Electron paramagnetic resonance (EPR) is an accurate and efficient technique to probe unpaired electrons in many applications across materials science, chemistry, and biology. Dynamic processes are investigated using EPR; however, these applications are limited by the use of resonator-based spectrometers such that the entire process must be confined to the resonator. The EPR-on-a-chip (EPRoC) device circumvents this limitation by integrating the entire EPR spectrometer into a single microchip. In this approach, the coil of a voltage-controlled oscillator (VCO) is used as the microwave source and detector simultaneously, operating under a protective coating such that the device may be placed in the sample solution directly. Additionally, improvements in sensitivity via rapid scan EPR (RS-EPR/RS-EPRoC) increase the accessible applications where SNR per measurement time is the fundamental limit. The herein reported device combines a dipstick EPRoC sensor with the enhanced sensitivity of frequency-swept frequency modulated rapid scan to measure triarylmethyl (trityl, Ox071) oxygen-sensitive probes dissolved in aqueous solutions. EPR spectra of Ox071 solutions were recorded using the RS-EPRoC sensor while varying the oxygen concentration of the solution between normal atmosphere and after purging the solution with nitrogen gas. We demonstrate that EPRoC may be employed to monitor dissolved oxygen in fluid solution in an online fashion.
