Continuous-flow electron spin resonance microfluidics device with sub-nanoliter sample volume

Abstract

This paper presents a novel continuous-flow electron spin resonance (ESR) microfluidic device designed for 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 microwave magnetic field hotspot while minimizing dielectric losses. The effective sample volume is ~0.06 nL, and the device supports standard microfluidic connectors, facilitating both continuous and stopped-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 for a 100-point spectrum acquired over 80 s, with a resonator quality factor of Q ~15-20. This corresponds to a spin sensitivity of ~1.04 × 10⁹ spins/√Hz/G. Pulsed ESR measurements, performed with 0.1 W microwave power and 10 ns π pulses, achieved an SNR of ~47 with 1 s of averaging, corresponding to a spin sensitivity of ~7.8 × 10⁸ spins/√Hz. A Rabi frequency of ~50 MHz was measured, indicating a microwave conversion efficiency of ~56 G/√W. Both the pulsed spin sensitivity and Rabi frequency are 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 enhancements 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.

Keywords: ESR; Micro resonators; Microfluidics; Single cell measurements.

Fig. 1.

The ESR microfluidic probehead. (a) Photograph of the fully assembled probehead. (b) 3D rendering of the complete probehead assembly. (c) Same as (b), but with the magnetic field modulation coils removed to reveal the internal structure. (d) Close-up view of the central region, showing the microfluidic assembly in transparent mode. (e) Further zoom-in with a side view of the microfluidic assembly. The microwave signal is delivered from the SMA connector to the microstrip feedline via a custom-made coaxial-to-microstrip adapter. Microwave energy is coupled from the microstrip line to the resonator (and vice versa) positioned above it. This coupling is adjusted by changing the position of the end of the feedline relative to the resonator using a manual linear nonmagnetic stage (Elliot Scientific model MDE261A-LM). Optimal coupling is typically achieved when the end of the microstrip line is aligned just below the center of the resonator.

Fig. 2.. Surface microresonator used in the microfluidic ESR device.

The resonator is fabricated by depositing 1 μm copper layer on 200 μm thick intrinsic silicon substrate [4,10]. Resonator diameter 4.63 mm. (a) Simulated microwave electric field (E-field) distribution at a height of 10 μm above the resonator surface for an input power of 1 W. (b) Simulated microwave magnetic field (H-field) distribution under the same conditions. (c) Magnified view of (a). (d) Magnified view of (b), showing that the H-field at the center of the resonator reaches approximately 15,000 A/m, corresponding to ~94.2 G/√W in the rotating frame, using the relation B₁=H₁ × 2π × 10⁻⁷ × 10⁴ (in Gauss). Simulations are carried out using CST Studio Suite finite element software (Dassault systems).

Fig. 3.. Quartz microfluidics chip.

(a) Schematic drawing of the chip structure (dimensions in mm). (b) Microscope image of the chip glued to the surface microresonator, with the microfluidic channel precisely aligned over the ParPar resonator’s “bridge” region—characterized by low microwave electric field and high microwave magnetic field. (c) Magnified view of the central region in (b), showing the resonator center at higher optical resolution.

Fig. 4.

The reflection coefficient of the resonator as a function of frequency without any sample (blue) and when introducing the trityl water solution (red).

Fig. 5.. CW ESR measurements of a 1 mM dFT aqueous solution.

(a) Photograph of the experimental setup showing the microfluidic ESR probe positioned inside the Bruker magnet, connected via coaxial (for microwave) and twinaxial (for field modulation) cables. (b) CW ESR spectra acquired at various magnetic field modulation amplitudes using 8 mW incident microwave power. Signal-to-noise ratio (SNR) and peak-to-peak linewidth (lw) values are indicated in the legend. The inset (lower left) shows the ESR signal intensity as a function of microwave power at a fixed modulation amplitude of 47 mG. Experimental parameters for all measurements: modulation frequency = 100 kHz; conversion time = 41 ms; integration time constant = 82 ms; 10 signal averages per acquisition. The inset (lower right) presents a reference measurement of the same radical sample placed in a 0.9 mm id glass capillary tube, performed under identical conditions of microwave power, conversion time, integration time, and field modulation, using a standard Bruker X-band cavity. The measured SNR in this case was approximately 5.4 × 10⁴, for a sample volume of 9.5 × 10⁹ μm³.

Fig. 6.. Pulsed ESR Hahn echo measurements of a 1 mM dFT aqueous solution.

(a) Frequency-domain representation of the signal (blue) and noise (red, acquired 100 G off-resonance). Data were recorded using a 20 MHz off-resonance detection offset. Acquisition parameters: excitation frequency = 9.1 GHz; π/2 and π pulse durations = 10 ns; interpulse delay = 200 ns; microwave power = 0.1 W. The signal was averaged over 1 s at a repetition rate of 200,000 shots per second. An 8-step phase cycling scheme (+/− and CYCLOPS) was employed to suppress artifacts. (inset) The same experiment, carried out with 2 W of power and 2 ns-long pulses. (b) Rabi oscillation curve showing the ESR signal as a function of the duration of the first pulse in the Hahn echo sequence, with the second pulse fixed at 10 ns for 0.1 W of microwave excitation power.