Using the CIQTEK EPR200M benchtop X-band EPR spectrometer, the team quantitatively resolved the binding behavior of Tau protein to negatively charged lipid membranes, extracted absolute concentrations of free and bound protein populations, and determined binding affinity with minimal experimental input. This work not only reveals key mechanistic insights into Tau–membrane interactions, but also demonstrates the power of CW EPR for quantitative analysis in complex biological systems.

Background: Why Quantifying Protein–Lipid Interactions Is So Difficult

Protein–lipid interactions play a central role in cellular signaling, membrane organization, and the aggregation of pathological proteins. In neurodegenerative diseases such as Alzheimer’s disease, the interaction between Tau protein and cellular membranes is believed to be a critical early event that triggers pathological aggregation.

Despite its importance, quantitative characterization of these interactions remains challenging. Biological membranes are heterogeneous, dynamic, and highly sensitive to experimental conditions. The interactions themselves are often weak, transient, and involve multiple conformational states. Conventional methods such as fluorescence or colorimetric assays typically provide relative signals and require calibration curves that introduce additional uncertainty.

EPR spectroscopy offers a fundamentally different approach. By directly probing the dynamics of spin-labeled molecules, quantitative EPR provides a sensitive and accurate window into molecular motion, binding, and conformational restriction, enabling precise determination of protein–lipid interactions.

From Spectral Line Shapes to Molecular Binding Dynamics

Tau is an intrinsically disordered protein, and its interaction with lipid membranes involves subtle changes in molecular mobility rather than large structural rearrangements. This makes CW EPR particularly well suited to the problem.

Tau protein was site-specifically labeled using site-directed spin labeling (SDSL). Continuous-wave EPR spectra were acquired on the CIQTEK EPR200M at room temperature and 150 K while increasing the concentration of POPS multilamellar vesicles (MLVs).

Free Tau shows a narrow, symmetric three-line spectrum corresponding to rapid isotropic motion (τc ≈ 0.383 ns), characteristic of intrinsically disordered proteins. As POPS concentration increases, spectral broadening and longer rotational correlation times (up to 2.25 ns) indicate progressive restriction of Tau motion upon membrane binding.

Figure 1. (A) Room-temperature CW-EPR spectra of spin-labeled Tau during titration with increasing concentrations of POPS MLVs, showing gradual line-shape changes.
(B) Frozen-state (150 K) EPR spectra and simulations of spin-labeled Tau in the absence (blue) and presence (red) of 25 mM POPS MLVs.
(C) Room-temperature EPR spectrum of Tau monomers in a free environment.
(D) Room-temperature EPR spectrum of Tau monomers in a restricted environment. Tau concentration was 50 μM. The free environment corresponds to Tau in buffer, while the restricted environment corresponds to Tau in solution containing MLVs.

Absolute Quantification via Two-Component Spectral Deconvolution

A key breakthrough of this work is the use of EPR signal intensity for absolute quantification. Signal intensity is directly proportional to the number of unpaired electrons, allowing precise determination of free and bound protein concentrations without calibration standards.

Using the high baseline stability and signal-to-noise ratio of the CIQTEK EPR200M, experimental spectra were linearly decomposed into free and membrane-bound components. This two-component deconvolution enables calculation of absolute protein concentrations, providing a solid foundation for constructing Hill-type binding models and determining apparent dissociation constants (K_D).

Figure 2. (A) Experimental EPR spectra of spin-labeled Tau at different POPS concentrations (black) and corresponding best-fit simulations (blue to red).
(B) Example of spectral decomposition into free (blue) and bound (orange filled) components.

Minimalist Strategy for Rapid and Accurate Affinity Determination

Error propagation analysis identified optimal conditions for accurate K_D determination: when approximately half of the Tau population is bound (θ ≈ 0.4–0.6), experimental uncertainty is minimized. A single EPR measurement using the EPR200M suffices to obtain apparent dissociation constants matching full titration results, reducing both experimental time and precious sample consumption.

Reliable Performance Trusted by International Users

The CIQTEK EPR200M demonstrates stable microwave performance, sensitive detection, and reliable room-temperature operation. These features support reproducible data acquisition for complex biological samples, from room-temperature dynamics to low-temperature hyperfine analysis.

The successful application at the University of Bordeaux confirms that CIQTEK instruments provide robust support for frontier life science research in Europe and globally.

CIQTEK EPR Solutions for Quantitative Research

CIQTEK offers a comprehensive range of EPR instruments:

• X-band CW & pulsed systems: benchtop or floor-standing, compatible with variable temperature, light, electrochemistry, and in situ modules.

• High-field Q- and W-band systems: higher magnetic fields and spectral resolution for materials, quantum states, and advanced biophysical studies.

With proven performance, CIQTEK EPR instruments are installed across Europe, North America, and Asia, supporting hundreds of research labs and industrial partners worldwide.

At Booth #307, CIQTEK will showcase its Electron Microscopy solutions, including:

• TEM for internal structure and nanostructure analysis

• FIB-SEM for site-specific sample preparation and 3D imaging

• FE-SEM for high-resolution imaging

• Tungsten filament SEM for general materials analysis

• Entry-level SEM for laboratories starting electron microscopy

These systems cover a wide range of materials characterization needs, from basic surface imaging to advanced studies.

Local Partner Presence

CIQTEK's U.S. distributor, JH Technologies, will also be at the booth.

For inquiries or to schedule time with CIQTEK U.S. team at TMS, please contact info.usa@ciqtek.com

We look forward to welcoming you at Booth #307 in San Diego to explore practical microscopy solutions.

For meeting requests or inquiries before and during the event, please contact: info.usa@ciqtek.com

We look forward to welcoming you at Booth #1113 in San Antonio!

At TSM 2026, CIQTEK will showcase its growing portfolio of electron microscopes (EM), designed to meet the evolving needs of academic research, shared facilities, and industrial laboratories.

CIQTEK's EM product line covers a wide range of applications, from routine imaging to advanced materials characterization, including:

• Transmission Electron Microscopes (TEM), enabling high-resolution structural and compositional analysis at the nanoscale

• FIB-SEM systems, delivering enhanced brightness and resolution for materials science and industrial inspection

• Field Emission SEM (FE-SEM), engineered for high-resolution imaging, analytical workflows, and demanding research environments

• Tungsten filament SEM, offering stable performance and cost-effective solutions for teaching labs and routine analysis

Across the portfolio, CIQTEK focuses on delivering reliable imaging performance, intuitive system operation, and long-term ownership value, helping laboratories achieve consistent results without unnecessary complexity.

Engaging with the Microscopy Community

CIQTEK views conferences like TSM not only as an opportunity to present instrumentation, but also as a platform for meaningful technical exchange. During the meeting, the CIQTEK team looks forward to discussing real-world microscopy challenges, system selection considerations, and long-term operational needs with users across materials science, life science, and industrial applications.

For attendees interested in learning more about CIQTEK EM solutions or exploring potential collaborations, CIQTEK warmly welcomes conversations during the meeting in Austin!

Local Support from CIQTEK USA

To better serve customers in North America, CIQTEK maintains a local branch in the United States, providing regional sales support, application consultation, and long-term service assistance for electron microscopy users.

For more information about CIQTEK EM solutions or to connect with the CIQTEK USA team, please contact us at info.usa@ciqtek.com.

Understanding the formation of radical intermediates is key to controlling electrochemical reaction rates and selectivity. These short-lived species at the electrode interface dictate outcomes, and relying solely on final products can lead to speculative mechanisms. With operando EPR using CIQTEK benchtop EPR200M, researchers can directly capture radicals in situ, mapping their formation sequence and structural fingerprints for robust mechanistic evidence.

A recent collaboration between Beijing University of Technology (Sun Zaicheng / Liu Yichang), Tsinghua University (Yang Haijun), and Wuhan University (Lei Aiwen) introduced a novel 3D-printed electrolytic cell tailored for in situ EPR. Fabricated with high-precision digital light processing (DLP), this flat cell enables reproducible integration with electrochemical systems. Their results, published in Chemical Engineering Journal under the title Bespoke electrolytic cell for operando EPR tests: Revealing the formation and accurate structures of amino and phenolic radicals, demonstrate the workflow’s ability to uncover radical structures across representative reactions.

Methodological Breakthrough: 3D-Printed Flat Electrolytic Cell for Reproducible Operando EPR

High-dielectric solvents commonly used in electrochemical cells reduce EPR signal-to-noise, making radical detection challenging. The flat cell design mitigates dielectric losses and enhances the resonator’s Q factor, improving operando EPR performance.

Beyond physics, the cell is engineered for reproducibility. Using DLP 3D printing, electrode channels, positioning structures, and short-circuit protection are fixed during fabrication. This eliminates manual variability, reduces system resistance, and improves signal quality, while maintaining mechanical strength, solvent compatibility, and cost efficiency.

This approach transforms operando EPR into a workflow of "standardized structural component + reproducible procedure", enabling cross-team and cross-system reproducibility and mechanistic comparison.

Time-Resolved Evidence Tracks Radical Formation in C–N Coupling

In situ EPR with time-resolved acquisition allows mapping radicals in real-time, showing which species appear first and how they evolve. This provides a reproducible evidence chain at the intermediate level, moving mechanistic understanding beyond product-based inference.

Cycloaddition Intermediates Reveal Reaction Selectivity

By comparing substrate-specific spectra and calculating spin density, EPR signals are directly translated into radical structural fingerprints. This forms a closed-loop framework for explaining regio- and chemo-selectivity in (3+2) cycloaddition reactions.

Solvent Effects Guide C–O Coupling Design

In situ EPR shows that the same radical exhibits distinct spectra in MeCN versus HFIP. Combined with NMR, the study links solvent, radical structure, and reaction selectivity, providing an experimental evidence chain for optimizing reaction conditions.

Integrated CIQTEK EPR200M Platform for Operando Electrochemical Studies

The study integrates the 3D-printed flat electrolytic cell with a CIQTEK EPR200M benchtop X-band CW spectrometer and an electrochemical workstation, synchronizing “power-on” with spectral acquisition. This modular design reduces dielectric loss and assembly variability, lowering the barrier for deploying electrochemical EPR. Data comparability improves, and mechanistic evidence chains can be reproduced across different teams and reaction systems.

Collaboration & Application Opportunities

For researchers interested in operando electrochemical EPR, 3D-printed cell solutions, or building radical intermediate evidence chains, CIQTEK can assist with device interfaces, test workflows, and data interpretation. This enables translating paper-level mechanistic insights into reproducible, actionable experimental capabilities.

Why a FIB-SEM Dual-Beam Microscope Matters

As materials science research continues to move toward smaller length scales and more dynamic processes, traditional sample preparation methods are no longer sufficient. Modern studies increasingly require site-specific preparation, in situ observation, and three-dimensional reconstruction at the micro- and nano-scale.

To meet these demands, the center introduced a FIB-SEM dual-beam electron microscope, supplied by CIQTEK. This advanced scientific instrumentation enables precise micro- and nano-fabrication while maintaining high-resolution imaging performance, making it an essential tool for frontier research.

"Our goal was very clear," Professor Gong explains. "We wanted to provide advanced experimental conditions that support breakthroughs in frontier science and engineering, while also offering a strong technical foundation for future industrial innovation."

CIQTEK FIBSEM at the High-End In Situ Electron Microscopy Joint Laboratory

Choosing CIQTEK: Technology, Reliability, and Collaboration

During the instrument selection process, the center focused on three core factors: system stability, performance precision, and long-term technical support.

"The core specifications of CIQTEK's FIB-SEM are already on par with world-leading systems," says Professor Gong. "That gave us confidence from the start. What truly convinced us, however, was CIQTEK's openness to collaboration."

CIQTEK worked closely with researchers to understand real experimental needs, offering flexible support in application development and software compatibility. This approach turned the dual-beam electron microscope into a platform that could continuously evolve with ongoing research rather than remain a fixed configuration.

More Than Equipment: A Long-Term Research Partner

After more than a year of daily operation, the CIQTEK FIB-SEM dual-beam electron microscope has proven to be stable and reliable under high-intensity research conditions.

"The overall experience has exceeded our expectations," says Yu Bai, engineer at the Engineering and Materials Science Experimental Center. "The system performs consistently well in both micro- and nano-fabrication and high-resolution imaging, which is essential for our in situ materials research."

Just as important, CIQTEK has continued to track user feedback and translate research challenges into concrete optimization and upgrade directions. This ongoing interaction ensures that the instrument remains aligned with evolving experimental needs.

Fast Response to Non-Standard Experimental Challenges

One example clearly illustrates the value of this collaboration. During a project that went beyond the standard application scenarios of the system, the research team encountered a critical technical bottleneck.

"CIQTEK's application engineers came on site immediately," Bai recalls. "They worked with us to refine the experimental approach and quickly delivered a customized software upgrade."

This rapid response allowed the team to complete the experiment successfully and demonstrated how university–industry collaboration can directly accelerate scientific progress.

"At that moment, we truly felt what it means to have a partner," Bai adds. "Not just an equipment supplier, but a team that stays with us throughout the innovation process."

Looking Ahead: Advancing In Situ Materials Research Together

The collaboration between the Engineering and Materials Science Experimental Center and CIQTEK offers a clear example of how advanced scientific instrumentation and close cooperation can support independent innovation.

As the High-End In Situ Electron Microscopy Joint Laboratory continues to develop, both sides will further focus on in situ materials research related to mechanics, micro- and nano-fabrication, and advanced experimental methodologies. Through continued collaboration, they aim to provide strong technical support for high-level research and future scientific breakthroughs.

With the support of CIQTEK Scanning NV Microscopy (SNVM), researchers at Tsinghua University have directly visualized nanoscale spin cycloid structures in multiferroic BiFeO₃. This work, published in Advanced Functional Materials, provides the missing microscopic evidence linking crystal symmetry, magnetic structure, and anisotropic magnon transport, highlighting SNVM as a decisive tool for magnonics and low-power spintronic research.

The study used the CIQTEK Scanning NV Probe Microscope (SNVM)

Research Background: Magnon Transport in Multiferroic Oxides

Magnon-mediated spin currents can propagate in magnetically ordered insulators with nearly zero energy dissipation, making them highly attractive for next-generation low-power spintronic devices. In multiferroic materials such as BiFeO₃, the coupling between ferroelectric and antiferromagnetic orders enables electric field control of magnons, a long-standing goal in spintronics.

Despite this promise, the microscopic origin of weakly anisotropic magnon transport in rhombohedral phase BiFeO₃, commonly referred to as R-BFO, has remained unresolved. Addressing this challenge requires direct real-space characterization of nanoscale magnetic structures, which has long been inaccessible using conventional techniques.

Technical Bottleneck: Lack of Direct Magnetic Structure Evidence

Theoretical studies have predicted that R-BFO hosts a cycloidal spin structure that plays a critical role in suppressing strong anisotropy in magnon transport. However, experimental confirmation has been elusive.

Traditional characterization techniques, such as X-ray magnetic linear dichroism, provide spatially averaged magnetic information and are unable to resolve nanoscale spin textures. As a result, the logical connection between crystal symmetry, magnetic structure, and magnon transport remained incomplete due to the absence of direct microscopic magnetic imaging.

CIQTEK SNVM Approach: Direct Nanoscale Magnetic Imaging

CIQTEK Scanning NV Microscopy (SNVM) overcomes these limitations by combining nanometer-scale spatial resolution with electron spin level magnetic field sensitivity. This enables non-invasive, quantitative imaging of local magnetic fields generated by complex spin textures inside functional materials.

In this work, the research teams led by Prof. Yi Di from the State Key Laboratory of New Ceramic Materials and Prof. Nan Tianxiang from the School of Integrated Circuits at Tsinghua University employed CIQTEK SNVM magnetic imaging to directly probe the intrinsic magnetic structure of R-BFO.

Key Findings Enabled by SNVM Magnetic Imaging

Using CIQTEK SNVM, the researchers clearly observed a uniform cycloidal spin structure within R-BFO, with a characteristic periodicity of approximately 70 nanometers. The high spatial resolution of SNVM allowed precise quantification of the cycloid wavelength and confirmed that the magnetic structure exists in a single-domain state.

By correlating SNVM nanoscale magnetic imaging with piezoresponse force microscopy, the team further demonstrated that the propagation vector k of the spin cycloid is perpendicular to the ferroelectric polarization direction P. This result provides direct experimental validation that the Dzyaloshinskii-Moriya interaction stabilizes the cycloidal spin structure in R-BFO.

These observations conclusively verify long-standing theoretical predictions and establish a complete experimental link between crystal symmetry, magnetic structure, and anisotropic magnon transport.

Magnetic structures of single-domain R-BFO and O-LBFO
a) X-ray magnetic linear dichroism (XMLD) spectra of the LSMO (22 nm)/R-BFO (10 nm) sample.
b) NV imaging of the LSMO (2 nm)/R-BFO (10 nm) sample. A 2 nm-thick LSMO layer was chosen to minimize interference from its stray magnetic fields.
d) XMLD spectra of the LSMO (22 nm)/O-LBFO (10 nm) sample.
e) X-ray magnetic linear dichroism–photoemission electron microscopy (XMLD-PEEM) imaging of the LSMO (22 nm)/O-BFO (10 nm) sample.

Scientific Impact and Application Value

Published in Advanced Functional Materials under the title Tuning Anisotropic Magnon Transport in Multiferroic Oxides by Crystal Symmetry, this study significantly advances the understanding of magnon transport mechanisms in multiferroic oxides.

More importantly, it demonstrates that Scanning NV Microscopy (SNVM) is not merely a supplementary technique, but a key enabling platform for modern condensed matter physics and functional materials research. Compared with traditional spectroscopic approaches, SNVM magnetic imaging uniquely provides direct, real-space access to complex magnetic textures at the nanoscale.

Looking forward, CIQTEK SNVM is expected to play an increasingly important role in multiferroic materials, antiferromagnetic devices, and magnon-based information processing, accelerating the development of energy-efficient spintronic technologies.

Experience CIQTEK SNVM for Nanoscale Magnetic Imaging

The CIQTEK Scanning NV Microscope (SNVM) is a state-of-the-art nanoscale magnetic field imaging system designed for advanced materials research. It supports temperatures from 1.8 to 300 K, vector magnetic fields up to 9 T out of plane and 1 T in plane, spatial resolution down to 10 nm, and magnetic sensitivity reaching 2 μT per square root Hertz.

CIQTEK Scanning NV Microscope (SNVM) has two versions: the ambient version and the cryogenic version

This milestone also highlights CIQTEK's expanding collaboration with the University of Strasbourg in Electron Paramagnetic Resonance (EPR) spectroscopy. CIQTEK will sponsor the ARPE EPR 10th Summer School, to be held in France from June 22–26, 2026.

During the event, researchers and students will gain hands-on experience with the CIQTEK EPR200M benchtop EPR spectrometer and explore CIQTEK’s advanced floor-stand EPR solutions through real-time remote demonstrations. More details coming soon!

Growing CIQTEK's Presence in France and Europe

Looking ahead, CIQTEK will further strengthen its presence in France and Europe, enhancing brand visibility, expanding collaborations with universities and laboratories, and delivering innovative EPR technologies that accelerate research in materials science, chemistry, and spin-related fields.

CIQTEK EPR Spectrometer Series

To address this, Associate Professor Peng Lei's team from the University of Science and Technology of China conducted an in-depth study using the CIQTEK scanning electron microscope (SEM) and dual-beam electron microscope. They built high-temperature magnetic-field steam corrosion and high-temperature water corrosion setups. Using SEM, EBSD, and FIB techniques, they analyzed oxide films formed on CLF-1 steel after 0–300 hours of steam corrosion at 400°C under 0T, 0.28T, and 0.46T magnetic fields, and after 1000 hours of high-temperature water corrosion at 300°C.

The study used CIQTEK SEM5000X ultra-high-resolution field-emission SEM and the FIB-SEM DB500

The study found that the oxide films form a multilayer structure, with a chromium-rich inner layer and an iron-rich outer layer. Film formation occurs in five stages: initial oxide particles, then floc-like structures, formation of a dense layer, growth of spinel structures on the dense layer, and finally, spinel cracking into laminated oxides. The presence of a magnetic field significantly accelerates corrosion, promotes the transformation of outer magnetite (Fe₃O₄) into hematite (Fe₂O₃), and enhances laminated oxide formation. This work was published in Corrosion Science, a top-tier journal in the field of corrosion and materials degradation, under the title: "Magnetic field effects on the high-temperature steam corrosion behavior of reduced activation ferritic/martensitic steel."

Surface Oxide Film Characterization

In high-temperature steam (HTS), CLF-1 steel surfaces show different corrosion states over time. On polished surfaces, early-stage oxidation (60 h) appears as small, dispersed particles. The Fe/Cr ratio is similar to the substrate, indicating that the oxide layer is not yet complete. By 120 h, floc-like oxides appear. At 200 h, a dense oxide layer forms, with new oxide particles and local spinel structures on top.

Rough surfaces corrode faster. Early floc-like oxides are finer and more evenly distributed. By 200 h, they transform into spinel structures, showing a stronger difference from polished surfaces. In high-temperature, high-pressure water (HTPW), polished surfaces display similar spinel structures. Spinel in HTPW is denser and more numerous, while spinel in HTS is larger in size.

When a magnetic field is applied (0.28 T on polished, 0.46 T on rough), corrosion changes further. After 60 h, oxide particles appear on both surfaces, more on rough surfaces. By 120 h, polished surfaces have particle-like oxides, while rough surfaces develop fine floc-like films. At 200 h, rough surfaces show spinel cracking and layered structures perpendicular to the surface, with many pores forming. By 240 h, layers become denser and well-aligned. EDS analysis shows that under magnetic fields, Fe/Cr decreases and oxygen increases over time. Cr content drops at 120 h, earlier than in non-magnetic conditions, showing that magnetic fields accelerate the formation of the iron-rich outer layer.

Figure 1. SEM images and EDS point scans (#1–#20) of CLF-1 surfaces under HTS and HTPW.

Figure 2. SEM images and EDS point scans (#1–#16) of CLF-1 surfaces exposed to magnetic fields: polished (0.28 T), rough (0.46 T).

Oxide Film Phase Analysis

Figures 3 and 4 show Raman spectra of CLF-1 steel oxide films in HTS, HTPW, and under magnetic fields. Without a magnetic field, films in both HTS and HTPW are mainly spinel structures composed of Fe₃O₄ and FeCr₂O₄. The Raman peaks (302, 534, 663, 685 cm⁻¹) overlap, making differentiation difficult. Hematite (Fe₂O₃) appears only on rough HTS surfaces after 240 h.

Under a magnetic field, oxidation accelerates. Polished surfaces show small Fe₂O₃ peaks only at 240 h, while rough surfaces show Fe₂O₃ as early as 120 h, increasing by 240 h. Meanwhile, Fe₃O₄ and FeCr₂O₄ peaks weaken, indicating faster hematite formation.

Figure 3. Raman spectra of oxide films on CLF-1 under HTS and HTPW: (a) polished; (b) rough.

Figure 4. Raman spectra under magnetic field HTS: (a) polished (0.28 T); (b) rough (0.46 T).

Cross-Section Oxide Film Characterization

EBSD analysis of rough surfaces after 300 h HTS corrosion (Figure 5a, b) shows a three-layer oxide structure: a thin, discontinuous Fe₂O₃ outer layer, a dense Fe₃O₄ middle layer, and a black chromium-rich layer between Fe₃O₄ and the substrate. FIB-prepared cross-sections (Figure 5c, d) and TEM/SAED analysis confirm that the chromium-rich layer is FeCr₂O₄, and the iron-rich layer is Fe₃O₄. Gaps at the interfaces indicate phase separation and pore formation during oxidation evolution.

Figure 5. Microstructure and phase distribution of cross-section oxide films on rough CLF-1 surfaces after 300 h HTS: (a) EBSD contrast; (b) EBSD phase map; (c) FIB cross-section; (d) dark-field TEM and SAED.

Figure 6 shows cross-sections under a magnetic field (HTS, 240 h). EBSD shows outer oxides composed of Fe₃O₄ and Fe₂O₃. Fe₃O₄ layers are vertically aligned with many pores, and Fe₂O₃ fills surface gaps. The chromium-rich layer between the outer layer and substrate is porous. Compared with non-magnetic conditions, films are looser with more pores, especially at layer interfaces and within the Fe-rich layer. SAED confirms that oxide films still consist of FeCr₂O₄ and Fe₃O₄ from inner to outer layers. Magnetic fields mainly affect film density and pore evolution, not phase composition.

Figure 6. Cross-section microstructure and phase distribution of rough CLF-1 surfaces under magnetic field HTS: (a) EBSD contrast; (b) EBSD phase map; (c) FIB cross-section; (d) dark-field TEM and SAED.

This study examines the effect of magnetic fields on CLF-1 steel corrosion after 300 h in 400°C HTS. It also compares oxide films formed under HTPW and HTS conditions. The findings provide important reference data for optimizing the corrosion resistance of fusion structural materials.

Researchers from Beihang University, led by Prof. Xiaomin Li and Prof. Wenhong Fan, used the CIQTEK scanning electron microscope (SEM) and electron paramagnetic resonance (EPR) spectrometer to conduct an in-depth investigation. They developed a new strategy using KOH-modified Arundo donax L. biochar to efficiently remove Ni-Cit from water. The modified biochar not only showed high removal efficiency but also enabled nickel recovery on the biochar surface. The study, titled “Removal of Nickel-Citrate by KOH-Modified Arundo donax L. Biochar: Critical Role of Persistent Free Radicals”, was recently published in Water Research.

Material Characterization

Biochar was produced from Arundo donax leaves and impregnated with KOH at different mass ratios. SEM imaging (Fig. 1) revealed:

• The original biochar (BC) exhibited a disordered rod-like morphology.

• At a 1:1 KOH-to-biomass ratio (1KBC), an ordered honeycomb-like porous structure was formed.

• At ratios of 0.5:1 or 1.5:1, pores were underdeveloped or collapsed.

• BET analysis confirmed the highest surface area for 1KBC (574.2 m²/g), far exceeding other samples.

SEM and BET characterization provided clear evidence that KOH modification dramatically enhances porosity and surface area—key factors for adsorption and redox reactivity.

Figure 1. Preparation and characterization of KOH-modified biochar.

Performance in Ni-Cit Removal

Figure 2.
(a) Removal efficiency of total Ni by different biochars;
(b) TOC variation during Ni–Cit treatment;
(c) Effect of Ni–Cit concentration on the removal efficiency of 1KBC;
(d) Effect of pH on the removal performance of 1KBC;
(e) Influence of coexisting ions on Ni–Cit removal by 1KBC;
(f) Continuous-flow removal performance of Ni–Cit by 1KBC.
(Ni–Cit = 50 mg/L, biochar dosage = 1 g/L)

Batch experiments demonstrated strong removal performance:

• At 50 mg/L Ni-Cit and 1 g/L material dosage, 1KBC removed 99.2% of total nickel within 4 hours, compared to 32.6% for BC.

• TOC removal reached 31% for 1KBC, confirming that Ni-Cit undergoes complex dissociation followed by Ni²⁺ adsorption.

• Even at 100 mg/L Ni-Cit, the removal efficiency remained above 93%.

• 1KBC maintained excellent performance across a wide pH range (pH > 5).

• Phosphate significantly inhibited removal due to solution acidification and competitive complexation with Ni²⁺.

• In continuous-flow tests, a 1KBC-packed fixed-bed reactor operated for 6900 minutes, treating 460 bed volumes, while maintaining effluent Ni < 0.5 mg/L.

Post-Treatment Material Characterization

Figure 3. Morphology and EDS comparison of the material before (a) and after (b) Ni–Cit removal;
(c) XPS spectra of surface Ni 2p after the removal process.

Recovered biochar (R1KBC) showed:

• No significant morphological changes.

• Uniform Ni distribution confirmed by EDS mapping.

• XPS spectra displayed both Ni²⁺ and Ni³⁺ peaks, direct evidence of oxidative complex dissociation.

EPR-Based Identification of ROS

Figure 4. EPR measurements:
(a) TEMP-trapped ¹O₂ generated by biochar;
(b, c) BMPO-trapped •OH and O₂•⁻ generated by biochar;
(d) Hyperfine splitting fitting analysis of the 1KBC signal in panel (c).

Using the CIQTEK EPR spectrometer, the team identified reactive oxygen species (ROS) generated on the biochar surface:

• ¹O₂: strong TEMP–¹O₂ triple signal (1:1:1, AN = 17.32 G) observed only in 1KBC.

• OH: BMPO–•OH quartet detected in both BC and 1KBC, but much stronger in 1KBC.

• O₂•⁻: identified through BMPO–•OOH signals in methanol-containing systems.

1KBC produced significantly higher levels of ¹O₂, •OH, and O₂•⁻ than BC, confirming the enhanced redox activity induced by KOH modification.

Free Radical Quenching Experiments

Figure 5.
(a) Effect of ¹O₂; (b) •OH; and (c) O₂•⁻ on Ni–Cit removal efficiency;
(d) Inhibition rates of different ROS on Ni–Cit removal.

By introducing quenchers, FFA (¹O₂), p-BQ (O₂•⁻), and methanol (•OH)—the team quantified the contributions of different ROS:

O₂•⁻ inhibition (55%) > ¹O₂ inhibition (17%) > •OH inhibition (12%)

This ranking indicates that O₂•⁻ plays the dominant role in Ni-Cit degradation and complex dissociation.

Role of PFRs and ROS Generation Mechanism

Figure 6.
(a) Detection of surface PFRs in biochar;
(b) Effect of PFR quenching on Ni–Cit removal by biochar;
(c) ¹O₂, (d) •OH, and (e) O₂•⁻ signals in 1KBC and TEA-treated samples;
(f) Schematic of ROS transformation pathways.

Persistent free radicals (PFRs) in biochar are closely linked to ROS formation. EPR results showed:

• 1KBC exhibited much higher PFR concentration than BC.

• PFRs had a g-value of 2.0034, characteristic of carbon-centered radicals adjacent to oxygen (e.g., phenoxy radicals).

• Triethylamine (TEA) effectively quenched PFRs, reducing Ni-Cit removal efficiency to ~50% and drastically lowering ROS levels.

The mechanism (Fig. 6f):

• Dissolved oxygen adsorbs onto the biochar surface.

• PFRs transfer electrons to O₂, forming O₂•⁻.

• O₂•⁻ initiates complex dissociation; subsequent ROS degrade the citrate ligand.

DFT Calculations and Mechanistic Insights

Figure 7.
(a) Optimized structure of Ni–Cit;
(b) Electrostatic potential (ESP) map;
(c) HOMO; (d) LUMO;
Fukui function isosurfaces of Ni–Cit:
(e) f⁻, (f) f⁺, (g) f⁰, (h) condensed dual descriptor (CDD), and (i) Fukui indices;
(j) Proposed degradation pathways of Ni–Cit.

Density functional theory (DFT) calculations clarified the molecular reaction pathways:

• Frontier molecular orbital and Fukui function analysis revealed that the Ni center is prone to nucleophilic attack, while the citrate ligand undergoes electrophilic reactions.

• O₂•⁻, with its strong nucleophilicity, targets the Ni center, breaking the Ni–Cit coordination.

• Citrate ligands degrade through two ROS-mediated pathways.

These theoretical results align with EPR findings and support the proposed mechanism.

KOH-modified biochar (1KBC) achieved 99.2% Ni removal from 50 mg/L Ni-Cit solution within 4 hours. The modification significantly enhanced porosity, surface functionality, and, critically, the concentration of persistent free radicals. These PFRs activated dissolved oxygen to generate ROS, among which O₂•⁻ acted as the primary species driving Ni-Cit dissociation. Subsequent ROS degraded the citrate ligand, while released Ni²⁺ was adsorbed onto the biochar.

This study demonstrates a sustainable "one-step dissociation and recovery" approach for treating metal–organic complexes, offering strong potential for future real-world applications.