Preparation of an Actinium-228 Generator.

Advances in targeted α-therapies have increased the interest in actinium (Ac), whose chemistry is poorly defined due to scarcity and radiological hazards. Challenges associated with characterizing Ac3+ chemistry are magnified by its 5f06d0 electronic configuration, which precludes the use of many spectroscopic methods amenable to small amounts of material and low concentrations (like EPR, UV-vis, fluorescence). In terms of nuclear spectroscopy, many actinium isotopes (225Ac and 227Ac) are equally "unfriendly" because the actinium α-, ß-, and γ-emissions are difficult to resolve from the actinium daughters. To address these issues, we developed a method for isolating an actinium isotope (228Ac) whose nuclear properties are well-suited for γ-spectroscopy. This four-step procedure isolates 228Ra from naturally occurring 232Th. The relatively long-lived 228Ra (t1/2 = 5.75(3) years) radioisotope subsequently decays to 228Ac. Because the 228Ac decay rate [t1/2 = 6.15(2) h] is fast, 228Ac rapidly regenerates after being harvested from the 228Ra parent. The resulting 228Ac generator provides frequent and long-term access (of many years) to the spectroscopically "friendly" 228Ac radionuclide. We have demonstrated that the 228Ac product can be routinely "milked" from this generator on a daily basis, in chemically pure form, with high specific activity and in excellent yield (∼95%). Hence, in the same way that developing synthesis routes to new starting materials has advanced coordination chemistry for many metals by broadening access, this 228Ac generator has the potential to broaden actinium access for the inorganic community, facilitating the characterization of actinium chemical behavior.

Scanning Electrochemical Microscopy Reveals That Model Silicon Anodes Demonstrate Global Solid Electrolyte Interphase Passivation Degradation during Calendar Aging.

Silicon is a promising next-generation anode to increase energy density over commercial graphite anodes, but calendar life remains problematic. In this work, scanning electrochemical microscopy was used to track the site-specific reactivity of a silicon thin film surface over time to determine if undesirable Faradaic reactions were occurring at the formed solid electrolyte interphase (SEI) during calendar aging in four case scenarios: formation between 1.5 V and 100 mV with subsequent rest starting at (1) 1.5 V and (2) 100 mV and formation between 0.75 V and 100 mV with subsequent rest starting at (3) 0.75 V and (4) 100 mV. In all cases, the electrical passivation of silicon decreased with increasing time and potential relative to Li/Li+ over a 3 day period. Along with the decrease in passivation, the homogeneity of passivation over a 500 µm2 area decreased with time. Despite some local "hot spots" of reactivity, the areal uniformity of passivation suggests global SEI failure (e.g., SEI dissolution) rather than localized (e.g., cracking) failure. The silicon delithiated to 1.5 V vs Li/Li+ was less passivated than the lithiated silicon (at the beginning of rest, the forward rate constants, kf, for ferrocene redox were 7.19 × 10-5 and 3.17 × 10-7 m/s, respectively) and was also found to be more reactive than the pristine silicon surface (kf of 5 × 10-5 m/s). This reactivity was likely the result of SEI oxidation. When the cell was only delithiated up to 0.75 V versus Li/Li+, the surface was still passivating (kf of 6.11 × 10-6 m/s), but still less so than the lithiated surface (kf of 3.03 × 10-9 m/s). This indicates that the potential of the anode should be kept at or below ∼0.75 V vs Li/Li+ to prevent decreasing SEI passivation. This information will help with tuning the voltage windows for prelithiation in Si half cells and the operating voltage of Si full cells to optimize calendar life. The results provided should encourage the research community to investigate chemical, rather than mechanical, modes of failure during calendar aging and to stop using the typical convention of 1.5 V as a cutoff potential for cycling Si in half cells.

Direct Characterization of Buried Interfaces in 2D/3D Heterostructures Enabled by GeO2 Release Layer.

Inconsistent interface control in devices based on two-dimensional materials (2DMs) has limited technological maturation. Astounding variability of 2D/three-dimensional (2D/3D) interface properties has been reported, which has been exacerbated by the lack of direct investigations of buried interfaces commonly found in devices. Herein, we demonstrate a new process that enables the assembly and isolation of device-relevant heterostructures for buried interface characterization. This is achieved by implementing a water-soluble substrate (GeO2), which enables deposition of many materials onto the 2DM and subsequent heterostructure release by dissolving the GeO2 substrate. Here, we utilize this novel approach to compare how the chemistry, doping, and strain in monolayer MoS2 heterostructures fabricated by direct deposition vary from those fabricated by transfer techniques to show how interface properties differ with the heterostructure fabrication method. Direct deposition of thick Ni and Ti films is found to react with the monolayer MoS2. These interface reactions convert 50% of MoS2 into intermetallic species, which greatly exceeds the 10% conversion reported previously and 0% observed in transfer-fabricated heterostructures. We also measure notable differences in MoS2 carrier concentration depending on the heterostructure fabrication method. Direct deposition of thick Au, Ni, and Al2O3 films onto MoS2 increases the hole concentration by >1012 cm-2 compared to heterostructures fabricated by transferring MoS2 onto these materials. Thus, we demonstrate a universal method to fabricate 2D/3D heterostructures and expose buried interfaces for direct characterization.

Evaluation of 134Ce as a PET imaging surrogate for antibody drug conjugates incorporating 225Ac.

INTRODUCTION: The in vivo generator 134Ce/134La has the potential to serve as a PET imaging surrogate for both alpha-emitting 225Ac and 227Th radionuclides due to the unique CeIII/CeIV redox couple and the relatively long half-life of 134Ce. The purpose of this study was to demonstrate the compatibility of 134Ce with DOTA-based antibody drug conjugates, which would act as therapeutic agents when incorporating 225Ac. METHODS: The in vivo biodistributions of [134Ce]Ce-DOTA and [134Ce]Ce-citrate were assayed by microPET imaging over 25 h in Swiss Webster mice to determine the in vivo stability of the [134Ce]Ce-DOTA complex. L3-edge X-ray absorption spectroscopy measurements were used to confirm the Ce oxidation state and the formation of a fully coordinated Ce-DOTA complex. The in vivo biodistribution of [134Ce]Ce-DOTA-Trastuzumab was assayed over 147 h by microPET imaging in SK-OV-3 tumor-bearing NOD SCID mice to evaluate tumor uptake and in vivo stability. Mice were euthanized at 214 h after administration of the radiolabeled antibody conjugate, and imaged 1 h later. An ex vivo biodistribution experiment was then performed in order to corroborate the PET images. RESULTS: [134Ce]Ce-DOTA displayed rapid renal elimination and high in vivo stability over 25 h, with negligible bone and liver uptake, in comparison to [134Ce]Ce-citrate. L3-edge X-ray absorption spectroscopy experiments confirmed the 3+ oxidation state within the stable Ce-DOTA complex. MicroPET images of [134Ce]Ce-DOTA-Trastuzumab displayed elevated tumor uptake over 214 h, with minimal bone and liver uptake analogous to previously reported [225Ac]Ac-DOTA-Trastuzumab biodistribution results, and the ex vivo biodistribution of [134Ce]Ce-DOTA-Trastuzumab corroborated the final PET images. CONCLUSION: These results demonstrate that 134Ce allows for long-term tumor targeting with DOTA-based antibody drug conjugates and may therefore be used to trace antibody drug conjugates incorporating 225Ac.