Stimuli-responsive hydrogel patches for resolving pH with magnetic particle spectrometry (MPS)

Magnetic particle spectrometry (MPS) enables the characterization of magnetic nanoparticles (MNPs) in terms of their nonlinear magnetic behavior, which also depends on the particles’ hydrodynamic mobility and, consequently, on the local environment. Therefore, MPS can be used to examine particle-matrix interactions. In this work, the pH value of a solution is resolved using MPS. The underlying principle is based on a pH-responsive magnetic hydrogel patch, which exhibits reversible volume alteration in response to a change in the surrounding pH value. This swelling behavior affects the particle-matrix interactions of embedded MNPs, resulting in a modified magnetic signal. A correlation was observed in which the hydrogel swelling increases with rising pH. In MPS measurements, the amplitudes of the spectra were found to correlate with the extent of swelling and, consequently, with the pH value. As a consequence, the measured MPS spectra can be assigned a specific pH value.