We describe the first example of redox catalysis using a dissolved electroactive nanoparticle, based on the oxidation of water by electrogenerated IrO(x) nanoparticles containing Ir(VI) states, in pH 13 solutions of 1.6 +/- 0.6 nm (dia.) Ir(IV)O(x) nanoparticles capped solely by hydroxide. At potentials (ca. +0.45 V) higher than the mass transport-controlled plateau of the nanoparticle Ir(V/IV) wave, rising large redox catalytic currents reflect electrochemical generation of Ir(VI) states, which by +0.55 V and onward to +1.0 V are shown by rotated ring disk electrode experiments to lead with 100% current efficiency to the oxidation of water to O(2). O(2) production at +0.55 V corresponds to an overpotential eta of only 0.29 V, relative to thermodynamic expectations of the four electron H(2)O-->O(2) reaction. The Ir site turnover frequency (TO, mol O(2)/Ir sites/s) is 8-11 s(-1). Controlled potential coulometry shows that all Ir sites in these nanoparticles (average 66 Ir each) are electroactive, meaning that the nanoparticles are small enough to allow the required electron and proton transport throughout. Both the overpotential and TO values are nearly the same as those observed previously for films electroflocculated from similar IrO(x) nanoparticles, providing the first comparison of electrocatalysis by nanoparticle films with redox catalysis by dissolved, diffusing nanoparticles.

The pH-dependent solution voltammetry (pH 1–13) of phosphate-stabilized, small (2 nm diameter) iridium oxide nanoparticles (IrIVO x NPs) is described and compared with that of (flocculated) films of the same size nanoparticles on electrodes. The IrIVO x NPs show waves with one electron/one proton formal potential dependency for the IrV/IV redox transformation and (below pH 6) for the IrIV/III reaction. Above pH 6, the IrIV/III reaction becomes a one-electron/two-proton process, unlike the one-electron/one-proton reactivity of the nanoparticles in films. The change is associated with surface oxide acid–base sites having pK A = 6 for solution phase nanoparticles that apparently are inactivated by the flocculation chemistry. Spectral isosbestic points are observed over the pH range of 5–8 for the dissolved nanoparticles. Controlled potential coulometry demonstrates that all of the Ir sites, throughout the nanoparticle, undergo the IrIV/III redox transformation. Whereas the IrIVO x NPs are stabilized at different pH values by phosphate ligation, the associated equilibria are somewhat sluggish, as indicated by small spectral differences for equi-pH nanoparticle solutions mixed with phosphate by different procedures. The IrIVO x NPs can also be capped with hydrophobic carboxylic acids, which allows extraction into nonpolar solvents such as CH2Cl2.

The pH-controlled assembly of gold nanoparticles within a polymer-gold nanoparticle (AuNP) composite material is described. Poly(allylamine) (PAAm) is used as a reducing and stabilizing agent to generate a stable water-soluble polymer-AuNPs composite. The optical properties and the morphology of the composite material are sensitive to the solution pH due to structural changes of the polymer. As a result, the nanoparticles undergo reversible assembly/dispersion processes controlled by the solution pH. The role of the PAAm in the formation of the assemblies is confirmed by infrared spectroscopy and place exchange reactions.

We report a simple route to produce fluorophore-encapsulated gold nanoparticles (AuNPs) in a single step under aqueous conditions using the fluorophore 1-pyrenemethylamine (PMA). Different amounts of PMA were used and the resulting core-shell gold nanoparticles were analyzed using UV-visible absorption spectroscopy, fluorescence spectroscopy, and transmission and scanning electron microscopy. Electron microscopy analysis shows nanoparticles consisting of a gold nanoparticle core which is encapsulated with a lower contrast shell. In the UV-visible spectra, we observed a significant red shift (37 nm) of the localized surface plasmon resonance (LSPR) absorption maximum (lambda(max)) compared to citrate-stabilized AuNPs of a similar size. We attribute the prominent LSPR wavelength shift for PMA-AuNP conjugates to the increase in the local dielectric environment near the gold nanoparticles due to the shell formation. This simple, aqueous-based synthesis is a new approach to the production of fluorophore-encapsulated AuNPs that could be applicable in biological sensing systems and photonic device fabrication.