Synthesis of shape-controlled gold and palladium plasmonic nanocrystals: reliable methods and good practices

This review reports reliable methods and good practices for the synthesis of shape-controlled plasmonic nanocrystals, focusing on Au and, comparatively, Pd. The authors attempted to reproduce most of the syntheses discussed below and report with high confidence the most reproducible protocols; some pioneering studies receive less emphasis where a more reliable route to the same shape has since been reported. Shape and interfacial chemistry, more than composition or size alone, govern light–matter interactions in plasmonic nanocrystals, dictating the position, intensity, and directionality of the localized surface plasmon resonance. Colloidal bottom-up synthesis is the method of choice for accessing this shape space, owing to its versatility and native compatibility with liquid media, an advantage for biomedical applications requiring dispersed, functionalized, stable aqueous nanoparticles. This versatility is also the field’s central challenge: anisotropic growth is an out-of-equilibrium process governed by a delicate interplay between reaction kinetics and facet-specific interfacial forces (capping agents, halides, underpotential-deposited metal adlayers), so small variations in precursor ratios, reagent purity, addition order, or reaction scale can have an outsized, often unpredictable impact on morphology, posing persistent obstacles to reproducibility and scale-up [25]. This review surveys shape-controlled synthesis of Au and Pd nanocrystals, for which a broad shape library has been achieved, including spheres, nanorods, mini-nanorods, cubes, concave and convex cubes, stars, bipyramids, plates, octahedra, icosahedra, truncated octahedra, and rhombohedra. By examining mechanistic principles common to these syntheses — seed crystallinity, ligand- and halide-mediated facet passivation, and deposition/diffusion kinetics — this work aims to provide a unified framework for understanding, and improving the reproducibility of, shape-controlled noble-metal nanocrystal synthesis.