An efficient strategy for unmodified nucleotide-mediated dispersion of magnetic nanoparticles, leading to a highly sensitive MRI-based mercury ion assay.

It is highly attractive to develop a detection system that is not only sensitive and selective but also simple, rapid, practical and cost-effective in operation. Here, we report an interesting observation that single-stranded oligonucleotide (ssDNA) can adsorb efficiently on carboxylic acid-functionalized magnetic nanoparticles (CAMNPs) and stabilize the nanoparticles against aggregation in weakly acidic solution. The adsorbing rate closely correlates with the pH of the solution, the temperature and the sequence length of ssDNA. On the basis of this observation, we have designed a highly sensitive, non-sandwich type magnetic relaxation-based detection system for quantitatively probing mercury ion. The assay is independent of the sample's optical properties, requires no covalent modification of the ssDNA or the CAMNPs surfaces, and can be used for high-throughput analysis. By varying the concentration of CAMNPs, four orders of dynamic response range and a detection limit of 0.3 nM for Hg(2+) are achieved. Moreover, we developed a multi-sample assay to detect Hg(2+) in real environmental samples with high sensitivity, selectivity and efficiency.