Visual detection of ultra-trace levels of uranyl ions using magnetic bead-based DNAzyme recognition in combination with rolling circle amplification

The authors describe a colorimetric method for the determination of ultra-trace levels of uranyl ion (UO22+) in beverage and milk. It employs (a) DNAzyme-functionalized magnetic beads (MBs) for UO22+ recognition, (b) horseradish peroxidase (HRP)-assisted catalytic oxidation of 3,3', 5,5'-tetramethylbenzidine (TMB) for signal generation, and (c) rolling circle amplification (RCA) for sensitivity improvement. The employment of DNAzyme-functionalized MBs facilitates the separation and collection of analyte from sample matrix. This results in more convenient operation, better selectivity and more strong resistibility to sample matrix. The RCA strategy realizes one UO22+-to-massive HRP effect, which strongly improves the sensitivity. The method has outstanding advantages including high sensitivity, convenient operation, strong resistibility to complex matrix, and good selectivity. It can be used to detect as low as 20.0 pg.mL(-1) (74 pmol.L-1) of UO22+ in milk and beverage by bare eye observation. Even lower concentrations (1.0 pg.mL(-1) or 3.7 pmol.L-1) of UO22+ can be detected with the method via UV-visible spectrometry at 650 nm. The method was applied to analyze spiked samples and gave recoveries of 98 to 105% and RSDs of +/- 7% (n = 6). The visual detection limit is much lower than the maximum allowable level of UO22+ in drinking water as defined by the Environmental Protection Agency of USA. This indicated that the method meets the requirement of simple, rapid and on-site detection of ultra-trace UO22+ in milk and beverage.