Gold nanoparticles (AuNps) are minute sized particle of gold with reduced dimentionality down to nanometers. Nano is a prefix which stands for 1/1 000 000 000 of a meter. They have synthesized for a long time, dating back to medieval times where different sized AuNps are exploited to stained glass windows in churches. Gold nanomaterials larger than 2nm possess metallic characteristics and only behave like a semi-conductor once the nanomaterials are reduced below 2 nm. The semi-conducting AuNps have been reported to induce fluorescence signal due to its valence bands. AuNps are most chemically inert out of the noble metals, stable and robust hence it is by far the preferred nanomaterials for wide range of applications, biomedical field especially for bioimaging and cancer therapies.
AuNps come in various sizes and shapes such as spheres, rods and cubes and their electric, optical and magnetic properties of AuNps differ from bulk gold. Furthermore, these properties are tunable by varying the sizes, shapes and structures of the AuNps via different nanofabrication routes published by an ever-increasing list of literatures. These include hexagonal plates, tetrahedral, octahedral, cubes and rods. These form the building blocks for much complicated morphology such as irregular, branched and multipods which are two to three dimensional clusters superlattices. However, all protocols are very unique with diverse conditions and there are no one method which is particularly reproducible or been rationalized. The electric , optical, transport and magnetic properties of these structures vary with the interaction and the coupling among the AuNps self-assembly. For instance, a thin film self-assembly involve self-organissation into monolayers whereas superlattice preparation require encapsulation of the AuNps by organic coating. The encapsulating agents function as interparticle molecular bonds in order to avoid coalescence. This is a variable which is adjustable to fine tune the amount of interparticle interaction desired through molecular chain length adjustments. Different syntheses of gold nanoparticles and their properties are explored, beginning with a summary of the basic properties of AuNps.
Electrons and protons are well accepted representation to describe matter in chemistry. Electrons interact according to Coulomb’s law and form the basis of bond forming and it is a key property to nanotechnology including AuNps.
F=Q1Q2 / r2
R is the distance between two atoms where F is the force present, Q1 and Q2 are the charges on the atoms. F is positive when two charges of the same sign, like charges repel; unlike charges attract and F is negative.
However, physics of nanomaterials are no longer dominated by Newton’s formulas and electrons behave as both waves and particles. This new theory is known as quantum mechanics. First of all, the surface to volume ratio progressively increase as bulk gold gets smaller in which the behaviors of surface atoms shift to quantum behaviors apply over the interior of the particle. This also influences the interaction with other materials. AuNps have long term chemical stability, increased strength and heat resistance due the large surface area of AuNps which allow them to interact efficiently in nanocomposites . Electron distributions over AuNps will cause a size effect and lead to quantized energy levels. Quantum dots are used to describe atoms with free/excited electrons confined to only occupy a small energy range. This controllable energy states has ignited the huge application researches of nanomaterials especially gold because of the ability to program a matter with desired properties. For example, the manipulation of light using minute sized AuNps allow AuNps to appear to be transparent once the dimensions of the AuNps are below the critical wavelength of light. This is particularly useful for coating and cosmetic applications.
This lead to optical properties of AuNps. AuNps can be seen in different colours according to different sizes and shapes. There are unique optical and electronic properties of AuNps are different from the bulk gold. Bulk gold has a yellow appearance oppose to the wine red colour in nanoparticulate gold attributed to “surface plasmonic resonance”. Plasmon refers to the oscillations of the electron cloud caused by the absorption of photons being converted to vibrations instead of reflection. There are 6 free electrons to each gold surface and they are known as the conduction electrons. Spherical AuNps show Plasmon resonance in the form of an absorption peak. An intense light absorption occurs when these electrons are excited by an electromagnetic field due to a synchronized vibration of electrons on surface of AuNps. These collective oscillations of conduction electrons within particles give strong interactions with visible light. The oscillation would dissipate as heat eventually whilst the reflected portion is a result of electric fields built up near the surface of AuNps and the expected maximum absorption peak at 510nm and is in the visible region of the electromagnetic spectrum. Both the width of Plasmon absorption band and the resonance frequencies depends on the nanoparticulate size of AuNps. Wavelengths increase with increasing size of AuNps. This resonant excitations behavior also enhance local electromagnetic field for particles near AuNps thus resulting in stronger light scattering by 2 to 6 orders of magnitude. This an important unique property in biolabelling applications using surface-enhanced Raman scattering in particular.