Colorimetric detection of lead(II) ions based on accelerating surface etching of gold nanorods to nanospheres: the effect of sodium thiosulfate

Abstract

A non-aggregation colorimetric sensing method based on the accelerated etching of gold nanorods (AuNRs) has been developed for detecting lead(II) ions (Pb²⁺). In this method, the addition of Pb²⁺ leads to the formation of a monolayer of AuPb₂ and AuPb₃ alloys on the gold surface, which results in a considerable decrease in the surface electrode potential. Therefore, the sodium thiosulfate induced dissolution rate of AuNRs has been accelerated. Because of this accelerated etching, the shape of the AuNRs changes rapidly to gold nanospheres. This morphology transformation leads to a blue shift and fade down of the longitudinal localized surface plasmon resonance (LSPR) absorption band of AuNRs until it merges into the transversal absorption. The qualitative spectral change from double bands to single band LSPR results in a distinct irreversible color change in the gold colloid from blue to red. This colorimetric sensing method can be used to detect Pb²⁺ with excellent selectivity and high sensitivity. Under the optimized conditions, the lowest limit of detection for Pb²⁺ observed by the naked eye is 0.1 μM and 20 nM measured by UV-Vis spectroscopy. In the concentration range from 25 to 300 nM of Pb²⁺, this sensing method exhibits a good linear relationship. This colorimetric sensing method for Pb²⁺ has promising applications in water and human serum analysis.

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1. Introduction

Pb²⁺ is one of the most toxic metallic ions associated with the damage to humans and the environment because of its non-degradability and accumulation.^1,2 Pb²⁺ could cause multiple metabolic pathway disorders such as hemachrome and cholesterol-induced hepatotoxicity. Several diseases, such as dystrophy and cognition impairment, have also been proven to be related to the long-term exposure to lead ions.^3,4 The U.S. Environmental Protection Agency (US EPA) reported that the concentration of Pb²⁺ cannot exceed 75 nM.⁵ Because Pb²⁺ can stunt the growth of children's brain, the U.S. Food and Drug administration has suggested that the lead level of children products should not be higher than 2.5 μM (518 μg L⁻¹).⁶ To date, the methods available for Pb²⁺ detection include atomic spectroscopy (e.g. atomic absorption spectroscopy (AAS), atomic emission spectroscopy (AES) and atomic fluorescence spectroscopy (AFS)), inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence spectrometry (XPF) and anodic stripping voltammetry (ASV). However, these traditional detection methods are less usable for rapid analysis because of the requirement of complex processes and expensive instruments. Therefore, it is necessary to develop a simple and rapid method for the determination of Pb²⁺.

Among the newly developed probes for the determination of lead ions, optical probes are particularly attractive.^7–10 Using covalent immobilization of dithizone on agarose membrane, Zargoosh et al. described an optical membrane for sensing Pb²⁺.⁷ Wang et al., developed a fluorescent probe for Pb²⁺ detection by quenching the fluorescence intensity of unmodified carbon dots.⁸ Recently, a large number of colorimetric probes based on gold nanoparticles has been reported. Gold particles exhibit unique localized surface plasmon resonance (LSPR) absorbance properties due to the size, shape, aggregation and dielectric condition-dependent absorption intensity and the peak wavelength.¹¹ For example, Liu et al. reported a facile method for the fabrication of novel black plasmonic colloidosomes assembled from gold nanospheres.¹² Because of the strong interparticle plasmonic coupling, the gold colloidosomes possessed hexagonal close-packed multilayer shells and showed low optical reflectivity and intense broadband absorption. Using laser irradiation-induced shape conversion and self-assembly, the rapid synthesis of monodisperse gold nanospheres was also developed.¹³ Through self-assembly utilizing their high monodispersity and perfect spherical shape, the gold nanoparticles could be fabricated into periodic monolayer arrays. Based on the plasmonic optical properties of gold colloid, the testing result of a colorimetric probe can be readout directly by the naked eye without the need for large-scale equipment, such as the detection of oligonucleotide,¹⁴ proteins,¹⁵ vitamin C,¹⁶ Cu²⁺,¹⁷ and heavy metallic ions, including Hg²⁺,^18–20 Cd²⁺ (ref. 21) and Cr(VI).^22,23 Using the LSPR of AuNRs, a non-aggregation colorimetric sensor for detecting vitamin C was reported by Wang et al.¹⁶ Colorimetric sensing of Cu²⁺ based on the catalytic etching of gold nanoparticles has also been studied. The color change induced by 40 nM Cu²⁺ can be observed easily by the naked eye.¹⁷ Based on the etching of AuNRs induced by Cr²⁺ under acid conditions, Li et al. developed a colorimetric sensor for detecting Cr(VI).²² The blue shift of LSPR is linear to the concentration of Cr(VI). However, the non-aggregation colorimetric sensor for Pb²⁺ has seldom been studied.

Most of colorimetric probes for the detection of Pb²⁺ were developed using DNA modified gold nanoparticles (AuNPs).^24,25 Lu et al. reported a series of DNAzyme–AuNPs sensor and the detectable concentrations were between 3 nM and 1 mM.^26–30 Dong et al., developed a DNAzyme-based sensor using AuNPs, and the lowest detectable concentration of Pb²⁺ was 0.5 μM.³¹ Although these DNAzyme-based sensors have high sensitivity and selectivity, they require complex fabrication and expensive materials. To date, using several substitutions of DNA-factionalized AuNPs, such as molecules, proteins and polymers for colorimetric sensing, have also attracted considerable attention. In the study of Thomas et al., Pb²⁺ was detected with a high sensitivity using gallic acid-modified AuNPs.³² Using the Pb²⁺ induced formation of Pb–gallic acid–Au, the aggregation of AuNPs could be obtained to improve the sensing system, and the limit of detection of this system was 25 nM.³³ Moreover, 11-mercaptoundecanoic acid,³⁴ glutathione³⁵ and thiol alkyl phosphate³⁶ was also used to modify the AuNPs for Pb²⁺ detection. However, these sensors demonstrate the poor practicability caused by the oxidation of thiol end groups. In addition to the colorimetric sensing based on the aggregation of AuNPs, the etching of gold nanoparticles could also result in a shift of the LSPR and be used for colorimetric sensing. Huang et al. developed a colorimetric probe for detecting Pb²⁺ based on the fact that Pb²⁺ can accelerate the leaching of gold by 2-mercaptoethanol and sodium thiosulfate and the limit of detection was 0.5 nM.³⁷ In addition, this analysis required 2 h for the reaction. In the study of Wu et al., the reaction time has been reduced to 30 min by using CTAB-modified AuNPs.³⁸ However, only the shade change of the color could be used for sensing. This sensing method has been improved further by Lin et al.; they use AuNRs to substitute AuNPs.³⁹ The colloidal color changed from red to blue when the concentration of Pb²⁺ was in the range of 0–0.5 μM, whereas it changed from blue, back to red when the concentration of Pb²⁺ exceeded 0.5 μM. Thus, this probe can cause an incorrect determination. Therefore, it is necessary to develop an accurate method to determine the concentration of Pb²⁺ based on an irreversible change of chromaticity.

In this study, we proposed a high sensitive and selective non-aggregation colorimetric probe for detecting Pb²⁺. By adding Pb²⁺ in the presence of Na₂S₂O₃, the etching rate of AuNRs could be accelerated within 5 min. The shape transformation of AuNRs to Au nanospheres leads to the shift, fading and disappearance of the longitudinal LSPR band. Therefore, the quantification of Pb²⁺ can be determined by the observation of unidirectional change in the colloid color. This study provides a cheap and accurate method to determine the concentration of Pb²⁺ with the naked eye.

2. Experimental

2.1. Materials

Cetyl trimethyl ammonium bromide (CTAB) and sodium thiosulfate (Na₂S₂O₃) were obtained from Sigma-Aldrich (USA). Chloroauric acid (HAuCl₄), silver nitrate (AgNO₃), carbonyl chloride (CoCl₂), cuprous chloride (CuCl), ferrous chloride (FeCl₂), manganese(II) chloride (MnCl₂), potassium phosphate (K₃PO₄), cadmium nitrate (Cd(NO₃)₂·4H₂O), chromium nitrate (Cr(NO₃)₃·9H₂O) and mercury chloride (HgCl₂) were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Sodium borohydride (NaBH₄), ascorbic acid (AA), Tris (base), lead nitrate (Pb(NO₃)₂) and zinc chloride (ZnCl₂) were obtained from Aladdin reagent Co. Ltd. (Shanghai, China). Hydrochloric acid (HCl) was purchased from Beijing Chemical Works (China). Magnesium chloride (MgCl₂) was obtained from Tianjin Kemiou Chemical Reagent Co. Ltd. (China). Calcium chloride (CaCl₂) was obtained from Chengdu Geleixi Chemical Reagent Co. Ltd. (China). Cupric chloride (CuCl₂), potassium carbonate (K₂CO₃), potassium sulphate (K₂SO₄), sodium oxalate (C₂Na₂O₄), ferric chloride (FeCl₃) and aluminum nitrate (Al(NO₃)₃) were obtained from Tianjin Tianli Chemical Reagent Co. Ltd. (China). Potassium dichromate (K₂Cr₂O₇) was purchased from Tianjin Basifu Chemical Reagent Co. Ltd. (China). All these chemicals were used as received without further purification. Tris (0.1 M) solutions were adjusted to pH 9.0 using 2.0 M HCl. Milli-Q water (18.2 MΩ cm) was used throughout the experiments.

2.2. Instrumentation

The absorbance spectra were obtained using a UV-3600 UV-Vis-NIR spectrophotometer (Shimadzu, Japan) in the range of 400–1000 nm. The particle shapes were determined by transmission electron microscopy (TEM) using a JEM-200CX transmission electron microscope (JEOL, Japan).

2.3. Preparation of AuNRs

In this system, we used the classic seed-growth method to obtain the AuNRs.^40,41 First, the gold seed was prepared. Subsequently, 0.25 mL HAuCl₄ (0.01 M) was added to a tube containing 5 mL CTAB (0.1 M). After shaking the tube to mix well, 0.6 mL ice-cold NaBH₄ (0.01 M) was added to the mix solution. The mixture was stirred violently until the color of the solution changed to yellowish-brown. The tube was placed into a water bath with a constant temperature of 27 °C for 2 hours. Second, the growth solution was prepared. Subsequently, 0.4 mL HAuCl₄ (0.1 M) was added to the tubes containing 0.01 M AgNO₃ (15 μL, 35 μL, 50 μL, and 75 μL). After shaking the tubes, 9.5 mL CTAB (0.1 M) was added to the tubes. After mixing by using a whirlpool mixer for 30 s, 1% HCl (250 μL) and 0.1 M AA (64 μL) were added to the tubes in turn. The tubes were shaken until the color of the solution changed from dark brown to colorless and then the growth solution was obtained. Finally, a 20 μL seed solution was injected into the mixed solution. The tubes were left to stand at 27 °C for 8 hours to allow the AuNRs to grow adequately.

2.4. Detection of Pb²⁺

First, the fresh AuNRs solution was centrifuged at 12 000 rpm for 15 min at 25 °C, and the supernatant was taken out to remove the excess CTAB. 50 μL of the above mentioned AuNRs solution and 150 μL of the freshly prepared Na₂S₂O₃ (0.12 M) solution were added to 3.8 mL of Tris–HCl buffer (10 mM, pH = 9.0). Second, 1 mL of a Pb(NO₃)₂ solution at different concentrations was added in the abovementioned mixture solutions to a final volume of 5 mL. The mixture solution was shaken and equilibrated at 50 °C. Five minutes later, the UV-Vis spectra of the Pb²⁺–S₂O₃²⁻–AuNRs system were scanned. From the spectra, the wavelength and intensity of the plasmonic absorption peaks of the test solution (λ, A) and the blank reagent (λ₀, A₀) were obtained. The blue shift Δλ (=λ₀ − λ) and the absorbance variable ΔA (=A₀ − A) could also be calculated.

3. Results and discussion

3.1. Sensing mechanism

The sensing strategy of this study is shown in Scheme 1. Thiosulfate is an alternative leachant for the extraction of gold. In this system, S₂O₃²⁻ first attaches to the surface of AuNRs via electrostatic interactions with the CTAB on the AuNRs surface. A redox reaction occurs at the solid–liquid interface and the Au(S₂O₃)₂³⁻ complex forms on the surface of the AuNRs in the presence of oxygen, which leads to the partial dissolution of AuNRs. However, the redox mediator forms a sulfur-like layer due to the decomposition of thiosulfate on the gold surface that hinders the dissolution of gold. Thus, the etching rate of AuNRs is decreased.³⁷

Scheme 1 Schematic of the colorimetric detection of Pb²⁺ using S₂O₃²⁻–AuNRs.

In the thiosulphate system, the addition of Pb²⁺ leads to the monolayer of AuPb₂ and AuPb₃ alloys form on the gold surface, which results in a considerable decrease in the electrode potential of gold. Therefore, the dissolution rate of AuNRs has been increased. In this reaction process, the AuNRs gradually become shorter and the gold nanoparticles gradually change from rods to spheres in the presence of S₂O₃²⁻ after adding Pb²⁺.^42,43 In response, the color of the AuNRs solution changes from blue to red, which is unidirectional. The longitudinal LSPR (LSPR_L) absorption peak shows a blue shift and decreases distinctly until it is merged into the transverse LSPR (LSPP_T) peak. The etching rate depends only on the concentration of Pb²⁺, which indicates that the concentration of Pb²⁺ can be determined by the color of the AuNRs solution.

3.2. The UV-Vis absorption spectra and TEM of Pb²⁺–S₂O₃²⁻–AuNRs system

Fig. 1 shows the color and the UV-Vis spectra of Pb²⁺ + S₂O₃²⁻, S₂O₃²⁻ + AuNRs and Pb²⁺ + S₂O₃²⁻ + AuNRs solutions. The mixture of Pb(NO₃)₂ (1 μM) and Na₂S₂O₃ (0.12 M) is completely colorless and transparent with no absorption peak in the 400–1000 nm wavelength range. Correspondingly, the color of the Na₂S₂O₃–AuNRs solution changes from blue to red because of the disappearance of the LSPR_L peak upon the addition of Pb²⁺ (1 μM). The surface electrode potential of AuNRs–S₂O₃²⁻ before adding Pb²⁺ is 17.327 mV (std error is 0.425 mV). After adding Pb²⁺, the surface electrode potential has dropped to 11.487 mV (std error is 0.577 mV). This considerable decrease in the surface electrode potential indicates that the addition of Pb²⁺ leads to the formation of a monolayer of AuPb alloy on the gold surface. The etching induced shape change of the gold nanoparticles could be demonstrated by the corresponding TEM images in Fig. 2. For AuNRs in the presence of Na₂S₂O₃, the nanorods have an aspect ratio of 1.64. As shown in Fig. 2a, the length of the AuNRs is 24.8 ± 4.2 nm and the diameter is 15.1 ± 2.6 nm. After adding Pb²⁺ (1 μM), the gold nanoparticles become spherical and the diameters of the gold nanospheres is 14.9 ± 2.4 nm, as shown in Fig. 2b. These results show that this sensor can be used to detect aqueous Pb²⁺ in solution.

Fig. 1 Comparison of the absorption spectra between Pb²⁺ + S₂O₃²⁻, S₂O₃²⁻ + AuNRs and Pb²⁺ + S₂O₃²⁻ + AuNRs solutions. The inset shows the corresponding color images.

Fig. 2 TEM image of the AuNRs (a) in the presence of 0.12 M Na₂S₂O₃ and (b) in the presence of 1 μM Pb(NO₃)₂ and 0.12 M Na₂S₂O₃.

The detailed change in the absorption spectrum is shown in Fig. 3. By comparing with the sample without Pb²⁺, the decrease and blue shift of the LSPR_L absorption are caused by the increasing of Pb²⁺ concentration. These spectral changes indicate that Pb²⁺ accelerates the etching rate of AuNRs. From the inset in Fig. 3a, one can also find that the color of the S₂O₃²⁻–AuNRs solution changes from blue to red with increasing Pb²⁺ concentration from 0 nM to 1 μM, which is irreversible. The shift of the LSPR_L peak and A_L/A_T value (the ratio of the absorption intensity of LSPR_L and LSPR_T) as a function of the Pb²⁺ concentration are shown in Fig. 3b. The A_L/A_T value is less than 1.0 when the level of Pb²⁺ is 0.1 μM. Correspondingly, the detection limit by the via naked eye is 0.1 μM, which can be observed from the contrast of the blank sample and the sample with 0.1 μM Pb²⁺, as shown in the inset in Fig. 3a. When the concentration of Pb²⁺ is increased to 275 nM, the LSPR_L peak disappears, and the corresponding color of the solution is completely turned to red, which proves that the AuNRs transforms into Au nanospheres. The absorption spectra show no obvious change and the colors of the solution remain red as the Pb²⁺ ions have been further increased. Thus, the detection range with high sensitivity is 100–275 nM by the naked eye.

Fig. 3 (a) Absorption spectra of the etching effect and the corresponding color images of AuNRs against Pb²⁺ ions in the range of 0 nM to 1 μM (the system contained 0.12 M Na₂S₂O₃ and the reaction was carried out at 50 °C for 5 min under pH 9.0). (b) Shift of the LSPR_L peak and A_L/A_T value (the ratio of the absorption intensity of LSPR_L and LSPR_T) as a function of Pb²⁺ concentration.

3.3. Optimization of the conditions for the Pb²⁺ detection

Because this probe is based on the catalytic etching of AuNRs, experimental conditions need to be optimized. Factors, including the concentration of Na₂S₂O₃, pH and ambient temperature, are tested using the working curves of the Pb²⁺ concentration-dependent wavelength shift and the decrease in intensity in the linear range of 0–100 nM Pb²⁺.

3.3.1. The effect of the concentration of Na₂S₂O₃. Fig. 4 shows the slopes of the Pb²⁺ concentration-dependent wavelength shift and decrease in intensity with different Na₂S₂O₃ concentrations. The slope of the Pb²⁺ concentration-dependent wavelength shift is denoted as K(λ) = Δλ_LSPR/Δρ_lead(II). The slope of the Pb²⁺ concentration-dependent absorbance decrease is denoted as K(A) = ΔA_LSPR/Δρ_lead(II). As the dosage of Na₂S₂O₃ is increased, the catalytic effect of Pb²⁺ on the etching rate of AuNRs increases, and the change in LSPR absorption accordingly enhances. The etching rate, which is corresponding to the slope, reaches a maximum when the concentration of Na₂S₂O₃ is 0.12 M. However, the etching rate has no obvious change when the concentration of Na₂S₂O₃ is above 0.12 M. Therefore, 0.12 M was chosen as the optimal concentration of Na₂S₂O₃ in this system with favorable sensitivity. The corresponding mechanism could be illuminated by the reaction equation in Scheme 1. The equation indicates that Na₂S₂O₃ and dissolved oxygen are involved in the redox reaction. Therefore, the increase in the concentration of Na₂S₂O₃ accelerates the etching rate of AuNRs. However, when the concentration increases gradually and reaches saturation, the redundant Na₂S₂O₃ will be oxidized by O₂, which results in a decrease in dissolved oxygen in solution. Therefore, the excess Na₂S₂O₃ is not conducive to the dissolution of the gold.

Fig. 4 Etching effects for the AuNRs in the presence of Pb²⁺ at different concentrations of Na₂S₂O₃ (the reaction was carried out at 25 °C for 5 min under pH 8.0).

3.3.2. The effect of pH. The effect of pH on the detection was tested over the range of 8–10. As shown in Fig. 5, the slopes of the Pb²⁺ concentration-dependent wavelength shift and intensity decrease with the pH. As the pH value is increased, the effect of Pb²⁺ on the etching rate of AuNRs increases first and the corresponding slopes reaches a maximum when the pH = 9. When the pH is further increased, the slopes decrease slightly again. Therefore, the optimal pH of 9.0 was used in subsequent tests. Na₂S₂O₃ will decompose to sulfide, sulfate, sulfite, tetrathionate, trithionate, polythionates and polysulfides under acidic condition, which leads to an unsatisfactory detection results. Under the alkalinity environment, the better stability of S₂O₃²⁻ and Na₂S₂O₃–AuNRs improves the etching rate and detection effect. However, when the pH value is further increased, Pb(OH)₂, PbO or Au(OH)₃ will be formed on the surface of the AuNRs and impede the dissolution of Au, which results in a lower etching rate.⁴²

Fig. 5 Etching effects for the AuNRs in the presence of Pb²⁺ at different pH (the system contained 0.12 M Na₂S₂O₃ and the reaction was carried out at 25 °C for 5 min).

3.3.3. The effect of temperature. In this study, the detection of Pb²⁺ is also affected significantly by the ambient temperature. The change in absorption and etching rate at different reaction temperatures over the range of 25–70 °C was investigated in Fig. 6. As the incubation temperature was increased, the etching rate (which corresponds to the slope of the curve) also increased initially and reached a maximum at 50 °C. The corresponding mechanism could be attributed to the enhancement of the rate of synthesis of the Pb–Au alloys, which is caused by the increasing collision frequency between Pb²⁺ and gold. However, the etching rate shows an insignificant decrease when the incubation temperature surpassed 50 °C. To achieve high sensitivity, an ambient temperature of 50 °C was selected in following experiments.

Fig. 6 Etching of the AuNRs in the presence of Pb²⁺ at different temperature (the system contained 0.12 M Na₂S₂O₃ and the reaction was carried out for 5 min at pH 9.0).

3.3.4. The effect of aspect ratio of AuNRs. Fig. 7 compares AuNRs with four aspect ratios at the same absorbance intensity to investigate the effect of the aspect ratio on the sensitivity of this probe. In Fig. 7a, the AuNRs have a small aspect ratio, and the corresponding initial LSPR_L peak is about 600 nm. The increasing Pb²⁺ concentration leads to the LSPR_L peak blue shift and fade down rapidly. When the addition of Pb²⁺ is increased to 300 nM, the LSPR_L peak merges to a LSPR_T peak. Therefore, only one absorption peak occurs in the spectrum, which indicates that the shape of AuNRs has changed to Au nanospheres. This qualitative change in the absorption spectrum also results in a distinct color change of the solution. Fig. 8 shows that the solution color changes from blue to red upon the addition of 300 nM Pb²⁺. In Fig. 7b–d, the AuNRs have a relative large aspect ratio, and the corresponding initial LSPR_L peaks are 680, 750 and 850 nm, respectively. In these cases, the increasing Pb²⁺ concentration also leads to the LSPR_L peak blue shift and fade down. However, the LSPR_L peak is always far from the LSPR_T peak. Thus, the LSPR_L peak will not merge into the LSPR_T peak and no qualitative spectral change occurs. Therefore, the AuNRs have not changed to Au nanospheres, and the corresponding solution color shows no noticeable change, as shown in Fig. 8. Therefore, the AuNRs with smaller aspect ratio can enhance the sensitivity of the probe. To better understand the effects of AuNR aspect ratio on the sensitivity of the probe, the peak wavelength separation and A_L/A_T value as a function of Pb²⁺ concentration with different aspect ratios are compared in Fig. 7e and f. As shown in Fig. 7e, the Pb²⁺ concentration-dependent absorbance ratio of the AuNRs with a large aspect ratio (the initial LSPR_L wavelength is 850 nm) is always greater than 3.8. Therefore, the two LSPR bands are always separated as the Pb²⁺ concentration is increased. On the other hand, the Pb²⁺ concentration-dependent peak separation of the AuNRs with a large aspect ratio is always greater than 280 nm, as shown in Fig. 7f. This also indicates that the shape of the AuNRs has not changed to a sphere. For AuNRs with a small aspect ratio (the initial LSPR_L wavelength is 600 nm), the absorbance ratio decreases from 1.1 to 0.8 as the Pb²⁺ concentration is increased, which indicates that the longitudinal LSPR band of AuNRs is merged to the transversal LSPR absorption. However, an increase in Pb²⁺ concentration also leads to a decrease in peak separation from 70 to 30 nm, which also indicates that the shape of the AuNRs has transformed to a sphere. This qualitative spectral change from double bands to single band LSPR results in a more distinct color change. Therefore, the color change of the AuNRs with the smaller aspect ratio is more obvious.

Fig. 7 Pb²⁺-dependent absorption spectra of the AuNRs with different aspect ratio, the LSPR_L peak is at (a) 600 nm, (b) 680 nm, (c) 750 nm and (d) 850 nm. The A_L/A_T value (e) and peak wavelength separation (f) as a function of the Pb²⁺ concentration with different aspect ration. The system contained 0.12 M Na₂S₂O₃ and the reaction was carried out at 50 °C for 5 min under pH 9.0.

Fig. 8 Pb²⁺-dependent color changes in the AuNRs solution with different aspect ratio, the LSPR_L peak is at 600 nm, 680 nm, 750 nm and 850 nm. The system contained 0.12 M Na₂S₂O₃ and the reaction was carried out at 50 °C for 5 min under pH 9.0.

3.4. Working curve, linear range and detection limit

The working curves for Pb²⁺ detection are shown in Fig. 9. Under the optimal conditions, the linear range of the S₂O₃²⁻–AuNRs detection system toward Pb²⁺ was 25–300 nM and the linear regression equations of the working curves could be expressed as Δλ = −0.65384 + 0.12949[Pb²⁺] (nM), R² = 0.9992, and ΔA = 1.61 × 10⁻³ + 5.29748 × 10⁻⁴[Pb²⁺] (nM), R² = 0.99942. The detection limit was 20.0 nM for Δλ and 24.5 nM for ΔA (calculated by 3S_b/k, herein the 3S_b/k referred to the quotient between the triple of the blank reagent's standard deviation and slope of the working curve, S_b referred to the standard deviation of 11 parallel analysis of the blank reagent, k is the slope of the working curve). The detection limit via the naked eyes was 0.1 μM, which can be observed from the contrast of blank sample and 0.1 μM Pb²⁺, as shown in the inset in Fig. 3a.

Fig. 9 Working curves of the (a) Δλ and the (b) ΔA vs. the concentrations of Pb²⁺. The system contained 0.12 M Na₂S₂O₃ and the reaction was carried out at 50 °C for 5 min under pH 9.0.

3.5. Selectivity of S₂O₃²⁻–AuNRs probe

To further evaluate the selectivity of this colorimetric probe towards Pb²⁺, the Pb²⁺ ions were replaced with other metal ions (Fe³⁺, Cu²⁺, Hg²⁺, Zn²⁺, Ca²⁺, Al³⁺, Mg²⁺, Cd²⁺, Cr²⁺, Cu⁺, Fe²⁺, Co²⁺, Mn²⁺, and Ag⁺ each at a concentration of 2 μM) and counter ions (NO₃⁻, Cl⁻, C₂O₄²⁻, Cr₂O₇²⁻, ClO₄⁻, PO₄³⁻, and SO₄²⁻ each at a concentration of 2 μM) under the same optimized conditions. Fig. 10a shows the absorbance intensity change and the wavelength shift of AuNRs upon the addition of 15 types of different metal ions. The absorbance intensity of the AuNRs shows a distinct decrease accompanied by a blue shift upon the addition of Pb²⁺, while there was very little change for Fe³⁺, Cu²⁺, Hg²⁺, Zn²⁺, Ca²⁺, Al³⁺, Mg²⁺, Cd²⁺, Cr²⁺, Cu⁺, Fe²⁺, Co²⁺, Mn²⁺ and Ag⁺. A distinct color change from blue to red could be observed due to the addition of Pb²⁺ but the other metal ions have little effect, as shown in the inset of Fig. 10a. Fig. 10b shows the change in the absorbance intensity and the shift in the wavelength of AuNRs upon the addition of 7 types of different counter ions. The absorbance intensity of the AuNRs showed a distinct decrease accompanied by a blue shift upon the addition of Pb²⁺, whereas there was very little change for NO₃⁻, Cl⁻, C₂O₄²⁻, Cr₂O₇²⁻, ClO₄⁻, PO₄³⁻ and SO₄²⁻. A distinct blue to red color change could be observed due to the addition of Pb²⁺ and the other counter ions were slightly affected, as shown in the inset of Fig. 10b. These results suggest that this proposed sensing method has good selectivity.

Fig. 10 Wavelength shift and intensity decrease of the AuNRs containing (a) different metal ions (2 μM Fe³⁺, Cu²⁺, Hg²⁺, Zn²⁺, Ca²⁺, Al³⁺, Mg²⁺, Cd²⁺, Cr²⁺, Cu⁺, Fe²⁺, Co²⁺, Mn²⁺, Ag⁺ and 0.2 μM Pb²⁺) and (b) different counter ions (2 μM NO₃⁻, Cl⁻, C₂O₄²⁻, Cr₂O₇²⁻, ClO₄⁻, PO₄³⁻, SO₄²⁻ and 0.2 μM Pb²⁺).

4. Conclusion

In summary, we designed a non-aggregation colorimetric assay for the detection of Pb²⁺ based on the catalytic etching of AuNRs. In this method, accelerated etching leads to a change in the shape of the gold nanorods to gold nanospheres. This morphological transformation leads to a qualitative spectral change from double band to single band LSPR, which further results in a distinct irreversible color change of the gold colloid from blue to red. These results are much different from the previous reports. This sensing method also exhibits many merits, such as simplicity, accuracy and rapidity, as modification of the AuNRs surface is not required. The concentration of Pb²⁺ can be determined by the change in LSPR absorption and the color of the AuNRs–S₂O₃²⁻–Pb²⁺ solution. Under the optimized conditions, this probe exhibits higher sensitivity towards Pb²⁺ compared to many other nanoparticle-based colorimetric probes and also exhibits high selectivity over other possible interference ions. This sensing system of Pb²⁺ has a promising application in water and human serum analysis. In the future, we hope that this system can be applied to an economical Pb²⁺ test paper.

Acknowledgements

This study was supported by the Fundamental Research Funds for the Central Universities under grant No. 2011jdgz17, xjj2015082 and the National Natural Science Foundation of China under grant No. 61178075.

References

1. Y. F. Li, C. Y. Chen, B. Li, J. Sun, J. X. Wang, Y. X. Gao, Y. L. Zhao and Z. F. Chai, J. Anal. At. Spectrom., 2006, 21, 94–96.
2. M. Meadows-Oliver, J. Pediatr. Health Care, 2012, 26, 213–215.
3. A. Mudipalli, Indian J. Med. Res., 2007, 126, 518–527.
4. C. Sobin, N. Parisi, T. Schaub, M. Gutierrez and A. X. Ortega, Arch. Environ. Contam. Toxicol., 2011, 61, 521–529.
5. J. W. Liu and Y. Lu, Anal. Chem., 2003, 75, 6666–6672.
6. J. A. Menezes-Filho, G. F. D. Viana and C. R. Paes, Environ. Monit. Assess., 2012, 184, 2593–2603.
7. K. Zargoosh and F. F. Babadi, Spectrochim. Acta, Part A, 2015, 137, 105–110.
8. Q. Wang, S. Zhang, H. Ge, G. Tian, N. Cao and Y. Li, Sens. Actuators, B, 2015, 207, 25–33.
9. C. V. Durgadas, V. Nair Lakshmi, C. P. Sharma and K. Sreenivasan, Sens. Actuators, B, 2011, 156, 791–797.
10. A. D'Agostino, A. Taglietti, B. Bassi, A. Dona and P. Pallavicini, J. Nanopart. Res., 2014, 16, 2683.
11. A. Moores and F. Goettmann, New J. Chem., 2006, 30, 1121–1132.
12. D. Liu, F. Zhou, C. Li, T. Zhang, H. Zhang, W. Cai and Y. Li, Angew. Chem., Int. Ed., 2015, 54, 9596–9600.
13. D. Liu, C. Li, F. Zhou, T. Zhang, H. Zhang, X. Li, G. Duan, W. Cai and Y. Li, Sci. Rep., 2015, 5, 7686.
14. S. Song, Z. Liang, J. Zhang, L. Wang, G. Li and C. Fan, Angew. Chem., Int. Ed., 2009, 48, 8670–8674.
15. P. M. Tessier, J. Jinkoji, Y. C. Cheng, J. L. Prentice and A. M. Lenhoff, J. Am. Chem. Soc., 2008, 130, 3106–3112.
16. X. X. Wang, J. M. Liu, S. L. Jiang, L. Jiao, L. P. Lin, M. L. Cui, X. Y. Zhang, L. H. Zhang and Z. Y. Zheng, Sens. Actuators, B, 2013, 182, 205–210.
17. R. L. Liu, Z. P. Chen, S. S. Wang, C. L. Qu, L. X. Chen and Z. Wang, Talanta, 2013, 112, 37–42.
18. Y. Li, P. Wu, H. Xu, Z. P. Zhang and X. H. Zhong, Talanta, 2011, 84, 508–512.
19. Y. Tao, Y. H. Lin, Z. Z. Huang, J. S. Ren and X. G. Qu, Talanta, 2012, 88, 290–294.
20. F. Q. Zhang, L. Y. Zeng, C. Yang, J. W. Xin, H. Y. Wang and A. G. Wu, Analyst, 2011, 136, 2825–2830.
21. Y. Xue, H. Zhao, Z. J. Wu, X. J. Li, Y. J. He and Z. B. Yuan, Analyst, 2011, 136, 3725–3730.
22. F. M. Li, J. M. Liu, X. X. Wang, L. P. Lin, W. L. Cai, X. Lin, Y. N. Zeng, Z. M. Li and S. Q. Lin, Sens. Actuators, B, 2011, 155, 817–822.
23. J. W. Xin, F. Q. Zhang, Y. X. Gao, Y. Y. Feng, S. G. Chen and A. G. Wu, Talanta, 2012, 101, 122–127.
24. H. Xu, B. X. Liu and Y. Chen, Microchim. Acta, 2012, 177, 89–94.
25. M. R. Knecht and M. Sethi, Anal. Bioanal. Chem., 2009, 394, 33–46.
26. J. W. Liu and Y. Lu, J. Am. Chem. Soc., 2003, 125, 6642–6643.
27. D. Mazumdar, J. W. Liu, G. Lu, J. Z. Zhou and Y. Lu, Chem. Commun., 2010, 46, 1416–1418.
28. Z. D. Wang, J. H. Lee and Y. Lu, Adv. Mater., 2008, 20, 3263–3267.
29. Z. D. Wang and Y. Lu, J. Mater. Chem., 2009, 19, 1788–1798.
30. J. Liu and Y. Lu, J. Am. Chem. Soc., 2005, 127, 12677–12683.
31. H. Wei, B. L. Li, J. Li, S. Dong and E. K. Wang, Nanotechnology, 2008, 19, 095501–095505.
32. K. Yoosaf, B. I. Ipe, C. H. Suresh and K. G. Thomas, J. Phys. Chem. C, 2007, 111, 12839–12847.
33. N. Ding, Q. A. Cao, H. Zhao, Y. M. Yang, L. X. Zeng, Y. J. He, K. X. Xiang and G. W. Wang, Sensors, 2010, 10, 11144–11155.
34. Y. J. Kim, R. C. Johnson and J. T. Hupp, Nano Lett., 2001, 1, 165–167.
35. F. Chai, C. G. Wang, T. T. Wang, L. Li and Z. M. Su, ACS Appl. Mater. Interfaces, 2010, 2, 1466–1470.
36. S. K. Kim, S. Kim, E. J. Hong and M. S. Han, Bull. Korean Chem. Soc., 2010, 31, 3806–3808.
37. Y. Y. Chen, H. T. Chang, Y. C. Shiang, Y. L. Hung, C. K. Chiang and C. C. Huang, Anal. Chem., 2009, 81, 9433–9439.
38. Y. J. Zhang, Y. M. Leng, L. J. Miao, J. W. Xin and A. G. Wu, Dalton Trans., 2013, 42, 5485–5490.
39. Y. J. Lan and Y. W. Lin, Anal. Methods, 2014, 6, 7234–7242.
40. T. K. Sau and C. J. Murphy, Langmuir, 2004, 20, 6414–6420.
41. X. Huang, S. Neretina and M. A. El-Sayed, Adv. Mater., 2009, 21, 4880–4910.
42. D. Feng and J. S. J. van Deventer, Hydrometallurgy, 2002, 64, 231–246.
43. G. Deschenes, R. Lastra, J. R. Brown, S. Jin, O. May and E. Ghali, Miner. Eng., 2000, 13, 1263–1279.

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