Gold Nanoparticles for Stimuli-Responsive Aggregate Assembly

Colloidal gold has been in use for making glass and decorating ceramics since the 5 th-4 th B.C. 1 , yet documented scientific study did not formally surge until the nineteenth century when Michael Faraday first reported on the optical properties of these solutions. 2 Since then, scientific interest resurged by the late 20 th century in terms of synthetic routes, stabilization, catalysis and sensing. 1 In recent years, there has been a surge in exploring gold nanoparticles (AuNPs) for self-assembly. Various other materials have been studied for self-assembly; including polymeric colloid systems and other inorganic nanoparticles, but AuNPs offer several advantages as a model system. 3 As a noble metal, AuNPs are highly chemically stable as compared to other inorganic nanoparticles that can be chemically reactive in a given environment. They are also well characterized for surface modifications, such as decorating with sulfur ligand monolayers 4-5 that can be used to direct assembly. Another strong advantage is the plasmon resonance band (PRB) for AuNPs below 100 nm that absorbs c.a. 530 nm and redshifts with increasing particle diameter. 1,6 This in turn can be easily used to track the size of nanoparticle assemblies by UV-Vis spectra. Recently, there has been increasing interest in making reversible AuNP assemblies. Reversibility becomes important for these systems to be used effectively in technological applications. Therefore, stimuli-responsive assemblies have gained interest due to their ability of reversing assembly when the stimuli is employed or removed. Stimuli that have been gained attention for their simplicity include temperature, redox activity, pH and light. These systems in general make use of a stimuli-responsive material that can be incorporated into the AuNP system albeit as a surface modification using self-assembled monolayers (SAMs) 4 or as an additive in the medium. Early examples by the groups of Meskers and Schenning make use of temperature sensitive π-π stacking surface ligands on AuNPs to make highly spherical assemblies that form at room temperature, but can be dissembled at higher temperatures (Figure 1). AuNPs of metallic diameters of ~2 and 4 nm functionalized with oligo(p-phenylene vinylene) (OPV) formed highly stable micrometer-sized spherical aggregates at room temperature. The aggregates formed by non-covalent interactions were even stable enough to be manipulated by atomic force microscopy (AFM) when drop-casted onto a substrate, demonstrating possibilities to use these aggregates for nano-plasmonics. Another system that has surged based on π-π stacking is that of redox stimulated pseudorotaxane …