Synthesis of Ag@SiO₂ yolk–shell nanoparticles for hydrogen peroxide detection

Abstract

Yolk–shell nanostructures are a potential platform for the application of sensors and detection. In this paper, Ag@SiO₂ yolk–shell nanoparticles (YSNs) were synthesized by a facile “two solvents” impregnation–reduction approach. XRD, SEM, TEM and N₂ adsorption characterization results revealed that the resultant Ag@SiO₂ YSNs possess distinctive structures, such as movable cores, perpendicular mesoporous channels, protective shells and hollow cavities. A nonenzymatic H₂O₂ sensor was constructed using Ag@SiO₂ YSNs as sensing interface. A three-electrode system was used for the measurement. Electrochemical results indicate that the Ag@SiO₂ YSNs modified electrode exhibits outstanding performance toward the H₂O₂ reduction, with a faster amperometric response, a lower detection limit (3.5 μM) and a wider linear range (0.1–15 mM) than that based on Ag@SiO₂ composites, which was synthesized by a direct impregnation method.

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1. Introduction

Noble metal nanoparticles, especially Ag, have recently attracted a great deal of scientific interest, due to their potential applications in imaging, catalysis, electronics, and antibacterial uses.^1–5 However, these nanoparticles are prone to aggregation and sintering, which cause deterioration of their chemical properties and decrease their reactivity.^6–8 Therefore, it is highly desirable to stabilize these small particles on a suitable support to eliminate aggregation and deactivation.

Significant efforts have been made to combine noble metal nanoparticles and porous support for the controlled synthesis of a variety of nanostructures with unique morphologies and properties.^9–15 Among different strategies, encapsulation of noble metal nanoparticles in hollow spheres, hollow silica/carbon sphere for example, to form a yolk–shell nanostructure has received considerable attention, which becomes another remarkable route to stabilize these nanoparticles.^16,17 With the appealing structures of movable cores, interstitial hollow spaces between the movable core and shell sections, and the functional shells, yolk–shell nanoparticles (YSNs) have played critical roles in modern science and technology as promising candidates for emerging applications, such as nanoreactors,^18–20 lithium-ion batteries,^21,22 drug delivery,^23,24 surface-enhanced Raman scattering,^25,26 and sensors.²⁷

Motivated by their promising prospects, many scientists have devoted to develop new synthetic approaches for these structures. Currently, the reported methods for YSNs include bottom-up or soft-templating methods,^28–30 Ostwald ripening or galvanic replacement process,^31,32 the Kirkendall effect based method,^33,34 selective etching^12,35–38 and ship-in-bottle approach.^39–42 On this basic, a series of noble metal nanoparticles (i.e. Au,¹⁵ Ag,^43,44 Pt,⁴⁵ Fe,⁴⁶ Ni⁴⁷) have been introduced into the cavity of hollow spheres for the construction of YSNs. Targeting a yolk–shell nanostructure with Ag nanoparticles as cores, methods are conventionally employed such as hard-templating route,⁴⁸ and a Stöber process.⁴⁹ For example, Kang and co-workers reported the preparation of silver@silica@PMAA core double shell hybrid nanoparticles by distillation–precipitation polymerization with silver@silica core–shell NPs from the sol–gel reaction as hard template.⁴⁸ Then, the Ag@PMAA YSNs were obtained by the removal of the silica shell by HF etching. The as-synthesized materials showed high catalytic activity for the degradation of p-nitrophenol. Although the hard template approach is successfully in producing YSNs, the procedure is a multiple step and complex process. Recently, a one-pot Stöber route was developed for the fabrication of Ag@carbon YSNs.⁴⁹ In this method, AgBr–silica–RF polymer core double shell nanoparticles were prepared with AgNO₃, TEOS, resorcinol, and formaldehyde as precursors. Then, Ag@C yolk–shell nanostructures are obtained by carbonization of RF and selective removal of the silica. The synthetic route reported here is expected to simplify the fabrication process of yolk–shell nanostructures, which usually entails multiple steps and previously synthesized metal nanoparticles and hard templates. However, synthesis of YSNs with Ag as cores by a simple and efficient way is barely reported and highly needed.

Herein, we describe a flexible and efficient route for the synthesis of Ag@SiO₂ YSNs. The obtained Ag@SiO₂ YSNs possess distinctive structures, such as dispersive Ag cores, perpendicular mesoporous channels, protective shells and hollow cavities. A nonenzymatic H₂O₂ sensor was conducted using Ag@SiO₂ YSNs as sensing interface. Electrochemical results indicate that Ag@SiO₂ YSNs modified electrode exhibits outstanding performance toward the H₂O₂ reduction, with a faster amperometric response, a lower detection limit (3.5 μM) and a wider linear range (0.1–15 mM) than that based on Ag@SiO₂ composites, which was synthesized by a direct impregnation method.

2. Experimental

2.1 Chemicals and materials

Anhydrous ethanol, concentrated ammonia aqueous solution (25 wt%), tetraethoxysilane (TEOS), sodium borohydride, methanol, n-hexane and sliver nitrate were purchased from Sinopharm Chemical Reagent Co., Ltd. Cetyltrimethylammonium bromide (CTAB) was purchased from Chengdu Kelong Chemical Co., Ltd. All chemicals were used as received without any further purification. Millipore water was used in all experiments.

2.2 Synthesis of hollow mesoporous silica spheres

Hollow mesoporous silica spheres (HMSS) were synthesized via a spontaneous self-transformation approach by the reported method.⁵⁰ The typical preparation of HMSS was described by the following procedure. 0.168 g of CTAB was added to a solution containing deionized water, anhydrous ethanol and ammonia solution (25 wt%). Then the resulting mixture was heated to 35 °C. 1 mL of TEOS was added under vigorous stirring. The molar ratio of the reaction mixture was 1.00TEOS : 0.092CTAB : 2.96NH₃ : 621H₂O : 79C₂H₅OH. After stirring at 35 °C for 24 h, the white product was collected by centrifugation at 5000 rpm for 10 min and washed three times with ethanol. Then, the as-made Stöber silica spheres were incubated in 160 mL of pure water at 70 °C for 12 h and then collected by centrifugation and washed three times with ethanol. Subsequently, the as-synthesized HMSS were washed three times with ethanol, dried and calcined at 550 °C in air. The obtained sample was donated as blank HMSS.

2.3 Synthesis of Ag@SiO₂ YSNs

The Ag@SiO₂ YSNs were prepared through a sequential “two solvents” impregnation–reduction approach. In a typical experiment, 1 g HMSS were added into 30 mL n-hexane with stirring until the HMSS were well dispersed. Then 60 μL of AgNO₃ aqueous solution (1.5 M) were added dropwise with stirring for 1 h. The mass ratio of Ag/SiO₂ was 0.1. Next, NaBH₄ aqueous solution was added under N₂ atmosphere. The molar ratio of B/Ag was 4/1 with adequate NaBH₄ for the growth of sliver nanoparticles. The obtained Ag@SiO₂ YSNs were collected by a magnet and washed with adequate methanol. The final products were dried under high vacuum, denoted as Ag@SiO₂-TS.

2.4 Synthesis of Ag@SiO₂ composites

For comparison, the Ag@SiO₂ composites were prepared by direct impregnation method. Briefly, 1 g HMSS was added into 5 mL AgNO₃ aqueous solution with stirring for 12 h. The mass ratio of Ag/SiO₂ was 0.1. Then the suspension was dried under vacuum at 45 °C, following with 10 mL NaBH₄ aqueous solution added with stirring. The molar ratio of B/Ag was 4/1 with adequate NaBH₄. The following producer was the same as above. The resulted products were denoted as Ag@SiO₂-DI.

2.5 Characterization

The X-ray diffraction (XRD) pattern were recorded on a Bruker AXS D8 advance powder diffraction system using Cu Kα (λ = 1.5418 Å) radiation. The XPS spectra were obtained by using a PHI QuanteraIIESCA System with Al Kα radiation at 1486.8 V. Scanning electron microscopy (SEM) images were collected on a JEOL JSM-6380LV microscope using gold-coated powder specimens. Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2100 microscope, operating at 200 kV. N₂ adsorption/desorption isotherms were measured using Micromeritics ASAP-2020 at liquid nitrogen temperature (−196 °C).

2.6 Electrochemical measurements

Electrochemical measurements were performed on a CHI 660E electrochemical workstation (Chenhua, Shanghai, China). A three-electrode system was used, including a glass carbon electrode (GCE, diameter 3 mm) as the working electrode, a platinum wire as the auxiliary electrode, and a Ag/AgCl (saturated KCl) electrode as the reference electrode. Ag@SiO₂-TS/GCE and Ag@SiO₂-DI/GCE were fabricated by casting 3 μL of concentrated Ag@SiO₂-TS and Ag@SiO₂-DI aqueous dispersions on polished GCE and dried in air. All of the electrochemical experiments were carried out in 0.1 M phosphate buffer solution (PBS, pH 6.8) under a high purity nitrogen atmosphere.

3. Results and discussion

Fig. 1 shows the low-angle and wide-angle XRD patterns of all samples. The low-angle XRD patterns of the different samples all show three diffraction peaks, which can be indexed to 100, 110, and 200 reflections of hexagonal symmetry with the space group p6mm (Fig. 1A). Therefore, the introduction of Ag has no apparent effect on the hexagonal structure of HMSS. From the wide-angle XRD patterns shown in Fig. 1B, it can be seen that except the blank HMSS, four obvious diffraction peaks at 38.1, 44.3, 64.4 and 77.3° can be observed in other two samples, which are indexed as (111), (200), (220), and (311) reflections, confirming the presence of silver. Meanwhile, the (111) diffraction peak of Ag is apparently weaker and broader in Ag@SiO₂-TS than that of Ag@SiO₂-DI, indicating the small size of sliver particles in Ag@SiO₂-TS. It can be contributed to the protective shells of Ag@SiO₂ YSNs, which prohibit the aggregation of sliver nanoparticles.

Fig. 1 (A) Small-angle and (B) wide-angle XRD patterns of (a) blank HMSS (b) Ag@SiO₂-TS (c) Ag@SiO₂-DI.

The surface Ag composition of the as-prepared samples is also investigated by XPS (Fig. 2). The photoelectron peaks at 367.2 eV and 373.2 eV represent the binding energies of Ag 3d_5/2 and 3d_3/2, respectively. Noticeably, intensity of Ag 3d in Ag@SiO₂-TS was much weaker than that of Ag@SiO₂-DI, which can be contributed to the isolation of silica shells and the limited probe depth of XPS,⁵¹ further confirming that silver nanoparticles in Ag@SiO₂-TS sample were mainly inside the cavity of HMSS.

Fig. 2 XPS spectra of Ag 3d region (a) Ag@SiO₂-TS (b) Ag@SiO₂-DI.

The morphology and structure of the three samples were investigated by SEM and TEM. As can be seen in Fig. 3a, the blank HMSS have a smooth surface and uniform diameter of about 400 nm. After the introduction of silver, the Ag@SiO₂ YSNs, fabricated by the sequential “two solvents” impregnation–reduction approach, retain the morphology and size of HMSS, with barely Ag particles on the outer shells (Fig. 3b). However, for the sample of Ag@SiO₂-DI (Fig. 3c), many silver particles can been observed, sticking on the outer shell of HMSS or even aggregating out the spheres. Therefore, we have reasons to believe that almost all of the Ag was introduced into the shell of HMSS by sequential “two solvents” impregnation–reduction approach. TEM was performed to further investigate the mesostructure and interior construction of the as-synthesized materials. A noticeable contrast between the cavity and the shell is observed in Fig. 3d–f, which verify the hollow structure of HMSS. The cavity of HMSS provides the void for the introduction of Ag core and also provides the zone for the catalytic reaction. The HMSS have a shell thickness of 80 nm and an average diameter of about 400 nm (Fig. 3d), consistent with the results of SEM. The high-magnification TEM image (Fig. S1†) reveals that the shells of HMSS display uniform and ordered mesoporous channels, which are radially oriented to the sphere surface. This result means that the mesochannels of the hollow spheres are readily accessible, which make the molecules easily penetrate the shell and reach the Ag core. Fig. 3e displays the TEM image of Ag@SiO₂-TS. Obviously, Ag cores, which are not located in center of the cavity, can be observed. This phenomenal deduces that the Ag cores are movable in the hollow space if it was filled with liquid,^52,53 which could provide more active sites for catalytic reactions. Meanwhile, the spherical shape and mesostructure (inset in Fig. 3e) are kept during the impregnation and reduction process, and the yield of the product is very high (Fig. S2†). For comparison, in the TEM image of Ag@SiO₂-DI (Fig. 3f), part of silver particles adhere to the outside surface of the shells or even appear aggregation out of the HMSS. The results obtained from TEM were consistent with the SEM and XRD characterization.

Fig. 3 SEM and TEM images of (a and d) blank HMSS (b and e) Ag@SiO₂-TS (c and f) Ag@SiO₂-DI.

Fig. 4A shows the nitrogen adsorption/desorption isotherms of the as-synthesized materials and Table 1 (Table 1) lists their textural parameters for comparison. All the samples exhibit a type IV isotherm with a type H2 hysteresis loop, characteristic of mesoporous structure. According to the Brunauer–Emmett–Teller (BET) method, the specific surface areas of blank HMSS and Ag@SiO₂-TS are 845.6 m² g⁻¹ and 769.4 m² g⁻¹, respectively. Therefore, after the incorporation of Ag cores, Ag@SiO₂-TS still kept a high specific surface area. However, the BET surface area of Ag@SiO₂-DI decrease more seriously than Ag@SiO₂-TS, which is caused by the pores blocking of the Ag particles, sticking on the outer shells of HMSS. The pore size distribution derived from the adsorption branch for all samples are shown in Fig. 4B. The as-synthesized samples show a primary mesoporous size centered at 2.5 nm. Noticeably, after the sequential “two solvents” impregnation–reduction approach and direct impregnation processes, the Ag/SiO₂ composites keep the same pore size and the mesoporous structure. Based on the morphology and structure characterizes, it can be confirmed that an Ag@SiO₂ yolk–shell nanostructure with movable cores, perpendicular mesochannels, protective shells and hollow cavities has been successfully prepared via the sequential “two solvents” impregnation–reduction approach.

Fig. 4 N₂ adsorption/desorption isotherms (A) and pore size distributions (B) of (a) blank HMSS (b) Ag@SiO₂-TS (c) Ag@SiO₂-DI.

Table 1 Textural parameters of as-synthesized samples

Samples	SBET/m^2 g^−1	Pore size/nm	Pore volume/cm^3 g^−1
HMSS	845	2.7	0.57
Ag@SiO2-TS	769	2.7	0.54
Ag@SiO2-DI	724	2.7	0.50

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Several reports have shown that YSNs with metal cores in the hollow shells have high catalytic activity in different heterogeneous catalystic reactions.^15–17,19 Meanwhile, the nanomaterials as nonenzymatic H₂O₂ sensors actually play the role of heterogeneous catalysts in the detection system.²⁷ Therefore, there is reason to believe that YSNs can be a promising candidate for nonenzymatic H₂O₂ sensors.

To demonstrate the catalytic performance of Ag/SiO₂-TS for H₂O₂ reduction, we designed an enzymeless H₂O₂ sensor by immobilization of the Ag@SiO₂-TS on a bare GCE surface (Ag@SiO₂-TS/GCE). Bare GCE and Ag@SiO₂-DI/GCE were also constructed for comparison. Fig. 5 shows the cyclic voltammograms (CVs) of bare GCE, Ag@SiO₂-TS/GCE and Ag@SiO₂-DI/GCE in the presence of H₂O₂ (1 mM) in PBS at pH 6.8. No obvious response is observed for bare GCE. Meanwhile, the response toward H₂O₂ for Ag@SiO₂-DI/GCE is remarkable (about 12.9 μA in intensity at −0.50 V), due to the existence of Ag. In contrast, the Ag@SiO₂-TS/GCE exhibits a more remarkable reduction current peak, about 17.1 μA in intensity at −0.50 V. This improved catalysis could be contributed to the following factors. First, the mesoporous shell supplies many exposed hot spots to integrate with target molecules, which is critical to highly sensing performance because the heterogeneous reactions take place on the surface of solid phase.^54,55 Meanwhile, the thin, mesoporous silica shell with a thickness of about 80 nm does not provide much resistance, and the rest porous system is open to the large hollow cavity. Then, the H₂O₂ molecules can easily penetrate through the mesoporous shells and reach the Ag cores.⁵⁶ Second, the movable cores inside the void space provide more exposed active sites for the reaction between Ag nanoparticles and H₂O₂ molecules. Third, the catalytic reaction is confined within the void space, in which chemical reaction possesses may present vast differences because of the confining effect and change in microenvironments.⁵⁷ The void space acts as a nanoreactor, which enriches the target molecules in the cavity and ensures the target molecules to be reacted as completely as possible. Moreover, it is found that the reduction peak current of Ag@SiO₂-TS/GCE is increased with the gradual addition of H₂O₂ (Fig. 6), indicating the catalytic property of the modified electrode in the reduction of H₂O₂.

Fig. 5 Cyclic voltammetries (CVs) of bare GCE, Ag@SiO₂-TS/GCE and Ag@SiO₂-DI/GCE in N₂-saturated 0.2 M PBS at pH 6.8 in the presence of 1.0 mM H₂O₂.

Fig. 6 CVs of Ag@SiO₂-TS/GCE in N₂-saturated 0.2 M PBS at pH 6.8 in the presence of different concentration of H₂O₂. Potential scan rate: 50 mV s⁻¹.

Fig. 7 shows a typical current–time plot of the Ag@SiO₂-TS/GCE in N₂-saturated 0.2 M PBS buffer (pH: 6.8) on consecutive step change of H₂O₂ concentrations under optimized condition. When an aliquot of H₂O₂ was dropped into the stirring PBS solution, the reduction current rose steeply to reach a stable value. The Ag@SiO₂-TS/GCE responded rapidly (achieve 95% of the steady state current within 5 s), indicating a fast amperometric response behavior. The inset (a) in Fig. 7 shows the calibration curve of the sensor, and the low concentration part of this line is shown in inset (b). The linear detection range and the detection limit were estimated to be from 0.1 mM to 15 mM (r = 0.999) and 3.5 μM at a signal-to-noise ratio of 3, indicating a promising property of the sensor. Several research groups have reported on the synthesis of Ag-based composites with other structures for enzyme-free H₂O₂ detection.^58–60 By comparison, the detection performance of Ag@SiO₂ YSNs was better than other Ag nanoparticles decorated structures, including Ag@TiO₂ composites,⁵⁸ Ag@SBA-15 (ref. 59) and Ag@graphene.⁶⁰ The superiority of Ag@SiO₂ YSNs may be contributed to the appealing properties of yolk–shell nanostructures in application of nanoreactors.

Fig. 7 Typical steady-state response of the Ag@SiO₂-TS/GCE to successive injection of H₂O₂ into the stirred N₂-saturated 0.2 M PBS at pH 6.8 (applied potential: −0.5 V). Inset: the calibration curve.

4. Conclusions

In conclusion, Ag@SiO₂ YSNs have been firstly prepared by a “two solvents” impregnation way. The as-synthesized Ag@SiO₂ YSNs possess distinctive structures, such as movable cores, perpendicular mesoporous channels, protective shells and hollow cavities. Furthermore, the Ag@SiO₂ YSNs were performed to fabricate a novel nonenzymatic H₂O₂ sensor in a three-electrode system. The electrochemical results show that the Ag@SiO₂ YSNs modified electrode performs enhanced catalysis for H₂O₂ detection than that based on Ag@SiO₂ composites, which was synthesized by a direct impregnation method. Moreover, the Ag@SiO₂-TS/GCE exhibits outstanding performance toward the H₂O₂ reduction with a fast amperometric response, a low detection limit and a wide linear range, which can be attributed to the appealing structures. Therefore, the Ag@SiO₂ YSNs provide a promising platform for the research of YSNs based materials in electrocatalysis, particularly, in the nonenzymatic amperometric sensor.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant no. 51478224). We acknowledge gratefully Professor Dan Shan and Mr Guangyao Zhang for useful discussion and suggestion.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra16061d

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