Chirality, the intriguing property of molecules having mirror-image isomers, holds pivotal importance across various scientific disciplines, with the biomedical industry being a particularly notable beneficiary. Yet, for scientists, the quest to detect single chiral molecules remains a formidable challenge.

In this narrative, we embark on a journey through the realms of nanophotonics, unveiling a revolutionary method for achieving the coveted single molecule sensitivity. Our story unfolds at the intersection of classical and quantum plasmonic effects, where nanophotonics takes center stage as the enabler of this breakthrough.

The Beginning of an Extraordinary Journey

Our story begins in the vast landscape of science and discovery, where the mysteries of chiral molecules have long fascinated and confounded researchers. These molecules, like left and right hands, are identical in composition but fundamentally different in their spatial arrangement. This property, known as chirality, influences their interactions with other molecules and plays a vital role in fields such as pharmaceuticals and biochemistry.

While the importance of chirality is evident, detecting single chiral molecules has remained a challenge of monumental proportions. Traditional techniques, like Raman spectroscopy and fluorescence spectroscopy, have their limitations in distinguishing enantiomers. It is here that we introduce a groundbreaking approach that promises to unlock new horizons in single molecule chiral sensing.

Nanophotonics: A Glimpse into the Extraordinary

Our journey of discovery takes us into the realm of nanophotonics, a field that explores the behavior of light at the nanoscale. It is in this world of extreme miniaturization that we find the key to achieving single molecule sensitivity in chiral molecule detection.

Imagine a landscape where structures are so small, they exist on the order of a few billionths of a meter. In this extraordinary world, nanophotonics offers the potential to manipulate light in ways that were once thought impossible.

The Oligoamide Sequences (OS): Pioneering the Breakthrough

In our quest to bridge the gap between classical and quantum plasmonic effects, we introduce a key player: the helical oligoamide sequences (OS). These OS are composed of quinoline-based octamers and contain a stereogenic center. Their unique design allows them to fold into helical superstructures with full P or M-helicity control.

This innovation serves as the bridge between the macroscopic classical world and the mysterious quantum realm. The OS's ability to form distinctive absorption and circular dichroism (CD) in the ultraviolet (UV) region is pivotal to our story. The OS can be further duplexed into enantiomers of double helices (DHs) via π-π interactions. Importantly, their racemic mixture shows no CD response across the entire spectrum. However, when these racemic OS molecules are introduced into ultrathin metal cavities of nanoparticle-on-mirror (NPoM) structures, a transformation occurs.

The NPoM Resonator: Unveiling Quantum Plasmonic Effects

The NPoM resonator, a remarkable nanophotonic construct, consists of an Au nanoparticle on a mirror surface with sub-nanometer-sized gaps. These gaps provide an intense electric field confinement, with an intensity that increases exponentially as the gap size decreases. This is where quantum plasmonic effects come into play.

In the quantum tunneling regime, which is often viewed as detrimental for plasmonic enhancement due to reduced local electric field strength, the NPoM system reveals an unexpected truth. Here, the OS molecules within the gaps interact with tunneling electrons, resulting in a significant increase in circular differential scattering (CDS) despite the challenges posed by the quantum tunneling effect. This generalized model of plasmon-enhanced chirality offers a profound understanding of the interactions between light and chiral matter at different scales.

The Potential for Single Molecule Sensitivity

One of the most remarkable outcomes of this work is the achievement of single molecule sensitivity. Using the NPoM system with OS molecules, the study demonstrates a great enhancement of chiral sensitivity, even in the quantum tunneling region, where the local electric field intensity decreases. The key to this breakthrough lies in the giant Coulomb interactions between the chiral OS molecules and the achiral NPoMs, coupled with the alignment of the chiral dipole to the local electric field with a small mode volume. These factors contribute to a g-factor of up to 0.3.

The implications of this achievement are monumental. The minimum number of detected molecules, based on a single gold (Au) particle, is approximately four. This means that the system has the potential to detect an enantiomeric excess within a sample, offering a promising capability for single molecule sensing. The NPoM system not only reveals racemates showing local chirality but also leverages additional chiral enhancement via tunneling electrons, opening new doors in the realm of chiral molecule sensing.

Local Chirality of Racemates: A Surprising Revelation

A fascinating aspect of this journey is the revelation of local chirality within racemates. These racemic mixtures, which appear achiral at first glance, can be locally resolved as enantiomer excess (ee) due to population fluctuations within the single NPoM gap. The study reveals that a significant portion of Au NPoMs exhibits diverse chiroptic responses. Some display CDS intensity of opposite signs, while others show no CDS response at all. This observation leads to a profound understanding of the local enantiomer excess within the NPoM system.

Contribution of Quantum Tunneling: Unveiling the Quantum Plasmonic Effects

To understand the contribution of quantum tunneling to the circular differential scattering effect, the paper investigates the effect of reducing the gap size. The findings indicate that electron tunneling enhances CDS intensity, even below the quantum tunneling limit. The reduction of the gap size leads to a change in the plasmon resonances of the Au NPoM, followed by a quantum correction. The study confirms that Coulomb interactions between the chiral OS and tunneling electrons further enhance the CDS intensity below the quantum tunneling limit, providing insights into the full description of chiral light-matter interactions.