Diversity of TiO<Subscript>2</Subscript> nanopowders’ characteristics relevant to toxicity testing

In this study, the physicochemical properties of several commercial ultrafine TiO₂ powders and their behaviour in the as-received form and colloidal suspensions were analysed. Besides the particle size, the morphology and agglomeration state of the dry powders, dispersibility, ζ-potential and sedimentation in water and in phosphate-buffered saline (PBS) were studied. Also, leaching of ions from the powders during ageing in physiological solution and the ability of the photoactivated powders to decompose organic substances were evaluated. The examined TiO₂ powders revealed diversified characteristics when dispersed in water. In general, while in dry conditions the particle size appeared in the nano-range (down to 32 nm), the particles were agglomerated in aqueous suspensions at pH ~7 and only a minor amount showed dimensions below 200 nm, but none below 100 nm. The inherent pH of the 3 % suspensions varies from 3.7 to 7.5 and the surface charge at these pH values varied from highly positive to highly negative values. In PBS, the surface charge is negative and relatively low for all the samples, which resulted in agglomeration. Five out of six powders exhibited significant photocatalytic activity when exposed to UV irradiation. This also includes one cosmetic-grade powder. Furthermore, during the immersion in aqueous media at physiological temperature, the powders released foreign ions, which might also contribute to the results of cytotoxicity tests. The results revealed the major role of the particle surface charge and its impact on particle dispersion or agglomeration. Due to the high ionic strength in the liquids relevant for cell-surface interaction tests, for all the examined titania powders the nanoparticulate character was lost. However, the presence of impurities and photocatalysis might further contribute to the results of cytotoxicity tests.     

Characterization of Sub‐Micron Perovskite Suspensions Formed Through Cavitation Processing

The size distribution within and electrokinetic properties of aqueous perovskite (LaCoO₃) suspensions, have been characterized as a function of processing conditions. Submicron–sized perovskite particles have been obtained using a cavitation technique in which the suspension is passed through a series of small orifices under extreme driving pressure drops. When no additives were used, the zeta potential of the particles was found to be positive over the entire pH range studied. Use of an acrylic copolymer surfactant with multiple negatively charged sites during the cavitation processing was found to improve dispersion stability. The observed variations in zeta potential and particle size for the suspensions are explained in terms of electrostatic interactions between particles, the tendency for the surfactant to adsorb onto the particles, and the degree of steric stabilization provided by the surfactant.     

Characterizing the dynamic behavior of nano-TiO<Subscript>2</Subscript> agglomerates in suspensions by photocorrelation spectroscopy

Metal oxide nanoparticles are small but easily form agglomerates in suspension, depending on the strength of particle–particle and particle–media interactions. To understand the agglomeration behavior of nanoparticles in media and relate to it to product performance testing, measurement methods are desired to characterize highly scattering metal oxide nanoparticle suspensions without dilution. In this article, we describe the advantages of using photocorrelation spectroscopy (PCS) in a backscattering detection configuration to carry out a realistic agglomerate size measurement in multiple scattering media found in most metal oxide nanoparticle suspensions. The dynamic behavior of nano-titanium dioxide (TiO₂) particles in buffer solutions of different chemical composition and pH values was investigated as a sample system using PCS. The resulting autocorrelation functions (AFs) at different time intervals, particle concentrations, and pH values were measured at several detection angles. The AF exhibits a multi-mode relaxation time feature and the calculated hydrodynamic diameters strongly depended on media composition and detection angle. This result indicates that the size and dispersion of nano-TiO₂ agglomerates are significantly affected by solution media. A measurement protocol for determining size and dispersion of metal oxide particles in media is proposed and related to a performance test found in industry.     

Particle size analysis: A comparison of various methods ii

In previous work, the relative performance of various methods used to characterize the particle size distribution of powders composed of fine irregularly shaped particles was assessed. It was found that methods employing Fraunhofer diffraction theory were inferior with respect to particle counting methods. Furthermore, calculated particle size distributions varied considerably between manufactures of Fraunhofer devices. It is well known that the Mie optical model can also be used to analyze the data collected by laser diffraction instruments. Here, we have compared particle size distributions collected using two Laser diffraction instruments to those determined by the Aerosizer. In our earlier work the Aerosizer was shown to produce results nearly identical to those determined by image analysis. The results of this study indicate that the use of the Mie optical model does not correct for deficiencies previously noted for laser diffraction methods. Considerable variation exists between the results obtained on laser diffraction instruments manufactured by different companies. Our earlier recommendation to use extreme caution when employing laser diffraction instruments to characterize fine powders continues to be supported in the present work.     

In-Line Particle Size Determination for Jet Agglomeration Processes

In jet agglomeration plants, powders are agglomerated to obtain good instant properties. The free-falling initial material is wetted in a spray cone by droplets or in a steam jet by condensation at the particle surface. In a subsequent region of high particle concentration, collision between particles occurs and agglomerates form, if the forces of adhesion are strong enough. A commercial measurement device, working according to the principle of Fraunhofer diffraction, was modified for in-line application. It was used to measure particle size distributions and concentrations of solid particles and droplets in jets. A model is presented to calculate local particle sizes by means of mass balances from integral measurements over large volumes. The results of in-line particle size and agglomerate size analyses show the practical importance of dry agglomeration during transport and lead to a better understanding of the subsequent wet agglomeration process.     

Cohesivity and Colloidal Properties Affecting the Dispersibility of Ni/Sb-Doped TiO2 Powders

The colloidal properties and cohesivity of Ni/SB-doped TiO₂ powders (normal sizes ranging from 0.7–1.3 (μm) have been studied. Pretreatment of the powders via washing proved to be necessary in order to obtain consistent and reproducible aqueous suspension behavior. The point-of-zero-charge of the powders was found to range from pH 5.8-7.8 from measurements of the sedimentation volume and particle size analyses performed for dispersions prepared at various pH values. Surface and mass titration studies corroborated these results. Stability characterization studied in nonaqueous solvents revealed the partial solubility parameters for the powders as well as the usefulness of drying the powder for the preparation of stable nonaqueous dispersions. In addition, tensile strength measurements revealed that drying the powders reduces their cohesivity. These results are useful for determining favorable processing strategies and optimal conditions for dispersibility in coatings formulations.     

Role of host liquid in optical limiting in ink suspensions

Optical limiting of 532 nm, 15 ns duration laser pulses in suspensions made by diluting black ink in water, ethanol, and isopropanol was studied and the strength of limiting was found to be quite different for the three suspensions. Microscopic investigations showed that the sizes of suspended aggregates were different for different host liquids. The suspension with largest aggregate was found to exhibit strongest optical limiting. Our work thus shows that host liquid influences optical limiting in suspensions not only through its thermodynamic properties but also by determining the particle size which has a strong effect on limiting strength.     

Characterization of size, surface charge, and agglomeration state of nanoparticle dispersions for toxicological studies

Characterizing the state of nanoparticles (such as size, surface charge, and degree of agglomeration) in aqueous suspensions and understanding the parameters that affect this state are imperative for toxicity investigations. In this study, the role of important factors such as solution ionic strength, pH, and particle surface chemistry that control nanoparticle dispersion was examined. The size and zeta potential of four TiO₂ and three quantum dot samples dispersed in different solutions (including one physiological medium) were characterized. For 15 nm TiO₂ dispersions, the increase of ionic strength from 0.001 M to 0.1 M led to a 50-fold increase in the hydrodynamic diameter, and the variation of pH resulted in significant change of particle surface charge and the hydrodynamic size. It was shown that both adsorbing multiply charged ions (e.g., pyrophosphate ions) onto the TiO₂ nanoparticle surface and coating quantum dot nanocrystals with polymers (e.g., polyethylene glycol) suppressed agglomeration and stabilized the dispersions. DLVO theory was used to qualitatively understand nanoparticle dispersion stability. A methodology using different ultrasonication techniques (bath and probe) was developed to distinguish agglomerates from aggregates (strong bonds), and to estimate the extent of particle agglomeration. Probe ultrasonication performed better than bath ultrasonication in dispersing TiO₂ agglomerates when the stabilizing agent sodium pyrophosphate was used. Commercially available Degussa P25 and in-house synthesized TiO₂ nanoparticles were used to demonstrate identification of aggregated and agglomerated samples.     

Characterization of Nano‐Particles Using Laser‐Induced Incandescence

Laser‐induced incandescence (LII) is introduced as a valuable tool for the characterization of nanoparticles. This optical measurement technique is based on the heating of the particles by a short laser pulse and the subsequent detection of the thermal radiation. It has been applied successfully for the investigation of soot in different fields of application, which is described here in the form of an overview with a focus on work done at the LTT‐Erlangen during the last 10 years. In laboratory flames the soot primary particle size, volume concentration, and relative aggregate size have been determined in combination with the number density of primary particles. Furthermore, the primary particle sizes of carbon blacks have been measured in situ and online under laboratory conditions and also in production reactors. Measurements with different types of commercially available carbon black powders, which were dispersed in a measurement chamber yielded a good correlation between LII results and the specified product properties. Particle diameters determined by LII in a furnace black reactor correlate very well with the CTAB‐absorption number, which is a measure for the specific surface area. It turned out that the LII method is not affected by variations of the aggregate structure of the investigated carbon blacks. The LII signal also contains information on the primary particle size distribution, which can be reconstructed by the evaluation of the signal decay time at, at least, two different time intervals. Additionally, soot mass concentrations have been determined inside diesel engines and online measurements were performed in the exhaust gas of such engines for various engine conditions simultaneously providing information about primary particle size, soot volume, and number concentration. The LII results exhibit good correlation with traditional measurement techniques, e.g., filter smoke number measurements. In addition to the soot measurements, primarily tests with other nanoparticles like TiO₂ or metal particles are encouraging regarding the applicability of the technique for the characterization of such different types of nanoparticles.     

Characterisation of silica nanoparticles prior to in vitro studies: from primary particles to agglomerates

The size, surface charge and agglomeration state of nanoparticles under physiological conditions are fundamental parameters to be determined prior to their application in toxicological studies. Although silica-based materials are among the most promising candidates for biomedical applications, more systematic studies concerning the characterisation before performing toxicological studies are necessary. This interest is based on the necessity to elucidate the mechanisms affecting its toxicity. We present here TEM, SAXS and SMPS as a combination of methods allowing an accurate determination of single nanoparticle sizes. For the commercial material, Ludox TM50 single particle sizes around 30 nm were found in solution. DLS measurements of single particles are rather affected by polydispersity and particles concentration but this technique is useful to monitor their agglomeration state. Here, the influence of nanoparticle concentration, ionic strength (IS), pH and bath sonication on the agglomeration behaviour of silica particles in solution has been systematically investigated. Moreover, the colloidal stability of silica particles in the presence of BSA has been investigated showing a correlation between silica and protein concentrations and the formation of agglomerates. Finally, the colloidal stability of silica particles in standard cell culture medium has been tested, concluding the necessity of surface modification in order to preserve silica as primary particles in the presence of serum. The results presented here have major implications on toxicity investigations because silica agglomeration will change the probability and uptake mechanisms and thereby may affect toxicity.     

Procedure for Predicting a Minimum Volume or Mass of Sample to Provide a Given Size Parameter Precision

Previous work to predict the minimum count or mass of particles to achieve a given repeatability of a size parameter has focussed on relatively narrow size distributions. Work by Masuda and Gotoh [1], based upon the log‐normal distribution by number, produce a prediction of the number of particles to be counted to achieve a defined accuracy of median volume mean MVD and mass median diameter MMD. The published paper limited the geometric standard deviations (s_g) to a maximum of 1.6. This equates to a size distribution covering just less than 1 decade of size. Modern laser diffraction units have optical arrangements enabling a range of particle sizes between 0.05 microns to 2000 microns to be determined. With the laser diffraction unit in mind the predictions of Masuda and Gotoh were extended to higher values of (s_g) where limitations were seen. An alternative prediction was then explored whose results compare very favourably with practical measurements of a characterised certified reference material CRM.     

Particles size effects of single domain CoFe2O4 on suspensions stability

Single domain magnetic CoFe₂O₄ nanoparticles with spinel structure were prepared by the coprecipitation method. Particles with size of 16, 20, 40 and 60 nm were synthesized by sintering the precursor at 500, 600, 800 and 900 °C, respectively. The magnetic hysteresis measurement of CoFe₂O₄ particles showed that particles were single domain particles with similar saturation magnetization (∼300 emu/cm³) at room temperature. The zeta potential study of suspensions (CoFe₂O₄-acetylacetone system) with various particle sizes showed the suspension systems had similar zeta potential values (∼40 mV). The effects of magnetic particle size on the suspension stability characterized by electrophoretic deposition yields and sediment volumes were studied. The suspension stability decreased with an increase in particle size and a flocculation threshold of particle radius a was found at 30 nm. A suspension stability theory approaching to the phenomenon was established. The theory based on the DLVO theory was developed by introducing an extra magnetic interaction force. Dormann model was adopted, in which the magnetic interactions of two spherical nanoparticles were investigated in terms of dipole-dipole interactions. Compared to DLVO, suspension's physical parameters not only zeta potential ζ and the Debye length 1/κ, but also particles' radius a brought about stable to flocculation transition in the theory.     

A Survey of Complementary Methods for the Characterization of Dense Colloidal Silica

The aim of this study is to enhance existing knowledge of different techniques developed for the characterization of stability and particle sizing of nanoparticles in dense dispersions subjected to interparticulate and hydrodynamic forces. Silica suspension, commercially known as Klebosol^® 30R50 and consisting of a particle size of 80 nm on average, was investigated in the study over a wide range of concentrations. The investigations were carried out using different optical and acoustic techniques such as laser diffraction, multiple light scattering, photon correlation spectroscopy and acoustic spectroscopy. The study details the capabilities and limitations of these modern techniques based on the different physical principles behind the characterization of the size distribution of particles in suspensions. The results are presented in terms of particle size ranges, solid concentration and technological aspects such as online and offline analysis. An important finding is that many of these modern techniques need to be improved for applications at higher concentrations since the standard models become practically invalid because of the complex interaction of acoustic and optical waves with particles in suspensions of silica.     

Influence of initial pH on the particle size and fluorescence properties of the nano scale Eu(III) doped yttria

Europium doped ytrrium oxide (Eu:Y₂O₃) was synthesized by a chemical wet method in the presence of tween-80 and ?-caprolactam in pH range 4-10. It has been observed that the variation in surface area, pore size, and pore volume of the final product, was strongly dependent on the initial pH of the solution. The powder with a large surface area (∼230 m²/g) and low pore diameter (∼16 nm) was obtained when the powder was processed at pH ∼4. The crystallite sizes of the powders processed at pH ∼4 and 10, were found to be 35 and 198 nm, respectively. The structural, chemical and thermal studies of the powders were characterized by X-ray diffraction (XRD), Fourier transformed infrared spectrophotometer (FTIR), Carbon analyzer and Thermogravimetry (TGA). High resolution transmission electron microscopic (HRTEM) study of heat treated powders showed a polygonal morphology with particle size of 40 nm when powder was derived at pH ∼4. Observations of fluorescence suggested that the ⁵D₀→⁷F₂ transition within europium was found to be highly dependent on the initial pH.     

Characterization of palmitic and lauric acid aerosols from rapid expansion of supercritical CO2 solutions

Palmitic acid aerosols and lauric acid aerosols were generated by rapid expansion of supercritical CO₂ solutions. The particle properties were analysed by rapid-scan infrared spectroscopy in situ, by X-ray powder diffraction, with a scanning mobility particle sizer, and by scanning electron microscopy. Particles with irregular elongated shapes were found. Most particles and agglomerates have sizes between 250 and 750 nm. Fewer agglomerates with sizes up to several microns are observed. Our investigation reveals that strong agglomeration takes place at the Mach disc. Palmitic and lauric acid particles are both crystalline and most particles crystallize in the C-form.     

Sintering and electrical conductivity of gadolinia-doped ceria

Bulk specimens of Ce_0.9Gd_0.1O_2-δ prepared with powders within a range of specific surface area were sintered in oxidizing, inert, and reducing atmospheres. The aim of this work is to investigate the effects of the sintering atmosphere on the microstructure and grain and grain boundary conductivities of the solid electrolyte. The lattice parameter determined by Rietveld refinement is 0.5420(1) nm, and the microstrain was found negligible in the powder materials. Specimens sintered in the Ar/4 % H₂ mixture display larger average grain sizes independent on the particle size of the starting powders. The grain and grain boundary conductivities of specimens sintered under reducing atmosphere are remarkably lower than those sintered under oxidizing and inert atmospheres. The activation energy (～0.90 eV) for total electrical conductivity remains unchanged with both the initial particle size and the sintering atmosphere.     

Synthesis and characterization of nano silver ferrite composite

We report the synthesis of nano sized silver ferrite composite having the empirical formula AgFeO₂ by a co-precipitation method. The resulting powders are thin platelets, transparent and a rich ruby red in color in transmission. The X-ray diffraction (XRD) powder data consisted of only nine reflections, and the analysis showed the unit cell to be rhombohedral. The powders showed extensive XRD line broadening and the sizes of the crystals are calculated to be in the range 4-36.5 nm. The morphology of the silver ferrite composite studied using scanning electron microscope showed nano sized particles. The particle size is found to increase with increase in annealing temperature. The magnetic behavior, measured using a vibrating sample magnetometer, indicated a change from paramagnetic to ferromagnetic with increase in particle size.     

Optimized method of dispersion of titanium dioxide nanoparticles for evaluation of safety aspects in cosmetics

Nanoparticles agglomerate when in contact with biological solutions, depending on the solutions’ nature. The agglomeration state will directly influence cellular response, since free nanoparticles are prone to interact with cells and get absorbed into them. In sunscreens, titanium dioxide nanoparticles (TiO₂-NPs) form mainly aggregates between 30 and 150 nm. Until now, no toxicological study with skin cells has reached this range of size distribution. Therefore, in order to reliably evaluate their safety, it is essential to prepare suspensions with reproducibility, irrespective of the biological solution used, representing the above particle size distribution range of NPs (30–150 nm) found on sunscreens. Thus, the aim of this study was to develop a unique protocol of TiO₂ dispersion, combining these features after dilution in different skin cell culture media, for in vitro tests. This new protocol was based on physicochemical characteristics of TiO₂, which led to the choice of the optimal pH condition for ultrasonication. The next step consisted of stabilization of protein capping with acidified bovine serum albumin, followed by an adjustment of pH to 7.0. At each step, the solutions were analyzed by dynamic light scattering and transmission electron microscopy. The final concentration of NPs was determined by inductively coupled plasma-optical emission spectroscopy. Finally, when diluted in dulbecco’s modified eagle medium, melanocytes growth medium, or keratinocytes growth medium, TiO₂–NPs displayed a highly reproducible size distribution, within the desired size range and without significant differences among the media. Together, these results demonstrate the consistency achieved by this new methodology and its suitability for in vitro tests involving skin cell cultures.     

The Effect of Temperature on the Particle Sizes and the Recrystallization of Silver Sulfide Nanopowders

The recrystallization of silver sulfide Ag₂S nanoparticles has been studied and the range of the thermal stability of the nanoparticle sizes has been determined. Nanopowders Ag₂S with particle sizes of 45–50 nm were obtained by chemical deposition from aqueous solutions. To study the thermal stability of the Ag₂S nanoparticle sizes, the nanocrystalline powders have been annealed in a vacuum of 0.01 Pa on heating from room temperature to 493 K and in argon at 623 K. Annealing up to a temperature of 453 K leads to insignificant nanoparticle growth and annealing of microstrains, which allows one to consider this temperature range as the region of thermal stability of the silver sulfide nanostate. The temperature range from 450 to 900 K, in which the particle size increases by a factor of 3–6, corresponds to the temperature of collective recrystallization of the silver sulfide nanopowder.     

Effect of Particle Size and Specific Surface Area on the Determination of the Density of Nanocrystalline Silver Sulfide Ag<Subscript>2</Subscript>S Powders

The effect of the particle size and specific surface area of silver sulfide powders on their density measured by the helium pycnometry method has been studied. Powders with different average particle sizes have been synthesized by hydrochemical deposition. The particle size of the silver sulfide powders has been determined by the Brunauer–Emmett–Teller method according to molecular nitrogen adsorption isotherms and, in the case of fine powders, by X-ray diffraction analysis according to diffraction reflection broadening. It has been shown that the density of the fine powders measured by helium pycnometry is underestimated compared with the true density, owing to the adsorption of helium by the highly developed surface of these powders.