Following Proteins One at a Time
Metasurface-enhanced Raman spectroscopy tracks label-free conformational changes in individual proteins under near-physiological conditions.
A metasurface-enhanced Raman platform has followed the structural dynamics of single proteins in liquid, without chemically modifying or immobilizing them.
The University of Twente-led team used a metasurface made from closely packed gold nanoparticles on a gold film to overcome the weakness of single-protein Raman scattering. Light concentrated into nanoscale electromagnetic hotspots, increasing the Raman signal from proteins passing through them by around 10 million times.
The researchers tested the approach on bovine serum albumin, a model for human serum albumin, the main transport protein in blood. Because the spectra were collected one protein at a time, the measurements captured structural variation that would be averaged out in bulk analysis. The resulting trajectories were used to construct free-energy landscapes, showing which conformations were most stable and how the protein moved between them.
The dominant transitions ran between α-helical and β-sheet structures, and between α-helical and random-coil forms. “Our method gives access to the dynamic behaviour of individual proteins in nearly physiological conditions,” says Femi Ojambati, who led the work, in a recent press release. “That matters, because we can learn more about a protein’s function from the energy landscape, and also from the pathways between structures.”
The team then modified the metasurface with methyl, carboxylate, and amine groups and varied the solution pH. At neutral pH, albumin generally retained its native α-helical structure, while other chemical environments shifted the balance toward β-sheet or random-coil conformations. The results linked those changes to electrostatic interactions between the protein, the surface chemistry, and the surrounding solution.
The researchers suggest that further metasurface improvements could capture faster conformational changes and extend the method to protein complexes and other biomolecules, while preserving a label-free view of single-molecule structural dynamics.
A Hidden State in 30 Femtoseconds
Time-resolved reflectance spectroscopy traces the transition through a previously unresolved bond-order wave intermediate.
A six-femtosecond time-resolved reflectance experiment has captured the formation of a photoinduced hidden state in a metal–organic framework, tracing the process through a previously unresolved intermediate electronic state.
Photoinduced hidden states are short-lived material configurations not accessed through ordinary thermal pathways. In metal–organic frameworks, the earliest electronic and structural changes can unfold within tens of femtoseconds, making the sequence difficult to resolve experimentally.
The researchers used ultrashort pump pulses to excite the framework and time-resolved reflectance spectroscopy to follow how its optical response evolved immediately afterward. The transient reflectance spectrum captured that rapid evolution, while comparison with theoretical calculations distinguished the initial electronic redistribution from the later structural response. The appearance of a new absorption feature within 30 fs marked the formation of the hidden state.
The transition was not direct. Theoretical analysis indicated that excitation first produced a bond-order wave state, in which electronic bonding between neighboring sites alternated between stronger and weaker coupling. Small atomic displacements then followed this redistribution, stabilizing the photoinduced structure.
The calculations also suggested that the resulting hidden state may be polar, with positive and negative charge distributed unevenly across the framework. This points to a route by which light could alter the material’s electronic properties without driving a conventional thermal phase transition.
“We found that the photoinduced hidden state forms within 30 fs through a previously unknown intermediate electronic state,” said lead author Tadahiko Ishikawa in a press release.
By resolving the electronic redistribution before the lattice response, the study gives future models a more specific sequence to reproduce. “By revealing intermediate states, our method could help design materials that can be efficiently controlled using light,” said Ishikawa.
An All-Attosecond View of Electron Motion
Table-top transient absorption resolves electronic coherences cycling in just over three femtoseconds using attosecond XUV pump and probe pulses.
Electronic coherences in xenon that cycle in just over three femtoseconds have been resolved with a table-top all-attosecond transient absorption experiment, using extreme-ultraviolet pulses for both excitation and probing.
Earlier attosecond measurements typically paired an extreme-ultraviolet (XUV) pulse with a femtosecond near-infrared pulse. But the infrared field could perturb the dynamics being measured, while its longer duration made electron motion on the 1–5 fs timescale difficult to resolve.
The new setup generated near-isolated attosecond XUV pulses by high-harmonic generation and split them into pump and probe replicas. Recording the transient absorption spectrum across photon energy and time delay gave the team both the temporal resolution to follow the motion and the spectral resolution to separate closely spaced ionic resonances.
In xenon, the pump pulse created a coherent superposition of spin–orbit states in Xe⁺. The absorption signal oscillated with a period of 3.1 ± 0.1 fs, matching the expected valence-hole motion between those states and resolving dynamics that had remained too fast for earlier XUV–infrared measurements.
The team also tracked longer-period electron wavepackets in argon, krypton, and neon. In neon, absorption features linked to inner- and outer-valence excitation oscillated out of phase. Calculations traced that contrast to changes in the shape of the hole wavefunction, which shifted between dumbbell-like and doughnut-like distributions as the wavepacket evolved.
The authors argue that high-harmonic generation offers a stable and more widely accessible route to all-attosecond transient absorption without relying on a free-electron laser. Extending the method to higher photon energies could bring the same combination of temporal and spectral resolution to core-level dynamics in molecules and solids.
Physics-Paired Raman Restoration
The self-supervised framework boosts signal-to-noise while preserving quantitative Raman contrast and three-dimensional structure.
A new physics-paired framework, named PHYSIQ, has improved the signal-to-noise ratio of fast volumetric stimulated Raman scattering microscopy, enabling label-free three-dimensional tracking of lipid droplets in living cells.
Fast three-dimensional SRS imaging is limited by short pixel dwell times, which leave chemically specific signals buried in shot noise and make conventional denoising difficult without clean references or repeated frames.
To address this, PHYSIQ generated two near-synchronous images of the same focal volume using in-phase and quadrature detection. The channels contained the same Raman signal but statistically independent shot noise, producing paired measurements for self-supervised restoration without clean ground truth or temporal averaging.
The method improved the effective signal-to-noise ratio by around 12.5 dB while preserving quantitative Raman contrast and structural detail. In tests on fixed cells and static microspheres, the reconstructed volumes approached the quality of longer averaged measurements while retaining sharper boundaries.
The team then targeted the lipid-associated CH₂ vibration near 2850 cm⁻¹ to track lipid droplets in living cells. The restored volumes allowed individual droplets to be segmented in three dimensions and their trajectories classified as directed, Brownian-like, or confined.
Those motility states shifted with cellular conditions. Oleic-acid loading increased lipid droplet abundance but reduced their overall spatial exploration, while preserving directed movement near the nucleus. Glycolytic inhibition suppressed long-range transport and shifted droplets toward shorter, more confined trajectories. During mitosis, confinement peaked in anaphase before directed transport partly returned in telophase.
The researchers argue that these condition-dependent trajectories provide a label-free readout of cellular physiological state. They also suggest that extending PHYSIQ across multiple Raman bands could add chemical composition to the measurements, allowing lipid organization and metabolic remodeling to be followed together.
Permethrin Persists
SERS detects permethrin on a feather specimen treated more than 20 years ago, while revealing strong material-dependent interference.
Historic pesticide treatments can remain hidden in museum collections long after records have been lost. Surface-enhanced Raman scattering (SERS) has now identified permethrin, a synthetic pyrethroid insecticide, on a real feather specimen treated more than 20 years ago, while also showing how strongly the underlying heritage material affects detection.
Organic pesticides are difficult to identify with routine collection-analysis methods. X-ray fluorescence cannot directly distinguish compounds such as permethrin, while chromatography requires specialist facilities and more involved sampling.
The team therefore compared silver- and gold-nanoparticle SERS substrates as a more accessible screening route. Gold nanoparticles gave the stronger enhancement, allowing permethrin to be detected in solution down to 0.1 ppm through its benzene-ring breathing band near 1002 cm⁻¹.
The method was then tested on contaminated cotton, balsa wood, feather, and buriti fiber mockups. Conventional Raman spectra did not resolve the pesticide, whereas SERS recovered the marker from cotton, wood, and feather. Cotton gave the clearest signal, while buriti fiber produced an overlapping band that prevented reliable identification. Feather also introduced nearby features that could obscure low concentrations.
The researchers next examined a feather item from the Museum of Archaeology and Ethnology at the University of São Paulo, labeled as having been treated with a commercial insecticide in 2004. SERS detected a band consistent with permethrin, and gas chromatography–mass spectrometry confirmed the assignment. The chromatographic analysis also found DDT, pointing to an additional undocumented treatment.
After gamma irradiation, the permethrin band disappeared from most mockups, although GC–MS showed that some residues remained. The authors therefore frame SERS as a screening and treatment-monitoring tool rather than a replacement for chromatography, with further work needed to improve detection limits and manage interference from complex heritage materials.
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