In rapid-scan EPR the magnetic field or frequency is repeatedly scanned through the spectrum at rates that are much faster than in conventional continuous wave EPR. The signal is directly-detected with a mixer at the source frequency. Rapid-scan EPR is particularly advantageous when the scan rate through resonance is fast relative to electron spin relaxation rates. In such scans, there may be oscillations on the trailing edge of the spectrum. These oscillations can be removed by mathematical deconvolution to recover the slow-scan absorption spectrum. In cases of inhomogeneous broadening, the oscillations may interfere destructively to the extent that they are not visible. The deconvolution can be used even when it is not required, so spectra can be obtained in which some portions of the spectrum are in the rapid-scan regime and some are not. The technology developed for rapid-scan EPR can be applied generally so long as spectra are obtained in the linear response region. The detection of the full spectrum in each scan, the ability to use higher microwave power without saturation, and the noise filtering inherent in coherent averaging results in substantial improvement in signal-to-noise relative to conventional continuous wave spectroscopy, which is particularly advantageous for low-frequency EPR imaging. This overview describes the principles of rapid-scan EPR and the hardware used to generate the spectra. Examples are provided of its application to imaging of nitroxide radicals, diradicals, and spin-trapped radicals at a Larmor frequency of ca. 250 MHz.
Measurement of thiol-disulfide redox status is crucial for characterization of tumor physiology. The electron paramagnetic resonance (EPR) spectra of disulfide-linked dinitroxides are readily distinguished from those of the corresponding monoradicals that are formed by cleavage of the disulfide linkage by free thiols. EPR spectra can thus be used to monitor the rate of cleavage and the thiol redox status. EPR spectra of 1H,14N- and 2H,15N-disulfide dinitroxides and the corresponding monoradicals resulting from cleavage by glutathione have been characterized at 250 MHz, 1.04 GHz, and 9 GHz and imaged by rapid-scan EPR at 250 MHz.
Integrated spectrometers provide the possibility of compact, low-cost portable spectroscopy sensing, which is the critical component of the lab-on-a-chip system. However, using existing on-chip designs is challenging to realize a high-resolution miniature spectrometer over a broad wavelength range. Here, we demonstrate an on-chip time-sampling narrowband filter array spectrometer that enables simultaneous acquisition of high-resolution spectra via optical Fabry−Peŕot cavities and a large spectral range with tunable free spectral range free filters. Two spectrometers consisting of five and seven filter cells with a compact footprint are fabricated and experimentally characterized, demonstrating a resolution of <0.43 and <0.51 nm and a spectral range of 73.2 and 102.7 nm, respectively. Unknown broad bandwidth input signal spectra can be successfully retrieved. Integrating more filter cells with thermal isolation trenches can dramatically boost the operational spectral range. Such spectrometers may open up new pathways toward spectral analytical applications.
Mid‐infrared (MIR) waveguide‐integrated photodetector is essential for various applications in the fields of sensing and optical communications. However, it is challenging to integrate traditional MIR photoactive materials such as HgCdTe or III−V compounds with complementary metal‐oxide‐semiconductor (CMOS)‐compatible silicon platform due to the lattice mismatch. Tellurium (Te), a novel van der Waals (vdW) material with a narrow bandgap, high carrier mobility, and great air stability, is a promising candidate for high‐performance MIR detection. Here, high‐quality Te nanosheets are synthesized using a hydrothermal method and their carrier dynamics are characterized by transient reflection spectrum. The effect of mobility anisotropy on response speed is investigated intuitively by a free space phototransistor. Combining the strong evanescent wave of a waveguide architecture with the synergy effect between the carrier collection path and the highest mobility crystal orientation in Te, an integrated Te photodetector with enhanced light–matter interaction and reduced carrier transit time is achieved. For the first time, the MIR waveguide‐integrated Te photodetector with a responsivity of 2.3 A W−1 and a bandwidth of 4 GHz at 2015 nm is realized, which is the highest speed MIR photodetector based on narrow bandgap vdW materials to date.
A method is presented for the determination of formaldehyde in air sample extracts containing aqueous sodium hydrogensulfite. Utilizing the unique properties of its hydrogensulfite complex, formaldehyde is separated from other sample components by ion-exclusion and ion-exchange chromatography, then selectively detected by amperometry at a silver electrode. Optimum sensitivity of detection was found to occur at +0.10 V versus a silver wire reference electrode using a strongly basic background electrolyte. Using ribose as an internal standard, a linear response (r2 > 0.99) was observed for aqueous formaldehyde concentrations in the range 0.02-10.0 mg 1-I; detection limits of < 1 ng for formaldehyde were obtained using a 50 pl sample loop. The short-term reproducibility was better than 5 % (as RSD). Analysis of laboratory air by collection in impingers containing aqueous NaHS03 yielded results consistent with previous literature values.
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