Enantiomers — mirror-image molecular twins with identical composition but often drastically different biological activity — must be detected in vanishingly small quantities during drug development, yet their intrinsic optical handedness signals are extremely weak. A 2017 twisted plasmonic metamaterial platform amplified molecular circular dichroism (CD) by roughly two orders of magnitude and discriminated as few as 55 zeptomoles of enantiomer, but the authors themselves flagged the conditions the method had not met: it measures only cumulative optical activity (no information about individual chiral centers), it relies on post-processing calibration to cancel the metamaterial's own large background CD against fabrication imperfections, its numerical model omits real substrate and adhesion-layer losses, its detection limit is set by detector noise rather than any demonstrated physical bound, and the envisioned nanofluidic flow-through geometry (molecules passing between the twisted layers) was not realized. The 2017–2026 follow-up literature advances the individual ingredients on adjacent platforms — racemic arrays with zero intrinsic background, absolute chiral-index measurements via signal reversals, zeptomole-level vibrational-CD fingerprints, collective-resonance enantioselective sensing, and few-molecule (about four molecules per particle) detection in quantum plasmonic nanocavities — while the single-molecule enantioselective limit, quantified calibration-free background suppression, general deconvolution of multi-chiral-center contributions, and flow-through implementation remain open. The problem asks for the completion of this sensing modality: reaching and establishing the few-/single-molecule detection limit, suppressing background without calibration-dependent post-processing under realistic fabrication tolerances, extracting chiral information beyond cumulative optical activity, and validating the result against a quantitative loss model in an integrated flow-through geometry.

Background

Many biomolecules — amino acids, sugars, nucleotides — are chiral: their two mirror-image forms, called enantiomers, share nearly all physical properties but can differ drastically in pharmacological potency and toxicity, so fast, unambiguous detection of molecular handedness in tiny samples is a standing need in pharmaceutical and biomedical analysis. The standard laboratory technique, circular dichroism (CD) spectroscopy, measures the difference in absorption of right- and left-handed circularly polarized light by a chiral sample; for small molecules this differential signal is intrinsically tiny (typically tens of millidegrees, usually in the ultraviolet), which forces minute-scale integration times, micromolar concentrations and sub-millilitre volumes. Nanophotonics attacks this weakness by concentrating chiral optical near fields — 'superchiral' regions where the local optical handedness density exceeds that of a circularly polarized plane wave — into nanometer-scale volumes around plasmonic (metal) or dielectric nanostructures, so that femtogram-scale molecular monolayers interact with enhanced fields. Early schemes had two failure modes: achiral plasmonic particles give intensity enhancement but handedness-ambiguous, orientation-dependent signals, while chiral metasurfaces (planar arrays of sub-wavelength nanostructures with engineered handedness) generate a huge background CD of their own that can swamp the weak molecular signal, and handedness-independent refractive-index shifts that contaminate spectral-shift-based readout.

The source work, Zhao, Askarpour, Sun, Shi, Li and Alù's 2017 Nature Communications paper, addressed the background problem with a twisted optical metamaterial: a planar plasmonic metasurface whose unit cells are twisted gold nanorod configurations, fabricated in an 'enantiomeric pair' of substrates with opposite twist angles (+60° and −60°) whose own CD spectra are equal and opposite. Because the two substrates enhance molecular CD with the same magnitude but opposite substrate background, summing the CD measured on the two substrates with the same analyte cancels the background and isolates the molecular signal with a sign that univocally identifies handedness. This platform detected enantiomers of 1,2-propanediol and of proteins down to 55 zeptomoles (a zeptomole is 10−2110^{-21} moles, roughly a few tens of thousands of molecules) at visible and near-infrared wavelengths, with sub-second acquisition. Robustness, however, came from a calibration step performed on the bare substrates, since fabrication imperfections leave small residual differences between the nominally enantiomeric substrates; simulations disagreed with experiment at short wavelengths because adhesion-layer and substrate losses were not modeled; and the readout, like all CD measurements, reports only the cumulative optical activity of the analyte, not the contribution of individual chiral centers (the stereocenters of a molecule at which handedness arises). Related threads in the field set the context for what follows: racemic arrays (balanced mixtures of left- and right-handed nanostructures) with zero net intrinsic CD; vibrational circular dichroism (VCD), the infrared analogue of CD that probes vibrational modes and therefore carries molecular-fingerprint information; collective resonances in self-assembled chiral nanoparticle crystals; and plasmonic nanocavities such as nanoparticle-on-mirror structures in which quantum tunnelling across sub-nanometer gaps boosts light–matter interaction. Several reviews of nanophotonic chirality sensing explicitly name single-molecule enantioselective detection as the field's ultimate goal, and clinical-oriented reviews note that cost-effective, reliable quantification of small chiral molecules at low concentration remains difficult. What was still missing in 2017 — and what the follow-up literature has only partially delivered — defines the problem stated below.

Problem Statement

Can enantiomer-selective optical detection based on nanophotonic enhancement of molecular circular dichroism be completed as a trusted, quantitative sensing modality? The originating demonstration — a pair of twisted plasmonic metamaterials with opposite twist angles whose summed CD cancels the substrate background and univocally reveals molecular handedness at zeptomole levels — left open the following jointly defining conditions, each addressing a distinct failure mode of the same measurement paradigm (sensitivity, systematic error, information content, sample delivery, and model fidelity, respectively):

  1. Sensitivity limit: determine and reach the practical detection limit of enhanced-CD enantioselective sensing — in particular the few-molecule and ultimately single-molecule regime — when combined with optimized detection instrumentation, rather than the detector-noise-limited performance of a proof-of-principle set-up.
  2. Background suppression: achieve cancellation of the metamaterial's own large background CD without relying on post-processing calibration of a fabricated enantiomeric substrate pair, and quantify the residual systematic error under realistic fabrication tolerances, so that the sign of the molecular chiral response is reliable without calibration.
  3. Information content: go beyond cumulative optical activity — extract chiral information specific to a molecule's structure, such as vibrational fingerprints or the contributions of distinct chiral centers, including for molecules with multiple stereocenters.
  4. Sample delivery: realize the flow-through geometry (molecules flowing between, rather than adsorbed atop, the twisted layers, integrated with nanofluidic systems) and establish its stability, throughput and sensing specificity.
  5. Model fidelity: establish a complete quantitative model of the platform, including substrate and adhesion-layer losses and their variability, that reproduces the measured spectra across the operating wavelength range.

References

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  2. Jose García-Guirado, Mikael Svedendahl, Joaquim Puigdollers, Romain Quidant, "Enantiomer-Selective Molecular Sensing Using Racemic Nanoplasmonic Arrays", Nano Letters (2018). DOI: 10.1021/acs.nanolett.8b02433, https://doi.org/10.1021/acs.nanolett.8b02433
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