The process of method development is at the heart of analytical chemistry, yet direct experimental experience with the process is rarely taught in the undergraduate laboratory. We report here an ...experiment that uses rapid, safe, and inexpensive paper-based microfluidics with cell phone detection in order to facilitate the development of a colorimetric method for the determination of iron with three different derivatizing agents (thiocyanate, 1,10 phenanthroline, or FerroZine). This lab has been developed with an inquiry-based pedagogy where students determine which figures of merit to measure, the order to measure them in, and their own procedures for measuring them. This experiment gives students the opportunity to measure linear range, sensitivity, selectivity, accuracy, precision, limit of detection, and optimal reaction time for each derivatizing agent. Students then compare and contrast the measured figures of merit to make an evaluation of which method is preferred for iron quantitation. The results are variable enough from group to group, and each derivatizing agent has different advantages and disadvantages so that there is no “right” answer. This experiment is very flexible and can be modified to fit into one or two lab periods, be delivered with students working in groups or individually, and be delivered in person or in a remote setting. Student learning was assessed using the Enhancing Learning by Improving Process Skills in STEM (ELIPSS) rubrics for management, critical thinking, and information processing. Students demonstrated growth in many skills and expressed significant appreciation for their own learning.
We previously described how thousands of the heme protein cytochrome c (cyt.c) self-organize into multilayered, roughly spherical superstructures as initiated by nucleation around one colloidal gold ...or silver nanoparticle. Within these superstructures, the protein is stabilized to unfolding in buffered media and survives superstructure encapsulation within silica gels and processing to form bioaerogels. We now report that Au∼cyt.c superstructures in buffered media are not simply static groupings of proteins, but that the Au core and protein corona exhibit dynamic electron-transfer reactions within the superstructure as verified by UV–visible and resonance Raman spectroscopy. Within the superstructure, hundreds to thousands of ferricytochrome c (FeIII-cyt.c) are reduced to ferrocytochrome c (FeII-cyt.c) following first-order kinetics with an average apparent forward rate constant of 1.9 ±0.4 × 10–5 s–1. The reducing power in the microheterogeneous medium is derived from two multielectron reductants: tannic acid used to stabilize the commercial gold sol and the Au nanoparticle at the center of the protein superstructure. Fluorescence monitoring of guanidinium chloride-induced unfolding reveals that superstructure-associated cyt.c is stabilized to unfolding before and after chemical reduction of FeIII-cyt.c to form FeII-cyt.c, indicating that the superstructures remain intact during microheterogeneous redox reactions. Smaller nucleating Au nanoparticles or lower ionic strength in the buffered medium yields a greater extent of cyt.c reduction. Partial oxidation of the cyt.c-associated nanoparticulate Au is verified by X-ray photoelectron spectroscopy. The Au nanoparticle at the heart of the superstructure functions as a direct electron donor to the heme with oxidized Au atoms being recycled back to Au(0) as long as residual tannic acid, derived from the Au sol mother liquor, is present in the aqueous microheterogeneous medium.
Sol-gel-derived aerogels are three-dimensional, nanoscale materials that combine large surface area with high porosity. These traits make them useful for any rate-critical chemical process, ...particularly sensing or electrochemical applications, once physical or chemical moieties are incorporated into the gels to add their functionality to the ultraporous scaffold. Incorporating biomolecules into aerogels, other than such rugged species as lipases or cellulose, has been challenging due to the inability of most biomolecules to remain structurally intact within the gels during the necessary supercritical fluid (SCF) processing. However, the heme protein cytochrome c (cyt.c) forms self-organized superstructures around gold (or silver) nanoparticles in buffer that can be encapsulated into wet gels as the sol undergoes gelation. The guest-host wet gel can then be processed to form composite aerogels in which cyt.c retains its characteristic visible absorption. The gold (or silver) nanoparticle-nucleated superstructures protect the majority of the protein from the harsh physicochemical conditions necessary to form an aerogel. The Au~cyt.c superstructures exhibit rapid gas-phase recognition of nitric oxide (NO) within the bioaerogel matrix, as facilitated by the high-quality pore structure of the aerogel, while remaining viable for weeks at room temperature. More recently, careful control of synthetic parameters (e.g., buffer concentration, protein concentration, SCF extraction rate) have allowed for the preparation of cyt.c-silica aerogels, sans nucleating nanoparticles; these bioaerogels also exhibit rapid gas-phase sensing while retaining protein structural stability.
The nucleation−growth−passivation Brust reaction has been modified so as to enrich the product in useful quantities of a 38-atom gold nanoparticle coated with a hexanethiolate monolayer. Two ...modifications are described, using −78 °C reduction temperature and a hyperexcess of thiol. Compositional evidence is presented that establishes the product as a Au38(C6)24 hexanethiolate monolayer protected cluster (MPC), based on transmission electron microscopy, laser ionization−desorption mass spectrometry, thermogravimetric analysis, and elemental analysis. Reverse phase HPLC confirms the relatively good monodispersity of the MPC products, but high-resolution double-column HPLC reveals that the MPCs are a mixture of closely related but chromatographically distinct products. Voltammetry, low energy spectrophotometry, and spectroelectrochemistry reveal, respectively, a 1.6 eV electrochemical energy gap between the first oxidation and the first reduction, an optical HOMO−LUMO energy absorbance edge at 1.3 eV, and a bleaching of optical absorbance near the 1.3 eV band edge that accompanies electrochemical oxidation of the nanoparticle.
The atomic metal core structures of the subnanometer clusters Au13PPh34S(CH2)11CH32Cl2 (1) and Au13PPh34S(CH2)11CH34 (2) were characterized using advanced methods of electron microscopy and X-ray ...absorption spectroscopy. The number of gold atoms in the cores of these two clusters was determined quantitatively using high-angle annular dark field scanning transmission electron microscopy. Multiple-scattering-path analyses of extended X-ray absorption fine structure (EXAFS) spectra suggest that the Au metal cores of each of these complexes adopt an icosahedral structure with a relaxation of the icosahedral strain. Data from microscopy and spectroscopy studies extended to larger thiolate-protected gold clusters showing a broader distribution in nanoparticle core sizes (183 ± 116 Au atoms) reveal a bulklike fcc structure. These results further support a model for the monolayer-protected clusters (MPCs) in which the thiolate ligands bond preferentially at 3-fold atomic sites on the nanoparticle surface, establishing an average composition for the MPC of Au180S(CH2)11CH340. Results from EXAFS measurements of a gold(I) dodecanethiolate polymer are presented that offer an alternative explanation for observations in previous reports that were interpreted as indicating Au MPC structures consisting of a Au core, Au2S shell, and thiolate monolayer.
This paper describes the effects of oxidative electronic charging of the Au cores of the monolayer-protected clusters (MPCs), Au140(S(CH2)5CH3)53 and Au38(SCH2CH2Ph)24, on nuclear magnetic resonance ...(NMR) spectra of their monolayer ligand shells. Previously unresolved fine structure in the 13C NMR hexanethiolate methyl and C5 methylene resonances is seen in spectra of solutions of monodisperse Au140(S(CH2)5CH3)53 MPCs, reflecting magnetically inequivalent ligand sites. Incremented increases in positive cluster core charge, effected by electrochemical charging, cause the spectral fine structure of the methyl resonance to coalesce, becoming a single peak at the Au140 3+ charge state. The spectral changes are reversible; charging back to the original core charge state regenerates the methyl 13C resonance fine structure. Adding an equimolar quantity of a Au(I) thiolate complex, AuISCH2(C6H4)C(CH3)3, to an uncharged Au140(S(CH2)5CH3)53 MPC solution in d 2 -methylene chloride causes partial spectral coalescence. 13C NMR spectra of Au38(SCH2CH2Ph)24 MPCs exhibit roughly comparable spectral changes upon positive core charging to the ‘0', ‘+1', and ‘+2' states. The NMR results indicate that exchange between magnetically inequivalent sites occurs at rates of 100 to 400 s-1, a rate believed to be too fast to be accountable by actual exchanges of ligands between different sites on the Au core. We also describe changes in core electronic spectra of Au140(S(CH2)5CH3)53 induced by positive charging, measured using spectroelectrochemistry.
Applications such as sensors, batteries, and fuel cells have been improved through the use of highly porous aerogels when functional compounds are encapsulated within the aerogels. However, few ...reports on encapsulating proteins within sol-gels that are processed to form aerogels exist. A procedure for encapsulating cytochrome c (cyt. c) in silica (SiO2) sol-gels that are supercritically processed to form bioaerogels with gas-phase activity for nitric oxide (NO) is presented. Cyt. c is added to a mixed silica sol under controlled protein concentration and buffer strength conditions. The sol mixture is then gelled and the liquid filling the gel pores is replaced through a series of solvent exchanges with liquid carbon dioxide. The carbon dioxide is brought to its critical point and vented off to form dry aerogels with cyt. c encapsulated inside. These bioaerogels are characterized with UV-visible spectroscopy and circular dichroism spectroscopy and can be used to detect the presence of gas-phase nitric oxide. The success of this procedure depends on regulating the cyt. c concentration and the buffer concentration and does not require other components such as metal nanoparticles. It may be possible to encapsulate other proteins using a similar approach making this procedure important for potential future bioanalytical device development.
The synthesis and characterization of the clusters Au13PPh34S(CH2)11CH32Cl2 (1) and Au13PPh34S(CH2)11CH34 (2) are described. These mixed-ligand, sub-nanometer clusters, prepared via exchange of ...dodecanethiol onto phosphine-halide gold clusters, show enhanced stability relative to the parent. The characterization of these clusters features the precise determination of the number of gold atoms in the cluster cores using high-angle annular dark-field scanning transmission electron microscopy, allowing the assignment of 13 gold atoms (±3 atoms) to the composition of both cluster molecules. Electrochemical and optical measurements reveal discrete molecular orbital levels and apparent energy gaps of 1.6−1.7 eV for the two cluster molecules. The electrochemical measurements further indicate that the Au13PPh34S(CH2)11CH32Cl2 cluster undergoes an overall two-electron reduction. The electrochemical and spectroscopic properties of the two Au13 cluster molecules are compared with those of a secondary synthetic product, which proved to be larger Au thiolate-derivatized monolayer-protected clusters with an average core of Au180. The latter shows behavior fully consistent with the adoption of metallic-like properties.
As part of a university-wide project to explore Shakespeare’s classic play, Romeo and Juliet, from a variety of perspectives, an interdisciplinary talk was presented to the university community on ...the chemistry of the potions and poisons referenced in Romeo and Juliet. To draw the multidisciplinary audience in and to teach about forensics as well as pharmaceutical herbs and chemicals, the presentation was given from the perspective of how a modern crime scene investigator would approach the famous play’s final death scene without any prior knowledge of the situation. An autopsy of Juliet’s body might have revealed the presence of the chemicals, hyoscine and atropine, that come from the plant Atropa belladonna. The autopsy could reveal whether the Friar had set out to sedate Juliet or if he had attempted to kill her. An autopsy of Romeo’s body might have revealed the presence of aconitine from the plant Aconitum napellus. Using a classic story to teach about chemistry, basic ideas were introduced about forensics and pharmacology, emphasizing the importance of dose when determining the effect of a drug on the human body.
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