Redox and Fluorophore Functionalization of Water-Soluble, Tiopronin-Protected Gold Clusters

Abstract

Place-exchange and amide-forming coupling reactions represent two facile and efficient routes to poly-functionalization of water-soluble nanoparticles. In this paper, place-exchange and amide-forming coupling reactions with water-soluble tiopronin-MPCs are described and their products characterized by 1H and 31P NMR, capillary electrophoresis, electrochemistry, and fluorescence spectroscopy. Place-exchange reactions of ligands with tiopronin-MPCs yield products with about half of the ligand exchange expected on the basis of solution stoichiometry and a nonselective exchange and were not noticeably affected by steric encumbrances. Tiopronin-MPCs to which viologens are coupled (avg 36/MPC) adsorb as monolayers on Au electrodes as shown in electrochemical quartz crystal microbalance experiments. Multilayer adsorption occurs on long experimental time scales. The strong adsorption of the viologen-functionalized clusters is ascribed to increased interaction and stability in the viologen reduction products. Capillary electrophoresis experiments with tiopronin-MPCs and viologen-functionalized tiopronin-MPCs reveal a number of separable core size/charge state combinations. Fluorescence measurements show ∼50% quenching of fluorescein upon attachment (avg 3.7/cluster) to tiopronin-MPCs, relative to the monomer under the same conditions. The results of this paper provide a pathway to explore poly-functionalized water-soluble nanoparticles in a variety of applications, including their use as biosensors.

Highly Fluorescent Noble-Metal Quantum Dots

“…In 1998, Wilcoxon et al reported blue emission at 440 nm from small gold nanoparticles (diameter < 2.5 nm) (67), however, due to the heterogeneity of the solution, the exact size of emissive species was undetermined. Near IR emission was also reported from glutathione encapsulated Au 28 clusters (9) and from tiopronin-capped ~1-nm Au species (68)(69)(70)(71)(72)(73)(74)(75), both in aqueous solution. Compared to well-studied alkali clusters (24,25), the size-dependent electronic structure of these noble metals remained poorly understood, yet a framework (9,23,76) for linking nanoparticle and cluster optical properties through free electron scalings was in place.…”

Monolayer-Protected Cluster Molecules

“…The few known precedents of poly-electron transfer reactions include soluble redox polymers 29 and redox-labeled dendrimers. 9 MPCs have been poly-functionalized with ferrocene (e25 sites per MPC), 22a,30 anthraquinone (e25), 31 phenothiazine (e7), 23 and viologen (e36) 26 groupings. Figure 4a illustrates voltammetry 30a of a Fc 9 -MPC solution; analogous ferrocenes have been examined as Langmuir monolayers at the air/water interface.…”

Toward Greener Nanosynthesis

“…This report focused on the viability of these water soluble nanoparticles as precursors for both ligand exchange and postsynthetic modification via amide coupling reactions. 40 In the same year, Chen and Kimura reported the synthesis of nanoparticles ranging in size from 1.0 to 3.4 nm by NaBH 4 reduction of HAuCl 4 in the presence of mercaptosuccinic acid, using methanol as a solvent. Although size evolution of the nanoparticle products in solution became apparent over time, the dried nanoparticle powders were stable and completely redispersible in water.…”

“…Candidates for postsynthetic modifications are generally produced by direct synthesis (ω-functionalized particles created by Brust preparations) or sometimes ligand exchange methods (usually involving the introduction of a ligand bearing a reactive pendant functional group). Modification reactions include polymerizations, [213][214][215][216] coupling reactions, 40,48,[217][218][219][220][221] or transformation of an existing chemical moiety. 48,[222][223][224] In all cases, the success of such modifications relies not only on the nanoparticles' tolerance for various reaction conditions but also on the overall reactivity and steric environment presented by functional groups that are constrained through binding to a nanoparticle.…”