1. Gold Nanoparticles
Gold nanoparticles are one type of metallic nanoparticle; others are Ni, and TiO2 nanoparticles.
It has advantages over other metal nanoparticles due to their biocompatibility and non-cytotoxicity.
Nanoparticles are nanometers in size.
These are 100 to 1000 times smaller than human cells.
The size of gold nanoparticles can be controlled during their synthesis and functionalization with different groups. Gold nanoparticles accumulate in the tumour cells and show optical scattering.

3. Characteristics of Gold Nanoparticles
Gold nanoparticles are chemically inert, These have greater biological compatibility, Optical properties like plasmon resonance are exhibited by gold nanoparticles, These exhibit versatility because of their ready functionalization through thiol linkages,
Gold nanoparticles provide microscopic probes for the study of the cancer cell, accumulate in the cancerous cell and show the cytotoxic effect i.e. apoptosis or necrosis of the specific cell and cell specific receptor, These have high stability due to the gold-sulphur bonds, Their photo physical properties can be exploited for drug release at remote place.

4. Types of gold nanoparticles
Gold nanoparticles (<50 nm) can be prepared with different geometries, such as
nanospheres
nanoshells
nanorods
nanocages
These particles show localized surface plasmon resonant properties, i.e. under the irradiation of light, the conduction electrons are driven by the associated electric field to a collective oscillation at a resonant frequency, thereby absorbing light and emitting photons with the same frequency in all directions.
Gold nanoparticles are excellent labels for biosensors because they can be detected by numerous techniques, such as optic absorption, fluorescence and electric conductivity.

5. Gold nanospheres
Gold nanospheres of 2 nm to over 100 nm in diameter
It can be synthesized by controlled reduction of an aqueous HAuCl4 solution using different reducing agents under varying conditions.
The most commonly used reducing agent is citrate, which can produce nearly monodisperse gold nanospheres.
The size of the nanospheres can be controlled by varying the citrate/gold ratio. Generally, smaller amount of citrate will yield larger nanospheres.

6. The major limitations of this method are the low yield and the restriction of using water as the solvent.
Many other methods have been investigated for gold nanosphere synthesis such as the use of other reductants or ligands.
There are a number of literature reports on the use of dendrimers as templates or stabilizers for gold nanosphere preparation