G-quadruplex-assisted enzyme strand recycling for amplified label-free fluorescent detection of UO22+

DNAzyme that can catalytically cleave of substrate DNA has shown to be attractive for amplified detection in biosensing events. During the catalytic process, the recycling of enzyme strand of DNAzyme is critically important. In this work, a G-quadruplex-assisted enzyme strand recycling strategy was developed for amplified label-free fluorescent detection of uranyl ion (UO₂²⁺). The DNAzyme was activated by the target UO₂²⁺ and further cleaved the substrate strand that contained the G-quadruplex sequence. The following formation of G-quadruplex helps the separation between the enzyme strand and the cleaved substrate strand, thus improving the recycle use of the enzyme strand. Such strategy allowed lablel-free detection of 0.2–200 ng/mL UO₂²⁺via SYBR green I (SG)-based fluorescence. The detection limit (3δ) is as low as 0.06 ng/mL (about 0.2 nmol/L), comparable to those obtained by ICP-MS and labeled DNAzyme. It was applied for detection of UO₂²⁺ in spiked environmental water samples with recoveries in the range of 96%–103%. This biosensor, with the advantages of simplicity and high sensitivity, is an appealing tool for fast detection of UO₂²⁺ in environmental water samples.