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The photoreduction of CO 2 to formate (HCOO − ) in sphalerite (ZnS) aqueous suspensions is systematically studied in the presence of Na 2 S hole scavenger. A series of cut-on filters at λ cut-on ≥ 280, 295, 305, 320, and 400 nm are used to measure the reaction rate of formate production. The dependence of the measured reaction rates on λ cut-on indicates that a wavelength of λ = 345 nm is associated with the actual bandgap of the semiconductor nanocrystallites suspended in water. The results from apparent quantum yield measurements during periodic illumination experiments suggest that (1) valence-band holes on the surface of ZnS disappear within deciseconds due to the oxidation of the scavenger while simultaneously pumping electrons to the conduction band, (2) excited electrons in the conduction band of ZnS are transferred to CO 2 to produce the intermediate CO 2 •−, and (3) CO 2•− abstracts a proton from water and undergoes further photoreduction on the surface of ZnS in an overall time scale for steps 2 + 3 of a few milliseconds. The separation of both process merges at ∼29 ms because it decreases exponentially with a drop in [Na 2 S] accompanied by a less negative surface potential. The behavior of the reaction rate at variable pH resembles the fraction of dissolved CO 2 , discarding the direct participation of bicarbonate and carbonate in the reaction. Combined chromatographic, mass spectrometry, and spectroscopic studies provide new insights to understand the role of surface chemistry on the photoreduction of CO 2 on ZnS nanocrystals.

Natural and anthropogenic emissions of aromatic hydrocarbons from biomass burning, agro-industrial settings, and fossil fuel combustion contribute precursors to secondary aerosol formation (SOA). How these compounds are processed under humid tropospheric conditions is the focus of current attention to understand their environmental fate. This work shows how catechol thin films, a model for oxygenated aromatic hydrocarbons present in biomass burning and combustion aerosols, undergo heterogeneous oxidation at the air-solid interface under variable relative humidity (RH = 0-90%). The maximum reactive uptake coefficient of O3(g) by catechol γO3 = (7.49 ± 0.35) × 10(-6) occurs for 90% RH. Upon exposure of ca. 104-μm thick catechol films to O3(g) mixing ratios between 230 ppbv and 25 ppmv, three main reaction pathways are observed. (1) The cleavage of the 1,2 carbon-carbon bond at the air-solid interface resulting in the formation of cis,cis-muconic acid via primary ozonide and hydroperoxide intermediates. Further direct ozonolysis of cis,cis-muconic yields glyoxylic, oxalic, crotonic, and maleic acids. (2) A second pathway is evidenced by the presence of Baeyer-Villiger oxidation products including glutaconic 4-hydroxy-2-butenoic and 5-oxo-2-pentenoic acids during electrospray ionization mass spectrometry (MS) and ion chromatography MS analyses. (3) Finally, indirect oxidation by in situ produced hydroxyl radical (HO(•)) results in the generation of semiquinone radical intermediates toward the synthesis of polyhydoxylated aromatic rings such as tri-, tetra-, and penta-hydroxybenzene. Remarkably, heavier polyhydroxylated biphenyl and terphenyl products present in the extracted oxidized films result from coupling reactions of semiquinones of catechol and its polyhydroxylated rings. The direct ozonolysis of 1,2,3- and 1,2,4-trihydroxybenezene yields 2- and 3-hydroxy-cis,cis-muconic acid, respectively. The production of 2,4- or 3,4-dihdroxyhex-2-enedioic acid is proposed to result from the sequential processing of cis,cis-muconic acid, 2- and 3-hydroxy-cis,cis-muconic acid. Overall, these reactions contribute precursors to form aqueous SOA from aromatics in atmospheric aerosols and brown clouds.

The reductive tricarboxylic acid (rTCA) cycle is an important central biosynthetic pathway that fixes CO 2 into carboxylic acids. Among the five reductive steps in the rTCA cycle, the two-electron reduction of fumarate to succinate proceeds nonenzymatically on the surface of photoexcited sphalerite (ZnS) colloids suspended in water. This model reaction is chosen to systematically study the surface photoprocess occurring on ZnS in the presence of [Na 2 S] (1−10 mM) hole scavenger at 15 °C. Experiments at variable pH (5−10) indicate that monodissociated fumaric acid is the primary electron acceptor forming the monoprotic form of succinic acid. The following reaction scheme is proposed: (1) photoexcitation of ZnS generates conduction band electrons and valence band holes, (2) the hole scavenger donates electrons while producing sulfur-containing intermediates en route to sulfate formation, (3) a first electron transfer occurs at the conduction band converting chemisorbed monoprotic fumaric acid at surface zinc sites into an adsorb radical anion, and (4) the radical anion accepts a second electron and forms an adsorbed carbanion, which (5) abstracts two protons consecutively from either hydronium ion (acidic condition) or water (neutral and basic condition) to be desorbed as monodissociated succinic acid. The apparent quantum yield measurement of succinate production (Φ s ) under periodic irradiation at λ ≥ 305 nm shows that the time scale of electron transfer on the conduction band (t 1 ) and valence band hole loss (t 2 ) are in the order of hundred microseconds and a few milliseconds, respectively. These transitions (t 1 and t 2 ) become undistinguishable at 520 μs for a zeta potential ζ = −22.09 mV corresponding to [Na 2 S] = 0.57 mM. Overall, this work provides new insights to model heterogeneous processes such as the reduction of CO 2 occurring on the surface of photocatalysts and advance present understanding of photocatalytic reactions.

A novel fluorescent probe based on N,S,P codoped carbon nanodots (N,S,P-CND) is very simple and quickly fabricated by a one-step hydrothermal pyrolysis of Saccharomyces cerevisiae and utilized for label-free and "on-off-on" sequential detection of manganese(VII) and l-ascorbic acid (l-AA). The fluorescence of N,S,P-CND can be effectively quenched by Mn(VII) based on an inner filter effect (IFE) and recovered upon the addition of l-AA due to the easy conversion of Mn(VII) to reduced states (i.e., Mn(IV), Mn(II), and Mn(0)) by l-AA. This probe exhibited favorable selectivity and sensitivity toward Mn(VII) and l-AA with detection limits of 50 nmol/L and 1.2 μmol/L, respectively. Simultaneously, an "AND" logic gate based on the as-fabricated N,S,P-CND has been constructed. Also, the as-proposed fluorescent probe was extended to detect Mn(VII) and l-AA in biosystems. Furthermore, the as-constructed fluorescent probe system was successfully applied to the analyses of Mn(VII) in tap water, Fenhe River water, and medicinal herb samples with satisfactory results. The proposed method is simple and easily accessible, demonstrating the great potential of N,S,P-CND in biosensing, disease diagnosis, cellular labeling, and environmental monitoring.