Real-time monitoring of light scattering and UV-vis profiles of four different Cp*Ir(III) precursors under various conditions give insight into nanoparticle formation during oxidation catalysis with NaIO(4) as primary oxidant. Complexes bearing chelate ligands such as 2,2'-bipyridine, 2-phenylpyridine, or 2-(2'-pyridyl)-2-propanolate were found to be highly resistant toward particle formation, and oxidation catalysis with these compounds is thus believed to be molecular in nature under our conditions. Even with the less stable hydroxo/aqua complex [Cp*(2)Ir(2)(μ-OH)(3)]OH, nanoparticle formation strongly depended on the exact conditions and elapsed time. Test experiments on the isolated particles and comparison of UV-vis data with light scattering profiles revealed that the formation of a deep purple-blue color (~580 nm) is not indicative of particle formation during oxidation catalysis with molecular iridium precursors as suggested previously.

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Organometallic iridium complexes bearing oxidatively stable chelate ligands are precursors for efficient homogeneous water-oxidation catalysts (WOCs), but their activity in oxygen evolution has so far been studied almost exclusively with sacrificial chemical oxidants. In this report, we study the electrochemical activation of Cp*Ir complexes and demonstrate true electrode-driven water oxidation catalyzed by a homogeneous iridium species in solution. Whereas the Cp* precursors exhibit no measurable O2-evolution activity, the molecular species formed after their oxidative activation are highly active homogeneous WOCs, capable of electrode-driven O2 evolution with high Faradaic efficiency. We have ruled out the formation of heterogeneous iridium oxides, either as colloids in solution or as deposits on the surface of the electrode, and found indication that the conversion of the precursor to the active molecular species occurs by a similar process whether carried out by chemical or electrochemical methods. This work makes these WOCs more practical for application in photoelectrochemical dyads for light-driven water splitting.

The addition of surface functional groups to single-walled carbon nanotubes (SWNTs) is realized as an opportunity to achieve enhanced functionality in the intended application. At the same time, several functionalized SWNTs (fSWNTs), compared to SWNTs, have been shown to exhibit decreased cytotoxicity. Therefore, this unique class of emerging nanomaterials offers the potential enhancement of SWNT applications and potentially simultaneous reduction of their negative human health and environmental impacts depending on the specific functionalization. Here, the percent cell viability loss of Escherichia coli K12 resulting from the interaction with nine fSWNTs, n-propylamine, phenylhydrazine, hydroxyl, phenydicarboxy, phenyl, sulfonic acid, n-butyl, diphenylcyclopropyl, and hydrazine SWNT, is presented. The functional groups range in molecular size, chemical composition, and physicochemical properties. While physiochemical characteristics of the fSWNTs did not correlate, either singularly or in combination, with the observed trend in cell viability, results from combined light scattering techniques (both dynamic and static) elucidate that the percent loss of cell viability can be correlated to fSWNT aggregate size distribution, or dispersity, as well as morphology. Specifically, when the aggregate size polydispersity, quantified as the width of the distribution curve, and the aggregate compactness, quantified by the fractal dimension, are taken together, we find that highly compact and narrowly distributed aggregate size are characteristics of fSWNTs that result in reduced cytotoxicity. The results presented here suggest that surface functionalization has an indirect effect on the bacterial cytotoxicity of SWNTs through the impact on aggregation state, both dispersity and morphology.

Cellulose nanofibrils (CNFs) present unique opportunities for rheology modification in complex fluids. Here we systematically consider the effect of ionic and non-ionic surfactants on the rheology of dilute CNF suspensions. Neat suspensions are transparent yield-stress fluids which display strong shear thinning and power-law dependence of modulus on concentration, G' ∼ c(2.1). Surfactant addition below a critical mass concentration cc produces an increase in the gel modulus with retention of optical clarity. Larger than critical concentrations induce significant fibril aggregation leading to the loss of suspension stability and optical clarity, and to aggregate sedimentation. The critical concentration was the lowest for a cationic surfactant (DTAB), cc ≈ 0.08%, while suspension stability was retained for non-ionic surfactants (Pluronic F68, TX100) at concentrations up to 8%. The anionic surfactant SDS led to a loss of stability at cc ≈ 1.6% whereas suspension stability was not compromised by anionic SLES up to 8%. Dynamic light scattering data are consistent with a scenario in which gel formation is driven by micelle-nanofibril bridging mediated by associative interactions of ethoxylated surfactant headgroups with the cellulose fibrils. This may explain the strong difference between the properties of SDS and SLES-modified suspensions. These results have implications for the use of CNFs as a rheology modifier in surfactant-containing systems.