Nanoplasmonic sensors based on surface-enhanced spectroscopies carry profound promise toward ultrasensitive detection of biomolecular analytes within miniaturized measurement footprints. High sensitivity in these sensors is achieved by intense electromagnetic (EM) enhancements at hot spots and colocalization of analytes with such hot spots. However, EM hot spots exhibiting high EM enhancements often present confined areas that render them inaccessible to large analytes such as biomolecules. Addressing this requires rational engineering of a nanoplasmonic interface that factors in the analyte surface concentrations and how they are distributed with respect to the EM hot spots. Here we demonstrate combination of metal-enhanced fluorescence (MEF) with a quartz crystal microbalance (QCM) through fabrication of highly resolved plasmonic nanoarrays directly on the QCM sensor. QCM and MEF are simultaneously employed to monitor in situ, real-time binding of fluorescently labeled protein to receptor-functionalized plasmonic arrays. The correlation between the QCM and MEF responses is used to quantify surface density of protein that contributes to the MEF signal intensities and obtain analyte distribution with respect to the EM hot spots by geometric modeling. Results further reveal the MEF assays to be sensitive down to ∼2 zmol of protein within measurement footprints that are 8 orders of magnitude smaller than that of the QCM.
Plasmonic metal nanoparticles (NPs) show promise in a variety of applications, ranging from theranostics to chemical sensing, with chemical sensing made possible by the sensitivity of the localized surface plasmon resonance (LSPR) to the surrounding dielectric environment. Au NPs have been the standard for LSPR sensing applications due to their narrow plasmon band, tunable LSPR maximum, and relative chemical stability; however, the comparatively low refractive index sensitivity (RIS) and morphological instability of Au nanostructures are limiting factors. Recent research has found that incorporating Pd into Au systems can increase RIS and impart multifunctionality, but how the distribution of Pd within Au-based nanostructures affects LSPR sensing is unclear. Here, Au-Pd heterostructures with different Au-Pd distributions were prepared to systematically study the effect of Pd distribution on RIS. Specifically, symmetrically branched Au nanocrystals with Oh symmetry (i.e., octopods) were selected as building blocks as their branch tips concentrate E-fields locally. Using these nanocrystals as seeds, Pd-tipped Au octopods and core@shell Au@Pd octopods were synthesized for comparison to alloy Au-Pd octopods and all-Au octopods. Through experiment and simulation, we show that RIS depends both on Pd loading and location in Au-Pd heterostructures, with the Pd-tipped Au octopods displaying the highest RIS while maintaining a moderate figure of merit. This systematic analysis highlights that localization of Pd at LSPR hotspots is critical to achieving the highest RIS, with this insight intended to guide design of future LSPR sensors that move beyond the all-Au standard.
We present an efficient approach to analyze CODEX-MAS exchange NMR data by comparing the experimental results with simulated data that are calculated from separately simulated motional trajectories rather than by application of the classic rate-equation approach. The trajectories can be calculated by random-walk simulations of dynamic processes of arbitrary motional geometries and distributions of exchange rates or taken from the result of MD simulations. The necessary computation time of this approach is independent of the complexity of the model and is particularly well-suited for the description of diffusive motions and/or complex models. It can also easily be expanded to intermediate motions. After comparison of trajectory and rate-equation approach, we discuss different dynamic models of discrete and diffusive motions and present applications to the model substance DMS and to scenarios that are applicable to polymeric or biological systems. As a proof of principle, we also calculate exchange-NMR data directly from a MD calculation of lipids; however, we adjusted its time scale to meet the dynamic range of the CODEX experiment.
Semiconductor-based heterogeneous photocatalysis has received much attention. A plasma-assisted method for synthesizing CuO was developed and used to decorate ZnO nanorods. High quality monoclinic CuO was synthesized in a highly excited oxygen-copper plasma generated by electron cyclotron resonance microwave discharge of O2 gas and pulsed laser ablation of a Cu target. The plasma and the CuO synthesis process were spectroscopically characterized by optical emission measurement. In the oxygen-copper plasma, surface decoration of hydrothermally grown ZnO nanorods with CuO was achieved. The CuO-decorated ZnO nanorods were systematically characterized for morphology, structure, photoexcitation, and light emission by field emission scanning electron microscopy, X-ray diffraction, Raman scattering spectroscopy, Fourier-transform infrared spectroscopy, light absorption, and photoluminescence measurements. Using the prepared samples as photocatalysts, degradation of organic rhodamine B dye was performed under ultraviolet or violet light illumination, showing that the CuO-decorated ZnO nanorods exhibit much higher photocatalytic activity than pure CuO films and bare ZnO nanorods. A Z-scheme electron transfer mechanism was proposed for the enhanced photocatalytic activity of nanostructured ZnO/CuO heterogeneous composites.
Predicting protein stability is a challenge due to the many competing thermodynamic effects. Through de novo protein design, one begins with a target structure and searches for a sequence that will fold into it. Previous work by Rocklin et al. introduced a data set of more than 16,000 miniproteins spanning four structural topologies with information on stability. These structures were characterized with a set of 46 structural descriptors, with no explicit inclusion of configurational entropy (Scnf). Our work focused on creating a set of 17 descriptors intended to capture variations in Scnf and its comparison to an extended set of 113 structural and energy model features that extend the Rocklin et al. feature set (R). The Scnf descriptors statistically discriminate between stable and unstable distributions within topologies and best describe EEHEE topology stability (where E = β sheet and H = α helix). Between 50 and 80% of the variation in each Scnf descriptor is described by linear combinations of R features. Despite containing useful information about minipeptide stability, providing Scnf features as inputs to machine learning models does not improve overall performance when predicting protein stability, as the R features sufficiently capture the implicit variations.
CuInS2 (CIS) quantum dots (QDs) would be attractive alternatives to the widely-investigated QDs containing toxic elements when the quantum yield (QY) of CIS QDs becomes comparable to that of typica...
The lattice dynamics of CsSnX3 (X = Cl, Br, and I) and CsPbI3, which are low-thermal-conductivity materials, are investigated using first-principles phonon calculations. Because of the strong latti...
Hydrogen-rich compounds are considered most likely to achieve room-temperature superconductivity since the critical temperature (T-c) above 250 K was observed in lanthanum hydride. Exploring the high-temperature super-conductivity in rare-earth metal hydrides becomes very interesting. Based on the particle swarm optimization for crystal structures and first-principles calculations, we investigate the crystal structures, phase stability, metallization, and possible superconducting properties of terbium hydride (TbHn, n = 1 - 12) under pressure. Our results show that terbium hydride is a potential high-temperature superconductor under high pressures. It stably exists at different pressure conditions by adjusting the H content. Specifically, the H atomic cage structure can be observed in most terbium hydrides, and the number of H atoms in the cage sublattice increases with the stoichiometry of H in TbHn. We demonstrate that the high T-c value is closely related to this cage sublattice and it increases with increasing H content in terbium hydride. The highest T-c above 270 K is predicted in TbH10 at 250 GPa for Fm (3) over barm and 310 GPa for R (3) over barm space group. This result indicates that the superconductivity with T-c close to or beyond lanthanum hydride can be achieved in other rare-earth metal hydrides.
A thermal responsive block copolymer made up of ethylene oxide (EO) and propylene oxide (PO) blocks was simulated with optimized atomistic potentials and enhanced sampling methods over a range of temperatures. The results for the L42 pluronic polymer (EO)4(PO)22(EO)4, which is known to undergo a transition in this temperature range, and the similarly sized (EO)30 polymer, which does not, are compared. The thermal responsive L42 polymers in a dilute solution tend to aggregate, and this tendency gets stronger as temperature increases. The poly(ethylene oxide) polymer shows no such tendency. The aggregation is stabilized by the hydrophobic contact of the propylene oxide methyl groups, which outweighs a small loss in hydrogen bonds between the ether oxygens and water.
CuWO4 is a ternary metal oxide semiconductor with promising properties for photoelectrochemical (PEC) water splitting and solar light conversion, due to its quite low band gap (2.3 eV) and high stability in an alkaline environment. Aiming at understanding the origin of the relatively low PEC efficiency attained with CuWO4 photoanodes, we here investigate transparent CuWO4 electrodes prepared by a simple solution-based method through the combination of femtosecond transient absorption spectroscopy with electrochemical, PEC, and photochromic characterizations. The very fast recombination dynamics of the charge carriers photogenerated in CuWO4, which is the reason for its low efficiency, is discussed in relation with its PEC performance and with the recently calculated band structure of this material, also in comparison with the behavior of other semiconductor oxides employed in PEC applications, in particular Fe2O3.
Many of graphene's remarkable properties arise from its linear dispersion of the electronic states, forming a Dirac cone at the K points of the Brillouin zone. Silicene, the 2D allotrope of silicon, is also predicted to show a similar electronic band structure, with the addition of a tunable bandgap, induced by spin-orbit coupling. Because of these outstanding electronic properties, silicene is considered as a promising building block for next-generation electronic devices. Recently, it has been shown that silicene grown on Au(111) still possesses a Dirac cone, despite the interaction with the substrate. Here, to fully characterize the structure of this 2D material, we investigate the vibrational spectrum of a monolayer silicene grown on Au(111) by polarized Raman spectroscopy. To enable a detailed ex situ investigation, we passivated the silicene on Au(111) by encapsulating it under few layers hBN or graphene flakes. The observed spectrum is characterized by vibrational modes that are strongly red-shifted with respect to the ones expected for freestanding silicene. By comparing low-energy electron diffraction (LEED) patterns and Raman results with first-principles calculations, we show that the vibrational modes indicate a highly (>7%) biaxially strained silicene phase.
Here, we report the effect of electron doping Zn for Cu on the physical properties of Cu₀.₅IrTe₂. Slight Zn doping concentration (x) becomes detrimental to the charge density wave (CDW) order, while the superconducting state can persist over a large change in chemical composition. The x dependence of the superconducting transition temperature (Tc) exhibits a weak dome-like shape with the highest Tc of 2.82 K at x = 0.5, whereas there is only a subtle change in Tc. The normalized electronic specific heat ΔCₑₗ./γTc value of 1.45 for the optimal doping composition Zn₀.₂₅Cu₀.₂₅IrTe₂ approaches the Bardeen–Cooper–Schrieffer (BCS) value (1.43), indicating the bulk nature of superconductivity. Magnetization and resistivity results further imply that our Zn-doped Cu₀.₅IrTe₂ are type II superconductors. We propose that these robust (ZnₓCu₁–ₓ)₀.₅IrTe₂ (0 ≤ x ≤ 0.9) superconductors may be suited to some research based on exfoliated crystal flakes and film devices.
When preparing nanostructured magnetic materials, the presence of an amorphous component is often considered a weakness of the synthesis method and a waste of material. This stems because the amorphous fraction is often pictured as a "dead" magnetic component, showing little to no contribution to the magnetic properties, for example, saturation magnetization. For this reason, additional steps are employed after the main synthesis process to reduce or isolate and remove the amorphous phase from the "optimal" crystalline product. Here, we propose a hybridstructured nanoarchitecture that combines crystalline cobalt ferrite and the amorphous parent material. The latter contributes partially to the total magnetic moment but exhibits a magnetic anisotropy much larger than the crystalline bulk parent material. With the information obtained from an in-depth structural and magnetic characterization, a micromagnetic model is created, allowing identifying the contribution of each component elucidating the active role of the amorphous phase. The extremely low cost, minimal complexity, and high yield of the synthesis process make this hybrid design of large interest for technological applications.
The study provides deep insight into the origin of photocatalytic deactivation of Nb2O5 after modification with ceria. Of particular interest was to fully understand the role of ceria species in diminishing the photocatalytic performance of CeO2/Nb2O5 heterostructures. For this purpose, ceria was loaded on niobia surfaces by wet impregnation. The as-prepared materials were characterized by powder X-ray diffraction, nitrogen physisorption, UV-visible spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. Photocatalytic activity of parent metal oxides (i.e., Nb2O5 and CeO2) and as-prepared CeO2/Nb2O5 heterostructures with different ceria loadings were tested in methanol photooxidation, a model gas-phase reaction. Deep insight into the photocatalytic process provided by operando-IR techniques combined with results of photoluminescence studies revealed that deactivation of CeO2/Nb2O5 heterostructures resulted from increased recombination of photo-excited electrons and holes. The main factor contributing to more efficient recombination of the charge carriers in the heterostructures was the ultrafine size of the ceria species. The presence of such highly dispersed ceria species on the niobia surface provided a strong interface between these two semiconductors, enabling efficient charge transfer from Nb2O5 to CeO2. However, the ceria species supported on niobia exhibited a high defect site concentration, which acted as highly active recombination centers for the photo-induced charge carriers.
Probing and understanding the local chemical environment of an active site is essential for designing high-performance single-atom catalysts (SACs). Density functional theory (DFT) calculations were performed to investigate the ligand configuration and site geometry of MgAl2O4-supported iridium single atoms (Ir-1) toward catalytic carbon monoxide (CO) oxidation. We employed MgAl2O4(111) and MgAl2O4(211) as the model substrates with adsorbed Ir single atoms of different site geometries. DFT calculations revealed that the Mg-site on MgAl2O4(111) and the step site on MgAl2O4(211) are the most stable adsorption sites for Ir single atoms. Irrespective of site choices, CO oxidation on supported Ir single atoms follows the Eley-Rideal (E-R) mechanism, in which the surface oxygen vacancies close to the Ir single atoms activate molecular O-2 with a negligible barrier and the rate-limiting step is the gas-phase CO directly attacking the O-Ir species that is modulated by a CO ligand. First-principles X-ray absorption near-edge spectra of reaction intermediates along with in situ/operando X-ray absorption spectroscopy (XAS) suggest that Ir single atoms adsorb primarily on the step sites of MgAl2O4. However, microkinetic modeling predicts that a higher activity can be attained on the equally stable Mg-site, maximizing the population of which in catalyst synthesis might prove fruitful in future studies. Electronic structure analysis indicates that the CO ligand increases the reactivity of adsorbed oxygen atoms bound to Ir single atoms by increasing the antibonding nature of the O-Ir bond.
Inverse ZnO/Cu catalysts are key systems in the conversion of CO2, a common atmospheric pollutant, into methanol, a high-value chemical and fuel. The chemistry of methanol and methoxy groups over inverse ZnO/Cu2O/Cu(111) catalysts was investigated employing ambient pressure X-ray photoelectron spectroscopy (AP-XPS), scanning tunneling microscopy (STM), and calculations based on density functional theory (DFT). The results of AP-XPS show that the adsorption of methanol on the binary oxide substrate at 300 K leads to formation of *CH3O and *HCOO species with a minor amount of *CHx. Most of the methoxy groups disappeared from the surface after heating to 450 K, the onset temperature for the formation of methanol during the hydrogenation of CO2. The results of AP-XPS, STM, and DFT point to preferential adsorption of methoxy on the ZnO regions of the binary oxide. On the supported ZnO or on a ZnO-Cu2O interface, the breaking of the O-H bond in methanol is an exothermic process with a negligible (1-2 kcal/mol) or non-existent energy barrier depending on the size and shape of the ZnO islands. STM shows large changes in the morphology of ZnO/Cu2O/Cu(111) surfaces upon reaction with methanol. The produced *CH3O, *HCOO, and *CHx species are localized in groups of active sites that have a dynamic nature and their structure changes during the adsorption/desorption processes.
Ce³⁺ and Eu²⁺ single- and double-doped CaMgSi₂O₆ phosphors have been prepared by a high-temperature solid-state reaction approach. The VUV–UV–vis luminescence properties are investigated at cryogenic temperatures. The dependencies of luminescence intensity and lifetime on temperature are discussed in detail, and the different thermal-quenching characteristics of luminescence of Ce³⁺ and Eu²⁺ in CaMgSi₂O₆ are revealed combined with the VRBE scheme. Because of the different energy barriers of the lowest 5d energy and the conduction band bottom, luminescence thermal quenching of Ce³⁺ does not occur below about 505 K, but that of Eu²⁺ arises at a temperature above ∼300 K. The energy transfer dynamics is then analyzed by using the Inokuti-Hirayama, Yokota-Tanimoto, and Burshtein models, respectively. The Ce³⁺–Eu²⁺ energy transfer is mainly through the electric dipole–dipole interaction with a critical distance of about 21.2 A, and the energy migration between Ce³⁺ ions in a fast or slow way is negligible. The different thermal-quenching behaviors of Ce³⁺ and Eu²⁺ luminescence and their energy transfer pave the way for the potential applications of the codoped samples in optical thermometry and anticounterfeiting.
In the present study, we investigate the surface acid-base properties of anatase TiO2 nanomaterials with dominant {101} and {001} facets via methanol titrations. Two anatase nanoparticles, TiO2(101) and TiO2(001), with well-defined morphology are prepared. TiO2(101) is predominantly enclosed by the {101} facets (>90%), and TiO2(001) contains similar to 46% {001} facets and similar to 54% {101} facets. Upon adsorption of methanol at 423 K, diffuse reflectance infrared Fourier transform spectroscopy measurements show that both molecular and dissociative adsorption occur on TiO2(101), while dissociative adsorption dominates on TiO2(001). During methanol temperature-programmed desorption, TiO2(001) mainly generates the acid-base product dimethyl ether and thermal cracking products CO and H-2, as anticipated. In contrast, substantial amounts of formaldehyde and methane also desorb from TiO2(101), suggesting strong participation of surface defects (e.g., oxygen vacancies).
With considerable attention focusing on metal nanoclusters, to explore the relationship of structures and their properties is of prime importance for designing novel functional nanoclusters and promoting their applications. Here we reported two novel alloy nanoclusters, [Pt1Ag24(SR)(20)](4-) and Pt1Ag14(SR)(6)(PPh3)(8) (H-SR: 2-chloro-4-fluorobenzenethiol), determined by X-ray crystallography. Interestingly, the two alloy nanoclusters have the same centered icosahedral Pt(1)Ag(12 )kernel, but shell structures are totally different due to the addition of PPh3 in the synthesis. Upon removal of the kernel, the shell of the Pt1Ag24 nanocluster is composed of two unique trefoil-like Ag-6(SR)(10 )motifs, while the Pt1Ag14 nanocluster has six PPh3 and two Ag(SR)(3)(PPh3) motifs. Furthermore, fluorescence spectros- copy and ultraviolet-visible absorption spectroscopy (UV-vis) were carried out to investigate the optical properties. The Pt1Ag24 nanocluster has near-infrared photoluminescence at 738 nm, whereas the emission of the Pt1Ag14 nanocluster is at 639 nm, demonstrating a blue shift of 99 nm. The two nanoclusters exhibit distinct optical fingerprints. Furthermore, the predicted partial density of states and UV-vis spectra assignments reveal the relationship of the crystal structures, electronic structures, and optical properties. These findings not only provide a feasible strategy to synthesize functional metal nanoclusters with distinctive structures but also give direct insight into understanding the structure-property correlations at the atomic level.
Delafossites are promising candidates for photocatalysis applications because of their chemical stability and absorption in the solar region of the electromagnetic spectrum. For example, CuAlO2 has good chemical stability but has a large indirect band gap, so that efforts to improve its absorption in the solar region through alloying are investigated. The effect of dilute alloying on the optical absorption of powdered CuAl1-xFexO2 (x = 0.0-1.0) is measured and compared to electronic band structures calculations using a generalized gradient approximation with Hubbard parameter and spin. A new absorption feature is observed at 1.8 eV for x = 0.01, which red-shifts to 1.4 eV for x = 0.10. This feature is associated with transitions from the L-point valence band maximum to the Fe-3d state that appears below the conduction band of the spin-down band structure. The feature increases the optical absorption below the band gap of pure CuAlO2, making dilute CuAl1-xFexO2 alloys better suited for solar photocatalysis.
