Noble metal nanoparticles (MNPs) have been intensively pursued in recent years, not only for their fundamental scientific interest1 but also for their technological applications, ranging from analytical sensors to catalysis and fuel cells.2 Recently, the attention paid to 1D nanomaterials has been increasing significantly, because of the need to fabricate alternative functional 1D nanostructures for applications in the fields of nanoelectronics and nanobiotechnology,3 due to the fact that they can act as interconnects between functional nanoscale components.4
Several experimental routes have been recently proposed to efficiently self-assemble preformed MNPs into 1D chains: methods involving hard,5 polymeric6 or surfactant-based7 templates, molecular recognition,8 specific functionalization,9 and surface- or solvent-induced phase separation10 have been successfully demonstrated.11 Chains of MNPs also have been prepared by using linear macromolecular templates, such as, DNA,12, 13 peptide,14, 15 insulin fibrils,16 protein fibrils17, 18 or carbon nanotubes.19
The growth mechanism of self-assembled metal nanostructures using (macro)molecular ligands has been also reported to exhibit common features with molecular step-growth polymerization.20 Similar to functional monomers, metal nanostructures assemble to form chains. The assembly was performed by small molecules (<2 nm), called molecular linkers, that contain at least two reactive ending groups, capable of attaching to a solid surface by chemisorption (thiol, amine) or interacting electrostatically with other functional groups (hydroxy, carboxyl, amine) present on the surface of nanoparticles (NPs).20 The governing factor in linker-mediated assembly of MNPs is the equilibrium between the attractive and repulsive forces.21 In particular, fabrication of anisotropic 1D noble MNP chains to obtain integrated optics operating below the diffraction limit of light has attracted much attention.22
Stellaci and co-workers10a have introduced anisotropic properties on ligand-stabilized AuNPs. Face-centered cubic (fcc) metallic NPs exhibit no intrinsic electric dipole, however, heterogeneities in surface charge and polarity, associated for example with the non-uniform spatial distribution of capping ligands on different crystal faces,23 or nanophase separation in mixed-ligand stabilization layers,24 are possible driving forces for anisotropic self-assembly.25 In the case of spherical NPs, controlling the surface chemistry of the fabricated NPs allows the creation of an anisotropic ligand organization.26 Enthalpy minimization, is obtained by promoting dipole alignment and reducing interdipole distances through the formation of linear chains of single NPs. This facilitates the orientation of specific interactions in one direction, which helps directing the selfassembly into 1D arrays. The self-assembly of the NPs into a well-defined 1D array is also influenced by interparticle chemical bonding, hydrogen bonding, van der Waals interactions, electrostatic forces, or any combination of these forces. In addition, entropy can be maximized at finite temperature by introducing some disorder in the linear chain, which corresponds to the incorporation of branching junctions and to chain reticulation that should be favored at elevated temperature.
Aggregation of NPs induces variations in absorption spectra accompanied by significant color changes of solutions.27 Similar color changes can be observed upon the addition of an analyte, which initiates the aggregation of noble MNPs, and this feature can be used for permitting their industrial application in biosensing, immunological, and biochemical investigations.28, 29 In the particular case of AgNPs, the geometrical shape also plays an important role in determining plasmon resonance properties.30 For example, triangular, pentagonal, and spherical silver particles are colored red, green, and blue, respectively. Consequently, it is important to develop approaches that can manipulate NPs into different shapes and dimensions.
While many studies have tackled the synthesis and characterization of gold dimers and networks with peculiar plasmon resonance behavior,31 organization of AgNPs in 2D superlattices is less common.32, 33 For example, Chang et al. reported a variety of 1D- and 2D-nanostructured assemblies formed from AgNPs by variations in pressure, temperature, and time in supercritical water (SCW) without the need for any external linking agents.34
To follow our interest in new emissive materials, and functionalized NPs and to explore their applications,35 herein, we investigate the mechanism of AuNP and AgNP chain assembly associated with the induction of electric dipole–dipole interactions. The nanoassembly capacity arises from the partial ligand exchange of surface-adsorbed negatively charged citrate ions, by covalently bound neutral molecular ligand L to produce a final mixed-ligand surface layer.
We show that exchange of surface adsorbed citrate with L, results in the formation of chain-like superstructures with topological features that are dependent on the extent of surface-ligand substitution. We determine the time-dependent structural changes associated with the formation of 1D NP superstructures. Morphological and optical characteristics of various nanostructures were investigated by TEM and UV/Vis.