Tailoring the aluminum nanocrystal surface oxide for all-aluminum-based antenna-reactor plasmonic photocatalysts.

Aluminum nanocrystals (AlNCs) are of increasing interest as sustainable, earth-abundant nanoparticles for visible wavelength plasmonics and as versatile nanoantennas for energy-efficient plasmonic photocatalysis. Here, we show that annealing AlNCs under various gases and thermal conditions induces substantial, systematic changes in their surface oxide, modifying crystalline phase, surface morphology, density, and defect type and concentration. Tailoring the surface oxide properties enables AlNCs to function as all-aluminum-based antenna-reactor plasmonic photocatalysts, with the modified surface oxides providing varying reactivities and selectivities for several chemical reactions.

Electrocatalytic activity and surface oxide reconstruction of bimetallic iron-cobalt nanocarbide electrocatalysts for the oxygen evolution reaction.

For renewable energy technology to become ubiquitous, it is imperative to develop efficient oxygen evolution reaction (OER) electrocatalysts, which is challenging due to the kinetically and thermodynamically unfavorable OER mechanism. Transition metal carbides (TMCs) have recently been investigated as desirable OER pre-catalysts, but the ability to tune electrocatalytic performance of bimetallic catalysts and understand their transformation under electrochemical oxidation requires further study. In an effort to understand the tunable TMC material properties for enhancing electrocatalytic activity, we synthesized bimetallic FeCo nanocarbides with a complex mixture of FeCo carbide crystal phases. The synthesized FeCo nanocarbides were tuned by percent proportion Fe (i.e. % Fe), and analysis revealed a non-linear dependence of OER electrocatalytic activity on % Fe, with a minimum overpotential of 0.42 V (15-20% Fe) in alkaline conditions. In an effort to understand the effects of Fe composition on electrocatalytic performance of FeCo nanocarbides, we assessed the structural phase and electronic state of the carbides. Although we did not identify a single activity descriptor for tuning activity for FeCo nanocarbides, we found that surface reconstruction of the carbide surface to oxide during water oxidation plays a pivotal role in defining electrocatalytic activity over time. We observed that a rapid increase of the (FexCo1-x)2O4 phase on the carbide surface correlated with lower electrocatalytic activity (i.e. higher overpotential). We have demonstrated that the electrochemical performance of carbides under harsh alkaline conditions has the potential to be fine-tuned via Fe incorporation and with control, or suppression, of the growth of the oxide phase.

Cool carriers: triplet diffusion dominates upconversion yield.

Perovskites have gained popularity both as the active material in photovoltaics and as bulk triplet sensitizers for solid-state triplet-triplet annihilation upconversion (TTA-UC). Prior to widespread implementation into commercial photovoltaics, an in-depth understanding of the environmental influences on device performance is required. To this point, the temperature-dependent structure-function properties of TTA-UC within methylammonium formamidinium lead triiodide (MAFA)/rubrene UC devices are explored. A strong temperature dependence of the underlying UC dynamics is observed, where the maximum UC efficiency is achieved at 170 K, reflecting the competition between triplet diffusion length, diffusion rate, and triplet-triplet encounter events. A combination of spectroscopic and structural methods and theoretical modelling illustrates that despite the significantly increased carrier lifetime of the perovskite at low temperatures, the TTA-UC dynamics are not governed by the underlying sensitizer properties but rather limited by the underlying triplet diffusion.

Influence of Al2O3 Overlayers on Intermolecular Interactions between Metal Oxide Bound Molecules.

Intermolecular interactions on inorganic substrates can have a critical impact on the electrochemical and photophysical properties of the materials and subsequent performance in hybrid electronics. Critical to the intentional formation or inhibition of these processes is controlling interactions between molecules on a surface. In this report, we investigated the impact of surface loading and atomic-layer-deposited Al2O3 overlayers on the intermolecular interactions of a ZrO2-bound anthracene derivative as probed by the photophysical properties of the interface. While surface loading density had no impact on the absorption spectra of the films, there was an increase in excimer features with surface loading as observed by both emission and transient absorption. The addition of ALD overlayers of Al2O3 resulted in a decrease in excimer formation, but the emission and transient absorption spectra were still dominated by excimer features. These results suggest that ALD may provide a post-surface loading means of influencing such intermolecular interactions.

Design and Synthesis of Kekulè and Non-Kekulè Diradicaloids via the Radical Periannulation Strategy: The Power of Seven Clar's Sextets.

This work introduces an approach to uncoupling electrons via maximum utilization of localized aromatic units, i.e., the Clar's π-sextets. To illustrate the utility of this concept to the design of Kekulé diradicaloids, we have synthesized a tridecacyclic polyaromatic system where a gain of five Clar's sextets in the open-shell form overcomes electron pairing and leads to the emergence of a high degree of diradical character. According to unrestricted symmetry-broken UCAM-B3LYP calculations, the singlet diradical character in this core system is characterized by the y0 value of 0.98 (y0 = 0 for a closed-shell molecule, y0 = 1 for pure diradical). The efficiency of the new design strategy was evaluated by comparing the Kekulé system with an isomeric non-Kekulé diradical of identical size, i.e., a system where the radical centers cannot couple via resonance. The calculated singlet-triplet gap, i.e., the ΔEST values, in both of these systems approaches zero: -0.3 kcal/mol for the Kekulé and +0.2 kcal/mol for the non-Kekulé diradicaloids. The target isomeric Kekulé and non-Kekulé systems were assembled using a sequence of radical periannulations, cross-coupling, and C-H activation. The diradicals are kinetically stabilized by six tert-butyl substituents and (triisopropylsilyl)acetylene groups. Both molecules are NMR-inactive but electron paramagnetic resonance (EPR)-active at room temperature. Cyclic voltammetry revealed quasi-reversible oxidation and reduction processes, consistent with the presence of two nearly degenerate partially occupied molecular orbitals. The experimentally measured ΔEST value of -0.14 kcal/mol confirms that K is, indeed, a nearly perfect singlet diradical.

A Single Source, Scalable Route for Direct Isolation of Earth-Abundant Nanometal Carbide Water-Splitting Electrocatalysts.

Single-phase MxCs (M = Fe, Co, and Ni) were prepared by solvothermal conversion of Prussian blue single source precursors. The single source precursor is prepared in water, and the conversion process is carried out in alkylamines at reaction temperatures above 200 °C. The reaction is scalable using a commercial source of Fe-PB. High-resolution transmission electron microscopy, X-ray photoelectron microscopy, and powder X-ray diffraction confirm that carbides have thin oxide termination but lack graphitic surfaces. Electrocatalytic activity reveals that Fe3C and Co2C are oxygen evolution reaction electrocatalysts, while Ni3C is a bifunctional [OER and hydrogen evolution reaction (HER)] electrocatalyst.

Effective Mass for Holes in Paramagnetic, Plasmonic Cu5FeS4 Semiconductor Nanocrystals.

The impact of a magneto-structural phase transition on the carrier effective mass in Cu5FeS4 plasmonic semiconductor nanocrystals was examined using Magnetic Circular Dichroism (MCD). Through MCD, the sample was confirmed as p-type from variable temperature studies from 1.8 - 75 K. Magnetic field dependent behavior is observed, showing an asymptotic behavior at high field with an m∗ value 5.98 m∗∕me at 10 T and 2.73 m∗∕me at 2 T. Experimentally obtained results are holistically compared to SQUID magnetization data and DFT results, highlighting a dependency on vacancy driven polaronic coupling, magnetocrystalline anisotropy, and plasmon coupling of the magnetic field all contributing to an overall decrease in the hole mean free path dependent on the magnetic field applied to Cu5FeS4.

Direct Measurement of the Selective Microwave-Induced Heating of Agglomerates of Dipolar Molecules: The Origin of and Parameters Controlling a Microwave Specific Superheating Effect.

Agglomerates of polar molecules in nonpolar solvents are selectively heated by microwave radiation. The magnitude of the selective heating was directly measured by using the temperature dependence of the intensities of the Stokes and anti-Stokes bands in the Raman spectra of p-nitroanisole (pNA) and mesitylene. Under dynamic heating conditions, a large apparent temperature difference (ΔT) of over 100 °C was observed between the polar pNA solute and the nonpolar mesitylene solvent. This represents the first direct measurement of the selective microwave heating process. The magnitude of the selective microwave heating was affected by the properties of the agglomerated pNA. As the concentration of the pNA increases, the magnitude of the selective heating of the pNA was observed to decrease. This is explained by the tendency of the pNA dipoles to orient in an antiparallel fashion in the aggregates as measured by the Kirkwood g value, which decreased with increasing concentration. This effect reduces the net dipole moment of the agglomerates, which decreases the microwave absorption. After the radiation was terminated, the effective temperature of the dipolar molecules returned slowly to that of the medium. The slow heat transfer was modeled successfully by treating the solutions as a biphasic solvent/solute system. Based on modeling and the fact that the agglomerate can be heated above the boiling temperature of the solvent, an insulating layer of solvent vapor is suggested to form around the heated agglomerate, slowing convective heat transfer out of the agglomerate. This is an effect unique to microwave heating.

Controlled Synthesis and Exploration of CuxFeS4 Bornite Nanocrystals.

Plasmonic semiconductor nanocrystals (NCs) are a new and exciting class of materials that enable higher control of their localized surface plasmon resonance (LSPR) than metallic counterparts. Additionally, earth-abundant and non-toxic materials such as copper iron sulfides are gaining interest as alternatives to heavy metal-based semiconductor materials. Colloidal bornite (Cu5FeS4) is an interesting but underexplored example of a heavy metal-free plasmonic semiconductor. This report details the hot-injection synthesis of bornite yielding NCs ranging from 2.7 to 6.1 nm in diameter with stoichiometric control of the copper and iron content. The absorbance spectra of bornite NCs with different Cu:Fe ratios change at different rates as the particles oxidize and develop LSPR in the near-infrared region. X-ray photoelectron spectroscopy results indicate that oxidation produces sulfates rather than metal oxides as well as a decrease in the iron content within the NCs. Additionally, increasing iron content leads to decreases in carrier density and effective mass of the carrier, as determined by the Drude model. This controlled synthesis, combined with a further understanding of the relationship between the particle structure and optical properties, will enable the continued development and application of these fascinating heavy metal-free plasmonic semiconductor nanoparticles.

Microwave Synthesis and Magnetocaloric Effect in AlFe2B2.

A promising magnetic refrigerant, AlFe2B2, has been prepared for the first time by microwave (MW) melting of a mixture of constituent elements. For comparison, samples of AlFe2B2 have been also prepared by arc-melting, traditionally used for the synthesis of this material, and by induction (RF) melting, which was used in the very first report on the synthesis of AlFe2B2. Although an excess of Al has to be used to suppress the formation of ferromagnetic FeB, the other byproduct, Al13Fe4, is easily removed by acid treatment, affording phase-pure samples of AlFe2B2. Our analysis indicates that the equimolar Fe/B ratio typically used for the preparation of AlFe2B2 might not provide the best synthetic conditions, as it does not account for the full reaction stoichiometry. Furthermore, we find that the initial Al/Fe loading ratio strongly influences magnetic properties of the sample, as judged by the range of ferromagnetic ordering temperatures (TC = 280-293 K) observed in our experiments. The TC value increases with the decrease in the Al/Fe ratio, due to the change in the Al/Fe antisite disorder. The use of the same Al/Fe loading ratio in the arc-, RF-, and MW-melting experiments leads to samples with a more consistent TC of 286-287 K and similar values of the magnetocaloric effect.

Structure-Function Correlation: Engineering High Quantum Yields in Down-Shifting Nanophosphors.

Lanthanides are routinely incorporated into quantum dots to act as down-shifting and up-converting phosphors in display and lighting applications due to their high photoluminescence quantum yields (PLQY). Recent efforts in the field have demonstrated that trivalent lanthanide, Ln(III), incorporated into ZnAl2O4 spinel nanocrystals can achieve PLQYs of 50% for down-shifting nanophosphors using earth abundant materials. The high PLQY is surprising as the Al(III) site in a spinel is centrosymmetric, which should lead to poor performance for these nanophosphors. However, spinels are prone to formation of an admixture of inverse and normal spinel lattices when the cation size ratio is not optimal. Such behavior can produce local cation disorder that can influence the phosphor performance. Herein, we describe the use of Tb(III) as an optical probe to evaluate the fractional population of the inverse and normal spinel structures within TbxZnAl2-xO4. The experimental data exhibits a Tb(III) concentration dependent change in the fractional population that results in a maximum PLQY of 37% with 3.56% Tb(III) incorporation. A decrease in the degree of inversion (cation disorder) leads to larger amounts of the cubic Fd3m phase resulting in the observed photoluminescence behavior. The correlation of NMR, pXRD, and optical methods provides direct insight into the high PLQY behavior for this class of nanophosphor.

Intracellular DNA Cargo Release from a Gold Nanoparticle Modulated by the Nature of the Surface Coupling Functionality.

Covalently coupling nucleic acids to a gold nanoparticle (AuNP) surface has been demonstrated for generating effective gene therapy agents to modify cellular protein expression. The therapeutic efficacy of the approach is anticipated to be impacted by the length of time the nucleic acid sequence resides in the endolysosomal pathway once transfected into a cell. It is believed that the dynamics of the processing should reflect the linkage chemistry of the DNA to the AuNP surface. In this manuscript the dynamics of nanotherapeutic uptake, nucleic acid release, and gene processing are investigated  in vitro for a AuNP-nucleic acid delivery platform transfected into A375 human melanoma cells, as a function of the nucleic acid-gold linkage chemistry. The dynamics of cell processing of the single monodentate thiol (SX), bidentate dual thiol (SS), or mixed bidentate thiol plus amine (SN) coordination of nucleic acids to the AuNP surface are evaluated using a multicolor nanosurface energy transfer bio-optical transponder (SET-BOT) technology. The use of live-cell fluorescence microscopy allows for the direct visualization of the uptake and localization of a lipofectamine-packaged SET-BOT using a dye (DL700) that is not quenched in the proximity of the AuNP, while fluorescence "turn on" of a dye that is proximally quenched by the AuNP (DL488) is used to report on the dynamics of release of the nucleic acid cargo within the cell. For protein expression following transcription of the gene, the emission signature of a red fluorescent protein, tdTomato, is monitored. The intracellular rates of DNA release from the AuNP surface once endosomally packaged within the A375 human melanoma cells were found to follow the binding activity series: bidentate thiol > bidentate thiol plus amine > monodentate thiol, consistent with the strength of multidentate chelation, paired with the stabilizing influence of π-backbonding of thiols compared to σ-donation in amines, when bound to a gold surface.

Selective Uptake Into Drug Resistant Mammalian Cancer by Cell Penetrating Peptide-Mediated Delivery.

Research over the past decade has identified several of the key limiting features of multidrug resistance (MDR) in cancer therapy applications, such as evolving glycoprotein receptors at the surface of the cell that limit therapeutic uptake, metabolic changes that lead to protection from multidrug resistant mediators which enhance degradation or efflux of therapeutics, and difficulty ensuring retention of intact and functional drugs once endocytosed. Nanoparticles have been demonstrated to be effective delivery vehicles for a plethora of therapeutic agents, and in the case of nucleic acid based agents, they provide protective advantages. Functionalizing cell penetrating peptides, also known as protein transduction domains, onto the surface of fluorescent quantum dots creates a labeled delivery package to investigate the nuances and difficulties of drug transport in MDR cancer cells for potential future clinical applications of diverse nanoparticle-based therapeutic delivery strategies. In this study, eight distinct cell penetrating peptides were used (CAAKA, HSV1-VP22, HIV-TAT, HIV-gp41, Ku-70, hCT(9-32), integrin-ß3, and K-FGF) to examine the different cellular uptake profiles in cancer versus drug resistant melanoma (A375 & A375-R), mesothelioma (MSTO & MSTO-R), and glioma (rat 9L and 9L-R, and human U87 & LN18) cell lines. The results of this study demonstrate that cell penetrating peptide uptake varies with drug resistance status and cell type, likely due to changes in cell surface markers. This study provides insight into developing functional nanoplatform delivery systems in drug resistant cancer models.

A Gold Nanoparticle Bio-Optical Transponder to Dynamically Monitor Intracellular pH.

A pH-sensitive bio-optical transponder (pH-BOT) capable of simultaneously reporting the timing of intracellular DNA cargo release from a gold nanoparticle (AuNP) and the evolving intracellular pH (pH i) during endosomal maturation is demonstrated. The pH-BOT is designed with a triple-dye-labeled duplex DNA appended to a 6.6 nm AuNP, utilizing pH-responsive fluorescein paired with DyLight405 as a surface energy transfer (SET) coupled dye pair to ratiometrically report the pH at and after cargo release. A non-SET-coupled dye, DyLight 700, is used to provide dynamic tracking throughout the experiment. The pH-BOT beacon of the cargo uptake, release, and processing was visualized using live-cell confocal fluorescent microscopy in Chinese hamster ovary cells, and it was observed that while maturation of endosomes carrying pH-BOT is slowed significantly, the pH-BOT is distributed throughout the endolysosomal system while remaining at pH ∼6. This observed decoupling of endosomal maturation from acidification lends support to those models that propose that pH alone is not sufficient to explain endosomal maturation and may enable greater insight into our understanding of the fundamental processes of biology.

Synthesis of Highly Uniform Nickel Multipods with Tunable Aspect Ratio by Microwave Power Control.

As the importance of anisotropic nanostructures and the role of surfaces continues to rise in applications including catalysis, magneto-optics, and electromagnetic interference shielding, there is a need for efficient and economical synthesis routes for such nanostructures. The article describes the application of cycled microwave power for the rapid synthesis of highly branched pure-phase face-centered cubic crystalline nickel multipod nanostructures with >99% multipod population. By controlling the power delivery to the reaction mixture through cycling, superior control is achieved over the growth kinetics of the metallic nanostructures, allowing formation of multipods consisting of arms with different aspect ratios. The multipod structures are formed under ambient conditions in a simple reaction system composed of nickel acetylacetonate (Ni(acac)2), oleylamine (OAm), and oleic acid (OAc) in a matter of minutes by selective heating at the (111) overgrowth corners on Ni nanoseeds. The selective heating at the corners leads to accelerated autocatalytic growth along the ⟨111⟩ direction through a "lightning rod" effect. The length is proprtional to the length and number of microwave (MW)-on cycles, whereas the core size is controlled by continuous MW power delivery. The roles of heating mode (cycling versus variable power versus convective heating) during synthesis of the materials is explored, allowing a mechanism into how cycled microwave energy may allow fast multipod evolution to be proposed.

Dielectric Properties for Nanocomposites Comparing Commercial and Synthetic Ni- and Fe3O4-Loaded Polystyrene.

Nanomaterial-loaded thermoplastics are attractive for applications in adaptive printing methods, as the physical properties of the printed materials are dependent on the nanomaterial type and degree of dispersion. This study compares the dispersion and the impact on the dielectric properties of two common nanoparticles, nickel and iron oxide, loaded into polystyrene. Comparisons between commercial and synthetically prepared samples indicate that well-passivated synthetically prepared nanomaterials are dispersed and minimize the impact on the dielectric properties of the host polymer by limiting particle-particle contacts. Commercial samples were observed to phase-segregate, leading to the loss of the low-k performance of polystyrene. The change in the real and imaginary dielectric was systematically studied in two earth abundant nanoparticles at the concentration between 0 and 13 vol % (0-50 wt %). By varying the volume percentage of fillers in the matrix, it is shown that one can increase the magnetic properties of the materials while minimizing unwanted contributions to the dielectric constant and dielectric loss. The well-dispersed nanoparticle systems were successfully modeled through the Looyenga dielectric theory, thus giving one a predictive ability for the dielectric properties. The current experimental work coupled with modeling could facilitate future material choices and guide design rules for printable polymer composite systems.

Microwave Enhancement of Autocatalytic Growth of Nanometals.

The desire for designing efficient synthetic methods that lead to industrially important nanomaterials has led a desire to more fully understand the mechanism of growth and how modern synthetic techniques can be employed. Microwave (MW) synthesis is one such technique that has attracted attention as a green, sustainable method. The reports of enhancement of formation rates and improved quality for MW driven reactions are intriguing, but the lack of understanding of the reaction mechanism and how coupling to the MW field leads to these observations is concerning. In this manuscript, the growth of a metal nanoparticles (NPs) in a microwave cavity is spectroscopically analyzed and compared with the classical autocatalytic method of NP growth to elucidate the underpinnings for the observed enhanced growth behavior for metal NPs prepared in a MW field. The study illustrates that microwave synthesis of nickel and gold NPs below saturation conditions follows the Finke-Watzky mechanism of nucleation and growth. The enhancement of the reaction arises from the size-dependent increase in MW absorption cross section for the metal NPs. For Ni, the presence of oxides is considered via theoretical computations and compared to dielectric measurements of isolated nickel NPs. The study definitively shows that MW growth can be modeled by an autocatalytic mechanism that directly leads to the observed enhanced rates and improved quality widely reported in the nanomaterial community when MW irradiation is employed.

Triangulating Nucleic Acid Conformations Using Multicolor Surface Energy Transfer.

Optical ruler methods employing multiple fluorescent labels offer great potential for correlating distances among several sites, but are generally limited to interlabel distances under 10 nm and suffer from complications due to spectral overlap. Here we demonstrate a multicolor surface energy transfer (McSET) technique able to triangulate multiple points on a biopolymer, allowing for analysis of global structure in complex biomolecules. McSET couples the competitive energy transfer pathways of Förster Resonance Energy Transfer (FRET) with gold-nanoparticle mediated Surface Energy Transfer (SET) in order to correlate systematically labeled points on the structure at distances greater than 10 nm and with reduced spectral overlap. To demonstrate the McSET method, the structures of a linear B-DNA and a more complex folded RNA ribozyme were analyzed within the McSET mathematical framework. The improved multicolor optical ruler method takes advantage of the broad spectral range and distances achievable when using a gold nanoparticle as the lowest energy acceptor. The ability to report distance information simultaneously across multiple length scales, short-range (10-50 Å), mid-range (50-150 Å), and long-range (150-350 Å), distinguishes this approach from other multicolor energy transfer methods.

Specific effects in microwave chemistry explored through reactor vessel design, theory, and spectroscopy.

Microwave chemistry has revolutionized synthetic methodology for the preparation of organics, pharmaceuticals, materials, and peptides. The enhanced reaction rates commonly observed in a microwave have led to wide speculation about the function of molecular microwave absorption and whether the absorption leads to microwave specific effects and enhanced molecular heating. The comparison of theoretical modeling, reactor vessel design, and dielectric spectroscopy allows the nuance of the interaction to be directly understood. The study clearly shows an unaltered silicon carbide vessel allows measurable microwave penetration and therefore, molecular absorption of the microwave photons by the reactants within the reaction vessel cannot be ignored when discussing the role of molecular heating in enhanced molecular reactivity for microwave synthesis. The results of the study yield an improved microwave reactor vessel design that eliminates microwave leakage into the reaction volume by incorporating a noble metal surface layer onto a silicon carbide reaction vessel. The systematic study provides the necessary theory and measurements to better inform the arguments in the field.

Cryopreservation of embryonic stem cell-derived multicellular neural aggregates labeled with micron-sized particles of iron oxide for magnetic resonance imaging.

Magnetic resonance imaging (MRI) provides an effective approach to track labeled pluripotent stem cell (PSC)-derived neural progenitor cells (NPCs) for neurological disorder treatments after cell labeling with a contrast agent, such as an iron oxide derivative. Cryopreservation of pre-labeled neural cells, especially in three-dimensional (3D) structure, can provide a uniform cell population and preserve the stem cell niche for the subsequent applications. In this study, the effects of cryopreservation on PSC-derived multicellular NPC aggregates labeled with micron-sized particles of iron oxide (MPIO) were investigated. These NPC aggregates were labeled prior to cryopreservation because labeling thawed cells can be limited by inefficient intracellular uptake, variations in labeling efficiency, and increased culture time before use, minimizing their translation to clinical settings. The results indicated that intracellular MPIO incorporation was retained after cryopreservation (70-80% labeling efficiency), and MPIO labeling had little adverse effects on cell recovery, proliferation, cytotoxicity and neural lineage commitment post-cryopreservation. MRI analysis showed comparable detectability for the MPIO-labeled cells before and after cryopreservation indicated by T2 and T2* relaxation rates. Cryopreserving MPIO-labeled 3D multicellular NPC aggregates can be applied in in vivo cell tracking studies and lead to more rapid translation from preservation to clinical implementation.