Microfluidic-Integrated Chip Resonators for Electron Spin Sensing in Submicromolar, Submicroliter Solutions

Published

October 15, 2024

Author(s)

Nandita Abhyankar, Megan Catterton, Gregory A. Cooksey, Veronika Szalai

Abstract

We use planar inverse anapole (PIA) microresonators to report the first experimental demonstration of continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy of sub-microliter volumes of sub-micromolar solutions of nitroxide radicals, with a demonstrated detection sensitivity of (330 ± 70) nmol L 1 and a concentration sensitivity limit of (920 ± 200) nmol L 1/mT√Hz ((92 ± 20) nmol L 1/G√Hz) for a signal-to-noise ratio (SNR) of 1. Compared to previous reports, our microresonator design improves the volume sensitivity by 100-fold and the concentration sensitivity by 10-fold for CW EPR measurements of undeuterated nitroxide spin labels, which are ubiquitously used to track protein environment and peptide mobility. To demonstrate ease of use while maintaining the smallest volume that can be dispensed with a micropipettor, we report two microfluidic device designs. The first microfluidic enables permanent placement of 100 nL of sample on the microresonator surface. The second design requires a larger total sample volume of approximately 300 nL to 500 nL but enables repeatable sample placement and removal from the microresonator surface. Both designs deposit a volume in the range of approximately 30 nL to 100 nL on the active surface of the microresonator. We provide experimental demonstration that the upper limit of volume sensitivity for the reported PIA microresonator is approximately 30 nL. These two novel microfluidic designs minimize dead volume, are fabricated via readily accessible techniques, and are integrable with a broad range of planar sensors for lab-on-a-chip devices. Together, these developments represent an advance not only in the sensitivity of EPR but also in the design of microfluidics that truly reduce the total required sample volume to a sub-microliter level. Lastly, we also demonstrate the utility of PIA microresonators for pulse EPR spectroscopy through measurements on polymethylmethacrylate (PMMA) films doped with 1, 3-bisdiphenylene-2-phenylallyl (BDPA): benzene complex.

Citation

Analytical Chemistry

Pub Type

Journals

Download Paper

https://doi.org/10.1021/acs.analchem.4c00464

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Citation

Abhyankar, N. , Catterton, M. , Cooksey, G. and Szalai, V. (2024), Microfluidic-Integrated Chip Resonators for Electron Spin Sensing in Submicromolar, Submicroliter Solutions, Analytical Chemistry, [online], https://doi.org/10.1021/acs.analchem.4c00464, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=956594 (Accessed December 21, 2024)