As it is well-known the optical properties of gold nanomaterials are highly size and shape dependant.
Particularly, in the case of spherical gold nanoparticles, they absorb light in the UV-Vis range (sensitive range for human eye) starting from 515 nm to longer wavelength (red shift) as the size of the gold nanoparticles increases. Moreover, larger gold spheres (>50 nm) scatter light owing to their larger optical cross sections, therefore large gold nanoparticle are a good candidate for labelling due to their high electronic contrast.
When light strikes a gold nanoparticle, the electron cloud of the metal will sense the the electromagnetic field of the incident beam and start to oscillate collectively at the same frequency as the incident light. This process is known as Localized Surface Plasmon Resonance (LSPR), and it could occur in any nanomaterial which an adequate density of free electrons. During this process the incident light can be re-radiated in all directions but with the same frequency, that is, the light is scattered.
On the other hand, the light is also converted into heat due to the simultaneous absorption of light. Both processes are responsible of the intensity attenuation of the incident light, and their combination is known as extinction. Furthermore, the interaction between light and matter leads to the production of electric field on the nanomaterial surface, and for instance this phenomena could be used to modify the optical signal of compound immobilized on the nanomaterial surface. All the previous interactions described can be employed as a tool in the development of new sensing platforms or to enhance the present bioapplications.
Apart from the optical properties described above, there are some features that make gold nanoparticles a crucial candidate for use in biomedical or clinical applications. For instance, the gold chemical inertness could be an essential advantage to perform in vivo assays or applications. Furthermore, the well-known strong bind between thiolated molecules or biomolecules to gold surface offers multiple possibilities to surface functionalization of nanoparticles that broaden the range of potential applications of this nanomaterial: sensing, optical imaging, drug delivery and cancer therapy.
An important aspect to be taken into account in the development of any application based on gold nanoparticles is their quality, that is, size distribution, uniformed shape or surface stabilization. Citrate stabilized gold nanoparticles are a good alternative since citrate is a easy displaceable ligand which could be exchanged to cover the gold surface with thiolated ligands or biomolecules such as proteins, peptides, antibodies or DNA. This versatile surface chemistry allows the spread the application of gold nanoparticles in bioapplications.
Another interesting advantage of gold nanoparticles is their irreversible aggregation which could be employed as a detection system in the development of sensing applications.
If gold nanoparticles of an adequate size aggregates, this aggregation cause an interparticle surface Plasmon coupling, that is, there is a colour change from red to blue visible by naked eye. This colour change is widely employed in the development of colorimetric sensing platforms that directly or indirectly induces nanoparticle aggregation or redispersion. Several applications have been developed in order to detect the presence of ions, small molecules or proteins. It must be highlighted the regular use of 40 nm gold nanoparticles (usually conjugated to an antibody that is specific to the target analyte) in the development of Lateral Flow Assays (LFA) because of nanoparticle high extinction coefficient, higher than an organic dye such as rhodamine.