No abstract

We describe unprecedented resolution of electrochemically observed quantized double layer (QDL) charging, attained with use of reduced solution temperatures and with an annealing procedure that produces hexanethiolate monolayer protected gold clusters (C6 MPCs) with a high level of monodispersity in charging capacitance, C(CLU). The spacing DeltaV = e/C(CLU) on the electrochemical potential axis between one electron changes in the electronic charge of nanoscopic metal particles is determined by their effective capacitance C(CLU). The high monodispersity of the C6 MPCs with Au(140) cores facilitates (a) detailed rotated disk and cyclic voltammetric measurements, (b) simulation of QDL waveshapes based on assumed reversible, multivalent redox-like behavior, (c) determination of nanoparticle diffusion rates, and (d) observation of as many as 13 changes in the MPC charge state, from MPC(6-) to MPC(7+). The single electron QDL charging peaks are quite evenly spaced (DeltaV constant) at potentials near the MPC potential of zero charge, but are irregularly spaced at more positive and negative potentials. The irregular spacing is difficult to rationalize with classical double layer capacitance ideas and is proposed to arise from a correspondingly structured (e.g., not smooth) density of electronic states of the nanoparticle core, resulting from its small HOMO/LUMO gap and incipiently molecule-like behavior.

This paper describes low-temperature voltammetry of purified hexanethiolate-coated monolayer-protected Au140 clusters (C6 MPCs). Lowered temperatures enhance the resolution of quantized double layer (QDL) charging peaks in differential pulse voltammetry (DPV) observations. As many as 13 resolved peaks are seen in illustrative voltammetry at 263 K in CH2Cl2 solvent, and the concept of voltammetric peak capacity is introduced. For the one-electron MPC charge steps surrounding the E(PZC) of the MPC (small numbers of electrons added or removed from the core), the capacitance C(CLU) of the MPCs (measured from the voltage spacing between charging peaks) increases by approximately 15% as the solvent temperature is lowered from 273 to 210 K. The experimental C(CLU) temperature dependency (d[ln(C(CLU))]/dT approximately -0.0025, in 0.1 M electrolyte) is discussed in light of temperature dependencies of the compact and diffuse double layer capacitances. It is concluded that the observed temperature dependence is probably a mixed diffuse, compact dependence. The regular voltage spacing of MPC charging peaks near the potential of zero charge is generally consistent with electrical double layer properties, but the irregular pattern of charging of the nanoparticles seen at higher charge states suggests intervention of the incipient molecular behavior of Au140 cores in the spacing of energies at which further electrons are added or removed.

Monolayer-protected Au clusters (MPCs) have been prepared with mixed monolayers of alkanethiolates and alkanethiolates terminally omega-functionalized with phenothiazine. The mixed monolayer MPCs can contain as many as 10 phenothiazines/MPC; these electron donors are electroactive in rapid, successive one-electron reactions. Surface adsorption of the functionalized MPCs is evident in cyclic voltammetry. Double-potential-step chronocoulometry with incremented potential steps was applied to unfunctionalized hexanethiolate-coated MPCs and to those functionalized with phenothiazine to analyze the coupling between the diffusion-controlled double-layer charging of the MPC cores and the oxidation of the phenothiazine centers. Apparent changes in ordering of the MPC alkanethiolate chains were observed with infrared spectroscopy in solutions of MPCs where alcohol, carboxylic acid, or phenothiazine moieties had been incorporated into the monolayer.

Recent emphasis on the science of food and cooking has been observed in our popular literature and media. As a result of this, a new non-science majors course, The Science of Food and Cooking, is being taught at our institution. We cover basic scientific concepts, which would normally be discussed in a typical introductory chemistry course, in the context of food and food preparation. A week-by-week description of the course curriculum is provided. Student assessment and results from student evaluations are reported.