Rapid-scan electron paramagnetic resonance using an EPR-on-a-Chip sensor

Abstract

Abstract. Electron paramagnetic resonance (EPR) spectroscopy is the method of choice to investigate and quantify paramagnetic species in many scientific fields, including materials science and the life sciences. Common EPR spectrometers use electromagnets and microwave (MW) resonators, limiting their application to dedicated lab environments. Here, novel aspects of voltage-controlled oscillator (VCO)-based EPR-on-a-Chip (EPRoC) detectors are discussed, which have recently gained interest in the EPR community. More specifically, it is demonstrated that with a VCO-based EPRoC detector, the amplitude-sensitive mode of detection can be used to perform very fast rapid-scan EPR experiments with a comparatively simple experimental setup to improve sensitivity compared to the continuous-wave regime. In place of a MW resonator, VCO-based EPRoC detectors use an array of injection-locked VCOs, each incorporating a miniaturized planar coil as a combined microwave source and detector. A striking advantage of the VCO-based approach is the possibility of replacing the conventionally used magnetic field sweeps with frequency sweeps with very high agility and near-constant sensitivity. Here, proof-of-concept rapid-scan EPR (RS-EPRoC) experiments are performed by sweeping the frequency of the EPRoC VCO array with up to 400 THz s−1, corresponding to a field sweep rate of 14 kT s−1. The resulting time-domain RS-EPRoC signals of a micrometer-scale BDPA sample can be transformed into the corresponding absorption EPR signals with high precision. Considering currently available technology, the frequency sweep range may be extended to 320 MHz, indicating that RS-EPRoC shows great promise for future sensitivity enhancements in the rapid-scan regime.

Monitoring the state of charge of vanadium redox flow batteries with an EPR-on-a-Chip dipstick sensor

“…Considering the different measurement times to acquire a spectrum using the two different instruments (resonator-based EPR: 60 s, EPRoC: 4593 s) it becomes clear that the SNR per second is between 20 and 400 for resonator-based EPR and about 1 for EPRoC, respectively. The sample volume used in the resonator-based EPR spectrometer measurements is about 50 μl, which is about 5 × 10 4 larger than the active volume of the EPRoC 52 (∼1 nl). Therefore, it can be concluded that the EPRoC shows an approximate 100–470 times ( cf.…”

“…With the EPRoC both dispersion-like 35,36,45 (frequencymodulated, FM) and absorption-like 46,47 (amplitude-modulated, AM) signals may be detected, while with resonator-based EPR usually only a field-modulated absorption EPR signal is measured. In this work, we only analyse the FM spectra as they benefit from the VCO-array concept, which will be discussed in Section 3 below.…”

Towards single-cell pulsed EPR using VCO-based EPR-on-a-chip detectors

“…So far almost all VCO-based EPR experiments presented in the literature have been conducted in the CW mode of operation, the only exceptions being the recent rapid scan experiments presented in [11,17,18]. In order to understand the interaction of the VCO and the electron spin ensemble during and after a pulsed excitation, the steady-state model discussed in the previous section is not sufficient.…”

X-band single chip integrated pulsed electron spin resonance microsystem

“…A promising approach for high sensitivity and low cost ESR spectroscopy on nanoliter and subnanoliter samples is the integration of the sensitivity relevant part of the spectrometer into a single chip having an area of less than 1 mm 2 [6]. Several single chip integrated ESR microsystems have been reported in the last 15 years, with operating frequencies from 8 to 260 GHz and operating temperatures from 1.4 to 300 K. Previously reported single chip integrated ESR microsystem are based on continuous wave (CW) [6][7][8][9][10][11][12][13][14][15], rapid scan [16], and pulsed excitations [17,18]. The reported sensitivity of these sensors are in range from 10 7 to 10 12 spins/Hz 1/2 depending on the operating frequency, temperature, and sensitive volume.…”