Enzyme-like catalysis of polyoxometalates for chemiluminescence: Application in ultrasensitive detection of H₂O₂ and blood glucose

Highlights

• Enzyme-like catalysis of POMs on luminol/H₂O₂ reaction for chemiluminescence detection.

• Accurate and ultrasensitive detection of H₂O₂ and blood glucose based on POM-catalytic reaction.

• Low limit of detection, wide detection range and simple assay procedure of the method.

• Potential applications in various bioassays with H₂O₂ as a product of oxidoreductase reactions.

Abstract

We show that vanadomolybdophosphoric heteropoly acid (H₅PMo₁₀V₂O₄₀, PMoV₂), one of the polyoxometalates (POMs), presents peroxidase-like activity that can catalyze the luminol/H₂O₂ reaction to generate chemiluminescence (CL) emission. Based on this foundation, we report an enzyme-free luminol/H₂O₂/PMoV₂ CL system that can be utilized for the ultrasensitive detection of H₂O₂. The CL intensity exhibits good linear dependence on H₂O₂ concentration in a wide range up to 5000 nM with a limit-of-detection of 5 nM. The method was successfully used for the determination of glucose in human serum. Our study paves the way for the application of functional POM materials in a variety of biosensing assays, such as the determination of cholesterol, uric acid, lactate, etc., in which H₂O₂ is a product of oxidoreductase enzymes.

Introduction

Chemiluminescence (CL) is a highly sensitive and robust detection method with low background interference and simple instrumentation that is widely used in various assays in the fields of bioanalysis, food safety, environmental monitoring and so on [[1], [2], [3], [4]]. The method relies on light emission from a catalytic chemical reaction that requires no external light source for excitation. The reaction between luminol and H₂O₂ is widely used in CL detection, producing luminescence emission centered at ~420 nm [[5], [6], [7]]. The commonly used catalysts for the reaction are some natural enzymes with peroxidase activity; for example, horseradish peroxidase (HRP) and catalase can catalyze the oxidation of luminol to its excited state 3-aminophthalate anion (APA*) with the simultaneous reduction of H₂O₂, producing a light signal for sensitive and selective detection [8]. In spite of their effective catalytic efficiency, natural enzymes have some drawbacks, including high costs for preparation, difficult storage and easy denaturation. Furthermore, the activity of enzymes will be affected by temperature, pH and harsh chemical environments. Therefore, efforts have been devoted to developing enzyme-free CL systems based on peroxidase mimetics instead of natural enzymes to improve the accuracy and stability of the CL detection method [9,10].

So far, many attractive materials have been reported that can catalyze the luminol/H₂O₂ system to generate CL [[11], [12], [13]]. Lim et al. reported a size-dependent catalyst using Pt dendrimer-encapsulated nanoparticles for the detection of several oxidases [14]. Willner’s group synthesized a series of Cu²⁺-modified nanomaterials as heterogeneous catalysts, including Cu²⁺-modified graphene oxide nanoparticles, Cu²⁺-functionalized carbon nitride nanoparticles and Cu²⁺-carbon dots, which were utilized to catalyze the luminol/H₂O₂ reaction to generate CL signals [15,16]. Luo et al. synthesized a novel solid catalyst by encapsulating Hemin into the HKUST-1 MOF materials, which was applied in the CL field [17]. These studies have shown the advantages of functional materials as peroxidase mimetics to establish an enzyme-free CL system for bioanalysis, including good enzyme-like activity, low cost and high stability.

As well-known nanosized metal oxide cluster compounds, polyoxometalates (POMs) possess many ideal properties for numerous applications in biological and medical fields, such as low-level toxicity, stability in biological conditions, rich structural versatility, superior redox activity, etc [[18], [19], [20], [21]]. It has been demonstrated that POMs exhibit peroxidase activities similar to HRP to catalyze oxidation reactions with a color change using H₂O₂ in colorimetric sensing [[22], [23], [24], [25], [26], [27]] and have been used in phosphorus determinations in aqueous solutions [[28], [29], [30], [31], [32]]. However, no information is available about the catalytic activity of POMs on the luminol/H₂O₂ reaction to generate CL. In this work, we observe that vanadomolybdophosphoric heteropoly acid (H₅PMo₁₀V₂O₄₀, PMoV₂, one of POMs) has peroxidase activity that is utilized to catalyze luminol to generate light using H₂O₂ as the oxidant. The CL signals exhibit good linear dependence on the concentration of H₂O₂. Thus, the enzyme-free CL method based on the luminol/H₂O₂/PMoV₂ system can be employed for simple, fast and ultrasensitive detection of H₂O₂. The method was successfully applied to determine blood glucose by monitoring the concentration of H₂O₂, the product of the glucose/O₂ reaction catalyzed by glucose oxidase (GOx). The proposed method could have potential applications in a variety of biosensing assays, such as the determination of cholesterol, uric acid, lactate, etc., in which H₂O₂ is a product of oxidoreductase enzymes.