Ce(III, IV)-MOF electrocatalyst as signal-amplifying tag for sensitive electrochemical aptasensing

Highlights

• An effective signal amplification according Ce(III, IV)-MOF as electrocatalyst.

• The electrocatalysis depends on spontaneous recycling of Ce(III)/Ce(IV) towards Thi.

• As-prepared aptasensor shows high specificity and sensitivity for thrombin detection.

Abstract

Metal–organic frameworks (MOFs) as a new class of porous materials have attracted increasing attention in the field of biomimetic catalysis. This study firstly reports a mixed valence state Ce-MOF possessing intrinsic catalytic activity towards thionine (Thi), and its application in constructing an amplified electrochemical aptasensor for thrombin detection. As noticed, the novel catalytic process combines the advantages of 3D infinite extension of the Ce(III, IV)-MOF skeleton containing large amounts of catalytic sites and spontaneous recycling of the Ce(III)/Ce(IV) for electrochemical reduction of Thi, thereby presenting amplified electrochemical signals. To further improve the aptasensor performance, the high selectivity of proximity binding-induced DNA strand displacement and high efficiency of exonuclease III-assisted recycling amplification were incorporated into the assay. The aptasensor was employed to detect thrombin in complex serum samples, which shows high sensitivity, specificity, stability and reproducibility. This work offers an opportunity to develop MOF-based electrocatalyst as signal-amplifying tag for versatile bioassays and catalytic applications.

Introduction

As an emerging and exciting research area of artificial enzymes, nanomaterial-based mimics have attracted considerable attention in the past decades. Since the first discovery of ferromagnetic nanoparticle (NP) with peroxidase-like activity, a great deal of excellent work on nanozyme has been carried out, such as Au (Hu et al., 2017, Liu et al., 2017), Fe₃O₄ (Zheng et al., 2014), CeO₂ (Wang et al., 2017a; Hu et al., 2015), carbon-based NPs (Yuan et al., 2016; Choudhary et al., 2017) and so on. Particularly, CeO₂NP, a classical nanozyme, is well known for its highly catalytic performance that probably is related to the coexistence of both Ce(III) and Ce(IV). Thus, CeO₂-based mimics have attracted special interest in biosensors because of the remarkable merits, such as enzyme-like activities (Montini et al., 2016, Yagati et al., 2013), non-toxicity (Tan et al., 2015) as well as good biocompatibility (Schweiger et al., 2014, Tian et al., 2015). However, most of the above mentioned NPs are prone to aggregate, poorly showing the complete catalytic ability. To overcome these drawbacks of NPs, the introduction of surface linker is highly desirable.

Metal–organic frameworks (MOFs), a new class of porous materials constructed by metal ions and organic ligands via coordination bonds, have attracted considerable attention recently. Some key features, such as 3D infinite extension of MOF skeletons, well-defined pores and exposed active sites that endows them with superior catalytic activities, have extended the application region of MOFs, including Fe(III)-based MIL-68 (Zhang et al., 2014), MIL-88A (Wang et al., 2016), porphyrin-based MOFs (Ling et al., 2016) and Cu-MOF (Shen et al., 2015) over other NPs. In 2015, Xiong and co-workers (Xiong et al., 2015) reported that a mixed valence state of Ce-MOF possessed an excellent mimics of catalytic activity toward 3,3′,5,5′-tetramethylbenzidine (TMB) and they established a sensitive colorimetric approach for the detection of biothiols. The catalytic mechanism may be attributed to the spontaneously recycling of Ce(III)/Ce(IV) system, which is similar to the Ce(III) ↔ Ce(IV) redox switch of CeO₂NPs. Nevertheless, there have not hitherto been referenced in electrochemical biosensors based on a mixed valence state Ce-MOF as electrocatalyst. Very recently, our work (Wang, et al., 2017b) reported that Co/Fe-MOFs exhibit highly electrocatalytic activities for thionine (Thi). Notably, different from most nanomaterials, such as Pd, Pt, Fe₃O₄, CeO₂-based peroxidase mimics/H₂O₂ system, Co/Fe-MOFs toward Thi catalysis not only retains the amplified current in electrochemical detection but also avoids the addition of any unstable substrate in solution, making the processes simpler and easier.

Inspired by reports on a mixed valence state Ce-MOF and our previous research, in this work, Ce(III, IV)-MOF electrocatalyst as signal-amplifying tag has been explored in constructing an electrochemical aptasensor for thrombin (Tb) detection. To further improve the detection selectivity and sensitivity, proximity binding-induced DNA strand displacement and Exo III-assisted recycling amplification are incorporated into our assay. As expressed in Scheme 1, in the presence of target protein (Tb), the proximity binding of two aptamers (S1, S2) can specifically bound with Tb, leading to the displacement of S3. Relying on its rational design, S3 was introduced to open the capture probe 1 (CP1), resulting in the duplex DNAs. Subsequently, with the aid of Exo III, the DNA duplex was digested from 3′ blunt terminus, leading to the S3 dropping from the DNA duplex to initiate the next cycle. As a result, with the proceeding of the Exo III-assisted S3 recycling, a large amount of single-stranded capture probes (partial CP1 sequences) are produced. Eventually, Au-Thi-Au@Ce(III, IV)-MOF labeled hairpin probe 1 (HP1) can be introduced to the sensing interface via binding with the above CP1 sequences, and thus, a significantly amplified current is obtained. Notably, the class of mixed valence state MOFs as electrocatalyst highlights the high catalytic activity and spontaneous recycling of the Ce(III)/Ce(IV) system for dye molecules catalysis even without substrate. The designed aptasensor is employed to detect Tb in complex serum samples down to the low fM level.