Visual sensing of Hg2+ using unmodified Au@Ag core–shell nanoparticles

Materials

Chloroauric acid (HAuCl ₄ ·4H ₂ O), silver nitrate (AgNO ₃ ) and trisodium citrate dehydrate (C ₆ H ₅ Na ₃ O ₇ ·2H ₂ O) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). l -ascorbic acid was from Aladdin Reagents Co, Ltd. (Shanghai, China). All other chemicals used in this work were analytical reagent grade. All solutions were prepared with twice-quartz-distilled water. The water sample was from Jinghu Lake (Wuhu, China) and tap water was from our laboratory.

Instruments

The absorption spectra were obtained on a U-3010 UV–vis-NIR spectrometer (Hitachi, Japan). The morphology and the size of the Au NPs and Au@Ag NPs were measured by a scanning electron microscope (SEM) using JE0LJSM-6700F microscope (Hitachi, Japan).

Preparation of Au NPs

Au NPs were prepared according to previously described process [ ²⁶ ]. In brief, a 50.0 mL of 0.01 % HAuCl ₄ was heated to boiling. Next, 2.0 mL of 1 % trisodium citrate was added under stirring vigorously, and kept boiling until the solution became wine red. Finally, the solution was cooled to room temperature while stirring. The diameter of the Au NPs was measured about 16 nm by SEM.

Preparation of Au@Ag NPs

Au@Ag NPs were prepared according to previously described method with a little modification [ ²⁷ ]. Simply, as-prepared Au NPs (1.0, 1.5, 2.0, 2.5 and 3.0 mL) were added to each of five 50 mL round-bottomed flasks containing 10 mL of water, respectively. Next, 0.1 mL of 38.8 mM trisodium citrate and 0.1 mL of 10 mM ascorbic acid were added dropwise to the above solution under stirring. Finally, 0.25 mL of 10 mM AgNO ₃ was added dropwise each flask while stirring. The resulting solutions were stirred for 30 min until the colors of the solutions changed from pink to yellow. The products obtained were collected for further used.

Visual detection of Hg2+

For visual detection of Hg ²⁺ , the pH of the system (2.0 mL of Au@Ag NPs) was adjusted to 4.5 with the citric acid–sodium hydrogen phosphate buffer saline. Then, different concentrations of Hg ²⁺ were added separately to the above solution; the color changes could be observed with naked eye; and the absorption curves were recorded at room temperature.

Analysis of real samples

To invest the practical application capability of this sensor, all the collected samples were centrifuged for 15 min at 9,000 rpm, and the supernatant was filtered through a 0.2 µm membrane. Then, a series of samples were prepared by spiking standard solutions of Hg ²⁺ to the lake water or tap water. The pretreated samples were added to 2.0 mL of Au@Ag NPs (pH 4.5) and then were analyzed using the developed sensing strategy.

Investigation of sensing mechanism

The sensing mechanism of Hg ²⁺ using Au@Ag NPs is probably related to a redox reaction between Ag-shell and Hg ²⁺ [the standard potential of 0.8 V (Ag ⁺ /Ag) and 0.85 V (Hg ²⁺ /Hg), respectively]. In the absence of Hg ²⁺ , the prepared Au@Ag NPs exhibited good spherical monodispersity (the monodispersity is related to excessive citrate ions in the solution) (seen in Fig.  ¹ c). In the presence of Hg ²⁺ , a redox reaction between Ag-shell and Hg ²⁺ would occur. The Ag-shell became thinner and the sizes of Au@Ag NPs decreased gradually due to the oxidation of Ag-shell. The color of the solution changed lighter (from yellow to faint yellow), the surface plasmon resonance (SPR) absorption peak blue-shifted gradually, and the intensity decreased. When the concentration of Hg ²⁺ was up to 10 µM, the color of the solution changed from yellow to purple red, and the SPR absorption peak red-shifted due to the strong affinity between Hg and Au NPs, the reduced Hg (0) would directly deposit onto the surface of Au-core to form Au–Hg alloys (Scheme  ¹ ). During these processes, citrate ions were kept off the surfaces of nanoparticles, leading to aggregation of nanoparticles (seen in Figs.  ¹ d, ⁴ a).

Fig. 1

Scheme 1

It was reported that citrate ions on the surface of Au NPs could also reduce Hg ²⁺ to form Au–Hg alloys and caused aggregation [ ²⁸ ]. To make it clear whether the redox reaction was from Hg ²⁺ and Ag-shell or Hg ²⁺ and citrate ions in our system, we design a contrast experiment to investigate the reaction mechanism. Namely, 20 µL of Hg ²⁺ (1 mM) was added to 2.0 mL solution of citrate-stabilized Au NPs and citrate-stabilized Ag NPs, respectively (the concentration of Hg ²⁺ was 10 µM). It was observed that the color of Au NPs solution changed nothing, the intensity and position in the SPR absorption peaks of Au NPs also did not change in the absence and presence of the Hg ²⁺ . On the contrary, in the presence of Hg ²⁺ , the solution color of Ag NPs changed from yellow to colorless, and the SPR absorption peaks of Ag NPs disappear. In addition, Energy-dispersive X-ray (EDX) analysis showed the elementary Ag in the agglomerates was basically no left while the content of Hg was higher (seen in Fig.  ¹ e). These facts showed that the formation of Au–Hg alloys was from the redox reaction between Ag-shell and Hg ²⁺ rather than between citrate ions and Hg ²⁺ .

The absorption spectra of Au@Ag NPs

When different amounts of Au seeds (1.0–3.0 mL) were added to the solution, the absorption spectra of Au@Ag NPs were located at 404, 398, 394, 392 and 389 nm, respectively (seen in Fig.  ¹ a).It could be observed that the intensity of SPR absorption decreased gradually as the concentration of Au seeds increased, accompanying with the blue shifts and broadening of absorption peak. The solution color of the prepared Au@Ag NPs changed from light yellow to deep yellow. The phenomenon could be explained that the optical properties of Au@Ag NPs were dominated by Ag at low concentration of Au seeds, and it was dominated by Au as the concentration of Au seeds increased.

Optimization of the experiment conditions

To optimize the experiment conditions, 20 µL Hg ²⁺ (1 mM) was injected into the five kinds of the different ratios of Au/Ag Au@Ag NPs solutions (2.0 mL), respectively. The solution color changed from yellow to purplish red, and a new absorption peak which appeared at about 520 nm accompanying with the original SPR absorption peaks disappeared. The intensity of the new absorption peak was increased as the concentration of Au seeds increased (Seen in Fig.  ¹ b). We observed that the color changes of Au@Ag NPs solution with 1.0, 1.5 mL Au seeds were not easy to be observed by naked eyes; in addition, we also observed that the two kinds of nanoparticles solution were difficult to centrifuge before and after addition of Hg ²⁺ , so Au@Ag NPs obtained from the 1.0, 1.5 mL Au seeds need not be adopted in this work.

From the view of peak shape, the color changes and the maneuverability, 2.0 mL Au seeds were chosen in this work. The SEM images of the Au@Ag NPs showed a good spherical monodispersity and the diameter was about 40 nm. EDX spectra showed that the molar ratios of Au/Ag were 1:3.

The effect of pH was also investigated in the absence and presence of Hg ²⁺ . The pH of the system was adjusted in the range of 3.5–6.0 using the citric acid–sodium hydrogen phosphate buffer saline (pH 2.2–7.0) and the results obtained are shown in Fig.  ² . It was observed that the stability of Au@Ag NPs was related to the pH of the system. Simply, if pH was lower than 3.5, the stability of Au@Ag NPs would decrease (seen in Fig.  ² a). As pH increases, the stability of Au@Ag NPs was improved; however, if the pH was higher than 6.0, the color of the solution did not change in the presence of Hg ²⁺ , and the decline of the SPR absorption intensity was not clear (seen in Fig.  ² b). From Fig.  ² c, it could be observed that Au@Ag NPs were stable and the changes of the SPR absorption spectra were obvious in presence of pH 4.5. Hence, a pH of 4.5 was employed in this study.

Fig. 2

Selectivity

To investigate selectivity of sensor, some foreign metal ions including Li ⁺ , Na ⁺ , K ⁺ , Mg ²⁺ , Ca ²⁺ , Ba ²⁺ , Zn ²⁺ , Mn ²⁺ Co ²⁺ , Fe ²⁺ ,Cu ²⁺ ,Cd ²⁺ Cr ³⁺ , Ni ²⁺ , Hg ²⁺ were selected for this study. Figure  ³ a shows the photographs of solutions in the presence of various foreign metal ions. It was clearly observed that the existence of about 50-fold of Li ⁺ , Na ⁺ , K ⁺ , Mg ²⁺ , Ca ²⁺ , Ba ²⁺ , Mn ²⁺ , Zn ²⁺ , Fe ²⁺ , Ni ²⁺ , and tenfold of Co ²⁺ , Cu ²⁺ , Pb ²⁺ and the same concentration of Cd ²⁺ and Cr ³⁺ did not interfere the determination of Hg ²⁺ . Hg ²⁺ was only the ion, which resulted in distinct color change from yellow to purplish red, indicating that this sensor has good selectivity.

Fig. 3

Analytic performance

To investigate the sensitivity of the sensor, the Au@Ag NPs were utilized for sensing Hg ²⁺ under the optimized conditions, and the results are shown in Fig.  ⁴ . As the concentration of Hg ²⁺ increased from 0 to 10 µM, the color of the Au@Ag NPs gradually changed from yellow to purplish red. In the meantime, the intensity of the absorption peak at 394 nm decreased gradually. Specially, when the concentration of Hg ²⁺ was up to 8 µM, a new absorption peak appeared at about 520 nm, indicating the formation of Au–Hg alloys. The decline of the intensity was linear with the concentration of Hg ²⁺ increased in the range of 0.1–1.0 µM. The linear regression equation was ΔA = 0.2178 + 0.2862c (Here, ΔA = A ₀ −A. A ₀ and A stand for the absorbance of solution in the absence and presence of Hg ²⁺ , respectively; the unit of concentration is µM) with the correlation coefficient of 0.996. The limit of detection was about 0.4 µM with the naked eye and 5.0 nM by UV–vis measurements.

Fig. 4

Analysis of real samples

To test the application of the proposed approach, we selected lake water and tap water samples for Hg ²⁺ detection. The results are shown in Table  ¹ , the recovery of Hg ²⁺ was within 82.4–112.0 % for tap water and 83.6–115.0 % for lake water, respectively, indicating that the sensor was suitable for the detection of Hg ²⁺ in real samples.