1. Introduction

With the high level of industrial development, lead and its compounds have been applied to various aspects including lead-acid batteries, industrial plating and smelting production [1]. But when enjoying the convenience brought by high-level industry, we have to suffer from the troubles of heavy metal contamination. For instance, waste water from the lead smelting production contains Pb²⁺. With improper treatment, Pb²⁺ will be enriched in water or land eventually and cannot be degraded, which is seriously detrimental to the environment. For humans, Pb²⁺ can cause varieties of adverse effects even at a very low level [2]. Human may suffer from anemia, neurological dysfunction, reproductive system damage and kidney damage induced by Pb²⁺ [3,4]. Blood lead levels, even as low as 50 µg/L, can still damage the baby's brain and cause lifelong health problems [5]. Therefore, Pb²⁺ has been listed as a notorious toxic environmental pollutant. Drinking water is a major source of lead exposure for human, and the world health organization (WHO) has set a standard of 10 µg/L for Pb²⁺ in drinking water [6]. Up to now, various Pb²⁺ trace detection techniques have been developed, such as atomic absorption spectrometry (AAS) [7–9], inductively coupled plasma mass spectrometry (ICP-MS) [10,11], inductively coupled plasma-atomic emission spectroscopy (ICP-AES) [12] and fluorescence spectroscopy [13–18]. Although these traditional methods exhibit excellent sensitivity, the requirement of cumbersome steps and the extended times limit their wide applications in Pb²⁺ detection on site. Thus, there is a great demand of new strategies for easy and rapid detection of Pb²⁺.

Colorimetric sensing approaches of heavy metal ions has been of interests because they do not depend on the advanced instruments and often response quickly. To date, Memon et al. reported that the truncated DNAzyme was split into single-stranded DNA fragments in the presence of Pb²⁺. The detached DNA fragments were then adsorbed onto gold NPs and protected against salt-induced aggregation, which would result in rapid color change from blue to pink [19]. Rong et al. designed a Pb²⁺ direct detection platform based on magnetic nano-enzyme, magnetic beads and gold nanoparticles, which can be used for colorimetric detection of lead ions [20]. Sang et al. presented a fast, simple and sensitive colorimetric sensor that can use tyrosine-functionalized gold NPs to simultaneously (or separately) detect Cr³⁺ and Pb²⁺ [21]. Guo et al. developed cost-effective and rapid colorimetric method for simultaneous detection of Hg²⁺, Pb²⁺and Cu²⁺ using papain enzyme-functionalized gold NPs with a detection limit of 0.2 µM [22]. However, colorimetric analysis may be prone to interference by the colors of samples. Moreover, the detection limit of some colorimetric methods is sometimes not satisfactory.

SERS which can amplify the Raman signals by several orders of magnitude (as high as 10¹⁵) using plasmonic nanostructures, shows to be another choice for highly selective and sensitive detection. For example, Olga Guselnikova et al. designed a surface plasmon-polariton based platform to realized selective and sensitive detection of heavy metal ions (with LOD of 10⁻¹⁴ M) [23]. They also developed a dual-mode functional chip for stereoselective discrimination of chiral amines using Wettability-Based mobile and portable SERS measurements [24]. Mark S. Frost et al. utilized a SERS-active citrate-functionalized gold sensor to detect Pb²⁺ through the metal affinity of citrate molecules [25]; Fu et al. proposed a highly sensitive and selective SERS method for determining Pb²⁺ based on a DNAzyme-linked plasmonic nanomachine and this method realized the lowest detection concentration of Pb²⁺ to 1.0 nM [26]; Shi et al. reported a kind of poly adenosine assisted SERS silicon wafer composed of core(Ag)-satellite(Au) NPs modified silicon wafer, which was used for high-performance Pb²⁺ detection [27]; Zou et al. used oligonucleotide functionalization and gold-plated polystyrene microspheres to construct a new conceptual DNA logic gate for synchronous detection of Hg²⁺ and Pb²⁺ [28]. Despite the impressive progress has been achieved, the applicability and sensitivity of Pb²⁺ sensing approaches still need improvement especially in the practical samples.

Taking the above-mentioned into account, herein, we present a dual-mode sensing method based on colorimetry and SERS for the detection of Pb²⁺. The principle of this method is illustrated in Fig. 1. Generally, gold NPs are first coated with silver nanoshells by the reduction of silver nitrate using glucose. Compared to Au or Ag NPs, Au@Ag core/shell structures have more advantages which combine the strong SERS activity of Ag and well stability of Au. In the growth of the silver shells, Gluc is adsorbed onto the surface of silver shells as the stabilizing agent [29–31] . The Gluc stabilized Au @ Ag NPs are then tagged with Raman reporter 2-NT through the Au-S bonds, which results in the formation of Gluc/2-NT@Au@Ag NPs. In such core-shell nanostructure, Au@Ag NPs are employed as SERS generators to boost the Raman signals of 2-NT. And the adsorbed Gluc can selectively form a complex with the lead ion due to the affinity between carboxyl and hydroxyl groups in Gluc and Pb²⁺ [32]. Upon the addition of Pb²⁺, the nanoprobes are aggregated and the hot spots are simultaneously formed, which consequently induced the obvious color change and significant enhancement of the Raman signals of 2-NT. This method combines the advantages of both colorimetry and SERS techniques, covering a broad response range with easy operation, fast readout and excellent sensitivity. This is particularly favorable with a large number of samples, as we can implement rapid preliminary screening with naked eyes and subsequently employ SERS spectra for quantitative analysis.

 

Fig. 1. (A) The synthesis principle of Gluc/2-NT@Au@Ag NPs. (B) Schematic illustration of SERS and colorimetric determination for Pb²⁺.

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2. Experimental section

2.1 Chemicals and reagents

Chloroauric acid tetrahydrate (HAuCl₄·4H₂O), sodium citrate dihydrate (C₆H₅Na₃O₇•2H₂O ≥ 99.0%), silver nitrate (AgNO₃), sodium hydroxide (NaOH), D (+)-Glucose (C₆H₁₂O₆) and D-Gluconic acid sodium salt (C₆H₁₁NaO₇, ≥99.0%) were purchased from MACKLIN (Shanghai, China). The chloride salts of Ni²⁺, Cu²⁺, Cr²⁺, Ca²⁺, Zn²⁺, K⁺, Hg²⁺, Na⁺ and Pb²⁺ were purchased from MACKLIN (Shanghai, China). 2-Naphthalenethiol (C₁₀H₇SH, ≥99.0%) were purchased from Sigma-Aldrich (Shanghai, China). All the reagents were used as received and deionized water with a resistivity of 18.25 M Ω · cm was used in all of the experiments. The real samples of tap water and drinking water were collected from Shanghai, China.

2.2 Instruments

Extinction spectra were recorded by a Shimadzu UV-3600 PC spectrophotometer with quartz cuvettes of 1 cm path length. Transmission electron microscopy (TEM) images were obtained with an JEM-2100F, JEOL electron microscope operating at 200 kV. SERS spectra were collected by a confocal Raman spectrometer (XploRA Plus, HORIBA) at the excitation of 638 nm. The scattering light was collected by a 10X objective lens to CCD. All the SERS spectra were acquired with two accumulations and for each accumulation, the exposure time was 4 seconds.

2.3 Preparation of Gluc/2-NT@Au@Ag NPs

According to the method of producing gold seeds with G. Frens [33], gold NPs were synthesized by reducing chloroauric acid with sodium citrate. Typically, 306 µL of HAuCl₄·4H₂O (10% w/w) was dissolved in 99.694 mL of deionized water. The mixture was heated to boiling. Then, 10 mL of 47.9 mM sodium citrate dihydrate was added into the boiled mixture. The mixed solution was kept on boiling for 15 min and then allowed to cool to room temperature.

6 mL of the resulting gold NPs were centrifuged at 9500 r/min for 35 min to remove excessive sodium citrate. The obtained gold NPs were dissolved in 54 mL of deionized water, and the mixture was then heated to 60°C under rapid agitation. Subsequently, 500 µL of 0.08 M D (+)-Glucose solution and 140 µL of 0.1 M AgNO₃ solution were added, followed by the addition of NaOH solution (1 M, 200 µL). After an interval of 3 minutes, 5 mL of 37.8 mM D- Gluconic acid sodium salt was added to the mixed solution at a rate of 200 µL every 10 seconds. The mixture was further heated for 30 minutes. After the color of the resulting solution became orange, the solution was cooled to room temperature and kept in the dark for at least one week. During this period, Gluc containing hydroxyl groups and carboxyl group could be bounded to the silver shell by physical adsorption more efficiently [34].

6 mL of the above Gluc/2-NT@Au@Ag NPs solution was added by 30 µL of 2-NT (100 mM). The mixture was oscillated for 2 h and kept in dark for later use.

2.4 Colorimetric and SERS detection of Pb²⁺

First of all, the detection of Pb²⁺ was conducted by colorimetric method. Specifically, 20 µL of aqueous solutions containing Pb²⁺ with different concentrations (ranging from 10⁻⁴ to 10⁻⁶ M) were added to 180 µL of the Gluc/2-NT@Au@Ag solution. The mixtures were then photographed and subjected to extinction detection by UV-3600 PC spectrophotometer. In the experiments, the color of the mixture with higher concentration of Pb²⁺ changed faster and more remarkably. Correspondingly, the recorded extinction spectra with an obvious change could be used for quantitative analysis.

Despite the colorimetric sensing is fast and efficient, the relatively low detection limit is not suitable for trace sensing. For trace sensing of the lead, SERS-based detection shows an advantage. In a specific experiment, aqueous solutions containing Pb²⁺ (10⁻⁶–10⁻¹¹ M, 20 µL) were added to 180 µL of the Gluc/2-NT@Au@Ag NPs solution. After 45 minutes, Raman spectra of the mixtures were recorded. Apparently, as the concentration of Pb²⁺ increased, the Raman signals of 2-NT were dramatically enhanced. The SERS spectra were collected for the quantitative sensing of Pb²⁺.

2.5 Selective detection of Pb²⁺

To test the selectivity of this presented method for the detection of Pb²⁺, several ubiquitous metal ions Ni²⁺, Cu²⁺, Cr²⁺, Zn²⁺, Ca²⁺, K⁺, Ag⁺, Hg²⁺ and Na⁺ aqueous solutions were used as control. In this experiment, the solutions of various metal ions (10⁻⁵ M, 20 µL) were added into 180 µL of the Gluc/2-NT@Au@Ag NPs solution. Only in the presence of Pb²⁺, the color and the extinction spectra of the solution was clearly changed. As for other metal ions, no obvious change was observed whether for the color or extinction spectra.

2.6 The detection of the real samples

To confirm the feasibility of the nanosensor for the application in real sample such as tap water and drinking water, the recovery experiments were performed under present conditions by adding Pb²⁺ spiked real samples into the Gluc/2-NT@Au@Ag solutions. The LSPR band at 417 nm and Raman band at 1621 cm⁻¹ of all the solutions were used to calculate Pb²⁺ concentration in the samples according to the calibration graphs, which could be treated as recovery results, and then the recovery rate was calculated by the formula (see Appendix A for details).

2.7 Statistical Analysis

Actually, for colorimetry sensing, 10 samples with Pb²⁺(10⁻⁶–10⁻⁴ M) were tested and the given spectrum for each sample was averaged from 15 measurements. While for SERS sensing, 6 samples with Pb²⁺(10⁻¹¹–10⁻⁶ M) were tested and the given spectrum for each sample was averaged from 15 measurements. All the experiments were repeated 5 time. Similarly, the blank sample and other metal ions samples were tested in the same way.

3. Results and discussion

3.1 Characterization of Gluc/2-NT@Au@Ag NPs

The preparation of Gluc/2-NT@Au@Ag NPs contained three steps. First, the gold NPs were synthesized through the reduction of HAuCl₄·4H₂O mediated by sodium citrate dihydrate. Au NPs were examined by TEM and UV-vis spectroscopy. Figure 2A showed that the gold NPs were spherical and monodispersed with a size about 12 nm. As indicated in Fig. 3A(a), an obvious absorption band at 520 nm appeared, which was typical for the surface plasmon resonance of gold NPs. And the gold colloid solution was burgundy. Then, Au NPs were coated by silver shells through the reduction of AgNO₃ by D (+)-Glucose, and simultaneously Gluc was acted as capping agent. After the silver shell coating, the color changed from burgundy to yellow. With the increase of the thickness of silver shell, the original absorption peak at 520 nm was blue-shifted to 411 nm (Fig. 3A(b)) due to the increased particle volume and the dielectric properties of silver [35]. Thus, Gluc capped Au-Ag core-shell nanostructures were synthesized (denoted as Gluc@Au@Ag). Gluc@Au@Ag NPs were then observed by TEM which are monodispersed with an averaged diameter of about 22 nm (Fig. 2B). Finally, the Raman reporter 2-NT was tagged onto the surface of Gluc@Au@Ag via a Ag-S bond. The obtained 2-NT tagged Gluc@Au@Ag was also observed by TEM (Fig. 2C) which showed a similar morphology as Gluc@Au@Ag NPs. Moreover, the SPR band of Gluc/2-NT@Au@Ag NPs red-shifts to 417nm (Fig. 3A(c)). The obtained Gluc/2-NT@Au@Ag NPs are expected to keep their colloidal stability before the utilization for sensing. As shown in Fig. 2(E) and 2(F), Gluc/2-NT@Au@Ag NPs preserved the hydrodynamic diameter and absorption band over time, which indicated that Gluc/2-NT@Au@Ag NPs were quite stable. SERS performance of Gluc/2-NT@Au@Ag NPs were then investigated, which proved such Gluc/2-NT@Au@Ag NPs as homogeneous substrates (Fig. 2(G) and 2(H)). Gluc/2-NT@Au@Ag NPs exhibited characteristic SERS bands of 2-NT at 422, 842, 1064, 1379 and 1621cm⁻¹ [33] (see Appendix B for detailed assignments). We thereafter conducted a preliminary experiment. While 7.5E-6 M Pb²⁺ was added into Gluc/2-NT@Au@Ag NPs, the NPs were found to aggregate quickly accompanied by a significant change of the color (Fig. 3A). This is reasonable because the Gluc on the silver shells can bind with Pb²⁺ to form a complex. Simultaneously, as shown in Fig. 3B, the SERS signals of Gluc/2-NT@Au@Ag in the presence of Pb²⁺ were observed to be intensely amplified which was caused by the hotspots occurred in the aggregates. The underlying mechanism of the hot-spots based Raman enhancing were then demonstrated by performing the simulation of electromagnetic field distributions in the Au@Ag NPs aggregates with FDTD solutions. According to TEM images in Fig. 2D, in the presence of the lead ions, the distances between Au@Ag NPs were counted to be around 1.72 nm. Then different possible the aggregation states of Au@Ag NPs were simulated at the interdistance of 1.72 nm(see Appendix C for the simulation of electromagnetic field distributions). Compared with one single metal particle, the aggregates exhibited the stronger electromagnetic coupling effect due to the inherent interparticle hot-spots. And as more particles formed the aggregates, more hot-spots were created which correspondingly amplify the Raman vibrations. The FDTD calculation is in agreement with experimental measurements, which provide some certain theoretical support for experiments. Such preliminary results facilitated the feasibility of using Gluc/2-NT@Au@Ag for colorimetric and SERS dual-mode sensing of the lead ions.

 

Fig. 2. TEM images of (A) Au NPs. (B) Gluc@Au@Ag NPs. (C) Gluc/2-NT@Au@Ag NPs. (D) The aggregations of Gluc/2-NT@Au@Ag NPs in the presence Pb²⁺ of 7.5E-6 M. (E) The extinction spectra of the Gluc/2-NT@Au@Ag NPs suspension (without the addition of Pb²⁺) for 5 days. (F) The hydrodynamic size of the Gluc/2-NT@Au@Ag NPs suspension (without the addition of Pb²⁺) for 5 days. (G)The histogram of SERS intensity of Gluc/2-NT-@Au@Ag NPs at 1621 cm⁻¹ band. (inset: SERS mapping image integrated at 1621 cm⁻¹ band and the corresponding bright-field image). (H) A serials of SERS spectra for all mapping blocks in (G) (RSD of the intensities of 1621 cm⁻¹ band is calculated to be 9.365%).

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Fig. 3. (A) Extinction spectra of (a) Au NPs, (b) Gluc@Au@Ag NPs, (c) Gluc/2-NT@Au@Ag NPs and (d) Gluc/2-NT@Au@Ag NPs aggregations in the presence Pb²⁺ of 7.5E-6 M. (inset: Corresponding photographs (Sequencing from left to right: Au NPs, Gluc@Au@Ag NPs, Gluc/2-NT@Au@Ag NPs, the aggregations of Gluc/2-NT@Au@Ag NPs in the presence Pb²⁺)). (B) The SERS spectra of (a) Gluc/2-NT@Au@Ag NPs aggregations in the presence Pb²⁺ of 7.5E-6 M. (b) Gluc/2-NT@Au@Ag NPs and (c) Gluc@Au@Ag NPs.

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3.2 Detection of Pb²⁺ based on Gluc/2-NT@Au@Ag

3.2.1 Colorimetric sensing of Pb²⁺

To analyze the quantitative relationship between various Pb²⁺ concentrations and corresponding extinction spectra intensity, the as-prepared nanoprobe (Gluc/2-NT@Au@Ag) were used to achieve the detection of Pb²⁺. First, time-sensitive extinction spectra of the dispersion were recorded after the addition of Pb²⁺. As shown in Fig. 4(A) and 4(B), Gluc/2-NT@Au@Ag NPs suffer from different aggregation stages. The relatively stable stage was between 30 min to 120 min. In our experiments, the extinction spectra at 45 min were used. Then, different concentrations of Pb²⁺ were added into the dispersion of the Gluc/2-NT@Au@Ag NPs. The response of Gluc/2-NT@Au@Ag was recorded. As shown in the Fig. 4D, after the addition of Pb²⁺ with concentrations ranging from 10⁻⁶ to 10⁻⁴ M, the colors of the dispersions of Gluc/2-NT@Au@Ag changed rapidly from yellow to orange and finally dark green, which were easily distinguishable by naked eyes. This is because the addition of Pb²⁺ induces the aggregation of Gluc/2-NT@Au@Ag NPs through the affinity between them. The extinction spectra of the dispersions were investigated after 45 min which further proved the occurrence of aggregation. It was noted that the SPR band of Gluc/2-NT@Au@Ag at 417 nm gradually decreased as the concentration of Pb²⁺ increased and a new broad band around 570–640 nm appeared in the extinction spectra (Fig. 4C). Herein, the linear dependence between the intensity of the absorption band at 417 nm and Pb²⁺ concentration was obtained. The calibration curve in the Pb²⁺ concentration ranging from 2.5E-5 to 1E-6 M was plotted in Fig. 4D. The relationship between the decrease in the SPR band intensity (Y) of Gluc/2-NT@Au@Ag at 417 nm and Pb²⁺ concentration (X) was regressed linearly as the following equation:

 

Fig. 4. (A) Extinction spectra of Gluc/2-NT@Au@Ag NPs in presence of 1E-5 M Pb²⁺ over time. (B) Plot of absorbance of SPR band at 417 nm with time in presence of different concentration of Pb²⁺ ions (1E-6 M, 5E-6 M and 1E-5 M). (C) Extinction spectral change of Gluc@Au@Ag NPs in presence of Pb²⁺ (10⁻⁴–10⁻⁶ M). (D) Linear plot of absorbance of SPR band at 417 nm at different concentrations of Pb²⁺. The error bars indicate the standard deviation of 15 measurements.

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Table 1. Linear plot of absorption band at 417 nm and SERS intensity of the 1621 cm⁻¹ band upon the addition of Pb²⁺ in different concentrations.

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When the Pb²⁺ concentration was down to 10⁻⁶ M, no obvious color change of the nanoprobe could be observed with the addition of Pb²⁺. Similarly, there is no apparent change in the SPR band of the nanoprobe after the addition of Pb²⁺ at a level of 10⁻⁶ M. Hence, a more sensitive sensing method is necessary for lower concentration detection of Pb²⁺.

3.2.2 SERS sensing of Pb²⁺

As illustrated before, the addition of Pb²⁺ can induce the aggregation of the nanoprobes. As the concentration of Pb²⁺ decreased, the lower aggregation degree was obtained corresponding with weaker SERS signals. To achieve the sensing of Pb²⁺, the SERS signals were estimated. The lead ions with a concentration range from 10⁻⁶ to 10⁻¹¹ M were added to Gluc/2-NT@Au@Ag dispersions, and the mixtures were then subject to SERS measurements by the confocal Raman spectrometer. As shown in Fig. 5A, the characteristic Raman peaks of 2-NT at 1621 cm⁻¹ could be clearly detected. As the Pb²⁺ concentrations increased, the Raman signals of the Gluc/2-NT@Au@Ag were observed to be enhanced at different levels. This is because the formation of hotspots between the Gluc/2-NT@Au@Ag NPs. And the results showed that hotpots responsive SERS signals were extremely sensitive to detect the Pb²⁺. In our experiments, the Raman intensity at 1621 cm⁻¹ showed good linear relationship with the logarithm of Pb²⁺ concentration from 10⁻⁶ M to 10⁻¹¹ M. A calibration curve was shown in Fig. 5B and it is linearly regressed as the equation:

 

Fig. 5. (A) Raman spectra of Gluc/2-NT@Au@Ag NPs in presence of Pb²⁺ (10⁻¹¹–10⁻⁶ M). (B) Linear plot of Raman intensity of the 1621 cm⁻¹ peak at various concentrations of Pb²⁺. The error bars indicate the standard deviation of 15 measurements.

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By combining the colorimetric and SERS methods, the detection of Pb²⁺ covered a wide range from 10⁻¹¹ to 10⁻⁵ M. Interestingly, for mass samples analysis on-site, those with higher concentration of Pb²⁺ (> 10⁻⁵ M) could be rapidly screened out by naked eyes. While for samples with lower concentration of Pb²⁺(10⁻⁶–10⁻¹¹ M), the accurate concentration of Pb²⁺ in the sample could be detected using SERS method. In order to test the reliability of the presented method, all experiments were repeated 5 times using different batches and the results follow the same law. Generally, our method has a lower detection limit and is more convenient to operate (Table 2). Hence, the presented dual-mode sensing method based on colorimeter and SERS combining rapid readout and excellent sensitivity showed its superiority in the detection of Pb²⁺ especially in field test.

Table 2. The comparison of the detection of Pb²⁺ with previously reported methods.

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3.3 Selectivity of the nanoprobe

The selectivity of the developed dual-mode sensing of Pb²⁺ was further examined by testing eight prevalent metal ions (Ni²⁺, Cu²⁺, Cr²⁺, Ca²⁺, Zn²⁺, K⁺, Ag⁺, Hg²⁺, Na⁺). Specifically, the above mentioned metal ions (10⁻⁵ M) were added to 180 µL of Gluc/2-NT@Au@Ag, and the mixtures were incubated for 45 min. Meanwhile, Gluc/2-NT@Au@Ag treated with 10⁻⁵ M Pb²⁺ under the same conditions was used as a positive control and Gluc/2-NT@Au@Ag without any treatment as a negative control. All mixtures were then photographed and subject to UV-vis absorption measurement. As revealed in Fig. 6A, only the color of the mixture containing Pb²⁺ changed obviously, while the colors of the mixtures containing other metal ions remained substantially the same as that of the negative control. Similarly, as indicated by the extinction spectra, the SPR band decrease of the nanoprobe was only observed in the presence of Pb²⁺, owing to the strong affinity between the gluconate ions on surface of the nanoprobe and the lead ions (Fig. 6A). Quantitative evaluation of the selectivity for different ions based on the intensity of SPR band at 417 nm was shown in Fig. 6B, which definitely showed the presented method had an outstanding selectivity for Pb²⁺, and could specifically distinguish Pb²⁺ from interfering metal ions. Similarly, SERS intensity of Gluc/2-NT@Au@Ag NPs with addition of Pb²⁺ (10⁻⁶ M) is significantly increased compared to that of Gluc/2-NT@Au@Ag NPs with other metal (10⁻⁶ M) (Fig. 6C and 6D), which prove that SERS sensing approach is also capable of selective detection of Pb²⁺. Therefore, the dual-detection method we proposed is capable of selectively detecting Pb²⁺.

 

Fig. 6. Selectivity test with various metal ions. (A) Histogram of absorbance of SPR band at 417 nm in the presence of different metal ions. (inset: Photographs of Gluc/2-NT@Au@Ag NPs in the presence of different metal ions of 10⁻⁵ M). (B) Extinction spectra of Gluc/2-NT@Au@Ag NPs in the presence of different metal ions. (C) Histogram of Raman intensity of the 1621 cm⁻¹ band in the presence of different metal ions. The error bars indicate the standard deviation of 15 measurements. (D) Raman intensity of the 1621 cm⁻¹ band in the presence of different metal ions of 10⁻⁶ M.

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3.4 Detection of Pb²⁺ in real sample

Tap water and drinking water were employed to further evaluate the application of presented method in real samples. The water sample was centrifuged to remove other impurities. The recovery experiments were performed from tap-water and drinking water samples spiked with 10⁻⁵ and 10⁻⁹ M of Pb²⁺, respectively. Then the results of the SPR bands and the Raman signals were respectively used to calculate Pb²⁺ concentration in the samples from the calibration graphs. The standard concentration, average detection concentration and recovery rate of each sample was listed in Table 3. Obviously, there was no significant difference between the determination results and the standard solution. A conclusion could be drawn that the recoveries of this presented method in real samples were in an acceptable range of 129.1% - 113.1%, indicating that such dual-mode sensing method had great potential in real samples.

Table 3. Determination of Pb²⁺ spiked in the real water sample

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4. Conclusions

In summary, a new strategy for the detection of Pb²⁺ using Gluc modified and 2-NT tagged Au-Ag core-shell nanoprobes had been developed. Upon the addition of Pb²⁺, the nanoprobes were induced to aggregate, correspondingly the color and SERS signals of nanoprobes changed. This facilitated the colorimetric and SERS dual-mode sensing of Pb²⁺. Combining both advantages of colorimeter and SERS, the detection method covered a wide range from 10⁻⁵ to 10⁻¹¹ M with a LOD of 0.185 pM. Moreover, this method had an outstanding selectivity for Pb²⁺. And the applicability in the real samples was also demonstrated with an excellent recovery rate. The results showed that such dual-mode method was extremely powerful for the detection of Pb²⁺ with a broad response range, rapid screening and high sensitivity.

Appendix A: Calculation method of recovery rate

The percentage of recovery values could be expressed by the following formula:

(3)$$\textrm{Recovery}(\%) = [(C_d/Cs)] \times 100\%$$

Where, “C_d” is the spiked concentration of Pb²⁺ in the real water samples; “C_s” is the average detection concentration of Pb²⁺ in the real water.

Appendix B: The fundamental vibrational wavenumbers of 2-naphthalenethiol

Band assignments of major peaks for 2-naphthalenethiol based on the literatures [36] are as follows in Table 4:

Table 4. Observed fundamental vibrational wavenumbers of 2-naphthalenethiol

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Appendix C: The simulation of the electric field distributions of the Au@Ag NPs

In the simulation, the excitation wavelength was set to be 638 nm. The diameter of Au@Ag NPs was set to be 12 nm for Au core and 5 nm for Ag shell. Besides, the light source was perpendicular to the alignment of the nanoparticles. And the distance between the simulated Au@Ag NPs in FDTD was set at 1.72 nm. Results are in Fig. 7.

 

Fig. 7. (A) Statistical results of interparticle distance of Au@Ag NPs. (B) –(H) The simulation of electric field distributions in different possible aggregation states of Au@Ag NPs with FDTD solutions.

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Appendix D: Calculation method of the LOD [27]

The LOD is estimated without Pb²⁺. And the colorimetric and SERS signal is at least three times higher than the background (deionized water). The calibration curve for Pb²⁺ is as follows:

(4)$$Y = A + K \times Log_{10}X$$

Among them, the meaning of Y in the SERS method is the SERS signal of 2-NT after adding Pb²⁺ into the probe; “X” is the final concentration of Pb²⁺. “A” and “K” are the variables obtained by linear regression of the least square root of the signal concentration curve, “A” is the intercept of the calibration curve, and “K” is the slope of the calibration curve.

The calculation method for the LOD of the calibration curve is:

(5)$$LOD = {10^{[[(Y_b + 3SD)/Yb - A]/K]}}$$

“Y_b” is the average SERS signal of the blank samples (deionized water is only added to the probe without adding any heavy metal ions), and “SD” is the standard deviation of the blank samples.

Appendix E: Calculation method of the SD

The SD of the blank signal was calculated according to the well-known formula:

$$SD = \sqrt {\left( {\frac{\textrm{1}}{{\textrm{n - 1}}} \times {{\sum\limits_{i = 1}^n {(Xi - Xaverage)} }^2}} \right)}$$

where, “n” is the total number of the blank sample. “X_i” is the “i” sample of the series of measurements. “X_average” is the average value of the SERS signals obtained for the specific series of identical samples repeated “n” times.

Funding

National Natural Science Foundation of China (Nos. 61805143); Shanghai Pujiang Program (Nos. 18PJ1408700); Shanghai Sailing Program (Nos. 19YF1435400).

Disclosures

The authors declare that there are no conflicts of interest related to this article.

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