Sensitive and selective amplified detection of silver ion based on NEase-aided target recycling

Abstract

G-quadruplex–hemin complexes are DNAzyme peroxidases that efficiently catalyze H₂O₂-mediated reactions, such as the oxidation of ABTS²⁻ (2,2′-azinobis(3-ethylbenzothiozoline)-6-sulfonate) by H₂O₂. On the other hand, nicking endonucleases are a special family of restriction endonucleases that can recognize a specific sequence of a double-stranded DNA (dsDNA) and cut one strand of this dsDNA. This function has been used to develop different nicking enzyme based amplified detection platforms for biosensing of different analytes. On the basis of this principle, a highly sensitive and selective Ag⁺-detection strategy was developed in this work. The strategy mainly consists of DNA1, DNA2, and DNA3. DNA1 includes two G-riched DNAzyme segments, the recognition sequence and cleavage site for nicking endonuclease (NEase), and the hybridization part for DNA2. DNA2 is designed to be partly complementary to both ends of DNA1. DNA3 is designed to be complementary to DNA1 with one mismatched cytosine–cytosine. The hybridization of DNA1 and DNA2 results in DNA1 forming a closed structure. Upon addition of Ag⁺, DNA1 can be cleaved into pieces by this NEase, which leads to the denaturation of the DNA1/DNA2/DNA3 complex. Therefore, segments of DNA1 folds into G-quadruplexs, which is able to effectively catalyze the H₂O₂-mediated oxidation of ABTS²⁻ in the presence of hemin, producing the free-radical ion ABTS˙⁻, which has a maximal absorption at 418 nm. The method allows simple detection of Ag⁺ with a detection limit of 0.01 nM. As a strong Ag⁺-binder, this Ag⁺-sensing system was also developed as a Cys-sensing system.