Editing Colloidal gold (section)

==== Effect of size, shape, composition and environment ====
As a general rule, the wavelength of light absorbed increases as a function of increasing nanoparticle size.<ref name="Link 4212–4217">{{Cite journal|last1=Link|first1=Stephan|last2=El-Sayed|first2=Mostafa A. | name-list-style = vanc |date=1999-05-01|title=Size and Temperature Dependence of the Plasmon Absorption of Colloidal Gold Nanoparticles |journal=The Journal of Physical Chemistry B|volume=103|issue=21|pages=4212–4217|doi=10.1021/jp984796o |citeseerx=10.1.1.596.6328}}</ref> Both the [[surface plasmon resonance]] frequency and scattering intensity depend on the size, [[Shape control in nanocrystal growth|shape composition]] and environment of the nanoparticles. This phenomenon may be quantified by use of the [[Mie scattering]] theory for spherical nanoparticles. Nanoparticles with diameters of 30–100&nbsp;nm may be detected easily by a microscope, and particles with a size of 40&nbsp;nm may even be detected by the naked eye when the concentration of the particles is 10<sup>−4</sup>&nbsp;M or greater. The scattering from a 60&nbsp;nm nanoparticle is about 10<sup>5</sup> times stronger than the emission from a [[fluorescein]] molecule.<ref>{{Cite journal |last1=Huang |first1=Xiaohua |last2=Jain |first2=Prashant K |last3=El-Sayed |first3=Ivan H |last4=El-Sayed |first4=Mostafa A |date=October 2007 |title=Gold nanoparticles: interesting optical properties and recent applications in cancer diagnostics and therapy |url=https://www.futuremedicine.com/doi/10.2217/17435889.2.5.681 |journal=Nanomedicine |language=en |volume=2 |issue=5 |pages=681–693 |doi=10.2217/17435889.2.5.681 |pmid=17976030 |issn=1743-5889|url-access=subscription }}</ref>