Highly Sensitive and Selective Colorimetric Sensors for Uranyl (UO22+): Development and Comparison of Labeled and Label-Free DNAzyme-Gold Nanoparticle Systems

摘要

Colorimetric uranium sensors based on uranyl (UO2²⁺) specific DNAzyme and gold nanoparticles (AuNP) have been developed and demonstrated using both labeled and label-free methods. In the labeled method, a uranyl-specific DNAzyme was attached to AuNP, forming purple aggregates. The presence of uranyl induced disassembly of the DNAzyme functionalized AuNP aggregates, resulting in red individual AuNPs. Once assembled, such a “turn-on” sensor is highly stable and worked in a single step at room temperature and had detection limit of 50 nM after 30 min. of reaction time. The label-free method, on the other hand, utilizes the different adsorption properties of single stranded and double stranded DNA on AuNPs, which affects the stability of AuNPs in the presence of NaCl. The presence of uranyl resulted in cleavage of substrate by DNAzyme, releasing a single stranded DNA that can be adsorbed on AuNPs and protect them from aggregation. Taking advantage of this phenomenon, a “turn-off” sensor was developed, which is easy to control through reaction quenching, and has 1 nM detection limit after 6 min. of reaction at room temperature. Both sensors have excellent selectivity over other metal ions and have detection limits below the maximum contamination level of 130 nM for UO2²⁺ in drinking water defined by the US Environmental Protection Agency (EPA). The study represents the first direct systematic comparison of these two types of sensor methods using the same DNAzyme and AuNPs, making it possible to reveal advantages, disadvantages, versatility, and limitations and potential applications of each method. The results obtained not only allow practical sensing application for uranyl, but also serve as a guide for choosing different method for designing colorimetric sensors for other targets.