Chiral Plasmonic Nanoparticles

• Chiral plasmonic nanoparticles are nanoscale metallic structures lacking mirror symmetry, enabling strong circular dichroism and optical rotary dispersion.
• Advanced fabrication techniques like DNA origami and top-down lithography yield sub-2 nm spatial precision with nearly 97% assembly accuracy.
• Their tunable optical properties drive applications in enantioselective sensing, catalysis, and active photonic devices with dynamic, switchable responses.

Chiral plasmonic nanoparticles are nanoscale metallic structures whose three-dimensional arrangements lack mirror and inversion symmetry, resulting in pronounced optical activity under circularly polarized light. These systems merge classical electromagnetism with molecular chirality concepts, utilizing engineered near-field interactions of conduction electrons—surface plasmons—to elicit giant chiroptical responses. Beyond being model systems for fundamental studies of light–matter interactions, chiral plasmonic nanoparticles are pivotal in applications ranging from enantioselective sensing and catalysis to optical information processing and the creation of artificial metamaterials with tailored functionality.

1. Synthesis and Structural Engineering

Two principal methods dominate the fabrication of chiral plasmonic nanoparticles: bottom-up self-assembly, notably via DNA origami, and top-down lithographic techniques. The DNA origami approach exemplifies spatial control at the nanometer scale, involving the folding of a long scaffold strand into a rigid 24-helix DNA bundle using hundreds of short staple strands programmed for precise spatial addresses. Metallic nanoparticles, most frequently gold with diameters around 10 nm, are conjugated with complementary DNA and positioned onto these scaffolds with spatial accuracy better than 2 nm (Kuzyk et al., 2011). Experimental verification by TEM confirms 97% yield on intended positions, yielding nearly perfect left- or right-handed helices.

Advances in materials integration further extend the optical tunability of these architectures. Sequential electroless deposition allows for gold nanoparticles to be coated with silver shells (~3 nm) or Au–Ag alloys. This compositional control enables spectral tuning across the visible band due to material-specific plasmon resonance shifts. Emerging methodologies utilize inorganic nanotube templates (e.g., WS₂ tubes) that provide chiral step-edges upon overgrowth, allowing for high-temperature assembly (>500 °C) and the functionalization with a range of metallic, semiconducting, or oxide nanoparticles. This route is not limited by the temperature sensitivity of organic templates and enables fabrication of robust chiral assemblies (Kachtík et al., 2023).

2. Optical Activity: Circular Dichroism and Optical Rotary Dispersion

The defining signatures of chiral plasmonic nanoparticles are their strong circular dichroism (CD) and optical rotary dispersion (ORD). The bisignate (peak-dip) line shapes of CD spectra are direct manifestations of the collective coupling between localized surface plasmon (LSP) modes in the helical (or otherwise chiral) configuration. The difference in absorbance for left- (LCP) and right-circularly polarized (RCP) light, $\Delta A = A_{LCP} - A_{RCP}$, is enhanced by coherent near-field coupling and can vastly exceed molecular CD signals (Kuzyk et al., 2011, Hentschel et al., 2021).

ORD arises macroscopically: a beam of linearly polarized light transmitted through a suspension (“metafluid”) of nanohelices undergoes a wavelength-dependent rotation, with the sense of rotation directly tied to the handedness of the nanoparticles. The observed response is independent of the propagation direction due to the isotropy of the suspension and the fundamental three-dimensionality of the nanohelices.

A critical scaling result quantifies the plasmonic CD signal (CD$_{plasmon}$) as a strong function of nanoparticle diameter ($a_{NP}$) and helix radius ($R_0$):

$$\mathrm{CD}_{\mathrm{plasmon}} \propto \frac{a_{NP}^{12}}{R_0}$$

Therefore, modest increases in $a_{NP}$ (e.g., from 10 to 16 nm) can result in orders-of-magnitude enhancement of the CD response (Kuzyk et al., 2011).

3. Coupling Phenomena and Mediated Chirality Transfer

The origin and morphology of plasmonic chirality are multifaceted. In DNA-assembled chains, achiral transmitters (e.g., gold nanospheres) placed between chiral-configured nanorods relay and enhance chiral energy transfer over distances up to ~100 nm (Martens et al., 2020). This mediated coupling amplifies the CD amplitude (by factors >3) and introduces new chiral spectral features aligned with nanoparticle resonances. The addition of a transmitter particle is empirically associated with a measurable redshift in the longitudinal plasmon resonance, indicative of strengthened electromagnetic coupling between rods.

Hybrid systems comprising plasmonic arrays embedded in chiral media demonstrate “chirality transfer” from molecular to plasmonic lattice modes, resulting in CD features at both localized and collective (surface lattice resonance, SLR) energies. This effect can be rigorously modeled using coupled-dipole theory with dyadic Green’s functions and Pasteur constitutive parameters, and is highly tunable with lattice periodicity (Goerlitzer et al., 2023).

In addition to purely structural chirality, recent work shows that the dielectric scaffold itself (such as the DNA origami trunk, modeled as a dielectric cylinder) can induce or modulate the chiral near-field distribution—an effect termed chiral plasmonic–dielectric coupling (Martens et al., 2021).

4. Switchable and Meso-Chiral Behavior

Dynamic regulation of plasmonic chirality is achieved via external stimuli. Hydrogenation of Mg-Au hybrid metamolecules reversibly switches the chiroptical response on and off, as magnesium transitions between metallic and dielectric magnesium hydride phases (Duan et al., 2018). Phase-change materials and optically or chemically gated DNA origami assemblies allow rapid and reversible control over the chiral spectral fingerprint, massively expanding the design space for active photonic devices (Hentschel et al., 2021).

A recently recognized phenomenon is “meso-chirality” [Editor's term]: particles where absorption and scattering CD responses are of equal amplitude but opposite sign, leading to hidden or “undetectable” chirality in standard extinction-based CD experiments. Multi-wound SiO₂/Au nanoparticles and gold helicoids represent system archetypes in which local chiral responses persist (manifest as heating under different circular polarizations), even when the g-factor in extinction, $g_{ext}$, is zero (Xie et al., 24 Sep 2025). The explicit relationship

$$g_{ext} = \frac{g_{abs}\cdot \langle S_{abs} \rangle + g_{scat}\cdot \langle S_{scat} \rangle}{\langle S_{abs} \rangle + \langle S_{scat} \rangle}$$

describes the weighted cancellation and highlights the necessity of characterizing absorption and scattering separately for accurate interpretation.

5. Analytical Models and Quantitative Descriptors

The macroscopic chiroptical response is underpinned by coherent coupling and multipole interference. Oscillator-based models describe metal nanoparticles as coupled harmonic oscillators with distinct eigenfrequencies and damping rates (Cheng et al., 2023). Symmetry-breaking (in oscillator parameters or spatial orientation), coupling strength (distance, orientation, environment), and coherent interference all contribute to the measured chirality in scattering, absorption, and photoluminescence. The chiral character can be heightened or reversed by slight modifications in geometry or material parameters.

For nonlinear regimes, plasmonic chiral helicoid nanoparticles exhibit second-harmonic generation (SHG) with nonlinear g-factors ($g_{NL}$) that can exceed $|1.6|$, surpassing their linear optical counterparts by nearly an order of magnitude due to the quadratic field dependence in nonlinear processes (Spreyer et al., 2022).

6. Applications: Sensing, Catalysis, and Information Processing

Chiral plasmonic nanoparticles are active components in ultrasensitive biosensing, enantiomeric recognition, and optical information protocols. 3D chiral metacrystals supporting surface lattice resonances (c-SLRs) integrate narrow spectral features (high $Q$) and intrinsic CD for refractive index sensing with sensitivities exceeding 500 nm/RIU (2207.14710). In nanophotonic platforms combining chiral nanoparticles and chiral metafilms, the surface-enhanced Raman spectroscopy (SERS) response is maximized for matched diastereomeric pairs (LL, RR), enabling discrimination at detection limits >$10^6$ times lower than standard CD-based probes (Kartau et al., 2022).

Plasmon-induced enhancement of molecular CD permits detection of otherwise undetectable chiral signals and can be harnessed for precise bioanalytics or drug development (Besteiro et al., 2017, Urban et al., 2021). In catalysis, polarization-sensitive hot electron generation in chiral plasmonic nanocrystals enables chiral-selective photochemical reactions and enantioselective crystal growth (Liu et al., 2019). The emergence of rotating chiral dipole states in engineered nanohelicoids supports enantio-sensitive, unidirectional forward scattering, a property essential for nanoscale light routing and the architectural design of optical nano-antennas and quantum photonic elements (Xie et al., 26 Aug 2024).

7. Outlook and Limitations

Ongoing challenges include achieving greater structural complexity with scalable, cost-effective fabrication, particularly for bulk metamaterials (Guglielmelli et al., 2023). Nonlinear, reconfigurable, and active chiral plasmonic systems will benefit from intelligent optimization (AI-driven geometric search spaces), improved theoretical tools incorporating full retardation and environment effects, and integration with a broader range of functional materials (e.g., phase-change alloys or molecular actuators). Advanced characterization techniques that decouple absorption and scattering, such as photothermal or angle-resolved spectroscopies, are critical for revealing hidden chiral properties, as meso-chirality can render conventional CD ambiguous (Xie et al., 24 Sep 2025).

The confluence of precise nanoscopic assembly, advanced modeling, and dynamic functionalization positions chiral plasmonic nanoparticles at the forefront of optical, chemical, and quantum device engineering, with continual progress anticipated as methods mature and new chiral phenomena are uncovered.