Enzyme-like activity of cobalt-MOF nanosheets for hydrogen peroxide electrochemical sensing

Highlights

• 2D-Co-MOF nanosheets display “oxidase” activity for the non-common catalytic oxidation of hydrogen peroxide.

• The current electrochemical sensor exhibits outstanding sensitivity, selectivity, stability, and durability at neutral pH.

• The calibration curves follow a Michaelis -Menten dependence.

• The particular coordination chemistry of the MOF is envisaged to play a key role.

Abstract

Metal-organic frameworks (MOFs) are receiving increased attention as new functional nanomaterials for the development of electrochemical sensors. Herein, we develop an electrochemical platform for non-enzymatic hydrogen peroxide detection built with a composite of two-dimensional cobalt MOF nanosheets and Nafion (2D-Co-MOF@Nafion). The feasibility of the 2D-Co-MOF@Nafion composite as active material for high performance hydrogen peroxide sensor was investigated by using cyclic voltammetry and chronoamperometry. Its voltammetric response reveals an efficient charge transport through the MOF composite, and rapid electron exchange between MOF and electrode. Notably, these MOF nanosheets exhibit enzyme-like activity for the non-common catalytic oxidation of hydrogen peroxide, leading to an electrochemical sensor with rapid quantitative detection, outstanding sensitivity, selectivity, stability, and durability at the desirable neutral pH. In particular, for a cobalt metal loading of 1.2 nanomol, the sensor yields amperometric H₂O₂ detection with characteristic electrocatalytic parameters of $i_{\mathit{max}}$= 5.7 mA cm⁻² and $K_M$ = 13 mM. Moreover, linear ranges of up to either 1 mM or 10 mM are achieved, with sensitivities as high as 570 ± 5 A cm⁻² mM⁻¹ or 395 ± 10 A cm⁻² mM⁻¹ for the low and high concentration ranges, respectively. The particular coordination chemistry of the MOF consisting of a regular arrangement of multiple Co(II) redox metal sites connected by appropriate organic ligands can provide inherent enzyme-mimicking properties, thereby explaining the higher oxidase-like activity of the present MOF. This work raises the new idea of using two-dimensional cobalt-based MOFs as active nanozymes, offering exciting opportunities in the design of non-enzymatic electrochemical sensing devices.

Introduction

The detection and quantification of H₂O₂ is important in the fields of clinical diagnostics, biotechnology, and industrial manufacture in environmental, food, and industrial analysis, because of its involvement as an oxidizing agent in many chemical, biological, pharmaceutical, and environmental processes. Its concentrations naturally range from micromolar to tens of millimolar [1]. Among other sensing techniques, electrochemical sensors show appealing performance in terms of low cost, fast response, ease of operation, and high sensitivity [2]. Electrochemical detection of H₂O₂ is categorized into two types, namely enzymatic and nonenzymatic. The use of enzymes, and in particular peroxidases, as electrocatalysts has been increasingly studied for the development of enzyme-based electrochemical H₂O₂ sensors, since they offer great benefits in terms of activity and specificity [1], [3], [4]. However, the use of these enzyme-based electrochemical sensors is strongly limited by their poor stability in non-optimal operating conditions and their inherent suicide inactivation reactions [1], [5], [6]. Alternatively, nanomaterials with enzyme-like characteristics, called “nanozymes”, are receiving increased attention as appealing non-enzymatic electrochemical platforms for sensing applications, since they can overcome the limitations of enzyme-based electrochemical biosensors [1], [7], [8], [9], [10]. A wide variety of nanostructured materials have proven enzyme-like activity, including carbon, metals and metal oxides [7], [8], [11], [12], [13], [14], [15], [16], [17]. In recent years, many works have been devoted to the synthesis of nanozymes with promising results for biosensing applications. However, the catalytic activity of most nanozymes remain much lower than that of the natural enzymes, so the design and development of highly active nanozyme materials is challenging [17].

Metal−organic frameworks (MOFs) are a novel class of porous materials that are emerging in the chemical sensing area due to their unique characteristics, such as ultrahigh specific surface area, regular porosity, controllable arrangement of isolated active sites and highly tunable structures, as well as excellent thermal and chemical stability. Besides, in a typical MOF, the enzyme-like catalytic capacity can originate from the added effect of metal redox couples binding to organic ligands that act as electron mediators [13], [18], [19], [20], [21], [22]. These ligands mimic the coordination environment of the metal centers in natural enzymes and catalyze the reduction (or oxidation) of substrates in a similar way. However, the research of MOFs as electrochemical sensors is still in an early stage and still constitutes a challenge in the bio- and chemical detection areas. Indeed, traditional MOFs have interconnected 3D structures that suffer from high mass transfer resistance, thereby diminishing the activity of MOF nanozymes [15]. One elegant strategy to solve this limitation is the development of related 2D materials, which provide more exposed electroactive sites, thus improving the electrical conductivity and lowering the diffusion barriers [15], [23], [24], [25], [26], [27], [28]. So far, few papers have been devoted to the fabrication of electrochemical H₂O₂ sensors based on cobalt MOFs, and most of them use oxy(hydroxide) species derived from the original MOF that operate only in the undesirable strong alkaline medium [10], [29], [30], [31], [32], [33]. H₂O₂ detection in typical bio- and chemical samples is carried out at circumneutral pHs, at which these cobalt oxide species are expected to decrease their catalytic activity.

Herein, we develop an electrochemical sensor for the detection of H₂O₂ at the desirable, but not commonly employed, neutral pH, consisting of a two-dimensional cobalt MOF (2D-Co-MOF) with a nitrogen coordination environment based on layered double nanosheets, and supported on a graphite electrode. It takes advantage of its previously proven superior catalytic activity for the water electroxidation reaction [34]. Interestingly, we demonstrate that this 2D-Co-MOF exhibits high oxidase-like activity at neutral pH, being able to mediate the catalytic oxidation of H₂O₂, whose novel catalytic properties are ascribed to the particular active metal coordination with several nitrogen-metal bonds. Moreover, this metal coordination in the secondary building unit of the MOF is preserved in the electrode, which is a differential feature with respect to most electrochemical H₂O₂ sensors based on MOFs, since these sensors operate under strongly alkaline conditions where the MOFs undergo a phase transition to form cobalt oxy(hydroxide) species. The so-developed electrode was shown to sense H₂O₂ with high sensitivity and selectivity, fast response time, wide linear range, and high stability, thus revealing enzyme-like activity. To the best of our knowledge, this electrocatalytic activity of as-synthesized Co-MOF for the electrooxidation of H₂O₂ is unprecedented.