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Platinum nanoparticles are usually in the form of a suspension or colloid of submicrometre-size particles of platinum in a fluid, usually water. A colloid is technically defined as a stable dispersion of particles in a fluid medium (liquid or gas).
Spherical Platinum nanoparticles can be made with sizes between about 2 and 100 nanometres (nm), depending on reaction conditions,. Platinum nanoparticles are suspended in the colloidal solution of brownish-red or black color. Nanoparticles come in wide variety of shapes including spheres, rods, cubes, and tetrahedra.
Platinum nanoparticles are the subject of substantial research, with potential applications in a wide variety of areas. These include catalysis, medicine, and the synthesis of novel materials with unique properties.
Platinum nanoparticles are typically synthesized either by the reduction of platinum ion precursors in solution with a stabilizing or capping agent to form colloidal nanoparticles, or by the impregnation and reduction of platinum ion precursors in a micro-porous support such as alumina.
Some common examples of platinum precursors include hexachloroplatinate (K2PtCl6) or platinous chloride (K2PtCl6) Different combinations of precursors, such as ruthenium chloride (RuCl3) and chloroplatinic acid (H2PtCl6), have been used to synthesize mixed-metal nanoparticles Some common examples of reducing agents include hydrogen gas (H2), sodium borohydride (NaBH4) and ethylene glycol (C2H6O2), although other alcohols and plant-derived compounds have also been used.
As the platinum metal precursor is reduced to neutral platinum metal (Pt0), the reaction mixture becomes supersaturated with platinum metal and the Pt0 begins to precipitate in the form of nanoscale particles. A capping agent or stabilizing agent such as sodium polyacrylic acid or sodium citrate is often used to stabilize the nanoparticle surfaces, and prevents the aggregation and coalescence of the nanoparticles.
The size of nanoparticles synthesized colloidally may be controlled by changing the platinum precursor, the ratio of capping agent to precursor, and/or the reaction temperature. The size of the nanoparticles can also be controlled with small deviation by using a stepwise seed-mediated growth procedure as outlined by Bigall et al. (2008). The size of nanoparticles synthesized onto a substrate such as alumina depends on various parameters such as the pore size of the support.
Platinum nanoparticles can also be synthesized by decomposing Pt2(dba)3 (dba = dibenzylideneacetone) under a CO or H2 atmosphere, in the presence of a capping agent. The size and shape distributions of the resulting nanoparticles depend on the solvent, the reaction atmosphere, the types of capping agents and their relative concentrations, the specific platinum ion precursor, as well at the temperature of the system and reaction time.
Shape and Size Control
Ramirez et al. reported the influence of ligand and solvent effects on the size and shape of platinum nanoparticles. Platinum nanoparticle seeds were prepared by the decomposition of Pt2(dba)3 in tetrahydrofuran (THF) under carbon monoxide (CO). These conditions produced Pt nanoparticles with weakly bound THF and CO ligands and an approximate diameter on 1.2 nm. Hexadecylamine (HDA) was added to the purified reaction mixture and allowed to displace the THF and CO ligands over the course of approximately seven days, producing monodispersed spherical crystalline Pt nanoparticles with an average diameter of 2.1 nm. After the seven-day period, an elongation of the Pt nanoparticles occurred. When the same procedure was followed using a stronger capping agent such as triphenyl phosphine or octanethiol, the nanoparticles remained spherical, suggesting that the HDA ligand affects particle shape.
When Pt2(dba)3 was decomposed in THF under hydrogen gas in the presence HDA, the reaction took much longer, and formed nanowires with diameters between 1.5 and 2 nm. Decomposition of Pt2(dba)3 under hydrogen gas in toluene yielded the formation of nanowires with 2-3 nm diameter independent of HDA concentration. The length of these nanowires was found to be inversely proportional to the concentration of HDA present in solution. When these nanowire syntheses were repeated using reduced concentrations of Pt2(dba)3, there was little effect on the size, length or distribution of the nanowires formed.
Platinum nanoparticles of controlled shape and size have also been accessed through varying the ratio of polymer capping agent concentration to precursor concentration. Reductive colloidal syntheses as such have yielded tetrahedral, cubic, irregular-prismatic, icosahedral, and cubo-octahedral nanoparticles, whose dispersity is also dependent on the concentration ratio of capping agent to precursor, and which may be applicable to catalysis. The precise mechanism of shape-controlled colloidal synthesis is not yet known; however, it is known that the relative growth rate of crystal facets within the growing nanostructure determines its final shape. Polyol syntheses of platinum nanoparticles, in which chloroplatinic acid is reduced to PtCl42- and Pt0 by ethylene glycol, have also been a means to shape-controlled fabrication. Addition of varying amounts of sodium nitrate to these reactions was shown to yield tetrahedra and octahedra at high concentration ratios of sodium nitrate to chloroplatinic acid. Spectroscopic studies suggest that nitrate is reduced to nitrite by PtCl42- early in this reaction, and that the nitrite may then coordinate both Pt(II) and Pt(IV), greatly slowing the polyol reduction and altering the growth rates of distinct crystal facets within the nanoparticles, ultimately yielding morphological differentiation.
An ecologically-friendly synthesis of platinum nanoparticles from chloroplatinic acid was achieved through the use of a leaf extract of Diospyros kaki as a reducing agent. Nanoparticles synthesized as such were spherical with an average diameter ranging from 2-12 nm depending on reaction temperature and concentration of leaf extract used. Spectroscopic analysis suggests that this reaction is not enzyme-mediated and proceeds instead through plant-derived reductive small molecules. Another eco-friendly synthesis from chloroplatinic acid was reported using leaf extract from Ocimum sanctum and tulsi as reducing agents. Spectroscopic analysis suggested that ascorbic acid, gallic acid, various terpenes, and certain amino acids were active in the reduction. Particles synthesized as such were shown through scanning electron microscopy to consist in aggregates with irregular shape. It has been shown that tea extracts with high polyphenol content may be used both as reducing agents and capping agents for platinum nanoparticle synthesis.
Properties and Applications
The chemical and physical properties of platinum nanoparticles (NP) make them applicable for a wide variety of research applications. Extensive experimentation has been done to create new species of platinum NPs, and study their properties. Platinum NP applications include electronics, optics, catalysts, and enzyme immobilization.
Catalytic properties of platinum nanoparticles
Platinum NPs are used as catalysts for proton exchange membrane fuel cell (PEMFC), for industrial synthesis of nitric acid, reduction of exhaust gases from vehicles and as catalytic nucleating agents for synthesis of magnetic NPs. The catalytic reactivity of the NP is dependent on the shape, size and morphology of the particle
One type of platinum NPs that have been researched on are colloidal platinum NPs. Monometallic and bimetallic colloids have been used as catalysts in a wide range of organic chemistry, including, oxidation of carbon monoxide in aqueous solutions, hydrogenation of alkenes in organic or biphasic solutions and hydrosilylation of olefins in organic solutions. Collodial platinum NPs protected by Poly(N-isopropylacrylamide) were synthesised and their catalytic properties measured. It was determined that they were more active in solution and inactive when phase separated due to its solubility being inversely proportional to temperature.
Optical properties of platinum nanoparticles
Platinum NPs exhibit fascinating optical properties. Being a free electron metal NP like Ag and Au, its linear optical response is mainly controlled by the surface plasmon resonance. Surface plasmon resonance occurs when the electrons in the metal surface are subject to an electromagnetic field that exerts a force on the electrons and cause them to displace from their original positions. The nuclei then exert a restoring force that results in oscillation of the electrons, which increase in strength when frequency of oscillations is in resonance with the incident electromagnetic wave.
Surface plasmon resonance (SPR) of platinum NPs occur in the ultraviolet region (215 nm). The resonance peak is very broad because the imaginary dielectric constant of platinum, Im(εPt), is large. Experiments were done and the spectra obtained are similar for most platinum particles regardless of size. However, there is an exception. Platinum NPs synthesized via citrate reduction do not have a surface plasmon resonance peak around 215 nm.
Modifying conductivity of other materials
Platinum NPs can be used to dope zinc oxide (ZnO) materials to improve their conductivity. ZnO has several characteristics that allow it to be used in several novel devices such as development of light-emitting assemblies and solar cells. However, because ZnO is of slightly lower conductivity than metal and indium tin oxide (ITO), it can be doped and hybridized with metal NPs like platinum to improve its conductivity. A method to do so would be to synthesize platinum NPs using methanol reduction and incorporate at 0.25 at.% platinum NPs. This boosts the electrical properties of ZnO films while preserving its transmittance for application in transparent conducting oxides.
One of the biological effects that researchers are currently investigating is how platinum nanoparticles affect lifespan. Research by Yusei Miyamoto at University of Tokyo, Japan found that 2-3 nm platinum nanoparticles have been found to increase the lifespan of the roundworm Caenorhabditis elegans by 20-25%. This is due to platinum nanoparticles having antioxidant properties. Antioxidant properties help to combat reactive oxygen species (ROS). ROS include, but are not limited superoxide anions, hydroxyl radicals, and hydrogen peroxide. The amount of ROS produced directly correlates with the metabolic rate and life span of an organism. An increase in oxidative stress has been observed in correspondence with age.
In the roundworm, C. elegans, it was noted that the intracellular level of lipofuscin increased with oxidative stress. Lipofuscin is a fluorescent molecule, and the fluorescence emitted was measured to show the amount of ROS in the roundworm. Platinum nanoparticle solutions were introduced to C. elegans in varying concentrations. It was observed that at a concentration of 0.1 mM the nano-Pt solution was too low to hinder aging or decrease the accumulation of lipofuscin. At 0.5 mM nano-Pt solution there was a significant decrease in the fluorescence observed, and the lifespan of C. elegans was extended by 22.3±2.8%. However, at a 1 mM concentration of nano-Pt solution the lifespan of C. elegans drastically decreased. This did not correspond with what the authors had initially hypothesized. They later theorized that the toxicity of the platinum nanoparticles could have attributed to the shortened lifespan of C. elegans.
In one study, mice injected with platinum nanoparticles of less than 1 nm size developed symptoms of liver damage (elevated ALT and AST levels) while mice injected with 15 nm platinum nanoparticles did not. In another study, mice that received a single injection of platinum nanoparticles of less than 1 nm size developed necrosis of tubular epithelial cells and urinary casts in the kidney, while mice repeatedly injected with 8 nm platinum nanoparticles did not. In yet another study, human cells exposed to platinum nanoparticles ~5–8 nm in size experienced DNA damage.
Nanoparticles are a new class of materials that are used commercially in cosmetic, pharmaceutical, and electronic industries. Nanoparticles are becoming increasingly more common for drug use. This is due to their ability to provide specific drug targeting and delivery. Nanoparticles have the benefits of being able to pass through cellular membranes, as well as to bind and stabilize proteins. It’s also been noted that generally, nanoparticles can decrease the overall toxicity of the incorporated drug. Initially, it wasn’t realized that the carrier system in drug delivery (nanoparticle) could be toxic in itself. This requires further study of the toxicology of nanoparticles.
It has become vital to research the toxicological impact of nanoparticles. This is primarily due to nanoparticle use in the pharmaceutical industry. Nanoparticles can have a high surface-volume ratio, which increases the reactivity and the toxicity of the surface. Small nanoparticles (<8 nm) have the ability to pass through cellular membranes, adversely affecting the human respiratory system, central nervous system, and the gastrointestinal tract, amongst other things. The inhalation of nanoparticles is considered the most antagonistic trait, because it can cause inflammation, and disease in the lungs. Nanotoxicology has been developed to study the adverse effects of nanoparticles. Currently, toxicological impacts haven’t been extensively studied in nanoparticles. Platinum nanoparticles in themselves have not been evaluated for the advantages or disadvantages they could bring to the table. Research needs to be further conducted to analyze all of the impacts that platinum nanoparticles contribute to a system.
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