Adsorption of Phosphate and Polyphosphate on Nanoceria Probed by DNA Oligonucleotides

Phosphate-containing molecules exist in many forms in biology and the environment, and their interaction with metal oxides is an important aspect of their chemistry and biochemistry. In this work, phosphates with different degrees of polymerization (e.g., orthophosphate, pyrophosphate (PPi), sodium triphosphate (STPP), sodium trimetaphosphate (STMP), and polyphosphate with 25 phosphate units) and phosphates with one or two capping groups were studied. CeO₂ nanoparticles (nanoceria) were used as a model metal oxide. DNA is also a polyphosphate, and a fluorescently labeled DNA oligonucleotide was mixed with nanoceria. These phosphate species were individually added to displace the adsorbed DNA. Longer phosphate chains were more efficient when each molecule was used at the same molar concentration, whereas PPi and STPP were most efficient at the same total phosphorus atom concentration. By capping the phosphate with organic groups, the affinity was significantly decreased. Isothermal titration calorimetry (ITC) was also performed to quantitatively measure thermodynamic parameters. Although STMP was very slow at displacing DNA, it was still adsorbed very strongly by nanoceria from ITC, indicating kinetic effects likely due to its ring structure. This observation allowed us to use the DNA as a probe to study the hydrolysis of STMP to form STPP. In summary, this study provides a systematic understanding of phosphate species interacting with metal oxides, and interestingly, it demonstrates an analytical application as well.