Novel neuron-network-like Cu–MoO₂/C composite derived from bimetallic organic framework for highly efficient detection of hydrogen peroxide

Highlights

• Neuron-network-like Cu–MoO₂/C was simply prepared by in-siu pyrolysis of bimetallic-organic framework.

• Non-enzymatic Cu–MoO₂/C/GCE electrochemical sensor for highly efficient detection of H₂O₂ is firstly fabricated.

• This sensor has high sensitivity and low detection limit to nM-level H₂O₂.

• This sensor exhibits practicality for detecting H₂O₂ in spiked human serum, commercial disinfector and contact lens solution.

Abstract

Fabrication of non-enzymatic electrochemical sensors based on metal oxides with low valence-state for nanomolar detection of H₂O₂ has been a great challenge. In this work, a novel neuron-network-like Cu–MoO₂/C hierarchical structure was simply prepared by in-situ pyrolysis of 3D bimetallic-organic framework [Cu(Mo₂O₇)L]_n [L: N-(pyridin-3-ylmethyl)pyridine-2-amine] crystals. Meanwhile, the MoO₂/C nano-aggregates were also obtained by liquid phase copper etching. Subsequently, two non-enzymatic electrochemical sensors were fabricated by simple drop-coating of the above two materials on the surface of glassy carbon electrode (GCE). Electrochemical measurements indicate that the Cu–MoO₂/C/GCE possesses highly efficient electrocatalytic H₂O₂ property during wider linear range of 0.24 μM–3.27 mM. At room temperature, the Cu–MoO₂/C composite displays higher sensitivity (233.4 μA mM⁻¹ cm⁻²) and lower limit of detection (LOD = 85 nM), which are 1 and 2.5 times larger than those of MoO₂/C material, respectively. Such excellent ability for trace H₂O₂ detection mainly originates from the synergism of neuron-network-like structure, enhanced electrical conductivity and increased active sites caused by low valence-state MoO₂ and co-doping of Cu and carbon, and even the interaction between Cu and Mo. In addition, the H₂O₂ detection in spiked human serum and commercially real samples indicates that the Cu–MoO₂/C/GCE sensor has certain potential application in the fields of environment and biology.

Introduction

Low valence state transition metal oxides (LVS-TMOs) are considered to be one promising candidate of diverse electrocatalysts owing to their variable d-band electronic configuration and rapid electron transfer characteristics [1,2]. MoO₂ with rutile phase has the advantages of high conductivity, good catalytic activity and stable layered structure, which has recently gained extensive attentions as electrocatalytic material [1,3]. So far, morphology regulation (such as nanorods [4], nanowires [5], nanosheets [6]) and dopant composite (such as noble metals [3], transition metals [1], carbon materials [6]) have been adopted to improve the electrocatalytic performance of MoO₂. The two strategies can not only improve the conductivity of MoO₂-based materials, but also facilitate the exposure of more catalytic active sites and tune the electronic state of Mo center, making them show preferable prospect in areas of energy [[7], [8], [9]] and catalysis [10,11]. For instance, chainmail-like MoO₂ core-shell prepared by radio-frequency induction method served as catalyst for converting syngas to higher alcohols [12]. MoO₂ nanotextiles tailored by hydrothermal and annealed method, which showed good electrochemical performance as anode of LIBs [13]. However, most of reported methods have the defects of complex, high cost and poor reproducibility, which seriously hinder their practical applications as an important catalyst [6,9,11]. Therefore, exploring a simple and controllable synthesis of MoO₂-based composites is beneficial to broaden their applications in electrocatalytic fields.

In comparison with the greater progress in new energy sources, researches on the application of MoO₂-based composites as electrochemical sensors in bioanalysis and environmental monitoring are rarely reported [[14], [15], [16]]. For instance, E. Fazio et al. fabricated the MoO_x/SPCE (SPCE: screen printed carbon paste electrode) sensor that used for the detection of dopamine with the LOD of 0.043 μM [14]; N. Tavakkoli et al. reported the preparation of MoO₂/Cu-NPGF (NPGF: nanoporous gold film) electrode, which realized the detection of methimazole with the LOD of 0.0035 μM [15]; Dong et al. synthesized Co,N co-doped MoO₂/MoC composites, showing good electrochemical sensitivity to acetaminophen and 4-aminophenol [16]. However, MoO₂-based composites serving as electrochemical H₂O₂ sensors have not been reported yet. As is known to all, H₂O₂, as a valuable green oxidant, has been extensively utilized in fields of food, medicine, chemical industry and so on. What’s more, it can participate in some important physiological functions in biological systems. The normal extracellular levels of H₂O₂ are found to be in the range of 0.001–0.1 μM, and the overproduction can inhibit the message transduction in cells, thus leading to gene mutation and causing serious diseases, such as diabetes, atherosclerosis and cancer [17,18]. Hence, detection of trace H₂O₂ is of great practical significance to environmental protection and medical treatment.

The portable electrochemical H₂O₂ sensor has caused widespread concern very recently due to their advantages of low cost, accurate analysis and good stability. Compared with expensive enzymes or noble metals, the non-enzymatic electrochemical H₂O₂ sensors based on TMOs have made great progress [2,19,20]. Especially in recent years, it has been realized that the electrocatalytic materials with good reduction property are more favorable for accurate detection of H₂O₂ on the basis of H₂O₂ participating in chemical reactions as a green oxidant. Consequently, the LVS-TMOs (such as Mn, Fe, Co, Cu, etc) materials with variable d-band electronic configuration and rapid electron transfer characteristics have aroused great interest [2,19,21,22]. For example, it can be seen from Table 1 that Ag–Cu₂O/N-RGO/GCE has the lowest LOD of 0.01 μM to H₂O₂ [2], and leaf-like CuO–CoO hetero-nanostructure possesses the highest response of 6349.0 μA mM⁻¹ cm⁻² towards H₂O₂. Unfortunately, this electrochemical CuO–CoO sensor exhibited a high LOD of 1.4 μM [21]. At present, the electrochemical H₂O₂ sensors based on LVS-TMOs have been widely investigated to date, and the ranges of sensitivity and LOD locate between 26.7 and 6349.0 μA mM⁻¹ cm⁻² and 0.01–2.80 μM, respectively. But many of these electrode materials still present low sensitivity and high LOD (Table 1), which then hindered their real applications for detecting nM-level H₂O₂ in food and cells. Meanwhile, the MoO₂-based materials with friendly biocompatibility are not reported in detection of H₂O₂. Accordingly, it is necessary to develop a simple method for preparing excellent electrocatalytic MoO₂-based materials and further realize its ultra-sensitive detection of H₂O₂.

Recently, isopolymolybdate based metal-organic frameworks (MOFs) were served as precursor to prepare molybdenum-based advanced functional materials with good electrochemical performance through simple pyrolysis [23,24]. In view of this concept and the excellent catalytic and conductive properties of Cu element [25,26], herein, a new 3D bimetallic-organic framework [Cu(Mo₂O₇) L]_n (L: N-(pyridin-3-ylmethyl)pyridine-2-amine) was synthesized, which was then in-situ pyrolyzed in N₂ atmosphere to fabricate hierarchically neuron-network-like Cu–MoO₂/C composite. Furthermore, the porous MoO₂/C nano-aggregates were also prepared by liquid phase copper etching. Then, the two materials were modified on the surface of bare GCE by facile drop-coating to fabricate non-enzymatic electrochemical H₂O₂ sensors. Especially, the Cu–MoO₂/C/GCE exhibits good electrocatalytic H₂O₂ property and can be adapted to monitor the H₂O₂ in real samples.