A compact fiber-integrated optofluidic platform for highly specific microRNA Förster resonance energy transfer detection

Enzyme-free and copper-free strategy based on cyclic click chemical-triggered hairpin stacking circuit for accurate detection of circulating microRNAs

Accurate detection of circulating microRNAs (miRNAs) plays a vital role in the diagnosis of various diseases. However, enzyme-free amplification detection remains challenging. Here, we report an enzyme-free fluorescence resonance energy transfer assay termed "3C-TASK" (cyclic click chemical-triggered hairpin stacking kit) for the detection of circulating miRNA. In this strategy, the miRNA could initiate copper-free click chemical ligation reactions and the ligated products then trigger another hairpin stacking circuit. The first signal amplification was achieved through the recycling of the target miRNA in the click chemical ligation circuit, and the second signal amplification was realized through the recycling of ligated probes in a hairpin stacking circuit driven by thermodynamics. The two-step chain reaction event triggered by miRNAs was quantified by the fluorescence signal value so that accurate detection of target miRNA could be achieved. The 3C-TASK was easily controlled because no enzyme was involved in the entire procedure. Although simple, this strategy showed sensitivity with a detection limit of 8.63 pM and specificity for distinguishing miRNA sequences with single-base variations. In addition, the applicability of this method in complex biological samples was verified by detecting target miRNA in diluted plasma samples. Hence, our method achieved sensitive and specific detection of miRNA and may offer a new perspective for the broader application of enzyme-free chemical reaction and DNA circuits in biosensing.

Fabrication of bifunctional G-quadruplex-hemin DNAzymes for colorimetric detection of apurinic/apyrimidinic endonuclease 1 and microRNA-21

G-quadruplex-based complexes have been widely used in various analytical methods due to their outstanding capabilities of generating colorimetric, fluorescent or electrochemical signals. However, since loop sequences in traditional G-quadruplex structures are quite short, it is difficult to establish biosensors solely using G-quadruplex-based complexes. Herein, we attempted to lengthen the loop sequences of G-quadruplex structures and found that G-quadruplex-hemin DNAzymes (G-DNAzymes) with long loops (even 30 nucleotides) maintain high peroxidase activity. In addition, the peroxidase activity is not affected by the hybridization of the long loop with its complementary counterpart. Consequently, G-DNAzyme can be endowed with an additional function by taking the long loop as a recognition element, which may facilitate the construction of diverse colorimetric biosensors. Furthermore, by designing an apurinic/apyrimidinic site or a complementary sequence of microRNA-21 (miRNA-21) in long loops, bifunctional G-DNAzymes can be split in the presence of apurinic/apyrimidinic endonuclease 1 (APE1) or miRNA-21, decreasing their peroxidase activities. Accordingly, APE1 and miRNA-21 are quantified using 3,3',5,5'-tetramethylbenzidine as a chromophore. Using the G-DNAzyme, APE1 can be detected in a linear range from 2.5 to 22.5 U mL^-1 with a LOD of 1.8 U mL^-1. It is to be noted that benefitting from duplex-specific nuclease-induced signal amplification, the linear range of the miRNA-21 biosensor is broadened to 5 orders of magnitude, while the limit of detection is as low as 73 fM. This work demonstrates that G-DNAzymes with long loops can both generate signals and recognize targets, providing an alternative strategy to design G-quadruplex-based analytical methods.