Electrolysis Can Be Used to Resolve Hydrogenation Pathways at Palladium Surfaces in a Membrane Reactor.

For common hydrogenation chemistries that occur at high temperatures (where H2 is adsorbed and activated at the same surface which the substrate must also adsorb for reaction), there is often little consensus on how the reactions (e.g., hydro(deoxy)genation) actually occur. We demonstrate here that an electrocatalytic palladium membrane reactor (ePMR) can be used to study hydrogenation reaction mechanisms at ambient temperatures, where the catalyst does not necessarily undergo structural reorganization. The ePMR uses electrolysis and a hydrogen-selective palladium membrane to deliver reactive hydrogen to a catalyst surface in an adjacent compartment for reaction with an organic substrate. This process forms the requisite metal-hydride surface for hydrogenation chemistry, but at ambient temperature and pressure, and without a H2 source. We demonstrate the utility of this analytical tool by studying the hydrogenation of benzaldehyde at palladium nanocubes with dimensions of 13-24 nm. This experimental design enabled us to resolve that the alcohol product forms at the facial sites, whereas the hydrodeoxygenation step occurs at edge sites. These observations enabled us to develop the first site-specific definition of how a carbonyl species undergoes hydro(deoxy)genation.

Facets and vertices regulate hydrogen uptake and release in palladium nanocrystals.

Crystal facets, vertices and edges govern the energy landscape of metal surfaces and thus the chemical interactions on the surface1,2. The facile absorption and desorption of hydrogen at a palladium surface provides a useful platform for defining how metal-solute interactions impact properties relevant to energy storage, catalysis and sensing3-5. Recent advances in in operando and in situ techniques have enabled the phase transitions of single palladium nanocrystals to be temporally and spatially tracked during hydrogen absorption6-11. We demonstrate herein that in situ X-ray diffraction can be used to track both hydrogen absorption and desorption in palladium nanocrystals. This ensemble measurement enabled us to delineate distinctive absorption and desorption mechanisms for nanocrystals containing exclusively (111) or (100) facets. We show that the rate of hydrogen absorption is higher for those nanocrystals containing a higher number of vertices, consistent with hydrogen absorption occurring quickly after ß-phase nucleation at lattice-strained vertices9,10. Tracking hydrogen desorption revealed initial desorption rates to be nearly tenfold faster for samples with (100) facets, presumably due to the faster recombination of surface hydrogen atoms. These results inspired us to make nanocrystals with a high number of vertices and (100) facets, which were found to accommodate fast hydrogen uptake and release.

Stabilizing Copper for CO2 Reduction in Low-Grade Electrolyte.

We demonstrate herein a CO2 reduction electrocatalyst regeneration strategy based on the manipulation of the Cu(0)/Cu2+ equilibrium with high concentrations of ethylenediaminetetraacetic acid (EDTA). This strategy enables the sustained performance of copper catalysts in distilled and tap water electrolytes for over 12 h. The deposition of common electrolyte impurities such as iron, nickel, and zinc is blocked because EDTA can effectively bind the metal ions and negatively shift the electrode potential of M/M n+. The Cu/Cu2+ redox couple is >600 mV more positive than the other metal ions and therefore participates in an equilibrium of dissolution and redeposition from and to the electrode in high concentrations of EDTA. These dynamic equilibria serve to further regenerate the surface copper catalyst to prevent the deactivation of catalytic sites. On the basis of this strategy, we show that >95% of initial hydrocarbon production activity can be maintained for 12 h in KHCO3 (99% purity) enriched distilled water and 6 h in KHCO3 (99% purity) enriched tap water.

Electrocatalytic Alloys for CO2 Reduction.

Electrochemically reducing CO2 using renewable energy is a contemporary global challenge that will only be met with electrocatalysts capable of efficiently converting CO2 into fuels and chemicals with high selectivity. Although many different metals and morphologies have been tested for CO2 electrocatalysis over the last several decades, relatively limited attention has been committed to the study of alloys for this application. Alloying is a promising method to tailor the geometric and electric environments of active sites. The parameter space for discovering new alloys for CO2 electrocatalysis is particularly large because of the myriad products that can be formed during CO2 reduction. In this Minireview, mixed-metal electrocatalyst compositions that have been evaluated for CO2 reduction are summarized. A distillation of the structure-property relationships gleaned from this survey are intended to help in the construction of guidelines for discovering new classes of alloys for the CO2 reduction reaction.

Simultaneous Enhancement of Photoluminescence, MRI Relaxivity, and CT Contrast by Tuning the Interfacial Layer of Lanthanide Heteroepitaxial Nanoparticles.

Nanoparticle (NP) based exogenous contrast agents assist biomedical imaging by enhancing the target visibility against the background. However, it is challenging to design a single type of contrast agents that are simultaneously suitable for various imaging modalities. The simple integration of different components into a single NP contrast agent does not guarantee the optimized properties of each individual components. Herein, we describe lanthanide-based core-shell-shell (CSS) NPs as triple-modal contrast agents that have concurrently enhanced performance compared to their individual components in photoluminescence (PL) imaging, magnetic resonance imaging (MRI), and computed tomography (CT). The key to simultaneous enhancement of PL intensity, MRI r1 relaxivity, and X-ray attenuation capability in CT is tuning the interfacial layer in the CSS NP architecture. By increasing the thickness of the interfacial layer, we show that (i) PL intensity is enhanced from completely quenched/dark state to brightly emissive state of both upconversion and downshifting luminescence at different excitation wavelengths (980 and 808 nm), (ii) MRI r1 relaxivity is enhanced by 5-fold from 11.4 to 52.9 mM-1 s-1 (per Gd3+) at clinically relevant field strength 1.5 T, and (iii) the CT Hounsfield Unit gain is 70% higher than the conventional iodine-based agents at the same mass concentration. Our results demonstrate that judiciously designed contrast agents for multimodal imaging can achieve simultaneously enhanced performance compared to their individual stand-alone structures and highlight that multimodality can be achieved without compromising on individual modality performance.

Direct Evidence for Coupled Surface and Concentration Quenching Dynamics in Lanthanide-Doped Nanocrystals.

Luminescence quenching at high dopant concentrations generally limits the dopant concentration to less than 1-5 mol% in lanthanide-doped materials, and this remains a major obstacle in designing materials with enhanced efficiency/brightness. In this work, we provide direct evidence that the major quenching process at high dopant concentrations is the energy migration to the surface (i.e., surface quenching) as opposed to the common misconception of cross-relaxation between dopant ions. We show that after an inert epitaxial shell growth, erbium (Er3+) concentrations as high as 100 mol% in NaY(Er)F4/NaLuF4 core/shell nanocrystals enhance the emission intensity of both upconversion and downshifted luminescence across different excitation wavelengths (980, 800, and 658 nm), with negligible concentration quenching effects. Our results highlight the strong coupling of concentration and surface quenching effects in colloidal lanthanide-doped nanocrystals, and that inert epitaxial shell growth can overcome concentration quenching. These fundamental insights into the photophysical processes in heavily doped nanocrystals will give rise to enhanced properties not previously thought possible with compositions optimized in bulk.

Compact Micellization: A Strategy for Ultrahigh T1 Magnetic Resonance Contrast with Gadolinium-Based Nanocrystals.

Paramagnetic gadolinium (Gd(3+))-based nanocrystals (NCs) with a large number of confined gadolinium ions can be expected to heavily enhance the longitudinal (T1) relaxation of water protons compared to clinical gadolinium complexes with only a single paramagnetic center. However, paramagnetic Gd(3+)-NCs reported to date show only a modest T1 relaxivity of ∼10 mM(-1) s(-1) per Gd(3+) at 1.5 T, only about 3-times higher than clinical Gd(3+) complexes. Here we demonstrate a strategy that achieves ultrahigh T1 relaxivity that is about 25-times higher than clinical Gd(3+) complexes by controlling the proximity of water protons to a paramagnetic NC surface. Using NaGdF4 NCs (∼3 nm) coated with PEG-ylated phospholipid (DSPE-PEG) micelles, we show that the distance of water protons to the NCs surface can be tuned by controlling the NC-micelle sizes. Increasing the ratio of DSPE-PEG to NCs during micellization decreases the size of NC-micelles, enhancing the proximity of water to the NC surface. Using this strategy, we have achieved compact NC-micelles (hydrodynamic diameter, HD ∼ 5 nm) with ultrahigh T1 relaxivity of ∼80 mM(-1) s(-1) per Gd(3+) at 1.41 T. The findings reported here demonstrate a nanostructured Gd(3+)-contrast agent (CA) that simultaneously achieves an ultrahigh T1 relaxivity approaching theoretical predictions, extremely compact size (HD < 5 nm), and a biocompatible surface. Our results show the hitherto unknown ultrahigh T1 relaxation enhancement of water protons in close proximity to a colloidal gadolinium-NC surface that is achievable by precise control of their surface structure.

Enhanced UV upconversion emission using plasmonic nanocavities.

Upconversion of near infrared (NIR) into ultraviolet (UV) radiation could lead to a number of applications in bio-imaging, diagnostics and drug delivery. However, for bare nanoparticles, the conversion efficiency is extremely low. In this work, we experimentally demonstrate strongly enhanced upconversion emission from an ensemble of ß-NaYF4:Gd3+/Yb3+/Tm3+ @NaLuF4 core-shell nanoparticles trapped in judiciously designed plasmonic nanocavities. In doing so, different metal platforms and nanostructures are systematically investigated. Our results indicate that using a cross-shape silver nanocavity, a record high enhancement of 170-fold can be obtained in the UV band centered at a wavelength of 345 nm. The observed upconversion efficiency improvement may be attributed to the increased absorption at NIR, the tailored photonic local density of states, and the light out-coupling characteristics of the cavity.

Lanthanide-based heteroepitaxial core-shell nanostructures: compressive versus tensile strain asymmetry.

Heteroepitaxial core-shell nanostructures have been proven advantageous in a wide variety of applications, ranging from luminescence enhancement, band gap engineering, multimodal theranostics, to catalysis. However, precisely tailoring the epitaxial growth is challenging, and a general understanding of the parameters that impact epitaxial growth remains unclear. Here we demonstrate the critical role of the sign of the lattice mismatch of the shell relative to the core (compressed/tensile) in generating lanthanide-based core-shell structures, a parameter conventionally not considered in heteroepitaxial design. We took advantage of the very gradual contraction of lanthanide ions along the series to control precisely both the magnitude and the sign of lattice mismatch and investigated multiple sodium lanthanide fluoride (NaLnF4) core-shell heterostructures of variable composition and size. We discovered that the tensile strained shells adapt to the core crystallite shape (i.e., conformal) and lattice structure (i.e., coherent), while under identical magnitude of mismatch, the compressively strained shells are neither conformal nor coherent to the core. This striking asymmetry between the tensile and compressively strained epitaxial growth arises from the fundamental anharmonicity of the interatomic interactions between the attractive and repulsive pairs. From a broader perspective, our findings redefine the a priori design consideration and provide a fundamental insight on the necessity to include the sign of lattice mismatch and not just its magnitude in designing heteroepitaxial core-shell nanostructures.

Engineering upconversion emission spectra using plasmonic nanocavities.

We show that the upconversion emission spectra of Tm³âº and Yb³âº codoped ß-NaYF4-NaYF4 core-shell nanoparticles can be judiciously modified by means of plasmonic nanocavities. Our analysis indicates that more than a 30-fold increase in conversion efficiency to the UV spectral band can be expected by engineering the NIR absorption and the local density of states. The effect of the nanocavity on the resulting radiation patterns is discussed. Our results are exemplified in cylindrical cavity geometries.

Ln(3+)-doped nanoparticles for upconversion and magnetic resonance imaging: some critical notes on recent progress and some aspects to be considered.

In this feature article we will critically discuss the synthesis and characterisation aspects of Ln(3+)-doped nanoparticles (NPs) that show upconversion, upon 980 nm excitation. Upconversion is a non-linear process that converts two or more low-energy photons, often near-infrared photons, into one of higher energy, e.g. blue and 800 nm from Tm(3+) and green and red from Er(3+) or Ho(3+). Nearly all researchers use the absorption of 980 nm light by Yb(3+) as the sensitiser for the co-doped emissive Ln(3+) ions. The focus will be on LnF(3) and MLnF(4) (M = alkali metal) as the host matrix, because most progress has been made with these. In particular we will argue that a detailed understanding of how the dopant ions and the host Ln(3+) ions are distributed (in the core) and how (doped) shell growth occurs is not well understood. Moreover, their use as optical and magnetic resonance imaging contrast agents will be discussed. We will argue that deep-tissue imaging beyond 600 µm with retention of optical resolution, i.e. to see fine structure such as blood capillaries in brain tissues, has not yet been achieved. Three key parameters have been identified as impediments: (i) the low absorption efficiency of the Yb(3+) sensitiser, (ii) the low quantum yield of upconversion, and (iii) the long-lived excited states. On the other hand, there are very encouraging results that suggest that these nanoparticles could be developed into very potent magnetic resonance imaging (MRI) contrast agents.

Self-focusing by Ostwald ripening: a strategy for layer-by-layer epitaxial growth on upconverting nanocrystals.

We demonstrate a novel epitaxial layer-by-layer growth on upconverting NaYF(4) nanocrystals (NCs) utilizing Ostwald ripening dynamics tunable both in thickness and composition. Injection of small sacrificial NCs (SNCs) as shell precursors into larger core NCs results in the rapid dissolution of the SNCs and their deposition onto the larger core NCs to yield core-shell structured NCs. Exploiting this NC size dependent dissolution/growth, the shell thickness can be controlled either by manipulating the number of SNCs injected or by successive injection of SNCs. In either of these approaches, the NCs self-focus from an initial bimodal distribution to a unimodal distribution (σ <5%) of core-shell NCs. The successive injection approach facilitates layer-by-layer epitaxial growth without the need for tedious multiple reactions for generating tunable shell thickness, and does not require any control over the injection rate of the SNCs, as is the case for shell growth by precursor injection.

An effective polymer cross-linking strategy to obtain stable dispersions of upconverting NaYF4 nanoparticles in buffers and biological growth media for biolabeling applications.

Ligands on the nanoparticle surface provide steric stabilization, resulting in good dispersion stability. However, because of their highly dynamic nature, they can be replaced irreversibly in buffers and biological medium, leading to poor colloidal stability. To overcome this, we report a simple and effective cross-linking methodology to transfer oleate-stabilized upconverting NaYF(4) core/shell nanoparticles (UCNPs) from hydrophobic to aqueous phase, with long-term dispersion stability in buffers and biological medium. Amphiphilic poly(maleic anhydride-alt-1-octadecene) (PMAO) modified with and without poly(ethylene glycol) (PEG) was used to intercalate with the surface oleates, enabling the transfer of the UCNPs to water. The PMAO units on the phase transferred UCNPs were then successfully cross-linked using bis(hexamethylene)triamine (BHMT). The primary advantage of cross-linking of PMAO by BHMT is that it improves the stability of the UCNPs in water, physiological saline buffers, and biological growth media and in a wide range of pH values when compared to un-cross-linked PMAO. The cross-linked PMAO-BHMT coated UCNPs were found to be stable in water for more than 2 months and in physiological saline buffers for weeks, substantiating the effectiveness of cross-linking in providing high dispersion stability. The PMAO-BHMT cross-linked UCNPs were extensively characterized using various techniques providing supporting evidence for the cross-linking process. These UCNPs were found to be stable in serum supplemented growth medium (37 °C) for more than 2 days. Utilizing this, we demonstrate the uptake of cross-linked UCNPs by LNCaP cells (human prostate cancer cell line), showing their utility as biolabels.

NaDyF4 Nanoparticles as T2 Contrast Agents for Ultrahigh Field Magnetic Resonance Imaging.

A major limitation of the commonly used clinical MRI contrast agents (CAs) suitable at lower magnetic field strengths (<3.0 T) is their inefficiency at higher fields (>7 T), where next-generation MRI scanners are going. We present dysprosium nanoparticles (ß-NaDyF4 NPs) as T2 CAs suitable at ultrahigh fields (9.4 T). These NPs effectively enhance T2 contrast at 9.4 T, which is 10-fold higher than the clinically used T2 CA (Resovist). Evaluation of the relaxivities at 3 and 9.4 T show that the T2 contrast enhances with an increase in NP size and field strength. Specifically, the transverse relaxivity (r2) values at 9.4 T were ∼64 times higher per NP (20.3 nm) and ∼6 times higher per Dy(3+) ion compared to that at 3 T, which is attributed to the Curie spin relaxation mechanism. These results and confirming phantom MR images demonstrate their effectiveness as T2 CAs in ultrahigh field MRIs.

Facile ligand-exchange with polyvinylpyrrolidone and subsequent silica coating of hydrophobic upconverting beta-NaYF(4):Yb(3+)/Er(3+) nanoparticles.

A facile ligand-exchange strategy with a water-soluble polymer, i.e. polyvinylpyrrolidone (PVP), to replace the surface passivating oleate ligands on the beta-NaYF(4) nanoparticle surface is reported. Highly monodisperse oleate-stabilized beta-NaYF(4) nanoparticles were synthesized and the oleates were exchanged with a commercially available PVP allowing the phase transfer of these nanoparticles. The exchanged nanoparticles are readily dispersible in water and other polar solvents. To show the effectiveness of the exchange reaction we used the affinity of the PVP chains to silica and coated the nanoparticles with a uniform, thin silica shell. The PVP exchanged and silica-coated nanoparticles show longer colloidal stability and no surfactant related problems as compared to the reverse microemulsion-based silica-coated nanoparticles, which show a high tendency to aggregate, when removed from the microemulsion. The optical properties of the ligand-exchanged nanoparticles dispersed in water were compared with that of the oleate-stabilized nanoparticles in organic solvents. A decrease in the upconversion emission intensity and a different relative ratio of the green and red upconverted light were observed for the particles dispersed in water after ligand-exchange. PVP is a highly biocompatible polymer and is reported to have a longer blood circulation time and very low accumulation in vital organs, two highly desired properties for in vivo studies. This ligand-exchange strategy opens a new pathway to study the use of beta-NaYF(4) for biological applications in vivo.