Spinel cobalt ferrite nanoparticles for sensing phosphate ions in aqueous media and biological samples.

Phosphate ions perform a variety of functions in metabolic processes and are essential for all living organisms. The determination of the concentration of phosphate ions is useful in the clinical diagnostics of various diseases, as an inadequate phosphate level could lead to many health problems. In the search for a more cost-effective method of fast monitoring, we investigated the use of cobalt ferrite nanoparticles (CoFeNPs) in the selective recognition of phosphate ions dissolved in aqueous media and more complex samples, such as human blood serum. We prepared these NPs by a chemical co-precipitation route and submitted them to annealing at 600˚C for 1 hour. The successful formation of the NPs was confirmed by FTIR spectroscopy, X-ray diffraction, Transmission Electron Microscopy and hysteresis loop measurements. The NPs presented exhibit a ferrimagnetic behavior, a spinel-type crystalline structure and hexagonal shape in the nanoscale range. We demonstrated that CoFeNPs containing immobilized fluorescent-labelled single-chain DNA (ssDNA*) probes can be applied for the fast-selective detection of phosphate ions dissolved in a liquid medium. We have explored the subjacent mechanism that phosphate groups can displace ssDNA* probes attached to the nanoparticles, causing a perceptible change in the fluorescent signal of the supernatant liquid. This detection method has been tested for the sensing of phosphate ions present both in aqueous solutions and in biological samples, with excellent selectivity and a low limit of detection (~ 1.75nM).