Role Of Particle Size Research Articles

Grazing by the Copepod Parvocalanus crassirostris on Picochlorum sp. at Harmful Bloom Densities and the Role of Particle Size

Harmful blooms of nanophytoplankton (2–20 μm) are increasingly common and sometimes severe, but requirements and controls of such blooms (e.g., water quality constraints, requirements for nutrients, and the presence of different guilds of grazers) are poorly understood. Laboratory grazing experiments were conducted to evaluate the potential for top-down control by the copepod Parvocalanus crassirostris on a small harmful algal species (an unidentified chlorophyte Picochlorum, 1–4 μm) and to test the effects of cell sizes on grazing rates. The Picochlorum sp. is a strain isolated from a long-lasting harmful algal bloom in the Indian River Lagoon that reached high densities (>1 × 106 cells ml–1). Experiments contrasted grazing on Picochlorum sp. with grazing on the palatable prymnesiophyte Isochrysis galbana (4–6 μm) in monocultures and mixed cultures. When presented in monocultures, grazing rates on Picochlorum sp. were lower than grazing rates on the palatable alternative. When Picochlorum sp. were presented alongside I. galbana, copepods essentially ceased feeding on the former. In additional experiments, P. crassirostris were fed plastic beads with diameters of 2.0–17.9 μm to control for differences in taste, toxicity, production of mucilage and shape of potential food. Paracalanus crassirostris fed most efficiently on beads with diameters of 7.0–7.9 μm. Results revealed that P. crassirostris can consume Picochlorum sp., but small size and the presence of palatable cells reduces the likelihood of top-down control of blooms of Picochlorum sp.

Engineering atorvastatin loaded Mg-Mn/LDH nanoparticles and their composite with PLGA for bone tissue applications

The impact of mixing method in conventional co-precipitation synthesis of layered double hydroxides (LDHs), on particle size, size distribution and drug loading capacity is reported. Synthesis of Mg (II)/Mn (III)-LDH nano-platelets was performed at constant pH using three different mixing systems, magnetic stirrer, mechanical mixer, and homogenizer at ambient temperature and a fixed Mg/Mn ratio of 3/1. The LDH characterization results showed that mechanical mixing and homogenization lead to production of very fine LDH nano-platelets (about 90–140 nm), with narrow particle size distribution. Amount of the intercalated drug was determined as about 60% and showed a significant increase in loading capacity of the LDH through homogenization and mechanical mixing compared to that of the magnetic stirring (about 35%). Our results also showed that in LDH preparation via co-precipitation, the mixing system plays a more influential role in particle size, size distribution, and drug loading control, than the mixing speed of each system. Drug loaded-LDH/PLGA composites were prepared via electrospinning to afford a bioactive/osteoinductive scaffold. A remarkable degree of cell viability on the scaffolds (drug-loaded-LDH/PLGA composite) was confirmed using MTT assay. Osteogenic differentiation of human ADMSCs, as shown by alkaline phosphatase activity and Alizarin Red staining assays, indicated that the scaffold with 5% drug loaded LDH(Mn-Mg-LDH/PLGA/AT5%) induced a remarkably higher level of the markers compared to the PLGA scaffold and therefore, it could be a valuable candidate for bone tissue engineering applications.

Polymer mediated synthesis of cationic silver nanoparticles as an effective anti-fungal and anti-biofilm agent against Candida species

“Superbugs”, multidrug-resistant microorganisms, have become a significant threat to the global healthcare sector over the past few decades. Among other diseases, invasive fungal infections caused by the Candida species are the most common cause of nosocomial infections with antifungal resistance. This work has employed a facile aqueous phase process to synthesize a stable, cationic silver nanoparticle using polyethyleneimine (PEI) as both a reducing and stabilizing agent. We determined that the polymer ratio plays a vital role in particle size and charge. As a result, the efficacy of antimicrobial activity was inversely proportional to the particle size from the disc diffusion and microdilution method. Furthermore, the smallest nanoparticles were shown to inhibit biofilm formation of Candida on silicon catheters and proved to be an efficient and potential anticandidal agent against the standard and clinical isolates of Candida. Therefore, the current investigation paves a new avenue for the green fabrication of nano‑silver based anticandidal agents.

Computational investigation of the plasmonic properties of TiN, ZrN, and HfN nanoparticles: the role of particle size, medium, and surface oxidation

Group 4 transition metal nitride (TMN) nanoparticles (NPs) display strong plasmonic responses in the visible and near-infrared regimes, exhibit high melting points and significant chemical stability, and thus are potential earth-abundant alternatives to Au and Ag based plasmonic applications. However, a detailed understanding of the relationship between TMN NP physical properties and plasmonic response is required to maximize their utility. In this study, the localized surface plasmon resonance (LSPR) frequency, bandwidth, and extinction of titanium nitride (TiN), zirconium nitride (ZrN), and hafnium nitride (HfN) NPs were examined as a function of the particle size, surface oxidation, and refractive index of the surrounding medium using finite element method (FEM). A linear redshift in the LSPR frequency and a linear increase in the associated full width at half maximum (FWHM) was observed with increasing the particle size, oxidation layer thickness, and medium refractive index. We show that the effect of surface oxidation on plasmonic properties of TMN NPs is strongly size-dependent with a significant LSPR redshift, intensity reduction, and broadening in small NPs compared with larger NPs. Furthermore, the performance and efficiency of HfN, ZrN, and TiN, as well as Au NPs for narrowband (photothermal therapy, PTT) and broadband (solar energy conversion) applications, was investigated in detail. The results indicate that narrowband and broadband photothermal performance of NPs strongly depend on the particle size, surface properties, and in case of narrowband absorption, excitation wavelength.

Size Effects of Brittle Particles in Aerosol Deposition\u2014Molecular Dynamics Simulation

Up to now, the role of particle sizes on the impact behavior of ceramic particles in aerosol deposition not yet fully understood. Hence, with the aim to supply a more general understanding, modeling series of low strain rate compression and high-speed impact were performed by molecular dynamics on single-crystalline particles in sizes of 10-300 nm that are tuned to match mechanical properties of TiO2-anatase. The modeling results reveal that particles with original diameter of 25-75 nm exhibit three different impact behaviors that could be distinguished as (i) rebounding, (ii) bonding and (iii) fragmentation, depending on their initial impact velocity. In contrast, particles larger than 75 nm do not exhibit the bonding behavior. Detailed stress and strain field distributions reveal that combination of “localized inelastic deformation” along the slip systems and “shear localization” cause bonding of the small and large particles to the substrate. The analyses of associated temperature rise by the inelastic deformation revealed that heat diffusion at these small scales depend on size. Whereas small particles could reach a rather homogeneous temperature distribution, the evolved heat in the larger ones keeps rather localized to areas of highest deformation and may support deformation and the formation of dense layers in aerosol deposition.

Self-Assembly of Polymer Grafted Nanoparticles within Spherically Confined Diblock Copolymers.

Geometric confinement plays an important role in the generation of interesting microstructures on account of structural frustration and confinement-induced entropy loss. In the present study, self-consistent field calculations have been performed to examine the self-assembly behavior of a mixture of diblock copolymers and polymer grafted nanoparticles within a spherical confinement. The analysis is aimed at obtaining the equilibrium distribution of nanoparticles with a high degree of order. The ordered mesophases of diblock copolymers provide useful templates to achieve ordering of nanoparticles in a selective domain. Self-assembly of nanoparticles within frustrated diblock copolymers is found to be very different from the bulk. A rich variety of equilibrium morphologies are observed depending on the degree of confinement and the extent of particle loading. In addition, the role of particle size and selectivity along with the length and the number of polymer chains grafted onto the surface of nanoparticles are analyzed to understand the self-assembly behavior. The specific interest is to obtain the chiral structures out of achiral block copolymers subjected to spherical confinement. The realization of various captivating microstructures, such as chiral ordering of nanoparticles, is highly essential to produce advanced nanomaterials with superior physical properties.

Tensile and fracture properties of epoxy alumina composite: role of particle size and morphology

Epoxy composites reinforced with three different shapes of Al2O3 nanoparticles were synthesized by instant cross-linking process. Out of three types of Al2O3 particles, two were of rod shape particles termed as large nano rod (diameter 2–4 nm and length 100–200 nm), and small nano rod (diameter of 5–10 nm and length less than 50 nm) and third particle was spherical in shape (diameter of 27–43 nm). 0.5, 1.0, and 1.5 wt.%. of reinforcement was maintained constant. Reinforcement of low (1.0) wt.% of spherical Al2O3 nanoparticles is more promising in enhancing tensile strength and failure strain of epoxy composite. At 1.0 wt.% reinforcement of spherical Al2O3 nanoparticles, tensile strength and failure strain are improved by 10% and 29%, respectively. Quasi-static fracture toughness of epoxy composite was improved by reinforcement of all three types of Al2O3 nanoparticles. Fracture toughness of epoxy composite was enhanced by 52% at 1.5 wt.% inclusion of large Al2O3 nano rod. Crack deflection and crack bridging were the micro-mechanics accountable for improved fracture toughness of spherical Al2O3 composite. Crack tilting and twisting, particle pull out, along with crack deflection were the main contributors for increased fracture toughness of composites of Al2O3 nano rods.

Identification of key aerosol types and mixing states in the central Indian Himalayas during the GVAX campaign: the role of particle size in aerosol classification

Studies in aerosol properties, types and sources in the Himalayas are important for atmospheric and climatic issues due to high aerosol loading in the neighboring plains. This study uses in situ measurements of aerosol optical and microphysical properties obtained during the Ganges Valley Aerosol eXperiment (GVAX) at Nainital, India over the period June 2011–March 2012, aiming to identify key aerosol types and mixing states for two particle sizes (PM1 and PM10). Using a classification matrix based on SAE vs. AAE thresholds (scattering vs. absorption Ångström exponents, respectively), seven aerosol types are identified, which are highly dependent on particle size. An aerosol type named “large/BC mix” dominates in both PM1 (45.4%) and PM10 (46.9%) mass, characterized by aged BC mixed with other aerosols, indicating a wide range of particle sizes and mixing states. Small particles with low spectral dependence of the absorption (AAE < 1) account for 31.6% and BC-dominated aerosols for 14.8% in PM1, while in PM10, a large fraction (39%) corresponds to “large/low-absorbing” aerosols and only 3.9% is characterized as “BC-dominated”. The remaining types consist of mixtures of dust and local emissions from biofuel burning and display very small fractions. The main optical properties e.g. spectral scattering, absorption, single scattering albedo, activation ratio, as well as seasonality and dependence on wind speed and direction of identified types are examined, revealing a large influence of air masses originating from the Indo-Gangetic Plains. This indicates that aerosols over the central Himalayas are mostly composed by mixtures of processed and transported polluted plumes from the plains. This is the first study that identifies key aerosol populations in the central Indian Himalayas based on in situ measurements and the results are highly important for aerosol-type inventories, chemical transport models and reducing the uncertainty in aerosol radiative forcing over the third pole.

Ceramic oxidation protection coatings for refractory alloys from filler-loaded preceramic polymers: The role of particle size and volume fraction of particulate fillers

Oxidation protection coatings on the base of perhydridopolysilazane (PHPS) polymer and Si/B filler materials were developed to prevent Mo-basis alloys against oxidation at 800 °C. Within this work the influence of the particle size and the volume fraction of particulate fillers on the oxidation protective effect of the coatings was investigated. Double and triple coatings were applied in order to determine the effect of layer thickness on the oxidation resistance properties.

How JRK’s 777 oil demolishes the dense, viscous herbal rich oils can be ineffective for psoriasis

The present study deals with the importance of herbal particle size in the AYUSH oil preparations for psoriasis in order to achieve greater absorption and therapeutic value. More often the viscosity of the oil, deep colour and the density of herbal constituents are associated with superior therapeutic value. But our study has clearly demonstrated that the dermal absorbable herbal constituents (under 40 µm) was less than 10% in most of the Wrightia tinctoria based AYUSH oil preparations sold for the treatment of psoriasis in India whereas Dr. JRK’s 777 oil contain more than 60% of the herbal constituents below 40 µm size. Further the role of particle size besides dermal absorption; in the therapeutic value also established by oxygen species scavenging assay. Findings reveal that smaller the particles greater was the antioxidant effect and hence such herbal constituents not only would offer superior therapeutic effect but also protection to the oil from going rancid. Details are presented in the paper. &#x0D; Keywords: AYUSH oils, Herbal oil, Psoriasis, dermal absorption, Wrightia tinctoria, rancid oil

The role of particle size and molar ratio of Ca2+/Na+ in the retention and remobilization of colloids in saturated porous media

The paper examined the effects of colloid size and molar ratio of Ca2+/Na+ on the retention and release of colloids under transients in ionic strength (IS). Column experiments were conducted to elucidate the transport behavior of three-sized polystyrene latex microspheres (300 nm, 500 nm, and 1000 nm) by varied molar ratios of Ca2+/Na+. Colloid concentration breakthrough curves showed the retention of colloids enhanced with the increase of molar ratio and the decrease of colloid size, which can be explained by Derjaguin–Landau–Verwey–Overbeek theory and the existence of nanoscale surface roughness. When the IS of the eluting solution was reduced from 10 to 1 mM, and when the eluting solution changed to ultrapure water, peak concentrations of colloid release were observed. Colloid recovery rates during the injection of ultrapure water were low, and substantial proportion of colloid still retained in the column even after the injection of ultrapure water, which suggested a stronger influence caused by the primary minimum compared with the secondary minimum on the retention of colloid. A modified numerical model considering the processes of Langmuirian dynamics blocking associated with a conservative tracer equation was successfully developed to simulate the transport process of all the experiments.

Radiodensity of Various Dental Biomaterials for Endodontics: The Role of Particle Size

Introduction: The thickness threshold for detecting endodontic biomaterials depends on many factors, such as the nature of the radiopacifier and the particle size. Aim: The aim of this study was to determine the effect of thickness on radiodensity of various endodontic biomaterials; and evaluate the impact of radiopacifier particle size on radiodensity. Materials and Methods: This in-vitro study was conducted between August 2018 to December 2019. The study was divided in two parts, in first part, Six endodontic biomaterials (AH26, EndoSequence, Endoseal Mineral Trioxide Aggregate (MTA), Nano-MTA, Endocem Zr, and MTA without radiopacifier) were selected and evaluated in different thicknesses, in second part, MTA mixed with Bismuth oxide 10 μm, 200 μm, 120 nm (Groups 1-3), and Zirconium oxide 5 μm, 1 μm and 20 nm (Groups 4-6) were placed in frames with 1 mm, 0.5 mm, 0.2 mm, 0.1 mm thicknesses to evaluate the radiopacity. Results: The mean radiodensity was significantly different among various thickness (p&lt;0.001) and materials (p&lt;0.001). The changes of the radiodensity in various thickness from one material to the other were not uniform (interaction p-value &lt;0.001). A 1 mm thickness had highest radiodensity (206.6±83.99), followed by 0.5 mm (68.9±24.6), 0.2 mm (17.9±4.9), and 0.1 mm thick material had least radiodensity (11.97±4.37). Materials of AH26 (99.1±103.2), Nano MTA (97.4±104.9), Endoseal MTA (87.86±101.4), Endosequence BC sealer (85.5±93.87) and Endocem Zr (71.88±77.67) were significantly different from the control group (16.38±10.85). The size of particles played important role in radiodensity (p&lt;0.001). The radiodensity of Fine GIII (100 nm) material (112.68±108.47) was significantly higher than other materials: Thin GII (200 nm) (100.9±102.4), Fine GVI (20-40 nm) (99.7±95.1), Coarse GI (10 μm) (76.66±74.75), Thin GV (1~3 μm) (63.19±67.3), Coarse GIV (5 μm) (49.66±51.59) and MTA without Radiopaque Agent GVII (100%) (23.67±19.68). The effect of the thickness on radiodensity was different for each biomaterial, with significant differences from the control group. Conclusion: One of the readily available methods for increasing radiodensity is to increase the amount of radiopacifier, which might compromise the physical properties of the material. Fine particle radiopacifier (120 nm) with 1 mm thickness has significantly higher radiodensity than any other biomaterials in this study. Within the limitations of the current study, it can be concluded that the radiopacifier particle size has a significant impact on the level of radiodensity of dental biomaterials. Finding the optimum distribution, size, and geometry of radiopacifier particles within the same fraction rate can enhance the radiodensity.

Role of particle size and surface functionalisation on the flexibility behaviour of switchable metal–organic framework DUT-8(Ni)

Functionalising the external surface of flexible MOF nanoparticles allows manipulating the gate opening/closing pressure.

Mechanisms that control the adsorption-desorption behavior of phosphate on magnetite nanoparticles: the role of particle size and surface chemistry characteristics.

Eutrophication caused by excessive phosphate discharge into surface water has raised wide concern, and the efficient removal of phosphates from wastewater using sorption methods is very important. In our study, magnetite particles with two different sizes and different surface characteristics were chosen as the sorbents to examine their adsorption and desorption behavior toward phosphate. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and N2 adsorption–desorption methods were used to characterize the morphological and surface chemical properties of the two differently sized magnetite particles. Adsorption kinetics and isotherm models (including the pseudo-first-order, Freundlich, Langmuir and Temkin models) were used to fit the experimental data, and to help with the mechanistic discussions. It was found that the nanometer-sized magnetite (nFe3O4) has a much higher surface area, larger pore volume, higher amounts of surface functional groups, and a lower point of zero charge (pHPZC) value than the micrometer-sized magnetite (Fe3O4). The adsorption kinetics show that reaching adsorption equilibrium in the case of nFe3O4 is much slower, and the particle size or surface characteristics of the magnetite may become the main factor determining the adsorption rate of the phosphate to magnetite in the rapid or slow adsorption step, respectively. nFe3O4 shows much stronger adsorption of phosphate compared to Fe3O4, which may be attributed to the larger surface area of the magnetite with a smaller particle size. In addition, the amount of functional groups and the surface electrical properties may also affect the adsorption of phosphate to magnetite by influencing the formation of the outer-sphere and/or inner-sphere complexes. The adsorption/desorption of phosphate to/from the magnetite decreases/increases with increasing pH, and the extent of change is more marked for nFe3O4. Increasing the ionic strength of the solution increases the adsorption of phosphate to the two differently sized magnetite particles, whereas the presence of humic acid only increases the adsorption of phosphate to Fe3O4. These trends may be caused by the different extents of change of the surface properties or the dispersion state of the two differently sized magnetite particles under different solution chemistry conditions. The results imply that when the synthesis of magnetite-based materials for phosphate sorption is performed, both the particle size and surface properties should be considered in order to realize the efficient and economical removal of phosphate from wastewater.

The Role of Particle Size in the Dispersion Engineering of Plasmonic Arrays

Grazing diffraction orders on metal gratings give rise to peculiar optical effects that were contemplated by Wood, Rayleigh and Fano. With plasmonic nanoparticles as resonant grating elements, the phenomenology of such surface lattice resonances becomes quite rich, including spectrally narrow extinction peaks and optical band gap formation. It has been observed that at perpendicular incidence either the higher or lower energy branch corresponding to the first grazing diffraction orders is bright, i.e., couples strongly to light. Reviewing the literature, it appears that particle size is the factor determining which dispersion branch lights up. However, a consistent explanation for this effect is lacking. After revisiting the effect experimentally and by numerical simulation, we clarify the underlying physics by analyzing nanoparticle gratings in terms of, first, an oscillator model and, second, a photonic crystal description. Both approaches reveal the central role of a particle-size-dependent phase shift...

Role of particle size on the cohesive behavior of limestone powders at high temperature

Thermal Energy Storage (TES) using granular solids is gaining momentum in the last years. With no degradation up to very high temperatures and very low price the use of some granular materials such as sand or SiC would be feasible for storing sensible heat at large scale. A further step beyond TES is thermochemical energy storage (TCES) wherein the granular solids undergo a highly endothermic reaction at high temperature. Energy can be in this way more efficiently stored in the long term and released on demand by means of the reverse exothermic reaction. The Calcium Looping process, based on the calcination/carbonation of CaCO3, is being actively investigated for this purpose. However, a caveat of using granular solids for energy storage is the possible increase of interparticle adhesive forces with temperature which would severely hamper the flowability of the solids in the process. The cohesiveness of granular materials is essentially determined by particle size. In this paper we investigate the dependence of the tensile yield strength and compressibility of CaCO3 powders on temperature and consolidation stress using samples of narrow particle size distribution in the relevant range between ∼30 and ∼80μm particle size and temperatures up to 500°C. Our experimental results show that powder cohesiveness is greatly increased with temperature especially in the case of the finest powders whose tensile yield strength can be increased by up 2 orders of magnitude. The increase of cohesiveness with temperature is further enhanced with a previously applied consolidation stress, which is particularly relevant for applications wherein large amounts of solids are to be stored at high temperature. Experimental data are consistent with the predictions by a contact mechanics model assuming that the solids deform plastically at interparticle contacts. A main conclusion from our work is that some mechanical properties of the solids, specially the mechanical hardness, and how they change with temperature, play a critical role on the flowability of the solids as affected by an increase of temperature.

Boron Carbide Based Biocide Compositions: A Study of Iodate Particle Size on Combustion and Iodine Output

AbstractThe ever‐present threat of biological weapons has encouraged the development of novel pyrotechnic formulations that release gaseous iodine upon combustion, which acts as a potent active agent capable of countering bio‐agents (e. g. anthrax). These formulations typically contain a fuel (Al, B, Al/B4C) and an iodate serving as an oxidizer. However, the percentage of free iodine generated is relatively low and aging of these formulations remains an issue. In this work, the role of particle size on the combustion velocities, iodine output, and combustion products are detailed for a boron carbide/sodium periodate formulation. Formulations are studied with and without the addition of binder (protectant) and the role of particle size on achieving 3‐D printable energetics is detailed. It is found that altering the fuel particle size (boron carbide) had little effect on the combustion velocity and iodine recovery. However, by reducing the particle size of the oxidizer, NaIO4, drastic improvements in iodine recovery are achieved. A reduction in mean particle size from 215 to 3 μm resulted in a 47 % increase in observed iodine recovery for powder formulations. Combustion velocities appear to be unaffected and were 1.4–1.6 mm/s. An in‐depth discussion of the decomposition pathway of pure NaIO4 as well as when paired with a B4C fuel source is also discussed.

Role of particle size on the multicycle calcium looping activity of limestone for thermochemical energy storage

The calcium looping process, based on the reversible reaction between CaCO3 and CaO, is recently attracting a great deal of interest as a promising thermochemical energy storage system to be integrated in Concentrated Solar Power plants (CaL-CSP). The main drawbacks of the system are the incomplete conversion of CaO and its sintering-induced deactivation. In this work, the influence of particle size in these deactivation mechanisms has been assessed by performing experimental multicycle tests using standard limestone particles of well-defined and narrow particle size distributions. The results indicate that CaO multicycle conversion benefits from the use of small particles mainly when the calcination is carried out in helium at low temperature. Yet, the enhancement is only significant for particles below 15 μm. On the other hand, the strong sintering induced by calcining in CO2 at high temperatures makes particle size much less relevant for the multicycle performance. Finally, SEM imaging reveals that the mechanism responsible for the loss of activity is mainly pore-plugging when calcination is performed in helium, whereas extensive loss of surface area due to sintering is responsible for the deactivation when calcination is carried out in CO2 at high temperature.

Glucose release from lentil flours digested in vitro: The role of particle size

AbstractBackground and objectivesRegular intake of pulses has been shown to offer numerous health benefits, including improved glycemic control. The objective of this study was to investigate the in vitro carbohydrate digestion rate of raw, baked, and moist‐heat cooked lentil flours of varying particle sizes: coarse (710–1,190 μm), regular (355–710 μm), fine (180–355 μm), very fine (80–180 μm), and superfine (&lt;80 μm) over 180 min using the Englyst method.FindingsThere was an effect of particle size (p &lt; .05), time (p &lt; .05), and a particle size‐by‐time interaction (p &lt; .05) on glucose release in raw, baked, and moist‐heat cooked lentils flours digested in vitro over 180 min. The magnitude of glucose release was lower for the coarse compared to finer grinds (p &lt; .05) in raw, baked, and moist‐heat cooked samples and could be attributed to the lower starch damage, lower amounts of available carbohydrates, and higher amounts of resistant starch.ConclusionsThe larger particle size of lentil flour is associated with reduced glucose release upon simulated gastrointestinal digestion which was partially retained after baking or moist‐heat cooking.Significance and noveltyThis is the first study demonstrating that lentil flours varying in their average particle sizes have distinct content of resistant and available carbohydrates that may predict their glycemic properties.

Insights into the reaction mechanism and particle size effects of CO oxidation over supported Pt nanoparticle catalysts

CO oxidation is an extensively studied reaction in heterogeneous catalysis due to its seeming simplicity and its great importance for emission control. However, the role of particle size and more specifically structure sensitivity in this reaction is still controversial. In the present study, colloidal “surfactant-free” Pt nanoparticles (NPs) in a size regime of 1–4 nm with narrow size distribution and control over particle size were synthesized and subsequently supported on Al2O3 to prepare model catalysts. CO oxidation was performed using Pt NPs catalysts with particles sizes of 1, 2, 3, and 4 nm at different reaction temperatures. It is shown that the reaction exhibits a particle size effect that depends strongly on the reaction conditions. At 170 °C, the reaction seems to proceed within the same kinetic regime for all particle sizes, but the surface normalized activity depends strongly on the particle size, with maximum activity for nanoparticles 2 nm in diameter. A temperature increase to 200 °C leads to a change of the kinetic regime that depends on the particle size. For Pt NPs 1 nm in diameter a reaction order of 1 for O2 was observed, indicating that O2 adsorbs molecularly and dissociates in a following step, which represents the generally accepted mechanism on Pt surfaces. The reaction order of −1 for CO demonstrates that the surface is saturated with CO under reaction conditions. With increasing particle size, the reaction orders of O2 and CO change. For particles 2 nm in size, an increase in temperature also results in reaction orders of 1 for O2 and −1 for CO; NPs of 3 and 4 nm, even at higher temperatures, show no clear kinetic behavior that can be explained by a single reaction mechanism. Instead, the Boudouard reaction between two adjacent adsorbed CO molecules was identified as an important additional reaction pathway that occurs preferentially on large particles and causes more complex kinetics.