A new methodology is described for the one-step aqueous preparation of highly monodisperse gold nanoparticles with diameters below 5 nm using thioether- and thiol-functionalized polymer ligands. The particle size and size distribution was controlled by subtle variation of the polymer structure. It was shown that poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) were the most effective stabilizing polymers in the group studied and that relatively low molar mass ligands (approximately 2500 g/mol) gave rise to the narrowest particle size distributions. Particle uniformity and colloidal stability to changes in ionic strength and pH were strongly affected by the hydrophobicity of the ligand end group. "Multidentate" thiol-terminated ligands were produced by employing dithiols and tetrathiols as chain-transfer agents, and these ligands gave rise to particles with unprecedented control over particle size and enhanced colloidal stability. It was found throughout that dynamic light scattering (DLS) is a very useful corroboratory technique for characterization of these gold nanoparticles in addition to optical spectroscopy and TEM.

The synthesis of fluorescent water-soluble gold nanoparticles by the reduction of a gold salt in the presence of a designed polymer ligand is described, the size and fluorescence of the particles being controlled by the polymer to gold ratio; the most fluorescent nanomaterial has a 3% quantum yield, a 1.1 nm gold core and a 6.9 nm hydrodynamic radius.

Au nanocrystals coated with thiol derivatives differing by the length of their alkyl chains are used to build 3D superlattices called supracrystals. In this study, we used two sets of Au nanocrystals differing by their sizes and size distributions. The average sizes are 5 nm (Au5) and 7 nm (Au7). From one experiment to the other, the size distribution slightly changes. For Au5 nanocrystals, it evolves from 6 to 8%, and for Au7 nanocrystals, it varies from 5 to 6%. The Au nanocrystals (Au5 and Au7) are first dispersed in toluene and produce fcc supracrystals by solvent evaporation. Here, by small-angle grazing X-ray diffraction, we observe a control in the average interparticle distance within the supracrystals. When the supracrystals are grown at zero toluene vapor pressure, the interparticle distances increase linearly with the alkyl chain length of the nanocrystals' coating agent regardless of their diameters. Furthermore, the dry supracrystals can swell and the interparticle distance within the superstructure be increased by subjecting the material to toluene vapor pressure after initial growth. This swelling process is reversible, and retraction occurs when the toluene vapor pressure drops. This indicates a strong ability of the dried supracrystals to trap toluene molecules. On increasing the toluene vapor pressure during the solvent evaporation process, the slope of the linear dependency of the interparticle distances to the alkyl chain length is markedly decreased and the interparticle distance reaches a quasi-plateau. This is explained by the influence of depletion forces created by the presence of thiol-containing molecules physisorbed on the coating molecules on the internal structure of these supracrystals. Recently, we demonstrated that, by using the same nanocrystals (Au5 and Au7), a hierarchy in the supracrystal growth process takes place from heterogeneous nucleation with the formation of a layer-by-layer film to homogeneous nucleation in solution with the formation of shaped supracrystals. Here it is shown that the interparticle distance is independent of the supracrystal growth mechanisms.

Fluorescence correlation spectroscopy (FCS) is a popular technique, complementary to cell imaging for the investigation of dynamic processes in living cells. Based on fluorescence, this single molecule method suffers from artifacts originating from the poor fluorophore photophysics: photobleaching, blinking, and saturation. To circumvent these limitations we present here a new correlation method called photothermal absorption correlation spectroscopy (PhACS) which relies on the absorption properties of tiny nano-objects. PhACS is based on the photothermal heterodyne detection technique and measures akin FCS, the time correlation function of the detected signals. Application of this technique to the precise determination of the hydrodynamic sizes of different functionalized gold nanoparticles are presented, highlighting the potential of this method.

Synthesizing stable Au and Ag nanocrystals of narrow size distribution from metal-N-heterocyclic carbene (NHC) complexes remains a challenge, particularly in the case of Ag and when NHC ligands with no surfactant-like properties are used. The formation of nanocrystals by one-phase reduction of metal-NHCs (metal = Au, Ag) bearing common NHC ligands, namely 1,3-diethylbenzimidazol-2-ylidene (L(1)), 1,3-bis(mesityl)imidazol-2-ylidene (L(2)), and 1,3-bis(2,6-(i)Pr2C6H3)imidazol-2-ylidene (L(3)), is presented herein. We show that both Au and Ag nanocrystals displaying narrow size distribution can be formed by reduction with amine-boranes. The efficiency of the process and the average size and size distribution of the nanocrystals markedly depend on the nature of the metal and NHC ligand, on the sequence in the reactant addition (i.e., presence or absence of thiol during the reduction step), and on the presence or absence of oxygen. Dodecanethiol was introduced to produce stable nanocrystals associated with narrow size distributions. A specific reaction is observed with Ag-NHCs in the presence of thiols whereas Au-NHCs remain unchanged. Therefore, different organometallic species are involved in the reduction step to produce the seeds. This can be correlated to the lack of effect of NHCs on Ag nanocrystal size. In contrast, alteration of Au nanocrystal average size can be achieved with a NHC ligand of great steric bulk (L(3)). This demonstrates that a well-defined route for a given metal cannot be extended to another metal.

The surface chemistry in colloidal nanocrystals on the final crystalline structure of binary superlattices produced by self-assembly of two sets of nanocrystals is hereby demonstrated. By mixing nanocrystals having two different sizes and the same coating agent, oleylamine (OAM), the binary nanocrystal superlattices that are produced, such as NaCl, AlB2, NaZn13, and MgZn2, are well in agreement with the crystalline structures predicted by the hard-sphere model, their formation being purely driven by entropic forces. By opposition, when large and small nanocrystals are coated with two different ligands [OAM and dodecanethiol (DDT), respectively] while keeping all other experimental conditions unchanged, the final binary structures markedly change and various structures with lower packing densities, such as Cu3Au, CaB6, and quasicrystals, are observed. This effect of the nanocrystals' coating agents could also be extended to other binary systems, such as Ag-Au and CoFe2O4-Ag supracrystalline binary lattices. In order to understand this effect, a mechanism based on ligand exchange process is proposed. Ligand exchange mechanism is believed to affect the thermodynamics in the formation of binary systems composed of two sets of nanocrystals with different sizes and bearing two different coating agents. Hence, the formation of binary superlattices with lower packing densities may be favored kinetically because the required energetic penalty is smaller than that of a denser structure.

Silver nanocrystals (NCs) stabilized using amine-terminated coating agents (oleylamine or dodecylamine), their size ranging between 2 and 12 nm in diameter, are synthesized by hot injection methods. Their dispersion in size is relatively low (typically below 10%) without the need for a postsynthesis size segregation process. The amine-terminated coating agents are replaced by thiol-terminated molecules (dodecanethiol or hexadecanethiol) by ligand exchange, allowing the formation of alkanethiol coated Ag colloids. All NCs with various surface coatings are dispersed in toluene. Regardless of the nature of the coating agent, the surface plasmon resonance (SPR) is red-shifted with decreasing the NC size. For a given size, the SPR peak of thiol-stabilized NCs is shifted to lower energies compared to that of amine-stabilized NCs. Furthermore, with thiol-stabilized Ag NCs, the position of the SPR peak was found to be sensitive to the length of the alkyl chains of the coating agent, whereas minor differences are detected for Ag NCs coated with amines terminated with differing alkyl chain lengths.

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