Most studies on iron oxide nanocrystals (NCs) suggest that the magnetic properties depend strongly on size for diameters below 10 nm, but there is less agreement about how the structure of the NC surface influences magnetic properties. Because the magnetic properties of iron oxide NCs hold promise for applications from cancer detection and therapeutics to environmental remediation, it is imperative to understand how size influences those properties. In most cases, the effective magnetic size is significantly lower than the measured physical size, a finding attributed to spin canting or disorder at the NC surface. A complicating factor is that the reaction conditions used to produce samples influence their magnetic properties. Thus, we employed a continuous growth method involving layer-by-layer addition of precursor to produce single-crystalline, spherical cores with subnanometer precision over a range of sizes under the same reaction conditions. Analysis of the NCs by small-angle X-ray scattering, transmission electron microscopy, and powder X-ray diffraction showed that the NCs possess the spinel structure (primarily maghemite) and are crystalline, defect-free, and uniform in size. The saturation magnetization values for a series of eight distinct diameters between 4 and 10 nm increase smoothly with increasing size, from 55 to 78 Am 2 /kg. Magnetic sizes of the NCs determined by fitting magnetization curves to the Langevin function are nearly identical to the physical sizes, suggesting low levels of strain-producing defects and a very thin nonmagnetic surface layer on the NCs. The results suggest that syntheses that permit slower growth at reduced temperatures through a single reaction mechanism can enhance, and offer fine control over, magnetic properties.

Spinel iron oxide nanocrystals (NCs) have been reported to have atomic-level core and surface structural features that differ from those of the bulk material. Recent advances in a continuous growth synthesis of metal oxide NCs make it possible to prepare a series of NCs with subnanometer control of size with diameters below 10 nm that are well-suited for investigating size-dependent structure and reactivity. Here, we study the evolution of size-dependent structure in spinel iron oxide and determine how nanoscale structure influences the growth of NCs. We synthesized spinel iron oxide NCs via a continuous growth method that permits layer-by-layer control of size in order to monitor nanoscale structure over 16 core sizes between 3 and 10 nm. X-ray total scattering data were collected and analyzed with pair distribution function (PDF) analysis in order to refine quantitative structural features including cation occupancies that could be used to detect changes both in the oxidation state and the presence of tetrahedrally coordinated cation vacancies in the NCs. We find that the average iron oxidation state increases as core diameters decrease from 8 down to 3 nm. The trend in iron oxidation state can be explained by the oxidation of surface layers in the NCs. For samples exposed to air for several weeks, oxidation appears to cease when a volume equivalent to that of an ∼1.3 nm shell is converted to the more oxidized maghemite. The number of tetrahedrally coordinated cation vacancies also increases as the NC core size decreases. The correlation between the number of these vacancies and the faster growth for smaller NCs suggests that these reactive vacancies may be responsible for the rapid growth observed for nanocrystals with diameters smaller than 8 nm.

The formation of ternary aqua complexes of metal-based diagnostics and therapeutics is closely correlated to their in vivo efficacy but approaches to quantify the presence of coordinated water ligands are limited. We introduce a general and high-throughput method for characterizing the hydration state of para- and diamagnetic coordination complexes in the gas phase based on variable-temperature ion trap tandem mass spectrometry. Ternary aqua complexes are directly observed in the mass spectrum and quantified as a function of ion trap temperature. We recover expected periodic trends for hydration across the lanthanides and distinguish complexes with several inner-sphere water ligands by inspection of temperature-dependent speciation curves. We derive gas-phase thermodynamic parameters for discernible inner- and second-sphere hydration events, and discuss their application to predict solution-phase behavior. The differences in temperature at which water binds in the inner and outer spheres arise primarily from entropic effects. The broad applicability of this method allows us to estimate the hydration states of Ga, Sc, and Zr complexes under active preclinical and clinical study with as-yet undetermined hydration number. Variable-temperature mass spectrometry emerges as a general tool to characterize and quantitate trends in inner-sphere hydration across the periodic table.

Recently, we pioneered the application of Cherenkov radiation (CR) of radionuclides for the in situ excitation of discrete Eu­(III) and Tb­(III) complexes. CR is produced by isotopes decaying under emission of charged particles in dielectric media and exhibits a maximum intensity below 400 nm. We have demonstrated that luminescent lanthanide antenna complexes are ideal acceptors for Cherenkov radiation-mediated energy transfer (CRET). Here, we develop and assess peptide-functionalized Tb­(III) and Eu­(III) complexes in conjunction with CRET excitation by the positron emissive radioisotope 18F for simultaneous, multiplexed imaging and in vivo optical imaging. This work demonstrates, for the first time, that the detection of the luminescence emission of a discrete Eu­(III) complex in vivo is feasible. Our results open possibilities for discrete luminescent lanthanide complexes to be used as diagnostic, optical tools for the intrasurgical guidance of tumor resection.

Luminescent lanthanides possess ideal properties for biological imaging, including long luminescent lifetimes and emission within the optical window. Here, we report a novel approach to responsive luminescent Tb(III) probes that...