Susceptibility of Stimuli-Responsive Hydrogels With Embedded Magnetic Microparticles for Inductively Wireless Chemical Sensing

Stimuli-responsive hydrogels with embedded magnetic microparticles (i.e., ferrogel) can be integrated with a planar LC circuit to remotely monitor environmental changes via electromagnetic resonant frequency measurements. To investigate the sensing mechanism from the point of view of magnetic susceptibility, we conducted magnetic characterizations of ferrogel swelling/deswelling in response to stimuli (i.e., pH), an indication of the extent of physical restriction of the particles. Within 4 hours, a
$2 \times 2$
mm
²
ferrogel can achieve an equilibrium swelling, reaching 10.8 times of its original weight when at pH 6, compared to 2.8 times when at pH 4. The magnetic remanence of the ferrogel is 3 emu/g, revealing a thermally blocked Néel relaxation; moreover, by comparing the complex susceptibility of the ferrogel between its equilibrium swelling and complete dry, the gel is found to have a Brown characteristic frequency at 2.2 Hz. When the gel switches from pH 4 to pH 6, the magnetic susceptibility of the gel increases from
$2.99\,\,\times10$
^-7
emu/oe to
$3.27\,\,\times10$
^-7
emu/oe, achieving a sensitivity of
$3.5\,\,\times10$
^-9
(emu/oe)/pH per swelling ratio between pH 4 and pH 6. These data verified that the ferrogel works as a magnetic core to tune the magnetic capacity (i.e., inductance) and thus resonance frequency of its incorporated sensing system through its volumetric response to external chemical stimuli.