Ralph Sperling of Philipps-University Marburg, Germany, explains why gold is so precious to biological scientists.

Gold particles can be real jewels - at least at the nano size they are in great demand by scientists. An inspiration to science from the time of Faraday, today gold nanoparticles are being used for an ever-growing number of applications.

Gold nanoparticles have many applications in biology, including immunostaining and delivering drugs or DNA into cells

A field that has showed fast growth over the past decades is the use of gold nanoparticles in biology, or life sciences. These bioapplications can be classified into four areas: labelling, delivery, heating, and sensing. 

For labelling, certain properties of the particles are exploited to generate contrast. For example in transmission electron microscopy, the strong electron absorbing properties of gold nanoparticles make them suitable as a stain for samples with poor contrast, such as tissue samples. Their small size and the possibility of functionalising the particles, for instance with antibodies (immunostaining), mean that they also provide extremely high spatial resolution and specificity in many labelling applications. Similarly, the particles' optical properties - strong absorption, scattering and especially plasmon resonance - make them of value for a large variety of light-based techniques including combined schemes such as photothermal or photo-acoustic imaging. In addition, gold nanoparticles can be radioactively-labelled by neutron activation, which allows for very sensitive detection, and used as an x-ray contrast agent.

Secondly, gold nanoparticles can serve as carriers for drug and gene delivery. Biologically active molecules adsorbed on the particle surfaces can be guided inside cells and released. DNA delivery, for instance, is the basis for gene therapy.

Thirdly, their strong light absorbing properties makes gold nanoparticles suitable as heat-mediating objects; the absorbed light energy is dissipated into the particles' surroundings, generating an elevated temperature in their vicinity. This effect can be used to open polymer microcapsules, for example, for drug delivery purposes. What's more, appropriately functionalised nanoparticles might bind specifically to certain cells, which might one day find a use in cancer targeting and hyperthermal therapy by heating the particle-loaded tissue in order to destruct the malignant cells. However, for such in vivo applications, the potential cytotoxicity of the nanoparticles might become an issue and should be investigated with care. So far very little is known about the implications for organisms or environmental systems in contact with nanosized materials.

Finally, gold nanoparticles can also be used as sensors. Their optical properties can change upon binding to certain molecules, allowing the detection and quantification of analytes. The absorption spectra of gold nanoparticles change drastically when several particles come close to each other. In the business of colloids aggregation is actually rather annoying but it can be exploited for very sensitive DNA detection, even of a single-base mismatch. 

Another strategy for sensing makes use of fluorescence quenching. Fluorescent molecules that are excited and in close proximity to a gold particle can transfer their energy to the metal, resulting in a non-radiative relaxation of the fluorophore. In several different detection schemes the analyte displaces the fluorescent molecules from the particle surface or changes their conformation, so that the optical emission of those reporter molecules is changed in the presence of the analyte.

Whilst many of the unique optical properties of gold nanoparticles have been exploited in recent applications, there is still plenty of room for new research. This should eventually lead to well-established, routinely-used assays for a variety of biological applications in the near future.

Read more in the critical review 'Biological applications of gold nanoparticles' in the thematic issue covering the topic of gold: chemistry, materials and catalysis in issue 9, 2008, of Chemical Society Reviews.