Richard E. Riman

Mechanochemical-hydrothermal synthesis of carbonated apatite powders at room temperature

Crystalline carbonate- and sodium-and-carbonate-substituted hydroxyapatite (CO3HAp and NaCO3HAp) powders were prepared at room temperature via a heterogeneous reaction between Ca(OH)2/CaCO3/Na2CO3 and (NH4)2HPO4 aqueous solution using the mechanochemical hydrothermal route. X-ray diffraction, infrared spectroscopy, thermogravimetry, and chemical analysis were performed. Room temperature products were phase-pure CO3HAp and NaCO3HAp containing 0.8-12 wt% of carbonate ions in the lattice. Dynamic light scattering revealed that the median agglomerate size of the room temperature CO3HAp and NaCO3HAp powders was in the range of 0.35-1.6 microm with a specific surface area between 82 and 121 m2/g. Scanning and transmission electron microscopy confirmed that the carbonated HAp powders consisted of mostly submicron aggregates of nanosized, approximately 20 nm crystals. The synthesized carbonated apatite powders exhibit chemical compositions and crystallinities similar to those of mineral constituents of hard tissues and therefore are promising for fabrication of bone-resembling implants.

Rare-earth-doped biological composites as in vivo shortwave infrared reporters

The extension of in vivo optical imaging for disease screening and image-guided surgical interventions requires brightly-emitting, tissue-specific materials that optically transmit through living tissue and can be imaged with portable systems that display data in real-time. Recent work suggests that a new window across the short wavelength infrared region can improve in vivo imaging sensitivity over near infrared light. Here we report on the first evidence of multispectral, real-time short wavelength infrared imaging offering anatomical resolution using brightly-emitting rare-earth nanomaterials and demonstrate their applicability toward disease-targeted imaging. Inorganic-protein nanocomposites of rare-earth nanomaterials with human serum albumin facilitated systemic biodistribution of the rare-earth nanomaterials resulting in the increased accumulation and retention in tumor tissue that was visualized by the localized enhancement of infrared signal intensity. Our findings lay the groundwork for a new generation of versatile, biomedical nanomaterials that can advance disease monitoring based on a pioneering infrared imaging technique.

Solution synthesis of hydroxyapatite designer particulates

Abstract This paper reviews our research program for intelligent synthesis of hydroxyapatite (HAp) designer particulates by low-temperature hydrothermal and mechanochemical–hydrothermal methods. Our common starting point for hydrothermal crystallization is the generation and validation of equilibrium diagrams to derive the relationship between initial reaction conditions and desired phase assemblage(s). Experimental conditions were planned based on calculated phase boundaries in the system CaO–P2O5–NH4NO3–H2O at 25–200 °C. HAp powders were then hydrothermally synthesized in stirred autoclaves at 50–200 °C and by the mechanochemical–hydrothermal method in a multi-ring media mill at room temperature. The synthesized powders were characterized using X-ray diffraction, infrared spectroscopy, thermogravimetry, chemical analysis and electron microscopy. Hydrothermally synthesized HAp particle morphologies and sizes were controlled through thermodynamic and non-thermodynamic processing variables, e.g. synthesis temperature, additives and stirring speed. Hydrothermal synthesis yielded well-crystallized needle-like HAp powders (size range 20–300 nm) with minimal levels of aggregation. Conversely, room-temperature mechanochemical–hydrothermal synthesis resulted in agglomerated, nanosized (∼20 nm), mostly equiaxed particles regardless of whether the HAp was stoichiometric, carbonate-substituted, or contained both sodium and carbonate. The thermodynamic model appears to be applicable for both stoichiometric and nonstoichiometric compositions. The mechanochemical–hydrothermal technique was particularly well suited for controlling carbonate substitution in HAp powders in the range of 0.8–12 wt.%. The use of organic surfactants, pH or nonaqueous solvents facilitated the preparation of stable colloidal dispersions of these mechanochemical–hydrothermal-derived HAp nanopowders.

Hydrothermal crystallization of ceramics

Abstract In broad terms, hydrothermal synthesis is a technology for crystallizing materials (chemical compounds) directly from aqueous solution by adept control of thermodynamic variables (temperature, pressure and composition). The objective of this chapter is to introduce the field of hydrothermal materials synthesis and slow how understanding solution thermodynamics of the aqueous medium can be used for engineering hydrothermal crystallization processes. In the first section, we will focus on hydrothermal synthesis as a materials synthesis technology by providing history, process definitions, technological merits and comments on its current implementation in industry. In the second section, we will describe how thermodynamic modeling is being developed as an engineering tool to predict equilibrium phase assemblages and use this predictive power as an engineering tool for development of hydrothermal technology for materials synthesis.

Hydrothermal Synthesis of Advanced Ceramic Powders

This paper briefly reviews hydrothermal synthesis of ceramic powders and shows how understanding the underlying physico-chemical processes occurring in the aqueous solution can be used for engineering hydrothermal crystallization processes. Our overview covers the current status of hydrothermal technology for inorganic powders with respect to types of materials prepared, ability to control the process, and use in commercial manufacturing. General discussion is supported with specific examples derived from our own research (hydroxyapatite, PZT, -Al2O3, ZnO, carbon nanotubes). Hydrothermal crystallization processes afford excellent control of morphology (e.g., spherical, cubic, fibrous, and plate-like) size (from a couple of nanometers to tens of microns), and degree of agglomeration. These characteristics can be controlled in wide ranges using thermodynamic variables, such as reaction temperature, types and concentrations of the reactants, in addition to non-thermodynamic (kinetic) variables, such as stirring speed. Moreover, the chemical composition of the powders can be easily controlled from the perspective of stoichiometry and formation of solid solutions. Finally, hydrothermal technology affords the ability to achieve cost effective scale-up and commercial production.

Intense Near‐IR Emission from Nanoscale Lanthanoid Fluoride Clusters

Lanthanoid-doped fluoride glasses are intense near-IR (NIR) emission sources because of the low-energy phonon characteristics of fluoride lattices. These materials are particularly useful in optical applications, because fluorides are absolutely air-stable. Unfortunately, the extreme insolubility of lanthanoid ions in the presence of fluoride sources has always presented a barrier to developing alternative synthetic approaches to LnFx materials, particularly in media that would preclude the incorporation of NIR-emission-quenching OH groups. Herein we demonstrate that by using unconventional chalcogen-based ligands we can dramatically alter the solubility characteristics of lanthanoid cations in the presence of fluoride anions. We describe the synthesis, structural characterization, and exceptional NIR emission properties of the largest known lanthanoid cluster. Exposure of in situ prepared Ln(SePh)3 to fluoride sources does not result in the immediate precipitation of solid LnF3. Metathesis reactions of Ln(SePh)3 with HgF2, CsF, or Me4NF have yet to deliver crystalline products, but reactions of Ln(SePh)3 with NH4F in pyridine with subsequent filtration and saturation of the solution (either by layering with hexane or slow cooling) result in the crystallization, in 5–20% yields, of nanoscale lanthanoid fluoride clusters that were shown, by low-temperature single crystal Xray diffraction, to be [(py)24Ln28F68(SePh)16] (Ln=Pr 1, Nd 2 ; py= pyridine; Equation (1)). An ORTEP diagram of the

Hydrothermal synthesis of perovskite materials: Thermodynamic modeling and experimental verification

Abstract A comprehensive thermodynamic model of hydrothermal reactions has been developed to predict the optimum conditions for synthesizing perovskite ferroelectric compounds. The model indicates the ranges of reagent concentrations, pH and temperature for which the desired product can be synthesized with the highest possible yield. The synthesis conditions for the Sr-Ti system have been compared with those for the Ba-Ti system. The predictions have been experimentally verified. As a result, phase-pure SrTiO3 has been obtained using Sr(NO3)2 or Sr(OH)2·8H2O as sources of strontium and hydrous and anhydrous TiO2 as a source of titanium.

Surfactant effects on efficiency enhancement of infrared-to-visible upconversion emissions of NaYF4:Yb-Er.

Infrared-to-visible rare earth doped upconversion phosphors that convert multiple photons of lower energy to higher energy photons offer a wide range of technological applications. The brightness (i.e., emission intensities) and energy efficiency of phosphors are important performance characteristics that determine which applications are appropriate. Optical efficiency can be used as a measure of the upconversion emission performance of these rare earth doped phosphors. In this work, hexagonal-phase NaYF(4):Yb-Er was synthesized using the hydrothermal method in the presence of surfactants like trioctylphosphine, polyethylene glycol monooleate, and polyvinylpyrrolidone. The upconversion emission optical efficiencies of NaYF(4):Yb-Er were measured to quantify and evaluate the effects of surface coatings and accurately reflect the brightness and energy efficiency of these phosphors. Polyvinylpyrrolidone-modified NaYF(4):Yb-Er particles were found to be ~5 times more efficient and brighter than the unmodified particles. The difference in efficiency was attributed to reduced reflectance losses at the particle-air interface via refractive index mismatch reduction between the core NaYF(4):Yb-Er particles and air using polyvinylpyrrolidone as a surfactant.

Synthesis and Cytotoxicity of Y2O3 Nanoparticles of Various Morphologies

As the field of nanotechnology continues to grow, evaluating the cytotoxicity of nanoparticles is important in furthering their application within biomedicine. Here, we report the synthesis, characterization, and cytotoxicity of nanoparticles of different morphologies of yttrium oxide, a promising material for biological imaging applications. Nanoparticles of spherical, rod-like, and platelet morphologies were synthesized via solvothermal and hydrothermal methods and characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), light scattering, surface area analysis, thermogravimetric analysis (TGA), and zeta potential measurements. Nanoparticles were then tested for cytotoxicity with human foreskin fibroblast (HFF) cells, with the goal of elucidating nanoparticle characteristics that influence cytotoxicity. Cellular response was different for the different morphologies, with spherical particles exhibiting no cytotoxicity to HFF cells, rod-like particles increasing cell proliferation, and platelet particles markedly cytotoxic. However, due to differences in the nanoparticle chemistry as determined through the characterization techniques, it is difficult to attribute the cytotoxicity responses to the particle morphology. Rather, the cytotoxicity of the platelet sample appears due to the stabilizing ligand, oleylamine, which was present at higher levels in this sample. This study demonstrates the importance of nanoparticle chemistry on in vitro cytotoxicity, and highlights the general importance of thorough nanoparticle characterization as a prerequisite to understanding nanoparticle cytotoxicity.

Low temperature hydrothermal synthesis and formation mechanisms of lead titanate (PbTiO3) particles using tetramethylammonium hydroxide: thermodynamic modelling and experimental verification

Abstract Thermodynamic modeling was used to predict the optimum synthesis conditions for precipitation of the phase-pure lead titanate (PbTiO 3 ) in the Pb–Ti-tetramethylammonium hydroxide (TMAH) system using a newly developed computer program for automatic generation of stability and yield diagrams. The thermodynamic model has been experimentally validated over a wide range of processing conditions. Like KOH-mineralized systems, it was determined that the pH of the hydrothermal reaction medium and the Pb/Ti ratio are critical factors in forming stoichiometric PbTiO 3 powder. Morphological evolution during the reaction suggests that the formation mechanism appears to be controlled by a dissolution and recrystallization process. Two possible growth mechanisms are proposed based on the magnitude of the Pb/Ti ratio. In the case of Pb/Ti ratio=1.1, at the early stage of the reaction (3 h) excess lead species promote the formation of spherical intermediate pyrochlore phase followed by the formation of primary cubic PbTiO 3 crystals. The growth of cubic PbTiO 3 crystals proceeds until the intermediate phase acting as a reservoir to provide precipitating ions is consumed. In case of Pb/Ti ratio=1.25, excess lead condition leads to the formation of a platelet-shaped intermediate pyrochlore phase. These platelet intermediate particles act as a template in which small cubic shaped PbTiO 3 grains grew on the surface of these platelets.