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 (Er) concentrations as high as 100 mol% in NaY(Er)F/NaLuF 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.

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.

b S Supporting Information ' INTRODUCTIONColloidal nanomaterials show unique properties and are widely explored for a variety of applications. 1À4 Lanthanidebased nanomaterials have versatile utility in biological applications, as they can be made either as luminescent, magnetic, or as dual probe by selective doping of lanthanide ions. 5 In particular, paramagnetic Gd 3+ -doped NPs show tremendous potential as contrast agents (CAs) for magnetic resonance imaging (MRI). 6,7 MRI is a powerful medical diagnostic tool, where the relaxation of water protons exposed to an external magnetic field is used to obtain morphological and anatomical information with unlimited tissue penetration and yet high spatial resolution. 8 CAs are used to improve the sensitivity, because they interact with the surrounding water protons and shorten their relaxation time to provide better contrast. Two types of CAs are clinically prevalent: (i) paramagnetic Gd 3+ chelates, which affect the longitudinal relaxivity (r 1 ), and are termed positive (T 1 ) CAs, because they enhance the contrast; 9 and (ii) superparamagnetic iron oxide (SPIO) NPs, which affect transverse relaxivity (r 2 ) and are referred to as negative (T 2 ) CAs, because they diminish the signal intensity at the region of interest. 7,10 T 1 contrast agents are preferred over the T 2 agents as their enhanced brightening effect can easily be used to differentiate the signal from other pathogenic or biological conditions. 7 Gd 3+ chelates that are used clinically have very low body circulation time, because of their low molecular weight and show limitations as molecular probes for long-term tracking. 6 They also provide very low local contrast, because each chelate has only one Gd 3+ ion. To increase the local contrast and relaxivity, second-generation agents have been developed by covalently anchoring Gd 3+ chelates to different nanostructure frameworks, 11 or bundling multiple Gd 3+ chelates together using polymers, dendrimers, liposomes, and viral capsids. 12 These structures have been shown to have high relaxivity and increased local contrast as multiple Gd 3+ ions are coupled to a single nanostructure. The main disadvantage of this class of agents concerns their functionalization, which is tedious, expensive, and the number of ions that can be loaded to a NP is further limited by the number of anchoring sites available. Moreover, some of these aggregates are too large to be clinically useful. 6,7 Recently,

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 (β-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.

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.

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.

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Size-Tunable, Ultrasmall NaGdF 4 Nanoparticles: Insights into Their T 1 MRI Contrast Enhancement. -Paramagnetic β-NaGdF4 nanoparticles are synthesized by addition of a MeOH solution containing NaOH and NH4F to a mixture of GdCl3, oleic acid, and octadecene. The size of the nanoparticles can precisely be controlled by varying reaction time and temperature. Particles of 2.5 nm sizes are obtained at 260°C for 10 min, 4.0 nm sized particles at 270°C for 40 min, 6.5 nm sized particles at 280°C for 90 min, and 8.0 nm particles at 285°C for 100 min. The prepared nanoparticles show magnetic resonance longitudinal relaxitivities per nanoparticle at 1.5 T which are 200-3000 times larger than that of the clinically used Gd-DTPA contrast agent, showing great potential as local contrast enhancement probes. β-NaGdF4 nanoparticles are good hosts for upconverting emission which is demonstrated by the preparation of luminescent ultrasmall β-NaGdF4:Yb 3+ /Tm 3+ nanoparticles as potential bimodal probes. -(JOHNSON, N. J. J.; OAKDEN, W.; STANISZ, G. J.; PROSSER, R. S.; VAN VEGGEL*, F. C. J. M.; Chem. Mater. 23 (2011) 16, 3714-3722, http://dx.