Localization and Dynamics of Long‐Lived Excitations in Colloidal Semiconductor Nanocrystals with Dual Quantum Confinement

Semiconductor nanocrystals consisting of a quantum dot (QD) core and a quantum well (QW) shell, where the QD and QW are separated by a tunneling barrier, offer a unique opportunity to engineer the photophysical properties of individual nanostructures. Using the thicknesses of the corresponding layers, the excitons of the first and second excited states can be separated spatially, localizing one state to the QD and the other to the QW. Thus the wave function overlap of the two states can be minimized, suppressing non‐radiative thermalization between the two wells, which in turn leads to radiative relaxation from both states. The molecular analogy to such dual emission would be the inhibition of internal conversion, a special case that violates Kasha′s rule. Using nanosecond time‐resolved spectroscopy of QDQW CdSe/ZnS onion‐like nanocrystals, an intermediate regime of exciton separation and suppressed thermalization is identified where the non‐radiative relaxation of the higher‐energy state is slowed, but not completely inhibited. In this intermediate thermalization regime, the temporal evolution of the delayed emission spectra resulting from trapped carriers mimic the dynamics of such states in nanocrystals that consist of only a QD core. In stark contrast, when a higher‐energy metastable state exists in the QW shell due to strongly suppressed interwell thermalization, the spectral dynamics of the long‐lived excitations in the QD and QW, which are spectrally distinct, are amplified and differ from each other as well as from those in the core‐only nanocrystals. This difference in spectral dynamics demonstrates the utility of exploiting well‐defined exciton localization to study the nature and spatial dependence of the intriguing photophysics of colloidal semiconductor nanocrystals, and illustrates the power of nanosecond gated luminescence spectroscopy in illuminating complex relaxation dynamics which are entirely masked in steady‐state or ultrafast spectroscopy.     

Estimation of Lattice Enthalpies of Ionic Liquids Supported by Hirshfeld Analysis

New measurements of vaporization enthalpies for 15 1:1 ionic liquids are performed by using a quartz‐crystal microbalance. Collection and analysis of 33 available crystal structures of organic salts, which comprise 13 different cations and 12 anions, is performed. Their dissociation lattice enthalpies are calculated by a combination of experimental and quantum chemical quantities and are divided into the relaxation and Coulomb components to give an insight into elusive short‐range interaction enthalpies. An empirical equation is developed, based on interaction‐specific Hirshfeld surfaces and solvation enthalpies, which enables the estimation of the lattice enthalpy by using only the crystal‐structure data.     

Overloading control of gas chromatographic systems

Increasing the interlaboratory reproducibility of gas chromatographic retention indices requires avoiding measurements distorted by overloading effects. Several criteria of evaluating the limits of the mass overloading of gas chromatographic systems are compared and reconsidered. The criteria mostly appropriate for practical purposes are based on (i) the dependences of factors of peak broadening (ratio of peak height and its width) vs. amount of analyte injected into the chromatographic column and (ii) the dependence of parameters characterizing the peak distortion (asymmetry factor) vs. the amount of analyte. Both these criteria provide mutually comparable evaluations of the overloading limits for analytes of different polarity. At the same time, the dependence of retention indices vs. amounts of analyte injected in the chromatographic column cannot be recommended for overloading control, because the parameters of the corresponding linear regressions indicate temperature dependence. The interpretation of certain gas chromatographic anomalies requires the correct evaluation of overloading limits. For example, the unusual temperature dependence of retention indices of polar analytes on non‐polar stationary phases and the dependence of retention indices on ratio of amounts of target analytes and reference compounds.     

Total synthesis and biochemical characterization of mirror image barnase

In this study we synthesized and characterized mirror image barnase (B. amyloliquefaciens ribonuclease). d-Barnase was identical to l-barnase, when analyzed by liquid chromatography and mass-spectrometry. Proteolysis of the mirror image enzyme revealed that in contrast to its native counterpart, d-barnase was completely stable to digestive proteases. In enzymatic assays, d-barnase had the reciprocal chiral specificity and was fully active towards mirror image substrates. Interestingly, d-barnase also hydrolyzed the substrate of the native chirality, albeit 4000 times less efficiently. This effect was further confirmed by digesting a native 112-mer RNA with the enzyme. Additional studies revealed that barnase accommodates a range of substrates with various chiralities, but the prime requirement for guanosine remains. These studies point toward using mirror image enzymes as modern agents in biotechnology.     

Steric effects and quantum interference in the inelastic scattering of NO(X) + Ar

Rotationally inelastic collisions of NO(X) with Ar are investigated in unprecedented detail using state-to-state, crossed molecular beam experiments. The NO(X) molecules are selected in the Ω = 0.5, j = 0.5, f state and then oriented such that either the ‘N’ or ‘O’ end of the molecule is directed towards the incoming Ar atom. Velocity map ion imaging is then used to probe the scattered NO molecules in well-defined quantum states. We show that the fully quantum state-resolved differential steric asymmetry, which quantifies how the relative efficiency for scattering off the ‘O’ and the ‘N’ ends of the molecule varies with scattering angle, is strongly affected by quantum interference. Significant changes in both integral and differential cross sections are found depending on whether collisions occur with the N or O ends of the molecule. The results are well accounted for by rigorous quantum mechanical calculations, in contrast to both classical trajectory calculations and more simplistic models that provide, at best, an incomplete picture of the dynamics.     

Protein ubiquitination and formation of polyubiquitin chains without ATP,E1 and E2 enzymes

Studying protein ubiquitination is difficult due to the complexity of the E1–E2–E3 ubiquitination cascade. Here we report the discovery that C-terminal ubiquitin thioesters can undergo direct transthiolation with the catalytic cysteine of the model HECT E3 ubiquitin ligase Rsp5 to form a catalytically active Rsp5∼ubiquitin thioester (Rsp5∼Ub). The resulting Rsp5∼Ub undergoes efficient autoubiquitination, ubiquitinates protein substrates, and synthesizes polyubiquitin chains with native Ub isopeptide linkage specificity. Since the developed chemical system bypasses the need for ATP, E1 and E2 enzymes while maintaining the native HECT E3 mechanism, we named it “Bypassing System” (ByS). Importantly, ByS provides direct evidence that E2 enzymes are dispensable for K63 specific isopeptide bond formation between ubiquitin molecules by Rsp5 in vitro. Additionally, six other E3 enzymes including Nedd4-1, Nedd4-2, Itch, and Wwp1 HECT ligases, along with Parkin and HHARI RBR ligases processed Ub thioesters under ByS reaction conditions. These findings provide general mechanistic insights on protein ubiquitination, and offer new strategies for assay development to discover pharmacological modulators of E3 enzymes.     

Selective glycoprotein detection through covalent templating and allosteric click-imprinting

Many glycoproteins are intimately linked to the onset and progression of numerous heritable or acquired diseases of humans, including cancer. Indeed the recognition of specific glycoproteins remains a significant challenge in analytical method and diagnostic development. Herein, a hierarchical bottom-up route exploiting reversible covalent interactions with boronic acids and so-called click chemistry for the fabrication of glycoprotein selective surfaces that surmount current antibody constraints is described. The self-assembled and imprinted surfaces, containing specific glycoprotein molecular recognition nanocavities, confer high binding affinities, nanomolar sensitivity, exceptional glycoprotein specificity and selectivity with as high as 30 fold selectivity for prostate specific antigen (PSA) over other glycoproteins. This synthetic, robust and highly selective recognition platform can be used in complex biological media and be recycled multiple times with no performance decrement.     

Oxygen deficient α-Fe2O3 photoelectrodes: a balance between enhanced electrical properties and trap-mediated losses

Intrinsic doping of hematite through the inclusion of oxygen vacancies (V_O) is being increasingly explored as a simple, low temperature route to preparing active water splitting α-Fe₂O_3–x photoelectrodes. Whilst it is widely accepted that the introduction of V_O leads to improved conductivities, little else is verified regarding the actual mechanism of enhancement. Here we employ transient absorption (TA) spectroscopy to build a comprehensive kinetic model for water oxidation on α-Fe₂O_3–x. In contrast to previous suggestions, the primary effect of introducing V_O is to block very slow (ms) surface hole – bulk electron recombination pathways. In light of our mechanistic research we are also able to identify and address a cause of the high photocurrent onset potential, a common issue with this class of electrodes. Atomic layer deposition (ALD) of Al₂O₃ is found to be particularly effective with α-Fe₂O_3–x, leading to the photocurrent onset potential shifting by ca. 200 mV. Significantly TA measurements on these ALD passivated electrodes also provide important insights into the role of passivating layers, that are relevant to the wider development of α-Fe₂O₃ photoelectrodes.     

3D electrogenerated chemiluminescence: from surface-confined reactions to bulk emission

Among luminescence techniques, electrogenerated chemiluminescence (ECL) provides a unique level of manipulation of the luminescent process by controlling the electrochemical trigger. Despite its attractiveness, ECL is by essence a 2D process where light emission is strictly confined to the electrode surface. To overcome this intrinsic limitation, we added a new spatial dimension to the ECL process by generating 3D ECL at the level of millions of micro-emitters dispersed in solution. Each single object is addressed remotely by bipolar electrochemistry and they generate collectively the luminescence in the bulk. Therefore, the entire volume of the solution produces light. To illustrate the generality of this concept, we extended it to a suspension of multi-walled carbon nanotubes where each one acts as an individual ECL nano-emitter. This approach enables a change of paradigm by switching from a surface-limited process to 3D electrogenerated light emission.