Au nanotriangles display interesting nanoplasmonic features with potential application in various fields. However, such applications have been hindered by the lack of efficient synthetic methods yielding sufficient size and shape monodispersity, as well as by insufficient morphological stability. We present here a synthesis and purification protocol that efficiently addresses these issues. The size of the nanotriangles can be tuned within a wide range by simply changing the experimental parameters. The obtained monodispersity leads to extended self-assembly, not only on electron microscopy grids but also at the air-liquid interface, allowing transfer onto centimeter-size substrates. These extended monolayers show promising performance as surface-enhanced Raman scattering substrates, as demonstrated for thiophenol detection.

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Most bacteria in nature exist as biofilms, which support intercellular signaling processes such as quorum sensing (QS), a cell-to-cell communication mechanism that allows bacteria to monitor and respond to cell density and changes in the environment. Because QS and biofilms are involved in the ability of bacteria to cause disease, there is a need for the development of methods for the non-invasive analysis of QS in natural bacterial populations. Here, by using surface-enhanced resonance Raman scattering spectroscopy, we report rationally designed nanostructured plasmonic substrates for the in-situ, label-free detection of a QS signaling metabolite in growing Pseudomonas aeruginosa biofilms and microcolonies. The in situ, non-invasive plasmonic imaging of QS in biofilms provides a powerful analytical approach for studying intercellular communication on the basis of secreted molecules as signals.

Optical manipulation of plasmonic nanoparticles provides opportunities for fundamental and technical innovation in nanophotonics. Optical heating arising from the photon-to-phonon conversion is considered as an intrinsic loss in metal nanoparticles, which limits their applications. We show here that this drawback can be turned into an advantage, by developing an extremely low-power optical tweezing technique, termed opto-thermoelectric nanotweezers (OTENT). Through optically heating a thermoplasmonic substrate, alight-directed thermoelectric field can be generated due to spatial separation of dissolved ions within the heating laser spot, which allows us to manipulate metal nanoparticles of a wide range of materials, sizes and shapes with single-particle resolution. In combination with dark-field optical imaging, nanoparticles can be selectively trapped and their spectroscopic response can be resolved in-situ. With its simple optics, versatile low-power operation, applicability to diverse nanoparticles, and tuneable working wavelength, OTENT will become a powerful tool in colloid science and nanotechnology.

Anisotropic (nonspherical) metal nanoparticles are of widespread research interest because changing the shape of metals at the nanoscale can provide access to materials with unique optical, electronic, and catalytic properties. The development of seeded growth syntheses has provided researchers unprecedented access to anisotropic metal nanocrystals (particularly, gold, silver, platinum, and palladium nanocrystals) with precisely controlled dimensions and crystallographic features. The mechanisms by which the various reagents present in seeded growth syntheses accomplish shape control, however, have yet to be fully elucidated. Recently, the role halide ions play in controlling metal nanocrystal shape has become a subject of particular interest. There are many ways in which the halide ions may direct the anisotropic growth of metal nanocrystals, including modulating the redox potentials of the metal ions, acting as face-specific capping agents, and/or controlling the extent of silver underpotential deposition at the nanocrystal surface. In this Perspective, we examine recent progress in elucidating and articulating the role halide ions play in seeded growth with particular emphasis on gold nanoparticles.

Plasmon modes of the exact same individual gold nanoprisms are investigated through combined nanometer-resolved electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) measurements. We show that CL only probes the radiative modes, in contrast to EELS, which additionally reveals dark modes. The combination of both techniques on the same particles thus provides complementary information and also demonstrates that although the radiative modes give rise to very similar spatial distributions when probed by EELS or CL, their resonant energies appear to be different. We trace this phenomenon back to plasmon dissipation, which affects in different ways the plasmon signatures probed by these techniques. Our experiments are in agreement with electromagnetic numerical simulations and can be further interpreted within the framework of a quasistatic analytical model. We therefore demonstrate that CL and EELS are closely related to optical scattering and extinction, respectively, with the addition of nanometer spatial resolution.

Successful synthesis of gold nanorods requires subtle combination of additives and reducing species. The latter are of major importance, as the reducing power determines the rate of metallic gold formation, which often defines the final shape and anisotropy. Ascorbic acid is a common reducing agent in the synthesis of gold nanorods, but its relatively strong reducing power limits the tunability of the final shape and optical response. We propose here a bimodal reducing agent system comprising a combination of salicylic and ascorbic acid. While salicylic acid pre-reduces Au(III) to Au(I) in the growth solution, ascorbic acid participates in the autocatalytic reduction of Au(I) to Au(0), selectively occurring on the metallic surface. This combination provides a fine control over gold reduction at any stage of nanorods formation, which in turn leads to improved monodispersity, better reduction yield and morphology control.

Reversible assembly of plasmonic nanoparticles can be used to modulate their structural, electrical, and optical properties. Common and versatile tools in nanoparticle manipulation and assembly are optical tweezers, but these require tightly focused and high-power (10-100 mW/μm) laser beams with precise optical alignment, which significantly hinders their applications. Here we present light-directed reversible assembly of plasmonic nanoparticles with a power intensity below 0.1 mW/μm. Our experiments and simulations reveal that such a low-power assembly is enabled by thermophoretic migration of nanoparticles due to the plasmon-enhanced photothermal effect and the associated enhanced local electric field over a plasmonic substrate. With software-controlled laser beams, we demonstrate parallel and dynamic manipulation of multiple nanoparticle assemblies. Interestingly, the assemblies formed over plasmonic substrates can be subsequently transported to nonplasmonic substrates. As an example application, we selected surface-enhanced Raman scattering spectroscopy, with tunable sensitivity. The advantages provided by plasmonic assembly of nanoparticles are the following: (1) low-power, reversible nanoparticle assembly, (2) applicability to nanoparticles with arbitrary morphology, and (3) use of simple optics. Our plasmon-enhanced thermophoretic technique will facilitate further development and application of dynamic nanoparticle assemblies, including biomolecular analyses in their native environment and smart drug delivery.