The Facile Synthesis of Branch-Trunk Ag Hierarchical Nanostructures and Their Applications for High-Performance H₂O₂ Electrochemical Sensors

Abstract

A novel branch-trunk Ag hierarchical nanostructure was synthesized via a galvanic replacement reaction combined with microwave-assisted synthesis using Te nanowire as a sacrificial template. The Te nanowire was synthesized via a hydrothermal process. We further investigated the potential application of the obtained hierarchical nanostructures in electrochemical sensor analysis. The results showed that the as-prepared sensor exhibited a wide linear range with 0.05 µM to 1.925 mM (R = 0.998) and the detection limit was estimated to be 0.013 µM (S/N = 3). These results indicate the branch-truck Ag hierarchical nanostructures are an excellent candidate material for sensing applications.

1. Introduction

Recently, three dimensional (3D) hierarchical nanostructure have provoked considerable interest due to their rich architectures, distinct properties and various novel applications [1,2,3,4]. Controlled synthesis of hierarchical nanostructure is highly desirable, owing to their structure merits. Among the myriad of possible nanostructures, branch-truck nanostructures have been studied in recent years because a great deal of branch nanostructure on a nanowire backbone could provide an even higher surface-to-volume ratio compared to single nanowires. For example, Her et al. proposed a simple two-step thermal vapor transport process to prepare brush-like p-Te/SnO₂ hierarchical nanostructures, which were used to detect NH₃ and exhibited excellent catalytic behavior [5]. Khoang et al. designed a branch-truck hierarchical SnO₂/ZnO nanostructure by combining a thermal evaporation method and a hydrothermal approach. These SnO₂/ZnO hierarchical nanostructures were systematically investigated for ethanol-sensing. These results revealed that the electrochemical sensors based on hierarchical nanostructures showed high sensibility and selectivity [6].

H₂O₂ is widely used in fields such as organic synthesis [7], environmental detection [8], biology [9], clinical [10], pharmaceutical [11], food production [12], sterilization [13] and so on [14,15]. In particular, it plays a role of considerable importance in the metabolic processes of organisms [16]. Therefore, it is critically important to develop an accurate, fast and reliable method for the detection of H₂O₂. Compared with other analytical methods for detecting of H₂O₂ [17,18,19,20,21], electrochemical sensors have become one of the hottest topics due to their higher sensitivity, simpler operation and lower cost [22,23,24] However, direct reduction of H₂O₂ at bare electrodes show poor sensitivity, low selectivity and poor reproducibility. Because of the high electrochemically active area as well as excellent electronic transfer capability, metal nanomaterials have been tried out as sensing materials to modify the electrode surface for developing H₂O₂ sensors with high sensitivity and good selectivity.

As a typical noble nanomaterial, Ag nanomaterial exhibits excellent conductivity as well as high catalytic properties. This was because nanoscale Ag show a rapid specific surface area increase and more atoms exposed on the surface make for increased active sites. In the catalytic decomposition of H₂O₂, Ag nanomaterials could accelerate the electron transfer rate, reduce the reaction energy and amplify the detection signal. For example, we have reported high sensitivity H₂O₂ sensors based on a perpendicular Ag nanowires array [25]. The catalytic properties of nanomaterials are closely related to their size, morphology and the types of composites. Therefore, it was important to prepare novel Ag nanostructures with superior function. Ag nanoparticles [26], nanowires [27], nanoplates [28], hollow hierarchical microspheres [29,30] have all been reported as sensing materials for developing H₂O₂ electrochemical sensors. However, to the best of our knowledge, up to now, no branch-truck Ag nanostructures have been reported for this purpose. Branch-trunk Ag hierarchical nanostructures provide a very high electrochemically active area and thereby lead to high detection sensitivity. They could also act as enhanced elements for effectively accelerating the electron transfer between the electrode and probe molecules, which would lead to a rapid current response and in the meantime reduce the overpotential of electrochemical reactions.

In this paper, we report the synthesis of branched-truck hierarchical nanostructures containing Ag nanowire trunks and Ag nanorod branches by a simple two-step synthesis pathway. High-quality single crystalline Te nanowire backbones were synthesized using a hydrothermal approach, whereas Ag nanorod branches were subsequently grown perpendicularly to the axis of Te nanowires via microwave-assisted synthesis (EG as solvent and reducing agent, PVP as polymer surfactant and CuCl₂ as control agent). The as-prepared branched-truck hierarchical nanostructure-modified electrode was used as a novel high-performance H₂O₂ sensor. The experimental data illustrated that the branch-truck Ag hierarchical nanostructure-modified electrodes demonstrated excellent catalytic performance for H₂O₂ reduction, while detection with a wide linear range, good selectivity and long-term stability was achieved.

2. Experimental

2.1. Materials

K₃Fe(CN)₆, K₄Fe(CN)₆, ammonium hydroxide, acetone, hydrazine (50%), KH₂PO₄, K₂HPO₄ were purchased from Tianjin Damao Chemical Factory (Tianjin, China). PVP [poly-(N-vinyl-2-pyrrolidone)] was obtained from Sigma-Aldrich (Darmstadt, Germany), AgNO₃, Na₂TeO₃ and H₂O₂ was obtained from Xiya Reagent (Chengdu, China). All the chemicals were of analytical grade. Distilled water was used in the experiments.

2.2. Apparatus

The electrochemical experiments were performed using a CHI660D electrochemical workstation (Chenhua, Shanghai, China). A standard three-electrode system (Ag branch-trunk hierarchical nanostructures modified electrode as work electrodes, Pt as the counter electrode and Ag/AgCl (saturated KCl) as reference electrode) was used in all the experiments. Scanning electron microscopy (SEM) analyses were performed using a 6701F instrument (JEOL, Tokyo, Japan). X-ray diffraction (XRD) analysis was carried out on an Ultima IV X-ray diffractometer (Rigaku, Tokyo, Japan). A microwave oven with 10–500 W power was used.

2.3. Synthesis of Branch-Truck Ag Hierarchical Nanostructures

Te nanowires were synthesized according to the reference [31]. Briefly, 1.0 g PVP and 0.0922 g of sodium tellurite were dissolved in 35 mL of double-distilled water to form a homogeneous solution under vigorous magnetic stirring at room temperature. Then 1.65 mL of hydrazine hydrate and 3.35 mL of aqueous ammonia solution were added into the previous solution. The final solution was transferred into a Teflon-lined stainless steel autoclave (50 mL in total volume), which was closed and maintained at 180 °C for 4 h, and then the sample was allowed to cool to room temperature. 110 mL of acetone was added into the final solution to precipitate the product, which was then centrifuged and washed with double-distilled water and absolute ethanol several times, respectively. Synthesis of branch-truck Ag hierarchical nanostructure: in brief, 15 mL of 1 mM CuCl₂/EG was prepared firstly. Then, 1.27 mg of Te nanowires and 1 g PVP was added into the above solution, followed by the addition of 5 mL of 20 mM AgNO₃/EG solution. The resulting solution was then transferred to a microwave for heating reaction at (temperature). After the reaction, the solution was cooled to room temperature. The final product was obtained, in order to remove the remaining CuCl₂, EG and PVP, after the solution was centrifuged and washed in acetone, alcohol and deionized water at a speed of 9000 r/min for 5 min each, respectively.

2.4. Fabrication of Branch-Truck Ag Hierarchical Nanosturture Sensor

First, bare glass electrode (GCE) was polished on suede with 1.0 and 0.3 μm Y-Al₂O₃ powder, respectively. The electrodes were washed with ethanol and double distilled water for 20 min, and then the performance of bare glass electrodes were detected in K₃Fe(CN)₆/K₄Fe(CN)₆. 1 mg branch-truck Ag hierarchical nanostructures were dispersed in 1 mL Nafion solution (1%) for 10 min. The suspension solution was dispersed onto a GCE surface with a microsyringe and dried at room temperature.

3. Results and Discussion

3.1. Characterization

Te nanowires were used as substrate and templates to prepare the branch-truck Ag hierarchical nanostructures. Figure S1 shows the SEM image of the as-prepared Te nanowires with a diameter of several hundred nanometers. XRD patterns of the Te nanowires were showed in Figure S2. All peaks can be indexed with the standard JCPDS card no. 36–1452 which confirms the hexagonal phase of tellurium nanowires. The peaks observed at 2θ values of 23.08°, 27.72°, 38.36°, 40.52°, 43.44°, 45.98°, 46.92°, 49.63°, 56.82°, 62.78°, 63.93°, 65.72°, 67.64°, 71.92°, and 75.24°are assigned to the diffractions from (100), (101), (102), (110), (111), (003), (200), (201), (202), (113), (210), (211), (104), (212) and (304) planes, respectively. Figure S3 show cyclic voltammograms of Te nanowires modified electrode in PBS solution with scan rate of 50 mVs⁻¹. A pair of sharp redox peaks at +0.264 V and −0.74 V are due to the self-redox of Te nanowires [32].

3.2. The Influence of Synthesis Condition:

When Te nanowires were used as precursor and template, the hierarchical Ag nanostructure was obtained via microwave-assisted synthesis by adjusting the synthetic parameters. In the presence of PVP and CuCl_2, the microwave power was set at 300 W and the reaction time was set at 30 s, Te-Ag core shell nanowire backbones were found to be fully covered by Ag thin layers with abundant flake-like Ag nanostructures (Figure 1A,B).

When the reaction time was prolonged to 60 s, complete nanorods with about 50–80 nm in width and 100–300 nm in length grew perpendicularly on the surface of the nanowires (Figure 1C,D). We further increased the reaction time to 90 s, and the branched Ag nanostructure was still growing in size. Ag nanorods eventually completely covered the nanowire backbones to form branch-stem Ag hierarchical nanostructures with an average diameter of ca. 100 nm and lengths of several hundreds of nanometers (Figure 1E,F). When the microwave power was set at 500 W and the microwave time was set at 90 s, vertical nanoplates will spring out of the Ag nanowire backbones to form Ag nanoplates on the surface of Te nanowire structures (Figure S4A,B). Vertical nanoplates can be generated either sparsely or densely by increasing the number of platelets at higher growth temperatures and lower pressures [33]. In the absence of PVP and CuCl₂, when the microwave power was set at 300 W and the microwave time was set at 90 s, Ag nanoparticles with irregular sizes and shapes were found to be generated on the surface of Te nanowires whereas no nanorods were observed on the Ag nanowires backbone (Figure S5A,B). This is because PVP, as a surfactant, will direct the growth of Ag to the nanorod structure form [34,35,36].

The XRD patterns of as-synthesized branch-stem Ag hierarchical nanostructures with various growth times are shown in Figure 2. The XRD patterns of 60 s branch-stem Ag hierarchical nanostructures is the mixture of diffraction peak of silver and tellurium. For comparison, the peaks of 90 s branch-stem Ag hierarchical nanostructures occurring at 38.14°, 44.32°, 64.43°, 77.56°, and 81.54° were attributed to diffractions from the (111), (200), (220), (311), and (222) crystal planes, which were consistent well with a face-centered cube of Ag. No diffraction peaks corresponding to Te were found, which suggested that the Te nanowire backbone have been completely replaced with Ag. The schematic illustration of the growth mechanism of Ag branches on the Ag nanowire backbones by microwave-assisted synthesis was shown in Figure 3.

An enormous amount of silver nanoparticles was synthesized via microwave-assisted synthesis. Ag cations replace Te in Te nanowires to form Ag nanoparticles at Te nanowires surface via a galvanic displacement reaction. The reaction between Te nanowires and Ag⁺ can be formulated as follows:

Ag⁺ + Te + 3H₂O = Ag + TeO₃²⁻ + 6H⁺

(1)

As the galvanic replacement reaction proceeded, the Ag nanoparticles would grow along Ag crystal seeds on the surface of the Ag nanowire to form nanospikes and nanoflakes. As the reaction continued, the Ag nanospikes and nanoflakes increased in size gradually and formed nanorods. Finally, high-quality Ag nanorods were produced and attached to the Ag nanowires.

Figure 4 depicts cyclic voltammograms (CVs) of bare GCE (a) and branch-truck Ag hierarchical nanostructures/GCE (b) in 0.1 M PBS (pH = 7.4) in the presence of 0.5 mM H₂O₂ with scan rate of 50 mVs⁻¹. A strong peak at about −0.48 V, attributed to the reduction of H₂O₂, was seen on branch-truck Ag hierarchical nanostructures modified GCE whereas a very small reduction peak was obtained at −0.61 V for H₂O₂ at bare GCE. The insert of Figure 4 shows the cyclic voltammogram of bare GCE in 0.1 M PBS (pH = 7.4) in the presence of 0.5 mM H₂O₂ with scan rate of 50 mVs⁻¹. Compared with bare GCE, a positively shifted peak potential of 110 mV was observed on the branch-truck Ag hierarchical nanostructures modified/GCE. There was an increase in current by increasing H₂O₂ concentration. When H₂O₂ concentration was increased from 2 mM to 8 mM, an increase reduction current was observed (Figure 5). Thus, branch-truck Ag hierarchical nanostructures could greatly reduce the overpotential and increase the catalytic current toward the reduction of H₂O₂, which was important for improving the high sensitive and selective detection for H₂O₂.

Direct electrochemical reduction of H₂O₂ at ordinary solid electrodes shows low electrode kinetics and high overpotential. Based on the available literature, the mechanism of H₂O₂ electroreduction can be described as follows [37]:

OH (aq) + e⁻ ⇋ OH⁻

(2)

H₂O₂ + e⁻ ⇋ OH (aq) + OH⁻

(3)

2OH⁻ + 2H⁺ ⇋ 2 H₂O

(4)

In the case of silver-catalyzed decomposition of H₂O₂, the reaction turned more irreversible [37]:

$${\mathrm{H}}_2{\mathrm{O}}_2 \stackrel{\mathrm{A}\mathrm{g}}{\to }1/2 {\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$

(5)

$${\mathrm{O}}_2+2{\mathrm{e}}^{-}+2{\mathrm{H}}^{+} \to {\mathrm{H}}_2{\mathrm{O}}_2$$

(6)

The O₂ produced in the catalytic reaction would induce detection signal on the electrode. According to some theories in the literature, the mechanism of the O₂ reduction on the electrode as described below [38,39]:

O₂ + e⁻ ⇋ [O^·−] _(ads)

(7)

[O^·−] _(ads) + H₂O ⇋ HO₂^·_(ads) +OH⁻

(8)

Then:

HO₂·_(ads) +O₂^·− ⇋ HO₂⁻ _(ads) +O₂

(9)

or:

HO₂^·_(ads) +e⁻ ⇋ HO₂⁻ _(ads)

(10)

Figure 6 shows the amperometric response of the branch-truck Ag hierarchical nanostructures/GCE with gradually additions of H₂O₂ to the PBS solution under stirring conditions. As the potential was set at −0.5 V, the branch-truck Ag hierarchical nanostructures/GCE responded rapidly and achieved steady-step current by adding H₂O₂ to the solution. The insert of Figure 6 shows the low concentration range. The corresponding linear calibration curve shown in the inset of Figure 7 reveals there were a good linear relationship between the reduction peak current and the H₂O₂ concentration with the range of 0.05 μM to 1.925 mM with a correlation coefficient of 0.998. The regression equation was I (μA) = −83.18 C − 2.1897 (C is the concentration of H₂O₂ in mM). The detection limit of H₂O₂ sensors was 0.013 μM (S/N = 3).

The sensitivity was calculated to be 325.52 μA mM⁻¹ cm⁻². The as-fabricated H₂O₂ sensors were compared with those of Ag nanomaterials-based non-enzymatic H₂O₂ sensors in

Table 1. Comparison of the fabricated sensor with other Ag material-based non-enzyme sensors for H₂O₂.

Electrode	Sensitivity (μA mM−1 cm−2)	Linear Range (μM)	LOD (μM)	Ref
Ag NPs/rGO/GCE	225	50–5000	10	[40]
Ag Y/CPE	650.7	20–5000	1.4	[41]
Ag NPs/rGO/GCE	---	10–6000	1.8	[42]
Ag-Au@Cu2O NWs/GCE	---	100–9000	1.1	[43]
AgNPs/SBA-16/GCE	816.6	20–8000	2.95	[44]
Ag@SiO2@Ag/GCE	56.07	5–24,000	1.7	[45]
AgNPs/X-CPE	60.6	20–11,760	9.1	[26]
Ag/boehmite NTs/rGO/GCE	80.14	0.5–10,000	0.17	[46]
branch-truck Ag HN/GCE	325.52	0.05–1925	0.013	our report

Where GCE—glassy carbon electrode, CPE—carbon paste electrode, Ag NPs—Ag nanoparticles, rGO—reduced grapheme oxide, Ag Y—Ag nanoparticles arrays, Ag-Au@Cu₂O NWs—Cu₂O nanocubes decorated by Ag-Au alloy nanoparticles, SBA-16—mesoporous SBA-16 nanoparticles, Ag@SiO₂@Ag—Ag@SiO₂@Ag, X—NaX nanozeolite, boehmite NTs—Ag decorated boehmite nanotubes, branch-truck Ag HN—branch-truck Ag hierarchical nanostructures.

. The proposed method was better than the previous reports. To illustrate the feasibility in practical sample analysis of the proposed branch-truck Ag hierarchical nanostructures/GCE the determination of H₂O₂ in milk samples were studied. The real samples were diluted 10 times with water and diluted milk samples were streamed into five groups separately (S. No 1, S. No 2, S. No 3, S. No 4, S. No 5), and then different concentrations of H₂O₂ (0.01, 0.05, 0.10, 0.15, and 0.20 mM) were spiked into five groups (S. No 1, S. No 2, S. No 3, S. No 4, S. No 5) of practical samples. The chronoamperometric responses were recorded of the spiked concentration, measured concentration of H₂O₂, and relative standard deviation (RSD) (three repeated experiments). The results, listed in

Table 2. The practical sample analysis result of hydrogen peroxide for the branch-truck Ag hierarchical nanostructures/GCE in pH 7 PBS (three repeated experiments).

S. No	Added Concentration (mM)	Found Concentration (mM)	Recovery (%)	RSD (%)
1	0.0100	0.01012	101.2	3.1
2	0.0500	0.05005	100.5	1.6
3	0.1000	0.09987	99.87	3.2
4	0.1500	0.15015	100.1	5.6
5	0.2000	0.01996	99.8	1.8

Where S. No 1—diluted milk sample 1, S. No 2—diluted milk sample 2, S. No 3—diluted milk sample 3, S. No 4—diluted milk sample 4, S. No 5—diluted milk sample 5.

, showed good reliability and reproducibility for the branch-truck Ag hierarchical nanostructures/GCE.

In practical samples analysis, some coexisting biomolecules can cause serious interference in the detection of a single component. Possible interfering species for detecting H₂O₂ such as dopamine (DA), ascorbic acid (AA), glucose and uric acid (UA) were investigated by amperometric response (Figure 8) with additions of 100 μL 1 mM H₂O₂, 1 mM H₂O₂, 10 mM AA, 10 mM DA, 10 mM UA and 10 mM glucose and 200 μL 1 mM H₂O₂. The branch-truck Ag hierarchical nanostructures/GCE showed a high sensitivity amperometric response to H₂O₂ whereas there is no obvious amperometric response for the interfering species AA, glucose and UA. Additionally, the interferent DA only produces a weak reduction current at −0.5 V. Therefore, at an applied potential of −0.5 V coexistint interferents do not interfere with the selective determination of H₂O₂.

As important evaluation sensor parameters, the repeatability and stability of the H₂O₂ sensors were investigated. The branch-truck Ag hierarchical nanostructures modified electrodes were used to detect 1 mM H₂O₂ with the applied potential range of −1.0 V to 1.0 V. The catalytic current decreased to 90.5% after 10 scanning cycles, illustrating that the as-prepared sensor had highly reproducibility (Figure 9). Furthermore, the relative standard deviation (RSD) for five branch-truck Ag hierarchical nanostructures modified electrodes to detect the same 0.5 mΜ H₂O₂ solution was 4.1%, further indicating the good reproducibility of the branch-truck Ag hierarchical nanostructures-based H₂O₂ sensor. To study the stability of the sensor, the branch-truck Ag hierarchical nanostructures modified GCE was tested after 21 days, 90% of its initial current response was remained. These results show the sensor has satisfactory performance.

4. Conclusions

Branch-truck Ag hierarchical nanostructures were synthesized via a combination of hydrothermal synthesis with microwave-assisted synthesis. The as-fabricated materials could be used as an excellent charge-transfer substrate for the electrocatalytic reduction of H₂O₂. The prepared electrochemical H₂O₂ sensor showed good catalytic performance with a wide linear range, low detection limit and good selectivity and reliability. Therefore, the sensor has potential application for the selective determination of H₂O₂ in real sample analysis.