G-quadruplex DNA-based colorimetric biosensor for the ultrasensitive visual detection of strontium ions using MnO₂...

Strontium-90 (⁹⁰Sr) is a major radioactive component that has attracted great attention, but its detection remains challenging since there are no specific energy rays indicative of its presence. Herein, a biosensor that is capable of rapidly detecting Sr²⁺ ions is demonstrated. Simple colorimetric method for sensitive detection of Sr²⁺ with the help of single-stranded DNA was developed by preparing MnO₂ nanorods as oxidase mimic catalysis 3,3',5,5'-tetramethylbenzidine (TMB). Under weakly acidic conditions, MnO₂ exhibited a strong oxidase-mimicking activity to oxidize colorless TMB into blue oxidation products (oxTMB) with discernible absorbance signals. Nevertheless, the introduction of a guanine-rich DNA aptamer inhibited MnO₂-mediated TMB oxidation and reduced oxTMB formation, resulting in blue fading and diminished absorbance. Upon the addition of strontium ions to the system, the aptamers formed a stable G-quadruplex structure with strontium ions, thereby restoring the oxidase-mimicking activity of MnO₂. Under the best experimental conditions, the absorbance exhibits a linear relationship with the Sr²⁺ concentration within the range 0.01-200 μM, with a limit of detection of 0.0028 µM. When the concentration of Sr²⁺ from 10^-8 to 10^-6 mol L^-1, a distinct color change gradient could be observed in paper-based sensor. We successfully applied this approach to determine Sr²⁺ in natural water samples, obtaining recoveries ranging from 97.6 to 103% with a relative standard deviation of less than 5%. By providing technical solutions for detection, our work contributed to the effective monitoring of transportation of radioactive Sr in the environment.