Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

Localized Surface Plasmon Resonance Sensors

Plasmons in strongly coupled metallic nanostructures.

Surface plasmons (SPs) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal -dielectric interface. The growing field of research on such light -metal interactions is known as ‘plasmonics’. 1-3 This branch of research has attracted much attention due to its potential applications in miniaturized optical devices, sensors, and photonic circuits as well as in medical diagnostics and therapeutics. 4-8

Nanostructured plasmonic sensors.

Colloidal metal nanoparticles are emerging as key materials for catalysis, plasmonics, sensing, and spectroscopy. Within these applications, control of nanoparticle shape lends increasing functionality and selectivity. Shape-controlled nanocrystals possess well-defined surfaces and morphologies because their nucleation and growth are controlled at the atomic level. An overall picture of shaped metal particles is presented, with a particular focus on solution-based syntheses for the noble metals. General strategies for synthetic control are discussed, emphasizing key factors that result in anisotropic, nonspherical growth such as crystallographically selective adsorbates and seeding processes.

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Shape Control of Colloidal Metal Nanocrystals

Gold nanoparticles of different shapes and sizes, including nanospheres, nanocubes, nanobranches, nanorods, and nanobipyramids, were dispersed into water-glycerol mixtures of varying volume ratios to investigate the response of their surface plasmon peaks to the refractive index of the surrounding medium. The refractive index sensitivities and figures of merit were found to be dependent on both the shape and the size of the Au nanoparticles. The index sensitivities generally increase as Au nanoparticles become elongated and their apexes become sharper. Au nanospheres exhibit the smallest refractive index sensitivity of 44 nm/RIU and Au nanobranches exhibit the largest index sensitivity of 703 nm/RIU. Au nanobipyramids possess the largest figures of merit, which increase from 1.7 to 4.5 as the aspect ratio is increased from 1.5 to 4.7.

Shape- and size-dependent refractive index sensitivity of gold nanoparticles.

Tailoring the longitudinal surface plasmon wavelengths (LSPWs), scattering, and absorption cross sections of gold nanorods has been demonstrated by combining anisotropic shortening and transverse overgrowth and judiciously choosing starting Au nanorods. Shortening yields Au nanorods with decreasing lengths but a fixed diameter, while overgrowth produces nanorods with increasing diameters but a nearly unchanged length. Two series of Au nanorods with LSPWs varying in the same spectral range but distinct extinction coefficients are thus obtained. The systematic changes in the LSPW and extinction for the two series of Au nanorods are found to be in good agreement with those obtained from Gans theory. Dark-field imaging performed on two representative nanorod samples with similar LSPWs shows that the scattering intensities of the overgrown nanorods are much larger than those of the shortened nanorods. The experimental results are found to be in very good agreement with those obtained from finite-difference tim...

Tailoring longitudinal surface plasmon wavelengths, scattering and absorption cross sections of gold nanorods.

Gold nanorods (NRs) have received much attention due to their size-dependent surface plasmon-related optical properties. A seed-mediated approach has recently been developed for the synthesis of Au NRs with varying length-to-diameter aspect ratios. With the introduction of silver ions in the growth solution, Au NRs of narrow size distributions can be produced in high yields. Herein we describe an approach for the continuous and selective shortening of Au NRs synthesized by the silver ion-assisted seed-mediated method through oxidation with environmentally benign oxygen at slightly elevated temperatures. UV−visible extinction measurements indicate that the longitudinal surface plasmon band of Au NRs decreases in intensity and blue-shifts as a function of the oxidation time. Transmission electron microscopy (TEM) imaging shows that the length of Au NRs decreases with oxidation and their diameter stays almost constant, which suggests that oxidation starts at the ends of Au NRs. The size distributions of shor...

Selective shortening of single-crystalline gold nanorods by mild oxidation.

Ordered gold nanostructure assemblies formed by droplet evaporation

Gold nanorods exhibit rich surface-plasmon-resonance (SPR)derived properties, which have made discrete nanorods useful for many interesting applications such as optical data storage, submicrometer metallic barcodes, sensing, biological imaging, and controlled gene delivery. Future scientific and technological applications of Au nanorods require the capability to assemble into complex one-, two-, or even three-dimensional (3D) functional architectures. The assembly of Au nanorods also allows for the utilization of their collective properties that result from the coupling of the optical and electronic properties between neighboring individual nanorods. Several approaches have been developed for the assembly of Au nanorods in either end-to-end (EE) or side-by-side (SS) orientations. They include i) assembly through electrostatic interactions, hydrogen bonding, or covalent bonding, ii) antibody/antigen and streptavidin/biotin biorecognitions, iii) use of carbon nanotubes and silica nanofibers as templates, and iv) interactions between functionalized polymers in selective solvents. Au nanorods assembled by these approaches are generally difficult to disassemble. Even though significant progress has beenmade in the organizationof nanomaterials, reversible assembly and disassembly of Au nanorods in either EE or SS orientations has remained a big challenge. So far, reversible aggregation of spherical Au nanoparticles has been demonstrated by functionalizing them with thiol-modified DNA oligomers. Here, we report on a robust strategy for the reversible assembly and disassembly of Au nanorods in both EE and SS fashion. Thiol-containing bifunctional molecules are selectively bound to the end or side surface of individual Au nanorods. The bound molecules induce the assembly of Au nanorods if the pH of the nanorod solution is adjusted within an optimal range. Outside the optimal pH range, Au nanorods are disassembled. This pH-controlled assembly and disassembly is reversible and can be repeated many times. Moreover, the distances between assembled nanorods are estimated to vary from 0.080 to 1.8 nm for different assemblingmolecules and assembly orientations.

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Xiaoshan Kou

Papers

Shape- and size-dependent refractive index sensitivity of gold nanoparticles.

Tailoring longitudinal surface plasmon wavelengths, scattering and absorption cross sections of gold nanorods.

Selective shortening of single-crystalline gold nanorods by mild oxidation.

Ordered gold nanostructure assemblies formed by droplet evaporation

pH-Controlled reversible assembly and disassembly of gold nanorods.