Noble metal-based nanoparticles

In this chapter, we first divide NIR NMs into five species carbon-based materials, quantum dots (QDs), noble metal-based nanoparticles (NPs), upconversion nanoparticles (UCNPs), and narrow-bandgap semiconductors. Then, we focus primarily on the progress of their toxicity studies in the past several years, discuss in detail how the biophysicochemical properties of NIR NMs influence their in vitro and in vivo toxicity, present a broad overview of the available in vitro and in vivo toxicity assessments of NIR NMs, and finally frame the future outlook for NIR NMs by highlighting areas of exceptional promise and challenges. Our emphasis here is mainly on discussion that could offer future opportunities to design and create NIR NMs with good biocompatibility as well as excellent functionalities, rather than attempting to provide a complete historical survey. 

Enhanced electric-field distribution is illustrated schematically in Figure 3.8, based on reported electromagnetic simulations, for a dimer of a noble metal spherical nanoparticle. The optical field enhancement at the gap site occurs only when the incident polarization is parallel to the interparticle axis of the dimer. 

Zero-valent metal atoms M then coalesce, adsorb on the surfaces, and in the presence of less noble metal ion, charge transfer between these two metals can occur. This method has been widely used for the preparation of metallic, alloyed, and core-shell nanoparticles (Ranita and Ranita 2010), especially of noble metals. Base metals such as Fe, Ni, Co, Cu, Zn, and others are prone to oxidation and can, therefore, form oxides after their reduction to the metallic state ... 

In general, non-noble metal alloy nanoparticles have shown some methanol tolerance effects, but their activity towards ORR is lower than that of Pt/C. Furthermore, it has been found that the non-precious metal catalysts do not present the required stability in the acidic environment, even in the case of Pd (at high potential). On the other hand, some works have shown that this instability (dissolution), mainly of the non-noble metal, can be overcome by the addition of small amounts of stabilizers like Au. Based on this, Mathiyarasu and Phani [30] examined the effect of the addition of Au, Ag and Pt on the activity and stability of several Pd-Co/C electrocatalysts. Results showed higher ORR activities for Pd-Co-Pt/C, equal to that of a commercial Pt/C electrocatalyst. 

The major drawback of the nickel catalyst is poisoning of Ni surface by the interaction with CO to form nickel carbonyl at low temperature. Noble metal based catalysts are more active and stable catalyst in comparison with Ni based catalysts [100]. Among Ru nanoparticle dispersed on various supports (Al Oj, MgAljO, MgO, C, etc ), Ru/Al Oj showed highest catalytic activity (turnover frequency, TOF = 16.5 x 10 s" ) [101,102]. Yttrium addition to the Ru-based catalyst enhances the activity and stability for methanation reaction [103], Pd/Mg-SiO and platinum titanate nanotubes were also foimd... 

The Pt alloy monolayer nanoparticle catalysts (e.g., Pt-Re layer on Pd cores) showed a clearly improved specific (Pt surface normalized) ORR activity their Pt mass-based electrocatalytic activity, however, exceeded that of pure Pt catalysts by an impressive factor of 18 x— 20 x. Their noble metal (Pt, Re, and Pd) mass-based activity improvement was still about a factor 4x. The Tafel slope in the 800-950 mV/RHE range suggested that the surface accumulation of Pt-OH species is delayed on the Pt monolayer catalyst. The enormous increase in Pt mass-based activity is obviously due to the small amount of Pt metal inside the Pt monolayer. 

An intense femtosecond laser spectroscopy-based research focusing on the fast relaxation processes of excited electrons in nanoparticles has started in the past decade. The electron dynamics and non-linear optical properties of nanoparticles in colloidal solutions [1], thin films [2] and glasses [3] have been studied in the femto- and picosecond time scales. Most work has been done with noble metal nanoparticles Au, Ag and Cu, providing information about the electron-electron and electron-phonon coupling [4] or coherent phenomenon [5], A large surface-to-volume ratio of the particle gives a possibility to investigate the surface/interface processes. 

The partial oxidation of alcohols, to afford carbonyl or carboxylic compounds, is another synthetic route of high industrial interest For this, scC02 was investigated as a reaction medium for the aerobic oxidation of aliphatic, unsaturated, aromatic and benzylic acids with different catalytic systems, mainly based on the use of noble metals, both in batch [58-64] and in continuous fixed-bed reactors [65-70]. In this context, very promising results have been obtained when studying the catalytic activity of supported palladium and gold nanoparticles in the oxidation of benzyl alcohol to benzaldehyde these allowed conversions and selectivities in excess of 90% to be achieved [71-73]. 

Another approach for the preparation of dendrimer-noble metal nanoparticles in toluene is a process driven by acid-base chemistry and ion pairing [35]. At first, palladium nanoparticles are prepared by reducing aqueous K2PdCl4 with sodium borohydride in the presence of G4 dendrimer where the pH of dendrimer solution is adjusted to about 2. The low pH protonates the exterior amines to a greater extent than the less basic interior tertiary amines. Accordingly, Pd2+ binds preferentially to the interior tertiary amines and upon reduction palladium particles form within the dendrimer interior. After the complete reduction, the pH of solutions is adjusted back to about 8.5. Then, these nanocomposites can be quantitatively transported from the aqueous phase into toluene containing 10-20% of dodecanoic acid. The transition is visualized by the color change brown aqueous solution of dendrimer-palladium nanoparticles becomes clear after addition of the acid, while the toluene layer turns brown. 

Based on a model on the features of the double-pulse technique, various structures of silver nanoparticles grown onto a thin ITO film covered glass plate were generated and characterized [30]. With this method, the conflict between both optimal conditions for nucleation and growth is partially defused. This is due to the amount of small seed additionally nucleated at the higher polarization and resolved as soon as the potential is switched over to the lower polarization of the growth pulse. The interaction of the pulse parameters was modeled, thus forming the basis for how the electrodeposition process of noble metal clusters can be variably controlled. 

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