Noble Metal Nanoparticles

Abstract

To synthesize noble metal nanomaterials in controlled sizes and dimensions, various approaches and mechanisms have been developed. The successful utilization of noble metal nanoparticles (NMNPs) relies on the availability of synthetic methods generating nanoparticles with the desired characteristics, namely high solubility in water, adequate morphology, and surface functionalities. Control over the shape and size of the nanoparticles is usually achieved through the careful selection of the experimental conditions, namely reducing agents, type and concentration of precursors, reaction time, temperature, UV light, (co)solvent, and capping agents. Depending on the reduction potentials of the metal precursor and the reducing-agent systems, reduction can occur at room temperature or at elevated temperatures. In general, citrate plays a role as a stabilizing agent with preparations of gold nanoparticles requiring relatively high temperatures due to its weak reducing strength. The use of amine–borane complexes is essential for the syntheses of monodisperse metallic nanoparticles. Upon the addition of strong reductants, such as NaBH4, metal cations are reduced rapidly, resulting in an immediate color change of the reaction mixture. Rationally designed molecular building blocks allow for the precise control of particle size and morphology of the supramolecular aggregate, and various defined structures, including spherical micelles, rodlike micelles, or vesicles. Molecules which control the overall crystal growth are known as “capping agents,” the term frequently used for specific adsorption of surface-active molecules on selective crystal planes of a particular geometry. Additives such as surfactants, polymers, foreign ions, ligands, and impurities present in the reaction medium have been observed to play important roles in controlling the morphology of particles produced. Surfactants, ligands, or polymers were commonly added as stabilizers to impart stability to nanoparticles against aggregation, since colloidal particles tend to aggregate to decrease the overall surface area and energy. In a typical liquid-phase synthesis, the nanoparticle formation process undergoes three distinct stages as follows: (1) reduction and generation of active nuclei; (2) formation of seed particles upon collision of active nuclei; and (3) formation of larger nanoparticles via a growth process, which may be Ostwald ripening or aggregation. The nanoparticle growth is generally categorized by two processes: diffusion-controlled Ostwald-ripening and aggregation/coalescence. Nowadays, a molecule which can act both as a reducing and capping agent is preferred so that the reaction takes place in one step and there is no need for an external reducing agent. Multifunctional amines and nitrogen-containing polymers have also been tested for the synthesis of nanoparticles. Poly(ethylene oxide)-poly(propylene oxide)-based block copolymers are well known as dispersion stabilizers and templates for the synthesis of mesoporous materials and nanoparticles. Coordination chemistry offers simplicity, stable bonding, and ligand-metal specificity, enabling ligand-bearing components to be assembled into supramolecular structures using appropriate metal ions. This approach is particularly compatible with surface chemistry, as binding of metal ions activates the surface toward ligand binding, and vice versa. Some stabilizing agents can also be used as a reducing agent. Ionic liquids (ILs) are a viable option as stabilizing agents because of their ionic character and can be easily made task-specific as phase-transfer catalysts due to their tunable nature. Ligand exchange reactions have proven a particularly powerful approach to incorporate functionality in the ligand shell of thiol-stabilized nanoparticles and are widely used to produce organic- and water-soluble nanoparticles with various core sizes and functional groups. NMNPs can be intercalated into the gallery regions of montmorillonite and formed hybrid framework. Models of particle (crystal) development consider two basic steps: nucleation and growth. The creation of a new phase from a metastable state is nucleation. Seed-mediated growth method has been demonstrated to be a powerful synthetic route to generate a range of different types of metal nanoparticles. This method separates the nucleation and growth stage of nanoparticle syntheses by introducing presynthesized small seed particles into a growth solution typically containing a metal precursor, reducing agent, surfactants, and some additives. Dissolution of silver nanoparticles, for example, occurs through oxidation of metallic Ag and release of Ag+ into solution (or dissolution rate is accelerated). Release of Ag+ is determined by intrinsic physicochemical properties of silver nanoparticles and by those of the solution. Parameters that either enhance or suppress silver nanoparticle dissolution are ionic strength, pH, dissolved oxygen concentration, temperature, dissolved complexing ligands (organic matter, sulfur, chlorine), silver surface coating, shape, and size. The surfactants find their way to various environmental segments and thus pose serious health hazards. Several different polymer-based anticancer agents have been approved for clinical use, for passive tumor targeting. Prominent examples of macromolecular drug carrier systems evaluated in patients are poly(ethylene glycol), poly(l-glutamic acid), poly[N-(2-hydroxypropyl)methacrylamide], and their copolymers. Copolymers based on N-(2-hydroxypropyl)methacrylamide (i.e., HPMA) were used to improve the tumor-directed delivery of doxorubicin.