Introduction

The wide use of lead in the fields of electric storage batteries, gasoline additives, paint pigments, pipes, ammunition, soldering activities, etc., for many years is attributed to its abundant physical properties. Thus, its wide usage makes lead one of the most important pollutants [1, 2]. Lead poisoning can cause a variety of diseases, such as muscle paralysis, memory loss, irritability, and anemia [3–5]. Therefore, developing rapid and sensitive methods to detect trace Pb2+ is crucial. Currently, many analytical methods to determine trace Pb2+ have been developed. The common methods used to detect Pb2+ include atomic absorption spectrometry [6] and inductively couples plasma optical emission spectrometry [7]. Nevertheless, these techniques are expensive, time-consuming, and require special laboratories. Colorimetric assay has received attentions because it can avoid these limitations [8, 9]. Moreover, the detection signal can be viewed by the naked eyes [10, 11]. Silver and gold nanoparticles are frequently employed as colorimetric probes for sensing biomolecules and metal ions because of their distance-dependent optical properties and extremely high extinction coefficients [12–16]. When ligands assembled on the surface of nanoparticles bond with an analyte, the aggregation of nanoparticles occurs. Consequently, the changes in color and absorption spectrum can be observed. Thus, the surface modification of nanoparticles is very important to improve the stability and sensing selectivity of nanoparticles [17–20]. For example, Li and his group synthesized glutathione-stabilized silver nanoparticles as colorimetric sensors for nickel ion [21] and aza-crown ether-modified silver nanoparticles as colorimetric sensors for Ba2+ [22]. Huang and his group reported polythymine-stabilized gold nanoparticles for the recognition of melamine [23]. Takarda and his group developed double-stranded DNA-carrying gold nanoparticles for detection of mercury ion [24]. Recently, Chai and his group [9] have reported glutathione modified gold nanoparticles based colorimetric assay for the detection of Pb2+ at 20 ppb level. Ray and his group also have reported glutathione conjugated gold nanoparticles probe for detection of Pb2+ from plastic toys, paints, and water samples [25].

Iminodiacetic acid (IDA) possesses not only good water-solubility but also strong binding ability with metal ions. In view of this, IDA and its derivatives have been employed to modify the surface of nanoparticles to sense metal ions effectively [26]. In the current study, we report a colorimetric assay for the determination of Pb2+ in aqueous solution based on Ag NPs coated with iminodiacetic acid. The functionalized IDA-Ag NPs was induced to aggregate in the presence of Pb2+ through electrostatic and metal-ligand interactions. Scheme 1 depicts the mechanism for the color change of Ag NPs induced by Pb2+.

Scheme 1
scheme 1

Pb2+induced the aggregation of IDA-Ag NPs resulting colorimetric responses

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Experimental

Materials

All chemicals were of analytical grade and used without further purification. All solutions were prepared using doubly distilled water. IDA is a product of Sigma-Aldrich Corp. (USA, http://www.sigmaaldrich.com). Silver nitrate (AgNO3), sodium borohydride (NaBH4), and the other metallic ions were purchased from Shanghai Qingxi Technology Co., Ltd. (Shanghai, China, www.ce-r.cn/sites/qingxi/).

Instruments

Absorption spectra were recorded on a UV–vis 2550 spectrophotometer (Shimadzu, Japan, www.shimadzu.com) using a 1.0 cm quartz cell. Infrared spectra were obtained with KBr pellets on a Nicolet 5700 FTIR spectrometer (Nicolet, USA, www.thermonicolet.com). Transmission electron microscopy (TEM) images were acquired on a JEM-2010 transmission electron microscope (JEOL Ltd. Japan, http://www.jeol.cn). Size distribution and zeta potential of IDA-Ag NPs in different pH value media were measured on a Hydrosol Nanoparticle Size Analyzerand Zeta Potential Analyzer (PSA NANO2590, Malvern Companies, UK, www.malvern.com). All pH measurements were made using a pHS-3 digital pH-meter (Shanghai REX Instrument Corp., Shanghai, China, http://en.lei-ci.com) combined with a glass-calomel electrode.

Preparation of IDA-Ag NPs and bare Ag NPs

IDA-Ag NPs were prepared through the method reported previously [27]. Briefly, 2.0 mL AgNO3 aqueous solution (0.01 mol L−1) mixed with 2.0 mL IDA (0.01 mol L−1) was diluted to 100 mL with doubly distilled water respectively. After 15 min, the solution added with sodium borohydride (NaBH4, 8.8 mg) reduced quickly under vigorous shaking. The mixture was stirred for 2 h in the dark at room temperature; bright yellow silver colloids were then obtained. The IDA-modified Ag NPs were used in the following experiments without further purification. The bare Ag NPs were prepared in the same way without IDA.

Results and discussion

Characterization of IDA-Ag NPs

Figure 1 shows the infrared and absorption spectra of pure IDA and IDA-Ag NPs. Comparing the IR spectra of them, significant features were observed: the characteristic peak of N-H located at 3100 cm−1 disappeared, indicating that IDA modified onto the surface of Ag NPs. View from Fig. 1b, it is clear that the absorption spectrum of IDA-Ag NPs underwent a little red-shift compared with that of bare Ag NPs, which also demonstrates that IDA modified onto the surface of Ag NPs. Similar spectral changes were found in Ag NPs modified with bipyridine [27–29]. It has been proved that the adsorption of organic compounds on the metal nanoparticles resulted in a red shift in the SPR band, mainly caused by the lessening of the plasma oscillation frequency or a change in the dielectric constant of the microenviroment around the nanoparticles.

Fig. 1
figure 1

a FT-IR spectra of IDA(A) and IDA-Ag NPs(B) with KBr pellets, b Absorption spectra of bare Ag NPs and IDA-Ag NPs in aqueous solution

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Stability and size of IDA-Ag NPs

The prepared Ag NPs solution modified with IDA was stable and highly dispersed. The TEM image of IDA-Ag NPs was obtained, as shown in part a of Fig. 2. Clearly, IDA-Ag NPs were highly dispersed in aqueous solution, with the size estimated as 8 nm. The size distribution of IDA-Ag NPs was also studied and presented in Fig. S1 (Electronic Supplementary Material, ESM). The size of 82 % IDA-Ag NPs particles was about 7 to 10 nm, which concurred with the appearance size of the TEM result. Further, note that, even if the solution was maintained for 30 days in 4 °C in a refrigerator, the graph of the size distribution changed little. Thus, we conclude that IDA-Ag NPs are highly dispersed and stabilized, making them promising for further application.

Fig. 2
figure 2

TEM images of dispersed IDA-Ag NPs (a) and aggregated IDA-Ag NPs induced by Pb2+ (b)

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Optimal experimental condition

Effect of IDA concentration

The IDA concentration not only affects the stability of the Ag NPs, but also affects the determination of the sensitivity. The optical IDA concentration was studied and the results showed in Fig. S2 (ESM). When the IDA concentration is low, it is very sensitive for detection smaller amounts of Pb2+. However, the concentration of IDA can’t be too low because the Ag NPs are not stable with less IDA and are aggregated by themselves. Meanwhile, the experiment results also display the size of IDA-Ag NPs increase little while the concentration of IDA increased (seen Fig. S1, ESM). So in this system, the molar concentration ratio of Ag+ / IDA was fixed at 1:1.

Effect of pH value on absorption spectrum of IDA-Ag NPs

The effect of pH value on the absorption spectrum of IDA-Ag NPs was investigated in our current work and shown in Fig. S4 (ESM). The results display the nanoparticles are considered stable at a pH range of 3.0 to 12.0 and are extremely stable at pH 5.0 to 9.0. Reportedly, the pKa of IDA are 2.98 and 9.89 [30]. The carboxyl groups of IDA are unprotonated and negatively charged, indicating that IDA-Ag NPs are charged particles under these pH conditions which is supported by the zeta potential (seen Fig. S4, ESM). Thus, the electrostatic force of nanoparticles surfaces was assumed the reason why nanoparticles were stable in this pH range.

Effect of reaction time on IDA-Ag NPs aggregation induced by Pb2+

The aggregation kinetics of our assay was examined with different concentration of Pb2+ by measuring the absorbance of IDA-Ag NPs at 650 nm at room temperature. Fig. S5 (ESM) shows that the absorbance of IDA-Ag NPs at 650 nm increased and then stabilized within 2 min after the addition of Pb2+. In other words, the aggregation of the IDA-Ag NPs occurred immediately after the addition of Pb2+, which was consequently completed in 2 min. Thus, the method was enabled to rapid detection of Pb2+ in water samples.

Metal ion recognition based on IDA-Ag NPs

To investigate the recognition ability of IDA-Ag NPs with metal ions, the absorption titration of IDA-Ag-NPs against 15 metal ions, including Pb2+, Cd2+, Cu2+, Ba2+, Ca2+, Zn2+, Co2+, Mn2+, Ag+, Fe3+, Mg2+, Hg2+, K+, Ca2+, and Al3+, was performed respectively. Upon interaction with various metal ions, both the absorption spectral changes and the absorbance ratio A650/A396 changes of IDA-Ag NPs are presented in Fig. 3. As shown Fig. 3a, the maximum absorption wavelength of IDA-Ag NPs peaked at 396 nm, and the presence of 10 μM Pb2+ caused the spectrum to change dramatically. The absorbance at 396 nm decreased, and a new absorption band appeared at approximately 650 nm, along with the color change from yellow to green. However, upon the addition of other metal ions, such as Cd2+, Cu2+, Ba2+, Ca2+, Zn2+, Co2+, Ag+, Fe3+, Mg2+, Hg2+, K+, Ca2+, and Al3+, the absorbance at 396 nm showed slight changes, and a new absorbance band did not appear. Only the concentration of Mn2+ was higher than 20 μM, a new absorption peak would be found at about 570 nm. Figure 3b displays the intensity ratio of A650nm/A396nm of IDA-Ag NPs when added with various metal ions. Clearly, IDA-Ag-NPs show high selectivity on Pb2+ over other ions.

Fig. 3
figure 3

UV–vis absorption spectra (a) and intensity ratio of A650nm/A396nm (b) of Ag NPs solution after mixing with 10 μM Pb2+ and other metal ions(10 μM for Mn2+, 20 μM for Cd2+, Cu2+ , Hg2+, Ba2+, 100 μM for Ca2+, Zn2+ and 200 μM for Co2+, Li+, Ag+, Fe3+, Mg2+, K+, Ca2+, Al3+) in pH 8 aqueous solution, respectively

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To evaluate the sensitivity of Pb2+ dependent on the colorimetric assay under the optimized detection conditions, the color change of the IDA-Ag NPs solution was recorded by digital camera, the absorption spectral change was monitored by UV–vis spectroscopy. The absorption spectral and solution color changes of IDA-Ag NPs upon the addition of Pb2+ are presented in Fig. 4. Figure 4a shows the color changes in IDA-Ag NPs after the addition of different concentrations of Pb2+. As the concentration of Pb2+ increased, the color of IDA-Ag NPs solution changed from yellow to yellow brown, yellow green, and deep green, which could be easily observed by naked eyes. Such changes can be easily used as a quantitative assay for trace Pb2+. Figure 4b illustrates the absorption spectral change of IDA-Ag NPs in the presence and absence of Pb2+. Upon the addition of increasing Pb2+, the absorbance at 396 nm decreased, and a new absorption peak appeared progressively at a longer wavelength range. Finally, the new absorption band red shifted to 650 nm, with the increasing concentration of Pb2+ maintained.

Fig. 4
figure 4

a Color changes in IDA-Ag NPs solution after the addition of different concentrations of Pb2+ ions. b The absorption spectra of Ag NPs solution with various concentrations of Pb2+ were 0, 0.4, 0.8, 2, 4, 6, and 8 × 10−6 M, respectively. The inset shows the dependence of R(A650/A396) values of IDA-Ag NPs on increasing concentration of Pb2+

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A good linear relationship between intensity ratio A650/A396 and the concentration of Pb2+ ranging from 0.4 μM to 8.0 μM was also obtained (seen in the inset plot of Fig. 4b). The regression equation was A650/A396 = −0.019 + 1.1 × 105 × CPb(molL−1). The limit of detection (LOD) was given as 13 nM by equation LOD = K × S0/S, where K is a numerical factor chosen according to the confidence level desired, S0 is the standard deviation (SD) of the blank measurements (n = 13, K = 3), and S is the slope of the calibration curve. Based on the investigation, a simple colorimetric assay of Pb2+ was introduced. Some similar methods for determination lead were listed in the Table 1. It is clear that the determination wavelength and the limit of detection of proposed method are better compared with the other methods.

Table 1 Comparison of analytical parameters of different methods for determination of Pb2+
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Analysis of real samples

To validate the reliability of our method, the detection of Pb2+ was performed in tap water and urea sample. The results are presented in Table 2. It is found that the Pb2+ content in the tested samples is too low to be detected by the IDA-Ag NPs. However, there is an obvious signal can be readout if different concentrations of Pb2+ are added into the sample. The mean recovery was 93.7 % to 98.6 %, and RSD was about 1.2–5.4 %.

Table 2 Determination of Pb2+ in tap water and urea sample by the standard addition method
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Mechanism

The spectral and color changes could be explained well by the aggregation of Ag NPs through the electrostatic interaction between Pb2+ and carboxylate anion of the IDA. The IDA-Ag NPs were stabilized in the solution because the IDA on the surface of Ag NPs protected them from aggregation. IDA has two free—COOH groups and one—NH group, which can be used to bind with metal ions. In our current assay, IDA is linked with Ag NPs through—NH linkage and as a result, there is no free—NH groups for the formation of complex with metal ions like, Fe2+, Zn2+, and Cd2+ which are well-known to bind to amino group [34, 35]. As a result,—COOH is the only binding site, which is known to bind strongly with Pb2+ [25, 34]. In the presence of Pb2+, negative carboxylate anion of IDA functionalized on the nanoparticles’ surface can be attracted together through a carboxylate-Pb2+-carboxylate linker at pH 8. The bridging interactions between Pb2+ and IDA can also be identified by the fact that the colorimetric change did not occur in the presence of EDTA (Fig. 5), which is a well-known strong chelator for divalent cations. Similar observation has been found in the reported references [11, 36, 37]. Based on interactions between carboxylate and Pb2+, the neighboring Ag NPs became closer and finally aggregated. The aggregation of IDA-Ag NPs was also confirmed by TEM (seen the part B of Fig. 2). The TEM image shows IDA-Ag NPs were dispersed uniformly in aqueous solution. The aggregation was clearly observed when Pb2+ was added into the solution.

Fig. 5
figure 5

Reversible binding of Pb2+ with IDA-Ag NPs in the presence of EDTA

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Conclusion

In summary, a low-cost, simple and fast colorimetric platform for the specific detection of trace Pb2+ using functionalized Ag NPs was introduced. The interaction between IDA capped on Ag-NPs and Pb2+ plays a key role in the aggregation of Ag-NPs. Thus, the selection and construction of modifier can significantly improve the selectivity of probes.