Chapter 14 - Synthesis, characterization, and optical response of gold nanoshells used to trigger release from liposomes.

Liposomes show great promise as intravenous drug delivery vehicles, but it is often difficult to combine stability in the circulation with rapid, targeted release at the site of interest. Targeting to specific tissues requires developing highly specific ligands with strong affinities to receptors overexpressed on diseased cells; a new cellular target requires developing new ligands and identifying new receptors. Novel photoactivated, hollow, gold nanoshell (HGN)/liposome composites provide a new approach to both controlled release and specific targeting. HGN are extremely efficient near infrared (NIR) light absorbers, and are not susceptible to photobleaching like conventional dyes. Near-complete liposome contents release can be initiated within seconds by irradiating HGNs with an NIR pulsed laser. Targeting the drug is limited only by the dimensions of the laser beam; no specific ligands or antibodies are required, so different tissues and cells can be targeted with the same HGN/liposomes. HGNs can be encapsulated within liposomes or tethered to the outer surface of liposomes for the most efficient drug release. HGNs in liposome solutions can also trigger release, but with lower efficiency. Drug release is induced by adsorbing femto- to nanosecond NIR light pulses that cause the HGNs to rapidly increase in temperature. The resulting large temperature gradients lead to the formation of vapor microbubbles in aqueous solutions, similar to the cavitation bubbles induced by sonication. The collapse of the unstable vapor bubbles causes liposome-membrane rupture and contents release, with minimal damage to the surroundings, and little overall heating of the solution.