Decoration of gold nanoparticles by a double-armed calix[4]pyrrole: a receptor-decorated nanoensemble for anion sensing and extraction. 
Gold nanoparticles decorated with a double-armed, deep-cavity calix[4]pyrrole were prepared and fully characterized.  Transmission electron microscopy imaging revealed that the average diameter of the particles was approximately 4 nm both before and after attachment of the receptor to the surface.  The calix[4]pyrrole-functionalized nanoparticles exhibited highly elevated sensing behavior (approximately 1000 times in dichloromethane) relative to its monomeric congener while maintaining its guest selectivity.  The receptor-nanoparticle conjugate (nanoreceptor) showed significant aggregation upon addition of the biphenolate anion, an effect ascribed to anion-mediated interparticle linking.  The receptor-nanoparticle conjugate is also capable of extracting the fluoride anion (as its tetrabutylammonium salt) from an aqueous layer to an organic medium.  Control experiments revealed that this extraction is not possible when using the analogous monomeric receptor.