Platinum nanoparticle
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Platinum nanoparticles are usually in the form of a suspension or
. 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
Platinum nanoparticles are the subject of substantial research,
Synthesis
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,[1][2][8] or by the impregnation and reduction of platinum ion precursors in a micro-porous support such as alumina.[9]
Some common examples of platinum precursors include potassium hexachloroplatinate (K2PtCl6) or platinous chloride (PtCl2)[1][8] Different combinations of precursors, such as ruthenium chloride (RuCl3) and chloroplatinic acid (H2PtCl6), have been used to synthesize mixed-metal nanoparticles[9] 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.[1][2][4][8][9][10][11][12]
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[1][2][8][9] 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.[1][8][9] 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).[1] The size of nanoparticles synthesized onto a substrate such as alumina depends on various parameters such as the pore size of the support.[9]
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.[2] 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.[2]
Shape and size control
Ramirez et al.
Oleylamine, oleic acid and
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.[16] 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.[16] 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.[17] 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.[17]
Green synthesis
An ecologically-friendly synthesis of platinum nanoparticles from chloroplatinic acid was achieved through the use of a leaf extract of
Properties
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
Platinum NPs are used as catalysts for
One type of platinum NPs that have been researched on are
Optical properties
Platinum NPs exhibit fascinating optical properties. Being a free electron metal NP like silver and gold, its linear optical response is mainly controlled by the surface plasmon resonance (SPR). 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.[23]
The SPR of platinum nanoparticles is found in the ultraviolet range (215 nm), unlike the other noble metal nanoparticles which display SPR in the visible range 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. Through experimentation, the resonance peak only showed slight variations with the change of size and synthetic method (while maintaining the same shape), with the exception of those nanoparticles synthesized by citrate reduction, which did not exhibit and SPR peak in this region..[24]
Through the control of percent composition of 2–5 nm platinum nanoparticles on SiO2, Zhang et al. modeled distinct absorption peaks attributed to platinum in the visible range, distinct from the conventional SPR absorption. This research attributed these absorption features to the generation and transfer of hot electrons from the platinum nanoparticles to the semiconductive material.[25] The addition of small platinum nanoparticles on semiconductors such as TiO2 increases the photocatalytic oxidation activity under visible light irradiation.[26] These concepts suggest the possible role of platinum nanoparticles in the development of solar energy conversion using metal nanoparticles. By changing the size, shape and environment of metal nanoparticles, their optical properties can be used for electrontic, catalytic, sensing, and photovoltaic applications.[24][27][28]
Applications
Fuel cells application
Hydrogen fuel cells
Among the precious metals, platinum is the most active toward the hydrogen oxidation reaction that occurs at the anode in hydrogen fuel cells. In order to meet cost reductions of this magnitude, the Pt catalyst loading must be decreased. Two strategies have been investigated for reducing the Pt loading: the binary and ternary Pt-based alloyed nanomaterials and the dispersion of Pt-based nanomaterials onto high surface area substrates.[29]
Methanol fuel cells
The
Electrochemical oxidation of formic acid
Formic acid is another attractive fuel for use in PEM-based fuel cells. The dehydration pathway produces adsorbed carbon monoxide. A number of binary Pt-based nanomaterial electrocatalysts have been investigated for enhanced electrocatalytic activity toward formic acid oxidation.[29]
Modifying conductivity of zinc oxide materials
Platinum NPs can be used to dope
Glucose detection applications
Enzymatic glucose sensors have drawbacks that originate from the nature of the enzyme. Nonenzymatic glucose sensors with Pt-based electrocatalysts offer several advantages, including high stability and ease of fabrication. Many novel Pt and binary Pt-based nanomaterials have been developed to overcome the challenges of glucose oxidation on Pt surfaces, such as low selectivity, poor sensitivity, and poisoning from interfering species.[29]
Other applications
Platinum catalysts are alternatives of automotive
Biological interactions
The increased reactivity of nanoparticles is one of their most useful properties and is leveraged in fields such as catalysis, consumer products, and energy storage. However, this high reactivity also means that a nanoparticle in a biological environment may have unintended impacts. For example, many nanoparticles such as silver, copper, and ceria interact with cells to produce reactive oxygen species or ROS which can cause premature cell death through apoptosis.[33] Determining the toxicity of a specific nanoparticle requires knowledge of the particle’s chemical composition, shape, size and is a field that is growing alongside advances in nanoparticle research.
Determining the impact of a nanoparticle on a living system is not straightforward. A multitude of
Drug delivery
A topic of research within the field of nanoparticles is how to use these small particles for
Toxicology
Toxicity stemming from platinum nanoparticles can take multiple forms. One possible interaction is cytotoxicity or the ability of the nanoparticle to cause cell death. A nanoparticle can also interact with the cell’s DNA or genome to cause genotoxicity.[37] These effects are seen in different levels of gene expression measured through protein levels. Last is the developmental toxicity that can occur as an organism grows. Developmental toxicity looks at the impact the nanoparticle has on the growth of an organism from an embryonic stage to a later set point. Most nanotoxicology research is done on cyto- and genotoxicity as both can easily be done in a cell culture lab.
Platinum nanoparticles have the potential to be toxic to living cells. In one case, 2 nm platinum nanoparticles were exposed to two different types of
Researchers have also compared the toxicity of Pt nanoparticles to other commonly used metallic nanoparticles. In one study, the authors compared the impact of different nanoparticle compositions on the
In a recent paper published in Nanotoxicology, the authors found that between silver (Ag-NP, d = 5–35 nm), gold (Au-NP, d = 15–35 nm), and Pt (Pt-NP, d = 3–10 nm) nanoparticles, the Pt nanoparticles were the second most toxic in developing zebrafish embryos, behind only the Ag-NPs.[40] However, this work did not examine the size dependence of the nanoparticles on their toxicity or biocompatibility. Size-dependent toxicity was determined by researchers at the National Sun Yat – Sen University in Kaohsiung, Taiwan. This group’s work showed that the toxicity of platinum nanoparticles in bacterial cells is strongly dependent on nanoparticle size and shape/morphology.[41] Their conclusions were based on two major observations. First, the authors found that platinum nanoparticles with spherical morphologies and sizes less than 3 nm showed biologically toxic properties; measured in terms of mortality, hatching delay, phenotypic defects and metal accumulation.[41] While those nanoparticles with alternative shapes—such as cuboidal, oval, or floral—and sizes of 5–18 nm showed biocompatibility and no biologically toxic properties.[41] Secondly, out of the three varieties of platinum nanoparticles which exhibited biocompatibility, two showed an increase in bacterial cell growth.[41]
The paper introduces many hypotheses for why these observations were made, but based on other works and basic knowledge of bacterial cell membranes, the reasoning behind the size dependent toxicity observation seems to be twofold. One: The smaller, spherically shaped nanoparticles are able to pass through cell membranes simply due to their reduced size, as well as their shape-compatibility with the typically spherical pores of most cell membranes.[41] Although this hypothesis needs to be further supported by future work, the authors did cite another paper which tracked the respiratory intake of platinum nanoparticles. This group found that 10 µm platinum nanoparticles are absorbed by the mucus of the bronchi and trachea, and can travel no further through the respiratory tract.[33] However, 2.5 µm particles showed an ability to pass through this mucus layer, and reach much deeper into the respiratory tract.[33] Also the larger, uniquely shaped nanoparticles are too large to pass through the pores of the cell membrane, and/or have shapes which are incompatible with the more spherically shaped pores of the cellular membrane.[41] In regards to the observation that the two largest platinum nanoparticles (6–8 nm oval, and 16–18 nm floral) actually increase bacterial cell growth, the explanation could originate in the findings of other works which have shown that platinum nanoparticles have demonstrated significant antioxidative capacity.[42][43] However, in order for these antioxidative properties to be exploited, the platinum nanoparticles must first enter the cells, so perhaps there is another explanation for this observation of increased bacterial cell growth.
Most studies so far have been size based using an in vivo mouse model. In one study, researchers compared the effects of sun 1 nm and 15 nm platinum nanoparticles on mice.
See also
- Colloidal gold
- Nanoparticles
- Nanomaterial based catalyst
- Nanotechnology
- Icosahedral twins
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