Μm Silica Particles Research Articles

High‐throughput proteome analysis using 50 cm long micro pillar array columns (μPAC&[trade

The practice of bottom‐up proteomics relies to a large extent on the separation performance that can be achieved with state‐of‐the art nano LC‐MS/MS equipment. Depending on the sample complexity or the instrument time that can be dedicated to a certain sample, different LC columns and corresponding LC‐MS/MS methods are required. When aiming for comprehensive proteome analysis with deep coverage, relatively long columns (lengths up to 75 cm) are typically operated with long and shallow solvent gradients, delivering the highest chromatographic performance. However, daily routine proteome analysis often deals with much less complex samples or demands increased sample throughput, making total analysis times above 120 min undesirable or even impossible. The LC columns typically used for this type of analyses do not exceed 25 cm in length, with a focus on workflow robustness and efficiency rather than on maximizing chromatographic performance or peak capacity (1).In the past few years, micro pillar array column (μPAC™) technology has evolved from a tool to perform fundamental LC research to a powerful alternative for classical nano LC columns. The inherent high permeability and low ‘on‐column’ dispersion obtained by the perfect order of the separation bed makes pillar array based chromatography unique in its kind. The peak dispersion originating from heterogeneous flow paths in the separation bed is drastically reduced and therefore components remain much more concentrated during separation (2). Apart from an improved efficiency, pillar array columns can also be designed with substantially lower flow resistances compared to packed bed columns. The distance between the pillars can be independently controlled from the pillar size, enabling the fabrication of columns over a range of external porosities.Using a 50 cm long reversed phase C18 μPAC™ nano LC column in combination with a Thermo Orbitrap Elite mass spectrometer for detection, we demonstrate extremely robust and high‐throughput proteome analysis at capillary flow rates up to 2 μl/min. At these flow rates, sample throughput can be increased to 20, 30, 60 and even 100 samples per day with corresponding peak capacity values (nC) of respectively 300, 250, 200 and 150. When comparing the chromatographic performance that could be obtained for single protein and cell lysate tryptic digest samples to state‐of‐the art packed bed nano LC columns (packed with sub 2 μm silica particles), average peptide peak widths could be reduced by a factor of 1.6. For complex HeLa cell digest samples, this resulted in an increase in peptide and protein group identifications of respectively 60 and 40%.

Interplays between elastic particles in an ultrasonic standing wave

Separated particles are agglomerated only by the secondary radiation force when trapped at the pressure node. However, the mechanism behind this phenomenon remains unclear. Here, interplays among elastic particles in a trapping process induced by an ultrasonic standing wave are numerically and experimentally investigated. The effects of multi-scattering on particle movement are analyzed by solving the comprehensive acoustic-structure interactions. The theoretical calculations well explain the observed inter-particle behaviors in experiments when they are at or approaching the pressure nodes. Ultrasonic trapping of 100 μm silica particles is experimentally showcased, and the result agrees well with the prediction.

Influence of stainless steel specimen topography on micro-abrasion and micro-abrasion-corrosion

This work carried out a systematic analysis to help understanding how surface roughness and relative orientation between surface topography and direction of abrasive entrainment can affect particle dynamics during both pure microabrasion and microabrasion-corrosion. For that, tests were carried out in AISI 304 austenitic stainless steel test specimens using a recently developed microabrasion-corrosion tester. A 3D load cell ensures controlled application and measurement of forces during the tests. For the microabrasion tests, a slurry composed of small ( ϕ=2.5 µm) silica particles dispersed in distilled water at a concentration of 20wt% was employed, whereas for the microabrasion corrosion tests the distilled water in the slurry was substituted by a corrosive solutions in distilled water (1 N H2SO4). Potentiodynamic curves varying from cathodic potentials into the transpassive region were obtained within the course of duration of each microabrasion-corrosion test. The specimens were ground using two different SiC papers (#80 and #4000) to vary their surface topography and were tested with the grooves oriented perpendicular and parallel to the direction of rotation of the ball. Although the effect of surface topography was slight under pure microabrasion conditions, it was more significant under microabrasion-corrosion conditions. It was proposed that under microabrasion conditions, the formation of a low friction passive layer on the specimens made particle entrainment more difficult and therefore the relative orientation between surface topography and ball rotation had a stronger effect on the amount of abrasives that entered the contact, affecting wear rates. Grooves parallel to the rotation of the sphere increased wear rates under microabrasion-corrosion conditions when compared with grooves oriented perpendicular to the contact. Parallel grooves also resulted in higher passivation currents than perpendicular grooves, probably due to more severe damage to the passive layer caused by more abrasives into the contact

Fast enantioseparation of indole phytoalexins in additive free supercritical fluid chromatography

A series of chiral indole phytoalexins with potential anticancer and antimicrobial activity were enantioseparated in supercritical fluid chromatography. Two polysaccharide-based chiral stationary phases composed of tris-(3,5-dimethylphenylcarbamate) derivatives of amylose or cellulose coated on 2.5 μm silica particles were successfully used. The influences of the polysaccharide backbone, co-solvent type and co-solvent content in the mobile phase on retention, enantioselectivity and enantioresolution of indole phytoalexins were investigated. Fast baseline separations were achieved for 26 from 27 tested compounds. Amylose‐based chiral stationary phase provided higher number of baseline resolutions of the indole phytoalexins than the cellulose-based one. However, certain complementary enantioresolution results towards the studied compounds were observed between the investigated columns. The relationship between structure of the indole phytoalexins and their chromatographic behavior in supercritical fluid chromatography was discussed.

A new tracking algorithm for multiple colloidal particles close to contact

In this paper, we propose a new algorithm based on radial symmetry center method to track colloidal particles close to contact, where the optical images of the particles start to overlap in digital video microscopy. This overlapping effect is important to observe the pair interaction potential in colloidal studies and it appears as additional interaction in the measurement of the interaction with conventional tracking analysis. The proposed algorithm in this work is simple, fast and applicable for not only two particles but also three and more particles without any modification. The algorithm uses gradient vectors of the particle intensity distribution, which allows us to use a part of the symmetric intensity distribution in the calculation of the actual particle position. In this study, simulations are performed to see the performance of the proposed algorithm for two and three particles, where the simulation images are generated by using fitted curve to experimental particle image for different sized particles. As a result, the algorithm yields the maximum error smaller than 2 nm for μm silica particles in contact condition.

Tunable two-dimensional assembly of colloidal particles in rotating electric fields

Tunable interparticle interactions in colloidal suspensions are of great interest because of their fundamental and practical significance. In this paper we present a new experimental setup for self-assembly of colloidal particles in two-dimensional systems, where the interactions are controlled by external rotating electric fields. The maximal magnitude of the field in a suspension is 25 V/mm, the field homogeneity is better than 1% over the horizontal distance of 250 μm, and the rotation frequency is in the range of 40 Hz to 30 kHz. Based on numerical electrostatic calculations for the developed setup with eight planar electrodes, we found optimal experimental conditions and performed demonstration experiments with a suspension of 2.12 μm silica particles in water. Thanks to its technological flexibility, the setup is well suited for particle-resolved studies of fundamental generic phenomena occurring in classical liquids and solids, and therefore it should be of interest for a broad community of soft matter, photonics, and material science.

In-column bonded phase polymerization for improved packing uniformity.

It is difficult to pack chromatographic particles having polymeric-bonded phases because solvents used for making a stable slurry cause the polymer layer to swell. Growth of the polymer inside the column (in situ) after packing was investigated and compared with conventional, ex situ polymer growth. The method of activators generated by electron transfer, along with atom-transfer radical polymerization, enabled polymerization under ambient conditions. Nonporous, 0.62μm silica particles with silane initiators were used. Polyacrylamide films with a hydrated thickness of 23nm in 75:25 water/isopropanol grew in 55min for both in situ and ex situ preparations, and the same carbon coverage was observed. Higher chromatographic resolution and better column-to-column reproducibility were observed for in situ polymer growth, as evaluated by hydrophilic interaction liquid chromatography for the model glycoprotein, ribonuclease B. In situ polymer growth was also found to give lower eddy diffusion, as shown by a narrower peak width for injected acetonitrile in 50:50 acetonitrile/water. When columns were packed more loosely, bed collapse occurred quickly for ex situ, but not for in situ, polymer growth. The higher resolution and stability for in situ polymer growth is explained by packing with hard, rather than soft, contacts between particles.

Enantioselective potential of polysaccharide-based chiral stationary phases in supercritical fluid chromatography.

The enantioselective potential of two polysaccharide-based chiral stationary phases for analysis of chiral structurally diverse biologically active compounds was evaluated in supercritical fluid chromatography using a set of 52 analytes. The chiral selectors immobilized on 2.5μm silica particles were tris-(3,5-dimethylphenylcarmabate) derivatives of cellulose or amylose. The influence of the polysaccharide backbone, different organic modifiers, and different mobile phase additives on retention and enantioseparation was monitored. Conditions for fast baseline enantioseparation were found for the majority of the compounds. The success rate of baseline and partial enantioseparation with cellulose-based chiral stationary phase was 51.9% and 15.4%, respectively. Using amylose-based chiral stationary phase we obtained 76.9% of baseline enantioseparations and 9.6% of partial enantioseparations of the tested compounds. The best results on cellulose-based chiral stationary phase were achieved particularly with propane-2-ol and a mixture of isopropylamine and trifluoroacetic acid as organic modifier and additive to CO2 , respectively. Methanol and basic additive isopropylamine were preferred on amylose-based chiral stationary phase. The complementary enantioselectivity of the cellulose- and amylose-based chiral stationary phases allows separation of the majority of the tested structurally different compounds. Separation systems were found to be directly applicable for analyses of biologically active compounds of interest.

Magnetofluidic Tweezing of Nonmagnetic Colloids.

Magnetofluidic tweezing based on negative magnetophoresis and microfabricated core-shell magnetic microtips allows controlled on-demand assembly of colloids and microparticles into various static and dynamic structures such as colloidal crystals (as shown for 3.2 μm silica particles).

Development and Validation of HPLC and HPTLC Methods for Estimation of Glabridin in Extracts of Glycyrrhiza glabra

Glabridin is a major bioactive phytoconstituent of licorice. This work discusses the development and validation of HPLC and HPTLC methods for analysis of glabridin in licorice. The HPLC separation was performed using a Purospher STAR RP-18e column (5 μm silica particle size, 250 mm × 4.6 mm inner diameter) with gradient elution of 0.2% acetic acid in water-acetonitrile. The flow rate was 1 mL/min. Quantification was performed at a detection wavelength of 280 nm. HTPLC separation was performed on precoated silica gel 60 F254 aluminum plate (10 × 10 cm, 250 μm thickness). A linear ascending development was done using a mobile phase of hexane-ethyl acetate-chloroform (5 + 4 + 3, v/v/v). After development, the plates were scanned at 285 nm. Both of the methods provided good separation of glabridin from other constituents of licorice extract. The methods were validated as per ICH guidelines. Comparison by Student t-test showed that there was a statistically insignificant difference between the mean glabridin content estimated by both methods at 95% confidence interval. The glabridin content in licorice extract was 3.90% by HPLC and 3.79% by HPTLC.

Aqueous size-exclusion chromatographic separations of intact proteins under native conditions: Effect of pressure on selectivity and efficiency.

The selectivity and separation efficiency of aqueous size-exclusion chromatographic separations of intact proteins were assessed for different flow rates, using columns packed with 3 and 5 μm silica particles containing 150 and 290 Å stagnant pores. A mixture of intact proteins with molecular weights ranging between 17 000 and 670 000 Da was used to construct the calibration curves. Both the model fit and the predictive properties, using a leave-one-out strategy, of different polynomial models (up to fifth order) were evaluated for different flow rates. The best compromise between model fit and predictive properties was obtained using a third-order polynomial model. The accuracy of the predictive properties decreased with 10% with an eightfold increase in the flow rate. No changes in retention factors (hence selectivity) were observed in the flow-rate range applied. A strong correlation between molecular weight and plate height was observed. Exclusion of large-molecular-weight proteins led to a significant reduction in the stationary-phase mass-transfer contribution to the total plate-height value, and this effect was also independent of the flow rate applied. The kinetic-performance limits, in terms of plate number and time, and optimal column-length particle-size combinations were determined at the maximum recommended operating pressure of the size-exclusion chromatography columns (20 MPa). Finally, the possibilities of method speed-up using ultra-high-pressure size-exclusion chromatography in combination with columns packed with sub-2 μm particles are discussed.

Introduction of KRI’s sol–gel technologies with reference to the shape classification of the materials

KRI has been successful in conducting many sol–gel related projects. In this article, we review KRI’s sol–gel projects that fall into the following four categories: zero dimensions (particle applications), one dimension (fiber applications), two dimensions (coating applications), and three dimensions (bulk applications). (1) Zero dimensions: at KRI, we have developed a new process to not only synthesize large-sized (over 10 µm) silica particles by using a microreactor device, but also synthesize core–shell particles that eliminate the photocatalytic effect of TiO2. (2) One dimension: KRI has developed the electrospinning deposition method which is a process used to form nanofibers by discharging raw material liquid from a syringe under high voltage. We have successfully formed polysilsesquioxane (PSQ) nanofibers for application in Li batteries as separators, as well as metal oxide nanofibers such as TiO2 and ZnO. (3) Two dimensions: KRI has developed organic–inorganic hybrid hard coatings. Recently, we have focused on PSQ materials for hydrophobic coatings and hydrophilic coatings. (4) Three dimensions: we have successfully developed large-sized silica glasses as well as organic–inorganic hybrids as transparent materials for bulk applications.

Effects of different particle deposition parameters on adhesion of resin cement to zirconium dioxide and phase transformation

This study evaluated the effect of air-abrasion parameters such as particle size, distance, and time on adhesion of resin cement to zirconium dioxide (Y-TZP) and t→m phase transformation. Y-TZP blocks (N = 80) (In-Ceram YZ, Vita) (4 mm3 × 4 mm3 × 3 mm3) were assigned into eight groups (n = 10): air-abrasion with 30 μm (CoJet Sand, S30) and 110 μm (Rocatec-Plus, S110) silica-coated alumina particles, applied for either for 10–20 s (T = time), from a distance of 10–20 mm (D = distance), composing the following groups: S30T10D10, S30T10D20, S30T20D10, S30T20D20, S110T10D10, S110T10D20, S110T20D10, and S110T20D20. Resin composite (RelyX ARC) was bonded to Y-TZP blocks in polyethylene molds. The specimens were aged (10,000 thermal cycles and water storage for 90 days) prior to shear bond test. Failure types were analyzed under stereomicroscope and SEM, and phase transformation was calculated. Data (MPa) were analyzed using 3-way ANOVA and Tukey’s tests. Air-abrasion with 110 μm silica particles (10.96) presented significantly higher bond strength (p = 0.0149) compared to 30 μm (8.96). Time (p = 0.403) and distance (p = 0.179) parameters did not affect the results significantly. Air-abrasion with 110 μm particles (12.3) promoted higher bond strength than that of 30 μm (6.4) when applied for 10 s from a distance of 10 mm (Tukey’s). Failure types were predominantly adhesive. Phase transformation ranged between 30.3 and 35.9% for 30 μm particles and 23.8–43.7% for 110 μm particles. While the size of silica-coated alumina particles were more relevant parameter for resin cement adhesion to Y-TZP, time (up to 20 s) and distance (up to 20 mm) appear to be less pertinent.

Mesoporus Silicon through Magnesium Reduction of Polymer Templated Silica for High Power Li-Ion Batteries

This work describes the synthesis and electrochemical characterization of mesoporous silicon for high power Li-ion battery applications. Silicon is an attractive Li-ion battery anode material for its high energy density, high elemental abundance and non-toxicity. However, full lithiation of silicon is accompanied by a 300% volume expansion which pulverizes the particles leading to rapid material failure. We have addressed these issues with silicon by developing a highly tunable synthesis method to produce mesoporous silicon from ordered mesoporous silica. This synthesis method leads to micron sized grains consisting of a 3-D interconnected silicon network that should accommodate the electrochemical induced volume expansion while the nanocystalline pore walls provide short ion diffusion lengths for high rate applications. The ordered mesoporous silica starting material is synthesized by evaporation induced self-assembly of sol-gel type precursors and a di-block copolymer. The mesoporous silicon is produced through reduction of the silica precursor with magnesium metal at 650 °C followed by selective chemical etching. The particle morphology and the pore size of both the silica and the silicon can be tuned over a broad range, and have been characterized by small angle scattering, high angle X-ray diffraction, nitrogen porosimetry, and SEM imaging. This synthesis technique produces 1-10 µm silica particles with a cubic lattice of 15 – 20 nm pores and 10 - 15 nm thick pore walls. After magnesium reduction, the porous polycrystalline silicon retains the microscale morphology and the porous nanostructure of the parent material. The silicon pore walls consist of ~30 nm crystallites which are optimal for high power density electrochemical applications Magnesium reduction of silica is an effective method to reduce silica while preserving the nanostructured architecture, but little is understood about reaction mechanism. In an effort to control the nanoscale architecture of silicon we have investigated the reaction mechanism. We have performed in-situ X-ray diffraction during the reduction process combined with Raman spectroscopy to reveal a multi-step reaction pathway which leads to the mesoporous crystalline silicon final product. This study demonstrates that the reaction of mesoporous silica and magnesium metal first forms amorphous silicon that further reacts to form crystalline magnesium silicide. The remaining mesoporous silica is reduced by both the magnesium metal and the magnesium silicide to produce mesoporous crystalline silicon. The electrochemical properties of mesoporous silicon have been characterized by galvanostatic cycling and cyclic voltammetry. Galvanostatic measurements show capacities on the order of 1700 mAh/g while the nanocrystalline silicon pore walls and interconnected 3-D framework led to an extremely high rate capability. Over 40% of the capacity, or 720 mAh/g, can be accessed in 150 seconds. In light of the high capacity and highest reported power density for mesoporous silicon, this material is an exciting candidate for the next generation of high performance Li-ion batteries. Figure 1

Toward enantioselective nano ultrahigh-performance liquid chromatography with Whelk-O1 chiral stationary phase.

In this study, a Whelk-O1 chiral stationary phase immobilized on 2.5μm silica particles was employed in nanoLC. Two nanocolumns (180 and 250mm long, 75μm id) with a single polymeric organic monolithic outlet frit were packed under high-pressure ultrasonic-assisted packing procedure. The monolithic outlet frit was prepared by thermal polymerization of methacrylate-based monomers affording high-mechanical stability and high-pressure resistance. Very efficient enantioseparations with more than 70000 plates/m were achieved in normal phase mode by eluting (+/-) acenaphthenol. Nanocolumns were also tested in RP mode by using on-line MS detection with nano-spray ESI ion source. Kinetic performances of columns in RP mode were comparable to those in normal phase-conditions.

Surface modification of silica particles with gold nanoparticles as an augmentation of gold nanoparticle mediated laser perforation.

Gold nanoparticle mediated (GNOME) laser transfection/perforation fulfills the demands of a reliable transfection technique. It provides efficient delivery and has a negligible impact on cell viability. Furthermore, it reaches high-throughput applicability. However, currently only large gold particles (> 80 nm) allow successful GNOME laser perforation, probably due to insufficient sedimentation of smaller gold nanoparticles. The objective of this study is to determine whether this aspect can be addressed by a modification of silica particles with gold nanoparticles. Throughout the analysis, we show that after the attachment of gold nanoparticles to silica particles, comparable or better efficiencies to GNOME laser perforation are reached. In combination with 1 µm silica particles, we report laser perforation with gold nanoparticles with sizes down to 4 nm. Therefore, our investigations have great importance for the future research in and the fields of laser transfection combined with plasmonics.

Mesoporus Silicon through Magnesium Reduction of Polymer Templated Silica for High Power Li-Ion Batteries

This work describes the synthesis and electrochemical characterization of mesoporous silicon for high power Li-ion battery applications. Silicon is an attractive Li-ion battery anode material for its high energy density, high elemental abundance and non-toxicity. However, full lithiation of silicon is accompanied by a 300% volume expansion which pulverizes the particles leading to rapid material failure. We have addressed these issues with silicon by developing a highly tunable synthesis method to produce mesoporous silicon from ordered mesoporous silica. This synthesis method leads to micron sized grains consisting of a 3-D interconnected silicon network that should accommodate the electrochemical induced volume expansion while the nanocystalline pore walls provide short ion diffusion lengths for high rate applications.The ordered mesoporous silica starting material is synthesized by evaporation induced self-assembly of sol-gel type precursors and a di-block copolymer. The mesoporous silicon is produced through reduction of the silica precursor with magnesium metal at 650 °C followed by selective chemical etching. The particle morphology and the pore size of both the silica and the silicon can be tuned over a broad range, and have been characterized by small angle scattering, high angle X-ray diffraction, nitrogen porosimetry, and SEM imaging. This synthesis technique produces 1-10 µm silica particles with a cubic lattice of 15 – 20 nm pores and 10 - 15 nm thick pore walls. After magnesium reduction, the porous polycrystalline silicon retains the microscale morphology and the porous nanostructure of the parent material. The silicon pore walls consist of ~30 nm crystallites which are optimal for high power density electrochemical applications.Magnesium reduction of silica is an effective method to reduce silica while preserving the nanostructured architecture, but little is understood about reaction mechanism. In an effort to control the nanoscale architecture of silicon we have investigated the reaction mechanism. We have performed in-situ X-ray diffraction during the reduction process combined with Raman spectroscopy to reveal a multi-step reaction pathway which leads to the mesoporous crystalline silicon final product. This study demonstrates that the reaction of mesoporous silica and magnesium metal first forms amorphous silicon that further reacts to form crystalline magnesium silicide. The remaining mesoporous silica is reduced by both the magnesium metal and the magnesium silicide to produce mesoporous crystalline silicon.The electrochemical properties of mesoporous silicon have been characterized by galvanostatic cycling and cyclic voltammetry. Galvanostatic measurements show capacities on the order of 1700 mAh/g while the nanocrystalline silicon pore walls and interconnected 3-D framework led to an extremely high rate capability. Over 40% of the capacity, or 720 mAh/g, can be accessed in 150 seconds. In light of the high capacity and highest reported power density for mesoporous silicon, this material is an exciting candidate for the next generation of high performance Li-ion batteries.

Impact of an oil coating on particle deposition and dust holding capacity of fibrous filters

A light coating of oil is known to increase the dust holding capacity of fibrous filter media. The underlying causes for this effect were investigated by performing filtration experiments with arrays of nylon and stainless steel fibers (diameters of 20, 30, and 44μm) coated with precisely defined amounts of oil. The single fiber efficiency in the inertial regime (Stokes numbers >0.5) was measured as a function of dust load, using 3.5μm polystyrene and 2.1μm silica particles in combination with various types of oils (0W-30, WD-40). Additionally, the influence of an oil film on the growth of particle deposit structures was investigated by optical microscopy.It was found that dust deposition on oily fibers occurs in two distinct stages: at first particles are immersed in the oil film without any appreciable increase in fiber efficiency. Once the film is saturated with particles, further deposition leads to quasi-normal dendritic growth typical of dust deposition on dry fibers, and a sharp increase in single fiber efficiency. The maximum packing density inside the film which is reached at the transition from particle immersion to regular growth, was approximately 46% by volume, regardless of film thickness. There were no indications of flow-induced particle rearrangement inside the film during the first stage.Comparative measurements were also made with standard paper media containing varying quantities of oil. The oil caused an increase in dust holding capacity by factors between about 1.5 and 3 compared to dry media, due to a delayed upswing of the filter pressure drop with dust loading. The penetration of dust through the media more than doubled.We conclude that particle immersion in the oil film is responsible for both the delayed increase in fiber efficiency and fiber drag, and that this delay is roughly proportional to the amount of oil loading.

One‐Pot Preparation of Conducting Polymer‐Coated Silica Particles: Model Highly Absorbing Aerosols

Near‐monodisperse 0.50 μm and 1.0 μm silica particles are surface‐modified using 3‐(trimethoxysilyl)propyl methacrylate (MPS) and subsequently coated by aqueous deposition of an ultrathin polypyrrole (PPy) overlayer to produce PPy‐coated silica particles. The targeted degree of MPS modification and PPy mass loading are systematically varied to optimize the colloidal stability and PPy coating uniformity. MPS surface modification is characterized by contact angle goniometry and the PPy overlayer uniformity is assessed by scanning electron microscopy. HF etching of the silica cores produces hollow PPy shells, thus confirming the contiguous nature of the PPy overlayer and the core–shell morphology of the original particles. Four‐point probe measurements and XPS studies indicate that the electrical conductivity of pressed pellets of PPy‐coated silica particles increases with PPy surface coverage. Colloidal stabilities of the bare, MPS‐modified, and PPy‐coated silica particles in aqueous solution are assessed using disk centrifuge photosedimentometry. MPS surface modification results in weak flocculation, with subsequent PPy deposition causing further aggregation. In contrast, white light aerosol spectrometry indicates a relatively high degree of dispersion for PPy‐coated silica particles in the gas phase. Such PPy‐coated silica particles are expected to be useful mimics for silica‐rich micrometeorites and may also serve as a model highly absorbing aerosol.

Limiting diffusion current at rotating disk electrode with dense particle layer

Exploiting the concept of diffusion permeability of multilayer gel membrane and porous multilayer we have derived a simple analytical equation for the limiting diffusion current at rotating disk electrode (RDE) covered by a thin layer with variable tortuosity and porosity, under the assumption of negligible convection in the porous film. The variation of limiting diffusion current with the porosity and tortuosity of the film can be described in terms of the equivalent thickness of stagnant solution layer, i.e., the average ratio of squared tortuosity to porosity. In case of monolayer of monodisperse spherical particles, the equivalent layer thickness is an algebraic function of the surface coverage. Thus, by means of cyclic voltammetry of RDE with a deposited particle monolayer we can determine the monolayer surface coverage. The effect of particle layer adsorbed on the surface of RDE increases non-linearly with surface coverage. We have tested our theoretical results experimentally by means of cyclic voltammetry measurements of limiting diffusion current at the glassy carbon RDE covered with a monolayer of 3 μm silica particles. The theoretical and experimental results are in a good agreement at the surface coverage higher than 0.7. This result suggests that convection in a monolayer of 3 μm monodisperse spherical particles is negligibly small, in the context of the coverage determination, in the range of very dense particle layers.