US Patent 7906223 - Permeable nanoparticle reflector

http://www.freepatentsonline.com/7906223.html

This patent from 3M teaches a type of optical indicator used in chemical and bio-sensing which uses metallic nanoparticles. Claim 1 reads:

1. A method for forming an optically-responsive multilayer reflective article, which method comprises

applying a dilute solution or suspension of metallic nanoparticles to an optically-responsive detection layer and

allowing the solution or suspension to dry to form a semicontinuous liquid- or vapor-permeable light-reflective layer that has the nanoparticles contacting each other in a stacked arrangement and that will permit a liquid or vapor analyte to pass through the light-reflective layer to cause an optically-responsive change in the detection layer in the presence of the analyte.