A wireless chemical sensor using ferroparticles embedded hydrogel

In this paper, we present a wireless chemical sensor using ferroparticles-embedded hydrogel (ferrogel). Chemically triggered reversible swelling and de-swelling of ferrogel immobilized on top of a planar inductor results in a modulated inductance through two competing mechanisms; ferroparticle density and magnetic field passing through the volume-changing ferrogel. Overall, the effect of the magnetic flux passing through the ferrogel dominates, thus increasing the inductance as the ferrogel swells. The resulting inductance change can be remotely monitored through wireless tracking of the coil self-resonant frequency. A pH sensor based on poly (methacrylic acid-co-acrylamide) hydrogel was fabricated and tested. The test was carried out in a laser machine acrylic chamber with buffers of varying pH values flowing through while monitoring the resonant frequency with a network analyzer. The sensor shows sensitivity of 200 kHz/pH between pH values of 6 to 5 and 105 kHz/pH from 5 to 4 with approximately 30 minutes of response time.