A terahertz platform built on an achiral gradient metasurface resolves the components, handedness, and mixing ratios of chiral biomolecule mixtures in a single broadband measurement.
(Nanowerk Spotlight) Many molecules come in two versions that are perfect mirror images of one another, like a left and a right hand. Chemically, the two look almost identical. Biologically, they can behave very differently. One version of a drug molecule may ease pain while its mirror twin causes birth defects. One form of an amino acid may nourish the body while its opposite is inert or toxic. This property, called chirality, sits at the center of pharmacology, nutrition, and food safety.
The standard tool for telling the two hands apart is a technique called circular dichroism. The idea behind it is simple enough. Light can be made to spiral as it travels, either clockwise or counterclockwise, and a chiral molecule absorbs one spiral direction slightly more strongly than the other. Measuring that small difference reveals the molecule’s handedness.
Each available version of the technique fails in its own way. Ultraviolet circular dichroism produces only faint signals and can destroy fragile biomolecules through light-induced breakdown. Visible and infrared versions, often enhanced by engineered metasurfaces, produce stronger signals. They miss the target, though, because the molecule’s own chiral vibrations do not live in those frequency bands.
Terahertz light fills a narrow slot in the spectrum between microwaves and infrared, and it happens to oscillate at the same rate as the collective low-frequency vibrations of whole biomolecules. These motions include the chiral phonons that encode a molecule’s handedness.
A terahertz circular dichroism measurement can therefore reveal not just that a molecule is chiral, but which specific molecule it is and which hand it belongs to. Previous work has shown what is possible with new high-resolution terahertz spectroscopy platforms.
The catch is that the intrinsic signal is tiny, typically only a few degrees. Researchers have tried to amplify it using chiral metasurfaces, which carry their own handedness. Those surfaces add their own background chirality to the measurement, contaminating the very signal they are supposed to reveal. Separating the sample’s fingerprint from the sensor’s own contribution requires heavy post-processing and often leaves ambiguity in the result.
Principles of the broadband THz eigenmode-fingerprint chiroptical spectroscopy (TEFCS) method for chiral biomolecules. (a) Plasmonic achiral metasurface and structural configuration of the unit cell. The structural parameters are as follows: the periods of the metasurface p1 and p2 along the x- and y-directions, respectively; the length of the short gold bar l2; the length of the long gold bar l1; and the width of all bars w. Here, the blue helix represents the LCP light, and the red helix represents the RCP light. (b) Normalized electric fields in the xz and xy planes bisecting the gold cross (Exz plane and Exy plane). (c) Schematics of the energy levels of the chiral biomolecule and metasurface. The typical energy level transitions from the ground (|0〉) state are depicted by different arrows. With respect to the energy levels of the metasurface, the electric field in the UV band is distinguished from those in the other bands because of the different mechanisms involved. (d) Schematic of the highly distinguishable broadband TEFCS approach for multiple chiral biomolecules. The period of the broadband metasurface in the y-direction is p′, and the distance between two structures is g. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
Their platform uses an achiral gradient metasurface, an array of microscopic gold crosses with differently sized arms patterned on a quartz substrate. Because the metasurface has no handedness, it produces zero background chirality when spiraling terahertz light strikes it head-on. Any signal the detector records must come from the sample sitting on top. The idea draws on earlier advances in chiral nanophotonics using anisotropic lattice designs.
The gradient in the design refers to a deliberate variation in resonator size across the surface. Each resonator rings at a slightly different terahertz frequency, and together they form a broadband mirror that stays highly reflective from roughly 0.5 to 1.8 THz. This wide window was the key technical move.
Earlier achiral metasurfaces worked only at a single narrow frequency, forcing researchers to tune the sensor to one molecule at a time. A broadband sensor can instead scan the full fingerprint region where different biomolecules reveal their chiral vibrations.
The measurement relies on a specific physical condition. When spiraling terahertz light strikes an anisotropic but non-chiral surface that carries a chiral coating, the surface converts part of the incoming polarization into its orthogonal counterpart. The conversion scales with the chirality of the coating, a quantity physicists call the Pasteur parameter. Comparing how the clockwise and counterclockwise reflections differ yields this parameter and with it the intrinsic chiral fingerprint of whatever molecule sits on the surface.
A coupled oscillator model that handles electric, magnetic, and magnetoelectric responses together predicts this behavior. The theoretical curves, the simulations, and the experimental spectra align closely, which supports the interpretation that the signal originates in the biomolecule rather than in any subtle asymmetry of the apparatus.
The team tested the system on eight amino acids: the left- and right-handed forms of histidine, tyrosine, glutamic acid, and glutamine. Each produced a distinct peak or valley in the circular dichroism spectrum at the frequency of its characteristic chiral vibration. The sign of the peak flipped between left- and right-handed forms, giving an unambiguous chirality readout. Signals reached roughly 15 degrees, an order of magnitude stronger than the same amino acids produced on a bare substrate.
The platform also handled mixtures. When two amino acids sat together on the sensor, each one registered its own characteristic peak at its own frequency. Both the identity and the individual handedness of each component could be read off the spectrum in one pass. The team then varied the mixing ratio between left- and right-handed versions of single amino acids.
The signal amplitude scaled linearly with the ratio, vanishing at the racemic halfway point and peaking at the pure forms. A single measurement therefore reports composition, handedness, and mixing ratio.
Sensitivity reached two orders of magnitude beyond previous terahertz chiral phonon detection, and the system detected down to roughly 10⁻² grams of amino acid. The gains come from the spectral overlap between the broadband metasurface resonances and the molecular chiral vibrations, which strengthens the coupling between photons and phonons in the biomolecule layer.
Some limitations remain. The broadband response is not perfectly flat, and the accessible range cuts off at 1.8 THz, leaving higher-frequency molecular modes out of reach. Closely spaced resonances in related molecules would also be hard to separate with the current bandwidth. The authors point toward wider broadband designs and optimized inter-resonator couplings as the next steps.
Even with these constraints, the work shows a useful inversion of the usual strategy. Removing chirality from the sensor and engineering the bandwidth instead yields a terahertz tool that reads molecular handedness in real mixtures in a single shot. The approach points toward practical instruments for pharmaceutical quality control, clinical diagnostics, and food authentication, extending the reach of label-free chirality detection methods into the terahertz band.
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