Editing Colloidal gold (section)

== Physical properties ==

=== Optical ===
[[File:Cross section vs. wavelength of Au nanoparticle cropped.png|thumb|upright=1.15|The variation of scattering [[Cross section (physics)|cross section]] of 100 nm-radius gold nanoparticle vs. the wavelength]]
Colloidal gold has been used by artists for centuries because of the nanoparticle’s interactions with visible light. Gold nanoparticles absorb and scatter light<ref>{{Cite journal|last1=Anderson|first1=Michele L.|last2=Morris|first2=Catherine A.|last3=Stroud|first3=Rhonda M.|last4=Merzbacher|first4=Celia I.|last5=Rolison|first5=Debra R. | name-list-style = vanc |date=1999-02-01|title=Colloidal Gold Aerogels: Preparation, Properties, and Characterization |journal=Langmuir|volume=15|issue=3|pages=674–681|doi=10.1021/la980784i }}</ref> resulting in colours ranging from vibrant reds (smaller particles) to blues to black and finally to clear and colorless (larger particles), depending on particle size, shape, local refractive index, and aggregation state. These colors occur because of a phenomenon called [[Localized surface plasmon|localized surface plasmon resonance]] (LSPR), in which conduction electrons on the surface of the nanoparticle oscillate in resonance with incident light.

==== 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>

==== Effect of local refractive index ====
Changes in the apparent color of a gold nanoparticle solution can also be caused by the environment in which the colloidal gold is suspended.<ref name="Ghosh 13963–13971">{{Cite journal|last1=Ghosh|first1=Sujit Kumar|last2=Nath|first2=Sudip|last3=Kundu|first3=Subrata|last4=Esumi|first4=Kunio|last5=Pal|first5=Tarasankar | name-list-style = vanc |date=2004-09-01|title=Solvent and Ligand Effects on the Localized Surface Plasmon Resonance (LSPR) of Gold Colloids |journal=The Journal of Physical Chemistry B|volume=108|issue=37|pages=13963–13971|doi=10.1021/jp047021q }}</ref><ref name="Underwood 3427–3430">{{Cite journal|last1=Underwood|first1=Sylvia|last2=Mulvaney|first2=Paul| name-list-style = vanc |date=1994-10-01|title=Effect of the Solution Refractive Index on the Color of Gold Colloids |journal=Langmuir |volume=10 |issue=10 |pages=3427–3430 |doi=10.1021/la00022a011 }}</ref> The optical properties of gold nanoparticles depend on the refractive index near the nanoparticle surface, so the molecules directly attached to the nanoparticle surface (i.e. nanoparticle ligands) and the nanoparticle [[solvent]] may both influence the observed optical features.<ref name="Ghosh 13963–13971"/> As the refractive index near the gold surface increases, the LSPR shifts to longer wavelengths.<ref name="Underwood 3427–3430"/> In addition to solvent environment, the [[Complex index of refraction#Complex refractive index|extinction peak]] can be tuned by coating the nanoparticles with non-conducting shells such as [[silica]], [[biomolecule]]s, or [[aluminium oxide]].<ref>{{Cite journal |last1=Xing |first1=Shuangxi |last2=Tan |first2=Li Huey |last3=Yang |first3=Miaoxin |last4=Pan |first4=Ming |last5=Lv |first5=Yunbo |last6=Tang |first6=Qinghu |last7=Yang |first7=Yanhui |last8=Chen |first8=Hongyu |s2cid=96293198 | name-list-style = vanc |date=2009-05-12|title=Highly controlled core/shell structures: tunable conductive polymer shells on gold nanoparticles and nanochains |journal=Journal of Materials Chemistry |volume=19 |issue=20 |doi=10.1039/b900993k |page=3286}}</ref>

==== Effect of aggregation ====
When gold nanoparticles aggregate, the optical properties of the particle change, because the effective particle size, shape, and [[dielectric]] environment all change.<ref>{{cite journal | vauthors = Ghosh SK, Pal T | s2cid = 46326525 | title = Interparticle coupling effect on the surface plasmon resonance of gold nanoparticles: from theory to applications | journal = Chemical Reviews | volume = 107 | issue = 11 | pages = 4797–862 | date = November 2007 | pmid = 17999554 | doi = 10.1021/cr0680282 }}</ref>