Editing Colloidal gold (section)

==== 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>