Quantification of the Photon Absorption, Scattering, and On-Resonance Emission Properties of CdSe/CdS Core/Shell Quantum Dots: Effect of Shell Geometry and Volumes.

Reliable quantification of the optical properties of fluorescent quantum dots (QDs) is critical for their photochemical, -physical, and -biological applications. Presented herein is the experimental quantification of photon scattering, absorption, and on-resonance-fluorescence (ORF) activities of CdSe/CdS core/shell fluorescent QDs as a function of the shell sizes and geometries. Four spherical QDs (SQDs) with different diameters and four rod-like QDs (RQDs) with different aspect ratios (ARs) have been analyzed using UV-vis, fluorescence, and the recent polarized resonance synchronous spectroscopic (PRS2) methods. All quantum dots are simultaneous absorbers and scatterers in the UV-vis wavelength region, and they all exhibit strong ORF emission in the wavelength regions where the QDs both absorb and emit. The absorption and scattering cross-sections of the CdS shell are linearly and quadratically, respectively, proportional to the shell volume for both the SQDs and RQDs. However, the effects of CdS shell coating on the core optical properties are different between SQDs and RQDs. For RQDs, increasing the CdS shell volume through the length elongation has no effect on either the peak wavelength or intensity of the CdSe core UV-vis absorption and ORF, but it reduces the QD fluorescence depolarization. In contrast, increasing CdS shell volume in the SQDs induces red-shift in the CdSe core peak UV-vis absorption and ORF wavelengths, and increases their peak cross-sections, but it has no effect on the SQD fluorescence depolarization. The RQD ORF cross-sections and quantum yields are significantly higher than their respective counterparts for the SQDs with similar particle sizes (volumes). While these new insights should be significant for the QD design, characterization, and applications, the methodology presented in this work is directly applicable for quantifying the optical activities of optically complex materials where the common UV-vis spectrometry and fluorescence spectroscopy are inadequate.

Colloidal Assembly of Au-Quantum Dot-Au Sandwiched Nanostructures with Strong Plasmon-Exciton Coupling.

Strong plasmon-exciton coupling could occur in hybrid metal-dye/semiconductor nanostructures, where the fast energy exchange between plasmons and excitons leads to two new eigenmodes of the system, known as Rabi splitting. In experiments, strongly coupled nanosystems are difficult to obtain because they require some strict conditions, such as low plasmonic damping, small plasmon mode volume, and good spectral overlap. This work demonstrates strongly coupled metal-semiconductor nanostructures can be constructed using colloidal assembly. Specifically, sandwiched Au-quantum dot-Au nanostructures were created through the assembly of Au nanoparticles and colloidal quantum dots (QDs). The sizes of the QDs and the assembly conditions were varied to control the mode volume of the plasmonic cavity formed between the two Au nanoparticles. With a decreased gap size, Rabi splitting was observed in both dark-field scattering and fluorescence spectra of single Au-QD-Au nanostructures. Theoretical simulations revealed that the strong coupling occurred between the excitons and the octupolar plasmon modes.

Degradation kinetics of hexachlorobenzene over zero-valent magnesium/graphite in protic solvent system and modeling of degradation pathways using density functional theory.

Hexachlorobenzene (HCB), like many chlorinated organic compounds, has accumulated in the environment from agricultural and industrial activity. Because of its health risks and adverse impact on various ecosystems, remediation of this contaminant is of vital concern. The objective of this study is to evaluate the proficiency of activated magnesium metal in a protic solvent system to accomplish reductive dechlorination of HCB. Experimental results were compared with those predicted by quantum chemical calculations based on Density Functional Theory (DFT). Multivariate analysis detected complete degradation of HCB within 30 min at room temperature, the reaction having a rate constant of 0.222 min-1. Dechlorination was hypothesized to proceed via an ionic mechanism; the main dechlorination pathways of HCB in 1:1 ethanol:ethyl lactate were HCB → PCBz → 1,2,4,5-TCB; 1,2,3,5-TCB → 1,2,4-TriCB; 1,3,5-TriCB → 1,4-DiCB; 1,3-DiCB. The direct relationship between the decreasing number of Cl substituents and dechlorination reaction kinetics agrees with the ΔG values predicted by the computational model. This methodology shows promise for the development of a practical and sustainable field application for the remediation of other chlorinated aromatic compounds.

Linear Extrapolation of the Analyte-Specific Light Scattering and Fluorescence Depolarization in Turbid Samples.

Anisotropy and depolarization are two interconvertible parameters in fluorescence and light scattering spectroscopy that describe the polarization distribution of emitted and scattered photons generated with linearly polarized excitation light. Whereas anisotropy is more frequently used in fluorescence literature for studying association/dissociation of fluorophore-bearing reagents, depolarization is more popular in the light-scattering literature for investigating the effect of scatterers' geometries and chemical compositions. Presented herein is a combined computational and experimental study of the scattering and fluorescence depolarization enhancement induced by light scattering in turbid samples. The most important finding is that sample light scattering and fluorescence depolarization increases linearly with sample light-scattering extinction. Therefore, one can extrapolate the analyte-specific scattering and fluorescence depolarization through linear curve fitting of the sample light scattering and fluorescence depolarization as a function of the sample concentration or the path length of the sampling cuvettes. An example application of this linear extrapolation method is demonstrated for quantifying the fluorophore-specific fluorescence depolarization and consequently its anisotropy for an aggregation-induced-emission sample. This work should be important for a wide range of macromolecular, supramolecular, and nanoscale fluorescent materials that are often strong light scatterers due to their large sizes.

Mechanistic and computational studies of PCB 151 dechlorination by zero valent magnesium for field remediation optimization.

Polychlorinated biphenyls (PCBs) are banned in the U.S. but are persistent in the environment; current regulations provide an urgent need to remediate PCBs in a cost-effective way. In prior work, a novel method of degradation of PCBs via hydrodehalogenation with ball milled zero-valent magnesium and activated carbon showed promising results even with water present in the system. In this research, a detailed study of the byproducts formed in the dechlorination process for PCB 151 (used as an example of hexa-chlorinated PCB) and a study of the mechanism involved in this reaction via density functional theory (DFT) computations are presented. It was demonstrated that these reactions are exothermic and involved two transition states, the formation of the ionic transition state being the rate limiting step of the reaction. The torsion angle of the PCB congeners was also shown to be an extremely important factor to be able to use activated carbon as part of the remediation process.