This article is not directly related to DNP-NMR spectroscopy, but shows the potential of miniaturizing the required instrumentation for EPR. Very impressive technology.
Schlecker, Benedikt, Alexander Hoffmann, Anh Chu, Maurits Ortmanns, Klaus Lips, and Jens Anders. “Towards Low-Cost, High-Sensitivity Point-of-Care Diagnostics Using VCO-Based ESR-on-a-Chip Detectors.” IEEE Sensors Journal 19, no. 20 (October 15, 2019): 8995–9003.
In this paper, we present a new architecture for VCO-based ESR detection for a future use in portable, point-ofcare ESR spectrometers. The proposed architecture is centered around an ASIC containing a VCO-based ESR detector with two distinct tuning ports with largely different VCO gains to enable wide frequency sweeps and small-signal frequency modulations while keeping the requirements on the digital-to-analog converter driving the ports manageable. Additionally, the proposed ASIC features a second VCO for an on-chip frequency downconversion via mixing. To allow for a precise derivation of the operating frequency from an external reference as it is required for quantitative ESR experiments, the two on-chip VCOs are embedded into an offset phase-locked loop. The proposed architecture is verified with ESR experiments on commonly used ESR standard samples (DPPH and BDPA). In these experiments, a spin sensitivity of 1.7*10^9 spins/(G sqrt(Hz)) has been achieved at B0 = 450mT, which is comparable to the state of the art, using a permanent magnet and low-cost signal processing on an FPGA. The presented proof-of-concept experiments clearly demonstrate the potential of the proposed VCO-based ESR detection system for future point-of-care applications.