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The sensitivity of electron spin resonance has been improved up to the quantum limit through the use of a Josephson parametric microwave amplifier combined with high-quality-factor superconducting microresonators cooled at millikelvin temperatures. The detection and characterization of paramagnetic species by electron spin resonance (ESR) spectroscopy is widely used throughout chemistry, biology and materials science 1 , from in vivo imaging 2 to distance measurements in spin-labelled proteins 3 . ESR relies on the inductive detection of microwave signals emitted by the spins into a coupled microwave resonator during their Larmor precession. However, such signals can be very small, prohibiting the application of ESR at the nanoscale (for example, at the single-cell level or on individual nanoparticles). Here, using a Josephson parametric microwave amplifier combined with high-quality-factor superconducting microresonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly four orders of magnitude 4 , 5 . We demonstrate the detection of 1,700 bismuth donor spins in silicon within a single Hahn 6 echo with unit signal-to-noise ratio, reduced to 150 spins by averaging a single Carr–Purcell–Meiboom–Gill sequence 7 . This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance. The detection volume of our resonator is ∼0.02 nl, and our approach can be readily scaled down further to improve sensitivity, providing a new versatile toolbox for ESR at the nanoscale.

We report an approach that extends the applicability of ultrasensitive force-gradient detection of magnetic resonance to samples with spin-lattice relaxation times (T ₁) as short as a single cantilever period. To demonstrate the generality of the approach, which relies on detecting either cantilever frequency or phase, we used it to detect electron spin resonance from a T ₁ = 1 ms nitroxide spin probe in a thin film at 4.2 K and 0.6 T. By using a custom-fabricated cantilever with a 4 μm-diameter nickel tip, we achieve a magnetic resonance sensitivity of 400 Bohr magnetons in a 1 Hz bandwidth. A theory is presented that quantitatively predicts both the lineshape and the magnitude of the observed cantilever frequency shift as a function of field and cantilever-sample separation. Good agreement was found between nitroxide T ₁ 's measured mechanically and inductively, indicating that the cantilever magnet is not an appreciable source of spin-lattice relaxation here. We suggest that the new approach has a number of advantages that make it well suited to push magnetic resonance detection and imaging of nitroxide spin labels in an individual macromolecule to single-spin sensitivity.

This review presents a chronological analysis of the scientific contributions of Professor Janko N. Herak (1937–2025), a Croatian biophysicist whose research played a major role in the development of electron para-magnetic resonance (EPR) spectroscopy in Croatia and significantly advanced the study of free radicals in biological systems. Beginning with his early work at Duke University under the mentorship of Prof. Walter Gordy, Herak investigated radiation-induced radicals in nucleic acid bases, nucleosides, and related biomolecules using EPR spectroscopy. During subsequent decades at the Ruđer Bošković Institute and later at the University of Zagreb Faculty of Pharmacy and Biochemistry, he conducted extensive studies of free radicals in irradiated single crystals of nucleic acid constituents, elucidating mechanisms of radical formation, hydrogen transfer, and structural rearrangements. His research later expanded to the investigation of oxidative processes in biological macromolecules, particularly the slow oxidation of plasma lipoproteins, where EPR spectroscopy and kinetic modelling provided insight into radical-mediated lipid peroxidation and the physicochemical basis of lipoprotein modification. In parallel, Herak explored long-range charge migration in nucleobase crystals and investigated metal-nucleobase interactions, integrating experimental EPR and ENDOR spectroscopy with theoretical modelling. These studies contributed to a deeper understanding of radical stabilisation, electronic structure, and energy transfer in ordered molecular systems. By situating Herak’s work within the broader development of radiation chemistry and magnetic resonance spectroscopy, this review highlights the methodological rigour, conceptual contributions, and lasting scientific impact of his research on free radicals and biologically relevant molecular systems.

Astaxanthin is a coloring agent which is used as a feed additive in aquaculture nutrition. Recently, potential health benefits of astaxanthin have been discussed which may be partly related to its free radical scavenging and antioxidant properties. Our electron spin resonance (ESR) and spin trapping data suggest that synthetic astaxanthin is a potent free radical scavenger in terms of diphenylpicryl-hydrazyl (DPPH) and galvinoxyl free radicals. Furthermore, astaxanthin dose-dependently quenched singlet oxygen as determined by photon counting. In addition to free radical scavenging and singlet oxygen quenching properties, astaxanthin induced the antioxidant enzyme paroxoanase-1, enhanced glutathione concentrations and prevented lipid peroxidation in cultured hepatocytes. Present results suggest that, beyond its coloring properties, synthetic astaxanthin exhibits free radical scavenging, singlet oxygen quenching, and antioxidant activities which could probably positively affect animal and human health.

Radiation therapy is one of the main modalities to treat cancer/tumor. The response to radiation therapy, however, can be influenced by physiological and/or pathological conditions in the target tissues, especially by the low partial oxygen pressure and altered redox status in cancer/tumor tissues. Visualizing such cancer/tumor patho-physiological microenvironment would be a useful not only for planning radiotherapy but also to detect cancer/tumor in an earlier stage. Tumor hypoxia could be sensed by positron emission tomography (PET), electron paramagnetic resonance (EPR) oxygen mapping, and in vivo dynamic nuclear polarization (DNP) MRI. Tissue oxygenation could be visualized on a real-time basis by blood oxygen level dependent (BOLD) and/or tissue oxygen level dependent (TOLD) MRI signal. EPR imaging (EPRI) and/or T1-weighted MRI techniques can visualize tissue redox status non-invasively based on paramagnetic and diamagnetic conversions of nitroxyl radical contrast agent. 13C-DNP MRI can visualize glycometabolism of tumor/cancer tissues. Accurate co-registration of those multimodal images could make mechanisms of drug and/or relation of resulted biological effects clear. A multimodal instrument, such as PET-MRI, may have another possibility to link multiple functions. Functional imaging techniques individually developed to date have been converged on the concept of theranostics.

The present study combined molecular and neuroimaging techniques to examine if free radical-mediated damage to barrier function in hypoxia would result in extracellular edema, raise intracranial pressure (ICP) and account for the neurological symptoms typical of high-altitude headache (HAH) also known as acute mountain sickness (AMS). Twenty-two subjects were randomly exposed for 18 h to 12% (hypoxia) and 21% oxygen (O2 (normoxia)) for collection of venous blood (0 h, 8 h, 15 h, 18 h) and CSF (18 h) after lumbar puncture (LP). Electron paramagnetic resonance (EPR) spectroscopy identified a clear increase in the blood and CSF concentration of O2 and carbon-centered free radicals (P > 0.05 versus normoxia) subsequently identified as lipid-derived alkoxyl (LO•) and alkyl (LC•) species. Magnetic resonance imaging (MRI) demonstrated a mild increase in brain volume (7.0 ± 4.8mL or 0.6% ± 0.4%, P > 0.05 versus normoxia) that resolved within 6 h of normoxic recovery. However, there was no detectable evidence for gross barrier dysfunction, elevated lumbar pressures, T2 prolongation or associated neuronal and astroglial damage. Clinical AMS was diagnosed in 50% of subjects during the hypoxic trial and corresponding headache scores were markedly elevated (P > 0.05 versus non-AMS). A greater increase in brain volume was observed, though this was slight, independent of oxidative stress, barrier dysfunction, raised lumbar pressure, vascular damage and measurable evidence of cerebral edema and only apparent in the most severe of cases. These findings suggest that free-radical-mediated vasogenic edema is not an important pathophysiological event that contributes to the mild brain swelling observed in HAH.

Klaus Möbius, Professor at the Free University of Berlin, was an outstanding physical chemist and biophysicist. He was a pioneer in the development of high-field/high-frequency EPR spectroscopy methods and their application in the study of photosynthesis. Among the most essential are the applications in studying the charge transfer kinetics and properties of the ion-radical pairs in photosynthetic reaction centers (RC). Under his leadership and with his direct participation a unique setup allowing registration of the kinetics of the electron transfer between the (bacterio)chlorophyll dimer and quinone in the bacterial photosynthetic RC and plant photosystem I (PSI) was created. This setup also allowed precise determining of the distance between separated charges based on measuring the frequencies of the Electron Spin Echo Envelope Modulation (ESEEM). This setup made it possible to prove that electron transfer in PSI occurs mainly along the A  branch of redox cofactors. The kinetics of backward electron transfer reaction (reoxidation of the phyllosemiquinone anion A 1 − and reduction of the photooxidized chlorophyll dimer P 700 + ) in PSI were measured under the same conditions. The essential data on the bioprotective effect of the disaccharide trehalose on the kinetics of forward and backward electron transfer in PSI complexes were obtained. A significant slowdown in the kinetics of electron transfer due to the restriction of protein conformational mobility, as well as long-term maintaining of functional activity of PSI dried in a vitreous trehalose matrix at room temperature (i.e., subjected to a reversible anhydrobiosis) was demonstrated. These results obtained in collaboration with Prof. Möbius and Prof. Venturoli (Bologna) allowed elucidating the role of hydrogen bond network and the conformational mobility of the protein subunits in facilitating electron transfer in the photosynthetic RC.

Site-directed spin labeling in combination with electron paramagnetic resonance spectroscopy has emerged as an efficient tool to elucidate the structure and conformational dynamics of biomolecules under native-like conditions. This article summarizes the basics as well as recent progress of site-directed spin labeling. Continuous wave EPR spectra analyses and pulse EPR techniques are reviewed with special emphasis on applications to the sensory rhodopsin-transducer complex mediating the photophobic response of the halophilic archaeum Natronomonas pharaonis and the photosynthetic reaction center from Rhodobacter sphaeroides R26.

Electron paramagnetic resonance (EPR)-spin trapping and flow cytometry were used to identify free radicals generated using argon-cold atmospheric plasma (Ar-CAP) in aqueous solutions and intracellularly in comparison with those generated by X-irradiation. Ar-CAP was generated using a high-voltage power supply unit with low-frequency excitation. The characteristics of Ar-CAP were estimated by vacuum UV absorption and emission spectra measurements. Hydroxyl (·OH) radicals and hydrogen (H) atoms in aqueous solutions were identified with the spin traps 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 3,3,5,5-tetramethyl-1-pyrroline-N-oxide (M4PO), and phenyl N-t-butylnitrone (PBN). The occurrence of Ar-CAP-induced pyrolysis was evaluated using the spin trap 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS) in aqueous solutions of DNA constituents, sodium acetate, and L-alanine. Human lymphoma U937 cells were used to study intracellular oxidative stress using five fluorescent probes with different affinities to a number of reactive species. The analysis and quantification of EPR spectra revealed the formation of enormous amounts of ·OH radicals using Ar-CAP compared with that by X-irradiation. Very small amounts of H atoms were detected whereas nitric oxide was not found. The formation of ·OH radicals depended on the type of rare gas used and the yield correlated inversely with ionization energy in the order of krypton > argon = neon > helium. No pyrolysis radicals were detected in aqueous solutions exposed to Ar-CAP. Intracellularly, ·OH, H2O2, which is the recombination product of ·OH, and OCl- were the most likely formed reactive oxygen species after exposure to Ar-CAP. Intracellularly, there was no practical evidence for the formation of NO whereas very small amounts of superoxides were formed. Despite the superiority of Ar-CAP in forming ·OH radicals, the exposure to X-rays proved more lethal. The mechanism of free radical formation in aqueous solutions and an intracellular milieu is discussed.

This minireview describes high-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy in the context of its application to bioinorganic chemistry, specifically to metalloproteins and model compounds. HFEPR is defined as frequencies above ~100 GHz (i.e., above W-band) and a resonant field reaching 25 T and above. The ability of HFEPR to provide high-resolution determination of g values of S = 1/2 is shown; however, the main aim of the minireview is to demonstrate how HFEPR can extract spin Hamiltonian parameters [zero-field splitting (zfs) and g values] for species with S > 1/2 with an accuracy and precision unrivalled by other physical methods. Background theory on the nature of zfs in S = 1, 3/2, 2, and 5/2 systems is presented, along with selected examples of HFEPR spectroscopy of each that are relevant to bioinorganic chemistry. The minireview also provides some suggestions of specific systems in bioinorganic chemistry where HFEPR could be rewardingly applied, in the hope of inspiring workers in this area.