Gold NPs and its alloys

Nanoparticles

Metal nanoparticles and alloy nanoparticles have remarkable optical, electronic and catalytic properties and have many different applications in biomedical and material sciences. In biomedicine, gold nanoparticles (AuNPs) are used in several purposes such as leukemia therapy, biomolecular immobilization, biosensor design and used as AuNPs as anti-angiogenesis, anti-malaria and anti-arthritic agents. Silver nanoparticles (AgNPs) are applied as selective coating agent for solar energy absorption, intercalation material for electric batteries, catalysts in chemical reactions and antimicrobial agents.

Physical methods

Preparation of nanometals using physical methods such as attrition and pyrolysis supply nanostructures with narrow and controlled size ranges, however, these methods require very expensive equipments and the final yield is low.

Biosynthesis

Microorganisms, both unicellular and multicellular, are known to produce inorganic materials often of nanoscale dimensions either intracellularly or extracellularly and the inherent uniformity of the biological structures can be combined with the functional properties of inorganic nanoparticles. For the biosynthesis of nanoparticles chloroauric acid (HAuCl4) and AgNO3 are used.

Crystallized and spherical-shaped Au and Au–Ag alloy nanoparticles have been synthesized and stabilized using a fungus, F. semitectum in an aqueous system. Aqueous solutions of chloroaurate ions for Au and chloroaurate and Ag+ ions (1 : 1 ratio) for Au–Ag alloy were treated with an extracellular filtrate of F. semitectum biomass.

Gold, silver and Au/Ag alloy nanoparticles can also be fabricated by dissolving pure enzyme in deionized water followed by adding aqueous concentration of HAuCl4, AgNO3 and Au/Ag.

Chemical reduction

AuNPs of various sizes and shapes can be synthesised by chemical reduction of gold salts such as hydrogen tetrachloroaurate (HAuCl4) using citrate as the reducing agent which produces monodisperse spherical AuNPs in the 10–20 nm diameter range. However, production of larger AuNPs (40–120 nm) by this method proceeds in low yields, often resulting in polydisperse particles. Monodisperse AuNPs can be synthesised by seeding approach to have diameters between 30 and 100 nm. This method uses the surface of AuNPs as a catalyst for the reduction of Au3+ by hydroxylamine.

Borohydride-reduced AuNPs seeds of 3 to 4 nm dia are mixed with gold salt growth solution, rod-shaped micellar template (cetyltrimethylammonium bromide; CTAB), reducing agent (ascorbic acid), and small amount of silver ions for shape induction to produce spheroid or rod-like gold nanoparticles.

Other methods

Other methods for the synthesis of AuNPs include physical reduction (hollow Au nanostructures in large-scale), photochemical reduction (cubic AuNPs), biological reduction (molecular hydrogels of peptide amphiphiles for producing various shapes of AuNPs), and solvent evaporation techniques (2D Au super lattices). A simple and potentially cost effective method is microwave irradiation approach for the synthesis of shape-controlled AuNPs by which irradiation of Au salt, reduced in CTAB micellar media, in the presence of alkaline 2,7-dihydroxy naphthalene (2,7-DHN), generate exclusively spherical, polygonal, rods, and triangular AuNPs within few seconds.

Bimetallic AuNPs

Bimetallic AuNPs, such as Au–Ag, have also attracted attention due to their interesting catalytic, structural and electronic properties, and the sensitivity of their surface plasmon resonance (SPR) properties. Accordingly, the development of simple and robust methods for the synthesis of bimetallic nanoparticles is currently of great interest.

Spherical Au/Ag alloy nanoparticles whose SPR band could easily be tuned by varying the molar fractions of gold could be obtained by reduction of Au and Ag salt with sodium citrate in refluxing aqueous solution. A seed-mediated approach to synthesize Au-Ag core-shell nanorods from silver ions, using gold nanorods as seeds, has also been reported. Other methods for the synthesis of bimetallic AuNPs include sputter deposition technique in ionic liquids, photochemical synthesis, deposition of Au/Ag on silica and reverse microemulsion method to prepare silica-coated Au–Ag nanoparticles.

The AuNPs have unique chemical and optical properties, easy to fabricate and used in various molecular imaging and delivery applications. The unique biodistribution of AuNPs within tumors have led to the discovery of gold-based nanosystems as delivery vehicles for chemotherapeutic agents.

For bio imaging

Researchers have used various exogeneous agents to visualize key subcellular compartments. Cell imaging is achieved through the generation of colorimetric contrast between different cells/subcellular organelles by these imaging agents. Conventional exogeneous imaging agents such as lanthanide chelates and organic fluorophores are prone to photobleaching, low quantum yields, and broad emission window.

The shortcomings of the conventional imaging agents have limited their applications as biomedical diagnostic tools and have stimulated interest in typical nanomaterials, such as magnetic nanoparticles, Q-dots and AuNPs since they eliminate most of the vulnerabilities of the conventional imaging agents. But AuNPs are unique exceptions because they are more tolerable and compatible with cellular environment and more useful in many in vitro and in vivo application. In addition, the colorimetric contrast observed within the AuNPs treated cells could be controlled by size, shape or even surface modification of the AuNPs due to a phenomenon called surface plasmon resonance (SPR).

When excited, the SPR of AuNPs could scatter and/or absorb light in the visible or the near-infrared (NIR) spectrum by which in vivo optical imaging such as photoacoustic and two-photon luminescence imaging can be done as it generates cellular contrast by tuning the SPR of the AuNPs to the NIR spectrum. Other noninvasive diagnostic tools such as MRI and X-ray computed tomography (X-ray CT) have utilized AuNPs as contrasting agent due to the ease of surface modifi cation and higher X-ray absorption coefficient, respectively.