Introduction

Environmental contamination by heavy metals is a topic of interest and great concern worldwide (López et al. 2020; Feitosa et al. 2021). Lead is among the most dangerous metals, since due to its chemical nature it represents a risk to human health and the environment (López et al. 2020). Although the origin of heavy metals may be natural, the high concentration in ecosystems is due to anthropic emissions (mining, agriculture and industry), which represents a risk of contamination. Lead (II) has been used in paints, gasoline, solder, plumbing, enamel, and many other applications. (Bratovcic 2020). Therefore, its distribution in the environment is wide. Studies published decades ago and recently demonstrated the wide distribution of lead. The heavy metals have been found in agricultural soils (Tian et al. 2022), in shooting ranges (Christou et al. 2022), sediments (Wakida et al. 2008), foods (Ortiz et al. 2017), springs or well water (Nieto et al. 2014) and rivers (López et al. 2020). In aqueous systems, lead exists predominantly in its divalent oxidation state (Pb2+) (Feitosa et al. 2021). A small amount of Pb2+ can cause damage to microorganisms (Wan et al. 2022) as well as to human health (Ortiz et al. 2017). It is highly toxic to the human body, having a negative impact on the kidneys (Chen et al. 2019), also associated with cardiovascular diseases. It even alters the expression of microRNA specifically miR-155 (Martinez et al. 2021).

According to World Health Organization (WHO) and Environmental Protection Agency (EPA, USA), the maximum permissible concentration of lead for, agricultural irrigation water is 2–5 μg/ml, for livestock consumption it is 0.1 μg/ml, water quality guidelines for recreational purposes is around 0.05 μg/ml and drinking water 0.015 μg/ml. Therefore, its detection with the naked eye is of vital importance. Several methods for the analysis of heavy metal ions have been developed in recent decades, including techniques based on atomic absorption spectroscopy (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma mass spectrometry (ICP-MS) in various aqueous samples including drinking water, saltwater, lakes, etc. (Güzel et al. 2021). Although the aforementioned methods offer excellent sensitivity and multi-element analysis, they are expensive, time-consuming, skill-intensive, and laboratory-intensive, making it necessary to suggest alternative detection methods. Colorimetric methods based on Cu, Au and Ag nanoparticles (CuNPs, AuNPs and AgNPs) for the detection and quantification of heavy metals have gained importance in recent years (Sengan and Veerappan 2019; Sonia and Seth 2020; Boruah et al. 2019) due to the detection sensitivity, low cost without the need for a sophisticated laboratory. The principle of this colorimetric technique is the aggregation of NPs in the presence of heavy metals, changing the color of the original nanoparticles. Metallic nanostructures possess the optical property of localized surface plasmon resonance (LSPR), which allows obtaining precise quantitative information on the target molecules (Prosposito et al. 2019). In addition, with the NP-based method, results are obtained quickly, efficiently, economically and with high reproducibility.

Copper nanostructures functionalized with N-myristoyltaurine were used to detect mercury ions Hg2+, the detection limit was 15 μM with the naked eye and by means of UV–Vis spectroscopy a limit of 0.1125 15 μM was recorded (Sengan and Veerappan 2019). Spherical Au nanoparticles ~ 30 nm conjugated with L-cysteine were prepared to detect concentrations of 10–40 ppm of cadmium in aqueous solutions (Sonia and Seth 2020). Nanometer-scale structures of silver functionalized with polyethylene glycol were used to specifically detect the arsenic ion III (Boruah et al. 2019). The nanosystem achieved a found detection limit of 1 ppb. It is important to mention that the colorimetric method uses functionalizing agents that help increase selectivity for heavy metals (Hyder et al. 2022). In this work, 2-mercaptoethanol (ME) was used to selectively detect lead ions at low concentrations. The ME linker is a short-chain molecule that has an –OH terminal at one end and a –SH at the other (Hong and Li 2013). It is well known that sulfhydryl functional groups bind to the surface of silver nanoparticles (Ramasamy et al. 2012). On the other hand, hydroxyls provide intermolecular binding forces to target molecules. The short length of ME ensures chemical and electromagnetic enhancements that amplify the optical transduction properties. The short length of ME ensures chemical and electromagnetic enhancements that amplify optical transduction, recorded either by UV–Vis or Raman spectroscopy (Hong and Li 2013; Filgueiras et al. 2016).

In this work, we present a new naked-eye colorimetric detection assay for the detection of Pb2+ in solution at low concentrations. AgNPs functionalized with 2-mercaptoethanol were used, which allowed observing the color change with the naked eye without the need for complex instrumentation.

Materials and methods

Materials

Silver nitrate AgNO3, sodium citrate dihydrate (Na3C6H5O7·2H2O) purchased from J. T. Baker. Sodium borohydride (NaBH4 ≥ 98.5%), 2-mercaptoethanol (C2H6OS ≥ 98%) and hydrochloric acid (NCl) acquired from Sigma-Aldrich. The test metal salts are listed below; lead nitrate (PbNO3), calcium chloride dihydrate (CaCl2·2H2O), sodium chloride (NaCl) and potassium chloride (KCl) were purchased from J. T. Baker. Cadmium chloride pentahydrate (CdCl2·5H2O) obtained of Sigma. A standard arsenic solution was used (1 mg/ml) from Baker. Finally, magnesium chloride hexahydrate (MgCl2·6H2O) and iron chloride hexahydrate (FeCl3·6H2O) purchased from Merck. The reagents used did not need further purification. Deionized water from a Milli-Q (18.2 MΩcm) purification system was used in all aqueous solution preparations. Previously, the glassware vessels were cleaned with aqua regia solution, followed by a wash with deionized water.

Synthesis and functionalization of silver nanoparticles

The chemical reduction method was used to obtain silver nanoparticles according to Li et al. (2014) with slight modifications. Briefly, 250 μl of a solution silver nitrate (0.1 mol/L) was added to an Erlenmeyer flask containing 100 ml of deionized water at a temperature of ~ 5 ºC, the solution was mixed vigorously. Then, 1 ml of sodium citrate (0.1 mol/L) was added, 30 ml of fresh sodium borohydride solution (0.005 mol/L) was quickly poured into the solution. The NaBH4 reduces silver ions (Ag+) to atomic silver (Agº). In addition, sodium citrate stabilizes AgNPs. The color of the reaction solution changes from colorless to yellow, indicative of the formation of silver nanoparticles. The reaction mixture was allowed to stir for 30 min, and then the functionalization process was continued. Surface modification of AgNPs was performed in a one step. 1 ml of 2-ME linker solution (0.15 mmol/L) was added to 100 ml of colloidal solution. The resulting solution was mixed for a few seconds and used for heavy metal detection tests. The incubation of silver nanoparticles with metals was carried out at room temperature without the need for darkness.

Instrumentation

The colloidal solutions were characterized in a UV–Vis (spectrophotometer Jenway 6715, Fisher Scientific). A quartz cell with a path length of 10 mm was used. For X-ray diffraction (XRD) measurements, the Rigaku Smartlab diffractometer was used in Bragg–Brentano configuration, the samples were analyzed using CuKα radiation (λ = 1.5418 Å), the diffractometer was operated at a scanning speed of 0.02°/ s in a 2θ range of 10–90°. The pH of the solutions was measured using a pH meter (model 2211 de Hanna, Instruments). The photograph was taken using high precision camera.

Results and discussion

Synthesis of silver nanoparticles

It is well known that silver nanoparticles have an absorption band in the visible region. In this case, the colloidal solution with mercaptoethanol presents a peak at 391 nm (shown in Fig. 1), which confirms the formation of nanoscale structures (Desai et al. 2012). Inset of Fig. 1 shows the yellow colloidal solution obtained.

Fig. 1
figure 1

UV–Vis spectrum of silver nanoparticles functionalized with 2-mercaptoethanol. Inset: colloidal solution obtained after the synthesis process

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On the other hand, the synthesized Ag nanoparticles were characterized by X-ray diffraction; the sample presented a typical XRD pattern as shown in Fig. 2. The crystal structure of the sample is well defined by the four sharp peaks at 2θ values of 38.5° (111), 44.7° (200), 65° (220) and 78.2° (311) related to the face cubic centered structure, according to Joint Committee on Powder Diffraction Standards (JCPDS card 04–0783) (Lange et al. 2010). The crystallite size D was estimated from the Debye–Scherer’s equation, \(D=\frac{0.9\lambda }{\beta *cos\theta }\) where λ = 0.15414 nm is the wavelength of the incident radiation, β is the full width at half maximum (FWHM) of the XRD peak (Lopantzi et al. 2022), in this case at 44.7°, finally θ is the Bragg angle. The D value was calculated around 10 nm.

Fig. 2
figure 2

XRD pattern of Ag particles synthesized from this work

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Selective colorimetric detection of lead (II)

Several heavy metal ions as well as other complementary metals (Pb2+, As3+, Cd2+, Mg2+, Fe3+ Ca2+, and Na+ y K+) were taken for the selectivity test. In centrifuge tubes, concentrations of up to 5 μg/mL of heavy metals were mixed with newly synthesized functionalized AgNPs. The color change is evident in Fig. 3. It was observed that only the Pb2+ containing centrifuge tube shows a prominent color change to bluish while others are pale yellow. From this observation with the naked eye, the strong selectivity of functionalized silver nanoparticles toward lead (II) was evidenced.

Fig. 3
figure 3

Photograph of centrifuge tubes containing heavy metal ions, where it is clarified that only the lead solution produces a color change in the original colloidal solution of AgNPs

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The aforementioned samples were characterized by UV–Vis spectroscopy at room temperature. Figure 4a shows the spectra obtained. All the samples containing AgNPs-metals (less lead) presented slight modifications in the characteristic band of the silver nanoparticles. The colloidal sample containing lead II recorded a significant decrease in the peak at 391 nm. This effect is associated with the aggregation of silver nanoparticles in the presence of the heavy metal (Su et al. 2021). The influence of the degree of acidity-basicity on selectivity for heavy metals was investigated. Therefore, the pH of the colloidal solution was adjusted to 7.4 (native pH of the AgNPs solution is 9.5) using HCl at 1N. No significant changes could be recorded by UV–Vis spectroscopy, the band associated with SRPL was the same with or without heavy metal (shown in Fig. 4b). For our system, an alkaline pH (9.5) allowed to selectively recognize lead ions, similar results have been published previously (Xing et al. 2018). It is worth mentioning that non-functionalized silver nanoparticles (at pH 7.4 and 9.5) were mixed with heavy metals, but no affinity for lead was demonstrated (data not presented).

Fig. 4
figure 4

UV–Vis spectra of the different test heavy metals mixed with solutions of silver nanoparticles at pH a alkaline 9.5 and b neutral 7.4

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Mechanism of color change

A simple technique for the detection of lead (II) in aqueous medium using silver nanoparticles functionalized with 2-mercaptoethanol is reported here. Two crucial properties were used. First, the localized surface plasmon resonance of silver nanoparticles that depends on the dielectric constant of their surrounding environment. When the dielectric constant changes, the absorption peak shifts to longer wavelengths. Second, the short length of ME ~ 4 nm that ensures electromagnetic enhancement by amplifying the optical properties of transduction (Hong and Li  2013). The ME contains a sulfhydryl functional group –SH that binds to the surface of the silver and at the other end contains an –OH group (Ramasamy et al. 2012). The hydroxyl provides solubility and negative charges to the particle surface that prevents its aggregation due to electrostatic or steric repulsion (Xavier et al. 2014). When lead was introduced to these functionalized silver nanoparticles, it interacts with the hydroxyl groups. Due to this interaction, the aggregation of silver nanoparticles occurs. This means that the environment of the silver nanoparticles changes. Therefore, there is a decrease in the peak at 391 nm and other bands appear at longer wavelengths. This effect produces a color change in the colloidal samples from yellow to bluish. Figure 5 shows the mechanism. Furthermore, we evaluated the selectivity of the colorimetric sensor at two different pH values. According to the results, we observe that at pH = 7.4 the AgNPs-ME do not precipitate in the presence of lead. From these observations, we suggest that, under neutral conditions, the hydroxyl does not completely deprotonate and avoid recognizing Pb2+ ions. The selectivity of –OH for heavy metal ions found here has also been reported with other functional groups with a negative nature such as carboxylic groups (–COOH) and amines (–NH2) (Anambiga et al. 2013).

Fig. 5
figure 5

Schematic representation of the proposed AgNPs-ME aggregation process in the presence of lead

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Reaction time on AgNPs-ME aggregation induced by Pb2+

Absorption spectra were taken periodically (every 2 min) to estimate the reaction rate. Figure 6 shows the UV–Vis spectra taken from minute 0 to 14 and at concentration of 2 μg/ml of lead. A decrease in the intensity of the absorption peak of λ391 could be observed from the first seconds of incubation with the AgNPs-Pb2+. The decrease in a LSPR band and formation of a shoulder at longer wavelengths was more evident with time. The spectra corresponding to minute 8, 10, 12, and 14 (pink, green, orange and gray spectrum of Fig. 6) have the same behavior. Which means that ME-functionalized AgNPs have a high affinity for lead ions. The reaction speed obtained in this work even exceeds other previously published works (Choudhury and Misra 2018).

Fig. 6
figure 6

a UV–Vis spectral behavior of AgNPs-Pb2+ (2 μg/ml) at different times

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Limit of detection and limit quantification

Limit of detection (LOD), linear range and limit of quantification (LOQ) were calculated because they are very important parameters in colorimetric sensors (Boruah et al. 2019). Various samples were evaluated at low Pb2+ concentrations to obtain the detectable range of lead II. For this, 1 ml of functionalized nanoparticles was mixed with 1 ml of deionized water containing 1–2 μg of Pb2+. After 8 min of storage at room temperature, the different samples were characterized by UV–Vis in a range of 200 to 800 nm (shown in Fig. 7a). A decrease in the absorption peak of λ391 and the formation of a second band at higher wavelengths were recorded as the concentration of heavy metals increased. On the other hand, a deconvolution was also performed in the 300–750 nm region. Figure 7b shows the results obtained by mixing 2 μg/ml of lead with AgNPs. It was possible to observe the formation of three subbands located at 380, 415 and 520 nm (green line), the sum (red line) and the experimental spectra (black line). These three subbands were recorded in all samples containing Pb2+ ions, data not shown. The band located at 520 nm was associated with the aggregation of nanostructures (Choudhury and Misra 2018) and consequently was used in subsequent measurements.

Fig. 7
figure 7

a Absorption vs different concentrations of lead, b deconvolution of the spectrum of Pb2+-AgNPs containing 2 μg/ml of the heavy metal and c linear response of colorimetric detection of A520/A391 vs Pb2+ concentration

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Absorbance ratio values of A520/391 were used to assess detection sensitivity in aqueous medium. The A520/391 value of the lead-free AgNPs was found to be 0.0387. When AgNPs are in the presence of lead at 1 and 1.8 μg/ml, the ratio value increased to 0.0689 and 0.2796, respectively (shown in Fig. 7c). The aforementioned results helped to calculate the linear range. Figure 7c shows a linearity of relationship 520/391 depending on the concentration of lead. An R2 = 0.99 was estimated between the values 1 and 1.8 μg/ml of lead, a slope of m = 0.26, with these data and the standard deviation σ it is possible to calculate the limit of detection (LOD) and the limit of quantification (LOQ) (Qi et al. 2012). The LOD y LOQ are obtained from the following relationship 3σ/m y 10σ/m being 0.04 and 0.12 μg/ml, respectively.

Jot-plot

To determine the union stoichiometry between Pb2+ and AgNP the Job graph was made (Fig. 8). Changes in absorbance at 391 nm were used for this estimation. In separate vials, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6 and 1.8 ml of heavy metal solutions at 1 mM were taken. To these lead solutions 1.8, 1.6, 1.4, 1.2, 1, 0.8, 0.6, 0.4 and 0.2 ml of AgNPs solution were added, respectively, as that the total volume of the solution was 2 ml. A binding stoichiometry of 2:3 was found between AgNPs y Pb2+, respectively. The results obtained here indicate that fewer moles of lead (II) are needed to precipitate silver particles compared to previous reports (Huang 1982; Wang and Zheng 2017).

Fig. 8
figure 8

Job plot determination for the binding stoichiometry between AgNP and Pb2+ monitored at 412 nm

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Here we report a simple, inexpensive, reproducible method without the need for expensive equipment to obtain a colorimetric sensor based on AgNPs-ME. The synthesis can be carried out under atmospheric conditions. Unlike to Noh et al. (2015), the authors synthesized an AgNPs nanosensor in the presence of nitrogen at 6 h of reaction. The selectivity of our system is high without the need for masking agents. Such as Hung et al. (2010) who detect Pb2+ ions with gold nanostructures functionalized with mercaptoethanol in the presence of sodium sulfide. The pH, reaction rate, binding stoichiometry, detection limits and quantification limit found in this work are comparable with others previously published. In this work, the high concentration tested was 5 µg/ml, so the nanosensor solution changed from yellow to bluish. It is worth mentioning that the colorimetric sensor developed here offers high sensitivity with a detected concentration of lead II at 0.04 µg/ml or 193 nM, lower values than other techniques such as voltammetric method reported by Huseinov et al. (2021). The authors used a platinum electrode to quantify Pb2+, the reported detection limit was 34.5 µM heavy metal. Table 1 summarizes the LOD values reported by other authors using colorimetric sensors.

Table 1 Comparison of colorimetric sensors based on silver nanoparticles to detect Pb2+
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Conclusion

A simple and easy method was developed to obtain a colorimetric sensor based on silver nanoparticles (with a crystallite size of ~ 10 nm) functionalized with 2-mercaptoethanol. Under alkaline conditions, ME helped to selectively detect Pb2+ among other heavy metals, with a detection speed of 8 min. To the naked eye, the nanosensor changed color from yellow to bluish in the presence of high concentrations of lead, such as 5 μg/ml. On the other hand, the sensitivity of the nanosystem made it possible to detect lead at low concentrations of 0.04 μg/ml, without the need for a sophisticated laboratory, and by means of UV–Vis spectroscopy. The values found here were below the maximum allowed for aquatic systems. The binding stoichiometry was 2:3 AgNPs:Pb2+, respectively. Therefore, compared to other spectroscopy methods, the colorimetric sensor developed here is competitive in terms of detection limit, simplicity, and convenience.