Biomedical applications of nanoparticles made by flame spray pyrolysis

The synthesis of functional nanoparticles via one-step flame spray pyrolysis (FSP), especially those of catalytic nature, has attracted the interests of scientists and engineers, as well as industries. The rapid and high temperature continuous synthesis yields nanoparticles with intrinsic features of active catalysts, that is, high surface area and surface energetics. For these reasons, FSP finds applications in various thermally inducible catalytic reactions. However, the design and synthesis of photocatalysts by FSP requires a knowledge set which is different from that established for thermal catalysts. Unknown to many, this has resulted in frustrations to those entering the field unprepared, especially since FSP appears to be an elegant tool in synthesising oxide nanoparticles of any elemental construct. From simple oxide to doped-oxide, and mixed metal oxide to the in situ deposition of noble metals, this Perspective gives an overview on the development of photocatalysts made by FSP in the last decade that led to a better understanding of the design criteria. Various challenges and opportunities are also highlighted, especially those beyond simple metal oxides, which perhaps contain the greatest potential for the exploitation of photocatalysts design by FSP.

Direct synthesis of strontium titanate phosphor particles with high luminescence by flame spray pyrolysis

Y2O3:Eu phosphor particles were prepared by flame spray pyrolysis and compared with the particles prepared by general spray pyrolysis. The particles prepared by flame spray pyrolysis had a spherical and dense morphology and were finer than the particles prepared by general spray pyrolysis. Flame temperature was an important factor in the preparation of the phosphor particles by flame spray pyrolysis. To obtain Y2O3:Eu particles with a uniformly dense structure, a sufficiently high temperature to form monoclinic phase was required. Too low flame temperature generated nonspherical and hollow particles with cubic phase because the particles did not melt completely, and too high flame temperature of flame generated many nanoparticles due to evaporation. After stepwise post-treatment of as-prepared particles with monoclinic phase and the dense structure, Y2O3:Eu phosphor particles with high brightness and cubic phase were obtained. The Y2O3:Eu phosphor particles prepared by flame spray pyrolysis showed 120% photoluminescence intensity in comparison with the particles prepared by general spray pyrolysis.

Two-nozzle frame spray analysis (FSP) synthesis of CoMo/Al2O3 where Co and Al are sprayed in separate flames was applied to minimize the formation of CoAl2O4 observed in one-nozzle flame spray pyrolysis (FSP) synthesis and the materials were characterized by N2-adsorption (BET), X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy, Raman spectroscopy, transmission electron microscopy, and catalytic performances in hydrotreating. By varying the flame mixing distances (81–175 mm) the amount of CoAl2O4 could be minimized. As evidenced by UV–vis spectroscopy, CoAl2O4 was detected only at short flame mixing distances, where the flame conditions resemble one-nozzle FSP. Raman spectroscopy revealed that β-CoMoO4 was a component of all the catalysts (in the as-prepared oxidic form) together with alumina supported MoO x surface species. The only phase detected with XRD was γ-Al2O3. The FSP synthesized oxidic catalysts were activated by sulfidation without further heat treatments. The hydrodesulfurization activity of the best two-nozzle FSP catalysts, compared to the one-nozzle FSP catalysts, improved from 75 to 91 % activity relative to a commercial reference catalyst and the hydrodenitrogenation activity improved from 70 to 90 % relative activity. This suggests that better promotion of the active molybdenum sulfide phase was achieved when using two-nozzle FSP synthesis, probably due to less formation of the undesired phase CoAl2O4, which makes Co unavailable for promotion.

Flame spray pyrolysis (FSP) is a versatile process for the production of inorganic nanoparticles featuring the advantage that the reagents are directly dissolved in the liquid fuel that is atomized to form the burning flame. A majority of previous studies on flame spray pyrolysis is focused on the formation and growth processes of the nanoparticles but neglect the preceding step of precursor atomization and spray formation. In this work an atomization concept for large‐scale nanoparticle production by flame spray pyrolysis is presented. A pressure swirl nozzle is applied for creating a liquid hollow cone, and in a second step, different dispersion gas nozzles are utilized to enhance the atomization of the liquid phase and to influence the spray cone formation and geometry. The relevant parameters influencing the atomization process (dispersion gas feed rate, liquid feed rate) are investigated (for air, water) in non‐burning (cold) spray conditions in order to access the utilization of the different atomizer concepts for the flame spray pyrolysis‐process. Measurements are conducted by applying high speed camera imaging (HSC), particle image velocimetry (PIV) and laser diffraction spectroscopy (LDS). Computational fluid dynamics (CFD) revealed further insight into the gas entrainment and the trajectory of droplets within the spray. Results show that the liquid volume flow rate (and thus the productivity of the process) may be increased significantly while still maintaining an appropriate droplet size compared to the conventional atomization process conditions in flame spray pyrolysis reactors.

Advances in aerosol technology over the past 10 years have enabled the generation and design of ultrafine nanoscale materials for many applications. A key new method is flame spray pyrolysis (FSP), which produces particles by pyrolyzing a precursor solution in the gas phase. FSP is a highly versatile technique for fast, single-step, scalable synthesis of nanoscale materials. New innovations in particle synthesis using FSP technology, including variations in precursor chemistry, have enabled flexible, dry synthesis of loosely agglomerated, highly crystalline ultrafine powders (porosity ≥ 90%) of binary, ternary, and mixed-binary-and-ternary oxides. FSP can fulfill much of the increasing demand, especially in biological applications, for particles with specific material composition, high purity, and high crystallinity. In this Account, we describe a strategy for creating nanoparticle libraries (pure or Fedoped ZnO or TiO₂) utilizing FSP and using these libraries to test hypotheses related to the particles' toxicity. Our innovation lies in the overall integration of the knowledge we have developed in the last 5 years in (1) synthesizing nanomaterials to address specific hypotheses, (2) demonstrating the electronic properties that cause the material toxicity, (3) understanding the reaction mechanisms causing the toxicity, and (4) extracting from in vitro testing and in vivo testing in terrestrial and marine organisms the essential properties of safe nanomaterials. On the basis of this acquired knowledge, we further describe how the dissolved metal ion from these materials (Zn²⁺ in this Account) can effectively bind with different cell constituents, causing toxicity. We use Fe-S protein clusters as an example of the complex chemical reactions taking place after free metal ions migrate into the cells. As a second example, TiO₂ is an active material in the UV range that exhibits photocatalytic behavior. The induction of electron-hole (e⁻/h⁺) pairs followed by free radical production is a key mechanism for biological injury. We show that decreasing the bandgap energy increases the phototoxicity in the presence of near-visible light. We present in detail the mechanism of electron transfer in biotic and abiotic systems during light exposure. Through this example we show that FSP is a versatile technique for efficiently designing a homologous library, meaning a library based on a parent oxide doped with different amounts of dopant, and investigating the properties of the resulting compounds. Finally, we describe the future outlook and state-of-the-art of an innovative two-flame system. A double-flame reactor enables independent control over each flame, the nozzle distances and the flame angles for efficient mixing of the particle streams. In addition, it allows for different flame compositions, flame sizes, and multicomponent mixing (a grain-grain heterojunction) during the reaction process.

Fine-sized, spherical BaMgAl10O17:Eu2+ phosphor particles with filled morphologies were prepared by flame spray pyrolysis. Precursor particles prepared by flame spray pyrolysis at a low-temperature diffusion flame and a short particle residence time within the diffusion flame were of large size and had hollow structures. On the other hand, precursor particles prepared at optimum preparation conditions were fine sized, had filled structures, and narrow size distributions. The melting of large-sized intermediate particles with hollow structures within the high-temperature diffusion flame formed fine-sized, spherical precursor particles with filled structures. The BaMgAl10O17:Eu2+ phosphor particles prepared by flame spray pyrolysis at optimum preparation conditions were fine-sized and had regular morphologies after post-treatment at 1400°C. The mean size of the BaMgAl10O17:Eu2+ phosphor particles was 1.0 μm. The photoluminescence intensity of fine-sized, spherical BaMgAl10O17:Eu2+ phosphor particles with filled structures was higher than that of large-sized phosphor particles with hollow structures excited by the vacuum ultraviolet light at the wavelength, 147 nm.

Morphologies and crystal structures of nano-sized Ba 1− xSr xTiO 3 primary particles prepared by flame spray pyrolysis

Synthesis and properties of Ce 1− xGd xO 2− x/2 solid solution prepared by flame spray pyrolysis

Rechargeable Mg batteries have gained considerable interest, as it possesses considerably higher volumetric capacity (3833 mAh/cm3) vs. Li metal (2046 mAh/cm3) due to the bivalency of Mg1. Other advantages of Mg is its abundancy in the earth's crust, low cost, environmentally friendly, low equilibrium potential (-2.31 V vs NHE2) and the benefit of using Mg directly as anode without dendrite formation thus alleviating the safety concerns as opposed to using Li metal as anode. Nevertheless, the development of rechargeable Mg batteries has yet not received strong commercial interest due major issues such as lack of high-voltage Mg intercalation cathodes with faster Mg insertion/extraction kinetics and passivation of metallic magnesium at the anode when exposed to oxygen, water and other protic solvents (including organics). These limitations result in restricted choice of electrolytes with poor anodic stability and potential window thus hindering the research on high energy density cathodes. The need for high energy density batteries as mentioned earlier would require cathode materials with high voltages (> 3 V). Much of the promising cathode materials for rechargeable Mg-ion batteries are metal oxides and sulfides. Of them, V2O5 is particularly interesting as it provides comparatively higher open circuit potential (OCV, about 2.66 V vs. Mg2+ in 1 M Mg(ClO4)2 in THF))2. Last year, we successfully demonstrated high specific capacity (245 mAh/g) for nanostructured V2O5 synthesized by flame spray pyrolysis (FSP)3. However, strong electrostatic interaction between Mg2+ ions and inorganic host materials hamper the kinetics of these oxides for reversible Mg2+ intercalation. Nevertheless, recently, amorphous structures are found to show better kinetics owing to their favourable interlayer spacing4. To this end, a series of amorphous V2O5-based cathode materials such as V2O5-SiO2 and V2O5-B2O3 are synthesized by flame spray pyrolysis (FSP). The effect of employing different solvents and flow rates during synthesis on the structure and morphology of the materials is studied. While XRD, SEM and BET are used to characterize the physical properties, 3-electrode cell assembly and coin cells study the electrochemical properties of the materials. The cathodes materials are coated on graphite sheet current collector with Mg plate and Mg(BH4)2in tetraglyme acting as anode and electrolyte, respectively. This work, as a whole will discuss the effect of synthesis parameters, heat treatment, composition and morphology of V2O5-based cathode materials (with varying degree of amorphicity) with respect to their electrochemical performance. H.D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pour and D. Aurbach, Energy Environ. Sci., 2013, 6, 2265M.M. Huie, D.C. Bock, E.S: Takeuchi, A.C. Marschilok and K.J. Takeuchi, Coordination Chemistry Reviews, 2015, 287, 15S. M. Hanetho, K. Jayasayee, P.I. Dahl, J. Kvello, J. R. Tolchard, A. Fossdal, T. Mokkelbost, F. Vullum-Bruer, "Mg Ion Batteries: V2O5 Cathode Materials by Flame Spray Pyrolysis", 227th ECS Meeting, 2015, Chicago, USAT.S Arthur, K. Kato, F. Mizuno and J. Germain, 20th International Conference on Solid State Ionics, June 14-19, 2015

Double flame spray pyrolysis as a novel technique to synthesize alumina-supported cobalt Fischer–Tropsch catalysts

Silver (Ag) nanoparticles dispersed in an amorphous silica (SiO2) matrix or coated by a SiO2 layer were synthesized by flame spray pyrolysis (FSP). The coated nanoparticles were produced by using a modified enclosed FSP setup, in which the SiO2 precursor was injected through a ring above the FSP nozzle at various burner-ring-distances (BRDs), after the core Ag nanoparticles had been formed. The produced nanoparticles were characterized by XRD, BET, TEM and UV/vis analysis. The Ag particle size was possible to be controlled by tuning the FSP parameters. For the SiO2 coated nanoparticles, larger Ag core sizes were obtained for higher BRDs. All the produced nanoparticles exhibited the characteristic plasmon resonance frequency of Ag nanoparticles.

Computational study of the effect of processing parameters on the formation and growth of ZrO2 nanoparticles in FSP process

Adsorption performance and mechanism of methyl orange by layered zinc hydroxide nitrate improved through flame spray pyrolysis method

Flame spray pyrolysis (FSP) is a technique for the synthesis of metal oxide nanoparticles by combusting precursor solutions in a spray flame. The combustion of certain precursor solutions is known to lead to severe droplet disruptions (μ-explosions) in the spray flame that are linked to the synthesis of homogeneous and phase-pure nanoparticles. In this work, a broad spectrum of suitable subsonic operating conditions for the synthesis of iron oxide nanoparticles by FSP is investigated to understand the influence of the jet Reynolds number and turbulence on the onset of μ-explosions and droplet dynamics in spray flames. In order to enable a coherent comparison between differently operated spray flames using an iron(III) nitrate nonahydrate solution, the gas-to-liquid mass ratio and, hence, the oxygen/fuel ratio have been kept constant in order to identify the influence of flow conditions on the droplet dynamics. From the analysis of the droplet sizes in the spray and in the spray flame, it is found that in all combusting sprays, the droplet sizes convert from unimodal (after atomization) to bimodal droplet size distribution (DSD) due to the presence of μ-explosions. The occurrence and evolution of the bimodal DSD reveal that high jet Reynolds numbers result in narrower DSD and in a sharper separation of both DSD probability peaks (modal values). A straightforward 1-step kinematic model is presented to describe the conversion of unimodal to bimodal DSD considering the evaporation of droplets as well as the disruption of droplets to mimic the effect of μ-explosions. The temporal evolution of droplets in FSP is investigated by spatially resolved velocity data that reveal the formation of a temporal self-similarity. The resulting iron oxide nanoparticle size decreases with increasing jet Reynolds number. The turbulent mixing and residence times in the flame, primarily set by the jet Reynolds number, are identified as key design parameters for FSP.