Gated hybrid delivery systems: En route to sensory materials with inherent signal amplification

• Nanoscopic porous architectures equipped with chemically or physically addressable gates are ideal for controlled delivery.
• Massive release of cargo molecules from voids controlled by few surface-bound gates exemplifies true (signal) amplification.
• If indicators are used as cargo and the release is fast, potent sensing systems can be created.
• Gating has been achieved by light, pH, metal ions, anions, antibodies or enzymes.

Hybrid nanoparticles with a large specific surface area are a particularly exciting vehicle for delivery applications. Such highly porous or container-like structures, usually prepared from silica and frequently from gold, can incorporate a large number of chemical substances such as drug and/or indicator molecules. When equipped with a chemically or physically addressable gating function at the openings of the voids, the release of the cargo can be controlled at will. Because many more molecules can be stored as cargo in the pores of the support than there are functional groups as anchoring sites for the gating entities attached to the outer surface (for efficient pore capping), the systems possess inherent features of (signal) amplification. The present article will introduce various design strategies for different types of physical (light, temperature, magnetism) and chemical (pH, metal ions, anion, small organic molecules, enzymes) stimuli in connection with drug and indicator release. We will highlight exciting aspects of combining both features in theranostic applications and will stress which requirements still have to be met by many of the systems to be readily applicable in a sensory context.

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