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The design and use of materials in the nanoscale size range for addressing medical and health-related issues continues to receive increasing interest. Research in nanomedicine spans a multitude of areas, including drug delivery, vaccine development, antibacterial, diagnosis and imaging tools, wearable devices, implants, high-throughput screening platforms, etc.using biological, nonbiological, biomimetic, or hybrid materials. Many of these developments are starting to be translated into viable clinical products. Here, we provide an overview of recent developments in nanomedicine and highlight the current challenges and upcoming opportunities for the field and translation to the clinic.

Rational design of elaborate, multicomponent nanomaterials is important for the development of many technologies such as optoelectronic devices, photocatalysts, and ion batteries. Combination of metal chalcogenides with different anions, such as in CdS/CdSe structures, is particularly effective for creating heterojunctions with valence band offsets. Seeded growth, often coupled with cation exchange, is commonly used to create various core/shell, dot-in-rod, or multipod geometries. To augment this library of multichalcogenide structures with new geometries, we have developed a method for postsynthetic transformation of copper sulfide nanorods into several different classes of nanoheterostructures containing both copper sulfide and copper selenide. Two distinct temperature-dependent pathways allow us to select from several outcomes─rectangular, faceted Cu2–xS/Cu2–xSe core/shell structures, nanorhombuses with a Cu2–xS core, and triangular deposits of Cu2–xSe or Cu2–x(S,Se) solid solutions. These different outcomes arise due to the evolution of the molecular components in solution. At lower temperatures, slow Cu2–xS dissolution leads to concerted morphology change and Cu2–xSe deposition, while Se-anion exchange dominates at higher temperatures. We present detailed characterization of these Cu2–xS–Cu2–xSe nanoheterostructures by transmission electron microscopy (TEM), powder X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning TEM–energy-dispersive spectroscopy. Furthermore, we correlate the selenium species present in solution with the roles they play in the temperature dependence of nanoheterostructure formation by comparing the outcomes of the established reaction conditions to use of didecyl diselenide as a transformation precursor.

Nanoparticle materials that consist of a solid solution between two end member compounds often have composition-dependent physical properties. Their synthesis can be challenging, as it requires balancing the competing reactivities of many different reagents to favor the formation of a single-phase product rather than a phase-segregated mixture of its end members. Here, we provide chemical insights into the synthesis of wurtzite CoxZn1–xS nanoparticle spheres, rods, and plates for x= 0.25, 0.50, and 0.75, which represent solid solutions of CoS and ZnS, by simultaneously exchanging the Cu+cations in roxbyite copper sulfide for Zn2+and Co2+. Density-functional theory calculations of 401 different prototypical structures and compositions spanning the CoxZn1–xS solid solution space confirm that they are metastable with positive mixing enthalpies and formation energies that are 100–150 meV per formula unit above the convex hull. Competition experiments reveal preferential exchange of Co2+vs Zn2+when both are present in excess. We balance their reactivities by controlling the ratio of total cations to copper sulfide, thereby avoiding the formation of cobalt sulfide byproducts. UV–vis–NIR absorption spectra reveal a decrease in band gap as xin CoxZn1–xS increases; x= 0.25 and x= 0.50 are semiconducting, while x= 0.75 is metallic. The optical properties of the CoxZn1–xS solid solutions differ from CoS–ZnS heterostructures, which have similar compositions but different mixing behavior. The CoxZn1–xS solid solution can also be integrated into heterostructured nanorods to combine their composition-tunable properties with other materials.

High-entropy oxides (HEOs) are of interest for their unique physical and chemical properties. Significant lattice distortions, strain, and tolerance for high-vacancy concentrations set HEOs apart from single-metal or mixed-metal oxides. Herein, we synthesized and characterized the structures and compositions, along with the optical, magnetic, and electrocatalytic properties, of two families of high-entropy mixed-metal tungsten and molybdenum oxides, AWO4and B2Mo3O8, where A and B are 3d transition metals. The HEOs A6WO4(A = Mn, Fe, Co, Ni, Cu, and Zn) and B25Mo3O8(B = Mn, Fe, Co, Ni, and Zn), as well as all accessible single-metal AWO4and B2Mo3O8parent compounds, were synthesized using high-temperature solid-state methods. X-ray photoelectron spectroscopy analysis of the surfaces revealed that the HEOs largely had the metal oxidation states expected from the bulk chemical formulas, but in some cases they were different than in the parent compounds. A6WO4exhibited antiferromagnetic (AFM) ordering with a Néel temperature of 30 K, which is less than the average of its AFM parent compounds, and had a narrow band gap of 0.24 eV, which is much lower than all of its parent compounds. B25Mo3O8was paramagnetic, despite the existence of AFM and ferromagnetic ordering in several of its parent compounds and had no observable band gap, which is analogous to its parent compounds. Both A6WO4and B25Mo3O8exhibited superior catalytic activity relative to the parent compounds for the oxygen evolution reaction, the oxidation half reaction of overall water splitting, under alkaline conditions, based on the overpotential required to reach the benchmark surface area normalized current density. Consistent with literature predictions of OER durability for ternary tungsten and molybdenum oxides, A6WO4and B25Mo3O8also exhibited stable performance without significant dissolution during 10 h stability experiments at a constant current.

Partial cation exchange reactions can be used to rationally design and synthesize heterostructured nanoparticles that are useful targets for applications in photocatalysis, nanophotonics, thermoelectrics, and medicine. Such reactions introduce intraparticle frameworks that define the spatial arrangements of different materials within a heterostructured nanoparticle, as well as the orientations and locations of their interfaces. Here, we show that upon heating to temperatures relevant to their synthesis and applications, the ZnS regions and Cu1.8S/ZnS interfaces of heterostructured ZnS–Cu1.8S nanorods migrate and restructure. We first use partial cation exchange reactions to synthesize a library of seven distinct samples containing various patches, bands, and tips of ZnS embedded within Cu1.8S nanorods. Upon annealing in solution or in air, ex situTEM analysis shows evidence that the ZnS domains migrate in different ways, depending upon their sizes and locations. Using differential scanning calorimetry, we correlate the threshold temperature for ZnS migration to the superionic transition temperature of Cu1.8S, which facilitates rapid diffusion throughout the nanorods. We then use in situthermal TEM to study the evolution of individual ZnS–Cu1.8S nanorods upon heating. We find that ZnS domain migration occurs through a ripening process that minimizes small patches with higher-energy interfaces in favor of larger bands and tips having lower-energy interfaces, as well as through restructuring of higher-energy Cu1.8S/ZnS interfaces. Notably, Cu1.8S nanorods containing multiple patches of ZnS thermally transform into ZnS–Cu1.8S heterostructured nanorods having ZnS tips and/or central bands, which provides mechanistic insights into how these commonly observed products form during synthesis.

Cation exchange reactions can modify the compositions of colloidal nanoparticles, providing easy access to compounds or nanoparticles that may not be accessible directly. The most common nanoparticle cation exchange reactions replace monovalent cations with divalent cations or vice versa, but some monovalent-to-monovalent exchanges have been reported. Here, we dissect the reaction of as-synthesized AgCuS nanocrystals with Au+to form AgAuS, initially hypothesizing that Au+could be selective for Cu+(rather than for Ag+) based on a known Au+-for-Cu+exchange and the stability of the targeted AgAuS product. Unexpectedly, we found this system and the putative cation exchange reaction to be much more complex than anticipated. First, the starting AgCuS nanoparticles, which match literature reports, are more accurately described as a hybrid of Ag and a variant of AgCuS that is structurally related to mckinstryite Ag5Cu3S4. Second, the initial reaction of Ag–AgCuS with Au+results in a galvanic replacement to transform the Ag component to a AuyAg1–yalloy. Third, continued reaction with Au+initiates cation exchange with Cu+in AuyAg1–y–AgCuS to form AuyAg1–y–Ag3CuxAu1–xS2and then AuyAg1–y–AgAuS, which is the final product. Crystal structure relationships among mckinstryite-type AgCuS, Ag3CuxAu1–xS2, and AgAuS help to rationalize the transformation pathway. These insights into the reaction of AgCuS with Au+reveal the potential complexity of seemingly simple nanoparticle reactions and highlight the importance of thorough compositional, structural, and morphological characterization before, during, and after such reactions.