Gold Helicoid Nanoparticles

• Gold helicoid nanoparticles are chiral, three-dimensional nanostructures whose geometry and plasmonic properties are precisely controlled via advanced bottom-up and top-down fabrication techniques.
• They feature unique core–shell architectures with tunable optical resonances near 570 nm and 950 nm, enabling applications in plasmonic sensing and chiral metamaterials.
• Emerging fabrication strategies—including EBID with plasma purification, self-assembly in soft matter, and DNA-templated dynamic assembly—offer versatile platforms for reconfigurable nanophotonics.

Gold helicoid nanoparticles are chiral, three-dimensional nanostructures whose geometry, crystallography, and plasmonic properties are controlled at the nanoscale, predominantly using bottom-up and top-down fabrication techniques. They serve as prototypical platforms for studying the interplay between structure-induced optical activity, localized surface plasmon resonance (LSPR), and the role of crystallographic and strain fields in functional nanophotonics and chiral plasmonics. Their unique core–shell architectures, controlled symmetry, and dynamic configurability enable a range of optical and sensing applications.

1. Fabrication Strategies

Gold helicoid nanoparticles can be synthesized via several routes, each imparting precise control over three-dimensional geometry, composition, and ultimately, plasmonic function.

Electron Beam Induced Deposition with Plasma Purification

Direct writing by electron beam induced deposition (EBID) is an established approach for fabricating three-dimensional gold helices with sub-50 nm features (Haverkamp et al., 2016). In this method, a metal-organic precursor (e.g., Me₂Au(acac)) is dissociated by a focused electron beam, resulting in deposition of gold nanocrystals within a carbonaceous matrix. Precise control (e.g., electron energy: 15 kV; beam current: 203 pA; pixel spacing: 1 nm for 3D constructs) permits helical morphogenesis.

To overcome the inherent carbon contamination of as-deposited EBID structures, a post-fabrication cold oxygen plasma purification step is performed. This process oxidizes and removes carbon from the outer regions, reducing the overall diameter by ~18 nm while maintaining geometric fidelity. The result is a core–shell nanostructure: a carbon-rich core clad by a ~20 nm thick gold shell.

Self-Assembly in Soft Matter Hosts

Helicoidal order at the colloidal scale is achieved by dispersing gold nanorods in lyotropic liquid crystal phases formed by oblate micelles (Liu et al., 2016). Anisotropic surface anchoring between the rods and the host leads to perpendicular alignment with respect to the local director. Addition of a chiral dopant (e.g., brucine sulfate heptahydrate) induces a cholesteric twist, yielding a helicoidal nanorod organization with a negative orientational order parameter.

DNA-Templated Dynamic Assembly

Recent work demonstrates the use of DNA origami scaffolds and “swingarm” mechanisms to create programmable, reconfigurable plasmonic helices from multiple gold nanoparticles (AuNPs) (Peil et al., 13 Jun 2025). Here, AuNPs tethered to flexible DNA arms are dynamically moved between defined binding sites along a 24-helix bundle via toehold-mediated strand displacement, enabling large translational “leaps” and reversible handedness switching.

2. Structural and Crystallographic Features

The structural integrity and crystalline configuration of gold helicoid nanoparticles are critical determinants of their optical and electronic performance.

Core–Shell Architecture and Dimensional Control

EBID-derived helices retain their shape through plasma purification provided the feature diameter exceeds ~60 nm (Haverkamp et al., 2016). The gold-enriched shell (~20 nm thick) exceeds the skin depth for visible light (typically ~15 nm for Au), ensuring bulk-like optical response. Transmission electron microscopy confirms the shell’s composition and thickness; smaller-diameter helices (<60 nm) are prone to deformation during plasma treatment.

High-Index Faceting and Chiral Gap Morphology

Bragg coherent X-ray diffraction imaging (BCDI) enables three-dimensional reconstruction of both morphology and lattice parameters in chiral concave gold nanoparticles (Choi et al., 2022). The concave gaps are characterized by a mixture of high-Miller-index planes and lower-index facets (e.g., {110}, {111}), producing a 432-symmetric structure. Stereographic projections show fourfold, threefold, and twofold symmetry along <100>, <111>, and <110> axes, respectively. The absence of symmetry-equivalent Miller indices across mirror planes evidences broken inversion symmetry and the presence of chirality.

Strain-Field Topology

BCDI mapping reveals localized strain fields adjacent to chiral gaps, with both tensile and compressive components reaching magnitudes of order 10⁻³ (Choi et al., 2022). For gaps facing <101>, compressive strain is concentrated at concave edges; adjacent facets experience tensile strain. These strain distributions influence local dielectric function and thus LSPR, establishing a direct link between atomic structure and optical performance.

3. Plasmonic and Chiroptical Properties

Gold helicoid nanoparticles exhibit rich plasmonic behavior arising from their geometric chirality, shell composition, and mesoscale arrangement.

Core–Shell Plasmonics

After oxygen plasma purification, the gold shell dictates the optical response due to the skin effect: only surface regions exceeding the optical skin depth contribute strongly at visible frequencies (Haverkamp et al., 2016). Finite element analysis predicts and experimental dark-field scattering confirms two principal resonances at ~570 nm and ~950 nm, in addition to a shoulder near 790 nm. The normalized scattering spectrum is given by

$$I_{\text{norm}} = \frac{I_{\text{scattering}} - I_{\text{dark}}}{I_{\text{source}}}$$

ensuring direct experiment-simulation correspondence.

Anisotropic Alignment and Negative Order Parameter

In soft matter composites, the orientational order of gold nanorods is quantified by

$$S_{\text{GNR}} = \left\langle \frac{3\cos^2\theta - 1}{2} \right\rangle$$

where $\theta$ is the angle between rod axis and the liquid crystal director. Experimentally, $S_{\text{GNR}} \approx -0.39$ indicates prevalent perpendicular alignment (ideal: $-0.5$) (Liu et al., 2016). The anisotropic surface anchoring energy follows the Rapini–Papoular form

$f_{\text{sa}} = \frac{W}{2} \sin^2\beta$

with $W$ the anchoring coefficient. The equilibrium orientation distribution arises from the Boltzmann factor,

$$f(\theta) \propto \exp\left(-\frac{8\pi L R W}{2 k_BT} \cos^2\theta\right)$$

(L, R: nanorod length and radius).

Tunable Chiroptical Response

DNA-swingarm-assembled helices allow for programmable and reversible switching between left- and right-handed helical arrangements, as detected by circular dichroism (CD) spectroscopy (Peil et al., 13 Jun 2025). Transitioning between six distinct geometric states modulates the CD signature—from featureless (disordered) in state 0 to bisignate peak-dip (LH) and dip-peak (RH) patterns in states I and VI, respectively. The optical response matches finite element simulations and is enhanced for larger AuNPs (~18 nm). This dynamic chiroptical control distinguishes swingarm systems from static DNA origami assemblies.

4. Characterization Techniques

Rigorous assessment of gold helicoid nanoparticles employs a diverse array of experimental and computational techniques:

Technique	Targeted Aspect	Notes
Transmission Electron Microscopy (TEM)	Shell thickness, morphology	Identifies core–shell formation and shape retention
Bragg Coherent X-ray Diffraction Imaging (BCDI)	3D crystallography and strain field mapping	Resolves high-index faceting and local lattice strain
Dark-Field Microscopy	Optical scattering, plasmonic resonance	Reveals resonances and chiral optical hotspots
Circular Dichroism Spectroscopy	Chiroptical activity	Tracks handedness and reconfigurability in DNA-assembled helices
Two-Photon Luminescence Imaging	Nanorod orientation, 3D pattern	Polarization-sensitive mapping in composite fluids

Additional tools include polarization-sensitive absorption/scattering for quantifying orientational order, and electromagnetic modeling (finite element method) for simulating spectral response and matching experimental observations.

5. Applications and Technological Potential

The combination of three-dimensional architecture, chiral symmetry, and plasmonic functionality enables a suite of advanced applications for gold helicoid nanoparticles:

• Plasmonic Sensing: Enhanced field localization and surface activity make these helices suited for detecting molecular adsorption and chiral analytes via resonant shifts or CD signal modulation.
• Chiral Metamaterials: Designed broadband circular polarization manipulation and chiral photonic metamaterials benefit from the pronounced CD and geometric tunability of gold helicoid motifs (Haverkamp et al., 2016).
• Reconfigurable Nanophotonics: Swingarm-mediated assembly permits real-time, reversible control over chiroptical response, supporting applications in dynamic photonic elements, optical switching, and information encoding (Peil et al., 13 Jun 2025).
• Soft-Matter Plasmonics: Nematiclike and helicoidal gold nanorod arrays function as tunable polarizers and effective media with programmable birefringence and dichroism (Liu et al., 2016).

Electrical characterization indicates a significant reduction in resistivity after plasma purification (~1.1 μΩ·m for the shell vs. 0.02 μΩ·m for bulk gold), enabling their integration into nanoelectronic schemes where moderate conductivity is acceptable (Haverkamp et al., 2016).

6. Comparative Analysis and Limitations

• Fabrication Fidelity: EBID-plasma strategies yield high geometric fidelity for diameters >60 nm, but deformation risk rises for smaller pillars during plasma exposure.
• Material Purity: Oxygen plasma effectively purifies only the outer shell; the core retains higher carbon content, although the skin-effect minimizes performance penalties in photonic use-cases.
• Configurability: DNA swingarm approaches surpass traditional DNA origami assemblies by enabling direct, large-step movement of AuNPs, minimizing detachment risk and maximizing reconfiguration speed and precision (Peil et al., 13 Jun 2025).
• Crystallographic Complexity: The interplay between high-Miller-index faceting, local strain, and optical function in chiral gaps is elucidated by BCDI, providing a platform to rationally tune LSPR and chirality (Choi et al., 2022).

A plausible implication is that further integration of atomic-level structure mapping (e.g., BCDI-derived strain topology) with programmable DNA nanotechnology could expand the parameter space for dynamic, application-specific gold helicoid assemblies.

7. Theoretical Frameworks and Formulas

The underlying physics governing the optical and structural functionalities is formalized via several equations:

• Plasmonic Scattering Normalization:

$$I_{\text{norm}} = \frac{I_{\text{scattering}} - I_{\text{dark}}}{I_{\text{source}}}$$

• Orientational Order Parameter:

$$S_{\text{GNR}} = \left\langle \frac{3\cos^2\theta - 1}{2} \right\rangle$$

• Surface Anchoring Energy Density:

$f_{\text{sa}} = \frac{W}{2} \sin^2\beta$

• Optical Chirality Density:

$$C(\mathbf{r}) = -\frac{\varepsilon_0 \omega}{2} \Im \left\{ \mathbf{E}_{\text{sc}}(\mathbf{r}) \cdot [\mathbf{H}_{\text{sc}}(\mathbf{r})]^* \right\}$$

• Strain-Modified Plasma Frequency:

$$\omega_{\text{strain}} = \sqrt{ \frac{4e^2}{\varepsilon_0 m_{\text{eff}} (a_{\text{strain}})^3} }$$

where symbols follow standard usage and definitions within the cited works.

All statements are substantiated by and traceable to the following works:

• (Haverkamp et al., 2016) Plasmonic Gold Helices for the visible range fabricated by oxygen plasma purification of electron beam induced deposits
• (Liu et al., 2016) Plasmonic complex fluids of nematiclike and helicoidal self-assemblies of gold nanorods with a negative order parameter
• (Choi et al., 2022) Strain and Crystallographic Identification of the Helically Concaved Surfaces of Nanoparticles
• (Peil et al., 13 Jun 2025) Transformable Plasmonic Helix with Swinging Gold Nanoparticles

These works collectively define the present technical understanding of gold helicoid nanoparticles, elucidating their structure–function relationships, fabrication, and utility across nanophotonics and chiral plasmonics.