A new study published in the journal Atmospheric Chemistry and Physics has taken a major step toward solving this puzzle. An international team of researchers led by Fobang Liu of Xi’an Jiaotong University and the Max Planck Institute for Chemistry, together with corresponding authors Kai Wang of China Agricultural University and Haijie Tong of Helmholtz-Zentrum Hereon, developed and applied a novel molecular classification framework called MCR-VK to link the molecular composition of organic aerosols directly to their capacity for generating reactive oxygen species. The work analyzed fine particulate matter collected from four Chinese megacities, a German semi-urban site, and a remote boreal forest, alongside laboratory-generated secondary organic aerosol from multiple precursors, providing an unprecedented molecular-level view of what makes some particles more oxidatively dangerous than others.

The central challenge the researchers faced is that atmospheric organic aerosol is chemically bewildering. Thousands of distinct molecular species coexist in any given sample, yet only between 10 and 30 percent of the organic aerosol mass can typically be identified as specific compounds using existing analytical techniques. Advances in ultrahigh-resolution mass spectrometry, particularly Fourier transform ion cyclotron resonance and Orbitrap instruments combined with soft ionization methods, have dramatically expanded scientists’ ability to assign molecular formulas to these complex mixtures. However, traditional molecular metrics such as double bond equivalents and aromaticity indices, while useful for characterizing structural features, do not fully capture the functional group chemistry that actually governs redox reactivity and ROS formation.

To overcome this limitation, the team built upon their recently developed maximum carbonyl ratio, or MCR, a metric that estimates the maximum number of carbonyl groups a molecule could contain based solely on its molecular formula. By incorporating MCR into the traditional Van Krevelen diagram, which plots the hydrogen-to-carbon ratio against the oxygen-to-carbon ratio, the researchers established the MCR-VK framework. This approach classifies individual organic compounds into two chemically meaningful categories: oxidized organic compounds, or OOC, with MCR values below 0.9, and unsaturated organic compounds, or UOC, with MCR values at or above 0.9. Higher MCR values indicate greater carbonyl functionality and electrophilic character, while lower MCR values reflect the dominance of highly oxidized, non-carbonyl species such as organic peroxides, both of which may contribute to oxidative stress in the respiratory system.

The researchers applied this framework to fine particulate matter samples collected during six field campaigns spanning dramatically different environments. These included the SMEAR II boreal forest station in Hyytiälä, Finland; an urban site in Mainz, Germany; and four major Chinese cities: Beijing, Xi’an, Shanghai, and Guangzhou. In parallel, they generated secondary organic aerosol in the laboratory from five precursors: isoprene, alpha-pinene, beta-pinene, limonene, and naphthalene, using both a quartz flow tube with ozone-driven ozonolysis and a potential aerosol mass chamber with hydroxyl radical photooxidation. Molecular composition was analyzed using an ultra-high-performance liquid chromatography system coupled to an electrospray ionization Orbitrap mass spectrometer, enabling the assignment of thousands of molecular formulas across all samples.

The results reveal striking compositional contrasts across environments. Isoprene-derived secondary organic aerosol exhibited the highest relative fraction of oxidized organic compounds at 98 percent, consistent with its dominance of highly oxygenated, low-MCR products such as 2-methyltetrols. Alpha-pinene and beta-pinene secondary organic aerosol were similarly dominated by oxidized compounds at 95 and 94 percent respectively, reflecting oxygen-rich ozonolysis products including pinonic acid, pinic acid, nopinone, and MBTCA. The boreal forest samples from Hyytiälä also showed a high oxidized fraction of 94 percent, comparable to monoterpene-derived laboratory aerosol, reflecting the dominant contribution of biogenic precursors at that site. In sharp contrast, naphthalene-derived secondary organic aerosol was characterized by a substantially higher fraction of unsaturated organic compounds at 64 percent, indicating strong contributions from aromatic, ring-retaining oxidation products such as quinones.

Urban fine particulate matter displayed a wide range of unsaturated compound fractions between 15 and 57 percent, reflecting the diversity of emission sources and atmospheric processing pathways in cities. The Chinese megacities of Beijing, Xi’an, Guangzhou, and Shanghai exhibited particularly high unsaturated fractions between 45 and 57 percent, indicating substantial contributions from aromatic and combustion-related organics. Mainz showed a considerably lower unsaturated fraction of just 15 percent, suggesting weaker aromatic emissions and a relatively greater biogenic influence compared with the Chinese megacities. Critically, the researchers validated their classification by demonstrating strong quantitative agreement with independent molecular indicators: the relative fraction of oxidized organic compounds correlated well with the fraction of highly oxygenated molecules, while the unsaturated fraction correlated strongly with the relative abundance of aromatic compounds.

When the team turned to reactive oxygen species measurements, a fascinating picture emerged. Hydrogen peroxide yields in aqueous particle extracts were quantified using a fluorometric probe, and radical yields were measured using electron paramagnetic resonance spectroscopy combined with a spin-trapping method. Across all sites, hydrogen peroxide dominated the ROS budget, accounting for approximately 78 to 97 percent of total ROS. Urban megacities such as Beijing, Guangzhou, Shanghai, and Xi’an exhibited substantially higher volume-normalized ROS yields, ranging from 57.8 to 260.5 picomoles per cubic meter of air, compared to just 13.0 picomoles per cubic meter at the boreal forest site. This spatial pattern tracked closely with fine particulate mass concentrations, indicating that environments with higher particle loadings contain greater amounts of ROS-generating substances per unit air volume.

However, when the researchers normalized ROS yields to particle mass rather than air volume, a completely different story unfolded. Mass-normalized ROS yields showed no correlation with particle loading whatsoever, ranging from 0.73 picomoles per microgram in Shanghai to 3.32 picomoles per microgram in Mainz, with the boreal forest site at Hyytiälä also exhibiting relatively high values of 2.49 picomoles per microgram despite its very low particle mass concentration. This divergence clearly demonstrated that intrinsic particle composition, rather than particle mass, governs ROS formation on a per-mass basis. Even more intriguingly, radical yields displayed a significant negative correlation with fine particulate mass concentration, meaning that although polluted environments generate higher total radical concentrations per unit air volume, individual particles in those environments are intrinsically less efficient at producing radicals than particles in clean, biogenic environments.

The most powerful result of the study is the establishment of the ratio between oxidized and unsaturated organic compound fractions as a robust predictor of ROS formation. Across urban sites, mass-normalized total ROS yields exhibited a strong positive exponential relationship with this ratio, with an R-squared value of 0.94, while hydrogen peroxide yields showed an even stronger correlation with an R-squared of 0.98. Laboratory-generated secondary organic aerosol further confirmed these composition-reactivity relationships, with total ROS yields following a strong positive linear relationship across all five precursors. Isoprene-derived aerosol, rich in oxidized compounds, exhibited exceptionally high ROS yields of 403 picomoles per microgram, while naphthalene-derived aerosol, dominated by unsaturated aromatic species, produced only 58 picomoles per microgram. These findings reveal that the molecular balance between oxygen-rich and unsaturated organic compounds provides a mechanistic basis for both the magnitude and spatial variability of ROS formation in atmospheric aerosols, offering a powerful new tool for assessing the health-relevant oxidative activity of fine particulate matter across diverse environments.

Subject of Research: Molecular composition of organic aerosols and reactive oxygen species formation in atmospheric fine particulate matter

Article Title: Linking molecular composition of organic aerosols to reactive oxygen species formation using an MCR-VK framework

Article References: Liu, F., Zhang, Y., Yue, S., Keskinen, H.-M. K., Petäjä, T., Kulmala, M., Fu, P., Hoffmann, T., Wang, K., & Tong, H. (2026). Linking molecular composition of organic aerosols to reactive oxygen species formation using an MCR-VK framework. Atmospheric Chemistry and Physics, 26(20), 14655-14669. https://doi.org/10.5194/acp-26-14655-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-14655-2026

Keywords: organic aerosol, reactive oxygen species, fine particulate matter, MCR-VK framework, secondary organic aerosol, hydrogen peroxide, electron paramagnetic resonance, ultrahigh-resolution mass spectrometry, oxidative stress, air quality, biogenic emissions, urban pollution

The appeal of SIFTER lies in what it can capture that its popular rival, PELDOR or DEER, cannot easily deliver. When two rigid spin labels are attached to a biomolecule, the dipolar coupling between their electron spins encodes not only the distance between them but also the mutual orientation of the labels, and therefore of the molecule itself. PELDOR experiments with rigid labels must be repeated many times, with the detection frequency shifted step by step across the spectrum, to assemble this orientation information. SIFTER, by contrast, is inherently a single-frequency technique that encodes orientation-dependent information directly in the time domain, so a single two-dimensional experiment can record the entire correlated data set at once. The catch has always been bandwidth: nitroxide radicals, the most common spin labels, have spectra far too broad for conventional rectangular microwave pulses to excite uniformly, and incomplete excitation degrades the signal and introduces unwanted artifacts.

The Frankfurt team, led by Paul Trenkler and Thomas Prisner, attacked this problem with chirped pulses, frequency-swept microwave pulses whose amplitude follows a smooth WURST profile, a shape borrowed originally from nuclear magnetic resonance. Fast arbitrary waveform generators capable of producing such pulses have been commercially available for over a decade, and chirp pulses are already routine as inversion pulses in PELDOR. Yet using them to generate broadband, phase-sensitive transverse magnetization, which is what SIFTER demands, has remained rare. The reason is that chirped pulses behave very differently from their rectangular cousins. Because spins at different frequency offsets are flipped into the transverse plane at different moments during the sweep, a chirped excitation pulse imprints a parabolic phase roll across the spectrum. In NMR this roll can be corrected after the fact, but EPR spectra are so broad that the resulting destructive interference destroys the signal outright unless the pulse sequence itself is designed to refocus it.

Using numerical Bloch vector simulations of a thousand independent spins, the researchers systematically dissected the phase behavior of chirped echoes. They distinguished several effects: the parabolic phase roll, an offset-dependent dynamic phase shift, and a constant phase shift at the center of the chirp, which they found depends nearly linearly on a key pulse parameter called the critical adiabaticity. Crucially, the slope of this dependence scales with the time bandwidth product of the pulse, the product of its duration and sweep width. Under realistic conditions, where the microwave field strength varies across the sample because of resonator inhomogeneity, these distributed phase shifts cause the echo intensity to collapse as the pulse amplitude is increased past an optimum. The team showed experimentally, using nitroxide radicals immobilized in trehalose, that shorter pulses and sequences with an even number of refocusing pulses dramatically reduce this signal loss, because pairs of pulses with matched properties compensate each other’s phase errors.

These insights fed directly into a calibration protocol for the two-dimensional SIFTER experiment at X-band frequencies. The procedure begins with verifying that every component of the spectrometer, from the arbitrary waveform generator through the traveling-wave tube amplifier to the detection electronics, operates linearly over the target bandwidth. The researchers found, for example, that their waveform generators could not maintain full output amplitude at the fastest modulation rates, forcing them to reduce the input amplitude and compensate with preamplifiers. They then corrected the shaped pulses with the transfer function measured from the resonator profile and swept the amplitudes of the refocusing pulses, observing strong oscillations in the SIFTER echo intensity. Their simulations explained these oscillations elegantly: the echo phase at the moment of the third pulse must align precisely for the sequence to work, and this condition is met only at specific values of the critical adiabaticity.

With the optimized pulses in hand, the team performed 2D-SIFTER measurements on a 20-base-pair RNA duplex carrying two rigid cytidine-derived spin labels, a construct previously characterized by orientation-selective PELDOR. The comparison was striking. The SIFTER experiment, including a background measurement, took only 24 hours, while the equivalent PELDOR data set had required five days of acquisition with fresh optimization at every frequency offset. The dipolar time traces from the two methods matched closely, and the SIFTER data showed considerably larger modulation depth, translating to better signal-to-noise ratios. Because the frequency resolution of SIFTER is limited only by the intrinsic linewidth of the nitroxide spectrum rather than by the excitation profile of the pulses, the two-dimensional data set resolved orientation selection far more finely than the serial PELDOR approach.

The two-dimensional Fourier transform of the SIFTER data yields a spectrum correlating the EPR frequency with the dipolar frequency, a rich map in which the dipolar oscillation frequency visibly disperses from the center to the edges of the nitroxide spectrum. When the researchers compared this experimental map with simulations built from molecular dynamics conformers of the RNA duplex, with the spin labels explicitly modeled, the agreement was excellent. Averaging over the full spectrum, meanwhile, produced a single orientation-unselected trace whose distance distribution agreed with conventional PELDOR analysis. In other words, a single 2D-SIFTER experiment delivers both the accurate distance distribution that rigid labels enable and the complete orientation information that normally demands an entire campaign of PELDOR measurements.

The study did not stop at optimizing the established sequence. The researchers derived new pulse length ratios by solving the phase-refocusing conditions computationally, even releasing a MATLAB script that generalizes the problem and automatically finds solutions for arbitrary pulse sequences. One novel four-pulse variant, with a pulse length ratio of 2:3:1:4, refocuses the dipolar coupling without frequency dispersion and may suit measurements of shorter distances, a regime where the standard sequence struggles. They also introduced a six-pulse SIFTER sequence incorporating two additional refocusing pulses in the spirit of Carr-Purcell dynamic decoupling, which suppresses transverse relaxation effects and, in their first tests, outperformed the classical four-pulse experiment while producing a flatter background signal.

For a technique that has lingered in the shadow of PELDOR for two decades, this work amounts to a compelling case for rehabilitation. By marrying modern broadband pulse technology with a rigorous understanding of chirped-pulse spin dynamics, the Frankfurt group has shown that 2D-SIFTER can deliver richer structural information in a fraction of the measurement time, at least for samples with rigid spin labels and sufficiently long relaxation times at cryogenic temperatures. Open questions remain, including a thorough characterization of the background signals in the new sequences and validation at shorter distances. But if the limits of the technique are, as the authors suggest, far from fully explored, structural biologists may soon have a genuinely two-dimensional window onto the shapes and motions of the molecules that underpin life.

Subject of Research: Optimized chirped microwave pulses for two-dimensional SIFTER pulsed EPR distance and orientation measurements

Article Title: Optimized shaped pulses for a 2D single-frequency technique for refocusing (SIFTER)

Article References: Trenkler, P. A. S., Endeward, B., Sigurdsson, S. T., & Prisner, T. F. (2025). Optimized shaped pulses for a 2D single-frequency technique for refocusing (SIFTER). Magnetic Resonance, 6(2), 281-315. https://doi.org/10.5194/mr-6-281-2025

Image Credits: AI Generated

DOI: 10.5194/mr-6-281-2025

Keywords: EPR spectroscopy, SIFTER, PELDOR, chirp pulses, spin labels, nitroxide radicals, RNA duplex, pulsed dipolar spectroscopy, arbitrary waveform generator, WURST pulses, orientation selection, biomolecular structure

The two compounds at the heart of the study are known as Cu6 and Cu8. Both belong to a family of spherical cages assembled from palladium ions and organic ligands, structures that self-organize into nanometer-scale spheres reminiscent of soccer balls. In Cu6, six copper(II) ions occupy positions corresponding to the vertices of an octahedron, while in Cu8 eight copper ions sit at what should be the corners of a cube. The parent cages, Pd12M6L(DMAP)24 and Pd12M8L(Pro)24, are attractive to chemists because they allow different metal ions to be introduced in a defined spatial arrangement, a property with obvious appeal for catalysis, where several metal centers acting in concert can enable reaction pathways unavailable to isolated atoms.

To read out the geometry of these metal clusters, the team turned to pulsed electron paramagnetic resonance, specifically the double electron-electron resonance experiment known as DEER, sometimes called PELDOR. In a DEER measurement, a sequence of microwave pulses interrogates a population of electron spins: observer pulses monitor the echo of one set of spins while a pump pulse flips a second set at a variable time. When the pump pulse flips a spin near an observed spin, the dipolar coupling between the two slightly shifts the echo frequency. Sweeping the pump pulse position across the time trace therefore produces an oscillating signal whose frequency encodes the distance between the spins. The technique is a workhorse for measuring nanometer-scale separations, routinely mapping distances between two nitroxide labels or two metal centers on proteins and synthetic molecules with sub-nanometer precision.

What makes the copper cages a compelling test case is that DEER theory was developed with isolated spin pairs in mind. When a molecule contains six or eight coupled spins, the observed signal becomes a superposition of many pairwise contributions, and additional phenomena appear. Multiple spins can be excited simultaneously, sum and difference frequencies can contaminate the distance distribution, and the modulation depth of the signal, the fraction by which the echo is modulated, no longer scales in the simple way it does for two spins. In principle, that complexity is an opportunity: if the multi-spin signature can be decoded, DEER might report not just distances but the number of spins in a cluster and their arrangement, turning a ruler into a three-dimensional structural probe.

The measurements did reveal clear evidence of multi-spin interaction in both cages. For Cu6, the analysis of the distance distribution allowed the team to establish an octahedral arrangement of the copper ions, consistent with the crystallographic model. For Cu8, two distinct distances were observed, and these were found to be consistent with two structural models that had been proposed for the compound, one of which places the eight copper ions at the vertices of a cube. In an ideal cube, there are two characteristic separations between vertices, the edge length and the face diagonal, so detecting two dominant distances is exactly what such a geometry would predict. The authors also measured a model compound containing just two copper centers, intended to serve as a calibration standard for relating modulation depth to the number of spins contributing to the signal.

However, the path from these observations to firm conclusions proved contentious. The preprint underwent open peer review on the Copernicus platform, and all three referees declined to support publication of the revised manuscript, which remains a discussion-stage preprint rather than an accepted paper. Their reports, published alongside the authors’ response, offer an unusually transparent window into how demanding multi-spin DEER is in practice, and they highlight technical issues that anyone attempting similar measurements will need to confront.

A central criticism concerned data quality. The referees noted that the phase memory time, Tm, which governs how long the dipolar oscillation can be followed and therefore how precisely distances can be resolved, is short for these copper compounds. Referee 1 pointed out that the measurements were performed at 20 kelvin, whereas the low-temperature maximum of Tm for such systems would be expected between 6 and 15 kelvin, and that because the achievable evolution time depends roughly exponentially on Tm, measuring at too high a temperature can severely degrade the data. The referees also emphasized that DEER experiments are conventionally run in deuterated solvents to extend Tm, and argued that the gain in signal-to-noise ratio from a deuterated solvent such as perdeuterated butyronitrile would have been substantial. With signal-to-noise ratios likely below ten to one, as Referee 2 estimated, quantifying complex distance distributions with multiple populations of different widths becomes hazardous, raising the risk of overinterpreting noise as structure.

The referees also dissected the physics of orientation selection, an effect the authors had struggled to assess. Copper(II) ions have a strongly anisotropic g-tensor, meaning their resonance frequency depends on how the molecule is oriented relative to the magnetic field, and the microwave pulses used in DEER excite only a subset of orientations. Referee 2 argued that in an octahedron, opposite vertices are related by inversion symmetry and therefore share identical g-tensor orientations, so if one copper of a pair is excited, its partner is too, which can inflate the modulation depth in ways that mimic a multi-spin signature. This could rationalize an observed one-to-one ratio of distance peak intensities where a four-to-one ratio was expected. The same referee argued that the two-copper model compound used for calibration, which carries distributed exchange coupling, was not a robust standard, since conformations with strong coupling fall outside the excitation bandwidth and do not contribute to modulation. In that referee’s assessment, the central conclusion about low effective spin numbers rested on a flawed calibration, while the genuinely strong orientation-selection effects went untested and unanalyzed.

Further technical points sharpened the critique. Referee 1 argued that shorter microwave pulses, perhaps 24 or even 20 nanoseconds instead of the 32 nanoseconds used, should have been achievable with the available hardware and would have widened the excitation bandwidth; that density functional theory calculations, feasible on cutouts of the cages because the spin population is localized, could have pinned down the relative orientations of the g-tensors; and that in ideal geometry the principal axes systems of diagonally opposite copper centers must coincide, so the missing cube-diagonal contribution in Cu8 could not be blamed on orientation selection. Referee 3 agreed that the low signal-to-noise ratio, particularly for Cu8, did not permit reliable conclusions from the fits and that a proposed discussion of structural flexibility in that compound was not convincingly supported. The authors submitted a combined point-by-point response in March 2026, but the revision was not accepted, and the discussion is now closed.

Even in its contested state, the study marks out the frontier. It demonstrates that DEER signals from six- and eight-spin copper clusters contain interpretable information, and that an octahedral arrangement can be recognized from the data, while also showing, through the review record, exactly where the method strains: short phase memory times, narrow excitation bandwidths, orientation selection amplified by molecular symmetry, and the absence of validated calibration standards for modulation-depth analysis all conspire to blur the line between genuine multi-spin effects and artifacts of a two-spin formalism pushed past its comfort zone. For the cages themselves, the motivation remains intact, since defined multi-metal arrangements inside self-assembled spheres are promising platforms for cooperative catalysis, and a spectroscopic method that could verify metal geometry in solution, independent of crystallography, would be a valuable quality control. The lesson of this preprint is that getting there will require colder samples, deuterated solvents, faster pulses, computational support from quantum chemistry, and a more careful theoretical treatment of how orientation correlation and exchange coupling reshape the DEER signal when a molecule carries a whole cage of spins rather than a lone pair.

Subject of Research: Pulsed DEER distance measurements between six and eight copper(II) ions in self-assembled spherical coordination cages

Article Title: Pulsed Electron Paramagnetic Resonance on two Cu(II)-Cage Compounds With Six, Respectively Eight Copper Ions

Article References: Passerini, L., Bobylev, E., de Zwart, F. J., Hintz, H., Godt, A., de Bruin, B., Reek, J., & Huber, M. (2025). Pulsed Electron Paramagnetic Resonance on two Cu(II)-Cage Compounds With Six, Respectively Eight Copper Ions. https://doi.org/10.5194/mr-2025-17

Image Credits: AI Generated

DOI: 10.5194/mr-2025-17

Keywords: electron paramagnetic resonance, DEER, copper(II), self-assembled cages, multi-spin systems, distance measurements, orientation selection, modulation depth, coordination chemistry, magnetic resonance, nanospheres, peer review

To understand why this matters, it helps to look at how conventional EPR spectrometers work. In almost all instruments, a single-mode cavity or dielectric resonator operates in reflection mode: the same microwave port is used both to excite the spins and to detect the signal they emit. The resonator amplifies the microwave magnetic field inside the sample, sharpening both the excitation pulses and the induced signal, and its performance largely determines the sensitivity of the whole instrument. But reflection mode carries an inherent penalty. After each excitation pulse, the resonator rings down, and a significant fraction of the microwave power reflects back toward the receiver. Protection switches must be deployed to shield the sensitive detection electronics from this reflected pulse, and every switch adds insertion loss and noise. In continuous-wave operation, microwave source noise leaking into the detection channel further erodes the signal-to-noise ratio. For decades, instrument builders have traded off these losses against the need to protect their amplifiers.

The bimodal cavity offers an elegant escape from this dilemma. The concept, first explored by James Hyde and coworkers in the late 1960s, exploits two microwave modes with orthogonal magnetic-field polarizations that resonate at the same frequency. The excitation pulse drives one mode, tipping the magnetization along one axis, while the circularly polarized induction signal emitted by the spins is picked up by the orthogonal mode. Because the two channels are physically decoupled, the powerful excitation pulse never travels down the detection path, and the ringing problem largely disappears. Bimodal cavities, bimodal loop-gap resonators and cross-loop resonators have all been built on this principle. What has been missing, however, is a way to combine this transmission-mode architecture with a modern low-noise amplifier at Q-band frequencies, roughly 34 gigahertz, in a package that drops straight into a commercial spectrometer.

The low-noise amplifier, or LNA, is the second half of the Frankfurt team’s strategy. Placing an LNA as the very first element after the resonator means that every downstream component, including circulators, switches and cables, contributes negligibly to the overall noise figure. This approach has gained momentum in recent years, with several groups demonstrating impressive sensitivity gains at X-band and beyond, including cryogenic amplifiers that approach the quantum limit of spin detection. The catch is compatibility: a strong reflected or ring-down pulse can saturate or even destroy an LNA, so most implementations require protection switches whose losses eat into the gain, or else demand invasive modifications to the spectrometer’s microwave bridge. The Frankfurt design sidesteps the problem by exploiting the isolation of the bimodal cavity itself. Because the excitation and detection modes are orthogonal, the LNA connected to the cavity output never sees the full excitation power and does not need a protective switch in front of it.

The new probehead is built around a bimodal cavity based on two polarization-crossed rectangular TE103 modes that share two half-wavelengths, scaled up from Hyde’s original design for Q-band operation. The cavity accepts sample volumes of 20 to 50 microliters, and the finished probe matches the dimensions of Bruker flexline Q-band resonators, fitting directly into an Oxford CF935 helium-flow cryostat for operation between 5 and 300 kelvin. A manual waveguide switch lets the user select reflection or transmission mode. In transmission mode, the signal from the cavity output passes through a uniline that guards against residual reflected power and then into a commercial LNA with a noise figure of 1.9 decibels at room temperature, equivalent to a noise temperature of 160 kelvin. A circulator with 0.13 decibels of insertion loss and 33 decibels of isolation routes the amplified signal back to the spectrometer receiver.

Finite-element simulations of the loaded cavity, performed with CST Suite for a frozen aqueous solution in a quartz tube, predicted an isolation of about 51 decibels between the input and output ports when both modes are tuned to 33.192 gigahertz. Network-analyzer measurements on real samples came close: the experimental decoupling reached approximately 46 decibels, a few decibels shy of the simulation, which the authors attribute to imperfections in the inner surfaces of the fabricated resonator and slight misalignment of the sample tube. The loaded quality factors were deliberately kept low, at 250 for the input mode and 180 for the output mode, to allow broad EPR excitation. The team also found that deliberately detuning the output mode by 134 megahertz increased the isolation by 5 decibels, a quirk they turned into a practical tool: the isolation level can be monitored indirectly from the spectrometer console to fine-tune the output mode against the input mode.

The performance tests delivered striking numbers. Using a BDPA:PS powder sample containing roughly 10^15 spins at room temperature, the bimodal probehead operating in transmission mode with the LNA showed a fourfold improvement in signal-to-noise ratio compared with the same probehead running in reflection mode without the amplifier. Against the commercial Bruker EN 5107D2 probehead available in the laboratory, measured with matched pulse conditions, the bimodal probe still delivered a twofold signal-to-noise advantage, thanks to the combination of the LNA and the isolation of the detection channel from the excitation path. Even when the commercial probe was driven at full microwave power with its optimal short pulses, the bimodal probe retained a twofold edge in peak echo amplitude, and the advantage persisted when integrated echo intensities were compared.

A second test at 80 kelvin, using a 0.1 millimolar solution of the nitroxide radical OXO TEMPO in toluene, pushed the enhancement to a factor of seven. Here the geometry of the bimodal cavity played a decisive role: its 2.8-millimeter sample tube holds about 20 microliters of solution, roughly 3.5 times the 6-microliter volume of the 1.6-millimeter tube required by the commercial probe. The authors therefore decompose the gain into a 3.5-fold contribution from the larger sample volume and an additional twofold contribution from the LNA and channel isolation. They also note a caveat that speaks to the honesty of the comparison: their EN 5107D2 probe is not new and may have a slightly degraded microwave power conversion factor compared with fresh commercial units, so the exact margin over commercial hardware may vary between laboratories.

The design is not yet at its ceiling. The 46-decibel isolation, while sufficient to protect the amplifier, still forces the experimenters to run with reduced microwave attenuation, meaning the full 150 watts of their traveling-wave-tube amplifier cannot yet be applied. The team plans to introduce tuning paddles into the resonator, a modification known from earlier bimodal designs, to push the isolation higher and unlock full-power operation. A cryogenic LNA placed inside the cryostat could cut the noise temperature from 160 kelvin to an estimated 10 to 50 kelvin, although strong magnetic fields can degrade such amplifiers through the Hall effect, requiring careful shielding or orientation, and repeated cooling cycles may shorten the amplifier’s lifetime. Even in its present form, the probehead is immediately attractive for time-resolved EPR methods that do not demand maximum microwave power, including transient EPR and non-adiabatic rapid-scan EPR, and the transmission-mode geometry may help suppress standing waves in experiments with broadband shaped pulses. As a proof of principle, the work demonstrates that two old ideas, crossed microwave modes and preamplification, can be married into a drop-in upgrade that makes every existing Q-band spectrometer quietly, dramatically better at hearing what electrons have to say.

Subject of Research: A bimodal Q-band EPR probehead with a low-noise amplifier that improves pulse electron paramagnetic resonance sensitivity

Article Title: Bimodal Q-band probehead with improved signal-to-noise ratio in pulse electron paramagnetic resonance

Article References: Denysenkov, V., Fedotov, A., Endeward, B., & Prisner, T. F. (2026). Bimodal Q-band probehead with improved signal-to-noise ratio in pulse electron paramagnetic resonance. Magnetic Resonance, 7(1), 21-28. https://doi.org/10.5194/mr-7-21-2026

Image Credits: AI Generated

DOI: 10.5194/mr-7-21-2026

Keywords: electron paramagnetic resonance, EPR spectroscopy, bimodal cavity, low-noise amplifier, Q-band, signal-to-noise ratio, pulse EPR, resonator design, instrumentation, spectrometer sensitivity, spin echo, microwave engineering

The research, led by Unnikrishnan Kannan and corresponding author Saji George at McGill University, together with colleagues at Manipal Academy of Higher Education, the Canadian Light Source, and Christ University, set out to test a fundamental assumption in the field. Toxicity studies of microplastics and nanoplastics have largely depended on primary plastic particles with smooth surfaces, yet the particles to which humans are predominantly exposed in food and environmental matrices are secondary particles, generated by fragmentation of larger plastic items. If those real-world particles behave differently from laboratory standards, then much of the existing hazard literature may need re-examination.

To create realistic test materials, the team mechanically fragmented two of the world’s most common plastics: high-density polyethylene, widely used in containers and packaging, and polyethylene terephthalate, the polymer of beverage bottles. From this process they generated microplastic particles in the 100 to 350 micrometer range and nanoplastic particles below 1000 nanometers. Molecular fingerprinting confirmed a crucial point: the parent polymer identity was retained after fragmentation. In other words, the fragments were still recognizably HDPE or PET. What changed was everything on and just beneath the surface.

Using an array of physicochemical characterization techniques, the researchers found that smaller particles showed greater surface roughness, altered surface charge, reduced crystallinity, chemical heterogeneity, and enhanced surface oxidation. These are not cosmetic differences. Crystallinity reflects how orderly the polymer chains are packed; mechanical grinding shatters that order, leaving disordered, strained, and chemically unsatisfied structures at the newly created surfaces. Those defect-rich surfaces are precisely where reactive chemistry happens. The two polymers diverged in instructive ways: HDPE nanoplastics remained mainly hydrocarbon rich but structurally disordered, whereas PET nanoplastics became oxygen enriched, with increased contributions from carbon-oxygen single bonds and ester groups of the O-C=O type.

The functional consequences of these surface changes were assessed with two complementary approaches. Electron paramagnetic resonance, a technique capable of detecting unpaired electrons and therefore free radicals, revealed stronger radical-associated activity in nanoplastics than in microplastics. Chemical probe assays told the same story from another angle, showing enhanced abiotic generation of reactive oxygen species, the aggressive oxygen-containing molecules that can damage lipids, proteins, and DNA. Reactive oxygen species produced by the particles themselves, without any biological catalyst, represent a form of intrinsic chemical hazard that travels with the particle wherever it goes.

The team then asked whether this heightened oxidative activity translates into biological damage. In experiments with erythrocyte membranes, the red blood cell membranes served as a model target, nanoplastics induced greater membrane lysis than their microplastic counterparts. Membrane lysis, the rupture of the lipid bilayer that encloses cells, is a direct physical-and-chemical assault that depends heavily on the particle surface contacting the membrane. Rougher, more oxidized, more reactive surfaces appear to make that contact more destructive.

Cellular studies reinforced the pattern. When Caco-2 cells, a widely used human intestinal epithelial model, were exposed to the particles, the nanoplastics triggered greater intracellular oxidative stress. Because Caco-2 cells model the gut lining, the first biological barrier that ingested plastic particles encounter, this finding is particularly relevant to dietary exposure. The result links the physical process of fragmentation, through altered surface chemistry, to a plausible mechanism of harm inside human-relevant cells.

One of the most nuanced findings of the study concerns the protein corona. When the researchers allowed bovine serum albumin, a common blood protein, to coat the particles before exposure, the cellular oxidation was attenuated. This indicates that the accessible particle surface, not merely the particle’s size or polymer type, drives biological reactivity. A protein layer acts as a buffer, masking the reactive sites that would otherwise generate radicals and attack membranes. The implication is double-edged: in protein-rich environments such as blood or digestive fluids, some of the particles’ oxidative aggression may be dampened, but the underlying reactive capacity remains, ready to manifest wherever the corona is absent or stripped away.

Taken together, the results carry a clear message for hazard assessment. The smaller a plastic particle becomes through mechanical fragmentation, the more reactive its surface grows, and the greater its potential to generate reactive oxygen species, lyse membranes, and stress cells. This means that size is not merely a matter of where particles can travel in the body, the usual focus of nanoplastic concern, but also of what those particles do chemically once they arrive. Secondary nanoplastics formed in the environment may be intrinsically more hazardous than the smooth primary particles that dominate laboratory test kits, meaning current risk assessments built on model beads could underestimate real-world toxicity.

The study also highlights the value of methodological rigor. By combining molecular fingerprinting, electron paramagnetic resonance, chemical probe assays, membrane lysis tests, and cell-based oxidative stress measurements, the researchers built a chain of evidence connecting a physical process, mechanical size reduction, to altered surface chemistry and finally to biological effect. The work, conducted with facilities including the Canadian Light Source and McGill’s electron microscopy and imaging platforms and supported by NSERC and Canada Foundation for Innovation funding, offers a template for how future toxicity studies might evaluate realistic secondary particles rather than idealized ones. As plastic pollution continues to fragment across ecosystems, from ocean surfaces to kitchen countertops, understanding that every crack and grind marks a fresh, reactive surface may reshape how scientists, regulators, and the public judge the true hazard potential of the plastic particles we all encounter daily.

Subject of Research: Surface reactivity and toxicity of mechanically fragmented secondary microplastics and nanoplastics

Article Title: Surface reactivity of plastic particles increases with size reduction during mechanical fragmentation: Implications for hazard potential

Article References: Kannan, U., Sangeeth, K., Wang, J., Lukose, J., Vinod, T. P., & George, S. (2026). Surface reactivity of plastic particles increases with size reduction during mechanical fragmentation: Implications for hazard potential. Microplastics and Nanoplastics. https://doi.org/10.1186/s43591-026-00231-3

Image Credits: AI Generated

DOI: 10.1186/s43591-026-00231-3

Keywords: microplastics, nanoplastics, mechanical fragmentation, surface reactivity, reactive oxygen species, free radicals, electron paramagnetic resonance, membrane lysis, Caco-2 cells, HDPE, PET, oxidative stress

Oxygen vacancies are missing oxygen atoms in an oxide lattice, and they are routinely introduced to tune the electronic structure of catalysts for fuel cells, metal–air batteries and green chemical synthesis. Conventionally, scientists count vacancies and assume more is better, or at least that all vacancies behave alike. The new work challenges that assumption at a fundamental level. When an oxygen atom leaves the lattice, it can leave behind electrons that either remain trapped at the vacancy site or are transferred to neighbouring metal cations. These two configurations correspond to distinct colour centres, known as F centres: an electropositive F1 centre, in which the electrons are not localized at the vacancy, and an electroneutral F2 centre, in which two electrons are trapped within the vacancy itself.

The challenge for the team, led by Xiaoyuan Zhang, Jingwen Sun and Junwu Zhu of Nanjing University of Science and Technology, was to isolate the effect of charge state from every other variable. To do this, they devised a template-assisted synthesis in which the atmosphere during preparation was carefully regulated, allowing them to produce zirconia samples, ZrO2−x, that contain predominantly F1 or predominantly F2 centres while keeping the crystal structure, particle morphology and vacancy concentration essentially unchanged. This clean experimental design meant that any difference in catalytic behaviour could be attributed directly to the charge state of the defects rather than to confounding structural differences.

Characterization confirmed the distinction. Electron paramagnetic resonance spectroscopy, which is sensitive to unpaired electrons, revealed the paramagnetic signature of the F1-type vacancies, while complementary measurements of the electronic structure showed the different local environments around zirconium cations adjacent to each vacancy type—F1-Zr4+ versus F2-Zr3+ configurations. X-ray absorption, electron energy-loss spectroscopy and photoluminescence measurements all supported the picture of two electronically distinct but structurally equivalent defect species. Crucially, electrochemical scanning transmission electron microscopy showed that the catalysts remained stable under operating conditions, ruling out structural reconstruction as the source of the differing reactivity.

The electrochemical consequences were striking. When the electroneutral F2 centres dominated, the catalyst favoured the two-electron oxygen reduction pathway, selectively converting oxygen into hydrogen peroxide. When the electropositive F1 centres dominated, the reaction instead proceeded toward full four-electron reduction, cleaving the oxygen–oxygen bond and producing water. Hydrogen peroxide electrosynthesis is a rapidly growing field because the compound is a green oxidant used in water treatment, disinfection and chemical manufacturing, and producing it on-site in an electrochemical cell could replace the energy-intensive anthraquinone process. A catalyst whose selectivity can be switched by defect charge state therefore has immediate practical appeal.

To understand the mechanism, the researchers deployed in situ electrochemical electron paramagnetic resonance, tracking the paramagnetism of the F centres while the reaction ran, together with in situ Raman spectroscopy to follow the evolution of reaction intermediates. The results overturned a common intuition. The electroneutral F2 centre, despite being the site where electrons are trapped, is not the primary adsorption site for oxygen. Instead, it acts through dynamic electronic compensation: it continuously donates and withdraws electron density to and from adjacent zirconium sites, stabilizing the adsorbed *OOH intermediate that is the hallmark of the two-electron pathway. It is this dynamism, rather than direct binding, that makes F2 centres the gatekeepers of peroxide selectivity.

The electropositive F1 centre behaves in an entirely different manner. It binds molecular oxygen directly at the vacancy site, and the interaction is strong enough to cleave the O–O bond, committing the reaction to the four-electron pathway. In the process, the F1 centre itself is quenched, its paramagnetic signature disappearing as the reaction proceeds. Density functional theory calculations reproduced both behaviours, showing favourable adsorption energetics for O2 at F1 sites and for *OOH at sites electronically modulated by neighbouring F2 centres, and the calculated free-energy landscapes matched the experimentally observed selectivity patterns.

Beyond zirconia, the study carries a broad message for the field. Oxygen vacancies have been invoked to explain catalytic behaviour in ceria, perovskites, cobalt and iron oxides, and countless other systems, but the charge state of those vacancies is rarely controlled or even measured. The authors argue that F-centre charge state should be regarded as an independent design lever, alongside vacancy concentration and position. Because the two charge states can be interconverted by atmosphere control during synthesis, and because in situ electron paramagnetic resonance can now monitor them under working conditions, the toolkit exists to rationally design vacancy chemistry rather than accept whatever defects a preparation happens to deliver.

The work also highlights the power of operando spectroscopy to catch catalysts in the act. Static characterization before and after a reaction can miss the transient electronic exchanges that actually govern selectivity; here, the decisive role of the F2 centre only became visible because its paramagnetism and the Raman signatures of intermediates were tracked simultaneously under electrochemical bias. As the energy transition drives demand for selective, precious-metal-free electrocatalysts for peroxide production, water treatment and chemical synthesis, the ability to dial in defect charge states could prove one of the more consequential ideas to emerge from defect engineering in recent years. What was once an invisible nuance of the oxide lattice has become a switch that chemists can now flip at will.

Subject of Research: Charge-state-dependent oxygen vacancy (F-centre) control of electrocatalytic oxygen reduction selectivity in zirconia

Article Title: F-centre charge state and dynamism govern oxide electrocatalytic selectivity

Article References: Zhang, X., Bukhvalov, D., Su, T., Fang, C., Dai, L., San, S., Duan, H., Wang, Y., Liu, K., Cui, J., Hua, Y., Xue, L., Hou, Z., Zhang, W., Xiong, P., Fu, Y., Sun, J., & Zhu, J. (2026). F-centre charge state and dynamism govern oxide electrocatalytic selectivity. Nature Chemistry. https://doi.org/10.1038/s41557-026-02256-w

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02256-w

Keywords: oxygen vacancies, F centres, zirconia, electrocatalysis, oxygen reduction reaction, hydrogen peroxide, selectivity, electron paramagnetic resonance, vacancy engineering, in situ Raman spectroscopy, defect chemistry, 2e- ORR pathway

The research team, led by Yu-Chieh Wang of National Sun Yat-sen University in Kaohsiung, Taiwan, together with colleagues from clinical and academic institutions in Taiwan and Thailand, focused their analysis on the PM1 fraction, particles with diameters of one micrometer or less. These ultrafine particles are of particular concern because they are small enough to penetrate deep into the lungs, past the body’s normal filtering defenses, and to deposit in the delicate alveolar regions where gas exchange takes place. Size-resolved aerosol samples were collected directly in operating rooms, capturing the real-world conditions in which surgeons, nurses, anesthesiologists and other staff work day after day.

Once collected, the PM1 samples underwent an unusually thorough chemical interrogation. The researchers measured the carbonaceous composition of the particles, distinguishing organic carbon from elemental carbon using established thermal-optical protocols, and quantified the concentrations of polycyclic aromatic hydrocarbons, a class of compounds formed during incomplete combustion that includes well-known carcinogens. They also determined the heavy metal content of the particles using high-resolution inductively coupled plasma mass spectrometry, a technique sensitive enough to detect trace elements at extremely low concentrations. Electrocautery, after all, works by heating tissue and metal electrodes to extreme temperatures, and previous work has shown that the procedure can even liberate molten metal particles from surgical implants.

The most striking findings, however, came from electron paramagnetic resonance spectroscopy, a technique that detects unpaired electrons and is therefore the gold standard for identifying free radicals. The EPR analysis confirmed the presence of persistent free radicals on the surgical particles, a category of long-lived radical species more commonly associated with environmental pollutants such as diesel exhaust, coal fly ash, incense smoke and biomass burning particles. Unlike ordinary free radicals, which react and disappear within fractions of a second, persistent free radicals can survive on particle surfaces for hours, days or longer, traveling intact through the air and into the respiratory tract.

The team went beyond simply detecting these radicals and quantified the intrinsic ability of the surgical aerosols to generate reactive oxygen species in aqueous solution. The ROS-generating capacity of the PM1 fraction was measured at 1.03 × 10¹⁸ ± 1.86 × 10¹⁷ spins per gram, a figure that places surgical smoke squarely within the range of recognized hazardous reference particulate materials. For comparison, the researchers noted that this value lies between those reported for SRM-2786, a fine urban dust standard issued by the US National Institute of Standards and Technology, and coal-fired power plant fly ash, a material whose oxidative toxicity has been extensively documented in laboratory animal studies.

This comparison carries a sobering implication. Environmental health scientists have spent decades establishing that the toxicity of airborne particulate matter correlates poorly with mass alone and much better with chemical reactivity, particularly the oxidative potential of the particles. Regulatory frameworks, however, still largely rely on mass-based metrics. The new results suggest that surgical smoke, which has historically escaped the kind of scrutiny applied to traffic or industrial pollution, deserves to be evaluated by the same reactive chemistry standards. The authors argue explicitly that surgical smoke should be regarded as a chemically reactive aerosol rather than an inert particulate contaminant, a reframing with direct consequences for how operating room air quality is monitored and controlled.

The mechanism by which these particles could harm the body follows a well-understood pathway. When inhaled particles carrying persistent free radicals come into contact with the aqueous lining fluid of the respiratory tract, the radicals can catalyze continuous cycles of redox reactions, generating a sustained flux of reactive oxygen species such as hydroxyl radicals and superoxide. These molecules attack lipids, proteins and DNA, triggering inflammatory signaling cascades. Because operating rooms are enclosed spaces with limited air exchange during procedures, and because surgical staff are exposed repeatedly across long careers, the authors warn that sustained inhalation of surgical aerosols may induce severe airway and inflammatory responses, underscoring the need for control measures to mitigate occupational exposure in surgical environments.

The study also situates itself within a growing body of literature on surgical smoke hazards. Earlier investigations have quantified particle concentrations during tonsillectomy and breast surgery, surveyed the inconsistent use of smoke evacuation systems in operating theaters, and catalogued the volatile organic compounds and viruses that can survive in surgical plumes. What distinguishes the new work is its integration of source-focused aerosol chemistry with radical spectroscopy and health risk assessment, providing a mechanistic bridge between what is emitted at the surgical site and what happens once those particles reach human tissue. The researchers describe the reactive aerosols as transient carriers of persistent free radicals that act as continuous sources of reactive oxygen species upon interaction with aqueous environments such as respiratory tract lining fluid.

The practical implications for hospitals are significant. Smoke evacuation systems, high-filtration respiratory protection and adequate room ventilation are already recommended by several professional bodies, but compliance remains uneven worldwide, partly because the hazard has been perceived as minor or poorly characterized. By demonstrating that surgical smoke exhibits oxidative potential comparable to regulated combustion pollutants, the study gives occupational health officers a concrete, quantitative basis for treating electrocautery plumes as a genuine respiratory hazard. The authors emphasize that the chemical reactivity of the particles, not merely their mass concentration, should determine how exposure risk is assessed, and that this sustained redox activity is central to understanding the health consequences of chronic exposure.

For the millions of healthcare workers who stand over electrocautery fields every day, the message of this research is clear: the haze that rises from the surgical site is chemically alive in the most unwelcome sense. It carries combustion-derived carcinogens, trace metals and long-lived radicals capable of generating oxidative stress long after the plume has dispersed. As the evidence accumulates, the operating room may come to be seen not as a pristine clinical environment with an occasional smoky inconvenience, but as an occupational setting with an air quality problem that demands the same rigor applied to factories, highways and power plants. The study was supported by the National Science and Technology Council, Taiwan, and the authors report no competing financial interests.

Subject of Research: Chemical characterization and health risks of electrocautery surgical smoke aerosols in operating rooms

Article Title: Emission characteristics of electrocautery surgical smoke in the operating room: polycyclic aromatic hydrocarbons, metals, persistent free radicals, reactive oxygen species generation, and health risk assessment

Article References: Wang, Y.-C., Ching, W.-C., Wang, C. C., Lee, C.-L., Khemawoot, P., & Ching, W.-M. (2026). Emission characteristics of electrocautery surgical smoke in the operating room: polycyclic aromatic hydrocarbons, metals, persistent free radicals, reactive oxygen species generation, and health risk assessment. Air Quality, Atmosphere & Health, 19(9), Article 206. https://doi.org/10.1007/s11869-026-02095-1

Image Credits: AI Generated

DOI: 10.1007/s11869-026-02095-1

Keywords: surgical smoke, electrocautery, persistent free radicals, reactive oxygen species, PM1, polycyclic aromatic hydrocarbons, heavy metals, operating room, occupational exposure, electron paramagnetic resonance, oxidative stress, air quality