concentration levels, filtered through a 0.22 μm membrane and then analyzed by SERS spectroscopy. The results are summarized in Table 3. It is can be seen from Table 3 that the recoveries of Hg2+ at two concentration levels are between 71% and 115%, the RSDs are between 0.3% and 2.3%. The results reveal that the present method can be applied to the analysis of real water samples.

Table 3 Analytical results for the three real samples
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Conclusion

The mesna-modified Ag NPs can be used as probe to determine mercury ions. This method will have several advantages: a) it can be 2–3 orders of magnitude more sensitive than the usual colorimetric method; b) it exhibits excellent selectivity; c) it takes only 6 min to determine mercury in aqueous solution; d) only a low-cost and portable device is needed. This method is simple and convenient. Therefore, this method might provide a potential tool for the sensitive determination of Hg2+ in biological and environmental samples.