Shape-controlled synthesis of gold nanoparticles has become a cornerstone of modern nanotechnology due to the unique optical, electronic, and catalytic properties imparted by their morphology. The ability to tailor nanoparticle shape allows precise tuning of localized surface plasmon resonance (LSPR), which is critical for applications in sensing, photonics, and catalysis. Several synthetic strategies have been developed to produce gold nanoparticles with well-defined geometries, including spheres, rods, cubes, stars, and other anisotropic structures. These methods rely on controlling nucleation, growth kinetics, and surface stabilization through capping agents, surfactants, and templating techniques.

Spherical gold nanoparticles are the most commonly synthesized morphology, typically produced via the citrate reduction method. In this approach, chloroauric acid (HAuCl4) is reduced by sodium citrate in an aqueous solution at elevated temperatures. The citrate acts as both a reducing agent and a stabilizer, adsorbing onto the nanoparticle surface to prevent aggregation. The size of spherical nanoparticles can be controlled by adjusting the citrate-to-gold ratio, with higher citrate concentrations yielding smaller particles due to increased nucleation sites. The LSPR of spherical gold nanoparticles typically appears around 520–530 nm, corresponding to their dipolar plasmon mode.

Gold nanorods represent a class of anisotropic nanoparticles with two distinct plasmon bands: a transverse mode around 520 nm and a longitudinal mode that varies with aspect ratio. The seed-mediated growth method is the most widely used approach for nanorod synthesis. In this process, small spherical gold seeds are first prepared, then added to a growth solution containing HAuCl4, a weak reducing agent (ascorbic acid), and surfactants such as cetyltrimethylammonium bromide (CTAB). CTAB plays a dual role as a stabilizing agent and a shape-directing surfactant, preferentially adsorbing onto certain crystal facets to promote anisotropic growth. The aspect ratio of nanorods can be tuned by varying the concentration of silver ions, which selectively block growth along specific directions.

Gold nanocubes are synthesized through a polyol process, where HAuCl4 is reduced by ethylene glycol in the presence of polyvinylpyrrolidone (PVP). PVP selectively binds to the {100} facets of gold, leaving the {111} facets more exposed for growth, resulting in cubic morphology. The size of the cubes can be controlled by adjusting reaction temperature and precursor concentrations. Gold nanocubes exhibit plasmonic properties distinct from spheres, with a red-shifted LSPR peak due to their sharp corners and edges.

Star-shaped gold nanoparticles, or nanostars, are synthesized using a seed-mediated approach with additional shape-directing agents. A common method involves reducing HAuCl4 with ascorbic acid in the presence of CTAB and silver nitrate, but with altered growth conditions that promote branching. The presence of iodide ions or other additives can further control branching density and tip sharpness. Nanostars exhibit multiple plasmon resonances due to their complex geometry, with strong near-infrared absorption useful for photothermal applications.

Other exotic morphologies, such as bipyramids, octahedra, and concave cubes, can be achieved through careful manipulation of growth conditions. Bipyramids are formed using a modified seed-mediated approach with specific surfactant mixtures, while octahedral particles grow preferentially when using certain halide additives. Concave cubes require precise control over reduction kinetics and surface passivation. Each morphology presents unique plasmonic characteristics due to variations in curvature, facet exposure, and symmetry breaking.

Capping agents and surfactants play a crucial role in shape control by selectively adsorbing onto specific crystal facets, altering surface energies and growth rates. CTAB is particularly effective for rod formation due to its preferential binding to {100} facets, while PVP favors cubic morphologies by stabilizing {111} planes. Thiolated molecules, such as alkanethiols, can also direct shape by forming strong Au-S bonds that passivate certain facets. The choice of reducing agent further influences shape; strong reductants like sodium borohydride favor rapid nucleation and isotropic growth, whereas weaker reductants like ascorbic acid allow for kinetic control over anisotropic structures.

Templating techniques provide an alternative route to shape control, where preformed structures guide nanoparticle growth. Hard templates, such as porous alumina or silica, can confine growth within defined channels or cavities. Soft templates, including micelles or liquid crystals, offer dynamic environments where surfactant assemblies direct nanoparticle morphology. Galvanic replacement reactions represent another templating approach, where sacrificial metal nanoparticles (e.g., silver) are partially replaced by gold, creating hollow or cage-like structures.

The plasmonic properties of gold nanoparticles are highly shape-dependent due to variations in electron oscillation modes. Spherical particles exhibit a single dipole plasmon resonance, while anisotropic structures like rods and stars support multiple resonances corresponding to different axes of symmetry. Sharp features such as tips or edges enhance local electromagnetic fields through lightning rod effects, making branched nanoparticles particularly effective for surface-enhanced Raman spectroscopy (SERS). The LSPR wavelength can be precisely tuned across the visible and near-infrared spectrum by adjusting aspect ratios or branching patterns, enabling applications in colorimetric sensing and photothermal therapy.

Shape also influences catalytic performance due to differences in exposed crystal facets and surface atom coordination. Cubic gold nanoparticles, with their {100} facets, show distinct catalytic behavior compared to octahedral particles dominated by {111} facets. High-index facets, often present on branched or concave structures, provide particularly active sites for reactions such as CO oxidation or electrochemical reduction processes.

In optoelectronic applications, shape-controlled gold nanoparticles serve as building blocks for plasmonic waveguides, metamaterials, and light-harvesting systems. Ordered arrays of anisotropic particles can exhibit collective plasmon modes with tailored optical responses. The ability to manipulate light at the nanoscale makes these materials valuable for advanced photonic devices and sensors.

The synthesis of shape-controlled gold nanoparticles continues to evolve with advances in mechanistic understanding and process optimization. Recent developments include the use of biomimetic approaches, where peptides or DNA sequences guide nanoparticle growth, and the application of microfluidic systems for improved reproducibility. These innovations promise even greater precision in nanomaterial design, expanding the possibilities for plasmonic technologies.

Challenges remain in achieving large-scale production with uniform shape distributions and in precisely controlling three-dimensional architectures. However, the fundamental principles governing shape-directed synthesis provide a robust framework for ongoing research. As synthetic methodologies advance, the integration of shape-controlled gold nanoparticles into functional devices will likely drive innovations across photonics, catalysis, and energy conversion technologies. The relationship between nanoparticle morphology and properties ensures that shape-controlled synthesis will remain a vital area of nanoscience research.