Switchable DNA tweezer and G-quadruplex nanostructures for ultrasensitive voltammetric determination of the K-ras gene fragment.

Voltammetric detection of the K-ras gene fragment was accomplished through the combined application of (a) a switchable DNA nanostructure, (b) the use of hairpin probe and exonuclease III (Exo III)-assisted signal amplification, (c) a split G-quadruplex, and (d) by exploiting the redox activity of DNAzyme. Three assistant oligonucleotides were designed to construct a DNA tweezer on a gold electrode. It is in "open state" in the absence of K-ras DNA. Then, a hairpin probe was introduced, whose stem-loop structure can be opened through hybridization with the K-ras DNA. Exo III is added which hydrolyzes the complementary region of the hairpin sequence to release a single-stranded rest fragment. The ssDNA hybridizes with the DNA tweezer on the electrode which thereby is switched to the "closed state". This leads to the formation of G-quadruplex due to the shortened distance of the split G-quadruplex-forming sequences in the tweezer. The voltammetric signal of the G-quadruplex-hemin complex, with a peak near -0.3 V vs. Ag/AgCl, is used as the signal output. Under the optimal conditions, the current response in differential pulse voltammetry (DPV) increases linearly with the concentration of K-ras DNA in the range of 0.01-1000 pM, and the detection limit is 2.4 fM. The assay can clearly discriminate K-ras DNA from a single-base mutation. The method has excellent selectivity and was applied to the determination of K-ras DNA in (spiked) serum samples. Graphical abstractSchematic representation of a method for the determination of the K-ras gene fragment through a combination of switchable DNA tweezer, split G-quadruplex, and exonuclease III (ExoIII)-assisted target recycling signal amplification.