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

Abstract

Colorimetric uranium sensors based on uranyl (UO 2 2+ ) 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 UO 2 2+ 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.

Surface Science of DNA Adsorption onto Citrate-Capped Gold Nanoparticles

“…Therefore, AuNPs have been used as a probe for DNA secondary structure and related analytical applications. [25][26][27][28][29][30][31][32] DNA adsorption has also been used to control the growth of AuNPs, 33,34 tune catalytic activities of AuNPs, 35,36 and improve the specificity of polymerase chain reactions (PCR). 37 47 Finally, applications related to drug delivery can also be envisioned.…”

Effects of Polyethylene Glycol on DNA Adsorption and Hybridization on Gold Nanoparticles and Graphene Oxide

“…23,25 For the other approach, ss-DNA is tightly adsorbed onto both nanomaterials, [18][19][20][21] where AuNPs adsorb DNA bases via strong chemical bonding interactions while GO adsorbs DNA via aromatic stacking and hydrophobic interactions. 17 Such adsorption has also been used in biosensor development, [18][19][20][21][26][27][28][29][30][31][32][33] PCR optimization, 34 and materials synthesis. 35,36 A recent research direction is to use these hybrid materials for cellular and in vivo systems.…”

Gold Nanoparticles in Biomedical Applications

“…84 Traditionally, uranium in the environment is detected using complex biophysical techniques such as fluorimetry, ICP-MS and atomic absorption spectroscopy. However, these methods are difficult to be used on-site.…”