Continuous-flow electron spin resonance microfluidics device with sub-nanoliter sample volume
Year: 2025
Type: article
Abstract: This paper presents a new continuous-flow electron spin resonance (ESR) microfluidic device designed to perform both continuous-wave (CW) and pulsed ESR measurements on sub-nanoliter liquid samples. The system integrates a planar surface microresonator (ParPar type) operating at ~9.4 GHz with a precision-fabricated quartz microfluidic chip, enabling spatial confinement of the sample within the resonator’s magnetic field hotspot while minimizing dielectric losses. The effective sample volume is ~0.06 nL, and the device supports standard microfluidic connectors, facilitating continuous flow experiments. Using a 1 mM aqueous solution of deuterated Finland trityl (dFT) radical, CW ESR measurements yielded a peak signal-to-noise ratio (SNR) of ~83 with 10 scans while the resonator quality factor was Q ~15–20. This corresponds to a spin sensitivity of ~4.5×10⁸ spins at SNR = 1 with an 80 s averaging time. Pulsed ESR measurements, performed with 0.1 W microwave power and 10 ns π pulses, achieved an SNR of ~33 within just 1 s of averaging, corresponding to a spin sensitivity of ~1.27×10⁸ spins. The Rabi frequency of ~50 MHz implies a microwave conversion efficiency of ~56 G/√W, consistent with simulated values. This device represents a significant step toward ESR-based detection of individual, slowly flowing cells—analogous to flow cytometry, but with magnetic resonance contrast. With future improvements such as higher operating frequencies, cryogenic integration, or optimized resonator geometries, the system is expected to enable practical ESR measurements at the single-cell level. Its simplicity, compatibility with commercial ESR systems, and promising sensitivity make it well suited for advancing microscale and dynamic ESR spectroscopy.
Language: English
Subfield: Biophysics
Domain: Life Sciences
Open Access status: gold
Estimated APC paid: $1,790
OpenAlex ID: W4412429361