A Symmetrically Split G-quadruplex DNAzymes Biosensor Based on Magnetic Nanoparticles for the Rapid Detection of Hg2+

A biosensor composed of magnetic nanoparticles and symmetrically split G-quadruplex DNAzymes was proposed for the rapid and sensitive detection of Hg2+. It was mainly based on the formation of a special thymine-Hg2+-thymine (T-Hg-T) structure by T-riched nucleic acid sequences and Hg2+, and then G-quadruplex DNAzymes catalyzed the oxidation of H2O2 and 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulphonate) (ABTS), so that the trace amount of Hg2+ in water can be successfully detected by colorimetry. The assembly of the biosensor was characterized by using of UV-Visible spectroscopy, CD spectroscopy and fluorescence microscopy imaging techniques respectively. The magnetic nanoparticles coated with T-riched short oligonucleotide (DNA1) via streptavidin-biodin interaction were employed as the capture elements. The long oligonucleotides (DNA2) consisting of the T-riched sequence in the middle and two G-riched fragments at the both ends were used as the sensing elements. The procedure of the protocol was divided into three steps. First: the magnetic nanoparticles covered with DNA1 (MNPs/DNA1) mixed with DNA2 were added into a water sample to capture the target Hg2+ by forming the special T-Hg-T structure. Incubating 2 h at 37 degrees C, two G-riched fragments at the ends of DNA2 were close to each other assembling the symmetrically G-quadruplex DNAzymes in the presence of hemin. Then, the supernatant was discarded by magnetic separation, and the residues were washed several times to resuspend in the mixture solution containing definite amount of H2O2 and ABTS. The catalytic signal of absorbance of ABTS(+) was recorded by UV-Visible spectrophotometer. The utilization of MNPs can greatly reduce the background signal, and the employment of symmetrically split DNAzymes made the design of the experiment more flexible and selectable. The biosensor was sensitive, and the calibration curve was identified in the range from 0.8 nmol/L to 20 nmol/L with a detection limit of 0.3 nmol/L. And the biosensor can detect Hg2+ without interference in the presence of a large number of competing ions. This sensor was simple, inexpensive, and renewable. Satisfactory values between 95.3% and 104.4% were obtained for the recovery experiments, and it can meet the requirement of detection of trace Hg2+ in natural water especially in drinking water.