In-situ synthesis of 3D Cu2O@Cu-based MOF nanobelt arrays with improved conductivity for sensitive photoelectrochemical detection of vascular endothelial growth factor 165
Introduction
In recent years, photoelectrochemical (PEC) has attracted extensive research as a promising analytical method (Hao et al., 2020; Xu et al., 2020). Compared with traditional electrochemistry, PEC possesses advantages of high sensitivity, great selectivity and low-cost determination due to the separation of the excitation light source and the output signal (Kong et al., 2020a; Sun et al., 2019; Liu et al., 2019; Zhu et al., 2019). More importantly, photoactive materials are critical for constructing PEC biosensing platforms with excellent analytical performance. The development of high-performance PEC materials is usually limited by the problems of unstable photocurrent and low electronic conversion efficiency. Therefore, in order to obtain more stable photoelectric conversion efficiency, it is necessary to develop new nanomaterials with larger contact area and strong stability under an excitation source to improve the electron transfer rate.
Metal organic frameworks (MOFs) are composed of metal ions/clusters and bridged ligands through coordination bonds with greater porosity and specific surface area (Liu et al., 2020; Wu et al., 2019; Zhao et al., 2018). Ligands in MOFs can absorb light energy, and photoelectrons can be further injected into the metal oxygen cluster through the charge transfer from the ligand to the cluster to generate electron-hole pairs, showing extraordinary semiconductor performance (Liu et al., 2018a; Li et al., 2019). However, the MOFs usually have disadvantages such as disordered orientation and low conductivity, which are not conducive to electron transfer (Cai et al., 2017; Deng et al., 2018). Therefore, there is an urgent need to develop new MOFs-based materials with high orientation and uniform interface structure to improve the photoactive performance.
Recently, MOFs-based materials have been widely reported for constructing of PEC biosensors. Liu's team established a PEC biosensor platform using PCN-224/rGO as the photoactive material for p-arsanilic acid detection (Peng et al., 2019). Yang's group constructed PEC sensor based on Au-NPs@Zn-MOF material to detect squamous cell carcinoma antigen (Wei et al., 2019). Actually, how to enhance the photoelectric conversion efficiency of MOFs is still the key problem for its development in the field of PEC bioanalysis. Three-dimensional (3D) nanomaterials can promote electron transfer and improve electrical conductivity due to their large specific surface area, uniform interface structure and high orientation (Guo et al., 2019; Kong et al., 2020b). More importantly, the heterostructure formed by combining 3D MOFs with other semiconductor materials shows better PEC performance due to band matching (Li et al., 2018a; Song et al., 2019; Zhan et al., 2013). For example, some metal oxides (TiO2, ZnO, Fe3O4, Cu2O) show excellent semiconductor performance in PEC applications (Li et al., 2018a; Wang et al., 2008; Zhang et al., 2018a). Therefore, we reasonably believe that the successful preparation of 3D metal oxide@MOFs nanoarray structures can effectively improve the photoelectric conversion efficiency.
In this paper, the Cu(OH)2 nanowire arrays (NWAs) grown on copper mesh (CM) by in-situ oxidation reaction are prepared and further used as a precursor to prepare Cu2O@Cu-MOF/CM nanocomposites successfully (Cao et al., 2019). The as-prepared 3D nanoarray materials have large specific surface area and excellent stability, which is favorable to improve the performance of PEC. On this basis, we propose a novel PEC aptasensor for ultrasensitive detection of vascular endothelial growth factor 165 (VEGF165; Scheme 1), considering that the expression of VEGF165 in vascular tissues plays a prominent role in normal and pathological angiogenesis (Da et al., 2018; Fu et al., 2020). Specifically, the captured DNA (S1) is modified on the surface of the Cu2O@Cu-MOF/CM electrode by Cu–S bond (Pakiari and Jamshidi, 2010). In the exonuclease III (Exo III)-assisted recycling strategy (Scheme 1A), the VEGF165 is introduced to obtain a large amount of single-stranded DNA (S2) which can be complementary hybridized with S1. In the presence of padlock probe, which can be hybridized with the tail sequence of S2, and the presence of T4 DNA ligase, phi29 polymerase and deoxyribonucleoside 5′-triphosphate (dNTPs) mixture, the rolling circle amplification (RCA) reaction is triggered. The resulting large amount of duplicate G-rich DNA sequences can specifically bind with hemin to form stable G-quadruplex structures with the function of DNAzyme. The DNAzyme can catalyze the bioprecipitation reaction of 4-chloro-1-naphthol (4-CN) to reduce the PEC signal, thus can be employed for the detection of VEGF165 (Scheme 1B). The successfully construction of the aptasensor not only provides a new path for sensitive determination of targets, but also extends the application of MOFs in the field of PEC sensing.
Section snippets
In-situ growth of Cu2O@Cu-MOF NBAs on copper mesh
CM treatment: ultrasound with dilute hydrochloric acid for 1 min, followed by sonicating with deionized water 2–3 times. Firstly, 3.2 g of NaOH and 0.91 g of (NH4)2S2O8 were dissolved in 20 mL of water, respectively. Subsequently, the (NH4)2S2O8 solution was poured into the NaOH solution, and the treated CM was put into the mixed solution for 20 min. The CM was taken out and washed with water and ethanol to obtain a dark blue Cu(OH)2 precursor.
Cu2O@Cu-MOF/CM were prepared by the above Cu(OH)2
Morphology characterization of the Cu2O@Cu-MOF nanobelt arrays
The morphology and structure of Cu(OH)2 nanoarrays and Cu2O@Cu-MOF composites were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figs. S1A and S1B, the Cu(OH)2 nanowire array is evenly distributed on the substrate surface. In addition, SEM image of Cu2O@Cu-MOF/CM synthesized with Cu(OH)2 NWAs as precursors shows a good alignment of the nanobelt arrays (Fig. 1A). TEM images (Fig. 1B) show that the nanobelts are successfully synthesized
Conclusion
In this work, using Cu(OH)2 as a precursor, a novel 3D nanobelt array structure Cu2O@Cu-MOF/CM have been successfully prepared by in-situ growth method. The material characterization experiments suggested that the synthesized Cu2O@Cu-MOF/CM NBAs have excellent PEC properties. Taking the above nanoarray material as the PEC electrode and combining with RCA and enzymatic biocatalytic precipitation strategies, an ultrasensitive PEC aptasensor for VEGF165 was constructed. Furthermore, the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21775089), Outstanding Youth Foundation of Shandong Province (ZR2017JL010), Taishan Scholars Program of Shandong Province (tsqn201909106), and the Key Research and Development Program of Jining City (2018ZDGH032).
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S bond and further oxidized by copper ions (Cu2+) to form L-Cystine (L-CySSyC). Then, the L-CySSyC serves as the electron acceptor to scavenge the photogenerated electron of Cu-MOF@Cu2O, resulting in the increase of photocurrent response. The linear concentration range for L-CySH detection is 0.5–500.0 μM, and the detection limit is 0.32 μM. Furthermore, this sensor also has been obtained satisfactory results for L-CySH detection in real urine samples.