Nanoparticles dissolution

Prockl et al. [60] measured the concentration of leached Pd species from palladium nanoparticles supported on a metal oxide via atomic absorption spectroscopy as a function of time in solution. The data indicated that the largest concentration of Pd species in solution (Pd " and/or Pd(0)) occurred during the reaction (Fig. 18.6). As the reaction neared 100% conversion, the soluble Pd concentration returned to the original value, presumably due to readsorption onto the metal oxide substrate. The process was concluded by the authors to have clearly involved heterogeneous reactions [60]. This data supports a catalytic mechanism that is heterogeneous in nature, where the reaction occurs at the interface and causes the dissolution of surface atoms into solution. This explanation is supported by the report of Prockl et al. that individual reactants did not initiate nanoparticle dissolution but that dissolution was observed during the reaction over the 25-50 min time interval when the conversion was the highest (Fig. 18.6). 

In vitro drug release kinetic was also performed in non-sink conditions using decane as solvent (to prevent drug loss from nanoparticles dissolution) and a laboratory designed release cell. About 10 mg of gliadins nanoparticles (containing 824 fig/g gliadins) were resuspended in 10 ml of decane. Aliquots were collected at successive time intervals and replaced by the same quantity of solvent in order to get a constant volume in the release cell. The samples were analysed by HPLC as described above for encapsulation experiments. 

Results discussed in this section reveal important trends in the stability of Pt nanoparticles. They identify the surface tension as a valid descriptor of nanoparticle stability. The surface tension must play an important role in the kinetic modeling of nanoparticle dissolution (Rinaldo et al., 2010, 2012). However, the main kinetic mechanisms that contribute to Pt nanoparticle dissolution proceed via formation and reduction of surface oxide intermediates at Pt. This well-founded observation suggests that stability studies, reported here for bare Pt nanoparticles evaluated in vacuo, should be expanded to Pt nanoparticles of varying surface oxidation state as well as conditions that mimic electrochemical conditions that the fuel cell catalyst is exposed to. 

Rinaldo, S. G., Stumper, J., and Eikerling, M. 2010. Physical theory of platinum nanoparticle dissolution in polymer electrolyte fuel cells. J. Phvs. Cham. C. 114(13), 5773-5785. 

In nanoparticle electrocatalysis, the area that Michael entered just some time ago in Munich, he and his coworkers rationalized the sensitivity of electrocatalytic processes to the stmcture of nanoparticles and interfaces. Studies of catalytic effects of metal oxide support materials revealed intriguing electronic structure effects on thin films of Pt, metal oxides, and graphene. In the realm of nanoparticle dissolution and degradation modeling, Michael s group has developed a comprehensive theory of Pt mass balance in catalyst layers. This theory relates surface tension, surface oxidation state, and dissolution kinetics of Pt. 

S.G. Rinaldo, J. Stumper, and M. Eikerling, Physical theory of platinum nanoparticle dissolution in polymer. Electrolyte Fuel Cells. J. Phys. Chem. C 114,5775-5783, 2010. 

Nanosized composites increase specific surface area, which is one of the aspects that affect biocompatibility. Nanoparticle dissolution and corrosion are important factors for biocompatibility, which are related to the specific surface area. For example, HA-collagen composite coatings can lead to the failure of a dental or bone implant. Apatite dust produced by dropout exfoliation delamination or abrasion can cause inflammation and the resorption of new bone and the coating of apatite. Another typical serious case of apatite dust is osteolysis which is caused by the inflammation induced by abraded particles (Watari et al., 2009). 

Nanoparticles of the semicondnctor titanium dioxide have also been spread as mono-layers [164]. Nanoparticles of TiOi were formed by the arrested hydrolysis of titanium iso-propoxide. A very small amount of water was mixed with a chloroform/isopropanol solution of titanium isopropoxide with the surfactant hexadecyltrimethylammonium bromide (CTAB) and a catalyst. The particles produced were 1.8-2.2 nm in diameter. The stabilized particles were spread as monolayers. Successive cycles of II-A isotherms exhibited smaller areas for the initial pressnre rise, attributed to dissolution of excess surfactant into the subphase. And BAM observation showed the solid state of the films at 50 mN m was featureless and bright collapse then appeared as a series of stripes across the image. The area per particle determined from the isotherms decreased when sols were subjected to a heat treatment prior to spreading. This effect was believed to arise from a modification to the particle surface that made surfactant adsorption less favorable. 

Another method to synthesize hollow nanocapsules involves the use of nanoparticle templates as the core, growing a shell around them, then subsequently removing the core by dissolution [30-32]. Although this approach is reminiscent of the sacrificial core method, the nanoparticles are first trapped and aligned in membrane pores by vacuum filtration rather than coated while in aqueous solution. The nanoparticles are employed as templates for polymer nucleation and growth Polymerization of a conducting polymer around the nanoparticles results in polymer-coated particles and, following dissolution of the core particles, hollow polymer nanocapsules are obtained. 

Photocatalytic Deposition and Plasmon-Induced Dissolution of Metal Nanoparticles on Ti02... 

In this chapter, photoelectrochemical control of size and color of silver nanoparticles, i.e., multicolor photo-chromism [1], is described. Silver nanoparticles are deposited on UV-irradiated Ti02 by photocatal5dic means [2]. Size of the nanoparticles can be roughly controlled in the photocatalytic deposition process. However, it is rather important that this method provides nanoparticles with broadly distributed sizes. The deposited silver nanoparticles are able to be dissolved partially and reduced in size by plasmon-induced photoelectrochemical oxidation in the presence of an appropriate electron acceptor such as oxygen. If a monochromatic visible light is used, only the particles that are resonant with the light are dissolved. That is, size-selective dissolution is possible [3]. This is the principle of the multicolor photochromism. 

The present technique enables light-induced redox reaction UV light-induced oxidative dissolution and visible light-induced reductive deposition of silver nanoparticles. Reversible control of the particle size is therefore possible in principle. The reversible redox process can be applied to surface patterning and a photoelectrochemical actuator, besides the multicolor photochromism. 

Visible Light-Induced Dissolution of Silver Nanoparticles... 

Figure 1. Mechanisms of photoelectrochemical deposition and dissolution of silver nanoparticles (Ag NPs) (a) UV light-induced deposition and (b) visible light-induced dissolution.

Figure 2. UV light-induced deposition of silver nanoparticles (al-a3) and wavelength-selective visible light-induced dissolution of silver nanoparticles (bl-b3).

This process occurs only at the silver nanoparticles of which resonance wavelength is in accordance with the incident light wavelength. Therefore, size-specific dissolution is possible (processes bl-b3 in Figure 2). 

In the meantime, to protect the silver nanoparticles from the photoelectrochemical dissolution, the particles may be coated with a polymer matrix or a hydrophobic thiol [10]. 

Silver nanoparticles can be deposited on Ti02 by UV-irradiation. Deposition of polydisperse silver particles is a key to multicolor photochromism. The nanoparticles with different size have different resonant wavelength. Upon irradiation with a monochromatic visible light, only the resonant particle is excited and photoelectrochemically dissolved, giving rise to a decrease in the extinction at around the excitation wavelength. This spectral change is the essence of the multicolor photochromism. The present photoelectrochemical deposition/dissolution processes can be applied to reversible control of the particle size. 

However, in the case of multimetallic catalysts, the problem of the stability of the surface layer is cmcial. Preferential dissolution of one metal is possible, leading to a modification of the nature and therefore the properties of the electrocatalyst. Changes in the size and crystal structure of nanoparticles are also possible, and should be checked. All these problems of ageing are crucial for applications in fuel cells. 

Self-assembled nanorods of vanadium oxide bundles were synthesized by treating bulk V2O5 with high intensity ultrasound [34]. By prolonging the duration of ultrasound irradiation, uniform, well defined shapes and surface structures and smaller size of nanorod vanadium oxide bundles were obtained. Three steps which occur in sequence have been proposed for the self-assembly of nanorods into bundles (1) Formation of V2O5 nuclei due to the ultrasound induced dissolution and a further oriented attachment causes the formation of nanorods (2) Side-by-side attachment of individual nanorods to assemble into nanorods (3) Instability of the self-assembled V2O5 nanorod bundles lead to the formation of V2O5 primary nanoparticles. It is also believed that such nanorods are more active for n-butane oxidation. 

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