Preparation of Au@Ag nanoparticles at a gas/liquid interface and their application for sensitive detection of hydrogen peroxide

Abstract

Au@Ag nanoparticles (NPs) were prepared by a seed-mediated growth procedure using spherical Au NPs as seeds at a gas/liquid interface and further used to fabricate a nonenzymatic hydrogen peroxide (H₂O₂) sensor. The as-prepared Au@Ag NPs were characterized by UV-visible spectroscopy, transmission electron microscopy (TEM) and dynamic light scattering (DLS). TEM and DLS analysis indicates that the obtained Au@Ag NPs are highly dispersed and possess narrow size distributions. The electrochemical behavior of the nonenzymatic sensor suggests that the Au@Ag NPs modified glassy carbon electrode exhibited a high sensitivity of 251.9 μA mM⁻¹ cm⁻² for electrochemical detection of H₂O₂ at a potential as low as −0.1 V. The results demonstrate that Au@Ag NPs should be promising materials for an electrochemical sensor in practical applications.

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1. Introduction

Over recent decades, nanomaterials have received great interest due to their applications in electronics, sensing, electrocatalysis, and biomedical research. To control their shape, composition and structure, widespread attention has been paid to developing new preparation methods. Interfacial reaction, which means chemical reactions that occur at an interface of two different phases, offers a new route for controlled preparation of materials effectively with unique properties.¹ Serving as a fertile medium for nanomaterials assembly, a liquid/liquid interface offers the potential to prepare nanomaterials.^2–4 In our previous study, fairly narrow Ag nanoparticles and three-dimensional network Fe₃O₄ were successfully prepared by a gas/liquid interfacial reaction and further used to fabricate an electrochemical sensor.^5–7 Compared with the liquid/liquid interface, the gas/liquid interface can be easily fabricated, because the reaction can be controlled by the reaction temperature, the gas pressure, the velocity of gas flow, and throughput.^8–10

Metallic NPs composed of noble metals including Au, Ag, Pt, and Pd have attracted widespread attention in recent years owing to their composition-dependent optical, electronic, and electrochemical catalytic properties. Ag NPs, as a typical noble metal, exhibits excellent electrocatalysis toward the reduction of H₂O₂.^11–13 It is worth noting that metallic NPs can easily aggregate in practical electrochemical applications, which results in an obvious decrease in catalytic activity. To prevent metallic NPs from aggregation and coagulum, various supports immobilizing metallic NPs are widely utilized.^14–18 However, the use of supports is limited by some disadvantages, such as a complex procedure for synthesis, a critical immobilization process, high cost, time-consuming processes. On the other hand, a great number of dispersed Ag NPs, prepared by chemical reduction of Ag⁺ in aqueous medium, easily aggregate and could lead the formation of aggregation. This can be explained by the fast self-nucleation, the poor balance between nucleation and growth processes, and the high surface energy.^19,20 Hence, it is highly desirable to develop a simple and effective method to prevent Ag NPs from aggregation in nucleation and growth processes and fabricate a highly sensitive non-enzymatic H₂O₂ sensor without supports.

In the present study, we propose a facile approach for the control preparation of highly dispersed Au@Ag NPs by seed-mediated growth procedure using Au NPs as seeds at a gas/liquid interface (Scheme 1). In this process, epitaxial growth of Ag shell onto Au core was conducted at a gas/liquid interface, in which volatilized CH₂O gas was used as reducing agent. Based on the highly dispersed Au@Ag NPs without aggregation, a highly sensitive non-enzymatic H₂O₂ sensor is fabricated and provide a new platform for the construction of H₂O₂ biosensors.

Scheme 1 Preparation of Au@Ag NPs at a gas/liquid interface by a seed-mediated growth procedure and its application for sensitive detection of hydrogen peroxide.

2. Experimental section

2.1. Chemicals and material

Formaldehyde (37%), chloroauric acid tetrahydrate (HAuCl₄·4H₂O) and silver nitrate (AgNO₃) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium borohydride (NaBH₄), L-ascorbic acid (AA), and cetyltrimethylammonium chloride (CTAC) were got from Sigma-Aldrich and used as received. Chitosan (CS, MW 5 to 6 × 10⁵, >90% deacetylation) and H₂O₂ (30%) was bought from Tianjin Tianli Chemistry Reagent Co., Ltd (Tianjin, China). All solutions are prepared with ultrapure water of 18.2 MΩ cm⁻¹.

2.2. Apparatus and measurements

Electrochemical measurements were performed on a CHI 660E electrochemical analyzer (Shanghai CH Instrument Co. Ltd., China) in a conventional three-electrode cell. Glassy carbon electrode (GCE, 3 mm in diameter) was got form GaossUnion Technology Co., Ltd. (Wuhan, China). All electrochemical experiments were conducted at room temperature (25 ± 2 °C). Transmission electron microscopic (TEM) images were obtained on Tecnai G2 F20 S-TWIN (FEI, USA). UV-visible spectra were recorded using a UV-2550 UV-visible spectrofluorophotometer (Shimadzu). The particle diameter distribution and the zeta potential were measured by dynamic light scattering (DLS) technique using a Malvern Zetasizer Nano ZS90 with a He-Ne laser (633 nm) and 90° collecting optics and treated with ultrasonic wave before testing.

2.3. Preparation of the sensor

2.3.1. Synthesis of spherical Au NPs as seeds. The synthesis procedure was carried out according to the previous literature^21,22 with some modification. First, to synthesize 3–5 nm Au nanoparticles, 1 mL of ice-cold NaBH₄ (10 mM) solution was added into a 10 mL aqueous solution containing HAuCl₄ (0.25 mM) and CTAC (100 mM). Then 5 mL of aqueous HAuCl₄ solution (10 mM), 10 mL of aqueous CTAC solution (200 mM), and 17.5 mL of aqueous AA solution (40 mM) were mixed, followed by the addition of 100 μL of the 3–5 nm Au nanoparticles. The Au seeds were centrifuged (12 000 rpm, 30 min) and redispersed in deionized water.

2.3.2. Preparation of Au@Ag NPs at a gas/liquid interface by seed-mediated growth. The growth of Ag shell onto Au core was conducted at a gas/liquid interface using CH₂O gas as the reducing agent. Briefly, the preparation of Ag[(NH₃)₂]⁺ solution described as follows: to 1 mL AgNO₃ solution (0.5 M) was added 1 mL aqueous ammonia (25–28%), followed by the addition of NaOH (0.1 M, 20 mL). Then the freshly synthetized Ag[(NH₃)₂]⁺ solution was diluted to 50 mL by doubly distilled water. 320 μL Au seeds, 80 μL as-synthesized Ag[(NH₃)₂]⁺ solution and 8 mL of CTAC aqueous solution (20 mM) were mixed in a 20 mL beaker. At last, the mixture and a vial that containing 20 mL of CH₂O solution were put in a closed container. The volatilized CH₂O gas can be used as reducing agent, and the gas/liquid interface reaction was performed for 6 h at room temperature with continuous stirring. The color of the mixture turned from pink to slightly yellow and finally deep yellow. The product, consisting of Au@Ag NPs, was centrifuged (6000 rpm, 10 min) two times for further use.

2.3.3. Preparation of Au@Ag NPs modified electrode. A glassy carbon electrode (GCE, 3 mm in diameter) was polished with slurries of 0.3 and 0.05 μm alumina to create a mirror finish, and sonicated with absolute ethanol and double-distilled water for about 1 min. After that, it was rinsed thoroughly with double-distilled water and dried with high-purity nitrogen steam. Then Au and Au@Ag suspensions (5 μL) were cast onto the surfaces of GCE respectively. After a few hours, chitosan solution (0.3 wt%) was cast onto the Au and Au@Ag NPs modified GCE respectively and dried at room temperature. The modified electrodes were denoted as Au NPs/GCE and Au@Ag NPs/GCE. A N₂-saturated 0.1 M PBS solution was used as the base electrolyte.

3. Results and discussion

3.1. Characterization of the nanoparticles

The formation of Au@Ag NPs was confirmed by TEM observation. Fig. 1A shows the TEM image of Au seeds which exhibit uniform spherical shape with an average size of 18 ± 2 nm. As shown in Fig. 1B, each particle is nearly uniform and average size of the Au@Ag NPs increased to 32 ± 3 nm, indicating the formation of Ag shell. A noteworthy detail about growth of core–shell heterogeneous structures is that the lattice mismatch between Au and Ag is rather small (0.17%). Therefore, the epitaxial growth of Ag shell occurred on the Au core and thus the boundary of Au core and Ag shell was observed from TEM image (Fig. 1B). Otherwise, large lattice mismatch could lead to non-epitaxial growth instead of epitaxial growth, so that the nanoislands rather than a complete shell is formed on the surface of the core.^23–25 Accordingly, the gray/light and dark color contrast difference observed in the Au core/Ag shell structure (Fig. 1B). For a bimetal core–shell nanoparticle, the metal with lower atomic number shows gray/light image and the metal with the higher atomic number usually shows dark image.²⁶

Fig. 1 TEM images of Au NPs (A) and Au@Ag NPs (B). Size distribution histograms of Au NPs (C) and Au@Ag NPs (D) dispersed in aqueous solution (pH 7.0).

Scheme 1 shows the strategy for the preparation of Au@Ag NPs by seed-mediated growth procedure at a gas/liquid interface and fabrication of nonenzymatic H₂O₂ sensor. Once volatilized CH₂O gas, used as reducing agent, reaches the surface of the Ag[(NH₃)₂]⁺ solution, it begins to dissolve into the solution and a chemical reaction occurs at the gas/liquid interface. Hence, the reaction proceeds under a mild reducing conditions and can stay in control for the slow reaction speed.^5–7 The particle size distribution of Au NPs and Au@Ag NPs suspension measured by DLS technique is shown in Fig. 1(C and D). They possessed narrow size distributions, indicating that no aggregation occurred during the entire growth process. Such stability of nanoparticles suspension can also be explained by the well-know DLVO theory²⁷ that the qualitative features of colloid stability as resulting from the interplay of van der Waals forces and electrostatic double layer interaction. The zeta potential is typically used as an index of the magnitude of electrostatic double layer interactions between colloidal particles. Particles with the absolute value of the zeta potential more than 15 mV are expected to be stabilized by electrostatic repulsion interaction.²⁸ A zeta potential of 34 ± 3 mV was observed from the as-prepared Au@Ag NPs suspension that is stabilized by the formation of an electrical double layer due to CTAC attached around each Au@Ag nanoparticle. These highly positive values further confirm the stability observed over a month for aqueous suspension of Au@Ag NPs.

Fig. 2 gives the corresponding UV-vis spectra taken from aqueous suspensions of Au NPs (dashed line) and Au@Ag NPs (solid line). The as-prepared Au NPs shows an obvious SPR peak at 520 nm, indicating a spherical shape of the Au NPs.^29,30 A new SPR band around 430 nm was observed, which implied the formation of Ag shell.^31,32 However, after Ag shell growth onto Au core, the peak at 520 nm for Au core disappeared, because the intensity of peak for Au core decreased as the thickness of the Ag shell increased. When the thickness of the shells increased to a critical point, the peak from the Au core disappeared, and only the peak for the Ag shells remained.³³

Fig. 2 UV-vis spectra of Au NPs (dashed line) and Au@Ag NPs (solid line) dispersed in aqueous solution.

3.2. Electrochemical behaviors of modified GCE toward the reduction of H₂O₂

In many biological and environmental processes, H₂O₂ is a very important intermediate.^34–36 Thus, many studies have focused on an accurate and sensitive determination of H₂O₂. The electrocatalytic activity of Au NPs and Au@Ag NPs modified GCE toward the reduction of H₂O₂ was studied using the typical cyclic voltammetry (CV). Fig. 3 shows the CV curves of Au NPs/GCE and Au@Ag NPs/GCE in N₂-saturated 0.1 M PBS solution (pH 7.2) in the absence and presence of H₂O₂ at a scan rate of 50 mV s⁻¹. Both Au NPs and Au@Ag NPs modified GCE (curve a, b, respectively) shows almost no response current in the absence of H₂O₂. For the Au NPs/GCE, after the addition of 3.0 mM H₂O₂ solution, a weak reductive current (curve c) was observed. Nevertheless, a remarkable reductive current (curve d) could be observed at −0.44 V with intensity of about 73 μA in the presence of 3.0 mM H₂O₂. Compared with Au NPs/GCE, Au@Ag NPs modified GCE has not only a higher electrocatalytic response, but also a more distinguishable current peak, demonstrating its superior electrocatalytic activity and kinetic toward the reduction of H₂O₂. This remarkable electrocatalytic activity may be attributed to the uniform morphology, well dispersion and high surface-to-volume ratio.

Fig. 3 CV curves of Au NPs/GCE obtained in the absence (a) and presence of 3 mM H₂O₂ (c), and Au@Ag NPs/GCE in the absence (b) and presence of 3 mM H₂O₂ (d).

According to the literature,³⁷ the mechanism of H₂O₂ electroreduction obeys the following illustration:

H₂O₂ + e⁻ ↔OH_(ads) + OH⁻

OH_(ads) + e⁻ ↔ OH⁻

2OH⁻ + 2H⁺ ↔ 2H₂O

Importantly, when Ag is introduced into the reaction, the reaction became more irreversible:

The electrochemical reduction of H₂O₂ becomes more complete after the introduction Ag shell onto Au core, and thus the detection signals would be amplified.

Electrochemical impedance spectroscopy (EIS) is also an effective tool for studying properties of surface modified electrodes. Generally, the semicircle diameter of the curves generally represents the electron transfer resistance (R_ct). By using [Fe(CN)₆]^4−/3− as the electrochemical probe, the Nyquist plots of different electrodes were shown in Fig. 4. The impedance spectra of the bare GCE (Fig. 4a) consisted of a small semicircle, implying that the bare GCE has a low resistance to the redox probe dissolved in electrolyte solution. However, when Au@Ag NPs were introduced onto the bare electrode, the value of R_ct decreased from 660 Ω (curve a) to 400 Ω (curve b), which is ascribed to the good conductivity of Ag NPs. This result suggests Au@Ag NPs could efficiently enhance the electron transfer.

Fig. 4 EIS of (a) bare GCE and (b) Au@Ag NPs/GCE in 5.0 mM [Fe(CN)₆]^4−/3− containing 0.1 M KCl from 1 × 10⁵ to 1 × 10⁻² Hz at amplitude of 5 mV.

From Fig. 5A, one can see that the cathodic peak current of Au@Ag NPs/GCE around −0.44 V dramatically increases with increasing H₂O₂ concentration at the range of 0–6.0 mM. Fig. 5B shows a good linear relationship between the peak current and H₂O₂ concentration from 0 mM to 6.0 mM (R = 0.9953). These results strongly suggest that the Au@Ag NPs modified GCE have a good electrocatalysis for H₂O₂ reduction. The corresponding CV curves of the Au@Ag NPs modified GCE with the increasing of scan rates from 40 mV s⁻¹ to 280 mV s⁻¹ are presented in Fig. 5C. As seen from Fig. 5D, the peak current increased linearly with the square root of the scan rate, which indicates that apparent electron diffusion of Au@Ag NPs/GCE can be controlled.

Fig. 5 (A) CV curves of Au@Ag NPs/GCE obtained in N₂-saturated 0.1 M PBS (pH 7.2) in the absence and presence of H₂O₂ with different concentration (from a to g: 0, 1, 2, 3, 4, 5 and 6 mM) at a scan rate of 50 mV s⁻¹. (B) Linear fitting program of the reduction peak current versus the H₂O₂ concentration. (C) CV curves of Au@Ag NPs/GCE in N₂-saturated 0.1 M PBS (pH 7.2) containing 3 mM H₂O₂ at different scan rates (from j to p: 40, 80, 120, 160, 200, 240 and 280 mV s⁻¹). (D) Linear fitting program of the reduction peak current versus the square root of scan rate.

The detection sensitivity on Au@Ag NPs modified GCE was further explored through amperometric study. Fig. 6A shows the stable amperometric current–time curve of the Au@Ag NPs/GCE with successive addition of varying concentrations of H₂O₂ in N₂-saturated 0.1 M PBS solution (pH = 7.2) under magnetic stirring. It can be observed that the Au@Ag NPs/GCE exhibits a quick response to the change of H₂O₂ concentration and can reach the steady-state current within 2 s. Although biggest reduction current appears at −0.44 V, −0.1 V is selected as the optimal determination potential mostly to ensure lower background, good signal-to-noise ratio and less interference of other electroactive species in solution.^38,39 A linear chronoamperometric response to H₂O₂ is showed in the concentration range from 5.0 μM to 15 mM (Fig. 6B) and the detection limit is estimated to be 1.3 μM (S/N = 3).

Fig. 6 (A) Amperometric response of Au@Ag NPs/GCE obtained upon the successive addition of H₂O₂ into N₂-saturated 0.1 M PBS (pH 7.2) with stirring, applied potential: −0.1 V. (B) Calibration curve of the amperometric determination of the concentration of H₂O₂.

The selectivity and anti-interference advantages of the Au@Ag NPs/GCE was studied. The potential influence of some other electroactive compounds of 0.05 mM ascorbic acid (AA), uric acid (UA) and dopamine (DA), on the detection of H₂O₂ was carried out at the potential of −0.1 V in N₂-saturated 0.1 M PBS (pH 7.2). As illustrated in Fig. 7, the relevant level of AA, UA and DA has no interference for H₂O₂ detection, but the addition of 0.05 mM H₂O₂ shows an obvious current. These results suggest that Au@Ag NPs/GCE has a selectivity towards H₂O₂ and a good ability of anti-interference to electroactive species.

Fig. 7 Amperometric response of Au@Ag NPs/GCE obtained to successive addition of H₂O₂, AA, UA and DA (0.05 mM, respectively) in N₂-saturated 0.1 M PBS (pH 7.2) at −0.1 V.

Long-term stability and repeatability is quite essential for nanomaterials modified electrode to fabricate a sensor. The reproducibility of the sensor was evaluated by 50 cycles of scanning in succession of 0.5 mM H₂O₂. Five independently Au@Ag NPs modified electrodes shows a good reproducibility with a deviation less than 4.5% (RSD) under the same conditions, suggesting that the Au@Ag NPs/GCE is high repeatability. Compared to the RSD of Au NPs/SGS/GCE (4.9%),⁴⁰ Ag NPs/MnOOH/GCE (5%),⁴² and Au–Ag nanotubes/GCE (8.4%),⁴⁶ the sensor designed here has better reproducibility. When the as-prepared Au@Ag NPs/GCE was stored at room temperature, it remained 85% of its initial current response over a month. Therefore, the Au@Ag NPs/GCE possess acceptable reproducibility and good stability.

Compared with several typical nonenzymatic H₂O₂ sensors (Table 1),^40–48 the Au@Ag NPs/GCE displays an excellent comprehensive performance, especially the high sensitivity (251.9 μA mM⁻¹ cm⁻²), a wide linear range from 5 μM to 15 mM and a low applied potential (−0.1 V). It is well-known that noble metallic nanoparticles such as Ag NPs, exhibiting excellent electrocatalytic properties, can easily aggregate in the chemical reduction of Ag⁺ to Ag NPs in aqueous medium, which results in an obvious decrease in electrochemical catalytic activity. In this work, Au@Ag NPs possess nearly uniform size without any aggregation (Fig. 1B, TEM patterns) for the following two reasons. Firstly, the aggregation caused by the fast self-nucleation of Ag NPs can be avoided by the seed-mediated growth procedure using spherical Au NPs as seeds instead of Ag seeds.^49–51 Herein, uniform spherical Au NPs with an average size of 18 ± 2 nm were employed as seeds during the preparation of Au@Ag NPs. Secondly, preparation of metal-based nanostructured materials with specific morphology and controlled size can be effectively controlled by a gas/liquid interface, because the gas/liquid interfacial reaction can be easily controlled by the reaction temperature, the gas pressures, velocity of gas flow and throughput. Consequently, a gas/liquid interfacial reaction has also been adopted in this work. Au@Ag NPs prepared via the combination of these two methods possess a high dispersion, nearly uniform morphology without aggregation, which helps to increase the surface-to-volume ratio area and the active site area that is beneficial for electrochemical catalysis. Further more, the sensitivity of electrocatalytic sensing can be significantly enhanced by the so-called “microelectrode arrays” that composed of Ag NPs separated from their nearest neighbours rather than aggregated together.⁵²

Table 1 Comparison of the performance of nonenzymatic H₂O₂ sensors

Nonenzymatic H2O2 sensors	Detective potential (V)	Linear range (mM)	Sensitivity (μA mM^−1 cm^−2)	Detection limit (μM)	Literature
Au nanoparticles/SGS/GCE	−0.2 (vs. SCE)	0.02–16	3.21	0.25	40
Ag nanoparticles/HNTs-MnO2/GCE	−0.2 (vs. SCE)	0.002–4.71	11.9	0.7	41
Ag nanoparticles/MnOOH/GCE	−0.3 (vs. SCE)	0.005–12.8	32.57	1.5	42
Ag nanowires/GCE	−0.3 (vs. AgCl)	0.02–3.62	—	2.3	43
Ag nanocrystal/ERGO/GCE	−0.5 (vs. AgCl)	0.02–10	183.5	3.0	44
Au–Ag nanoparticles/rGO/GCE	−0.4 (vs. AgCl)	0.1–5.0	—	1.0	45
Au–Ag nanotubes/GCE	−0.2 (vs. SCE)	0.008–1.3	—	3.18	46
Ag@C@Ag/GCE	−0.4 (vs. AgCl)	0.07–10	—	23	47
Au@Ag nanorods/GCE	−0.3 (vs. AgCl)	0.02–7.02	—	0.67	48
Au@Ag nanoparticles/GCE	−0.1 (vs. SCE)	0.005–15	251.9	1.3	This work

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Briefly, all of these benefits mentioned above contribute to enhanced electrochemical response of Au@Ag NPs modified electrode, indicating the gas/liquid interface reaction by the seed-mediated growth procedure is a reliable way to prepare Au@Ag NPs for highly sensitive detection of H₂O₂.

4. Conclusions

In summary, we have successfully prepared Au@Ag NPs by a seed-mediated growth procedure at a gas/liquid interface and thus a non-enzymatic H₂O₂ sensor is fabricated. Compared with other reports, the Au@Ag NPs/GCE shows remarkable electrocatalytic performance toward the reduction of H₂O₂, such as a high sensitivity and a low applied potential. This is because of the aggregation from Au@Ag NPs constituted the sensing interface can be avoided through the seed-mediated growth procedure and the gas/liquid interfacial reaction, which helps to increase the specific surface area and the catalytic site area of Au@Ag NPs modified electrode. Therefore, the Au@Ag NPs might be used as an advanced electrochemical sensing platform to fabricate various sensors.

Acknowledgements

The authors gratefully acknowledge the financial support of this project by the National Science Foundation of China (21575113 and 21275116), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20126101120023), the Natural Science Foundation of Shaanxi Province in China (2013JM2006, 2013KJXX-25 and 2012JM2013), the Scientific Research Foundation of Shaanxi Provincial Key Laboratory (2010JS088, 11JS080, 12JS087, 13JS097, 13JS098) and the Fund of Shaanxi Province Educational Committee of China (12JK0576).

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