Editing Polyelectrolyte (section)

== Multilayers ==

Polyelectrolytes have been used in the formation of new types of materials known as '''polyelectrolyte multilayers''' ('''PEMs'''). These thin films are constructed using a '''layer-by-layer'' ('''LbL''') deposition technique. During LbL deposition, a suitable growth substrate (usually charged) is dipped back and forth between dilute baths of positively and negatively charged polyelectrolyte solutions. During each dip, a small amount of polyelectrolyte is adsorbed, and the surface charge is reversed, allowing the gradual and controlled build-up of electrostatically [[cross-link]]ed films of polycation-polyanion layers. Scientists have demonstrated thickness control of such films down to the single-nanometer scale. LbL films can also be constructed by substituting charged species such as [[nanoparticle]]s or [[Zeolite|clay platelet]]s<ref>{{cite journal|title=Layer-By-Layer Assembly Of Zeolite Crystals On Glass With Polyelectrolytes As Ionic Inkers|doi=10.1021/ja010517q|year=2001|last1=Lee|first1=Goo Soo|last2=Lee|first2=Yun-Jo|last3=Yoon|first3=Kyung Byung|journal=Journal of the American Chemical Society|volume=123|issue=40|pages=9769–79|pmid=11583538}}</ref> in place of or in addition to one of the polyelectrolytes. LbL deposition has also been accomplished using [[hydrogen bond]]ing instead of [[electrostatic]]s. For more information on multilayer creation, please see [[polyelectrolyte adsorption]]. 
[[File:Polyelectrolyte multilayer formation.jpg|thumbnail|right|Formation of 20 layers of PSS-PAH polyelectrolyte multilayer measured by multi-parametric surface plasmon resonance]]
An LbL formation of PEM (PSS-PAH (poly(allylamine) hydrochloride)) on a gold substrate can be seen in the Figure. The formation is measured using [[multi-parametric surface plasmon resonance]] to determine adsorption kinetics, layer thickness, and optical density.<ref>{{cite journal|title=Characterizing Ultrathin and Thick Organic Layers by Surface Plasmon Resonance Three-Wavelength and Waveguide Mode Analysis|doi=10.1021/la401084w|year=2013|last1=Granqvist|first1=Niko|last2=Liang|first2=Huamin|last3=Laurila|first3=Terhi|last4=Sadowski|first4=Janusz|last5=Yliperttula|first5=Marjo|last6=Viitala|first6=Tapani|journal=Langmuir|volume=29|issue=27|pages=8561–71|pmid=23758623}}</ref>

The main benefits of PEM coatings are the ability to conformably coat objects (that is, the technique is not limited to coating flat objects), the environmental benefits of using water-based processes, reasonable costs, and the utilization of the particular chemical properties of the film for further modification, such as the synthesis of [[metal]] or [[semiconductor]] nanoparticles, or [[porosity]] phase transitions to create [[anti-reflective coating]]s, optical [[shutter (photography)|shutters]], and [[superhydrophobic]] coatings.