Introduction

Nitrite (NO2 −) has undesirable effects on both environmental protection and human health [1]. As a common food additive and preservative, nitrite is widely applied in industry and can be found in food and the environment [2]. It is also present in water by chlorination reactions inducing the formation of nitrite in water system [3]. According to the World Health Organization, the maximum limit of nitrite in drinking water is 3 mg L−1 [4]. Nitrite contamination in drinking water would cause different diseases, including “Blue Baby Syndrome”, methemoglobinemia, and stomach cancer caused by the formation of carcinogenic N-nitrosamines when it reacts with amine in stomach [5]. Thus, it is significant to develop a simple, rapid and sensitive method to determine nitrite for public health and environmental security [6].

Diverse detection techniques have been developed for the determination of nitrite, including spectrophotometry [7], fluorescence spectroscopy [8], and high performance liquid chromatography [9]. However, these methods are insensitive, time-consuming, and even toxicity by using toxic agents [4].

Electroanalysis has high sensitivity and selectivity, simplicity, and low cost [10]. Much work has been focused on developing a rapid electrochemical method for nitrite detection, without sample pretreatment and interference from other readily reducible compounds (such as nitrate ions and molecular oxygen).

Nanomaterial-based electrochemical nitrite sensors were fabricated to overcome the above limitations [11, 12]. Bimetallic nanostructures have been extensively investigated as electrode materials because of their good biocompatibility and conductivity, enlarged specific surface area, and enhanced catalytic activity [13]. Wang et al. have investigated the electrochemical sensing activities of nanoporous palladium-iron alloy toward nitrite with the aim to construct a highly sensitive and stable electrochemical sensor [14].

Particularly, bimetallic Au-based alloys possess remarkable catalytic activity as compared with their mono-counterparts. Several examples (e.g., AuPt [15], AuPd [16], and AuCu [17] bimetallic alloys) demonstrate much higher catalytic activity relative to monometallic Au particles. Among them, AuCu alloys have attracted considerable attention for the construction of electrochemical sensors, owing to the relatively low cost and superior catalytic activity [18]. For example, Chen et al. fabricated a potential pH sensor based on AuCu nanoclusters with pH-dependent photoluminescence property [19]. In another example, Wang and co-workers constructed a highly sensitive H2O2 sensor based on AuCu nanowires [20].

Herein, gold-copper nanochain networks (AuCu NCNs) were rapidly fabricated by a simple one-step wet-chemical approach with the help of metformin, which was explored for the fabrication of a sensor by the detection of nitrite as a model system.

Experimental section

Chemicals

Copper chloride (CuCl2), chloroauric acid (HAuCl4), sodium borohydride (NaBH4), metformin hydrochloride, ethanol and sodium nitrite were obtained from Shanghai Aladdin Chemical Reagent Company (Shanghai, China, www.chemicalbook.com). Other chemicals were of analytical grade. Twice-distilled water was employed throughout all the experiments.

Synthesis of AuCu NCNs

Typical preparation of AuCu NCNs was described as follows: Firstly, 0.041 g of metformin was dissolved into 8.0 mL of water, followed by mixing with 823 μL of HAuCl4 (24.3 mM) and 1.0 mL of CuCl2 (20 mM) together in ice-bath (0 °C). After stirring for 10 min, 200 μL of the freshly-prepared NaBH4 solution (1.0 M) was drop-wise added to the mixed solution under stirring. Finally, the resulting black precipitates were separated by centrifugation, thoroughly washed with water and ethanol, and then dried at 60 °C. For comparison, control samples were also prepared with the Au/Cu molar ratio of 1:3 and 3:1 (defined as Au1Cu3 and Au3Cu1), respectively. And Au nanoparticles were synthesized according to the previous report [21].

Preparation of AuCu NCNs/GCE

To prepare AuCu NCNs modified electrode, 2.0 mg of the sample was dispersed into 1.0 mL of water under ultrasonication for 20 min. Subsequently, 6 μL of the resulting suspension was dropped on the clean glassy carbon electrode (GCE, 3.0 mm in diameter) and allowed to dry in air. The resulting electrode was denoted as AuCu NCNs/GCE. And Au nanoparticles/GCE was prepared for comparison in a similar way. The details of instrumental characterization and electrochemical experiments were provided in the Electronic Supplementary Material.

Results and discussion

Characterization

Figure 1A and B show transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images of the product. A lot of uniform nanochain networks are observed across the whole section. The nanochains are consisted of branched nanoparticles with an averaged diameter of 5.1 ± 0.2 nm as building units, which are assembled to form branched nanochains extending to hundreds of nanometers in length. As shown in Fig. S1 (Electronic Supplementary Material, ESM), the chain-like structure for AuCu products is well kept instead of a phase separation between Au and Cu atoms after two months, indicating long-term stability of the nanochains. And their polycrystalline nature is manifested by the selective area electron diffraction pattern (inset in the Fig. 1B).

Fig. 1
figure 1

TEM (A and B) and HR-TEM (C) images of AuCu NCNs. Inset in B shows the corresponding SAED pattern

Full size image

Moreover, HR-TEM image (Fig. 1C) exhibits well-resolved lattice fringes with the inter-planar distance of 0.23 ± 0.002 nm, corresponding to the (111) planes of the face-centered cubic (fcc) AuCu alloy [22], indicating the predominant crystal growth along the direction of the (111) planes. This observation is identical with the previous study [23].

To analyze the elemental distribution in AuCu NCNs, high angle annular dark-field scanning TEM (HAADF-STEM) mapping images and cross-section compositional line profiles were supplied (Fig. 2). The resultant patterns exhibit uniform distribution of Au and Cu elements in AuCu NCNs, strongly verifying the formation of AuCu alloy [24].

Fig. 2
figure 2

HAADF-STEM-EDS mapping images (a-d), and cross-sectional compositional line profiles (e) of AuCu NCNs. Inset in e shows the corresponding HAADF-STEM image

Full size image

The crystalline nature of AuCu NCNs was investigated by X-ray diffraction (XRD, Fig. 3). There are five characteristic peaks emerged at around 38.5°, 44.5°, 65.1°, 77.8°, and 82.3°, which are assigned to the (111), (200), (220), (311) and (222) planes of AuCu NCNs [25], respectively. The diffraction peaks are slightly shifted and lie between pure Au (JCPDS-04-0784) and Cu (JCPDS-04-0836), indicating the formation of AuCu alloy [26]. In addition, the peaks detected at 35.5° and 48.8° might be originated from CuO, owing to the incomplete reduction of CuCl2 or surface Cu oxidation under ambient conditions [27].

Fig. 3
figure 3

XRD pattern of AuCu NCNs. The standard patterns of bulk Au (JCPDS-04-0784) and Cu (JCPDS-04-0836) are provided for comparison

Full size image

The oxidation state and surface composition of AuCu NCNs were surveyed by XPS (Fig. 4). The two prominent peaks observed at 83.82 and 87.51 eV are indexed to 4f7/2 and 4f5/2 feature of Au0 (Fig. 4a) [28], respectively. Nevertheless, there is no obvious peak of Au3+ appeared, revealing the complete reduction of AuCl4 −. As illustrated in Fig. 4b, two characteristic peaks emerge at around 932.3 and 952.2 eV corresponding to the binding energies of 2p1/2 and 2p3/2 of Cu0 [29]. A shake-up is observed at 943 eV, confirming the existence of very minimal Cu2+ in AuCu NCNs generated from unreacted CuCl2 [30]. Additionally, it’s highly significant to mention that the 2p3/2 binding energy of Cu0 is only ~0.1 eV different from Cu+. Thus, the valence states of Cu herein may lie between 0 and +1 [31].

Fig. 4
figure 4

High-resolution XPS spectra of Au 4f (a) and Cu 2p (b) in AuCu NCNs

Full size image

Formation mechanism

To better understand the formation mechanism of AuCu NCNs, a series of control experiments were carried out, including the metformin concentrations, the type of the reducing agent, and the molar ratio of Au/Cu. As shown in Fig. S2 (ESM), the absence of metformin yields a great deal of irregular nanoparticles with seriously aggregation (Fig. S2A, ESM). This is attributed to the formation of bare crystal planes without any capping agent to prevent them from aggregation [32]. The presence of 10 mM metformin produces chain-like nanocrystals with poor quality (Fig. S2B, ESM), and the best ones are produced with 25 mM metformin (Fig. 1). However, further increasing the concentration of metformin inhibits the chain length growth, causing the formation of short chains with aggregation (e.g., 50 mM, Fig. S2C, ESM). These results demonstrate metformin as a growth-directing agent and a weak stabilizing agent in the present synthesis [33].

Similarly, different reducing agent was also found to be essential in controlling the morphology of AuCu alloy nanocrystals. When ascorbic acid is used, the product contains severely aggregated AuCu nanoparticles with broad size distribution, rather than AuCu NCNs (Fig. S3A, ESM). By reducing the precursors with hydrazine hydrate (Fig. S3B, ESM), the product contains many poor nanochain networks. These results apparently demonstrate that the morphology of the product can be well controlled by the reaction kinetics [34]. Moreover, the Au/Cu molar ratio was also confirmed to be a main factor in shaping the final products, as displayed in Fig. S4 (ESM). When the Au/Cu molar ratio is 1:3 (Fig. S4A, ESM), only some irregular and agglomerated AuCu nanoparticles are obtained, while aggregated nanochains are observed with the Au/Cu molar ratio of 3:1 (Fig. S4B, ESM). It reveals that the Au/Cu molar ratio of the two precursors is strongly related to the nucleation and growth kinetics, therefore affecting the final morphology of the products [26].

The formation mechanism of AuCu NCNs can be described by a three-step growth model: rapid nucleation, selective adsorption, and oriented attachment growth. As shown in Fig. 5, numerous Au and Cu atoms are rapidly generated upon the addition of NaBH4 at the very early stage, owing to the extremely strong reducing ability of NaBH4 [35]. Immediately, the newly-formed Au and Cu atoms transform to AuCu nuclei, which are selectively adsorbed by metformin molecules on the (100) and (110) crystal planes with lower energies, facilitating the predominant growth along the (111) directions [19, 36, 37], as strongly confirmed by the associated HR-TEM images (Fig. 1C). Finally, AuCu NCNs are formed with the electrostatic interactions and oriented attachment with the assistance of metformin.

Fig. 5
figure 5

Schematic illustration of the formation mechanism of AuCu NCNs

Full size image

Electrocatalytic oxidation of nitrite at AuCu NCNs/GCE

The electrocatalytic behaviors of AuCu NCNs/GCE (curve a), Au nanoparticles/GCE (curve b), GCE (curve c), Au1Cu3 nanoparticles/GCE (curve d), and Au3Cu1 nanoparticles/GCE (curve e) were studied by cyclic voltammetry (CV) in 1.0 mM nitrite (Fig. 6A ). Apparently, the oxidation peak current of nitrite for AuCu NCNs/GCE (40.33 ± 0.012 μA, at 0.851 V vs. SCE) is about 1.54 ± 0.01, 1.68 ± 0.017, 6.06 ± 0.02, and 6.50 ± 0.019 times larger than those at bare GCE (26.13 ± 0.02 μA, at 1.081 V vs. SCE) and Au nanoparticles/GCE (23.94 ± 0.027 μA, at 0.788 V vs. SCE), Au1Cu3 nanoparticles/GCE (6.655 ± 0.021 μA, at 0.834 V vs. SCE), and Au3Cu1 nanoparticles/GCE (6.208 ± 0.017 μA, at 0.818 V vs. SCE), respectively. Besides, the anodic peak for AuCu NCNs/GCE at 0.207 V is attributed to the conversion of Cu (0)–Cu (I) [38]. The cathodic peaks at 0.155 V and 0.419 V are attributed to the transition of Cu (II) to Cu (I) and of Cu (I) to Cu (0), respectively [39]. The improved peak current and more negative peak potential is ascribed to the enhanced specific surface area of AuCu NCNs, high conductivity, and synergistic effects between Au and Cu in AuCu NCNs.

Fig. 6
figure 6

A Cyclic voltammograms of 1.0 mM nitrite on AuCu NCNs/GCE (curve a), Au nanoparticles/GCE (curve b), and bare GCE (curve c) in 0.1 M phosphate buffer (pH 7.0) at a scan rate of 50 mV s−1. (B) Cyclic voltammograms of AuCu NCNs/GCE in 0.1 M phosphate buffer (pH 7.0) containing 1.0 mM nitrite at different scan rates from 10 to 100 mV s−1. Inset shows the linear relationship between the oxidation peak current of nitrite and the square root of scan rate at 0.78 V (vs. SCE)

Full size image

The electrocatalytic behaviors of AuCu NCNs/GCE were examined in 1.0 mM nitrite at various scan rates (Fig. 6B). It is observed that the oxidation peak currents increase gradually with the scan rates (10 ~ 100 mV s−1), accompanied with the positive shift of the peak potentials. However, the oxidation peak current of nitrite is in proportion to the square root of scan rate with a correlation coefficient (R2) of 0.9988 (inset in Fig. 6B), which exhibits the diffusion-controlled process in the determination of nitrite [40].

Fig. S5 (ESM) shows the effects of pH values on the electrocatalytic behavior toward 1.0 mM nitrite oxidation at AuCu NCNs/GCE by varying the pH values from 3.0 to 9.0. The peak current achieves the maximum at pH 7.0 and then decreases by further extending the pH values. This is attributed to the fact that nitrite is instable in acidic media [41] and the nitrite oxidation becomes more difficulty with the lack of protons in alkaline media [42]. Therefore, pH 7.0 was employed in the subsequent experiments.

Figure 7 displays the oxidation peak currents from the differential pulse voltammetry with various nitrite concentrations at AuCu NCNs/GCE under the optimized conditions. Clearly, the peak current is proportional to the nitrite concentration with the linear range of 0.01 ~ 4.0 mM. The linear calibration equation was I p (μA) = 0.0176 C (μM) - 0.0176 (correlation coefficient, R 2 = 0.9994). The sensitivity is 0.0176 μA μM−1 with the limit of detection (LOD) of 0.2 μM (S/N = 3). Clearly, the presence of AuCu NCNs improves the catalytic performance for nitrite sensing in terms of sensitivity and LOD as compared to those in previous reports (Table 1) [2, 40, 43–45].

Fig. 7
figure 7

Differential pulse voltammetry curves of AuCu NCNs/GCE in the presence of different nitrite concentrations at 0.68 V (vs. SCE)

Full size image
Table 1 Comparison of this sensor for nitrite determination with those in the literature
Full size table

The reproducibility and stability of AuCu NCNs/GCE were determined in 1.0 mM nitrite. The fabrication reproducibility for five electrodes prepared independently was calculated by the responses to 1.0 mM nitrite with the relative standard deviation (RSD) of 1.7 %. Moreover, the RSD of current responses with the same electrode was 1.9 % for five successive measurements. These results suggest good reproducibility of this approach. After storing in refrigerator at 4 °C for 35 days, the peak current responses were still retained 98.60 % of the initial values on AuCu NCNs/GCE, revealing good stability of the sensor for the detection of nitrite. The excellent long-term stability and reproducibility of AuCu NCNs/GCE make them attractive in the field of analytical applications.

The selectivity was studied by adding some potentially interfering ions to the phosphate buffer (pH 7.0) containing 0.5 mM nitrite. It is found that 100 folds of NaCl, CH3COONa, NaNO3, NaCO3, NaSO4, and CH3OH have no interference on nitrite determination, as shown in Fig. S6 (ESM). These results reveal this sensor has high selectivity for nitrite, which is attributed to the strong affinity between nitrite and metallic nanocrystals [46].

To evaluate the feasibility of nitrite detection in real samples, AuCu NCNs/GCE was employed to determine nitrite in water samples. As listed in Table S1 (ESM), the recoveries ranged from 99.8 % to 103.4 % for five parallel measurements, with RSD below 4.1 %. Therefore, AuCu NCNs/GCE can be utilized to determine nitrite in real water samples.

Conclusion

In summary, AuCu NCNs were successfully prepared and employed for the construction of a highly selective electrochemical sensor for the detection of nitrite. This sensor exhibited significantly enhanced catalytic performance toward nitrite oxidation as compared with the referenced sensors. The wide linear range, high selectivity, good stability, low detection limit and high reproducibility make it a potential candidate for practical applications.