Water-soluble poly(N-isopropylacrylamide) nanoparticles grafted with trivalent lanthanide complexes as highly sensitive ratiometric nanothermometers

Nanothermometry became a very active research field, particularly in the biomedical area, given the interrelationship between abnormal temperatures and pathological conditions, as well as the development of heating-mediated therapies. In order to provide an accurate assessment of temperature in the submicron range, several thermosensitive optical materials have been developed. In this work, a new water-soluble, optical ratiometric nanothermometer, composed of pNIPAM nanoparticles grafted to lanthanide complexes, was designed and synthesized. A chelator monomer derivative of dipicolinic acid was successfully synthesized and incorporated into the pNIPAM network through free radical emulsion polymerization. The further coordination of trivalent lanthanide ions (Ln(III) = Tb and/or Eu) to the obtained nanoparticles yielded luminescent thermosensitive polymers with a tunable temperature-responsive range, depending on the adopted synthesis conditions. A synergy between the pNIPAM thermosensitive response and the susceptibility of the lanthanide ion emission to be suppressed through multiphonon relaxation was explored, giving rise to optical temperature probes with reversible response, high relative thermal sensitivity, low temperature uncertainties, and excellent repeatability in aqueous solution. The obtained results indicate that the system has great potential for temperature probing in aqueous media, showing one of the highest relative thermal sensitivities so far described in the literature for water-soluble, lanthanide-based systems with ratiometric response within the biological temperature range.