Lanthanide-coated gold nanoparticles for biomedical applications

• The route to the development of water-soluble gold nanoparticles for functionalisation.
• Advantages of metal complexes as biological imaging agents.
• Approaches to coating nanoparticles with luminescent metal complexes.
• Gadolinium coated gold nanoparticles for magnetic resonance imaging.

The use of gold nanoparticles as a scaffold on which to assemble molecular architectures presents a versatile approach to produce nanoprobes with modalities defined by the properties of the capping architectures at the molecular level. Lanthanide ions, with their characteristic magnetic and luminescent properties, are ideal probes for imaging applications. Gold nanoparticles represent an inert, biocompatible and rigid scaffold on which to assemble molecular complexes using clean, facile chemistries. The size of the gold nanoparticles can be easily tuned using robust and established chemistry. The combination of the stable, long-lived luminescence signal or characteristic magnetic properties afforded by the trivalent lanthanide ions with the size-tuneable inert gold core allows the formation of tailored nanoprobes ideally suited to the investigation of biological systems and biomedical applications. In this review the different methods for attachment of lanthanide complexes onto gold nanoparticles are presented for the production of nanoprobes with luminescence and magnetic resonance signal outputs. A short introduction outlining the development of the preparation methods for water-soluble gold nanoparticles is presented, followed by the approaches to the functionalisation of gold nanoparticles with metal-complexes which set the scene for the lanthanide studies. Two approaches of introducing lanthanides onto gold nanoparticles are identified and the systems are grouped in the discussion under the attachment of pre-assembled lanthanide complexes and the employment of ligands on nanoparticles that bind lanthanides. The preparation and properties of lanthanide-coated nanoparticles are then discussed in detail as luminescent visible and near infrared probes. Finally, the development of gold nanoparticles as a platform on which to assemble Gd(III) complexes for magnetic resonance imaging applications is discussed.