Cobalt phosphide nanowires: an efficient electrocatalyst for enzymeless hydrogen peroxide detection

Hydrogen peroxide (H₂O₂) has been generally applied to environmental protection, clinical studies, food, pharmaceutical, chemistry and many other fields [1–3]. Specially, H₂O₂ is also a common reactive oxygen species [4] and the presence of excess H₂O₂ in cells may lead to oxidative stress that can induce cancer, Alzheimer's disease and Parkinson's disease [5–7]. Therefore, it is highly necessary to sensitively and selectively detect H₂O₂ under physiological conditions. Many techniques have been developed for H₂O₂ detection, such as chemiluminescence [8], fluorescence [9, 10], colorimetry [11] and electrochemical methods [12, 13], etc. Among them, the electrochemical technique has received considerable attention with its merits of rapid response, high sensitivity, good selectivity and simple apparatus [14, 15].

Enzymes such as horseradish peroxidase [16], cytochrome [17] and myoglobin [18] are efficient for electroreduction of H₂O₂ and thus used widely for electrochemical H₂O₂ sensing. Unfortunately, the enzyme-based sensors suffer from several drawbacks associated with complex fabrication procedures, environmental instability, low reproducibility and high cost, limiting their large-scale applications [19–21]. To overcome these obstacles, it is highly desirable to develop non-enzymatic electrochemical H₂O₂ sensors.

In recent years, noble metal materials have been intensively utilized in the construction of electrochemical H₂O₂ sensors [22, 23]. Given the deficiency and high cost of the precious noble metals, the large-scale practical utilization of noble metal-based sensors for H₂O₂ detection has been severely impeded. In contrast, transition metal-based nanomaterials have received increasing interest in electrochemical H₂O₂ sensors [24, 25], but suffer from limited electrical conductivity. It is well known that transition metal phosphides (TMPs) are an important class of compounds with metalloid characteristics and good electrical conductivity [26], but their use as an electrocatalyst for enzymeless H₂O₂ detection has not been reported before.

In this letter, we report on our recent finding that cobalt phosphide nanowires (CoP NWs) behave as an efficient noble metal-free catalyst for electrochemical reduction of H₂O₂. This nonenzymatic H₂O₂ sensor exhibits a fast amperometric response time of less than 5 s, a linear range of 0.001–12 mM and a low detection limit of 0.48 μM at a signal-to-noise ratio of 3. It also shows favorable selectivity due to its high anti-interference property and excellent resistance to chloride poisoning.

2.1. Chemicals

2.2. Preparation of Co(CO₃)_0.5(OH) · 0.11H₂O nanowires and CoP NWs

2.3. Fabrication of H₂O₂ biosensors based on CoP NWs

2.4. Characterizations

2.5. Electrochemical measurements

CoP NWs were derived from its Co(CO₃)_0.5(OH) · 0.11H₂O precursor via low-temperature phosphidation reaction. Figures 1(a) and (b) show the x-ray diffraction (XRD) patterns of the precursor and phosphided product, respectively. As shown in figure 1(a), the diffraction peaks of the precursor are in accordance with the standard diffraction pattern of Co(CO₃)_0.5(OH) · 0.11H₂O (JCPDS No. 48−0083). The XRD pattern for the phosphided product (figure 1(b)) can be well indexed to the (011), (111), (112), (211), (103) and (020) planes of CoP (JCPDS No. 65–1474), confirming the successful conversion of Co(CO₃)_0.5(OH) · 0.11H₂O into CoP. The scanning electron microscopy (SEM) image shows the precursor materials consist of nanowires that are several micrometers in length, as shown in figure 1(c). Also note that phosphided product still preserves the 1D morphology (figure 1(d)). The energy-dispersive x-ray (EDX) spectrum (figure 1(e)) verifies a 1:1.1 atomic ratio for Co:P, consistent within experimental error with the expected 1:1 stoichiometry of CoP. The high resolution transmission electron microscopy (HRTEM) image taken from one single CoP nanowire (figure 1(f)) reveals lattice fringes with an interplanar distances of 0.26 nm, corresponding to the (211) plane of CoP. Figure S1(a) displays the x-ray photoelectron spectroscopy (XPS) spectra in the Co(2p) and P(2p) regions for CoP NWs. As shown in figure S1(a), The Co 2p XPS spectrum shows a doublet containing Co 2p_3/2 and Co 2p_1/2 at 779.1 and 793.9 eV [27]. The peaks at 785.6 and 803.2 eV are attributed to two shake-up satellites [28]. The binding energies (BEs) of P 2p_3/2 and P 2p_1/2 appear at 129.5 and 130.4 eV (figure S1(b)) [29, 30]. The peaks at 781.9 and 133.8 eV are assigned to oxidized Co and P species resulting from superficial oxidation of CoP [31]. The Co 2p BEs of 779.1 and 793.9 eV are positively shifted from that of Co metal (778.1−778.2 eV), and the P 2p_3/2 BE of 129.5 eV is negatively shifted from that of elemental P (130.2 eV) [32]. According to the XPS analysis, the Co in CoP has a partial positive charge (δ⁺) while the P has a partial negative charge (δ⁻), implying transfer of electron density from Co to P, which is in good agreement with previous report [33].

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To investigate the electrocatalytic activity of the CoP NWs toward H₂O₂ reduction, we designed a non-enzymatic H₂O₂ sensor by depositing the CoP NWs on a bare GCE surface. Figure 2(a) shows the CV curves of bare GCE and CoP NWs/GCE in saturated 0.1 M PBS at pH 7.4 with and without 1 mM H₂O₂, respectively. As observed, the bare GCE exhibits negligible response to H₂O₂ reduction with pretty low reduction current. In contrast, the CoP NWs/GCE exhibits a remarkable catalytic current peak about 141 μA in intensity at −0.53 V. It is also important to note that the CoP NWs/GCE exhibits no electrochemical response in the absence of H₂O₂. All the above observations indicate that the CoP NWs/GCE exhibits a notable catalytic performance for H₂O₂ reduction. Furthermore, the effect of scan rate on the CoP NWs/GCE was also examined. As shown in figure 2(b), the peak currents increase accordingly with the scan rates in the range of 10–200 mV s⁻¹. The peak currents increase linearly with the square root of the scan rate, demonstrating a diffusion-controlled process of H₂O₂ reduction on the CoP NWs/GCE electrode.

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Figure 3(a) shows the amperometric responses of the CoP NWs/GCE upon successive addition of various concentrations of H₂O₂ into the stirring 0.1 M PBS (pH 7.4) at –0.5 V. It can be observed that the CoP NWs/GCE exhibits quick response to the change of H₂O₂ concentration and achieves the maximum steady-state current within 5 s. The inset of figure 3(a) displays the amperometric responses to low H₂O₂ concentration. Figure 3(b) shows the calibration curve of the sensor. The linear detection range is from 0.001–12 mM. As shown in figure S2, CoP NWs/GCE displays a multi-linear range for the concentration of H₂O₂ from 0.01 mM–0.1 mM, 0.1 mM−0.2 mM, 0.2 mM−1 mM and 1 mM−12 mM with a detection sensitivity of 3463, 7029, 375 and 26.8 μA cm⁻² mM^–1, respectively. The existence of more than one linear range on nonenzymatic metallic devices has been observed for H₂O₂ detection [34, 35]. The detection limit is calculated to be 0.48 μM at a signal-to-noise ratio of 3, which is lower than most of the previous reported nanomaterials based biosensors, as shown in table S1.

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One important influence factor for electrochemical analysis application is the interference from electrochemical signals. The influence of some potentially interfering reagents, such as ascorbic acid (AA), lactose, glucose, uric acid (UA), fructose and urea on the detection of H₂O₂ were introduced one by one during the H₂O₂ sensing process. In addition, chloride may poison the electrode [36–38]. The influence from NaCl was also investigated to evaluate the interference. As shown in figure 3(c), a quick and obvious current response is presented after the addition of 0.1 mM H₂O₂ at −0.5 V into 0.1 M PBS under stirring, whereas no further significant current change is observed with the continuous addition of 10 mM ascorbic acid (AA), lactose, glucose, uric acid (UA), fructose, urea and NaCl. Nevertheless, the current increases again with another addition of 0.1 mM H₂O₂. These results indicate both electro-active species and chloride ions show no interference on the response of this electrode toward H₂O₂ detection, which proves a good selectivity and an excellent resistance to chloride poisoning of this fabricated CoP NWs/GCE.

The long-term stability of the CoP NWs electrode was evaluated by testing the current response at −0.5 V to 0.1 mM H₂O₂ every week. As shown in figure 4(a), the CoP NWs/GCE retains 90.18% of its initial response after four weeks of storage at room temperature. Moreover, a relative standard deviation of 5.59% was obtained for the responses at potential of −0.5 V to 0.1 mM H₂O₂ recorded on five independently fabricated CoP NWs modified electrodes (figure 4(b)), indicating good reproducibility.

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In summary, CoP NWs have been demonstrated as an efficient catalyst toward electrochemical reduction of H₂O₂. As a non-enzymatic H₂O₂ sensor, it shows a fast amperometric response time of less than 5 s, a linear range of 0.001−12 mM and a low limit of detection of 0.48 μM at a signal-to-noise ratio of 3 with favorable selectivity. Our present study is important because it opens new opportunities in exploring the use of TMPs nanostructures, a big family of water-splitting catalysts [32, 33, 39–44], for electrochemical detection of small molecules.