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  • br Materials and methods br Results and discussion br Conclu


    Materials and methods
    Results and discussion
    Acknowledgements This work was supported financially by the LIAONING S&T Project (No. 20180550107) and Basic Scientific Research Funds of DLNU (0918110143).
    Introduction Since 1970s, numerous artificial mimetic enzymes have been extensive applied in biosensing areas owing to its outstanding properties and advantages, compared with natural enzymes [[1], [2], [3]]. Among them, hemin (Fe (III)-protoporphyrin IX), the catalytic center element of many proteins and enzymes, can catalyze a variety of oxidation reactions like peroxidases and has become potential candidate of enzyme mimetics in biosensing application [4,5]. However, direct application of hemin as an oxidation catalyst is a significant challenge due to its tendency to aggregate into inactive dimers in aqueous solution, which causes passivation of its catalytic activity [[6], [7], [8]].Recent years, various materials, such as oligonucleotides [9], nanomaterials [10], peptide [11], metal-organic framework materials [12], were employed to combine with hemin to overcome its low solubility in aqueous and realize superior peroxidase catalytic capacity. G-quadruplexes (G4) are classical nucleic Cephalexin structures with stacked G-tetrads assembled by Hoogsteen hydrogen-bonding [13]. The G4 structure associates with hemin to form peroxidase mimicking G4/hemin DNAzyme which is one of the most significant artificial enzymes and powerful catalytic toolkit in biosensing [14,15]. With the outstanding catalytic performance, G4 DNAzyme was frequently embedded into DNA self-assembly circuit as a signal transducer to develop various biosensors and molecular machines [[16], [17], [18], [19]]. For example, Xu [20] designed an aptamer sensing approach based on catalytic hairpin assembly (CHA) and split G4/hemin DNAzyme; Chen [21] reported a G4-based hybridization chain reaction (HCR) fluorescent biosensor for small molecules; Willner I [22] designed a hemin/G4-crosslinked catalytic hydrogel to mimic the function of horseradish peroxidase and exhibit switchable ability. As a common signal transducer, G4/hemin DNAzymes possess advantages of easy synthesis, facile manipulation and amenability to design of allosteric control in various DNA self-assembly circuit. Thus, these G4/hemin DNAzyme-based DNA assembly system showed high catalytic capacity for protein enzyme-free signal amplification, presenting a powerful catalytic toolkit in biosensing, biomaterials and bio-molecular devices [23]. Nevertheless, there are still several drawbacks of these systems to limit their further and more comprehensive application, (і) the combination of G4 sequences with DNA self-assembly structures would increase the complexity of sequence design. (іі) additional time and reaction condition were needed to further assemble activated G4/hemin DNAzyme. And the free hemin in the system would bring high background in some catalysis reaction [24,25]. (ііі) G4/hemin DNAzymes are susceptible to oxidative inactivation during peroxidation reactions because of the damage of guanine with the H2O2 [26,27]. The covalent linkage of hemin with nucleic acids is one of the alternative strategies for catalytic capacity improvement of hemin, which has been proved to significantly improve hemin’s solubility and recover peroxidase activity [28]. In previous work, we have investigated the catalytic properties and analytical performance of DNA-grafted hemin (DNA-hemin) and G4/hemin as artificial enzyme mimetics for fluorescent biosensing. The results revealed that DNA-hemin had lower background, higher catalytic rate and better oxidation tolerance as compared with G4/hemin [29]. This work provided direct evidences that DNA-hemin presents an outstanding artificial enzyme for protein enzyme-free transducer and would become a potential tool for extensive biosensing application. Herein, we further established a dynamic DNA self-assembly activated hemin-mimetic enzymes system by embedding DNA-hemin into an entropy-driven DNA-assembly for fluorescent biosensing. The entropy-driven dynamic DNA-assembly is one of the toehold-mediated isothermal strand displacement reactions, which is driven forward by increases in the entropy of the system and possesses the advantages of rapid and effective amplification performance, high thermostability, flexible sequence design and decreased reversible conversion [30,31]. Hence, we utilize this strategy as an efficient dynamic DNA self-assembly to activate the DNA-hemin mimetic enzymes for enzyme-free signal amplification and biosensing. Hemin dimmers with suppressed catalytic activity were firstly formed due to the complementary of DNA strands. In the presence of target DNA, the dynamic DNA-assembly amplification reaction was initiated, along with the separation of hemin dimmers into monomers to modulate the DNA-hemin enzyme activity. Once the catalytic activity of hemin-mimetic enzymes was activated, the substrate tyramine can be catalyzed into fluorescent dityramine, producing a dramatically enhanced fluorescence signal. In this system, the DNA-hemin not only was utilized as the module for dynamic DNA-assembly but also as the tunable mimetic enzyme, providing a simple, fast and enzyme-free signal amplification strategy. The proposed strategy was applied to detect the specific pathogenic gene of Group B Streptococci (GenBank Accession no.1012782) and small molecule cocaine, proving its simplicity, practicability for versatile fluorescent biosensing.